Substituent Effects on the Stability of Thallium and

molecules Article Article

Substituent Effects on the Stability of Thallium and Effects on the Stability of Thallium and ThalliumSubstituent andPhosphorus Triple Bonds: A Density Functional Triple Bonds: A Density FunctionalPhosphorus Study Functional Study Jia-Syun Lu 1, Ming-Chung Yang 1 and Ming-Der Su 1,2,* 1 , Ming-Chung Yang 1 and Ming-Der Su 1,2, * Jia-Syun 1LuDepartment of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan;

[email protected] (J.-S.L.); [email protected] (M.-C.Y.) Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan; 2 Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, [email protected] (J.-S.L.); [email protected] (M.-C.Y.) 2 Taiwan iversity, Kaohsiung 80708, Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan * Correspondence: [email protected]; Tel.: +886-5-2717964 * Correspondence: [email protected]; Tel.: +886-5-2717964

004, Taiwan;

1

12 June 2017; Accepted: 29 June 2017; Published: 5 July 2017 Received:Received: 12 June 2017; Accepted: 29 June 2017; Published: 5 July 2017

Abstract: Three computational (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/ Abstract: Three computational methodsmethods (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/ LANL2DZ+dp) were used to study the effect of substitution on the potential energy surfaces W91/Def2-TZVP and B3LYP/ LANL2DZ+dp) were used to study the effect of substitution on the potential energy surfaces of RTl≡PR 3, SiH 3, SiMe(SitBu )2, SiiPrDis 2, Tbt 6H2-2,4,6-(CH(SiMe )3),and and Ar* F,OH, OH,H,H, ntial energy surfacesRTl of ≡ RTl≡PR PR(R(R == F, CHCH (=C6(=C H2 -2,4,6-(CH(SiMe 3 , SiH 3 , SiMe(SitBu 3 )2 , 3SiiPrDis 2 , Tbt 3 )2 3))32), (=C 6 H 3 -2,6-(C 6 H 2 -2, 4,6-i-Pr 3 ) 2 )). The theoretical results show that these triply bonded RTl≡PR 2,4,6-(CH(SiMe3)2)3),Ar* and (=CAr* H -2,6-(C H -2, 4,6-i-Pr ) )). The theoretical results show that these triply bonded RTl ≡ PR 6 3 6 2 3 2 ◦ ◦ ∠ ∠ compounds have a preference for a bent geometry (i.e., R Tl P ≈ 180° and Tl P R ≈ 120°). these triply bonded RTl≡PR have a preference for a bent geometry (i.e., ∠R–Tl–P ≈ 180 and ∠Tl–P–R ≈ 120 ). Two compounds valence bond models used interpretthe thebonding bondingcharacter characterofofthe theTlTl≡P 80° and ∠Tl P R ≈Two 120°).valence Two bond models areare used totointerpret ≡P triple bond. One is [I], which Thisinterprets interprets the the bonding bonding conditions f the Tl≡P triple bond. is One One is model which isisbest bestdescribed describedasasTlTl P.P.This conditions for for RTl≡PR feature small ligands. TheThe other is model [II], which is bestisrepresented as Tl P. onding conditions RTl for ≡ RTl≡PR PRmolecules moleculesthat that feature small ligands. other is model [II], which best represented This bondingcharacter characterof RTl of ≡ RTl≡PR molecules that feature large substituents. best represented asas TlTl P. P. Thisexplains explains the bonding PR molecules that feature large substituents. Irrespective of the of substituents for≡the the theoretical investigations t feature large substituents. Irrespective of the types oftypes substituents used forused the RTl PRRTl≡PR species,species, the theoretical investigations (based on the natural bond orbital, the natural resonance the decomposition charge decomposition , the theoretical investigations (based on the natural bond orbital, the natural resonance theory, theory, and the and charge analysis) demonstrate that their Tl≡Pbonds triple are bonds veryHowever, weak. However, the theoretical nd the charge decomposition analysis) demonstrate that their Tl≡ P triple veryare weak. the theoretical results results predict that only bulkier substituents greatly stabilize the triply bonded RTl≡PR species, owever, the theoretical results predict that only bulkier substituents greatly stabilize the triply bonded RTl≡PR species, from thefrom the kinetic viewpoint. ded RTl≡PR species, fromviewpoint. the kinetic Keywords: triply bonded molecules; tripleacetylene; bond; acetylene; substituent Keywords: triply bonded molecules; triple bond; substituent effects effects

ent effects

1. Introduction 1. Introduction

The preparation and characterization of triply bonded heavier main main groupgroup element (E14 = The preparation and characterization of triply bonded heavier element (ESi, 14 = Si, Ge, Ge, Sn, and Pb) molecules (i.e., RE ≡ E R) is a popular field of study in inorganic chemistry [1–41]. ain group element (E14 = Si,Sn, Ge,and Pb) molecules (i.e.,14RE14≡E 14 14R) is a popular field of study in inorganic chemistry [1–41]. From From the valence electron viewpoint, the triply bonded RE13 ≡RE E1513R≡E compound is isoelectronic to the to the the valence electron viewpoint, the triply bonded 15R compound is isoelectronic rganic chemistry [1–41]. From RE14 ≡to ERE R species. However, the former has been the subject of much less study than the latter, inlatter, the in the 14 ≡E 14 R species. However, the former has been the subject of much less study than the pound is isoelectronic the 14 ofinsynthetic chemistry. Therefore, the levelthe of understanding of the chemistry of RE13 ≡E R is field chemistry. Therefore, level of understanding of the chemistry of15RE 13≡E15R is ess study than thefield latter, the of synthetic that for that group less-coordinate alkyne analogues. lower than for14group 14 less-coordinate alkyne analogues. the chemistry of lower RE13≡Ethan 15 R is In the group family, is more issimilar its diagonal relative,relative, carbon, carbon, than to than to In the 15 group 15 phosphorus family, phosphorus more to similar to its diagonal nitrogen [42]. is alsoisknown to betomonovalent andand hashas an an ionic radius nitrogen [42]. Thallium also known be monovalent ionic radiusthat thatisissimilar similar to that onal relative, carbon, than to Thallium to thattoofthat potassium,sosoititis is often presumed to abe a pseudo [43]. The isolation and potassium, often presumed to be pseudo alkalialkali metalmetal [43]. The isolation and characterization nic radius that is similar characterization of the singly bonded organothallium phosphorus molecule, (Me SiCH ) Tl–P(SiMe of the singly bonded organothallium phosphorus molecule, (Me e isolation and characterization 3 3SiCH 2 3 2)3Tl P(SiMe 3 )33),3, was was3)experimentally reported about twenty yearsyears ago [44]. that contain the reported about twenty agoTwo [44].other Two novel other compounds novel compounds that contain the (Me3SiCH2)3Tl P(SiMe 3, experimentally was single bond have also been identified [45,46]. If both thallium and phosphorus phosphorus single bond have also been identified [45,46]. If both thallium and phosphorus el compounds thatthallium–phosphorus containthallium the could be stabilized using ausing single bond bond to connect them,them, it might be possible to extend elements could be stabilized a single to connect it might be possible to extend this both thallium andelements phosphorus field to the study of other triply bonded RTl≡ PR inorganic molecules. This work reports thethe first field to the study of other triply bonded RTl≡PR inorganic molecules. This work reports ight be possible tothis extend this first theoretical study of the possible synthesis of the RTl ≡ PR molecule, which may be isolable theoretical study of the possible synthesis of the RTl≡PR molecule, which may be isolable as a es. This work reports the first as a long-lived TheThe study determines potential inorganic complexes thatthat can can stabilize long-lived compound. study determines potential inorganic complexes stabilize the which may be isolable as a compound. mplexes that can stabilizeMolecules the 2017, 22, 1111; doi:10.3390/molecules22071111 www.mdpi.com/journal/molecules Molecules 2017, 22, 1111; doi:10.3390/molecules22071111 www.mdpi.com/journal/molecules

www.mdpi.com/journal/molecules

Molecules 2017, 22, 1111

2 of 14

the thallium≡phosphorus triple bond, to demonstrate the theoretical possibility that these unusual acetylene inorganic analogues can be synthesized. 2. Methodology Using the Gaussian 09 program package [47], all geometries are fully optimized at the M06-2X [48], B3LYP [49,50], and B3PW91 [51,52] levels of theory, in conjunction with the Def2-TZVP [53] and LANL2DZ+dp [54–58] basis sets. These DFT calculations are signified as M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ+dp, respectively. In order to confirm that the reactants and products have no imaginary frequencies and that the transition states possess only one imaginary frequency, frequency calculations were performed for all structures. Thermodynamic corrections to 298 K, heat capacity corrections and entropy corrections (∆S) are applied to the three levels of DFT. The relative free energy (∆G) at 298 K is also computed at the same levels of theory. Next, (SiiPrDis2 )Tl≡P(SiiPrDis2 ), (Tbt)Tl≡P(Tbt), and (Ar*)Tl≡P(Ar*) are the model reactants for this study. It is known that the B3LYP functional fails to describe non-valent interactions, such as the London dispersion correctly. As a result, for large ligands, calculations were performed using dispersion-corrected M06-2X method [48]. Because of the limitations of the available memory size and CPU time, frequencies are not computed at the dispersion-corrected M06-2X/Def2-TZVP level of theory for the triply bonded R´Tl≡PR’ systems that have bulky ligands (R’), so the zero-point energies and the Gibbs free energies that are derived using the dispersion-corrected M06-2X/Def2-TZVP cannot be used for these systems. 3. General Considerations Two interaction models that describe the chemical bonding of the triply bonded RTl≡PR, which serve as a basis for discussion, are given in this section. For convenience, the RTl≡PR molecule is divided into two fragments: Tl–R and P–R. On the basis of theoretical results (see below), three computational methods (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ+dp) all indicate that the Tl–R and P–R fragments are respectively calculated to be in the singlet ground state and the triplet ground state. In model [I], electron promotion energy (∆E1 ) forces the P–R moiety from the triplet ground state to the singlet excited state, so the electronic structure of RTl≡PR can be described in terms of the dimerization of singlet Tl–R and singlet P–R fragments, as shown in Figure 1. From the chemical bonding viewpoint, model [I] shows that the Tl≡P triple bond consists of one σ–donation of Tl→P and two π–donations of Tl←P. In model [II], the electron advancement energy (∆E2 ) promotes the Tl–R unit from the singlet ground state to the triplet excited state. Accordingly, the bonding structure of RTl≡PR can also be represented as the dimerization of triplet Tl–R and triplet P–R fragments, as shown in Figure 1. From the bonding structure viewpoint, model [II] shows that the Tl≡P triple bond is composed of one Tl←P π–bond, one regular σ–bond and one π–bond. It is schematically shown in Figure 1 that the formation of the triply bonded RTl≡PR molecule can be regarded as either [Tl–R]1 + [P–R]1 → [RTl≡PR]1 (model [I]) or [Tl–R]3 + [P–R]3 → [RTl≡PR]1 (model [II]). It is worthy of note that since the lone pair of phosphorus has significant amount of s character, this could reduce the bonding overlaps between Tl and P elements (see the black lines in model [I] and model [II] in Figure 1). As a consequence, the Tl≡P triple bond should be very weak, which is in contrast to the traditional triple bond of acetylene. This prediction is confirmed in the following section. Both models are used in this study clearly show that the Tl≡P triple bond is mostly attributed to electron donation from the lone pair of P to the empty p-orbital of Tl. This bonding analysis is used to interpret the bonding properties of the triply bonded RTl≡PR molecule in the next section.

Molecules 2017, 22, 1111

Molecules 2017, 22, 1111

3 of 14

3 of 13

Figure1.1.The Theinteraction interactionmodels, models,[I][I]and and[II], [II],for forthe thetriply triplybonded bondedRTl RTl≡PR molecule. Figure ≡PR molecule.

Resultsand andDiscussion Discussion 4.4.Results 4.1.Small SmallLigands Ligandson onSubstituted SubstitutedRTl RTl≡PR 4.1. ≡PR Theeffect effectofofsmall smallsubstituents substituentson onthe thestability stabilityofofthe thetriply triplybonded bondedRTl RTl≡PR speciesisisdiscussed discussed The ≡PR species from the kinetic and the thermodynamic viewpoints. Five small substituents (R = H, F, OH, CHCH 3 and from the kinetic and the thermodynamic viewpoints. Five small substituents (R = H, F, OH, 3 SiH3SiH ) are used for the RTl≡PR model molecule. The important geometrical parameters for the and ) are used for the RTl ≡ PR model molecule. The important geometrical parameters for 3 RTl≡PR compounds areare calculated computational methods methods (M06-2X/Def2-TZVP, (M06-2X/Def2-TZVP, the RTl≡PR compounds calculatedatatthe the three three computational B3PW91/Def2-TZVP and B3LYP/LANL2DZ+dp) and the results are listed in Table TheCartesian Cartesian B3PW91/Def2-TZVP and B3LYP/LANL2DZ+dp) and the results are listed in Table 1.1.The coordinatesfor forthe thetriply triplybonded bondedminima minimaare aregiven givenininthe theSupplementary SupplementaryInformation. Information. coordinates

Molecules 2017, 22, 1111

4 of 14

Table 1. The important geometrical parameters, the natural charge densities (QTl and QP), the binding energies (BE), the HOMO-LUMO energy gaps and the Wiberg Bond Index (WBI) for RTl≡PR using the M06-2X/Def2-TZVP, B3PW91/Def2-TZVP (in round brackets) and B3LYP/LANL2DZ+dp (in square brackets) levels of theory. R

F

OH

H

CH3

SiH3

Tl≡P (Å)

2.422 (2.425) [2.455]

2.437 (2.443) [2.480]

2.320 (2.327) [2.331]

2.339 (2.349) [2.360]

2.313 (2.336) [2.337]

R-P-Tl (◦ )

179.7 (179.7) [178.5]

179.1 (176.5) [177.9]

179.1 (178.5) [178.2]

175.2 (174.5) [171.3]

174.6 (175.7) [179.1]

P-Tl-R (◦ )

94.63 (96.59) [94.22]

98.92 (101.5) [100.1]

86.51 (86.82) [86.36]

100.4 (102.2) [102.6]

94.76 (92.71) [90.78]

R-P-Tl-R (◦ )

180.0 (180.0) [180.0]

179.4 (178.8) [179.2]

179.1 (179.2) [179.8]

178.0 (178.8) [179.9]

177.0 (179.1) [179.4]

QP

(1)

0.16 (0.17) [0.096]

0.076 (0.13) [0.021]

−0.63 (−0.60) [−0.62]

−0.37 (−0.33) [−0.39]

−0.83 (−0.72) [−0.76]

QTl

(2)

1.19 (1.11) [1.25]

1.14 (1.03) [1.17]

1.12 (0.87) [0.99]

1.07 (0.99) [1.13]

0.82 (0.75) [0.89]

∆EST for Tl–R (kcal/mol) (3)

102.1 (103.7) [102.2]

83.57 (80.69) [83.15]

84.85 (85.69) [83.05]

66.82 (67.38) [67.94]

75.96 (77.63) [74.40]

∆EST for P–R (kcal/mol) (4)

−28.91 (−33.35) [−31.76]

−17.53 (−21.29) [−20.24]

−30.75 (−35.49) [−33.16]

−26.43 (−30.26) [−29.21]

−15.84 (−18.68) [−14.46]

HOMO—LUMO (kcal/mol)

184.1 (131.6) [182.5]

167.6 (118.1) [169.1]

210.6 (212.0) [215.4]

151.2 (149.3) [146.5]

142.1 (145.1) [148.5]

BE (kcal/mol) (5)

95.58 (95.74) [93.43]

83.57 (82.10) [83.15]

84.85 (85.69) [83.05]

66.82 (67.38) [67.94]

75.96 (77.63) [74.40]

WBI (6)

1.159 (1.194) [1.191]

1.162 (1.197) [1.178]

1.456 (1.491) [1.475]

1.382 (1.415) [1.403]

1.404 (1.417) [1.372]

(1)

The natural charge density on the central phosphorus atom; (2) The natural charge density on the central thallium atom; (3) ∆EST (kcal mol−1 ) = E(triplet state for R–Tl) – E(singlet state for R–Tl); (4) ∆EST (kcal mol−1 ) = E(triplet state for R–P) – E(singlet state for R–P); (5) BE (kcal mol−1 ) = E(singlet state for R–Tl) + E(triplet state for R–P) – E(singlet for RTl≡PR); (6) The Wiberg bond index (WBI) for the Tl≡P bond: see reference [59–61].

There are four noteworthy features of Table 1: (1) The central Tl≡P triple bond distances (Å) for R = F, OH, H, CH3 and SiH3 are respectively estimated to be 2.313–2.422 Å, 2.336–2.443 Å and 2.331–2.480 Å, at the M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ+dp levels of theory. As mentioned in the Introduction, neither experimental nor theoretical results for the triply bonded RTl≡PR species are available to allow a definitive comparison. However, to the author’s best knowledge, there are only a few published reports concerning the singly bonded R3 Tl–PR3 molecules and these report the Tl–P bond length to be 2.922 Å [44], 3.246–3.301 Å [45] and 3.032–3.168 Å [46]. These single bond distances are all longer than the sum of the covalent radii (i.e., 2.62 Å) [62] for the Tl and P elements. (2) The three DFT calculations shown in Table 1 demonstrate that the R–Tl and R–P components have a singlet and triplet ground state, respectively. The three DFT computational results also show that the singlet-triplet energy differences (∆EST ) for R–Tl and R–P fragments are estimated to be at

Taiwan * Correspondence: [email protected]; Tel.: +886-5-2717964 Received: 12 June 2017; Accepted: 29 June 2017; Published: 5 July 2017

Abstract: Three computational methods (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/ 5 of 14 LANL2DZ+dp) were used to study the effect of substitution on the potential energy surfaces of RTl≡PR (R = F, OH, H, CH3, SiH3, SiMe(SitBu3)2, SiiPrDis2, Tbt (=C6H2-2,4,6-(CH(SiMe3)2)3), and Ar* (=C−6H 6H2-2, 4,6-i-Pr3)2)).These The energy theoretical results show that that these triply least +67 and 153-2,6-(C kcal/mol, respectively. values strongly suggest model [I],bonded which RTl≡PR ∠R Tlcharacters ∠Tl bonded a preference bent geometrythe (i.e., P ≈ 180° and P R ≈ 120°). Two is shown incompounds Figure 1, is have superior to modelfor [II]a in describing bonding of triply valence bond modelssmall are used to interpret the bonding character the Tl≡Pstructure triple bond. RTl≡PR molecules that feature substituents (R). Model [I] shows that theofbonding of One is model is best described asasTlTl P. P. the bonding conditions for RTl≡PR the triple bond in[I], RTlwhich ≡PR can be represented It This mustinterprets be noted that the fact that the lone molecules that feature small ligands. Thepother is of model [II], which is bestsmaller represented as Tl P. pair of phosphorus has s character and the valence orbital phosphorus is much than that explains the bonding character affect of RTl≡PR molecules that featurephosphorus large substituents. of thalliumThis means that both factors can vigorously the bonding overlaps between Irrespective of the types substituents for thebond RTl≡PR species, the investigations and thallium atoms. Therefore, it is of anticipated thatused the triple in these RTl≡ PRtheoretical species is very (based on the natural bond orbital, the natural resonance theory, and the charge decomposition weak. This prediction is confirmed by the three DFT calculations shown in Table 1. All of the values analysis) that their triple bonds are the theoretical for the Wiberg bond demonstrate index (WBI) [59–61] areTl≡P a little bit higher thanvery 1.0, weak. rather However, than 2.0. That is to say, results predict thatsmall only electropositive bulkier substituents greatly stabilize the triplyare bonded RTl≡PR species, regardless of whether or small electronegative groups attached, the RTl ≡PR from the kinetic viewpoint. systems possess a quite weak Tl≡P triple bond. (3) As already shown, model [I] describes the bonding characters in triply bonded RTl≡PR molecules; bond;[II]. acetylene; effects compoundsKeywords: that featuretriply smallbonded substituents better triple than model This, insubstituent turn, strongly implies that an ◦ ◦ acute bond angle ∠Tl–P–R (close to 90 ) and a linear bond angle ∠R–Tl–P (close to 180 ) is favored in the triply bonded RTl≡PR molecule, which is verified by the three DFT calculations as shown in Table 1. The nearly perpendicular angle on the P center can also be attributed to the “orbital non-hybridization 1. Introduction effect” [63–66] and the “inert s-pair effect” [63–66] as discussed previously. (4) The binding energies (BE) that are required toof cleave centralheavier Tl≡P bond, to one The preparation and characterization triplythe bonded mainwhich groupleads element (E14 = Si, Ge, R–Tl and one R–P Pb) fragment in the(i.e., singlet and in field the triplet ground state, respectively, Sn, and molecules RE14ground ≡E14R) isstate a popular of study in inorganic chemistry are [1–41]. From summarized Table 1. electron The calculated BE values for the ≡15PR the range to the the invalence viewpoint, the (kcal/mol) triply bonded RERTl 13≡E R molecules compoundareisinisoelectronic of 67–96, 67–96 68–93, atHowever, the M06-2X, andbeen B3LYP ofof theory, This data RE14≡Eand 14R species. theB3PW91 former has thelevels subject muchrespectively. less study than the latter, in the confirms that thallium and phosphorus atoms substituted RTl PR chemistry compounds fieldthe of central synthetic chemistry. Therefore, the levelinofthe understanding of≡the of are RE13≡E15R is strongly bonded. lower than that for group 14 less-coordinate alkyne analogues. Considering of family, RTl≡PR,phosphorus the theoretical resultssimilar for the to potential energyrelative, surfacescarbon, of the than to In the stability group 15 is more its diagonal model molecule, (R = F, OH, H, CH SiH ), are described Figure 2. This figure a to that nitrogenRTlPR [42]. Thallium is also known be3 monovalent andinhas an ionic radius thatshows is similar 3 andto number ofofstationary points exist, including local minima that correspond to RTl ≡ PR, R Tl=P, Tl=PR potassium, so it is often presumed to be a pseudo alkali metal [43]. The isolation and characterization 2 2 and the transition connect them. The three DFT computational show2)3that all of 3)3, was of the states singly that bonded organothallium phosphorus molecule,results (Me3SiCH Tl P(SiMe the triply experimentally bonded RTl≡PR compounds feature small substituents immediately transfer that to the reported aboutthat twenty years ago [44]. Two other novel compounds contain the corresponding doubly bonded species facile 1,2-migration reactions.[45,46]. In other the theoretical thallium phosphorus singlevia bond have also been identified If words, both thallium and phosphorus evidence shows thatcould triplybebonded RTl≡ PR species that feature small ligands bothbe kinetically and elements stabilized using a single bond to connect them, itare might possible to extend this thermodynamically unstable, regardless of whether they are electronegative or electropositive, so it is field to the study of other triply bonded RTl≡PR inorganic molecules. This work reports the first unlikely that they could be prepared or synthesized in of a laboratory. theoretical study of the possible synthesis the RTl≡PR molecule, which may be isolable as a long-lived compound. 0The study determines potential inorganic complexes that can stabilize the 4.2. Large Ligands on Substituted R Tl≡PR0 Molecules 2017, 22, 1111

Molecules 2017, 22, 1111; doi:10.3390/molecules22071111

www.mdpi.com/journal/molecules

As previously mentioned, in order to stabilize R0 Tl≡PR0 from the kinetic viewpoint, three types of large substituents (R´) are used in this study. These are SiMe(SitBu3 )2 , SiiPrDis2 , Tbt (=C6 H2 -2,4,6(CH(SiMe3 )2 )3 ), and Ar* (=C6 H3 -2,6-(C6 H2 -2,4,6-i-Pr3 )2 ) [67,68], as shown in Figure 3. The geometrical structures of R0 Tl≡PR0 are optimized at the dispersion-corrected M06-2X/Def2-TZVP [53] level of theory. Their important calculated parameters are listed in Table 2. Table 2. The Bond Lengths (Å), Bond Angels (◦ ), Singlet—Triplet Energy Splitting (∆EST), Natural Charge Densities (QTl and QP), Binding Energies (BE), the HOMO-LUMO Energy Gaps, the Wiberg bond index (WBI), and Some Reaction Enthalpies for R0 Tl≡PR0 at the dispersion-corrected M06-2X/Def2-TZVP Level of Theory. See also Figure 4. R0

SiMe(SitBu3 )2

SiiPrDis2

Tbt

Ar*

Tl≡P (Å) ∠R0 –Tl–P (◦ ) ∠Tl–P–R0 (◦ ) ∠R0 –Tl–P–R0 (◦ ) QTl (1)

2.386 166.9 122.3 171.4 0.975

2.384 166.4 113.7 179.5 0.739

2.385 168.9 116.2 173.9 1.166

2.336 161.2 115.6 174.4 1.218

Molecules 2017, 22, 1111

6 of 14

Table 2. Cont. R0 (2)

QP ∆EST for Tl—R0 (kcal/mol) (3) ∆EST for P—R0 (kcal/mol) (4) HOMO—LUMO (kcal/mol) (5) Molecules 2017, 22,BE 1111(kcal/mol) ∆H1 (kcal/mol) (6) (6) (6) ΔH22(kcal/mol) ∆H (kcal/mol) (7) (7) WBI WBI

SiMe(SitBu3 )2

−0.860 35.91 −43.10 71.27 80.24 91.34 73.98 73.98 2.116 2.116

72.83 2.273

SiiPrDis2

Tbt

Ar*

−0.826 35.52 −37.47 27.21 85.43 90.49 72.83 2.273

−0.344 31.27 −39.74 58.05 62.51 89.22 71.27 2.127

−0.257 30.24 −40.52 39.34 67.896 of 13 87.11 74.01 74.01 2.201 2.201

71.27 2.127

(1) The natural charge density on the central thallium atom; (2) The natural charge density on the The natural charge density on the central thallium atom; (2) The natural charge density on the central phosphorus central phosphorus atom; (3) ΔEST (kcal mol−1) = E(triplet state for R′⎼Tl) – E(singlet state for R′⎼Tl); atom; (3) ∆EST (kcal mol−1 ) = E(triplet state for R0 –Tl) – E(singlet state for R0 –Tl); (4) ∆EST (kcal mol−1 ) = E(triplet (4) ΔEST (kcal mol−1) = E(triplet state for R′⎼P) – E(singlet state for R′⎼P); (5) BE (kcal mol−1) = E(triplet state for R0 –P) – E(singlet state for R0 –P); (5) BE (kcal mol−1 ) = E(triplet state for R0 –Tl) + E(singlet state for R0 –P) – (6) See Figure 4; (7) The Wiberg bond 0 Tl≡PR 0 ); (6) See state (7) The statefor forRR′⎼Tl) + E(singlet for4;R′⎼P) – E(singlet for R′Tl≡PR′); E(singlet Figure Wiberg bond index (WBI) for the Tl≡P bond: see reference [59–61]. index (WBI) for the Tl≡P bond: see reference [59–61].

(1)

Figure 2. The Relative Gibbs free energy surfaces for RTl≡PR (R = F, OH, H, CH3 and SiH3). These

Figure 2. The Relative Gibbs free energy surfaces for RTl≡PR (R = F, OH, H, CH3 and SiH3). These energies are in kcal/mol and are calculated at the M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and energies are in kcal/mol andofare calculated at see thethe M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp levels theory. For details text and Table 1. B3LYP/LANL2DZ+dp levels of theory. For details see the text and Table 1.

Molecules 2017, 22, 1111

7 of 14

Molecules 2017, 22, 1111

7 of 13

Figure 3. 3. Four Fordetails, details,see see references [66,67]. Figure Fourbulky bulkygroups. groups. For references [66,67].

0 molecules Figure 4. The potential energy surfacefor forthe the1,2-migration 1,2-migration reaction R′Tl≡PR′ molecules with with Figure 4. The potential energy surface reactionofofthe the R0 Tl≡PR 0 groups balkybalky groups (R ).(R′).

Five important conclusions can be drawn from these theoretical results:

Five important conclusions can be drawn from these theoretical results: (i) The results presented in Table 2 predict that the Tl≡P triple bond lengths (Å) are about 2.386 (i) The presented in Table 2 predict that 33the Tl≡P triple bond are about 2.386 Å, Å, 2.384results Å, 2.385 Å, and 2.336 Å, for (SiMe(SitBu )22)Tl≡P(SiMe(SitBu 33)22), lengths (SiiPrDis(Å) 22)Tl≡P(SiiPrDis 22), 2.384(Tbt)Tl≡P(Tbt), Å, 2.385 Å, and 2.336 Å, for (SiMe(SitBu ≡P(SiMe(SitBu (SiiPrDis and (Ar*)Tl≡P(Ar*), respectively. These estimated are shorter than the 3 )2 )Tltheoretically 3 )2 ),values 2 )Tl≡P(SiiPrDis 2 ), experimentally P singlerespectively. bond distance, as mentioned previously [44–46]. Similarly to the than (Tbt)Tl ≡P(Tbt), andreported (Ar*)TlTl ≡P(Ar*), These theoretically estimated values are shorter case for small substituents, the single DFT optimized results show that all ofpreviously the triply bonded the experimentally reported Tl–P bond distance, as mentioned [44–46].R′Tl≡PR′ Similarly to molecules that feature bulky ligands studied adopt a bent structure, as shown in Table 2. the case for small substituents, the DFT optimized results show that all of the triply bonded R0 Tl≡PR0

molecules that feature bulky ligands studied adopt a bent structure, as shown in Table 2. (ii) If the R´Tl≡PR’ compound is cut in half, the Tl–R’ and P–R0 two fragments are obtained. The DFT results shown in Table 2 demonstrate that the ∆EST for the Tl–R0 unit is greater than 30 kcal/mol

edical University, Kaohsiung 80708,

4

017 Molecules 2017, 22, 1111

8 of 14

VP, B3PW91/Def2-TZVP and B3LYP/ n the potential energy surfaces of RTl≡PR (=C6H2-2,4,6-(CH(SiMe 3)2)modulus 3), and of Ar* and the ∆E for the P–R0 moiety is greater than 37 kcal/mol. That is to say, the promotion Molecules 2017, 22, 1111 ST 8 of 13 ow that these triply bonded RTl≡PR energy from the singlet ground state to the triplet excited for Tl–R0 is smaller than the energy that is (ii) If theTwo R´Tl≡PR´ compound is cut(Table in half,1). theThe Tl bonding R´ and P model R′ two fragments are obtained. Tl P ≈ 180° and ∠ Tl P R ≈for 120°). required promotion from that for Tl–R that is shown in FigureThe 1 shows 0 0 molecules DFTbond. results shown in Table demonstrate that the ΔEcharacter ST for the Tl unit is greaterR than kcal/mol aracter of the Tl≡Pthat triple is used model [II]One can be to 2interpret the bonding in R′ triply bonded Tl≡30PR 0 is best and the modulus of ΔER ST0 for the P R′ moiety is greater than 37 kcal/mol. That is to ets the bonding conditions forbulky RTl≡PR that feature ligands, . Namely, the bonding structure of the triple bond in R0 Tlsay, ≡PRthe Article promotion energy from the singlet ground state to the triplet excited for Tl R′ is smaller than the which is best represented asas TlTl P. P. described In this model, the electrons that are donated from the lone pair of phosphorus have energy that is required for promotion from that for Tl R (Table 1). The bonding model that is shown ules that features character, large substituents. as shown in Figure 1. Moreover, the size of 2p orbital of P is also much smaller than the 6p in Figure 1 shows that model [II] can be used to interpret the bonding character in triply bonded 0 species. R species, the theoretical investigations orbital of Tl. These twothat factors combined produce a weak Tl ≡bonding P triple structure bond in the R0 Tl ≡PR R´Tl≡PR′ molecules feature bulky ligands, R′. Namely, the of the triple bond 0 0 theory, and the charge Supporting theoretical evidence in shows that the for R that Tl≡PR is 2.21, 2.37, and 2.20 indecomposition R´Tl≡PR′ is best described as Table Tl P.2In this model, theWBI electrons are donated from2.13, the lone weak. However, the results for theoretical Rpair = SiMe(SitBu ) , SiiPrDis , Tbt, and Ar*, respectively. These WBI values are much smaller of phosphorus as shown in Figure 1. Moreover, the size of 2p orbital of P is also than 3 2 have s character, 2 riply bonded RTl≡PR species, from the the the value acetylene (2.99). muchfor smaller than 6p orbital of Tl. These two factors combined produce a weak Tl≡P triple bond 1 and Ming-Der Su 1,2,* Jia-Syun Lu 1, Ming-Chung Yang in the Supporting theoretical in Table 2 shows that of thetriply WBI for R′Tl≡PR′ is≡PR0 (iii) InR´Tl≡PR′ order tospecies. determine the effect of bulkyevidence substituents on the stability bonded R0 Tl 2.21, 2.37,the 2.13, and 2.20 for R = SiMe(SitBu 3)2, SiiPrDis2, Tbt, and Ar*, ThesetoWBI values the 1 Departmentcompounds, dispersion-corrected M06-2X/Def2-TZVP level ofrespectively. theory is used determine of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan; are much smaller than the value for acetylene (2.99). substituent effects potential energy surfaces for the isomerization(M.-C.Y.) reaction. As shown in Table 2, the triply bonded [email protected] (J.-S.L.); [email protected] In order to determine the effect of bulky substituents on the stability of triply bonded 2 Department of and Applied Kaohsiung Medical University, 80708, lower than that for R0 TlMedicinal ≡PR0(iii) molecules haveChemistry, values that are at least 87 (∆H (∆H2 ) kcal/mol 1 ) and 71Kaohsiung R′Tl≡PR′ compounds, the dispersion-corrected M06-2X/Def2-TZVP level of theory is used to determine Taiwan the corresponding doubly bonded isomers. Therefore, the theoretical results show that a triply bonded the potential energy surfacesTel.: for the isomerization reaction. As shown in Table 2, the triply bonded * Correspondence: [email protected]; +886-5-2717964 R0 Tl≡ PR0 compound bulky substituents more stable than its corresponding doubly R′Tl≡PR′ moleculesthat havefeatures values that are at least 87 (ΔHis 1) and 71 (ΔH2) kcal/mol lower than that for 0 Tl=P: and: Tl=PR0 isomers, from the kinetic viewpoint. bonded R Received: 12 June 2017; Accepted: 29 June 2017;2 bonded Published: 5 July 2017 2 the corresponding doubly isomers. Therefore, the theoretical results show that a triply (iv) In verify the conclusion frombulky point substituents (ii), “charge decomposition analysis” (CDA), reported bonded compound that features is more stable than its corresponding avier main group element (Eorder 14R′Tl≡PR′ = Si,toGe, Abstract: Three computational methods (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/ by Dapprich and Frenking [69] is used in the present study. For instance, the computational results doubly bonded R′ 2 Tl=P: and: Tl=PR′ 2 isomers, from the kinetic viewpoint. dy in inorganic chemistry [1–41]. From (iv) In order to verify the conclusion from point (ii), “charge decomposition analysis” (CDA), LANL2DZ+dp) were used to study the effect of substitution on the potential energy surfaces of RTl≡PR (SiMe(SitBu 5R compound is concerning isoelectronic to the 3 )2 )Tl≡P(SiMe(SitBu3 )2 ) based on the dispersion-corrected M06-2X/Def2-TZVP by Dapprich and Frenking [69] in(=C the6H present study. For instance, the computational 3, collected SiH SiMe(SitBu , SiiPrDis 2the , used Tbt 2-2,4,6-(CH(SiMe 3)2)3), and Ar*R0 –Tl to R0 –P is (R less = F, OH, H,reported CH method are in Table 3.3)2As seen inis X column, the biggest contribution from f much study than the latter, in3, the results concerning (SiMe(SitBu 3)2)Tl≡P(SiMe(SitBu3)2) based on the dispersion-corrected M06-2X/ 0 0 3-2,6-(C 6H2-2,of(HOMO–1) 4,6-i-Pr The theoretical show that these from triplyR –P bonded RTl≡PR No.227 orbital. However,results the largest contribution to R –Tl is No.228 (HOMO) nding(=C of6H the chemistry RE13≡E3)152)). R is Def2-TZVP method are collected in Table ∠ 3. As seen in the X column, the biggest contribution from 0 0 ∠ compounds have a preference for a bent geometry (i.e., R Tl P ≈ 180° and Tl P R ≈ 120°). Two orbital. As a result, the net electron transfer (−0.213) is from R –P to R –Tl, which is shown in the s. R′ Tl to R′ P is No.227 (HOMO orbital. However, the largest contribution from R′ P to R′ Tl is 0 –P1) 0 –Tl valence bond models are than usedNamely, to interpret bonding character the Tl≡Ptotriple One is (X – Y) column. the Rthe unit donates moreofelectrons the Rbond. unit. The theoretical its diagonal relative, carbon, No.228 (HOMO)toorbital. As a result, the net electron transfer (−0.213) is from R′ P to R′ Tl, which is model [I], which is best described as Tl P. This interprets the bonding conditions for RTl≡PR evidence is in good agreement with the valence-electron bonding model (Figure 1; model [II]) The as stated as an ionic radius that shown is similar to that in the (X – Y) column. Namely, the R′ P unit donates more electrons to the R′ Tl unit. 0 0 0 0 that feature smallevidence ligands. other is modelofwith [II], which is best as TlTl (Figure earlier. Consequently, the bonding nature R Tl ≡PR can berepresented considered R PRP.. 1; model l [43].molecules The isolation and characterization theoretical isThe in good agreement the valence-electron bondingas model This explains the bonding character of RTl≡PR molecules that feature large substituents. (v) The NBO [59–61] and NRT [70–72] are also used to determine the bonding properties [II]) as stated earlier. Consequently, the bonding nature of R′Tl≡PR′ can be considered as R′Tl PR′.of the lecule, (Me3SiCH2)3Tl P(SiMe3)3, was 0 0 Irrespective of the types of substituents used for the RTl≡PR species, the theoretical investigations electronic structures the Rand Tl≡ PR [70–72] molecules, asused shown in Table 4. tableproperties clearly shows The NBO [59–61] NRT are also to determine theThis bonding of the that her novel compounds that(v) contain the of electronic structures of thenatural R′Tl≡PR′ molecules, as shownfrom in Table 4. Thisdonation table clearly shows thattothe the bond orbital, the resonance and the charge decomposition thenatural major bonding character between Tl and Ptheory, comes electron from 2p(P) 6p(Tl), 5,46].(based If bothon thallium and phosphorus major bonding character between Tl and Pvery comes fromHowever, electron donation from )2p(P) to 6p(Tl),for which analysis) demonstrate that their Tl≡P triple bonds are weak. the theoretical results which is denoted as 6p(Tl) ← 2p(P). In the (SiMe(SitBu ) )Tl ≡ P(SiMe(SitBu ) molecule, instance, 3 2 3 2 hem, it might be possible to extend this is denoted as 6p(Tl) ←M06-2X/Def2-TZVP 2p(P). In the (SiMe(SitBu 3)2)Tl≡P(SiMe(SitBu3)2) molecule, for instance, the predictThis thatwork only bulkier substituents greatly stabilize the triply bondedshow RTl≡PR the the dispersion-corrected calculations thatspecies, the Tl≡from P π bonding occurs as molecules. reports the first dispersion-corrected M06-2X/Def2-TZVP calculations show that the Tl≡P π bonding occurs as 4.77 1.42 kinetic viewpoint. follows: π⊥ (Tl≡P) =a 0.3114(sp 4.77)Tl + 0.9503(sp1.42 )P. That is, a polarized π⊥ bond exists between Tl molecule, which may be isolable as follows: π⊥ (Tl≡P) = 0.3114(sp )Tl + 0.9503(sp )P. That is, a polarized π⊥ bond exists between Tl and can P, which arises from the donation of the P lone pair to the empty Tl p orbital. As seen in Table 4, ganic complexes that the and stabilize P, which arises from the donation of the P lone pair to the empty Tl p orbital. As seen in Table 4, Keywords: triply bonded molecules; triple bond; acetylene; substituent effects the Tlthe ≡PTl≡P π⊥ π bonding orbitals natural orbitals and 90% natural P orbitals (Figure bonding orbitalscomprise comprise 9.7% 9.7% natural TlTlorbitals and 90% natural P orbitals (Figure 5). 5). ⊥ www.mdpi.com/journal/molecules The similar theoretical results can also be found in the Tl ≡ P π bonding orbitals as already represented The similar theoretical results can also be found in the kTl≡P π‖ bonding orbitals as already in Table 4. represented in Table 4.

Substituent Effects on the Stability of Thallium and Phosphorus Triple Bonds: A Density Functional Study

1. Introduction The preparation and characterization of triply bonded heavier main group element (E14 = Si, Ge, Sn, and Pb) molecules (i.e., RE14≡E14R) is a popular field of study in inorganic chemistry [1–41]. From the valence electron viewpoint, the triply bonded RE13≡E15R compound is isoelectronic to the RE14≡E14R species. However, the former has been the subject of much less study than the latter, in the field of synthetic chemistry. Therefore, the level of understanding of the chemistry of RE13≡E15R is lower than that for group 14 less-coordinate alkyne analogues. In the group 15 family, phosphorus is more similar to its diagonal relative, carbon, than to nitrogen [42]. Thallium is also known to be monovalent and has an ionic radius that is similar to that (a) π⊥ of potassium, so it is often presumed to be a pseudo alkali metal [43]. The isolation and characterization of the singly bonded organothallium phosphorus molecule, (Me3SiCH2)3Tl P(SiMe3)3, was Figure 5. Cont. experimentally reported about twenty years ago [44]. Two other novel compounds that contain the thallium phosphorus single bond have also been identified [45,46]. If both thallium and phosphorus elements could be stabilized using a single bond to connect them, it might be possible to extend this field to the study of other triply bonded RTl≡PR inorganic molecules. This work reports the first theoretical study of the possible synthesis of the RTl≡PR molecule, which may be isolable as a long-lived compound. The study determines potential inorganic complexes that can stabilize the

Molecules 2017, 22, 1111

9 of 14

Molecules 2017, 22, 1111

9 of 13

(b) π‖ Figure 5. The natural Tl≡P π bonding orbitals ((a) and (b)) for (SiMe(SitBu3)2)Tl≡P(SiMe(SitBu3)2).

Figure 5. The natural Tl≡P π bonding orbitals ((a) and (b)) for (SiMe(SitBu3 )2 )Tl≡P(SiMe(SitBu3 )2 ). For comparison, see also Figure 3. For comparison, see also Figure 3. Table 3. The charge decomposition analysis (CDA) (a) for R′Tl≡PR′ (R′ = SiMe(SitBu3)2) system based (a) for R0 Tl≡PR0 (R0 = SiMe(SitBu ) ) system based Tableon3.M06-2X The charge decomposition analysis (CDA) orbitals, where the X term indicates the number of electrons donated from R′⎼Tl 3 2 fragment to R′⎼P orbitals, fragment,where the Y term the number of electrons back donated from R′⎼P to on M06-2X the Xindicates term indicates the number of electrons donated fromfragment R0 –Tl fragment 0 0 fragment the andYthe Q term indicates numberofofelectrons electrons back involved in repulsive polarization. to R R′⎼Tl –P fragment, term indicates thethe number donated from R –P fragment to (a),(b) and the Y terms are bolded for easier comparison. R0 –TlSignificant fragmentXand Q term indicates the number of electrons involved in repulsive polarization.

Significant X and Y terms are bolded for easier X comparison. (a),(b) Orbital Occupancy Y

HOMO LUMO

HOMO sum LUMO (a)

218 219 Orbital 220 218 221 219 222 220 223 221 224 222 225 223 226 224 227 225 228 226 229 227 230

2.000000 2.000000 Occupancy 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 0.000000 2.000000 0.000000 2.000000 456.000000

0.000757 0.001036 X 0.000932 0.000757 0.000026 0.001036 0.001151 0.000932 0.000081 0.000026 0.000037 0.001151 0.001777 0.000081 0.000477 0.000037 0.008445 0.001777 −0.005339 0.000477 0.000000 0.008445 0.000000 −0.028853 0.005339

0.000586 0.000522 Y 0.000539 0.000586 0.004350 0.000522 −0.000164 0.000539 0.003145 0.004350 0.002403 −0.000164 0.029263 0.003145 0.013735 0.002403 0.068258 0.029263 0.003033 0.013735 0.000000 0.068258 0.000000 0.003033 0.241774

X–Y 0.000171 0.000513 X–Y 0.000394 0.000171 −0.004325 0.000513 0.001315 0.000394 −0.003064 −0.004325 −0.002366 0.001315 −0.027486 −0.003064 −0.013259 −0.002366 −0.059813 −0.027486 −0.008432 −0.013259 0.000000 − 0.059813 0.000000 − 0.008432 −0.212922

Q −0.002462 −0.004450 Q −0.006342 −0.002462 −0.002504 −0.004450 −0.001354 −0.006342 −0.001960 −0.002504 −0.000054 −0.001354 −0.030329 −0.001960 −0.007124 −0.000054 −0.018272 −0.030329 −0.004437 −0.007124 0.000000 −0.018272 0.000000 − 0.004437 −0.107250

228 229 0.000000 0.000000 0.000000 0.000000 0.000000 For clearness, only list the X, Y, and Q terms for HOMO (No.228) ⎼10 ~ LUMO+2. (b) Summation of 230 0.000000 0.000000 0.000000 0.000000 0.000000 contributions from all unoccupied and occupied orbitals. sum 456.000000 0.028853 0.241774 −0.212922 −0.107250

(a)

(b) ForTable clearness, onlynatural list the bond X, Y, and Q terms for HOMO ~LUMO+2. of contributions 4. The orbital (NBO) and the(No.228) natural–10 resonance theorySummation (NRT) analysis for from all unoccupied and occupied orbitals. R′Tl≡PR′ molecules that feature ligands (R′ = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar*) at the dispersion-corrected M06-2X/Def2-TZVP level of theory (1,2).

Table 4. The natural bond orbital (NBO) and the natural resonance theory (NRT) analysis for R0 Tl≡PR0 NBO Analysis NRT Analysis molecules that feature ligands (R0 = SiMe(SitBu3 )2 , SiiPrDis2 , Tbt, and Ar*) at theTotal/ dispersion-corrected R′Tl≡PR′ WBI Resonance (1,2) Occupancy Polarization Covalent/ M06-2X/Def2-TZVP level of theory . Hybridization Weight σ = 2.21

R0 Tl≡PR0 R′ = SiMe(SitBu3)2

WBI 2.11

Occupancy π⊥ = 1.84 π 1.92 σ‖== 2.21

R0 = SiMe(SitBu3 )2 R′ = SiiPrDis2

2.11 2.37

πσ⊥ ==1.83 1.84 π⊥ = 1.92

πk = 1.92 π‖ = 1.93

σ = 1.83 R0

R′ = Tbt

= SiiPrDis2

2.13

2.37

σ = 1.77

π⊥ = 1.92 πk = 1.93

Ionic 26.18% (Tl) NRT Analysis 73.82% (P) Tl⎼P: 23.17% Total/Covalent/ Resonance 9.70% (Tl) 4.77 1.42 Hybridization Polarization2.22/1.55/0.67 π⊥: 0.3114 Tl (sp ) + 0.9503 P (sp ) Ionic Tl=P: 66.87% Weight 90.30% (P) Tl≡P: 9.94% 5.69% (Tl) (Tl) 26.18% 99.87 99.99) 2.07 1.27 πσ ‖: :0.6833 Tl (sp ) + 0.7556 P (sp 0.5116 Tl (sp ) + 0.8592 P (sp ) 94.31% (P) 73.82% (P) Tl–P: 23.17% 41.24% (Tl) 9.70% (Tl) 2.22/1.55/0.67 σ : 0.6422 Tl (sp0.86)4.77 + 0.7665 P (sp20.18) 1.42 Tl=P: 66.87% π⊥ : 0.3114 Tl (sp ) + 0.9503 P (sp ) 58.76% (P) 90.30% (P) Tl≡P: 9.94% Tl⎼P: 17.35% 16.51% (Tl) π⊥: 0.4064 Tl (sp99.99 ) + 0.9137 P (sp44.72) 99.99 Tl=P: 71.14% 5.69% (Tl) 2.59/0.83/1.76 99.87 83.49% (P) πk : 0.6833 Tl (sp ) + 0.7556 P (sp ) Tl≡P: 11.51% 94.31% (P) 14.79% (Tl) π‖: 0.4551 Tl (sp99.99) + 0.8997 P (sp94.99) 41.24% 85.21% (P) (Tl) σ : 0.6422 Tl (sp0.86 ) + 0.7665 P (sp20.18 ) 58.76% 47.45% (Tl) (P) Tl⎼P: 27.42% Tl–P: 17.35% 2.08/1.59/0.49 σ : 0.6888 Tl (sp0.94) + 0.7249 P (sp38.46) Tl=P: 63.76% 52.55% (P) (Tl) 16.51% 2.59/0.83/1.76 Tl=P: 71.14% 1.27) + 0.8592 P (sp2.07) σ : 0.5116 Tl (sp NBO Analysis

π⊥ : 0.4064 Tl (sp99.99 ) + 0.9137 P (sp44.72 ) πk : 0.4551 Tl

(sp99.99 )

+ 0.8997 P

(sp94.99 )

83.49% (P) 14.79% (Tl) 85.21% (P)

Tl≡P: 11.51%

Molecules 2017, 22, 1111

10 of 14

Article

y of Thallium and sity Functional Molecules 2017, 22, 1111 R0 Tl≡PR0

NBO Analysis

WBI Occupancy σ = 1.77

R0 = Tbt

2.13

π⊥ π= 1.94 = 1.94 ⊥

π‖ π = 1.90 = 1.90 k

hiayi 60004, Taiwan; M.-C.Y.) R′R=0 80708, Ar* = Ar* dical University, Kaohsiung

Table 4. Cont.

Substituent Effects on the Stability of Thalliu Phosphorus Triple Bonds: A Density Function 10 of 13 Study

σ = 1.96 σ = 1.96 2.202.20

π⊥ = 1.77 π⊥ = 1.77 π‖ π = 1.92 = 1.92 k

Hybridization

NRT Analysis

Polarization

Total/Covalent/ Ionic

Resonance Weight

47.45% (Tl) 52.55% (P) Tl–P: 8.82% 27.42% 23.43% (Tl) Tl≡P: 87.83) 23.43% (Tl) π⊥: 0.4133 Tl (sp135.51 ) + 0.9244 P (sp 2.08/1.59/0.49 1,2, Tl=P: 63.76% 1 and π⊥ : 0.4133Lu Tl (sp ) + 0.9244 P (sp87.83 ) 82.74% Jia-Syun , 35.51 Ming-Chung Yang *Tl≡P: 8.82% (P) 82.74% (P)Ming-Der Su 17.28% (Tl) 99.99) 17.28% (Tl) 0.4118 Tl (sp99.89 ) + 0.9077 P (spChemistry, 99.89 99.99 ) 1π‖π:Department + 0.9077 P (sp National Chiayi University, Chiayi 60004, Taiwan; 82.72% k : 0.4118 Tl (sp of )Applied 82.72%(P) (P) 54.20% (Tl) [email protected] (J.-S.L.); [email protected] (M.-C.Y.) 54.20% (Tl) σ: 0.7362 Tl (sp0.040.04 ) + 0.6767 P (sp64.96) σ: 0.7362 Tl (sp ) + 0.6767 P (sp64.96 ) 45.80% 2 Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiu 45.80%(P) (P) Tl⎼P: Tl–P:19.82% 19.82% 10.09% (Tl) 99.99) 10.09% (Tl) π⊥Taiwan : 0.3177 Tl (sp99.99 ) + 0.9482 P (sp 2.17/1.66/0.51 Tl=P: 71.69% 2.17/1.66/0.51 Tl=P: 71.69% π⊥ : 0.3177 Tl (sp99.99 ) + 0.9482 P (sp99.99 ) 89.91% (P) 89.91% (P) Tl≡P:8.49% 8.49% Tl≡P: * Correspondence: [email protected]; Tel.: +886-5-2717964 16.67% (Tl) π‖: 0.4083 Tl (sp99.99 ) + 0.9128 P (sp99.99) 16.67% (Tl) πk : 0.4083 Tl (sp99.99 ) + 0.9128 P (sp99.99 ) 83.33% (P) Received: 12 June 2017; Accepted: 2983.33% June(P) 2017; Published: 5 July 2017 σ: 0.6888 Tl (sp0.94 ) + 0.7249 P (sp38.46 )

The of the theWiberg Wibergbond bondindex index (WBI) Tl–P bond the occupancy the corresponding The value value of (WBI) forfor thethe Tl–P bond andand the occupancy of theofcorresponding σ and σ π (2) NRT; see reference [70–72]. (2)[59–61]). and π bonding (see [59–61]). reference bonding NBO (seeNBO reference NRT; see reference [70–72]. Abstract: Three computational methods (M06-2X/Def2-TZVP, B3PW91/Def2-TZV

(1) (1)

17

LANL2DZ+dp) were used to study the effect of substitution on the potential energy sur P, B3PW91/Def2-TZVP and B3LYP/ 5. Conclusions (R = F, OH, H, CH3, SiH3, SiMe(SitBu3)2, SiiPrDis2, Tbt (=C6H2-2,4,6-(CH(SiMe the potential energy surfaces of RTl≡PR In summary, the theoretical observations support that electronic and (=C 6H3-2,6-(C 6strongly H2-2, support 4,6-i-Pr 3)the 2)). the Theidea theoretical results show that these triply b (=C6H2-2,4,6-(CH(SiMe3)2)3summary, ), and Ar* the theoretical observations strongly idea that both both electronic and steric steric effects determine the relative stability ofhave molecules that contain a PTl≡P triple bond, asRwell as≈ 180° and ∠Tl P compounds a that preference for geometry (i.e., Tl w that these triply bonded RTl≡PR effects determine the relative stability of molecules contain aa Tlbent ≡ triple bond, as ∠ well as P its its doubly bonded isomers.The Thesimple simple bonding models schematically illustrated valence bond models are used to interpret the bonding character l P ≈ 180° and ∠Tl Pcorresponding R ≈ 120°). Two corresponding doubly bonded isomers. bonding models schematically illustrated in of the Tl≡P tripl Figure 1 show that model [I], whose bonding character is symbolized by Tl P, better interprets the model [I], which is best symbolized described asby TlTl P, P. This interprets the bonding conditi racter of the Tl≡P triple bond. One is bonding character triple bond RTl≡PR feature [II], whose is is best represente thatsubstituents. feature smallModel ligands. The otherbonding is modelproperty [II], which ts the bonding conditions for in RTl≡PR RTl ≡PRspecies species that thatmolecules feature small small substituents. Model [II], whose 0 0 typifiedasas P,P. better describes the triple bond in R′Tl≡PR′ molecules that feature bulky ligands This explains the bonding character of RTl≡PR molecules that feature larg which is best represented TlTl P, better describes the triple bond in R Tl≡PR molecules regardless of ofIrrespective whether the theof substituents insubstituents triplybonded bonded RTl≡PR compound are the types of used for the RTl≡PR species, the theoretica les that feature (Figure large substituents. 6). However, regardless whether substituents in triply RTl ≡PR large or triple bonds areare quite Two effects can can explain thesethese phenomena. The and the charge (based onquite theweak. natural bond orbital, theexplain natural resonance theory, species, the theoretical investigations or small, small,their theirTl≡P Tl≡P triple bonds weak. Two effects phenomena. different sizessizes of the orbitals in the P elements mean thatthat their overlapping are However, the the analysis) that their Tl≡P triple bonds populations arepopulations very weak. heory, and the charge decomposition The different of pthe p orbitals in Tl theand Tl demonstrate and P elements mean their overlapping pretty small and thethe lone pair of of the phosphorus atom has significant sscharacter, whichbonded RTl≡PR sp predict that only bulkier greatlyof stabilize the triply weak. However, the theoretical results are pretty small and lone pair the phosphorus atom hassubstituents significantamount amount of character, results in poor overlaps between thallium kinetic and viewpoint. ply bonded RTl≡PR species, from the thallium and phosphorus. phosphorus. ItIt is is hoped hoped that the results of experimental synthesis and structural characterization will confirm confirm these these predictions. predictions. Keywords: triply bonded molecules; triple bond; acetylene; substituent effects ubstituent effects

1. Introduction

The preparation and characterization of triply bonded heavier main group eleme Sn, and Pb) molecules (i.e., RE14≡E14R) is a popular field of study in inorganic chemist vier main group element (E14 = Si, Ge, the valence electron viewpoint, the triply bonded RE13≡E15R compound is isoel y in inorganic chemistry [1–41]. From RE14≡E14R species. However, the former has been the subject of much less study than t R compound is isoelectronic to the field of synthetic chemistry. Therefore, the level of understanding of the chemistry much less study than the latter, in the lower than that for group 14 less-coordinate alkyne analogues. ding of the chemistry of RE13≡E15R is Figure structure on theoretical calculations. Figure 6. 6. The The predicted predicted geometrical geometrical structure based on the the present present theoretical calculations. In the groupbased 15 family, phosphorus is more similar to its diagonal relative, c nitrogen [42]. Thallium is also known to be monovalent and has an ionic radius that i its diagonal relative, carbon, than to Supplementary Materials: Supplementary materials are available online. The CDA and NRT results concerning the of potassium, it available is often presumed be aand pseudo [43]. The isolation and s an ionic radius that is similar toMaterials: that Supplementary Supplementary materialssoare online. ThetoCDA NRT alkali resultsmetal concerning 2)Tl≡P(SiiPrDis2), (Tbt)Tl≡P(Tbt), and (Ar*)Tl≡P(Ar*) molecules are collected in the Supporting Information. (SiiPrDis and (Ar*)Tl ≡P(Ar*) molecules are collected in the Supporting of ≡P(Tbt), the singly bonded organothallium phosphorus molecule, (Me3SiCH2)3Tl P [43]. The isolation the and(SiiPrDis characterization 2 )Tl≡P(SiiPrDis2 ), (Tbt)Tl Information. grateful to thereported National about Centertwenty for High-Performance experimentally years ago [44]. Computing Two other of novel compounds t cule, (Me3SiCH2)Acknowledgments: 3Tl P(SiMe3)3, wasThe authors are Taiwan for generousThe amounts of computing time, and the Ministry of Science and Technology of Taiwan for the Acknowledgments: authors are grateful to the National Center for High-Performance Computing of Taiwan thallium phosphorus single bond have also been identified [45,46]. If both thallium a er novel compounds that contain the financial support. Special thanks aretime, also due forScience very help comments. for generous amounts of computing and to thereviewers Ministry of andsuggestions Technologyand of Taiwan for the financial elements could be stabilized using a single bond to connect them, it might be possibl ,46]. If both thallium and phosphorus support. Special thanks are also due to reviewers for very help suggestions and comments. AuthortoContributions: Ming-Chung Yang the theoretical calculations; Ming-Der Su field to the study ofperformed other triply bonded RTl≡PR inorganic molecules. This work r em, it might be possible extend this Jia-Syun Lu and Author Contributions: Jia-Syun Lu and Ming-Chung Yang performed the theoretical calculations; Ming-Der Su wrote the paper. theoretical study of the possible synthesis of the RTl≡PR molecule, which may b molecules. This work reports the first wrote the paper. long-lived compound. olecule, which may be isolable as a Conflicts of Interest: The authors declare no conflict of interest.The study determines potential inorganic complexes that ca Conflicts of Interest: The authors declare no conflict of interest. anic complexes that can stabilize the Molecules 2017, 22, 1111; doi:10.3390/molecules22071111 www.mdpi.com References www.mdpi.com/journal/molecules

1. 2.

Bino, A.; Ardon, M.; Shirman, E. Formation of a carbon-carbon triple bond by coupling reactions in aqueous solution. Science 2005, 308, 234–235. Su, P.; Wu, J.; Gu, J.; Wu, W.; Shaik, S.; Hiberty, P.C. Bonding conundrums in the C2 molecule: A valence

Molecules 2017, 22, 1111

11 of 14

References 1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20. 21.

22. 23.

Bino, A.; Ardon, M.; Shirman, E. Formation of a carbon-carbon triple bond by coupling reactions in aqueous solution. Science 2005, 308, 234–235. [CrossRef] [PubMed] Su, P.; Wu, J.; Gu, J.; Wu, W.; Shaik, S.; Hiberty, P.C. Bonding conundrums in the C2 molecule: A valence bond study. J. Chem. Theory Comput. 2011, 7, 121–130. [CrossRef] [PubMed] Ploshnik, E.; Danovich, D.; Hiberty, P.C.; Shaik, S. The nature of the idealized triple bonds between principal elements and the σ origins of trans-bent geometries—A valence bond study. J. Chem. Theory Comput. 2011, 7, 955–968. [CrossRef] [PubMed] Danovich, D.; Bino, A.; Shaik, S. Formation of carbon–carbon triply bonded molecules from two free carbyne radicals via a conical intersection. J. Phys. Chem. Lett. 2013, 4, 58–64. [CrossRef] [PubMed] Power, P.P. π-Bonding and the lone pair effect in multiple bonds between heavier main group elements. Chem. Rev. 1999, 99, 3463–3504. [CrossRef] [PubMed] Jutzi, P. Stable systems with a triple bond to silicon or its homologues: Another challenge. Angew. Chem. Int. Ed. 2000, 39, 3797–3800. [CrossRef] Weidenbruch, M. Some recent advances in the chemistry of silicon and its homologues in low coordination states. J. Organomet. Chem. 2002, 646, 39–52. [CrossRef] Power, P.P. Silicon, germanium, tin and lead analogues of acetylenes. Chem. Commun. 2003, 2091–2101. [CrossRef] Weidenbruch, M. Molecules with a genuine Si-Si triple bond and a stable derivative of [SiH]+ . Angew. Chem. Int. Ed. 2005, 44, 514–516. [CrossRef] [PubMed] Power, P.P. Synthesis and some reactivity studies of germanium, tin and lead analogues of alkynes. Appl. Organomet. Chem. 2005, 19, 488–493. [CrossRef] Lein, M.; Krapp, A.; Frenking, G. Why do the heavy-atom analogues of acetylene E2 H2 (E = Si−Pb) exhibit unusual structures? J. Am. Chem. Soc. 2005, 127, 6290–6299. [CrossRef] [PubMed] Sekiguchi, A.; Ichinohe, M.; Kinjo, R. The chemistry of disilyne with a genuine Si–Si triple bond: Synthesis, structure, and reactivity. Bull. Chem. Soc. Jpn. 2006, 79, 825–828. [CrossRef] Power, P.P. Bonding and reactivity of heavier group 14 element alkyne analogues. Organometallics 2007, 26, 4362–4366. [CrossRef] Sekiguchi, A. Disilyne with a silicon-silicon triple bond: A new entry to multiple bond chemistry. Pure Appl. Chem. 2008, 80, 447–451. [CrossRef] Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Interaction of π-bonds of the silicon–silicon triple bond with alkali metals: An isolable anion radical upon reduction of a disilyne. Synt. Met. 2009, 159, 773–780. [CrossRef] Fischer, R.C.; Power, P.P. π-Bonding and the lone pair effect in multiple bonds involving heavier main group elements: Developments in the new millennium. Chem. Rev. 2010, 110, 3877–3882. [CrossRef] [PubMed] Peng, Y.; Fischer, R.C.; Merrill, W.A.; Fischer, J.; Pu, L.; Ellis, B.D.; Fettinger, J.C.; Herber, R.H.; Power, P.P. Substituent effects in ditetrel alkyne analogues: Multiple vs. single bonded isomers. Chem. Sci. 2010, 1, 461–478. [CrossRef] Sasamori, T.; Han, J.S.; Hironaka, K.; Takagi, N.; Nagase, S.; Tokitoh, N. Synthesis and structure of stable 1,2-diaryldisilyne. Pure Appl. Chem. 2010, 82, 603–611. [CrossRef] Sekiguchi, A.; Kinjo, R.; Ichinohe, M. A stable compound containing a silicon–silicon triple bond. Science 2004, 305, 1755–1758. [CrossRef] [PubMed] Wiberg, N.; Vasisht, S.K.; Fischer, G.; Mayer, P.Z. Disilynes. III [1] a relatively stable disilyne RSi≡SiR (R = SiMe(SitBu3 )2 ). Anorg. Allg. Chem. 2004, 630, 1823–1831. [CrossRef] Sasamori, T.; Hironaka, K.; Sugiyama, T.; Takagi, N.; Nagase, S.; Hosoi, Y.; Furukawa, Y.; Tokitoh, N. Synthesis and reactions of a stable 1,2-diaryl-1,2-dibromodisilene: A precursor for substituted disilenes and a 1,2-diaryldisilyne. J. Am. Chem. Soc. 2008, 130, 13856–13868. [CrossRef] [PubMed] Stender, M.; Phillips, A.D.; Wright, R.J.; Power, P.P. Synthesis and characterization of a digermanium analogue of an alkyne. Angew. Chem. Int. Ed. 2002, 41, 1785–1789. [CrossRef] Stender, M.; Phillips, A.D.; Power, P.P. Formation of [Ar*Ge{CH2 C(Me)C(Me)CH2 }CH2 C(Me)=]2 (Ar* = C6 H3 -2,6-Trip2 ; Trip = C6 H2 -2,4,6-i-Pr3 ) via reaction of Ar*GeGeAr* with 2,3-dimethyl-1,3-butadiene: Evidence for the existence of a germanium analogue of an alkyne. Chem. Commun. 2002, 1312–1313. [CrossRef]

Molecules 2017, 22, 1111

24.

25.

26. 27.

28.

29. 30.

31.

32. 33. 34. 35. 36. 37. 38. 39. 40. 41.

42. 43. 44. 45. 46.

12 of 14

Pu, L.; Phillips, A.D.; Richards, A.F.; Stender, M.; Simons, R.S.; Olmstead, M.M.; Power, P.P. Germanium and tin analogues of alkynes and their reduction products. J. Am. Chem. Soc. 2003, 125, 11626–11632. [CrossRef] [PubMed] Sugiyama, Y.; Sasamori, T.; Hosoi, Y.; Furukawa, Y.; Takagi, N.; Nagase, S.; Tokitoh, N. Synthesis and properties of a new kinetically stabilized digermyne: New insights for a germanium analogue of an alkyne. J. Am. Chem. Soc. 2006, 128, 1023–1031. [CrossRef] [PubMed] Spikes, G.H.; Power, P.P. Lewis base induced tuning of the Ge–Ge bond order in a “digermyne”. Chem. Commun. 2007, 85–88. [CrossRef] [PubMed] Phillips, A.D.; Wright, R.J.; Olmstead, M.M.; Power, P.P. Synthesis and characterization of 2,6-Dipp2 H3 C6 SnSnC6 H3 -2,6-Dipp2 (Dipp = C6 H3 -2,6-Pri 2 ): A tin analogue of an alkyne. J. Am. Chem. Soc. 2002, 124, 5930–5931. [CrossRef] [PubMed] Pu, L.; Twamley, B.; Power, P.P. Synthesis and characterization of 2,6-Trip2 H3 C6 PbPbC6 H3 -2,6-Trip2 (Trip = C6 H2 -2,4,6-i-Pr3 ): A stable heavier group 14 element analogue of an alkyne. J. Am. Chem. Soc. 2000, 122, 3524–3525. [CrossRef] Karni, M.; Apeloig, Y.; Schröder, D.; Zummack, W.; Rabezzana, R.; Schwarz, H. Nachweis von HCSiF und HCSiCl als die ersten Moleküle mit formalen C≡Si-Bindungen. Angew. Chem. Int. Ed. 1999, 38, 332–335. Danovich, D.; Ogliaro, F.; Karni, M.; Apeloig, Y.; Cooper, D.L.; Shaik, S. Silynes (RC identical with SiR0 ) and disilynes (RSi identical with SiR0 ): Why are less bonds worth energetically more? Angew. Chem. Int. Ed. 2001, 40, 4023–4027. [CrossRef] Gau, D.; Kato, T.; Saffon-Merceron, N.; Cozar, A.D.; Cossio, F.P.; Baceiredo, A. Synthesis and structure of a base-stabilized C-phosphino-Si-amino silyne. Angew. Chem. Int. Ed. 2010, 49, 6585–6589. [CrossRef] [PubMed] Lühmann, N.; Müller, T. A compound with a Si-C triple bond. Angew. Chem. Int. Ed. 2010, 49, 10042–10046. [CrossRef] [PubMed] Liao, H.-Y.; Su, M.-D.; Chu, S.-Y. A stable species with a formal Ge≡C triple bond–a theoretical study. Chem. Phys. Lett. 2001, 341, 122–131. [CrossRef] Wu, P.-C.; Su, M.-D. The effect of substituents on the stability of triply bonded galliumuantimony molecules: A new target for synthesis. Dalton. Trans. 2011, 40, 4253–4260. [CrossRef] [PubMed] Wu, P.-C.; Su, M.-D. Triply bonded stannaacetylene (RC≡SnR): Theoretical designs and characterization. Inorg. Chem. 2011, 50, 6814–6822. [CrossRef] [PubMed] Wu, P.-C.; Su, M.-D. A new target for synthesis of triply bonded plumbacetylene (RC≡PbR): A theoretical design. Organometallics 2011, 30, 3293–3303. [CrossRef] Paetzold, P. Iminoboranes. Adv. Inorg. Chem. 1987, 31, 123–132. Paetzold, P. Boron Chemistry. In Proceedings of the 6th International Meeting on Born Chemistry, Bechyne, Slovakia, 22–26 June 1987; Hermanek, S., Ed.; World Scientific: Singapore, 1987; p. 446. Paetzold, P. New perspectives in boron-nitrogen chemistry-I. Pure Appl. Chem. 1991, 63, 345–363. [CrossRef] Paetzold, P. Boron-Nitrogen analogues of cyclobutadiene, benzene and cyclooctatetraene: Interconversions. Phosphorus Sulfur Silicon Relat. Elem. 1994, 93, 39–50. [CrossRef] Wright, R.J.; Phillips, A.D.; Allen, T.L.; Fink, W.H.; Power, P.P. Synthesis and characterization of the monomeric imides Ar0 MNAr” (M = Ga or In; Ar0 or Ar” = terphenyl ligands) with two-coordinate gallium and indium. J. Am. Chem. Soc. 2003, 125, 1694–1696. [CrossRef] [PubMed] Dillon, K.B.; Mathey, F.; Nixon, J.F. Phosphorus: The Carbon Copy: From Organophosphorus to Phospha-Organic Chemistry; Wiley: New York, NY, USA, 1998; p. 2. Mosha, D.M.S. The monomeric pentacyanocobaltate (II) anion: Preparation and properties of thallium (I) pentacyanocobaltate (II). J. Chem. Educ. 1982, 59, 1057–1068. [CrossRef] Baldwin, R.A.; Wells, R.L.; White, P.S. Synthesis and characterization of an organothallium-phosphorus adduct: Crystal structure of (Me3 SiCH2 )3 Tl·P(SiMe3 )3 . Main Group Chem. 1997, 2, 67–75. [CrossRef] Francis, M.D.; Jones, C.; Deacon, G.B.; Delbridge, E.E. Synthesis and structural characterization of a novel polyheterocyclopentadienyl thallium (I) complex. Organometallics 1998, 17, 3826–3832. [CrossRef] Fox, A.R.; Wright, R.J.; Rivard, E.; Power, P.P. Tl2 [Aryl2 P4 ]: A thallium complexed diaryltetraphosphabutadienediide and its two-electron oxidation to a diaryltetraphosphabicyclobutane, Aryl2 P4 . Angew. Chem. Int. Ed. 2005, 44, 7729–7733. [CrossRef] [PubMed]

Molecules 2017, 22, 1111

47.

48. 49. 50. 51. 52. 53.

54. 55. 56. 57. 58.

59.

60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71.

13 of 14

Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; et al. Gaussian 09, version Revision D.01; Gaussian, Inc.: Wallingford, CT, USA, 2013. Zhao, Y.; Truhlar, D.G. Density functionals with broad applicability in chemistry. Acc. Chem. Res. 2008, 41, 157–167. [CrossRef] [PubMed] Becke, A.D. Density-Functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A 1988, 38, 3098–3100. [CrossRef] Becke, A.D. Density-Functional thermochemistry. J. Chem. Phys. 1993, 98, 5648–5652. [CrossRef] Lee, C.; Yang, W.; Parr, R.G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37, 785–789. [CrossRef] Perdew, J.P.; Wang, Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys. Rev. 1992, B45, 13244–13249. [CrossRef] Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. [CrossRef] [PubMed] Dunning, T.H., Jr.; Hay, P.J. Modern Theoretical Chemistry; Schaefer, H.F., III, Ed.; Plenum: New York, NY, USA, 1976; pp. 1–28. Hay, P.J.; Wadt, W.R. Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270–283. [CrossRef] Hay, P.J.; Wadt, W.R. Ab initio effective core potentials for molecular calculations. Potentials for main group elements Na to Bi. J. Chem. Phys. 1985, 82, 284–298. Hay, P.J.; Wadt, W.R. Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitals. J. Chem. Phys. 1985, 82, 299–310. [CrossRef] Check, C.E.; Faust, T.O.; Bailey, J.M.; Wright, B.J.; Gilbert, T.M.; Sunderlin, L.S. Addition of polarization and diffuse functions to the LANL2DZ basis set for p-block elements. J. Phys. Chem. A 2001, 105, 8111–8116. [CrossRef] The Wiberg Bond Index, Which is Used to Screen Atom Pairs for the Possible Bonding in the Natural Bonding Orbital (NBO) Search, are Performed with the NBO Program (version NBO 6.0; University of Wisconsin System: Madison, WI, USA, 2013). Available online: http://www.chem.wisc.edu/~nbo5 (accessed on 4 July 2017). Wiberg, K.B. Application of the pople-santry-segal CNDO method to the cyclopropylcarbinyl and cyclobutyl cation and to bicyclobutane. Tetrahedron 1968, 24, 1083–1096. [CrossRef] Reed, A.E.; Curtiss, L.A.; Weinhold, F. Intermolecular interactions from a natural bond orbital, donor-acceptor viewpoint. Chem. Rev. 1998, 88, 899–993. [CrossRef] Emsley, J. The Elements, 2nd ed.; Clarendon Press: Oxford, UK, 1991; p. 285. Pyykko, P.; Desclaux, J.-P. Relativity and the periodic system of elements. Acc. Chem. Res. 1979, 12, 276–282. [CrossRef] Kutzelnigg, W. Chemical bonding in higher main group elements. Angew. Chem. Int. Ed. Engl. 1984, 23, 272–276. [CrossRef] Pyykko, P. Relativistic effects in structural chemistry. Chem. Rev. 1988, 88, 563–623. [CrossRef] Pyykko, P. Strong closed-shell interactions in inorganic chemistry. Chem. Rev. 1997, 97, 597–636. [CrossRef] [PubMed] Kobayashi, K.; Nagase, S. Silicon−Silicon triple bonds: Do substituents make disilynes synthetically accessible? Organometallics 1997, 16, 2489–2495. [CrossRef] Kobayashi, K.; Takagi, N.; Nagase, S. Do bulky aryl groups make stable silicon−silicon triple bonds synthetically accessible? Organometallics 2001, 20, 234–236. [CrossRef] Dapprich, S.; Frenking, G. Investigation of donor-acceptor interactions: A charge decomposition analysis using fragment molecular orbitals. J. Phys. Chem. 1995, 99, 9352–9367. [CrossRef] Glendening, E.D.; Weinhold, F. Natural resonance theory: I. General formalism. J. Comput. Chem. 1998, 19, 593–606. [CrossRef] Glendening, E.D.; Weinhold, F. Natural resonance theory: II. Natural bond order and valency. J. Comput. Chem. 1998, 19, 610–623. [CrossRef]

Molecules 2017, 22, 1111

72.

14 of 14

Glendening, E.D.; Weinhold, F.; Badenhoop, J.K. Natural resonance theory. III. Chemical applications. J. Comput. Chem. 1998, 19, 628–632. [CrossRef]

Sample Availability: Not available. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).