Heterobimetallic SilverâIron Complexes Involving ... - ACS Publications

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Heterobimetallic Silver−Iron Complexes Involving Fe(CO)5 Ligands Guocang Wang,† Yavuz S. Ceylan,‡ Thomas R. Cundari,*,‡ and H. V. Rasika Dias*,† †

Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, United States Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, Denton, Texas 76203, United States

‡

S Supporting Information *

ABSTRACT: Iron(0) pentacarbonyl is an organometallic compound with a long history. It undergoes carbonyl displacement chemistry with various donors (L), leading to molecules of the type Fe(CO)x(L)5−x. The work reported here illustrates that Fe(CO)5 can also act as a ligand. The reaction between Fe(CO)5 with the silver salts AgSbF6 and Ag[B{3,5-(CF3)2C6H3}4] under appropriate conditions resulted in the formation of [(μ-H2O)AgFe(CO)5]2[SbF6]2 and [B{3,5-(CF3)2C6H3}4]AgFe(CO)5, respectively, featuring heterobimetallic {Ag− Fe(CO)5}+ fragments. The treatment of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 with 4,4′-dimethyl-2,2′-bipyridine (Me2Bipy) and Fe(CO)5 afforded a heterobimetallic [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] species with a Ag−Fe(CO)5 bond and a heterotrimetallic [{Fe(CO)5}2(μAg)][B{3,5-(CF3)2C6H3}4] with a (CO)5Fe−Ag−Fe(CO)5 core, respectively, illustrating that it is possible to manipulate the coordination sphere at silver while keeping the Ag−Fe bond intact. The chemistry of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 with Et2O and PMes3 (Mes = 2,4,6-trimethylphenyl) has also been investigated, which led to [(Et2O)3Ag][B{3,5-(CF3)2C6H3}4] and [(Mes3P)2Ag][B{3,5-(CF3)2C6H3}4] with the displacement of the Fe(CO)5 ligand. X-ray structural and spectroscopic data of new molecules as well as results of computational analyses are presented. The Fe−Ag bond distances of these metal-only Lewis pairs range from 2.5833(4) to 2.6219(5) Å. These Ag−Fe bonds are of primarily an ionic/electrostatic nature with a modest amount of charge transfer between Ag+ and Fe(CO)5. The ν(CO) bands of the molecules with Ag−Fe(CO)5 bonds show a ̅ notable blue shift relative to those observed for free Fe(CO)5, indicating a significant reduction in Fe→CO back-bonding upon its coordination to silver(I).

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Warner,44−48 Lewis basicity of transition metal adducts has been surprisingly underappreciated and a less discussed topic in relation to their Lewis acidity. We seek to investigate whether this feature can be exploited to prepare heterometallic complexes such as those involving silver and iron. It is noteworthy that isolable and structurally authenticated heterodi- and tri-metallic complexes that feature Ag−Fe bonds without the help of bridging ligands are rather limited and include (Figure 1) dinuclear [(p-Tol)Ph2P]AgFe(CO)3(PMe3)(SiPh2Me) (1),30 and (IPr)AgFe(Cp)(CO)2 (2)31 and the trinuclear [{Fe(CO)5}2(μ-Ag)][Al{OC(CF3)3}4] (3)32 and [{Fe(PMe3)2(CO)3}2(μ-Ag)][B(3,5-Cl2C6H3)4] (4).33 Furthermore, structurally characterized compounds containing the simple, well-known metal carbonyl complex Fe(CO)5 acting as a ligand are even more rare.12,32 One of these is the trinuclear complex [{Fe(CO)5}2(μ-Ag)][Al{OC(CF3)3}4] (3) noted above, which involves two Fe(CO)5 moieties on silver (with an Fe−Ag−Fe core), while the other is an Fe−Ga bonded compound, (CO)5FeGaCl3.12 Such complexes are of interest not only in terms of structure and bonding but also as a means of tuning the basicity of a transition metal site and as potential catalysts containing earth abundant iron.49−51 In this paper, we describe the synthesis of

INTRODUCTION Heterometallic complexes with direct metal−metal interactions have been investigated widely for their unusual M−M′ bonding and diverse structural features including metallophilic interactions and supramolecular chemistry.1−16 These complexes display fascinating physical properties such as luminescence, magnetism and electrical conductivity,2,4,17,18 and photochemistry. Such complexes are also of relevance in biology, for example, in the active site of hydrogenase enzymes.19,20 Heterometallic complexes are also important in small molecule activation chemistry and catalysis, particularly through harnessing the cooperativity between metal sites of different properties including the transition-metal-only frustrated Lewis pair concept.15,21−29 An area of research focus in our laboratory concerns the use of weakly coordinating ligands and anions such as [HB(3,5(CF3)2Pz]− (Pz = pyrazolyl) and [SbF6]− in conjunction with coinage metal ions (M″(I) = Cu(I), Ag(I), Au(I)) to stabilize reactive or labile M″−L species involving these d10 metal atoms (e.g., M″(I)-CO and M″(I)-ethylene adducts) as thermally stable complexes.34−42 In these complexes, the metal M″ functions as a Lewis acidic site, which is well recognized. A number of transition metal moieties also display Lewis basic properties. Although it is an important trait relevant to homogeneous catalysis5,43 and was noted in early seminal contributions by the groups of Vaska, Collman, Shriver, and © XXXX American Chemical Society

Received: August 12, 2017

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DOI: 10.1021/jacs.7b08595 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Scheme 1. Synthesis of [(μ-H2O)AgFe(CO)5]2[SbF6]2 (5) Using Fe(CO)5 as a Ligand

temperature under nitrogen. It shows IR bands at 2018 (broad), 2082, and 2135 cm−1 corresponding to CO stretches (Table 1), typical for terminal metal−CO groups.59 The X-ray structure of [(μ-H2O)AgFe(CO)5]2[SbF6]2 (5) shows that it is a dimer of [(H2O)AgFe(CO)5] with water molecules serving as the bridge (forming a Ag2O2 core) and sits on an inversion center (Figure 2). Each silver atom coordinates to Fe(CO)5 with an Fe−Ag distance of 2.6028(14) Å as well as to two oxygen atoms of the bridging water molecules (with significantly different Ag−O distances). In addition, there is a somewhat long Ag−F contact (2.64 Å) involving a nearby fluorine atom of a [SbF6]− counterion. This interaction is significant enough to slightly pyramidalize the silver sites (sum of the angles at silver involving O2AgFe = 352.9°). Iron centers have slightly distorted octahedral coordination environments, and silver atoms adopt a distorted trigonal planar geometry. In view of the low thermal stability and solubility of compound [(μ-H2O)AgFe(CO)5]2[SbF6]2, we set out to investigate additional silver(I) adducts of Fe(CO)5 using a more soluble and larger [B{3,5-(CF3)2C6H3}4]− counteranion instead of [SbF6]−. Gratifyingly, [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (6) can be synthesized quite easily by treating Ag[B{3,5(CF3)2C6H3}4] with one equivalent of Fe(CO)5 in CH2Cl2 at −20 °C (Scheme 2). It was isolated as a colorless crystalline solid and characterized by FT-IR, multinuclear NMR spectroscopy, and X-ray crystallography. Although it is not soluble in hexanes or benzene, it is more soluble than compound 5 in dichloromethane. The crystals of 6 are stable in air for several hours at room temperature (at least 4 h with only a trace of decomposition noticeable on the surface). It is also stable in solution under nitrogen at room temperature but decomposes slowly if these solutions are exposed to air. The IR spectrum of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 showed several peaks (2140, 2084, 2061, 2047 cm−1) in the typical metal carbonyl region that can be assigned to CO vibrational frequencies. In CD2Cl2, the 1H NMR spectrum displayed two peaks at δ 7.88 and 7.73 ppm, while the 19F and 11B NMR spectra showed singlets at δ −63.37 and −7.63 ppm, respectively, which are assigned to the [B{3,5-(CF3)2C6H3}4]− ion. Single crystals of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 suitable for X-ray diffraction analysis were obtained from the saturated solutions of CH2Cl2 at −20 °C. The molecular structure reveals that [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 has the same Ag− Fe(CO)5 moiety (Figure 3) observed in [(μ-H2O)AgFe(CO)5]2[SbF6]2. The [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 however is a monomeric, dinuclear species and features a [B{3,5(CF3)2C6H3}4]− that coordinates to the silver atom via ipsocarbons of two phenyl groups in η1-fashion (with Ag−C distances of 2.5183(18) Å). The [B{3,5-(CF3)2C6H3}4]AgFe-

Figure 1. Structurally characterized heterodi- and tri-metallic complexes that feature unsupported Ag−Fe bonds.30−33

the first heterodinuclear, metal-only Fe−Ag Lewis pairs (MOLPs)5 with Fe(CO)5 as the Lewis base. We also demonstrate that it is possible to alter the coordination sphere at silver using certain Lewis bases while keeping the Ag−Fe bond intact.

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RESULTS AND DISCUSSION The first Ag−Fe compound, [(μ-H2O)AgFe(CO)5]2[SbF6]2, of the group we are reporting here was discovered rather unexpectedly during the attempted synthesis of a silver carbonyl adduct. We have been exploring the chemistry of coinage metal carbonyl complexes34−37 for some time and have reported the successful isolation of Cu(I) and Au(I) carbonyl complexes using an N-heterocyclic carbene (NHC) ligand in combination with a weakly coordinating [SbF6]− counterion.52−54 During an attempted synthesis of a silver(I)−carbonyl using the silver−NHC precursor [(SIPr)Ag][SbF6] (SIPr = 1,3bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and CO gas, we obtained trace quantities of a quite reactive, colorless crystalline product of which the IR analysis showed several bands in the 2018 to 2135 cm−1 region, indicating the formation of a metal carbonyl species. This finding however was not consistent with the targeted terminal Ag(I)-CO complex containing an NHC ligand support, as there were several bands and the CO stretch values were somewhat low.52,54−56 Fortunately, this product forms good-quality crystals, and the X-ray analysis revealed that it was indeed not a Ag(I)-CO species, but rather an even more interesting molecule with a Ag−M bond with a composition [(μH2O)AgM(CO)5]2[SbF6]2. Although the identity of “M” was initially baffling, after some careful work, M was pinpointed as Fe, present in the form of Fe(CO)5 as a trace impurity in carbon monoxide cylinders.57,58 The identity of this metal adduct was confirmed by preparing [(μ-H2O)AgFe(CO)5]2[SbF6]2 (5) via a more conventional route from the reaction between one equivalent of freshly distilled Fe(CO)5 and AgSbF6 at −40 °C in dichloromethane that has not been dried (Scheme 1). The compound [(μ-H2O)AgFe(CO)5]2[SbF6]2 precipitated from the solution as a white solid and was obtained in 74% yield. It is a highly air-sensitive compound. Upon exposure to air, the white solid rapidly decomposed to a black material. [(μ-H2O)AgFe(CO)5]2[SbF6]2 is not soluble in hexane, benzene, or toluene and slightly soluble in CH2Cl2 but very unstable in both solution and the solid state at room B


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Table 1. Observed νC̅ O (cm−1) in the IR Spectra for Fe(CO)5,66 [Fe(CO)6][SbF6]2,67 (CO)5FeGaCl3,12 and Molecules with a Ag−Fe(CO)5 Moiety Fe(CO)5

[Fe(CO)6][SbF6]2

5

6

7

8

3

(CO)5FeGaCl3

2022 2000

2242 2219 2205

2135 2082 2018

2140 2084 2061 2047

2132 2051 2041

2133 2090 2073 2054 2037

2137 2093 2084 2076 2066 2063 2048 2038

2086 2022 1986

Figure 2. Molecular structure of [(μ-H2O)AgFe(CO)5]2[SbF6]2 (5). Selected bond lengths (Å) and angles (deg): Ag−Fe 2.6028(14), Ag− O6 2.363(8), Ag−O6i 2.527(8), O6−Ag−O6i 74.6(3).

Figure 3. Molecular structure of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (6). Selected bond lengths (Å) and angles (deg): Ag−Fe 2.6219(5), Ag−C4 2.5183(18), C4−Ag−C4i 63.15(8).

(CO)5 has a 2-fold rotation axis along the B···Ag−Fe. Although the iron center adopts a distorted octahedral coordination environment, the silver features a perfect trigonal planar geometry. The Ag−C distances and [B{3,5-(CF3)2C6H3}4]− coordination modes of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 are not unusual. For example, [(2.2′-bipy)Ag][B{3,5(CF3)2C6H3}4] has both η1- and η2-bonded aryl groups with Ag−C distance of the η1-interaction at 2.640(3) Å and much shorter Ag−C contacts for the η2-bonded aryl moiety (2.424(3) and 2.493(3) Å).60 Good solubility and stability of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 in CH2Cl2 make it ideal for further investigations. We probed the reactivity of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 toward 4,4′-dimethyl-2,2′-bipyridine (Me2Bipy), Fe(CO)5, diethyl ether, and trimesityl phosphine (PMes3). Remarkably, the treatment of the [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 adduct with one equivalent of Me 2Bipy afforded a dinuclear compound, [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] (7,

Scheme 3). This demonstrates that it is possible to alter the coordination sphere of these compounds at silver without breaking the Ag−Fe bond. It can also be synthesized via a separate route by reacting [(Me 2 Bipy)Ag][B{3,5(CF3)2C6H3}4] with one equivalent of Fe(CO)5 (Scheme 3). The 1H NMR spectrum in CD2Cl2 at room temperature shows two peaks at δ 7.77 and 7.55 ppm assignable to the protons on the [B{3,5-(CF3)2C6H3}4]− ion and four resonances at δ 8.33, 7.99, 7.40, and 2.52 ppm corresponding to the silver-bound 4,4′-dimethylbipyridine moiety protons. 13C NMR and 19F NMR spectra displayed a signal at δ 207.3 and −63.18 ppm assignable to carbonyl carbons and fluorines of the trifluoromethyl groups, respectively, indicating a rather fluxional molecule in solution at room temperature. Single crystals of [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] suitable for X-ray crystallography were obtained from saturated solutions of CH2Cl2 and hexane at −20 °C. It features a discrete [(Me2Bipy)AgFe(CO)5]+ cation and [B{3,5-

Scheme 2. Synthesis of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (6) Using Ag[B{3,5-(CF3)2C6H3}4] and Fe(CO)5

C


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Scheme 3. Synthesis of [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] (7) and [{Fe(CO)5}2(μ-Ag)][B{3,5-(CF3)2C6H3}4] (8)

Figure 4. Molecular structure of [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] (7). Selected bond lengths (Å) and angles (deg): Ag−Fe 2.5833(4), Ag−N1 2.279(2), Ag−N2 2.267(2), N1−Ag−N2 73.46(7).

(CF3)2C6H3}4]− counterion (Figure 4). The iron atom, as in complexes 5 and 6, has a distorted octahedral coordination geometry, while silver adopts an essentially trigonal planar arrangement (sum of the bond angles at silver = 357.4°). The two Ag−N distances are similar at 2.279(2) and 2.267(2) Å, which are comparable to the Ag−N distances observed in molecules such as [(2.2′-bipy)Ag][B{3,5-(CF3) 2C6H3}4] (2.292(3), 2.281(3) Å) or [(2,2′-bipy)Ag(NCMe)][BF4] (2.262(2) and 2.271(2) Å).60,61 The treatment of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (6) with an additional equivalent of Fe(CO)5 afforded heterotrimetallic [{Fe(CO)5}2(μ-Ag)][B{3,5-(CF3)2C6H3}4] (8, Scheme 3). This again demonstrates, under certain conditions, the possibility of doing coordination chemistry at silver without

breaking the Ag−Fe(CO)5 bond. The X-ray crystal structure of this molecule is illustrated in Figure 5, and it contains discrete [{Fe(CO)5}2(μ-Ag)]+ cations with a trinuclear Fe−Ag−Fe core and [B{3,5-(CF3)2C6H3}4]− counterions. The silver atom of the Ag{Fe(CO)5}2 moiety sits on an inversion center (with two equal Fe−Ag distances at 2.2925(2) Å), while the boron of the [B{3,5-(CF3)2C6H3}4]− ion lies on a 2-fold rotation axis. Analogous [{Fe(CO)5}2(μ-Ag)][Al{OC(CF3)3}4] (3) containing a different counterion has been reported.32 The [{Fe(PMe3)2(CO)3}2(μ-Ag)][B(3,5-Cl2C6H3)4] (4) adduct also has an Fe−Ag−Fe core but the bis(trimethylphosphine)tris(carbonyl) iron adduct is expected to be a better Lewis base than Fe(CO)5.33 The [{Fe(CO)5}2(μ-Ag)]+ cations of both 3 and 8 are essentially identical, and the carbonyl groups D


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characterized compounds of the form M−Fe(CO)5 involving metal-only Lewis pairs.12,32 The stretching frequencies are in the range expected for typical terminal carbonyls, which corroborate the X-ray structural data. In addition, almost all the ν̅CO bands of 5−8 show a notable blue shift (appear at higher frequencies) relative to those observed for free Fe(CO)5 (ν̅CO = 2022, 2000 cm−1 in n-hexane or 2002, 1979, 1989 cm−1 for pure liquid).66 This is expected, as iron sites of these compounds are bonded to fairly Lewis acidic silver(I), which should reduce the Fe→CO back-bonding relative to those expected in free Fe(CO)5. However, these values (blue shifts) are not as high as those observed for the nonclassical56 carbonyl cation [Fe(CO)6]2+, which has essentially “σ-only” Fe−CO bonding (see Table 1).67 This indicates that, as also pointed out by Malinowski and Krossing for [{Fe(CO)5}2(μ-Ag)][Al{OC(CF3)3}4], electron density at the central iron atoms of 5−8 is intermediate between the neutral iron carbonyl Fe(CO)5 and [Fe(CO)6]2+.32 Key structural parameters extracted from X-ray structural data of compounds 5−8 are summarized in Tables S19 and S20 (Supporting Information). As noted above, molecular structures show that these adducts feature Ag−Fe bonds. Although one of the partners of this interaction is the same (i.e., Fe(CO)5), the coordination number and geometry as well as the type of donor atoms at silver varies: a “pseudo”-fourcoordinate, distorted trigonal planar geometry with an FeO2F coordination sphere in 5; three-coordinate, trigonal planar arrangement with FeC2 and FeN2 coordination spheres, respectively, in 6 and 7; and a two-coordinate, linear geometry with two Fe donors in 8. Thus, it is somewhat difficult to compare and contrast the Ag−Fe bond distances of the new adducts reported here. Nevertheless, the Ag−Fe bond distances found in complexes 5−8 (which range from 2.5833(4) to 2.6219(5) Å) are significantly shorter than the sum of the experimentally derived covalent radii of Ag and Fe (2.77 Å),68 indicating a strong interaction. These heterodinuclear metal atom distances can also be compared to other Ag−Fe complexes. For example, [{Fe(CO) 5 } 2 (μ-Ag)][Al{OC(CF 3 ) 3 } 4 ] (3) and [{Fe(PMe 3 ) 2 (CO) 3 } 2 (μ-Ag)][B(3,5Cl2C6H3)4] (4) have average Ag−Fe bond lengths of 2.5981(6) and 2.628(7) Å, respectively, while [(p-Tol)Ph2P]AgFe(CO)3(PMe3)(SiPh2Me) (1)30 and (IPr)AgFe(Cp)(CO)2 (2)31 have Ag−Fe bonds at the shorter end of distances at 2.581(1) and 2.5215(11) Å, respectively. Note that compounds 1 and 2 are based on formally anionic, significantly more nucleophilic69 [Fe(CO)3(PMe3)(SiPh2Me)]− and [Fe(Cp)(CO)2]− compared to neutral Fe(CO)5; thus shorter Ag− Fe contacts in the latter two molecules are not surprising. Interestingly, the Ag−Fe complex 4 containing more electronrich Fe(PMe3)2(CO)3 ligands has longer Ag−Fe bonds than the corresponding distances of 8 (2.5925(2) Å) with less nucleophilic Fe(CO)5 ligands. This could be either a result of having four relatively large PMe3 groups on the two iron atoms (instead of all carbonyls as in 8) and/or closer Ag···CO contacts in 4 (range of 2.601−2.745 Å compared to 2.747− 3.106 Å in 8). The iron atoms of the AgFe(CO)5 moiety in 5−8 adopt a distorted octahedral coordination with four CO ligands that are perpendicular to the Ag−Fe−Ct axis (Ct = carbon atom of the CO trans to silver, whereas Cc = carbon atoms of carbonyls cis to Ag) significantly bent toward the Ag atom. This is evident from the Ag−Fe−Cc angles of 5−8 (see Table S20), most of which deviate from the ideal 90° angle. The closest Ag···Cc

Figure 5. Molecular structure of [{Fe(CO)5}2(μ-Ag)][B{3,5-(CF3)2C6H3}4] (8). Selected bond lengths (Å) and angles (deg): Ag−Fe 2.5925(2), Ag−Fei 2.5925(2), Fe−Ag−Fei 180.0.

perpendicular to the Fe−Ag−Fe axis surprisingly adopt an eclipsed configuration. Recrystallization of [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (6) from Et2O/CH2Cl2/hexane (3:1:3) afforded a three-coordinate, diethyl ether complex of silver, [(Et 2 O) 3 Ag][B{3,5(CF3)2C6H3}4] (9), with the loss of the Fe(CO)5 ligand. The 13 C NMR spectra of 5, 6, and 8 taken in deuterated DMSO at room temperature show a resonance corresponding to carbonyl carbons at an essentially identical position (δ ∼210 ppm), indicating likely displacement of Fe(CO)5 from silver by DMSO. Addition of only very limited quantities of diethyl ether (∼3 equiv) to 6 appears to produce a compound that retains the Ag−Fe(CO)5 fragment based on IR analysis of the reaction mixture, but we have not yet been able to isolate such a species in pure form. The latter experiments also suggest that it may be possible to stabilize cationic [Ag−Fe(CO)5]+ with appropriate oxygen-containing ligands under certain conditions, as observed with [(μ-H2O)AgFe(CO)5]2[SbF6]2 (5). Treatment of [B{3,5(CF3)2C6H3}4]AgFe(CO)5 (6) with one equivalent of Mes3P leads to the loss of the Fe(CO) 5 ligand, producing [(Mes3P)2Ag][B{3,5-(CF3)2C6H3}4] (10), rather than a silver-monophosphine adduct with the Ag−Fe bond intact. Both 9 and 10 have been characterized by NMR spectroscopy and X-ray crystallography (Supporting Information). It is interesting to note that three-coordinate silver adducts involving diethyl ether are rare. A search of the Cambridge Structural database62 produced only [(Et2O)3Ag][N{CN·B(C6F5)3}2] as a related example with a [(Et2O)3Ag]+ moiety.63 Molecules involving [(Mes3P)2Ag]+ are not unusual, and structural data for adducts with different counterions are known.64 In addition to the coordination modes of CO (e.g., terminal vs bridging), carbonyl stretching frequencies of metal carbonyls provide valuable information about the electron density at the metal.56,59,65 The positions of the ν̅CO bands observed in the IR spectra for compounds 5−8 as well as for several related compounds are summarized in Table 1. This includes trimetallic [{Fe(CO)5}2(μ-Ag)][Al{OC(CF3)3}4] and dimetallic (CO)5FeGaCl3, representing the only other structurally E


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Figure 6. DFT-calculated geometries of “naked” [Ag−Fe(CO)5]+ and [{Fe(CO)5}2(μ-Ag)]+ cations. Bond lengths in Å, and angles in degrees.

Ag atom in [{Fe(CO)5}2(μ-Ag)]+ was coordinated by two Fe atoms linearly (180.0°) and equally distant (Ag−Fe = 2.64 Å), Figure 6. Experimental results for 8 yielded Ag−Fe = 2.59 Å and Fe−Ag−Fe = 180.0°, Table S19. Calculations also reproduced the angles to the cis carbonyls: Ag−Fe−Cc = 75.0° (74.7° and 75.4°, exptl) with the other two cis carbonyls at Ag−Fe−Cc = 87.0° (85.7° and 89.2°, exptl). The “naked” mono-Fe(CO)5 cation, [Ag−Fe(CO)5]+, yields nearly perfect C4v symmetry upon geometry optimization, Ag−Fe = 2.59 Å, Ag−Fe−Cc = 83.0°, Figure 6. Carbonyl stretching frequencies of metal carbonyls provide valuable information about the electron density at the metal. DFT-calculated reporter ligand CO frequencies of [Fe(CO)6]2+, [Fe(CO)5]+, [Ag−Fe(CO)5]+, [{Fe(CO)5}2(μAg)]+, Fe(CO)5, and (IPr)AgFe(Cp)(CO)2 models were calculated at the same level of theory to assess the bonding in the newly characterized complexes reported here. DFT analyses were adopted to optimize and calculate (ν̅CO) frequencies of these complexes. Frequencies were averaged to simplify the analysis as per the recent work of Wolczanski,77 which resulted in νC̅ O = 2190, 2113, 2090, 2066, 2023, and 1902 cm−1 for [Fe(CO)6]2+, [Fe(CO)5]+, [Ag−Fe(CO)5]+, [{Fe(CO) 5 } 2 (μ-Ag)] + , Fe(CO) 5 , and (IMe)AgFe(Cp)(CO)2,31 respectively, the latter being an electron-rich FeAg complex based on a compound reported by Mankad and coworkers (Dipp substituents on the NHC were replaced with methyl groups). Similar calculations were completed for the neutral [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 complex, with an average νC̅ O of 2053 cm−1, 37 cm−1 lower than the naked [Ag− Fe(CO)5]+ cation. It is interesting that [{Fe(CO)5}2(μ-Ag)]+ (ν̅CO = average 2066 cm−1) has a lower ν̅CO or red shift of 24 cm−1 relative to [Ag−Fe(CO)5]+ (νC̅ O = average 2090 cm−1), indicating greater Fe→CO π-back-bonding in the former, but it is not as high as that calculated for [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (ν̅CO = average 2053 cm−1). The average Fe−C bond distances based on X-ray data of the mono-Fe(CO)5 adducts 5−7 (1.827, 1.833, 1.822 Å, respectively) are shorter than the corresponding value of the bis-Fe(CO)5 adduct 8 (1.835 Å), which is consistent with expectations from computed vibrational data. All complexes, apart from the electron-rich (IMe)AgFe(Cp)(CO)2, have a higher than average calculated ν̅CO of Fe(CO)5 (2023 cm−1). Hence, by this metric, all the newly reported complexes display less Fe→ CO π-back-bonding than Fe(CO)5. One may also surmise that the [Fe(CO)5] moiety in these complexes carries a partial positive charge, as the average ν̅CO, while greater than neutral Fe(CO)5, lies below the computed value for cationic [Fe(CO)5]+.

distance among the Ag−Fe(CO)5 complexes 5−8 is found in the heterotrimetallic [{Fe(CO)5 }2 (μ-Ag)][B{3,5-(CF3 ) 2C6H3}4] (8) adduct at 2.747 Å, while the closest Ag···Cc separation in the heterodimetallic complexes is present in complex 7 (2.756 Å). For comparison, silver(I) carbonyl adducts such as [Ag(CO)2]+ and [HB(3,5-(CF3)2Pz]AgCO with terminal CO groups have much shorter Ag−C distances (2.156 and 2.037 Å, respectively),39,70 while the Bondi’s van der Waals contact distance between Ag and C is 3.42 Å.71,72 It is also possible to calculate the bridge asymmetry parameter (α) for compounds 5−8 using Fe−CO and Ag···CO distances as described by Curtis et al.73,74 (Table S21). The term α can provide a way to differentiate terminal, bridging, and semibridging carbonyls in metal adducts. According to this system, carbonyls with α ≥ 0.6 are considered essentially terminal, whereas α ≤ 0.1 are bridging carbonyls. Carbonyls with α values in-between these two cutoff points could be considered as semibridging carbonyls. The α-values of compounds 5−8 show that most of them are above or near 0.6, while a few may be considered as semibridging under this classification (lowest with α = 0.51 in 8). For comparison, the lowest α-value in compound 3 and (CO)5FeGaCl3 are 0.52 and 0.54, respectively (Table S21). The Fe−C distances of compounds 5−8 range from 1.851(3) to 1.812(3) Å. They show a notable lengthening relative to the corresponding distances found in free Fe(CO)5 (1.801−1.811 Å)75 but significantly shorter than in [Fe(CO)6]2+ (1.903−1.917 Å).76 Complex [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (6) was chosen as a reference complex in order to understand the nature of the Ag−Fe bond. Utilizing identical DFT methods to those of Malinowski and Krossing,32 experimental geometries were well reproduced; upon geometry optimization the [B{3,5(CF3)2C6H3}4]AgFe(CO)5 complex showed that Ag, Fe, and the boron of the borate anion were aligned linearly, 179.8° (180.0°, exptl), with distances of 2.62 Å (2.62 Å, exptl) and 3.08 Å (3.13 Å, exptl) for Ag−Fe and Ag−B, respectively. The calculations indicated that two of the four cis CO ligands in [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 were perpendicular relative to the Ag−Fe bond axis (89.6° vs 85.1°, exptl, see Table S20), while the other two CO ligands were bent toward the Ag atom (70.5° vs 78.2°, exptl). The [B{3,5-(CF3)2C6H3}4]Ag + Fe(CO)5 → [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 reaction has a computed ΔH value of −33 kcal/mol, pointing to a strong driving force for the formation of 6 with a surprisingly strong Ag−Fe bond. For comparison, the “naked” [Ag−Fe(CO)5]+ and [{Fe(CO)5}2(μ-Ag)]+ cations were geometry optimized at the same level of theory as [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (6). The F


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Figure 7. Total electron density of [Ag−Fe(CO)5]+ (left) and [{Fe(CO)5}2(μ-Ag)]+ (right). Isovalue = 0.10 (fine grid, full density matrix).

Table 2. Analysis of the AgFe σ Orbitals in [Ag−Fe(CO)5]+ and [{Fe(CO)5}2(μ-Ag)]+ via a Mulliken Analysis of CAS(4,4) Natural Orbitals

The total density of the complex was analyzed and plotted in Figure 7 for [Ag−Fe(CO)5]+ and [{Fe(CO)5}2(μ-Ag)]+. Note the lack of density in the Ag−Fe internuclear region at the 0.1 contour value, which is a typical value used to represent bond density, and the near-spherical nature of the Ag electron density. This clearly points to an ionic/electrostatic interaction. In light of the blue shifting of the ν̅CO relative to Fe(CO)5 and the red shifting of ν̅CO for [Ag−Fe(CO)5]+ versus the borate salt 6, one may surmise a modest degree of charge transfer from the Fe(CO)5 to Ag+, which is ameliorated to some extent by the counterion. It is also worth noting that the electron density plots of the analyzed complexes indicated minimal covalent bonding between the carbons of COs and the silver ion, Figure 7. There is no obvious buildup of electron density between Ag and the cis-carbonyls, suggestive of minimal Ag···cCO interaction. This finding is consistent with the experimentally observed and computed CO stretching frequencies, which lie in a region where typical terminal CO bands are observed, and are much higher than typical bridging carbonyl stretches. Geometry optimization of [HFe(CO)5]+ also showed acute H−Fe−cCO angles of 83° (and an average CO stretching frequency of 2132 cm−1). There are also additional data from other sources pointing to the insignificant nature of these silver ion and CO interactions. For example, computational studies show a barrier-free Fe(CO)5 rotation around the Fe−Ag bond in the [{Fe(CO)5}2(μ-Ag)]+ cation.32 The bending of OCc ligands toward gallium has also been noted in the main group adduct (CO)5FeGaCl3.12 Furthermore, studies on copper adducts such as (IMes)CuFe(Cp*)(CO)2 that feature even shorter M···Cc distances (2.408(1) Å)25 indicate that such contacts are weak and possibly dictated by crystal packing forces. To further probe the bonding between Ag and Fe, CASSCF (complete active space SCF) (4-orbital, 4-electron) calculations were carried out at the DFT-optimized geometries using the GAMESS package.78 According to the CAS(4,4) wave function, highly polarized AgFe σ-bonds exist for [Ag−Fe(CO)5]+ and [{Fe(CO)5}2(μ-Ag)]+. For [Ag−Fe(CO)5]+, the AgFe σ natural orbital was 62% Fe and 17% Ag (ca. 4:1 ratio, Table 2) with the remainder ligand character via a Mulliken analysis, Figure 8 (left). The [{Fe(CO)5}2(μ-Ag)]+ in-phase σFeAg is nearly identical61% Fe, 18% Ag, with the remainder ligand character, Figure 8 (right)in its composition to the AgFe σ natural orbital in the mono-Fe(CO)5 complex. Hence, the AgFe bonds are very polarized, suggestive of an electrostatic or ionic interaction.

natural orb.

nat. orb. occ. (e−)

Fe pop. (e−)

Ag pop. (e−)

+

AgFe σ AgFe σ* AgFe σ AgFe σ*

[Ag−Fe(CO)5] 1.90 1.18 0.10 0.06 [{Fe(CO)5}2(μ-Ag)]+ 1.90 0.58 on each Fe 0.10 0.03 on each Fe

0.33 0.02 0.34 0.01

Figure 8. CAS(4,4) σFeAg natural orbitals for [Ag−Fe(CO)5]+ (left) and [{Fe(CO)5}2(μ-Ag)]+ (in-phase combination, right). Isovalue = 0.045.

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CONCLUSIONS The Fe(CO)5 is a molecule with a more than 125-year history.79 It most typically undergoes carbonyl displacement chemistry with a variety of donors (L), leading to molecules of the type Fe(CO)x(L)5−x.12,59,80−82 Compounds such as [(μH2O)AgFe(CO)5]2[SbF6]2 (5), [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (6), [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] (7), and [{Fe(CO)5}2(μ-Ag)][B{3,5-(CF3)2C6H3}4] (8) demonstrate a different facet of Fe(CO)5 in which it acts as a Lewis base. X-ray crystal structures show that these molecules feature heterometallic Ag−Fe(CO)5 bonds. It is also possible to perform ligand substitution chemistry on silver or have different donors on silver without breaking the Ag−Fe bond. The ν̅CO bands of these molecules show a notable blue shift relative to those observed for free Fe(CO)5, indicating a significant reduction in Fe→CO back-bonding as a result of bonding to the Lewis acidic silver sites. A primarily electrostatic FeAg interaction with a modest amount of charge transfer between Ag+ and Fe(CO)5 is concluded from scrutiny of CASSCF wave function and DFT density plots, which is further corroborated by experimental and computational analysis of CO stretching frequencies. We are presently exploring the chemistry of these as well as other metal-only Lewis pairs. G


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as a colorless crystalline solid (228 mg, 65% yield). 1H NMR (CD2Cl2, 300.53 MHz, 298 K) δ: 8.33 (d, J = 5.1 Hz, 2H), 7.99 (s, 2H), 7.77 (s, 8H), 7.55 (s, 4H), 7.40 (d, J = 5.1 Hz, 2H), 2.52 (s, 6H, CH3). 13 C{1H} NMR (CD2Cl2, 125.77 MHz, 298 K) δ: 207.3 (CO). 162.0 (q, JCB = 49 Hz, CB), 152.1 (d), 150.4 (s), 135.0 (s), 129.5 (q, JCF = 29 Hz, CF3), 128.1 (s), 127.1 (s), 126.0 (s), 123.8 (s), 121.7 (s), 118.0 (s), 21.6 (s). 19F NMR (CD2Cl2, 282.78 MHz, 298 K) δ: −63.18 (s, CF3). FT-IR (solid samples on ATR), ν̅(CO) cm−1: 2132 (CO), 2051 (CO), 2041 (CO). Anal. Calcd for C49H24AgBF24FeN2O5: C, 43.56; H, 1.79; N, 2.07. Found: C, 43.24; H, 1.64; N, 2.05. Compound 7 can also be synthesized from 6. A solid 4,4′-dimethyl-2,2′-bipyridine (3.1 mg, 0.017 mmol) sample was added to a solution of 6 (20 mg, 0.017 mmol) in CD2Cl2 (0.5 mL) in an NMR tube at room temperature. The resulting colorless solution was analyzed by 1H and 13C NMR spectroscopy, which indicated the formation of [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] in >90% yield. [{Fe(CO)5}2(μ-Ag)][B{3,5-(CF3)2C6H3}4] (8). A yellow solution of Fe(CO)5 (196 mg, 1 mmol) in CH2Cl2 (1.7 mL) was added dropwise to a colorless solution of Ag[B{3,5-(CF3)2C6H3}4] (485.5 mg, 0.5 mmol) in CH2Cl2 (10 mL) at −20 °C. The resulting colorless solution was stirred for 10 min. The mixture was concentrated under reduced pressure and kept in a refrigerator at −20 °C overnight to obtain [{Fe(CO)5}2(μ-Ag)][B{3,5-(CF3)2C6H3}4] as colorless crystals (580 mg, 85% yield). FT-IR (solid samples on ATR), ν(CO) cm−1: 2133 ̅ (CO), 2090 (CO), 2073 (CO), 2054 (CO), 2037 (CO). Compound 8 can also be synthesized from compound 6. A yellow solution of Fe(CO)5 (19.6 mg, 0.1 mmol) in CH2Cl2 (1.0 mL) was added dropwise to a colorless solution of 6 (117 mg, 0.1 mmol) in CH2Cl2 (4 mL) at −20 °C. The resulting mixture was stirred for 10 min and kept in a refrigerator at −20 °C overnight to obtain [{Fe(CO)5}2(μAg)][B{3,5-(CF3)2C6H3}4] as a colorless crystalline solid (86 mg, 63% yield). [(Et2O)3Ag][B{3,5-(CF3)2C6H3}4] (9). A solution of [B{3,5(CF3)2C6H3}4]AgFe(CO)5 (117 mg, 0.1 mmol) in Et2O/CH2Cl2/ hexane (3 mL, 3:1:3) was kept in a freezer at −20 °C to obtain [(Et2O)3Ag][B{3,5-(CF3)2C6H3}4] as a colorless crystalline solid (100 mg, 84% yield). 1H NMR (CD2Cl2, 300.53 MHz, 298 K) δ: 7.81 (s, 8H), 7.70 (s, 4H), 3.54 (m, 12H, OCH2CH3), 1.16 (m, 18H, OCH2CH3). 19F NMR (CD2Cl2, 282.78 MHz, 298 K) δ: −62.9 (s, CF3). Anal. Calcd for C32H12AgBF24·OEt2: C, 41.37; H, 2.12. Found: C, 40.38; H, 2.25. [(Mes3P)2Ag][B{3,5-(CF3)2C6H3}4] (10). A solution of [B{3,5(CF3)2C6H3}4]AgFe(CO)5 (180 mg, 0.154 mmol) and PMes3 (60 mg, 0.154 mmol) in CH2Cl2/hexane (1:2, 3 mL) was kept in a freezer at −20 °C to obtain [(Mes3P)2Ag][B{3,5-(CF3)2C6H3}4] as a crystalline solid (109 mg, 81% yield). 1H NMR (CD2Cl2, 500.16 MHz, 298 K) δ: 7.74 (s, 8H), 7.58 (s, 4H), 6.88 (s, 12H, Mes), 2.36 (s, 18H, CH3), 2.11 (s, 18H, CH3), 1.82 (s, 18H, CH3). 19F NMR (CD2Cl2, 470.62 MHz, 298 K) δ: −62.82 (s, CF3). 31P NMR (CD2Cl2, 121.65 MHz, 298 K) δ: −27.65 (1J(P, 107Ag) = 513 Hz, 1J(P, 109Ag) = 592 Hz). The spectroscopic data of [(Mes3P)2Ag]+ agree well with the data from the [(Mes3P)2Ag][BF4] analogue.84 X-ray Data Collection and Structure Determinations. A suitable crystal covered with a layer of hydrocarbon/Paratone-N oil was selected and mounted on a Cryo-loop and immediately placed in the low-temperature nitrogen stream. The X-ray intensity data were measured at 100(2) K on a Bruker D8 Quest with a Photon 100 CMOS detector equipped with an Oxford Cryosystems 700 series cooler, a Triumph monochromator, and a Mo Kα fine-focus sealed tube (λ = 0.710 73 Å). Intensity data were processed using the Bruker Apex program suite. Absorption corrections were applied by using SADABS.85 Initial atomic positions were located by SHELXT,86 and the structures of the compounds were refined by the least-squares method using SHELXL87 within Olex2 GUI.88 All the non-hydrogen atoms were refined anisotropically. The H atoms were included in their calculated positions and refined as riding on the atoms to which they are joined. X-ray structural figures were generated using Olex2.88 Additional information including figures and CIF for [(μ-H2O)AgFe(CO) 5 ] 2 [SbF 6 ] 2 (5), [B{3,5-(CF 3 ) 2 C 6 H 3 } 4 ]AgFe(CO) 5 (6), [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] (7), [{Fe(CO)5}2(μ-

EXPERIMENTAL PROCEDURES

All manipulations were carried out under an atmosphere of purified nitrogen using standard Schlenk techniques or in an MBraun LABMaster glovebox equipped with a −10 °C refrigerator. Solvents were purchased from commercial sources and purified by conventional methods prior to use. Glassware was oven-dried at 150 °C overnight. The NMR spectra were recorded at room temperature on a JEOL Eclipse 500 and a JEOL Eclipse 300 spectrometer (1H: 500.16 and 300.53 MHz; 13C: 125.77 and 75.57 MHz; 19F: 470.62 and 282.78 MHz; 11B: 96.42 MHz, 31P: 121.65 MHz). Chemical shifts for 1H and 13 C spectra are referenced to the solvent peak (1H; CD2Cl2, δ 5.32, DMSO-D6, δ 2.50; 13C; CD2Cl2, δ 53.84, DMSO-D6, δ 39.52). 19F NMR chemical shifts were referenced relative to external CFCl3. IR spectra were collected at room temperature on a Shimadzu IRPrestige21 FTIR containing an ATR attachment at 2 cm−1 resolution. Elemental analyses were performed at Intertek USA, Whitehouse, NJ. AgSbF6 was purchased from Sigma-Aldrich Company and was used as received. Fe(CO)5 was obtained from Sigma-Aldrich Company and distilled prior to use. Deuterated solvents were purchased from Acros Organics and Cambridge Isotope Laboratories, respectively. Ag[B{3,5(CF3)2C6H3}4] was prepared according to literature procedures.83 [(μ-H2O)Ag(Fe(CO)5)]2[SbF6]2 (5). Fe(CO)5 (36 mg, 0.18 mmol) in CH2Cl2 (1 mL) was added dropwise to a solution of AgSbF6 (62 mg, 0.18 mmol) in wet CH2Cl2 (10 mL; taken directly from the CH2Cl2 bottle without drying) at −40 °C. The mixture was allowed to slowly warm to −20 °C to obtain [(μ-H2O)Ag(Fe(CO)5)]2[SbF6]2 as a crystalline white solid, which was separated by filtration (74 mg, 74% yield). Single crystals suitable for X-ray diffraction were obtained from a saturated CH2Cl2 solution at −20 °C. FT-IR (single crystals on ATR), ν̅(CO) cm−1: 2135 (CO), 2082 (CO), 2018 (CO). The compound is not soluble in hexane or benzene but is slightly soluble in CH2Cl2. It is quite unstable even as a solid state at room temperature. Elemental analysis of crystalline material was attempted, but due to fast sample decomposition at room temperature, satisfactory data could not be obtained. [B{3,5-(CF3)2C6H3}4]AgFe(CO)5 (6). A yellow solution of Fe(CO)5 (80 mg, 0.41 mmol) in CH2Cl2 (1 mL) was added dropwise to Ag[B{3,5-(CF3)2C6H3}4] (400 mg, 0.41 mmol) in CH2Cl2 (10 mL) at −20 °C. The resulting pale-yellow solution was stirred for 10 min and kept in a refrigerator at −20 °C overnight to obtain [B{3,5(CF3)2C6H3}4]AgFe(CO)5 as colorless crystals (306 mg, 64% yield). 1 H NMR (CD2Cl2, 300.53 MHz, 298 K) δ: 7.88 (s, 8H), 7.73 (s, 4H). 19 F NMR (CD2Cl2, 282.78 MHz, 298 K) δ: −63.37 (s, CF3). 11B NMR (CD2Cl2, 96.42 MHz, 298 K) δ: −7.70 (s). ESI−/MS: m/z 863.03 ([B{3,5-(CF3)2C6H3}4]−). FT-IR (solid samples on ATR), ν(CO) cm−1: 2140 (CO), 2084 (CO), 2061 (CO), 2147 (CO). Anal. ̅ Calcd for C37H12AgBF24FeO5: C, 38.08; H, 1.04. Found: C, 37.93; H, 0.84. The compound is not soluble in hexane or benzene but is more soluble than compound 5 in CH2Cl2. [(Me2Bipy)Ag][B{3,5-(CF3)2C6H3}4]. A mixture of Na[B{3,5(CF3)2C6H3}4] (232 mg, 0.262 mmol), AgSbF6 (90 mg, 0.262 mmol), and 4,4′-dimethyl-2,2′-bipyridine (48 mg, 0.262 mmol) in CH2Cl2 (10 mL) was stirred overnight. After filtration, 10 mL of hexane was added to the colorless filtrate and kept in a 5 °C refrigerator to give colorless crystals of [(Me2Bipy)Ag][B{3,5(CF3)2C6H3}4] (218 mg, 72% yield). 1H NMR (CD2Cl2, 300.53 MHz, 298 K) δ: 7.95 (s, 8H), 7.86 (d, J = 5.1 Hz, 2H), 7.79 (s, 2H), 7.58 (s, 4H), 7.27 (d, J = 5.1 Hz, 2H), 2.45 (s, 6H, CH3). 19F NMR (CD2Cl2, 282.78 MHz, 298 K) δ: −63.4 (s, CF3). Anal. Calcd for C44H24AgBF24N2: C, 45.74; H, 2.09; N, 2.42. Found: C, 45.31; H, 2.04; N, 2.35. See Supporting Information for the X-ray crystal structure. [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] (7). A yellow solution of Fe(CO)5 (50 mg, 0.260 mmol) in CH2Cl2 (0.6 mL) was added dropwise to a solution of [(Me2Bipy)Ag][B{3,5-(CF3)2C6H3}4] (300 mg, 0.260 mmol) in CH2Cl2 (10 mL) at −20 °C. The resulting colorless solution was stirred for 10 min, and hexane (10 mL was added. The solution was concentrated and kept in refrigerator at −20 °C overnight to obtain [(Me2Bipy)AgFe(CO)5][B{3,5-(CF3)2C6H3}4] H


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Journal of the American Chemical Society Ag)][B{3,5-(CF3)2C6H3}4] (8), [(Et2O)3Ag][B{3,5-(CF3)2C6H3}4] (9), and [(Mes3P)2Ag][B{3,5-(CF3)2C6H3}4] (10) are presented in the Supporting Information. In addition, CCDC 1568632−1568637 contain the supplementary crystallographic data. Computational Methods. The Gaussian 0989 package was employed to perform all simulations; DFT with dispersion correction D3 was used to investigate and optimize the geometries of complexes. The BP86 functional was used along with the def2-TZVPP effective core potential for Fe and Ag, and the 6-311++G(d,p) basis set was used for main group elements; this level of theory was used in a study of a bare silver complex with Fe(CO)5 as ligands.32 Furthermore, natural bond orbital analysis and MCCSF computations (4-orbital, 4electron CASSCF) wave functions were carried out with the GAMESS package78 to investigate the bonding character among the Ag and Fe for all complexes studied. Orbitals were plotted graphically with GaussView590 or MacMolplt.91 All systems were fully optimized to obtain vibrational frequencies in the gas phase at 1 atm and 298.15 K. The same computational methods have been used in a related study involving a bare silver complex with Fe(CO)5 as ligands.32

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08595. Tables of selected bond distances and angles, additional figures and details, X-ray crystallographic data and molecular structure of [(Me 2 Bipy)Ag][B{3,5(CF3)2C6H3}4], and computational data, including Cartesian coordinates (PDF) X-ray crystallographic data for 5−10 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

H. V. Rasika Dias: 0000-0002-2362-1331 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Robert A. Welch Foundation (Grant Y-1289) and the National Science Foundation (CHE1265807). The authors thank the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, for support of this research via grant DE-FG02-03ER15387. The authors also acknowledge the National Science Foundation for their support of the UNT Chemistry CASCaM high performance computing facility through grant CHE-1531468.

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