Synthesis, crystal structure and DFT calculations of a

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Journal of Molecular Structure 1169 (2018) 110e118

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Synthesis, crystal structure and DFT calculations of a cyanido-bridged dinuclear zinc(II) complex of cis-1,2-diaminocyclohexane (Dach) containing a dinuclear cyanidozincate(II) anion, [Zn2(Dach)4(CN)] [Zn2(CN)7]$2CH3OH Muhammad Monim-ul-Mehboob a, Tobias Rüffer b, Heinrich Lang b, Wiktor Zierkiewicz c, Abdul Rauf d, Muhammad Amin d, Anvarhusein A. Isab e, Omar Ajouyed f, Saeed Ahmad f, * a

Department of Chemistry, Government Dyal Singh College, Lahore, Pakistan Inorganic Chemistry Division, Institute of Chemistry, Faculty of Natural Sciences, Technische Universitaet Chemnitz, 09107, Chemnitz, Germany c Faculty of Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370, Wroclaw, Poland d Department of Chemistry, University of Engineering and Technology, Lahore, 54890, Pakistan e Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia f Department of Chemistry, College of Sciences and Humanities, Prince Sattam bin Abdulaziz University, Al-Kharj, 11942, Saudi Arabia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 December 2017 Received in revised form 24 April 2018 Accepted 15 May 2018 Available online 19 May 2018

A zinc(II)-cyanide complex of cis-1,2-diaminocyclohexane (Dach) containing two dinuclear units, [Zn2(Dach)4(CN)][Zn2(CN)7]·2CH3OH (1) was prepared and characterized by IR and NMR spectroscopy as well as by single crystal X-ray crystallography. Complex 1 is composed of two dinuclear ionic units, cationic [Zn2(Dach)4(CN)]3þ and anionic [Zn2(CN)7]3- and two non-coordinating molecules of methanol. The Zn atoms in [Zn2(Dach)4(CN)]3þ cation adopt a square-pyramidal environment with CN ligand occupying the apical position, while the Zn atoms in [Zn2(CN)7]3- anion exhibit a tetrahedral geometry. The dinuclear units are associated to each other through hydrogen bonding interactions to form a three dimensional network in the solid state. The presence of, a band around 2200 cm1 in IR spectrum and a resonance at 140 ppm in 13C NMR indicated the presence of cyanide in the complex. The structures of 1 and two of its analogues, [Zn(Dach)(CN)2]4 (2) and {[Zn(Dach)2][Zn(CN)4]}2 (3) were predicted using theoretical (DFT) method. The DFT calculated values of the Gibbs free energies of the model complexes reveal that 1 is more stable than the analogous predicted complexes, 2 and 3 by 5.16 and 3.47 kcal mol1, respectively. © 2018 Elsevier B.V. All rights reserved.

Keywords: Zinc(II) cis-1,2-Diaminocyclohexane Cyanide X-ray structure DFT calculations

1. Introduction Cyanide ion has strong affinity for metals and forms complexes with almost any metal [1,2]. It may coordinate either through C atom as a monodentate ligand, or through both C and N atoms as a bridging ligand, forming polymeric compounds with a one-, twoor three-dimensional networks [3,4]. Zinc is used to complex cyanide for the isolation of precious metals in the cyanadation process [5,6]. Therefore, the complexation of Zinc(II) with cyanide is very important from metallurgical point of view. In spite of this importance there are only very few reports on the crystal structures of

* Corresponding author. E-mail address: [email protected] (S. Ahmad). https://doi.org/10.1016/j.molstruc.2018.05.049 0022-2860/© 2018 Elsevier B.V. All rights reserved.

zinc cyanide-dia(i)mine systems [7e10] although other zinc(II) complexes of diamines have been studied extensively [11e29]. Except (2,9-dimethyl-1,10-phenanthroline)dicyanidozinc(II) [7], which is characterized by a Zn(CN)2 unit, in the other reported complexes, zinc cyanide exists in the anionic form as [Zn(CN)4]2[8e10,30,31]. In this paper, we report the crystal structure of a new dinuclear zinc(II) cyanide complex of cis-1,2-diaminocyclohexane (Dach), [Zn2(Dach)4(CN)][Zn2(CN)7]$2CH3OH (1), which is the first report describing the formation of a discrete dimeric zinc cyanide anion, [Zn2(CN)7]3-. However, CN bridging in other [Zn(CN)4]2complexes is already known [9,10]. DFT optimized structures of 1 and two related complexes, [Zn(Dach)(CN)2]4 (2) and {[Zn(Dach)2] [Zn(CN)4]}2 (3) are also presented. We have recently reported the crystal structures of the related zinc(II) chloride and thiocyanate

M. Monim-ul-Mehboob et al. / Journal of Molecular Structure 1169 (2018) 110e118

complexes of cis-1,2-diaminocyclohexane, [Zn(Dach)2][ZnCl4] [11] and [Zn(Dach)(NCS)2] [12]. 2. Experimental 2.1. Chemicals ZnCl2, cis-1,2-diaminocyclohexane (Dach) and potassium cyanide were obtained from Merck Chemical Co., Germany. 2.2. Synthesis A solution of 0.14 g (1 mmol) cis-1,2-diaminocyclohexane in 15 mL methanol was added to 0.14 g ZnCl2 (1 mmol) in 5 mL distilled water. A clear solution was obtained on mixing. After stirring for 15 min, 0.13 g KCN (2 mmol) in 10 mL water was added to this solution. The colorless solution was stirred for 15 min and then filtered. The filtrate was kept for crystallization at room temperature. After 3 days, light yellow crystals were obtained (M.p. ¼ 162e164  C). Anal. for 1, Found: C, 40.91; H, 6.28; N, 22.36%. Calc.: C, 41.23; H, 6.51; N, 22.62. 2.3. IR and NMR measurements The solid state FT-IR spectrum of the complex was recorded on a Nicolet 6700 FTIR spectrophotometer over the range 4000e400 cm1. The spectra were collected from 16 scans at a spectral resolution of 2 cm1. NMR measurements were carried out in DMSO on a Jeol JNM-LA 500 NMR spectrophotometer at 297 K. The 1H NMR spectra were recorded at a frequency of 500.00 MHz. The 13C NMR spectra were obtained at 125.65 MHz with 1H broadband decoupling and referenced relative to TMS. The spectral conditions were: 32 k data points, 0.967 s acquisition time and 1.00 s pulse delay. IR Data (n, cm1): [Zn2(Dach)4(CN)][Zn2(CN)7]$2CH3OH (1), 3341, 3168 (N‒H), 2935, 2866 (C-H), 2192 and 2149 (CN), 1045 (C‒ N); Dach, 3363, 3288 1591, 1092. 1 H NMR (DMSO, TMS, ppm): 1, d ¼ 1.23, 1.38, 1.49, 1.52, 2.80; Dach, d ¼ 1.12, 1.28, 1.69, 1.85, 2.23. 13 C NMR (DMSO, TMS, ppm): 1, d ¼ 20.78, 27.61, 51.50, 140.14; Dach, d ¼ 26.36, 35.26, 58.20. 2.4. X-ray structure determination Single crystal data collection of 1 was performed at 120 K on an Oxford Gemini S diffractometer and using CuKa radiation. The structures were solved by direct methods with SHELXL-2013 and refined by full-matrix least-squares procedures on F2 using SHELXL-2013 [32]. Crystal data and structural refinement details of the data collection are summarized in Table 1. Both cationic [Zn2(Dach)4(CN)]3þ and anionic [Zn2(CN)7]3- units exhibit crystallographically imposed inversion symmetry, with the inversion centers in the middle between the respective Zn atoms. Due to that, the m-cyanido-1kN:2kC ligands are statistically disordered and have been refined on two positions with occupation factors of 0.5/0.5. The atom C1M of the methanol molecules has been refined disordered to split occupancies of 0.56/0.44. All non-hydrogen atoms were refined anisotropically and riding models were used for the isotropic refinement of C- and O-bonded hydrogen atoms. The positions of N-bonded hydrogen atoms were taken from difference Fourier maps. 2.5. Theoretical (DFT) calculations Theoretical studies were performed for [Zn2(Dach)4(CN)]

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[Zn2(CN)7]·2CH3OH (1) as well as for three model compounds; [Zn2(Dach)4(CN)][Zn2(CN)7], [Zn(Dach)(CN)2]4 (2) and {[Zn(Dach)2] [Zn(CN)4]}2 (3). In case of [Zn2(Dach)4(CN)][Zn2(CN)7]·2CH3OH, the fragment of the crystallographic structure was used as the initial geometry in the optimization procedure. Vibrational frequency calculations confirmed that the optimized structures correspond to the minima in the potential energy surfaces. The calculations were performed by using B3LYP-D3 method (standard hybrid density functional B3LYP method [33,34] with dispersion correction D3 [35]) and combined basis sets LanL2DZ [36] for Zn atoms in conjunction with the D95V(d,p) [37] basis set for all other atoms. The electronic energies (DEele), enthalpies (DH) and Gibbs free energies (DG) of [Zn2(Dach)4(CN)][Zn2(CN)7] and its analogues, 2 and 3 were calculated. Natural bond orbital (NBO) analysis was performed for complex 1 [38,39]. All computations were carried out with Gaussian 09 set of programs [40]. 3. Results and discussion The reaction of ZnCl2 with Dach in a 1:1 molar ratio lead to the formation of a chlorido complex, [Zn(Dach)2][ZnCl4], the structure of which has been reported recently [11]. The subsequent addition of 2 equivalents of potassium cyanide resulted in a product of empirical composition, [Zn(Dach)(CN)2].0.5CH3OH. The complex was characterized by usual techniques: elemental analysis, IR, 1H & 13 C NMR and X-ray crystallography. The elemental analysis data is consistent with the predicted composition. The X-ray diffraction analysis established that the prepared complex existed as, [Zn2(Dach)4(CN)][Zn2(CN)7]$2CH3OH. 3.1. IR and NMR studies Fig. 1 illustrates the experimental and theoretically calculated infrared spectra of 1. The DFT calculated vibrational frequencies and IR intensities as well as the bands observed in the experimental infrared spectrum along with their assignments are collected in Table 2. The vibrational assignment of the experimental spectrum was performed by examination of the calculated atomic displacements and visualization of the normal modes of the model

Table 1 Crystal data and refinement details for compound 1. Formula Formula Weight Crystal system Space Group a, b, c (Å) a, b, g (deg) V (Å3) Z rcalc (g cm3) Wavelength (Å) m(CuKa) (mm1) F(000) Crystal size (mm) Temperature (K) q range (deg) h, k, l limits Reflections; collected/Uniq. Reflections: observed [I > 2s (I)] Tmax, Tmin Data/restraints/parameters Goodness of fit on F2 R1, wR2 ([I > 2s (I)]) R1, wR2 (all data) Largest diff. peak, hole (e Å3)

C34H64N16O2Zn4 990.49 monoclinic C2/c 18.5594(2), 12.4809(1), 20.6921(2) 90, 100.079(1), 90 4719.11(8) 4 1.394 1.54184 2.681 2064 0.3  0.3 x 0.2 120(2) 4.290e65.991  21:21, e14:14, e24:24 20044/4101 (Rint ¼ 0.0213) 1167 1.00000, 0.85227 4101/10/284 1.077 0.0392, 0.1155 0.0403, 0.1165 1.819, 0.694

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Fig. 1. Comparison between the experimental IR Spectrum (a) of [Zn2(Dach)4(CN)][Zn2(CN)7]$2MeOH (1) and the theoretical spectrum (b) calculated at the DFT level of theory.

Table 2 Experimental IR data for [Zn2(Dach)4(CN)][Zn2(CN)7]$2CH3OH (1) and, DFT calculated frequencies (cm1) and intensities (km mol1) for the model complex (intensities are given in parentheses). Assignmenta

IR Frequencies Exp.

B3LYP-D3

3341 s 3168 m 2935 s 2866 m 2192 m 2149 m 1608 s 1448 m 1398 w 1132 m 1046 s 1009 s 978 s 906 w 874 w 688 m 624 w 549 w 434 m

3479 (614) 3262 (821) 3100 (63) 2999 (96) 2263 (33) 2217 (27) 1672 (116) 1492 (20) 1433 (9) 1134 (55) 1086 (57) 1073 (217) 970 (137) 916 (14) 886 (16) 697 (36) 632 (55) 550 (23) 432 (10)

a

n(OH) þ n(NH) n(NH) þ n(OH) n(CH)Dach nsym(CH3) n(CN) n(CN) d(NH2) d(CH) d(CH) þ d(NH2) d(CH) þ d(NH2) n(CN)Dach n(CO) þ d(NH2) d(OH) d(CH) n(CC) d(NH2) d(NH2) d(NH2) Ring def

n, stretching; d, bending; m, medium; s, strong; w, weak.

Fig. 2. Above/Below: ORTEP Diagrams (50% ellipsoid probability) of the molecular structures of [Zn2(Dach)4(CN)]3þ and two different perspective views on the structure of [Zn2(CN)7]3e. Of the statistically disordered m-cyanido-1kN:2kC ligands only one atomic position is displayed. All C-bonded hydrogen atoms and the MeOH molecules were omitted for clarity. Symmetry codes: “A” ¼ ex þ 2, ey, ez þ 1; “B” ¼ ex þ 3/2, ey þ ½, ez þ 1.

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[Zn2(Dach)4(CN)][Zn2(CN)7]$2CH3OH complex. As follows from the IR spectrum (Fig. 1) and data presented in Table 2, two clear bands due to the C^N stretching mode were observed at 2192 and 2149 cm1. The calculated frequencies of these modes are 2263 and 2217 cm1, respectively. The splitting of n(C^N) suggests the presence of both the bridged and terminal CN ligands. Generally, the stretching frequency of terminal cyanide is lower relative to that of bridging one [9,10,41]. The presence of Dach is indicated by several characteristic absorption bands. The N-H stretching vibrations of Dach appeared at 3341 and 3168 cm1. As shown in Table 2, according to the calculations two modes at 3479 and 3262 cm1 involve a significant contribution from the n(NH) and n(OH) vibrations. The peak at 1608 cm1 was assigned to NH2 bending. The C-H bands were observed at 2935 and 2866 cm1. The signal due to C-N stretch of diamine was detected at 1046 cm1 and calculated at the 1086 cm1. The strong experimental band at 1009 cm1 is assigned to the n(CO) and d(NH2) vibrations. On the basis of the theoretical calculations, the strong peak at 978 cm1 is assigned to the d(OH) bending vibration. The three peaks at 688, 624 and 549 cm1 are assigned to the d(NH2) vibrations, in agreement with the theoretical results. In the 1H NMR spectrum of [Zn2(Dach)4(CN)][Zn2(CN)7]·2CH3OH (1), the dach protons were identified as two sets of signals, a singlet at 2.80 ppm and a multiplet between 1.23 and 1.53 ppm. The C-H protons of methanol were detected at 3.14 ppm. The signals of NC-H protons of Dach shifted towards downfield position compared to those in free ligand. In the 13C NMR spectrum of 1, the C-N, C-CN and C-CC carbons of dach are observed at 51.50, 27.61 and 20.78 ppm, respectively. The eCH2 resonances of diamine shifted upfield upon coordination. The CN resonance was detected at 140.1 ppm. The signal at 48.9 ppm indicated the presence of

methanol. The 1H and 13C NMR spectra of complex 1 are shown in Figs. S1 and S2 respectively. 3.2. Structure of 1 in the solid state The molecular structure of 1 along with the atomic numbering scheme is illustrated in Fig. 2, while the selected bond distances and angles are listed in Table 3. Complex 1 consists of two dinuclear ionic species, a cyanido-bridged Zn-Dach cation, [Zn2(Dach)4(CN)]3þ and a zinc cyanide anion, [Zn2(CN)7]3-, and two molecules of methanol. Both ionic species exhibit crystallographically imposed inversion symmetry with the inversion centers in the middle between the atoms Zn1 and Zn1A for [Zn2(Dach)4(CN)]3þ and between the atoms Zn2 and Zn2B for [Zn2(CN)7]3-. In the cationic part, the symmetry related zinc(II) atoms (Zn1 & Zn1A) exhibit a distorted square pyramidal geometry attained by four N atoms of two cis-1,2-diaminocyclohexane molecules at the equatorial positions and a cyanide ion occupying the axial position. The degree of distortion from square pyramid is reflected in the axial and equatorial bond angles. The basal cis NeZneN angles vary from 81.61(9) to 95.1(2) , while the trans NeZneN angles are in the range of 138.53(10) - 166.47(10) . The large variation in bond angles may be associated with the steric constraints of the cyclic Dach ligands. The central CN ion exhibits a nearly linear environment and is disordered over two positions. In the anionic component, zinc is coordinated by four carbon atoms or three carbon and one nitrogen atoms of cyanide groups adopting a distorted tetrahedral geometry. The bond angles around Zn2 vary from 101.1(18) to 116.60(12) . The CN-Zn moieties are nearly linear with the bond angles ranging from 174.5(3) to 178.3(3) . The Zn-N and Zn-C bond distances in both units are similar to the corresponding values in

Table 3 Selected bond distances (Å) and bond angles  () for 1. Bond Distance Zn(1)-N(1) Zn(1)-N(2) Zn(1)-N(3) Zn(1)-N(4) Zn(1)-N(8) Zn(1)-C(8N) Zn(2)-C(9N) Zn(2)-N(9) Zn(2)-C(13) Zn(2)-C(14) Zn(2)-C(15) C(1)-N(1) C(6)-N(2) C(8N)-N(8)A N(8)-N(8)A C(8N)-C(8N)A C(9N)-N(9)B C(13)-N(6) C(14)-N(7) C(15)-N(5)

Bond angles 2.153(3) 2.089(3) 2.087(3) 2.180(2) 2.011(8) 2.059(11) 2.04(7) 2.03(5) 2.034(3) 2.021(3) 2.034(3) 1.479(4) 1.483(4) 1.138(6) 1.248(16) 1.189(19) 1.145(13) 1.138(4) 1.145(4) 1.135(4)

Symmetry transformations: “A” ¼ ex þ 2, ey, ez þ 1. “B” ¼ ex þ 3/2, ey þ ½, ez þ 1.

N(1)-Zn(1)-N(2) N(1)-Zn(1)-N(3) N(1)-Zn(1)-N(4) N(1)-Zn(1)-N(8) N(1)-Zn(1)-C(8N) N(2)-Zn(1)-N(3) N(2)-Zn(1)-N(4) N(2)-Zn(1)-N(8) N(2)-Zn(1)-C(8N) N(3)-Zn(1)-N(4) N(3)-Zn(1)-N(8) N(3)-Zn(1)-C(8N) N(4)-Zn(1)-N(8) N(4)-Zn(1)-C(8N) N(8)-Zn(1)-C(8N) C(13)-Zn(2)-C(14) C(13)-Zn(2)-C(15) C(13)-Zn(2)-C(9N) C(13)-Zn(2)-N(9) C(14)-Zn(2)-N(9) C(14)-Zn(2)-C(9N) C(14)-Zn(2)-C(15) C(15)-Zn(2)-N(9) C(15)-Zn(2)-C(9N) N(9)-Zn(1)-C(9N) C(1)-N(1)-Zn(1) C(8N)A-N(8)-Zn(1) N(8)A-C(8N)-Zn(1) C(9N)B-N(9)-Zn(2) N(9)B-C(9N)-Zn(2) N(6)-C(13)-Zn(2) N(7)-C(14)-Zn(2) N(5)-C(15)-Zn(2)

81.74(11) 92.16(10) 166.47(10) 98.4(2) 102.1(2) 138.53(10) 94.95(10) 105.78(18) 117.7(2) 81.61(9) 115.69(17) 103.7(2) 95.1(2) 91.1(2) 12.3(3) 116.60(12) 111.77(12) 102(2) 101.1(8) 108.2(13) 107.0(16) 110.51(11) 107.8(11) 108.7(14) 1(2) 108.49(19) 174.4(5) 170.9(5) 175.5(12) 175(3) 174.5(3) 177.3(3) 178.3(3)

M. Monim-ul-Mehboob et al. / Journal of Molecular Structure 1169 (2018) 110e118 Table 4 Geometrical details of hydrogen bonds of 1 (Å, deg). No./DH…A

DH

H…A

D… A

: DH…A

1/N1H2/N6i 2/N2H4/N5ii 3/N4H7/N5ii 4/N4H8/N6iii 5/N3H5/N7iv 6/N1H1/N7iv 7/N3H6/O1Mi 8/O1MH1M…O1Mv

0.92(3) 0.93(2) 0.91(3) 0.94(2) 0.93(2) 0.94(2) 0.91(4) 0.82

2.41(3) 2.20(2) 2.51(3) 2.60(3) 2.20(3) 2.55(3) 2.51(4) 2.40

3.316(4) 3.079(4) 3.353(4) 3.520(4) 3.124(3) 3.423(3) 3.236(6) 3.015(9)

167(3) 158(3) 155(2) 168(3) 170(3) 156(3) 137(3) 132

Symmetry codes: i ¼ 2  x, y, 1  z; ii ¼ 2  x, 1  y, 1  z; iii ¼ 3/2  x, ½ y; iv ¼ ½ þ x, ½  y, ½ þ z; 1  z; v ¼ 2  x, y, ½  z.

the related compounds [8e12,16e19,30]. However, in complex 1 one of the Zn-N bonds (Zn1-N4) is elongated than the other three. The structure of compound 1 is very much different from the known zinc(II) complexes of Dach and cyanide. The reported ZnDach complexes exist in the mononuclear forms as [Zn(Dach)X2]

115

[12,16,17] or [Zn(Dach)2]X2 [11,18,19] with zinc atoms adopting a tetrahedral geometry. Complex 1 is the first example, where a ligand (cyanide) links the two Zn(II)-Dach units to form a bimetallic moiety. Likewise, the bonding situation of zinc-cyanide system is different from the related complexes, which possess the characteristic [Zn(CN)4]- ion [8,9,30,31]. The unique feature of complex 1 is that [Zn(CN)4]- ion is transformed into a dinuclear anion, [Zn2(CN)7]3-. Most likely, the existence of Zn-CN anion in the dimeric form, [Zn2(CN)7]3- is conferred by the high stability of [Zn2(Dach)4(CN)]3þ moiety. The two ionic dinuclear units are linked to each other through hydrogen bonding interactions to form a 3D supramolecular network. The hydrogen bonds are formed between the N-H groups of Dach and the terminal nitrogen atoms of [Zn2(CN)7]3- anions. The methanol hydrogen and oxygen atoms are also involved in hydrogen bonding. Geometrical data of the hydrogen bonds are summarized in Table 4. The 3D network is composed of layers, oriented along the crystallographic a- and b-axes (Fig. S3, Suppl. Material) and due to hydrogen bonds No. 1 to 4 (Table 4). As

Fig. 3. Graphical representation of a selected part of the 3D network of 1 in the solid state. Dotted lines oriented along the crystallographic c-axes indicate hydrogen bonds between layers of 1. All C-bonded hydrogen atoms and MeOH molecules were omitted for clarity.

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M. Monim-ul-Mehboob et al. / Journal of Molecular Structure 1169 (2018) 110e118 Table 5 Selected structural parameters of [Zn2(Dach)4(CN)][Zn2(CN)7]$2CH3OH. Experimentala and bTheoretical values calculated at the DFT (B3LYP-D3) level. Parameter

Bond distances (Å) Zn(1)-N(1) Zn(1)-N(2) Zn(1)-N(3) Zn(1)-N(4) Zn(1)-N(8) Zn(1)-C(8N) Zn(2)-C(9N) Zn(2)-N(9) Zn(2)-C(13) Zn(2)-C(14) Zn(2)-C(15) C(1)-N(1) Bond angles (o) N(1)-Zn(1)-N(2) N(1)-Zn(1)-N(3) N(1)-Zn(1)-N(4) N(1)-Zn(1)-N(8) N(1)-Zn(1)-C(8N) N(2)-Zn(1)-N(3) N(2)-Zn(1)-N(4) C(13)-Zn(2)-C(14) C(13)-Zn(2)-C(15) C(13)-Zn(2)-C(9N) C(13)-Zn(2)-N(9) C(14)-Zn(2)-N(9)

Fig. 4. DFT-fully optimized structure of [Zn2(Dach)4(CN)][Zn2(CN)7]$2CH3OH (1) with numbering of the selected atoms.

shown in Fig. 3 the layers are connected with each other by hydrogen bonds No. 5 and 6 (Table 4) along the crystallographic caxes. The hydrogen bonding pattern plays an important role in stabilizing the solid state structure of the complex.

Exp.a

Theor.b

2.153(3) 2.089(3) 2.087(3) 2.180(2) 2.011(8) 2.059(11) 2.04(7) 2.03(5) 2.034(3) 2.021(3) 2.034(3) 1.479(4)

2.216 2.175 2.212 2.186 2.170 2.147 2.198 2.147 2.047 2.155 2.109 1.481

81.74(11) 92.16(10) 166.47(10) 98.4(2) 102.1(2) 138.53(10) 94.95(10) 116.60(12) 111.77(12) 102(2) 101.1(8) 108.2(13)

78.6 88.8 130.17 98.2 131.7 132.96 96.4 112.2 129.1 112.1 110.0 99.5

3.3.1. NBO analysis The NBO charges on the selected atoms of [Zn2(Dach)4(CN)] [Zn2(CN)7]·2CH3OH are reported in Table 7. As shown in the table, the charge on the Zn1 cation surrounded by the nitrogen atom of cis-1,2-diaminocyclohexane is larger by 0.378 e than that calculated on Zn2 surrounded by three nitrogen and one carbon atoms. With respect to the other zinc atom of [Zn2(CN)7]3-, which is surrounded

3.3. DFT calculations The fragment of the crystallographic structure of 1 was taken as the starting geometry in the optimization procedures. Fig. 4 illustrates the fully optimized structure of [Zn2(Dach)4(CN)] [Zn2(CN)7]·2CH3OH (1) complex. The selected structural parameters (interatomic distances, and bond angles) of [Zn2(Dach)4(CN)][Zn2(CN)7]$2CH3OH (1), determined experimentally and DFT calculated are given in Table 5. As follows from these results, the calculated bond distances are slightly overestimated with respect to the corresponding experimental X-ray data. The differences range from 0.002 to 0.159 Å for C(1)-N(1) and Zn(1)-N(8), respectively. Similar results were obtained at the same level of theory in the cases of other zinc(II) complexes with diamines reported in the literature [11e13]. The structures of 1 without methanol molecules and two of its analogues, [Zn(Dach)(CN)2]4 (2) and {[Zn(Dach)2][Zn(CN)4]}2 (3) were predicted using theoretical (DFT) method. Fig. 5 illustrates the structures of these complexes optimized at the BLYP-D3 level. Table 6 lists the relative values of electronic energies (DEele), enthalpies (DH) and Gibbs free energies (DG) of the investigated complexes, calculated with the B3LYP-D3 functional. According to the data presented in this table, the smallest value of the Gibbs free energy was calculated in the case of the model complex 1. It means that [Zn2(Dach)4(CN)][Zn2(CN)7] (1) is more stable than its analogous complexes, 2 and 3 by 5.16 and 3.47 kcal mol1, respectively.

Fig. 5. Fully optimized structures of [Zn2(Dach)4(CN)][Zn2(CN)7] (1), [Zn(Dach)(CN)2]4 (2) and {[Zn(Dach)2][Zn(CN)4]}2 (3) complexes.

M. Monim-ul-Mehboob et al. / Journal of Molecular Structure 1169 (2018) 110e118 Table 6 Relative values of electronic energies (DEele), enthalpies (DH) and Gibbs free energies (DG) of [Zn2(Dach)4(CN)][Zn2(CN)7] (1), [Zn(Dach)(CN)2]4 (2) and {[Zn(Dach)2][Zn(CN)4]}2 (3) complexes calculated at the B3LYP-D3/D95V(d,p)/ LanL2DZ level of theory. All values in kcal mol1. Complex

DEele

DH

DG

1 2 3

0.00 11.56 3.25

0.00 12.03 3.54

0.00 5.16 3.47

Table 7 NBO charges on selected atoms [q in e] of [Zn2(Dach)4(CN)][Zn2(CN)7]$2CH3OH (1) complex. Atom

Charge

Zn1 N1 N2 N3 N4 Zn2 C13 C14 C15 N8 C8 N9 C9

1.289 1.001 1.023 1.014 1.014 0.911 0.155 0.004 0.048 0.667 0.000 0.657 0.097

by four nitrogens the charge on this cation is 0.820 e. The results in the table show that the absolute values of the charges on the N atoms of the Dach ligands range from 1.023 to 1.001 e. They are almost twice larger than those found on the N atoms of CN bridged ligands (N8 and N9). With respect to the charges on the carbon atoms they oscillate around zero.

4. Conclusion The present paper deals with the spectroscopic characterization and the crystal structure of a zinc(II) complex of 1,2diaminocyclohexane and cyanide, [Zn2(Dach)4(CN)][Zn2(CN)7]$ 2CH3OH (1). Each zinc atom in the cationic part of 1 adopts a distorted square pyramidal geometry having cyanide nitrogen at the apical position while in the anionic part, zinc atoms are tetrahedrally coordinated. The structure is stabilized through H-bonding between cationic and anionic species or methanol. Based on the calculated thermodynamic properties of [Zn2(Dach)4(CN)] [Zn2(CN)7] and its two analogues: [Zn(Dach)(CN)2]4 (2) and {[Zn(Dach)2][Zn(CN)4]}2 (3), it is concluded that the complex 1 is the most stable complex. The NBO analysis reveals that the charge on zinc(II) surrounded by the nitrogen atoms of cis-1,2diaminocyclohexane is larger by 0.378 e than that found on zinc(II) surrounded by three nitrogen and one carbon atoms. The study provides useful information about the structure and bonding in zinc(II)-cyanide complexes of diamines.

Acknowledgments This work was financed in part by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Technology. A generous computer time from the Wroclaw Supercomputer and Networking Center as well as the Poznan Supercomputer and Networking Center is acknowledged.

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