Steric Effects of Alkyl Substituents at N-Donor Bidentate Amines Direct the Nuclearity, Bonding and Bridging Modes in Isothiocyanato-Copper(II) Coordination Compounds

Abstract

A series of Cu(II)-isothiocyanato coordination compounds derived from sterically hindered N-donor diamines were synthesized and characterized: catena-[Cu(Me₃en)(μ-NCS)(NCS)] (1), catena-[Cu(NEt₂Meen)(μ-NCS)(NCS)] (2), catena-[Cu(N,N,2,2-Me₄pn)(μ-NCS)(NCS)] (3), the dimeric: [Cu₂(N,N′-isp₂en)₂(µ-NCS)₂(NCS)₂] (4) and the monomeric compound [Cu(N,N′-t-Bu₂en)(NCS)₂] (5), where Me₃en = N,N,N′-trimethylethylenediamine, NEt₂Meen = N,N-diethyl-N′-methylethylenediamine, N,N,2,2-Me₄pn = N,N,2,2-tetramethylpropylenediamine, N,N′-isp₂en = N,N′-diisopropylethylenediamine and N,N′-t-Bu₂en = N,N′-di(tert-butyl)ethylenediamine. The coordination compounds were characterized by elemental microanalyses, IR, and UV–Vis spectroscopy as well as single crystal X-ray crystallography. Density Functional Theory (DFT) was used to evaluate the role of steric effects in compounds 4 and 5 and how this may affect the adaption of a specific geometry, NCS-bonding mode, and the dimensionality of the resulting coordination compound.

1. Introduction

Pseudohalides are a class of compounds that are able to simultaneously bind two or more metal ions leading to the formation of di- or poly-nuclear coordination compounds of different nuclearity and clusters, and coordination polymers (CPs) of different dimensionality (1D, 2D, 3D) and topology [1,2,3,4,5,6,7,8,9,10]. However, the formation of these compounds depends largely on a number of factors which include the electronic nature and oxidation state of the metal ion, the nature of the coordinated ancillary co-ligand(s), its skeletal structure, and the steric hindrance imposed by the blocking co-ligand(s) surrounding the central metal ion [11,12]. Steric effects caused by the coordinating blocking ligands in coordination compounds have been reported to produce a dramatic effect on the structural and magnetic behavior of pseudohalide coordination compounds [1,2,3,4,5,6,7,8,9,10,11,13,14,15,16] but also in the biological catalytic reactions of the P–O bond cleavage in DNA and phosphodiester hydrolysis [17,18,19,20].

Isothiocyanato coordination compounds are considered as among the most studied systems in coordination chemistry due to the diverse bonding properties of the NCS⁻ ion as an ambidentate and bridging ligand that can bind two or more metal ions simultaneously as well as its ability to propagate magnetic interaction between bridged paramagnetic metal ions. In general, several bonding modes were reported in bridging-isothiocyanato-polynuclear and/or -polymeric coordination compounds. These include the bonding modes μ_1,3-NCS⁻ [1,5,6,7,10,21,22,23,24,25,26,27,28,29,30,31], μ_1,1-NCS⁻ [32,33,34], μ_3,3-NCS⁻ [35,36,37], μ_1,1,3-NCS⁻ [38], μ_1,3,3-NCS⁻ [39], and μ_1,3,3,3-NCS⁻ [40]. In compounds where the coordinated isothiocyanate ion is acting as a terminal ligand, then according to Hard Soft Acid Base (HSAB) concept, soft Lewis acid metal ions prefer the soft Lewis S-site and hard metals prefer the soft site [41]. Interestingly, although nitrogen in NCS⁻ is classified as borderline Lewis base, most hard metals such as Fe²⁺, Ni²⁺, Cu²⁺, Co²⁺, and Zn²⁺ show high preference to the N-site [42,43].

Herein, we report the synthesis and structural characterization of a series of Cu(II)-isothiocyanato coordination compounds derived from sterically hindered N-donor diamines with different alkyl substituents as co-ligands: the coordination 1D-polymers: catena-[Cu(Me₃en)(μ-NCS)(NCS)] (1), catena-[Cu(NEt₂Meen)(μ-NCS)(NCS)] (2), catena-[Cu(N,N,2,2-Me₄pn)(μ-NCS)(NCS)] (3), the dimeric [Cu₂(N,N′-isp₂en)₂(µ-NCS)₂(NCS)₂] (4), and the monomeric compound [Cu(N,N′-t-Bu₂en)(NCS)₂] (5). This should allow us to evaluate the role of the steric effect imposed by the coordinated amine co-ligands in adapting a specific NCS-bonding mode as well as the selected dimensionality. The structures of the ligands used in this study together with other related diamine derivatives are illustrated in Scheme 1. The Density Functional Theory (DFT) computations were performed in an attempt to account for the observed geometrical and dimensionality of 4 and 5 compounds.

2. Results and Discussion 2.1. Synthetic Aspects

The isolated coordination compounds: catena-[Cu(Me₃en)(μ-NCS)(NCS)] (1), catena-[Cu(NEt₂Meen)(μ-NCS)(NCS)] (2), catena-[Cu(N,N,2,2-Me₄pn)(μ-NCS)(NCS)] (3), [Cu₂(N,N′-isp₂en)₂(µ-NCS)₂(NCS)₂] (4), and [Cu(N,N′-t-Bu₂en)(NCS)₂] (5) were synthesized in reasonably good yields (71%–92%) by the reaction of a methanolic solution containing equimolar amounts of Cu(NO₃)·3H₂O or Cu(ClO₄)₂·6H₂O and the corresponding diamine ligand with four equivalents of NH₄NCS solution dissolved in H₂O or MeOH. Crystals suitable for X-ray analysis were obtained from dilute solutions or by recrystallization from CH₃CN or anhydrous MeOH. The purity of the coordination compounds was confirmed by elemental microanalyses (see experimental section). Molar conductivities, Л_M measured in CH₃CN, are within the range of 7–12 Ω⁻¹cm²mol⁻¹ which reflect their non-electrolytic nature [44]. The coordination compounds were also characterized by IR, UV–Vis, and by single crystal X-ray crystallography. Probably, it is interesting to mention that although the μ-1,3- and μ-1,1-bonding modes are common in bridging-azido-metal(II) coordination compounds [8,9,11,12,14,15,16], the corresponding μ-1,1-bonding in the bridging-isothiocyanato compounds is rare [32,33,34] and in most cases alternating μ-1,3-bonding chains are observed [1,2,3,4,5,6,7,17,21,22,23,24,25,26,27,28,29,30,31].

2.2. IR and UV–VIS Spectra of the Coordination Compounds

The IR asymmetric stretching frequencies of the isothiocyanato-Cu(II) coordination compounds under investigation and their corresponding UV-Vis spectra in acetonitrile solution are collated in Table 1. It has been stated that the ν_as(C≡N) frequencies could be used as a criterion to differentiate between the terminally N-bonded isothiocyanato anions which in most cases display a value below 2100 cm⁻¹, whereas in the corresponding S-bonded and/or µ-1,3-bridging isothiocyanato anions, the stretching vibrations are observed above 2100 cm⁻¹ [12,28,29,30,31,42,43,45]. The IR spectra of the compounds 1–4 display very strong bands over the 2095–2131 cm⁻¹ region due to ν_as(C≡N) of the bridged-isothiocyanato ligands. In addition, the same series of coordination compounds as well as 5 showed a strong to medium intensity band over the 2059–3086 cm⁻¹ region which can be attributed to the N-bonded terminal isothiocyanate ligand(s). This data are fully consistent with N-bonded isothiocyanate in all coordination compounds and µ-1,3-bridging isothiocyanate in 1–4. These were also confirmed by X-ray structural analysis (Section 2.3).

Inspection of the acetonitrile solutions of the UV–Vis spectral data of the coordination compounds 1–4, shown in Table 1, reveals some common spectral features as they display broad maxima in the 615–656 nm region. This feature is consistent with five-coordinate Cu(II) compounds with square pyramidal geometry (SP) which may be associated with a low-energy shoulder at λ > 800 nm [28,29,30,31,42,43,45,46,47,48,49,50]. The presence of the small intense maxima at 974 nm in compound 1 may reflect the increased distortion towards the trigonal bipyramidal geometry (TBP) [45]. Obviously, compound 4 does not belong to the observed geometry but instead it has one visible maxima at ~766 nm, resulting from d–d transition; ²E←²T in tetrahedral environment [51]. The very strong intense band located at 469 nm can be assigned to ligand–metal charge transfer transition (L→M CT), whereas the 299 nm maxima is most likely attributable to electronic transition within the NCS⁻ ligand (π→π*). The solid spectra of complexes 1–4 showed a similar spectral pattern with two maxima located at 480–520 and 630–690 nm regions and a shoulder around 730–750 nm (Figures S1–S4 in the Supplementary Materials for 1–4 compounds, respectively). The geometrical finding around the Cu(II) ion in acetonitrile solution was retained in the solid state as it was supported by the X-ray structural data.

2.3. Description of the Structures

2.3.1. catena-[Cu(Me₃en)(μ-NCS)(NCS)] (1), catena-[Cu(NEt₂Meen)(μ-NCS)(NCS)] (2), and catena-[Cu(N,N,2,2-Me₄pn)(μ-NCS)(NCS)] (3)

Perspective views with the atom numbering scheme for the polymeric coordination compounds 1–3 are presented in Figure 1, Figure 2 and Figure 3 and Tables S1–S3 summarize main bond parameters. The copper(II) centers are linked by single μ_1,3-isothiocyanato bridges to form polymeric chains of polyhedra (1D). Each metal center is penta-coordinated by two N-donor atoms of the amine ligand and two N-atoms of terminal and bridging N,S-NCS⁻ anions (atoms with minor split occupancy in partially disordered crystal structure of compound 1 were ignored). The CuN₄S chromophores may be described as distorted SP geometry. The basal Cu–N bond distances are in the range of 1.9487(13) to 2.106(7) Å. The S-atoms of the bridging NCS⁻ groups are located in apical positions [Cu–S from 2.6421(11) to 2.8910(16)]. The Cu···Cu’ distances within the chain are 5.4245(11), 5.7755(9), and 5.7679(15) Å, for 1–3, respectively. The shortest metal–metal separations between the chains are 6.7646(16), 6.8602(10), and 6.9132(16) Å, for 1–3, respectively. Hydrogen bonds of type N–H···S are formed between N-amine ligand to S atoms of neighboring Cu(II)-polyhedra (Table 2, Figures S5–S7) to form a supramolecular 2D system in case of 1 and 3, respectively.

2.3.2. [Cu₂(N,N′-isp₂en)₂(µ-NCS)₂(NCS)₂] (4)

Compound 4 consists of centrosymmetric dimeric [Cu₂(N,N′-isp₂en)₂(μ-NCS)₂(NCS)₂] units (Figure 4). Each Cu(II) center is penta-coordinated by two N-atoms of N,N′-isp₂en ligand, two N-atoms of terminal and bridging NCS⁻ anions and an S-atom of the bridging NCS⁻ anion. The CuN₄S chromophore is described as a distorted polyhedron with intermediate geometry between SP and TBP and a τ value of 0.50. The apical site is occupied by the S(1’) atom [Cu(1)-S(1’) = 2.9037(6) Å]. The basal Cu–N bonds vary from 1.9487(13) to 2.0388(12) Å. The μ_1,3-isothiocyanato bridges connect two Cu(II) polyhedron to centrosymmetric dimeric units. Their eight-membered Cu(NCS)₂Cu rings are almost planar (“chair angle” θ which is defined as acute angle between plane of the di-μ_1,3-isothiocyanato bridges and the Cu/N(1)/S(1) plane is 2.5°). The Cu(1)···Cu(1’) distance within the Cu(NCS)₂Cu ring is 5.8794(8) Å, and the inter-dimer separation is 6.6979(9) Å. The Cu–N···S–Cu’ torsion angle is 23.2°, the Cu–N–C–S torsion angles are −139.4 and 87.9°. Hydrogen bonds of type N–H···S are formed between N(3) atoms of N,N′-isp₂en ligand to the S(1) atom of neighboring Cu-polyhedra to generate a supramolecular 2D system oriented along the b- and c-axis of the unit cell (Table 2, Figure S8).

2.3.3. [Cu(N,N′-t-Bu₂en)(NCS)₂] (5)

Compound 5 consists of monomeric neutral [Cu(N,N′-t-Bu₂en)(NCS)₂] units where the Cu(II) centers are tetrahedrally coordinated by four nitrogen donor atoms: two belong to the N,N′-t-Bu₂en ligand and two belong to N-terminal isothiocyanato anions (Figure 5). The Cu–N bond distances are in the range from 1.9299(17) to 2.0108(16) Å. The distorted coordination tetrahedra form N–Cu–N bond angles in the range from 97.03(7) to 140.14(7)°. The shortest Cu···Cu separation is 6.4592(9) Å. Hydrogen bonds of type N–H···S are formed between N(3) and N(4) atoms of N,N′-t-Bu₂en ligand to S(1) atoms of neighboring Cu-polyhedra to generate a supramolecular 2D system oriented along the b- and c-axis of the unit cell (Table 2, Figure S9).

We think it would be interesting to compare our structural results of compounds 1–5 with the literature data of Cu(II)-isothiocyanato coordination compounds which are derived from related bidentate amines and these compounds together with ours are collated in Table 3. Inspection of this data may show some general trends about the coordination behavior of these compounds. The isothiocyanato compounds which are constructed from ethylenediamine (en) [52,53,54,55,56], mono-N-alkyl ethylenediamine (Me-en, propen) [57,58], less hindered di(N,N′-alkyl)-ethylenediamine (N,N′-Me₂en) [59] and 1,5-diazacyclooctane (DACO) [60] resulted in the formation of mono-nuclear Cu(II)-bis(diamine) compounds with or without N–NCS-coordination. With very bulky di(N,N′-alkyl)-ethylenediamine; N,N′-t-Bu₂en, only mono-nuclear Cu(II)-(diamine), [Cu(N,N′-t-Bu₂en)(NCS)₂] (5) was formed with N–NCS-coordination. However, when one of the methyl groups in each of the tert-butyl moieties in N,N′-t-Bu₂en was replaced by hydrogen in order to moderate and relieve the steric environment (N,N′-isp₂en), the dinuclear bridged-NCS coordination compound [Cu₂(N,N′-isp₂en)₂(µ₂-NCS)₂(NCS)₂] (4) was isolated. On the other hand, the isothiocyanato coordination compounds which were constructed from N,N-dialkyl, N,N,N′-trialkyl- and/or tetra-N,N,N′,N′-tetraalky-diamine ligands afforded five-coordinate 1D-polymeric chains, where the Cu(II)-(diamine) moieties are μ_N,S-NCS bridged and the coordination geometry is achieved by another N–NCS group [61,62,63,64] as this was the case observed in the 1–3 compounds under investigation.

2.4. The DFT Computational Results

DFT was used in order to garner insights into the aggregation properties of the various Cu coordination compounds. In particular, we were interested in the intrinsic reasons for the ligand specific variations in adapting a certain mono- or polynuclear chain size. Figure 6 shows the geometries and energetics associated with various oligomeric arrangements of 5 and 4. Figure 6a shows the ground state minimum energy geometry of Cu(I), Cu(II), and Cu(III) oxidation states of monomeric 5; The N,N′-t-Bu₂en ligand tends to form a mononuclear tetrahedral arrangement around the metal center which is in good agreement with the X-ray crystal structure results. The Cu(I) the lowest energy charge of 5 is −1, which can be understood by considering that the entire coordination compound is closed-shell when Cu is in the +1 oxidation state. Attempts were made to dimerize 5 with overall charges of neutral, +1 and −1. Upon optimization, the neutral coordination compound associates and forms a stable dimer, the +1 and −1 charged 5 dimers dissociate and form a monomer pair at an asymptotic separation. The neutral compound forms a stable dimer but it is at a higher energy state when compared to the −1 coordination compound of 5. Thus, the Cu(I) coordination compound 5 is the lowest energy oxidation state of the metal-center and yields a coordination compound with an overall charge of −1; The −1 charge of 5 leads to prompt dissociation of a dimer upon geometry optimization—forming two monomers.

Analogous computations on 4 also returned Cu(I) as the lowest energy oxidation state of the metal-center. Attempts to dimerize 4 were successful for the lowest energy triplet state of Cu(II) (henceforth ³Cu(II)) oxidation state and, unlike 5, did not dissociate to form two monomers upon optimization. The dimer form of triplet-Cu(II) of 4 is lower that the monomer forms by ~16 kcal/mol. All other oxidation states of Cu of 4 failed to converge to a dimer and yielded two monomers. However, further attempts to trimerize 4 were unsuccessful and yielded a dimer + monomer at an infinite separation upon optimization. This observation is again in line with the observed dimeric crystal structure of 4. In comparing 5 with 4, we postulate that the bulky tert-butyl moiety in 5 shows an enhanced steric hindrance, thus inhibiting its ability to sustain a polymeric structure. The loss of two methyl groups in 5 leads to some reduction in such steric hindrance. As a result, 4 is able to sustain a dimeric crystal structure but unable to maintain a larger oligomeric arrangement since (en)₂ remains a bulky group that introduces steric hindrance. With the available resources and large molecular sizes, coordination compounds 1, 2, and 3 produced computationally challenging structures. However, using the available computational data for 5 and 4 we were able to postulate that similar steric arguments would hold in the cases of 1–3 since the bulky N-alkyl substituents at the ethylenediamine co-ligand becomes progressively less favorable across this series. 3. Experimental Section 3.1. Materials and Physical Measurements N,N,N′-Trimethylethylenediamine (Me₃en), N,N-diethyl-N′-methylethylenediamine (Et₂Meen), N,N,2,2-tetramethylpropylenediamine (N,N,2,2-Me₄pn), N,N′-diisopropylethylene-diamine (N,N′-isp₂en), and N,N′-di(tert-butyl)ethylenediamine (N,N′-t-Bu₂en) were purchased from TCI-America. All other commercially available chemicals were used without further purification. Infrared spectra of solid samples were measured on a Cary 630 (FT IR-ATR) spectrometer. Electronic spectra in acetonitrile were measured using an Agilent 8453 HP diode array UV–Vis spectrophotometer, whereas the corresponding solid spectra of 1–4 compounds were performed with an LS 950 Perkin-Elmer Lamda-spectrometer in the 350–950 nm range. A Mettler Toledo Seven Easy conductivity meter was used for conductivity measurements. For its calibration a 1413 μS/cm conductivity standard was used. Elemental analyses were performed by the Atlantic Microlaboratory, Norcross, Georgia, USA. 3.2. Syntheses of the Coordination Compounds Catena-[Cu(Me₃en)(μ-NCS)(NCS)] (1). A mixture of Cu(ClO₄)₂·6H₂O (0.186 g, 0.5 mmol), N,N,N′-trimethylethylenediamine (Me₃en) (0.051 g, 0.50 mmol), and NH₄NCS (0.152 g, 2.0 mmol) was dissolved in MeOH (20 mL) and the resulting dark green solution was heated for 5 min, filtered while hot, and allowed to stand at room temperature. The precipitate obtained after 2 h was collected and crystallized from CH₃CN to afford navy blue single crystals. These were collected by filtration, washed with propan-2-ol, Et₂O, and air dried (yield: 0.105 g, 74%). Anal. Calcd for C₇H₁₄CuN₄S₂ (MM = 281.89 g/mol): C, 29.83, H, 5.01, N, 19.88%. Found: C, 29.92, H, 4.86, N, 19.92%. Selected IR bands (ATR–IR, cm⁻¹): 3178 (w) (N–H stretching), 2921 (w) (aliphatic C–H stretching), 2131 (vs), 2086 (vs) (C≡N stretching of NCS⁻). UV–Vis spectrum {λ_max, nm (ε, M⁻¹cm⁻¹)} in CH₃CN: 615 (153), 974 (59). Л_M (CH₃CN) = 8 Ω⁻¹cm²mol⁻¹. Catena-[Cu(Et₂Meen)(μ-NCS)(NCS)] (2). To a mixture containing Cu(NO₃)₂·3H₂O (0.241g, 1.0 mmol), N,N-diethyl-N′-methylethylenediamine (Et₂Meen) (0.130 g, 1.0 mmol) dissolved in MeOH (25 mL), an aqueous solution of NH₄NCS (0.152 g, 2.0 mmol dissolved in 5 mL H₂O) was added drop by drop and the reaction mixture was heated for 5 min, filtered through celite and then allowed to crystallize at room temperature. The green precipitate which separated was recrystallized from anhydrous MeOH and the resulting long needle crystals were collected by filtration, washed with propan-2-ol, Et₂O, and air dried (yield: 0.227 g, 73%). Anal. Calcd for C₉H₁₈CuN₄S₂ (MM = 309.94 g/mol): C, 34.88; H, 5.85; N, 18.08%. Found: C, 35.02, H, 5.97, N, 17.89%. Selected IR bands (ATR–IR, cm⁻¹): 3168 (w) (N–H stretching), 2974 (w), 2933 (vw), 2878 (vw) (aliphatic C–H stretching), 2095 (vs) (C≡N stretching of NCS⁻). UV–Vis spectrum {λ_max, nm (ε, M⁻¹cm⁻¹)} in CH₃CN: 655 (159). Л_M (CH₃CN) = 11 Ω⁻¹cm²mol⁻¹. Catena-[Cu(N,N,2,2-Me₄pn)(μ-NCS)(NCS)] (3). This compound was isolated as green single crystals and prepared using a similar procedure to that described for 2, except N,N,2,2-tetramethylpropylenediamine was used instead of Et₂Meen (yield: 92%). Anal. Calcd for C₉H₁₈CuN₄S₂ (MM = 309.94 g/mol): C, 34.88; H, 5.85; N, 18.08%. Found: C, 34.67; H, 5.55; N, 18.30%. Selected IR bands (ATR–IR, cm⁻¹): 3291 (w), 3225 (N–H stretching), 2995 (vw), 2965 (w), 2886 (vw), 2842 (vw) (aliphatic C–H stretching), 2099 (vs), 2068 (vs) (C≡N stretching of NCS⁻). UV–Vis spectrum {λ_max, nm (ε, M⁻¹cm⁻¹)} in CH₃CN: ~656 (136). Л_M (CH₃CN) = 12 Ω⁻¹cm²mol⁻¹. [Cu₂(N,N′-isp₂en)₂(µ₂-NCS)₂(NCS)₂] (4). Green single crystals were obtained from a methanolic solution using a procedure similar to that described for 2, with N,N′-diisopropylethylenediamine (N,N′-isp₂en) used instead of Et₂Meen (yield: 79%). Anal. Calcd for C₂₀H₄₀Cu₂N₈ S₄ (MM = 647.94 g/mol): C, 37.08; H, 6.22; N, 17.29%. Found: C, 36.80; H, 6.06; N, 17.25%. Selected IR bands (ATR–IR, cm⁻¹): 3213 (w), 3158 (w) (N–H stretching), 2975 (w), 2964 (vw), 2925 (vw), 2876 (vw) (aliphatic C–H stretching), 2128 (s), 2073 (vs) (C≡N stretching of NCS⁻). UV–Vis spectrum {λ_max, nm (ε, M⁻¹cm⁻¹)} in CH₃CN: ~655 (120). Л_M (CH₃CN) = 12 Ω⁻¹cm²mol⁻¹. [Cu(N,N′-t-Bu₂en)(NCS)₂] (5). Green single crystals were separated from a methanolic solution using a procedure similar to that described for 2, with N,N′-di(tert-butyl)ethylenediamine (N,N′-t-Bu₂en) used instead of Et₂Meen (yield: 71%). Anal. Calcd for C₁₂H₂₄CuN₄S₂ (MM = 352.02 g/mol): C, 40.94; H, 6.87; N, 15.92%. Found: C, 41.08; H, 6.72; N, 16.21%. %. Selected IR bands (ATR–IR, cm⁻¹): 3242 (m) (N–H stretching), 2972 (w), 2859 (vw) (aliphatic C–H stretching), 2059 (vs) (C≡N stretching of NCS⁻). UV–Vis spectrum {λ_max, nm (ε, M⁻¹cm⁻¹)} in CH₃CN: ~766 (266), 469 (2160), 299 (3090). Л_M (CH₃CN) = 7 Ω⁻¹cm²mol⁻¹. 3.3. X-ray Crystal Structure Analysis and Refinement

A Bruker-AXS APEX II CCD diffractometer operating at 100(2) K was used for single-crystal X-ray measurements of compounds 1–5. Table 4 and Table 5 summarize the crystallographic data, parameters for intensity data collection and structure refinement. Mo-Kα radiation (λ= 0.71073 Å) was used for data collection. The computer programs SAINT (Version 7. 23; Bruker AXS Inc.: Madison, WI, USA, 2005), APEX (Version 2. 0-2; Bruker AXS Inc.: Madison, WI, USA, 2006), and SADABS [67] were used for data processing, and corrections for Lorentz-Polarisation and absorption effects. The SHELXTL program package [68] with implemented direct methods as well as full-matrix least-squares refinement procedures operating on F² was used for structure solutions and refinements. Anisotropic displacement parameters were applied to non-hydrogen atoms. The positions of hydrogen atoms were taken from difference Fourier maps and their isotropic displacement factors U_iso set to 1.2 to 1.5 times U_eq of parent C or N atom. Geometrical constraints (HFIX) were applied for H atoms bonded to C atoms. The MERCURY program [69] was used to perform molecular plots. In compound 1, the central Cu atom and the S atom of the terminal isothiocyanate group are disordered over two positions and refined with 90/10 split occupancy using free variables to refine the respective occupancy of the affected fragment (PART) [70]. ISOR and EADP were applied to the disordered atoms to make ADP values for these atoms more reasonable. In compound 1, the N1–H1 distance in the Me₃en donor was restrained using DFIX 0.87 0.02, whereas structure refinements of 2–5 were performed without any restraints for H atoms involved in hydrogen bonds. Compound 1 was refined as a 2-component inversion twin (-100 0-10 00-1) using the TWIN option in SHELXL resulting in BASF = 0.33. Deposition numbers: CCDC 1878247-1878251 for 1–5, respectively.

3.4. The Computational Methodology

The ground state minimum energy geometry of the various structures was optimized with density functional theory (DFT), using the Becke-3 parameter-Lee-Yang-Parr (B3LYP) functional [71] and the 6-31G(d) Pople basis set [72]. The increasing size and molecular coordination commodity of the oligomeric metal coordination compounds precluded the use of higher levels of theory. The B3LYP/6-31G(d) level is therefore an adequate compromise between accuracy and computational expense and allows for a qualitative comparison between polymers of varying size. The individual Cu-compounds may associate to form oligomers of varying chain length. Association energies were calculated by optimizing a given polymer and their associated monomers and smaller oligomers that make up the polymers. Computations were performed on both Cu(I) and Cu(III) oxidation states in order to determining the dominant oxidation state in a given oligomer. Computations of the normal mode wavenumbers were proved to be computationally too expensive due mostly to the large molecular sizes and the heavy metal effects. The Gaussian 09 computational package was used to perform all computations [73].

4. Conclusions Five different Cu(II)-isothiocyanate coordination compounds were constructed from N-donors bidentate amine ligands including the polymeric 1D-chains catena-[Cu(Me₃en)(μ-NCS)(NCS)] (1), catena-[Cu(NEt₂Meen)(μ-NCS)(NCS)] (2), catena-[Cu(N,N,2,2-Me₄pn)(μ-NCS)(NCS)] (3), the dimeric [Cu₂(N,N′-isp₂en)₂(µ-NCS)₂(NCS)₂] (4), and the monomeric coordination compound [Cu(N,N′-t-Bu₂en)(NCS)₂] (5), where the NCS⁻ ions bridge the metal centers in 1–4 with N–NCS terminal binding to the Cu²⁺ ion in 5. The extent of nuclearity (monomer, dimer or polymer) and the isothiocyanate bonding mode (N–NCS vs. µ_N,S-NCS-bridging) depend entirely on the steric hindrance imposed by the N-alky groups introduced into the N-donors amines. In general, when N,N-dialkyl, N,N,N′-trialkyl- and/or tetra-N,N,N′,N′-tetraalkyl-diamines were employed as ligands, Cu(II)-(diamine)-(µ₂-NCS)-(N-NCS) 1D-polymeric chains were obtained (1–3 coordination compounds). With high sterically hindered N,N′-dialkyl bidentate amines, only Cu(II)-(diamine)(N-NCS)₂ as was the case in compound 5, whereas with moderate steric environment such as in N,N′-isp₂en, similar species to those 1–3 were produced but with dinuclear composition, [Cu₂(N,N′-isp₂en)₂(µ₂-NCS)₂(NCS)₂] (4).

Probably, we should mention that geometrical tetrahedral Cu(II) coordination compounds similar to 5 are very rare because the vast majority of Cu(II) compounds display distorted octahedral, trigonal bipyramidal, square pyramidal, and in a few cases square planar geometries. However, monomeric tetrahedral coordination compounds, [Cu(L)₂(NCS)₂] (L = substituted pyridine derivatives) with a composition similar to 5 have been recently reported [74]. In addition, tetrahedral Cu(II) coordination compounds with two bidentate ligands of bis(3,5-dibromo-N-p-tolyl-salicylaldiminate) [75], and with a tridentate N₃ ligand and the fourth coordination site occupied with a labile ligand in a C_3v symmetry [76] have also been structurally recognized.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4352/9/1/38/s1. Selected bond lengths (Å) and angles (°) for the compounds under investigation are given in Tables S1-S5 and their corresponding packing views are shown in Figures S5-S9 for 1-5 coordination compounds, respectively. UV-VIS spectra of solid compounds 1-4 are presented as Figures S1-S4, respectively.

Author Contributions

F.A.M., R.C.F. and A.T. performed the X-ray structural analysis. S.S.M., M.M.H., A.M., H.D. and F.R.L. contributed in the synthesis and spectral characterization of the designed compounds. T.N.V.K. performed the computational study. F.A.M., S.S.M., F.R.L. and T.N.V.K. contributed to the writing of the manuscript.

Funding

This research received no external funding.

Acknowledgments

S.S.M. acknowledges the financial support of this research by the Department of Chemistry University of Louisiana at Lafayette. F.A.M. thanks J. Baumgartner (TU Graz) for assistance.

Conflicts of Interest

The authors declare no conflict of interest.

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