Studies of structural diversity due to inter-/intra-molecular hydrogen bonding and photoluminescent properties in thiocarboxylate Cu(I) and Ag(I) complexes

Abstract

Seven new copper(I) thiocarboxylate and two silver thiocarboxylate complexes, containing 2-mercaptobenzimidazole (MB) and 2-mercapto-2-thiazoline (MT) have been synthesized and characterized by elemental analysis, IR, ¹H NMR, ¹³C NMR and UV-Visible spectroscopic techniques. Molecular structures of all the complexes have been examined by single crystal X-ray diffraction analysis. In the case of complexes 1 ([(PPh₃)₂Cu(SCOMe)MB]) and 3 ([(PPh₃)₂Cu(SCOth)MB]), the direction of hydrogen bonding is changed from intra-molecular to inter-molecular by increasing the size of the R-group of the thiocarboxylate ligands. 7 ([Cu₂(μ-SCOPh)₂(μ-MT)(MT)]₂) is a tetranuclear complex in which the distance between the two copper atoms is shorter than the sum of the covalent radii of the two atoms without having a formal covalent bond between them, as evidenced by NBO (DFT) calculation and bond critical point calculations using the AIM theory. In complex 8 two different molecules, [(PPh₃)₂Ag(SCOPh)MT] and [(PPh₃)₂Ag(SCOPh)] co-crystallized in the same lattice. The unit cell of complex 9 also possesses two structurally different molecules with the same molecular formula. Emission spectra of the complexes have been studied in both solution and solid states. Electronic spectral behaviors of the complexes 1 and 7 have been explained by TDDFT calculations.

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Introduction

Sulfur containing organic molecules are versatile ligands for a wide range of metals. High polarizability, low electronegativity and the presence of lone pairs of electrons offer a rich sulfur based chemistry of the complexes.¹ Cu(I) and Ag(I) complexes of sulfur ligands display a wide variety of structural formats ranging from mononuclear discrete molecules to polynuclear clusters.²

The coordination chemistry of heterocyclic thioamides has seen spectacular expansion mainly because of the relevance of these compounds to biological systems.³ The key feature of this class of compounds, with respect to their ligating properties along with donor atoms for hydrogen bonding (Scheme 1), is the ability to bind to a metal centre adopting several coordination modes, thus giving rise to a rich variety of complexes with variable nuclearities and often unusual geometries.^3a,4

Scheme 1 Sulfur ligands used.

Metal complexes with a d¹⁰ configuration,⁵ particularly those containing both phosphine and a sulfur ligand (thiolate, thiocarboxylate, dithiocarbamate and dithioxanthate ligands)^2f,6 exhibit fascinating photophysical and photochemical properties. Complexes of copper(I) and silver(I) with d¹⁰ configuration have no low-energy ligand-field transitions, however, low-energy charge-transfer excited states are available in these cases, which is an important property of luminescent materials.^5a,b,6d

Recently, we have observed that Cu(II) complexes of thiocarboxylate ligands undergo spontaneous desulfurization where as those of Cu(I) are quite stable.⁷ Thiocarboxylate ligand (having both S and O donor sites) form primary bonding with the metal through S atom while the O remains free which can be exploited for hydrogen bonding. In view of these facts we thought it will be worthwhile to study the structural features of copper and silver complexes containing heterocyclic thioamides as well as thiocarboxylates.

Experimental

Reagents and general procedures

All the solvents were dried according to standard procedures and distilled before use. [(PPh₃)₂Cu(SCOth)],⁷ thiophene-2-thiocarboxylic acid, furan-2-thiocarboxylic acid (FuCOSH),⁸ bis(triphenylphosphine)copper(I) nitrate⁹ and bis(triphenylphosphine)silver(I) nitrate¹⁰ were prepared by reported procedures. Thiobenzoic acid, thioacetic acid, 2-mercapto-2-thiazoline (MT) and 2-mercaptobenzimidazole (MB) were purchase from Aldrich and used as received.

Instrumentation

IR spectra were recorded using Varian-3100 FTIR instruments. NMR spectra were obtained using a JEOL AL300 FT NMR spectrometer. Electronic absorption spectral measurements were carried out using a Shimadzu UV-1700 PharmaSpec Spectrometer. Emission spectra were recorded from VARIAN, CARY Eclipse Fluorescence spectrometer. Absorption and emission spectra of the complexes have been recorded in chloroform solution (∼10⁻⁵ M). Elemental analyses were performed by the EAT Exeter Analytical Inc. CE-440, elemental analyzer.

Single crystal X-ray analysis

Single crystal X-ray data of all the complexes were collected on a Xcalibur Eos Oxford diffractometer using graphite monochromated MoKα radiation (λ = 0.7107 Å). Data collections were carried out at room temperature. Structures were solved by the direct method and then refined on F² by the full matrix least square technique with SHELX-97 software¹¹ using the WinGX program package.¹² Crystal data of complexes are given in Table S1 (ESI†). The crystals of a few complexes showed some disorders. The disordered atoms of the thiophene (th) rings were split in two parts and then some geometrical restraints were applied (whenever necessary) for refinement. Similarly, disordered atoms present elsewhere have also been refined. In case of complexes 7 and 9 some a-level could not be removed. These alerts are generated because there is a large amount of disorder in the solvent molecule.

Computational details

All the calculations were performed using GAUSSIAN 03W software.¹³ Initial atomic co-ordinates were obtained from X-ray crystallographic data which were used as such for NBO and TDDFT¹⁴ calculations. The time dependent and NBO calculations were performed using 6-31G** and 6-31+G** respectively for all the atoms at B3LYP level. All the molecular orbital plots were generated by Gauss View program.

Syntheses

Synthesis of [(PPh₃)₂Cu(SCOMe)MB] (1). To the stirred methanolic solution (10.0 mL) of sodium thioacetate [prepared in situ by mixing thioacetic acid (0.076 g, 1.0 mmol) and sodium methoxide (0.054 g, 1.0 mmol)] and 2-mercaptobenzimidazole (MB) (0.150 g, 1.0 mmol), a chloroform solution (10 mL) of (PPh₃)₂Cu(NO₃) (0.650 g, 1.0 mmol) was added to get a colourless solution. The reaction mixture was stirred for three hours. The solvent was evaporated under reduced pressure and the residue was dissolved in CHCl₃ (20 mL) and filtered off to separate out NaNO₃. The filtrate was evaporated under reduced pressure. The colorless product was dried under vacuum for 1 h. Rectangular crystals were obtained from mixture of chloroform and toluene solution (1 : 1) at room temperature. Yield: 0.659 g (81%). Elemental analysis: cal. for C₄₅H₃₉N₂OS₂P₂Cu: C, 66.44, H, 4.83, N, 3.44. Found: C, 66.27, H, 4.78, N, 3.47. IR spectra (KBr, cm⁻¹): 3054 ν(N–H), 1582 ν(CO), 1121, 953 ν(C–S). NMR (CDCl₃, δ ppm): ¹H NMR; 2.49 (Me), 7.14–7.37 (Ph and MB ring), 11.42 (NH). ¹³C NMR; 36.86 (Me), 110.7, 123.2 (MB), 128.2–134.5 (Ph and MB ring), 164.5 (N₂CS), 199.2 (COS).

Synthesis of [(PPh₃)Cu(SCOth)MT₂] (2). To a chloroform (10 mL) solution of [(PPh₃)₂Cu(SCOth)] (0.731 g, 1.0 mmol), added a solution of 2-mercapto-2-thiazoline (MT) (0.238 g, 2.0 mmol) in 5 mL of methanol with stirring. The reaction mixture was stirred for three hours and evaporated under reduced pressure. The yellow residue was dissolved in mixture of chloroform and methanol (2 : 1) and kept for crystallization. Yellow block shaped crystals were obtained after few days. Yield: 0.558 g (79%) (when [(PPh₃)₂Cu(SCOth)] and MT taken in 1 : 1 ratio then the same product was obtained in low yield). Elemental analysis: cal. for C₂₉H₂₈N₂OS₆PCu: C, 49.23, H, 3.99, N, 3.96. Found: C, 49.12, H, 3.89, N, 4.03. IR spectra (KBr, cm⁻¹): 3063 ν(N–H), 1525 ν(CO), 1042, 936 ν(C–S). NMR (CDCl₃, δ ppm): ¹H NMR; 3.46 (CH₂), 3.91 (CH₂), 6.94–7.74 (Ph and th ring), 10.09 (NH). ¹³C NMR; 33.6 (CH₂), 51.9 (CH₂), 125.2–149.3 (Ph and th ring), 186.3 (COS), 199.2 (NSCS).

Synthesis of [(PPh₃)₂Cu(SCOth)MB] (3). To a chloroform (10 mL) solution of [(PPh₃)₂Cu(SCOth)] (0.731 g, 1.0 mmol), added a solution of MB (0.150 g, 1.0 mmol), in 5 mL of methanol with stirring. The reaction mixture was stirred for three hours and evaporated under reduced pressure. The yellow coloured residue was dissolved in mixture of chloroform and methanol (2 : 1) and kept for crystallization. Yellow block shaped crystals were obtained after few days. Yield: 0.731 g (83%). Elemental analysis: cal. for C₄₈H₃₉N₂OS₃P₂Cu: C, 65.40, H, 4.46, N, 3.18. Found: C, 65.28, H, 4.42, N, 3.21. IR spectra (KBr, cm⁻¹): 3068 ν(N–H), 1558 ν(CO), 1093, 910 ν(C–S). NMR (CDCl₃, δ ppm): ¹H NMR; 6.94–7.79 (Ph, th and MB ring), 11.64 (NH). ¹³C NMR; 110.7, 123.2 (MB), 127.1–150.2 (Ph, th and MB ring), 164.4 (N₂CS), 199.8 (COS).

Synthesis of H[(PPh₃)₂Cu(SCOth)(SCOFu)] (4). The same procedure was followed as mentioned for 3 except the fact that furan-2-thiocarboxylic acid (0.128 g, 1.0 mmol) was used in place of 2-mercaptobenzimidazole. Red rectangular crystals were obtained from toluene solution. Yield: 0.748 g (87%). Elemental analysis: cal. for C₄₆H₃₇O₃S₃P₂Cu: C, 64.28, H, 4.34. Found: C, 64.09, H, 4.32. IR spectra (KBr, cm⁻¹): 1585, 1558 ν(CO), 1039, 1017 ν(C–S). NMR (CDCl₃, δ ppm): ¹H NMR; 6.44–7.69 (Ph, th and Fu ring), 1.93 (OH). ¹³C NMR; 127.6–141.3 (Ph, th and Fu ring), 190.4, 197.9 (COS).

Synthesis of H[(PPh₃)₂Cu(SCOth)(SCOPh)] (5). The same procedure was followed as mentioned for 3 except the fact that thiobenzoic acid (0.138 g, 1.0 mmol) was used in place of 2-mercaptobenzimidazole. Red rectangular crystals were obtained from toluene solution. Yield: 0.809 g (93%). Elemental analysis: cal. for C₄₈H₃₉O₂S₃P₂Cu: C, 66.30, H, 4.52. Found: C, 66.23, H, 4.47. IR spectra (KBr, cm⁻¹): 1584, 1508 ν(CO), 1094, 999 ν(C–S). NMR (CDCl₃, δ ppm): ¹H NMR; 6.89–8.09 (Ph and th ring), 2.11 (OH). ¹³C NMR; 128.4–140.1 (Ph and th ring), 189.1, 193.9 (COS).

Synthesis of [(PPh₃)Cu(μ-SCOPh)₂(μ-MB)Cu(PPh₃)] (6). The same procedure was followed as mentioned for 1 except the fact that thiobenzoic acid (0.138 g, 1.0 mmol) was used in place of thioacetic acid. Yellow rectangular crystals were obtained from mixture of chloroform and toluene solution (1 : 1). Yield: 0.828 g (77%). Elemental analysis: cal. for C₅₇H₄₆N₂O₂S₃P₂Cu₂: C, 63.61, H, 4.31, N, 2.60. Found: C, 63.57, H, 4.29, N, 2.63. IR spectra (KBr, cm⁻¹): 3053 ν(N–H), 1536, 1497 ν(CO), 1094, 948, 923 ν(C–S). NMR (CDCl₃, δ ppm): ¹H NMR; 7.18–7.42 (Ph ring), 11.90 (NH). ¹³C NMR; 110.9, 123.4 (MB), 127.5–142.4 (Ph and MB ring), 164.1 (N₂CS), 207.8 (COS).

Synthesis of [Cu₂(μ-SCOPh)₂(μ-MT)(MT)]₂ (7). To a mixture of sodium thiobenzoate [prepared in situ by mixing PhCOSH (0.138 g, 1.0 mmol) and sodium methoxide (0.054 g, 1.0 mmol)] and MT (0.119 g, 1 mmol) in 10 mL of methanol, added solid cuprous chloride (0.099 g, 1.0 mmol) with stirring. The reaction mixture was stirred further for five hours. The yellow precipitate formed, was filtered and dried under vacuum. The yellow residue was dissolved in mixture of chloroform and toluene (3 : 1) and kept for crystallization. Yellow block shaped crystals were obtained after few days. Yield: 0.235 g (73%). Elemental analysis: cal. for C₄₀H₄₀N₄O₄S₁₂Cu₄: C, 37.54, H, 3.15, N, 4.38. Found: C, 37.44, H, 3.13, N, 4.40. IR spectra (KBr, cm⁻¹): 2996 ν(N–H), 1592, 1569 ν(CO), 1049, 993, 917 ν(C–S). NMR (CDCl₃, δ ppm): ¹H NMR; 3.51 (CH₂), 3.92 (CH₂), 7.14–7.97 (Ph ring), 10.09 (NH). ¹³C NMR; 32.8 (CH₂), 51.4 (CH₂), 125.2–137.3 (Ph ring), 167.2 (COS), 199.9 (NSCS).

Synthesis of {[(PPh₃)₂Ag(SCOPh)MT][(PPh₃)₂Ag(SCOPh)]·toluene} (8). The same procedure was followed as mentioned for 1 except the fact that (PPh₃)₂Ag(NO₃) (0.694 g, 1.0 mmol) and MT (0.119 g, 1 mmol) were used in place of (PPh₃)₂Cu(NO₃) and MB. Colorless rectangular crystals were obtained from mixture of chloroform and toluene solution (1 : 1). Yield: 0.596 g (69%). Elemental analysis: cal. for C₉₇H₈₄NO₂S₃P₄Ag₂: C, 67.28, H, 4.89, N, 0.81. Found: C, 67.35, H, 4.82, N, 0.87. IR spectra (KBr, cm⁻¹): 3052 ν(N–H), 1584, 1542 ν(CO), 1093, 997, 919 ν(C–S). NMR (CDCl₃, δ ppm): ¹H NMR; 2.35 (Me of toluene), 3.51 (CH₂), 3.98 (CH₂), 7.15–8.14 (Ph ring), 9.78 (NH). ¹³C NMR; 33.9 (CH₂), 51.7 (CH₂), 127.4–133.9 (Ph ring), 207.4, 200.5, (COS), 199.6 (N₂CS).

Synthesis of [(PPh₃)₂Ag(SCOPh)MB]·toluene (9). The same procedure was followed as mentioned for 8 except the fact that MB (0.150 g, 1.0 mmol) was used in MT. Colorless rectangular crystals were obtained from mixture of chloroform and toluene solution (1 : 1). Yield: 0.699 g (76%). Elemental analysis: cal. for C₅₀H₄₁N₂OS₂P₂Ag: C, 65.29, H, 4.49, N, 3.05. Found: C, 64.97, H, 4.42, N, 3.13. IR spectra (KBr, cm⁻¹): 3056 ν(N–H), 1582, 1537 ν(CO), 1188, 1166, 924 ν(C–S). NMR (CDCl₃, δ ppm): ¹H NMR; 2.28 (Me of toluene), 7.00–8.18 (Ph and MB ring), 11.72 (NH). ¹³C NMR; 110.6, 123.2 (MB), 127.4–144.3 (Ph and MB ring), 166.6 (N₂CS), 207.9 (COS).

Results and discussions

Syntheses and characterizations

The complexes 1–6 have been synthesized either by direct reaction of bis(triphenylphosphine)copper(I) nitrate with sodium salt of thiocarboxylate and 2-mercaptobenzimidazole/2-mercapto-2-thiazoline/thiocarboxylic acid or by synthesizing the thiocarboxylate complexes of bis(triphenylphosphine)copper(I) and then reacting it with 2-mercaptobenzimidazole/2-mercapto-2-thiazoline/thiocarboxylic acid (Scheme 2). The products obtained are of general formula [(PPh₃)₂Cu(SCOR)L] (1, 3–5) except in the case of 2 and 6 where a phosphine ligand is also replaced. In complex 2, the substitution is by another 2-mercapto-2-thiazoline molecule whereas in case of 6 there is no new substituent molecule, however, the 2-mercaptobenzimidazole bridges between two copper atoms, thus keeping the coordination number of Cu unchanged.

Scheme 2

The complex 7 was synthesized by reaction of cuprous chloride with sodium salt of thiocarboxylate and 2-mercapto-2-thiazoline in equimolar ratio (Scheme 3).

Scheme 3

Complexes 8 and 9 were synthesized by reaction of bis(triphenylphosphine)silver(I) nitrate with sodium thiobenzoate and 2-mercapto-2-thiazoline or 2-mercaptobenzimidazole respectively (Scheme 4).

Scheme 4

All the complexes were isolated in good yields and were found to be quite stable under ambient conditions. The complexes were characterized by elemental analysis, IR, ¹H NMR, ¹³C NMR, and UV-Visible spectroscopic techniques. In the IR spectra of all complexes three distinct absorption bands at 2996–3068, 1592–1597 and 1121–910 cm⁻¹ were obtained corresponding to the N–H, C–O and C–S stretchings respectively. ¹H NMR spectra of complexes 1–3 and 6–9 showed a broad peak at 9.78–11.64 ppm which can be assigned due to NH protons where as complexes 4 and 5 showed broad peaks respectively at 1.93 and 2.11 ppm for OH protons. Complexes 2, 7 and 8 showed two peaks at 3.46–3.51 and 3.91–3.98 ppm for the methylene protons of thiazoline. Other than these peaks a bunch of peaks were obtained in the range of 6.44–8.14 ppm due to the protons attached to aromatic rings. The ¹³C NMR spectra were also consistent with the ¹H NMR features (given in Experimental section).

Crystal and molecular structures

All the complexes have been characterized by single crystal X-ray diffraction techniques. A summary of crystallographic data and structure solutions are listed in Table S1 (ESI†). Selected bond lengths and bond angles are given in Table S2 (ESI†). The coordination geometry around copper atom in complexes 1–5 can be defined as distorted tetrahedral (Fig. 1a–e) and all the bond lengths and bond angles are comparable to reported values for thiocarboxylate and heterocyclic thioamide complexes of copper(I).^6a,7 In the complexes 1 and 3 copper atom is coordinated by the two phosphorus of triphenylphosphine and sulfur of 2-mercaptobenzimidazole besides the sulfur of thiocarboxylate ligands whereas in 2 (Fig. 1b) copper is coordinated by a phosphorus of triphenylphosphine and two sulfur from MT ligand in addition to the thiocarboxylate. Complexes 4 and 5 have similar coordination environments as observed in the case of 1 (Fig. 1d and e).

Fig. 1 Molecular structure of complexes 1–5.

In the complexes 1–3, strong hydrogen bonds are present in between NH of heterocycle and O of thiocarboxylate (Table S3, ESI†). In case of 1 and 2 intra-molecular hydrogen bonding are present. On changing the R group of thiocarboxylate (from Me to th) the hydrogen bonding changed from intra-molecular to inter-molecular type (as seen in the case of 3). In case of 1 besides intra-molecular hydrogen bonding, strong inter-molecular NH⋯S (2.700 Å) and CH⋯π (2.711 Å) interactions are also present. Due to these interactions a helical structure is formed (Fig. 2a). Presence of inter-molecular hydrogen bonding (NH⋯O) in 3 results in a linear chain of molecules (Fig. 2b). In case of complex 4 the hydrogen of thiocarboxylic acid is present in between two oxygens of thiocarboxylate groups but slightly near to the O1 (1.102 Å) and have strong hydrogen bonding with O2 (1.351 Å). The two C–S bonds (1.673 and 1.688 Å) and the two C–O bonds (1.280 and 1.274 Å) of thiocarboxylate groups are respectively comparable and possibly the H atom oscillates between the two oxygen atoms⁷ (Fig. 1d). Complex 5 also has similar hydrogen bonding as in 4 except the fact that the two C–O bonds are of different bond lengths (1.312 and 1.259 Å).

Fig. 2 Arrangement of 1 (a) and 3 (b) due to intermolecular H-bonding.

Complex 6 is bimetallic molecule in which two copper centres are bridged by two thiocarboxylate and one sulfur of 2-mercaptobenzimidazole. Molecular structure of 6 is depicted in Fig. 3. In this complex two different types of binding modes of thiobenzoate ligands are observed; O, S (bidentate) bridging which represents the classical binding mode of the ligands and S bridging. Besides these bridging ligands a strong Cu–Cu interaction is also responsible for holding the two metals together. The Cu–Cu distance [2.7180(18) Å] is slightly longer than the sum of the covalent radii of the two atoms (2.64 Å). Both the copper atoms have tetrahedral geometries with different degrees of distortions.

Fig. 3 Molecular structure of 6.

Besides the intra-molecular hydrogen bonding between N1H and O2 inter-molecular hydrogen bonding is also present in between N2H and S3 (Table S3, ESI†). Pairing of molecules occurred due to presence of these inter-molecular hydrogen bondings (Fig. 4).

Fig. 4 A pair of 6 due to intermolecular H-bonding.

Complex 7 is crystallized in triclinic system with P1̄ space group. A solvent (disordered) molecule co-crystallized with 7 which could not be identified. The molecular structure of complex depicted in Fig. 5a. Complex 7 is a tetra-nuclear (Cu₄) compound, with a planar six membered ring constituted by two copper atoms and one sulfur atom alternatively (Fig. 5b). Distance between the two adjacent copper atoms (2.598 Å) are very short even shorter than sum of covalent radii (2.64 Å) but bond angles around metal centres do not suggest the existence of a covalent bond between the two. As revealed by the bond angles around Cu₁ (which are very close to 120°), it possesses a trigonal planar geometry with slight deviation (0.018 Å) from the plane constituted by S1, S2 and S3 atoms. Notably, the geometry around Cu₂ is highly deviated from trigonal planar it is situated above the plane constituted by S1, S2′ and S5 atoms by 0.417 Å. This deviation may be because of the strong interaction between of Cu₂ and S3 atoms (2.812 Å) which is shorter than the sum of van der Waals radii of two atoms (3.20 Å). Complex 7 also have two intra-molecular NH⋯O hydrogen bonding with distances 1.991 and 2.053 Å respectively.

Fig. 5 (a) Molecular structure of 7 and (b) its core structure.

In the complex 8 two different types of molecules co-crystallized together along with a toluene molecule (Fig. 6). Both the molecules are connected with toluene by CH⋯π interactions with distances of 3.445 and 2.686 Å respectively. In the first molecule, silver having distorted tetrahedral geometry is bonded with two phosphorus of triphenylphosphine and sulfur of 2-mercapto-2-thiazoline besides the sulfur of thiobenzoate. All the bond lengths and bond angles are comparable to reported values for thiobenzoate and heterocyclic thioamide complexes of silver(I).^2e,15 An intra-molecular hydrogen bonding also present in between NH of heterocycle and oxygen of thiocarboxylate (Table S3, ESI†).

Fig. 6 Molecular structure of 8.

In the second molecule, silver is surrounded by two phosphorus of triphenylphosphine and sulfur of thiobenzoate group. In this case also all the bond lengths and bond angles are comparable to the analogous bond lengths and angles reported for silver thiobenzoate.^2e The geometry around Ag2 atom is trigonal planar and the Ag atom is slightly tipped above (0.011 Å) the plane constituted by S4, P3 and P4 atoms.

Complex 9 also has two types of molecules both of which have distorted tetrahedral geometry. Each Ag(I) is surrounded by two phosphorus of triphenylphosphine and one sulfur of 2-mercaptobenzimidazole besides sulfur of thiobenzoate. Notably, the orientation of thiocarboxylate groups are not identical. In one rotamer oxygen of thiobenzoate is facing towards the benzimidazole ring whereas in the other it is facing away from benzimidazole ring (Fig. 7).

Fig. 7 Molecular structure of 9.

The two molecules are interconnected by intra and inter-molecular hydrogen bonding between NH of heterocyle and O of thiocarboxylate as well as NH⋯S hydrogen bondings (Table S3, ESI†). A linear chain of molecules are formed due to the presence of NH⋯O and pair of NH⋯S hydrogen bonds (Fig. 8).

Fig. 8 A linear chain of 9 due to intra and intermolecular H-bonding.

Theoretical calculations

To get further insight into the nature of Cu–Cu interaction in 6 and 7, we have carried out density functional calculations. The results of second order perturbation analysis of Fock matrix (NBO) reveal that there are significant intra-molecular interactions. In case of 6 electron transfer from Lp* orbitals of Cu1 to Lp* of Cu2 amounts to 227 kcal mol⁻¹ of energy lowering. It may therefore, be concluded that the short Cu–Cu distance is not merely a consequence of steric constrains imposed by the ligands but is a result of strong bonding interactions between the two atoms.

In case of 7 two Cu–Cu distances (2.598 and 2.987 Å) are observed. In the first case distance between two copper atoms (2.598 Å) is even shorter than the sum of covalent radii of the two atoms indicating the existence of a covalent bond between the two atoms. Surprisingly, a look into the maximum stabilisation energy associated with electron transfers between a pair of Cu atoms revealed that the pair with a longer Cu–Cu distance has a stronger interaction (Lp*Cu48 → Lp*Cu62 = −43 kcal mol⁻¹) as compared to that between the other two (Cu53 to Cu48 = −36.9 kcal mol⁻¹).

To get further insight about the Cu–Cu interaction in 7, bond critical points (bcp) were calculated using the Atoms In Molecules (AIM) theory.¹⁶ The bond critical points were not observed between the pairs copper atoms (Cu56–Cu62 and Cu48–Cu53) with shorter Cu–Cu distances, thus confirming the absence of any bond between these pairs of Cu atoms (Fig. 9). However, the presence of bond critical point in between Cu48 and Cu62, shows the presence of weak interaction in between these two copper atoms. However, the distance between these two atoms is slightly longer than the sum of their van der Waals radii. The value of electron density (ρbcp); Laplacian electron density (∇2ρbcp); bond ellipticity (ε) and total energy density (H) at the bond critical point for Cu⋯Cu interactions in 7 are +0.012891, +0.035751, +0.258915 and 0.000176 au respectively. It is evident that the electron density at bond critical point (ρbcp) for Cu48–Cu62 being less than +0.10 au, is indicative of weak closed shell Cu–Cu interactions. Additionally, the Laplacian of the electron density ∇2ρbcp in this case being positive indicates the depletion of electron density in the region of contact between the Cu–Cu atoms which is characteristic of closed shell interactions. The bond ellipticity (ε) measures the extent to which the density is preferentially accumulated in a given plane containing the bond path. The ε values for 7 indicate that Cu⋯Cu interactions are cylindrically non-symmetrical in nature.

Fig. 9 Molecular graphs for the 7 by AIM calculations using B3LYP/6-31G** level of theory (bond critical points are shown by green spots).

Electronic absorption and photoluminescent spectra

Absorption spectra of 1, 3, 6 and 9 have similar absorption bands in which 2-mercaptobenzimidazole coordinated to the metal (Fig. S1 and S2, ESI†). Complex 1 shows bands at 244, 254, 280 and 309 nm. The spectra of 2, 4, 5 and 8 have similar absorptions bands listed in Table 1. The spectrum of complex 7 shows bands at 249, 279 and 366 nm. In general, lower energy peaks appear because of the metal to ligand (or ligand to metal) charge transfers while peak below 300 nm are due to inter- or intra-ligand charge transfers.

Table 1 Absorption bands of complexes

Complex	Absorption bands (nm)
1	309, 280 (broad), 254, 244
2	345 (broad), 279, 253
3	310, 255, 244
4	335 (broad), 261, 242
5	333 (broad), 257, 244
6	309, 302, 245
7	366 (broad from 310–404), 279, 249
8	317 (broad), 276 (shoulder), 245
9	309, 300 (shoulder), 247

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For unambiguous assignment of the absorption bands time dependent density functional theory (TDDFT) calculations have been performed for the complexes 1 and 7 at B3LYP level. The orbital transition plots of 1 are shown in Fig. 10. The calculated absorptions at 299 and 311 nm are due to electron transfers from lone pair of sulfur, phosphorous, oxygen and copper atoms to the phenyl rings of triphenylphosphine and benzimidazole ring (n → π*). Other absorption peaks at 280, 254 and 244 nm are due to intra- and inter-ligand charge transfers.

Fig. 10 Selected orbital transitions in 1 (orbital contour value 0.05).

In case of complex 7 calculated band at 372 nm is due to charge transfers from the lone pair of sulfur of thiobenzoate and copper atoms to phenyl rings of thiobenzoate ligands (n → π*) as shown in Fig. 11.

Fig. 11 Selected orbital transitions in 7 (orbital contour value 0.05).

Emission spectra of all the complexes recorded in solution (Fig. S3 and S4, ESI†) as well as in solid state (Fig. S5 and S6, ESI†). The emission and excitation data of complexes are summarized in Table 2. Upon excitation at 375 nm (calculated excitation wavelength) in chloroform solution at room temperature, complexes 1 give rise to an intense emission band at 435 nm other than a shoulder emission band at 412 nm, while in solid state emission band obtained at 406 nm and 491 nm (shoulder) (Fig. 12).

Table 2 Emission bands of complexes in solution and solid state

Complex	Excitation band (nm)	Emission band (nm)	Emission band (nm)
	Solution state		Solid state
1	375	412 (shoulder), 435	406, 491 (shoulder)
2	379	436, 463 (shoulder)	396 (intense band), 308, 509, 604 (weak band)
3	375	410 (shoulder), 434	398 (intense band), 307, 488, 602 (weak band)
4	371	408, 431	393 (intense band), 306, 604 (weak band)
5	375	409, 433	390 (intense band), 307, 605 (weak band)
6	374	436, 460 (shoulder)	396 (intense band), 306, 510, 602 (weak band)
7	371	424	396 (intense band), 305, 491, 602 (weak band)
8	358	436	393 (intense band), 305, 603 (weak band)
9	337	407	410 (intense band), 306, 601 (weak band)

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Fig. 12 Emission spectra of 1 in solution and solid state.

Similar emission bands are also observed in case of complexes 2–6 given in Table 2 (Fig. S3 and S5, ESI†). In solution state, complex 7 showed weak and broad emission band (Fig. S3, ESI†) at 424 nm when it was excited at 371 nm where as in solid state an intense emission band observed at 396 nm along with weak emission bands at 305, 491 and 602 nm. Complexes 8 and 9 showed emission bands (Fig. S4 and S6, ESI†) at 436 and 407 nm respectively in solution state and intense emission bands at 393 and 410 nm respectively in solid state along with three weak emission bands (Table 2). Though the assignments of the emissive excited states in the Cu(I) complexes is a difficult task, yet a few inferences could be drawn. The strong emission peaks around 400 nm are similar to that observed in a large number of Cu(PPh₃) complexes, which indicates that the origin of these emissions involves emissive state derived from ligand-centered [π–π*] transition.¹⁷ All the complexes (except 1) in solid state exhibit a low-energy emission with k_em at ∼600 nm, which is due to M–S (thiocarboxylate) charge transfer. Interestingly, another emission (centered between 488 and 510 nm) is observable in the spectra of the Cu(I) complexes except 4 and 5 originated due to Cu(I)–S (heterocyclic thioamides) charge transfer. Since the Cu(I) ion express strong tendency for charge transfer of this type, the emissions cannot be considered purely to be intra-ligand origin.¹⁷

Conclusions

Seven new copper(I) thiocarboxylate complexes, [(PPh₃)₂Cu(SCOR)L] [when L = MB and R = Me (1), th (3), R = th and L = FuCOSH (4) and PhCOSH (5)], [(PPh₃)Cu(SCOth)MT₂], (2) [(PPh₃)Cu(μ-SCOPh)₂(μ-MB)] (6), [Cu₂(μ-SCOPh)₂(μ-MT)(MT)]₂ (7) and two silver thiocarboxylate complexes, {[(PPh₃)₂Ag(SCOPh)MT][(PPh₃)₂Ag(SCOPh)]·C₇H₈} (8), [(PPh₃)₂Ag(SCOPh)MB]·C₇H₈ (9) containing 2-mercaptobenzimidazole and 2-mercapto-2-thiazoline have been synthesized and structurally characterized. All the complexes have either intra or inter-molecular hydrogen bonding between oxygen/sulfur of thiocarboxylate and hydrogen of heterocyclic thioamides. From single crystal structures of complexes it is clear that direction of hydrogen bonding can be customized by modifying the size of terminal R-groups of thiocarboxylate and heterocyclic ring. 7 is a tetranuclear copper(I) complex in which two copper atoms are forced to sit very close (but without any direct Cu–Cu bonding interaction) within their covalent radii by bridging of sulfur donor ligand and intra-molecular hydrogen bonding. In case of complex 8 two different types of molecules co-crystallized in same lattice and they are connected by weak interactions (CH⋯π) through a toluene molecule. In case of complex 9 also two different molecules are present in lattice having same molecular formula but are different rotamers. All the complexes showed photoluminescent property in both solution and solid states, these may find applications as fluorescent sensors.

Acknowledgements

The authors are grateful to Dr Abhinav Kumar, Department of Chemistry, Lucknow University, India for his help in AIM calculation. Financial supports from the Council of Scientific and Industrial Research, India in the forms of a scheme to S.B. and a fellowship to S.S. are gratefully acknowledged.

Notes and references

1. (a) C. G. Young, J. Inorg. Biochem., 2007, 101, 1562–1585; (b) C. S. Clarke, D. A. Haynes and J. M. Rawson, Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem., 2008, 104, 124–133; (c) J. R. Dilworth, P. Arnold, D. Morales, Y. L. Wong and Y. Zheng, Modern Coordination Chemistry, ed. G. J. Leigh and N. Winterton, Royal Society of Chemistry, Cambridge, UK, 2002, p. 217; (d) Handbook of Chalcogen Chemistry: New Perspective of Sulfur, Selenium and Tellurium, F. Devillanaova, Royal Society Chemistry, London, England, 2006.
2. (a) J. R. Dilworth and N. Wheatley, Coord. Chem. Rev., 2000, 199, 89–158; (b) D. K. Joshi, K. B. Mishra, V. K. Tiwari and S. Bhattacharya, RSC Adv., 2014, 4, 39790–39797; (c) J. T. Sampanthar, J. J. Vittal and P. A. W. Dean, J. Chem. Soc., Dalton Trans., 1999, 3153–3156; (d) T. C. Deivaraj and J. J. Vittal, J. Chem. Soc., Dalton Trans., 2001, 329–335; (e) S. Singh, J. Chaturvedi, S. Bhattacharya and H. Nöth, Polyhedron, 2011, 30, 93–97; (f) C.-H. Li, S. C. F. Kui, I. H. T. Sham, S. S.-Y. Chui and C.-M. Che, Eur. J. Inorg. Chem., 2008, 2421–2428.
3. (a) R. Mitra, S. Das, S. V. Shinde, S. Sinha, K. Somasundaram and A. G. Samuelson, Chem.–Eur. J., 2012, 18, 12278–12291; (b) K. D. Karlin and J. Zubieta, Copper Coordination Chemistry: Biochemical and Inorganic Perspectives, Adenine Press, New York, 1983, and references therein; (c) B. Krebs and G. Henkel, Angew. Chem., Int. Ed. Engl., 1991, 30, 769–788.
4. (a) P. Aslanidis, S. Kyritsis, M. Lalia-Kantouri, B. Wicher and M. Gdaniec, Polyhedron, 2012, 48, 140–145; (b) E. Guerrero, S. Miranda, S. Luttenberg, N. Frohlich, J.-M. Koenen, F. Mohr, E. Cerrada, M. Laguna and A. Mendía, Inorg. Chem., 2013, 52, 6635–6647; (c) E. S. Raper, Coord. Chem. Rev., 1985, 61, 115–184; (d) E. S. Raper, Coord. Chem. Rev., 1994, 129, 91–151.
5. (a) P. C. Ford, E. Cariati and J. Bourassa, Chem. Rev., 1999, 99, 3625–3647; (b) V. W.-W. Yam and K. K.-W. Lo, Chem. Soc. Rev., 1999, 28, 323–334; (c) C.-M. Che and S.-W. Lai, Coord. Chem. Rev., 2005, 249, 1296–1309; (d) D. L. Phillips, C.-M. Che, K. H. Leung, Z. Mao and M.-C. Tse, Coord. Chem. Rev., 2005, 249, 1476–1490.
6. (a) V. W.-W. Yam, C.-K. Li and C.-L. Chan, Angew. Chem., Int. Ed., 1998, 2857–2859; (b) S. Singh, J. Chaturvedi, A. S. Aditya, N. R. Reddy and S. Bhattacharya, Inorg. Chim. Acta, 2013, 396, 6–9; (c) S. Singh, J. Chaturvedi and S. Bhattacharya, Inorg. Chim. Acta, 2013, 407, 31–36; (d) G. Rajput, V. Singh, S. K. Singh, L. B. Prasad, M. G. B. Drew and N. Singh, Eur. J. Inorg. Chem., 2012, 3885–3891.
7. S. Singh, J. Chaturvedi and S. Bhattacharya, Dalton Trans., 2012, 41, 424–431.
8. S. Singh, S. Bhattacharya and H. Noeth, Eur. J. Inorg. Chem., 2010, 5691–5699.
9. G. J. Kubas, Inorg. Synth., 1979, 19, 93.
10. T. C. Deivaraj, J.-H. Park, M. Afzaal and J. J. Vittal, Chem. Mater., 2003, 15, 2383–2391.
11. G. M. Sheldrick, SHELX 97, Program for Crystal Structure Refinement from Diffraction Data, University of Gottingen, 1997.
12. L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837–838.
13. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr, T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03, Revision D.01, Gaussian, Inc., Wallingford, CT, 2004.
14. J. Tomasi, B. Mennucci and E. Cances, J. Mol. Struct.: THEOCHEM, 1999, 464, 211–226.
15. J. P. Fackler Jr, C. A. Lopez, R. J. Staples, S. Wang, R. E. P. Winpennya and R. P. Lattime, J. Chem. Soc., Chem. Commun., 1992, 146–148.
16. R. F. W. Bader, Atoms in Molecules: A Quantum Theory, Oxford University, New York, NY, 1990.
17. Q.-M. Qiu, M. Liu, Z.-F. Li, Q.-H. Jin, X. Huang, Z.-W. Zhang, C.-L. Zhang and Q.-X. Meng, J. Mol. Struct., 2014, 1062, 125–132.

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Footnote

† Electronic supplementary information (ESI) available: Tables S1–S3 and Fig. S1–S6. CCDC 935064–935072. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra07883g

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