Synthesis and spectral characterization of bimetallic metallomacrocyclic structures [M^II₂-μ²-bis-{(κ²S,S-S₂CN(R)C₆H₄)₂O}] (M = Ni/Zn/Cd): density functional theory and host–guest reactivity studies

Abstract

A series of bimetallic dithiocarbamate metallomacrocyclic complexes [M^II₂-μ²-bis-{(κ²S,S-S₂CN(R)C₆H₄)₂O}]·L {R = benzyl; M = Ni^II 1, Zn^II 2, Cd^II 3; R = 1-naphthylmethyl; M = Ni^II 4, Zn^II 5, Cd^II 6 and L = Et₃N for 1, 4 only} were efficiently synthesized through a self-assembly process involving 4,4′-bis(arylmethylamino)diphenyl ethers L¹, L², CS₂ and M(OAc)₂. The compounds were characterized by micro-, relevant spectroscopic (ESI-MS, FT-IR, ¹H, ¹³C and ¹H DOSY NMR, UV-visible absorption, fluorescence) and TGA/DTA analyses. The geometry of all the complexes has been optimized by a DFT method with B3LYP/LanL2DZ basis sets. Notably, the fluorescence properties of L¹ and L² were enhanced upon complexation with Ni, Zn or Cd metal ions in the binuclear complexes 1–6. Ni^II complexes 1 and 4 gave stable residual masses of 20.1% and 29.5% in their thermogravimetric analyses which correspond to NiS (calc. 14.18% for 1 and 12.26% for 4) plus char, respectively. A Job plot experiment reveals the ability of macrocycles 1–4 to form host–guest complexes in 1 : 1, 1 : 2 and 2 : 1 stoichiometry, depending on the relative sizes of the hosts and guests and their electronic nature.

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Introduction

Coordination-driven self-assembly provides a robust tool for the preparation of a broad range of supramolecular structures that includes macrocyclic cages or polymers with fascinating physicochemical properties.^1–7 Among these structures, metallomacrocyclic structures with varied cavity sizes are of particular interest due to their potential applications in catalysis,⁴ host–guest chemistry,^2,5 molecular and ion sensing,⁶ separation, transport, and storage.⁷ The insights of changes induced by external perturbations on self-assembly can facilitate the design and development of macrocyclic compounds with potential applicability in drug delivery, two-phase transport and biosensing.⁸ In this connection, the ability of dithiocarbamate ligands to display varied binding modes while stabilizing various transition/non-transition metal ions present in different oxidation states, makes them highly promising for the development of diversified self-assembled molecular structures.^{2,4e,5a,b,9,10}

Besides, transition metal complexes with sulphur rich ligands exhibit a wide range of applications in the area of electrical conductivity, molecular magnetism, electrochemical, optoelectronic properties and biological processes.^11,12 Their potential uses in medicine reportedly arise due to the existence of the dithiocarbamate moiety in a variety of biologically active molecules.¹³ These complexes have also been used as a single source precursor for the synthesis of high-quality semi-conductor nanoparticles.¹⁴ Their widespread industrial applications such as foam rubber, fungicides, effective heat stabilizers, antioxidant action, reprocessing of polymers have been advocated by thermogravimetric study¹⁵ which indeed suggests the suitability of the complexes to be used as single source precursors for the synthesis of metal sulphide nanoparticles and thin films.¹⁶ The size and shape of the metal sulphide nanoparticles greatly depends on the nature of the ligand framework of the complexes which in turn affects the fundamental properties such as optical, electrical and mechanical.¹⁷

Recently our group has utilized 4,4′-diaminodiphenyl ether to derive a number of bimetallic metallomacrocyclic structures and systematically investigated these derivatives from medicinal perspectives.¹⁸ Earlier Professor N. Yoshida et al.¹⁹ had demonstrated the use of bis-N,O-bidentate Schiff-base ligands derived from 4,4′-diaminodiphenyl methane/4,4′-diaminodiphenyl sulfone to bind two separate metal ions owing to the fact that the bridging group (–C₆H₄CH₂C₆H₄–/–C₆H₄SO₂C₆H₄–) prevents the metal ions from forming 1 : 1 four coordinate complexes.

Herein, we report on the synthesis, spectroscopic characterization, DFT calculations and host–guest reactivity studies of a series of binuclear M^II dithiocarbamate macrocyclic complexes [M^II₂-μ²-bis-{(κ²S,S-S₂CN(R)C₆H₄)₂O}]·L {R = benzyl; M = Ni^II 1, Zn^II 2, Cd^II 3; R = 1-naphthylmethyl; M = Ni^II 4, Zn^II 5, Cd^II 6 and L = Et₃N for 1, 4 only}. The ease of synthesis and their potential to form intramolecular 1 : 1/1 : 2 inclusion complexes with bidentate guests and the formation of transition metal sulphide during thermal degradation would add merit to the present series of macrocyclic complexes.

Results and discussion

Syntheses and characterization

The one-pot reaction protocol involving self-assembly of the diamine precursor L¹ or L², CS₂ and corresponding metal acetate in Et₃N affords access to a series of binuclear M^II dithiocarbamate macrocyclic scaffolds [M^II₂-μ²-bis-{(κ²S,S-S₂CN(R)C₆H₄)₂O}]·L 1–6 (Scheme 1). The one-pot synthetic methodology has been utilized extensively by Beer and his coworkers to develop self-assembled dithiocarbamate–transition metal macrocycles. Such a process that involves several chemical operations simultaneously in a single vessel has several distinctive advantages such as, it can reduce the purification of the intermediate compounds, especially in cases of unstable intermediates and may eventually save valuable resources. The green coloured binuclear Ni^II dithiocarbamate macrocycles (1, 4), white coloured binuclear Zn^II dithiocarbamate macrocycles (2) and yellow coloured binuclear Cd^II dithiocarbamate macrocycles (3) exhibit good solubility in polar organic solvents such as DMSO/DMF. The products isolated appear to be air stable in the solid and in solution state over a period of days.

Scheme 1 One-pot synthetic strategy used for binuclear metallomacrocyclic dithiocarbamate complexes 1–6.

Although these compounds could not yield single crystals suitable for single crystal XRD analysis, their composition and structures were elucidated by microanalysis, various spectroscopic data and corroborated by DFT study. The elemental analysis data for metallomacrocyclic dithiocarbamate complexes 1–6 are in good agreement with their compositions as per the proposed chemical formula, which is mutually supported by subsequent spectroscopic data and thereafter, theoretical studies. The characterization data for 1–6 is summarized in the experimental section. Electrospray mass spectrometry (ESMS) in acetonitrile solutions confirmed the formation of all dinuclear macrocyclic structures. Binuclear M^II dithiocarbamate complexes 1–4 (except 3) gave m/z peaks at 1279.7, 1213.3 and 1343.3 which correspond to [M·Et₃N + H]⁺, [M + Na]⁺ and [(M − S) + H]⁺ respectively, in the positive-ion mode. However, complex 3 displayed a m/z peak, in negative ion mode at 1320.5, corresponding to the [M + Cl]⁻ molecular ion. Additionally, a summary of the calculated values of ES-MS (other than [M·Et₃N + H]⁺ for 1, [M + Na]⁺ for 2, [M + Cl]⁻ for 3 and [(M − S) + H]⁺ for 4 ion peaks) which match well with the observed ones on MS spectra of 1–4, are provided in the ESI as Table S1.† The IR spectra of 1–6 clearly showed the disappearance of one of the ν(N–H) vibration bands (3390–3370 cm⁻¹) associated with amine moieties and the appearance of two new single sharp medium intensity bands in the regions 1502–1495 cm⁻¹ and 1110–963 cm⁻¹ due to ν(N–CSS) and ν_assy(CSS) stretching vibrations, respectively. These two regions are of particular interest in the IR spectral study of dithiocarbamate complexes and confirm the bidentate coordination of dithiocarbamate ligands.²⁰ A significant enhancement in the ν(C–N) frequency of the complexes by comparison to that found in free diamine precursors, indicates the importance of electron delocalization over coordinated ligand moieties (R = N⁺–CS₂⁻). Notably, a strong band at ∼860 cm⁻¹ appeared due to the aromatic ν(C–H) out-of plane bending vibrations, which is a characteristic feature of para-disubstituted benzene rings.²¹ Reportedly, the ν(M − S) band normally occurs in the far-IR region²² and this depends significantly on the nature of the metal ion and the ancillary part of the ligand. In agreement with earlier observations,²² we have observed medium to weak intensity ν(M − S) bands in the 554–454 cm⁻¹ range. All the complexes display characteristic ¹H and ¹³C NMR signals associated with various molecular sub-units with appropriate splitting thus confirming their composition and purity. A significant shifting of the N-methylene signals of the complexes, compared to those of the free diamine precursors reinforce the formation of proposed structures as they are very sensitive to any kind of chemical change at the amine functionality. The binuclear M^II dithiocarbamate macrocycles 1–4 display downfield resonances in the δ 208.5 to 212.0 ppm range, characteristic of coordinated dithiocarbamates (–N¹³CS₂).²³ We could not detect the –N¹³CS₂ signals for complexes 5 and 6 even after an increased number of scans mainly due to their inadequate solubility and the longer relaxation times of the N¹³CS₂. The relatively poor solubility of complexes 5 and 6 could be due to the presence of several aromatic and dithiocarbamate moieties, leading to a large number of intermolecular non-covalent interactions such as CH⋯π, π⋯π and M⋯S interactions. The appearance of sharp signals with resolved splitting patterns, instead of multiple sets of overlapped peaks in the NMR spectra of binuclear complexes 1–6 indicates the formation of discrete macrocyclic complexes. In the case of complex 1, signals appearing at 1.43 (t, J = 7.2 Hz; CH₃), 3.23 (q, J = 7.2 Hz; CH₂) correspond to Et₃N whereas the additional CH₂ signals correspond to the complex, which are in fact merged peaks and not split due to coupling as clearly indicated by the ¹H COSY NMR spectrum (Fig. 1). Additional or merged signals for CH₂ and N¹³CS₂ groups have appeared in most of instances because of the different stereochemical environment of these groups in the molecules 1–6. DOSY of compound 1 clearly differentiates the peaks corresponding to Et₃N from those of complex 1. The ¹H DOSY NMR spectra (Fig. 2) unambiguously show the presence of a single species in solution and rule out the possibility of the formation of oligomers or coordination polymers.

Fig. 1 ¹H COSY NMR spectrum of complex 1.

Fig. 2 ¹H DOSY NMR spectra of complexes 1 (2A) and 5 (2B).

The loss of a triethylamine fragment at a much higher temperature than its boiling point in thermogravimetric analysis confirms the association of Et₃N with Ni^II complexes 1 and 4 which is indeed consistent with the microanalysis and NMR data. The thermogravimetric analysis revealed that the Ni^II complexes 1 and 4 gave a stable residual mass of 20.1% and 29.5% which corresponds to NiS (calc. 14.18% for 1 and 12.26% for 4) plus char, respectively (ESI†).

The UV-visible absorption spectra of L¹, L² exhibit two prominent bands at shorter wavelengths ∼250 nm and ∼280 nm, attributable to π → π* (phenyl) and π → π* (amine) transitions, respectively.^18a All the binuclear complexes show two principal bands at 261–286 nm region and at 388–478 nm region, attributable to π → π* (phenyl) and charge transfer transitions, respectively, except Zn(II) complexes 2 and 5 which exhibit featureless electronic spectra presenting a broad absorption at ∼270 nm. The diamagnetism along with UV-visible absorption bands suggest a square planar/distorted square planar environment around Ni^II and a tetrahedral/distorted tetrahedral environment around Zn^II/Cd^II centres in their respective complexes.²⁴ Besides, fluorescence study evidently depicted an enhancement in the fluorescence properties of binuclear complexes 1–6, compared to their respective ligand precursors L¹ and L².

All the complexes 1–6 exhibit maximum fluorescence emissions at 372, 371, 437, 533, 371 and 330 nm upon excitation at 261, 267, 282, 326, 271 and 265 nm with concomitant Stokes shifts of 111, 96, 155, 207, 100 ≈ 65 nm, respectively (Fig. 3). Notably, among 1–6, complex 1 bearing a Ni^II centre exhibits the maximum fluorescence which may be attributed to the reduction of the photoinduced electron transfer process on complex formation.²⁵ Moreover, the values of optical band gap energy (E_g) obtained for an entire group of compounds 1–6 (ESI, Fig. S28†) falls in the range of 2.1–2.6 eV and exhibits the features of a direct band gap semiconductor.²¹

Fig. 3 UV-visible emission spectra of compounds L¹, L² and 1–6 from 10⁻⁵ M DMSO solution.

Host–guest reactivity study (Jobs plot)

The interaction of Et₃N molecules with compounds 1 and 4 in a 1 : 1 stoichiometry has been clearly revealed by microanalysis, NMR later by thermogravimetric analysis. This has encouraged us to explore the host–guest binding ability of some of these macrocyclic complexes towards a number of guest species viz. 1,4-dioxane, 4,4′-bipyridine, piperazine and triethylamine by using UV-vis absorption spectrophotometry.

Job plot experiments revealed that 1–4 form host–guest complexes in 1 : 1, 1 : 2 and 2 : 1 stoichiometries, depending on the relative sizes of the host–guest and their electronic nature (ESI, Fig. S31†). Evidently, the binding of bidentate guest species outside the receptor cavity is unaffected by the size of the macrocyclic cavities. For instance, in spite of the large macrocyclic cavity sizes of receptors 1–4, they fail to accommodate all the bidentate guests (see previously) to form a 1 : 1 inclusion complex, instead receptor 1 predominantly forms a 1 : 2 complex while receptor 2 forms a 2 : 1 complex exclusively with all the bidentate guests, whereas receptors 3 and 4 form such a complex only with piperazine and 4,4′-bipyridine, respectively. The binding behavior of these bidentate guests is consistent with the behavior of terephthalate and isonicotinate with ditopic dinuclear zinc(II) dtc macrocyclic receptors.¹⁰ The host–guest binding stoichiometry for receptor 4 with bidentate guests 1,4-dioxane ∼ (2.9 × 4.8) Ų and piperazine ∼ (4.73 × 4.87) Ų was found to be 1 : 1, indicating the formation of intramolecular inclusion complexes (Fig. 2) in Job plot experiments. Although, the size of this host is much larger (12.2 × 6.1) Ų than the guest species receptors 4, however the formation of such inclusion complexes may be associated with the flexibility of –[Ar–O–Ar]– spacer group causing variation in the transannular Ni–Ni distance and thereby the cavity size and shape of the receptor. Moreover, the unique bowl shaped architecture adopted by this receptor 4 (Fig. 4) may allow small sized bidentate guests to enter into the molecular cavity (bowl) and forms 1 : 1 complex, whereas bulkier 4,4′-bipyridine guest failed to form such complex, instead it showed 2 : 1 complex. All the receptors form 1 : 2 complexes with Et₃N, except receptor 1 suggesting cavity size independent binding of this guest with receptors. The presence of a M^II dithiocarbamate Lewis acid centre in a cyclic structural framework, allows the efficient interaction with Lewis base guests, probably converting the square planar Ni^II dithiocarbamate and tetrahedral Zn^II/Cd^II dithiocarbamate moiety into a five-coordinate square pyramidal complex, respectively.²⁶

Fig. 4 A representation of interacting modes of receptor 4 with various guests based on a Job’s plot experiment.

Computational investigations

To get a better understanding of the spectroscopic results, a DFT level calculation has been performed on diamine precursors L¹, L² and complexes 1–6. Such calculations have been widely used in recent years due to their ability to provide reasonably good results for large molecular structures, including transition metal complexes.²⁷ All the calculations were performed using the Gaussian 03 program suite²⁸ and molecular orbitals were generated by the GaussView 3.0 program. Full geometry optimizations of diamine precursors L¹, L² and complexes 1–6 were performed using density functional theory (DFT) at B3LYP/6-31G (d,p) and B3LYP/LanL2DZ basis sets, respectively. Such a type of basis set has been used with good success in a number of studies involving similar species, having a good agreement with experimental results.²⁷ An optimized geometry for the minimum energy conformation of L¹, L² and binuclear complexes 1–6 are given in Fig. 5 whereas structural parameters are summarized in Table 1.

Fig. 5 An optimized geometry for the minimum energy conformation of (a) L¹, (b) 1, (c) 2, (d) 3, (e) L², (f) 4, (g) 5 and (h) 6.

Table 1 Summary of DFT study performed on L¹, L² at the DFT B3LYP/6-31G (d,p) level and binuclear metallomacrocyclic dithiocarbamate complexes 1–6 at the DFT B3LYP/LanL2DZ level

Entry	Energy of optimized geometry (Hartree)	Coordination geometry	Transannular M⋯M distance (Å)	EHOMO, ELUMO (eV)	EHOMO–LUMO (eV)	λmax cal. (exp) nm
L^1	−1189.9755	—	—	−4.7782	4.5814	271 (277)
				−0.1967
L^2	−1497.2624	—	—	−4.8478	3.7430	331 (313)
				−1.1048
1	−6054.1712	Distorted square planar	9.959	−5.4797	3.6774	237 (233)
				−1.8023
2	−5846.7443	Distorted tetrahedral	10.402	−5.8823	4.8028	258 (267)
				−1.0795
3	−5811.6669	Distorted tetrahedral	10.494	−5.7836	4.7118	263 (281)
				−1.0719
4	−6668.6825	Distorted square planar	12.205	−5.7251	3.4649	358 (337)
				−2.2602
5	−6461.2899	Distorted tetrahedral	10.476	−5.8285	4.5376	273 (260)
				- 1.2909
6	−6426.1713	Intermediate geometry between tetrahedral and square planar	13.682	−5.5921	3.7738	329 (350)
				−1.8183

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For complexes 1–6, optimized geometries clearly suggest that each dithiocarbamate ligand bridges two metal centres via chelating dithiocarbamate moieties, resulting in the formation of binuclear macrocyclic architectures. The optimized structures (Fig. 5) clearly suggest the distorted square planar coordination geometry around both the Ni^II nuclei in the binuclear dithiocarbamate complexes 1, 4; distorted tetrahedral coordination geometry around both the Zn^II and Cd^II centres in their corresponding dithiocarbamate complexes 2, 3, 5, 6. The study also points out nearly coplanar macrocyclic rings as minimum energy conformations for binuclear M^II dithiocarbamate complexes 1–3 and 5. Among four N-alkyl substituents, three are projected towards either side of the molecular plane and one leans to the opposite side of the molecular plane in 1–3, while in the case of 5, each set of two substituents is oriented towards either side of the molecular plane. Interestingly, complexes 4 and 6 exhibit bowl shaped molecular architectures (Fig. 6) as minimum energy conformations. Among the four 1-naphthylmethyl groups, two are projected towards the upper side, one is projected below and the fourth one is arranged parallel to the molecular plane to compensate for stereo-electronic repulsion.

Fig. 6 An illustration of a bowl shaped molecular architecture for (a) 4 and (b) 6 in capped stick and spacefill models.

The dithiocarbamate functionalities are most vulnerable to any change in the electronic environment induced by the aromatic substituents as they are present in the close vicinity. This could be the basis for magnetic non-equivalence of dithiocarbamate moieties present in a molecular framework which is also experimentally supported by more than one ¹³C NMR signal for the –N¹³CS₂ moiety. The selected structural parameters for the optimized geometries of 1–6 are summarized in Table 2. It appears from the optimized geometries that there is a gradual increase in the bond lengths associated with M^II dithiocarbamate moieties from Ni–Zn–Cd in all the complexes 1–6 and this trend is indeed unaffected by various N-substituents; however a significant effect of these substituents can be seen on the C–O–C bond angles. For instance, a gradual decrease in the C–O–C bond angle is observed for complexes Ni–Zn–Cd bearing N-benzyl substituents whereas a reverse trend is observed for those bearing N-(1-naphthyl methyl) substituents.

Table 2 Selected geometrical parameters obtained from the optimized geometry of complexes 1–6

Selected bond	Bond lengths (Å)	Selected bonds	Bond angles (°)
L^1
N–C	1.400–1.457	C–O–C	119.73
L^2
N–C	1.402–1.454	C–O–C	119.74
1
N–C	1.346–1.348	S–Ni–S (chelate)	78.227–78.281
C–S	1.726–1.734	S–Ni–S (trans S)	174.98–176.64
Ni–S	2.266–2.293	C–O–C	117.91–117.98
2
N–C	1.349	S–Zn–S (chelate)	75.75–75.80
C–S	1.738–1.749	S–Zn–S (trans S)	118.02–118.58, 129.79–133.31
Zn–S	2.442–2.455	C–O–C	117.68
3
N–C	1.352	S–Cd–S (chelate)	70.36–70.39
C–S	1.738–1.750	S–Cd–S (trans S)	119.62–137.83
Cd–S	2.619–2.639	C–O–C	117.65
4
N–C	1.347–1.357	S–Ni–S (chelate)	78.21–78.38
C–S	1.727–1.739	S–Ni–S (trans S)	156.83–175.42
Ni–S	2.274–2.283	C–O–C	118.99, 120.26
5
N–C	1.349, 1.353	S–Zn–S (chelate)	75.73–75.77
C–S	1.738–1.749	S–Zn–S (trans S)	117.77–134.63
Zn–S	2.443–2.452	C–O–C	118.11, 118.15
6
N–C	1.352, 1.360	S–Cd–S (chelate)	69.40–69.80
C–S	1.737–1.751	S–Cd–S (trans S)	103.06–174.71
Cd–S	2.632–2.652	C–O–C	125.07, 122.75

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Further, structural parameters (Table 2) obtained from optimized geometries were compared with those obtained from an X-ray study (Table 3) of closely related compounds.²⁹ It appears that in the case of M^II dithiocarbamate complexes 1–6, C–S, M–S, N–CS₂ bond distances and S–M–S (chelate), S–M–S (trans) angles are somewhat overestimated, compared to the experimental results reported. This slight disparity in the structural parameters could be explained by the mutual effect of differential stereoelectronic factors and the existence of extensive non-covalent interactions.

Table 3 Selected geometrical parameters obtained from X-ray structures²⁹

Selected bond	Bond lengths (Å)	Selected bonds	Bond angles (°)
Binuclear Ni^II dithiocarbamate complex
N–C	1.318–1.326	S–Ni–S (chelate)	79.39–79.47
C–S	1.710–1.722	S–Ni–S (trans S)	176.47–179.04
Ni–S	2.200–2.208	Coordination geometry	Square planar
Binuclear Zn^II dithiocarbamate complex
N–C	1.340–1.361	S–Zn–S (chelate)	77.23–79.01
C–S	1.683–1.801	S–Zn–S (trans S)	117.44–136.16
Zn–S	2.348–2.406	Coordination geometry	Distorted tetrahedral

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The molecular electrostatic potential (MESP) of any chemical species provides valuable information about the electronic environment of the molecule, useful for the prediction of its properties and potential sites for reactivity, including biological systems.³⁰ The localization of slight negative potential around the respective metal centres in complexes 1–3 and 5 whereas as the positive potential around metal centres in bowl shaped complexes 4 and 6 could be clearly revealed from mapping of the electrostatic potential surface (ESI, S32†). This generates scope for fine tuning the reactivity of any ditopic receptors bearing identical spacer moieties, through modification at the metal ion and aromatic substituents resulting in a varied cavity size and electronic environment for effective interaction with various guest molecules.

The calculated HOMO–LUMO energy gaps (isovalue = 0.02) for ligand precursor L¹, L² and binuclear M^II dithiocarbamate complexes 1–6 are given in Table 1 and their localization is illustrated in Fig. 7.

Fig. 7 Frontier molecular orbitals (isovalue = 0.02) for (a) L¹ and 1–3, (b) L² and 4–6.

It may be noted that the delocalization of LUMO (Fig. 7b) in complex 4 over ethereal phenyl rings, coordinated dithiocarbamate functionality and one N-(1-naphthylmethyl) moiety, forming a bowl shaped molecular framework, enhancing the Lewis acidic character of this cavity and hence complex 4 is exclusively involved in formation of 1 : 1 host–guest inclusion complexes with bidentate guests (see previously). The HOMO–LUMO energy differences for the Ni^II dithiocarbamates 1 (3.6774 eV) and 4 (3.4649 eV) are significantly lower compared to their Zn^II and Cd^II analogues. The substitution of N-benzyl substituents from 1–3 by N-(1-naphthylmethyl) in 4–6 causes remarkable decreases in HOMO–LUMO energy gaps due to extended conjugation. Interestingly, the HOMO–LUMO energy gaps for bimetallic dithiocarbamate complexes 1–6 is found in the range 3.4649–4.7118 eV; which are significantly higher than the optical band gaps determined experimentally by using UV-visible transmittance measurements in the solid state (ESI†). This is presumably because massive numbers of non-covalent interactions can be expected in the solid state, causing a lowering in the band gaps. Thus, it adds further merit to this class of compounds towards their potential applicability as semiconducting materials. λ_max values (ESI, Table S2†) determined experimentally by means of UV-visible absorptions for complexes 1–6 are comparable with HOMO–LUMO gaps obtained by computational study which validate the computational investigations.

Conclusions

This study allows us to conclude that a series of bimetallic metallomacrocyclic dithiocarbamate complexes [M^II₂-μ²-bis-{(κ²S,S-S₂CN(R)C₆H₄)₂O}]·L (1–6) can efficiently be synthesized in a single-pot reaction involving 4,4′-bis(arylmethylamino)diphenyl ether precursors L¹ or L², CS₂ and Ni(II), Zn(II) or Cd(II) metal ion. The composition and purity of these compounds were confirmed by elemental analysis which was further supported by ESI-MS, FT-IR, ¹H, ¹³C and ¹H DOSY NMR. The diamagnetism along with UV-visible absorption bands suggest square planar/distorted square planar environment around Ni^II and tetrahedral/distorted tetrahedral environment around Zn^II/Cd^II centres in their respective complexes. Further, geometry of 1–6 has been optimized by DFT method with B3LYP/LanL2DZ basis sets and UV-visible transmittance measurement evidences their wide band-gap semiconducting behavior. Interestingly, the fluorescence property of L¹ and L² apparently enhanced upon complexation with Ni, Zn or Cd metal ions in the binuclear complexes 1–6. The thermogravimetric analysis indicates that complexes 1 and 4 can be suitably used as single source precursors for the synthesis of metal sulphide nanoparticles. Remarkably, Job plot experiments reveal the ability of 1–4 to form host–guest complexes with various guest species viz. 1,4-dioxane, 4,4′-bipyridine, piperazine and triethylamine. The stoichiometries 1 : 1, 1 : 2 or 2 : 1 of these complexes apparently depend on the relative sizes and electronic nature of the host and guest species.

Experimental section

Material and physical measurements

Diamine precursors 4,4′-bis(benzylamino)diphenyl ether (L¹) and 4,4′-bis(naphthylmethylamino)diphenyl ethers (L²) were synthesized following the literature procedure.²¹ Reactions and manipulations were performed under an inert atmosphere. All the chemicals and solvents used in this work were of laboratory grade available at various commercial sources and used without further purification. Melting points were recorded in open capillaries and uncorrected. Thin Layer Chromatography was performed on Merck 60 F254 aluminium coated plates. Elemental analyses (C, H, N) were performed on a Perkin-Elmer 2400 analyzer. Mass spectra were obtained on AB SCIEX 3200 Q TRAP LCMS instrument. FT-IR (KBr pellets) spectra were recorded in the 4000–400 cm⁻¹ range using a Perkin-Elmer FT-IR spectrometer. The NMR experiments were carried out on a Bruker AV-III 400 MHz spectrometer in CDCl₃/d⁶-DMSO solvents as per the solubility and chemical shifts are reported in parts per million (ppm). UV-visible absorption spectra were recorded on a Perkin Elmer Lambda 35 UV-visible spectrophotometer and the optical characterization of solid samples was performed by using UV-visible transmittance measurements. Fluorescence was recorded on a JASCO spectrofluorometer model FP-6300. TGA/DTA plots were obtained using SII TG/DTA 6300 in flowing N₂ with a heating rate of 10 °C min⁻¹. All the geometry optimizations were performed with the Gaussian 03 program suite²⁸ and molecular orbitals were generated using GaussView 3.0 program.

Synthesis of binuclear M^II dithiocarbamate macrocyclic complexes 1–6

To a triethylamine solution of 1 equivalent of respective diamine precursors (200 mg, 0.526 mmol) of L¹ or (252 mg, 0.526 mmol) of L²), an excess amount of carbon disulphide (∼10 equivalent; ∼0.5 mL) was added with vigorous stirring. The mixture was stirred further for 12 h at room temperature, during this time period, a change in colour from colourless to pale yellow was observed. To this reaction mixture, Ni^II(C₂H₃O₂)₂·4H₂O (157 mg, 0.6312 mmol), Zn^II(C₂H₃O₂)₂·2H₂O (138.5 mg, 0.6312 mmol) or Cd^II(C₂H₃O₂)₂·4H₂O (168.2 mg, 0.6312 mmol) dissolved in a minimum amount of distilled water was added with rigorous stirring and the reaction was allowed to continue for 8 h at room temperature. The reaction mixture was dried under vacuum and the sticky residue was washed several times with rigorous stirring by distilled water, followed by n-hexane and diethyl ether. At the end, a free flowing powder obtained was dried under vacuum for several hours to yield the corresponding products 1–6 in 80–93% yields, stored under a nitrogen atmosphere and samples were taken for analysis. The synthetic methodology is outlined in Scheme 1.

[Ni^II-μ²-bis-(κ²-S,S′-dtcL¹)] (1). Green; yield: ca. 286 mg, 85%; m.p. > 110 °C dec. Elemental analysis: found: C, 58.35; H, 4.72; N, 5.50; S, 19.97. Calcd for C₆₂H₅₉N₅Ni₂O₂S₈ (1280.07): C, 58.17; H, 4.65; N, 5.47; S, 20.04. Infrared spectrum (KBr disc, cm⁻¹): 2990w, 2975w, 2960w, 2360m, 1592m, 1495s, 1448s, 1352m, 1240s, 1199s, 1161m, 1076m, 1028w, 1009w, 962s, 873w, 833s, 722m, 697s, 630m, 552m. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 1.43 (t, J = 7.2 Hz; CH₃), 3.23 (q, J = 7.2 Hz; CH₂), 4.33, 5.06, 5.088, 5.110 (s; CH₂), 6.62–6.94 (m; Ph), 7.25–7.39 (m; Ph); ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 9.23 (CH₃), 46.42, 48.72, 56.41 (CH₂), 113.79, 117.07, 119.58, 121.72, 127.33–129.01, 133.01–159.46 (Ph), 210.72, 210.82, 211.04, 211.14 (–N¹³CS₂).

[Zn^II-μ²-bis-(κ²-S,S′-dtcL¹)] (2). White; yield: ca. 291 mg, 93%; m.p. > 140 °C dec. Elemental analysis: found: C, 56.45; H, 3.75; N, 4.69; S, 21.50. Calcd for C₅₆H₄₄N₄O₂S₈Zn₂ (1192.25): C, 56.41; H, 3.72; N, 4.70; S, 21.52. Infrared spectrum (KBr disc, cm⁻¹): 3028w, 2977w, 2924w, 2360w, 2350w, 1591m, 1550m, 1496s, 1441m, 1393s, 1351m, 1239s, 1201s, 1161m, 1078m, 1011w, 962m, 873w, 832m, 724m, 698s, 650w, 554w, 514w. ¹H NMR (400 MHz, CDCl₃): 5.42, 5.45 (s; CH₂), 6.62–7.03, 7.29–7.41 (m; Ph), ¹³C NMR (400 MHz, DMSO-d6): δ (ppm) 46.16, 47.32 (CH₂), 113.60–158.51 (Ph), 209.07, 209.08, 209.25, 209.27 (–N¹³CS₂).

[Cd^II-μ²-bis-(κ²-S,S′-dtcL¹)] (3). Pale yellow; yield: ca. 301 mg, 89%; m.p. > 90 °C dec. Elemental analysis: found: C, 52.35; H, 3.57; N, 4.33; S, 19.90. Calcd for C₅₆H₄₄Cd₂N₄O₂S₈ (1286.32): C, 52.29; H, 3.45; N, 4.36; S, 19.94. Infrared spectrum (KBr disc, cm⁻¹): 3027w, 2977w, 2666w, 1602w, 1495s, 1450w, 1434w, 1385m, 1448w, 1238s, 1200s, 1160m, 1078m, 963m, 873w, 833m, 725w, 699m, 647w, 553w, 454w. ¹H NMR (400 MHz, DMSO-d₆): δ (ppm) 4.22, 4.24, 5.48 (s; CH₂), 6.23 (t, Ph), 6.58–6.82, 7.03–7.11, 7.23–7.38 (m; Ph), ¹³C NMR (400 MHz, DMSO-d6): δ (ppm) 46.23, 47.40 (CH₂), 113.57–148.86 (Ph), 211.90 (–N¹³CS₂).

[Ni^II-μ²-bis-(κ²-S,S′-dtcL²)] (4). Green; yield: ca. 307 mg, 81%; m.p. > 150 °C dec. Elemental analysis: found: C, 63.50; H, 4.75; N, 4.70; S, 17.29. Calcd for C₇₈H₆₇N₅Ni₂O₂S₈ (1480.31): C, 63.29; H, 4.56; N, 4.73; S, 17.33. Infrared spectrum (KBr disc, cm⁻¹): 3042w, 2976m, 2939m, 2738w, 2675m, 2603m, 2495m, 2360w, 1598m, 1497s, 1398w, 1358w, 1233s, 1162m, 1104w, 1090w, 1014m, 871w, 833m, 785s, 778s, 735w, 607s, 532m, 408w. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 1.41 (t, J = 7.2 Hz; CH₃), 3.14 (m; CH₂), 4.74, 5.56, 6.02 (s; CH₂), 6.65–7.02, 7.44–7.74, 7.77–8.31 (m; Ph). ¹³C NMR (400 MHz, DMSO-d6): δ (ppm) 8.98 (CH₃), 45.88 (CH₂), 52.47, 53.76, 53.80, 53.88, 56.30, 56.34 (CH₂), 113.42, 113.51, 119.48–159.27 (Ph), 208.89, 208.92, 208.96, 208.98 (–N¹³CS₂).

[Zn^II-μ²-bis-(κ²-S,S′-dtcL²)] (5). Off white; yield: ca. 322 mg, 88%; m.p. > 110 °C dec. Elemental analysis: found: C, 62.11; H, 3.75; N, 4.00; S, 18.50. Calcd for C₇₂H₅₂N₄O₂S₈Zn₂ (1392.49): C, 62.10; H, 3.76; N, 4.02; S, 18.42. Infrared spectrum (KBr disc, cm⁻¹): 3051w, 2910w, 2851w, 2360m, 2336m, 1611m, 1596m, 1502s, 1440m, 1396m, 1313m, 1227s, 1161m, 1110s, 1068m, 869m, 797s, 775s, 736w, 490w, 475w. ¹H NMR (400 MHz, DMSO-d6): δ (ppm) 4.65, 466 (s; CH₂), 6.05 (t, J = 5.6 Hz), 6.61(d), 6.69(d), 7.44(s), 7.46 (d, J = 8 Hz), 7.56(m), 7.83 (d, J = 8 Hz), 7.96 (dd, J = 8.4 Hz, 1.6 Hz), 8.14 (d, J = 8 Hz) (Ph); ¹³C NMR (400 MHz, DMSO-d6): δ (ppm) 45.71, 46.18 (CH₂), 113.44, 119.50, 124.13, 125.56, 125.92, 126.15, 126.51, 127.74, 128.94, 131.64, 133.85, 135.62, 145.17, 148.91 (Ph).

[Cd^II-μ²-bis-(κ²-S,S′-dtcL²)] (6). Yellow; yield: ca. 324 mg, 83%; m.p. > 120 °C dec. Elemental analysis: found: C, 58.25; H, 3.50; N, 3.80; S, 17.35. Calcd for C₇₂H₅₂Cd₂N₄O₂S₈ (1487.99): C, 58.17; H, 3.53; N, 3.77; S, 17.26. Infrared spectrum (KBr disc, cm⁻¹): 3045w, 2962w, 1596m, 1495s, 1393w, 1356w, 1233s, 1161m, 1072w, 1010m, 990m, 960m, 850w, 832m, 777s, 632w, 512w. ¹H NMR (400 MHz, DMSO-d6): δ (ppm) 4.66 (m, CH₂), 5.95 (s, CH₂), 6.19–6.99, 7.41–7.55, 7.70–7.94, 8.15 (m; Ph). ¹³C NMR (400 MHz, DMSO-d6): δ (ppm) 46.20 (CH₂), 113.42–146.35 (Ph).

Host–guest reactivity study (Jobs plot)

Stock solutions of the binuclear M^II dithiocarbamate macrocyclic receptors 1–4 were studied with DMSO solution (1 × 10⁻⁵ M). The guest (1,4-dioxane, 4,4′-bipyridine, piperazine and triethylamine) solutions (1 × 10⁻⁵ M) in DMSO solvent were used to evaluate the effect of different guests on macrocyclic receptors. The UV-visible spectrum was taken for each of the different solutions containing a total of 3.0 mL of the receptor and guest species solutions in the following ratios: 3.0 : 0, 2.5 : 0.5, 2.0 : 1.0, 1.5 : 1.5, 1.0 : 2.0, 0.5 : 2.5 and 0.0 : 3.0. The data was plotted to explore the impact of guest species on receptors.

Acknowledgements

VKS acknowledges CSIR, New Delhi, India for the financial support (Project No. 01/2733/13/EMR-II). One of the authors, R. Kadu acknowledges UGC, New Delhi, India for the financial support in the form of fellowship. Authors are thankful to Dr Sanjio Zade for his help in the geometry optimization study.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22175g

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