Influence of the Substituents on the Electronic and Electrochemical Properties of a New Square-Planar Nickel-Bis(quinoxaline-6,7-dithiolate) System: Synthesis, Spectroscopy, Electrochemistry, Crystallography, and Theoretical Investigation

Abstract

We describe the synthesis, crystal structures, electronic absorption spectra, and electrochemistry of a series of square-planar nickel-bis(quinoxaline-6,7-dithiolate) complexes with the general formula [Bu(4)N](2)[Ni(X(2)6,7-qdt)(2)], where X = H (1a), Ph (2a), Cl (3), and Me (4). The solution and solid-state electronic absorption spectral behavior and electrochemical properties of these compounds are strongly dependent on the electron donating/accepting nature of the substituent X, attached to the quinoxaline-6,7-dithiolate ring in the system [Bu(4)N](2)[Ni(X(2)6,7-qdt)(2)]. Particularly, the charge transfer (CT) transition bands observed in the visible region are greatly affected by the electronic nature of the substituent. A possible explanation for this influence of the substituents on electronic absorption and electrochemistry is described based on highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gaps, which is further supported by ground-state electronic structure calculations. In addition to this, the observed CT bands in all the complexes are sensitive to the solvent polarity. Interestingly, compounds 1a, 2a, 3, and 4 undergo reversible oxidation at very low oxidation potentials appearing at E(1/2) = +0.12 V, 0.033 V, 0.18 V, and 0.044 V vs Ag/AgCl, respectively, in MeOH solutions, corresponding to the respective couples [Ni(X(2)6,7-qdt)(2)](-)/[Ni(X(2)6,7-qdt)(2)](2-). Compounds 1a, 3, and 4 have been characterized unambiguously by single crystal X-ray structural analysis; compound 2a could not be characterized by single crystal X-ray structure determination because of the poor quality of the concerned crystals. Thus, we have synthesized the tetraphenyl phosphonium salt of the complex anion of 2a, [PPh(4)](2)[Ni(Ph(2)6,7-qdt)(2)]·3DMF (2b) for its structural characterization.

Bis(quinoxaline‐dithiolato)nickel(III) Complexes [Bu4N][NiIII(6,7‐qdt)2] and [Ph4P]­[NiIII(Ph26,7‐qdt)2]·CHCl3 (6,7‐qdt = Quin­oxaline‐6,7‐dithiolate; Ph26,7‐qdt = Diphenylquinoxaline‐6,7‐dithiolate): Synthesis, Spectroscopy, Electrochemistry, DFT Calculations, Crystal Structures and Hirshfeld Surface Analysis

“…Therefore, in the case of compound 2, the major absorption at 880 nm can be assigned to "mixed-metal-ligand-to-ligand" (MMLL) charge-transfer transitions. [7] As observed in compound 1, a similar scenario of linear com-bination of AOs is also observed in the case of compound 2, where the 3p z AOs of both nickel and sulfur are the participating orbitals.…”

“…We mentioned in our earlier report [7] that the parent compounds [Bu 4 [7] as well as in DMF (see the Supporting Information), we can assign these reductive responses of compounds 1 and 2 to the [Ni III (6,7- thus, the relevant metal centre would be more easily reduced. The low reduction potential value of compound 2 compared with that of compound 1 can also be justified from the perspective of theoretical calculations, which show that the energy gap between the HOMO and LUMO is less in the case of compound 2 compared with that of compound 1.…”

“…As both the HOMOs (Figure 2, B and C) include mixed-metal-ligandbased orbitals, we can say that the major transition (853 nm) is a "mixed-metal-ligand-to-ligand" (MMLL) charge-transfer transition. [7] Although the oxidation state of nickel in compound 1 is formally +3 (confirmed from ESR spectroscopy and electrochemical studies), electron density corresponds to the 3p y atomic orbital (AO) of nickel is situated at the nickel centre in the β-spin HOMO-2 MO. All 3p y AOs of the surrounding four sulfur donor atoms (all sulfur atoms have positive MO coefficients) and the central nickel 3p y AO form a linear combination to create the π-bonding-type β-spin HOMO-2 ( Figure 2, B).…”

“…[7] The electrochemical studies of these two Ni II compounds demonstrate that they can be oxidized at a low potential to the corresponding one-electron oxidized compounds [Bu 4 2 ], respectively, in DMF solutions to mixed-metal-ligand-to-ligand charge-transfer transitions based on DFT calculations because relevant the HOMOs include mixed metal-ligand-based orbitals and the LUMOs were defined as ligand-based π-MOs. [7] Careful analysis of these spectra reveals the appearance of weak features beyond 800 nm, which were explained by the presence of oxidized impurities (corresponding to Ni III complexes), formed by oxidation with air of [Bu 4 Figure 1 (a). Similarly for compound 2, absorption bands appear at approximately 880 nm (ε = 8160 L mol -1 cm -1 ) and approximately 554 nm (ε = 12440 L mol -1 cm -1 ) as shown in Figure 1 …”

“…[7] These ligands, because of their non-innocent behaviour, can stabilize coordination complexes in several oxidation states of the pertinent metal. [4] Generally, the dianionic state (M II oxidation state) is the most stable state for bis(dithiolene) complexes, but in some cases they can be isolated as monoanionic (M III oxidation state) or even as neutral complexes (M IV oxidation state).…”