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

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 [Bu4N]2[Ni­(X26,7-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 [Bu4N]2[Ni­(X26,7-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 E1/2 = +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(X26,7-qdt)2]−/[Ni­(X26,7-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, [PPh4]2[Ni­(Ph26,7-qdt)2]·3DMF (2b) for its
structural characterization.