Synthesis, structure and properties of new unsymmetrical nickel dithiolene complexes useful as near-infrared dyes

The new unsymmetrical [Bu4N][Ni(Pri2timdt)(dmit)] 4 and [Bu4N][Ni(Pri2timdt)(mnt)] 5 dithiolene complexes (Pri2timdt is the mono-anion of 1,3-diisopropylimidazolidine-2,4,5-trithione; dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate; mnt = maleonitriledithiolate) have been prepared from the reaction of the neutral complex [Ni(Pri2timdt)2] 1 with the bi-anionic nickel dithiolene complexes [Bu4N]2[Ni(dmit)2] 2 or [Bu4N]2[Ni(mnt)2] 3, respectively. A strong near-IR electronic absorption is observed in the spectra of 4 and 5 in CH3CN at λmax = 1194 and 1070 nm, respectively. Diiodine oxidation of 4 and 5 affords the [Ni(Pri2timdt)(dmit)] 6 and [Ni(Pri2timdt)(mnt)] 7 neutral compounds (λmax at 883 nm in CH3CN). Features in the electronic spectra suggest that the properties of unsymmetrical nickel dithiolene complexes as near-infrared dyes can be tuned by a combination of the ligands and by the charge on the complexes. A crystal of 6 has been analysed by X-ray diffraction methods. Structural data show that the nickel ion exhibits square-planar co-ordination. Chains along b are formed through contacts between the terminal sulfur atoms of the Pri2timdt and dmit ligands [S⋯S 3.243(2) A]. The formal charge of the ligands in 4 and 6 is estimated from the frequency of the Raman CC stretching vibration.