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

The new unsymmetrical [Bu4N][Ni(Pr(2)(i)timdt)(dmit)] 4 and [Bu4N][Ni(Pr(2)(i)timdt)(mnt)] 5 dithiolene complexes (Pr(2)(i)timdt 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(Pr(2)(i)timdt)(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 lambda (max)=1194 and 1070 nm, respectively. Diiodine oxidation of 4 and 5 affords the [Ni(Pr(2)(i)timdt)(dmit)] 6 and [Ni(Pr(2)(i)timdt)(mnt)] 7 neutral compounds (lambda (max) 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 Pr(2)(i)timdt and dmit ligands [S . . .S 3.243(2) Angstrom]. The formal charge of the ligands in 4 and 6 is estimated from the frequency of the Raman C=C stretching vibration.