Magnetic properties and fluxional behaviour of heteroleptic cyclopentadienyl–dithiolene d1 and d0 niobium complexes

Abstract

Paramagnetic d¹ niobium complexes of general formula [Nb(η-C₅H₄R)₂(dithiolene)][R = SiMe₃ or Bu^t; dithiolene = C₃S₅^2–(4,5-disulfanyl-1,3-dithiole-2-thionate), C₃OS₄^2–(4,5-disulfanyl-1,3-dithiol-2-onate) or dddt^2–(5,6-dihydro-1,4-dithiine-2,3-dithiolate)] have been prepared and their redox properties studied. The crystal structure determination of [Nb(η-C₅H₄SiMe₃)₂(C₃S₅)] showed that the folding angle of the NbS₂C₂ plane along the S–S axis is 34(2)°, an intermediate value compared with those of analogous d⁰(45–50°) and d²(0–10°) complexes, as rationalized by extended-Hückel calculations. The Curie-type temperature dependence of the spin susceptibility of [Nb(cp)₂(C₃S₅)], [Nb(cp)₂(C₃OS₄)] and [Nb(η-C₅H₄R)₂(C₃S₅)](cp =η-C₅H₅) demonstrates that the paramagnetic complexes do not interact with each other in the solid state. Cationic d⁰ complexes were obtained either by chemical oxidation (tetracyanoquinodimethane) of the d¹ molecules or from the direct reaction of dddt^2– with d⁰[Nb(η-C₅H₄R)₂Cl₂][PF₆]. The fluxional behaviour of these d⁰ complexes has been investigated by temperature-dependent NMR studies and shown to be comparable with that of isoelectronic d⁰[Ti(cp)₂(dithiolene)] complexes.