Synthesis and structures of monocyclopentadienylniobium complexes with bi- and tri-dentate dithiolate ligands

Reactions of cyclopentadienyl and methylcyclopentadienyl-tetrachloroniobium with thallium(I) 1,3-propanedithiolate yield compounds [NbCpx (1,3-S2C3H6)2] [1, Cpx=-C5H5(Cp); 2, Cpx=-C5H4Me (Cp)]; similar reactions of the thallium(I) derivative of 2,2-thiodiethanethiol afford [NbCpxCl2 {(SC2H4)2S}](3, Cpx= Cp; 4, Cpx= Cp). Reaction of [NbCpCl4] with thallium(I) 2,2-oxydiethanethiolate produces both [NbCpCl2{(SC2H4)2O}]5 and [NbCpO{(SC2H4)2O}]6. Products have been characterised analytically and by NMR spectroscopy; the solid-state structures of 1, 3, 5 and 6 have been established by XRD. Compound 1 is five-coordinate, with a distorted four-legged piano-stool geometry, compound 3 is six-coordinate with tridentate thiolate and cis-equatorial Cl ligands, compound 5 is also six-coordinate but with trans-equatorial Cl ligands, and compound 6 is five-coordinate, approximating to a trigonal bipyramidal geometry with an equatorial oxo ligand. Compounds 1 and 2 are fluxional in solution due to non-rigidity of the twist conformations of the six-membered chelate rings, as shown by VT 1H NMR spectroscopic studies with G= 42.7 ± 1.4 kJ mol–1(at 231 K) and G= 43.1 ± 1.3 kJ mol–1(at 233 K), respectively, for ring inversion in toluene solutions. Cyclic voltammetric studies on 1 and 2 showed two consecutive, quasi-reversible, reduction processes, attributable to the sequential formation of mono- and di-anionic derivatives; reductions of the other compounds showed less evidence for reversibility. ESR spectra of the initial products of chemical reduction of 1 and 2 in thf, using either a sodium film or cobaltocene as reductant, are typical of those from Nb(IV) species and assigned to the respective mono-anions.