Bis(η5-cyclopentadienyl) complexes of niobium(IV) and tantalum(IV); electron spin resonance and electrochemical studies and the molecular structure of [Ta(SCOPh)2(η5-C5H5)2]

Abstract

Bis(η⁵-cyclopentadienyl) dihalides of niobium(IV) and tantalum(IV), [MX₂(cp)₂](M = Nb or Ta; X = Cl, Br, or I; cp =η⁵-C₅H₅) have been prepared by known methods or via metathetical displacement of chloride ligands in [MCl₂(cp)₂] by bromide or iodide ions. The new, characterised bis(monothiobenzoate) complexes, [M(SCOPh)₂(cp)₂](M = Nb or Ta), have been synthesised from [MCl₂(cp)₂] and TI(SCOPh) in acetone. The molecular structure of [Ta(SCOPh)₂(cp)₂], determined by X-ray diffraction of a single crystal [space group P2₁2₁2, a= 7.8458(22), b= 19.903(4), c= 7.028(3)Å, Z= 2, R= 0.031, R′= 0.037] comprises a normal, bent sandwich geometry with monodentate, S-bonded monothiobenzoate ligands. The angle S–Ta–S, 79.4(8)°, is relatively acute for a complex of type [MX₂(cp)₂] with a d¹ electronic configuration. Comparative cyclic voltammetric studies have been performed for [MX₂(cp)₂](M = Nb or Ta; X = Cl, Br, I, or SCOPh) in dichloromethane solutions. Oxidation to [MX₂(cp)₂]⁺ is essentially reversible for these systems and ease of oxidation decreases with ligand X in the order SCOPh > Cl > Br > I. Reduction is irreversible in dichloromethane at room temperature for the dihalide complexes, whereas a primary, reversible reduction of [M(SCOPh)₂(cp)₂] may be attributed to the formation of the respective monoanions. All complexes [MX₂(cp)₂] have been investigated by e.s.r. spectroscopy in solutions and in frozen glasses of 2-methyltetrahydrofuran–dichloromethane. Solution spectra yield isotropic parameters g_isoand A_iso(M)(M =⁹³Nb or ¹⁸¹Ta) and the variation of these parameters with the nature of X in [MX₂(cp)₂] is discussed: values of g_isoor A_iso(M) increase or decrease, respectively, with change of X in the sequence Cl, Br, I. Glass spectra of [MX₂(cp)₂] have been analysed in terms of the anisotropic e.s.r. parameters.