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(qs-cyclopentadienyl) dihalides of niobium(iv) and tantalum(rv), [MX,(cp),] (M = Nb or Ta; X = CI, Br, or I; cp = q5-C5H5) have been prepared by known methods or via metathetical displacement of chloride ligands in [ MCl,(cp),] by bromide or iodide ions. The new, characterised bis(monothiobenzoate) complexes, [M(SCOPh),(cp),](M = Nb or Ta), have been synthesised from [MCl,(cp),] and TI(SC0Ph) in acetone. The molecular structure of [Ta(SCOPh),(cp),], determined by X-ray diffraction of a single crystal [space group P2,2,2, a = 7.8458( 22), b = 19.903(4), c = 7.028(3) A, Z = 2, R = 0.031, R' = 0.0371 comprises a normal, bent sandwich geometry with monodentate, Sbonded monothiobenzoate ligands. The angle S-Ta-S, 79.4(8)", is relatively acute for a complex of type [MX,(cp),] with a d1 electronic configuration. Comparative cyclic voltammetric studies have been performed for [MX,(cp),] (M = N b or Ta; X = CI, Br, I, or SCOPh) in dichloromethane solutions. Oxidation to [ MX,(cp),] + is essentially reversible for these systems and ease of oxidation decreases with ligand X in the order SCOPh > CI > Br > I. Reduction is

Synthesis and characterisation of monocyclopentadienyl dithiolato complexes of niobium and tantalum; crystal structures of [Nb2(μ-O)(μ-η2∶η2-S2C2H4)(η5-C5H4Me)2Cl4] and [Nb2(μ-η1∶η2-S2C2H4)2(η5-C5H4Me)2O(S2C2H4)] †

“…The paramagnetic species are likely to be the anions [NbCp(S 2 C 6 H 3 R) 2 ] Ϫ and the g and A values are consistent with an unpaired electron primarily located on Nb IV with a little delocalisation onto the ligands. 15 Similar ESR spectra have been reported for bis(cyclopentadienyl)niobium() complexes with sulfur ligands. 14b- 17 Reaction of 1 with an equimolar amount of cobaltocene in thf also forms an orangebrown soluble product which gives an ESR spectrum with parameters identical to the product of sodium reduction and , but this reaction also produces some insoluble material.…”

“…been reported for bis(cyclopentadienyl) derivatives of niobium: complexes with unsaturated dithiolate ligands, [NbCp 2 (dithiolate)], show two reversible 1-electron processes linking niobium-() and -() states, 14 and other niobium() species, [NbCp 2 -X 2 ], undergo 1-electron oxidations and reductions although chemical irreversibility has been noted in some systems. 15 Brief reactions of complexes 1 or 2 in thf with a sodium mirror in an evacuated ESR cell caused the deep green solutions to become deep red-orange, and the resulting solutions from either precursor gave similar ten-line ESR spectra typical of d 1 niobium() species with g iso = 1.991 and A iso ( 93 Nb) = 0.0090 cm Ϫ1 (see Fig. 1).…”

Halide Effects in Transition Metal Catalysis

“…When the effects of halide ligands on oxidative processes are considered, it is the ability to participate in π donation that predominates. For example, electrochemical studies on [Cp 2 MX 2 ] (M=Nb, Ta; Cp=C 5 H 5 )22 and [Cp 2 WX(R)]23 revealed that the ease of oxidation increased I<Br<Cl. In another study,24 it was found that reversible oxidation of [Cp*Fe(dppe)X] (dppe=bis(diphenylphosphino)ethane; Cp*=C 5 Me 5 ) to the mono‐ and dicationic species occurs most readily for the most electronegative but best π‐donating halide, F, and becomes more difficult down the series.…”

Der Einfluss von Halogenidionen in der Übergangsmetallkatalyse

“…Rechnungen zur ‹berlappung zwischen dem s-Donororbital der verschiedenen Halogenidliganden und den 6p s -Orbitalen von Pt ergaben, dass die ‹berlappung von Cl nach I zunimmt. So ergaben elektrochemische Untersuchungen an [Cp 2 MX 2 ] (M Nb, Ta; Cp C 5 H 5 ) [22] und [Cp 2 WX(R)], [23] dass die Komplexe in der Reihe I < Br < Cl zunehmend leichter oxidiert werden kˆnnen. Halogenidliganden und stˆchiometrische Reaktionen 3.1.…”