Niobium−Mercury Heterometallic Compounds as Sources of Niobium(II): Radical Paths to Organoniobium Species

Abstract

The niobium−mercury compounds [Cp‘2Nb(L)]2Hg (Cp‘ = η5-C5H4SiMe3, L = CO (4), PMe3 (5), or CNtBu (6)) serve as stable precursors to the short-lived Nb(II) radicals of general formula [Cp‘2Nb−L]. The homolysis of the Nb−Hg bond may result from a slow thermal reaction (generating low concentrations of radicals) or from a photochemical process in which mercury extrusion is more rapid. Since the Nb(II) species show no evidence for Nb−Nb bond formation, they are useful in the synthesis of a variety of Nb(III) and Nb(IV) species. Reactions of 4 with dimeric species such as [CpFe(CO)2]2, [CpNi(CO)]2, Co2(CO)8, or RSe−SeR give rise to new Nb−M or Nb−Se compounds, while reactions with potential π donors such as formaldehyde or azobenzene lead to displacement of the ligand L and formation of the Nb(IV) complexes. Crystallographic and variable-temperature NMR studies on the Nb−Fe compound Cp‘2Nb(μ-CO)2Fe(CO)Cp (10) are consistent with a low-energy fluxional process involving exchange of bridging and terminal carbony...