Monocyclopentadienyl complexes of niobium and tantalum.

Abstract

New complexes of Nb(III) and Ta(III) of the type, CpMCl$\sb2$L$\sb{\rm n}$ (L = CO, phosphines, and butadiene) from reductions of $(\eta\sp5$-C$\sb5$H$\sb5$)MCl$\sb4$ and ($\eta\sp5$-C$\sb5$Me$\sb5$)MCl$\sb4$ with Al/HgCL$\sb2$ or Na/Hg have been prepared. The carbonyl derivative ((C$\sb5$H$\sb5$)NbCl$\sb2$(CO)$\sb2\rbrack\sb2$ (1) reacts with excess phenylacetylene to give triphenylbenzene. Acetylene is polymerized to a mixture of cis and trans-polyacetylene by THF solutions of 1. A stoichiometric amount of disubstituted alkynes, phosphines, and acrylates cleanly displace CO from 1 to give the monomeric complexes (C$\sb5$H$\sb5$)NbCl$\sb2$(RC$\equiv$CR), (C$\sb5$H$\sb5$)NbCl$\sb2$(PMe$\sb3)\sb3$, and (C$\sb5$H$\sb5$)NbCl$\sb2$(acrylates). The pentamethycyclopentadienyl complex (C$\sb5$Me$\sb5$) TaCl$\sb2$(CO)$\sb2$(THF) (2) reacts also with alkynes, phosphines, and bis-olefins to yield the complexes (C$\sb5$Me$\sb5$)TaCl$\sb2$(RC$\equiv$CR) (3), (C$\sb5$Me$\sb5$)TaCl$\sb2$(CO)(dmpe), (C$\sb5$Me$\sb5$)TaCl$\sb2$(CO)$\sb2$(PMe$\sb3$), (C$\sb5$Me$\sb5$)TaCl$\sb2$(butadiene), and (C$\sb5$Me$\sb5$)TaCl$\sb2$(methyl acrylate) respectively. The convenient and high yield syntheses of the dicarbonyl complexes, ((C$\sb5$H$\sb5$)NbCl$\sb2$(CO)$\sb2\rbrack\sb2$ and (C$\sb5$Me$\sb5$)TaCl$\sb2$(CO)$\sb2$(THF) have led us to prepare a variety of complexes of niobium and tantalum in an intermediate oxidation state by simple and clean displacement reactions with no side products in the reaction mixture. Easily accessible alkyne and diolefin complexes allowed us to study interesting transformations of organic moieties. Reduction of the Ta(III) complexes, 2 and 3, with the Na/amalgam in toluene afforded the Ta(II) metal-metal bonded dimers. These compounds, especially single bonded compound ((C$\sb5$Me$\sb5$)TaCl(CO)$\sb2$) $\sb2$, will be promising starting materials for further reduction chemistry to prepare metal-metal multiply-bonded complexes of group V metals.