SYNTHESIS, STRUCTURE, AND REDOX REACTIVITY OF A SUBSTITUTED NIOBOCENEFORMALDEHYDE COMPLEX - IMPORTANCE OF HYDROGEN-BONDING IN THE REDOX CHEMISTRY

The niobocene chloride [Cp'Nb-2(mu-Cl)](2) (1, Cp' = eta(5)-C(5)H(4)Si Me(3)) reacts with a suspension of paraformaldehyde to yield an eta(2) -formaldehyde complex Cp'Nb-2(Cl)(eta(2)-CH2O) (2), the crystal struct ure of which was determined by X-ray diffraction. This compound underg oes facile reduction with sodium amalgam to produce the Nb(IV) radical Cp'Nb-2(eta(2)-CH2O) (3); the latter is unstable toward isolation but was found stable in dilute solution and characterized with ESR and in frared techniques. Ifa similar reduction of 2 is carried out in the pr esence of added alcohol, the result is instead the diamagnetic hydride Cp'Nb-2(H)(eta 2-CH2O) (4); the process thus constitutes a sodium/eth anol reduction used to convert a halide to a hydride. Radical. 3 was c onfirmed to reside on the path to 4, and 3 is only susceptible to subs equent reduction if alcohol is present. This has been attributed to hy drogen-bond activation, and the reduction of the resulting intermediat e (5) was observed in voltammetric studies.