Investigation into the reactivity of oxoniobocene complexes [Cp*Nb-2(=O)R](Cp* = eta(5)-C5Me5; R = H, OH, OMe) towards heterocumulenes: formation ofcarbamato and thiocarbamato complexes and catalytic cyclization of PhNCO

The reaction of [Cp-2*NbCl2] (Cp* = eta (5)-C5Me5) with KOH or Ba(OH)(2).8H (2)O in THF was investigated under slightly modified conditions. In additio n to the known complex [Cp-2*Nb(=O)H] (1), the new compound [Cp-2*Nb(=O)OH] (2) was formed. The reaction of 2 with PhNCS gave yellow [Cp-2*Nb(=O){SC(O )NHPh}] (3), while PhNCO formed yellow [Cp-2*Nb(=O){OC(O)NHPh}] (4). Comple xes 3 and 4 were analytically and spectroscopically characterized. X-ray di ffraction analyses of both compounds show that they contain either eta (1)- S-thiocarbamato (3) or eta (1)-O-carbamato ligand (4) along with a terminal Nb=O group. The reaction of 1 with one equivalent of PhNCO mainly gave ora nge [Cp-2*NbH{OC(O)NPh}] (5) along with some 4. The molecular structure of 5 contains a niobocene unit comprising eta (2)-N,O-carbamato chelate, which formally is a [2 + 2] cycloaddition product of the Nb=O group and the hete rocumulene. A hydride ligand completes the coordination sphere around Nb. R eaction of l with excess PhNCO gave a mixture of heterocycles (PhNCO)(2) (6 ) and (PhNCO)(3) (7) in the approximate ratio 3:2. By contrast, the reactio n of [Cp-2*Nb(=O)OMe] with PhNCO in molar ratios from 1:3 to 1:100 gave nea rly pure triphenylisocyanurate 7. (C) 2001 Published by Elsevier Science B. V.