Investigation into the reactivity of oxoniobocene complexes [Cp*2Nb(O)R] (Cp*=η5-C5Me5; R=H, OH, OMe) towards heterocumulenes: formation of carbamato and thiocarbamato complexes and catalytic cyclization of PhNCO

Abstract The reaction of [Cp* 2 NbCl 2 ] (Cp*=η 5 -C 5 Me 5 ) with KOH or Ba(OH) 2 ·8H 2 O in THF was investigated under slightly modified conditions. In addition 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 ). Complexes 3 and 4 were analytically and spectroscopically characterized. X-ray diffraction analyses of both compounds show that they contain either η 1 - S -thiocarbamato ( 3 ) or η 1 - O -carbamato ligand ( 4 ) along with a terminal NbO group. The reaction of 1 with one equivalent of PhNCO mainly gave orange [Cp* 2 NbH{OC(O)NPh}] ( 5 ) along with some 4 . The molecular structure of 5 contains a niobocene unit comprising η 2 - N , O -carbamato chelate, which formally is a [2+2] cycloaddition product of the NbO group and the heterocumulene. A hydride ligand completes the coordination sphere around Nb. Reaction of 1 with excess PhNCO gave a mixture of heterocycles (PhNCO) 2 ( 6 ) and (PhNCO) 3 ( 7 ) in the approximate ratio 3:2. By contrast, the reaction of [Cp* 2 Nb(O)OMe] with PhNCO in molar ratios from 1:3 to 1:100 gave nearly pure triphenylisocyanurate 7 .