Reactions of unisolable homoleptic aryls of niobium and tantalum with PMe2Ph and ButNC. Crystal structures of M(2,5-Me2C6H3)2(η2-2,5-Me2C6H2)(PMe2Ph)2, M = Nb or Ta, and Ta(η2-PhCNBut)(η2-Ph2CNBut)2(CNBut)
The tetraaryls, MR4, M = Nb or Ta, R = 2-MeC6H4 or 2,5-Me2C6H3, made in situ by interaction of MCl4 and LiR or MgRbr react with PMe2Ph to give the η2-aryne compounds MR2(η2-aryne)(PMe2Ph)2. From the reaction of TaCl5 and LiPh in Et2O in the presence of PMe3 the similar tantalum(IV) benzyne, TaPh2(η2-C6H4)(PMe3)2, is obtained. Interaction of a green solution probably of TaPh5 with ButNC produces the tantalum(V) compound, Ta(PhCNBut)(Ph2CNBut)2(CNBut), via multiple insertion reactions of ButNC into the Ta–C bonds. The X-ray structures of M(2,5-Me2C6H3)2(η2-2,5-Me2C6H2)(PMe2Ph)2(M = Nb or Ta) and Ta(η2-PhCNBut)(η2-Ph2CNBut)(CNBut) have been determined. The isostructural arynes can be regarded as having a square-pyramidal structure with the C–C aryne bond occupying the axial site. The tantalum(V) compound contains one η2-iminoacyl and two η2-imine ligands.