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)

Abstract

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 Bu^tNC produces the tantalum(V) compound, Ta(PhCNBu^t)(Ph₂CNBu^t)₂(CNBu^t), via multiple insertion reactions of Bu^tNC 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-PhCNBu^t)(η²-Ph₂CNBu^t)(CNBu^t) 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.