Aromatic interactions in hydrotris(3-methylindazolyl) borate organoniobium complexes:: control of an alkyne ligand orientation in the crystal

Reaction of NbCl3(MeOCH2CH2OMe)(PhC=CMe) with KTp(4Bo,3Me) in THF produces a 68% yield of the dichloro - phenylpropyne complex Tp(4Bo,3Me)NbCl(2)(PhC=CMe) [Tp(4Bo,3Me) = hydrotris(3-methylindazol-1-yl) borate]. As observed by solution NMR spectroscopy, the four-electron donor alkyne sits in the molecular mirror plane and restricted rotation of the alkyne ligand allows the observation of an equilibrium between two rotamers. The conformation of the alkyne ligand in the major isomer is such that the phenyl group is proximal to Tp(4Bo,3Me). Unexpectedly, the minor rotamer in solution, that with the distal phenyl group, is observed in the crystal. Analysis of the possible interactions suggest that aromatic interactions are responsible for this unusual observation.