Ortho-metalation, rotational isomerization, and hydride-hydride coupling at rhenium (V) polyhydride complexes stabilized by aromatic amine ligands

摘要

An ortho-metalated rhenium (V) polyhydride complex has been prepared through the reaction of ReH_7(PPh_3)_2 with 2-phenylpyridine. Additionally, a small series of neutral rhenium (V) pentahydride complexes, each of which is stabilized by an aromatic amine ligand, has been prepared. E and Z rotational isomers of the ReH_5(PPh_3)_2(aromatic amine) complexes have been observed at low temperatures by NMR spectroscopy. The E and Z rotational isomers arise from a combination of the lack of a mirror plane sym_metry element orthogonal to the aromatic ring in the aromatic amine ligands and the restricted rotation about the Re-N bond in such complexes. Restricted rotation about the Re-N bond in the related complex, ReH_5(PPh_3)_2(Py) has previously been observed by Crabtree et al. The restricted rotation about the Re-N bond seems to result from π-donation of the lone electron pair on the rhenium (V) center to the π sys_tem of the aromatic amine ligands. Different populations of the E and Z rotational isomers arise from interactions of substituents on the aromatic ring with the other ligands bound to rhenium. The values of ΔG~_ for the restricted rotation about the Re-N bonds, for the complexes containing 4-phenylpyrimi_dine, 2-aminopyrimidine, or 2-aminopyridine, range from 9.9 to 11.3 kcal mole. One of the new com_pounds reported herein, ReH_5(PPh_3)_2[1-(2-NH_2Pyr)] is the first rhenium (V) polyhydride complex to display hydride-hydride coupling in its ~1H NMR spectrum.