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

An ortho-metalated rhenium (V) polyhydride complex has been prepared through the reaction of ReH7(PPh3)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 ReH5(PPh3)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 symmetry 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, ReH5(PPh3)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∗ system 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-phenylpyrimidine, 2-aminopyrimidine, or 2-aminopyridine, range from 9.9 to 11.3 kcal/mole. One of the new compounds reported herein, ReH5(PPh3)2[1-(2-NH2Pyr)] is the first rhenium (V) polyhydride complex to display hydride–hydride coupling in its 1H NMR spectrum.

Aromatic amine ligands which lack a plane of symmetry orthogonal to the aromatic ring cause E and Z isomers for rhenium (V) pentahydride complexes. Values of ΔG‡ for Re–N bond rotation lie between 9.9 and 11.3 kcal/mol. The E isomer of ReH5(PPh3)2[1-(2-NH2Pyr)] exhibits 1H NMR hydride–hydride coupling.Figure optionsDownload as PowerPoint slide