Synthesis and dynamic NMR studies of Rhenium(I) tricarbonyl bromide complexes of benzimidazolylpyridine ligands.: Crystal structure of [ReBr(CO)3{2,6-bis(1′,5′,6′-trimethylbenzimidazol-2′-yl)pyridine}]

Pentacarbonylbromorhenium(I) reacted with 2,6-bis(benzimidazol-2'-yl)pyridine (bbip), 2,6-bis(1'-methylbenzimidazol-2'-yl)pyridine (bmbip), 2,6-bis(5',6'-dimethylbenzimidazol-2'-yl)pyridine (bdmbip), and 2,6-bis(1',5',6'-trimethylbenzimidazol-2'-yl)pyridine (btmbip) to form stable octahedral complexes of type fac[ReBr(CO)(3)L] (L = bbip, bmbip, bdmbip or btmbip) in which L is acting as a bidentate chelate ligand. At above-ambient temperatures in solution the complexes are fluxional with the nitrogen ligand oscillating between equivalent bidentate bonding modes. Rates and activation energies for these fluxions have been investigated by NMR methods. At low temperatures rotation of the uncoordinated benzimidazolyl unit is restricted and for fac-[ReBr(CO)(3)(btmbip)] in CD2Cl2 solution, two preferred rotamers exist in 89:11% relative abundances. A rotational energy barrier has been estimated. The X-ray crystal structure of fac-[ReBr(CO)(3))(btmbip)] confirms the bidentate chelate bonding of btmbip with a N-Re-N angle of 74.2 degrees. The pendant benzimidazole unit is inclined at an angle of 60.7 degrees to the pyridine ring. (C) 1998 Elsevier Science Ltd. All rights reserved.