PHOTOCHEMICAL METAL-TO-OXO MIGRATIONS OF ARYL AND ALKYL LIGANDS

Photolysis of the oxo-phenyl complex (HBpz(3))ReO(Ph)Cl (HBpz(3) = hydrotris(1-pyrazolyl)borate) in the presence of pyridine or other donor ligands (MeCN, OPMe(3)) slowly gives paramagnetic rhenium(III) phenoxide complexes (HBpz(3))Re(OPh)(Cl)(L). Phenyl-to-oxo migration is predominantly intramolecular, as indicated by a crossover experiment involving photolysis of (HBPz(3))Re(O-18)(C6H5)(Cl) and (HBPz(3))Re(O-16)(C6D5)(Cl) The radical traps, MeCN and PhSH, have little influence on the reaction, further ruling out the involvement of free phenyl radicals. Migration of substituted aryl ligands in (HBpz(3))ReO(Cl)(Ar) (Ar = p-anisyl, p-phenoxyphenyl, 2,4-dimethylphenyl, 2,5-dimethylphenyl) occurs exclusively with carbon-oxygen bond formation to the ipso carbon of the aryl ligand. The reactions are therefore simple [1,2]-migrations; they are proposed to take place via nucleophilic attack of the oxo ligand on the aryl group in the excited state. Photochemical ethyl-to-ore migration also occurs in (HBpz(3))ReO(C2H5)Cl to give (HBpz(3))Re(OC2H5)Cl(py). But net ethyl migration occurs by a process involving free ethyl radicals, as indicated by facile trapping by PhSH. Thus two different mechanisms are involved in these processes, the first clear examples of [1,2]-migration of hydrocarbon ligands from a metal to an oxo group.