On the Mechanism of C−H Bond Activation in the Photochemical Arylation of Rhenium(V) Oxo Iodide Complexes

Photolysis of the Re(V) oxo−iodide compound (HBpz3)ReO(I)Cl (1) in arene solvents gives the aryl complexes (HBpz3)ReO(Ar)Cl. Substituted arenes react with electrophilic selectivity exclusively at aromatic C−H bonds, and a variety of functional groups are tolerated. Yields are improved when photolysis is carried out in the presence of pyridine. Photolysis of the diiodide complex, (HBpz3)ReOI2 (2), gives a mixture of mono- and disubstituted Re−aryl complexes, and the diphenyl derivative (HBpz3)ReOPh2 has been structurally characterized. The isotope effect found when 1 is photolyzed with a 1:1 mixture of C6H6 and C6D6 was found to depend on the concentration of 1. Lower concentrations (∼0.5 mM) show a 1.2(2):1 ratio of d0 to d5 products, while higher concentrations (∼20 mM) show higher d0:d5 ratios of 1.8(2):1. The apparent isotope effects increase with increasing conversion and are also affected by the presence of additives such as pyridine or iodine. Photolysis of 1 with 1,3,5-trideuteriobenzene shows a 4.0(4):1 ratio of C−H vs C−D activation, which is independent of reaction conditions. A mechanism for arene activation is proposed that involves initial arene binding, which discriminates intermolecularly between arenes with a low isotope effect, followed by C−H bond cleavage, which discriminates intramolecularly within an arene with a higher isotope effect. The reactive species appears to be a rhenium(IV) oxo complex which can add aromatic C−H bonds across the ReO linkage.