Design of Transition Metal Complexes with High Quantum Yields for Ligand Substitution: Efficient Photochemical Chelate Ring Closure in Cyclopentadienylmanganese Tricarbonyl Derivatives

The quantum yields for photosubstitution of (η 5 -C 5 H 4 R)Mn(CO) 3 (R = H, COCH 3, COCH 2 OCH 3, COCH 2 SCH 3, CO(CH 2 ) 2 SCH 3, CH 2 CO 2 CH 3, (CH 2 ) 2 CO 2 CH 3 )) in heptane for 337 nm irradiation are 0.67, 0.82, 0.64, 1.00, 0.82, 0.80, and 1.05, respectively. The yields of ring-closed products were determined for the sulfur-containing complexes while the yields of substitution with dispersed ligands were determined for the remaining complexes since the ring-closed products were not stable. In contrast to a previous study for cyclopentadienylmanganese complexes the quantum yields were found to change with the structure of a substituent. The results for R = COCH 2 SCH 3 and (CH 2 ) 2 CO 2 CH 3 are apparently the first bona fide examples of unit quantum yields for organometallic complexes in solution. It is proposed that after CO dissociation that selected ring substituents can trap the metal center before CO can recombine with the metal.