PHOTOSUBSTITUTION OF 2 IRON PENTACARBONYL COS IN SOLUTION VIA A SINGLE-PHOTON PROCESS - DEPENDENCE ON DISPERSED LIGANDS AND ROLE OF TRIPLETINTERMEDIATES

Photolysis of Fe(CO)s in cyclohexane at 337 nm in the presence of PR3 (R = Et, Me, n-Bu, Ph) pyridine, CNC-(CH3)3, and CH3CN was performed u nder conditions where secondary photolysis of photoproducts was neglig ible. Depending on the dispersed ligand, both 1 and 2 (Fe(CO)4L and Fe (CO)3L2, respectively) were formed. At low concentrations of dispersed ligand relative to Fe(CO)5, we report the first observation of quantu m yields greater than unity for noncatalytic reactions leading to the formation of 1 and 2. The mechanism of reaction is proposed to involve the formation of multinuclear species via the reaction of Fe(CO)5 wit h coordinatively-unsaturated intermediates. At high ligand concentrati ons, the formation of multinuclear species is inhibited and the total quantum yield is 0.8. When investigated for the specific case of PEt3, the product ratio of 2/1 did not change for 1-10% conversion of Fe(CO )5. It is concluded that, at high concentrations of dispersed ligand, a single-photon process produces 2. The role of triplet intermediates was probed using triplet sensitizers and quenchers. The product ratio was unchanged when Fe(CO)5 was triplet-sensitized with xanthone in the presence of PEt3. Disubstituted product was formed when Fe(CO)4PEt3 w as triplet-sensitized with benzophenone in the presence of PEt3. Final ly, the distribution of products was changed with the addition of trip let quenchers. It is proposed that Fe(CO)5 photolysis produces a tripl et Fe(CO)4. Further reaction with L forms triplet Fe(CO)4L, which diss ociates CO in competition with intersystem crossing to the ground stat e.