Kinetics and mechanism of ring closure for a series of photoproduced [M(CO)5L] intermediates (M = Cr, Mo, or W; L = pyridine 2-carbaldehyde imine ligand)

Abstract

Light irradiation of a solution of [M(CO)₆](M = Cr, Mo, or W) in the presence of a pyridine 2-carbaldehyde imine ligand 2-C₅H₄NCHNR (L; R = Ph, Bu^t, Buⁿ, or Prⁱ) produced a transient complex [M(CO)₅L], in which the normally bidentate ligand is co-ordinated in a monodentate fashion. The photoproduced complex [M(CO)₅L] complex subsequently extrudes CO thermally to form a bidentate product [M(CO)₄L]; this process is relatively slow and its time-dependent behaviour has been monitored with a diode-array u.v.–visible spectrophotometer for each of the ligands L. Kinetic data obtained for these chelation reactions indicate that the pyridine nitrogen atom in L preferentially scavenges the photoproduced [M(CO)₅] intermediate. Rate and activation energy parameters closely relate to the nature of the ligand L and imply varying degrees of associative character in the ring-closure transition state.