Photoactivation of methane by η5-cyclopentadienyl and substituted η5-cyclopentadienyl Group 8 metal dicarbonyl complexes, [M(η5-C5R5)(CO)2](M = Rh or Ir, R = H or Me), and dicarbonyl(η5-indenyl)iridium: a matrix isolation study

Abstract

Infrared spectroscopic evidence, including deuterium labelling, is presented to show that photolysis of [M(η⁵-C₅R₅)(CO)₂](M = Rh or Ir, R = H or Me) complexes in CH₄ matrices at ca. 12 K leads primarily to [M(η⁵-C₅R₅)(CO)(Me)(H)] complexes. Comparative photolysis experiments in CH₄, Ar, and ¹³CO-doped (5%) matrices provided evidence that the initial step is the loss of CO to form the ‘naked’ 16-electron species, [M(η⁵-C₅R₅)(CO)]. Additional photoproducts in CH₄ matrices are tentatively assigned as the CH₄ complexes [M(η⁵-C₅R₅)(CO)⋯ CH₄]. Cobalt analogues failed to activate CH₄ but did afford very weak bands which could possibly be assigned to the CH₄ complexes [Co(η⁵-C₅R₅)(CO)⋯ CH₄](R = H or Me). The indenyl complex, [Ir(η⁵-C₉H₇)(CO)₂], behaves similarly to the cyclopentadienyl complexes. The mechanism of CH₄ activation in matrices in discussed and related to the activation of CH₄ in solution at ambient temperatures. The lack of CH₄ activation by Co analogues seems to be best rationalised on thermodynamic grounds.