Photochemistry of pentacarbonyl(cyclohepta-1,3,5-triene-7-carbonyl)manganese and pentacarbonyl(σ-cycloheptatrienyl)rhenium in frozen gas matrices at ca. 12 K: infrared spectroscopic evidence for carbon monoxide ejection with concomitant polyene ring hapticity changes

Abstract

Infrared spectroscopic evidence, including ¹³CO labelling and energy-factored force-field fitting, is presented to show that once a CO ligand has been photochemically ejected from [Mn(C₇H₇CO)(CO)₅] to yield [Mn(σ-C₇H₇)(CO)₅]via a 16-electron species, [Mn(C₇H₇CO)(CO)₄], subsequent photolyses of [M (σ-C₇H₇)(CO)₅] complexes (M = Mn or Re) leads to sequential formation of the 18-electron species [M(η³-C₇H₇)(CO)₄], [M(η⁵-C₇H₇)(CO)₃], and [M(η-C₇H₇)(CO)₂] in Ar, CH₄, N₂, and CO matrices at ca. 12 K. The fact that no 16-electron species were observed prior to ring slippage even at such low temperatures indicated that such ring slips occur extremely readily or that they occur as a concerted process with CO ejection. The facility of the ring-slippage process was demonstrated by the failure of intermediate species [M(σ-C₇H₇)(CO)₅] and [M(η³-C₇H₇)(CO)₄] to undergo exchange with ¹³CO in doped matrices or to take up a N₂ ligand when photolyses are carried out in nitrogen matrices. Exchange with ¹³CO and substitution of CO by N₂ does, however, occur once the ring-slippage process is nearing completion for Mn affording [Mn(η⁵-C₇H₇)(¹²CO)_n(¹³CO)_{3 –n}] and [Mn(η⁵-C₇H₇)(CO)₂(N₂)]. The electronic spectra of [Mn(η⁷-C₇H₇)(CO)₂] shows vibrational splitting which indicates a symmetric structure, cf.[Co(η⁵-C₅H₅)(CO)₂], rather than a bent geometry. The photophysical and photochemical processes associated with the observed reactions are discussed.