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

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.