Photochemistry of some manganese and chromium dinuclear metal carbonyl complexes in frozen gas matrices at ca. 12 K

Abstract

Infrared spectroscopic evidence is presented to show that photolysis of the non-metal–metal bonded dimers [Mn₂(µ-η⁵:η⁵′-C₁₀H₈)(CO)₆], [Mn₂(µ-η⁵:η⁵′-C₅H₄CH₂C₅H₄)(CO)₆], [Cr₂(µ-η⁶:η⁶′-C₁₂H₁₀)(CO)₆], and [Cr₂(µ-η⁶:η⁶′-C₁₄H₁₄)(CO)₆] together with its mononuclear analogue [Cr(η⁶-C₇H₈)(CO)₃] in gas matrices at ca. 12 K results in CO ejection to yield [M(CO)₂] molecular fragments rather than detachment of polyene rings or the formation of new M–M linkages via M–M bonding and/or CO bridging. The reactivity of the resultant [M(CO)₂] molecular fragments is demonstrated by the recombination with CO when matrices (Ar, CH₄) were irradiated with light of a longer wavelength and by the reactions with N₂ and ¹³CO (5% in Ar or CH₄) to afford [M(CO)₂(N₂)] and [M(¹²CO)_n(¹³CO)_{3 –n}] fragments, respectively. The failure to form new M–M linkages is attributed to the isolation of the dimers in pseudo-trans configurations in the matrix cages such that even the excess photochemical energy is insufficient to both cleave M–CO bonds and supply the energies required to rotate the bulky fragments in the rigid matrix cages. These findings are discussed in relation to solution photochemical reactions.