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It is now generally well established that photodissociation of a ligand from many transition metal complexes results in the formation of a solvent-stabilized metal center within picoseconds of ligand loss, even in saturated hydrocarbon solvents.1-6 For example, Rayner and co-workers have determined that, in the gas phase, interaction between the tungsten center and hydrocarbons (RH) in (CO)sW(RH) can be as high as 12 kcal/mol,1 234567 and Yang and co-workers have established an approximately 8 kcal/mol binding energy for the first-row metal complex Cp(CO)3Mn(n-heptane) in the liquid alkane.8 Even in unconventional solvents such as liquid Xe and Kr, there is compelling evidence for such metal-solvent interactions following the loss of a CO ligand from the metal complex.9•10The interaction energy between a metal center and a noble gas atom (Kr and Xe) has been estimated to be on the order of 5-10 kcal/mol, with the M-Xe bond a few kilocalories stronger than the M-Kr bond. Therefore, while noble gas atoms appear to bind to unsaturated metal centers somewhat less strongly than do hydrocarbons, the metal-noble gas bond energies are significant.Consistent with the above observations, recently we obtained strong evidence for the formation of Cp*Rh(CO)(Xe) and Cp*Rh(CO)(Kr) complexes after photolysis of Cp*Rh(CO)2 in liquid Xe and Kr, respectively.11•12 These molecules reacted with cyclohexane (or neopentane) to give an uninserted metalalkane complex, Cp*Rh(CO)(C6Hi2), which in a unimolecular step proceeded to yield the C-H activation product, Cp*Rh(CO)-(HXC5H11). In analogy to the rhodium (and analogous iridium) system, irradiation of the isoelectronic cobalt complex CpCo-(CO)2 results in CO loss. However, the intermediate generated in the cobalt reaction does not result in overall activation of the C-H bonds of hydrocarbons. Assuming that (in analogy to the Cr(CO)5, W(CO)s, and CpMn(CO)3 fragments studied earlier) cobalt-noble gas and cobalt-alkane complexes were probably formed in these reactions but were stable to further reaction, we decided to study the photochemistry of CpCo(CO)2 in liquid Kr and Xe in the presence of hydrocarbons in order to estimate directly the relative strengths of the CpCo(CO)-noble gas and CpCo(CO)-alkane interactions.8 However, we have found that in dramatic contrast to Cp*Rh(CO) and the coordinatively unsaturated organometallic complexes summarized above, CpCo(CO) and Cp*Co(CO) do not bind delectably to either a noble gas atom or cyclohexane.

The manganese tricarbonyl complex fac-Mn(Br)(CO)3((i)Pr2Ph-DAB) (1) [(i)Pr2Ph-DAB = (N,N'-bis(2,6-di-isopropylphenyl)-1,4-diaza-1,3-butadiene)] was synthesized from the reaction of Mn(CO)5Br with the sterically encumbered DAB ligand. Compound 1 exhibits rapid CO release under low power visible light irradiation (560 nm) suggesting its possible use as a photoCORM. The reaction of compound 1 with TlPF6 in the dark afforded the manganese(I) tetracarbonyl complex, [Mn(CO)4((i)Pr2Ph-DAB)][PF6] (2). While 2 is comparatively more stable than 1 in light, it demonstrates high thermal reactivity such that dissolution in CH3CN or THF at room temperature results in rapid CO loss and formation of the respective solvate complexes. This unusual reactivity is due to the large steric profile of the DAB ligand which results in a weak Mn-CO binding interaction.

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