Photochemistry bridged intermediate

Another dinuclear carbonyl which presents interesting problems is ](ri C5H5)Fe(CO) ] 2 Does the photochemistry proceed exclusively through homolytic fission to produce two (13 -05 )Fe-(C0)2 radicals or by other possible routes The discussion of this reaction has involved mechanistic and synthetic studies (77), flash photolysis (78) and low-temperature photolysis (29) - the latter work, in THF or ethyl chloride at -78°C, invokes an intermediate in which the Fe-Fe direct bond is broken but the two halves of the molecule are held together by a CO bridge. Clearly such an intriguing problem merits more detailed investigations. 

The formation of the unsaturated intermediate Run(OEP) from the hydrido-complex RumH(OEP) is supposed to occur from a LMCT excited state. As a consequence, the dimeric [RuH(OEP)]2 with a metal-metal interaction is formed [245]. Irradiation of some systems containing Fein(Por)N3 leads to p-nitrido bridged binuclear mixed-valence complexes [(Por)Fem-N-Fe,v(Por)] [134, 162], In both cases photochemistry was used as a conventional preparative route for synthesis of the binuclear complexes. 

A detailed study on the photochemical behavior of l,3,3-trimethylbicyclo[2.2.2]octa-5,7-dien-2-ones has revealed that under triplet-sensitized conditions, and despite the potential for competition from ODPM-based processes, only DPM-derived photoproducts are observed. In fact, it has been suggested that the rates and regioselectivities of the DPM reactions involved are enhanced by the presence of the carbonyl moiety embedded within the framework. hi a related vein, a study of the photochemistry of a series of l-aza-2-vinyl-l,4-dienes has established that DPM- and ADPM-based rearrangement reactions can conpete with one another, the precise outcome being dictated by the nature of the substituents attached to the substrate (by virtue of their influence on the relative stabilities of the 1,4-bridged biradical intermediates, e.g., 4, that would be involved in the competing pathways). 

Turner and co-workers have compared the photochemistry of Mn2(CO),o, MnRe(CO),Q and Re2(CO),o in liquid argon and xenon. The former two are quite similar, but Re2(CO)j(, does not appear to form a bridged species. The low-temperature studies in inert gases with IR spectroscopy also revealed the intermediates shown in Scheme 7.14. 

Curley JJ, Cook TR, Reece SY et al (2008) Shining light on dinitrogen cleavage structural features, redox chemistry, and photochemistry of the key intermediate bridging dinitrogen complex. J Am Chem Soc 130 9394-9405... 

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