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The Photoisomerisation Reactions of the Cluster [Os 3 (CO) 9 ( μ 3 ‐ η 2 : η 2 : η 2 ‐C 6 H 6 )] and Some of its Derivatives
Author(s) -
Johnson Brian F. G.,
Nairn Jacqueline G. M.,
Brown David B.,
Lewis Jack,
Gallop Mark,
Parker David G.
Publication year - 1995
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19950010409
Subject(s) - chemistry , osmium , isomerization , protonation , photochemistry , photodissociation , phosphine , metal , diradical , bond cleavage , medicinal chemistry , crystallography , excited state , catalysis , atomic physics , physics , singlet state , ion , organic chemistry , ruthenium , biochemistry
The highly efficient photoinduced isomerisation of [Os 3 (CO) 9 ( μ 3 ‐ η 2 : η 2 : η 2 ‐C 6 H 6 )] ( 1 ) to the previously characterised μ 3 ‐benzyne complex [H 2 Os 3 (CO) 9 ( μ 3 ‐ η 2 : η 1 : η 1 ‐C 6 H 4 )] ( 2 ) is described. We find that for the more electron‐rich clusters [Os 3 (CO) 9‐n (PPh 3 ) n ( μ 3 ‐ η 2 : η 2 : η 2 ‐C 6 H 6 )] ( n = 1 or 2) the efficiency of the photolytic process is improved. We suggest that photolysis generates a vacant site at osmium (mimicking the inherent coordinative unsaturation of a metal complex) and that this transformation offers a compelling model for the interconversion of associatively and dissociatively chemisorbed benzene. Moreover, we can fine‐tune our molecular “work function” (really the separation of HOMO/LUMO energies) by protonation or by phosphine substitution for CO at the metal. Thus, the cation [HOs 3 (CO) 9 ( μ 3 ‐ η 2 : η 2 : η 2 ‐C 6 H 6 )] + does not undergo internal metalation, while photoisomerisation of the electron‐rich phosphine derivatives proceeds more readily than for 1 . We find that these photolytic reactions are easily monitored by IR spectroscopy for samples suspended in polymethylmethacrylate films under ambient conditions. This method permits “matrix isolation” experiments under ideal conditions. Finally, we speculate that the mechanism of interconversion almost certainly involves the symmetric cleavage of a OsOs bond (σ‐σ* transition) to produce an extremely reactive diradical intermediate.