| Abstract
| - Both diastereoisomeric forms of [Ru2(dppcb)(bipy)4]4+, where dppcb is cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane, show a completely unexpected photochemical reactivity. In acetonitrile, a ligand exchange reaction produces [Ru2(dppcb)(bipy)2(MeCN)4]4+. To the best of our knowledge, dppcb is the first phosphine leading to this reaction type. A careful examination of the X-ray structures of the diastereoisomerically pure forms of [Ru2(dppcb)(bipy)4]4+ and of [Ru2(dppcb)(bipy)2(MeCN)4]4+ reveals that the release of steric strain observable as the disappearance of mechanical coupling is responsible for this reactivity.
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