Formation of organometallic exciplexes of the type [Hg(η2-arene)] in mercury photosensitized reactions of aromatic compounds

The formation of an organometallic exciplex, 3[Hg(η2-C6H6)], between triplet mercury (Hg∗) and the arene ring is proposed on the basis of experimental and MP2 PCI-80 theoretical studies to account for the unusual products in the Hg-photosensitized dehydrodimerization of a series of aromatic substrates. Apart from the usual CH bond cleavage, CC cleavage is also seen for PhCH2CH3 and related systems. In this paper we demonstrate that these bond cleavage reactions occur, probe their mechanism, and explore ways to prevent or promote them. In particular, we propose that exciplex formation leads to triplet sensitization of the arene, which is followed by CC bond cleavage via a pathway investigated by Otsuji and coworkers.