Rh(III)-Photosensitized Interconversion of Norbornadiene and Quadricyclane

The utility of two Rh(III) diimine complexes, Rh(phen)3 and Rh(phi)2(phen) (phen ) 1,10-phenanthroline, phi ) 9,10-phenanthrenequinone diimine), as sensitizers for the interconversion of norbornadiene (N) and quadricyclane (Q) has been investigated using steady-state photochemical and laser flash photolysis (LFP) techniques. Irradiation of acetonitrile solutions of Rh(phen)3 and N causes slow conversion to Q. The reaction is reversible; irradiation of Rh(phen)3 in the presence of Q leads to N. Irradiation of acetonitrile solutions of Rh(phi)2(phen) and Q yields N. However, this reaction is irreversible; irradiation of the Rh(III) complex in the presence of N fails to afford Q. Irradiation of methanol solutions of either Rh(III) complex in the presence of N or Q affords minor amounts of two methanol-C7 adducts but fails to quench the N-Q interconversion reaction. The results are consistent with N-Q interconversion via an exciplex intermediate. The Rh(III)-sensitized deazatization of two cyclic azoalkane derivatives (Azo-N, Azo-Q) of N andQ was also investigated. Deazatization was achieved by Rh(phen)3 but not Rh(phi)2(phen) sensitization. The results are consistent with a mechanism involving triplet energy transfer, but the involvement of exciplex intermediates cannot be ruled out. Bimolecular rate constants for quenching of the Rh(III) excited states by N, Q, Azo-N, and Azo-Q were determined by LFP.