Competition between aquation and hydrogen/deuterium exchange in 1064-nm laser excitation of cobalt(III)-amine complexes: a search for vibrational photochemistry in solution

Abstract

Spectra of the complex ions (Co(NH/sub 3/)/sub 6/)/sup 3 +/, (Coen/sub 3/)/sup 3 +/, and (Co(NH/sub 3/)/sub 5/Me/sub 2/SO)/sup 3 +/ have been determined in the wavelength range 700-1100 nm covering the region of spin-forbidden d-d transitions and the N-H stretching second vibrational overtones. Kinetic studies are reported of H/D exchange in (Co(NH/sub 3/)/sub 6/)/sup 3 +/, and the competition between trans NH/sub 3/ H/D exchange and aquation in (Co(NH/sub 3/)/sub 5/Me/sub 2/SO)/sup 3 +/. Nd:YAG laser excitation of the complexes at 1064 nm led to photoaquation (normal electronic photochemistry) or H/D exchange (ascribed to vibrational photochemistry following state-selective N-H (nu = 3) preparation of a hot ground state). For (Co(NH/sub 3/)/sub 6/)/sup 3 +/, H/D exchange occurred with phi = (2 +/- 2) x 10/sup -4/, and a t test showed a probability greater than 80% for a nonzero quantum yield. This laser stimulated proton-transfer reaction is probably the first example of vibrational photochemistry for a transition-metal complex in solution. For (Coen/sub 3/)/sup 3 +/ and (Co(NH/sub 3/)/sub 5/Me/sub 2/SO)/sup 3 +/, photoaquation was observed with quantum yields (6 +/- 2) x 10/sup -5/ (en loss) and (16 x 6) x 10/sup -5/ (Me/sub 2/SO loss), respectively.