LIGAND MIGRATION DURING PHOTOAQUATION OF BIS(AM(M)INE)(1,4,8,11-TETRAAZACYCLOTETRADECANE)CHROMIUM(III) COMPLEXES

The cis-(cyclam)(A-A)chromium(III) complexes (cyclam = 1,4,8,11-tetraazacyclotetradecane with A-A equal to ethylenediamine (en) and diammine were prepared and their photochemistry and doublet properties studied in acidic aqueous solution. The en complex is photoactive with a quantum yield at 436 nm of 0.091 +/- 0.003, that is, just over half that for the diammine at 0.160 +/- 0.009. In contrast, the room-temperature lifetimes of the doublet states are 8.4 +/- 0.2 and 1.7 +/- 0.1-mu-s, respectively, and their temperature dependencies correspond to activation energies for doublet depopulation of 41 and 37 kJ mol-1, respectively. Chromatographic product analysis shows that the diammine complex photolyses to produce the cis-aquo ammine product. Similarly, the en complex gives only a single chromatographic product peak (under all conditions studied), suggesting the two complexes react by the same mechanism. By comparison of the photobehavior to that of other similar systems, it is argued that, after an initial Cr-N bond breaking, the entire ethylenediamine ligand migrates in the photochemical reaction. The photophysical results show that the excited-state reaction is slower for the en complex than for the diammine.