STEADY-STATE AND TRANSIENT STEREORETENTIVE PHOTOAQUATION OF [TRANS-CR(1,9-DIAMINO-3,7-DIAZANONANE)(CN)2]CLO4

In order to study the stereochemistry of equatorial photosubstitution in Cr(III) complexes, [trans-Cr(2,3,2-tet)-(CN)2]ClO4(2,3,2-tet = 1,9- diamino-3,7-diazanonane) has been synthesized and characterized. Its p hotoaquation has been studied by proton uptake and cyanide release mea surements. The molecule photoaquates one end of the 2,3,2-tet ligand w ith a quantum yield of 0.09 +/- 0.01, while any photochemical cyanide loss has a quantum yield of less-than-or-equal-to 0.02. That the photo product has the cyanide ligands in the trans configuration is strongly indicated by its thermal recoordination at pH 6 to the starting compl ex, as well as by the UV-vis spectrum of the photoproduct. These resul ts show that photosubstitution of the equatorial ligand in this comple x is stereoretentive, in contrast to the stereochemical change that ac companies loss of the equatorial ligand in other Cr(III) complexes. Na nosecond laser flash photolysis experiments with conductivity detectio n showed that 100% of the reaction occurs as a single-exponential grow th with a lifetime of 26 +/- 1 mus, in good agreement with the observe d doublet state emission lifetime at 21-degrees-C of 28 +/- 1 mus. The results are discussed in the context of data on analogous cyano compl exes, the angular overlap theory of chromium(III) photochemistry inclu ding the Jahn-Teller distortions of the intermediates, and the behavio r of photoinert cyclam complexes.