Stereochemistry Of Thiocyanate Photoanation In Trans-Crcl(Nh3)(1,3-Diaminopropane)(2)(2+)

The compound trans-CrCl(NH3)(tn)2(2+) (where tn = 1,3-diaminopropane) has been synthesized, characterized and the structure determined by X-ray single-crystal methods. The crystal is monoclinic, P2(1)/n (No. 14), with chemical composition trans-[CrCl(NH3)(tn)2]2(ClO4)3(NO3).H2O, M(r) = 883.8, and a = 13.516(1) angstrom, b = 9.749(2) angstrom, c = 26.533(3) angstrom, beta = 93.02(1)-degrees, V(cell) = 3491.3 angstrom3, and Z = 4 (asymmetric unit was Cr2Cl2N11C12H48O16). In acidic aqueous solution at room temperature, the compound loses chloride at a slow rate (rate constant <5 X 10(-7) s-1 at 28-degrees-C), but photolysis leads to aquation of ammonia with a wavelength independent quantum yield of 0.33 +/- 0.01 to give cis-CrCl(H2O)(tn)2(2+) as the only observable product, the total chloride yield having an upper limit of 0.003. When photolyzed in the presence of thiocyanate ion, cis-CrCl(NCS)(tn)2+ is produced in competition with the aquation, and the quantum yield of ammonia loss remains unchanged over the range 0-3 M NCS-. These results show that photoanation competes with photoaquation and that both processes occur with entry of the substituting nucleophile trans to the displaced ammonia ligand. Chromatographic studies of the products of thermal and photoaquation followed by thermal thiocyanate anation of trans- and cis-CrCl2(tn)2+ were used to identify the photoproduct. This work additionally revealed that, thermally, direct displacement of chloride by thiocyanate can occur in competition with anation, particularly for cis-CrCl(H2O)(tn)22+.