CO(TACN)(NCS)(3) SYNTHESIS AND THIOCYANATE PHOTOSUBSTITUTION - YIELD ENHANCEMENT BY ADDED THIOCYANATE

The complex Co(tacn)(NCS)(3) (tacn = 1,4,7-triazacyelononane) has been prepared and characterized by elemental analysis, NMR, IR, and W-vis spectroscopy, and conductivity measurements. It is thermally stable in dmso and aqueous/dmso solution but, on irradiation of the latter solu tions at 360 nm, undergoes parallel photosubstitution processes to for m dmso- and aqua-substituted products with a quantum yield of 0.012 +/ - 0.005. Under these conditions, no photoredox reaction was detected. On addition of thiocyanate ion to the solutions, the product yields we re increased in an almost linear fashion. For 1 M thiocyanate solution , the quantum yield for disappearance of the starting complex rose to 0.022 +/- 0.002 and a small redox yield of 0.0008 +/- 0.0003 was found . Under these same conditions, nanosecond laser flash photolysis at 35 5 nm revealed a transient absorption owing to (NCS)2(-.), which was pr oduced with an estimated quantum yield of 0.036. These results were in terpreted in terms of scavenging of radical pair species by thiocyanat e ion followed by back electron transfer to give photosubstituted prod uct. Comparison with results for other ammine thiocyanate systems show s that the distribution of redox versus substitution products depends on the rate of loss of the strong field ligands from the Co(II) fragme nt of the radical pair. A radical pair yield of 0.29 and lifetime of 1 2 ps was derived from a kinetic model of the scavenging by thiocyanate . The final recombination step between the Co(II) fragment and (NCS)2( -.) was shown to be second-order and, hence, that it was a bulk recomb ination process with k = 6 x 10(9) M(-1) s(-1).