BETA-ALPHA PHOTOISOMERIZATION OF COBALOXIME COMPLEXES IN THE SOLID-STATE .3. ACCELERATED REACTION-RATE BY HYDROGEN-BOND

The crystal structures of two cobaloxime complexes with different axial base ligands have been determined by X-ray analysis at room temperature. I: (3-Aminopyridine)(2-cyanoethyl)bis(dimethylglyoximato)cobalt(III); the cystal is monoclinic, the space group being C 2/c, Z = 8 with a = 28.849(8), b = 7.746(2), c = 18.429(5) angstrom, and beta = 105.71(3)-degrees. II: (Aniline)(2-cyanoethyl)bis(dimethylglyoximato)cobalt(III) aniline solvate; the crystal is monoclinic the space group being P 2(1)/c, Z = 4 with a = 14.228(7), b = 11.426(4), c = 14.178(3) angstrom, and beta = 91.92(3)-degrees. Both structures were refined by a full-matrix least-squares method to final R values of 0.070 and 0.074 for 3886 and 3311 observed reflections, respectively. Their photoisomerization rates in the solid state obtained from changes in the IR spectra, 3.2 and 4.2 x 10(-4) s-1, respectively, are significantly greater than those of the related compounds. The hydrogen bonds between the reactive 2-cyanoethyl groups and the atoms of the neighboring molecules in the two crystals should be responsible for the greater reaction rates. Taking into account the role of the hydrogen bonds, a probable isomerization mechanism is proposed.