BETA-ALPHA PHOTOISOMERIZATION OF COBALOXIME COMPLEXES IN THE SOLID-STATE .5. REACTION-RATE REDUCED BY A HYDROGEN-BOND

The crystal structures of three cobaloxime complexes with different ax ial base ligands have been analyzed by Xray analysis at room temperatu re. I: imethylglyoximato)((S)-phenylalaninol)cobalt(III); the crystal is orthorhombic, the space group being P2(1)2(1)2(1), Z=4 With a=9.416 (3), b=27.184(3), c=9.184(3) Angstrom. II: anoethyl)-bis(dimethylglyox imato)((S)-phenyl-alani methyl ester)cobalt(III); the crystal is monoc linic, the space group being P21, Z=4 with a=14.376(2), b=11.988(2), c =14.874(1) Angstrom, and beta=101.05(1)degrees. III: ylglyoximato)-(me thyldiphenylphosphine)cobalt(III) ; the crystal is monoclinic, the spa ce group being P2(1)/n, 2=4 with a=11.558(4), b=15.542(3), c=14.393(5) Angstrom, and beta=91.02(4)degrees. The structures were refined by th e full-matrix least-squares method to final R values of 0.041, 0.052, and 0.044 for 2340, 3287, and 4400 observed reflections, respectively. The photoisomerization rates of I, II, and III in the solid state wer e obtained from the changes in the IR spectra. The rate of I was insig nificantly small, but the rates of II and III were calculated to be 0. 1x10(-4) and 1.9x10(-4) s(-1), respectively, assuming first-order kine tics. The 2-cyanoethyl groups in the three complex crystals take perpe ndicular conformations to their cobaloxime planes. The 2-cyanoethyl gr oup of I makes a hydrogen bond with the N-H group of the neighboring m olecule. The nonreactivity of I may be brought about by the hydrogen b ond.