CHIRAL METAL-COMPLEXES .36. STEREOSELECTIVITY ENHANCED BY N,N'-DIMETHYLATION OF A CHIRAL N4 TETRADENTATE, INCLUDING THE CRYSTAL-STRUCTURE OF DELTA-ALPHA-1,ALPHA-2-([N,N'-DIMETHYL-3R-METHYL-1,6-DI(2-PYRIDYL)-2,5-DIAZAHEXANE)(S-ALANINATO)]-COBALT(III)) PERCHLORATE

N,N'-dimethylation of the N4 linear tetradentate R-picpn (3R-methyl-1,6-di(2-pyridyl)-2,5-diazahexane) in DMF in the presence of excess formaldehyde and sodium cyanoborohydride produces, in high yield, R-picpnMe2 (N,N'-dimethyl-3R-methyl-1,6-di(2-pyridyl)-2,5-diazahexane). Reaction of this ligand with cobalt(III) in the presence of excess chloride ion under non-aqueous conditions produces stereospecifically the DELTA-cis-alpha dichloro complex in high yield. The chloride donor atoms undergo replacement by water, nitrite, oxalate or S-alaninate with retention of this stereochemistry. The alaninato complex crystallises in the orthorhombic space group P2(1)2(1)2(1), with a = 10.502(3), b = 13.623(7), c = 18.655(7) angstrom and Z = 4. The structure was refined by least-squares methods to R = 0.048 for 2085 diffractometer data collected at -130-degrees-C. The cobalt atom has octahedral coordination with the N4 tetradentate adopting DELTA-alpha topology and with the aminoacidate completing the coordination sphere. The S-ala Co-N and Co-O bond distances are 1.965(7) and 1.887(6) angstrom, and the Co-N(py) and Co-N(tert-amine) distances average 1.95(1) and 1.990(7) angstrom, respectively. This latter bond distance is significantly longer than Co-N(sec-amine) distances observed for analogous complexes with cis-beta topology. The two geometric isomers which are possible for this asymmetric complex form, defined herein as cis-alpha1 and cis-alpha2, are observed to co-crystallise. In the crystal structure the cation shows positional disorder for the methyl substituent on the central link of the tetradentate, and the indicated 2:1 ratio for these isomers is consistent with NMR data.