SYNTHESIS, CHARACTERIZATION AND STEREOCHEMICAL PREFERENCES OF COBALT(III) TERNARY AMINO-ACID COMPLEXES CONTAINING N-((2-PYRIDYL)METHYL)-2-((2-AMINOETHYL)THIO)ACETAMIDE, A STEREOSPECIFIC LINEAR NSNN TETRADENTATE LIGAND

The preparation, isolation and characterization of cis-beta-[Co(pyge)(AA)]PF6 [pygeH = N-((2-pyridyl)methyl)-2-((2-aminoethyl)thio)acetamide; AA = gly, L-ala, L-ile, L-leu, sarcosine (sar), alpha-aminoisobutyric acid (aiba) and beta-ala] are described. H-1 and C-13 NMR spectra of the new cobalt complexes strongly suggest that one geometrical isomer is obtained with very high stereoselectivity. beta-1-[Co(pyge)(gly)]PF6.2H2O (1) crystallized in the monoclinic space group P2(1/c) with a = 14.518(2), b = 14.583(2), c = 19.783(3) angstrom, beta = 98.51(1)-degrees and Z = 8 and was refined to R = 0.080. In both of the two independent cations in the asymmetric unit, the gly chelate had the beta-1 configuration with respect to the pyge ligand. The complex beta-[Co(pyge)(Cl)(NO2)].H2O was prepared; recrystallization of this complex provided single crystals of beta-[Co(pyge)(Cl)(NO2)].[Co(pyge)(NO2)2].2H2O (2). Complex 2 crystallized in the triclinic space group P1BAR with a = 7.850(1), b = 13.346(2), c = 15.998(2) angstrom, alpha = 106.45(1), beta = 99.07(1), gamma = 104.78(1)-degrees and Z = 2, and was refined to R = 0.052. The chlorine ligand was found to be trans to the amido donor of the pyge ligand, suggesting that the beta-1 configuration found for the amino acid complexes stems from kinetically-based substitutions. The complex beta-[Co(pyge)Cl2].H2O promotes the hydrolysis of amide and ester bonds of dipeptides and amino acid esters to again give beta-1-[Co(pyge) (AA)]PF6. Finally, evidence is presented for sar and L-ala, that the diastereomeric beta-1 complex that predominates at equilibrium is the isomer with the ligand methyl group hydrophobically interacting with the pyridyl moiety of the pyge ligand.