STRUCTURAL COMPARISON OF 5-COORDINATE THIOLATE-LIGATED M-II = FE-II, CO-II, NI-II, AND ZN-II IONS WRAPPED IN A CHIRAL HELICAL LIGAND

The syntheses and structures of two analogous five-coordinate mixed ni trogen/thiolate-ligated Co2+ and Fe2+ complexes are described and comp ared to their previously reported Zn2+ and Ni2+ analogues. The linear, single-chain [(S2N3)-N-R2(Pr,Pr)](2-) (R = H, Me) ligands examined in this study wrap themselves around metal ions in both a clockwise and counterclockwise manner to afford a racemic mixture of chiral, helical molecules. [(FeS2N3)-S-II(Pr,Pr)] (1) crystallizes in the monoclinic space group P2(1)/c with a = 7.853(2) Angstrom, b = 8.667(2) Angstrom, c = 26.079(5) Angstrom, beta = 90.37(3)degrees, V = 1775.0(7) Angstro m(3), and Z = 4. [(CoS2N3)-S-II-N-Me2(Pr,Pr)] (2) crystallizes in the monoclinic space group P2(1)/c with a = 9.389(2) Angstrom, b = 19.706( 3) Angstrom, c = 12.165(2) Angstrom, beta = 103.67(2)degrees. V = 2186 (1) Angstrom(3), and Z = 3. Trends in helicity and angular distortions in the Fe2+, Co2+, Ni2+, Zn2+ series which correlate with ionic radiu s are described. It is suggested that ligand constraints are responsib le for the increasing distortion observed (Fe similar to Zn < Co much less than Ni) in these structures and that similar constraints may alt er the geometries of metalloenzyme active sites.