Structural Comparison of Five-Coordinate Thiolate-Ligated M^II = Fe^II, Co^II, Ni^II, and Zn^II Ions Wrapped in a Chiral Helical Ligand

Abstract

The syntheses and structures of two analogous five-coordinate mixed nitrogen/thiolate-ligated Co2+ and Fe2+ complexes are described and compared to their previously reported Zn2+ and Ni2+ analogues. The linear, single-chain [S2R2N3(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. [FeIIS2N3(Pr,Pr)] (1) crystallizes in the monoclinic space group P21/c with a = 7.853(2) Å, b = 8.667(2) Å, c = 26.079(5) Å, β = 90.37(3)°, V = 1775.0(7) Å3, and Z = 4. [CoIIS2Me2N3(Pr,Pr)] (2) crystallizes in the monoclinic space group P21/c with a = 9.389(2) Å, b = 19.706(3) Å, c = 12.165(2) Å, β = 103.67(2)°, V = 2186(1) Å3, and Z = 4. Trends in helicity and angular distortions in the Fe2+, Co2+, Ni2+, Zn2+ series which correlate with ionic radius are described. It is suggested that ligand constraints are responsible for the increasing distortion observed (Fe ∼ Zn < Co ≪ Ni) in these structures and that similar constraints may alter the geometries of metalloenzyme active sites.