Stepwise Reduction of Dinitrogen Bond Order by a Low-Coordinate Iron Complex

This article discusses stepwise reduction of dinitrogen bond order by a low-coordinate iron complex

Cobalt-Dinitrogen Complexes with Weakened N-N Bonds

Article discussing research on cobalt-dinitrogen complexes with weakened N-N bonds

Methanethiol Binding Strengths and Deprotonation Energies in Zn(II)–Imidazole Complexes from M05-2X and MP2 Theories: Coordination Number and Geometry Influences Relevant to Zinc Enzymes

Zn­(II) is used in nature as a biocatalyst in hundreds of enzymes, and the structure and dynamics of its catalytic activity are subjects of considerable interest. Many of the Zn­(II)-based enzymes are classified as hydrolytic enzymes, in which the Lewis acidic Zn­(II) center facilitates proton transfer(s) to a Lewis base, from proton donors such as water or thiol. This report presents the results of a quantum computational study quantifying the dynamic relationship between the zinc coordination number (CN), its coordination geometry, and the thermodynamic driving force behind these proton transfers originating from a charge-neutral methylthiol ligand. Specifically, density functional theory (DFT) and second-order perturbation theory (MP2) calculations have been performed on a series of [(imidazole)_<i>n</i>Zn–S­(H)­CH₃]²⁺ and [(imidazole)_<i>n</i>Zn–SCH₃]⁺ complexes with the CN varied from 1 to 6, <i>n</i> = 0–5. As the number of imidazole ligands coordinated to zinc increases, the S–H proton dissociation energy also increases, (i.e., −S­(H)­CH₃ becomes less acidic), and the Zn–S bond energy decreases. Furthermore, at a constant CN, the S–H proton dissociation energy decreases as the S–Zn–(ImH)_<i>n</i> angles increase about their equilibrium position. The zinc-coordinated thiol can become more <i>or</i> less acidic depending upon the position of the coordinated imidazole ligands. The bonding and thermodynamic relationships discussed may apply to larger systems that utilize the [(His)₃Zn­(II)–L] complex as the catalytic site, including carbonic anhydrase, carboxypeptidase, β-lactamase, the tumor necrosis factor-α-converting enzyme, and the matrix metalloproteinases