The N−H···S Hydrogen Bond in (TACN)2Fe2S6 (TACN = Triazacyclononane) and in Model Systems Involving the Persulfido Moiety: An ab Initio and DFT Study

Abstract

Extended Huckel, DFT, and ab initio MP2 calculations have been carried out to rationalize the unprecedented structural characteristics of the recently synthesized complex (μ−η1-S2)3 (Fe-TACN)2 (TACN = triazacyclononane). The orbital interaction diagram between the metal−macrocycle dimer and the three disulfide ligands accounts for some of the observed properties of the complex: diamagnetism, existence of an Fe−Fe single bond, nucleophilicity of the terminal sulfur atoms. The unprecedented occurrence of a M2S6 core, as the very unusual μ−η1 coordination of the S2 ligands, however, requires further analysis. It was assumed that the key to the structural singularities of this complex should be sought in the network of intramolecular H···S bonds revealed by the crystallographic analysis and involving all six NH groups and all three terminal S atoms. We therefore report the first quantitative theoretical investigation of the energetics of intramolecular H···S bonds. Geometry optimizations have been carried ou...