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 Hu ¨ckel, DFT, and ab initio MP2 calculations have been carried out to rationalize the unprecedented structural characteristics of the recently synthesized complex (µ-η 1 -S 2 ) 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 M 2 S 6 core, as the very unusual µ-η 1 coordination of the S 2 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 out by means of the Density Functional Theory (DFT) on two configurations of (µ-η 1 -S 2 ) 3 (Fe-TACN) 2 deduced one from another by inverting the pyramidality at the proximal sulfur atoms. The experimental conformation 1, characterized by six "strong" N-H‚‚‚S bonds (d H‚‚‚S ) 2.31 Å), is more stable by 10.0 kcal‚mol -1 than the hypothetic structure 2 with "weak" hydrogen bonds (d H‚‚‚S ) 2.65 Å). Replacing persulfide by sulfoxide ligands leads to a similar energy difference (11.4 kcal‚mol -1 ), despite the well-documented tendency to obtain stronger H bonds with oxygen than with sulfur. Those results are rationalized by means of a systematic investigation, at the DFT and MP2 levels, of the N-H‚‚‚S and N-H‚‚‚O interactions in the model systems C 2 H 4 SX‚‚‚HNH 2 (X ) S, O). The strength of the N-H‚‚‚X interaction is shown to be highly dependent on the directionality of the hydrogen bond characterized by the angle θ ) H-X-S. For X ) S, the optimal interaction is obtained for θ ∼ 80°, which almost exactly reproduces the angular parameter optimized in 1 (76.7°). For X ) O, the interaction is most favorable for higher values of θ (∼115°) which cannot be obtained in the hypothetic compound (µη 1 -SO) 3 (Fe-TACN) 2 because of structural constraints. Finally, varying the H‚‚‚X distance in the models shows that those interactions are extremely far-reaching. The energy difference between conformations 1 and 2 then accounts for only a small part of the global stabilization assigned to the hydrogen bond network, which could hence represent the key to the stabilization of the Fe 2 S 6 core.

Hydrogen Bonding with Sulfur

“…Therefore, the resulting S‚‚‚H-N hydrogen-bond distances of 2.37-2.48 Å coincide with fairly strong to medium hydrogen bonds according to the published literature. 15,16 Structural Characterization. TMT was chosen as a substrate since the D 3h symmetry of the molecule (fully deprotonated) could provide a potential point of organization for the formation of supramolecular assemblies.…”

Controlling the Conformation of Arylamides: Computational Studies of Intramolecular Hydrogen Bonds between Amides and Ethers or Thioethers

“…Recently François et al [12] described the effects of NÀ H···S hydrogen bonds on the stability of an Fe III complex. They found that six NÀH···S hydrogen bonds stabilized the complex by about 10 kcal mol À1 relative to a suitable control; this suggested the strength of each N À H···S hydrogen bond to be about 1.7 kcal mol À1 .…”

“…Also it was found that the S-containing hydrogen bonds helped stabilize the complex better than NÀH···O hydrogen bonds. [12] C-C-N-C torsional potentials of acetanilide (2) versus benzanilide: Since the target oligomer structure 1 has another benzene ring attached after the carbonyl group, we also calculated the 1 torsional potential for benzanilide, which is ] for geometries with constrained ring-amide C-C-N-C torsion angle 1 for 6 and 2. Some of the constrained structures are shown: the ca.…”

Computational Study of the Interaction between TIBO Inhibitors and Y181 (C181), K101, and Y188 Amino Acids

“…In the literature, there are some works reporting that, according to the AIM method, interactions of the N-H‚‚‚S and O-H‚‚‚S type have all the features of a hydrogen bond. 69,71,72,83 Intermolecular interactions O4‚‚‚H22′ and S4′‚‚‚H11 were also observed in 8 Cl-TIBO/K101 (M) dimer (Table 10 and Figure S1 (Supporting Information) for details on atom numbering. b Electron density in the nearest RCP from the corresponding BCP, indicated in Figure S2 (Supporting Information) by capital letters.…”

“…intermolecular interaction in 9 Cl-TIBO/ K101 dimer. the ability of the sulfur atom to act as proton acceptor [69][70][71][72]. Nevertheless, this is the first time that the presence of the hydrogen bond is reported to be involved in the stabilization of the TIBO inhibitor with K101.The sum of the stabilization energies of the amino acid fTIBO interactions gives values of 32.02, 21.59, and 14.94 kcal/ mol for 8 Cl-TIBO/K101 (W), 8 Cl-TIBO/K101 (M), and 9 Cl-TIBO/K101 dimers, respectively.…”