Chalcogens as Terminal Ligands to Iron:  Synthesis and Structure of Complexes with Fe^III−S and Fe^III−Se Motifs

Abstract

Metal complexes with terminal chalcogenido ligands are known for the early transition-metal complexes, yet for the heavier congeners (e.g., sulfido and selenido), there are no analogous examples for the late 3d metal ions. Reported herein is the isolation and characterization of monomeric iron(III) complexes containing sulfido and selenido ligands; isolation was accomplished using the tripodal ligand tris[(N'-tert-butylureaylato)-N-ethylene]aminato ([H3buea]3-). The FeIII-E (E = S2-, Se2-) complexes were prepared from the iron(II) precursor, [FeII(H3buea)]2-, and the elemental forms of the chalogen. The formulation of [FeIIIH3buea(S)]2- and [FeIIIH3buea(Se)]2- as monomeric complexes with Fe-E units is supported by spectroscopic, analytical, and X-ray diffraction studies. For instance, X-band EPR spectra contain well-resolved axial signals, which are consistent with each complex having S = 5/2 ground states. The solid-state molecular structures reveal FeIII-E bond lengths of 2.211(1) and 2.355(1) A for [FeIIIH3buea(S)]2- and [FeIIIH3buea(Se)]2-, respectively. The primary coordination sphere for each complex also contains three deprotonated urea nitrogen atoms from [H3buea]3-; the apical amine nitrogen atom weakly interacts with the iron centers at distances of greater than 2.6 A. The terminal chalcogenido ligands appear to weakly hydrogen-bond with the urea NH groups of the [H3buea]3-; however, open H-bond cavities are observed for [FeIIIH3buea(S)]2- and [FeIIIH3buea(Se)]2-, which may contribute to their observed long-term instability.

Trigonal bipyramidal iron(III) and manganese(III) oxo, sulfido, and selenido complexes. An electronic-structural overview

“…Overall, as shown in Table 1, the optimized Fe/Mn-O/ S/Se distances agree well with experiment [7,8,12]. However, for the Fe-N 1 bonds (trans to the Fe III -E unit), our calculations rather badly (>0.1 Å in certain cases) overestimate the bond lengths, relative to experiment.…”

“…We have seen similar failures of DFT to correctly describe structural trans effects in Fe(II) and Fe(III) nitro (N-nitrito) complexes [ bond acceptor in an almost perfectly symmetrical manner. In contrast, while perfect C 3 geometries could be obtained as apparent local minima in geometry optimizations, the lowest-energy optimized geometries of [Fe III (L 3 )(S)] 2À and [Fe III (L 3 )(Se)] 2À exhibit small but significant deviations from C 3 symmetry [12]. Unfortunately, given the large sizes and numbers of molecules examined here, we could not undertake vibrational analyses so as to rigorously characterize the nature of the stationary points obtained by geometry optimizations.…”

The effects of hydrogen bonds on metal-mediated O2 activation and related processes

“…One area that we have had success was in the formation of iron complexes with terminal sulfido and selenido ligands. Fe–E–M (E = S 2− ,Se 2− ; M = metal ion) units are common motifs, especially in protein cofactors,20 yet to our knowledge, iron complexes with terminal sulfido and selenido ligands were unknown prior to our work in 2004 10,21. The lack of these iron complexes has been attributed in part to the propensity of the chalcogenides, especially the heavier congeners, to bridge metal centers 10.…”

“…We were successful in preparing [Fe III H 3 buea(S)] 2− and [Fe III H 3 buea(Se)] 2− following the procedure outlined for the M–O complexes but used molecular S 8 and Se instead of dioxygen 21. X-Band EPR spectra on frozen solutions are consistent with both complexes having nearly axial symmetry around high-spin Fe( III ) centers.…”

X-ray Absorption Spectroscopy and Density Functional Theory Studies of [(H3buea)FeIII-X]n- (X = S2-, O2-, OH-): Comparison of Bonding and Hydrogen Bonding in Oxo and Sulfido Complexes

“…This study is further extended to consider the effects H-bonding in a hypothetical Fe IV soxo system having the same [H 3 buea] 3-ligand; these results are compared to those for heme-oxo species and used to define the features that may be present in biologically relevant ferryl species. 17,23 XAS Data Collection. X-ray absorption spectra were recorded at the Stanford Synchrotron Radiation Laboratory (SSRL) on the 31-pole wiggler beam line 10-1 and bending magnet beam line 8-2 (Fe L-edges), bending magnet beam line 2-3 (Fe K-edges), and 54-pole wiggler beam line 6-2 (S K-edge) under ring operating conditions of 70-100 mA and 3 GeV.…”