Halide coordinated homoleptic [Fe4S4X4](2-) and heteroleptic [Fe4S4X2Y2](2-) clusters (X, Y = Cl, Br, I)--alternative preparations, structural analogies and spectroscopic properties in solution and solid state

New facile methods to prepare iron sulphur halide clusters [Fe4S4X4](2-) from [Fe(CO)5] and elemental sulphur were elaborated. Reactions of ferrous precursors like tetrahalidoferrates(ii) or simple ferrous halides with [Fe(CO)5] and sulphur turned out to be efficient methods to prepare homoleptic [Fe4S4X4](2-) (X = Cl, Br) and heteroleptic clusters [Fe4S4X4-nYn](2-) (X = Cl, Br; Y = Br, I). Solid materials were obtained as salts of BTMA(+) (= benzyltrimethylammonium); the new compounds containing [Fe4S4Br4](2-) and [Fe4S4X2Y2](2-) (X, Y = Cl, Br, I) were all isostructural to (BTMA)2[Fe4S4I4] (monoclinic, Cc) as inferred from synchrotron X-ray powder diffraction. While the solid materials contain defined heteroleptic clusters with a halide X : Y ratio of 2 : 2, dissolving these compounds leads to rapid scrambling of the halide ligands forming mixtures of all five possible [Fe4S4X4-nYn](2-) clusters as could be shown by UHR-ESI MS. The variation of X and Y allowed assignment of the absorption bands in the visible and NIR; the long-wavelength bands around 1100 nm were tentatively assigned to intervalence charge transfer (IVCT) transitions.

A continuous symmetry measure of [4Fe-4S]+ core distortions and analysis of supramolecular synthons in crystal structures of (Et4N)3[Fe4S4Cl4].Et4NCl at 100 and 295 K

Distortions of the [4Fe-4S](+) cores of synthetic models from T(d) symmetry are analyzed in terms of the continuous symmetry measures (CSM, S(T(d))), and these are related to lattice effects in terms of the supramolecular synthon terminology common to crystal engineering of small molecule structures. The small tetragonal compression to D(2d) from idealized T(d) symmetry observed at low temperature is attributed to environmental factors. New members of the isomorphous series of compositional variations of double salts of the air-sensitive reduced cluster (Et(4)N)(3)[Fe(4)S(4)Cl(4)].Et(4)NCl (1) are prepared in modest yield by treatment of FeCl(2) with NaHS, or (Et(4)N)(2)[FeCl(4)] with Et(4)NSH and a base. The crystals are isomorphous with the corresponding HS(-) ligated cluster. Crystal data: tetragonal, P42(1)c, Z = 2, a = b = 12.2550 (4), c = 16.278 (1) A at 100 K, and a = b = 12.385 (1), c = 16.344 (2) A at 295 K. The crystallographically imposed S(4) symmetry obtained with sterically unincumbering ligands affords a better view of the intrinsic geometry of the core structure. The cocrystallization of the halide ion affords the opportunity to compare three types of weak C-H...X hydrogen bonds, or hydrogen bridges, between tetraalkylammonium cations and anions within the same crystal lattice. The C...Cl(-) distances (3.590 and 3.634 at 100 K increase to 3.616 and 3.655 A, respectively, at 295 K) are virtually temperature independent, indicative of hard hydrogen bridges, whereas the C...Cl-Fe distances are 3.702-3.718 A at 100 K but are 3.753-3.764 A at room temperature, suggesting a softer hydrogen bridge. A similar trend applies to the two sets of C...mu(3)-S distances (3.766-3.788 A and 3.594-3.604 A at 100 K and 3.821-3.848 A and 3.614-3.676 A at room temperature). The longer hydrogen bridges are more linear (170 degrees ) than the shorter ones (134 degrees ). The core distortions are correlated with spatial distribution of cations around the clusters.