α- and β-[Fe2(μ-StBu)2(StBu)4]2−: Coexistence of Two Bond-Stretch Isomers of a Classical Bitetrahedral Metal Chalcogenolate Compound
✍ Scribed by Bernd Hammann; Changneng Chen; Ulrich Flörke; Ralf Hauptmann; Eckhard Bill; Sebastian Sinnecker; Gerald Henkel
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 381 KB
- Volume
- 45
- Category
- Article
- ISSN
- 0044-8249
No coin nor oath required. For personal study only.
✦ Synopsis
on the occasion of his 75th birthday
Eight years ago, we reported homoleptic binuclear iron(II) thiolate complexes with the coordination number three in which sterically demanding ligands prevented the metal center from adopting the tetrahedral coordination environment typically found with sulfur ligands. [1] Besides these coordinatively unsaturated species, several other structurally related metal(II) chalcogenolate complexes are known which also contain three-coordinate metal centers as a result of the use of bulky ligands. [2] Homobinuclear thiolate and selenolate complexes of more conventional design, however, are much more numerous. Characterized by four chalcogen donor functions surrounding each metal ion, typical examples include the iron alkane thiolate complexes [Fe 2 (SMe)
. They consist of two FeS 4 tetrahedra that share a common edge. [3] This architecture leads to a central cyclic Fe 2 S 2 unit which is normally planar in the case of monofunctional ligands and which has the form of a rhomb with acute Fe-S-Fe (and obtuse S-Fe-S) angles.
Similar planar M 2 E 2 rhombs (E = S, Se) are also present in the binuclear complexes with coordination number three.
However, from direct comparison of this unit in [Fe 2 {SC 6 H 3 -2,6-(SiMe 3 ) 2 } 4 ] and [Fe 2 (SiPr) 6 ] 2À , typical examples for threeand four-coordinate complexes, respectively, it became evident that the Fe 2 S 2 rhomb within [Fe 2 {SC 6 H 3 -2,6-(SiMe 3 ) 2 } 4 ] has acute angles at the iron atoms-in clear contradiction to an expected widening as a result of the trigonal-planar coordination with ideal-typical valence angles of 1208. [1, 3b] Even more surprising, however, are the structural features of the complex [Fe 2 {SC 6 H 3 -2,6-(SiMe 3 ) 2 } 4 (MeCN) 2 ] which has been obtained by addition of two acetonitrile molecules to [Fe 2 {SC 6 H 3 -2,6-(SiMe 3 ) 2 } 4 ]. [1] In the course of this reaction, the coordination geometry of the iron centers changed from distorted trigonal planar to distorted tetrahedral resulting in a further lowering of the S-Fe-S angle within the central Fe 2 S 2 rhomb from 84.3(1) to 79.8(1)8.
At least as surprising as this extremely small value is that the exocyclic (opposing) S-Fe-N angle does not compensate for this shortening by being larger than the ideal tetrahedral value of 109.58, but is rather, with 100.3(1)8, significantly smaller.
From these observations it seemed likely that classical binuclear homoleptic chalcogenolate complexes of coordination number four can occur in at least one further conformation not previously observed, which corresponds to a defined minimum on the potential energy surface.
In search for such a conformation, which has clearly been hidden by the metal/ligand combinations used to date, we reexamined various reaction systems. These investigations focused on ligands with highly nucleophilic sulfur donor atoms but that are small enough to allow tetrahedral coordination. Thus, the tert-butanethiolate ligand appears to be especially suitable. In our previous studies based on this ligand and its selenium homologue, we were able to synthesize various complexes with completely novel chemical and structural features, for example the bitetrahedral nickel and iron complexes [Ni 2 (StBu) 6 ] 2À and [Fe 2 (StBu) 5 ] À which show, respectively, the first pair of NiS 4 tetrahedra sharing a common edge and the first pair of FeS 4 tetrahedra sharing a common face. [4] Examination of the reaction system directed towards the synthesis of [Fe 2 (StBu) 6 ] 2À resulted in the discovery of a novel conformation in addition to a species of identical composition but of conventional design, which together represent an unprecedented pair of bond-stretch isomers.
Both isomers form, together with tetramethylammonium cations, a complex salt of composition [Me 4 N] 4 {a-[Fe 2 -(StBu) 6 ]}{b-[Fe 2 (StBu) 6 ]}. [5, 6] a-[Fe 2 (StBu) 6 ] 2À (1 a) is the isomer with the conventional structure (Figure 1). Its centroid is a crystallographic inversion center, and the overall symmetry comes very close to the point group C 2h with the 2-fold axis passing through the iron atoms.
Within the central planar Fe 2 S 2 unit, the angles at the iron atoms are 95.01(2)8 and thus are obtuse, whereas they are acute at the sulfur atoms (84.99(2)8). This geometric feature brings 1 a in line with other alkane thiolate complexes of general formula [Fe 2 (SR) 6 ] 2À , for example, with R = Me, Et, and iPr the angles at the iron atoms are 104.1(1), 102.3(1), and 99.2(1)8, and at the sulfur atoms 75.9(1), 77.7(1), and 80.8(1)8, respectively. Furthermore, the tendency of these angles to