Square-planar coordination is an ideal structure for tetracoordinate complexes in transition metal chemistry. The presence of high spin square-planar complexes is unusual for most of the d metal ions that have a d electron configuration ≥ 4, including but not limited to d4 Cr(II) and d6 Fe(II) due to the relatively large energy separation of dx2 -y2 orbital from the rest of the d orbitals in square-planar geometry.1,2 There are several examples containing high spin Cr(II) and Fe(II) square-planar coordination in the literature, however such examples are uncommon.3,4 The Cr(II) and Fe(II) analogues of the gillespite group of metal tetrasilicates, ABSi4O10 (A= Ca, Sr, Ba and B= Cr, Fe, Cu), are noteworthy examples of rare, high spin Cr(II) and Fe(II) in square-planar coordination. Among these tetrasilicate phases, the synthesis of ACrSi4O10 has been known for several decades yet much of its chemistry remains unexplored.5,6
In this study, we focus on the chemistry of ACrSi4O10 through optimized synthetic routes, improved ACrSi4O10 crystal growth, and nanostructuring into nanosheets by exfoliation. For both solid state and flux-based routes, we utilize excess metallic Cr to maintain a low oxygen fugacity in the reaction environment to avoid the formation of highly oxidized Cr species. 5,6 For flux-based routes, we additionally utilize a Cr foil envelope for the same purpose. We further extend the chemistry of metal tetrasilicates to include mixed transition metals in the B-site of BaBSi4O10. The incorporation of mixed metal centers in ABSi4O10 phase is new territory in metal tetrasilicate chemistry, and Ba(Fe,Cr)Si4O10 is the first example of transition metal substitution in any of the known ABSi4O10 systems. We find that a targeted stoichiometry of BaFe0.5Cr0.5Si4O10 yields an experimentally determined composition with up to a ~40-60% incorporation of Fe(II), which appears to constitute the maximum incorporation of Fe(II) into BaCrSi4O10 via reactions between preformed tetrasilicate precursors BaFeSi4O10 or BaCrSi4O10 and Cr or Fe sources, respectively. These reactions provide the first demonstration of direct metal ion exchange in an ABSi4O10 system, which has been considered largely chemically inert. Importantly, these results opens the possibility of substitution by other M(II), and this work includes experiments that indicate the successful incorporation of other first row transition metals, such as Mn(II) and Co(II), into tetrasilicate phases through a templated growth process. Finally, nanostructuring of ACrSi4O10 is discussed.
1. Cirera, J.; Ruiz, E.; Alvarez, S. Stereochemistry and Spin State in Four-Coordinate Transition Metal Compounds. Inorg. Chem. 2008, 47, 2871-2889.
2. Cirera, J.; Alemany, P.; Alvarez, S. Mapping the Stereochemistry and Symmetry of Tetracoordinate Transition-Metal Complexes. Chem. Eur. J. 2004, 10, 190-207.
3. Eremenko, L.; Pasynskii, A. A.; Kalinnikov, V. T. Synthesis and Molecular Structure of Bis(2,6- di-methylpyridine)bis-(trifluoroacetate)Cr(II) with an Unusual Square Planar Environment of the Chromium Atom. Inorganica Chim. Acta 1981, 54, L85-L86.
4. Wurzenberger, X.; Piotrowski, H.; Klüfers, P. A Stable Molecular Entity Derived from Rare Iron(II) Minerals: The Square-Planar High-Spin-d6 FeIIO4 Chromophore. Angew. Chem. Int. Ed. 2011, 50, 4974-4978.
5. Belsky, H. L.; Rossman, G. R.; Prewitt, C. T.; Gasparik, T. Crystal Structure and Optical Spectroscopy (300 to 2200 nm) of CaCrSi4O10. Am. Mineral. 1984, 69, 771-776.
6. Miletich, R.; Allan, D. R.; Angel, R. The Synthetic Cr2+ Silicates BaCrSi4O10 and SrCrSi4O10: The Missing Links in the Gillespite-type ABSi4O10 Series. Am. Mineral. 1997, 82, 697-707.