Tetrasilicates with Cr(II), Fe(II) and Other Divalent Transition Metals in Square-Planar Coordination

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 BaFe_0.5Cr_0.5Si₄O₁₀ 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 Fe^IIO4 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.