Facile and rapid synthesis of crystalline quadruply bonded Cr(ii) acetate coordinated with axial ligands

Abstract

A quadruple bond formed between d-block or f-block atoms is an interesting research topic due to its unique nature including a supershort bonding distance and narrow energy gap between δ and δ*. Among various multiply bonded complexes, quadruply bonded Cr(II) acetates are considered useful to control the δ–δ* energy gap by the Lewis basicity of additional ligands. However, the synthesis and preparation of the high-quality, large-sized crystals of Cr(II) acetates coordinated with axial ligands (Cr₂(OAc)₄L₂) have been difficult due to their vulnerability to O₂, a representative oxidizing agent under aerobic conditions. In this study, we report a facile synthesis of sub-millimeter-scale crystals of Cr₂(OAc)₄L₂ by simple dissolution of Cr₂(OAc)₄ in ligand solvents L. To obtain stably ligated Cr₂(OAc)₄L₂, anhydrous Cr(II) acetates (Cr₂(OAc)₄) were dissolved in the ligand solvents L, which was degassed of dissolved O₂. Also, sub-millimeter-scale single crystals of Cr₂(OAc)₄L₂ were produced rapidly for less than an hour by the drop-drying process. The single-crystalline phase of the synthesized Cr(II) complexes was measured by X-ray diffraction techniques, confirming the dependency of Lewis basicity of the additional axial ligands on the Cr–Cr quadruple bond distance. Further, the Raman peaks of the quadruple bonds in Cr₂(OAc)₄L₂ were observed to be red-shifted with the increased basicity of the axial ligands.