A dianionic C₃-symmetric scorpionate: synthesis and coordination chemistry

Introducing charges into ligand systems fine-tunes their electronic properties and influences the solubility of their metal complexes. Herein, we present a synthesis of a dianionic, C3-symmetric ligand combining three anionic N-donors tethered to a positively charged phosphonium center. The tris-skatylmethylphosphonium (TSMP) ligand, isolated in the form of its dipotassium salt TSMPK2, is the first dianionic homoscorpionate capable of metal exchange. The potassium cations in TSMPK2 are exchangeable for other metals, which results in rich coordination chemistry. Thus, the ligand displays a bridging μ2:κ2:κ1 coordination mode with trigonal planar Cu(i) centers in the tetrameric complex [(TSMP)Cu]44-. The κ3 mode is accessed upon addition of 1 equiv. of P(OEt)3 per Cu(i) to yield the tetrahedral monomeric complex [(TSMP)CuP(OEt)3]-. Both Fe(ii) and Ni(ii) in pyridine give octahedral high-spin κ3 complexes with composition (TSMP)M(Py)3 (M = Fe, Ni). Displacement of three pyridine ligands in (TSMP)Fe(Py)3 for a second equivalent of TSMP gives a high-spin pseudotetrahedral 2 : 1 complex [(TSMP)2Fe]2- with the ligands in κ2 coordination mode. The reduction in coordination number is likely due to electrostatic repulsion of the negatively-charged indolides as well as their weaker π-accepting character as compared to pyridine.