A dianionic C3-symmetric scorpionate: synthesis and coordination chemistry

Abstract

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.