L/Z-ligand type amphoterism of an acridinium unit

Abstract

As part of our study of ambiphilic carbenium-based ligands, we now report on the coordination chemistry of the known acridinium phosphine ligand [(o-Ph₂P(C₆H₄)Acr)]⁺ ([L^acr]⁺, Acr = 9-N-methylacridinium) towards rhodium(I) reagents. While the simple coordination complex [L^acrRh(COD)Cl]⁺ ([1]⁺) is obtained from [L^acr]⁺ and [Rh(COD)Cl]₂ (COD = 1,5-cyclooctadiene) in CH₂Cl₂, the use of [Rh(COE)₂Cl]₂ (COE = cyclooctene) in MeCN affords [(o-Ph₂P(C₆H₄)Acr)Rh(MeCN)₂Cl]₂²⁺ ([2]₂²⁺), a chloride-bridged dimeric complex in which each rhodium atom is involved in a Rh→C_carb dative bond (C_carb = acridinium C9 atom), indicating Z-type behavior of the acridinium unit. The same reaction in CH₂Cl₂ affords [(o-Ph₂P(C₆H₄)Acr)RhCl]₂²⁺ ([3]₂²⁺), a chloride-bridged dimeric square planar complex in which the acridinium assumes the role of an η²-heteroarene, L-type ligand. All complexes, which have been structurally characterized as their triflate salts, can be interconverted into one another via simple ligand exchange or displacement reactions. These experimental results, supported by computational analyses, show that the acridinium unit of [L^acr]⁺ displays ligand-type amphoterism as it easily and reversibly switches from L-type to Z-type in response to changes in the coordination sphere of the metal center.