Synthesis of Phosphaphenalenium Salts via P-C Reductive Elimination at a Ru(II) Center and Their Fluorescence Properties

Rare examples of P-C reductive elimination at a Ru(II) center to form phosphonium salts have been developed. A five-membered phospharuthenacycle derived from P(1-Naph)Ph-2 and [(eta(6)-C6Me6) RuCl2](2) is converted to eta(4)-phosphaphenalenium complexes [(eta(6)-C6Me6)Ru(eta(4)-phosphaphenalenium)]-[BAr4F] via alkyne insertion followed by P-C reductive elimination. This reaction sequence including P-C reductive elimination is applicable to various dialkyl and diarylalkynes, although the eta(4)-phosphaphenalenium complexes with electron-rich aryl groups are not stable enough to be isolated. The reaction of the eta(4)-phosphaphenalenium complexes with I-2, CO or CuCl2 resulted in the liberation of the phosphaphenalenium salts. In contrast to the highly distorted phosphacycle in the Ru complex, the phosphaphenalenium cations have nearly planar pi-conjugated structure, and their BAr4F salts display green to yellow and blue to green fluorescence in solution and in the solid state, respectively, depending on the substituents on the phenalene skeleton. Because of the cationic character of the phosphacycle as well as their large counteranion, pi-pi stacking of the phosphacycles in the crystalline state is effectively suppressed, leading to emission with shorter wavelength in the solid state than in solution. Electronic structures of phosphaphenalenium ions as well as effects of substituents were investigated by TD-DFT calculations.