Synthesis, spectroscopy, and photochemistry of bis[bis(diphenylphosphino)-methane]bis(2,5-di-isocyano-2,5-dimethylhexane) dirhodium(I)

Reaction between [Rh₂L₄]²⁺ (L = 2,5-di-isocyano-2,5-dimethylhexane) and dppm [bis(diphenylphosphino)methane] in acetonitrile yielded [Rh₂L₂(dppm)₂]²⁺. Its optical spectrum displays an intense absorption at 595 nm, which is assigned to the transition ¹A_1g[(d_σ*)²] → ¹B_1u [(d_σ*)¹(p_σ)¹]. Room-temperature fluorescence at 677 nm, from the ¹B_1u excited state, is observed in fluid solution. The ³B_1u state of the rhodium complex is long-lived with a measured lifetime of 1.0 ± 0.1 μs. Studies on the quenching reactions with a series of pyridinium acceptors established that the excited reduction potential [E°(Rh₂ ³⁺-Rh₂ ²⁺*)] is -1.13 V vs. saturated calomel electrode. The mechanism of the photoreactions of the rhodium complex with halogenocarbons is discussed.