Ortho phosphorylation of PPh₃ to give a diphosphine and formation of a "butterfly" structure on a tetrairidium framework

A brief thermolysis of the bis-phosphine-substituted tetrairidium cluster Ir₄(CO)₁₀(PPh₃)₂ (1) in chlorobenzene (CB) converts it rapidly to the diphosphine cluster Ir ₄(CO)₇(μ-CO)₃{κ²-Ph ₂P(o-C₆H₄)PPh₂} (2) in 53% yield, providing evidence for ortho phosphorylation at the tetrairidium cluster framework. Treatment of 2 with excess C₆₀ affords the "butterfly" Ir₄-C₆₀ complex Ir ₄(CO)₆)(μ-CO){μ₃-κ²- Ph₂P(o-C₆H₄)P(η¹-o-C ₆H₄)}(μ₃-η²: η²η²-c₆₀ (3) in 71% yield. Interestingly, compound 3 can be prepared directly from the thermolysis of 1 with excess C₆₀ in moderate yield (52%). Compounds 2 and 3 have been characterized by micro-analysis, spectroscopy, and single-crystal X-ray diffraction studies. A molecular structure determination reveals that complex 2 adopts a regular tetrahedral geometry with three edge-bridging CO groups around the basal triangle and a diphosphine κ²-Ph₂P(o- C₆H₄)PPh₂ ligand chelating one of the basal iridium atoms. In contrast, complex 3 possesses a "butterfly" geometry in which the C₆₀ ligand is coordinated to the lower wing of the "butterfly" by an arene type μ₃-η²: η²:η²-C₆₀ bonding mode. One unique feature of 3 is the presence of both ortho phosphorylated and ortho-metalated phenyl rings forming five-membered [Ir-P-C-C-P] and [Ir-Ir-P-C-C] metallocycles joined in a spiro union at the iridium-indium-bridged phosphorus atom. A crossover experiment confirms that the conversion of 1 to 2 is intramolecular. Plausible reaction pathways for the formation of 2 and 3 are proposed.