Cyclobutadiene Arene Complexes of Rhodium and Iridium

Reactions of [(C₂H₄)₂RhCl]₂ or [(coe)₂RhCl]₂ (coe = cyclooctene) with AgPF₆ and arenes, followed by addition of 3-hexyne, give the cyclobutadiene complexes [(C₄Et₄)­Rh­(arene)]⁺ in 40–65% yield (arene = tert-butylbenzene, p-xylene, mesitylene, 4-mesitylbutanoic acid). In the absence of arenes, the hexaethylbenzene complex [(C₄Et₄)­Rh­(C₆Et₆)]⁺ is formed in 70% yield as a result of cyclotrimerization of 3-hexyne in the coordination sphere of rhodium. Similar reaction of [(coe)₂IrCl]₂ with AgPF₆ and 3-hexyne leads to [(C₄Et₄)­Ir­(C₆Et₆)]⁺, which is apparently the first reported cyclobutadiene iridium complex. DFT calculations suggest that formation of the model cyclobutadiene complex [(C₄Me₄)­Rh­(C₆H₆)]⁺ from bis­(alkyne) intermediate [(C₂Me₂)₂Rh­(C₆H₆)]⁺ can proceed via a metallacycle transition state with a low energy barrier of 14.5 kcal mol^–1.