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.