Cyclobutadiene Arene Complexes of Rhodium and Iridium

Reactions of [(C2H4)(2)RhCl](2) or [(coe)(2)RhCl](2) (coe = Cydooctene) 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 [(C(4)Bt(4))Rh(C6Et6)](+) is formed in 70% yield as a result of cyclotrimetization of 3-hexyne in the coordination sphere of rhodium. Similar reaction of [(coe)(2)IrCl](2) with AgPF6 and 3-beltyne 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)(2)Rh(C6H6)(+) can proceed via a metallacycle transition state with a low energy barrier of 14.5 kcal mol(-1).