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Cyclobutadiene Arene Complexes of Rhodium and Iridium
Reactions
of [(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>RhCl]<sub>2</sub> or [(coe)<sub>2</sub>RhCl]<sub>2</sub> (coe = cyclooctene)
with AgPF<sub>6</sub> and arenes, followed by addition of 3-hexyne,
give the cyclobutadiene complexes [(C<sub>4</sub>Et<sub>4</sub>)ÂRhÂ(arene)]<sup>+</sup> in 40–65% yield (arene = <i>tert</i>-butylbenzene, <i>p</i>-xylene, mesitylene, 4-mesitylbutanoic acid). In the absence
of arenes, the hexaethylbenzene complex [(C<sub>4</sub>Et<sub>4</sub>)ÂRhÂ(C<sub>6</sub>Et<sub>6</sub>)]<sup>+</sup> is formed in 70% yield
as a result of cyclotrimerization of 3-hexyne in the coordination
sphere of rhodium. Similar reaction of [(coe)<sub>2</sub>IrCl]<sub>2</sub> with AgPF<sub>6</sub> and 3-hexyne leads to [(C<sub>4</sub>Et<sub>4</sub>)ÂIrÂ(C<sub>6</sub>Et<sub>6</sub>)]<sup>+</sup>, which
is apparently the first reported cyclobutadiene iridium complex. DFT
calculations suggest that formation of the model cyclobutadiene complex
[(C<sub>4</sub>Me<sub>4</sub>)ÂRhÂ(C<sub>6</sub>H<sub>6</sub>)]<sup>+</sup> from bisÂ(alkyne) intermediate [(C<sub>2</sub>Me<sub>2</sub>)<sub>2</sub>RhÂ(C<sub>6</sub>H<sub>6</sub>)]<sup>+</sup> can proceed
via a metallacycle transition state with a low energy barrier of 14.5
kcal mol<sup>–1</sup>