Synthesis and crystal structures of cis- and trans-dimethyl-di-µ-methylene-bis(pentamethylcyclopentadienyl)dirhodium( IV )

Reaction of [{Rh(C5Me5)Cl}2(µ-Cl)2](1) with Li4Me4 or Al2Me6 gives, after air-oxidation, cis- or trans-[{Rh(C5Me5)Me}2(µ-CH2)2][(2a)(28%) and (2b)(15%) respectively] which are formally of RhIV. The structures were elucidated spectroscopically and were confirmed by a single-crystal X-ray determination of the cis isomer (2a). This showed a dinuclear complex with each rhodium bound to one methyl and one η5-C5Me5, and with two bridging methylenes about a rhodium–rhodium bond [2.620(1)Å]. The two hydrogens on each methylene bridge are inequivalent as can be seen from the 1 H n.m.r. spectrum. Nuclear Overhauser effect experiments, interpreted in the light of the X-ray structure, showed that it was only the equatorial hydrogen which was coupled to the rhodiums. The cis isomer (2a) was converted into trans-(2b) on reaction with Lewis acids (e.g. Al2Me6); (2b) also reacted with AI2Et6 to give trans-[{Rh(C5Me5)Et}2(µ-CH2)2] suggesting a common pathway for substitution and isomerisation. When the yellow solution obtained by reaction of (1) and AI2Me6 in hydrocarbons was reacted with acetone at –78 °C the cis isomer (2a) was obtained in good yield; if the reaction was carried out at higher temperature the trans isomer (2b) was isolated in 89% yield. The results of experiments when perdeuterio-(1) was reacted with AI2Me6 and the reactions quenched (a) with air and (b) with acetone are discussed.