Rhodium-stabilized o-quinone methides:: Synthesis, structure, and comparative study with their iridium congeners

The parent o-quinone methide (o-QM) cannot be observed in condensed phases above -100 degreesC, but in this work, o-QM and its dimethyl derivative are stabilized by complexation to Cp*Rh. Precursor oxo-dienyl rhodium complexes [(eta (5)-Cp*)Rh(eta (5)-2-alkyl-oxodienyl)] [BF4] (5a,b) were isolated as yellow microcrystalline solids in 85-90% yields and fully characterized. In addition the X-ray molecular structure of 5b is reported. Selective alkyl deprotonation by t-BuOK in CH2Cl2 provided the first series of rhodium-o-QM complexes [(eta (5)-Cp*)Rh(eta (4)(R)(2)C7H4O)] (R = H, 6a; R = Me, 6b). The o-QM is stabilized through eta (4)-coordination to the metal center, as shown by the X-ray structure of 6b. Structural and reactivity studies on 6a,b show that the rhodium-o-QM complexes are less stable than their iridium congeners.