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

Abstract

The parent o-quanone methide (o-QM) cannot be observed in condensed phases above -100°C, but in this work, o-QM and its dimethyl derivative are stabilized by complexation to Cp*Rh. Precursor oxo-dienyl rhodium complexes [(η5-Cp*)Rh(η5-2- 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 [(η5-Cp*)Rh(η4-(R)2C7H4O)] (R = H, 6a; R = Me, 6b). The o-QM is stabilized through η4-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.