Methylene Arenium Cations via Quinone Methides and Xylylenes Stabilized by Metal Complexation

The quinone methide (QM) rhodium complex of 3,5-bis(di- tert -butylphosphinomethyl)-2,6-dimethyl-4-methylene-3,5-cyclohexadien-1-one (L) ( 1 ) was protonated by trifluromethanesulfonic acid (triflic acid) at the quinonoid carbonyl group giving the unique methylene arenium complex (Cl)Rh[LH + ]CF 3 SO 3 - ( 2 ). Complexes 2 and its (trimethyl)silyl analogue ( 3 ) were fully characterized spectroscopically, and complex 2 was also characterized by single-crystal X-ray analysis. The crystallographic studies on 2 have revealed that the positive charge is delocalized between the carbon atoms of the ring with most of it being at the para- and ortho-carbon atoms. The electron deficient QM complex (CO)Rh + [L] CF 3 SO 3 - ( 4 ), which has also been crystallographically characterized, is less basic, requiring excess of triflic acid to obtain the methylene arenium complex (CO)Rh + [LH + ]2CF 3 SO 3 - ( 5 ), demonstrating a dramatic effect of the electron density on the metal center on the stability of the methylene arenium species. When 1 was reacted with 2−3 equiv of MeLi formation of two complexes, MeRh[3,5-bis(di- tert -butylphosphinomethyl)-2,6-dimethyl-4-methylene-3,5-cyclohexadien-1-en] ( 6 ) and its ortho-xylylene isomer ( 7 ), took place. Complexes 6 and 7 represent the first example of thermally stable xylylenes coordinated via only one of the exocyclic double bonds. Both 6 and 7 undergo protonation by CF 3 SO 3 H giving as a single product the arenium complex CF 3 SO 3 Rh[3,5-bis(di- tert -butylphosphinomethyl)-1,2,6-trimethyl-4-methylene-3,5-cyclohexadienyl] + CF 3 SO 3 - ( 8 ). 13 C NMR studies performed on the arenium complexes shows no para-substituent effect on the chemical shift of the coordinated CH 2 group and that this group does not participate in the positive charge delocalization. Thus, the reported methylene arenium compounds can be viewed as a resonance form of a benzyl cation stabilized by metal complexation.