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+]CF3SO3- (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]CF3SO3- (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 CF3SO3H giving as a single product the arenium complex CF3SO3Rh[3,5-bis(di-tert-butylphosphinomethyl)-1,2,6-trimethyl-4-methylene-3,5-cyclohexadienyl]+CF3SO3- (8). C-13 NMR studies performed on the arenium complexes shows no para-substituent effect on the chemical shift of the coordinated CH2 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.