Metal-Stabilized Methylene Arenium and σ-Arenium Compounds: Synthesis, Structure, Reactivity, Charge Distribution, and Interconversion

Protonation of rhodium and iridium complexes of formula ClM(CH 3 )[C 6 H(CH 3 ) 2 (CH 2 P(t-Bu) 2 ) 2 ] (M = Rh ( 1 ), Ir ( 3 )) with a strong acid (HOTf, trifluoromethanesulfonic acid) results in clean formation of the new methylene arenium metal complexes ClM[CH 2 C 6 H(CH 3 ) 2 (CH 2 P(t-Bu) 2 ) 2 ] + OTf - (M = Rh ( 2a ), Ir ( 4 ), respectively), which have been fully characterized including an X-ray single-crystal analysis. This new method can be applied to complexes having both electron-donating and electron-withdrawing substituents in the aromatic ring. The methylene arenium complexes bear most of the positive charge in the ring, resulting in their high CH acidity. Deprotonation of these complexes with NEt 3 gives the new metal xylylene complexes 12 (M = Rh) and 13 (M = Ir), which can be converted back to the methylene arenium complexes by reaction with HOTf. Reaction of the cationic complexes - OTf + Rh(R)[C 6 H(CH 3 ) 2 (CH 2 P(t-Bu) 2 ) 2 ] (R = CH 3, PhCH 2 ) with CO gives the new alkyl σ-arenium rhodium complexes Rh(CO)[R−C 6 H(CH 3 ) 2 (CH 2 P(t-Bu) 2 ) 2 ] + OTf - (R = CH 3 ( 9a ), PhCH 2 ( 9b )). On the basis of the NMR data, the X-ray crystal structure analysis, and the reactivity, these Rh complexes were shown to have much less positive charge in the ring as compared to the methylene arenium complexes, most probably due to stabilization of an arene form by an agostic interaction of the arene−alkyl C ipso −C bond with the metal center. The nature of the reported phenomena is discussed. The interconversion of the methylene arenium and alkyl σ-arenium metal complexes is also presented.