Chemical Behavior of a Pair of (COD)CpRh and −Ir Complexes with Pendant Peripheral −B(C₆F₅)₂ Groups

Hydroboration of (COD)Rh(η5-C5H4-allyl) (7) with the strongly electrophilic reagent HB(C6F5)2 yields (COD)Rh[η5-C5H4-(CH2)3B(C6F5)2] (8). Nucleophilic N-heterocyclic reagents (1-methylimidazole or 1-methylbenzimidazole) add to the boron atom of the bifunctional complex to yield the respective adducts (9a, 9b). Both were characterized by X-ray diffraction. Addition of HB(C6F5)2 to the corresponding (COD)Ir(η5-C5H4-allyl) complex (10) eventually results in the formation and isolation of the cycloborylated zwitterionic (COD)(η5-C5H3-cyclo-(CH2)3B(C6F5)2]IrH product (13) (also characterized by X-ray crystal structure analysis). However, 13 seems to thermally equilibrate in a reverse electrophilic aromatic substitution reaction with the thermodynamically disfavored open-chain (COD)Ir[η5-C5H4-(CH2)3B(C6F5)2] isomer (11). This follows from reactions of 13 with 1-methylimidazole or 1-methylbenzimidazole, which rapidly yield the corresponding open-chain adducts 14a and 14b at room temperature (both characterized by X-ray diffraction). In a slow reaction complex 13 even eventually reacts with pyrrole by ring opening to yield the open-chain product 15, which features the 2H-pyrrole isomer coordinated to the strong −B(C6F5)2 Lewis acid at the end of the trimethylene tether. Complex 15 was also characterized by an X-ray crystal structure analysis.