Chemical behavior of a pair of (COD)CpRh and -Ir complexes with pendant peripheral -B(C6F5)2 groups

Hydroboration of (COD)Rh(eta(5)-C5H4-allyl) (7) with the strongly electrophilic reagent HB(C6F5)(2) yields (COD)Rh [eta(5)-C5H4-(CH2)(3)B(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(eta(5)-C5H4-allyl) complex (10) eventually results in the formation and isolation of the cycloborylated zwitterionic (COD)(eta(5)-C5H3-cyclo-(CH2)(3)B(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[eta(5)-C5H4-(CH2)(3)B(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.