Interplay of Linker, N-Substituent, and Counterion Effects in the Formation and Geometrical Distortion of N-Heterocyclic Biscarbene Complexes of Rhodium(I)

A new series of (CH2)n-linked N-heterocyclic biscarbene (bis-NHC) complexes of rhodium(I) have been synthesized via in situ generation of the silver carbene complexes from bis(imidazolium) halide salts and silver(I) oxide followed by transmetalation to [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene). With a large t-Bu N-substituent, short linkers favor the chelate and long linkers favor the 2:1 complex, contrary to the previous findings with the small n-Bu substituent. In addition, the use of a noncoordinating anion for the AgNHC favors chelate formation. The chelate complex [Rh(COD)L2]PF6 (L2 = [methylenebis(N-(tert-butyl)imidazol-2-ylidene)]) and its dicarbonyl analogue [Rh(CO)2L2]PF6 were structurally characterized. They showed distortion both of the square plane, an electronic effect, and of the metal−NHC moiety, a steric effect.