Rhodium Complexes of a Chelating Ligand with Imidazol-2-ylidene and Pyridin-2-ylidene Donors: The Effect of C-Metalation of Nicotinamide Groups on Uptake of Hydride Ion

Rhodium complexes of the imidazolylidene (C-im) N-heterocyclic carbene (NHC) ligand, C-im-pyH+, bearing a nicotinamide cation substituent (pyH+) have been targeted for ligand-centered uptake and delivery of hydride ion. This work reveals that rhodium­(I) complexes such as [Rh­(C-im-pyH+)­(COD)­X]­[PF6] (1, a: X = Cl, b: X = I) undergo facile C-metalation of the nicotinamide ring to afford rhodium complexes of a novel chelate ligand, C,C′-im-py, with coordinated imidazolylidene (Cim) and pyridylidene (Cpy) NHC-donors. Seven examples were characterized and include rhodium­(III) monomers of the general formula [Rh­(C,C′-im-py)­LxI2]z+ (2: z = 1, L = H2O or solvent, x = 2; 3, 5, 7: z = 0, L = carboxylate, x = 1) and novel rhodium­(II) dimers, the anti/syn-isomers of [Rh2(C,C′-im-py)2(μOAc)2I2] (4-anti/syn). The NMR data, backed by DFT calculations, is consistent with attribution of the C,C′-im-py ligand as a bis­(carbene) donor. Single crystal X-ray diffraction studies are reported for 2, 3, 4-anti, 4-syn and 7. Consistently, within the each complex, the Rh–Cim bond length is shorter than the Rh–Cpy bond length, which is the opposite trend to that expected based on simple electronic considerations. It is proposed that intramolecular steric interactions imposed by different rings in the rigid C,C′-im-py chelate ligand dictate the observed Rh–CNHC bond lengths. Attempts to add hydride to the C-metalated nicotinamide ring in 3 were unsuccessful. The redox behavior of 3 and 4 and, for comparison, an analogous bis­(imidazolylidene)­rhodium­(III) monomer (8), were characterized by cyclic voltammetry, electron paramagnetic resonance (EPR), and UV–vis spectroelectrochemistry. In 3 and 4, the C-metalated nicotinamide ring is found to exhibit a one-electron reduction process at far lower potential (−2.34 V vs. Fc+/Fc in acetonitrile) than the two-electron nicotinamide cation-dihydronicotinamide couple found for the corresponding nonmetalated ring (−1.24 V). The C,C′-ligand is electrochemically silent over a large potential range (from −2.3 V to the anodic solvent limit), thus for both 3 and 4 the first reduction processes are metal-centered. For 4-anti, the cyclic voltammetry and UV–vis spectrochemical results are consistent with a diamagnetic [Rh­(I)­Rh­(II)]2 tetrameric reduction product. Density functional theory (DFT) calculations were used to further probe the uptake of hydride ion by the nicotinamide ring, both before and after C-metalation. It is found that C-metalation significantly decreases the ability of the nicotinamide ring to take up hydride ion, which is attributed to the “carbene-like” character of a C-metalated pyridylidene ring.