Charge-Delocalized κC-2,N-NHC-Amine Complexes of Rhodium, Iridium, and Ruthenium

The development of a novel set of complexes bearing an NHC-amine ligand (C-NHC-NH2) is described. M(cod) complexes (M = 1r, Rh) and a Ru complex have been synthesized in which three different coordination modes of the ligand were established: monodentate, neutral bidentate, and anionic bidentate. The anionic bidentate coordination mode of the anionic C-NHC-NH- ligand arises from deprotonation of the amine moiety of the neutral C-NHC-NH2 ligand. Ligand deprotonation proved to be reversible for the Rh and 1r complexes, as was shown by subsequent treatment of the complexes with base and acid. The structural parameters of these differently coordinated ligands were examined, and it was shown that the conjugation of the aniline ring plays a major role in determining the ligand properties. Structural parameters derived from DFT calculations confirm delocalization of the anionic charge over the ligand framework, as is clear from a comparison of the (hypothetical) neutral bidentate complexes [M(cod)(kappa C-2,N-{C-NHC-NH2})](+) with those of the (synthesized) monoanionic complexes [M(cod)(kappa C-2,N-{C-NHC-NH})] (M = Rh, Ir). A similar trend in the structure and bond lengths of the aniline rings was found in the solid-state structure of the novel dimeric complex [(Ru(kappa C-2,N-{C-NHC-NH})(kappa C-2,N-{C-NHC-NH2})Cl)2(mu-Cl)](PF6). The octahedral d5 ruthenium(III) centers in this complex both contain a neutral bidentate C-NHC-NH2 ligand as well as an anionic bidentate ligand. Quite remarkably, the complex is diamagnetic, arising from antiferromagnetic coupling of the two low-spin ruthenium(III) centers over the chloride linker. DFT calculations indeed confirm that the open-shell singlet electronic structure is most stable.