Ru(II) Complexes with Protic- and Anionic-Naked-NHC Ligands for Cooperative Activation of Small Molecules

A set of ruthenium(II)-protic-N-heterocyclic carbene complexes, [Ru(NNCH)(PPh3)2(X)]Cl (1, X=Cl and 2, X=H) and their deprotonated forms [Ru(NNC)(PPh3)2(X)] (1ʹ, X=Cl and 2ʹ, X=H) are reported where NNC is a new unsymmetrical pincer ligand. The four complexes are interconvertible by simple acid-base chemistry. The combined theoretical and spectroscopic investigations indicate charge segregation in anionic-NHC complexes (1ʹ and 2ʹ) and can be described from a Lewis pair perspective. The chemical reactivity of deprotonated complex 1ʹ shows cooperative small molecule activation. Complex 1ʹ activates H-H bond of hydrogen, C(sp3)-I bond of iodomethane, and C(sp)-H bond of phenylacetylene. The activation of CO2 using anionic NHC complex 1ʹ at moderate temperature and ambient pressure and subsequent conversion to formate is also described. All the new compounds have been characterized using ESI-MS, 1H, 13C, and 31P NMR spectroscopy. Molecular structures of 1, 2, and 2ʹ have also been determined with single-crystal X-ray diffraction. The cooperative small molecule activation perspective broadens the scope of potential applications of anionic-NHC complexes in small molecule activation, including the conversion of carbon dioxide to formate, a much sought after reaction in the renewable energy and sustainable development domains.