Ru(II)-Anionic-Naked-NHC Complexes as Cooperative Lewis Pairs: Parallels in Structure and FLP-type Chemical Reactivity

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)] (1', X=Cl and 2', 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 (1' and 2') and can be described from a Lewis pair perspective. The chemical reactivity of deprotonated complex 1' shows parallels with the FLP chemistry. Complex 1' 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 1' 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. Structures of 1, 2, and 2' have also been determined with single-crystal X-ray diffraction. The FLP-type (cooperative Lewis pair) perspective broadens the scope of potential applications of anionic-NHC complexes in small molecule activation.