Synthesis and properties of [Ru(tpy)(4,4′-X2bpy)H]+ (tpy=2,2′:6′,2″-terpyridine, bpy=2,2′-bipyridine, X=H and MeO), and their reactions with CO2

A novel type of hydrido complex [Ru(tpy)(4,4′-X2bpy)H]+ (X=H and MeO) was synthesized. The stronger hydridic character of the complexes compared with [Ru(bpy)2(L)H]+ type complexes (L=CO, PPh3 and AsPh3) was demonstrated by the relatively high chemical shifts of RuH in the 1H NMR spectra and by higher reactivities with CO2. The reactions of [Ru(tpy)(4,4′-X2bpy)H]+ with CO2 occurred at second-order rate constants varying from (4.69±0.02) to (5.51±0.04)×10−3 M−1 s−1 depending on both solvent and X, giving the formato complexes [Ru(tpy)(4,4′-X2bpy)(OCHO)]+ quantitatively. The rate constant was increased with the increase of solvent acceptor number, and the reaction of [Ru(tpy){4,4′-(MeO)2bpy}H]+ with CO2 was found to be 3.6 times faster than that of [Ru(tpy)(bpy)H]+. These results suggest that nucleophilic attack of the hydride ligand to the carbon atom of CO2 is the rate determining step for the formation of the formato complex. The structure of the formato complex [Ru(tpy)(bpy)(OCHO)](PF6) was determined by X-ray crystallographic analysis.