Transition Metal Complexes Coordinated by an NAD(P)H Model Compound and their Enhanced Hydride-Donating Abilities in the Presence of a Base

The ruthenium(II) and rhenium(I) complexes containing an NAD(P)H model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), as ligand, [Ru(tpy)(bpy)(BNAH)] 2 + (1a) and [Re(bpy)(CO) 3 (BNAH)] + (1b), were quantitatively produced by the reaction of the corresponding metal hydrido complexes with BNA + (1-benzyl-nicotinamidium cation). In the presence of base with pK a =8.9, la and 1b have much greater reducing power than "free" BNAH. The oxidation potentials of la in the absence and the presence of triethylamine were 0.55 V and -0.04 V, respectively, versus Ag/ AgNO 3 , whereas that of "free" BNAH was 0.30 V. Spectroscopic results clearly showed that the base extracts a proton from the carbamoyl group on 1a and 1b to give the deprotonated BNAH coordinating to the transition-metal complexes [Ru(tpy)(bpy)-(BNAH-H + )] + (3 a) and [Re-(bpy)(CO) 3 (BNAH-H + )] (3b); this deprotonation underlies the enhancement in reducing ability. The deprotonated forms 3a and 3b can efficiently reduce other NAD(P) models to give the corresponding 1,4-dihydro form, resulting in the deprotonated BNA + being coordinated to the metal complexes [Ru(tpy)(bpy)(BNA + -H + )] 2 + (2a) and [Re(bpy)(CO) 3 (BNA + -H + )] + (2b); "free" BNAH and the protonated adducts la and 1b cannot act in this way. X-ray crystallography was performed on the PF 6 - salt of 2a, and showed that the deprotonated nitrogen atom on the carbamoyl group coordinates to the ruthenium(II) metal center with a bond length of 2.086(3) A. Infrared spectral data suggested that the deprotonated carbamoyl group on the reduced forms 3a and 3b is converted to the imido group, and that the oxygen atom coordinates to the metal center.