Transition Metal Complexes Coordinated by an NAD(P)H Model Compound and their Enhanced Hydride-Donating Abilities in the Presence of a Base
✍ Scribed by Atsuo Kobayashi; Hideo Konno; Kazuhiko Sakamoto; Akiko Sekine; Yuji Ohashi; Masashi Iida; Osamu Ishitani
- Publisher
- John Wiley and Sons
- Year
- 2005
- Tongue
- English
- Weight
- 367 KB
- Volume
- 11
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
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+^ (1 a) and [Re(bpy)(CO)~3~(BNAH)]^+^ (1 b), were quantitatively produced by the reaction of the corresponding metal hydrido complexes with BNA^+^ (1‐benzylnicotinamidium cation). In the presence of base with p__K__~a~=8.9, 1 a and 1 b have much greater reducing power than “free” BNAH. The oxidation potentials of 1 a 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 1 a and 1 b 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^+^)] (3 b); this deprotonation underlies the enhancement in reducing ability. The deprotonated forms 3 a and 3 b 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+^ (2 a) and [Re(bpy)(CO)~3~(BNA^+^−H^+^)]^+^ (2 b); “free” BNAH and the protonated adducts 1 a and 1 b cannot act in this way. X‐ray crystallography was performed on the PF~6~^−^ salt of 2 a, 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) Å. Infrared spectral data suggested that the deprotonated carbamoyl group on the reduced forms 3 a and 3 b is converted to the imido group, and that the oxygen atom coordinates to the metal center.