Development of an Efficient and Durable Photocatalytic System for Hydride Reduction of an NAD(P)⁺ Model Compound Using a Ruthenium(II) Complex Based on Mechanistic Studies

Abstract

The mechanism of photocatalytic reduction of 1-benzylnicotinamidium cation (BNA(+)) to the 1,4-dihydro form (1,4-BNAH) using [Ru(tpy)(bpy)(L)](2+) (Ru-L(2+), where tpy = 2,2':6',2''-terpyridine, bpy = 2,2'-bipyridine, and L = pyridine and MeCN) as a photocatalyst and NEt(3) as a reductant has been clarified. On the basis of this mechanistic study, an efficient and durable photocatalytic system for selective hydride reduction of an NAD(P)(+) model compound has been developed. The photocatalytic reaction is initiated by the formation of [Ru(tpy)(bpy)(NEt(3))](2+) (Ru-NEt(3)(2+)) via the photochemical ligand substitution of Ru-L(2+). For this reason, the production rate of 1,4-BNAH using [Ru(tpy)(bpy)(MeCN)](2+) (Ru-MeCN(2+)) as a photocatalyst, from which the quantum yield of photoelimination of the MeCN ligand is greater than that of the pyridine ligand from [Ru(tpy)(bpy)(pyridine)](2+) (Ru-py(2+)), was faster than that using Ru-py(2+), especially in the first stage of the photocatalytic reduction. The photoexcitation of Ru-NEt(3)(2+) yields [Ru(tpy)(bpy)H](+) (Ru-H(+)), which reacts with BNA(+) to give 1:1 adduct [Ru(tpy)(bpy)(1,4-BNAH)](2+) (Ru-BNAH(2+)). In the presence of excess NEt(3) in the reaction solution, a deprotonation of the carbamoyl group in Ru-BNAH(2+) proceeds rapidly, mainly forming [Ru(tpy)(bpy)(1,4-BNAH-H(+))](+) (Ru-(BNAH-H(+))(+)). Although photocleavage of the adduct yields 1,4-BNAH and the cycle is completed by the re-coordination of a NEt(3) molecule to the Ru(II) center, this process competes with hydride abstraction from Ru-(BNAH-H(+))(+) by BNA(+) giving 1,4-BNAH and [Ru(tpy)(bpy)(BNA(+)-H(+))](2+). This adduct was observed as the major complex in the reaction solution after the photocatalysis was depressed and is a dead-end product because of its stability. Based on the information about the reaction mechanism and the deactivation process, we have successfully developed a new photocatalytic system using Ru-MeCN(2+) with 2 M of NEt(3) as a reductant, which could reduce more than 59 equivalent amounts of an NAD(P)(+) model, 1-benzyl-N,N-diethylnicotinamidium cation, selectively to the corresponding 1,4-dihydro form in a 6 x 10(-4) quantum yield using 436-nm light.

Photocatalytic Reduction of Artificial and Natural Nucleotide Co-factors with a Chlorophyll-Like Tin-Dihydroporphyrin Sensitizer

“…This value (calculated here as a lower limit by assuming only one mole of incident photons required per mole of product molecule formed) 47 is in the order of magnitude of other photocatalytic co-factor recycling systems reported recently, which were powered by blue-light radiation. 43,48 …”

Unusually Efficient Pyridine Photodissociation from Ru(II) Complexes with Sterically Bulky Bidentate Ancillary Ligands

“…This process occurs with Φ < 10 –5 for L = py but is orders of magnitude more efficient for L = CH 3 CN, with Φ = 0.006 (λ irr = 436 nm) under similar experimental conditions. 25 Due to the inefficiency and exhaustive photolysis required, applications involving the photodissociation of pyridine-containing ligands coordinated to Ru(II) has been largely impractical to date.…”

New RuII Complex for Dual Activity: Photoinduced Ligand Release and 1O2 Production

“…[23,28] For example,astarkdifferenceisobserved in the efficiency of the exchange of the Ll igand from [Ru(tpy)(bpy)(L)] 2 + (tpy = 2,2':6',2''terpyridine;L= CH 3 CN, py) in DMF,w ith quantum yields, F 436 , of 0.006 and < 10 À5 for L = CH 3 CN and py,r espectively. [29] In an effort to enhance the pyridine photodissociationp rocess, the bpy ligand was replaced by sterically demanding 6,6'-dimethyl-2,2'-bipyridine (Me 2 bpy) and biq, resulting in significantly greater quantum yields of photoinduced py exchange in [Ru-(tpy)(Me 2 bpy)(py)] 2 + and[ Ru(tpy)(biq)(py)] 2 + with CH 3 CN solvent, with F 500 values of 0.16(1) and 0.033(1), respectively. [30] The lower efficiency measuredf or [Ru(tpy)(biq)(py)] 2 + is attributed to the lower energy Ru!biq 3 MLCT state in this complex relative to the Ru!tpy 3 MLCT state in [Ru(tpy)(Me 2 bpy)(py)] 2 + , such that there is al arger energy gap for the thermalp opulation of the dissociative 3 LF state in the former.…”