Photochemical Properties and Reactivity of a Ru Compound Containing an NAD/NADH-Functionalized 1,10-Phenanthroline Ligand

An
NAD/NADH-functionalized ligand, benzo­[b]­pyrido­[3,2-f]­[1,7]-phenanthroline (bpp), was newly synthesized. A Ru
compound containing the bpp ligand, [Ru­(bpp)­(bpy)2]2+, underwent 2e– and 2H+ reduction,
generating the NADH form of the compound, [Ru­(bppHH)­(bpy)2]2+, in response to visible light irradiation in CH3CN/TEA/H2O (8/1/1). The UV–vis and fluorescent
spectra of both [Ru­(bpp)­(bpy)2]2+ and [Ru­(bppHH)­(bpy)2]2+ resembled the spectra of [Ru­(bpy)3]2+. Both complexes exhibited strong emission, with quantum
yields of 0.086 and 0.031, respectively; values that are much higher
than those obtained from the NAD/NADH-functionalized complexes [Ru­(pbn)­(bpy)2]2+ and [Ru­(pbnHH)­(bpy)2]2+ (pbn = (2-(2-pyridyl)­benzo­[b]-1.5-naphthyridine,
pbnHH = hydrogenated form of pbn). The reduction potential of the
bpp ligand in [Ru­(bpp)­(bpy)2]2+ (−1.28
V vs SCE) is much more negative than that of the pbn ligand in [Ru­(pbn)­(bpy)2]2+ (−0.74 V), although the oxidation potentials
of bppHH and pbnHH are essentially equal (0.95 V). These results indicate
that the electrochemical oxidation of the dihydropyridine moiety in
the NADH-type ligand was independent of the π system, including
the Ru polypyridyl framework. [Ru­(bppHH)­(bpy)2]2+ allowed the photoreduction of oxygen, generating H2O2 in 92% yield based on [Ru­(bppHH)­(bpy)2]2+. H2O2 production took place via singlet oxygen
generated by the energy transfer from excited [Ru­(bppHH)­(bpy)2]2+ to triplet oxygen.

本研究新合成了一种经烟酰胺腺嘌呤二核苷酸/烟酰胺腺嘌呤二核苷酸还原态（NAD/NADH）修饰的配体：苯并[b]吡啶并[3,2-f][1,7]菲咯啉（benzo[b]pyrido[3,2-f][1,7]-phenanthroline，简称bpp）。一种含bpp配体的钌（Ru）配合物[Ru(bpp)(bpy)₂]²⁺（bpy为2,2'-联吡啶（2,2'-bipyridine）的缩写），在乙腈（CH₃CN）/三乙胺（TEA）/水（H₂O）体系（体积比8/1/1）中经可见光照射后，可发生2电子（2e⁻）与2质子（2H⁺）的还原反应，生成该配合物的NADH型形式[Ru(bppHH)(bpy)₂]²⁺。
[Ru(bpp)(bpy)₂]²⁺与[Ru(bppHH)(bpy)₂]²⁺的紫外-可见光谱及荧光光谱均与三(2,2'-联吡啶)合钌(II)（[Ru(bpy)₃]²⁺）的光谱相似。两种配合物均具有较强的发射特性，其量子产率分别为0.086与0.031，远高于经NAD/NADH修饰的配合物[Ru(pbn)(bpy)₂]²⁺和[Ru(pbnHH)(bpy)₂]²⁺的量子产率（其中pbn为2-(2-吡啶基)苯并[b]-1,5-萘啶，pbnHH为pbn的氢化形式）。
[Ru(bpp)(bpy)₂]²⁺中bpp配体的还原电位为-1.28 V（相对于饱和甘汞电极（SCE）），远低于[Ru(pbn)(bpy)₂]²⁺中pbn配体的还原电位（-0.74 V）；尽管bppHH与pbnHH的氧化电位基本相等（0.95 V）。上述结果表明，NADH型配体中二氢吡啶结构域的电化学氧化与包括钌多吡啶骨架在内的π共轭体系相互独立。
[Ru(bppHH)(bpy)₂]²⁺可实现氧气的光还原，以[Ru(bppHH)(bpy)₂]²⁺为基准计，过氧化氢（H₂O₂）的生成产率可达92%。H₂O₂的生成路径为：激发态的[Ru(bppHH)(bpy)₂]²⁺通过能量转移将三线态氧（基态氧气）转化为单线态氧，进而生成H₂O₂。