Electronic Structure and Multicatalytic Features of Redox-Active Bis(arylimino)acenaphthene (BIAN)-Derived Ruthenium Complexes

The article examines the newly designed and structurally characterized redox-active BIAN-derived [Ru­(trpy)­(R-BIAN)­Cl]­ClO4 ([1a]­ClO4–[1c]­ClO4), [Ru­(trpy)­(R-BIAN)­(H2O)]­(ClO4)2 ([3a]­(ClO4)2–[3c]­(ClO4)2), and BIAO-derived [Ru­(trpy)­(BIAO)­Cl]­ClO4 ([2a]­ClO4) (trpy = 2,2′:6′,2′′-terpyridine, R-BIAN = bis­(arylimino)­acenaphthene (R = H (1a+, 3a2+), 4-OMe (1b+, 3b2+), 4-NO2 (1c+, 3c2+), BIAO = [N-(phenyl)­imino]­acenapthenone). The experimental (X-ray, 1H NMR, spectroelectrochemistry, EPR) and DFT/TD-DFT calculations of 1an–1cn or 2an collectively establish {RuII–BIAN0} or {RuII–BIAO0} configuration in the native state, metal-based oxidation to {RuIII–BIAN0} or {RuIII–BIAO0}, and successive electron uptake processes by the α-diimine fragment, followed by trpy and naphthalene π-system of BIAN or BIAO, respectively. The impact of the electron-withdrawing NO2 function in the BIAN moiety in 1c+ has been reflected in the five nearby reduction steps within the accessible potential limit of −2 V versus SCE, leading to a fully reduced BIAN4– state in [1c]4–. The aqua derivatives ({RuII–OH2}, 3a2+–3c2+) undergo simultaneous 2e–/2H+ transfer to the corresponding {RuIVO} state and the catalytic current associated with the RuIV/RuV response probably implies its involvement in the electrocatalytic water oxidation. The aqua derivatives (3a2+–3c2+) are efficient and selective precatalysts in transforming a wide variety of alkenes to corresponding epoxides in the presence of PhI­(OAc)2 as an oxidant in CH2Cl2 at 298 K as well as oxidation of primary, secondary, and heterocyclic alcohols with a large substrate scope with H2O2 as the stoichiometric oxidant in CH3CN at 343 K. The involvement of the {RuIVO} intermediate as the active catalyst in both the oxidation processes has been ascertained via a sequence of experimental evidence.

本研究考察了全新设计并经结构表征的氧化还原活性双(芳基亚氨基)苊（BIAN，bis(arylimino)acenaphthene）衍生配合物[Ru(trpy)(R-BIAN)Cl]ClO₄（[1a]ClO₄–[1c]ClO₄）、[Ru(trpy)(R-BIAN)(H₂O)](ClO₄)₂（[3a](ClO₄)₂–[3c](ClO₄)₂）以及[N-(苯基)亚氨基]苊酮（BIAO，[N-(phenyl)imino]acenapthenone）衍生配合物[Ru(trpy)(BIAO)Cl]ClO₄（[2a]ClO₄），其中trpy为2,2':6',2''-三联吡啶（2,2':6',2''-terpyridine），R-BIAN的取代基R分别为H（对应1a+、3a2+）、4-OMe（对应1b+、3b2+）、4-NO₂（对应1c+、3c2+）。
通过X射线衍射、氢核磁共振谱（¹H NMR）、光谱电化学、电子顺磁共振谱（EPR）实验手段以及密度泛函理论（DFT）/含时密度泛函理论（TD-DFT）计算，对1a–1c及2a类配合物开展系统表征后证实：基态下目标配合物均呈现{RuII–BIAN⁰}或{RuII–BIAO⁰}的电子构型；金属中心发生氧化时，构型转化为{RuIII–BIAN⁰}或{RuIII–BIAO⁰}；电子后续依次被α-二亚胺配体片段捕获，再分别被BIAN或BIAO配体中的三联吡啶（trpy）与萘π体系接收。
1c+中BIAN骨架上的吸电子硝基（NO₂）官能团，使其在相对于饱和甘汞电极（SCE）-2 V的可测电势范围内展现出五步相邻还原过程，最终在[1c]⁴–中得到完全还原的BIAN⁴–配体状态。
水合配合物（即{RuII–OH₂}型物种3a2+–3c2+）可同时经历2e–/2H+转移过程，转化为对应的{RuIV=O}物种；其与RuIV/RuV氧化还原对相关的催化电流表明，该活性物种可能参与电催化水氧化反应。
在298 K的二氯甲烷（CH₂Cl₂）体系中，以二乙酸碘苯（PhI(OAc)₂）为氧化剂，3a2+–3c2+可作为高效且选择性优异的预催化剂，将多种烯烃转化为对应环氧化物；在343 K的乙腈（CH₃CN）体系中，以过氧化氢（H₂O₂）为化学计量氧化剂时，该类配合物同样可高效催化一级、二级及杂环醇的氧化反应，底物适用范围广泛。
通过一系列实验证据证实，{RuIV=O}中间体是上述两类氧化反应中的活性催化物种。