Hazari Arijit Singha, Das Ankita, Ray Ritwika, Agarwala Hemlata, Maji Somnath, Mobin Shaikh M, Lahiri Goutam Kumar
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
‡Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Indore 452017, India.
Inorg Chem. 2015 May 18;54(10):4998-5012. doi: 10.1021/acs.inorgchem.5b00615. Epub 2015 Apr 30.
This article deals with a class of ruthenium-BIAN-derived complexes, [Ru(II)(tpm)(R-BIAN)Cl]ClO4 (tpm = tris(1-pyrazolyl)methane, R-BIAN = bis(arylimino)acenaphthene, R = 4-OMe ([1a]ClO4), 4-F ([1b]ClO4), 4-Cl ([1c]ClO4), 4-NO2 ([1d]ClO4)) and Ru(II)(tpm)(OMe-BIAN)H2O (3a2). The R-BIAN framework with R = H, however, leads to the selective formation of partially hydrolyzed BIAO ([N-(phenyl)imino]acenapthenone)-derived complex [Ru(II)(tpm)(BIAO)Cl]ClO4 ([2]ClO4). The redox-sensitive bond parameters involving -N═C-C═N- or -N═C-C═O of BIAN or BIAO in the crystals of representative [1a]ClO4, 3a2, or [2]ClO4 establish its unreduced form. The chloro derivatives 1a(+)-1d(+) and 2(+) exhibit one oxidation and successive reduction processes in CH3CN within the potential limit of ±2.0 V versus SCE, and the redox potentials follow the order 1a(+) < 1b(+) < 1c(+) < 1d(+) ≈ 2(+). The electronic structural aspects of 1a(n)-1d(n) and 2(n) (n = +2, +1, 0, -1, -2, -3) have been assessed by UV-vis and EPR spectroelectrochemistry, DFT-calculated MO compositions, and Mulliken spin density distributions in paramagnetic intermediate states which reveal metal-based (Ru(II) → Ru(III)) oxidation and primarily BIAN- or BIAO-based successive reduction processes. The aqua complex 3a(2+) undergoes two proton-coupled redox processes at 0.56 and 0.85 V versus SCE in phosphate buffer (pH 7) corresponding to {Ru(II)-H2O}/{Ru(III)-OH} and {Ru(III)-OH}/{Ru(IV)═O}, respectively. The chloro (1a(+)-1d(+)) and aqua (3a(2+)) derivatives are found to be equally active in functioning as efficient precatalysts toward the epoxidation of a wide variety of alkenes in the presence of PhI(OAc)2 as oxidant in CH2Cl2 at 298 K, though the analogous 2(+) remains virtually inactive. The detailed experimental analysis with the representative precatalyst 1a(+) suggests the involvement of the active {Ru(IV)═O} species in the catalytic cycle, and the reaction proceeds through the radical mechanism, as also supported by the DFT calculations.
本文研究了一类钌 - BIAN 衍生的配合物，[Ru(II)(tpm)(R - BIAN)Cl]ClO₄（tpm = 三(1 - 吡唑基)甲烷，R - BIAN = 双(芳基亚氨基)苊，R = 4 - OMe（[1a]ClO₄）、4 - F（[1b]ClO₄）、4 - Cl（[1c]ClO₄）、4 - NO₂（[1d]ClO₄））以及Ru(II)(tpm)(OMe - BIAN)H₂O（3a₂）。然而，当 R = H 时，R - BIAN 骨架会导致选择性形成部分水解的 BIAO（[N - (苯基)亚氨基]苊酮）衍生的配合物[Ru(II)(tpm)(BIAO)Cl]ClO₄（[2]ClO₄）。在代表性的[1a]ClO₄、3a₂或[2]ClO₄晶体中，涉及 BIAN 或 BIAO 的 -N═C - C═N - 或 -N═C - C═O 的氧化还原敏感键参数确定了其未还原形式。氯代衍生物 1a(+) - 1d(+)和 2(+)在 CH₃CN 中相对于 SCE 在±2.0 V 的电位范围内表现出一个氧化和连续还原过程，氧化还原电位顺序为 1a(+) < 1b(+) < 1c(+) < 1d(+) ≈ 2(+)。通过紫外 - 可见和电子顺磁共振光谱电化学、密度泛函理论计算的分子轨道组成以及顺磁中间态的 Mulliken 自旋密度分布对 1a(n) - 1d(n)和 2(n)（n = +2、+1、0、 - 1、 - 2、 - 3）的电子结构方面进行了评估，结果揭示了基于金属（Ru(II)→Ru(III)）的氧化以及主要基于 BIAN 或 BIAO 的连续还原过程。水合配合物 3a(2+)在磷酸盐缓冲液（pH 7）中相对于 SCE 分别在 0.56 和 0.85 V 处经历两个质子耦合的氧化还原过程，分别对应{Ru(II) - H₂O}/{Ru(III) - OH}和{Ru(III) - OH}/{Ru(IV)═O}。发现在 298 K 下于 CH₂Cl₂中，当以 PhI(OAc)₂作为氧化剂时，氯代（1a(+) - 1d(+)）和水合（3a(2+)）衍生物作为各种烯烃环氧化的高效预催化剂具有同等活性，尽管类似的 2(+)实际上仍然无活性。对代表性预催化剂 1a(+)的详细实验分析表明催化循环中涉及活性{Ru(IV)═O}物种，并且反应通过自由基机理进行，密度泛函理论计算也支持这一点。
