Leipzig Benjamin K, Rees Julian A, Kowalska Joanna K, Theisen Roslyn M, Kavčič Matjaž, Poon Penny Chaau Yan, Kaminsky Werner, DeBeer Serena, Bill Eckhard, Kovacs Julie A
The Department of Chemistry, University of Washington , Box 351700, Seattle, Washington 98195-1700, United States.
Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
Inorg Chem. 2018 Feb 19;57(4):1935-1949. doi: 10.1021/acs.inorgchem.7b02748. Epub 2018 Feb 7.
Considerable effort has been devoted to the development of first-row transition-metal catalysts containing redox-active imino-pyridine ligands that are capable of storing multiple reducing equivalents. This property allows abundant and inexpensive first-row transition metals, which favor sequential one-electron redox processes, to function as competent catalysts in the concerted two-electron reduction of substrates. Herein we report the syntheses and characterization of a series of iron complexes that contain both π-donating thiolate and π-accepting (α-imino)-N-heterocycle redox-active ligands, with progressively larger N-heterocycle rings (imidazole, pyridine, and quinoline). A cooperative interaction between these complementary redox-active ligands is shown to dictate the properties of these complexes. Unusually intense charge-transfer (CT) bands, and intraligand metrical parameters, reminiscent of a reduced (α-imino)-N-heterocycle ligand (L), initially suggested that the electron-donating thiolate had reduced the N-heterocycle. Sulfur K-edge X-ray absorption spectroscopic (XAS) data, however, provides evidence for direct communication, via backbonding, between the thiolate sulfur and the formally orthogonal (α-imino)-N-heterocycle ligand π*-orbitals. DFT calculations provide evidence for extensive delocalization of bonds over the sulfur, iron, and (α-imino)-N-heterocycle, and TD-DFT shows that the intense optical CT bands involve transitions between a mixed Fe/S donor, and (α-imino)-N-heterocycle π*-acceptor orbital. The energies and intensities of the optical and S K-edge pre-edge XAS transitions are shown to correlate with N-heterocycle ring size, as do the redox potentials. When the thiolate is replaced with a thioether, or when the low-spin S = 0 Fe(II) is replaced with a high-spin S = 3/2 Co(II), the N-heterocycle ligand metrical parameters and electronic structure do not change relative to the neutral L ligand. With respect to the development of future catalysts containing redox-active ligands, the energy cost of storing reducing equivalents is shown to be lowest when a quinoline, as opposed to imidazole or pyridine, is incorporated into the ligand backbone of the corresponding Fe complex.
人们付出了巨大努力来开发含氧化还原活性亚氨基吡啶配体的第一排过渡金属催化剂，这些配体能够储存多个还原当量。这一特性使丰富且廉价的第一排过渡金属（其有利于连续的单电子氧化还原过程）能够在底物的协同双电子还原反应中作为有效催化剂发挥作用。在此，我们报告了一系列铁配合物的合成与表征，这些配合物同时含有供π电子的硫醇盐和吸π电子的（α-亚氨基）-N-杂环氧化还原活性配体，且N-杂环的环逐渐增大（咪唑、吡啶和喹啉）。结果表明，这些互补的氧化还原活性配体之间的协同相互作用决定了这些配合物的性质。异常强烈的电荷转移（CT）带以及配体内的度量参数，让人联想到还原的（α-亚氨基）-N-杂环配体（L），最初表明供电子的硫醇盐使N-杂环发生了还原。然而，硫K边X射线吸收光谱（XAS）数据提供了证据，表明通过反馈键合，硫醇盐硫与形式上正交的（α-亚氨基）-N-杂环配体π轨道之间存在直接相互作用。密度泛函理论（DFT）计算为键在硫、铁和（α-亚氨基）-N-杂环上的广泛离域提供了证据，含时密度泛函理论（TD-DFT）表明强烈的光学CT带涉及混合的Fe/S供体与（α-亚氨基）-N-杂环π受体轨道之间的跃迁。光学和S K边预边XAS跃迁的能量和强度与N-杂环的环大小相关，氧化还原电位也是如此。当硫醇盐被硫醚取代时，或者当低自旋S = 0的Fe(II)被高自旋S = 3/2的Co(II)取代时，N-杂环配体的度量参数和电子结构相对于中性L配体没有变化。关于未来含氧化还原活性配体催化剂的开发，结果表明，当将喹啉而非咪唑或吡啶引入相应铁配合物的配体主链时，储存还原当量的能量成本最低。
