Bimetallic Complexes Supported by a Redox-Active Ligand with Fused Pincer-Type Coordination Sites

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• 作者：Denan Wang ; Sergey V. Lindeman ; Adam T. Fiedler
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：September 8, 2015
• 年：2015
• 卷：54
• 期：17
• 页码：8744-8754
• 全文大小：575K
• ISSN：1520-510X

文摘

The remarkable chemistry of mononuclear complexes featuring tridentate, meridionally chelating 鈥減incer鈥?ligands has stimulated the development of ligand frameworks containing multiple pincer sites. Here, the coordination chemistry of a novel pentadentate ligand (L^N3O2) that provides two closely spaced NNO pincer-type compartments fused together at a central diarylamido unit is described. The trianionic L^N3O2 chelate supports homobimetallic structures in which each M(II) ion (M = Co, Cu, Zn) is bound in a meridional fashion by the bridging diarylamido N atom and O,N-donors of the salicyaldimine arms. The metal centers are also coordinated by a mono- or bidentate auxiliary ligand (L^aux), resulting in complexes with the general form [M₂(L^N3O2)(L^aux)₂]⁺ (where L^aux = 1-methyl-benzimidazole (1MeBI), 2,2鈥?bipyridine (bpy), 4,4鈥?dibromo-2,2鈥?bipyridine (bpy^Br2), or (S)-2-(4-isopropyl-4,5-dihydrooxazolyl)pyridine (S-^iPrOxPy)). The fused nature of the NNO pincer sites results in short metal鈥搈etal distances ranging from 2.70 脜 for [Co₂(L^N3O2) (bpy)₂]⁺ to 3.28 脜 for [Zn₂(L^N3O2) (bpy)₂]⁺, as revealed by X-ray crystallography. The complexes possess C₂ symmetry due to the twisting of the aryl rings of the 渭-NAr₂ core; spectroscopic studies indicate that chiral L^aux ligands, such as S-^iPrOxPy, are capable of controlling the helical sense of the L^N3O2 scaffold. Since the four- or five-coordinate M(II) centers are linked solely by the amido moiety, each features an open coordination site in the intermetallic region, allowing for the possibility of metal鈥搈etal cooperativity in small-molecule activation. Indeed, the dicobalt(II) complex [Co₂(L^N3O2) (bpy^Br2)₂]⁺ reacts with O₂ to yield a dicobalt(III) species with a 渭-1,2-peroxo ligand. The bpy-containing complexes exhibit rich electrochemical properties due to multiple metal- and ligand-based redox events across a wide (3.0 V) potential window. Using electron paramagnetic resonance (EPR) spectroscopy and density functional theory (DFT), it was determined that one-electron oxidation of [Co₂(L^N3O2) (bpy)₂]⁺ results in formation of a S = 1/2 species with a L^N3O2-based radical coupled to low-spin Co(II) centers.