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Synthesis of Binucleating Macrocycles and Their Nickel(II) Hydroxo- and Cyano-Bridged Complexes with Divalent Ions: Anatomical Variation of Ligand Features

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posted on 20.02.2016, 09:04 by Xiaofeng Zhang, Deguang Huang, Yu-Sheng Chen, R. H. Holm
The planar NNN-pincer complexes [MII(pyN2Me2)­(OH)]1– (MII = Ni, Cu) fix CO2 in η1-OCO2H complexes; results for the copper system are described. MnII, FeII, CoII, and ZnII behave differently, forming [MII(pyN2Me2)2]2– with N4O2 coordination. Incorporation of the NiII pincer into binucleating macrocycle 2 containing a triamino MII locus connected by two 1,3-biphenylene groups affords proximal NiII and MII sites for investigation of the synthesis, structure, and reactivity of Ni–X–M bridge units. This ligand structure is taken as a reference for variations in MII atoms and binding sites and bridges X = OH and CN to produce additional members of the macrocyclic family with improved properties. Macrocycle 2 with a 22-membered ring is shown to bind MII = Mn, Fe, and Cu with hydroxo bridges. Introduction of the 4-BuiO group (macrocycle 3) improves the solubility of neutral complexes such as those with NiII–OH–CuII and NiII–CN–FeII bridges. Syntheses of macrocycle 5 with a 7-Me-[12]­aneSN3 and macrocycle 6 with a 1,8-Me2-[14]­aneN4 MII binding site are described together with hydoxo-bridged Ni/Cu and cyano-bridged Ni/Fe complexes. This work was motivated by the presence of a Ni···(HO)–Fe bridge grouping in a reactive state of carbon monoxide dehydrogenase. Attempted decrease in Ni–(OH)–M distances (3.70–3.87 Å) to smaller values observed in the enzyme by use of macrocycle 4 having 1,2-biphenylene connectors led to a mononuclear octahedral NiII complex. Bridge structural units are summarized, and the structures of 14 macrocyclic complexes including 8 with bridges are described.