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

The planar NNN-pincer complexes [M^II(pyN₂^Me2)­(OH)]^1– (M^II = Ni, Cu) fix CO₂ in η¹-OCO₂H complexes; results for the copper system are described. Mn^II, Fe^II, Co^II, and Zn^II behave differently, forming [M^II(pyN₂^Me2)₂]^2– with N₄O₂ coordination. Incorporation of the Ni^II pincer into binucleating macrocycle 2 containing a triamino M^II locus connected by two 1,3-biphenylene groups affords proximal Ni^II and M^II 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 M^II 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 M^II = 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 Ni^II–OH–Cu^II and Ni^II–CN–Fe^II bridges. Syntheses of macrocycle 5 with a 7-Me-[12]­aneSN₃ and macrocycle 6 with a 1,8-Me₂-[14]­aneN₄ M^II 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 Ni^II complex. Bridge structural units are summarized, and the structures of 14 macrocyclic complexes including 8 with bridges are described.