Zigzag-Chain and Cyclic-Tetrameric Compounds Derived by Deprotonation of Mononuclear Copper(II) Complexes with N,N‘-Bis(2-substituted-imidazol-4-ylmethyl- idene)-1,4-diaminobutane (2-Substituent = H, Me): Synthesis, Characterization, Structure, Substituent Effect, and Interconvertibility

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Abstract
Two copper(II) complexes with N,N'-bis(2-substituted-imidazol-4-ylmethylidene)-1,4-diaminobutane (2-substituent = H, 1; Me, 2) were synthesized, and their deprotonation assembly behavior was studied. X-ray analyses of 1 and 2 revealed that the molecules assume a butterfly shape bent by a line of N(2)-Cu-N(4) where the angle of N(3)-Cu-N(5) representing the extent of the molecular bend is 150.9(2) degrees and 105.66(8) degrees for 1 and 2, respectively. Due to the molecular bend, two imidazole moieties (sites A and B) are recognized as two inequivalent groups for the deprotonation process in which the imidazole group of site A is in an equatorial CuN(3) plane, while that of site B is in a CuN(2) plane bent from the equatorial coordination plane. 1 dissociates a proton of site B first to give an infinite zigzag-chain compound 1', while 2 dissociates a proton of site A first to give a cyclic-tetranuclear compound 2'. 2' undergoes a further deprotonation to give an electrically neutral di-deprotonated complex 2". The interconversion between the protonated monomer and the deprotonated oligomer/polymer was investigated by pH-dependent potentiometric and absorption spectrometric titrations.

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