pH-Dependent Monomer ↔ Oligomer Interconversion of Copper(II) Complexes with N-(2-R-imidazol-4-ylmethylidene)-2-aminoethylpyridine (R = Methyl, Phenyl)

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Abstract
The monomer ↔ oligomer interconversion of the reported metal complexes is generated by proton abstraction/supply as a common external information input. The mononuclear copper(II) complexes 1 and 2 with [CuCl2(HLn)] chemical formula have been prepared (HL1 = N-(2-methylimidazol-4-ylmethylidene)-2-aminoethylpyridine; HL2 = N-(2-phenylimidazol-4-ylmethylidene)-2-aminoethylpyridine). The crystal structures were determined. 1·H2O, C12H16N4OCl2Cu: a = 13.773(2) Å, b = 8.245(2) Å, c = 13.861(2) Å, β = 110.10(1)°, monoclinic, P21/n, and Z = 4. 2, C17H16N4Cl2Cu: a = 7.6659(7) Å, b = 16.287(1) Å, c = 14.103(1) Å, β = 95.058(7)°, monoclinic, P21/c, and Z = 4. Complexes 1·H2O and 2 assume a pentacoordinated square pyramidal geometry with a N3Cl2 donor set consisting of the nitrogen atoms of the protonated tridentate ligand and two chloride ions in the solid state, while in aqueous solution the Cu(II) ion is tetracoordinated (N3Ow donor set). When 1 and 2 are treated with an equimolar amount of sodium hydroxide or triethylamine, the deprotonation of the imidazole moiety promotes a self-assembly process, arising from coordination of the imidazolate nitrogen atom to a Cu(II) ion of an adjacent unit, to yield compounds 1 ‘·4H2O as the perchlorate salt, and 2 ‘ a·6H2O as the perchlorate salt and2 ‘ b as the hexafluorophosphate salt, respectively. 1 ‘·4H2O, C12H15N4O5ClCu: a = b = 13.966(2) Å, c = 33.689(3) Å, tetragonal, I41/a, and Z = 16. 2 ‘ a·6H2O, C51H51N12O15Cl3Cu3: a = 15.177(3) Å, b = 15.747(3) Å, c = 14.128(3) Å, α = 100.06(2)°, β = 110.37(2)°, γ = 63.54(1)°, triclinic, P1̄, and Z = 2. 2 ‘ b, C17H15N9F6PCu: a = b = 29.812(5) Å, c = 11.484(3) Å, trigonal, R3̄, and Z = 18. The nuclearity of the self-assembled molecules and their detailed structure were confirmed to be cyclic imidazolate-bridged tetranuclear for 1 ‘·4H2O and hexanuclear for 2 ‘ a·6H2O and 2 ‘ b, respectively, through single-crystal X-ray analyses and FAB-MS spectra. Variable-temperature experimental magnetic susceptibility data were well reproduced by using the Heisenberg model based on a cyclic tetranuclear structure for 1 ‘ and a cyclic hexanuclear structure for 2 ‘ a and 2 ‘ b. The reversible interconversion between the protonated monomeric and deprotonated oligomeric species were confirmed by pH-dependent potentiometric and electronic spectral titrations in aqueous solution, whereas the Pd(II) complex did not show a perfect disassembly process.

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