New pyridine carboxamideligands and their complexation to copper(ii). X-Ray crystal structures of mono-, di, tri- and tetranuclear copper complexes

Abstract

Seven new pyridine dicarboxamide ligands H₂L^1–7 have been synthesised from condensation reactions involving pyridine-2,6-dicarboxylic acid (H₂dipic), pyridine-2,6-dicarbonyl dichloride or 2,6-diaminopyridine with heterocyclic amine or carboxylic acid precursors. Crystallographic analyses of N,N′-bis(2-pyridyl)pyridine-2,6-dicarboxamide monohydrate (H₂L⁸·H₂O), N,N′-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide and N,N′-bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide monohydrate revealed extensive intramolecular hydrogen bonding interactions. 2,6-Bis(pyrazine-2-carboxamido)pyridine (H₂L⁶) and 2,6-bis(pyridine-2-carboxamido)pyridine (H₂L⁷) reacted with copper(II) acetate monohydrate to give tricopper(II) complexes [Cu₃(L)₂(μ₂-OAc)₂]. X-Ray crystallography confirmed deprotonation of the amidic nitrogen atoms and that the (L^6,7)²⁻ ligands and acetate anions hold three copper(II) ions in approximately linear fashion. H₂L⁸. Reacted with copper(II) tetrakis(pyridine) perchlorate to give [Cu(L⁸)(OH₂)]₂·2H₂O, in which (L⁸)²⁻ was tridentate through the nitrogen atoms of the central pyridine ring and the deprotonated carboxamide groups at one copper centre, with one of the terminal pyridyl rings coordinating to the other copper atom in the dimer. The corresponding reaction using H₂L⁷ gave [Cu₃(L⁷)₂(py)₂][ClO₄]₂, which transformed during an attempted recrystallisation from ethanol under aerobic conditions to a tetracopper(II) complex [Cu₄(L⁷)₂(L⁷–O)₂].