Secondary Sphere Hydrogen Bonding in Monocopper Complexes of Potentially Dinucleating Bis(carboxamide) Ligands

Reaction of a macrocyclic ligand precursor comprising two bis(carboxamido)pyridine units (H₄L4) connected by ethylene linkers with NMe₄OH and CuX₂ (X = Cl, OAc, or OTf) yielded monocopper complexes [NMe₄][(H₂L4)Cu(X)] [X = Cl (3), OAc (4), or OH (5)], in contrast to the results of previous work on a related ligand with ortho-phenylene linkers wherein dicopper compounds were isolated. X-ray structures of the complexes revealed hydrogen bonding from the free carboxamide N-H groups in the doubly protonated form of the ligand (H₂L4^2-) to the monodentate fourth ligand coordinated to the Cu^II ion. Similar secondary sphere hydrogen bonding interactions were identified in multinuclear compounds [NMe₄]₂[{(H₂L4)Cu}_n(CO₃)] (n = 2 or 3) that were isolated from exposure of 5 to air. Cyclic voltammetry revealed oxidations of 3 and 5 at potentials about 300 mV higher than those of analogous monocopper complexes of bis(arylcarboxamido)pyridine ligands, which lack the intramolecular hydrogen bonds, consistent with removal of electron density from the metal center by the hydrogen bonding array. Another ligand variant (H₄L5) with ortho-phenylene linkers and only one bis(carboxamido)pyridine moiety yielded monocopper complexes [NMe₄][(H₂L5)Cu(OAc)]·DMF (8) and [NMe₄][(H₂L5)CuCl]·CH₃CN (9), but the X-ray structures revealed a different hydrogen bonding arrangement to the solvate molecules. Nonetheless, a high redox potential for 9 was observed, consistent with intramolecular hydrogen bonding interactions in solution.