Dicopper(II) Metallacyclophanes with <i>N,N</i>′‑2,6-Pyridinebis(oxamate): Solution Study, Synthesis, Crystal Structures, and Magnetic Properties

The complexing ability of copper­(II) in solution by the ligand <i>N,N</i>′-2,6-pyridinebis­(oxamic acid) (H₄mpyba, H₄L) was determined through potentiometric and UV–vis spectroscopy at 25 °C and 0.15 M NaCl. The logarithms of the equilibrium constants for its copper­(II) complexes according to the eqs 2H₂L + 2Cu ⇆ [Cu₂(H₂L)₂], 2H₂L + 2Cu ⇆ [Cu₂(H₂L) (HL)] + H, 2H₂L + 2Cu ⇆ [Cu₂(HL)₂] + 2H, 2H₂L + 2Cu ⇆ [Cu₂(HL)­(L)] + 3H, and 2H₂L + 2Cu ⇆ [Cu₂L₂] + 4H were 12.02(7), 8.04(5), 1.26(6), −7.51(6), and −16.36(6), respectively. The knowledge of the solution behavior has supported the synthesis of three new compounds bearing the common building block Cu₂L₂^4–. Their formulas are (Me₄N)₄[Cu₂(mpyba)₂(H₂O)₂]·H₂O (<b>1</b>), (Me₄N)₄[K₂Na₂Cu₄(mpyba)₄(H₂O)_6.8]·1.6H₂O (<b>2</b>), and [Na₆Cu₂(mpyba)₂Cl₂(H₂O)₈]·7H₂O (<b>3</b>) (Me₄N⁺ = tetramethylammonium cation). The [Cu₂(mpyba)₂(H₂O)₂]^4– tetraanionic unit, which is present in <b>1</b>, has a [3,3] metallacyclophane-type motif connected by two N–Cu–N bonds. In <b>2</b>, a heterotrimetallic decanuclear nanocage is formed through front-to-front assembly of two [Cu₂(mpyba)₂]^4– units, which also coordinate to potassium­(I) and sodium­(I) cations by means of carboxylate oxygens from oxamate. The structure of <b>3</b> consists of heterobimetallic layers of formula [Na₆Cu₂(mpyba)₂Cl₂(H₂O)₈] and crystallization water molecules, which are interlinked by hydrogen bonds leading to a supramolecular three-dimensional network. The investigation of the magnetic properties of <b>1</b>–<b>3</b> in the temperature range 1.9–300 K shows the occurrence of ferromagnetic interactions between the dicopper­(II) metallacyclophane unit [<i>J</i> = +6.85 (<b>1</b>), +7.40 (<b>2</b>), and +7.90 cm^–1 (<b>3</b>); <i><b>H</b></i> = −<i>J<b>S</b></i>_Cu1·<i><b>S</b></i>_Cu2, where <i>S</i>_Cu1 = <i>S</i>_Cu2 = 1/2]. Theoretical calculations on <b>1</b>–<b>3</b> were carried to substantiate the nature and magnitude of the involved magnetic interactions and to support the occurrence of a spin polarization mechanism accounting for them