Crystal Engineering Applied to Modulate the Structure and Magnetic Properties of Oxamate Complexes Containing the [Cu(bpca)]⁺ Cation

This work deals with the crystal engineering features of four related copper­(II)-based compounds with formulas {[{Cu­(bpca)}₂­(H₂ppba)]·1.33DMF­·0.66DMSO}_n (2), [{Cu­(bpca)­(H₂O)}₂­(H₂ppba)] (3), [{Cu­(bpca)}₂­(H₂ppba)]­·DMSO (4), and [{Cu­(bpca)}₂­(H₂ppba)]­·6H₂O (5) [H₄ppba = N,N′-1,4-phenylenebis­(oxamic acid) and Hbpca = bis­(2-pyridylcarbonyl)­amide] and how their distinct molecular and crystal structures translate into their different magnetic properties. 2 and 3 were obtained through the hydrolytic reaction of the double-stranded oxamato-based dipalladium­(II) paracyclophane precursor of formula [{K₄(H₂O)₂}­{Pd₂(ppba)₂}] (1) with the mononuclear copper­(II) complex [Cu­(bpca)­(H₂O)₂]⁺, either in a water–DMSO–DMF solvent mixture or in water, respectively. The straightforward reaction of the neutral H₄ppba molecule with [Cu­(bpca)­(H₂O)₂]⁺ in a water–DMSO mixture afforded compound 4, whereas compound 5 resulted from the reaction between the copper­(II) complex and the K₂ppba salt in water. The [Pd₂(ppba)₂]^4– tetraanionic unit which is present in 1 has a [3,3] metallacyclophane-type motif connected by two N–Pd–N bonds. This entity acts as a ligand toward partially hydrated potassium­(I) cations through its outer oxamate oxygens leading to a neutral three-dimensional network. The structure of 2 consists of neutral chains made up of double oxo­(carboxylate-oxamate)-bridged di­[{bis­(2-pyridylcarbonyl)­amidate}­copper­(II)] units are which connected by the extended H₂ppba^2– ligand, each of its oxamate fragments adopting a bidentate/outer monodentate coordination mode. Compounds 2–5 are neutral and centrosymmetric dicopper­(II) complexes which have in common the presence of peripheral bpca ligands and H₂ppba^2– as a bridge with each of its monoprotonated oxamate groups exhibiting rare monodentate (3) and bis-bidentate (4 and 5) coordination modes. Compounds 2, 4, and 5 share the same basic [{Cu­(bpca)}₂­(H₂ppba)] unit, but besides the difference in the cocrystallization solvent molecules and synthetic strategies, they feature very different crystal structures. To better understand the role of palladium­(II) ions in the formation of 2 and 3, some studies were carried out using different mixtures of solvents such as water, DMSO, and DMF which revealed a major importance of DMF in the formation of 2 and the dependence on the palladium­(II) ions in the formation of 3. A reaction pathway leading to the formation of 2 and 3 is then proposed. The variable-temperature (2.0–300 K) magnetic susceptibility measurements of 2, 4, and 5 revealed the occurrence of weak ferro- [J = +0.70 cm^–1 (2)] and antiferromagnetic interactions [J = −0.90 (4) and −0.79 cm^–1 (5)], the spin Hamiltonian being defined as H = −JS₁·S₂. The different nature and strength of the magnetic coupling along this unique series of compounds are discussed in the light of the structural data, and they are compared with those of related dicopper­(II) systems.