Magnetic Ordering in Two Molecule-Based (10,3)-a Nets Prepared from a Copper(II) Trinuclear Secondary Building Unit

Two new molecule-based materials of formulas 3D-{[K(H2O)(6)](0.5)[K(18-crown-6)](0.5)[MnCu3(Hmesox)(3)]center dot 5.25H(2)O} (1) and 3D-{(Ph4P)(2)[MnCu3(Hmesox)(3)Cl]center dot 3.5H(2)O} (2) have been prepared from a tricopper(II) secondary building unit (SBU), [Cu-3(Hmesox)(3)](3-) (H(4)mesox = mesoxalic acid, 2-dihydroxymalonic acid). Compound 1 is obtained by means of the reaction of the SBU with manganese(II) acetate in the presence of potassium cations and the 18-crown-6 ether, whereas compound 2 is obtained by means of the reaction of the SBU with manganese(II) acetate in the presence of Ph4PCl. The [MnCu3(Hmesox)(3)](-) and [MnCu3(Hmesox)(3)Cl](2-) moieties in compounds 1 and 2, respectively, yield chiral 3-connected three-dimensional (3D) anionic (10,3)-a (srs, SrSi2) nets. In the cubic and centrosymmetric structures (Pa (3) over bar) of 1, two inversion-symmetry-related anion nets interpenetrate to a racemic structure. The Ph4P+ cations in 2 are organized in a supramolecular (10,3)-a net through the 6-fold phenyl embrace. In 2, both the cationic and anionic nets are homochiral and enantiopure with opposite handedness and form interpenetrating supramolecular and covalent (10,3)-a nets in the noncentrosymmetric Sohncke space group P 2(1)2(1)2(1). Both compounds display ferrimagnetic interaction with long-range magnetic ordering below 2.5 and 15.2 K for 1 and 2, respectively. A dehydrated phase of 2 exhibits a T-c of 21.8 K. The saturation of magnetization, M-s, indicates two different ground states, S = 1/2 and 3/2, for the tricopper(II) units in 1 and 2, respectively. The different spin states of the tricopper(II) unit in 1 and 2 has been explained by means of a density functional theory (DFT) study performed in the [Cu-3(Hmesox)(3)](3-) and [Cu-3(Hmesox)(3)Cl](4-) fragments, for 1 and 2, respectively. A further DFT study has allowed one to analyze the structural parameters that lead to the different spin ground states for the trinuclear units in both compounds.