Influence of the Synthetic Conditions on the Structural Diversity of Extended Manganese−Oxalato−1,2-bis(4-pyridyl)ethylene Systems

We report herein the synthesis and physicochemical characterization of eight new manganese−oxalato compounds with 1,2-bis(4-pyridyl)ethylene (bpe): {(Hbpe)2[Mn2(μ-ox)3]·∼0.8(C2H5OH)·∼0.4(H2O)}n (1), {[Mn(μ-ox)(μ-bpe)]·xH2O}n (2), [Mn2(μ-ox)2(μ-bpe)(bpe)2]n (3), [Mn(μ-ox)(μ-bpe)]n (4a and 4b), and {[Mn4(μ-ox)3(μ-bpe)4(H2O)4]·(X)2·mY}n with X = NO3− (5a), Br− (5b), and ClO4− (5c) and Y = solvation molecules. The appropriate selection of the synthetic conditions allowed us to control the crystal structure and to design extended 2D and 3D frameworks. Compound 1 is obtained at acid pH values and its crystal structure consists of stacked [Mn2(μ-ox)3]2− layers with cationic Hbpe+ molecules intercalated among them. Compound 2 was obtained at basic pH values with a manganese/bpe ratio of 1:1, and the resulting 3D structure consists of an interpenetrating framework in which metal−oxalato chains are bridged by bpe ligands, leading to a microporous network that hosts a variable number of water molecules (between 0 and 1) depending on the synthetic conditions. Compound 3, synthesized with a manganese/bpe ratio of 1:3, shows a 2D framework in which linear metal−oxalato chains are joined by bis-monodentate 1,2-bis(4-pyridyl)ethylene ligands. The thermal treatment of compound 3 permits the release of one of the bpe molecules, giving rise to two new 2D crystalline phases of formula [Mn(μ-ox)(μ-bpe)]n (4a and 4b) depending on the heating rate. The open structures of 5a−5c were synthesized in a medium with a high concentration of nitrate, perchlorate, or bromide salts (potassium or sodium as cations). These anions behave as templating agents directing the crystal growing toward a cationic porous network, in which the anions placed in the voids and channels of the structure present high mobility, as inferred from the ionic exchange experiments. Variable-temperature magnetic susceptibility measurements show an overall antiferromagnetic behavior for all compounds, which are discussed in detail.