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Crystals of the title compound, (C 5 H 8 N 3 ) 3 [VO 2 (C 2 O 4 ) 2 ]·2.5H 2 O, a mononuclear V V complex, were obtained by slow evaporation at room temperature of an aqueous solution containing vanadium pentoxide, oxalic acid and 2,6-di-amino-pyridine. The asymmetric unit contains one bis-(oxalato)dioxovanadate(V) anionic complex, three 2,6-di-amino-pyridinium cations and two and a half uncoordinated water mol-ecules. The mononuclear vanadium(V) anions are connected to the organic cations and water mol-ecules through a strong N-H⋯O and O-H⋯O hydrogen-bond network, consolidated by π-π stacking inter-actions, to form a three-dimensional structure.

The title organic-inorganic hybrid salt, (C7H11N2)2[V(C2O4)2O(H2O)]·2H2O, shows a distorted octa-hedral coordination environment for the vanadium(IV) atom in the anion (point group symmetry 2), with four O atoms from two symmetry-related chelating oxalate dianions and two O atoms in trans configuration from a coordinating water mol-ecule and a terminal vanadyl O atom. In the crystal, (001) layers of cations and anions alternate along [001]. The anionic layers are built up by inter-molecular O-H⋯O hydrogen bonds involving the coordinating and solvent water mol-ecules. The cationic layers are linked to the anionic layers via N-H⋯O hydrogen bonds between the pyridinium group and the non-coordinating O atoms of the oxalate group. The 4-(di-methyl-amino)-pyridinium cations are also engaged in π-π stacking with their anti-parallel neighbours [centroid-to-centroid distance = 3.686 (2) Å]. Considering all supra-molecular features, a three-dimensional network structure is accomplished.

As part of our studies on the synthesis and the characterization of oxalate-bridged compounds M-ox-M (ox = oxalate dianion and M = transition metal ion), we report the crystal structure of a new oxalate-bridged Mn(II) phase, {(CH6N3)[Mn(C2O4)Cl(H2O)]·H2O} n . In the compound, a succession of Mn(II) ions (situated on inversion centers) adopting a distorted octa-hedral coordination and bridged by oxalate ligands forms parallel zigzag chains running along the c axis. These chains are inter-connected through O-H⋯O hydrogen-bonding inter-actions to form anionic layers parallel to (010). Individual layers are held together via strong hydrogen bonds involving the guanidinium cations (N-H⋯O and N-H⋯Cl) and the disordered non-coordinating water mol-ecule (O-H⋯O and O-H⋯Cl), as well as by guanidinium π-π stacking. The structural data were confirmed by IR and UV-Visible spectroscopic analysis.