Bis(imidazo[4,5-f][1,10]phenanthroline)dinitratolead(II)

In the title compound, [Pb(NO3)2(C13H8N4)2], the PbII atom (site symmetry 2) is hexacoordinated by four N atoms from two N,N′-bidentate imidazo[4,5-f][1,10]phenanthroline (L) ligands and two O atoms from two weakly coordinated nitrate ions [Pb—O = 2.872 (5) Å] in an irregular arrangement, which may be ascribed to the stereochemically active lone pair of electrons on the metal ion. In the crystal, intermolecular bifurcated N—H⋯(O,O) hydrogen bonds connect the molecules into chains propagating along [100]. Adjacent chains interact by strong aromatic π–π stacking interactions, with a centroid–centroid distance of 3.483 (2) Å.

In the title compound I, each Pb atom is hexacoordinated by four N atoms (N1, N1 i , N2, N2 i ) from two L ligand and two O (O1, O1 i ) atoms from two NO 3 ions ( Fig. 1). The Pb-N bond lengths are normal (Table 1), however, O1 and O1 i could be described as being semicoordinated with long Pb-O lengths of 2.872 (5) Å. The PbN 4 O 2 coordination polyhedron approximates a distorted octahedral configuration.
In the crystal structure, the mononuclear complex molecules are linked via intermolecular N-H···O and N-H··· N hydrogen bonds (Table 2) forming one-dimensional chains along [100] (Fig. 2). Withal, one-dimensional chains are stabilized by π-π interactions between the L rings planes of neighboring molecules with distances of 3.442 Å (Fig. 3), leading to a three-dimensional network.

Experimental
The L ligand was synthesized according to the literature method of Steck & Day (1943). A mixture of Pb(NO 3 ) 2 (0.5 mmol) and the ligand (1 mmol) was dissolved in 15 ml distilled water, then followed by addition of NaOH until the pH value of the system approximately 6.0. Finally the resulting solution was sealed into a 23 ml Teflon-lined stainless steel autoclave and heated at 413 K for 3 d under autogenous pressure. The reaction autoclave slowly cooled to room temperature, yielding yellow block-like crystals of (I) in 62% yield based on Pb.

Refinement
All H atoms on C atoms were positioned geometrically (C-H = 0.93 Å) and refined as riding, with U iso (H)= 1.2U eq (C).
The hydrogen atoms of water molecules were located from difference Fourier maps and their positions and U iso values were refined freely.