Di-μ-azido-diazidodi-μ-oxalato-dihistaminetetracopper(II) 0.9-hydrate

The title compound, [Cu4(C2O4)2(N3)4(C5H9N3)2]·0.9H2O, contains a tetranuclear CuII-based molecule composed of two oxalate-bridged CuII dimers linked through end-on azide ions and related by an inversion center. The tetranuclear unit contains two crystallographically independent CuII ions. One CuII ion coordinates to two N atoms of a histamine molecule, two O atoms of a bridging oxalate ligand, and an N atom of an end-on bridging azide ligand, leading to an elongated square-pyramidal coordination geometry in which the azide ion occupies the axial position. The other CuII ion, which has a square-planar coordination geometry, is coordinated by two O atoms of a bridging oxalate ligand and two N atoms of two different azide ligands, one which is bridging. In the crystal, a two-dimensional network parallel to (010) is formed by N—H⋯N and N—H⋯O hydrogen bonds. A partially occupied solvent water molecule refined to an occupancy of 0.447 (5). Two of the azide ligands were refined as disordered over two sets of sites with refined occupancies in the ratios 0.517 (8):0.483 (8) and 0.553 (5):0.447 (5).

The title compound, [Cu 4 (C 2 O 4 ) 2 (N 3 ) 4 (C 5 H 9 N 3 ) 2 ]Á0.9H 2 O, contains a tetranuclear Cu II -based molecule composed of two oxalate-bridged Cu II dimers linked through end-on azide ions and related by an inversion center. The tetranuclear unit contains two crystallographically independent Cu II ions. One Cu II ion coordinates to two N atoms of a histamine molecule, two O atoms of a bridging oxalate ligand, and an N atom of an end-on bridging azide ligand, leading to an elongated squarepyramidal coordination geometry in which the azide ion occupies the axial position. The other Cu II ion, which has a square-planar coordination geometry, is coordinated by two O atoms of a bridging oxalate ligand and two N atoms of two different azide ligands, one which is bridging. In the crystal, a two-dimensional network parallel to (010) is formed by N-HÁ Á ÁN and N-HÁ Á ÁO hydrogen bonds. A partially occupied solvent water molecule refined to an occupancy of 0.447 (5). Two of the azide ligands were refined as disordered over two sets of sites with refined occupancies in the ratios 0.517 (8):0.483 (8) and 0.553 (5):0.447 (5).

Comment
The title compound is a polynuclear coordination complex involving mixed bridging ligands, both oxalate (Coronado et al. 2003;Pardo et al. 2010;Sun et al., 1997) and azide (Ribas, et al. 1999) anions. The tetrameric unit in the title compound is centrosymmetric (Fig.1), so pairs of equivalent ligands lie trans to each other. Two of the four azide ions within each tetramer are in end-on bridging mode, while the other two are non-bridging but form long N4A-Cu[1-x, 2-y, 1-z] (two-dimensional) bonds of 2.486 (3) Å with Cu II ions on neighbouring tetramers and therefore link tetramers along the crystallographic b axis to produce one-dimensional chains (Fig.2). The distance between the two Cu II ions linked by this long Cu-N bond is 3.2261 (8) Å.

Experimental
An aqueous solution (15 ml) of copper(II) nitrate trihydrate (4.0 mmol, 0.97 g) was slowly added to an aqueous solution (50 ml) containing histamine dihydrochloride (4.0 mmol, 0.74 g), sodium oxalate (2.0 mmol, 0.27 g), sodium azide (4.0 mmol, 0.26 g), and sodium hydroxide (8.0 mmol, 0.32 g). The mixture was stirred for 10 minutes and allowed to stand in air. Green platelet crystals were collected after a few days and washed with deionized water and dried in air.
Methyl H atoms were allowed to rotate around the corresponding C-C.
The N2-N3 and N5-N6 moieties were refined a disordered and each refined in two parts against N2′-N3′ and N5′-N6′. A partial water molecule alternates with the N5′-N6′ moiety in occupying the same space. The two N9 protons were obtained from a difference Fourier map but did not refine properly thus they were refined in idealized positions and were riding on their parent atom. The N9 proton is refined freely. All disordered parts have properly refined occupancy factors.

Figure 2
One-dimensional chain of tetramers of the title compound.