Dibromido(4′-phenyl-2,2′:6′,2′′-terpyridyl)copper(II) hemihydrate

The title CuII complex, [CuBr2(C21H15N3)]·0.5H2O, was obtained by the hydrothermal reaction of copper(II) bromide, 4′-phenyl-2,2′:6′,2′′-terpyridyl (4′-Ph-terpy or L) and sodium citrate in water in 31% yield. There are two unique complex molecules and a water molecule in the asymmetric unit. The CuII cation is ligated by three N atoms of L and two bromide anions, forming an irregular CuN3Br2 polyhedron with a distorted square-pyramidal coordination geometry. In the crystal structure, O—H⋯Br hydrogen bonds link the molecules in a three-dimensional network.


Comment
The design and synthesis of photo-luminescent metal coordination compounds bearing terpy ligands have attracted a considerable interest. Examples include various terpy complexes of Pd(II), Pt(II), Zn(II) and Ag(I), (Bugarcic et al., 2004;Ma et al., 2009;Yam et al., 2003). Despite a preparative route which involves the presence of sodium citrate in the hydrothermal reaction mixture, the single crystal structure of the title complex exhibits a neutral mononuclear unit with a copper(II) cation, one 4'-Ph-terpy ligand and two bromide ions in its coordination sphere. There is no evidence of coordination by the citrate anion to the central metal ion.
There are two independent complex molecules of [CuBr 2 (C 21 H 15 N 3 )] 2 .H 2 O in the asymmetric unit of the triclinic unit cell ( Figure 1). Each copper (II) cation is coordinated by the three nitrogen atoms of the 4'-phenyl-2,2':6',2''-terpyridyl ligand, L, and two bromide anions, forming an irregular distorted square pyramidal CuN 3 Br 2 polyhedron. In both unique molecules, the two bromide ions occupy the apical [Br(2) and Br(4)] and equatorial [Br(1) and Br (3)] positions. The other three basal coordination positions are occupied by the three nitrogen atoms of L. This geometry is confirmed by the angles between the ligand donor atoms in the equatorial plane of the square pyramid (Table 1). The angles between the two apical bromide ions and the three terpy nitrogen atoms are in the range 95.37 -101.39 ° for one molecule and 94.30 -102.29° for the other. The terpyridyl molecules in the two compounds are nearly planar (with RMS deviations 0.1446 for one compound and 0.0292 for the other). The phenyl rings of the terpy ligands are twisted to make angles of 17.1 (1) and 25.3 (2)° respectively with the CuN 3 coordination planes. Similar irregular CuN 3 Br 2 polyhedra have been found in other copper(II) complexes (Arriortua et al., 1988;Kickelbick et al., 2002;Koo et al., 2003). A lattice water is contained in the asymmetric unit, which originates from water used in the synthesis and is also involved in weak Cu-Br···H 2 O hydrogen bonding with the neighboring bromide ligands (Br(1), Br(2)) and Br(3)), respectively.

Experimental
Free L was prepared by a reported procedure. (Constable et al., 1990). The title compound was synthesized by reaction of copper(II) bromide, sodium citrate and L in hydrothermal conditions as follows: A mixture of CuBr 2 .4H 2 O (0.044 g, 0.14 mmol), L (0.040 g, 0.071 mmol), sodium citrate (Na 3 C 6 H 5 O 7 .2H 2 O) (0.021 g, 0.071 mmol), and distilled water (20 ml) was sealed in a 25 ml stainless steel reactor with a Teflon liner and heated at 110 °C for 3 days. Blue crystals of 1, suitable for X-ray characterization, were isolated by mechanical separation from a mixture which included an unidentified powder.
The yield of 1 was 31 % based on the ligand.

Refinement
Hydrogen atoms bonded to the ligands were positioned geometrically and refined using a riding model with C-H = 0.93 -0.97 Å and with Uiso(H) = 1.2 times Ueq(C).These hydrogen atoms were assigned isotropic thermal parameters and allowed to ride on their respective parent atoms before the final cycle of least-squares refinement. Oxygen-bound hydrogen

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )