Monomers, dimers, and trimers of [Au(CN)2]− in a Ba(diaza-18-crown-6)2+ coordination polymer

The structure of the title compound, poly[triaquatetra-μ-cyanido-tetracyanidobis(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane)dibarium(II)tetragold(I)], [Au4Ba2(CN)8(C12H26N2O4)2(H2O)3]n, displays O—H⋯N hydrogen bonding between water molecules and cyano ligands and an unusual pattern of aurophilic interactions that yields a monomer, dimer, and trimer of [Au(CN)2]− within the same crystal structure. In two of the five Au positions, the atom resides on a center of inversion. The overall arrangement is that of a coordination polymer assisted by aurophilic and hydrogen-bonded interactions.


Experimental
Crystal data [Au 4 Ba 2 (CN) 8 À124.22 (13) Symmetry code: (i) Àx þ 2; Ày; Àz þ 2. Table 2 Hydrogen-bond geometry (Å , ).  Fig. 2, the polymer is connected through a combination of coordination of the [Au(CN) 2 ]nitrogen atoms to barium and aurophilic interactions. All of the hydrogen atoms of the three coordinated waters behave as hydrogen bond donors to N3, N4, N6, N7 and N8 of the cyanide groups (see Table 2). Fig. 3 depicts how a portion of the polymeric structure is supported by these hydrogen bonds.
The bariums, Ba1 and Ba2, have coordination numbers of 9 and 10, respectively. Ba1 is six-coordinated by the diaza-18-crown-6, two [Au(CN) 2 ]anions and one water molecule. It is 0.56 (2) Å out of the N 2 O 4 plane of the crown, giving endo and exo faces. One dicyanidoaurate is coordinated to each face while the water molecule coordinates on the exo face.
The coordination environment of Ba2 is different. Ba2, which is 0.71 (2) Å out of the N 2 O 4 plane of the crown, is also coordinated by two dicyanidoaurates, but both are found on the exo face. Two water molecules are coordinated to Ba2, one on each face. Four of the eight independent cyanide groups are coordinated through their cyanide N atom to a barium (N5, N6, N9, N12). Interestingly, even though the dimer and trimer differ in their Au···Au distances, they show similar C-Au-Au-C torsion angles that are intermediate between eclipsed and staggered. The average value of the two smaller angles is 53° for the trimer and 55° for the dimer (see Table 1 for details). Hydrogen atoms on water and aza-N atoms were located in a difference map and subsequently refined with U iso = 1.2U eq (N or O) and distance restraints of 0.84 (1) Å for O-H, 0.88 Å for N-H and H···H of 1.32 (3) Å for water. The C-H geometry was determined by a riding model with idealized geometry and a C-H distance of 0.99 Å. The largest difference map peaks are due to a small amount of conformational disorder in one of the aza crown rings but this was not modeled. The disorder is reflected in somewhat elongated thermal ellipsoids in the cation involving Ba2. Fig. 1. A drawing of the asymmetric unit of the title compound. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 R-supplementary materials sup-4 factors(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.