Crystal structure of (N^C) cyclometalated AuIII diazide at 100 K

The title compound, an (N^C)-cyclometalated gold(III) diazide, namely, diazido[5-ethoxycarbonyl-2-(5-ethoxycarbonylpyridin-2-yl)phenyl-κ2 C 1,N]gold(III), [Au(C17H16NO4)(N3)2] or Au(ppyEt)(N3)2, was synthesized by reacting Au(ppyEt)Cl2 with NaN3 in water for 24 h. The complex features a gold center with a square-planar environment.


Chemical context
Among gold azide complexes, Au I have dominated over Au III azides (Del Castillo et al., 2011;Powers et al., 2015;Partyka et al., 2007). Until now, only three examples of Au III azide complexes have been reported (Fig. 1). The reported compounds feature the N-heterocyclic carbene complex and pyridine coordinated Au-triazide groups (Schuh et al., 2016;Peng et al., 2019) as well as cationic cyclometalated monoazide (Roth et al., 2016). To the best of our knowledge, a cyclometalated phenyl pyridine Au III azide complex has not been reported before.

Structural commentary
The molecular structure of Au(ppy Et )(N 3 ) 2 (2) is shown in Fig. 2. The complex forms monoclinic crystals belonging to the space group P2 1 /c and crystallizes with one molecule in the asymmetric unit. The solid-state structure of 2 displays a square-planar coordination geometry, as expected for the d 8 electron configuration of the Au III center. The Au-N bond length trans to the pyridine N atom [2.042 (2) Å ] is shorter than the one trans to the C atom [2.067 (2) Å ], indicating the stronger trans influence of the phenyl carbon atom. N-N bond distances in the azide ligands are in line with reported literature values (Dori et al., 1973) with shorter terminal N-N bond lengths compared to the internal ones (1.150 vs 1.200 Å , on average). The N-N-N angles [174.7 (3) and 173.8 (3) ] deviate only slightly from the expected linear arrangement and the Au-N-N angles of 118.7 (2) and 119.2 (2) for the azide groups trans to N and C, respectively, indicate the expected bent coordination of these ligands. The azide groups are twisted by 56.2 (2) with respect to each other, and point in-and-out of the plane with distances of 1.092 (2) Å for the terminal N atom trans to C and 0.975 (2) Å for the terminal N atom trans to the pyridine N atom (Fig. 3). The pyridine and benzene rings are essentially coplanar, the angle formed by their mean planes being 3.64 (10) .

Supramolecular features
The title crystal structure features infinite stacking chains along the [100] direction. The neighboring molecules within the stack are related by inversion. The mean plane of the core of the complex molecule including the Au atom, both aromatic rings and two N atoms of azide groups attached to the Au atom form an angle with the a-axis direction of 69.53 (2) . The distances between these planes of neighboring molecules within the stack are 3.331 (1) and 3.314 (1) Å (Fig. 4).

Database survey
A search was performed in the Cambridge Structural Database (CSD version 5.41; Groom et al., 2016) with the following constraints: an Au III complex featuring a phenylpyridine backbone and two additional non-cyclic ligands bonding to Au through N or C. Fourteen structures were found to match this motif. The features of the title structure resemble those observed in the structures found in this database survey, e.g. an observable trans effect (distance Au-L trans to N is always shorter than that trans to C), Au-C bond lengths are shorter than the Au-N ones and angles around the Au III center are close to 90 .

Synthesis and crystallization
The reaction scheme for the synthesis of the title compound is provided in Fig. 5. The gold complex Au(ppy Et )Cl 2 (1) was prepared according to previously published procedure (Levchenko et al., 2020). Complex 1 (70 mg, 0.124 mmol, 1 equiv.) was stirred with sodium azide (64.5 mg, 1 mmol, 8 equiv.) in water for 24 h at room temperature. The solids were recovered by filtration, washed with large excess of water and dried in air giving 50 mg (70%) of 2 as a white solid. Needle-like crystals were obtained by slow diffusion of cyclohexane into a solution of the product in CH 2 Cl 2 containing few drops of acetone. 1 H NMR (600 MHz, DMSO- Mutual orientation of the azide groups with respect to the metalacycle plane.

Figure 4
Crystal packing of the title compound viewed along the a axis.

Figure 2
Molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. OLEX2 was used as user interface (Dolomanov et al., 2009). All hydrogen atoms were placed in calculated positions with C-H = 0.95-0.99 Å and refined as riding with fixed isotropic displacement parameters [U iso (H) = 1.2-1.5U eq (C)].

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.