Structure and NMR properties of the dinuclear complex di-μ-azido-κ4 N 1:N 1-bis[(azido-κN)(pyridine-2-carboxamide-κ2 N 1,O)zinc(II)]

The crystal structure of a dimeric ZnII complex with picolinamide and azido ligands is characterized in the solid state and in solution.

The new diamagnetic complex, [Zn 2 (N 3 ) 4 (C 6 H 6 N 2 O) 2 ] or [Zn 2 (pca) 2 ( 1,1 -N 3 ) 2 -(N 3 ) 2 ] was synthesized using pyridine-2-carboxamide (pca) and azido ligands, and characterized using various techniques: IR spectroscopy and single-crystal X-ray diffraction in the solid state, and nuclear magnetic resonance (NMR) in solution. The molecule is placed on an inversion centre in space group P1. The pca ligand chelates the metal centre via the pyridine N atom and the carbonyl O atom. One azido ligand bridges the two symmetry-related Zn 2+ cations in the end-on coordination mode, while the other independent azido anion occupies the fifth coordination site, as a terminal ligand. The resulting five-coordinate Zn centres have a coordination geometry intermediate between trigonal bipyramidal and square pyramidal. The behaviour of the title complex in DMSO solution suggests that it is a suitable NMR probe for similar or isostructural complexes including other transition-metal ions. The diamagnetic nature of the complex is reflected in similar 1 H and 13 C NMR chemical shifts for the free ligand pca as for the Zn complex.

Chemical context
Polynuclear complexes have received the attention of coordination chemists as they are ideal candidates for developing new functional molecular materials. In the design and preparation of such systems, a number of synthetic strategies have been used for propagating new motifs, affording a large number of polynuclear complexes with potential applications (Miller & Drillon, 2002). Complexes based on Zn 2+ ions are of interest because of the versatility of this transition metal towards different kinds of chelating ligands, and its ability to bind ligands with different coordination numbers, ranging from two to six (Sakai et al., 2006). Some complexes have been proposed as models for the active sites of zinc-containing enzymes (Parkin, 2000;Dö ring et al., 2002), while others have been studied for their catalytic properties (Dey et al., 2002) or for the purpose of producing OLED devices (Sano et al., 2000;Tokito et al., 2000;Ray et al., 2012).
Upon coordination of a ligand to a metal centre, the ligand properties, such as electrophilic or nucleophilic character, acidity, susceptibility to oxidation or reduction, can be significantly altered, thereby enhancing or inhibiting its reactivity (Konidaris et al., 2012). Co-ligands are also important for the structure and properties of the complex, especially if they can bridge metal centres. Among them, the azido ligand, N 3 À , ISSN 2056-9890 has been widely used in the building of molecular magnetic materials with a rich diversity of topologies (Ribas et al., 1999;Hong & Chen, 2009). The challenging aspect of N 3 À is its great coordination flexibility, which turns out to be rather a drawback since structures are poorly predictable. However, the correlation between the structures of polynuclear complexes including azido bridges and their magnetic properties is now well understood (Husain et al., 2012;Yu et al., 2007).
The azido ion can link two or more metal ions in different configurations. The most representative are the end-to-end (EE) mode, in which two terminal N atoms bridge the metals, and the end-on (EO) mode, in which only one terminal N atom is used (Dori & Ziolo, 1973;Mautner et al., 2013). Based on a survey of the CSD (Groom et al., 2016), the prevalence of the EO mode is much higher than the EE mode, by a factor of about ten. Mixed species having both terminal (i.e. nonbridging) and EE/EO bridging azides are known, but are not so common. Several architectures occur depending on whether EE or EO bridges are present, which can be symmetric or asymmetric, single or multiple, and associated or not with other bridges (Goher et al., 2000;Maji et al., 2001).
In this context, our group has paid attention to the synthesis of Zn 2+ complexes including azido ligands, with the aim of using these diamagnetic compounds as NMR probes for other structurally related or analogous complexes. Herein, we report the molecular structure of a dinuclear complex with bridging and non-bridging azido ligands, synthesized with picolinamide, a pyridine derivative with an amido group, suitable for the chelation of transition metals (Ðaković et al., 2008).

Structural commentary
The dinuclear complex [Zn 2 (N 3 ) 4 (pca) 2 ], where pca is picolinamide (IUPAC name: pyridine-2-carboxamide), crystallizes in the triclinic space group P1, with the molecule placed on the inversion centre (Fig. 1). The central [Zn 2 N 2 ] core is thus planar by symmetry, with azido ligand N3/N4/N5 bridging the metal centres in the EO configuration. The double bridge is asymmetric, with Zn-N3 bond lengths of 2.057 (3) and 2.218 (3) Å (Table 1). These bond lengths are comparable to those observed in other Zn 2+ complexes bearing Schiff bases (Ray et al., 2012;Ðaković et al., 2015;Sheng et al., 2014), and are in agreement with IR spectroscopy data (You et al., 2009;Qian & You, 2011). The pca molecule behaves as a 2 -N,Ochelating ligand, forming a common five-membered metallacycle. This mode of coordination is almost universally found in other complexes including pca as ligand: there are very few occurrences of 2 -N,N-pca ligands reported so far in the CSD. Finally, each Zn centre coordinates one terminal azido ion, N6/N7/N8, with the short distance Zn-N6 = 1.991 (4) Å . Both independent azido ligands are nearly linear, and the bridging azido has a bent coordination with the metal centre. In the dinuclear complex, the ZnÁ Á ÁZn separation is 3.2760 (11) Å .
The IR spectrum of the solid shows the stretching modes of coordinated pca ligands (Fig. 2). The band at 1678 cm 1 is assigned to the C O vibration, which is shifted towards lower energy because of the C O bond lengthening upon coordination [C6 O1: 1.250 (4) Å ]. In contrast, the N-H stretching band of the amide group is not displaced in comparison to the free ligand, indicating that the NH 2 group does not coordinate to Zn 2+ ions (Konidaris et al., 2012). The medium intensity band at 1296 cm À1 can be attributed to the C . . . N vibration in the pyridyl ring. The most useful IR vibrations are those related to azido ligands, which are clearly split over two frequencies, at 2094 and 2065 cm À1 (Fig. 2, inset). Based on previous reports in the literature, the former can be assigned to bridging-EO azido ligands and the latter to Acta Cryst. (2021). E77, 111-116 research communications Figure 1 Molecular structure of the title compound, showing 50% probability displacement ellipsoids for non-H atoms. Non-labelled atoms are generated by the symmetry operation 1 À x, 1 À y, 1 À z. Table 1 Selected geometric parameters (Å , ).  (12) terminal azido ligands (Ðaković et al., 2015;Forster & Horrocks, 1966). Similar intensities for these bands are in agreement with the X-ray structure. Finally, Zn-N vibrations give a low-intensity band at 412 cm À1 (Majumder et al., 2006). The resulting dinuclear complex has five-coordinate Zn 2+ ions, for which the Addison geometric parameter is 5 = 0.55, midway between an ideal square-pyramidal ( 5 = 0) and a trigonal-bipyramidal geometry ( 5 = 1; Addison et al., 1984). The strain caused by the five-membered metallacycle formed by the pca ligand [bite angle: 77.87 (12) ], together with the geometric restraint imposed by the central [Zn 2 N 2 ] ring [N3-Zn1-N3 i angle: 80.02 (12) ] account for the observed trigonal distortion. Such distortion has been observed in other similar dinuclear five-coordinate Zn 2+ complexes bearing both terminal and bridging azido ligands: for nine complexes retrieved from the CSD, the Addison parameter ranges from 5 = 0.40 (Sun & Wang, 2007) to 5 = 0.93 (Wang et al., 2004).
Non-covalent intermolecular interactions are present in the crystal structure. Given that the NH 2 groups in the pca ligands are not engaged in coordination, they form instead weak intermolecular N-HÁ Á ÁN hydrogen bonds with terminal N atoms of azide groups (Table 2). These bonds form a 2D framework parallel to plane (100) in the crystal. The molecules are then arranged in such a way that pyridyl rings are stacked in the [100] direction, with an offset face-to-face arrangement characterized by centroid-to-centroid distances for pyridyl rings of 4.702 (3) and 5.141 (3) Å along a stack (Fig. 3).

NMR measurements and chemical shift calculations
Using DMSO-d 6 solutions of the free ligand pca and the title complex, 1 H and 13 C-NMR spectra were recorded on a Bruker Avance III 500 MHz spectrometer. Computationally, the geometry for the complex was optimized with the BLYP functional (Becke, 1993) and the 6-31+G(2d,p) basis to correlate the experimental structural information, timedependent DFT, and NMR chemical shift estimations. Bond lengths and angles are similar in the DFT-optimized structure and in the X-ray crystal structure, validating the correctness of the calculations (GAUSSIAN09; Frisch et al., 2009). The shielding scales were converted to chemical shift scales by applying reference shielding of 32.0531 and 178.5112 ppm for 1 H and 13 C in TMS, respectively.

Figure 3
Part of the crystal structure of the title complex showing the arrangement of N-HÁ Á ÁN hydrogen bonds. The proximity between systems is reflected in the intermolecular C4Á Á ÁC4 separations, as measured using Mercury (  The presence of two NH broad signals with short relaxation times is due to the presence of the N and Zn atoms, which are more electronegative than H. The proton signals are slightly deshielded upon complexation, with the magnitude of deshielding decreasing while the distance from the metal centre increases. As seen in Fig. 4, the 3d 10 cation does not affect the position of the signals very much. The most affected signals are those corresponding to the amide NH groups, which are shifted by ca 0.2 ppm and broadened upon coordination. This behaviour is probably related to different hydrogen-bonding schemes involving the NH 2 group: free pca is strongly stabilized in the solid state by R 2 2 (8) ring motifs (É vora et al., 2012), which are no longer present once the molecule is coordinated to the metal centre. The small influence of the metal centres on NMR properties is confirmed by experimental 13 C-NMR chemical shifts, which are almost identical for pca and the title complex (Fig. 5). However, a broadening is observed for the quaternary carbon atom C5, which is located in the close vicinity of the N and Zn sites, resulting in a very short relaxation time.
These data corroborate that proton chemical shifts for pca are only marginally affected by coordination to a diamagnetic metal centre as Zn 2+ . Very different spectra would be expected with paramagnetic centres, such as Mn 2+ , Co 2+ , or Cu 2+ . Most often, NMR spectra are difficult to interpret for these complexes, due to their broad and out of tune signals. However, our NMR data do not allow determination of whether the complex survives as a dimeric compound in solution, and whether the hydrogen bonding scheme observed in the crystal structure is retained in solution. Experimental 13 C-NMR spectra of pca (blue) and [Zn 2 (N 3 ) 4 (pca) 2 ] (red) in DMSO-d 6 . Chemical shifts are given in Table 3.  Experimental 1 H-NMR spectra of pca (blue) and [Zn 2 (N 3 ) 4 (pca) 2 ] (red) in DMSO-d 6 . Chemical shifts and coupling constants are given in Table 3.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All C-bound H atoms were placed in calculated positions and refined as riding on their carrier C atoms, while amide H atoms were found in a difference map and refined with free orientation. The geometry of the NH 2 group was restrained with distance targets N-H = 0.87 (2) Å , and isotropic displacement parameters for these H atoms were calculated as U iso (H) = 1.2U eq (N2).  (Farrugia, 2012) and Mercury (Macrae et al., 2020); software used to prepare material for publication: publCIF (Westrip, 2010).