Co(NCS)2(abpt)2 and Ni(NCS)2(abpt)2 [abpt is 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole]: structural characterization of polymorphs A and B

The synthesis and structures of two polymorphs, A and B, of Co(NCS)2(abpt)2 and Ni(NCS)2(abpt)2 are reported. The polymorph structures of A with CoII or NiII were found to be isostructural, as were the corresponding pair of polymorph B structures with the different metals.


Introduction
The bidentate ligand 4-amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole (abpt) has been found to form mononuclear complexes, as well as single-or double-bridged dinuclear complexes, with a variety of metals (for examples, see Dupouy et al., 2008;White et al., 2009;Li et al., 2011). Amongst these, a number of Fe II complexes have been synthesized and studied because of their interesting polymorphism and spin-crossover behaviour. Perhaps the most studied is the Fe(NCS) 2 (abpt) 2 complex, of which there are four known polymorphs, denoted A-D, all of which display different magnetic behaviour. Three of the polymorphs, i.e. A (Moliner et al., 1999;Sheu et al., 2009;Mason et al., 2016), C (Sheu et al., 2009;Shih et al., 2010) and D (Sheu et al., 2009(Sheu et al., , 2012Mason et al., 2021), undergo at least a partial thermal spin crossover under ambient pressure, while polymorph B (Gaspar et al., 2003) only undergoes a thermal spin crossover at pressures above 4.4 kbar (1 bar = 10 5 Pa). All of the three polymorphs which display at least a partial thermal spin crossover also show light-induced excited-spinstate trapping (LIESST) at low temperature. While three of the polymorphs (A, B and D) are known to undergo a pressure-induced spin crossover at room temperature (Mason et al., 2016(Mason et al., , 2021, polymorph C has not been studied under pressure at room temperature. To date, Co(NCS) 2 (abpt) 2 is the only other M(NCS) 2 (abpt) 2 complex containing a transition metal for which any structures have been reported. Like the Fe analogue, this has also been found to display polymorphism, with two different polymorphs of Co(NCS) 2 (abpt) 2 reported at room temperature. These will be referred to as Co(NCS) 2 (abpt) 2 polymorphs B (Peng et al., 2006) and D (Chen & Peng, 2007) throughout, as they are isostructural with Fe(NCS) 2 (abpt) 2 polymorphs B and D. The structures of two polymorphs, A and B, of both Co(NCS) 2 (abpt) 2 and Ni(NC-S) 2 (abpt) 2 are reported herein (see Scheme 1).

Synthesis
The synthesis of M(NCS) 2 (abpt) 2 , where M is Co or Ni, was carried out using a slow-diffusion method with methanolwater solutions as reported previously (Sheu et al., 2009).
All chemicals were obtained from Sigma-Aldrich and used as supplied. CoSO 4 Á7H 2 O (1 mmol, 0.281 g) or NiSO 4 Á6H 2 O (1 mmol, 0.263 g) and KNCS (2 mmol, 0.194 g) were stirred in methanol (10 ml) for 15 min. A pale-yellow insoluble K 2 SO 4 precipitate was removed by filtration and deionized water (10 ml) was added to the remaining clear solution. Abpt (2 mmol, 0.477 g) was dissolved in methanol (20 ml) and placed in a narrow (<5 cm) Schlenk tube. The M 2+ /NCX À solution was very carefully pipetted at the bottom of the Schlenk tube to form a lower more dense layer below the abpt solution. Immediately, a coloured band formed at the interface between the two layers containing the target complex. The Schlenk tube was left undisturbed and single crystals suitable for X-ray diffraction studies had formed within one week to one month later.

Refinement
Details of the crystallographic data collections are given in Table 1. All H atoms, apart from the N-H hydrogens, were positioned geometrically and refined using a riding model. The N-H hydrogens were located in a difference Fourier map (FDM) wherever feasible.

Results and discussion
The structure of Co(NCS) 2 (abpt) 2 polymorph B has already been reported at room temperature and is consistent with that reported here (Peng et al., 2006). The main structural features of all four structures are very similar: they all crystallized in the monoclinic space group P2 1 /n with half a molecule in the  Table 1 Experimental details. For all structures: monoclinic, P2 1 /n, Z = 2. Experiments were carried out at 120 K with Mo K radiation. Absorption was corrected for by multi-scan methods (SADABS ;Bruker, 1999Bruker, -2013. Refinement was on 202 parameters. H atoms were treated by a mixture of independent and constrained refinement.  (7), 16.3774 (11) 11.4978 (5), 9.5235 (4), 12.7179 (5) 8.4041 (7), 10.0681 (9), 16.2360 (14) 11.5860 (14) asymmetric unit (Z 0 = 0.5) (Fig. 1). Each of the four complexes consists of an approximately octahedrally coordinated metal centre (Co II or Ni II ) coordinated to six N atoms, one from each of the NCS À ligands and two from each abpt ligand (one pyridyl and one triazole N atom). Each of the structures contains an intramolecular N-HÁ Á ÁN hydrogen bond between the NH 2 group on the triazole ring and the N atom of the uncoordinated pyridyl ring, as well as two intramolecular C-HÁ Á ÁN interactions, one between a pyridyl C-H group and the N atom of the NH 2 group attached to the triazole ring, and a second between a pyridyl C-H group and the uncoordinated N atom on the triazole group (Table 2). The pair of A polymorphs of the Co II or Ni II structures are isostructural with each other, and are also isostructural with the previously reported Fe(NCS) 2 (abpt) 2 polymorph A structure (Moliner et al., 1999;Sheu et al., 2009;Mason et al., 2016). In addition to the previously mentioned N-HÁ Á ÁN hydrogen bonding and C-HÁ Á ÁN interactions, the structures contain intermolecularstacking between pairs of molecules and involving the two pyridyl rings at each end of the abpt ligand interacting with the two pyridyl rings on an adjacent abpt ligand, creating a one-dimensional chain through the structure (Table 3 and Fig. 2). As seen for the pair of polymorph A structures, the two polymorph B structures were also isostructural with each other and with the previously reported Fe(NCS) 2 (abpt) 2 polymorph B structure (Gaspar et al., 2003;Mason et al., 2021). The structures of polymorph B also displayinteractions, but in this case each of the pyridyl rings on the abpt ligand is involved in ainteraction to a pyridyl ring on a different abpt ligand, creating a threedimensional network of interactions in the structure (Table 3 and Fig. 2). Along with the difference in the form of theinteractions between the polymorph A and polymorph B structures, the other main difference is the twist between the two rings on the abpt ligands. In the case of A, the twist between the rings is $9 , while for B, Illustration of the structures of Co(NCS) 2 (abpt) 2 polymorphs (a) A and (b) B, and Ni(NCS) 2 (abpt) 2 polymorphs (c) A and (d) B, with the atomic numbering schemes depicted. H atoms have been omitted for clarity. [Symmetry code: (i) Àx + 1, Ày + 1, Àz + 1.] Table 2 Hydrogen-bond geometry (Å , ) for Co(NCS) 2 (abpt) 2 and Ni(NCS) 2 (abpt) 2 at 120 (2)  Symmetry codes: (i) Àx + 1, Ày + 1, Àz + 1; (ii) Àx + 1, Ày + 1, Àz + 1.
the twist between the rings is $35 (Table 4). This is likely to be the reason for the significantly differentstacking, as the larger twist in B would prevent both rings on one abpt ligand being correctly orientated to interact with both rings on a single abpt ligand on an adjacent molecule.
The Hirshfeld fingerprint plots (Turner et al., 2017) for the two polymorphs highlight the differences between the two structures ( Fig. 3). The plots are only shown for the Co polymorphs A and B, as the plots for the Ni polymorphs A and B were essentially identical to those of the respective Co polymorphs. The shapes of the two plots are clearly slightly different, although given that the structures are polymorphs, it is unsurprising that they show the same main short contacts. For both polymorphs, the SÁ Á ÁH contacts are quite pronounced, with a similar shape and position. However, in the case of A, the CÁ Á ÁH contacts are more pronounced than is seen for B, while the HÁ Á ÁH contacts for A are less pronounced than observed for B.
Examining the Hirshfeld surfaces for both compounds, the greater number of red spots on the surface of A than for B indicates that A has more short contacts.

Figure 3
The Hirshfeld surface plot and fingerprint plot for (a) polymorph A and (b) polymorph B for Co(NCS) 2 (abpt) 2 . The Ni plots for the same respective polymorphs are essentially identical.

Table 4
Twist and fold angles between planes calculated through the six atoms of the two rings on the abpt ligand at 120 (2) K.  Co-N distances for Co(NCS) 2 (abpt) 2 at 120 (2) and 300 (2) K.
Polymorph information). Examining the Co-N bond lengths showed them to be essentially identical to the 120 (2) K structure and indicate that no spin transition had occurred over this temperature range (Table 5). In the case of Ni II , the complex is d 8 so no spin transition would be possible.

Conclusions
The synthesis and structures of Co(NCS) 2 (abpt) 2 and Ni-(NCS) 2 (abpt) 2 are reported. Two polymorphs were identified for each of the complexes, A and B, and the pairs of polymorphs with the different metal centres were found to be isostructural. All of the structures contained intramolecular N-HÁ Á ÁN hydrogen bonding, intramolecular C-HÁ Á ÁN interactions andstacking. There are identifiable differences between the two polymorph structures. Firstly, the twist angle between the two six-membered rings on one abpt ligand was $9 for polymorph A and $35 for polymorph B.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Co1 0.500000 0.500000 0.500000 0.01882 (13) S1 0.26353 (8) 0.71891 (7) 0.25547 (4) Hydrogen-bond geometry (Å, º) (3) 136 (3)   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.

Crystal data
[Ni(NCS) 2 (C 12 H 10 N 6 ) 2 ] M r = 651.39 Monoclinic, P2 1 /n a = 8.4041 (7)  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.

Bis[4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole-κ 2 N 2 ,N 3 ]bis(thiocyanato-κN)nickel(II) (Ni_B_120K)
Crystal data 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.

Special details
Experimental. The data collection nominally covered a full sphere of reciprocal space by a combination of 3 sets of ω scans each set at different φ and/or 2θ angles and each scan (12 s exposure) covering -0.300° degrees in ω. The crystal to detector distance was 4.424 cm. 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.

Special details
Experimental. The data collection nominally covered a full sphere of reciprocal space by a combination of 3 sets of ω scans each set at different φ and/or 2θ angles and each scan (5 s exposure) covering -0.300° degrees in ω. The crystal to detector distance was 4.424 cm. 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.