Crystal structure at 100 K of bis[1,2-bis(diphenylphosphanyl)ethane]nickel(II) bis(trifluoromethanesulfonate): a possible negative thermal expansion molecular material

Description and comparison of the crystal structure of (Ni(1,2–bis(diphenylphosphanyl)ethane)2)(CF3SO3)2 at 100 K with its nitrate and bromide analogues.

Triflates (trifluromethanesulfonates, CF 3 SO 4 À ) are known as precursors of a wide range of compounds due to their lability (Lawrence, 1986). Therefore, we compare the title structure, 1, to the structures reported with the other two counter-ions to evaluate the effect of introducing the triflate. As we describe below, the crystal structure at room ISSN 2056-9890 temperature (see supplementary material) shows disorder of the anion that is reduced, but not completely eliminated at 100 K. In addition, the structure shows negative thermal expansion (NTE) (Liu et al., 2018) based on the unit-cell volume at the two measured temperatures.

Structural commentary
The geometry of the cation formed by Ni (site symmetry 1) with the two dppe ligands is square planar (Fig. 1). We might expect the Ni-P distances to be the same (the ligand is symmetric); however, they are different. The corresponding distances are listed in Table 1 for the structure collected at 296 and 100 K and compared to the ones from VASCIB (Williams, 1989) and XUQYOZ (Higgs et al., 2010). As this structure is formed by chelation of a simple bidentate ligand, the counterion has a limited effect on it, and as in the two previous structures reported, the triflate ions remain outside of the coordination sphere, being blocked from the metal center by the phenyl rings. However, there is an effect on the P-C-C-P torsion angle of the chelate ring, which is probably dependent on the size of the counter-ion (Table 1).
The bulky cation formed and the lack of strong interactions with the counter-ions lead to presumed dynamic disorder of the triflate ion at room temperature (296 K), which was also observed in the case of VASCIB (Williams, 1989). XUQYOZ on the other hand was acquired at a lower temperature (85 K) and no reference to any disorder was reported (Higgs et al., 2010).
For 1 at 296 K, the triflate anion is disordered over two sets of sites with 65% occupancy for the major component, which is the one with the shortest distance to the Ni atom (Fig. 2). The distance between the disordered structures is as follows, for the carbon atoms 0.744 (15) and for the S atoms 0.34 (4) Å (Fig. 2). For 1 at 100 K, the disorder is reduced although not eliminated completely (Fig. 2 ORTEP rendering of 1 at 100 K with displacement ellipsoids drawn at the 50% probability level. Hydrogen atoms and the disordered parts of the anion were omitted for clarity. Atoms with the suffix A are generated by the symmetry operation (1 À x, 1 À y, 1 À z). Table 1 Comparison of selected geometric parameters (Å , ) for 1 at 296 and 100 K, VASCIB and XUQYOZ.

Figure 2
Ball and stick rendering of the trifluoromethanesulfonate ion for 1 at 100 K (left) and at 296 K (right) showing both disorder components.
Open bonds indicate the minor disorder component.

Figure 3
View parallel to the coordination plane of the Ni and P atoms, showing the counter-ions blocked by the phenyl rings. A space-filling rendering was used for the phenyl groups, the Ni atom and the oxygen atom pointed towards Ni. The disordered part of the anion is omitted for clarity. . This could be analysed in two ways: the disorder is also static or the temperature is not low enough to eliminate completely the dynamic disorder. Surprisingly, a negative thermal expansion was observed (Liu et al., 2018). The Ni-P bond distances for 1 at 100 K (Table 1) are elongated by 1.08 and 1.20% in comparison to the values for 1 at 296 K, very close values to the volumetric expansion of the unit cell of 1.25 (12)%. With respect to the unit cell, the a and b axes are affected most in comparison with c, with coefficients of linear expansion ( l ) of À29 (4) Â 10 À6 , À30 (4) Â 10 À6 , and À6(4) Â 10 À6 K À1 respectively. Based on two temperatures, the volumetric thermal expansion coefficient for the title compound is À63 (6) Â 10 À6 K À1 .
Another feature of the anion-cation interaction is that the NiÁ Á ÁO long-distance interaction is not perpendicular to the mean plane formed by Ni and the four P atoms but tilted at an angle of 74 (Fig. 3). This tilted orientation is also present in the crystal structures of VASCIB (Williams, 1989) and XUQYOZ (Higgs et al., 2010) with angles of 73 and 71 , respectively.
A packing diagram of 1 at 100 K viewed down [100] is shown in Fig. 4; there are C-HÁ Á ÁX (X = O, F) interactions, but because of the disorder of the triflate ion they are not described in detail.

Database survey
Dppe is a very common ligand: more than 2800 structures are reported in the Cambridge Structural Database (CSD version 5.38, updated ofMay2017; Groom et al., 2016), 240 of them are with nickel, and only one (LUCLOK; Uehara et al., 2002) has triflate as counter-ion. In this example, as in other reports of nickel with different ligands (e.g. Lyubartseva et al., 2013), the triflate anions are outside the coordination sphere as is the case with the title compound and with the two reports with different counter-ions: NO 3 À (VASCIB; Williams, 1989) and Cl À (XUQYOZ; Higgs et al., 2010).
For comparison, compounds with similar structures to the title compound and the same metallic group (group 10: P II , Pt II ) with bis[1,2-bis(diphenylphosphanyl)ethane], show almost an ideal square-planar geometry and also counter-ions outside the coordination sphere (see, for example, Engelhardt et al., 1984).
With respect to the Ni-P distances, we found in the CSD that both equivalent and non-equivalent Ni-P distances occur for Ni(+2)-bis(diphosphines), although it is hard to discern a pattern: for example, the Ni complexes formed with the 1-para-X-phenyl-3,6-triphenyl-1-aza-3,6-diphosphacycloheptane ligand, X = Cl (IFOFOA) or Br (IFOFEQ), are isostructural compounds that crystallize in space group P1 (Stewart et al., 2013), but one has equivalent Ni-P bonds while the other does not.

Synthesis and crystallization
The title compound was prepared in two steps. First, 1,2-bis(diphenylphosphanyl)ethane and nickel(II) chloride hexahydrate (molar ratio 1:2) were reacted in hot ethanol. The product obtained, dichloro-bis[1,2-bis(diphenylphosphanyl)ethane]nickel(II), was then reacted with silver(I) trifluoromethanesulfonate in dichloromethane (molar ratio 1:2). The product of this second reaction was filtered off and purified using a Soxhlet system with dichloromethane in which the by product, silver(I) chloride, was insoluble (Cano, 2012).
The crystallization process was carried out by dissolution of the purified compound in the minimum volume of methanol at 323 K (' 2.5 mg mL À1 ). When the solution reached room temperature, it was transferred to a chamber saturated with diethyl ether. Diffusion of diethyl ether into the solution over a three-week period led to the formation of translucent intensely yellow block-like crystals at the bottom and on the walls of the vessel.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were positioned geometrically and refined as riding with C-H = 0.95-0.99 Å and U iso (H) = 1.2U eq (C). Packing view of 1 at 100 K along the a axis.

Bis[1,2-bis(diphenylphosphanyl)ethane]nickel(II) bis(trifluoromethanesulfonate)
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.