Copper(II) Halide Salts with 1-(4′-Pyridyl)-Pyridinediium

The compounds [1,4′-bipyridine]-1,1′-diium [CuCl4] (1) and [1,4′-bipyridine]-1,1′-diium [CuBr4] (2) were prepared and their crystal structures and magnetic properties are reported. The compounds are isomorphous and crystallize in the monoclinic space group C2/c. The cation crystallizes in a two-fold disordered fashion with the terminal nitrogen and carbon atoms exhibiting 50% occupancies. This results in a crystal packing arrangement with significant hydrogen bonding that is very similar to that observed in the corresponding 4,4′-bipyridinediium complexes. Temperature dependent magnetic susceptibility measurements and room temperature EPR spectroscopy indicate the presence of very weak antiferromagnetic exchange. The data were fit to the Curie–Weiss law and yielded Weiss constants of −0.26(5) K (1) and −1.0(1) K (2).


Introduction
The importance of hydrogen bonding to crystal engineering has been noted in a number of recent studies [1][2][3]. The ability to predict or design an approximate packing structure using restricted avenues for strong hydrogen bonds is highly desirable in a variety of fields including crystal engineering [4], bioinorganic chemistry [5], and magnetism [6][7][8][9].
Compounds with the doubly cationic [1,4′-bipyridine]-1,1′-diium are here described. While it was evident that the modification of the (BH)2[CuX4] model to the (BH)[CuX4] scaffold could result in less well-isolated layers, chains, ladders or honeycombs, due to the loss of one bulky cation per CuX4 2− unit, it was posited that some unique structures could be discovered, since the use of doubly cationic molecules is much less common. Thus, by exploring the use of [1,4′-bipyridine]-1,1′-diium (B1H), a sterically long, doubly cationic, and versatile counter-cation, it was proposed that design features stemming from the directionality of the hydrogen bond donors would present themselves. The compounds [1,4′-bipyridine]-1,1′-diium [CuCl4] (1), [1,4′-bipyridine]-1,1′-diium [CuBr4] (2) are described here regarding the effect of hydrogen bonding on crystal packing and the effect of B1H as a design agent in the synthesis of magnetic materials.

Synthesis
The synthesis of 1 and 2 may be achieved directly in the appropriate concentrated acid solution with stoichiometric ratios of the starting materials. The synthetic method is summarized in Scheme 1 for 1 and 2. Both compounds grow as flat plate crystals, with the expected colors for copper(II) halide salts (yellow, 1; purple, 2). The asymmetric unit of 2 contains half of a CuBr4 2− unit and half of a disordered B1H dication. The molecular unit of 2 is shown in Figure 1; the CuX4 distorted tetrahedra and full B1H dications are generated by 2-fold rotation axes running though Cu1 and the central bond of the B1H dication. The CuX4 2− unit in 2 may be comparably described as a flattened tetrahedron or a distorted square planar unit with a mean trans angle [28] in 1 = 144.9° and in 2 = 142.9° (the corresponding τ4 parameters are 0.53 (1) and 0.50 (2) [29]. The Cu-X bond lengths are those expected for a CuX4 2− unit [13][14][15][16][17][18][19][20][21][22][23][24]; selected bond lengths and angles are given in Table 2. The Jahn-Teller distortion characteristic of four-coordinate copper(II) compounds causes variation of the bond angles from purely tetrahedral. The very close similarity in the structures of 1 and 2 is easily seen in the overlay drawing ( Figure S3). The orientation of the organic group B1H is two-fold disordered in the lattice. Any given dication is hydrogen bonded to the CuBr4 2− unit via N11-H11. A superposition of the two orientations of the dication is shown in Figure 2 for 1. Atoms N11/C24 and N21/C14 occupy identical positions in the lattice and each have 0.5 occupancy as required by symmetry. Atoms C12, C13, C15 and C16 are not disordered and have full occupancies. Labels A, B and C indicate symmetry generated atoms. The fact that the organic moiety is in fact disordered in the lattice indicates that the orientation must indeed be random; if the orientation were to alternate is some fashion, the crystals could exhibit a different space group and a larger unit cell to accommodate the additional symmetry.   (19) The N11-H11/C24-H24 hydrogen bond donors from B1H are involved in a bifurcated hydrogen bond to X1 and X2, with parameters shown below in Table 3 and shown in Figure 3. The symmetry equivalent disordered C24-H24 hydrogen bonding parameters are identical; however, the hydrogen bonding strength is presumed to be much weaker due to the lower polarity of the C-H bond. The angle between the normals to the symmetry equivalent pyridine rings in B1H is 45.9° (2) (48.2°, 1) as expected based on similar compounds [30][31][32]. Additional short C-H···Cl interactions are present in the structures (average dC···Cl~3.6 Å; average ∡ C-H···Cl~149° for 1). Complexes using B1H as the counterion have been previously reported including halido compounds of molybdenum and tungsten [31], the tetrachloridoplatinate and pentachloridostibnate salts [33] and a hexacyanidoferrate complex [34]. No structures involving tetrahedral metalate anions have been reported to our knowledge. The dihedral angles observed in the B1H cations are typically ~38° [30,31,33], significantly smaller than observed here, but in the ferrate species [34] that angle is 44°, comparable to what is observed for 1 and 2. None of the reported complexes exhibit the disorder observed for 1 and 2.  Table 3 for hydrogen bonding data. The crystal packing is such that the CuBr4 2− and B1H dications alternate in three dimensions as shown in Figure 4a (Figure S2 for 1). Due to the fact that there is only one B1H dication per CuBr4 2− unit, the CuBr4 2− units are close enough to each other to propagate weak magnetic interactions in three dimensions [1 − dCl···Cl = 4.36 Å, ∡Cu-Cl···Cl = 119° and 109°, ∡Cu-Cl···Cl-Cu = 71°; 2 − dBr···Br = 4.47 Å, ∡Cu-Br···Br = 120° and 108°, ∡Cu-Br···Br-Cu = 73°] [28]. It has been previously demonstrated in tetrahalidocuprate systems that the magnetic exchange is primarily dependent upon the halide···halide separation, rather than the distance between Cu(II) ions [35].  The crystal packing in 1 and 2 is very similar to that of (4,4′-bipyridinediium) [CuCl4] and 4,4′-(H2diazastilbene) [CuCl4] [10], each of which are composed of the ABAB ribbons with bifurcated hydrogen bonds controlling the packing pattern. The limited directionality of the hydrogen bond donors controls the packing of these compounds, thus enabling control of the 3D structure.

Magnetism
M(H) plots for 1 and 2 are shown in Figure 5. Comparison of the two plots indicates that the chloride analog exhibits weaker internal superexchange interactions as shown by the increased rate at which M(H) of 1 is nearing saturation at 50 kOe; 2 is still ~15% below saturation at that field. The internal interactions in compound 1 are sufficiently weak that the magnetization was ably fit to the Brillouin function, giving an average g-factor of 2.11. The susceptibility versus temperature plots of 1 and 2 are void of structure and resemble the susceptibility of paramagnetic complexes. As demonstrated by the slight downturn in the χT(T) plots at low temperatures ( Figure 6), there are very weak antiferromagnetic interactions. Given the weakness of the interactions, the 1/χ(T) data from 25 K to 310 K were fit to the Curie-Weiss model [36]. The fitted parameters for 1 and 2 are shown in Table 4 and indicate very weak coupling as expected. Weaker coupling in the chloride than the bromide complex is observed as expected of twohalide pathways of isostructural pairs of this type [28] and as expected from the M(H) data (vide supra).

Powder X-Band EPR
Despite the very weak nature of superexchange in these complexes, no hyperfine structure is observed in the powder EPR spectra. Both compounds 1 and 2 display axial g-tensors with largely Lorentzian line shapes as determined by fits using EasySpin [37]. The powder spectra are shown in Figure 7; the fitted g-values are g⊥ = 2.0594, g‖ = 2.3363 (gave = 2.1517) (1) and g⊥ = 2.0820, g‖ = 2.2361 (gave = 2.1334) (2). The broadening of g‖ is much larger in 1 (G-strain = 0.0523) than in 2 (G-strain = 0.0064), both because Δge is larger and because the interactions are much weaker, such that the effect of collapsed hyperfine coupling on the linewidth is larger. The average g-factor from EPR studies is considered more accurate than that from the fits to the magnetization and 1/χ(T).
(a) (b) Figure 7. The X-band powder EPR spectra of 1 (a) and 2 (b) at room temperature with fits shown displaced from the data by −5 intensity units in red.
The average g-factor found from the fit to the Brillouin function agrees within 2% of that found from EPR studies, and that from 1/χ(T) is within about 1% for 1. The average g-factor from the fit to 1/χ(T) for 2 is within 1.5% of that found from EPR studies. Similar to the findings of Farra et al. [38], g‖ for the bromide salt is much smaller than that of the chloride salt and the g⊥ is much larger for the bromide salt compared to the chloride salt, most likely due to the spin orbit coupling and ligand field effects of the bromide ion versus the chloride ion [39].

Materials and Methods
[1,4′-bipyridine]-1,1′-diium dichloride x-hydrate was purchased from TCI and was used as received except for the synthesis of 2 for which a halide exchange (vide infra) was performed. Copper(II) chloride dihydrate was purchased from Allied Chemical Corp. and copper(II) bromide was purchased from Alfa Aesar and used without further purification. Powder X-ray diffraction data were collected with a Bruker D8 Focus diffractometer. IR spectra were obtained with a Perkin Elmer Spectrum 100 FT-IR spectrometer via ATR. Elemental analyses were performed at the Marine Science Institute, University of California-Santa Barbara, CA, USA.

Magnetism
The magnetizations of 1 and 2 were collected on a Quantum Design MPMS SQUID magnetometer from 0 to 50 kOe at 1.8 K, with several data points collected from 50 kOe to 0 kOe to check for hysteresis. As expected of copper(II) complexes, none was observed. The magnetic susceptibilities of 1 and 2 were collected in a 1 kOe applied field from 1.8 K to 310 K. The data were corrected for the temperature independent paramagnetism of the Cu(II) ion and the diamagnetic moment of the constituent atoms as estimated from Pascals constants [41].

Crystal Structure Data Collection, Solution and Refinement
Single crystal X-ray diffraction data for 1 and 2 were collected at 150 K with Bruker APEX2 software (APEX2-2014, Bruker AXS Inc., Madison, WI, USA) employing ϕ and ω scans. The data reduction and refinement of the cell constants were performed with Bruker SAINT software [42]. Absorption corrections were made via SADABS [43]. The structures were solved using SHELXS-97 [44] and refined using SHELXL-2016 [45]. The disorder in the orientation of B1H in 1 and 2 was treated by constraining the anisotropic displacement parameters and positions of the disordered C and N atoms to be the same value/position (C14 and N21, and C24 and N11 occupy the same coordinates). The disorder occupancies were fixed at 50% to agree with the required symmetry in the final refinement; free refinement of the occupancies gave 53(2)% (1)/50(2)% (2). Details of the X-ray data collection and refinement are shown in Table 1. The data have been deposited with the CCDC: 1967063 (1), 1967066 (2).

Conclusions
The compounds (B1H) [CuX4] were synthesized and characterized via X-ray crystallography and magnetic studies, revealing the presence of very weak antiferromagnetic interactions in 3D between the CuX4 2− units which pack in an ABAB ribbon with the counter-dication B1H. Despite the loss of one strong N-H hydrogen bond donor in the ribbon, these structures resemble quite closely those of related compounds such as (4,4′-bipyridinediium) [CuCl4]. The disorder in the orientation of B1H may play an important role in the conservation of this ribbon structure. In future studies, the cation B1H can be modified to incorporate hydrogen bonding in different directions, modifying the crystal packing to achieve low dimensional magnetic systems with increased control. Further, the use of B1 as a monocationic ligand is under investigation.