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Pyridine-based complexes of copper(II) chloride and bromide: ligand
conformation effects on crystal structure. Synthesis, structure and
magnetic behavior of Cu(2-Cl- 3 - X’py) 2 X 2 [X, X’= Cl, Br]
Robert J. DuBois,a^ Christopher P.Landee,b^ Melanie Rademeyerc^ and Mark M. Turnbulla a (^) Carlson School of Chemistry and Biochemistry and bDept. of Physics, Clark University, 950 Main St., Worcester, Massa- chusetts 01610 USA. c) Dept. of Chemistry, University of Pretoria, Private Bag X20, Hatfield 0028 , South Africa.
Abstract : Reaction of copper(II) chloride or bromide with 2-chloro- 3 - bromopyridine (2 or 2,3-dichlo- ropyridine generates a family of compounds of the general formula L 2 CuX 2 ( 1 - 4 ). X-Ray crystallog- raphy shows that the bromide complexes (3-bromo- 2 - chloropyridine)dibromidocopper(II) ( 1 ) and (2,3- dichloropyridine)dibromidocopper(II) ( 3 ) are particularly unusual in that they crystallize with both the syn- and anti-conformation structures in the same crystal. A review of the literature on complexes of the formula (substituted-pyridine) 2 CuX 2 suggests that these are the first examples of such complexes. The members of the family show a variety of magnetic behaviors and variable temperature magnetic susceptibility data indicate that 1 is essentially paramagnetic (θ = - 0.9(1) K) while 3 is weakly ferro- magnetic (J = 2.9(1) K). Compound 2 [(3-bromo- 2 - chloropyridine)dichloridocopper(II)] is well fit by the uniform one-dimensional antiferromagnetic model (J = - 19.6(1) K), while compound 4 [(2,3-dichlo- ropyridine)dichloridocopper(II) exhibits weak anti-ferromagnetic interactions (J = - 3.68(3) K).
INTRODUCTION Transition metal complexes are very interesting regarding their magnetic properties and offer a wide field of study. Recent work on transition metal complexes in areas such as spin-crossover [ 1 ], sin- gle-molecule magnetism [ 2 ], the magnetocaloric effect [ 3 ] and magnetic nanoparticles [ 4 ] shows the continuing interest and potential in such materials. One interesting aspect of such complexes arises from their lattice properties such as local geometry and the presence of non-bonding interactions be- tween molecular units such as hydrogen and halogen bonding. One particular family of these complexes is the pyridine-based copper(II) halide complexes [(sub-py) 2 CuX 2 ,^ sub-py = substituted pyridine-based ligand, X = Cl, Br]. The azophilicity of the Cu(II) ion allows for a very wide variety of pyridine-based ligands, in terms of their electron-donating ability and steric requirements, to coordinate to the metal. The resulting compounds adopt two general forms; square planar and tetrahedral. Significant distortion of the tetrahedral geometry is observed in virtually all cases due to the Jahn-Teller effect.
Within each of these coordination geometries, there also exists the possibility of conformational geometric differences with respect to the pyridine-based ligand if the ligand is non-symmetrically sub- stituted. As illustrated in Figure 1, the substituents can adopt a conformation where the proximal sub- stituents lie on the same face of the copper coordination plane (the syn-conformation, Fig. 1a) or on op- posite faces of the copper coordination plane (the anti-conformation, Fig. 1b). For square planar com- plexes, the pyridine ligand is not observed to lie in the copper coordination plane in the absence of che- lating ligands.
a) b) Figure 1. The a) syn- and b) anti-conformations of a (sub-py) 2 CuX 2 complex.
Although a truly tetrahedral complex would not have a ‘coordination plane,’ the Jahn-Teller distortion at Cu(II) is sufficient to make definition of a mean coordination plane unambiguous and thus the termi- nology is still useful for distorted tetrahedral systems. We have been interested in this family of complexes for some time for the development of mag- netostructural correlations [ 5 ]. There are two common superexchange pathways observed in these mate- rials, the bi-halide pathway and the two-halide pathway (see Figure 2). In the bi-halide pathway, a bridge is formed between neighboring Cu(II) ions via a pair of bridging halide ions. The Cu-X dis- tances within a bridge are generally not symmetric, but rather exhibit one traditional Cu-X bond and one semi-coordinate interaction of variable length. The important structural parameters are the Cu-X dis- tances, the Cu-X-Cu angle and the planarity of the system [ 6 ]. In the two-halide pathway, magnetic ex- change between neighboring Cu(II) ions is achieved via overlap of the van der Waals spheres of the hal- ide ions. Here, the important structural parameters are the inter-halide distance, the Cu-X…X bond an- gles and the Cu-X…X-Cu torsion angle [ 7 ].
a) b) Figure 2 angle (θ) and the planarity of the Cu. The a) bi-halide and b) two halide superexchange pathways. In the bi 2 X 2 ring appear to control the superexchange. In the two-halide pathway, the distances d and d’, the-halide pathway, the distance (d), angles (θ 1 and θ 2 ) and torsion angle (τ) are the important parameters in the superexchange pathway.
N
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Cu
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N
X
X
N
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NX
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Cu
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Cu Br Br Cu
d
Bis( 3 - bromo- 2 - chloropyridine)dichloridocopper(II) (2). 3 - Bromo- 2 - chloropyridine (0.191 g, 1.0 mmol) was dissolved in 4.0 ml of 1 - propanol. Copper(II) chlo- ride dihydrate (0.089 g, 0.5 0 mmol) was dissolved in 4.0 ml of 1-propanol to give a light green solution. The copper(II) chloride dihydrate solution was added dropwise to the ligand solution with stirring and the resulting solution was left for slow evaporation. After 16 days, a mixture of green crystals and re- crystallized 3-bromo- 2 - chloropyridine was observed. The mixture was slurried in a small amount of 1- propanol to dissolve the unreacted ligand and quickly vacuum filtered to give green crystals of 2 in 32 % yield (0.084 grams). IR: 3086 w, 1577 s, 1424 w, 1394 s, 1155 m, 1049 s, 803 s, 753 m, 714 s, 678 m. CHN for C 10 H 6 N 2 Cl 2 Br 4 Cu, found (calc.): C, 22.67(23.13); H, 1.14(1.16); N, 5.53(5.28).
Bis (2,3-dichloropyridine)dibromidocopper(II) ( 3 ) Copper(II) bromide (0.111 g, 0.50 mmol) was dissolved in 10.0 ml of 1-propanol with warming result- ing in a dark red solution. 2, 3 - Dichloropyridine (0.148 g, 1.0 mmol) was then dissolved in 4.0 ml of 1- propanol. The 2, 3 - dichloropyridine solution was then added to the copper(II) bromide solution. The so- lution was left on the desktop for slow evaporation. After 14 days, a mass of black thin rectangular crys- tals were present in a minimal volume of solution. These crystals were isolated by vacuum filtration and washed with a small amount of 1-propanol to give a 62. 8 % yield (0.163 g). IR: 3089 w, 3068 w, 1590 w, 1578 m, 1555 w, 1429 w, 1412 m, 1399 s, 1255 w, 1221 m, 1165 s, 1128 w, 1075 s, 1035 w, 985 w, 938 w, 806 s, 792 s, 774 m, 716 s 691 m. CHN for C 10 H 6 N 2 OCl 4 Br 2 Cu, found (calc.): C, 22.85(23.13); H, 1.00(1.16); N, 5.20(5.39).
Bis (2, 3 - dichloropyridine)dichloridocopper(II) ( 4 ). 2 ,3-Dichloropyridine (0.248 g. 1.7 mmol) was dissolved in 15.0 ml of acetonitrile with warming. Cop- per(II) chloride dihydrate (0.153 g, 0.9 g) was added directly, resulting in a brown solution after stirring for ca. 10 minutes. The solution was then allowed to evaporate slowly at room temperature. After eight days a mass of dark black crystals were present in solution which were isolated via vacuum filtration and washed with a small amount of acetonitrile to give a 34.5% yield (0.132 g). IR: 3093 w, 3075 w, 1581 m, 1556 w, 1407 s, 1250 w, 1225 m, 1163 m, 1071 s, 991 w, 948 w, 806 s, 775 w, 718 s, 687 m. CHN for C 10 H 6 N 2 Cl 6 Cu, found (calc.): C, 20.83(21.27); H, 1.03(1.03); N, 4.82(4.96).
Magnetic data Magnetic data for compounds 1 - 4 were collected using a Quantum Design MPMP-XL SQUID magne- tometer. Powdered crystalline samples were loaded in a gelatin capsule which was then placed in a straw and mounted in the instrument. Magnetic moments were measured between 0 and 50 kOe at 1.
K. In order to determine if hysteresis was present, several data points were collected as the magnetic field returned to 0 kOe. No hysteresis was observed. M(H) for all samples was linear to at least 3 kOe. Magnetization was then measured between 1.8- 3 10 K in an applied field of 1 .0 kOe. Background cor- rections for the gelatin capsule and straw (measured independently) and the diamagnetic contributions of the constituent atoms (estimated via Pascal’s constants [ 8 ]) were made. After magnetic measure- ments, powder X-ray diffraction was carried out on all SQUID samples and compared to the single crys- tal structure to confirm the phase of the complexes matched the single crystal structure for 1 - 3 ; no impu- rities were detected.
Single-crystal X-ray diffraction data Data collections were carried out on a Bruker D8 Venture diffractometer fitted with a Photon 100 CMOS detector employing graphite-monochromated Mo-Kα radiation, using φ and ω scans. Using the SAINT+ software [ 9 ], the data were reduced and absorption corrections were made using the SADABS program [ 10 ]. Structure solution was carried out using the program SHELXS- 97 [ 11 ] while refinements were performed using SHELXL- 2014 [ 12 ]. Non-hydrogen atoms were refined using anisotropic thermal parameters. Hydrogen atoms were placed in their geometrically calculated positions and refined as a rid- ing model with fixed isotropic thermal displacement parameters, Uiso(H) = 1.2 Ueq(C, N). Crystallo- graphic information for compounds 1 and 3 can be found in Table 1. Single crystal X-ray data was col- lected multiple times on crystals of 2. All data collections resulted in the same crystal parameters, but the crystals were unstable and diffracted poorly. Although the general coordination environment of the Cu(II) ion and conformational geometry of the complex was clear, suitable refinements could not be ob- tained (minimum R 1 ~ 1 4 %). We report here only the cell constants [ 13 ] and general geometry of the complex. The structures of 1 and 3 have been deposited with the CCDC as deposit numbers: 1896820 , 1 ; 1896822 , 3.
Table 1 : Crystal data for compounds 1 and 3. 1 3 Empirical Formula C 10 H 6 N 2 Br 4 Cl 2 Cu C 10 H 6 N 2 Br 2 Cl 4 Cu MW 608.25 519. T(K) 150(2) 150 (2) Wavelength(Å) 0.71073 0. Crystal system Monoclinic Monoclinic Space group C2/c C2/c a (Å) 18.9530(17) 18.9284(16) b (Å) 6.3155(6) 6.2543(5)
Table 3 : Comparison of two-halide superexchange parameters of 1 and 3 1 3 X···X distance (Å) 4.945 4. Cu-X···X angle (˚) 80.4˚ 79.9˚ Table 4 : Comparison of bi-bridged chain parameters of 1 and 3 1 3 X···Cu distance (Å) 5.192 5. Cu-X···Cu angle (˚) 108.1˚ 107.8˚
RESULTS AND DISCUSSION Literature review of compounds with the general formula (sub-py) 2 CuX 2 : Criteria used for the selection of compounds: A search of the Cambridge Crystallographic Database [ 14 ] was conducted using the following search parameters: a) Copper(II) complexes with two coordinated halide ions (no restriction on the identity of the halide ion) and two coordinated pyridine-based moieties (those containing a pyridine ring bonded through the N-atom to the Cu(II) ion). No restrictions were placed on the coordination number of the Cu(II) ion to allow for the possibility of bridging halide ions. However, complexes with bridging or chelating pyri- dine-based substituents were not generally included in the final list as they restricted the geometry of the copper coordination sphere, virtually always generating cis-configurations in the case of chelating lig- ands. Some exceptions were included ( vide infra ) as they represented particular and unusual coordina- tion spheres within the general constraints. Further, all examples found were either copper(II) chloride or bromide complexes. We assume that this arises due to the generally poor solubility of copper(II) flu- oride complexes, making crystallization and purification difficult, and the observation that iodide ion tends to reduce Cu(II) to Cu(I), although some exceptions are known.
b) Complexes with more than four independent ligands were not included in the final selections as they restricted the ability of the complex to adopt either syn- or anti-geometries and reduced/prevented the complexes from developing bridging halide ion linkages.
c) Complexes with excessively large substituents on the pyridine ring were not generally included as the final crystal packing could be controlled solely by the stearic bulk and/or additional interactions be- tween the ancillary groups (intermolecular) rather than by the central core of the complex.
d) Only square-planar compounds, or mildly distorted square-planar compounds, were considered as the lack of a well-defined coordination plane renders the definition of the syn- and anti-geometries moot.
In all, 101 compounds were identified and categorized based on the conformational geometry of the pyr- idine substituents (syn or anti) and the nature of the connectivity between molecular units (bihalide bridged chain, two-halide bridged chain, bihalide bridged dimer) if any. The following discussion is or- ganized by the substitution pattern on the pyridine moiety and then, within each of those categories, by the nature of the conformation about the Cu(II) ion and type of intermolecular interaction.
1) 2-Substituted pyridine moieties. A significant number of copper(II) halide complexes have been reported where the pyridine moiety con- tains a single substituent in the 2-position. The syn/anti conformations of rings with a substituent in the 2 - position and no substituent in the 6-position are perhaps the most likely to be affected by steric con- siderations because the difference between the two sides of the pyridine ring, close to the metal center, is the most distinct and most likely to be controlled by direct interactions between the substituents. In this category we find simple alkyl substituted pyridine moieties such as bis(2-methylpyridine)dichlo- ridocopper(II)^15 and bis(2-methylpyridine)dibromidocopper(II).^16 In both cases, the complexes crystal- lize as the syn-conformers (see Fig. 3a),^17 although the ligands are not co-planar (due to steric repulsions between the methyl substituents). Syn-conformation molecules may form dimeric structures via short Cu…Cl contacts (3.382 Å) as observed in these molecules (Fig. 3b). One face of the molecule is blocked by the 2-substitutents, effectively preventing close contact to a neighboring molecule on that face, but the other is free and this is the most common intermolecular structural synthon in such mole- cules if the metal geometry is planar ( vide infra ).
a) b) Figure 3. a) A model of bis(2-methylpyridine)dichloridocopper(II)^1515 showing the syn-ligand conformation. b) The dimer structure formed by short Cu…Cl contacts.
Figure 5. Two molecules of bis(2-methoxypyridine)dichloridocopper(II) showing the distorted planar geometry at the metal ion and the two-halide linkage, forming dimers.
Finally, in the 2-substituted pyridine category, it is worth mentioning a small number of di-pyridyl com- pounds where the bridge between the pyridine rings is long enough to permit the formation of either cis- or trans-configurations. Most of the reported complexes show trans-configurations with syn-confor- mations^23 (as expected due to the chelation effect) and generate bihalide-bridged dimers with the oppo- site face blocked by the bridge linking the two pyridine rings [trans-dichlorido-(4,5-bis(2-pyridylethyl- sulfanyl)-1,3-dithiole- 2 - thione)copper(II)^23 b^ is shown as an example in Figure 6a]. However, in the case of bis[dichlorido-(μ2-1,3-di(2-pyridyl)propane-N,N')-copper(II)]^24 , the 1,3-dipyridylpropane ligands bridge between pairs of copper chloride units, holding the Cu(II) ions more than 7 Å apart in a pseudo- syn conformation (Figure 6b). A complex layer network is then formed by short two-halide contacts between the chloride ions.
a) b) Figure 6. a) The dimeric structure of trans-dichlorido-(4,5-bis(2-pyridylethylsulfanyl)-1,3-dithiole- 2 - thione)copper(II). b) The structure of bis[dichloro-(μ2-1,3-di(2-pyridyl)propane-N,N')-copper(II)].
We will now consider complexes with 2,n-substitution on the pyridine ring, where n ≠ 6. With respect to the interactions near the metal center, these compounds may be expected to behave similarly to their 2 - substitutent counterparts, but intermolecular interactions may be quite different due to the ancillary groups. A significant number of 2,3-disubstituted pyridine-based complexes have been reported. The
bis(2,3-dimethylpyridine)X 2 Cu complexes (X = Cl, Br) both crystalize as anti-conformers,^25 but no hal- ide…halide or metal…halide contacts link the molecules into more extended motifs. Although these are the only reported 2,3-dialkylpyridine compounds, a number of compounds with one methyl group and one halogen have been reported. The bis(2-methyl- 3 - X’-pyridine)X 2 Cu complexes with X, X’ = Br, Cl are known.^26 Of these, only the X’ = Cl, X = Cl compound crystalizes in the syn-conformation and it forms the expected bihalide bridged dimer with Cu…Cl = 2.727 Å. The remaining three complexes crystalize in the anti-conformation and all form bihalide bridged chains with Cu...X distances between 4.3-4.7 Å (Figure 7).
Figure 7. Bihalide-bridged chain structure of bis(3-bromo- 2 - methylpyridine)dichloridocopper(II).
Reversing the positions of the substituents generated a very similar family of complexes, but with its own variations.^27 Bis(2-chloro- 3 - methylpyridine)dichloridocopper(II) crystallizes in the syn-confor- mation and forms the expected bihalide bridged dimeric structure (Cu…Cl = 3.579 Å), while the bis(2- bromo- 3 - methylpyridine)X 2 copper(II) (X = Br, Cl) compounds are both in the anti-form with the bro- mide compound forming two-halide bridged chains (Cu…Br = 4.482 Å) and the chloride analogue fail- ing to exhibit such intermolecular contacts. The most interesting member of this family is the bis(2- chloro- 3 - methylpyridine)dibromidocopper(II) complex which exhibits polymorphs where one is in the syn-conformation while the other is anti (Figure 8). Crystals of the syn-complexes exhibit the typical dimeric structure (Cu…Br = 3.579 Å), while those of the anti-conformation form two-halide bridged chains (Br…Br = 4.505 Å).
trend as well.^33 However, several of the complexes show significant distortions toward tetrahedral ge- ometry (Figure 9b). In these cases, the dimer structure does not form and intermolecular interactions take place predominately through the two-halide pathway.
a) b) Figure 9. a) The dimeric structure of bis(μ-chloro)-dichlorido-tetrakis(2,5-dichloropyridine)-di-copper(II).^32 b^ b) The struc- ture of dichlorido-bis(2-chloro- 5 - iodopyridine)-copper(II).^32 a
Here again, we see the existence of polymorphs for some of the compounds with the anti-conformer also known. This is true for bis(2-bromo- 5 - fluoropyridine)dichloridocopper(II), bis(2,5-dichloro- pyridine)dichloridocopper(II) and bis(5-bromo- 2 - chloropyridine)dichloride-copper(II).^32 The intermo- lecular contacts for these compounds occur via the two-halide pathway. Bis(2-chloro- 5 - methylpyri- dine)dichloridocopper(II) crystallizes in the same pattern.^34
The copper chloride and bromide complexes of 2-amino- 5 - trifluoromethylpyridine provide an excellent example of the diversity that arises from the plasticity of the Cu(II) ion.^35 The bromide complex crystal- lizes in the anti-conformation (Figure 10a) and exhibits the formation of bihalide-bridged chains (simi- lar to that shown in Figure 7), one of the common motifs for anti-conformation complexes. The chlo- ride compound, however, provides a rare example of an anti-conformation compound which crystallizes as bihalide bridged dimers (Figure 10b). While for most anti-conformation molecules, the interactions between the pyridine substituents lead to destabilization of the dimer structure in favor of the more sepa- rated bibridged chains (greater X…Cu distances), the possibility of halogen bonding across the dimer exists between the fluorine substituents (shortest F…F distance is 2.99 Å) which may explain the stabil- ity of this unique compound.
a) b) Figure 10. The a) bromide and b) chloride copper(II) complexes of 2-amino- 5 - trifluoromethylpyridine.
We also note one example of a (2,5-disubstitutedpyridine) 2 CuCl 2 complex which is perhaps best de- scribed as distorted square-planar, but where the pyridine ligands are cis, rather than trans.^36 This very uncommon configuration likely arises from the very bulky substituents on the pyridine ring (the ligand is ethyl 6-((4,4-dimethyl-3,4-dihydro-2H-thiochromen- 6 - yl)ethynyl)nicotinate). Possibly due to the length of the ligand, steric repulsions between molecules override the normal steric constraints at the metal center, which result in the bulk of members of this group of compounds having trans-configura- tions. Finally, a small family of 2,3,5-substituted pyridine complexes have been reported. The four bis(2- amino-3,5-X’ 2 pyridine) 2 CuX 2 compounds (X, X’ = Cl, Br)^37 all crystallize in the anti-conformation and pack into chains via bihalide linkages with Cu…X distances of ~ 3.0 Å for the copper chloride com- plexes and ~ 4.1 Å for the copper bromide complexes. The copper bromide and copper chloride com- plexes of 2-amino- 5 - chloro- 3 - fluoropyridine, however, show different coordination conformations and structures.^38 The CuCl 2 complex crystallizes in the anti-conformation and generates halide bibridged chains (Cu…Cl = 2.97 Å), similar to the 2-amino-3,5-X’ 2 pyridine complexes, while the bromide ana- logue occurs in the syn-conformation and generates bibromide bridged dimers similar to Figure 8a (Cu…Br = 2.67 Å). In the latter case, fluorine-fluorine halogen bonds (F…F = 3.1 Å) further link the dimers into chains.
A small group of compounds have been reported with substituents in both the 2- and 6-positions (and more substituents in some cases) and as one might expect, the severe steric crowding around the metal center substantially reduces the possibilities for interactions. Dibromido- and dichloridobis(2,6-dime- thylpyridine)copper(II) are both known^39 and while the bromide complex exhibits weak interactions via the two-halide pathway (Br…Br = 4.86 Å), no such interactions are seen in the chloride analogue. (2,4,6-Trimethylpyridine) 2 CuCl 240 also exhibits weak interactions via the two-halide pathway (Cl…Cl =
4.03Å), the other generates halide bibridged dimers which may be stabilized by halogen bonds between fluorine atoms on adjacent perfluorophenyl rings within the dimers (F…F = 2.92 Å). Two 3,5-disubsti- tutedpyridine complexes in this category have been reported. Dibromidobis(3,5-dimethyl)copper(II) form chains via bibromide bridges (Cu…Br = 3.29 Å)^56 while the packing of bis(4-(t-butyl(oxido)aza- nyl)-3,5-dichloropyridine)dichloridocopper(II) does not produce significant potential superexchange in- teractions between the metal coordination spheres.^57
Synthesis and structure of bis-2,3-dihalopyridinedihalidocopper(II) complexes: Reaction of one equivalent of CuX 2 with two equivalents of 3 - X’- 2 - chloropyridine, where X’ is chlorine or bromine, produced four compounds; (3-bromo- 2 - chloropyridine) 2 CuBr 2 ( 1 ), (3-bromo- 2 - chloro- pyridine) 2 CuCl 2 ( 2 ), (2, 3 - dichloropyridine) 2 CuBr 2 ( 3 ) and (2, 3 - dichloropyridine) 2 CuCl 2 ( 4 ). Good quality crystals of 1 and 3 were isolated from the reaction mixture for single x-ray diffraction data collection. Only poor quality crystals of 2 were obtained after multiple attempts using a variety of techniques and solvents and no crystals of 4 of X-ray quality could be prepared in our hands.
Figure 11. Preparation of compounds 1 - 4.
X-ray Structure of Bis(3-bromo- 2 - chloropyridine)dibromidocopper(II)(1). Compound 1 crystalizes in the monoclinic space group C 2/c. The molecular unit is shown in Figure 1 2. Two independent molecules are seen in the unit cell, one in the anti-conformation (Cu1) and one in the syn-conformation (Cu2). Selected bond lengths and angles are given in Table 2. The Cu 1 ion sits on an inversion center, creating a planar coordination environment and forcing trans bond angles to be 180 as required by symmetry; the Br1-Cu1-N11 angle is 91.02(0). The pyridine rings are nearly planar with a mean deviation of the constituent atoms of 0.0044Å. They are inclined 78.1 relative to the Cu1 coordi- nation plane. Cu2 sits on a two-fold rotation axis and adopts a highly distorted tetrahedral environment with a mean trans angle [ 7 ] of 152.4. The two independent N-Cu-Br angles are just over 90. The N21-pyridine rings are only slightly less planar (mean deviation = 0. 0 116 Å) than seen for the N11 ring
N
X'
Cl +^ CuX^2 N
X' Cl CuX 2 1 : X', X = Br^2 23 : X' = Br, X = Cl: X' = Cl, X = Br 4 : X', X = Cl
ligands and adopt a syn-conformation. The N21 and N21A rings are canted 60.1 relative to each other, presumably to reduce steric hindrance between the halogen substituents. The Cu-Br and Cu-N distances are within the normal parameters for copper(II) bromide and copper(II) pyridine bond lengths as de- scribed in the literature review (vide supra).
Figure 12. tion spheres are labeled. Hydrogen atoms are shown as spheres of arbitrary size and not labeled. Molecular unit of 1 shown as 50% probability thermal ellipsoids. Only the asymmetric unit and copper coordina-
As described in the literature review above, close contacts may be seen amongst nearest neighboring bromide ions and bromine and chlorine atoms on the pyridine ring which influence crystal lattice (Fig- ure 13). Cu1 and Cu2 molecules are related by unit cell translations and stack parallel to the b - axis. The stacks then form layers parallel to the bc - plane which alternate between Cu1 containing layers and Cu containing layers in the a - direction. Halogen bonds provide significant stabilization. Br1 ions interact with Br23 atoms forming Type II halogen bonds [ 58 ] [dBr1..Br23A = 3.421 Å, Cu1-Br1…Br23A = 87.4, Br1…Br23A-Cu2A = 169.3, Symm. Op. A: x+0.5, y+1.5, z] which link Cu1 molecules to Cu2 molecules.
Figure 13. Packing of 1 viewed parallel to the b - axis. Dashed lines represent halogen bonds (see text for details). Further, Cu2 molecules are linked to Cu2 molecules (parallel to the b - axis) via Type II halogen bonds between Br2 ions and Cl22 atoms [dBr 2 ..Cl22A = 3. 766 Å, Cu 2 - Br 2 …Cl 22 A = 108. 7 , Br2…Cl 22 A-C 2 2A =
- 5 , Symm. Op. A: 1.5– x, 1.5-y, - z]. A number of weaker halogen bonds are also observed in the
intermolecular halide…halide and halide…copper contacts are nearly 5Å or greater suggesting that only weak magnetic interactions are expected.
Structure of (3-bromo- 2 - chloropyridine)dichloridocopper(II)(2). Repeated attempts failed to produce crystals of sufficient quality and stability to allow for full data col- lection and refinement of and X-ray structure for compound 2. However, repeated attempts did produce a reproducible unit cell (see Ref. 13) and well a defined coordination sphere for the complex. Com- pound 2 is the chloride analogue of compound 1 , but unlike 1 , only a syn-conformation, distorted tetra- hedral environment is observed, rather than both syn- and anti-conformations (Figure 15). The mean trans angle at Cu1 is 150 and the angle between the pyridine rings is ~ 53.
Figure 15 pra). – The coordination geometry of compound 2. Full refinement of the X-ray structure was not possible (vide su-
The general packing arrangement of the molecules can also be confirmed from the data available, but distances and angles must be considered to have potentially large esds given the poor quality of the re- finement. Short Cl…Cl contacts [dCl1…Cl2A ~ 3.7 ÅCu1-Cl1…Cl2A ~157°, Cl1…Cl2A-Cu1A ~ 167°, Cu1- Cl1…Cl2A-Cu1A ~^3 °,^ Symm. Opp. A: x+1, y,z] create a uniform chain superexchange pathway parallel to the a - axis in the crystal. Additional halogen bonding is present in the structure (the closest Br…Br dis- tance is ~ 3.5 Å), but detailed analysis will have to wait for higher quality, stable crystals.
Magnetic behavior Magnetic susceptibility data for compounds 1 - 4 were obtained on a Quantum Design MPMP-XL SQUID magnetometer using powdered samples. To verify that the powdered sample used for magnetic data collection was the same phase as the bulk material, IR analysis was performed on the sample prior to magnetic data collection. To verify the purity of the SQUID samples, powder X-ray diffraction pat- terns were collected on 1 - 3 after being run in the SQUID and compared to theoretical patterns deter-
mined from the single crystal data (calculated powder patterns were obtained using Mercury [ 17 ]). Pa- rameters for the best fits to the data shown in Table 5. Models were taken from Reference 59 and fit us- ing the Η = −𝐽 ∑^ 𝑛𝑛 𝑆𝑖 ∙ 𝑆𝑖+ 1 Hamiltonian
Table 5: (^) = Weiss constant, p = paramagnetic impurity]Fitted values for the magnetic susceptibility data of compound 1 - 4. [C = Curie Constant, J = exchange constant, Compound Model C Oe) (emu-K/mol- J (K) (K) P (%) 12 Curie1D-AFM-Weiss 0.429(5)0.433(1) (^) - 19.6(1) - 0.9(1) (^1) 0.38(2)* 34 1D1D--FMAFM 0.416(2)0.414(11) (^) - 3.682.9(1)(3) 0.29(5) 0.08(9) 1 *
- Paramagnetic impurity value fixed at 1%.
Bis(3-bromo- 2 - chloropyridine)dibromidocopper(II)( 1 ). Magnetic interactions in compound 1 are vanishingly weak. No maximum is seen in χ(T) down to 1.8 K (Figure 16) and only a slight downturn is visible in the data at low temperatures when plotted as χT(T). The data were fit to the Curie-Weiss law from 5- 31 0 K and yielded a very weak antiferromagnetic inter- action (θ = - 0.9(1) K).
Figure 16 χ vs. T (□) and 1/ vs. T (∆) plots for compound 1. Solid lines represent the best fit to the Curie-Weiss law.
