Crystal structure of [tris(pyridin-2-ylmethyl)amine-κ4 N]copper(II) bromide

The complex [tris(pyridin-2-ylmethyl)amine]copper(II) bromide adopts a trigonal–bipyramidal coordination geometry about the CuII ion. The outer sphere bromine counter-ions are severely disordered.

In the asymmetric unit of the title compound, [CuBr(C 18 H 18 N 4 )]Br, there are three crystallographically independent cations. One of the cations exhibits positional disorder of the pyridin-2-ylmethyl groups over two sets of sites with refined occupancies of 0.672 (8) and 0.328 (8). The outer-sphere bromine counter-ion is severely disordered over multiple sites. In each cation, the Cu II ion is coordinated by the four N atoms of the tris(pyridin-2-ylmethyl)amine ligand and one bromine and adopts a slightly distorted trigonal-bipyramidal geometry.

Chemical context
Atom Transfer Radical Addition (ATRA) reactions involve the formation of carbon-carbon bonds through the addition of saturated poly-halogenated hydrocarbons to alkenes (Eckenhoff & Pintauer, 2010). First reported by Kharasch in the 1940s (Kharasch et al., 1945), the reaction incorporates halogen-group functionalities within products which can be used as starting reagents in further functionalization reactions (Iqbal et al., 1994). Subsequently, ATRA reactions have emerged as some of the most atom-economical methods for simultaneously forming C-C and C-X bonds, leading to the production of more attractive molecules (Eckenhoff & Pintauer, 2010). Most ATRA reactions proceed in the presence of a free-radical precursor or transition metal complex (catalyst), as the halogen-atom transfer agent and have been efficiently catalyzed by complexes incorporating nickel, ruthenium, iron, and copper (Eckenhoff et al., 2008). Studies suggest that the type of ligands used in ATRA reactions significantly influence the behavior of the catalyst generated due to different steric and electronic interactions with the metal atom (Matyjaszewski et al., 2001). Copper complexes made with tetradentate nitrogenbased ligands such as tris [2-(dimethylamino)ethyl]amine (Me 6 TREN), 1,4,8,4,8,, and tris(pyridin-2-ylmethyl)amine (TPMA) are currently some of the most active multi-dentate ligand structures used in atom-transfer radical reactions (Tang et al., 2008). Given the significance and application of complexes made from these tetradentate ligands, we report on the synthesis and crystal structure of the title compound [CuBr(C 18 H 18 N 4 )]Br (I) which incorporates tris(pyridin-2-ylmethyl)amine. ISSN 2056-9890

Structural commentary
There are three crystallographically independent copper(II) atoms within the asymmetric unit reported herein (Fig. 1). Each of the atoms adopts a slightly distorted trigonal-bipyramidal geometry and is coordinated by the four nitrogen atoms of the tris(pyridin-2-ylmethyl)amine ligand and one bromine atom (Table 1). The amine nitrogen and bromine atoms adopt the apical positions of the coordination environment and the pyridine nitrogen atoms are located in the equatorial plane. Derived metrics (bond lengths and angles) from the copper atoms to their respective coordinating atoms are typical (MOGUL analysis; Bruno et al., 2004). The -5 values for Cu1, Cu2 and Cu3 are 0.99, 0.99 and 0.89, respectively (Addison et al., 1984); the latter deviates the most from ideal geometry due to the disorder present in that molecule.
One of the three independent cations exhibits positional disorder of the pyridin-2-ylmethyl groups (see Refinement below for specific details). Despite this disorder, the connec-tivity is unequivocal. Unlike the polymorphic structure (Eckenhoff et al., 2008) that has crystallographically imposed symmetry on the pyridin-2-ylmethyl arms, the pyridin-2-ylmethyl groups on the cations reported here have geometries independent of the others. Furthermore, the structure here is mixture of Á and Ã conformations of the ligand, whereas Eckenhoff's structure has chirally resolved upon crystallization.

Supramolecular features
The prominent feature of the crystal packing within this structure is the excessive positional disorder of the outersphere bromine anions. These are observed in a channel within the lattice (Fig. 2) that presumably has unresolvable solvent of crystallization also present. Because there are no prominent charge surfaces, packing is solely due to van der Waals interactions.

Database survey
There are six reported copper(II) bromide structures deposited in the Cambridge Structure Database incorporating the tris(pyridin-2-ylmethyl)amine ligand derivatives (Groom et al., 2016; CSD Version 5.37 plus one update). Of those six structures, one is a dimer incorporating two bridging bromine ligands (Maiti et al., 2007) and the remaining five are monomers. Out of the five monomer structures, three incorporate methyl or methoxy electron-withdrawing groups (Kaur et al., 2015), while one incorporates hydroxyl electron-donating groups (He et al., 2000). The final structure is a polymorph of that presented here: it incorporates an unsubstituted TPMA ligand framework but adopts a different space group (cubic, P2 1 3) and unit-cell parameters (a = 12.633 Å ) due to lack of Labeling scheme for [tris(pyridin-2-ylmethyl)amine]copper(II) bromide. Atomic displacement ellipsoids depicted at 50% probability and H atoms as spheres of arbitrary radius. Some labels are omitted for clarity.
disorder in the ligand framework (Eckenhoff et al., 2008). Of the six total reported structures, four adopt similar distorted five-coordinate geometries as observed in complex (I), while two adopt a distorted six-coordinate geometry about the metal atom.

Synthesis and crystallization
Synthesis of tris(pyridin-2-ylmethyl)amine (TPMA) ligand: the TPMA ligand was synthesized according to modified literature procedures (Britovsek et al., 2005) Synthesis of tris(pyridin-2-ylmethyl)amine copper(II) bromide complex: TPMA (0.500 g, 1.72 mmol) was dissolved in 15 mL methanol in a 100 mL round-bottom flask. Copper(II) bromide (0.384 g, 1.72 mmol) was added to the flask to give a greenish-blue-colored solution. The reaction was allowed to mix for one hour then 30 mL of diethyl ether was transferred into the flask, facilitating the precipitation of the desired complex as a green powder. The mixture was filtered and the precipitate washed with excess diethyl ether solvent. The precipitate was dried under vacuum for 30  UV-Vis: max (MeOH) = 700 nm. Green-colored single crystals suitable for X-ray analysis were obtained by slow diffusion of diethyl ether into a concentrated complex solution made in methanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The two ordered cations, the major occupancy component of the disordered cation and all outersphere bromine atoms were modeled with anisotropic atomic displacement parameters. The minor occupancy component of the disordered cation was modeled with isotropic atomic displacement parameters. Hydrogen atoms were included in geometrically calculated positions with C-H = 0.99 (methylene) and 0.95 Å (aromatic) and U iso (H) = 1.2U eq (C).
The disorder of the pyridin-2-ylmethyl groups was observed as residual electron density oriented in approximately a mirror to the major occupancy components. The occupancies of the two components were refined summed to unity, yielding an approximately 0.67:0.33 ratio. The pyridine rings for both components were constrained to an ideal hexagon, with C-C = 1.39 Å .
All of the outer-sphere, non-coordinating bromine counterions were found to be disordered over multiple sites. Initially, occupancies were refined freely to identify possible site pairings. One bromine (Br4) was found to be nearly fully located at one site. In subsequent refinement cycles, residual density adjacent to the site was revealed and ultimately modeled as a bromine disordered over two sites with occupancies 0.80:0.20. Two bromine sites whose occupancies refined independently to nearly 50% were both set to 50% occupancy and assumed to be disorder of the same bromine atom (Br5/5A). Final residual electron density ranging from 8 to 13 e Å À3 was observed. Because an additional bromine was required for charge balance and there were no other counter-ions used during synthesis, it was assumed that the final bromine was disordered over multiple sites, presumably in concert with solvent from crystallization. Ultimately, seven locations were refined as partial-occupancy bromine atoms with a total occupancy summed to unity, yielding a 0.13:0.17:0.17:0.20:0.11:0.12:0.10 ratio of sites. The solvent contribution could not be reliably modeled.  Computer programs: APEX3 and SAINT (Bruker, 2015), SHELXT2014/2 (Sheldrick, 2015a), SHELXL2014/7 (Sheldrick, 2015b), XP (Bruker, 2015) and publCIF (Westrip, 2010 Data collection: APEX3 (Bruker, 2015); cell refinement: SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to solve structure: SHELXT-2014/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: XP (Bruker, 2015); software used to prepare material for publication: publCIF (Westrip, 2010).

[Tris(pyridin-2-ylmethyl)amine-κ 4 N]copper(II) bromide
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. Refinement. The outer sphere bromine anion atoms were all found to be disordered over multiple sites. Br4/4A was found to occupy two sites close to each other and was refined with occupancies summed to unity yielding an approximate 0.83:0.17 ratio. Br5/5A was modeled as two half occupancy bromine atoms from an initial, independent, refinement of the occupancies for these sites. Br6 is disordered over multiple sites. Occupancies of the sites were refined summed to unity yielding an approximately 0.14:0.17:0.17:0.20:0.11:0.12:0.09 ratio of site occupancies. Attempts to model this disorder as undifferentiated solvent did not meet with success. Furthermore, because the electron density associated with this is located within the enveloped developed by SQUEEZE, this routine could not be employed. The result is that there is some additional residual electron density that cannot be reliably accounted for.
Presumably there is solvent of crystallization present at the sites when they are not occupied by anions. This was not modeled.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.