Crystal structure of {2,2′-[N,N′-bis(pyridin-2-ylmethyl)cyclohexane-trans-1,2-diyldi(nitrilo)]diacetato}cobalt(III) hexafluoridophosphate

In the title compound, a CoIII center is coordinated by four N atoms and two O atoms, with the monodentate acetate groups of the ligand oriented trans with respect to each other, whereas the pyridine N atoms are coordinated in a cis configuration.

The title compound [Co(C 22 H 26 N 4 O 4 )]PF 6 , commonly known as [Co(bpcd)]PF 6 , where bpcd 2À is derived from the historical ligand name N,N 0 -bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N 0 -diacetate, crystallized by slow evaporation of a saturated acetonitrile solution in air. The cation of the hexafluoridophosphate salt has the Co III atom in a distorted octahedral coordination geometry provided by an N 4 O 2 donor atom set. The acetate groups, which are oriented trans with respect to each other, exhibit monodentate coordination whereas the pyridyl N atoms are coordinating in a cis configuration. The geometry of the cation is compared to the geometries of other diamino diacetate complexes with Co III .
The ligand precursor, H 2 bpcd, belongs to a relatively small group of diamino diacetic acids that contain softer aromatic nitrogen donor groups ( Fig. 1) (Caravan et al., 1997a;Heitzmann et al., 2009;Kissel et al., 2014). The preorganized ligand precursor H 2 bpcd is of interest as a novel candidate for selective and efficient actinide(III)/lanthanide(III) separations. Preorganization of a ligand can reduce the pre-orientation energy required for metal ion complexation and provide improved metal-ligand complex stability (Rizkalla et al., 1987;Choppin et al., 2006;Ogden et al., 2012). The addition of aromatic functionalities, such as pyridine and pyrazine, may increase ligand selectivity for softer metal ions and provide ISSN 2056-9890 greater stability towards radiolysis (Heitzmann et al., 2009). The members of this group of diacetic acids, however, differ in the nature of the diamine backbone.
The ethylenediamine backbone is a classic scaffold that has been used for the construction of many polydentate ligands. The amine N atoms are ideal for functionalization, which allows different donor atom groups to be incorporated into a ligand's design. The close proximity of the diamine nitrogens also maximizes the number of possible five-and six-membered chelate rings capable of forming upon metal ion complexation. H 2 bped (A) is a hexadentate 2-pyridylmethyl-substituted diacetic acid based on this classic scaffold (Lacoste et al., 1965;Caravan et al., 1997a). gem-H 2 bped (B) is a very closely related 2-pyridylmethyl-substituted diacetic acid that is also based on the ethylenediamine scaffold. In this case, however, both pyridine substituents are bonded to the same amine N atom (Heitzmann et al., 2009). The C-C chain length between the N atoms in the diamine backbone of these ligands allows for the formation of five-membered chelate rings. Hancock has shown the formation of five-membered chelate rings to be more favourable for larger metal ions than for smaller metal ions (Hancock & Martell, 1989). The ligand precursor, H 2 bpcd (C), for the title compound is similar to A and B, but it incorporates the ethylenediamine backbone into a cyclohexyl group. Restricted rotation about the C-C bonds in the cyclohexane ring fixes the positions of the trans diamine nitrogen atoms and favourably preorganizes these donor groups for metal ion complexation. Consequently, the trans amine groups are constrained into a conformation that is preoriented favorably for binding and results in a complex of increased stability (Rizkalla et al., 1987;Choppin et al., 2006;Ogden et al., 2012). In contrast, H 2 bppd (D) features a 1,3diaminopropane backbone that provides greater flexibility compared to A, B, or C with their shorter backbones. Further, the increased chain length of the propylene linker allows a sixmembered chelate ring to form upon metal complexation. Formation of six-membered chelate rings in complexes with smaller metal ions has been shown to increase the stability of the complex relative to five-membered rings (Hancock & Martell, 1989). Here, we report the structure of a Co III complex with bpcd 2À , C.

Structural commentary
The structure of the [Co(bpcd)] + cation in the title compound is shown in Fig. 2 and selected geometric parameters are listed in Table 1. The cation is very similar to the structures of the [Co(bped)] + and [Co(bppd)] + complex ions. Nearly all of the Co-O ac bond lengths for the five structures given in Table 1 are within experimental error of each other. One of the Co-O ac bond lengths in the [Co(bppd)] + cation, however, is slightly shorter than the others. The C-O and C O bond lengths are also quite similar. There are, however, some variations in the bond lengths and angles as shown in Tables 1 and 2. The Co-N am bond length in the [Co(bpcd)] + cation is slightly shorter than the Co-N am bond lengths reported for the two [Co(bppd)] + cations given in Table 1. They are, Figure 1 The diamino diacetic acids, H 2 bped (A) and gem-H 2 bped (B), where bped stands for bis(2-pyridylmethyl)-1,2-diaminoethane diacetate, H 2 bpcd (C), and H 2 bppd (D), where bppd stands for bis(2-pyridylmethyl)-1,3-diaminopropane diacetate.

Supramolecular features
The structure of the title compound ( Fig. 3) exists in the solid state as an intricate network of anions and cations closely associated through many short interactions. Hydrogenbonding interactions are listed in Table 3. Each PF 6 À anion is in close contact with six cations: three of the four unique F atoms interact with two neighboring cations while the remaining atom, F4, has a long interaction (2.29 Å ) with only the C-H9A bond of the cyclohexyl ring of one cation. This   View of the molecular components of the title structure, [Co(bpcd)]PF 6 .
[Symmetry code: (i) Àx + 1, Ày + 1 2 , z.]  (2) 114.32 (9) 115.33 (10) 114.57 (7)  There are also several interactions between pyrdidyl ring H atoms and carboxylate O atoms from neighboring cations, i.e. a 2.408 Å interaction with Co-bound oxygen O1, and a 2.700 Å interaction with terminal oxygen O2. The short interaction has a C-HÁ Á ÁO angle of 140.7 so it does not appear in Table 3. There also existsstacking for each of the two pyridyl rings with neighboring cations stacked antiparallel. Each has a distance of 3.829 (13) Å between ring centroids.

Database survey
There is very little information in the literature about H 2 bpcd and its metal complexes. There is a structurally characterized heptacoordinate [Fe II (H 2 bpcd)(C 3 H 6 O)](ClO 4 ) 2 complex with trans pyridine N atoms and cis carboxylic acid groups (Oddon et al., 2012). In that case, Fe II is coordinated in a distorted pentagonal-bipyramidal geometry with an unusual N 4 O 3 donor atom set, including a bound acetone molecule. The carboxylic acid moieties are fully protonated with the H 2 bpcd ligand coordinating through the carbonyl O atoms, which reside in the equatorial plane. The coordinating amine N atoms also lie in this plane, whereas the pyridyl N atoms are coordinating at the axial positions. This unique arrangement results in longer Fe-O and Fe-N py bonds than are typically observed. In the present case, a fully deprotonated bpcd 2À ligand binds Co III in a pseudo-octahedral fashion with trans acetate groups to form a hexacoordinate complex. Although only one structure of a metal-H 2 bpcd complex has been reported in the literature, there are several structures reported for related pseudo-octahedral Co III complexes with bis-2-pyridylmethyl substituted diamino diacetic acids, i.e. H 2 bped (A) and H 2 bppd (D) in Fig. 1

Synthesis and crystallization
H 2 bpcd (C) was prepared from trans-1,2-diaminocyclohexane using the procedure reported for H 2 bppd (D) (Kissel et al., 2014). The title compound was prepared using methods analogous to those previously reported for [Co (bppd)

{2,2′-[N,N′-Bis(pyridin-2-ylmethyl)cyclohexane-trans-1,2-diyldi(nitrilo)]diacetato}cobalt(III)
hexafluoridophosphate Crystal data [Co(C 22  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.66 e Å −3 Δρ min = −0.52 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. There is a small amount of disorder that can be modeled for the PF 6 anion. F2 and F3 can be moved in the plane, as one might imagine. R1 can be reduced to 0.0252 by modeling it, but the occupancy is less than 10% and results in a less chemically satisfactory PF 6 anion. Therefore, the disorder was not modeled.