Stereosopecificity in [Co(sep)][Co(edta)]Cl22H2O

The X-ray structure of racemic [Co(sep)][Co(edta)]Cl2·2H2O is reported and reveals heterochiral stereospecificity in the interactions of [Co(sep)]3+ with [Co(edta)]−. Hydrogen-bonding along the molecular C2-axes of both complexes accounts for the stereospecificity. The structure of Λ[Co(en)3]∆-[Co(edta)]2Cl·10H2O has been re-determined. Previous structural data for this compound were collected at room temperature and the model did not sufficiently describe the disorder in the structure. The cryogenic temperature used in the present study allows the disorder to be conformationally locked and modeled more reliably. A clearer inspection of other, structurally interesting, interactions is possible. Again, hydrogen-bonding along the molecular C2-axis of [Co(en)3] and the equatorial carboxylates of [Co(edta)]− is the important interaction. The unique nature of the equatorial carboxylates and molecular C2-axis in [Co(edta)]−, straddled by two pseudo-C3-faces where the arrangement of the carboxylate groups conveys the same helicity, is highlighted. Implications of these structures in understanding stereoselectivity in ion-pairing and electron transfer reactions

Our interest derives from the useful correlation between the chiral recognition in these ion pairs and the chiral induction in the electron transfer reactions of [Co(edta)] − and other complexes with the pseudo-C3 face with [Co(en)3] 2+ and derivatives [7][8][9][10][11][12][13]. A conclusion might be that the ion pairs serve as reasonable analogues for the precursor complex for the electron transfer process, despite the difference in the charge on the cation. The [Co(edta)] − system is particularly apt, as the complex has two very distinct sides; the carbon CH2-backbone of the ligand that is not capable of forming hydrogen-bonds, and   Quantitative studies of ion pairing by conductivity, notably by Tatehata and coworkers, are consistent with this simple explanation [4], although the discrimination is at However, in [Co(edta)] − , it must be noted that the chirality resulting from the arrangement of the carboxylate groups of the equivalent pseudo-C3 faces straddle the molecular C2 axis, see Figure 2. Consequently, unlike in the case of the tris-bidentate chelates, the helicity conveyed by the ligands along the principal C2-axis and the pseudo-C3 faces is the same [5].
Our interest derives from the useful correlation between the chiral recognition in these ion pairs and the chiral induction in the electron transfer reactions of [Co(edta)] − and other complexes with the pseudo-C3 face with [Co(en)3] 2+ and derivatives [7][8][9][10][11][12][13]. A conclusion might be that the ion pairs serve as reasonable analogues for the precursor complex for the electron transfer process, despite the difference in the charge on the cation. The [Co(edta)] − system is particularly apt, as the complex has two very distinct sides; the carbon CH2-backbone of the ligand that is not capable of forming hydrogen-bonds, and Quantitative studies of ion pairing by conductivity, notably by Tatehata and coworkers, are consistent with this simple explanation [4], although the discrimination is at However, in [Co(edta)] − , it must be noted that the chirality resulting from the arrangement of the carboxylate groups of the equivalent pseudo-C 3 faces straddle the molecular C 2 axis, see Figure 2. Consequently, unlike in the case of the tris-bidentate chelates, the helicity conveyed by the ligands along the principal C 2 -axis and the pseudo-C 3 faces is the same [5].
Our interest derives from the useful correlation between the chiral recognition in these ion pairs and the chiral induction in the electron transfer reactions of [Co(edta)] − and other complexes with the pseudo-C 3 face with [Co(en) 3 ] 2+ and derivatives [7][8][9][10][11][12][13]. A conclusion might be that the ion pairs serve as reasonable analogues for the precursor complex for the electron transfer process, despite the difference in the charge on the cation. The [Co(edta)] − system is particularly apt, as the complex has two very distinct sides; the carbon CH 2 -backbone of the ligand that is not capable of forming hydrogen-bonds, and the molecular C 2 -carboxylate axis surrounded by the two-pseudo-C 3 faces, all, in the parlance of Yoneda, with the same handedness. Dipole considerations dictate that it is this latter side that should interact with hydrogen bonding cations.
Information derived from crystal structures on the interactions of [Co(edta)] − with [Co(en) 3 ] 3+ and its derivatives or their analogues is very limited. In an earlier study, the structure of Λ-[Co(en) 3 ]∆-[Co(edta)] 2 Cl·10H 2 O was reported [13]. In the present paper, this structure has been re-determined at cryogenic temperature to provide a better model for a chelate ring conformational disorder and improve an understanding of the hydrogen bonding between the complexes. Indeed, a characteristic of the chiral induction in electron transfer with [Co(edta)] − and diastereomeric derivatives of [Co(en) 3 ] 3+ is a dependence on chelate ring conformation. The structure of racemic [Co(sep)][Co(edta)]Cl 2 ·2H 2 O has been determined for the first time. The relevance of these static structures in understanding electron transfer is discussed.

Materials and Methods
The compounds [Λ-(+) D -[Co(en) 3 3 ]Cl 3 , in 3 mL water as previously described [13]. An arbitrary sphere of data was collected on a violet rod-like crystal, having approximate dimensions of 0.267 × 0.062 × 0.040 mm, on a Bruker APEX-II diffractometer using a combination of ωand ϕ-scans of 0.5 • [15]. For both structures, data were corrected for absorption and polarization effects, and analyzed for space group determination [16]. The structures were solved by dual-space methods and expanded routinely [17]. Models were refined by full-matrix least-squares analysis of F 2 against all reflections [18]. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters.
Atomic displacement parameters for hydrogen atoms in Λ-[Co(en) 3 ]∆-[Co(edta)] 2 Cl· 10H 2 O were modeled as a mixture of refined and constrained geometries. Hydrogen atoms on the Co complexes were modeled with atoms riding on the coordinates of the atom to which they are bonded with atomic displacement parameters tied to that of the atom to which they are bonded (U iso (H) = 1.2 U eq (C/N)). Water hydrogen atoms were included in positions located from a difference Fourier map. Most water hydrogen atoms were refined freely; several that did not model well were modeled with atomic displacement parameters tied to that of the oxygen to which they are bonded (U iso (H) = 1.5 U eq (O)).
In [Co(sep)][Co(edta)]Cl 2 ·2H 2 O, one water was found to be disordered over two positions during refinement. In the final structure, this atom was modeled with two, halfoccupancy oxygen atoms, and concomitant hydrogen atoms, at sites suggested as the loci of the original extended displacement parameters. Residual electron density (2.24 e − /Å 3 ) is located near (0.87 Å) Co4 of one of the [Co(edta)] − anions. It is unclear what this residual density might be, and is likely due to Fourier ripple or a very small amount of otherwise unresolvable molecular disorder. Hydrogen atoms for [Co(sep)][Co(edta)]Cl 2 ·2H 2 O were treated as a mixture of freely refined and geometrically constrained atoms. Hydrogen atoms bonded to carbon and nitrogen were treated as riding models with U iso (H) = 1.2 U eq (C). Water hydrogen atoms were modeled at locations initially located from a difference Fourier map and subsequently tied to the coordinates of the oxygen to which they are bonded. Atomic displacement parameters for water hydrogen atoms in this model were restrained to U iso (H) = 1.5 U eq (O).
The disorder in Λ-[Co(en) 3 ]∆-[Co(edta)] 2 Cl·10H 2 O was resolved more thoroughly with a cryogenic measurement of the data for the complex. The ethane-1,2-diamine was observed to be a twist disorder of the ethylene backbone, across the crystallographic twofold axis that bisects the ethylene chain. Only carbon atom C3 is the unique atom in the model. The two sites were modeled from density observed in a Fourier difference map and the site occupancy ratios summed to unity yielding an approximately 0.77:0.23 ratio. The major component was refined with anisotropic displacement parameters and the minor component with an isotropic atom. Hydrogen atoms about the disorder (on nitrogen and the disordered carbon) were modeled using routine methodology (occupancies tied to the disorder component, riding atom positions, and displacement parameters). The cations have a cobalt atom encapsulated in an octahedral fashion by a sepulchrate ligand. The cobalt is coordinated by the amine nitrogen atoms, that retain their hydrogen atoms. The "apical" nitrogen atoms of the sepulchrate are non-coordinating. The conformation of the complex is ∆(λ,λ,λ) or Λ(δ,δ,δ) (lel 3 ). In the anion, the cobalt is chelated by edta 4− in a six-coordinate coordination geometry with geometry ∆Λ∆ or Λ∆Λ, abbreviated ∆ and Λ, respectively.

Results
The  (1) Å for the two independent pairs. The chlorine atoms form hydrogen bonds with two neighboring sepulchrate amide nitrogen atoms, Table 1, occupying two of the molecular C 2 -axes of the complex cation, but do not participate in further H-bonding. Each chloride atom is hydrogen bonded by two N-H atoms from a single sepulchrate. The third molecular C 2 -axis of the [Co(sep)] 3+ is occupied in stereospecific fashion by [Co(edta)] − with a pair of N-H hydrogen bonds to the equatorially coordinated G-ring oxygens of its neighboring [Co(edta)] − anion (graph set notation R 2 2 (8)), see Figure 3. Thus, they also do not propagate the hydrogen bonded network. The disorder in Λ-[Co(en)3]∆-[Co(edta)]2Cl·10H2O was resolved more thoroughly with a cryogenic measurement of the data for the complex. The ethane-1,2-diamine was observed to be a twist disorder of the ethylene backbone, across the crystallographic twofold axis that bisects the ethylene chain. Only carbon atom C3 is the unique atom in the model. The two sites were modeled from density observed in a Fourier difference map and the site occupancy ratios summed to unity yielding an approximately 0.77:0.23 ratio. The major component was refined with anisotropic displacement parameters and the minor component with an isotropic atom. Hydrogen atoms about the disorder (on nitrogen and the disordered carbon) were modeled using routine methodology (occupancies tied to the disorder component, riding atom positions, and displacement parameters).
The cations have a cobalt atom encapsulated in an octahedral fashion by a sepulchrate ligand. The cobalt is coordinated by the amine nitrogen atoms, that retain their hydrogen atoms. The "apical" nitrogen atoms of the sepulchrate are non-coordinating.
The conformation of the complex is ∆(λ,λ,λ) or Λ(δ,δ,δ) (lel3). In the anion, the cobalt is chelated by edta 4− in a six-coordinate coordination geometry with geometry ∆Λ∆ or Λ∆Λ, abbreviated ∆ and Λ, respectively. for the two independent pairs. The chlorine atoms form hydrogen bonds with two neighboring sepulchrate amide nitrogen atoms, Table 1, occupying two of the molecular C2-axes of the complex cation, but do not participate in further H-bonding. Each chloride atom is hydrogen bonded by two N-H atoms from a single sepulchrate. The third molecular C2axis of the [Co(sep)] 3+ is occupied in stereospecific fashion by [Co(edta)] − with a pair of N-H hydrogen bonds to the equatorially coordinated G-ring oxygens of its neighboring [Co(edta)] − anion (graph set notation 8 ), see Figure 3. Thus, they also do not propagate the hydrogen bonded network.   (4) 3.757 (4)  The hydrogen bonded network is extended through the structure with water molecules linking non-coordinated acetate oxygen atoms of [Co(edta)] − . The arrangement of molecules results in a 2D sheet of H-bonded molecules parallel to the b/c plane. Each [Co(edta)] − accepts four hydrogen bonds and is the "corner" of a 4-connected square. Located in the center of each square is a [Co(sep)] 3+ from an adjacent sheet, related by inversion symmetry. Due to the orientation of the ligands, these sheets are bi-layers, with hydrophobic regions between layers, see Figure 4.
The chloride ions are coordinated by eight water molecules, part of an extensive hydrogen bonding network forming channels, parallel to the c-axis. The amine nitrogen atoms of the [Co(en) 3 ] 3+ cations also participate, with alternating [Co(en) 3 ] 3+ and Cl − in the ac-plane. The [Co(edta)] − anions alternate in orientation in a parallel ac-plane, completing a layered structure along the b-axis. The layers are held together by hydrogen bonding between the [Co(en) 3 ] 3+ and [Co(edta)] − . The two [Co(edta)] − anions interact with [Co(en) 3 ] 3+ in different fashion. One interaction, with a Co-Co distance of 5.844(1) Å lies roughly along a molecular C 2 -axis of [Co(en) 3 ] 3+ . There is hydrogen-bonding between one N-H proton along the C 2 -axis, and a second nitrogen on the C 3 -face of [Co(en) 3 ] 3+ with the coordinated and un-coordinated oxygen atoms of the carboxylate group of one of the G-rings on [Co(edta)] − (graph set notation R 2 2 (8)). The interaction with the C 2 -axis nitrogen involves the disordered ethane-1,2-diamine ring on [Co(en) 3 ] 3+ , and examination of the linearity of the hydrogen bonds for the two conformations reveals that the λ conformation is preferred, Table 1, and that the 5.844 Å interaction favors Λ(λ,λ,λ) (ob 3 ), see Figure 5.
The other interaction with a Co-Co distance of 7.509(1) Å shows a hydrogen bonding interaction between an uncoordinated carbonyl oxygen of an out-of-plane R-ring of [Co(edta)] − with an N-H proton from the disordered 1,2-diaminoethane ring on [Co(en) 3 ] 3+ . Again, examination of the linearity of the hydrogen bonds for the two conformations reveals that the δ conformation is preferred, and that the 7.509 Å interaction favors Λ(δ,λ,λ) (lelob 2 ). linearity of the hydrogen bonds for the two conformations reveals that the λ conformation is preferred, Table 1, and that the 5.844 Å interaction favors Λ(λ,λ,λ) (ob3), see Figure 5. The other interaction with a Co-Co distance of 7.509(1) Å shows a hydrogen bonding interaction between an uncoordinated carbonyl oxygen of an out-of-plane R-ring of [Co(edta)] − with an N-H proton from the disordered 1,2-diaminoethane ring on [Co(en)3] 3+ . Again, examination of the linearity of the hydrogen bonds for the two conformations reveals that the δ conformation is preferred, and that the 7.509 Å interaction favors Λ(δ,λ,λ) (lelob2).

Discussion
As expected, bond distances and angles within the molecular ions in both structures are comparable with related studies. However, the focus of this communication is the interactions between the metal-ion complexes.
It has been a generally accepted concept that the chiral discriminations of tris-bidentate chelate complexes such as [Co(en)3] 3+ are the result of different orientations of hydrogen-bonding interactions since the C3 axis has the opposite helicity to the C2 axis [2][3][4]. Thus, ∆-[Co(en)3] 3+ is P(C3)M(C2) using the nomenclature for P or positive helicity referring to a right-handed screw and M to the left-handed screw. Ion pairing discrimination studies with metal-complex carboxylate liganded anions carried out by chromatography and conductivity measurements have highlighted the importance of the hydrogenbonded match of the C3 and C2-axes of [Co(en)3] 3+ with a pseudo-C3 carboxylate face where three carboxylate groups form a facial motif that is not subtended by a chelate ring. The helicity of the pseudo-C3 carboxylate face in the anion projects to the axis with the same helicity in the cation. However, the hexa-coordinated complex ion, [Co(edta)] − , differs in symmetry from a tris-bidentate chelate. The two pseudo-C3 carboxylate faces flank the C2 axis, and the ligand arrangement is such that all three present the same overall helicity. What is notable in both of the structures reported here is that the interactions involving the closest Co-Co distances involve the C2 axis or the in-plane G-rings of the anion, strongly suggesting a

Discussion
As expected, bond distances and angles within the molecular ions in both structures are comparable with related studies. However, the focus of this communication is the interactions between the metal-ion complexes.
It has been a generally accepted concept that the chiral discriminations of tris-bidentate chelate complexes such as [Co(en) 3 ] 3+ are the result of different orientations of hydrogenbonding interactions since the C 3 axis has the opposite helicity to the C 2 axis [2][3][4]. Thus, ∆-[Co(en) 3 ] 3+ is P(C 3 )M(C 2 ) using the nomenclature for P or positive helicity referring to a right-handed screw and M to the left-handed screw. Ion pairing discrimination studies with metal-complex carboxylate liganded anions carried out by chromatography and conductivity measurements have highlighted the importance of the hydrogen-bonded match of the C 3 and C 2 -axes of [Co(en) 3 ] 3+ with a pseudo-C 3 carboxylate face where three carboxylate groups form a facial motif that is not subtended by a chelate ring. The helicity of the pseudo-C 3 carboxylate face in the anion projects to the axis with the same helicity in the cation. Thus the pseudo-C 3 carboxylate face of ∆-[Co(edta)] − interacts preferentially with the N-H hydrogens on the C 3 -axes of ∆-[Co(chxn) 3 ] 3+ where the C 2 -axes are sterically encumbered, but with the N-H hydrogens on C 2 -axes of Λ-[Co(sep)] 3+ where the C 3 -axes are encumbered.
However, the hexa-coordinated complex ion, [Co(edta)] − , differs in symmetry from a tris-bidentate chelate. The two pseudo-C 3 carboxylate faces flank the C 2 axis, and the ligand arrangement is such that all three present the same overall helicity. What is notable in both of the structures reported here is that the interactions involving the closest Co-Co distances involve the C 2 axis or the in-plane G-rings of the anion, strongly suggesting a more important role for the helicity conveyed along the C 2 -axis of [Co(edta)] − in determining the discriminations. This observation is also consistent with the solution NMR structure of the ion pair, {[Cr(en) 3 ] 3+ [Co(edta)] − }, where by symmetry, the paramagnetic cation straddles the C 2 -axis of [Co(edta)] − [5].
There is a structural comparison with ∆-[Ni(en) 3 ]∆-[Ni(edta)]·4H 2 O that is also relevant [23]. The cation and anion share two interactions. The closest Ni-Ni distance is 5.40 Å and reveals a direct hydrogen bond formed between the non-coordinated G-ring oxygen of [Ni(edta)] 2− and an N-H on the C 3 -axis of [Ni(en) 3 ] 2+ with a second interaction involving the coordinated O of the other G-ring, a bridging water molecule, and a second N-H on the C 3 -axis of [Ni(en) 3 ] 2+ (Graph set notation R 2 2 (12)). There is no direct pseudo-C 3 carboxylate interaction involving the three N-H groups of the C 3 -axis of [Ni(en) 3 ] 2+ . Instead, it is the in-plane G-ring carboxylates that again play a dominant role. A longer Ni-Ni distance at 6.14 Å involves a non-coordinated R-ring oxygen of [Ni(edta)] 2− with two N-H protons on the C 3 -axis of [Ni(en) 3 ] 2+ (Graph set notation R 1 2 (6)). While the distinction between the ligand arrangement conveying the same helicity through the C 2 -axis and pseudo-C 3 faces in [Co(edta)] − with the same helicity may seem semantic, it should be noted that the dipole moment of [Co(edta)] − projects along the C 2 -axis. The idea that discriminations by [Co(edta)] − can be projected through the C 2axis and not only by the pseudo-C 3 -faces has implications in the interpretation of the extensive data on related outer-sphere stereoselective electron transfer. The reductions of [Co(edta)] − with both [Co(en) 3 ] 2+ and [Co(sep)] 2+ have been shown to occur by an outer-sphere mechanism [7,9].
Computational work on the effects of distance and orientation in outer-sphere electrontransfer reactions between metal ion complexes has focused predominantly on the [Fe(OH 2 ) 6 ] 3+/2+ and [Ru(OH 2 ) 6 ] 3+/2+ self-exchange reactions [24][25][26]. Increasingly, sophisticated calculations [27][28][29][30][31] have not markedly changed the conclusions first reached that face-to-face interactions along the C 3 -axes represent the closest approach of the two metal centers, at distances roughly 5-6 Å, and the most favorable configuration for overlap of the donor and acceptor orbitals resulting in electron transfer. Other orientations over a range of distances provide less favorable pathways with super-exchange mechanisms involving the ligands more likely at distances in excess of 6 Å.
Unlike the self-exchange reactions, the outer-sphere oxidations of [Co(en) 3 ] 2+ and [Co(sep)] 2+ by [Co(edta)] − involve complexes with opposite charges, and the additional electrostatic attraction can provide a more intimate interaction, generally at hydrogenbonded distances. An attractive model for a C 3 -C 3 interaction is provided by the structure of [Cr(en) 3 ] 3+ [Cr(ox) 3 ] 3+ where the metal-metal distance is 4.98 Å [32], shorter than the closest approach distances found in the present structural studies with [Co(en) 3 ] 3+ (5.84 Å) and [Co(sep)] 3+ (5.17 Å). Although the electron transfer precursor is dynamic and comparisons with static structures are fraught with problems, were distance the only factor, then for [Co(edta)] − projecting discrimination through the pseudo-C 3 -face, one might well expect the reaction stereoselectivity to reflect a dominant C 3 -C 3 interaction and hence a homochiral, ∆∆ or ΛΛ, preference. That is not what is observed, providing a further piece of evidence that the discrimination by [Co(edta)] − is more complex.
Further, a detailed analysis of structural, charge, and dipolar effects on the stereoselective electron transfer data also revealed [33] that there is a distinction between [Co(edta)] − and the oxalate containing reagents that possess the pseudo-C 3 motif such as C 1 -cis(N)-[Co(gly) 2 (ox)] − (gly − = glycinate(−1)). Stereoselectivity and chiral discriminations involving [Co(edta)] − should be considered in a class of their own.

Conclusions
In conclusion, the structures presented highlight an important role for hydrogen bonding involving the unique C 2 -axis of [Co(edta)] − in chiral discriminations with [Co(en) 3 ] 3+ and derivatives. Stereoselectivity and chiral discriminations involving [Co(edta)] − should be considered in a class of their own. This has implications in the interpretation of data for related stereoselective electron transfer reactions and suggest that generalizations should be avoided.