Chiral heterocyclic ligands. XII. Metal complexes of a pyrazine ligand derived from camphor

The synthesis and X-ray crystal structures of copper(II) nitrate, copper(I) iodide and zinc(II) bromide complexes of the chiral ligand 2 are described.


Introduction
Chiral heterocyclic ligands have found many applications in chemistry, most notably in the area of asymmetric catalysis. 1Such compounds are usually synthesised from readily available, naturally occurring compounds from the chiral pool. 2 Monoterpenes serve as a useful source of inexpensive synthons for such studies. 3For example, von Zelewsky and co-workers have prepared a vast library of chelating and bridging heterocyclic ligands which contain a fused pinane subunit within their structures. 4Similarly, we have synthesised many chiral ligands, using camphor as a source of the chirality. 5Accordingly, by fusing a pyrazole ring to the bornane skeleton we have prepared many bidentate and tridentate chelating ligands, as well as a number of bridging ligands containing this subunit. 6

N
More recently, we have turned our attention to fusing the bornane skeleton to azine rings and have reported the synthesis of the first chiral 2,2'-bipyrimidine. 7We have also fused bornane units to a pyrazine ring and have described the preparation of some chiral coordination polymers, using the C 2 -symmetric ligand 1 as a bridging ligand. 8In contrast, the C 1 -symmetric ligand 2 proved less useful for the construction of chiral coordination polymers, 8 because of the difficulty for this ligand to faithfully assemble in a single orientation, due to the similar, but subtly different, nature of the two nitrogen donors.However, this ligand can successfully be used as a monodentate ligand for the construction of discrete, rather than polymeric, coordination compounds.In this context, we have studied the coordination chemistry of 2 and the related quinoxaline 3 with various transition metals and now report the synthesis and X-ray crystal structures of copper and zinc mononuclear complexes and a tetranuclear copper complex in which ligand 2 acts as a monodentate donor.We believe that ligands such as these offer considerable potential as auxiliaries in the topical context of asymmetric synthesis.

Results and Discussion
Ligands 2 and 3 were prepared from (1R)-(+)-camphor (4), as shown in Scheme 1, via camphorquinone (5).Although 4 can be oxidised directly to 5 using selenium dioxide, 9 we decided to avoid the use of this toxic reagent and chose to carry out this conversion in two steps via nitrosation to an intermediate quinone-monoxime, 10 followed by hydrolysis, 11 in a procedure that is both more efficient and environmentally friendly.The quinone was then condensed with ethylenediamine to give a dihydropyrazine 12 followed by oxidation to 2, in 69% overall yield. 13ondensation of 5 with o-phenylenediamine furnished 3 directly in 54% yield. 13
The coordination chemistry of 2 and 3 was explored with various transition metal reagents.No complexes of 3 were able to be isolated, possibly due to the highly hindered nature of both ARKAT nitrogen donors.However, three crystalline products were isolated in good yields from reactions of 2. Reaction with a methanolic copper(II) nitrate solution produced very thin blue plates of complex 6.Reaction with copper(I) iodide in acetonitrile gave yellow crystals of complex 7.The 1 H NMR spectrum of 7 showed only one set of signals for the organic ligand.A colourless complex 8 was obtained by reaction with a methanol solution of zinc(II) bromide.The structures of these complexes were determined by X-ray crystallography.
The light-blue crystals of the copper(II) nitrate complex, 6, were very thin and diffracted only weakly.Nevertheless, the structure of 6 was unambiguously established as that shown in Figure 1.The complex crystallizes in the orthorhombic space group C222 1 , with the asymmetric unit containing half a copper atom, one molecule of 2, a coordinated monodentate nitrate and a coordinated water molecule, which is disordered over two sites.The copper atom lies on a twofold rotation axis that generates the other half of the complex.

ARKAT
The copper(I) iodide complex, 7, crystallizes in the monoclinic space group C2, with four molecules of 2 and four copper iodides in the asymmetric unit.The complex consists of Cu 4 I 4 clusters with each of the copper atoms coordinated by one molecule of 2. The asymmetric unit contains two independent half Cu 4 L 4 I 4 clusters, one of which is shown in Figure 2. The four copper and four iodine atoms of each cluster form a distorted cube-like structure with the copper atoms forming a tetrahedron.The two independent clusters each sit astride a two-fold axis and have similar geometries, with Cu•••Cu distances in the range 2.670(3) -2.738(3) Å for one cluster, and 2.648(3) -2.776(3) Å for the other.The copper atoms all have tetrahedral coordination geometry, and are each coordinated by three iodine atoms, with Cu-I bond lengths in the range 2.655(2) -2.769(2) Å.The remaining site of the tetrahedral copper is occupied by the less hindered nitrogen atom of 2, with Cu-N distances between 2.030(9)Å and 2.043(9) Å.
The distorted cube-like cluster is the most common structure found for tetranuclear copper(I) halide complexes, and is more common for iodide complexes than for those of other halides. 14The cube-like Cu 4 I 4 cluster has potential S 4 point symmetry, and the formation of the clusters occupying crystallographic S 4 sites has been observed for a number of complexes utilising nitrogen-donor ligands. 15However, the coordination to the copper atoms of the chiral ARKAT ligand, 2, precludes any possible S 4 symmetry.The two independent copper clusters, which are not related by symmetry, differ principally in the relative orientation of the molecules of 2 that are coordinated to the copper atoms.Similar differences have been observed in the copper(I) iodide complex of pyridine, which crystallizes in the orthorhombic space group P2 1 2 1 2 1 with one cluster in the asymmetric unit. 16he zinc dibromide complex, 8, crystallizes in the chiral monoclinic space group P2 1 , with four molecules of 2 and two zinc dibromide moieties in the asymmetric unit.The complex consists of two independent zinc atoms, each coordinated by two chiral ligand molecules and two bromine atoms, one unit of which is shown in Figure 3.The zinc atoms are coordinated by two molecules of 2, through the least hindered nitrogen atoms, with Zn-N bond lengths of 2.080(6) and 2.086(7) Å for one Zn atom, and 2.089(7) and 2.105(6) Å for the other.The zinc atoms are also coordinated by two bromine atoms, with Zn-Br bond lengths in the range 2.3439(2) -2.3585(2) Å.The zinc atoms have slightly distorted tetrahedral coordination environments, with the largest distortion being the angles between the bromine atoms, which are 119.54(6)ºand 118.20(6)º for Zn1 and Zn2, respectively.The bond lengths and bond angles of this complex are similar to those found in the tetrahedral zinc dibromide complex of the less sterically hindered molecule pyrazine. 17However, this complex is a one-dimensional polymer, which in the present case is not formed, presumably due to the more sterically hindered coordination environment of the non-coordinating nitrogen atom.
In conclusion, we have shown that the chiral ligand 2 can bind to transition metals to form discrete complexes in which 2 acts as a monodentate ligand, with coordination through the less hindered of the two nitrogen atoms.

X-Ray Crystallography
Data were collected with a Siemens SMART CCD area detector, using graphite monochromatized MoKα radiation (λ = 0.71073 Å).The intensities were corrected for Lorentz and polarization effects and for absorption. 19The structure was solved by direct methods using SHELXS 20 and refined on F 2 , using all data, by full-matrix least-squares procedures using SHELXTL. 21All non-hydrogen atoms were refined with anisotropic displacement parameters.Hydrogen atoms were included in calculated positions, with isotropic displacement parameters 1.
5(9)°].The coordinated oxygen atoms are in a plane, with the nitrate anions being cis and having a Cu-O distance of 2.13(1) Å and an O-Cu-O angle of 89.6(6)°.The coordinated water molecules are disordered over two sites, with the major contributing water molecule occupied 60% of the time.The major contributing water molecule has a Cu-O bond length of 1.96(6) Å.The minor contributing water molecule is further away from the copper atom, with a Cu-O distance of 2.48(8) Å.