Synthesis , X-ray Structural and Thermal Analysis of a Novel Copper Compound Containing Dissymmetric Independent 5-and 6-coordinate CuL ( H 2 O ) n ( n = 2 , 3 ) Units and Hydrogen bond Bridges

The synthesis, thermogravimetric and X-ray structure analysis of a highly unusual copper compound prepared from a chiral tridentate Schiff base ligand is reported. The title compound [CuL(H2O)2] · [CuL(H2O)3] · H2O crystallizes in the orthorhombic crystal system in space group P212121 with Z = 4. The two independent CuL(H2O)n complexes are present in the same unit cell, along with an uncoordinated water molecule. The five-coordinated copper(II) centre occupies a distorted square-pyramidal geometry whereas the six-coordinated copper(II) centre has a distorted octahedral geometry. The independent copper complexes and the uncoordinated water molecule are joined together with intermolecular hydrogen bonds in a two-dimensional supramolecular network which lies parallel to the ab plane.


Introduction
][6][7][8][9][10][11][12] Although different structures are often adopted, it can reasonably be assumed that these properties are partly due to the availability of sites of coordinative unsaturation at the metal centre and the presence of amino acid residues.In particular, a defining property of Cu(II) ions is that, unless a tridentate ligand is flexible enough to complement its distorted coordination sphere, 13 Cu(II) will preferably bind only one ligand. 14Thus, for copper(II) complexes containing only solvent molecules in addition to a tridentate ONO Schiff base ligand, although a 4-coordinate square planar geometry is possible, 15 in the majority of cases the Cu(II) ion adopts a 5-coordinate square pyramidal geometry where the fourth and fifth coordination sites are occupied by solvent or bridging ligands.These structural constraints allow the formation of mononuclear, [16][17][18] oligo-nuclear, [19][20][21] or polymeric structures. 22Similar observations have been made for related complexes containing an additional ligand instead of a coordinated solvent molecule, 23,24 and for reduced Schiff base ligands. 14Therefore, we were interested in the investigation of the copper(II) complexation behaviour towards a tridentate Schiff base ligand (L) derived from phenylalanine and 2,4-dihydroxybenzaldehyde resulted in a structure that contains two independent Cu(II)L(H 2 O) n units.In one independent unit, the Cu(II) centre adopts the expected square pyramidal geometry.However, in the second independent unit, an additional water molecule is coordinated to afford an octahedral geometry.Although a square pyramidal/octahedral arrangement has previously been observed with the highly flexible dipicolylamine ligand, 25 this example is perhaps more striking as the different geometries are created simply by a differing degree of solvent associaton.We consider this to be a clear demonstration of the versatility of Cu(II) and its ability to coordinate and uncoordinate solvent or substrate molecules which is believed to be a fundamental and necessary property of copper-based enzymes. 12

1. Synthesis of the Title Complex
To a solution of D-phenylalanine (1.0 mmol, 165.2 mg) in 5 mL methanol, NaOH (1.0 mmol, 40 mg) in 3 mL methanol was added.Then, 2,4-dihydroxybenzaldehyde (1.0 mmol, 138 mg) was added and the reaction mixture was stirred at room temperature for 2 h.

X-ray Structure Determination
Intensity datasets were collected from the selected crystal at room conditions using an Agilent Diffraction Xcalibur diffractometer equipped with an Eos-CCD detector with Mo-Kα (λ = 0.71073 Å) radiation and a grap-  hite monochromator.Data were absorption-corrected within the CrysAlis program. 26The structure was solved by SHELXS-97 and refined by means of SHELXL-97 program 27 incorporated in the OLEX2 program package. 28All non-H atoms were refined anisotropically.A summary of crystallographic data, experimental details, and refinement results for the complex are given in Table 1.

1. Crystal Structure
The asymmetric unit of the complex consists of three independent molecules; a six-coordinate copper complex, a five-coordinate copper complex, and an uncoordinated water molecule.Fig. 1 shows an ORTEP plot of the asymmetric unit. 29n the six-coordinate copper(II) complex, the equatorial plane of the metal is occupied by the chelating tridentate (N1, O1, O2) ligand atoms and one oxygen atom (O1w) of a water molecule, while two water molecules (O2w, O3w) are in the axial positions.The Cu1 ion strays from the equatorial plane with a deviation of 0.057(2) Å towards the oxygen atom of the more strongly bound axial water molecule (O2w).As expected, due to the Jahn-Teller effect, 30,31 Cu-O distances in the axial directions [Cu1-O2w = 2.473(6) Å, Cu1-O3w = 2.593( 6) Å] are longer than those in the equatorial plane, leading to distorted octahedral geometry. 32,33However, in the five-coordinate copper(II) complex, the geometry around the Cu2 atom is a distorted square-pyramidal.A nitrogen atom (N2), a phenolate oxygen atom (O6) and a carboxylate oxygen atom (O5) along with a coordinated water molecule (O4w) complete the basal plane.A second water molecule (O5w) is coordinated in the apical position.This apical Cu-O bond length [Cu2-O5w = 2.320(4) Å] is within the expected range for apically bonded water molecules, 16,17 and is significantly shorter than the axial Cu-O distances observed in the octahedral unit.In this case, the Cu2 ion was found to stray from the basal plane with a deviation of 0.117(2) Å towards the axial O5w.
5][36][37] The benzylic phenyl groups are not quite parallel to the planar ONO backbone of the ligand.The dihedral angles between the five-membered chelate rings and the phenyl rings are 42.06(3)° for Cu1 and 26.88(2)° for Cu2, respectively.The two five-membered chelate rings are in the envelope conformation, i.e. the C2 atom is deviated from the plane by 0.064(4) Å.The relative configuration at the C2 and C18 chiral centers are confirmed to be (R,R).3, Fig. 2a, 2b).In the asymmetric unit (x, 1 + y, z), the cutoff distances are 1.85 Å for H4 ... O3 and 1.92 Å for H8A ... O7.These distances are considerably shorter than the van der Waals atomic radii (2.72 Å) given by Bondi and Pauling. 38,39There are intermolecular O-H ... O hydrogen bonds observed between adjacent copper molecules (Table 3, entries 2, 5, 6, 8, 10) forming supramolecular ribbons along the a axis (Fig. 2c).Furthermore, the uncoordinated water molecule is connected to copper molecules via O-H ... O hydrogen bonds to form a two-dimensional su-pramolecular network which lies parallel to the ab plane.This plane and the packing of the complexes can be seen in Fig. 3.

2. Thermogravimetric Analysis
With the aim to determine the relative ease of loss of the coordinated and non-coordinated water molecules, thermogravimetric analysis was carried out.The TGA and DTA curves of the product are given in Figure 4.It was noticed that loss of water began immediately on the initiation of heating and occurred in two stages.Thus, between 29 °C and 95 °C, a weight loss of 9.0% had occurred (9.0% calculated for loss of 4 H 2 O).Subsequently, between 95 °C and 235 °C, a further weight loss of 4.5% had occurred.Thus it seems reasonable to assume that initially, the water of crystallisation and three coordinated water molecule had been lost affording two square planar LM(H 2 O) fragments, from which the final water molecules were considerably more difficult to remove.

Conclusions
X-ray structural analysis of a novel compound containing pentacoordinated and hexacoordinated copper complexes shows that the axially coordinated water ligands of the octahedral complex [bond lengths of 2.473(6) Å and 2.593(6) Å] are bound in a significantly weaker fashion than the apically coordinated water molecule [bond length of 2.320(4) Å] of the pentacoordinate complex.Overall, these results experimentally confirm that in this case, and possibly for many other Cu(II) complexes, there is only a very small energy difference between octahedral and square pyramidal geometries, which in turn, demonstrates how pentacoordinated copper(II) complexes can readily associate substrate moieties.

Scheme 1 .
Scheme 1.The synthetic method for the preparation of Cu 2 L 2 (H 2 O) 5 • H 2 O.

Fig 1 .
Fig 1.The molecular structure of the title compound in the asymmetric unit, with the atom-numbering scheme.Displacement ellipsoids are drawn at the 20% probability level and H atoms are shown as small spheres of arbitrary radii.Dashed line indicates the inter-molecular hydrogen bond.

Fig 2 .
Fig 2. Intermolecular O-H ... O hydrogen bonding: (a) and (b) link the molecules into infinite chains along the b axis and (c) link the two adjacent copper molecules into infinite chains along the a axis (seeTable 3 for further details).

Fig 3 .
Fig 3. 2D supramolecular network of the compound along the ab plane (viewed along the b axis).Selected H atoms have been omitted for clarity