(Acetonitrile-κN)aqua[N,N′-bis(pyridin-2-ylmethyl)ethane-1,2-diamine-κ4 N,N′,N′′,N′′′]zinc(II) perchlorate

The structure of acetonitrileaqua[1,2-bis(pyridin-2-ylmethyl)ethane-1,2-diamine]zinc(II) perchlorate contains a six-coordinate cation consisting of the tetradentate bispicen ligand, coordinated water, and coordinated acetonitrile with the latter two ligands adopting a cis configuration.


Chemical context
One of the greatest challenges in synthetic chemistry is the selective conversion of non-activated C-H bonds to useful functional groups (Gunay & Theopold, 2010). Coordination complexes have been extensively explored due to their potential to catalyze such transformations. The ligand's chelation around the metal ion determines the number and relative orientation of vacant coordination sites where terminal oxidants and/or substrates can bind. Installed steric bulk or substrate binding groups can either preclude certain molecules from accessing the active site (Chen & White, 2010) or attract compounds with specific shapes or functional groups (Das et al., 2006). These benefits rely upon the ability to understand, predict, and control the coordination geometry of the polydentate ligand.
The 1,2-bis(pyridin-2-ylmethyl)ethane-1,2-diamine (bispicen) ligand and other tetradentate ligands with reduced imine linkages have been observed to wrap around single transition metal ions in primarily two fashions: cis-and cis-(Scheme 2) (Chen et al., 2002). The cis-, in which the two pyridine groups are trans to each other, has been the only conformation heretofore observed with bispicen and its methylated derivatives (Goodson et al., 1990(Goodson et al., , 1991. The cisconformation, in which the two pyridine groups are cis to each other, has been observed most often with ligands with propane-1,3-diamine backbones (England et al., 2007;Hureau et al., 2005a,b).
A third conformational possibility, alternately described as trans or planar (Scheme 2), has been structurally observed most commonly for tetradentate ligands with imine linkages, such as salens (Jacobsen et al., 1991). In tetradentate ligands with reduced imine linkages, the trans conformation has been observed rarely and only with ligands with either severely strained bridges or longer alkyl linkages between the amines (Mas-Ballesté et al., 2006). Consequently, the trans conformer is rarely mentioned as a plausible isomer in reactivity studies involving bispicen derivatives.
In view of the fact that Zn II is a d 10 system and thus has a relatively plastic coordination environment it is of interest to determine which of the possible conformations the bispicen ligand adopts upon coordination to this metal ion. There is only one previous structural study of a bispicen derivative of Zn (Parajó n- Costa et al., 2013). This study is in continuation of our past studies on the role of zinc in hydrolytic enzymes (Gultneh et al., 1996(Gultneh et al., , 1999, in particular the role of Zn in lowering the pK a of coordinated water molecules.

Structural commentary
In the structure of the title compound ( Fig. 1), the six-coordinate cation consists of the tetradentate bispicen ligand, coordinated water, and coordinated acetonitrile, with the latter two ligands adopting a cis conformation. There are two complete formula units in the asymmetric unit. Both cations show almost identical structural features with the bispicen ligand adopting the more common cis-conformation, in which the two pyridine groups are cis to each other with the H 2 O and CH 3 CN ligands in trans position to the N-H groups. One of the four perchlorate anions is disordered over two positions with occupancies of 0.9090 (15) and 0.0910 (15). A related complex containing a [cis-Zn(bispicen)Cl(H 2 O)] + cation and a [ZnCl 4 ] 2À anion has been published recently (Parajó n-Costa et al., 2013). In contrast to previous studies, bond lengths for the two types of Zn-N bonds (Zn-N py and Zn-N N-H ) show very similar values, with the exception of those which are trans to the coordinated acetonitrile (Table 1). The latter are significantly longer [2.2056 (7) and 2.2066 (7) Å ]. Interestingly, Zn-OH 2 bond lengths are relatively short [2.1333 (7) and 2.1279 (7) Å ] reflecting a strong Zn-O bond. Further studies will be made to see the effect of this on the pK a of the coordinated water. There is extensive inter-ionic hydrogen bonding (Table 2) between the perchlorate anions and O-H and N-H groups in the cations including a bifurcated hydrogen bond between an N-H group and two O atoms of one perchlorate anion. As a result of this extended hydrogen-bond network the ions are linked into a complex three-dimensional array.

Supramolecular features
There is a complex array of hydrogen bonds between the O-H and N-H groups in the cations and the O atoms of the anions. In addition, there are weak C-HÁ Á ÁO interactions between the CH 3 groups of the coordinated acetonitrile

Figure 1
Diagram of the Zn-containing cation, showing the atom labeling. Anions have been omitted for clarity. Atomic displacement parameters are at the 30% probability level. moieties and the adjoining O atoms of the perchlorate anions. These link the ions into a complex three-dimensional array (Fig. 2).

Synthesis and crystallization
Pyridine-2-carbaldehyde (2.3996 g, 0.0022 mol) was added to a reaction flask and dissolved in 50 ml methanol. Ethylenediamine (0.6732 g, 0.0012 mol) was added to the solution. The mixture was stirred for 3 d before refluxing the reaction for 1 h. The contents of the reaction flask were cooled to 268 K. 4 equivalents of NaBH 4 (1.6646 g, 0.0044 mol) were added to the reaction mixture which was then allowed to reach room temperature. The mixture was stirred overnight. Methanol was evaporated under reduced pressure. The contents were redissolved in water (50 ml) before being extracted with chloroform (4 Â 50 ml). Moisture was removed using anhydrous MgSO 4 . Chloroform was evaporated under reduced pressure producing the ligand as a brown oil (yield 68%). The zinc(II) complex was prepared by reacting 0.4356 g (0.0018 mol) of the ligand in 50 ml acetonitrile with Zn(ClO 4 ) 2 Á6H 2 O (0.4251 g, 0.0018 mol). The mixture was stirred at room temperature overnight and layered with 50 ml diethyl ether. The container was sealed and diethylether allowed to diffuse into the solution for 3 d to give yellow crystals which were filtered and dried [yield based on Zn(ClO 4 ) 2 : 55%].

(Acetonitrile-κN)aqua[N,N′-bis(pyridin-2-ylmethyl)ethane-1,2-diamine-κ 4 N,N′,N′′,N′′′]zinc(II) perchlorate
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.