Crystal structure of diaquabis(7-diethylamino-3-formyl-2-oxo-2H-chromen-4-olato-κ2 O 3,O 4)zinc(II) dimethyl sulfoxide disolvate

A near-perfect octahedral zinc(II) complex coordinated to two coumarin fluorophores.


Chemical context
Fluorescent molecular probes have been utilized in the monitoring of anions, cations, and neutral species in many applications in supramolecular analytical chemistry (Lee et al., 2015). In particular, derivatives of 1,2-benzopyrone (commonly known as coumarin) have been used extensively as fluorescent chemosensors for a wide range of applications due to their unusual photo-physical properties in different solvent systems and using theoretical calculations (Lanke & Sekar, 2015;Liu et al., 2013). There is a plethora of coumarin dyes and their derivatives that have been used as colorimetric and fluorescent sensors (Lin et al., 2008;Ray et al., 2010). In fact our own group has used a coumarin-enamine organic compound as a chemosensor for the detection of cyanide ions, via a Michael addition approach (Davis et al., 2014). Additionally, we have utilized a small family of the coumarin chemosensors to discriminate metal ions as their chloride salts utilizing Linear Discriminant Analysis (Mallet et al., 2015).
The detection of one particular metal ion, Zn II , is of special interest to our group. The Zn II ion is ubiquitous in nature, playing important biological roles, and acting as a Lewis acid in the hydrolysis process involving carboxypeptides. Zinc also plays many structural roles and is often found accompanied with cysteine and histidine residues (the classic zinc finger motif; Osredkar & Sustar, 2011). As a consequence of the filled d shell with its d 10 electron configuration, the zinc ion is found in all geometrical arrangements, with the tetrahedral and octahedral being the two most common motifs. Additionally Zn II is spectroscopically silent, therefore direct monitoring of this ion is challenging, especially in aqueous media. Our intention was to synthesize a planar molecular chemosensor with a high degree of conjugation which can be easily perturbed to produce a spectroscopic response upon the coordination of Zn II ions. In this paper we report the synthesis and the supramolecular architecture of [Zn(7-diethylamino-3formyl-chromen-2,4-dione) 2 (H 2 O) 2 ], (1).

Structural commentary
The molecular structure of (1) is shown in Fig. 1. The coumarin ligand is planar and is coordinated to the Zn II ion in a chelating fashion by the two carbonyl functional groups that form a pseudo--diketone motif. This is indicated by the short C O bond of the dione (O3-C4) and the C O bond length of the formyl moiety (O4-C9), with values of 1.2686 (10) and 1.2603 (10) Å , respectively. The Zn-O bonds complete the stable six-membered chelating motif, which is favorable for smaller metal ions (Hancock & Martell, 1989 (Dong et al., 2010). The metal ion is located on an inversion center. The axial positions are occupied with two water molecules, the Zn1-O5 bond length is at 2.1624 (7) Å slightly longer than that in other hydrated Zn II coordination complexes, whereby the average Zn-O (aqua ligand) distance is 2.09 Å (Nimmermark et al., 2013). The coordination sphere of the Zn II ion is a near perfect octahedron with all of the bond angles close to 90 , ranging from 86.82 (3) to The molecular structure of the title compound, showing displacement ellipsoids at the 50% probability level, with a single DMSO molecule hydrogen bonded to a water molecule coordinating to the zinc cation. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x À 1; y; z; (ii) Àx þ 2; Ày þ 1; Àz þ 1; (iii) x þ 1; y; z.

Figure 2
The crystal packing of the title compound highlighting the extensive hydrogen-bond network.

Supramolecular features
The crystal structure of the title compound shows an extensive array of hydrogen-bonding interactions (Table 1) forming hydrogen-bond ring systems and infinite chains (Fig. 2). The encapsulated DMSO solvent molecule forms a hydrogenbonding interaction with a single water molecule that is coordinating to the Zn II ion S1-O6Á Á ÁH52-O5 [1.983 (9) Å ]. Interestingly, there are also two C-HÁ Á ÁO hydrogen-bonding interactions from the methyl moiety of DMSO; one with the O atom on the formyl functional group in the equatorial position (H13AÁ Á ÁO4 = 2.52 Å ) and an additional hydrogen-bonding interaction from the carbonyldione group occupying another equatorial position (H12BÁ Á ÁO3 = 2.62 Å ). Together these two interactions form three R 2 2 (8) systems. Furthermore, the DMSO solvent molecule encapsulated within the crystal structure forms a single hydrogen-bonding interaction with an adjacent DMSO molecule H13CÁ Á ÁO6(x + 1, y, z) (2.29 Å ), forming an infinite chain.
It is well known that coumarin crystal packing displaysstacking motifs as a consequence of the planarity of the organic framework (Guha et al., 2013). Interestingly, the crystal packing of the title compound is influenced by off-set interactions between the electron deficient coumarin ring system of one molecule (ring system O1-C8A) and the electron-rich region of the second coumarin ring system (C4A-C8A) of an adjacent compound, whereby the centroids are 3.734 Å apart (Fig. 3). This is in good agreement with otherstacking motifs (Wallace et al., 2005). As a consequence, the packing arrangement shows a distinct zigzag pattern (Fig. 4).

Database survey
For coumarin-derived molecular probes for the detection of neutral compounds, see: Wallace et al. (2006). A coumarinbased chemosensor for the detection of copper(II) ions was prepared by Xu et al. (2015)     Chemical structures used in the CSD similarity search. crystal structure of a coumarin-cyanide adduct. There are over 25,000 zinc(II) coordination complexes in the Cambridge Structure Database (CSD; Groom et al., 2016), both the tetrahedral and octahedral environments. Therefore, the authors carried out a refined structure search based on the structures shown in Figs. 5(a) and 5(b); however, these did not yield any results. Therefore a modification of the search by specifically searching structures that have a bidentate chelating -diketone motif coordinated to the zinc(II) in the equatorial position, with two water molecules in the axial position, as shown in Fig. 5(c) was carried out. This refined search yielded two similar structures with Zn II octahedrally coordinated, the first by Solans et al., whereby two 1,3-bis(2hydroxyphenyl)propane-1,3-dionate ligands coordinate to the Zn II ion, with the remaining two coordination sites occupied by two ethanol molecules (Solans et al., 1983). The other similar structure was reported by Dong et al. (2010) who incorporated two 2-(4-benzoyloxy-2-hydroxybenzoyl)-1phenylethenolate ligands that were bound to the metal ion in the equatorial position and two ethanol molecules situated in the axial postions.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms on C were idealized with a C-H distance of 0.95 Å for Csp 2 , 0.99 Å for CH 2 , and 0.98 Å for methyl groups. Those on O atoms were assigned from difference maps, and their positions refined, with O-H distances restrained to 0.86 (1) Å . U iso values for H atoms were assigned as 1.2 times U eq of the attached atoms (1.5 for methyl and water groups).

Computing details
Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015). 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.