Crystal structure and identification of resonance forms of diethyl 2-(3-oxoiso-1,3-dihydrobenzofuran-1-ylidene)malonate

The dominant resonance form of the title compound was identified from bond lengths and may correlate with a short intramolecular O⋯O contact.


Chemical context
The structural analysis of diethyl 2-(3-oxoisobenzofuran-1(3H)-ylidene)malonate (I) was undertaken as part of a study into the synthesis of new reagents for the recovery of trivalent lanthanide metal ions by liquid-liquid extraction. We intended to prepare 2,2 0 -phthaloylbis (N,N,N 0 ,N 0 -tetrabutylmalonamide) (II), which is similar to the reported earlier 2,2 0 -[1,2phenylenebis(methylene)]bis (N,N,N 0 ,N 0 -tetrabutylmalonamide) (III) (Tyumentsev et al., 2016), from the respective tetraethyl 2,2 0 -phthaloyldimalonate (IV). In turn (IV) was to be made by the reaction of diethyl malonate with phthaloyl chloride. It is already known that acid chlorides react with diethyl malonate when treated with a combination of triethylamine and a mild Lewis acid (magnesium chloride) in acetonitrile (Rathke & Cowan, 1985). Instead of (IV), an organic product, which contained two ethyl groups in different electronic environments, was obtained in this reaction. Crystals of this compound were grown and examined with singlecrystal X-ray diffractometry, and the product was found to be the title compound, (I). The formation of (I) can be rationalized by the nucleophilic attack of the oxygen atom (in an enol form) of the keto-diethylmalonate group on the carbon atom of the unreacted acid chloride group. The mechanism of the formation of (I) was proposed by Naik et al. (1988), who obtained this compound by another reaction.
The C2-O1 bond distance of 1.394 (1) Å perfectly matches the corresponding distances in phthalic anhydride [1.396 (5) Å and 1.393 (6) Å (Bates & Cutler, 1977)]. The shorter C7-O1 distance of 1.385 (1) Å strongly suggests that the bond between the endocyclic oxygen atom O1 and the non-carbonyl carbon atom C7 has an order greater than 1. In the diethyl malonate fragment the distances C8-C9 [1.489 (2) Å ] and C8-C12 [1.507 (1) Å ] are different most likely due to the particular conformation adopted by the molecule. The bond lengths for the atoms, associated with both the furan ring of the isobenzofuran unit and the diethyl malonate fragment, indicate that the dipolar resonance form (Ia) of (I) makes a considerable contribution to its overall molecular electronic structure (Fig. 2).
According to the structure of the resonance form (Ia) a partial positive charge is localized on the oxygen atom of the heterocyclic furan ring, and one of the carbonyl oxygen atoms of the diethyl malonate fragment carries a partial negative charge, which should lead to an electrostatic attraction of these two oxygen atoms. In the structure of (I) the O1 and O9 atoms are nearly coplanar (the torsion angles C7-C8-C9-O9 and C9-C8-C7-O1 equal to À10.6 (2) and 0.7 (2) , respectively), and the distance O1Á Á ÁO9 is 2.756 (2) Å . It can Chemical diagram of a molecule (I) and its possible resonance forms (Ia) and (Ib).
be argued that simple electrostatic attraction is responsible for this close contact.

Supramolecular features
The possible non van der Waals contact in the crystal of (I) is a very weak C-HÁ Á ÁO interaction (Table 1), which links the molecules into [100] C(8) chains. A parallel-displacedstacking interaction between molecules of (I) is observed with an interplanar distance of 3.423 Å (Fig. 3) and intermolecular furan-benzene and benzene-benzene centroid-to-centroid distances of 3.5379 (13) and 3.7859 (14) Å , respectively.

Synthesis and crystallization
The title compound was prepared by the reaction of diethyl malonate with phthaloyl chloride in acetonitrile in the presence of triethylamine and magnesium chloride (Rathke & Cowan, 1985). The reagents for the synthesis were purchased from Aldrich and were used as supplied. The crude product was washed with petroleum ether on filter paper and recrystallized from cyclohexane solution as colorless crystals (69%   yield

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were refined using the riding model with C-H = 0.95-0.99 Å and U iso (H) = 1.2U eq (C).

Diethyl 2-(3-oxo-1,3-dihydro-2-benzofuran-1-ylidene)propanedioate
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.25 e Å −3 Δρ min = −0.18 e Å −3 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. Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt).