Crystal structure, Hirshfeld surface analysis and DFT study of 2,2′′-({[(1E,1′E)-(diselanediyl)bis(2,1-phenylene)]bis(methaneylylidene)}bis(azaneylylidene))bis[3′,6′-bis(diethylamino)-4a’,9a’-dihydrospiro[isoindoline-1,9′-xanthen]-3-one]

The X-ray crystal and molecular structures, DFT study and Hirshfeld surface analysis of a novel spirobicyclic diselenide are reported.

The title compound, C 70 H 70 N 8 O 4 Se 2 , is a spiro bicyclic diselenide, made up of two [SeC 6 H 4 CH=N-N(CO)C 6 H 4 (C)C 6 H 3 NEt 2 (O)C 6 H 3 NEt 2 ] units related by a twofold crystallographic symmetry element bisecting the diselenide bond. The compound crystallizes in a non-centrosymmetric polar space group (tetragonal, P4b2) and the structure was refined as an inversion twin. The two diethyl amine groups and their attached phenyl groups of the xanthene ring are disordered over two orientations, with occupancies of 0.664 (19)/0.336 (19) and 0.665 (11)/ 0.335 (11), respectively. The dihedral angles between the mean planes of the central isoindoline and the phenyl rings are 26.8 (2) and 2.5 (4) , respectively. The mean plane of the central xanthene ring forms dihedral angles of 2.0 (5), 8.8 (5), 1.7 (5) and 7.9 (6) with the peripheral phenyl rings. The isoindoline and xanthene rings subtend a dihedral angle of 89.8 (2) . The molecular conformation is stabilized by an intramolecular C-HÁ Á ÁO hydrogen bond generating an S(6) ring motif. In the crystal, molecules are linked by C-HÁ Á ÁO hydrogen bonds together with C-HÁ Á Á (ring) interactions, forming a threedimensional network. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions to the crystal packing are from HÁ Á ÁH (68.1%), CÁ Á ÁH/HÁ Á ÁC (21.2%) and OÁ Á ÁH/HÁ Á ÁO (8.7%) contacts. The optimized structure calculated using density functional theory (DFT) at the B3LYP/6 -31 G(d) level is compared with the experimentally determined molecular structure in the solid state. The HOMO-LUMO behaviour was used to determine the energy gap and the molecular electrostatic potential (MEP) of the compound was investigated.

Chemical context
Diaryl diselenides and aryl selenolates have been previously used as ligand precursors for the synthesis of transition-metal complexes (Khandelwal & Gupta, 1989;Gupta & Parihar, 1995. Selenospirocyclic compounds are a class of heterocyclic compounds with a wide variety of uses in organic synthesis (Aho et al., 2005;Kotha et al., 2009;James et al., 1991), biological activities (Mugesh et al., 2001;Nogueira et al., 2004;Press et al., 2008;Alberto et al., 2009) and photoluminescence properties (Singh et al., 2011;Shi et al., 2010). However, the formation of spirobicyclic diselenides is rare and to the best of our knowledge, not reported in the literature. There are very few reports of the formation of selenospirocyclic derivatives which have been structurally characterized (Singh et al., 2011;Shi et al., 2010). Very recently, organoselenium compounds containing both N and Se have Diagram showing: (a) a half molecule showing the disorder, (b) the major component of the title compound [symmetry operation: 1 2 + y, À 1 2 + x, 1 À z]. Displacement ellipsoids are shown at the 30% probability level. distance of 2.940 (9) Å leading to an S(6) ring motif (Bernstein et al., 1995) is also present. Furthermore, there exists a C-HÁ Á Á interaction between the H25C atom of the methyl carbon C25 and the centroid of the C16-C21 phenyl ring; symmetry code 1 À y, À1 + x, 1 À z. These interactions play a vital role in stabilizing the crystal packing within the crystal structure.

Hirshfeld surface analysis
Hirshfeld surface (HS) calculations (Spackman & Jayatilaka, 2009) were performed on the title compound to further investigate the intermolecular interactions. The Hirshfeld surface plotted over d norm in the range À1.0432 to + 2.0960 a.u. generated using CrystalExplorer 21.5 (Spackman et al., 2021) is shown in Fig. 3. The red spots that appear around O1 are caused by the intermolecular C7-H7Á Á ÁO1 and C12-H12Á Á ÁO1 interactions, which are important in the packing of the title molecule. An intramolecular C-HÁ Á ÁO hydrogen bond is also indicated by the red spots near the hydrogen and oxygen atoms (Fig. 3b). Bright-red spots on top and bottom of the HS near N3 indicate an intermolecular C-HÁ Á Á (ring) interaction involving H25B of the C25 methyl group and a benzene ring (Fig. 3c).
Starting geometries were taken from the X-ray refinement data. Theoretical and experimental results related to bond lengths and angles are in good agreement (Table 3). Calculated molecular orbital energies (eV) for the surfaces of the frontier molecular orbitals of the title compound are shown in Fig Table 3 Comparison of selected (X-ray and DFT) bond lengths and angles (Å , )..

Figure 5
Calculated frontier molecular orbitals of the title compound. unoccupied molecular orbital) as an electron acceptor.
Calculated numerical values for the title compound including, electronegativity (c), hardness (h), ionization enthalpy (IE), dipole moment (m), electron gain enthalpy (EE), electrophilicity (!) and softness (s), are collated in Table 4. The significance of h and s is to evaluate both the reactivity and stability.
As shown in Fig. 5, the HOMO is mainly located on the xanthene phenyl ring and diethyl amine groups whereas the LUMO is distributed on the phenyl ring attached to selenium, azomethine and carbonyl group. In HOMO -1, electron clouds are distributed on the azomethine group, the phenyl ring attached to selenium and the diethyl amine groups on the other side of the molecule. In LUMO + 1, electron clouds are located on the isoindoline and azomethine groups of both sides of the molecule whereas in LUMO + 2, it involves the selenium atom, phenyl ring, azomethine and isoindoline groups on one side of the molecule. The energy band gap [ÁE = E LUMO À E HOMO ] of the molecule is 3.7536 eV, and the frontier molecular orbital energies, E HOMO and E LUMO , are À5.0048 and À1.2512 eV, respectively.
The molecular electrostatic potential (MEP) map ( Fig. 6) was calculated at the B3LYP/6-31G(d) level of theory. In the MEP diagram, the molecular electrostatic potential is in the range À0.0833 to 0.0321 a.u. and the different electrostatic potentials at the surface of the molecule are represented by different colours. Electrostatic potentials increase in the order red < yellow < green < blue, and red indicates the electron rich region and blue indicates the electron-deficient region. As shown in Fig. 6, the carbonyl groups are surrounded by negative charges, indicating some possible nucleophilic sites, whereas the positive charge regions are located on the H atoms indicating possible electrophilic sites.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.42, update May 2021; Groom et al., 2016) for the basic skeleton of this compound gave no hits. However, a CSD search on phenyl-Se-Se-phenyl compounds gave 152 hits and 199 observations with the Se-Se distance ranging from 2.287 to 3.051 Å (with a mean value of 2.393 Å and a standard deviation 0.162). In the structures of CATWEB01, REDGAK, REDGEO and REDGUE (Panda et al., 2012), the typical torsional angles of the selenium-attached phenyl ring (C-Se-Se-C) are ca 81 and those of CIDXET and CIDXUJ (Kulcsar et al., 2007) are 80.9 and 114.0 , respectively.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5 (11). All atoms in the diethyl amine groups (N3/C18/C19/C20/C22/C23/C24/C25 and N4/ C32/C33/C34/C35) were subject to displacement and positional restraints using SIMU and SAME instructions. For the SIMU command the esd's used were 0.005 while for the SAME command the esd's used were 0.003.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (