Synthesis and Study of Charge-transfer Complexes for 5 , 6-dimethyl-2 , 1 , 3-benzoselenadiazole

The synthesis of new solid charge-transfer (CT) complexes using 5,6-dimethyl-2,1,3benzoselnadiazoles as donors with 1,4-naphthaquinone; 1,8-dihydroxyanthraquinone, 2hydroxy-1,4-naphthaquinone and 2,3-dichloro-4,5-dicyano-p-benzoquinone (DDQ) as acceptor are reported. These complexes are characterized by elemental analysis, IR, UV-V, and HNMR spectroscopic data. Solid conductivity for 1, 2 and 3 compounds revel that they are not conductors. The calculated HOMO is largely localized on the selnadiazoles fragment, while the calculated LUMO of the studied molecule is seen to be substantially localized along the C–C axis of the conjugated system.


Introduction
It is well known that the most reactions in the preparation of selenadiazole are based on the reaction between diamine and SeO 2 [1][2][3].This very reproducible reaction has been used for absorption or spectroscopic detection of small amounts of selenium in chemical or natural media [4].Charge-transfer complexes of selenadiazoles have been synthesized and spectroscopically characterized [5][6][7][8][9][10].Recently there has been considerable interest in the studies on the charge-transfer complexes of 5, 6-dimethyl-2, 1, 3benzoselenadiazole with tetracyano-p-quinodimethane (TCNQ), tetrachloro-pbenzoquinone (TCQ) and tetracyanoethylene (TCNE) [6] which act as electron acceptors.Physical studies of these new complexes are reported.

Charge-transfer complexes
A common procedure was used for the preparation of charge-transfer complexes [5].To a solution of 2 mmol of selenadiazole compound in acetonitrile (30 ml) was added a solution of 2 mmol of quinone in acetonitrile 30 ml.The reaction mixture was refluxed for 3 hrs and the solvent was removed by rotary evaporator to give colored crystals.The product was washed with small amounts of acetonitrile and dried in vacuum.

Physical measurements
IR spectra as KBr discs in the range (200-4000) cm -1 were recorded on a Pye-Unicam SP3-300s IR spectrometer.Electronic spectra were recorded on a Pye-Unicam SP8-100 spectrophotometer in DMSO solution. 1HNMR spectra in DMSO-d 6 were recorded on Joel EX-90 FT using TMS as an internal standard [8].Melting point was measured on Gallenkamp melting point apparatus and is uncorrected.The carbon hydrogen and nitrogen analyses were carried out with Perkin-Elmer240M elemental analyzer.

Conductivity measurements
A sample of the CT complexes was compressed into a disc of 10 mm diameter and 1mm thickness.Two aluminum electrodes were deposited on the disc by evaporation under reduced pressure (10 -6 torr).Copper wires were attached by silver paint.The induced currents at different applied voltages were measured at room temperature to establish the characteristics of the studied complexes (I-IV) [5][6][7][8].

Computational method
Semi-empirical methods are done on HYPERCHEM program version 7.5 [11] running on a windows XP workstation with a Pentium IV PC.The HOMO and LUMO of the studied complexes were done by performing semi-empirical methods.Semi-empirical selfconsistent field molecular orbital (SCF-MO) method at PM3 level [12] within the restricted Hartree-Fock (RHF) [13] formalism has been considered.Geometry optimization is carried out by using a conjugate gradient method (Polak-Ribiere algorithm) [14].The SCF convergency is set to 0.001 kcal mol -1 , while the RMS gradient is set to 0.001 kcal/(Å mol) in the calculations.

Results and Discussions
The reaction of 5, 6-dimethyl-2, 1, 3-benzoselenadiazole as donor with 1,4naphthaquinone; 2-hydroxynaphthalene-1,4-dione; 1,8-dihydroxy anthraquinone and DDQ as an acceptors in acetonitrile solution gives solid complexes 1:1 stoichiometry.Table 1 shows the analytical and physical properties of these complexes.The IR spectra of (CT) complexes are compared with the spectra of their components; the spectra of (CT) complexes are similar to the superposition of the spectra of the donors and acceptors.There is a shift in frequency [8] and some alteration in the intensity, indicating the molecular association.The degree of charge-transfer of complexes was monitored by examination of ν C=O for quinones and ν (CO) and ν (CN) in DDQ [6,15] complex.Slight changes in the ν (CO) presumably is caused by charge-transfer (Table 2).
The UV-visible spectra were recorded for 200-750 nm.There are two types of electronic transitions, viz., n-π* and the transitions.The second transition is absorption.
The electronic spectral data of 1, 2 and 3 complexes are in DMSO solvent (Table 2).The electronic spectra of CT complexes show complexity, arising from overlapping between n-π* and π-π* transitions (Fig. 1).In general the n-π* transitions undergo a considerable blue shift upon formation of CT complexes.This change can be attributed to the formation of complexes and the deviation of electron cloud around selenium atom.The electron donors, the quinones, decrease the n-π* and π-π* conjugate effect in chromophoric group and increase the energy needed for the n-π* and π-π* transitions [17][18][19][20], resulting shifting in the absorption band toward shorter wave length.
1 H NMR of complex (IV) in DMSO solvent shows a chemical shift at 2.3 ppm (singlet) for methylene group and at 7.55 for benzene ring of selenadiazole; while chemical shift at 2.5 ppm (singlet) belongs to DMSO solvent.The conductivity for the solid I, II and III complexes shown in Table 3 reveals that these complexes are non conductors.The calculated frontier bonding molecular orbitals (MO) are shown in Fig. 2. The calculated HOMO is largely localized on the selnadiazoles fragment.The electronic character is also of interest with respect to its reactivity, especially towards nucleophilic agents.The calculated LUMO (electron acceptor orbital) of the studied complexes is seen to be substantially localized along the C-C axis of the conjugated system.

Table 1 .
Analytical and physical properties of CT-complexes (I-IV).

Table 2 .
Electronic, IR data for the new CT complexes (I-IV).

Table 3 .
The Electrical conductivity of the solid complexes I to III.