4,5-Dicyano-3,6-diethylbenzo-1,2-diselenete, a Highly Stable 1,2-Diselenete: Its Preparation, Structural Characterization, Calculated Molecular Orbitals, and Complexation with Tetrakis(triphenylphosphine)palladium

Abstract

The first isolable benzo-1,2-diselenete, 4,5-dicyano-3,6-diethylbenzo-1,2-diselenete (4), was prepared by the reaction of 4,5-(o-xylylenediseleno)-3,6-diethylphthalonitrile (3) with aluminum chloride in toluene. X-ray crystallographic analysis demonstrated that 4 contains a trapezoidal diselenide ring rather than a benzo-1,2-diselenone structure. In crystal form, 4 undergoes self-assembly and generates structures based on layered molecular sheets since the unit cell contains only one molecule. While the cyclic voltammogram of 4 exhibited only one irreversible peak (Ep = 1.59 V) during oxidation and two quasireversible couples during reduction, three peaks were observed in the differential pulse voltammogram of the reduction couples (E1/2 = -1.19, -0.75, and -0.69 V). Although a THF solution of 4 in the presence of sodium metal was EPR silent, various signals were readily observed in its (1)H, (13)C, and (77)Se NMR spectra. Molecular orbital calculations for 4 demonstrated that the HOMO orbital is primarily localized at the two selenium atoms and four of the benzene carbon atoms while the LUMO orbital is situated solely on the diselenete ring. It appears that the HOMO and LUMO orbitals of 4 receive significant stabilization from the nitrile groups compared to the level of stabilization in the unsubstituted benzo-1,2-diselenete (BDS). The reaction of 4 with tetrakis(triphenylphosphine)palladium in benzene was found to produce a dinuclear palladium complex (8), and the structure of this complex was determined by X-ray crystallographic analysis. The central four membered ring of 8 consists of the Pd1, Se2, Pd2, and Se3 atoms and is not planar but rather adopts a folded arrangement.

Preparation and reactivity of benzo‐1,2‐dichalcogenete derivatives and their bis(triphenylphosphine)platinum complexes

“…As stable benzo‐1,2‐diselenete, compound 2c was prepared by treatment of 4,5‐( o ‐xylylenediseleno)‐3,6‐diethylphthalonitrile ( 1c ) with aluminum chloride in toluene (Scheme ) [8]. However, when 4,5‐( o ‐xylylenedithio)‐3,6‐diethylphthalonitrile ( 1a ) was similarly treated, 3,6‐diethyl‐4,5‐dicyanobenzene‐1,2‐dithiol ( 2a ) produced as a ring‐opened form.…”

“…Since it is expected that the effect of the nitrile groups is important to stabilize 2c ,[8] the nitrile group was exchanged with one or two bromine atoms. When 4,5‐( o ‐xylylenediseleno)‐3,6‐diethyl‐1,2‐dibromobenzene was treated with aluminum chloride in toluene, the products were 4,7‐diethyl‐5,6‐dibromobenzo‐1,2,3‐triselenole and elemental selenium.…”

“…In a previous paper, when 2c was treated with sodium metal in deuterated tetrahydrofuran, the corresponding dianion in the reddish‐brown solution was observed by 1 H and 77 Se NMR spectra. Compound 2c was recovered after quenching the tetrahydrofuran solution with hydrochloric acid [8]. To determine the reactivity of the species, methyl iodide was added to the reddish‐brown solution.…”

“…Kuropatov et al reported the platinum complex of benzo‐1,2‐dithiete [6c,9]. We also prepared the dinuclear palladium complex upon treatment of benzo‐1,2‐diselenete 2c with tetrakis(triphenylphosphine)palladium [8]. In addition, there are many reports of disulfide, diselenide, and ditelluride complexes coordinating to zerovalent palladium and platinum atoms [10,11]…”

“…However, there have been several reports of four‐membered selenium derivatives, 1,2‐diselenetes and benzo‐1,2‐diselenetes [7]. Recently, we could prepare highly stable benzo‐1,2‐diselenete, 3,6‐diethyl‐4,5‐dicyanobenzo‐1,2‐diselenete ( 2c ), as reddish‐purple crystals [8]. To stabilize the four‐membered diselenide ring of 2c , the buttressing effect of the two ethyl groups is important to keep the bridged ring structure and the two nitrile groups as an electron withdrawing group are also effective [5,6c]…”