Studies of the dihedral angle of a crowded diselenide by X-ray crystallography and ultraviolet spectroscopy

Abstract

, 2749 (1983). Bis(di-tert-butylmethyl) diselenide (4) is the first example of a diselenide with an abnormal obtuse dihedral angle resulting from steric crowding. Its value was determined to be 112.1(3)" by X-ray crystallography, as compared with typical dihedral angles of 74-87" for other diselenides. The uv spectrum of 4 supports a similar conformation in solution. ence for conformations where the dihedral angle (C-Se-Se/ Se-Se-C) ( Fig. 1) is acute and lies within the range of 74"-87", except when constrained by the imposition of a cyclic structure. Thus, the cyclic diselenide 2 displays a dihedral angle of only 56" (7).The effects of very bulky substituents on the conformations of diselenides have not been investigated by crystallographic methods. To our knowledge, even the most hindered diselenide with a known crystal structure, bis(diphenylmethy1) diselenide (3), exhibits a normal, acute dihedral angle of 82" (2). More recently, Barton and co-workers (8) reported the preparation of bis(di-tert-butylmethyl) diselenide (4), a stable, crystalline substance. An examination of a model of this compound indicated that steric interactions might be severe enough to result in significant departure from the usual preferred diselenide geometry. Steric congestion could be relieved either through an increase in the dihedral angle or by lengthening of the bonds in the C-Se-Se-C moiety. It is also interesting to note that the analogous triselenide 5, whose crystal structure has been reported (9), displays normal C-Se and Se-Se bond lengths and moderate extension of the two C-Se-Se/Se-Se-Se dihedral angles to 100.6" and 86.9". It is evident, however, that FIG. I . Conformation of a typicaI diselenide.the interposition of the third contiguous selenium atom. To ascertain the extent of such effects in diselenide 4, we undertook its X-ray crystallographic structure determination. We now report the first example of a diselenide with an obtuse dihedral angle. Since molecular conformations may differ in the solid state and in solution, we also endeavoured to confirm that diselenide 4 has an abnormal dihedral angle in solution. Compression of the dihedral angle from the preferred value of 74-87" raises the energy of the ground state of a diselenide but has little effect on the excited state (10). Thus, cyclic diselenides such as 2 exhibit strong bathochromic shifts as a result of the lowered excitation energy (1 1). A similar phenomenon might be expected in the case of a diselenide in which destabilization of the ground state results from the enlargement rather than the compression of the dihedral angle, although such an effect has not yet been documented. We believe that diselenide 4 constitutes the first such example, as it displays a marked bathochromic shift compared to other, less crowded acyclic diselenides.any steric effects resulting from the interaction of the bulky Experimental substituents are greatly reduced in the triselenide Bis(di-tert-butylmethyl) diselenide (4) was by the method of Barton and co-workers (8) int...

A convenient synthesis of some symmetrical and unsymmetrical diarylmethyl sulfur and selenium compounds: X-ray crystal structure of diphenylmethylseleno-2-propene and bis[p-chlorophenyl(phenyl)methyl] diselenide

“…However, the C-Se bond is slightly longer [2.015(4) and 1.995(4) Å ] than the expected bond length (1.93-1.97 Å ). Unlike few other sterically hindered diselenides [17], the CSe-Se-C dihedral angle is acute [82.62 (18)] and close to the theoretical value [18] of 90°which means that the steric strain is being relieved through bond lengthening rather than widening up of dihedral angle. One common observation in both the crystal structures is that, one of the aryl rings (phenyl or p-chlorophenyl) is more closely disposed towards the selenium atom than the other.…”

Synthesis, Characterization and Photo Response Behaviour of InSe and CuInSe2 Nanostructures Using Tris(5‐methyl‐2‐pyridylselenolato)indium(III) as Molecular Precursor

“…. The C−Se and Se−Se bond lengths are also in the range of what found in most of the organic diselenides, i. e. (1.91–1.97 Å) and (2.285–2.33 Å), respectively …”

The synthesis and characterisation of masked phosphonioalkyl selenoates: Potential ligands for the production of functionalised gold nanoparticles

“…Although the molecular structures of a number of diorgandiselenides have been reported, the majority contain aryl groups or sterically demanding alkyl groups as substituents [17]. Previous studies of diorganodiselenides have shown that the Se-Se and Se-C bond lengths are largely independent of the organic substituents and steric strain is relieved by increasing the C-Se-Se-C dihedral angle [17,18]. The phosphonioalkyl groups in 5a are not particularly sterically demanding and the dihedral angle C(22)-Se(2)-Se(1)-C(21) [62.80(4)°] is significantly lower than those with bulkier groups, which typically lie in the range 73-104° [17].…”