Isolablep- andm-[(^tBu₂MeSi)₂Si]₂C₆H₄: Disilaquinodimethane vs Triplet Bis(silyl radical)

Abstract

Isomeric p- and m-disilaquinodimethanes 2 and 4 were synthesized by the reductive dehalogenation of the corresponding p- and m-bis(halosilyl)benzenes 1 and 3, respectively, and were isolated and structurally characterized. The X-ray diffraction and solid-state NMR studies of 2 revealed its singlet quinodimethane structure featuring two exocyclic Si═C double bonds with some singlet biradical contribution. In contrast, the X-ray crystallography and EPR measurements of 4 disclosed its biradical nature, described as a triplet ground state bis(silyl radical).

Conversion of a Singlet Silylene to a stable Biradical

“…It contains a four-coordinate silicon atom ( Figure 1). In contrast to the known triplet biradical, [7] compound 2 has only one silicon atom, the coordination of which is completely different. The silicon dichloride unit is coordinated by two cyclic alkyl(amino)carbene (L 1 D) ligands to adopt a distorted tetrahedral geometry around the Si atom.…”

“…In contrast, stable triplet biradicals of main group elements are rare. Recently Sekiguchi et al [7] reported a triplet diradical with two silicon atoms prepared by reduction of a meta-substituted bissilyl phenyl derivative with KC 8 . Power [8] and Lee et al reviewed persistent stable free radicals of Group 14.…”

“…The UV absorption at 569 nm may be compared with that of the tert-butylnitroxides (450 nm) [12] or that of the bissilyl phenyl biradical (555 nm). [7] The powder sample enriched with polymorph I melts at 167-168 8C and decomposes at 172-173 8C. .…”

“…The Si À C carbene bond distances in 2 are 184.55(16) (SiÀC1) and 184.82(17) pm (Si1ÀC21), respectively, and distinctly shorter (by about 14 pm) than that in three-coordinate LDSiCl 2 (1; SiÀC = 198.5(4) pm). [14] The SiÀC bonds in 2 are shorter than typical SiÀC aryl single bonds (187.9 pm), [16] but slightly longer than Si À C bonds (181.74(14) pm) of 3,6-bis[bis(di-tertbutylmethylsilyl)silylidene]cyclohexa-1,4-diene, [7] and much longer than Si À C double bonds (170.2-177.5 pm). [17] The Cl1-Si1-Cl2 bond angle in 2 (105.51 (3)8) is larger than that of 1 ((97.25 (6)8) and can be explained by the stereochemically inactive lone pair.…”

Theoretical Study on the Enhancement of the Second Hyperpolarizabilities of Si-, Ge-Disubstituted Quinodimethanes: Synergy Effects of Open-Shell Nature and Intramolecular Charge Transfer

“…4,5,13,27 On the other hand, in 2011, the synthesis of the Sidisubstituted para-and meta-quinodimethane derivatives was reported. 28 It is experimentally found that the para-derivative exhibits singlet ground state together with the smaller bondlength alternation (BLA) of the central benzene ring (0.053 Å) as compared to that of the all-carbon analogue, Thiele's hydrocarbon (0.103 Å). 29 This experimental observation evokes the presence of larger contribution of the open-shell resonance structure in the para-quinodimethane framework for this synthesized system.…”