A crystalline carbene–silylene adduct 1,2-C6H4[N(R)]2C-Si[N(R)]2C6H4-1,2 (R = CH2But); synthesis, structure and bonding in model compounds†

Zwitterionic Silenes: Interesting Goals for Synthesis?

“…Moreover, 21 A is a silene with a silicon ± carbon single bond and with a structure intermediate between that of a regular SiC bonded silene and the weak carbene ± silylene complex of Lappert and co-workers. [11] It should largely be described by the resonance structures of type II, and we therefore call it a zwitterionic silene.…”

“…[9] In Lappert×s silylene ± carbene complex a distance as long as 2.162 ä was observed. [11] The extended silicon ± carbon bonds in these compounds are contrasted by the silicon ± carbon distances of 1.7039 and 1.692 ä for H 2 Si CH 2 and Me 2 Si CH 2 measured by millimeter and microwave spectroscopy, [12,13] and the silicon ± carbon distances of 1.702 ± 1.741 ä in the aryl-, alkyl-, and/or silyl-substituted silenes reported by Apeloig et al and Wiberg et al [14,15] Moreover, Si pyramidalizations were calculated for the parent 2-silenolate, [8] variously substituted 4-silatriafulvenes, [5] and in 1,1-bis(trimethylsilyl)-2-amino-2-siloxysilenes [7] and observed in our recent 2-silenolate, [9] in H 2 SiCO, [10] and in the [1,2-C 6 H 4 [N(R)] 2 C ± Si[N(R)] 2 C 6 H 4 -1,2 (R CH 2 tBu)] complex. [11] It was also concluded that negative charge in the silylene ± carbene complex is located at Si, [11] and its long silicon ± carbon bond can be rationalized by very extensive reversed polarization.…”

“…Indeed, our recent X-ray crystal structure of 1-tBu-2,2-bis(trimethylsilyl)-2-silenolate revealed that this 2-silenolate is SiÀC singly bonded, [9] proving that they are strongly affected by reversed polarization. The silaketene H 2 SiCO studied by Maier and co-workers [10] and the [1,2-C 6 H 4 [N(R)] 2 C ± Si[N(R)] 2 C 6 H 4 -1,2 (R CH 2 tBu)] silylene ± carbene complex of Lappert and co-workers [11] should also be noted, since these formally could be written as SiC bonded; however, in reality these compounds are donor ± acceptor complexes with the utmost weak silicon ± carbon bonds.…”

“…In contrast to methyl/silyl substituents at Si that do not lead to further geometric distortion of an already zwitterionic silene, strong p electron donor substituents at the Si of a zwitterionic silene further elongate and weaken the silicon ± carbon bond. [11] This results from stabilization of the singlet state of the silylene leading to an even larger SDE ST than in the silenes presently studied.…”

“…[11] We reason that alkyl and silyl substitution at Si in 21 gives singlebonded silenes that are stable to dissociation, even though the SiC bond dissociation energies in 21 A ± 21 C are only 45 ± 60 % when compared to that of a normal SiÀC single bond (% 90 kcal mol…”