Expanded ring N-heterocyclic carbene adducts of group 15 element trichlorides: synthesis and reduction studies

Abstract

Reactions of the expanded ring N-heterocyclic carbene, 6-Dip (:C{N(Dip)CH2}2CH2, Dip = 2,6-diisopropylphenyl), with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl3] (E = P, As or Sb), two examples of which (E = P and Sb) have been crystallographically characterised. Reduction of [(6-Dip)PCl3] with KC8 yielded the unusual tetraphosphorus dicationic complex, [(6-Dip)2(μ-P4)]Cl2, the X-ray crystal structure of which shows it to be an ion-separate salt. The compound can also be prepared from the direct reaction of excess 6-Dip with PCl3. Treatment of the cyclic amidinium salt, [6-MesH]Br (6-MesH = [HC{N(Mes)CH2}2CH2](+), Mes = mesityl) with KC8, leads to reductive coupling of the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)2. An X-ray crystallographic analysis of (6-MesH)2 shows the compound to have a long central C-C bond, while an electrochemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane solutions.

Synthesis, Structure, and Reactivity of Ga‐Substituted Distibenes and Sb‐Analogues of Bicyclo[1.1.0]butane

“…Unfortunately, any attempts to separate the different species through fractional crystallization failed. However, our studies clearly demonstrate the high synthetic potential of the insertion‐reduction method using LGa as compared to the coordination‐reduction approach using N‐heterocyclic carbenes (NHCs); the attempted synthesis of an NHC‐stabilized Sb 4 complex by reduction of a NHC‐SbCl 3 adduct failed ,. However, the three‐electron reduction of the adduct CAAC‐SbCl 3 (CAAC=cyclic alkyl(amino)carbene) with 3 equiv.…”

London Dispersion Interactions in Pnictogen Cations [ECl2]+ and [E=E]2+ (E=P, As, Sb) Supported by Anionic N‐Heterocyclic Carbenes

“…Naturally, the E−C carbene bond lengths increase from P−C1=1.842(1)/1.841(3) Å in 2 a / 2 b to As−C1=1.978(2) Å in 3 and Sb−C1=2.179(2) Å in 4 . These bonds are shorter than those found in hypervalent (or hypercoordinate) (NHC)EX 3 adducts, for example, (IDipp)PCl 3 , (IDipp)AsCl 3 , (IDipp)PBr 3 , and (IDipp)SbBr 3 , which generally feature disphenoidal (seesaw) molecular geometries with one shorter and two longer E−X bonds. Similar E−C carbene and E−Cl bond lengths, however, were found in cationic pnictanes such as [(IDipp)PCl 2 ]OTf and [( Me IPr)ECl 2 ]OTf (E=As, Sb) .…”

“…Although NHC‐stabilized diantimony and dibismuth were also calculated to be stable entities, the isolation of (IDipp) 2 E 2 (E=Sb, Bi) by reduction of the carbene adducts (IDipp)ECl 3 failed, suggesting “that the ′carbene‐stabilization′ strategy may be less effective with the heavier pnictogens.” Likewise, the attempted reduction of a series of six‐membered tetrahydropyrimidin‐2‐ylidene adducts of group 15 element trichlorides only returned a mixture of products, including significant amounts of the free carbene . Cyclic (alkyl)(amino)carbenes (CAACs), however, which were also employed for the preparation of the (CAAC) 2 E 2 species X (E=P) and XI (E=As), allowed the preparation of (CAAC) 2 Sb 2 ( XI ) by three‐electron reduction of (CAAC)SbCl 3 with KC 8 (Figure ) .…”

“…[29] Likewise, the attempted reduction of as eries of sixmembered tetrahydropyrimidin-2-ylidenea dducts of group 15 elementt richlorides only returned am ixture of products, including significant amountso ft he free carbene. [31] Cyclic (alkyl)(amino)carbenes (CAACs), [32] however,w hichw ere also employed for the preparation of the (CAAC) 2 E 2 species X (E = P) and XI (E = As), [33,34] allowed the preparation of (CAAC) 2 Sb 2 (XI) by three-electron reduction of (CAAC)SbCl 3 with KC 8 (Figure 1). [35] It is noteworthy that XI represents the only compound reported to date in whichadiantimony unit is flanked by two carbene ligands.…”

A Gallium‐Substituted Distibene and an Antimony‐Analogue Bicyclo[1.1.0]butane: Synthesis and Solid‐State Structures

“…28 8)i ss ignificantly larger than that in 3 (89.359-(10) 8 8). Note that the attempted synthesis of an NHC stabilized Sb 4 complex by reduction of an NHC-SbCl 3 adduct failed, [42] which clearly illustrates the high synthetic potential of the insertion-reduction method using monovalent RGa compared to the widely established coordinationreduction approach by use of NHCs.…”