Deprotonation of Group 14 Metal Amide Complexes Bearing Ditopic Carbanionic N-Heterocyclic Carbene Ligands. Constitutional Isomerism and Dynamic Behavior

Abstract

The reactivity of the lithiated N-heterocyclic carbene [:C[N(2,6-i Pr2C6H3)]2(CH)CLi]∞ with two-coordinate homoleptic group 14 amides E[N(SiMe3)2]2 (E = Sn, Pb) is described. Solutions of these mixtures readily result in the formal loss of one equivalent of bis(trimethylsilyl)amine, HN(SiMe3)2, to afford novel metallacycles. Reactions involving Sn[N(SiMe3)2]2 initially give rise to the anionic complex [{:C[N(2,6i Pr2C6H3)]2C(CH)}Sn{N(SiMe3)2}2] − (1), which eventually affords the novel distannane [{(THF)2Li:C[N(2,6-i Pr2C6H3)]2(CH)C}Sn{μ-κ 1 :κ 1 -N(SiMe3)(SiMe2CH2)}Sn{C(CH)[N(2,6i Pr2C6H3)]2C:}{N(SiMe3)2}] (2) in which one of the trimethylsilyl substituents has been deprotonated and bridges the Sn-Sn bond affording a five-membered metallacycle. By contrast, reactions involving Pb[N(SiMe3)2]2 give rise to a species that may be considered a constitutional isomer of 2, [{(THF)2Li:C[N(2,6i Pr2C6H3)]2(CH)C}Pb{N(SiMe3)}2{C(CH)[N(2,6-i Pr2C6H3)]2C:}Pb{κ 2 -N(SiMe3)(SiMe2CH2)}] (3), featuring a base-stabilised plumbylene species. The dynamic behaviour of 3 in solution suggests stereochemical inversion at the plumbylene moiety, Pb{κ 2 -N(SiMe3)(SiMe2CH2)}, a process which can be retarded at low temperatures allowing for the observation of the two inequivalent proton environments of the methylene bridge.This process is believed to involve rapid intermolecular transfer of the plumbylene fragment between the ditopic carbanionic carbene ligands of the plumbyl core.

NHCs in Main Group Chemistry

“…Recently, Goicoechea and colleagues reported the distannane 48 (cf., Figure ) that is coordinated by two abnormal NHCs. Formation of 48 from the abnormal ( a NHC)Li adduct 34a and Sn­[N­(SiMe 3 ) 2 ] 2 proceeds via an intermediary a NHC stannylene adduct 843 , observed by 1 H NMR spectroscopy, which further undergoes dimerization and rearrangement reaction (Scheme ).…”

“…Both C a NHC –Sn bond lengths (2.186 and 2.256 Å) are within the observed range of abnormal bonded NHC tin complexes. With 2.870 Å, the Sn–Sn bond is slightly longer than the sum of the covalent radii and longer than that in hexaphenylditin; however, it still remains in the range of Sn–Sn single bonds …”

“… Plumbyliumylidene 48a , b , which can also be directly prepared by addition of 0.5 equiv of Pb­[N­(SiMe 3 ) 2 ] 2 to 35a and 45 , features two nearly equal C NHC –Pb bonds (2.363(5), 2.339(5) Å). In 2017, Goicoechea and co-workers converted 48a with another equivalent of Pb­[N­(SiMe 3 ) 2 ] 2 , giving compound 49 as an adduct of 48b and a carbene-coordinated four-membered cyclic plumbylene, formed upon deprotonation of the N­(SiMe 3 ) 2 moiety . In solution, the plumbylene moiety shows rapid intermolecular exchange resulting in inversion of the configuration, which was investigated by variable-temperature NMR, and further evidenced by two conformations of 48a in the XRD analysis.…”

“…129 Very recently, Goicoechea and co-workers also reported the synthesis of the bis(aNHC)-ditin complex 47, where one of the NHC moieties is also coordinated to a lithium cation in the "regular" C2 position (Figure 8). 130 The complex was obtained by reaction of Sn[N(SiMe 3 ) 2 ] 2 with NHDC 34a.…”

Polar organometallic strategies for regioselective C–H metallation of N-heterocyclic carbenes

“…(1•THF) (left) and section of the polymeric structure of 1•THF established by X-ray crystallography (right). 7 Exhibiting a unique polymeric structure, where the {Li(THF)} cations are connected by anionic NHC bridges through their normal C2 and C4 (so-called abnormal) positions, 1 has become a versatile precursor for accessing anionic carbene complexes of main group elements such as B, 7,12 Al, 7 Ga, 13 Zn 14 or Sn 15 amongst others (2a-f, Scheme 2). These reactions are driven by the stronger electronegative (carbophilic) character of the main group element, which undergoes fast transmetallation with 1 at its C4 position, with Li being trapped by coordination to its normal carbenic site (Scheme 2).…”