Coinage Metal Complexes of the Carbenic Tautomer of a Conjugated Mesomeric Betaine Akin to Nitron

Abstract

This study was motivated by our recent observation that the analytical reagent Nitron (2) is an “instant carbene”, whose reaction with coinage metal salts MX afforded complexes of its carbenic tautomer 1,4-diphenyl-3-phenylamino-1,2,4-triazol-5-ylidene (2′). Our aim was to establish an alkyl homologue of 2 in order to achieve a carbenic tautomer of higher donicity. For this purpose 1-tert-butyl-4-methyl-1,2,4-triazol-4-ium-3-tert-butylaminide (6) was synthesized. Its reactions with MX afforded complexes of the carbenic tautomer 1-tert-butyl-3-tert-butylamino-4-methyl-1,2,4-triazol-5-ylidene (6′). With a stoichiometric ratio of 1:1 complexes of the type [MX(6′)] were obtained. A ratio of 2:1 furnished complexes of the type [MX(6′)2] or [M(6′)2]X. 6′ is a better σ-donor and less electrophilic than 2′ according to NMR spectroscopic data of 6H[BF4] and 6′ = Se, respectively, and IR spectroscopic data of [RhCl(6′)(CO)2] confirm that its net electron donor capacity is superior to that of 2′. A comparison of the complexes of 2′ and 6′ reveals two pronounced structural differences. [CuX(6′)2] (X = Cl, Br) exhibit more acute C–Cu–C bond angles than [CuX(2′)2]. In contrast to [CuCl(2′)], [CuCl(6′)] aggregates through Cu···Cu contacts of ca. 2.87 Å, compatible with cuprophilic interactions. These differences may be explained by the complementary steric requirements of the t-Bu and the Me substituent of 6′.

Base‐Ionizable Anionic NHC Ligands in Pd‐catalyzed Reactions of Aryl Chlorides

“…Among these ligands, Nitron (R = R′ = R′′ = Ph), a well‐known analytic reagent for gravimetric determination of nitrate ions, [13] has attracted attention as an NHC proligand more than 10 years ago [12a] . Complexes of Pd, [12g–k] Rh, [12a,e,g] Ru, [12b,g] Ir, [12g,j] Ni, [9d,12j] and coinage metals (Cu, Ag and Au) [12c,e] with Nitron and its analogs have been reported in the literature, some of which have been studied as catalysts. For example, complexes of Ir with Nitron catalyzed dehydrogenation of formic acid, aldehyde hydrosilylation, dehydrocoupling of hydrosilanes and alcohols, and ketone reduction via transfer hydrogenation [12j] .…”

N-Heterocyclic Carbene Complexes of Nickel, Palladium, and Iridium Derived from Nitron: Synthesis, Structures, and Catalytic Properties

“…Conceptually related to Enders carbene is a derivative that features an exocyclic 4-NHPh rather than 4-Ph substituent (Figure ); however, while this NHC has not been isolated, its long-known mesoionic tautomer, (1,4-diphenyl-1,2,4-triazol-4-ium-3-yl)­phenylazanide (Nitron), − exhibits reactivity that provides access to the carbene form (Figure ). − For example, Nitron reacts with elemental sulfur to afford a thione derivative; likewise, Nitron has been used to obtain several metal complexes of the type (Nitron NHC )­ML n (M = Ru, Rh, Cu, Ag, Au) that feature the 1,2,4-triazol-5-ylidene ligand, Nitron NHC . − In view of this reactivity, Nitron has been referred to as a “crypto-NHC”. , Here, we describe further applications of Nitron as a crypto-NHC to afford 1,2,4-triazolylidene compounds of nickel, palladium, and iridium, together with their structural and catalytic properties.…”

(Benz-)imidazolin-2-ylidene-benzimidazolatonickel(II) Chelates: Syntheses, Structures, and Tuning of Noninnocent Chelate Ligand Behavior

“…Imidazolium-benzimidazolates ( 1 H) are readily available conjugated mesomeric betaines, which were recently established by us as convenient sources of donor-functionalized N-heterocyclic carbenes (NHCs) . Their “instant carbene” behavior can be ascribed to a betaine–carbene tautomerization ( 1 H ⇄ 1′ H; Scheme , top), which was first recognized for the analytical reagent Nitron. − We have extended this work to benzimidazolium-benzimidazolates ( 2 H) (Scheme , bottom).…”