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′.
Highlights
IInnttrroodduuctionThey are in good agreement with the values reported for 1,2,4-triazol-5-ylidene complexes of the dicoordinate type [Ag(NHC)2]X containing less nucleophilic anions (for example, X = NO3 [110], ClO4 [27], OTf [89]), including the recently reported complexes of the “Enders carbene”, [Ag(1)2]X (X = BF4, PF6) [22] as well as our Nitron-derived examples [Ag(2 )2][BF4] and [Ag(2 )2](OTf) [33]
Neat dimethylselenide was used as external standard (δ = 4 ppm) [112]
[CuCl(6 )] forms dimeric 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 tert-butyl and the methyl substituent of 6, which allow a comparatively closer approach of two such ligands, both intra- and intermolecularly
Summary
They are in good agreement with the values reported for 1,2,4-triazol-5-ylidene complexes of the dicoordinate type [Ag(NHC)2]X containing less nucleophilic anions (for example, X = NO3 [110], ClO4 [27], OTf [89]), including the recently reported complexes of the “Enders carbene”, [Ag(1)2]X (X = BF4, PF6) [22] as well as our Nitron-derived examples [Ag(2 )2][BF4] and [Ag(2 )2](OTf) [33] The latter compound is the homologue of [Ag(6 )2](OTf) synthesized and structurally characterized in this work. The anion of [Ag(6 )2](OTf) appears to be weakly hydrogen-bonded to the exocyclic amino unit of one of the 1,2,4-triazol-5-ylidene ligands, as is suggested by an NH···O contact of ca. 2.51 Å, which is ca. 7% shorter than the sum of the van der Waals radii of H and O (2.70 Å) [73]
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