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

摘要

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′.