Transition metal mediated formation of dicationic diselenides stabilised by N-heterocyclic carbenes: designed synthesis

Abstract

Reactions of the N-heterocyclic carbene derivatives of selenium(II) dihalides, LSeX₂ (X = Cl, Br; L = 1,3-dibutylbenzimidazol-2-ylidene, C₁₅H₂₂N₂) (4b, 4b′), L′SeX₂ (X = Cl, Br; L′ = 1,3-diⁿpropylbenzimidazol-2-ylidene, C₁₃H₁₈N₂) (4c, 4c′) and L′′SeX₂ (X = Cl, Br; L′′ = 1,3-diⁱpropylbenzimidazol-2-ylidene, C₁₃H₁₈N₂) (4d, 4d′) with iron led to the formation of dicationic diselenides, [L₂Se₂²⁺]₂[{FeCl₄}⁻]₃Cl⁻ (7), [L₂Se₂²⁺][FeBr₄]⁻Br⁻ (8), [L′₂Se₂²⁺][FeCl₄]⁻Cl⁻ (9), [L′₂Se₂²⁺][FeBr₄]⁻Br⁻ (10), [L′′₂Se₂²⁺][FeCl₄]⁻Cl⁻ (11) and [L′′₂Se₂²⁺][FeBr₄]⁻Br⁻ (12). The reactions of LSeX₂ with copper gave selone (LSe) complexes of cuprous halides, [(LSe)₂CuBr] (13) and [(LSe)₂(CuI)₂] (14) respectively. However, LSeX₂ (X = Cl, Br, I) (4b–4b′′) on treatment with zinc gave both the dicationic diselenides, [L₂Se₂²⁺]Y [Y = {ZnCl₃(H₂O)₂}²⁻ (15), {ZnBr₄}²⁻ (17)] and complexes, [(LSe)₂ZnX₂] (16, 18, 20). The plausible mechanism for the formation of dicationic diselenides, [L₂Se₂²⁺], has been proposed and is supported by performing various test reactions. To validate these mechanisms, DFT calculations have been carried out. Interestingly, the reactions of the mixture of LSeX₂/L′SeX₂/L′′SeX₂ and L′Se⁰/L′Se⁰/L′′Se⁰ with metal halides (FeCl₃, ZnX₂; X = Cl, Br, I; CdI₂, HgI₂, PdCl₂, and PtCl₂) produced dicationic diselenides, 7–12, 15, 17 and [L₂Se₂²⁺]Y [Y = {ZnI₄}²⁻ (19), {CdI₄}²⁻ (22), {HgI₄}²⁻ (25), {PdCl₄}²⁻ (26), {PtCl₄}²⁻ (27)]. The methodology is more general than the previous method and gives better yields of [L₂Se₂²⁺]Y. The controlled oxidation of complexes, [(LSe)₂MX₂] (M = Zn, Cd, Hg; X = Cl, Br, I) (16, 18, 20, 28–32) with halogenating agents (SOCl₂/Br₂/I₂) also gave dicationic diselenides, 15, 17, 19, 22, 25 and [L₂Se₂²⁺]Y [Y = {CdCl₄}²⁻ (21), {Hg₂Cl₆}²⁻ (23), {HgBr₄}²⁻ (24)], proving to be the most general method for the synthesis of [L₂Se₂²⁺]. Attempted syntheses of unsymmetrical dicationic diselenides, [LSe-SeL′]²⁺[FeCl₄]⁻Cl⁻ and dicationic ditelluride, [L₂Te₂²⁺]₂[{FeCl₄}⁻]₃Cl⁻ by the reaction of LSeX₂/LTeX₂ (X = Cl, Br, I) with Fe metal were unsuccessful. This observation has been corroborated with DFT calculations.