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

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-di(n)propylbenzimidazol-2-ylidene, C13H18N2) (4c, 4c') and L''SeX2 (X = Cl, Br; L'' = 1,3-di(i)propylbenzimidazol-2-ylidene, C13H18N2) (4d, 4d') with iron led to the formation of dicationic diselenides, [L2Se2(2+)]2[{FeCl4}(-)]3Cl(-) (7), [L2Se2(2+)][FeBr4](-)Br(-) (8), [L'2Se2(2+)][FeCl4](-)Cl(-) (9), [L'2Se2(2+)][FeBr4](-)Br(-) (10), [L''2Se2(2+)][FeCl4](-)Cl(-) (11) and [L''2Se2(2+)][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, [L2Se2(2+)]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, [L2Se2(2+)], 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'Se(0)/L'Se(0)/L''Se(0) with metal halides (FeCl3, ZnX2; X = Cl, Br, I; CdI2, HgI2, PdCl2, and PtCl2) produced dicationic diselenides, 7-12, 15, 17 and [L2Se2(2+)]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 [L2Se2(2+)]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 [L2Se2(2+)]Y [Y = {CdCl4}(2-) (21), {Hg2Cl6}(2-) (23), {HgBr4}(2-) (24)], proving to be the most general method for the synthesis of [L2Se2(2+)]. Attempted syntheses of unsymmetrical dicationic diselenides, [LSe-SeL'](2+)[FeCl4](-)Cl(-) and dicationic ditelluride, [L2Te2(2+)]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.