Insights into the Halogen Oxidative Addition Reaction to Dinuclear Gold(I) Di(NHC) Complexes

Gold(I) dicarbene complexes [Au-2(MeIm-Y-ImMe)(2)](PF6)(2) (Y = CH2 (1), (CH2)(2) (2), (CH2)(4) (4), MeIm = 1-methylimidazol-2-ylidene) react with iodine to give the mixed-valence complex [Au(MeIm-CH2-ImMe)(2)AuI2](PF6)(2) (1a(I)) and the gold(III) complexes [Au2I4(MeIm-Y-ImMe)(2)](PF6)(2) (2c(I) and 4c(I)). Reaction of complexes 1 and 2 with an excess of ICl allows the isolation of the tetrachloro gold(III) complexes [Au2Cl4(MeIm-CH2-ImMe)(2)](PF6)(2) (1c(Cl)) and [Au2Cl4(MeIm-(CH2)(2)-ImMe)(2)](Cl)(2) (2c(Cl)-Cl) (as main product); remarkably in the case of complex 2, the X-ray molecular structure of the crystals also shows the presence of I-Au-Cl mixed-sphere coordination. The same type of coordination has been observed in the main product of the reaction of complexes 3 or 4 with ICl. The study of the reactivity towards the oxidative addition of halogens to a large series of dinuclear bis(dicarbene) gold(I) complexes has been extended and reviewed. The complexes react with Cl-2, Br-2 and I-2 to give the successive formation of the mixed-valence gold(I)/gold(III) na(X) and gold(III) nc(X) (excluding compound 1c(I)) complexes. However, complex 3 affords with Cl-2 and Br-2 the gold(II) complex 3b(X) [Au2X2(MeIm-(CH2)(3)-ImMe)(2)](PF6)(2) (X=Cl, Br), which is the predominant species over compound 3c(X) even in the presence of free halogen. The observed different relative stabilities of the oxidised complexes of compounds 1 and 3 have also been confirmed by DFT calculations.