Dinuclear N-Heterocyclic Dicarbene Gold Complexes in I-III and III-III Oxidation States: Synthesis and Structural Analysis

A series of dinuclear N-heterocyclic bis-dicarbene gold(III) complexes of the general formula [Au2Br4(RIm-Y-ImR)2] (PF6)2 (Im = imidazol-2-ylidene; 1b, R = Me, Y = CH 2; 2b, R = Me, Y = (CH2)2; 3b, R = Me, Y = (CH2)3; 4b, R = Me, Y = (CH2)4; 5b, R = Cy, Y = CH2; 6b, R = Me, Y = m-xylylene) were successfully synthesized by oxidative addition of bromine to the corresponding dicarbene gold(I) complexes 1a-6a. The stability of the digold(III) complexes depends on the length of the bridge Y between the carbene units. The complex with Y = CH2 undergoes a partial reductive elimination, giving the first example of the mixed-valence gold(I)/gold(III) dinuclear bis-dicarbene complex 1c, together with a minor quantity of the neutral digold(III) mono-dicarbene complex [Au2Br6(MeIm-CH2-ImMe)] (1d). The X-ray crystal structures of complexes 1c,d, 3b, and 6b were determined. Besides complex 3b, the addition of bromine to complex 3a gives complex 3b?, a coordination metallopolymer, formed by an infinite chain of AuBr2 units bridged by the dicarbene ligand.