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 [Au(2)Br(4)(RIm-Y-ImR)(2)]-(PF(6))(2) (Im = imidazol-2-ylidene; 1b, R = Me, Y = CH(2); 2b, R = Me, Y = (CH(2))(2); 3b, R = Me, Y = (CH(2))(3); 4b, R = Me, Y = (CH(2))(4); 5b, R = Cy, Y = CH(2); 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 = CH(2) 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 [Au(2)Br(6)(MeIm-CH(2)-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 AuBr(2) units bridged by the dicarbene ligand.