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

Abstract

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 = CH2; 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.

Synthesis of dinuclear N-heterocyclic dicarbene Au(III)/Au(III) and Au(II)/Au(II) complexes via oxidative addition of chlorine or bromine to Au(I)/Au(I) species

“…The mean coordination planes of the metal centres are almost orthogonal forming a dihedral angle of 87.68 (2) . The AueC bond distances span from 1.99(1) to 2.08(1) Å, in agreement with those found in other AueC(carbene) bond distances involving Au(I) and Au(III) centres [9]; the AueBr bond distances are of 2.5101(4) and 2.4982(4) Å for Au1eBr1 and Au2eBr2 respectively, longer than those found in [9] and in agreement with those found in bis [(m 2 -2,2-diphenylphosphoniapropane-1,3-diyl)-bromo-gold(II)] complex (2.516(2), 2.504(2) Å) [15]. The two linearly coordinated carbene rings to the Au1 or Au2 metal centre are slightly twisted (dihedral angles 16.14(2) (Au1) and 17.59 (2) (Au2)).…”

“…Two sets of NMR signals are observed in a 1/9 ratio: the first one can be attributed to 3b by analogy with the spectra of complexes 1b, 2b, 4b and with the similar complex obtained with bromine [9]. (3b) 1 Slight modification of the procedure reported in [9]. Bromine (0.30 mmol, 0.3 mL of 1 M solution in acetonitrile) was added to a solution of the gold(I) bis(hexafluorophosphate) complex (0.12 mmol) in acetonitrile (15 mL).…”

“…Found: C,21.98;H,2.74;N,9.20;Cl 6.99%. Two sets of NMR signals are observed in a 1/9 ratio: the first one can be attributed to 3b by analogy with the spectra of complexes 1b, 2b, 4b and with the similar complex obtained with bromine [9]. (3b) 1 Slight modification of the procedure reported in [9].…”

“…On the basis of the above, we have re-evaluated the nature of the products obtained with bromine as oxidant for 3a [9]. In the reported experimental conditions, i.e.…”

“…We have recently reported the synthesis of N-heterocyclic dicarbene dinuclear gold(III) complexes via oxidative addition of bromine to the corresponding gold(I) complexes [9]. These last ones differ mainly for the length of the bridge between the carbene units and for the wingtip substituents at the nitrogen atoms of the heterocyclic ring (Chart 1).…”