Cationic, linear Au(i) N-heterocyclic carbene complexes: synthesis, structure and anti-mitochondrial activity

Abstract

Six linear, two-coordinate cationic Au(I) N-heterocyclic carbene complexes of the form [(R₂Im)₂Au]⁺ (R = Me 1, Me, Et 2, i-Pr 3, n-Bu 4, t-Bu 5 and Cy 6) have been prepared by the reaction of two equivalents of the appropriate dialkylimidazol-2-ylidene (R₂Im) with (Me₂S)AuCl in dmf. Single crystal structural studies for 1·PF₆, 2·PF₆, 3·Cl and 4–6·PF₆ show that for all six complexes the gold(I) centres have quasi-linear C–Au–C coordination, with quasi-parallel pairs of aromatic imidazole planes, except in 5·PF₆ where they are quasi-normal; in the latter, Au–C are 2.038(3), 2.033(3) Å, cf. (e.g.) 3 2.027(2) Å. Inter-cation Au⋯Au are close at 3.487(2), 3.525(2) Å in 1·PF₆ and 2·PF₆. The structural studies and low temperature NMR experiments provide no supportive evidence for the presence of π back-bonding within this series of complexes. The lipophilicities of the six compounds, as estimated from the logarithm of the n-octanol–water partition coefficients (log P), varied across the series within the range −1.09 to 1.73. To investigate their potential as possible anti-mitochondrial anti-tumour agents, five of the compounds have been evaluated for their propensities to induce mitochondrial membrane permeabilization (MMP) in isolated rat liver mitochondria. At concentrations between 1–10 µM compounds 1·Br and 3–6·Cl induced dose-dependent, Ca²⁺-sensitive mitochondrial swelling at rates that increased with the lipophilicities of the complexes, with the most lipophilic compounds inducing the most rapid onset of swelling. The swelling was completely inhibited by cyclosporin A, the specific inhibitor of the mitochondrial permeability transition pore.