Electrochemical properties of gold(III) complexes with 2,2′-bipyridine and oxygen ligands

Comparing Metal-Halide and -Oxygen Adducts in Oxidative C/O-H Activation: AuIII-Cl versus AuIII-OH

High-valent metal–halides have come to prominence as highly effective oxidants. A direct comparison of their efficacy against that of traditional metal–oxygen adducts is needed. [Au^III(Cl)(terpy)](ClO₄)₂ (1; terpy = 2,2′:6′,2-terpyridine) readily oxidized substrates bearing O–H and C–H bonds via a hydrogen atom transfer mechanism. A direct comparison with [Au^III(OH)(terpy)](ClO₄)₂ (2) showed that 1 was a kinetically superior oxidant with respect to 2 for all substrates tested. We ascribe this to the greater thermodynamic driving force imbued by the Cl ligand versus the OH ligand.

We report a direct comparison of the efficacy of metal−halide oxidants to those of traditional metal−oxygen adducts. Au^III−Cl and Au^III−OH oxidants, supported by the same ancillary ligand, readily oxidized substrates via a hydrogen atom transfer mechanism. The Au^III−Cl oxidant was kinetically superior for all substrates. We ascribed this to the greater thermodynamic driving force imbued by the Cl ligand versus the OH ligand.

Hydrogen Atom Transfer Oxidation by a Gold-Hydroxide Complex

Au^III-oxygen adducts have been implicated as intermediates in homogeneous and heterogeneous Au oxidation catalysis, but their reactivity is under-explored. Complex 1, ([Au^III(OH)(terpy)](ClO₄)₂, (terpy = 2,2':6',2-terpyridine), readily oxidized substrates bearing C-H and O-H bonds. Kinetic analysis revealed that the oxidation occurred through a hydrogen atom transfer (HAT) mechanism. Stable radicals were detected and quantified as products of almost quantitative HAT oxidation of alcohols by 1. Our findings highlight the possible role of Au^III-oxygen adducts in oxidation catalysis and the capability of late transition metal-oxygen adducts to perform proton coupled electron transfer.

Self-association and columnar liquid crystalline phase of cationic alkyl-substituted-bipyridine benzenedithiolato gold(III) complexes

The introduction of a Au(III) ion into a mesogenic core, [M(Bdt)(Cnbpy)](+) (Bdt = 1,2-benzenedithiolato and Cnbpy = 4,4'-di-alkyl-2,2'-bipyridine (n = 13 (4,4'-di-tridecyl-2,2'-bipyridine (C13bpy)) and 8,10 (4,4'-di-(3-octyltridecyl)-2,2'-bipyridine (C8,10bpy)))), leads to the formation of ionic molecular assemblies in crystalline and mesophases. Successive syntheses of precursor complexes, [AuCl2(Cnbpy)]PF6 (n = 13 (1) and 8,10 (2)), followed by the target complexes, [Au(Bdt)(Cnbpy)]PF6 (n = 13 (3) and 8,10 (4)), were achieved. The crystallographic analysis of 3 revealed that the central cationic cores form a characteristic one-dimensional columnar structure, which is an essential property for the formation of columnar structures in the liquid crystalline phase. The central cores of the [Au(Bdt)(C13bpy)](+) cations in 3 stack alternatively so as to cancel out their dipole moments and the counteranions lie between the cationic columns. Furthermore, the introduction of the branched alkyl tails in 4 induces the formation of a rectangular columnar liquid crystalline phase (Col(r)) with C2/m symmetry, and it melts to an isotropic liquid at 69 °C. The Col(r) phase of 4 is partially similar in its liquid crystalline structure to the neutral [Pt(Bdt)(C8,10bpy)] liquid crystal in a hexagonal columnar phase reported previously, while complex 4 shows a lower clearing point than that of the Pt analogue. Based on these results, the present study demonstrates the designability of the liquid crystalline structures of the [M(Bdt)(C8,10bpy)] skeleton together with their assembled structures and physico-chemical properties.

Structural and Solution Chemistry, Antiproliferative Effects, and DNA and Protein Binding Properties of a Series of Dinuclear Gold(III) Compounds with Bipyridyl Ligands

A series of six dinuclear gold(III) oxo complexes with bipyridyl ligands, of general formula [Au2(N,N)2(mu-O)2][PF6]2 (Auoxo1-Auoxo6) [where N,N = 2,2'-bipyridine (1), 4,4'-di-tert-butyl- (2), 6-methyl- (3), 6-neopentyl- (4), 6-(2,6-dimethylphenyl)- (5), 6,6'-dimethyl-2,2'-bipyridine (6)], were investigated as potential cytotoxic and anticancer agents, and their antiproliferative properties were evaluated in vitro toward the reference A2780 human ovarian carcinoma cell line. While five compounds manifested moderate cytotoxic properties (with IC50 approximately 10-30 microM), the sixth one (Auoxo6), turned out to be approximately 5-15 times more active against both cell lines and will merit further pharmacological studies. The interactions of Auoxo1 and Auoxo6 with a few model proteins (serum albumin, cytochrome c, ubiquitin) and with calf thymus DNA were analyzed in detail by various spectroscopic methods. Both tested compounds manifested a high and peculiar reactivity toward the mentioned model proteins; specific differences were detected in their reactivity with DNA. The mechanistic implications of these results are discussed.

Gold(III) complexes with bipyridyl ligands: solution chemistry, cytotoxicity, and DNA binding properties

Gold(III) compounds generally exhibit significant cytotoxic effects on cancer cell lines and are of potential interest as antitumor drugs. We report here on the solution chemistry, the cytotoxicity, and the DNA binding properties of two new bipyridyl gold(III) compounds: [Au(bipy)(OH)(2)][PF(6)] (1) and the organometallic compound [Au(bipy(c)-H)(OH)][PF(6)] (2) (bipy(c) = 6-(1,1-dimethylbenzyl)-2,2'-bipyridine). Both compounds are sufficiently soluble, and stable for hours, within a physiological buffer at 37 degrees C; [Au(bipy)(OH)(2)][PF(6)], at variance with [Au(bipy(c)-H)(OH)][PF(6)], is quickly and quantitatively reduced by ascorbate. Both compounds showed relevant cytotoxic effects toward the A2780S, A2780R, and SKOV3 tumor cell lines; lower effects were detected on the CCRF-CEM/S and CCRF-CEM/R lines. In most cases the mechanisms of resistance to CDDP are only marginally effective against these gold(III) complexes. The interactions of [Au(bipy)(OH)(2)][PF(6)] and [Au(bipy(c)-H)(OH)][PF(6)] with calf thymus DNA were investigated in vitro by various techniques to establish whether DNA represents a primary target for these compounds. Addition of saturating amounts of DNA did not affect appreciably the visible spectra of these gold(III) complexes. Some slight modifications of the CD spectra of calf thymus DNA and of the DNA melting parameters were observed; in any case, ultrafiltration experiments showed that binding of these gold(III) complexes to DNA is weak and reversible. The mechanistic implications of these findings are discussed.