Electronic and steric effects in gold(i) phosphine thiolate complexes

The unusual yellow color of Au(2)(dppm)(SR)(2) (R = 4-tolyl; dppm = diphenylphosphinomethane) is attributed to a red-shift in the S-->Au charge transfer caused by destabilization of the sulfur highest occupied molecular orbital (HOMO). Variable temperature experiments show two broad bands at -80 degrees C in the (31)P{(1)H} NMR spectrum of Au(2)(dppm)(SR)(2) and the activation energy for interconversion is 10 kcal/mol. Only one sharp band is observed down to -80 degrees C in the spectrum of the white complex, Au(2)(dppe)(SR)(2) (dppe = diphenylphosphinoethane). Molecular mechanics calculations on Au(2)(dppm)(SR)(2) and Au(2)(dppe)(SR)(2) reveal that, for Au(2)(dppe)(SR)(2), a series of maxima and minima, separated by 2.5 kcal/mol, occur every 120 degrees which is consistent with rotation around an unhindered carbon-phosphorus single bond. The Au atoms are not within bonding distance in any conformation. Computational results for Au(2)(dppm)(SR)(2) indicate one minimum energy structure in which the Au-P bonds are anti. There is a high energy conformation (9 kcal/mol above the global minimum) where overlap between golds is maximized. The implications of gold-gold bonding in this complex are discussed. The steric influence of the thiolate ligand has been examined by synthesizing a series of dinuclear gold(I) complexes in which the steric properties of the thiolate are varied: Au(2)(dppm)(SR)(2) (R = 2,6-dichlorophenyl; 2,6-dimethylphenyl; 3,5-dimethylphenyl). The 2,6-disubstituted complexes are white, while the 3,5-dimethyl complex is yellow. These results, along with VT-NMR experiments, are consistent with the conclusion that the more sterically-bulky thiolates hinder the close approach of the golds in the dinuclear complexes.     

Antitumor activity of gold(i), silver(i) and copper(i) complexes containing chiral tertiary phosphines

The in vitro cytotoxicities of a number of gold(I), silver(I) and copper(I) complexes containing chiral tertiary phosphine ligands have been examined against the mouse tumour cell lines P815 mastocytoma, B16 melanoma [gold(I) and silver(I) compounds] and P388 leukaemia [gold(I) complexes only] with many of the complexes having IC(50) values comparable to that of the reference compounds cis-diamminedichloroplatinum(ll), cisplatin, and bis[1,2-bis(diphenylphosphino) ethane]gold(I) iodide. The chiral tertiary phosphine ligands used in this study include (R)-(2-aminophenyl)methylphenylphosphine; (R,R)-, (S,S)- and (R(*),R(*))-1,2-phenylenebis(methylphenylphosphine); and (R,R)-, (S,S)- and (R(*),R(*))-bis{(2-diphenylphosphinoethyl)phenylphosphino}ethane. The in vitro cytotoxicities of gold(I) and silver(I) complexes containing the optically active forms of the tetra(tertiary phosphine) have also been examined against the human ovarian carcinoma cell lines 41M and CH1, and the cisplatin resistant 41McisR, CH1cisR and SKOV-3 tumour models. IC(50) values in the range 0.01 - 0.04 muM were determined for the most active compounds, silver(I) complexes of the tetra(tertiary phosphine). Furthermore, the chirality of the ligand appeared to have little effect on the overall activity of the complexes: similar IC(50) data were obtained for complexes of a particular metal ion with each of the stereoisomeric forms of a specific ligand.     

Structures of gold(i) and silver(i) thiolate complexes of medicinal interest: a review and recent results

We review the crystal structures, electrospray ionization mass spectra (ESI-MS) data and chiral HPLC data for the racemic and optically pure mononuclear AuL2 complexes, and for racemic [AuLI]n and optically pure [AgLII]n polymers (LI = thiomalate, LII = D-penicillamine). We postulate an equilibrium between polymeric, mononuclear and free ligand species for [AuLII]n (gold sodium thiomalate or GST). The ESI-MS results clearly show a tetrameric principal species in the 1:1 gold polymers, [AuLI]n. For the 1:1 silver: D-penicillamine complex, [AgLII]n, a non-molecular crystal of double helical structure, the ESI-MS results show multi-ligand-silver species, including tetramers, pentamers and hexamers. Other, relevant gold, silver and copper complexes are compared.     

Redox chemistry of gold(i) phosphine thiolates: sulfur-based oxidation

The redox chemistry of mononuclear and dinuclear gold(I) phosphine arylthiolate complexes was recently investigated by using electrochemical, chemical, and photochemical techniques. We now report the redox chemistry of dinuclear gold(I) phosphine complexes containing aliphatic dithiolate ligands. These molecules differ from previously studied gold(I) phosphine thiolate complexes in that they are cyclic and contain aliphatic thiolates. Cyclic voltammetry experiments of Au(2) (LL)(pdt) [pdt = propanedithiol; LL = 1,2-bis(diphenylphosphino)-ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), 1,4-bis(diphenylphosphino)butane (dppb), 1,5-bis(diphenylphosphino)pentane (dpppn)] in 0.1 M TBAH/CH(3)CN or CH(2)Cl(2) solutions at 50 to 500 mV/sec using glassy carbon or platinum electrodes, show two irreversible anodic processes at ca. +0.6 and +1.1 V (vs. SCE). Bulk electrolyses at +0.9 V and +1.4 V result in n values of 0.95 and 3.7, respectively. Chemical oxidation of Au(2)(dppp)(pdt) using one equivalent of Br(2) (2 oxidizing equivalents) yields 1,2-dithiolane and Au(2)(dppp)Br(2). The reactivity seen upon mild oxidation /= +1.3 V) is consistent with oxidation of gold(I) to gold(III). Structural and electrochemical differences between gold(I) aromatic and aliphatic thiolate oxidation processes are discussed.     

Electronic structure of dinuclear gold(i) complexes

Cyclic voltammetry (CV) experiments on LL(AuSR *)(2) complexes [LL = diphenylphosphinomethane (dppm), diphenylphosphinopentane (dpppn); R(*) = p-SC(6)H(4)CH(3)] show anodic sweeps that broaden by about 25 mV on going from the longer (dpppn) to the shorter (dppm) bidentate phosphine ligand. Changing concentrations had no effect on the shape of the waveform. The result suggests a weak intramolecular metal-metal interaction in dppm(AuSR *)(2) that correlates well with rate acceleration occurring in the reaction of dppm(AuSR *)(2) with organic disulfides. Quantum yields for cis-dppee(AuX)(2) [dppee = 1,2-bis(diphenylphosphino)ethylene; X = Cl, Br, I] complexes, (disappearance) Phi , are significantly higher in complexes with a softer X ligand, a trend that correlates well with aurophilicity. This result also suggests that electronic perturbation caused by Au(I)-Au(I) interactions is important in explaining the reactivity of some dinuclear gold(I) complexes. The crystal structure for cis-dppee(Aul)(2) shows short intramolecular Au(I)-Au(I) interactions of 2.9526 (6) A degrees , while the structure of trans-dppee(AuI)(2) , shows intermolecular Au(I)-Au(I) interactions of 3.2292 (9) A degrees . The substitution of .As for P results in a ligand, cis-diphenylarsinoethylene (cis-dpaee), that is photochemically active, in contrast to the cis-dppee ligand. The complexes, cis-dpaee(AuX)(2), are also photochemically active but with lower quantum yields than the cis-dppee(AuX)(2) complexes.     

Reactions of organic disulfides and gold(i) complexes

Gold-thiolate/disulfide exchange reactions of (p-SC(6)H(4)Cl)(2) with Ph(3)PAu(SC(6)H(4)CH(3)), dppm(AuSC(6)H(4)CH(3))(2), and dppe(AuSC(6)H(4)CH(3)) (2) were investigated. The rate of reactivity of the gold-thiolate complexes with (p-SC(6)H(4)Cl)(2) is: dppm(AuSC(6)H(4)CH(3))(2)> dppe(AuSC(6)H(4)CH(3))(2)>Ph(2)PAu (SC(6)H(4)CH(3)). This order correlates with conductivity measurements and two ionic mechanisms have been evaluated. (1)H NMR experiments demonstrate that in the reaction of dppm(AuSC(6)H(4)CH(3))(2) with (p-SC(6)H(4)Cl)(2), the mixed disulfide, ClC(6)H(4)SSC(6)H(4)CH(3), forms first, followed by the formation of (p-SC(6)H(4)CH(3))(2). The rate law is first order in (pp-SC(6)H (4)Cl)(2) and partial order in dppm(AuSC(6)H(4)CH(3))(2). Results from electrochemical and chemical reactivity studies suggest that free thiolate is not involved in the gold-thiolate/disulfide exchange reaction. A more likely source of ions is the dissociation of a proton from the methylene backbone of the dppm ligand which has been shown to exchange with D(2)O. The implications of this are discussed in terms of a possible mechanism for the gold-thiolate/disulfide exchange reaction.     

New gold(I) and silver(I) complexes of sulfamethoxazole: Synthesis,X-ray structural characterization and microbiological activities of triphenylphosphine(sulfamethoxazolato-N2)gold(I) and (sulfamethoxazolato)silver(I)

Sulfamethoxazole (SMTZ) reacts with Ph₃PAuCl and AgCl in methanol/triethylamine to give [Ph₃PAu(SMTZ_-1H+)] (SMTZ_-1H+ = sulfamethoxazolato anion) (1) and [Ag(SMTZ_-1H+)] (2). While the lattice of 1 contains single molecules with linear N–Au–P bonds, compound 2 comprises a two-dimensional polymeric assembly of the deprotonated SMTZ ligand and silver ions, which are coordinated by one oxygen and three nitrogens in a distorted tetrahedral array. The microbiologic activities (Mueller–Hinton broth dilution tests) of 1 and 2 were determined in relation to free sulfamethoxazole.     

Copper, silver, and gold complexes in hydrosilylation reactions

The reduction of diverse functional groups is an essential protocol in organic chemistry. Transition-metal catalysis has been successfully applied to the reduction of olefins, alkynes, and many carbonyl compounds via hydrogenation or hydrosilylation; the latter presenting several advantages over hydrogenation. Notably, hydrosilylation generally occurs under mild reaction conditions, and consequently over-reduced products are rarely detected. Moreover, the great majority of hydrosilanes employed in this reaction are easily handled, inexpensive, or both. A large number of multiple bonds can be involved in this context, and the hydrosilylation reaction can be regarded as a useful method for the synthesis of silicon-containing organic molecules or a convenient way of reducing organic compounds. Furthermore, the silyl group can also be retained as a protecting group, a strategy that can be of great usefulness in organic synthesis. Since the first Wilkinson's catalyst-mediated hydrosilylation of ketones in 1972, metals such as rhodium and iridium have attracted most of the attention in this area. A wide array of catalytic systems for hydrosilylation reactions is nowadays available, which has allowed for a great expansion of the synthetic scope of this transformation. After having been overlooked in the early years, group 11 metals (Cu, Ag, and Au), especially copper, have emerged as appealing alternatives for hydrosilylation. The use of a stabilized form of copper hydride, the hexameric [(Ph3P)CuH]6, by Stryker represented a breakthrough in copper-catalyzed reduction reactions. Nowadays, several copper-based catalytic systems compare well with a variety of reported rhodium-based catalysts, which generally suffer from the high cost of the catalyst. Tertiary phosphine ligands are the most widely used in these transformations. However, other families such as N-heterocyclic carbenes (NHCs) have shown promising activities. Compared with copper, little attention has been paid to silver- or gold-based catalysts. Silver salts have been considered inert towards hydrosilylation, and they are often employed as innocent anion exchange reagents for the in situ generation of cationic transition metal catalysts. Despite the rare reports available, they have already shown interesting reactivity profiles, for example, in the chemoselective reduction of aldehydes in the presence of ketones. Furthermore, 1,2-hydride delivery is favored over 1,4-reductions for alpha,beta-unsaturated carbonyl compounds, in contrast with most copper-based systems.     

Macrocyclic silver and gold complexes containing bis (N-heterocyclic carbene) ligand: Synthesis and structural characterization

Silver and gold complexes, [Ag₂(L)₂](PF₆)₂ and [Au₂(L)₂](PF₆)₂, supported by but-2-yne-1,4-diyl linked bis(N-heterocyclic carbene) ligand have been prepared and structurally characterized. The complexes display novel twisted macrocyclic conformation and weak intramolecular metal–metal interaction. The complexes are intensely emissive in their solid states.     

Cytotoxicity of triorganophosphinegold(i) complexes of thiobenzoate

The preparation and characterization of two triorganophosphinegold(I) complexes containing the anion derived from thiobenzoic acid are described. The cytotoxicity of these complexes has been investigated along with that of triphenylphosphinegold(I) mercaptopurinate, a known anti-tumor compound, against a variety of human cell lines. The complexes showed moderate to high cytotoxicity (ID(50) 250 - 2500 ng/ml).     

Syntheses and structures of two- and three-dimensional coordination networks generated from silver complexes and hmt (hmt=hexamethylenetetramine)

Ag(CO₂PhCO₂)Ag reacts with hexamethylenetetramine (hmt) in acetonitrile solution in the ratio 1:2 to yield the network [Ag₂(μ₃-hmt)₂(μ-O₂CPhCO₂)]·7H₂O, in which AgCO₂PhCO₂Ag units intercalate between the two-dimensional honeycomb-like layers to give a three-dimensional open network, while Ag(MePhSO₃) reacts with hmt in the ratio 2:1 to produce the coordination network Ag₂(μ₄-hmt)(MePhSO₃)₂, consisting of two-dimensional grids with square cavities. The X-ray diffraction studies show that these two metal complexes can be used as molecular building blocks in the synthesis of silver–hmt networks.     

Reactions of phenylbis(2-pyridyl)phosphine with oxorhenium(v) complexes

[ReOCl₃(PPh₃)₂] reacts with phenylbis(2-pyridyl)phosphine (PPhpy₂) in boiling THF under the formation of the rhenium(III) compound [ReCl₃(PPh₃)(OPPhpy₂-O,N)]. Reduction of the metal can be avoided by milder reaction conditions, and an asymmetric, oxo-bridged rhenium(V) dimer of the composition (NBu₄)[Re₂O₂Cl₅(μ-PPhpy₂-P,N,N′)(μ-O)] is obtained from the reaction of PPhpy₂ and (NBu₄)[ReOCl₄] in THF at room temperature. The products have been characterized spectroscopically and by X-ray structure analyses.     

Luminescent palladium(0) complexes bearing N-heterocyclic carbene and phosphine ligands

Palladium(0) complexes bearing a monodentate phosphine ligand and an N-heterocyclic carbene ligand have been prepared. In these complexes, photophysical properties of the complexes, [Pd(IPr)(PPh₃)] and [Pd(IPr)(P(o-tol)₃)] have been studied (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). The emissive excited states have been tentatively assigned to ³MLCT. In addition to the results of the luminescent complexes, the synthesis of the related complexes, [Pd(O₂)(IPr)(P(m-tol)₃)] and [PdCl(CH₂Cl)(IPr)(P(MeOPh)₃)] (P(MeOPh)_3 = tris(pmethoxyphenyl)phosphine) have been studied and the structures were characterized by X-ray.     

Synthesis and structural studies of a 1:2 adduct of silver(I) tetrakis(pyrazolyl)borate(III) with a tertiary phosphine

The synthesis and spectroscopic (IR, far-IR, ¹H and ³¹P NMR) characterization of the new complex Ag(pzTp)(PPh₃)₂ is reported, with a single crystal X-ray structure determination showing it to assume the form [(Ph₃P)₂Ag(pz₂Bpz₂)]. Although the silver atom is essentially four-coordinate P₂AgN₂, Ag–P 2.4154(4) Å, 2.5123(4) Å, Ag–N 2.315(1) Å, 2.357(1) Å at ca. 150 K, NMR spectroscopy clearly distinguishes between the bonding character of the two different phosphorus atoms.     

Electrochemistry of transition metal complex catalysts. Part 9. One- and two-electron oxidation of iridium complexes with cyclohexane-derived tripod phosphine ligands

The redox chemistry of Ir tripod-type tri-phosphine complexes in dichloromethane is investigated by cyclic voltammetry, hold-ramp experiments, and preparative electrolysis at Pt electrodes. Products are identified by spectroscopic data, as well as EDX and EXAFS results. Complexes with the Ir central atom in the oxidation states +I, +II and +III are detected and several follow-up reactions are possible from those. Most of the intermediates and products are characterized. In particular, experiments in the presence of CO contribute to the assignment of peaks in the cyclic voltammograms. The experimental results for the individual steps are summarized in a comprehensive redox reaction mechanism (mesh scheme) for which most steps are characterized by redox potentials.     

Characterisation and in vitro cytotoxicity of triorganophosphinegold(i) 2-mercaptobenzoate complexes

The preparation and full NMR ((1)H, (13)C and (31)P) characterisation of three [R(3)PAu(2mba)] complexes, Where R = Et, Ph and Cy, and 2mba is the anion derived from 2-mercaptobenzoic acid is reported. An interesting solvent dependence in the (1)H spectra is rationalised in terms of competing intra- and inter-molecular hydrogen bonding. An X-ray analysis of the [Ph(3) PAu(2mba)] species reveals a linear P-Au-S arrangement and association in the lattice via the familar carboxylic acid dimer motif. The in Vitro cytotoxicity against seven human tumout lines is also described. The complexes display moderate to very high activity. Particularly noteworthy is their greater activity against the H226 cell line (non-small cell lung cancer) compared with that displayed by a range of cytotoxic drugs.     

A three-dimensional silver(I) coordination polymer involving a new bridging mode of saccharinate

A three-dimensional silver(I)-saccharinato (sac) coordination polymer [Ag₂(μ-sac)₂]_n has been synthesized and characterized by elemental analysis, IR and single crystal X-ray diffraction. The silver(I) ions are doubly bridged by the imino N and carbonyl O atoms of two sac ligands, leading to a dimer [Ag₂(μ-sac)₂] with a very short Ag–Ag contact of 2.8622(6) Å. The dimeric units are linked by unique Ag–C_sac (η¹) bonds into two-dimensional ladder-like layers, which are further assembled into a three-dimensional network by weak Ag?O(sulphonyl) and π(sac)?π(sac) interactions. Complex 1 is the first example of a polymeric network involving the η¹ Ag–C bonds. In addition to the new coordination mode of sac, the IR spectrum and thermal properties of the complex were discussed.