Reactivity and flexibility in platinum metal clusters

The synthesis, structural properties, and fluxional behaviour of platinum-triosmium and platinum-triruthenium clusters derived from Os₃Pt(-H)₂ (CO)₁₀(PR₃) and Ru₃Pt(-H)(-CC ^t Bu)(CO)₉ (dppe) and related species are described.     

An experimental proof for negative oxidation states of platinum: ESCA-measurements on barium platinides

ESCA-measurements on barium platinides provide the first spectroscopic evidence for negative oxidation states of platinum and are in excellent agreement with theoretical predictions based on quantum-chemical calculations.     

Formation of negative oxidation states of platinum and gold in redox ionic liquid: Electrochemical evidence

The electrochemical reduction of noble metal electrodes in the presence of redox ionic liquid, 1-ferrocenylethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [FcEMIM][TFSI], was investigated by cyclic voltammetry. Our experiments suggest the formation of metal with negative oxidation states, in the cases of platinum and gold electrodes [M_n⁻, FcEMIM⁺]. By analogy with the previous work, the formation of these phases is concomitant with the insertion of the supporting electrolyte; which correspond in our experimental condition to the redox cation of the ionic liquid. As an exciting result, the electrochemical investigations of the reduced electrode in electrolytic solution, containing solvent and supporting electrolyte, evidence the presence of the ferrocene groups at the electrode surface. Moreover, the reduced electrode exhibits the presence of the ferrocene even after, contact with air, after ultrasound, and after physical polishing, highlighting the large stability of this organo-metallic phases formed in this media. The AFM investigations demonstrate the morphological change of the platinum surface after the reduction process. Finally, our works bring a formal electrochemical proof of the presence of the ionic liquid cation inside the electrode material after the cathodic treatment in this media.     

Adventures with substances containing metals in negative oxidation states

A brief history of substances containing s,p- and d-block metals in negative oxidation states is described. A classification of these species and discussions of formal oxidation state assignments for low-valent transition metals in complexes are included, along with comments on the innocent and noninnocent character of ligands in metalates. Syntheses of highly reduced carbonyl complexes formally containing transition metals in their lowest known oxidation states of III- and IV- are discussed. Atmospheric-pressure syntheses of early-transition-metal carbonyls involving alkali-metal polyarene-mediated reductions of non-carbonyl precursors have been developed. In the absence of carbon monoxide, these reactions afford homoleptic polyarenemetalates, including the initial species containing three aromatic hydrocarbons bound to one metal. In several instances, these metalates function as useful synthons for "naked" spin-paired atomic anions of transition metals.     

Gas-phase negative ions of platinum metal fluorides. I. Electron affinity of platinum metal tetrafluorides

Knudsen cell mass spectrometry was used in studies of a number of ion/molecule equilibria involving platinum metal tetrafluorides. Using a combined ion source made it possible to extend considerably the series of tetrafluorides under investigation. This yielded electron affinities of 4.75±0.28, 5.43±0.28, 3.87±0.24, 4.67±0.29 and 5.50±0.25 eV ¹ for Ru, Rh, Os, Ir, and Pt tetrafluorides, respectively.     

Gas-phase negative ions of platinum metal fluorides. II. Electron affinity of platinum metal hexafluorides

Reactions of platinum metal hexafluoride negative ions were studied by Knudsen cell mass spectrometry. The electron affinities were measured for OsF₆(5.93±0.28 eV), IrF₆ (6.50±0.38 eV), PtF₆(7.00±0.35 eV), and RuF₆ (6.47±0.31 eV).The relevant data on electron affinities of platinum metal hexafluorides are discussed.     

The assembly of organic radical anions between metal-borate scaffolds

Metal-organic frameworks based on the Pb[B(Im)(4)](+) unit form layered structures analogous to those observed in clays and double layered hydroxide minerals. These layers can act as scaffolds for the organization of anionic organic guests. In this report, we use this scaffold to assemble TEMPO and PROXYL carboxylates in the interlayer spacings of Pb[B(Im)(4)](4-carboxy-TEMPO) 1 and Pb[B(Im)(4)](3-carboxy-PROXYL)(H(2)O)2, respectively. The resultant materials are paramagnetic, and the organization of the radical units differs between the two compounds. This results in changes in electronic structure of the radical unit, as observed by EPR spectroscopy.     

Effect of anions on the oxidation of organic compounds with ultrasonically activated persulfate

The effect of anions typically present in natural and waste waters on the oxidation of the azo dye methyl orange with persulfate activated with high-frequency ultrasound was studied. At a chloride concentration of 1 mmol/L, the rate constant of substrate oxidation increased 1.5-fold, but further increase in the chloride content retarded the process. The addition of nitrates, carbonates, and hydrogen carbonates to the solution inhibited the process (NO₃^- < HCO₃^- ~ CO₃^2-). These tendencies were in good agreement with the results obtained on a real water matrix of the natural surface water from Lake Baikal.     

Using metal complex reduced states to monitor the oxidation of DNA

Metallointercalating photooxidants interact intimately with the base stack of double-stranded DNA and exhibit rich photophysical and electrochemical properties, making them ideal probes for the study of DNA-mediated charge transport (CT). The complexes [Rh(phi)(2)(bpy')](3+) (phi = 9,10-phenanthrenequinone diimine; bpy' = 4-methyl-4'-(butyric acid)-2,2'-bipyridine), [Ir(ppy)(2)(dppz')](+) (ppy = 2-phenylpyridine; dppz' = 6-(dipyrido[3,2-a:2',3'-c]phenazin-11-yl)hex-5-ynoic acid), and [Re(CO)(3)(dppz)(py')](+) (dppz = dipyrido[2,3-a:2',3'-c]phenazine; py' = 3-(pyridin-4-yl)-propanoic acid) were each covalently tethered to DNA to compare their photooxidation efficiencies. Biochemical studies show that upon irradiation, the three complexes oxidize guanine by long-range DNA-mediated CT with the efficiency: Rh > Re > Ir. Comparison of spectra obtained by spectroelectrochemistry after bulk reduction of the free metal complexes with those obtained by transient absorption (TA) spectroscopy of the conjugates suggests that the reduced metal states form following excitation of the conjugates at 355 nm. Electrochemical experiments and kinetic analysis of the TA decays indicate that the thermodynamic driving force for CT, variations in the efficiency of back electron transfer, and coupling to DNA are the primary factors responsible for the trend observed in the guanine oxidation yields of the three complexes.     

Chemiluminescence in the oxidation of phenyl-substituted silole radical anions and dianions

The emittor in the chemiluminescent electron transfer oxidation of the radical anions and dianions of 1,1-dimethyl-2,5-diphenyl- and 1,1-dimethyl-2,3,4,5-tetraphenylsilole is shown to be the parent compound. Chemiluminescence is reported in the oxidation of the radical anions of 1,2,5-triphenylphosphole, 2,3,4,5-tetraphenylthiophene-5-dioxide and 1,1-diphenyldibenzosilole.     

The metal complexes using organic ligands with photo-excited high-spin states

In this work, 4-pyridyl-phenylanthracene-iminonitroxide radical 2 was synthesized, which can make the coordination to metal ions. It was confirmed by the time-resolved ESR experiments that 2 has a photo-excited quartet (S = 3/2) high-spin state. Cu(II)(hfac)₂(2)₂ and Mn(II)(hfac)₂(2)₂ were synthesized by using 2 as a ligand. Their magnetic properties on the ground states were analyzed by three-spincluster model S₁ − S_M − S₂ (S₁ = S₂ = S_M = 1/2 for Cu(II)(hfac)₂(2)₂ and S₁ = S₂ = 1/2, S_M = 5/2 for Mn(II)(hfac)₂(2)₂). The exchange interactions (J/k_B) between 2 and the metal ions were very weak (J/k_Bs were ferromagnetic for Cu(II)(hfac)₂(2)₂ and antiferromagnetic for Mn(II)(hfac)₂(2)₂). The molecular orbital calculations of 2 have suggested the strong interaction between the paramagnetic center of the metal ions and the photo-excited quartet high-spin state.     

Reactivity of DNA guanyl radicals with phenolate anions

Guanine bases are the most easily oxidized sites in DNA. Electron-deficient guanine species are major intermediates produced in DNA by the direct effect of ionizing radiation (ionization of the DNA itself) because of preferential hole migration within DNA to guanine bases. By using thiocyanate ions to modify the indirect effect (ionization of the solvent), we are able to produce these single-electron-oxidized guanine radical species in dilute aqueous solutions of plasmid DNA where the direct effect is negligible. The guanyl radical species produce stable modified guanine products. They can be detected in the plasmid by converting them to strand breaks after incubation with a DNA repair enzyme. If a phenol is present during irradiation, the yield of modified guanines is decreased. The mechanism is reduction of the guanine radical species by the phenol. It is possible to derive a rate constant for the reaction of the phenol with the guanyl radical. The pH dependence shows that phenolate anions are more reactive than their conjugate acids, although the difference for guanyl radicals is smaller than with other single-electron-oxidizing agents. At physiological pH values, the reduction of a guanyl radical entails the transfer of a proton in addition to the electron. The relatively small dependence of the rate constant on the driving force implies that the electron cannot be transferred before the proton. These results emphasize the potential importance of acidic tyrosine residues and the intimate involvement of protons in DNA repair.     

Synthesis of selenoxides by oxidation of selenides with superoxide radical anions and 2-nitrobenzenesulfonyl chloride

Alkyl phenyl selenoxides were produced in excellent yields by oxidation of the corresponding selenides with 2-nitrobenzenesulfonyl chloride and potassium superoxide in dry acetonitrile at −15 °C.     

Dibenz[b,f]oxepin and thiepin radical anions. Conjugative properties of sulfur in its different oxidation states

The radical anion of dibenz[b,f]oxepin has been investigated by esr spectroscopy and the relative coupling constants compared with dibenzo[b,f]thiepin. In both cases assignments were obtained using a theoretical method (INDO). In addition radicals from dibenzo[b,f]thiepin sulfoxide and sulfone could be detected and, unusually, the free electron density on carbon atoms of the radical anion containing a sulfonyl group is larger than that on carbons of radical anions containing a sulfide or a sulfoxide group.     

Reactivity indices as a measure of rate constants for protonation of radical anions and dianions

The DFT-based reactivity indices were used to describe protonation reactions of radical anions (RA) and dianions (DA) of aromatic compounds. A correlation between the experimental rate constants for protonation and the global reactivity indices was found. The indices were expressed through the electron affinities and ionization energies computed at the B3LYP level of theory. The protonation reactions of RA and DA of aromatic compounds are correctly described by the reactivity indices calculated as the inverse of the difference between the formal formation potential of RA (or DA) and the formal reduction potential of the proton donor.     

Reactivity of cyclometallated platinum complexes with chiral ligands

The reaction of [Pt₂Me₄(μ-SMe₂)₂] with 3-substituted iminic thiophenes and 2-phenylpyridine gives platinum (II) [C,N] cyclometallated complexes which contain a labile ligand (SMe₂ or CH₃CN). Several platinum (II) complexes have been synthesized by substitution reactions with phosphine or sulfoxide ligands to introduce, in most cases, a second chiral center. The new complexes’ reactions with methyl iodide were subsequently studied and showed results that are dependent on the steric and electronic effects of both the cyclometallated ligand and the ancillary phosphine or sulfoxide ligand. The structure of [PtMe((R)-C₁₀H₇CHMeNCHC₄H₂S)(CH₃CN)], a synthetic precursor, is also reported.     

Tertiary-poly-amine ligands as stabilisers of transition metal complexes with uncommon oxidation states

Abstract

Tertiary-amine ligands are known to be poorer [sgrave] donor ligands than the corresponding primary- or secondary-amine ligands. They are known to shift the redox potentials of given couples to the anodic direction relative to the corresponding complexes with primary- or secondary-amine ligands. A review of data in the literature and of recent results on nickel complexes with tetra-aza-macrocyclic ligands and copper complexes with open chain polyamine ligands suggests that the major source for these effects is the poorer solvation of the complexes with the tertiary-amine complexes due to the lack of hydrogen bonding between the complexes and the solvent, or the counter ions. Thus the stabilisation of low valent transition metal complexes by tertiary-amine ligands is due to thermodynamic reasons. On the other hand, tertiary-amine-macrocyclic ligands stabilise high valent complexes because the route to the formation of imine groups is kinetically inhibited in these complexes.     

C-H oxidation by platinum group metal oxo or peroxo species

While C-H oxidation by ruthenium oxo compounds has been broadly applied in organic synthesis, examples of C-H oxidation by metal oxo complexes from the rest of the platinum group are still rare. We survey the preparation and reactivity of these late-transition metal oxo and peroxo complexes in this tutorial review.