Mechanism of Pt(IV) sonochemical reduction in formic acid media and pure water

Sonochemical synthesis of platinum nanoparticles (Pt?NPs) in formic acid solutions and pure water was investigated using a 20?kHz ultrasonic irradiation. The obtained results gave new insights on the underneath Pt(IV) reduction mechanism in formic acid media under argon and in pure water under Ar/CO atmosphere. It was shown that in pure water sonochemical reduction of platinum ions occurs by hydrogen issued from homolytic water molecule split. Pt(IV)?ion reduction appears to be a very slow process under argon atmosphere in pure water due to formation of oxidizing species like OH radicals and H(2)O(2) leading to reoxidation of intermediate Pt(II) ions. Sonochemical reduction is accelerated manifold in the presence of formic acid or Ar/CO gas mixture. Solution and gas-phase analyses reveal that both CO and HCOOH act as OH(.) radical scavenger and reducing agent under ultrasonic irradiation. Their ability to reduce platinum ions at room temperature is enhanced due to the local heating in the liquid shell surround the cavitation bubble. An innovative synthesis route for monodispersed Pt?NPs in pure water without any templates or capping agents in the presence of Ar/CO gas mixture is then proposed. Obtained Pt?NPs within the range of 2-3?nm exhibited a strong stability towards sedimentation in water. Since Ar/CO atmosphere is the only restriction of the process, this procedure can be applied in various media and is also compatible with a large array of experimental conditions.     

An influence of pretreatment conditions on surface structure and reactivity of Pt(100) towards CO oxidation reaction

We present a combined electrochemical and in situ STM study of the surface structure of Pt(100) single crystal electrodes in dependence on the cooling atmosphere after flame annealing. The following cooling conditions were applied: Ar/H₂ and Ar/CO mixtures (reductive atmosphere), argon (inert gas) and air (oxidative atmosphere). Surface characterization by in-situ STM allows deriving direct correlations between surface structure and macroscopic electrochemical behavior of the respective platinum electrodes. We investigated the influence of defect type and density as well as long range surface order on the kinetics of the CO electro-oxidation reaction. The defect-rich Pt(100) electrodes as cooled in air or Ar, and followed by immersion in the hydrogen adsorption region display higher activities as compared to the rather smooth Pt(100)-(1 × 1) electrode cooled in an Ar/H₂-atmosphere.     

Photolysis and Thermolysis of Platinum(IV) 2,2′‐Bipyridine Complexes Lead to Identical Platinum(II)–DNA Adducts

Two Pt^IV and two Pt^II complexes containing a 2,2′‐bipyridine ligand were treated with a short DNA oligonucleotide under light irradiation at 37 °C or in the dark at 37 and 50 °C. Photolysis and thermolysis of the Pt^IV complexes led to spontaneous reduction of the Pt^IV to the corresponding Pt^II complexes and to binding of Pt^II 2,2′‐bipyridine complexes to N7 of guanine. When the reduction product was [Pt(bpy)Cl₂], formation of bis‐oligonucleotide adducts was observed, whereas [Pt(bpy)(MeNH₂)Cl]⁺ gave monoadducts, with chloride ligands substituted in both cases. Neither in the dark nor under light irradiation was the reductive elimination process of these Pt^IV complexes accompanied by oxidative DNA damage. This work raises the question of the stability of photoactivatable Pt^IV complexes toward moderate heating conditions.     

In situ PEI and formic acid directed formation of Pt NPs/MWNTs hybrid material with excellent electrocatalytic activity

A hybrid material based on Pt nanoparticles (Pt NPs) and multi-walled carbon nanotubes (MWNTs) was fabricated with the assistance of PEI and formic acid. The cationic polyelectrolyte PEI not only favored the homogenous dispersion of carbon nanotubes (CNTs) in water, but also provided sites for the adsorption of anionic ions PtCl₄²⁻ on the MWNTs’ sidewalls. Deposition of Pt NPs on the MWNTs’ sidewalls was realized by in situ chemical reduction of anionic ions PtCl₄²⁻ with formic acid. The hybrid material was characterized with TEM, XRD and XPS. Its excellent electrocatalytic activity towards both oxygen reduction in acid media and dopamine redox was also discussed.     

Solvent-specific reduction of Na<Subscript>2</Subscript>PtI<Subscript>6</Subscript> in acetone

Oxidation state of iodide complexes of Pt^II and Pt^IV in large excess of NaI with respect to platinum in water and acetone solution has been determined by means of ¹⁹⁵Pt NMR spectroscopy. In acetone, iodide complexes of Pt^IV are almost quantitatively reduced into Pt^II, and iodine is bound in a poorly soluble polymeric complex with sodium iodide and acetone. Iodometric titration has revealed the formation of equivalent amount of iodine. Reduction of platinum has not been observed in aqueous medium.     

A one‐dimensional iodine‐bridged PtII/PtIV mixed‐valence complex,catena‐poly[[[bis­(ethyl­ene­diamine)­platinum(II)]‐μ‐iodo‐[bis­(ethyl­ene­di­amine)platinum(IV)]‐μ‐iodo] hydrogenphosphate dihydrogenphosphate iodide trihydrate]

The title compound, {[Pt^IIPt^IVI₂(C₂H₈N₂)₄](HPO₄)(H₂PO₄)I·3H₂O}_n, has a chain structure composed of square‐planar [Pt(en)₂]²⁺ and elongated octa­hedral trans‐[PtI₂(en)₂]²⁺ cations (en is ethyl­ene­diamine) stacked alternately along the c axis and bridged by the I atoms; a three‐dimensionally valence‐ordered system exists with respect to the Pt sites. The title compound also has a unique cyclic tetra­mer structure composed of two hydrogenphosphate and two dihydrogenphosphate ions connected by strong hydrogen bonds [O⋯O = 2.522 (10), 2.567 (10) and 2.569 (11) Å]. The Pt and I atoms form a zigzag ⋯I—Pt^IV—I⋯Pt^II⋯ chain, with Pt^IV—I bond distances of 2.6997 (7) and 2.6921 (7) Å, inter­atomic Pt^II⋯I distances of 3.3239 (8) and 3.2902 (7) Å, and Pt^IV—I⋯Pt^II angles of 154.52 (3) and 163.64 (3)°. The structural parameters indicating the mixed‐valence state of platinum, expressed by δ = (Pt^IV—I)/(Pt^II—I), are 0.812 and 0.818 for the two independent I atoms.     

A one‐dimensional chloride‐bridged PtII/PtIV mixed‐valence complex with a 4‐[(4‐hydroxyphenyl)diazenyl]benzenesulfonate counter‐ion

The title compound, catena‐poly[[[bis(ethylenediamine‐κ²N,N′)platinum(II)]‐ μ‐chlorido‐[bis(ethylenediamine)platinum(IV)]‐μ‐chlorido] tetrakis{4‐[(4‐hydroxyphenyl)diazenyl]benzenesulfonate} dihydrate], {[Pt^IIPt^IVCl₂(C₂H₈N₂)₄](HOC₆H₄N=NC₆H₄SO₃)₄·2H₂O}_n, has a linear chain structure composed of square‐planar [Pt(en)₂]²⁺ (en is ethylenediamine) and elongated octahedral trans‐[PtCl₂(en)₂]²⁺ cations stacked alternately, bridged by Cl atoms, along the b axis. The Pt atoms are located on an inversion centre, while the Cl atoms are disordered over two sites and form a zigzag ...Cl—Pt^IV—Cl...Pt^II... chain, with a Pt^IV—Cl bond length of 2.3140 (14) Å, an interatomic Pt^II...Cl distance of 3.5969 (15) Å and a Pt^IV—Cl...Pt^II angle of 170.66 (6)°. The structural parameter indicating the mixed‐valence state of the Pt atom, expressed by δ = (Pt^IV—Cl)/(Pt^II...Cl), is 0.643.     

Observation of the Properties of a Single Unit of a Limited CDW Structure in a PtII/PtIV Host–Guest Binary System Based on a Square‐Shaped Complex

A macrocyclic tetranuclear platinum(II) complex [Pt(en)(4,4′‐bpy)]₄(NO₃)₈ ( 1 ?(NO₃)₈; en=ethylenediamine, 4,4′‐bpy=4,4′‐bipyridine) and a mononuclear platinum(IV) complex [Pt(en)₂Br₂]Br₂ ( 2 ?Br₂) formed two kinds of Pt^II/Pt^IV mixed valence assemblies when reacted: a discrete host–guest complex 1 ? 2 ?Br₁₀ ( 3 ) and an extended 1‐D zigzag sheet 1 ?( 2 )₃?Br₈(NO₃)₆ ( 4 ). Single crystal X‐ray analysis showed that the dimensions of the assemblies could be stoichiometrically controlled. Resonance Raman spectra suggested the presence of an intervalence interaction, which is typically observed for quasi‐1‐D halogen‐bridged M^II/M^IV complexes. The intervalence interaction indicates the presence of an isolated {Pt^II???X? Pt^IV? X???Pt^II} moiety in the structure of 4 . On the basis of electronic spectra and polarized reflectance measurements, we conclude that 4 exhibits intervalence charge transfer (IVCT) bands. A Kramers–Kronig transformation was carried out to obtain an optical conductivity spectrum, and two sub‐bands corresponding to slightly different Pt^II–Pt^IV distances were observed.     

Organic Photovoltaic Catalyst with σ-π Anchor for High-Performance Solar Hydrogen Evolution

Efficient in situ deposition of metallic cocatalyst, like zero-valent platinum (Pt), on organic photovoltaic catalysts (OPCs) is the prerequisite for their high catalytic activities. Here we develop the OPC (Y6CO), by introducing carbonyl in the core, which is available to σ-π coordinate with transition metals, due to the high-energy empty π* orbital of carbonyl. Y6CO exhibits a stronger capability to anchor Pt species and reduce them to metallic state, resulting in more Pt⁰ deposition, relative to the control OPC without the central σ-π anchor. Single-component and heterojunction nanoparticles (NPs) employing Y6CO show enhanced average hydrogen evolution rates of 230.98 and 323.22 mmol h⁻¹ g_[OPC]⁻¹, respectively, under AM 1.5G, 100 mW cm⁻² for 10 h, and heterojunction NPs yield the external quantum efficiencies of ca. 10 % in 500–800 nm. This work demonstrates that σ-π anchoring is one efficient strategy for integrating metallic cocatalyst and OPC for high-performance photocatalysis.     

A Dogma in Doubt: Hydrolysis of Equatorial Ligands of PtIV Complexes under Physiological Conditions

Due to their high kinetic inertness and consequently reduced side reactions with biomolecules, Pt^IV complexes are considered to define the future of anticancer platinum drugs. The aqueous stability of a series of biscarboxylato Pt^IV complexes was studied under physiologically relevant conditions. Unexpectedly and in contrast to the current chemical understanding, especially oxaliplatin and satraplatin complexes underwent fast hydrolysis in equatorial position (even in cell culture medium and serum). Notably, the resulting hydrolysis products strongly differ in their reduction kinetics, a crucial parameter for the activation of Pt^IV drugs, which also changes the anticancer potential of the compounds in cell culture. The discovery that intact Pt^IV complexes can hydrolyze at equatorial position contradicts the dogma on the general kinetic inertness of Pt^IV compounds and needs to be considered in the screening and design for novel platinum‐based anticancer drugs.     

Electrocatalysis and redox behavior of Pt2+ ion in CeO2 and Ce0.85Ti0.15O2: XPS evidence of participation of lattice oxygen for high activity

Electronic states of CeO₂, Ce_1????em>x Pt _x O_2????em>δ , and Ce_{1????em>x????em>y} Ti _y Pt _x O_2????em>δ electrodes have been investigated by X-ray photoelectron spectroscopy as a function of applied potential for oxygen evolution and formic acid and methanol oxidation. Ionically dispersed platinum in Ce_1????em>x Pt _x O_2????em>δ and Ce_{1????em>x????em>y} Ti _y Pt _x O_2????em>δ is active toward these reactions compared with CeO₂ alone. Higher electrocatalytic activity of Pt²⁺ ions in CeO₂ and Ce_1????em>x Ti _x O₂ compared with the same amount of Pt⁰ in Pt/C is attributed to Pt²⁺ ion interaction with CeO₂ and Ce_1????em>x Ti _x O₂ to activate the lattice oxygen of the support oxide. Utilization of this activated lattice oxygen has been demonstrated in terms of high oxygen evolution in acid medium with these catalysts. Further, ionic platinum in CeO₂ and Ce_1????em>x Ti _x O₂ does not suffer from CO poisoning effect unlike Pt⁰ in Pt/C due to participation of activated lattice oxygen which oxidizes the intermediate CO to CO₂. Hence, higher activity is observed toward formic acid and methanol oxidation compared with same amount of Pt metal in Pt/C.     

A one‐dimensional platinum mixed‐valence complex with bridging thiocyanate S atoms: [[PtII(en)2](μ‐SCN)[PtIV(en)2](μ‐SCN)](ClO4)4 (en is ethane‐1,2‐diamine)

A new one‐dimensional platinum mixed‐valence complex with nonhalogen bridging ligands, namely catena‐poly[[[bis(ethane‐1,2‐diamine‐κ²N,N′)platinum(II)]‐μ‐thiocyanato‐κ²S:S‐[bis(ethane‐1,2‐diamine‐κ²N,N′)platinum(IV)]‐μ‐thiocyanato‐κ²S:S] tetrakis(perchlorate)], {[Pt₂(SCN)₂(C₂H₈N₂)₄](ClO₄)₄}_n, has been isolated. The Pt^II and Pt^IV atoms are located on centres of inversion and are stacked alternately, linked by the S atoms of the thiocyanate ligands, forming an infinite one‐dimensional chain. The Pt^IV—S and Pt^II...S distances are 2.3933 (10) and 3.4705 (10) Å, respectively, and the Pt^IV—S...Pt^II angle is 171.97 (4)°. The introduction of nonhalogen atoms as bridging ligands in this complex extends the chemical modifications possible for controlling the amplitude of the charge‐density wave (CDW) state in one‐dimensional mixed‐valence complexes. The structure of a discrete Pt^IV thiocyanate compound, bis(ethane‐1,2‐diamine‐κ²N,N′)bis(thiocyanato‐κS)platinum(IV) bis(perchlorate) 1.5‐hydrate, [Pt(SCN)₂(C₄H₈N₂)₂](ClO₄)₂·1.5H₂O, has monoclinic (C2) symmetry. Two S‐bound thiocyanate ligands are located in trans positions, with an S—Pt—S angle of 177.56 (3)°.     

Human Serum Albumin Conjugated Nanoparticles for pH and Redox‐Responsive Delivery of a Prodrug of Cisplatin

Platinum anticancer drugs are particularly in need of controlled drug delivery because of their severe side effects. Platinum(IV) agents are designed as prodrugs to reduce the side effects of platinum(II) drugs; however, premature reduction could limit the effect as a prodrug. In this work, a highly biocompatible, pH and redox dual‐responsive delivery system is prepared by using hybrid nanoparticles of human serum albumin (HSA) and calcium phosphate (CaP) for the Pt^IV prodrug of cisplatin. This conjugate is very stable under extracellular conditions, so that it protects the platinum(IV) prodrug in HSA. Upon reaching the acidic and hypoxic environment, the platinum drug is released in its active form and is able to bind to the target DNA. The Pt–HSA/CaP hybrid inhibits the proliferation of various cancer cells more efficiently than cisplatin. Different cell cycle arrests suggest different cellular responses of the Pt^IV prodrug in the CaP nanocarrier. Interestingly, this delivery system demonstrates enhanced cytotoxicity to tumor cells, but not to normal cells.     

Coating Platinum Nanoparticles with Methyl Radicals: Effects on Properties and Catalytic Implications

It was recently reported that the reaction of methyl radicals with Pt⁰ nanoparticles (NPs), prepared by the reduction of Pt(SO₄)₂ with NaBH₄, is fast and yields as the major product stable (Pt⁰‐NPs)?(CH₃)_n and as side products, in low yields, C₂H₆, C₂H₄, and some oligomers. We decided to study the effect of this coating on the properties of the Pt⁰‐NPs. The results show that the coating can cover up to 75 % of the surface Pt⁰ atoms. The rate constant of the reaction, k( ^. CH₃+Pt⁰‐NPs), decreases with the increase in the surface coverage, leading to competing reaction paths in the solution, which gradually become dominant, affecting the composition of the products. The methyl coating also affects the zeta potential, the UV spectra, and the electrocatalytic reduction of water in the presence of the NPs. Thus, the results suggest that binding alkyl radicals to Pt⁰ surfaces might poison the NPs catalytic activity. When the Pt⁰‐NPs are prepared by the reduction of a different precursor salt, PtCl₆^2?, nearly no C₂H₄ and oligomers are formed and the methyl coating covers a larger percentage of the surface Pt⁰ atoms. The difference is attributed to the morphology of the Pt⁰‐NPs: those prepared from Pt(SO₄)₂ are twinned nanocrystals, whereas those prepared from PtCl₆^2? consist mostly of single crystals. Thus, the results indicate that the side products, or most of them at least, are formed on the twinned Pt⁰ nanocrystal edges created between (111) facets. In addition, the results show that Pt⁰‐NPs react very differently compared with other noble metals, for example, Au⁰ and Ag⁰; this difference is attributed in part to the difference in the bond strength, (M⁰‐NP)?CH₃, and should be considered in heterogeneous catalytic processes involving alkyl radicals as intermediates.     

Carbon‐Supported Divacancy‐Anchored Platinum Single‐Atom Electrocatalysts with Superhigh Pt Utilization for the Oxygen Reduction Reaction

Maximizing the platinum utilization in electrocatalysts toward oxygen reduction reaction (ORR) is very desirable for large‐scale sustainable application of Pt in energy systems. A cost‐effective carbon‐supported carbon‐defect‐anchored platinum single‐atom electrocatalysts (Pt₁/C) with remarkable ORR performance is reported. An acidic H₂/O₂ single cell with Pt₁/C as cathode delivers a maximum power density of 520 mW cm^?2 at 80 °C, corresponding to a superhigh platinum utilization of 0.09 g_Pt kW^?1. Further physical characterization and density functional theory computations reveal that single Pt atoms anchored stably by four carbon atoms in carbon divacancies (Pt‐C₄) are the main active centers for the observed high ORR performance.     

Controlling the Oxidation State of Pt Single Atoms for Maximizing Catalytic Activity

Single-atom catalysts (SACs) have emerged as promising materials in heterogeneous catalysis. Previous studies reported controversial results about the relative level in activity for SACs and nanoparticles (NPs). These works have focused on the effect of metal atom arrangement, without considering the oxidation state of the SACs. Here, we immobilized Pt single atoms on defective ceria and controlled the oxidation state of Pt SACs, from highly oxidized (Pt⁰: 16.6 at %) to highly metallic states (Pt⁰: 83.8 at %). The Pt SACs with controlled oxidation states were then employed for oxidation of CO, CH₄, or NO, and their activities compared with those of Pt NPs. The highly oxidized Pt SACs presented poorer activities than Pt NPs, whereas metallic Pt SACs showed higher activities. The Pt SAC reduced at 300 °C showed the highest activity for all the oxidations. The Pt SACs with controlled oxidation states revealed a crucial missing link between activity and SACs.     

Synthesis and characterization of platinum(II) and (IV) complexes containing hexamethyleneimine ligand: crystal structure of [PtII(hexamethyleneimine)2(cyclobutanedicarboxylato)]·H2O

A series of new platinum(II) and platinum(IV) complexes of the type [Pt^II(HMI)₂X] (where HMI=hexamethyleneimine, X=dichloro, sulfato, 1,1-cyclobutanedicarboxylato [CBDCA], oxalato, methylmalonato, or tatronato) and [Pt^IV(HMI)₂Y₂Cl₂] (where Y=hydroxo, acetato, or chloro) were synthesized and characterized by infrared (IR) spectroscopy, ¹³C and ¹⁹⁵Pt nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. Among the complexes synthesized, [Pt^II(hexamethyleneimine)₂(1,1-cyclobutanedicarboxylato)]·H₂O was examined by single-crystal X-ray diffraction. The slightly distorted square planar coordination environment of the platinum metal includes the amino group of the hexamethyleneimine (HMI) molecule and the oxygen atoms of the carboxylato ligand. The cyclobutanedicarboxylic acid (CBDCA) molecule adopts six-member chelating rings with platinum. Hydrogen bonding plays an important part in holding the crystal lattice together.     

Photoinduced Reduction of PtIV within an Anti‐Proliferative PtIV–Texaphyrin Conjugate

In an effort to increase the stability and control the platinum reactivity of platinum–texaphyrin conjugates, two Pt^IV conjugates were designed, synthesized, and studied for their ability to form DNA adducts. They were also tested for their anti‐proliferative effects using wild‐type and platinum‐resistant human ovarian cancer cell lines (A2780 and 2780CP, respectively). In comparison to an analogous first‐generation Pt^II chimera, one of the new conjugates provided increased stability in aqueous environments. Using a combination of ¹H NMR spectroscopy and FAAS (flameless atomic‐absorption spectrometry), it was found that the Pt^IV center within this conjugate undergoes photoinduced reduction to Pt^II upon exposure to glass‐filtered daylight, resulting in an entity that binds DNA in a controlled manner. Under conditions in which the Pt^IV complex is reduced to the corresponding Pt^II species, these new conjugates demonstrated potent anti‐proliferative activity in both test ovarian cancer cell lines.     

Der Sintermechanismus von hochdispersem Pt auf Al2O3 in Sauerstoff

Mechanism of Sintering of Highly Dispersed Pt on Al₂O₃ in Oxygen Sintering of highly dispersed Pt on alumina in O₂ consists in a slow thermal decomposition of a Pt^IV surface compound, followed by a fast coalescence of the highly mobile, zerovalent Pt, formed during the decomposition. Sintering according to this mechanism is inhibited by O₂. The slower Ostwald ripening, promoted by O₂, comes into effect only after complete decomposition of the Pt^IV surface compound.     

Synthesis and Characterization of Novel Trans-Mixed Diamine Platinum(II) and Platinum(IV) Complexes

A series of novel trans-mixed diamine platinum(II) and platinum(IV) complexes of type trans-[Pt^II(R-NH₂)(R'-NH₂)Cl₂] and trans -[Pt^IV(R-NH₂)(R'-NH₂)Cl₄] (where R-NH₂ = ethylamine or butylamine and R'-NH₂ = methylamine, propylamine, isopropylamine, pentylamine, or hexylamine) was synthesized and characterized using elemental analysis and infrared and ¹⁹⁵Pt nuclear magnetic resonance spectroscopic techniques.