Photochemistry of (n-Bu4N)2[Pt(NO3)6] in acetonitrile.

Photochemistry of the (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile was studied by means of stationary photolysis and nanosecond laser flash photolysis. The primary photochemical process was found to be an intramolecular electron transfer followed by an escape of an •NO3 radical to the solution bulk. The spectra of two successive Pt(III) intermediates were detected in the microsecond time domain, and their spectral and kinetic characteristics were determined. These intermediates were identified as PtIII(NO3)52- and PtIII(NO3)4- complexes. Disproportionation of Pt(III) species resulted in formation of final Pt(II) products.

Primary processes in photophysics and photochemistry of a potential light-activated anti-cancer dirhodium complex.

Photophysics and photochemistry of a potential light-activated cytotoxic dirhodium complex [Rh2(µ-O2CCH3)2(bpy)(dppz)](O2CCH3)2, where bpy = 2,2'-bipyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine (Complex 1 or Rh2) in aqueous solutions was studied by means of stationary photolysis and time-resolved methods in time range from hundreds of femtoseconds to microseconds. According to the literature, Complex 1 demonstrates both oxygen-dependent (due to singlet oxygen formation) and oxygen-independent cytotoxicity. Photoexchange of an acetate ligand to a water molecule was the only observed photochemical reaction, which rate was increased by oxygen removal from solutions. Photoexcitation of Complex 1 results in the formation of the lowest triplet electronic excited state, which lifetime is less than 10 ns. This time is too short for diffusion-controlled quenching of the triplet state by dissolved oxygen resulting in 1O2 formation. We proposed that singlet oxygen is produced by photoexcitation of weakly bound van der Waals complexes [Rh2…O2], which are formed in solutions. If this is true, no oxygen-independent light-induced cytotoxicity of Complex 1 exists. Residual cytotoxicity deaerated solutions are caused by the remaining [Rh2…O2] complexes.

Platinum(IV) Carbonato Complexes: Formation via the Addition of CO2 to the [Pt(OH)6]2- Anion and Generation of Platinum(IV) Oxide Nanoparticles for the Preparation of Catalysts.

A combination of multinuclear nuclear magnetic resonance spectroscopy and theoretical calculation based on density functional theory was used for a speciation study of Pt in solutions prepared either by the interaction of [Pt(OH)6]2- with gaseous CO2 in an alkaline solution of platinum(IV) hydroxide ([Pt(OH)4(H2O)2]) or by the dissolution of [Pt(OH)4(H2O)2] in an aqueous KHCO3 solution. The formed solutions contained coexisting Pt(IV) carbonato complexes with κ1- and κ2-coordination modes. The gradual condensation of mononuclear Pt species in such bicarbonate solutions resulted in the formation of PtO2 nanoparticles aggregating into a solid precipitate on prolonged aging. The deposition of PtO2 particles from bicarbonate solutions was adapted for the preparation of Pt-containing heterogeneous catalysts: bimetallic Pt-Ni catalysts were prepared using various supporting materials (CeO2, SiO2, and g-C3N4) and tested for the activity in hydrazine-hydrate decomposition. All prepared materials showed high selectivity with respect to H2 production from the hydrazine-hydrate with PtNi/CeO2 showing the highest rate of H2 evolution. In the long-range evaluation, the PtNi/CeO2 catalyst operating at 50 °C showed an exceptional turnover number value of 4600 producing hydrogen at a 97% selectivity level and with a mean turnover frequency value of about 470 h-1. In the case of the PtNi/g-C3N4 catalyst, for the first time, the photodriven decomposition of hydrazine-hydrate was shown to enhance the productivity of the catalyst by 40%.

Highly Active Visible Light-Promoted Ir/g-C3N4 Photocatalysts for the Water Oxidation Reaction Prepared from a Halogen-Free Iridium Precursor.

A combination of the exceptional stability of fac-[Ir(H2O)3(NO2)3] together with thermolability of nitro and aqua ligands and high solubility in various solvents makes it promising as a brand-new chlorine-free precursor of iridium for the preparation of heterogeneous catalysts. In the current work, a new technique of fac-[Ir(H2O)3(NO2)3] preparation based on hydrothermal treatment of (NH4)3[Ir(NO2)6] was developed. For this purpose, the influence of reaction parameters such as the reaction time, temperature, and pH of the solution on the process of hexanitroiridate salt hydrolysis was investigated. The synthesized fac-[Ir(H2O)3(NO2)3] solution in this optimized way was used for the preparation of the series of Ir/g-C3N4 catalysts, which were evaluated in the water oxidation reaction with NaIO4 utilized as a sacrificial reagent. A 20-fold enhancement of the oxygen evolution reaction (OER) activity was found to take place under visible light (λ = 411 nm) illumination of the systems. The highest rate of the photoinduced OER per iridium center was achieved by the Ir0.005/g-C3N4 (air, 400°C) catalyst with an exceptional turnover frequency value of 967 min-1 approaching the activity of known homogeneous iridium OER catalysts. The leaching experiments have shown that aquated Ir species are generated in a solution after prolonged functioning of the catalysts. Despite this, in the closed system the photodriven OER activity persists at a steady-state level evidencing an equilibrium achieved between dissolved and anchored Ir species forming catalytic tandem with the g-C3N4.

Sulfuric Acid Solutions of [Pt(OH)4(H2O)2]: A Platinum Speciation Survey and Hydrated Pt(IV) Oxide Formation for Practical Use.

The systematic study of the platinum speciation in sulfuric acid solutions of platinum (IV) hydroxide {[Pt(OH)4(H2O)2], HHPA} was performed with the use of a combination of methods. Depending on the prevailing Pt form, the three regions of H2SO4 concentration were marked: (1) up to 3 M H2SO4 forms unstable solutions gradually generating the PtO2·xH2O particles; (2) 4-12 M H2SO4, where the series of mononuclear aqua-sulfato complexes ([Pt(SO4)n(H2O)6-n]4-2n, where n = 0···4) dominate; and (3) 12 M and above, where, along with [Pt(SO4)n(H2O)6-n]4-2n species, the polynuclear Pt(IV) species and complexes with a bidentate coordination mode of the sulfato ligand are formed. For the first time, the salts of the aqua-hydroxo Pt(IV) cation [Pt(OH)2(H2O)4]SO4 (triclinic and monoclinic phases) were isolated and studied with a combination of methods, including the single-crystal X-ray diffraction. The formation of PtO2·xH2O particles in sulfuric acid solutions (1-3 M) of HHPA and their spectral characteristics and morphology were studied. The deposition of PtO2·xH2O was highlighted as a convenient method to prepare various Pt-containing heterogeneous catalysts. This possibility was illustrated by the preparation of Pt/g-C3N4 catalysts, which show an excellent performance in catalytic H2 generation under visible light irradiation with a quantum efficiency up to 5% and a rate of H2 evolution up to 6.2 mol·h-1 per gram of loaded platinum.

Hydrolysis of [PtCl6]2- in Concentrated NaOH Solutions.

The transformations of Pt complex species in concentrated NaOH solutions (1-12 M) of Na2[PtCl6] were studied with a combination of methods, including 195Pt nuclear magnetic resonance, ultraviolet-visible, and Raman spectroscopy. The two-step process was observed under the following conditions: (1) formation of the [Pt(OH)5Cl]2- anion that proceeds relatively fast even at room temperature and (2) further slow substitution of the last chlorido ligand with the formation of the [Pt(OH)6]2- anion. Overall, it was determined that the [PtCl6]2- to [Pt(OH)6]2- transformation (especially the first stage) is greatly accelerated under blue light (455 nm) irradiation. The structures of [Pt(OH)Cl5]2- and [Pt(OH)5Cl]2- were determined using the single-crystal X-ray diffraction data of the corresponding salts isolated for the first time. Analysis of the [Pt(OH)Cl5]2- reactivity showed that under analogous conditions, its hydrolysis proceeds 2 orders of magnitude slower than that of [PtCl6]2-, indicating that the formation of [Pt(OH)5Cl]2- from [PtCl6]2- (stage 1) does not follow a simple sequential substitution pattern. A model for [Pt(OH)5Cl]2- anion formation that includes the competing reaction of direct Cl ligand substitution and the self-catalyzed second-order reaction caused by a redox process is proposed. The influence of Pt speciation in alkaline solutions on the reductive behavior is shown, illustrating its impact on the preparation of Pt nanoparticles.

Comprehensive Review on g-C3N4-Based Photocatalysts for the Photocatalytic Hydrogen Production under Visible Light.

Currently, the synthesis of active photocatalysts for the evolution of hydrogen, including photocatalysts based on graphite-like carbon nitride, is an acute issue. In this review, a comprehensive analysis of the state-of-the-art studies of graphic carbon nitride as a photocatalyst for hydrogen production under visible light is presented. In this review, various approaches to the synthesis of photocatalysts based on g-C3N4 reported in the literature were considered, including various methods for modifying and improving the structural and photocatalytic properties of this material. A thorough analysis of the literature has shown that the most commonly used methods for improving g-C3N4 properties are alterations of textural characteristics by introducing templates, pore formers or pre-treatment method, doping with heteroatoms, modification with metals, and the creation of composite photocatalysts. Next, the authors considered their own detailed study on the synthesis of graphitic carbon nitride with different pre-treatments and respective photocatalysts that demonstrate high efficiency and stability in photocatalytic production of hydrogen. Particular attention was paid to describing the effect of the state of the platinum cocatalyst on the activity of the resulting photocatalyst. The decisive factors leading to the creation of active materials were discussed.

Tetranitratopalladate(II) Salts with Tetraalkylammonium Cations: Structural Aspects, Reactivity, and Applicability toward Palladium Deposition for Catalytic Applications.

A series of salts (R4N)2[Pd(NO3)4] (R = CH3, C2H5, n-C3H7; 1-3) were synthesized in high yield from a nitric acid solution of palladium. The salts were characterized by a combination of physicochemical methods, and their crystal structures were determined by X-ray diffraction. The conformation of the [Pd(NO3)4]2- anion was studied in detail using crystal structure data and density functional theory calculations. A combination of nonhygroscopicity and stability under normal conditions, together with thermolability, high solubility in various solvents, and the lability of nitrato ligands, makes salts 1-3 valuable starting materials for the synthesis of Pd compounds and the preparation of Pd-containing catalysts. In this work, these applications were illustrated by the synthesis of heteroleptic Pd(II) nitrato complexes with N-donor ligands and the preparation of Pd0.1Ni0.9/SiO2 catalysts, which worked well in H2 generation from hydrazine hydrate. Generally, it was shown that up to several weight percent of Pd can be deposited on various oxide/hydroxide supports using a straightforward chemisorption procedure from acetone solutions of 1-3.

Photochemistry of hexachloroosmate(IV) in ethanol.

The photochemistry of the OsIVCl62- complex in ethanol was studied by means of stationary photolysis, nanosecond laser flash photolysis, ultrafast pump-probe spectroscopy and quantum chemistry. The direction of the photochemical process was found to be wavelength-dependent. Irradiation in the region of the d-d and LMCT bands results in the photosolvation (with the wavelength-dependent quantum yield) and photoreduction of Os(iv) to Os(iii), correspondingly. The characteristic time of photosolvation is ca. 40 ps. Photoreduction occurs in the micro- and millisecond time domains via several Os(iii) intermediates. The nature of intermediates and the possible mechanisms of photoreduction are discussed. We believe that the lability of the photochemically produced Os(iv) and Os(iii) intermediates determines the synthetic potential of OsIVCl62- photochemistry.

Photoinduced Deposition of Platinum from (Bu4N)2[Pt(NO3)6] for a Low Pt-Loading Pt/TiO2 Hydrogen Photogeneration Catalyst.

An efficient method for the deposition of ionic platinum species PtOx onto a TiO2 surface was developed on the basis of light-induced activation of the [Pt(NO3)6]2- anion. The deposited PtOx species with an effective Pt oxidation state between +4 and +2 have an oxygen-made environment and include single ion centers {PtOn} and polyatomic ensembles {PtnOm} connected to a TiO2 surface with Pt-O-Ti bonds. The resulting PtOx/TiO2 materials were tested as photocatalysts for the hydrogen evolution reaction (HER) from a water ethanol mixture and have shown uniquely high activity with the rate of H2 evolution achieving 11 mol h-1 per gram of Pt, which is the highest result for such materials reported to date. A combination of spectral methods shows that, under HER conditions, reduction of the supported PtOx species leads to the formation of well-dispersed nanoparticles of metallic platinum attached on the surface of TiO2 by Ti-O-Pt bonds. The high activity of the PtOx/TiO2 materials is believed to result from a combination of uniform distribution of small platinum nanoparticles over the titania surface and their close interaction with TiO2.

Rh(III) hydroxocomplexes speciation using HPLC-ESI-MS.

A mixture of rhodium(iii) hydroxocomplexes formed during the polycondensation process in alkaline media has been fully characterized by the hyphenated high performance liquid chromatography with electrospray ionization mass spectrometry (HPLC-ESI-MS). The baseline separation was achieved using reverse phase ion-pair chromatography (RP-IP-HPLC) in gradient elution mode. When using SDS as the ion pair, the formation of all types of its associates with the molecules of acetonitrile, as well as oligomers of rhodium aquahydroxocomplexes, etc. was taken into consideration. As a result, it was shown that the test mixture is presented by [Rh(H2O)6]3+, [Rh2(µ-OH)2(H2O)8]4+, [Rh3(µ-OH)4(H2O)10]5+, [Rh4(µ-OH)6(H2O)12]6+.

Mechanistic study of the trans,cis,cis-[RuCl2(DMSO)2(H2O)2] complex photochemistry in aqueous solutions.

It is known that trans,cis,cis-[RuCl2(DMSO)2(H2O)2] (1a) complexes, which are formed upon dissolution of trans-[RuCl2(DMSO)4] in water, demonstrate light-induced cytotoxicity. The mechanistic study of 1a photochemistry has been performed using ultrafast pump-probe spectroscopy, laser flash photolysis and stationary photolysis. The first stage of 1a photochemistry is the photoexchange of a DMSO ligand to a water molecule; its quantum yield is wavelength-dependent (estimating by values 0.3 and 0.04 upon irradiation at 308 and 430 nm, respectively). The mechanism of photoexchange is complicated involving at least four Ru(ii) intermediates. Two tentative mechanisms of the process are proposed.

The study of Rh(iii) hydroxocomplexes using capillary zone electrophoresis with a UV-Vis detector: the development of the method.

The expediency of capillary zone electrophoresis application in the study of a mixture of rhodium(iii) hydroxocomplexes is shown for the species formed during the thermal treatment of [Rh(OH)6]3- at 60 °C in alkaline media. As these compounds are unstable in solution because of polycondensation at pH > 10 and the formation of insoluble rhodium(iii) hydroxides at pH 5-10, their acidic derivatives with terminal aqua ligands acted as the objects of the study. Optimal separation conditions were achieved using sodium perchlorate as a background electrolyte, a voltage of +20 kV and pH 2.6. Rhodium(iii) species with different nuclearities were identified in accordance with their UV spectra and the regularities of electrophoretic migration in the capillary. The presence of a dimer was also verified by the spiking of the mixture under investigation with a previously synthesised complex [Rh2(µ-OH)2(H2O)8](NO3)4, which confirms the correct identification.

Tetraalkylammonium Salts of Platinum Nitrato Complexes: Isolation, Structure, and Relevance to the Preparation of PtO x/CeO2 Catalysts for Low-Temperature CO Oxidation.

A series of tetraalkylammonium salts with anionic platinum nitrato complexes (Me4N)2[Pt2(µ-OH)2(NO3)8] (1), (Et4N)2[Pt2(µ-OH)2(NO3)8] (2), ( n-Pr4N)2[Pt2(µ-OH)2(NO3)8] (3b), ( n-Pr4N)2[Pt(NO3)6] (3a), and ( n-Bu4N)2[Pt(NO3)6] (4) were isolated from nitric acid solutions of [Pt(H2O)2(OH)4] in high yield. The structures of salts 2, 3a, 3b, and 4, prepared for the first time, were characterized by X-ray diffraction. The sorption of [Pt(NO3)6]2- and [Pt2(µ-OH)2(NO3)8]2- complexes onto the ceria surface from acetone solutions of salts 4 and 1 was examined. The dimeric anion was shown to quickly and irreversibly chemisorb onto the CeO2 carrier, selectively transforming into Pt(II) centers after thermal treatment, becoming active in the low-temperature CO oxidation reaction ( T50% = 110 °C at a space velocity of 240 000 h-1). By contrast, the homoleptic complex [Pt(NO3)6]2- did not interact with the ceria, which may be attributed to the substitutional inertness of the [Pt(NO3)6]2- anion. We believe that the strategy based on the sorption of polynuclear platinum nitrato complexes is an effective route to prepare ionic platinum species uniformly distributed on an oxide carrier for various catalytic applications.

A cis,fac-[RuCl2(DMSO)3(H2O)] complex exhibits ultrafast photochemical aquation/rearrangement.

It is known that both cis,fac-[RuCl2(DMSO)3(H2O)] (1a) and trans,cis,cis-[RuCl2(DMSO)2(H2O)2] (2a) complexes, which are formed on the dissolution of trans and cis-isomers of [RuCl2(DMSO)4] in water, demonstrate light-induced anticancer activity. The first stage of 1a photochemistry is its transformation to 2a occurring with a rather high quantum yield, 0.64 ± 0.17. The mechanism of the 1a → 2a phototransformation was studied by means of nanosecond laser flash photolysis and ultrafast pump-probe spectroscopy. The reaction occurs in the picosecond time range via the formation and decay of two successive intermediates interpreted as Ru(ii) complexes with different sets of ligands. A tentative mechanism of phototransformation is proposed.

Spectroscopic and DFT Study of RhIII Chloro Complex Transformation in Alkaline Solutions.

The hydrolysis of [RhCl6]3- in NaOH-water solutions was studied by spectrophotometric methods. The reaction proceeds via successive substitution of chloride with hydroxide to quantitatively form [Rh(OH)6]3-. Ligand substitution kinetics was studied in an aqueous 0.434-1.085 M NaOH matrix in the temperature range 5.5-15.3 °C. Transformation of [RhCl6]3- into [RhCl5(OH)]3- was found to be the rate-determining step with activation parameters of ΔH† = 105 ± 4 kJ mol-1 and ΔS†= 59 ± 10 J K-1 mol-1. The coordinated hydroxo ligand(s) induces rapid ligand substitution to form [Rh(OH)6]3-. By simulating ligand substitution as a dissociative mechanism, using density functional theory (DFT), we can now explain the relatively fast and slow kinetics of chloride substitution in basic and acidic matrices, respectively. Moreover, the DFT calculated activation energies corroborated experimental data that the kinetic stereochemical sequence of [RhCl6]3- hydrolysis in an acidic solution proceeds as [RhCl6]3- → [RhCl5(H2O)]2- → cis-[RhCl4(H2O)2]-. However, DFT calculations predict in a basic solution the trans route of substitution [RhCl6]3- → [RhCl5(OH)]3- → trans-[RhCl4(OH)2]3- is kinetically favored.

Primary photochemical processes for Pt(iv) diazido complexes prospective in photodynamic therapy of tumors.

Diazide diamino complexes of Pt(iv) are considered as prospective prodrugs in oxygen-free photodynamic therapy (PDT). Primary photophysical and photochemical processes for cis,trans,cis-[Pt(N3)2(OH)2(NH3)2] and trans,trans,trans-[Pt(N3)2(OH)2(NH3)2] complexes were studied by means of stationary photolysis, nanosecond laser flash photolysis and ultrafast kinetic spectroscopy. The process of photolysis is multistage. The first stage is the photosubstitution of an azide ligand to a water molecule. This process was shown to be a chain reaction involving redox stages. Pt(iv) and Pt(iii) intermediates responsible for the chain propagation were recorded using ultrafast kinetic spectroscopy and nanosecond laser flash photolysis. The mechanism of photosubstitution is proposed.

Polynuclear Hydroxido-Bridged Complexes of Platinum(IV) with Terminal Nitrato Ligands.

For the first time the polynuclear hydroxido-bridged platinum(IV) nitrato complexes with nuclearity higher than two were isolated from nitric acid solutions of [Pt(H2O)2(OH)4] and crystallized as supramolecular compounds of macrocyclic cavitands cucurbit[n]uril (CB[n], n = 6,8) and 18-crown-6 ether: [Pt4(µ3-OH)2(µ2-OH)4(NO3)10]·CB[6]·25H2O (I), [Pt6(µ3-OH)4(µ2-OH)6(NO3)12](NO3)2·CB[8]·50H2O (II), and [H3O⊂18-crown-6]2[Pt2(µ2-OH)2(NO3)8][Pt4(µ3-OH)2(µ2-OH)4(NO3)10] (III). The isolation of the compounds in the single crystalline state allows the determination of the structure of the tetranuclear and hexanuclear complexes [Pt4(µ3-OH)2(µ2-OH)4(NO3)10] and [Pt6(µ3-OH)4(µ2-OH)6(NO3)12](2+), which have been previously unknown in the solid state. Stability of Ptx(OH)y cores of the polynuclear nitrato complexes toward alkaline hydrolysis was verified by (195)Pt NMR spectroscopy. Analysis of (195)Pt NMR spectra of the compound III reveals that addition of every Pt(µ-OH)2Pt ring results in ∼260 ppm downfield shift relative to the mononuclear form, which allows the prediction of signal positions for complexes of higher nuclearity.

Speciation of platinum(IV) in nitric acid solutions.

The speciation of platinum(IV) ions in nitric acid (6-15.8 M) solutions of H2[Pt(OH)6] has been studied by (195)Pt NMR and Raman spectroscopy. Series of aqua-hydroxo-nitrato complexes [Pt(L)(x)(NO3)(6-x)] (L = H2O or OH(-); x = 0, ..., 6) were found to exist in such solutions. The pair additivity model of chemical shifts and statistical theory were used to assign signals in NMR spectra to particular [Pt(L)(x)(NO3)(6-x)] species. Mononuclear hexanitratoplatinates(IV) have been isolated in solid state in substantial yield as pyridinium salt (PyH)2[Pt(NO3)6] and characterized by single-crystal X-ray diffraction. Aging of the platinum nitric acid solutions for more than 5-6 h results in oligomerization of [Pt(L)(x)(NO3)(6-x)] species and the formation of oligonuclear aqua-hydroxo-nitrato complexes with OH(-) and NO3(-) bridging ligands. Oligomeric platinum(IV) complexes with two and four nuclei were unambiguously detected by NMR on (195)Pt -enriched samples. Oligomers with even higher nuclearity were also detected. Dimeric anions [Pt2(µ-OH)2(NO3)8](2-) have been isolated as single crystals of tetramethylammonium salt and characterized by X-ray diffraction.