Speciation of Platinum(IV) in Nitric Acid Solutions

Coordination of Pt(IV) by {P8W48} Macrocyclic Inorganic Cavitand: Structural, Solution, and Electrochemical Studies

Hydrothermal reaction of a macrocyclic inorganic POM cavitand Li₁₇(NH₄)₂₁H₂[P₈W₄₈O₁₈₄] with [Pt(H₂O)₂(OH)₄] results in coordination of up to six {Pt(H₂O)_x(OH)_4-x} fragments to the internal surface of the polyoxoanion. The product was isolated as K₂₂(NH₄)₉H₃[{Pt(OH)₃(H₂O)}₆P₈W₄₈O₁₈₄]·79H₂O (1) and characterized by multiple techniques in the solid state (SCXRD, XRPD, XPS, FTIR, and TGA) and in solution (NMR, ESI-MS, and HPLC-ICP-AES). Electrochemical properties were studied both in solution and as components of the paste electrode. The complex shows electrocatalytic activity in water oxidation.

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.

All-inorganic ferric wheel based on hexaniobate-anion linkers

Utilizing the inherent ability of Lindquist-type hexaniobate cluster-anions, [Nb₆O₁₉]^8-, to serve as oxo-donor ligands in complexes with transition-metal cations, we report the synthesis and characterization of the first all-inorganic "ferric" wheel, Li₄₈[(Nb₆O₁₉)₈Fe₈(OH)₈]·88H₂O, comprised of eight Fe atoms linked by eight hexaniobate cluster-anion ligands. Bond valence sum analysis of the X-ray structure and the synthesis conditions themselves indicate that the Fe atoms are in the +3 oxidation state. This is confirmed by magnetic susceptibility and electron paramagnetic resonance (EPR) measurements which indicate the presence of high spin (S = 5/2) Fe(III) ions. In addition, magnetic susceptibility measurements reveal long-range superexchange antiferromagnetic interactions between the hexaniobate-ligand separated Fe³⁺ ions (J = -0.22 cm^-1). More generally, the results suggest the use of hexaniobate cluster-anions as linkers in the synthesis of other two- or three-dimensional polyoxometalate framework structures.

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.

Speciation and separation of platinum(iv) polynuclear complexes in concentrated nitric acid solutions

Understanding the solution chemistry of Pt(iv) is crucial for the hydrometallurgy of precious metals. To gain such an understanding, the speciation and separation of Pt(iv) complexes in concentrated HNO₃ solutions were investigated via Pt L_III edge X-ray absorption fine structure (XAFS) spectroscopy. The XAFS results for concentrated HNO₃ solutions of Na₂Pt(OH)₆ revealed the dominant presence of Pt polynuclear complexes, wherein the formation of Pt(iv) polynuclear complexes depended on the metal concentration and the Na₂Pt(OH)₆ dissolution temperature. The dominant species present in a heated nitrate solution of 0.90 g-Pt L^-1 and a non-heated nitrate solution of 3.2 g-Pt L^-1 were dinuclear Pt(iv) complexes, whereas those in a heated solution of 3.0 g-Pt L^-1 were predominantly larger polynuclear complexes, such as, tetra- and hexa-nuclear complexes. The presence of larger Pt(iv) complexes was confirmed via XAFS spectroscopy, wherein the adsorption of Pt(iv) ions from a 10 M HNO₃ solution by a chelating resin functionalised with iminodiacetic acid and a strongly basic anion-exchange resin bearing trimethyl ammonium nitrate was examined. The adsorption of 50 mg L^-1 of Pt(iv) by the two resins was tested using aqueous solutions diluted from heated HNO₃ solutions with varying metal concentrations, and also from a non-heated solution. We found that Pt(iv) complexes from heating solutions containing high Pt(iv) concentrations displayed high adsorption percentages. In addition, the selective adsorption of Pt(iv) over Pd(ii), Ag(i), Cu(ii), Ni(ii), and Fe(iii) from a 10 M HNO₃ solution was achieved using a strongly basic anion-exchange resin.

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 PtO_x onto a TiO₂ surface was developed on the basis of light-induced activation of the [Pt(NO₃)₆]^2- anion. The deposited PtO_x species with an effective Pt oxidation state between +4 and +2 have an oxygen-made environment and include single ion centers {PtO_n} and polyatomic ensembles {Pt_nO_m} connected to a TiO₂ surface with Pt-O-Ti bonds. The resulting PtO_x/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 PtO_x 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 PtO_x/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.

Complexation and bonding studies on [Ru(NO)(H2O)5]3+ with nitrate ions by using density functional theory calculation

Complexation reactions of ruthenium–nitrosyl complexes in HNO₃ solution were investigated by density functional theory (DFT) calculations in order to predict the stability of Ru species in high-level radioactive liquid waste (HLLW) solution. The equilibrium structure of [Ru(NO)(NO₃)₃(H₂O)₂] obtained by DFT calculations reproduced the experimental Ru–ligand bond lengths and IR frequencies reported previously. Comparison of the Gibbs energies among the geometrical isomers for [Ru(NO)(NO₃)_x(H₂O)_5−x]^(3−x)+/− revealed that the complexation reactions of the ruthenium–nitrosyl complexes with NO₃⁻ proceed via the NO₃⁻ coordination to the equatorial plane toward the Ru–NO axis. We also estimated Gibbs energy differences on the stepwise complexation reactions to succeed in reproducing the fraction of Ru–NO species in 6 M HNO₃ solution, such as in HLLW, by considering the association energy between the Ru–NO species and the substituting ligands. Electron density analyses of the complexes indicated that the strength of the Ru–ligand coordination bonds depends on the stability of the Ru species and the Ru complex without NO₃⁻ at the axial position is more stable than that with NO₃⁻, which might be attributed to the difference in the trans influence between H₂O and NO₃⁻. Finally, we demonstrated the complexation kinetics in the reactions x = 1 → x = 2. The present study is expected to enable us to model the precise complexation reactions of platinum-group metals in HNO₃ solution.

Density functional study on the complexation of [Ru(NO)(H₂O)₅]³⁺ with NO₃⁻ ions reproduced the stabilities of the geometrical isomers and the stepwise substitution reactivities by combining the association energy with the leaving/entering ligands.

Synthesis of macrocyclic polyphenol resin by methylene crosslinked calix[4]arene (MC-[4]H) for the adsorption of palladium and platinum ions

A novel macrocyclic polyphenol resin, a calix[4]arene derivative, namely, a methylene crosslinked calix[4]arene (MC-[4]H), was successfully synthesised by demethylation of MC-[4]CH₃.

Intrinsic 37/35Cl and 18/16O isotope shifts in 195Pt and 103Rh NMR of purely inorganic Pt and Rh complexes as unique spectroscopic fingerprints for unambiguous assignment of structure

Well-resolved intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope shifts (where M = ¹⁹⁵Pt or ¹⁰³Rh) are visible in the ¹⁹⁵Pt NMR peak profiles of relatively kinetically inert [PtCl_n(H₂O)_6-n]^4-n (n = 1-6) complexes, their corresponding hydroxido [PtCl_6-n(OH)_n]^2- (n = 1-5/6) anions, and [RhCl_n(H₂O)_6-n]^3-n (n = 3-6) complexes in aqueous solutions at ca. 293 K. Although some such isotope effects have been previously reported, there are very limited published data in the open literature, and the first systematic studies of such intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope effects are reviewed in this perspective. In high magnetic-field NMR spectrometers, the ¹⁹⁵Pt and ¹⁰³Rh NMR peak profiles acquired within a relatively narrow temperature range (288-300 K) constitute unique 'spectroscopic fingerprints', which allow unambiguous structural assignment in solution. Available data for Pt(iv) and Rh(iii) complexes give rise to intrinsic isotope ¹Δδ¹⁹⁵Pt/¹⁰³Rh(^37/35Cl) profiles, which are extraordinarily sensitive to the structure of a particular complex or its geometric isomer. The profiles of aquated Pt(iv) and Rh(iii) complexes in acidic solutions may be resolved at either an isotopologue level only or at both an isotopologue and an isotopomer level depending on the structure. By contrast, in the series of [PtCl_6-n(OH)_n]^2- (n = 1-6) anions, ¹Δδ¹⁹⁵Pt(^37/35Cl) isotope shifts are resolved only at an isotopologue level. Relatively larger ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts obtained by the partial ¹⁸O enrichment of both the [PtCl_n(H₂O)_6-n]^4-n (n = 1-6) and [PtCl_6-n(OH)_n]^2- (n = 1-6) series give rise to remarkable ¹⁹⁵Pt NMR peak profiles showing both ^37/35Cl and ^18/16O shifts. In the [PtCl_6-n(OH)_n]^2- (n = 1-5/6) anions a typical NMR peak profile spanning ∼2 ppm only may be resolved at both the isotopologue and isotopomer levels, depending on whether ^18/16OH^- ions are coordinated trans to chloride ions or not. The potential utility of such ¹Δ¹⁹⁵Pt(^37/35Cl) and ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts in selected practical applications involving such complexes is briefly illustrated.

Selective speciation improves efficacy and lowers toxicity of platinum anticancer and vanadium antidiabetic drugs

Improving efficacy and lowering resistance to metal-based drugs can be addressed by consideration of the coordination complex speciation and key reactions important to vanadium antidiabetic drugs or platinum anticancer drugs under biological conditions. The methods of analyses vary depending on the specific metal ion chemistry. The vanadium compounds interconvert readily, whereas the reactions of the platinum compounds are much slower and thus much easier to study. However, the vanadium species are readily differentiated due to vanadium complexes differing in color. For both vanadium and platinum systems, understanding the processes as the compounds, Lipoplatin and Satraplatin, enter cells is needed to better combat the disease; there are many cellular metabolites, which may affect processing and thus the efficacy of the drugs. Examples of two formulations of platinum compounds illustrate how changing the chemistry of the platinum will result in less toxic and better tolerated drugs. The consequence of the much lower toxicity of the drug, can be readily realized because cisplatin administration requires hospital stay whereas Lipoplatin can be done in an outpatient manner. Similarly, the properties of Satraplatin allow for development of an oral drug. These forms of platinum demonstrate that the direct consequence of more selective speciation is lower side effects and cheaper administration of the anticancer agent. Therefore we urge that as the community goes forward in development of new drugs, control of speciation chemistry will be considered as one of the key strategies in the future development of anticancer drugs.

Prediction of (195) Pt NMR chemical shifts of dissolution products of H2 [Pt(OH)6 ] in nitric acid solutions by DFT methods: how important are the counter-ion effects?

(195) Pt NMR chemical shifts of octahedral Pt(IV) complexes with general formula [Pt(NO3 )n (OH)6 - n ](2-) , [Pt(NO3 )n (OH2 )6 - n ](4 - n) (n = 1-6), and [Pt(NO3 )6 - n  - m (OH)m (OH2 )n ](-2 + n - m) formed by dissolution of platinic acid, H2 [Pt(OH)6 ], in aqueous nitric acid solutions are calculated employing density functional theory methods. Particularly, the gauge-including atomic orbitals (GIAO)-PBE0/segmented all-electron relativistically contracted-zeroth-order regular approximation (SARC-ZORA)(Pt) ∪ 6-31G(d,p)(E)/Polarizable Continuum Model computational protocol performs the best. Excellent second-order polynomial plots of δcalcd ((195) Pt) versus δexptl ((195) Pt) chemical shifts and δcalcd ((195) Pt) versus the natural atomic charge QPt are obtained. Despite of neglecting relativistic and spin orbit effects the good agreement of the calculated δ (195) Pt chemical shifts with experimental values is probably because of the fact that the contribution of relativistic and spin orbit effects to computed σ(iso) (195) Pt magnetic shielding of Pt(IV) coordination compounds is effectively cancelled in the computed δ (195) Pt chemical shifts, because the relativistic corrections are expected to be similar in the complexes and the proper reference standard used. To probe the counter-ion effects on the (195) Pt NMR chemical shifts of the anionic [Pt(NO3 )n (OH)6 - n ](2-) and cationic [Pt(NO3 )n (OH2 )6 - n ](4 - n) (n = 0-3) complexes we calculated the (195) Pt NMR chemical shifts of the neutral (PyH)2 [Pt(NO3 )n (OH)6 - n ] (n = 1-6; PyH = pyridinium cation, C5 H5 NH(+) ) and [Pt(NO3 )n (H2 O)6 - n ](NO3 )4 - n (n = 0-3) complexes. Counter-anion effects are very important for the accurate prediction of the (195) Pt NMR chemical shifts of the cationic [Pt(NO3 )n (OH2 )6 - n ](4 - n) complexes, while counter-cation effects are less important for the anionic [Pt(NO3 )n (OH)6 - n ](2-) complexes. The simple computational protocol is easily implemented even by synthetic chemists in platinum coordination chemistry that dispose limited software availability, or locally existing routines and knowhow. Copyright © 2016 John Wiley & Sons, Ltd.

Investigations on preferential Pu(IV) extraction over U(VI) by N,N-dihexyloctanamide versus tri-n-butyl phosphate: evidence through small angle neutron scattering and DFT studies

Straight chain amide N,N-dihexyloctanamide (DHOA) has been found to be a promising alternative extractant to tri-n-butyl phosphate (TBP) for the reprocessing of irradiated uranium- and thorium-based fuels. Unlike TBP, DHOA displays preferential extraction of Pu(IV) over U(VI) at higher acidities (≥3 M HNO3) and poor extraction at lower acidities. Density functional theory (DFT) based calculations have been carried out on the structures and relative binding energies of U(VI) and Pu(IV) with the extractant molecules. These calculations suggest that the differential hardness of the two extractants is responsible for the preferential binding/complexation of TBP to uranyl, whereas the softer DHOA and the bulky nature of the extractant lead to stronger binding/complexation of DHOA to Pu(IV). In conjunction with quantum chemical calculations, small angle neutron scattering (SANS) measurements have also been performed for understanding the stoichiometry of the complex formed that leads to relatively lower extraction of Th(IV) (a model for Pu(IV)) as compared to U(VI) using DHOA and TBP as the extractants. The combined experimental and theoretical studies helped us to understand the superior complexation/extraction behavior of Pu(IV) over U(VI) with DHOA.