Wilhelmus J. Gerber

195Pt NMR isotopologue and isotopomer distributions of [PtCln(H2O)6 − n]4 − n (n = 6,5,4) species as a fingerprint for unambiguous assignment of isotopic stereoisomers

A detailed analysis of the (35)Cl/(37)Cl isotope shifts induced in the 128.8 MHz (195)Pt NMR resonances of [PtCl(n)(H(2)O)(6 - n)](4 - n) complexes (n = 6,5,4) in acidic solution at 293 K, shows that the unique isotopologue and isotopomer distribution displayed by the resolved (195)Pt resonances, serve as a fingerprint for the unambiguous identification and assignment of the isotopic stereoisomers of [PtCl(5)(H(2)O)](-) and cis/trans-[PtCl(4)(H(2)O)(2)].

A robust method for speciation, separation and photometric characterization of all [PtCl6−nBrn]2− (n = 0–6) and [PtCl4−nBrn]2− (n = 0–4) complex anions by means of ion-pairing RP-HPLC coupled to ICP-MS/OES, validated by high resolution 195Pt NMR spectroscopy

A robust reversed phase ion-pairing RP-HPLC method has been developed for the unambiguous speciation and quantification of all possible homoleptic and heteroleptic octahedral platinum(IV) [PtCl(6-n)Br(n)](2-) (n=0-6) as well as the corresponding platinum(II) [PtCl(4-n)Br(n)](2-) (n=0-4) complex anions using UV/Vis detection. High resolution (195)Pt NMR in more concentrated solutions of these Pt(II/IV) complexes (≥50 mM) served to validate the chromatographic peak assignments, particularly in the case of the possible stereoisomers of Pt(II/IV) complex anions. By means of IP-RP-HPLC coupled to ICP-MS or ICP-OES it is possible to accurately determine the relative concentrations of all possible Pt(II/IV) species in these solutions, which allows for the accurate determination of the photometric characteristics (λ(max) and ɛ) of all the species in this series, by recording of the UV/Vis absorption spectra of all eluted species, using photo-diode array, and quantification with ICP-MS or ICP-OES. With this method it is readily possible to separate and estimate the concentrations of the various stereoisomers which are present in these solutions at sub-millimolar concentrations, such as cis- and trans-[PtCl(4)Br(2)](2-), fac- and mer-[PtCl(3)Br(3)](2-) and cis- and trans-[PtCl(2)Br(4)](2-) for Pt(IV), and cis- and trans-[PtCl(2)Br(2)](2-) in the case of Pt(II). All mixed halide Pt(II) and Pt(IV) species can be separated and quantified in a single IP-RP-HPLC experiment, using the newly obtained photometric molar absorptivities, ɛ, determined herein at given wavelengths.

A kinetic and thermodynamic study of the unexpected comproportionation reaction between cis-[OsVIIIO4(OH)2] 2- and trans-[OsVIO2(OH)4] 2- to form a postulated [OsVIIO3(OH) 3]2- complex anion

A kinetic study of [OsO(4)] reduction by aliphatic alcohols (MeOH and EtOH) was performed in a 2.0 M NaOH matrix at 298.1 K. The rate model that best fitted the UV-VIS data supports a one-step, two electron reduction of Os(VIII) (present as both the [Os(VIII)O(4)(OH)](-) and cis-[Os(VIII)O(4)(OH)(2)](2-) species in a ratio of 0.34:0.66) to form the trans-[Os(VI)O(2)(OH)(4)](2-) species. The formed trans-[Os(VI)O(2)(OH)(4)](2-) species subsequently reacts relatively rapidly with the cis-[Os(VIII)O(4)(OH)(2)](2-) complex anion to form a postulated [Os(VII)O(3)(OH)(3)](2-) species according to: cis-[Os(VIII)O(4)(OH)(2)](2-) + trans-[Os(VI)O(2)(OH)(4)](2-) (k+2) <−> (k-2) 2[Os(VII)O(3)(OH)(3)](2-). The calculated forward, k(+2), and reverse, k(-2), reaction rate constants of this comproportionation reaction are 620.9 ± 14.6 M(-1) s(-1) and 65.7 ± 1.2 M(-1) s(-1) respectively. Interestingly, it was found that the postulated [Os(VII)O(3)(OH)(3)](2-) complex anion does not oxidize MeOH or EtOH. Furthermore, the reduction of Os(VIII) with MeOH or EtOH is first order with respect to the aliphatic alcohol concentration. In order to corroborate the formation of the [Os(VII)O(3)(OH)(3)](2-) species predicted with the rate model simulations, several Os(VIII)/Os(VI) mole fraction and mole ratio titrations were conducted in a 2.0 M NaOH matrix at 298.1 K under equilibrium conditions. These titrations confirmed that the cis-[Os(VIII)O(4)(OH)(2)](2-) and trans-[Os(VI)O(2)(OH)(4)](2-) species react in a 1:1 ratio with a calculated equilibrium constant, K(COM), of 9.3 ± 0.4. The ratio of rate constants k(+2) and k(-2) agrees quantitatively with K(COM), satisfying the principle of detailed balance. In addition, for the first time, the molar extinction coefficient spectrum of the postulated [Os(VII)O(3)(OH)(3)](2-) complex anion is reported.

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.

A DFT Mechanistic Study of the trans-[OsVIO2(OH)4]2– and [OsVIIIO4(OH)n]n− (n = 1, 2 cis) Comproportionation Proton-Coupled Electron Transfer Reaction

Herein, we present a DFT computational study of the trans-[OsVIO2(OH)4]2- and [OsVIIIO4(OH) n] n- ( n = 1, 2 cis) comproportionation reaction mechanism that occurs in a basic aqueous matrix. The reaction pathway where [OsVIIIO4(OH)]- reacts with trans-[OsVIO2(OH)4]2- via an intermediate mediated concerted electron-proton transfer yielded the best agreement with experiment (Δ‡ H°, Δ‡ S° and Δ‡ G° experimental data for the forward reaction are 10.3 ± 0.5 kcal mol-1, -2.6 ± 1.6 cal mol-1 K-1, and 11.1 ± 0.9 kcal mol-1 and for the reverse reaction are -6.7 ± 1.0 kcal mol-1, -63.6 ± 3.4 cal mol-1 K-1, and 12.2 ± 2.0 kcal mol-1, respectively, where at the PBE-D3 level for the forward reaction are 11.3 kcal mol-1, -9.8 cal mol-1 K-1, and 14.2 kcal mol-1 and for the reverse reaction are -11.8 kcal mol-1, -80.7 cal mol-1 K-1, and 12.3 kcal mol-1, respectively) and consists of (i) formation of a (singlet spin state) noncovalent adduct, [OsVIII═O···HO-OsVI]3-, (ii) spin-forbidden, concerted electron-proton transfer (i-EPT) from the trans-[OsVIO2(OH)4]2- donor to the OsVIII acceptor to form a second (triplet spin state) noncovalent adduct, [OsVII-OH···O═OsVII]3-, (iii) separation of the OsVII monomers, and finally (iv) interconversion of the separated species to form trans-[OsVIIO3(OH)2]- and mer-[OsVIIO3(OH)3]2- stereoisomer species. i-EPT from OsVI to the OsVIII species was found to be the rate-determining step, which corroborated the experimental evidence (kinetic isotope effect) that the rate-determining step involves the transfer of a proton.