Halogenated rhenacarboranes: optoelectronic behavior of the iodinated rhenacarborane complex anion [3,3,3-(CO)3-8-I-closo-3,1,2-ReC2B9H10]

Treatment of the long-known compound Cs[3,3,3-(CO)₃-closo-3,1,2-ReC₂B₉H₁₁] with [N(C₆H₄Br-4)₃][SbCl₆] in the presence of Me₃NO in THF (tetrahydrofuran) has led to the formation of [HNMe₃][3,3-(CO)₂-3,3-Cl₂-closo-3,1,2-ReC₂B₉H₁₁] in good yield. Structural characterization of this compound has identified it as a four-legged piano stool half-sandwich complex anion resulting from oxidation of rhenium by both the aminium and hexachloroantimonate ions and CO displacement promoted by Me₃NO in the presence of by-product chloride ions. Direct iodination of Cs[3,3,3-(CO)₃-closo-3,1,2-ReC₂B₉H₁₁] has yielded [3,3,3-(CO)₃-3-I-closo-3,1,2-ReC₂B₉H₁₁], which undergoes iodide migration in donor solvents from the metal to the β-B vertex in the coordinating face of the cage. The resulting complex anion, [3,3,3-(CO)₃-8-I-closo-3,1,2-ReC₂B₉H₁₀]⁻, whose structure has been confirmed by X-ray diffraction, has been shown to be luminescent in MeTHF at 77 K (λ_em = 455 nm) and electroactive in solution at ambient temperatures, undergoing a quasi-reversible two-electron oxidation to a proposed Re^III cationic rhenacarborane species in MeCN solutions. By contrast, two fully reversible sequential one-electron oxidations have been observed in CH₂Cl₂ solutions.     

Mono- and bis-cobalt complexes with carbonyl, nitrosyl, and monocarbollide ligands

Reaction of [Co(CO)₃(NO)] with [2-NMe₃-closo-2-CB₁₀H₁₀] in refluxing CH₂Cl₂ affords the mono- and di-cobalt complexes [1-NMe₃-2-CO-2-NO-closo-2,1-CoCB₁₀H₁₀] (3) and [2,7-{Co(CO)(NO)}-7-(μ-H)-1-NMe₃-2-CO-2-NO-closo-2,1-CoCB₁₀H₉] (4), respectively, of which 4 contains formally both Co(I) and Co(-I) centers. Compound 4 reacts with CO to give 3, or with donor ligands L in the presence of Me₃NO to afford simple substituted species, [1-NMe₃-2-L-2-NO-closo-2,1-CoCB₁₀H₁₀] (compounds 5; L = PEt₃, PPh₃, CNBu^t).     

Synthesis of the manganese-monocarbollide dianion [1-NHBu-2,2,2-(CO)3-closo-2,1-MnCB10H10]: Reactions with transition metal cations

Reaction of [Mn(NCMe)₃(CO)₃][PF₆] with Li₃[7-NHBu^t-nido-7-CB₁₀H₁₀] in THF (THF = tetrahydrofuran) affords the twelve-vertex manganacarborane dianion [1-NHBu^t-2,2,2-(CO)₃-closo-2,1-MnCB₁₀H₁₀]²⁻, isolated as the bis-[N(PPh₃)₂]⁺ salt (5a). This species reacts with {Pt(dppe)}²⁺ (dppe = Ph₂PCH₂CH₂PPh₂) to afford the bimetallic complex [1-NH₂Bu^t-2,3-{Pt(dppe)}-2,2,2-(CO)₃-closo-2,1-MnCB₁₀H₉] (7) which has an Mn-Pt bond. In contrast, with {Cu(PPh₃)}⁺ the anion of 5a yields a CuMnCu trimetallic compound [1-{NH(Bu^t)Cu(PPh₃)}-2,3,7-{Cu(PPh₃)}-3,7-(μ-H)₂-2,2,2-(CO)₃-closo-2,1-MnCB₁₀H₈] (8) in which one of the Cu centers is bonded to Mn, whilst the other is attached to the pendant NHBu^t group. Upon treatment with Ag⁺, compound 5a is oxidized giving the very unusual Mn(III)-carbonyl complex [1,2-μ-NHBu^t-2,2,2-(CO)₃-closo-2,1-MnCB₁₀H₁₀] (9a) in which the carborane ligand formally acts as an eight-electron donor to manganese. The novel structural features of compounds 7, 8, and 9a have been confirmed by X-ray diffraction studies.     

Salts of cationic platinum dithiolenes with anionic platinum complexes: structural characterization of [Pt(Me2pipdt)2][Pt(SCN)4] (Me2pipdt = N,N-dimethyl-piperazine-2,3-dithione)

[Pt(Me₂pipdt)₂](BF₄)₂ salts [Me₂pipdt = N,N^′-dimethyl-piperazine-2,3-dithione] bearing cationic dithiolene complexes react with (Bu₄N)₂[Pt(X)₄] (X = SCN, CN) to form [Pt(Me₂pipdt)₂][Pt(SCN)₄ ] (1) and [Pt(Me₂pipdt)₂][Pt(CN)₄] (2) salts by metathesis. Black crystals of 1 have been structurally characterized showing that the two metals lie on inversion centers and exhibit a square planar coordination. The Pt-S bond distances in the anion complex (2.324(2) Å) are longer than in the cation complex (2.280(2) Å) whereas the C-S bond distances are shorter in SCN⁻ (average 1.669 Å) than in Me₂pipdt (average 1.694 Å). The chelating Me₂Pipdt ligand is found disordered in the λ/δ conformations with site occupancies of 50/50, respectively. The cation and anion complexes run parallel to a.     

An improved synthesis, crystal structures, and metallochromism of salts of [Ru(tolyl-terpy)(CN)3]

The previously reported complex [Ru(ttpy)(CN)₃]⁻ [ttpy = 4′(p-tolyl)-2,2′:6′,2″-terpyridine] is conveniently synthesised by reaction of ttpy with Ru(dmso)₄Cl₂ to give [Ru(ttpy)(dmso)Cl₂], which reacts in turn with KCN in aqueous ethanol to afford [Ru(ttpy)(CN)₃]⁻ which was isolated and crystallographically characterised as both its (PPN)⁺ and K⁺ salts. The K⁺ salt contains clusters containing three complex anions and three K⁺ cations connected by end-on and side-on cyanide ligation to the K⁺ ions. The solution speciation behaviour of [Ru(ttpy)(CN)₃]⁻ was investigated with both Zn²⁺ and K⁺ salts in MeCN, a solvent sufficiently non-competitive to allow the added metal cations to associate with the complex anion via the externally-directed cyanide lone pairs. UV-Vis spectroscopic titration of (PPN)[Ru(ttpy)(CN)₃] with Zn(ClO₄)₂ showed a blue shift of 2900 cm⁻¹ in the ¹MLCT absorption manifold due to the ‘metallochromism’ effect; a series of distinct binding events could be discerned corresponding to formation of 4:1, 1:1 and then 1:3 anion:cation adducts, all with high formation constants, as the titration proceeded. In contrast titration of (PPN)[Ru(ttpy)(CN)₃] with the more weakly Lewis-acidic KPF₆ resulted in a much smaller blue-shift of the ¹MLCT absorptions, and the titration data corresponded to formation of 1:1 and then 2:1 cation:anion adducts with weaker stepwise association constants of the order of 10⁴ and then 10³ M⁻¹. Although association of [Ru(ttpy)(CN)₃]⁻ resulted in a blue-shift of the ¹MLCT absorptions, the luminescence was steadily quenched, as raising the ³MLCT level makes radiationless decay via a low-lying ³MC state possible.     

Reactions of alkyl and aryl sulfides with a monocarbon platinacarbaborane anion

The reaction of the monocarbon carbaborane complex Na[Pt(PEt₃)₂(η⁵-CB₁₀H₁₁)] with some diaryl- and dialkyl disulfides has been investigated. With Ph₂S₂, two new cage substituted products are formed, [Pt(SPh)(PEt₃)(η⁵-9-SPh-7-CB₁₀H₁₀)] (1) and [Pt(SPh)(PEt₃)(η⁵-8-SPh-11-SPh-7-CB₁₀H₉)] (2), whereas with S₂ the main product is the metal substituted complex, [Pt(SBu^t)(PEt₃)(η⁵-7-CB₁₀H₁₁)] (4). All three new molecules have been identified spectroscopically (¹H, ¹³C, ³¹P, ¹¹B NMR) and through single crystal X-ray diffraction.     

Structural diversity in thallium chemistry, IV. Further examples of solid state bromothallate(III) derivatives obtained by employing certain classes of alkyl diammonium cations

Six new bromothallate(III)-containing salts with different alkyl diammonium cations have been prepared from bromide containing solutions and studied by single-crystal X-ray crystallographic analyses. The N,N′-diethyl-N,N,N′,N′-tetramethyl-1,2-ethylenediammonium, N-methyl-1,3-propanediammonium, N,N,N′,N′-tetramethyl-1,3-propanediammonium and N,N,N′,N′-tetraethyl-1,2-ethylenediammonium cations yield complexes (I, II, III and IV, respectively) with the [TlBr₅]²⁻ anionic stoichiometry. For I and II, both complexes contain the [TlBr₅]²⁻ anion. In complex II, this appears as a distorted octahedron with one long Tl?Br2′ contact of 3.632(4) Å from an adjacent anion, thus completing the hexacoordination about an otherwise distorted square pyramid. On the other hand, for III and IV, both complexes contain a tetrahedral [TlBr₄]⁻ anion together with an isolated, but hydrogen-bonded, Br⁻ anion. The 1,5-hexanediammonium complex (V) contains tetrahedral [TlBr₄]⁻, slightly distorted octahedral [TlBr₆]³⁻ and Br⁻ anions. The asymmetric unit of the N,N-diethyl-1,3-propanediammonium salt (VI) contains one cation and half of each of a [TlBr₄]⁻ and an axially compressed octahedral [TlBr₆]³⁻ anion. Extensive hydrogen-bonded networks exist in complexes II-VI. NH?Br hydrogen bonds generally have a significant influence on the nature of the anions present in species with the formal [TlBr₅] stoichiometry.     

Synthesis and structural investigation of sandwich polyoxotungstates containing cerium (III/IV) and mono-lacunary Keggin tungstophosphate units

The two isostructural anions, [Ce^III(α-PW₁₁O₃₉)₂]¹¹⁻ and [Ce^IV(α-PW₁₁O₃₉)₂]¹⁰⁻, both composed of Ce^III/IV sandwiched by two mono-lacunary Keggin units [α-PW₁₁O₃₉]⁷⁻, were isolated as dimethylammonium salts and structurally characterized by single-crystal X-ray diffraction analysis. The structures of the [Ce^III/IV(α-PW₁₁O₃₉)₂]^11−/10− anions exhibit two enantiomers d- and l-forms due to C₂ (2) symmetry from the square-antiprismatic coordination of the central Ce^III/IVO₈ polyhedron. The Ce^III compound crystallizes in achiral space group P2₁/n, while the Ce^IV analog in chiral space group P2₁ shows spontaneous resolution without any chiral auxiliaries. The correlation between the ionic radius of the central metal (M) and twist angle of the two mono-lacunary units was studied for a series of [M(α-PW₁₁O₃₉)₂]ⁿ⁻ (M = Ce^III/IV, Eu^III, Zr^IV, Hf^IV) anions and explained in terms of steric repulsion between the O atoms in the mono-lacunary units.     

Reactions of the pentamethylcyclopentadienyl trisulfido tungsten with M′Cl2 (M′ = Zn, Cd, Hg): Isolation and structural characterization of [PPh4][(η-C5Me5)WS3(ZnCl2)] and [{(η-C5Me5)WS3}2Hg]

Reactions of [PPh₄][(η⁵-C₅Me₅)WS₃] with equimolar M′Cl₂ (M′ = Zn, Cd) in MeCN or 0.5 equiv. of HgCl₂ in DMF afforded two binuclear clusters [PPh₄][(η⁵-C₅Me₅)WS₃(M′Cl₂)] (1: M′ = Zn; 2: M′ = Cd) and one trinuclear cluster [{(η⁵-C₅Me₅)WS₃}₂Hg] (3). Compounds 1-3 were characterized by elemental analysis, IR, UV-Vis, ¹H NMR and X-ray crystallography. Compound 1 may be viewed as a 1:1 composite of [PPh₄][(η⁵-C₅Me₅)WS₃] and ZnCl₂, in which one [(η⁵-C₅Me₅)WS₃]⁻ anion binds a ZnCl₂ moiety via two μ-S atoms. In the structure of 3, two [(η⁵-C₅Me₅)WS₃]⁻ anions coordinate the central Hg atom via two μ-S atoms, forming an unique bent linear structure. In addition, internal redox reactions of [PPh₄][(η⁵-C₅Me₅)WS₃] under the presence of M′Cl₂ (M′ = Zn, Cd, Hg) in high concentrations were discussed.     

Shape selectivity in outer-sphere electron transfer reactions

Chiral induction has been examined in the four diastereomeric products formed in a series of outer-sphere electron transfer reactions between the oxidants [Co(ox)₃]³⁻, [Co(edta)]⁻, [Co(gly)(ox)₂]²⁻, C₁-cis(N)-[Co(gly)₂(ox)]⁻, [Co(en)(ox)₂]⁻, C₂-cis(N)-[Co(gly)₂(ox)]⁻ and trans(N)-[Co(gly)₂(ox)]⁻ with [Co((RR,SS)-chxn)₃]²⁺ and [Co((R, S)-pn)₃]²⁺ as reductants. The products; [Co((RR,SS)-chxn)₃-lel₃]³⁺, [Co((RR,SS)-chxn)₃-lel₂ob]³⁺, [Co((RR,SS)-chxn)₃-lelob₂]³⁺, [Co((RR,SS)-chxn)₃-ob₃]³⁺ and corresponding species for [Co((R, S)-pn)₃]³⁺ show patterns of selectivity which are analyzed in terms of the size and structure of the reactants. The presence of a pseudo-C₃ carboxylate face on the oxidant enhances selectivity but the pattern is quite different for those oxidants that contain oxalate as one of their ligands compared with non-oxalate containing species such as [Co(edta)]⁻. A very simple model is developed in which the reductant employs a limited set of interactions corresponding to the major symmetry axes. The unrestricted reductant has very low aggregate selectvity. Steric and hydrogen bonding patterns in both oxidant and reductant enhance individual interactions resulting in the observed selectivities.     

Cation effect on energy transfer reactions from [Tb(2,6-pyridinedicarboxylate)3] to anionic chromium(III) and neodymium(III) complexes in aqueous solutions

Cation effects are studied on the excitation energy transfer reaction between anionic complexes, i.e., [Tb(dpa)₃]³⁻ (dpa=2,6-pyridinedicarboxylate) quenched by [Cr(ox)₃]³⁻ (ox=oxalate ion), [Cr(mal)₃]³⁻ (mal=malonate ion) and [Nd(dpa)₃]³⁻ in aqueous solutions in the presence of alkali metal ions added for adjustments of ionic strengths. In the quenching reaction of [Cr(ox)₃]³⁻, magnitudes of quenching rate constants (k_q) and energy transfer rate constant in encounter complex (k₁) are changed by the cations in the order of Li⁺ < Na⁺ < K⁺ ≈ Rb⁺ ≈ Cs⁺, that is quite contrary of the cation effect on energy transfer reaction between [Ru(N-N)₃]²⁺ and [Cr(ox)₃]³⁻, reported in the previous paper. On the other hand, the rate constants in quenching reactions by [Cr(mal)₃]³⁻ and [Nd(dpa)₃]³⁻ remain almost constant. This result indicates that more separated donor-acceptor pair is not sensitive to coexisting cations.     

Experimental and theoretical investigations of magnetic properties for two low-dimensional spin systems based on bis(2-thioxo-1,3-dithiole-4,5-dithiolato)nickelate monoanion building blocks

Two complexes of [Ni(dmit)₂]⁻ (dmit²⁻ = 2-thioxo-1,3-dithiole-4,5-dithiolate) with nonmagnetic Schiff base cations, 1-(4-bromobenzylideneamino)pyridinium (4-BrBz-1-APy⁺; 1) and 1-(3-nitrobenzylideneamino)pyridinium (3-NO₂Bz-1-APy⁺; 2), have been characterized structurally. Their striking structural feature is the deviation of the [Ni(dmit)₂]⁻ anion from the square-planar environment around the Ni atom with 11.42° and 6.57° dihedral angles (between the mean molecular planes of two dmit²⁻ ligands) in 1 and 2, respectively. These twists arise from the molecular packing interactions between the superimposed anion and cation. In 1, the magnetic [Ni(dmit)₂]⁻ anions are arranged into a wave-shaped regular spin chain, whose magnetism was well fitted by one-dimensional (1D) Heisenberg uniform linear antiferromagnetic chain with |J/k_B| = 66 K. In 2, 1D ladder-shape [Ni(dmit)₂]⁻ chains are formed through lateral-to-lateral S?S contacts between the adjacent anions, which are further aligned into a two-dimensional (2D) anion layer via van der Waals forces. Complex 2 shows Curie-Weiss-type paramagnetic behavior with Curie constant C = 0.421 emu K mol⁻¹ and Weiss constant θ = −1.279 K. The broken-symmetry DFT approach was utilized to evaluate the magnetic coupling nature for 1 and 2, the theoretical analyses performed at ubpw91/lanl2dz level and concerned the so-called “weak bonding” regime approaches qualitatively explained the magnetic behaviors of 1 and 2.     

A multifrequency high-field EPR (9–285 GHz) investigation of a series of dichloride mononuclear penta-coordinated Mn(II) complexes

The electronic structure of a series of 11 penta-coordinated dichloride mononuclear Mn(II) complexes [Mn(L)Cl₂] (L = Cl-terpy, Br-terpy, OH-terpy, phenyl-terpy, tolyl-terpy, mesityl-terpy, EtO-terpy, Me₂N-terpy, tBu₃-terpy, py-phen, and dpya) has been investigated by a multifrequency EPR study (9–285 GHz). The X-ray structures of [Mn(Br-terpy)Cl₂], [Mn(EtO-terpy)Cl₂], [Mn(Me₂N-terpy)Cl₂] and [Mn(tolyl-terpy)Cl₂] are described. The spin Hamiltonian parameters have been determined for all complexes and show that the steric and electronic effects of the N-tridentate ligand L do not induce appreciable variations on the zero field splitting parameters. The magnitude of D, close to 0.3 cm⁻¹, is governed by the chloride anion. High-field EPR spectroscopy allows the determination of electronic parameters of mononuclear Mn(II) complexes characterized by relatively large magnitudes of D and the unambiguous interpretation of the X-band spectra of these kinds of complexes.     

[S2], [S3], and [N3] coordination modes of hydrotris(thioxotriazolyl)borato. Zinc, nickel, cobalt, and bismuth complexes

The ligand hydrotris(1,4-dihydro-3-methyl-4-phenyl-5-thioxo-1,2,4-triazolyl)borato (Tr^Ph,Me) was synthetized as natrium salt and the complexes [Zn(Tr^Ph,Me)₂] · 7.5H₂O · 1.5CH₃CN (2a), [Zn(Tr^Ph,Me)₂] · 8DMF (2b), [Co(Tr^Ph,Me)₂] · 8DMF (3a), [Ni(Tr^Ph,Me)₂] · H₂O · 6DMSO (4a), [Bi(Tr^Ph,Me)₂]NO₃ (5), have been isolated and structurally characterized by X-ray diffraction. In the zinc derivatives the ligand adopts different denticity and coordination modes, η² and [S₂] for 2a and η³ and [N₃] for 2b, depending on the crystallization solvent, giving rise to tetrahedral and octahedral geometry, respectively. In the octahedral cobalt and nickel complexes the ligand is η³ and [N₃] coordinated whereas in the bismuth complex the η³ and [S₃] coordination is exhibited.     

Oxidation of [Ir(η-C5Me5)(C3S5)] [C3S5 = 4,5-disulfanyl-1,3-dithiole-2-thionate(2−)] and X-ray crystal structure of [IrBr(η-C5Me5)(μ-C2S4)IrBr(η-C5Me5)]

[Ir(η⁵-C₅Me₅)(C₃S₅)] [C₃S₅²⁻ = 4,5-disulfanyl-1,3-dithiole-2-thionate(2−)] was prepared by a reaction of [NMe₄]₂[C₃S₅] with [Ir(η⁵- C₅Me₅)Cl₂]₂ in ethanol. It was reacted with bromine to afford a paramagnetic species [IrBr(η⁵-C₅Me₅)(C₃S₅)] with the Ir-Br bond and in the one-electron-oxidized state, and a diamagnetic dinuclear species [IrBr(η⁵-C₅Me₅)(μ-C₂S₄)IrBr(η⁵-C₅Me₅)]. ESR spectra for the one-electron-oxidized species in solution are discussed. The X-ray crystal structural analysis for the latter complex revealed the geometry consisting of dinuclear IrBr(η⁵-C₅Me₅) moieties bridged by the C₂S₄²⁻ ligand.     

Curcuminoids as potential new iron-chelating agents: spectroscopic, polarographic and potentiometric study on their Fe(III) complexing ability

The pK_a values of curcumin and diacetylcurcumin are, here doubtless, determined by means of spectroscopic and potentiometric measurements, and the enolic proton is the more acidic one. The interaction of Fe³⁺ with curcumin and diacetylcurcumin, in water/methanol 1:1 solution, leads to the formation of the complex species [FeH₂CU(OH)₂] and [FeDCU(OH)₂] (H₂CU and DCU=curcumin or diacetylcurcumin monoanion, respectively) which prevails near pH 7. At more basic condition the prevailing species are [FeH₂CU(OH)₃]⁻ and [FeDCU(OH)₃]⁻, which prevent metal hydroxide precipitation. ¹H NMR data state that the dissociated β-diketo moiety of the ligands is involved in metal chelation. The pK_a value of the deprotonation reaction is strongly anticipated by the metal ion, as shown by UV spectral data. The stability constants, evaluated from potentiometric data, are near to that of desferrioxamine, which is, by now, the only iron-chelating agent for clinical use.     

Coordinated nitroxyl anion is produced and released as nitrous oxide by the decomposition of iridium-coordinated nitrosothiols

The aqueous decomposition of the iridium coordinated nitrosothiols (RSNOs) trans-K[IrCl₄(CH₃CN)NOSPh] (1), and K₂[IrCl₅(NOECyS)] (2, ECyS = cysteine ethyl ester), was studied by MS analysis of the gaseous products, ESI-MS, NMR, and UV-Vis spectroscopy. Bent NO (NO⁻, nitroxyl anion), sulfenic acids and nitrite were observed as coordinated products in solution, while nitrous oxide (N₂O) and nitrogen were detected in the gas phase. The formation of coordinated NO⁻ and N₂O, a nitroxyl dimerization product, allows us to propose the formation of free nitroxyl (HNO) as an intermediate. Complex 1 decomposes 300 times slower than free PhSNO does. In both cases (1 and 2) kinetic results show a first order decomposition behavior and a very negative ΔS^≠, which strongly indicates an associative rate-determining step. A proposed decomposition mechanism, supported by the experimental data and DFT calculations, involves, as the first step, nucleophilic attack of H₂O on to the sulfur atom of the coordinated RSNO, producing an NO⁻ complex and free sulfenic acid, followed by two competing reactions: a ligand exchange reaction of this NO⁻ with the sulfenic acid or, to a minor extent, coordination of N₂O to produce an NO⁻/N₂O complex which finally renders free N₂ and coordinated NO₂⁻. Some of the produced NO⁻ is likely to be released from the metal center producing nitroxyl by protonation and finally N₂O by dimerization and loss of H₂O. In conclusion, the decomposition of these coordinated RSNOs occurs through a different mechanism than for the decomposition of free RSNOs. It involves the formation of sulfenic acids and coordinated NO⁻, which is released from the complexes and protonated at the reaction pH producing nitroxyl (HNO), and ultimately N₂O.     

Kinetics and mechanism for reduction of halo- and haloam(m)ine platinum(IV) complexes by l-ascorbate

Reduction of the model platinum(IV) complexes cis-[PtCl₄(NH₃)₂] (1), trans-[PtCl₄(NH₃)₂] (2), trans-[PtCl₂(en)₂]²⁺ (3), trans-[PtBr₂(NH₃)₄]²⁺ (4), [PtCl₆]²⁻ (5), and [PtBr₆]²⁻ (6) with l-ascorbic acid (H₂Asc) in 1.0 M aqueous medium at 25 °C in the region 1.75≤pH≤7.20 has been investigated using stopped-flow spectrophotometry. The redox reactions follow the rate law: −d[Pt(IV]/dt=k[H₂Asc]_tot[Pt(IV)] where k is a pH-dependent second-order rate constant and [H₂Asc]_tot, the total concentration of ascorbic acid. The pH-dependence of k is attributed to parallel reduction of Pt(IV) by the protolytic species HAsc⁻ and Asc²⁻. Analysis of the kinetics data reveals that the ascorbate anion Asc²⁻ is up to seven orders of magnitude more reactive than HAsc⁻ while H₂Asc is unreactive. Electron transfer from HAsc⁻/Asc²⁻ to the Pt(IV) compounds is suggested to take place by a mechanism involving a reductive attack on any one of the mutually trans-halide ligands by Asc²⁻ and/or HAsc⁻ forming a halide-bridged activated complex. The rapid reduction of these complexes supports the assumption that ascorbate Asc²⁻ might be an important reductant at physiological conditions for anticancer active Pt(IV) pro-drugs capable of undergoing reductive trans elimination. The parameters ΔH^≠ and ΔS^≠ for reduction of Pt(IV) with Asc²⁻ have been determined from the study of the temperature dependence of k.     

Reaction of platinum(II) diamine and triamine complexes with selenomethionine

We have reacted [Pt(dien)Cl]Cl, [Pt(en)(D₂O)₂]²⁺, and [Pt(Me₄en)(D₂O)₂]²⁺ [Me₄en = N,N,N′,N′-tetramethylethylenediamine] with selenomethionine (SeMet). When [Pt(dien)Cl]Cl is reacted with SeMet, [Pt(dien)(SeMet-Se)]²⁺ is formed; two Se-CH₃ resonances are observed due to the different chiralities at the Se atom upon platination. In a reaction of [Pt(dien)Cl]Cl with an equimolar mixture of SeMet and Met, the SeMet product forms more quickly though a slow equilibrium with approximately equal amounts of both products is reached. [Pt(Me₄en)(D₂O)₂]²⁺ reacts with SeMet to form [Pt(Me₄en)(SeMet-Se)(D₂O)]²⁺ initially but forms [Pt(Me₄en)(SeMet-Se,N)]⁺ ultimately. One stereoisomer of the chelate, assigned to the R chirality at the Se atom, dominates within the first few minutes of reaction. [Pt(en)(D₂O)₂]²⁺ forms a variety of products depending on reaction stoichiometry; when one equivalent or less of SeMet is added, the dominant product is [Pt(en)(SeMet-Se,N)]⁺. In the presence of excess SeMet, [Pt(en)(SeMet-Se)₂]²⁺ is the dominant initially, but displacement of the en ligand occurs leading to [Pt(SeMet-Se,N)₂] as the eventual product. Displacement of the en ligand from [Pt(en)(SeMet-Se,N)]⁺ does not occur. In reactions of K₂PtCl₄ with two equivalents of SeMet, [Pt(SeMet-Se,N)₂] is formed, and three sets of resonances are observed due to different chiralities at the Se atoms. Only the cis geometric isomers are observed by ¹H and ¹⁹⁵Pt NMR spectroscopy.