Solution luminescence from chloro(2,2′:6′,2″-terpyridine)platinum(II) chloride in micelles

Previously characterized as being non-luminescent in room-temperature fluid solution, the coordination compound chloro(2,2′:6′,2″-terpyridine)platinum(II) chloride can display two types of luminescence in certain microenvironments. In aqueous solutions of anionic and neutral surfactants having concentrations near or above their critical micelle concentration, [Pt(terpy)Cl]Cl (10-50 μM) displays broad emission centered at ∼610 nm that is characterized as metal-to-ligand charge transfer phosphorescence (³MLCT). In high concentration (10-100 mM) solutions having no surfactant, [Pt(terpy)Cl]Cl aggregates form. Excitation in the 470-540 nm region results in a long-wavelength emission centered at ∼720 nm that is characterized as metal-metal-to-ligand charge transfer phosphorescence (³MMLCT). This emission can also be detected in lower concentration solutions (10-50 μM) with surfactant concentration below its critical micelle concentration. Enhancement of ³MLCT luminescence is also found for the related phenylacetylide complex cation [Pt(terpy)(CCPh)]⁺ in micelles of the anionic surfactant sodium dodecyl sulfate.     

Platinum(II) triammine antitumour complexes: structure-activity relationship with guanosine 5′-monophosphate (5′-GMP)

The multinuclear (¹H, ¹⁵N, ³¹P and ¹⁹⁵Pt) NMR spectroscopies, ES-MS and HPLC have been employed to investigate the structure-activity relationship for the reactions between guanosine 5′-monophosphate (5′-GMP) and the platinum(II)-triamine complexes of the general formulation cis-[Pt(NH₃)₂(Am)Cl]NO₃ (where Am represents a substituted pyridine). The order of reaction rate of the reactions was found to be: 3-phpy > 4-phpy > py > 4-mepy > 3-mepy > 2-mepy. The two basic factors, steric and electronic, were attributed to the order of the binding rate constants. A possible mechanism of the reaction of cis-[Pt(NH₃)₂(Am)Cl]⁺ with 5′-GMP suggested that the reactions proceed via direct nucleophilic attack and no loss of ammonia. cis-[Pt(NH₃)₂(Am)Cl]⁺ binds to the N7 nitrogen of the guanine residue of 5′-GMP to form a coordinate bond with the Pt metal centre. This mechanism is apparently different from that of cisplatin. The pK_a value of cis-[Pt(NH₃)₂(4-mepy)(H₂O)](NO₃)₂ (5.63) has been determined at 298 K by the use of distortionless enhancement by polarization transfer (DEPT) ¹⁵N NMR spectroscopy and compared to the pK_a value of cis-[PtCl(H₂O)(NH₃)₂]⁺.     

Two new 4′,4′′-disubstituted dipyrazolylpyridine derivatives, and the structures and spin states of their iron(II) complexes

Reaction of 2 equiv of the sodium salt of ethyl pyrazole-4-carboxylate, with 1 equiv of 2,6-dibromopyridine, in diglyme at 130 °C for 5 days yields 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L¹), with 2-bromo-6-[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L²) as a significant byproduct. Reduction of L¹ with excess NaBH₄ in thf affords 2,6-di[4-(hydroxymethyl)pyrazol-1-yl]pyridine (L³) in low yield. The crystalline complex [Fe(L¹)₂][BF₄]₂ · 2CF₃CH₂OH is low-spin at 150 K, while bulk samples with this formula are approximately 10% high-spin and 90% low-spin at room temperature. This ratio does not vary significantly on cooling from its magnetic susceptibility, suggesting that the material might be contaminated by a second, minor high-spin phase. Single crystals of [Fe(L³)₂][BF₄]₂·1.4CH₃CN have a mixed spin-state population, with the low-spin state predominating at 150 K. The [Fe(L³)₂(BF₄)]⁺ moieties in the lattice associate into 1-D chains through intermolecular O-H?O and O-H?F hydrogen bonding. Bulk samples of [Fe(L³)₂][BF₄]₂ · H₂O are fully low-spin below 200 K, but the magnetic data imply the onset of a gradual thermal spin-transition centred above room temperature. DSC and TGA measurements imply that this transition is centred at 322 K, and involves loss of lattice water. Both complexes undergo spin-crossover in (CD₃)₂CO solution, with transition midpoints near 250 K.     

Synthesis, structures and spectroscopic properties of platinum(II), copper(I) and zinc(II) complexes bearing 4-(p-dimethylaminophenyl)-6-phenyl-2,2′-bipyridine ligand

The reactions of 4-(p-dimethylaminophenyl)-6-phenyl-2,2′-bipyridine (HL) with three metal salts of platinum(II), copper(I) and zinc(II) provide the new complexes [Pt(L)(PPh₃)]ClO₄ (1), [Cu(HL)₂]BF₄ (2), [Cu(HL)(PPh₃)]BF₄ (3) and [Zn(HL)₂](ClO₄)₂ (4). All the structures of these four complexes have been characterized by single crystal X-ray diffraction, and their spectroscopic properties were investigated. Especially for complex 1, upon protonation, the excited state can be tuned from the intraligand charge transfer (ILCT) to the metal-to-ligand charge transfer (MLCT), and such switching in the excited state is acid/base reversible. The time-dependent density functional theory (TD-DFT) calculation was used to interpret the absorption spectra of complex 1, and the calculated result is consistent with those of experiments results. In contrast with 1, the lowest energy absorption at 410-650 nm of complexes 2 and 3 can be assigned to MLCT excited state. In solid state or solution complex 4 exhibits intense photoluminescence attributed to a ILCT transition in nature.     

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.     

Platinum(II) and palladium(II) saccharinato complexes with 2,2′:6′,2″-terpyridine: Synthesis, characterization, crystal structures, photoluminescence and thermal studies

[Pd(sac)(terpy)](sac)·4H₂O (1), [Pt(sac)(terpy)](sac)·5H₂O (2), [PdCl(terpy)](sac)·2H₂O (3) and [PtCl(terpy)](sac)·2H₂O (4) (sac = saccharinate, and terpy = 2,2′:6′,2″-terpyridine) have been synthesized and characterized by elemental analysis, FT-IR, ¹H NMR and ¹³C NMR. In 1 and 2, a tridentate terpy ligand together with an N-coordinated sac ligand form the square-planar geometry around the palladium(II) or platinum(II) ions, while one sac anion remains outside the coordination sphere as a counter-ion. X-ray single crystal studies show that the [M(sac)(terpy)]⁺ ions in 1 and 2 reside in the centers of a hydrogen bonded honeycomb network formed by the uncoordinated sac ions and the lattice water molecules. Complexes 3 and 4 are isostructural and consist of a [M(Cl)(terpy)]⁺ cation, a sac anion and two lattice water molecules. The [M(Cl)(terpy)]⁺ ions interact with each other via M-M and π-π stacking interactions and these π interacted units are assembled to a 2D network by water bridges involving the sac ions and lattice water molecules. Convenient synthetic paths for 1-4 are also presented, and spectral, luminescence and thermal properties were discussed.     

Photolysis of the ion pair [Pt(NH3)5Cl]S2O8: reduction of platinum(IV) induced by outer-sphere charge transfer excitation

The ion pair [Pt^IV(NH₃)₅Cl]³⁺S₂O₈²⁻ shows a S₂O₈²⁻ → [Pt(NH₃)₅Cl]³⁺ outer-sphere charge transfer (OSCT) absorption at λ_max=267 nm. OSCT excitation leads to the reduction of Pt(IV) by S₂O₈²⁻ to Pt(II) with φ=3×10⁻³ at λ_irr=280 nm.     

Synthesis and luminescence properties of [Pt{4′-(Np1)-trpy}(CCPh)]SbF6 and [Pt{4′-(Np1)-trpy}{CC(CH2)2CH3}]SbF6 [4′-(Np1)-trpy = 4′-(1-naphthyl)-2,2′:6′,2″-terpyridine]

The synthesis and characterisation of [Pt{4′-(Np1)-trpy}(CCPh)]SbF₆ (1) and [Pt{4′-(Np1)-trpy}{CC(CH₂)₂CH₃}]SbF₆ (2) [4′-(Np1)-trpy = 4′-(1-naphthyl)-2,2:6′,2′-terpyridine] are described. Complexes 1 and 2 exhibit unimolecular ³MLCT (MLCT = metal-to-ligand charge transfer) emission in acetonitrile and in a low concentration 77 K glass solution in butyronitrile. The high concentration glass emission as well as the emission in the solid state is from a ³MMLCT (MMLCT, metal-metal-to-ligand charge transfer) excited state, reflecting the presence of interactions in these media.     

Non-covalent interactions between cations in the crystal structure of [Pt{4′-(p-tolyl)trpy}Cl]SbF6, where trpy is 2,2′:6′,2″-terpyridine, underpin the salt’s complex solid-state luminescence spectrum

The synthesis and characterization of [Pt{4′-(p-tolyl)trpy}Cl]SbF₆ is described where trpy is 2,2′:6′,2″-terpyridine. A single crystal X-ray structure determination at 100 K shows that the cations are stacked in columns that comprise cations arranged in a staircase motif. Successive cations within a column are linked by π(trpy)-π(phenyl) stabilizing interactions; and each cation in one column is linked to a cation in an adjacent column by a weakly stabilizing Pt···Pt interaction. The Pt···Pt distance is 3.434(1) Å. The metrics governing non-covalent interactions between [Pt{4′-(aryl)trpy}Cl]⁺ cations have been analyzed for the present structure and related structures in the CSD (Cambridge Structural Database). Cation dimers cluster into three distinct groups based on their lateral shifts and, to a lesser extent, the angular parameters governing their relative displacements; the dominant grouping exhibits Pt···Pt and π(trpy)-π(trpy) stabilizing interactions. An emission spectrum recorded at 77 K on a solid sample of the compound is best interpreted as arising from the decay of three photoexcited states: a ³MLCT (MLCT = metal-to-ligand charge transfer) state; a ³MMLCT (MMLCT = metal-metal-to-ligand charge transfer) state, and an excimeric ³π-π^∗ state.     

Optical investigation of spin-crossover in cobalt(II) bis-terpy complexes

The spin transition of the [Co(terpy)₂]²⁺ complex (terpy = 2,2′:6′,2″-terpyridine) is analysed based on experimental data from optical spectroscopy and magnetic susceptibility measurements. The single crystal absorption spectrum of [Co(terpy)₂](ClO₄)₂ shows an asymmetric absorption band at 14 400 cm⁻¹ with an intensity typical for a spin-allowed d-d transition and a temperature behaviour typical for a thermal spin transition. The single crystal absorption spectra of suggest that in this compound, the complex is essentially in the high-spin state at all temperatures. However, the increase in intensity observed in the region of the low-spin MLCT transition with increasing temperature implies an unusual partial thermal population of the low-spin state of up to about 10% at room temperature. Finally, high-spin → low-spin relaxation curves following pulsed laser excitation for [Co(terpy)₂](ClO₄)₂ dispersed in KBr discs, and as a comparison for the closely related [Co(4-terpyridone)₂](ClO₄)₂ spin-crossover compound are given.     

Electron transfer between plastocyanin and benzimidazolic coordination compounds in DMSO-H2O

The electron transfer reactions between several chromium(III), iron(III) and cobalt(III) coordination compounds and spinach plastocyanin (PC) were studied. The ligands coordinated to the metal ions are derivatives of benzimidazole. Kinetic studies were carried out in dimethyl sulphoxide-H₂O (25:75%) and the reaction mechanism is discussed. For comparison with previous studies, the reaction of [Co(phen)₃]³⁺ with PC was studied both in aqueous buffer solution and in dimethyl sulphoxide-H₂O (25:75%); the results indicated that the electron transfer is accelerated in reactions carried out in the latter medium. [Fe(2gb)₃](NO₃)₃, 2gb=2-guanidinobenzimidazole, and [Fe(ntb)Cl₂]Cl, ntb=tris(benzimidazolyl)methylamine oxidised PC following a simple second order outer sphere mechanism. The rate constants for electron transfer are 1.4×10⁴±1.1×10² and 706.2±12.7 M⁻¹ s⁻¹, respectively. Cobalt(III) and chromium(III) benzimidazolic compounds behaved as inhibitors to the electron transfer process. NMR studies indicated that the conformation of the protein does not change in DMSO-H₂O (25:75% v/v) when compared with that in aqueous buffer.     

Anaerobic oxidation of cysteine to cystine by manganese(III) in aqueous acetic acid

The anaerobic oxidation of cysteine, Cys, by Mn(III) in acetic acid solutions has been followed by use of a stopped-flow spectrophotometric method at a temperature of 20 °C. The formation and disappearance of the [Mn(OAc)₂Cys]⁻ complex was monitored at 350 nm. The rate depends strongly on the acetic acid concentration (and hence also on pH) and led to the conclusion that more than one cysteine-containing species was involved. These mono-cysteinyl complexes are formed by the loss of two protons from the cysteine - one from the - SH and the other from either the -NH₃⁺ or, more likely, the -COOH which is partially protonated at the low pH values involved (0.5-2.5). The rate-determining reprotonation of the bound -COO⁻ (or -NH₂) is then accompanied by internal electron transfer yielding Mn(II) and the cysteinyl radical, Cys•, which then dimerises to form (inactive) cystine. At high acetic acid concentrations (60-90% AcOH) the tris-acetato species, [Mn(OAc)₃], predominates together with some of the bis-complex, [Mn(OAc)₂]⁺, and the active species is [Mn(OAc)₂Cys]⁻ which decomposes with a rate constant of k₂=16.8±0.9 M⁻¹ s⁻¹. At low acetic acid concentrations (20-30% AcOH) the mono-acetato species predominates and the reactive species is [Mn(OH)Cys] for which the rate of decomposition=k₂^′=(1.32±0.11)×10⁴ M⁻¹ s⁻¹. The relative values of the rate constants obtained are discussed, as is the bonding of cysteine to manganese(III).     

Synthesis and characterization of Pt(II) and Pt(II)/Ru(II) complexes with the bidentate bridging ligand dipyrido(2,3-a:3′,2′-h)phenazine (dpop)

Three new complexes [Pt(dpop)(Cl)₂], [(Cl)₂Pt(dpop)Pt(Cl)₂] and [(bpy)₂Ru(dpop)Pt(Cl)₂](PF₆)₂ (dpop = dipyrido(2,3-a:3′,2′-h)phenazine) were prepared and studied. The electronic absorption spectra of the complexes display Pt dπ → dpop π* and Ru dπ → dpop π* MLCT transitions at longer wavelengths than for previously reported similar complexes. Results of cyclic voltammograms show reversible dpop centered reductions while for the mixed metal [(bpy)₂Ru(dpop)Pt(Cl)₂]²⁺ an irreversible Pt(II) oxidative wave precedes the Ru(II) oxidation/reduction couple. Spectroelectrochemical results show that all oxidative and reductive processes are completely reversible. The [(Cl)₂Pt(dpop)Pt(Cl)₂] complex cleaves in solution with pseudo-first order kinetics resulting in loss of the Pt dπ → dpop π* MLCT transition at 545 nm.     

Synthesis and characterization of Os(II)(dpop′) (dpop′ = dipyrido(2,3-a;3′,2′-j)phenazine) complexes with 2,2′-bipyridine(bpy); 2,2′-bipyrimidine(bpm) and 2,3-bis(2-pyridyl)pyrazine(dpp)

New Os(II) complexes including [Os(dpop′)₂](PF₆)₂ (dpop′= dipyrido(2,3-a;3′,2′-j)phenazine) and a series of mixed ligand [Os(dpop′)(N-N)Cl]PF₆ (N-N = 2,2′-bipyridine(bpy); 2,2′-bipyrimidine(bpm) and 2,3-bis(2-pyridyl)pyrazine(dpp)) were synthesized. The Os dπ → dpop′ π^∗ MLCT transitions for [Os(dpop′)₂]²⁺ are observed at lower energy than for Os dπ → tpy π^∗ (tpy = 2,2′:6′,2″-terpyridine) and Os dπ → tppz π^∗ (tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) (The ligand abbreviations tpd, tpp and tpypz have also appeared in the literature for 2,3,5,6- tetrakis(2-pyridyl)pyrazine in addition to tppz.) MLCT transitions in the comparative [Os(tpy)₂]²⁺ and [Os(tppz)₂]²⁺ complexes. The Os dπ → dpop′ π^∗ MLCT transitions are observed at lower energy in mixed bidentate ligand N-N systems compared with [Os(dpop′)₂]²⁺. Cyclic voltammetry shows more positive osmium oxidation, and less negative ligand reduction potentials for [Os(dpop′)₂]²⁺ as compared to [Os(tpy)₂]²⁺ and [Os(tppz)₂]²⁺ complexes. The osmium oxidation potentials in mixed ligand [Os(dpop′)(N-N)Cl]⁺ complexes are at less positive potential than for the [Os(dpop′)₂]²⁺ ion. NMR results show different chemical shifts for ring protons either trans or cis to dpop′ in mixed ligand systems, and also show two geometrical isomers for the [Os(dpop′)(dpp)Cl]⁺ complex. The [Os(dpop′)(dpp)Cl]⁺ geometric isomer with the pyrazine ring of dpp trans to dpop′ is found more predominate by 1.0/0.7 over the isomer with the pyrazine ring of dpp cis to dpop′ and that inter-conversion of geometric isomers does not occur in room temperature solution on the NMR timescale.     

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.     

Photophysics of ruthenium(II) complexes carrying amino acids in the ligand 2,2′-bipyridine and intramolecular electron transfer from methionine to photogenerated Ru(III)

New ruthenium(II) complexes carrying methionine and phenylalanine in the bipyridine ligand, [Ru(bpy)₂(4-Me-4′-(CONH-l-methionine methyl ester)-2,2′-bipyridine)](PF₆)₂ (IV) and [Ru(bpy)₂(4-Me-4′-(CONH-l-phenylalanine ethyl ester)-2,2′-bpy)](PF₆)₂(V) have been synthesized and characterized and their photophysical properties studied. Flash photolysis measurements of complex IV, in the presence of an electron acceptor, methyl viologen (MV²⁺) show that an intermolecular electron transfer from the excited state of Ru(II) in complex IV, to MV²⁺ takes place, forming Ru(III) and the methyl viologen cation radical, MV⁺. The formation of MV⁺ in this system is confirmed using time-resolved transient absorption spectroscopy. This intermolecular electron transfer is followed by intramolecular electron transfer from the thioether moiety (methionine) to the photogenerated Ru(III), regenerating Ru(II).     

Kinetics and mechanism of the complex formation of [Pd(NNN)Cl] with pyridines in methanol: synthesis and crystal structure of [Pd(terpy)(py)](ClO4)2

The kinetics of the complex-formation reactions between monofunctional palladium(II) complexes [Pd(NNN)Cl]⁺, where NNN is 2,2^′:6^′,2″-terpyridine (terpy), diethylenetriamine (dien) or bis(2-pyridylmethyl)amine (bpma), with pyridine, 4-methylpyridine, 4-acetylpyridine, 4-cyanopyridine and 4-aminopyridine, have been studied in methanol at 25 °C using stopped-flow spectrophotometry. The highest reactivity was observed for the [Pd(terpy)Cl]⁺ complex, whereas 4-aminopyridine is the strongest nucleophile. The results, compared with those previously published on the [Pt(NNN)Cl]⁺ complexes, are discussed in terms of reactivity and discrimination ability of the reaction centre. The crystal structure of [Pd(terpy)(py)](ClO₄)₂ has been determined by X-ray diffraction. Crystals are triclinic, space group , and consist of distorted square planar [Pd(terpy)(py)]²⁺ cations and perchlorate anions. The Pd-N bond length to the central atom of terpy ligand is well below 2.0 Å and significantly shorter than any of the other M-N distances. The pyridine plane forms a dihedral angle of 61.9(2)° with the coordination N4 donors.     

Interaction of Pt(II) and Pd(II) complexes of terpyridine with 1-methylazoles: A combined experimental and density functional study

The synthesis and characterization of several complexes of the composition [{M(terpy)}_n(L)](ClO₄)_m (M = Pt, Pd; L = 1-methylimidazole, 1-methyltetrazole, 1-methyltetrazolate; terpy = 2,2′:6′,2″-terpyridine; n = 1, 2; m = 1, 2, 3) is reported and their applicability in terms of a metal-mediated base pair investigated. Reaction of [M(terpy)(H₂O)]²⁺ with 1-methylimidazole leads to [M(terpy)(1-methylimidazole)](ClO₄)₂ (1: M = Pt; 2: M = Pd). The analogous reaction of [Pt(terpy)(H₂O)]²⁺ with 1-methyltetrazole leads to the organometallic compound [Pt(terpy)(1-methyltetrazolate)]ClO₄ (3) in which the aromatic tetrazole proton has been substituted by the platinum moiety. For both platinum(II) and palladium(II), doubly metalated complexes [{M(terpy)}₂(1-methyltetrazolate)](ClO₄)₃ (4: M = Pt; 5: M = Pd) can also be obtained depending on the reaction conditions. In the latter two compounds, the [M(terpy)]²⁺ moieties are coordinated via C5 and N4. X-ray crystal structures of 1, 2, and 3 are reported. In addition, DFT calculations have been carried out to determine the energy difference between fully planar [Pd(mterpy)(L)]²⁺ complexes Ip-IVp (mterpy = 4′-methyl-2,2′:6′,2″-terpyridine; L = 1-methylimidazole-N3 (I), 1-methyl-1,2,4-triazole-N4 (II), 1-methyltetrazole-N3 (III), or 3-methylpyridine-N1 (IV)) and the respective geometry-optimized structures Io-IVo. Whereas this energy difference is larger than 70 kJ mol⁻¹ for compounds I, II, and IV, it amounts to only 0.8 kJ mol⁻¹ for the tetrazole-containing complex III, which is stabilized by two intramolecular C-H?N hydrogen bonds. Of all complexes under investigation, only the terpyridine-metal ion-tetrazole system with N3-coordinated tetrazole appears to be suited for an application in terms of a metal-mediated base pair in a metal-modified oligonucleotide.     

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.     

pH dependence of charge transfer between tryptophan and tyrosine in dipeptides

Time-resolved absorption spectroscopy has been employed to study the directionality and rate of charge transfer in W-Y and Ac-W-Y dipeptides as a function of pH. Excitation with 266-nm nanosecond laser pulses produces both W⋅ (or [⋅WH]⁺, depending on pH) and Y⋅. Between pH 6 and 10, W⋅ to was found to oxidize Y with k_X⋅=9.0×10⁴ s⁻¹ and 1.8×10⁴ s⁻¹ for the W-Y and Ac-W-Y dipeptide systems, respectively. The intramolecular charge transfer rate increases as the pH is lowered over the range 6>pH>2. For 10<pH<12, the rate of radical transport for the W-Y dipeptide decreases and becomes convoluted with other radical decay processes, the timescales of which have been identified in studies of control dipeptides Ac-F-Y and W-F. Further increases in pH prompt the reverse reaction to occur, W-Y⋅→W⋅-Y⁻ (Y⁻, tyrosinate anion), with a rate constant of k_X⋅=1.2×10⁵ s⁻¹. The dependence of charge transfer directionality between W and Y on pH is important to the enzymatic function of several model and natural biological systems as discussed here for ribonucleotide reductase.