Deogratius Jaganyi

Substitution of aqua ligands from alkyldiamine-bridged dinuclear Pt(II) complexes using azole nucleophiles

The rate of substitution of aqua ligands in dinuclear Pt(II) complexes, which are bridged by alkyldiamine linkers of variable chain lengths, and their mononuclear Pt(II) analogues were studied under pseudo-first order conditions as a function of concentration and temperature using azoles. The results indicate that substitution of aqua ligands of the dinuclear Pt(II) complexes occurs simultaneously and increases as the alkyl chain length of the diamine bridge increases. Steric hindrance due to the C2h conformational symmetry, whose influence decreases as the length of alkylamine linker increases, appears to be the dominant factor controlling the reactivity of the dinuclear Pt(II) complexes. Prop, a homologue which has a 1,3-propanediamine bridge and a C2v conformation, shows an unusual high reactivity. Weak sigma donicity due to the α,ω-alkyldiamine bridge is evident when the reactivity of dinuclear species is compared to their mononuclear analogues. Mononuclear Pt(II) complexes are more reactive than the dinuclear Pt(II) complexes with their reactivity increasing with increasing chain length of the alkylamine tail. The nucleophilicity of the azoles decreases in the order Pz > Tz > mPz. This is in accord with the basicity of the coordinating nitrogen donor in the case of Pz and Tz, while a steric hindrance to the approach of 1-methylpyrazole due to the ortho N-methyl substituent on the ring is evident for mPz. The substitution of aqua ligands by azoles remains associatively-activated.

Understanding the role of flexible 4′-functionalized polyethylene glycoxy chains on the behavior of platinum(II) (4′-(ethylene glycoxy)-2,2′:6′,2′′-terpyridine: a kinetic and a mechanistic study

The ligand substitution kinetics of 4′-functionalized mononuclear Pt(II) (4′-(ethylene glycoxy)-2,2′:6′,2′′-terpyridine complexes, [Pt(nY-tpy)Cl)Cl] (where Y = ethylene glycoxy, n = number of ethylene, glycoxy units = 1, 2, 3, and 4, and tpy = 2,2′:6′,2′′-terpyridine), with thiourea, 1,3-dimethyl-2-thiourea, 1,1,3,3-tetramethyl-2-thiourea, and iodide were investigated under pseudo-first-order conditions as a function of concentration and temperature by conventional stopped-flow technique. The observed first-order rate constants followed the simple rate law kobs = k2[Nu]. The data obtained show that the ethylene glycoxy pendant, trans to the leaving group, acts as a σ-donor into the terpyridine ligand and is effective only up to n = 1, beyond which the substitution reactivity of the complexes are controlled by the steric influence of the appended ethylene glycoxy pendant units, which decreases with increase in the number of ethylene glycoxy units. The activation parameters obtained support an associative mechanism, where bond formation in the transition state is favored. The observed reactivity trends were supported by density functional theory calculations. Graphical Abstract The polyethylene glycoxy pendant, trans to the leaving group acts as a σ-donor into the terpyridine ligand and is effective only up to n = 1, beyond which the substitution reactivity of the complexes are controlled by the steric influence of the appended ethylene glycoxy pendant units, which decreases with increase in the number of ethylene glycoxy units.

Tuning the π-backbonding and σ-trans effect of N^C^N coordinated Pt(II) complexes. Kinetic and computational study

The nucleophilic substitution reaction of cyclometallated complexes; [PtL2Cl] (L2 = 3,5-di(2-pyridinyl)-fluorobenzene), [PtL3Cl] (L3 = 2,4-di(2-pyridinyl)-fluorobenzene), and [PtL4Cl] (L4 = 3,5-di(2-pyridinyl)-toluene) with a series of neutral nucleophiles with different steric properties, thiourea (TU), N,N-dimethylthiourea (DMTU), and N,N,N′,N′-tetramethylthiourea (TMTU), was studied under pseudo-first-order conditions in methanol solution of an ionic strength of 0.1 M (0.09 M LiCF3SO3 and 0.01 M LiCl). The rate of substitution of the chloro ligand was studied as a function of nucleophile concentration and temperature using UV–visible and stopped-flow spectrophotometric techniques. The observed pseudo-first-order rate constants for the substitution reactions obey the rate law kobs = k2[Nu] + k−2. The reactivity of the investigated complexes when [PtL1Cl] is used as a reference follows the order [PtL2Cl] > [PtL3Cl] > [PtL4Cl] > [PtL1Cl]. The lability of the chloro group is dependent on the extent of π-backbonding and the σ-trans effect of the ligand backbone. [PtL2Cl] and [PtL3Cl], which have a common electron-withdrawing fluoride on the ligand trans to the leaving group, have a higher reaction rate compared to [PtL4Cl], which has an electron-donating methyl group attached to the ligand backbone. The position of the substituent on the phenyl group trans to the leaving group also influences the overlap of frontier molecular orbitals which result in controlling the reactivity of the fluoro complexes. In general, the results show that the nature of the substituent, either electron withdrawing or electron donating, results in an increase in the rate of substitution. Second-order kinetics and large negative activation entropies (ΔS#) support an associative substitution mechanism. The experimental data are supported by DFT calculations. The mechanistic pathway for the substitution reaction of a strong trans labilizing phenyl of N^C^N platinum(II) complexes by biorelevant thiourea nucleophiles go through the solvent associated pathway.

The role of diaminocyclohexane and diaminobenzene linking bridges on the aqua substitution of chelated dinuclear Pt(II) complexes by nitrogen donor heterocycles. A kinetic and mechanistic study

The substitution of the aqua ligands from six Pt(II) complexes, viz., [Pt(H2O)(N,N-bis(2-pyridylmethyl)cyclohexylamine](ClO4)2 (Pt1); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-trans-1,4-cyclohexyldiamine)](ClO4)4 (Pt2); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-methylenedicyclohexyldiamine)](ClO4)4 (Pt3); [Pt(H2O)N,N-bis(2-pyridylmethyl)phenylamine)](ClO4)2 (Pt4); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,4-phenyldiamine](ClO4)4 (Pt5); and [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-methylenediphenyldiamine)](ClO4)4 (Pt6), by nitrogen heterocyclic ligands{viz., pyrazole (Pz); 3-methylpyrazole (mPz); 1,2,4-triazole (Tz) and pyrazine (Pzn)} were studied in an aqueous 0.01 M perchloric acid medium. The substitutions were investigated under pseudo-first-order conditions as a function of the concentration of nucleophiles and reaction temperature using UV–visible spectrophotometry. The substitution of the aqua ligands by all the nitrogen donor heterocycles proceeded via a single step whose rate decreased in the respective orders: Pt1 > Pt3 > Pt2 and Pt4 > Pt6 > Pt5 in the two sets of complexes. Of the nucleophiles used in this study, pyrazine was the most reactive and the complete order of the rate of aqua substitution was Pzn >> Pz > Tz > mPz. The large and negative activation entropies and low but positive enthalpies of activation values support a significant contribution from bond making in the transition state of the substitution process. Graphical Abstract

Aqua substitution from mononuclear Pt(II) complexes with 2-(pyrazol-1-ylmethyl)quinoline non-leaving ligands

Abstract The rates of aqua substitution from [Pt{2-(pyrazol-1-ylmethyl)quinoline}(H2O)2](ClO4)2, [Pt(H2Qn)], [Pt{2-(3,5-dimethylpyrazol-1-ylmethyl)quinoline}(H2O)2](ClO4)2, [Pt(dCH3Qn)], [Pt{2-[(3,5-bis(trifluoromethyl)pyrazol-1-ylmethyl]quinoline}(H2O)2](ClO4)2, [Pt(dCF3Qn)], and [Pt{2-[(3,5-bis(trifluoromethyl)pyrazol-1-ylmethyl]pyridine}(H2O)2](ClO4)2, [Pt(dCF3Py)], with three sulfur donor nucleophiles were studied. The reactions were followed under pseudo-first-order conditions as a function of nucleophile concentration and temperature using a stopped-flow analyzer and UV/visible spectrophotometry. The substitution reactions proceeded sequentially. The second-order rate constants for substituting the aqua ligands in the first substitution step increased in the order Pt(dCH3Qn) < Pt(dCF3Qn) < Pt(H2Qn) < Pt(dCF3Py), while that of the second substitution step was Pt(dCH3Qn) < Pt(dCF3Qn) < Pt(dCF3Py) < Pt(H2Qn). The reactivity trends confirm that the quinoline substructure in the (pyrazolylmethyl)quinoline ligands acts as an apparent donor of electron density toward the metal center rather than being a π-acceptor. Measured pKa values from spectrophotometric acid–base titrations were Pt(H2Qn) (pKa1 = 4.56; pKa2 = 6.32), Pt(dCH3Qn) (pKa1 = 4.88; pKa2 = 6.31), Pt(dCF3Qn) (pKa1 = 4.07; pKa2 = 6.35), and Pt(dCF3Py) (pKa1 = 4.76; pKa2 = 6.27). The activation parameters from the temperature dependence of the second-order rate constants support an associative mechanism of substitution. Graphical Abstract

A kinetic and mechanistic study of dinuclear Pt(II) 2,2′:6′,2″-terpyridine compounds bridged with polyethyleneglycol ether flexible linkers

A series of dinuclear Pt(II) complexes bridged with polyethyleneglycol ether of the type [ClPt(tpy)O(CH2CH2O)n(tpy)PtCl]Cl2 where n = 1 (Ptdteg), 2 (Ptdtdeg), 3 (Ptdtteg), 4 (Ptdttteg), and linker-free complex, (Ptdt) (where tpy = 2,2′:6′,2″-terpyridine), were synthesized and characterized to investigate the role of bridging polyethyleneglycol ether linker on the substitution reactivity of dinuclear Pt(II) complexes. Substitution reactions were studied using thiourea nucleophiles, viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU) under pseudo-first-order conditions as a function of concentration and temperature by conventional stopped-flow reaction analyzer. The reactions gave single exponential fits following the rate law kobs = k2[Nu]. Introduction of polyethyleneglycol ether linker decreases the electrophilicity of the platinum center and the whole complex. The results obtained indicate that the rate of substitution is controlled by both electronic and steric hindrance which increases with the length of the linker. Experimental results are supported by density functional theory calculations and structures obtained at B3LYP/LANL2DZ level. The order of the reactivity of the nucleophiles is TU > DMTU > TMTU. The magnitude and the size of the enthalpy of activation and entropy of activation support an associative mode of mechanism, where bond formation in the transition state is favored. Graphical Abstract Introduction of ethyleneglycol ether linker decreases the electrophilicity of the platinum center and the whole complex. The rate of substitution reactions is controlled by both electronic and steric hindrance which increases with the length of the linker

Oxidation of Methionine by Colloidal MnO2 in Aqueous and Micellar Media: A Kinetic Study

The oxidation of methionine by freshly prepared colloidal manganese dioxide in aqueous as well as micellar media was studied spectrophotometrically at 35°C. The reaction between methionine and MnO2 in both media exhibits 1:1 stoichiometry (methionine:MnO2). The oxidation reaction is first order with regard to the MnO2 concentration, but is fractional-order in the methionine concentration and HClO4 concentrations. A catalytic effect of nonionic surfactant TX-100 on the rate of oxidation was observed and reaction rate was found to be proportional to {k′ + k″ [TX-100]}, where k′ and k″ are the rate constants in absence and presence of surfactant, respectively. The use of surfactant micelles is highlighted as, in favorable cases; the micelles help the redox reactions by bringing the reactants in a close proximity through hydrogen bonding. The oxidation reaction in aqueous and micellar media is shown to proceed via methionine–MnO2 and methionine–MnO2–TX-100 complexes, respectively, which decomposes slowly in a rate determining step to give methionine sulfoxide as the product. A suitable mechanism is proposed for these observations.

Meridional anchorage of coordinate occupancy by a planar tridentate ligand and its effect on ligand substitution reactions of octahedral ruthenium(II) complexes

A kinetic study of the substitution behavior of octahedral [Ru(terpy)(bipy)(OH2)]2+ and [Ru(terpy)(tmen)(OH2)]2+ {terpy = 2,2:6′,2″-terpyridine, bipy = 2,2′-bipyridine and tmen = N,N,N′,N′-tetramethylethylenediamine} with thiourea, 1,3′-dimethyl-2-thiourea, and acetonitrile nucleophiles (Nu) as a function of concentration in pH of 4.0 aqueous media using UV-Vis spectroscopy has been made. The reactions are first order in both the concentration of the Nu and the ruthenium complex in accordance to the two-term rate law k obs = k 2[Nu] + k− 2. The ligand effect of the cis-coordinated bidentates (NN) on the substitutional lability of the aqua leaving group in the [Ru(terpy)(NN)(OH2)]2+ complexes increases in the order: NN = dppro < dopro < phen ≈ bipy < tmen < diox < Me2phen. This order reflects the steric as well as the electronic properties of the bidentate ligand where the meridionally coordinated terpy enacts stereoelectronic rigidity on the bidentate ligand in addition to providing an efficient drainage of electron density at the metal centers. In the tmen complex, the retardation of the incoming groups caused by a dominant cis σ-effect from the tmen toward the metal center controls the rate of the reaction, as a result of the induced weakening of the scorpionatic effect of the steric tmen ligand due to the strong π-repulsive backbone of the meridionally coordinated terpy.

Octahedral iron(II) complexes with pyridyl triazine and bipyridine ligands – synthesis, computational studies, mechanisms and kinetics with 1,10-phenanthroline and 2,2′,6,2″-terpyridine

Abstract [Bis(3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine)(2,2′-bipyridine)iron(II)], [Fe(PDT)2(bpy)]2+ (1), [bis(3-(4-phenyl-2-pyridyl)-5,6-diphenyl-1,2,4-triazine)(2,2′-bipyridine)iron(II)], [Fe(PPDT)2(bpy)]2+ (2), [bis(2,2′-bipyridine)(3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine)iron(II)], [Fe(PDT)(bpy)2]2+ (3), and [bis(2,2′-bipyridine)(3-(4-phenyl-2-pyridyl)-5,6-diphenyl-1,2,4-triazine)iron(II)], [Fe(PPDT)(bpy)2]2+ (4) have been synthesized and characterized. Substitution of the triazine and bipyridine ligands from the complexes by nucleophiles (nu), namely 1,10-phenanthroline (phen) and 2,2′,6,2″-terpyridine (terpy) was studied in a sodium acetate-acetic acid buffer over the pH range 3–6 at 25, 35, and 45°C under pseudo-first order conditions. Reactions are first order in the concentration of complexes 1–4. The reaction rates increase with increasing [nu] and pH whereas ionic strength has no effect on the rate. Straight-line plots with positive slopes are observed when the kobs values are plotted against [nu] or 1/[H+]. The substitution reactions proceed by dissociative as well as associative paths and the latter path is predominant. Observed low Ea values and negative ΔS# values support the dominance of the associative path. Phenyl groups on the triazine ring modulate the reactivity of the complexes. The π-electron cloud on the phenyl rings stabilizes the charge on metal center by inductive donation of electrons toward the metal center, resulting in a decrease in reactivity of the complex and the order is 1 < 2 < 3 < 4. Density functional theory (DFT) calculations also support the interpretations drawn from the kinetic data.

Characterization of Triangular Gold Nanoparticles Using Aloe arborescens Leaf Extract: A Green Synthesis Approach

Herein, the authors show the use of the Aloe arborescens leaf extract for green synthesis of gold nanoparticles in water at room temperature under very mild conditions. The synthesis of the gold nanoparticles was complete in several minutes, and no extra stabilizing or capping agents were necessary. The size of the nanoparticles could be controlled by varying the concentration of the leaf extract. The silver nanoparticles and reaction process were characterized by UV-vis spectrometer, Fourier transform infrared spectroscopy, and transmission electron microscopy. The UV-visible spectra of synthesized nanoparticles showed absorption maxima at 540 nm. Transmission electron microscopy images confirm that the nanoparticles are spherical and triangular in shape.