Allen Mambanda

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.

Synthesis, characterization and electrocatalytic behavior of cobalt and iron phthalocyanines bearing chromone or coumarin substituents

Cobalt and iron phthalocyanines (Pcs) bearing peripherally tetra-substituted chromone (chr) or coumarin (cou) moieties were formulated and characterized by UV–Vis and FTIR spectroscopy, ESI-TOF mass spectrometry, and elemental analysis. The structural elucidations of the ligands, 4-(chromone-7-oxy)phthalonitrile (1) and 4-(4-(trifluoromethyl)-coumarin-7-oxy)phthalonitrile (2) were complemented by NMR spectroscopy and single crystal X-ray analysis (for 1). The redox properties of the complexes were investigated via voltammetry and the subsequent voltammetric assignments were corroborated by UV–Vis spectroelectrochemistry. Each metal complex displayed four redox processes of which their Pc ring oxidations are irreversible and the remaining redox couples are quasi-reversible. Utilizing the respective metallophthalocyanines, modified working electrodes were prepared by electropolymerization and their electrocatalytic activities toward nitrite oxidation were explored. All the metal complexes showed an increase in nitrite oxidation currents and a minor decrease in oxidation potentials which is indicative of electrocatalysis. The trend of electrocatalytic activity was found to be as follows: CoPc–chr (3) > FePc–cou (4) > CoPc–cou (5).

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

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.

Role of a 2,3-bis(pyridyl)pyrazinyl chelate bridging ligand in the reactivity of Ru(II)–Pt(II) dinuclear complexes on the substitution of chlorides by thiourea nucleophiles – a kinetic study

Chloride substitution from [(1,10-phenanthroline)2Ru(II)(μ-2,3-bis(2-pyridyl)pyrazine)Pt(II)dichloride]2+ (RuPt1), [(1,10-phenanthroline)2Ru(II)(μ-2,3-bis(2-pyridyl)quinoxaline)Pt(II)dichloride]2+ (RuPt2) and [(1,10-phenanthroline)2Ru(II)(μ-2,3-bis(2-pyridyl)benzo[g]quinoxaline)Pt(II)dichloride]2+ (RuPt3) by thiourea (TU), 1,3-dimethyl-2-thioura (DMTU) and 1,1,3,3-tetra methyl-2-thiourea (TMTU) was studied in a methanol medium (I = 0.10 M) under pseudo-first-order conditions. The rate of substitution was investigated as a function of concentration of nucleophile and temperature using the stopped-flow technique. Two consecutive substitution steps were observed. The first and fastest step was ascribed to the simultaneous substitution of the two chloride co-ligands by incoming nucleophiles according to the rate law: k1stobs = k1st2[Nu]. The subsequent step was assigned to the dechelation of the rigid 2,3-bis(pyridyl)pyrazinyl bridging ligand from the Pt(II) centres of the substituted intermediates to give Pt(Nu)42+ and (phen)2Ru(II)(2,3-bis(pyridyl)pyrazinyl) groups as products. The rate law for this step is k2ndobs = k2nd2[Nu] + k2nd−2. The second-order kinetics and large negative entropies for both steps support an associative mechanism of substitution. The rate of chloride substitution was RuPt1 ≪ RuPt2 DMTU > TMTU, in accordance with their steric bulk.

N,N-Bis(2-pyridylmeth-yl)-tert-butyl-amine.

In the title compound, C16H21N3, the dihedral angle between the two pyridine rings is 88.11 (9)°. In the crystal, molecules are linked through intermolecular C—H⋯π interactions, forming a layer expanding parallel to the (10) plane.

N,N-Bis(pyridin-2-ylmeth­yl)cyclo­hexa­namine

The pyridine rings of the title compound, C18H23N3, are in a nearly perpendicular orientation relative to the plane defined by the three amino-bonded C atoms, making dihedral angles of 87.4 (1) ° and 84.2 (1) °. One of the pyridine N atoms acts as an hydrogen-bond acceptor for two pyridine C—H groups. By means of these intermolecular hydrogen bonds, the molecules form a two-dimensional network parallel to the ab plane.