Browsing by Author "Reddy, D."

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    (1H-Pyrazole-[kappa]N2)(2,2':6',2''-ter­pyridine-[kappa]3N,N',N'')platinum(II) bis­(perchlorate) nitro­methane monosolvate
    (Structural Chemistry, 2011) Akerman, M.; Akerman, K.; Jaganyi , D.; Reddy, D.
    The reaction between [PtCl(terpy)]·2H2O (terpy is 2,2':6',2''-terpyridine) and pyrazole in the presence of two equivalents of AgClO4 in nitro­methane yields the title compound, [Pt(C3H4N2)(C15H11N3)](ClO4)2·CH3NO2, as a yellow crystalline solid. Single-crystal X-ray diffraction shows that the dicationic platinum(II) chelate is square planar with the terpyridine ligand occupying three sites and the pyrazole ligand occupying the fourth. The torsion angle subtended by the pyrazole ring relative to the terpyridine chelate is 62.4 (6)°. Density functional theory calculations at the LANL2DZ/PBE1PBE level of theory show that in vacuo the lowest-energy conformation has the pyrazole ligand in an orientation perpendicular to the terpyridine ligand (i.e. 90°). Seemingly, the stability gained by the formation of hydrogen bonds between the pyrazole NH group and the perchlorate anion in the solid-state structure is sufficient for the chelate to adopt a higher-energy conformation.
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    A kinetic and mechanistic study into the substitution behaviour of platinum(ii) polypyridyl complexes with a series of azole ligands
    (Dalton Transactions, 2013-04-04) Shaira, A.; Reddy, D.; Jaganyi, D.
    The kinetics of chloride substitution from a series of square-planar platinum(II) complexes, viz. [Pt(terpyridine)Cl]+, (Pt1), [Pt{2-(2′-pyridyl)-1,10-phenanthroline}Cl]Cl, (Pt2), [Pt{4′-(2′′′-CH3-phenyl)-2,2′:6′,2′′-terpyridine}Cl]CF3SO3 (Pt3) and [Pt{4′-(2′′′-CH3-phenyl)-6-(3′′-isoquinoyl)-2,2′bipyridine}Cl]SbF6 (Pt4) were studied using a series of five-membered heterocyclic neutral nitrogen donor nucleophiles, viz. pyrazole (Pz), triazole (Tz), imidazole (Im), 1-methylimidazole (MIm) and 1,2-dimethylimidazole (DMIm) under pseudo-first-order conditions in methanol using UV/Visible spectrophotometry and conventional stopped-flow techniques. The observed second-order rate constants, k2, followed a two term rate law kobs = k2[nucleophile] + ks except for DMIm with Pt1, Pt3, Pt4 and Pz, Tz with Pt1. Increasing the π-conjugation in the cis position decreases the rate of chloride substitution by decreasing the π-acceptor property of the terpy moiety. However, increasing the π-conjugation in the cis/trans position increases the rate of substitution by enhancing the π-acceptor property within the ligand framework whereby increasing the reactivity of the metal centre. The observed trend for the reactivity was Pt2 > Pt1 > Pt3 > Pt4. The substitution kinetics was influenced by the basicity of the incoming nucleophiles except for the sterically hindered nucleophile, DMIm. The general trend observed for the reactivity of the nucleophiles is MIm > Im > DMIm > Pz > Tz.
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    Controlling the extent of π-backbonding in platinum(ii) terpyridyl systems: a detailed kinetic, mechanistic and computational approach
    (Dalton Transactions, 2008) Reddy, D.; Jaganyi, D.
    The rate of substitution of the chloride ligand from [Pt(terpy)Cl]+ (Pt1) (where terpy = 2,2′:6′,2″-terpyridine) and its corresponding analogue [Pt(tBu3terpy)Cl]+ (Pt2) (where tBu3terpy = 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine) by a series of neutral and anionic nucleophiles, viz.thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU), iodide (I−) and thiocyanate (SCN−), was determined under pseudo first-order conditions as a function of concentration and temperature using standard stopped-flow spectrophotometric techniques. The observed pseudo first-order rate constants for the substitution reactions obeyed the simple rate law kobs = k2[nucleophile]. Second-order kinetics and negative activation entropies support an associative mode of activation. The rate of substitution of chloride is observed to decrease with an increase in the steric bulk of the neutral nucleophiles, whilst rate of substitution by I− was observed to be faster than that by SCN−, in correlation with their polarizability and the softness of the metal centre. A comparison of the second-order rate constants, k2, at 298 K, obtained for the substitution reactions of Pt1 and Pt2 shows that the introduction of strong σ-donating groups on the periphery of the terpyridyl backbone in Pt2 results in a corresponding decrease in the reactivity. DFT calculations at the B3LYP/LACVP** level of theory for the two complexes, Pt1 and Pt2, and a series of similar analogues containing either electron-donating or electron-withdrawing groups in the periphery positions demonstrate that the introduction of electron-donating groups decreases the positive charge on the metal centre and increases energy separation of the frontier molecular orbitals (EHOMO−ELUMO) of the ground state platinum(II) complexes leading to a less reactive metal centre whilst the introduction of electron-withdrawing groups has an opposite effect leading to increased reactivity of the metal centre.
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    Influence of the bridging azine ligand on the rate of ligand substitution in a series of dinuclear platinum(II) complexes
    (International journal of chemical kinetics, 2011-04) Reddy, D.; Jaganyi, D.
    A series of azine-bridged dinuclear platinum(II) complexes of the type [{trans-Pt(NH3)2(OH2)}2(μ-azn)](ClO4)4 (where azn = pyrazine (pzn, Pt1), 2,3-dimethylpyrazine (2,3-pzn, Pt2), and 2,5-dimethylpyrazine (2,5-pzn, Pt3)) were synthesized to investigate the influence of the bridging azine ligand on the reactivity of the platinum(II) centers. The pKa values of the complexes were determined via acid–base titration, and the rate of substitution of the aqua moiety by a series of neutral nucleophiles, viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), and 1,1,3,3-tetramethyl-2-thiourea (TMTU), was determined under pseudo-first-order conditions as a function of concentration and temperature using standard spectrophotometric techniques. The introduction of the methyl groups to the bridging azine linker in Pt2 and Pt3 leads to a moderate increase in the pKa values obtained for the first and second deprotonation steps, respectively, as a result of the increased σ-donor capacity of the bridging azine ligand trans to the aqua moiety. A comparison of the rate constants, k1 and k2, at 298 K, obtained for the substitution of the aqua moieties from Pt1, Pt2, and Pt3 by TU, shows that the introduction of the σ-donating methyl groups on the bridging azine ligand in Pt2 and Pt3 results in a corresponding decrease in the reactivity, by ca. five times for the first substitution step and ca. 10 times for the second substitution step. Density functional theory calculations at the B3LYP/LACVP** level of theory for the complexes demonstrate that the introduction of electron-donating methyl groups results in (i) increased steric hindrance over the metal centers and (ii) decreased the positive charge on the metal center and increases energy separation of the frontier molecular orbitals (EHOMO – ELUMO) of the ground-state platinum(II) complexes, leading to a less-reactive metal center. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 161–174, 2011