Browsing by Author "Reddy, Desigan."

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    A kinetic investigation into the rate of chloride substitution from chloro terpyridine platinum(II) and analogous complexes by a series of azole nucleophiles
    (Transition Metal Chemistry, 2011-06-25) Reddy, Desigan.; Akerman, Kate J.; Akerman, Matthew P.; Jaganyi, Deogratius.
    The substitution kinetics of the complexes [Pt(terpy)Cl]Cl·2H2O (PtL1), [Pt(tBu3terpy)Cl]ClO4 (PtL2), [Pt{4′-(2′′′-CH3-Ph)terpy}Cl]BF4 (PtL3), [Pt{4′-(2′′′-CF3-Ph)terpy}Cl]CF3SO3 (PtL4), [Pt{4′-(2′′′-CF3-Ph)-6-Ph-bipy}Cl] (PtL5) and [Pt{4′-(2′′′-CH3-Ph)-6-2′′-pyrazinyl-2,2′-bipy}Cl]CF3SO3 (PtL6) with the nucleophiles imidazole (Im), 1-methylimidazole (MIm), 1,2-dimethylimidazole (DIm), pyrazole (Pyz) and 1,2,4-triazole (Trz) were investigated in a methanolic solution of constant ionic strength. Substitution of the chloride ligand from the metal complexes by the nucleophiles was investigated as a function of nucleophile concentration and temperature under pseudo first-order conditions using UV/Visible and stopped-flow spectrophotometric techniques. The reactions follow the rate law . The results indicate that changing the nature or distance of influence of the substituents on the terpy moiety affects the π-back-donation ability of the chelate. This in turn controls the electrophilicity of the metal centre and hence its reactivity. Electron-donating groups decrease the reactivity of the metal centre, while electron-withdrawing groups increase the reactivity. Placing a strong σ-donor cis to the leaving group greatly decreases the reactivity of the complex, while the addition of a good π-acceptor group significantly enhances the reactivity. The results indicate that the metal is activated differently by changing the surrounding atoms even though they are part of a conjugated system. It is also evident that substituents in the cis position activate the metal centre differently to those in the trans position. The kinetic results are supported by DFT calculations, which show that the metal centre is less electrophilic when a strong σ-donor is cis to the leaving group and more electrophilic when a good π-acceptor group is part of the ring moiety. The temperature dependence studies support an associative mode of activation. An X-ray crystal structure of Pyz bound to PtL3 was obtained and confirmed the results of the DFT calculations as to the preferred N-atom as a binding site.
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    Electrostatic repulsion between the cations of (1-methyl-1H-imidazole-κN3)(2,2′:6′,2′′-terpyridine-κ3N,N′,N′′)platinum(II) perchlorate nitro­methane monosolvate prevents Pt⋯Pt inter­actions
    (Crystallographic Communications, 2011-08) Akerman, Matthew.; Akerman, Kate.; Jaganyi, Deogratius.; Reddy, Desigan.
    The reaction between [Pt(terpy)Cl]·2H2O (terpy = 2′,2′′:6′,2′′-terpyridine) and 1-methyl­imidazole (MIm) in the presence of two equivalents of AgClO4 in nitro­methane yields the title compound, [Pt(C15H11N3)(C4H6N2)](ClO4)2·CH3NO2. The dicationic complexes are arranged in a staggered configuration. The torsion angle subtended by the 1-methyl­imidazole ring relative to the terpyridine ring is 114.9 (5)°. Inter­molecular C—H⋯O inter­actions between the perchlorate anions and the H atoms of the terpy ligand are observed. Consideration of related phenyl­bipyridyl complexes of platinum(II), which are monocationic, leads to the conclusion that the electrostatic repulsion between the dicationic chelates prevents the formation of Pt⋯Pt inter­actions. These inter­actions are a common feature associated with the monocationic species.
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    Understanding the role of flexible alkyl-α,ω-diamine linkers on the substitution behaviour of dinuclear trans-platinum(II) complexes: A kinetic and mechanistic study
    (Inorganica Chimica Acta, 2021-08-01) Olusegun, Moses Ariyo.; Reddy, Desigan.; Jaganyi, Deogratius.
    An investigation of the role of flexible alkyl-α,ω-diamine linkers on the substitution behaviour of dinuclear trans-platinum(II) complexes. The substitution reactions of four dinuclear trans-platinum(II) complexes viz. [PtNH3Cl2]2-µ-NH2(CH2)2NH2] (P12), [PtNH3Cl2]2-µ-NH2(CH2)3NH2] (P13), [PtNH3Cl2]2-µ-NH2(CH2)4NH2] (P14) and [PtNH3Cl2]2-µ-NH2(CH2)5NH2] (P15) with three neutral thiourea-based nucleophiles specifically: thiourea (TU), N-methyl-2-thiourea (MTU) and N,N-dimethyl-2-thiourea (DMTU) were studied quantitatively under pseudo-first-order condition as function of concentration and temperature by conventional UV–Visible and stopped-flow spectrophotometers. The ligand substitution reaction of the complexes proceeds in veritably three consecutive steps. Each step follows first-order kinetics with the respective complex and nucleophile. The pseudo first order rate constants, kobs(1/2/3), for sequential substitution of the chlorido ligands, the ammine, and subsequent displacement of the linker obeyed the rate law: kobs(1/2/3) = k(1/2/3)[NU]. The ligand substitution reactions were driven by both electronic and steric factors. However, our findings revealed that upon the substitution of the chlorido ligands by the nucleophiles at the platinum centres, the σ-donor capacity via inductive effect of these electron-rich nucleophiles over compensate the steric strain imposed by the nucleophiles and by the alkanediamine linker at the substitution sites. Consequently, electronic factors governed the overall reaction pattern of these complexes. 195Pt NMR results confirmed the simultaneous substitution of all the chlorido ligands by thiourea-based nucleophiles, followed by the subsequent but successive displacement of the ammine groups and the flexible alkanediamine linker from the metal centres. The order of reactivity of the nucleophiles with the complexes decreases with the increase in steric bulk in the nucleophiles: TU > MTU > DMTU. The small positive enthalpy and the large but negative entropy confirm the associative mode of activation for all the studied complexes. Computational modelling using density functional theory (DFT) calculations was employed to rationalise the kinetic trends.