Variable-Temperature and -Pressure Kinetics and Mechanism of the Cyclopalladation Reaction of Imines in Aprotic Solvent

The substitution reactions of sulfide by phosphines on Pt(IV) complexes having a cyclometalated imine ligand, two methyl groups in a cis geometrical arrangement, and a halogen and a sulfide as ligands, [Pt (Me)(2)X(CN)(SR2)], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines, sulfides, X-1 and CN. In most of these cases, a limiting dissociative mechanism has been found, where the dissociation of the sulfide ligand corresponds to the rate-determining step. The intermediate species formed behaves as a true pentacoordinated Pt(IV) compound in a steady-state concentration only for the systems with SMe2; for the bulkier SEt2 and SBzl(2) leaving ligands the rate constants and activation parameters show an important degree of solvent dependence, which correlates with the ability of the solvent to form hydrogen bonds, The X-ray crystal structure of one of the dibenzyl sulfide complexes has been determined, and the geometrical arrangement of the ligands has been determined by NOE NMR measurements at low temperature. The nature of the solvent, imine, sulfide, and halogen ligands produces differences in the reaction rates, which can be quantified very well by the corresponding AS' values that move from +48 to -90 J K-1 mol(-1). The reaction on [Pt(Me)(2)F(C-5(C) under barF(4)CH (N) under bar CH2Ph) (SME2)] has been found to take place via a mechanism that depends strongly on the bulkiness of the substituting phosphine, While for PCy3 the reaction is dissociative, for smaller entering ligands the first associatively activated substitution mechanisms on organometallic Pt(IV) complexes have been established with values of DeltaH(double dagger) and DeltaS(double dagger) in the 28-44 kJ mol(-1) and -120 to -83 J K-1 mol(-1) ranges. Important intramolecular hydrogen bonding in the starting material can be held responsible for this difference with the remaining systems.

Substitution reactions on cyclometalated Pt(IV) complexes. Associative tuning by fluoro ligands and fluorinated substituents.

The outer-sphere redox behaviour of a series of [LnCoIII-NCFeII(CN)(5)](-) (L-n = n-membered pentadentate aza-macrocycle) complexes have been studied as a function of pH and oxidising agent. All the dinuclear complexes show a double protonation process at pH approximate to 2 that produces a shift in their UV/Vis spectra. Oxidation of the different non-protonated and diprotonated complexes has been carried out with peroxodisulfate, and of the non-protonated complexes also with trisoxalatocobaltate(III). The results are in agreement with predictions from the Marcus theory. The oxidation of [Fe(phen)(3)](3+) and [IrCl6](2-) is too fast to be measured, although for the latter the transient observation of the process has been achieved at pH = 0. The study of the kinetics of the outer-sphere redox process, with the S2O82- and [Co(ox)(3)](3-) oxidants, has been carried out as a function of pH, temperature, and pressure. As a whole, the values found for the activation volumes, entropies, and enthalpies are in the following margins, for the diprotonated and non-protonated dinuclear complexes, respectively: DeltaV(not equal) from 11 to 13 and 15 to 20 cm(3) mol(-1); DeltaS(not equal) from 110 to 30 and -60 to -90 J K-1 mol(-1); DeltaH(not equal) from 115 to 80 and 50 to 65 kJ.mol(-1). The thermal activation parameters are clearly dominated by the electrostriction occurring on outer-sphere precursor formation, while the trends found for the values of the volume of activation indicate an important degree of tuning due to the charge distribution during the electron transfer process. The special arrangement on the amine ligands in the isomer trans[(L14CoNCFeII)-N-III(CN)(5)](-) accounts for important differences in solvent-assisted hydrogen bonding occurring within the outer-sphere redox process, as has been established in redox reactions of similar compounds. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).

Discrete Cyanide‐Bridged Mixed‐Valence Co/Fe Complexes: Outer‐Sphere Redox Behaviour

Substitution reactions of the type trans-[FeH(H2)(DPPE)2]+ + L → trans-[FeHL(DPPE)2]+ + H2 (L = MeCN, PhCN, DMSO) occur in a single measurable kinetic step. Although the observed rate constants, ko...

Unexpected Mechanism for Substitution of Coordinated Dihydrogen in trans-[FeH(H2)(DPPE)2]+

The kinetics and mechanism of the redox reaction between [FeII(CN)6]4− and the macrocyclic ligand complex [CrClL15]2+ (L15 = 6-methyl-1,4,8,12-tetraazacyclopentadecane-6-amine) has been studied at different pH values. In acidic solution, the expected redox process occurs with no formation of any of the possible CrIII/FeII mixed valence complexes, as those seen for the CoIII species of the same family, due to the enhanced lability of the CrII species formed on FeII to FeIII oxidation. In alkaline conditions, the formation of the complex [Cr(L15)(OH)2]+ takes place as an initial step that precedes a simple substitution process producing the expected cyano-bridged CrIII/FeII complex. In this species the potentially pentadentate ligand, L15, has a tetradentate coordination mode with a protonated exocyclic primary amine group and the redox potential is shifted to more negative values, thus disfavouring a redox driven reaction; the equivalent complex [CrCl(HL14)(H2O)]3+ (L14 = 6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine) has been prepared by the same method and characterized by X-ray crystallography. The final [FeII(CN)6]4− substituted complex, [{(HL15)(OH)CrIIINC}FeII(CN)5]− shows pKa values of 3.8 and 7.4, as expected for the aqua and amino ligands, respectively. Its characterization indicated its Class II mixed valence character with a very intense MMCT band at 350 nm showing a much larger extinction coefficient than that observed for the CoIII complexes of the same family. This fact is in good agreement with the much larger CrIII–FeII (t2g–t2g) coupling through cyanide bridging ligands expected for these complexes. The fully mixed metal/valence/ligand trimetallic complex [{(HL15)(OH)CrIIINC}{L13CoIIINC}FeII(CN)4]2+ has been prepared following the same procedures and the results are comparable. The final complex has the same Class II mixed valence character and its electronic spectrum shows the characteristics of both the FeII-to-CrIII and FeII-to-CoIII CT bands. The study allows the application of the redox and/or substitutionally driven methodology, already described and kinetically designed, for the preparation of discrete mixed valence complexes of inert centres, so avoiding the unpredictable nature of other approaches seen in the past.

https://espace.library.uq.edu.au/view/UQ:185766/UQ185766_OA.pdf

Mechanistic aspects of the chemistry of mononuclear Cr(III) complexes with pendant-arm macrocyclic ligands and formation of discrete Cr(III)/Fe(II) and Cr(III)/Fe(II)/Co(III) cyano-bridged mixed valence compounds.

The activation volumes were obtained for the aquation reactions of [M(RNH2)5L]3+(M = Cr, R = Me, L = Me2SO, HCONMe2 or MeCONMe2; M = Rh, R = H or Me, L = Me2SO or HCONMe2); they are in accord with the existence of a dissociative shift in the substitution mechanism, not only with the size of the substituent R, but also with the size of the leaving ligands L.

Dalton communications. Effect of amine substituents and neutral leaving groups on the activation volume for aquation of octahedral pentaamine complexes of CrIII and RhIII

Elisa Rodríguez

Papers

Dalton communications. Effect of amine substituents and neutral leaving groups on the activation volume for aquation of octahedral pentaamine complexes of CrIII and RhIII

Effect of amine substituents and neutral leaving groups on the activation volume for aquation of octahedral pentaamine complexes of criii and rhiii

Steric Hindrance in Substitution Reactions on Arsenic Acid by Pentaam(m)ine Complexes of CoIII and CrIII