Arene transition metal chemistry : III. Arene exchange phenomena

Arene exchange between free arene and an arene—transition metal complex is reviewed with respect to structure, thermodynamics, kinetics and reaction mechanism. Catalysis of such arene exchange is also examined. Experimental results are presented for arene exchange between arene and the arene—metal complexes: η⁶-arene-1,5-cyclooctadieneruthenium, the cationic η⁶-arene-1,5-cyclooctadieneiridium and η⁶-arenetricarbonylmolybdenum. Mechanistic features of these exchange reactions are discussed.     

Hydrogen-deuterium exchange reactions catalyzed by zerovalent phosphinepalladium complexes

Complexes of the type [Pd(PR₃)_n] (n = 3 or 4), which may be conveniently generated in situ, catalyze hydrogen-deuterium exchange reactions between D₂O and various organic substrates, at ambient temperature. The reactions are most rapid for nitroalkanes, where exchange occurs exclusively at the C(1) position and allows the isolation of specifically deuterium-labeled compounds.     

Application of iridium pincer complexes in hydrogen isotope exchange reactions

Iridium pincer complex catalyzed hydrogen to deuterium exchange could be achieved using aromatic and heteroaromatic substrates. The reactions proceed under mild conditions and with high regioselectivity. The efficiency of the hydrogen isotope exchange reaction depends on the electronic properties of the pincer complex catalyst.     

Arene complexes of beta-diketiminato supported organoscandium cations: mechanism of arene exchange and alkyne insertion in solvent separated ion pairs

A family of isolable solvent separated organoscandium methyl cations stabilized by beta-diketiminato ligands (Ar)NC(CH3)CHC(CH3)N(Ar) (Ar=2,6-iPr-C6H3, LMe) has been prepared by reaction of LMeScR2 with [CPh3][B(C6F5)4] in the presence of an arene solvent. Arenes such as bromobenzene, benzene, toluene, para-xylene and mesitylene bind the scandium center in an eta6-bonding mode, yielding cations 1 a-e. Their solution and solid-state structures have been explored using multinuclear NMR spectroscopy and X-ray crystallography. Mechanistic studies on arene exchange reactions and the insertion of diphenylacetylene indicate that these processes occur via arene intermediates of lower hapticity, followed by binding of the incoming reagent. Which of the two steps is rate limiting depends on the arene being displaced and/or the nature of the incoming substrate. The experiments present a unified view of these mechanisms, which have relevance to propagation processes in olefin polymerizations mediated by such cations.     

Spin coherence in crystalline complexes. Naphthalene: 2(1,4-diiodotetrafluorobenzene)

The weak crystalline complex of naphthalene : 2(1,4-diiodotetrafluorobenzene), which exhibits naphthalene phosphorescence with a lifetime in milliseconds, is conveniently investigated by a combination of conventional and pulse techniques of optically detected magnetic resonance. The D + E and D ? E resonances, investigated here, reveal no transferred hyperfine interaction, but increased D and E values because of the external spin-orbit coupling which is found to be anisotropic. Measurements are made of T₂, T₂*, and T₂? for this complex. This coherence study finds T₂ to be similar in magnitude to what has been observed for mixed organic crystals. T₂*, when compared with T₂, confirms the earlier suggestion that the crystalline complex is highly ordered. T_2? measurement suggests interaction with the nuclear spins of C₆I₂F₄.     

Ortho-Metallation reactions V. Reactions of rhodium complexes with azobenzene

Hydrated rhodium(III) chloride reacts with azobenzene (HAzb) affording RhCl₃(PhNH₂)₂ and the dimeric [(Azb)₂RhCl]₂. The latter reacts with donor ligands to give (Azb)₂RhCl(L), (L=PPh₃, tetrahydrofuran). With [Rh(CO)₂Cl]₂, azobenzene affords an unusual Rh^I---Rh^III complex, [(Azb)₂RhCl₂Rh(CO)₂], which can also be obtained from [Rh(CO)₂Cl]₂ and [(Azb)₂RhCl]₂. These complexes contain the ortho-metallated (phenylazo)phenyl-2C,N′ ligand, and their spectroscopic properties are summarised.     

Metal exchange reactions in cadmium complexes with porphyrins of various structures

The reaction kinetics of the metal exchange Cd(II)-Cu(II) and Cd(II)-Zn(II) in the cadmium complexes (CdP) with porphyrin ligands (H₂P) differing by degree of stiffness (tetraphenylporphin, N-methyltetraphenylporphin, and tetraphenyltetrabenzoporphin) in the DMSO medium, was studied using spectrophotometric method. The rate of metal exchange reaction depends on the nature of the non-planatrity of H₂P in the structure of CdP complexes, as well as on the additional screening of the reaction center MN₄ by the extra-ligands and substituents. The reduction of the coordinating ability of the anion X⁻ in the structure of the solvate-salt of incoming metal M’X₂(Solv) _n−2 in a series: acetylacetonate > acetate > chloride > nitrate favors the metal exchange. In the most studied cases the reaction of CdP proceeds along a combined associative-dissociative mechanism. The order of the metal exchange reaction is found to be depending on temperature indicating a change in the contributions of associative and combined routes. The “pure” associative reaction mechanism in a medium of DMSO was for the first time found for the labile complex CdTPTBP with the saddle-type nonplanar ligand.     

Some peculiarities of metal exchange reactions in porphyrin and phthalocyanine complexes

Kinetics of metal exchange reaction Cd(II) Zn(II) and Cd(II) Cu(II) in Cd complexes with tetraphenylporphyrin in DMSO is studied. Reaction with Cu(II) nitrate occurs in both cases more vigorously as compared to that with Zn(II) nitrate. Conditions for metal exchange reactions are studied depending on the nature of metal porphyrinate, a salt (nitrates, acetates, and chlorides of Zn(II), Cu(II), and Co(II), and of organic solvent (DMSO, CH₃CN). It is shown that Zn(II) complexes with nonplanar porphyrins do not show metal exchange Zn(II) Cu(II) or Zn(II) Co(II) under mild conditions in DMSO and CH₃CN.Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 2, 2005, pp. 104–109. Original Russian Text Copyright © 2005 by D. Berezin, Shukhto, Nikolskaya, B. Berezin.     

Magnetic exchange coupling in imido bimetallic uranium(V) complexes. A relativistic DFT study

Magnetic exchange coupling between uranium U(V) ions, in the case of the two para- and meta-imido diuranium complexes, respectively [(C₅H₅)₃U]₂(μ-1,4-N₂C₆H₄) and [(C₅H₅)₃U]₂(μ-1,3-N₂C₆H₄) exhibiting the 5f¹-5f¹ configuration, have been investigated using relativistic DFT calculations, combined with the broken symmetry (BS) approach. Using the B3LYP functional, the singlet BS state of the para complex has been found of lower energy than the high spin (HS) triplet one, in agreement with the observed antiferromagnetic character of the complex. On the contrary the BP86 functional fails to predict this magnetic property. The spin density distributions and MO analysis explain well the antiferromagnetic character of the para complex and clarify the failure of the BP86 functional. The effective participation of the 5f metal orbitals in bonding with the imido ligand plays a key role for electronic and magnetic communication between the two active U(V) 5f¹ electrons. The same technique led us to explain the ferromagnetic character of the meta isomer in agreement with experiment. For both isomers the spin polarization mechanism explains well their observed magnetic behaviour.     

Some new fluorine exchange reactions of perfluoroalkyliodine(V)fluorides

The perfluoroalkyliodine (V) fluorides CF₃IF₄ and CF₃IOF₂ react with many alcohols, acids and anhydrids in solution to give fluorine exchange. In all these reactions we observed equilibria, e.g.:

These reactions depend strongly on the kind of the reacting compounds and on the conditions, and they can be followed by n.m.r. spectroscopy. When trying to isolate the new derivatives always decomposition takes place.These reactions, the n.m.r. spectra, the properties of the new compounds and the decomposition reactions will be reported.     

Investigation of ligand exchange reactions in aqueous uranyl carbonate complexes using computational approaches

Carbonate anion exchange reactions with water in the uranyl-carbonate and calcium-uranyl-carbonate aqueous systems have been investigated using computational methods. Classical molecular dynamics (MD) simulations with the umbrella sampling technique were employed to determine potentials of mean force for the exchange reactions of water and carbonate. The presence of calcium counter-ions is predicted to increase the stability of the uranyl-carbonate species in accordance with previous experimental observations. However, the free energy barrier to carbonate exchange with water is found to be comparable both in the presence and absence of calcium cations. Possible implications of these results for uranyl adsorption on mineral surfaces are discussed. Density functional theory (DFT) calculations were also used to confirm the trends observed in classical molecular dynamics simulations and to corroborate the validity of the potential parameters employed in the MD scheme.     

Catalysis of methyl for halogen exchange reactions in platinum(II) complexes

The symmetrisation reaction between cis-[PtMe₂ (PMe₂Ph)₂] and cis-[PtCl₂-(PMe₂Ph)₂] to give cis-[PtClMe(PMe₂Ph)₂] is catalysed by [Pt₂Cl₂ (μ-Cl)₂-(PMe₂Ph)₂].     

Heterogeneous reactions of solid nickel(II) complexes,V

The kinetics of the thermal decomposition of solid complexes of the type Ni(NCS)₂L₂ (L=pyridine,β-picoline and quinoline), of pseudooctahedral configuration, were studied by using isothermal methods, on the basis of losses of weight, in the temperature range 90–191?. The most suitable reaction order for all the complexes under investigation was found to ben=2/3, i.e. the total decomposition rate is determined by the chemical process proper. The calculated values ofE _a(in kcal · mole^?1) decrease in the following order: Ni(NCS)₂py₂ (29.4)>Ni(NCS)₂(β-pic)₂ (27.6)>Ni(NCS)₂Q₂ (24.3). With increasing volume of the ligand L the reaction rate also increases, and this suggests that the reaction proceeds by dissociative activation. For all the investigated complexes it was found that δH>E _A; this may be explained by a several-step mechanism and the complex Ni(NCS)₂L is then considered an intermediate.     

Effect of chelate dynamics on water exchange reactions of paramagnetic aminopolycarboxylate complexes

Because of our interest in evaluating a possible relationship between complex dynamics and water exchange reactivity, we performed (1)H NMR studies on the paramagnetic aminopolycarboxylate complexes Fe (II)-TMDTA and Fe (II)-CyDTA and their diamagnetic analogues Zn (II)-TMDTA and Zn (II)-CyDTA. Whereas a fast Delta-Lambda isomerization was observed for the TMDTA species, no acetate scrambling between in-plane and out-of-plane positions is accessible for any of the CyDTA complexes because the rigid ligand backbone prevents any configurational changes in the chelate system. In variable-temperature (1)H NMR studies, no evidence of spectral coalescence due to nitrogen inversion was found for any of the complexes in the available temperature range. The TMDTA complexes exhibit the known solution behavior of EDTA, whereas the CyDTA complexes adopt static solution structures. Comparing the exchange kinetics of flexible EDTA-type complexes and static CyDTA complexes appears to be a suitable method for evaluating the effect of ligand dynamics on the overall reactivity. In order to assess information concerning the rates and mechanism of water exchange, we performed variable-temperature and -pressure (17)O NMR studies of Ni (II)-CyDTA, Fe (II)-CyDTA, and Mn (II)-CyDTA. For Ni (II)-CyDTA, no significant effects on line widths or chemical shifts were apparent, indicating either the absence of any chemical exchange or the existence of a very small amount of the water-coordinated complex in solution. For [Fe (II)(CyDTA)(H 2O)] (2-) and [Mn (II)(CyDTA)(H 2O)] (2-), exchange rate constant values of (1.1 +/- 0.3) x 10 (6) and (1.4 +/- 0.2) x 10 (8) s (-1), respectively, at 298 K were determined from fits to resonance-shift and line-broadening data. A relationship between chelate dynamics and reactivity seems to be operative, since the CyDTA complexes exhibited significantly slower reactions than their EDTA counterparts. The variable-pressure (17)O NMR measurements for [Mn (II)(CyDTA)(H 2O)] (2-) yielded an activation volume of +9.4 +/- 0.9 cm (3) mol (-1). The mechanism is reliably assigned as a dissociative interchange (I d) mechanism with a pronounced dissociation of the leaving water molecule in the transition state. In the case of [Fe (II)(CyDTA)(H 2O)] (2-), no suitable experimental conditions for variable-pressure measurements were accessible.     

The reactions of diphenylcarbazide and diphenylcarbazone with cations : Part V. Extraction dissociation constants of the carbazone complexes

Extraction experiments in the system water-toluene on the diphenylcarbazone complexes of Mn(II), Fe(II) and (III). Co(II), Ni(II), Cu(ll), Zn(ll), Cd(II), Hg(I) and (II), Sn(II) and Pb(II) are described. Only uncharged complexes are formed, the formulae of which are for mercury HgD, Hg(HD)₂, Hg₂D and Hg₂(HD)₂ and for the other ions mentioned M(HD)_n, depending on the valence _n of the cation. The extraction dissociation constants, the molar extinction coefficients and the partition coefficients of the complexes funned by the cations studied were obtained, The complexes prove to be far less stable than the corresponding dithizone compounds so that diphenylcarbazone is less suitable for general analytical use than its sulphur analogue.