Oh the mechanism of stereoregulation in the allylnickel complex catalyzed butadiene polymerization

After a brief account of the present knowledge on the mechanism of the allylnickel complex catalyzed stereospecific butadiene polymerization, synthesis and catalytic properties of the cationic allylnickel(II) complexes, [C₃H₅NiL₂]PF₆ (L = P(OAlk)₃, P(OAr)₃, PPh₃, AsPh₃, SbPh₃, MeCN, Me₃CNC, 1,5–COD), are described. From kinetic and ³¹P-NMR spectroscopic investigations, especially with [C₃H₅Ni(P(OPh)₃)₂]PF₆ as a typical trans-catalyst, the definite course of the catalytic stereoregulation has been elucidated.     

Cationic polymerization with titanium(iv) compounds: Living polymerization and possibility of stereoregulation

This paper deals with the development of living cationic polymerizations and the possibility of stereoregulation therein based on the modulation of the Lewis acid activators by their ligands. For this, titanium(IV) chlorides [TiCl₄-n(OR)n] with various alkoxy or aryloxy groups were synthesized and employed for the cationic polymerizations of isobutyl vinyl ether (IBVE) in conjunction with HCl-IBVE adduct. Living polymerizations were feasible with the titanium chlorides [TiCl₂(OR)₂] disubstituted with isopropoxy or phenoxy groups in CH₂Cl₂ at −15°C. The meso (isotactic) contents of the polymers obtained with TiCl₂(OR)₂ at −78°C became larger (up to 86%) with bulkier o-substituents of phenoxy groups on the titanium compounds.     

Spin crossover in a four-coordinate iron(II) complex

The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)(3)Fe-N═PPh(3) undergoes an S = 0 to S = 2 spin transition with T(C) = 81 K, as determined by variable-temperature magnetic measurements and Mo?ssbauer spectroscopy. Variable-temperature single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is associated with an increase in the Fe-C and Fe-N bond distances and a decrease in the N-P bond distance. These structural changes have been interpreted in terms of electronic structure theory.     

Kinetics and mechanism of oxidation in the diaqua(nitrilotriacetato)cobaltate(II)/N-bromosuccinimide reaction: evidence for iron(II) catalysis

Summary Oxidation of the diaqua(nitrilotriacetato)cobaltate(II) complex, [CoIInta(H2O)2]-, by NBS has been studied in aqueous medium. The kinetics of the reaction in the presence of an iron(II) catalyst obey the rate law: Catalysis by iron(II) is believed to be due to the oxidation of iron(II) to iron(III), which acts as the oxidizing agent. The thermodynamic activation parameters were calculated and we propose that electron transfer proceeds through an inner-sphere mechanism. This revised version was published online in June 2006 with corrections to the Cover Date.     

Spin transition in a chainlike supramolecular iron(II) complex

A one-dimensional supramolecular head-to-tail N+ -H...N-type hydrogen-bonded chain of the complex [FeII(L)2H](ClO4)3.MeOH [L = 4'-(4'-pyridyl)-1,2':6'1'-bis(pyrazolyl)pyridine] exhibits a reversible, thermally driven spin transition at 286 K with a hysteresis loop of ca. 2 K.     

Bis(imino)pyridine iron(II) alkyl cations for olefin polymerization

Treatment of the five-coordinate ferrous dialkyl complex, (iPrPDI)Fe(CH2SiMe3)2 (iPrPDI = ((2,6-CHMe2)2C6H3N=CMe)2C5H3N), with [PhMe2NH][BPh4] in the presence of diethyl ether or tetrahydrofuran furnished the corresponding alkyl cations, where the donor ligand is coordinated in the basal plane of a distorted square pyramidal iron(II) alkyl cation. Performing the same reaction with the neutral Lewis acid, B(C6F5)3, induced methide abstraction from a silicon atom followed by rearrangement to afford the base free ferrous alkyl cation, [(iPrPDI)Fe(CH2SiMe2CH2SiMe3)][MeB(C6F5)3]. This complex is active for the polymerization of ethylene and yields polymers that are of higher molecular weight and narrower polydispersity than traditional methylalumoxane-activated catalysts.     

Separation and preconcentration of iron(II) and iron(III) from natural water on a melamine-formaldehyde resin

A combined method for the preconcentration and selective spectrophotometric determination of both valencies of iron, i.e., Fe(II) and Fe(III), down to 0.4 mug l(-1) has been developed. Iron(III) from synthetic and natural water samples has been concentrated on a melamine-formaldehyde resin at pH 5; iron(II) was not retained under identical conditions. The oxidized iron was concentrated on a second resin column. The iron in both columns was eluted with 1 M HCl solution and separately analyzed by the 1,10-phenanthroline-citrate spectrophotometric method. The effect of pH, adsorption and elution rates, and interferences on the developed procedure were investigated. Metal ions that can be retained by the resin at moderate concentrations, e.g., Al(3+), do not cause interference in more dilute solutions encountered in natural water samples. At least 160-fold volume enrichment can be easily obtained using an adsorption flowrate of 50 ml min(-1). A hydrothermal water sample was analyzed by the recommended procedure and by a literature method, and the results were statistically compared by t- and F-tests.     

Mechanistic insight into N=N cleavage by a low-coordinate iron(II) hydride complex

The reaction pathways of high-spin iron hydride complexes are relevant to the mechanism of N2 reduction by nitrogenase, which has been postulated to involve paramagnetic iron-hydride species. However, almost all known iron hydrides are low-spin, diamagnetic Fe(II) compounds. We have demonstrated that the first high-spin iron hydride complex, LtBuFeH (LtBu = bulky beta-diketiminate), reacts with PhN=NPh to completely cleave the N-N double bond, giving LtBuFeNHPh. Here, we disclose a series of experiments that elucidate the mechanism of this reaction. Crossover and kinetic experiments rule out common nonradical mechanisms, and support a radical chain mechanism mediated by iron(I) species including a rare eta2-azobenzene complex. Therefore, this high-spin iron(II) hydride can break N-N bonds through both nonradical and radical insertion mechanisms, a special feature that enables novel reactivity.     

Free-radical polymerization of methyl methacrylate in the presence of a ferrocenyl-containing iron(II) clathrochelate complex and initiators of various natures

Benzoyl peroxide-, lauryl peroxide-, and AIBN-initiated free-radical polymerization of methyl methacrylate has been studied in bulk and solution in the presence of macrobicyclic iron(II) bis(ferrocenyl borate) tris(nioximate). It has been found that the ferrocenyl-containing iron(II) clathrochelate forms efficient initiating systems with peroxides, whereas, in the case of AIBN, its presence has no effect on the kinetic parameters of the process and the properties of the resulting polymer. The use of clathrochelate complex-peroxide initiator systems accelerates the polymerization of methyl methacrylate and decreases the molecular mass of the polymer. The kinetic parameters of the process have been determined.     

Spin transition charted in a fluorophore-tagged thermochromic dinuclear iron(II) complex

The first crystal structures of a dinuclear iron(II) complex with three N1,N2-1,2,4-triazole bridges in the high-spin and low-spin states are reported. Its sharp spin transition, which was probed using X-ray, calorimetric, magnetic, and (57)Fe Mossbauer analyses, is also delineated in the crystalline state by variable-temperature fluorimetry for the first time.     

Effect of co-catalyst system on α-olefin polymerization with rac- and meso-[dimethylsilylenebis(2,3,5-trimethyl-cyclopentadienyl)]zirconium dichloride

Propene, 1-butene and 1-hexene polymerization was conducted with a mixture of rac- and meso-[dimethylsilylenebis((2,3,5-tetramethyl-cyclopentadienyl))]zirconium dichloride (Me₂Si(2,3,5-Me₃Cp)₂ZrCl₂) ( 1 ) combined with methylaluminoxane (MAO), triethylaluminium (AlEt₃)/triphenylcarbenium tetrakis(pentafluorophenyl)borate (Ph₃CB(C₆F₅)₄) ( 2 ) and triisobutylaluminium (AliBu₃)/Ph₃CB(C₆F₅)₄, respectively, as co-catalyst systems. The ratios of polymerization rates R_p(rac)/R_p(meso) were changed with the combined cocatalysts. It was found that in the case of using trialkylaluminium/ 2 as co-catalyst R_p(rac)/R_p(meso) is lower than when using MAO in any kind of α-olefin polymerization.     

Synthesis and photochemical properties of a photochromic iron(II) complex of hexaarylbiimidazole

The photochromic ligand bis(terpyridyl)hexaarylbiimidazole (bistpy-HABI) and the Fe(II) complex of bistpy-HABI with formula [{Fe(tpy)}2.bistpy-HABI](PF6)4.4H2O were synthesized and characterized. Bistpy-HABI is readily cleaved into a pair of terpyridyltriphenylimidazolyl radicals (tpy-TPI*) on irradiation with UV light. This photochemical reaction is completely reversible, and the light-induced radicals can thermally recombine to form bistpy-HABI in the dark. [{Fe(tpy)}2.bistpy-HABI]4+ is the first example of a transition-metal complex of an HABI derivative and was found to show photochromic reaction in solution. The spin state of the light-induced radical pair in a frozen matrix was investigated by ESR spectroscopy. The triplet state of the light-induced radical pair from [{Fe(tpy)}2.bistpy-HABI]4+, as well as that from bistpy-HABI, was confirmed to be a ground state or nearly degenerated with a singlet state. Kinetic studies on the radical recombination reaction in solution elucidated the decrease in the activation energy by forming the Fe(II) complex. This is the first observation of a decrease in the activation energy of the radical recombination reaction by the formation of a metal-coordinated radical complex. The syntheses, photochemical properties, and spin states of bistpy-HABI and [{Fe(tpy)}2.bistpy-HABI](PF6)4 are discussed.     

Kinetics and mechanism of oxidation of the chromium(III)-dl-valine complex/periodate reaction. Evidence for iron(II) catalysis

Oxidation of the chromium(III)-dl-valine complex [CrIII(L)2(H2O)2]+ by periodate has been investigated in aqueous medium. The kinetics of the reaction in aqueous medium in the presence of iron(II) as catalyst obeyed the rate law:Catalysis by iron(II) is believed to be due to the oxidation of iron(II) to iron(III), which acts as the oxidizing agent. The thermodynamic activation parameters were calculated and we propose that electron transfer proceeds through an inner-sphere mechanism via coordination of IO4– to chromium(III).     

Spectroscopic, redox and magnetic properties of a tetraiminediphenolate iron(II) macrocyclic complex: a model compound for iron proteins

The title complex [Fe(II)(tidf-H2)(H2O)2](ClO4)2*H(2)O (tidf-H2 = tetraiminediphenolate ligand) has been prepared from a transmetallation reaction between [Mg2(tidf)](NO3)2*4H2O and an iron(II) salt in methanolic solutions under inert atmosphere conditions. It was characterized by analytical, magnetic and spectroscopic methods (Mossbauer, FTIR, UV-vis), by cyclic voltammetry as well as spectroelectrochemistry.     

Gas-phase and solution-phase polymerization of epoxides by Cr(salen) complexes: evidence for a dinuclear cationic mechanism

The gas-phase reactions of a series of mass-selected mononuclear and dinuclear Cr(salen) complexes with propylene oxide suggest that the enhanced reactivity of the dinuclear complexes in gas-phase and in solution may derive from a dicationic mechanism in which the alkoxide chain is mu(2)-coordinated to two Lewis acidic metal centers. The double coordination is proposed to suppress backbiting, and hence chain-transfer in the gas-phase homopolymerization of epoxides.     

Kinetics and mechanism of oxidation of the chromium(III)-DL-aspartic acid complex/periodate reaction. Evidence for iron(II) catalysis

The kinetics of oxidation of the chromium(III)-DL- aspartic acid complex, [CrIIIHL]+ by periodate have been investigated in aqueous medium. In the presence of Fe^II as a catalyst, the following rate law is obeyed:

Separation of antimony(V) from iron(III), copper(II), cobalt(II) and cadmium(II) by ion-exchange

With excess of tartaric acid, antimony(V) forms a stable anionic complex that completely escapes adsorption on a cation-exchange ream, Zeo-Carb-225, while Cu(II), Fe(IIl), Co(II) or Cd(II) is quantitatively retained. Antimony(V) can be separated from these ions in this way.