The characterization and kinetics of N-formyl-N-methylaminomethenylpentaaquochromium(III) ion

Anet was the first to isolate a stable sigma bonded organochromium (III) complex.1 This was the benzylpentaquochromium (III) ion obtained by the reduction of benzyl halide with chromium (II) perchlorate. The mechanism of this and similar reactions has been thoroughly studied and is believed to be2,3,4 [see PDF file for formula] where R is an alkyl group and X is a halogen other than fluorine. All complexes reported in the literature show chromium sigma bonded to an sp3hybridized carbon atom. D. Biggs, of L. Speer\u27s research group, attempted to synthesize a complex containing chromium sigma bonded to an sp2 hybridized carbon atom.5 This was attempted by reacting chromium (II) perchlorate with parabromopyridine in the presence of N,N dimethylformamide (DMF). Amides have been shown in the literature to enhance the reductive power of chromium (II) salts.3,6,7 An orange-brown complex was obtained with Biggs attributed to parapyridylpentaaquochromium (III) ion. An attempt was made to characterized this complex, maily through the infrared (IR) and nuclear magnetic resonance (NMR) spectra of a solid mercury (II) derivative of the complex. Mercury (II) halides have been shown to react with organochromium (III) complexes via bimolecular displacement to yield the corresponding organomercury (II) halide salt which can be precipitated from solution:8,9 [see PDF file for formula] The IR and NMR spectra of a solid precipitate, obtained after the reaction of mercury (II) nitrate with the orange colored solution described above and subsequent precipitation with chloride ion, led Biggs to believe that he had prepared parapyridylpentaaquochromium (III) ion. Little effort was done to determine the stoichiometry, or the kinetics, of the aquation of the complex. The results presented here are the result of an attempt to establish the stoichiometry and the kinetics of aquation of the reported complex in acidic media. It will be shown that Biggs\u27 assignment of parapyridylpentaaquochromium (III) as the structure of the complex is in error and another structure will be given

Formation And Kinetics Of Dissociation Of Some Pentaaquotrihalomethyl Chromium(Iii) Ions.

The present.study was an investigation of the formation and kinetic stability of store complexes contain.ir)ing a chromium-carbon bond in an aqueous medium. An orange-brown pentaaquotrifluoromethylchromium(III) ion was obtained by the reduction of trifluororrethyl iodide with Cr(II) and is the most inert of any organochrotnium(III) complex known. The initial aquation rates of trifluoromethylchromium(III) ion were described by the differential rate law -dln [(H2O)5CrCF32+]/dt = ko + k1(H+) The products of the initial reaction were hexaaquochromium (III), carbon monoxide and HF. The values of ko and k1 at 45° were 5.21 x 10-1 and 9.38 x 10M-1sec-1 respectively. The activation parameters for the acid independent and acid dependent pathways were: ΔH+ = 19.7 kcal/more, ΔS+ = -27.4 cal-deg-1mole-1; and ΔHǂ1 = 23.4 kcal/mole and ΔSǂ1 = -12.7 cal-deg-1mole-1. The acceleration in the aquation with time was observed indicating autocatalysis by a product of the reaction. An intermediate, (Cr( OH2)4FCrCF31+ ), was separated from the aquation reaction solution; this same ion was also formed immediately when fluoride ion was added to the (H2)5CrCF32+ ion in 0.05M perchloric acid. The added fluoride ion also increased the rate of aquation. Rapid formation of (Cr(OH2)4FCrCF31+) ion was attributed to the strong stabilizing effect of the trifluoromethyl ligand on water molecules in the inner coordination sphere of chromium( III). A solvent-assisted mechanism for the aquation of pentaaquotrifluoromethyl-chromium(III) ion was proposed in which an activated complex was formed which dissociated in a concerted manner to give the products. Also, a reaction sequence was proposed for the complete aquation reaction which was consistent with the rate laws, the effect of added fluoride ion and the activation parameters. Pink organochromium(III) species of 2+ charge was isolated in the reduction of carbon tetrachloride and carbon tetrabromide with chromium( II). he pink species have ·chromium:carbon:halide ratios of 1:1:1 and their uv-visible spectra showed unusually high absorbance values in the 500 nm region. The pink organochromium(III) reacted very rapidly with oxygen. The products of dissociation were carbon monoxide, formic acid, formaldehyde, bexaaquochromium(III), chromium( II) halide ions. The kinetics of aquation were studied, and initial rate coefficients and activation parameters were obtained. The identity dt these organochromium(III) species was not established but most possible structures were proposed

Oxidative and electrophilic pathways in the reactions of organochromium(III) complexes with nitrous acid and tris(2,2\u27-bipyridyl)ruthenium(III)

The kinetics and mechanisms of the reactions of nitrous acid with pentaaquoorganochromium(III) complexes were studied. Nitrous acid reacts with (H(,2)O)(,5)CrR(\u272+) complexes via an NO(\u27+) intermediate by either an electrophilic or electron transfer mechanism depending on the nature of the R group. The rate constants for the aliphatic organochromium complexes can be correlated using a linear free energy correlation which implies a common transition state for these reactions. The lack of steric effects for the reactions of the alkyl-chromium complexes, and the detection of Cr(\u272+) as an intermediate in the reactions of the (alpha)-hydroxy- and (alpha)-alkoxyalkylchromium complexes suggests that these complexes react with HONO by an electron transfer mechanism. A mechanism has been proposed in which NO(\u27+) is formed which then oxidizes CrR(\u272+) by one electron to give CrR(\u273+). This species decomposes to Cr(\u273+) and R(.) when R is an alkyl group, but forms Cr(\u272+) and ROH when R is an (alpha)-hydroxy- or (alpha)-alkoxyalkyl group. R(.) and Cr(\u272+) react with the NO formed in the reaction to give RNO and CrNO(\u272+), respectively;In contrast, aralkylchromium(III) complexes react with nitrous acid by an electrophilic mechanism as indicated by the products and correlations of the rate constants with other electrophilic reactions in the literature;Photochemically generated tris(2,2\u27-bipyridyl)-ruthenium(III) oxidizes a range of pentaaquoorganochromium(III) complexes by one electron. The trends in the rate constants and the usual behavior of Ru(bpy)(,3)(\u273+) suggest that electron transfer is occurring by an outer-sphere mechanism. In the case of CrCH(,2)CH(,3)(\u272+), the intermediate;formed, CrCH(,2)CH(,3)(\u273+), decays to Cr(\u273+) and (.)CH(,2)CH(,3) as shown by kinetic studies and a product analysis; (\u271)DOE Report IS-T-1193. This work was performed under contract W-7405-Eng-82 with the Department of Energy

Cr[CH(SiMe3)2]3/SiO2 catalysts for ethene polymerization: The correlation at a molecular level between the chromium loading and the microstructure of the produced polymer

Several spectroscopic techniques have been applied in a concerted way to investigate the local structure and the function of the chromium sites in a series of Cr[CH(SiMe3)2]3/SiO2 catalysts for ethene polymerization that differ in chromium loading and to correlate them with the microstructure of the produced polyethylene (PE). According to previous studies, the density of the produced PE is proportional to the Cr loading: the higher the Cr loading, the lower the PE density, because of an increase in the content of short-chain branching. Here, the nature of the branches and their enchainment in the polymer have been further investigated to determine the different properties of the produced polymers, and the differences detected have been traced back by spectroscopic analysis to the presence of two different types of Cr sites working in tandem: monografted [tbnd]SiO[sbnd]Cr[CH(SiMe3)2]2 species that are responsible for ethene oligomerization, and bis-grafted ([tbnd]SiO[sbnd])2CrCH(SiMe3)2 species that account for ethene homo- and co-polymerization. The relative abundance of the two species depends on the chromium loading. Moreover, we have also found that carbon monoxide selectively poisons the sites responsible for ethene oligomerization, indicating a possible strategy for the fine regulation of the properties of the produced PE

Kinetics and mechanisms of the reactions of alkyl radicals with oxygen and with complexes of Co(III), Ru(III), and Ni(II)

The kinetics of the reactions of [superscript]•C[subscript]2H[subscript]5 with Co(NH[subscript]3)[subscript]5X[superscript]2+, Ru(NH[subscript]3)[subscript]5X[superscript]2+, and Co(dmgH)[subscript]2(X) (Y) (X = Br, Cl, N[subscript]3, SCN; Y = H[subscript]2O, CH[subscript]3CN) complexes were studied using laser flash photolysis of ethylcobalt complexes. The kinetics were obtained by the kinetic probe method using ABTS[superscript]• - and IrCl[subscript]6[superscript]2-. Some relative rate constants were also determined by a competition method based on ethyl halide product ratios. The products of these reactions are largely (\u3e90%) the ethyl halide and ethyl thiocyanate, substantiating an inner-sphere mechanism. Minor but regular yields of C[subscript]2H[subscript]4 are also found (\u3c10%), suggesting a small contribution from the outer-sphere oxidation of [superscript]•C[subscript]2H[subscript]5;The kinetics of colligation reactions of a series of alkyl radicals with [beta]-Ni(cyclam)[superscript]2+ were studied using laser flash photolysis of alkylcobalt complexes. The kinetics were obtained by employing the kinetic probe competition method (with ABTS[superscript]• - or MV[superscript]• +);The kinetics of the unimolecular homolysis of a series of RNi(cyclam)H[subscript]2O[superscript]2+ were studied using ABTS[superscript]• -, MV[superscript]• + or oxygen as radical scavenging agents. Activation parameters were obtained for the unimolecular homolysis of C[subscript]2H[subscript]5Ni(cyclam)H[subscript]2O[superscript]2+;The colligation and homolysis rate constants are strongly influenced by steric and electronic factors. Kinetic and thermodynamic data obtained from these reactions were compared with those for other [sigma]-bonded organometallic complexes;The kinetics of the unimolecular homolysis of a series of RNi(cyclam)H[subscript]2O[superscript]2+ complexes were studied by monitoring the formation of the oxygen insertion product RO[subscript]2Ni(cyclam)H[subscript]2O[superscript]2+. The rate constants for the reactions of alkyl radicals with oxygen, k[subscript] O, were obtained from the analysis of the homolysis rate equation which contains the known rate constants for the colligation of R[superscript]• with [beta]-Ni(cyclam)[superscript]2+, k[subscript] col, and for the homolysis of RNi(cyclam)H[subscript]2O[superscript]2+, k[subscript] hom. The higher rate constants for the reactions of alkyl radicals with oxygen in solution, as compared with those measured in the gas phase, were discussed

Unravelling the photochemical transformations of chromium(I) 1,3 Bis(diphenylphosphino), [Cr(CO)4(dppp)]+, by EPR spectroscopy

UV-induced photochemical transformations of the paramagnetic [Cr(CO)4(Ph2PCH2CH2CH2PPh2)]+ complex (abbreviated [Cr(CO)4(dppp)]+) in dichloromethane was investigated by CW EPR spectroscopy. Room-temperature UV irradiation results in the rapid transformation of [Cr(CO)4(dppp)]+ into trans-[Cr(CO)2(dppp)2]+. However, low-temperature (77–120 K) UV irradiation reveals the presence of an intermediate mer-[Cr(CO)3(κ1-dppp)(κ2-dppp)]+ complex which photochemically transforms into trans-[Cr(CO)2(dppp)2]+. The derived spin Hamiltonian parameters for these complexes were confirmed by DFT calculations. The photoinduced reaction is shown to be concentration-dependent, leading to a distribution of the three complexes ([Cr(CO)4(dppp)]+, mer-[Cr(CO)3(κ1-dppp)(κ 2-dppp)]+, and trans-[Cr(CO)2(dppp)2]+). A bimolecular photoinduced mechanism is proposed to account for the formation of mer-[Cr(CO)3(κ1-dppp)(κ2-dppp)]+ and trans-[Cr(CO)2(dppp)2]+