Base hydrolysis of mono-alkylsubstituted chloropentaamminecobalt(III) complexes

Summary The preparation of the series ofcis- andtrans-[Co(NH₃)₄(RNH₂)Cl]²⁺ complexes (withcis, R = Me orn-Pr andtrans, R = Me, Et,n-Pr,n-Bu ori-Bu) is described. The u.v-visible spectra indicate a decrease of the ligand field on increasing chain length. Infrared spectra show an enhanced Co-Cl bond strength compared to the pentaammine. Partial molar volumes of the complex cations do not reveal steric compression. From proton exchange studies in D₂O it follows that [Co(NH₃)₅Cl]²⁺ and thecis- andtrans-[Co(NH₃)₄-(CH₃NH₂)C1]²⁺ complexes exchange the amine protons on the grouptrans to the chloro faster than those on thecis. A coordinated methylamine group exchanges its amine protons slower than a corresponding NH₃ group in the parent pentaammine, but the methyl introduction accelerates the exchange of the other NH₃ groups. The aquation of thetrans-alkylamine complexes (studied at 52° C) is acceleratedca. 10 times compared to the parent pentaammine, irrespective of the nature of the alkyl group. Thecis complexes do not show this acceleration of aquation. In base hydrolysis (studied at 25° C) thecis complexes are the most reactive (a factor 20 over the parent ion). Thecis/trans product ratio in base hydrolysis and the competition ratio in the presence of azide ions were calculated from the 500 MHz¹H n.m.r. spectra, which display distinctly different alkyl resonances for each individual complex. Thecis ions react under stereochemical retention of configuration; thetrans compounds give 10±1%trans tocis rearrangement. The ionic strength (4 mol dm^–3) and the pH do not affect this result. The same product ratio is obtained in methanol-water and DMSO-water mixtures. Ammoniation in liquid ammonia gives the same ratios as in base hydrolysis, base-catalyzed solvolysis in neat methylamine gives stereochemical retention for both thecis- andtrans-methylamine ion. The product competition ratio (Co-N₃)/(Co-OH₂) for thecis compounds and the bulkier amines (R =n- andi-Bu), 15–25% at 1 mol dm^–3N ₃ ^– , isca. twice that of thetrans compounds and the pentaammine. The results are interpreted in the classical conjugate base mechanism, and discussed in the context of current ideas about stereochemistry of base hydrolysis.Prof. C. R. Píriz Mac-Coll from Uruguay is a guest at the Free University of Amsterdam.     

Selective formation of thecis isomer of the nitridotechnetium(V) complex with 2,2′:6′,2″-terpyridine

The reaction of [TcNCl₂(PPh₃)₂] with 2,2′:6′,2″-terpyridine producedcis-[TcNCl₂(terpy)] selectively. The resulting complexes were characterized by¹H NMR and IR spectroscopy. The geometries of thecis andtrans isomers were estimated by theoretical calculations following a density functional method. Thecis isomer is likely more stable than thetrans one with respect to thetrans influence of the nitrido ligand. Furthermore, the behavior of nitridotechnetium complexes in polar solvents was compared to Os-analogues.     

Excess heat capacities and orientational order in systems containing hexadecane isomers of different molecular structure

Using the Picker flow microcalorimeter, excess heat capacities have been obtained at 25°C throughout the concentration range for 2,2-dimethylbutane,n-hexane, and cyclohexane each mixed with a series of hexadecane isomers of increasing degrees of orientational order, as determined by depolarized Rayleigh scattering. The isomers are 2,2,4,4,6,8,8-heptamethylnonane, 6-, 4-, and 2-methylpentadecane, andn-hexadecane. Thec _p ^E values are negative, increasing rapidly in magnitude with increase of orientational order, and are not predicted by the Prigogine—Flory theory which neglects order. Values ofc _p ^E are obtained at 10, 25, and 55°C for cyclohexane +6-, 4-, and 2-methylpentadecane which with other literature data lead to the temperature dependence of the thermodynamic excess functions for cyclohexane solutions of the five C₁₆ isomers. The excess enthalpy and entropy vary with the C₁₆ isomer and with temperature, but the corresponding variation of the excess free energy is small, indicating a high degree of enthalpy-entropy compensation. This is consistent with a rapid decrease with temperature of orientational order in the C₁₆ isomers.     

Kinetics and mechanisms of the reactions of thiosulphato-amine complexes of cobalt(III). Part I. Isomerization and base hydrolysis ofcis andtrans isomers of dithiosulphatobis(ethylenediamine) cobalt(III) ion in aqueous solution

Summary Investigations were carried out on the isomerization and base hydrolysis ofcis andtrans forms of dithiosulphatobis-(ethylenediamine)cobalt(III) ions. Thecis form isomerizes to thetrans form in neutral aqueous medium, rates being 1.15, 2.30 and 4.0×10^–5s^–1, respectively at 42, 50 and 58 °C. Thetrans complex isomerizes to thecis form in basic solution only, the rate varying with pH in a sigmoid pattern. In presence of OH^–, an acid-base equilibrium of the complex ion sets in, but only the basic form takes part in the isomerization reaction. Hydrolysis of thecis isomer proceeds through a base-dependent path only, but that of thetrans isomer proceeds both through base-dependent and base-independent paths. The mechanisms are associative in nature. Thetrans form reacts faster thancis in all cases.     

Study of radical cations ofcis- andtrans-decalin in nonpolar solutions by the MARY and optically detected ESR spectroscopy

Radical cations ofcis- andtrans-decalin in nonpolar solvents were studied by optically detected ESR and magnetically affected reaction yield (MARY) spectroscopy. The observed differences in the spectra ofcis- andtrans-decalin are in agreement with the assumption of the existence of temperature-activated intramolecular dynamic transitions in the radical cation oftrans-decalin. Using MARY spectroscopy, the signals of the corresponding radical cations were detected at room temperature in diluted solutions containingcis- andtrans-decalin molecules as acceptors. Under these conditions, the total recovery of dynamic transitions in the radical cation oftrans-decalin is observed. Radical cations of bothcis- andtrans-decalin participate in the reaction of the ion-molecular charge transfer to a neutral molecule; the rate constant of this reaction is close to the diffusion-controlled one. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 968–973, May 1997.     

The effect of pressure on order destruction and order creation in linear or branched alkane mixtures

The pressure dependence of the excess enthalpy H ^E , dH ^E /dP, has been calculated from experimental excess volumes V ^E and dV ^E /dT using dH ^E /dP=V ^E –TdV ^E /dT. dH ^E /dP at zero pressure are reported at 25°C and equimolar concentration for the mixtures: cyclohexane with the series of normal alkanes (n-C _n , where n=6,8,10,12,14 and 16) and with the series of highly branched alkanes (br-C _n , where n=6,8,12 and 16), benzene, toluene and p-xylene +n-C _n and 1-chloronaphthalene +n-C _n and br-C _n . Experimental and Flory theory dH ^E /dP values are in good agreement for the whole cyclohexane +br-C _n series. For the n-C _n series, dH ^E /dP becomes increasingly positive deviating from the Flory predictions. This discrepancy is due to the presence of short-range orientational order in the higher n-C _n pure liquids which makes dH/dP more negative and which, upon mixing, is destroyed producing a positive contribution to dH ^E /dP not accounted for by the theory. The discrepancy between theoretical and experimental dH ^E /dP is large for benzene, but progressively smaller for toluene, p-xylene and 1-chloronaphthalene. These results are consistent with creation of order between the aromatic plate-like molecule and the long n-C _n in solution. For 1-chloronaphthalene +n-C _n , this order creation process produces a negative contribution to dH ^E /dP which balances the positive order-destruction contribution originated by the rupture, upon mixing, of short-range orientational order in pure n-C _n .     

Oxidative addition of tetrabromo-1,2-benzoquinone totrans-carbonylchlorobis(triphenylphosphine)rhodium andtrans-carbonyliodiobis(triphenylphosphine)rhodium

Summary The oxidative addition of tetrabromo-1,2-benzoquinone to Rh(CO)X(PPh₃)₂ (X = Cl or I) has been studied. With the square planar complex Rh(CO)Cl(PPh₃)₂, two new isomeric hexacoordinated compounds; withcis andtrans PPh₃ ligands, have been isolated and their structures are discussed on the basis of spectroscopic data. Thecis isomer in acetone solution quickly converts into thetrans. Such a conversion presumably proceeds through the dissociation of a triphenylphosphine molecule as seems indicated by the isolation of the pentacoordinated intermediate species Rh(CO)I(PPh₃)(1,2-O₂C₆Br₄), which has been identified by elemental analysis and spectroscopically characterized.     

Thermodynamische Untersuchungen ann-Alkan/n-Alkin-Systemen

Heats of mixing (H ^E)ofn-octane with 1,2,3,4-isomers ofn-octyne,n-heptane withn-1-heptyne andn-nonane withn-1-nonyne at temperatures from 298.15 to 318.15 K were measured with aCalvet-type microcalorimeter. The results are presented in tables and correlated with theRedlich-Kister equation.

     

Head to head polymers. XVII: Head to head polypropylene

Head to head polypropylene was prepared by catalytic hydrogenation of eithercis-1,4-poly(2,3-dimethylbutadiene) ortrans-1,4-poly(2,3-dimethylbutadiene) with cobalt 2-ethylhexanoate/triethylaluminium as the hydrogenation catalyst in decahydronaphthalene solution. The hydrogenation occurred predominantly bycis hydrogen addition, but was not stereospecific. The samples of head to head polypropylene were characterized by IR and NMR, particularly by¹³C-NMR spectroscopy. The polymers were amorphous and exhibited glass transition temperatures about 20°C lower than that of head to tail poly-propylene; the glass transition temperatures were measured by DSC and varied somewhat from sample to sample (sufficiently high molecular weight) according to their stereochemistry. TheT _gvalues were confirmed by Rheovibron measurements. The thermal stability of head to head polypropylene is not significantly different from that of either atactic or isotactic head to tail polypropylene.Part XVI:Grossman S., Yamada A., Vogl O., J. Macromol. Sci.-Chem.A 16, 897 (1981).     

Thermodynamics of mixtures of hexane and heptane isomers with normal and branched hexadecane

Molar excess mixing enthalpies h ^E , Gibbs free energies g ^E and hence entropies s ^E have been obtained using calorimetry and the vapor sorption method at 25°C for hexane isomers+2,2,4,4,6,8,8-heptamethylnonane, a highly branched C ₁₆ . The h ^E and g ^E are negative while Ts ^E are positive, but small. The values are explained by the Prigogine-Flory theory through negative free volume contributions to h ^E and Ts ^E , counterbalanced in the case of Ts ^E by the positive combinatiorial Ts ^E for mixing molecules of different size. No contribution is seen from the interaction between methyl and methylene groups. The excess quantities are also obtained for hexane and heptane isomers mixed with n-hexadecane. Values of h ^E and Ts ^E are now strongly positive, while those of g ^E are only slightly less negative. The interpretation requires two recently advanced contributions in addition to those of the Prigogine-Flory theory: 1) a decrease of order when correlations of orientations between n-C ₁₆ molecules in the pure liquid are replaced in the solution by weaker correlations whose strengths depend on the shapes of the lower alkane isomers. For lower alkane isomers of the same shape, but highly sterically hindered, h ^E and Ts ^E are small, manifesting, 2) a negative contribution, ascribed to a rotational ordering of n-C ₁₆ segments on the sterically-hindered molecule. Enthalpy-entropy compensation is observed for these new contributions, arising from their rapid fall-off with increase of temperature.