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C. Biedma e P. G. D'Alfonso - "Le langage du dessin"
Full text linkC. Biedma e P. G. D'Alfonso - "Le langage du dessin" - Test de Wartegg-BiedmaVersion modifiée du testde Wartegg. Delachaux & Niestle S. A. - Neuchatel e Paris, 1955, 144 pg., 22 Pranchas e fôlhas de Registro em separado
Anionic oligomerization as a route to chain clusters
No full textThr combination of two old concepts (the "sigma-donor" capability of M-H bonds and the propensity of unsaturated complexes for addition reactions) has led to the first example of addition oligomerization of an organometallic analogue of olefins: chains containing cis-[Re(CO)(4)] units joined by Re-H-Re interactions have been obtained through the oligomerization of the unsaturated complex [Re-2(mu-H)(2)(CO)(8)] promoted by anionic "initiators". The limits of the method and the possible extension to other substrates are discussed
EASILY REVERSIBLE IONIC FRAGMENTATION OF A TETRAHEDRAL UNSATURATED RHENIUM CLUSTER, A REAPPRAISAL OF THE REACTION OF {[RE-4(MU(3)-H)(4)(CO)(12)] WITH METHANOL}
No full textThe unsaturated tetrahedral cluster {[Re-4(mu(3)-H)(4)(CO)(12)] in the
presence of weak donor molecules L, like methanol or acetone, reversibly
fragments to unsaturated triangular cluster anions and to mononuclear
cations, probably via spiked-triangular [Re-4(mu-H)(4)(CO)(12)L(4)]
intermediates.
KINETICS OF THE REACTION OF DICHLORO-TRANS-RAC-1,2-DIAMINOCYCLOHEXANEPLATINUM(II) AND DICHLORO-CIS-MESO-1,2-DIAMINOCYCLOHEXANEPLATINUM(II) WITH PYRIDINE
No full textThe kinetics of the reaction [Pt(dach)Cl2] + py- [Pt(dach)(py)CI]Cl (dach = either trans-rac- or cis-meso-1,2-diaminocyclohexane, py = pyridine) has been studied in dimethylformamide (dmf) solution, under pseudo-first-order conditions (excess of pyridine), in the range 50-80-degrees-C, by measuring the increase in conductance of the solution. In dmf the products behave like weak electrolytes, with dissociation constants of about 1.4 x 10(-3) mol dm-3 . The reactions were found to be reversible under the experimental conditions. Owing to the uncertainty in evaluating the equilibrium concentrations, the kinetic constants k(obs) have been calculated from the initial slopes (up to 1 5% conversion) of first-order plots. Their values obey the equation k(obs) = k1 + k2[py], k1 being about two orders of magnitude lower than k2. The values of k(obs) for the cis derivative are always slightly but significantly higher than those of the trans. The activation parameters have been evaluated only for k2, since the k1 values are affected by too high uncertainties: DELTAH(double dagger) trans 76(1), cis 79(1) kJ mol-1; DELTAS(double dagger) trans -101(3), cis -92(3) J K-1 mol-1. The higher rate for the cis isomer is interpreted in terms of higher flexibility of the diamine chelate ring
1H and 19F investigation of the reaction of B(C6F5)3 with water in toluene solutions
No full textTitrations of B(C6F5)3 (1) with water, in toluene-d8 solution, monitored by 19F and 1H NMR at 196 K, showed first the formation of the adduct [(C6F5)3B(OH2)] (2) and then its stepwise transformation into the two aqua species [(C6F5)3B(OH)]·H2O (3) and [(C6F5)3 B(OH2)]·2H2O (4) containing, respectively, one or two water molecules hydrogen-bonded to the protons of the B-bound water molecule. The NMR data show that in each titration step only two species were present in significant concentration: 1 and 2 up to 1 equiv, 2 and 3 between 1 and 2 equiv, 3 and 4 between 2 and 3 equiv. Above 3 equiv the solutions rapidly attained saturation and phase separation occurred (although there was evidence of interaction of 4 with more water molecules). Titrations at room temperature indicated an analogous stepwise course. Variable-temperature experiments demonstrated water exchange between the different aqua species and between the different water sites in the adducts 3 and 4 ("internal" or B-bound and "external" or H-bound). The rate of these processes increased with the amount of water bonded to B(C6F5)3. The exchange of B-bound water among the different B(C6F5)3 molecules (resulting in the 1 ⇔ 2 interconversion) caused the averaging of the 19F resonances of 1 and 2, above 273 K. Band shape analysis in the temperature range 235-312 K provided the kinetic constants, whose dependence on the concentration revealed a dissociative mechanism (ΔH‡ 67(2) kJ mol-1, ΔS‡ 58(7) J mol-1 K-1). For the adduct [(C6F5)3B(OH2)]·H2 O (3), four different dynamic processes have been recognized: (i) the exchange of H-bound water among different [(C6F5)3B(OH2)] adducts (the 2 ⇔ 3 exchange) or (ii) among different [(C6F5)3 B(OH2)]̇H2O adducts (the 3 ⇔ 4 exchange), (iii) the exchange between H-bound and B-bound water, (iv) the hopping of H-bound water between the two protons of B-bound water. This process was so fast that an averaged signal for the protons of internal water was observed even at 187 K. The rate of the process (i) increased with the concentration of 2, so that separate 19F and 1H signals for 2 and 3 were observed only in very dilute solutions at the lowest temperatures. Linear plots of the kinetic constants (estimated from 1H NMR spectra in the near fast exchange region, temperature range 188-214 K) vs the concentration of 2 allowed the estimation of the constant for the dissociative pathway (4 orders of magnitude faster than for the exchange of B-bound water) and for the bimolecular pathway [ΔH‡ 30(2) kJ mol-1, ΔS‡ 3(10) J mol-1 K-1]. Process (ii) was too fast on the NMR time scale to allow any kinetic investigation. Process (iii) caused the parallel broadening of both the 1H signals of 3 at T > 225 K, with a rate quite close to that of the dissociative exchange of water among different B(C6F5)3 molecules. The activation parameters (ΔH‡ 55(2) kJ mol-1, ΔS‡ 7(3) J mol-1 K-1, temperature range 233-273 K) allowed no discrimination between the exchange of an entire water molecule and the mere exchange of protons. Even small amounts of 4 accelerated process (iii), due to the occurrence of two much faster processes: the 3 ⇔ 4 exchange and the exchange between the protons of internal and external water in 4. The study of any kind of water mobility concerning the trihydrate 4 was prevented by the occurrence of proton exchange processes (so fast as to broaden the signals of internal and external water even at 188 K), possibly favored by the acidic dissociation of the protons of the B-bonded water molecule of 4
Asymmetric Hydrogenation of Chiral Aminophosphine-phosphinite Rhodium Complexes
No full textThe ligands (S)-N-(diphenylphosphino)-2-diphenylphosphinoxymethylpyrrolidine, (S)-prolophos, and (S)-1-diphenylphosphinoxy-2-N-diphenylphosphinoaminobutane, (S)-butaphos, have been prepared. The Rh(I) complexes of these phosphines act as efficient homogenous hydrogenation catalysts at ambient temperature and pressure for A-N-acetaminoacrylic acid and itaconic acid
C-13 NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE PROPERTIES OF THE UNSATURATED ANION {[RE3(MU-H)4(CO)9(NCME)]- AT LOW-TEMPERATURE}
No full textThe carbonyl resonances of [Re3(μ-H)4(CO)9(NCMe)]- have been assigned and the 13C spin-lattice relaxation times measured at different field strengths (20.15,50.31, and 75.43 MHz) at 238 K. The results indicate that scalar coupling of the second kind with rhenium isotopes is the dominant relaxation mechanism at this temperature. Values of T1Q (Re) and 1J(187Re-C) were calculated. The presence on the H2Re(CO)3(NCMe) moiety of a poor π-acceptor ligand doubles 1J(187Re-C) with respect to the unsubstituted H2Re(CO)4 moiety in the precursor [Re3(μ-H)4(CO)10]-
1,2-eq,eq-[Re-2(CO)(8)(THF)(2)]: A reactive Re-2(CO)(8) fragment that easily activates H-H and C-H bonds
No full textThe reaction of [Re-2(mu-H)(2)(CO)(8)] (1) with diazomethane at 193 K in THF-d(8) gives the unstable [Re-2(mu-H)(mu-CH3)(CO)(8)] derivative (2) containing a methyl group arising from the insertion of CH2 into a Re-H-Re bond. Isotopic perturbation of the equilibria by partial deuteration demonstrated that the methyl bridges the Re-Re bond in an unsymmetrical way, with a fast exchange between one agostic and two terminal C-H bonds. At temperatures higher than 253 K, 2 decomposes, in THF solution, with CH4 elimination to give the novel red complex 1,2-eq,eq-[Re-2(CO)(8)(THF)(2)] (3), which was characterized by NMR spectroscopy and X-ray analysis. In the solid form a staggered conformer of Ca symmetry was found. C-13 NMR analysis revealed the presence, in wet THF, of the aquo complexes [Re-2(CO)(8)(THF)(H2O)] (4) and [Re-2(CO)(8)(H2O)(2)] (5), whose formation is favored at low temperature (Delta H degrees for the formation of 5 from 4: -14.4(2) kJ mol(-1)). In solution, due to the lability of the THF ligands, 3 behaves as a "lightly stabilized" Re-2(CO)(8) fragment, capable of activating different E-H bonds. Reaction with HCl in THF leads to [Re-2(mu-H)(mu-Cl)(CO)(8)], while with H-2 the unsaturated starting material [Re-2(mu-H)(2)(CO)(8)] is obtained. In THF solution, at room temperature, reactions with phenylacetylene, styrene, and acetaldehyde give the derivatives of C-H activation [Re-2(mu-H)(mu-C drop CPh)(CO)(8)], [Re-2(mu-H)(mu-CH=C(H)Ph)(CO)(8)], and [Re-2(mu-H)(mu-eta(2)-C(Me)O)(CO)(8)]. Moreover, the activation of an sp(3) C-H bond in ethyl acetate occurs slowly when 3 is dissolved in the reactant itself, the resultant product being [Re-2(mu-H)(mu-eta(2)-CH2C(O)OEt)(CO)(8)]
Spectroscopic titration of {[Re-4(mu(3)-H)(4)(CO)(12)] with chloride ions. An electron-precise tetranuclear cluster as intermediate in the transformation of a tetranuclear unsaturated cluster into trinuclear unsaturated anions}
No full textThe unsaturated tetrahedral cluster {[Re-4(mu(3)-H)(4)(CO)(12)] easily
added halide ions giving [Re(4)H(4)X(CO)(12)](-) anions, which in the
presence of even poor donor species underwent fragmentation to
unsaturated triangular cluster anions.
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