1,721,004 research outputs found
Disclosing the multi-faceted world of weakly interacting inorganic systems by means of NMR spectroscopy
No full textThe potential of NMR spectroscopy to investigate inorganic systems assembled by, or whose reactivity is affected by, non-covalent interactions is described. Subjects that have received particular attention in recent years (halogen bonding and Frustrated Lewis Pairs) and more classical subjects that remain under-explored (self-aggregation of ion pairs in low polar solvents, behavior of MAO containing metallocenium ion pairs, and hydrogen bonding/ion pairing effects in Au(I) catalysis) are considered, using an innovative approach, always focusing on the crucial information that can be provided by NMR
A PGSE NMR approach to the characterization of single and multi-site halogen-bonded adducts in solution
Full text linkWe demonstrate here that the Pulsed field Gradient Spin Echo (PGSE) NMR diffusion technique can be effectively used as a complementary tool for the characterization of mono- and multi-site intermolecular halogen bonding (XB) in solution. The main advantage of this technique is that it provides the possibility of unambiguously determining the stoichiometry of the supramolecular adduct, information that is particularly important when multi-site molecular systems are studied. As an example, PGSE NMR measurements in chloroform indicate that hexamethylenetetramine (HMTA), a potentially four-site XB acceptor, actually exploits only two sites for the interaction with the XB donor N-bromosuccinimide (NBS), leaving the other two nitrogen sites unoccupied. Charge displacement calculations suggest that this is due also to the anti-cooperativity of the XB interaction between HMTA and NBS
NMR studies on the dynamic behavior of zirconaaziridinium ion pairs in solution
No full textThe fluxional behavior of zirconaaziridinium ion pairs [Cp2Zr(η2-CH2NMePh][X] (X– = MeB(C6F5)3– (1a), B(C6F5)4– (1b)) in solution has been investigated by means of 2D EXSY and line shape analysis NMR methods at different temperatures. It is found that two dynamic processes are operative: (i) the inversion of the absolute configuration of the coordinated nitrogen atom (PI process) and (ii) the dynamic symmetrization of the ion pair occurring through a 180° rotation around Zr–CH2 bond and anion relocation from one side of the cation to the other (BS process). Analysis of the kinetic data indicates that both processes necessitate nitrogen decoordination from the metal that, by the way, is not the rate-determining step; the nature of the borate anion has a negligible effect on the rates of both PI and BS processes
Probing the association of frustrated phosphine-borane Lewis pairs in solution by NMR spectroscopy
No full text19F,1H HOESY, diffusion, and temperature-dependent 19F and 1H NMR studies allowed us to unequivocally probe the association between the frustrated PR3/B(C6F 5)3 (1, R = CMe3; 2, R = 2,4,6-Me 3C6H2) Lewis pairs in aromatic solvents. No preferential orientation is favored, as deduced by combining 19F,1H HOESY and DFT results, suggesting association via weak dispersion rather than residual acid/base interactions. The association process is slightly endoergonic [K = 0.5 M-1, ΔG 0(298 K) = +0.4 kcal/mol for 2], as derived from diffusion NMR measurements. © 2013 American Chemical Society
An NMR study on the reaction of substituted dimethyl zirconocenes with dimethylanilinium borate
No full textThe reaction of a series of dimethyl zirconocenes [Me2Si(Cp)2ZrMe2, 1; Cpt−bu2ZrMe2, 2; Cpn−bu2ZrMe2, 3; Ind2ZrMe2, 4; CpMe42ZrMe2, 5; Cp∗2ZrMe2, 6] with [HNMe2Ph][B(C6F5)4] was investigated by means of NMR spectroscopy. It was found that protonolysis of a Zr–Me group occurred generating a coordinative vacancy at the metal center and methane. Cations coming from 1–4 dimethyl precursors bound NMe2Ph, liberated from the protonation process, and formed zirconaaziridinium ion pairs {[Me2Si(Cp)2Zr(η2–CH2NMePh)][B(C6F5)4], 7; [Cpt−bu2Zr(η2–CH2NMePh)][B(C6F5)4], 8; [Cpn−bu2Zr(η2–CH2NMePh)][B(C6F5)4], 9; [Ind2Zr(η2–CH2NMePh)][B(C6F5)4], 10}, reasonably as a consequence of CH activation of one Me group of coordinated NMe2Ph and methane elimination. The intramolecular/interionic structures and dynamics of 7–10 ion pairs were investigated by 1H, 13C and 19F 1D-and 2D-NMR techniques. The reactions of 7 and 10 ion pairs with 2-methyl-1-heptene afforded stable diastereoisomeric ion pairs bearing a five-member azametallacycle
An Integrated NMR and DFT Study on the Single Insertion of α-Olefins into the M-C Bond of Group4 Metallaaziridinium Ion Pairs
No full textThe reactions of metallaaziridinium ion pairs [Cp2M(η2-CH2NMePh)][B(C6 F5)4] [M=Zr (1), Hf (2)] with 1-hexene and 2-methyl-1-heptene were investigated by means of NMR spectroscopy and DFT calculations. We found that a single insertion of the olefin into the M-C bond led to the formation of stable five-membered heterocycles (3-6). The reactions were 1,2-regioselective and afforded two diastereoisomers whose ratio was mainly controlled by the nature of the olefin, whilst the nature of the metal only played a marginal role. In particular, the reaction with 1-hexene was found to be not only more favored than that with 2-methyl-1-heptene from a thermodynamic point of view but it was also more diastereoselective. In all cases, the observed diastereoisomeric ratio was dictated by thermodynamics because the products interconverted into each other, as indicated by EXSY NMR spectroscopy and by DFT calculations. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Mechanistic aspects of water oxidation catalyzed by organometallic iridium complexes
No full textThe reactions of three iridium water‐oxidation catalysts {[Cp*IrL1L2L3]Xn; 1: L1, L2 = 2,2‐bipyridine (bpy), L3 = Cl, n = 1, X = Cl; 2: L1, L2 = 2‐benzoylpyridine (bzpy), L3 = NO3; 3: L1 = L2 = L3 = H2O, n = 2, X = NO3; Cp* = pentamethylcyclopentadienyl} with cerium ammonium nitrate (CAN) and NaIO4 (sacrificial oxidants, SOs) have been studied by Clark electrode measurements (both in solution and in the gas phase), on‐line mass spectrometry, manometry and UV/Vis spectroscopy. Furthermore, cyclic voltammetry has been applied to evaluate the relative tendency of 1 and 2 to be oxidized. The turnover frequency (TOF) increases as the ratio (R) between the concentration of the SO and that of the catalyst increases. O2 production with CAN is observed in experiments with R = 20 for 1 and 3, whereas O2 becomes detectable with 2 only when R = 40. Catalyst 2 has the highest tendency to be oxidized to IrIV and forms a blue intermediate I characterized by a UV/Vis band at 574 nm. The formation of I occurs with the same velocity as that of the production of O2, which indicates that I is a species directly involved in the catalytic cycle. The disappearance of I, when O2 evolution is finished, is a second‐order process more than one order of magnitude slower than O2 production and is strongly accelerated by the presence of benzyl alcohol. This suggests that I is a molecular species that slowly undergoes disproportion when catalysis is over. Experiments in which multiple aliquots of SO (CAN) were added (R = 20 and 40) indicate that catalysts 1–3 can reinitiate the catalytic cycle once they have been kept in a dormant state for 0–9 min; the TOFs of the second and third additions are approximately equal and higher than that of the first addition. By combining manometry and on‐line mass spectrometry measurements, it was found that O2 evolution is parallel to the production of a small amount of CO2 owing to catalyst degradation. The TOFs of the experiments performed with NaIO4 as the SO are about 2–3 times lower than those with CAN, but the same reactivity order is found 3 > 2 > 1. The activation parameters were evaluated with NaIO4 for all catalysts and with CAN for 2 at 10–45 °C. ΔG# is practically the same in all situations (25–26 kcal mol–1), whereas ΔH# is appreciably lower for 2 (13.1 kcal mol–1 with CAN and 13.3 kcal mol–1 with NaIO4) than for 1 (16 kcal mol–1) and 3 (16.9 kcal mol–1). The lowest enthalpic cost with 2 is balanced by the highest entropic cost (–41 cal mol–1 K–1) that approaches that typical for an associative bimolecular process
Low-temperature kinetic NMR studies on the insertion of a single olefin molecule into a Zr-C bond: Assessing the counterion-solvent interplay
No full textSticky counterions: Low-temperature kinetic NMR studies were performed to determine ΔH ≠ and ΔS ≠ values for the insertion of a single 2-methyl-1-heptene molecule into a Zr-C bond of [Cp 2Zr(η 2-CH 2NMePh)][X] (1a: X -=MeB(C 6F 5) 3 -, 1b: B(C 6F 5) 4 -) in [D 8]toluene and a 1:1 mixture of [D 8]toluene and [D 5]chlorobenzene. Both activation parameters critically depend on the interplay of the counterion and the solvent. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Assessing the Role of Counterion in Gold-Catalyzed Dearomatization of Indoles with Allenamides by NMR Studies
Full text linkThe counterion effect in the gold(I)-catalyzed
dearomative condensation
of indoles with allenamides is unveiled by means of 1D- and 2D-NMR
investigation. The different coordination ability and hydrogen bonding
tendency of TFA<sup>–</sup> and OTf<sup>–</sup> led
to specific interactions with the reaction partners dictating the
regiodivergent outcome
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