1H, 19F, and 15N NMR study of the interaction between bis(pentafluorophenyl)borinic acid and nitrogen bases

Bis(pentafluorophenyl)borinic acid ((ArF)2BOH, ArF = C6F5, 1) is a molecule that, due to its multiple reactivity, can behave in different and not always straightforward ways in the presence of nucleophiles. It has been already shown its behaviour in dicloromethane solution in the presence of water, THF and methanol. Briefly, it exists as monomer and trimer (1m and 1t, see Chart 1), and the presence of nucleophiles strongly influence the thermodynamic and the kinetic of the monomer-trimer interconversion. Moreover, according to the nature of the nucleophile, 1 shows a camaleonic nature by forming several, sometimes unexpected, species.2,3 We have studied now the reaction of 1 in the presence of nitrogen bases, that can act as Lewis and Br\uf8nsted bases. Two nitrogen bases were studied, namely 1,8-bis(dimethylamino)naphthalene (DMAN), which can act as Br\uf8nsted base only, and pyridine, which is a good Lewis base but weak Br\uf8nsted base. First of all, the Br\uf8nsted base DMAN has been proved to be more efficient than oxygenated Lewis bases in catalyzing the trimerization process. Indeed it is enough a catalytic amount of base to cause the complete trimerization of 1. The so obtained trimeric deprotonated anion 2 (Chart 2) is unstable with respect to dearylation reactions, leading eventually (in the presence of more than 0.33 equiv of DMAN and at higher temperatures) to the tetrarylic species 3 (Chart 2). On the other hand, the behaviour of pyridine is quite different, since 0.33 equiv of base are requested to complete the trimerization of 1, and also in this case the obtained trimer is anionic. Moreover, the presence of 1 equivalent of pyridine does not cause the dearylation process but gives rise to the monomeric neutral 1:1 adduct 4 (Chart 2). Both the identification and the characterization of the involved species were performed by multiparametric and multinuclear low temperature NMR spectroscopy

1H NMR Characterization of Organic and Inorganic Nanoparticles

Drug delivery employing nano-object as liposomes, polymer conjugates, and nanoparticles suspended in solution is a subject of high current interest [1]. The characterization of the size and the surface functionalization of these nanoparticles is of primary importance. Microscopy techniques give information on deposited colloidal samples, after solvent evaporation, so that the correspondence with the nature of the species in solution is not granted. Dynamic Light Scattering (DLS), which is usually used to estimate the size of a colloidal sample in solution, can overestimate the radii of very small nanoparticles [2]. Diffusion NMR, and in particular Pulsed gradient spin-echo (PGSE) technique, has recently emerged as a valuable tool for colloids characterization [3], complementary to DLS from the point of view of the size evaluation, being highly reliable for the measurement of the smallest particles. Moreover, NMR provide information not only on the size, but also on the interaction between the capping ligands and the nanoparticle surface. In this work, we present the characterization through 1H PGSE NMR measurements of the size of spherical and rod-like TiO2/oleic acid nanoparticles and of conjugates between Re complexes and polyamidoamine nanoparticles. Moreover, by 1H NMR experiments the interaction between TiO2 and the capping oleic acid (OA) has been characterized. [1] a) A. H. Faraji, P. Wipf, Bioorganic and Medicinal Chemistry 2009, 17, 2950-2962; b) S. M. Garg, A. V. Deshmukh, Nano Science and Nano Technology 2007, 1(2), 45-58; c) C. Khemtong, C. W. Kessinger, J. Gao, Chem. Commun. 2009, 3497-3510. [2] C. Sanna, C. La Mesa, L. Mannina, P. Stano, S. Viel, A. Segre, Langmuir 2006, 22, 6031-6041. [3] a) F. Ribot, V. Escax, C. Roiland, C. Sanchez, J. C. Martins, M. Biesemans, I. Verbruggen, R. Willem, Chem. Commun. 2005, 1019-1021; b) M. Valentini, A. Vaccaro, A. Rehor, A. Napoli, J. A. Hubbell, N. Tirelli, J. Am. Chem. Soc. 2004, 126, 2142-2147

Mesit­yl(2,4,6-trimeth­oxy­phen­yl)borinic acid

In the title mol­ecule, C18H23BO4, the dihedral angle between the least-squares planes of the aromatic rings is 84.88 (3)°. The B atom deviates by 0.202 (1) Å from the least-squares plane of the mesityl ring. All of the meth­oxy groups are approximately coplanar with the 2,4,6-trimeth­oxy­phenyl ring, whereas the BOH group is twisted with respect to it by 19.5°. The borinic OH group is engaged in an intra­molecular O—H⋯O hydrogen bond with one of ortho-meth­oxy groups. The mol­ecular structure is stabilized by weak C—H⋯O contacts. In the crystal, mol­ecules are linked by weak C—H⋯O and C—H⋯π inter­actions, generating a three-dimensional network

Lipophilic Cage Ligands: Synthesis, Spectroscopic Properties and Applications

Lipophilic Cage Ligands: Synthesis, Spectroscopic Properties and Application

Solution conformation and dynamics of the ion pairs originating from the reaction of B(C6F5)(3) with bisindenyl dimethyl zirconium complexes

The two ion pairs [(4,7-Me(2)indenyl)(2)ZrMe](+)[MeB(C6F5)(3)](-) (1b) and [(indenyl)(2)ZrMe](+) [MeB(C6F5)(3)](-) (2b) have been generated in situ by reaction of stoichiometric B(C6F5)(3) with the corresponding dimethyl zirconocenes. It has been shown that molecular mechanics computations, guided by experimental H-1/H-1 NOE correlations, can provide information on the conformers present in solution. The dynamics of the ion pairs has also been investigated, showing the occurrence of both the processes previously characterized for this class of compounds, namely the B(C6F5)(3) migration between the two methyl groups and dissociation-recombination of the whole [MeB(C6F5)(3)](-) anion, the latter process being much faster than the first one (about three order of magnitude). Moreover, it has been shown that in certain conditions intermolecular processes can occur, which mimic the above-mentioned dissociative exchanges. In particular, the presence of species containing loosely bound [MeB(C6F5)(3)](-) anion fastens the exchange of this anion, while the presence of free B(C6F5)(3) accelerates its exchange between the two methyl sites

Competition studies on the activation of the C-H bond of diazines by the unsaturated triangular cluster anion [Re3(\u3bc-H) 4(CO)10]

The reactions of the unsaturated cluster anion [Re3(\u3bc-H) 4(CO)10]- (1) with 1,2-, 1,3- and 1,4-diazines (used as solvents) have been investigated. The reaction with 1,2-diazine at room temperature gives quantitatively the anion [Re3(\u3bc-H) 2(\u3bc-\u3b72-N2C4H 4)(CO)10]- (3), containing a diazine bridging through the two N atoms, characterised by single crystal X-ray analysis. The orthometallated isomer [Re3(\u3bc-H)3(\u3bc-\u3b7 2-N2C4H3)(CO)10] - (4) was obtained by refluxing 3 in toluene. The reaction of [NEt4]1 in molten 1,4-diazine (60\ub0C) affords the anion [Re 3(\u3bc-H)3(\u3bc-\u3b72-N2C 4H3)(CO)10]- (5), containing orthometallated pyrazine. The slow reaction of [NEt4]1 with 1,3-diazine at room temperature gives two orthometallated isomers 6, corresponding to the metallation of carbon C2 (6a) and C6 (6b), respectively (6b/6a ratio ca. 1.2). Differently from what previously found for the analogous cluster anion containing orthometallated pyridine, the metallation reaction of diazines showed little reversibility, and poor conversion of 5 and 6 to the starting anion 1 was observed after several days under 100 atm of H 2. Competition experiments, performed by dissolving [NEt 4]1 in equimolar mixtures of pyridine/pyrazine or pyridine/pyrimidine, showed that the anion with metallated pyridine was the kinetically preferred product and that at longer times it slowly converted to the derivatives with metallated diazines (5 or 6). \ua9 2003 Elsevier Science B.V. All rights reserved

Aggregation and ionization equilibria of bis(pentafluorophenyl)borinic acid driven by hydrogen-bonding with tetrahydrofuran

Bis(pentafluophenyl)borinic acid, Ar2BOH (1, Ar = C6F5), in dichloromethane solution is present as an equilibrium mixture of monomeric (1m) and trimeric (1t) forms. Previous studies showed that water affects both the position and the rate of this equilibrium. Here, the behavior of 1 in the presence of tetrahydrofuran (THF), a nucleophile able to behave as a Lewis base and H-bond acceptor only, has been studied, by monitoring with H-1 and F-19 NMR the course of titrations performed directly into NMR tubes. The addition, at 183 K, of 0.33 equiv of THF caused the instantaneous and quantitative formation of the hydrogen-bonded adduct between the trimer 1t and one molecule of THF. Homo- and heteronuclear 2D NMR correlation experiments led to a solution structure consistent with the C2-optimized geometry obtained by PM3 computations. The H-bonding of the THF molecule causes major deformations of the molecular geometry of the trimer, so that only one molecule of THF can interact with the trimer, in spite of its three OH groups. Intra- and intermolecular exchange processes involving this adduct have been investigated by 2D EXSY experiments, showing flopping of the cycle conformation, rotation of the aromatic rings around their B-C bonds, and exchange of THF among the three OH groups, in addition to the exchange between free 1t and the adduct. When the amount of added THF was higher than 0.33 equiv, an unexpected ionization process occurred, leading to the cation [Ar2B(OH2)2]+ and to deprotonated 1t, i.e., to the anion [Ar6B3O3H2]- of Cs symmetry. On increasing the temperature, progressive partial fragmentation of the trimeric species was observed. Both B-11 NMR evidence and PM3 computations indicated that, at variance with what is observed in the interaction with H2O, the interaction between THF and 1m occurs preferentially via an H-bonded adduct, Ar2BO-H...THF, rather than a Lewis acid-base complex, Ar2B(OH)(THF). This confirms the poor Lewis acidity of the boron atom of 1m