¹H and ¹⁹F NMR Investigation of the Reaction of B(C₆F₅)₃ with Water in Toluene Solution

Titrations 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(OH2)]·H2O (3) and [(C6F5)3B(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)]·H2O (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)3B(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

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 1H and 19F 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 11B 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

Bis(pentafluorophenyl)borinic Acid: a Cyclic Trimer in the Solid State and a Monomer, with Hindered Rotation around the B−OH Bond, in Solution

The title molecule in the solid state exists as a cyclic trimer, with B−O(H)−B bridges and a cyclohexane-like structure (C2 twist-boat conformation); dissolution in toluene-d8 affords the B(C6F5)2OH monomer, in which the low-temperature 19F NMR data reveal restricted rotation of the OH substituent around the Ar2B−OH bond (Ea = 39 kJ mol-1), as a result of the partial double-bond character of this interaction

The Role of Water in the Oligomerization Equilibria Involving Bis(pentafluorophenyl)borinic Acid in Dichloromethane Solution

The 1H, 19F, and 11B NMR data indicated that in CD2Cl2 solution monomeric bis(pentafluorophenyl)borinic acid, (C6F5)2BOH (1m), is in equilibrium with the cyclic trimer (1t) observed in the solid state. The position of the association equilibrium shifted to the right on increasing the concentration, on decreasing the temperature, and on decreasing solvent polarity, in the series CD2Cl2, CDCl3, CCl4, in agreement with the higher polarity of the monomer (2.38 D for 1m and 0.65 D for 1t, according to PM3 computations). At temperatures lower than 210 K the 1H and 19F NMR spectra revealed the simultaneous (reversible) formation of two novel compounds, which have been formulated as the (C6F5)2BOB(C6F5)2 anhydride (2) and the trimeric species [(C6F5)2BOH]3·OH2 (3), of C2 symmetry, with a water molecule formally inserted into a B−O(H)−B bridge of 1t, to give a very strong BO(H)···HO(H)B hydrogen bond (δ 18.6). 1H and 19F EXSY experiments at 184 K revealed exchange between 3 and 1m, and not 1t. The data showed that the formation of 3, observed at temperatures where the monomer−trimer equilibrium is frozen, occurs by aggregation of monomeric units and not by cycle opening from 1t. The stabilization of the water molecule in 3 is strong enough to promote the dehydration of 1 to give the anhydride 2; for entropic reasons, the reaction occurs only at very low temperatures and is reversed on raising the temperature. At higher temperatures, the position of the monomer−trimer equilibrium is affected by the amount of water, which stabilizes the trimeric form, owing to the formation of a hydrogen-bond adduct 4 containing exocyclic water. At low temperatures, in the presence of the monomer, this species progressively dehydrated, due to the formation of 3. The amount of water present in solution also affected the rate of attainment of the 1m/1t equilibrium, the oligomerization being exceedingly slow in anhydrous conditions. The catalytic role of water can be attributed to the increased nucleophilicity of the BOH group upon water coordination, which allows alternative aggregation pathways. Semiempirical computations, at the PM3 level, provided a picture of the oligomerization in the presence and in the absence of water that well agrees with the experimental findings

Electrochemical, Computational, and Photophysical Characterization of New Luminescent Dirhenium–Pyridazine Complexes Containing Bridging OR or SR Anions

A series of [Re2(μ-ER)2(CO)6(μ-pydz)] complexes have been synthesized (E = S, R = C6H5, 2; E = O, R = C6F5, 3; C6H5, 4; CH3, and 5; H, 6), starting either from [Re­(CO)5O3SCF3] (for 2 and 4), [Re2(μ-OR)3(CO)6]− (for 3 and 5), or [Re4(μ3-OH)4(CO)12] (for 6). Single-crystal diffractometric analysis showed that the two μ-phenolato derivatives (3 and 4) possess an idealized C2 symmetry, while the μ-benzenethiolato derivative (2) is asymmetrical, because of the different conformation adopted by the phenyl groups. A combined density functional and time-dependent density functional study of the geometry and electronic structure of the complexes showed that the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are the two lowest-lying π* orbitals of pyridazine, whereas the highest occupied molecular orbitals (HOMOs) are mainly constituted by the “t2g” set of the Re atoms, with a strong Re–(μ-E) π* character. The absorption spectra have been satisfactorily simulated, by computing the lowest singlet excitation energies. All the complexes exhibit one reversible monoelectronic reduction centered on the pyridazine ligand (ranging from −1.35 V to −1.53 V vs Fc+|Fc). The benzenethiolato derivative 2 exhibits one reversible two-electron oxidation (at 0.47 V), whereas the OR derivatives show two close monoelectronic oxidation peaks (ranging from 0.85 V to 1.35 V for the first peak). The thioderivative 2 exhibits a very small electrochemical energy gap (1.9 eV, vs 2.38–2.70 eV for the OR derivatives), and it does not show any photoluminescence. The complexes containing OR ligands show from moderate to poor photoluminescence, in the range of 608–708 nm, with quantum yields decreasing (ranging from 5.5% to 0.07%) and lifetimes decreasing (ranging from 550 ns to 9 ns) (3 > 4 > 6 ≈ 5) with increasing emission wavelength. The best emitting properties, which are closely comparable to those of the dichloro complex (1), are exhibited by the pentafluorophenolato derivative (3)

Exploiting Ultrashort α,β-Peptides in the Colloidal Stabilization of Gold Nanoparticles

Colloidal gold nanoparticles (GNPs) have found wide-ranging applications in nanomedicine due to their unique optical properties, ease of preparation, and functionalization. To avoid the formation of GNP aggregates in the physiological environment, molecules such as lipids, polysaccharides, or polymers are employed as GNP coatings. Here, we present the colloidal stabilization of GNPs using ultrashort α,β-peptides containing the repeating unit of a diaryl β2,3-amino acid and characterized by an extended conformation. Differently functionalized GNPs have been characterized by ultraviolet, dynamic light scattering, and transmission electron microscopy analysis, allowing us to define the best candidate that inhibits the aggregation of GNPs not only in water but also in mouse serum. In particular, a short tripeptide was found to be able to stabilize GNPs in physiological media over 3 months. This new system has been further capped with albumin, obtaining a material with even more colloidal stability and ability to prevent the formation of a thick protein corona in physiological media

Electrochemical, Computational, and Photophysical Characterization of New Luminescent Dirhenium–Pyridazine Complexes Containing Bridging OR or SR Anions

A series of [Re2(μ-ER)2(CO)6(μ-pydz)] complexes have been synthesized (E = S, R = C6H5, 2; E = O, R = C6F5, 3; C6H5, 4; CH3, and 5; H, 6), starting either from [Re­(CO)5O3SCF3] (for 2 and 4), [Re2(μ-OR)3(CO)6]− (for 3 and 5), or [Re4(μ3-OH)4(CO)12] (for 6). Single-crystal diffractometric analysis showed that the two μ-phenolato derivatives (3 and 4) possess an idealized C2 symmetry, while the μ-benzenethiolato derivative (2) is asymmetrical, because of the different conformation adopted by the phenyl groups. A combined density functional and time-dependent density functional study of the geometry and electronic structure of the complexes showed that the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are the two lowest-lying π* orbitals of pyridazine, whereas the highest occupied molecular orbitals (HOMOs) are mainly constituted by the “t2g” set of the Re atoms, with a strong Re–(μ-E) π* character. The absorption spectra have been satisfactorily simulated, by computing the lowest singlet excitation energies. All the complexes exhibit one reversible monoelectronic reduction centered on the pyridazine ligand (ranging from −1.35 V to −1.53 V vs Fc+|Fc). The benzenethiolato derivative 2 exhibits one reversible two-electron oxidation (at 0.47 V), whereas the OR derivatives show two close monoelectronic oxidation peaks (ranging from 0.85 V to 1.35 V for the first peak). The thioderivative 2 exhibits a very small electrochemical energy gap (1.9 eV, vs 2.38–2.70 eV for the OR derivatives), and it does not show any photoluminescence. The complexes containing OR ligands show from moderate to poor photoluminescence, in the range of 608–708 nm, with quantum yields decreasing (ranging from 5.5% to 0.07%) and lifetimes decreasing (ranging from 550 ns to 9 ns) (3 > 4 > 6 ≈ 5) with increasing emission wavelength. The best emitting properties, which are closely comparable to those of the dichloro complex (1), are exhibited by the pentafluorophenolato derivative (3)

¹⁹F-NMR spectra recorded after the interaction of c-FABPL with increasing amounts of dexamethasone, from 0.5 to 2 equivalents (traces a-d).

<p>The spectrum of the ligand in the absence of the protein is reported in trace e. Operating conditions: phosphate buffer pH 7.4, 7 T, 298 K.</p

Tuning Polyamidoamine Design To Increase Uptake and Efficacy of Ruthenium Complexes for Photodynamic Therapy

In this work, we report the synthesis of [Ru­(phen)32+]-based complexes and their use as photosensitizers for photodynamic therapy (PDT), a treatment of pathological conditions based on the photoactivation of bioactive compounds, which are not harmful in the absence of light irradiation. Of these complexes, Ru-PhenISA and Ru-PhenAN are polymer conjugates containing less than 5%, (on a molar basis), photoactive units. Their performance is compared with that of a small [Ru­(phen)32+] compound, [Ru­(phen)2BAP]­(OTf)2 (BAP = 4-(4′-aminobutyl)-1,10-phenanthroline, OTf = triflate anion), used as a model of the photoactive units. The polymer ligands, PhenISA and PhenAN, are polyamidoamines with different acid–base properties. At physiological pH, the former is zwitterionic, the latter moderately cationic, and both intrinsically cytocompatible. The photophysical characterizations show that the complexation to macromolecules does not hamper the Ru­(phen)32+ ability to generate toxic singlet oxygen upon irradiation, and phosphorescence lifetimes and quantum yields are similar in all cases. All three compounds are internalized by HeLa cells and can induce cell death upon visible light irradiation. However, their relative PDT efficiency is different: the zwitterionic PhenISA endowed with the Ru-complex lowers the PDT efficiency of the free complex, while conversely, the cationic PhenAN boosts it. Flow cytometry demonstrates that the uptake efficiency of the three agents reflects the observed differences in PDT efficacy. Additionally, intracellular localization studies show that while [Ru­(phen)2BAP]­(OTf)2 remains confined in vesicular structures, Ru-PhenISA localization is hard to determine due to the very low uptake efficiency. Very interestingly, instead, the cationic Ru-PhenAN accumulates inside the nucleus in all treated cells. Overall, the results indicate that the complexation of [Ru­(phen)2BAP]­(OTf)2 with a cationic polyamidoamine to give the Ru-PhenAN complex is an excellent strategy to increase the Ru-complex cell uptake and, additionally, to achieve accumulation at the nuclear level. These unique features together make this compound an excellent photosensitizer with very high PDT efficiency