Supramolecular Silanol Chemistry in the Gas Phase. Topological (AIM) and Population (NBO) Analyses of Hydrogen-Bonded Complexes between H3SiOH and Selected O- and N-Acceptor Molecules

Thermally stable polyfluorinated monoalkoxysilanetriols and dialkoxydisiloxanetetrols

The reaction of the polyfluorinated lithium triarylmethanolates Ar₃COLi (Ar = C₆F₅, 3,5-(CF₃)₂C₆H₃) with SiCl₄ provided the monosubstituted products Ar₃COSiCl₃ (1a, Ar = C₆F₅; 1b, Ar = 3,5-(CF₃)₂C₆H₃). The hydrolysis of 1a and 1b produced the silanetriols Ar₃COSi(OH)₃ (2a, Ar = C₆F₅; 2b, Ar = 3,5-(CF₃)₂C₆H₃) without the aid of an HCl scavenger. The reaction of two equivalents of Ar₃COLi (Ar = C₆F₅, 3,5-(CF₃)₂C₆H₃) with (Cl₃Si)₂O afforded the disubstituted products [(C₆F₅)₃COSiCl₂]₂O (3a) and {[(3,5-(CF₃)₂C₆H₃)₃CO]SiCl₂}₂O (3b), the hydrolysis of which gave the corresponding disiloxanetetraols [(C₆F₅)₃COSi(OH)₂]₂O (4a) and [(3,5-(CF₃)₂C₆H₃)₃COSi(OH)₂]₂O (4b). At high concentrations in the presence of HCl, 2b undergoes controlled condensation to yield 4b. In the solid-state, 2a, 4a and 4b are mainly associated by hydrogen bonds of the type SiO-H⋯O(H)Si whereas the competing SiO-H⋯O(C)Si type was not observed.

Perfluorinated Trialkoxysilanol with Dramatically Increased Brønsted Acidity

The Brønsted acidity of the perfluorinated trialkoxysilanol {(F₃C)₃CO}₃SiOH is more than 13 orders of magnitude higher than that of orthosilicic acid, Si(OH)₄, and even more for most previously known silanols. It is easily deprotonated by simple amines and pyridines to give the conjugate silanolates [OSi{OC(CF₃)₃}₃]⁻, which possess extremely short Si−O bonds, comparable to those of silanones.

Inter-molecular inter-actions in a phenol-substituted benzimidazole

Hydrogen bonding plays an important role in the design of solid-state structures and gels with desirable properties. 1-(4-Hydroxybenzyl)-2-(4-hydroxyphenyl)-5,6-dimethyl-1H-benzimidazole was isolated as the acetone disolvate, C22H20N2O2·2C3H6O. O-H⋯N hydrogen bonding between benz-imidazole mol-ecules results in chains parallel to [010]. One of the acetone solvate mol-ecules participates in O-H⋯O hydrogen bonding with the benzimidazole derivative. C-H⋯π inter-actions are observed in the extended structure. Hirshfeld surface analysis was used to explore the inter-molecular inter-actions and density functional theory was used to estimate the strength of the hydrogen bonds.

Covalency and Ionicity Do Not Oppose Each Other-Relationship Between Si-O Bond Character and Basicity of Siloxanes

Covalency and ionicity are orthogonal rather than antipodal concepts. We demonstrate for the case of siloxane systems [R₃ Si-(O-SiR₂ )_n -O-SiR₃ ] that both covalency and ionicity of the Si-O bonds impact on the basicity of the Si-O-Si linkage. The relationship between the siloxane basicity and the Si-O bond character has been under debate since previous studies have presented conflicting explanations. It has been shown with natural bond orbital methods that increased hyperconjugative interactions of LP(O)→σ*(Si-R) type, that is, increased orbital overlap and hence covalency, are responsible for the low siloxane basicity at large Si-O-Si angles. On the other hand, increased ionicity towards larger Si-O-Si angles has been revealed with real-space bonding indicators. To resolve this ostensible contradiction, we perform a complementary bonding analysis, which combines orbital-space, real-space, and bond-index considerations. We analyze the isolated disiloxane molecule H₃ SiOSiH₃ with varying Si-O-Si angles, and n-membered cyclic siloxane systems Si₂ H₄ O(CH₂ )_n-3 . All methods from quite different realms show that both covalent and ionic interactions increase simultaneously towards larger Si-O-Si angles. In addition, we present highly accurate absolute hydrogen-bond interaction energies of the investigated siloxane molecules with water and silanol as donors. It is found that intermolecular hydrogen bonding is significant at small Si-O-Si angles and weakens as the Si-O-Si angle increases until no stable hydrogen-bond complexes are obtained beyond φ_SiOSi =168°, angles typically displayed by minerals or polymers. The maximum hydrogen-bond interaction energy, which is obtained at an angle of 105°, is 11.05 kJ mol^-1 for the siloxane-water complex and 18.40 kJ mol^-1 for the siloxane-silanol complex.

Ordered hybrids from template-free organosilane self-assembly

Despite considerable achievements over the last two decades, nonporous organic-inorganic hybrid materials are mostly amorphous, especially in the absence of solvothermal processes. The organosilane self-assembly approach is one of the few opportunities for creating a regular assembly of organic and inorganic moieties. Additionally, well-established organosilicon chemistry enables the introduction of numerous organic functionalities. The synthesis of periodically ordered hybrids relies on mono-, bis-, or multisilylated organosilane building blocks self-assembling into hybrid mesostructures or superstructures, subsequently cross-linked by siloxane Si-O-Si condensation. The general synthesis procedure is template-free and one-step. However, three concurrent processes underlie the generation of self-organized hybrid networks: thermodynamics of amphiphilic aggregation, dynamic self-assembly, and kinetically controlled sol-gel chemistry. Hence, the set of experimental conditions and the precursor structure are of paramount importance in achieving long-range order. Since the first developments in the mid-1990s, the subject has seen considerable progress leading to many innovative advanced nanomaterials providing promising applications in membranes, pollutant remediation, catalysis, conductive coatings, and optoelectronics. This work reviews, comprehensively, the primary evolution of this expanding field of research.

The Nature of Hydrogen Bonding Involving the Siloxane Group

Variation of the Si–O–Si angle in siloxane compounds is a way to tune their basicity from highly hydrophobic systems at linear geometry to hydrophilic systems at small angles. This has great potential in the design of new siloxane materials with properties distinct from those of known silicones. We investigate hydrogen bonds with the siloxane linkage as an acceptor in a large range of Si–O–Si angles for the two hydrogen-bonded complexes disiloxane⋯silanol [(H3Si)2O⋯HOSiH3] and disiloxane⋯water [(H3Si)2O⋯HOH] with free disiloxane [H3SiOSiH3] as reference in a quantum-mechanical ab-initio study. Geometry, electron density, and the electron localizability indicator provide several complementary indicators of hydrogen bonding which show how Si–O–Si angle variation affects the nature and strength of these unusual hydrogen bonds.

Molecular orbital model of the influence of interaction between O2 and aluminosilicate sites on the triplet-singlet energy gap and reactivity

The behavior of O(2) molecule in models of acid aluminosilicate sites on any kind of material was investigated using reliable QM ab initio calculations. The triplet-singlet energy gap of isolated O(2) was calculated at confident levels of theory with different basis sets as a reference. Models of aluminosilicate active sites interacting with oxygen in their singlet and triplet electronic states were considered for two kinds of O(2) arrangements. Geometry optimizations were performed on both non-corrected and corrected BSSE potential energy surfaces, realizing that good modeling of heavy atom-hydrogen interactions is sensitive to BSSE corrections during these processes. Energies were further evaluated at higher level of theory to test tendencies. Singlet oxygen appears more attractive to active aluminosilicate sites than those calculated with triplet oxygen, indicating a source of oxidative efficiency for designed nanostructures containing such molecular residues. It was clearly seen that aluminosilicate groups, appearing ubiquitously in several materials, could reduce the O(2) triplet-singlets energy gap by at least 10 kJ/mol. Some elegant features of oxygen interactions with such sites were further analyzed by means of the atoms in molecules (AIM) theory.

Formation and hydrogen bonding of a novel POSS-trisilanol

The formation of t-butyl substituted polyhedral silsesquioxanes (POSS) starting from the corresponding silanetriol is reported. Several intermediates involved in this complex condensation process could be identified spectroscopically. In addition, the initial condensation intermediate t-Bu(2)Si(2)O(OH)(4), as well as t-Bu(6)Si(6)O(9) and t-Bu(7)Si(7)O(9)(OH)(3), could be characterized by X-ray crystallography. Owing to the high crystal quality it was possible to experimentally determine the geometric parameters of the unusual cyclic hydrogen bond network in a dimeric POSS trisilanol for the first time. Furthermore, we investigated the structural changes concomitant with dimer formation/dissociation of the t-butyl substituted POSS trisilanol and its methyl analogue.

Bonding in tropolone, 2-aminotropone, and aminotroponimine: no evidence of resonance-assisted hydrogen-bond effects

The properties of the intramolecular hydrogen bond (IMHB) in tropolone, aminotropone, and aminotroponimine have been compared with those in the corresponding saturated analogues at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In general, all those compounds in which the seven-membered ring is unsaturated exhibit a stronger IMHB than their saturated counterparts. Nevertheless, this enhanced strength is not primarily due to resonance-assisted hydrogen-bond effects, but to the much higher intrinsic basicity and acidity of the hydrogen-bond acceptor and donor groups, respectively, in the unsaturated compounds. These acidity and basicity enhancements have a double origin: 1) the unsaturated nature of the moiety to which the hydrogen-bond donor and acceptor are attached and 2) the cyclic nature of the compounds under scrutiny. As has been found for hydroxymethylene and aminomethylene cyclobutanones, and cyclobutenones and their nitrogen-containing analogues, the IMHB strength follows the [donor, acceptor] trend: [OH, C=NH]>[OH, C=O]>[NH(2), C=NH]>[NH(2), C=O] and fulfills a Steiner-Limbach correlation similar to that followed by intermolecular hydrogen bonds.

Non-resonance-assisted hydrogen bonding in hydroxymethylene and aminomethylene cyclobutanones and cyclobutenones and their nitrogen counterparts

The characteristics of intramolecular hydrogen bonds (IMHB) have been systematically analyzed for a series of 32 different enols of derivatives of cyclobutane, cyclobutene, and cyclobutadiene bearing oxygen and nitrogen functionalities, at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In those cases where two tautomers (interconnected by a hydrogen shift through the IMHB) exist, tautomer a, in which the HB-donor group (YH) is attached to the four-membered ring, is less stable than tautomer b, in which is the HB-acceptor (X) is the one attached to the four-membered ring. As expected the OH group behaves as a better HB-donor than the NH(2) group and the C==NH group as a better HB-acceptor than the C==O group, although the first effect clearly dominates. Accordingly, the expected IMHB strength follows the [donor, acceptor] trend: [OH,C==NH]>[OH,C==O]>[NH(2),C==NH]>[NH(2),C==O]. Quite unexpectedly, in all those compounds in which the functionality exhibiting the IMHB is unsaturated, the IMHB is weaker than in their saturated counterparts, verifying that the primary effect on the strength of the IMHB is the structure of the sigma-skeleton of the system. In the conjugated systems investigated here, the severe constraints imposed by the four-membered ring force the HB donor and acceptor to be far apart and the IMHB is rather weak. These geometrical constraints are less severe for the saturated analogues and the IMHB becomes stronger, confirming that the characteristics of the sigma-skeleton, and not the resonance-assisted hydrogen bond (RAHB) phenomenon, are the primary contributors to the strength of the IMHB in conjugated compounds.