Lewis Acid Behavior of SF4: Synthesis, Characterization, and Computational Study of Adducts of SF4with Pyridine and Pyridine Derivatives

Illuminating the multiple Lewis acidity of triaryl-boranes via atropisomeric dative adducts

Using the principle that constrained conformational spaces can generate novel and hidden molecular properties, we challenged the commonly held perception that a single-centered Lewis acid reacting with a single-centered Lewis base always forms a single Lewis adduct. Accordingly, the emergence of single-centered but multiple Lewis acidity among sterically hindered and non-symmetric triaryl-boranes is reported. These Lewis acids feature several diastereotopic faces providing multiple binding sites at the same Lewis acid center in the interaction with Lewis bases giving rise to adducts with diastereomeric structures. We demonstrate that with a proper choice of the base, atropisomeric adduct species can be formed that interconvert via the dissociative mechanism rather than conformational isomerism. The existence of this exotic and peculiar molecular phenomenon was experimentally confirmed by the formation of atropisomeric piperidine-borane adducts using state-of-the-art NMR techniques in combination with computational methods.

Fluoride-Ion Donor Properties of AsF5

Arsenic pentafluoride undergoes ligand-induced autoionization in the presence of 1,10-phenanthroline (phen) in a SO2ClF solution to form the donor-stabilized [AsF4(phen)][AsF6] salt. Reacting [AsF4(phen)][AsF6] with the strong Lewis acid SbF5·SO2 yields the mixed arsenic-antimony salt [AsF4(phen)][Sb2F11]. These salts are the first examples of crystallographically characterized donor-stabilized [AsF4]+ cations. The analogous reaction of AsF5 and 2,2'-bipyridine (bipy) does not result in autoionization but leads to the formation of the neutral 2:1 adduct (AsF5)2·bipy. The gas-phase and solution fluoride-ion affinities of [AsF4]+ and [SbF4]+ were calculated, revealing them to be incredibly strong Lewis acids. Density functional theory calculations and natural bond orbital analysis show that significant electron-pair donation from phen to the As center in [AsF4(phen)]+ occurs and quenches the extreme electrophilicity of the [AsF4]+ cation.

Square tetravalent chalcogen bonds in dimeric aggregates: a joint crystallographic survey and theoretical study

Square tetravalent chalcogen bonds were systematically investigated through a combination of crystal structure analysis and DFT calculations.

Recent advances in sulfur tetrafluoride chemistry: syntheses, structures, and applications

Sulfur and fluorine occupy crucial positions in main group chemistry because these two elements form a variety of compounds with versatile bond modalities and unique functionalities. Among sulfur-fluorine compounds, the importance of SF₄ and its derivatives is recognized in the literature. The amphoteric nature of SF₄ results in its rich Lewis acidic and basic reactivities; the reactions with F^- acceptors and donors yield [SF₃]⁺ and [SF₅]^- salts, respectively. Lewis basic molecules can also form adducts with SF₄via various interaction motifs. The deoxofluorinating properties of SF₄ have been used by organic chemists to selectively introduce fluorine atoms in specific substrates, extending also to industrial applications. Although the properties and reactivity of SF₄ have been studied since its first synthesis, the recent progress in the SF₄-related chemistry is striking, involving various fields of chemistry. In this Frontier article, recent advances, mainly the last ten years, in syntheses and structures of SF₄-related compounds including its cationic and anionic derivatives and adducts with Lewis bases are concisely reviewed. Their uses in fundamental and applied inorganic chemistries are also described.

Chalcogen versus Dative Bonding in [SF3]+ Lewis Acid-Base Adducts: [SF3(NCCH3)2]+, [SF3(NC5H5)2]+, and [SF3(phen)]+ (phen = 1,10-phenanthroline)

The Lewis-acid behavior of [SF₃][MF₆] (M = Sb, As) salts toward mono- and bidentate nitrogen bases was explored. Reactions of [SF₃][MF₆] with excesses of CH₃CN and C₅H₅N yielded [SF₃(L)₂]⁺ (L = CH₃CN, C₅H₅N) salts, whereas the reaction of [SF₃][SbF₆] with equimolar 1,10-phenanthroline (phen) in CH₃CN afforded [SF₃(phen)][SbF₆]·2CH₃CN. Salts of these cations were characterized by low-temperature X-ray crystallography and Raman spectroscopy in the solid state as well as by ¹⁹F NMR spectroscopy in solution. In the solid state, the geometries of [SF₃(NC₅H₅)₂]⁺ and [SF₃(phen)]⁺ are square pyramids with negligible cation-anion contacts, whereas the coordination of CH₃CN and [SbF₆]^- to [SF₃]⁺ in [SF₃(NCCH₃)₂][SbF₆] results in a distorted octahedral coordination sphere with a minimal perturbation of the trigonal-pyramidal SF₃ moiety. ¹⁹F NMR spectroscopy revealed that [SF₃(L)₂]⁺ is fluxional in excess L at -30 °C, whereas [SF₃(phen)]⁺ is rigid in CH₂Cl₂ at -40 °C. Density functional theory (DFT-B3LYP) calculations suggest that the S-N bonds in [SF₃(NC₅H₅)₂]⁺ and [SF₃(phen)]⁺ possess substantial covalent character and result in a regular AX₅E VSEPR geometry, whereas those in [SF₃(NCCH₃)₂]⁺ are best described as S···N chalcogen-bonding interactions via σ-holes on [SF₃]⁺, which is consistent with the crystallographic data.

Participation of S and Se in hydrogen and chalcogen bonds

The heavier chalcogen atoms S, Se, and Te can each participate in a range of different noncovalent interactions. They can serve as both proton donor and acceptor in H-bonds. Each atom can also act as electron acceptor in a chalcogen bond.

Reactions of Molybdenum and Tungsten Oxide Tetrafluoride with Sulfur(IV) Lewis Bases: Structure and Bonding in [WOF4]4, MOF4(OSO), and [SF3][M2O2F9] (M = Mo, W)

The structure of [WOF₄]₄ has been reinvestigated by low-temperature X-ray crystallography and DFT (MN15/def2-SVPD) studies. Whereas the W₄F₄ ring of the tetramer is planar and disordered in the solid state, the optimized gas-phase geometry prefers a disphenoidally puckered W₄F₄ ring and demonstrates asymmetric fluorine bridging. Dissolution of MOF₄ (M = Mo, W) in SO₂ and SF₄ results in the formation of MOF₄(OSO) and [SF₃][M₂O₂F₉], respectively. Both SO₂ adducts and [SF₃][Mo₂O₂F₉] have been characterized by X-ray crystallography. The crystal structure of [SF₃][Mo₂O₂F₉] reveals dimerization of the ion pair that results in a rare heptacoordinate sulfur center. Optimization of the {[SF₃][M₂O₂F₉]}₂ dimers in the gas phase, however, results in the elongation of one contact such that the sulfur centers are effectively hexacoordinate. Meanwhile, the crystal structure of [SF₃][W₂O₂F₉]·HF instead demonstrates hexacoordinate sulfur centers and a highly unusual coordination to [SF₃]⁺ from [W₂O₂F₉]^- through an oxido ligand. While [SF₃][W₂O₂F₉] does not decompose at ambient temperature, MOF₄(OSO) and [SF₃][Mo₂O₂F₉] are unstable toward evolution of SO₂ or SF₄. Computational studies reveal that the monomerization of [WOF₄]₄ in the gas phase at 25 °C is thermodynamically unfavorable using SO₂, but favorable using SF₄, consistent with the relative thermal stabilities of WOF₄(OSO) and [SF₃][W₂O₂F₉].

Reactivity of Binary and Ternary Sulfur Halides towards Transition-Metal Compounds

Binary sulfur fluorides exhibit an interesting reactivity towards transition metal complexes. They open up routes for the generation of sulfur‐containing building blocks. Often ligands with particular properties can be constructed. This includes their ability to transfer sulfur atoms or polysulfide units as well as fluorination reactions. This Minireview provides an insight into the reactivity of the binary and ternary sulfur halides S₂Cl₂, SCl₂, SF₄, SF₆ and SF₅Cl towards transition‐metal compounds.

Chalcogen bonding of two ligands to hypervalent YF4 (Y = S, Se, Te, Po)

The ability of two NH₃ ligands to engage in simultaneous chalcogen bonds to a hypervalent YF₄ molecule, with Y = S, Se, Te, Po, is assessed via quantum calculations. The complex can take on one of two different geometries. The cis structure places the two ligands adjacent to one another in a pseudo-octahedral geometry, held there by a pair of σ-hole chalcogen bonds. The bases can also lie nearly opposite one another, in a distorted octahedron containing one π-hole and one strained σ-hole bond. The cis geometry is favored for Y = S, while Te, and Po tend toward the trans structure; they are nearly equally stable for Se. In either case, the binding energy rises rapidly with the size of the Y atom, exceeding 30 kcal mol^-1 for PoF₄.

Effects of Halogen, Chalcogen, Pnicogen, and Tetrel Bonds on IR and NMR Spectra

Complexes were formed pairing FX, FHY, FH2Z, and FH3T (X = Cl, Br, I; Y = S, Se, Te; Z = P, As, Sb; T = Si, Ge, Sn) with NH3 in order to form an A⋯N noncovalent bond, where A refers to the central atom. Geometries, energetics, atomic charges, and spectroscopic characteristics of these complexes were evaluated via DFT calculations. In all cases, the A-F bond, which is located opposite the base and is responsible for the σ-hole on the A atom, elongates and its stretching frequency undergoes a shift to the red. This shift varies from 42 to 175 cm-1 and is largest for the halogen bonds, followed by chalcogen, tetrel, and then pnicogen. The shift also decreases as the central A atom is enlarged. The NMR chemical shielding of the A atom is increased while that of the F and electron donor N atom are lowered. Unlike the IR frequency shifts, it is the third-row A atoms that undergo the largest change in NMR shielding. The change in shielding of A is highly variable, ranging from negligible for FSnH3 all the way up to 1675 ppm for FBr, while those of the F atom lie in the 55-422 ppm range. Although smaller in magnitude, the changes in the N shielding are still easily detectable, between 7 and 27 ppm.

The Chalcogen Bond in Crystalline Solids: A World Parallel to Halogen Bond

The distribution of the electron density around covalently bonded atoms is anisotropic, and this determines the presence, on atoms surface, of areas of higher and lower electron density where the electrostatic potential is frequently negative and positive, respectively. The ability of positive areas on atoms to form attractive interactions with electron rich sites became recently the subject of a flurry of papers. The halogen bond (HaB), the attractive interaction formed by halogens with nucleophiles, emerged as a quite common and dependable tool for controlling phenomena as diverse as the binding of small molecules to proteinaceous targets or the organization of molecular functional materials. The mindset developed in relation to the halogen bond prompted the interest in the tendency of elements of groups 13-16 of the periodic table to form analogous attractive interactions with nucleophiles. This Account addresses the chalcogen bond (ChB), the attractive interaction formed by group 16 elements with nucleophiles, by adopting a crystallographic point of view. Structures of organic derivatives are considered where chalcogen atoms form close contacts with nucleophiles in the geometry typical for chalcogen bonds. It is shown how sulfur, selenium, and tellurium can all form chalcogen bonds, the tendency to give rise to close contacts with nucleophiles increasing with the polarizability of the element. Also oxygen, when conveniently substituted, can form ChBs in crystalline solids. Chalcogen bonds can be strong enough to allow for the interaction to function as an effective and robust tool in crystal engineering. It is presented how chalcogen containing heteroaromatics, sulfides, disulfides, and selenium and tellurium analogues as well as some other molecular moieties can afford dependable chalcogen bond based supramolecular synthons. Particular attention is given to chalcogen containing azoles and their derivatives due to the relevance of these moieties in biosystems and molecular materials. It is shown how the interaction pattern around electrophilic chalcogen atoms frequently recalls the pattern around analogous halogen, pnictogen, and tetrel derivatives. For instance, directionalities of chalcogen bonds around sulfur and selenium in some thiazolium and selenazolium derivatives are similar to directionalities of halogen bonds around bromine and iodine in bromonium and iodonium compounds. This gives experimental evidence that similarities in the anisotropic distribution of the electron density in covalently bonded atoms translates in similarities in their recognition and self-assembly behavior. For instance, the analogies in interaction patterns of carbonitrile substituted elements of groups 17, 16, 15, and 14 will be presented. While the extensive experimental and theoretical data available in the literature prove that HaB and ChB form twin supramolecular synthons in the solid, more experimental information has to become available before such a statement can be safely extended to interactions wherein elements of groups 14 and 15 are the electrophiles. It will nevertheless be possible to develop some general heuristic principles for crystal engineering. Being based on the groups of the periodic table, these principles offer the advantage of being systematic.

Unexpected chalcogen bonds in tetravalent sulfur compounds

Combined CSD analysis and theoretical calculations show the importance of the polarizability in chalcogen bonding interactions. We provide evidence that the Lewis base has a preference in some cases for the σ-hole that is opposite to the more polarizable group instead of the more electron withdrawing one.

Chalcogen bonding in synthesis, catalysis and design of materials

Chalcogen bonding is a type of noncovalent interaction in which a covalently bonded chalcogen atom (O, S, Se or Te) acts as an electrophilic species towards a nucleophilic (negative) region(s) in another or in the same molecule. In general, this interaction is strengthened by the presence of an electron-withdrawing group on the electron-acceptor chalcogen atom and upon moving down in the periodic table of elements, from O to Te. Following a short discussion of the phenomenon of chalcogen bonding, this Perspective presents some demonstrative experimental observations in which this bonding is crucial for synthetic transformations, crystal engineering, catalysis and design of materials as synthons/tectons.

S(vi) Lewis acids: fluorosulfoxonium cations

Avenues to S-based Lewis acids were developed via the oxidation of aryl-sulfoxides with XeF₂, giving difluorodiarylsulfoxides which react via fluoride abstraction to afford Lewis acidic fluorosulfoxonium cations; this acidity is derived from the S-F σ* orbital and has been probed both experimentally and computationally.

Synthesis and Characterization of Adducts between SF4 and Oxygen Bases: Examples of O···S(IV) Chalcogen Bonding

Lewis acid-base adducts between SF₄ and the oxygen bases tetrahydrofuran, cyclopentanone, and 1,2-dimethoxyethane were synthesized and characterized by Raman spectroscopy and X-ray crystallography. Crystal structures of (SF₄·OC₄H₈)₂, SF₄·(OC₄H₈)₂, SF₄·CH₃OC₂H₄OCH₃, and SF₄·(O═C₅H₈)₂ show weak S···O chalcogen bonding interactions ranging from 2.662(2) to 2.8692(9) Å. Caffeine, which has three Lewis basic sites, was reacted with SF₄ and one aliquot of HF forming C₈H₁₀N₄O₂·2SF₄·HF, which was also characterized by X-ray crystallography. Density functional theory calculations aided in the assignment of the vibrational spectra of (SF₄·OC₄H₈)₂, SF₄·(OC₄H₈)₂, SF₄·CH₃OC₂H₄OCH₃, and SF₄·(O═C₅H₈)₂. Bonding was studied by natural bond order and the quantum theory of atoms in molecules analyses.

Supramolecular macrocycles reversibly assembled by Te(…)O chalcogen bonding

Organic molecules with heavy main-group elements frequently form supramolecular links to electron-rich centres. One particular case of such interactions is halogen bonding. Most studies of this phenomenon have been concerned with either dimers or infinitely extended structures (polymers and lattices) but well-defined cyclic structures remain elusive. Here we present oligomeric aggregates of heterocycles that are linked by chalcogen-centered interactions and behave as genuine macrocyclic species. The molecules of 3-methyl-5-phenyl-1,2-tellurazole 2-oxide assemble a variety of supramolecular aggregates that includes cyclic tetramers and hexamers, as well as a helical polymer. In all these aggregates, the building blocks are connected by Te(…)O-N bridges. Nuclear magnetic resonance spectroscopic experiments demonstrate that the two types of annular aggregates are persistent in solution. These self-assembled structures form coordination complexes with transition-metal ions, act as fullerene receptors and host small molecules in a crystal.