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

Kinetics of sulfur-transfer from titanocene (poly)sulfides to sulfenyl chlorides: rapid metal-assisted concerted substitution

The kinetics of sulfur transfer from titanocene (poly)sulfides (RCp2TiS5, Cp2TiS4CMe2, Cp2Ti(SAr)2, Cp2TiCl(SAr)) to sulfenyl chlorides (S2Cl2, RSCl) have been investigated by a combination of stopped-flow UV-Vis/NMR reaction monitoring, titration assays, numerical kinetic modelling and KS-DFT calculations. The reactions are rapid, proceeding to completion over timescales of milliseconds to minutes, via a sequence of two S-S bond-forming steps (k 1, k 2). The archetypical polysulfides Cp2TiS5 (1a) and Cp2TiS4C(Me2) (2a) react with disulfur dichloride (S2Cl2) through rate-limiting intermolecular S-S bond formation (k 1) followed by a rapid intramolecular cyclization (k 2, with k 2 ≫ k 1 [RSCl]). The monofunctional sulfenyl chlorides (RSCl) studied herein react in two intermolecular S-S bond forming steps proceeding at similar rates (k 1 ≈ k 2). Reactions of titanocene bisthiophenolates, Cp2Ti(SAr)2 (5), with both mono- and di-functional sulfenyl chlorides result in rapid accumulation of the monothiophenolate, Cp2TiCl(SAr) (6) (k 1 > k 2). Across the range of reactants studied, the rates are relatively insensitive to changes in temperature and in the electronics of the sulfenyl chloride, moderately sensitive to the electronics of the titanocene (poly)sulfide (ρ (Ti-(SAr)) ≈ -2.0), and highly sensitive to the solvent polarity, with non-polar solvents (CS2, CCl4) leading to the slowest rates. The combined sensitivities are the result of a concerted, polarized and late transition state for the rate-limiting S-S bond forming step, accompanied by a large entropic penalty. Each substitution step {[Ti]-SR' + Cl-SR → [Ti]-Cl + RS-SR'} proceeds via titanium-assisted Cl-S cleavage to generate a transient pentacoordinate complex, Cl-[Cp2TiX]-S(R')-SR, which then undergoes rapid Ti-S dissociation.

Nonheme binuclear transition metal complexes with hydrosulfide and polychalcogenides

The intriguing chemistry of chalcogen (S, Se)-containing ligands and their capability to bridge multiple metal centres have resulted in a plethora of reports on transition metal complexes featuring hydrosulfide (HS-) and polychalcogenides (En2-, E = S, Se). While a large number of such molecules are strictly organometallic complexes, examples of non-organometallic complexes featuring HS- and En2- with N-/O-donor ligands are relatively rare. The general synthetic procedure for the transition metal-hydrosulfido complexes involves the reaction of the corresponding metal salts with HS-/H2S and this is prone to generate sulfido bridged oligomers in the absence of sterically demanding ligands. On the other hand, the synthetic methods for the preparation of transition metal-polychalcogenido complexes include the reaction of the corresponding metal salts with En2- or the two electron oxidation of low-valent metals with elemental chalcogen, often at an elevated temperature and/or for a long time. Recently, we have developed new synthetic methods for the preparation of two new classes of binuclear transition metal complexes featuring either HS-, or Sn2- and Sen2- ligands. The new method for the synthesis of transition metal-hydrosulfido complexes involved transition metal-mediated hydrolysis of thiolates at room temperature (RT), while the method for the synthesis of transition metal-polychalcogenido complexes involved redox reaction of coordinated thiolates and exogenous elemental chalcogens at RT. An overview of the synthetic aspects, structural properties and intriguing reactivity of these two new classes of transition metal complexes is presented.

Expanding the Frontier of Linear Drug Design: Cu-Catalyzed Csp -Csp 3 -Coupling of Electron-Deficient SF4 -Alkynes with Alkyl Iodides

Despite the attractive properties of tetrafluorosulfanyl (SF4 ) compounds in drug discovery, medicinal research on SF4 molecules is hindered by the scarcity of suitable synthetic methodologies. Drawing inspiration from the well-established Sonogashira cross-coupling of terminal alkynes under Pd-catalysis, it is envisioned that SF4 -alkynes can serve as effective coupling partners. To overcome the challenges associated with the electron-deficient nature of SF4 -alkynes and the lability of the SF4 unit under transition-metal catalysis, an aryl radical mediated Csp -Csp 3 cross-coupling reaction is successfully developed under Cu catalysis. This methodology facilitates the coupling of SF4 -alkynes with alkyl iodides, leading to the immediate synthesis of SF4 -attached drug-like molecules. These findings highlight the potential impact of SF4 -containing molecules in the drug industry, paving the way for further research in this emerging field.

Generation and Reactivity of Polychalcogenide Chains in Binuclear Cobalt(II) Complexes

A series of six binuclear Co(II)-thiolate complexes, [Co2(BPMP)(S-C6H4-o-X)2]1+ (X = OMe, 2; NH2, 3), [Co2(BPMP)(μ-S-C6H4-o-O)]1+ (4), and [Co2(BPMP)(μ-Y)]1+ (Y = bdt, 5; tdt, 6; mnt, 7), has been synthesized from [Co2(BPMP)(MeOH)2(Cl)2]1+ (1a) and [Co2(BPMP)(Cl)2]1+ (1b), where BPMP1- is the anion of 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol. While 2 and 3 could allow the two-electron redox reaction of the two coordinated thiolates with elemental sulfur (S8) to generate [Co2(BPMP)(μ-S5)]1+ (8), the complexes, 4-7, could not undergo a similar reaction. An analogous redox reaction of 2 with elemental selenium ([Se]) produced [{Co2(BPMP)(μ-Se4)}{Co2(BPMP)(μ-Se3)}]2+ (9a) and [Co2(BPMP)(μ-Se4)]1+ (9b). Further reaction of these polychalcogenido complexes, 8 and 9a/9b, with PPh3 allowed the isolation of [Co2(BPMP)(μ-S)]1+ (10) and [Co2(BPMP)(μ-Se2)]1+ (11), which, in turn, could be converted back to 8 and 9a upon treatment with S8 and [Se], respectively. Interestingly, while the redox reaction of the polyselenide chains in 9a and 11 with S8 produced 8 and [Se], the treatment of 8 with [Se] gave back only the starting material (8), thus demonstrating the different redox behavior of sulfur and selenium. Furthermore, the reaction of 8 and 9a/9b with activated alkynes and cyanide (CN-) allowed the isolation of the complexes, [Co2(BPMP)(μ-E2C2(CO2R)2)]1+ (E = S: 12a, R = Me; 12b, R = Et; E = Se: 13a, R = Me; 13b, R = Et) and [Co2(BPMP)(μ-SH)(NCS)2] (14), respectively. The present work, thus, provides an interesting synthetic strategy, interconversions, and detailed comparative reactivity of binuclear Co(II)-polychalcogenido complexes.

Advances in Continuous Flow Fluorination Reactions

Fluorination reactions are important in constructing organofluorine motifs, which contribute to favorable biological properties in pharmaceuticals and agrochemicals. However, fluorination reagents and reactions are associated with various problems, such as their hazardous nature, high exothermicity, and poor selectivity and scalability. Continuous flow has emerged as a transformative technology to provide many advantages relative to batch syntheses. This review article summarizes recent continuous flow techniques that address the limitations and challenges of fluorination reactions. Approaches based on different flow techniques are discussed, including gas-liquid reactions, packed-bed reactors, in-line purifications, streamlined multistep synthesis, large-scale reactions well as flow photoredox- and electrocatalysis.

Reduction of SF5CF3via iridium catalysis: radical trifluoromethylation of aromatics

The greenhouse gas SF₅CF₃ acts as CF₃ source for the photocatalytic trifluoromethylation of arenes on using [Ir(dtbbpy)(ppy)₂]PF₆ (4,4'-di-tert-butyl-2,2'-dipyridyl, ppy = 2-phenylpyridine) as catalyst. The trifluoromethylation of C₆D₆ in the presence of 1-octanol results in the concomitant generation of 1-fluorooctane, presumably by intermediate SF₄.

Synthesis of a Hexachloro Sulfate(IV) Dianion Enabled by Polychloride Chemistry

The preparation and structural characterization of [NEt₃ Me]₂ [SCl₆ ] is described, which is the first example of a [SCl₆ ]^2- dianion and of a halosulfate anion of the type [S_x X_y ]^z- in general. This dianion belongs to the group of 14-valence electron AB₆ E systems and forms an octahedral structure in the solid-state. Interestingly, co-crystallization with CH₂ Cl₂ affords [NEt₃ Me]₂ [SCl₆ ]⋅4 CH₂ Cl₂ containing [SCl₆ ]^2- dianions with C_4v symmetry. As suggested by quantum-chemical calculations, the distortion of the structure is not caused by a stereochemically active lone pair but by enhanced hydrogen bonding interactions with CH₂ Cl₂ . At elevated temperatures, [NEt₃ Me]₂ [SCl₆ ] decomposes to various sulfur chlorine compounds as shown by Raman spectroscopy. Cooling back to room temperature results in the selective formation of [NEt₃ Me]₂ [SCl₆ ] which is comparable to the well-studied SCl₄ .

Metal-Free SF6 Activation: A New SF5 -Based Reagent Enables Deoxyfluorination and Pentafluorosulfanylation Reactions

The activation of SF₆ , a potent greenhouse gas, under metal-free and visible light conditions is reported. Herein, mechanistic investigations including EPR spectroscopy, NMR studies and cyclic voltammetry allowed the rational design of a new fluorinating reagent which was synthesized from the 2-electron activation of SF₆ with commercially available TDAE. This new SF₅ -based reagent was efficiently employed for the deoxyfluorination of CO₂ and the fluorinative desulfurization of CS₂ allowing the formation of useful fluorinated amines. Moreover, for the first time we demonstrated that our SF₅ -based reagent could afford the mild generation of Cl-SF₅ gas. This finding was exploited for the chloro-pentafluorosulfanylation of alkynes and alkenes.

Repurposing of F-gases: challenges and opportunities in fluorine chemistry

Fluorinated gases (F-gases) are routinely employed as refrigerants, blowing agents, and electrical insulators. These volatile compounds are potent greenhouse gases and consequently their release to the environment creates a significant contribution to global warming. This review article seeks to summarise: (i) the current applications of F-gases, (ii) the environmental issues caused by F-gases, (iii) current methods of destruction of F-gases and (iv) recent work in the field towards the chemical repurposing of F-gases. There is a great opportunity to tackle the environmental and sustainability issues created by F-gases by developing reactions that repurpose these molecules.

Polysulfido Chain in Binuclear Zinc(II) Complexes

The synthesis and a detailed reactivity study of a binuclear zinc(II) bis(benzenethiolate) complex, [Zn₂(BPMP)(SPh)₂]⁺ (4), and an unprecedented binuclear zinc(II) pentasulfido complex, [Zn₂(BPMP)(μ₂-S₅)]⁺ (6), are presented. While one-electron oxidation of the coordinated benzenethiolate ligands in 4 by Cp₂Fe⁺ produces diphenyl disulfide and [Zn₂(BPMP)(μ₂-OH)]²⁺ (5), a two-electron redox reaction between coordinated benzenethiolate ligands in 4 and elemental S (S₈) generated diphenyl disulfide and the binuclear zinc(II) pentasulfido complex 6. Complex 6 features a chelating, dianionic, pentasulfido (S₅^2-) chain and can consume up to a maximum of 3 equiv of PPh₃ to generate Ph₃PS and 5, while the reaction of 6 with 1 equiv of diphenylphosphinoethane allowed the isolation of [Zn₂(BPMP)(μ₂-S₄)]⁺ (7). A proteolysis reaction of the coordinated S₅^2- chain in 6 with fluoroboric acid (HBF₄), benzoic acid (PhCOOH), and thioacetic acid (MeCOSH) generates the complexes [Zn₂(BPMP)(MeCN)₂]³⁺ (1), [Zn₂(BPMP)(μ₂-PhCOO)₂]⁺ (8), and [Zn₂(BPMP)(μ₂-SCOMe)₂]⁺ (9), respectively, while the protonated S₅^2- chain liberates S₈ and hydrogen sulfide (H₂S). Finally, the transfer of the coordinated benzenethiolate ligands in 4 and the S₅^2- chain in 6 to selected organic compounds, namely, PhCH₂Br and PhC(O)Cl, for the generation of various organosulfur compounds is demonstrated.

Fluorination Reactions at a Platinum Carbene Complex: Reaction Routes to SF3 , S(=O)F and Fluorido Complexes

The electron-rich Pt complex [Pt(IMes)₂ ] (IMes: [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolinylidine]) can be used as precursor for the syntheses of a variety of fluorido ligand containing compounds. The sulfur fluoride SF₄ undergoes a rapid oxidative addition at Pt⁰ to yield trans-[Pt(F)(SF₃ )(IMes)₂ ]. A photolytic reaction of SF₆ at [Pt(IMes)₂ ] in the presence of IMes gave the fluorido complexes trans-[Pt(F)₂ (IMes)₂ ] and trans-[Pt(F)(SF₃ )(IMes)₂ ] along with trans-[Pt(F)(SOF)(IMes)₂ ] and trans-[Pt(F)(IMes')(IMes)] (IMes': cyclometalated IMes ligand), the latter being products produced by reaction with adventitious water. trans-[Pt(F)(SOF)(IMes)₂ ] and trans-[Pt(F)₂ (IMes)₂ ] were synthesized independently by treatment of [Pt(IMes)₂ ] with SOF₂ or XeF₂ . A reaction of [Pt(IMes)₂ ] with a HF source gave trans-[Pt(H)(F)(IMes)₂ ], and an intermediate bifluorido complex trans-[Pt(H)(FHF)(IMes)₂ ] was identified. Compound trans-[Pt(H)(F)(IMes)₂ ] converts in the presence of CsF into trans-[Pt(F)(IMes')(IMes)].

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.

Complete deconstruction of SF6 by an aluminium(I) compound

The room-temperature activation of SF₆, a potent greenhouse gas, is reported using a monovalent aluminium(i) reagent to form well-defined aluminium(iii) fluoride and aluminium(iii) sulfide products. New reactions have been developed to utilise the aluminium(iii) fluoride and aluminium(iii) sulfide as a nucleophilic source of F⁻ and S²⁻ for a range of electrophiles. The overall reaction sequence results in the net transfer of fluorine or sulfur atoms from an environmentally detrimental gas to useful organic products.

The room-temperature activation of SF₆, a potent greenhouse gas, is reported using a monovalent aluminium(i) reagent to form well-defined aluminium(iii) fluoride and aluminium(iii) sulfide products.

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.