Polydentate chalcogen bonding: anion trapping with a water-stable host compound carrying Se-CF3 functions.

Bidentate and tetradentate chalcogen bonding host systems with SeCF3 functions as σ-hole donors in close proximity at the alkyne functions of 1,8-diethynylanthracene and its syn-dimer were prepared in quantitative yield by tin-selenium exchange reactions of the corresponding trimethylstannyl precursors with ClSeCF3. The bidentate system shows chalcogen bonding interactions with THF, but does not bind halide ions. The tetradentate system cooperatively chelates chloride, bromide and iodide ions with its four CC-SeCF3 units by rotating the four σ-holes towards the halide ion. The structures of these halide ion adducts were determined by X-ray diffraction. The hydrobromide and -iodide salts of the ethyl derivative of Schwesinger's phosphazene superbase served as halide salts with very weakly coordinating cations.

The First Main Group Element Lewis Acid Thionyl chloride Adduct and its Chemistry.

Although the adduct of aluminum trichloride with thionyl chloride has been reported, no thionyl chloride adduct of a main group element Lewis acid or organometallic compound has been structurally characterized. In this communication we present the synthesis and reactivity of the structurally ascertained adduct of thionyl chloride with tris(pentafluoroethyl)gallane as a representative of a main group element Lewis acid. Gallium and indium compounds with electron withdrawing groups, e.g. the pentafluoroethyl ligand, display versatile properties. While gallates and indates, [MR4]-, behave as weakly coordinating anions, neutral gallanes and indanes, MR3, are strong Lewis acids. Salts with the tetrakis(pentafluoroethyl)gallate and -indate, [M(C2F5)4]- (M=Ga, In), have recently been studied in detail. In contrast to this, work on the syntheses of the free Lewis superacids M(C2F5)3 (M= Ga, In) is scarce and underdeveloped. The hydrates [M(C2F5)3(OH2)2] proved to be suitable starting materials, particularly due to their thermal stability. Herein we report on synthesis and characterization of reactive adducts, [M(C2F5)3D], with the weak donor molecules (D) SOCl2 and Me3SiF. The effective Lewis acidities of Ga(C2F5)3 and In(C2F5)3 were experimentally deter-mined by the (modified) Gutmann-Beckett method and their catalytic potential is showcased.

Ice nucleation in aqueous solutions of short- and long-chain poly(vinyl alcohol) studied with a droplet microfluidics setup.

Poly(vinyl alcohol) (PVA) has ice binding and ice nucleating properties. Here, we explore the dependence of the molecular size of PVA on its ice nucleation activity. For this purpose, we studied ice nucleation in aqueous solutions of PVA samples with molar masses ranging from 370 to 145 000 g mol-1, with a particular focus on oligomer samples with low molar mass. The experiments employed a novel microfluidic setup that is a follow-up on the previous WeIzmann Supercooled Droplets Observation on a Microarray (WISDOM) design by Reicher et al. The modified setup introduced and characterized here, termed nanoliter Bielefeld Ice Nucleation ARraY (nanoBINARY), uses droplet microfluidics with droplets (96 ± 4) µm in diameter and a fluorinated continuous oil phase and surfactant. A comparison of homogeneous and heterogeneous ice nucleation data obtained with nanoBINARY to those obtained with WISDOM shows very good agreement, underpinning its ability to study low-temperature ice nucleators as well as homogeneous ice nucleation due to the low background of impurities. The experiments on aqueous PVA solutions revealed that the ice nucleation activity of shorter PVA chains strongly decreases with a decrease in molar mass. While the cumulative number of ice nucleating sites per mass nm of polymers with different molar masses is the same, it becomes smaller for oligomers and completely vanishes for dimer and monomer representatives such as 1,3-butanediol, propan-2-ol, and ethanol, most likely because these molecules become too small to effectively stabilize the critical ice embryo. Overall, our results are consistent with PVA polymers and oligomers acting as heterogeneous ice nucleators.

Synthesis and Characterization of Tetrakis(pentafluoroethyl)indate Salts.

In the rising field of organoindium chemistry little is known about the perfluoroorganyl-substituted indium compounds. The increasing use of indium reagents is explained by their high stability and tolerance towards water and functional groups. Here we report on the synthesis of the novel tetrakis(pentafluoroethyl)indate, [In(C2 F5 )4 ]- , and its characterization in salts with the cations [Li(thf)3 ]+ , Cs+ , [PPh4 ]+ and [H14 O6 ]2+ (thf=tetrahydrofuran). To the best of our knowledge, these salts constitute the first perfluoroalkylindates and, in general, the first isolated perfluoroalkylindium compounds which are fully characterized by NMR spectroscopy, mass spectrometry, elemental analysis and X-ray diffraction analysis.

Triorganylphosphoranides: Realization of an Unusual Structural Motif Utilizing Electron Withdrawing Pentafluoroethyl Groups.

Phosphoranides comprising three phosphorus carbon bonds are scarcely represented in the literature. We now utilized tris(pentafluoroethyl)phosphane, P(C2 F5 )3 , and cyanobis(pentafluoroethyl)phosphane, P(C2 F5 )2 (CN), featuring electron withdrawing pentafluoroethyl groups, to synthesize such compounds. Metal fluorides MF (M=Ag, Cs) add to P(C2 F5 )3 yielding respective M[P(C2 F5 )3 F] salts. Those M[P(C2 F5 )3 F] subsequently suffer a loss of C2 F4 , furnishing M[P(C2 F5 )2 F2 ]. The cesium salt decomposes instantaneously when warmed to rt, whereas the silver salt decomposes slowly over several days at rt. Treatment of Ag[P(C2 F5 )3 F] with 2,2'-bipyridine facilitated the isolation and structural characterization of [Ag(bipy){P(C2 F5 )3 F}]. With P(C2 F5 )2 (CN), AgF reacts under substitution of the cyano group yielding P(C2 F5 )2 F, rather than phosphoranide formation. However, [K(18-crown-6)]F adds to P(C2 F5 )2 (CN) furnishing [K(18-crown-6)][P(C2 F5 )2 (CN)F]. Its structural characterization was successful, despite its tendency to undergo an exchange of substituents, yielding [K(18-crown-6)][P(C2 F5 )2 F2 ] and presumably [K(18-crown-6)][P(C2 F5 )2 (CN)2 ]. The latter forms an equilibrium with [K(18-crown-6)]CN and P(C2 F5 )2 (CN) which lies well on side of the phosphane and cyanide salt.

Free Difluorobis(pentafluoroethyl)phosphoranide Ion and Its Ligand Properties.

In this contribution, we report on the "free" [P(C2F5)2F2]- ion and its ligand properties in transition metal complex chemistry. For this purpose, Ag[P(C2F5)2F2] was treated with [{(Et2N)3P═N}3P═NHC(CH3)3]Cl ([EtP4H]Cl) to yield [EtP4H][P(C2F5)2F2], featuring a weakly coordinating phosphazenium cation. Due to the weak interaction between the cation and anion, the [P(C2F5)2F2]- ion meets the so-called pseudo-gas-phase conditions. To determine the Tolman electronic parameter, [EtP4H][Ni(CO)3{P(C2F5)2F2}] was prepared from [EtP4H][P(C2F5)2F2] and [Ni(CO)4], facilitating the classification of the P(C2F5)2F2 moiety as a moderately π-acidic ligand. By treatment of [EtP4H][P(C2F5)2F2] with [AuCl(tht)], the neutral tetrahydrothiophene ligand was substituted by the phosphoranide ion, yielding [EtP4H][AuCl{P(C2F5)2F2}]. When Ag[P(C2F5)2F2] was treated with [AuCl(tht)], on the other hand, the chloride was substituted. Transmetalation reactions of this type proved to be an efficient transfer method of the P(C2F5)2F2 moiety, as further demonstrated by the reactions of Ag[P(C2F5)2F2] with [FeCl(CO)2Cp], [FeCl(CO)2Cp*], and [PdCl2(NCMe)2]. Surprisingly, P(C2F5)2F demonstrated fluorinating abilities toward [FeCl(CO)2Cp], [FeCl(CO)2Cp*], [AuCl(tht)], and [PdCl2(NCMe)2]. Apparently, fluorido transition metal complexes were generated in situ under the formation of P(C2F5)2Cl. The fluorido iron and palladium complexes transfer their fluoride ions onto P(C2F5)2F, yielding the respective phosphoranido complexes, featuring the P(C2F5)2F2 moiety.

Oxidative Additions of C-F Bonds to the Silanide Anion [Si(C2 F5 )3 ].

Compounds exhibiting main group elements in low oxidation states were found to mimic the reactivity of transition metal complexes. Like the latter, such main group species show a proclivity of changing their oxidation state as well as their coordination number by +2, therefore fulfilling the requirements for oxidative additions. Prominent examples of such main group compounds that undergo oxidative additions with organohalides R-X (R=alkyl, aryl, X=F, Cl, Br, I) are carbenes and their higher congeners. Aluminyl anions, which like carbenes and silylenes oxidatively add to strong σ-bonds in R-X species, have been recently discovered. We present the first anion based upon a Group 14 element, namely the tris(pentafluoroethyl)silanide anion, [Si(C2 F5 )3 ]- , which is capable of oxidative additions towards C-F bonds. This enables the isolation of non-chelated tetraorganofluorosilicate salts, which to the best of our knowledge had only been observed as reactive intermediates before.

Synthesis, Reactivity and Structural Properties of Trifluoromethylphosphoranides.

Phosphoranides are interesting hypervalent species which serve as model compounds for intermediates or transition states in nucleophilic substitution reactions at trivalent phosphorus substrates. Herein, the syntheses and properties of stable trifluoromethylphosphoranide salts are reported. [K(18-crown-6)][P(CF3 )4 ], [K(18-crown-6)][P(CF3 )3 F], and [NMe4 ][P(CF3 )2 F2 ] were obtained by treatment of trivalent precursors with sources of CF3 - or F- units. These [P(CF3 )4-n Fn ]- (n=0-2) salts exhibit fluorinating (n=1-2) or trifluoromethylating (n=0) properties, which is disclosed by studying their reactivity towards selected electrophiles. The solid-state structures of [K(18-crown-6)][P(CF3 )4 ] and [K(18-crown-6)][P(CF3 )3 F] are ascertained by single crystal X-ray crystallography. The dynamics of these compounds are investigated by variable temperature NMR spectroscopy.

Difluorobis(pentafluoroethyl)phosphoranide: A Promising Building Block for Phosphoranidometal Complexes.

Despite the fact that different metal tetrafluorophosphoranides, M[PF4] (M = Cs, Ag, K), decompose readily, we successfully enhanced the stability of such species by the replacement of two fluorine atoms with electron withdrawing pentafluoroethyl groups. Thus, AgF adds to P(C2F5)2F, yielding Ag[P(C2F5)2F2], which served as a starting material for the synthesis of mono-, bis-, and tris[difluorobis(pentafluoroethyl)phosphoranido]silver complexes. Addition of 2,2'-bipyridine allowed for the isolation of stable [Ag(bipy){P(C2F5)2F2}], whereas the reaction with the chlorides [NMe4]Cl and CoCl2 afforded the bis- and trisphosphoranidoargentates [NMe4][Ag{P(C2F5)2F2}2(OEt2)] and [Co(NCMe)6][Ag{P(C2F5)2F2}3]·2MeCN, respectively. Altogether, the difluorobis(pentafluoroethyl)phosphoranido moiety serves as a novel, small, noncyclic phosphoranido ligand. It provided access to the first homoleptic phosphoranidometal complex, [Co(NCMe)6][Ag{P(C2F5)2F2}3]·2MeCN, which itself features the unusual structural motif of an [AgX3]2- ion.

Salts of Tris(pentafluoroethyl)silylchalcogenolates [Si(C2F5)3E]- with E = S, Se, and Te: Synthesis, Structure, and Reactivity.

Unlike silanolates [SiR3O]- (R = alkyl, aryl), which have been intensely studied, the heavier derivatives [SiR3E]- (E = S, Se, Te) have been much less examined. Among such species, virtually nothing is known about perfluoroalkyl-substituted silylchalcogenolates. In this contribution, a convenient synthesis of tris(pentafluoroethyl)silylchalcogenolate salts [{(Et2N)3P═N}3PN(H)tBu][Si(C2F5)3E] (E = S, Se, Te; tBu = tert-butyl) is presented. All representatives were isolated and fully characterized by multinuclear NMR spectroscopy, IR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction studies. Furthermore, first reactivity studies of these novel species toward selected metal halide complexes were performed. In this course, metal complexes [HgPh{SSi(C2F5)3}] (2) and [Au(PPh3){SSi(C2F5)3}] (3) were isolated and characterized.

Tetrakis(pentafluoroethyl)gallate, [Ga(C2 F5 )4 ]- , Ionic Liquids.

We synthesized new imidazolium-based tunable aryl alkyl ionic liquids (TAAILs) with the weakly coordinating tetrakis(pentafluoroethyl)gallate anion, [Ga(C2 F5 )4 ]- . Phenyl and phenyl derivatives (2-Me, 4-OMe, 2,4-F) were combined with varying alkyl chain lengths at the imidazolium core leading to TAAILs, which were investigated with regard to their viscosity, conductivity, and electrochemical window and compared to EMIM and BMIM standard cations. Remarkable low viscosities of 29 cP at 25 °C for [BMIM][Ga(C2 F5 )4 ] were achieved. However, the EMIM and BMIM gallates show electrochemical instability, releasing pentafluoroethane at a voltage of 1.5 V. The 2-Me-substituted gallate-TAAILs slowly decompose over several weeks, whereas all other gallate-TAAILs showed no decomposition at all. With electrochemical windows of up to 5.15 V and low viscosities in a range of 66-162 cP, the gallate-TAAILs are promising candidates as electrolytes in electrochemical applications.

Tris(pentafluoroethyl)silanol Derivatives and the Lewis Amphoteric Tris(pentafluoroethyl)silanolate Anion, [Si(C2 F5 )3 O].

While alkyl-substituted siloxanes are widely known, virtually nothing is known about perfluoroalkyl siloxanes and their congener species, the silanols and silanolates. We recently reported on the tris(pentafluoroethyl)silanide ion, [Si(C2 F5 )3 ]- , which features Lewis amphoteric character deriving from the pentafluoroethyl substituents and their strong electron-withdrawing properties. Transferring this knowledge, we investigated the Lewis amphoteric behavior of the tris(pentafluoroethyl)silanolate, [Si(C2 F5 )3 O]- . In order to examine such Lewis amphoteric behavior, we first developed a strategy for the synthesis of the corresponding silanol Si(C2 F5 )3 OH, which readily condenses at room temperature to the hexakis(pentafluoroethyl)disiloxane, (C2 F5 )3 SiOSi(C2 F5 )3 . Deprotonation of Si(C2 F5 )3 OH employing a sterically demanding phosphazene base allows the characterization of the first example of a dimeric triorganosilanolate: the dianionic hexakis(pentafluoroethyl)disilanolate, [{Si(C2 F5 )3 O}2 ]2- , implies Lewis amphoteric character of the monomeric [Si(C2 F5 )3 O]- anion.

Phosphorus-Containing Superbases: Recent Progress in the Chemistry of Electron-Abundant Phosphines and Phosphazenes.

The renaissance of Brønsted superbases is primarily based on their pronounced capacity for a large variety of chemical transformations under mild reaction conditions. Four major set screws are available for the selective tuning of the basicity: the nature of the basic center (N, P, …), the degree of electron donation by substituents to the central atom, the possibility of charge delocalization, and the energy gain by hydrogen bonding. Within the past decades, a plethora of neutral electron-rich phosphine and phosphazene bases have appeared in the literature. Their outstanding properties and advantages over inorganic or charged bases have now made them indispensable as auxiliary bases in deprotonation processes. Herein, an update of the chemistry of basic phosphines and phosphazenes is given. In addition, due to widespread interest, their use in catalysis or as ligands in coordination chemistry is highlighted.

Evidence of the Lewis-Amphoteric Character of Tris(pentafluoroethyl)silanide, [Si(C2 F5 )3 ].

According to a first view on the geometrical and electronic structure of the tris(pentafluoroethyl)silanide, this anion appears as a Lewis base. Quantum chemical calculations on perfluoroalkylated silanides show significantly lower HOMO and LUMO energy levels in comparison to their non-fluorinated counterparts, which implies reduced Lewis basicity and increased Lewis acidity of the [Si(C2 F5 )3 ]- ion. With these findings and a HOMO-LUMO gap of 4.80 eV similar to N-heterocyclic silylenes (NHSis), perfluoroalkyl silanides are predestined to exhibit Lewis-amphoteric character similar to silylenes. Deprotonation of Si(C2 F5 )3 H with sterically demanding phosphazene bases afforded thermally stable phosphazenium salts of the [Si(C2 F5 )3 ]- anion, which add to benzaldehyde, benzophenone, CS2 , and CO2 in various manners. This behavior also mirrors the reactivity of silylenes towards ketones as well as heterocumulenes and is rationalized by Lewis amphotericity being inherent in these silanides.

Non-coordinated and Hydrogen Bonded Phenolate Anions as One-Electron Reducing Agents.

In this work, the syntheses of non-coordinated electron-rich phenolate anions via deprotonation of the corresponding alcohols with an extremely powerful perethyl tetraphosphazene base (Schwesinger base) are reported. The application of uncharged phosphazenes renders the selective preparation of anionic phenol-phenolate and phenolate hydrates possible, which allows for the investigation of hydrogen bonding in these species. Hydrogen bonding brings about decreased redox potentials relative to the corresponding non-coordinated phenolate anions. The latter show redox potentials of up to -0.72(1) V vs. SCE, which is comparable to that of zinc metal, thus qualifying their application as organic zinc mimics. We utilized phenolates as reducing agents for the generation of radical anions in addition to the corresponding phenoxyl radicals. A tetracyanoethylene radical anion salt was synthesized and fully characterized as a representative example. We also present the activation of sulfur hexafluoride (SF6 ) with phenolates in a SET reaction, in which the nature of the respective phenolate determines whether simple fluorides or pentafluorosulfanide ([SF5 ]- ) salts are formed.

The Influence of Weakly Coordinating Cations on the O-H⋠⋠⋠O- Hydrogen Bond of Silanol-Silanolate Anions.

The reaction of a saline phosphazenium hydroxide hydrate with siloxanes led to a novel kind of silanol-silanolate anions. The weakly coordinating behavior of the cation renders the formation of silanol-silanolate hydrogen bonds possible, which otherwise suffer from detrimental silanolate-oxygen cation interactions. We investigated the influence of various weakly coordinating cations on silanol-silanolate motifs, particularly with regard to different cation sizes. While large cations favor the formation of intramolecular hydrogen bonds resulting in cyclic structures, the less bulky tetramethyl ammonium cation encourages the formation of polyanionic silanol-silanolate chains in the solid state.

Non-Coordinated Phenolate Anions and Their Application in SF6 Activation.

The reaction of the strong monophosphazene base with the weakly acidic phenol leads to the formation of a phenol-phenolate anion with a moderately strong hydrogen bond. Application of the more powerful tetraphosphazene base (Schwesinger base) renders the isolation of the corresponding salt with a free phenolate anion possible. This compound represents the first species featuring the free phenolate anion [H5 C6 -O]- . The deprotonation of phenol derivatives with tetraphosphazene bases represents a great way for the clean preparation of salts featuring free phenolate anions and in addition allows the selective syntheses of hydrogen bonded phenol-phenolate salts. This work presents a phosphazenium phenolate salt with a redox potential of -0.72 V and its capability for the selective activation of the chemically inert greenhouse gas SF6 . The performed two-electron reduction of SF6 leads to phosphazenium pentafluorosulfanide ([SF5 ]- ) and fluoride salts.

Synthesis and Characterization of Tetrakis(pentafluoroethyl)aluminate.

While perfluorinated aryl, aryloxy and alkoxy aluminum species are well-established as weakly coordinating anions (WCAs), corresponding perfluoroalkyl aluminum derivatives are virtually unknown. Reaction of Si(C2 F5 )3 CH3 with Li[AlH4 ] afforded the tetrakis(pentafluoroethyl)aluminate, [Al(C2 F5 )4 ]- . Several salts of the [Al(C2 F5 )4 ]- ion were synthesized and characterized by NMR spectroscopic methods, mass spectrometry, X-ray diffraction studies and elemental analysis.

Synthesis and Reactivity of the First Isolated Hydrogen-Bridged Silanol-Silanolate Anions.

We report on the first examples of isolated silanol-silanolate anions, obtained by utilizing weakly coordinating phosphazenium counterions. The silanolate anions were synthesized from the recently published phosphazenium hydroxide hydrate salt with siloxanes. The silanol-silanolate anions are postulated intermediates in the hydroxide-mediated polymerization of aryl and alkyl siloxanes. The silanolate anions are strong nucleophiles because of the weakly coordinating character of the phosphazenium cation, which is perceptible in their activity in polysiloxane depolymerization.

Generation and Applications of the Hydroxide Trihydrate Anion, [OH(OH2 )3 ]- , Stabilized by a Weakly Coordinating Cation.

The reaction of a strongly basic phosphazene (Schwesinger base) with water afforded the corresponding metastable hydroxide trihydrate [OH(OH2 )3 ]- salt. This is the first hydroxide solvate that is not in contact with a cation and furthermore one of rare known water-stabilized hydroxide anions. Thermolysis in vacuum results in the decomposition of the hydroxide salt and quantitative liberation of the free phosphazene base. This approach was used to synthesize the Schwesinger base from its hydrochloride salt after anion exchange in excellent yields of over 97 %. This deprotonation method can also be used for the phosphazene-base-catalyzed preparation of the Ruppert-Prakash reagent Me3 SiCF3 using fluoroform (HCF3 ) as the trifluoromethyl building block and sodium hydroxide as the formal deprotonation agent.