Chlorogermane (C2F5)3GeCl with very electronegative pentafluoroethyl groups was converted with LiCH2P(tBu)2 to obtain the intramolecular frustrated Lewis pair (FLP) (C2F5)3GeCH2P(tBu)2, a neutral, ...germanium‐based FLP. Its reactivity was compared to its silicon homologue (C2F5)3SiCH2P(tBu)2. Both FLPs cleave NO but give cyclic (Si) and open‐chain oxides (Ge). In reactions with HCl both FLPs gave the same adduct type in the solid state, while the proton seems more mobile in solution in the germanium case. Reactions with PhCNO and Me3SiCHN2 result in ring‐type adducts. The structures of (C2F5)3GeCH2P(tBu)2 and of five adducts with substrates were elucidated by X‐ray diffraction. The study clearly showed the germanium compound to have a more moderate Lewis acidity compared to the silicon analogue.
Frustrated as well is the germanium Lewis Pair (C2F5)3GeCH2P(tBu)2 but, compared to its silicon homologue, it shows contrasting behavior in its reactivity to certain substrates as HCl and NO featuring the germanium function to be less Lewis acidic.
The new oxygen‐bridged geminal Si/P Frustrated Lewis Pair (FLP) tBu2P−O−Si(C2F5)3 (2) is able to reversibly bind carbon dioxide at ambient temperature. We compared its reactivity towards benzil, ...but‐3‐en‐2‐one, nitriles and phenylacetylene to that of the Al/P FLP tBu2P−O−AlBis2 (Bis=−CH(SiMe3)2) (1). When reacted with benzil, both, 1 and 2, form the 1,2‐addition product, but in the Si/P FLP 2, the second carbonyl function additionally binds to the silicon atom. With but‐3‐en‐2‐one 2 forms the 1,2‐addition product, while 1 binds in 1,4‐position. The reaction with acetonitrile yielded an unexpected etheneimine adduct for both systems, while only 1 reacted with tert‐butylnitrile. With benzonitrile and acrylonitrile, 2 showed reversible addition to the C≡N bond and 1 forms a stable adduct with benzonitrile. Solely 1 shows reactivity towards phenylacetylene affording a mixture of the CH deprotonation adduct tBu2P(H)−O−AlBis2(CCPh) and the FLP −C≡C 1,2‐addition adduct under ring formation. All compounds were characterized by multinuclear NMR spectroscopy, XRD and elemental analysis.
Variations in reactivity are observed for the geminal Si−O−P and Al−O−P Frustrated Lewis Pairs tBu2P−O−Si(C2F5)3 and tBu2P−O−Al(CH(SiMe3)2)2 towards CO2, nitriles and alkynes regarding regioselectivity, structure and stability of the corresponding adducts.
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.
Silanol–silanolate anions that are devoid of any contacts to the counterion were synthesized by treatment of siloxane species with a phosphazenium hydroxide hydrate salt. Because of the weakly coordinating cation, the silanolate anions exhibit strong silanol–silanolate hydrogen bonding. The D3OH− salt was successfully tested in the depolymerization of polydimethylsiloxane with an outstanding activity relative to NaOH or KOH.
The divinyldiarsene radical cations {(NHC)C(Ph)}As2(GaCl4) (NHC=IPr: C{(NDipp)CH}2 3; SIPr: C{(NDipp)CH2}2 4; Dipp=2,6‐iPr2C6H3) and dications {(NHC)C(Ph)}As2(GaCl4)2 (NHC=IPr 5; SIPr 6) are readily ...accessible as crystalline solids on sequential one‐electron oxidation of the corresponding divinyldiarsenes {(NHC)C(Ph)}As2 (NHC=IPr 1; SIPr 2) with GaCl3. Compounds 3–6 have been characterized by X‐ray diffraction, cyclic voltammetry, EPR/NMR spectroscopy, and UV/vis absorption spectroscopy as well as DFT calculations. The sequential removal of one electron from the HOMO, that is mainly the As−As π‐bond, of 1 and 2 leads to successive elongation of the As=As bond and contraction of the C−As bonds from 1/2→3/4→5/6. The UV/vis spectrum of 3 and 4 each exhibits a strong absorption in the visible region associated with SOMO‐related transitions. The EPR spectrum of 3 and 4 each shows a broadened septet owing to coupling of the unpaired electron with two 75As (I=3/2) nuclei.
One‐by‐one electron oxidation of diarsenes As2 featuring very efficient π‐donor N‐heterocyclic vinyl substituents with GaCl3 leads to the formation of radical cations As2+. and dications As2+ as crystalline solids. Experimental and computational studies revealed the delocalization of unpaired electron over the π‐conjugated CAs2C framework.
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.
Effect of hydrogen bonding on redox properties: The deprotonation of phenol derivatives with a tetraphosphazene base delivers non‐coordinated phenolates with zinc‐like redox potentials of up to −0.72(1) V vs. SCE, which are capable in radical anion synthesis and SF6 activation. The addition of phenol or water allows for the selective formation of phenolate adducts and enables the study of the effect of hydrogen bonding on phenolate redox properties.
The olefinic C−H bond functionalization of (NHC)CHPh (NHC=IPr=C{(NAr)CH}2 1; SIPr=C{(NAr)CH2}2 2; Ar=2,6‐iPr2C6H3), derived from classical N‐heterocyclic carbenes (NHCs), with PCl3 affords the ...dichlorovinylphosphanes {(NHC)C(Ph)}PCl2 (NHC=IPr 3, SIPr 4). Two‐electron reduction of 3 and 4 with magnesium leads to the formation of the divinyldiphosphenes {(NHC)C(Ph)}P2 (NHC=IPr 5, SIPr 6) as crystalline solids. Unlike literature‐known diphosphenes, which are mostly yellow or orange, 5 is a green whereas 6 is a purple solid. Although the P=P bond lengths of 5 (2.062(1)) and 6 (2.055(1) Å) are comparable to those of the known diphosphenes (2.02–2.08 Å), the C−P bond lengths of 5 (1.785(1)) and 6 (1.797(1) Å) are, however, considerably shorter than a Csp2
−P single bond length (1.85 Å), indicating a considerable π‐conjugation between C=C and P=P moieties. The HOMO–LUMO energy gap for 5 (4.15) and 6 (4.52 eV) is strikingly small and thus the narrowest among the diphosphenes (>4.93 eV) reported as yet. Consequently, 5 readily undergoes P=P bond cleavage at room temperature on treatment with sulfur to form the unique dithiophosphorane {(IPr)C(Ph)}P(S)2 7. Interestingly, reaction of 5 with selenium gives the selenadiphosphirane {(IPr)C(Ph)}P2Se 8 with an intact P−P bond.
P=P π‐Conjugation: A modular synthetic route to the divinyldiphosphenes III with the smallest HOMO–LUMO energy gap (4.15–4.52 eV) has been established. Compounds III are accessible as colored crystalline solids on reduction of the corresponding dichlorides II with magnesium. A divinyldiphosphene readily undergoes P=P bond cleavage reaction on treatment of S8 to give the rare vinyldithiophosphorane IV.
Seven derivatives of 1,2‐dicarbadodecaborane (ortho‐carborane, 1,2‐C2B10H12) with a 1,3‐diethyl‐ or 1,3‐diphenyl‐1,3,2‐benzodiazaborolyl group on one cage carbon atom were synthesized and ...structurally characterized. Six of these compounds showed remarkable low‐energy fluorescence emissions with large Stokes shifts of 15100–20260 cm−1 and quantum yields (ΦF) of up to 65 % in the solid state. The low‐energy fluorescence emission, which was assigned to a charge‐transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage CC bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660–18090 cm−1 for the CT bands and 1960–5540 cm−1 for the high‐energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with ΦF values between 8–32 %. Two distinct excited singlet‐state (S1) geometries, denoted S1(LE) and S1(CT), were observed computationally for the benzodiazaborolyl‐ortho‐carboranes, the population of which depended on their orientation (ψ). TD‐DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C‐diazaborolyl‐ortho‐carboranes were viewed as donor–acceptor systems with the diazaborolyl group as the donor and the ortho‐carboranyl group as the acceptor.
Official D–A business: Visible low‐energy fluorescence emissions with large Stokes shifts were observed for a series of C‐benzodiazaborolyl‐ortho‐carboranes owing to relaxation by cage CC bond‐lengthening. Experimental and computational data showed that these emissions were due to a charge transfer (CT) that involved the cluster and the borolyl group.
(C6F5)Te(CH2)3NMe2 (1), a perfluorophenyltellurium derivative capable of forming intramolecular N⋅⋅⋅Te interactions, was prepared and characterized. The donor‐free reference substance (C6F5)TeMe (2) ...and the unsupported adduct (C6F5)(Me)Te⋅NMe2Et (2 b) were studied in parallel. Molecular structures of 1, 2 and 2 b were determined by single‐crystal X‐ray diffraction and for 1 and 2 by gas‐phase electron diffraction. The structure of 1 shows N⋅⋅⋅Te distances of 2.639(1) Å (solid) and 2.92(3) Å (gas). Ab initio plus NBO and QTAIM calculations show significant charge transfer effects within the N⋅⋅⋅Te interactions and indicate σ‐hole interactions.
Willing to interact are the tellurium and nitrogen atoms of Me2N(CH2)3Te(C6F5) as was experimentally proven both in the solid state and the gas phase.
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.
Silanol‐silanolate anions: The reactions of hydroxide salts featuring weakly coordinating cations with cyclic dimethylsiloxanes were investigated and afforded salts of the corresponding silanol‐silanolate anions. The formed hydrogen bonds are strongly affected by the size and coordination ability of the cation. While cyclic D3OH− anions evolve in the presence of phosphazenium cations, linear polyanionic silanol‐silanolate strands are formed with NMe4+.
Weakly coordinating anions (WCAs) are important for academic reasons as well as for technical applications. Tetrakis(pentafluoroethyl)gallate, Ga(C2F5)4−, a new WCA, is accessible by treatment of ...GaCl3(dmap) (dmap=4‐dimethylaminopyridine) with LiC2F5. The anion Ga(C2F5)4− proved to be reluctant towards deterioration by aqueous hydrochloric acid or lithium hydroxide. Various salts of Ga(C2F5)4− were synthesized with cations such as PPh4+, CPh3+, (O2H5)2(OH2)22+, and Li(dec)2+ (dec=diethyl carbonate). Thermolysis of (O2H5)2(OH2)2Ga(C2F5)42 gives rise to a dihydrate of tris(pentafluoroethyl)gallane, Ga(C2F5)3(OH2)2. All products were characterized by NMR and IR spectroscopy, mass spectrometry, X‐ray diffraction, and elemental analysis. Furthermore, an outlook for the application of Li(dec)2Ga(C2F5)4 as a conducting salt in lithium‐ion batteries is presented.
Gallium chemistry: The new weakly coordinating anion Ga(C2F5)4− is accessible in the form of various salts, including a hydronium dication, by the reaction of GaCl3(dmap) (dmap=4‐dimethylaminopyridine) with LiC2F5. Thermolysis of (O2H5)2(OH2)2Ga(C2F5)42 yields a dihydrate of a tris(pentafluoroethyl)gallane, Ga(C2F5)3(OH2)2. Li(dec)2Ga(C2F5)4 (dec=diethyl carbonate) was investigated for its application in lithium‐ion batteries.
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