Nikolai Ignat'ev

The Hexacyanodiborane(6) Dianion [B2(CN)6]2−

Diborane(6) dianions with substituents that are bonded to boron via carbon are very reactive and therefore only a few examples are known. Diborane(6) derivatives are the simplest catenated boron compounds with an electron-precise B-B σ-bond that are of fundamental interest and of relevance for material applications. The homoleptic hexacyanodiborane(6) dianion [B2 (CN)6 ](2-) that is chemically very robust is reported. The dianion is air-stable and resistant against boiling water and anhydrous hydrogen fluoride. Its salts are thermally highly stable, for example, decomposition of (H3 O)2 [B2 (CN)6 ] starts at 200 °C. The [B2 (CN)6 ](2-) dianion is readily accessible starting from 1) B(CN)3 (2-) and an oxidant, 2) [BF(CN)3 ](-) and a reductant, or 3) by the reaction of B(CN)3 (2-) with [BHal(CN)3 ](-) (Hal=F, Br). The latter reaction was found to proceed via a triply negatively charged transition state according to an SN 2 mechanism.

Deprotonation of a Hydridoborate Anion

The first deprotonation of a borohydride anion was achieved by treatment of [BH(CN)3 ]- with strong non-nucleophilic bases, which resulted in the formation of alkali-metal salts of the tricyanoborate dianion B(CN)32- in up to 97 % yield and 99.5 % purity. [BH(CN)3 ]- is less acidic than (Me3 Si)2 NH but a stronger acid than iPr2 NH. Less sterically hindered, more nucleophilic bases such as PhLi and MeLi mostly attack a CN group under formation of imine dianions [RC(N)B(CN)3 ]2- , which can be hydrolyzed to ketones of the [RC(O)B(CN)3 ]- type. The boron-centered nucleophile B(CN)32- reacts with CO2 and CN+ reagents to give salts of the [B(CN)3 CO2 ]2- dianion and the tetracyanoborate anion [B(CN)4 ]- , respectively, in excellent yields.

Synthesis of Tris‐ and Tetrakis(pentafluoroethyl)silanes

The synthesis and complete characterization of functional, highly Lewis acidic tris(pentafluoroethyl)silanes as well as tetrakis(perfluoroalkyl)silanes Si(C2F5)4 and Si(C2F5)3 CF3 by direct fluorination is described. The reaction of SiCl4 with LiC2F5 invariably affords (pentafluoroethyl)fluorosilicates. To avoid silicate formation by fluoride transfer from LiC2F5 the Lewis acidity of the silane has to be decreased by electron-donating substituents, such as dialkylamino groups. The easily accessible Si(C2F5)3 NEt2 is a valuable precursor for a series of tris(pentafluoroethyl)silanes.

Synthesis of five- and six-coordinate tris(pentafluoroethyl)fluorosilicates.

The research area of perfluoroalkylsilanes is still in its infancy. Although there are already many examples of difluorotriorganylsilicates, the first example of a completely characterized trifluorotriorganylsilicate is presented, the dianion [Si(C2 F5 )3 F3 ](2-) . The strongly electron-withdrawing influence of the pentafluoroethyl groups appears to be a fundamental cause of the stability of this compound. This dianion is also the first structurally characterized example of a tris(pentafluoroethyl)silicon compound. The synthesis and complete characterization of [PPh4 ]2 [Si(C2 F5 )3 F3 ] and [PPh4 ][Si(C2 F5 )3 F2 ] along with the precursor [H(OEt2 )2 ][Si(C2 F5 )3 F2 ] was achieved from SiCl4 and LiC2 F5 .

Synthesis of Functional Phosphates [P(C2F5)3F2X]− from the Phosphorane Adduct [P(C2F5)3F2(dmap)]

Weakly coordinating anions (WCAs) are of great academic as well as industrial interest. To advance the fluoroalkyl phosphate anion [P(C2F5)3F3](-), which is ideally suited for technical applications, functional substituents X are attached to the Lewis acid (C2F5)3PF2. In keeping with this, the reaction of (C2F5)3PF2 with 4-(dimethylamino)pyridine (DMAP) yields the corresponding adduct [P(C2F5)3F2(dmap)]. Treatment of this adduct with Brønsted acids (HX) leads to the selective formation of the corresponding phosphate anions [P(C2F5)3F2X](-) by trapping the proton with the liberated base DMAP. In this way, OH functions can be transformed in one step into WCAs, slightly increasing the coordination properties of the corresponding anion. An ion exchange to the corresponding [PPh4] salts results in solids, whereas the corresponding imidazolium salts are obtained as room-temperature ionic liquids.

Synthesis of Tris(pentafluoroethyl)germanes

The synthesis of tris(pentafluoroethyl)germanium derivatives is described. The reaction of germanium tetrachloride with three equivalents of the pentafluoroethylation reagent LiC2 F5 does not lead selectively to the formation of tris(pentafluoroethyl)chlorogermane, (C2 F5 )3 GeCl. Here the introduction of a diethylamino function as a protecting group was beneficial. Thus, treatment of Cl3 GeNEt2 with LiC2 F5 smoothly afforded (C2 F5 )3 GeNEt2 . The replacement of the amino substituent by halides was accomplished by reaction with HBr or HCl on a multigram scale. The combination of (C2 F5 )3 GeCl with Ag2 CO3 gave rise to the formation of the digermoxane [(C2 F5 )3 Ge]2 O. An obtuse Ge-O-Ge angle of 150.2(1)° was determined by X-ray diffraction. Attempted hydrolysis of the digermoxane leads to an equilibrium mixture of the precursor, (C2 F5 )3 GeOH, and water.

Synthesis of Unsymmetrically Substituted Phosphane Oxides (R1R2P(O)H) and Phosphinous Acids (R1R2POH)

This paper describes the synthesis of unsymmetrically substituted phosphinous acids and phosphane oxides featuring at least one electron-withdrawing pentafluoroethyl group. The presence of a diethylamino function as a protecting group allows a selective reaction of RClPNEt2 (R=CF3 , C6 F5 , C6 H5 ) with LiC2 F5 . On treatment with para-toluenesulfonic acid the isolated aminophosphanes R(C2 F5 )PNEt2 are readily converted into the corresponding phosphinous acids or phosphane oxides, respectively. Investigation of the tautomeric equilibrium between oxide and acid tautomer revealed (CF3 )(C2 F5 )POH as a stable phosphinous acid, whereas the pentafluorophenyl and phenyl derivatives constitute a solvent dependent equilibrium between the acid and the oxide tautomer.

Ionic Liquids—Advanced Reaction Media for Organic Synthesis

Abstract The advantages in the application of ionic liquids as reaction media in organic synthesis, i.e., in the preparation of chromane derivatives, substituted pyrazines, 4-aminofuran-2(5H)-ones, or in bromination of Levulinic acid or dehydration of alcohols, saccharides, and polysaccharides, have been demonstrated on several examples. Ionic liquids with Brønsted acidity have been shown to possess catalytic activity and provide access to convenient technologies for the preparation of various useful compounds.

Transition Metal Complexes of Phosphinous Acids Featuring a Quasichelating Unit: Synthesis, Characterization, and Hetero-bimetallic Complexes

Diorganophosphane oxides were employed as preligands for the synthesis of catalytically active transition metal complexes of the phosphinous acids (CF3)2POH and (C2F5)2POH. Their reactions with solid PtCl2 and PdCl2 led to the formation of mononuclear phosphinous acid complexes [Cl2M{P(R(f))2OH}2] (M = Pd, Pt; R(f) = C2F5, CF3), which can be crystallized, for example, as its pyridinium salts, 2[HPy](+)[Cl2Pd{P(CF3)2O}2](2-). In vacuo HCl is liberated from the neutral palladium complexes affording mixtures of di- and polynuclear complexes. Moreover, (C2F5)2POH was reacted with several β-diketonato complexes of palladium, platinum, and nickel yielding air- and moisture-stable complexes [(acac)M{[P(R(f))2O]2H}], featuring a quasichelating phosphinous acid phosphinito unit {P(R(f))2O···H···O(R(f))2P}(-). Treatment of [Ni(Cp)2] (Cp = cyclopentadienyl) and [(cod)RhCl]2 (cod = 1,5-cyclooctadiene) with (C2F5)2POH leads to the substitution of one Cp or chloro ligand by a quasichelating unit. The novel coordination compounds were characterized by NMR and IR spectroscopies, mass spectrometry, and X-ray diffraction analysis. The platinum complex [(acac)Pt{[P(C2F5)2O]2H}] (acac = acetylacetonato) was used for the construction of hetero-bimetallic complexes by the treatment with [(cod)RhCl]2 and [Ni(Cp)2]. The trinuclear bimetallic complex [{(acac)Pt[P(C2F5)2O]2}2Ni] is the first structurally characterized hetero-bimetallic species containing a bis(perfluoroalkyl)phosphinito bridge.

Tris(pentafluoroethyl)germane: Deprotonation and Hydrogermylation Reactions

Owing to the highly electron-withdrawing C2 F5 groups, the tris(pentafluoroethyl)germane (C2 F5 )3 GeH represents an interesting Brønsted acidic compound. The germane is accessible by the treatment of the corresponding halogenogermanes (C2 F5 )3 GeX (X=Cl, Br) with Bu3 SnH. After clarifying its molecular structure in the solid state by X-ray diffraction, the acidity of (C2 F5 )3 GeH was examined by treatment with different bases, for example, 1,8-bis(dimethylamino)naphthalene. The resulting germanate(II) ion [Ge(C2 F5 )3 ]- serves as a germyl group transfer reagent. The reaction with main-group and transition-metal complexes or organohalides opens access to a variety of different compounds, the structures of which were mostly determined by X-ray diffraction. The corresponding tricarbonylnickelate(0) complex [(CO)3 NiGe(C2 F5 )3 ]- also gives information about the π-acceptor properties of the [Ge(C2 F5 )3 ]- ligand. Furthermore, hydrogermylation reactions of (C2 F5 )3 GeH with alkynes afforded different stereoisomers (α,β-cis, β-trans) depending on the respective reaction conditions and substrates.