Mathias Lehmann

Bissilylated Halonium Ions: [Me3SiXSiMe3][B(C6F5)4] (X=F, Cl, Br, I)

Ions of the type [R X R], where X is any halogen, are referred to as halonium ions. They may be open-chain or cyclic and are an important class of onium ions. They play a major role in preparative chemistry as reaction intermediates, for example in Friedel–Crafts alkylation or Lewis acid catalyzed halogenation reactions. Dialkyl chloro-, bromo-, and iodonium ions can be prepared as stable, long-lived ions and even isolated as stable salts by treating an excess of haloalkane with strong Lewis acids in low-nucleophilicity solvents, mostly superacidic media (Scheme 1A). However,

Cyclic Distiba‐ and Dibismadiazenium Cations

Four-membered pnictogen-nitrogen heterocycles of the type [XE(m-NR)]2 (E = element of Group 15, X = halogen) are known as 1,3-dihalogeno-cyclo-1,3-dipnicta(III)-2,4-diazanes (Scheme 1, species A). The phosphorus species in particular play a major role in preparative phosphorus–nitrogen chemistry, for example in the preparation of macrocycles, polymers, main-group complexes, and ring transformation reactions or the generation of cyclic binary PN cations. In recent years, new examples for cyclo-1,3-dipnicta(III)-2,4diazanes were prepared by introduction of bulky R substituents and characterized, with the focus on kinetic stabilization. Only recently, the synthesis of salts bearing the highly reactive cyclo-1,3-dipnicta(III)-2,4-diazenium cations [ClE(mNTer)2E] + (E = P and As; Ter = terphenyl = 2,6-bis(2,4,6trimethylphenyl)phenyl; Scheme 1, species B) was reported. The generation of such cations was achieved by halide abstraction upon addition of strong Lewis acids, such as GaCl3. [9] Utilization of the bulky terphenyl group leads to the formation of separated ion pairs in the crystal with large interionic distances.

Chlorine/methyl exchange reactions in silylated aminostibanes: a new route to stibinostibonium cations.

The Lewis acid assisted triflate/methyl, azide/methyl, and chlorine/methyl exchange reactions between silicon and antimony have been studied in the reaction of R(Me(3)Si)N-SbCl(2) (R = Ter) with AgOTf, AgN(3), KOtBu, GaCl(3), and Me(3)SiN(3)/GaCl(3), resulting in the formation of different methylantimony compounds. Furthermore, R(Me(3)Si)N-SbCl(2) (R = SiMe(3)) was reacted with GaCl(3) at low temperatures to yield a hitherto unreported amino(chloro)stibenium cation, the proposed intermediate in methyl exchange reactions. Tetrachloridogallate salts bearing different stibinostibonium cations such as [(Me(3)Sb)SbMe(2)](+) and [(Me(3)Sb)(2)SbMe](2+) along with the GaCl(3) adduct of SbMe(3) were isolated from such R(Me(3)Si)N-SbCl(2)/GaCl(3) mixtures (R = SiMe(3)) at ambient temperatures, depending on the reaction parameters.

An Unusual Isomerization to Tetraazastiboles

(RN5; R is usually an aryl group). They form a class of highly endothermic and explosive compounds. The existence of an all-nitrogen aromatic azole ring, RN5 (R = C6H5), was considered by Clusius and Hurzeler and shown by Huisgen and Ugi in the reaction of phenyldiazonium chloride, [C6H5N2] Cl , and lithium azide, LiN3, leading to the intermediate formation of acyclic phenyldiazonium azide (65 %) and phenyl pentazole (35 %), in which the pentazole ring is strongly stabilized by conjugation with the phenyl ring. Substitution of one nitrogen atom in pentazoles by a heavier element of Group 15 leads to isovalent tetraazapnictoles (Scheme 1), RN4E (E = P, As, Sb, Bi), which also have electronic structures that are related to those of aromatic hydrocarbons with (4n + 2)p electrons. Only recently, two examples of tetraazapnictoles (E = P, R = Mes* = 2,4,6-tritert-butylphenyl or m-Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl; 5] E = As, R = Mes*) have been isolated and fully characterized. Two different synthetic routes to tetraazapnictoles have been described: 6] 1) The reaction of iminopnictanes with trimethylsilylazide (Scheme 2, route A) gives, in the presence of a Lewis acid such as GaCl3, the corresponding tetraazapnictole RN4E (E = P, As) stabilized as GaCl3 adducts; 2) utilization of Me3Si-substituted aminodichloropnictanes, which can be regarded as “disguised” kinetically stabilized iminopnictanes, gives the desired RN4E in high yields when added to Me3SiN3 (Scheme 2, route B). This reaction only occurs when a Lewis acid is added, as it induces Me3SiCl elimination and thus the release of the reactive iminopnictane. 8] Substitution of two nitrogen atoms in pentazoles by heavier Group 15 elements leads to triazadipnictoles, RN3E2, of which only the triazadiphosphole 9, 10] was reported (E = P, R = N(SiMe3)2, Mes*). All other classes of heterocycles consisting only of Group 15 atoms, namely RN2E3, RNE4, and RE5, remain unknown. To the best of our knowledge, analogous Group 15 heterocycles involving Sb and Bi have not been reported. Following our interest in Group 15 element nitrogen compounds with a high nitrogen content, we describe herein the synthesis, isolation, and full characterization of a tetraazastibole, RN4Sb (R = Mes*), stabilized as B(C6F5)3 adduct. As illustrated in Scheme 2, both synthetic procedures (method A and B) were studied to isolate the tetraazastibole Mes*N4Sb. At first we started with an investigation of the equilibrium between cyclic distibadiazane and its monomer, the iminostibane (Scheme 2), by means of H NMR spectroscopy. However, in contrast to the situation found for E = P, As, only the dimer was detected. In agreement with these experimental data, quantum chemical calculations show that the heavier the pnictogen, the more stable the dimer is: D298H(monomer!dimer): + 49.7 (P)<+ 14.4 (As)< 95.4 (Sb)< 121.5 kJmol 1 (Bi). 12] As the monomer is needed for the cyclization step, no cycloaddition occurred when Me3SiN3 was added in the presence of GaCl3 (Scheme 2 and Scheme 3). In a next series of experiments, route B was followed, which included the synthesis of the Me3Si-substituted aminodichlorostibane Mes*N(SiMe3)SbCl2. [13] HowScheme 1. Five-membered aromatic heterocycles consisting exclusively of Group 15 atoms (E).

Synthesis of Blue Imino(pentafluorophenyl)phosphane

The reaction of AgC(6)F(5) with monomeric iminophosphanes of Mes*-N═P-X (X = Cl, I) in CH(2)Cl(2) at ambient temperature gives imino(pentafluorophenyl)phosphane, Mes*N═P(C(6)F(5)) (1), in almost quantitative yield (96%), which could be isolated as a highly viscous blue oil. The same reaction with LiC(6)F(5) results in the formation of imino(amino)phosphane (C(6)F(5))(2)P-N(Mes*)-P═NMes* (2) (yield 93%). In the second series of experiments the analogous reaction of MC(6)F(5) (M = Ag, Li) with dimeric [Cl-P(μ-N-Dipp)](2) was studied, leading to the formation of [R-P(μ-N-Dipp)](2) (R = C(6)F(5)) (3) for M = Ag, while only decomposition products such as P(C(6)F(5))(3) were observed in the reaction with the Li salt. Highly labile Mes*-N═P-C(6)F(5) (1) decomposes at ambient temperatures, forming among other products the diphosphane (C(6)F(5))(2)P-P(C(6)F(5))(2) (4). Reaction of 1 with Fe(2)(CO)(9) yields the iron carbonyl complexes Mes*-N═P(C(6)F(5))·Fe(CO)(4) (5) and [Mes*-N═P(C(6)F(5))](2)·Fe(CO)(3) (6). The structure, bonding, and potential energy surface are discussed on the basis of B3LYP/6-31G(d,p) computations. According to time-dependent B3LYP calculations, the blue color of 1 arises from an n → π* electronic transition.