Alexander Villinger

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,

Synthesis, structure, and bonding of weakly coordinating anions based on CN adducts.

The addition of alkali or silver salts of dicyanoamide (dca), tricyanomethanide (tcm) and tetracyanoborate (tcb) to a solution of B(C(6)F(5))(3) in diethyl ether affords salts containing very voluminous B(C(6)F(5))(3) adduct anions of the type [E(CN)(n)(-)] x [B(C(6)F(5))(3)](n): E = N (dca_nb with n = 1, 2; b = B(C(6)F(5))(3)); E = C (tcm_nb with n = 1, 2, 3), and E = B (tcb_nb with n = 1, 2, 3, 4). Salts bearing these anions such as B[(CN) x B(C(6)F(5))(3)](4)(-) (= [B(CN)(4)(-)] x [B(C(6)F(5))(3)](4)), C[(CN) x B(C(6)F(5))(3)](3)(-) (= [C(CN)(3)(-)] x [B(C(6)F(5))(3)](3)), and N[(CN) x B(C(6)F(5))(3)](2)(-) (=[N(CN)(2)(-)] x [B(C(6)F(5))(3)](2)) can be prepared in good yields. They are thermally stable up to over 200 degrees C and dissolve in polar organic solvents. Depending on the stoichiometry mono-, di-, tri-, or tetraadduct formation is observed. The solid state structures of dca_2b, tcm_3b and tcb_4b salts show only long cation...anion contacts and thereby weak interactions, large anion volumes and only small distortions of the dca, tcm or tcb core enwrapped between B(C(6)F(5))(3) groups. That is why these anions can be regarded as weakly coordinating anions. On the basis of B3LYP/6-31+G(d) computations the energetics, structural trends and charge transfer of the adduct anion formation were studied. Since tcm_3b and tcb_4b are easily accessible and can also be prepared in large quantities, these anions may be utilized as a true alternative to other widely used weakly coordinating anions. Moreover, for both steric and electronic reasons it seems reasonable to expect that as counterions for cationic early transition metal catalysts such anions may show reduced ion pairing and hence increased catalytic activity.

Reactions of group 4 metallocene alkyne complexes with carbodiimides: experimental and theoretical studies of the structure and bonding of five-membered hetero-metallacycloallenes.

The reaction of the low-valent metallocene(II) sources Cp(2)Ti(η(2)-Me(3)SiC(2)SiMe(3)) (7) and Cp(2)Zr(py)(η(2)-Me(3)SiC(2)SiMe(3)) (11, Cp = η(5)-cyclopentadienyl, py = pyridine) with carbodiimides RN═C═NR (R = Cy, i-Pr, p-Tol) leads to the formation of five membered hetero-metallacycloallenes Cp(2)M{Me(3)SiC═C═C[N(SiMe(3))(R)]-N(R)} (9M-R) (M = Ti, R = i-Pr; M = Zr, R = Cy, i-Pr, p-Tol). Elimination of the alkyne (as the hitherto known reactivity of titanocene and zirconocene alkyne complexes would suggest) was not observed. The molecular structures of the obtained complexes were confirmed by X-ray studies. Moreover, the structure and bonding of the complexes 9Zr-Cy and 9Zr-p-Tol was investigated by DFT calculations.

Binary Bismuth(III) Azides: Bi(N3)3 , [Bi(N3)4]−, and [Bi(N3)6]3−

Binary azides of Group 15 elements form a class of highly endothermic compounds. Whereas numerous binary azides of the heavier Group 15 elements arsenic and antimony have been reported and characterized, such as As(N3)3, [1] [As(N3)4] , [As(N3)4] , As(N3)5, [3] [As(N3)6] , 4] Sb(N3)3, [1c,5] [Sb(N3)4] , [Sb(N3)4] , Sb(N3)5, [3] and [Sb(N3)6] , 3] no binary bismuth azides have been isolated to date. In the only report on binary bismuth triazide, the authors assumed the formation of Bi(N3)3 in the reaction of BiI3 with AgN3 in acetonitrile on the basis of IR data, but they were not able to isolate Bi(N3)3 from the reaction mixture. [6] Crystal structures of binary Group 15 azides are only known for As(N3)3, [1c] Sb(N3)3, [1c] [As(N3)6] , and [Sb(N3)6] , but no experimentally observed structural data are available for binary bismuth azides and Group 15 azides of the type [E(N3)4] (E = pnictogen). The energetic azido moiety adds about 70 kcal mol 1 to the energy content of a molecule. Thus we started the search for high-nitrogen-content Bi N compounds with the synthesis of bismuth triazide. In a second series of experiments, the synthesis of binary tetra-, penta-, and hexaazido bismuth compounds, which are highly endothermic polyazide compounds, was carried out in which the energy content increases with an increasing number of azido ligands. Following our interest in Group 15 element nitrogen compounds with a high nitrogen content, we describe herein the synthesis, isolation, and full characterization of binary tri-, tetra-, and hexaazido bismuth compounds for the first time. The synthesis of the binary compound Bi(N3)3 (1) was first achieved in the reaction of a solution of bismuth triiodide (BiI3) in tetrahydrofuran (THF) and neat, carefully dried silver azide (AgN3) at ambient temperature (Scheme 1). The resulting yellow suspension was stirred for two hours, resulting in an off-white suspension (mixture of AgI and 1). Removal of solvent and drying in vacuo gives a pale brownish residue. The major problem is the isolation of 1 from this residue. The isolation procedure includes re-suspension and extraction of the residue in THF at 50 8C. Finally, removal of solvent from the filtrate in vacuo gives Bi(N3)3·THF as a colorless solid in low yields. The solvent molecule cannot be removed at elevated temperatures in vacuo. An improved synthesis starts from bismuth trifluoride (BiF3) and an excess trimethylsilylazide (Me3SiN3) in THF, and also yields Bi(N3)3·THF (yield> 98 %). Solvent-free 1 (Figure 1) can be isolated when this reaction is carried out in

Activation of Small Molecules by Phosphorus Biradicaloids

The reactivity of biradicaloid [P(μ-NTer)]2 was employed to activate small molecules bearing single, double, and triple bonds. Addition of chalcogens (O2 , S8 , Sex and Tex ) led to the formation of dichalcogen-bridged P2 N2 heterocycles, except from the reaction with molecular oxygen, which gave a P2 N2 ring featuring a dicoordinated P(III) and a four-coordinated P(V) center. In formal [2πe+2πe] addition reactions, small unsaturated compounds such as ethylene, acetylene, acetone, acetonitrile, tolane, diphenylcarbodiimide, and bis(trimethylsilyl)sulfurdiimide are readily added to the P2 N2 heterocycle of the biradicaloid [P(μ-NTer)]2 , yielding novel heteroatom cage compounds. The synthesis, reactivity, and bonding of the biradicaloid [P(μ-NTer)]2 were studied in detail as well as the synthesis, properties, and structural features of all addition products.

Stable Heterocyclopentane‐1,3‐diyls

Diphosphadiazanediyl, [(μ-NR)P]2 (R=Ter=2,6-dimesitylphenyl), is known to readily activate small molecules with multiple bonds. CO is an especially intriguing species for activation, because either 1,1- or 1,2-bridging mode would lead to a [1.1.1]bicycle or a carbene, respectively. The activation of CO with diphosphadiazanediyl already occurs at ambient temperatures (1 bar, 25 °C). However, CO is involved in an unprecedented ring expansion reaction under preservation of the biradical character, which leads to the formation of the first stable cyclopentane-1,3-diyl analogue displaying photochromic molecular switch characteristics.

Cyclic arsenic-nitrogen cations.

A series of different cyclo-diarsa-diazenium salts bearing several bulky groups such as supermesityl (Mes* = 2,4,6-tBu(3)C(6)H(2)) and m-terphenyl (2,6-Mes(2)-C(6)H(3), Mes = 2,4,6-Me(3)C(6)H(2)) and anions such as triflate (OTf = SO(3)CF(3) = trifluoromethylsulfonate) and tetrachloridogallate (GaCl(4)(-)) were synthesized and fully characterized. The novel 1-chloro-cyclo-1,3-diarsa-2,4-diazenium cation represents the first example of a binary cyclic As(III)/N four-membered heterocyclic cation, with a di- and tricoordinated As atom and a delocalized pi bond along the NAs((+))N unit. The addition of excess Me(3)SiN(3) yields the fully characterized cationic arsenic azide, 1-azido-cyclo-1,3-diarsa-2,4-diazenium-mu-azido-hexachlorido-digallate. The Cl(-)/N(3)(-) exchange is triggered by the action of the Lewis acid GaCl(3). Depending on the Me(3)SiN(3) stoichiometry, different mu-azido-hexachlorido-digallate salts with either 1-chloro- or 1-azido-cyclo-1,3-diarsa-2,4-diazenium cations or even a mixture of both are observed. Moreover, it was of special interest to study the distances between the cationic arsenic center and the anion in cyclo-diarsa-diazenium salts. A correlation between the color of the salt and the anion/cation distance, ranging between 2 and 8 A in cyclo-diarsa-diazenium salts of the type [R(2)N(2)As(2)Y](+)X(-) depending on the bulky group R (R = Mes*, Ter), the substituent Y (Y = Cl, N(3), OTf), and the anion X(-) (X = OTf, GaCl(4), Cl(3)Ga-N(3)-GaCl(3)), was established.

Regioselective and Guided C–H Activation of 4-Nitropyrazoles

A divergent and regioselective approach to 5-aryl-4-nitro-1H-pyrazoles was developed by guided transition-metal-catalyzed arylation of 4-nitro-1H-pyrazoles. This method provides a convenient tool for the functionalization of the pharmacologically relevant pyrazole scaffold. The scope and limitations of the methodology were studied.

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.

A mixed arsenic-phosphorus centered biradicaloid.

Main-group singlet biradicaloids have been thoroughly investigated in the past two decades, especially derivatives of cyclobutane-1,3-diyl. However, in each of the known examples, the radical centers are identical. Therefore, we sought to prepare a mixed dipnictadiazanediyls with P and As bearing the radical character. To achieve this goal, the unprecedented cyclodichloro arsaphosphadiazane [ClP(μ-NTer)2AsCl] had to be prepared first. Treatment of [ClP(μ-NTer)2AsCl] with a halide-abstracting agent led to the novel cyclic cation [P(μ-NTer)2AsCl](+), while reduction with magnesium afforded the first arsaphosphadiazanediyl [P(μ-NTer)2As].