The element boron is known to have a variety of ways to relieve its inherent electron deficiency. The acceptance of an electron pair (Lewis acidity) has applications in catalysis [1] and activation of element-element bonds (frustrated Lewis pairs). [2] The combination of boron with p-donating substituents (e.g. BF 3 ) and its incorporation into organic p-conjugated systems allows the empty p z orbital of boron to participate in p bonding and p conjugation, respectively, and the latter enables the use of boron in optoelectronic materials with unique properties. [3] The absence of p-donating substituents at the boron center may result in multiple-center bonding to form nonclassical frameworks (e.g. B 2 H 6 or clusters). In addition, organoboranes and -diboranes(4) are prone to accept a single electron by chemical reduction. [4] Likewise, hydrogen atom abstraction from N-heterocyclic carbene (NHC)-stabilized boranes (NHC-BH 3 ) can lead to neutral, persistent [4f] boryl radicals of the type NHC-BH 2 C, [5] which have been studied by means of cyclic voltammetry, EPR, and UV/Vis spectroscopy as well as trapping reactions. [4][5][6] However, examples of isolated boron radicals are rare owing to the reactive nature of the species, and only little is known about their structural properties. Steric protection of the boron center combined with spin delocalization over the organic substituents, both achieved by substitution with mesityl groups (Mes = 2,4,6-trimethylphenyl), has occasionally enabled isolation and structural characterization of radical anions such as [Li ([12]crown-4) 2 ][BMes 3 ] (1) or [K([18]crown-6)-(thf) 2 ][Mes 2 BB(Ph)Mes] (2). [7]Our group has recently studied a persistent radical anion as an intermediate in the stepwise reduction of 1-ferrocenyl-2,3,4,5-tetraphenylborole (3). [8] Boroles are a class of antiaromatic compounds with interesting chemical and photophysical properties [9,10] that are well-known for their ability to accept two electrons with formation of an aromatic borole dianion. [11,12] Encouraged by these recent results on the radical anion [3]C À , which indicated the presence of a highly unusual C 4 B p system bearing five electrons, [8] we set out to isolate and characterize a stable borol radical anion. As we report here, this was possible by choice of steric protection and an appropriate reducing agent.The synthesis of MesBC 4 Ph 4 (1-mesityl-2,3,4,5-tetraphenylborole, 4) by means of the commonly employed tin-boron exchange reaction [11][12][13][14] was unsuccessful because of the low reactivity of dihalo(mesityl)boranes (MesBX 2 ; X = Cl, Br). However, 4 was obtained in 41 % yield by functionalization of the boron center in 1-chloro-2,3,4,5-tetraphenylborole (5) through nucleophilic displacement of the chlorine ligand with LiMes (Scheme 1). [14b] A more efficient alternative was found to be the salt-elimination reaction of MesBCl 2 with 1,4dilithio-2,3,4,5-tetraphenylbuta-1,3-diene (6) which provided 4 in 66 % yield. Formation of a Lewis acid-base adduct with Et 2 O, as previou...

The taming of borylene: Dehalogenation of BHCl2⋅IMe (IMe=1,3‐dimethylimidazol‐2‐ylidene) leads to a carbene‐stabilized elusive BH borylene. A syn‐selective [2+1] cycloaddition with naphthalene enabled the isolation and full characterization of the resulting diastereomeric products. These findings along with calculations provide clear evidence for the existence of borylenes (crystal structures: B violet, C black, H gray, N blue).

Borole systems tend to undergo various reactions driven by the disruption of its destabilizing antiaromatic character. As a consequence, the isolation and characterization of free boroles is challenging, especially when the substituents around the C 4 B framework are sterically less demanding. In the present paper we report the synthesis of 1-bromo-2,3,4,5tetraphenylborole. The title compound readily undergoes a dimerization/rearrangement reaction in analogy to the previously reported 1-chloro-2,3,4,5-tetraphenyborole to form an isostructural product identified by X-ray crystallography. Additionally we present the formation of Lewis acidÀbase adducts of the title compound with 3,5-lutidine, PCy 3 , N-hetrocyclic carbene, cyclic (amino)(alkyl)carbene, and THF. The latter compounds were analyzed by single-crystal X-ray diffraction and compared.

The first cationic metallaborylene complexes, [{(OC)5Mn}2(μ‐B)][BArf4] and [{(η5‐C5H4R)(OC)2Fe}2(μ‐B)][BArf4] (R=H, Me; see structure) were synthesized by halide abstraction from bridged haloborylene complexes and structurally characterized. The boron atom is located in the coordination sphere of two transition‐metal centers and is highly unsaturated.

Syntheses of the first heteroleptic N-heterocyclic carbene (NHC)-phosphane platinum(0) complexes and formation of the corresponding Lewis acid-base adducts with aluminum chloride is reported. The influence of N-heterocyclic carbenes on tuning the Lewis basic properties of the metal complexes was judged from spectroscopic, structural, and computational data. Conclusive experimental evidence for the enhanced Lewis basicity of NHC-containing complexes was provided by a transfer reaction.

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A novel one-pot method was developed for the preparation of [Ti(η(5)-C(5)H(5))(η(7)-C(7)H(7))] (troticene, 1) by reaction of sodium cyclopentadienide (NaCp) with [TiCl(4)(thf)(2)], followed by reduction of the intermediate [(η(5)-C(5)H(5))(2)TiCl(2)] with magnesium in the presence of cycloheptatriene (C(7)H(8)). The [n]troticenophanes 3 (n=1), 4, 8, 10 (n=2), and 11 (n=3) were synthesized by salt elimination reactions between dilithiated troticene, [Ti(η(5)-C(5)H(4)Li)(η(7)-C(7)H(6)Li)]⋅pmdta (2) (pmdta = N,N',N',N'',N''-pentamethyldiethylenetriamine), and the appropriate organoelement dichlorides Cl(2)Sn(Mes)(2) (Mes = 2,4,6-trimethylphenyl), Cl(2)Sn(2)(tBu)(4), Cl(2)B(2)(NMe(2))(2), Cl(2)Si(2)Me(4), and (ClSiMe(2))(2)CH(2), respectively. Their structural characterization was carried out by single-crystal X-ray diffraction and multinuclear NMR spectroscopy. The stanna[1]- and stanna[2]troticenophanes 3 and 4 represent the first heteroleptic sandwich complexes bearing Sn atoms in the ansa bridge. The reaction of 3 with [Pt(PEt(3))(3)] resulted in regioselective insertion of the [Pt(PEt(3))(2)] fragment into the Sn-C(ipso) bond between the tin atom and the seven-membered ring, which afforded the platinastanna[2]troticenophane 5. Oxidative addition was also observed upon treatment of 4 with elemental sulfur or selenium, to produce the [3]troticenophanes [Ti(η(5)-C(5)H(4)SntBu(2))(η(7)-C(7)H(6)SntBu(2))E] (6: E=S; 7: E=Se). The B-B bond of the bora[2]troticenophane 8 was readily cleaved by reaction with [Pt(PEt(3))(3)] to form the corresponding oxidative addition product [Ti(η(5)-C(5)H(4)BNMe(2))(η(7)-C(7)H(6)BNMe(2))Pt(PEt(3))(2)] (9). The solid-state structures of compounds 5, 6, and 9 were also determined by single-crystal X-ray diffraction.