Krzysztof Radacki

Ambient-Temperature Isolation of a Compound with a Boron-Boron Triple Bond

B-B Bond Alkynes contain carbon-carbon triple bonds and represent a diverse class of organic compounds. In principle, valence rules suggest that the boron analog of an alkyne, with a B-B triple bond, ought to be accessible by appending a two-electron donor to each B atom. Braunschweig et al. (p. 1420; see the Perspective by Frenking and Holzmann) now present the synthesis, isolation, and crystallization of a solid, triple-bonded diboryne, with N-heterocyclic carbenes as the terminal substituents, which contains the expected linear bonding geometry. A boron analog of an alkyne has been synthesized by reduction of a brominated precursor. Homoatomic triple bonds between main-group elements have been restricted to alkynes, dinitrogen, and a handful of reactive compounds featuring trans-bent heavier elements of groups 13 and 14. Previous attempts to prepare a compound with a boron-boron triple bond that is stable at ambient temperature have been unsuccessful, despite numerous computational studies predicting their viability. We found that reduction of a bis(N-heterocyclic carbene)-stabilized tetrabromodiborane with either two or four equivalents of sodium naphthalenide, a one-electron reducing agent, yields isolable diborene and diboryne compounds. Crystallographic and spectroscopic characterization confirm that the latter is a halide-free linear system containing a boron-boron triple bond.

Multiple complexation of CO and related ligands to a main-group element

The ability of an atom or molecular fragment to bind multiple carbon monoxide (CO) molecules to form multicarbonyl adducts is a fundamental trait of transition metals. Transition-metal carbonyl complexes are vital to industry, appear naturally in the active sites of a number of enzymes (such as hydrogenases), are promising therapeutic agents, and have even been observed in interstellar dust clouds. Despite the wealth of established transition-metal multicarbonyl complexes, no elements outside groups 4 to 12 of the periodic table have yet been shown to react directly with two or more CO units to form stable multicarbonyl adducts. Here we present the synthesis of a borylene dicarbonyl complex, the first multicarbonyl complex of a main-group element prepared using CO. The compound is additionally stable towards ambient air and moisture. The synthetic strategy used—liberation of a borylene ligand from a transition metal using donor ligands—is broadly applicable, leading to a number of unprecedented monovalent boron species with different Lewis basic groups. The similarity of these compounds to conventional transition-metal carbonyl complexes is demonstrated by photolytic liberation of CO and subsequent intramolecular carbon–carbon bond activation.

Oxoboryl complexes: boron-oxygen triple bonds stabilized in the coordination sphere of platinum.

One B, One O Boron has a tendency to share electrons with multiple different atoms, hence exhibiting a rich cluster chemistry, in contrast to the more traditional two-center, two-electron bonds prevailing in the compounds of most other light elements. Braunschweig et al. (p. 345) have coaxed boron into a more confined setting and made a boron analog of carbon monoxide as a triply bonded BO anion that was stabilized by coordination to a platinum center. The product formed easily in room temperature solution from a precursor substituted with a silyl group on the oxygen and a bromide on the boron and exhibited surprising stability toward heating and photolysis. The BO anion is a fundamental binary material, isoelectronic with CO, CN-, and NO+, which have been the key binary ligands in organometallic and coordination chemistry for more than 50 years. A mild synthetic method yields a boron analog of the widely studied carbon monoxide ligand. Monomeric oxoboranes have hitherto been detected only as short-lived species in gas-phase or low-temperature matrix experiments. Here, we report formation of the oxoboryl complex trans-[(Cy3P)2BrPt(B≡O)] (Cy being cyclohexyl) by means of reversible liberation of trimethylsilylbromide from the boron–bromine oxidative addition product of dibromo(trimethylsiloxy)borane and [Pt(PCy3)2] in room-temperature toluene solution. The platinum complex is inert toward oligomerization, even under photolytic conditions and at elevated temperatures. The bromide was substituted by thiophenolate, and spectral parameters of both products as well as results of computational and x-ray diffraction studies are in agreement with the formulation of a triple bond between boron and oxygen. The boron–oxygen distance of 120.5(7) picometers shows a bond shortening of 7.2% as compared with a double bond, which is similar to the shortening observed in carbon–carbon analogs.

An Isolable Radical Anion Based on the Borole Framework

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 and activation of element–element bonds (frustrated Lewis pairs). The combination of boron with p-donating substituents (e.g. BF3) and its incorporation into organic p-conjugated systems allows the empty pz 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. The absence of p-donating substituents at the boron center may result in multiple-center bonding to form nonclassical frameworks (e.g. B2H6 or clusters). In addition, organoboranes and -diboranes(4) are prone to accept a single electron by chemical reduction. Likewise, hydrogen atom abstraction from N-heterocyclic carbene(NHC)-stabilized boranes (NHC-BH3) can lead to neutral, persistent boryl radicals of the type NHC-BH2C, [5] which have been studied by means of cyclic voltammetry, EPR, and UV/Vis spectroscopy as well as trapping reactions. 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][BMes3] (1) or [K([18]crown-6)(thf)2][Mes2BB(Ph)Mes] (2). [7] Our group has recently studied a persistent radical anion as an intermediate in the stepwise reduction of 1-ferrocenyl2,3,4,5-tetraphenylborole (3). Boroles are a class of antiaromatic compounds with interesting chemical and photophysical properties that are well-known for their ability to accept two electrons with formation of an aromatic borole dianion. Encouraged by these recent results on the radical anion [3]C , which indicated the presence of a highly unusual C4B 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 MesBC4Ph4 (1-mesityl-2,3,4,5-tetraphenylborole, 4) by means of the commonly employed tin–boron exchange reaction was unsuccessful because of the low reactivity of dihalo(mesityl)boranes (MesBX2; 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). A more efficient alternative was found to be the salt-elimination reaction of MesBCl2 with 1,4-

Generation of a carbene-stabilized bora-borylene and its insertion into a C-H bond.

A novel NHC adduct of a dihalodiborane(4), 1, is reduced by KC(8) with formation of the five-membered boracycle 2. The reaction most likely proceeds via C-H insertion of an intermediate NHC-stabilized free bora-borylene species.

Isolation of a Neutral Boron‐Containing Radical Stabilized by a Cyclic (Alkyl)(Amino)Carbene

Utilizing a cyclic (alkyl)(amino)carbene (CAAC) as a ligand, neutral CAAC-stabilized radicals containing a boryl functionality could be prepared by reduction of the corresponding haloborane adducts. The radical species with a duryl substituent was fully characterized by single-crystal X-ray structural analysis, EPR spectroscopy, and DFT calculations. Compared to known neutral boryl radicals, the isolated radical species showed larger spin density on the boron atom. Furthermore, the compound that was isolated is extraordinarily stable to high temperatures under inert conditions, both in solution and in the solid state. Electrochemical investigations of the radical suggest the possibility to generate a stable formal boryl anion species.

Controlled homocatenation of boron on a transition metal

Only a handful of elements are able to be controllably homocatenated (that is, to be formed into one- or two-dimensional chains or rings of the element), because most have weak element-element bonds. Boron forms strong B-B bonds, but its favourable cluster formation makes homocatenation very difficult. Recently, the coupling of borylene (:BR) ligands on a metal was predicted computationally. We have brought this prediction to fruition experimentally, and extended it by adding two further borylene units, stepwise forming a B(4) chain bound to a metal under mild conditions. This complex is a useful model for understanding the metal-boron interactions required to promote transition of the boron atoms from borylene ligands to oligoborane networks bound side-on. The concept shows great promise for the controlled construction of one-dimensional boron chains.

Borylene-Based Direct Functionalization of Organic Substrates: Synthesis, Characterization, and Photophysical Properties of Novel π-Conjugated Borirenes

Room temperature photolysis of aminoborylene complexes, [(CO)(5)M=B=N(SiMe(3))(2)] (1: M = Cr, 2: Mo) in the presence of a series of alkynes and diynes, 1,2-bis(4-methoxyphenyl)ethyne, 1,2-bis(4-(trifluoromethyl)phenyl)ethyne, 1,4-diphenylbuta-1,3-diyne, 1,4-bis(4-methoxyphenyl)buta-1,3-diyne, 1,4-bis(trimethylsilylethynyl)benzene and 2,5-bis(4-N,N-dimethylaminophenylethynyl)thiophene led to the isolation of novel mono and bis-bis-(trimethylsilyl)aminoborirenes in high yields, that is [(RC=CR)(mu-BN(SiMe(3))(2)], (3: R = C(6)H(4)-4-OMe and 4: R = C(6)H(4)-4-CF(3)); [{(mu-BN(SiMe(3))(2)(RC=C-)}(2)], (5: R = C(6)H(5) and 6: R = C(6)H(4)-4-OMe); [1,4-bis-{(mu-BN(SiMe(3))(2)(SiMe(3)C=C)}benzene], 7 and [2,5-bis-{(mu-BN(SiMe(3))(2) ((C(6)H(4)NMe(2))C=C)}-thiophene], 8. All borirenes were isolated as light yellow, air and moisture sensitive solids. The new borirenes have been characterized in solution by (1)H, (11)B, (13)C NMR spectroscopy and elemental analysis and the structural types were unequivocally established by crystallographic analysis of compounds 6 and 7. DFT calculations were performed to evaluate the extent of pi-conjugation between the electrons of the carbon backbone and the empty p(z) orbital of the boron atom, and TD-DFT calculations were carried out to examine the nature of the electronic transitions.

Antiaromaticity to Aromaticity: From Boroles to 1,2‐Azaborinines by Ring Expansion with Azides

We have exploited the reactivity of antiaromatic boroles, gaining access to aryl-substituted monocyclic 1,2-azaborinines. The observed ring-expansion reaction of inherently electron-deficient boroles with organometallic and organic azides is demonstrated for representative examples. This substance class is expected to provide a new avenue into 1,2-azaborinine chemistry, especially in the area of functional organoboron materials. Our results are based on NMR and UV/Vis spectroscopy as well as single-crystal X-ray crystallography and provide a virtually quantitative approach that also offers numerous points of variation.

The reduction chemistry of ferrocenylborole.

In the chemistry of functionalized metallocenes, borylated ferrocenes have drawn great attention owing to their potential applications as electron sponges, anion chemosensors, and redox-active macromolecules. Electrochemical stimulation of the Fe/Fe couple can lead to significant changes in the molecular structure, and to the anion binding properties of the system. One distinct structural feature of borylated ferrocenes is the bending of the boryl group towards the iron atom. Both experimental and theoretical studies of these molecules reveal a direct through-space interaction between the filled iron 3d orbital and the empty 2p orbital at boron. This “dip” angle can be reduced by incorporation of p-donating substituents to the boron atom, coordination with Lewis bases, by increasing the number of boryl functionalities on the Cp rings, or oxidation of the ferrocene to ferrocenium. The structural changes induced by electronic reduction of the boryl group, however, have not received much attention. Investigations on the reduction chemistry of organoboranes have led to the isolation of various interesting boroncentered radical species. The reduction potential of the boron center can be tuned by substitution with fluorinated aryl groups, introduction of a second boryl moiety, attachment of cationic functionalities, or by incorporating the boron atom into an antiaromatic ring system. In line with our interests in antiaromatic boracycles and metallocene chemistry, we now report the reduction chemistry of 1-ferrocenylborole and the isolation of the resulting reduced species. In our previous work, the most striking structural feature of 1-ferrocenyl-2,3,4,5-tetraphenylborole (1) is the large dip angle of 29.48. This observation is attributed to the strong Fe–B interaction resulting from the antiaromatic nature of the borole moiety. The presence of electro-active ferrocene (Fc) and borole units in 1 prompted us to investigate its electrochemistry. The redox behavior of 1 in CH2Cl2 and in THF was studied by cyclic voltammetry (referenced against the Fc/Fc couple). Compound 1 displays one broad irreversible oxidation peak around Epa = 0.5 V in CH2Cl2. (see Figure S1 of the Supporting Information). The oxidation behavior of 1 is distinctly different from that observed for 9-ferrocenyl borafluorene, which shows a reversible Fe/Fe redox couple at 0.01 V (vs. Fc/Fc). The oxidation process of 1 is thus anodically shifted as a result of the stronger Fe–B interaction in 1. The oxidation event is also more positively shifted than that of the ferrocenylboron dication (E1/2 = 0.24 V), in which the effect on Fe/Fe couple is solely inductive. According to the study of 9-ferrocenyl borafluorene, one-electron oxidation results in the formation of a ferrocenium cation and a dip angle decrease from 25.58 to 6.38. However, the synthetic procedure for ferrocenium borafluorene is not applicable for the ferrocenium borole. This difference is because the transformation of a neutral borole to a cationic borole greatly enhances the electron deficiency and Lewis acidity of the boron center, and leads to undesired coordination and/or decomposition of the resulting cationic molecule. Most revealingly, two well-separated reduction waves for 1 were identified in THF solution. As shown in Figure 1, 1 displays a quasi-reversible reduction event centered at E1/2 = 1.96 V indicating the formation of stable borole radical anion, [1]C . Note that in contrast to the well-studied borole dianions, the paramagnetic 5p-electron radical anion of