Analysis of Why Boron Avoids sp2 Hybridization and Classical Structures in the BnHn+2 Series

We performed global minimum searches for the B_nH_n+2 (n=2‐5) series and found that classical structures composed of 2c–2e B? H and B? B bonds become progressively less stable along the series. Relative energies increase from 2.9 kcal mol^?1 in B₂H₄ to 62.3 kcal mol^?1 in B₅H₇. We believe this occurs because boron atoms in the studied molecules are trying to avoid sp² hybridization and trigonal structure at the boron atoms, as in that case one 2p‐AO is empty, which is highly unfavorable. This affinity of boron to have some electron density on all 2p‐AOs and avoiding having one 2p‐AO empty is a main reason why classical structures are not the most stable configurations and why multicenter bonding is so important for the studied boron–hydride clusters as well as for pure boron clusters and boron compounds in general.     

Bonding in Diborane–Metal Complexes: A Quantum‐Chemical and Experimental Study of Complexes Featuring Early and Late Transition Metals

The coordination chemistry of the doubly base‐stabilised diborane(4), [HB(hpp)]₂ (hpp=1,3,4,6,7,8‐hexahydro‐2H‐pyrimido‐[1,2‐a]pyrimidinate), was extended by the synthesis of new late transition‐metal complexes containing Cu^I and Rh^I fragments. A detailed experimental study was conducted and quantum‐chemical calculations on the metal–ligand bonding interactions for [HB(hpp)]₂ complexes of Group 6, 9, 11 and 12 metals revealed the dominant B? H? M interactions in the case of early transition‐metal fragments, whereas the B? B? M bonding prevails in the case of the late d‐block compounds. These findings support the experimental results as reflected by the IR and NMR spectroscopic parameters of the investigated compounds. DFT calculations on [MeB(hpp)]₂ and model reactions between [B₂H₄ ? 2NMe₃] and [Rh(μ‐Cl)(C₂H₄)₂] showed that the bicyclic guanidinate allows in principle for an oxidative addition of the B? B bond. However, the formation of σ‐complexes is thermodynamically favoured. The results point to the selective B? H or B? B bond‐activation of diborane compounds by complexation, depending on the chosen transition‐metal fragment.     

B−B Bond Nucleophilicity in a Tetraaryl μ‐Hydridodiborane(4) Anion

The tetraaryl μ‐hydridodiborane(4) anion [ 2 H]^? possesses nucleophilic B?B and B?H bonds. Treatment of K[ 2 H] with the electrophilic 9‐H‐9‐borafluorene (HBFlu) furnishes the B₃ cluster K[ 3 ], with a triangular boron core linked through two BHB two‐electron, three‐center bonds and one electron‐precise B?B bond, reminiscent of the prominent [B₃H₈]^? anion. Upon heating or prolonged stirring at room temperature, K[ 3 ] rearranges to a slightly more stable isomer K[ 3 a ]. The reaction of M[ 2 H] (M⁺=Li⁺, K⁺) with MeI or Me₃SiCl leads to equimolar amounts of 9‐R‐9‐borafluorene and HBFlu (R=Me or Me₃Si). Thus, [ 2 H]^? behaves as a masked [:BFlu]^? nucleophile. The HBFlu by‐product was used in situ to establish a tandem substitution‐hydroboration reaction: a 1:1 mixture of M[ 2 H] and allyl bromide gave the 1,3‐propylene‐linked ditopic 9‐borafluorene 5 as sole product. M[ 2 H] also participates in unprecedented [4+1] cycloadditions with dienes to furnish dialkyl diaryl spiroborates, M[R₂BFlu].     

Reversible Dihydrogen Activation by Reduced Aryl Boranes as Main‐Group Ambiphiles

A new approach to main‐group H₂ activation combining concepts of transition‐metal and frustrated Lewis pair chemistry is reported. Ambiphilic, metal‐like reactivity toward H₂ can be conferred to 9,10‐dihydro‐9,10‐diboraanthracene (DBA) acceptors by the injection of two electrons. The resulting [DBA]^2? ions cleave the H?H bond with the formation of hydridoborates under moderate conditions (T=50–100 °C; p<1 atm). Depending on the boron‐bonded substituents R, the addition is either reversible (R=C≡CtBu) or irreversible (R=H). The reaction rate is strongly influenced by the nature and the coordination behavior of the countercation (Li⁺ slower than K⁺). Quantum‐chemical calculations support the experimental observations and suggest a concerted, homolytic addition of H₂ across both boron atoms. As proven by the successful conversion of Me₃SiCl into Me₃SiH, the system Li₂[DBA]/H₂ appears generally relevant for the hydrogenation of element–halide bonds.     

First‐Row Transition‐Metal–Diborane and –Borylene Complexes

A combined experimental and quantum chemical study of Group 7 borane, trimetallic triply bridged borylene and boride complexes has been undertaken. Treatment of [{Cp*CoCl}₂] (Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl) with LiBH₄ ? thf at ?78 °C, followed by room‐temperature reaction with three equivalents of [Mn₂(CO)₁₀] yielded a manganese hexahydridodiborate compound [{(OC)₄Mn}(η⁶‐B₂H₆){Mn(CO)₃}₂(μ‐H)] ( 1 ) and a triply bridged borylene complex [(μ₃‐BH)(Cp*Co)₂(μ‐CO)(μ‐H)₂MnH(CO)₃] ( 2 ). In a similar fashion, [Re₂(CO)₁₀] generated [(μ₃‐BH)(Cp*Co)₂(μ‐CO)(μ‐H)₂ReH(CO)₃] ( 3 ) and [(μ₃‐BH)(Cp*Co)₂(μ‐CO)₂(μ‐H)Co(CO)₃] ( 4 ) in modest yields. In contrast, [Ru₃(CO)₁₂] under similar reaction conditions yielded a heterometallic semi‐interstitial boride cluster [(Cp*Co)(μ‐H)₃Ru₃(CO)₉B] ( 5 ). The solid‐state X‐ray structure of compound 1 shows a significantly shorter boron–boron bond length. The detailed spectroscopic data of 1 and the unusual structural and bonding features have been described. All the complexes have been characterized by using ¹H, ¹¹B, ¹³C NMR spectroscopy, mass spectrometry, and X‐ray diffraction analysis. The DFT computations were used to shed light on the bonding and electronic structures of these new compounds. The study reveals a dominant B?H?Mn, a weak B?B?Mn interaction, and an enhanced B?B bonding in 1 .     

The Na⋅⋅⋅B Bond in NaBH3−: A Different Type of Bond

A newly introduced Na−B bond in NaBH₃⁻ has been a challenge for the chemical bonding community. Here, a series of MBH₃⁻ (M=Li, Na, K) species and NaB(CN)₃⁻ are studied within the context of quantum chemical topology approaches. The analyses suggest that M–B interaction cannot be classified as an ordinary covalent, dative, or even simple ionic interaction. The interactions are controlled by coulombic forces between the metals and the substituents on boron, for example, H or CN, more than the direct M–B interaction. On the other hand, while the characteristics of the (3, −1) critical points of the bonds are comparable to weak hydrogen bonds, not covalent bonds, the metal and boron share a substantial sum of electrons. To the best of the author's knowledge, the characteristics of these bonds are unprecedented among known molecules. Considering all paradoxical properties of these bonds, they are herein described as ionic-enforced covalent bonds.     

Metal‐Free Oxidative B−N Coupling of nido‐Carborane with N‐Heterocycles

A general method for the oxidative substitution of nido‐carborane (7,8‐C₂B₉H₁₂^?) with N‐heterocycles has been developed by using 2,3‐dichloro‐5,6‐dicyanobenzoquinone (DDQ) as an oxidant. This metal‐free B?N coupling strategy, in both inter‐ and intramolecular fashions, gave rise to a wide array of charge‐compensated, boron‐substituted nido‐carboranes in high yields (up to 97 %) with excellent functional‐group tolerance under mild reaction conditions. The reaction mechanism was investigated by density‐functional theory (DFT) calculations. A successive single‐electron transfer (SET), B?H hydrogen‐atom transfer (HAT), and nucleophilic attack pathway is proposed. This method provides a new approach to nitrogen‐containing carboranes with potential applications in medicine and materials.     

Observation of Transition‐Metal–Boron Triple Bonds in IrB2O− and ReB2O−

Multiple bonds between boron and transition metals are known in many borylene (:BR) complexes via metal d_π→BR back‐donation, despite the electron deficiency of boron. An electron‐precise metal–boron triple bond was first observed in BiB₂O^? [Bi≡B?B≡O]^? in which both boron atoms can be viewed as sp‐hybridized and the [B?BO]^? fragment is isoelectronic to a carbyne (CR). To search for the first electron‐precise transition‐metal‐boron triple‐bond species, we have produced IrB₂O^? and ReB₂O^? and investigated them by photoelectron spectroscopy and quantum‐chemical calculations. The results allow to elucidate the structures and bonding in the two clusters. We find IrB₂O^? has a closed‐shell bent structure (C_s, ¹A′) with BO^? coordinated to an Ir≡B unit, (^?OB)Ir≡B, whereas ReB₂O^? is linear (C_∞v, ³Σ^?) with an electron‐precise Re≡B triple bond, [Re≡B?B≡O]^?. The results suggest the intriguing possibility of synthesizing compounds with electron‐precise M≡B triple bonds analogous to classical carbyne systems.     

The chemical composition and bonding structure of B–C–N–H thin films deposited by reactive magnetron sputtering

The chemical composition and bonding structures of B–C–N–H films fabricated by medium frequency magnetron sputtering, with N₂+CH₄+Ar gas mixture sputtering the boron target, were investigated. XPS and FTIR spectrometric analyses show that the increase of CH₄ flow rate during deposition causes an increase of the C content in the films. The increase in the CH₄ flow rate promotes an increase in the B–C, C–N single and C?N double bonds which are the components of the hybridized B–C–N bonding structure. From the results of Raman spectroscopy analysis, it is seen that the intensity of the D band of the films' Raman spectrum decreases with increasing CH₄ flow rate, indicating a decrease of the sp²‐phase content or the sp² C cluster size. The decreases of I_D/I_G also reflect the formation of more boron‐ or nitrogen‐ bound sp³‐coordinated carbons in the films. Copyright © 2009 John Wiley & Sons, Ltd.     

Evidence for Extensive Single‐Electron‐Transfer Chemistry in Boryl Anions: Isolation and Reactivity of a Neutral Borole Radical

Despite the synthesis of a boryl anion by Yamashita et al. in 2006, compounds that show boron‐centered nucleophilicity are still rare and sought‐after synthetic goals. A number of such boryl anions have since been prepared, two of which were reported to react with methyl iodide in apparent nucleophilic substitution reactions. One of these, a borolyl anion based on the borole framework, has now been found to display single‐electron‐transfer (SET) reactivity in its reaction with triorganotetrel halides, which was confirmed by the isolation of the first neutral borole‐based radical. The radical was characterized by elemental analysis, single‐crystal X‐ray crystallography, and EPR spectroscopy, and has implications for the understanding of boron‐based nucleophilic behavior and the emergent role of boron radicals in synthesis. This radical reactivity was also exploited in the synthesis of compounds with rare B? Sn and B? Pb bonds, the latter of which was the first isolated and structurally characterized compound with a “noncluster” B? Pb bond.     

A Series of Isolable Silanoic Thio‐, Seleno‐, and Telluroesters (LSi(X)OR) with Donor‐Supported SiX Double Bonds (L=β‐Diketiminate; X=S,Se, Te)

The first silicon analogues of carbonic (carboxylic) esters, the silanoic thio‐, seleno‐, and tellurosilylesters 3 (Si?S), 4 (Si?Se), and 5 (Si?Te), were prepared and isolated in crystalline form in high yield. These thermally robust compounds are easily accessible by direct reaction of the stable siloxysilylene L(Si:)OSi(H)L′ 2 (L=HC(CMe)₂[N(aryl)₂], L′=CH[(C?CH₂)‐CMe][N(aryl)]₂; aryl=2,6‐iPr₂C₆H₃) with the respective elemental chalcogen. The novel compounds were fully characterized by methods including multinuclear NMR spectroscopy and single‐crystal X‐ray diffraction analysis. Owing to intramolecular N→Si donor–acceptor support of the Si?X moieties (X=S, Se, Te), these compounds have a classical valence‐bond N⁺–Si–X^? resonance betaine structure. At the same time, they also display a relatively strong nonclassical Si?X π‐bonding interaction between the chalcogen lone‐pair electrons (n_π donor orbitals) and two antibonding Si? N orbitals (σ*_π acceptor orbitals mainly located at silicon), which was shown by IR and UV/Vis spectroscopy. Accordingly, the Si?X bonds in the chalcogenoesters are 7.4 ( 3 ), 6.7 ( 4 ), and 6.9 % ( 5 ) shorter than the corresponding Si? X single bonds and, thus, only a little longer than those in electronically less disturbed Si?X systems (“heavier” ketones).     

Isolation of a Bis(oxazol‐2‐ylidene)–Phenylborylene Adduct and its Reactivity as a Boron‐Centered Nucleophile

An isolable phenylborylene species supported by two oxazol‐2‐ylidene ligands was synthesized and structurally characterized. Computational studies revealed the presence of lone‐pair electrons on the boron atom in this molecule; therefore, there are eight electrons around the three‐coordinate boron center. The nucleophilic property was confirmed by the reactions with trifluoromethanesulfonic acid and [(thf)Cr(CO)₅], which gave the corresponding conjugate acid and a chromium–borylene complex, respectively.     

Boron-Phosphorus Compounds and Multiple Bonding

Boron-phosphorus compounds have not been as thoroughly studied as their boron-nitrogen counterparts. Until recently many classes of B-P compounds that had been well established in B-N chemistry were either unknown or poorly characterized. This statement is particularly true for compounds involving possible multiple bonding between boron and phosphorus. For example, detailed structural information on simple monomeric phosphino-boranes, R₂BPR′₂, did not become available until 1986 even though the isoelectronic SiC double bonded species, the silenes, had already been reported. However, new work has shown that it is possible to prepare and characterize several novel types of boron-phosphorus compounds with varying degrees of multiple B—P bonding. These include not only monomeric phosphinoboranes but also phosphanediylborates (borylphosphides), three- and four-membered rings (diphosphadi-boretanes), boron phosphorus analogues of borazine, B-P skeletal analogues of allyl cations and anions, butadiene and cage compounds. Structural, spectroscopic (mainly NMR) and theoretical studies reveal some important differences between B-P and B-N compounds which in many cases can be traced to the presence of a high inversion barrier at phosphorus that reduces the π interaction. This usually causes compounds such as R₂BPR′₂ to associate through σ bonding between B and P. Supporting evidence for this view comes from species that involve phosphorus and nitrogen in competitive π bonding with a boron p orbital in which the dative interaction between B and N is dominant and the phosphorus center remains pyramidal. Recently published work has shown that steric and electronic factors can be used to favor π bonding and give an approximately planar system. Furthermore, theoretical studies reveal that p? p π overlap in a planar B-P system is of similar efficiency to its B-N analogue. Good examples are seen in the phosphanediyl borates, the boron-phosphorus analogues of borazine and the π-allyl cations, whose molecular configurations and B—P bond lengths support strong boron—phosphorus π bonding.     

Correlation between electron delocalization and structural planarization in small water rings

The discovery of the covalent‐like character of the hydrogen bonding (H‐bonding) system [Science 342 , 611(2013)] has promoted a renewal of our understanding of the electronic and geometric structures of water clusters. In this work, based on density functional theory calculations, we show that the preferential formation of a stable quasiplanar structure of (H₂O)_n(n = 3–6) is closely related to three kinds of delocalized molecular orbitals (MOs; denoted as MO‐I, II, and III) of water rings. These originate from the 2p lone pair electrons of oxygen (O), the 2p bond electrons of O and the 1s electrons of H and the 2s electrons of O and 1s electrons of H, respectively. To maximize the orbital overlaps of the three MOs, geometric planarization is needed. The contribution of the orbital interaction is more than 30% in all the water rings according to our energy decomposition analysis, highlighting the considerable covalent‐like characters of H‐bonds. © 2015 Wiley Periodicals, Inc.     

Bismuth–Boron Multiple Bonding in BiB2O− and Bi2B−

Despite its electron deficiency, boron is versatile in forming multiple bonds. Transition‐metal–boron double bonding is known, but boron–metal triple bonds have been elusive. Two bismuth boron cluster anions, BiB₂O⁻ and Bi₂B⁻, containing triple and double B−Bi bonds are presented. The BiB₂O⁻ and Bi₂B⁻ clusters are produced by laser vaporization of a mixed B/Bi target and characterized by photoelectron spectroscopy and ab initio calculations. Well‐resolved photoelectron spectra are obtained and interpreted with the help of ab initio calculations, which show that both species are linear. Chemical bonding analyses reveal that Bi forms triple and double bonds with boron in BiB₂O⁻ ([Bi≡B−B≡O]⁻) and Bi₂B⁻ ([Bi=B=Bi]⁻), respectively. The Bi−B double and triple bond strengths are calculated to be 3.21 and 4.70 eV, respectively. This is the first experimental observation of Bi−B double and triple bonds, opening the door to design main‐group metal–boron complexes with multiple bonding.     

C?H Functionalizations by Means of Direct Borane–Hydrocarbon Dehydrogenations and Dehydrocarbonations

The C? H functionalization of methane by means of direct C? H borations with BH₃ or MeBH₂ is compared computationally (using the B3LYP/6‐311⁺⁺G** method) to C? H lithiations with LiH or LiMe as well as to other analogue C–metal (Be, Na, Mg, Al) formations. For the borations only, this internal electrophilic substitution at carbon (S_Ei) relies more on the electrophilicity of boron than on the basicity of the internal base Y, that is, H or Me. Such direct borations of methane are more favored for dehydrogenations than for dehydrocarbonations. Due to decreased electrophilicity, substituents at boron disfavor such borations. Hence, the BH₂ group appears to be most efficient for C? H functionalizations by means of direct hydrocarbon borations.     

Cation–Cation Pairing by NCH⋅⋅⋅O Hydrogen Bonds

The pairing of ions of opposite charge is a fundamental principle in chemistry, and is widely applied in synthesis and catalysis. In contrast, cation–cation association remains an elusive concept, lacking in supporting experimental evidence. While studying the structure and properties of 4‐oxopiperidinium salts [OC₅H₈NH₂]X for a series of anions X^? of decreasing basicity, we observed a gradual self‐association of the cations, concluding in the formation of an isolated dicationic pair. In 4‐oxopiperidinium bis(trifluoromethylsulfonyl)amide, the cations are linked by N? H???O?C hydrogen bonds to form chains, flanked by hydrogen bonds to the anions. In the tetra(perfluoro‐tert‐butoxy)aluminate salt, the anions are fully separated from the cations, and the cations associate pairwise by N? C? H???O?C hydrogen bonds. The compounds represent the first genuine examples of self‐association of simple organic cations based merely on hydrogen bonding as evidenced by X‐ray structure analysis, and provide a paradigm for an extension of this class of compounds.     

Protonenresonanzuntersuchungen an Aminoboranen—II: Einflüsse von para-Phenylsubstituenten auf die Rotationsbarriere um die N?B-Bindung

The electronic influence of substituents on the free enthalpy of rotation around the N? B bond in aminoboranes was investigated in two series of compounds: (a) (CH₃)₂N?BCl (phenyl-p-X), containing the para-phenyl substituent at the boron atom, and (b) (p-X-phenyl)CH₃N?B(CH₃)₂, containing the para-phenyl substituent at the nitrogen atom of the N? B linkage (X = ? NR₂, ? OCH₃, ? C(CH₃)₃, ? Si(CH₃)₃, ? H, ? F, ? Cl, ? Br, ? I, ? CF₃ and ? NO₂). By comparing the rotational barriers in corresponding compounds of both series, a reverse effect of the substituents could be observed. Electron-withdrawing substituents in the para position of the phenyl ring increase the ΔG_c^≠ if the phenyl group is attached to the boron atom; on the other hand, a lower ΔG_c^≠ is observed if the phenyl ring is bonded to the nitrogen atom of the N? B system. Substitution of the phenyl ring with electron-donating substituents in the paraposition exerts the opposite effect. Within each series of compounds, the differences of ΔG_c^≠ values [δ(ΔG_c^≠) = ΔG_c^≠ (X) ? ΔG_c^≠ (X = H)] between substituted and unsubstituted compounds can be explained in terms of inductive and mesomeric effects of the ring substituents and can be correlated with the Hammett σ constant of each substituent. A comparison of the slopes of the plotted lines shows that the influence of the ring substituents is more pronounced in compounds with N-phenyl-p-X than in those with B-phenyl-p-X.     

Nucleophilic substitution in <Emphasis Type="Italic">closo</Emphasis>-decaborate [B<Subscript>10</Subscript>H<Subscript>10</Subscript>]<Superscript>2−</Superscript> in the presence of carbocations

Nucleophilic substitution at the exo-polyhedral boron atoms of claso-decaborate [B₁₀H₁₀]^2- in the presence of carbocations, which were generated in situ from various halocarbons (triphe-nylmethyl chloride, 1-bromoadamantan, n-butyl bromide), was studied. The reactions carried out in nucleophilic solvents (cyclic ethers and thioethers, N,N-disubstituted amides, and car-boxylic acids) and in the presence of halocarbons afforded mono-and disubstituted compounds with the exo-polyhedral B—O and B—S bonds, which contained a molecule of the solvent as substituent. The structures of the compounds synthesized were confirmed by the IR, mass, and ¹H, ¹¹B, and ¹³C NMR spectra.     

Tuning the Photoisomerization of a N^C‐Chelate Organoboron Compound with a Metal?Acetylide Unit

To examine the impact of metal moieties that have different triplet energies on the photoisomerization of B(ppy)Mes₂ compounds (ppy=2‐phenyl pyridine, Mes=mesityl), three metal‐functionalized B(ppy)Mes₂ compounds, Re‐B , Au‐B , and Pt‐B , have been synthesized and fully characterized. The metal moieties in these three compounds are Re(CO)₃(tert‐Bu₂bpy)(C?C), Au(PPh₃)(C?C), and trans‐Pt(PPh₃)₂(C?C)₂, respectively, which are connected to the ppy chelate through the alkyne linker. Our investigation has established that the Re^I unit completely quenches the photoisomerization of the boron unit because of a low‐lying intraligand charge transfer/MLCT triplet state. The Au^I unit, albeit with a triplet energy that is much higher than that of B(ppy)Mes₂, upon conjugation with the ppy chelate unit, substantially increases the contribution of the π→π* transition, localized on the conjugated chelate backbone in the lowest triplet state, thereby leading to a decrease in the photoisomerization quantum efficiency (QE) of the boron chromophore when excited at 365 nm. At higher excitation energies, the photoisomerization QE of Au‐B is comparable to that of the silyl–alkyne‐functionalized B(ppy)Mes₂ ( TIPS‐B ), which was attributable to a triplet‐state‐sensitization effect by the Au^I unit. The Pt^II unit links two B(ppy)Mes₂ together in Pt‐B , thereby extending the π‐conjugation through both chelate backbones and leading to a very low QE of the photoisomerization. In addition, only one boron unit in Pt‐B undergoes photoisomerization. The isomerization of the second boron unit is quenched by an intramolecular energy transfer of the excitation energy to the low‐energy absorption band of the isomerized boron unit. TD‐DFT computations and spectroscopic studies of the three metal‐containing boron compounds confirm that the photoisomerization of the B(ppy)Mes₂ chromophore proceeds through a triplet photoactive state and that metal units with suitable triplet energies can be used to tune this system.