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The transfer hydrogenation of NHC-supported diborenes with dimethylamine borane proceeds with high selectivity for the trans-1,2-dihydrodiboranes(6). DFT calculations suggest a stepwise proton-first-hydride-second reaction mechanism via an intermediate μ-hydrodiboronium dimethylaminoborate ion pair.
Treatment of an anionic dimanganaborylene complex ([{Cp(CO)\(_2\)Mn}\(_2\)B]\(^-\)) with coinage metal cations stabilized by a very weakly coordinating Lewis base (SMe\(_2\)) led to the coordination of the incoming metal and subsequent displacement of dimethylsulfide in the formation of hexametalladiborides featuring planar four-membered M\(_2\)B\(_2\) cores (M = Cu, Au) comparable to transition metal clusters constructed around four-membered rings composed solely of coinage metals. The analogies between compounds consisting of B\(_2\)M\(_2\) units and M\(_4\) (M = Cu, Au) units speak to the often overlooked metalloid nature of boron. Treatment of one of these compounds (M = Cu) with a Lewis-basic metal fragment (Pt(PCy\(_3\))\(_2\)) led to the formation of a tetrametallaboride featuring two manganese, one copper and one platinum atom, all bound to boron in a geometry not yet seen for this kind of compound. Computational examination suggests that this geometry is the result of d\(^{10}\)-d\(^{10}\) dispersion interactions between the copper and platinum fragments.
The reductive coupling of an NHC-stabilized aryldibromoborane yields a mixture of trans- and cis-diborenes in which the aryl groups are coplanar with the diborene core. Under dilute reduction conditions two diastereomers of a borirane-borane intermediate are isolated, which upon further reduction give rise to the aforementioned diborene mixture. DFT calculations suggest a mechanism proceeding via nucleophilic attack of a dicoordinate borylene intermediate on the aryl ring and subsequent intramolecular B-B bond formation.
Herein, we describe the selective formation of a stable neutral spiroborate radical by one-electron oxidation of the corresponding tetraorganoborate salt Li[B(C\(_4\)Ph\(_4\))\(_2\)], formally containing a tetrahedral borate centre and a s-cis-butadiene radical cation as the spin-bearing site. Spectroscopic and computational methods have been used to determine the spin distribution and the chromism observed in the solid state.
Despite the prevalence of stable π-complexes of most d\(^{10}\) metals, such as Cu(I) and Ni(0), with ethylene and other olefins, complexation of d\(^{10}\) Zn(II) to simple olefins is too weak to form isolable complexes due to the metal ion's limited capacity for π-backdonation. By employing more strongly donating π- ligands, namely neutral diborenes with a high-lying π(B=B) or- bital, monomeric 16-electron M(II)-diborene (M = Zn, Cd) π- complexes were synthesized in good yields. Metal–B2 π- interactions in both the solid and solution state were confirmed by single-crystal X-ray analyses and their solution NMR and UV-vis absorption spectroscopy, respectively. The M(II) centers adopt a trigonal planar geometry and interact almost symmetrically with both boron atoms. The MB2 planes significantly twist out of the MX\(_2\) planes about the M-centroid(B–B) vector, with angles rang- ing from 47.0° to 85.5°, depending on the steric interactions be- tween the diborene ligand and the MX\(_2\) fragment.
A series of 22 new bis(phosphine), bis(carbene) and bis(isonitrile) tetrahalodiborane adducts has been synthesized, either by direct adduct formation with highly sensitive B2X4 precursors (X = Cl, Br, I) or by ligand exchange at stable B2X4(SMe2)2 precursors (X = Cl, Br) with labile dimethylsulfide ligands. The isolated compounds have been fully characterized using NMR spectroscopic, (C,H,N)- elemental and, for 20 of these compounds, X-ray crystallographic analysis, revealing an unexpected variation in the bonding motifs. Besides the classical B2X4L2 diborane(6) adducts, some of the more sterically demanding carbene ligands induce a halide displacement leading to the first halide-bridged monocationic diboron species, [B2X3L2]A (A = BCl4, Br, I). Furthermore, low-temperature 1:1 reactions of B2Cl4 with sterically demanding N-heterocyclic carbenes led to the formation of kinetically unstable mono-adducts, one of which was structurally characterized. A comparison of the NMR and structural data of new and literature-known bis-adducts shows several trends pertaining to the nature of the halides and the stereoelectronic properties of the Lewis bases employed.
no abstract available
In the molecular structure of the title compound, C34H58B2N2, each B atom of the diborane(4) is connected to one dimethylamino group and one Tip ligand (Tip = 2,4,6-triisopropylphenyl). These findings indicate that the increased steric demand of the Tip groups exerts influence solely on the B—B separation but not on the overall geometry of the title compound.
Bis(μ-diisopropyl-phosphanido-\(κ^2\)P:P)bis-[hydrido(triisopropyl-phosphane-κP)platinum(II)]
(2012)
In the centrosymmetric molecular structure of the title compound \([Pt_2(C_6H_{14}P)_2H_2)(C_9H_{21}P)_2]\), each \(Pt^{II}\) atom is bound on one side to a phosphane ligand \((PiPr_3)\) and a hydrido ligand. On the other side, it is bound to two phosphanide ligands \((μ-PiPr_2)\), which engage a bridging position between the two \(Pt^{II}\) atoms, forming a distorted square-planar structure motif. The PtPt distance is 3.6755(2)Å. A comparable molecular structure was observed for bis-(μ-di-tert-butyl-phosphanido)bis-[hydrido(triethyl-phosphane)platinum(II)] [Itazaki et al. (2004 ). Organometallics, 23, 1610-1621].
In the molecular structure of the dinuclear title compound \([η^5-(C_5(CH_3)_5)(CO)Fe{(μ-BCl)(μ-CO)}PtCl(P(C_6H_{11})_3)]·C_6H_6\), the two metal atoms, iron(II) and platinum(II), are bridged by one carbonyl (μ-CO) and one chloridoborylene ligand (μ-BCl). The \(Pt^{II}\) atom is additionally bound to a chloride ligand situated trans to the bridging borylene, and a tricyclohexylphosphane ligand \((PCy_3)\) trans to the carbonyl ligand, forming a distorted square-planar structural motif at the \(Pt^{II}\) atom. The \(Fe_{II}\) atom is bound to a pentamethylcyclopentadienyl ligand \([η^5-C_5(CH_3)_5]\) and one carbonyl ligand (CO), forming a piano-stool structure. Additionally, one benzene solvent molecule is incorporated into the crystal structure, positioned staggered relative to the pentamethylcyclopentadienyl ligand at the \(Fe^{II}\) atom, with a centroid–centroid separation of 3.630 (2) Å.
Five compounds containing boron–boron multiple bonds are shown to undergo hydrophosphination reactions with diphenylphosphine in the absence of a catalyst. With diborenes, the products obtained are highly dependent on the substitution pattern at the boron atoms, with both 1,1- and 1,2- hydrophosphinations observed. With a symmetrical diboryne, 1,2-hydrophosphination yields a hydro(phosphino)diborene. The different mechanistic pathways for the hydrophosphination of diborenes are rationalised with the aid of density functional theory calculations.
Room temperature hydrogenation of an SIDep-stabilized diboryne (SIDep = 1,3-bis(diethylphenyl)-4,5-dihydroimidazol-2-ylidene) and a CAAC-supported diboracumulene (CAAC = 1-(2,6- diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) provided the first selective route to the corresponding 1,2-dihydrodiborenes. DFT calculations showed an overall exothermic (ΔG = 19.4 kcal mol\(^{-1}\) two-step asynchronous H\(_2\) addition mechanism proceeding via a bridging hydride.
Among the numerous routes organic chemists have developed to synthesize benzene derivatives and heteroaro- matic compounds, transition-metal-catalyzed cycloaddition reactions are the most elegant. In contrast, cycloaddition reactions of heavier alkene and alkyne analogues, though limited in scope, proceed uncatalyzed. In this work we present the first spontaneous cycloaddition reactions of lighter alkene and alkyne analogues. Selective addition of unactivated alkynes to boron–boron multiple bonds under ambient con- ditions yielded diborocarbon equivalents of simple aromatic hydrocarbons, including the first neutral 6 π-aromatic dibora- benzene compound, a 2 π-aromatic triplet biradical 1,3-dibor- ete, and a phosphine-stabilized 2 π-homoaromatic 1,3-dihydro- 1,3-diborete. DFT calculations suggest that all three com- pounds are aromatic and show frontier molecular orbitals matching those of the related aromatic hydrocarbons, C\(_6\)H\(_6\) and C\(_4\)H\(_4\)\(^{2+}\), and homoaromatic C\(_4\)H\(_5\)\(^+\).
Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens
(2016)
The reactions of a diborene with elemental selenium or tellurium are shown to afford a diboraselenirane or diboratellurirane, respectively. These reactions are reminiscent of the sequestration of subvalent oxygen and nitrogen in the formation of oxiranes and aziridines; however, such reactivity is not known between alkenes and the heavy chalcogens. Although carbon is too electronegative to affect the reduction of elements with lower relative electronegativity, the highly reducing nature of the B B double bond enables reactions with Se0 and Te0. The capacity of multiple bonds between boron atoms to donate electron density is highlighted in reactions where diborynes behave as nucleophiles, attacking one of the two Te atoms of diaryltellurides, forming salts consisting of diboratellurenium cations and aryltelluride anions.
The title compound, [Zr(3)(C(14)H(20)Si(2))(3)O(3)], consists of three disila-bridged zirconocene units, which are connected via an oxide ligand, forming a nearly planar six-membered ring with a maximum displacement of 0.0191 (8) A. The compound was isolated as a by-product from a mixture of [(C(5)H(4)SiMe(2))(2)ZrCl(2)] and Li[AlH(4)] in Et(2)O.
Whereas the reduction of N-heterocyclic carbene (NHC)-stabilised cymantrenyldibromoboranes, (NHC)BBr\(_2\)Cym, in benzene results in formation of the corresponding diborenes (NHC)\(_2\)B\(_2\)Cym\(_2\), a change of solvent to THF yields a borylene of the form (NHC)\(_2\)BCym, stabilised through its boratafulvene resonance form.
2,2′-Bipyridyl is shown to spontaneously abstract a borylene fragment (R–B:) from various hypovalent boron compounds. This process is a redox reaction in which the bipyridine is reduced and becomes a dianionic substituent bound to boron through its two nitrogen atoms. Various transition metal–borylene complexes and diboranes, as a well as a diborene, take part in this reaction. In the latter case, our results show an intriguing example of the homolytic cleavage of a B═B double bond.
N‐heterocyclic olefins (NHOs), relatives of N‐heterocyclic carbenes (NHCs), exhibit high nucleophilicity and soft Lewis basic character. To investigate their π‐electron donating ability, NHOs were attached to triarylborane π‐acceptors (A) giving donor (D)–π–A compounds 1–3. In addition, an enamine π‐donor analogue (4) was synthesized for comparison. UV–visible absorption studies show a larger red shift for the NHO‐containing boranes than for the enamine analogue, a relative of cyclic (alkyl)(amino) carbenes (CAACs). Solvent‐dependent emission studies indicate that 1–4 have moderate intramolecular charge‐transfer (ICT) behavior. Electrochemical investigations reveal that the NHO‐containing boranes have extremely low reversible oxidation potentials (e.g., for 3, \(E^{ox}_{1/2}\) =−0.40 V vs. ferrocene/ferrocenium, Fc/Fc\(^+\), in THF). Time‐dependent (TD) DFT calculations show that the HOMOs of 1–3 are much more destabilized than that of the enamine‐containing 4, which confirms the stronger donating ability of NHOs.
Molecules containing multiple bonds between atoms—most often in the form of olefins—are ubiquitous in nature, commerce, and science, and as such have a huge impact on everyday life. Given their prominence, over the last few decades, frequent attempts have been made to perturb the structure and reactivity of multiply-bound species through bending and twisting. However, only modest success has been achieved in the quest to completely twist double bonds in order to homolytically cleave the associated π bond. Here, we present the isolation of double-bond-containing species based on boron, as well as their fully twisted diradical congeners, by the incorporation of attached groups with different electronic properties. The compounds comprise a structurally authenticated set of diamagnetic multiply-bound and diradical singly-bound congeners of the same class of compound.