A rigid redox-active-ligand-supported bis(germylene) as a two-centre six-electron donor.

A naphthyridine diimine (NDI) supported bis(germylene) NDI-Ge2 containing two dicoordinate, coplanar Ge(II) atoms has been synthesised. Computational investigations on NDI-Ge2 indicated the two Ge(II) atoms are nearly independent. The EDA-NOCV analysis of the [NDI-Ge2][Fe2(CO)6] complex revealed the six-electron donor behavior of NDI-Ge2, the first example for group-14-element-based bidentate ligands.

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry.

A one-pot synthesis of a CAAC-stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two-electron reduction steps from an adduct of CAAC and B2 Br4 (SMe2 )2 . Theoretical studies revealed that this diborene has a considerably smaller HOMO-LUMO gap than those of reported NHC- and phosphine-supported diborenes. Complexation of the diborene with [AuCl(PCy3 )] afforded two diborene-AuI π complexes, while reaction with DurBH2 , P4 and a terminal acetylene led to the cleavage of B-H, P-P, and C-C π bonds, respectively. Thermal rearrangement of the diborene gave an electron-rich cyclic alkylideneborane, which readily coordinated to AgI via its B=C double bond.

Controlled Synthesis of Oligomers Containing Main-Chain B(sp2 )-B(sp2 ) Bonds.

A number of novel alkynyl-functionalized diarylbis(dimethylamino)diboranes(4) are prepared by salt metathesis, and the appended alkynyl groups are subjected to hydroboration. Their reactions with monohydroboranes lead to discrete boryl-appended diborane(4) species, while dihydroboranes induce their catenation to oligomeric species, the first known examples of well-characterized macromolecular species with B-B bonds. The oligomeric species were found to comprise up to ten repeat units and are soluble in common organic solvents. Some of the oligomeric species have good air stability and all were characterized by NMR and vibrational spectroscopy and size-exclusion chromatography techniques.

cAAC-Stabilized 9,10-diboraanthracenes-Acenes with Open-Shell Singlet Biradical Ground States.

Narrow HOMO-LUMO gaps and high charge-carrier mobilities make larger acenes potentially high-efficient materials for organic electronic applications. The performance of such molecules was shown to significantly increase with increasing number of fused benzene rings. Bulk quantities, however, can only be obtained reliably for acenes up to heptacene. Theoretically, (oligo)acenes and (poly)acenes are predicted to have open-shell singlet biradical and polyradical ground states, respectively, for which experimental evidence is still scarce. We have now been able to dramatically lower the HOMO-LUMO gap of acenes without the necessity of unfavorable elongation of their conjugated π system, by incorporating two boron atoms into the anthracene skeleton. Stabilizing the boron centers with cyclic (alkyl)(amino)carbenes gives neutral 9,10-diboraanthracenes, which are shown to feature disjointed, open-shell singlet biradical ground states.

Synthesis of fused B,N-heterocycles by alkyne cleavage, NHC ring-expansion and C-H activation at a diboryne.

The addition of alkynes to a saturated N-heterocyclic carbene (NHC)-supported diboryne results in spontaneous cycloaddition, with complete B[triple bond, length as m-dash]B and C[triple bond, length as m-dash]C triple bond cleavage, NHC ring-expansion and activation of a variety of C-H bonds, leading to the formation of complex mixtures of fused B,N-heterocycles.

Stable Lewis Base Adducts of Tetrahalodiboranes: Synthetic Methods and Structural Diversity.

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 B2 X4 precursors (X=Cl, Br, I) or by ligand exchange at stable B2 X4 (SMe2 )2 precursors (X=Cl, Br) with labile dimethylsulfide ligands. The isolated compounds have been fully characterized using NMR spectroscopy, elemental analysis, and, for 20 of these compounds, single-crystal X-ray diffraction, revealing an unexpected variation in the bonding motifs. In addition to the classical B2 X4 L2 diborane(4) bis-adducts, certain more sterically demanding carbene ligands induce a halide displacement which led to the first halide-bridged monocationic diboron species, [B2 X3 L2 ]A (A=BCl4 , Br, I). Furthermore, low-temperature 1:1 reactions of B2 Cl4 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 spectra 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.

Isolation of diborenes and their 90°-twisted diradical congeners.

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.

Scope of the Thermal Ring-Expansion Reaction of Boroles with Organoazides.

Electronic and steric factors have been investigated in the thermal ring expansion of boroles with organic azides, a reaction that provides access to highly arylated 1,2-azaborinines, BN analogues of benzene. Reactions of a variety of boroles and organic azides demonstrate that the synthetic method is quite general in furnishing 1,2-azaborinines, but the respective reaction rates reveal a strong dependence on the substituents on the two reactants. The products have been characterized by UV/Vis, electrochemical, NMR, and X-ray diffraction methods, clarifying their constitutions and highlighting substituent effects on the electronic structure of the 1,2-azaborinines. Furthermore, analysis of several possible mechanistic pathways for 1,2-azaborinine formation, as studied by DFT, revealed that a two-step mechanism involving azide-borole adduct formation and nitrene insertion is favored.

Uncatalyzed Hydrogenation of First-Row Main Group Multiple Bonds.

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 H2 addition mechanism proceeding via a bridging hydride.

Cyclic (Alkyl)(Amino)Carbene Complexes of Rhodium and Nickel and Their Steric and Electronic Parameters.

N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) are of great interest, as their electronic and steric properties provide a unique class of ligands and organocatalysts. Herein, substitution reactions involving novel carbonyl complexes of rhodium and nickel were studied to provide a deeper understanding of the fundamental electronic factors characterizing CAAC(methyl) , which were compared with the large array of data available for NHC and sterically more demanding CAAC ligands.

Neutral zero-valent s-block complexes with strong multiple bonding.

The metals of the s block of the periodic table are well known to be exceptional electron donors, and the vast majority of their molecular complexes therefore contain these metals in their fully oxidized form. Low-valent main-group compounds have recently become desirable synthetic targets owing to their interesting reactivities, sometimes on a par with those of transition-metal complexes. In this work, we used stabilizing cyclic (alkyl)(amino)carbene ligands to isolate and characterize the first neutral compounds that contain a zero-valent s-block metal, beryllium. These brightly coloured complexes display very short beryllium-carbon bond lengths and linear beryllium coordination geometries, indicative of strong multiple Be-C bonding. Structural, spectroscopic and theoretical results show that the complexes adopt a closed-shell singlet configuration with a Be(0) metal centre. The surprising stability of the molecule can be ascribed to an unusually strong three-centre two-electron π bond across the C-Be-C unit.

The Synthesis of B2(SIDip)2 and its Reactivity Between the Diboracumulenic and Diborynic Extremes.

A new compound with the formula L-B2-L wherein the stabilizing ligand (L) is 1,3-bis[diisopropylphenyl]-4,5-dihydroimidazol-2-ylidene (SIDip) has been synthesized, isolated, and characterized. The π-acidity of the SIDip ligand, intermediate between the relatively non-acidic IDip (1,3-bis[diisopropylphenyl]imidazol-2-ylidene) ligand and the much more highly acidic CAAC (1-[2,6-diisopropylphenyl]-3,3,5,5-tetramethylpyrrolidin-2-ylidene) ligand, gives rise to a compound with spectroscopic, electrochemical, and structural properties between those of L-B2-L compounds stabilized by CAAC and IDip. Reactions of all three L-B2-L compounds with CO demonstrate the differences caused by their respective ligands, as the π-acidities of the CAAC and SIDip carbenes enabled the isolation of bis(boraketene) compounds (L(OC)B-B(CO)L), which could not be isolated from reactions with B2(IDip)2. However, only B2(IDip)2 and B2(SIDip)2 could be converted into bicyclic bis(boralactone) compounds.

Experimental assessment of the strengths of B-B triple bonds.

Diborynes, molecules containing homoatomic boron-boron triple bonds, have been investigated by Raman spectroscopy in order to determine the stretching frequencies of their central B≡B units as an experimental measure of homoatomic bond strengths. The observed frequencies between 1600 and 1750 cm(-1) were assigned on the basis of DFT modeling and the characteristic pattern produced by the isotopic distribution of boron. This frequency completes the series of known stretches of homoatomic triple bonds, fitting into the trend established by the long-known stretching frequencies of C≡C and N≡N triple bonds in alkynes and dinitrogen, respectively. A quantitative analysis was carried out using the concept of relaxed force constants. The results support the classification of the diboryne as a true triple bond and speak to the similarities of molecules constructed from first-row elements of the p block. Also reported are the relaxed force constants of a recently reported diborabutatriene, which again fit into the trend established by the vibrational spectroscopy of organic cumulenes. As part of these studies, a new diboryne with decreased steric bulk was synthesized, and a computational study of the rotation of the stabilizing ligands indicated alkyne-like electronic isolation of the central B2 unit.

Metal-free binding and coupling of carbon monoxide at a boron-boron triple bond.

Many metal-containing compounds, and some metal-free compounds, will bind carbon monoxide. However, only a handful of metal-containing compounds have been shown to induce the coupling of two or more CO molecules, potentially a method for the use of CO as a one-carbon-atom building block for the synthesis of organic molecules. In this work, CO was added to a boron-boron triple bond at room temperature and atmospheric pressure, resulting in a compound into which four equivalents of CO are incorporated: a flat, bicyclic, bis(boralactone). By the controlled addition of one CO to the diboryne compound, an intermediate in the CO coupling reaction was isolated and structurally characterized. Electrochemical measurements confirm the strongly reducing nature of the diboryne compound.

Ambient-temperature isolation of a compound with a boron-boron triple bond.

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.

Synthesis and structure of distanna and tristanna ansa half-sandwich complexes of ruthenium and nickel.

The synthesis and structural characterization of the first tin-bridged ansa half-sandwich complexes via a two-step protocol from Na[η(5)-C(5)H(5)Ru(CO)(2)] and in situ generated Na[η(5)-C(5)H(5)Ni(CO)] are presented. Both compounds are characterized by multinuclear NMR spectroscopy and single-crystal diffraction.

Synthesis, structure and reactivity of disila- and distanna ansa half-sandwich complexes of molybdenum and tungsten.

We present the synthesis and characterization of disila- and distanna ansa half-sandwich complexes of Group 6 transition metals. These compounds exhibit high ring strain within the ansa bridge, which is the key factor for the insertion of elemental chalcogens.

[2]borametallocenophanes of group 4 metals: synthesis and structure.

We report a series of [2]borametallocenophanes of Ti, Zr, and Hf with various ligand systems. The ligands have been synthesized in high yields starting from 1,2-dibromo-1,2-bis(dimethylamino)diborane(4) upon reaction with Na[C5H5] and Li[C13H9], respectively. All compounds were fully characterized by multinuclear NMR spectroscopy and, for selected examples, by X-ray analysis.