Rian D. Dewhurst

Electron-precise coordination modes of boron-centered ligands.

3. Boryl Ligands 3933 3.

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.

Single, Double, Triple Bonds and Chains: The Formation of Electron-Precise BB Bonds

The construction of boron-boron bonds, despite the intense synthetic interest in diboranes and the high B-B bond enthalpy, is still difficult, uncontrollable, and unpredictable. Methods for the construction of B-B multiple bonds are rarer still. These problems have witnessed some progress in recent years; this Minireview attempts to provide a background to the history of B-B bond synthesis and summarize the recent results in the area.

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.

Bond-strengthening π backdonation in a transition-metal π -diborene complex

Transition-metal catalysis is founded on the principle that electron donation from a metal to a ligand is accepted by an antibonding orbital of the ligand, thereby weakening one of the bonds in the ligand. Without this, the initial step of bond activation in many catalytic processes would simply not occur. This concept is enshrined in the well-accepted Dewar-Chatt-Duncanson model of transition-metal bonding. We present herein experimental and computational evidence for the first true violation of the Dewar-Chatt-Duncanson bonding model, found in a π-diborene complex in which an electron-rich group 10 metal donates electrons into an empty bonding π orbital on the ligand, and thereby strengthens the bond. The complex is also the first transition-metal complex to contain a bound diborene, a species not isolated before, either in its free form or bound to a metal.

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.

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.

Direct Hydroboration of BB Bonds: A Mild Strategy for the Proliferation of BB Bonds

Synthetic access to electron-precise boron chains is hampered by the preferential formation of nonclassical structures. The few existing strategies for this involve either strongly reducing reagents or transition-metal catalysts, both with distinct disadvantages. The synthesis of new furyl- and thienyl-substituted diborenes is presented, along with their direct hydroboration with catecholborane (CatBH) to form a new electron-precise B-B bond and a B3 chain. The reaction is diastereoselective and proceeds under mild conditions without the use of strong reducing agents or transition-metal catalysts commonly used in B-B coupling reactions.

A Trimetallic Gold Boride Complex with a Fluxional Gold–Boron Bond

Stable compounds featuring gold–boron bonds are rare, and until 2006, were limited to those containing hypercoordinate boron ligands, for example, polymetallic monoboron clusters and complexes in which a {LAu} fragment formally bridges an apical boron atom of a carborane cluster and another metal atom. Linking low-coordinate boron to gold has proved a challenging task, but this has recently been shown to reflect little more than a synthetic roadblock. Traditional methods for constructing transition-metal–boron bonds (based on B/M polarity) were found to be unsuitable: Au appears to lack sufficient reducing strength or nucleophilicity to perturb boron–halide bonds or to donate to an nonbridging Lewis acidic borane. Synthetic strategies based on the opposite polarity (B /M) were unthinkable until very recently, because of the absence of boron-based nucleophiles. In the last three years, success has been found with both of these strategies, and two new boron–gold coordination modes have been discovered, namely the borane gold complexes (Au!B), and the boryl gold complexes (Au !B), by the groups of Bourissou, and Yamashita and Nozaki, respectively. Unlike the earlier complexes, these two new bonding patterns are considered to be classical two-center-two-electron interactions, adding to their novelty. The borane gold complexes feature one or more tethered phosphine groups binding orthogonally to the boron–gold axis (B-Au-L: 79–848, L = chelating phosphino group), while in the boryl gold complexes, the B-Au-L (L = phosphine, Nheterocyclic carbene) axis is effectively linear. 5] There is a correspondingly large difference between the Au B distances of borane gold complexes (2.32–2.66 ) and those of boryl gold complexes (2.08–2.09 ). These structural patterns correlate well with the generally accepted understanding of the boryl ligand as a pure s-donor, and the borane ligand as a pure s-acceptor. Herein we experimentally and theoretically examine the structure of an unusual dimanganese boryl gold complex with a bonding situation that is not described correctly by either of the abovementioned patterns. The anionic complex [Li(dme)3] [{(h C5H4Me)(OC)2Mn}2B] (1, dme = 1,2-dimethoxyethane; Scheme 1), and its nucleophilic reactivity with methyl