Tetrakis(N-heterocyclic Carbene)-Diboron(0): Double Single-Electron-Transfer Reactivity

Stabilization of [(N5)2BX]2- and [(N5)2B2X2]2- (X = H, F, Cl, Br) by Conjugation and Hyperconjugation Effects

The isolation of nucleophilic boron bases has led to a paradigm shift in boron chemistry. Previous studies of the bis(carbene) borylene complexes revealed that these compounds possess strong donor abilities, and their reaction inertness is due to the large steric hindrance between boron reagents and reactant. In the present study, we have theoretically studied the [(N5)2BX]2- and [(N5)2B2X2]2- compounds (X = H, F, Cl, Br). Their electronic structures and properties are discussed by using the NBO, LOL, and ELF methods. We found that both π-conjugation and hyperconjugation effects can effectively stabilize the substituted nucleophilic anionic boron compounds [(N5)2BX]2- and [(N5)2B2X2]2-. Substituents, especially X = H, stabilize the boron center through highly delocalized π-bonding, involving the formally "empty" in-plane p orbitals of the boron atom. While the halogen substituents have high electron withdrawal ability, leading to systems being less stable, we suggest the borinium anions [(N5)2BH]2- and [(N5)2B2H2]2- as possible synthetic targets of novel environmentally friendly catalysts.

Ambient Temperature Isolation of a Monatomic Boron(0) Complex

The first bottleable example of a neutral Group 13 atom bound only by neutral donor ligands (L) has been fully characterized by spectroscopic methods and its structure determined by a single-crystal X-ray diffraction study. A two-coordinate paramagnetic L2B0 complex can readily be accessed through a facile reduction reaction and is stabilized by π-accepting cyclic (alkyl)(amino)carbene (CAAC) ligands. Further reduction of (CAAC)2B leads to the isolation of a stable diamagnetic boride anion. In turn, oxidation leads to the putative formation of a transient two-coordinate cationic borylene, which has been trapped to form a stable boron(I) complex. Density functional theory calculations support the formulation of (CAAC)2B as a boron(0) complex stabilized by strong multiple bonding.

Reductive coupling of azonaphthalenes for the synthesis of BINAMs via a diboron-enabled [5,5]-sigmatropic rearrangement

The [5,5]-sigmatropic rearrangement is a less-studied reaction and may be strategically utilized to devise unique synthetic processes. Herein, we document a diboron-enabled [5,5]-sigmatropic rearrangement for practical synthesis of BINAM derivatives. Mechanistically, a concerted activation of azonaphthalenes by diboron creates a unique ten-membered transition state, which subsequently triggers a [5,5]-sigmatropic rearrangement. The reaction occurs under mild conditions, and offers operational simplicity, remarkable chemo- and regioselectivities, and good scalability (>10 grams).

Crystalline Silylene-Stabilized Diboryne and Siladiborirene

The exploration of main group compounds with multiple bonds has significantly enhanced our understanding of chemical bonding and expanded transition-metal-free bond activation and catalysis. Diborynes, characterized by a boron-boron triple bond (B≡B), represent a particularly challenging area due to boron's limited valence electrons. Here, we report the synthesis and characterization of a silylene-stabilized diboryne (7), expanding the frontier of diboryne stabilization. X-ray diffraction analysis and density functional theory calculations confirm the presence of a B≡B bond in the diboryne. Remarkably, this diboryne undergoes [1 + 2] cycloaddition reactions with dichlorosilylene and terphenyl isocyanide, producing unprecedented siladiborirene (8) and diborirenimine (9), respectively. X-ray diffraction studies show that 8 and 9 bear a three-membered boron-containing heterocycle with a B═B bond, and computational studies reveal their distinct aromatic character.

A Digermanium(III) 1,2-Dication Stabilized by Amidinate and cAAC-Phosphinidenide Ligands

A digermanium(III) 1,2-dication comprises two cationic centers located at two interconnected Ge atoms. The strong Coulombic repulsion between two positively charged germanium cations hinders their bond formation. Balancing these two oppositions was achieved by using amidinate and cyclic (alkyl)amino carbene (cAAC)-phosphinidenide ligands, where an amidinato cAAC-phosphinidenidogermylene complex, [LGeP(cAACMe)] (2, where L = PhC(NtBu)2, cAACMe = :C{C(Me)2CH2C(Me)2NAr}, and Ar = 2,6-iPr2C6H3), underwent one-electron oxidation with a bis(phosphinidene) radical cation, [(cAACMe)P]2•+, to form a digermanium(III) 1,2-dication, [LGeP(cAACMe)]22+, in compound 4.

Carbon dioxide capture and functionalization by bis(N-heterocyclic carbene)-borylene complexes

Derivatives of free monocoordinated borylenes have attracted considerable interest due to their ability to exhibit transition-metal-like reactivity, in particular small molecules capture. However, such complexes are rare as the formation is either endergonic, or the resulting adduct is a transient intermediate that is prone to reaction. Here, we present the synthesis of two bis(N-heterocyclic carbene)-borylene complexes capable of capturing and functionalizing carbon dioxide. The capture and subsequent functionalization of CO2 by the bis(NHC)-disilylamidoborylene 1 is demonstrated by the formation of the bis(NHC)-isocyanatoborylene-carbon dioxide complex 3. Reversible capture of CO2 is observed using the bis(NHC)-mesitylborylene 2, and the persistent bis(NHC)-mesitylborylene-carbon dioxide adduct 4 can be stabilized by hydrogen bonding with boric acid. The reactions of 4 with ammonia-borane and aniline demonstrate that the captured CO2 can be further functionalized.

Single electron reduction of NHC-CO2-borane compounds

The carbon dioxide radical anion [CO2˙-] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO2 to generate [CO2˙-] occurs at very negative reduction potentials, which is often a limiting factor for applications. Here, we show that NHC-CO2-BR3 species - generated from the Frustrated Lewis Pair (FLP)-type activation of CO2 by N-heterocyclic carbenes (NHCs) and boranes (BR3) - undergo single electron reduction at a less negative potential than free CO2. A net gain of more than one volt was notably measured with a CAAC-CO2-B(C6F5)3 adduct, which was chemically reduced to afford [CAAC-CO2-B(C6F5)3˙-]. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO2 moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO2˙-] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO2˙-]-like species.

Carbone stabilized B2 and B22+ - isoelectronic analogues to diborabutyne and diborabutatriene

It has been reported that various unusual main group compounds can be stabilized by coordinating with ligands. Here, we report the use of carbone ligands in stabilizing diboron in its neutral and dicationic states by computational quantum mechanical calculations. The neutral [(L2C)·B2·(CL2)] (L = CO, NHC, PMe3, and cAAC) has singlet non-planar cumulenic-type equilibrium geometry where CL2 groups are almost orthogonal to each other. MO analysis indicates that the [(L2C)·B2·(CL2)] can be considered as formed by the interaction of the B2 fragment in the 1Σg+ excited state with two CL2 ligands having σ- and π-type lone pairs. Accordingly, the π delocalization in the C-B-B-C skeleton consists of two mutually orthogonal allylic anionic-type delocalizations along the C-B-B chain. Since one of the π-delocalized MOs of allylic anionic C-B-B is majorly localized on the carbone carbon atom, the carbone ligands formally act as two-electron ligands. On the other hand, the ground state of [(L2C)·B2·(CL2)]2+ shows a singlet planar/pseudo-planar cumulenic geometry when L = NHC and PMe3. The MO analysis indicates that the C-B-B-C skeleton is similar to that of butatriene, viz. one localized B-B π MO, and two delocalized C-B-B-C π MOs, indicating that each carbone acts as a four-electron ligand. Since CO and cAAC are good π-acceptor ligands, [(L2C)·B2·(CL2)]2+ ions (L = CO and cAAC) have triplet non-planar cumulenic ground states.