A Brief Survey of our Contribution to Stable Carbene Chemistry

An air-persistent oxyallyl radical cation with simple di(methyl)amino substituents

We report an experimental and theoretical study of the 1,1,3,3-tetrakis-di(methylamino)oxyallyl radical cation. Despite simple substituents with minimal steric hindrance, this radical was found to be stable under an inert atmosphere and persistent for several hours in well-aerated solutions.

Experimental and quantum chemical studies of anionic analogues of N-heterocyclic carbenes

A combination of quantum chemical and synthetic/crystallographic methods have been employed to probe electronic structure in two series of anionic ligands related to the well-known NHC donor class.

1,3,2-Diazaborole-derived carbene complexes of boron

Reaction of 2-bromo-1,3,2-diazaborole (1) with excess BBr₃ induces 1,2-hydrogen migration, giving 1,3,2-diazaborole-derived carbene complexes of boron bromide (2). Compound 2 exists in a dynamic solution equilibrium with 1. The ¹H NMR study shows that the equilibrium lies to the right side of the dissociation reaction of 2. Parallel reaction of 1 with excess BI₃ gives the corresponding 1,3,2-diazaborole-derived carbene boron iodide complex (3). Notably, in contrast to 2, the dissociation reaction of 3 largely lies to the left side, favouring the formation of 3. The dynamic solution equilibrium behaviours of 2 and 3 are probed by both experimental and theoretical methods.

Coordination Behavior of a Planar Chiral Cyclic (Amino)(Ferrocenyl)Carbene Ligand in Iridium Complexes

Iridium complexes bearing a new type of chiral carbene ligand, cyclic (amino)(ferrocenyl)carbene (CAFeC), were prepared. The complexes, which are quite stable in air and moisture, were fully characterized by NMR spectroscopy and X-ray diffraction analysis. The NMR spectra of the dicarbonyl complex [IrCl(CO)₂ (CAFeC)] showed signals for two isomers in solution, which were assigned to diastereomeric rotamers arising from the rotation of the chiral CAFeC ligand around the axes of the Ir-C_carbene bond. IR spectroscopy of the dicarbonyl complex revealed that the donor strength of CAFeC is much higher than those of classic N-heterocyclic carbenes (NHCs) and comparable to those of cyclic amino alkyl carbenes (CAACs).

Accessing Low-Valent Inorganic Cations by Using an Extremely Bulky N-Heterocyclic Carbene

The extremely bulky N-heterocyclic carbene (NHC), ITr (ITr=[(HCNCPh₃ )₂ C:]) featuring sterically shielding umbrella-shaped trityl (CPh₃ ) substituents was prepared. This NHC features the highest percent buried volume (%V_bur ) to date, and was used to form a thermally stable quasi one-coordinate thallium(I) cation [ITr-Tl]⁺ . This Tl^I adduct and the corresponding lithium complex [ITr⋅Li(OEt₂ )]⁺ are versatile "all-in-one" transmetalation/ligation reagents for preparing low-coordinate inorganic species inaccessible by pre-existing routes.

Exceptionally Strong Electron-Donating Ability of Bora-Ylide Substituent vis-à-vis Silylene and Silylium Ion

Electropositive boron-based substituent (phosphonium bora-ylide) with an exceptionally strong π- and σ-electron donating character dramatically increases the stability of a new type of N-heterocyclic silylene 2 featuring amino- and bora-ylide-substituents. Moreover, the related silylium ion 4 and transition-metal-silylene complexes, with trigonal-planar geometries around the silicon center, are also well stabilized. Therefore, the N,B-heterocyclic silylene 2 can be used as a strongly electron-donating innocent ligand in coordination chemistry similarly to N-heterocyclic carbenes.

α-Radical Phosphines: Synthesis, Structure, and Reactivity

A series of phosphines featuring a persistent radical were synthesized in two steps by condensation of dialkyl-/diarylchlorophosphines with stable cyclic (alkyl)(amino)carbenes (cAACs) followed by one-electron reduction of the corresponding cationic intermediates. Structural, spectroscopic, and computational data indicate that the spin density in these phosphines is mainly localized on the original carbene carbon from the cAAC fragment; thus, it remains in the α-position with respect to the central phosphorus atom. The potential of these α-radical phosphines to serve as spin-labeled ligands is demonstrated through the preparation of several Au^I derivatives, which were also structurally characterized by single-crystal X-ray diffraction.

Computational Study on the Mechanisms of Multiple Complexation of CO and Isonitrile Ligands to Boron

The recent experimental realization of compound Tripp-B(CO)₂ (denoted as 2a), where Tripp is 2,6-di(2,4,6-triisopropylphenyl)-phenyl), breaks through conventional knowledge that only transition metals can bind more than one CO to form multicarbonyl adducts. Compound 2a is stable in air but liberates CO under light. The B-CO bonds of 2a are considered to be similar to donor-acceptor bonds of transition metal complexes. To address the formation mechanism and chemical bonding of this novel type of boron compounds, we present a density functional theory study on the formation and photolysis of 2a and similar compounds. The results suggest that the formation of 2a is facile by three consecutive additions of CO to the terminal borylene metal complex, that is, the boron source of the synthesis. These CO additions can be practically accomplished via two different paths: CO direct addition and CO migration followed by addition. Such mechanisms can be excellently rationalized by the donor-acceptor bonding model of the terminal borylene complex, which in turn suggests that using donor-acceptor bonds for 2a is natural for understanding the mechanisms. Liberation of CO from 2a and its similar compounds has higher energy barriers at the ground states than that at the triplet states by 40 kcal/mol. These energy barriers explain the experimentally observed air stability and photolysis of these compounds. The results for the first time provide mechanistic insights for the unprecedented chemical processes; they allow evaluation of the applicability of donor-acceptor bonding in main-group compounds from the new perspective of chemical reactions.

Indium-Arsenic Molecules with an In≡As Triple Bond: A Theoretical Approach

The effect of substitution on the potential energy surfaces of RIn≡AsR (R = F, OH, H, CH3, and SiH3 and R' = SiMe(SitBu3)2, SiiPrDis2, and N-heterocyclic carbene (NHC)) is determined using density functional theory calculations (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp). The computational studies demonstrate that all of the triply bonded RIn≡AsR species prefer to adopt a bent geometry, which is consistent with the valence electron model. The theoretical studies show that RIn≡AsR molecules that have smaller substituents are kinetically unstable with respect to their intramolecular rearrangements. However, triply bonded R'In≡AsR' species that have bulkier substituents (R' = SiMe(SitBu3)2, SiiPrDis2, and NHC) occupy minima on the singlet potential energy surface, and they are both kinetically and thermodynamically stable. That is, the electronic and steric effects of bulky substituents play an important role in making molecules that feature an In≡As triple bond viable as a synthetic target. Moreover, two valence bond models are used to interpret the bonding character of the In≡As triple bond. One is model [A], which is best represented as . This interprets the bonding conditions for RIn≡AsR molecules that feature small ligands. The other is model [B], which is best represented as . This explains the bonding character of RIn≡PAsR molecules that feature large substituents.

Triply-bonded indiumphosphorus molecules: theoretical designs and characterization

The theoretical results indicate the connected substituents (R) play a decisive role in determining both the kinetic and the thermodynamic stability of triple-bonded RInPR molecules.

The effect of substituents on the stability of triply bonded galliumantimony molecules: a new target for synthesis

The M06-2X, B3PW91 and B3LYP computational results show that, from the kinetic viewpoint, only bulkier substituents have a significant stabilizing effect on the triply bonded RGaSbR compounds.

A redox-switchable ring-closing metathesis catalyst

A ring-closing metathesis catalyst was arrested upon reduction of a redox-active ligand; subsequent oxidation restored catalytic activity.

Facile routes to abnormal-NHC-cobalt(ii) complexes

Deprotonation of 1 with Co{N(SiMe₃)₂}₂ affords aNHC-Co(ii) complex 2, whereas carbene transfer from 3 to Co{N(SiMe₃)₂}₂ enables access to complex 4.

Copper-catalyzed dehydrogenative borylation of terminal alkynes with pinacolborane

LCuOTf complexes [L = cyclic (alkyl)(amino)carbenes (CAACs) or N-heterocyclic carbenes (NHCs)] selectively promote the dehydrogenative borylation of C(sp)-H bonds at room temperature. It is shown that σ,π-bis(copper) acetylide and copper hydride complexes are the key catalytic species.

Borocation catalysis

The synthesis, stability and catalytic reactivity of borocations are described in the context of their reaction in frustrated Lewis pair-type processes.

C4-Ferrocenylsilyl-bridged and -substituted N-heterocyclic carbenes: complexation of germanium chloride

C4-Ferrocenylsilyl-bridged and -substituted N-heterocyclic carbenes and their germanium chloride complexes have been prepared and characterized.

Hg(ii) and Pd(ii) complexes with a new selenoether bridged biscarbene ligand: efficient mono- and bis-arylation of methyl acrylate with a pincer biscarbene Pd(ii) precatalyst

New selenoether (CSeC) bridged carbene complexes of Hg(ii) and Pd(ii) have been prepared; a pincer Pd(ii) complex showed catalytic activity in mono- and double-Heck reactions.

Azido- and amido-substituted gallium hydrides supported by N-heterocyclic carbenes

The synthesis of azido- and amido-gallanes supported by hindered N-heterocyclic carbene donors is reported.

Metal-Free Reduction of CO2 to Methoxyborane under Ambient Conditions through Borondiformate Formation

An abnormal N-heterocyclic carbene (aNHC) based homogeneous catalyst has been used for the reduction of carbon dioxide to methoxyborane in the presence of a range of hydroboranes under ambient conditions and resulted in the highest turnover number of 6000. A catalytically active reaction intermediate, [aNHC-H⋅9BBN(OCOH)₂ ] was structurally characterized and authenticated by NMR spectroscopy. A detailed mechanistic cycle of this catalytic process via borondiformate formation has been proposed from tandem experimental and computational experiments.

Isolation of a Cyclic (Alkyl)(amino)germylene

A 1,4-addition of a dichlorogermylene dioxane complex with α,β-unsaturated imine 1 gave a dichlorogermane derivative 2 bearing a GeC₃N five-membered ring skeleton. By reducing 2 with KC₈, cyclic (alkyl)(amino)germylene 3 was synthesized and fully characterized. Germylene 3 readily reacted with TEMPO, N₂O and S₈, producing the 1:2 adduct 4, the oxo-bridged dimer 5 and the sulfido-bridged dimer 6, respectively.