Experimental Observation of a Terminal Borylene-Dinitrogen Adduct via Cleavage of a 1,2,3,4,5-Diboratriazoline.

While azides do not react with simple alkenes except under harsh conditions, a diboron alkene analogue, the doubly cyclic alkyl(amino)carbene (CAAC)-stabilized dicyanodiborene 1, reacts spontaneously with organic azides (7-10 equiv.) at room temperature to yield two equivalents of stable CAAC-stabilized imino(cyano)boranes (2-R). NMR-spectroscopic monitoring of the reaction mixtures shows the initial formation of a 1:1 mixture of 2-R and a relatively long-lived intermediate (Int), which in the presence of excess azide is converted into a second equivalent of 2-R. In the absence of excess azide, however, Int decomposes to 3, the product of an intramolecular C‒H activation by a putative dicoordinate borylene intermediate "(CAAC)B(CN)". Mechanistic insights from trapping experiments, NMR-spectroscopic and high-resolution mass spectrometry data, as well as DFT computations reveal that Int is the terminal borylene end-on-dinitrogen adduct [(CAAC)B(CN)(η1-N2)]. The formation of the iminoboranes 2-R from diborene 1 and RN3 proceeds via an azide-diborene Huisgen-type [3+2] cycloaddition reaction, followed by a retro-[3+2] cycloaddition, yielding 2-R and [(CAAC)B(CN)(η1-N2)]. The latter then undergoes either N2 extrusion and intramolecular C‒H activationto generate 3, or a Staudinger-type reaction with a second equivalent of azide to generate a second equivalent of the iminoborane 2-R.

Bismuth in Dynamic Covalent Chemistry: Access to a Bowl-Type Macrocycle and a Barrel-Type Heptanuclear Complex Cation.

Dynamic covalent chemistry (DCvC) is a powerful and widely applied tool in modern synthetic chemistry, which is based on the reversible cleavage and formation of covalent bonds. One of the inherent strengths of this approach is the perspective to reversibly generate in an operationally simple approach novel structural motifs that are difficult or impossible to access with more traditional methods and require multiple bond cleaving and bond forming steps. To date, these fundamentally important synthetic and conceptual challenges in the context of DCvC have predominantly been tackled by exploiting compounds of lighter p-block elements, even though heavier p-block elements show low bond dissociation energies and appear to be ideally suited for this approach. Here we show that a dinuclear organometallic bismuth compound, containing BiMe2 groups that are connected by a thioxanthene linker, readily undergoes selective and reversible cleavage of its Bi-C bonds upon exposure to external stimuli. The exploitation of DCvC in the field of organometallic heavy p-block chemistry grants access to unprecedented macrocyclic and barrel-type oligonuclear compounds.

Trapping of a Transient Base-Stabilised Alumylene and Alumylene-Type Reactivity of a Self-Stabilising Dialumene towards Organic Azides.

The reduction of a carbene-coordinated, sterically encumbered terphenyl-substituted aluminium diiodide, (LRAlI2 ), yielded a "masked" dialumene (LRAl=AlRL), self-stabilised through [2+2] cycloaddition with a peripheral aromatic group. During the course of the reaction, a carbene-stabilised arylalumylene (LRAl:) was generated in situ, which was trapped using an alkyne, generating an aluminacyclopropene or a C-H activated product thereof, depending on the steric bulk of the alkyne. The masked dialumene also underwent intramolecular cycloreversion and dissociation into alumylene fragments, which reacted with various organic azides to yield monomeric or dimeric iminoalanes depending on the sterics of the azide substituent. The thermodynamics of monomeric and dimeric iminoalane formation were probed by theoretical calculations.

Salicylaldimines: Formation via Ring Contraction and Synthesis of Mono- and Heterobimetallic Alkali Metal Heterocubanes.

The formation of salicylaldimine derivatives via ring contraction as byproducts in 2-aminotropone syntheses has been investigated. Salicylaldiminate (SAI) complexes of the alkali metals Li-K have been synthesized and transformed into heterobimetallic complexes. Important findings include an unusual double heterocubane structure of the homometallic sodium SAI, an unprecedented ligand-induced E/Z isomerization of the aldimine functional group in the homometallic potassium SAI, and the first example of a structurally authenticated mixed-metal SAI based on s-block central atoms. Rapid equilibria have been shown to play a crucial role in the solution phase chemistry of mixed-metal SAIs. Analytical techniques applied in this work include (heteronuclear) NMR spectroscopy, VT- and DOSY NMR spectroscopy, high-resolution mass spectrometry, single-crystal X-ray diffraction analysis, and DFT calculations.