Organic Mixed Valence Compounds Derived from Cyclic (Alkyl)(amino)carbenes

Carbene derived diradicaloids - building blocks for singlet fission?

Organic singlet diradicaloids promise application in non-linear optics, electronic devices and singlet fission. The stabilization of carbon allotropes/cumulenes (C1, C2, C4) by carbenes has been equally an area of high activity. Combining these fields, we showed recently that carbene scaffolds allow as well for the design of diradicaloids. Herein, we report a comprehensive computational investigation (CASSCF/NEVPT2; fractional occupation DFT) on the electronic properties of carbene-bridge-carbene type diradicaloids. We delineate how to adjust the properties of these ensembles through the choice of carbene and bridge and show that already a short C2 bridge results in remarkable diradicaloid character. The choice of the carbene separately tunes the energies of the S1 and T1 excited states, whereas the bridge adjusts the overall energy level of the excited states. Accordingly, we develop guidelines on how to tailor the electronic properties of these molecules. Of particular note, fractional occupation DFT is an excellent tool to predict singlet-triplet gaps.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Kekulé diradicaloids derived from a classical N-heterocyclic carbene

The direct double carbenylation of 1,4-diiodobenzene and 4,4'-dibromobiphenyl with a classical N-heterocyclic carbene, SIPr (1) (SIPr = :C{N(2,6-iPr2C6H3)}2CH2CH2), by means of nickel catalysis gives rise to 1,3-imidazolinium salts [(SIPr)(C6H4)(SIPr)](I)2 (2) and [(SIPr)(C6H4)2(SIPr)](Br)2 (3) as off-white solids. Two-electron reduction of 2 and 3 with KC8 cleanly yields Kekulé diradicaloid compounds [(SIPr)(C6H4)(SIPr)] (4) and [(SIPr)(C6H4)2(SIPr)] (5), respectively, as crystalline solids. Structural parameters and DFT as well as CASSCF calculations suggest the closed-shell singlet ground state for 4 and 5. Calculations reveal a very low singlet-triplet energy gap ΔES-T for 5 (10.7 kcal mol-1), while ΔES-T for 4 (29.1 kcal mol-1) is rather large.

An air-stable N-heterocyclic carbene iminoxyl borate radical zwitterion

The presented N-heterocyclic carbene iminoxyl borate radical zwitterion shows remarkable stability toward air, moisture, and silica gel.

The serendipitous discovery of a readily available redox-bistable molecule derived from cyclic(alkyl)(amino)carbenes

A simple redox bistable system is available in one step from a stable carbene and a bis(acyl chloride).