This tutorial explores the diversity and characteristics of C-donor ligands. Particular emphasis is put on the conceptual design of the electronic properties for applications in coordination ...chemistry. More specifically, the implications of both σ and π effects are discussed. Cyclic (alkyl)(amino)carbenes as well as methanediides and carbones are brought in perspective to “conventional” N-heterocylic carbenes, mesoionic carbenes, and Fischer as well as Schrock carbene ligands.
Unconventional singlet fission materials Ullrich, Tobias; Munz, Dominik; Guldi, Dirk M
Chemical Society reviews,
03/2021, Letnik:
5, Številka:
5
Journal Article
Recenzirano
Singlet fission (SF) is a photophysical downconversion pathway, in which a singlet excitation transforms into two triplet excited states. As such, it constitutes an exciton multiplication generation ...process, which is currently at the focal point for future integration into solar energy conversion devices. Beyond this, various other exciting applications were proposed, including quantum cryptography or organic light emitting diodes. Also, the mechanistic understanding evolved rapidly during the last year. Unfortunately, the number of suitable SF-chromophores is still limited. This is
per se
problematic, considering the wide range of envisaged applicability. With that in mind, we emphasize uncommon SF-scaffolds and outline requirements as well as strategies to expand the chromophore pool of SF-materials.
Singlet fission (SF) is a photophysical downconversion pathway, in which a singlet excitation transforms into two triplet excited states.
A versatile methodology, involving readily available starting materials, allows for the synthesis of stable hemilabile bidentate cyclic (alkyl)(amino)carbenes (CAACs) featuring alkene, ether, ...amine, imine, and phosphine functionalities. The stability of the free carbenes has been exploited for the synthesis of copper(I) and gold(I) complexes. It is shown that the pendant imine moiety stabilizes the gold(III) oxidation state and enables the C–C bond oxidative addition of biphenylene to the corresponding cationic gold(I) complex. The latter and the corresponding copper(I) complex show high catalytic activity for the hydroarylation of α-methylstyrene with N,N-dimethylaniline, and the copper(I) complex promotes the anti-Markovnikov hydrohydrazination of phenyl acetylene with high selectivity.
Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', ...photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e
chemistry or switching.
We thank the Fonds der Chemischen Industrie (Liebig fellowship for D.M.), the German‐American Fulbright Commission (Fulbright‐Cottrell Award for D.M.) as well as the Bavarian Equal Opportunities ...Sponsorship – Realization of Equal Opportunities for Women in Research and Teaching (fellowship for A.G.) for financial support. We thank the RRZ Erlangen for computational resources.
Metal−nitrogen formally multiple bonded complexes, i. e. nitrenes, imidyls, and imido ligands, are key intermediates in organic catalysis and group transfer chemistry. Complexes of the group 9–11 metals are emerging as excellent catalysts and the isolation and characterization of these highly covalently bonded metal complexes can shed light on their structural and electronic features. Herein, we summarize the developments in the coordination chemistry of late transition metal imido‐ and nitrene complexes since 2013. Based on the computational analysis of their electronic structure, we highlight the implications on stochiometric reactivity and catalytic applications.
We report a detailed mechanistic study on the CH functionalization of alkanes by palladium complexes with chelating bis(N‐heterocyclic carbene) (NHC) complexes. The experimental results are ...complemented by detailed DFT calculations, which allow us to rationalize the regioselectivity and the catalytic activity. The study includes a library of catalysts with different electronic and steric properties, kinetic data, and isotope effects. The combined experimental and computational results favor a mechanism involving organometallic palladium(IV) intermediates. Furthermore, it is shown that at high halide loadings a different mechanism is operative.
No propane, no gain: The mechanism for the functionalization of propane with palladium(NHC) catalysts was investigated in detail. The combined experimental and computational (DFT) results favor a mechanism with CH activation by palladium(II) and oxidation by bromine to palladium(IV). It is proposed that the oxidation of palladium(II) to palladium(IV) alkyl species proceeds faster for iso‐alkyl than for n‐alkyl complexes.
The reaction of the cobalt(I) complex (TIMMNmes)CoI(BPh4) (2) (TIMMNmes=tris‐2‐(3‐mesitylimidazolin‐2‐ylidene)methylamine) with 1‐adamantylazide yields the cobalt(III) imido complex ...(TIMMNmes)CoIII(NAd)(BPh4) (3) with concomitant release of dinitrogen. The N‐anchor in diamagnetic 3 features an unusual, planar tertiary amine, which results from repulsive electrostatic interaction with the filled d(z2)‐orbital of the cobalt ion and negative hyperconjugation with the neighboring methylene groups. One‐electron oxidation of 3 with FeCp2(OTf) provides access to the rare, high‐valent cobalt(IV) imido complex (TIMMNmes)CoIV(NAd)(OTf)2 (4). Despite a half‐life of less than 1 h at room temperature, 4 could be isolated at low temperatures in analytically pure form. Single‐crystal X‐ray diffractometry and EPR spectroscopy corroborate the molecular structure and the d5 low‐spin, S=1/2
, electron configuration. A computational analysis of 4 suggests high covalency within the CoIV=NAd bond with non‐negligible spin density located at the imido moiety, which translates into substantial triplet nitrene character.
Straightforward access to a cobalt(IV) terminal imido complex was provided by one‐electron oxidation of a cobalt(III) terminal imido precursor. The cobalt(IV) monoimido complex could be isolated at low temperatures in analytically pure form. Single‐crystal X‐ray diffractometry and EPR spectroscopy corroborate the molecular structure and the d5 low‐spin, S=1/2
, electron configuration.
The synthesis, characterization, and reactivity of a series of cobalt terminal imido complexes supported by an N-anchored tripodal tris(carbene) chelate is described, including a Co-supported ...singlet nitrene. Reaction of the CoI precursor (TIMMNmes)CoI(PF6) (TIMMNmes = tris-2-(3-mesityl-imidazolin-2-ylidene)-methylamine) with p-methoxyphenyl azide yields a CoIII imide (TIMMNmes)CoIII(NAnisole)(PF6) (1). Treatment of 1 with 1 equiv of FeCp2(PF6) at −35 °C affords a formal CoIV imido complex (TIMMNmes)Co(NAnisole)(PF6)2 (2), which features a bent Co–N(imido)–C(Anisole) linkage. Subsequent one-electron oxidation of 2 with 1 equiv of AgPF6 provides access to the tricationic cobalt imido complex (TIMMNmes)Co(NAnisole)(PF6)3 (3). All complexes were fully characterized, including single-crystal X-ray diffraction (SC-XRD) analyses, infrared (IR) vibrational, ultraviolet/visible (UV/vis) electronic absorption, multinuclear NMR, X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD XAS). Quantum chemical calculations provide additional insight into the electronic structures of all compounds. The dicationic CoIV imido complex 2 exhibits a doublet ground state with considerable imidyl character as a result of covalent Co–NAnisole bonding. At room temperature, 2 readily converts to a CoII amine complex involving intramolecular C–H bond amination. Electronically, tricationic complex 3 can be understood as a singlet nitrene bound to CoIII with significant CoIV imidyl radical character. Verifying the pronounced electrophilicity, nucleophiles such as H2O and t BuNH2 add to 3analogous to the parent free nitrenein the para position of the aromatic substituent, thus, clearly corroborating singlet nitrene-type reactivity.
Herein, we present a new class of singlet fission (SF) materials based on diradicaloids of carbene scaffolds, namely cyclic (alkyl)(amino)carbenes (CAACs). Our modular approach allows the tuning of ...two key SF criteria: the steric factor and the diradical character. In turn, we modified the energy landscapes of excited states in a systematic manner to accommodate the needs for SF. We report the first example of intermolecular SF in solution by dimer self‐assembly at cryogenic temperatures.
Diradicaloids derived from cyclic carbenes are singlet‐fission molecules with exceptionally large diradicaloid character, which self‐assemble at cryogenic temperatures in solution.
The cobalt‐catalyzed oxidation of methane to methyl trifluoroacetate by molecular oxygen in trifluoroacetic acid has been studied in detail. Yields of up to 50 % based on methane were obtained. The ...catalytic activities were highly dependent on the anions of the cobalt salts (CoII, CoIII) under investigation. Deactivation by precipitation of the cobalt catalyst could be prevented by the addition of trifluoroacetic anhydride.
The selective cobalt‐catalyzed oxidation of methane to methyl trifluoroacetate by dioxygen has been studied in detail. The catalytic activities of different cobalt precatalysts were investigated, with cobalt(II) nitrate proving to be the most efficient. The effects of solvent, reaction time, temperature, and catalyst loading have been studied.
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