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.
A Pair of Cobalt(III/IV) Terminal Imido Complexes Mao, Weiqing; Fehn, Dominik; Heinemann, Frank W. ...
Angewandte Chemie (International ed.),
July 19, 2021, Letnik:
60, Številka:
30
Journal Article
Recenzirano
Odprti dostop
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.
Singlet Fission in Carbene‐Derived Diradicaloids Ullrich, Tobias; Pinter, Piermaria; Messelberger, Julian ...
Angewandte Chemie (International ed.),
May 11, 2020, Letnik:
59, Številka:
20
Journal Article
Recenzirano
Odprti dostop
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.
The isolation of terminal oxo complexes of the late transition metals promises new avenues in oxidation catalysis like the selective and catalytic hydroxylation of unreactive CH bonds, the activation ...of water, or the upgrading of olefins. While terminal oxo ligands are ubiquitous for early transition metals, well-characterized examples with group 10 metals remain hitherto elusive. In search for palladium terminal oxo complexes, the relative stability/reactivity of such compounds are evaluated computationally (CASSCF/NEVPT2; DFT). The calculations investigate only well-known ligand systems with established synthetic procedures and relevance for coordination chemistry and homogeneous catalysis. They delineate and quantify, which electronic properties of ancillary ligands are crucial for taming otherwise highly reactive terminal oxo intermediates. Notably, carbene ligands with both strong σ-donor and strong π-acceptor properties are best suited for the stabilization of palladium(ii) terminal oxo complexes, whereas ligands with a weaker ligand field lead to highly reactive complexes. Strongly donating ligands are an excellent choice for high-valent palladium(iv) terminal oxo compounds. Low coordinate palladium(ii) as well as high-valent palladium(iv) complexes are best suited for the activation of strong bonds.
Atomically precise nanoclusters hold promise for supramolecular assembly and (opto)electronic‐ as well as magnetic materials. Herein, this work reports that treating palladium(0) precursors with a ...triphosphirane affords strongly colored Pd8(PDip)6 that is fully characterized by mass spectrometry, heteronuclear and Cross‐Polarization Magic‐Angle Spinning (CP‐MAS) NMR‐, infrared (IR), UV–vis, and X‐ray photoelectron (XP) spectroscopies, single‐crystal X‐Ray diffraction (sc‐XRD), mass spectrometry, and cyclovoltammetry (CV). This coordinatively unsaturated 104‐electron Pd(0) cluster features a cubic Pd8‐core, µ4‐capping phosphinidene ligands, and is air‐stable. Quantum chemical calculations provide insight to the cluster's electronic structure and suggest 5s/4d orbital mixing as well as minor Pd─P covalency. Trapping experiments reveal that cluster growth proceeds via insertion of Pd(0) into the triphosphirane. The unsaturated cluster senses ethylene and binds isocyanides, which triggers the rearrangement to a tetrahedral structure with a reduced frontier orbital energy gap. These experiments demonstrate facile cluster manipulation and highlight non‐destructive cluster rearrangement as is required for supramolecular assembly.
An air‐stable Pd8 hexaphosphinidene nanocluster is reported. Spectroscopic investigations and calculations substantiate zerovalency, whereas the coordinative unsaturation allows for facile post‐functionalization.
The development of efficient aerobic oxidation methods remains a challenge for the selective functionalization of CH bonds in alkanes. Herein we report the development of a CH functionalization ...procedure for propane by using a palladium catalyst with chelating bis(N‐heterocyclic carbene) ligands in trifluoroacetic acid together with a vanadium co‐catalyst. Halides play a decisive role in the reaction. The experimental results are presented together with supporting kinetic data and an isotope effect. The reaction can be run with dioxygen as the oxidant if vanadium salts and halides are present in the reaction mixture. Experimental as well as computational results favor a mechanism involving CH activation by palladium(II), followed by oxidation to palladium(IV) by bromine.
Reoxidation by dioxygen: The combination of CH activation by a homogeneous catalytic palladium complex with a vanadiumoxo co‐catalyst allows the selective aerobic oxidation of propane with dioxygen (see scheme).
The isolation of terminal oxo complexes of the late transition metals promises new avenues in oxidation catalysis like the selective and catalytic hydroxylation of unreactive CH bonds, the activation ...of water, or the upgrading of olefins. While terminal oxo ligands are ubiquitous for early transition metals, well-characterized examples with group 10 metals remain hitherto elusive. In search for palladium terminal oxo complexes, the relative stability/reactivity of such compounds are evaluated computationally (CASSCF/NEVPT2; DFT). The calculations investigate only well-known ligand systems with established synthetic procedures and relevance for coordination chemistry and homogeneous catalysis. They delineate and quantify, which electronic properties of ancillary ligands are crucial for taming otherwise highly reactive terminal oxo intermediates. Notably, carbene ligands with both strong σ-donor and strong π-acceptor properties are best suited for the stabilization of palladium(
ii
) terminal oxo complexes, whereas ligands with a weaker ligand field lead to highly reactive complexes. Strongly donating ligands are an excellent choice for high-valent palladium(
iv
) terminal oxo compounds. Low coordinate palladium(
ii
) as well as high-valent palladium(
iv
) complexes are best suited for the activation of strong bonds.
The isolation of terminal oxo complexes of the late transition metals promises new avenues in oxidation catalysis like the selective and catalytic hydroxylation of unreactive CH bonds, the activation of water, or the upgrading of olefins.
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