We report the synthesis and the reactivity of 1,2,3-triazolin-5-imine type mesoionic imines (MIIs). The MIIs are accessible by a base-mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O-stretching frequencies in MIIÀ CO 2 and À Rh(CO) 2 Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N-heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituentinduced fluorine-specific interactions and undergo CÀ H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO-LUMO gap compared to other related ligands. MIIs are potentially able to act as both π-donor and π-acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.
Invited for the cover of this issue are Biprajit Sarkar and co‐workers at the University of Stuttgart and University of Freiburg. In the image, the solar flare represents the non‐innocence (fluorine‐specific interactions) of the counterion, and the black hole at the metal center illustrates the oxidation/electron deficiency of the Cr‐center, while the electron “gets lost” in the space (oxidation agent). Read the full text of the article at 10.1002/chem.202301205.
The control of catalytic properties through ligand design is at the heart of homogeneous catalysis. Herein we report on ligand effects in six 1,3,4-substituted 1,2,3-triazolylidene gold complexes as precatalysts for the intermolecular hydroamination catalysis of alkynes with amines. Mainly silver-free protocols have been followed for the synthesis of the complexes and for the catalytic reactions. All of the novel gold complexes have been fully characterized, one of them being, to the best of our knowledge, the first 1,2,3-triazolylidene-based mesoionic carbene (MIC) gold complex with a weakly coordinating second ligand. In contrast to expectations, the gold complexes bearing the most strongly electron donating MICs did not display the best conversion in catalysis. According to our investigations the stability of the complex is a determining factor in the present case. The MIC gold bistriflimide complex shows activity in hydroamination catalysis without the need for an activating additive.
Two homoleptic Fe(II) complexes in different spin states
bearing
superbasic terpyridine derivatives as ligands are investigated to
determine the relationship between spin state and electrochemical/spectroscopic
behavior. Antiferromagnetic coupling between a ligand-centered radical
and the high-spin metal center leads to an anodic shift of the first
reduction potential and results in a species that shows mixed valency
with a moderately intense intervalence-charge-transfer band. The differences
afforded by the different spin states extend to the electrochemical
reactivity of the complexes: while the low-spin species is a precatalyst
for electrocatalytic CO2 reduction and leads to the preferential
formation of CO with a Faradaic efficiency of 37%, the high-spin species
only catalyzes proton reduction at a modest Faradaic efficiency of
approximately 20%.
We present here new synthetic strategies for the isolation
of a
series of Ru(II) complexes with pyridyl-mesoionic carbene ligands
(MIC) of the 1,2,3-triazole-5-ylidene type, in which the bpy ligands
(bpy = 2,2′-bipyridine) of the archetypical [Ru(bpy)3]2+ have been successively replaced by one, two, or three
pyridyl-MIC
ligands. Three new complexes have been isolated and investigated via
NMR spectroscopy and single-crystal X-ray diffraction analysis. The
incorporation of one MIC unit shifts the potential of the metal-centered
oxidation about 160 mV to more cathodic potential in cyclic voltammetry,
demonstrating the extraordinary σ-donor ability of the pyridyl-MIC
ligand, while the π-acceptor capacities are dominated by the
bpy ligand, as indicated by electron paramagnetic resonance spectroelectrochemistry
(EPR-SEC). The replacement of all bpy ligands by the pyridyl-MIC ligand
results in an anoidic shift of the ligand-centered reduction by 390
mV compared to the well-established [Ru(bpy)3]2+ complex. In addition, UV/vis/NIR-SEC in combination with theoretical
calculations provided detailed insights into the electronic structures
of the respective redox states, taking into account the total number
of pyridyl-MIC ligands incorporated in the Ru(II) complexes. The luminescence
quantum yield and lifetimes were determined by time-resolved absorption
and emission spectroscopy. An estimation of the excited state redox
potentials conclusively showed that the pyridyl-MIC ligand can tune
the photoredox activity of the isolated complexes to stronger photoreductants.
These observations can provide new strategies for the design of photocatalysts
and photosensitizers based on MICs.
Wir berichten über die Synthese und die Reaktivität mesoionischer Imine (MIIs) des 1,2,3‐Triazolin‐5‐imin Typs. Die MIIs sind über eine Basen‐mediierte Cycloaddition eines substituierten Acetonitrils mit einem aromatischen Azid, Methylierung nach etablierten Methylierungsrouten und anschließender Deprotonierung zugänglich. C=O‐Streckschwingungen in MII−CO2− und −Rh(CO)2Cl‐Komplexen wurden bemüht um die Gesamtdonorstärke zu bestimmen. Die MIIs sind stärkere Donoren als N‐Heterocyclische Imine (NHIs). MIIs sind exzellente Liganden für Hauptgruppenelemente und Übergangsmetalle, in welchen sie Substituenten‐induzierte Fluor‐spezifische Wechselwirkungen zeigen oder C−H‐Aktvierungen eingehen. DFT‐Berechnungen haben Einblicke in die Grenzorbitale der MIIs gegeben. Die Rechnungen sagen im Vergleich zu verwandten Liganden eine relative kleine HOMO–LUMO Lücke vorraus. MIIs können potentiell sowohl als π‐Donor‐ als auch π‐Akzeptorliganden wirken. Dieser Bericht demonstiert das Potential von MIIs spannende Eigenschaften mit einem gewaltigen Zukunftspotential zu entwickeln.
Here we present stable and crystalline chromium(I) tetracarbonyl complexes with pyridyl-MIC (MIC = mesoionic carbene) ligands and weakly coordinating anions (WCA = [Al(OR F ) 4 ] À , R F = C(CF 3 ) 3 and BAr F = [B(Ar F ) 4 ] À , Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ). The complexes were fully characterized via crystallographic, spectroscopic and theoretical methods. The influence of counter anions on the IR and EPR spectroscopic properties of the Cr I complexes was investigated, and the electronic innocence versus non-innocence of WCAs was probed. These are the first examples of stable and crystalline [Cr(CO) 4 ] + complexes with a chelating pÀ accepting ligand, and the data presented here are of relevance for both the photochemical and the electrochemical properties of these classes of compounds.
An azido‐1,2,3‐triazolium salt is a useful synthon that reacts either photochemically or thermally to deliver mesoionic carbene‐stabilized imino‐phosphoranes and mesoionic carbene‐stabilized triazenes, which can be converted to stable radicals. More information can be found in the Research Article by B. Sarkar and co‐workers (DOI: 10.1002/chem.202300771).
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