Π-acceptor Ligands Research Articles

The selective adsorption of Ni2+ over Co2+ from aqueous solutions in surface functionalized metal–organic frameworks

In this study, two metal-organic frameworks (MOFs), zirconium (IV)-based MOF-808 and chromium (III)-based MIL-101 were investigated for the selective adsorption of nickel (Ni2+) over cobalt (Co2+) and the role of the nitrogen (N)-containing functional groups was evaluated using pyrazole (PyC) and amine (–NH2). Selective adsorption and adsorption rates were quantified using batch adsorption experiments. The post-reaction MOFs after Co2+ and Ni2+ adsorption were characterized with attenuated total reflectance Fourier transform infrared (ATR-FTIR) and X-ray photoelectron (XPS) spectroscopies to identify the active adsorption sites and to infer the adsorption mechanism. The overall adsorption of Co2+ and Ni2+ by MOF-808 and MIL-101 increased after their surface functionalization with PyC and –NH2. However, selective Ni2+ adsorption was only observed with MOF-808-PyC. The pseudo-second-order model fit to the kinetic adsorption data suggests chemically driven adsorption of Co2+ and Ni2+ onto all examined MOFs. The ATR-FTIR and XPS analyses of MOFs with adsorbed Co2+ and Ni2+ indicate that the N sites in PyC and –NH2 are the reactive sites. ATR-FTIR analyses of aqueous PyC solutions with metal cations show a greater shift in the N–H vibrational band of PyC when interacting with Ni2+ compared to Co2+, which is linked to a stronger interaction between Ni2+ and PyC, resulting in the Ni2+ selectivity by PyC-functionalized MOF-808. We propose that PyC, as a π-acceptor ligand, exhibits greater affinity for Ni2+ compared to Co2+ due to i) the greater electronegativity of Ni2+ and ii) the greater stability of Ni2+complex with PyC ligand based on the Irving-Williams series and ligand field stabilization energy based on the electron configuration differences between Co2+ (3d7) and Ni2+ (3d8).

Divergent Interaction of (Iso)nitriles with a Linear Iron(I) Silylamide─A Combined Structural, Spectroscopic, and Computational Study.

Nitriles and isonitriles are important σ-donor ligands in coordination chemistry. Isonitriles also function in low-valent complexes as π-acceptor ligands similar to CO. Herein we present the unusual behavior of the highly reducing, high-spin iron(I) complex [Fe(hmds)2]- toward these compound classes. Rare examples of side-on coordination of nitriles to the metal center are observed. Insights gained by 57Fe Mössbauer spectroscopy as well as DFT and CASSCF calculations give an interplay between the resonance structures of not only an iron(I) π-complex and an iron(III) metallacycle but also point to the importance of an iron(II) nitrile radical anion. For an aromatic isonitrile end-on coordination is observed, which is best described as an iron(I) complex with only minor unpaired spin transfer onto the isonitrile. For aliphatic isonitriles, the selective R-CN bond cleavage occurs and yields stoichiometric mixtures of alkyl iron(II) and cyanido iron(II) complexes. Attempts to isolate presumed (iso)nitrile radical anions void of 3d-metal coordination give for the reaction of an aromatic isonitrile with KC8 facile reductive coupling to the corresponding diamido acetylene.

An isolable, chelating bis[cyclic (alkyl)(amino)carbene] stabilizes a strongly bent, dicoordinate Ni(0) complex

Chelating ligands have had a tremendous impact in coordination chemistry and catalysis. Notwithstanding their success as strongly σ-donating and π-accepting ligands, to date no chelating bis[cyclic (alkyl)(amino)carbenes] have been reported. Herein, we describe a chelating, C2-symmetric bis[cyclic (alkyl)(amino)carbene] ligand, which was isolated as a racemic mixture. The isolation and structural characterization of its isostructural, pseudotetrahedral complexes with iron, cobalt, nickel, and zinc dihalides featuring eight-membered metallacycles demonstrates the binding ability of the bis(carbene). Reduction of the nickel(II) dibromide with potassium graphite produces a dicoordinate nickel(0) complex that features one of the narrowest angles measured in any unsupported dicoordinate transition metal complexes.

Influence of strong π-acceptor ligands on Cr-K-edge X-ray absorption spectral signatures and consequences for the interpretation of surface sites in the Phillips catalyst.

X-ray absorption spectroscopy (XAS) has been central to the study of the Phillips polymerization catalyst (CrO3/SiO2). As Cr K-edge XAS signatures are sensitive to the oxidation state, geometry and types of ligands on surface (active) sites, the superposition of these effects makes their interpretation challenging. Notably, CO has been particularly used as a reductant to generate low valent Cr sites from CrO3/SiO2 and as a structural IR probe for analysing reduced Cr surface sites. Hence, it is essential to establish a solid understanding of the spectroscopic impact of CO on low-valent Cr sites. We thus built a series of fully characterized low-valent Cr molecular compounds bearing isoelectronic isocyanide ligands in place of CO, with the goal of understanding the effect of the coordination of π-acceptor ligands on the XANES signature of Cr sites. Cr K-edge spectra supplemented with DFT calculations elucidate the effect of the coordination of π-acceptor ligands on XAS signatures, giving a sharp resonance at the white line while modifying the fine structure due to short Cr-C distances and stability of low-spin Cr(ii/iii) species. The isocyanide references allow the deconvolution of the XAS spectra of the reduced CrO3/SiO2 catalyst by evaluating the types of surface species and relative amounts of bound CO at different CO pressures and temperatures.

Luminescence and Excited-State Reactivity in a Heteroleptic Tricyanido Fe(III) Complex.

Harnessing sunlight via photosensitizing molecules is key for novel optical applications and solar-to-chemical energy conversion. Exploiting abundant metals such as iron is attractive but becomes challenging due to typically fast nonradiative relaxation processes. In this work, we report on the luminescence and excited-state reactivity of the heteroleptic [FeIII(pzTp)(CN)3]- complex (pzTp = tetrakis(pyrazolyl)borate), which incorporates a σ-donating trispyrazolyl chelate ligand and three monodentate σ-donating and π-accepting cyanide ligands. Contrary to the nonemissive [Fe(CN)6]3-, a broad emission band centered at 600 nm at room temperature has been recorded for the heteroleptic analogue attributed to the radiative deactivation from a 2LMCT excited state with a luminescence quantum yield of 0.02% and a lifetime of 80 ps in chloroform at room temperature. Bimolecular reactivity of the 2LMCT excited state was successfully applied to different alcohol photo-oxidation, identifying a cyanide-H bonding as a key reaction intermediate. Finally, this research demonstrated the exciting potential of [Fe(pzTp)(CN)3]- as a photo-oxidant, paving the way for further exploration and development of emissive Fe-based photosensitizers competent for photochemical transformations.

Luminescent rhenium(I) complex with cyanopentafluorophosphate ligand: Synthesis, characterization, and photophysics

A tricarbonyl Re(I) phenanthroline complex (1) with anionic cyanopentafluorophosphate [NCPF5]– ligand has been synthesized and characterized. Although the structure of the complex has also been determined by X-ray crystallography, the identity of the [NCPF5]– needs to be confirmed by the vibrational spectroscopic study. The cyanopentafluorophosphate Re(I) phenanthroline complex 1 also exhibits phosphorescence derived from a 3MLCT excited state. Importantly, as the MLCT absorption and emission energies of 1 are comparable to those of the structurally related complexes with the strongest π-accepting anionic isocyanoborate ligands, it reveals the exceptionally strong π-accepting ability of [NCPF5]–. This result suggests that [NCPF5]– is a promising alternative to isocyanoborate ligands for designing phosphorescent materials, catalysts and photocatalysts.

The Best of Both Worlds: Combining the Power of MICs and WCAs To Generate Stable and Crystalline CrI -Tetracarbonyl Complexes with π-Accepting Ligands.

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 Best of Both Worlds: Combining the Power of MICs and WCAs To Generate Stable and Crystalline CrI -Tetracarbonyl Complexes with π-Accepting Ligands.

Here we present stable and crystalline chromium(I) tetracarbonyl complexes with pyridyl-MIC (MIC=mesoionic carbene) ligands and weakly coordinating anions (WCA=[Al(ORF )4 ]- , RF =C(CF3 )3 and BArF =[B(ArF )4 ]- , ArF =3,5-(CF3 )2 C6 H3 ). 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 CrI 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 accepting ligand, and the data presented here are of relevance for both the photochemical and the electrochemical properties of these classes of compounds.

Electronic Structures and Photoredox Chemistry of Tungsten(0) Arylisocyanides.

ConspectusThe high energy barriers associated with the reaction chemistry of inert substrates can be overcome by employing redox-active photocatalysts. Research in this area has grown exponentially over the past decade, as transition metal photosensitizers have been shown to mediate challenging organic transformations. Critical for the advancement of photoredox catalysis is the discovery, development, and study of complexes based on earth-abundant metals that can replace and/or complement established noble-metal-based photosensitizers.Recent work has focused on redox-active complexes of 3d metals, as photosensitizers containing these metals most likely would be scalable. Although low lying spin doublet ("spin flip") excited states of chromium(III) and metal-to-ligand charge transfer (MLCT) excited states of copper(I) have relatively long lifetimes, the electronic excited states of many other 3d metal complexes fall on dissociative potential energy surfaces, owing to the population of highly energetic σ-antibonding orbitals. Indeed, we and other investigators have shown that low lying spin singlet and triplet excited states of robust closed-shell metal complexes are too short-lived at room temperature to engage in bimolecular reactions in solutions. In principle, this problem could be overcome by designing and constructing 3d metal complexes containing strong field π-acceptor ligands, where thermally equilibrated MLCT or intraligand charge transfer excited states might fall well below the upper surfaces of dissociative 3d-3d states. Notably, such design elements have been exploited by investigators in very recent work on redox-active iron(II) systems. Another approach, one we have actively pursued, is to design and construct closed-shell complexes of earth-abundant 5d metals containing very strong π-acceptor ligands, where vertical excitation of 5d-5d excited states at the ground state geometry would require energies far above minima in the potential surfaces of MLCT excited states. As this requirement is met by tungsten(0) arylisocyanides, these complexes have been the focus of our work aimed at the development of robust redox-active photosensitizers.In the following Account, we review recent work on homoleptic tungsten(0) arylisocyanides. Originally reported by our group 45 years ago, W(CNAr)6 complexes have exceptionally large one- and two-photon absorption cross-sections. One- or two-photon excitation produces relatively long-lived (hundreds of nanoseconds to microsecond) MLCT excited states in high yields. These MLCT excited states, which are very strong reductants with E°(W+/*W0) = -2.2 to -3.0 V vs Fc[+/0], mediate photocatalysis of organic reactions with both visible and near-infrared (NIR) light. Here, we highlight design principles that led to the development of three generations of W(CNAr)6 photosensitizers; and we discuss likely steps in the mechanism of a prototypal W(CNAr)6-catalyzed base-promoted homolytic aromatic substitution reaction. Among the many potential applications of these very bright luminophores, two-photon imaging and two-photon-initiated polymerization are ones we plan to pursue.

Effect of an amine functionalization of phenanthroline ligand associated with an ester-pyridine ligand on excited-state properties of Re(I) complex. Interplay between two 3MLCT

In this work, the synthesis, characterization and photophysical properties of rhenium(I) polypyridyl compound, fac-[Re(et-isonic)(NH2phen)(CO)3]PF6, where NH2phen = 5-amino-1,10-phenanthroline and et-isonic = ethyl isonicotinate, are reported. The compound exhibited a broad absorption band around 300–500 nm that by Time-Dependent Density Functional Theory (TD-DFT) corresponds to a mixture of ILNH2phen and MLCTRe→NH2phen transitions and a contribution of the charge transfer transition MLCTRe→et-isonic. Contrary to what is expected, the change of Cl (λmax = 490 and 565 nm; ϕ = 0.002; τ = 0.49 ± 0.07 μs and < 0.020 μs) for et-isonic ligand (λmax = 590 nm; ϕ = 0.003; τ = 0.43 ± 0.03 μs and 0.044 ± 0.014 μs), a π-acceptor ligand, promoted a bathochromic shift of the emission maxima in acetonitrile solution. This distinct behavior can be rationalized in terms of the inversion of the lowest-lying excited state of the usual observed MLCTRe→NN to the unusual MLCTRe→et-isonic. On the other hand, the contribution of ligand-based excited state in the deactivation process cannot be ruled out. Additionally, as a triplet emitter, a high quantum yield for the singlet oxygen generation (0.50 ± 0.03) can be associated with either, 3MLCT or a 3LC state. The photophysics ally to the higher singlet oxygen generation reported herein provides new fundamental insights into the understanding of substituent groups on the polypyridyl ligands that are relevant to the practical development of the scientific field.

Theoretical Measurements of Quantitative Effects Caused by Spectator Ligands on Palladium-catalyzed C–H Activation

Ligands can definitely influence C–H activation at the metal center. A ligand not directly participating in the reaction is called a spectator ligand. We attempt to quantitatively characterize the effects of diverse spectator ligands on C-H activation at palladium. We designed a model palladium catalyst and selected an array of spectator ligands, such as methoxyl, amide, methyl, phenyl, cyanide, fluorine, chlorine, and several neutral ligands, and performed density functional theory calculations on the mechanism and energetics of C–H activation reactions of benzene with different catalysts. Univalent ligands have substantially larger effects than neutral ligands, and strongly σ-donating ligands (e.g., methyl and phenyl) severely hinder the C-H activation in progress. A ligand trans to the reaction site influences C–H activation more than that cis to the reaction site, indicating electronic effects to be at work. For example, the existence of a methyl ligand raises the barrier height of C-H activation by 6.4 or 14.4 kcal/mol when it is placed at the position cis or trans to the C–H activation site. The effects of poorly σ-donating ligands are not significant and similar to those of the κ1-acetate ligand. Some σ-donating and π-accepting ligands, such as cyanide and isonitrile, hinder the C–H activation trans to them but appear to facilitate the C–H activation cis to them. On the basis of molecular orbital analyses, a chemical model is proposed to understand the observed ligand effects. Lastly, the conclusions are applied to explain the plausible mechanism of the dehydrogenative Heck coupling.

Added Complexity!-Mechanistic Aspects of Heterobimetallic Complexes for Application in Homogeneous Catalysis.

Inspired by multimetallic assemblies and their role in enzyme catalysis, chemists have developed a plethora of heterobimetallic complexes for application in homogeneous catalysis. Starting with small heterobimetallic complexes with σ-donating and π-accepting ligands, such as N-heterocyclic carbene and carbonyl ligands, more and more complex systems have been developed over the past two decades. These systems can show a significant increase in catalytic activity compared with their monometallic counterparts. This increase can be attributed to new reaction pathways enabled by the presence of a second metal center in the active catalyst. This review focuses on mechanistic aspects of heterobimetallic complexes in homogeneous catalysis. Depending on the type of interaction of the second metal with the substrates, heterobimetallic complexes can be subdivided into four classes. Each of these classes is illustrated with multiple examples, showcasing the versatility of both, the types of interactions possible, and the reactions accessible.

Two-Electron-Induced Reorganization of Cobalt Coordination and Metal-Ligand Cooperative Redox Shifting Co(I) Reactivity toward CO2 Reduction.

Electrochemical reorganization of complex structures is directly related to catalytic reactivity; thus, the geometric changes of catalysts induced by electron transfer should be considered to scrutinize the reaction mechanism. Herein, we studied electron-induced reorganization patterns of six-coordinate Co complexes with neutral N-donor ligands. Upon two-electron transfer into a Co center enclosed within a bulky π-acceptor ligand, the catalytic site exhibited different reorganization patterns depending on the ligand characteristics. While a bipyridyl ligand released Co-bound solvent (CH3CN) to open a reaction site, a phenanthroline ligand caused Co-Narm (side "arm" of NNN-ligand) bond dissociation. The first electron transfer occurred in the Co(II/I) reduction step and the second electron entered the bulky π-acceptor, of which redox steps were assigned from cyclic voltammograms, magnetic moment measurements, and DFT calculations. In comparison, the Co complex of [NNNNCH3-Co(CH3CN)3](PF6)2 ([1-(CH3CN)3](PF6)2) showed a high H2 evolution reactivity (HER), whereas a series of Co complexes with bulky π-acceptors such as [NNNNCH3-Co(L)(CH3CN)](PF6)2 (L = phen ([2-CH3CN](PF6)2), bpy ([3-CH3CN](PF6)2), [NNNNCH3-Co(tpy)](PF6)2 ([4](PF6)2), and [NNNCH2-Co(phen)(CH3CN)](PF6)2 ([5-CH3CN](PF6)2)) suppressed the HER but rather enhanced the CO2 reduction reaction. The metal-ligand cooperative redox steps enabled the shift of Co(I) reactivity toward CO2 reduction. Additionally, the amine pendant attached to the NNNNCH3-ligand could stabilize the CO2 reduction intermediate through the hydrogen-bonding interaction with the Co-CO2H adduct.

PTABS: A Unique Water-Soluble π-Acceptor Caged Phosphine

AbstractCaged phosphines have unique structures and provide many advantageous properties that can be fine-tuned to develop efficient catalytic systems. Our research group recently introduced a highly water-soluble caged phosphine: PTABS (KapdiPhos), which is a derivatized form of triazaphosphaadamantane, and explored its applicability as a strongly π-accepting ligand in combination with metals such as Pd or Cu in a variety of cross-coupling reactions of biologically relevant halonucleosides as well as chloroheteroarenes. This account details our journey from ideation to the various catalytic applications of the ligands and eventually to its commercialization.1 Introduction2 Derivatization of PTA to PTABS and Its Applications2.1 Nucleoside Modification2.2 Heteroarene Modification3 Conclusion4 Summary and Future Outlook

Diphenylacetylene stabilised alkali-metal nickelates: synthesis, structure and catalytic applications.

Whilst low-valent nickelates have recently been proposed as intermediates in Ni-catalysed reactions involving polar organometallics, their isolation and characterisation is often challenging due to their high sensitivity and reactivity. Advancing the synthetic, spectroscopic and structural insights of these heterobimetallic systems, here we report a new family of alkyne supported alkali-metal nickelates of the formula Li4(solv)n(Ar)4Ni2{μ2:η2,η2-Ph-CC-Ph} (where solv = Et2O, THF; Ar = Ph, o-Tol, naphthyl, 4-tBu-C6H4) which can be accessed through the combination of Ni(COD)2, Ph-CC-Ph and the relevant lithium aryl in a 2 : 1 : 4 ratio. Demonstrating the versatility of this approach, the sodium and potassium nickelates can also be accessed when using PhNa or via alkali-metal exchange with AMOtBu (AM = Na, K). When employing bulky or structurally constrained aryl-lithiums, mononickel complexes of the formula Li2(solv)n(Ar)2Ni{η2-Ph-CC-Ph} are instead obtained, highlighting the structural diversity of alkali-metal nickelates bearing alkyne ligands. Expanding the catalytic potential of these systems, their ability to promote the catalytic cyclotrimerisation of diphenylacetylene to hexaphenylbenzene was explored, with mononickel compounds bearing electron rich aryl-substituents displaying the best performance.

Spin-State Splittings in 3d Transition-Metal Complexes Revisited: Benchmarking Approximate Methods for Adiabatic Spin-State Energy Differences in Fe(II) Complexes.

The CASPT2+δMRCI composite approach reported in a companion paper has been extended and used to provide high-quality reference data for a series of adiabatic spin gaps (defined as ΔE = Equintet - Esinglet) of [FeIIL6]2+ complexes (L = CNH, CO, NCH, NH3, H2O), either at nonrelativistic level or including scalar relativistic effects. These highly accurate data have been used to evaluate the performance of various more approximate methods. Coupled-cluster theory with singles, doubles, and perturbative triples, CCSD(T), is found to agree well with the new reference data for Werner-type complexes but exhibits larger underestimates by up to 70 kJ/mol for the π-acceptor ligands, due to appreciable static correlation in the low-spin states of these systems. Widely used domain-based local CCSD(T) calculations, DLPNO-CCSD(T), are shown to depend very sensitively on the cutoff values used to construct the localized domains, and standard values are not sufficient. A large number of density functional approximations have been evaluated against the new reference data. The B2PLYP double hybrid gives the smallest deviations, but several functionals from different rungs of the usual ladder hierarchy give mean absolute deviations below 20 kJ/mol. This includes the B97-D semilocal functional, the PBE0* global hybrid with 15% exact-exchange admixture, as well as the local hybrids LH07s-SVWN and LH07t-SVWN. Several further functionals achieve mean absolute errors below 30 kJ/mol (M06L-D4, SSB-D, B97-1-D4, LC-ωPBE-D4, LH12ct-SsirPW92-D4, LH12ct-SsifPW92-D4, LH14t-calPBE-D4, LHJ-HFcal-D4, and several further double hybrids) and thereby also still overall outperform CCSD(T) or uncorrected CASPT2. While exact-exchange admixture is a crucial factor in favoring high-spin states, the present evaluations confirm that other aspects can be important as well. A number of the better-performing functionals underestimate the spin gaps for the π-acceptor ligands but overestimate them for L = NH3, H2O. In contrast to a previous suggestion, non-self-consistent density functional theory (DFT) computations on top of Hartree-Fock orbitals are not a promising path to produce accurate spin gaps in such complexes.

Trigonal Copper(I) Complexes with Cyclic (Alkyl)(amino)carbene Ligands for Single-Photon Near-IR Triplet Emission.

Molecular near-IR (NIR) triplet-state emitters are of importance for the development of new, organic-electronics-based telecommunication technologies as optical fibers operating in the corresponding spectral bands allow for data transfer over much longer distances due to the significantly lower attenuation. However, achieving such low-energy triplet excited states with good radiative rate constants is very challenging, and studies regarding the single-photon emission of organometallics in this energy range are scarce. We have prepared a series of trigonal CuI CAAC complexes bearing chelating ligands with O, N, S, and Se donor atoms and studied their photophysical properties in this context. The compounds show weak low-energy absorption in solution between 400 and 500 nm due to mixed Cu → CAAC 1MLCT/LLCT states, resulting in yellow-green to orange appearance, which we have also correlated to the 15N NMR resonances of the π-accepting carbene ligand. In the solid state, phosphorescence from dominant 3(Cu → CAAC) CT states is observed at room temperature. The emission of the complexes is bathochromically shifted in comparison to structurally related linearly coordinated copper(I) CAAC complexes due to structural reorganization in the excited state to a T-shape. For [Cu(dbm)(CAACMe)], the broad phosphorescence with outstanding λmax = 760 nm tailors out to ca. 1100 nm and leads to its proof-of-concept application as a nonclassical single-photon light source, constituting key functional units for the implementation of tap-proof data transfer.

Current Developments of N-Heterocyclic Carbene Au(I)/Au(III) Complexes toward Cancer Treatment.

Since their first discovery, N-heterocyclic carbenes have had a significant impact on organometallic chemistry. Due to their nature as strong σ-donor and π-acceptor ligands, they are exceptionally well suited to stabilize Au(I) and Au(III) complexes in biological environments. Over the last decade, the development of rationally designed NHCAu(I/III) complexes to specifically target DNA has led to a new “gold rush” in bioinorganic chemistry. This review aims to summarize the latest advances of NHCAu(I/III) complexes that are able to interact with DNA. Furthermore, the latest advancements on acyclic diamino carbene gold complexes with anticancer activity are presented as these typically overlooked NHC alternatives offer great additional design possibilities in the toolbox of carbene-stabilized gold complexes for targeted therapy.

Mesoionische Imine (MIIs): Starke Donoren und vielseitige Liganden für Übergangsmetalle und Hauptgruppensubstrate

AbstractWir 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.

Mesoionic Imines (MIIs): Strong Donors and Versatile Ligands for Transition Metals and Main Group Substrates.

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 MII−CO2 and −Rh(CO)2Cl 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 substituent‐induced fluorine‐specific interactions and undergo C−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.