Synthesis of a copper-supported triplet nitrene complex pertinent to copper-catalyzed amination

Chasing isolable late d-block metal nitrenes and imidyls

Imido complexes of late transition metals are of fundamental interest as transient species in bond activation catalysis. In this perspective article, we present late imido metal complexes in uncommon and highly reactive states, concentrating on structurally authenticated imido metal complexes with higher spin states and imidyl or even nitrene character. Further, the few examples of analogous metalla nitridyls and metallanitrenes are discussed. This work serves as an overview of the existing array of these compounds and provides insights into their unequivocal identification or ambiguity, offering an entry point for interested molecular organic/inorganic chemists to contribute to this area.

Switchable Closed-Shell and Open-Shell Biradical States in Bis-Palladium Complexes of Tetrathiadodecaphyrin via Coordination Rearrangement

A figure-eight tetrathiadodecaphyrin (1), featuring two porphyrin-like sub-pockets separated by central carbazolylenes was synthesized. Metalation of the thiaporphyrinoid ligand with Pd(OAc)2 produces two distinct bis-Pd(II) complexes with different coordination environments. Complex 2, adopting an {NNCS} metalation mode, exhibits a closed-shell electronic structure, whereas complex 3, with an {NNCC} coordination environment, exists as a ligand-centered organic biradicaloid with two magnetically independent spins (S = 1/2). Biradical formation is attributed to single-electron transfer from each ligand sub-pocket to the Pd(II) center accommodated in a d8 square-planner coordination geometry. Notably, the complexes are interconvertible through doubly one-electron redox processes, demonstrating a reversible metal coordination rearrangement via thiophene ring flipping within a porphyrinoid framework. This work establishes the first example of such tunable metal coordination, offering a precise strategy for modulating closed-shell and open-shell biradical states. In addition, while complex 2 displays intense absorption and photoacoustic responses to the first near-infrared (NIR-I) light in water after encapsulation within nanoparticles, the nanocomposites encapsulating biradicaloid 3 exhibits enhanced responsiveness in the second near-infrared (NIR-II) region.

An Open-Shell FeIV Nitrido

We report the photogeneration and characterization of an open-shell, terminal iron nitrido (EmL)Fe(N) using a sterically encumbered dipyrrin ligand environment. The Fe-N distance in the solid-state, zero-field 57Fe Mössbauer spectrum, and computational analysis are consistent with a triplet electronic ground state of the iron nitrido. Notably, the attenuation of Fe-N multiple bond character through occupying π*Fe-N enables (i) primary C(sp3)-H amination, (ii) H2 cleavage, (iii) aromatic C-C cleavage, and (iv) photocatalytic N-atom transfer reactivity. These modes of reactivity have not previously been observed in low-spin Fe(N) analogues.

Copper and Silver Trispyrazolylborate-Phosphinoazide Complexes: Synthesis, Characterization, and Nitrene Generation

Phosphinoazide complexes of the composition TpBr3M-L (M = Cu, Ag, and L = 2-azido-1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaphosphole) have been synthesized and structurally characterized. Their thermal decomposition led to cyclodiphosphazenes as a result of the metal-mediated coupling of two nitrene units in a process that takes place in both a stoichiometric and catalytic manner. Experimental data have allowed proposing a mechanistic pathway for this new transformation.

Investigating the formation of metal nitride complexes employing a tetradentate bis-carbene bis-phenolate ligand

The synthesis of MnV and CrV nitride complexes of a pro-radical tetradentate bis-phenol bis-N-heterocyclic carbene ligand H2LC2O2 was investigated. Employing either azide photolysis of the MnIII precursor complex MnLC2O2(N3) or a nitride exchange reaction between MnLC2O2(Br) and the nitride exchange reagent Mnsalen(N) failed to provide a useful route to the target nitride MnLC2O2(N). Experimental results support initial formation of the target nitride MnLC2O2(N), however, the nitride rapidly inserts into a Mn-CNHC bond. A second insertion reaction results in the isolation of the doubly inserted ligand product [H2LC2O2(N)]+ in good yield. In contrast, the Cr analogue CrLC2O2(N) was readily prepared and characterized by a number of experimental methods, including X-ray crystallography. Theoretical calculations predict a lower transition state energy for nitride insertion into the M-CNHC bond for Mn in comparison to Cr, and in addition the N-inserted product is stabilized for Mn while destabilized for Cr. Natural bond order (NBO) analysis predicts that the major bonding interaction (π MN → σ* M-CNHC) promotes nucleophilic attack of the nitride on the carbene as the major reaction pathway. Finally, one-electron oxidation of CrLC2O2(N) affords a relatively stable cation that is characterized by experimental and theoretical analysis to be a metal-oxidized d0 CrVI species.

Isolation and characterization of a triplet nitrene

Nitrene radical compounds are short-lived intermediates in a variety of nitrogen-involved transformations. They feature either a singlet or a triplet ground state, depending on the electronic properties of the substituents. Triplet nitrenes are highly reactive and their isolation in the condensed phase under ambient conditions is challenging. Here we report the synthesis and isolation of a triplet arylnitrene supported by a bulky hydrindacene ligand. The arylnitrene is fully characterized by various spectroscopic and structural techniques including electron paramagnetic resonance spectroscopy and single-crystal X-ray diffraction. Its high stability is largely attributed to the steric hindrance and effective electron delocalization provided by the supporting ligand. Electron paramagnetic resonance spectroscopy in conjunction with highly correlated wavefunction-based ab initio calculations provides support for a triplet ground state nitrene with axial zero-field splitting D = 0.92 cm-1 and vanishing rhombicity E/D = 0.002.

High-temperature carbon dioxide capture in a porous material with terminal zinc hydride sites

Carbon capture can mitigate point-source carbon dioxide (CO2) emissions, but hurdles remain that impede the widespread adoption of amine-based technologies. Capturing CO2 at temperatures closer to those of many industrial exhaust streams (>200°C) is of interest, although metal oxide absorbents that operate at these temperatures typically exhibit sluggish CO2 absorption kinetics and instability to cycling. Here, we report a porous metal-organic framework featuring terminal zinc hydride sites that reversibly bind CO2 at temperatures above 200°C-conditions that are unprecedented for intrinsically porous materials. Gas adsorption, structural, spectroscopic, and computational analyses elucidate the rapid, reversible nature of this transformation. Extended cycling and breakthrough analyses reveal that the material is capable of deep carbon capture at low CO2 concentrations and high temperatures relevant to postcombustion capture.

C-H Insertion from Isolable Copper Benzylidenes

Despite the utility of copper catalysts for the insertion of carbene moieties into C-H bonds, the copper carbene intermediate often invoked in these transformations has not been isolated. Herein, we describe the synthesis and structural characterization of a series of copper benzylidenes utilizing the sterically encumbered dipyrrin ligand (EmL)H. These isolated copper carbenes demonstrate intramolecular insertion into the primary C(sp3)-H bond of the ligand (EmL)H and intermolecular insertion into ethereal and allylic C-H bonds. The copper carbenes isolated are best described as Cu(I) carbene adducts akin to canonical Fischer carbenes, given their diamagnetic ground state and electrophilic carbene reactivity. Furthermore, the insertion chemistry can be rendered catalytic utilizing a more sterically exposed dipyrrin ligand (ArFL)H. The ability to isolate and observe stoichiometric C-H insertion and olefin cyclopropanation from well-characterized copper benzylidenes illuminates their viability as catalytic intermediates and their participation in potential catalyst deactivation pathways.

C=C Dissociative Imination of Styrenes by a Photogenerated Metallonitrene

Photolysis of a platinum(II) azide complex in the presence of styrenes enables C=C double bond cleavage upon dissociative olefin imination to aldimido (PtII-N=CHPh) and formimido (PtII-N=CH2) complexes as the main products. Spectroscopic and quantum chemical examinations support a mechanism that commences with the decay of the metallonitrene photoproduct (PtII-N) via bimolecular coupling and nitrogen loss as N2. The resulting platinum(I) complex initiates a radical chain mechanism via a dinuclear radical-bridged species (PtII-CH2CHPhN•-PtII) as a direct precursor to C-C scission. The preference for the PtI mediated route over styrene aziridination is attributed to the distinct nucleophilicity of the triplet metallonitrene.

Synthesis and Characterization of a Copper Dinitrogen Complex Supported by a Weakly Coordinating Anion

We report the synthesis and full characterization of the copper dinitrogen complex [(η1-N2)Cu{Al(ORF)4}] 2 (RF=C(CF3)3) prepared by a cascade metathesis reaction of Ag[Al(ORF)4] with CuI-excess in iso-perfluorohexane (i-pfh) under N2 atmosphere. Title compound 2 features an extraordinarily high N2 stretching frequency at 2313/2314 cm-1 (IR/Raman) and was characterized by single-crystal and powder X-ray diffractometry. Quantum chemical charge displacement analysis based on natural orbitals of chemical valence (CD-NOCV) indicates that the copper-dinitrogen interaction is still governed by weak π-backdonation, but is significantly reduced compared to all literature-known transition metal dinitrogen complexes.

Analyzing the Intensities of K-Edge Transitions in X2 Molecules (X = F, Cl, Br) for Use in Ligand K-Edge X-ray Absorption Spectroscopy

Ligand K-edge X-ray absorption spectroscopy (XAS) is regularly used to determine the ligand contribution to metal-ligand bonds. For quantitative studies, the pre-edge transition intensities must be referenced to an intensity standard, and pre-edge intensities obtained from different ligand atoms cannot be compared without standardization due to different cross sections at each absorption edge. In this work, the intensities of the 1s → σ* transitions in F2, Cl2, and Br2 are analyzed for their use as references for ligand K-edge XAS. We show that the intensities of these transitions are equal to the intensities of the 1s → np transitions in the unbound halogens. This finding is supported by a comparison between the normalized experimental intensities for the molecules and the calculated oscillator strengths for the atoms. These results highlight the potential for these molecules to be used as intensity standards in F, Cl, and Br K-edge XAS experiments.

Solid Handmade Ternary Coinage Metal Mentors for Organic Bond-Making and Bond-Breaking Applications

A comparative approach is employed for the novel synthesis of a magnetically recoverable ternary nanocomposite consisting of g-C3N4-supported Fe3O4 decorated with coinage metals (Au, Ag, and Cu). This synthesis is achieved through a straightforward and convenient one-step grinding protocol. In situ, the nanoparticles were grown on the g-C3N4-assist Fe3O4 matrix (GCFM), and the agglomeration of these nanoparticles on the matrix creates a pathway for the formation of the nanocomposite (NC). The as-formed CNC was confirmed with the help of characterization analyses, namely XRD, FT-IR, HR-TEM, FE-SEM, XPS, VSM, UV-vis, and NMR studies. Together with NPs and GCFM, with the quantum consequence, the activity of the NC shows better electron transfer via transfer of electrons, which grabs tremendous attention toward it, resulting in enhanced plausible photocatalytic degradation toward pharmaceutical compounds, dyes, and anthropogenic pollutants. The activity of the C-NC hikes at 88% for ciprofloxacin (CX) and 90% for paracetamol (PM); furthermore, the activity of the C-NC hikes at 88% and 87% for xylene Cyanol FF (XCF) and malachite green (MLG), respectively. Interestingly, an added advantage is the formation of a C-C bond (homocoupling reaction) in phenylboronic acid (PA) via a greener solvent under ambient conditions. The yield percentage of the conversion product shows satisfactory results, and its reproducibility was good for the prepared ternary NC. The conversion treatment of anthropogenic pollutants, namely 4-nitrophenol, grasps a high percentage (98%). In addition, the NC shows good activity toward both types of bacteria. The reproducibility of the composite also shows virtuous activity against pharmaceutical as well as toxic contaminants. The as-prepared CNC was specifically engineered to perform both bond formation and bond cleavage of organic molecules under ambient conditions for multiple cycles.

Square-planar imido complexes of cobalt: synthesis, reactivity and computational study

Treatment of [Co(N2)(tBuPNP)] (tBuPNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole) with one equivalent of an aryl azide generates the four-coordinate imido complexes [Co(NAr)(tBuPNP)] (Ar = mesityl, phenyl, or 4-tBu-phenyl). X-ray crystallographic analysis of the compounds shows an unusual square-planar geometry about cobalt with nearly linear imido units. In the presence of the hydrogen atom donor, TEMPOH, [Co(NPh)(tBuPNP)] undergoes addition of the H atom to the imido nitrogen to generate the corresponding amido complex, [Co(NHPh)(tBuPNP)], whose structure and composition were verified by independent synthesis. Despite the observation of H atom transfer reactivity with TEMPOH, the imido complexes do not show catalytic activity for C-H amination or aziridination for several substrates examined. In the case of [Co(NPh)(tBuPNP)], addition of excess azide produced the tetrazido complex, [Co(N4Ph2)(tBuPNP)], whose bond metrics were most consistent with an anionic Ph2N4 ligand. Density Functional Theory (DFT) investigations of the imido and tetrazido species suggest that they adopt a ground state best described as possessing a low-spin cobalt(II) ion ferromagnetically coupled to an iminyl radical.

Synthesis of a stable crystalline nitrene

Nitrenes are a highly reactive, yet fundamental, compound class. They possess a monovalent nitrogen atom and usually a short life span, typically in the nanosecond range. Here, we report on the synthesis of a stable nitrene by photolysis of the arylazide MSFluindN3 (1), which gave rise to the quantitative formation of the arylnitrene MSFluindN (2) (MSFluind is dispiro[fluorene-9,3'-(1',1',7',7'-tetramethyl-s-hydrindacen-4'-yl)-5',9''-fluorene]) that remains unchanged for at least 3 days when stored under argon atmosphere at room temperature. The extraordinary life span permitted the full characterization of 2 by single-crystal x-ray crystallography, electron paramagnetic resonance spectroscopy, and superconducting quantum interference device magnetometry, which supported a triplet ground state. Theoretical simulations suggest that in addition to the kinetic stabilization conferred by the bulky MSFluind aryl substituent, electron delocalization across the central aromatic ring contributes to the electron stabilization of 2.

Terminal Copper Nitrenoid Formation and Reactivity Induced by Absorption to an Antenna Ligand

Copper nitrenoids are key intermediates in copper-catalyzed direct C-H amination reactions. Further development of this important reaction relies on knowing the properties and reactivity of the nitrenoid intermediates. This work utilizes antenna ligands to form copper nitrenoid complexes and monitor the consecutive C-H amination reactions under well-defined single-molecule conditions in the gas phase. The [Cu(Lphoto)(Lazide)]+ precursors (Lphoto is a bidentate antenna ligand, and Lazide is an organic azide) were stored in an ion trap at 3.5 K and irradiated by visible light, which resulted in denitrogenation of the complex. Further irradiation of the copper nitrenoid led to the consecutive C-H amination of the antenna ligand. The nitrenoid complexes, as well as the products of the C-H amination, were characterized by helium tagging IRPD spectroscopy, and the mechanism was described by DFT calculations. This research demonstrates that the antenna ligands can be used to promote the denitrogenation of metal azides in the gas phase and also channel the internal energy to promote further reactivity, which opens a new way to study the reactivity of highly reactive species under well-defined conditions.

Lewis Acid Supported Nickel Nitrenoids

Metalation of the polynucleating ligand F,tbs LH6 (1,3,5-C6 H9 (NC6 H3 -4-F-2-NSiMe2 t Bu)3 ) with two equivalents of Zn(N(SiMe3 )2 )2 affords the dinuclear product (F,tbs LH2 )Zn2 (1), which can be further deprotonated to yield (F,tbs L)Zn2 Li2 (OEt2 )4 (2). Transmetalation of 2 with NiCl2 (py)2 yields the heterometallic, trinuclear cluster (F,tbs L)Zn2 Ni(py) (3). Reduction of 3 with KC8 affords [KC222 ][(F,tbs L)Zn2 Ni] (4) which features a monovalent Ni centre. Addition of 1-adamantyl azide to 4 generates the bridging μ3 -nitrenoid adduct [K(THF)3 ][(F,tbs L)Zn2 Ni(μ3 -NAd)] (5). EPR spectroscopy reveals that the anionic cluster possesses a doublet ground state (S =1 / 2 ${{ 1/2 }}$ ). Cyclic voltammetry of 5 reveals two fully reversible redox events. The dianionic nitrenoid [K2 (THF)9 ][(F,tbs L)Zn2 Ni(μ3 -NAd)] (6) was isolated and characterized while the neutral redox isomer was observed to undergo both intra- and intermolecular H-atom abstraction processes. Ni K-edge XAS studies suggest a divalent oxidation state for the Ni centres in both the monoanionic and dianionic [Zn2 Ni] nitrenoid complexes. However, DFT analysis suggests Ni-borne oxidation for 5.

Photoinduced Metallonitrene Formation by N2 Elimination from Azide Diradical Ligands

Transition-metal nitrides/nitrenes are highly promising reagents for catalytic nitrogen-atom-transfer reactivity. They are typically prepared in situ upon optically induced N2 elimination from azido precursors. A full exploitation of their catalytic potential, however, requires in-depth knowledge of the primary photo-induced processes and the structural/electronic factors mediating the N2 loss with birth of the terminal metal-nitrogen core. Using femtosecond infrared spectroscopy, we elucidate here the primary molecular-level mechanisms responsible for the formation of a unique platinum(II) nitrene with a triplet ground state from a closed-shell platinum(II) azide precursor. The spectroscopic data in combination with quantum-chemical calculations provide compelling evidence that product formation requires the initial occupation of a singlet excited state with an anionic azide diradical ligand that is bound to a low-spin d8 -configured PtII ion. Subsequent intersystem crossing generates the Pt-bound triplet azide diradical, which smoothly evolves into the triplet nitrene via N2 loss in a near barrierless adiabatic dissociation. Our data highlight the importance of the productive, N2 -releasing state possessing azide ππ* character as a design principle for accessing efficient N-atom-transfer catalysts.

Nitrene transfer from a sterically confined copper nitrenoid dipyrrin complex

Despite the myriad Cu-catalyzed nitrene transfer methodologies to form new C-N bonds (e.g., amination, aziridination), the critical reaction intermediates have largely eluded direct characterization due to their inherent reactivity. Herein, we report the synthesis of dipyrrin-supported Cu nitrenoid adducts, investigate their spectroscopic features, and probe their nitrene transfer chemistry through detailed mechanistic analyses. Treatment of the dipyrrin CuI complexes with substituted organoazides affords terminally ligated organoazide adducts with minimal activation of the azide unit as evidenced by vibrational spectroscopy and single crystal X-ray diffraction. The Cu nitrenoid, with an electronic structure most consistent with a triplet nitrene adduct of CuI, is accessed following geometric rearrangement of the azide adduct from κ1-N terminal ligation to κ1-N internal ligation with subsequent expulsion of N2. For perfluorinated arylazides, stoichiometric and catalytic C-H amination and aziridination was observed. Mechanistic analysis employing substrate competition reveals an enthalpically-controlled, electrophilic nitrene transfer for primary and secondary C-H bonds. Kinetic analyses for catalytic amination using tetrahydrofuran as a model substrate reveal pseudo-first order kinetics under relevant amination conditions with a first-order dependence on both Cu and organoazide. Activation parameters determined from Eyring analysis (ΔH‡ = 9.2(2) kcal mol-1, ΔS‡ = -42(2) cal mol-1 K-1, ΔG‡298K = 21.7(2) kcal mol-1) and parallel kinetic isotope effect measurements (1.10(2)) are consistent with rate-limiting Cu nitrenoid formation, followed by a proposed stepwise hydrogen-atom abstraction and rapid radical recombination to furnish the resulting C-N bond. The proposed mechanism and experimental analysis are further corroborated by density functional theory calculations. Multiconfigurational calculations provide insight into the electronic structure of the catalytically relevant Cu nitrene intermediates. The findings presented herein will assist in the development of future methodology for Cu-mediated C-N bond forming catalysis.

Ruthenium(v) terminal arylimido corroles: isolation, spectroscopic characterization and reactivity

Terminal Ru(v)-imido species are thought to be as reactive to group transfer reactions as their Ru(v)-oxo homologues, but are less studied. With the electron-rich corrole ligand, relatively stable and isolable Ru(v)-arylimido complexes [Ru(tBu-Cor)(NAr)] (H3(tBu-Cor) = 5,15-diphenyl-10-(p-tert-butylphenyl)corrole, Ar = 2,4,6-Me3C6H2 (Mes), 2,6-(iPr)2C6H3 (Dipp), 2,4,6-(iPr)3C6H2 (Tipp), and 3,5-(CF3)2C6H3 (BTF)) can be prepared from [Ru(tBu-Cor)]2 under strongly reducing conditions. This type of Ru(v)-monoarylimido corrole complex with S = ½ was characterized by high-resolution ESI mass spectrometry, X-band EPR, resonance Raman spectroscopy, magnetic susceptibility, and elemental analysis, together with computational studies. Under heating/light irradiation (xenon lamp) conditions, the complexes [Ru(tBu-Cor)(NAr)] (Ar = Mes, BTF) could undergo aziridination of styrenes and amination of benzylic C(sp3)-H bonds with up to 90% product yields.

A low-coordinate iron organoazide complex

A labile organoazide iron complex is reported. Under ambient conditions, the azide adduct is subject to a dissociation equilibrium in solution, yet also undergoes intramolecular C-H bond amination. Single-crystal irradiation of the azide at 80 K leads to partial N2-extrusion and formation of a putative imido iron intermediate, which was computationally identified as a highly covalent {FeNR}8 species.

Mechanistic snapshots of rhodium-catalyzed acylnitrene transfer reactions

Rhodium acylnitrene complexes are widely implicated in catalytic C-H amidation reactions but have eluded isolation and structural characterization. To overcome this challenge, we designed a chromophoric octahedral rhodium complex with a bidentate dioxazolone ligand, in which photoinduced metal-to-ligand charge transfer initiates catalytic C-H amidation. X-ray photocrystallographic analysis of the Rh-dioxazolone complex allowed structural elucidation of the targeted Rh-acylnitrenoid and provided firm evidence that the singlet nitrenoid species is primarily responsible for acylamino transfer reactions. We also monitored in crystallo reaction of a nucleophile with the in situ generated Rh-acylnitrenoid, providing a crystallographically traceable reaction system to capture mechanistic snapshots of nitrenoid transfer.

Iron-Catalyzed Intermolecular C-N Cross-Coupling Reactions via Radical Activation Mechanism

A concept for intermolecular C-N cross-coupling amination has been discovered using tetrazoles and aromatic and aliphatic azides with boronic acids under iron-catalyzed conditions. The amination follows an unprecedented metalloradical activation mechanism that is different from traditional metal-catalyzed C-N cross-coupling reactions. The scope of the reaction has been demonstrated by the employment of a large number of tetrazoles, azides, and boronic acids. Moreover, several late-stage aminations and a short synthesis of a drug candidate have been showcased for further synthetic utility. Collectively, this iron-catalyzed C-N cross-coupling should have wide applications in the context of medicinal chemistry, drug discovery, and pharmaceutical industries.

Scrutinizing formally NiIV centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory

Nickel K- and L2,3-edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L2,3-edge XAS reveals that the physical d-counts of the formally NiIV compounds measured lie well above the d6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF62- is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d6 NiIV center. The reactivity of NiIV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers.

Copper-catalysed difluorocarbene transfer enables modular synthesis

The use of metal catalysts to produce and control the reactivity of carbenes has long offered a powerful approach to organic synthesis; however, difluorocarbene transfer catalysed by metal is an outlier and remains a substantial challenge. In that context, copper difluorocarbene chemistry has been elusive so far. Here we report the design, synthesis, characterization and reactivity of isolable copper(I) difluorocarbene complexes, which enable the development of a copper-catalysed difluorocarbene transfer reaction. The method offers a strategy for the modular synthesis of organofluorine compounds from simple and readily available components. This strategy facilitates a modular difluoroalkylation by coupling difluorocarbene with two inexpensive feedstocks, silyl enol ethers and allyl/propargyl bromides, in a one-pot reaction via copper catalysis, providing a diversity of difluoromethylene-containing products without laborious multistep synthesis. The approach enables access to various fluorinated skeletons of medicinal interest. Mechanistic and computational studies consistently reveal a mechanism involving nucleophilic addition to an electrophilic copper(I) difluorocarbene.

Prospects and challenges for nitrogen-atom transfer catalysis

Conversion of C-H bonds to C-N bonds via C-H amination promises to streamline the synthesis of nitrogen-containing compounds. Nitrogen-group transfer (NGT) from metal nitrenes ([M]-NR complexes) has been the focus of intense research and development. By contrast, potentially complementary nitrogen-atom transfer (NAT) chemistry, in which a terminal metal nitride (an [M]-N complex) engages with a C-H bond, is underdeveloped. Although the earliest examples of stoichiometric NAT chemistry were reported 25 years ago, catalytic protocols are only now beginning to emerge. Here, we summarize the current state of the art in NAT chemistry and discuss opportunities and challenges for its development. We highlight the synthetic complementarity of NGT and NAT and discuss critical aspects of nitride electronic structure that dictate the philicity of the metal-supported nitrogen atom. We also examine the characteristic reactivity of metal nitrides and present emerging strategies and remaining obstacles to harnessing NAT for selective, catalytic nitrogenation of unfunctionalized organic small molecules.

Structural and Spectroscopic Characterization of Copper(III) Complexes and Subsequent One-Electron Oxidation Reaction and Reactivity Studies

The formation of Cu(III) species are often invoked as the key intermediate in Cu-catalyzed organic transformation reactions. In this study, we synthesized Cu(II) (1) and Cu(III) (3) complexes supported by a bisamidate-bisalkoxide ligand consisting of an ortho-phenylenediamine (o-PDA) scaffold and characterized them through an array of spectroscopic techniques, including UV-visible, electron paramagnetic resonance, X-ray crystallography, and ¹H nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy. The Cu-N/O bond distances in 3 are ∼0.1 Å reduced compared to 1, implying a significant increase in 3's overall effective nuclear charge. Further, a Cu(III) complex (4) of a bisamidate-bisalkoxide ligand containing a trans-cyclohexane-1,2-diamine moiety exhibits nearly identical Cu-N/O bond distances to that of 3, inferring that the redox-active o-PDA backbone is not oxidized upon one-electron oxidation of the Cu(II) complex (1). In addition, a considerable difference in the 1s → 4p and 1s → 3d transition energy was observed in the X-ray absorption near-edge structure data of 3 vs 1, which is typical for the metal-centered oxidation process. Electrochemical measurements of the Cu(II) complex (1) in acetonitrile exhibited two consecutive redox couples at -0.9 and 0.4 V vs the Fc⁺/Fc reference electrode. One-electron oxidation reaction of 3 further resulted in the formation of a ligand-oxidized Cu complex (3a), which was characterized in depth. Reactivity studies of species 3 and 3a were explored toward the activation of the C-H/O-H bonds. A bond dissociation free energy (BDFE) value of ∼69 kcal/mol was estimated for the O-H bond of the Cu(II) complex formed upon transfer of hydrogen atom to 3. The study represents a thorough spectroscopic characterization of high-valent Cu complexes and sheds light on the PCET reactivity studies of Cu(III) complexes.

Iminyl-Radical-Mediated Formation of Covalent Au-N Bonds for Molecular Junctions

The interaction between organic radicals and transition metals plays a crucial role in radical-mediated chemical reactions, functional devices, and biocatalysis. Characterizing such interactions, however, remains a long-standing challenge due to the inherently high reactivity of radical species. Here, using a scanning tunneling microscope breaking junction (STM-BJ) technique, we are able to detect the interaction mode between iminyl radicals and the gold surface at a single molecule level. We show that the free iminyl radicals generated through photochemical N-O bond homolysis of oxime esters react toward the gold electrode surface and produce covalent Au-N bonds. Intriguingly, we find that the Au-N bonding reactions lead to the formation of robust and highly conductive single-molecule junctions. These findings provide not only insights into the mechanism of iminyl-radical-involved reactions but also a facile photolysis method to create a new type of covalent electrode-molecule bonding contact for molecular devices.

From Divalent to Pentavalent Iron Imido Complexes and an Fe(V) Nitride via N-C Bond Cleavage

As key intermediates in metal-catalyzed nitrogen-transfer chemistry, terminal imido complexes of iron have attracted significant attention for a long time. In search of versatile model compounds, the recently developed second-generation N-anchored tris-NHC chelating ligand tris-[2-(3-mesityl-imidazole-2-ylidene)-methyl]amine (TIMMN^Mes) was utilized to synthesize and compare two series of mid- to high-valent iron alkyl imido complexes, including a reactive Fe(V) adamantyl imido intermediate en route to an isolable Fe(V) nitrido complex. The chemistry toward the iron adamantyl imides was achieved by reacting the Fe(I) precursor [(TIMMN^Mes)Fe^I(N₂)]⁺ (1) with 1-adamantyl azide to yield the corresponding trivalent iron imide. Stepwise chemical reduction and oxidation lead to the isostructural series of low-spin [(TIMMN^Mes)Fe(NAd)]^0,1+,2+,3+ (2^Ad-5^Ad) in oxidation states II to V. The Fe(V) imide [(TIMMN^Mes)Fe(NAd)]³⁺ (5^Ad) is unstable under ambient conditions and converts to the air-stable nitride [(TIMMN^Mes)Fe^V(N)]²⁺ (6) via N-C bond cleavage. The stability of the pentavalent imide can be increased by derivatizing the nitride [(TIMMN^Mes)Fe^IV(N)]⁺ (7) with an ethyl group using the triethyloxonium salt Et₃OPF₆. This gives access to the analogous series of ethyl imides [(TIMMN^Mes)Fe(NEt)]^0,1+,2+,3+ (2^Et-5^Et), including the stable Fe(V) ethyl imide. Iron imido complexes exist in a manifold of different electronic structures, ultimately controlling their diverse reactivities. Accordingly, these complexes were characterized by single-crystal X-ray diffraction analyses, SQUID magnetization, and electrochemical methods, as well as ⁵⁷Fe Mössbauer, IR vibrational, UV/vis electronic absorption, multinuclear NMR, X-band EPR, and X-ray absorption spectroscopy. Our studies are complemented with quantum chemical calculations, thus providing further insight into the electronic structures of all complexes.

Engineering Catalysis within a Saturated In(III)-Based MOF Possessing Dynamic Ligand-Metal Bonding

Metal-organic frameworks have developed into a formidable heterogeneous catalysis platform in recent years. It is well established that thermolysis of coordinated solvents from MOF nodes can render highly reactive, coordinatively unsaturated metal complexes which are stabilized via site isolation and serve as active sites in catalysis. Such approaches are limited to frameworks featuring solvated transition-metal complexes and must be stable toward the formation of "permanent" open metal sites. Herein, we exploit the hemilability of metal-carboxylate bonds to generate transient open metal sites in an In(III) MOF, pertinent to In-centered catalysis. The transient open metal sites catalyze the Strecker reaction over multiple cycles without loss of activity or crystallinity. We employ computational and spectroscopic methods to confirm the formation of open metal sites via transient dissociation of In(III)-carboxylate bonds. Furthermore, the amount of transient open metal sites within the material and thus the catalytic performance can be temperature-modulated.

Non-innocent Role of the Halide Ligand in the Copper-Catalyzed Olefin Aziridination Reaction

In the context of copper-catalyzed nitrene transfer to olefins, many systems operate upon mixing a CuX salt (X = halide, OTf) and a polydentate N-based ligand, assuming that the X ligand is displaced from the coordination sphere toward a counterion position. Herein, we demonstrated that such general assumption should be in doubt since studies carried out with the well-defined copper(I) complexes (TTM)CuCl and [(TTM)Cu(NCMe)]PF6 (TTM = tris(triazolyl)methane ligand) demonstrate a dual behavior from a catalytic and mechanistic point of view that exclusively depends on the presence or absence of the chloride ligand bonded to the metal center. When coordinated, the turnover-limiting step corresponds to the formation of the carbon-nitrene bond, whereas in its absence, the highest barrier corresponds to the formation of the copper-nitrene intermediate.

Theoretical Exploration of Energy Transfer and Single Electron Transfer Mechanisms to Understand the Generation of Triplet Nitrene and the C(sp3)-H Amidation with Photocatalysts

Mechanistic explorations and kinetic evaluations were performed based on electronic structure calculations at the CASPT2//CASSCF level of theory, the Fermi's golden rule combined with the Dexter model, and the Marcus theory to unveil the key factors regulating the processes of photocatalytic C(sp³)-H amidation starting from the newly emerged nitrene precursor of hydroxamates. The highly reactive nitrene was found to be generated efficiently via a triplet-triplet energy transfer process and to be benefited from the advantages of hydroxamates with long-range charge-transfer (CT) excitation from the N-centered lone pair to the 3,5-bis(trifluoromethyl)benzoyl group. The properties of the metal-to-ligand charge-transfer (MLCT) state of photocatalysts, the functionalization of chemical moieties for substrates involved in the charge-transfer (CT) excitation, such as the electron-withdrawing trifluoromethyl group, and the energetic levels of singlet and triplet reaction pathways may regulate the reaction yield of C(sp³)-H amidation. Kinetic evaluations show that the triplet-triplet energy transfer is the main driving force of the reaction rather than the single electron transfer process. The effects of electronic coupling, molecular rigidity, and excitation energies on the energy transfer efficiency were further discussed. Finally, we investigated the inverted behavior of single-electron transfer, which is correlated unfavorably to the catalytic efficiency and amidation reaction. All theoretical explorations allow us to better understand the generation of nitrene with visible-light photocatalysts, to expand highly efficient substrate sources, and to broaden our scope of available photosensitizers for various cross-coupling reactions and the construction of N-heterocycles.

Metal-Support Interactions in Molecular Single-Site Cluster Catalysts

This study provides atomistic insights into the interface between a single-site catalyst and a transition metal chalcogenide support and reveals that peak catalytic activity occurs when edge/support redox cooperativity is maximized. A molecular platform MCo₆Se₈(PEt₃)₄(L)₂ (1-M, M = Cr, Mn, Fe, Co, Cu, and Zn) was designed in which the active site (M)/support (Co₆Se₈) interactions are interrogated by systematically probing the electronic and structural changes that occur as the identity of the metal varies. All 3d transition metal 1-M clusters display remarkable catalytic activity for coupling tosyl azide and tert-butyl isocyanide, with Mn and Co derivatives showing the fastest turnover in the series. Structural, electronic, and magnetic characterization of the clusters was performed using single crystal X-ray diffraction, ¹H and ³¹P nuclear magnetic resonance spectroscopy, electronic absorption spectroscopy, cyclic voltammetry, and computational methods. Distinct metal/support redox regimes can be accessed in 1-M based on the energy of the edge metal's frontier orbitals with respect to those of the cluster support. As the degree of electronic interaction between the edge and the support increases, a cooperative regime is reached wherein the support can deliver electrons to the catalytic site, increasing the reactivity of key metal-nitrenoid intermediates.

Umpolung in a Pair of Cobalt(III) Terminal Imido/Imidyl Complexes

Reaction of the CoI complex [(TIMMNmes )CoI ](PF6 ) (1) (TIMMNmes =tris-[2-(3-mesityl-imidazolin-2-ylidene)-methyl]amine) with mesityl azide yields the CoIII imide [(TIMMNmes )CoIII (NMes)](PF6 ) (2). Oxidation of 2 with [FeCp2 ](PF6 ) provides access to a rare CoIII imidyl [(TIMMNmes )Co(NMes)](PF6 )2 (3). Single-crystal X-ray diffractometry and EPR spectroscopy confirm the molecular structure of 3 and its S= 1 / 2 ground state. ENDOR, X-ray absorption spectroscopy and computational analyses indicate a ligand-based oxidation; thus, an imidyl-radical electronic structure for 3. Migratory insertion of one ancillary NHC to the imido ligand in 2 gives the CoI N-heterocyclic imine (4) within 12 h. Conversely, it takes merely 0.5 h for 3 to transform to the CoII congener (5). The migratory insertion in 2 occurs via a nucleophilic attack of the imido ligand at the NHC to give 4, whereas in 3, a nucleophilic attack of the NHC at the electrophilic imidyl ligand yields 5. The reactivity shunt upon oxidation of 2 to 3 confirms an umpolung of the imido ligand.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation

Oxidation of a series of Cr^V nitride salen complexes (Cr^VNSal^R) with different para-phenolate substituents (R = CF₃, tBu, NMe₂) was investigated to determine how the locus of oxidation (either metal or ligand) dictates reactivity at the nitride. Para-phenolate substituents were chosen to provide maximum variation in the electron-donating ability of the tetradentate ligand at a site remote from the metal coordination sphere. We show that one-electron oxidation affords Cr^VI nitrides ([Cr^VINSal^R]⁺; R = CF₃, tBu) and a localized Cr^V nitride phenoxyl radical for the more electron-donating NMe₂ substituent ([Cr^VNSal^NMe2]^•+). The facile nitride homocoupling observed for the Mn^VI analogues was significantly attenuated for the Cr^VI complexes due to a smaller increase in nitride character in the M≡N π* orbitals for Cr relative to Mn. Upon oxidation, both the calculated nitride natural population analysis (NPA) charge and energy of molecular orbitals associated with the {Cr≡N} unit change to a lesser extent for the Cr^V ligand radical derivative ([Cr^VNSal^NMe2]^•+) in comparison to the Cr^VI derivatives ([Cr^VINSal^R]⁺; R = CF₃, tBu). As a result, [Cr^VNSal^NMe2]^•+ reacts with B(C₆F₅)₃, thus exhibiting similar nucleophilic reactivity to the neutral Cr^V nitride derivatives. In contrast, the Cr^VI derivatives ([Cr^VINSal^R]⁺; R = CF₃, tBu) act as electrophiles, displaying facile reactivity with PPh₃ and no reaction with B(C₆F₅)₃. Thus, while oxidation to the ligand radical does not change the reactivity profile, metal-based oxidation to Cr^VI results in umpolung, a switch from nucleophilic to electrophilic reactivity at the terminal nitride.

Introducing the Catalytic Amination of Silanes via Nitrene Insertion

The direct functionalization of Si–H bonds by the nitrene insertion methodology is described. A copper(I) complex bearing a trispyrazolylborate ligand catalyzes the transfer of a nitrene group from PhI=NTs to the Si–H bond of silanes, disilanes, and siloxanes, leading to the exclusive formation of Si–NH moieties in the first example of this transformation. The process tolerates other functionalities in the substrate such as several C–H bonds and alkyne and alkene moieties directly bonded to the silicon center. Density functional theory (DFT) calculations provide a mechanistic interpretation consisting of a Si–H homolytic cleavage and subsequent rebound to the Si-centered radical.

Palladium Terminal Imido Complexes with Nitrene Character

Whereas triplet-nitrene complexes of the late transition metals are isolable and key intermediates in catalysis, singlet-nitrene ligands remain elusive. Herein we communicate three such palladium terminal imido complexes with singlet ground states. UV-vis-NIR electronic spectroscopy with broad bands up to 1400 nm as well as high-level computations (DFT, STEOM-CCSD, CASSCF/NEVPT2, EOS analysis) and reactivity studies suggest significant palladium(0) singlet-nitrene character. Although the aliphatic nitrene complexes proved to be too reactive for isolation in analytically pure form as a result of elimination of isobutylene, the aryl congener could be characterized by SC-XRD, elemental analysis, IR-, NMR spectroscopy, and HRMS. The complexes' distinguished ambiphilicity allows them to activate hexafluorobenzene, triphenylphosphine, and pinacol borane, catalytically dehydrogenate cyclohexene, and aminate ethylene via nitrene transfer at or below room temperature.

Reversible C-C Bond Cleavage of a Cobalt Diketimide into an Elusive Cobalt Aryl Nitrenoid Complex

The reactivity of (^Tr L)Co (^Tr L=5-mesityl-1,9-(trityl)dipyrrin) toward various aryl azides was examined to elucidate the electronic structure and reactivity of dipyrrinato cobalt aryl nitrenoid complexes. Herein, we demonstrate the synthesis of a Co^II diketimide complex [(^Tr L)Co(NC₆ F₅ )]₂ and its reversible C-C bond cleavage to yield a monomeric Co nitrenoid complex (^Tr L)Co(NC₆ F₅ ). Exposure of [(^Tr L)Co(NC₆ F₅ )]₂ to an excess amount of an H-atom donor cleanly affords the Co^II anilide complex (^Tr L)Co(NHC₆ F₅ ). The half-order decay of [(^Tr L)Co(NC₆ F₅ )]₂ via H-atom abstraction (HAA) reveals saturation kinetic behavior indicating a pre-equilibrium between [(^Tr L)Co(NC₆ F₅ )]₂ and (^Tr L)Co(NC₆ F₅ ) prior to HAA. Furthermore, (^Tr L)Co(NC₆ F₅ ) undergoes reductive coupling with another equivalent of azide to furnish the four-coordinate tetrazido complex (^Tr L)Co(κ² -N₄ (C₆ F₅ )₂ ), expulsion of a fluorine atom to afford (^Tr L)CoF, and N-group transfer reactivity to PPh₃ .

Nitrogen Atom Transfer Catalysis by Metallonitrene C-H Insertion: Photocatalytic Amidation of Aldehydes

C-H amination and amidation by catalytic nitrene transfer are well-established and typically proceed via electrophilic attack of nitrenoid intermediates. In contrast, the insertion of (formal) terminal nitride ligands into C-H bonds is much less developed and catalytic nitrogen atom transfer remains unknown. We here report the synthesis of a formal terminal nitride complex of palladium. Photocrystallographic, magnetic, and computational characterization support the assignment as an authentic metallonitrene (Pd-N) with a diradical nitrogen ligand that is singly bonded to PdII . Despite the subvalent nitrene character, selective C-H insertion with aldehydes follows nucleophilic selectivity. Transamidation of the benzamide product is enabled by reaction with N3 SiMe3 . Based on these results, a photocatalytic protocol for aldehyde C-H trimethylsilylamidation was developed that exhibits inverted, nucleophilic selectivity as compared to typical nitrene transfer catalysis. This first example of catalytic C-H nitrogen atom transfer offers facile access to primary amides after deprotection.

A Combined Spectroscopic and Computational Study on the Mechanism of Iron-Catalyzed Aminofunctionalization of Olefins Using Hydroxylamine Derived N-O Reagent as the "Amino" Source and "Oxidant"

Herein, we study the mechanism of iron-catalyzed direct synthesis of unprotected aminoethers from olefins by a hydroxyl amine derived reagent using a wide range of analytical and spectroscopic techniques (Mössbauer, Electron Paramagnetic Resonance, Ultra-Violet Visible Spectroscopy, X-ray Absorption, Nuclear Resonance Vibrational Spectroscopy, and resonance Raman) along with high-level quantum chemical calculations. The hydroxyl amine derived triflic acid salt acts as the “oxidant” as well as “amino” group donor. It activates the high-spin Fe(II) (S_t = 2) catalyst [Fe(acac)₂(H₂O)₂] (1) to generate a high-spin (S_t = 5/2) intermediate (Int I), which decays to a second intermediate (Int II) with S_t = 2. The analysis of spectroscopic and computational data leads to the formulation of Int I as [Fe(III)(acac)₂-N-acyloxy] (an alkyl-peroxo-Fe(III) analogue). Furthermore, Int II is formed by N–O bond homolysis. However, it does not generate a high-valent Fe(IV)(NH) species (a Fe(IV)(O) analogue), but instead a high-spin Fe(III) center which is strongly antiferromagnetically coupled (J = −524 cm^–1) to an iminyl radical, [Fe(III)(acac)₂-NH·], giving S_t = 2. Though Fe(NH) complexes as isoelectronic surrogates to Fe(O) functionalities are known, detection of a high-spin Fe(III)-N-acyloxy intermediate (Int I), which undergoes N–O bond cleavage to generate the active iron–nitrogen intermediate (Int II), is unprecedented. Relative to Fe(IV)(O) centers, Int II features a weak elongated Fe–N bond which, together with the unpaired electron density along the Fe–N bond vector, helps to rationalize its propensity for N-transfer reactions onto styrenyl olefins, resulting in the overall formation of aminoethers. This study thus demonstrates the potential of utilizing the iron-coordinated nitrogen-centered radicals as powerful reactive intermediates in catalysis.

A New Bis(thioether)-Dipyrrin N2S2 Ligand and Its Coordination Behaviors to Nickel, Copper and Zinc

Tetradentate N2S2 coordination platforms are widespread in biological system and have endowed the metalloenzymes and metalloproteins with abundant reactivities and functions. However, there have only three types of N2S2 scaffolds...

Electronic structure studies reveal 4f/5d mixing and its effect on bonding characteristics in Ce-imido and -oxo complexes

This study presents the role of 5d orbitals in the bonding, and electronic and magnetic structure of Ce imido and oxo complexes synthesized with a tris(hydroxylaminato) [((2-^tBuNO)C₆H₄CH₂)₃N]³⁻ (TriNO_x³⁻) ligand framework, including the reported synthesis and characterization of two new alkali metal-capped Ce oxo species. X-ray spectroscopy measurements reveal that the imido and oxo materials exhibit an intermediate valent ground state of the Ce, displaying hallmark features in the Ce L_III absorption of partial f-orbital occupancy that are relatively constant for all measured compounds. These spectra feature a double peak consistent with other formal Ce(iv) compounds. Magnetic susceptibility measurements reveal enhanced levels of temperature-independent paramagnetism (TIP). In contrast to systems with direct bonding to an aromatic ligand, no clear correlation between the level of TIP and f-orbital occupancy is observed. CASSCF calculations defy a conventional van Vleck explanation of the TIP, indicating a single-reference ground state with no low-lying triplet excited state, despite accurately predicting the measured values of f-orbital occupancy. The calculations do, however, predict strong 4f/5d hybridization. In fact, within these complexes, despite having similar f-orbital occupancies and therefore levels of 4f/5d hybridization, the d-state distributions vary depending on the bonding motif (Ce Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O vs. CeN) of the complex, and can also be fine-tuned based on varying alkali metal cation capping species. This system therefore provides a platform for understanding the characteristic nature of Ce multiple bonds and potential impact that the associated d-state distribution may have on resulting reactivity.

Ce(iv) complexes with multiple bonds display similar f⁰ fractions, but different f/d hybridization, 5d-orbital energies, and TIP levels.