Ligand Redox Noninnocence in [CoIII(TAML)]0/- Complexes Affects Nitrene Formation.

The redox noninnocence of the TAML scaffold in cobalt-TAML (tetra-amido macrocyclic ligand) complexes has been under debate since 2006. In this work, we demonstrate with a variety of spectroscopic measurements that the TAML backbone in the anionic complex [CoIII(TAMLred)]- is truly redox noninnocent and that one-electron oxidation affords [CoIII(TAMLsq)]. Multireference (CASSCF) calculations show that the electronic structure of [CoIII(TAMLsq)] is best described as an intermediate spin (S = 1) cobalt(III) center that is antiferromagnetically coupled to a ligand-centered radical, affording an overall doublet (S = 1/2) ground-state. Reaction of the cobalt(III)-TAML complexes with PhINNs as a nitrene precursor leads to TAML-centered oxidation and produces nitrene radical complexes without oxidation of the metal ion. The ligand redox state (TAMLred or TAMLsq) determines whether mono- or bis-nitrene radical complexes are formed. Reaction of [CoIII(TAMLsq)] or [CoIII(TAMLred)]- with PhINNs results in the formation of [CoIII(TAMLq)(N•Ns)] and [CoIII(TAMLq)(N•Ns)2]-, respectively. Herein, ligand-to-substrate single-electron transfer results in one-electron-reduced Fischer-type nitrene radicals (N•Ns-) that are intermediates in catalytic nitrene transfer to styrene. These nitrene radical species were characterized by EPR, XANES, and UV-vis spectroscopy, high-resolution mass spectrometry, magnetic moment measurements, and supporting CASSCF calculations.

Examination of Protonation-Induced Dinitrogen Splitting by in Situ EXAFS Spectroscopy.

The splitting of dinitrogen into nitride complexes emerged as a key reaction for nitrogen fixation strategies at ambient conditions. However, the impact of auxiliary ligands or accessible spin states on the thermodynamics and kinetics of N-N cleavage is yet to be examined in detail. We recently reported N-N bond splitting of a {Mo(µ2:η1:η1-N2)Mo}-complex upon protonation of the diphosphinoamide auxiliary ligands. The reactivity was associated with a low-spin to high-spin transition that was induced by the protonation reaction in the coordination periphery, mainly based on computational results. Here, this proposal is evaluated by an XAS study of a series of linearly N2 bridged Mo pincer complexes. Structural characterization of the transient protonation product by EXAFS spectroscopy confirms the proposed spin transition prior to N-N bond cleavage.

[FeFe]-Hydrogenase Mimic Employing κ2-C,N-Pyridine Bridgehead Catalyzes Proton Reduction at Mild Overpotential.

Two novel κ2-C,N-pyridine bridged [FeFe]-H2ase mimics (1 and 2) have been prepared and are shown to function as efficient molecular catalysts for electrocatalytic proton reduction. The elemental and structural composition of the complexes are confirmed by NMR and IR spectroscopy, high-resolution mass spectrometry and single-crystal X-ray diffraction. Electrochemical investigations reveal that the complexes reduce protons at their first reduction potential, resulting in the lowest overpotential (120 mV) ever reported for [FeFe]-H2ase mimics in proton reduction catalysis when mild acid (phenol) is used as proton source.

Electronically Asynchronous Transition States for C-N Bond Formation by Electrophilic [CoIII(TAML)]-Nitrene Radical Complexes Involving Substrate-to-Ligand Single-Electron Transfer and a Cobalt-Centered Spin Shuttle.

The oxidation state of the redox noninnocent tetra-amido macrocyclic ligand (TAML) scaffold was recently shown to affect the formation of nitrene radical species on cobalt(III) upon reaction with PhI=NNs [van Leest N. P.; J. Am. Chem. Soc.2020, 142, 552-563]. For the neutral [Co III (TAML sq )] complex, this leads to the doublet (S = 1/2) mono-nitrene radical species [Co III (TAML q )(N • Ns)(Y)] (bearing an unidentified sixth ligand Y in at least the frozen state), while a triplet (S = 1) bis-nitrene radical species [Co III (TAML q )(N • Ns) 2 ] - is generated from the anionic [Co III (TAML red )] - complex. The one-electron-reduced Fischer-type nitrene radicals (N•Ns-) are formed through single (mono-nitrene) or double (bis-nitrene) ligand-to-substrate single-electron transfer (SET). In this work, we describe the reactivity and mechanisms of these nitrene radical complexes in catalytic aziridination. We report that [Co III (TAML sq )] and [Co III (TAML red )] - are both effective catalysts for chemoselective (C=C versus C-H bonds) and diastereoselective aziridination of styrene derivatives, cyclohexane, and 1-hexene under mild and even aerobic (for [Co III (TAML red )] -) conditions. Experimental (Hammett plots; [Co III (TAML)]-nitrene radical formation and quantification under catalytic conditions; single-turnover experiments; and tests regarding catalyst decomposition, radical inhibition, and radical trapping) in combination with computational (density functional theory (DFT), N-electron valence state perturbation theory corrected complete active space self-consistent field (NEVPT2-CASSCF)) studies reveal that [Co III (TAML q )(N • Ns)(Y)], [Co III (TAML q )(N • Ns) 2 ] -, and [Co III (TAML sq )(N • Ns)] - are key electrophilic intermediates in aziridination reactions. Surprisingly, the electrophilic one-electron-reduced Fischer-type nitrene radicals do not react as would be expected for nitrene radicals (i.e., via radical addition and radical rebound). Instead, nitrene transfer proceeds through unusual electronically asynchronous transition states, in which the (partial) styrene substrate to TAML ligand (single-) electron transfer precedes C-N coupling. The actual C-N bond formation processes are best described as involving a nucleophilic attack of the nitrene (radical) lone pair at the thus (partially) formed styrene radical cation. These processes are coupled to TAML-to-cobalt and cobalt-to-nitrene single-electron transfer, effectively leading to the formation of an amido-γ-benzyl radical (NsN--CH2-•CH-Ph) bound to an intermediate spin (S = 1) cobalt(III) center. Hence, the TAML moiety can be regarded to act as a transient electron acceptor, the cobalt center behaves as a spin shuttle, and the nitrene radical acts as a nucleophile. Such a mechanism was hitherto unknown for cobalt-catalyzed hypovalent group transfer and the more general transition-metal-catalyzed nitrene transfer to alkenes but is now shown to complement the known concerted and stepwise mechanisms for N-group transfer.

Spectroscopic Investigation of the Activation of a Chromium-Pyrrolyl Ethene Trimerization Catalyst.

1-Hexene is an important α-olefin for polyethylene production and is produced from ethene. Recent developments in the α-olefin industry have led to the successful commercialization of ethene trimerization catalysts. An important industrially applied ethene trimerization system uses a mixture of chromium 2-ethylhexanoate, AlEt3, AlEt2Cl, and 2,5-dimethylpyrrole (DMP). Here, we have studied the activation of this system using catalytic and spectroscopic experiments (XAS, EPR, and UV-vis) under conditions employed in industry. First, chromium 2-ethylhexanoate was prepared and characterized to be [Cr3O(RCO2)6(H2O)3]Cl. Next, the activation of chromium 2-ethylhexanoate with AlEt3, AlEt2Cl, and DMP was studied, showing immediate reduction (<5 ms) of the trinuclear Cr(III) carboxylate and formation of a neutral polynuclear Cr(II) carboxylate complex. Over time, this Cr(II) carboxylate complex is partially converted into a chloro-bridged dinuclear Cr(II) pyrrolyl complex. In cyclohexane, small quantities of an unknown Cr(I) complex (∼1% after 1 h) are observed, while in toluene, the quantity of Cr(I) is much higher (∼23% after 1 h). This is due to the formation of cationic bis(tolyl)Cr(I) complexes, which likely leads to the observed inferior performance using toluene as the reaction solvent. Catalytic studies allow us to exclude some of the observed Cr(I) and Cr(II) complexes as the active species in this catalytic system. Using this combination of techniques, we have been able to structurally characterize complexes of this selective Cr-catalyzed trimerization system under conditions which are employed in industry.