The catalytic hydrofunctionalization of alkenes through radical-polar crossover metal hydrogen atom transfer (MHAT) offers a mild pathway for the introduction of functional groups in sterically congested environments. For M = Co, this reaction is often proposed to proceed through secondary alkylcobalt(IV) intermediates, which have not been characterized unambiguously. Here, we characterize a metastable (salen)Co-(isopropyl) cation, which is capable of forming C−O bonds with alcohols as proposed in the catalytic reaction. Electron nuclear double resonance (ENDOR) spectroscopy of this formally cobalt(IV) species establishes the presence of the cobalt−carbon bond, and accompanying DFT calculations indicate that the unpaired electron is localized on the cobalt center. Both experimental and computational studies show that the cobalt(IV)−carbon bond is stronger than the analogous bond in its cobalt(III) analogue, which is opposite of the usual oxidation state trend of bond energies. This phenomenon is attributable to an inverted ligand field that gives the bond Co δ− −C δ+ character and explains its electrophilic reactivity at the alkyl group. The inverted Co−C bond polarity also stabilizes the formally cobalt(IV) alkyl complex so that it is accessible at unusually low potentials. Even another cobalt(III) complex, [(salen)Co III ] + , is capable of oxidizing (salen)Co III (iPr) to the formally cobalt(IV) state. These results give insight into the electronic structure, energetics, and reactivity of a key reactive intermediate in oxidative MHAT catalysis.

We recently reported a reaction sequence that activates C−H bonds in simple arenes as well as the N−N triple bond in N 2 , delivering the aryl group to N 2 to form a new N−C bond (Nature 2020, 584, 221). This enables the transformation of abundant feedstocks (arenes and N 2 ) into N-containing organic compounds. The key N−C bond forming step occurs upon partial silylation of N 2 . However, the pathway through which reduction, silylation, and migration occurred was unknown. Here, we describe synthetic, structural, magnetic, spectroscopic, kinetic, and computational studies that elucidate the steps of this transformation. N 2 must be silylated twice at the distal N atom before aryl migration can occur, and sequential silyl radical and silyl cation addition is a kinetically competent pathway to a formally iron(IV)−NN(SiMe 3 ) 2 intermediate that can be isolated at low temperature. Kinetic studies show its first-order conversion to the migrated product, and DFT calculations indicate a concerted transition state for migration. The electronic structure of the formally iron(IV) intermediate is examined using DFT and CASSCF calculations, which reveal contributions from iron(II) and iron(III) resonance forms with oxidized NNSi 2 ligands. The depletion of electron density from the Fe-coordinated N atom makes it electrophilic enough to accept the incoming aryl group. This new pathway for the N−C bond formation offers a method for functionalizing N 2 using organometallic chemistry.

We investigate the utility of the driven molecular dynamics (DMD) approach to complex molecular vibrations by applying it to linear clusters with several degenerate vibrational modes and infrared (IR) intense combination bands. Here, the prominent features in N 4 H + and N 4 D + IR spectra, reported and described by others previously, have been characterized for the first time by DMD using recently published high-level potential and dipole moment surfaces. Namely, the calculations closely correlate the parallel proton stretch vibration in N 4 H + , at 750 cm −1 , with the one observed experimentally at 743 cm −1 . Second, the intense IRactive combination bands found in experimental spectra within 900−1100 cm −1 have been properly recovered by DMD at 950 cm −1 as strongly IR-active and confirmed as consisting of H + asymmetric stretch and N 2 •••N 2 intermolecular symmetric stretch modes. Furthermore, we show that certain combination bands involving overtone transitions may be recovered by DMD using a hard-driving regime, such as the 1409 cm −1 band measured in N 4 H + , revealed by DMD at 1375 cm −1 , and assigned to a progressive combination of the parallel H + stretch and two quanta of N 2 •••N 2 stretch, in agreement with quantum mechanical studies reported previously by others.

We recently reported a reaction sequence that activates C–H bonds in simple arenes as well as the N–N triple bond in N2, delivering the aryl group to N2 to form a new N–C bond. This enables the transformation of abundant feedstocks (arenes and N2) into N-containing organic compounds. The key N–C bond forming step occurs upon partial silylation of N2, which then can accept the aryl fragment. However, the pathway through which reduction, silylation, and migration occurred was unknown. Here we describe synthetic, structural, magnetic, spectroscopic, kinetic, and computational studies that elucidate the steps of this transformation. N2 must be silylated twice at the distal N atom before aryl migration can occur, and sequential silyl radical and silyl cation addition is a kinetically competent pathway to a formally iron(IV) intermediate with an NN(SiMe3)2 ligand. It can be isolated at low temperature. Kinetic studies show its first-order conversion to the migrated product, and DFT calculations indicate a concerted transition state for migration. The electronic structure of the formally iron(IV) intermediate is examined using DFT and CASSCF calculations, which reveal contributions from iron(II) and iron(III) resonance forms with oxidized NNSi2 ligands. The depletion of electron density from the Fe-coordinated N atom makes it electrophilic enough to accept the incoming aryl group. This unprecedented pathway for N–C bond formation offers a method for functionalizing N2 with organometallic chemistry.

SCS pincer ligands have an interesting combination of strong-field and weak-field donors that is also present in the nitrogenase active site. Here, we explore the electronic structures of iron(II) and iron(III) complexes with such a pincer ligand, bearing a monodentate phosphine, thiolate S donor, amide N donor, ammonia, or CO. The ligand scaffold features a protonresponsive thioamide site, and the protonation state of the ligand greatly influences the reduction potential of iron in the phosphine complex. The N-H bond dissociation free energy can be quantitated as 56 ± 2 kcal/mol. EPR spectroscopy and SQUID magnetometry measurements show that the iron(III) complexes with S and N as the fourth donors have an intermediate spin (S = 3/2) ground state with large zero field splitting, and X-ray absorption spectra show high Fe-S covalency. The Mössbauer spectrum changes drastically with the position of a nearby alkali metal cation in the iron(III) amido complex, and DFT calculations explain this phenomenon through a change between having the doubly-occupied orbital as dz2 or dyz, as the former is more influenced by the nearby positive charge.