Reactivity and Mössbauer Spectroscopic Characterization of an Fe(IV) Ketimide Complex and Reinvestigation of an Fe(IV) Norbornyl Complex

Thermolysis of Fe­(NC^tBu₂)₄ (<b>1</b>) for 8 h at 50 °C generates the mixed valent Fe­(III)/Fe­(II) bimetallic complex Fe₂(NC^tBu₂)₅ (<b>2</b>) in moderate yield. Also formed in this reaction are <i>tert</i>-butyl cyanide, isobutane, and isobutylene, the products of ketimide oxidation by the Fe⁴⁺ center. Reaction of <b>1</b> with 1 equiv of acetylacetone affords the Fe­(III) complex, Fe­(NC^tBu₂)₂(acac) (<b>3</b>), concomitant with formation of bis­(<i>tert</i>-butyl)­ketimine, <i>tert</i>-butyl cyanide, isobutane, and isobutylene. In addition, the Mössbauer spectra of <b>1</b> and its lower-valent analogues [Li­(12-crown-4)₂]­[Fe­(NC^tBu₂)₄] (<b>5</b>) and [Li­(THF)]₂[Fe­(NC^tBu₂)₄] (<b>6</b>) were recorded. We also revisited the chemistry of Fe­(1-norbornyl)₄ (<b>4</b>) to elucidate its solid-state molecular structure and determine its Mössbauer spectrum, for comparison with that recorded for <b>1</b>

Reactivity and Mössbauer Spectroscopic Characterization of an Fe(IV) Ketimide Complex and Reinvestigation of an Fe(IV) Norbornyl Complex

Thermolysis of Fe­(NC^tBu₂)₄ (<b>1</b>) for 8 h at 50 °C generates the mixed valent Fe­(III)/Fe­(II) bimetallic complex Fe₂(NC^tBu₂)₅ (<b>2</b>) in moderate yield. Also formed in this reaction are <i>tert</i>-butyl cyanide, isobutane, and isobutylene, the products of ketimide oxidation by the Fe⁴⁺ center. Reaction of <b>1</b> with 1 equiv of acetylacetone affords the Fe­(III) complex, Fe­(NC^tBu₂)₂(acac) (<b>3</b>), concomitant with formation of bis­(<i>tert</i>-butyl)­ketimine, <i>tert</i>-butyl cyanide, isobutane, and isobutylene. In addition, the Mössbauer spectra of <b>1</b> and its lower-valent analogues [Li­(12-crown-4)₂]­[Fe­(NC^tBu₂)₄] (<b>5</b>) and [Li­(THF)]₂[Fe­(NC^tBu₂)₄] (<b>6</b>) were recorded. We also revisited the chemistry of Fe­(1-norbornyl)₄ (<b>4</b>) to elucidate its solid-state molecular structure and determine its Mössbauer spectrum, for comparison with that recorded for <b>1</b>

Oxidation and Reduction of Bis(imino)pyridine Iron Dinitrogen Complexes: Evidence for Formation of a Chelate Trianion.

Oxidation and reduction of the bis­(imino)­pyridine iron dinitrogen compound, (^iPrPDI)­FeN₂ (^iPrPDI = 2,6-(2,6-ⁱPr₂–C₆H₃–NCMe)₂C₅H₃N) has been examined to determine whether the redox events are metal or ligand based. Treatment of (^iPrPDI)­FeN₂ with [Cp₂Fe]­[BAr^F₄] (BAr^F₄ = B­(3,5-(CF₃)₂-C₆H₃)₄) in diethyl ether solution resulted in N₂ loss and isolation of [(^iPrPDI)­Fe­(OEt₂)]­[BAr^F₄]. The electronic structure of the compound was studied by SQUID magnetometry, X-ray diffraction, EPR and zero-field ⁵⁷Fe Mössbauer spectroscopy. These data, supported by computational studies, established that the overall quartet ground state arises from a high spin iron­(II) center (<i>S</i>_Fe = 2) antiferromagnetically coupled to a bis­(imino)­pyridine radical anion (<i>S</i>_PDI = 1/2). Thus, the oxidation event is principally ligand based. The one electron reduction product, [Na­(15-crown-5)]­[(^iPrPDI)­FeN₂], was isolated following addition of sodium naphthalenide to (^iPrPDI)­FeN₂ in THF followed by treatment with the crown ether. Magnetic, spectroscopic, and computational studies established a doublet ground state with a principally iron-centered SOMO arising from an intermediate spin iron center and a rare example of trianionic bis­(imino)­pyridine chelate. Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, (^iPrBPDI)­Fe­(N₂)₂ resulted in isolation of both the mono- and dianionic iron dinitrogen compounds, [(^iPrBPDI)­FeN₂]⁻ and [(^iPrBPDI)­FeN₂]^2‑, highlighting the ability of this class of chelate to serve as an effective electron reservoir to support neutral ligand complexes over four redox states

Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

The bis­(arylimidazol-2-ylidene)­pyridine cobalt methyl complex, (^iPrCNC)­CoCH₃, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H₂ pressure, (^iPrCNC)­CoCH₃ is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as <i>trans</i>-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, (^iPrCNC)­CoH, was accomplished by hydrogenation of (^iPrCNC)­CoCH₃. Over the course of 3 h at 22 °C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H₂-^iPrCNC)­CoN₂. Similar alkyl migration was observed upon treatment of (^iPrCNC)­CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis­(arylimidazol-2-ylidene)­pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active

Electronic Structure Determination of Pyridine N‑Heterocyclic Carbene Iron Dinitrogen Complexes and Neutral Ligand Derivatives

The electronic structures of pyridine N-heterocyclic dicarbene (^iPrCNC) iron complexes have been studied by a combination of spectroscopic and computational methods. The goal of these studies was to determine if this chelate engages in radical chemistry in reduced base metal compounds. The iron dinitrogen example (^iPrCNC)­Fe­(N₂)₂ and the related pyridine derivative (^iPrCNC)­Fe­(DMAP)­(N₂) were studied by NMR, Mössbauer, and X-ray absorption spectroscopy and are best described as redox non-innocent compounds with the ^iPrCNC chelate functioning as a classical π acceptor and the iron being viewed as a hybrid between low-spin Fe(0) and Fe­(II) oxidation states. This electronic description has been supported by spectroscopic data and DFT calculations. Addition of <i>N</i>,<i>N</i>-diallyl-<i>tert</i>-butylamine to (^iPrCNC)­Fe­(N₂)₂ yielded the corresponding iron diene complex. Elucidation of the electronic structure again revealed the CNC chelate acting as a π acceptor with no evidence for ligand-centered radicals. This ground state is in contrast with the case for the analogous bis­(imino)­pyridine iron complexes and may account for the lack of catalytic [2π + 2π] cycloaddition reactivity

Electronic Structure Determination of Pyridine N‑Heterocyclic Carbene Iron Dinitrogen Complexes and Neutral Ligand Derivatives

The electronic structures of pyridine N-heterocyclic dicarbene (^iPrCNC) iron complexes have been studied by a combination of spectroscopic and computational methods. The goal of these studies was to determine if this chelate engages in radical chemistry in reduced base metal compounds. The iron dinitrogen example (^iPrCNC)­Fe­(N₂)₂ and the related pyridine derivative (^iPrCNC)­Fe­(DMAP)­(N₂) were studied by NMR, Mössbauer, and X-ray absorption spectroscopy and are best described as redox non-innocent compounds with the ^iPrCNC chelate functioning as a classical π acceptor and the iron being viewed as a hybrid between low-spin Fe(0) and Fe­(II) oxidation states. This electronic description has been supported by spectroscopic data and DFT calculations. Addition of <i>N</i>,<i>N</i>-diallyl-<i>tert</i>-butylamine to (^iPrCNC)­Fe­(N₂)₂ yielded the corresponding iron diene complex. Elucidation of the electronic structure again revealed the CNC chelate acting as a π acceptor with no evidence for ligand-centered radicals. This ground state is in contrast with the case for the analogous bis­(imino)­pyridine iron complexes and may account for the lack of catalytic [2π + 2π] cycloaddition reactivity

Oxidative Addition of Carbon–Carbon Bonds with a Redox-Active Bis(imino)pyridine Iron Complex

Addition of biphenylene to the bis­(imino)­pyridine iron dinitrogen complexes, (^iPrPDI)­Fe­(N₂)₂ and [(^MePDI)­Fe­(N₂)]₂(μ₂-N₂) (^RPDI = 2,6-(2,6-R₂C₆H₃NCMe)₂C₅H₃N; R = Me, ⁱPr), resulted in oxidative addition of a CC bond at ambient temperature to yield the corresponding iron biphenyl compounds, (^RPDI)­Fe­(biphenyl). The molecular structures of the resulting bis­(imino)­pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mössbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis­(imino)­pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis­(imino)­pyridine ligand

Oxidative Addition of Carbon–Carbon Bonds with a Redox-Active Bis(imino)pyridine Iron Complex

Addition of biphenylene to the bis­(imino)­pyridine iron dinitrogen complexes, (^iPrPDI)­Fe­(N₂)₂ and [(^MePDI)­Fe­(N₂)]₂(μ₂-N₂) (^RPDI = 2,6-(2,6-R₂C₆H₃NCMe)₂C₅H₃N; R = Me, ⁱPr), resulted in oxidative addition of a CC bond at ambient temperature to yield the corresponding iron biphenyl compounds, (^RPDI)­Fe­(biphenyl). The molecular structures of the resulting bis­(imino)­pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mössbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis­(imino)­pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis­(imino)­pyridine ligand

Quantitative Evidence for Lanthanide-Oxygen Orbital Mixing in CeO₂, PrO₂, and TbO₂

Understanding the nature of covalent (band-like) vs ionic (atomic-like) electrons in metal oxides continues to be at the forefront of research in the physical sciences. In particular, the development of a coherent and quantitative model of bonding and electronic structure for the lanthanide dioxides, LnO₂ (Ln = Ce, Pr, and Tb), has remained a considerable challenge for both experiment and theory. Herein, relative changes in mixing between the O 2p orbitals and the Ln 4f and 5d orbitals in LnO₂ are evaluated quantitatively using O K-edge X-ray absorption spectroscopy (XAS) obtained with a scanning transmission X-ray microscope and density functional theory (DFT) calculations. For each LnO₂, the results reveal significant amounts of Ln 5d and O 2p mixing in the orbitals of t_2g (σ-bonding) and e_g (π-bonding) symmetry. The remarkable agreement between experiment and theory also shows that significant mixing with the O 2p orbitals occurs in a band derived from the 4f orbitals of a_2u symmetry (σ-bonding) for each compound. However, a large increase in orbital mixing is observed for PrO₂ that is ascribed to a unique interaction derived from the 4f orbitals of t_1u symmetry (σ- and π-bonding). O K-edge XAS and DFT results are compared with complementary L₃-edge and M_5,4-edge XAS measurements and configuration interaction calculations, which shows that each spectroscopic approach provides evidence for ground state O 2p and Ln 4f orbital mixing despite inducing very different core–hole potentials in the final state

Covalency in Lanthanides. An X‑ray Absorption Spectroscopy and Density Functional Theory Study of LnCl₆^<i>x</i>– (<i>x</i> = 3, 2)

Covalency in Ln–Cl bonds of <i>O</i>_<i>h</i>-LnCl₆^<i>x</i>– (<i>x</i> = 3 for Ln = Ce^III, Nd^III, Sm^III, Eu^III, Gd^III; <i>x</i> = 2 for Ln = Ce^IV) anions has been investigated, primarily using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L_3,2-edge and M_5,4-edge XAS were also used to characterize CeCl₆^<i>x</i>– (<i>x</i> = 2, 3). The M_5,4-edge XAS spectra were modeled using configuration interaction calculations. The results were evaluated as a function of (1) the lanthanide (Ln) metal identity, which was varied across the series from Ce to Gd, and (2) the Ln oxidation state (when practical, i.e., formally Ce^III and Ce^IV). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (<i>t</i>_2<i>g</i>* and <i>e</i>_<i>g</i>*) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing Ce^III to Ce^IV had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl₆^3– (formally Ln^III), the 4f-orbitals participated only marginally in covalent bonding, which was consistent with historical descriptions. Surprisingly, there was a marked increase in Cl 3p- and Ce^IV 4f-orbital mixing (<i>t</i>_1<i>u</i>* + <i>t</i>_2<i>u</i>*) in CeCl₆^2–. This unexpected 4f- and 5d-orbital participation in covalent bonding is presented in the context of recent studies on both tetravalent transition metal and actinide hexahalides, MCl₆^2– (M = Ti, Zr, Hf, U)