Bimolecular Reductive Elimination of Ethane from Pyridine(diimine) Iron Methyl Complexes: Mechanism, Electronic Structure, and Entry into [2+2] Cycloaddition Catalysis.

The application of bimolecular reductive elimination to the activation of iron catalysts for alkene-diene cycloaddition is described. Key to this approach was the synthesis, characterization, electronic structure determination, and ultimately solution stability of a family of pyridine(diimine) iron methyl complexes with diverse steric properties and electronic ground states. Both the aryl-substituted, (MePDI)FeCH3 and (EtPDI)FeCH3 (RPDI = 2,6-(2,6-R2-C6H3N═CMe)2C5H3N), and the alkyl-substituted examples, (CyAPDI)FeCH3 (CyAPDI = 2,6-(C6H11N═CMe)2C5H3N), have molecular structures significantly distorted from planarity and S = 3/2 ground states. The related N-arylated derivative bearing 2,6-di-isopropyl aryl substituents, (iPrPDI)FeCH3, has an idealized planar geometry and exhibits spin crossover behavior from S = 1/2 to S = 3/2 states. At 23 °C under an N2 atmosphere, both (MePDI)FeCH3 and (EtPDI)FeCH3 underwent reductive elimination of ethane to form the iron dinitrogen precatalysts, [(MePDI)Fe(N2)]2(µ-N2) and [(EtPDI)Fe(N2)]2(µ-N2), respectively, while (iPrPDI)FeCH3 proved inert to C-C bond formation. By contrast, addition of butadiene to all three iron methyl complexes induced ethane formation and generated the corresponding iron butadiene complexes, (RPDI)Fe(η4-C4H6) (R = Me, Et, iPr), known precatalysts for the [2+2] cycloaddition of olefins and dienes. Kinetic, crossover experiments, and structural studies were combined with magnetic measurements and Mössbauer spectroscopy to elucidate the electronic and steric features of the iron complexes that enable this unusual reductive elimination and precatalyst activation pathway. Transmetalation of methyl groups between iron centers was fast at ambient temperature and independent of steric environment or spin state, while the intermediate dimer underwent the sterically controlled rate-determining reaction with either N2 or butadiene to access a catalytically active iron compound.

High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex.

Among Earth-abundant catalyst systems, iron-carbene intermediates that perform C-C bond forming reactions such as cyclopropanation of olefins and C-H functionalization via carbene insertion are rare. Detailed descriptions of the possible electronic structures for iron-carbene bonds are imperative to obtain better mechanistic insights and enable rational catalyst design. Here, we report the first square-planar iron-carbene complex (MesPDPPh)Fe(CPh2), where [MesPDPPh]2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. The compound was prepared via reaction of the disubstituted diazoalkane N2CPh2 with (MesPDPPh)Fe(thf) and represents a rare example of a structurally characterized, paramagnetic iron-carbene complex. Temperature-dependent magnetic susceptibility measurements and applied-field Mössbauer spectroscopic studies revealed an orbitally near-degenerate S = 1 ground state with large unquenched orbital angular momentum resulting in high magnetic anisotropy. Spin-Hamiltonian analysis indicated that this S = 1 spin system has uniaxial magnetic properties arising from a ground MS = ±1 non-Kramers doublet that is well-separated from the MS = 0 sublevel due to very large axial zero-field splitting (D = -195 cm-1, E/D = 0.02 estimated from magnetic susceptibility data). This remarkable electronic structure gives rise to a very large, positive magnetic hyperfine field of more than +60 T for the 57Fe nucleus along the easy magnetization axis observed by Mössbauer spectroscopy. Computational analysis with complete active space self-consistent field (CASSCF) calculations provides a detailed electronic structure analysis and confirms that (MesPDPPh)Fe(CPh2) exhibits a multiconfigurational ground state. The majority contribution originates from a configuration best described as a singlet carbene coordinated to an intermediate-spin FeII center with a (dxy)2{(dxz),(dz2)}3(dyz)1(dx2-y2)0 configuration featuring near-degenerate dxz and dz2 orbitals.

Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron Dialkyl Complexes.

Two methods for the synthesis of bis(imidazol-2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe(CH2SiMe3)2, have been developed. The first route consists of addition of two equivalents of LiCH2SiMe3 to the iron dihalide complex, (CNC)FeBr2, while the second relies on addition of the free CNC ligand to readily-prepared (py)2Fe(CH2SiMe3)2 (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type ( Ar CNC)Fe(CH2SiMe3)2(N2) ( Ar CNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site trans to the pyridine of the CNC-chelate. In contrast, the alkyl-substituted variant, (tBuACNC)Fe(CH2SiMe3)2 (tBuACNC = 2,6-(tBu-imidazol-2-ylidene)2pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the ( Ar CNC)Fe(CH2SiMe3)2(N2) derivatives to an H2 atmosphere resulted in formation of the corresponding iron hydride complexes ( Ar CNC)FeH4. These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H2, generating isotopologues and isotopomers of ( Ar CNC)Fe(H n )(D4-n ) (n = 0-4). When (3,5-Me2 MesCNC)Fe(CH2SiMe3)2(N2) (3,5-Me2 MesCNC = 2,6-(2,4,6-Me3-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) was treated successively with H2 and then N2, the corresponding reduced dinitrogen complex (3,5-Me2 MesCNC)Fe(N2)2 was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me2 MesCNC)Fe(CH2SiMe3)2(N2).

Reactivity of Pyridine Dipyrrolide Iron(II) Complexes with Organic Azides: C-H Amination and Iron Tetrazene Formation.

Reaction of (MesPDPPh)Fe(thf) (H2MesPDPPh = 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine) with organic azides has been studied. The identity of the azide substituent had a profound impact on the transformation type and nature of the observed products. Reaction with aromatic p-tolyl azide, N3Tol, resulted in exclusive formation of the corresponding iron tetrazene complex (MesPDPPh)Fe(N4Tol2). In contrast, the use of bulky 1-adamantyl azide led to clean intramolecular C-H amination of one of the benzylic C-H bonds of a mesityl substituent on the pyridine dipyrrolide, PDP, supporting ligand. The smaller aliphatic substituent in benzyl azide allowed for the isolation of two different compounds from distinct reaction pathways. One product is the result of double C-H amination of the PDP ligand via nitrene transfer, while the second one contains a dibenzyltetrazene and a benzaldimine ligand. All isolated complexes were characterized using a combination of X-ray crystallography, solid state magnetic susceptibility measurements, 1H NMR and 57Fe Mössbauer spectroscopy, and density functional theory (DFT), and their electronic structures were elucidated. Potential electronic structures for putative iron(IV) imido or iron(III) imidyl radical complexes were explored via DFT calculations.

Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand.

Two square-planar high-spin FeII complexes bearing a dianionic pyridine dipyrrolate pincer ligand and a diethyl ether or tetrahydrofuran ligand were synthesized and structurally characterized, and their electronic structures were elucidated by a combined spectroscopic and computational approach. In contrast to previous examples, the S = 2 ground states of these square-planar FeII complexes do not require an overall anionic charge of the compounds or incorporation of alkali metal cations. The tetrahydrofuran complex exhibits an equilibrium between four- and five-coordinate species in solution, which was supported by 1H NMR and 57Fe Mössbauer spectroscopy and comparison to a structurally characterized five-coordinate pyridine dipyrrolate iron bis-pyridine adduct. A detailed computational analysis of the electronic structures of the four- and five-coordinate species via density functional theory provides insight into the origins of the unusual ground state configurations for FeII in a square-planar ligand field and explains the associated characteristic spectroscopic parameters.

Increasing the rate of hydrogen oxidation without increasing the overpotential: a bio-inspired iron molecular electrocatalyst with an outer coordination sphere proton relay.

Oxidation of hydrogen (H2) to protons and electrons for energy production in fuel cells is currently catalyzed by platinum, but its low abundance and high cost present drawbacks to widespread adoption. Precisely controlled proton removal from the active site is critical in hydrogenase enzymes in nature that catalyze H2 oxidation using earth-abundant metals (iron and nickel). Here we report a synthetic iron complex, (CpC5F4N)Fe(PEtN(CH2)3NMe2 PEt)(Cl), that serves as a precatalyst for the oxidation of H2, with turnover frequencies of 290 s-1 in fluorobenzene, under 1 atm of H2 using 1,4-diazabicyclo[2.2.2]octane (DABCO) as the exogenous base. The inclusion of a properly tuned outer coordination sphere proton relay results in a cooperative effect between the primary, secondary and outer coordination spheres for moving protons, increasing the rate of H2 oxidation without increasing the overpotential when compared with the analogous complex featuring a single pendant base. This finding emphasizes the key role of pendant amines in mimicking the functionality of the proton pathway in the hydrogenase enzymes.

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 N,N-diallyl-tert-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.

Solvent and electrolyte effects on Ni(P(R)(2)N(R')2)(2)-catalyzed electrochemical oxidation of hydrogen.

We report solvent and electrolyte effects on the electrocatalytic oxidation of H2 using Ni(P(Cy)2N(R')2)2 (R = Bn, (t)Bu) complexes. A turnover frequency of 46 s(-1) for Ni(P(Cy)2N(Bn)2)2 was obtained using 0.2 M [(n)Bu4N][BF4] in THF. A turnover frequency of 51 s(-1) was observed for Ni(P(Cy)2N(tBu)2)2 using 0.2 M [(n)Bu4N][B(C6F5)4] in fluorobenzene. These observations, in conjunction with previous studies, indicate nitrile binding inhibits catalysis supported by Ni(P(Cy)2N(Bn)2)2.

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, ((iPr)CNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2 pressure, ((iPr)CNC)CoCH3 is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-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, ((iPr)CNC)CoH, was accomplished by hydrogenation of ((iPr)CNC)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-(iPr)CNC)CoN2. Similar alkyl migration was observed upon treatment of ((iPr)CNC)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.

Reactivity and Mössbauer spectroscopic characterization of an Fe(IV) ketimide complex and reinvestigation of an Fe(IV) norbornyl complex.

Thermolysis of Fe(N═C(t)Bu2)4 (1) for 8 h at 50 °C generates the mixed valent Fe(III)/Fe(II) bimetallic complex Fe2(N═C(t)Bu2)5 (2) in moderate yield. Also formed in this reaction are tert-butyl cyanide, isobutane, and isobutylene, the products of ketimide oxidation by the Fe(4+) center. Reaction of 1 with 1 equiv of acetylacetone affords the Fe(III) complex, Fe(N═C(t)Bu2)2(acac) (3), concomitant with formation of bis(tert-butyl)ketimine, tert-butyl cyanide, isobutane, and isobutylene. In addition, the Mössbauer spectra of 1 and its lower-valent analogues [Li(12-crown-4)2][Fe(N═C(t)Bu2)4] (5) and [Li(THF)]2[Fe(N═C(t)Bu2)4] (6) were recorded. We also revisited the chemistry of Fe(1-norbornyl)4 (4) to elucidate its solid-state molecular structure and determine its Mössbauer spectrum, for comparison with that recorded for 1.

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, ((iPr)PDI)Fe(N(2))(2) and [((Me)PDI)Fe(N(2))](2)(µ(2)-N(2)) ((R)PDI = 2,6-(2,6-R(2)-C(6)H(3)-N═CMe)(2)C(5)H(3)N; R = Me, (i)Pr), resulted in oxidative addition of a C-C bond at ambient temperature to yield the corresponding iron biphenyl compounds, ((R)PDI)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.

High-Activity Iron Catalysts for the Hydrogenation of Hindered, Unfunctionalized Alkenes.

The activity of aryl-substituted bis(imino)pyridine and bis(arylimidazol-2-ylidene)pyridine iron dinitrogen complexes has been evaluated in a series of catalytic olefin hydrogenation reactions. In general, more electron donating chelates with smaller 2,6-aryl substituents produce more active iron hydrogenation catalysts. Establishment of this structure-activity relationship has produced base metal catalysts that exhibit high turnover frequencies for the hydrogenation of unfunctionalized, tri- and tetrasubstituted alkenes, one of the most challenging substrate classes for homogenous hydrogenation catalysts.

Synthesis of aryl-substituted bis(imino)pyridine iron dinitrogen complexes.

The synthesis and characterization of dimeric, aryl-substituted bis(imino)pyridine iron dinitrogen complexes is described. In contrast to reduction with sodium amalgam where bis(chelate) iron compounds were isolated, stirring ((Ar)PDI)FeBr(2) or ((Me)BPDI)FeBr(2) (PDI = 2,6-(ArN=CMe)(2)C(5)H(3)N; Ar = 2,6-Et(2)-C(6)H(3)N ((Et)PDI), 2,6-Me(2)-C(6)H(3)N ((Me)PDI), 2-(i)Pr,6-Me-C(6)H(3)N ((Me,iPr)PDI); (Me)BPDI = 2,6-(2,6-Me(2)-C(6)H(3)N=CPh)(2)C(5)H(3)N) with sodium naphthalenide resulted in isolation of the desired iron dinitrogen compounds as diamagnetic solids. Two examples, [((Et)PDI)Fe(N(2))](2)(mu(2)-N(2)) and [((Me)BPDI)Fe(N(2))](2)(mu(2)-N(2)), were characterized by X-ray diffraction. The solid state metrical parameters, in combination with infrared and Mossbauer spectroscopic data, establish ferrous compounds with doubly reduced chelates. Each new bis(imino)pyridine iron dinitrogen compound was screened for the catalytic hydrogenation of ethyl-3-methylbut-2-enoate, and the compound bearing the smallest aryl substituent, [((Me)PDI)Fe(N(2))](2)(mu(2)-N(2)), offers significant improvement over the original ((iPr)PDI)Fe(N(2))(2) pre-catalyst and is one of the most active iron pre-catalysts known.