Oxidation and reduction of bis(imino)pyridine iron dicarbonyl complexes

The oxidation and reduction of a redox-active aryl-substituted bis(imino)pyridine iron dicarbonyl has been explored to determine whether electron-transfer events are ligand- or metal-based or a combination of both. A series of bis(imino)pyridine iron dicarbonyl compounds, [((iPr)PDI)Fe(CO)(2)](-), ((iPr)PDI)Fe(CO)(2), and [((iPr)PDI)Fe(CO)(2)](+) [(iPr)PDI = 2,6-(2,6-(i)Pr(2)C(6)H(3)N═CMe)(2)C(5)H(3)N], which differ by three oxidation states, were prepared and the electronic structures evaluated using a combination of spectroscopic techniques and, in two cases, [((iPr)PDI)Fe(CO)(2)](+) and [((iPr)PDI)Fe(CO)(2)], metrical parameters from X-ray diffraction. The data establish that the cationic iron dicarbonyl complex is best described as a low-spin iron(I) compound (S(Fe) = ?) with a neutral bis(imino)pyridine chelate. The anionic iron dicarbonyl, [((iPr)PDI)Fe(CO)(2)](-), is also best described as an iron(I) compound but with a two-electron-reduced bis(imino)pyridine. The covalency of the neutral compound, ((iPr)PDI)Fe(CO)(2), suggests that both the oxidative and reductive events are not ligand- or metal-localized but a result of the cooperativity of both entities.     

O2 activation by bis(imino)pyridine iron(II)-thiolate complexes

The new iron(II)-thiolate complexes [((iPr)BIP)Fe(II)(SPh)(Cl)] (1) and [((iPr)BIP)Fe(II)(SPh)(OTf)] (2) [BIP = bis(imino)pyridine] were prepared as models for cysteine dioxygenase (CDO), which converts Cys to Cys-SO(2)H at a (His)(3)Fe(II) center. Reaction of 1 and 2 with O(2) leads to Fe-oxygenation and S-oxygenation, respectively. For 1 + O(2), the spectroscopic and reactivity data, including (18)O isotope studies, are consistent with an assignment of an iron(IV)-oxo complex, [((iPr)BIP)Fe(IV)(O)(Cl)](+) (3), as the product of oxygenation. In contrast, 2 + O(2) results in direct S-oxygenation to give a sulfonato product, PhSO(3)(-). The positioning of the thiolate ligand in 1 versus 2 appears to play a critical role in determining the outcome of O(2) activation. The thiolate ligands in 1 and 2 are essential for O(2) reactivity and exhibit an important influence over the Fe(III)/Fe(II) redox potential.     

Synthesis and hydrogenation of bis(imino)pyridine iron imides

Treatment of the iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (iPrPDI = (2,6-iPr2C6H3N=CMe)2C5H3N), with a series of aryl azides resulted in loss of 3 equiv of N2 and formation of the corresponding four-coordinate iron imide compounds, (iPrPDI)Fe(NAr). These complexes, two of which (Ar = 2,6-iPr2-C6H3 and 2,4,6-Me3-C6H2) have been characterized by X-ray diffraction, are significantly distorted from planarity. The metrical parameters in combination with M?ssbauer spectroscopic and SQUID magnetic data suggest an intermediate spin iron(III) center antiferromagnetically coupled to a ligand-centered radical. Nitrene group transfer has been accomplished by addition of 1 atm of CO, yielding aryl isocyanates, ArNCO, and (iPrPDI)Fe(CO)2. Hydrogenation of the more sterically hindered members of the series furnished free aniline and the previously reported iron dihydrogen complex. Catalytic aryl azide hydrogenation has also been achieved, and the observed relative rates are consistent with N-H bond formation as the rate-determining step in aniline formation.     

Neutral-ligand complexes of bis(imino)pyridine iron: synthesis, structure, and spectroscopy

A family of bis(imino)pyridine iron neutral-ligand derivatives, ((iPr)PDI)FeL(n) ((iPr)PDI = 2,6-(2,6-iPr2-C6H3N=CMe)2C6H3N), has been synthesized from the corresponding bis(dinitrogen) complex, ((iPr)PDI)Fe(N2)2. When L is a strong-field ligand such as tBuNC or a chelating alkyl diphosphine such as DEPE (DEPE = 1,2-bis(diethylphosphino)ethane), a five-coordinate, diamagnetic compound results with no spectroscopic evidence for mixing of paramagnetic states. Reducing the field strength of the neutral donor to principally sigma-type ligands such as tBuNH2 or THT (THT = tetrahydrothiophene) also yielded diamagnetic compounds. However, the 1H NMR chemical shifts of the in-plane bis(imino)pyridine hydrogens exhibit a large chemical shift dispersion indicative of temperature-independent paramagnetism (TIP) arising from mixing of an S = 1 excited state via spin-orbit coupling. Metrical data from X-ray diffraction establish bis(imino)pyridine chelate reduction for each structural type, while M?ssbauer parameters and NMR spectroscopic data differentiate the spin states of the iron and identify contributions from paramagnetic excited states.     

Bis(imino)pyridine iron(II) alkyl cations for olefin polymerization

Treatment of the five-coordinate ferrous dialkyl complex, (iPrPDI)Fe(CH2SiMe3)2 (iPrPDI = ((2,6-CHMe2)2C6H3N=CMe)2C5H3N), with [PhMe2NH][BPh4] in the presence of diethyl ether or tetrahydrofuran furnished the corresponding alkyl cations, where the donor ligand is coordinated in the basal plane of a distorted square pyramidal iron(II) alkyl cation. Performing the same reaction with the neutral Lewis acid, B(C6F5)3, induced methide abstraction from a silicon atom followed by rearrangement to afford the base free ferrous alkyl cation, [(iPrPDI)Fe(CH2SiMe2CH2SiMe3)][MeB(C6F5)3]. This complex is active for the polymerization of ethylene and yields polymers that are of higher molecular weight and narrower polydispersity than traditional methylalumoxane-activated catalysts.     

Bis(imino)pyridine iron complexes for aldehyde and ketone hydrosilylation

Bis(imino)pyridine iron dinitrogen and dialkyl complexes are well-defined precatalysts for the chemo- and regioselective reduction of aldehydes and ketones. Efficient carbonyl hydrosilylation is observed at low (0.1-1.0 mol %) catalyst loadings and with 2 equiv of either PhSiH(3) or Ph(2)SiH(2), representing one of the most active iron-catalyzed carbonyl reductions reported to date.     

High activity acetylene polymerisation with a bis(imino)pyridine iron(II) catalyst

A catalyst system composed of a 2,6-bis(arylimino)pyridineiron(II) dichloride complex and methylaluminoxane is found to be extremely active for acetylene polymerisation. The formation of poly(acetylene) gels and surface films occurs at very low catalyst concentrations, around three orders of magnitude lower than traditional catalyst systems.     

Synthesis and electronic structure of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes: evaluation of redox activity in single-component ethylene polymerization catalysts

A family of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes has been prepared, and their electronic and molecular structures have been established by a combination of X-ray diffraction, Mo?ssbauer spectroscopy, magnetochemistry, and open-shell density functional theory. For the cationic complexes, [((iPr)PDI)Fe-R][BPh(4)] ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N═CMe)(2)C(5)H(3)N; R = CH(2)SiMe(3), CH(2)CMe(3), or CH(3)), which are known single-component ethylene polymerization catalysts, the data establish high spin ferrous compounds (S(Fe) = 2) with neutral, redox-innocent bis(imino)pyridine chelates. One-electron reduction to the corresponding neutral alkyls, ((iPr)PDI)Fe(CH(2)SiMe(3)) or ((iPr)PDI)Fe(CH(2)CMe(3)), is chelate-based, resulting in a bis(imino)pyridine radical anion (S(PDI) = 1/2) antiferromagnetically coupled to a high spin ferrous ion (S(Fe) = 2). The neutral neopentyl derivative was reduced by an additional electron and furnished the corresponding anion, [Li(Et(2)O)(3)][((iPr)PDI)Fe(CH(2)CMe(3))N(2)], with concomitant coordination of dinitrogen. The experimental and computational data establish that this S = 0 compound is best described as a low spin ferrous compound (S(Fe) = 0) with a closed-shell singlet bis(imino)pyridine dianion (S(PDI) = 0), demonstrating that the reduction is ligand-based. The change in field strength of the bis(imino)pyridine coupled with the placement of the alkyl ligand into the apical position of the molecule induced a spin state change at the iron center from high to low spin. The relevance of the compounds and their electronic structures to olefin polymerization catalysis is also presented.     

Enantiopure C1-symmetric bis(imino)pyridine cobalt complexes for asymmetric alkene hydrogenation

Enantiopure C(1)-symmetric bis(imino)pyridine cobalt chloride, methyl, hydride, and cyclometalated complexes have been synthesized and characterized. These complexes are active as catalysts for the enantioselective hydrogenation of geminal-disubstituted olefins.     

Synthesis, electronic structure, and ethylene polymerization activity of bis(imino)pyridine cobalt alkyl cations

A new spin on polymers: the title cations comprise low-spin Co(II) centers with neutral bis(imino)pyridine chelating ligands. These complexes serve as single-component ethylene polymerization catalysts and offer insight into the mechanism of chain growth and catalyst deactivation, which occurs by forming inactive cationic bis(imino)pyridine cobalt complexes with a diethyl ether ligand.     

Pyridine bis(imino) iron and cobalt complexes for ethylene polymerization: influence of the aryl imino substituents

In the present paper, the synthesis of new pyridine bis(imine) ligands modified with halogens (Cl, Br, CF₃) or alkyl groups (Heptyl, tert-butyl, Phenyl, …) is reported. When coordinated with iron or cobalt dichloride, they yielded complexes which were associated to methylaluminoxane (MAO) to achieve the polymerization of ethylene. It was shown that cobalt catalysts are generally more sensitive to the ligand substitutions than the iron ones. The addition of a chlorine atom on the ligand frame is generally unfavorable. On the contrary, the presence of a bromine atom seems more favorable. Phenyl rings lead to almost completely inactive catalysts, probably because of a too weak coordination to the metal. It was also demonstrated that a mono-substitution of the aryl groups with an electron-withdrawing group (-CF₃) is sufficient to yield polymers, whereas, considering the bulkiness of this substituent only, oligomers would have been expected.     

New iron bis(imino)pyridyl complexes containing dendritic wedges for alkene oligomerisation

The synthesis, characterisation and catalytic behaviour of new iron bis(imino)pyridyl complexes containing dendritic wedges, as well as the synthesis of bis(para-hydroxyphenylimino)pyridines is described. The hydroxyl functionality of the bis(para-hydroxyphenylimino)pyridines was used to attach dendritic wedges of the carbosilane type as well as the benzylphenyl ether type. After attachment of the dendritic wedges, complexation of these ligands to iron(II) chloride was achieved. The resulting dendritically functionalised bis(imino)pyridyl iron complexes were tested in the catalytic oligomerisation of ethene.     

Square planar vs tetrahedral geometry in four coordinate iron(II) complexes

The geometric preferences of a family of four coordinate, iron(II) d6 complexes of the general form L2FeX2 have been systematically evaluated. Treatment of Fe2(Mes)4 (Mes = 2,4,6-Me3C6H2) with monodentate phosphine and phosphite ligands furnished square planar trans-P2Fe(Mes)2 derivatives. Identification of the geometry has been accomplished by a combination of solution and solid-state magnetometry and, in two cases (P = PMe3, PEt2Ph), X-ray diffraction. In contrast, both tetrahedral and square planar coordination has been observed upon complexation of chelating phosphine ligands. A combination of crystallographic and magnetic susceptibility data for (depe)Fe(Mes)2 (depe = 1,2-bis(diethylphosphino)ethane) established a tetrahedral molecular geometry whereas SQUID magnetometry and M?ssbauer spectroscopy on samples of (dppe)Fe(Mes)2 (dppe = 1,2-bis(diphenylphosphino)ethane) indicated a planar molecule. When dissolved in chlorinated solvents, the latter compound promotes chlorine atom abstraction, forming tetrahedral (dppe)Fe(Mes)Cl and (dppe)FeCl2. Ligand substitution reactions have been studied for both structural types and are rapid on the NMR time scale at ambient temperature.     

Unique catalytic behavior of chromium complexes having halogenated bis(imino)pyridine ligands for ethylene polymerization

Reactions of CrCl₃(thf)₃ with bis(imino)pyridines gave a series of {bis(imino)pyridine}chromium(III) trichloride complexes, {2,6‐(RN?CMe)₂C₅H₃N}CrCl₃ [R = C₆HPr₂‐2,6 ( 1 ), C₆H₃Et₂‐2,6 ( 2 ), C₆H₃Me₂‐2,6 ( 3 ), C₆H₂Me₃‐2,4,6 ( 4 ), C₆H₃Me₂‐3,5 ( 5 ), C₆H₅ ( 6 ), cyclohexyl ( 7 ), 2‐methyl‐1‐naphthyl ( 8 ), C₆H₃F₂‐2,6 ( 9 ), C₆H₃Br₂‐2,6 ( 10 ), C₆F₅ ( 11 )]. Pseudo‐octahedral geometries of 6 , 10 , and 11 were revealed by X‐ray crystallography. The complexes having bulky substituents such as 1 – 4 showed high activity for ethylene polymerization in combination with modified methylaluminoxane (MMAO) to give linear polyethylenes. In sharp contrast, the pentafluorophenyl complex 11 /modified methylaluminoxane system was found to be moderately active for ethylene homopolymerization to give moderately branched polyethylene with only ethyl branches. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3368–3375, 2005     

Kinetic regularities of catalytic ethylene polymerization on single- and multi-site cobalt and vanadium bis(imino)pyridine complexes

The number of active centers C _p in the homogeneous complexes LCoCl₂ and LVCl₃ (L = 2,6-(2,6-R₂C₆H₃N=CMe)₂C₅H₃N; R = Me, Et, ^t Bu) and the propagation rate constants k _p have been determined by the radioactive ¹⁴CO quenching of ethylene polymerization on these complexes in the presence of the methylaluminoxane (MAO) activator. For the systems studied, a significant portion of the initial complex (up to 70%) transforms into polymerization-active centers. The catalysts based on the cobalt complexes are single-site, and the constant k _p in these systems is independent of the volume of substituent R in the ligand, being (2.4?3.5) × 10³ L mol^?1 s^?1 at 35°C. The much larger molecular weight of the polymer formed on the complex with the tert-butyl substituent in the aryl rings of the ligand compared to the product formed on the complex with the methyl substituent is due to the substantial (～11-fold) decrease in the rate constant of chain transfer to the monomer. At the early stages of the reaction (before 5 min), the vanadium complexes contain active centers of one type only, for which k _p = 2.6 × 10³ L mol^?1 s^?1 at 35°C. An increase in the polymerization time to 20 min results in the appearance, in the vanadium systems, of new, substantially less reactive centers on which high-molecular-weight polyethylene forms. The number of active centers C _p in the 2,5-tBu₂LCoCl₂ and 2,6-Et₂LVCl₃ systems with the MAO activator increases as the polymerization temperature is raised from 25 to 60°C. The activation energies of the chain propagation reaction (E _p) have been calculated. The value of E _p for complex 2,5-tBu₂LCoCl₂ is 4.5 kcal/mol. It is assumed that the so-called “dormant” centers form in ethylene polymerization on the 2,6-Et₂LVCl₃ complex, and their proportion increases with a decrease in the polymerization temperature. Probably, the anomalously high value E _p = 14.2 kcal/mol for the vanadium system is explained by the formation of these “dormant” centers.     

NMR and EPR studies of the bis(pyridine) and bis(tert-butyl isocyanide) complexes of iron(III) octaethylchlorin

The NMR and EPR spectra of a series of pyridine complexes [(OEC)Fe(L)2]+ (L = 4-Me2NPy, Py, and 4-CNPy) have been investigated. The EPR spectra at 4.2 K suggest that, with a decrease of the donor strength of the axial ligands, the complexes change their ground state from (d(xy))2 (d(xz)d(yz))3 to (d(xz)d(yz))4 (d(xy))1. The NMR data from 303 to 183 K show that at any temperature within this range the chemical shifts of pyrrole-8,17-CH2 protons increase with a decrease in the donor strength of the axial ligands. The full peak assignments of the [(OEC)Fe(L)2]+ complexes of this study have been made from COSY and NOE difference experiments. The pyrrole-8,17-CH2 and pyrroline protons show large chemical shifts (hence indicating large pi spin density on the adjacent carbons which are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from molecular orbital calculations, both Hückel and DFT; the DFT calculations additionally show close energy spacing of the highest five filled orbitals (of the Fe(II) complex) and strong mixing of metal and chlorin character in these orbitals that is sensitive to the donor strength of the axial substituents. The pattern of chemical shifts of the pyrrole-CH2 protons of [(OEC)Fe(t-BuNC)2]+ looks somewhat like that of [(OEC)Fe(4-Me2NPy)2]+, while the chemical shifts of the meso-protons are qualitatively similar to those of [(OEP)Fe(t-BuNC)2]+. The temperature dependence of the chemical shifts of [(OEC)Fe(t-BuNC)2]+ shows that it has a mixed (d(xz)d(yz))4 (d(xy))1 and (d(xy))2 (d(xz),d(yz))3 electron configuration that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 5/2 excited state that lies somewhat more than 2kT at room temperature above the ground state; the observed pattern of chemical shifts is the approximate average of those expected for the two S = 1/2 electronic configurations, which involve the a-symmetry SOMO of a planar chlorin ring with the unpaired electron predominantly in the d(yz) orbital and the b-symmetry SOMO of a ruffled chlorin ring with the unpaired electron predominantly in the d(xy) orbital. A rapid interconversion between the two, with calculated vibrational frequency of 22 cm(-1), explains the observed pattern of chemical shifts, while a favoring of the ruffled conformation explains the negative chemical shift (and thus the negative spin density at the alpha-pyrroline ring carbons), of the pyrroline-H of [TPCFe(t-BuNC)2]CF3SO3 (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (OEC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. The contact shifts of the pyrrole-CH2 and meso protons of the high-spin complex depend on both sigma and pi spin delocalization due to contributions from three of the occupied frontier orbitals of the chlorin ring.     

Photolysis and thermolysis of bis(imino)pyridine cobalt azides: C-H activation from putative cobalt nitrido complexes

A series of planar aryl-substituted bis(imino)pyridine cobalt azide complexes were prepared and evaluated as synthetic precursors for the corresponding cobalt nitrido compounds. Thermolysis or photolysis of two examples resulted in intramolecular C-H activation of the benzylic positions of the aryl substituents. For the mesityl-substituted compound, C-H activation by the putative nitride resulted in formation of a neutral imine ligand and modification of the chelate by hydrogen transfer to the imine carbon.