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.     

Iron-catalyzed [2pi + 2pi] cycloaddition of alpha,omega-dienes: the importance of redox-active supporting ligands

The bis(imino)pyridine iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-(2,6-iPr2C6H3NCR)2C5H3N), serves as an efficient precursor for the catalytic [2pi + 2pi] cycloaddition of alpha,omega-dienes to yield the corresponding bicycles. For amine substrates, the rate of catalytic turnover increases with the size of the nitrogen substituents, demonstrating competing heterocycle coordination and product inhibition. In one case, a bis(imino)pyridine iron azobicycloheptane product was characterized by X-ray diffraction. Preliminary mechanistic studies highlight the importance of the redox activity of the bis(imino)pyridine ligand to maintain the ferrous oxidation state throughout the catalytic cycle.     

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.     

Iron-catalyzed intermolecular [2π+2π] cycloaddition

The bis(imino)pyridine iron dinitrogen compounds, ((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 = (i)Pr, Me), promote the catalytic intermolecular [2π + 2π] cycloaddition of ethylene and butadiene to form vinylcyclobutane. Stoichiometric experiments resulted in isolation of a catalytically competent iron metallocycle intermediate, which was shown to undergo diene-induced C-C reductive elimination. Deuterium labeling experiments establish competitive cyclometalation of the bis(imino)pyridine aryl substituents during catalytic turnover.     

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.     

Electronic structure of bis(imino)pyridine iron dichloride, monochloride, and neutral ligand complexes: a combined structural, spectroscopic, and computational study

The electronic structure of a family of bis(imino)pyridine iron dihalide, monohalide, and neutral ligand compounds has been investigated by spectroscopic and computational methods. The metrical parameters combined with M?ssbauer spectroscopic and magnetic data for ((i)PrPDI)FeCl(2) ((i)PrPDI = 2,6-(2,6-(i)Pr(2)C(6)H(3)N=CMe)(2)C(5)H(3)N) established a high-spin ferrous center ligated by a neutral bis(imino)pyridine ligand. Comparing these data to those for the single electron reduction product, ((i)PrPDI)FeCl, again demonstrated a high-spin ferrous ion, but in this case the S(Fe) = 2 metal center is antiferromagnetically coupled to a ligand-centered radical (S(L) = (1)/(2)), accounting for the experimentally observed S = (3)/(2) ground state. Continued reduction to ((i)PrPDI)FeL(n) (L = N(2), n = 1,2; CO, n = 2; 4-(N,N-dimethylamino)pyridine, n = 1) resulted in a doubly reduced bis(imino)pyridine diradical, preserving the ferrous ion. Both the computational and the experimental data for the N,N-(dimethylamino)pyridine compound demonstrate nearly isoenergetic singlet (S(L) = 0) and triplet (S(L) = 1) forms of the bis(imino)pyridine dianion. In both spin states, the iron is intermediate spin (S(Fe) = 1) ferrous. Experimentally, the compound has a spin singlet ground state (S = 0) due to antiferromagnetic coupling of iron and the ligand triplet state. Mixing of the singlet diradical excited state with the triplet ground state of the ligand via spin-orbit coupling results in temperature-independent paramagnetism and accounts for the large dispersion in (1)H NMR chemical shifts observed for the in-plane protons on the chelate. Overall, these studies establish that reduction of ((i)PrPDI)FeCl(2) with alkali metal or borohydride reagents results in sequential electron transfers to the conjugated pi-system of the ligand rather than to the metal center.     

Synthesis, electronic structure, and catalytic activity of reduced bis(aldimino)pyridine iron compounds: experimental evidence for ligand participation

The two-electron reduction chemistry of the aryl-substituted bis(aldimino)pyridine iron dibromide, ((iPr)PDAI)FeBr(2) ((iPr)PDAI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N═CH)(2)C(5)H(3)N), was explored with the goal of generating catalytically active iron compounds and comparing the electronic structure of the resulting compounds to the more well studied ketimine derivatives. Reduction of ((iPr)PDAI)FeBr(2) with excess 0.5% Na(Hg) in toluene solution under an N(2) atmosphere furnished the η(6)-arene complex, ((iPr)PDAI)Fe(η(6)-C(7)H(8)) rather than a dinitrogen derivative. Over time in pentane or diethyl ether solution, ((iPr)PDAI)Fe(η(6)-C(7)H(8)) underwent loss of arene and furnished the dimeric iron compound, [((iPr)PDAI)Fe](2). Crystallographic characterization established a diiron compound bridged through an η(2)-π interaction with an imine arm on an adjacent chelate. Superconducting quantum interference device (SQUID) magnetometry established two high spin ferrous centers each coupled to a triplet dianionic bis(aldimino)pyridine chelate. The data were modeled with two strongly antiferromagnetically coupled, high spin iron(II) centers each with an S = 1 [PDAI](2-) chelate. Two electron reduction of ((iPr)PDAI)FeBr(2) in the presence of 1,3-butadiene furnished ((iPr)PDAI)Fe(η(4)-C(4)H(6)), which serves as a precatalyst for olefin hydrogenation with modest turnover frequencies and catalyst lifetimes. Substitution of the trans-coordinated 1,3-butadiene ligand was accomplished with carbon monoxide and N,N-4-dimethylaminopyridine (DMAP) and furnished ((iPr)PDAI)Fe(CO)(2) and ((iPr)PDAI)Fe(DMAP), respectively. The molecular and electronic structures of these compounds were established by X-ray diffraction, NMR and Mo?ssbauer spectroscopy, and the results compared to the previously studied ketimine variants.     

Bis(imino)pyridine iron alkyls containing beta-hydrogens: synthesis, evaluation of kinetic stability, and decomposition pathways involving chelate participation

Bis(imino)pyridine iron alkyl complexes bearing beta-hydrogens, ((iPr)PDI)FeR (((iPr)PDI = 2,6-(2,6-(i)Pr2-C6H3N=CMe)2C5H3N; R = Et, (n)Bu, (i)Bu, CH2 (cyclo)C5H 9; 1-R), were synthesized either by direct alkylation of ((iPr)PDI)FeCl (1-Cl) with the appropriate Grignard reagent or more typically by oxidative addition of the appropriate alkyl bromide to the iron bis(dinitrogen) complex, ((iPr)PDI)Fe(N2)2 (1-(N2)2). In the latter method, the formal oxidative addition reaction produced ((iPr)PDI)FeBr (1-Br), along with the desired iron alkyl, 1-R. Elucidation of the electronic structure of 1-Br and related 1-R derivatives by magnetic measurements, structural studies and NMR spectroscopy established high spin ferrous compounds antiferromagnetically coupled to chelate radical anions. Thus, the formal oxidative process is bis(imino)pyridine ligand-based (one electron is formally removed from each chelate, not the iron) during oxidative addition. The kinetic stability of each 1-R compound was assayed in benzene-d6 solution and found to produce a mixture of the corresponding alkane and alkene. The kinetic stability of the iron alkyl complexes was inversely correlated with the number of beta-hydrogens present. For example, the iron ethyl complex, 1-Et, underwent clean loss of ethane over the course of three hours, whereas the corresponding 1-(i)Bu compound had a half-life of over 12 h under identical conditions. The mechanism of the decomposition was studied with a series of deuterium labeling experiments and support a pathway involving initial beta-hydrogen elimination followed by cyclometalation of an isopropyl methyl group, demonstrating an overall transfer hydrogenation pathway. The relevance of such pathways to chain transfer in bis(imino)pyridine iron catalyzed olefin polymerization reactions is also presented.     

Bis(diisopropylphosphino)pyridine iron dicarbonyl, dihydride, and silyl hydride complexes

Treatment of the bis(diisopropylphosphino)pyridine iron dichloride, ((iPr)PNP)FeCl2 ((iPr)PNP = 2,6-(iPr2PCH2)2(C5H3N)), with 2 equiv of NaBEt3H under an atmosphere of dinitrogen furnished the diamagnetic iron(II) dihydride dinitrogen complex, ((iPr)PNP)FeH2(N2). Addition of 1 equiv of PhSiH3 to ((iPr)PNP)FeH2(N2) resulted in exclusive substitution of the hydride trans to the pyridine to yield the silyl hydride dinitrogen compound, ((iPr)PNP)FeH(SiH2Ph)N2, which has been characterized by X-ray diffraction. The solid-state structure established a distorted octahedral geometry where the hydride ligand distorts toward the iron silyl. Both ((iPr)PNP)FeH2(N2) and ((iPr)PNP)FeH(SiH2Ph)N2 form eta2-dihydrogen complexes upon exposure to H2. The iron hydrides and the eta2-H2 ligands are in rapid exchange in solution, consistent with the previously reported "cis" effect, arising from a dipole/induced dipole interaction between the two ligands. Taken together, the spectroscopic, structural, and reactivity studies highlight the relative electron-donating ability of this pincer ligand as compared to the redox-active aryl-substituted bis(imino)pyridines.     

Computational study of iron bis(dithiolene) complexes: redox non-innocent ligands and antiferromagnetic coupling

The molecular and electronic structure of monomeric ([Fe(S2C2H2)2](z), [Fe(S2C2(C6H4-p-OCH3)2)2](z)) and dimeric ([{Fe(S2C2H2)2}2](z)) iron bis(dithiolene) complexes, and of their phosphine adducts ([(PH3)Fe(S2C2H2)2](z), [(P(C6H5)3)Fe(S2C2H2)2](z), [(PH3)Fe(S2C2(C6H4-p-OCH3)2)2](z)), carrying various charges (z = 0, 1-, 2-), have been investigated by density functional theory (DFT). Net total spin polarization values S of zero, two, and four have been considered for all neutral model compounds and their dianions, whereas all monoanions have been examined with net total spin polarization values S of one, three, and five. The DFT calculations utilized the pure functional BP86, as well as the hybrid functionals B3LYP and B3LYP*. For the monomers, the calculations reveal the presence of redox non-innocent dithiolene ligands and antiferromagnetic coupling between the ligands and the metal center. For the dimers, complexes with antiferromagnetically coupled iron centers have been found to represent structures of low energy, if not lowest energy structures. The spin-coupling constant of [{Fe(S2C2H2)2}2](2-) is calculated as J = -230 cm(-1). On the basis of the computational results, a model for reversible, electrochemically controlled binding and release of phosphine ligands to iron bis(dithiolene) complexes is proposed. Only BP86 and B3LYP* results, but not those of B3LYP calculations, are in qualitative agreement with experimental findings. BP86 calculations provide the best quantitative match in comparison with the experiment.     

Bis(imino)pyridine ligand deprotonation promoted by a transient iron amide

Addition of 2 equiv of LiNMe(2) to the bis(imino)pyridine ferrous dichloride, ((i)(Pr)PDI)FeCl(2) ((i)(Pr)PDI = (2,6-(i)()Pr(2)-C(6)H(3)N=CMe)(2)C(5)H(3)N), resulted in deprotonation of the chelate methyl groups, yielding the bis(enamide)pyridine iron dimethylamine adduct, ((i)(Pr)PDEA)Fe(NHMe(2)) ((i)(Pr)PDEA = (2,6-(i)Pr(2)-C(6)H(3)NC=CH(2))(2)C(5)H(3)N). Performing a similar procedure with KN(SiMe(3))(2) in THF solution afforded the corresponding bis(THF) adduct, ((i)(Pr)PDEA)Fe(THF)(2). ((i)(Pr)PDEA)Fe(NHMe(2)) has also been prepared by addition of the free amine to the iron dialkyl complex, ((i)(Pr)PDI)Fe(CH(2)SiMe(3))(2), implicating formation of a transient iron amide that is sufficiently basic to deprotonate the bis(imino)pyridine methyl groups. Deprotonation of the amine ligand in ((i)(Pr)PDEA)Fe(NHMe(2)) has been accomplished by addition of amide bases to afford the ferrous amide-ate complexes, [((i)(Pr)PDEA)Fe(mu-NMe(2))M] (M = Li, K).     

Catalytic reduction of dinitrogen to ammonia by molybdenum: theory versus experiment

Molybdenum complexes that contain the triamidoamine ligand [(RNCH(2)CH(2))(3)N](3-) (R = 3,5-(2,4,6-iPr(3)C(6)H(2))(2)C(6)H(3)) catalyze the reduction of dinitrogen to ammonia at 22 degrees C and 1 atm with protons from 2,6-dimethylpyridinium and electrons from decamethylchromocene. Several theoretical studies have been published that bear on the proposed intermediates in the catalytic dinitrogen reduction reaction and their reaction characteristics, including DFT calculations on [(HIPTNCH(2)CH(2))(3)N]Mo species (HIPT =hexaisopropylterphenyl = 3,5-(2,4,6-iPr(3)C(6)H(2))(2)C(6)H(3)), which contain the actual triamidoamine ligand that is present in catalytic intermediates. Recent theoretical findings are compared with experimental findings for each proposed step in the catalytic reaction.     

Coordination of a complete series of N2 reduction intermediates (N2H2, N2H4, and NH3) to an iron phosphine scaffold

The series of dinitrogen reduction intermediates (N(2)H(2), N(2)H(4), and NH(3)) coordinated to the Fe(DMeOPrPE)(2)H(+)(DMeOPrPE = 1,2-[bis(dimethoxypropyl)phosphino]ethane) scaffold has been synthesized or generated. The synthesis of trans-[Fe(DMeOPrPE)(2)(NH(3))H][BPh(4)] and generation of trans-[Fe(DMeOPrPE)(2)(N(2)H(4))H][BPh(4)] were achieved by substitu tion of the dinitrogen ligand on trans-[Fe(DMeOPrPE)(2)(N(2))H][BPh(4)]. The trans-[Fe(DMeOPrPE)(2)(N(2)H(2))H](+) complex and its deprotonated conjugate base, trans-Fe(DMeOPrPE)(2)(N(2)H)H, were observed by (31)P and (1)H NMR from decomposition of trans-[Fe(DMeOPrPE)(2)(N(2)H(4))H](+) in the presence of excess hydrazine. Attempts to chemically oxidize trans-[Fe(DMeOPrPE)(2)(N(2)H(4))H](+) to trans-[Fe(DMeOPrPE)(2)(N(2)H(2))H][BPh(4)] with a variety of oxidizing agents yielded only decomposition products consistent with the intermediate formation of trans-[Fe(DMeOPrPE)(2)(N(2)H(2))H](+) prior to decomposition.     

High-frequency and -field EPR of a pseudo-octahedral complex of high-spin Fe(II): bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II)

A pseudo-octahedral complex of high-spin Fe(II), bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II), which has a cis-FeN'2N4 chromophore, has been investigated by high-frequency, high-field electron paramagnetic resonance (HFEPR). Complementary M?ssbauer and DC magnetic susceptibility studies were also performed. HFEPR spectra of powder samples were recorded at frequencies up to 700 GHz and over a magnetic field range of 0-25 T. Analysis of the field-frequency data set yields the following set of spin Hamiltonian parameters for S = 2: D = +12.427(12) cm-1, E = +0.243(3) cm-1; gx = 2.147(3), gy = 2.166(3), gz = 2.01(1). The parameters are analyzed by use of a simple crystal-field model. This study represents the first precise determination by HFEPR of spin Hamiltonian parameters in six-coordinate high-spin Fe(II) and indicates the applicability of HFEPR to the study of high-spin Fe(II) in coordination complexes and biological model compounds.     

Bis(pyrazol-1-yl)acetates as tripodal heteroscorpionate ligands in iron chemistry: syntheses and structures of iron(II) and iron(III) complexes with bpza, bdmpza, and bdtbpza ligands

The molecular structure of the previously reported species "[Fe(bdtbpza)Cl]" has been revealed by X-ray structure determination to be a ferrous dimer [Fe(bdtbpza)Cl](2) (2c) [bdtbpza = bis(3,5-di-tert-butylpyrazol-1-yl)acetate]. The syntheses of ferrous 2:1 complexes [Fe(bpza)(2)] (3a) and [Fe(bdtbpza)(2)] (3c) as well as ferric 1:1 complexes [NEt(4)][Fe(bpza)Cl(3)] (4a) and [NEt(4)][Fe(bdmpza)Cl(3)] (4b) [bpza = bis(pyrazol-1-yl)acetate, bdmpza = bis(3,5-dimethylpyrazol-1-yl)acetate] are reported. Complexes 3a, previously reported [Fe(bdmpza)(2)] (3b), and 3c are high-spin. No spin crossover to the low-spin state was observed in the temperature range of 5-350 K. 4a and 4b are synthesized in one step and in high yield from [NEt(4)](2)[Cl(3)FeOFeCl(3)]. 4a and 4b are iron(III) high-spin complexes. Crystallographic information: 2c (C(24)H(39)ClFeN(4)O(2).CH(2)Cl(2).CH(3)CN) is triclinic, P1, a = 12.171(16) A, b = 12.851(14) A, c = 13.390(13) A, alpha = 98.61(9) degrees, beta = 113.51(11) degrees, gamma = 108.10(5) degrees, Z = 2; 3a (C(8)H(7)Fe(0.5)N(4)O(2)) is monoclinic, P2(1)/n, a = 7.4784(19) A, b = 7.604(3) A, c = 16.196(4) A, beta = 95.397(9) degrees, Z = 4; 3c (C(24)H(39)Fe(0.5)N(4)O(2)) is monoclinic, P2(1)/n, a = 9.939(6) A, b = 18.161(10) A, c = 13.722(8) A, beta = 97.67(7) degrees, Z = 4; 4b (C(20)H(35)Cl(3)FeN(5)O(2)) is monoclinic, C2/c, a = 30.45(6) A, b = 12.33(2) A, c = 16.17(3) A, beta = 118.47(5) degrees, Z = 8.     

Mechanistic studies on oxidation of hydrazine by a mu-oxo diiron(III,III) complex in aqueous acidic media-proton coupled electron transfer

[Fe2(mu-O)(phen)4(H2O)2]4+ (1), one of the simplest mu-oxo diiron(III) complexes, quantitatively oxidises hydrazine to dinitrogen and itself is reduced to two moles of ferroin, [Fe(phen)3]2+ in presence of excess phenanthroline. The weak dibasic acid, 1 (pKa1= 3.71 +/- 0.05 and pKa2= 5.28 +/- 0.10 at 25.0 degrees C, I= 1.0 mol dm(-3)(NaNO3)) and its conjugate bases, [Fe2(mu-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(mu-O)(phen)4(OH)2]2+ (3) are involved in the redox process with the reactivity order 1 > 2 > 3 whereas N2H4 and not N2H5+ was found to be reactive in the pH interval studied 3.45-5.60. Cyclic voltammetric studies indicate poor oxidizing capacity of the title substitution-labile diiron complex, yet it oxidizes N2H4 with a moderate rate--a proton coupled electron transfer (1e, 1H+) drags the energetically unfavourable reaction to completion. The rate retardation in D2O media is substantially higher at higher pH due to the increasing basicity of the oxo-ligand in the order 3 > 2 > 1. Marcus calculations result an unacceptably high one-electron self-exchange rate for the iron center indicating an inner-sphere nature of the electron-transfer.     

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.