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.     

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).     

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.     

Catalytic C-H bond amination from high-spin iron imido complexes

Dipyrromethene ligand scaffolds were synthesized bearing large aryl (2,4,6-Ph(3)C(6)H(2), abbreviated Ar) or alkyl ((t)Bu, adamantyl) flanking groups to afford three new disubstituted ligands ((R)L, 1,9-R(2)-5-mesityldipyrromethene, R=aryl, alkyl). While high-spin (S=2), four-coordinate iron complexes of the type ((R)L)FeCl(solv) were obtained with the alkyl-substituted ligand varieties (for R=(t)Bu, Ad and solv=THF, OEt(2)), use of the sterically encumbered aryl-substituted ligand precluded binding of solvent and cleanly afforded a high-spin (S=2), three-coordinate complex of the type ((Ar)L)FeCl. Reaction of ((Ad)L)FeCl(OEt(2)) with alkyl azides resulted in the catalytic amination of C-H bonds or olefin aziridination at room temperature. Using a 5% catalyst loading, 12 turnovers were obtained for the amination of toluene as a substrate, while greater than 85% of alkyl azide was converted to the corresponding aziridine employing styrene as a substrate. A primary kinetic isotope effect of 12.8(5) was obtained for the reaction of ((Ad)L)FeCl(OEt(2)) with adamantyl azide in an equimolar toluene/toluene-d(8) mixture, consistent with the amination proceeding through a hydrogen atom abstraction, radical rebound type mechanism. Reaction of p-(t)BuC(6)H(4)N(3) with ((Ar)L)FeCl permitted isolation of a high-spin (S=2) iron complex featuring a terminal imido ligand, ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))), as determined by (1)H NMR, X-ray crystallography, and (57)Fe Mo?ssbauer spectroscopy. The measured Fe-N(imide) bond distance (1.768(2) ?) is the longest reported for Fe(imido) complexes in any geometry or spin state, and the disruption of the bond metrics within the imido aryl substituent suggests delocalization of a radical throughout the aryl ring. Zero-field (57)Fe Mo?ssbauer parameters obtained for ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))) suggest a Fe(III) formulation and are nearly identical with those observed for a structurally similar, high-spin Fe(III) complex bearing the same dipyrromethene framework. Theoretical analyses of ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))) suggest a formulation for this reactive species to be a high-spin Fe(III) center antiferromagnetically coupled to an imido-based radical (J = -673 cm(-1)). The terminal imido complex was effective for delivering the nitrene moiety to both C-H bond substrates (42% yield) as well as styrene (76% yield). Furthermore, a primary kinetic isotope effect of 24(3) was obtained for the reaction of ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))) with an equimolar toluene/toluene-d(8) mixture, consistent with the values obtained in the catalytic reaction. This commonality suggests the isolated high-spin Fe(III) imido radical is a viable intermediate in the catalytic reaction pathway. Given the breadth of iron imido complexes spanning several oxidation states (Fe(II)-Fe(V)) and several spin states (S=0→(3)/(2)), we propose the unusual electronic structure of the described high-spin iron imido complexes contributes to the observed catalytic reactivity.     

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.     

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.     

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.     

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.     

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.     

Monovalent iron in a sulfur-rich environment

A series of low-coordinate, paramagnetic iron complexes in a tris(thioether) ligand environment have been prepared. Reduction of ferrous {[PhTt(tBu)]FeCl}2 [1; PhTt(tBu) = phenyltris((tert-butylthio)methyl)borate] with KC8 in the presence of PR3(R = Me or Et) yields the high-spin, monovalent iron phosphine complexes [PhTt(tBu)]Fe(PR3) (2). These complexes provide entry into other low-valent derivatives via ligand substitution. Carbonylation led to smooth formation of the low-spin dicarbonyl [PhTt(tBu)]Fe(CO)2 (3). Alternatively, replacement of PR 3 with diphenylacetylene produced the high-spin alkyne complex [PhTt(tBu)]Fe(PhCCPh) (4). Lastly, 2 equiv of adamantyl azide undergoes a 3 + 2 cycloaddition at 2, yielding high-spin dialkyltetraazadiene complex 5.     

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.     

High-spin diimine complexes of iron(II) reject binding of carbon monoxide: theoretical analysis of thermodynamic factors inhibiting or favoring spin-crossover

A new series of Fe(II) complexes, FeCl2[N(R)=C(Me)C(Me)=N(R)], containing diimine ligands with hemilabile sidearms R (R = CH2(CH2)2NMe2, 1, CH2(CH2)2OMe, 2, CH2(CH2)2SMe), 3) were synthesized. The crystal structure of 1 showed 6-coordination where both amine arms were attached, whereas 2 was a 5-coordinate 16e species with one methoxy arm dangling free. Extensive attempts were made to bind CO to these species to synthesize precursors for dihydrogen complexes but were unsuccessful. Reaction of 1 with 1 or 2 equiv of AgOTf under CO atmosphere resulted in isolation of only a 6-coordinate bis(triflate)-containing product [Fe[N(R)=C(Me)C(Me)=N(R)](OTf)2] (R = CH2(CH2)2NMe2), 5. Reaction of 5-coordinate 2 with AgSbF6 under CO did not give a CO adduct but afforded instead a dicationic dinuclear complex [Fe[N(R)=C(Me)C(Me)=N(R)](mu-Cl)]2[SbF6]2 (R = CH2(CH2)2OMe), 4, containing a weakly bound SbF6. Thus coordination of hard-donor anions to iron was favored over CO binding. The unexpected rejection of binding of CO is rationalized by the iron being in a high-spin state in this system and energetically incapable of spin crossover to a low-spin state. Theoretical calculations on CO interaction with Fe(II) centers in spin states S = 0, 1, and 2 for both the 16e complexes and their CO adducts aid further understanding of this problem. They show that interaction of CO with a high-spin 5-coordinate Fe model diimine complex is essentially thermoneutral but is exergonic by about 48 kcal/mol to a comparable but low-spin diphosphine fragment. Spin crossover is thus disfavored thermodynamically rather than kinetically (e.g. a "spin block" effect); i.e., the ligand field strengths of the primarily N-donor groups are apparently insufficient to give a low-spin CO adduct.     

Electronic structure of bispidine iron(IV) oxo complexes

The electronic structure, based on DFT calculations, of a range of FeIV=O complexes with two tetra- (L1 and L2) and two isomeric pentadentate bispidine ligands (L3 and L4) is discussed with special emphasis on the relative stability of the two possible spin states (S = 1, triplet, intermediate-spin, and S = 2, quintet, high-spin; bispidines are very rigid diazaadamantane-derived 3,7-diazabicyclo[3.3.1]nonane ligands with two tertiary amine and two or three pyridine donors, leading to cis-octahedral [(X)(L)FeIV=O]2+ complexes, where X = NCCH3, OH2, OH-, and pyridine, and where X = pyridine is tethered to the bispidine backbone in L3, L4). The two main structural effects are a strong trans influence, exerted by the oxo group in both the triplet and the quintet spin states, and a Jahn-Teller-type distortion in the plane perpendicular to the oxo group in the quintet state. Due to the ligand architecture the two sites for substrate coordination in complexes with the tetradentate ligands L1 and L2 are electronically very different, and with the pentadentate ligands L3 and L4, a single isomer is enforced in each case. Because of the rigidity of the bispidine ligands and the orientation of the "Jahn-Teller axis", which is controlled by the sixth donor X, the Jahn-Teller-type distortion in the high-spin state of the two isomers is quite different. It is shown how this can be used as a design principle to tune the relative stability of the two spin states.     

Iron and cobalt ethylene polymerization catalysts: variations on the central donor

Three classes of ligands, designed to explore the effect of variations on the central pyridine donor core in bis(imino)pyridine iron and cobalt ethylene polymerization catalysts of the general formula [LMCl(2)] (M = Fe or Co), have been prepared. The first class comprises six-membered N-heterocycles (pyrimidine and triazine) and the second class five-membered heterocycles (furan and thiophene) as the central donor core. In the third class of ligands, the imine donor arm has been extended by one carbon to give anionic tridentate ligands based on carbazole and neutral analogues based on dibenzofuran and dibenzothiophene. The coordination behavior of these ligands upon reaction with FeCl(2) or CoCl(2) has been investigated, whereby only in the case of the neutral pyrimidine or the anionic carbazolide unit as the central donor core have stable complexes been obtained. Ethylene polymerization results are compared with the parent bis(imino)pyridine iron and cobalt catalyst systems.     

Mononuclear, dinuclear, and pentanuclear [[N,S(thiolate)]iron(II)] complexes: nuclearity control, incorporation of hydroxide bridging ligands, and magnetic behavior

The mixed N3S(thiolate) ligand 1-[bis[2-(pyridin-2-yl)ethyl]amino]-2-methylpropane-2-thiol (Py2SH) was used in the synthesis of four iron(II) complexes: [(Py2S)FeCl] (1), [(Py2S)FeBr] (2), [(Py2S)4Fe5II(mu-OH)2](BF4)4 (3), and [(Py2S)2Fe2II(mu-OH)]BF4 (4). The X-ray structures of 1 and 2 revealed monomeric iron(II)-alkylthiolate complexes with distorted trigonal-bipyramidal geometries. The paramagnetic 1H NMR spectra of 1 and 2 display resonances from delta = -25 ppm to +100 ppm, consistent with a high-spin iron(II) ion (S = 2). Spectral assignments were made on the basis of chemical shift information and T1 measurements and show the monomeric structures are intact in solution. To provide entry into hydroxide-containing complexes, a novel synthetic method was developed involving strict aprotic conditions and limiting amounts of H2O. Reaction of Py2SH with NaH and Fe(BF4)2.6 H2O under aprotic conditions led to the isolation of the pentanuclear, mu-OH complex 3, which has a novel dimer-of-dimers type structure connected by a central iron atom. Conductivity data on 3 show this structure is retained in CH2Cl2. Rational modification of the ligand-to-metal ratio allows control over the nuclearity of the product, yielding the dinuclear complex 4. The X-ray structure of 4 reveals an unprecedented face-sharing, biooctahedral complex with an [S2O] bridging arrangement. The magnetic properties of 3 and 4 in the range 1.9-300 K were successfully modeled. Dinuclear 4 is antiferromagnetically coupled [J = -18.8(2) cm(-1)]. Pentanuclear 3 exhibits ferrimagnetic behavior, with a high-spin ground state of S(T) = 6, and was best modeled with three different exchange parameters [J = -15.3(2), J' = -24.7(3), and J' = -5.36(7) cm(-1)]. DFT calculations provided good support for the interpretation of the magnetic properties.     

Non-heme iron(II) complexes containing tripodal tetradentate nitrogen ligands and their application in alkane oxidation catalysis

A series of iron(II) bis(triflate) complexes containing tripodal tetradentate nitrogen ligands with pyridine and dimethylamine donors of the type [N(CH(2)Pyr)(3-n)()(CH(2)CH(2)NMe(2))(n)] [n = 0 (tpa, 1), n = 1 (iso-bpmen, 3), n = 2 (Me(4)-benpa, 4), n = 3 (Me(6)-tren, 5)] and the linear tetradentate ligand [(CH(2)Pyr)MeN(CH(2)CH(2))NMe(CH(2)Pyr), (bpmen, 2)] has been prepared. The preferred coordination geometry of these complexes in the solid state and in CH(2)Cl(2) solution changes from six- to five-coordinate in the order from 1 to 5. In acetonitrile, the triflate ligands of all complexes are readily displaced by acetonitrile ligands. The complex [Fe(1)(CH(3)CN)(2)](2+) is essentially low spin at room temperature, whereas ligands with fewer pyridine donors increase the preference for high-spin Fe(II). Both the number of pyridine donors and the spin state of the metal center strongly affect the intensity of a characteristic MLCT band around 400 nm. The catalytic properties of the complexes for the oxidation of alkanes have been evaluated, using cyclohexane as the substrate. Complexes containing ligands 1-3 are more active and selective catalysts, possibly operating via a metal-based oxidation mechanism, whereas complexes containing ligands 4 and 5 give rise to Fenton-type chemistry.