A Mononuclear Nonheme Iron(IV)-Oxo Complex of a Substituted N4Py Ligand: Effect of Ligand Field on Oxygen Atom Transfer and C-H Bond Cleavage Reactivity

Article

pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX −XXX

A Mononuclear Nonheme Iron(IV)-Oxo Complex of a Substituted

N4Py Ligand: Eﬀect of Ligand Field on Oxygen Atom Transfer and

C−H Bond Cleavage Reactivity

Reena Singh,*^,†Gaurab Ganguly,^†Sergey O. Malinkin,^‡,∥Serhiy Demeshko,^§Franc Meyer,^§

Ebbe Nordlander,^‡and Tapan Kanti Paine*^,†

^†School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata

700032, India

^‡Chemical Physics, Department of Chemistry, Lund University, Box 124, SE-22100 Lund, Sweden

§

̈

Universitat Gottingen, Institut fur Anorganische Chemie, Tammanstrasse 4, D-37077 Gottingen, Germany

S

* Supporting Information

ABSTRACT: A mononuclear iron(II) complex

[Fe^II(N4Py^Me2)(OTf)](OTf)(1), supported by a new penta-

dentate ligand, bis(6-methylpyridin-2-yl)-N,N-bis((pyridin-2-

yl)methyl)methanamine (N4Py^Me2), has been isolated and

characterized. Introduction of methyl groups in the 6-position

of two pyridine rings makes the N4Py^Me2a weaker ﬁeld ligand

compared to the parent N4Py ligand. Complex 1 is high-spin in

the solid state and converts to [Fe^II(N4Py^Me2)(CH₃CN)]-

(OTf)₂(1a) in acetonitrile solution. The iron(II) complex in

acetonitrile displays temperature-dependent spin-crossover

behavior over a wide range of temperature. In its reaction with m-CPBA or oxone in acetonitrile at −10 °C, the iron(II)

complex converts to an iron(IV)-oxo species, [Fe^IV(O)(N4Py^Me2)]²⁺(2). Complex 2 exhibits the Mossbauer parameters δ =

̈

0.05 mm/s and ΔE_Q= 0.62 mm/s, typical of N-ligated S = 1 iron(IV)-oxo species. The iron(IV)-oxo complex has a half-life of

only 14 min at 25 °C and is reactive toward oxygen-atom-transfer and hydrogen-atom-transfer (HAT) reactions. Compared to

the parent complex [Fe^IV(O)(N4Py)]²⁺, 2 is more reactive in oxidizing thioanisole and oxygenates the C−H bonds of aliphatic

substrates including that of cyclohexane. The enhanced reactivity of 2 toward cyclohexane results from the involvement of the S

= 2 transition state in the HAT pathway and a lower triplet-quintet splitting compared to [Fe^IV(O)(N4Py)]²⁺, as supported by

DFT calculations. The second-order rate constants for HAT by 2 is well correlated with the C−H bond dissociation energies of

aliphatic substrates. Surprisingly, the slope of this correlation is diﬀerent from that of [Fe^IV(O)(N4Py)]²⁺, and 2 is more reactive

only in the case of strong C−H bonds (>86 kcal/mol), but less reactive in the case of weaker C−H bonds. Using oxone as the

oxidant, the iron(II) complex displays catalytic oxidations of substrates with low activity but with good selectivity.

heme iron(IV)-oxo complexes supported by polydentate

ligands have been reported over the last couple of

decades.^{22,23,25,30,31}These complexes display varying degrees

of stability and exhibit versatile reactivity such as C−H bond

activation and oxo-atom-transfer reactions. The iron(IV)-oxo

complex supported by the 1,4,8,11-tetramethyl-1,4,8,11-

tetraazacyclotetradecene (TMC) ligand can only oxidize

substrates with C−H bond dissociation energies (BDEs) <80

kcal/mol; whereas those based on N4Py (N4Py = N,N-bis(2-

pyridylmethyl)-N-bis(2-pyridyl)methylamine), Bn-tpen (Bn-

tpen = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diamino-

ethane), and Me₃NTB (tris(N-methylbenzimidazol-2-yl)-

methyl)amine) have been reported to oxidize C−H bonds as

strong as those of cyclohexane (C−H BDE 99.3 kcal/

mol).³²⁻³⁶While enzymatic systems involve high-spin (S =

INTRODUCTION

■

A large variety of iron enzymes activate dioxygen (O₂) to

catalyze biologically important oxidation reactions.¹⁻⁵For

many of these oxidation reactions, the reductive activation of

dioxygen at the reduced iron center leads to the generation of

high-valent iron-oxo species as the active oxidant. In heme

biochemistry, high-valent iron(IV)-oxo species coupled to a

radical (e.g., iron(IV)-oxo(porphyrin radical cation)) have

been identiﬁed as active oxidants.⁶⁻⁸Diiron(IV) intermediates,

generated from dinuclear nonheme iron enzymes, have been

postulated to have strong oxidizing power, permitting them to

cleave strong hydrocarbon C−H bonds.^1,9−12The character-

ization of high-spin nonheme iron(IV)-oxo intermediates in

taurine α-ketoglutarate dioxygenase (TauD) and in other

nonheme iron oxygenases such as prolyl hydroxylase, tyrosine

hydroxylases, and in halogenases¹³⁻¹⁹has fueled interest in

synthesis and reactivity studies of nonheme high-valent iron-

oxo complexes.²⁰⁻²⁹Consequently, many mononuclear non-

Received: September 11, 2018

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Scheme 1. Synthesis of the N4Py^Me2Ligand and Its Iron(II) Complex

2) iron(IV)-oxo oxidants, the majority of the well-charac-

terized iron(IV)-oxo complexes possess an S = 1 ground spin

state.^{20,21,24,25,27,31,36−51}Recently, the isolation and reactivity

studies of a few high-spin iron(IV)-oxo complexes have been

reported.⁵²⁻⁶⁰Theoretical calculations suggest that S = 2

iron(IV)-oxo species are more reactive toward C−H bond

activation than the S = 1 species,^61,62which would be an

obvious reason for the presence of the S = 2 spin state in

intermediates for both mononuclear and dinuclear nonheme

enzyme sites in contrast to the low-spin form that is found in

heme enzymes. To stabilize high-spin iron(IV)-oxo complexes,

weak ﬁeld ligands are required. The supporting ligands should

either enforce C_4vsymmetry with weak equatorial donors or

should engender C_3vsymmetry to enable high-spin conﬁg-

uration.²⁴Furthermore, appropriate design of the supporting

ligand is required to tune the stability and reactivity of the

resulting iron-oxo complexes.

The S = 1 iron(IV)-oxo complex of the N4Py ligand has

been reported to exhibit C−H bond activation and oxo-atom-

transfer reactivity.³⁸In spite of its long half-life (60 h at 298

K), the complex can slowly oxidize the C−H bonds of

cyclohexane. Computational modeling indicates that a small

energy gap between the S = 1 and S = 2 states enables the

complex to react via the accessible S = 2 state.⁶²If the energy

gap between the two states can be reduced by tuning the

supporting ligand, the more easily accessible S = 2 state is

predicted to make an iron(IV)-oxo species highly reactive. A

pentadentate ligand based on the N4Py platform incorporating

phenyl substituents on the dipyridyl unit has been reported to

decrease the energy separation between the S = 1 and S = 2

states compared to that for the parent N4Py system.⁶³

However, due to the orientation of the phenyl rings in close

proximity to the iron-oxo units, the intermediate was prone to

intramolecular aromatic C−H hydroxylation. Substitution of

phenyl groups on the pyridine rings by 2,6-diﬂuorophenyl

groups allowed the trapping of the corresponding S = 1

iron(IV)-oxo complex for structural characterization. Interest-

ingly, the iron(IV)-oxo species was capable of carrying out

intramolecular aromatic C−F bond hydroxylation.^64,65Re-

placement of the two pyridine donors of the bis(pyridyl-2-

methyl)arms of the N4Py scaﬀold by N-methylbenzimidazole

or quinoline groups has been reported to enhance the

reactivity of the corresponding iron(IV)-oxo complexes toward

hydrogen-atom and oxygen-atom-transfer reactions.^66,67Very

recently, the X-ray crystal structures of the iron(IV)-oxo

complexes of the N4Py-type ligands with two N-methylbenzi-

midazole or quinoline donors revealed longer average Fe−N

distances than those of the corresponding complex of the

parent N4Py ligand. The steric eﬀects of the quinolyl donors

not only increased the average Fe−N bond length but also

tilted the FeO unit away from the donor groups.⁶⁷With an

objective to develop highly reactive iron-oxo complexes, we

have been exploring the eﬀect of alkyl substitution on the

pyridine donors of the N4Py ligand on the reactivity and

stability of the corresponding iron(IV)-oxo complexes. We

report herein the synthesis and characterization of a

mononuclear iron(II) complex [Fe^II(N4Py^Me2)(OTf)](OTf)-

(1), supported by the new bis(6-methylpyridin-2-yl)-N,N-

bis((pyridin-2-yl)methyl)methanamine (N4Py^Me2) ligand

(Scheme 1). The identity of the complex in acetonitrile

solution has been established using variable-temperature (VT)

̈

NMR, magnetic, and Mossbauer studies. The generation of an

iron(IV)-oxo complex from 1 in acetonitrile and its reactivity

toward external substrates along with the selectivity for

aliphatic C−H bond cleavage and oxygen-atom-transfer

reactions are presented in this work.

RESULTS AND DISCUSSION

■

Synthesis and Characterization. The pentadentate

ligand N4Py^Me2was prepared by a reductive amination

reaction between bis(6-methylpyridin-2-yl)methanamine⁶⁸

and pyridine-2-carboxaldehyde in dichloromethane. The iron-

(II) complex [Fe^II(N4Py^Me2)(OTf)](OTf) (1) was isolated as

a light yellow solid from the reaction of equimolar amounts of

Fe(OTf)₂·2MeCN and ligand in dichloromethane, followed by

washing the crude product with a mixture of dichloromethane

and diethyl ether (Scheme 1 and Experimental Section).

The ESI-mass spectrum (positive ion mode in acetonitrile)

of complex 1 exhibits an ion peak at m/z 225.6 with the

isotope distribution pattern calculated for [Fe(N4Py^Me2)]²⁺

(Figure S1, Supporting Information). In the optical spectrum

in acetonitrile, the complex displays charge-transfer bands at

455 nm (sh), 430 nm (sh), and 360 nm (ε ∼ 1200 M⁻¹cm⁻¹).

These bands may be attributed to metal-to-ligand charge-

transfer (MLCT) transitions (Figure S2, Supporting Informa-

tion). The intensity of the charge-transfer bands is however

weaker compared to other reported iron(II) complexes of

N4Py-type ligands, indicating that the complex is high-

spin.^66,69

Magnetic susceptibility measurements for a solid sample of 1

display a χ_MT value of 3.6 cm³K mol⁻¹in the region between

80 and 295 K, as expected for a high-spin iron(II) complex

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(Figure S3, Supporting Information). With decreasing temper-

ature below 80 K, the χ_MT value slowly decreases to 1.94 cm³

K mol⁻¹, likely due to the presence of zero ﬁeld splitting (|D| =

8.3 cm⁻¹). The ⁵⁷Fe Mossbauer spectrum of a solid sample of 1

̈

measured in zero-ﬁeld at 80 K with an isomer shift δ = 1.20

mm/s and quadrupole splitting ΔE_Q= 3.18 mm/s conﬁrms the

high-spin nature of the complex (Figure 1). Decreasing the

temperature to 10 K does not change the spectral pattern of

the solid sample (Figure S4, Supporting Information).

57

̈

Figure 1. Zero-ﬁeld Fe Mossbauer spectrum of a solid sample of 1

measured at 80 K.

Figure 2. Zero-ﬁeld ⁵⁷Fe Mossbauer spectrum of a frozen sample of 1

in acetonitrile measured at 80 K (top) and 200 K (bottom).

̈

Having understood the spin state of 1 in the solid state, the

spin state in solution was evaluated by VT ¹H NMR,

̈

Mossbauer, and magnetic measurements. In CD₃CN at 298

K, the H NMR spectrum exhibits paramagnetically shifted

1

proton resonances in the region from −30 ppm to 70 ppm

(Figure S5, Supporting Information), typical of a high-spin

nature of the complex. No appreciable change in the spectral

pattern is observed except slight shifting of some signals upon

decreasing the temperature to 233 K (Figure S5, Supporting

Information). Thus, the complex remains high-spin within the

accessible low-temperature limit for NMR measurements in

̈

CD₃CN. Interestingly, when Mossbauer measurements are

carried out on a frozen sample of the complex in acetonitrile in

zero ﬁeld at 80 K, two species with nearly 1:1 ratio are

observed: a high-spin species (54%) with δ = 1.16 mm/s and

ΔE_Q= 2.79 mm/s and a low-spin species (46%) with δ = 0.51

mm/s and ΔE_Q= 0.39 mm/s (Figure 2). Upon increasing the

temperature to 200 K, the proportion of the high-spin species

increases to 82%, indicating the presence of spin-crossover

(SCO) behavior in frozen solution. At 10 K, the high- and low-

spin species remain in identical ratio as at 80 K (Figure S6,

Supporting Information).

Figure 3. Variable-temperature magnetic data of 1 in acetonitrile

(ZFS = zero-ﬁeld splitting; SCO = spin crossover).

All the results discussed above support the idea that the

species in acetonitrile can exhibit temperature-dependent SCO

behavior, whereas the solid sample of 1 is purely high-spin. To

̈

Since the Mossbauer measurements cover only the frozen

solution temperature range, while NMR spectroscopy is

restricted to the liquid solution phase, magnetic susceptibility

measurements with a SQUID magnetometer were carried out

on an acetonitrile solution of 1 to cover the entire temperature

range between 295 and 2 K. The magnetic data are in

evaluate the nature of the species generated in acetonitrile, ¹⁹

F

NMR spectra of the sample were measured in two diﬀerent

solvents. The triﬂate anion, being a labile ligand, may easily be

replaced by solvent molecules, leading to the formation of

diﬀerent species. In a coordinating solvent such as CH₃CN, the

¹⁹F NMR spectrum of the complex displays a sharp resonance

at −78 ppm (typical for free triﬂate anion), while a relatively

broad signal at −50 ppm is observed in CD₂Cl₂(Figures S7

and S8, Supporting Information, respectively). These results

indicate that in acetonitrile, the triﬂate is noncoordinating,⁷⁰

while a fast exchange between coordinated and free triﬂate

anion takes place in dichloromethane. It is thus assumed that

̈

accordance with both NMR and Mossbauer spectral data and

provide a complete picture. The complex in acetonitrile is

purely high-spin above ca. 220 K, but upon lowering the

temperature, a gradual and incomplete SCO occurs until the

ratio of high- and low-spin species becomes almost 1:1 at

around 80 K (Figure 3). A further decrease of χ_MT is attributed

to the zero-ﬁeld splitting eﬀect of the remaining high-spin

species (cf. Figure S3, Supporting Information).

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the triﬂate-coordinated high-spin species [Fe^II(N4Py^Me2)-

(OTf)](OTf) is present in CH₂Cl₂solution and also in solid

material of 1 that has been thoroughly washed with CH₂Cl₂(in

agreement with elemental analysis data). In contrast, complex

1 in acetonitrile converts to [Fe^II(N4Py^Me2)(CH₃CN)](OTf)₂

(1a) as the dominant species. It remains unclear, however, why

the SCO of 1a is incomplete and reaches only a 1:1 ratio of

high-spin and low-spin species at low temperatures; no

evidence for dimerization of 1a has been observed.

For structural characterization of the complex in the solid

state, eﬀorts were made to grow single crystals of 1.

Unfortunately, all attempts to isolate single crystals of 1 or

1a with triﬂate counterion(s) were unsuccessful. However, X-

ray quality single crystals of the perchlorate salt of 1a,

[Fe(N4Py^Me2)(CH₃CN)](ClO₄)₂(1b), were grown by

recrystallization of the complex (isolated from the reaction of

iron(II) perchlorate hydrate and N4Py^Me2in acetonitrile) from

a solvent mixture of acetonitrile and diethyl ether. A molecular

view of the cationic part of complex 1b is shown in Figure 4.

Table 1. Comparison of Crystallographic Bond Distances

(Å) and Angles (deg) of [Fe^II(N4Py^Me2)(CH₃CN)]²⁺(1b)

and [Fe^II(N4Py)(CH₃CN)]²⁺

bond distances and

angles

[Fe^II(N4Py^Me2

)

a

(CH₃CN)]²⁺(1b)

[Fe^II(N4Py)(CH₃CN)]²⁺

Fe1−N1

Fe1−N2

Fe1−N3

Fe1−N4

Fe1- N5

Fe1−N6

1.977(8)

2.045(8)

2.043(8)

1.982(9)

1.977(8)

1.889(10)

81.6(4)

81.8(3)

83.2(4)

84.6(4)

178.6(4)

87.2(3)

88.5(4)

166.0(4)

97.1(4)

164.8(4)

92.9(3)

98.6(4)

87.8(4)

96.4(4)

96.7(4)

1.961(3)

1.976(3)

1.967(3)

1.968(3)

1.975(3)

1.915(3)

N1−Fe1−N2

N1−Fe1−N3

N1−Fe1−N4

N1−Fe1−N5

N1−Fe1−N6

N2−Fe1−N3

N2−Fe1−N4

N2−Fe1−N5

N2−Fe1−N6

N3−Fe1−N4

N3−Fe1−N5

N3−Fe1−N6

N4−Fe1−N5

N4−Fe1−N6

N5−Fe1−N6

82.58(12)

82.56(11)

85.37(12)

85.29(12)

177.32(11)

87.12(11)

90.20(11)

167.71(13)

94.95(12)

167.87(13)

89.37(11)

96.32(12)

90.78(11)

95.69(12)

97.15(12)

a

Ref 71.

adequate coordination of the 6-Me-pyridyl moieties to the

metal and thus leads to lengthened bonds.

All the data for complex 1 discussed above are consistent

with the introduction of the two methyl groups on the pyridine

rings of the N4Py ligand weakening the ligand ﬁeld, resulting

in the formation of a high-spin iron(II)-triﬂate complex (1).

Similar observations have been made for iron(II) complexes of

weak ﬁeld N-methylbenzimidazolyl/quinolyl-substituted N4Py

ligands.^66,67Of note, the iron(II)-acetonitrile complex of the

parent N4Py ligand remains low-spin in both solid state and in

solution (Figure S9, Supporting Information). In contrast, the

iron(II)-acetonitrile complex (1a) with a small energy gap

between the high-spin and low-spin states exhibits temper-

ature-dependent SCO behavior. This observation is consistent

with that reported for substituted TPA ligands (TPA = tris(2-

pyridylmethyl)amine), where methyl substitution on one of

the pyridine rings of TPA is suﬃcient to cause a change in spin

state from low-spin in [Fe^II(TPA)(CH₃CN)₂]²⁺to high-spin in

[Fe^II(6-MeTPA)(CH₃CN)₂]²⁺.⁷³

Generation and Characterization of an Iron(IV)-Oxo

Complex. Complex 1, upon treatment with a slight excess of

m-CPBA in acetonitrile at −10 °C, produces a pale green

intermediate [Fe^IV(O)(N4Py^Me2)]²⁺(2) with an absorption

band at λ_max= 740 nm (ε ∼ 220 M⁻¹cm⁻¹) (Figure 5). A

titration experiment with 1 and m-CPBA in acetonitrile at −40

°C suggests that 5 equiv of m-CPBA is required for maximum

formation of 2 (Figure S10, Supporting Information). Thus,

lowering the temperature does not change the yield of 2. The

formation of 2 is also observed at 25 °C using oxone (2 equiv)

as an oxidant. Complex 2 may also be generated in water upon

reaction with ceric ammonium nitrate at 25 °C; however, a

colloidal suspension is formed immediately after generation of

2, making further investigations diﬃcult. Moreover, the

iron(IV)-oxo complex can be prepared from 1 using excess

Figure 4. ORTEP plot of the complex cation of [Fe^II(N4Py^Me2)-

(CH₃CN)]²⁺(1b) with 30% thermal ellipsoids. All hydrogen atoms

have been omitted for clarity.

The coordination environment around the iron center is

distorted octahedral where the central iron is surrounded by

ﬁve nitrogen donors from the ligand and the sixth coordination

site is occupied by an acetonitrile molecule. The axial positions

are occupied by the amine nitrogen (N1) and the nitrogen

atom (N6) of the solvent molecule with the N1−Fe1−N6

angle being 178.6(4)° (Table 1). The four pyridine nitrogen

donors (N2, N3, N4, and N5) from the pentadentate ligand

make the equatorial plane. The Fe−N bond distances vary in

the range of 2.04−1.88 Å, and are in good agreement with the

reported low-spin iron(II) complexes supported by N4Py-type

ligands (Table 1).^63,69,71The Fe−N_pybonds for the pyridine

moieties bearing 6-methyl groups are lengthened in the

equatorial plane. There are thus two distinct sets of Fe−N_py

distances; the two Fe−N_py‑6‑Medistances average approx-

imately 2.04 Å, while the two Fe−N_pydistances average

approximately 1.98 Å. The elongation of the Fe−N_py‑6‑Me

distances in 1b relative to the analogous distances in the

parent N4Py complex is ca. 0.07 Å (cf. Table 1). Similar

elongated Fe−N_py‑6‑Medistances (average ca. 2.08 Å) have

been observed in the related iron(II)-acetonitrile complex of

the N4Py-type ligand, bis(pyridin-2-yl)-N,N-bis((6-methylpyr-

idin-2-yl)methyl)methanamine ([(N2Py)(N2CH₂Py^Me)]).⁷²

The steric hindrance of the 6-Me substituents prevents

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oxo complex of the parent N4Py ligand (λ_max= 695 nm)

further supports that the N4Py^Me2ligand engenders a weak

ligand ﬁeld in the equatorial plane in 2.

The oxidation state and spin state of the iron center of 2

57

̈

were evaluated by zero-ﬁeld Fe Mossbauer spectroscopy

using a partially (20%) ⁵⁷Fe-enriched sample in frozen

acetonitrile solution at 80 K. The main species (green

subspectrum, 60%) in the spectrum (Figure 6) exhibits the

Figure 5. Optical spectrum of complex 2 (green line) generated upon

treatment of 1 (orange line, 1 mM) with m-CPBA (5 equiv in

acetonitrile) at −10 °C. Inset: (a) Spectrum showing the region 500−

1100 nm. (b) ESI-MS spectrum (positive ion mode in acetonitrile) of

2. Spikes marked with * in the near-IR region likely arise from

artifacts in the data collection process.

(20 equiv) solid PhIO at −40 °C but with low conversion.

Since the precursor iron(II) complex (1) in acetonitrile

remains purely high-spin in the temperature range mentioned

above (Figure 3), the S = 2 iron(II) species leads to the

formation of the iron(IV)-oxo species. In analogy with the

optical spectra for reported iron(IV)-oxo complexes, the

absorption band in the near IR region may be attributed to

ligand ﬁeld (d−d) transitions of the iron(IV) ion.⁷⁴The ESI-

mass spectrum (positive ion mode) of the green species

displays ion peaks at m/z 233.5 with the isotope distribution

patterns calculated for [Fe(O)(N4Py^Me2)]²⁺(C₂₅H₂₅FeN₅O

expected at m/z 233.5) (Figure 5, inset and Figure S11, inset a,

Supporting Information). The half-life (t_1/2) of the inter-

mediate species at 25 °C is found to be 14 min (Figure S12,

Supporting Information), demonstrating thermal instability of

the species compared to the previously reported S = 1

iron(IV)-oxo complexes bearing mixed amine/pyridine donor

pentadentate ligands (Table 2).^{33−35,42,66,75}Of note, the

[Fe(O)(N4Py)]²⁺complex generated in acetonitrile with m-

CPBA shows a half-life (t_1/2) value of 2 h at 25 °C (Table 2),

which is much shorter than the reported value of 60 h for the

species generated with PhIO.³³The shift of the near IR

absorption band to lower energy compared to the iron(IV)-

Figure 6. Zero-ﬁeld ⁵⁷Fe Mossbauer spectrum of a frozen acetonitrile

solution of 2 measured at 80 K.

̈

Mossbauer parameters δ = 0.05 mm/s and ΔE_Q= 0.62 mm/s,

which are typical for N-ligated S = 1 iron(IV)-oxo complexes.²⁵

Thus, the pale green intermediate is an S = 1 iron(IV)-oxo

species [Fe^IV(O)(N4Py^Me2)]²⁺(2). The remaining 40%

species indicate the presence of decomposition products. In

̈

analogy with the Mossbauer parameters of an oxo-bridged

diiron(III) species generated from the [Fe^IV(O)(ⁿBu-P2DA)]

complex (δ = 0.46 mm/s and ΔE_Q= 1.65 mm/s),⁷⁶one of the

iron(III) byproducts (δ = 0.50 mm/s and ΔE_Q= 1.44 mm/s)

is tentatively assigned to [(N4Py^Me2)Fe−O−Fe((N4Py^Me2)]⁴⁺,

while the second one should be a mononuclear high-spin ferric

species. No direct experimental evidence of a possible

exchange of the oxygen atom of the iron(IV)-oxo species (2)

with water was obtained.⁷⁷Treatment of an acetonitrile

solution of 2 with H₂¹⁸O (98% ¹⁸O enriched) did not result in

the formation of any ¹⁸O-incorporated species, as detected by

mass spectrometry. Instead, the ESI-MS displays an ion peak at

m/z 234.0 indicating partial decay of the iron(IV)-oxo to the

Table 2. Optical Spectral Properties and Stability of Diﬀerent Fe(IV)-Oxo Complexes Supported by Diﬀerent Pentadentate

Ligands

complex

λ_max, nm (ε, M⁻¹s⁻¹

)

t_1/2at 298 K

refs

[Fe^IV(O)(N4Py^Me2)]²⁺

[Fe^IV(O)(N4Py)]²⁺

740 (220)

695 (400)

14 min

60 h

this work

33

a

[Fe^IV(O)(Bn-tpen)]²⁺

[Fe^IV(O)(TMC-Py)]²⁺

[Fe^IV(O)(Py₂TACN)]²⁺

[Fe^IV(O)(BisPi1)]²⁺

739 (400)

834 (260)

740(340) 900 (200)

728(400)

6 h

7 h

not reported

40 h

33

42

35

75, 99

75

66

67

[Fe^IV(O)(BisPi2)]²⁺

728(380)

708 (400)

725 (380)

770 (380)

[Fe^IV(O)(N₃Py-(NMB)]²⁺

[Fe^IV(O)(N₂Py-(NMB)₂]²⁺

[Fe^IV(O)(N₂Py2Q]²⁺

2.5 h

a

The iron(IV)-oxo complex generated from the precursor iron(II) complex with m-CPBA in acetonitrile exhibits a t_1/2value of 2 h at 298 K.

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corresponding iron(III)-hydroxide species, [Fe(OH)-

(N4Py^Me2)]²⁺(Figure S11, inset b, Supporting Information).

However, no intraligand hydroxylated product was observed in

the self-decay pathway. The thermal instability of 2 does not

permit the observation of any intermediate to a full extent due

to the partial decay of the complex in the process of

iron-oxo species supported by mixed amine/pyridine donor

pentadentate ligands.³⁵A comparison of oxo-atom-transfer

reaction rates of 2 and other reported synthetic S = 1 iron(IV)-

oxo complexes supported by mixed amine/pyridine donor

ligands is shown in Table 3.

A series of alkane substrates having C−H BDEs ranging

from 77 to 99.3 kcal/mol were used to test the reactivity of 2

toward hydrogen atom abstraction. Under pseudo-ﬁrst-order

reaction conditions, the band at 740 nm decays rapidly in the

presence of 9,10-dihydroanthracene (5 equiv) at −10 °C

(Figure S13, Supporting Information). The linear dependence

of rate on substrate concentration yields the second-order rate

constant values of 1.7 × 10⁻¹M⁻¹s⁻¹and 2.0 M⁻¹s⁻¹at −10

and 25 °C, respectively (Figure S14e, Supporting Information

and Table 3).

Other substrates such as 1,4-cyclohexadiene (1,4-CHD),

ethylbenzene (PhEt), toluene (PhCH₃), 2,3-dimethylbutane

(2,3-DMB), and cyclohexane (cyclo-C₆H₁₂) were also used to

test the reactivity of 2 toward hydrogen atom abstraction.

Second-order rate constants (k₂) at 298 K for these substrates

were determined (Table 4, and Figure S14, Supporting

̈

transferring the sample for Mossbauer measurements.

Reactivity of the Iron(IV)-Oxo Complex (2). The

observed short lifetime of 2 indicates that the complex

would exhibit enhanced reactivity toward external substrates in

comparison to the parent N4Py complex. The oxygen-atom-

transfer (OAT) ability of 2 was tested using methyl phenyl

sulﬁde (PhSMe) as a substrate at −10 °C. The fast decay of

the band at 740 nm compared to the self-decay rate of 2

suggests oxidation of phenyl methyl sulﬁde. During the course

of the reaction, 2 was converted into its iron(II) precursor

complex 1 with a clear isosbestic point at 660 nm (Figure 7).

Table 4. Comparison of Second-Order Rate Constant for

the Reaction of [Fe^IV(O)(N4Py^Me2)]²⁺and

[Fe^IV(O)(N4Py)]²⁺with Various Alkane Substrates at 298 K

[Fe^IV(O)

substrate (BDE in

kcal/mol)

(N4Py^Me2)]²⁺k₂

(N4Py)]²⁺

a

b

(M⁻¹s⁻¹

)

k₂(M⁻¹s⁻¹

)

k₂(M⁻¹s⁻¹

)

9,10-DHA (77)

1,4-CHD (78)

PhEt (87)

PhCH₃(90)

2,3-DMB (96.5)

cyclo-C₆H₁₂(99.3)

2.0

0.79

7.62 × 10⁻³

3.54 × 10⁻³

1.78 × 10⁻³

6.73 × 10⁻⁴

18.0

10.69

4.0 × 10⁻³

6.3 × 10⁻⁴

1.2 × 10⁻⁴

5.5 × 10⁻⁵

13.1

10.4

3.0 × 10⁻³

8.3 × 10⁻⁴

1.6 × 10⁻⁴

6.7 × 10⁻⁵

a

Data taken from refs 31, 33, and 35 except that for 1,4-CHD.

Values obtained with the iron(IV)-oxo species generated with m-

b

Figure 7. Decay of 2 in the presence of thioanisole (5 equiv) with

concomitant formation of 1 at −10 °C in acetonitrile. Inset: (a)

Spectrum in the region between 500 and 1100 nm. (b) Time course

of the decay monitored at 740 nm. Spikes marked with * likely arise

from artifacts in the data collection process.

CPBA in acetonitrile.

Information) which aﬀorded the corrected k₂′ based on the

number of abstractable weak C−H bonds for each substrate.

The iron-oxo species 2 is eﬃcient in C−H bond cleavage of

aliphatic substrates and shows a reasonable linear correlation

between log k₂′ and the C−H BDE of the substrates (Figure

8). This linear correlation indicates that the reactions of 2 with

the substrates take place via the rate-determining transfer of a

hydrogen atom, as established earlier for other related

iron(IV)-oxo complexes.^32,33,78From the plot in Figure 8, it

The reaction follows a pseudo-ﬁrst-order behavior in the

presence of excess substrate (5−20 equiv). The observed rate

constant was found to depend linearly on the substrate

concentration enabling extraction of the second-order rate

constant (k₂). The k₂value of 1.03 ( 0.03) is at least 3−100-

fold higher than the values obtained for most of the reported

Table 3. Oxygen-Atom-Transfer and Hydrogen Atom Abstraction Reactivity of Diﬀerent Fe(IV)-Oxo Complexes Supported by

Diﬀerent Pentadentate Ligands

substrate, complex

thioanisole (263 K) k₂(M⁻¹s⁻¹

)

cyclohexane (298 K) k₂(M⁻¹s⁻¹

6.73 × 10⁻⁴

)

9,10-DHA k₂(M⁻¹s⁻¹

2 ( 0.07) at 298 K [0.17 ( 0.01) at 263 K] this work

)

ref

[Fe^IV(O)(N4Py^Me2)]²⁺(2)

[Fe^IV(O)(N4Py)]²⁺

1.03 ( 0.03)

0.014

0.33

0.004

0.024

a

6.7 × 10⁻⁵

13.10 at 298 K

100 at 298 K

7.4 at 298 K

1.1 at 298 K

40 at 298 K

not reported

1.4 (at 243 K)

not reported

31, 35

35

66

67

[Fe^IV(O)(Bn-tpen)]²⁺

[Fe^IV(O)(Py₂TACN)]²⁺

[Fe^IV(O)(BisPi1)]²⁺

3.9 × 10⁻⁴

not reported

3.0 × 10⁻⁴

2.9 × 10⁻³

2.9 × 10⁻²

[Fe^IV(O)(BisPi2)]²⁺

2.4

[Fe^IV(O)(N₃Py-(NMB)]²⁺

[Fe^IV(O)(N₂Py-(NMB)₂]²⁺

[Fe^IV(O)(N₂Py2Q]²⁺

0.033 (at 243 K)

0.31 (at 243 K)

7.4 (at 233 K)

a

Values obtained with the iron(IV)-oxo species generated with m-CPBA in acetonitrile.

F

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Supporting Information). The observed KIE value is lower

than that reported for [Fe^IV(O)(N4Py)]²⁺,⁷⁹but similar to

those for the iron(IV)-oxo complexes of N4Py-derived

ligands.⁶⁶Thus, the nonclassical KIE value is indicative of a

reaction pathway that is very similar to that proposed for HAT

reactions by iron(IV)-oxo complexes.

It has been demonstrated that weakening the ligand ﬁeld of

the iron(IV)-oxo complex by replacing one of the four nitrogen

donors of the tetramethylcyclam (TMC) or 1,4,7,11-

tetramethyl-1,4,7,11-tetraazacyclotetradecane (TMCN) li-

gands by an oxygen donor dramatically increases the OAT

and HAT reactivity.⁸⁰Similar observations have been made

with iron(IV)-oxo complexes based on N4Py-type ligands with

N-methylbenzimidazole or quinoline donors.^66,67The weak-

ening of the ligand ﬁeld in 2 increases the electrophilicity of

the iron-oxo unit resulting in faster HAT reactivity, especially

in case of less acidic C−H groups. When compared with

iron(IV)-oxo complexes with other N5 ligands bearing mixed

amine and pyridine donors, the rates of HAT by 2 with

aliphatic substrates having C−H BDE > 86 kcal/mol are faster

than the [Fe^IV(O)(N4Py)]²⁺complex, whereas the reactivity

of 2 in cyclohexane oxidation is almost comparable to that of

[Fe^IV(O)(BnTPEN)]²⁺(Table 3). However, 2 is less reactive

than the [Fe^IV(O)(N2Py2Q)]²⁺complex (N2Py2Q = 1,1-

di(pyridin-2-yl)-N,N-bis(quinolin-2-ylmethyl)-

methanamine).⁶⁷The latter is almost 75-fold more reactive

toward thioanisole and 40-fold more reactive in cyclohexane

hydroxylation. Interestingly, complex 2 is 70-fold more reactive

in thioanisole oxidation at −10 °C and 10-fold more reactive in

hydroxylating cyclohexane at 25 °C than the parent [Fe^IV(O)-

(N4Py)]²⁺complex but is 7-fold slower in oxidizing 9,10-

DHA. Given the short half-life (14 min) of 2, this is a

surprising trend that is diﬃcult to rationalize. It may be argued

that the slow HAT observed for oxidation of 9,10-DHA and

1,4-CHD may be due to restricted orientations in the approach

of the substrates to 2 due to steric interactions, but this is

Figure 8. Plot of log k₂′ versus C−H BDEs of diﬀerent substrates in

the oxidation by [Fe^IV(O)(N4Py ^Me2)]²⁺(blue squares) and

2+

[Fe^IV(O)(N4Py)]

(red circles). k₂′ is obtained by dividing the

second-order rate constant by the number of abstractable C−H bonds

on the substrate.

is clear that the diﬀerences in hydrogen-atom-transfer (HAT)

rate between 2 and the [Fe^IV(O)(N4Py)]²⁺complex diminish

as the substrate C−H bonds become weaker, resulting in a

smaller slope for 2. Notably, the two lines cross at around 86

kcal/mol, and the hydrogen atom aﬃnity of the iron(IV)-oxo

unit of 2 is lower with decreasing C−H BDE. The almost

similar values of the second-order rate constants obtained with

9,10-DHA and 1,4-CHD substrates indicate that the steric

factor of 6-Me substitution does not play a major role in the

HAT reactivity of 2.

The second-order rate constants for separate reactions

between intermediate 2 and toluene and toluene-d₈yielded a

primary kinetic isotope eﬀect value of 14 (Figure S15,

Figure 9. Relative condensed phase free energy proﬁle for the reaction of 2 (triplet and quintet) with cyclohexane (C₆H₁₂) at the UB3LYP-

D3(CPCM)/6-31++G(d,p)/Fe(SDD)//UB3LYP/6-31++G(d,p)/Fe(SDD) level of theory. The free energy values given in brackets for transition

states indicate the values for the corresponding spin states of [Fe^IV(O)(N4Py)]²⁺.

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contradicted by the fact that the observed HAT rates for the

two substrates are very similar (vide supra) and that the HAT

rates for the iron(IV)−O complexes with the sterically

hindered bis-quinoline and bis(methyl)benzimidazolyl deriva-

tives of N4Py are more reactive across the board in reactions

with the same substrates, relative to the parent N4Py complex

(Table 3).^66,67,81

To understand the mechanism of the substrate oxidation by

complex 2, the reaction pathways were investigated by density

functional theory (DFT) calculations (see Experimental

Section for details). Oxygenation of C−H bonds by a high

valent iron-oxo species has been reported to take place via two

steps: The ﬁrst step involves the HAT from the targeted C−H

bond followed by an oxygen rebound in the second step. In

almost all cases, the HAT step is rate determining, and the

observed KIE (vide supra) indicates that this is the case also

for 2.^61,62,82,83HAT pathways for the reaction of 2 with

cyclohexane were computed on the triplet and quintet surfaces

and compared with those of the [Fe^IV(O)(N4Py)]²⁺complex

(Figure 9). In both cases, the triplet state was found to be the

ground state. The triplet-quintet energy gap was predicted to

be 9.4 kcal/mol for [Fe^IV(O)(N4Py)]²⁺, whereas for 2, the gap

reduces to 6.9 kcal/mol (Supporting Information). The

corresponding condensed phase free energy gaps, ΔG_TQ

(sol), are 7.4 and 5.0 kcal/mol for [Fe^IV(O)(N4Py)]²⁺and

2, respectively.

Figure 10. Energy optimized geometries of the transition states of C−

H bond activation of cyclohexane by [Fe^IV(O)(N4Py^Me2)]²⁺in (a)

triplet spin state and (b) in quintet spin state at the UB3LYP/6-31+

+G(d,p)/Fe(SDD) level of theory.

9,10-DHA takes place at the opposite side of the 6-Me groups

of the ligand in 2. As observed experimentally, the steric factor

is less likely to play any role on the rate of HAT with 9,10-

DHA. For both the iron(IV)-oxo complexes, [Fe^IV(O)-

(N4Py^Me2)]²⁺(2) and [Fe^IV(O)(N4Py)]²⁺, the activation

barriers for the quintet transition states are negative with

respect to the quintet states of the corresponding iron-oxo

complexes (Figure S17). However, the barriers for the triplet

states are found at 8.4 and 7.2 kcal/mol for 2 and

[Fe^IV(O)(N4Py)]²⁺, respectively. In the TSR model, the

kinetic barrier is predicted to be a weighted average of the

barrier in the ground triplet and the corresponding excited

quintet states.^61,62,83As the barriers for the TS on the quintet

surface are negative, the rate-determining C−H bond

activation rate may be primarily governed by the triplet state

indicating slightly faster C−H activation by [Fe^IV(O)-

(N4Py)]²⁺as observed experimentally. However, this small

energy gap cannot be reliably predicted considering the error

limit of the DFT level of theory. Thus, further investigations to

characterize the electronic structure of the iron(IV) complex

(2) are warranted to unravel the unusual trend in HAT

reactivity.

The relative strengths of iron-oxo bonds in the complexes

[Fe^IV(O)(N4Py)]²⁺and [Fe^IV(O)(N4Py^Me2)]²⁺(2) were

estimated from the FeO stretching frequency. The infrared

(IR) and Raman intensities associated with all normal modes

for both the noncentrosymmetric molecules were computed at

the B3LYP/6-31++G(d,p) level of theory. The computed

symmetric (with respect to the Fe−N_transbond) FeO

stretching frequencies were identiﬁed by the marked Raman

intensities over IR intensities and the FeO stretching (with

respect to the trans Fe−N_transbond) frequencies were observed

at 938 and 919 cm⁻¹for [Fe^IV(O)(N4Py)]²⁺and [Fe^IV(O)-

(N4Py^Me2)]²⁺, respectively (Table S1, Supporting Informa-

tion). It is important to mention here that the DFT predicted

FeO stretching frequency is larger than the experimentally

reported value (841 cm⁻¹)⁸⁴for the [Fe^IV(O)(N4Py)]²⁺

complex. However, it has been reported that the B3LYP

functional systematically overestimates FeO stretching

frequencies by ∼50 cm⁻¹,^84,85and therefore considering the

intrinsic methodical error, the B3LYP-predicted frequency of

919 cm⁻¹closely scales to the experimentally reported value of

nonheme iron(IV)-oxo complexes. Nevertheless, the presence

of 6-methyl groups on the pyridine rings of N4Py^Me2ligand

HAT from the σ(C−H) of an alkane to the LUMO of

[Fe^IV(O)(L)] species may take place either in the quintet or in

triplet spin state. The singly occupied π*d_xz/yzand the

2

unoccupied σ*d_zfrontier orbitals of the triplet iron(IV)-oxo

species are involved in the HAT step. Because of the presence

of the π* d_xz/yzorbital, the π-attack requires a FeO···HC

bond angle of ∼120° (bent TS) for better overlap between the

σ(C−H) and the p_x/yof the oxygen atoms of the oxo unit. In

2

the quintet spin state, the LUMO turned out to be the σ*d_z

orbital. Therefore, the C−H σ-bond vertically approaches the

FeO unit with a Fe−O···HC angle of ∼180° (linear TS) to

2

ensure the best overlap between σ(C−H) and σ*d_zduring σ-

attack.

In the case of C−H activation by the iron(IV)-oxo

complexes, the calculated transition state on the quintet

surface is lower than that on the triplet surface, that is, a

crossover of spin states in the transition state takes place

(Figures 9 and 10 and Figure S16, Supporting Information).

The low triplet−quintet gap (ΔE_TQ) makes the excited quintet

state more accessible, resulting in a lower kinetic barrier for the

C−H cleavage. For 2, the barriers for HAT in the triplet and

quintet states turned out to be 18.7 and 9.6 kcal/mol,

respectively. For [Fe^IV(O)(N4Py)]²⁺, those values are much

higher at 28.9 and 12.8 kcal/mol, respectively (Figure 9).

Thus, the lower barriers in case of 2 clearly support the faster

reactivity of the complex compared to [Fe^IV(O)(N4Py)]²⁺.

After C−H bond activation, the O−C bond formation takes

place via a rebound mechanism, which is a thermodynamically

downhill process. Of note, a radical pair is not observed in the

quintet surface. Following the HAT step, the rebound step

takes place without any distinct barrier (Figure 9), which

ultimately leads to a hydroxylated product. The computational

modeling of cyclohexane oxidation is in agreement with the

two-state reactivity (TSR) model.⁶¹

The activation barriers for 9,10-DHA are computed to be

quite low compared to those of cyclohexane, which are

consistent with the experimental observation. The approach of

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weakens the iron-oxo bond relative to the parent N4Py

complex, as reﬂected in the corresponding stretching

frequencies.

The DFT-optimized structural data for 2 (Table S2,

Supporting Information) were considered further to under-

stand the diﬀerence in HAT and OAT reactivity of the

iron(IV)-oxo complexes of N4Py-type ligands. It has recently

been suggested that the higher reactivity of [Fe^IV(O)-

(N2Py2Q)]²⁺arises from the 10° tilt of the FeO unit,

observed in its crystal structure, away from the z-axis of the

complex due to steric interactions with the quinoline donors.⁶⁷

On the other hand, the DFT calculated structure of 2 reveals

that the FeO unit is only tilted 3° from the z axis and in the

opposite direction. It may be noted that no such tilt is

observed in [Fe^IV(O)(N4Py)]²⁺complex.³⁸The tilt of the

FeO unit toward the more sterically hindering 6-Me-pyridyl

donor moieties appears counterintuitive, but is almost

negligible. A similar tilt of the acetonitrile ligand toward the

6-Me-Py moieties is also observed in the crystal structure of 1b

(vide supra). A close scrutiny of this structure shows that once

the Fe−N distances to the 6-Me-Py groups are elongated, the

steric encumbrance by these moieties is no greater than that of

the pyridyl moieties. As shown in Figure 11, the distances

Figure 11. (a) A Mercury plot of the molecular structure of 1b,

showing the hydrogens of the 6-methyl substituents and the 6-H

hydrogens of the nonsubstituted pyridyl groups. (b) A corresponding

space-ﬁlling plot, viewed down the molecular z-axis; the hydrogen

atoms of the acetonitrile ligand have been omitted for the sake of

clarity.

between the 6-H hydrogens of the nonsubstituted pyridyl

moieties and the nitrogen atom in the acetonitrile ligand are

the same as the shortest distances between that nitrogen and

the hydrogens of the 6-methyl substituents (2.67 Å) (Table S3,

Supporting Information). The tilt of the acetonitrile ligand in

complex 1b and the corresponding computed tilt in 2 (vide

supra) are thus not unreasonable.

Comparing the crystallographically determined structures of

the iron(IV)-oxo complexes [Fe^IV(O)(N4Py)]²⁺,³⁸[Fe^IV(O)-

(N₂Py-(NMB)₂)]²⁺, and [Fe^IV(O)(N2Py2Q)]²⁺(N₂Py-

(NMB)₂= N,N-bis((1-methyl-1H-benzoimidazol-2-yl)-

methyl)-1,1-di(pyridin-2-yl)methanamine) to those of the

analogous iron(II)-acetonitrile complexes (Table 5),^66,67,81it

may be noted that there is a direct correlation between the

structural trends in the two types of complexes with respect to

equatorial Fe−N distances and tilt of axial ligand (both degree

and direction of ligand tilt). Assuming that this correlation

holds also for other N4Py-type ligands, it may be predicted

that the iron(IV)-oxo complex of bis(pyridin-2-yl)-N,N-bis((6-

methylpyridin-2-yl)methyl)methanamine, (N2Py)-

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(N2CH₂Py^Me),⁷²shall exhibit a reactivity on par with that of

the most reactive of the Fe(IV)O complexes, [Fe^IV(O)-

(N2Py2Q)]²⁺, since the iron(II)-acetonitrile complexes of the

two ligands exhibit very similar structural parameters (Table

6), including tilts of the axial ligands that exceed 10°. We are

currently investigating this matter.

From the decay patterns of 2 in the presence of thioanisole

and 9,10-DHA, it is clear that the iron(II) complex is

regenerated after the reaction (Figure 7 and Figure S13,

Supporting Information). This prompted us to investigate the

reactivity of 1 for catalytic oxidations using oxone (KHSO₅) as

the oxidant. For these catalytic reactions, use of m-CPBA was

avoided in order to exclude that this strong oxidant would

cause direct oxidation of substrates without the mediation of a

metal-based oxidant. The catalytic abilities of 1 and

[Fe^II(N4Py)(CH₃CN)]²⁺were compared under similar

reaction conditions using diﬀerent substrates. With complex

1, cyclohexane oxidation resulted in cyclohexanol and

cyclohexanone with an overall yield of 25% (based on

oxone) and with an alcohol to ketone ratio (A/K) of 2.7. It

may be noted that poor yields of oxidation products (10%) and

a low A/K ratio (0.5) were obtained for cyclohexane oxidation

by [Fe^II(N4Py)(CH₃CN)]²⁺under similar experimental

conditions. In addition, 1 was found to oxidize adamantane

with a C3/C2 normalized selectivity of 22. Such high C3/C2

selectivity (11−48) was observed for oxidations with PhIO

catalyzed by P450 mimics.⁸⁶Other substrates such as 9,10-

DHA, ﬂuorene, ethylbenzene, toluene, and 1,4-cyclohexadiene

(1,4-CHD) were also oxidized by 1 using oxone as the oxidant

(Table 7).

The observed reactivity for 2 and the computational results

are in agreement with an oxygen rebound mechanism in C−H

bond activation reactions. With cyclohexene, however, allylic

oxidation products were obtained, and the eﬃciency with

respect to consumption of oxidant was around 90%. The

alcohol to ketone ratio (A/K) of 3.4 is greater than the A/K

ratio of 1.7 obtained for the less selective [Fe^II(N4Py)-

(CH₃CN)]²⁺system. The observation of allylic oxidation

products in cyclohexene oxidation suggests that this reaction

may proceed via a non-rebound mechanism.⁸⁷⁻⁸⁹Substrates

such as 1-octene and styrene were converted to 1-

octeneepoxide and benzaldehyde, respectively. Complex 1

also carries out the oxidation of benzyl alcohol and thioanisole

to aﬀord benzaldehyde and thioanisole oxide, respectively. The

yields of the products from diﬀerent substrates are listed in

Table 7. For aliphatic substrates, some degree of catalytic

activity supports that HAT reactions can reform the iron(II)

precursor. While the catalytic activity of the complex is low, the

selectivity for substrate oxidation is higher compared to other

related complexes. The detection of oxo-bridged diiron(III)

species or mononuclear iron(III) complex as decay products in

̈

the Mossbauer spectrum indicates their formation during

decay of the iron(IV)-oxo complex (2). Therefore an active

iron(IV)-oxo species in the catalytic reaction may form inactive

dimeric species or may undergo degradation to some

unproductive pathway, resulting in low catalytic turnover.

SUMMARY AND CONCLUSION

■

We have reported the isolation and characterization of an

iron(II) complex supported by a substituted N4Py ligand. The

ligand forms a high-spin iron(II)-triﬂate complex in the solid

state. In acetonitrile solution, however, the triﬂate is replaced

by an acetonitrile ligand and the complex displays temperature-

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Table 7. Catalytic Activities of [Fe^IV(O)(N4Py^Me2)]²⁺(2) and [Fe^IV(O)(N4Py)]²⁺Towards Various Substrates

[Fe^IV(O)(N4Py^Me2)]²⁺(2)

[Fe^IV(O)(N4Py)]²⁺

a

b

c

d

a

b

c

d

substrate (mmol)

cyclohexane (5)

product

TON

eﬃciency (%)

A/K

2.7

3°/2°

TON

eﬃciency (%)

A/K

0.4

3°/2°

cyclohexanol

cyclohexanone

1- adamantanol

2- adamantanol

2- adamantanone

benzaldehyde

anthracene

1-phenylethanol

acetophenone

ﬂuorenone

1.9

0.7

1.9

0.1

0.2

0.6

8.3

0.3

0.6

2.6

0.6

3.4

6.9

2.0

4.3

1.1

26

−

0.3

0.7

2.6

0.1

0.5

0.3

7.0

0.3

0.6

0.9

0.5

0.2

5.2

3.1

3.6

1.0

10

−

22

−

22

32

−

14

adamantane (0.25)

toluene (0.5)

9,10-DHA (0.25)

6.0

83

9

−

2.7

70

9

−

ethylbenzene (0.5)

ﬂuorene (0.25)

1,4-CHD (0.5)

1-octene (0.5)

26

6

34

89

−

3.4

−

9

5

2

−

benzene

1-octene oxide

cyclohexenol

cyclohexenone

benzaldehyde

thioanisole oxide

83

1.7

cyclohexene (5)

benzyl alcohol (0.5)

thioanisole (0.5)

43

11

−

36

10

−

c

−

a

b

Turnover number (TON) = mol of product/mol of catalyst. Eﬃciency (%) = moles of total products/mol of oxidant × 100. A/K = (mol of

alcohol)/(mol of ketone). Ratio of tertiary alcohol to secondary alcohol (3°/2°) = 3[1-adamantanol/2-adamantanol +2-adamantanone].

d

spectra were collected on a Bruker Avance 500 MHz spectrometer.

dependent spin crossover. The iron(II) complex reacts with m-

CPBA or oxone in acetonitrile to generate an S = 1 high-valent

iron-oxo species, [Fe^IV(O)(N4Py^Me2)]²⁺(2), exhibiting a

characteristic absorption band at 740 nm. The steric hindrance

engendered by the 6-Me substituents on the pyridine rings

results in weakening of the ligand ﬁeld of the N4Py^Me2ligand

in the equatorial plane. The iron-oxo species is relatively

unstable with a half-life (t_1/2) of only 14 min at ambient

temperature. Complex 2 is one of the most reactive oxo

transfer agents among the reported iron(IV)-oxo complexes of

pyridine-rich N5 ligands. In addition, the high-valent iron

complex is capable of oxygenating relatively strong C−H

bonds of aliphatic substrates including that of cyclohexane

(C−H BDE 99.3 kcal/mol). For cyclohexane oxidation, a 10-

fold increase in the reactivity is observed with respect to that of

the parent N4Py ligand. Surprisingly, complex 2 is less reactive

than [Fe^IV(O)(N4Py)]²⁺toward substrates with weak C−H

bonds (<86 kcal/mol; 9,10-dihydroanthracene, 1,4-cyclo-

hexadiene). In the oxidation of toluene, the intermediate 2

shows a KIE value of 14, clearly indicating the C−H bond

abstraction by the iron-oxo species being the rate-limiting step.

DFT calculations on the HAT reactions with cyclohexane

reveal a lower triplet-quintet splitting in case of 2 compared to

parent [Fe^IV(O)(N4Py)]²⁺and support the involvement of the

quintet transition state in the reaction pathway, in agreement

with the two-state reactivity concept. Moreover, the complex

displays catalytic OAT and HAT transfer reactivity, albeit with

moderate activity.

57

Fe Mossbauer spectra were recorded with a ⁵⁷Co source in a Rh

̈

matrix using an alternating constant acceleration Wissel Mossbauer

spectrometer operated in the transmission mode and equipped with a

Janis closed-cycle helium cryostat. Isomer shifts are given relative to

iron metal at ambient temperature. Simulation of the experimental

data was performed with the Mfit program (E. Bill, Max-Planck

Institute for Chemical Energy Conversion, Mulheim/Ruhr, Ger-

̈

many). Temperature-dependent magnetic susceptibility measure-

ments were carried out with a Quantum-Design MPMS-XL-5

SQUID magnetometer equipped with a 5 T magnet in the range

from 2 to 295 K at a magnetic ﬁeld of 0.5 T. GC-MS measurements

were carried out with a PerkinElmer Clarus 680 GC and SQ8T MS,

using Elite 5 MS (30 m × 0.25 mm × 0.25 μm) column with

maximum temperature set at 300 °C. The labeling experiment was

carried out with H₂O¹⁸(98 atom %) from Icon Services Inc.

Synthesis. Bis(6-methylpyridin-2-yl)methanone, bis(6-methylpyr-

idin-2-yl)methanone oxime, and bis(6-methylpyridin-2-yl)-

methanamine were prepared according to the procedures reported

in literature.⁶⁸Caution! Although no problem was encountered during

the synthesis of the complex, perchlorate salts are potentially explosive and

should be handled with care!

Ligand N4Py^Me2. To a mixture of bis(6-methylpyridin-2-yl)-

methanamine (0.50 g, 2.35 mmol) and sodium triacetoxyborohydride

(3.44 g, 16.2 mmol) in dichloromethane (50 mL) was added

pyridine-2-carboxaldehyde (0.47 mL, 4.9 mmol). The mixture was

allowed to stir for 48 h at ambient temperature. Subsequently, a

saturated aqueous sodium hydrogen carbonate solution was added,

followed by stirring for an additional 1 h. The organic product was

then extracted with ethyl acetate (3 × 50 mL). The combined organic

extracts were dried over Na₂SO₄and evaporated to dryness to yield

pale yellow oil. Yield: 0.45 g (48%). ¹H NMR (500 MHz, DMSO-d₆,

298 K): δ(ppm) 8.42 (d, 2H), 7.80−7.69 (m, 4H), 7.57 (d, 2H), 7.40

(d, 2H), 7.18 (t, 2H), 7.1 (d, 2H), 5.15 (s, 1H), 3.84 (s, 4H), 2.39 (s,

6H). ESI-MS (+ve ion mode, acetonitrile): m/z = 396.2 (100%, [M +

H]⁺), 418.2 (85%, [M + Na]⁺), 434.2 (20%, [M + K]⁺).

EXPERIMENTAL SECTION

■

All chemicals and reagents were obtained from commercial sources

and were used without further puriﬁcation unless otherwise noted.

Solvents were distilled and dried before use. Preparation and handling

of air-sensitive materials were carried out under an inert atmosphere

in a glovebox. Fourier transform infrared spectroscopy on KBr pellets

was performed on a Shimadzu FT-IR 8400S instrument. Elemental

analyses were performed on a PerkinElmer 2400 series II CHN

analyzer. Electro-spray ionization (ESI) mass spectra were recorded

with a Waters QTOF Micro YA263 instrument. Solution electronic

spectra (single and time-dependent) were measured on an Agilent

8453 diode array spectrophotometer. All room-temperature NMR

[Fe^II(N4Py^Me2)(OTf)](OTf) (1). Fe(OTf)₂·2MeCN (0.11 g, 0.25

mmol) was added to a dichloromethane solution (5 mL) of the ligand

(0.10 g, 0.25 mmol), and the resulting solution was allowed to stir for

6 h under nitrogen atmosphere. The solvent was then removed

completely under vacuum, and the residue was washed thoroughly

with a mixture of dichloromethane and diethyl ether (1:5) to isolate a

light yellow solid. Yield: 0.12 g (58%). Anal. calcd for 1·CH₂Cl₂,

C₂₈H₂₇Cl₂F₆FeN₅O₆S₂(834.42 g/mol): C, 40.30; H, 3.26; N 8.39;

K

DOI: 10.1021/acs.inorgchem.8b02577

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Found: C 40.06, H 3.35, N 8.06. ESI-MS (+ve ion mode in CH₃CN):

m/z = 225.6 (100%) [Fe(N4Py^Me2)]²⁺. UV−vis (CH₃CN): λ_max(ε,

M⁻¹cm⁻¹); 455 nm (sh), 430 nm (sh), and 360 nm (1200). ¹H NMR

(500 MHz in CD₃CN): δ (ppm) 69.2 (Py-α-H), 55.2 (Py−CH₂),

51.3, 49.5 (Py-H_βand Py-H_β′), 36.3 (Py-H_γ), 23.5 (Me-Py-H_βand

Me-Py-H_β′), 13.5 (Me-Py-H_γ), 7.3 (N−CH), −25.1 (Me-Py-CH₃). X-

ray quality single crystals of [Fe(N4Py^Me2)(CH₃CN)](ClO₄)₂(1b)

were grown by recrystallization of the complex (isolated from the

reaction of iron(II) perchlorate hydrate and N4Py^Me2in acetonitrile)

from a solvent mixture of acetonitrile and diethyl ether.

using the UBLYP⁹³⁻⁹⁵hybrid density functional method in

combination with Pople’s 6-31++G(d,p) basis set. The 1997 Stuttgart

relativistic small-core eﬀective core-potential (Stuttgart RSC 1997

ECP, denoted as SDD)^96,97was employed for the core electrons of

the Fe center, and the (311111/22111/411/1) basis set was used for

its valence electrons. Harmonic vibrational frequency calculations

were performed at the same level of theory to characterize the

structures as intermediates (all real frequencies) and transition states

(one imaginary mode) and also to extract thermo-chemical

information. Single point condensed phase calculations were also

performed using the CPCM⁹⁸solvent model at the uB3LYP/6-31+

+G(d,p)/Fe(SDD) level of theory with empirical dispersion added to

it (B3LYP-D3) using the universal force ﬁeld (UFF) radii. The

dielectric constant in the CPCM calculations was set to 35.688 to

simulate acetonitrile.

Generation of [Fe^IV(O)(N4Py^Me2)]²⁺(2). To an acetonitrile

solution (1 mM) of [Fe^II(N4Py^Me2)(OTf)](OTf) (1) was added m-

CPBA (5 equiv) in acetonitrile at −10 °C. Immediately a pale green

species showing an absorption maximum at 740 nm (ε∼220

M⁻¹cm⁻¹) was observed. The pale green intermediate could also be

generated using other oxidants such as PhIO and oxone. When oxone

(2 equiv), dissolved in H₂O (50 μL), was added to an acetonitrile

solution 1 (1 mM) at 25 °C, the same species was generated. When

excess solid PhIO (20 equiv) was used to generate the green species

at −40 °C, a relatively low yield of 2 was observed.

Hydrogen-Atom-Transfer Reactions. Kinetic experiments were

performed by adding appropriate amounts of substrates (0.01−0.4 M)

to 1 mM solution of 2 (generated by addition of 5 equiv of m-CPBA)

in CH₃CN solution under anaerobic condition. The decay of the band

at 740 nm was monitored spectrophotometrically at 25 °C. Rate

constants, k_obs, were determined by pseudo-ﬁrst-order ﬁtting of the

absorbance decay at 740 nm. Second-order rate constants k₂were

obtained from the slopes of the linear ﬁts of k_obsvs concentration of

substrates.

Reactions with Thioanisole and 9,10-DHA at −10 °C. To a

solution of 2 generated from complex 1 (2 mM in CH₃CN) was

added thioanisole or 9,10-DHA (0.01−0.04 M), and the decay of the

740 nm band was monitored. Following the same methodology as

described above, second-order rate constants k₂were obtained.

Isotope Labeling Experiments. The isotope labeling experiment

was carried out by adding 20 μL H₂¹⁸O (98% ¹⁸O-enriched) to an

acetonitrile solution of 2 at −10 °C.

Catalysis Experiments. The catalytic oxidation reactions were

carried out using oxone (KHSO₅) as oxidant. In a typical experiment,

oxone (0.05 mmol dissolved in 50 μL H₂O) was added to an

acetonitrile solution (2 mL) of the complex (0.005 mmol)

[Fe^II(N4Py^Me2)(CH₃CN)]²⁺or [Fe^II(N4Py)(CH₃CN)]²⁺containing

appropriate amounts of external substrates (Table 7). The reaction

mixture was stirred in air for 1 h at 298 K. To isolate the organic

products, the solution at the end of the reaction was passed through a

small silica column (to remove Fe-catalyst), followed by washing with

ethyl acetate. The ethyl acetate solution was then analyzed directly by

GC-MS (with naphthalene as quantiﬁcation standard). The

thioanisole oxidation product was identiﬁed by ¹H NMR and

quantiﬁed using 2,4-di-tert-butylphenol as an internal standard.

X-ray Crystallographic Data Collection, Reﬁnement, and

Solution of the Structure of 1b. X-ray single-crystal data for 1b

were collected at 223 K using Mo Kα (λ = 0.7107 Å) radiation on a

SMART-APEX diﬀractometer equipped with CCD area detector.

Data collection, data reduction, structure solution, and reﬁnement

were carried out using the software package of APEX II.⁹⁰The

structure was solved by direct methods and subsequent Fourier

analyses and reﬁned by the full-matrix least-squares method based on

F²with all observed reﬂections.⁹¹The non-hydrogen atoms were

treated anisotropically.

ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.8b02577.

■

S

Spectroscopic data, DFT optimized coordinates, and

crystallographic data in CIF format (PDF)

Accession Codes

CCDC 1824770 contains the supplementary crystallographic

data for this paper. These data can be obtained free of charge

via www.ccdc.cam.ac.uk/data_request/cif, or by emailing

data_request@ccdc.cam.ac.uk, or by contacting The Cam-

bridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: ictkp@iacs.res.in (T.K.P.).

*E-mail: icrs@iacs.res.in (R.S.).

■

ORCID

Franc Meyer: 0000-0002-8613-7862

Tapan Kanti Paine: 0000-0002-4234-1909

Present Address

^∥Department of Chemistry, Taras Shevchenko National

University of Kyiv, 64/13Volodymyrska Street, City of Kyiv,

Ukraine, 01601

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

The authors gratefully acknowledge the Swedish Research

Links Programme of the Swedish Research Council (T.K.P.,

E.N.), and the Science and Engineering Research Board

(SERB) (T.K.P. and R.S.) for ﬁnancial support. R.S. thanks the

Council of Scientiﬁc and Industrial Research (CSIR), India, for

a research associateship. The authors thank Mr. Sridhar

̈

Banerjee for his help with Mossbauer studies and Dr. Arup

Sinha for assistance in analysis of crystal structure data.

Crystal Data of 1b. MF = C₂₇H₂₈Cl₂FeN₆O₈, M_r= 691.30,

monoclinic, space group P21/c, a = 18.312(7), b = 12.860(5), c =

13.500(5) Å, α = 90.00°, β = 106.411(6)°, γ = 90.00°, V = 3050.0(2)

Å³, Z = 4, ρ = 1.506 mg m⁻³, μ Mo−Kα = 0.728 mm⁻¹, F(000) =

1424, GOF = 1.070. A total of 10,806 reﬂections were collected in the

range 1.16 ≤ θ ≤ 23.46, 4399 of which were unique (R_int= 0.2075).

R₁(wR₂) = 0.0863 (0.1924) for 400 parameters and 4399 reﬂections

(I > 2σ(I)).

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A Mononuclear Nonheme Iron(IV)-Oxo Complex of a Substituted N4Py Ligand: Effect of Ligand Field on Oxygen Atom Transfer and C-H Bond Cleavage Reactivity

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