Assessing the impact of electronic and steric tuning of the ligand in the spin state and catalytic oxidation ability of the FeII(Pytacn) family of complexes

Article

pubs.acs.org/IC

Assessing the Impact of Electronic and Steric Tuning of the Ligand in

the Spin State and Catalytic Oxidation Ability of the Fe^II(Pytacn)

Family of Complexes

Irene Prat,^†Anna Company,*^,†Teresa Corona,^†Teodor Parella,^‡Xavi Ribas,^†and Miquel Costas*^,†

^†Grup de Química Bioinorgan

de Química, Universitat de Girona, Campus Montilivi, E17071 Girona, Catalonia, Spain

^‡Servei de Ressonan

cia Magnetica Nuclear, Universitat Autonoma de Barcelona, Bellaterra, E08193 Barcelona, Catalonia, Spain

̀ ̀

ica i Supramolecular (QBIS), Institut de Química Computacional i Catalisi (IQCC) and Departament

̀

S

* Supporting Information

ABSTRACT: A family of iron complexes with the general

formula [Fe^II(^R,R′Pytacn)(X)₂]ⁿ⁺is described, where ^R,R′Pytacn

is the tetradentate ligand 1-[(4-R′-6-R-2-pyridyl)methyl]-4,7-

dimethyl-1,4,7-triazacyclononane, R refers to the group at the

α-position of the pyridine, R′ corresponds to the group at the

γ-position, and X denotes CH₃CN or CF₃SO₃. Herein, we

study the inﬂuence of the pyridine substituents R and R′ on

the electronic properties of the coordinated iron center by a

combination of structural and spectroscopic characterization

1

using X-ray diﬀraction, H NMR and UV−vis spectroscopies,

and magnetic susceptibility measurements. The electronic properties of the substituent in the γ-position of the pyridine ring (R′)

modulate the strength of the ligand ﬁeld, as shown by magnetic susceptibility measurements in CD₃CN solution, which provide a

direct indication of the population of the magnetically active high-spin S = 2 ferrous state. Indeed, a series of complexes

[Fe^II(^H,R′Pytacn)(CD₃CN)₂]²⁺exist as mixtures of high-spin (S = 2) and low-spin (S = 0) complexes, and their eﬀective

magnetic moment directly correlates with the electron-releasing ability of R′. On the other hand, the substitution of the hydrogen

atom in the α-position of the pyridine by a methyl, chlorine, or ﬂuorine group favors the high-spin state. The whole family of

complexes has been assayed in catalytic C−H and CC oxidation reactions with H₂O₂. These catalysts exhibit excellent

eﬃciency in the stereospeciﬁc hydroxylation of alkanes and in the oxidation of oleﬁns. Remarkably, R′-substituents have little

inﬂuence on the eﬃciency and chemoselectivity of the catalytic activity of the complexes, but the selectivity toward oleﬁn cis-

dihydroxylation is enhanced for complexes with R = Me, F, or Cl. Isotopic labeling studies in the epoxidation and cis-

dihydroxylation reactions show that R has a deﬁnitive role in dictating the origin of the oxygen atom that is transferred in the

epoxidation reaction.

complexes have been developed and tested during the past

decade in the challenging oxidation of C−H and CC

bonds.⁷⁻⁹Selected families are capable of eliciting enzyme-like

reactivity, meaning that they can mediate oxidations through

metal-centered processes without a signiﬁcant involvement of

free diﬀusing radicals.^5a,bSuch studies have shown that the

activity of these complexes as oxidation catalysts depends, in a

very delicate manner, on a number of aspects. Some of them

are rather obvious such as metal nuclearity, ligand denticity,

strength of metal binding, presence of available coordination

sites, nature of the ligand donor set, or the oxidatively robust

nature of the ligand. However, other subtler aspects have been

identiﬁed, which include ligand ﬁeld and coordination top-

ology.¹⁰Presumably, interconnections among multiple aspects

exist, and this complexity makes structure−activity correlations

challenging.

INTRODUCTION

■

There is a growing interest in the development of transition-

metal complexes that can catalyze the selective oxidation of C−

H and CC bonds using green oxidants such as O₂or

peroxides with novel and better selectivities and eﬃciencies

than traditional less-sustainable methodologies.¹In this regard,

iron is a particularly attractive metal for catalyst development

because of its high availability and lack of toxicity.²Because

several highly selective oxygenases contain iron in their active

site,^3,4nature constitutes a source of inspiration for the design

of such iron oxidation catalysts.⁵For example, Rieske

oxygenases are enzymes that bear a mononuclear nonheme

iron center in their active center, and they use O₂to catalyze

the hydroxylation of alkyl aromatic moieties and the cis-

dihydroxylation of arenes in the ﬁrst step of the degradation of

persistent pollutants.⁶

Taking into consideration the ability of mononuclear iron

sites to mediate oxidation reactions in a selective manner,

several families of bioinspired mononuclear nonheme iron

Received: February 15, 2013

A

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Scheme 1. Ligands and Complexes Employed in This Work

Most iron complexes that carry out selective C−H and CC

oxidation reactions share some structural features that include:

(i) exchangeable coordination sites that can engage in fast

substitution reactions with the oxidant, and (ii) ligand sets that

can support iron centers in high oxidation states. Because of

that, electronic factors that inﬂuence the lability of the

complexes and the redox properties of the iron centers might

have a signiﬁcant impact on the catalytic activity. Electronic

properties of the iron center can be modulated not only by the

electron-donating ability of the ligand but also by the strength

of the ligand ﬁeld, as this directly determines the spin state.

Spin state determines the lability of the complexes because

high-spin complexes are inherently faster than their low-spin

analogues in substitution reactions.

We have previously reported a family of iron complexes

containing tetradentate ligands composed of a 1,4,7-triaza-

cyclononane arm derivatized with a methylpyridine arm.

Complexes [Fe(CF₃SO₃)₂(^H,HPytacn)] (^H,H1) and [Fe-

(CF₃SO₃)₂(^Me,HPytacn)] (^Me,H1) (Scheme 1) exhibit eﬃcient

catalytic activity in the stereospeciﬁc hydroxylation of alkanes

and the epoxidation and cis-dihydroxylation of alkenes using

H₂O₂as an oxidant.¹¹As ascertained by several mechanistic

probes, these oxidative transformations occur through the

mediation of a high-valent iron−oxo species.^11,12Moreover,

compound ^H,H1 is highly active in the oxidation of water to

generate O₂using cerium as the sacriﬁcial oxidizing reagent in a

process that mimics what occurs in photosystem II.¹³

site. In order to evaluate how the substitution in the pyridine

ring aﬀects the catalytic activity of the resulting iron complexes,

we have studied their catalytic performance in the oxidation of

cyclohexane and cyclooctene, which constitute model sub-

strates for C−H hydroxylation and CC oxidation reactions,

respectively.

RESULTS AND DISCUSSION

■

Synthesis of Ligands and Complexes. A family of

tetradentate ligands (^R,R′Pytacn, Scheme 1), which contains a

functionalized triazacyclononane ring that has diﬀerent

methylpyridine derivatives with distinctive electronic and steric

properties, has been prepared. These ligands constitute an

extension of the previously described ^H,HPytacn ligand, whose

iron¹¹⁻¹³and manganese¹⁴complexes were reported as robust

and eﬃcient oxidation catalysts. The introduction of several R′

groups with diﬀerent electronic properties in the γ-position of

the pyridine heterocycle (R′ = NMe₂, OMe, Me, H, Cl, CO₂Et,

NO₂) is aimed at evaluating putative electronic eﬀects without

interference caused by steric eﬀects. Instead, modiﬁcation of the

α-position of the pyridine ring (R = H, Me, F, Cl) not only

aﬀects the electronic properties but also enables tuning of the

steric crowding around the iron metal site (Scheme 1).

^R,R′Pytacn ligands were prepared by alkylation of 1,4-

dimethyl-1,4,7-triazacyclononane with the corresponding sub-

stituted picolyl chloride arm in acetonitrile and were obtained

as pale yellow oils in good yields (67−73%).

In this work, the basic structure of ^H,H1 has been modiﬁed by

introducing a range of substituents in the α- and γ-positions of

the pyridine ring with diﬀerent electronic and steric properties.

We aim at evaluating the inﬂuence of these substituents in the

electronic structure, as well as in the spectroscopic and

structural features of the resulting iron(II) compounds, by

means of several spectroscopic techniques both in the solid

state and in solution. Herein, we show how the straightforward

substitution of the pyridine ring of the primary Pytacn structure

leads to important modiﬁcations in the electronic properties of

the iron(II) center, which in some cases is translated into

substantial changes in the spin-crossover properties of the metal

Reaction of the tetradentate ^R,R′Pytacn ligands with [Fe-

(OTf)₂(CH₃CN)₂] (OTf = triﬂuoromethanesulfonate anion)

in THF or CH₂Cl₂under anaerobic conditions aﬀorded the

desired bis-triﬂato complexes, [Fe(OTf)₂(^R,R′Pytacn)] (^R,R′1).

Slow diﬀusion of diethyl ether over saturated CH₂Cl₂solutions

aﬀorded ^R,R′1 as a crystalline material in 27−77% yields

(Scheme 1). Following the synthetic procedure previously

described for the preparation of [Fe(CH₃CN)₂(^H,HPytacn)]-

(PF₆)₂(^H,H2PF₆),^11a^H,H2SbF₆and ^Me,H2SbF₆were synthesized

in a one-pot reaction by mixing ^H,HPytacn or ^Me,HPytacn with 1

equiv of FeCl₂in acetonitrile and with posterior addition of 2

equiv of AgSbF₆in acetonitrile. Subsequent removal of the

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Figure 1. (Top) X-ray structures of ^F,H1, ^H,NO21, ^H,NMe21, and ^Me,Me1. Hydrogen atoms and triﬂate groups have been omitted for clarity except for the

oxygen atoms directly bound to the iron center. (Bottom) Space-ﬁlling diagrams for complexes ^F,H1, ^H,NO21, ^H,NMe21, and ^Me,Me1. Triﬂate groups

have been omitted for clarity, but the oxygen atoms directly bound to the iron center are represented.

precipitated AgCl and slow diethyl ether diﬀusion over the

resulting solutions aﬀorded the desired complexes as crystalline

solids.

Despite the similarities in the structure for the whole range of

bis-triﬂato complexes ^R,R′1, some systematic diﬀerences arise

when they are analyzed in more detail. The Fe−N_pybond

length changes systematically depending on the substitution in

the α-position of the pyridine ring; this value is 2.21 in ^Me,Me1,

2.26 in ^Cl,H1, 2.20 in ^F,H1, and it decreases to 2.15 0.02 Å in

^H,R′1, in which there is no substituent in the α-position of the

pyridine ring. This observation strongly suggests that the

bulkiness of the substituent in the α-position of the pyridine

ring has a signiﬁcant and systematic inﬂuence on the Fe−N_py

distance, being longer for bulkier-substituted pyridines.

Substitution in the γ-position of the pyridine only provides

slight changes in the Fe−N_pybond.

Solid-State Characterization. The solid-state molecular

structure of complexes ^R,R′1 can be established by X-ray

diﬀraction analysis. The crystal structures of ^H,H1, the diaquo

complex [Fe^II(H₂O)₂(^Me,HPytacn)](OTf)₂, and ^H,H2PF₆have

been previously reported,^11,12while the structures of ^H,NMe21,

^H,OMe1, ^H,Me1, ^H,Cl1, ^H,CO2Et1, ^H,NO21, ^Me,Me1, ^Cl,H1, and ^F,H1 are

described for the ﬁrst time in the present work. Compound

^Me,H1 crystallizes as very thin needles not suitable for X-ray

diﬀraction. Instead, the corresponding bis-acetonitrile complex

^Me,H2SbF₆forms colorless rectangles, which can be measured by

X-ray diﬀraction at diﬀerent temperatures (100 and 300K),

showing a spin transition in the solid state (vide infra).

As representative examples of the bis-triﬂato complexes, the

X-ray structures of complexes ^F,H1, ^H,NO21, ^H,NMe21, and ^Me,Me1

are depicted in Figure 1, experimental details of their crystal-

structure determination are collected in Table 1, and a list of

selected bond distances and angles can be found in Table 2.

The same information for ^Cl,H1, ^H,Me1, ^H,Cl1, ^H,CO2Et1, and

^H,OMe1 is found in the Supporting Information (Tables S1−S4

and Figures S1 and S2). These complexes crystallize in the

monoclinic or orthorhombic crystal system, and they contain

an iron(II) center in a distorted octahedral coordination

geometry. Four coordination sites are occupied by the nitrogen

atoms of the ligands, with the three N atoms of the

triazacyclononane (tacn) macrocycle bound facially. That

leaves two coordination sites in a relative cis conﬁguration

that are accessible for binding to triﬂate anions. The pyridine

arm binds trans to one of the N-methyl groups of the ligand,

and the other two nitrogen atoms of the tacn ligand are trans to

the triﬂate ligands. Average Fe−N bond distances for these

complexes are 2.1−2.2 Å, characteristic of high-spin Fe^II

complexes.¹⁵

As previously reported,^11athe X-ray structure of the dark red

compound ^H,H2PF₆is characteristic of a low-spin (LS) iron(II)

center, as evidenced by the short Fe−N distances at around 2.0

Å.^15,16The situation is diﬀerent for the bis-acetonitrile complex

^Me,H2SbF₆, whose structure was solved at 300 and 100 K

because an evident and reversible change in color from

colorless to dark blue was observed when the crystal sample

was cooled. Figure 2 shows a representation of the X-ray

structure of ^Me,H2SbF₆, Table 3 collects the experimental details

of the crystal-structure determination, and a list of selected

bond distances can be found in Table 4. A free acetonitrile

molecule is present in the unit cell, and the coordination

geometry of the cationic [Fe(^Me,HPytacn)(CH₃CN)₂]²⁺mole-

cule is the same as in the parent triﬂato analogues; therefore, it

will not be described further. The X-ray structure at 300 K

presents an iron(II) center with an average Fe−N distance of

2.20 Å, indicative of a high-spin (HS) electronic conﬁguration

(Figure 2).^15b,16a,17A signiﬁcant contraction of the unit cell

occurs when the temperature is reduced from 300 to 100 K. It

corresponds to a unit-cell-volume decrease of ∼7% (from

3365.1 to 3105.9 Å³). This is accompanied by a contraction of

the coordination sphere so that the average Fe−N distance at

100 K becomes 2.02 Å, a value typical of a low-spin (LS) Fe^II

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Table 1. Crystal Data for ^F,H1, ^H,NO21, ^H,NMe21, and ^Me,Me1

^F,H1

^H,NO21

^H,NMe21

^Me,Me1

empirical formula

formula weight

temperature

C₁₆H₂₃F₇FeN₄O₆S₂

620.35

C₁₆H₂₃F₆FeN₅O₈S₂

647.36

C₁₈H₂₉F₆FeN₅O₆S₂

645.43

C₁₈H₂₈F₆FeN₄O₆S₂

630.41

100(2) K

wavelength

0.71073 Å

orthorhombic

P2₁2₁2₁

crystal system

space group

monoclinic

P2₁/c

unit cell dimensions

a = 15.926(4) Å

α = 90°

a = 9.4350(8) Å

α = 90°

a = 8.745(8) Å

α = 90°

a = 8.8046(14) Å

α = 90°

b = 9.434(2) Å

β = 115.519(4)°

c = 17.376(4) Å

γ = 90°

2356.0(9) Å³

1.749 g·cm⁻³

0.914 mm⁻¹

1264

b = 14.8017(12) Å

β = 95.722(2)°

c = 17.4272(14) Å

γ = 90°

2421.7(3) Å³

1.776 g·cm⁻³

0.895 mm⁻¹

1320

b = 25.56(2) Å

β = 124.53(4)°

c = 14.522(10) Å

γ = 90°

b = 13.591(2) Å

β = 90.00°

c = 21.920(3) Å

γ = 90°

2622.9(7) Å³

1.596 g·cm⁻³

0.818 mm⁻¹

1296

volume

2674(4) Å³

density (calculated)

absorption coeﬃcient

F(000)

1.603 g·cm⁻³

0.805 mm⁻¹

1328

cell formula units_Z

crystal size

4

0.6 × 0.1 × 0.07 mm

2.36−28.52°

−21 ≤ h ≤21

−12 ≤ k ≤12

−22 ≤ l ≤22

34 373

0.35 × 0.30 × 0.12 mm

2.169−28.339°

−12 ≤ h ≤12

−19 ≤ k ≤19

−22 ≤ l ≤22

37 799

0.35 × 0.2 × 0.1 mm

2.332−28.308°

−11 ≤ h ≤11

−33 ≤ k ≤32

−19 ≤ l ≤19

23 266

0.3 × 0.2 × 0.15 mm

2.39−28.34°

−11 ≤ h ≤11

−18 ≤ k ≤18

−29 ≤ l ≤29

40 513

Θ range for data collection

limiting indices

reﬂections collected

independent reﬂections

5817

5956

6504

[R(int) = 0.0752]

97.1% (Θ = 28.52°)

full-matrix least-squares on F²

5817/0/327

1.024

[R(int) = 0.0398]

99.8% (Θ = 25.242°)

full-matrix least-squares on F²

5956/9/345

1.056

[R(int) = 0.0872]

99.9% (Θ = 25.242°)

full-matrix least-squares on F²

6504/0/347

1.042

[R(int) = 0.0327]

99.8% (Θ = 28.34)

full-matrix least-squares on F²

6504/0/339

completeness to Θ

reﬁnement method

data/restraints/parameters

goodness-of-ﬁt on F²

ﬁnal R indices [I > 2σ(I)]

1.034

R1 = 0.0591

wR2 = 0.1498

R1 = 0.1133

wR2 = 0.1764

1.750 and −0.701 e·Å⁻³

R1 = 0.0439

wR2 = 0.1163

R1 = 0.0532

wR2 = 0.1233

1.862 and −0.844 e·Å⁻³

R1 = 0.0701

wR2 = 0.1305

R1 = 0.1098

wR2 = 0.1450

0.885 and −0.473 e·Å⁻³

R1 = 0.0226

wR2 = 0.0554

R1 = 0.0254

R indices (all data)

wR2 = 0.0565

0.361 and −0.207 e·Å⁻³

largest diﬀ peak and hole

Table 2. Selected Bond Lengths (Å) and Angles (°) for ^F,H1, ^H,NO21, ^H,NMe21, and ^Me,Me1

^F,H1

^H,NO21

^H,NMe21

^Me,Me1

Fe−N1

Fe−N2

Fe−N3

Fe−N4

Fe−O2

Fe−O4

N1−Fe−N2

N1−Fe−N4

N1−Fe−O4

N1−Fe−O1

N4−Fe−N2

N4−Fe−N3

N2−Fe−O4

N4−Fe−O1

N3−Fe−O1

N2−Fe−N3

N3−Fe−O4

O4−Fe−O1

2.196(4)

2.201(4)

Fe−N1

Fe−N4

Fe−N5

Fe−N3

Fe−O3

Fe−O6

N4−Fe−O3

N4−Fe−N5

N1−Fe−O3

N1−Fe−O6

N4−Fe−N3

N5−Fe−N3

N4−Fe−O6

N5−Fe−O3

N1−Fe−N5

N1−Fe−N3

N3−Fe−O6

O3−Fe−O6

2.183(2)

2.208(2)

2.220(2)

2.188(2)

2.0365(19)

2.1415(19)

92.16(8)

80.79(8)

110.21(8)

84.04(8)

80.29(8)

81.38(8)

98.53(8)

96.28(8)

94.71(8)

77.31(8)

93.64(8)

88.67(8)

Fe−N1

Fe−N3

Fe−N4

Fe−N5

Fe−O1

Fe−O4

N1−Fe−N3

N1−Fe−N5

N1−Fe−O4

N1−Fe−O1

N5−Fe−N3

N5−Fe−N4

N3−Fe−O4

N5−Fe−O1

N4−Fe−O1

N3−Fe−N4

N4−Fe−O4

O4−Fe−O1

2.153(4)

2.220(4)

Fe−N1

Fe−N2

Fe−N3

Fe−N4

Fe−O1

Fe−O4

N1−Fe−N2

N1−Fe−N3

N1−Fe−O1

N1−Fe−O4

N2−Fe−N4

N3−Fe−N4

N4−Fe−O1

N4−Fe−O4

N2−Fe−O1

N2−Fe−N3

N3−Fe−O4

O1−Fe−O4

2.2112(13)

2.1720(13)

2.2419(13)

2.2247(13)

2.1910(12)

2.0513(11)

76.37(5)

2.218(4)

2.219(4)

2.211(4)

2.258(4)

2.043(3)

2.093(3)

2.149(3)

2.151(3)

76.11(13)

92.43(15)

95.24(13)

106.49(13)

79.99(14)

80.53(16)

92.66(14)

96.29(14)

96.35(15)

80.82(16)

88.74(14)

90.60(13)

78.65(13)

94.28(13)

93.34(13)

108.94(13)

79.52(14)

80.40(14)

94.37(14)

93.74(14)

92.49(13)

79.67(14)

89.67(13)

91.15(14)

107.38(5)

86.36(5)

107.11(5)

80.28(5)

79.52(5)

86.81(5)

97.19(5)

100.62(5)

80.36(5)

93.84(5)

84.61(5)

center (Table 4).^15b,16a,17This contraction in bond distance is

indicative of a spin transition in the solid state from high- (300

K) to low-spin (100 K).¹⁸The complete spin transition was

also evident by measurement of the molar susceptibility (χ_m) as

a function of the temperature using a SQUID magnetometer

(Figure S3). ^Me,H2SbF₆presents a gradual spin-crossover

occurring over a temperature range of approximately 100 K

and centered at around 170 K.

D

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Table 4. Selected Bond Lengths (Å) and Angles (°) for

^Me,H2SbF₆·CH₃CN at 100 and 300 K

^Me,H2SbF₆·CH₃CN (100 K)

^Me,H2SbF₆·CH₃CN (300 K)

Fe−N1

Fe−N2

Fe−N3

Fe−N4

Fe−N5

Fe−N6

N1−Fe−N2

N1−Fe−N4

N1−Fe−N5

N1−Fe−N6

N3−Fe−N2

N3−Fe−N4

N3−Fe−N5

N3−Fe−N6

N2−Fe−N4

N2−Fe−N5

N5−Fe−N6

N4−Fe−N6

2.075(5)

1.986(5)

2.039(5)

2.064(5)

1.946(5)

1.942(5)

82.9(2)

94.2(2)

87.5(2)

101.2(2)

84.8(2)

84.9(2)

93.1(2)

91.1(2)

85.0(2)

93.8(2)

88.6(2)

92.6(2)

Fe−N1

Fe−N2

Fe−N3

Fe−N4

Fe−N5

Fe−N6

N1−Fe−N2

N1−Fe−N4

N1−Fe−N5

N1−Fe−N6

N3−Fe−N2

N3−Fe−N4

N3−Fe−N5

N3−Fe−N6

N2−Fe−N4

N2−Fe−N5

N5−Fe−N6

N4−Fe−N6

2.251(6)

2.179(6)

2.213(6)

2.248(6)

2.188(6)

2.194(6)

2.191(6)

2.182(7)

2.218(6)

2.234(6)

2.167(7)

2.184(6)

2.175(4)

2.181(6)

2.221(6)

2.231(7)

2.184(7)

2.172(7)

Figure 2. Representation of the cationic part of the X-ray structure of

^Me,H2SbF₆·CH₃CN at 100 K. The table shows the Fe−N_py, the average

Fe−N_tacn, and the average Fe−N_acnbond lengths for this compound at

a given temperature (100 and 300 K). Hydrogen atoms and the

noncoordinated acetonitrile molecule found in the unit cell have been

omitted for clarity.

¹H NMR Spectroscopy. As representative examples of the

family of bis-triﬂato complexes, ¹H NMR spectra of ^H,H1, ^Me,H1,

^H,NO21, and ^H,NMe21 in CD₂Cl₂are depicted in Figure 3. Figures

spectra of all complexes (^R,R′1). These compounds exhibit

spectral windows that expand from −40 to 200 ppm, which is

1

S4 and S5 in the Supporting Information collect the H NMR

Table 3. Crystal Data for ^Me,H2SbF₆·CH₃CN at 100 and 300 K

^Me,H2SbF₆·CH₃CN (100 K)

^Me,H2SbF₆·CH₃CN (300 K)

empirical formula

formula weight

temperature

C₂₁H₃₅F₁₂FeN₇Sb₂

912.91

C₂₁H₃₅F₁₂FeN₇Sb₂

912.91

100(2) K

300(2) K

wavelength

0.71073 Å

crystal system

space group

monoclinic

P2₁/c

unit cell dimensions

a = 13.0482(17) Å

α = 90°

a = 13.3058(18) Å

α = 90°

b = 12.2015(16) Å

β = 101.023(2)°

c = 19.875(3) Å

γ = 90°

3105.9(7) Å³

1.952 g·cm⁻³

2.286 mm⁻¹

1784

b = 12.4573(17) Å

β = 100.276(2)°

c = 20.633(3) Å

γ = 90°

3365.1(8) Å³

2.802 g·cm⁻³

2.110 mm⁻¹

1784

volume

density (calculated)

absorption coeﬃcient

F(000)

cell formula units_Z

crystal size

4

0.35 × 0.35 × 0.08 mm

2.062−28.358°

−17 ≤ h ≤17

−16 ≤ k ≤16

−26 ≤ l ≤26

46 648

0.35 × 0.35 × 0.08 mm

2.257−28.271°

−17 ≤ h ≤17

−16 ≤ k ≤16

−27 ≤ l ≤27

51 823

Θ range for data collection

limiting indices

reﬂections collected

independent reﬂections

7689

8279

[R(int) = 0.0779]

99.9% (Θ = 25.242°)

full-matrix least-squares on F²

7689/0/394

1.246

[R(int) = 0.0842]

99.9% (Θ = 25.242°)

full-matrix least-squares on F²

8279/0/394

1.006

completeness to Θ

reﬁnement method

data/restraints/parameters

goodness-of-ﬁt on F²

ﬁnal R indices [I > 2σ(I)]

R1 = 0.0634

wR2 = 0.1204

R1 = 0.0799

wR2 = 0.1253

1.599 and −1.352 e·Å⁻³

R1 = 0.0513

wR2 = 0.1219

R1 = 0.1087

wR2 = 0.1494

1.166 and −0.689 e·Å⁻³

R indices (all data)

largest diﬀ peak and hole

E

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1

Figure 3. H NMR spectra (400 MHz) of ^H,H1, ^Me,H1, ^H,NO21, and ^H,NMe21 in CD₂Cl₂.

indicative of octahedral t_2g⁴e_gFe^IIparamagnetic species.

2

1

Table 5. H NMR Chemical Shifts (ppm) for β and γ

R,R

Spectra show mainly very broad signals because of the close

proximity of the corresponding protons to the paramagnetic

center. Along this line, in most of the cases the spectra are

rather simple, with a relatively small number of signals,

suggesting that some protons give signals that are too broad

to be eﬀectively observed because of fast relaxation. Lowering

the temperature causes a narrowing of the NMR signals as

exempliﬁed for complex ^Me,H1 (Figure S6), whose protons

exhibit chemical shifts linearly dependent on 1/T, which is

characteristic of a Curie behavior (Figure S7).

In spite of the width of the NMR signals at room

temperature, it is possible to clearly identify β and γ protons

of the pyridine, which appear as relatively sharp signals (Table

5). They can be assigned on the basis of their relative

integration, by comparison of the spectra along the whole series

of complexes and because polypyridyl Fe^IIcomplexes present

typical NMR patterns.^15c,16,19This assignment explains the

absence of a sharp signal at 15−20 ppm, corresponding to the γ

proton for compounds ^H,R′1 and ^Me,Me1, which bear a

substituent in the para-position of the pyridine ring and the

lack of the two sharp signals between 30 and 70 ppm in the ¹H

NMR spectrum of ^H,OMe1, for which the pyridine ring presents

methyl groups in the two β-positions. Assignment of the α-

methylpyridine group in ^Me,Me1 and ^Me,H1 can also be made

because of the characteristic paramagnetic upﬁeld shift of these

protons arising from Fermi contact interactions between

protons and the ferrous center, dominated by a spin

polarization mechanism (Figure S5).^16a,20Resonances belong-

ing to the N−CH₃groups could be easily identiﬁed in the

spectra of ^H,H1 and ^H,NMe21 as a very broad signal between 80

and 100 ppm on the basis of their relative integration. The two

Protons of the Pyridine Ring of Complexes ′1 in CD₂Cl₂

complex

H_β

H_γ

^H,H1

49.2

42.7

36.3

39.5

17.7

^H,NMe21

^H,OMe1

^H,Me1

^H,Cl1

^H,CO2Et1

^H,NO21

^Me,H1

^Me,Me1

^Cl,H1

47.7

47.8

49.7

47.8

53.1

52.6

60.3

68.8

35.6

33.8

34.9

33.2

38.8

39.0

32.4

29.7

16.5

18.6

18.5

^F,H1

N−CH₃groups of each [Fe(Pytacn)] unit are chemically

nonequivalent, but they accidentally overlap in the spectra of

^H,H1. This situation is changed in the spectra of ^Me,H1 and

^H,NO21, where the two N−CH₃groups appear as two diﬀerent

signals at 65/95 and 95/110 ppm (Figure 3), and also in the

spectra of ^R,H1 (R = Me, Cl, and F in Figure S5) and ^H,R′1 (R′

= OMe, Me, Cl, and CO₂Et in Figure S4) in CD₂Cl₂.

¹H NMR spectra of ^R,R′1 were also measured in CD₃CN

(Figures 4 and 5 and S8−S11). Similar to what is reported for

other iron(II) complexes,^15b,21this solvent acts as a

coordinating ligand, and it displaces triﬂate groups to form

solvato complexes [Fe(^R,R′Pytacn)(CH₃CN)₂]^{2+ R,R}′2OTf), in

(

F

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1

Figure 4. (a) H NMR spectra (400 MHz) of ^H,H2SbF₆and ^H,H1 in CD₃CN. The latter aﬀords the bis-acetonitrile complex ^H,H2OTf upon being

dissolved. The partial assignation of the ¹H NMR signals for complex ^H,H2SbF₆was based on integration values and COSY experiment (Figure S8b).

¹H NMR spectra (400 MHz) of ^Me,H2SbF₆and ^Me,H1 in CD₃CN. The latter aﬀords the bis-acetonitrile complex ^Me,H2OTf upon being dissolved. For

complex ^Me,H2SbF₆, the assignation was made by comparison with similar complexes described in the literature^16a,22and according to the integration

values (Figure S9).

which two acetonitrile molecules and the tetradentate ^R,R′-

Pytacn ligand constitute the coordination sphere of the iron(II)

center. This phenomenon was conﬁrmed by comparing the ¹H

NMR spectra of ^H,H1 and ^Me,H1 in CD₃CN (^H,H2OTf and

^Me,H2OTf) with those of the synthetically isolated bis-

acetonitrile complexes ^H,H2SbF₆and ^Me,H2SbF₆(Figure 4).

complexes without any substituent in the α-position (class I)

and complexes with a substituent in this position (class II).

On the one hand, class I complexes, that is ^H,R′2OTf, exhibit

a compact spectral window compared to their bis-triﬂato

counterparts (^H,R′1). It expands from 0 to 15 ppm, except for

^H,NMe22OTf, which shows signals up to 45 ppm (Figure S10).

The contraction of the spectral window is related to a

modiﬁcation of the electronic properties of the iron complex,

which becomes mainly LS upon substitution of the triﬂate

anions by acetonitrile ligands. This has been previously

observed for other iron complexes^10a,15a,d,23, and it can be

explained by simple crystal ﬁeld theory because the acetonitrile

ligand is a stronger ligand ﬁeld than triﬂate anion. Qualitative

analysis of the NMR spectra allows the evaluation of the

relative contribution of the HS state. It is particularly interesting

to notice that complex ^H,NO22OTf, with a nitro group in the γ-

position, does not show signals above 11 ppm, which indicates

1

For these two compounds, the H NMR spectrum exhibited

much sharper signals than the corresponding bis-triﬂato

complexes ^H,H1 and ^Me,H1 in CD₃CN, but chemical shifts and

relative integrals were essentially the same. This observation is

indicative of a fast exchange between triﬂate and acetonitrile

binding, which gives rise to broader signals for ^H,H2OTf and

^Me,H2OTf.^10a

1

The spectral window of the H NMR spectrum of each

complex in CD₃CN reﬂects the spin state of ^R,R′2OTf.

Important diﬀerences arise upon analysis of these NMR spectra

so that the complexes might be classiﬁed into two categories:

G

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Figure 5. H NMR spectra (400 MHz) of ^H,H1, ^Me,H1, ^H,NO21, and ^H,NMe21 in CD₃CN. The triﬂate anions are replaced by acetonitrile molecules,

thus, the bis-acetonitrile complexes ^H,H2OTf, ^Me,H2OTf, ^H,NO22OTf, and ^H,NMe22OTf are formed in solution.

a

R,R

Table 6. Selected Physical Data for ′2OTf

λ_max,nm (ε, M^‑1·cm^‑1)

b

complex

^H,H2OTf

π−π*

239 (13 900)

MLCT

d−d

μ_eff, μ_B

385 (3200)

327 (4700)

370 (5400)

370 (2900)

403 (3900)

455 (6100)

526 (2400)

340 (255)

344 (220)

410(950)

550 (65)

1.26

2.62

2.09

1.24

1.71

0.78

0

^H,NMe22OTf

^H,OMe2OTf

^H,Me2OTf

^H,Cl2OTf

^H,CO2Et2OTf

^H,NO22OTf

^Me,H2OTf

^Me,Me2OTf

^Cl,H2OTf

240 (> 20 000)

250 (13300)

240 (10500)

241 (15800)

275 (6150)

231 (14000)

267 (5100)

260 (4700)

269 (7000)

540 sh (100)

547 (120)

553 (70)

547 (45)

4.95

4.75

4.87

4.63

^F,H2OTf

237 (5600), 254 (5600)

368 (900)

578 (14)

a

b

Compounds ^R,R′2OTf were prepared by dissolving the bis-triﬂato complexes ^R,R′1 in CH₃CN or CD₃CN. Determined by the Evans’ method.

that in this case the compound is purely diamagnetic and must

be described as a pure LS iron(II) center (Figure 5). Instead,

for ^H,NMe22OTf, which has a strongly electron-donating

dimethylamino substituent in the γ-position, there is an

important inﬂuence of the HS state, and the spectral window

expands up to 45 ppm (Figure 5). Thus, it seems clear that the

substituent in the para-position of the pyridine directly

inﬂuences the electronic properties of the iron(II) center. In

contrast, class II catalysts, that is ^R,R′2OTf, where R = Me, Cl,

or F, show spectral windows that expand from 0 to 150 ppm,

indicating a HS conﬁguration of the iron(II) center in

acetonitrile (Figure S11).

troscopy, could be quantiﬁed by measuring their eﬀective

magnetic moment (μ_eff) using the Evans’ method (see below).

Eﬀective Magnetic Moment. The measurement of the

eﬀective magnetic moment (μ_eff) allows the quantiﬁcation of

the contribution of the LS and HS states in iron(II) complexes.

This information is specially important in the present family of

complexes, for which both spin states are close in energy.

Values of μ_efffor ^R,R′2OTf were measured using the Evans′

NMR method by dissolving ^R,R′1 in CD₃CN (Table 6).

For class I complexes, μ_effvalues are clearly inﬂuenced by the

electronic properties of the group in the γ-position. As detailed

in Table 6, μ_effincreases when the electron-donating properties

of this substituent are increased. The extreme cases correspond

to ^H,NO22OTf, with a highly electron-withdrawing group and no

The diﬀerent spin states of the iron(II) center in class I and

class II bis-acetonitrile complexes, evidenced by NMR spec-

H

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eﬀective magnetic moment, indicative of a LS Fe^II(in

agreement with its diamagnetic ¹H NMR spectrum) and

^H,NMe22OTf, with the strongest electron-donating γ-substituent

and a μ_effof 2.62 μ_B, implying a signiﬁcant population of the HS

state. The inﬂuence of the γ-substituent in the μ_effis clearly

observed by the good correlation of this value with the

corresponding Hammett σ_pconstant (Figure 6a). These values

Taking 298 K as the initial temperature, it was observed that

increasing the temperature to 338 K caused a signiﬁcant

increase in the μ_eff. The opposite was observed upon cooling.

As stated above, ^H,NO22OTf already has a diamagnetic behavior

at room temperature, while for ^H,H2OTf the pure LS state was

reached at 273 K. In contrast, complete conversion to the LS

state does not occur for ^H,NMe22OTf in the temperature range

accessible with acetonitrile, but at 233 K a small μ_effof 0.8 μ_B

was measured, which is indicative of a major contribution of the

LS state.

The situation is completely diﬀerent for compounds

belonging to class II, which bear a substituent in the α-position

of the pyridine ring (^R,H2OTf). The measured μ_effvalues of

4.80 0.17 μ_Bare very close to the theoretical spin-only value

of 4.90 μ_Bof a HS iron(II) center with four unpaired electrons.

This result fully agrees with the paramagnetism observed by ¹H

NMR for the compounds in this class (Figure S11). It is likely

that the α-substituent in the pyridine sterically interacts with

the iron center, which disfavors the formation of the shorter

Fe−N bond characteristic of the LS conﬁguration.^16aHowever,

it is interesting to notice that the μ_effvalue measured for

^F,H2OTf is slightly lower (4.63 μ_B) than what would be

expected for a pure HS center with four unpaired electrons. It

was observed that the μ_effvalues for ^F,H2OTf decreased upon

lowering the temperature, which is consistent with a spin

transition phenomenon in solution. Ranges of μ_efffrom 4.77 μ_B

at 320 K to 0.92 μ_Bat 235 K (Figure 6b) indicate an almost

complete crossover, reminiscent to that observed for

structurally related systems.^21a,b,22This spin transition could

also be followed by UV−vis spectroscopy (Figure 8). In order

to exclude the occurrence of the decoordination of the 6-ﬂuoro-

pyridine and the posterior coordination of acetonitrile solvent

molecules upon lowering the temperature, which could also

explain the change in the spin state, we carried out variable-

temperature ¹⁹F NMR (298 to 230 K) experiments. No

signiﬁcant changes in the ¹⁹F NMR spectra were observed in

the temperature range studied, and only a single sharp signal at

−79 ppm assigned to the noncoordinated triﬂate counterions

was observed.^21a,cIf decoordination of 6-ﬂuoropyridine

occurred, the presence of a ﬂuorine sharp signal around −65

ppm would be expected.²⁴The ﬂuorine substituent in the sixth

position of the pyridine ring was not detected, likely because

pyridine coordination places this atom in close proximity to the

paramagnetic iron(II) center. This causes short relaxation times

and signiﬁcant signal broadening, a phenomenon that has been

previously reported in structurally related complexes with

ﬂuoro α-substituted pyridine-based ligands.^21cSpin-crossover in

solution was not observed for the other class II complexes, that

is, ^Me,H2OTf, ^Cl,H2OTf, and ^Me,Me2OTf, which remain HS when

the temperature is decreased.

Figure 6. (a) Representation of the eﬀective magnetic moment of

^H,R′2OTf in CD₃CN at 298 K in front of the Hammett constants. (b)

Representation of the eﬀective magnetic moment of ^H,H2OTf,

^F,H2OTf, ^H,NO22OTf, and ^H,NMe22OTf as a function of temperature.

^R,R′2OTf complexes were obtained by dissolving ^R,R′1 in CD₃CN. The

eﬀective magnetic moment was measured in CD₃CN solutions using

the Evans’ method.

1

fully agree with the increased spectral window of the H NMR

spectra upon increasing the electron-donating abilities of the

para-substituent (Figure S10). The eﬀect of the pyridine

substitution on the μ_effcan be understood on the basis of

simple ligand ﬁeld theory, by assuming that this value reﬂects

diﬀerent stages of a spin transition equilibrium. According to

this theory, the magnitude of the ligand ﬁeld (i.e., the t_2g-e_g

orbital splitting) is higher when increasing the π-acceptor

character of the ligands. Introduction of NO₂and NMe₂groups

into the pyridine causes, respectively, a decrease and an increase

of the energy of the pyridine π orbitals, with respect to the

hydrogen-, methyl-, or chloro-substituted ligands. Therefore,

the NO₂group makes the pyridine a better π-acceptor, while

the opposite happens with the NMe₂group. Overall, electron-

withdrawing groups increase the ligand ﬁeld, favoring the LS

state, while the opposite occurs for electron-releasing groups,

which populate the HS state.

Class I complexes exhibit an incomplete spin-crossover

phenomenon with the temperature, as previously observed for

structurally related iron(II) complexes.^21aThis behavior was

studied for complexes ^H,H2OTf, ^H,NMe22OTf, and ^H,NO22OTf,

as representative examples of this class, bearing a hydrogen

atom, an electron-donating group, and an electron-withdrawing

group in the γ-position of the pyridine, respectively (Figure 6b).

UV−Vis Spectroscopy. The UV−vis spectra of the bis-

acetonitrile complexes ^R,R′2OTf, formed by dissolving the

corresponding bis-triﬂato complexes ^R,R′1 in acetonitrile, were

analyzed to obtain further information about the electronic

structure of the complexes (Table 6 and Figure 7).

Compound ^H,H2OTf is a prototypical example of class I

complexes. The UV−vis spectrum of this compound is

characterized by intense bands at 239 (ε = 13 900

M⁻¹·cm⁻¹), 385 (ε = 3200 M⁻¹·cm⁻¹), and a relatively weak

band at 550 nm (ε = 65 M⁻¹·cm⁻¹), which is responsible for

the pink-yellowish color of the solution (Figure 7 and Table 6).

By analogy to the UV−vis spectra of other iron(II) complexes

of pyridine-alkylamino ligands,^21a,b,25the two high-intensity

I

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Figure 7. (a) UV−vis spectra (298 K) in acetonitrile of complexes ^H,R′2OTf. (b) UV−vis spectra (298 K) in acetonitrile of ^R,R′2OTf where R = Me,

Cl, or F. ^R,R′2OTf complexes were obtained upon dissolving ^R,R′1 in acetonitrile.

bands in the UV region can be assigned to a ligand-centered

π−π* transition and to a metal-to-ligand charge transfer

(MLCT), respectively. The latter arises from a transition

between the iron(II) t_2gorbitals and the π* pyridine orbitals,²⁶

thus, being more intense for low-spin iron(II) compounds. In

the particular case of ^H,H2OTf, the intensity of the MLCT band

is consistent with a LS iron(II) center. Finally, the low-intensity

broad band at 550 nm is assigned to a d−d transition that is

characteristic of an octahedral LS iron(II) complex (¹T₁←

¹A₁).

Compound ^F,H2OTf has a slight contribution of the LS state

at room temperature, as evidenced by the presence of a low-

intensity band at 578 nm in its UV−vis spectrum, which

strongly resembles that d−d transition described above for LS

iron(II) complexes (Figure 8). The intensity of this band, as

well as that of MLCT, and the π−π* transitions at 237 and 368

nm depend on the temperature. Lowering the temperature

results in an increase of their molar absorptivity without

saturation in the temperature range studied (from 20 to −40

°C) (Figure 8). This behavior indicates a spin-crossover

The position of the intense MLCT band (ε ≈ 2400−6100

M⁻¹·cm⁻¹) is directly related to the electronic properties of the

γ-substituent. An increase in the electron-withdrawing character

of the γ-group causes a bathochromic shift (Figure 7a). Thus,

the energy of this band decreases in the following order: NMe₂

> OMe ≈ Me > H > Cl > CO₂Et > NO₂. This phenomenon

has been previously observed for other iron complexes, ^21aand

it can be rationalized by the tuned π-acceptor character of the

pyridine ring. The shift in the λ_maxis so signiﬁcant for

^H,NO22OTf and ^H,CO2Et2OTf that the weak d−d transition is

masked by the overlap with this intense MLCT band.

For class II catalysts, the UV−vis spectrum of ^Me,H2OTf,

which is very similar to that of ^Cl,H2OTf and ^Me,Me2OTf, is

taken as the reference. This complex exhibits a band at 267 nm

(ε ∼ 5100 M⁻¹·cm⁻¹), corresponding to a ligand-centered

π−π* transition and a shoulder at 340 nm (ε ∼ 255 M⁻¹·cm⁻¹)

that is related to a MLCT. The low ε value for the latter is

characteristic of a HS iron(II) center (Figure 7b).^25a,26a

Figure 8. UV−vis spectra of ^F,H1 in CH₃CN at diﬀerent temperatures

(from 20 to −40 °C). The triﬂate anions are replaced by acetonitrile

molecules, thus, the bis-acetonitrile complex ^F,H2OTf is formed in

solution.

J

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Table 7. Oxidation of Cyclohexane and cis-Cyclooctene with H₂O₂Using Complexes [Fe(OTf)₂(^R,R′Pytacn)] (^R,R′1) as

a

Catalysts

b

c

d

e

f

g

catalyst

^H,H1

H₂O₂(equiv)

cyclohexane A + K (A/K)

cyclooctene D + E (D/E)

¹⁶O¹⁸O

97

¹⁸O

77

10

100

10

6.5 (12.3)^11a

39 (2.6)^11a

4.3 (8.9)²⁷

31 (3.8)

6.2 (10.2)²⁷

47 (2.8)

6.5 (10.5)²⁷

48 (2.3)

8.1 (1.0)^11a

99 (1.0)^11a

8.2 (2.3)

69 (1.4)

7.6 (1.1)

83 (1.5)

9.3 (2.6)

85 (1.0)

9.5 (1.5)

85 (2.1)

8.2 (1.5)

82 (1.2)

7.8 (1.5)

50 (1.1)

7.1 (5.5)^11a

86 (6.2)^11a

8.6 (5.2)

81 (3.5)

8.4 (2.6)

63 (4.7)

7.5 (0.8)

73 (1.0)

^H,NMe21

^H,OMe1

^H,Me1

95

97

99

78

80

89

74

71

72

67

63

66

5

100

10

100

10

100

10

^H,Cl1

5.9 (8.3)²⁷

40 (4.3)

100

10

^H,CO2Et1

^H,NO21

^Me,H1

5.7 (9.2)²⁷

43 (3.2)

100

10

5.3 (8.1)²⁷

34 (3.9)

100

10

7.6 (10.2)^11a

64 (4.3)^11a

6.1 (9.3)²⁷

60 (5.1)

100

10

^Me,Me1

^Cl,H1

4

100

10

6.8 (6.7)²⁷

28 (3.0)

5.9 (8.3)²⁷

26 (2.7)

9

100

10

^F,H1

4

100

a

1000 equiv of substrate (100 equiv for isotope labeling studies) with respect to catalyst. Final catalyst concentration = 1 mM. Reactions were

performed by slow-syringe-pump addition over 30 min of an acetonitrile solution of H₂O₂(10 or 100 equiv) into a solution of catalyst and substrate

b

c

at room temperature. Turnover number (mols of products/mols of catalyst), A = cyclohexanol, K = cyclohexanone. A/K = mols of alcohol/mols

d

e

of ketone. Turnover number (mol product/mol catalyst), D = syn-cyclooctane-1,2-diol, E = cyclooctene epoxide. D/E = mols of diol/mols of

f

epoxide. Percentage of syn-diol ¹⁶O¹⁸O-labeled when the oxidation of cyclooctene was carried out in the presence of 1000 equiv of H₂¹⁸O.

g

Percentage of epoxide ¹⁸O-labeled when the oxidation of cyclooctene was carried out in the presence of 1000 equiv of H₂¹⁸O.

phenomenon from HS to LS upon lowering the temperature,

which was further corroborated by measurement of the eﬀective

magnetic moment (see Figure 6b).

obtained, which is consistent with the involvement of a highly

selective metal-centered oxidant as the active species in these

reactions.²⁸When the H₂O₂concentration was increased up to

100 equiv, the eﬃciency of the oxidations slightly decreased,

giving yields down to 26−60%, and the A/K ratio also

decreased (Table 7). However, these catalysts are still among

the most eﬃcient for such transformation,^5b,29and up to now,

the catalytic results disclosed here have only been surpassed by

two previously reported iron complexes under analogous

conditions.³⁰

Careful analysis of the catalytic results indicates that both the

catalytic eﬃciency and the A/K ratio are essentially the same

for all complexes belonging to class I (^H,R′1), meaning that the

electronic properties of the γ-substituent in the pyridine ring do

not signiﬁcantly inﬂuence the catalytic activity. Instead, the

introduction of a methyl substituent in the α-position of the

pyridine (^Me,H1 and ^Me,Me1) gives catalysts that aﬀord minimal

depletion of eﬃciency when the amount of oxidant is increased.

This is exempliﬁed for catalyst ^Me,Me1, which converts hydrogen

peroxide into products in 60% yield with either 10 or 100 equiv

of H₂O₂. Therefore, these catalysts appear to be good

candidates to catalyze C−H hydroxylation under preparative

scale conditions. As a matter of fact, some of us have recently

demonstrated that ^Me,H1 is a remarkably eﬃcient and selective

Catalytic Oxidations. In order to evaluate the electronic/

steric inﬂuence imposed by the Pytacn-based ligands in the

catalytic activity of the corresponding [Fe(OTf)₂(^R,R′Pytacn)]

complexes (^R,R′1), we have tested their behavior as catalysts in

oxidation reactions using H₂O₂as the oxidant. In particular, we

have targeted the oxidation of cyclohexane and cis-cyclooctene

as model substrates for C−H bond and CC bond oxidation

reactions (Table 7). Indeed, in previous work, the catalytic

activity of ^R,R′1 in some of these reactions has been studied

under very particular conditions.^11a,27

In a typical experiment, 10−100 equiv of H₂O₂diluted in

acetonitrile was delivered via syringe pump for 30 min into an

acetonitrile solution containing the iron catalyst (1 equiv) and

the substrate (100−1000 equiv). The ﬁnal concentrations of

the reactants were 1 mM catalyst, 10−100 mM H₂O₂, and 100

mM−1 M substrate. Oxidant-limiting conditions were used in

all the experiments in order to limit product overoxidation.

The oxidation of cyclohexane with 10 equiv of H₂O₂

catalyzed by complexes ^R,R′1 aﬀorded excellent conversion of

H₂O₂into oxidation products (cyclohexanol and cyclo-

hexanone) that ranged from 43 to 76%.²⁷Remarkably, large

alcohol/ketone ratios (A/K) between 6.7 and 12.3 were

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catalyst in alkane C−H hydroxylation under preparative

conditions.^11b

When this mechanism is operative, in contrast to the Fe^III/Fe^V

pathway, the two oxygen atoms in the syn-diol originate from

the oxidant. Therefore, the isotopic labeling pattern observed in

the cis-dihydroxylation of oleﬁns contains essential information

about the nature of the oxidizing species.

The oxidation of cis-cyclooctene with 10 equiv of H₂O₂by

complexes ^R,R′1 aﬀorded high yields (between 72 and 95%) of

products as a mixture of syn-diol (D)²⁷and epoxide (E). Even

when the amount of H₂O₂was increased up to 100 equiv, the

yields remained at the same level, with the single exception of

^H,NO21 that showed modest eﬃciency under these conditions.

Such high eﬃciencies in the conversion of oxidant into oxidized

products indicate that the complexes described in this work

could be potentially suitable for alkene oxidation for synthetic

purposes. The diol/epoxide ratio (D/E) was modest for most

complexes (D/E = 1−2), except for ^Me,H1, ^Cl,H1, and ^Me,Me1

belonging to class II, which exhibited D/E values up to 6.2.

This observation suggests that these three complexes could be

used as catalysts for the preparation of syn-diols.³¹

Isotopic Labeling Studies in the Oxidation of Cyclo-

octene. Isotopic labeling experiments were devised to

investigate if this set of complexes operates through a common

reaction mechanism. Two main mechanisms of action have

been proposed for iron catalysts that mediate oleﬁn cis-

dihydroxylation with H₂O₂(Scheme 2). On the one hand,

Labeling experiments have been carried out by oxidizing cis-

cyclooctene (100 equiv) in the presence of H₂¹⁸O (1000 equiv)

using H₂O₂as oxidant (10 equiv) (Table 7). For all catalysts,

most of the syn-diol product contains one oxygen atom coming

from water (89

10% singly ¹⁸O-labeled diol).²⁷The other

oxygen atom incorporated into the syn-diol product originates

from H₂O₂, as previously reported for catalysts ^H,H1 and ^Me,H1

using H₂¹⁸O₂as oxidant.^11a,32dClear-cut diﬀerences between

class I and class II complexes arise when the labeling results for

the other product of the reaction, cyclooctene oxide, are

analyzed. Incorporation of water into the epoxide product is

very high for class I complexes (70 7%), while for class II

complexes the amount of ¹⁸O-labeled epoxide dramatically

decreases to 7

3% (Table 7). A similar trend in the ¹⁸O-

content (i.e., high and low incorporation for class I and class II,

respectively) has been previously observed in alcohol products

obtained in the hydroxylation of alkanes in the presence of

H₂¹⁸O.²⁷Therefore, by analyzing the isotope labeling in cis-

dihydroxylation and the epoxidation products, we conclude that

class I and class II catalysts operate via a water-assisted pathway

with a common Fe^V(O)(OH) oxidant. However, while the oxo

ligand transferred to cyclooctene in class I catalysts mainly

originates from water, it almost exclusively comes from H₂O₂in

class II, as ascertained from the labeling results in the epoxide

product. Some involvement of the nonwater-assisted mecha-

nism (Scheme 2, right) cannot be ruled out, particularly for

class II complexes for which a lower percentage of ¹⁸O-atom

incorporation into the cis-diol is observed (78−89% for class II

compared to 95−99% for class I).

Scheme 2. Water-Assisted (Fe^III/Fe^V) and Non-Water-

Assisted (Fe^II/Fe^IV) Mechanisms To Explain the Oxidation

of Oleﬁns by Mononuclear Non-Heme Iron Catalysts

It is also important to notice that within class I catalysts the

percentage of water incorporation into the epoxide is slightly

tuned by the electronic properties of the γ-substituent in the

pyridine ring, though this eﬀect is much subtler. Catalysts with

no substituent (^H,H1) or an electron-rich group in the γ-

position (^H,Me1, ^H,NMe21, and ^H,OMe1) aﬀord epoxide with an

¹⁸O-content roughly 10% higher compared to catalysts with

electron-withdrawing groups (^H,Cl1, ^H,NO21, and ^H,CO2Et1) (74

3 versus 65 2%).

In a previous mechanistic study on the oleﬁn epoxidation

reaction catalyzed by ^H,H1, we concluded that because the two

coordination sites where oxo and hydroxide ligand binding

occur are not identical, the Fe^V(O)(OH) species (4) can exist

as two tautomers, which are related by a fast water-assisted

equilibrium (Scheme 3).^32dThe diﬀerence between these two

isomers is the relative position of the oxo and the hydroxide

ligand. While in the isomer Fe^V(O_A)(O_BH) (4a), the oxo

ligand binds in position A, which is trans to the NCH₂Py unit,

in isomer Fe^V(O_B)(O_AH) (4b), the oxo group is located trans

to one of the N-methyl groups (position B). As hydrogen

peroxide binding in the preceding Fe^III(OOH)(H₂O) inter-

mediate (3) takes place preferentially in the B-position,²⁷water

binding occurs mainly in position A. Thus, the relative reactivity

of 4a and 4b determines the percentage of oxygen from water

that is incorporated into the oxidized product. In the particular

case of ^H,H1, this value was found to be dependent on the

nature of the oleﬁn. Cis-oleﬁns were epoxidized with ∼60−70%

incorporation of water, while trans- and terminal oleﬁns

incorporated substantially smaller amounts of water (∼30%).^32d

some catalysts operate through an Fe^III/Fe^Vcatalytic cycle with

Fe^V(O)(OH) as the active species, which is formed via water-

assisted O−O cleavage of Fe^III(OOH)(OH₂).^23,32Conse-

quently, the oxygen inventory in Fe^V(O)(OH) is composed

of one oxygen atom that comes from the oxidant (H₂O₂), and

the second one is derived from water. Rapid oxo−hydroxo

tautomerism explains that oxygen atoms originally coming from

water end up being incorporated as terminal oxo ligands. Initial

electrophilic attack of the hydroxyl ligand of Fe^V(O)(OH)

toward an oleﬁn results in a cis-dihydroxylation reaction^32bso

that the two oxygen atoms of the Fe^V(O)(OH) species end up

in the corresponding syn-diol product. C−H hydroxylation and

oleﬁn epoxidation by Fe^V(O)(OH) are initiated by the oxo

ligand and occur with stereoretention. On the other hand, other

catalysts operate via an Fe^II/Fe^IVcatalytic cycle where

Fe^IV(O)(OH₂) or Fe^IV(OH)₂is the key oxidizing species.³³

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Scheme 3. Proposed Mechanism for Alkene Oxidation, Where the Fast Equilibrium between the Two Tautomers 4a and 4b Is

Represented

The present analysis indicates that the relative reactivity of

the two tautomers is only slightly modiﬁed when the electronic

properties of the pyridine are altered by introducing electron-

releasing or electron-withdrawing groups at the γ-position. This

conclusion is not surprising if it is taken into account that the

pyridine ligand is disposed in a relative cis-position with respect

to the two coordination sites occupied by the oxo and

hydroxide ligands (Scheme 3). Ligands in relative cis-position

are expected to have a smaller eﬀect than those in trans because

the latter facilitates electronic communication between the two

ligands. Instead, the introduction of sterically more-demanding

groups at the α-position of the pyridine has a profound impact

on the relative reactivity of 4a/4b and, in turn, in the epoxide

labeling result. Indeed, a space-ﬁlling analysis (Figure 1) shows

that position A, which falls in the same plane as the pyridine

ring, is in close proximity to the α-pyridine substituent and

presumably becomes less accessible to the substrate. As a

consequence, isomer 4a becomes less reactive, and the

percentage of water incorporated into the products is

diminished. With these premises, we suggest that steric

interactions produce a more-sensitive modiﬁcation of the

relative reactivity of the two tautomers. However, further

clariﬁcation of this point is under study.

the γ-substituent. On the other hand, the presence of

substituents in the α-position of the pyridine (class II

compounds) introduces steric constraints that cause dramatic

modiﬁcations in the properties of the iron center by favoring

the HS conﬁguration over the LS conﬁguration.

Although all complexes presented herein behave as highly

eﬃcient catalysts in the oxidation of C−H and CC bonds

using H₂O₂as the oxidant through the intermediacy of a

common Fe^V(O)(OH) oxidant, there are some clear diﬀer-

ences among them. The presence of a methyl group in the α-

position of the pyridine ring aﬀords catalysts that maintain their

activity at higher oxidant concentrations and exhibit remarkably

high D/E ratios in the oxidation of cyclooctene. Diﬀerences

between class I and class II catalysts are evidenced in the ¹⁸O-

content of the epoxide product for experiments carried out in

the presence of H₂¹⁸O. Complexes without any substituent in

the α-position (class I) exhibit much higher levels of oxygen

incorporation from water into the oxidized products than those

bearing a substituent in this position (class II). Although the

eﬀect of the introduction of a substituent in the α-position is

important, the electronic properties of the group in the γ-

position of the pyridine only induce minimal modiﬁcations.

Thus, steric demands in close proximity to the iron site emerge

from this study as key structural aspects deﬁning not only the

relative reactivity of the two tautomeric active Fe^V(O)(OH)

species but also the chemoselectivity toward cis-dihydroxylation

over epoxidation.

CONCLUSIONS

■

In the present work, we have described the synthesis and the

characterization of a broad family of mononuclear iron

complexes containing a derivatized Pytacn ligand structure.

We have demonstrated that the introduction of diﬀerent

substituents in the pyridine ring causes modest but systematic

eﬀects in the geometric parameters of the resulting complexes

and substantial changes in the electronic properties of the iron

center. Modiﬁcation of the electronic properties of the iron

center are achieved by introducing diﬀerent substituents in the

γ-position of the pyridine ring (class I complexes). These

modiﬁcations become important to systematically and con-

sistently deﬁne the spin state of the iron center in this class of

complexes, for which the LS and HS conﬁgurations are

energetically very close. In particular, for ^H,R′2OTf complexes,

the eﬀect of the γ-substituent can be understood in accordance

with simple ligand ﬁeld theory; the LS contribution is more

important upon increasing the π-acceptor character of the

pyridine, which is modulated by the electron-releasing ability of

EXPERIMENTAL SECTION

■

Materials. Reagents and solvents were of commercially available

reagent quality unless otherwise stated. H₂¹⁸O (95% ¹⁸O-enriched)

was received from ICON Isotopes. Solvents were purchased from SDS

and Scharlab.

Physical Measurements. IR spectra were taken in a Mattson-

Galaxy Satellite FT-IR spectrophotometer using a MKII Golden Gate

single reﬂection ATR system. UV−vis spectroscopy was performed on

a Cary 50 Scan (Varian) UV−vis spectrophotometer with 1 cm quartz

cells. NMR spectra were taken on a Bruker DPX200, 300 or 400 MHz

spectrometer using standard conditions. Elemental analyses were

performed using a CHNS-O EA-2400 serie II from Perkin-Elmer. The

ESI-MS experiments were performed on a HPLC/MS chromatograph,

HPLC from Agilent 1100 Series and MS from Bruker Daltonics,

Esquire6000 Ion Trap or on a Bruker micrOTOF Q (II) spectrometer

using acetonitrile as a mobile phase. Product analyses were performed

M

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on an Agilent 7820A gas chromatograph (HP5 column, 30m or

Cyclosil-B column, 30m) and a ﬂame ionization detector. GC-MS

spectral analyses were performed on an Agilent 7890A gas

chromatograph interfaced with an Agilent 5975c MS mass

spectrometer. A 50% NH₃/CH₄mix was used as the ionization gas

for chemical ionization analyses. The products were identiﬁed by

comparison of their GC retention times and GC/MS with those of

authentic compounds. Variable-temperature magnetic susceptibility

was obtained on microcrystalline samples with a Quantum Design

MPMS-XL SQUID magnetometer. Solution magnetic susceptibility

measurements were measured by NMR by the Evans’ method³⁴using

special coaxial insert tubes purchased from Wilmad Glass, Co.

Temperature-dependent density of the solvent acetonitrile has been

taken into account for the measurement of the magnetic moments

(Evans’ method) and UV−vis spectra at variable temperature.

Crystal-Structure Determination. The measurements were

carried out on a BRUKER SMART APEX CCD diﬀractometer

using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å)

from an X-ray tube. Programs used: data collection, Smart version

5.631 (Bruker AXS 1997−02); data reduction, Saint + version 6.36A

(Bruker AXS 2001); absorption correction, SADABS version 2.10

(Bruker AXS 2001). Structure solution and reﬁnement were done

using SHELXTL version 6.14 (Bruker AXS 2000−2003) and

SHELXT version 2013_3. The structures were solved by direct

methods and reﬁned by full-matrix least-squares methods on F². The

non-hydrogen atoms were reﬁned anisotropically. The H atoms were

placed in geometrically optimized positions and forced to ride on the

atom to which they are attached. Although crystal data indicate that

the X-ray structures of ^H,NMe21 and ^F,H1 are not fully satisfactory, they

are of suﬃcient quality for the discussion of the crystallographic

parameters made in the present work.

to a vigorously stirred solution of ^H,ClPytacn (101 mg, 0.36 mmol) in

THF (1.5 mL). After a few seconds the solution became cloudy and a

pale yellow precipitate appeared. After being stirred for 2 h the

solution was ﬁltered oﬀ and the resulting yellow solid dried under

vacuum. This solid was dissolved in CH₂Cl₂and ﬁltered through

Celite. Slow diethyl ether diﬀusion over the resulting solution aﬀorded,

in a few days, 143 mg of yellow crystals (0.22 mmol, 63%). Anal. Calcd

for C₁₆H₂₃ClF₆FeN₄O₆S₂: C, 30.18; H, 3.64; N, 8.80; S, 10.07. Found:

C, 30.09; H, 3.62; N, 8.88; S, 9.95. FT-IR (ATR) ν, cm⁻¹: 2960−2885

1

(C−H)_sp3, 1281 (py), 1222, 1161, 1027, 634 (CF₃SO₃). H NMR

(CD₃CN, 400 MHz, 300 K) δ, ppm: 12.44, 9.53, 7.07, 5.45, 3.95, 3.43,

1

2.18, 1.13. H NMR (CD₂Cl₂, 400 MHz, 300 K) δ, ppm: 157.29,

119.78, 100.05, 89.73, 47.813, 33.75. ESI-MS (m/z): 487.0 [M −

OTf]⁺(100), 168.9 [M − OTf]²⁺(25).

[Fe(OTf)₂(^H,CO2EtPytacn)] (^H,CO2Et1). Prepared in analogous manner

to ^H,Cl1. Yield = 43%. Anal. Calcd for C₁₉H₂₈F₆FeN₄O₈S₂: C, 33.84; H,

4.18; N, 8.31. Found: C, 33.48; H, 3.97; N, 8,63. FT-IR (ATR) ν,

cm⁻¹: 2982−2865 (C−H)_sp3, 1732 (CO), 1284 (py), 1284, 1224,

1

1164, 1026, 633 (CF₃SO₃). H NMR (CD₃CN, 400 MHz, 300 K) δ,

1

ppm: 10.84, 8.89, 5.76, 4.83, 4.36, 4.39, 3.04, 2.11, 1.97. H NMR

(CD₂Cl₂, 400 MHz, 300 K) δ, ppm: 164.22, 103.84, 90.26, 49.67,

39.01. ESI-MS (m/z): 525.1 [M − OTf]⁺(25), 188.0 [M − 2OTf]²⁺

(100).

[Fe(OTf)₂(^H,NO2Pytacn)] (^H,NO21). Prepared in analogous manner to

^H,Me1. Yield =73%. Anal. Calcd for C₁₆H₂₃F₆FeN₅O₈S₂: C, 29.69; H,

3.58; N, 10.82; S, 9.91. Found: C, 29.87; H, 3.70; N, 10.81; S, 9.76.

FT-IR (ATR) ν, cm⁻¹: 2917 (C−H)_sp3, 1279 (py), 1223, 1159, 1028,

1

633 (CF₃SO₃). H NMR (CD₃CN, 400 MHz, 300 K) δ, ppm: 10.47,

1

8.73, 4.93, 4.17, 7.78, 2.86, 2.64, 2.14, 1.13. H NMR (CD₂Cl₂, 400

MHz, 300 K) δ, ppm: 174.18, 136.30, 108.77, 92.12, 47.82, 33.18. ESI-

MS (m/z): 498.0 [M − OTf]⁺(100), 174.5 [M − 2OTf]²⁺(100).

[Fe(OTf)₂(^Me,MePytacn)] (^Me,Me1). Prepared in analogous manner to

^H,Me1. Yield = 59%. Anal. Calcd for C₁₈H₂₈F₆FeN₄O₆S₂: C, 34.30; H,

Synthesis of Ligands. Syntheses and full characterization of

ligands ^R,R′Pytacn are included in the Supporting Information.

Synthesis of Complexes. Synthesis of Bis-triﬂato Complexes

[Fe(OTf)₂(^R,R′Pytacn)] (^R,R′1). ^R,R′1 complexes were prepared in a

glovebox ([O₂] < 1 ppm, [H₂O] < 1 ppm). [Fe(OTf)₂(^H,HPytacn)]

1

4.48; N, 8.89. Found: C, 34.37; H, 4.11; N, 8.83. H NMR (CD₃CN,

200 MHz, 300 K) δ, ppm: 138.63, 84.64, 54.02, 47.91, 42.83, 31.14,

1

13.71, −32.78. H NMR (CD₂Cl₂, 200 MHz, 300 K) δ, ppm: 176.16,

(

^H,H1)¹²and [Fe(OTf)₂(^Me,HPytacn)] (^Me,H1)^11awere synthesized as

130.18, 101.05, 95.33, 60.52, 52.75, 38.97, −1.70, −37.25. ESI-MS (m/

previously reported. Single crystals of compounds ^R,R′1 suitable for X-

ray analyses were obtained by slow diethyl ether diﬀusion into CH₂Cl₂

solutions of the complexes.

z): 166.1 [M − 2OTf]²⁺, 481.12 [M − OTf]⁺.

[Fe(OTf)₂(^Cl,HPytacn)] (^Cl,H1). Prepared in analogous manner to

^H,Cl1. Yield = 66%. Anal. Calcd for C₁₆H₂₃F₆ClFeN₄O₆S₂: C, 30.18; H,

3.64; N, 8.80. Found: C, 29.98; H, 3.58; N, 8.68. FT-IR (ATR) ν,

cm⁻¹: 3357−3262 (C−H)_sp3, 1281, 1226, 1163, 1027, 630 (CF₃SO₃).

¹H NMR (CD₃CN, 200 MHz, 300 K) δ, ppm: 100.19, 60.14, 39.41.

¹H NMR (CD₂Cl₂, 400 MHz, 300 K) δ, ppm: 155.97, 121.95, 106.25,

101.89, 89.16, 75.50, 60.30, 44.68, 32.45, 18.57. ESI-MS (m/z): 169.0

[M − 2OTf]²⁺, 487.1 [M − OTf]⁺.

[Fe(OTf)₂(^H,MePytacn)]

( A suspension of [Fe-

^H,Me1).

(OTf)₂(CH₃CN)₂] (176 mg, 0.40 mmol) in CH₂Cl₂(2 mL) was

added dropwise to a vigorously stirred solution of ^H,MePytacn (106 mg,

0.40 mmol) in CH₂Cl₂(1 mL). The iron triﬂate salt quickly solubilized

aﬀording a dark red solution, which was stirred for 3 h. Afterward, the

solution was ﬁltered and slow diethyl ether diﬀusion aﬀorded 190 mg

of yellow crystals (0.31 mmol, 77%). Anal. Calcd for

C₁₇H₂₆F₆FeN₄O₆S₂: C, 33.13; H, 4.25; N, 9.09. Found: C, 33.29; H,

[Fe(OTf)₂(^F,HPytacn)] (^F,H1). Prepared in analogous manner to ^H,Cl1.

Yield = 27%. Anal. Calcd for C₁₆H₂₃F₇FeN₄O₆S₂: C, 30.98; H, 3.74; N,

9.03. Found: C, 31.06; H, 3.43; N, 9.28. FT-IR (ATR) ν, cm⁻¹: 2927

(C−H)_sp3, 1309, 1213, 1158, 1015, 630 (CF₃SO₃). ¹H NMR

(CD₃CN, 200 MHz, 300 K) δ, ppm: 104.67, 84.41, 70.10, 50.59,

1

4.07; N, 9.12. H NMR (CD₃CN, 200 MHz, 300 K) δ, ppm: 14.66,

1

11.17, 7.73, 5.46. H NMR (CD₂Cl₂, 200 MHz, 300 K) δ: 145.68,

119.76, 92.89, 48.61, 35.44, −11.56. ESI-MS (m/z): 159.1 [M −

2OTf]²⁺.

1

[Fe(OTf)₂(^H,NMe2Pytacn)] (^H,NMe21). Prepared in analogous manner to

^H,Me1. Yield = 69%. Anal. Calcd for C₁₈H₂₉F₆FeN₅O₆S₂: C, 33.50; H,

4.53; N, 10.85. Found: C, 33.67; H, 4.33; N, 10.82. FT-IR (ATR) ν,

cm⁻¹: 2916−2872 (C−H)_sp3, 1298 (py), 1219, 1158, 1027, 1010, 632

(CF₃SO₃). ¹H NMR (CD₃CN, 400 MHz, 300 K) δ, ppm: 44.52,

28.53, −2.91 . H NMR (CD₂Cl₂, 200 MHz, 300 K) δ, ppm: 142.65,

122.50, 108.05, 80.16, 69.76, 29.99, 18.95. ESI-MS (m/z): 161.0 [M −

2OTf]²⁺, 471.1 [M − OTf]⁺.

[Fe(^H,HPytacn)(CH₃CN)₂](SbF₆)₂^H,H2SbF₆). FeCl₂(51 mg, 0.40

(

mmol) was added directly as a solid to a vigorously stirred solution of

^H,HPytacn (100 mg, 0.40 mmol) in CH₃CN. The initially pale yellow

solution became gradually bright orange as the FeCl₂got dissolved.

After being stirred for 30 min, AgSbF₆(277 mg, 0.80 mmol) was

added which caused the immediate precipitation of AgCl and an

evident color change of the solution from bright orange to dark brown.

After being stirred for 30 min, the solution was ﬁltered through Celite.

Slow diethyl ether diﬀusion over the resulting solution aﬀorded, in a

few days, 245 mg of red-brown crystals (0.29 mmol, 71% yield). Anal.

Calcd for C₁₈H₃₀F₁₂FeN₆Sb₂: C, 25.20; H, 3.53; N, 9.80. Found: C,

1

34.32, 29.21, 20.73, 18.80, 12.11, 6.90. H NMR (CD₂Cl₂, 400 MHz,

300 K) δ, ppm: 120.50, 108.63, 83.25, 42.69, 39.49. ESI-MS (m/z):

496.0 [M − OTf]⁺(100), 173.5 [M − 2OTf]²⁺(25).

[Fe(OTf)₂(^H,OMePytacn)] (^H,OMe1). Prepared in analogous manner to

^H,Me1. Yield = 38%. Anal. Calcd for C₁₉H₃₀F₆FeN₄O₇S₂: C, 34.55; H,

4.58; N, 8.48. Found: C, 34.59; H, 4.43; N, 8.64. FT-IR (ATR) ν,

cm⁻¹: 2968−2888 (C−H)_sp3, 1468 (py), 1250, 1157, 1026, 635

(CF₃SO₃). ¹H NMR (CD₃CN, 400 MHz, 300 K) δ, ppm: 13.41,

1

11.60, 8.80, 6.49, 3.64. H NMR (CD₂Cl₂, 400 MHz, 300 K) δ, ppm:

1

120.38, 114.44, 94.31. ESI-MS (m/z): 511.1 [M − OTf]⁺(25), 181.0

[M − 2OTf]²⁺(100).

25.44; H, 3.47; N, 10.15. H NMR (CD₃CN, 400 MHz, 300 K) δ,

ppm: 12.68, 9.73, 9.59, 9.10, 7.33, 6.70, 3.82, 2.17, 1.96. HRMS (m/z):

152.0668 [M − 2CH₃CN−2SbF₆]²⁺, 161.0718 [M − 2CH₃CN−

2SbF₆+ H₂O]²⁺, 539.0312 [M − 2CH₃CN−SbF₆]⁺.

[Fe(OTf)₂(^H,ClPytacn)] (^H,Cl1)_.A solution of [Fe(OTf)₂(CH₃CN)₂]

(155 mg, 0.36 mmol) in anhydrous THF (2 mL) was added dropwise

N

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Inorganic Chemistry

Article

[Fe(^Me,HPytacn)(CH₃CN)₂](SbF₆)₂^Me,H2SbF₆). FeCl₂(11 mg, 0.09

(

AUTHOR INFORMATION

Corresponding Author

* E-mail: miquel.costas@udg.edu (M.C.), anna.company@udg.

edu (A.C.).

■

mmol) was added directly as a solid to a vigorously stirred solution of

^Me,HPytacn (21 mg, 0.09 mmol) in CH₃CN. The initially pale yellow

solution became gradually bright orange as the FeCl₂got dissolved.

After being stirred for 30 min, AgSbF₆(60 mg, 0.17 mmol) was added

which caused the immediate precipitation of AgCl and an evident

color change of the solution from bright orange to yellowish. After

being stirred for 30 min, the solution was ﬁltered through Celite. Slow

diethyl ether diﬀusion over the resulting solution aﬀorded, in a few

days, 54 mg of colorless crystals which were suitable for X-ray

diﬀraction (0.05 mmol, 69% yield). Anal. Calcd for

C₁₉H₃₂F₁₂FeN₆Sb₂: C, 26.18; H, 3.70; N, 9.64%. Found: C, 26.16;

H, 3.45; N, 8.94% (repeated attempts to improve the accuracy of the

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

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Financial support for this work was provided by the Spanish

Ministry of Science (Project CTQ2012−37420-C02-01/BQU

and Consolider Ingenio Ingenio/CSD2010-00065 to M.C. and

X.R. and project CTQ2012-32436 to T.P.) and the European

Research Council (ERC-2009-StG-239910 to M.C.). M.C. and

X.R. acknowledge Generalitat de Catalunya for ICREA-

Academia Awards and 2009-SGR637. A.C. acknowledges the

European Commission for a Career Integration Grant (FP7-

PEOPLE-2011-CIG-303522). The Spanish Ministry of Science

1

elemental analysis were unsuccessful). H NMR (CD₃CN, 400 MHz,

300 K) δ, ppm: 143.94, 84.76, 56.30, 49.10, −7.74, −32.53. ESI-MS

(m/z): 159.0759 [M − 2CH₃CN−2SbF₆]²⁺, 553.0462 [M −

2CH₃CN−SbF₆]⁺.

Reaction Conditions for Catalysis. In a typical reaction, 0.36 mL of

a 70 mM (25 μmol) or 700 mM (250 μmol) H₂O₂solution (diluted

from a 35% H₂O₂aqueous solution) in CH₃CN was delivered by

syringe pump over 30 min at 25 °C to a vigorously stirred CH₃CN

solution (2.14 mL) containing the iron catalyst (2.5 μmols) and the

substrate (2500 μmols). The ﬁnal concentrations of reagents were 1

mM iron catalyst, 10 mM/100 mM H₂O₂, and 1 M substrate. After

syringe pump addition, the resulting solution was stirred for another

10 min. For the oxidation of cyclohexane, biphenyl (internal standard)

was added at this point and the iron complex was removed by passing

the solution through a short path of basic alumina or silica followed by

elution with 2 mL of AcOEt. Finally, the solution was subjected to GC

analysis. The organic products were identiﬁed by comparison with

authentic compounds.

For the oxidation of cyclooctene, the reaction was performed as

described above but under a N₂atmosphere. After the oxidant was

added by syringe pump and the mixture was stirred for 10 min, 1 mL

of acetic anhydride together with 0.1 mL of 1-methylimidazole was

added to aﬀord the esteriﬁcation of the diol product. After being

stirred for 15 min at room temperature, ice was added and the mixture

was stirred for about 30 min. Biphenyl (internal standard) was added

and the mixture was extracted with 2 mL of CHCl₃. The organic layer

was washed with 2 mL H₂SO₄1M, 2 mL sat. NaHCO₃and 2 mL H₂O,

dried with MgSO₄and subjected to GC analysis. The organic products

were identiﬁed by comparison with authentic compounds.

́

is acknowledged for a FPU PhD grant to I.P. and for a Ramon y

Cajal contract to A.C. We thank Catexel for a generous gift of

tritosyl-1,4,7-triazacyclononane. X.R. is grateful for ﬁnancial

support from INNPLANTA project IPN-2011-0059-PCT-4200

0-ACT1.

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Isotopic Labeling Experiments Using H₂¹⁸O. The experimental

procedure was similar to the one described above for a typical

oxidation reaction of cyclooctene. In these experiments, 0.36 mL of a

70 mM (25 μmols) H₂O₂solution were delivered by syringe pump

over a solution containing the catalyst (2.5 μmols), 50 μL of H₂¹⁸O

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In order to account for the isotopic purity of H₂¹⁸O and its dilution

because of the use of aqueous H₂O₂solutions, corrected values were

obtained by dividing the simulated percentage of ¹⁸O-labeled epoxide

and ¹⁶O¹⁸O-labeled syn-diol by 0.94.

ASSOCIATED CONTENT

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S

* Supporting Information

Description of the synthesis of ^R,R′Pytacn ligands, solid-state

characterization of ^H,OMe1, ^H,Me1, ^H,Cl1, ^H,CO2Et1, and ^Cl,H1,

SQUID measurement for ^Me,H2SbF₆, paramagnetic H NMR

1

spectra of ^R,R′1 and ^R,R′2, and variable-temperature para-

1

magnetic H NMR of ^Me,H1. This material is available free of

charge via the Internet at http://pubs.acs.org.

O

dx.doi.org/10.1021/ic4004033 | Inorg. Chem. XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/ic4004033 | Inorg. Chem. XXXX, XXX, XXX−XXX

Article Doi

Assessing the impact of electronic and steric tuning of the ligand in the spin state and catalytic oxidation ability of the FeII(Pytacn) family of complexes

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