A Pseudotetrahedral Terminal Oxoiron(IV) Complex: Mechanistic Promiscuity in C−H Bond Oxidation Reactions [original]

Bioinorganic Chemistry Hot Paper

A Pseudotetrahedral Terminal Oxoiron(IV) Complex: Mechanistic

Promiscuity in CH Bond Oxidation Reactions

Katrin Warm, Alice Paskin, Uwe Kuhlmann, Eckhard Bill, Marcel Swart, Michael Haumann,

Holger Dau, Peter Hildebrandt, and Kallol Ray*

Dedicated to Professor Wolfgang Kaim on the occasion of his 70th birthday

Abstract: S=2 oxoiron(IV) species act as reactive intermedi-

ates in the catalytic cycle of nonheme iron oxygenases. The few

available synthetic S=2Fe

IV=O complexes known to date are

often limited to trigonal bipyramidal and very rarely to

octahedral geometries. Herein we describe the generation and

characterization of an S=2 pseudotetrahedral FeIV=O com-

plex 2supported by the sterically demanding 1,4,7-tri-tert-

butyl-1,4,7-triazacyclononane ligand. Complex 2is a very

potent oxidant in hydrogen atom abstraction (HAA) reactions

with large non-classical deuterium kinetic isotope effects,

suggesting hydrogen tunneling contributions. For sterically

encumbered substrates, direct HAA is impeded and an

alternative oxidative asynchronous proton-coupled electron

transfer mechanism prevails, which is unique within the

nonheme oxoiron community. The high reactivity and the

similar spectroscopic parameters make 2one of the best

electronic and functional models for a biological oxoiron(IV)

intermediate of taurine dioxygenase (TauD-J).

Introduction

High-valent oxoiron(IV) intermediates act as the active

oxidants in the catalytic cycles of a variety of mononuclear

non-heme iron oxygenases.[1] These high-valent species have

been characterized by rapid freeze quench methods in few

cases[1] and were unambiguously shown by UV/Vis, Mçssba-

uer, and X-ray absorption spectroscopic methods to contain

high-spin (S=2) iron(IV) centres. However, the available

experimental data could not reveal other important structural

features, such as the number, identity, and disposition of

ligands in the FeIV coordination sphere. Density functional

theoretical (DFT) studies[2] on the taurine:aKG dioxygenase

(TauD) system have shown that the spectroscopic properties

of the hydrogen-abstracting oxoiron(IV) key intermediate

(TauD-J) are consistent with both suggested structural

models (Scheme 1), that is, with trigonal bipyramidal (TBP)

as well as distorted octahedral (Oh) coordinations. Significant

synthetic efforts in the past decade have led to the generation

of oxoiron(IV) cores in both TBP and Ohgeometries

(Scheme 1). Although the majority of the synthetic complexes

exhibit S=1 ground states in Ohgeometry,[3] DFT-studies

predicted stabilization of the more reactive[4] S=2 oxoiron-

(IV) units[5] either by enforcing a TBP geometry at the

iron(IV) centre[5a–d] or by weakening the equatorial donation

in Ohgeometry.[5e]

Results and Discussion

In the context of the existing ambiguity related to the

coordination number of iron in biological oxoiron(IV)

intermediates,[2] and the limitation of the synthetic S=2

oxoiron(IV) cores to mainly TBP and in rare cases to Oh

geometries, we have now sought to identify a tripodal ligand

that allows for trapping an FeIV=O core in a geometry

different from the known TBP or Ohgeometries. Herein we

report the synthesis and characterization of the S=2 pseu-

dotetrahedral [FeIV(O)(tBu3tacn)]2+(2,tBu3tacn[6] =1,4,7-tri-

tert-butyl-1,4,7-triazacyclononane) complex, which exhibits

spectroscopic and reactivity properties distinct from the

oxoiron(IV) cores in TBP or Ohgeometries. In particular, in

direct contrast to the vast majority of previous oxoiron(IV)

cores,[3a–g,5a–e] where the reactivity with substrates containing

CH bonds is controlled by the CH bond dissociation

[*] M. Sc. K. Warm, M. Sc. A. Paskin, Prof. Dr. K. Ray

Institut fr Chemie, Humboldt-Universitt zu Berlin

Brook-Taylor-Str. 2, 12489 Berlin (Germany)

E-mail: [email protected]

Dr. U. Kuhlmann, Prof. Dr. P. Hildebrandt

Institut fr Chemie, Technische Universitt Berlin, Fakultt II

Straße des 17. Juni 135, 10623 Berlin (Germany)

Dr. E. Bill

Max-Planck-Institut fr Chemische Energiekonversion (CEC)

Stiftstraße 34–36, 45470 Mlheim (Germany)

Prof. Dr. M. Swart

Institut de Qumica Computacional i Catlisi, Universitat de Girona,

Campus Montilivi (Cincies)

Maria Aurlia Capmany i Farns, 69, 17003 Girona (Spain)

and

ICREA

Pg. Llus Companys 23, 08010 Barcelona (Spain)

Dr. M. Haumann, Prof. Dr. H. Dau

Institut fr Physik, Freie Universitt Berlin

Arnimallee 14, 14195 Berlin (Germany)

Supporting information and the ORCID identification number(s) for

the author(s) of this article can be found under:

https://doi.org/10.1002/anie.202015896.

 2020 The Authors. Angewandte Chemie International Edition

published by Wiley-VCH GmbH. This is an open access article under

the terms of the Creative Commons Attribution Non-Commercial

NoDerivs License, which permits use and distribution in any

medium, provided the original work is properly cited, the use is non-

commercial and no modifications or adaptations are made.

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International Edition: doi.org/10.1002/anie.202015896

German Edition: doi.org/10.1002/ange.202015896

6752  2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021,60, 6752 – 6756

energies (BDEC-H), complex 2demonstrates a mechanistic

promiscuity in its CH oxidation reactions. Sterically less

hindered CH bonds are oxidized via a conventional direct

hydrogen atom abstraction (HAA; Scheme 2) mechanism

that is characterized by large deuterium kinetic isotope effects

(KIEs), which are greater than the semi-classical limit of 7,

implying a significant contribution of hydrogen tunnelling.[7]

In contrast, for sterically encumbered substrates, where the

direct access to the FeIV=O core is blocked, the CH

oxidation reaction proceeds with a significantly lower KIE

and presumably involves a proton-coupled electron transfer

(PCET) mechanism along a spectrum of “asynchronicity”[8] in

which the transition state for the net H-atom transfer contains

more electron transfer character (Scheme 2; Oxidative asyn-

chronous PCET).

Combination of equimolar amounts of the previously

reported tBu3tacn ligand[6,9] and FeII(OTf)2(CH3CN)2in

CH2Cl2afforded [FeII(tBu3tacn)(OTf)](OTf) (1), whose crys-

tal structure (Figure S1; Tables S1,S2) exhibited a distorted

tetrahedral geometry (N-Fe-N angles of 86.5–88.3

) with an

Fe-O distance of 1.935(2)  and three Fe-N distances of

2.105(2)–2.124(2) . The zero-field Mçssbauer spectrum of

1(Figure S2) revealed a single doublet with an isomer shift

(d) of 0.97 mms1and a large quadrupole splitting (DEQ=

1.98 mms1), consistent with an S=2 spin state, which is also

supported by DFT[11] (Table S3). Reaction of 1in pure CH2Cl2

or butyronitrile (PrCN) at 90

C with 2-(tert-butylsulfonyl)-

iodosobenzene (sPhIO)[12] yielded a transient species 2(Fig-

ure 1A; half-life at 70

C=20 min) with electronic absorp-

tion features centered at lmax =356 nm (e=7500 M1cm1)

and 780 nm (e=150 M1cm1). Notably, the presence of

a well-defined strong absorption band in the near-UV region

is typical of S=2 oxoiron(IV) cores (Table S4);[5a–d] in 2this

band at lmax =356 nm is slightly red-shifted (Table 1) relative

to that of TauD-J(lmax =318 nm).[1a] The S=2 spin state of 2

was additionally corroborated by the Evans[13] NMR method

(Figure S3) at 90

C which yielded the magnetic moment

meff =4.50 mB(theoretical value for S=2: 4.90 mB). An elec-

tron spray ionization mass spectrum (Figure S4) of 2exhib-

Scheme 1. Left: Proposed structures of S=2TauD-Jbased on DFT studies;[2] middle: selected examples of S=1 and S=2 oxoiron(IV) cores in

TBP and Ohgeometries; right: A pseudotetrahedral S=2oxoiron(IV) complex 2reported in this work; in the inset is shown the DFT calculated

structure of 2in the S=2 state.

Scheme 2. Mechanisms of net hydrogen atom transfer.

Figure 1. A) UV/Vis spectra of 1(dashed line) and 2(solid line) in

CH2Cl2at 90

C; inset shows the rRaman spectra of 16O- (solid line)

and 18O-labelled (dashed line) 2(4 mM solution) in CH2Cl2upon

406 nm irradiation at 90

C; solvent signals are indicated by an

asterisk; B) Zero-field Mçssbauer spectrum (grey) of a frozen sample

of 2in PrCN/CH2Cl2(10:1) and simulation with d=0.11 mms1and

DEQ=0.96 mms1for the main species (solid line, 87%). The minor

species (dashed line) with d=0.97 mms1and DEQ=1.98 mms1

corresponds to unreacted 1.

Table 1: Comparison of the spectroscopic properties of TauD-Jand 2.

TauD-J[1a,b,2,10] 2

lmax [nm] 318 356

R(Fe-O) [] 1.62 1.66

nFe=O[cm1] 821 802

d[mm1s1] 0.31 0.11

DEQ[mm1s1] 0.88 0.96

Axx,Ayy,Azz [T] S=2:

18.4, 17.6, 31

S=2:

10.1, 3.3, 36.1

Eo[eV] 7123.8 7123.2

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ited a signal at m/z=518.7, consistent with its formulation as

[FeIV(O)(tBu3tacn)(OTf)]+(m/zcalc=518.2). However, the

19F-NMR spectrum (Figure S5) of 2displayed a single reso-

nance at 77.0 ppm, which confirmed that the triflate (OTf)

anion is not bound to the Fe-centre in 2. This observation

together with the same UV-vis spectrum (Figure S6) of 2in

both coordinating (PrCN) and non-coordinating (CH2Cl2)

solvents corroborates the absence of any exogeneous ligands

binding to the iron centre. Thus, the 4-coordinate geometry

found in 1is retained in 2, leading us to formulate the latter as

[FeIV(O)(tBu3tacn)]2+.

The 4-coordinate geometry of 2was also supported by

Extended X-ray Absorption Fine Structure (EXAFS) anal-

ysis (Figure S7A, Table S5), which yielded a good fit with an

oxygen ligand at 1.66 , assigned to the Fe=O bond, and

a further shell of three nitrogen ligands at 2.06 , corre-

sponding to the N donors of tBu3tacn. The Fe K-edge X-ray

absorption spectrum (Figure S7B) of 2reveals an edge energy

of 7123.2 eV (vs. 7119.7 eV for 1), which is within the range of

values found for synthetic FeIV=O complexes.[3b,5a–c] Further-

more, in contrast to the pre-edge features of existing S=1

complexes that can be modelled with a single Gaussian,[13] in

the pre-edge region of 2two spectral features at 7115 and

7117 eV are tentatively discernible (Figure S7A,C), which

may be rationalized in terms of a splitting of the aand bdz2

orbitals by spin polarization in the S=2 oxoiron(IV) cor-

es.[5a,b]

Resonance Raman spectroscopy revealed a n(Fe=O)

stretching mode at 802 cm1in 2(Figure 1A, inset) that

shifted to 767 cm1upon 18O-labelling. The observed n(Fe=O)

mode has one of the lowest energies reported to date for

oxoiron(IV) cores. This may be attributed to the high spin

(S=2) ground state of 2as this would (in a simplified

pseudotetrahedral ligand field) require a d(x2y2)1d(xy)1d-

(xz,yz)2d(z2)0electronic configuration with an FeO bond

order (BO) of 2.0.[15] Notably, the high-spin ground state of 2

is unique for a pseudotetrahedral geometry; previously

reported pseudotetrahedral M-X (X=O2,NR

2,orN

3)

complexes,[16] including the recent CoIIIO complex,[8,17] all

possess a low-spin ground state with a M-X BO of 3. The zero

field Mçssbauer spectrum of 2exhibits a doublet (87% yield)

with a quadrupole splitting, DEQ=0.96 mms1, and an isomer

shift, d=0.11 mms1(Figure 1B). Although, the DEQvalue is

very close to the value reported for TauD-J(Table 1),[1b] the

d-value is significantly lower, which may reflect the nitrogen-

rich character in 2in contrast to the harder oxygen-containing

ligand sphere in TauD-J.In applied magnetic fields, the

spectra of 2exhibit paramagnetic hyperfine structures, which

were analysed by assuming an S=2 center yielding a non-

axial A-tensor with Axx/gnbn=10.1 T, Ayy/gnbn=3.3 T and

Azz/gnbn=36.1 T (Figure S8). The structure of 2as obtained

by DFT calculations (Scheme 1, inset) reveals an off-axis tilt

of the oxo ligand resulting in a deviation from the C3

symmetry, which may account for the non-axial A-tensor

determined from magnetic Mçssbauer studies. The quintet

state was calculated to be more stable than the triplet and the

singlet states by 0.8 and 6.6 kcalmol1, respectively (Ta-

ble S3). Furthermore, among all spin states, the calculated

spectroscopic properties of the S=2 state provide the best

description of the experimental data. The calculated Fe=O

and FeN bond distances (1.63 and 2.06 , respectively), Fe=

O stretching mode frequency (893 cm1,18O isotope shift

36 cm1), and Mçssbauer d-value (0.06 mms1), on the

ground S=2 state are in satisfactory agreement with experi-

ments (Table S3). Notably, the calculated data for the S=1

and S=0 states deviate significantly from the experiments,

such that we take the calculations as a further support for the

S=2 ground state in 2.

The oxidative reactivity of 2(Figures S9–S18; Table S6)

has been investigated with several substrates in oxygen atom

transfer (OAT) and HAA reactions and the second order rate

constants derived from these studies in CH2Cl2are compared

with three of the most reactive high-valent Fe-oxo inter-

mediates reported to date (namely the [(TQA)FeIV(O)-

(CH3CN)]2+(TQA=tris(2-quinolylmethyl)amine),[5e]

[(Me3NTB)FeIV(O)]2+(Me3NTB=tris((N-methyl-benzimi-

dazol-2-yl)methyl)amine)[3c] and [(TMCO)FeIV(O)-

(CH3CN)]2+(TMCO=4,8,12-trimethyl-1-oxa-4,8,12-triaza-

cyclotetradecane)[3f] complexes (Table 2). In reactions with

ethylbenzene, 1,4-cyclohexadiene (1,4-CHD), and toluene, 2

is a stronger oxidant than [(TMCO)FeIV(O)(CH3CN)]2+, but

comparable to [(TQA)FeIV(O)(CH3CN)]2+and

[(Me3NTB)FeIV(O)]2+. Interestingly, the reactivity trend is

reversed in reactions with 9,10-dihydroanthracene (DHA),

where 2exhibits the least reactivity. Furthermore, when the

logarithms of the statistically corrected second order rate

constants (k2) were plotted vs. the BDEC-H values of the

substrates (Figure 2A, Figure S20A), the linear correlation

typically observed for oxoiron(IV) cores[3a–i,5] is found to be

not valid for 2. While the respective log(k2) values associated

with 2for the oxidation of 1,4-CHD, 1,3-cyclohexadiene (1,3-

CHD), ethylbenzene, cyclohexene and toluene fall on a line

(Figure 2A, black points), xanthene, DHA, indene and

fluorene substrates (Figure 2A, inset) deviate from this

pattern and exhibit significantly lower rates than predicted

by the linear relationship. Particularly interesting is the large

rate difference of two orders of magnitude for DHA and 1,4-

CHD, which are known to have small difference in BDEC-H

values.[18] Furthermore, large deuterium KIEs of 7 (Fig-

ure S9), 12 (Figure S10), and 53 (Figure 2C, Figure S11) were

Table 2: Comparison of the reaction rate constants k2’(normalized to the

number of equivalent H atoms) at 40

C for the CH activation reaction

of 2and the highly reactive intermediates (TMCO)FeIV=O,

(Me3NTB)FeIV=O and (TQA)FeIV=O towards a selection of substrates.

Substrate

(BDEC-H, kcal/mol)

k2’[M1s1]

2(TMCO)

FeIV=O

(Me3NTB)

FeIV=O

(TQA)

FeIV=O

1,4-CHD (76.0) 1.0102[a] nd 7.8102nd

DHA (76.3) 1.6[b] Too fast (90

C) 2.4102nd

Ethylbenzene (85.4) 3.3[b] 0.10[c] 0.75 1.1

Toluene (89.7) 0.43[b] 0.0044[c] 0.16 0.21

nd=rate not determined;k2’values at 40

C were calculated from the

values measured at [a] 90

C; [b] 70

C; [c] 60

C; [d] 50

C and

corrected for the temperature difference by doubling the rate for every 10

degrees rise in temperature.

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recorded for toluene, 1,4-CHD, and ethylbenzene reactions,

respectively, suggesting a HAA mechanism with significant

contribution of hydrogen-tunnelling, as is frequently pro-

posed in CH bond activation reactions of FeIV=O species.[7]

In contrast, significantly reduced KIEs of 1.2 (Figure 2D,

Figure S12) and 2.1 (Figure S13) were determined for DHA

and xanthene, respectively, thereby pointing to a change of

mechanism. Further mechanistic insights were obtained by

plotting the rate constants against the pKaand the ionization

energies (IE) of the substrates. The log(k2) vs. IE plot

(Figure 2B, Figure S20B) revealed that for reactions of 2with

xanthene, DHA, indene and fluorene the rate decreased

linearly with increasing IE, whereas the rates for 1,4-CHD,

1,3-CHD, ethylbenzene, cyclohexene and toluene scatter

irregularly. Furthermore, no linear trend was observed in

the log(k2)vs.pKaplot (Figure S20C) for all the investigated

substrates. Thus, the tBu3tacn ligand blocks the HAA pathway

by presumably impeding access of the bulkier polycyclic

hydrocarbons to the Fe=O unit in 2. An alternative oxidative

asynchronous PCET mechanism (Scheme 2) prevails in such

cases, which are typically characterized by low KIEs and

a linear correlation of the reaction rates to IEs.

Conclusion

Taken together the results presented herein unequivocally

validate the formation of a terminal oxoiron(IV) complex 2in

a pseudotetrahedral geometry. The computational and ex-

perimental analyses are consistent with the presence of an S=

2Fe

IV=O core in 2. Complex 2represents the only example of

a high-spin complex with metal-ligand multiple bond charac-

ter in a pseudotetrahedral geometry; notably, a pseudotetra-

hedral oxoiron(IV) complex has been very recently demon-

strated to possess an S=0 state in the gas-phase.[19] The

absorption spectrum, Mçssbauer DEQ, Fe K-edge energy, and

the n(Fe=O) mode of 2(Table 1) bear very close resemblance

to the corresponding spectroscopic properties of TauD-J.2

also exhibits the distinct high-reactivity features known from

the strongly oxidizing iron-oxo cores in biology and accord-

ingly possesses one of the most reactive oxoiron(IV) cores

that have been synthesized to date. Furthermore, a large KIE

of 53 has been determined for the reaction of 2with

ethylbenzene, which compares well with the KIE of 57[1]

determined for the oxidation of taurine by TauD-J.The

uniqueness of 2within the non-heme oxoiron family is,

however, emphasized in its ability to oxidize sterically

hindered CH bonds by an IE-driven asynchronous PCET

mechanism. Although limited examples of CH oxidation by

a basicity controlled PCET mechanism (Scheme 2) are

known,[8,20] evidence of oxidative PCET mechanism has

stayed elusive prior to this study. In conclusion, the high

reactivity and the similar spectroscopic parameters of 2and

TauD-Jmake 2one of the best structural, electronic and

functional models for TauD-J.

Acknowledgements

This work was funded by the Deutsche Forschungsgemein-

schaft (DFG, German Research Foundation) under Germa-

nys Excellence Strategy—EXC 2008–390540038—UniSys-

Cat to K.R., P.H., and H.D., and the Heisenberg-Professor-

ship to K.R., and MINECO (CTQ2017-87392-P) and FED-

ER (UNGI10-4E-801) to M.S. K.W. also thanks Einstein

Foundation Berlin (ESB)—Einstein Center of Catalysis

(EC2) for its support. Open access funding enabled and

organized by Projekt DEAL.

Conflict of interest

The authors declare no conflict of interest.

Keywords: bioinorganic chemistry · enzyme models ·

high-valent iron · hydrogen atom abstraction · electron transfer

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Manuscript received: November 29, 2020

Accepted manuscript online: December 21, 2020

Version of record online: February 15, 2021

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