Vibrational characterization of a diiron bridging hydride complex – a model for hydrogen catalysis [original]

Vibrational characterization of a diiron bridging

hydride complex –a model for hydrogen catalysis†

Leland B. Gee, ‡

Vladimir Pelmenschikov, ‡*

Hongxin Wang, ‡

Nakul Mishra,

Yu-Chiao Liu,

Yoshitaka Yoda,

Kenji Tamasaku,

Ming-Hsi Chiang *

and Stephen P. Cramer *

A diiron complex containing a bridging hydride and a protonated terminal thiolate of the form [(m,k

bdtH)(m-PPh

)(m-H)Fe

(CO)

]

has been investigated through

Fe nuclear resonance vibrational

spectroscopy (NRVS) and interpreted using density functional theory (DFT) calculations. We report the

Fe–mH–Fe wagging mode, and indications for Fe–mD stretching vibrations in the D-isotopologue,

observed by

Fe-NRVS. Our combined approach demonstrates an asymmetric sharing of the hydride

between the two iron sites that yields two nondegenerate Fe–mH/D stretching vibrations. The studied

complex provides an important model relevant to biological hydrogen catalysis intermediates. The

complex mimics proposals for the binuclear metal sites in [FeFe] and [NiFe] hydrogenases. It is also an

appealing prototype for the ‘Janus intermediate’of nitrogenase, which has been proposed to contain

two bridging Fe–H–Fe hydrides and two protonated sulfurs at the FeMo-cofactor. The signiﬁcance of

observing indirect eﬀects of the bridging hydride, as well as obstacles in its direct observation, is

discussed in the context of biological hydrogen intermediates.

Introduction

Biological hydrogen catalysis is driven by a ubiquitous and

diverse set of enzymes called hydrogenases

1–4

and to a lesser

extent by the nitrogenase family.

Hydrogenases reversibly

convert molecular hydrogen into reducing power at binuclear

[FeFe], binuclear [NiFe],

or mononuclear [Fe] active sites.

Similarly, hydrogen evolution in Mo-nitrogenase occurs at

a unique Fe

MoS

C cluster called the FeMo-cofactor, and

analogous chemistry likely occurs at the Fe(V,Fe)-cofactors.

8,9

The activities of all these enzymes are presumably related by the

formation of metastable Fe–H



hydrides and protonation of

nearby sulfurs during catalysis (Fig. 1).

6,10,11

Synthesis and

characterization of model complexes that can simulate

biochemical intermediates and their proton transfer capabil-

ities by the stabilization of transition metal hydrides (M–H



)

and nearby thiols (–SH) are key to understanding hydrogenase

and nitrogenase active site hydrogen catalysis.

10,12–16

Fig. 1 Schematic structures of (a) the mHS(Me/H) synthetic complexes

studied in this work; (b) the [NiFe] hydrogenase Ni–R intermediate; (c)

the [FeFe] hydrogenase H

hyd

intermediate; (d) the nitrogenase Janus

intermediate. The iron hydrides and sulfur protonations are labelled in

red.

Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, CA

94305, USA

Institut f¨

ur Chemie, Technische Universit¨

at Berlin, Strasse des 17 Juni 135, 10623

Berlin, Germany. E-mail: pelmentschikov@tu-berlin.de

SETI Institute, 189 Bernardo Avenue, Mountain View, CA 94043, USA. E-mail:

[email protected]

Department of Chemistry, University of California, Davis, One Shields Ave, Davis, CA

95616, USA

Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan

Department of Medicinal and Applied Chemistry, Kaohsiung Medical University,

Kaohsiung 807, Taiwan

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-

gun, Hyogo 679-5198, Japan

RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan

†Electronic supplementary information (ESI) available: Materials and methods,

Fig. S1–S12, Table S1, atomic Cartesian coordinates, animated vibrational

modes. See DOI: 10.1039/d0sc01290d

‡These authors contributed equally.

Cite this: Chem. Sci., 2020, 11, 5487

All publication charges for this article

have been paid for by the Royal Society

of Chemistry

Received 2nd March 2020

Accepted 1st May 2020

DOI: 10.1039/d0sc01290d

rsc.li/chemical-science

This journal is © The Royal Society of Chemistry 2020 Chem. Sci.,2020,11,5487–5493 | 5487

Chemical

Science

EDGE ARTICLE

Open Access Article. Published on 06 May 2020. Downloaded on 6/10/2021 8:13:50 AM.

This article is licensed under a

Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

View Article Online

View Journal

| View Issue

These special –SH/M–H



interactions take various forms. In

[FeFe] hydrogenases there is a conserved cysteine situated at the

end of a proton transfer chain formed by hydrophilic amino

acid residues and water molecules, with the –SH thiol group

adjacent to an azadithiolate (ADT) bridge of the [2Fe]

sub-

cluster (Fig. 1c). There is considerable evidence that the ADT

bridge plays a critical role in proton acceptance and relay to the

catalytic Fe

site.

Moreover, disruption of the proton supply

chain in the enzymes from Chlamydomonas reinhardtii

(CrHydA1) or Desulfovibrio desulfuricans (DdHydAB) has allowed

the transient H

hyd

catalytic state to be trapped and unequivo-

cally shown to involve a terminal Fe

–H



hydride (Fig. 1c).

14,18–21

For [NiFe] hydrogenases, the catalytic Ni–R state has been

shown to feature a bridging hydride, with a Ni-bound proton-

ated cysteine sulfur (Fig. 1b).

Finally, in nitrogenases the E

state is presumed to have one bridging Fe–H



–Fe hydride with

a nearby protonated sulfur capable of H

release, while two

hydrides capable of reductive elimination have been proposed

for the E

or ‘Janus intermediate’of the Fe(Mo,V,Fe)-cofactor

(Fig. 1d).

To model the aforementioned biological hydrides, previous

eﬀorts produced and characterized synthetic protonated thiols

coordinated to diiron centers.

22–25

Here we extend this work by

exploring

Fe-specic vibrational dynamics for a diiron center

with a bridging hydride and a protonated thiol ligand. Speci-

cally, we have characterized the [(m,k

-bdtH)(m-PPh

)(m-H)

(CO)

]

(bdt ¼1,2-benzenedithiolate) model compound,

referred hereaer as ‘mHSH’(Fig. 1a), and as well its deuterated

isotopologue [(m,k

-bdtD)(m-PPh

)(m-D)Fe

(CO)

]

,mDSD, using

nuclear resonance vibrational spectroscopy (NRVS). We also

recorded the spectrum of the S-methylated complex: [(m,k

bdtCH

)(m-PPh

)(m-H)Fe

(CO)

]

, here referred to as ‘mHSMe’.

Fe-NRVS is a synchrotron-based technique that observes

excitation of an

Fe nucleus together with the excitation/

deexcitation of vibrational modes.

26–28

NRVS is essentially the

recoil fraction that steals intensity from the recoil-free

M¨

ossbauer eﬀect, with the measured intensity for a given

normal mode proportional to the

Fe kinetic energy in that

mode, ultimately yielding an

Fe partial vibrational density of

states (PVDOS). The experimental frequencies and intensities

can be directly compared with normal mode calculations from

density functional theory (DFT) or even empirical force elds.

The asymmetric diferrous compound mHSH is relevant to the

protonated and bridging hydride-bound states proposed for H

production in [NiFe] hydrogenases and to the nitrogenase E

and E

intermediates. It is also a good benchmark for

comparison with the NRVS for intermediates in [FeFe] hydrog-

enase proposed to contain a m-hydride ligand bridging the

(II) and Fe

(II) sites (Fig. S1†).

In line with a recent inter-

pretation that [FeFe] hydrogenase operates exclusively with

bridging CO intermediates,

the m-hydride intermediates were

attributed to the enzyme's ‘slow cycle’.

Results

NRVS spectra and qualitative assignments

The overall NRVS-derived

Fe-PVDOS for the mHSH,mDSD and

mHSMe samples are compared in Fig. 2. Qualitative assign-

ments can be made based on comparison with previous spectra

for Fe–S proteins and model compounds. The low energy

<200 cm

1

portion of the spectrum is related to acoustic and

torsional modes largely involving the bulky Ph and bdt

subunits, as well as bending modes at the Fe sites. The next

region up to 400 cm

1

comprises predominantly Fe–S and Fe–P

stretching modes. The 400–650 cm

1

region has prominent

features from modes that are a mix of nFe–CO stretching and

dFe–CO bending.

At higher energies, a band at 745 cm

1

is particularly

signicant –it represents an Fe–H–Fe wagging mode. This

assignment is based on comparison with results for similar

Fe–mH



–Fe synthetic complexes and their computational

models,

31,32

as well as Ni–mH



–Fe models and the Ni–R state of

[NiFe] hydrogenase.

The signicant weakening at this position

in the mDSD spectrum supports this assignment; residual

Fig. 2 Top: the NRVS-derived

Fe-PVDOS spectra for mHSH ( ) and its deuterium isotopologue mDSD ( ), vertically oﬀset by 1 10

3

cm from

each other in the high-energy region. Bottom: the

Fe-PVDOS spectra for mHSMe ( ). In all cases the high-energy region intensity above

650 cm

1

is multiplied by 5 for visibility. The bands are labelled with their top positions, with those above 800 cm

1

assigned tentatively.

5488 |Chem. Sci.,2020,11,5487–5493 This journal is © The Royal Society of Chemistry 2020

Chemical Science Edge Article

Open Access Article. Published on 06 May 2020. Downloaded on 6/10/2021 8:13:50 AM.

This article is licensed under a

Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

View Article Online

intensity likely results from an incomplete H-to-D exchange.

Below the 745 cm

1

band is a second candidate wagging mode

feature at 701 cm

1

. Although this energy region coincides

with benzene-dithiol modes, their Fe motion is predicted to be

low. The energy is also consistent with the predicted hydride

wagging motion of a deprotonated species, mHS



, based on DFT

calculations as described below. For the mHSMe complex,

a slight downshito 733 cm

1

is observed for the main wagging

mode feature.

From previous NRVS work on Fe–H/D



complexes,

11,31,32

well as guidance from the conventional vibrational spectros-

copy literature,

we expect observation of the Fe–H stretching

modes at vibrational energies above 1100 cm

1

. There are hints

of such features at 1165 cm

1

and 1403 cm

1

, but the signal-to-

noise is not convincing. Even in the best of cases, these modes

are extremely weak in NRVS because they primarily involve the

hydride nucleus motion with an almost static Fe partner. Below,

we will use DFT calculations to investigate additional factors

such as sample heterogeneity and intermolecular coupling that

conspire to split the already weak Fe–H stretches into a multi-

tude of even weaker modes that are below the current detection

limit.

DFT calculations and quantitative comparisons

The

Fe-PVDOS predictions from the DFT models for mHSH,

mDSD, and mHSMe (Fig. S3a and S4†) are compared in Fig. 3, S8

and S9.†The key experimental feature at 745 cm

1

, attributed to

the Fe–H–Fe wagging mode, is reproduced very well in terms of

energy and intensity by the calculated band at 746 cm

1

. In this

mode, the calculated mH



hydride motion is normal to the

‘Fe–H–Fe’plane, as shown in Fig. S5 (see additionally ESI for

animated vibrational modes, and Fig. S8 for an overlay of the

NRVS-observed and DFT-predicted spectra).†A corresponding

Fe–D–Fe wagging is not specically resolved in the NRVS of the

deuterated sample.

To better understand the diﬀerences in these two spectra, we

calculated the PVDOS for the bridging H and D nuclei, as shown

in Fig. 4 and S7d.†Inspection of these curves reveals three

distinct hydride bands, whereas there are only two distinct

deuteride bands above 650 cm

1

. The intensity for the third

deuteride band (corresponding to the wagging motion) is

redistributed throughout the 400–600 cm

1

region, indicating

signicant coupling in the motions of the D nucleus with the

ve CO ligands. A comparison of the single wagging mode in

mHSH with three prominent mDSD modes involving coupled

motions is also depicted in Fig. S5.†

The computed spectrum of mDSD sees shis of the Fe–H

stretching modes at 1424/1196 cm

1

to respectively 1015/

855 cm

1

. The

Fe-PVDOS intensities of the Fe–D(vs. Fe–H)

bands are enhanced due to the amplied

Fe displacements

when the heavier deuteride is the bridging ligand, which allows

the Fe–D stretching signals to raise above the NRVS noise level

(Fig. S11 and S12†). For future NRVS observation of such

stretching modes, Fe–D complexes are clearly favored over Fe–H

isotopologues.

Asymmetric hydride sharing by Fe

and Fe

We also calculated separate PVDOS for the

and

nuclei, as shown in Fig. 4 and S7c.†We use this notation for

the Fe sites respectively proximal and distal to the thiol,

following the notation commonly employed for [FeFe]

hydrogenases, see Fig. 1 and S1.†Although both Fe atoms

contribute to the vibrational motion, we can discriminate

between the two Fe

p/d

–mH stretching modes by the Fe–H

bond that aligns closest to the mH displacement vector, as

shown in Fig. S6 and visualized in the ESI†animations.

Breakdown of the DFT

Fe-PVDOS into individual

p/d

contributions demonstrates that the Fe–Hmodesare

asymmetrically comprised of mostly the Fe

motion. The Fe

–

mH stretching mode is predicted for the high-end of the

mHSH spectrum at 1424 cm

1

,whilethepredictedFe

–mH

Fig. 3 DFT-based

Fe-PVDOS for diﬀerent chemical models and their isomers. Left (a): mHSH or mDSD. Right, top to bottom: (b) mHS



or mDS



or mDSMe

, (d) [mHSH]

or [mDSD]

, (e) mHSH

vs. mHSH (¼mHSH

out

)ormDSD

vs. mDSD (¼mDSD

out

). The high energy region

>650 cm

1

(b–e) is multiplied by 2 for visibility. In (a), the intensities <210 cm

1

are based on the [mHSH]

or [mDSD]

dimer calculations, as

explained in the main text and shown in Fig. 5.

This journal is © The Royal Society of Chemistry 2020 Chem. Sci.,2020,11,5487–5493 | 5489

Edge Article Chemical Science

Open Access Article. Published on 06 May 2020. Downloaded on 6/10/2021 8:13:50 AM.

This article is licensed under a

Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

View Article Online

stretch is at a lower energy of 1196 cm

1

. Experimentally

these hydride modes coincide with intensities at 1403 and

1165 cm

1

respectively (Fig. 2 and S7†), however the signal-

to-noise is low for these features despite several long acqui-

sition experiments at high incident ux (see ESI Methods†).

On the other hand, the two most intense experimental bands

at 245/280 cm

1

(Fig. 2) are reproduced by DFT at 251/

281 cm

1

(Fig. S7†)andidentied as mostly Fe

–S/P

stretching in character –the quantitative agreement with

the NRVS experiment indicates a proper modeling of the

electronic structure by a low-spin 2Fe(II) core, also consistent

with the recorded M¨

ossbauer spectrum, see Fig. S2 and ESI

Discussion.†

The deduction that the Fe

site contributes more NRVS

intensity than the Fe

site in the wagging and stretching hydride

modes is supported by the diﬀering Fe–H bond lengths revealed

in the DFT-optimized geometry of mHSH,Fe

–H¼1.73 

Avs.

–H¼1.65

A; comparable values and their variations are seen

as well in the X-ray structure

(Table S1†). The 0.1 

Adiﬀerence

in the Fe

p/d

–H bond lengths as well rationalizes the 230 and

160ð 230=

ﬃﬃﬃ

pÞcm1splittings in the predicted Fe

p/d

–mH

and Fe

p/d

–mD stretching frequencies.

Conformational heterogeneity of the Fe thiol

We also considered the possibility of alternate structural

conformations having an eﬀect on the NRVS proles. We found

from DFT calculations that a local minimum was produced by

reorientation of the protonated S

–H

thiolate ligand of Fe

towards the bridging hydride, yielding a ‘mHSH

’(or ‘mDSD

’)

structure (Fig. S3a†). The calculated energy diﬀerence was

a trivial +0.3 kcal mol

1

compared to the above described best-

t model having the S

–H

moiety oriented away from the

hydride, referred to simply as mHSH (or mDSD). We thus expect

mHSH (¼mHSH

out

) and mHSH

species to reach an equilibrium

and co-exist in the sample crystals. The calculated

Fe-PVDOS

for mHSH

produced an Fe–H–Fe wag at 752 cm

1

, upshied

by 6 cm

1

from the spectrum for mHSH (Fig. 3 and S8†). Slightly

larger upshisof7cm

1

and 10 cm

1

were predicted for the

Fe–D and Fe–H stretching modes, respectively.

Chemical heterogeneity of the Fe thiolate

From previous work with this compound, we know that

deprotonation of the S

–H

moiety occurs readily (for example,

upon solvation of mHSH in THF), and we refer to the resultant

thiolate species as ‘mHS



’(Fig. S3a†). DFT calculations on this

candidate predict a 35 cm

1

red-shiin the Fe–H–Fe wagging

mode to 711 cm

1

(Fig. S8b†). The mHS



model is thus

a possible explanation for the weak feature observed at

701 cm

1

in the experimental spectrum of mHSH –although

a similar, but weaker, feature was observed in mHSMe, implying

the source of this intensity need not be limited to the mHS



species in the spectrum of mHSH (Fig. 2, S7a, and S9†).

An even higher degree of hydride sharing asymmetry is

calculated for the mHS



/mDS



model (Table S1†) and, conse-

quently, larger energy splittings between the predicted bands

for the Fe

p/d

–mH/D stretches (Fig. 3b and S8b†). Specically, the

–mH/D stretches calculated respectively at 818/1138 cm

1

are

signicantly red-shied when compared to the best-tmHSH/

mDSD model, due to a stronger Fe

–S

bond strength which in

turn leads to a weaker adjacent Fe

–mH



bonding. Notably, the

deprotonation of mHSH to mHS



leads to a predicted upshiby

15 cm

1

(332 to 347 cm

1

, Fig. S8b†) for the band observed at

330 cm

1

and having the strongest Fe

–S

stretching character

(see ESI†for these modes animated). Finally, to produce a more

realistic simulation of the observed NRVS spectrum, in Fig. 6

and S12†we provide an average of the three DFT models

considered above.

Intermolecular coupling –the dimer model

A common weakness of DFT simulations of the NRVS spectra of

solids is the lack of predicted intensity in the low-energy regions

(<100 cm

1

). In solid model complexes, the experimental modes

in this region are due to solid matrix motions, and lattice or

acoustic modes. In larger biological samples, this energy region

corresponds primarily to torsional/dihedral modes and larger

scale motions of the protein, and it provides an indicator of

coupling between the motion of the

Fe-enriched bioinorganic

core and its environment.

18,20,35,36

To partially account for these

modes in our calculations, we have used a DFT model based on

Fig. 4 Top: comparison of DFT-predicted

and

PVDOS for models mHSH and mDSD. Bottom: comparison of DFT-predicted mH/D

PVDOS for the same models. The relative intensity multiplication factors () are applied for visibility.

5490 |Chem. Sci.,2020,11,5487–5493 This journal is © The Royal Society of Chemistry 2020

Chemical Science Edge Article

Open Access Article. Published on 06 May 2020. Downloaded on 6/10/2021 8:13:50 AM.

This article is licensed under a

Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

View Article Online

a dimeric form [mHSH]

which is directly available from the

crystallographic unit cell in the X-ray structure (Fig. S3b†);

this

approach is justied as the NRVS experiment employed a poly-

crystalline powder sample.

The vibrational calculation on [mHSH]

shows an increased

density of normal modes and signicant improvement in the

low-energy <200 cm

1

region of the predicted NRVS spectrum

over that of the mHSH monomer only (Fig. 5 and S10†). Of note

is reproduction of the broad experimental feature centered at

35 cm

1

produced by highly-delocalized expansions of the

dimer, see Fig. 5 and ESI†for the representative normal mode at

38 cm

1

animated. The intermolecular dispersion interaction

plays an important role here, included as a correction to our

DFT setup on [mHSH]

(see ESI Methods†). The dimer compu-

tational model does not capture all of the features in the low-

energy region –specically the prominence at 70 cm

1

–

which may be reproduced by models accounting for more

molecules and/or their structural periodicity, however such

a computational eﬀort is beyond the scope of this work.

Notably, the dimer is composed of mHSH and its mirror-

image isomer mHSH

, or the enantiomer, yet the [mHSH]

structure itself does not exhibit a symmetry (higher than C

Deviation from the exact mirror-image symmetry between

mHSH and mHSH

eﬀectively splits the Fe–mH/D stretching

normal modes >800 cm

1

, which are particularly sensitive to

even minimal Fe–mH bond length changes of 0.01 

A (Table

S1†), in line with the hydride mode variations indicated above

for the monomer models. This results in a more faithful

simulation of their low PVDOS intensities –see the calculated

[mHSH]

/[mDSD]

vs. mHSH/mDSD spectra compared to the

observed H/D sample NRVS data in Fig. 6, S10, and S12.†

Discussion

Our results on the bridging hydride samples show both the

strengths and limitations of NRVS for observation of Fe–Hnormal

modes. The experimental and calculated observations demon-

strate that the Fe–H–Fe wag mode is sensitive to the chemistry at

the Fe–Sligand,reected by the species-dependent redistribution

of intensity in the 690–760 cm

1

region. The DFT calculations

suggest the wag mode is even sensitive to the conformation of the

SH proton, shiing 6 cm

1

between the ‘in’and ‘out’conformers.

In the range of the models considered, 40 cm

1

upshiof the

predicted wag band position is approximately correlated with

Fig. 5 (A) The normal mode calculated at 38 cm

1

showing relative

displacements of the two enantiomers comprising the [mHSH]

dimer.

Actual amplitude of this [mHSH]–[mHSH]

vibration is 0.05



A. (B)

Low-frequency (<350 cm

1

)

Fe-PVDOS spectra for the mHSH

compound from NRVS experiment (blue) and DFT calculations using

[mHSH]

dimer (black) and mHSH monomer (orange) models; for the

full-range spectra, see Fig. S10.†

Fig. 6 DFT

Fe-PVDOS spectra for the (a) H- (top) and (b) D- (bottom) isotopologues in the iron-hydride bands region >650 cm

1

:mHSH

monomer (blue), averaged between the three mHSH/mHSH

/mHS



monomers (green), and the [mHSH]

dimer (red) models. The relative intensity

multiplication factors () are applied for visibility.

Edge Article Chemical Science

Open Access Article. Published on 06 May 2020. Downloaded on 6/10/2021 8:13:50 AM.

This article is licensed under a

Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

View Article Online

0.01



A increase in the adjacent Fe–Sbondlength(TableS1†).

This sensitivity makes NRVS a useful structural probe, but it also

means that intensity can be lost by division into multiple channels

in cases of sample heterogeneity.

The DFT calculations reveal that Fe–H stretching modes are

even more sensitive to variability at the Fe–S ligand than the

Fe–H–Fe wag modes. In contrast to the well-dened wag mode,

current S/N limitations unfortunately prevented unambiguous

experimental assignments of the stretching modes. The pre-

dicted position of the lower frequency Fe

–H stretch mode is

especially sensitive to the chemistry at the sulfur ligand adja-

cent to Fe

, varying by almost 70 cm

1

in the range of the

considered alternatives (S



: 1138 cm

1

,SH

out

: 1195 cm

1

SMe

: 1204 cm

1

,SH

: 1206 cm

1

Another result from the DFT calculations is the inequitable

sharing of the bridging hydride. This is most directly seen in the

optimized 0.1



AFe–H distance diﬀerences. Similar diﬀerences

have been predicted by DFT previously in the bimetallic cofac-

tors of [FeFe] and [NiFe] hydrogenases and their models.

11,20,32

The diﬀerence in Fe

p/d

–mH bonding is also reected in the

prediction of two distinct Fe–H stretching modes split by as

much as 322 cm

1

. Even greater asymmetry was observed in

aNi–mH–Fe model complex, where Ni–H and Fe–H stretches

were seen respectively at 954 cm

1

and 1468/1532 cm

1

, the

latter split by 64 cm

1

due to a conformational heterogeneity in

the m-pdt ligand.

It is noteworthy that the asymmetry is

modulated by the status of the iron-bound sulfur –a larger

asymmetry occurs when the sulfur is deprotonated.

The model studies have relevance to proposed [FeFe]

hydrogenase intermediates. A terminal hydride state H

hyd

has

been convincingly assigned using NRVS.

18–20,29

However,

bridging hydride species have also been proposed for interme-

diates named H(s)red using other methods,

29,37

and a recent

report has investigated the Hred state using a combination of

Fe-NRVS and other spectroscopies, supported by QM/MM

calculations.

It was proposed that the Hred state contains

a bridging Fe

–mH



–Fe

hydride with its wag distributed among

the 600–650 cm

1

(Fe–)CO/CN



/ADT motions, thus prohibit-

ing its direct observation.

Our experimental results on model compounds found

wagging modes at higher frequencies, ranging from 694–

702 cm

1

in a sterically congested diiron hydride

to 733–

745 cm

1

for the current samples to 758 cm

1

in a Ni–mH–Fe

model complex.

Of course, structural diﬀerences can conspire

to move wagging modes to lower frequencies –in a recent study

of a complex with an Fe(m-H)

Fe core the wagging modes were

identied at 455 and 587 cm

1

, mostly screened by other NRVS

bands.

For future work with improved S/N, observation of

Fe–D and Fe–H stretching modes would be a more rigorous way

to test various structural hypotheses.

Summary and outlook

In this work we have characterized the mHSH and mHSMe (and

likely mHS



) synthetic models with NRVS experiments and DFT

calculations. Foremost, we have directly identied the well-

separated Fe–H–Fe wagging modes both in the experimental

and DFT-simulated spectra. The related Fe–D–Fe wagging

modes were found to be heavily mixed with Fe–CO vibrations.

The wagging mode displays signicant variability that makes it

a sensitive test of molecular structure.

Fe–H and Fe–D stretching modes were also scrutinized by

DFT modeling for the isomeric mHSH

in/out

and deprotonated

mHS



forms of mHSH, as well as a [mHSH]

dimer relevant to the

polycrystalline NRVS sample. These modes exhibit larger

frequency and intensity shis, making them an even more

delicate probe of iron-hydride bonding. Their routine applica-

tion awaits improvements in sample preparation, beamline

ux, monochromators, and detectors.

Conﬂicts of interest

There are no conicts to declare.

Acknowledgements

This work was funded by National Institutes of Health Grant

GM-65440, National Science Foundation grant CHE 1308384,

the Einstein Foundation Berlin grant number EVF-2016-277 (S.

P. C.), the Deutsche Forschungsgemeinscha(DFG, German

Research Foundation) under Germany's Excellence Strategy –

EXC 2008 –390540038 –UniSysCat (V. P.), and the Ministry of

Science and Technology of Taiwan and Academia Sinica (AS-SS-

108-02-1) (Y.-C.L., M.-H.C). Some computational work was per-

formed under the XSIM project on the CORI computing system

at NERSC a U.S. Department of Energy Oﬃce of Science User

Facility operated under contract no. DE-AC02-05CH11231.

Synchrotron experiments were performed at SPring-8 under

proposal numbers 2014A2056, 2015A0103-2016B0103,

2016A1154, 2017A1115, 2018A1409 (under JASRI) and

20150048, 20160063 (under RIKEN).

Notes and references

1 P. M. Vignais, B. Billoud and J. Meyer, FEMS Microbiol. Rev.,

2001, 25, 455–501.

2 P. M. Vignais and B. Billoud, Chem. Rev., 2007, 107, 4206–

4272.

3 W. Lubitz, H. Ogata, O. Rudiger and E. Reijerse, Chem. Rev.,

2014, 114, 4081–4148.

4 J. W. Peters, G. J. Schut, E. S. Boyd, D. W. Mulder,

E. M. Shepard, J. B. Broderick, P. W. King and

M. W. W. Adams, Biochim. Biophys. Acta, Mol. Cell Res.,

2015, 1853, 1350–1369.

5 D. Lukoyanov, N. Khadka, Z. Y. Yang, D. R. Dean,

L. C. Seefeldt and B. M. Hoﬀman, J. Am. Chem. Soc., 2016,

138, 1320–1327.

6 H. Ogata, K. Nishikawa and W. Lubitz, Nature, 2015, 520,

571–575.

7 G. F. Huang, T. Wagner, M. D. Wodrich, K. Ataka, E. Bill,

U. Ermler, X. L. Hu and S. Shima, Nat. Catal., 2019, 2, 537–

543.

8 D. F. Harris, D. A. Lukoyanov, S. Shaw, P. Compton,

M. Tokmina-Lukaszewska, B. Bothner, N. Kelleher,

Chemical Science Edge Article

Open Access Article. Published on 06 May 2020. Downloaded on 6/10/2021 8:13:50 AM.

This article is licensed under a

Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

View Article Online

D. R. Dean, B. M. Hoﬀman and L. C. Seefeldt, Biochemistry,

2018, 57, 701–710.

9 D. F. Harris, D. A. Lukoyanov, H. Kallas, C. Trncik, Z. Y. Yang,

P. Compton, N. Kelleher, O. Einsle, D. R. Dean,

B. M. Hoﬀman and L. C. Seefeldt, Biochemistry, 2019, 58,

3293–3301.

10 D. Schilter, J. M. Camara, M. T. Huynh, S. Hammes-Schiﬀer

and T. B. Rauchfuss, Chem. Rev., 2016, 116, 8693–8749.

11 H. Ogata, T. Kr¨

amer, H. Wang, D. Schilter,

V. Pelmenschikov, M. van Gastel, F. Neese,

T. B. Rauchfuss, L. B. Gee, A. D. Scott, Y. Yoda, Y. Tanaka,

W. Lubitz and S. P. Cramer, Nat. Commun., 2015, 6, 7890.

12 Y. Ohki, Bull. Chem. Soc. Jpn., 2014, 87,1–19.

13 S. Ogo, Coord. Chem. Rev., 2017, 334,43–53.

14 S. Rumpel, C. Sommer, E. Reijerse, C. Fares and W. Lubitz, J.

Am. Chem. Soc., 2018, 140, 3863–3866.

15 J. A. Denny and M. Y. Darensbourg, Chem. Rev., 2015, 115,

5248–5273.

16 N. A. Arnet, N. Bhuvanesh and M. Y. Darensbourg, J. Biol.

Inorg Chem., 2019, 24, 909–917.

17 J. F. Duan, M. Senger, J. Esselborn, V. Engelbrecht,

F. Wittkamp, U. P. Apfel, E. Hofmann, S. T. Stripp,

T. Happe and M. Winkler, Nat. Commun., 2018, 9, 4726.

18 C. C. Pham, D. W. Mulder, V. Pelmenschikov, P. W. King,

M. W. Ratzloﬀ, H. Wang, N. Mishra, E. E. Alp, J. Zhao,

M. Y. Hu, K. Tamasaku, Y. Yoda and S. P. Cramer, Angew.

Chem. Int. Ed., 2018, 57, 10605–10609.

19 E. J. Reijerse, C. C. Pham, V. Pelmenschikov, R. Gilbert-

Wilson, A. Adamska-Venkatesh, J. F. Siebel, L. B. Gee,

Y. Yoda, K. Tamasaku, W. Lubitz, T. B. Rauchfuss and

S. P. Cramer, J. Am. Chem. Soc., 2017, 139, 4306–4309.

20 V. Pelmenschikov, J. A. Birrell, C. C. Pham, N. Mishra,

H. X. Wang, C. Sommer, E. Reijerse, C. P. Richers,

K. Tamasaku, Y. Yoda, T. B. Rauchfuss, W. Lubitz and

S. P. Cramer, J. Am. Chem. Soc., 2017, 139, 16894–16902.

21 D. W. Mulder, Y. Guo, M. W. Ratzloﬀand P. W. King, J. Am.

Chem. Soc., 2017, 139,83–86.

22 W. Dong, M. Wang, X. Liu, K. Jin, G. Li, F. Wang and L. Sun,

Chem. Commun., 2006, 305–307, DOI: 10.1039/B513270C.

23 S. Ezzaher, A. Gogoll, C. Bruhn and S. Ott, Chem. Commun.,

2010, 46, 5775–5777.

24 Y.-C. Liu, K.-T. Chu, Y.-L. Huang, C.-H. Hsu, G.-H. Lee,

M.-C. Tseng and M.-H. Chiang, ACS Catal., 2016, 6, 2559–

2576.

25 U.-P. Apfel, D. Troegel, Y. Halpin, S. Tschierlei,

U. Uhlemann, H. G¨

orls, M. Schmitt, J. Popp, P. Dunne,

M. Venkatesan, M. Coey, M. Rudolph, J. G. Vos, R. Tacke

and W. Weigand, Inorg. Chem., 2010, 49, 10117–10132.

26 T. Petrenko, W. Sturhahn and F. Neese, Hyperne Interact.,

2007, 175, 165–174.

27 W. R. Scheidt, J. F. Li and J. T. Sage, Chem. Rev., 2017, 117,

12532–12563.

28 L. B. Gee, H. X. Wang and S. P. Cramer, in Fe-S Cluster

Enzymes, Part B, ed. S. S. David, Academic Press,

Cambridge, MA, 1st edn, 2018, vol. 599, ch. 14, pp. 409–425.

29 S. Mebs, J. Duan, F. Wittkamp, S. T. Stripp, T. Happe,

U.-P. Apfel, M. Winkler and M. Haumann, Inorg. Chem.,

2019, 58, 4000–4013.

30 J. Birrell, V. Pelmenschikov, N. Mishra, H. Wang, Y. Yoda,

K. Tamasaku, T. B. Rauchfuss, S. P. Cramer, W. Lubitz and

S. DeBeer, J. Am. Chem. Soc., 2020, 142, 222–232.

31 V. Pelmenschikov, L. B. Gee, H. Wang, K. C. MacLeod,

S. F. McWilliams, K. L. Skubi, S. P. Cramer and

P. L. Holland, Angew. Chem., Int. Ed., 2018, 57, 9367–9371.

32 M. R. Carlson, D. L. Gray, C. P. Richers, W. Wang, P.-H. Zhao,

T. B. Rauchfuss, V. Pelmenschikov, C. C. Pham, L. B. Gee,

H. Wang and S. P. Cramer, Inorg. Chem., 2018, 57, 1988–

2001.

33 K. Nakamoto, Infrared & Raman Spectra of Inorganic &

Coordination Compounds, Wiley-Interscience, New York, 5th

edn, 1997.

34 Y. C. Liu, K. T. Chu, R. L. Jhang, G. H. Lee and M. H. Chiang,

Chem. Commun., 2013, 49, 4743–4745.

35 Y. Xiao, H. Wang, S. J. George, M. C. Smith, M. W. W. Adams,

J. F. E. Jenney, W. Sturhahn, E. E. Alp, J. Zhao, Y. Yoda,

A. Dey, E. I. Solomon and S. P. Cramer, J. Am. Chem. Soc.,

2005, 127, 14596–14606.

36 Y. Xiao, M.-L. Tan, T. Ichiye, H. Wang, Y. Guo, M. C. Smith,

J. Meyer, W. Sturhahn, E. E. Alp, J. Zhao, Y. Yoda and

S. P. Cramer, Biochemistry, 2008, 47, 6612–6627.

37 P. Chernev, C. Lambertz, A. Brunje, N. Leidel,

K. G. V. Sigfridsson, R. Kositzki, C. H. Hsieh, S. L. Yao,

R. Schiwon, M. Driess, C. Limberg, T. Happe and

M. Haumann, Inorg. Chem., 2014, 53, 12164–12177.

38 V. Pelmenschikov, L. B. Gee, H. Wang, K. C. MacLeod,

S. F. McWilliams, K. L. Skubi, S. P. Cramer and

P. L. Holland, Angew. Chem., Int. Ed., 2018, 130, 9511–9515.

Edge Article Chemical Science

Open Access Article. Published on 06 May 2020. Downloaded on 6/10/2021 8:13:50 AM.

This article is licensed under a

Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

View Article Online