Controlling O2 Reactivity in Synthetic Analogues of [NiFeS]- And [NiFeSe]-Hydrogenase Active Sites

Article abstract of DOI:10.1021/jacs.9b07448

Strategies for limiting, or reversing, the degradation of air-sensitive, base metal catalysts for the hydrogen evolution/oxidation reaction on contact with adventitious O₂ are guided by nature′s design of hydrogenase active sites. The affinity

Full text of DOI:10.1021/jacs.9b07448

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Article
2
Controlling O Reactivity in Synthetic Analogues of
[NiFeS]- and [NiFeSe]-Hydrogenase Active Sites
Xuemei Yang, Lindy C. Elrod, Trung Le, Valeria S. Vega, Haley Naumann, Yohannes
Rezenom, Joseph H Reibenspies, Michael B. Hall, and Marcetta Y. Darensbourg
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07448 • Publication Date (Web): 29 Aug 2019
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Controlling O2 Reactivity in Synthetic Analogues of [NiFeS]- and
[NiFeSe]-Hydrogenase Active Sites
Xuemei Yanga, Lindy C. Elroda, Trung Lea, Valeria S. Vegaa, Haley Naumanna,
Yohannes Rezenoma, Joseph H. Reibenspiesa, Michael B. Halla, and Marcetta Y. Darensbourga*
a Texas A&M University, Department of Chemistry, College Station, TX 77843, USA
Fax: (internat.) +1-979-845-0158
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E-mail: marcetta@mail.chem.tamu.edu
Abstract:
Strategies for limiting, or reversing, the degradation of air-sensitive, base metal catalysts for the
hydrogen evolution/oxidation reaction on contact with adventitious O2 are guided by Nature’s
design of hydrogenase active sites. The affinity of oxygen for sulfur and selenium, in [NiFeS]-and
[NiFeSe]-H2ase, yields oxygenated chalcogens under aerobic conditions, and delays irreversible
oxygen damage at the metals by maintaining the NiFe core structures. To identify the controlling
features of S-site oxygen uptake, related Ni(µ-EPhX)(µ-S’N2)Fe (E = S or Se, Fe = (η5-
C5H5)FeII(CO)) complexes were electronically tuned by the para substituent on µ-EPhX (X = CF3,
Cl, H, OMe, NMe2), and compared in aspects of communication between Ni and Fe. Both single
and double O-atom uptake at the chalcogens led to the conversion of the 4-membered core, Ni(µ-
EPhX)(µ-S’N2)Fe, to a 5-membered Ni-O-E-Fe-S’, where an O atom inserts between E and Ni. In
the E = S, X = NMe2 case, the 2-oxygen uptake complex was isolated and characterized as the
sulfinato species with the second O of the O2SPh-NMe2 unit pointing out of the 5-membered
Ni-O-S-Fe-S’ ring. Qualitative rates of reaction and ratios of oxygen-uptake products correlate
with Hammett parameters of the X substituent on EPhX. DFT computational results support the
observed remote effects on the NiFe core reactivity; the more electron-rich sulfurs are more O2
responsive in the SPhX series; the selenium analogues were even more reactive with O2. Mass
spectral analysis of the sulfinato products using a mixture of 18O2/ 16O2 suggests a concerted
mechanism in O2 addition. Deoxygenation, by reduction or O-atom abstraction reagents, occurs
for the 1-O addition complexes, while the 2-O, sulfinato, analogues are inert. The abstraction of
oxygen from the 1-O, sulfenato species, is related to oxygen repair in soluble, NAD+-reducing
[NiFe]-H2ase. (Horch, M., Lauterbach, L., et al., J. Am. Chem. Soc. 2015, 137, 2555-2564.)
Introduction
The deleterious effect of O2 comprises a major challenge in technological development of
molecular catalysts for H+ reduction based on abundant transition metals, needed for sustainable
electron conversion to H2.1-2 Oxygen as a poison is also well known to the organisms dependent
on Hydrogenase enzymes for H2 production for its use as an energy vector in many biological
pathways.3 Evolution over billions of years has developed various strategies for protection from
O2 as a competing, degrading substrate for Hydrogenase enzymes as well as possible self-repair
mechanisms. Among the different types of Hydrogenases, only the [NiFe]-H2ases provide
examples of the capability to operate in the presence of O2; These are examples of oxygen tolerance.
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At least three natural strategies have emerged to protect [NiFe]-H2ases’ active sites from oxygen
exposure: (i) a narrow hydrophobic gas channel that hinders diffusion of the bulkier O2 into the
protein-enclosed active site;4-5 (ii) the presence of an unusual [4Fe-3S] subunit located at the
proximal cluster of the O2-tolerant membrane-bound [NiFe]- H2ase such as that of MBH from the
hyperthermophilic bacterium Aquifex aeolicus and Ralstonia eutropha, which provides “an
electron-rich environment for O2 detoxification”;6-11 and (iii) the change of a terminal cysteine
into selenocysteine in the O2-resistant [NiFeSe]-H2ases.12
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Known to be superior to the all-sulfur analogue, the [NiFeSe]-H2ase subfamily shows
higher activity in HER; reduced inhibition by the product H2; and, when damaged by adventitious
O2, a more rapid recovery.13-15 The last feature is likely the greatest contributor to its reputation
for O2 tolerance. The overall protein as well as the active sites of [NiFeS]- and [NiFeSe]-H2ase are
structurally analogous. In the same position as a terminal cysteine sulfur in the former that acts as
a proton shuttle to the hydride-loaded NiFe unit, the selenocysteine appears to be poised for the
same function in [NiFeSe]-H2ase.16 Although SeR– is a poorer Brønsted-Lowry base than SR–,
and expected to be less prone to proton binding, its larger size renders it a better proton shuttle or
directing agent, as it both attracts and releases.12, 17 Nevertheless, its incorporation into synthetic
molecular catalysts for proton reduction has been thusfar limited.15, 18-19
Upon exposure of a [NiFeS]-H2ase to O2, the active site deactivates to the so-called “Ni-
A” (unready) and “Ni-B” (ready) states.8 The more O2-tolerant [NiFeS]-H2ases are known to form
“Ni-B” displayed as a bridging hydroxo ligand between Ni(III) and Fe(II), Figure 1.20 When
electron and protons from the normal reductive processes are pumped into this ready state, the
oxygen is quickly removed (t < 1 min) as H2O and catalytic activity is recovered.2,21-24 However,
the “Ni-A” state, with a possible hydroxo bridge between Ni(III) and Fe (II) as well as a bridging
sulfenate, requires longer times (t > 1 h) to be reactivated.25, 2 Several structures of these
oxygenates have been determined by crystallography, and signals of their presence have long been
known from EPR studies which identified Ni(III) as the redox level in these off-cycle species.26-
27, 1Studies by Lenz and Zebger, et al., show that the conversion of Ni-A to Ni-B in soluble NAD+-
reducing [NiFe] hydrogenase could proceed via oxygenation of the bridging sulfur in Ni-B, whose
active site is a structure similar to another unready state found by Fontecilla-Camps.28-29 It should
be pointed out that some 15 to 20 structures with varying degrees of oxygenation at metal or
chalcogen in [NiFeS] and [NiFeSe]-H2ases active sites are found in the protein data bank (PDB)
as of this publication. We selected a subset of these to describe in Figure 1. Undoubtedly there will
be more in the future.
In contrast to the [NiFeS]-H2ase, oxygen-damage of [NiFeSe]-H2ase results in various O-
uptake levels, none of which feature paramagnetic Ni(III); however selenium and/or sulfur are
found oxygenated or oxidized as dichalcogenides in the structures.30-31 Such oxygenation products
of [NiFeSe]-H2ase, as well as of [NiFeS]-H2ase active sites, can be interpreted as prevention or
protection, avoiding further metal oxidation and degradation of the Ni-Fe core structures in
each.19a,b,30,31 Reductive repair processes return the enzymes’ function in both cases. 32
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Recent reports from Pereira, et al., have provided key experiments that constrain the
differences in activity of [NiFeSe]- and [NiFeS]-H2ases to selenium itself rather than any structure
changes in the protein.33 Thus the simplest explanations for the greater hydrogenase activity and
easier reactivation of oxygen-degraded [NiFeSe]-H2ase as contrasted to the all-sulfur analogue lie
in the greater polarizability of selenium, and the weaker Se-O bonds as contrasted to S-O.17 As
such soft descriptions are difficult to quantify we have pursued relevant structure/function analyses
in well-characterized heterobimetallic synthetic analogues containing S and Se. Our ultimate goal
is to interpret the clues from synthetic models and from nature that might guide development of
robust, oxygen-tolerant and cheap molecular catalysts for the hydrogen evolution/oxidation
reaction.
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Examples of S-oxygenated thiolates are plentiful in monomeric complexes containing
nickel bound within a rigid tetradentate N2S2 ligand field; several examples are displayed in Figure
1.34-38 In fact, the single oxy-sulfur species, or sulfenate, was used to assemble the first reported
biomimetic of S-oxygenated [NiFe]-H2ase, using FeBr2 as the iron receiver.39 The synthesis of
nickel-iron bimetallic complexes containing both sulfur and selenium, thus providing faithful
synthetic analogues of the active sites of [NiFeS]- and [NiFeSe]-H2ases, represents a considerable
challenge.19, 40-41 A strategy found to be successful in the preparation of synthetic analogues of the
nickel superoxide dismutase, and for address of other questions in the bioinorganic chemistry of
2+
nickel, is the splitting of dimeric [NiN2S]2 by exogeneous thiolates.30, 42-43 Adaptations of this
strategy are responsible for the results presented below.
Examples of nickel sulf-oxygenates within rigid N2S2 ligand fields:34-38
R
R
R
R
R
R
R
R
R
R
R
O
O
O
O
O
N
N
S
S
S
N
N
N
N
N
N
S
S
N
N
O
O
Ni
Ni
Ni
Ni
O
Ni
S
S
S
S
S
O
R
R
R
R
R
R
R
R
R
R = H or CH3
R = H or CH3
R = CH3
R = CH3
Selected [NiFeS]- and [NiFeSe]-Hydrogenase oxygenated active sites:8, 20, 26-27
Cys
H
Cys
S
Cys
N
Cys
S
O
N
N
N
O
C
C
C
C
S
S
C
y
s
-
Cys-S
Cys-S
S
C
y
S
N
s
-
S
O
C
O
III
II
Fe
II
C
O
II
II
Ni
O
FeII
C
C
II
Ni
Fe
Ni
NiII
S
Cys
Fe
O
O
Cys
C
C
C
C
S
N
O
N
S
N
N
O
Se
Cys
Se
Cys
Se
Cys
S
O
Cys
O
Cys
Cys
Ni-A
Figure 1. Selected S-oxygenated thiolates in monomeric nickel complexes, and examples from
crystallography of oxygen-damaged [NiFe]-H2ase active sites.8, 20, 26-27, 34-38 Note: these examples of the
latter are taken from some 15 to 20 reported structures.
Results and Discussion
Minimal models of [NiFe]-H2ase active site core structure are accessed from the cleavage
of dimeric [NiN2S]22+ complexes by various nucleophiles,43 including aryl chalcogenides, EPhH-, E
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= S and Se.41 Unlike the stable S-oxygenates described in Figure 1, the resulting monomeric
Ni(EPhX)(S’N2) complexes are air-sensitive resulting in degradation. Nevertheless, when combined
+
with (η5-C5H5)FeII(CO)(CH3CN)2 , displacing the CH3CN labile ligands, the resultant Ni-Fe
complexes provide examples of stable products of O2 uptake, Figure 2. While these models are
imperfect structural analogues, our NiFe small molecules offer a paradigm for contrasting S and
Se in relevant O2-addition and repair processes representative. Moreover, the arylchalcogenides
are susceptible to modifications by para-substituents on the arene,44 giving clues regarding
electronic effects operative on oxygen uptake and product distribution. A summary of the
reactions explored in this study is found in Figure 2.
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S
N
Ni
S
N
X
+
S
+
e
F
S
Ni
O
N
e
O
S
F
S
Ni
O
C
O
N
N
C
O
X
N
X
O2
=
O2
N
F 3
M
C
e2
,
,
X
H
H
,
=
C
N
F3
X
+
S
S
N
Ni
Fe
N
C
O
PMe3
)
g
(
O
C
-
2
+
Ph
Se
S
S
N
1/2
+
Ni
S
N
N
+
Ni
Fe
Fe
Ni
N
+
C
S
C
S
N
PMe3
O
N
O
PMe3
C
O
+
X
S
X
N
Se
Ni
Fe
N
C
O
Figure 2. Synthesis of NiFe complexes containing para-substituted arylthiolates and various reactions. L
+
in the CpFe(CO)L2 synthon, 12 o’clock arrow position, is CH3CN as labile ligand. In all products the Ni
and Fe are in +2 oxidation states.
Synthesis and Characterizations. The synthetic methods and characterizations (mass spectra,
elemental analyses, 1H NMR spectra, UV and CV spectra, and XRD crystal structures) are further
detailed in the supplementary information. Monomeric Ni(SPhH)(S’N2),41 and para-substituted aryl
derivatives, Ni(SPhX)(S’N2), (X = CF3, Cl, H, OMe, NMe2) complexes, were derived from the
[NiN2S]22+ ([1-(2-mercaptoethyl)-methyl-1,4-diazacycloheptane] nickel(II)) dimer cleaved by the
-
appropriate SPhX. The Ni(SPhX)(S’N2) complexes readily displaced acetonitrile in the (η5-
+
C5H5)FeII(CO)(MeCN)2 precursor to generate diamagnetic, thermally stable Ni(µ-SPhX)(µ-
S’N2)Fe, (Fe = (η5-C5H5)FeII(CO)+; X = CF3, Cl, H, OMe, NMe2) as rudimentary synthetic
analogues of the [NiFeS]-H2ase active sites. Figure 2 summarizes the synthetic scope as well as
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reactivities explored in this study. The reaction profiles, including O2 uptake, will be described in
separate sections below. A selection of analogous Ni(µ-SePhX)(µ-S’N2)Fe, (X = CF3, H, NMe2)
complexes were similarly prepared and isolated. See the S.I. for details.
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Molecular structures.
Dark purple, X-ray
quality, block crystals of the monomeric
Ni(SPhX)(S’N2) complexes were obtained by
diethyl ether vapor diffusion into a solution of
CH3CN. The heterobimetallic, cationic Ni(µ-
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-
SPhX)(µ-S’N2)Fe complexes were isolated as BF4
salts and crystallized as dark brown blocks from a
pentane-layered CH2Cl2 solution at -35 °C. The X
substituents on the aryl ring do not substantially
modify the structures. The monomeric Ni
complexes crystallize in the P21/c (X = CF3), P-1
(X = Cl) and Pbcn (X = NMe2) space groups and
feature minimally distorted NiN2S2 square planes.
The full structural reports of the complexes in
Figure 3 are deposited in the Cambridge Data
Base, and selected metric data are tabulated in the
Supporting Information (Figure S43).
The
Ni(µ-SPhX)(µ-S’N2)Fe
complex
structures feature square planar Ni(SPhX)(S’N2)
units connected by chalcogenide bridges into the
typical piano-stool geometry about the [(η5-
C5H5)Fe(CO)]+ unit, resulting in butterfly-like
Ni-S-Fe-S’ cores. The hinge angles, defined as the
intersection of the best N2SS’ plane with the SS’Fe
plane, are in the range of 135-141°. The NiII - - FeII
distances of 3.1 – 3.2 Å, are beyond the possibility
of a metal-metal bond. Earlier we reported the
XRD structures of the Ni(µ-SePhH)(µ-S’N2)Fe with the NiII - - FeII distance = 3.253 Å, and the
mono-oxy derivative of the phenyl-selenolate. The expanded 5-membered metallocycle seen in
the latter, the 2 o’clock position of Figure 2, has a slightly larger NiII - - FeII distance, 3.568 Å.
In the current study, only the X = NMe2 derivative provided x-ray quality crystals from reaction
of Ni(µ-SPhX)(µ-S’N2)Fe with O2 . The structure displays a sulfinate unit in the 5-membered Ni-O-
S(=O)Fe-S’ ring, with NiII - - FeII distance = 3.395 Å. In summary, neither the 2-oxy or the mono-
oxy products show significant modification of the Ni-Fe bimetallic structures.
Figure 3.
Molecular XRD structures
determined for monomeric Ni and for the Ni(µ-
SPhX)(µ-S’N2)Fe complexes. Full listings of
metric data are in the Supplementary
Information.
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IR spectra and electrochemical characterizations. Correlations of the Hammett p parameter
with (CO) IR values and NiII/I reduction potentials derived from cyclic voltammetry, for the
series of NiFe complexes are presented as plots in Figure 4.44 Both theoretical (DFT-derived
values) and experimental data of the (CO) in Ni(µ-SPhX)(µ-S’N2)Fe and Ecathode potentials for
NiII/NiI in monomeric Ni(SPhX)(µ-S’N2) complexes conform with the Hammett parameters of the
X substituents on the -SPhX ligands. Specifically, more electron-donating substituents result in
lower (CO) values, illustrating electronic communication over 5 bonds and the influence on π-
backbonding from FeII to the CO. The Ni(µ-SePhX)(µ-S’N2)Fe series shows (CO) responses to
X similar to the sulfur analogues, however moderated in value. Interestingly, the phenyl
derivative with the most electron-donating substituent, Ni(µ-SPhNMe2)(µ-S’N2)Fe, gives same
(CO) value (1934 cm-1) as found in Ni(µ-SePhH)(µ-S’N2)Fe. There is not such a match in the
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NiII/I reduction potential.
S
X
N
X
c)
Ni
b)
a)
E
N
+
+
S
Se
S
S
N
Ni
Fe
N
IR Exp
Ni
-1.3
-1.4
-1.5
-1.6
-1.7
-1.8
-1.9
-2
IR, Exp
Fe
N
C
N
C
O
IR Calc
O
IR, Calc
More e- donating
E , Exp
Red Exp
E =Se, X = H
c
NMe2
OMe
H
1934
1934
E , Calc
c
Red Calc
Cl
CF3
1939
1937
1941
1943
Less e- donating
2100
2050
2000
1950
1850
1800
Wavenumber (cm1-1900
)
-0.9
-0.4
0.1
0.6
V vs. Fc/Fc+
σ
Figure 4. a) Cyclic voltammograms in Ec region for NiII/NiI and monomeric Ni complexes; b) The ν(CO) IR
spectra of Ni(µ-EPhX)(µ-S’N2)Fe; c) Correlations of Hammett  parameters with experimental and calculated
ν(CO) values and Ec values of NiII/NiI in Ni(µ-SPhX)(µ-S’N2)Fe.
Computational Section I. DFT calculations were performed using the TPSSTPSS functional with
the 6-311++G(d,p) basis set on the nonmetal atoms and the 6-311+G basis set on nickel and iron
in the Guassian 16 suite.45 The molecular structures from XRD presented above were used as
geometric starting points with all other structures made by the appropriate atomic substitutions in
the AMPAC Graphical User Interface (AGUI)46. All structures were optimized in solvent by using
the SMD solvation model with acetonitrile as the solvent. Vibrational frequencies were calculated
‘in solvent’ and all species were confirmed to be minimum energy structures by the absence of an
imaginary frequencies. Standard statistical mechanical and solvation corrections were applied to
the electronic energy of the optimized structures to give free energy values (standard states were
not converted to mol/L).
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The computational methods yielded the structures and energies of the Ni(µ-SPhX)(µ-
S’N2)Fe complexes as well as their singly and doubly oxygenated forms. These calculations aimed
to examine how the properties of the model complexes depended on the chalcogen identity, sulfur
vs. selenium, and with para substituents, X, that modified the electron-donating properties of the
E-PhX. Our computational method accurately reproduces the trends seen in the experimental data
for structures (where available), the trends in v(CO) IR values (absolute values are underestimated),
and the positions of NiII/I reduction potentials (Figure 4).
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Mentioned above, the Ni(-EPhX)(-S’N2)Fe complexes with E = Se, X = H and with E =
S, X = NMe2 display the same v(CO) values in experiment (1934 cm-1) and from theory (1895
cm-1). These equivalent values indicate that, as relayed by iron to the carbon monoxide ligand, the
-SePhH and the -SPhNMe2 are equally strong electron donors. Similarly, the calculated v(CO) stretch
for the singly oxygenated Ni(-O-µ-SePh)(-S’N2)Fe (1919 cm-1) is comparable to that of Ni(-O-
SPhNMe2)(-S’N2)Fe (1921 cm-1). In contrast with the doubly oxygenated complexes the v(CO)
value calculated for the two-oxy species, the Ni(-O-(O=)SePhH)(-S’N2)Fe complex (1936 cm-1)
is now most similar to the sulfur analogue with the electron withdrawing X = CF3, i.e., Ni(-(-
O-S(=O)PhCF3)(-S’N2)Fe. There are minor differences in for the three di-oxy selenium species.
Table S1 lists v(CO) values of all species that were derived experimentally and also the DFT
calculated values.
Chemical Reactivity. Various chemical reactions of Ni(µ-SPhX)(µ-S’N2)Fe complexes are
presented in Figure 2. In the Ni(µ-SPhX)(µ-S’N2)Fe series, the -SePhH ligand is found to replace the
-SPhX to form Ni(µ-SePhH)(µ-S’N2)Fe in low yield along with degradation products. Under CO(g),
2+
the Ni(µ-SPhX)(µ-S’N2) complexes cleanly convert into dimeric [NiN2S]2
and (η5-
C5H5)Fe(CO)2SPhX. With PMe3, the Ni(µ-SPhX)(µ-S’N2)Fe also easily cleaves into two products:
Ni(S’N2)(PMe3) and (η5-C5H5)Fe(CO)(PMe3)SPhX. In summary, a) the reactivity of Ni(µ-SPhX)(µ-
S’N2)Fe with the poor nucleophile, CO, is controlled by SPhX- shifting to Fe with concomitant
release of [Ni(S’N2)]+, readily scavenged by another of itself forming the thermodynamically stable
2+
[NiN2S]2 ; b) with the good nucleophile, PMe3, both the Ni and Fe products contain PMe3. The
fact that -SPhX prefers the FeII rather than NiII is consistent with the observation of oxygen insertion
-
between Ni and SPh that leaves the sulfur of SPhX bound to Fe, vide infra.
Reactions with O2. Three Ni(µ-SPhX)(µ-S’N2)Fe and three Ni(µ-SePhX)(µ-S’N2)Fe complexes were
selected for studies of O2 reactivity: X = NMe2 (most electron-donating); X = H; and X = CF3
(most electron-withdrawing). Details of the experimental protocol are deposited in the Supporting
Information (p.S4, Synthesis and Characterization Section). Dichloromethane solutions of the
NiFe complexes were sparged with O2 for 30 min at 22 °C. The reactions were monitored by ν(CO)
FTIR spectroscopy until no further changes, followed by celite filtration and isolation of the S-
oxygenated products by solvent removal. Mass spectroscopy (+ESI) was used to identify the
components in the product mixture. A summary of results is found in Figure 5. In all cases, the
2+
byproducts were the disulfide, XPhS-SPhX, and the dimeric species, [NiN2S]2 . The presence of
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an FeIII species isolated from residual solids was established by addition of aqueous Na+SCN- with
formation of the blood red [Fe(SCN)(H2O)5]2+ complex.
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Under identical conditions for Ni(µ-SPhX)(µ-S’N2)Fe derivatives of three arene substituents,
we compared the O2 reaction times and yields of the principal products. In the case of X = H, 7 h
are required to complete the O2 reaction, yielding a mixture of the mono- and di-oxy species with
overall yield of 37%.41 In contrast, when X = CF3, the reaction required 24 h to maximize the
products from the reduced form, with ν(CO) at 1943 cm-1, to oxygenated products (a mixture of
1-oxy and 2-oxy species) that displayed as an unresolved broad ν(CO) band at 1971 cm-1. The
isolated yield was ca. 30%. With the Me2N substituent, the major product is the di-oxy species,
isolated in 55% yield after 3.5 h.
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For comparison, the Ni(µ-SePhH)(µ- S’N2)Fe completed reaction with O2 after only 2 h,
giving a 56% yield of sulf-oxygenated products predominantly of the mono-oxy type.41
–
Modifications using CF3 and NMe2 as para-substituents in SePhX gave slower (3.5 h) and faster
(0.8 h) reactions with O2, respectively. In summary, while selenium analogues are more reactive
than sulfur, both the selenolate and thiolate bridges respond to the X substituent on the aryl groups.
The more electron-donating substituent in the aryl-substituted chalcogenide ligands, Ni(µ-EPhX)(µ-
S’N2)Fe, gave greater yields and more rapid E-oxygenation reactions. The reaction time profile is
as follows:
0.8 h (SePh-NMe2) < 2 h (SePh-H) < 3.5 h (SePh-CF3) ≈ 3.5 h (SPh-NMe2) < 7 h (SPh-H) < 24 h (SPh-CF3).
X
+
Fe(III) species
XPh-S-S-PhX
S
e
F
E(O)
Ni
O
N
+
O2
C
n
S
+
+
N
O
N
E
Ni
Fe
DCM
N
C
n = 0, 1
O
?
X
E-X
σ
Reaction
time
Main Product
Combined Yield of
Oxygenate(s)
S-NMe2
S-H
-0.830
0.000
3.5 h
7 h
2-Oxy Species
Mixture (n = 0, 1)
Mixture (n = 0, 1)
1-Oxy Species
55 %
37 %
30 %
61 %
56 %
40 %
S-CF3
+0.540
-0.830
0.000
24 h
0.8 h
2 h
Se-NMe2
Se-H
1-Oxy Species
Se-CF3
+0.540
3.5 h
Mixture (n = 0, 1)
Figure 5. The reactions of Ni(µ-EPhX)(µ-SN2)Fe complexes dissolved in CH2Cl2 under 1 atm O2 at room
temperature. Reaction time is defined as that required to reach a plateau of the product ν(CO) band.
Attempts to separate or determine the distribution in the mixtures of 1- and 2-oxy products were
unsuccessful. Components of product mixtures identified by +ESI-Mass spectrometry.
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The identical ν(CO) absorptions (1934 cm-1) of the Ni(µ-SePhH)(µ-S’N2)Fe and the Ni(µ-
SPhNMe2)(µ-S’N2)Fe complexes are reasonably connected to their oxygen reactivity which is
significantly greater than the congeners in the series. While the electron density reported by π-
back-bonding of iron to CO appears to be the same in the two complexes, a difference exists in
reactivity (time to completion) and product distribution. A single O-uptake for Ni(µ-SePhH)(µ-
S’N2)Fe) leads to the selenoate, Ni-O-Se-Fe-S’ bridge between Ni and Fe; a 2-oxy addition, with
production of a bridging sulfinato complex, Ni-O-S(=O)-Fe, is seen for product ( >90% of the 2-
oxy species) from Ni(µ-SPhNMe2)(µ- S’N2)Fe.
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Computational Section II. The O-uptake distinctions in the Ni—Fe complexes inspired further
DFT computations that addressed thermodynamic driving forces for O2 reactions and the two types
of products. Summarized in Figure 6 are free energies, G , for the sulfur and selenium single
oxygenation reactions found to be similar at -14.3 and -15.6 kcal/mol, respectively. The double
oxygenation reactions however show a greater difference; the G in the selenium case is -25.2
kcal/mol whereas the sulfur case is favored by -35.5 kcal/mol. The selenium 2-oxy species is more
stable than the 1-oxy but the energy gap between the levels (9.6 kcal/mol) is approximately half
as large as the energy gap for the sulfur analogues (21.1 kcal/mol). While the reaction energies
indicate in both cases the 2-oxy species should be the thermodynamic product, only the sulfur
displays the sulfinate. In the absence of a mechanism for the O2 uptake reaction we suggest
possible working hypotheses: #1) the selenium 2-oxy product is less kinetically accessible than
the 1-oxy product; or #2) the 2-oxy product is formed but the weak terminal Se=O, see below,
allows the complex to undergo comproportionation with the mixed chalcogenide precursor to form
two equivalents of 1-oxy products. This type of reactivity has no direct analog in enzymes due to
the enclosed nature of the active site.
Reasonable support for hypothesis #2 is that the difference in the G for the 2-oxy
complexes correlates with the strength of the  bond to the terminal oxygen for sulfur vs. selenium.
The Natural Bond Orbital (NBO)47 analysis (second order perturbation) reveals the total
stabilization energy coming from interactions of the lone pairs on the terminal oxygen with the
chalcogen to be 24.4 kcal/mol for the sulfur and 13.0 kcal/mol for the selenium variant. While the
natural atomic orbitals that make up the S=O and Se=O π interaction (details in the SI) appear
nearly identical, the greater electronegativity difference for Se and the greater orbital size
mismatch for Se results in the weaker  bond with the terminal oxygen as compared to sulfur.
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Figure 6: DFT calculated free energy values, G , for comparison of oxygen-uptake reactions of Ni(µ-
EPhH)(µ-S’N2)Fe complexes, E = S and Se, in kcal/mol.
Mechanism of O2 addition—isotopic labeling.
Figure 7. Predicted concerted and sequential mechanisms for the reactions of Ni(µ-SPhX)(µ-S’N2)Fe
complexes with 18O2.
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Notably, the addition of O2 in the case of X = NMe2 in Ni(µ-SPhX)(µ-S’N2)Fe, cleanly yields the
2-oxy-sulfur, or sulfinato, complex as the main oxygenate. We addressed the question of concerted
addition of the oxygen atoms from one O2 molecule or stepwise addition from separate O2
molecules by isotopic labeling/crossover experiments, as shown in Figure 7. A mixture of
18O2/16O2 (in ratio of 38:62) gas was added to a
160%
a
b
c
Label retention
Label scrambling
Experimental data
CH2Cl2 solution of Ni(µ-SPhNMe2)(µ-S’N2)Fe
complex, and isolated products were subjected
to mass spectrometric analysis in order to
determine the isotope distribution in the
product sulfinato complex. If a concerted
mechanism prevails, the di-oxy product should
retain the labels of the O2 substrate; if stepwise
addition, there should be evidence of label
scrambling, the 16O18O sulfinato product.
Isotopic bundle analysis finds an isotopomer at
the mass peak of 566 m/z which is distinctive
as an indicator for label scrambling. The
theoretical distributions in the ion bundle are
shown in Figure 8 along with the experimental
result for the specific mixture of 18O2/16O2.
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140%
120%
100%
80%
60%
40%
20%
0%
562 563 564 565 566 567 568 569 570 571
m/ z
Figure 8. Theoretical and experimental ion abundances
for the mass spectrum in the [M + 2O]+ region from the
reaction of Ni(µ-SPhNMe2)(µ-S’N2)Fe with a 62:38 mixture
of 16O2:18O2 (a) by label retention; (b) by label
scrambling; and (c) the experimental results.
From the lack of a match of the m/z 566 signal we conclude that the experimental reaction mixture
best fits label retention; i.e., the isotopomeric di-oxy products are Ni16O16O SFe and Ni18O18O SFe,
and the two oxygens on sulfur are from one oxygen molecule. This agrees with previous studies on
monomeric NiN2S2 complexes featuring terminal cis-dithiolates, Figure 1, that have shown that the
addition of 3 O2 proceeds primarily through a concerted mechanism to produce both bissulfenates
(RSO-) as well as mono- and bissulfinates (RSO2). 35, 38
Oxygenated chalcogenide repair. Earlier we determined that the O atom in the mono-oxy, µ-O,
µ-EPh, complexes could be rapidly removed by PR3 (R = Me or o-tolyl) in both the sulfur and
selenium cases, regaining the µ-EPh.41 However, electrochemical reductions in the presence of acid
were ineffective towards removal of the O-atom as H2O. In the current study, Cp2Co was adopted
as an electron source and HBF4 as proton source. A dark reddish-brown CH2Cl2 solution of Ni-O-
SePhH-Fe, with (CO) 1954 cm-1, was cooled to -78 C and transferred into a pre-cooled flask
containing 2 equiv of Cp2Co powder whereupon a reduced, dark green, CO-containing species of
unknown composition was formed, Figure 9. On subsequent addition of 2 equiv of HBF4 a gradual
color change back to dark brown was observed over 1.5 h, along with a shift in the (CO) to 1934
cm-1, indicating with O-atom removal and a spectroscopic yield of 60%. The +ESI-Mass spectrum
confirmed the deoxygenation and return to Ni(µ-SePhH)(µ-S’N2)Fe. Infrared and proton NMR
spectroscopies indicated that the oxygen was removed as H2O. An experiment with the 1-oxy-
sulfur analogue, Ni-O-SPhH-Fe as a mixture with the 2-oxy species, indicated reduction of the
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former, however the sulfinato species was not affected. Addition of excess Cp2Co resulted in
overall degradation/decomposition.
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To further examine the O-removal from the sulfinate, we took the isolated two-oxy species
or sulfinato complex, Ni-O-S(=O)Fe in the case of X = NMe2 and attempted the “repair” using
Cp2Co and HBF4. After adding Cp2Co, the intensity of (CO) 1960 cm-1 decreased but no new
band was observed even after 6 h, or with increased amounts of HBF4. Further analysis of +ESI-
Mass spectrum indicates a low intensity signal at m/z 564 for the 2-oxy species but no indication
of the reduced species, either 1-oxy or the NiSFe parent complex.
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S
e
S
F
2 CoCp2
2 HBF4
Ni
O
N
N
Se
Ni Se
Dark green
+ H2O
Fe
C
O
DCM , 1.5h
-78 ºC to RT
DCM, 2h
-78 ºC
N
N
C
O
Dark reddish brown
Dark brown, ~ 60%
Figure 9. Oxygen removal reaction using Cp2Co as reductant and HBF4.
The model repair process shown in Figure 9 is of relevance to the soluble, NAD+-reducing
[NiFe]-H2ase from R. eutropha, whose S-oxygenated active site has been suggested to be
reversible under O2 and NADH/H+.28,52 In the proposed mechanism for the latter, the S-mono-
oxygenate is reduced by NADH; with added H+ producing H2O. Similarly, in our repair process,
the reductant Cp2Co analogous to NADH, and along with the H+, removes the O atom as H2O.
Conclusions and Final Remarks
The salient features of this study follow:
a)
The nominal models of [NiFeS]- and [NiFeSe]-H2ase active sites described above with
bridging chalcogenides function as a probe of O2 reactivity that yield isolable NiFe complexes
where thiolate and selenolate are converted into sulf- and seleno-oxygenates. The presence of a
carbon monoxide reporter ligand on Fe offered opportunity to explore “electronic alchemy”
through remote effects of substituents on the EPhX ligand that effectively (electronically)
transformed S into Se. Preliminary Mossbauer studies find simple quadrupole doublets and nearly
identical isomer shifts for the parent Ni-Fe-S and Ni-Fe-Se reduced complexes. Thus, the
increased electron density from the Se that influences the v(CO) IR values via FeII has no effect
on the iron nuclei.
b)
The stability of these NiFe complexes, even under siege by O2, is impressive.
Crystallography finds only minimal changes in the coordination sphere of the bimetallic complex;
the NiFe core is maintained with marginal differences in the Ni--Fe distances even though the -
EPhX bridging ligand has been expanded into an Ni-O-S-Fe or Ni-O-Se-Fe unit.
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c)
Supported by earlier DFT computations,41 we surmise that the rigidity of the tridentate N2S
“pincer” type ligand guides production of E-oxygenates at the more mobile, mono-dentate,
bridging EPhX ligand site. Consistent with this conclusion are results from the Ogo group using
NiN2S2 (with N2S2 as a fixed tetradentate binding site for NiII) as metalloligand to Cp*FeII, bearing
an open site on iron.48-50 Under O2 such Ni-Fe complexes yield isolable FeIV(peroxo) species, with
O2 side-on bound to Fe in [NiN2S2-Fe(O2)Cp*]+ rather than any of the S-oxygenates displayed
2-
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in Figure 1.
d)
The oxidation states of Ni and Fe in the product oxygenates of our study remain at NiII and
FeII for both the selenium and the sulfur derivatives. However we note that low temperature (0 °C)
monitors of the O2 reactions with the Ni-Fe containing the µ-SPhNMe2 bridging ligand found a
buildup of a transient (but long-lived) EPR-active species as the reaction proceeded; a signal at
gavg ≈ 2.09 is assigned to NiIII while one at g = 4.19 is likely FeIII, see Supporting Information
(Figure S60-62). At reaction’s end, oxygenated sulfurs were produced and the (presumed) NiIII
signal had disappeared. Some byproduct containing iron(III) is found in the oxidized residue from
these reactions. These observations are reminiscent of the early EPR studies of [NiFe]-H2ase redox
poised in different levels, which gave rise to signals for Ni-A and Ni-B. 1, 26-27
e)
The preference of O-atom bridged S and Ni, that we observed here, is seen in the sulfenato
complex of O-damaged [NiFeS]-H2ase, (Figure 1). The Ni-O-Se-Fe as a bridging unit is also
observed here, but it is opposite to the Ni-Se-O-Fe arrangement found in one of the forms of O-
damaged [NiFeSe]-H2ase.30-31 In fact, protein crystallography has uncovered a variety of
chalcogen-oxgenates and myriad binding modes in the structures of oxygen-damaged [NiFe]-
H2ase enzyme active sites; such a display is likely a benefit of reaction within a restrictive enzyme
active site cavity that partially accounts for the longevity of these species. In contrast, in
oxygenated solutions containing our small molecule active-site analogues, serious oxygen
exposure and damage is likely to lead to intractable metal oxides.
f) While there are discernible variations in oxygen uptake and product distributions that show
correlations with electronic differences in the para-substituent series, the possible causes are many
and expected to be intricately interrelated. For example, enhancement of electron-rich character
at E in EPhX from the para-substituent effect, increases the likelihood for O2 binding both to E, S
or Se, as well as to the metals which they bridge. Assuming that the affinity for the EPhX ligand
by FeII continues to be greater than to the NiII, then the O2-uptake activity should be limited to the
two sites, NiII and EPhX. Whether the O2 activation by Ni precedes O-atom attachment to E is
unknown at this point; whether the mono-oxy species result from an initial di-oxy species in all
cases, is also unclear. A full computational mechanistic study will address such questions.
–
We have seen in these studies, consistent with the enzyme studies or results, that compared
to sulfur in nearly identical chemical environment, selenium exhibits more facility for O2 uptake.17
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The observed oxygenated Se product is a single oxy species (we cannot discount a di-oxy species
as intermediate), and O-atom removal is facile for selenium. Excellent commentaries regarding
“Why nature chose- - ”17, 51 heavier elements in the chalcogen or pnictogen family for numerous
biological processes clearly point to their intricate interactions with oxygen. In hydrogenase
enzyme chemistry, the many benefits of selenium incorporation include not only enhancement of
catalytic activity through proton shuttling and hydrogen expulsion rates, but also of protection of
the active sites from the poisonous O2. Such benefits apparently outweigh the added cost to the
organism of the biosynthesis. As of now, there are few synthetic HER or ORR molecular catalysts2
that target selenium substitution and explore possible paybacks for the minimal synthetic
expenditure. We hope that our results point to new directions in this regard.
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Associated Content
Supplementary Information
Experimental, characterizations, additional spectroscopic, preliminary Mössbauer and EPR studies,
and computational details (PDF)
X-ray Crystallographic data (CIF)
This material is available free of charge via the Internet at http://pubs.acs.org.
Author Information
Corresponding Authors:
*marcetta@chem.tamu.edu
*hall@science.tamu.edu
Notes
The authors declare no competing financial interest.
Acknowledgement
This work was financial supported by the National Science Foundation (CHE-1300787, CHE-
1664866 to M.B.H. and CHE-1665258 to M.Y.D.) and the Robert A. Welch Foundation (A-0648
to M.B.H. and A-0924 to M.Y.D.). We thank Kyle Burns, Waseem Vali, and Prof. Paul Lindahl
for the preliminary Mössbauer studies.
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TOC graphic:
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Ni
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Ni
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