Effects of Thiolate Ligation in Monoiron Hydrogenase (Hmd): Stability of the {Fe(CO)2}2+ Core with NNS Ligands

Article abstract of DOI:10.1021/acs.inorgchem.8b01185

In this work, we report the effects of NNS-thiolate ligands and nuclearity (monomer, dimer) on the stability of iron complexes related to the active site of monoiron hydrogenase (Hmd). A thermally stable iron(II) dicarbonyl motif is the core feature of th

Full text of DOI:10.1021/acs.inorgchem.8b01185

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Effects of Thiolate Ligation in Monoiron Hydrogenase (Hmd):
2+
Stability of the {Fe(CO) } Core with NNS Ligands
2
Zhu-Lin Xie, Doran L. Pennington, Dylan G. Boucher, James Lo, and Michael J. Rose*
Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States
*
S Supporting Information
ABSTRACT: In this work, we report the effects of NNS-thiolate ligands and nuclearity (monomer, dimer) on the stability of
iron complexes related to the active site of monoiron hydrogenase (Hmd). A thermally stable iron(II) dicarbonyl motif is the
core feature of the active site, but the coordination features that lead to this property have not been independently evaluated for
their contributions to the {Fe(CO)2}2 stability. As such, non-bulky and bulky benzothiazoline ligands (thiolate precursors)
were synthesized and their iron(II) complexes characterized. The use of non-bulky thiolate ligands and low-temperature
+
Ph
crystallizations result in isolation of the dimeric species [(NNS) Fe (CO) (I) ] (1), [(N NS) Fe (CO) (I) ] (2), and
2
2
2
2
2
2
2
2
Me
[
( NNS) Fe (CO) (I) ] (3), which exhibit dimerization via thiolato (μ -S) bridges. In one particular case (unsubstituted
2 2 2 2 2 2
NNS ligand), the pathway of decarbonylation and oxidation from 1 was crystallographically elucidated, via isolation of the
half-bis-ligated monocarbonyl dimer [(NNS) Fe (CO)]I (4) and the fully decarbonylated and oxidized mononuclear
3
2
[
(NNS) Fe]I (5). The transformations of dicarbonyl complexes (1, 2, and 3) to monocarbonyl complexes (4, 6, and 7) were
2
monitored by UV/vis, demonstrating that 1 and 3 exhibit longer t1/2 (80 and 75 min, respectively) than 2 (30 min), which is
attributed to distortion of the ligand backbone. Density functional theory calculations of isolated complexes and putative
intermediates were used to corroborate the experimentally observed IR spectra. Finally, dimerization was prevented using a
Ph
DMPh
bulky ligand featuring a 2,6-dimethylphenyl substituent, which affords mononuclear iron dicarbonyl complex, [(N NS
)-
Fe(CO) Br] (8), identified by IR and NMR spectroscopies. The dicarbonyl complex decomposes to the decarbonylated
2
Ph
DMPh
[
(N NS
) Fe] (9) within minutes at room temperature. Overall, the work herein demonstrates that the thiolate moiety
2
2
+
does not impart thermal stability to the {Fe(CO)2} unit formed in the active site, further indicating the importance of the
organometallic Fe-C(acyl) bond in the enzyme.
INTRODUCTION
As the depletion of fossil fuels approaches and the need for
renewable energy sources increases, attention has turned to
Scheme 1. Reaction Catalyzed by Hmd
■
1
−3
the production and utilization of dihydrogen (H2).
The
development of biomimetic catalysts that efficiently perform
H -related reactions, such as H activation and water oxidation,
2
2
are of particular interest. Nature has developed three types of
hydrogenases that activate and utilize H using different mech-
2
anisms. These enzymes include [FeFe], [FeNi], and the most
recently discovered mono-[Fe] hydrogenase (also called hydro-
gen-forming methylene-tetrahydromethanopterin dehydrogen-
ase (Hmd)), which is endogenously expressed in methanogens
[
Fe] hydrogenase is an intermediate step in the methanogenic
+
4
−6
reduction of CO to CH , where H MPT serves as a C carrier
in the metabolic cycle.
2
4
4
1
such as Methanocaldococcus jannaschii.
The unique feature
4
−6
of the mono-[Fe] hydrogenase is that it catalyzes the hetero-
+
−
lytical splitting of H and produces H and H , the latter of which
2
is transferred to the substrate, methenyltetrahydromethanopterin
Received: April 30, 2018
+
(
H MPT , Scheme 1). This H cleavage and hydride transfer by
4
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01185
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Inorganic Chemistry
Article
The structure of the active site of Hmd was determined
crystallographically by Shima and co-workers. As shown in
Scheme 2, the active site contains an Fe(II) ion ligated by a
mononuclear iron(II) carbonyl species resembling the active
7
,8
14−16
site to lesser or greater extents, respectively (Scheme 3).
2
+
While both systems replicated the cis-{Fe(CO)2} motif, the
Schiff base ligand did not provide thermal stability (decarbon-
ylation occurred above −20 °C in coordinating solvent), and
the pincer−CNS complexwhile stable up to 50 °C in coor-
Scheme 2. Active Site of Hmd Detailing the Two Possible
Protonation States: Pyridinol (left) versus Pyridone (right)
dinating solventsdid not exhibit any reactivity with H or
2
model substrates. In fact, in the latter case (pincer), CO
dissociation was only observed when using a strong decarbon-
16
ylation reagent such as Me NO. Overall, on these premises, it
3
was not clear whether the inclusion of a thiolate donor (in the
absence of an acyl-C donor) would provide (i) thermal sta-
2
+
bility to the cis-{Fe(CO)2} unit, or (ii) promote reactivity
with H and model substrates. Therefore, in this work we
2
incorporated a thiolate into the donor set to evaluate its
structural preferences and thermal stability. One possible
outcome of thiolate metalation ligands is the formation of
dimeric or multinuclear complexes due to the bridging ten-
dency of thiolato-S (Scheme 4). By increasing the bulkiness of
bidentate acyl-pyridone/pyridinol ligand, two terminal CO
groups in cis orientation, and a cysteinyl sulfur (Cys176). The
coordination site trans to the acyl unit is occupied by solvent
(
H O) in its resting state and is the putative binding and
activation site for H . Although the exact mechanism of H
2
2
2
activation remains unknown, researchers agree that either the
Scheme 4. Targeted Ligands and Dicarbonyl Complex
pyridone oxygen or cysteine sulfur serves as the “pendant base”
9
−11
in cooperation with the Lewis acidic Fe(II) ion.
As a result,
the kinetic barrier of H cleavage has been calculated to be
2
‡
11
drastically lowered (ΔG ≈ 2 kcal/mol).
One seemingly innocuous feature of the active site warrants
some discussion: the cis-{Fe(CO)2}2+ motif. While hundreds
of examples of structurally characterized iron(II) carbonyl
complexes can be found in the Cambridge Crystallographic
Data Centre (CCDC), there are relatively few phosphine-free
2
+
coordination complexes containing cis-{Fe(CO)2} motif.
More specific to the enzyme, synthetic models of [Fe]-
hydrogenase have been developed to understand the mech-
anistic details, reactivity, and bonding of the active site.
Previously, Hu and co-workers reported a series of methylene-
acyl containing iron complexes exhibiting structural relevance
the thiolate, we attempted to avoid the bridging thiolate motif
and thus afford mononuclear complex, as we demonstrated in
17
our anthracene-base CNS(thiolate) model. Herein, we report
the synthesis of Schiff base thiolate iron complexes and,
ultimately, control of nuclearity of these complexes by varying
the steric hindrance of the thiolate ligands.
12
to the Hmd active site (Scheme 3). Pickett and co-workers
Scheme 3. Synthetic Models for [Fe] Hydrogenase
RESULTS AND DISCUSSION
■
Syntheses. Non-bulky Ligands. The non-bulky ligands
were derived from condensation of the substituted pyridine
carboxaldehyde or ketone with 2-aminobenzenethiol, based on
18,19
the literature report (Scheme 5).
The thermodynamically
more favorable benzothiazoline products were obtained as a
result of cyclization after condensation (Scheme 5), wherein
the benzothiazoline features an S/N heterocyclic five-membered
20
ring that can be easily opened by metal ions or organic bases.
The formation of the benzothiazoline was demonstrated by
1
NMR and IR spectroscopies as follows. The H NMR spectra
for L1−L3 in CDCl showed a single product isolated from
3
each reaction. The benzothiazoline methine −CH− proton of
L1 and L2 are identified at 6.41 and 6.38 ppm, respectively,
which fall within the expected range of methine resonances, rather
18,19
than the CHN resonance (pyridine-imines ∼8.0 ppm).
The two ligands exhibit amine NH protons at 5.06 and
.12 ppm, and the methyl proton of L2 resonates at 2.55 ppm
5
synthesized a series of “carbamoyl” (amide-acyl) iron com-
plexes that mimicked the active site. And while the afore-
mentioned models replicated most features of the first coor-
dination sphere, none exhibited the enzyme-like functionality
as expected for 2-methylpyridine. Although L3 does not have
a CH proton, its existence was indicated by NH resonance
at 6.17 ppm. None of the ligands exhibit a thiol SH
21
13
resonance, usually observed at ∼3 ppm. The C NMR
13
of H cleavage in the absence of the protein environment.
spectra in CDCl for the three ligands are also consistent with
2
3
Previously, we utilized Schiff base (NNS) and carbamoyl-
pincer (CNS) ligands bearing thioether moieties to generate
the benzothiazoline structures. For example, all three ligands
show the hallmark features for methine carbon (CH) near
B
DOI: 10.1021/acs.inorgchem.8b01185
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Scheme 5. Synthetic Pathways for Non-bulky Benzothiazoline Ligands
a
Scheme 6. Synthetic Pathways for Iron Complexes Derived from Non-bulky Benzothiazoline (A for L1; B for L2 and L3)
a
See text above for discussion of 1 → 4 conversion stoichiometry.
Ph
7
0 ppm (70.08 ppm for L1, 70.26 ppm for L2, 82.24 ppm
dicarbonyl complexes [(NNS) Fe (CO) (I) ] (1), [(N NS) Fe -
2
2
2
2
2
2
2
2
Me
for L3). It has been speculated that an equilibrium of
benzothiazoline and imine may be present in the solution.
However, in the present study no such equilibrium was evident
in the H or C NMR spectra. This result is consistent
with the study of Tyler and co-workers, who also proposed
the formation of benzothiazoline as the only product in
both solid and solution.
the three ligands exhibit a feature typical for the ν(SH)
(CO) (I) ] (2), and [( NNS) Fe (CO) (I) ] (3). These dark
2 2 2 2 2 2
20
green products are stable for weeks in solid state at −30 °C
under inert atmosphere; when dissolved in weakly coordinat-
ing solvents such as MeCN or tetrahydrofuran (THF), these
complexes are stable for days at −30 °C. At room temperature,
the dimeric dicarbonyl complexes in MeCN rapidly transform
to the dinuclear monocarbonyl complex, [(NNS) Fe (CO)]I
1
13
18,19
Lastly, none of the IR spectra of
3
2
Ph
Me
(4), [(N NS) Fe (CO)]I (6), and [( NNS) Fe (CO)]I (7),
3
2
3
2
−
1
23
stretch (generally ∼2500 cm ). All of the above character-
ization data support the benzothiazoline formation for L1, L2,
and L3.
as evidenced by UV/vis and X-ray diffraction (vide infra).
Complex 4 can be independently synthesized first by treatment
of L1 with [Fe(CO) (I) ] at −30 °C, followed by crystalliza-
4
2
Metal Complexes of Non-bulky Ligands. The syntheses of
the iron carbonyl complexes of the non-bulky ligands L1−L3
are summarized in Scheme 6. The metalation of L1−L3 with
tion with a trace amount of dimethylformamide (DMF) in
MeCN. The green crystals of 4 are stable in solid state under
inert atmosphere, and a MeCN solution of 4 can be stored
under light at ambient temperature without loss of CO. The
sharp contrast of stability between 1 and 4 indicates that the
NNS ligandspecifically, inclusion of the thiolate moiety
does not generate a stable iron dicarbonyl core. Additionally,
[
Fe(CO) (I) ] was pursued in MeCN at −30 °C to thermally
4
2
stabilize the CO ligands bound to the iron center. The bridging
promiscuity of the thiolate donor promotes dimerization of
the iron complexes in all three cases, yielding the dimeric
C
DOI: 10.1021/acs.inorgchem.8b01185
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Inorganic Chemistry
Article
a
Scheme 7. Synthesis of Bulky Ligand, L , Ultimately Derived from Isomerization of the Intended Schiff Base Ligand
B
a
Reaction conditions: (i) NH Br, 30 wt % H O , AcOH, rt; (ii) 2,6-dimethylphenyl boronic acid, K CO , Pd(dba) , XPhos, THF/H O (5:1, v/v),
4
2
2
2
3
2
2
reflux; (iii) H SO , NaNO , H O/acetone, 0 °C; KSCN, CuSCN, rt; (iv) LiAlH , THF, reflux; (v) 2-benzoylpyridine, AcOH, r.t.
2
4
2
2
4
Scheme 8. Synthesis of the Iron Complexes 8 and 9 Derived from the Bulky Thiolate Ligand
as indicated by the 1 → 4 conversion in trace DMF, strongly
coordinating solvents easily replace one of the CO ligands,
generating the thermodynamically more favorable monocar-
bonyl complexes, for example [(NNS) Fe (CO)]I (4). In terms
of the stoichiometry of the 1 → 4 conversion, complex 1 likely
first loses one CO ligand. Then two-thirds of the decarbonylated
intermediate obtains a third ligand, provided by the other one-
thiolate complex. In this strategy, a bulky substituent
(2,6-dimethylphenyl) was installed ortho to the thiolate donor.
The synthetic pathway is represented in Scheme 7. Starting
with 4-methyl-2-nitroaniline, bromination at the 6-position was
performed in acetic acid using NH Br/H O as a bromine
3
2
4
2
2
source. The purpose of starting with 4-methyl-2-nitroaniline,
rather than 2-nitroaniline, was to prevent the bromination at
the 4-position, which otherwise decreased the yield and pre-
vented clean separation. Subsequently, Suzuki coupling of a
with 2,6-dimethylphenyl boronic acid afforded the dimethyl-
phenyl-appended nitroaniline b; the reaction proceeded with
good yields only with bis(dibenzylideneacetone)palladium(0)/
XPhos as catalyst. The preparation of c was performed via
diazotization (acetone/H O, 0 °C, NaNO , H SO ) followed
third of the intermediate, thus forming complex 4. The 1/3
−
equiv of Fe I core (plus two released I ) then forms 2 equiv of
2
2
an unrecovered Fe(I) (solv) species:
2
3
L Fe I (CO) → 2[L Fe (CO)]I + 2FeI + 4CO
(1)
2
2 2
2
3
2
2
On the one hand, attempts to further decarbonylate 4 to
2
2
2
4
prepare the bis-ligated Fe(II) complex [(NNS) Fe] with UV
2
by the addition of KSCN and CuSCN. The subsequent one-
pot reduction of the nitro and thiocyanate groups was achieved
light were unsuccessful (for comparison, [(NNS) Fe] was syn-
2
thesized separately); the single CO is tightly bound to the Fe
center. On the other hand, an MeCN solution of 4 is oxidized
by air within seconds, turning the dark green solution to gray.
Crystallization of this species afforded green crystals of the bis-
ligated Fe(III) complex [(NNS) Fe]I (5), whose BPh salt was
with LiAlH in dry THF, producing the expected product d.
4
Finally, condensation of d with 2-benzoylpyridine in acetic
acid afforded the target ligand L in its “apo-form” as the
B
1
benzothiazoline, as evidenced by H NMR (N-H = 4.71 ppm)
2
4
1
3
24
and C NMR (methine C-H = 83.70 ppm).
previously reported by Mascharak. The extreme sensitivity of
to oxygen is also indicated by the fact that single crystals of 5
Similar to the method described above, metalation of
4
the bulky ligand LB with [Fe(CO) (Br) ]/[Fe(CO) (I) ]
were on occasion isolated in attempts to prepare 4 in the
drybox, and a minority presence of 5 is evident in the ele-
mental analysis of 4. As no reactivity or further measurements
4
2
4
2
(
Scheme 8) in THF at −30 °C was attempted. However, these
conditions resulted in crude products with ν(CO) stretches
exclusively above 2000 cm ; this is distinct from complexes
1−4, which exhibit ν(CO) features in the 1950 cm region.
Such high-energy ν(CO) features are consistent with the
ligation of a neutral thioether donor (e.g., benzothiazoline),
rather than the intended thiolate. We thus inferred that, for the
bulky ligand, base might be necessary to promote the desired
−
1
(
other than routine characterization) were performed on 4, the
−
1
minority presence of 5 in bulk crystalline samples of 4 proved
inconsequential.
Bulky Ligand and Its Metal Complex. The dimerization
issue encountered above prompted us to increase the steric
bulk of the thiolate to facilitate formation of a mononuclear
14
D
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Article
Ph
Figure 1. ORTEP diagrams (30% thermal ellipsoids) for [(NNS) Fe (CO) (I) ] (1) and [(N NS) Fe (CO) (I) ] (2). H atoms are omitted for
2
2
2
2
2
2
2
2
clarity.
ring-opening reaction prior to rather than during metalation.
The monocarbonyl intermediate was not isolated, nor was it
spectroscopically identified (vide infra).
We subsequently found that, when using [Fe(CO) (Br) ] as a
4
2
+
+
metal source, anionic bases such as acetate (K or NEt salt)
Characterization of the Non-bulky Thiolate Complexes.
4
resulted in decarbonylation of the iron starting salt, whereas
X-ray Structures. The complex [(NNS) Fe (CO) (I) ] (1)
2
2
2
2
bulky and neutral nitrogen bases such as Hu
̈
nig’s base and
(Figure 1) crystallized in monoclinic C2/c. The structure is a
neutral dimeric complex with an [Fe (S ) ] core. The two
proton sponge successfully promoted ring-opening and thiolate
2
thiolate 2
−1
ligation, vis a vis generation of ν(CO) features (below 2000 cm ).
monomers are symmetric to each other with respect to a C2
axis. Each Fe(II) ion exhibits pseudo-octahedral geometry,
consisting of a NNS ligand coordinated in meridional fashion,
a carbonyl, an iodide ion, and another (bridging) thiolate
Ultimately, it was determined that 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU) provided the cleanest product, as evidenced by two
ν(CO) features in the IR. This set of ν(CO) featuresone
−
1
above and one below 2000 cm is consistent with the for-
sulfur. The N −Fe, Fe−C(O), and CO distances are
py
mulation of a mononuclear dicarbonyl species ligated by an
1.979(5), 1.789(8), and 1.114(8) Å, respectively, which are
10,12,25,26
14,16,27
anionic ligand frame.
By comparison, iron(II) dicar-
within the normal range for a low-spin Fe(II) ion.
bonyls ligated by neutral NNS ligands (with methylthioether
In addition, there are two unique Fe−S distances: the primary
NNS thiolate bound to Fe (2.2889(17) Å) and the S-donor
from the neighboring NNS ligand (2.3276(18) Å).
−1
14
donor) exhibit both ν(CO) stretches above 2000 cm . Thus,
Ph
DMPh
the product was postulated as the target complex, [(N NS
)-
Ph
Fe(CO) Br] (8); further spectroscopic evidence for 8 is provided
The structure of [(N NS) Fe (CO) (I) ] (2) (Figure 1) is
2
2
2
2
2
below. Comparatively, the reactions of [Fe(CO) (I) ] with the
also dimeric, and the Fe(II) centers exhibit pseudo-octahedral
geometry. The coordination environment of the complex is the
same as that of 1, and the bond lengths are within the expected
4
2
bulky ligand only afforded product with ν(CO) stretches above
−
1
2
000 cm , regardless of the addition of bases. This may indi-
14,16,27
cate that, in the presence of [Fe(CO) (I) ], the pK of NH is
range for low-spin Fe(II) center (Table S1).
However,
4
2
a
not decreased as with [Fe(CO) (Br) ], and stronger base may
the phenyl ring on the Schiff base linkage slightly changes the
structure of the complex, compared with 1. The Schiff base
phenyl rings are tilted away from the plane of ligand backbone,
with dihedral angles of 63.52° and 67.68°. Notably, because of
the steric hindrance of Schiff base phenyl rings, the planes of
two ligand backbones are not parallel, and the dihedral angle
between them in 2 (34.93°) is larger than that of 1 (21.29°).
Another subtle difference between 2 and 1 is the “bite angle”
4
2
be required to convert the ligand into Schiff base; such con-
ditions were not pursued upon obtaining the successful result
with [Fe(CO) (Br) ].
4
2
Concerning the above observations, the use of a base in the
reaction proved imperative to generate the desired bulky ring-
opened Schiff base ligand. In contrast to the non-bulky ligands
(no base required), the base-specificity when metalating with
the bulky L is likely due to its large steric hindrance, which
provided by the NNS chelate. In unsubstituted 2, the N −
B
py
likely prevents the metal source from pre-emptively binding to
either the N or S moiety, thus precluding a decrease in NH pKa
that would be expected upon such “pre”-complexation. The
importance of choosing the proper solvent for metalation was
also noticed in this reaction. THF evidently provided a suitable
Fe−S bite angle of 98.5° is slightly more acute versus that for 1
(101.5°). Correspondingly, the spatial distance between the
Npy and S donors in 2 (4.169 Å) is notably shorter than that
for 1 (4.229 Å). Thus, the phenyl substituent in 2 causes a
slight “pinching” effect in the ligand frame that is not evident
“
middle ground” in terms of coordinating ability. The same
in 1. As a result, the Fe−N and Fe−S distances in 2 are
py
reactions in MeCN or dichloromethane (DCM) afforded prod-
ucts with multiple or no ν(CO) features, respectively.
shorter than those of 1 (Fe−N : 1.963(11) and 1.945(11) Å
py
in 2, vs 1.979(5) Å in 1; Fe−S: 2.259(4) Å and 2.306(4) Å, vs
Upon crystallization in THF at −30 °C, the complex 8 gradu-
2.2889(17) Å); the Fe−N bonds follow the opposite trend
SB
ally decomposed, affording a diamagnetic bis-ligated Fe(II)
complex, [(N NS
(1.994(10) in Å 2 vs 1.982(5) Å in 1).
Ph
DMPh
)2Fe] (9). The structure is shown in
Complex [(NNS) Fe (CO)]I (4) (Figure 2) crystallized in
3
2
Figure S29, and the selected bond distances and angles
are tabulated in the Supporting Information (Table S1).
monoclinic P2 /c and is a cationic di-iron monocarbonyl com-
plex with charge balance provided by an outer sphere iodide.
1
E
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The Fe−N distances are 2.015(8) and 2.016(8) Å, and the
py
Fe−S distances are 2.212(3) and 2.222(3) Å, which are
comparable with the analogous structure of [(NNS) Fe]BPh
2
4
24
obtained by Mascharak and co-workers, with Fe−N
=
py
2
.015, 1.977 Å and Fe−S = 2.223, 2.211 Å. For comparison,
the Fe(II) bis-ligated complex [(NNS) Fe] was synthesized
2
and crystallized independently in Pna2 . The Fe−N distances
1
py
are shorter than those of 5 (1.973(9) and 1.961(10) Å), and
Fe−S distances are longer than those of 5 (2.290(3) and
2
.294(3) Å). The changes in bond distances are likely due
to the higher oxidation state, which facilitates the bonding
between anionic-S donor with the cationic Fe(III).
IR Spectroscopy. The solid-state IR spectra of the non-bulky
thiolate complexes were recorded under N atmosphere and
2
are shown in the Supporting Information (Figures S16−S20);
the values are tabulated in Table 1. The CO peaks for 1, 2, and 3
Table 1. Selected ν(CO) IR Features for the Non-bulky and
Bulky Thiolate Complexes
Figure 2. ORTEP diagram (30% thermal ellipsoids) for
[
(NNS) Fe (CO)]I (4). H atoms are omitted for clarity.
3 2
cmpd
1
2
3
4
experimental
calculated
1941, 1926
2028, 2005
1955, 1932
1951, 1927
1940
2023
Unlike 1, the two Fe ions in 4 are in different coordination
environments. One of the Fe ions is bis-ligated by two NNS
ligands. The other Fe center is bridged by the thiolates from
the bis-ligation unit, as well as coordinated by a third NNS
ligand (the only non-bridging thiolate observed with L1-L3).
The coordination geometry is completed by CO. Notably, 4
can be regarded as an intermediate in the transformation
−
1
are observed below 2000 cm , which correspond to the
symmetric (higher wavenumber) and asymmetric stretches of
the two CO ligands. The similar wavenumbers of CO peaks in
1, 2, and 3 show negligible change of the electron density at
the metal center, whereas modification of the ligand backbone
(i.e., adding methyl group on the ortho position of pyridine or
phenyl group on the imine carbon) decreases the intensity of
the asymmetric stretching of CO (1932 cm for 2 and 1927 cm
for 3). Small Δν (<50 cm ) and different intensity between
symmetric and asymmetric CO stretches suggest that the CO
from 1 to 5. The N −Fe distances are within the expected
py
range for low-spin Fe(II) (N1−Fe1 = 1.961(3) Å, N3−Fe1 =
14,16,27
1
.964(3) Å, N5−Fe2 = 1.988(3) Å).
The Fe−C(O)
−1
−1
distance (1.774(5) Å) is slightly shorter than that of 1 but still
within the normal range for low-spin Fe(II). The CO
distance (1.115(5) Å) in 4 is comparatively longer, due to
increased π back-bonding from the Fe center (compared to 1)
due to the additional ligation of the non-bridging thiolate
donor.
−
1
2
6
ligands do not bind to Fe center in a cis orientation. These
values are in line with the density functional theory (DFT)-
−
1
calculated IR frequency (2028 cm for symmetric vibration;
−
1
The Fe−S bonds in 4 can be categorized into three types:
2005 cm for asymmetric vibration). For 4, a single CO
−
1
Fe1−S
, Fe2−S
, Fe2−S
. The Fe2−S
vibration is observed at 1940 cm , which is consistent with
the X-ray structure of 4 as a monocarbonyl complex.
Experimental and DFT Characterization of the Bulky
Thiolate Monomer 8. Although the instability of the com-
plex 8 prevented its crystallization, the formation of the mono-
nuclear dicarbonyl complex [(N NS
was evidenced by spectroscopic data (IR and C NMR) and
bridging
bridging
nonbridging
nonbridging
distance is the shortest [2.2733(10) Å], followed by Fe1−
S
[2.2888(11) and 2.2972(10) Å], and finally the longest,
bridging
Fe2−S
[2.3061(10) and 2.4026(11) Å].
bridging
Complex [(NNS) Fe]I (5) (Figure 3) was crystallized in
2
Ph
DMPh
P2 /n and consists of two NNS units ligated to a single Fe(III) ion
)Fe(CO) Br] (8)
1
2
1
3
in pseudo-octahedral geometry; an outer sphere iodide provides
charge balance, and a molecule of I was also present as solvate.
DFT calculation. The IR spectrum (Figure 4) of 8 shows two
2
Figure 3. ORTEP diagrams (30% thermal ellipsoids) for [(NNS) Fe]I·I (5·I ) and [(NNS) Fe]. H atoms are omitted for clarity.
2
2
2
2
F
DOI: 10.1021/acs.inorgchem.8b01185
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Inorganic Chemistry
Article
1
3
Figure 4. Low-temperature (−30 °C) C NMR spectrum of 8 (solvent: THF. The complex was generated in situ at −30 °C under CO
atmosphere). (inset) Solid-state IR spectrum of 8.
−
1
carbonyl peaks at 2036 and 1985 cm , which are blue-shifted
−
1
∼
90 cm compared with those of the dimer complexes (1, 2,
and 3). The equal intensity and Δν of the two CO frequencies
−
1
is ∼51 cm , larger than those of the dimers. Both of the above
features suggest that the two COs are bound to one iron center
2
6
in a cis fashion. Compared with the CO stretch of FeGP
2
8
−1
26
cofactor (solid: 2004 and 1934 cm ; solution: 2031 and
−
1
1
972 cm ), the CO frequencies are blue-shifted, indicating
that the electron density of the Fe center in the NNS complex
is less than that in the enzyme active site.
1
3
The C NMR spectrum of 8 was obtained at −30 °C with
the in situ prepared complex in THF. The NMR tube was
charged with CO gas (1 atm) to prevent decomposition. The
1
3
C NMR exhibits two resonances in the far downfield region
at 215.2 and 211.9 ppm, which are assigned as the two chem-
ically inequivalent COs. The chemical shifts of the COs are
2
+
comparable with other reported complexes with cis-{Fe(CO) }
motif (195−220 ppm).
2
12,29
Figure 5. DFT-calculated structure and IR spectrum of 8.
The possibility of the presence of 8
as a dinuclear dicaronyl complex can be excluded, as in those
cases the two terminal COs are chemically equivalent and
would only display a single resonance in the C NMR
spectrum.
two CO vibrations are almost equally intense, which is in con-
trast to the calculated CO intensities for [(NNS) Fe (CO) I ]
1
3
2
2
2 2
(1), where the CO symmetric vibrations are significantly more
intense than the asymmetric vibration (Figures S31 and S32).
In addition, the wavenumber of CO vibrations for 8 is blue-
shifted by 20 cm , compared to the calculated value of 1. The
same trend is evident in the experimental data.
Attempts of observing 8 in mass spectrometry were unsuc-
cessful, as the dicarbonyl complex decomposed during electro-
spray ionization (ESI) or chemical ionization (CI) process.
Instead, the ESI-MS analysis shows two peaks with m/z =
−
1
4
63.0944 and 565.0036, consistent with the formulations of
On the basis of the above data, the increase of the steric
hindrance of the NNS ligand considerably affects the product
afforded from the metalation. The bulky aryl substituent 2,6-
dimethylphenyl (DMPh) facilitates the formation of the
mononuclear cis-dicarbonyl complex 8.
Thermal Stability of the Dicarbonyl Complexes.
We demonstrated the different ligation properties of the
non-bulky and bulky NNS ligands. The thermal instability of
the dicarbonyl complexes (1, 2, 3, and 8) are also observed
qualitatively in the synthesis. To quantitatively compare
the ability of the NNS ligands to stabilize the carbonyl com-
plexes, time-dependent UV/vis and IR experiments were
performed.
Ph
DMPh
+
Ph
DMPh
+
monomeric [(N NS
respectively.
)Fe] and [(N NS
)FeBr+Na] ,
To further postulate the formation of the mononuclear iron(II)
dicarbonyl complex 8, DFT calculations were performed at the
level of B3LYP/6-311G** for C, H, O, N, S, Br and B3LYP/
SDD for Fe. Geometry optimization provided a converged mono-
nuclear diamagnetic Fe(II) structure featuring cis terminal
carbonyls (Figure 5). The selected bond lengths and angles are
shown in the Supporting Information (Table S1). The cal-
culated IR of 8 (Figure 5) also exhibits two CO stretching
−
1
features at 2044 and 2020 cm , corresponding to the sym-
metric and asymmetric vibrations, respectively. Notably, the
G
DOI: 10.1021/acs.inorgchem.8b01185
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Inorganic Chemistry
Article
The dimeric dicarbonyl complexes 1−3 were synthesized
in situ in MeCN at low temperature (−30 °C) with a concen-
tration of ∼0.5 mM. The solutions were quickly transferred to
an air-free cuvette, and the changes in UV/vis absorption spectra
were monitored at room temperature in the dark (Figure S38).
All the samples gradually converted to the corresponding mono-
carbonyl complexes; the conversions are plotted in Figure 6.
Figure 7. Conversion of 8 to the bis-ligated FeII complex 9 as
determined by changes in the solution IR spectra in THF (inset).
considerably shorter than those dimeric dicarbonyl complexes.
The conversion of 8 to 9 is complete within 15 min, with t1/2
=
2
min. During the conversion, complex 8 loses two CO ligands
simultaneously, and the dimeric dicarbonyl complex was not
observed (Figure 7 inset).
Overall, it is evident that the NNS(thiolate) ligand frame is
unable to stabilize the cis-dicarbonyl motifregardless of mono-
or dinuclearityleading to decarbonylation and the (half)bis-
ligated complexes. This suggests that one of the critical roles of the
organometallic Fe−C(acyl) moiety in the Hmd enzyme active site
is to prevent thermalization of the CO ligands under biological
temperature and conditions. An analogous effect is also observed
14,16
in our previous research,
whereas the incorporation of car-
bamoyl unit to the Fe center resulted in a series of stable organo-
NH py Me
15,16
metallic complexes of type [(OC N S )Fe(CO) L].
2
CONCLUSIONS
■
(
1) This series of iron carbonyl complexes featuring Schiff-
Figure 6. Conversion of 1, 2, and 3 to the corresponding Fe(II)
monocarbonyls 4, 6, and 7, respectively, as determined by changes in
the UV/vis spectra (changes of absorbance were monitored at the
wavelength of 731 nm for 1, 766 nm for 2, and 430 nm for 3).
base pyridine/(NNS) thiolate have been prepared via
one-step metalation using benzothiazoline ligand pre-
cursors and iron tetracarbonyl halides.
(
2) Upon metalation, the non-bulky NNS thiolate ligands
produce diiron dicarbonyl complexes due to the thiolate
bridging effect. The products of stepwise decarbonylation
of diiron dicarbonyl complexes are diiron monocarbonyl
complexes; finally, the oxidized iron(III) bis-ligated
complexes were isolated.
The variation of the ligand backbone gave rise to different t1/2
values. Complex 2 shows the shortest t1/2 (30 min), whereas
complexes 3 and 1 exhibit similar stabilities (t1/2: 75 min for 3,
8
0 min for 1). The difference in t1/2 can be attributed to the
structural distortion eident in the crystal structures. As the
structure of 2 shows, the phenyl moiety on the imine carbon
prevents ligand backbone from attaining ideal planarity in the
crystal (see X-ray section), which decreases the stability of 2.
In contrast, in 3 the methyl group at the ortho position of
pyridine does not twist the ligand backbone, thus rendering a
similar t1/2 as 1. In contrast, the monocarbonyl complexes (4, 6,
and 7) are quite stable at room temperature in the inert atmo-
sphere; no change in the UV/vis spectra was observed at room
temperature over the course of several days.
(3) A bulky thiolate ligand with a 2,6-dimethylphenyl group
ortho to the (aryl)thiolate was developed. This functional
group prevents the dimerization across the thiolate,
thereby affording a mononuclear dicarbonyl complex.
This strategy could be applied to the synthesis of future
structural and functional model of [Fe]-hydrogenase.
(4) The stability study has shown that the thiolate (without
the acyl unit) is not sufficient to stabilize the cis-
2
+
{Fe(CO)2} core and that ensuing decarbonylation leads
to the formation of a bis-ligated complex. This indi-
cates that one of functions of the acyl unit in the active
site of [Fe]-hydrogenase is to prevent thermalization of
the CO ligands bound to the Fe center.
Attempts to monitor the decomposition of the mono-
Ph
DMPh
nuclear dicarbonyl complex [(N NS
)Fe(CO) Br] (8) by
2
UV/vis proved unsuccessful, as 8 is unstable at room tem-
perature, and the absorption spectrum of the bis-ligated
complex 9 completely obscures the trace of 8 (Figure S39).
Thus, we instead monitored the solution IR of 8 to study its
thermal stability. A THF solution of 8 was incubated in the
dark at room temperature, and an IR spectrum was acquired
every minute. As shown in Figure 7, the thermal stability of 8 is
EXPERIMENTAL SECTION
General Procedures and Reagents. All organic starting mate-
rials were purchased from Acros Organics or Sigma-Aldrich and used
without further purification. The Fe(II) starting salt [Fe(CO) (I) ]
■
4
2
was prepared by reaction of [Fe(CO) ] (Strem) with I according to
5
2
H
DOI: 10.1021/acs.inorgchem.8b01185
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
0
the published procedure. The Fe(II) starting salt [Fe(CO) (Br) ]
added consecutively to reaction mixture, which then was stirred for
68 h. The mixture was poured into an aqueous solution of 1 M sodium
carbonate, and the precipitate was collected by filtration and dried in
vacuo to afford a as a pure, bright orange powder. Yield: 7.13 g
(93.8%). IR (neat, cm ) 3481 m, 3466 m, 3367 s, 3355 s, 1621 m,
1573 s, 1551 s, 1504 s, 1445 m, 1395 m, 1344 s, 1323 s, 1251 s,
4
2
was prepared by reaction of [Fe(CO) ] (Strem) with Br , according
to the published procedure, but purified by recrystallization from
CH Cl at −20 °C instead of sublimation. All iron complexes were
prepared inside the glovebox under dinitrogen atmosphere in the
dark, unless otherwise indicated. High-performance liquid chromatog-
raphy (HPLC)-grade solvents were purchased from EMD, Fisher,
Macron, or J.T. Baker and dried through an alumina column sys-
5
2
25
2
2
−
1
1237 s, 1201 s, 1086 s, 935 s, 865 s, 791 m, 761 s, 727 s, 551 s, 458 m;
1
H NMR (400 MHz, CDCl ) δ = 7.93 (s, 1H), 7.55 (d, J =
3
1
3
tem (Pure Process Technology). Deuterated solvent (CDCl ) was
purchased from Cambridge Isotopes and used as received.
2.0 Hz,1H), 6.46 (s, 2H), 2.27 (s, 3H); C NMR (100 MHz, CDCl )
3
3
δ = 140.3, 140.1, 132.7, 126.6, 125.6, 112.0, 20.0; MS (ESI, m/z): 231
+
Ligand Syntheses. NNS 2-(Pyridin-2′-yl)benzothiazoline (L1).
[MH] .
The synthesis of L1 was a modified version of a published pro-
2-(2′,6′-Dimethylphenyl)-4-methyl-6-nitroaniline (b). Under
dinitrogen atmosphere, a (5.00 g, 21.64 mmol), 2,6-dimethylphenyl
boronic acid (4.00 g, 26.67 mmol), potassium carbonate (4.00 g,
28.94 mmol), 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl
31
cedure. Under dinitrogen atmosphere, 2-pyridinecarboxaldehyde
1.000 g, 9.337 mmol) was mixed with 2-aminobenzenethiol (1.167 g,
.337 mmol), upon the addition of which a pale yellow precipitate
formed immediately. Then, 2 mL of methanol was added, and
the reaction was stirred for 10 min. The methanol was decanted, and
the residue was washed with pentane. The product was collected as a
(
9
(Xphos) (309 mg, 0.649 mmol), and Pd(dba) (373 mg, 0.649 mmol)
2
were mixed in 120 mL of degassed THF and 24 mL of degassed water.
The reaction was refluxed for 24 h, after which an additional amount of
2,6-dimethylphenyl boronic acid (2.00 g, 13.34 mmol) and potassium
carbonate (2.00 g, 14.47 mmol) were added and refluxed for another
24 h. After it cooled to room temperature, THF was removed in
vacuo, and the mixture was extracted with ethyl acetate (50 mL × 3).
The combined organic phase was washed with brine and dried over
Na SO . The product was purified by column chromatography (silica
−
1
pale yellow solid. Yield: 1.930 g (96.5%). IR (neat, cm ) 3185 m,
3
1
1
7
5
1
7
7
5
1
166 m, 3067 w, 3015 w, 2953 w, 1589 m, 1577 m, 1467 s, 1434 m,
421m, 1348 m, 1306 m, 1268 w, 1250 m, 1158 m, 1146 m, 1120 m,
098 m, 1071 m, 1048 m, 1017 m, 996 w, 957 w, 911 m, 905 w,
94 m, 768 m, 745 s, 733 s, 716 s, 685 s, 646 m, 620 s, 572 w, 562 w,
29 m,478 s; H NMR (400 MHz, CDCl ) δ = 8.56 (ddd, J = 4.9, 1.8,
1
3
2
4
.0 Hz, 1H), 7.69 (ddd, J = 7.4, 1.8, 0.4 Hz, 1H), 7.58 (dtd, J =
.9, 1.1, 0.5 Hz, 1H), 7.22 (dddd, J = 7.5, 4.8, 1.2, 0.4 Hz, 1H), 7.12−
.02 (m, 1H), 7.01−6.91 (m, 1H), 6.86−6.75 (m, 2H), 6.41 (s, 1H),
gel, EtOAc/hexane = 1:8) to afford b as an orange oil. Yield: 4.33 g
−
1
(78%). IR (neat, cm ): 1716 m, 1629 m, 1567 s, 1517 m, 1442 m,
1253 m, 1230 s, 1083 w, 1031 w, 935 m, 872 m, 768 s, 709 m;
.06 (s, 1H); 13C NMR (100 MHz, CDCl ) δ = 160.4, 149.2, 146.7,
1
3
H NMR (400 MHz, CDCl ): δ 7.97 (d, J = 1.2 Hz, 1H), 7.25−7.21
3
37.4, 127.7, 125.6, 123.3, 121.9, 121.6, 121.1, 111.7, 70.1; MS (ESI,
m/z): 215 [MH] , 237 [MNa] .
(m, 1H), 7.17 (d, J = 7.3 Hz, 2H), 7.02 (d, J = 2.1 Hz, 1H), 5.81
(s, 2H), 2.30 (s, 3H), 2.04 (s, 6H); C NMR (100 MHz, CDCl ):
+
+
13
3
Me
NNS 2-(6′-Methyl-pyridin-2′-yl)benzothiazoline (L2). Under dini-
δ = 140.3, 137.5, 137.3, 135.1, 132.2, 129.7, 128.5, 128.1, 126.0,
+
+
trogen atmosphere, 6-methyl-2-pyridinecarboxaldehyde (0.985 mg,
124.5, 20.2, 20.0; MS (ESI, m/z): 257 [MH] , 279 [MNa] .
2-(2′,6′-Dimethylphenyl)-4-methyl-6-nitrobenzenethiocyanate
(c). To a solution of b (4.33 g, 16.89 mmol) in 300 mL of acetone was
8
8
.131 mmol) was mixed with 2-aminobenzenethiol (1.017 g,
.131 mmol), upon the addition of which a pale yellow precipitate
formed immediately. Then, 1 mL of methanol was added, and the
reaction was stirred for 10 min. The methanol was removed in vacuo.
added 10.7 mL of sulfuric acid (202.59 mmol). NaNO (5.82 g,
2
84.47 mmol) in 150 mL of water was added dropwise to the mixture
at 0 °C. The reaction was stirred at 0 °C for 30 min. Then, copper
thiocyante (5.34 g, 43.92 mmol) and potassium thiocyanate (16.42 g,
168.9 mmol) in 150 mL of water were added to the reaction at 0 °C.
The reaction was stirred at 0 °C for 1 h and at room temperature, for
4 h. A 10 M NaOH aqueous solution was added dropwise to adjust
the pH to ∼7, and acetone was removed under reduced pressure. The
mixture was filtered, and the filtrate was extracted with ethyl acetate
(20 mL × 3). The combined organic phase was washed with brine and
dried over Na SO . The product was purified with column chromatog-
The resulting orange oil was triturated with Et O/pentane (v/v = 1/4,
2
5
mL) and washed with pentane (3 mL), which afforded the product
−
1
as a pale yellow solid. Yield: 1.429 g (76%). IR (neat, cm ) 3136 br,
3
1
1
066 m, 3007 w, 2935 w, 2884 m, 1591 s, 1571 s, 1498 w, 1446 s,
374 w, 1305 w, 1275 w, 1251 w, 1149 m, 1114 w, 1085 m 1056 m,
032 w, 1018 m, 989 m, 808 m, 731 s, 690 m, 421 w; H NMR
1
(400 MHz, CDCl ) δ = 7.58 (t, J = 7.7 Hz, 1H), 7.37 (d, J = 7.7 Hz,
3
1
2
H), 7.11−7.05 (m, 2H), 6.99−6.93 (m, 1H), 6.80 (d, J = 7.8 Hz,
13
H), 6.38 (s, 1H), 5.12 (s, 1H), 2.55 (s, 3H); C NMR (100 MHz,
2
4
CDCl ) δ = 159.4, 158.04, 146.9, 137.5, 128.2, 125.5, 122.9, 121.8,
raphy (silica gel, EtOAc/hexane = 1:8) to afford c as an orange solid.
3
+
−1
1
2
21.6, 118.1, 111.9, 70.3, 24.5; MS (ESI, m/z): 229 [MH] ,
Yield: 1.92 g (38%). IR (neat, cm ): 2170 m (SCN), 1531 s, 1463 s,
+
1
51 [MNa] .
1379 m, 1355 s, 1081 w, 770 s, 710 m; H NMR (400 MHz, CDCl ):
3
Ph
N NS 2-(Pyridin-2′-yl)-2-phenyl-benzothiazoline (L3). Under dini-
δ 7.82 (s, 1H), 7.35 (m, 1H), 7.30 (m, 1H), 7.19 (d, J = 7.9 Hz, 2H), 2.53
1
3
trogen atmosphere, 2-benzoylpyridine (1.000 g, 5.460 mmol) was
mixed with 2-aminobenzenethiol (0.750 g, 6.000 mmol), and 10 mL
of acetic acid was added. The reaction was stirred at room temperature
for 24 h. The acetic acid was removed in vacuo. The resulting oil was
(s 3H), 2.04 (s 6H); C NMR (101 MHz, CDCl ): δ = 152.5, 147.5,
3
143.3, 137.1, 136.5, 136.0, 129.2, 128.3, 125.4, 114.9, 108.1, 21.3,
+
20.8; MS (CI, m/z): 272 [M without CN].
2-Mercapto-3-(2′,6′-dimethylphenyl)-5-methyl-aniline (d).
triturated with ethanol (3 × 3 mL), Et O (3 mL), and pentane (3 mL),
Under dinitrogen atmosphere, c (500 mg, 1.676 mmol) in 15 mL
2
which afforded the product as a pale yellow solid. Yield: 1.130 g (71%).
of dry THF was added to LiAlH (508 mg, 13.407 mmol) slowly at
4
−
1
IR (neat, cm ) 3258 m, 3058 w, 2979 m, 2875 w, 1607 w, 1582 m,
0 °C and then refluxed for 16 h. After it was cooled to 0 °C, the reac-
tion was quenched with degassed water and filtered under dinitrogen. The
residue was washed with degassed THF (5 mL) and water (2 × 5 mL).
A 1 M HCl aqueous solution was added to the filtrate until the pH was
∼3. The THF was removed in vacuo, and the compound d precipitated as
a yellow solid, which was washed with water (3 × 10 mL) and pentane
(3 × 5 mL), and used for the next step without further purification.
1
7
474 w, 1458 m, 1446 m, 1275 m, 1212 w, 1119 w, 1032 w, 904 w,
63 s, 749 s, 730 m, 714 m, 576 w, 454 w; H NMR (400 MHz,
1
CDCl ) δ = 8.67−8.51 (m, 1H), 7.84 (d, J = 7.9 Hz, 1H), 7.69 (td,
3
J = 7.7, 1.8 Hz, 1H), 7.65−7.50 (m, 2H), 7.47−7.27 (m, 2H), 7.25
(
1
6
d, J = 7.3 Hz, 1H), 7.22−7.12 (m, 1H), 7.06 (dd, J = 7.6, 1.3 Hz,
H), 6.95 (td, J = 7.6, 1.3 Hz, 1H), 6.83 (dd, J = 7.9, 1.2 Hz, 1H),
1
3
−1
.77 (td, J = 7.5, 1.2 Hz, 1H), 6.17 (s, 1H); C NMR (100 MHz,
Yield: 257 mg (63%). IR (neat, cm ): 3383 w, 3309 w, 2961 w, 2584 w
CDCl ) δ = 163.0, 148.8, 146.1, 144.5, 136.8, 128.3, 127.8, 127.1,
(S−H), 2558 w (S−H), 1620 s, 1566 s, 1453 s, 1408 m, 1375 m, 1329
3
1
1
25.6, 122.5, 121.8, 121.3, 121.2, 111.8, 84.2; MS (ESI, m/z): 291
m, 1259 s, 1084 s, 1027 s, 798 m, 768 m, 567 w; H NMR (400 MHz,
+
+
[
MH] , 313 [MNa] .
-Bromo-4-methyl-6-nitrobenzenamine (a). 4-Methyl-2-nitroani-
CDCl ): δ = 7.18 (dd, J = 8.6, 6.5 Hz, 1H), 7.13−7.08 (m, 2H), 6.58
3
2
(dd, J = 1.8, 0.7 Hz, 1H), 6.39 (dd, J = 1.8, 0.7 Hz, 1H), 4.02 (s, 2H),
+
line (5.00 g, 32.90 mmol) was dissolved in acetic acid (50 mL). Finely
ground ammonium bromide (5.16 g, 52.60 mmol) and 30 wt %
2.61 (s, 1H), 2.26 (s, 3H), 2.00 (s, 6H); MS (CI, m/z): 244 [MH] .
Ph
DMPh
N NS
2-(Pyridin-2′-yl)-2-phenyl-4-(2″,6″-dimethylphenyl)-
solution of hydrogen peroxide (in H O) (5.25 mL, 52.60 mmol) were
6-methylbenzothiazoline (L ). Under dinitrogen atmosphere,
2
B
I
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
d (257 mg, 1.056 mmol) and 2-benzoylpyridine (174 mg, 0.950 mmol)
were mixed in 5 mL of acetic acid. The reaction was stirred at room
temperature for 24 h, and a large amount of off-white precipitate was
observed. The acetic acid was removed in vacuo. The oil that resulted
was treated with pentane (30 mL), which afforded the product as an
filtrate was subjected to vapor diffusion of diethyl ether at room
temperature in air, which resulted in small dark green crystals suitable
for X-ray diffraction. Yield: 23 mg (32%). IR (neat, cm ): 1599 s,
−
1
1577 s, 1472 s, 1454 m, 1437 m, 1352 m, 1256 m, 1151 m, 1063 m,
896 w, 763 s, 709 m, 590 s, 433 m. High-resolution mass spectrometry
−
1
+
off-white solid. Yield: 233 mg (54%). IR (neat, cm ): 1577 m,
(HR-MS) (ESI+): m/z 482.0328 [M] . UV/Vis in MeCN, λ
max
−
1
−1
1
5
4
7
7
464 m, 1435 m, 1409 m, 1287 w, 1210 w, 995 w, 839 m, 760 s, 701 s,
(in nm) (ε in M cm ): 563 (3730), 413 (5230), 340 (15 400),
292 (27 800), 245 (32 500). Anal. calcd for C H FeIN S ·0.25I : C
42.85, H 2.70, N 8.33%; found: C 43.14, H 2.92, N 8.41%.
1
95 m, 551 m; H NMR (400 MHz, CDCl , 25 °C): δ = 8.53 (d, J =
3
24 18
4
2
2
.6 Hz, 1H), 7.80 (d, J = 8.0 Hz, 1H), 7.65 (td, J = 7.6, 1.6 Hz, 1H),
.50 (dd, J = 8.2, 1.0 Hz, 2H), 7.26 (s, J = 15.0 Hz, 1H), 7.20 (d, J =
.0 Hz, 1H), 7.18−7.11 (m, 2H), 7.07 (d, J = 7.7 Hz, 2H), 6.64
[(NNS) Fe]. Under dinitrogen atmosphere, 50 mg (0.23 mmol) of
2
L1 was dissolved in 2 mL of MeCN, and 40 μL (0.23 mmol) of N,N-
diisopropylethylamine was added. The mixture was stirred for 10 min.
(
(
s, 1H), 6.36 (s, 1H), 6.29 (s, 1H), 4.71 (s, 1H), 2.26 (s, 3H), 2.04
s, 3H), 1.97 (s, 3H); C NMR (101 MHz, CDCl , 25 °C): δ = 163.3,
1
3
In a separate vial, FeI (36 mg, 0.12 mmol) was dissolved in 2 mL of
3
2
1
1
8
48.7, 146.5, 144.6, 140.0, 136.6, 136.2, 136.0, 135.8, 134.3, 128.2,
27.5, 127.4, 127.3, 127.2, 126.7, 123.5, 122.3, 122.0, 121.7, 111.0,
MeCN and then added to the solution. The resultant solution was
stirred for 30 min at room temperature. The solvent was removed in
+
+
3.7, 21.3, 20.2, 20.2. MS (ESI, m/z): 409 [MH] , 431 [MNa] .
Complex Syntheses. [(NNS) Fe (CO) (I) ] (1). Under dinitrogen
vacuo, and the residue was washed with Et O, which afforded a dark
2
green powder. Yield: 43 mg (76%). The dark green crystal suitable for
2
2
2
2
atmosphere, 0.099 g (0.234 mmol) of [Fe(CO) (I) ] in a vial was
X-ray diffraction was obtained from MeCN solution subjected to
diethyl ether vapor diffusion. IR (neat, cm ): 1657 m, 1587 s, 1568 s,
1481 s, 1438 m, 1421 m, 1250 s, 1144 s, 1062 s, 1024 m, 909 m,
4
2
−
1
dissolved in 4 mL of acetonitrile as a dark red solution. In a separate
vial, 0.050 g (0.234 mmol) of L1 was dissolved in another 4 mL of
acetonitrile to generate a yellow solution. At −30 °C, the addition of
the [Fe(CO) (I) ]/acetonitrile solution to the ligand solution immedi-
834 s, 759 s, 735 s, 654 s, 600 m, 439 s. UV/Vis in MeCN, λ
(in nm) (ε in M cm ): 614 (2210), 441 (5000), 319 (12 600), 281
max
−
1
−1
4
2
ately generated a dark green solution, which was stirred at −30 °C for
(15 300).
In situ preparation of [(N NSDMPh)Fe(CO)
Ph
(Br)] (8). Under dinitrogen
was dissolved in 1 mL of
4
h. The resulting solution was filtered, and the filtrate was subjected
2
to vapor diffusion of diethyl ether at −30 °C, which resulted in small
atmosphere, 0.010 g (0.0245 mmol) of L
B
dark brown crystals suitable for X-ray diffraction. Yield: 52 mg (53%).
THF as a pale yellow solution in a vial. Then, 3.65 μL (0.0245 mmol)
of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was added to the same
vial and stirred for 20 min. In a separate vial, 0.008 g (0.0245 mmol)
−
1
IR (neat, cm ): 1941 s, 1926 s, 1601 m, 1469 m, 1456 m, 1432 m,
1
4
149 m, 777 s, 757 s, 743 s, 580 s. UV/vis in MeCN, λmax (in nm):
65, 309, 245.
of [Fe(CO)
(Br)
4
2
] was dissolved in another 1 mL of THF to generate
(Br) ]
2
Ph
[
(N NS) Fe (CO) (I) ] (2). This complex was prepared according to
a dark red solution. At −30 °C, the addition of [Fe(CO)
4
2
2
2
2
the procedure for [(NNS) Fe (CO) (I) ] (1): L3 (0.050 g,
solution to the ligand solution immediately generated a dark green
solution, which was stirred at −40 °C for 2 h. The resulting product
was not stable. After the product was subjected to vapor diffusion
of diethyl ether at −30 °C, dark green crystals suitable for X-ray dif-
2
2
2
2
0
0
.172 mmol) in 4 mL of acetonitrile and [Fe(CO) (I) ] (0.073 g,
.172 mmol) in 4 mL of acetonitrile. The dark brown crystals suitable
4 2
for X-ray diffraction were obtained from acetonitrile solution subjected
to diethyl ether vapor diffusion at −30 °C. Yield: 17 mg (21%).
Ph
DMPh
fraction were isolated, which proved to be [(N NS
) Fe] (9). The
2
−
1
13
IR (neat, cm ): 1955 s, 1932 m, 1588 m, 1573 s, 1458 m, 1442 m,
C NMR spectrum of 8 was acquired with in situ synthesized product,
and the J-Y tube was charged with CO gas and kept at −30 °C to
avoid product decomposition. (Caution! Decomposition could lead to
1
327 m, 1017 w, 745 s, 714 s, 617 s, 580 s, 568 s, 440 m. UV/vis in
MeCN, λ (in nm) = 465, 309, 245.
max
Me
−1
[
( NNS) Fe (CO) (I) ] (3). This complex was prepared according
explosion, handle with care.) IR for 8 (neat, cm ): 2036 m, 1985 m,
2
2
2
2
to the procedure for [(NNS) Fe (CO) (I) ] (1): L2 (0.050 g,
1643 s, 1581 m, 1441 m, 1321 m, 1206 w, 1063 m, 858 m, 775 m,
2
2
2
2
1
3
0
0
.219 mmol) in 4 mL of acetonitrile and [Fe(CO) (I) ] (0.092 g,
.219 mmol) in 4 mL of acetonitrile. The resulting acetonitrile
699 w. C NMR (126 MHz, THF, −30 °C): δ = 215.2, 210.9, 166.4,
163.0, 155.7, 153.8, 145.5, 143.3, 140.8, 139.4, 137.1, 136.2, 131.4,
131.2, 130.5, 128.9, 128.8, 128.4, 127.8, 127.6, 127.1, 123.8, 109.9,
20.7, 20.4.
4
2
solution was subjected to diethyl ether vapor diffusion at −30 °C,
−
1
which afforded black powder. Yield: 36 mg (38%). IR (neat, cm ):
951 s, 1927 m, 1605 m, 1471 s, 1375 m, 1329 m, 1098 m, 794 m,
66 s, 736 s, 567 s, 424 m. UV/vis in MeCN: λmax (in nm): 465, 309, 245.
(NNS) Fe (CO)]I (4). Method A. This complex was prepared
[(N NSDMPh)2Fe] (9) (Method B). Under dinitrogen atmosphere, LB
(30 mg, 0.074 mmol) in 1 mL of PhF was mixed with
N,N-diisopropylethylamine (7.0 μL, 0.040 mmol). In a separate vial,
Ph
1
7
[
3
2
according to the procedure for [(NNS) Fe (CO) (I) ], except that
FeBr (8.7 mg, 0.040 mmol) was dissolved in 1 mL of PhF and then
2
2
2
2
2
after the reaction, the filtrate was subjected to vapor diffusion of
diethyl ether at ambient glovebox temperature, which resulted in small
dark green crystals suitable for X-ray diffraction. Yield: 48 mg (69%). IR
was added to L . The solution turned dark green immediately and was
B
stirred for 1 h. The solvent was removed in vacuo, and the residue was
washed with diethyl ether and pentane. The dark green crystals suit-
able for X-ray diffraction were obtained from PhF solution subjected
to diethyl ether vapor diffusion at −30 °C. Yield: 21 mg (68%) IR
−1
(
neat, cm ): 1940 s, 1661 s, 1651 s, 1596 m, 1504 m, 1467 m, 1434 m,
384 m, 1297m, 1287 m, 1151 s, 764 m, 712 m, 572 m. UV/vis in
1
MeCN, λmax (in nm) (ε in M− cm ): 727 (4360), 627 (3790), 424
1
−1
−1
(neat, cm ): 1582 m, 1471 m, 1436 s, 1422 m, 1324 m, 1308 m,
(
8470), 317 (25 500), 244 (42 000). Note: Elemental analysis for
1275 m, 1228 m, 1153 w, 1128 w, 937 m, 743 s, 686 s, 643 s, 593 m.
MS (ESI, m/z): 870 [M] .
Stability Studies. Non-bulky Complexes. In two separate vials,
+
bulk crystalline material of 4 indicated the presence of 5 (∼25%),
which immediately forms from solutions of 4 exposed to oxygen.
Although samples of 4 were synthesized and crystallized in a drybox,
we attribute the presence of 5 (estimated ∼25%) to two factors: first,
[Fe(CO) (I) ] and the specific NNS ligand were dissolved in MeCN,
4
2
respectively. The solutions were cooled to −30 °C and then mixed
together in the dark (the concentrations of product were ∼0.5 mM).
The resultant solution was immediately transferred to an air-free
cuvette, and UV/vis spectra were acquired every 5 min.
residual O in the drybox, and second, with the identical charge
2
−
(monocation) and counterion (I ) of the two complexes. Anal. calcd
for C H Fe N OS I (4, 75%) plus C H FeIN S (5, 25%):
37
27
2
6
3
24 18
4 2
C 48.75, H 3.00, N 9.25%; found: C 46.50, H 3.73, N 9.76%.
Bulky Complexes. In two separate vials, [Fe(CO) (I) ] and L
4
2
B
Method B. The [(NNS) Fe (CO)]I was prepared according to the
ligand were dissolved in THF, respectively (DBU was added to the
ligand). The solutions were cooled to −30 °C and then mixed
together in the dark (the concentrations of product were ∼0.5 mM).
The resultant solutions were immediately transferred to an air-free
cell, and IR spectra were obtained at a rate of one per minute.
Physical Measurements. NMR spectra were collected on Varian
400 MHz spectrometer, and chemical shifts were referenced to
3
2
procedure for [(NNS) Fe (CO) (I) ], except that, after the reaction,
2
2
2
2
the filtrate was treated with several drops of DMF and subjected to
vapor diffusion of diethyl ether at −30 °C, which resulted in small
dark green crystals suitable for X-ray diffraction.
[
(NNS) Fe]I·0.25I (5). This complex was prepared according to the
2 2
procedure for [(NNS) Fe (CO)]I, except that after the reaction, the
3
2
J
DOI: 10.1021/acs.inorgchem.8b01185
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
CDCl . Solid-state infrared spectra were recorded on a Bruker Alpha
spectrometer equipped with a diamond attenuated total reflectance
3
ACKNOWLEDGMENTS
■
This research was supported by the Robert A. Welch
Foundation (F-1822), the American Chemical Society PRF
program (53542-DN13), and the UT Austin College of
Natural Sciences.
(
ATR) crystal. Mass spectra (MS) were acquired on either Thermo
Scientific TSQ (CI) or Thermo Finnigan TSQ with Dionex Ultimate
3
2
000 LC (ESI). The UV/vis absorption spectra were obtained at
98 K with an Agilent Cary 60 spectrophotometer. Microanalytical
(C, H, N) data were provided by Midwest Microlabs.
X-ray Diffraction Data Collection and Crystal Structure
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ASSOCIATED CONTENT
Supporting Information
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in the Active Site of [Fe]-Hydrogenase: Hints from a Model Study.
■
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AUTHOR INFORMATION
Corresponding Author
E-mail: mrose@cm.utexas.edu.
■
*
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ORCID
(18) Carlson, L. J.; Welby, J.; Zebrowski, K. A.; Wilk, M. M.; Giroux,
Michael J. Rose: 0000-0002-6960-6639
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The authors declare no competing financial interest.
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