A Thermodynamic Model for Redox-Dependent Binding of
Carbon Monoxide at Site-Differentiated, High Spin Iron Clusters
Charles H. Arnett
,
Matthew J. Chalkley
, and
Theodor Agapie
*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Abstract
Binding of N
2
and CO by the FeMo-cofactor of nitrogenase depends on the redox level of the
cluster, but the extent to which pure redox chemistry perturbs the affinity of high spin iron clusters
for
π
-acids is not well understood. Here, we report a series of site-differentiated iron clusters
which reversibly bind CO in redox states Fe
II
4
through Fe
II
Fe
III
3
. One electron redox events result
in small changes in the affinity for (at most ~400-fold) and activation of CO (at most 28 cm
−1
for
ν
CO
). The small influence of redox chemistry on the affinity of these high spin, valence-localized
clusters for CO is in stark contrast to the large enhancements (10
5
-10
22
fold) in
π
-acid affinity
reported for monometallic and low spin bimetallic iron complexes, where redox chemistry occurs
exclusively at the ligand binding site. While electron-loading at metal centers
remote
from the
substrate binding site has minimal influence on the CO binding energetics (~1 kcal·mol
−1
), it
provides a conduit for CO binding at an Fe
III
center. Indeed, internal electron transfer from these
remote
sites accommodates binding of CO at an Fe
III
, with a small energetic penalty arising from
redox reorganization (~ 2.6 kcal·mol
−1
). The ease with which these clusters redistribute electrons
in response to ligand binding highlights a potential pathway for coordination of N
2
and CO by
FeMoco, which may occur on an oxidized edge of the cofactor.
Graphical Abstract
*
Corresponding Author
agapie@caltech.edu.
ASSOCIATED CONTENT
Synthetic procedures, characterization data and crystal structures, spectroscopic and electrochemical results, CO reactivity studies.
This material is available free of charge via the Internet at
http://pubs.acs.org
.
HHS Public Access
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J Am Chem Soc
. Author manuscript; available in PMC 2019 April 25.
Published in final edited form as:
J Am Chem Soc
. 2018 April 25; 140(16): 5569–5578. doi:10.1021/jacs.8b01825.
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INTRODUCTION
The Mo-nitrogenase enzyme mediates the multielectron reductions of N
2
1
and CO
2
at a
unique heterometallic [7Fe-9S-Mo-C-
R
-homocitrate] active site, the iron-molybdenum
cofactor (FeMoco, Figure 1a).
3
–
4
In both cases, catalysis involves an electron loading phase
prior to substrate binding, suggesting that coordination of both N
2
and CO is sensitive to the
redox level of the cofactor. While atomic level details remain elusive, binding of N
2
does not
occur until FeMoco has been reduced by at least three electrons relative to its resting state,
5
whereas only one or two reducing equivalents are required to initiate CO binding.
6
–
8
As
both a substrate and reversible inhibitor of catalysis, CO is an excellent reporter of substrate
interactions with FeMoco. While an N
2
-bound form of the cofactor has yet to be
unambiguously characterized, both terminal and bridging CO adducts of FeMoco have been
spectroscopically detected during turnover.
9
–
11
One of these intermediates has recently been
crystallographically characterized, demonstrating that CO bridges between Fe2 and Fe6.
12
Several spectroscopic and biochemical studies support a central role for these two belt iron
sites in binding of CO in several proposed intermediates,
7
,
11
,
13
as well as other substrates,
14
–
16
including perhaps N
2
.
17
Despite progress in their spectroscopic and structural characterization, no information is
currently available about the distribution of oxidation states in CO-bound forms of FeMoco.
In addition to controlling substrate access to the cofactor,
18
it has been suggested that the
local protein environment can induce some degree of valence localization within the cluster.
Notably, spatially resolved anomalous dispersion refinement of FeMoco in its resting state
revealed that the specific iron centers which have been implicated as CO binding sites lie on
a more oxidized edge of the cofactor.
19
Depending on the location of hydride accumula-tion,
which has been proposed to occur during the electron loading phase of catalysis,
10
internal
electron transfer events may be required for CO to bind at this oxidized edge.
Although clearly electron loading of FeMoco plays a key role in allowing the cofactor to
bind
π
-acids, it is challenging to untangle the effects of pure redox chemistry from
concomitant structural changes that may occur upon reduction. Moreover, the energetic
consequences of internal redox rearrangements which may accommodate substrate binding
have not been experimentally determined. Despite capturing essential structural features of
the biological system,
20
–
22
synthetic high spin iron(II/III) clusters generally lack site-
differentiation due to reliance on self-assembly strategies, complicating studies of ligand
binding at discrete reactive site(s). Furthermore, large structural changes and redistribution
of ligands often occur upon redox changes or CO binding in iron cluster models.
23
–
26
While
well-defined multimetallic systems which exhibit reactivity relevant to nitrogenase have
been reported,
21
,
27
–
38
to date there are no reported studies on the energetics of CO binding
in multiple, isostructural redox states of a synthetic, high spin iron cluster.
In order to evaluate the influence of redox chemistry on ligand binding and activation
phenomena, our group has recently developed synthetic strategies to access site-
differentiated tetranuclear clusters featuring a coordinatively unsaturated metal center.
39
–
41
Here, we report the synthesis of a redox series of high spin, site-differentiated iron clusters
which reversibly bind CO in four redox states (Fe
II
4
through Fe
II
Fe
III
3
). We observe that
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coordination of CO to both the Fe
II
2
Fe
III
2
and Fe
II
Fe
III
3
redox state of the cluster involves
an internal redox reorganization; binding of CO at the apical Fe
III
site induces an internal
electron transfer from a distal Fe
II
center. Studying the energetics of CO binding, we observe
only small enhancements (at most ~400-fold) in the affinity for CO due to pure redox
chemistry in these high spin, valence localized iron clusters, in contrast to the large
enhancements (>10
5
-fold) in
π
-acid affinity reported for monometallic and low spin
bimetallic iron complexes, where redox chemistry occurs exclusively at the ligand binding
site. Deconvoluting the effect of redox at specific sites within the cluster, we demonstrate
that electron-loading at metal centers
remote
from the substrate binding site has a relatively
small influence on the CO binding energetics. Additionally, a small energetic cost is
associated with redistribution of electrons in response to ligand binding which explains why
coordination of CO at an oxidized face of the cluster remains facile.
RESULTS AND DISCUSSION
A Redox Series of Site-Differentiated, Tetranuclear Iron Clusters.
In order to evaluate the effect of electron loading and (re)distribution on CO binding in high
spin iron clusters, we targeted the synthesis of imidazolate bridged congeners of our
previously reported
41
pyrazolate bridged iron clusters. The differences in the electronic
properties of the ligands was probed by DFT calculations (B3LYP/6–31G+(d,p)) for 3-
methylpyrazolate and 1-methylimidazolate as simplified models. The frontier orbitals of 3-
methylpyrazolate include two N-based donor molecular orbitals (MOs) of
σ
-symmetry
(HOMO-3 and HOMO-4) with respect to interactions with individual ligands. Nearly equal
contributions from atomic orbitals localized on either nitrogen atom (Figure 2A) are
observed. This is in contrast to 1-methylimidazolate where the analogous
σ
-donor orbitals
are spatially distinct, with the HOMO largely localized on C (Figure 2B). Moreover, the
energy separation between the two
σ
-donor orbitals (relative to the HOMO) is larger for 1-
methylimidazolate and, due to the lower electronegativity of C, these orbitals lie at higher
energy than those of 3-methylpyrazolate. By tuning the steric bulk of the imidazolate to
orient the ligand with its C-donors binding the apical metal, this electronic desymmetrization
of the bridging ligand was anticipated to enhance electron density of the apical metal
(relative to the distal triiron core). This electronic effect increases the propensity to oxidation
with imidazolate compared to pyrazolate ligands at the apical metal site.
The desired clusters are accessible in three steps (Scheme 1) from the triiron precursor
LFe
3
(OAc)
3
.
42
Complete acetate removal was effected by treatment of LFe
3
(OAc)
3
with an
excess of Me
3
SiOTf in dichloromethane, affording the precursor LFe
3
(OTf)
3
(
1
) with more
labile triflate ligands (Supplementary Fig. 86). Addition of 1-phenyl imidazole (PhIm-H, 3.3
equiv.) and iodosobenzene (PhIO) to a suspension of
1
in tetrahydrofuran affords the PhIm-
H coordinated species [LFe
3
O(PhIm-H)
3
][OTf]
3
(
2
, Supplementary Fig. 87). Deprotonation
of
2
with sodium hexamethyldisilazide (Na[N(SiMe
3
)
2
], 3.2 equiv.) followed by addition of
FeCl
2
affords the desired species [LFe
3
O(PhIm)
3
Fe][OTf]
2
(
3
). A single crystal X-ray
diffraction study confirms the formation of a tetranuclear iron cluster (Figure 3a), where the
bond metrics within the Fe
4
(μ
4
-O) motif are diagnostic of metal oxidation states.
39
–
41
For
the structurally homologous pyrazolate bridged clusters [LFe
3
O(PhPz)
3
Fe][OTf]
n
(n = 1–3),
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the distances between the distal, six-coordinate iron centers (Fe1, Fe2, Fe3, respectively) and
the interstitial oxygen atom (O1) elongate upon reduction (average Fe1/2/3-O1 distances:
1.96 Å for Fe
III
and 2.07 Å for Fe
II
).
41
The observation of two long (2.1480(19) and
2.093(2) Å) and one short (1.983(2) Å) bond distance between the interstitial oxygen (O1)
and the iron centers Fe1, Fe2, and Fe3 suggests a valence localized [Fe
II
2
Fe
III
] assignment
for the basal triiron core of
3
. This indicates an Fe
III
assignment for the apical Fe4 center,
consistent with its short Fe4-O1 distance (1.8128(19) Å, Supplementary Table 4).
For comparison, the isoelectronic pyrazolate bridged cluster [LFe
3
O(PhPz)
3
Fe][OTf]
2
features a significantly longer Fe4-O1 distance (1.972(2) Å), consistent with its assignment
as Fe
II
based on
57
Fe Mössbauer spectroscopy.
41
This indicates that, unlike
3
, both of the
ferric centers in [LFe
3
O(PhPz)
3
Fe][OTf]2 are localized within the basal triiron core (Fe1-
O1: 1.932(2) Å, Fe2-O1: 1.998(2) Å for [LFe
3
O(PhPz)
3
Fe][OTf]2).
41
Consistent with our
computational studies, these results demonstrate that substitution of the 3-phenyl pyrazolate
ligands by 1-phenyl imidazolate indeed makes the apical binding site more electron rich,
facilitating oxidation at Fe4. For the pyrazolate bridged clusters [LFe
3
O(PhPz)
3
Fe][OTf]
n
(n
= 1–3), oxidation of the apical Fe4 center was not observed in the absence of an additional
anionic donor.
41
In order to interrogate the effect of the imidazolate ligands on the electronic properties of the
cluster as a whole, the CV of
3
was recorded in dichloromethane (Figure 4). Three (quasi)-
reversible one electron redox events are observed at −1.013 V, −0.200 V, and +0.450 V
(Supplementary Table 1, all vs. Fc/Fc
+
). The first two electrochemical events are assigned to
the Fe
II
3
Fe
III
/Fe
II
2
Fe
III
2
(−1.013 V) and Fe
II
2
Fe
III
2
/Fe
II
Fe
III
3
(−0.200 V) redox couples.
These potentials are cathodically shifted by 286 mV and 182 mV, respectively, relative to the
analogous redox events for the pyrazolate bridged homolog [LFe
3
O(PhPz)
3
Fe][OTf]
2
(Supplementary Table 1),
41
demonstrating the enhanced donor properties of 1-phenyl
imidazolate relative to 3-phenyl pyrazolate (Figure 5). The final quasi-reversible
electrochemical event at +0.450 V is assigned to the Fe
II
Fe
III
3
/Fe
III
4
couple. Notably, the
corresponding oxidation was not observed in the CV of [LFe
3
O(PhPz)
3
Fe][OTf]
2
at
potentials up to 1 V. However, the CV of [LFe
3
O(PhPz)
3
Fe][OTf]
2
in dichloromethane
exhibits an additional reduction at −1.733 V assigned to the Fe
II
4
/Fe
II
3
Fe
III
redox event.
41
At similar potentials, the CV of
3
exhibits a large reductive wave (Supplementary Fig. 46),
suggesting that the all-ferrous cluster reacts with dichloromethane. Notwithstanding, the
Fe
II
4
/Fe
II
3
Fe
III
redox event becomes (quasi)-reversible (−1.868 V) when the CV of
3
is
recorded in tetrahydrofuran (Supplementary Fig 49).
Consistent with its electrochemical behavior, treatment of
3
with [Fc][OTf] in
dichloromethane affords a new paramagnetic species which, following crystallization, was
structurally characterized as [LFe
3
O(PhIm)
3
Fe][OTf]
3
(
4
). Addition of Cp
2
Co to a solution
of
3
in dichloromethane cleanly affords the reduced species [LFe
3
O(PhIm)
3
Fe][OTf] (
5
).
Further reduction of
5
with sodium napthalenide (Na[C
10
H
8
]) in tetrahydrofuran affords an
insoluble blue powder, assigned as the all-ferrous cluster, [LFe
3
O(PhIm)
3
Fe] (
6
), on the
basis of Mössbauer spectroscopy (Supplementary Fig. 75).
44
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The solid-state structures of
4
and
5
demonstrate that the basic geometric features of
3
are
maintained throughout the redox series (Supplementary Fig. 90 and 91), where the bond
metrics within the Fe
4
(μ
4
-O) motif reveal the primary locus of redox chemistry
(Supplementary Table 4). Oxidation of
3
to
4
results in a significant contraction of the Fe3-
O1 distance from 2.092(2) Å to 1.983(4)Å, consistent with oxidation within the basal triiron
core. Conversely, reduction of
3
to
5
results in an elongation of the Fe4-O1 distance from
1.8128(19) Å to 1.883(4) Å, suggesting reduction of the apical iron from Fe
III
to Fe
II
. The
insolubility of
6
precludes structural characterization.
The crystallographic assignment of redox distributions in
3
-
6
are further corroborated by
their zero field
57
Fe Mössbauer spectra. The 80 K Mössbauer spectrum of
3
(Supplementary
Fig. 64) was best fit with four quadrupole doublets, corresponding to four inequivalent iron
centers. Two quadrupole doublets with isomer shifts of 1.03 mm/s and 1.14 mm/s (|ΔE
Q
| of
3.13 mm/s and 3.22 mm/s, respectively) are characteristic of six-coordinate, high spin
ferrous centers, while the quadrupole doublet with an isomer shift of 0.39 mm/s (|ΔE
Q
| =
0.37 mm/s) is consistent with the presence of one octahedral ferric ion.
39
–
41
This results in
an assignment of the core oxidation level as [Fe
II
2
Fe
III
], which is identical to that inferred
from the solid state structure. The remaining quadrupole doublet, with an isomer shift of
0.19 mm/s (|ΔE
Q
| = 1.11 mm/s), is attributed to the apical iron. Similar parameters have been
observed for four coordinate, high spin ferric centers.
20
Compared to the spectrum of
3
, the relative intensity of the diagnostic basal core Fe
II
resonance near 3 mm/s decreases in
4
, consistent with oxidation within the triiron core. The
spectrum of
4
was best fit with four quadrupole doublets with parameters indicating the
presence of only one six-coordinate, high spin ferrous center, maintenance of the apical,
high spin Fe
III
, and two high spin, six-coordinate ferric centers (Supplementary Fig. 72).
Conversely, upon reduction of
3
to
5
, there is no change in the relative intensity of the
Lorentzian feature near 3 mm/s (Supplementary Fig. 74). Instead, a substantial change in the
isomer shift of the quadrupole doublet assigned to the apical iron is observed (
δ
= 0.19 mm/s
in
3
vs.
δ
= 0.89 mm/s in
5
), suggesting one electron reduction at Fe4.
Electronic Structure of 3.
In order to confirm the high spin assignment of the apical, four-coordinate Fe
III
centers of
3
and
4
inferred by Mössbauer spectroscopy, additional spectroscopic studies were
undertaken, with a focus on
3
which features the shortest Fe4-O1 bond length. To assess the
nature of the exchange coupling and the spin ground state, variable temperature (VT)
magnetic susceptibility and variable temperature-variable field (VTVH) magnetization data
were collected. The VT magnetic susceptibility data for
3
obtained between 1.8 K and 300 K
at 0.1 T (Figure 6a) indicate overall ferromagnetic coupling and an
S
= 4 spin ground state.
A plateau in the susceptibility is observed between 10–20 K at a value of ~9.1 cm
3
K mol
−1
which decreases gradually to 6.4 cm
3
K mol
−1
at 300 K. Below 10 K, a drop in
χ
M
T is also
observed, likely a result of zero-field splitting. The susceptibility data for
3
was fit between
1.8 and 300 K according to the spin Hamiltonian
H =
Σ
{D(S
z,i
2
−1/3(S
i
(S
i
+1)+g
μ
B
S
i
·H)} –
2J
ij
(S
i
·
S
j
)
. A satisfactory simulation of the experimental data is obtained assuming all metal
centers are
locally
high spin with isotropic exchange constants:
J
14
= −29.2 cm
−1
,
J
24
=
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−63.9 cm
−1
,
J
34
= −28.8 cm
−1
,
J
12
=
J
23
= −8.2 cm
−1
and
J
13
= −9.5 cm
−1
(for additional
details see Supplementary Fig. 61). From these simulated parameters, the observed
ferromagnetic behavior may be rationalized. Strong antiferromagnetic interactions of the
apical Fe
III
(Fe4) with each of the metal centers of the triiron core (|
J
apical-core
| ≥ 3|
J
core-core
|)
results in ferromagnetic alignment of the spins on Fe1/Fe2/Fe3 at low temperatures,
affording an
S
= 4 ground state.
Consistent with this spin coupling scheme, VTVH magnetization data collected between 1.8
and 9 K at fields of 1 to 7 T (Figure 6b) were well simulated with the system spin
Hamiltonian
H = DS
z
2
+ E(S
x
2
+ S
y
2
) + gμ
B
S·H
. Due to the presence of zero field splitting,
the VTVH magnetization data for
3
saturates near 5.4μ
B
at 1.8 K and 7 T, below the
expected M =
gS
limit for
g
= 2.0. However, the experimental data is well reproduced
assuming an
S
= 4 ground state with
g
= 2.00,
D
= −3.65 cm
−1
, and
|E/D|
= 0.33. Consistent
with its assignment as a non-Kramer’s system with
D
< 0,
45
the Mössbauer spectrum of
3
at
2.3 K exhibits pronounced magnetic hyperfine splitting with well-resolved features between
−7 and 8 mm/s in an applied field of only 50 mT (Supplementary Fig. 66). The parallel
mode EPR spectrum of
3
in a propionitrile/butryonitrile (4:5) glass exhibits a sharp feature
with
g
~17.2 at 4.5 K which is assigned to a transition within the
Ms
= +/− 4 doublet
(Supplementary Fig. 63).
CO Binding Equilibria of 3.
Having confirmed the high spin assignment of the apical Fe
III
center in
3
, we explored its
reactivity with CO (Figure 7). In this regard, variable temperature IR spectroscopy indicated
the formation of both mono- (
3
-
CO
) and dicarbonyl (
3
-
(CO)
2
) adducts of
3
(Supplementary
Fig. 18). The IR spectrum of
3
measured at 195 K in CO-saturated dichloromethane
following an Ar purge exhibited an intense feature at 1944 cm
−1
(
3-CO
) in addition to
weaker features at 2014 cm
−1
and 1960 cm
−1
(
3-(CO)
2
). Warming the solution to 273 K
with stirring under Ar results in loss of the features at 2014 and 1960 cm
−1
and a decrease of
intensity at 1944 cm
−1
. Upon further warming to room temperature, no CO vibrational
features were observed.
The temperature dependent formation of both
3-CO
and
3-(CO)
2
was confirmed by
1
H-
NMR studies. Cooling solutions of
3
in either dichloromethane-
d
2
(Supplementary Fig. 27)
or acetone-
d
6
(Supplementary Fig. 37) under an atmosphere of CO from room temperature
initially affords
3-CO
as the major species, though an additional species simultaneously
grows in. Further cooling results in the loss of
3-CO
and complete conversion to this more
asymmetric species, assigned as
3-(CO)
2
. Confirmation of this assignment was obtained by
crystallization from solutions of
3
at low temperature under an atmosphere of CO, which
afforded crystals of
3-(CO)
2
suitable for XRD. The solid state structure of
3-(CO)
2
confirms
that both CO ligands bind Fe4 (Figure 3c). Warming solutions of
3-(CO)
2
from 198 K back
to room temperature confirms that these temperature dependent CO binding events are fully
reversible.
In the absence of redox reorganization, binding of CO by
3
would afford an apical Fe
III
-CO
unit in
3-CO
(Table 1). However, with few exceptions,
46
–
49
Fe
III
centers generally display
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no affinity for CO.
50
Alternatively, we envisioned that an internal electron transfer (
i
-ET)
from a distal Fe
II
site might accommodate coordination of CO (Figure 7a). Based on the
diagnostic features associated with the basal core Fe
II
centers in these clusters
39
–
41
,
Mössbauer spectroscopy serves as a convenient tool to determine whether redox
reorganization accompanies CO binding.
51
The zero field Mössbauer spectrum (80 K)
obtained by freezing a CO-saturated solution of
3
in 2,6-lutidine (
f.p.
= −5 °C) reveals a
significant loss of basal Fe
II
intensity (Supplementary Fig. 77). The spectrum can be
satisfactorily fit to a mixture of
3-(CO)
n
(61%) and
3
(39%) (Supplementary Fig. 78). The
Mössbauer spectrum of
3-(CO)
n
(Figure 7b, bottom) obtained following subtraction of
residual
3
reveals a single quadrupole doublet (25% total iron) with an isomer shift near 1
mm/s (
δ
= 1.05 mm/s, |ΔE
Q
| = 3.22 mm/s), indicating the presence of a single core ferrous
center and a change in the core redox level from [Fe
II
2
Fe
III
] to [Fe
II
Fe
III
2
] following binding
of CO. The simulated Mössbauer parameters associated with the apical iron center of
3-
(CO)
n
(
δ
= 0.10 mm/s, |ΔE
Q
| = 3.22 mm/s) are consistent with the formation of an S = 1
trigonal bipyramidal Fe
II
-CO complex following internal electron transfer (Supplementary
Table 3).
52
In contrast to the well-defined reactivity of
3
, reactions of CO with synthetic,
high spin iron(II/III) clusters typically result in cluster fragmentation and the formation of
reduced, low spin iron carbonyl clusters,
23
–
24
further illustrating the advantages of
employing robust ligand scaffolds to interrogate chemistry relevant to nitrogenase.
29
,
36
Reversible CO Binding Across Four Redox States.
Encouraged by the reactivity of
3
with CO, we investigated the dependence of CO binding
on the redox state of the cluster. Remarkably, binding of CO remains reversible for
4
–
6
.
Cooling solutions of
4
in dichloromethane-
d
2
under an atmosphere of CO affords
4-CO
(Supplementary Fig. 31), an assignment confirmed by the observation of a single CO
stretching frequency (
ν
CO
= 1966 cm
−1
) in its IR spectrum (CO-saturated dichloromethane
at 195 K, Figure 7c). Oxidation of
4
with [N(C
6
H
4
Br-4)
3
][OTf] in dichloromethane-
d
2
affords the all-ferric cluster [LFe
3
O(PhIm)
3
Fe][OTf]
4
(
7
), whose
1
H-NMR (Supplementary
Fig. 44) and UV-Vis (Supplementary Fig. 20) spectral features are identical under N
2
or CO,
suggesting that at least one Fe
II
center is necessary for CO binding.
Under an atmosphere of CO,
5
converts predominately to [LFe
3
O(PhIm)
3
Fe(CO)][OTf] (
5-
CO
) at room temperature based on IR (
ν
CO
= 1916 cm
−1
, Supplementary Fig. 14) and
1
H-
NMR (Supplementary Fig. 12) spectroscopy. Further cooling converts
5-CO
to
5-(CO)
2
(Supplementary Fig. 34), which exhibits diagnostic features at 1994 and 1944 cm
−1
in its
low temperature IR spectrum (CO-saturated dichloromethane at 195 K, Figure 7c). By
1
H-
NMR spectroscopy, heating
5-CO
under CO in chlorobenzene-
d
5
(Supplementary Fig. 41)
or exposure to an atmosphere of N
2
returns
5
, demonstrating that binding of CO is
reversible. Single crystals of
5-CO
amenable to XRD were obtained from solutions of
5
under CO and confirm its identity as a monocarbonyl adduct featuring a trigonal bipyramidal
coordination environment at Fe4 (Figure 3b).
Unfortunately, the insolubility of
6
precludes direct solution monitoring of its reactivity with
CO. However, changes in the ATR-IR spectrum following addition of an atmosphere of CO
to a suspension of
6
in tetrahydrofuran supports the formation of both mono- (
6-CO
,
ν
CO
=
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1899 cm
−1
) and dicarbonyl (
6-(CO)
2
,
ν
CO
= 1980 and 1891 cm
−1
) adducts (Supplementary
Fig. 17). The formation of these CO bound species is reversible; removing the CO
atmosphere results in gradual loss of the Fe-CO stretching frequencies for both
6-CO
and
6-
(CO)
2
and formation of an insoluble blue material with IR spectral features indicative of
6
.
For the monocarbonyl complexes described herein, shifts in
ν
CO
of only 20–30 cm
−1
are
observed per redox event (
6-CO
: 1899 cm
−1
,
5-CO
: 1916 cm
−1
,
3-CO
: 1944 cm
−1
,
4-CO
:
1966 cm
−1
, Figure 5c). These shifts are similar in magnitude to those which arise from
remote
redox chemistry in related tetranuclear iron nitrosyl clusters
39
,
41
and are significantly
smaller than expected for redox chemistry centered at the Fe-CO unit (~100 cm
−1
per redox
event).
48
–
49
,
53
,
54
Moreover, the observed Fe-CO stretching frequencies are within the range
reported for other trigonal bipyramidal Fe
II
monocarbonyl complexes (see Supplementary
Table 3). Combined with the observation of a change in the core redox level of
3
by
Mössbauer spectroscopy,
55
these results suggest an Fe
II
-CO assignment across the redox
series (
3-CO
to
6-CO
). This implies that coordination of CO induces an internal electron
transfer from one of the distal Fe
II
centers to the apical Fe
III
site in both
3
and
4
. Ligand-
induced redox reorganizations (LIRR) related to those observed for
3
and
4
have been
reported for monometallic compounds featuring redox active supporting ligands,
56
–
59
as
well as complexes with pendant ferrocenyl substituents.
60
–
61
Notwithstanding, we are not
aware of precedence for a reversible, internal electron transfer involving metal centers within
a multinuclear cluster which is induced by small molecule binding. Changes in the identity
of an ancillary ligand (DMF, MeCN, or
-
CN) have been shown to modulate the extent of
valence delocalization in a series of hexairon clusters.
62
However, the site-differentiated
nature of the clusters examined here allows us to distinguish the effects of CO binding on
the electronic properties of the binding site from those on
remote
metal centers.
CO Binding Energetics.
In order to quantify the effect of redox chemistry on the affinity of
3
–
6
for CO, we evaluated
their CO binding energetics by
1
H-NMR spectroscopy, which facilitated accurate
identification of speciation in the reaction mixtures.
63
At 303 K, the CO binding constant for
3
(
K
1
(
3
) = 0.15 atm
−1
, dichloromethane-
d
2
,
P
CO = 1 atm.) is at least 10
3
-fold lower than for
most Fe
II
complexes (Table 2),
64
–
66
though a sterically encumbered, trigonal monopyrimidal
Fe
II
complex with a similar affinity for CO (
K
298K
= 6.9 atm
−1
) has been reported.
67
The
thermodynamic parameters associated with the formation of
3-CO
(Δ
H
= −13.6(8) kcal·mol
−1
, Δ
S
= −48(3) cal·mol
−1
·K
−1
) suggest that this low CO affinity derives from an unusually
large entropic penalty, which we attribute to loss of rotational freedom in the flanking aryl
substituents upon CO binding. While a complete study on the energetics of forming
5-CO
in
dichloromethane (
b.p.
= 39.6 °C) was not possible due to temperature constraints, at 303 K
the affinity of the apical Fe
II
of
5
for CO (
K
1
(
5
)) was determined to be 59 atm
−1
, an
enhancement of only ~400-fold (ΔΔ
G
303K
~ 3.6 kcal·mol
−1
) relative to
3
, which features an
apical Fe
III
.
In contrast to the relatively small difference in the CO affinities of
3
and
5
(~400-fold),
significantly larger enhancements (>10
5
-fold) in binding affinities have previously been
reported to accompany 1e
-
redox chemistry (Table 2). For example, reduction of a square
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planar iron tetraphosphine complex from iron(II) to iron(I) and then to iron(0) results in
successive ~10
22
-fold and 10
5
-fold enhancements in its affinity for N
2
.
68
Reduction of a low
spin (N
2
)Fe
II
(μ-H)
2
Fe
II
complex to its valence-delocalized (N
2
)Fe
I
(μ-H)
2
Fe
II
congener
results in a 10
6
-fold enhancement in its affinity for a second molecule of N
2
.
35
Notably,
computational studies revealed that the SOMO of both (N
2
)Fe
I
(μ-H)
2
Fe
II
and (N
2
)Fe
I
(μ-
H)
2
Fe
II
(N
2
) complexes are valence-delocalized, suggesting that minimal redox
reorganization accompanies N
2
binding, and the large effect on binding is due to the formal
difference in oxidation state at the N
2
binding site.
The small influence which reduction of
3
to
5
has on the CO binding energetics seems
inconsistent with the low affinity Fe
III
typically exhibits toward CO
50
and the large changes
in binding affinity seen in other systems upon 1e
-
reduction. We propose instead that the
internal electron transfer (
i
-ET) which accompanies coordination of CO to
3
facilitates this
otherwise unfavorable binding event. From this perspective,
3
contains a masked apical Fe
II
site whose affinity for CO is modulated relative to
5
by two terms, one accounting for the
energetic cost of redox reorganization and the other for the effect of changes in redox states
of the remote metals (Figure 8). Although our data for these and related clusters
39
–
41
,
69
–
70
is
most consistent with a valence-localized assignment, an analogous scheme can be
constructed for a valence-delocalized system, where the internal electron transfer (
i
-ET)
term is replaced by a term accounting for the energetic penalty of trapping an electron at
Fe4, assuming the CO bound product is valence-localized.
Despite the simplicity of this thermodynamic model, it adequately accounts for trends in the
energetics of CO binding in
3
-
5
. The difference in enthalpy (ΔΔ
H
) for the second CO
binding event in
3
and
5
, the formation of
3-(CO)
2
and
5-(CO)
2
, respectively, is only 0.9(6)
kcal·mol
−1
. This small
ΔΔ
H
reflects the relatively small influence that the redox states of the
remote
metal sites have on CO binding in these high spin, valence-localized iron clusters in
the absence of redox reorganization. In contrast, the first CO binding event for
3
and
5
, the
formation of
3-CO
and
5-CO
, respectively, has a larger ΔΔ
H
(3.6 kcal·mol
−1
). Assuming
that changes in the redox state of the
remote
metals have an effect on CO binding similar to
that observed in the dicarbonyl series (~1 kcal·mol
−1
), the redox reorganization penalty must
be on the order of 2.6(6) kcal·mol
−1
(
RRE
= -nFΔ
E
, Δ
E
~ 110 mV).
Standard state: 1 atm. CO unless noted otherwise. a. Data taken from ref. 64,
K
303K
calculated from ln
K
= Δ
S
/R - Δ
H
/RT. b. Data taken from ref. 67.
K
measured at 298 K. c.
Data taken from ref. 68.
K
measured directly or determined electrochemically at 298 K. d.
Data taken from ref. 35. Standard state: 1 M CO.
K
(M
−1
) measured directly or determined
electrochemically at 298 K. e. Calculated at 303 K from ln
K
= Δ
S
/R - Δ
H
/RT. See
Supplementary Tables 6–7 for measured values. f. Estimated from ΔΔ
G =
3.6 kcal·mol
−1
assuming ΔΔ
S
~ 0 cal·mol
−1
·K
−1
for
3
vs.
5
. g. The smaller entropic penalty for the
formation of the dicarbonyl adducts suggests that CO binding is cooperative, perhaps due to
rotational “locking” of the aryl substituents upon formation of the corresponding
monocarbonyl adducts. Note, independent measurements for
3
in acetone-
d
6
and
5
in
chlorobenzene-
d
5
confirm the observed trends. Despite a 1500-fold difference in the CO
affinity of
5
compared to
3
at 298 K, the CO binding constants of
5-CO
and
3-CO
differ by
a factor of only ~1.6. For additional details, see the Supporting Information.
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