Thermodynamics of Proton and Electron Transfer in Tetranuclear
Clusters with Mn–OH
2
/OH Motifs Relevant to H
2
O Activation by
the Oxygen Evolving Complex in Photosystem II
Christopher J. Reed
and
Theodor Agapie
*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Abstract
We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a
single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation
of Fe
III/II
3
Mn
II
–OH
2
clusters leads to internal reorganization resulting in formal oxidation at Mn
to generate Fe
III/II
3
Mn
III
–OH.
57
Fe M
ӧ
ssbauer spectroscopy reveals that oxidation state changes
(three for Fe
III/II
3
Mn–OH
2
and four for Fe
III/II
3
Mn–OH clusters) occur exclusively at the Fe
centers; the Mn center is formally Mn
II
when water is bound and Mn
III
when hydroxide is bound.
Experimentally determined p
K
a
(17.4) of the [Fe
III
2
Fe
II
Mn
II
–OH
2
] cluster and the reduction
potentials of the [Fe
3
Mn–OH
2
] and [Fe
3
Mn–OH] clusters were used to analyze the O–H bond
dissociation enthalpies (BDE
O–H
) for multiple cluster oxidation states. BDE
O–H
increases from
69, to 78, and 85 kcal/mol for the [Fe
III
Fe
II
2
Mn
II
-OH
2
], [Fe
III
2
Fe
II
Mn
II
-OH
2
], and [Fe
III
3
Mn
II
-
OH
2
] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of
the [Fe
3
Mn–OH
x
] system was obtained by constructing a potential–p
K
a
diagram; the shift in
reduction potentials of the [Fe
3
Mn–OH
x
] clusters in the presence of different bases supports the
BDE
O–H
values reported for the [Fe
3
Mn–OH
2
] clusters. A lower limit of the p
K
a
for the hydroxide
ligand of the [Fe
3
Mn–OH] clusters was estimated for two oxidation states. These data suggest
BDE
O–H
values for the [Fe
III
2
Fe
II
Mn
III
–OH] and [Fe
III
3
Mn
III
–OH] clusters are greater than 93
and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe
3
Mn=O] in this
and related multinuclear systems.
Graphical Abstract
*
Corresponding Author
agapie@caltech.edu.
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
Supplementary Data (PDF)
Crystallographic data files (CIFs)
The authors declare no competing financial interests.
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J Am Chem Soc
. 2018 August 29; 140(34): 10900–10908. doi:10.1021/jacs.8b06426.
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INTRODUCTION
During photosynthesis, water oxidation is catalyzed at the active site of Photosystem II
(PSII) by a CaMn
4
O
5
cluster known as the Oxygen Evolving Complex (OEC).
1
The
catalytic mechanism is outlined by the Kok cycle, with the cluster transitioning through five
distinct so-called S-states (S0, S1, ..., S4).
2
Four sequential oxidations of the cluster occur
(S
0
→ →
S
4
), followed by the O–O bond forming step, with concomitant loss of O
2
and
binding of H
2
O to complete the cycle (S
4
→
S
0
). Protons are sequentially released from the
active site during the S-state cycle; deprotonation of bound H
2
O in this stepwise manner
prevents the buildup of significant charge at the active site, facilitating the further oxidation
of the CaMn
4
O
5
cluster.
3
PSII utilizes a nearby tyrosine radical (Y
z•
) as a mediator to
transfer electrons/protons away from the OEC during turnover; because of the nature of the
tyrosine radical, proton-coupled electron transfer (PCET) of the H
2
O-derived ligands bound
to the OEC is considered to play an important role in the Kok cycle.
3
–
4
Due to the wealth of
information available in X-ray crystallographic
1b–d
,
5
, EPR
6
, and X-ray absorption
2b
,
6d
,
7
spectroscopic techniques, much is known about the Mn oxidation states and electronic
environment of the OEC in the S
0
through S
3
states of the Kok cycle. More challenging has
been understanding the precise protonation state of H
2
O ligands and relevant neighboring
amino acid residues of any S-state; computational studies of the OEC have considered a
variety of possible protonation states.
4c
,
8
Experimentally, time-resolved IR spectroscopy has
been helpful in gaining insight to the dynamics of protons at the active site during turnover.
6j
,
9
Furthermore, multiple computational models of the OEC mechanism invoke a terminal
Mn-oxo as a crucial part of the O–O bond forming S
4
intermediate
2c
,
10
. Therefore, there is
significant interest in understanding the chemistry of a Mn–OH
2
species undergoing
multiple proton and electron transfers to reach a reactive terminal Mn-oxo.
The chemistry of synthetic Mn-aquo, -hydroxo, and -oxo motifs has been a subject of
interest for inorganic chemists, particularly within the context of gaining insight into the
thermodynamic basis of Mn–OH
x
PCET reactivity and how it relates to the mechanism of
the OEC.
11
,
12
,
13
Reported mononuclear systems have been able to probe the roles of Mn
oxidation state
12c, 12d, 12i
, ligand field
11a, 11f
,
13b
, and oxygen ligand protonation state
11j
,
12j
on PCET reactions and the intrinsic O–H bond dissociation enthalpies (BDE) of Mn–OH
x
moieties. There are fewer examples of such studies with multinuclear Mn complexes, with
most of the reports examining the PCET chemistry of bridging oxo moieties
14
, as opposed to
terminally bound OH
x
ligands.
15
Most of these reports are limited to binuclear Mn
complexes or systems where only a single redox couple could be examined. The PCET
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reactivity of a synthetic Mn
III
3
Mn
IV
O
3
(OH) cubane cluster has been examined, where the
BDE
O–H
of the
μ
3
-OH could be estimated to be >94 kcal/mol; however, precise
determination of the thermodynamic bond strength was complicated by subsequent
decomposition of the protonated cubane.
14d
A report of proton and electron transfer at a
terminal Mn-OH
x
moiety with an adjacent Mn center over three oxidation states (Mn
III
2
,
Mn
III
Mn
IV
, and Mn
IV
2
) represents a very rare example of thermodynamic studies of a
terminal Mn-OH
x
in a multinuclear system.
15a
Access to a suitable synthetic platform to
interrogate the effects of
multiple
neighboring redox-active metal centers on the chemistry of
a terminal Mn–OH
x
motif may facilitate a more complete picture of the dynamics of proton
and electron transfer of the OEC leading up to its reactive S
4
state, and more generally lead
to a better understanding of the behavior of metal clusters in reactions involving water,
dioxygen, and multi-electron transformations.
Our group has demonstrated the utility of rationally-designed, well-defined molecular
clusters for probing structure-function relationships in multinuclear first-row transition metal
complexes, acting as models of of complex active sites found in biology.
16
,
17
Recently, we
have studied a family of tetranuclear Fe and Mn complexes composed of three
coordinatively-saturated metal ions bridged to a fourth (apical) metal center through
substituted pyrazolate (or phenyl-imidazolate) ligands and a
μ
4
-single atom ligand.
18
The
apical metal has a coordination site available for exogenous ligands, allowing for the study
of substrate binding and reactivity by a molecular cluster. With bulky and nonpolar phenyl
substituents in the 3 position of the pyrazolate ligands coordination of bulk-ier ligands
remains inhibited, and intramolecular ligand activation had been encountered.
18d–f
In
contrast, we have established that amino-phenylpyrazolate ligands, which are more open and
facilitate hydrogen bonding interactions, support oxo-bridged tetramanganese clusters
bearing a Mn
III
–OH moiety and are competent for catalyzing electro-chemical water
oxidation, demonstrating O-O bond formation function relevant to the OEC.
19
Here, clusters
with unsubstituted bridging pyrazolate ligands (Pz
−
) were synthesized to further promote
intermolecular reactivity between apical Mn–OH
x
groups and external acids, bases, or
hydrogen atom donors/acceptors. A heterometallic cluster composition, in this case Fe
3
Mn,
was targeted to provide a spectroscopic handle of metal oxidation states within the cluster,
via
57
Fe Mössbauer spectroscopy. The thermodynamic aspects of the PCET reactivity of
these LFe
3
O(Pz)
3
Mn(OH
x
)
n+
clusters were investigated through examination of the discrete
electron and proton transfers taking place over multiple redox states. The results herein
establish the significant influence redox changes at distal metal sites in a cluster have on a
Mn–OH
x
motif and, conversely, how this motif’s protonation state can modulate the electron
distribution between metals in the cluster.
RESULTS AND DISCUSSION
Synthesis and Characterization of Pyrazolate Bridged [Fe
3
Mn] Clusters.
The [Fe
III
2
Fe
II
Mn
II
] cluster (
2-[OTf]
) can be prepared via one-pot synthesis, starting from
previously reported
[LFe
3
(OAc)(OTf)][OTf]
complex.
18a
Sequential addition of Ca(OTf)
2
(which serves to sequester the equivalent of acetate in the starting material, to avoid mixtures
of counterions), potassium pyrazo-late, iodosobenzene (PhIO), and manganese (II)
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trifluoromethanesulfonate bis-acetonitrile solvate (Mn(OTf)
2
• 2 MeCN) allows for isolation
of the desired complex (Figure 1C; see Experimental Section in Supporting Information for
complete synthetic details).
1
H NMR and Mössbauer spectra of
2-[OTf]
are similar to our
previously reported [LFe
3
O(PhPz)
3
Mn][OTf]
2
cluster which was synthesized using sodium
phenyl pyrazolate, supporting the assignment that the apical metal is Mn (Figures S3 and
S48).
18e
The structure of
2-[OTf]
was confirmed by single crystal X-ray diffraction (Figure
S62; see Figure 1A for isostructural
1-[OTf]
); the cluster geometry is analogous to the
substituted pyrazolate and imidizolate tetranuclear clusters, with a single
μ
4
-interstitital
ligand and pyrazolates bridging each Fe center of the tri-nuclear core to the apical Mn.
18
In
the case of the previously reported clusters, the apical metal typically adopts a four-
coordinate, trigonal pyramidal geometry since the sterics of the substituted pyrazolate
ligands disfavor binding of one of the triflate counterions to the apical metal. Here, the
apical Mn is ligated by one triflate counterion with a trigonal bipyramidal geometry,
indicative of the increased steric accessibility of the apical metal with the unsubstituted
pyrazolates.
Cyclic voltammetry (CV) data of
2-[OTf]
in MeCN show a quasi-reversible oxidation at
−0.11 V, a quasi-reversible reduction wave at −0.84 V, and an irreversible reductive process
below −1.50 V (Figure S25; all potentials vs Fc/Fc
+
). The one electron reduced (
1-[OTf]
),
and one electron oxidized (
3-[OTf]
) clusters were prepared via chemical reduction/oxidation
of
2-[OTf]
with cobaltocene (CoCp
2
) and silver trifluoromethanesulfonate (AgOTf),
respectively. The X-ray crystal structures of these three compounds all have identical
coordination modes for the metal centers (Figure 1A, Figures S61–S63). Bond distances
between the metals and the
μ
4
-oxo are consistent with the redox processes taking place at the
Fe centers, with the apical Mn maintaining a +2 oxidation state across the series
1-[OTf]
–
3-[OTf]
(Table 1). Mössbauer data corroborate these oxidation state assignments, and are
similar to our previously characterized clusters (Figures S46–S49).
18a–e
Preparation of Mn–OH
2
and Mn–OH Clusters.
Binding of water to these clusters was investigated; however, the coordination of triflate to
the apical Mn complicates direct access to the Mn–OH
2
moiety for all oxidation states of the
cluster. The triflate ligand in
2-[OTf]
is sufficiently labile to allow for isolation of the Mn–
OH
2
cluster as single crystals by slow diffusion of Et
2
O into a MeCN/5% H
2
O solution of
the cluster, and its structure was confirmed via X-ray crystallography (
2-[OTf] (H2O)
;
Figure 2B). Attempts to obtain crystals of the analogous reduced Mn-OH
2
cluster (
1-[OTf]
(H
2
O)
) were unsuccessful; we postulate that the difficulty lies in poor crystallinity of the
complex, as opposed to an inability to coordinate H
2
O over triflate. Crystallization attempts
of
3-[OTf]
in MeCN/5% H
2
O solutions produced crystals of triflate coordinated clusters
(Figure S63), demonstrating the complication of preparing Mn–OH
2
clusters across these
oxidation states with the triflate counterions. The structure of
2-[OTf] (H
2
O)
displays H
2
O
coordinated to the apical Mn, with a long Mn–O distance of 2.163(6)
Ǻ
, consistent with a
Mn
II
–OH
2
assignment
13e
,
20
; furthermore, both triflate counterions are hydrogen bonding to
each proton of the Mn–OH
2
moiety through one of the sulfonate oxygens (O
aquo
–O
OTf
distances of 2.787 and 2.695
Ǻ
; Figure S63).
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To ensure that H
2
O remained coordinated to the cluster in solution, experiments were
performed in THF, a less coordinating solvent than MeCN, and triflate counterions were
replaced with the non-coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([BAr
F
4
])
anion. This was accomplished by reducing the dicationic cluster,
2-[OTf]
, with Na/Hg
amalgam in THF to obtain the neutral all M
II
cluster,
4
, as a blue solid (Figure 1C). Similar
to the related neutral phenyl pyrazolate clusters,
18a
4
is either insoluble or unstable in most
organic solvents, so its chemistry towards H
2
O was not pursued. Oxidation of
4
with 1 and 2
equivalents of Ag[BAr
F
4
] • 2 MeCN affords
1-[BAr
F
4
]
and
2-[BAr
F
4
]
, respectively (Figure
1C). The [Fe
III
3
Mn
II
] cluster,
3-[BAr
F
4
]
, was prepared by oxidation of
2-[BAr
F
4
]
with
acetyl-ferrocenium ([
Ac
Fc][BAr
F
4
]). All these clusters are highly soluble in THF and bind
H
2
O under conditions where it is present in ~100 molar equivalents (Figures S7 - S9).
Significant decomposition is observed when H
2
O concentrations above ~1000 equivalents
were used; therefore, all the studies described herein were performed on ca. 2 mM of a
cluster with
−
[BAr
F
4
] counterions in THF solution with 250 mM H
2
O, as these conditions
displayed
1
H NMR spectra consistent with complete or near complete binding of H
2
O to the
apical Mn.
Deprotonation of the Mn–OH
2
moiety in the [Fe
III
2
Fe
II
Mn
II
] cluster,
2-[BAr
F
4
]
, was
accomplished by addition of 1 equivalent of a relatively strong organic base, 1,8-
diazabicyclo(5.4.0)undec-7-ene (DBU; p
K
a
(THF) = 19.1)
21
, or by stirring a THF solution of
2-[BAr
F
4
]
over solid KOH for 30 minutes. Both reactions lead to the same species based on
the
1
H NMR and UV-Vis absorbance features, assigned to the Mn–OH cluster,
6-[BAr
F
4
]
.
Structural confirmation of this species was obtained by performing analogous reactions on
2-[OTf]
to prepare the triflate salt,
6-[OTf]
, which displays the same
1
H NMR features
(Figure S11). This species could be crystallized from MeCN solution by slow diffusion of
Et
2
O, and was characterized via X-ray diffraction (Figure 2C). The structure of
6-[OTf]
is
similar to
2-[OTf] (H
2
O)
, displaying Mn binding to the hydroxide ligand with a trigonal
bipyramidal geometry. Notably, the Mn–O distance to the hydroxide ligand is contracted by
approximately 0.3 Å relative to
2-[OTf] (H
2
O)
(1.843(9) vs 2.163(6) Å). Furthermore, the
distance of the apical Mn to the interstitial
μ
4
-O in the cluster is also shortened significantly
(1.838(8) vs 2.064(5) Å in
2-[OTf] (H
2
O)
; Table 1); both of these observations are
consistent with a Mn
III
–OH assignment.
22
The Mn–OH and Mn–
μ
4
-O distances of ~1.8
Ǻ
are similar to the bond metrics observed in our previously reported hydroxide-bound
tetramanganese cluster in the [Mn
III
2
Mn
II
2
] oxidation state.
19
There, the Mn–OH bond is
slightly longer (1.872(2) Å) due to hydrogen bonding to the pendant
tert
-butyl-phenyl-
aminopyrazolate ligands. The structural data for
6-[OTf]
are consistent to an oxidation state
assignment of [Fe
II
2
Fe
III
Mn
III
] for the cluster; corroborated by the M
ӧ
ssbauer spectrum of
6-[BAr
F
4
]
(Figure S58). Deprotonation of the Mn
II
–OH
2
in
2
to form
6
leads to
rearrangement of the redox states of the metals in the cluster to produce a Mn
III
–OH site.
Similar ‘valence tautomerizations’ have been observed in Mn
V
(O)-corrole systems, where
protonation or binding a Lewis acid to the oxo moiety leads to reversible formation of a
Mn
IV
(O–X)-(corrole-radical cation) complexes.
23
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Electrochemistry.
The electrochemistry of the [Fe
3
OMn–OH
2
] and [Fe
3
OMn–OH] clusters was investigated
by cyclic voltammetry of
2-[BAr
F
4
]
and
6-[BAr
F
4
]
. Two quasi-reversible redox events were
observed in
2-[BAr
F
4
]
: an oxidation at −0.02 V and a reduction at −0.90 V (all potentials vs
Fc/Fc
+
; Figure 3, red trace). These redox events were assigned to the [Fe
III
2
Fe
II
Mn
II
]
→
[Fe
III
3
Mn
II
] and [Fe
III
Fe
II
2
Mn
II
]
→
[Fe
III
2
Fe
II
Mn
II
] couples. Mössbauer spectra collected on
1-[BAr
F
4
]
–
3-[BAr
F
4
]
in THF with 250 mM H
2
O show that both oxidation state changes
occur at the Fe centers (Figures S50 – S54), leading to the conclusion that the apical Mn
remains divalent when bound to H
2
O across all the oxidation states observed in the CV
experiment, and only the distal Fe centers change oxidation states. The hydroxide-bound
cluster,
6-[BAr
F
4
]
, displays two oxidations: a quasi-reversible couple at −0.49 V
([Fe
III
Fe
II
2
Mn
III
]
→
[Fe
III
2
Fe
II
Mn
III
]), and an irreversible event at +0.26 V ([Fe
III
2
Fe
II
Mn
III
]
→
[Fe
III
3
Mn
III
]). A quasi-reversible reduction for the [Fe
III
Fe
II
2
Mn
III
]
→
[Fe
II
3
Mn
III
] couple
is also observed at −1.34 V. The Mössbauer spectra of
5 – 7-[BAr
F
4
]
(Figures S56 – S60)
are again consistent with redox changes occurring exclusively at Fe. Notably, no catalytic
oxidation wave is observed at high potentials for
6-[BAr
F
4
]
, in contrast to our previously
reported tetramanganese cluster bridged with ter-butyl-phenylaminopyrazolates.
19
Reasons
for this difference may be the ~100 mV negative shift in reduction potentials for the
[Fe
3
Mn–OH] clusters, along with the lower concentration of H
2
O. The lack of
electrocatalytic oxidation by
6-[BAr
F
4
]
could also suggest the importance of pendant basic
groups near the Mn
III
–OH moiety for water oxidation catalysis.
Determination of p
K
a
of H
2
O Ligand in [Fe
3
Mn
II
–OH
2
] Clusters.
The p
K
a
of the aquo-ligand bound to 2-[BAr
F
4
] was measured by mixing this cluster with
various concentrations of 1,1,3,3-tetramethyl-2-phenylguanidine (PhTMG; p
K
a
(THF) =
16.5).
21
The ratio of
2-[BAr
F
4
]
and
6-[BAr
F
4
]
was examined by UV-Vis absorbance
spectroscopy as a function of PhTMG concentration, and a p
K
a
value of 17.5 for
2-[BAr
F
4
]
was obtained (Figure S24). Analogous experiments were attempted on the oxidized aquo-
cluster,
3-[BAr
F
4
]
; however, the changes in UV-Vis spectral features upon deprotonation are
minor (Figures S21 and S23). The pKa of
3-[BAr
F
4
]
could be obtained by examining its
1
H
NMR resonances in the presence of a suitable exogenous base, 2,6-Me
2
-pyridine (Figure
S13; p
K
a
(THF) = 9.5).
21
As expected, the acidity of the [Fe
3
Mn–OH
2
] cluster increases
upon oxidation; a p
K
a
value of 9.2 was obtained for
3-[BAr
F
4
].
While a titration on the
reduced [Fe
III
Fe
II
2
Mn
II
] cluster,
1-[BAr
F
4
]
, was not conducted, we determined that its p
K
a
is significantly higher than the other clusters investigated, since it does not react with excess
DBU (Figure S7; p
K
a
(THF) = 19.1).
21
BDE
O–H
and PCET Reactivity of the Different Redox States.
The homolytic bond dissociation enthalpy of the aquo O–H (BDE
O–H
) were determined for
the three cluster oxidation states observed (
1-[BAr
F
4
] – 3-[BAr
F
4
]
) by analyzing the p
K
a
and reduction potentials of the aquo- and hydroxide-bound clusters (Figure 4). This
calculation combines the energies of the discrete proton and electron transfers involved,
along with the energy of formation of the hydrogen atom in THF (
C
; 66 kcal/mol
24
):
25
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BDE
O−H
(kcal/mol) = 1.37p
K
a
+ 23.06
E
° +
C
(1)
Therefore, summing the energy of the oxidation of
1-[BAr
F
4
]
(−0.90 V; −20.6 kcal/mol) and
the energy of deprotonation of
2-[BAr
F
4
]
(17.5; 24.0 kcal/mol) with
C
establishes an energy
of 69 kcal/mol for the formal H-atom transfer from
1-[BAr
F
4
]
to
6-[BAr
F
4
]
. Likewise, the
oxidation of
2-[BAr
F
4
]
(−0.02 V; −0.5 kcal/mol) followed by the deprotonation of
3-
[BAr
F
4
]
(9.2; 12.6 kcal/mol) leads to a BDE
O–H
of 78 kcal/mol for the aquo-ligand of
2-
[BAr
F
4
]
(formal HAT to form
7-[BAr
F
4
]
). An alternate way to determine the BDE
O–H
of
2-
[BAr
F
4
]
is from the p
K
a
of
2-[BAr
F
4
]
and the reduction potential of
6-[BAr
F
4
]
, leading to a
similar bond enthalpy of 78.7 kcal/mol. The same square scheme analysis can be done to
obtain a BDE
O–H
of 85 kcal/mol for
3-[BAr
F
4
]
. With these measured values, the p
K
a
of
1-
[BAr
F
4
]
could be estimated; a BDE
O–H
of 69 kcal/mol for
1-[BAr
F
4
]
means the enthalpy of
deprotonation for this cluster is expected to be ~34 kcal/mol (p
K
a
= 24.9), by combining this
energy with the oxidation of 5 (−1.34 V; −30.9 kcal/mol).
The BDE
O–H
values of these clusters were evaluated by investigating their proton-coupled
electron transfer (PCET) reactivity towards different organic radicals. The PCET reagents
employed were (2,2,6,6,-tetramethylpiperidin-1yl)oxyl (TEMPO; BDE
O–H
= 70 kcal/mol)
and 2,4,6-tri-tert-butylphenoxy radical (2,4,6-TBPR; BDE
O–H
= 82 kcal/mol).
25c
Formal
hydrogen atom transfer from
1-[BAr
F
4
]
to form
6-[BAr
F
4
]
could be accomplished using
either one equivalent of TEMPO or 2,4,6-TBPR, consistent with a BDE
O–H
less than 70
kcal/mol (Scheme 1; Figure S14).
2-[BAr
F
4
]
reacts with 1 equivalent 2,4,6-TBPR to form
7-
[BAr
F
4
]
, but no reaction is observed between this cluster and TEMPO, indicative of a
BDE
O–H
that is between 70 and 82 kcal/mol (Figure S15).
3-[BAr
F
4
]
does not react with
either PCET reagent, which supports the assignment of a BDE
O–H
greater than 82 kcal/mol
(Figure S16).
Potential–p
K
a
Diagram of [Fe
3
Mn–OH
x
] Clusters.
Further insight into the basis of PCET reactivity of these clusters was obtained by
investigating the effect of external bases on the reduction potentials of the aquo- and
hydroxide-bound clusters. Typically, this type of analysis is conducted under aqueous
conditions, where the reduction potentials can be measured as a function of solution pH;
data are presented as a potential–pH plot, known as a Pourbaix diagram.
26
Aqueous
Pourbaix diagrams have been helpful in understanding the speciation of a number of
molecular Ru/Mn water oxidation catalysts and related complexes.
15b
,
27
Recently, Pourbaix
theory has been applied to nonaqueous solvents, where the reduction potential of PCET will
depend on the p
K
a
of an external acid/base (and the concentration of this acid/base relative
to its conjugate base/acid at Nernstian equilibrium).
28
For the system reported here, a
potential– p
K
a
plot was constructed as a means of providing experimental support for the
aquo-ligand p
K
a
and BDE
O–H
values of
1-[BAr
F
4
]
-
3-[BAr
F
4
]
and to gain information
about the thermochemistry of PCET with the Mn–OH clusters to form a terminal Mn-oxo
moiety.
Reed and Agapie
Page 7
J Am Chem Soc
. Author manuscript; available in PMC 2018 December 03.
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Measuring the CV of
2-[BAr
F
4
]
with one equivalent of various organic bases, with p
K
a
values of their conjugate acids in THF ranging from 7.5 to 28.1, produced the potential–p
K
a
plot depicted in Figure 5 (blue points; see Table S3 and Figures S31 – S42 for individual
CVs). For relatively weak bases (p
K
a
< 10), the reduction potentials of
2-[BAr
F
4
]
do not
significantly deviate from their potentials in the absence of any base. As the strength of the
base increases, the reduction potential for the oxidation of
2-[BAr
F
4
]
is lowered as a
function of the conjugate acid p
K
a
, consistent with PCET occurring between the p
K
a
range
10–17. The data points in this p
K
a
range follow the diagonal line calculated for the
2-
[BAr
F
4
]
→
7-[BAr
F
4
]
PCET process, based on the reduction potentials of
2-[BAr
F
4
]
and
6-
[BAr
F
4
]
and the p
K
a
values of
2-[BAr
F
4
]
and
3-[BAr
F
4
]
. Observing the predicted linear
decrease in reduction potential for
3-[BAr
F
4
]
in the p
K
a
range 10 – 17 supports the p
K
a
values reported for
2-[BAr
F
4
]
and
3-[BAr
F
4
]
in Figure 4. Similar support is given to the p
K
a
of
1-[BAr
F
4
]
(24.9) when using strong bases (p
K
a
> 17.5). Evidence for the
1-[BAr
F
4
]
→
6-
[BAr
F
4
]
PCET process was observed under these conditions, again with the data roughly
matching the diagonal line with an intercept at −1.34 V and a p
K
a
of 24.9. Furthermore,
when a base was employed with a p
K
a
> 24.9, the reduction potentials observed were nearly
identical to the potentials reported for
6-[BAr
F
4
]
in the absence of any acid or base.
As expected, deviations of the data from the calculated diagonal line occur as the base p
K
a
approaches the p
K
a
of the cluster (see
2-[BAr
F
4
]
→
7-[BAr
F
4
]
around p
K
a
of 10, for
example), based on predicted potential–p
K
a
relationships for ET-PT or PT-ET reaction
mechanisms.
28
Further deviations from the predicted solid lines in Figure 5 could be due to
incompatibility of the chosen base with this system (too coordinating, electrochemically
unstable, etc.). Ultimately, inconsistencies between the potential–p
K
a
data of
2-[BAr
F
4
]
and
the BDE
O–H
values reported in Figure 4 only amount to a difference of ~3 kcal/mol, which
is a reasonable uncertainty for these bond energy determinations.
25c
Based on the PCET
reactivity of these complexes towards TEMPO and TBPR (vide supra), it is unlikely that
these BDE
O–H
values could deviate more than a few kcal/mol.
Potential–p
K
a
data were also obtained for
6-[BAr
F
4
]
in the presence of relatively strong
organic bases in attempts to observe a PCET process accessing Mn=O clusters, since this
technique has been previously useful for gaining insight into the proton and electron transfer
thermodynamics for unstable species.
28
Based on the potential–p
K
a
plot constructed in
Figure 5, PCET to form a Mn-oxo cluster could be possible at high potentials with a strong
base (top right area of the diagram). The CV of
6-[BAr
F
4
]
with one equivalent tert-
butylimino-tri(pyrrolidino)phosphorene (
t
-BuP
1
(pyrr); p
K
a
(THF) = 22.8)
21
shows no shift in
the Mn–OH cluster reduction potentials. Similarly, no change is observed with the
5
→
6-
[BAr
F
4
]
and
6-[BAr
F
4
]
→
7-[BAr
F
4
]
reduction potentials with 1-ethyl-2,2,4,4,4-
pentakis(dimethylamino)2-
λ
5
,4
λ
5-
catenadi(phosphazene) (EtP
2
(dma); p
K
a
(THF) = 28.1).
21
,
29
These experimental observations provide a lower limit to the p
K
a
values of
8-[BAr
F
4
]
and
7-[BAr
F
4
]
, respectively. With these values, the BDE
O–H
of
7-[BAr
F
4
]
and
8-[BAr
F
4
]
are predicted to be greater than 93 and 103 kcal/mol, respectively (Figure 6).
30
These
BDE
O–H
estimates are higher than Mn complexes, where these bond strengths have been
reported.
11c
,
12d, 12i, 12k
The large BDE
O–H
values for these hydroxide clusters suggest that if
these terminal oxo moieties could be accessed, they would be highly reactive. Indeed,
Reed and Agapie
Page 8
J Am Chem Soc
. Author manuscript; available in PMC 2018 December 03.
Author Manuscript
Author Manuscript
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