Redox-inactive metals modulate the reductionpotential in heterometallic manganese–oxidoclustersEmily Y. Tsui1, Rosalie Tran2, Junko Yano2 and Theodor Agapie1*
Redox-inactive metals are found in biological and heterogeneous water oxidation catalysts, but, at present, their rolesin catalysis are not well understood. Here, we report a series of high-oxidation-state tetranuclear-dioxido clusterscomprising three manganese centres and a redox-inactive metal (M). Crystallographic studies show an unprecedentedMn3M(m4-O)(m2-O) core that remains intact on changing M or the manganese oxidation state. Electrochemical studiesreveal that the reduction potentials span a window of 700 mV and are dependent on the Lewis acidity of the second metal.With the pKa of the redox-inactive metal–aqua complex as a measure of Lewis acidity, these compounds demonstrate alinear dependence between reduction potential and acidity with a slope of ∼100 mV per pKa unit. The Sr21 and Ca21
compounds show similar potentials, an observation that correlates with the behaviour of the oxygen-evolving complex ofphotosystem II, which is active only if one of these two metals is present.
Redox-inactive metal ions are critical components in manybiological electron transfer reactions1,2. For example, Ca2þ isessential for activity in the oxygen-evolving complex (OEC) of
photosystem II (PSII), although its exact role in catalysis remainsunclear3–5. There have been numerous studies of electron transferto synthetic organic substrates1,6, but fewer studies of the electrontransfer to metal–oxo complexes relevant to the active sites of anumber of metalloenzymes, possibly due to the challenge of isolatingcomplexes with bound metal ions. Recently, a non-haem FeIVOcomplex with a bound Sc3þ ion was characterized crystallo-graphically, and it was found that the presence of Sc3þ or Ca2þ insolution allowed the two-electron reduction of the complexusing ferrocene7. The addition of other Lewis-acidic metal ions to adifferent FeIVO compound greatly enhanced electron transfer rates,although the adducts were not structurally characterized8. With amonomeric MnII complex, faster rates of oxygen reduction wereobserved in the presence of Ca2þ, and a MnIII 2 (m-OH) 2 CaII
complex was isolated as the product9. Substitution of Sr2þ for Ca2þ
in this complex showed a similar reduction potential, whilesubstitution with Ba2þ resulted in a more negative reductionpotential10. Oxygen atom transfer from some manganese–oxo com-plexes has also been promoted by the binding of redox-inactivemetals such as lithium11.
Redox-inactive metal ions also play a role in non-biological elec-tron transfer reactions such as water oxidation catalysed by hetero-geneous cobalt12–14 and manganese oxides15–17 containing alkali oralkali earth metals. In these examples, there has been speculationthat the redox-inactive metal is associated with the transition-metal catalyst in cubane-like structures reminiscent of the crystallo-graphically determined structure of the OEC, an oxide-bridgedCaMn4 cluster in which the calcium centre is associated withthree of the manganese centres in a cubane motif (Fig. 1)18,19. Arecent study varying the redox-inactive metal (Kþ, Ca2þ, Sr2þ,Mg2þ) in layered heterogeneous manganese oxides showed that,as in the OEC, the presence of Ca2þ allows for the highest catalyticactivity15. Additionally, heterogeneous mixed oxides of cobalt show
different water oxidation behaviour that is dependent on the natureof the redox-inactive metal present in the mixture20. The role of theredox-inactive metal on electron transfer and catalysis within thematerial remains unclear in these systems.
Our recent report of a structural model of the CaMn3 subsiteof the OEC containing a high-oxidation-state heterometallicMnIV
3CaO4 moiety, and comparison to a tetramanganese analogue,suggests a significant influence of the calcium centre on the redoxproperties of the cluster21. To study the scope and chemical basisof this phenomenon in multimetallic oxide clusters such as thosefound in the OEC and in heterogeneous systems, access to well-defined and structurally related heterometallic oxido clusters ofredox-active and inactive metals is desirable. Based on a multinucleat-ing ligand-based synthetic strategy developed by our group22–24,we targeted heteronuclear clusters supported by the hexapyridyl tris-alkoxido 1,3,5-triarylbenzene ligand (L). In the present work, wedescribe the synthesis of a series of tetranuclear heterometallic tri-manganese dioxo clusters [Mn3M(m4-O)(m2-O)] containing aredox-inactive cation bridging via oxido moieties to manganesecentres. Electrochemical characterization reveals that large changes
CoO
Mn
O
MnO
Mn
M
O
Co
Co
M
O
O
OO
OO
O
O
Co
CoM
Co
Co
Mn
O
O
Mn
O
MnO
Mn
M
OMn
O
Figure 1 | Proposed structures of water oxidation catalysts containing
redox-inactive metals (M) in the OEC (left, middle) and in heterogeneous
cobalt oxide water oxidation catalysts (right)12,43. The OEC is known to
contain a Mn3M core: one major model is based on extended X-ray
absorption fine structure (EXAFS) and electron paramagnetic resonance
(EPR) studies (left)44–46 and one on X-ray crystallography (middle)18,19. Bold
bonds emphasize the Mn3M and Co3M cluster cores.
1Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard MC 127-72, Pasadena, California 91125,USA, 2Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. *e-mail: [email protected]
ARTICLESPUBLISHED ONLINE: 3 MARCH 2013 | DOI: 10.1038/NCHEM.1578
in the Lewis acidity of the redox-inactive metal have a systematic effecton the redox properties of the cluster.
Results and discussionHeterometallic clusters have been targeted by several groups as pro-posed structural models of the OEC as evidence has emerged that itcontained a mixed Mn–Ca–oxido cluster25–31. Calcium–manganeseclusters remain uncommon, although recently a number ofsuch complexes have been isolated and structurally character-ized9,21,26–31. Because heterometallic clusters are often synthesizedby self-assembly, controlling the composition and relative arrange-ment of metals has been a challenge. Furthermore, these syntheticprotocols are not necessarily extended easily to the incorporationof other redox-inactive metals instead of calcium. To develop
general syntheses of heterometallic clusters, we used a multinu-cleating ligand that affords versatile trimetallic (MII
3) precursors23.These MnII
3 species could be elaborated to site-differentiated tetra-manganese cubanes, Mn4O4, as well as to heteronuclear Mn3CaO4clusters21. Strategies for general synthetic protocols to related het-eronuclear complexes were then explored.
A dicationic calcium-bridged hexamanganese complex([LMnIII
2MnIIO(OAc)3]2Ca(OTf)2) in which each trimanganeseunit is coordinated by a m3-oxide was prepared and proposed to bean intermediate in the synthesis of LMnIV
3CaO4(OAc)3(THF)21.In an effort to isolate other manganese clusters of high oxidationstate relevant for the preparation of structural mimics of the OEC,[LMnIII
2MnIIO(OAc)3]2Ca(OTf)2 was treated with Ca(OTf)2 andPhIO in 1,2-dimethoxyethane (DME) to form a red-purple
O
OO
O
Mn
MnN
NN
NN
N
O
O
O
Me
Ca
O
O
Me
OMe
OO
O
MnMn
Mn
N
NN
NN
N
O
O
O
MeO
Me
O
O Me
OO
O
Mn Mn
Mn
N
NN
NN
N
OAcO
AcO O
Me
0.5 equiv. Ca(OTf)2
PhIO
{[LMnIIMnIII2O(OAc)3]2Ca}2+
2+
NaOTfPhIO
DME
[1-M(DME)(OTf)]2+ (M = Ca, Sr)or [2-Y(DME)(OTf)]2+
M(OTf)n (1.5 equiv.)PhIO (2 equiv.), DME
(M = Ca, Sr, Y)
2+
THF
0.5
LMnII3(OAc)3
OO
O
Mn Mn
MnN
NN
NN
N
O
NaO
OO
Me
O
OMe
4+
OO
O
MnMn
MnN
NN
NN
N
O
NaO
OO
Me
O
OMe
3+
(M = Ca)
[1-Na]24+
[1-Zn(CH3CN)]3+
Zn(OTf)2
CH3CN
Ca(OTf)2
PhIODME
OO
O
Mn Mn
MnN
NN
NN
N
O
MO
OTfO
O
O
OO
Me
Me+
[2-M(DME)(OTf)]+ (M = Ca, Sr)
Cp *2 Fe
AgOTf
OH
N
N HO
N N
HON
H3L
OHHO
HO
N
NN
NN N
1 = LMnIVMnIII2O2(OAc)2 2 = LMnIII
3O2(OAc)2
Me
Me
O
OO
O
Mn Mn
MnN
NN
NN
N
O
MO
O
TfO
O
O
OO
Me
Me
Me
Me
O
OO
O
Mn Mn
MnN
NN
NN
N
O
ZnO
O
NCCH3
O
OMe
Me
[1-Ca(OH2)3]3+
3+
O
OO
O
Mn Mn
MnN
NN
NN
N
O
CaO
O
H2O
O
O
OH2
OH2
Me
Me
DME
H2O
O
O
Mn
N
Figure 2 | Synthesis of tetrametallic trimanganese dioxido complexes. The reduced precursor LMn3(OAc)3 was oxidized in the presence of metal triflate
salts to form [LMn3MO2(OAc)2] compounds (M¼Na, Ca, Sr, Y). Complex [1–Ca(DME)(OTf)]2þ (1¼ LMnIVMnIII2O2(OAc)2) undergoes reversible chemical
reduction, substitution of Zn2þ for Ca2þ, or exchange of the DME ligand for coordinated water molecules.
compound (Fig. 2). The same compound was also independentlysynthesized in high yield (84%) in one step from the morereduced LMnII
3(OAc)3 precursor (Fig. 2). A single-crystal X-ray dif-fraction (XRD) study of this species confirmed the material to be acalcium trimanganese dioxo complex ([1–Ca(DME)(OTf)]2þ (1¼LMnIVMnIII
2O2(OAc)2), Fig. 3). The metal oxidation states wereassigned based on crystallographic, X-ray absorption spectroscopy(XAS) and magnetism data (vide infra). In this complex, as inLMnII
3(OAc)3, the three manganese centres are bridged by threealkoxide donors from L, forming a six-membered ring, and the pyr-idine nitrogens of each dipyridyloxymethyl moiety coordinate toadjacent metal centres. Ca2þ is bridged to the trimanganesecluster by a m4-oxido, to the MnIV centre by a m2-oxido, and tothe remaining MnIII centres by bridging acetate moieties. TheCa2þ is further coordinated by a bidentate DME ligand and a tri-fluoromethanesulfonate anion. Two trifluoromethanesulfonateions remain outer-sphere. The isolated compounds reported heredisplay diagnostic 1H NMR spectra, although the paramagneticallybroadened and shifted signals have not been assigned. The additionof excess water to a CD2Cl2 solution of [1–Ca(DME)(OTf)]2þ
forms a new species by 1H NMR spectroscopy (SupplementaryFig. S2) that was identified by XRD as the tris(aqua) complex [1–Ca(OH2)3]3þ (Supplementary Fig. S26). Addition of DME to theCD2Cl2 solution of [1–Ca(OH2)3]3þ converts the complex back to[1–Ca(DME)(OTf)]2þ (1H NMR spectroscopy, SupplementaryFig. S9). In the solid state, [1–Ca(DME)(OTf)]2þ is stable forweeks under ambient conditions.
Treatment of a DME suspension of [1–Ca(DME)(OTf)]2þ withthe one-electron reductant decamethylferrocene yields the singly
The chemical reversibility of this conversion is demonstrated by treat-ment of [2–Ca(DME)(OTf)]þ with AgOTf to convert back to[1–Ca(DME)(OTf)]2þ. 1H NMR analysis is consistent with cleaninterconversion of these two clusters. An XRD study of [2–Ca(DME)(OTf)]þ reveals a cluster isostructural to [1–Ca(DME)(OTf)]2þ with only slight bond distance changes (see below), indicat-ing little rearrangement of the cluster upon reduction.
The variable-temperature magnetic susceptibilites of [1–Ca(DME)(OTf)][OTf]2 and [2–Ca(DME)(OTf)][OTf] were studied(Supplementary Fig. S11). For [1–Ca(DME)(OTf)][OTf]2, dominantferromagnetic coupling between Mn ions is observed (seeSupplementary Table S1 for fitting parameters). At 14 K, the xMTvalue (xM¼molar magnetic susceptibility) increases to a maximumof 18.7 cm3 mol21 K, which is close to the expected spin-only valueof an S¼ 11/2 system (17.9 cm3 mol21 K, g¼ 2 (g is the effectivegyromagnetic ratio)). Different values would be expected for a moreoxidized S¼ 5 system (15 cm3 mol21 K, g¼ 2) or a more reducedS¼ 6 system (21 cm3 mol21 K, g¼ 2). These results support theoxidation state assignment of [1–Ca(DME)(OTf)]2þ as MnIVMnIII
2.The xMT value of [2–Ca(DME)(OTf)][OTf] approaches10.2 cm3 mol21 K at 300 K, which is near the expected spin-onlyvalue of three uncoupled MnIII ions (S¼ 2 spins, 3 cm3 mol21 K,g¼ 2). The xMT value decreases at low temperatures, reaching2.6 cm3 mol21 K at 4 K, which shows a dominant antiferromagneticinteraction (Supplementary Table S1). To further confirm the oxi-dation state assignment of the isolated species, Mn X-ray absorptionnear edge spectra (XANES) were collected for [1–Ca(DME)(OTf)](OTf)2 and [2–Ca(DME)(OTf)](OTf) (Fig. 4a).
N2O3
N3
N4
N6
Mn3 Mn2
Mn1O1
N1 O2
O7
O9
C2
C1
C3
C4
O8
Ca1
C8
C9
C5 C7
O6
O13
O14A
O11
O12S1A
F2A
F1A
F3A
C6
O10
N5
O5O4
Ca1
a
e f g h
b c d
O2O1
Mn3
Mn2
Mn1
O5O4
O9
O3O8O7
C4C1
C2
C9B
O6
O14B
S1B O12
C8
C5 C7
F3BO13
F2BF1B
O11
Ca1
N1N6
Mn1Mn3
Mn2
N5
N3
N4O2
O1
O3
N2
F2
F3
F1 O12
O14
O9
O13
O6C1
O7
O9
N5
Mn3 Mn2
O5N4
N3 N2
Mn1
O4O3
N6
N1 O2
C8Sr1
O11
O10C7
C6
O1
O8C4
C3
C2
S1
C5
O13
C9
C4
C8
C7
C6
C5O11
O10
Y1C3O8
O6O9
N5
N6
Mn3Mn2
Mn1
O7 O1O2
O3
O5O4
N1
N2N3
N4
C2
C1
F1F3
F2
O12
O14S1
C4
C3C2
O8
O6
O3
O5O4
O2O1
C1O7
O9
Zn1
N7B
C5B
C6B
N1
N2
N3
N4
N5
N6
Mn3
Mn2Mn1
C4
C3
C1
Na
N1
N5
N6
N4
C2
O8
O9
O7
O5O4 O3
O2O1
Mn1Mn2Mn3
N2N3
O6
F2AF3A
F1AO13A
O14A
O12A
S1A
C4
C3
O9 O6
O7 O1N1
O2
Mn1Mn2
Mn3N5
N6
O5 O3
N4
N3
N2O4
C1
C2O8
Sr1
O11
C8
C7
C6
O10
C5
C9A
Figure 3 | Solid-state structures of reported complexes (thermal ellipsoids shown at 50% level). Hydrogen atoms and outer-sphere anions not shown
for clarity. a, Full structure of [1–Ca(DME)(OTf)][OTf]2. b, Truncated view of [1–Ca(DME)(OTf)][OTf]2. c, Truncated view of [2–Ca(DME)(OTf)][OTf].
d, Truncated view of [1–Sr(DME)(OTf)][OTf]2. e, Truncated view of [2–Sr(DME)(OTf)][OTf]. f, Truncated view of [1–Na]2[OTf]4. g, Truncated view
of [1–Zn(CH3CN)][OTf]3. h, Truncated view of [2–Y(DME)(OTf)][OTf]2.
The rising edge energy, taken as a zero crossing point of the secondderivative spectrum (Fig. 4b), is shifted to a higher energy by�1.0 eV from [2–Ca(DME)(OTf)](OTf) (6,548.66 eV) to [1–Ca(DME)(OTf)](OTf)2 (6,549.76 eV). Such an edge shift is consistentwith a one-electron oxidation state change in redox-active Mn com-plexes32, when the geometry and type of ligands are highly conserved.This result, correlated with charge balance in the solid-state structure,magnetism and chemical reactivity, therefore supports the formal oxi-dation state assignments for [1–Ca(DME)(OTf)](OTf)2 and [2–Ca(DME)(OTf)](OTf) as MnIVMnIII
2 and MnIII3 respectively.
To study the effects of the redox-inactive centre on the propertiesof the [Mn3O2] core, the analogous Sr2þ-, Y3þ-, Naþ- and Zn2þ-capped trimanganese dioxo complexes were targeted (Figs 2and 3). Treatment of LMnII
3(OAc)3 with PhIO and M(OTf)n(M¼Na, Sr, Y) led to new species with 1H NMR spectroscopic
characteristics similar to compounds [1–Ca(DME)(OTf)]2þ and[2–Ca(DME)(OTf)]þ. Complexes [1–Sr(DME)(OTf)]2þ and [2–Y(DME)(OTf)]2þ are structurally analogous to [1–Ca(DME)(OTf)]2þ, with the redox-inactive metal bridged by two oxido moi-eties to the trimanganese cluster and further coordinated by a DMEmolecule and a triflate anion (Fig. 3d,h). The yttrium-capped dioxocompound was isolated in the more reduced MnIII
3 state rather thanthe MnIVMnIII
2 state observed under the same reaction conditionsfor the calcium and strontium dioxo compounds. Similar to thereduction of [1–Ca(DME)(OTf)]2þ to [2–Ca(DME)(OTf)]þ, thereduced strontium compound [2–Sr(DME)(OTf)]þ was preparedby treating [1–Sr(DME)(OTf)]2þ with decamethylferrocene. TheNa-capped dioxo complex [1–Na]2
4þ was isolated in the solidstate as a dimer of Mn3NaO2 moieties via acetate bridges. EachMn3NaO2 core is structurally similar to the Ca2þ, Sr2þ and Y3þ ana-logues, with the Naþ cation bridged by two oxidos to the trimanga-nese moiety (Fig. 3f). Preparation of the Zn-capped compound[1–Zn(CH3CN)]3þwas accomplished by the addition of Zn(OTf)2to a CH3CN solution of [1–Ca(DME)(OTf)]2þ, leading to substi-tution of Ca2þ with Zn2þ. An XRD study of [1–Zn(CH3CN)]3þ
grown from an acetonitrile/diethyl ether mixture shows one aceto-nitrile coordinated to the Zn2þ centre. The Mn3ZnO2 core is struc-turally similar to the other Mn3MO2 moieties reported here.The magnetic susceptibilities of the MnIVMnIII
2 complexes[1–Sr(DME)(OTf)][OTf]2, [1–Na]2[OTf]4 and [1–Zn(CH3CN)][OTf]3 were measured, and all demonstrate ferromagnetic couplingof spins close to that of [1–Ca(DME)(OTf)][OTf]2, supporting thesame formal oxidation state assignment (Supplementary Fig. S11).
Analysis of the solid-state structures show that the distances fromthe redox-inactive metals to the bridging oxido moieties vary asexpected based on the ionic radius of the ion M, with the M−O dis-tances increasing from [1–Zn(CH3CN)]3þ (2.008(3), 2.090(3) Å)to [1–Sr(DME)(OTf)]2þ (2.510(5), 2.651(4) Å) (Table 1). Uponreduction of the MnIVMnIII
2 clusters [1–Ca(DME)(OTf)]2þ and[1–Sr(DME)(OTf)]2þ to form the MnIII
3 complexes [2–Ca(DME)(OTf)]þ and [2–Sr(DME)(OTf)]þ, the M–(m4-O) distances con-tract, indicating a stronger interaction with the redox-inactivemetal as the interaction of the m4-oxido with the trimanganesecore weakens due to increased population of s-antibonding orbitalsin d4, MnIII centres. As expected, the Mn−O distances increaseupon reduction (compounds 2 versus 1), but only smallvariations are observed based upon the redox-inactive metal.[2–Y(DME)(OTf)]2þ displays the longest Mn–oxido average dis-tance, consistent with the yttrium centre, the most Lewis-acidicmetal of the series, drawing more electron density from the oxidomoieties and weakening the Mn−O interactions. Notably, thebond distances of the Mn3O2 core are essentially the same in
6,530 6,540 6,550 6,560 6,570 6,580
X-ray energy (eV)
X-ray energy (eV)
0
1.0
0.5
1.5N
orm
aliz
ed in
tens
ity
[1-Ca(DME)(OTf)]2+,MnIII
2MnIV
[2-Ca(DME)(OTf)]+, MnIII3
–0.06
0
0.06
6,530 6,540 6,550 6,560 6,570 6,580
d2(F
/I 0)/
dE2
a
b
Figure 4 | The shift in the rising edge energy in the Mn XANES spectra.
The shift for [1–Ca(DME)(OTf)][OTf]2 (solid line) and [2–
Ca(DME)(OTf)][OTf] (dashed line) (6,549.76 and 6,548.66 eV,
respectively) suggests a one-electron oxidation state change. a,b, Mn
XANES spectra (a) and second derivative spectra (b).
Table 1 | Selected bond lengths (in Å) for reported complexes.
The Mn–O bond lengths vary depending on manganese oxidation state, but not significantly due to the redox-inactive metal M. M–O bond lengths vary with ionic radius. All estimated standard deviations werecalculated rigorously from the full covariance matrix42.
[1–Ca(DME)(OTf)]2þ and [1–Sr(DME)(OTf)]2þ. These observa-tions are consistent with XAS studies of Sr-substituted PSII,indicating no significant structural change in the OEC fromCa-substituted PSII3,33.
We and others have reported studies that suggest a significanteffect of redox-inactive metals on the electron transfer properties ofmetal–oxido species, a phenomenon of particular relevance in bio-logical systems and heterogeneous mixed metal oxides7,8,15,21,24,34–36.With a series of well-defined and structurally analogous Mn3MO2complexes in hand, we investigated the effect of metal M on thereduction potential of the clusters. The cyclic voltammogram (CV)of a 10:1 CH2Cl2/DME solution of [1–Ca(DME)(OTf)]2þ (0.1 MNBu4PF6) shows two quasireversible redox couples at 270 and2530 mV versus the ferrocene/ferrocenium couple (Fc/Fcþ)(Supplementary Fig. S15). Because [1–Ca(DME)(OTf)]2þ is chemi-cally reduced to [2–Ca(DME)(OTf)]þ by the addition of 1 equiv.of decamethylferrocene (EO ≈ 20.48 V versus Fc/Fcþ inCH2Cl2), the couple centred at 270 mV is assigned as the[MnIVMnIII
2/MnIII3] couple. A 10:1 CH2Cl2/DME solution of
[1–Ca(DME)(OTf)]2þ (0.05 M LiOTf electrolyte) was electrolysedat a potential of 20.25 V versus Ag/Agþ to form a new species by1H NMR spectroscopy. The amount of current passed at that poten-tial supports the assignment of the wave at 270 mV as the one-electron reduction of [1–Ca(DME)(OTf)]2þ. Independently prepared
[2–Ca(DME)(OTf)]þ reacts with excess LiOTf to cleanly form aproduct whose 1H NMR spectrum matches that of the controlledpotential electrolysis product described above (SupplementaryFig. S13). Under the same conditions, [1–Ca(DME)(OTf)]2þ doesnot react with LiOTf. The more negative redox couple at 2530 mVis presumed to correspond to the [MnIII
3/MnIII2MnII] couple, but
the more reduced product has not yet been isolated.As the electrochemical studies are presumed to be of intact
clusters, the solution stability of the [MMn3O2] core was studied.Isotopically labelled LCaMn3
18O2(OAc)2(DME)(OTf)3 ([1*–Ca(DME)(OTf)][OTf]2) was prepared and mixed with 1 equiv.of natural abundance [1–Ca(DME)(OTf)]2þ in dichloromethaneat room temperature, and the mixture was analysed over timeusing electrospray ionization mass spectrometry (ESI-MS).Less than 20% isotopic scrambling occurs on the timescale ofthe electrochemical experiments (�18% after 1 h at roomtemperature). Additionally, methylene chloride solutions of[1–Ca(DME)(OTf)]2þ are stable at room temperature under anhy-drous conditions for days (1H NMR spectroscopy). The addition of10 equiv. of Ca(OTf)2 to the solution of [1–Ca(DME)(OTf)]2þ doesnot change the reduction potential of the complex (SupplementaryFig. S17), indicating that any equilibrium towards dissociation of theclusters lies towards Ca2þ association. To interrogate the effect ofthe Ca-coordinated ligands, under the same conditions, thereduction of [1–Ca(OH2)3]3þ occurs within 30 mV of that of [1–Ca(DME)(OTf)]2þ (Supplementary Fig. S16), suggesting that thecapping ligands on the Ca2þ centre do not significantly affect thereduction potentials of the clusters.
The CVs of [1–Sr(DME)(OTf)]2þ, [2–Y(DME)(OTf)]2þ, [1–Zn(CH3CN)]3þ and [1–Na]2
4þ all display the [MnIVMnIII2/MnIII
3]couple observed for [1–Ca(DME)(OTf)]2þ, although at differentpotentials (Fig. 5a). Although the peak-to-peak separations of theobserved couples are large when measured at a glassy carbon elec-trode (DEp ≈ 400 mV), indicating slow electron transfer to thecomplexes, the E1/2 values calculated from the CVs of [1–Ca(DME)(OTf)]2þ, [1–Sr(DME)(OTf)]2þ and [1–Na]2
4þ are ingood agreement with CVs collected using a hanging drop mercuryelectrode, with DEp ≈ 200 mV (Supplementary Fig. S14). The E1/2corresponding to the reduction of [1–Ca(DME)(OTf)]2þ to[2–Ca(DME)(OTf)]þ was also measured by monitoring the elec-tronic absorption spectrum upon titration with dimethylferrocene(Supplementary Fig. S18), and the calculated values (20.1 V versusFc/Fcþ) are close to the value from the CV (20.07 V versus Fc/Fcþ).
As structural characterization was obtained only for clusters dis-playing the MnIVMnIII
2 and MnIII3 oxidation states, these are the
reduction potentials that are compared below. Moreover, the morehighly oxidized species are pertinent to the moieties present inthe OEC and proposed for water oxidation catalysts. The E1/2values of the [MnIVMnIII
2/MnIII3] couple become more positive
as the charge of the redox-inactive metal increases. This trendsuggests that the increased charge of the proximal redox-inactivecation facilitates reduction of the manganese centres. The E1/2value for the Zn2þ complex appears at potentials �230 mV morepositive compared to the Ca2þ and Sr2þ species. Although[1–Zn(CH3CN)]3þ is tricationic and [1–Ca(DME)(OTf)]2þ
and [1–Sr(DME)(OTf)]þ are dicationic, the E1/2 of the[MnIVMnIII
2/MnIII3] couple of [1–Ca(OH2)]3þ, also a tricationic
complex, is more positive than that of [1–Ca(DME)(OTf)]2þ, byless than 30 mV. The more positive potential of the Zn2þ compoundis inconsistent with a purely electrostatic explanation of the changein reduction potentials, as proposed for oxo-bridged manganesedimers with alkali and alkali earth metal ions associated via salenligands modified with crown ether moieties35.
The effect of redox-inactive metals on the kinetics of electrontransfer to a non-haem FeIVO species has been linked previouslyto the Lewis acidity of the metal8,37. The E1/2 values of the
8 12 16–0.4
–0.2
0
0.2
0.4
0.6
pKa of M(aqua)n+ ion
E1/
2 ve
rsus
Fc/
Fc+
(V
)
Y3+
Zn2+
Ca2+ Sr2+
Na+
b
–1 0 1
E/V (versus Fc/Fc+)
15 µA Na+
Sr2+
Ca2+
Zn2+
Y3+
a
Figure 5 | The redox potentials of the [MMn3O2] complexes are correlated
with the Lewis acidity of the redox-inactive metal. a, CVs of reported
complexes in 0.1 M NBu4PF6 10:1 CH2Cl2/DME (CV of [1–Na]24þ in
CH2Cl2) using a glassy carbon disc electrode at a scan rate of 100 mV s21.
b, Dependence of E1/2 of MnIVMnIII2/MnIII
3 couple on pKa of M(aqua)nþ
ion36. Error bars correspond to the standard deviation of the reduction
potentials measured from three independent samples.
3] couples measured above in organic solventswere plotted against the pKa of the metal aqua ions, M(aqua)nþ, inwater38, used here as a measure of the Lewis acidity of the metal M.A remarkable linear correlation is observed (Fig. 5b) that clearlylinks the effect of the redox-inactive metal to the cluster reductionpotential in terms of the Lewis acidity of the metal. The slope providesa quantitative measure of this correlation, with each pKa unit shiftingthe potential by �100 mV. This effect is probably a consequence ofthe interaction between the oxido moieties and redox-inactivemetals versus manganese centres. The stronger Lewis acid is expectedto draw more electron density from the oxido ligands and to destabi-lize the higher-oxidation-state manganese centres.
The above findings have implications for biological and hetero-geneous metal catalysts for water oxidation and other redox pro-cesses. Catalysis occurs at discrete multinuclear sites, consisting offive metal centres for the OEC in PSII and fewer than ten metalcentres for heterogeneous manganese and cobalt oxides. The poten-tial of the cluster is expected to vary based on structure and numberof oxido ligands. For example, the reduction potential of the[MnIVMnIII
2CaO2/MnIII3CaO2] couple reported here is more
positive than that of the [MnIV3CaO4/MnIV
2MnIIICaO4] coupleof the CaMn3 cubane cluster21, despite the higher manganese oxi-dation states in the latter complex, probably due to the greaternumber of oxido ligands. Changing the nature of the redox-inactive metal component of the cluster, without a structuralchange of the cluster, allows for drastic variation of the reductionpotential in both directions, potentially from values that render thechemistry of interest (for example, water oxidation) thermo-dynamically unfavourable to values that make it favourable. Tuningof the reduction potentials of mixed metal-oxide clusters by Lewisacids is an appealing strategy for designing practical catalysts forwater splitting.
It is notable in the present series that the Ca2þ and Sr2þ variantshave essentially the same potentials, which is consistent with theobservation that substitution of Sr2þ for Ca2þ in PSII retains reac-tivity and a similar electronic structure33,39. Although a previouslyproposed role of the Ca2þ (or Sr2þ) of the OEC in facilitating theattack of a water or hydroxide moiety on an electrophilic manganeseoxo is still possible40, the reported results support a significant rolein redox tuning of the cluster.
In summary, a series of tetrametallic dioxo complexes containingredox-inactive metal ions in the þ1 to þ3 oxidation states were syn-thesized using a trinucleating ligand framework. The structuralcharacteristics of these complexes, with the oxido ligands bridgingthe redox-inactive metals and the manganese centres, make them par-ticularly relevant to biological and heterogeneous metal–oxido clus-ters. Electrochemical studies of these compounds show that thereduction potentials are highly dependent upon the Lewis acidity ofthe redox-inactive metal, identifying the chemical basis for theobserved differences in electrochemistry. This correlation provides evi-dence for the role of the Ca2þ ion in modulating the redox potential ofthe OEC and of other redox-inactive ions in tuning the redox poten-tials of other metal-oxide electrocatalysts. The observed linear depen-dence between cluster potential and Lewis acidity provides a rationalstrategy for tuning the redox properties of heterometallic metal–oxidoclusters of interest for catalysis. Current studies are focused on furtherunderstanding the relationship between the structures of mixed-metalmanganese–oxido clusters and their reactivity.
MethodsUnless indicated otherwise, reactions were carried out in oven-dried glassware in aglovebox under a nitrogen atmosphere. Anhydrous tetrahydrofuran (THF) waspurchased from Aldrich in 18-litre Pure-Pac containers. Anhydrousdichloromethane, diethyl ether and THF were purified by sparging with nitrogen for15 min and then passing under nitrogen pressure through a column of activated A2alumina. Anhydrous DME was dried over sodium/benzophenone ketyl andvacuum-transferred onto molecular sieves. CD2Cl2 was purchased from Cambridge
Isotope Laboratories, dried over calcium hydride, then degassed by three freeze–pump–thaw cycles and vacuum-transferred before use.
Iodosobenzene was prepared according to literature procedures41. LMn3(OAc)3and [LMn3O(OAc)3]2Ca(OTf)2 were prepared according to previously publishedprocedures21,23. (Caution! Iodosobenzene is potentially explosive and should be usedonly in small quantities.)
The syntheses of all reported compounds, characterization methods andequipment are described in the Supplementary Information.
Synthesis of [1–Ca(DME)(OTf )][OTf ]2. In the glovebox, a round-bottom flaskequipped with a stir bar was charged with LMn3(OAc)3 (2.0 g, 1.67 mmol) andCa(OTf)2 (0.90 g, 2.67 mmol, 1.6 equiv.). DME (200 ml) was added, and the yellowsuspension was stirred at room temperature for 5 min. Iodosobenzene (0.81 g,3.68 mmol, 2.2 equiv.) was added as a solid, and the mixture was stirred at roomtemperature for 4 h, turning from yellow to purple. The purple solid was collectedvia filtration, washed with DME, then extracted with dichloromethane. The red-purple solution was concentrated in vacuo to yield the product as a red-purplesolid (2.45 g, 84%). 1H NMR (CD2Cl2, 300 MHz): d 77.8, 76.5, 72.3, 69.6, 60.5, 53.7,48.4, 38.6, 37.0, 24.2, 19.9, 17.5, 15.9, 8.1, 3.3, 2.9, 219.3, 223.5, 224.9, 226.4,229.1 ppm. 19F NMR (CD2Cl2): d 274.4 ppm. UV-Vis (CH2Cl2, lmax (1)): 498(1,410 M21 cm21), 846 (640 M21 cm21) nm. Anal. Calcd. For C68H55CaF9Mn3N6O20S3:C, 46.72; H, 3.17; N, 4.81. Found: C, 46.92; H, 3.27; N, 4.89.
Synthesis of [2–Ca(DME)(OTf )][OTf ]. In the glovebox, a round-bottom flaskequipped with a stir bar was charged with [1–Ca(DME)(OTf)]2þ (0.750 g,0.429 mmol) and decamethylferrocene (0.140 g, 0.429 mmol, 1 equiv.). DME (30 ml)was added, and the purple mixture was stirred at room temperature for 1 h. The grey-purple precipitate was collected on a fritted glass funnel and washed with DME, thenextracted with cold THF (40 ml). The purple filtrate was concentrated to �20 mlin vacuo, then cooled to 235 8C to precipitate out more Cp*2Feþ, which was filteredoff over Celite. The purple filtrate was concentrated in vacuo to a purple solid, thenrecrystallized from DME/CH2Cl2/hexanes to yield the product as a purple solid(0.405 g, 59%). 1H NMR (CD2Cl2, 300 MHz): d 65.1, 57.8, 50.7, 41.6, 35.3, 29.6,15.7, 15.0, 14.2, 8.2, 3.0, 29.6, 212.6, 217.0, 217.9 ppm. 19F NMR (CD2Cl2):d 2 74.6 ppm. UV-Vis (CH2Cl2, lmax (1)): 495 (710 M21 cm21), 860(310 M21 cm21) nm. Anal. Calcd. for C67H55CaF6Mn3N6O17S2: C, 50.32; H, 3.47;N, 5.26. Found: C, 50.04; H, 3.63; N, 5.06.
Electrochemistry. Electrochemical measurements were recorded using a PineInstrument Company AFCBP1 bipotentiostat using the AfterMath softwarepackage. CVs were recorded on �1 mM solutions of the relevant complexes in theglovebox, at 20 8C, with an auxiliary Pt-coil electrode, a Ag/Agþ reference electrode(0.01 M AgNO3, 0.1 M nBu4NPF6 in CH3CN) and a 3.0 mm glassy carbon electrodedisc (BASI). The electrolyte solutions were 0.1 M nBu4NPF6 in CH2Cl2 ([1–Na]2
4þ)or 10:1 CH2Cl2/DME ([1–Ca(DME)(OTf)]2þ, [1–Sr(DME)(OTf)]2þ, [1–Zn(CH3CN)]3þ and [2–Y(DME)(OTf)]2þ). For [1–Ca(DME)(OTf)]2þ, [1–Sr(DME)(OTf)]2þ and [1–Na]2
4þ, CVs were also recorded in an electrolyte solutionof 0.1 M nBu4NPF6 in CH2Cl2 using a hanging mercury drop electrode (BASICGME). Using the average mass of the mercury drop (0.037 g), the surface area ofthe drop (assumed to be a sphere) was calculated to be 0.095 cm2. All reported valuesare referenced to an internal Fc/Fcþ couple.
Received 29 August 2012; accepted 22 January 2013;published online 3 March 2013
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AcknowledgementsThis work was supported by the California Institute of Technology, the Searle ScholarsProgram, an NSF CAREER award (CHE-1151918 to T.A.) and the NSF Graduate ResearchFellowship Program (to E.Y.T.). The authors thank L.M. Henling and D.E. Herbert forassistance with crystallography, and P-H. Lin for assistance with magnetic susceptibilitystudies. The Bruker KAPPA APEXII X-ray diffractometer was purchased with an NSFChemistry Research Instrumentation award to Caltech (CHE-0639094). The X-rayspectroscopy work was supported by the NIH (grant no. F32GM100595 to R.T.) and by theDirector of the Office of Basic Energy Science (OBES), Division of Chemical Sciences,Geosciences, and Biosciences, DOE (contract no. DE-AC02-05CH11231 to J.Y.).Synchrotron facilities were provided by the Stanford Synchrotron Radiation Lightsource(SSRL), operated by the DOE, OBER.
Author contributionsE.Y.T. and T.A. designed the research. E.Y.T. and R.T. performed the experiments. R.T. andJ.Y. provided XANES characterization. E.Y.T., R.T., J.Y. and T.A. analysed data. E.Y.T. andT.A. wrote the paper.
Additional informationSupplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints. Correspondence andrequests for materials should be addressed to T.A.
Competing financial interestsThe authors declare no competing financial interests.
