Heterometallic Triiron-Oxo/Hydroxo Clusters: Effect of Redox-
Inactive Metals
David E. Herbert
,
Davide Lionetti
,
Jonathan Rittle
, and
Theodor Agapie
*
Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E
California Blvd, Pasadena, California 91125
Abstract
A series of tetranuclear oxo/hydroxo clusters comprised of three Fe centers and a redox-inactive
metal (M) of various charge is reported. Crystallographic studies show an unprecedented
Fe
3
M(
μ
4
-O)(
μ
2
-OH) core that remains intact upon changing M or the oxidation state of iron.
Electrochemical studies reveal that the reduction potentials (
E
1/2
) span a window of 500 mV and
depend upon the Lewis acidity of M. Using the p
K
a
of the redox-inactive metal aqua complex as a
measure of Lewis acidity, these compounds display a linear dependence between
E
1/2
and acidity
with a slope of
ca
. 70 mV per p
K
a
unit. The current study of [Fe
3
MO(OH)] and previous ones of
[Mn
3
MO
n
] (n = 2, 4) moieties support the generality of the above relationship between the
reduction potentials of heterometallic oxido clusters and the Lewis acidity of incorporated cations,
as applied to clusters of different redox-active metals.
The chemistry of synthetic and biological redox centers is affected by Lewis acidic metal
ions.
1
A fascinating case in biology is the role of Ca
2+
, a redox inactive metal in the
catalytic site of photosynthetic water oxidation, the heterometallic CaMn
4
O
x
oxygen-
evolving complex (OEC) of photosystem II (PSII).
2
Synthetic Fe
IV
–oxo complexes show
enhanced electron transfer rates and more positive reduction potentials upon addition of
redox-inactive Lewis acids such as Sc
3+
or Ca
2+
.
3
Group 2 metal ions enhance the rates of
dioxygen activation by monometallic Mn
II
and Fe
II
complexes.
4
Trivalent redox-inactive
Lewis acids (Sc
3+
, Y
3+
) facilitate O–O bond cleavage in non-heme iron(III)-peroxo
species,
5
and Sc
3+
modulates O- and H-atom transfer reactivity of a Mn
IV
-oxo complex.
6
Valence tautomerism is induced by the addition of Zn
2+
was to Mn
V
O-porphyrinoid
complexes.
7
The ligation of redox-inactive metals to pendant donors also affects the
reduction potential of oxo-bridged dimanganese species.
8
Alkali and alkali earth metals have
also been proposed as components of catalytic clusters in heterogeneous water oxidation by
cobalt and manganese oxides.
9
Synthetic access to well-defined isostructural multimetallic complexes containing different
redox-
inactive
metal ions allows systematic investigation of their effects upon the redox-
active
metallic constituents. Our group recently reported a series of heterometallic
trimanganese dioxido clusters [Mn
3
M(
μ
4
-O)(
μ
2
-O)] (M = Na
+
, Ca
2+
, Sr
2+
, Zn
2+
, and Y
3+
)
and demonstrated that the reduction potentials of the clusters are linearly correlated with the
Lewis acidity of the redox-inactive metal.
10
A similar trend was observed for a series of [Mn
IV
3
MO
4
] cubane complexes (M = Ca
2+
,
Sr
2+
, Zn
2+
, Sc
3+
, Mn
3+
), supported by a multinucleating ligand framework (H
3
L, Scheme
*
Corresponding Author: agapie@caltech.edu.
Supporting Information. Experimental procedures, spectroscopic characterization, and crystallographic (CIF) data. This material is
available free of charge via the Internet at
http://pubs.acs.org
.
NIH Public Access
Author Manuscript
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
Published in final edited form as:
J Am Chem Soc
. 2013 December 26; 135(51): 19075–19078. doi:10.1021/ja4104974.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
1), that are structurally related to the CaMn
3
O
4
cubane subsite of the OEC.
11
These studies
suggest a role for the Ca
2+
center in tuning the reduction potential of the active site in PSII.
Moreover, the similar trends observed for different cluster structures indicate that this is a
general phenomenon in manganese chemistry. It is of interest to determine if the effects
discovered in manganese chemistry extend to other transition metals because of the variety
of metal oxides studied as catalysts for water oxidation and O
2
reduction.
12
Herein, we
describe the synthesis and redox chemistry of a series of heterometallic tetranuclear clusters
of iron.
Following synthetic protocols developed with manganese, oxidized heterometallic clusters
were targeted from an all-ferrous precursor, LFe
3
(OAc)
3
,
13
supported by a triarylbenzene
architecture appended with pyridine and alkoxide donors.
13–14
Treatment of a 1,2-
dimethoxyethane (DME) suspension of LFe
II
3
(OAc)
3
and M(OTf)
2
(M = Ca, Sr; OTf =
trifluoromethanesulfonate) with iodosobenzene (PhIO), followed by crystallization from a
CH
2
Cl
2
/DME solution layered with Et
2
O, afforded the all-ferric M-capped complexes
1
-M
(M = Ca, Sr) as orange-brown solids (Scheme 1). Single crystal X-ray diffraction (XRD)
studies of
1
-M (M = Ca, Sr) revealed that in these complexes, as in LFe
II
3
(OAc)
3
, the three
iron centers are bridged by three alkoxide donors from L, forming a six-membered ring, and
the pyridine nitrogens of each dipyridyloxymethyl moiety coordinate to adjacent metal
centers. The apical metal (M) is bridged to the triiron cluster by a
μ
4
-oxido, to one unique
iron center by a
μ
2
-hydroxo, and to the remaining Fe
III
centers by bridging acetate moieties.
In addition, M is further coordinated by a bidentate DME ligand and a [OTf]
−
anion (Fig.
1a–b). Two [OTf]
−
ions remain outer-sphere.
The isolated compounds reported here display diagnostic proton nuclear magnetic resonance
(
1
H NMR) spectra, although the paramagnetically broadened and shifted signals have not
been assigned (see SI). The zero-field
57
Fe Mössbauer spectra of
1
-M (M = Ca, Sr) show
features at 80 K that are best modeled as two quadrupole doublets in a 2:1 ratio, consistent
with two distinct ferric sites (Fig. 2, S9; Table S1). The long Fe–
μ
2
-O(5) bond distances [
1
-
Ca, 1.881(2);
1
-Sr, 1.884(2) Å] and spectral properties supported the assignment of O(5) as
a hydroxo moiety coordinated to an Fe
III
center of a Fe
III
3
MO(OH) moiety. In comparison,
the Fe–O bond distances for a series of
μ
2
-hydroxo bridges between Fe
III
and redox-inactive
dications (Ca
2+
, Sr
2+
, Ba
2+
) are between 1.859(2) and 1.872(2) Å.
4b
By contrast, the Fe–O
distance in a linear
μ
2
-oxo bridge between Fe and Sc centers is significantly shorter at
1.754(3) Å.
3a
The synthetic protocol above likely involves the transfer of two O-atoms from PhIO. This
generates a highly reactive Fe
IV
intermediate capable of H-atom abstraction to form
1-
M.
Under similar reaction conditions, related Mn precursors give Mn
III
2
Mn
IV
MO
2
clusters,
likely due to the less oxidizing character of the Mn cluster. The scandium analog of
1
-Ca
and
1
-Sr was isolated in the reduced Fe
II
Fe
III
2
oxidation state (
2
-Sc). A single crystal XRD
study of
2
-Sc revealed an Fe
3
MO(OH) core analogous to
1
-Ca and
1
-Sr (Fig. 1c). The
assignment of the iron oxidation states was based on the absence of a fourth triflate
counteranion, as well as on the observation of a disparity in the Fe–
μ
4
-O distances in
2
-Sc –
two of which (2.005(3), 1.931(3) Å) were similar to those in
1
-Ca and a longer one
(2.211(4) Å), consistent with one of the two Fe centers
not
bound to the
μ
2
-O(H) being more
reduced. The presence of a ferrous ion was further confirmed by the zero-field
57
Fe
Mössbauer spectrum, which showed three distinct features best modeled as one ferrous (
δ
:
1.135
mm
/s) and two ferric (
δ
: 0.466
mm
/s, 0.477
mm
/s) quadrupole doublets in a 1:1:1 ratio
(Fig. S8), in good agreement with literature values for Fe
II/III
compounds bearing N/O
ligands.
15
The Fe(2)–O(5)H bond in
2
-Sc is elongated compared to
1
-Ca and
1
-Sr likely due
to a combination of a more reduced Fe
3
core and a stronger interaction of the bridging
moieties with the more Lewis acidic Sc
3+
. The one-electron reduced Ca compound (
2
-Ca)
Herbert et al.
Page 2
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
was obtained by the chemical reduction of
1
-Ca using one equivalent of cobaltocene
(CoCp
2
;
E
O
~ −1.33 V vs. Fc/Fc
+
) in CH
2
Cl
2
(Scheme 1). Crystallization from CH
2
Cl
2
/
Et
2
O afforded the reduced compound as confirmed by an XRD study (Fig. 1e). The
observed changes in Fe–O distances in
2
-Ca are similar to those of
2
-Sc, with an elongated
Fe(2)–O(5)H bond and one long (> 2.1 Å) Fe–
μ
4
-O distance. The zero-field Mössbauer
spectrum collected at 80 K revealed features similar to those of
2
-Sc – two quadrupole
doublets in a 1:2 ratio, consistent with one ferrous (
δ
: 1.166
mm
/s) and two ferric sites (
δ
:
0.475
mm
/s; Fig. 2).
Complexes containing other redox-inactive metal ions could not be isolated by analogous
procedures, possibly due to solubility differences. However, when
1
-Ca was treated with
Zn(OTf)
2
in CH
3
CN (Scheme 1), electrospray ionization mass spectrometry (ESI-MS) of
the reaction mixture showed a new species at 1476
m/z
corresponding to [LZnFe
3
O(OH)
(OAc)(OTf)
2
]
+
and the disappearance of signals corresponding to
1
-Ca. A single crystal
XRD study of
1
-Zn shows that
1
-Zn retains the [MFe
3
O(OH)] moiety, structurally related to
1
-Ca and
1
-Sr, although the smaller zinc center is five-coordinate and binds an aceto-nitrile
solvent ligand in place of DME and [OTf]
−
(Fig. 1d). Similar to
2
-Sc,
1
-Zn shows a slightly
longer Fe(2)–O(5) distance [1.923(4) Å] relative to
1
-Ca and
1
-Sr suggesting that the
stronger interaction between
μ
2
-OH and the more Lewis acidic Zn
2+
results in a weaker
interaction between
μ
2
-OH and Fe. Under the same reaction conditions using La(OTf)
3
instead of Zn(OTf)
2
resulted in a product with
1
H NMR and Mössbauer spectroscopic
features similar to
1
-Ca,
1
-Sr and
1
-Zn (Table S1). This La species was reduced with one
equivalent of CoCp
2
in CH
2
Cl
2
to obtain the one-electron reduced Fe
II
Fe
III
2
cluster,
2
-La,
which was crystallographically characterized (Fig. 1f). Across the series of Fe
3
MO(OH)
complexes structurally characterized, Fe(2)-M (and other Fe-M) distances [
1
-Zn, 2.0207(15)
Å;
2
-Sc, 3.1638(11) Å;
2
-Ca, 3.3104(18) Å;
1
-Ca, 3.3541(6) Å;
2
-La 3.4159(9) Å;
1
-Sr,
3.5456(4) Å] correlate with the trend of effective ionic radii (Zn
2+
< Sc
3+
< Ca
2+
< La
3+
<
Sr
2+
).
16
With these complexes in hand, the effect of changing the redox-inactive metals in the
clusters was studied electrochemically. Cyclic voltammograms (CV) in CH
2
Cl
2
/DME (9:1)
with 0.1 M NBu
4
PF
6
showed quasireversible redox processes assigned as the
[MFe
III
3
O(OH)]/[MFe
III
2
Fe
II
O(OH)] couple at potentials of −490 (
1
-Ca), −490 (
1
-Sr), −210
(
1
-Zn), −80 (
1
-La), and +70 mV (
2
-Sc) vs. the ferrocene/ferrocenium couple (Fc/Fc
+
) (Fig.
3). Although both
2
-La and
2
-Sc share the same core structure, contain tricationic redox-
inactive metals, and have the same overall charge, their reduction potentials differ by ca. 150
mV. The reduction potentials of
1
-Ca and
1
-Sr are similar (
E
1/2
= −490 mV vs. Fc/Fc
+
),
while the reduction potential of
1
-Zn is more positive by greater than 300 mV (
E
1/2
= −210
mV), even though Zn
2+
is also a dication. Although there are structural differences at the
redox-inactive metal between
1
-Ca/Sr and
1
-Zn (Fig. 1a, b, d), studies of CaMn
3
O
2
clusters
indicated that changes in the coordination sphere at Ca
2+
do not have a significant effect on
the redox chemistry of the cluster.
10
The variation in redox potential observed here for the
iron clusters is therefore inconsistent with a purely electrostatic effect. The similarity of the
redox potentials of the Ca and Sr variants in comparison to those of the other analogs
correlates with the observation that both Sr
2+
and Ca
2+
generate catalytically active OEC in
PSII (although the activity of the Sr
2+
-reconstituted active site is lower than that of the
native protein).
17
The
E
1/2
values of the [Fe
III
3
MO(OH)]/[MFe
III
2
Fe
II
O(OH)] and those of previously
prepared [Mn
3
MO
2
] and [Mn
3
MO
4
] complexes
10–11
were plotted against the p
K
a
of the
metal aqua ions measured in water,
18
used here as a measure of the Lewis acidity of cation
M. In all cases, a linear correlation is observed (Fig. 4). Hence, the reduction potentials of
the clusters can be tuned by the Lewis acidity of the incorporated redox inactive metal. The
Herbert et al.
Page 3
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
positive shift in reduction potential with increasing Lewis acidity is likely due to the
increased electron-withdrawing effect on the bridging oxido/hydroxo ligands, which
stabilizes the more reduced iron oxidation state. The distinct effects of the redox-inactive
metals is apparent in the different Fe-O(H) distances for both reduced and oxidized clusters
(Table S4).
The change in slope between the Fe and Mn [M
′
3
MO
2
(H)] clusters (70 vs. 90 mV per p
K
a
unit, respectively) may reflect the differences in number of oxido ligands, protonation state,
metal identity, and oxidation state of the redox-active component. Further studies are
necessary for distinguishing these possibilities. The intercepts of the two series are different
by
ca
. 400 mV, with the [Fe
III
3
MO(OH)] complexes having more negative reduction
potentials than the corresponding [Mn
IV−
Mn
III
2
MO
2
] complexes, consistent with the lower
oxidation states for the iron species. The similar linear dependences upon Lewis acidity of
the dioxido trimanganese and the oxo/hydroxo triiron complexes suggest that a general
correlation exists between the redox potentials of mixed metal oxides and the Lewis acidity
of incorporated redox-inactive metals. Such a relationship may provide a quantitative
method for tuning the potentials of both homogeneous and heterogeneous metal oxide
electrocatalysts by changing the redox-inactive metal in isostructural compounds. The wide
range of reduction potentials found within the [Fe
3
MO(OH)] clusters demonstrates that a
large change in the thermodynamics of a catalyst can be effected by redox-inactive metal
substitution.
In summary, [Fe
3
MO(OH)] clusters substituted with divalent and trivalent redox-inactive
metals were prepared. A systematic study of the electrochemical effect of the Lewis acidic
metal ions on the iron cluster reduction potentials was carried out. Varying the Lewis acidity
of the capping metal from Ca
2+
to Sc
3+
shifted the redox potentials of these clusters by over
500 mV. These results support the generality of the role redox-inactive metals can play in
modulating the redox potential of the redox-active centers via
μ
4
-oxo and/or
μ
2
-hydroxo
ligands. Current studies are focused on the effects of redox-inactive metals on the physical
properties and chemical reactivity of other metal-oxido compounds of varying structure,
metal character, oxidation state and oxido content to better understand the fundamental basis
for multi-electron, multi-proton catalysis by complex metal clusters.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
This work was supported by the California Institute of Technology, the Searle Scholars Program, the NSF
CAREER CHE-1151918 (T.A.), a Camille & Henry Dreyfus Environmental Chemistry Fellowship (D.E.H.), and a
Resnick Sustainability Institute graduate fellowship (D.L.). T.A. is a Sloan and Cottrell Fellow. We thank Larry M.
Henling and Dr. Michael Takase for assistance with crystallography and Prof. Jonas C. Peters for use of a
Mössbauer spectrometer. The Bruker KAPPA APEXII X-ray diffractometer was purchased via an NSF Chemistry
Research Instrumentation award to Caltech (CHE -0639094). We acknowledge the Gordon and Betty Moore
Foundation, the Beckman Institute, and the Sanofi-Aventis BRP at Caltech for their generous support of the
Molecular Observatory at Caltech. Operations at SSRL are supported by the US DOE and NIH.
References
1. (a) Fukuzumi S, Ohkubo K. Coord Chem Rev. 2010; 254:372.(b) Fukuzumi, S. In Prog Inorg Chem.
Karlin, KD., editor. Vol. 56. John Wiley & Sons Inc; New York: 2009. p. 49
2. (a) Yocum CF. Coord Chem Rev. 2008; 252:296.(b) Umena Y, Kawakami K, Shen JR, Kamiya N.
Nature. 2011; 473:55. [PubMed: 21499260] (c) Ferreira KN, Iverson TM, Maghlaoui K, Barber J,
Iwata S. Science. 2004; 303:1831. [PubMed: 14764885]
Herbert et al.
Page 4
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
3. (a) Fukuzumi S, Morimoto Y, Kotani H, Naumov P, Lee YM, Nam W. Nat Chem. 2010; 2:756.
[PubMed: 20729896] (b) Morimoto Y, Kotani H, Park J, Lee YM, Nam W, Fukuzumi S. J Am
Chem Soc. 2011; 133:403. [PubMed: 21158434]
4. (a) Park YJ, Ziller JW, Borovik AS. J Am Chem Soc. 2011; 133:9258. [PubMed: 21595481] (b)
Park YJ, Cook SA, Sickerman NS, Sano Y, Ziller JW, Borovik AS. Chem Sci. 2013; 4:717.
[PubMed: 24058726]
5. (a) Li F, Van Heuvelen KM, Meier KK, Muenck E, Que L Jr. J Am Chem Soc. 2013; 135:10198.
[PubMed: 23802702] (b) Lee YM, Bang S, Kim YM, Cho J, Hong S, Nomura T, Ogura T,
Troeppner O, Ivanovic-Burmazovic I, Sarangi R, Fukuzumi S, Nam W. Chem Sci. 2013; 4:3917.
6. Chen J, Lee YM, Davis KM, Wu X, Seo MS, Cho KB, Yoon H, Park YJ, Fukuzumi S, Pushkar YN,
Nam W. J Am Chem Soc. 2013; 135:6388. [PubMed: 23324100]
7. Leeladee P, Baglia RA, Prokop KA, Latifi R, de Visser SP, Goldberg DP. J Am Chem Soc. 2012;
134:10397. [PubMed: 22667991]
8. (a) Horwitz CP, Ciringh Y, Weintraub ST. Inorg Chim Acta. 1999; 294:133.(b) Horwitz CP, Ciringh
Y. Inorg Chim Acta. 1994; 225:191.
9. (a) Risch M, Klingan K, Ringleb F, Chernev P, Zaharieva I, Fischer A, Dau H. Chem Sus Chem.
2012; 5:542.(b) Zaharieva I, Najafpour MM, Wiechen M, Haumann M, Kurz P, Dau H. Energy
Environ Sci. 2011; 4:2400.(c) Wiechen M, Zaharieva I, Dau H, Kurz P. Chem Sci. 2012; 3:2330.(d)
Najafpour MM, Pashaei B, Nayeri S. Dalton Trans. 2012; 41:4799. [PubMed: 22382465] (e)
Najafpour MM, Ehrenberg T, Wiechen M, Kurz P. Angew Chem Int Ed. 2010; 49:2233.
10. Tsui EY, Tran R, Yano J, Agapie T. Nat Chem. 2013; 5:293. [PubMed: 23511417]
11. (a) Tsui EY, Agapie T. Proc Natl Acad Sci USA. 2013; 110:10084. [PubMed: 23744039] (b)
Kanady JS, Tsui EY, Day MW, Agapie T. Science. 2011; 333:733. [PubMed: 21817047]
12. (a) Singh A, Spiccia L. Coord Chem Rev. 2013; 257:2607.(b) Bockris JO, Otagawa T. J
Electrochem Soc. 1984; 131:290.(c) Cheng FY, Chen J. Chem Soc Rev. 2012; 41:2172. [PubMed:
22254234] (d) Neburchilov V, Wang HJ, Martin JJ, Qu W. J Power Sources. 2010; 195:1271.
13. Tsui EY, Kanady JS, Day MW, Agapie T. Chem Commun. 2011; 47:4189.
14. Tsui EY, Day MW, Agapie T. Angew Chem Int Ed. 2011; 50:1668.
15. (a) Reynolds RA, Coucouvanis D. Inorg Chem. 1998; 37:170.(b) Schmitt W, Anson CE, Pilawa B,
Powell AK. Z Anorg Allg Chem. 2002; 628:2443.(c) Chardon-Noblat S, Horner O, Chabut B,
Avenier F, Debaecker N, Jones P, Pecaut J, Dubois L, Jeandey C, Oddou JL, Deronzier A, Latour
JM. Inorg Chem. 2004; 43:1638. [PubMed: 14989656] (d) Singh AK, Jacob W, Boudalis AK,
Tuchagues JP, Mukherjee R. Eur J Inorg Chem. 2008:2820.(e) Lalia-Kantouri M, Papadopoulos
CD, Hatzidimitriou AC, Bakas T, Pachini S. Z Anorg Allg Chem. 2010; 636:531.
16. Shannon RD. Acta Crystallogr A. 1976; 32:751.
17. Ghanotakis DF, Babcock GT, Yocum CF. FEBS Lett. 1984; 167:127.
18. Perrin, DD. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. Pergamon
Press; New York: 1982.
Herbert et al.
Page 5
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Figure 1.
Truncated solid-state structures of (a)
1
-Ca, (b)
1
-Sr, (c)
2
-Sc, (d)
1
-Zn, (e)
2
-Ca and (f)
2
-
La. Portions of the ligand (L), hydrogen atoms and outer-sphere anions are omitted for
clarity. Thicker lines emphasize the [MFe
3
O
2
] moiety.
Herbert et al.
Page 6
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Figure 2.
Zero-field
57
Fe Mössbauer spectra for
1
-Ca and
2
-Ca: (80 K, data: black dots, spectral fit:
green line, deconvolution: red and blue lines; residual: grey dots.
Herbert et al.
Page 7
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Figure 3.
Cyclic voltammograms corresponding to the [MFe
III
3
O(OH)]/[MFe
III
2
Fe
II
O(OH)] redox
couple (M = Sc
3+
, La
3+
, Zn
2+
, Ca
2+
, and Sr
2+
) in 0.1 M NBu
4
PF
6
in CH
2
Cl
2
/1,2-DME
(9:1). Scan rate of 200 mV/s. Potentials are referenced to Fc/Fc
+
.
Herbert et al.
Page 8
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Figure 4.
Reduction potentials of MFe
3
O(OH) complexes (green circles), MMn
3
O
2
complexes
10
(blue
diamonds), and MMn
3
O
4
complexes
11a
(red squares) vs. p
K
a
of the corresponding
M(aqua)
n
+
ion as a measure of Lewis acidity. Potentials were referenced to Fc/Fc
+
.
Herbert et al.
Page 9
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Scheme 1.
Synthesis of complexes
1
-M (M = Ca, Sr, Zn),
2
-M (M = Ca, Sc, La).
Herbert et al.
Page 10
J Am Chem Soc
. Author manuscript; available in PMC 2014 December 26.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript