The effects of redox-inactive metal ions on the activation of dioxygen: Isolation and characterization of a heterobimetallic complex containing a MnIII-(μ-OH)-CaII core

Article abstract of DOI:10.1021/ja203458d

Rate enhancements for the reduction of dioxygen by a Mn^II complex were observed in the presence of redox-inactive group 2 metal ions. The rate changes were correlated with an increase in the Lewis acidity of the group 2 metal ions. These studie

Full text of DOI:10.1021/ja203458d

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The Effects of Redox-Inactive Metal Ions on the Activation of
Dioxygen: Isolation and Characterization of a Heterobimetallic
Complex Containing a MnIIIꢀ(μ-OH)ꢀCaII Core
Young Jun Park, Joseph W. Ziller, and A. S. Borovik*
Department of Chemistry, University of California-Irvine, 1102 Natural Sciences II, Irvine, California 92697, United States
S Supporting Information
b
secondary coordination spheres of metal ions. Most of our
ABSTRACT: Rate enhancements for the reduction of
dioxygen by a MnII complex were observed in the presence
of redox-inactive group 2 metal ions. The rate changes were
correlated with an increase in the Lewis acidity of the group
2 metal ions. These studies led to the isolation of hetero-
bimetallic complexes containing MnIIIꢀ(μ-OH)ꢀMII cores
(MII = CaII, BaII) in which the hydroxo oxygen atom is
derived from O2. This type of core structure has relevance to
the oxygen-evolving complex within photosystem II.
ligands contain H-bond donors, such as the urea-based ligand
[H3buea]3ꢀ, whose inward placement of NH groups produces
a positively polarized cavity (Figure 1). To reverse the polarity
of the cavity, we designed the sulfonamido-based tripodal li-
gand N,N0,N00-[2,20,200-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimeth-
ylbenzenesulfonamido) ([MST]3ꢀ, Figure1)15 andshowedthat the
SO2R groups can serve as H-bond acceptors. Previously reported
ligands with sulfonamido groups are almost exclusively C2-
symmetric, such as those of Walsh,16 who showed that both
nitrogen and oxygen atoms are capable of coordinating to a metal
ion. During the course of our studies with the MnII complex
[MnIIMST]ꢀ, we discovered that the oxygen atoms of the SO2Ar
groups can bind group 2 metal ions, which appears to be a
requirement for the acceleration of the dioxygen activation
process.
etal ions that function as Lewis acids are known to have a
M
major influence on a variety of chemical transformations.1
Often they are used in combination with redox-active transi-
tion-metal complexes to promote a variety of reactions invol-
ving the transfer of electrons.2 This effect is typified in
metalloproteins such as the copperꢀzinc superoxide dismu-
tases, in which both metal ions have been proposed to be
functionally active.3 In addition, it is now widely accepted that a
redox-inactive CaII ion is necessary for oxidation of water to
dioxygen in the oxygen-evolving complex (OEC).4,5 Examples of
these effects are also found in synthetic systems: Yu showed that
group1metalionspromoteCꢀHbondactivationinaseriesofPdII
complexes;6 Lau illustrated that Lewis acidic metal ions enhance
the reactivity of metalꢀoxo complexes toward oxidation of
alkanes;7 and Collins demonstrated that secondary ions activate
O-atom transfer reactions of MnVdO complexes.8 Moreover,
Fukuzumi and Nam recently reported that the rates of electron
transfer from FeIVdO complexes are significantly altered in the
presence of Lewis acidic metal ions, which led to the isolation of a
complex containing an FeIVdO---ScIII unit.9 Fukuzumi further
demonstrated that the rate of the one-electron conversion of O2 to
superoxide ion is correlated with the Lewis acidities of redox-
inactive metal ions,10 implying that redox-inactive metal ions may
play a role in the activation of dioxygen.11 We report herein that the
rates of O2 reductionbyamonomeric MnII complex are accelerated
in the presence of group 2 metal ions. These findings led to the
isolation and structural characterization of a discrete heterobime-
The preparation of [MnIIMST]ꢀ was accomplished by treat-
ment of a N,N-dimethylacetamide (DMA) solution of H3MST
with 3 equiv of NaH followed by the addition of Mn(OAc)2.17
Metathesis with [NMe4](OAc) afforded [NMe4][MnIIMST] in
90% yield after workup (Scheme 1). This salt showed limited
reactivity with dioxygen at 25 °C, with an initial rate of 6.2 ꢁ
10ꢀ6 sꢀ1 as assayed by optical spectroscopy (Figure 2). The low
rate of the reaction prevented isolation of any oxidized species.
However, we found that the rate of the reaction with dioxygen
was accelerated in the presence of group 2 metal ions. For
instance, an initial rate of 5.8 ꢁ 10ꢀ4 sꢀ1 was obtained when the
mixture of [NMe4][MnIIMST] and dioxygen was treated with 1
equiv of Ca(OTf)2/15-crown-5 (Figures 2 and 3). Addition of
larger amounts of Ca(OTf)2/15-crown-5 did not change this rate
substantially, as only a small decrease in the initial rate was
observed (Figure S1 in the Supporting Information).17 The
initial rate for this reaction was also affected when the
[NMe4][MnIIMST]/dioxygen mixture was treated with 1 equiv
of Ba(OTf)2/18-crown-6, which gave an initial rate of 1.8 ꢁ
10ꢀ5 sꢀ1 18 Although the role of the group 2 metal ions remains
.
to be clarified, these results clearly indicate that the rate of O2
activation by [MnIIMST]ꢀ is dependent upon the presence of
redox-inactive metal ions.
The major product of the reaction with Ca(OTf)2/15-crown-
5 was obtained in yields of 50ꢀ60% and exhibited a visible
absorption spectrum with features at λmax/nm (εM/cmꢀ1 Mꢀ1) =
450 (340), 640 (600), and 800 (sh) (Figure 3 inset) that matched
the final spectrum obtained from the reaction. Analytical data
tallic complex containing a MnIIIꢀ(μ-OH)ꢀCaII core,12,13
a
construct that is related to the active site of the OEC in
photosystem II.
Our group has been investigating the influences of the
secondary coordination sphere on metal-mediated processes.14
In particular, we have developed ligands that promote intramo-
lecular hydrogen-bonding (H-bonding) networks within the
Received: April 14, 2011
Published: May 19, 2011
r
2011 American Chemical Society
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|

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Figure 1. Examples of systems containing intramolecular H-bonds.
Scheme 1. Reactions Involving [MnIIMST]ꢀ
Figure 3. Electronic absorbance spectra for the reduction of dioxygen in
the presence of [MnIIMST]ꢀ and Ca(OTf)2/15-crown-5 in CH2Cl2 at
25 °C. Spectra were recorded every 30 min. The inset shows the spectrum
of the isolated product, [15-crown-5⊃CaIIꢀ(μ-OH)ꢀMnIIIMST]þ.
the hydroxo ligand undergoes water/hydroxo exchange reac-
tions (Figure S3).17
The molecular structure of [15-crown-5⊃CaIIꢀ(μ-OH)ꢀ
MnIIIMST]þ was determined by X-ray diffraction, which re-
vealed the predicted heterobimetallic complex (Figure 4). The
MnIII center has an N4O primary coordination sphere with
trigonal-bipyramidal geometry. The nitrogen atoms of the
[MST]3ꢀ ligand coordinate to the MnIII ion with a Mn1ꢀN1
bond length of 2.075(2) Å and an average Mn1ꢀNeq bond
distance of 2.052(2) Å. The primary sphere of the MnIII center is
completed by a hydroxo ligand having a Mn1ꢀO7 bond distance
of 1.829(2) Å and a N1ꢀMn1ꢀO7 bond angle of 176.9(1)°.
Notably, each SO2Ar group adopts a conformation in which one
of the SdO bonds is positioned nearly perpendicular to the
equatorial plane, with an average MnꢀNꢀSꢀO torsion angle of
22.8(2)°. This configuration of the SO2Ar groups produces a
cavity that forms an intramolecular H-bond between the
MnIIIꢀOH unit and O5 of the [MST]3ꢀ ligand [O7 O5
3 3 3
distance = 2.693(2) Å].19
The most striking feature of the structure is the placement of a
CaII ion within the secondary coordination sphere of the MnIII
center. The CaII ion interacts with the complex through O7 of the
MnIIIꢀOH unit and O1 and O3 from two of the sulfonamide
groups of the [MST]3ꢀ ligand; it appears that these oxygen
atoms are positioned in such a way that they form an auxiliary
face-capping site for the binding of additional metal ions. The O7
atom of the hydroxo ligand bridges the two metal centers with a
Figure 2. Initial rate data for the reaction of [NMe4][MnIIMST]ꢀ and
O2 alone (black 9, 6.2 ꢁ 10ꢀ6 sꢀ1) and in the presence of Ca(OTf)2/
(gray b, 1.8 ꢁ 10ꢀ5 sꢀ1). Reactions were performed in CH2Cl2 at 25 °C.
Mn1ꢀO7ꢀCa1 bond angle of 127.49(9)° and a CaII MnIII
3 3 3
separation of 3.748(2) Å. The Ca1---O7 bond distance is
2.342(2) Å, which is similar to the Ca1---O1 and Ca1---O3 bond
lengths of 2.333(2) and 2.370(2) Å. The five oxygen atoms of the
crown ether fill the remaining coordination sites on Ca1, and the
average Ca1---O15-crown-5 distance is 2.48 Å. Also, the 15-crown-5
ligand adopts an ‘‘inverted umbrella’’ structure, whereby the CaII
ion is displaced out of the crown ether toward the [MST]3ꢀ
ligand.20
15-crown-5 (black [, 5.8 ꢁ 10ꢀ4
s
ꢀ1) or Ba(OTf)2/18-crown-6
suggested a molecular formulation of [15-crown-5⊃CaIIꢀ
(μ-OH)ꢀMnIIIMST]OTf, which was supported by electrospray
ionization mass spectrometry (ESI-MS) experiments. The ESI-
MSþ spectrum contained a strong ion signal at m/z 1021.27
when the complex was prepared with 16O2; the mass and
calculated isotopic distribution were consistent with [15-crown-
5⊃CaIIꢀ(μ-OH)ꢀMnIIIMST]þ (Figure S2).17 When the reac-
tion was performed under 18O2, the molecular ion peak shifted
by 2 mass units to m/z 1023.27, indicating that the oxygen atom
of the hydroxo ligand was derived from the activation of
dioxygen. Moreover, our mass spectral studies indicated that
To our knowledge, [15-crown-5⊃CaIIꢀ(μ-OH)ꢀMnIIIMST]þ
represents the first example of a structurally characterized species
containing a MnIIIꢀ(μ-OH)ꢀCaII core. In fact, discrete
CaIIꢀOH systems are rare, with the LCaIIꢀ(μ-OH)2ꢀCaIIL
homodimer of Roesky the only other structurally characterized
complex.21 In that complex, each calcium center is five-coordinate,
and the CaꢀO(H) distances (<2.25 Å) are significantly shorter
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Scheme 2. Proposed Role of the Redox-Inactive Metal Ions
in the Activation of Dioxygen
Figure 4. Thermal ellipsoid diagram depicting the molecular structure of
[15-crown-5⊃CaIIꢀ(μ-OH)ꢀMnIIIMST]þ. Ellipsoids are drawn at the
50% probability level, and for clarity, only the hydroxo hydrogen atom has
been shown. Selected bond lengths (Å) and angles (deg): Mn1ꢀO7,
1.829(2); Mn1ꢀN1, 2.075(2); Mn1ꢀN2, 2.019(2); Mn1ꢀN3, 2.107(2);
Mn1ꢀN4, 2.029(2); Mn1---Ca1, 3.7478(6); Ca1ꢀO1, 2.332(2);
Ca1ꢀO3, 2.369(2); Ca1ꢀO7, 2.342(2); Ca1ꢀO8, 2.490(3); Ca1ꢀO9,
2.508(2); Ca1ꢀO10, 2.492(2); Ca1ꢀO11, 2.486(2); Ca1ꢀO12,
2.453(2); O7ꢀMn1ꢀN2, 96.95(8); O7ꢀMn1ꢀN4, 96.58(8); N2ꢀ
Mn1ꢀN4, 131.81(10); O7ꢀMn1ꢀN1, 176.94(9); N2ꢀMn1ꢀN1,
81.83(9); N4ꢀMn1ꢀN1, 82.25(9); O7ꢀMn1ꢀN3, 101.09(8); N2ꢀ
Mn1ꢀN3, 109.38(9); N4ꢀMn1ꢀN3, 112.93(9); N1ꢀMn1ꢀN3,
81.97(9); Mn1ꢀO7ꢀCa1, 127.49(9).
heterobimetallic MnIICaII complex that coordinates dioxygen
and then facilitates electron transfer to form a putative super-
oxido adduct. The rate of intramolecular electron transfer to
generate a putative MnIIIꢀ(superoxo)ꢀMII species would de-
pend on the Lewis acidity of the group 2 metal ion, which would
help alleviate the buildup of negative charge. This possible
function of the redox-inactive metal ion is reminiscent of the
role played by the H-bonding cavities in the metal complexes of
our urea-based tripods (Figure 1). In these systems, the positively
polarized cavities promote the binding and activation of dioxygen
through the formation of intramolecular H-bonds.14,23 The
results found with [MnIIMST]ꢀ illustrate the possibility that
redox-inactive metal ions can also be used to facilitate the
activation of dioxygen, which in turn may provide another way
to control and enhance the reactivity of transition-metal
complexes.
than those observed in [15-crown-5⊃CaIIꢀ(μ-OH)ꢀMnIIIMST]þ.
We are also aware of only three other structurally characterized
molecular systems containing calcium/manganese bridged units:
the Mn13Ca2 cluster prepared by Christou,22a the Mn4Ca system
of Powell,22b and the Mn3CaNa cluster of Reedijk.22c
Studies of the properties of heterometallic complexes with
manganese/calcium cores have been sparked by the OEC of
photosystem II. Sufficient information is available to show that
the active site of the OEC contains a Mn4Ca cluster that is
surrounded by a network of H-bonds. Most current mechanistic
proposals stipulate that the calcium center has a direct role in water
oxidation,4c,5 a premise that is supported by a recent X-ray structure
of the OEC at a resolution of 1.9 Å.4f This structure shows that the
CaII ion has two bound water molecules and is connected to the
manganese centers through oxygen-containing bridging ligands,
some of which could possibly be oriented in a manner similar to
that found in [15-crown-5⊃CaIIꢀ(μ-OH)ꢀMnIIIMST]þ. More-
over, the isolation of [15-crown-5⊃CaIIꢀ(μ-OH)ꢀMnIIIMST]þ
indicates that inclusion of sulfonamido groups into ligands may
provide a new synthetic method for the preparation of these types
of heterometallic complexes.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, support-
b
ing tables and figures, and crystallographic data (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
aborovik@uci.edu
We have presented data showing that group 2 metal ions have a
marked influence on the rate of dioxygen activation by a mono-
meric MnII complex. Our findings showed an increase in the initial
rate in the presence of CaII and BaII ions, with the reaction in the
presence of CaII ions being faster by a factor of 35. These results
complement those reported by Fukuzumi,10a who showed that the
rate of electron transfer from [CoII(tetraphenylporphyrin)] to O2
is also accelerated in the presence of CaII and BaII ions; in this
one-electron transfer process, CaII ions increased the rate by a
factor of 22 relative to BaII ions. The similarity of those rate data
to the data for our systems offers a possible explanation of why
dioxygen activation with [MnIIMST]ꢀ depends on the presence
of group 2 ions (Scheme 2). The reduction of dioxygen to
superoxide could be dependent on the prior formation of a
’ ACKNOWLEDGMENT
Financial support from the NIH (GM050781) is acknowl-
edged. Y.J.P. also acknowledges EXAX Inc. for partial support.
We thank R. Zarkesh for collecting the X-ray data and Professor
L. Doerrer for helpful advice.
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