EXAFS Spectroscopic Analysis of Heterobinuclear TiOMn Charge

ARTICLE pubs.acs.org/JPCC

EXAFS Spectroscopic Analysis of Heterobinuclear TiOMn Charge-Transfer Chromophore in Mesoporous Silica H. S. Soo, M. L. Macnaughtan, W. W. Weare,† J. Yano, and H. Frei* Physical Biosciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, United States

bS Supporting Information ABSTRACT: Analysis of extended X-ray absorption fine structure (EXAFS) measurements of binuclear TiIVOMnII visible light charge-transfer chromophores anchored on silica nanopore surfaces reveals an oxo-bridged structure of a Ti and a Mn center. For TiMn-SBA-15 samples with 1 mol % for each metal, Ti and Mn K-edge EXAFS curve fitting indicates a common TiMn distance of 3.3 Å (Ti edge: 3.36 ( 0.05 Å; Mn edge: 3.25 ( 0.07 Å) and a bond angle of 111°. The first sphere coordination of the two metal centers, a distorted tetrahedral for Ti and a pseudo octahedral for Mn, is largely preserved upon formation of the linkage of the binuclear unit. Increasing the Ti loading from 1 to 3% does not introduce titania clusters, whereas loading of Mn beyond 2% leads to some Mn oxide cluster formation. The binuclear unit retains its structural integrity upon prolonged exposure to air or heating at high temperature (350 °C) in the presence of oxygen. The oxidation state increase of the Mn center upon calcination is accompanied by a shortening of the oxo bridge. The results provide the first detailed structural information on the TiIVOMnII MMCT unit, which is a promising candidate as a visible light charge-transfer chromophore for driving multielectron catalysts for artificial photosynthesis.

1. INTRODUCTION The development of robust charge-transfer chromophores that absorb visible light to drive multielectron catalysts for water oxidation, hydrogen evolution, or carbon dioxide reduction is a critical challenge for developing artificial photosynthetic systems.1,2 Although many organic chromophores have high quantum efficiencies, they offer challenges in terms of long-term stability under exposure to intense sunlight in ambient environments. All-inorganic heterobinuclear units linked by μ-oxo bridges in inert supporting scaffolds like mesoporous silica exhibit tunability of redox properties similar to organic molecules, while showing the robustness of inorganic matter.312 With a judicious choice of the two metal components, the metalto-metal charge-transfer (MMCT) units can absorb light across the visible spectrum, resulting in a transient charge-transfer excited state that can drive chemical reactions.5,8,9,11,12 Proper selection of donor or acceptor metal centers allows us to closely match the redox potentials of the chromophore and catalyst, which is essential for converting a maximum fraction of the photon energy to chemical energy of the product. By nature of their oxidative and thermal stability, MMCT chromophores are suitable candidates as photosensitizers in scalable, integrated, artificial photosynthetic systems.1,312 With their wide range of oxidation states and redox potentials, MMCT units can serve as tunable single photon, single electron pumps for driving catalysts. As a demonstration of their synthetic accessibility, MMCT units containing TiIV or ZrIV acceptors r 2011 American Chemical Society

bridged to donors such as CrIII,5,6 MnII,3,12 FeII,10 CoII,4,6 and CeIII6,9 have been prepared recently. The MMCT units were shown to transfer holes to iridium oxide clusters,5 which were capable of driving visible light water oxidation at remarkable efficiency. Moreover, photoexcitation of the MMCT state of ZrIVOCuI units anchored in mesoporous silica scaffolds was found to result in carbon dioxide reduction to carbon monoxide.8 By appropriately selecting the donor or acceptor metals, the redox potentials of the MMCT can be matched to drive a desired catalytic reaction. Details of the geometrical and electronic structure of the heterobinuclear units on nanoporous silica developed thus far (oxo bridge, oxidation state of donor and acceptor metal centers, coordination geometry) are mainly based on observations with UVvis, FT-IR, FT-Raman, EPR, and L-edge and K-edge X-ray absorption spectroscopy. For example, observation of the MMCT optical absorption of TiOCoII in MCM41 mesoporous silica was found to require covalent linkage of the donor and acceptor center.4 Moreover, the CuIO infrared mode of the ZrOCuI unit was directly observed by action spectroscopy when CuIO was oxidized to CuIIO upon MMCT-induced photoreaction with CO2.8 The infrared asymmetric stretch mode of a TiOFeII unit in SBA-15 reported by Nakamura et al.10 also provides direct evidence for the formation Received: August 31, 2011 Revised: November 2, 2011 Published: November 02, 2011 24893

dx.doi.org/10.1021/jp208419q | J. Phys. Chem. C 2011, 115, 24893–24905

The Journal of Physical Chemistry C

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Table 1. Ti K-Edge EXAFS Curve-Fitting Results for Ti-1-SBA, Ti-1-MCM, and Ti-3-MCM sample Ti-1-SBA

r (Å)

N (S02 = 0.83)

σ2 (10‑3 Å2)

E0 (eV)

R factor (10‑3)

TiO

1.85 ( 0.01

3.4 ( 0.3

6.7

0.87

1.1

TiSi

3.17 ( 0.02

2.0 ( 0.3

6.2

TiOSia

3.29 ( 0.02

2.0 ( 0.3a

0.50

9.9

6.7

1.3

shell

138° Ti-1-MCM

21

b

TiO

1.83 ( 0.02

3.6 ( 0.6

2.3

TiSi

3.19 ( 0.05

3.0 ( 0.4

5.1

TiOSia

3.36 ( 0.08

3.0 ( 0.4a

22

b

Ti-3-MCM

TiO

129° 3.58 ( 0.09

3.0 ( 0.4

2.8

TiO

1.87 ( 0.01

4.0 ( 0.8

2.2

TiO

2.12 ( 0.04

1.5 ( 0.1

6.0

TiSi

3.19 ( 0.02

2.8 ( 0.7

TiOSia

3.32 ( 0.02

2.8 ( 0.7a

5.5 20

135° b a

TiOSi three-leg multiple scattering (MS) path with the coordination number set to be identical as the coordination number as the TiSi single scattering path. b Angle derived by application of the trigonometric cosine rule, by using the MS path length to calculate the SiO bond length and using the three known distances in the TiOSi triangle to obtain the TiOSi angle.

of an oxo-bridged unit. Optical, EPR, XANES, and L- and K-edge X-ray absorption measurements have revealed detailed information on the oxidation state and coordination geometry of TiOCoII,4,6 TiOCeIII,6,9 TiOFeII,10 TiOCu,I7 ZrOCu,I8 TiOSnII,7 and TiOCrIII,5,6 units anchored on MCM-41 or SBA-15 silica nanopores. Recently, our group reported the synthesis and characterization of a TiIVOMnII MMCT unit anchored in mesoporous silica MCM-41 and SBA-15.12 The chromophore absorbs throughout the visible region with an onset around 650 nm. L-edge X-ray absorption spectroscopy showed that the Ti center of the binuclear unit is a TiIV with distorted tetrahedral coordination, whereas the Mn center is in oxidation state II, confirming previous results from EPR and optical data.3 Using transient optical absorption spectroscopy, we determined the lifetime and back electron transfer kinetics after excitation of the TiIVOMnII MMCT unit, and found an unusually long lifetime of 1.8 ( 0.3 μs at room temperature.3 The fact that MnIII in the excited state has sufficient potential to drive a water oxidation catalyst while transient TiIII can be coupled to a CO2 reduction catalyst makes this a particularly promising chromophore. Therefore, we sought detailed structural characterization by extended X-ray absorption fine structure (EXAFS) spectroscopy. This is an established technique for the elucidation of bonding parameters in both crystalline and amorphous substrates,1317 including the natural photosynthetic oxygen-evolving complex of photosystem II.18 Both XANES and EXAFS spectroscopy have been applied to resolve the oxidation states and structural parameters of metal centers substituted or anchored in mesoporous silica such as MCM-4119 or SBA-1520 including titanium2127 and manganese.2832 According to previous reports, calcined and thoroughly dehydrated samples contain Ti coordinated to silica framework oxygens in a distorted tetrahedral geometries.2124 Extra framework TiO2 clusters in octahedral environments are observed only at relatively high Ti loading levels (>3%).27 In the work reported here, we have exploited the element specific nature of Ti and Mn K-edge X-ray absorption and EXAFS measurements and analysis to probe more closely the coordination environment around both Ti and Mn. In particular,

detailed structural information on the oxo-bridged heterobinuclear unit was sought by EXAFS spectroscopy and curve fitting analysis. Both XANES and EXAFS for Ti and Mn show that 1 mol % loading of the metals yield mostly isolated TiIVOMnII sites, whereas neighboring Ti or Mn clusters is postulated for loadings of 2 mol percent or higher. Furthermore, the structural integrity of the TiOMn unit under oxidation conditions was demonstrated.

2. EXPERIMENTAL SECTION The synthesis of mesoporous silica SBA-15 or MCM-41 featuring isolated TiIV or MnII centers, or containing binuclear TiIVOMnII units has been reported previously.12 The characterization includes optical diffuse reflectance spectroscopy (DRS), EPR, Mn K-edge XANES, and XRD.12 X-ray absorption spectroscopy (XAS) was performed at Beamline 7-3 (Mn K-edge) and Beamline 4-3 (Ti K-edge) at Stanford Synchrotron Radiation Lightsource (SSRL). The synchrotron ring SPEAR operates at 3.0 GeV with a beam current in the range of 100200 mA. Details of the Mn K-edge measurements at Beamline 7-3 are described in previous reports.11,12 Ti K-edge XAS measurements were conducted at Beamline 4-3 using a liquid-N2-cooled Si(111) double crystal monochromator. Spectra were collected with a silicon drift detector (Vortex ME4) at 40 K, using a liquid He cryo-stream for sample cooling at ambient pressure. The intensity of the incident X-ray beam was monitored by a He-filled ion chamber (I0) in front of the sample. For energy calibration, a Ti foil was used with E0 = 4966.0 eV, corresponding to the first peak of the first derivative on the Ti0 edge. Data processing and analysis were carried out as reported before using SixPACK version 0.67 and IFEFFIT version 1.2.10a EXAFS analysis software package. Data processing, including energy calibration and averaging of scans, was performed using SixPACK. Energy alignment, final data averaging by aligning and merging of scans, background subtraction (including pre-edge subtraction and k3-weighted spline correction), as well as forward Fourier transform (FT) of k3-weighted data (with a Hanning window) were performed using Athena version 0.8.054.33 For the energy (eV)-to-wave vector (k, Å1) axis conversion, E0 was 24894

dx.doi.org/10.1021/jp208419q |J. Phys. Chem. C 2011, 115, 24893–24905


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Figure 1. (a) FT and fit of k3-weighted Ti K-edge EXAFS of Ti-1-SBA. (b) Ti K-edge XANES of Ti-1-MCM (black), Ti-3-MCM (red), Ti-1-SBA (blue), TiMn-1,1-SBA (green), and calcined TiMn-1,1-SBA (gray). The inset is the expanded data of Ti pre-edge region. (c) FT and fit of k3-weighted Ti K-edge EXAFS of Ti-3-MCM. (d) Fit of k3-weighted Ti K-edge EXAFS for TiMn-1,1-SBA. In panels a, c, and d, the insets are the Ti K-edge fits in k-space up to k = 13.5, 11.5, and 15.0, respectively.

defined as 4977.0 eV for Ti and 6547.6 eV for Mn. The k-space data were truncated near the zero crossings at low k (typically 2.03.0 Å1) and at high k (typically 11.815.0 Å1), depending on the quality of the data set. Ti or Mn K-edge EXAFS data fittings were performed using Artemis version 0.8.011. Artemis works within the framework of FEFF’s multiple-scattering path expansion by describing data as a summation of one or more scattering paths.13,15,33 The EXAFS equation can be described as follows: χðkÞ ¼ S20

N

∑j krj2j f eff ðπ, k, rj Þe2σ k e2r =λ ðkÞ sinð2krj þ ϕijðkÞÞ 2 2 j

j

j

j

ð1Þ

The coordination sphere around the central atom(s) is divided into j shells, with all atoms possessing the same (or adjacent) atomic numbers and distance from the central atom grouped into a single shell. Within each shell, the coordination number Nj is defined as the number of atoms in shell j at a distance of rj from the central atom, i. feffj (π,k,rj) is the ab initio calculated2 amplitude 2 function for shell j, and the DebyeWaller term e2σj k accounts for the amplitude attenuation due to static and thermal disorder in the measured absorber-backscatter distances. The term e(2rj)/(λj(k)) can be attributed to losses arising from inelastic scattering, where λj(k) is the electron mean free path. The oscillations in the EXAFS spectrum are reflected in the term sin(2krj + ϕij(k)), where ϕij(k) is the ab initio phase calculated for shell j. The sinusoidal term provides the direct correspondence between the frequency of the EXAFS oscillations in k-space, and the absorber-back scatterer distance rj. The EXAFS equation was used to fit the Fourier reduced experimental data, with N, r, and EXAFS DebyeWaller factor

(σ2) as variable parameters (see Tables 15). Each shell has been labeled as M-X (M = Ti, Mn; X = N, O, Si, M) to define the jth coordination sphere around the central metal M. E0 is a variable in the Artemis program corresponding to the energy shift from k = 0. S02 is an amplitude reduction factor due to shakeup/shake-off processes at the central atom(s) and the values have been set at 0.83 for Ti and 0.85 for Mn, based on fits to model compounds from previously published work.11,18,26 The data were fit in R-space, with typical R-ranges of 1.04.0 Å and k-ranges of 2.015.0 Å1. Estimations of the goodness of fit and the uncertainty in the parameters of the fits are described in detail in the Supporting Information. The fit numbers listed in Table 3 correspond to distinct models that can be directly compared based on their reduced χ2 values.

3. RESULTS 3.1. Structure of Anchored TiIV and MnII Centers. X-ray

absorption measurements were conducted up to the EXAFS region of both the Mn and the Ti K-edge involving samples with varying Ti and Mn concentrations. For these experiments, samples of SBA-15 structure were mainly used.20 The FT of the Ti K-edge EXAFS of 1 mol % Ti loaded in SBA-15, designated Ti-1-SBA (Figure 1a) is best fit with a first-shell TiO distance of 1.85 Å and an average coordination number of 3.4. The curve fitting results are summarized in Table 1. These bond parameters are consistent with a distorted tetrahedral coordination environment of TiIV anchored in the silica framework and agree with EXAFS analysis of isolated Ti center in other porous silica materials.2125 Although the pre-edge intensity of 24895



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Figure 2. (a) Mn K-edge XANES of Mn-1-SBA (black), Mn-2-SBA (red), oxidized Mn-2-SBA (blue), and calcined TiMn-1,1-SBA (green). (b) k3weighted Mn K-edge EXAFS fit of Mn-1-SBA. (c) k3-weighted Mn K-edge EXAFS fit of Mn-2-SBA. (d) k3-weighted Mn K-edge EXAFS fit for TiMn-1,1SBA. In panels bd, the insets are the Mn K-edge fits in k-space up to k = 11.5, 12.0, and 12.0, respectively.

the Ti-1-SBA sample is low (Figures 1b) the coordination number derived from the EXAFS curve fit points to a distortion by the silica scaffold as the cause, rather than coordination of water molecules.2123 This is plausible because the Ti-1-SBA samples were calcined at 550 °C and dehydrated at 250300 °C prior to EXAFS measurements. The broad signal in the FT of the Ti K-edge EXAFS centered around an apparent distance of 3 Å (Figure 1) does not fit a single shell of either Si or O; two subshells are needed to obtain reasonable agreement.21,2325 In fact, the data could not be fit with independent Si and O subshells but required a Si subshell at 3.19 Å and an associated 3-leg multiple scattering (MS) Ti SiO subshell at 3.29 Å (Table 1 and Figure S1).1315,23,25 The 3-leg MS path consists of both a TiO and an OSi component each but is constrained to the same coordination number as the Si subshell. The distance and DebyeWaller factor parameter were independently refined since the MS path is longer than a TiSi 2-leg path and will have corresponding larger uncertainties; physically realistic values were obtained as detailed in Table 1.1315 The average TiOSi angle derived from these distances is 138°. Comparison with the angles of 128°168° previously reported for TiO multiply bonded species in titanium siloxide complexes3437 indicates that 138° requires substantial π interactions from the silica O to the electron-deficient TiIV center in a distorted tetrahedral environment. Although two additional independent shells of O at 3.52 Å and Si at 3.90 Å visually improved the fits, the extra parameters did not significantly improve the R factor and χ2 of the fit and have been omitted from the best fit data in Table 1. The best fit model of the EXAFS for 1 mol % loading of Ti in mesoporous silica of MCM-41

structure (designated hereon Ti-1-MCM for brevity) are also summarized in Table 1 (Figure S2 shows fit components). The parameters obtained for Ti-1-MCM do not have statistically significant disparities with the structural model from the Ti-SBA-15 sample measurements. Moreover, the broad feature centered at 3.7 Å for the FT of Ti-1-MCM could not be adequately modeled with physically reasonable variables containing O, Si, or Ti subshells. The pre-edge and XANES features of averaged data from two independently prepared samples of SBA-15 loaded with 1 mol % Mn (Mn-1-SBA) are shown in Figure 2a. The FT of the Mn K-edge is modeled well with two MnO shells, namely one shell at 2.11 Å with coordination number of 5.0 and one shell at 2.71 Å with coordination number of 2.0 (Figure 2b and Table 2). An additional shell corresponding to Si at a distance of 3.30 Å with a coordination number of 2.0 completes the model (Figure S3). Some peaks in the FT-EXAFS of the Mn K-edge with apparent distances between 3 and 5 Å could not be modeled with physically reasonable O, Si, or Mn subshells using an acceptable number of independent variables. Up to 2 mol % Mn has also been loaded with the same grafting protocol. However, attempts to fit the Mn-2-SBA data using the identical paths used for Mn-1-SBA were not successful and the fit quality is substantially lower (Figure 2c and Table 2). Introduction of an additional shell of Mn did not improve the fit. Plausible reasons for the poorer fit are the presence of Mn μ-oxo clusters or greater heterogeneity in the sample produced by the higher Mn loadings. 3.2. Elucidating the Local Environment of the TiIVOMnII MMCT Chromophore. The FT of the Ti K-edge EXAFS and the 24896



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Table 2. Mn K-Edge EXAFS Curve-Fitting Results for Mn-1-SBA and Mn-2-SBA sample Mn-1-SBA

Mn-2-SBA

σ2 (10‑3 Å2)

N (S02 = 0.85)

r (Å)

shell MnO

2.11 ( 0.02

5.0 ( 1.2

MnO

2.71 ( 0.04

2.0 ( 0.3

MnSi

3.30 ( 0.05

2.0 ( 0.3

12

MnO

2.18 ( 0.02

5.6 ( 0.4

11

MnO

2.80 ( 0.09

0.39 ( 0.11

MnSi

3.45 ( 0.06

4.0 ( 0.2

11

E0 (eV)

R factor (10‑3)

6.1

0.34

6.8 0.85

11

1.9 27

Table 3. Ti and Mn K-Edge EXAFS Curve-Fitting Results for TiMn-1,1-SBA sample

fit

TiMn-1,1-SBA

1

shell

r (Å)

σ2 (103 Å2)

Na

E0 (eV)

R factor (10‑3)

reduced χ2 (102)

4.2

3.6

2.5

4.4

6.3

3.0

1.0

0.21

0.48

0.70

2.2

0.50

0

3.4

0.84

Ti K-edge TiO TiSi

1.84 ( 0.07 3.19 ( 0.10

3.5 ( 0.3 3.0 ( 0.3

3.7 12

TiOSib

3.29 ( 0.07

3.0 ( 0.3b

27

142°

TiMn-1,1-SBA

2

c

TiMn

3.36 ( 0.05

0.85 ( 0.14

3.1

TiO

3.54 ( 0.05

1.5 ( 0.2

2.7

TiO

1.84 ( 0.07

3.2 ( 0.2

3.0

TiSi

3.20 ( 0.02

3.1 ( 0.3

5.6

TiOSib

3.36 ( 0.01 131° c

3.1 ( 0.3b

TiO

3.55 ( 0.04

1.4 ( 0.2

MnO

2.16 ( 0.01

6.2 ( 0.8

MnO

2.71 ( 0.03

0.85 ( 0.14

20 0.9

Mn K-edge TiMn-1,1-SBA

TiMn-1,1-SBA

TiMn-1,1-SBA

1

2

3

12 5.4

MnSi

3.26 ( 0.08

1.8 ( 0.7

7.9

MnTi

3.25 ( 0.07

0.95 ( 0.07

5.3

TiOMnd MnO

111° 2.16 ( 0.01

6.2 ( 0.8

MnO

2.72 ( 0.03

1.0 ( 0.1

12 7.7

MnSi

3.37 ( 0.03

1.8 ( 0.1

11

MnO

2.17 ( 0.02

6.5 ( 1.1

13

MnO

2.76 ( 0.07

1.0 ( 0.2

12

MnTi

3.19 ( 0.03

0.95 ( 0.09

10

a

For Ti, S02 = 0.83; for Mn, S02 = 0.85. b TiOSi three-leg multiple scattering (MS) path with the coordination number set to be identical as the coordination number as the TiSi single scattering path. c Angle derived by application of the trigonometric cosine rule, by using the MS path length to calculate the SiO bond length and using the three known distances in the TiOSi triangle to obtain the TiOSi angle. d Angle derived by application of the trigonometric cosine rule, by using the known TiO and MnO distances, and assuming an average of 3.3 Å for the TiMn distance in the TiOMn triangle to obtain the TiOMn angle.

best fit for 1 mol % loading each of Ti and Mn in SBA-15 (TiMn1,1-SBA) are shown in Figure 1d. The XANES spectrum of TiMn-1,1-SBA, included in Figure 1b shows a pre-edge peak height of about 45% relative to the plateau between the K-edge and the onset of the EXAFS region.39 The height is consistent with tetrahedral coordination for the majority of the Ti centers.2125 Similar to the data for Ti-1-SBA, the FT consists predominantly of one principal peak, which fits well with a shell of 3.5 O at 1.84 Å (Table 3). The broad peak spanning an apparent distance of 2.5 to 3.5 Å for the TiMn-1,1-SBA FTEXAFS curve can be modeled with the same components as for Ti-1-SBA, with the outcome of a shortened TiOSi distance resulting in an increase of the TiOSi angle to 142° from 138° (Table 3 and Figure S1). However, the R factor of the fit almost

halved when a shell of one Mn is introduced (Table 3, fit 1 with TiMn distance included, compared to fit 2 without the Mn shell in the model). Fits with k-weights of 3, favoring heavier elements, improve substantially when a second shell Mn is introduced.1517,33 The TiMn distance of 3.36 ( 0.05 Å is within the range of a single μ-oxo bridge between Ti and Mn.2830,32,38 We performed this fit on EXAFS measurements conducted for five independently prepared TiMn-1,1-SBA samples and obtained excellent agreement, indicating that the preparation is reproducible and reliable over several batches of mesoporous silica samples. The result described in Table 3 is the averaged data set of all five samples. The analysis of the Mn K-edge EXAFS data for several independently prepared samples gave a similar consistent model 24897



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Table 4. Ti and Mn K-Edge EXAFS Curve-Fitting Results for TiMn-1,2-SBA (without NEt3), TiMn-1,2-SBA, TiMn-1,0.5-SBA, and TiMn-1,3-SBA sample

shell

σ2 (10‑3 Å2)

Na

r (Å)

E0 (eV)

R factor (10‑3)

Ti K-edge TiMn-1,2-SBA

TiO

1.85 ( 0.01

3.2 ( 0.3

1.4

(without NEt3)

TiSi TiMn

3.26 ( 0.08 3.34 ( 0.06

3.0 ( 0.2 0.84 ( 0.24

7.7 2.4

3.38 ( 0.09

3.0 ( 0.2b

TiOSib

2.2

13

32

137° c TiMn-1,2-SBA

TiO

1.84 ( 0.02

4.4 ( 0.9

TiSi

3.28 ( 0.14

3.0 ( 0.2

3.2

1.1

6.9

4.5

1.2

2.2

2.7

8.6

1.4

12

TiMn

3.37 ( 0.05

0.90 ( 0.06

TiOSib

3.37 ( 0.17

3.0 ( 0.2b

1.7 40

143° c Mn K-edge TiMn-1,0.5-SBA

TiMn-1,2-SBA

TiMn-1,3-SBA

MnO

2.18 ( 0.02

5.8 ( 1.2

14

MnO

2.63 ( 0.08

1.0 ( 0.2

32

MnTi

3.21 ( 0.03

1.0 ( 0.2

11

MnO

2.14 ( 0.02

6.1 ( 0.3

13

MnO

2.75 ( 0.04

1.0 ( 0.1

MnTi

3.17 ( 0.06

0.51 ( 0.15

MnO

2.20 ( 0.01

7.4 ( 0.9

MnO MnTi

2.65 ( 0.02 3.22 ( 0.02

1.0 ( 0.1 0.29 ( 0.05

4.2 8.4 21 5.5 2.7

a

For Ti, S02 = 0.83; for Mn, S02 = 0.85. b TiOSi three-leg multiple scattering (MS) path with the coordination number set to be identical as the coordination number as the TiSi single scattering path. c Angle derived by application of the trigonometric cosine rule, by using the MS path length to calculate the SiO bond length and using the three known distances in the TiOSi triangle to obtain the TiOSi angle.

for 1% loading of Ti and Mn. The best fits of the Mn K-edge FT in R and k space for TiMn-1,1-SBA are illustrated in Figures 2d. Using the results of Mn-1-SBA as a first approximation, two subshells of O at 2.16 (N = 6.2) and 2.71 (N = 0.85) are required to model the predominant first peak in the FT of the Mn EXAFS. The broad feature with an apparent distance of 2.4 to 3.4 Å can be fit with only one shell of Si at 3.37 Å and a coordination number of about 1.8 (Fit 2 of the Mn K-edge sample in Table 3 and Figure S4b). Alternatively, a model with one shell of Ti at 3.19 A and a coordination number of 0.95 is also satisfactory (Fit 3 of the Mn sample in Table 3 and Figure S4c). However, the R factor improves more than 10-fold when the second shell peak is fit to a combination of both one subshell of 0.95 Ti at 3.25 ( 0.07 Å and one subshell of 1.8 Si also at 3.26 ( 0.08 Å (Fit 1 of the Mn K-edge sample in Table 3 and Figure S4a). Importantly, the MnTi distance extracted from the Mn K-edge EXAFS data is in agreement with the TiMn distance obtained from Ti K-edge fits within experimental error, supporting a TiMn distance of 3.3 Å with modest dispersion attributed to the heterogeneity of the disordered silica ligand environment. From the TiMn distance of 3.3 Å, a TiOMn bond angle of 111° is derived. Moreover, the ligand environment surrounding Mn appears to be similar for both Mn-1-SBA and TiMn-1,1-SBA, indicating that the synthetic protocol for anchoring Mn to SBA-15 provides uniform anchoring of Mn in various different environments. These results constitute the first structural model for a heterobinuclear MMCT unit prepared in nanoporous silica. We find that the ligand environment for Ti and Mn of the binuclear units compared to monometallic Ti or Mn in SBA-15 is structurally preserved to a considerable degree.

To further probe the robustness and consistency of this result, the relative concentrations of Ti and Mn were varied. Experimentally, the presence or absence of the base triethylamine (NEt3) during the synthesis of the binuclear units influenced the Ti/Mn ratio of a given sample (the base was removed after the synthesis by heating the TiMn-SBA sample under vacuum).12 Briefly, NEt3 facilitates reaction of the Mn precursor species with surface OH groups, resulting in higher concentrations of anchored Mn centers. On the other hand, avoiding the use of NEt3 for attachment of donor metal precursors to Ti centers in mesoporous silica enhances the selectivity of the reaction, i.e. the ratio of binuclear units compared to Mn anchored on the pore surface as isolated centers.6,9,10 The higher propensity for attachment of the Mn precursor to TiOH rather than to SiOH is attributed to the higher acidity, and hence increased reactivity of TiOH. In the presence of a base like NEt3, the preference for reaction with the more acidic TiOH group is diminished because of the availability of reactive siloxide (SiO) sites.6,9,10 Nakamura et al. made a similar observation when using very mild bases for preparing TiIVOFeII units.9 Ti K-edge EXAFS was first analyzed for 2 mol % Mn loaded into Ti-1-SBA in the presence and absence of NEt3. We find that, for the first and second shell coordination around Ti, the TiMn1,2-SBA sample prepared in the absence of NEt3 (Table 4) gives a strikingly similar result as the TiMn-1,1-SBA sample (with NEt3) (Table 3). As shown in Figure 3a (for components see Figure S5a), the Ti edge FT of this TiMn-1,2-SBA sample exhibits only two main peaks centered around apparent distances of 1.5 and 3.0 Å. A key result of the comparison of the model for TiMn-1,2-SBA (absence of NEt3) and TiMn-1,1-SBA 24898



Figure 3. (a) Fit of k3-weighted Ti K-edge EXAFS for TiMn-1,2-SBA (no NEt3 used). (b) k3-weighted Ti K-edge EXAFS fit for TiMn-1,2SBA. The insets are the Ti K-edge fit up to k = 15.0 and 12.6 for panels a and b, respectively.

(with NEt3) is that the TiMn distance of 3.34 ( 0.06 Å (N = 0.84 Mn) remains unchanged (Tables 3 and 4). On the other hand, TiMn-1,2-SBA samples prepared with NEt3 show a broader 3 Å peak than without base (Figures 3b and S5b) suggesting that a more selective loading procedure for Mn (absence of NEt3) leads to less heterogeneity in the Mn coordination environment. The curve fit for the TiMn-1,2-SBA (with NEt3) sample yields one Mn subshell (N = 0.90) at 3.37 Å (Figure S5b and Table 4). Moreover, the broader 3 Å feature, which is not completely modeled with the same parameters as the sample without NEt3, may be attributable to Mn μ-oxo clusters or a less homogeneous distribution of Si and Mn shells around the Ti center. The FT of the Mn edge EXAFS for 2 mol % Mn loaded on Ti1-SBA (with NEt3) is depicted in Figure 4a. The data fit well with two O subshells at 2.14 (N = 6.1) and 2.75 (N = 1.0) Å, and one shell of Ti at 3.17 Å (N = 0.51) (Table 4), which are similar to the parameters obtained for the less concentrated TiMn-1,1-SBA (Table 3 and Figure 2d). At higher concentrations of up to 3 mol % Mn, the resulting FT of the Mn K-edge of TiMn-1,3-SBA (Figure S6c) has distinct differences compared to TiMn-1,2-SBA. Nevertheless, the structural parameters of the best fits of the two samples remain very close, including the fits for a shell of 0.29 Ti at 3.22 Å (Table 4 and Figure S6c). The coordination numbers suggest that at 3 mol % Mn loading, around 1/3 of the Mn form MMCT units with Ti. Both the 2 and 3 mol % Mn loaded samples have features beyond 3.5 A that cannot be adequately modeled with improved parameters. We conclude that TiIVOMnII units are assembled consistently with relatively uniform TiMn distances as Mn loading is increased, but MnOx clusters may form at concentrations significantly above 1%.

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Using the set of data acquired for samples containing different ratios of Ti and Mn, a linear combination analysis of the XANES was carried out for the Mn K-edge of the samples TiMn-1,2-SBA and TiMn-1,1-SBA. The XANES spectra for the data sets involved were normalized and are presented in Figure 4c. The analysis is based on the assumption that TiMn-1,0.5-SBA has the highest fraction of TiIVOMnII MMCT units because there is a redundancy of Ti for Mn to link up to (Ti has twice the concentration of Mn). This assertion is reflected in the XANES spectrum (Figure 4c), where increasing relative ratios of Ti to Mn (from Ti:Mn = 0:1 in Mn-1-SBA to Ti:Mn = 2:1 in TiMn-1,0.5SBA) results in a small shift to higher energy (black line to red) for the edge-jump. Indeed, the possible combinations for fitting the XANES of TiMn-1,2-SBA with Mn-1-SBA and TiMn-1,0.5SBA components (Figures S7a and Table S1) suggest that TiMn1,2-SBA contains a mixture of TiIVOMnII MMCT units and isolated Mn sites. TiMn-1,1-SBA is found to consist predominantly of MMCT units since it comprises about 89% TiMn-1,0.5SBA and only 11% Mn-1-SBA (Figure S7b and Table S1), confirming the higher yield of heterobinuclear units at lower Mn concentrations. At higher Mn concentrations of 2 mol %, the Mn precursor has a greater tendency to anchor randomly, resulting in approximately 25% yield of MMCT units rather than the expected 50% (Table S1). An important insight of the concentration study emerges from the comparison of the FTEXAFS curves for the Mn edge (with the same forward Fourier transform window) for samples Mn-1-SBA, TiMn-1,2-SBA, TiMn-1,1-SBA, and TiMn-1,0.5-SBA (increasing Ti:Mn ratio). As can be seen from Figure 4d, a reduction in the peak corresponding to the Si subshell and a concomitant increase in the signal due to the Ti subshell is observed as the ratio of Ti to Mn increases from 0 to 0.5, 1.0, and 2.0. Hence, the FT-EXAFS function reveals distinct peaks for second shell scattering centers Ti and Si. There is also a corresponding reduction in the O subshell (2.62.8 Å) as the silica environment around Mn becomes more ordered due to the higher relative concentration of the Ti. 3.3. TiOMn Units with Mn in Higher Oxidation State. When samples loaded with 2 mol % of Mn in SBA are oxidized by exposure to ambient air over a period of several weeks, the material acquires a brown color consistent with conversion of MnII to higher oxidation states. The XANES spectrum for oxidized Mn-2-SBA clearly shows a small shift of the Mn K-edge to higher energy compared to freshly prepared Mn-2-SBA after heat treatment under vacuum (Figure 2a), suggesting at least partial oxidation of the MnII sites.11 The FT of the Mn K-edge EXAFS also contains an additional subshell at a shorter apparent distance than the tallest peak (Figure 5). Fitting of these two peaks gave two subshells (Table 5) at 1.90 (N = 1.4) and 2.17 Å (N = 5.3). The latter distance is in the range observed for all other MnO distances reported in this paper for a MnII center, but the short MnO distance of 1.90 Å provides evidence for an oxidized Mn(μ-O)xMn (x = 1 2) of a MnIII or MnIV center (Table 5), as previously reported for several MnOx species.11,28,32,38 Due to the higher mol % loading of Mn, oxidation of the samples under air can result in formation of Mn μ-oxo clusters due to the excess Mn anchored. These clusters are anticipated to have shorter MnO distances,11,28,32,38 compatible with the 1.90 A derived from the data. Furthermore, besides a shell of Si at 3.40 Å, the fit quality dramatically improves with the inclusion of a shell of about 1.0 Mn at 2.84 Å (Table 5 and Figure S8). The short MnMn distance is fully consistent 24899



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Figure 4. (a) k3-weighted Mn K-edge EXAFS fit for TiMn-1,2-SBA. The inset is the Mn K-edge fit up to k = 11.5 for TiMn-1,2-SBA. (b) k3-weighted Mn K-edge EXAFS fit for TiMn-1,0.5-SBA. The inset is the Mn K-edge fit up to k = 11.8 for TiMn-1,0.5-SBA. (c) Normalized Mn K-edge XANES of Mn-1SBA (black), TiMn-1,2-SBA (green), TiMn-1,1-SBA (blue), and TiMn-1,0.5-SBA (red). (d) FT of Mn-1-SBA, TiMn-1,2-SBA, TiMn-1,1-SBA, and TiMn-1,0.5-SBA, showing the reduction in Si and the increase in Ti contribution to the Mn K-edge EXAFS. The inset shows the expanded view of the reduction in Si and the increase in Ti contribution to the Mn K-edge EXAFS with the same color scheme as panel c.

Figure 5. k3-weighted Mn K-edge EXAFS fit for air-oxidized Mn-2SBA. The inset shows the Mn K-edge fit up to k = 11.7 for air-oxidized Mn-2-SBA.

with the presence of Mn u-oxo clusters although the mild oxidation at room temperature results in only partial conversion of the isolated sites to the bridging MnOx units. Similar to the data for Mn-2-SBA before exposure to oxygen that had features above 3.5 A that are not adequately accounted for, Mn μ-oxo clusters may have grown in number for the samples exposed to air. Although the TiIV centers cannot be oxidized, exposure of TiMn-1,1-SBA to air results in a discernible reorganization of

the Ti coordination sphere to give oxidized TiMn-1,1-SBA (Figure 6). The best fit of the FT for the Ti EXAFS in oxidized TiMn-1,1-SBA is mostly similar to the results for TiMn-1,1-SBA, except that the TiMn distance is significantly reduced from about 3.36 Å to 3.03 Å (Table 5 and Figure S9). With the TiO distance remaining unchanged within experimental error at 1.83 Å, the main reorganization upon air oxidation must arise from shortening of the MnO distance due to an increase in the Mn oxidation state, reduction in the TiOMn angle, or both. Nonetheless, since distances to Si centers have not altered significantly, the steric environment around Ti precludes large changes in the TiOMn angle. On calcination of TiMn-1,1-SBA in air, similar rearrangements take place around the Ti center (calcined TiMn-1,1-SBA, Figure 7), although they seem more pronounced. The XANES spectrum of calcined TiMn-1,1-SBA is included in Figures 1b (gray line) and shows a pre-edge intensity of about 56% relative to the plateau between the K-edge and the onset of the EXAFS region.39 The TiO distance remains unchanged around 1.87 Å, whereas the TiMn distance has reduced to 3.07 Å as expected (Table 5). Interestingly, a shell of O at 3.59 A no longer improves the fit, and the distances have increased marginally after calcination (Table 5 and Figure S9). These structural changes suggest substantial reorganization of the coordination sphere around Ti, which can be related to oxidation of the MnII in close proximity that results in a shortening of the MnO bond length and a 24900



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Table 5. Ti and Mn K-Edge EXAFS Curve-Fitting Results for Oxidized Mn-2-SBA, Oxidized TiMn-1,1-SBA, and Calcined TiMn-1,1-SBA sample oxidized Mn-2-SBA

shell

Na

r (Å)

σ2 (10‑3 Å2)

E0 (eV)

R factor (103)

0.53

8.2

2.8

7.8

6.0

3.5

2.4

6.1

MnO

1.90 ( 0.04

1.4 ( 0.7

5.8

MnO MnMn

2.17 ( 0.03 2.84 ( 0.12

5.3 ( 0.8 1.0 ( 0.3

6.9 20

MnSi

3.40 ( 0.05

2.7 ( 0.7

7.4

TiO

1.83 ( 0.01

3.4 ( 0.3

3.6

TiMn

3.03 ( 0.06

1.0 ( 0.1

7.3

TiSi

3.17 ( 0.19

3.0 ( 0.3

16

TiOSib

3.32 ( 0.10

3.0 ( 0.3b

25

Ti K-edge oxidized TiMn-1,1-SBA

132° c calcined TiMn-1,1-SBA

TiO TiO

3.59 ( 0.06 1.87 ( 0.02

1.2 ( 0.3 3.3 ( 0.4

0.4 4.1

TiMn

3.07 ( 0.10

1.0 ( 0.1

8.9

TiSi

3.18 ( 0.12

3.3 ( 0.2

TiOSib

3.34 ( 0.06

3.3 ( 0.2b

9.0 19

130° c Mn K-edge calcined TiMn-1,1-SBA

MnO

1.90 ( 0.02

2.0 ( 0.6

1.5

MnO

2.23 ( 0.09

0.50 ( 0.22

4.2

MnO MnTi

2.88 ( 0.09 3.17 ( 0.09

1.4 ( 0.6 1.0 ( 0.3

6.1 12

a

For Ti, S02 = 0.83; for Mn, S02 = 0.85. b TiOSi three-leg multiple scattering (MS) path with the coordination number set to be identical as the coordination number as the TiSi single scattering path. c Angle derived by application of the trigonometric cosine rule, by using the MS path length to calculate the SiO bond length and using the three known distances in the TiOSi triangle to obtain the TiOSi angle.

Figure 6. Best fit of air-oxidized TiMn-1,1-SBA for k3-weighted Ti K-edge EXAFS. The inset is the Ti K-edge fit up to k = 15.0 for airoxidized TiMn-1,1-SBA.

decrease of the TiOMn angle. The MMCT core of calcined TiMn-1,1-SBA probably attains a di-μ-oxo structure, as corroborated by the Mn K-edge data. The XANES of the Mn K-edge of calcined TiMn-1,1-SBA (Figure 2a) and the FT of the EXAFS (Figure 7b) illustrate the effects of oxidation and structural rearrangement around the Mn center. The Mn K-edge in the XANES spectrum of calcined TiMn-1,1-SBA is shifted to higher energy compared to the MnII samples and the partially oxidized samples, as reported previously by our group.12 The O coordination sphere around Mn can also be resolved into three subshells at 1.90, 2.23, and 2.88 Å with

coordination numbers of 2.0, 0.50, and 1.4, respectively. The latter two distances have been observed in the MnII samples described above. However, the two short MnO ligands at 1.90 Å likely constitute a geometry close to a di-μ-oxo “diamond core” configuration.11,28,32,38 The reduction in the total coordination number to around four ligands surrounding Mn is fully consistent with oxidation of the Mn site. Fitting of the MnTi distance to 3.17 ( 0.09 Å also confirms the shortened TiMn distance of 3.07 ( 0.10 Å seen from the Ti EXAFS (Figure S9c). Moreover, a shell of Si around 3.3 Å is no longer necessary for a good fit probably because the reduced coordination shell and larger distribution in MnSi distances after calcination, resulting in too broad a signal. As mentioned for the Ti data of the calcined TiMn-1,1- sample, the uncertainties in the coordination numbers and distances of the Mn EXAFS have also increased slightly relative to the TiMn-1,1-SBA samples synthesized in the absence of air.

4. DISCUSSION 4.1. Coordination Environment of Ti Center. In the light of extensive K-edge EXAFS studies reported by other groups on the coordination of Ti centers in silica nanopores,2125 we will discuss our results in the context of the most recent points raised in the literature. One such point is the presence of a small peak at an apparent distance of about 1.9 Å and one or more broad peaks centered at an apparent distance of 3 Å in the FT spectrum. We concur with the literature reports that the peak at 1.9 Å is likely an artifact of the FT for the first O shell at 1.84 Å, and is not 24901



Figure 7. (a) k3-weighted Ti K-edge EXAFS fit of calcined TiMn-1,1SBA. (b) k3-weighted Mn K-edge EXAFS fit of calcined TiMn-1,1-SBA. The insets are the fits in k-space for the Ti K-edge (up to 12.0) and Mn K-edge (up to 11.8).

due to edge-shared TiSi distances of the siloxide ligands.25 Previous authors also postulated that the broad peaks can only be accounted for by the inclusion of both Si and O subshells, but those studies were conducted prior to the advent of full MS techniques for FEFF calculations.1315,23,25 We have modeled the broad peaks and find that they arise from the partial destructive interference of one Si shell and an associated threeleg MS TiOSi path (Figures S1 and S2 and Table 1). The coordination number of the TiOSi path has been modeled to have the same coordination number as the Si shell because Si is a necessary component of this MS path, but with a degeneracy of two, for the equivalent TiOSi and TiSiO legs. The distance and DebyeWaller parameters were independently optimized, depending on the TiOSi angle, because the three-leg MS path consisting of both a TiO and an OSi component, will necessarily have longer distances and larger uncertainties than a two-leg path.1315 Appropriately, the DebyeWaller factors for the TiOSi MS legs are larger than the correlated TiSi single scattering paths, which are physically reasonable because each MS path should have a larger distribution of distances. These three- or more-legged MS paths are usually important only when the AB-C (for any elements A C) angles exceed 120°, and can even have greater amplitudes than the corresponding single-scattering paths when the AB-C angle is close to 180°. Using the TiO, TiSi, and TiOSi path lengths derived from our data, geometrical manipulation yields values of TiOSi bond angles that exceed 129° in all the samples examined, confirming their non-negligible contribution to the

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EXAFS fits. As a corollary to this insight, there must be substantial π bonding interactions between the bound siloxide oxygens and the electrophilic, tetrahedral TiIV centers.3437 Typical angles around O with purely σ-donation do not exceed 120°, whereas π-donation enlarges the bond angle around O because of contributions from one or both the 2p lone pairs on oxygen to the bonding. Since Ti is a d0 transition metal and can accommodate up to 18 electrons in its valence molecular orbitals, a TiIV center with four purely σ-donating ligands in a tetrahedral geometry will be highly electrophilic. To compensate for their electron-deficiency in a solvent- and air-free environment, the TiIV centers form multiple bonding interactions with the bound siloxide oxygens. The fact that all of our samples containing only Ti and most of the ones containing TiIVOMnII units can be modeled with both the TiSi and the TiOSi MS shells is strong evidence that the local geometry around the TiIV centers is highly ordered and rigid over several independently synthesized samples. 4.2. Coordination Environment of Mn Center. A similarly ordered coordination environment does not exist for Mn, which we believe is principally due to the electronic structure of MnII in a distorted octahedral environment. The second O shell at 2.71 A with 2.0 O (Table 2) is occasionally seen in X-ray diffraction structures containing MnII bound by general Lewis acidbase interactions with O of siloxane (SiOSi) linkages in the silica.4048 When 2 mol % Mn or more is loaded, some of the Mn cations can form linkages to TiOH or SiOH functional groups that are in close proximity, or even form Mn μ-oxo molecular complexes on the silica nanopore surface. After thermal treatment, additional Mn μ-oxo clusters deposit in the nanopores. Therefore, these processes can lead to μ-oxo clusters and greater heterogeneity in the sample, giving rise to a poorer quality fit to the data (Tables 2 and 5). The greater propensity toward formation of MnOx clusters at as low as 2 mol % Mn28,38 compared to formation of TiOx clusters (only around 3 mol % Ti) could be due to the steric protection provided by the Cp ligands of the Ti precursor upon anchoring and before removal by calcination.24,27 4.3. Structure of the TiOMn Unit. After loading 13 mol % Mn into Ti-1-SBA, there is some reorganization of the silica ligand environment as inferred from the Ti K-edge EXAFS (Figure 3 and Table 4). The broad peak centered at an apparent distance of 3 Å has its relative intensity shifted to a longer distance, and the best fit includes a shell of Mn at 3.37 Å with a coordination number of 0.90. The TiSi single scattering and the TiOSi three-leg MS path alters slightly so that the average TiOSi bond remains around 140° in Ti-1-SBA, TiMn-1,1SBA, and TiMn-1,2-SBA prepared with and without NEt3 (Tables 1, 3, and 4, respectively). This large bond angle confirms the increased π donation from O that can effectively π bond to the electrophilic TiIV metal center. Substantial local reorganization of the silica ligand sphere may be necessary for Mn to anchor on the nanopore surface while forming a linkage to Ti through the μ-oxo bridge, with a plausible case shown in Scheme 1. The average bond angle of TiOMn is about 111°, close to the ideal bond angle for O of 105109° when oxygen is a pure σdonor. The TiMn distance and the TiOMn angle support the formulation of a single bridging μ-oxo ligand between Ti and Mn for the unoxidized samples, instead of a di μ-oxo core. However, the curvature within the nanopores of SBA-15 may not be able to accommodate both Ti and Mn anchored in the silica surface with such a small bond angle. Therefore, after Mn loading and subsequent thermal treatment in vacuum, the silica 24902



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Scheme 1. Proposed Mechanism for Loading and Anchoring of Mn Precursor to form TiIVOMnII Metal-to-Metal Charge Transfer Unit

Scheme 2. Proposed Mechanism for the Reorganization of TiIVOMnII Unit to Oxidized TiIV(μ-O)2MngIII Diamond Core upon Calcination at 350 °C

framework may rearrange around the TiIV center by widening the TiOSi angle to relieve the structural strain of the MMCT units and the local silica environment. The Mn K-edge EXAFS data also demonstrates similar restructuring, where the coordination number of the first shell of O increases from 5.0 to 6.2 from Mn-1-SBA to TiMn-1,1-SBA, and the second shell of O decreases from 2.0 to 0.85 (Tables 2 and 3). These two parameters suggest that Ti preorganizes the silica surface or restricts the Mn to coordinate to a more rigid environment, where the Mn center forms stronger and shorter bonding interactions with the SBA-15 nanopore surface and fewer long, Lewis acidbase type interactions with siloxanes on the silica surface.4048 Thus, Mn anchors more tightly to Ti and the silica scaffold. Nonetheless, the MnTi distance of 3.25 ( 0.07 Å points to a larger uncertainty and wider distribution of distances for the Mn local coordination sphere, compared to the TiMn distance of 3.36 ( 0.05 Å obtained for the fit of the Ti EXAFS in TiMn-1,1-SBA. When 2 mol % of Mn is loaded, the Ti K-edge data for TiMn-1,2-SBA suggests that some excess Mn may be anchored in the vicinity of Ti, perhaps because the limited number of accessible SiOH groups are saturated with both Ti and Mn. The closest neighboring SiOH groups may be only about 4 Å apart,4951 so that the excess Mn may anchor near Ti without forming a μ-oxo bridge. As described for Mn-2-SBA, the loading of as low as 2 mol % Mn with NEt3 as the base can result in small Mn μ-oxo clusters. Nevertheless, it is noteworthy that the TiMn distances from the Ti K-edge data of 3.36 ( 0.05 Å for TiMn-1,1-SBA and 3.37 ( 0.05 Å for TiMn-1,2-SBA remain consistent with all the other TiIVOMnII MMCT samples reported in this paper (Tables 3 and 4). 4.4. Structural Integrity of the TiMn Unit under Harsh Treatment. For samples that were exposed to air for prolonged periods or underwent calcination, curve fitting indicates structural rearrangements of both the Ti and Mn centers. The most marked

observation for TiMn-1,1-SBA is a reduction of the TiMn distance (Ti EXAFS data) to 3.03 A (oxidation in air at RT) and 3.07 Å (calcination at 300 °C) (Table 5). Since the first shell TiO distances remain practically unchanged at 1.83 and 1.87 Å in the two samples, the substantial shortening of TiMn distance must originate from a corresponding reduction in the MnO bond length of the μ-oxo bridge and a reduction in the TiOMn angle.2830,32,42 The shorter TiMn distance can be attributed to the conversion of a TiOSi linkage to form a Ti(O)2Mn di μ-oxo diamond core structure (Scheme 2). It is interesting that the uncertainties in distances for the oxidized samples have increased slightly (Table 5). Both observations indicate a loss of structural order and sample homogeneity around Ti, more π donation by the siloxide to TiIV, and a need for Ti to distort further from its tetrahedral ground state in TiMn-1,1-SBA to adjust to the constraints imposed by a di μ-oxo bridge with Mn in the calcined sample (Scheme 2). Upon exposure of Mn-2-SBA to air, the most significant structural changes are a new, short MnO shell at 1.90 Å (N = 1.4), and the observation of a MnMn shell at 2.84 Å (Table 5). The shell of O at 1.90 Å is suggestive of a short, possibly terminal MndO due to partial oxidation of some of the Mn centers in the sample, or more likely Mn μ-oxo clusters from the excess Mn loaded. The short MnMn distance also supports partial oxidation and hence shortening of some MnO bond lengths.11,2830,32,38 The Mn μ-oxo clusters most probably remain part of the silica framework and are not aggregates of extraframework MnO2, since the larger clusters have been reported to contain short MnMn distances of about 2.85 Å associated with di-μ-oxo bridged units between MnIV centers. The most significant confirmation of reorganization and oxidation of Mn can be derived from the EXAFS fitting results of calcined TiMn-1,1-SBA. The best fit for the Mn edge FT now includes three shells of O (Table 5). The MnO distance of 1.90 Å is in close agreement with the MnO distances in colloidal MnOx clusters that have been studied by XAS as well.32,38 The model proposed in previous reports consists of intercalated di μ-oxo clusters. According to the fit of TiMn-1,1SBA, the Mn is oxidized to form a Ti(O)2Mn di μ-oxo core, with up to two more MnO bonds to the silica framework at 2.26 Å and 2.90 Å.4048 The MnTi distance has shortened to 3.17 ( 0.09 Å, which is within experimental uncertainty of the TiMn distance of 3.07 ( 0.10 Å in the calcined sample of TiMn-1,1SBA (Table 5).11,28,32,38 A plausible mechanism for these structural rearrangements is proposed in Scheme 2. The slightly 24903


The Journal of Physical Chemistry C larger uncertainties in bond distances reflect the heterogeneity of the samples after calcination.

5. CONCLUSIONS With the insights derived from EXAFS curve fitting of metal K-edge measurements, a self-consistent, reproducible, structural model has been developed for TiIVOMnII MMCT units anchored on silica nanopore surfaces. The result confirms the oxobridged nature of photoredox active heterobinuclear units in nanoporous silica previously concluded from optical, infrared, and EPR spectroscopic observations by our group and the Hashimoto group.1,312 The TiMn distance of the oxo-bridged unit determined through both Ti and Mn K-edge EXAFS fitting is 3.3 Å (12 mol % Ti and 13 mol % Mn loading). After formation of the linkage between the two metals, the TiIV centers retain their highly ordered, tetrahedral coordination, while the MnII sites acquire a more uniform structure. The binuclear unit does not disintegrate upon prolonged exposure to air or heating at high temperature under oxygen (calcination). Rather, the oxidation of the Mn center to the III state (or higher) is accompanied by a shortening of the oxo bridge and formation of a TiMn di-μ-oxo structure. For monometallic Ti or Mn silica materials, the TiIV centers have distorted tetrahedral geometry after the material is rigorously calcined and dehydrated before the X-ray absorption measurement. Small amounts of extraframework anatase TiO2 are deposited in the nanopores only when more than 3 mol % TiCp2Cl2 precursor is used. The coordination environment around Mn is less regular with oxygens bound at different distances in a distorted octahedral geometry. Greater uncertainty in the fits are observed, especially in the presence of triethylamine base at loading levels as low as 2 mol % Mn precursor. We associate the higher tendency toward small cluster formation in the case of Mn compared to Ti to the lack of protection provided by the Cp capping ligand of Ti during anchoring. Through analysis of as-synthesized materials after Mn grafting in the presence of NEt3, we propose a mechanism where Mn complexes link to Ti via a μ-oxo bridge and form additional bonds to surface Si after thermal treatment in vacuum, creating isolated TiIVOMnII sites with pronounced MMCT absorption. The fact that the MMCT bands gain intensity after anchoring suggests that a distinct geometry is necessary for maximum overlap of the d orbitals of the two metal centers. Our results provide the first detailed structural information on the extensively characterized TiIVOMnII MMCT unit, which is an attractive candidate as a visible light charge-transfer pump for driving artificial photosynthetic systems. ’ ASSOCIATED CONTENT Supporting Information. Description of curve fitting procedure, figures showing components of the EXAFS fits, XANES linear combination fitting results, and additional results for samples with higher Ti concentrations. This material is available free of charge via the Internet at http://pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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Present Addresses †

Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695.

’ ACKNOWLEDGMENT This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical, Geological and Biosciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program. The authors thank Prof. Carlo Segre (Illinois Institute of Technology), Prof. Scott Calvin (Sarah Lawrence College), and Dr. Shelly Kelly (Argonne National Laboratory) for guidance on using the SixPACK and IFEFFIT software packages for XANES and EXAFS analysis. ’ REFERENCES (1) Frei, H. Chimia 2009, 63, 721. (2) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729. (3) Cuk, T.; Weare, W. W.; Frei, H. J. Phys. Chem. C 2010, 114, 9167. (4) Han, H. X.; Frei, H. Microporous Mesoporous Mater. 2007, 103, 265. (5) Han, H. X.; Frei, H. J. Phys. Chem. C 2008, 112, 16156. (6) Han, H. X.; Frei, H. J. Phys. Chem. C 2008, 112, 8391. (7) Lin, W. Y.; Frei, H. J. Phys. Chem. B 2005, 109, 4929. (8) Lin, W. Y.; Frei, H. J. Am. Chem. Soc. 2005, 127, 1610. (9) Nakamura, R.; Okamoto, A.; Osawa, H.; Irie, H.; Hashimoto, K. J. Am. Chem. Soc. 2007, 129, 9596. (10) Okamoto, A.; Nakamura, R.; Osawa, H.; Hashimoto, K. Langmuir 2008, 24, 7011. (11) Weare, W. W.; Pushkar, Y.; Yachandra, V. K.; Frei, H. J. Am. Chem. Soc. 2008, 130, 11355. (12) Wu, X.; Weare, W. W.; Frei, H. Dalton Trans. 2009, 10114. (13) Newville, M.; Kas, J. J.; Rehr, J. J. J. Phys.: Conf. Ser. 2009, 190, 012023. (14) Poiarkova, A. V.; Rehr, J. J. Phys. Rev. B 1999, 59, 948. (15) Rehr, J. J.; Albers, R. C. Rev. Mod. Phys. 2000, 72, 621. (16) Stern, E. A. Phys. Rev. B 1974, 10, 3027. (17) Stern, E. A. Contemp. Phys. 1978, 19, 289. (18) Robblee, J. H.; Messinger, J.; Cinco, R. M.; McFarlane, K. L.; Fernandez, C.; Pizarro, S. A.; Sauer, K.; Yachandra, V. K. J. Am. Chem. Soc. 2002, 124, 7459. (19) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olsen, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (20) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (21) Blasco, T.; Corma, A.; Navarro, M. T.; Pariente, J. P. J. Catal. 1995, 156, 65. (22) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina, A.; Boscherini, F.; Buffa, F.; Genoni, F.; Leofanti, G.; Petrini, G.; Vlaic, G. J. Phys. Chem. 1994, 98, 4125. (23) Gleeson, D.; Sankar, G.; Catlow, C. R. A.; Thomas, J. M.; Spano, G.; Bordiga, S.; Zecchina, A.; Lamberti, C. Phys. Chem. Chem. Phys. 2000, 2, 4812. 24904



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EXAFS Spectroscopic Analysis of Heterobinuclear TiOMn Charge

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