Synthesis and Characterization of Cerium-Oxo Clusters Capped by Acetylacetonate

Cerium-oxo clusters have applications in fields ranging from catalysis to electronics and also hold the potential to inform on aspects of actinide chemistry. Toward this end, a cerium-acetylacetonate (acac1–) monomeric molecule, Ce(acac)4 (Ce-1), and two acac1–-decorated cerium-oxo clusters, [Ce10O8(acac)14(CH3O)6(CH3OH)2]·10.5MeOH (Ce-10) and [Ce12O12(OH)4(acac)16(CH3COO)2]·6(CH3CN) (Ce-12), were prepared and structurally characterized. The Ce(acac)4 monomer contains CeIV. Crystallographic data and bond valence summation values for the Ce-10 and Ce-12 clusters are consistent with both clusters having a mixture of CeIII and CeIV cations. Ce L3-edge X-ray absorption spectroscopy, performed on Ce-10, showed contributions from both CeIII and CeIV. The Ce-10 cluster is built from a hexameric cluster, with six CeIV sites, that is capped by two dimeric CeIII units. By comparison, Ce-12, which formed upon dissolution of Ce-10 in acetonitrile, consists of a central decamer built from edge sharing CeIV hexameric units, and two monomeric CeIII sites that are bound on the outer corners of the inner Ce10 core. Electrospray ionization mass spectrometry data for solutions prepared by dissolving Ce-10 in acetonitrile showed that the major ions could be attributed to Ce10 clusters that differed primarily in the number of acac1–, OH1–, MeO1–, and O2– ligands. Small angle X-ray scattering measurements for Ce-10 dissolved in acetonitrile showed structural units slightly larger than either Ce10 or Ce12 in solution, likely due to aggregation. Taken together, these results suggest that the acetylacetonate supported clusters can support diverse solution-phase speciation in organic solutions that could lead to stabilization of higher order cerium containing clusters, such as cluster sizes that are greater than the Ce10 and Ce12 reported herein.


■ INTRODUCTION
−7 These applications, together with the relative abundance and low cost of cerium, have motivated efforts focused on structure−property relationships and hence materials development.Among cerium compounds, CeO 2 (ceria) stands out as having uniquely diverse properties.One of the most notable industrial applications of ceria is its use in three-way catalysts (TWCs); ceria can serve several purposes for TWCs, including acting as an oxygen storage component due to its reduction potential. 8,9−12 Largely motivated to (1) better understand the catalytic behavior of CeO 2 and (2) develop novel Ce-based catalysts that can be fine-tuned for specific applications, the chemistry community has become interested in cerium-oxo clusters.These species can be regarded as molecular scale nanoparticles of CeO 2 , and their properties seem to depend on the nuclearity and composition of the Ce cluster. 13As a testament, Christou et al. recently demonstrated the ability of Ce-oxo clusters to scavenge reactive oxygen species, and the activity of these clusters was dependent on the number of Ce III atoms present in the cluster. 14It seems likely that the catalysis applications space for Ce-oxo clusters is wide and diverse because there are a large number of Ce-oxo clusters that can be prepared.For example, synthetic chemists have discovered reproducible synthetic methods for molecules that have between two cerium atoms (homometallic dimers; Ce 2 ) and clusters with up to one hundred cerium atoms (Ce 100 ).In an attempt to further diversify the landscape of Ce-oxo cluster chemistry, new XAS data were collected in transmission mode using an in-house XAS spectrometer at Los Alamos National Laboratory. 49Fifty scans were obtained and averaged per cerium sample, and four scans were obtained and averaged for the Cr foil (used for energy calibration).Data manipulation and analysis were conducted as previously described by Solomon and co-workers. 50Further details on the instrument configuration, data collection, and data analysis are available in the Supporting Information.
Solution State Characterization Methods.Small Angle X-ray Scattering (SAXS).X-ray scattering data were collected on an Anton Paar SAXS instrument using Cu−Kα radiation (1.54 Å) equipped with line collimation.A 2-D image plate was used for data collection in the q = 0.018−2.5Å −1 range.The lower q resolution is limited by the beam attenuator.The solution obtained by dissolving Ce-10 in acetonitrile was filtered with a 0.45 μm membrane filter and then filled in a 1.5 mm glass capillary (Hampton Research) for the SAXS measurement.Scattering data of neat solvent were also collected for background subtraction.Scattering was measured for 30 min for each experiment.SAXSQUANT software was used for data collection and post processing (normalization, primary beam removal, background subtraction, desmearing, and smoothing to remove extra noise created by the desmearing routine).Data were analyzed using IRENA macros with IgorPro 6.3 (Wavemetrics) software. 51Simulated scattering patterns of the Ce-10 and Ce-12 clusters were generated using SolX utilizing structural files (.xyz) containing a selected portion of the structure that did not include solvent or coordinated ligands. 52 1  Nuclear Magnetic Resonance Spectroscopy.Crystals of Ce-10 were dissolved in deuterated acetonitrile (CD 3 CN) and filtered through Celite.A 1 H NMR spectrum was then collected using a Varian 400-MR NMR.The spectrum (Figure S22) and peak assignments (Table S6) are provided as Supporting Information.
Electrospray Ionization Mass Spectrometry.Mass spectra were collected by using nanospray ionization and a quadrupole time-offlight instrument (QStar XL, Sciex, Ontario, Canada).Crystals of Ce-10 (4.5 mg) were dissolved in acetonitrile, and the solution was diluted to approximately 50 μM.The sample solution was then loaded into a pulled borosilicate capillary (1 mm o.d.0.75 mm i.d.pulled to 5 μm tip size) with its tip placed ∼1 cm from the curtain plate inlet of the mass spectrometer.A platinum electrode was inserted into the back of the capillary to make electrical contact with the solution for nanospray generation.Voltages of 2200−2400 V were applied to the solution while the spectrometer plate was held at 1100 V.The instrument was operated in TOF mode without any collision-induced dissociation gas in Q2 using DP1 = 50 V and DP2 = 10 V.These conditions produced a gentle ion sampling while helping desolvate the noncovalently bound solvent molecules.S2).Overall, the structure is built from a decameric [Ce 6 IV Ce 4 III O 8 (OMe) 4 ] 16+ cluster core (Figure 2), wherein ten Ce sites are bridged by eight μ 3 /μ 4 -oxo dianions and four μ 3 -OMe 1− monoanions.Bond valence summation values, which are the sum of the individual valences for an atom that add up to the oxidation state, 54 calculated for the Ce-10 cluster were consistent with six Ce IV and four Ce III sites (Table S7).−70 By comparison, the Ce III sites form methoxy-bridged dinuclear [Ce 2 (OMe) 2 ] species and two of these structural units "cap" the hexamer via bridging μ 4 -oxo groups to generate the decameric cluster shown in Figure 2. Fourteen acac 1− ligands, two OMe 1− , and two MeOH ligands bind the cluster core (Figure 2b).

Inorganic Chemistry
and two acetate groups to give a neutral cluster (Figure 3).Within the cluster, there are six crystallographically unique Ce sites, one of which (Ce1) is Ce III and five of which (Ce2−Ce6) are Ce IV based on BVS values (Table S10).As shown in Figure 3a, the five Ce IV metal centers, together with their symmetry equivalent sites, are bridged through two μ 2 -hydroxo, two μ 3hydroxo, ten μ 3 -oxo, and two μ 4 -oxo groups to form a decameric unit that can be described as two edge-sharing Ce 6 octahedra.These octahedra are linked through two μ 4 -oxo sites.The two Ce III metal centers are located at opposite sides of the Ce 10 core (related by an inversion center) to yield the Ce 12 structural unit; the Ce III cations bind through two μ 3 -oxo ions (of the Ce 10 unit), which become μ 4 -oxo via coordination of the Ce III sites.Location of the Ce III metal centers at the periphery of the cluster is consistent with previous reports of Ce cluster chemistry, 24,71 as well as the structure of compound Ce-10.As shown in Figure 3b, the [Ce IV 10 Ce III 2 O 12 (OH) 4 ] 18− cluster is further coordinated to 16 acac 1− and two acetate ligands, with the latter presumably forming in situ through oxidative cleavage of the acac 1− . 72Additionally, six acetonitrile molecules are present in the lattice (Figure S11).Further details on metal ion coordination numbers, Ce−O bond distances, and Ce−OH bond lengths are provided in the Supporting Information (Table S3).
Relationship to Other Ce-Oxo Clusters.−78 The Ce-10 and Ce-12 clusters exhibit unique arrangements of Ce metal centers that previously have not been reported.Nonetheless, some features of the clusters compare well with those of other known clusters.As summarized in the Supporting Information (Table S12), the most common core motif reported for Ce is the hexameric unit  of composition [Ce 6 O 4 (OH) 4 ] 12+ . 14,18,24,77,79This cluster has most frequently been isolated using carboxylate donors, and the Ce sites are usually tetravalent.Notably, the pervasiveness of the hexanuclear entity is similarly reflected in the cluster chemistry of other +3 and +4 metal ions including Bi III , Zr IV , Hf IV , Th IV , U IV , Np IV , and Pu IV . 35,64,65,80he Ce 6 octahedral core manifests in both the Ce-10 and Ce-12 clusters reported herein.As noted above, the Ce 10 assembly in Ce-10 consists of a central Ce IV 6 11− , that is perhaps best described as two hexameric units bridged via oxo groups. 81−84 These Ce 10 cluster cores vary in the arrangement of the Ce metal centers and range from edge-sharing Ce 6 octahedra to three Ce 3 units surrounding a central Ce site., adopt the same decameric unit that is observed in Ce-12.Such units have likewise been o b s e r v e d f o r t h e a c t i n i d e s . 8 5 , 8 6F o r e x a m p l e , [U 10 O 8 (OH) 6 (PhCO 2 ) 14 I 4 (H 2 O) 2 (MeCN) 2 ] consists of edge-sharing octahedra. 86−91 Yet there are only a handful of homo-and heterometallic Ce 12 units reported. 5,81,87,88,92Perhaps most notably, Ce 12 (C 2 ) 3 I 17 adopts a similar core to that observed in compound Ce-12. 87However, although the cluster in Ce-12 exists as an isolated structural unit, Ce 12 (C 2 ) 3 I 17 adopts extended chains with C 2 units in interstitial positions.
Finally, it is worth noting the location of the Ce III and Ce IV sites in the clusters.It is fairly well established in the cerium literature that for mixed oxidation state clusters, Ce III and Ce IV tend to locate to the cluster surface and core, respectively. 18ndeed, the oxidation state assignments of the Ce sites in Ce-10 and Ce-12 based on bond valence summation values are consistent with literature precedence. 71For Ce-10, the Ce IV sites form a hexanuclear core that is capped by Ce III dimers.Compound Ce-12 has a similar arrangement of Ce III and Ce IV sites, with ten Ce IV forming edge-sharing octahedra, and two Ce III located at the periphery of the cluster.This tendency is also manifested in heterometallic clusters.For  ] for which the central position in the cluster core was occupied by Ce IV and the surface was composed of two Ce III and eight Mn III sites. 94Nonetheless, it is important to note that this is not the only arrangement observed.For example, Kogerler et al. reported a Ce decamer that consisted of Ce IV cations surrounding a central Ce III . 20 Synthetic Considerations.An interesting attribute associated with the formation of Ce-10 was that its formation was dependent on the solvent identity and the cerium starting material used in the reaction.For example, monomeric Ce(acac) 4 was isolated from ethanolic solutions.Meanwhile, Ce-10 that consists of methoxy bridged units was isolated from methanolic solutions.We rationalized these results based on the realization that methanol is more acidic than ethanol, which can explain why a methoxide is formed and the ethoxide equivalent does not form. 95Interestingly, the Ce-10 cluster was pervasive across a range of reaction conditions as detailed in the Supporting Information.The structural unit was found to precipitate irrespective of the Ce source.For example, Ce IIIchloride, nitrate, triflate, and sulfate salts as well as Ce IV (SO 4 ) 2 all generated the Ce-10 cluster.Thus, Ce-10 formed irrespective of the Ce oxidation state in the starting materials, with oxidation of Ce III possibly occurring by action of ambient O 2 , 96 and reduction of Ce IV plausibly occurring via oxidation of acac 1− , as reported previously. 97,98In addition to Ce-10, two other structures built from Ce-10 were isolated (see Supporting Information).These phases differ primarily in packing due to differences in solvent inclusion into the lattice.Finally, Ce-12 was prepared through dissolution of Ce-10 in MeCN, and efforts to synthesize the phase from cerium salts were unsuccessful.The 1 H NMR and SAXS data are consistent with rearrangement of the structural units in solution and ligand dissociation from the cluster.Peaks in the 1 H NMR spectrum (Figure S22; Table S6) are observed at approximately 3.6, 5.45, and 5.6 ppm and are attributed to free Hacac (3.6 and 5.6 ppm) and bound acac 1− (5.45 ppm, 5.6 ppm). 99he presence of acetate ligands in Ce-12 may result from oxidative cleavage of acac 1− , in situ.Such oxidative cleavage has previously been reported in MeCN using mild catalysts such as quaternary ammonium iodide and H 2 O 2 . 72The oxidative cleavage of acac 1− to yield acetate in this work points to the reactivity of the Ce clusters.
Vibrational Spectroscopy.The infrared (IR) spectra for Ce-10 (Figure S20) and Ce-12 (Figure S21) are reported.The spectra are dominated by vibrations attributed to acac 1− .Assignments are provided in Tables S4 and S5.

H Nuclear Magnetic Resonance Spectroscopy.
A 1 H NMR spectrum was collected for the solution obtained by adding Ce-10 to deuterated acetonitrile (Figure S22), mimicking the procedure used to obtain Ce-12.The peak observed at approximately 3.6 ppm is consistent with the keto form of free Hacac, and the peak at 5.45 ppm is assigned to bound acac 1− .The peak at approximately 5.6 ppm may be attributed to the enol tautomer of Hacac or bound acac 1− .These assignments point toward evidence of ligand dissociation of Ce-10 when dissolved in solution, and SAXS, discussed below, points to species larger than Ce-10 or Ce-12 in solution.Crytals of Ce-12 precipitate from the solution, along with an amorphous material that is likely cerium oxyhydroxide.
Small Angle X-ray Scattering (SAXS).We conducted SAXS experiments to determine if any information could be gleaned about the stability of Ce-10 in acetonitrile and/or the formation of Ce-12, which (along with amorphous precipitate) was deposited from the MeCN solution of Ce-10.Certainly, rearrangement is necessary given the differences in topology and the differences in the Ce IV :Ce III ratio of Ce-10 and Ce-12.

Inorganic Chemistry
Comparison of the experimental scattering data for the Ce-10/ MeCN solution and the simulated scattering for both Ce-10 and Ce-12 suggest that Ce-10 reacts in MeCN to make clusters that are larger than Ce-10 and Ce-12.As shown in Figure 4a, the simulated scattering curves of Ce-10 and Ce-12 are similar given the similar size and shape of the clusters.Yet the simulated scattering curve for Ce-10 suggests a slightly larger size (based on the slight shift to lower q of the Guinier elbow at q ≈ 0.32 Å −1 ), despite its smaller nuclearity.In fact, for Ce-12, the distance between the periphery Ce III ions is 10.Clearly there is a mismatch in the Guinier region between the experimental scattering and the simulated scattering for both Ce-10 and Ce-12; the experimental scattering shows a shift to lower q, indicating a larger average size species than both of the crystallized clusters or some aggregation.Fourier transform of the scattering data gives a pair distance distribution function (PDDF) representation (Figures 4b and S19), which is a probability distribution map of scattering vectors through the scattering species, and also provide shape information based on characteristic PDDF profiles. 100he PDDFs of the simulated Ce-10 and Ce-12 give Gaussian distribution of scattering vectors, as expected for approximately spherical, dense particles, and respective radii of gyration (R g ) of 4.15 and 4.05 Å (Figure S19).R g is the sizeindependent, root−mean−squared average of the electrons from the center of the cluster, similar to the radius and related  by ∼√(5/3)R g = radius for a spherical particle.The distance (R) where the probability goes to zero is ∼11 Å for both Ce-10 and Ce-12, consistent with the longest Ce−O distance within the cluster core.On the other hand, the PDDF for Ce-10 dissolved in acetonitrile is consistent with the presence of clusters associated as dimers in solution, and an average R g of 6.9 Å. 100 This sort of association is reminiscent of V 10 dimerization via H-bonding, observed by SAXS in solution (also observed in the solid state). 101Because the acac 1− ligands are noted to dissociate from the cluster core via 1 H NMR in acetonitrile, we suspect the SAXS data indicates that cluster− cluster association in solution is important to the Ce-10 to Ce-12 conversion.

Electrospray Ionization Mass Spectrometry (ESI-MS).
To further interrogate the identity of the Ce species present in solution upon the dissolution of Ce-10 in MeCN, ESI-MS data were collected (Figure 5).Ions above 600 m/z were attributed to Ce clusters based on isotopic patterns.Three prominent groups of peaks were observed in the spectrum: peaks around m/z 950, around m/z 1490, and around m/z 3080 corresponded to triply, doubly, and singly charged ions, respectively, based on isotopic pattern spacings.Examination of the ions within these groups showed peaks that were spaced by mass differences corresponding to water and methanol.This was interpreted as the presence of clusters that differed in the number of OH − , MeO − , and O 2− ligands.To find potential molecular formulas, a combinatorial search was conducted using constraints of 5−15 cerium nuclearity, 7−30 acac ligands, 0−20 OH − ligands, 0−20 O 2− ligands, and 0−20 MeO − ligands.Only formulas that satisfied the ion charge (based on a combination of Ce III and Ce IV ) and were within 30 ppm of the experimental monoisotopic mass were retained.
For the major triply charged ion with monoisotopic peak at m/z 948.87 (Figure 5   clusters in the original list of 14 had poorer isotopic matching with the experimental data compared to that of Ce 10 clusters.Further filtering the list using a constraint of 6 Ce IV centers based on crystallographic data resulted in 4 formulas, all of which were Ce 10 clusters (Table S11).Accordingly, it was concluded that the triply charged ion at m/z 948.8753 represents a Ce 10 cluster.The inset of Figure 5  X-ray Absorption Spectroscopy (XAS).Ce-10 was examined via Ce L 3 -edge X-ray absorption spectroscopy (XAS).Note that limited sample size or impurities precluded similar investigations for Ce-1 and Ce-12.The data were comparatively evaluated against two oxidation state standards: ceria (Ce IV O 2 ) and cerium(III) acetylacetonate [Ce III (acac) 3 ] (Figure 6a).The spectrum from Ce-10 was hybridic in nature, meaning that it had attributes associated with both oxidation state standard extremes.It showed the double white line characteristic of cerium in a +4 oxidation state.However, the rising edge inflection point 5724.1(1)eV and peak maximum 5726.7(1)eV for the first feature were lower in energy than we expected for a Ce IV complex and suggested Ce III .Another metric for deciphering Ce III vs Ce IV content was the branching ratio between the two absorption features of the double white l i n e p e a k : ( ) Low Energy Feature Intensity Low Energy Feature Intensity High Energy Feature Intensity + . 24,102−106 The +4 oxidation state standard, Ce IV O 2 , had a branching ratio of 0.51.In contrast, the double white line branching ratio (determined via curve fitting analysis) for Ce-10 was 0.67.The high branching ratio in Ce-10 reflected that the low energy absorption feature was substantially more intense than the high energy feature.Higher branching ratios can be indicative of compounds that contain both Ce III and Ce IV , which was consequently how we interpreted this spectrum. 107n Ce-10, there are eight oxo dianions, 14 acac 1− monoanions, and six methoxide ligands; the total number of negative charges is 36.The presence of a double white line absorption feature in the Ce L 3 -edge X-ray absorption spectrum unambiguously refuted homovalent Ce III models of the spectrum.It was also difficult to rationalize that the XAS data could originate from a cluster that only had Ce IV cations because of (1) the low energy associated with the rising edge inflection point, (2) the low energy for the first peak maximum, and (3) the relative intensities from the two features (high branching ratio).Instead, the experiments suggested that two contributions to the Ce 10 XAS spectrum existed: one from cerium in the +3 oxidation state and another from cerium in the +4 oxidation state.Under the aforementioned designation (with six methoxide ligands), there was one way to charge balance and generate a neutral cluster: four Ce III and six Ce IV .We fit the data from Ce-10 as a summation of spectra from the two oxidation state standards to test the validity of this description.This was achieved using the following equation: Here, Ce(acac) 3(spectrum) was the L 3 -edge XAS data from Ce(acac) 3 , CeO 2(spectrum) was the L 3 -edge XAS data from CeO 2 , and N was a linear combination mixing coefficient.The resulting model fit excellently the features, relative intensities, and overall line shape of the Ce L 3 -edge XAS data from Ce-10 (R-factor = 3.6%).The model consisted of a 44(2)% contribution from the +3 standard [Ce(acac) 3 ] and 56(2)% contribution from the +4 standard CeO 2 (Figure 6b).These experimentally determined +3 vs +4 contribution values were equivalent to the charge balanced description of Ce-10 mentioned above and the bond valence summation values from the structural analysis: four Ce III cations and six Ce IV cations. .Bond valence summation performed for both clusters showed Ce III /Ce IV mixed oxidation state cluster cores, with XAS data confirming this assignment for the Ce-10 cluster.The stability of Ce-10 in solution was probed using SAXS and ESI-MS.The ESI-MS exhibits peaks consistent with decanuclear (Ce 10 ) species.SAXS identifies the presence of clusters in solution (either Ce-10 and/or Ce-12) that are associated via dispersive or hydrophobic interactions, which may be important for the conversion of Ce-10 to Ce- 12.  Overall, this study points to the utility of β-diketonate ligand scaffolds in stabilizing novel cluster cores and the role that solvent identity has on cluster formation.Note, in methanol, we isolated methoxy bridged polynuclear species, and in ethanol, we isolated monomeric structural units.Given ongoing interest in Ce-oxo cluster chemistry in catalysis application spaces and for advancing fundamental insight into lanthanide and actinide oxo cluster chemistry, our ongoing efforts center on identifying solution stable species and probing their chemical reactivity and catalytic behavior.Additionally, we are excited about the prospect of extending these results to actinide systems and characterizing similarities and/or differences between Ce and Pu cluster chemistry.
Crystallographic refinement details, thermal ellipsoid plots, packing diagrams for Ce-1, Ce-10, and Ce-12, descriptions of Ce coordination chemistry in Ce-10 and Ce-12, powder X-ray diffraction patterns, small-angle Xray scattering PDDF, IR and NMR spectra, bond valence summation values, XAS sample preparation, instrument configuration, data analysis, summary of previously reported homometallic cerium-oxo clusters (PDF)

■
RESULTS AND DISCUSSION Structure Descriptions.Compound Ce-1, Ce(acac) 4 , crystallized in the monoclinic space group, C2/c.The structure consists of a Ce IV monomeric unit, Ce(acac) 4 , that is composed of an eight-coordinate cerium(IV) metal center bound to oxygen atoms from four acac 1− ligands as shown in Figure 1.The acac 1− ligands bind in a bidentate fashion, with Ce−O bond distances ranging from 2.311(2)−2.348(2)Å.Note that Ce-1 is isostructural with a Ce(acac) 4 monomer reported by Matkovic and Grdenic; 53 however, the compound crystallizes with a different unit cell.Compound Ce-10, [Ce 10 O 8 (acac) 14 (CH 3 O) 6 (CH 3 OH) 2 ]• 10.5 MeOH, crystallized in the P2 1 /n space group.Five crystallographically unique Ce sites constitute the structure of Ce-10.Further details on metal ion coordination numbers and Ce−O bond lengths are provided in the Supporting Information (Table

Figure 1 .
Figure 1.Ball and stick illustration of the Ce(acac) 4 monomer.Ce is shown in yellow, O in red, and C in black.Hydrogen atoms have been removed for clarity.

Figure 2 . 4 IIIO 8 (
Figure 2. Illustration of the Ce 10 cluster of Ce-10 (a) highlighting the [Ce 6 IV Ce 4 III O 8 (Ome) 4 ] 16+ core; Ce III and Ce IV sites are shown in orange and yellow, respectively, and (b) showing the acac 1− decorated [Ce 10 O 8 (acac) 14 (CH 3 O) 6 (CH 3 OH) 2 ] cluster.Ce is shown in yellow and orange, O in red, and C in black.

Figure 3 .
Figure 3. Illustration of Ce-12 highlighting (a) the [Ce IV 10 Ce III 2 O 12 (OH) 4 ] 18+ cluster core that consists of both Ce III (orange) and the Ce IV (yellow) sites and (b) the acac 1− and acetate decorated [Ce 12 O 12 (OH) 4 (acac) 16 (CH 3 COO) 2 ] cluster.Ce is shown in yellow and orange, O in red, and C in black.Acetonitrile molecules that reside in the lattice have been omitted for clarity.
8 Å.On the other hand, comparing the longest Ce−Ce distance within the Ce 10 core of Ce-12 (8.2 Å) to the longest Ce−Ce distance of Ce-10 (9.3 Å) shows that the Ce-10 core is actually bigger.This is because the Ce-10 core contains six Ce IV and four Ce III , while the Ce-12 core contains ten Ce IV , with commensurate shorter Ce−O bond distances, on average.

Figure 4 .
Figure 4. (a) Scattering spectra from Ce-10 dissolved in acetonitrile, along with simulated scattering for Ce-10 and Ce-12.(b) Pair distance distribution function (PDDF) of the experimental and simulated scattering data.

Figure 5 .
Figure 5. Mass spectra from 600 to 3600 m/z of 50 μM Ce-10 in a MeCN solution.The boxes with dotted lines highlight the singly, doubly, and triply charged groups of ions.The inset shows the major triply charged ion with a monoisotopic peak at m/z 948.87, with the experimental data from the Ce-10 solution in blue and the simulated data of Core Ce-10(3+) in red.
inset), the constraints above resulted in 14 formulas, among which 3 were Ce 11 and 11 were Ce 10 , indicating more likelihood of Ce 10 clusters.Moreover, Ce 11

Figure 6 .
Figure 6.(A) Room temperature background subtracted and normalized Ce L 3 -edge XAS spectra (298 K) from Ce-10 (pink trace).For comparison, spectra collected under analogous conditions from +4 and +3 cerium oxidation state standards were included, namely Ce IV O 2 (blue trace) and Ce III (acac) 3 (green trace).(B) The normalized Ce L 3 -edge spectrum from Ce-10 (○) and a linear combination analysis fit (purple trace) comprised contributions from the +4 and +3 cerium oxidation state standards, specifically Ce IV O 2 and Ce III (acac) 3 .Contributions from the +4 standard were 56(2)% and contributions from the +3 standard were 44(2)%.(C) Deconvolution of room temperature, background subtracted, and normalized Ce L 3 -edge spectra from CeO 2 (top), Ce-10 (middle), and Ce(acac) 3 (bottom).Experimental data (○) were overlaid on the fit (purple trace) and functions used to generate the model.These included Gaussian functions (brown, green, blue, and pink traces) used to model the X-ray absorption peaks and a step function (1:1 combination of an arctangent and error function; gray trace).
depicts isotopic envelope matching between the experimental data and one of t h e 4 p o t e n t i a l f o r m u l a s : Ce 10 (CH 3 COCHCOCH 3 ) 12 (OH) 5 O 7 (CH 3 O) 2 3+ denoted as Core Ce-10(3+).The doubly and singly charged ion groups in Figure 5 can be interpreted in relation to the triply charged ion at m/z 948.8753 via variations in number of ligands and are further discussed in the Supporting Information (Figures S23− S25).

A d i s
c r e t e C e -o x o c l u s t e r , Ce 10 O 8 (acac) 14 (CH 3 O) 6 (CH 3 OH) 2 ]•10.5 MeOH (Ce-10), and a monomeric molecule, Ce(acac) 4 , were isolated from methanol and ethanol solutions, respectively.Dissolution of Ce-10 in acetonitrile led to the isolation of another discrete Ce-oxo cluster, [Ce 12 O 12 (OH) 4 (acac) 16 (CH 3 COO) 2 ]-(MeCN) 6 (Ce-12) oxo bridged moiety that is effectively capped by two methoxy bridged Ce III dimers to generate a decamer.By comparison, Ce-12 is composed of edge-sharing Ce IV octahedra; two Ce III monomers are located at opposite ends of the core.The observation of the Ce 6 unit as a component of larger assemblies is similarly reflected in the literature.For example, C h r i s t o u e t a l .r e c e n t l y r e p o r t e d (pyH) 8 [Ce 10 O 4 (OH) 4 (O 3 PPh) 12 (NO 3 ) 12 ] that consisted of a face capped [Ce 6 O 4 (OH) 4 ] 12+ core. 77Additionally, Loiseau et al. reported a Ce dodecamer, [{Ce 6 example, Gupta et al. characterized [Ce 6 Mn 12 O 17 (O 2 CPh) 26 ], which consisted of four {Mn 3 (μ 3 -O) 2 } surrounding a Ce IV 6 core. 93Likewise, Thuijs et al. reported [Ce 3 Mn 8 O 8 (O 2 CPh) 18 (HO 2 CPh) 2 CCDC 2257002−2257006 contain the supplementary crystallographic data for this paper.These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The