Coordination of {Mo142} Ring to La3+ Provides Elliptical {Mo134La10} Ring with a Variety of Coordination Modes

A 28-electron reduced C2h-Mo-blue 34Ǻ outer ring diameter circular ring, [Mo142O429H10(H2O)49(CH3CO2)5(C2H5CO2)]30- (≡{Mo142(CH3CO2)5(C2H5CO2)}) comprising eight carboxylate-coordinated (with disorder) {Mo2} linkers and six defect pockets in two inner rings (four and three for each, respectively), reacts with La3+ in aqueous solutions at pH 3.5 to yield a 28-electron reduced elliptical Ci-Mo-blue ring of formula [Mo134O416H20(H2O)46{La(H2O)5}4{La(H2O)7}4{LaCl2(H2O)5}2]10- (≡{Mo134La10}), isolated as the Na10[Mo134O416H20(H2O)46{La(H2O)5}4{La(H2O)7}4{LaCl2(H2O)5}2]·144 H2O Na+ salt. The elliptical structure of {Mo134La10} showing 36 and 31 Å long and short axes for the outer ring diameters is attributed to four (A-D) modes of LaO9/LaO7Cl2 tricapped-trigonal-prismatic coordination (TTP) geometries. Two different LaO2(H2O)7 and one LaO2(H2O)2Cl2 TTP geometries (as A-C modes) for each of two inner rings result from the coordination of all three defect pockets of the inner ring for {Mo142(CH3CO2)5(C2H5CO2)}, and two LaO4(H2O)5 TTP geometries (as D mode) result from the displacement of two (acetate/propionate-coordinated) binuclear {Mo2} linkers with La3+ in each inner ring. The isothermal titration calorimetry (ITC) of the ring modification from circle to ellipsoid, showing the endothermic reaction of [La3+]/[{Mo142(CH3CO2)5(C2H5CO2)}] = 6/1 with ΔH = 22 kJ⋅mol-1, ΔS = 172 J⋅K-1⋅mol-1, ΔG = −28 kJ⋅mol-1, and K = 9.9 × 104 M-1 at 293 K, leads to the conclusion that the coordination of the defect pockets to La3+ precedes the replacement of the {Mo2} linkers with La3+. 139La- NMR spectrometry of the coordination of {Mo142(CH3CO2)5(C2H5CO2)} ring to La3+ is also discussed.


Introduction
The Mo VI →Mo V reduction of isopolyoxomolybdates in strongly acidic aqueous solutions containing trivalent lanthanide cations (Ln 3+ ) provides for the incorporation of Ln 3+ into the Mo-blue ring circles with a resultant modification of the ring shape to a Japanese rice-ball shape: [Mo 120 {Pr(H 2 O) 5 [2] have been prepared by the hydrazinium-dichloride reduction of the Na 2 MoO 4 / Pr(NO 3 ) 3 /HCl system at 70 °C and the UV-photolysis of the [NH 4 ] 6 [Mo 7 O 24 ]/LaCl 3 /p-CH 3 C 6 H 4 SO 2 Na system at pH 1.2, respectively, and both complexes have been successfully characterized as Japanese rice-ball structured 24-electron reduced Mo-blues [1][2]. In 4 ] system at pH 1.0 and the hydrazinium-dichloride reduction of the K 2 MoO 4 /EuCl 3 /HCl system at 60-65 °C have been also characterized as 28-and 24-electron reduced elliptical Mo-blue rings, respectively. As well as the above Japanese rice-ball-shaped Mo-blues, the former has shown that the coordination of Ln 3+ within the inner ring is the same as the one of the binuclear {Mo 2 } linker which coordinates four O atoms belonging to MoO 6 octahedra of two head and two shoulder Mo sites (as a 4-fold ligand), as shown in Figure 1a [4][5][6][7]. Such a coordination of two sets of head and shoulder MoO 6 -octahedral sites to Ln 3+ , yielding the LnO 4 (H 2 O) 5 tricapped-trigonal-prismatic coordination (TTP) geometry, arises from a strong electrostatic interaction with the negatively charged inner-ring compared with the {Mo 2 } ( = [Mo 2 O 5 (H 2 O) 2 ] 2+ ) linker to lead to the elliptical structure due to a larger molecular curvature, if we considered that Ln 3+ is smaller in size than the {Mo 2 } linker [2]. On the other hand, the latter is a dimer of two elliptical rings ( 6 TTP geometry (Figure 1b), and other Eu atoms are in the EuO 2 (H 2 O) 7 TTP geometry without coordination of the O atom belonging to the neighboring ring [3].
The presence of Ln 3+ in the self-assembly process to the Mo-blue rings around at pH = 1 not only provides for the incorporation of Ln 3+ into the inner ring, but also generates the Mo-blue ringaggregates resulting from the dehydrated condensation between the two inner rings of neighboring  [9]. In our continuing work on the molecular design of the Mo-blue nano-rings by a use of Ln 3+ , the reaction of the Mo-blue rings with Ln 3+ has been investigated in order to discuss the competitive coordination to Ln 3+ between the displacement of the {Mo 2 } linker and the defect pocket within the inner ring, which is expected for 28-electron reduced defect ring, [Mo 142 O 429 H 10 (H 2 O) 49 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )] 30− (≡{Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )}), comprising eight carboxylate-coordinated (with disorder) {Mo 2 } linkers ( = {Mo 2 (carboxylate)}) and six defect pockets in two inner rings (four and three for each, respectively) [10]. In this work we describe that the coordination of{Mo 142 (CH 3 CO 2 ) 5  O, and discuss that the coordination of the defect pockets to La 3+ precedes the displacement of {Mo 2 } linkers within the inner ring, with a help of the isothermal titration calorimetry (ITC) and 139 La-NMR spectrometry. The obtained results give a good method for the modification of the ring shape from circle to ellipsoid.

Figure 1.
Environments of the La-coordination [2] in {Mo 150 La 2 } (a) and the Eucoordination [3] in {Mo 128 Eu 4 } 2 (b). Pink-, blue-and yellow colored Mo atoms are at shoulder, head and linker Mo sites, respectively. 10  assumed to be due to the removal of all the aqua ligands and crystal water molecules. The estimated number of the crystal water molecules is close to the one (144) based on the X-ray crystal structure analysis.  [11] indicated the coordination of 104 aqua ligands (for ∑s < 0.4) to the anion. Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen Germany (Fax: (+49)7247-808-666; E-Mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request for deposited data.html), on quoting the depository number CSD-421263.

Isothermal Titration Calorimetry (ITC)
Isothermal calorimetric titration of the aqueous solution containing {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} by LaCl 3 was performed with an Omega isothermal titration calorimeter (MicroCal, Northampton, MA, USA). The solutions were prepared with 0.05 ionic-strength of NaCl in deionized distilled water. The aqueous solutions containing 0.02 mM {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} was placed in the sample cell, and the reference cell was filled with ultra-pure water. To avoid the generation of the heat of neutralization, the pH level of the LaCl 3 solution was adjusted to pH 3.5 (natural pH level of the {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} solution) by adding HCl. Aliquots of the LaCl 3 solution (10 μL, 3 mM) were injected into the sample cell at 3.5-min intervals. The titration curves were analyzed with a help of Origin software (ver. 5.0) provided by MicroCal. Experimental data were fitted to theoretical curves for determining the binding enthalpy per mole of LaCl 3 (ΔH), the binding constant (K), and the number of binding La 3+ ions per {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} ring (n) as the reaction stoichiometry. In order to determine the molar enthalpy for the reaction of La 3+ with the Mo-blue ring, the heat of dilution of La 3+ , which was easily estimated by adding the aqueous solution of LaCl 3 into the aqueous solution of 0.05 M NaCl adjusted to pH 3.5, was subtracted from the experimental value on each addition. The goodness of the curve fitting is evaluated by the minimization (<10,000) of χ 2 , which is simply expressed as where n eff = the total number of experimental points used in the fitting, p = total number of adjustable parameters, y i = experimental data points, and f(x i ; p 1 , p 2 ,…) = fitting function [12]. The best fit for each titration curve was determined based on χ 2 and error values: χ 2 = 871 for the titration curve at 278 K, χ 2 = 1650 for 293 K, χ 2 = 3270 for 313 K, and χ 2 = 6549 for 333 K.

NMR Measurements
139 La-NMR spectra were measured at 298 ± 1 K using Φ = 10-mm internal diameter NMR tubes on a JEOL AL-300 spectrometer at 42.5 MHz with 90° pulses. A line-broadening factor of 1 Hz was applied before FT. Chemical shifts were referenced to an external LaCl 3 solution (1 M) for which the resonance was taken as δ = 0 ppm.

Crystal Structure of {Mo 134 (La) 10 }
The C 2h -symmetric 28-electron-reduced {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} circular ring (with an outerring diameter of 34 Ǻ) reacts with La 3+ in aqueous solutions at pH 3.5 (without any pH adjustment) to yield the elliptical ring of [ Figure 2a shows an elliptical C i -symmetric structure of {Mo 134 La 10 } and the long and short axes diameters for the outer ring of which are 36 and 31 Å, respectively. The manganometric redox titration analysis indicates the presence of 28 Mo V centers in {Mo 134 La 10 }, indicating that 28 electrons injected in {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} remain also in {Mo 134 La 10 } through the structural change from circular to elliptical ring. Interestingly, all the acetates and the propionate ligands coordinated in {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} were removed through the coordination to ten La 3+ ions within two inner rings (five for each) of {Mo 134 La 10 }.

Calorimetry for the Coordination of {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} to La 3+
The thermodynamic parameters for the La 3+ -induced change of the ring conformation of the {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} circle to the {Mo 134 La 10 } ellipsoid are obtained by ITC technique. The ITC system let us directly measure the quantity of the heat evolved or absorbed in liquid samples on injecting reactants. The quantity of the evolved heat can be estimated by using the differential feedback power between the reference cell and the sample cell. An injection which results in the absorption of heat in the sample cell causes a positive change in the power, since the heat absorption requires feedback power. Thus, the La 3+ titration of the aqueous solution of {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} at pH 3.5 indicates the endothermic behavior which is deduced by plotting the integrated quantities of the reaction heat against the molar ratio of La 3+ to {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} and by applying a model based on a single set of identical sites. At 293 K, binding constant (K) = 9.9 × 10 4 M −1 , number of La 3+ ions binding with a {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} molecule (n) = 6.1, and molar enthalpy of La 3+ ion (ΔH) = +22 kJ·mol −1 are estimated by using the best fit curve based on the model, as shown in Figure 3. The molar entropy of La 3+ ion (ΔS) = +172 J·mol −1 ·K −1 is calculated from K and ΔH value (ΔG = −RTlnK = ΔH−TΔS ). Table 1 lists the thermodynamic parameters obtained at various temperatures. At these temperatures, the coordination of {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} to La 3+ endothermically proceeded: with increasing temperature, the K value increases from 8.7 × 10 4 M −1 at 278 K to 6.1 × 10 5 M −1 at 333 K, in contrast to other parameters which indicate no significant change of n ≈ 6 and ΔH ≈ +22 kJ·mol -1 .  Since for the ring modification of circle to ellipsoid the ΔG value (ΔG = ΔH−TΔS) must be negative, the ring modification of {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} to {Mo 134 La 10 } should occur through the sufficient entropy gain, which is an important factor for the ring modification from circle to ellipsoid. In the diluted aqueous solution of LaCl 3 (<0.3 M), most of La 3+ ions exist in the hydrated complex with {La(H 2 O) 9 } 3+ nine-fold coordination [13]. In the {Mo 134 La 10 } formation system under the 1-mM concentration of LaCl 3 , thus, {La(H 2 O) 9 } 3+ reacts with the {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} ring to yield the A-D modes of the TTP geometries as revealed by the above structural analysis of {Mo 134 (La) 10 } through the displacement of two (for A-C modes) or four (for D mode) aqua ligands with head and shoulder MoO 6 octahedral O atoms. Such a dehydratation process on the coordination to La 3+ seems to provide a large entropy gain, if we consider that the coordination of {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} to ten La 3+ occurs through the liberation of twenty-eight aqua ligands. It is notable that the estimated binding number (n) of 6 implies that the reaction with a large heat change occurs under La 3+ /{Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} = 6:1 which corresponds to the number of defect pockets in {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )}. Thus, the ITC result for the La 3+ / {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} system strongly supports the preferential coordination of all the defect pockets of {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} in two inner rings to six La 3+ ions to yield a two sets of the A-C modes of the TTP geometries with the liberation of twelve aqua ligands, which follows the displacement of four {Mo 2 (carboxylate)} linkers with four La 3+ to yield the D-mode of the TTP geometries with the liberation of sixteen aqua ligands. The occurrence of the displacement of the carboxylates-coordinated {Mo 2 } linkers with La 3+ as a second step seems to be along with the fact that four {Mo 2 } linkers (two for each inner ring) still remain in {Mo 134 La 10 }. The removal of all the carboxylates coordinated in {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} through the reaction with LaCl 3 in the aqueous solutions is probably due to the kinetically controlled displacement of the carboxylates with aqua ligands.

139 La-NMR Spectrometry
The 139 La nuclide has high natural abundance (99.9 %) and is a very sensitive nuclide for NMR measurements. It has relatively high quadrupole moments: I = 7/2, Q = 0.22 × 10 −24 cm 2 ; therefore, the line width of the signal strongly depends on the site symmetry at La atoms. In our trial of 139 La-NMR spectrometry of the {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} coordination to La 3+ the aqueous solution of {Mo 134 La 10 } showed no observable 139 La−NMR signal. This arises from the inhomogeneous broadening due to the second order quadrupole interaction for the La 3+ sites of the A-D modes in {Mo 134 La 10 }. Interestingly, the perfect oxidation of {Mo 134 La 10 } after adding a small amount of HNO 3 (less than 10% v/v) gives rise to a signal around at −2.7 ppm which is assigned to the perfectly hydrated [La(H 2 O) 9

Conclusions
The ring modification of the defect-containing C 2h -{Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} circle to the C i -{Mo 134 La 10 } ellipsoid occurs through the coordination to La 3+ in aqueous solutions. Together with the ITC calorimetry results, the structural comparison of two rings between {Mo 142 (CH 3 CO 2 ) 5 (C 2 H 5 CO 2 )} and {Mo 134 La 10 } indicates that the coordination of the defect pockets to La 3+ precedes the displacement of the carboxylates-coordinated {Mo 2 9 ] 3+ results in a reduction (from 30-to 16-) of the negative charge of the anion ring, which seems to induce easier displacement of positively-charged (carboxylate-coordinated){Mo 2 } linkers with La 3+ . The release of all the hydrophobic carboxylate ligands from the two inner rings on the coordination to La 3+ ions indicates an increase of the hydrophilic La 3+ -TTP sites within the inner surface of the ring, suggesting the change of the ring surface from hydrophobic to hydrophilic property.