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

Abstract

A 28-electron reduced C_2h-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 C_i-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.

1. Introduction

The Mo^VI→Mo^V reduction of isopolyoxomolybdates in strongly acidic aqueous solutions containing trivalent lanthanide cations (Ln³⁺) provides for the incorporation of Ln³⁺ into the Mo-blue ring circles with a resultant modification of the ring shape to a Japanese rice-ball shape: [Mo₁₂₀{Pr(H₂O)₅}₆O₃₆₆H₁₂(H₂O)₄₈]⁶⁻ (≡{Mo₁₂₀Pr₆}) [1] and [Mo₁₂₀O₃₆₆H₁₄(H₂O)₄₈{La(H₂O)₅}₆]⁴⁻ (≡{Mo₁₂₀La₆}) [2] have been prepared by the hydrazinium-dichloride reduction of the Na₂MoO₄/ Pr(NO₃)₃/HCl system at 70 °C and the UV-photolysis of the [NH₄]₆[Mo₇O₂₄]/LaCl₃/p-CH₃C₆H₄SO₂Na 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 addition, the species [Mo₁₅₀O₄₅₂H₂(H₂O)₆₆{La(H₂O)₅}₂]²⁴⁻ (≡{Mo₁₅₀La₂}) [2] and [{Mo₁₂₈Eu₄O₃₈₈H₁₀(H₂O)₈₁}₂]²⁰⁻ (≡{Mo₁₂₈Eu₄}₂) [3] prepared by the UV-photolysis of [NH₄]₈[Mo₃₆O₁₁₂(H₂O)₁₆]/LaCl₃/[ⁱPrNH₃][ClO₄] system at pH 1.0 and the hydrazinium-dichloride reduction of the K₂MoO₄/EuCl₃/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 Ln³⁺ within the inner ring is the same as the one of the binuclear {Mo₂} linker which coordinates four O atoms belonging to MoO₆ 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 MoO₆-octahedral sites to Ln³⁺, yielding the LnO₄(H₂O)₅ tricapped-trigonal-prismatic coordination (TTP) geometry, arises from a strong electrostatic interaction with the negatively charged inner-ring compared with the {Mo₂} ( = [Mo₂O₅(H₂O)₂]²⁺) linker to lead to the elliptical structure due to a larger molecular curvature, if we considered that Ln³⁺ is smaller in size than the {Mo₂} linker [2]. On the other hand, the latter is a dimer of two elliptical rings (due to the insertion of two [Mo₂O₇(H₂O)]^2- into the outer ring) linked by Eu³⁺ through two Mo-O-Eu-O-Mo bonds: four Eu atoms in the elliptical ring coordinate two O atoms belonging to head and shoulder MoO₆ octahedra, and one of them is also bound to the O atom belonging to the MoO₆ octahedron of the binuclear {Mo₂} linker of the neighboring ring with a resultant EuO₃(H₂O)₆ TTP geometry (Figure 1b), and other Eu atoms are in the EuO₂(H₂O)₇ TTP geometry without coordination of the O atom belonging to the neighboring ring [3].

The presence of Ln³⁺ in the self-assembly process to the Mo-blue rings around at pH = 1 not only provides for the incorporation of Ln³⁺ into the inner ring, but also generates the Mo-blue ring-aggregates resulting from the dehydrated condensation between the two inner rings of neighboring Mo-blue rings, as exemplified by compounds like {[Mo₁₅₄O₄₅₈H₁₂(H₂O)₆₆]^8-}_∞ (nano-tube) [8] and {[Mo₁₄₆O₄₄₂H₆(H₂O)₆₀{La(H₂O)}₂}]^8-}_∞ (nano-chain) [9]. In our continuing work on the molecular design of the Mo-blue nano-rings by a use of Ln³⁺, the reaction of the Mo-blue rings with Ln³⁺ has been investigated in order to discuss the competitive coordination to Ln³⁺ between the displacement of the {Mo₂} linker and the defect pocket within the inner ring, which is expected for 28-electron reduced defect ring, [Mo₁₄₂O₄₂₉H₁₀(H₂O)₄₉(CH₃CO₂)₅(C₂H₅CO₂)]³⁰⁻ (≡{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}), comprising eight carboxylate-coordinated (with disorder) {Mo₂} linkers ( = {Mo₂(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{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to La³⁺ at pH 3.5 provides a novel elliptical Mo-blue ring [Mo₁₃₄O₄₁₆H₂₀(H₂O)₄₆{La(H₂O)₅}₄{La(H₂O)₇}₄{LaCl₂(H₂O)₅}₂]^10- (≡{Mo₁₃₄La₁₀}, with C_i symmetry, isolated as the corresponding Na⁺ salt, Na₁₀[Mo₁₃₄O₄₁₆H₂₀(H₂O)₄₆{La(H₂O)₅}₄{La(H₂O)₇}₄{LaCl₂(H₂O)₅}₂]·144H₂O, and discuss that the coordination of the defect pockets to La³⁺ precedes the displacement of {Mo₂} linkers within the inner ring, with a help of the isothermal titration calorimetry (ITC) and ¹³⁹La-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 {Mo₁₅₀La₂} (a) and the Eu-coordination [3] in {Mo₁₂₈Eu₄}₂ (b). Pink-, blue- and yellow colored Mo atoms are at shoulder, head and linker Mo sites, respectively.

Figure 1. Environments of the La-coordination [2] in {Mo₁₅₀La₂} (a) and the Eu-coordination [3] in {Mo₁₂₈Eu₄}₂ (b). Pink-, blue- and yellow colored Mo atoms are at shoulder, head and linker Mo sites, respectively.

2. Experimental Section

2.1. Synthesis of Na₁₀[Mo₁₃₄O₄₁₆H₂₀(H₂O)₄₆{La(H₂O)₅}₄{La(H₂O)₇}₄{LaCl₂(H₂O)₅}₂]·144H₂O

[NH₄]₂₇[(CH₃)₃NH]₃[Mo₁₄₂O₄₂₉H₁₀(H₂O)₄₉(CH₃CO₂)₅(C₂H₅CO₂)]·150 ± 10H₂O was synthesized according to the published procedure [10]. The precursor [NH₄]₂₇[(CH₃)₃NH]₃[Mo₁₄₂O₄₂₉H₁₀(H₂O)₄₉(CH₃CO₂)₅(C₂H₅CO₂)]·150 ± 10H₂O (10.5 mg, 0.4 μmol) was added to a solution containing NaCl (29.7 mg, 510 μmol) and LaCl₃·7H₂O (5.7 mg, 15 μmol) in 15 mL of H₂O. The resulting rhombohedral dark−blue colored crystals of Na₁₀[Mo₁₃₄O₄₁₆H₂₀(H₂O)₄₆{La(H₂O)₅}₄{La(H₂O)₇}₄{LaCl₂(H₂O)₅}₂]·144H₂O were precipitated at 4 °C within two weeks with a yield of 4.5 mg (40 % based on Mo). Calcd.: Na, 0.90; La, 5.46; Mo, 50.53; Cl, 0.56; and H, 2.04 %. Found: Na, 0.87; La, 5.09; Mo, 50.82; Cl, 0.52; and H 1.93 %. IR (KBr pellet): ν(cm⁻¹) = 1,622{m, δ(H₂O)}, 965 (m), 903 (w), 872 (w), 634 (s), 561 (s). Manganometric redox titration showed the presence of 28 ± 1 Mo^V centers in {Mo₁₃₄La₁₀}. TG analysis performed on a Rigaku Thermoflex TG-DGC instrument, showing almost monotonous decrease up to 200 °C, indicated that the crystal water content per molecule was approximately 151, when the gravimetric loss observed at 200 °C was 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.

2.2. Crystal Data for Na₁₀[Mo₁₃₄O₄₁₆H₂₀(H₂O)₄₆{La(H₂O)₅}₄{La(H₂O)₇}₄{LaCl₂(H₂O)₅}₂]·144H₂O

Na₁₀Mo₁₃₄La₁₀O₆₆₄H₅₁₆Cl₄, M = 25760.40 gmol⁻¹, space group P2₁/c (No.14), a = 35.530(16), b = 37.05(2), c = 36.11(2) Å, β = 119.85(2) °, V = 41227(38) ų, Z = 2, ρ = 2.075 gcm⁻³, μ = 25.74 cm⁻¹, F(000) = 24408. Crystal size = 0.20 × 0.15 × 0.10 mm. Transmission factors = 0.51–0.77. Crystal was coated with paraffin oil and mounted in a loop. The intensity data were collected on a Rigaku RAXIS−RAPID imaging plate diffractometer with a graphite monochromatized MoK_α (λ = 0.71069 Å) radiation at −100 °C. Lorentz polarization effects and numerical absorption corrections (using the program Numabs and Shape, T. Higashi, Program for absorption correction, Rigaku Corporation, Tokyo, 1999) were applied to the intensity data, and H atoms were not included in the calculation. A total of 388109 reflections (ω scan and 2θ_max = 55.0 °) were collected of which 94422 unique reflections (R_int = 0.102) were used. All of the calculations were performed using the CrystalStructure software package (CrystalStructure 3.8: Crystal Structure Analysis Package, Rigaku and Rigaku/MSC 2000−2003). The structure was solved by direct methods (SHELXS−97) and refined by SHELXL−97 (1915 parameters) to R₁ = 0.092 for 52411 reflections with I > 2σ(I), and R = 0.161 and R_w = 0.283 for all data. Anisotropic temperature factors were applied to all La, Na and Mo atoms except four Mo atoms (Mo57, Mo61, Mo62, and Mo67). All O, Cl, and the four Mo atoms were refined isotropically. The positional parameters of the four Mo atoms (Mo57, 61, 62, and 67) and ten O atoms (O72–75, O84–90, O102–104) were not refined. The thermal parameters of Mo57, Mo67, O72–75, and O102–104 were kept to be unchanged throughout a structure refinement. The thermal parameters of Mo61, Mo62, and O84–90 were also unchanged. The maximum and minimum residual electron densities were 8.97 and −6.13 eÅ⁻³, respectively. Seven large reflection peaks remained around four Mo atoms: 8.97 eÅ⁻³ at the distance of 2.33 Å from Mo57, 7.87 and 7.60 eÅ⁻³ at 0.89 and 0.83 Å respectively from Mo61, 7.79 and 7.29 eÅ⁻³ at 0.79 and 0.67 Å respectively from Mo62, and 7.25 and 6.31 eÅ⁻³ at 0.79 and 0.67 Å respectively from Mo67. The calculation of the bond valence sums (∑s) for all the oxygen atoms for La−O and Mo−O bonds [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.

2.3. Isothermal Titration Calorimetry (ITC)

Isothermal calorimetric titration of the aqueous solution containing {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} by LaCl₃ 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 {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} 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 LaCl₃ solution was adjusted to pH 3.5 (natural pH level of the {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} solution) by adding HCl. Aliquots of the LaCl₃ 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 LaCl₃ (ΔH), the binding constant (K), and the number of binding La³⁺ ions per {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} ring (n) as the reaction stoichiometry. In order to determine the molar enthalpy for the reaction of La³⁺ with the Mo-blue ring, the heat of dilution of La³⁺, which was easily estimated by adding the aqueous solution of LaCl₃ 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 χ², which is simply expressed as

χ² = (1/n^eff−p) ∑ [y_i−f(x_i; p₁, p₂,…)]²

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; p₁, p₂,…) = fitting function [12]. The best fit for each titration curve was determined based on χ² and error values: χ² = 871 for the titration curve at 278 K, χ² = 1650 for 293 K, χ² = 3270 for 313 K, and χ² = 6549 for 333 K.

2.4. NMR Measurements

¹³⁹La-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 LaCl₃ solution (1 M) for which the resonance was taken as δ = 0 ppm.

3. Results and Discussion

3.1. Crystal Structure of {Mo₁₃₄(La)₁₀}

The C_2h-symmetric 28-electron-reduced {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} circular ring (with an outer-ring diameter of 34 Ǻ) reacts with La³⁺ in aqueous solutions at pH 3.5 (without any pH adjustment) to yield the elliptical ring of [Mo₁₃₄O₄₁₆H₂₀(H₂O)₄₆{La(H₂O)₅}₄{La(H₂O)₇}₄{LaCl₂(H₂O)₅}₂]^10- ({Mo₁₃₄La₁₀}) with C_i symmetry, isolated as the corresponding Na⁺ salt of Na₁₀[Mo₁₃₄O₄₁₆H₂₀(H₂O)₄₆{La(H₂O)₅}₄{La(H₂O)₇}₄{LaCl₂(H₂O)₅}₂]·144H₂O. Figure 2a shows an elliptical C_i-symmetric structure of {Mo₁₃₄La₁₀} 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 {Mo₁₃₄La₁₀}, indicating that 28 electrons injected in {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} remain also in {Mo₁₃₄La₁₀} through the structural change from circular to elliptical ring. Interestingly, all the acetates and the propionate ligands coordinated in {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} were removed through the coordination to ten La³⁺ ions within two inner rings (five for each) of {Mo₁₃₄La₁₀}.

Figure 2. Structure of {Mo₁₃₄La₁₀} (a), four (A-D) modes for the coordination geometries of five La³⁺ ions at the half side of {Mo₁₃₄La₁₀}ring (b), and details of distorted TTP geometries for five La³⁺ ions in {Mo₁₃₄La₁₀} (c). Symmetry codes: I –x+2, -y+2, -z+3.

Figure 2. Structure of {Mo₁₃₄La₁₀} (a), four (A-D) modes for the coordination geometries of five La³⁺ ions at the half side of {Mo₁₃₄La₁₀}ring (b), and details of distorted TTP geometries for five La³⁺ ions in {Mo₁₃₄La₁₀} (c). Symmetry codes: I –x+2, -y+2, -z+3.

No La³⁺ exists around an environment of {Mo₁₃₄La₁₀} in the lattice. All the La³⁺ sites of eight LaO₉ (with La−O bond distances in 2.44(2)–2.71(2) Å) and two LaO₇Cl₂ (with La−Cl distances in 2.44(2)–2.71(2) Å) polyhedra in {Mo₁₃₄La₁₀} shows the distorted TTP geometries as well as other cases of elliptical {Mo₁₅₀La₂} and Japanese rice-ball {Mo₁₂₀La₆} species. However, one can remark four (A-D) modes of the TTP geometries within the inner ring with a resultant formation of C_i-symmetric ring, different from the case (only D mode) for C₂-symmetric {Mo₁₂₀La₆} and C_2h-symmetric {Mo₁₅₀La₂}with the coordination of four O atoms belonging to MoO₆ octahedra at two head and two shoulder Mo sites (as a 4-fold ligand for the ring) (Figure 1a) [8]. A-D modes of the TTP geometries in {Mo₁₃₄La₁₀} are depicted in Figure 2b where a half side of the {Mo₁₃₄La₁₀} ring is shown. A-mode indicates the LaO₂(H₂O)₇ TTP geometry comprising two O atoms belonging to MoO₆ octahedra at two shoulder sites, B-mode indicates the LaO₂(H₂O)₅Cl₂ TTP geometry with the same as A-mode but with the displacement of two aqua ligands with two Cl^-, C-mode indicates the LaO₂(H₂O)₇ TTP geometry comprising two O atoms belonging to the corner-sharing MoO₆ octahedra at one head and one shoulder sites, and D-mode indicates the LaO₄(H₂O)₅ TTP geometries of the same as for {Mo₁₅₀La₂} and {Mo₁₂₀La₆} (in Figure 1). All the 9-fold coordination geometries for A-D modes in the half side (Figure 2b) of the {Mo₁₃₄La₁₀} ring are schematically shown in Figure 2c, where each of the A-D modes indicates the distorted TTP geometry.

3.2. Calorimetry for the Coordination of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to La³⁺

The thermodynamic parameters for the La³⁺-induced change of the ring conformation of the {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} circle to the {Mo₁₃₄La₁₀} 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 La³⁺ titration of the aqueous solution of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} 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 La³⁺ to {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} and by applying a model based on a single set of identical sites. At 293 K, binding constant (K) = 9.9 × 10⁴ M⁻¹, number of La³⁺ ions binding with a {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} molecule (n) = 6.1, and molar enthalpy of La³⁺ 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 La³⁺ 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 {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to La³⁺ endothermically proceeded: with increasing temperature, the K value increases from 8.7 × 10⁴ M⁻¹ at 278 K to 6.1 × 10⁵ M⁻¹ at 333 K, in contrast to other parameters which indicate no significant change of n ≈ 6 and ΔH ≈ +22 kJ∙mol^-1.

Figure 3. Binding isotherm for the titration of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} with LaCl₃ at 293 K. An 20 μM solution of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂) was titrated with 10-μL 3 mM injections of LaCl₃. Diagram of the enthalpy produced in each of injections of LaCl₃ is shown in upper panel. The area under each injection signal was integrated and plotted against molar ratio in lower panel. The red-colored curve represents a nonlinear least squares fit of the reaction heat for the injection with the assumption of a single binding site.

Figure 3. Binding isotherm for the titration of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} with LaCl₃ at 293 K. An 20 μM solution of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂) was titrated with 10-μL 3 mM injections of LaCl₃. Diagram of the enthalpy produced in each of injections of LaCl₃ is shown in upper panel. The area under each injection signal was integrated and plotted against molar ratio in lower panel. The red-colored curve represents a nonlinear least squares fit of the reaction heat for the injection with the assumption of a single binding site.

Table 1. Thermodynamic parameters obtained for the coordination of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to La³⁺ ions at various temperatures.

Temperature (K)	278	293	313	333
n	5.6	6.1	6.2	5.8
K (M-1)	8.7 × 104	9.9 × 104	3.0 × 105	6.1 × 105
ΔH (kJ∙mol-1)	19	22	22	23
ΔS (J·mol−1·K−1)	164	172	175	182
ΔG (kJ∙mol-1)	−28	−28	−33	−37

Table 1. Thermodynamic parameters obtained for the coordination of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to La³⁺ ions at various temperatures.

Temperature (K)	278	293	313	333
n	5.6	6.1	6.2	5.8
K (M-1)	8.7 × 104	9.9 × 104	3.0 × 105	6.1 × 105
ΔH (kJ∙mol-1)	19	22	22	23
ΔS (J·mol−1·K−1)	164	172	175	182
ΔG (kJ∙mol-1)	−28	−28	−33	−37

Since for the ring modification of circle to ellipsoid the ΔG value (ΔG = ΔH−TΔS) must be negative, the ring modification of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to {Mo₁₃₄La₁₀} 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 LaCl₃ (<0.3 M), most of La³⁺ ions exist in the hydrated complex with {La(H₂O)₉}³⁺ nine-fold coordination [13]. In the {Mo₁₃₄La₁₀} formation system under the 1-mM concentration of LaCl₃, thus, {La(H₂O)₉}³⁺ reacts with the {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} ring to yield the A-D modes of the TTP geometries as revealed by the above structural analysis of {Mo₁₃₄(La)₁₀} through the displacement of two (for A-C modes) or four (for D mode) aqua ligands with head and shoulder MoO₆ octahedral O atoms. Such a dehydratation process on the coordination to La³⁺ seems to provide a large entropy gain, if we consider that the coordination of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to ten La³⁺ 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 La³⁺/{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} = 6:1 which corresponds to the number of defect pockets in {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}. Thus, the ITC result for the La³⁺/ {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} system strongly supports the preferential coordination of all the defect pockets of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} in two inner rings to six La³⁺ 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 {Mo₂(carboxylate)} linkers with four La³⁺ 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 {Mo₂} linkers with La³⁺ as a second step seems to be along with the fact that four {Mo₂} linkers (two for each inner ring) still remain in {Mo₁₃₄La₁₀}. The removal of all the carboxylates coordinated in {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} through the reaction with LaCl₃ in the aqueous solutions is probably due to the kinetically controlled displacement of the carboxylates with aqua ligands. Figure 4 shows two steps for the coordinations of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to ten La³⁺ ions which reveal the ring modification from circle to ellipsoid. The coordination of all the defect pockets to six La³⁺ ions (for A-C modes) with the liberation of twelve aqua ligands from [La(H₂O)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 four positively (1+)-charged {Mo₂(carboxylate)} linkers with four La³⁺ ions as well as an easy liberation (to more-positively (2+)-charged {Mo₂}) of the carboxylates from four remaining {Mo₂(carboxylate)} linkers.

Figure 4. Two steps of the coordination of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to La³⁺ in aqueous solutions. The removal of the coordinated carboxylates from the Mo-blue ring is governed by the kinetically controlled displacement of the carboxylates with aqua ligands. The La³⁺-coordination at six defect pockets (a→b) precedes the displacement of four {Mo₂(carboxylate)} linkers (b→c): structure of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} ring (a); {Mo₁₃₄(CH₃CO₂)₅(C₂H₅CO₂)(La)₆} ring with two sets of A-C (one for each) modes (b); {Mo₁₃₄La₁₀} ring with additional two sets of two D modes (c).

Figure 4. Two steps of the coordination of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to La³⁺ in aqueous solutions. The removal of the coordinated carboxylates from the Mo-blue ring is governed by the kinetically controlled displacement of the carboxylates with aqua ligands. The La³⁺-coordination at six defect pockets (a→b) precedes the displacement of four {Mo₂(carboxylate)} linkers (b→c): structure of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} ring (a); {Mo₁₃₄(CH₃CO₂)₅(C₂H₅CO₂)(La)₆} ring with two sets of A-C (one for each) modes (b); {Mo₁₃₄La₁₀} ring with additional two sets of two D modes (c).

3.3. ¹³⁹La-NMR Spectrometry

The ¹³⁹La 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 cm²; therefore, the line width of the signal strongly depends on the site symmetry at La atoms. In our trial of ¹³⁹La-NMR spectrometry of the {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} coordination to La³⁺ the aqueous solution of {Mo₁₃₄La₁₀} showed no observable ¹³⁹La−NMR signal. This arises from the inhomogeneous broadening due to the second order quadrupole interaction for the La³⁺ sites of the A-D modes in {Mo₁₃₄La₁₀}. Interestingly, the perfect oxidation of {Mo₁₃₄La₁₀} after adding a small amount of HNO₃ (less than 10% v/v) gives rise to a signal around at −2.7 ppm which is assigned to the perfectly hydrated [La(H₂O)₉]³⁺ species formed by the oxidative decomposition of {Mo₁₃₄La₁₀}. Furthermore, the ¹³⁹La-NMR spectrum of the [La³⁺]/[{Mo₁₃₄La₁₀}] = 10/1 system showed the broadened signal with ν_1/2 = 703 Hz around at −0.3 ppm. Figure 5 shows ¹³⁹La-NMR spectra of the aqueous solutions of {Mo₁₃₄La₁₀} (a), after oxidation (b), and the [La³⁺]/[{Mo₁₃₄La₁₀}] = 10/1 system (c). Together with the above readily coordination of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to La³⁺ in aqueous solutions, the results in Figure 5a-c let us measure the ¹³⁹La-NMR spectra of the aqueous solutions with the variety of [La³⁺]/[{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}] ratio. Figure 5d-h show ¹³⁹La-NMR spectra of [La³⁺]/[{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}] = 10/1 (d), 15/1 (e), 20/1 (f), and 50/1(g). The [La³⁺]/[{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}] = 10/1 system exhibits no signal, probably due to the perfect coordination of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} to ten La³⁺ ions to yield {Mo₁₃₄La₁₀} (Figure 5d). This can be also supported by the appearance of the peak around at −2.3 ppm when the [La³⁺]/[{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}] = 10/1 system is perfectly oxidized by HNO₃ (h). With increasing the ratio of La³⁺ a single broadened signal appears around at −0.6 ppm and its intensity increases with ν_1/2 = 646 Hz for [La³⁺]/[{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}] = 50/1. However, the intensities of broadened signals for [La³⁺]/[{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}] = 15/1 and 20/1 are not sufficiently strong to estimate the linewidths.

Figure 5. ¹³⁹La-NMR spectra: 0.5 mM {Mo₁₃₄La₁₀} (a); oxidization of (a) with HNO₃ (b); 0.5 mM {Mo₁₃₄La₁₀} and 5 mM LaCl₃ (c); 0.1 mM {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} and 1 mM LaCl₃ (d); and 15 mM LaCl₃ (e); and 20 mM LaCl₃ (f); and 50 mM LaCl₃ (g); oxidization of the solution of (d) with HNO₃ (h).

Figure 5. ¹³⁹La-NMR spectra: 0.5 mM {Mo₁₃₄La₁₀} (a); oxidization of (a) with HNO₃ (b); 0.5 mM {Mo₁₃₄La₁₀} and 5 mM LaCl₃ (c); 0.1 mM {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} and 1 mM LaCl₃ (d); and 15 mM LaCl₃ (e); and 20 mM LaCl₃ (f); and 50 mM LaCl₃ (g); oxidization of the solution of (d) with HNO₃ (h).

4. Conclusions

The ring modification of the defect-containing C_2h-{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} circle to the C_i-{Mo₁₃₄La₁₀} ellipsoid occurs through the coordination to La³⁺ in aqueous solutions. Together with the ITC calorimetry results, the structural comparison of two rings between {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} and {Mo₁₃₄La₁₀} indicates that the coordination of the defect pockets to La³⁺ precedes the displacement of the carboxylates-coordinated {Mo₂} linkers with La³⁺. Four modes (A-D) of LaO₉/LaO₇Cl₂ -tricapped-trigonal-prismatic coordination (TTP) geometries (each one for A-C modes and two for D mode within each inner ring of {Mo₁₃₄La₁₀}) are revealed: A,{LaO₂(H₂O)₇} (comprising the coordination of two O atoms at two shoulder MoO₆ sites); B, {LaO₂(H₂O)₅Cl₂} (two O atoms at two shoulder); C, {LaO₂(H₂O)₇} (two atoms at head and shoulder); and D, {LaO₄(H₂O)₅} (four O atoms from two sets of head and shoulder). The coordination of all the defect pockets to six La³⁺ ions (for A-C modes) with the liberation of twelve aqua ligands from [La(H₂O)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){Mo₂} linkers with La³⁺. The release of all the hydrophobic carboxylate ligands from the two inner rings on the coordination to La³⁺ ions indicates an increase of the hydrophilic La³⁺-TTP sites within the inner surface of the ring, suggesting the change of the ring surface from hydrophobic to hydrophilic property.