Mixed Metal Phosphonates: Structure and Proton Conduction Manipulation through Various Alkaline Earth Metal Ions

: Three new layered mixed metal phosphonates [CoMg(notpH 2 )(H 2 O) 2 ]ClO 4 · nH 2 O ( CoMg · nH 2 O ),[Co 2 Sr 2 (notpH 2 ) 2 (H 2 O) 5 ](ClO 4 ) 2 · nH 2 O( CoSr · nH 2 O ),and[CoBa(notpH 2 )(H 2 O) 1.5 ]ClO 4 ( CoBa


Introduction
Metal phosphonates (MPs) are a subclass of organic-inorganic hybrid materials, often combined with high water and thermal stability, showing potentially valuable properties such as ion exchange, sorption, catalysis, and proton conduction [1][2][3][4][5].Incorporating mixed metal species can further expand the structural diversity of MPs and tune their properties through different metal components [6,7].However, synthesizing mixed metal phosphonates is often challenging in terms of controlling, predicting, and crystallizing the final product.The coordination-driven self-assembly is highly influenced by numerous factors, such as temperature, pH value, solubility, coordination modes of the ligand, and other weak interactions [8].Generally, there are three synthetic approaches to producing mixed metal phosphonates: (1) directly reacting different metal salts with ligands; (2) using ditopic or polytopic ligands that consist of two or more metal ion receptors; (3) using well-defined metal complexes as the metalloligands.The metalloligand approach is more controllable among the three approaches and has developed remarkably in the synthesis of mixed metal coordination polymers [9,10].However, such a strategy is limited to mixed metal phosphonates due to rare phosphonate-based metalloligands compared to carboxylate-based, azole-based, and pyridine-based metalloligands [11].
In our previous work, we explored a neutral mononuclear complex Co(notpH 3 ) based on tripodal phosphonic acid [notpH 6 = C 9 H 18 N 3 (PO 3 H 2 ) 3 ] as the metalloligand, which can serve as a bi, tri, or tetradentate ligand to bind various metal cations such as Ag(I), Ca(II), Co(II), Ni(II), and Ln(III) [11][12][13].In these Co(notpH x ) 3-x -based mixed metal phosphonates, the selected second metal ions can influence the structural dimensionality, the degree of protonation in phosphonate groups, and the amount of coordinated water.Such variable factors agree with the design considerations (proton sources and proton transfer pathways) of intrinsic proton-conductive materials and are ideal for systematically understanding the relationship between proton conductivity and structure [14,15].Recently, isostructural [M II  3 Co III 2 (notp) 2 (H 2 O) 12 ]•2H 2 O (M = Co or Ni) provided an example of the effect of hydrated metal ions on proton conduction, demonstrating the enhancement of proton conductivity as a result of the stronger Lewis acidity of Co(II) over Ni(II) [13].As we know, the Lewis acid strengths of divalent alkaline earth metal ions follow the tendency: Be > Mg > Ca > Sr > Ba [16], and their unusual coordination geometries are often observed [17].The reported [CoCa(notpH 2 )(H 2 O) 2 ]ClO 4 •nH 2 O (CoCa•nH 2 O) shows a moderate proton conductivity of 1.55 × 10 −5 S cm −1 at 25 • C and 95% relative humidity (RH) [15].It would be interesting to further study how the various alkaline earth metal nodes affect the topology and proton-conducive properties in related compounds.Herein, we use the metalloligand Co(notpH 3 ) to react with Mg(II), Sr(II), and Ba(II) ions to obtain three new Co-M phosphonates: 1).The synthesis, crystal structures, and proton-conductive properties of these compounds are reported.phosphonates, the selected second metal ions can influence the structural dimensionality, the degree of protonation in phosphonate groups, and the amount of coordinated water.Such variable factors agree with the design considerations (proton sources and proton transfer pathways) of intrinsic proton-conductive materials and are ideal for systematically understanding the relationship between proton conductivity and structure [14,15].(CoBa) (Figure 1).The synthesis, crystal structures, and proton-conductive properties of these compounds are reported.

Materials and Methods
The metalloligand Co(notpH3)•3H2O was synthesized using the literature procedure [12].All other starting materials, reagents, and solvents were obtained from commercial suppliers and were used without further purification.The infrared spectra were recorded on a Bruker Tensor 27 spectrometer using KBr pellets, and the powder X-ray diffraction patterns were obtained with a Bruker D8 advance diffractometer using Cu-Kα radiation (λ = 1.5406Å).Thermogravimetric analyses (TGA) were conducted with a Mettler Toledo TGA/DSC instrument from 25 to 500 °C, with a heating rate of 5 °C min −1 under a nitrogen atmosphere.The conductivities of the sample pellets were obtained by AC impedance measurements, which were carried out under different environmental conditions by the conventional quasi-four-probe method with a Solartron SI 1260 Impedance/Gain-Phase Analyzer and 1296 Dielectric Interface in the frequency range 1.0 MHz-0.1 Hz.The electrical contacts were prepared using the gold paste to attach the 50 μm-diameter gold wires to the 2.5 mm-diameter compressed pellets or selected single crystals.Exposure of the samples to a humid environment (40% to 95%) at different temperatures (15 to 45 °C) was performed using a GSJ-100 (Su-Ying Corp.) humidity-controlled oven.

Materials and Methods
The metalloligand Co(notpH 3 )•3H 2 O was synthesized using the literature procedure [12].All other starting materials, reagents, and solvents were obtained from commercial suppliers and were used without further purification.The infrared spectra were recorded on a Bruker Tensor 27 spectrometer using KBr pellets, and the powder X-ray diffraction patterns were obtained with a Bruker D8 advance diffractometer using Cu-K α radiation (λ = 1.5406Å).Thermogravimetric analyses (TGA) were conducted with a Mettler Toledo TGA/DSC instrument from 25 to 500 • C, with a heating rate of 5 • C min −1 under a nitrogen atmosphere.The conductivities of the sample pellets were obtained by AC impedance measurements, which were carried out under different environmental conditions by the conventional quasi-four-probe method with a Solartron SI 1260 Impedance/Gain-Phase Analyzer and 1296 Dielectric Interface in the frequency range 1.0 MHz-0.1 Hz.The electrical contacts were prepared using the gold paste to attach the 50 µm-diameter gold wires to the 2.5 mm-diameter compressed pellets or selected single crystals.Exposure of the samples to a humid environment (40% to 95%) at different temperatures (15 to 45 • C) was performed using a GSJ-100 (Su-Ying Corp.) humidity-controlled oven.

Structure Determinations
For CoMg•4H 2 O and CoBa, single crystals with dimensions of 0.30 × 0.10 × 0.05 mm 3 and 0.30 × 0.30 × 0.05 mm 3 were respectively selected and sealed in the mother solution for data collection on a Bruker SMART APEX II diffractometer using graphite-monochromated Mo-K α radiation (λ = 0.71073 Å) at room temperature (296 K).For CoSr•2H 2 O, a single crystal with dimensions of 0.40 × 0.10 × 0.10 mm 3 was used for data collection on a Bruker D8 diffractometer using graphite-monochromated Mo-K α radiation (λ = 0.71073 Å) at 123 K.The numbers of collected and observed independent [I > 2σ(I)] reflections were 16662 and 5773 (R int = 0.075) for CoMg•4H 2 O, 16662 and 11247 (R int = 0.054) for CoSr•2H 2 O, and 10899 and 1384 (R int = 0.049) for CoBa.The data were integrated using the Siemens SAINT program [18].Adsorption corrections were applied.The structures were solved by direct methods and refined on F 2 by full-matrix least-squares using SHELXTL [19,20].Anisotropic temperature factors were used to refine all atoms, excluding hydrogen.All hydrogen atoms bound to carbon were refined isotropically in the riding mode; hydrogen atoms of water molecules were detected in the experimental electron density and then refined isotropically with reasonable restriction of O-H bond distances and H-O-H angles.The crystallographic data are given in Table S1, and selected bond lengths and angles are in Tables S2-S4.

Structural Describes
A single crystal of CoMg•4H 2 O was sealed in the mother solutions to keep the saturated lattice water content, and these were used for the X-ray single-crystal structural determination at room temperature.Structural analysis reveals that CoMg•4H 2 O is isostructural to CoCa•4H 2 O [15].It crystallizes in monoclinic system space groups P2 1 /n (Table S1) and contains one [Co(notpH 2 )] − ligand, one Mg 2+ , one ClO 4 − , and two coordinated and four lattice water molecules in the asymmetric unit (Figure 2a).The [Co(notpH 2 )] − links four Mg atoms via the phosphonate oxygen atoms O2, O5, O8, and O9 as a tetradentate metalloligand.The two phosphonate oxygen atoms (O3 and O6) of the [Co(notpH 2 )] − unit are protonated.Each Mg atom is six-coordinated, with four sites provided by four phosphonate oxygens and two water molecules.termination at room temperature.Structural analysis reveals that CoMg•4H2O is isostruc-tural to CoCa•4H2O [15].It crystallizes in monoclinic system space groups P21/n (Table S1) and contains one [Co(notpH2)] − ligand, one Mg 2+ , one ClO4 − , and two coordinated and four lattice water molecules in the asymmetric unit (Figure 2a)

Thermal Stability
The thermogravimetric analyses were performed on CoMg•nH2O, CoSr•nH2O, and CoBa to compare their thermal stabilities.As shown in Figure 5

Proton Conduction
The proton conductivities of CoMg•nH2O, CoSr•nH2O, and CoBa were evaluated by impedance measurements (Figures S5-S13) using pressed pellets (1.0 Gpa) with thicknesses of 0.76, 0.75, and 0.69 mm, respectively.As shown in Figure 6a, all three samples and the reported compound, CoCa•nH2O, exhibit humidity-dependent proton-conductivities under 25 °C.At 40% RH, the proton conductivities of CoMg•nH2O, CoCa•nH2O, CoSr•nH2O, and CoBa are 1.83 × 10 −7 , 1.08 × 10 −9 , 1.14 × 10 −8 , and 4.28 × 10 −8 S cm −1 , respectively.All samples' conductivities increased with relative humidity, reaching maximum values at 95% RH.Furthermore, the conductivity at 95 % RH and 25 °C followed the sequence: The temperature dependence of the conductivities was measured at 95% RH from 15 to 45 °C at 10 °C intervals.Figure 6b shows the ln(σT) plots vs. 1000/T for all samples.The activation energies, Ea, are estimated to be 0.80 eV for CoMg•nH2O, 0.76 eV for CoSr•nH2O, and 0.54 eV for CoBa.CoMg•4H2O has a continuous hydrogen-bonding network involving ClO4 − anions, -PO3H groups, and water molecules.Therefore, the large proton conduction activation energy of CoMg•4H2O can arise from the rotation and movement of the ClO4 − anions, as is the case for CoCa•4H2O.In CoSr•nH2O and CoBa, the hydrogen bonds are isolated, and the proton might migrate through the medium via the vehicle-type mechanism, corresponding to the large activation energies.

The Effects of Varying the Alkaline Earth Metal Nodes on Proton Conduction
Sufficient acidic proton concentration and continuous hydrogen-bonding networks play critical roles in efficient proton conduction.Coordination water, the acidic moieties of the frameworks, and acidic guest molecules can act as proton sources.For CoMg•nH2O, CoCa•nH2O, CoSr•nH2O, and CoBa, the equal number of protonated phosphonate oxygen atoms is 2 per ligand, and the numbers of coordinated water are 2, 2, 2.5, and 1,5, respectively.However, the lack of continuous hydrogen bonds in CoSr•nH2O and CoBa leads to poorer proton conduction (Figure 3d).It is worth noting that CoBa contains no lattice water but exhibits humidity-dependent proton conduction, which could be attributed to the effect of the grain boundary using a pellet for measurement [21].In isostructural CoMg•nH2O and CoCa•nH2O, the proton sources and proton pathways are identical (Figure 8), but CoMg•nH2O exhibits a 28-fold enhanced value for proton conductivity compared with CoCa•nH2O at 95% RH and 25 °C.The only difference between the structures of both of the compounds is the different dihydrated metal centers: Mg(II) and Ca(II).The pKa values of the aqueous metal ions Mg(H2O)6 2+ and Ca(H2O)7 2+ are 11.2 and 12.7 [22], respectively.This indicates that the water binding to the Mg(II) ion can provide more acidic protons, agreeing with the higher proton conductivity of CoMg•nH2O.3d).It is worth noting that CoBa contains no lattice water but exhibits humidity-dependent proton conduction, which could be attributed to the effect of the grain boundary using a pellet for measurement [21].2+ and Ca(H 2 O) 7 2+ are 11.2 and 12.7 [22], respectively.This indicates that the water binding to the Mg(II) ion can provide more acidic protons, agreeing with the higher proton conductivity of CoMg•nH 2 O.

Crystals 2022 , 13 Figure 3 .
Figure 3.The crystal structure of CoSr•2H2O; the asymmetric unit (a), one inorganic layer (b), the three-dimensional packing diagram (c), and hydrogen bonds among the water molecules, phosphonate groups, and perchlorate anions (d).All hydrogen atoms are omitted for clarity, except in the protonated phosphonate groups or water molecules.

Figure 4 .
Figure 4.The crystal structure of CoBa; the asymmetric unit (a), one hexagonal inorganic layer (b), and the ABCABC type of the three-dimensional packing diagram (c).All hydrogen atoms are omitted for clarity, except in the protonated phosphonate groups or water molecules.
, the CoMg•nH2O stored in the air (ca.50% RH) shows a two-step decomposition process from 25 to 500 °C.Dehydration occurs below 130 °C, and the weight loss of 11.2% agrees with releasing two lattice water and two coordinated water molecules (calc.10.9% for CoMg•2H2O).It indicates that the collected product of CoMg•nH2O lost two lattice water molecules per formula unit in the air and formed the dihydrate phase, CoMg•2H2O, which is confirmed by the Pawley fitting of the PXRD pattern (Figure S1).The fitted unit cell parameters of CoMg•2H2O (P21/n, a = 13.14Å, b = 9.93 Å, c = 18.58 Å, β = 109.2°,V = 2289.3Å 3 ) are identical to those of

Figure 4 .
Figure 4.The crystal structure of CoBa; the asymmetric unit (a), one hexagonal inorganic layer (b), and the ABCABC type of the three-dimensional packing diagram (c).All hydrogen atoms are omitted for clarity, except in the protonated phosphonate groups or water molecules.
It could be interesting to compare the different coordination modes of tripodal metalloligand Co(notpHx) x−3 with Mg(II), Ca(II), Sr(II), and Ba(II), respectively (Figure7).Steric factors commonly govern the coordination geometry of the alkaline earth metal cations.In CoSr•2H2O, 7-coordinate {SrO7} is the distorted capped triangular prism.The smaller Mg(II) and Ca(II) ions are 6-coordinated to form the distorted octahedron in CoMg•4H2O and CoCa•4H2O.In contrast to Sr(II), the larger Ba(II) ion is 9-coordinated to form the distorted tricapped triangular prism {BaO9} in CoBa.Moreover, water molecules occupy three vertices on the prism's three triangular faces.The coordination number increases with the radius of the alkaline earth metal ions; consequently, the tripodal metalloligand Co(notpHx) x − 3 adopts variable bonding modes.In CoMg•4H2O and CoCa•4H2O, the Co(notpH2) − offers four oxygen atoms to bind four Mg(II) or Ca(II) ions, and the other two oxygen atoms are protonated.In CoSr•2H2O, two coordination modes for Co(notpH2) -are observed, and two μ-O atoms bridge the two Sr(II) ions and adjacent Co(III) and Sr(II) ions, respectively.Interestingly, Co(notpH2) -chelates the Ba(II) ion in CoBa using three oxygen atoms, which are coordinated with the Co(III) ion.Furthermore, each equivalent phosphonate group of Co(notpH3) is monoprotonated (occupancy of H is 2/3), and the other three oxygen atoms bind to three Ba(II) ions.Such a coordination mode differs from the reported Co(notpHx) x−3 -based mixed metal phosphonates[11][12][13].

Figure 6 .
Figure 6.Proton-conductive properties of CoMg•nH 2 O, CoSr•nH 2 O, and CoBa; (a) plots of log(σ) vs. RH at 25 • C (increasing RH: filled circle; decreasing RH: open circle).(b) Plots of ln(σT) vs. 1000/T at 95% RH.The data of CoCa•nH 2 O are taken from Bao et al. [15].The temperature dependence of the conductivities was measured at 95% RH from 15 to 45 • C at 10 • C intervals.Figure 6b shows the ln(σT) plots vs. 1000/T for all samples.The activation energies, E a , are estimated to be 0.80 eV for CoMg•nH 2 O, 0.76 eV for CoSr•nH 2 O, and 0.54 eV for CoBa.CoMg•4H 2 O has a continuous hydrogen-bonding network involving ClO 4 − anions, -PO 3 H groups, and water molecules.Therefore, the large proton conduction activation energy of CoMg•4H 2 O can arise from the rotation and movement of the ClO 4 − anions, as is the case for CoCa•4H 2 O.In CoSr•nH 2 O and CoBa, the hydrogen bonds are isolated, and the proton might migrate through the medium via the vehicle-type mechanism, corresponding to the large activation energies.4.Discussion4.1.The Effects of Varying the Alkaline Earth Metal Nodes on the StructuresIt could be interesting to compare the different coordination modes of tripodal metalloligand Co(notpH x ) x−3 with Mg(II), Ca(II), Sr(II), and Ba(II), respectively (Figure7).Steric factors commonly govern the coordination geometry of the alkaline earth metal cations.In CoSr•2H 2 O, 7-coordinate {SrO 7 } is the distorted capped triangular prism.The smaller Mg(II) and Ca(II) ions are 6-coordinated to form the distorted octahedron in CoMg•4H 2 O and CoCa•4H 2 O.In contrast to Sr(II), the larger Ba(II) ion is 9-coordinated to form the distorted tricapped triangular prism {BaO 9 } in CoBa.Moreover, water molecules occupy three vertices on the prism's three triangular faces.The coordination number increases with the radius of the alkaline earth metal ions; consequently, the tripodal metalloligand Co(notpH x ) x−3 adopts variable bonding modes.In CoMg•4H 2 O and CoCa•4H 2 O, the Co(notpH 2 ) − offers four oxygen atoms to bind four Mg(II) or Ca(II) ions, and the other two oxygen atoms are protonated.In CoSr•2H 2 O, two coordination modes for Co(notpH 2 ) − are observed, and two µ-O atoms bridge the two Sr(II) ions and adjacent Co(III) and Sr(II) ions, respectively.Interestingly, Co(notpH 2 ) − chelates the Ba(II) ion in CoBa using three oxygen atoms, which are coordinated with the Co(III) ion.Furthermore, each equivalent phosphonate group of Co(notpH 3 ) is monoprotonated (occupancy of H is 2/3), and the other three oxygen atoms bind to three Ba(II) ions.Such a coordination mode differs from the reported Co(notpH x ) x−3 -based mixed metal phosphonates[11][12][13].

Figure 7 .
Figure 7. Diversities of coordination geometry (a) at the Mg(II), Ca(II), Sr(II), and Ba(II) centers, and coordination modes (b) in the tripodal metalloligand Co(notpH 2 ) − .4.2.The Effects of Varying the Alkaline Earth Metal Nodes on Proton Conduction Sufficient acidic proton concentration and continuous hydrogen-bonding networks play critical roles in efficient proton conduction.Coordination water, the acidic moieties of the frameworks, and acidic guest molecules can act as proton sources.For CoMg•nH 2 O, CoCa•nH 2 O, CoSr•nH 2 O, and CoBa, the equal number of protonated phosphonate oxygen atoms is 2 per ligand, and the numbers of coordinated water are 2, 2, 2.5, and 1,5, respectively.However, the lack of continuous hydrogen bonds in CoSr•nH 2 O and CoBa leads to poorer proton conduction (Figure3d).It is worth noting that CoBa contains no lattice water but exhibits humidity-dependent proton conduction, which could be attributed to the effect of the grain boundary using a pellet for measurement[21].In isostructural CoMg•nH 2 O and CoCa•nH 2 O, the proton sources and proton pathways are identical (Figure8), but CoMg•nH 2 O exhibits a 28-fold enhanced value for proton conductivity compared with CoCa•nH 2 O at 95% RH and 25 • C. The only difference between the structures of both of the compounds is the different dihydrated metal centers: Mg(II) and Ca(II).The pK a values of the aqueous metal ions Mg(H 2 O) 62+ and Ca(H 2 O) 7 2+ are 11.2 and 12.7[22], respectively.This indicates that the water binding to the Mg(II) ion can provide more acidic protons, agreeing with the higher proton conductivity of CoMg•nH 2 O.
In isostructural CoMg•nH 2 O and CoCa•nH 2 O, the proton sources and proton pathways are identical (Figure 8), but CoMg•nH 2 O exhibits a 28-fold enhanced value for proton conductivity compared with CoCa•nH 2 O at 95% RH and 25 • C. The only difference between the structures of both of the compounds is the different dihydrated metal centers: Mg(II) and Ca(II).The pK a values of the aqueous metal ions Mg(H 2 O) 6 The average Mg-O bond length is 2.108(3) Å