Tetracarbonatodiruthenium Fragments and Lanthanide(III) Ions as Building Blocks to Construct 2D Coordination Polymers

Two-dimensional coordination polymers of [Pr(DMSO)2(OH2)3][Ru2(CO3)4(DMSO)(OH2)]·5H2O (Prα) and [Ln(OH2)5][Ru2(CO3)4(DMSO)]·xH2O (Ln = Sm (Smβ), Gd (Gdβ)) formulae have been obtained by reaction of the corresponding Ln(NO3)3·6H2O dissolved in dimethyl sulphoxide (DMSO) and K3[Ru2(CO3)4]·4H2O dissolved in water. Some DMSO molecules are coordinated to the metal atoms reducing the possibilities of connection between the [Ru2(CO3)4]3− and Ln3+ building blocks giving rise to the formation of two-dimensional networks. The size of the Ln3+ ion and the synthetic method seem to have an important influence in the type of two-dimensional structure obtained. Slow diffusion of the reagents gives rise to Prα that forms a 2D net that is built by Ln3+ ions as triconnected nodes and two types of Ru25+ units as bi- and tetraconnected nodes with (2-c)(3-c)2(4-c) stoichiometry (α structure). An analogous synthetic procedure gives Smβ and Gdβ that display a grid-like structure, (2-c)2(4-c)2, formed by biconnected Ln3+ ions and two types of tetraconnected Ru25+ fragments (β structure). The magnetic properties of these compounds are basically explained as the sum of the individual contributions of diruthenium and lanthanide species, although canted ferrimagnetism or weak ferromagnetism are observed at low temperature.

The very stable carbonate anion, [Ru 2 (CO 3 ) 4 ] 3− , has two ruthenium atoms joined by four bridging carbonate ligands forming the typical paddlewheel structure with a Ru-Ru bond order of 2.5 ( Figure 1). This anion is very similar to the diruthenium [Ru 2 (O 2 CR) 4 ] + cation and, in accordance with the theoretical calculations carried out by Norman et al. [3], a configuration σ 2 π 4 δ 2 π* 2 δ* 1 is assumed. Due to the near degeneracy of the π* and δ* orbitals, the diruthenium(II,III) complexes with paddlewheel structure usually present three unpaired electrons (S = 3/2) [3][4][5][6]. A magnetic study shows that this complex presents a canted ferrimagnet behavior below 4.2 K [7]. The influence of chloride and bromide anions on the self-assembling of [Ru2(CO3)4] 3− and Co 2+ or Cu 2+ ions in aqueous solution has been studied. This self-assembling leads to layer structures with different composition:

Materials and Physical Measurements
K 3 [Ru 2 (CO 3 ) 4 ]·4H 2 O was prepared following a published procedure [2]. The rest of the reagents were purchased from commercial sources and used as received without further purification. Elemental analyses were done by the Microanalytical Services of the Universidad Complutense de Madrid. FTIR spectra were measured using a Perkin-Elmer Spectrum 100 with a universal ATR accessory in the 4000-650 cm −1 spectral range. Thermogravimetric measurements were perfomed using a PerkinElmer Pyris 1 TGA instrument under nitrogen atmosphere with a heating rate of 5 • C min -1 . A Quantum Design MPMSXL Superconducting Quantum Interference Device (SQUID) magnetometer was used to obtain the variable temperature magnetic susceptibility data of finely ground crystals in the temperature range 2-300 K under 1 T. Magnetization measurements were collected at 2 K from −5 to 5 T. All data were corrected taking into account the signal of the sample holder and the diamagnetic contributions of the samples. The molar diamagnetic corrections were calculated on the basis of Pascal's constants. Single crystal X-ray diffraction measurements were carried out with a Bruker Smart-CCD diffractometer at room temperature using a Mo Kα (λ = 0.71073 Å) radiation and a graphite monochromator. CCDC 1894711-1894714 contain the crystallographic data for the new compounds described in this work. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. A summary of some crystal and refinement data are shown in Table 1. Powder X-ray diffraction (PXRD) measurements were carried out by the X-ray service of the UCM using a PANalytical X'Pert MPD diffractometer.  Pr3D: FT-IR (cm −1 ): 1651w, 1597w, 1494s, 1461s, 1320w, 1252m, 1051m, 814w, 762w, 716w. Sm3D: FT-IR (cm −1 ): 1649w, 1598w, 1503s, 1463s, 1255m, 1051w, 814w, 716w.

Synthesis
As previously reported, the equimolecular reaction of K 3 [Ru 2 (CO 3 ) 4 ]·4H 2 O and Ln(NO 3 ) 3 ·xH 2 O in water under different conditions (direct mixture, layering synthesis or solvothermal synthesis with or without microwave radiation) leads to the formation of 3D structures with [Ln(OH 2 ) 4 ][Ru 2 (CO 3 ) 4 (OH 2 )]·xH 2 O composition [21]. This approach has been also successfully employed to prepare Pr3D and Sm3D in this work. In order to reduce the dimensionality of that structure, the same reaction was assayed by dissolving the lanthanide salts in a strong donor solvent such as dimethyl sulphoxide (DMSO) with the aim of blocking some coordination positions of the metals. The layering method was selected to prepare the new compounds because it was successfully used to obtain single crystals of [Ln(OH 2 ) 4 ][Ru 2 (CO 3 ) 4 (OH 2 )]·xH 2 O. Changing the solvent of the rare earth salt solutions was sufficient to form two other structures, one with LnRu 2 (CO 3 ) 4 ·3DMSO·9H 2 O composition for the lighter lanthanide (Pr) and LnRu 2 (CO 3 ) 4 ·DMSO·8H 2 O composition for the heavier ones (Sm, Gd and Dy). The decrease of the lanthanide radius could be the explanation of that change.
Before adding the water solution of K 3 [Ru 2 (CO 3 ) 4 ]·4H 2 O, 10 mL of neat water was added to avoid the precipitation of the compounds at the interface of both solutions. This procedure permits the slow diffusion of the reactants leading to the direct formation of single crystals, suitable for X-ray diffraction analysis, with acceptable yields. It should be also taken into account that the insolubility of the compounds prevents their recrystallization. Powder X-ray diffraction measurements show that Calculated powder X-ray diffractograms for the 2D structures simulated from the single crystal data of Prα (α structure, green) and Smβ (β structure, black). Experimental powder X-ray diffraction pattern obtained for a bulk sample of Prα (pink), Smβ (red) and Gdβ (blue).
The direct mixing of the reagents, K3[Ru2(CO3)4]·4H2O in water and Ln(NO3)3·xH2O in DMSO, instantaneously produces precipitation of a solid that presents the same single phase for Pr, Sm and Gd, as demonstrated by powder X-ray diffraction analysis ( Figure 3). This phase is the same as the one obtained for praseodymium by the layering method (α structure). However, no completely satisfactory elemental analyses have been obtained for these samples. Nevertheless, these results point out that LnRu2(CO3)4·3DMSO·9H2O is the kinetic compound, whereas LnRu2(CO3)4·DMSO·8H2O is thermodynamically more stable, as least when Ln = Sm and Gd.  Calculated powder X-ray diffractograms for the 2D structures simulated from the single crystal data of Prα (α structure, green) and Smβ (β structure, black). Experimental powder X-ray diffraction pattern obtained for a bulk sample of Prα (pink), Smβ (red) and Gdβ (blue).

Structural Description
The direct mixing of the reagents, K 3 [Ru 2 (CO 3 ) 4 ]·4H 2 O in water and Ln(NO 3 ) 3 ·xH 2 O in DMSO, instantaneously produces precipitation of a solid that presents the same single phase for Pr, Sm and Gd, as demonstrated by powder X-ray diffraction analysis ( Figure 3). This phase is the same as the one obtained for praseodymium by the layering method (α structure). However, no completely satisfactory elemental analyses have been obtained for these samples. Nevertheless, these results point out that LnRu 2 (CO 3 ) 4 ·3DMSO·9H 2 O is the kinetic compound, whereas LnRu 2 (CO 3 ) 4 ·DMSO·8H 2 O is thermodynamically more stable, as least when Ln = Sm and Gd. Calculated powder X-ray diffractograms for the 2D structures simulated from the single crystal data of Prα (α structure, green) and Smβ (β structure, black). Experimental powder X-ray diffraction pattern obtained for a bulk sample of Prα (pink), Smβ (red) and Gdβ (blue).
The direct mixing of the reagents, K3[Ru2(CO3)4]·4H2O in water and Ln(NO3)3·xH2O in DMSO, instantaneously produces precipitation of a solid that presents the same single phase for Pr, Sm and Gd, as demonstrated by powder X-ray diffraction analysis ( Figure 3). This phase is the same as the one obtained for praseodymium by the layering method (α structure). However, no completely satisfactory elemental analyses have been obtained for these samples. Nevertheless, these results point out that LnRu2(CO3)4·3DMSO·9H2O is the kinetic compound, whereas LnRu2(CO3)4·DMSO·8H2O is thermodynamically more stable, as least when Ln = Sm and Gd.
The  Five water molecules per formula that do not belong to the 2D network have been found in the crystal structure of Prα. These water molecules establish multiple hydrogen bonds with neighbor water molecules and carbonate, DMSO and water ligands belonging to the 2D network. Interestingly, the thermogravimetric analysis of Prα (heating rate of 5 • C min -1 , Figure S8) shows a weight loss in the 35-65 • C range that corresponds to ca. five water molecules per formula. Then, a plateau is observed until 90 • C, when a loss that corresponds to 3-4 water molecules is observed. The framework is stable until 190 • C when it begins to decompose.

) 4 (DMSO) 2 ] 3− and [Ru 2 (CO 3 ) 4 ] 3− units, with a (2-c)(3-c)
Three crystallization water molecules per formula have been found in the structure of Smβ, while only two have been found in the structure of Gdβ. These water molecules form hydrogen bonds with neighbour water molecules and with carbonate and water ligands of the polymeric structure. The thermogravimetric analyses of Smβ and Gdβ (heating rate of 5 • C min -1 , Figures S9 and S10) show a gradual decomposition in the 35-200 • C range. Three crystallization water molecules per formula have been found in the structure of Smβ, while only two have been found in the structure of Gdβ. These water molecules form hydrogen bonds with neighbour water molecules and with carbonate and water ligands of the polymeric structure. The thermogravimetric analyses of Smβ and Gdβ (heating rate of 5 °C min -1 , Figures S9 and S10) show a gradual decomposition in the 35-200 °C range.   Smβ and Gdβ display a grid-like structure, formed by biconnected nodes, [Ln(OH2)5] 3+ , and two types of tetraconnected nodes, [Ru2(CO3)4(DMSO)2] 3− and [Ru2(CO3)4] 3− units, with a (2-c)(3-c)2(4-c) stoichiometry (Figure 7).
Three crystallization water molecules per formula have been found in the structure of Smβ, while only two have been found in the structure of Gdβ. These water molecules form hydrogen bonds with neighbour water molecules and with carbonate and water ligands of the polymeric structure. The thermogravimetric analyses of Smβ and Gdβ (heating rate of 5 °C min -1 , Figures S9 and S10) show a gradual decomposition in the 35-200 °C range.

Magnetic Properties
The temperature dependence of the magnetic susceptibility of Prα, Smβ, Gdβ, Pr3D and Sm3D was measured between 300 and 2 K at 1 T. The plots of the χ M T vs. temperature are displayed in Figure 8. Compounds with identical lanthanide present approximately the same χ M T values at room temperature despite their different crystal structure. Those values (4.10, 2.56, 10.40, 3.89 and 2.76 emu mol −1 K for Prα, Smβ, Gdβ, Pr3D and Sm3D, respectively) are slightly higher than the value expected from the sum of independent Ru 2 5+ and Ln 3+ ions (3,48, 1.97 and 9.76 emu mol −1 K, respectively, for Pr 3+ , Sm 3+ and Gd 3+ with Ru 2 5+ ).

Magnetic Properties
The temperature dependence of the magnetic susceptibility of Prα, Smβ, Gdβ, Pr3D and Sm3D was measured between 300 and 2 K at 1 T. The plots of the χMT vs. temperature are displayed in Figure 8. Compounds with identical lanthanide present approximately the same χMT values at room temperature despite their different crystal structure. Those values (4.10, 2.56, 10.40, 3.89 and 2.76 emu mol −1 K for Prα, Smβ, Gdβ, Pr3D and Sm3D, respectively) are slightly higher than the value expected from the sum of independent Ru2 5+ and Ln 3+ ions (3,48, 1.97 and 9.76 emu mol −1 K, respectively, for Pr 3+ , Sm 3+ and Gd 3+ with Ru2 5+ ). The χMT values for Prα, Smβ, Pr3D and Sm3D descend smoothly until ≈80 K. Below this temperature a sharper decrease is observed until 2 K for Prα and until 18, 12.6 and 12.7 K for Smβ, Sm3D and Pr3D, when the χMT values increase and a maximum in the curves is observed at 5 K. However, there is almost no variation in the χMT values for Gdβ until 60 K, even a slight increase can be detected. Then, the χMT values decrease until 30 K and at lower temperatures they increase to reach a maximum of 12.10 emu mol −1 K at 5.4 K. Finally, χMT values abruptly descend. This is the same pattern observed for [Gd(H2O)4][Ru2(CO3)4(H2O)2]·2.5H2O (Gd3D) [21] although the χMT maximum is 10.43 emu mol −1 K at 4.6 K (Figure 8).
The decrease of χMT has been observed in other heteronuclear tetracarbonatodiruthenium(II,III) derivatives in which the Ru2 5+ centers are the sole magnetic species [7,9,13,14,19] and it has been ascribed to a large zero field splitting (ZFS) associated with the Ru2 5+ species [7]. In Prα, Smβ, Pr3D and Sm3D this decrease is due to the sum of the ZFS of the Ru2 5+ units and the depopulation of the MJ sublevels of the Ln(III) ions produced by the splitting of the ground state by the ligand field [22].
The Gdβ compound does not present an important temperature dependence of χMT until low temperatures and, therefore, the contribution of Gd(III) to χMT at high temperatures comes basically from the 7 unpaired electrons of the lanthanide ion that arise a 8 S7/2 ground state, without first order spin-orbit coupling [23].
Intramolecular exchange coupling in lanthanide compounds is usually very weak due to the radially contracted nature of 4f orbitals [24]. Therefore, the increase in χMT values at low temperatures could be ascribed to a canted ferrimagnetism produced by the diruthenium species. Actually, this phenomenon has been reported for several tetracarbonatodiruthenium compounds without other magnetic centers [7,9,13,14,19,21]. Interestingly, it was only observed for compounds in which two Ru2 5+ species are connected by a carbonate ligand in the same fashion found in Smβ, Gdβ, Pr3D and Sm3D. However, a continuous lowering of χMT values with temperature was observed when the axial position of the diruthenium species was occupied by other ligands. This is also the case for Prα.
The field dependence of the magnetization at 2 K between −5 and 5 T of compounds Smβ and Gdβ (Figures S11 and S12) shows almost saturation of the magnetization for Gdβ while the value of the magnetization is far from saturation at 5 T for Smβ.  The χ M T values for Prα, Smβ, Pr3D and Sm3D descend smoothly until ≈80 K. Below this temperature a sharper decrease is observed until 2 K for Prα and until 18, 12.6 and 12.7 K for Smβ, Sm3D and Pr3D, when the χ M T values increase and a maximum in the curves is observed at 5 K. However, there is almost no variation in the χ M T values for Gdβ until 60 K, even a slight increase can be detected. Then, the χ M T values decrease until 30 K and at lower temperatures they increase to reach a maximum of 12.10 emu mol −1 K at 5.4 K. Finally, χ M T values abruptly descend. This is the same pattern observed for [Gd(H 2 O) 4 ][Ru 2 (CO 3 ) 4 (H 2 O) 2 ]·2.5H 2 O (Gd3D) [21] although the χ M T maximum is 10.43 emu mol −1 K at 4.6 K (Figure 8).
The decrease of χ M T has been observed in other heteronuclear tetracarbonatodiruthenium(II,III) derivatives in which the Ru 2 5+ centers are the sole magnetic species [7,9,13,14,19] and it has been ascribed to a large zero field splitting (ZFS) associated with the Ru 2 5+ species [7]. In Prα, Smβ, Pr3D and Sm3D this decrease is due to the sum of the ZFS of the Ru 2 5+ units and the depopulation of the M J sublevels of the Ln(III) ions produced by the splitting of the ground state by the ligand field [22]. The Gdβ compound does not present an important temperature dependence of χ M T until low temperatures and, therefore, the contribution of Gd(III) to χ M T at high temperatures comes basically from the 7 unpaired electrons of the lanthanide ion that arise a 8 S 7/2 ground state, without first order spin-orbit coupling [23].
Intramolecular exchange coupling in lanthanide compounds is usually very weak due to the radially contracted nature of 4f orbitals [24]. Therefore, the increase in χ M T values at low temperatures could be ascribed to a canted ferrimagnetism produced by the diruthenium species. Actually, this phenomenon has been reported for several tetracarbonatodiruthenium compounds without other magnetic centers [7,9,13,14,19,21]. Interestingly, it was only observed for compounds in which two Ru 2 5+ species are connected by a carbonate ligand in the same fashion found in Smβ, Gdβ, Pr3D and Sm3D. However, a continuous lowering of χ M T values with temperature was observed when the axial position of the diruthenium species was occupied by other ligands. This is also the case for Prα. The field dependence of the magnetization at 2 K between −5 and 5 T of compounds Smβ and Gdβ (Figures S11 and S12) shows almost saturation of the magnetization for Gdβ while the value of the magnetization is far from saturation at 5 T for Smβ. These measurements suggest the existence of predominant ferromagnetic interactions in Gdβ as in Mn 4 (H 2 O) 16 [18] and other tetracarbonatodiruthenium compounds without other magnetic centers [7,9,13,19,21]. In fact, Ru-O-Gd-O-Ru fragments with Ru-Gd distances of 4.352 and 4.445 Å are found in the structure of Gdβ.
The equation to simulate the magnetic contribution of Pr 3+ ions in Prα or Pr3D should consider the depopulation of the M J sublevels which requires too many Hamiltonian Crystal Field parameters [22]. Therefore, we have used as an approximation the same model above employed for the Gd 3+ derivatives.
The best data obtained from the fits are shown in Table 2 and the figures can be found in the SI (Figures S13-S18). The fits were made with the χ M T values from room temperature until the minimum of the χ M T vs. T curves. The g Ru and D values obtained from the fits are within the normal range observed for diruthenium(II,III) compounds and are close to those for K 3 [Ru 2 (CO 3 ) 4 ]·4H 2 O, which were estimated to be 2.20 and 70 cm −1 [7]. However, the D value obtained for Gdβ was lower (39 cm −1 ) than expected. For this reason, a new fit was done with a fixed D value of 70 cm −1 . In these cases, a higher θ and a lower g values were obtained.

Conclusions
The use of different solvents allows one to control the dimensionality of coordination polymers made from the reaction between K 3 [Ru 2 (CO 3 ) 4 ]·4H 2 O and Ln(NO 3 ) 3 ·xH 2 O (Ln 3+ = Pr, Sm and Gd). The use of neat water leads to the formation of 3D coordination polymers while a H 2 O/DMSO mixture leads to the formation of 2D structures. A different 2D phase can be obtained depending of the reaction method. Thus, slow diffusion of the reagents gives a net made by triconnected Ln 3+ nodes and two different Ru 2 5+ units that are bi-and tetraconnected when the Ln 3+ ion is Pr 3+ (α-structure, Prα).
A grid-like net formed by biconnected Ln 3+ nodes and two different tetraconnected Ru 2 5+ is obtained when the Ln 3+ ions are Sm or Gd (β structure, Smβ and Gdβ). Direct mixing of the reagents leads to the α-structure in all cases. The magnetic behavior of the complexes is consistent with the sum of the individual contributions of diruthenium and lanthanide species. The increase in χ M T at low temperatures is associated with a weak canted ferrimagnetism from the diruthenium species and weak ferromagnetic interaction between Ru 2 5+ and lanthanide ions.