Poly[[tetra-μ3-acetato-hexa-μ2-acetato-diaqua-μ2-oxalato-tetra­praseodymium(III)] dihydrate] (2024)

Lanthanides are characterized by a high affinity for ligands with hard donoratoms such as O and/or N (Brunet et al.,2007). The compounds thusformed may possess unique photophysical and magnetic properties that make themsuitable for applications in advanced materials (Steckl & Zavada, 1999; de Saet al., 2000; Kido & Okamoto, 2002). The construction of extendedstructures containing praseodymium or other f-elements bridged bycarboxylate groups is of interest because of the large variety ofarchitectures that result from the high and variable coordination numbers ofthe lanthanides, as well as the versatility of carboxylate ligands.

Ionic liquids (ILs) are a group of ionic compounds having a broad temperaturerange at which they remain liquid combined with very low vapour pressures, andthey have been proposed as potential replacements for conventional organicsolvents. ILs are generally composed of one organic cation (such asimidazolium, pyridinium or pyrrolidinum) and an anion (such BF₄^-,PF₆^-, Cl^- etc.); they have been shown to act both as solvents andas reactants which may be incorporated into the final reaction product (forinstance, behaving as a charge neutralizer; Cocalia et al., 2006). Theuse of ILs in the in situ synthesis of ligands in coordinationcompounds has increased in the last few years, along with efforts to elucidatethe mechanisms of the reactions involved (Reichert et al., 2006).

Reported herein is the title new polymeric compound, (I), obtained underhydrothermal conditions via praseodymium acetate and the IL1-butyl-3-methylimidazolium chloride, [bmim][Cl]. Fig. 1 shows a molecularview of (I), where each ligand is identified by a unique trailing number:acetates 1-5; oxalate 6.

The tetranuclear complex lies acoss an inversion centre located at the centreof the bridging oxalate ligand, so that the asymmetric unit is composed of twoPr^III cations, one water ligand, five acetate anions and half of an oxalateunit; a solvent water molecule completes the asymmetric unit. The two Pr^IIIcations (Pr1 and Pr2) receive bonds from all five acetate anions, while theoxalate anion coordinates only to Pr1 and the water ligand coordinates only toPr2. As a result, the metal cations present different PrO₉ (Pr1) and PrO₁₀(Pr2) environments (Table 1), with Pr—O distances in the range2.417(3)–2.698(3) Å for Pr1 and 2.388(3)–2.786(3) Å for Pr2, and bondvalence sums of 3.33 and 3.22, respectively. The five acetate ligands, inturn, bind in three different bridging ways: as µ₂-κ²O,O'(simple bridge, acetate 2), as µ₂-κ²O:O',O' (simplebridge–simple chelate, acetates 3 and 5) and asµ₃-κ²O,O:O',O') (double bridge–simplechelate, acetates 1 and 4). The oxalate anion, as usual, binds in aµ₂-κ⁴O:O',O'':O'''(simple bridge–doublechelate) mode.

This intricate bonding scheme results in a tight two-dimensional mesh ofinterlinked Pr^III coordination polyhedra, with the metals joined by avariety of `short' (Pr—O—Pr) and `long' (Pr—O—C—O—Pr) bridges togive Pr···.Pr distances of 4.0618(3) (Pr1···Pr2, three short bridges),4.2224(3) (Pr1···Pr2ⁱ, two short bridges), 6.4045(3) (Pr1···Pr1ⁱⁱ, twolong oxalate bridges) and 4.4995(3) Å (Pr2···Pr2ⁱⁱⁱ, two short bridges)[symmetry codes: (i) -x + 1, y - 1/2, -z + 3/2; (ii)-x + 1, -y + 1, -z + 2; (iii) -x + 1, -y +2, -z + 2].

Fig. 2(a) shows a view of one such mesh, parallel to (100). It can be seen thatthe cations, water ligands and acetate anions generate an infinite layer(represented by light lines) with large centrosymmetric `holes' in it, wherethe oxalate anions (heavy lines) lie, linking symmetry-related Pr1 centrestogether. These strongly connected planar structures are further stabilized bya strong `intra-planar' hydrogen bond (Table 2, first entry), while beinglinked to each other via the other three `interplanar' hydrogen bonds(Table 2, entries 2–4). Figs. 2(a) and 2(b) depict thesehydrogen bonds in full detail, while Fig. 2(c) sketches the way inwhich they operate in the interplanar linkage, through a rather simple chainmade out of `conjugated' hydrogen-bonded rings (A and B)threading through a line of inversion centres across the planes. The rings,characterized by graph-set descriptors (Bernstein et al., 1995)R⁴₆(12) (ring A) and R²₂(8) (ring B), areformed by just four non-H atoms (O1W, O2W, Pr2 and O25; Fig.2c).

Fig. 3 shows the IR and Raman spectra of (I), in the range 500–3000 cm^-1, inwhich the oxalate and acetate bands can be observed. The peaks are assignedaccording to previously reported data (Frost, 2004; Ito & Bernstein, 1956).The range 1500–1400 cm^-1 in the Raman spectrum shows the bands associatedwith double bonds, i.e. C═O stretching of both acetate and oxalate.The antisymmetric C═O stretching extends to 1700 cm^-1 in the IRspectrum. The bands between 1100 and 500 cm^-1 correspond to C—Cstretching. The acetate can be distinguished from the oxalate group in therange 3100–2900 cm^-1, in which only the CH stretching vibrations of theacetate are seen.

As a final remark, it should be stated that the reaction responsible for thepresence of the oxalate bridge in the final product is not yet fullyunderstood. This situation is by no means unique; in situ oxalate-anionformation under hydrothermal conditions has already been reported, and hasbeen related to the rearrangement or cleavage of organic compounds (Knope &Cahill, 2007; Zhang et al., 2006). However, the mechanisms that lead tooxalate-bridge formation remain to be understood.