Crystal structures of binuclear complexes of gadolinium(III) and dysprosium(III) with oxalate bridges and chelating N,N′-bis(2-oxidobenzyl)-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine (bbpen2−)

Oxalate-bridged, centrosymmetric binuclear complexes of gadolinium(III) and dysprosium(III) with hexadentate bbpen2– (H2bbpen = N,N′-bis(2-hydroxybenzyl)-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine) are isotypic and were synthesized for structural and magnetic susceptibility studies. The dimeric molecules of the complexes crystallize together with water and methanol molecules, with which they form a variety of weak and medium-strength hydrogen bonds.


Structural commentary
Compounds 1 and 2 are isostructural and crystallize in the P1 space group, with four methanol and four water molecules per lanthanide dimer. Crystals contain the neutral [Ln 2 (-ox)-(bbpen) 2 ] molecules ( Fig. 1) in which gadolinium(III) (1) or dysprosium(III) (2) are eight-coordinate; the [Ln(bbpen)] + units are connected to one another by oxalate bridging in the usual bis(bidentate) coordination mode. The ox 2ligand lies about an inversion centre. The coordination sphere of the lanthanide(III) ion is formed by an N 4 O 2 donor set from the bbpen 2ligand and two oxygen atoms from the bridging oxalate. In 1 and 2 each metal cation has a distorted squareantiprismatic coordination environment (Fig. 2  View of [{Gd(bbpen)} 2 (-ox)]Á4CH 3 OHÁ4H 2 O (compound 1), with the atom-numbering scheme. Hydrogen atoms have been omitted for clarity. Displacement ellipsoids are drawn at the 50% probability level Unlabelled atoms are generated by the symmetry operation Àx, Ày + 1, Àz + 1.

Figure 2
Plot of the coordination sphere about the lanthanide(III) atom in the structure of 1 [symmetry code: (i) Àx, Ày + 1, Àz + 1]. by general inspection of atom positions and bond angles, and confirmed from the crystallographic data by the use of the SHAPE program (Llunell et al., 2005). The average Ln-N bonds are ca 2.60 and 2.58 Å for 1 and 2, respectively, while the average Ln-O distances are ca 2.27 (1) and 2.24 Å (2). The non-bonding DyÁ Á ÁDy distance in 2, 6.1488 (17) Å , is close to the analogous distance of 6.14 Å in [Dy 2 (-ox)-(HBpz 3 ) 4 ]Á2CH 3 CNÁCH 2 Cl 2 (Xu et al., 2010). The O3-Ln-O4 i angles of approximately 68 in both 1 and 2 [symmetry code: (i) Àx, 1 À y, 1 À z] are also similar to those reported for the dysprosium(III)-hydrotris(pirazolylborate) dimer mentioned above. The slightly decreased crystal volume of the Dy compound [1626.3 (7) Å 3 ] compared with that of the Gd compound [1633.7 (3) Å 3 ] is a perfect match with the smaller effective ionic radius of eight-coordinate Dy III versus Gd III (1.027 and 1.053 Å , respectively; Shannon, 1976), and is in line with the lanthanide contraction. Structural representations provided in this paper are for compound 1; the dysprosium(III) product 2 gives rise to very similar results.

Supramolecular features
In both structures, the hydrogen atoms from the crystallizing solvents (water and methanol) participate in an extensive three-dimensional hydrogen-bonding network that may be described as medium-strength intermolecular interactions (Tables 1 and 2).
The solvating (methanol and water) molecules, half of which refine well and are depicted in Fig. 3, participate in intermolecular interactions with the dimeric complexes 1 and 2. As seen in Fig. 3, one water and one methanol molecule are hydrogen-bonded to one another and to the phenolate oxygen atoms in the ligands, generating an O1Á Á ÁH-O30Á Á ÁH-O1W-HÁ Á ÁO2 iii 'bridge', as well as a symmetry-related chain on both sides of the plane formed by the metal and oxalate ions. The water molecules in these chains also connect one dimer to another through weak C1-H1BÁ Á ÁO1W ii interactions ( Fig. 4; Tables 1 and 2).
The other half of the solvent molecules in the unit cell, the electron densities of which have been removed with the SQUEEZE routine in PLATON (Spek, 2015) because of being highly disordered, also contribute to the overall hydrogen-bonding network. This is inferred from the positions of the four main electron-density peaks, which have been assigned to oxygen atoms from the disordered solvents and may give rise to medium-strength to weak hydrogen-bond interactions. For 1, OÁ Á ÁO distances involving three of these peaks amount to 2. Symmetry codes: (ii) Àx þ 1; Ày þ 1; Àz þ 1; (iii) Àx; Ày þ 1; Àz þ 1.

Figure 4
Representation of the dimeric molecules of 1 viewed approximately down the b axis of the unit cell. The binuclear complexes are linked through medium-strength hydrogen bonds to solvating water and methanol molecules, and through weak C1-H1BÁ Á ÁO1W ii -HÁ Á ÁO phenolate interactions to one another [symmetry code: (ii) Àx + 1, Ày + 1, Àz + 1.].
concerned, with O1W acting as a potential electron-density acceptor, and are larger than 3.1 Å for OÁ Á ÁO30 (numbering scheme in Fig. 3). For 2, in turn, the corresponding distances are longer than for 1 at 2.88-3.84 Å for OÁ Á ÁO1W, and even larger (> 4.6 Å ) for OÁ Á ÁO30. On the other hand, any possible interaction involving the phenolate oxygen atoms would be very weak, with the shortest OÁ Á ÁO contact with the disordered solvents being longer than 4.0 Å .

Synthesis and crystallization
LnCl 3 Á6H 2 O (Ln III = Gd or Dy) and K 2 C 2 O 4 ÁH 2 O were purchased from Aldrich and used without purification. (Neves et al., 1992) and the [Ln(bbpen)Cl] precursors, with Ln = Gd or Dy (Liu et al., 2016), were prepared using adapted procedures described in the literature. Methanol and diethyl ether (Vetec) were used without treatment. Ultrapure water (Milli-Q, Millipore type 1, resistivity of 18.2 M cm at 298 K) was employed as described below.
A solution of 8.11 mg (0.0440 mmol) of K 2 C 2 O 4 ÁH 2 O in 1.0 ml of water was slowly added to a methanol solution of 61.1 mg (0.0947 mmol) of [Gd(bbpen)Cl]. The colourless reaction mixture was stirred at room temperature for ca 5 min, and was then cooled down to 277 K to give block-shaped colourless crystals after four days. These were isolated by filtration, washed with diethyl ether and dried.

Refinement
Crystal data, data collection and structure refinement details for the two structures are summarized in Table 3. Both 1 and 2 showed high susceptibility to the loss of the crystallization solvent molecules once removed from the mother liquor. Hydrogen atoms in 1 and 2 were included in idealized positions with methyl, methylene and aromatic C-H distances set at 0.98, 0.99 and 0.95 Å , respectively, and O-H at 0.84 Å and refined as riding with U iso (H) = 1.2-1.5U eq (C,O). Hydrogen atoms on the water molecules were located in difference-Fourier maps and were refined with distance restraints (DFIX O-H = 0.82 Å for 1 and 2, DANG = 1.45 Å for 2).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq

(µ-Oxalato)bis{[N,N′-bis(2-oxidobenzyl-κO)-N,N′-bis(pyridin-2-ylmethyl-κN)ethylenediamineκ 2 N,N′]dysprosium(III)}-methanol-water (1/4/4) (2)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq