Crystal structures of two erbium(III) complexes with 4-aminobenzoic acid and 4-chloro-3-nitrobenzoic acid

In the structures of two ErIII compounds with 4-aminobenzoic acid and 4-chloro-3-nitrobenzoic acid, discrete centrosymmetric bridged dinuclear complex units are present giving an overall three-dimensional hydrogen-bonded structure in the first complex and a one-dimensional coordination polymer in the second.


Chemical context
The coordination chemistry of the rare earth (RE) metals has been investigated extensively and the structures of a large number of complexes with various ligand types are known (Sastri et al., 2003). Of interest is the lanthanide contraction across the series and 4-aminobenzoic acid (4-ABAH) has provided a valuable ligand for this purpose in a comprehensive study of this effect with the RE 3+ (La-Y) series of complexes (Sun et al., 2004). Within this series there are two sub-sets of isotypic complexes, one monoclinic (P2 1 /n) (La-Tb as well as Dy and Er), in which the structures are twodimensional, the second triclinic (P1) forming dinuclear structures (Yb, Lu, Y, as well as Tb). The solvatomorphism of ISSN 2056-9890 the Tb member {monoclinic, [Tb 2 (4-ABA) 6 (H 2 O) 2 ]; triclinic [[Tb 2 (4-ABA) 6 (H 2 O) 2 ]Á2H 2 O]} is of interest and its occurrence was indicated as being dependent on pH control in the preparation.
It was considered that some of the other later members of the RE series (predominantly triclinic) might also show the same effect so this was tested with Er in a reaction of erbium(III) acetate with 4-ABA in aqueous ethanol under mild reaction conditions, with no additional pH control. The title triclinic complex [Er 2 (C 7 H 6 NO 2 ) 6 (H 2 O) 4 ]Á2H 2 O, (I), was obtained. For (I), the preliminary unit-cell data (Table 1) suggested a possible solvatomorphic variant of the previously reported polymeric monoclinic Er 3+ complex with 4-ABA (Sun et al., 2004), and this was confirmed in the X-ray structural analysis. The comparative cell data for the triclinic Tb 3+ complex with 4-ABA are a = 9.0964 (1), b = 11.0117 (1), c = 12.7430 (2) Å , = 89.372 (5), = 72.0360 (6), = 75.0730 (7) , V = 1169.97 (2) Å 3 , confirming that the two are isotypic.

Structural commentary
In the title centrosymmetric dinuclear structure of compound (I) (Fig. 1) et al., 2004), in which the extending Er-N bond is somewhat elongated at 2.660 (3) Å , with (I), there is no reasonable Er-N bonding contact. The monodentate water molecule O2W in (I) replaces the bridging amino N-donor site which is present in the 8-coordination sphere about Er in the solvatopolymorph. Within the dinuclear complex unit of (I), an intra-dimer O-HÁ Á ÁO carboxylate hydrogen bond is present between one of the the coordinating water molecules (O1W) and an inversion- Symmetry code: (i) Àx þ 1; Ày þ 1; Àz þ 1.

Figure 1
The molecular configuration and atom-naming scheme for the centrosymmetric dinuclear title complex and water molecules of solvation in (I), with displacement ellipsoids drawn at the 40% probability level. For symmetry code (i), see Table 1.
related carboxylate O-atom (O11A i ) ( Table 2). This structure is similar to the triclinic isotypic Tb 3+ complex with 4-ABA (Sun et al., 2004). In (I), the 4-ABA ligand species show some variation in the conformation of the carboxylate groups. In one of the bidentate O,O 0 -chelate ligands (A) and the bridging ligand (C), the groups are essentially coplanar with the benzene ring [torsion angles C2A/C-C1A/C-C11A/C-O11A/C = 171.2 (4) and 174.8 (4) , respectively], while in the second bidentate chelate ligand (B) the group is rotated out of the plane [corresponding torsion angle = 155.9 (4) ]. Such a 'planar' conformation is also found in the structure of the parent acid (Gracin & Fischer, 2005) and in molecular adducts with aromatic carboxylic acids (Chadwick et al., 2009).

Figure 3
The packing of the one-dimensional polymeric chain structure of (II) in the unit cell, viewed approximately along [001]. H atoms have been omitted.

Supramolecular features
In the crystal structure of compound (I), extensive inter-unit O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen-bonding interactions are present, involving both the coordinating water molecules as well as the solvent water molecules, with carboxylate O-atom acceptors and amine N-atom acceptors (Table 2). These, together with amine N-HÁ Á ÁO water and O carboxyl hydrogen bonds give a three-dimensional network structure (Figs. 4 and 5). One H atom of each of the amine groups on the three 4-ABA ligand components of the complex is not involved in hydrogen-bonding. Also present in the supramolecular structure are weakinteractions between A ligands [ringcentroid separation AÁ Á ÁA vii = 3.711 (3) Å ] and C ligands [CÁ Á ÁC viii = 3.676 (3) Å ] (for symmetry codes, see Table 2). This dimeric carboxylate-bridged complex mode is similar to that found in the erbium acetate complex [Er 2 (CH 3 CO 2 ) 6 - et al., 1984). With (II), present are two weak intra-polymer C-HÁ Á ÁO hydrogen bonds involving methyl H atoms and both a DMSO O-atom acceptor and a Cl-atom acceptor (Table 4).

Synthesis and crystallization
The title compounds were synthesized by warming together for 10 min, a solution obtained by mixing 5 ml of ethanolic 4-aminobenzoic acid (1 mmol: 135 mg) [for (I)] or 4-chloro-3nitrobenzoic acid (1 mmol: 200 mg) [for (II)], with 10 ml of aqueous erbium(III) acetate hexahydrate (0.3 mmol: 216 mg). Partial room-temperature evaporation of these solutions provided pale-pink block-like single crystals of (I), suitable for X-ray analysis while a colourless powder was obtained from the preparation of (II). Recrystallization using the slow diffusion of water into a DMSO solution gave minor small crystals of (II), suitable for X-ray analysis.

Refinement details
Crystal data, data collection and structure refinements for (I) and (II) are summarized in Table 5. Hydrogen atoms on all water molecules and the amine groups of the 4-ABA ligands in (I) were located by difference methods and positional parameters were refined with restraints [O-H bond length = 0.85 (2) Å and N-H = 0.88 (2) Å ], with U iso (H) = 1.5U eq (O) or 1.2U eq (N). Other H atoms were included in the refinement at calculated positions [C-H(aromatic) = 0.95 Å or C-H(methyl) = 0.96 Å , with U iso (H) = 1.2U eq (C)(aromatic) or 1.5U eq (C)(methyl)], using a riding-model approximation. In the refinement of (II), a number of large difference electron density residual peaks (5-7 e Å À3 ) located within 1.0 Å of the Er1 site were present. These are possibly due to poor crystal quality coupled to effects of an insufficient absorption correction. The dimeric complex (I) in the unit cell, viewed approximately down [100], showing intra-and interdimer hydrogen-bonding extensions as dashed lines. Non-associative H atoms have been omitted. For symmetry codes, see Table 2.

Figure 5
The three-dimensional hydrogen-bonded structure of (I) in the unit cell, viewed along [100]. Non-associative H atoms have been omitted.

(II) Poly[hexakis(µ 2 -4-chloro-3-nitrobenzoato-κ 2 O:O′)bis(dimethyl sulfoxide-κO)dierbium(III)]
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 6.83 e Å −3 Δρ min = −2.41 e Å −3 Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

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