Comment

The structural chemistry of anionic five-membered nitrogen heterocyclic ligands, especially pyrazolate and 1,2,4-triazolate ligands, has received much attention because of the structural diversity and variety of the coordination modes of these compounds (Zheng et al., 2003; Kobrsi et al., 2005, 2006; Werrett et al., 2011). Pyrazolate ligands are among the most versatile of ligands, with 20 different coordination modes identified so far (Halcrow, 2009). 1,2,4-Triazolate ligands are frequently used to construct metal-organic frameworks (MOFs) with various transition metal ions (Yang et al., 2007, 2009; Mahata et al., 2009; Wei et al., 2010). However, to the best of our knowledge, there is only one reported example of a lanthanide complex containing 1,2,4-triazolate anions, namely the tetranuclear complex [Cp'₂Yb(-¹:²-Tz)]₄·2THF (Tz = 1,2,4-triazolate and Cp' = methylcyclopentadienyl; Zhang et al., 2007). Monomeric lanthanide complexes are typically obtained in the presence of sterically demanding ligands. We report here the synthesis and crystal structures of two monomeric lanthanum complexes containing ²-3,5-diphenylpyrazolate (Ph₂Pz) and ²-3,5-diphenyl-1,2,4-triazolate (Ph₂Tz) ligands, namely tris(3,5-diphenylpyrazolato-²N,N')tris(tetrahydrofuran-O)lanthanum(III) tetrahydrofuran monosolvate, (I), and tris(3,5-diphenyl-1,2,4-triazolato-²N¹,N²)tris(tetrahydrofuran-O)lanthanum(III), (II).

Complex (I) crystallizes as a monomeric complex with three ²-Ph₂Pz ligands and three coordinated tetrahydrofuran (THF) molecules in the asymmetric unit, together with one solvent molecule of THF (Fig. 1), while (II) is monomeric with three ²-Ph₂Tz ligands and three coordinated THF molecules, and no solvent (Fig. 2). The average La-N [2.531 (3) Å] and La-O [2.612 (2) Å] bond lengths in (I) do not differ significantly from those in (II) [La-N = 2.535 (2) Å and La-O = 2.603 (2) Å] (Tables 1 and 3). The coordination geometries about atom La1 in both (I) and (II) can be described as distorted octahedral if the centres of the N-N bonds are treated as monodentate donors (Pfeiffer et al., 1999). However, complex (I) has a mer-distorted octahedral geometry, while complex (II) has a fac-distorted geometry (Fig. 3). There are a few other lanthanide complexes reported so far with the formula [ML₃X₃] (M = lanthanide, X = THF and L = pyrazolate or pseudo-pyrazolate ligands), for instance, [Nd(²-Ph₂Pz)₃(THF)₃]·THF (Cosgriff et al., 1993) and [Sm(²-N,N-3,5-Ph₂dp)₃(THF)₃]·THF (3,5-Ph₂dp = 3,5-diphenyl-1,2,4-diazaphospholide; Pi et al., 2008); both of these are isostructural with (I). All of the complexes with a mer configuration are solvated with THF in the crystal structure, while (II), having a fac configuration, is not solvated.

Since bonding between lanthanide ions and ligands is highly electrostatic and nondirectional, the coordination geometries of lanthanide complexes are often very irregular, depending mainly on steric factors associated with the ligands (Marques et al., 2002; Aspinall, 2007). From this viewpoint, the structure of (II) might be expected to be similar to that of (I). It has been demonstrated recently that intramolecular noncovalent interactions also play important roles in the geometries of the metal centres of lanthanide complexes (Yuasa et al., 2011), and we suggest that the stability of complexes (I) and (II) could depend on intramolecular C-HN/O interactions, in addition to steric factors. In both (I) and (II), C-HN/O interactions are formed between the ligands (Tables 2 and 4). In (I), one of the ²-Ph₂Pz ligands (containing atoms H16 and H30) forms C-HN interactions with the triazole rings of the other two ²-Ph₂Pz ligands, while the other two ²-Ph₂Pz ligands form C-HO interactions with the THF ligands (Fig. 4). One ²-Ph₂Pz ligand forms a C1-H1O3 interaction and a much longer C15-H15O2 contact (HO = 3.22 Å), while another ²-Ph₂Pz ligand forms a C31-H31O2 interaction and no apparent contact with atom O3. In (II), the fac coordination geometry permits each of the ²-Ph₂Tz ligands to form one C-HN contact in a threefold cyclic arrangement (Fig. 5). All of the C-HO contacts to the THF ligands in (II) are much longer (HO > 3 Å).

Although C-HN/O interactions are generally considered to be weak, they appear to have an impact on the La-N bond lengths. For all three ²-Ph₂Tz ligands in (II), the two La-N bonds have significantly different lengths (Table 3), and the shorter length in each case is at the same end of the ligand as the C-HN contact. In (I), a similar asymmetry in the La-N bond lengths is observed for the two ²-Ph₂Pz ligands forming C-HO contacts, while the ligand forming C-HN contacts at both of its ends (H16 and H30) displays the most symmetrical La-N distances (La1-N3/N4; Table 1). It has previously been reported that weak C-HN interactions produce asymmetric ²-bonding between five-membered heterocyclic ligands and Ba²⁺ or K⁺ ions (Kobrsi et al., 2005, 2006).

Experimental

All manipulations were carried out using standard Schlenk line or dry-box techniques under a nitrogen atmosphere. The THF, hexane and deuterated benzene (C₆D₆) solvents were refluxed over sodium and distilled.

For the synthesis of (I), La[N(SiMe₃)₂]₃ (1.0374 g, 1.673 mmol) in THF (10 ml) was added to a THF solution (20 ml) of Ph₂PzH (1.1055 g, 5.019 mmol) at room temperature. The solution was stirred for 24 h and then evaporated to dryness under vacuum to afford a pale-yellow solid. The residue was extracted with a mixture of THF and hexane. Complex (I) was obtained from the filtered extract at 243 K as colourless crystals by slow evaporation (yield 1.49 g, 82%). Analysis calculated for C₆₁H₆₅LaN₆O₄: C 67.52, H 6.04, N 7.74%; found: C 67.46, H 6.13, N 7.61%. ¹H NMR (C₆D₆, 300 MHz): 8.15 (d, J = 7.6 Hz, 12H), 7.46 (s, 3H), 7.21-7.26 (m, 12H), 7.07-7.12 (m, 6H), 3.44 (m, 16H), 1.04 (m, 16H).

Complex (II) was obtained by protolysis of La[N(SiMe₃)₂]₃ (0.6598 g, 1.064 mmol) with Ph₂TzH (0.7063 g, 3.192 mmol), following a similar procedure to that for (I). Colourless crystals of (II) were obtained by slow evaporation from a mixture of THF and hexane (yield 0.91 g, 84%). Analysis calculated for C₅₄H₅₄LaN₉O₃: C 63.84, H 5.36, N 12.41%; found: C 63.75, H 5.43, N 12.34%. ¹H NMR (C₆D₆, 300 MHz): 8.62 (d, J = 6.4 Hz, 12H), 7.19-7.24 (m, 12H), 7.08-7.13 (m, 6H), 3.30 (m, 12H), 0.85 (m, 12H); ¹³C{H} NMR (C₆D₆, 75 MHz): 160.6, 132.7, 128.9, 128.7, 126.8, 69.8, 24.9.

All H atoms were placed geometrically and treated as riding atoms, with C-H = 0.95 (pyrazole and phenyl) or 0.99 Å (CH₂) and U_iso(H) = 1.2U_eq(C). Some of the coordinated THF ligands in both complexes show orientational disorder. Atom C51 of (I) is split over two sites (C51 and C51') with refined site occupancies of 0.900 (12) and 0.100 (12), respectively. In complex (II), two sets of positions are defined by atoms C44/C45/C46 and C44'/C45'/C46', with refined site occupancies of 0.347 (5) and 0.653 (5), respectively. Atom C49 of (II) is also split over two sites (C49 and C49'), with refined site occupancies of 0.48 (3) and 0.52 (3), respectively. The free THF molecule in (I) is refined as disordered over two positions, with site occupancies of 0.82 (3) and 0.18 (3). The geometries of the disordered groups were restrained to be similar to one another or to specific distances and displacement parameters were either restrained to be similar for neighbouring atoms, or to have approximate isotropic behaviour, or constrained to be equivalent for the corresponding atoms in the disordered parts. The crystal of (I) was refined as an inversion twin, with a refined twin ratio of 0.550 (8):0.450 (8).

For both compounds, data collection: CrystalClear (Rigaku, 2007); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009); software used to prepare material for publication: SHELXTL.

---

Supplementary data for this paper are available from the IUCr electronic archives (Reference: BI3050 ). Services for accessing these data are described at the back of the journal.

---