Formation and structural characterization of a europium(II) mono(scorpionate) complex and a sterically crowded pyrazabole

Reaction of EuI2(THF)2 with K[HB(3,5-iPr2pz)] (= KTpiPr2, pz = pyrazolyl) afforded the new europium(II) scorpionate complex (KTpiPr2)(3,5-iPr2pzH)2EuIII (1) in addition to the sterically crowded pyrazabole derivative trans-{(3,5-iPr2pz)HB(μ-3,5-iPr2pz)}2 (2) which were both structurally characterized through X-ray diffraction.

The starting material EuI 2 (THF) 2 was prepared from Eu metal and 1,2-diiodoethane using an established literature procedure (Girard et al., 1980). The reaction of EuI 2 (THF) 2 with 1.5 equiv. of KTp iPr2 in THF produced a fluorescent, neon-yellow solution and a white precipitate of potassium iodide. Crystallization from n-pentane solvent afforded brightyellow, air-sensitive crystals, which turned out to be the unexpected europium(II) mono(scorpionate) complex (Tp iPr2 )(3,5-iPr2 pzH) 2 Eu II I (1). The 78% isolated yield of 1 was surprisingly high. The coordinated neutral 3,5-diisopropylyrazole ligands clearly resulted from fragmentation of the Tp iPr2 ligand. Ln-induced fragmentation of substituted Tp ligands is well documented (Domingos et al., 2002, and references cited therein), but it seems to be even more prevalent in the sterically highly demanding Tp iPr2 system, as seen in some recently reported Ln(Tp iPr2 )-derived polysulfide complexes (Kü hling et al., 2016). Despite its paramagnetic nature, interpretable NMR spectra could be obtained for 1. A single resonance at À5.3 ppm in the 11 B NMR spectrum proved the presence of a single boron-containing species. A high-intensity peak at m/z 769 in the mass spectrum of 1 could be assigned to the fragment ion [Eu(Tp iPr2 )( iPr2 pz)] + , while a peak at m/z 616 corresponds to the ion [Eu(Tp iPr2 )] + .

Supramolecular features
In both compounds 1 and 2, no unusually short intermolecular contacts have been observed. In 1, the bulky i Pr groups at the molecule's surface does not allow for intermolecular N-HÁ Á ÁN hydrogen bonding.
For general information on scorpionate ligands, see: Kitajima et al. (1992), Pettinari (2008), Trofimenko (1966Trofimenko ( , 1999 The molecular structure of compound 1 in the crystal, showing orientational disorder of two isopropyl groups. Displacement ellipsoids are drawn at the 40% probability level, H atoms attached to C atoms omitted for clarity.

Figure 3
The molecular structure of compound 2 in the crystal, showing orientational disorder of one isopropyl group. Displacement ellipsoids drawn at the 50% probability level, H atoms attached to C atoms omitted for clarity. [Symmetry code: For general information on the chemistry and structures of pyrazaboles, see: Cavero et al. (2008), Niedenzu & Niedenzu (1984), Niedenzu & Nö th (1983), Trofimenko (1966).

Synthesis and crystallization
All operations were performed under an argon atmosphere using standard Schlenk techniques. THF, hexamethyldisiloxane (HMDSO), and n-pentane were distilled from sodium/benzophenone under argon prior to use. NMR spectra were recorded on a Bruker DPX400 ( 1 H: 400 MHz) spectrometer in THF-D 8 at 295 (2) K. The 11 B NMR spectra were obtained by using inverse gated decoupling on a Bruker Avance 400 NMR spectrometer, operating at 128.4 MHz. The external standard was 15 wt-% BF 3 ÁOEt 2 in CDCl 3 ( B = 0 ppm). IR spectra were measured on a Bruker Vertex V70 spectrometer equipped with a diamond ATR unit, electron impact mass spectra on a MAT95 spectrometer with an ionization energy of 70 eV. Elemental analyses (C, H and N) were performed using a VARIO EL cube apparatus. The starting materials EuI 2 (THF) 2 (Girard et al. 1980) and KTp iPr2 (Kitajima et al. 1992) were prepared according to published procedures.
Preparation of (Tp iPr2 )(3,5-iPr2 Hpz) 2 Eu II I (1) and trans-{(3,5-iPr2 pz)HB(-3,5-iPr2 pz)} 2 (2): In a 250 mL Schlenk flask, THF (150 mL) was added to a mixture of EuI 2 (THF) 2 (2.36 g, 4.29 mmol) and KTp iPr2 (3.20 g, 6.33 mmol), and the resulting suspension was stirred for 12 h at r.t. A white precipitate (KI) was removed by filtration and the neon-yellow, fluorescent filtrate was evaporated to dryness. The residue was extracted with n-pentane (3 Â 50 mL), the combined extracts filtered again and concentrated in vacuo to a total volume of ca 30 mL. Cooling to 277 K afforded bright-yellow, air-sensitive crystals of 1 (3.64 g, 78%), which were suitable for X-ray diffraction. The mother liquid was taken to dryness, and the slightly sticky residue was redissolved in ca 5 mL of THF. Addition of dry hexamethyldisiloxane (ca 50 mL) followed by cooling to 277 K for several days afforded ca 0.5 g of 2 as colorless, cube-like single-crystals.