Crystal structure of tris[bis(2,6-diisopropylphenyl) phosphato-κO]pentakis(methanol-κO)europium methanol monosolvate

The crystal structure of the complex {Eu[O2P(O-2,6-iPr2C6H3)2]3(CH3OH)5}·CH3OH, which exhibits intra- and intermolecular O—H⋯O hydrogen bonding, and its luminescent properties have been studied.


Structural commentary
The asymmetric unit of (1) contains the complex [Eu{O 2 P(O-2,6-i Pr 2 C 6 H 3 ) 2 } 2 (CH 3 OH) 5 ] and one non-coordinating methanol molecule (Fig. 1). Selected bond distances in complex (1) are given in Table 1. The Eu 3+ cation is coordinated by five methanol molecules and three diarylphosphate ligands displaying the terminal 1 O-coordination mode, which leads to the Eu 3+ coordination number of eight. Two phosphate ligands are located close to each other (atoms P1, P2), but the third phosphate ligand (atom P3) is separated from them by the methanol molecules. The complex itself does not have any symmetry element (the C 1 point group), but in a rough approximation, the EuO 8 core might be thought of as belonging to the C s point group with a mirror plane passing through atoms Eu1,O9 and O16. This supports the conclusions drawn from photophysical studies about the Eu 3+ environment (see x4).
The Eu-O P distances are on average 0.11 Å shorter than Eu-O MeOH (Table 1), being in agreement with ion-ion and ion-dipole Ln-ligand interaction types, accordingly. The phosphorous atoms are in a distorted tetrahedral environment. The smallest O-P-O bond angle in each ligand corresponds to the O C -P-O C angle between bulky aryl substituents [99.08 (8) for O2-P1-O3; 100.80 (9) for O6-P2-O7, 101.24 (8) for O10-P3-O11], whereas the largest bond angles are for O Ln -P O [114.89 (9) for O1-P1-O4, 116.23 (9) for O5-P2-O8, 116.11 (9) for O9-P3-O12].  (Minyaev et al., 2017). A roughly single-bond character for both the O-C ipso and P-O C bonds indicates no conjugation between the aryl fragments and the phosphorus atom and consequently prevents charge transfer from aryl groups to Eu 3+ . Therefore, the chosen organophosphate is inapplicable as an 'antenna' ligand, which is in agreement with the rather low quantum yield of the complex (see x4).

Luminescence studies
The steady-state luminescence excitation spectrum of (1) (Fig. 3a) was recorded in the spectroscopic range from 250 to 600 nm with emission monitored on the hypersensitive 5 D 0 ! 7 F 2 transition at 612 nm. This spectrum consists of narrow bands assigned to the 4f-4f intraconfigurational transitions and a broad band centered around 350 nm. The latter could be tentatively assigned to an interligand charge-transfer (ILCT) band due to the presence of the anion-assisted strong hydrogen bonding between coordinated methanol molecules and oxygen atoms at the O P bonds of the organophosphate ligands (see x3 and Fig. 2). A similar charge-transfer band was observed in the case of lanthanide triflates, where the charge redistribution caused by intermolecular hydrogen bonds resulting in an additional CT state was found and confirmed by combined research of luminescence data and the experimental electron density distribution function analysis (Nelyubina et al., 2014). The emission spectrum of (1) (Fig. 3b), recorded in the range from 400 to 720 nm under excitation at 394 nm ( 7 F 0 ! 5 L 6 transition), exhibits intense narrow bands corresponding to the 5 D 0 ! 7 F J transitions (J = 0-4). These electronic transitions display the maximum possible number of Stark components pointing to a low site symmetry for Eu 3+ , i.e. equal to or lower than C 2v . Generally, the intensities and Stark splittings of the 5 D 0 ! 7 F J transitions are influenced by the Intra-and intermolecular O-HÁ Á ÁO bonding in the crystal structure of complex (1). Only core atoms and hydroxy H atoms are shown. Atomic displacement parameters are set to the 50% probability level. Table 2 Hydrogen-bond geometry (Å , ). strength and symmetry of the ligand. A forbidden 5 D 0 ! 7 F 0 transition (region 570-585 nm) of the Eu 3+ cation is presented by a relatively intense symmetric line that indicates the presence of only one type of Eu environment. The integrated intensity of this transition is 0.13, which corresponds to a relatively strong deviation of the Eu 3+ site symmetry from C i . The electric dipole 5 D 0 ! 7 F 2 transition (region 600-620 nm) is extremely sensitive to the symmetry of the europium surroundings and called hypersensitive, and so the ratio of integrated intensities of the 5 D 0 ! 7 F 2 transition to 5 D 0 ! 7 F 1 is a measure of the symmetry of the coordination sphere. In a centrosymmetric environment the magnetic dipole 5 D 0 ! 7 F 1 transition is dominating and the above ratio is < 1, while the distortion of the symmetry around the ion causes an intensity enhancement of the 5 D 0 ! 7 F 2 transition. In (1), this ratio equals 5, which points to a remarkable deviation from a centrosymmetric environment of the Eu 3+ ion. These facts correlate with the found site symmetry for Eu 3+ from the X-ray data (see Figs. 1 and 2). The high intensity of the first Stark component of the 5 D 0 ! 7 F 2 transition at 300 K can potentially be used for obtaining a relatively high colour purity (the line at 610 nm, $50% of the total integrated intensity). Furthermore, a weak broad band was observed in this spectrum in the region 400-550 nm, indicating the residual luminescence of the ligands. Consequently, the overall quantum yield is quite low for the complex ($2.5%), which prevents the use of complex (1) in luminescent applications.

Synthesis
Complex (1) was obtained as a minor product in the reaction of lithium bis(2,6-diisopropylphenyl) phosphate with EuCl 3 (H 2 O) 6 in a 3:1 ratio in methanol at room temperature ( Fig. 4). Only a few single crystal samples were represented by analytically pure (1), whereas the precipitated bulk microcrystalline product was a mixture and mainly contained {Eu[O 2 P(O-2,6-i Pr 2 C 6 H 3 ) 2 ] 2 Cl(CH 3 OH) 4 }ÁCH 3 OH (2), according to IR and C/H analysis. The structure and photophysical properties of (2) will be reported elsewhere. Attempts to isolate (1) as the only product in this reaction failed. Furthermore, attempts to synthesize and grow single crystals of the analogous Tb and Gd tris(phosphate) complexes failed as well. Therefore, the isostructural complexes {Ln[O 2 P(O-2,6-i Pr 2 C 6 H 3 ) 2 ] 3 (CH 3 OH) 5 }ÁCH 3 OH can only be obtained for lanthanides from La to Eu.

General experimental remarks
The synthesis of (1) was carried out under an argon atmosphere. Methanol was distilled over Ca/Mg alloy and stored over molecular sieves (4 Å ). The salt [{(2,6-i Pr 2 C 6 H 3 -O) 2 POO}Li(MeOH) 3 ]ÁMeOH was prepared according to the literature (Minyaev et al., 2015). C/H elemental analysis was performed with a PerkinElmer 2400 Series II elemental analyser. Steady-state luminescence and excitation measurements in the visible region were performed with a Fluorolog FL 3-22 spectrometer from Horiba-Jobin-Yvon-Spex, which has a 450 W xenon lamp as the excitation source and an R-928 photomultiplier. The quantum yield measurements were carried out on solid samples with a Spectralone-covered G8 integration sphere (GMP SA, Switzerland) under ligand excitation, according to the absolute method by Wrighton (Wrighton et al., 1974;de Mello et al., 1997;Greenham et al., 1995).

Synthetic procedure
(3.315 g, 6.00 mmol) in methanol (12 ml) was added to a stirred solution of EuCl 3 Á6H 2 O (0.733 g, 2.00 mmol) in methanol (5 ml). Then, the reaction mixture was allowed to stand overnight at room temperature. Some single crystals ($150 mg) that had formed on the walls of the flask were taken for X-ray studies and elemental analysis, which showed that their composition corresponds to (1). Analysis found (calculated for C 78 H 126 EuO 18 P 3 ) (%): C 58.79 (58.67), H 8.02 (7.95).
The remaining reaction mixture was kept at room temperature for 2 days and for 1 day in a freezer (255 K). The formed precipitate was filtered off, washed with cold (268 K) methanol (3 Â 5 ml), then dried under vacuum to provide 1.861 g of a microcrystalline product.   product contains (2) with some impurities of (1) and possibly of the starting lithium salt.
Numerous attempts to obtain (1) as a single product by varying the reaction conditions failed.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The positions of all non-H and hydroxy H atoms were found from difference electron-density maps. All other H atoms were also found from difference-Fourier maps (with the exception of the disordered fragments) but were positioned geometrically (C-H = 0.95 Å for aromatic, 0.98 Å for methyl, 1.00 Å for tertiary hydrogen atoms) and refined as riding atoms with U iso (H) = 1.5U eq (Cmethyl) and 1.2U eq (C) for other H atoms. A rotating group model was applied for the methyl groups. Reflection 100 was affected by the beam stop, and omitted from the final refinement. Atoms C8, C9 and C47, C48 and corresponding H atoms were disordered over two positions in two isopropyl fragments. Since the residual electron density was not enough to properly position minor components of the disordered isopropyl groups, initial positions for corresponding carbon atoms were taken from isostructural compounds (Minyaev et al., 2018a). This allowed the disorder to be resolved success-fully [the disorder ratios are 0.921 (5):0.079 (5) for atoms C8A, C9A / C8B, C9B and 0.879 (6):0.121 (6) for C47A, C48A / C47B, C48B] and to improve the crystallographic model slightly.   Data collection: APEX2 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT2013 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication:

Special details
Experimental. moisture sensitive 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.