Structure of triaquatris(1,1,1-trifluoro-4-oxopentan-2-olato)cerium(III) as a possible fluorescent compound

The title complex has two kinds of ligands, trifluoroacetylacetonate and water. As a result of the presence of F atoms in the 1,1,1-trifluoro-4-oxopentan-2-olate ligand, the metal–metal distance is longer than in the case of the analogous yttrium complex containing an acetylacetonato ligand and also for the analogous lanthanum complex containing acetylacetonate.

Luminescence due to the d-f transition of Ce 3+ is quite rare in metal-organic complexes where concentrate quenching frequently occurs. One of the possible ways to avoid this is to design an architecture with elongated metal-metal distances. In the structure of the title complex, triaquatris(1,1,1-trifluoro-4oxopentan-2-olato-2 O,O 0 )cerium(III), [Ce(C 5 H 4 F 3 O 2 ) 3 (H 2 O) 3 ], the Ce III complex is linked to neighbouring ones by hydrogen bonding. Within the complex, the Ce III atom is coordinated by nine O atoms from three 1,1,1trifluoro-4-oxopentan-2-olate (tfa) anions as bidentate ligands and three water molecules as monodentate ligands. Thus, the coordination number of Ce III atom is nine in a monocapped square-antiprismatic polyhedron. The F atoms of all three independent CF 3 groups in tfa are disordered over two positions with occupancy ratios of about 0.8:0.2. The intermolecular hydrogen bonds between the ligands involve tfa-water interactions along the [110] and [110] directions, generating an overall two-dimensional layered network structure. The presence of the F atoms in the tfa anion is responsible for an increased intermolecular metal-metal distance compared to that in the analogous acetylacetonate (acac) derivatives. Fluorescence from Ce 3+ is, however, not observed.

Chemical context
-diketonate ligands have been used widely in metal-organic complexes involving rare earth elements because of their simple usage as organic bidentate ligands (Binnemans, 2005). The nature of the ligand used is important for a possible enhancement of the luminescence efficiency and intensity; for example, acac is known to have a possible effect on the 4f-4f transition emission of Eu 3+ (Kuz'mina et al., 2006). Tb(acac) 3 was first used as an active light-emitting layer material in LEDs based on the emission from the lanthanide complex (Kido et al., 1990). Recently, a lanthanide complex containing Tb 3+ and Eu 3+ , hexafluoroacetylacetonate (hfa) and 4,4 0 -bis(diphenylphosphoryl)biphenyl (dpdp), [Tb 0.99 Eu 0.01 (hfa) 3 -(dpdp)] n , was reported to exhibit an expression thermosensing emission, called chameleon luminophore (Miyata et al., 2013;. The hfa anion can absorb efficiently a visible light excitation and transfer the excited energy from hfa to Tb 3+ , because the energy of the triplet state of hfa (22 000 cm À1 ) is very close to an energy level of Tb 3+ (20 500 cm À1 ; Katagiri et al., 2004). However, the proximity of the levels causes a back-energy transfer from Tb 3+ to hfa. The probability of three types of energy transfer from hfa to Tb 3+ , from Tb 3+ to Eu 3+ and from Tb 3+ to hfa is temperature dependent. As a result, the complex can show green, yellow, orange and red emissions despite the 4f-4f transition.
The nature of the ligand is important in the design of fluorescent metal-organic complexes. The F atoms in hfa are larger than the H atoms in acac, which means that the hfa ligand can reduce the energy loss due to thermal vibrations and could increase the intermolecular distance between the central lanthanide atoms. This may control the concentration quenching.
A considerable number of metal-organic complexes containing Ce 3+ have been reported so far, but the examples of emission based on the 5d-4f transition of Ce 3+ in metalorganic complexes are scarce. [Ce(triRNTB) 2 ](CF 3 SO 3 ) 3 [NTB = N-substituted tris(N-alkylbenzimidazol-2-ylmethyl)amine] and 1 3 [Ce(Im) 3 (ImH)]ÁImH (Zheng et al., 2007;Meyer et al., 2015) are some of the rare cases. One of the reasons for the small number of fluorescent metal-organic complexes containing Ce 3+ is the too short distance between the Ce 3+ ions, causing luminescence quenching by the energy transfer between Ce 3+ ions. [Ce(triRNTB) 2 ](CF 3 SO 3 ) 3 can show a blue emission thanks to a long Ce-Ce distance of about 17-18 Å . The use of more bulky ligands such as NTB is favourable for a longer Ce-Ce distance. 1 3 [Ce(Im) 3 (ImH)]ÁImH also shows a blue fluorescence emission despite a relatively short separation between the Ce 3+ cations of 7 Å . Emission occurs more frequently in 3D structures with isolated complexes than in framework structures.
This study reports structural data on a newly synthesized Ce 3+ complex with functional ligands of tfa.

Structural commentary
The title complex crystallizes in the orthorhombic space group Pcab with eight formula units of [Ce(C 5 Each molecule is isolated individually, i.e. the crystal structure is not a framework structure. The central Ce atom is coordinated by nine O atoms of three hfa and three water molecules ( Fig. 1). Thus, the Ce atom has a monocapped square-antiprismatic coordination. The Ce-O bond lengths can be classified into two categories; the first is involved in interactions with a bidentate hfa, and the second in interactions with monodentate water molecules. All distances are comparable with those reported for tfa complexes (Nakamura et al., 1986). The trifluoromethyl groups of tfa coordinating the Ce 3+ ion are all disordered on the F atoms, as is frequently observed in trifluoroacetate and tetrafluoroborate complexes (Hamaguchi et al., 2011;Strehler et al., 2015).

Figure 1
View of the molecular structure of the title complex, with displacement ellipsoids for non-H atoms drawn at the 30% probability level.
hydrogen bonds, two types of which are cross-linked to each other, building up two-dimensional networks (Fig. 2). The functional hydrophobic groups of -CF 3 and -CH 3 are located on the outside of the layer, resulting in the stabilization of the stacking layers by intermolecular forces. Such a layer structure is also observed in the acetylacetonate complex, (Cunningham et al., 1967) (Fig. 3). This yttrium complex also contains an isolated water in the structure, different from the title compound, but the water molecule can act as a hydrogen-bond linker because it exists within a molecular layer. As a result, the hydrogen bonds make a two-dimensional layered network, as in the title compound. The Ln-Ln distance of nearest neighbours in this complex is longer than that of [Y(C 5 H 7 O 2 ) 3 (H 2 O) 3 ], the shortest distance in the former being 6.141 Å and in the latter 6.035 Å . This difference is mainly caused by atomic size difference between F and H atoms, even taking into account the atomic size difference between La and Y. The shortest Koizumi et al., 2017) is slightly longer than that of the title compound. The fact that the present complex does not show any luminescence from Ce 3+ can certainly be attributed to an insufficient metal-metal separation. Based on previous studies and the present work, the minimum metal-metal separation is expected to be more than 7 Å .

Synthesis and crystallization
Yellow plate-like crystals were obtained by slow evaporation from an acetone solution of Ce(NO 3 ) 3 Á6H 2 O and trifluoroacetylacetone (1:3 molar ratio). The products were filtered off and dried at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to a C atom were positioned geometrically after each cycle in idealized locations and refined as riding on their parent C atoms with C-H = 0.93 Å and U iso (H) = 1.2U iso (C atom). All hydrogen atoms bonded to a water O atom were located in a difference-Fourier map and refined isotropically with a distance restraint of 0.85 (2)       program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012).

Triaquatris(1,1,1-trifluoro-4-oxopentan-2-olato-κ 2 O,O′)cerium(III)
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.