Perfluoroalkyl Chain Length Effect on Crystal Packing and [LnO8] Coordination Geometry in Lanthanide-Lithium β-Diketonates: Luminescence and Single-Ion Magnet Behavior

Functionalized perfluoroalkyl lithium β-diketonates (LiL) react with lanthanide(III) salts (Ln = Eu, Gd, Tb, Dy) in methanol to give heterobimetallic Ln-Li complexes of general formula [(LnL3)(LiL)(MeOH)]. The length of fluoroalkyl substituent in ligand was found to affect the crystal packing of complexes. Photoluminescent and magnetic properties of heterobimetallic β-diketonates in the solid state are reported. The effect of the geometry of the [LnO8] coordination environment of heterometallic β-diketonates on the luminescent properties (quantum yields, phosphorescence lifetimes for Eu, Tb, Dy complexes) and single-ion magnet behavior (Ueff for Dy complexes) is revealed.


Introduction
Advances in lanthanide coordination chemistry continues to be the cornerstone in the development of promising sensitizers, agents for theranostics, catalysts, and optical and magnetic materials [1][2][3][4][5][6][7][8]. Exploring the luminescent and magnetic properties of Ln III compounds allows the structure-property correlations to be determined for further optimization of the ligand environment.
The current work aims to investigate the correlations between the structure and magneto-optical properties of Ln III complexes with a similar [LnO 8 ] coordination environment. By varying the fluoroalkyl substituent in the β-diketonates (from L 1 (R F = CF 3 ) to L 4 (R F = C 4 F 9 )), we have demonstrated how the crystal packing, intermolecular interactions, and minor changes of [LnO 8 ] geometry influence the properties of luminescent and magnetoactive molecules.

Synthesis of Dinuclear [Dy-Li] β-Diketonates
Ln III -Li I diketonates 1-16 were synthesized through the reaction of functionalized fluorinated diketonates with corresponding lanthanide(III) chloride in methanol or ethanol as a solvent (Schemes 1 and 2), Table 1) according to the previous reports [66,68,70]. Changing the solvent from methanol to ethanol leads to the formation of heterometallic diketonate 16, in which the geometry of the [LnO 8 ] coordination polyhedron and crystal packing differ from those of complex 4 [66].
In the series of complexes 1-16, the coordination sphere of Ln atoms includes eight oxygen atoms O(1) → O(8) from four β-diketonate molecules. Three ligand molecules form three six-membered 4f-metallocycles due to the β-diketonate-anions and result in a tris-diketonate lanthanide fragment ( Figure 5a). The fourth β-diketonate molecule is an initial LiL, which coordinates the lanthanide ion via one methoxy group (O(7)) and oxygen atom of the β-diketonate-anion (O(8)) to give a heterometallic complex structure (Figure 5b). Similarly, one of the molecules from the tris-diketonate fragment coordinates with the Li atom. Methanol molecule completes the coordination sphere of penta-coordinated Li(I) ion. The values of the O-Ln-O angles in the five-and six-membered metallocycles of complexes 1-16 and the bond lengths between the Ln and O(1)-O(8) atoms are given in SI (Tables S2-S9).      (Figures S3-S5). However, in the case of L 2 -L 4 diketonates, we observe linear 1D chains, which lead to layered crystal structures of complexes ( Figures S7-S9). The shortest distance between the two adjacent molecules of complex 16 is determined by the presence of intermolecular hydrogen bonds of the coordinated water without molecule chain motif.
Using the SHAPE program [73,74], we have calculated the geometry of coordination [LnO 8 ] polyhedra of synthesized heterometallic β-diketonates 1-16 (Table 2, also see Tables S10-S13). Table 2 shows that increasing the length of fluoroalkyl group in diketonates strongly distorts the triangular dodecahedron (TDD-8) geometry of [LnO 8 ] polyhedron in Eu and Gd complexes. Calculated values of coordination polyhedra of Tb and Dy complexes are closer to those of the ideal TDD-8 geometry, except for C 3 F 7 containing diketonate. In compound 16, the calculated values of the distorted geometry of the Dy coordination environment correspond better to TDD-8 and biaugmented trigonal prism (BTPR-8).
To gain more insight into the solid structure analysis of obtained complexes, we have compared the dihedral angles between the LnOO and β-diketonate planes (Table S14). The length of the fluoroalkyl group in diketonates greatly affects the planarity of the five-and six-membered chelate cycles. As follows from Table S14, the sum of the dihedral angles between the LnOO and β-diketonate planes increases from CF 3 (L 1 ) to C 4 F 9 (L 4 ) complexes. The exception is C 2 F 5 β-diketonates (L 2 ), for which the deviations from the planarity of the chelate cycles are the smallest in this series. For the five-membered chelate cycles of the complexes, the values of dihedral angles vary irregularly with increasing fluorine atoms in β-diketonates (Table S14).  The IR spectra of 1-16 are similar and display strong absorption bands in the range of 1645-1633 and 1541-1468 cm −1 , which is typical for β-diketonates, that can be attributed to enolate C=O and C=C vibrations, respectively (Figure S18) [16]. Typical C-F vibrations are observed in the range of 1198-1122 cm −1 (the most intensive peaks are at 1137-1122 cm −1 ), and C-H vibrations can be seen between 3000 and 2840 cm −1 .

Mechanoluminescence
Previously, we have observed the mechanoluminescence (ML) of polycrystalline trifluoromethyl Ln-Li diketonates 1, 3 and 4 [66]. However, complexes based on ligands with R F ≥ C 2 F 5 are ML-inactive 5, 7-9, 11, 12, 14, and 15. This indicates the crucial role of structural differences in the crystal packing for ML activity of the complexes.
The crystal packing of trifluoromethyl diketonates 1, 3, 4 consists of zigzag 1D chains of isolated molecules of complexes with the shortest distance between Ln atoms ( Figures S7-S9). In complexes based on ligands with R F ≥ C 2 F 5 , molecules are arranged lengthwise to form linear 1D chains. The coordinated water molecule in complex 16 forms hydrogen bonds to give dimers as the structural unit with the shortest Dy···Dy distance ( Figure S17). Therefore, the molecular herringbone arrangement is a feature of ML activity that is consistent with the reported examples of Ln III tris-diketonates [13].

Photoluminescence
The photoluminescence properties of Eu 3+ , Tb 3+ and Dy 3 + complexes are studied in the solid state. Complexes under illumination by a standard laboratory UV lamp at 365 nm clearly showed visible red, green and yellow luminescence depending on the nature of the lanthanide ion. Broad excitation band in the UV region of the spectrum with the maximum at 320-330 nm indicates the ligand-centered absorbance resulted from π* ← π absorption of β-diketonate fragment L ( Figure 6). Upon excitation at 340 nm, complexes showed the characteristic emission bands of the corresponding Ln III ion originating from the following transitions: 5     We have calculated the radiative (A rad ) and non-radiative (A nrad ) decay rates, as well as the 5 D 0 intrinsic quantum yield (Q Ln Ln ) of the complexes using the following Equations: where A MD,0 is the spontaneous emission probability of the magnetic dipole 5 D 0 → 7 F 1 transition, which is equal to 14.65 s −1 ; n is the refractive index (considered to be 1.5 for solids); (I total /I MD ) is the ratio of the total integrated area of the corrected Eu 3+ emission spectrum to the area of the 5 D 0 → 7 F 1 band [13,75]; and τ obs is measured 5 D 0 luminescence lifetime of [EuL 3 )(LiL)(MeOH)] 1, 5, 9, 12 (Table 3, Figures S24, S27, S30 and S32). Sensitization efficiency ( sens ) for the Eu III complex was estimated as the ratio of the measured 5 D 0 luminescence quantum yield (Q L Ln ) to the calculated intrinsic quantum yield (Table 3): To elucidate the energy transfer process of the lanthanide(III) complexes, the triplet energy levels of the ligands were estimated. Since the lowest lying excited level ( 6 P 7/2 → 8 S 7/2 ) of Gd 3+ is located at 32,150 cm −1 , we can determine the 3 Because of the large difference of energy levels in Eu III complexes, the energy transfer from E 0-0 (3ππ*) to 5 D 2 is the most probable process for them [75]. In Tb III complexes, the energy levels E 0-0 (3ππ*) and 5 D 4 are close, thus quenching of the luminescence intensity is possible due to the reverse energy transfer from the excited level of Tb III to ligand.
Since the differences in the triplet levels of the ligands are insignificant, we consider the geometry of the Ln coordination environment as the key factor influencing the luminescence properties. It was previously reported that a low-symmetry ligand environment around Ln 3+ ion (from D 4d , D 2d to C 3v , C 2v ) improves the luminescence efficiency [15]. This correlation is observed in the case of Eu III and Dy III complexes. In C 2 F 5 -diketonate-based complexes, the geometry of the coordination environment [EuO 8 ] is closest to that of TDD-8 (D 4d ), which explains the difference of luminescence characteristics in this series of complexes. The largest deviation of the coordination polyhedron [DyO 8 ] from the ideal TDD-8 geometry in the case of CF 3 -diketonate leads to a significant increase in the quantum yield. In contrast, a number of diketonates with C 2 F 5 /C 4 F 9 substituents have a high quantum yield due to the [TbO 8 ] geometry close to C 2v symmetry.

Magnetic Properties
The effective magnetic moment µ eff temperature dependences for polycrystalline samples of complexes 8, 15, 16 are shown in Figure 9. The room temperature values of µ eff are 10.74-10.85 µ B , in accordance with the theoretical value of 10.64 µ B for Dy III free ion ( 6 H 15/2 ground state with g J = 4/3). Under cooling, µ eff gradually decreases for 15, 16 and increases for 8 to 10 K below, and the moment drops to a value of 8-8.9 µ B by 2 K. Field dependencies of magnetization are nonlinear at low temperatures ( Figures S38 and S39). The decrease of µ eff at low temperatures suggests the mixture of anisotropy and possible intermolecular exchange interactions. AC magnetic measurements were performed for Dy III complexes 8, 15 and 16 ( Figures 10, S36 and S37). Fast magnetic relaxation is observed under zero dc field due to quantum tunneling magnetization. Applying dc field of 1000 Oe quenches QTM processes, and complexes 8, 15 and 16 exhibit slow relaxation of the magnetization. Frequency dependences of the in-phase (χ ) and out-of-phase (χ") AC magnetic susceptibility were analyzed using the generalized Debye model: where χ T -adiabatic susceptibility, χ S -isothermal susceptibility, τ-relaxation time and α-relaxation times distribution width parameter. The best fit values of χ T , χ S , τ and α are listed in SI (Tables S15-S17).  (Figure 11), which implies more than just the Orbach relaxation mechanism (τ −1 = τ −1 0 e −U eff /k B T ). Raman (τ −1 = CT n ) and Direct (τ −1 = BH 2 T) processes are possible as alternative relaxation mechanisms, and the best fit of the experimental data for 8, 15 and 16 was achieved by taking into account Orbach and Raman relaxation mechanisms [34,35,37]. Optimal parameter values τ 0 and U eff for Orbach and C and n for Raman processes are listed in Table 4. Dashed lines in Figure 11 are simulated curves with the obtained best fit parameters for individual Orbach and Raman processes. Fitting the linear region of ln(τ) vs. T dependencies, considering only the Orbach relaxation mechanism, allows us to estimate energy barrier values U eff , which are close to those listed in Table 4. The obtained U eff values for 8, 15 and 16 are significantly lower than those for 4 (37.7 cm −1 ). In the case of 16, the strong linear dependence of ln(τ) at a low temperature implies a domination of the Orbach relaxation mechanism. However, its curvature at a higher temperature indicates the competing relaxation processes, which makes it difficult to obtain Raman process parameters properly. Therefore, n = 7 was fixed to avoid overparameterization for 16. The obtained values of n are expected for Kramer Ln III ions, for which n may be in the range of 5-9; the deviation may be caused by differences in the crystal field strength [76].
IR diffuse-reflectance spectra were recorded with a Perkin-Elmer Spectrum One FTIR instrument in the range 400-4000 cm −1 . Fluorescence and phosphorescence spectra were recorded in the solid state on a Varian Cary Eclipse fluorescence spectrophotometer with mutually perpendicular beams. The emission lifetimes (τ obs ) and quantum yields (Q L Ln ) have been measured using FS5 Edinburgh Instruments spectrofluorometer at room temperature with absolute error ±2%; excitation was performed through a ligand, and the absolute method in the integration sphere was used. Elemental analysis was performed using a Perkin Elmer (Waltham, MA, USA) PE 2400 Series II analyzer.
The magnetic susceptibility of the polycrystalline samples was measured with a Quantum Design MPMSXL SQUID magnetometer in the temperature range 2-300 K in the magnetic field of 5 kOe. Diamagnetic corrections were made using the Pascal constants. The effective magnetic moment was calculated as µ eff (T) = [(3k/NAµB2)χT]1/2 ≈ (8χT) 1/2 , where k is Boltzman constant, N A -Avogadro's number and µ B -Bohr magneton. Check of the field dependence ac-susceptibility in the range 200-2000 Oe revealed that optimal dc magnetic field is 1 kOe. Therefore, frequency-dependent ac susceptibilities for complexes 8, 15 and 16 were measured under 1 kOe dc field at various temperatures.

Synthesis of the Compounds 1-16 (General Method)
For a solution of LiL (1 mmol) in 15 mL of methanol (for 1-15) or ethanol (for 16), the Ln III salt (0.25 mmol) was added and the mixture was stirred at room temperature for 1 h. The resulting solution was slowly evaporated, and solids were washed with water and cold methanol. The polycrystalline products were recrystallized from the corresponding alcohol (MeOH or EtOH) and filtered off through Celite ® 545 to afford a clear solution. Its slow evaporation at 5-10 • C gave colorless or slightly colored crystals suitable for single-crystal X-ray diffraction structure analysis.

Conclusions
Based on acetal-containing β-diketonates of variable perfluoroalkyl chain length, the discrete heterometallic [Ln-Li] complexes were synthesized. The fluoroalkyl group affects both the crystal packing structure and the distortion of the coordination geometry in complexes. Increasing the number of fluorine atoms in the β-diketonates has no significant impact on the triplet-state energy level of the ligand. By increasing the length of fluoroalkyl substituent in the ligand, the distortion of the coordination geometry [LnO 8 ] from ideal symmetry in [Ln -Li] complexes is irregular and depends on the nature of the Ln III ion. In the series of Eu III and Dy III complexes, PLQY increases when the coordination polyhedron [LnO 8 ] changes from TDD-8 to BTRP-8 geometry. However, the opposite correlation was observed in case of Tb III complexes 7, 14: the closer the [TbO 8 ] environment is to TDD-8 symmetry, the higher the PLQY (60-64%). It has been shown that decreasing the S Q (SAPR- Acknowledgments: X-ray diffraction analysis (complexes 5, 6, 8-14), were performed using the equipment at the Center for Collective Use of the Kurnakov Institute RAS, which operates with the support of the state assignment of the IGIC RAS in the field of fundamental scientific research. CHN and IR-spectral analyses, photo-physical measurements of complexes 1-16 were carried out using the equipment of the Center for Joint Use "Spectroscopy and Analysis of Organic Compounds" at the Postovsky Institute of Organic Synthesis UB RAS. Magnetochemistry and SC XRD analysis of complexes 8 and 15 were carried out within the state assignment of ITC SB RAS (theme No. 121012290037-2). The authors are grateful to Alexandra A. Musikhina for her help in preparing the graphical abstract for this manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.