Structural Characterization, Magnetic and Luminescent Properties of Praseodymium(III) ‐ 4,4,4 ‐ Trifluoro ‐ 1 ‐ (2 ‐ Naphthyl)Butane ‐ 1,3 ‐ Dionato(1 ‐ ) Complexes

: Four new Pr(III) mononuclear complexes of formula [Pr(ntfa) 3 (MeOH) 2 ] ( 1 ), [Pr(ntfa) 3 (bipy) 2 ] ( 2 ), [Pr(ntfa) 3 (4,4` ‐ Mt 2 bipy)] ( 3 ) and [Pr(ntfa) 3 (5,5` ‐ Me 2 bipy)] ( 4 ), where ntfa = 4,4,4 ‐ trifuoro ‐ 1 ‐ (naphthalen ‐ 2 ‐ yl)butane ‐ 1,3 ‐ dionato(1 ‐ ), 5,5` ‐ Me2bipy = 5,5` ‐ dimethyl ‐ 2,2` ‐ dipyridine, 4,4` ‐ Mt2bipy = 4,4 ′‐ dimethoxy ‐ 2,2` ‐ dipyridine, have been synthesized and structurally characterized. The complexes display the coordination numbers 8 for 1, 3 and 4 , and 10 for 2 . Magnetic measurements of complexes 1–4 were consistent with a magnetically uncoupled Pr 3+ ion in the 3 H 4 ground state. The solid state luminescence studies showed that the ancillary chelating bipyridyl ligands in the 2 – 4 complexes greatly enhance the luminescence emission in the visible and NIR regions through efficient energy transfer from the ligands to the central Pr 3+ ion; behaving as “antenna” ligands.

The high surface positive charge density of Ln 3+ ions (charge/ionic radius) makes them behave as hard acids and consequently they prefer the coordination to hard Lewis bases. Although the intermolecular interaction forces of Ln 3+ with chelated ligand(s) are electrostatic in nature, steric factors rather than electronic ones [44] dominate the geometry of the complexes and as a result, the Ln 3+ complexes containing the same ligand are usually all isostructural. The interactions of β-diketonato ligands with Ln 3+ produces thermodynamically stable complexes [9,17,40,41,43,[45][46][47][48], where the coordinated βdiketonates act as efficient antenna ligands for lanthanides emitting in the visible and NIR region [1][2][3][4]6,17,36]. The resulting Ln(III)-β-diketonates complexes may display coordination numbers (C.N.) up to 12, but 8 and 9 are the most frequently observed C.N. with the Ln 3+ ions. In most of these complexes, the central Ln 3+ ion is coordinated to 3 bidentate-β-diketonate anions, resulting in the presence of several vacant coordination positions, which may be available for the interaction with various solvent molecules and/or ancillary ligands [36,[48][49][50][51].

Single Crystal X-Ray Diffraction Analysis
Single-crystal data of 1-4 were measured on an APEX II CCD diffractometer (Bruker-AXS). Table 1 summarizes crystallographic data, intensity data collection, and structure refinement specifications. Data collections were performed at 100(2) K with Mo-Kα radiation (λ = 0.71073 Å); computer programs APEX and SADABS [52,53] were used for data reduction, LP, and absorption corrections. The program library SHELX [54,55] was used for solution (direct methods) and refinement (full-matrix least-squares methods on F 2 ). Anisotropic displacement parameters were applied to all non-hydrogen atoms. H atoms (Uiso) were obtained from difference Fourier maps. HFIX geometrical constraints were applied only for C-H bonds. Additional software: Mercury [56]; PLATON [57]. CCDC deposition numbers: CCDC 2054083 for 1, CCDC 2054084 for 2, CCDC 2054085 for 3, and CCDC 2054086 for 4.

Magnetic Measurements
Magnetic measurements were performed on solid polycrystalline samples in a Quantum Design MPMS-XL SQUID magnetometer at the Magnetic Measurements Unit of the Universitat de Barcelona. Pascal's constants were used to estimate the diamagnetic corrections, which were subtracted from the experimental susceptibilities to give the corrected molar magnetic susceptibilities.

Luminescence Measurements
Solid state fluorescence spectra were recorded on a Horiva Jobin Yvon SPEX Nanolog fluorescence spectrophotometer equipped with a three slit double grating excitation and emission monochromator with dispersions of 2.1 nm/mm (1200 grooves/mm) at room temperature. The steady-state luminescence was excited by unpolarized light from a 450 W xenon CW lamp and detected at an angle of 90° for solid state measurement by a redsensitive Hamamatsu R928P photomultiplier tube. Spectra were reference corrected for both the excitation source light intensity variation (lamp and grating) and the emission spectral response (detector and grating). Near infra-red spectra were recorded at an angle of 90° using a liquid nitrogen cooled, solid indium/gallium/arsenic detector (850-1600 nm).

Synthesis and Spectra
The  4), respectively in moderate yields (50-63%). The isolated complexes 1-4 were structurally characterized by single crystal X-ray crystallography as well as by elemental microanalyses and by IR spectroscopy. Furthermore, their purity was checked by Powder X-ray diffraction (PXRD).
As expected, the IR spectra of the complexes 1-4 display a general characteristic feature. The strong vibrational band observed over the frequency range 1605-1615 cm -1 is typically assigned to the coordinated carbonyl stretching frequency, ν(C=O) and the weak broad band observed around 3450 cm -1 in 1 reveals the ν(O-H) stretching frequency of MeOH. The calculation of the degree of distortion of the PrO8 coordination polyhedron for 1 with respect to ideal eight-vertex polyhedra, using the continuous shape measure theory and SHAPE software [58,59] (Tables S1-S4). Interestingly in the packing of 3 a stacking sequence of four naphthyl moieties is observed where the ‧‧‧ ring‧‧‧ring interactions connect the 4 mononuclear complexes within the unit cell to a secondary building unit (SBU) ( Figure S7).

Description of the Crystal Structures (1-4)
There exist two types of conformers with respect to the relative orientations of the three ntfa ligands. In complexes 1 and 3 with conformer A the three ntfa ligands are oriented in the same direction, whereas in 2 and 4 one ntfa ligand has opposite orientation compared to the other two ones (conformer B).

Magnetic Properties
Powder samples of complexes 1-4 were measured under applied magnetic fields of 0.3 T over the temperature rang 300-2 K. The data are plotted as χMT products versus T in Figure 5. The Magnetization dependence of the applied field at 2 K were also recorded and shown in Figure 6

Luminescence Properties
The luminescence properties of the compounds under investigation were studied in the solid state at room temperature. In all compounds, the excitation spectra reveal the presence of an intense band around 350 nm, which corresponds to the π→π* transition from the ligands. The emission spectra were recorded at the excitation wavelengths (λex) of 350 nm for 1 and 4, 330 nm for 2 and 340 nm for 3. The excitation emission spectra of all compounds are illustrated in Figure 8.
For [Pr(ntfa)3(MeOH)2] (1), very weak emission bands were observed around 596 and 606 nm, which are attributed to the f-f transitions in the Pr 3+ ion and a broad intense band at 454 nm corresponding to the ligand emission showing a non-effective antenna effect. The presence of two coordinated methanol molecules bonded to the central Pr 3+ ion in the coordination sphere of 1 and their involvement in H-bonding in the vicinity of the metal center are strongly quench the luminescence through non-radiative relaxation processes, especially through the vibrational quenching caused by O-H vibrations [62].
The luminescence emission spectra of the complexes 2-4 display general characteristic features, where four emission bands were observed around 596, 610, 618 nm and a fourth one was located at 635, 632 and 625 nm for complexes 2, 3 and 4, respectively ( Figure 8). These emission bands result from the f-f transitions of the Pr 3+ ion in these compounds. The two intense bands centered at 610 and 618 nm were assigned to the 1 D2 → 3 H4 and 3 P0 → 3 H6, respectively in the three complexes 2-4, whereas the medium intense band located around 630 ± 5 nm was most likely assigned to the 3 P0 → 3 F2 transition. In addition, each complex displays a pair of emission bands in the near Infrared (NIR) region at 1025, 1061 nm for complex 2, 1010, 1061 nm for 3 and 615, 1061 nm for 4. These bands arise from the 1 D2 level and can be assigned to the 1 D2 → 3 F4 transition for the lower energy band and tentatively to 1 D2 → 3 F2 transition for the former band in each complex [63]. Probably, it should be noted that luminescence band located at 1061 nm was previously assigned to 1 G4 → 3 H4 transition but due to the absence of conclusive information, it was assigned to 1 D2 → 3 F4 transition [64]. Similar NIR emission bands with very weak intensities were also detected in 1.  The assignments of the emission bands in the four complexes were based on comparison of the f-f transitions of the Pr 3+ ion in compounds 2-4 with some similar chelated Pr(β-diketonato)3 systems [65,66] and their corresponding f-f energy levels scheme [67]. Moreover, it has been reported that the excited Pr 3+ may lose energy in the cross-relaxation process Pr 3+ (1D 2 ) + Pr 3+ (3H 4 ) → Pr 3+ (1G 4 ) + Pr 3+ (3F 4 ) [68,69], in addition the luminescence is generated by two different excited states, 3 P0 and 1 D2. This is common for Pr 3+ ions when the ligand triplet state is considerably higher than the 3 P0 and 1 D2 energy levels. Thus, in the compounds 2-4, there is an efficient intramolecular energy transfer from the ligand's triplet state to the excited states of Pr 3+ ion. The results obtained here are similar to those reported in related Pr 3+ compounds and Pr 3+ -β-diketonates systems [60,63,70,71].
The magnetic measurements of the four complexes 1-4 revealed that the χMT values at 300 K are within the range of 1.60 cm 3 ‧mol −1 ‧K, which is predicted for magnetically uncoupled Pr(III) compounds (4f 2 ) in the 3 H4 ground state. Similar magnetic behavior was observed in [Pr(hfac)3(NITFumbis)] {NITFumbis = (2,5-bis-(1-oxido-4,4`,5`,5`tetramethyl-4,5-hydro-1H-imidazol-2-yl)furan} molecules, where the C.N. = 8 [41]. The luminescence studies of the complexes showed that binding the "antenna" ligands to the Pr 3+ ion resulted in a strong absorption of the ligand in the UV region and efficient energy transfer from the ligands to the central Pr(III) ion. Replacement of the coordinated MeOH in 1 by strong chelating bipyridyl ligands (bipy, 4,4`-Mt2bipy or 5,5`-Me2bipy), significantly enhances the luminescent intensities in the visible and NIR regions, where the complexes 2-4 revealed strong emission bands at 610, 618 and 1061 nm, which are almost independent of the nature of the ancillary bipyridyl ligands. The luminescence emission and magnetic results reported here indicate that these properties are not significantly affected by either the C.N. of the Pr(III) complexes nor their local symmetry, however, this is not the case in the late lanthanide series [41].  Acknowledgments: A.T., A.F. and R.V. acknowledge the financial support: "Open Access Funding by the Graz University of Technology".

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