Article pubs.acs.org/IC
Investigation of Praseodymium Fluorides: A Combined MatrixIsolation and Quantum-Chemical Study Thomas Vent-Schmidt† and Sebastian Riedel*,‡ †
Institut für Anorganische und Analytische Chemie, Albert-Ludwigs-Universität Freiburg, Albertstr.21, 79104 Freiburg, Germany Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstr.34-36, 14195 Berlin, Germany
‡
S Supporting Information *
ABSTRACT: The chemistry of the lanthanides is mostly dominated by compounds in the oxidation state +III. Only few compounds of Ce, Pr, and Tb are known with the metal in the +IV oxidation state. Removal of the last f-electron on praseodymium +IV would lead to a closed-shell system with formal oxidation state V. In this work we investigated the stability of the PrF5 molecule by theory and matrix-isolation techniques through the reaction of laser-ablated praseodymium atoms with fluorine in excess of neon, argon, krypton, or neat fluorine. Besides the known PrF3 molecule, unreported IR bands for PrF4 could be observed, and there is evidence for the formation of PrF and PrF2 but not for the formation of PrF5.
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functional theory (DFT) level using the B3LYP9,10 hybrid functional and Dunning’s correlation consistent triple-ζ basis set (denoted as aVTZ) for fluorine11 and the 28 electron core quasirelativistic pseudopotential from the Stuttgart/Kö ln group12 (denoted as SDD) for praseodymium. All possible spin multiplicities were considered. Structure optimizations at Coupled-Cluster level (CCSD(T)) were performed for the ground states of all molecules within the restrictions of their respective point groups. The 4s, 4p, and 4d orbitals of praseodymium as well as the 1s orbital of fluorine were kept frozen in all CC calculations. Structure optimizations as well as harmonic frequency calculations were performed with Gaussian0913 at DFT level and with the CFOUR program package14 at CCSD(T) level. Natural bond order (NBO) analysis was performed with the NBO program implemented in Gaussian09. Multireference CASPT2 calculations were performed using the Molpro 2006.1 program package.15 Matrix-isolation experiments: Fluorine (99.8%, Solvay) from a stainless steel cylinder, which was cooled to 77 K during the experiments to freeze out impurities, was premixed with neon (99.999%, Air Liquide), argon, or krypton (both 99.999%, Sauerstoffwerk Friedrichshafen) in another stainless steel cylinder. The mixing vessel was connected to a self-made matrix chamber by a stainless steel capillary. The reactants were condensed onto a CsI window cooled to 4 K for neon experiments and 10 K for argon, krypton, and fluorine using a closed-cycle helium cryostat (Sumitomo Heavy Industries, RDK-205D) inside the vacuum chamber. For the laser-ablation experiments, the 1064 nm fundamental of a Nd:YAG laser
INTRODUCTION Over the last decades the lanthanides have become key-metals for high-tech applications in modern techniques, such as electronics, wind power, or magnets. Because of their electropositive character they had often been used as cheap redox agents in the past but became interesting because of their optic and magnetic properties. Praseodymium is used as an alloy with magnesium, cobalt, or iron or as color agent in glasses. The dominant oxidation state of praseodymium is +III, but there are also some oxides1 and the tetrafluoride2,3 with praseodymium in the +IV oxidation state. Moreover, the +II oxidation state is known from the praseodymium iodide PrI24 or the praseodymium dihydride.5 The synthesis of the tetrafluoride is possible at room temperature by reaction of oxides with KrF23 or in aqueous HF under UV photolysis.2 However, no data about the vibrational spectrum of PrF4 is known so far. The electron configuration of the +IV state in this compound is 6s05d04f1. Removal of the last f-electron would lead to a closed-shell system with praseodymium in the formal oxidation state +V. In this study we predict the stability of the PrF5 molecule by quantum-chemical calculations. Furthermore, we report the reaction products of laser-ablated praseodymium atoms with fluorine in different noble gases. The use of the laser-ablation technique represents a different methodology in studying the lanthanide fluorides than the commonly used Knudsen cell evaporation and has been shown to be a suitable method to study high oxidation states.6−8
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THEORETICAL AND EXPERIMENTAL DETAILS Quantum chemical calculations: The structures of all molecules were fully optimized (by relaxing all parameters) at density © XXXX American Chemical Society
Received: June 1, 2015
A
DOI: 10.1021/acs.inorgchem.5b01175 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
and 524.7 cm−1 could be found. This compound is a common product of the laser-ablation process with fluorine and has been observed many times.6,16 Also a band for OOF at 589.4 cm−1 could be identified by comparison to other experimental results.17 Very recently Mikulas et al. investigated the formation of lanthanide oxyfluorides through the reaction of laser-ablated lanthanide atoms with OF2.18 By comparison with these spectra bands of PrOF at 451.0 and 767.6 cm−1 and for PrOF2 at 694.1, 519.8, and 476.9 cm−1 could be assigned. Also PrO2 at 730.3 cm−1 could be observed, which coincides with earlier observations.1 These products derive from the laser-ablated oxidized surface of the praseodymium target. Therefore, these byproducts are much more prominent in the early recorded spectra than in the later ones. IR bands of knudsen cell evaporated praseodymium trifluoride in solid argon and krypton but not for neon were reported by Wesley et al.19 and agree well with our measured bands; see Table 2. Neon values for PrF3 at 552.4 and 542.0 cm−1 were assigned due to the same experimental behavior on annealing and photolysis compared to the obvious PrF3 mode at 470.9 cm−1. However, the assignment of the 542.0 cm−1 band must be regarded as tentative although the argon to neon shift agree well compared to the shift observed for the 470.9 cm−1 band in neon. Besides the PrF3 and impurities bands several other bands could be found, which belong to currently unknown compounds. To the best of our knowledge there are no reported IR frequencies for the PrF4 molecule. We assigned bands at 537.7, 547.6, 552.4, and 559.1 cm−1 in argon and 547.0, 556.6, 561.2, and 564.0 cm−1 in neon to this molecule due to the following reasons. First, all possible bands decrease on UV irradiation, while the bands of PrF3 increase. Second, with increasing fluorine concentration the ratio between PrF3 and PrF4 band intensities show an increase in PrF4 formation; see Figure S2 in Supporting Information. We also performed some experiments in a neat fluorine matrix, which show PrF4 as the only product; see Figure S1. Third, on annealing the bands for PrF4 increase, and the bands of PrF3 decrease, which can be easily recognized in argon spectra. Because of the lower annealing temperatures this behavior is not that pronounced in neon matrices but can be clearly identified by difference spectra. These observations indicate the presence of a higher binary praseodymium fluoride. Obviously, the symmetry of the molecule is lower than a regular tetrahedral structure, which would have T2 and A1 symmetric modes in this spectral region, of which only the T2 mode is IR active. However, it is very difficult to determine the symmetry of the molecule based on the measured vibrational modes. There are three possibilities. First, a D2d symmetric structure is possible, which could be derived from the tetrahedral structure by Jahn−Teller distortion resulting in a splitting of the singly occupied t2 HOMO in a set of e and b2 symmetric orbitals. The T2 vibrational mode would then also be splitted in an E and B2 symmetric mode. It is likely that the E symmetric mode is also distorted in the matrix environment, which would explain the additional bands in the spectra. The second possibility is a more distorted structure of C2v symmetry, which would lead to a splitting of the T2 mode in three modes of A1, B1, and B2 symmetry. This structure would explain the four bands like a C1 symmetric structure would do, which is the third possibility. If a D2d symmetric structure is assumed the experimentally observed splitting between the two modes is 9.6 cm−1 in neon, 14.7 cm−1 in argon, and 16.1 cm−1 in fluorine,
(Continuum, Minilite II, 10 Hz repetition rate and 6 ns pulse width) was used, which was focused onto a rotating praseodymium target (99.9%, ChemPur) through a hole in the cold window. Matrix samples were prepared by the laserablation process of praseodymium atoms together with codeposited F2 (different concentrations ranging from 0.5 to 3% and 100%) under excess of rare gas for 1 h. The infrared spectra were recorded on a Bruker Vertex 70 spectrometer purged with dry air at 0.5 cm−1 resolution in the region between 4000 and 430 cm−1 using a liquid nitrogen-cooled MCT detector. The matrix samples were irradiated by a mercury arc street lamp (Osram HQL 250) with the outer globe removed.
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RESULTS AND DISCUSSION The reaction of laser-ablated praseodymium atoms with fluorine in various concentrations yielded PrF3 and PrF4 as the main products; see Figure 1 for neon spectra and Figure 2
Figure 1. Pr + 0.5% F2 in neon. (a) Deposition. (b) Annealing to 11 K. (c) 15 min of photolysis. (d) Annealing to 11 K.
Figure 2. Pr + 2% F2 in Argon. (a) Deposition. (b) Annealing to 20 K. (c) Annealing 30 K. (d) UV photolysis above 220 nm for 30 min.
for argon. Some bands could be identified as impurities or praseodymium compounds other than the binary fluorides. In all experiments known bands for the [F3]− molecule at 510.3 B
DOI: 10.1021/acs.inorgchem.5b01175 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. NBO Analysis of Praseodymium Fluorides at the B3LYP Levela praseodymium
a
F
Pr−F
molecule
6s
6p
5d
4f
charge
charge
BOb
PrF PrF2 PrF3 (C3v) PrF4 (C2v) PrF5 (C4v)
0.93 0.34 0.01 0.03 0.05
0.04 0.06 0.05 0.11 0.17
0.26 0.36 0.59 0.88 1.00
2.95 2.70 2.23 1.83 2.00
+0.83 +1.56 +2.10 +2.09 +1.79
−0.83 −0.78 −0.70 −0.53/−0.51 −0.39/−0.35
0.35 0.43 0.56 0.86/0.88 1.05/1.10
Basis sets; Pr: SDD, F: aVTZ. bMayer Bond order.
unassigned bands in the area 560−600 cm−1 that might fit the predicted upper bands of A1 and E symmetry. It seems also possible that the E mode is underneath the bands of PrF4. To get more information about the complex Pr/F2 systems, electronic structure calculations as well as the frequency evaluation were performed. The electronic structure of all monomer binary praseodymium fluorides were calculated at DFT and high-level coupled-cluster level. Optimized structures are shown in Figure 3. Total energies of all molecules used in
respectively, thus showing that the distortion depends on the host/guest interaction in the matrix. This observation coincides with the expected trend where neon is known to show only weak interactions with guests, while argon and fluorine are less inert. The most intense band of PrF4 at 552.4 cm−1 in argon and 556.6 cm−1 in neon shows also a second effect. On annealing the band shifts to lower wave numbers, while on photolysis it is back-shifted. This process is completely reversible and could be cycled in the experiment. The shift is 1.6 cm−1 in argon and only 0.6 cm−1 in neon and therefore seems also dependent on the matrix to host interaction. The effect might be due to a change in the electronic structure of PrF4 resulting in a small structural change or changes in the matrix environment, but there is no proof for that. Unfortunately, this shift could neither be observed for the other bands of PrF4 or the matrix splittings nor for the possible B2 mode of a D2d symmetric structure at 537.7 cm−1 in argon. However, the shift of the 537.7 cm−1 band might interfere with another band increasing on annealing above 25 K, which is located at 533.8 cm−1. This band is also reported in the spectra of Wesley et al.,19 and they assigned it as a matrix site of PrF3. Since the band increases on annealing it is unlikely that this band is a matrix site. We think that it belongs to the PrF3 dimer, although we found no corresponding stretching mode for the bridging Pr−F−Pr unit in the far-infrared (FIR) region. However, this might be due to a broad band with low intensity or the quite low sensitivity of our FIR La-DTGS detector. In addition to the new bands for PrF4 we also found bands that could tentatively be assigned to the low-valent binary fluorides PrF and PrF2. We assigned the band at 504.3 cm−1 in argon to PrF and the bands at 481.5 and 487.7 cm−1 with matrix site bands at 478.8 and 491.1 cm−1 to PrF2 due to the behavior on annealing where these bands rapidly vanish. This observation is in line with low-valent fluorides. Note that the lower band of PrF2 is very close to the 476.9 cm−1 band of PrOF2, but it could be clearly identified by comparison with argon spectra showing more prominent oxyfluoride bands. There is another band at 527.3 cm−1 in neon, 520 cm−1 in argon, and 511.7 in krypton, which could not be assigned properly. Note that there is a band of the PrOF2 molecule at 519.8 cm−1 in argon, but comparison between two spectra of which one shows oxyfluoride formation, while the other one does not, the intensity of this band stays nearly the same. On the basis of these observations we do not consider this band to a corresponding oxyfluoride. The band is nearly unaffected by annealing but vanishes on photolysis, which would be expected for a high-valent fluoride. Also the band shift to lower wavenumbers than the PrF4 bands is in line with a less stable molecule. The calculated frequencies for PrF5, shown in Table 1, give no clear answer, but an assignment to an A1 symmetric band of the C4v isomer might be possible. There are also
Figure 3. Optimized structures of binary praseodymium fluorides at different levels of theory using the SDD/aVTZ basis set combination. Bond lengths are in picometers, and angles are in degrees. Highest T1 and T2 amplitudes for CCSD(T) minimum structures: PrF3 (C3v) 0.02 (T1), 0.01(T2); PrF4 (C2v) 0.08 (T1), 0.02(T2); PrF5 (C4v) 0.12(T1), 0.05(T2). For PrF and PrF2 see text.
this study are summarized in Table S1 in Supporting Information. Recently it was shown that spin−orbit effects are of minor importance for the description of the lanthanide trifluorides and that reliable structures can be obtained by including only scalar-relativistic corrections.20 Because of the lower occupation of the 4f shell in PrF4 and PrF5 we assume that this observation holds also for the higher oxidation states. Therefore, we neglected spin−orbit coupling in our present calculations. It is well-known that the lanthanide trifluorides show a large number of low-lying electronic states with high multiconfigurational character.21 However, the 4f orbitals are energetically well-separated from the 5d and 6s shells, and therefore the 4f electrons usually do not participate in chemical bonding.21 Therefore, the multiconfigurational character of the ground state becomes of minor importance for an accurate description of the structure, the frequencies, and the thermochemistry, and a single reference method treatment seems to be feasible. Our calculated bond length of 208.5 pm (B3LYP) and 209.0 pm (CCSD(T)) for PrF3 is in excellent agreement with the effective rotationally and vibrationally averaged internuclear distance rg of 209.1 pm determined C
DOI: 10.1021/acs.inorgchem.5b01175 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry experimentally by electron diffraction22 and thus gives evidence that the structure is reliably described by these methods. The symmetry of the structure of PrF3 is still controversial and discussed in literature,12,19,22−31 and we are also not able to give a clear answer based on our present calculations; however, some points are noteworthy. One difficulty in determining the structure is an unusual splitting of the v3 mode resulting in two modes at 543.3 and 456.8 cm−1 in argon. A combined Raman and IR matrix study give evidence for a D3h symmetric structure and that the splitting might be caused by a coupling of a degenerate ground state with a low-lying electronic state of the same symmetry.19,24 However, this possibility has never been studied theoretically, which will require multiconfigrational methods and a treatment of spin−orbit coupling. These demanding calculations are beyond the scope of this study. Nevertheless our spectra of laser-ablated praseodymium atoms with fluorine in neon are comparable to previously reported argon and krypton spectra and show that the splitting is not a matrix effect. The splitting is smallest in neon (81.5 cm−1) and somewhat higher in argon (86.5 cm−1) and krypton (84.9 cm−1) indicating only little dependence on the host/guest interaction. A D3h symmetric structure is also found in the gas phase by electron diffraction studies.25 Our calculations show that the D3h structure is slightly higher in energy than the C3v minimum by only 0.3 kJ mol−1 at the CCSD(T) level in accordance with previous reports on a shallow hyper surface of the compound.21 Therefore, the low barrier for inversion might lead to an averaged planar structure especially under the conditions necessary to study the molecule in the gas phase. Evidence for the D3h structure in the matrix is also given by a theoretical investigation of the matrix effects to LnF3 host molecules.29,30 In these studies the authors show that the gradual addition of rare gas atoms around the LaF3 molecule, which is known to be C3v symmetric due to the participation of d-orbitals in the bonding,23 leads to a preference of the D3h isomer. Similar to the reports in literature29 we added two argon atoms along the threefold axis and optimized the structure again; however, the PrF3 molecule is still calculated to be C3v symmetric. In contrast to the predictions of a D3h symmetric isomer, the structures of LnF3 (Ln = La, Ce, Nd, Eu, Gd, Tb, Ho, Yb) were found to be nonplanar by matrix IR experiments.23,32 For the known tetrafluoride our calculations result in a C2v symmetric structure that emerges from Jahn−Teller-distortion of the tetrahedral structure due to the unpaired electron. Its electronic state is 2B2. The D2d symmetric structure is calculated to have approximately the same energy at the CCSD(T) level, while it is slightly lower in energy than the C2v structure by 6.1 kJ mol−1 at the B3LYP level and 0 K; see also table S1 in Supporting Information. At 298 K the C2v symmetric structure is also more stable than the D2d structure. Our calculations at DFT level show no evidence that the symmetry must be further reduced to C1 symmetry, and a CASSCF calculation shows no multireference character of the wave function. Two isomers of the hypothetical structure of PrF5 were considered whereat the C4v symmetric structure was found to be slightly more stable than the D3h symmetric isomer by 5.1 kJ mol−1 at the CCSD(T) level. The symmetry of the PrF2 molecule is C2v, and its electronic ground state is 4A2. Linear PrF2 is 36.5 kJ mol−1 higher in energy than the bent ground state. The lowest energy doublet state, which has also C2v symmetry, is 52.5 kJ mol−1 higher in energy. As the slightly high values of the T2 amplitude in the
Coupled Cluster calculation for PrF2 (0.18 for 4f,4f into 4f,5d double excitation) and the far too high T1 amplitude for PrF (1.06 for 5d into 4f excitation) indicates a multireference character of the wave function,33 we do not consider the results from these calculations. We further investigated the electronic structures of PrF and PrF2 at the CASPT2 level. For PrF the 6s, 4f, and 5d orbitals were included into the active space, and the ground state exhibits a 6s15d14f2 electron configuration with two leading configurations that differ in the 4f occupation. There is a second low-lying state that also has a 6s15d14f2 occupation and which is only 0.5 kJ mol−1 higher in energy. The optimized Pr−F bond length at the CASPT2 level is 202.3 pm and thus substantially shorter than the DFT value. The 5d orbitals tend to be of minor importance for the description of PrF2, and thus only the 6s and 4f orbitals were treated as active space. The two lowest states have essentially the same energy, and both have a 6s14f3 occupation. This quasi degeneracy was also found at the DFT level, where the energy difference is calculated to be only 0.3 kJ mol−1. We were only able to optimize the structure if both of these states were treated and if the resulting Pr−F bond length is 204.4 pm and the F−Pr−F angle is 120.0°. These values differ again significantly from the DFT values but are in close agreement with the above-mentioned CCSD(T) calculation. Unfortunately, we neither were able to calculate the frequencies at the CASPT2 level for PrF2 nor for PrF due to conversion issues although several multistate CASPT2 calulations and different options were tried. Therefore, we cannot further support our tentative assignment of PrF and PrF2 by theory, and the assignment remains in question. The bent structure indicates participation of d-orbital contribution as known for being the reason why BaF2 is bent and LaF3 is pyramidal.32,34 Another reason for the bent structure of PrF2 and perhaps for the pyramidal C3v structure of PrF3 might be the steric demand of the unpaired f-electrons. This was very recently shown to play a significant role in the bonding angle in HUF compared to HThF.35 To get a deeper understanding of the bonding in the praseodymium fluorides we performed an NBO analysis, which is summarized in Table 1. Note that the values for PrF and PrF2 should only be used for qualitative comparison with the higher praseodymium fluorides due to the limits of the single reference DFT description. The calculated NPA charges and the bond orders show that the Pr−F bond in PrF, PrF2, and PrF3 is best described as strongly ionic, while for the higher fluorides PrF4 and PrF5 covalent contribution might also become important. In this context it is interesting to recognize the high values for the 4f and 5d orbital population in PrF4 and PrF5. The NBO analysis shows that these high occupations arise from back-donation from F(2p) orbitals into empty 5d and 4f orbitals, which has previously been reported for the series of LnF3 molecules.20 It is anticipated that the population of the 5d orbital and to a lesser extent also of the 4f orbitals lead to structural changes, and in fact the main contribution in PrF2 is the back-donation from F(2p) orbitals into the Pr(5dx2−y2) orbital. Since the spin density, see Figure S3 in Supporting Information, is rather symmetric this 2p/5d interaction is probably the reason for the bent structure of PrF2. If the d-orbital contributions are responsible for the C3v symmetric structure of PrF3 one would expect back bonding mainly into the dxz, dz2, and dyz orbitals. However, the donation into the dxy and dx2−y2 orbitals is slightly higher than in the dxz, dz2, and dyz orbitals and is comparable to D
DOI: 10.1021/acs.inorgchem.5b01175 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 2. Predicted Frequencies of Different Isomers of PrF5, PrF4, and PrF3. PrF5 C4v A1 A1 A1 B1 B2 B2 E E E
138.6(26) 523.4(111) 588.8(23) 187.4(0) 64.3(0) 468.1(0) 131.5(1) 172.8(8) 559.3(199) PrF4 C2v 80.8 (14) 126.3 (16) 542.6 (192) 584.2 (8)
A1 A1 A1 A1 A2
79.3 243.9 29.9 548.8 84.7 520.3
(0) (0) (28) (205) (14) (238) PrF3 C3v 11.2 (67) 534.9 (0) 53.9 (33) 490.5 (279)
B1 B1 B2 B2 A1 A1 E E a
PrF5 D3h
B3LYPa
mode
CCSD(T)a
mode
B3LYPa
CCSD(T)a
115.0(28) 527.5(84) 606.5(91) 185.3(0) 81.5(0) 595.8(0) 126.9(1) 157.3(6) 548.5(109)
A1′ A1′ A2″ A2″ E′ E′ E′ E″
474.3(0) 571.7(0) 150.9(10) 570.5(187) 25.1(10) 162.4(11) 539.6(181) 173.5(0)
422.1(0) 626.2(0) 147.8(7) 516.5(51) 33.7(10) 138.8(14) 577.4(232) 174.2(0)
112.3 134.1 580.8 624.6
(5) (14) (286) (0)
A1 A1 B1 B2 B2 E E
109.3 558.3 119.2 567.5
(30) (262) (31) (258)
49.9 551.0 103.2 510.4
(69) (10) (21) (264)
PrF4 D2d 126.4(0) 581.5(0) 91.3 (0) 135.8 (0) 533.4 (178) 130.3 (22) 556.7 (222)
121.5 624.7 336.3 120.3 563.2 114.5 563.6
(0) (0) (0) (32) (266) (22) (267)
PrF3 D3h (0) (67) (33) (278)
546.9 −13.9 109.9 509.2
(0) (0) (21) (270)
A1′ A2″ E′ E′
534.9 11.2 54.4 490.6
SDD/aVTZ basis sets. Frequencies in inverse centimeters and (intensities) in kilometers per mole.
Table 3. Comparison of Calculated and Measured Frequencies of Praseodymium Fluoridesa PrF PrF2 PrF3
PrF4f
mode
B3LYPb
∑g A1 B2 E A1 E E B1 B2 B2 A1
491.3(114) 472.0(191) 477.6(207) 504.0 (246)c 546.0 (17) 504.0 (246)c 556.7(222) 548.8 (205) 520.3 (238) 533.4(178) 542.6 (192)
CCSD(T)b
510.4 (264)c 551.0 (10) 510.4 (264)c 563.6 (267) 558.3 (262) 567.5 (258) 563.2 (266) 580.8 (286)
Ne
Ar
508.7 487.7 493.2 470.9 542.0 552.4 556.6 561.2 564.0 547.0
504.3 481.5, 478.5 487.7, 491.1 456.8d 526.3d 543.3d 552.4 559.1 547.6 537.7
Kr 477−489 453.0e 522.3e 537.9e 545.7 550.0
Frequencies in inverse centimeters and (intensities) in kilometers per mole. bSDD/aVTZ basis sets. cSplitting of ν3 mode. d542, 525, 458 from ref 19. e538, 522, 455 from ref 19. fBold values for C2v structure, italic for D2d. a
those in the D3h isomer. Therefore, the NBO analysis gives no further evidence whether PrF3 is C3v or D3h symmetric. The predicted frequencies of both isomers as well as both isomers of PrF3 and PrF4 are shown in Table 2, and a comparison of our experimental values with the calculated ones is given in Table 3. We also considered charged species in our calculations to assign the unknown bands. The cationic species PrFn+ (n = 2−4) show predicted frequencies in the region above 600 cm−1, while the frequencies of the anionic species PrFn− (n = 2−5) are expected in the region below 440 cm−1. Therefore, an assignment in the region in between seems not plausible. The reliability of our frequency calculations are best validated by comparison of the calculated ν1 frequency with the
experimental neon value. The shift between calculated and observed frequency is 4.0 cm−1 at the B3LYP level and 9.0 cm−1 at the CCSD(T) level, which shows good performance of the two methods. Furthermore, the comparison of experimental and calculated values for PrF4 are in good agreement. We therefore expect the methods to be appropriate to reliably describe the structures and frequencies of the praseodymium fluorides and therefore support our tentative assignment of PrF and PrF2. Nevertheless it is not possible to assign the modes of PrF4 without doubt. The theoretically found structure of C2v symmetry would have three bands in the 520−580 cm−1 with an intensity distribution of approximately 1:1:1. However, these calculated intensities do not fit our experimental values quite well. Indeed the calculated intensity distribution at B3LYP level E
DOI: 10.1021/acs.inorgchem.5b01175 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry for the D2d symmetric structure would fit much better, especially for the experimental values obtained in our neat fluorine matrix. However, the intensity found in experiment might be superimposed by other modes in this region. To get information about the stability of the binary fluorides a thermochemical analysis was performed. Two possible decomposition pathways for all compounds were taken into account: the concerted elimination of F2 and the homolytic bond cleavage of one praseodymium−fluorine bond. For PrF4 and PrF5 also the bimolecular reaction is considered. The bimolecular reaction is often a preferred decomposition pathway in the gas phase as it is for PrF5 (see Table 4);
of Pr4+ might be also of interest to estimate the accessibility of the +V oxidation state. Our calculated value at the CCSD(T) level is 59.51 eV, which is in excellent agreement with a recently reported value of 59.89 eV.36 Comparison with the formation of an Ir(+IX) compound through the reaction of laser-ablated iridium atoms with oxygen, which has a calculated eighth ionization energy of 121.20 eV (CCSD(T)/aug-cc-pVTZ-PP), shows that the +V oxidation state of praseodymium is accessible under the conditions of laser ablation.
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CONCLUSION In this work we presented the reaction products of laser-ablated praseodymium atom with fluorine in different rare gas matrices and in neat fluorine and reported the neon matrix spectrum of PrF3 as well as the first IR spectrum of PrF4. Furthermore, we predicted that the pentafluoride of praseodymium is thermochemically stable under cryogenic conditions. We found unassigned bands at 520 cm−1 and in the 560−600 cm−1 region in argon that show the behavior of a higher fluoride on annealing and photolysis and which therefore might be due to PrF5. But, the calculated reaction free enthalpy for the decomposition to PrF4 and F atoms is calculated to be slightly endergonic at the B3LYP level, which might indicate that the formation of the pentafluoride is not likely to occur in the gas phase. Nevertheless, a reliable statement based on this DFT calculation seems not to be possible. To give further experimental evidence for the stability of the +V oxidation state, Raman matrix isolation studies or the vibrational analysis of mass selected ions would be helpful. In addition, the formation of praseodymium oxide cations formed in the gas phase would be another possible way to reach the +V oxidation state, as the formation of the [IrO4]+ cation recently shows.37
Table 4. Calculated Thermochemistry of Praseodymium Fluorides at Different Levels of Theory reactiona
ΔrH (0 K) B3LYPb,c
ΔrH (0 K) CCSD(T)b,c
ΔrG (298 K) B3LYPb,c
PrF5→ PrF4 + F PrF5 → PrF3 + F2 2 PrF5 → 2 PrF4 + F2 PrF4 → PrF3 + F PrF4 → PrF2 + F2 2 PrF4 → 2 PrF3 + F2 PrF3 → PrF2 + F PrF3 → PrF + F2 PrF2 → PrF + F PrF2 → Pr + F2 PrF → Pr + F
33.9 107.8 −81.2 222.9 651.9 296.8 578.0 1010.2 581.2 1007.5 575.2
38.6 141.7 −69.8 250.1 734.9 353.3
−9.5 62.5 −141.7 194.7 616.0 266.8 543.9
a
The energies of the following structures were considered for the thermochemical analysis: PrF5 (C4v), PrF4 (C2v), PrF3 (C3v). bBasis sets; Pr: SDD, F: aVTZ. cValues in kilojoules per mole and ZPE corrected.
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ASSOCIATED CONTENT
S Supporting Information *
however, it is unlikely to occur in the matrix. The calculated thermochemistry (see Table 4) predicts the low-valent fluorides PrF and PrF2 to be stable against fluorine elimination and homolytic bond breaking. The calculations predict PrF4 as the most stable compound in the system at 0 K, which is in line with our experimental results. The data for PrF5 shows that this molecule is stable against F2 elimination and homolytic bond cleavage. The lowest decomposition channel for PrF5 is the homolytic bond cleavage being endothermic by only 38.6 kJ mol−1. Comparison with the predicted stability of HgF4 shows that it would be possible to stabilize PrF5 by matrix isolation techniques.8 Since the formation of such high-valent species often occurs in the gas phase before deposition we also calculated the reaction free enthalpies at 298 K; see Table 4, right column. While PrF5 is still stable against the elimination of fluorine the reaction to PrF4 and F atoms becomes exergonic by −9.5 kJ mol−1 at the B3LYP level. Thus, the formation of the pentafluoride would only be possible in the matrix; however, we do not know the barriers for the two possible reactions. Nevertheless if the formation of PrF5 seems not to be possible in the gas phase before deposition, bands for this compound are not expected to appear in the deposition scan. Hence the basis of our tentative assignment based on the experimental behavior of the unknown bands would be gone. Nevertheless, we must note that this value is computed at the DFT level, and it might be possible that the error of the calculation is more than 10 kJ mol−1. Therefore, a reliable statement about the possible formation of PrF5 is so far impossible. In this context the ionization energy
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01175. Total energies used in this study, comparison of IR spectra of praseodymium fluorides in different noble gases and different concentrations, illustrated spin densities calculated at B3LYP level. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from Fond der Chemischen Industrie to T.V.-S, and S.R. thanks the DFG graduate research training group, 1582/2 “Fluorine as a Key Element”. We thank Prof. Krossing for computational resources and Dr. Ludwig for the preparation of the praseodymium target.
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REFERENCES
(1) Gabelnick, S. D.; Reedy, G. T.; Chasanov, M. G. J. Chem. Phys. 1974, 60, 1167−1171. (2) Mazej, Z. J. Fluorine Chem. 2002, 118, 127−129.
F
DOI: 10.1021/acs.inorgchem.5b01175 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
(31) Tsukamoto, S.; Mori, H.; Tatewaki, H.; Miyoshi, E. Chem. Phys. Lett. 2009, 474, 28−32. (32) Hastie, J. W.; Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1971, 3, 56−72. (33) Schlöder, T.; Riedel, S. In Comprehensive Inorganic Chemistry; Alvarez, S., Ed.; Elsevier: Amsterdam, The Netherlands, 2013; Vol. 9, pp 227−243. (34) Kaupp, M.; Schleyer, P. v. R.; Stoll, H.; Preuss, H. J. Am. Chem. Soc. 1991, 113, 6012−20. (35) Vent-Schmidt, T.; Metzger, J.; Andrews, L.; Riedel, S. J. Fluorine Chem. 2015, 174, 2−7. (36) Ma, C. G.; Brik, M. G.; Li, Q. X.; Tian, Y. J. Alloys Compd. 2014, 599, 93−101. (37) Wang, G.; Zhou, M.; Goettel, J. T.; Schrobilgen, G. J.; Su, J.; Li, J.; Schloder, T.; Riedel, S. Nature 2014, 514, 475−477.
(3) Spitsyn, V. I.; Kiselev, Y. M.; Martynenko, L. I.; Prusakov, V. N.; Sokolov, V. B. Dokl. Akad. Nauk SSSR 1974, 219, 621−624. (4) Warkentin, E.; Baernighausen, H. Z. Anorg. Allg. Chem. 1979, 459, 187−200. (5) Willson, S. P.; Andrews, L. J. Phys. Chem. A 2000, 104, 1640− 1647. (6) Schloeder, T.; Vent-Schmidt, T.; Riedel, S. Angew. Chem., Int. Ed. 2012, 51, 12063−12067. (7) Gong, Y.; Zhou, M.; Kaupp, M.; Riedel, S. Angew. Chem., Int. Ed. 2009, 48, 7879−7883. (8) Wang, X.; Andrews, L.; Riedel, S.; Kaupp, M. Angew. Chem., Int. Ed. 2007, 46, 8371−8375. (9) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (10) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (11) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007−1023. (12) Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1989, 90, 1730− 1734. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al.; Gaussian 09, Revision C.01; Gaussian, Inc: Wallingford, CT, 2009. (14) Stanton, J. F.; Gauss, J.; Harding, M. E.; Szalay, P. G.; Auer, A. A.; Bartlett, R. J.; Benedikt, U.; Berger, C.; Bernholdt, D. E.; Bomble, Y. J.; Christiansen, O.; Heckert, M.; Heun, O.; Huber, C.; Jagau, T.-C.; Jonsson, D.; Jusélius, J.; Klein, K.; Lauderdale, W. J.; Matthews, D. A.; Metzroth, T.; O’Neill, D. P.; Price, D. R.; Prochnow, E.; Ruud, K.; Schiffmann, F.; Stopkowicz, S.; Tajti, A.; Vázquez, J.; Wang, F.; Watts, J. D. CFour, 1.2 ed.; Mainz, 2010. (15) Werner, H.-J.; Knowles, G.; Knizia, F. R.; Manby, M.; Schütz, P. Celani, T.; Korona, R.; Lindh, A.; Mitrushenkov, G.; Rauhut, K. R.; Shamasundar, T. B.; Adler, R. D.; Amos, A.; Bernhardsson, A.; Berning, D. L.; Cooper, M. J. O.; Deegan, A. J.; Dobbyn, F.; Eckert, E.; Goll, C.; Hampel, A.; Hesselmann, G.; Hetzer, T.; Hrenar, G.; Jansen, C.; Köppl, Y.; Liu, A. W.; Lloyd, R. A.; Mata, A. J.; May, S. J.; McNicholas, W.; Meyer, M. E.; Mura, A.; Nicklaß, D. P.; O’Neill, P.; Palmieri, D.; Peng, K.; Pflüger, R.; Pitzer, M.; Reiher, T.; Shiozaki, H.; Stoll, A. J.; Stone, R.; Tarroni, T.; Thorsteinsson, M.; Wang, M. MOLPRO, 2006.1, a package of ab initio programs; University College Cardiff Consultants Limited: Cardiff, U.K., 2006. (16) Riedel, S.; Koechner, T.; Wang, X.; Andrews, L. Inorg. Chem. 2010, 49, 7156−7164. (17) Arkell, A.; Reinhard, R. R.; Larson, L. P. J. Am. Chem. Soc. 1965, 87, 1016−1020. (18) Mikulas, T.; Chen, M.; Dixon, D. A.; Peterson, K. A.; Gong, Y.; Andrews, L. Inorg. Chem. 2014, 53, 446−456. (19) Wesley, R. D.; DeKock, C. W. J. Chem. Phys. 1971, 55, 3866− 3877. (20) Xu, W.; Ji, W.-X.; Qiu, Y.-X.; Schwarz, W. H. E.; Wang, S.-G. Phys. Chem. Chem. Phys. 2013, 15, 7839−7847. (21) Dolg, M.; Cao, X. In Computational Inorganic and Bioinorganic Chemistry; Solomon, E. I., Scott, R. A., King, R. B., Eds.; John Wiley & Sons, Ltd: Hoboken, NJ, 2009. (22) Hargittai, M. Coord. Chem. Rev. 1988, 91, 35−88. (23) Hauge, R. H.; Hastie, J. W.; Margrave, J. L. J. Less-Common Met. 1971, 23, 359−65. (24) Lesiecki, M.; Nibler, J. W.; DeKock, C. W. J. Chem. Phys. 1972, 57, 1352−1353. (25) Kaiser, E. W.; Falconer, W. E.; Klemperer, W. J. Chem. Phys. 1972, 56, 5392−8. (26) Hastie, J. W.; Hauge, R. H.; Margrave, J. L. J. Less-Common Met. 1975, 39, 309−34. (27) Joubert, L.; Picard, G.; Legendre, J.-J. Inorg. Chem. 1998, 37, 1984−1991. (28) Tsuchiya, T.; Taketsugu, T.; Nakano, H.; Hirao, K. J. Mol. Struct.: THEOCHEM 1999, 461−462, 203−222. (29) Lanza, G.; Minichino, C. J. Phys. Chem. A 2005, 109, 2127− 2138. (30) Lanza, G.; Minichino, C. ChemPhysChem 2009, 10, 507−511. G
DOI: 10.1021/acs.inorgchem.5b01175 Inorg. Chem. XXXX, XXX, XXX−XXX
