Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Bidentate Sulfur Dioxide Complexes of Scandium, Yttrium, and Lanthanum Difluorides Rui Wei,†,‡ Xiuting Chen,†,‡ and Yu Gong*,† †
Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China
‡
Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on March 27, 2019 at 23:05:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: The SO2 complexes of scandium, yttrium, and lanthanum difluorides [MF2(O2S)] were prepared via the reactions of laser-ablated metal atoms and SO2F2 upon UV−vis irradiation in cryogenic matrixes. The presence of bidentate SO2 ligand in the products was demonstrated by the characteristic infrared absorptions as well as isotopic frequency ratios from both S18O2F2 and 34SO2F2 experiments and is further supported by DFT calculations. All three product molecules were predicted to have nonplanar C2v symmetry with the SO2 ligand bound to the metal center through both oxygens. The computed S−O bond length and stretching frequencies of ligated SO2 approach those of SO2− as a result of electron transfer from metal center to the 1π* orbital of SO2, in agreement with the results from bonding analysis. On the basis of DFT calculations, fluorine transfer from SO2F2 to the metal center to form the MF2(O2S) complexes is highly exothermic. Although a proposed intermediate in the form of MF(O2SF) was predicted to be stable, it was not observed in the experiments, presumably because of the low energy barrier for further isomerization to MF2(O2S).
■
INTRODUCTION Transition metal sulfur dioxide complexes have been the subject of numerous studies because of their important roles in catalytic sulfur dioxide activation and functionalization processes.1−5 From the structural point of view, SO2 is an ambiphilic ligand that can either donate electrons from its highest occupied molecular orbital (HOMO) to an available unoccupied orbital of the metal center or accept electrons from the metal into its lowest unoccupied molecular orbital (LUMO), which strongly depends on the nature of metal center and coordination environment.6,7 It has been established that noble metals such as ruthenium, rhodium, platinum, and osmium form sulfur-bound (η1-S) SO2 complexes,8−14 and stable and metastable side-on (η2-SO) SO2 complexes can be isolated under different conditions.11−15 The single oxygen bound end-on (η1-OSO) coordination mode was identified in the metastable photoinduced linkage isomers of the ruthenium complexes.12,14 Recent spectroscopic studies also demonstrated that copper, silver, and gold react with SO2 to form either sulfur-bound (η1-S) complexes or bidentate complexes with both oxygen atoms bound to the metal center.16 Compared with the rich studies on the SO2 complexes of noble metals, experimental results on the SO2 complexes of early transition metals received much less attention.17−20 The X-ray crystal structures of the polymeric compounds {[M(SO2)n](AsF6)3}m (M = Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb) revealed that SO2 is bound to the metal center in an η1-OSO fashion.21 Dinuclear lanthanide complexes in which the two metal centers are bridged through oxygen of SO2 were identified in both solution and the solid state.22 A recent gasphase mass spectrometric study revealed the formation of a © XXXX American Chemical Society
series of lanthanide−SO2 complex anions upon collisioninduced dissociation (CID) of Ln(CH3SO2)4−, and both oxygens of SO2 were found to bind the lanthanum and lutetium centers on the basis of density functional theory (DFT) calculations.23 A similar coordination mode was found in the SO2 complexes of titanium, zirconium, hafnium, cerium, and thorium.24,25 However, it is still unclear whether such a coordination mode is preserved in the neutral SO2 complexes of scandium, yttrium, and lanthanum because of the limited number of known examples. In this paper, the bidentate SO2 complexes of group 3 metal difluorides were produced via the reactions of laser-ablated metal atoms and SO2F2 in cryogenic matrixes. These complexes were identified by their characteristic infrared absorptions and isotopic frequency ratios. Theoretical calculations were employed to support the assignments and gain further insights into the structure, bonding, and formation process of the products.
■
EXPERIMENTAL AND COMPUTATIONAL METHODS
The SO2 complexes of group 3 metal difluorides, MF2(O2S), were prepared via the reactions of metal atoms with SO2F2 in excess neon or argon at 4 K. Details of the experimental apparatus and procedure have been described previously.26−28 The Nd:YAG laser fundamental (Continuum II, 1064 nm, 10 Hz repetition rate, 6 ns pulse width, 4−8 mJ/pulse) was focused onto a rotating scandium, yttrium, or lanthanum target (Metallium, 99%). Laser-ablated Sc, Y, or La atoms were codeposited with argon (99.999%, Xiangkun Special Gas, Received: February 7, 2019
A
DOI: 10.1021/acs.inorgchem.9b00365 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry China) or neon (99.999%, Dalian Special Gas, China) containing 0.5% SO2F2 (99.9%, Maotoo Gas, China) onto a cesium iodide window at a temperature of 4 K that was maintained using a closedcycle helium refrigerator. In general, the samples were deposited for 60 min (Ar) or 30 min (Ne). For the isotopic samples, we first synthesized S18O2 and 34SO2 via the reactions of 18O2 (99.7%, Taiyo Nippon Sanso, Japan) with sulfur powder (99.9%, Sinopharm Chemical Reagent, China) and 34S powder (99.3%, ISOFLEX) with O2 (99.999%, Xiangkun Special Gas, China) at 300 °C. The S18O2F2 (90% enriched 18O) and 34SO2F2 (99% enriched 34S) samples were then synthesized by the reactions of S18O2 or 34SO2 with AgF2 (99.9%, Sigma-Aldrich) following the procedures reported by Fristrom.29 The gaseous products were purified by several freeze− pump−thaw cycles using liquid nitrogen before use. Infrared spectra were recorded on a Bruker Vertex 70 V FT-IR spectrometer at 0.5 cm−1 resolution with a DLaTGS detector. Matrix samples were annealed at different temperatures and cooled back to 4 K for spectral acquisition, and selected samples were subjected to λ > 220 nm UV− vis irradiation using a 250 W mercury arc lamp with the outer globe removed. Geometry optimizations and frequency calculations were performed using the hybrid B3LYP density functional as implemented in Gaussian 09.30−32 The 6-311+G(3df) basis set was employed for oxygen, fluorine, sulfur, and scandium, and the 28-electron core SDD pseudopotential was used for yttrium and lanthanum.33−38 The geometrical parameters for all of the products were fully optimized. Harmonic vibrational frequencies of the optimized structures were obtained analytically, and zero-point energies were derived. The twodimensional localized orbital locator (LOL) maps were plotted using Multiwfn,39 and the wave functions for LOL analysis were generated by Gaussian 09 at the B3LYP/6-311+G(3df)/SDD level. Natural bond orbital (NBO) analysis was done using the NBO6 program at the B3LYP//6-311+G(d)/SDD level.40
Figure 2. Infrared spectra in the product absorption regions from the reactions of laser-ablated yttrium atoms with SO2F2 in excess neon at 4 K: (a) Y + 0.5% SO2F2 codeposited for 30 min; (b) after annealing to 6 K; (c) after λ > 220 nm irradiation; (d) after annealing to 8 K.
■
RESULTS AND DISCUSSION Infrared Spectra. The reactions of laser-ablated group 3 metal atoms with SO2F2 were studied using different laser energies and various SO2F2 concentrations, and no significant change was observed for the product absorptions. The infrared spectra of the group 3 metal and 0.5% SO2F2 reaction products in excess neon are shown in Figures 1−3, and the spectra in excess argon are shown in Figures S1−S3. Besides the intense precursor SO2F2 absorptions, two groups of metal-independent absorptions were observed in all the experiments, and they have been assigned to SO2F2− and SO2 produced during
Figure 3. Infrared spectra in the product absorption regions from the reactions of laser-ablated lanthanum atoms with SO2F2 in excess neon at 4 K: (a) La + 0.5% SO2F2 codeposited for 30 min; (b) after annealing to 6 K; (c) after λ > 220 nm irradiation; (d) after annealing to 8 K.
sample deposition.41 Weak absorptions due to scandium, yttrium, and lanthanum monoxide molecules were observed upon sample deposition as well.28,42,43 The infrared spectra from codeposition of scandium atoms and SO2F2 in neon are shown in Figure 1. New product absorptions were observed at 972.9, 969.4, 708.6, 667.7, and 611.2 cm−1 after sample deposition. These bands slightly sharpened during subsequent annealing to 6 K, but their intensities were substantially increased when the sample was subjected to λ > 220 nm irradiation. The new product bands sharpened again upon further annealing to 8 K. To help identify the structure of the reaction product, experiments with isotopically labeled 34SO2F2 and S18O2F2 samples were carried out under similar conditions, and the infrared spectra are shown in Figure 4. The product absorptions were observed at 964.4, 960.7, 708.5, 667.5, and 609.3 cm−1 and at 938.1, 933.5, 708.4, 664.5, and 581.5 cm−1 when scandium reacted with 34SO2F2 and S18O2F2, respectively. The behaviors of these bands upon sample annealing and UV−vis irradiation were the same as those observed in the SO2F2 experiment. Figures 2 and 3 show the infrared spectra from the reactions of yttrium and lanthanum atoms with SO2F2 in solid neon. In each case, four new product absorptions were observed after
Figure 1. Infrared spectra in the product absorption regions from the reactions of laser-ablated scandium atoms with SO2F2 in excess neon at 4 K: (a) Sc + 0.5% SO2F2 codeposited for 30 min; (b) after annealing to 6 K; (c) after λ > 220 nm irradiation; (d) after annealing to 8 K. B
DOI: 10.1021/acs.inorgchem.9b00365 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 4. Infrared spectra in the product absorption regions from the reactions of laser-ablated scandium atoms with isotopically labeled SO2F2 samples in excess neon at 4 K: (a) 0.5% SO2F2; (b) 0.5% 34 SO2F2; (c) 0.5% S18O2F2. Spectra were taken after λ > 220 nm irradiation followed by annealing to 8 K. The absorption labeled with the asterisk and its adjacent absorption of the product are most likely a Fermi resonance doublet.
Figure 6. Infrared spectra in the product absorption regions from the reactions of laser-ablated lanthanum atoms with isotopically labeled SO2F2 samples in excess neon at 4 K: (a) 0.5% SO2F2; (b) 0.5% 34 SO2F2; (c) 0.5% S18O2F2. Spectra were taken after λ > 220 nm irradiation followed by annealing to 8 K.
Table 1. Observed Vibrational Frequencies of Group 3 MF2(O2S) Complexes in Neon Matrixes
sample deposition, and their behaviors upon sample annealing and irradiation were similar to those of the scandium product bands. The yttrium and lanthanum product absorptions exhibit specific isotopic shifts in the experiments with isotopically labeled SO2F2 samples (Figures 5 and 6), providing solid
frequency (cm−1) mode antisym. O−S−O str. sym. O−S−O str. antisym. F−Sc−F str. sym. F−Sc−F str. SO2 bend antisym. O−S−O str. sym. O−S−O str. antisym. F−Y−F str. sym. F−Y−F str./SO2 bend sym. F−Y−F str./SO2 bend antisym. O−S−O str.c sym. O−S−O str.c sym. F−La−F str./SO2 bend sym. F−La−F str./SO2 bend antisym. F−La−F str.c
SO2F2 ScF2(O2S) 972.9 969.4 708.6 667.7 611.2 YF2(O2S) 981.5 972.2 585.5 b
567.1 LaF2(O2S) 989.7 972.7
SO2F2
S18O2F2
964.4 960.7 708.5 667.5 609.3
938.1 933.5 708.4 664.5a 581.5
970.8 963.3 585.5
947.5 935.5 585.3
b
b
566.1
547.9
978.6 963.8
955.8 935.6
34
b
b
b
518.1 504.8
517.6 504.8
510.5 504.8
A satellite arising from Fermi resonance was observed at 660.9 cm−1. Too weak to be observed. cMajor band of multiple matrix site splittings. a
Figure 5. Infrared spectra in the product absorption regions from the reactions of laser-ablated yttrium atoms with isotopically labeled SO2F2 samples in excess neon at 4 K: (a) 0.5% SO2F2; (b) 0.5% 34 SO2F2; (c) 0.5% S18O2F2. Spectra were taken after λ > 220 nm irradiation followed by annealing to 8 K.
b
in argon than in neon and are not always well-resolved in argon matrixes. The product absorptions observed in argon are listed in Table S1. It can be seen that the neon matrix frequencies of the scandium, yttrium, and lanthanum products are higher than the argon matrix values by less than 15 cm−1, suggesting very weak interactions between the new product molecules and argon atoms.44,45 ScF2(O2S) Assignment. The new absorptions at 972.9, 969.4, 708.6, 667.7, and 611.2 cm−1 exhibited the same behaviors when the Sc/SO2F2/Ne sample was annealed and irradiated with a UV−vis lamp (Figure 1), indicating that they should be due to different vibrational modes of the same molecule. The 708.6 and 667.7 cm−1 bands correspond to the 695.2 and 655.6 cm−1 bands observed in argon, which are very close to the band positions of the F−Sc−F stretches of ScF2,
evidence for the identification of the product structures. All of the new absorptions observed in the experiments are listed in Table 1. It should be noted that most of the lanthanum product bands exhibit double or triple splittings arising from different matrix trapping sites, and only the major site of each band is listed. The new product absorptions observed in argon matrixes (Figures S1−S3) possess behaviors during sample annealing and UV−vis irradiation analogous to those observed in neon matrixes, and the infrared spectra from the reactions of metal atoms and isotopically substituted SO2F2 samples in argon are shown in Figures S4−S6. It should be noted that the two product absorptions between 950 and 1000 cm−1 are broader C
DOI: 10.1021/acs.inorgchem.9b00365 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 7. Optimized geometries (bond lengths in angstroms and bond angles in degrees) of ground state (2B1, C2v) ScF2(O2S), YF2(O2S) and LaF2(O2S) complexes at the B3LYP/6-311+G(3df) level of theory. Oxygen, fluorine and sulfur atoms are in red, green and yellow.
Table 2. Comparison of the Calculated and Experimental Frequencies (in cm−1) and Isotopic Frequency Ratios of the Product Molecules freq. (cm−1) mode
a
calcd
antisym. O−S−O str. sym. O−S−O str. antisym. F−Sc−F str. sym. F−Sc−F str. SO2 bend
961.9 967.2 703.8 663.5 606.5
(194) (110) (264) (191) (126)
antisym. O−S−O str. sym. O−S−O str. antisym. F−Y−F str. sym. F−Y−F str./SO2 bend sym. F−Y−F str./SO2 bend
971.8 969.9 593.8 590.0 572.7
(210) (124) (214) (0) (222)
antisym. O−S−O str. sym. O−S−O str. sym. F−La−F str./SO2 bend sym. F−La−F str./SO2 bend antisym. F−La−F str.
981.1 974.6 555.7 508.9 501.2
(207) (129) (15) (209) (240)
32
S/34S
exptl
b
ScF2(O2S) 972.9 969.4 708.6 667.7 611.2 YF2(O2S) 981.5 972.2 585.5 c
567.1 LaF2(O2S) 989.7 972.7 c
518.1 504.8
16
O/18O
calcd
exptl
calcd
exptl
1.0112 1.0096 1.0000 1.0002 1.0030
1.0088 1.0091 1.0001 1.0003 1.0031
1.0365 1.0396 1.0001 1.0005 1.0524
1.0371 1.0385 1.0003 1.0048 1.0511
1.0114 1.0096 1.0000 1.0020 1.0014
1.0110 1.0092 1.0000
1.0361 1.0399 1.0000 1.0139 1.0386
1.0359 1.0392 1.0003
1.0117 1.0093 1.0032 1.0010 1.0000
1.0018 1.0114 1.0092 1.0010 1.0000
1.0358 1.0405 1.0383 1.0131 1.0002
1.0350 1.0355 1.0397 1.0149 1.0000
a
Infrared intensities (in km/mol) are given in parentheses. bExperimental frequencies in neon. cToo weak to be observed.
OScF2. and CH2ScF2,46−48 and we assign these two bands to the antisymmetric and symmetric F−Sc−F stretching modes of the new product. This is also consistent with the very small 34S and 18O shifts (
