Bidentate SO₂ Complexes of Zirconium and Hafnium Difluorides with Highly Activated S–O Bonds

Abstract

Transition metal–SO2 complexes with different S–O bond lengths serve as ideal models for the understanding of catalytic activation of SO2 at different stages. Herein, sulfur dioxide complexes of zirconium and hafnium difluorides [MF2(O2S)] with highly activated S–O bonds were prepared in cryogenic matrixes. The structures of these complexes were identified by infrared spectroscopy and density functional theory calculations. Both ZrF2(O2S) and HfF2(O2S) were predicted to have singlet ground states and nonplanar C s geometries with the metal center coordinated by both oxygens of SO2. The much lower O–S–O stretching vibrational frequencies (650–730 cm–1) in the ligated complexes indicate that the SO2 ligand should be considered as a singlet SO2 2–, which is consistent with the rather long S–O bond lengths (∼1.7 Å) as a result of the two-electron transfer from the metal center to the 1π* orbital of SO2 upon formation of the complexes.

Sulfur Dioxide Complexes of Main-Group Elements: from SO20 to SO2– and SO22– upon Coordination to Aluminum and Silicon Difluorides

“…In comparison with the frequencies of AlF 2 (O 2 S), the two O−S−O stretches of SiF 2 (O 2 S) are even lower and were observed at 744.3 and 668.4 cm −1 . Both frequencies are close to the observed and computed frequencies of SO 2 in ZrF 2 (O 2 S) and HfF 2 (O 2 S) (around 700 cm −1 ), 23 which is also in support of the presence of a singlet SO 2 2− in the SiF 2 (O 2 S) complex. Natural population analysis (NPA) (Table S4) gave a negative charge of −0.65 e on the SO 2 ligand in AlF 2 (O 2 S), while the Al center is positively charged by 2.12 e. The negative charge carried by SO 2 is close to the values (∼0.75 e) Molecular orbital analyses reveal that the singly occupied molecular orbital (SOMO) of AlF 2 (O 2 S) is mainly composed of the 1π* orbital of SO 2 (Figure 8), which is unoccupied in neutral SO 2 .…”

“…The remaining bands at 744.3, 668.4, and 522.5 cm –1 are therefore assigned to the symmetric and antisymmetric O–S–O and SO 2 bending modes of ligated SO 2 . Such low stretching frequencies have been observed in the cases of TiF 2 (O 2 S), ZrF 2 (O 2 S), and HfF 2 (O 2 S) with bidentately coordinated SO 2 , , which is consistent with the absence of terminal SO stretches in the infrared spectra.…”

“…Different from the aluminum case, the S−O bond length in SiF 2 (O 2 S) is 1.711 Å, much longer than that in AlF 2 (O 2 S), but approaches the same bond length in ZrF 2 (O 2 S) and HfF 2 (O 2 S) containing singlet SO 2 2− ligands. 23 The SiF 2 (O 2 S) complex is the refore con sidered as [SiF 2 ] 2+ [SO 2 ] 2− with the Si center retaining the IV oxidation state. Other geometric parameters such as the Si−F and Si−O distances are within the ranges for typical single bonds.…”

“…The coordination chemistry of SO 2 has been widely studied, and diverse coordination modes of SO 2 were discovered when it was coordinated to transition metals. In addition to the sulfur-bound (η 1 -S), side-on (η 2 -SO), and end-on (η 1 -OSO) modes that have been established for mononuclear transition metals on the basis of crystallography, − a series of sulfur dioxide complexes with a bidentate coordination mode were also prepared in noble-gas matrixes, and the product structures were determined by infrared spectroscopy and density functional theory (DFT) calculations. − Notably, the reactions of scandium and titanium group metal atoms with SO 2 F 2 afforded the SO 2 complexes of metal difluorides in which the SO 2 ligands can be considered as SO 2 – and SO 2 2– depending on the nature of the metal. − Inspired by the successful preparation of these SO 2 complexes, we were interested in knowing whether SO 2 can be coordinated to light main-group elements in an analogous bidentate fashion and how charge transfer depends on the element, which offers an opportunity to get a fundamental understanding of the stepwise reduction of SO 2 induced by light main-group elements.…”

Preparation of Group 3 Metal Sulfur Monoxide Complexes via Oxidation of Metal Atoms using SOF2 in Cryogenic Matrixes

“…[11][12][13][14] It has been recently demonstrated that sulfuryl fluoride (SO 2 F 2 ) can serve as a precursor for the convenient synthesis of metal-sulfur dioxide complexes when it reacts with metal atoms in cryogenic matrixes. [15][16][17] In particular, sulfur monoxide complexes were obtained in some cases as long as there are enough valence electrons from the metal center to support one SO and one oxo ligand as well as two fluoro ligands. [18,19] The reactions of molybdenum and tungsten atoms with SO 2 F 2 in solid matrixes gave rise to side-on OMoF 2 (η 2 -SO) and OWF 2 (η 2 -SO), which are considered as peroxo-like metal complexes, (OMoF 2 ) 2 + (SO) 2À and (OWF 2 ) 2 + (SO) 2À .…”