Replacement of Molybdenum by Tungsten in a Biomimetic Complex Leads to an Increase in Oxygen Atom Transfer Catalytic Activity

Abstract

Upon replacement of molybdenum by tungsten in DMSO reductase isolated from the Rhodobacteraceae family, the derived enzyme catalyzes DMSO reduction faster. To better understand this behavior, we synthesized two tungsten(VI) dioxido complexes [W VI O 2 L 2 ] with pyridine- (PyS) and pyrimidine-2-thiolate (PymS) ligands, isostructural to analogous molybdenum complexes we reported recently. Higher oxygen atom transfer (OAT) catalytic activity was observed with [WO 2 (PyS) 2 ] compared to the Mo species, independent of whether PMe 3 or PPh 3 was used as the oxygen acceptor. [W VI O 2 L 2 ] complexes undergo reduction with an excess of PMe 3 , yielding the tungsten(IV) oxido species [WOL 2 (PMe 3 ) 2 ], while with PPh 3 , no reactions are observed. Although OAT reactions from DMSO to phosphines are known for tungsten complexes, [WOL 2 (PMe 3 ) 2 ] are the first fully characterized phosphine-stabilized intermediates. By following the reaction of these reduced species with excess DMSO via UV–vis spectroscopy, we observed that tungsten compounds directly react to W VI O 2 complexes while the Mo analogues first form μ-oxo Mo(V) dimers [Mo 2 O 3 L 4 ]. Density functional theory calculations confirm that the oxygen atom abstraction from W VI O 2 is an endergonic process contrasting the respective reaction with molybdenum. Here, we suggest that depending on the sacrificial oxygen acceptor, the tungsten complex may participate in catalysis either via a redox reaction or as an electrophile.

Dioxygen Activation by a Bioinspired Tungsten(IV) Complex

“…The complex exists as a single isomer in solution, as confirmed by 1 H and 13 C NMR spectroscopy. The IR stretching characteristic for WO bonds were detected at 907 and 949 cm –1 , comparable to similar W VI O 2 complexes …”

“…In benzene, compound 1 is stable for at least 24 h, even when purged with dioxygen. However, it is not stable in a DMSO solution where the bidentate ligands dissociate and oxidize to the respective disulfide (6-MePyS) 2 accompanied by the reduction of DMSO to dimethyl sulfide and the formation of unidentified tungsten oxides (WO 3 ) n , a behavior previously observed for [WO 2 (PymS) 2 ] (PymS = pyrimidine-2-thiolate) . The complex exists as a single isomer in solution, as confirmed by 1 H and 13 C NMR spectroscopy.…”

“…However, it is not stable in a DMSO solution where the bidentate ligands dissociate and oxidize to the respective disulfide (6-MePyS) 2 accompanied by the reduction of DMSO to dimethyl sulfide and the formation of unidentified tungsten oxides (WO 3 ) n , a behavior previously observed for [WO 2 (PymS) 2 ] (PymS = pyrimidine-2-thiolate). 14 The complex exists as a single isomer in solution, as confirmed by 1 H and 13 C NMR spectroscopy. The IR stretching characteristic for W�O bonds were detected at 907 and 949 cm −1 , comparable to similar W VI O 2 complexes.…”

“…The IR stretching characteristic for W�O bonds were detected at 907 and 949 cm −1 , comparable to similar W VI O 2 complexes. 14 Single crystals suitable for X-ray diffraction analysis were obtained from CH 2 Cl 2 /MeCN at −37 °C. The molecular structure (Figure 1) reveals a distorted octahedral geometry of 46 However, the W�O bonds are with 1.717(3) and 1.727(2) Å, respectively, slightly elongated in the tungsten analogue 1, as previously observed for comparable tungsten and molybdenum systems.…”

“…14,46,47 literature data of similar complexes. 14,48 Furthermore, the purity of the sample was confirmed by elemental analysis. The solid-state structure of 2 was determined by singlecrystal X-ray diffraction analysis (Figure 2).…”