Inspired by Nature—Functional Analogues of Molybdenum and Tungsten-Dependent Oxidoreductases

Abstract

Throughout the previous ten years many scientists took inspiration from natural molybdenum and tungsten-dependent oxidoreductases to build functional active site analogues. These studies not only led to an ever more detailed mechanistic understanding of the biological template, but also paved the way to atypical selectivity and activity, such as catalytic hydrogen evolution. This review is aimed at representing the last decade’s progress in the research of and with molybdenum and tungsten functional model compounds. The portrayed systems, organized according to their ability to facilitate typical and artificial enzyme reactions, comprise complexes with non-innocent dithiolene ligands, resembling molybdopterin, as well as entirely non-natural nitrogen, oxygen, and/or sulfur bearing chelating donor ligands. All model compounds receive individual attention, highlighting the specific novelty that each provides for our understanding of the enzymatic mechanisms, such as oxygen atom transfer and proton-coupled electron transfer, or that each presents for exploiting new and useful catalytic capability. Overall, a shift in the application of these model compounds towards uncommon reactions is noted, the latter are comprehensively discussed.

Dioxygen Activation by a Bioinspired Tungsten(IV) Complex

“…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.…”

“…Characterization of 2 by NMR spectroscopy revealed a complex solution behavior. 1 H NMR spectroscopy of a solution of in situ formed 2 in CDCl 3 , which contains an excess of PMe 3 (10 equiv), confirms the formation of a species with two equiv of coordinated PMe 3 molecules. Next to the resonance for uncoordinated PMe 3 , all other signals are assignable to the symmetric hepta-coordinated [WO(6-MePyS) 2 (PMe 3 ) 2 ] with two PMe 3 ligands (Figure S6).…”

“…Over the past decades, tungsten(VI) and especially molybdenum(VI) dioxido motifs have drawn much attention as models for different metalloenzymes which catalyze oxygen atom transfer (OAT) reactions. 1 − 5 The major drawback of molybdenum OAT catalysts is their high tendency to form relatively inert molybdenum(V) μ-oxo dimers, which appear as a result of comproportionation of molybdenum(IV) oxido and molybdenum(VI) dioxido species. 6 − 8 The dinucleation can be inhibited by increasing the steric bulk of the ligands 9 or immobilizing the active species to a polymeric support.…”

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

“… 12 , 13 To better understand the mechanism under which the metalloenzymes perform the OAT, many molybdenum model compounds were synthesized and investigated, while tungsten modeling chemistry is far less explored. 1 , 14 , 15 Frequently used model reactions for OAT usually employ the biological substrate DMSO and different tertiary phosphines as sacrificial oxygen acceptors. 16 − 18 In general, if the catalyst is based on a higher-valent metal center (M VI O 2 2+ ), OAT model reactions involve the concomitant two-electron reduction coupled with oxygen abstraction by the oxygen acceptor ( Scheme 1 a).…”

“…Generally, in tungsto- and molybdoenzymes, the metal center in oxidation state +IV or +VI is coordinated by one or two of any variations of the metallopterin moiety. , However, the activity of the metalloenzymes is strongly dependent on the metal ion situated in their active site. , Some molybdoenzymes, such as sulfite oxidase or nitrate reductase, − are less active or not active at all upon replacement of Mo by W. Conversely, when molybdenum is replaced with tungsten in DMSO reductase (DMSOr) from Rhodobacter capsulatus or Rhodobacter sphaeroides, the derived enzyme reduces DMSO at a higher rate but also is inactive in catalyzing the reverse DMS oxidation. , Similarly, trimethylamine− N –oxide reductase (TMAOr) from Escherichia coli shows a slight increase in catalytic activity when molybdenum is substituted by tungsten . Both DMSOr and TMAOr catalyze biochemical transformations known as oxygen atom transfer (OAT) which are widespread reactions for molybdenum and tungsten oxidoreductase enzymes. , To better understand the mechanism under which the metalloenzymes perform the OAT, many molybdenum model compounds were synthesized and investigated, while tungsten modeling chemistry is far less explored. ,, Frequently used model reactions for OAT usually employ the biological substrate DMSO and different tertiary phosphines as sacrificial oxygen acceptors. − In general, if the catalyst is based on a higher-valent metal center (M VI O 2 2+ ), OAT model reactions involve the concomitant two-electron reduction coupled with oxygen abstraction by the oxygen acceptor (Scheme a) . Reduced species (M IV O 2+ ) can further undergo oxidation to recover the catalyst by abstracting the oxygen from DMSO or any related substrate (Scheme b) …”