Electronic Fine‐Tuning of Oxygen Atom Transfer Reactivity of cis‐Dioxomolybdenum(VI) Complexes with Thiosemicarbazone Ligands

Abstract

A series of six cis‐dioxomolybdenum(VI) complexes with thiosemicarbazone ligands was synthesized and characterized. The ligands were obtained by reacting ethyl thiosemicarbazide with salicylaldehydes substituted with a selection of electron‐withdrawing and electron‐donating groups. The crystal structures, IR, NMR spectroscopic data and oxygen atom transfer activities of the complexes revealed that the electronic effects of the substituents located in the para‐position of the phenolate donor are transmitted through to the molybdenum center, as reflected by linear relationships between Hammett constants and key properties of the complexes, including the molybdenum–phenolate bond lengths and the coordination shift of the imine proton resonance. Compared with the unsubstituted catalyst, electron‐withdrawing substituents increase the rate of oxygen atom transfer from dimethyl sulfoxide to triphenylphosphine, whereas electron‐donating groups have the opposite effect. The highest rate enhancement was achieved through the introduction of a strongly electron‐withdrawing NO2 substituent in the p‐position of the phenolate donor.

Oxygen Atom Transfer Reactivity of Molybdenum(VI) Complexes Employing Pyrimidine- and Pyridine-2-thiolate Ligands

“…As exemplified in this work, the positioning of a methyl group initiates remarkable reactivity differences. Therefore, the influence of various substituents in different positions ought to be more intensively investigated in the future . Furthermore, our results show that complexes 1 – 4 are capable of catalyzing the most challenging step of nitrate reduction: namely that from nitrate to nitrite.…”

“…Molybdenum is the only second-row transition metal that is required by most living organisms, where it is found as a mononuclear metal center in the active site of many enzymes. , With the exception of nitrogenase and related proteins, the active site of all molybdoenzymes contains a pyranopterin-dithiolene cofactor in which the metal is coordinated by the dithiolene moiety. − The dimethyl sulfoxide reductase (DMSOR) superfamily is structurally and catalytically the largest and most diverse family of molybdoenzymes, and reactions catalyzed by its members frequently involve oxygen atom transfer (OAT). − With these naturally occurring systems as inspiration, dioxidomolybdenum­(VI) complexes have been extensively investigated in catalytic OAT reactions. − Since an aqueous environment has not been feasible for the majority of those complexes, a widespread model reaction, which was developed in the 1980s, is the OAT from Me 2 SO to PPh 3 , yielding Me 2 S and POPh 3 . − The ligands utilized in this model reaction are either dithiolene or non-dithiolene type systems. , Although bidentate non-dithiolene ligands with S,N, O,O, or N,O donor sets are structurally obviously different from the molybdopterin cofactor present in molybdoenzymes, they were found to be generally more suitable for molybdenum-catalyzed OAT reactions. − Even more different tri- and tetradentate ligands with various donor sets − have been successfully utilized, with the class of scorpionate ligands being outstanding. − Furthermore, various theoretical studies have been performed to assess the influence of the protein ligand , or charge differences and to compare active sites containing either molybdenum or tungsten. , Apart from sulfoxides, molybdenum complexes could also be applied in the deoxygenation of more challenging substrates: nitrate is reduced by naturally occurring nitrate reductases belon...…”

Dioxidomolybdenum(VI) Complexes of Tripodal Tetradentate Ligands for Catalytic Oxygen Atom Transfer between Benzoin and Dimethyl Sulfoxide and for Oxidation of Pyrogallol

“…Several synthetic models based on the cis ‐[MoO 2 ] 2+ core have been proposed and evaluated for their oxygen atom transfer properties and related mechanistic pathways. Most of these studies involve tertiary phosphines as ideal substrates because their properties can be tuned readily by substitution and the reactions can be followed by 31 P NMR spectroscopy . The generally accepted pathway for oxygen atom transfer (OAT) reactions with tertiary phosphines involves an associative mechanism, , initiated by the nucleophilic attack of the phosphine onto the more accessible and labile oxido atom, and the other oxido ligand acts as the “spectator group” and serves to strengthen the Mo=O bond through the formation of a formal Mo≡O bond during the oxygen transfer process , …”

Acid-facilitated product release from a Mo(IV) center: relevance to oxygen atom transfer reactivity of molybdenum oxotransferases

“…For this purpose, small molecule molybdenum model compounds have been developed, with several reviews recently published on this topic [8–10]. Representative examples of cis -Mo VI O 2 complexes exhibiting OAT reactivities are supported by different ligands including bis(dithiocarbamate) [11, 12], bis(dithiolene) [13, 14], bis(dithiolatopyridine) [15], trispyrazolylborate [16, 17], thiosemicarbazone [18, 19], or iminophenolate [20]. OAT reaction mechanisms of model cis -Mo VI O 2 compounds [21–25] lent credence to the proposal that the sulfite lone pair attacks the equatorial Mo VI =O group to afford a Mo(IV)-sulfate species, followed by product liberation to complete the reductive half-reaction of SO [26–28].…”