Photo-induced Oxygen Atom Transfer with molybdenum and tungsten dioxo complexes

To date, only a few examples of photo-catalysed oxygen atom transfer (OAT) with molybdenum and tungsten containing complexes have been reported, and these suffer from poor performance or require extensive synthesis. This work investigates new simple molybdenum and tungsten complexes that overcome these shortcomings.
Sixteen molybdenum (VI) dioxo and two tungsten (VI) dioxo complexes with substituted Schiff base SAP ligands (SAP = salicylidene amino phenol) were synthesised and characterised by NMR-, ATR-IR- and UV-Vis spectroscopies, mass spectrometry and elemental analysis. Substituent position and stereo-electronic properties were varied to study their influence on the metal complex properties and catalytic activity. UV-Vis and emission spectroscopic measurements revealed little difference between complexes except when additional chromophores were present. Excited state lifetimes of selected complexes were in the range of 2 to 6 ns.
The ability of the complexes to catalyse OAT between DMSO and PPh3 under irradiation was screened. Conversions up to 78 % after 3 h were observed, with some complexes being inactive.
The most active complex (Mo-2) was used to study the substrate scope. Several different phosphines and sulfoxides were tested. While conversion changed with the substrate a clear trend was not observed.
Further information on the mechanism was obtained from the stoichiometric reaction between Mo-2 and PPh3. Analysis of the reaction mixture after irradiation suggested the formation of molybdenum (IV), likely in the triplet state.
Reaction kinetics were found to depend strongly on the solvent used and, particularly, on the concentration of Mo-2. Below 1 mM the reaction rate increased linearly with catalyst concentration while above 1mM it decreased exponentially.
Three mechanisms for photoactivated OAT have been discussed expanding on the mechanism found in thermal OAT, one involving the formation of radical molybdenum and triphenyl phosphine species. None of the mechanisms can fully explain the results obtained and further work is needed.