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Dioxomolybdenum(VI) Complexes for Oxygen Atom Transfer
In inorganic chemistry, the dioxomolybdenum(VI) functional group (MoVIO2) is a transition metal oxo complex motif that is studied for its role in oxygen atom transfer catalysis. Coordination complexes synthesized for this reactivity often utilize the MoVIO2 functional group for its ability to donate an oxygen atom, using the MoVI / MoIV oxidation state couple in order to facilitate selective oxidations with capable donor or acceptor substrates, e.g. selective epoxidation of olefins, oxidation of tertiary phosphines, and the oxidation of sulfides. In catalysis, the reduced MoIVO species is observed to deoxygenate small, oxygen-containing biomolecules such as DMSO and nitrates. While the bioenzymes known to contain molybdenum utilize a dithiolate cofactor, known as molybdopterin, to perform oxygen atom transfer reactions, the use of synthetic polydentate donor ligands with all- or mixed- donor atom environments containing oxygen, nitrogen, and/or sulfur atoms has allowed for the study of the MoVIO2 functional group in synthetic oxygen atom transfer reactions. The use of bidentate iminophenolate ligands has been shown to support Mo(VI)O2 complexes capable of catalytically activating O2, a desirable reaction in pursuing sustainable chemistry.

Mo2VO2(μ-O) Dimer
It has been shown through the use of EXAFS and EPR studies that mononuclear oxygen atom transfer catalysis occurs by a series of single electron reduction events, traversing from MoVIO2 to a MoV oxido intermediate with the acceptor substrate (MoVIO2 to MoVO(OA)) followed by a second reductive elimination process to release the oxygenated product and the highly reactive monomeric MoIVO intermediate (MoVO(OA) to MoIVO + OA). The continuation of catalysis at this stage is dependent on the reformation of the active species, MoVIO2, by deoxygenation of oxygen-donor substrate. However, within these reactions, the formation of a purple, binuclear Mo2VO2(μ-O) dimer containing a bridging oxo ligand has been observed to halt reactivity due to the calculated kinetic stability of this species. Formation of this dimer is attributed to the instability of the reduced MoIVO intermediate species, reacting unselectively to reduce oxygen-containing molecules which, in these reactions, may be the target oxygen-donor substrate or an unreacted MoVIO2 monomer. This catalytically inactive Mo2VO2(μ-O) species is the byproduct that occurs when a highly reactive MoIVO intermediate reduces an unreacted MoVIO2 catalyst. This dimerization is also observed when isolated MoIVO species are reacted with H2O, leading to a comproportionation reaction and the formation of the stable Mo2VO2(μ-O) dimer, indicating a water-intolerance catalytic cycle and the need for air-free conditions for successful catalysis.

Comproportionation of Mo(VI)O2 and Mo(IV)O
Mo(IV)O + Mo(VI)O2 --> Mo(V)O2(μ-O)

Mo(IV)O + OD --> Mo(VI)O2 + D

Avoiding or Reversing Formation of Mo2VO2(μ-O) Dimer
A study from 1996 from N. J. Cooper illustrates that attempts to reform a monomeric complex from a (μ-O) bridging Mo(V) species does not produce a disproportionation effect, meaning that cleavage of the dimer does not lead to a formation of catalytically active Mo(VI) and/or Mo(IV) monomeric species. Instead, photolysis of the dimer leads to a redistribution of the monodentate ligands with the product being an inactive monomeric MoVO complex. This illustrates the importance of avoiding formation of the inactive dimer rather than trying to reverse the dimerization.

In the work of R. H. Holm, circumventing the formation of the (μ-O) bridging Mo(V) dimer was achieved by increasing the dentation of the ligand from a bidentate {S, S} donor to a tridentate {S, N, S} pincer-type ligand which contains a neutral, coordinating pyridine N atom. In this system, it was observed that stabilization of the MoIVO species was brought about by avoiding under coordination, with the observation of an additionally coordinated electron-donor solvent molecule, dimethylformamide (DMF). However, increasing the dentation of the ligand from a bichelate to a tris-chelate does not guarantee that dimerization is blocked, shown by a study from Young et. al. in which the tridentate TpiPr ligand (hydrotris(3-isopropylpyrazol-1-yl)borate) was used to support a MoVIO2 complex capable of oxygen atom transfer but was still observed to dimerize in the presence of water.

The use of coordinating tertiary phosphines in conjunction with bidentate ligands has shown to be a successful method for maintaining a high coordination number in the MoIVO catalytic intermediate, stabilizing this highly reactive reduced species. By reacting [MoVIO2L2] complexes in the presence of sterically unhindered phosphines such as PMe3 in excess, oxidation of the phosphine substrate is achieved as well as the isolable [MoIVOL2 (PMe3)] complex, with no formation of the inactive dimer, [Mo2VO3L4]. Application of this procedure has been shown to allow for the activation of molecular oxygen as the oxygen atom donor substrate in catalysis.