Oxygen activation and catalytic aerobic oxidation by Mo(iv)/(vi) complexes with functionalized iminophenolate ligands

Abstract

Synthesis of molybdenum(vi) dioxido complexes 1-3, coordinated by one or two functionalized iminophenolate ligands HL1 or HL2, bearing a donor atom side chain or a phenyl substituent, respectively, allowed for systematic investigation of the oxygen atom transfer (OAT) reactivity of such complexes towards phosphanes. Depending on stoichiometry and employed phosphane (PMe3 or PPh3), different molybdenum(iv) and molybdenum(v) complexes 4-7 were obtained. Whereas molybdenum(iv) complexes 4 and 5, bearing a terminal PMe3 ligand, readily reacted with molecular O2 to form oxido peroxido complexes 8 and 9, phosphane free μ-oxido bridged dinuclear molybdenum(v) complexes 6 and 7 proved to be stable towards oxidation with molecular O2 under ambient conditions. Single-crystal X-ray diffraction analyses revealed different isomeric structures in the solid state for dioxido complexes 1 and 2 in comparison with oxido phosphane complex 5, dinuclear oxido μ-oxido complex 6 and oxido peroxido complexes 8 and 9, pointing towards an isomeric rearrangement during OAT. Compounds 1 and 2 were furthermore tested for their ability to catalyze the aerobic oxidation of PMe3 and PPh3. A significant difference in catalytic activity has been observed in the oxidation of PMe3, where complex 1 bearing donor atom functionalized ligands led to higher conversion and selectivity than complex 2 coordinated by phenyl iminophenolate ligands. In the oxidation of PPh3, complex 2 leads to higher conversion compared to 1. In a control experiment, phenyl-based dinuclear μ-oxido complex 7, derived from complex 2, was found to be catalytically active, which suggests a lower energy barrier for disproportionation into [MoO(L)2] and [MoO2(L)2] in comparison with methoxypropylene based compound 6, a prerequisite for subsequent reactivity toward molecular O2.

Activation of Molecular Oxygen by a Molybdenum(IV) Imido Compound

“…This is in contrast to the related oxido peroxido compound where the higher reactivity of the dioxido compound prevented its observation during OAT. 15,16 However, addition of a further equivalent of PMe3 to the above mixture led to the expected mixture of 3a and [Mo(NtBu)(PMe3)(L1)2], 2a (Scheme 3). This suggests a similar but generally lower reactivity of the imido peroxide system.…”

“…Presumably for this reason, complexes 1a and 1b, [MoO(NtBu)(L)2], both were essentially inactive under catalytic conditions, § which is in contrast to the dioxido analogue [MoO2(L2)2], 4. 16 This may be attributed to the remarkable stability of the corresponding peroxides towards reduction. Furthermore, we did not observe a transfer of the imido group to the phosphane substrate for 1a, 1b or the peroxide 3a, suggesting that the imido transfer reaction is also energetically disfavoured.…”

“…[6][7][8]15,16,30,31 In general, the more electron donating imido group leads to a slight elongation of Mo=O as well as Mo-(O-O) bonds in comparison to a second oxido ligand, reflecting weaker σ-bonds to the less electrophilic metal center ( Table 1). The O-O bond lengths on the other hand are essentially identical (1.4399(17) in 3a vs. 1.4425(18) in 5), 16 further corroborating an absent π-backbonding to the peroxido unit. Figures S17 and S18, SI) revealed a further interesting observation as their geometric arrangement in solid state is different, namely N,O in 1a and N,N in 1b.…”

“…Whereas the activation of molecular oxygen by molybdenum(IV) complexes has been established by our group and others, concomitant oxygen atom transfer is restricted to very few substrates. [11][12][13][14][15][16] All molybdenum complexes capable of activating molecular oxygen are of the general structure [MoOLn] and lead to the formation of [MoO(O2)Ln] complexes, with L being either CNor bidentate monoanionic ligands. The oxido ligand renders the metal center electron poor, thus leading to strong σ-bonds but simultaneously inhibiting any π-backbonding to the peroxido unit.…”

Dioxygen Activation with Molybdenum Complexes Bearing Amide-Functionalized Iminophenolate Ligands

“…Several transition metals, including molybdenum, are known to activate dioxygen [23,24,25,26,27,28,29,30,31,32]. In our group, molybdenum(IV) complexes based on β-ketiminate and iminophenolate ligand systems were investigated regarding their usability in dioxygen activation [33,34,35,36]. As a result, we were able to demonstrate the catalytic aerobic oxidation of phosphanes and determine the influence of steric and electronic properties of the ligands.…”