Direct Proof for a Lower Reactivity of Monomeric vs. Dimeric Oxidovanadium Complexes in Alcohol Oxidation

Abstract

Previous attempts to synthesize and isolate (thiobisphenolate) vanadium(V) dioxido complexes had always provided their dimers containing [O=V(μ‐O)2V=O]2+ cores, and these also dominate the solution reactivity. Hence, the behavior of their parent monomers, which represent the major species in solution, has remained uncertain. Herein we report the development of a synthetic route that allowed for the successful isolation, spectroscopic investigation, and structural characterization of the monomer PPh4[SLVO2] (3) [SL2– = 2′2‐thiobis(2, 4‐di‐tert‐butylphenolate)]. For this purpose PPh4[SLVOCl2] (1) had to be accessed first in order to convert it to the ethoxido compound PPh4[SLVO(OEt)2] (2), which is more prone to hydrolysis. Treatment of 2 with stoichiometric amounts of water followed by immediate cooling to –30 °C led to crystals of 3. After its dissolution NMR spectra were recorded that were identical with those obtained after dissolution of its dimer, thus confirming the monomer/dimer equilibrium postulated previously. The molecular structure of 3 revealed the absence of a V···S interaction, which, however, stabilizes its dimer, and thus suggested the employment of a bisphenolate ligand lacking a bridging sulfur atom to obtain an analogue, which does not undergo dimerization in solution. In EtL2– the sulfur atom is replaced by an ethylmethine unit and indeed the corresponding complex NBu4[EtLVO2] (4) proved to be stable as a monomer. Investigation of its potential as a catalyst for the oxidative dehydrogenation of 9‐fluorenol confirmed a much lower reactivity in comparison to dimeric complexes, which is discussed.

Reversible PCET and Ambient Catalytic Oxidative Alcohol Dehydrogenation by {V=O} Perfluoropinacolate Complexes

“…Cinnamyl, benzyl, and α-methylbenzyl alcohols were oxidized by 3a to their corresponding aldehydes/ketones as their sole reaction products in 40, 7, and 3% yield, respectively, a range similar to that observed for Limberg's V-thiacalixarene system. 48,50 In each case, upon substrate addition, the initial yellow solution of 3a immediately turned brown/orange; as catalysis progressed, the solution became slightly green. These color changes suggest an initial binding event of the substrate at the oxophilic V center(s), followed by the transient formation of blue 4a during the catalytic process.…”

“…Additionally, the substrate scope listed in Table encompasses allylic, primary, and secondary benzylic alcohols. Cinnamyl, benzyl, and α-methylbenzyl alcohols were oxidized by 3a to their corresponding aldehydes/ketones as their sole reaction products in 40, 7, and 3% yield, respectively, a range similar to that observed for Limberg’s V-thiacalixarene system. , …”

“…Not only do these systems catalyze OAD, but mechanistic investigations have been carried out that corroborate Mayer’s findings concerning the relative HAT reactivities of terminal versus bridging oxo moieties. Studies of the VO thiacalixarene system revealed that upon dissolution, the (μ-O)-bridged V­(V) dimer enters into an equilibrium with a monomeric form, and experiments confirm that the dimeric complex is solely responsible for catalysis, indicating the μ-O bridge as necessary in HAT …”

“…Studies of the VO thiacalixarene system revealed that upon dissolution, the (μ-O)-bridged V(V) dimer enters into an equilibrium with a monomeric form, and experiments confirm that the dimeric complex is solely responsible for catalysis, indicating the μ-O bridge as necessary in HAT. 50 Recent work by Doerrer and co-workers has focused on the formation of Earth-abundant transition-metal complexes featuring oxidatively robust highly fluorinated alkoxide ligands. 51−62 As weak π-donors, these ligands are significantly less nucleophilic than their hydrogenated counterparts and can be air and water stable.…”

Catalytic Applications of Vanadium: A Mechanistic Perspective

“…In their follow up to the original report, Limberg and co-workers provided compelling evidence to support the role of the dimeric species in solution. 175 This is achieved by substituting the sulfur atom in the diphenolate ligand with an ethyl linker, which prevents the stabilizing sulfur−vanadium interaction in the dimeric species (Scheme 24b). This modest ligand substitution translates into over an order of magnitude decrease in rate, underscoring the importance of dimer formation in decreasing the reorganization energy during hydrogen atom abstraction.…”