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

Previous attempts to synthesize and isolate (thiobisphenolate) vanadium(V) dioxido complexes had always provided their dimers containing [O=V(μ‐O) 2 V=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 PPh 4 [ S LVO 2 ] ( 3 ) [ S L 2– = 2′2‐thiobis(2, 4‐di‐ tert ‐butylphenolate)]. For this purpose PPh 4 [ S LVOCl 2 ] ( 1 ) had to be accessed first in order to convert it to the ethoxido compound PPh 4 [ S LVO(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 Et L 2– the sulfur atom is replaced by an ethylmethine unit and indeed the corresponding complex NBu 4 [ Et LVO 2 ] ( 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.