Probing Hydrogen Atom Transfer at a Phosphorus(V) Oxide Bond Using a “Bulky Hydrogen Atom” Surrogate: Analogies to PCET

Abstract

Recent computational studies suggest that the phosphate support in the commercial vanadium phosphate oxide (VPO) catalyst may play a critical role in initiating butane C–H bond activation through a mechanism termed reduction-coupled oxo activation (ROA) similar to proton-coupled electron transfer (PCET); however, no experimental evidence exists to support this mechanism. Herein, we present molecular model compounds, (Ph2N)3VN–P­(O)­Ar2 (Ar = C6F5 (2a), Ph (2b)), which are reactive to both weak H atom donors and a Me3Si• (a “bulky hydrogen atom” surrogate) donor, 1,4-bis­(trimethylsilyl)­pyrazine. While the former reaction led to product decomposition, the latter resulted in the isolation of the reduced, silylated complexes (Ph2N)3V–NP­(OSiMe3)­Ar2 (3a/b). Detailed analyses of possible reaction pathways, involving the isolation and full characterization of potential stepwise square-scheme intermediates, as well as the determination of minimum experimentally and computationally derived thermochemical values, are described. We find that stepwise electron transfer (ET) + silylium transfer (ST) or concerted EST mechanisms are most likely. This study provides the first experimental evidence supporting a ROA mechanism and may inform future studies in homogeneous or heterogeneous C–H activation chemistry, as well as open up a possible new avenue for main group/transition metal cooperative redox reactivity.

Reductive Silylation Using a Bis‐silylated Diaza‐2,5‐cyclohexadiene

“…However, because it has been shown that bis‐silyation of O 2 with variants of this silylating reagent occurs stepwise, it is premature to generalize about mechanisms of reductive silylation. In the end, different substrates may proceed by different mechanisms, as shown recently …”

“…In the end, different substrates mayp roceed by different mechanisms, as shownr ecently. [19] Broader application can be envisioned. This silylation reagent has potential fors toichiometricu se in derivatizationo f N 2 -derived nitride complexes, and in am echanistically simpler manner than use of KC 8 with Me 3 SiCl.…”

A Spectroscopic Study of Supported‐Phosphate‐Catalysts (SPCs): Evidence of Surface‐mediated Hydrogen‐Transfer

“…Similarly, elsewhere in the literature hydrogen-transfer was reported on a P=O moiety in a molecular complex. [11] Over the P=O containing complex, hydrogen-transfer was observed and studied mecha-nistically, providing first experimental evidence of reductioncoupled oxo activation (ROA) mechanism, which is similar to the proton-coupled electron transfer (PCET) mechanism. [11] Based on results in the present study and the findings from the literature, it is likely that the P=O surface sites are the active sites for the surface-mediated hydrogen-transfer reaction in SPCs.…”

“…[10] Little direct evidence corroborating an active role of surface phosphate sites in the hydrogen-transfer reaction can be found in the literature. In one instance, hydrogen-transfer was studied mechanistically on P=O containing moiety in a molecular complex [11] providing the first experimental evidence of reduction-coupled oxo activation (ROA) mechanism, which is similar to the proton-coupled electron transfer (PCET) mechanism. [11] Elsewhere, an IR study of SiO 2 -supported phosphate catalysts reported the formation of new surface À OH sites upon NH 3 adsorption on the catalyst but did not expand upon the observed phenomenon and the underlying mechanism.…”

“…In one instance, hydrogen-transfer was studied mechanistically on P=O containing moiety in a molecular complex [11] providing the first experimental evidence of reduction-coupled oxo activation (ROA) mechanism, which is similar to the proton-coupled electron transfer (PCET) mechanism. [11] Elsewhere, an IR study of SiO 2 -supported phosphate catalysts reported the formation of new surface À OH sites upon NH 3 adsorption on the catalyst but did not expand upon the observed phenomenon and the underlying mechanism. [12] Therefore, it is critical to elucidate whether surface phosphate sites in SPCs can participate in hydrogenation (via hydrogen-transfer or hydrogen-spillover) since numerous reactions catalyzed by SPCs include hydrogenation step(s).…”