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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.

+ = ferrocenium). Electrochemical analysis of this reaction revealed a possible ECE mechanism that includes prior and subsequent electron transfer to this dimerization. The electronic structure of the dimer 4 was probed by SQUID magnetommetry and X-band EPR spectroscopy (100 K and 4 K). The dimer was found to contain two antiferromagnetically coupled V IV centers, as well as a small portion of monomeric V IV species, 2 + . In contrast to 2, oxidation of 1 resulted in an EC mechanism, the product of which remains unknown. Preliminary reactions with compounds 1 and 2 bearing free P=O groups were performed using H-atom donors to test their ability to undergo H-atom transfer in the context of the proposed "reduction-coupled oxo activation" mechanism; however, no clear reaction pathway supporting this mechanism was yet observed.Considerable interest lies in the structure and bonding of vanadium phosphate complexes due to their varied applications, such as in size-selective inorganic hosts, ion exchangers, and magnetic systems.1-4 From the industrial perspective, DuPont's "butox" process utilizes a surface vanadium phosphate oxide (VPO) catalyst for the oxidative conversion of butane to maleic anhydride (MA) on a 500 kt scale (Scheme 1a). The commodity chemical is used as a resin, a food additive, an agricultural chemical, as well as in pharmaceuticals. 5 The conversion of butane to MA is considered one of the most complex in industry, involving the abstraction of 8 H-atoms and the insertion of 3 O-atoms, as part of an overall 14 e -process. As such, this process has been the subject of extensive mechanistic investigations over the decades, 5-9 and continues to be the subject of considerable interest today.10-14 However, in spite of these extensive studies, no consensus mechanism yet exists as to the exact steps of this transformation.Recent gas-phase studies by Schwarz and co-workers have described the ability of simple oxidized oxide clusters, such as [P4O10] •+ or [V4O10]•+ , to abstract a H atom from methane or other simple hydrocarbons, through an H-atom transfer (HAT) process. It was further found that the in situ-generated [P4O10] •+ was more reactive than the metallic analog, [V4O10] •+ . [15][16] The increased reactivity upon P-incorporation into the clusters was further demonstrated by comparison of the smaller oxide cations, [VPO4] •+ and [V2O4]•+ , wherein the former was more reactive towards HAT than the latter. These experimental conclusions were further supported by theoretical work by Goddard and coworkers who proposed that the terminal P=O linkages in VPO are responsible for C-H functionalization at butane (Scheme 1b-c). The mechanism, coined "reduction-coupled oxo activation" (ROA), occurs as a result of the strong basicity at the P=O bond coupled with the neighboring high-valent, oxidative V V centers which undergoes reduction, 10, 12 and is in many respects analogous to well-studied proton-coupled electron transfer (PCET) reactions common to both biological and artificial systems. [17][18]...

Transient Cu–H monomers have long been invoked in the mechanisms of substrate insertion in Cu–H catalysis. Their role from Cu–H aggregates has been mostly inferred since ligands to stabilize these monomeric intermediates for systematic studies remain limited. Within the last decade, new sterically demanding N-heterocyclic carbene (NHC) ligands have led to isolable Cu–H dimers and, in some cases, spectroscopic characterization of Cu–H monomers in solution. We report an NHC ligand, IPr*R, containing para R groups of CHPh2 and CPh3 on the ligand periphery for the isolation of a Cu–H monomer for insertion of internal alkenes. This reactivity has not been reported for (NHC)­CuH complexes despite their common application in Cu–H-catalyzed hydrofunctionalization. Changing from CHPh2 to CPh3 impacts the relative concentration of Cu–H monomers, rate of alkene insertion, and reaction of a trisubstituted internal alkene. Specifically, for R = CPh3, monomeric (IPr*CPh3)­CuH was isolated and provided >95% monomer (10 mM in C6D6). In contrast, for R = CHPh2, solutions of [(IPr*CHPh2)­CuH]2 are 80% dimer and 20% (IPr*CHPh2)­CuH monomer at 25 °C based on 1H, 13C, and 1H–13C HMBC NMR spectroscopy. Quantitative 1H NMR kinetic studies on cyclopentene insertion into Cu–H complexes to form the corresponding Cu–cyclopentyl complexes demonstrate a strong dependence on the rate of insertion and concentration of the Cu–H monomer. Only (IPr*CPh3)­CuH, which has a high monomer concentration, underwent regioselective insertion of a trisubstituted internal alkene, 1-methylcyclopentene, to give (IPr*CPh3)­Cu­(2-methylcyclopentyl), which has been crystallographically characterized. We also demonstrated that (IPr*CPh3)­CuH catalyzes the hydroboration of cyclopentene and methylcyclopentene with pinacolborane.

In this Article, we outline the synthesis of B(CF)-coordinated mono-, di-, and trivanadocene phosphorus oxide complexes, CpVOP(OB(CF))Ph (2), (CpVO)P(OB(CF))Ph (3), and (CpVO)P(OB(CF)) (4), respectively (Cp = η-cyclopentadienyl). The complexes were synthesized from the known reagents, CpVF and PhP(O)OSiMe (for 2) or PhP(O)(OSiMe) (for 3) or (MeSiO)PO (for 4), via MeSiF elimination and in the presence of B(CF). The multimetallic complexes (3 and 4) could not be synthesized without the capping B(CF) Lewis acid, whereas the uncapped version of 2, CpVOP(O)Ph (1), has previously been reported by us. Spectroscopic and crystallographic analyses of 2-4 support an increasingly Lewis basic P═O bond upon substitution of -Ph for -OVCp fragments (2-4). The increased metal nuclearity also results in increasingly reducing complexes as evidenced by cyclic voltammetry (CV). Magnetic measurements (SQUID) further revealed high-spin complexes with negligible magnetic exchange between V centers. Chemical oxidation of 2 with 0.5 equiv of [Ag][B(CF)] resulted in a ligand rearrangement reaction producing the V product, CpV(OP(OB(CF))Ph) (7). In contrast, the oxidation of 4 with the trityl salt, [PhC][B(CF)], resulted in the isolation of a mixed-valent V/V dimetallic species, (CpVOP(OB(CF))OVCp (9). Both oxidations likely produce the [CpV][B(CF)] byproduct and evidence for its formation is presented. The synthesis and characterization of the mono- and dimetallic species, CpVOP(OBPh)Ph (8) and (CpVO)P(OB(CF))OSiMe (5), is also reported.

A C3v-symmetric triarylphosphine oxide locked into conformation by H-bonding and displaying an extended MOF-like solid-state structure is reported.

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