Reexamining Oxidation States during the Synthesis of 2-Rhodaoxetanes from Olefins

Abstract

Herein, we report experimental, spectroscopic, and computational data that indicate that a rhodium ethylene complex, formally described as rhodium(I) and which forms a 2-rhoda(III) oxetane following reaction with H2O2, is more accurately described as a rhodium(III) metallacyclopropane. X-ray absorption spectroscopy clearly demonstrates a change in the oxidation state at rhodium following ligand coordination with tris(2-pyridylmethyl)amine. Both NMR and density functional theory studies suggest a high energy barrier to rotation of the coordinated ethylene, which is attributed to large geometric and electronic reorganization resulting from the loss of π-back-bonding. These results imply that the role of H2O2 in the formation of 2-rhoda(III) oxetanes is to oxidize the C2H4 fragment rather than the metal center, as has been previously suggested.

Strong Electronic Coupling of Molecular Sites to Graphitic Electrodes via Pyrazine Conjugation

“…Based on literature precedent, the reduction of Rh III to Rh I is expected to shift the rising portion of the K-edge to lower energy by 2-3 eV. 19,20 Indeed, we observe a significant difference in both the position and shape of the K-edge between [Rh III Cp*(phen)Cl] + and Rh I Cp*(phen) ( Figure S20). The Rh K-edge for [Rh III Cp*(phen)Cl] + has an inflection point at 23,226.5 eV, while the Rh K-edge for Rh I Cp*(phen) onsets earlier and has two inflection points at 23,225.6 and 23,231.7 eV ( Figure S21).…”

The Importance of Ligand‐Induced Backdonation in the Stabilization of Square Planar d10 Nickel π‐Complexes

“…Experimentally,t he 31 P{ 1 H} NMR spectrum of 1 up to 110 8Cr eveals no dynamic processes, indicatingt hat the barrier to carbonyl rotation is greater than 70 kJ mol À1 .W eh ave previously reported similar high barriers to rotation with ar hodium-olefin system. [8] Indeed, the metallaepoxide electromer of related nickel complexes have recently been invoked by the groups of Doyle [42] and Ogoshi [37] based on reactivity studies, andi sa lso shown explicitly in Group 4c omplexes that display similars tructural parameters to the nickel species discussed here. [58][59][60][61][62][63] Ambiguity in the electronic structureo ft hese nickel p-complexesh inderse fforts towards the rational design of nickel-catalysed processes.…”

“…[87] Given that the electron density of these systems are well described from DFTcalculations, we approached this same issue by applying natural resonance theory (NRT) [109][110][111] to expose different contributions to the overall electronic description ( Table 5). [8] In all cases, the Ni II metallacycle contributesl ittle to the overall electronic structure. TheN i 0 p-adduct and Ni I intermediate resonance structures account for > 80 %o ft he electronic structure in all cases.…”

“…In previous work, we noted that the 31 P{ 1 H} NMR spectroscopic data of an umber of (dtbpe)Ni [dtbpe = 1,2-bis(di-tertbutyl)phosphinoethane] p-complexes werec onsistent with typical d 10 Ni 0 complexes (Table 1). [10,11,57] In contrast, we also noted that the distorted squarep lanar geometry with significant elongation of the p-bond was mostc onsistentw ith ad 8 Ni II formulation,i nk eeping with the metallaepoxide extreme of the Dewar-Chatt-Duncanson (DCD) model of bonding (Scheme 1). In addition, preliminaryd ensity functional theory (DFT) calculations (vide infra) revealed prohibitively high barriers to rotationo ft he p-ligand (i.e.,8 0-100 kJ mol À1 ), demonstratingt hat these complexes have as trong preference for the square planar geometry despite stericc onstraints.…”

“…[5] In contrast to these open-shell systems, closed-shell systems that rely on the ligand-accepting and/or-donating electron pairs are less common.Anotable example of this type of reactivity is the zirconium system reported by Heyduk, which allowsf or ap utative "oxidativea ddition" reaction to occur at a Zr IV ,d 0 metal centre. [6] In another example, we have recently shownt hat the oxidation of (TPA)Rh olefin complexes (TPA = tris(2-pyridylmethyl)amine) with H 2 O 2 to form 2-rhodaoxetanes [7] is more accurately described as al igand-centred [8] rather than metal-centred oxidation. [9] In this case, the p-ligand acts as at wo-electron redoxcentre.…”