Oxidative Cleavage of CH₃ and CF₃ Radicals from BOXAM Nickel Complexes

Abstract

Oxidation of the nickel complexes [(BOXAM)-Ni(R)] (HBOXAM = bis((4-isopropyl-4,5-dihydrooxazol-2-yl)phenyl)amine) with R = CF 3 , CH 3 , Cl leads to the corresponding radical cationic complexes, which rapidly undergo homolytic Ni−R bond splitting. The corresponding radicals R • can be generated by either anodic (electrochemical) or aerial oxidation and were detected by spin trapping experiments using N-tert-butyl-α-phenylnitrone (PBN). The splitting reaction was also monitored by UV−vis−near-IR absorption spectroelectrochemistry, showing the formation of the "byproduct" complex [(BOXAM)Ni(THF)] + in THF solution. Additionally, the undissociated complex radicals [(BOXAM)Ni(R)] •+ were studied by EPR and UV−vis−near-IR spectroelectrochemistry, revealing inter alia largely ligand-centered oxidation. DFT calculations on ground and oxidized states support this finding.

Nickel–Alkyl Complexes with a Reactive PNC‐Pincer Ligand

“…Furthermore, 2CH 3 and 2CF 3 were anodically oxidized in the presence of the spin trap PBN ( N ‐ tert ‐butyl‐α‐phenylnitrone) in the EPR cavity but no signals of PBN adducts of either alkyl fragment, indicative for the formation of · CH 3 or · CF 3 radicals, were observed. Such signals have been observed upon oxidation of the complexes [Ni(BOXAM)(alkyl)] (BOXAM = bis((4‐isopropyl‐4,5‐dihydrooxazol‐2‐yl)phenyl)amine) bearing either CH 3 or CF 3 as alkyl ligand, which suggest formation of a species containing Ni II and the oxidized BOXAM ligand (aminyl radical) …”

“…In late transition metal complexes, the CF 3 substituent is usually strongly bound to the metal center and is therefore often not susceptible to reductive elimination . It was proposed that Ni could be an active catalyst for this transformation,, and the precise understanding of Ni–CF 3 bonding and reactivity of these complexes have been subject to several studies, usually in comparison to Ni–CH 3 bonding . The main findings show that the lone pair of the carbanionic CF 3 ligand has increased C 2s character and a stronger donation to the metal compared to the methyl analog.…”

Multielectron Redox Chemistry of Transition Metal Complexes Supported by a Non‐Innocent N3P2 Ligand: Synthesis, Characterization, and Catalytic Properties

“…Given its S = 1/2 ground state, there are three possible electronic configurations for 3 ox : (i) low‐spin Ni III bound to a closed‐shell L N3P2 anion, (ii) high‐spin Ni II antiferromagnetically coupled to a L N3P2 ‐based radical, or (iii) low‐spin Ni II bound to a L N3P2 ‐based radical. The presence of a Ni III center is not consistent with the small g‐anisotropy (Δ g = g 1 – g 3 ) of ≈ 0.04 observed in the EPR spectrum, as true Ni III complexes generally exhibit Δ g ‐values greater than 0.20 . In addition, DFT calculations of 3 ox determined that the lowest‐energy model ( 3 ox ‐LS ) consists of a low‐spin Ni II center bound to a L N3P2 ‐based radical.…”

Circumventing Intrinsic Metal Reactivity: Radical Generation with Redox‐Active Ligands

“…Stepping away from alkyl radicals, generation of CF 3 . radicals from nickel complexes has been reported by Vicic through homolytic splitting of Ni−CF 3 bond in BOXAM and terpyridine nickel complexes. Interestingly, in the former case, study of the singly oxidized radical complex before dissociation showed a high extent of ligand‐based oxidation at the BOXAM ligand.…”