Two-Electron Redox Tuning of Cyclopentadienyl Cobalt Complexes Enabled by the Phenylenediamide Ligand

Abstract

Achieving multielectron activity at first-row transition-metal complexes has important implications for homogeneous catalysis using earth-abundant metals. Here, we report a family of cobalt–phenylenediamide complexes that undergo reversible 2e– oxidation regardless of the ligand substituents, enabling unprecedented multielectron redox tuning over 0.5 V and, in each case, affording the dicationic Co­(III)-benzoquinonediimine species. The neutral complexes are best described as delocalized systems with π-bonding in the metallocycle, consistent with a closed-shell singlet ground state predicted by density functional theory (DFT) calculations. Our DFT results also predict an ECE pathway for 2e– oxidation (ECE = electrochemical step, chemical step, electrochemical step), where the first 1e– step involves redox-induced electron transfer to yield a Co­(II) intermediate. Disruption of the metallocycle bonding in this state enables a change in the coordination geometry through association of an addition ligand, which is critical for accessing the potential inversion. The electronic properties of the phenylenediamide ligand govern whether the second electron is lost from the ligand or metal, providing a remarkable example of tunable 2e– behavior at first-row systems.

Redox-active ligand promoted electrophile addition at cobalt

“…This result is in stark contrast to [1-MeCN] + in which the radical spin density was calculated by DFT to be mainly centred on cobalt. 5 The 1e À reduction of 2 can also be monitored by UV-vis spectroelectrochemistry (Fig. S26b, ESI †).…”

“…[DBT-CF 3 ] + , where [1-MeCN] 2+ may be generated from the disproportionation of [1] + . 5 To probe the possibility of radical trifluoromethylation of other organic substrates, we tested the reactivity of 1 in the presence of [DBT-CF 3 ] + and 3-methyl-1H-indole at 25 1C. Indeed, the formation of 3-methyl-2-(trifluoromethyl)-1Hindole was observed by 1 H and 19 F NMR within 20 min (Fig.…”

“…A limited number of first-row metal complexes bearing redox-active ligands exhibit a 2e − redox couple, 4,5 for which a change in coordination geometry is often required to favour potential inversion 6 and access the multielectron pathway.…”

“…Among these examples, we recently reported a series of cobalt complexes based on the redox-active o -phenylenediamide ligand (opda). 5 By cyclic voltammetry (CV), the isopropyl derivative, complex 1 , was shown to undergo a reversible 2e − oxidation at –0.17 V vs. Fc +/0 in MeCN, yielding the dicationic complex [1–MeCN] 2+ in which the opda ligand has been fully oxidized to the benzoquinonediimine form (bqdi; Scheme 1). This 2e − oxidation can be achieved electrochemically or by treatment of 1 with a chemical oxidant.…”

“…The N–C phenylene bond length in 2 is 1.311(5) Å, comparable to the ligand backbone in [1–MeCN] 2+ . 5 Overall, 2 is formulated as a coordinatively saturated Co( iii ) centre with a fully oxidized bqdi ligand and anionic trifluoromethyl ligand; thus, the formation of 2 involves formal ligand-to-substrate redox transfer.…”