Nicolas Leconte

Lithium batteries are considered as the most promising electricity storage solution to support the energy transition but still suffer for cost and sustainable issues related to the use of transition metal‐based electrode materials. Due to their reduced environmental footprint and low cost, organic active materials such as carbonyl family (quinone, anhydride, diimide) are some interesting candidates to replace them but electrochemical performances in particular redox potential have to be increased in order to become a viable alternative. Here benefiting from tunability of organic structure and guided by rational design rules based on Hammett theory, new strategies are proposed to increase the redox potential of molecular diimide by introducing electron‐withdrawing groups such as bromine, cyano, or nitro groups. This work can be considered as a first step to develop some competitive positive electrodes materials based on organic molecules or polymers using diimide derivatives.

The N,N‐bis(2‐amino‐3,5‐di‐tert‐butylphenyl)amine pincer ligand was coordinated to copper. Depending on the copper source, a mononuclear complex 1⁺ or a trimer 2 could be isolated and were structurally characterized. Complex 1⁺ consists of two deprotonated iminobenzoquinone ligands coordinated to a Cu(I) center. Complex 2 is trinuclear with a (3:3) (M:L) stoichiometry, featuring a three‐fold repetition of a unit made of a Cu(II) center coordinated to a tridentate ligand radical‐dianion. In 2, the metal ions are bridged by an anilido nitrogen. The coordination sphere of each copper is completed to four by a neighboring iminosemiquinone moiety. Complex 1⁺ belongs to an electron‐transfer series. The paramagnetic complexes 1 and 1²⁺ were generated and characterized by EPR and Vis‐NIR spectroscopy. Complex 1 exhibits an isotropic resonance at g = 2.00, which is reminiscent of Cu(I) iminosemiquinone species. The dication 1²⁺ exhibits a metal‐based ground spin state and hence is described as a Cu(II) iminobenzoquinone complex. Both 1 and 1⁺ show a NIR band (954, 980 nm) of high intensity (> 20 mm^–1 cm^–1) assigned to ligand‐based charge transfer transitions. Two‐electron reduction of 1 produces 2 via ligand release and disproportionation. Conversely, oxidation of 2 affords 1⁺. Finally, carbon‐nanotube‐supported complex 2 is active towards electrocatalytic reduction of H₂O₂.

The bis(di-tert-butyl-aminophenyl)amine ligand H₃L^N,N,N was reacted with Mn(II), Co(II), Fe(III) and Cu(II) salts in air. The ligand undergoes oxidative transformations, which involve intra and intermolecular C–N and N–N bond formations. A rare aromatic C–N bond cleavage leading to a C–O bond has also been observed.

A well-defined diradical cobalt(iii) complex derived of the N,N′-bis(2-aminophenyl)-o-phenylenediamine catalyses the cycloisomerization of alkynoic acids.

The sterically hindered bis(phenol)–dipyrrin ligands ^HLH₃ and ^PhLH₃ were reacted with 1 equiv of copper(II) under ambient conditions to produce the copper radical complexes [Cu(^HL)] and [Cu(^PhL)]. Their X-ray crystal structures show relatively short C–O bond distances (mean bond distances of 1.287 and 1.291 Å), reminiscent of mixed pyrrolyl–phenoxyl radical species. Complexes [Cu(^HL)] and [Cu(^PhL)] exhibit rich electronic spectra, with an intense near-IR (NIR) band (ε > 6 mM^–1 cm^–1) at 1346 and 1321 nm, respectively, assigned to a ligand-to-ligand charger-transfer transition. Both show a reversible oxidation wave (E_1/2^1,ox = 0.05 and 0.04 V), as well as a reversible reduction wave (E_1/2^1,red = −0.40 and −0.56 V versus ferrocenium/ferrocene, respectively). The cations ([Cu(^HL)]⁺ and [Cu(^PhL)]⁺) and anions ([Cu(^HL)]^– and [Cu(^PhL)]^–) were generated. They all display an axial (S = ¹/₂) signal with a copper hyperfine structure in their electron paramagnetic resonance spectra, consistent with ligand-centered redox processes in both reduction and oxidation. Complex [Cu(^HL)](SbF₆) was cocrystallized with [Cu(^HL)]. Oxidation is accompanied by a slight contraction of both the C–O bonds (mean bond distance of 1.280 Å) and the C–C bonds connecting the peripheral rings to the dipyrrin. The cations show vis–NIR bands of up to 1090 nm due to their quinoidal nature. The anions do not show a significant band above 700 nm, in agreement with their bis(phenolate)–dipyrrin character. The radical complexes efficiently catalyze the aerobic oxidation of benzyl alcohol, 1-phenylethanol, and unactivated 2-phenylethanol in basic conditions.

The pincer ligand H₃L^3N, bis(2‐amino‐3,5‐di‐tert‐butylphenyl)amine}, was chelated to Ni^II, Pd^II, and Pt^II. Square‐planar complexes of stoichiometry 2:1 (ligand/metal) formed, wherein the ligand coordinates in a bidentate fashion. By X‐ray diffraction we established that each ligand molecule coordinates in its diiminosemiquinone radical form in the neutral Ni^II, Pd^II, and Pt^II complexes (1, 2, and 3, respectively). The resulting systems are singlet diradical species, with a predicted J value in the range –1062 to –1764 cm^–1. These diradicals display an intense LLCT transition involving the diiminosemiquinone moieties: 860, 852, and 760 nm for 1, 2, and 3, respectively. Complex 1, 2, and 3 exhibit reversible oxidation waves in their CV curves. Cations 1⁺, 2⁺, and 3⁺ are (S = 1/2) systems with g values close to 2. The increase in the intensity of the resonance in the order 1⁺ < 2⁺, 3⁺ is consistent with the easy comproportionation of 1⁺. X‐ray diffraction analysis discloses that 3⁺ is a mixed‐valent radical mononuclear complex. An intense IVCT is observed at low energy (1536, 1656, and 1256 nm for 1⁺, 2⁺, and 3⁺, respectively), confirming that this classification applies to all the cations. Dications 1²⁺, 2²⁺, and 3²⁺ are closed‐shell bis(diiminobenzoquinone) species. The crystal structure of 3²⁺ shows that this complex exhibits the shortest mean C–N bond lengths within the series.

Three copper(II) complexes of the (R,R)‐N,N′‐bis(3,5‐di‐tert‐butyl‐2‐aminobenzylidene)‐1,2‐diaminocyclohexane ligand, namely [Cu(^NL)], [Cu(^NLH)]⁺ and [Cu(^NLH₂)]²⁺, were prepared and structurally characterized. In [Cu(^NLH₂)]²⁺ the copper ion lies in an octahedral geometry with the aniline groups coordinated in equatorial positions. In [Cu(^NL)] the anilines are deprotonated (anilido moieties) and coordinated to an almost square‐planar metal ion. Complex [Cu(^NL)] displays two oxidation waves at E_1/2^ox, 1=−0.14 V and E_1/2^ox, 2=0.36 V vs. Fc⁺/Fc in CH₂Cl₂. Complex [Cu(^NLH₂)]²⁺ displays an irreversible oxidation wave at high potential (1.21 V), but shows a readily accessible and reversible metal‐centered reduction at E_1/2^red=−0.67 V (Cu^II/Cu^I redox couple). Oxidation of [Cu(^NL)] by AgSbF₆ produces [Cu(^NL)](SbF₆), which was isolated as single crystals. X‐ray structure analysis discloses a contraction of the coordination sphere by 0.05 Å upon oxidation, supporting a metal‐centered process. Complex [Cu(^NL)](SbF₆) displays an intense NIR band at 1260 nm corresponding to an anilido‐to‐copper(III) charge transfer transition. This compound slowly evolves in CH₂Cl₂ solution towards [Cu(^NLH)](SbF₆), which is a copper(II) complex comprised of both anilido and aniline groups coordinated to the metal center. The copper(III) complex [Cu(^NL)](SbF₆) is an efficient catalyst for benzyl alcohol oxidation, with 236 TON in 24 h at 298 K, without additives other than oxygen and a base.

The sterically hindered bis(2-aminophenyl)dipyrrin ligand H₃^NL was prepared. X-ray diffraction discloses a bifurcated hydrogen bonding network involving the dipyrrin and one aniline ring. The reaction of H₃^NL with one equivalent of nickel(II) in the air produces a paramagnetic neutral complex, which absorbs intensively in the Vis-NIR region. Its electron paramagnetic resonance spectrum displays resonances at g₁ = 2.033, g₂ = 2.008, and g₃ = 1.962 that are reminiscent of an (S = 1/2) system having a predominant organic radical character. Both the structural investigation (X-ray diffraction) and density functional theory calculations on [Ni^II(^NL^•)] points to an unprecedented mixed “pyrrolyl-anilinyl” radical character. The neutral complex [Ni^II(^NL^•)] exhibits both a reversible oxidation wave at −0.28 V vs Fc⁺/Fc and a reversible reduction wave at −0.91 V. The anion was found to be highly air-sensitive, but could be prepared by reduction with cobaltocene and structurally characterized. It comprises a Ni(II) ion coordinated to a closed-shell trianionic ligand and hence can be formulated as [Ni^II(^NL)]⁻. The cation was generated by reacting [Ni^II(^NL^•)] with one equivalent of silver hexafluoroantimonate. By X-ray diffraction we established that it contains an oxidized, closed-shell ligand coordinated to a nickel(II) ion. We found that a reliable hallmark for both the oxidation state of the ligand and the extent of delocalization within the series is the bond connecting the dipyrrin and the aniline, which ranges between 1.391 Å (cation) and 1.449 Å (anion). The cation and anion exhibit a rich Vis-NIR spectrum, despite their nonradical nature. The low energy bands correspond to ligand-based electronic excitations. Hence, the HOMO–LUMO gap is small, and the redox processes in the electron transfer series are exclusively ligand-centered.

The electron-rich pincer ligand 1 has been synthesized and chelated to manganese. The octahedral Mn(IV) bis(diiminosemiquinonate) and Mn(V) (diiminobenzoquinone) (diiminosemiquinonate) radicals were structurally characterized.

The tetradentate ligand dppH₃, which features a half-porphyrin and two electron-rich phenol moieties, was prepared and chelated to manganese. The mononuclear Mn(III)-dipyrrophenolate complex 1 was structurally characterized. The metal ion lies in a square pyramidal environment, the apical position being occupied by a methanol molecule. Complex 1 displays two reversible oxidation waves at 0.00 V and 0.47 V vs. Fc⁺/Fc, which are assigned to ligand-centered processes. The one-electron oxidized species 1⁺ SbF₆⁻ was crystallized, showing an octahedral Mn(III) center with two water molecules coordinated at both apical positions. The bond distance analysis and DFT calculations disclose that the radical is delocalized over the whole aromatic framework. Complex 1⁺ SbF₆⁻ exhibits an S_tot = 3/2 spin state due to the antiferromagnetic coupling between Mn(III) and the ligand radical. The zero field splitting parameters are D = 1.6 cm⁻¹, E/D = 0.18(1), g_⊥ = 1.99 and g_∥ = 1.98. The dication 1²⁺ is an integer spin system, which is assigned to a doubly oxidized ligand coordinated to a Mn(III) metal center. Both 1 and 1⁺ SbF₆⁻ catalyze styrene oxidation in the presence of PhIO, but the nature of the main reaction product is different. Styrene oxide is the main reaction product when using 1, but phenylacetaldehyde is formed predominantly when using 1⁺ SbF₆⁻. We examined the ability of complex 1⁺ SbF₆⁻ to catalyze the isomerization of styrene oxide and found that it is an efficient catalyst for the anti-Markovnikov opening of styrene oxide. The formation of phenylacetaldehyde from styrene therefore proceeds in a tandem E–I (epoxidation–isomerization) mechanism in the case of 1⁺ SbF₆⁻. This is the first evidence of control of the reactivity for styrene oxidation by changing the oxidation state of a catalyst based on a redox-active ligand.