A hybrid bioinspired catechol-alloxazine triangular nickel complex stabilizing protons and electrons

Abstract

A new class of redox-active ligands merging catechol and alloxazine structures is reported. A trimetallic triangular complex is formed upon complexation to nickel.

A Single Bioinspired Hexameric Nickel Catechol–Alloxazine Catalyst Combines Metal and Radical Mechanisms for Alkene Hydrosilylation

“…Due to its distinctive structure, proligand 6 bears two sites for metal coordination: a catechol O,O site and an alloxazine N,O site. We have reported the preparation of an unusual Ni 3 L 3 triangular structure upon coordination to nickel of a closely related ligand with methyl instead of benzyl groups [18] . This complex can stabilize protons and electrons on its structure, thus establishing this hybrid catechol‐alloxazine motif as a redox‐active ligand.…”

“…These observations hint towards a semiquinone (SQ) redox state for the alloxazine subunit. It is worth noting that the two binding sites are not independent and that delocalisation occurs over the whole ligand through its flat structure [18] …”

“…We have reported the preparation of an unusual Ni 3 L 3 triangular structure upon coordination to nickel of a closely related ligand with methyl instead of benzyl groups. [18] This complex can stabilize protons and electrons on its structure, thus establishing this hybrid catechol-alloxazine motif as a redox-active ligand. However, the low solubility of the triangular complex precludes any applications in homogeneous catalysis, which led us to investigate alternative structures, the benzyl groups were introduced to prevent the stacking observed with the triangular complex.…”

“…They can promote bis-electronic steps at first-row transition metals [61] but also sustain outer-sphere SET to generate radicals. [62][63][64][65][66] We show here that a new class of bioinspired catechol-alloxazine redox-active ligands [18] can promote a dual mechanism involving outer-sphere and inner-sphere reactivities.…”

“…It is worth noting that the two binding sites are not independent and that delocalisation occurs over the whole ligand through its flat structure. [18] The UV-vis absorption properties of 6 and 7 in DMSO (Figure S7) show that the broad absorption band of proligand 6 at 400 nm (ɛ = 2.0 × 10 4 M À 1 cm À 1 ) undergoes a bathochromic shift upon metal complexation, yielding a sharper absorption band at 454 nm for 7 (ɛ = 1.7 × 10 5 M À 1 cm À 1 ). The hexameric structure of complex 7 was further confirmed by highresolution mass spectrometry (HRMS), elemental analysis, and the stability of the structure in solution was ascertained by DOSY experiments (see below).…”