The Formazanate Ligand as an Electron Reservoir: Bis(Formazanate) Zinc Complexes Isolated in Three Redox States

Abstract

Metal-mediated redox processes are of fundamental importance in a wide variety of bond formation and cleavage reactions. The utility of transition metal catalysts in this type of reactions stems from their ability to switch between two (or more) oxidation states. Recently, there has been increased interest in redox processes that do not occur at the metal centre, but instead take place within the ancillary ligand framework (so-called 'redox-active' or 'non-innocent' ligands). [1] The use of organic ligands as redox equivalents is of key importance in biological (enzymatic) transformations, [2] and has been shown to open new reactivity pathways in catalysis. [1c] The most studied ligands of this class are dithiolenes and dioxolenes, while recent work has focussed on α-diimines [3] and bis(imino)pyridines. [4] A class of ligands that has found widespread utility in the synthesis of metal complexes across the periodic table are the monoanionic β-diketiminates. [5] These are generally considered as stable ligand scaffolds without involvement in redox-chemistry. Recently, examples of β-diketiminate metal complexes were reported in which the ligand was either reduced to a metal-bound di-or trianion, [6] or oxidized to a neutral radical species. [7] However, the limited stability of β-diketiminate complexes upon changing oxidation state prevents widespread application. [8] Formazanates (1,2,4,5-tetraazapentadienyls), [9] which are close analogues of β-diketiminates, have received comparatively little attention as ligands in coordination chemistry. [10] The stability of organic 6-membered heterocyclic radicals (verdazyls) that are derived from formazans prompted us to explore the use of formazanates as potential redox-active ligands (Chart 1). Here we show that zinc complexes with formazanate ligands engage in remarkably facile and reversible redox-chemistry which allows the full characterization of bis(formazanate) L 2 Zn complexes in charge neutral, anionic or dianionic redox states.

Alkali Cation Effects on Redox-Active Formazanate Ligands in Iron Chemistry

“…The two [Na(THF) 3 ] + ions have interactions with the internal N2 atoms of the five membered chelates and with nearby phenyl groups. The N–Na distances of 2.373(3) Å and 2.372(3) Å are comparable to those observed for non-isomerized formazanates bound to B (2.35 – 2.39 Å) 28 and Zn (2.41 – 2.52 Å) 29 . The formazanate Fe–N bond lengths (2.006(3) – 2.067(3) Å) are all elongated compared to the solid state structure of 2 (1.935(5) – 1.958(5) Å), consistent with the higher coordination number of iron in 3-Na , which is four-coordinate in contrast to three-coordinate 2 .…”

“…Metal complexes with multiple redox-active ligands can display ligand-based mixed valence, which is often reflected in distinct differences in intraligand metric parameters. 29,30 Although the two formazanate ligands in 3-Na are not crystallographically related, there are no significant differences in bond lengths and angles (Table 1), indicating that both ligands are in the same oxidation state. Analysis of the intraligand bond lengths of the formazanate core shows an elongation of the N–N bonds (1.372(4) – 1.393(4) Å) compared to 1 (1.317(2) – 1.325(2) Å), similar but more pronounced to what is observed in 2 (1.377(6) and 1.345(6) Å), suggestive of a ligand-centered reduction (Chart 1).…”

Metal–Ligand Cooperation Facilitates Bond Activation and Catalytic Hydrogenation with Zinc Pincer Complexes

“… 30 − 32 Other examples with Group 13 metals have also been introduced and explored with similar application. 33 In the case of zinc, we are only aware of reports in which the ligand itself is implicated as a proton or electron reservoir, 34 , 35 and very recently, MLC was proposed to facilitate methanol activation en route to a characterized zinc methylcarbonate species in a novel CO 2 reduction process. 36 Nonetheless, direct observation of reversible H-X bond activation across any main group metal and ligand via aromatization/dearomatization is unknown, and main group metal–ligand cooperation remains an underexplored area.…”

Reactivity of Two-Electron-Reduced Boron Formazanate Compounds with Electrophiles: Facile N–H/N–C Bond Homolysis Due to the Formation of Stable Ligand Radicals

“…Intrigued by work from the Hicks group on formazanate ligands as nitrogen-rich, redox-active analogues of the well-known β-diketiminates, 26 our group has started a research program to explore the coordination chemistry, redox behavior, and reactivity of complexes with formazanate ligands. 27 Although some early reports on related complexes exist, 28 , 29 it is only recently that interest in this class of ligands has gained momentum. Concurrent with our work, the Gilroy group and others have reported related complexes with formazanate ligands and studied the properties of these compounds 30 , 31 and materials derived thereof.…”