Metal–Ligand Redox Cooperativity in Cerium Amine-/Amido-Phenolate-Type Complexes

Abstract

The synthesis and characterization of two cerium complexes of redox-active amine/amido-phenolate-type ligands are reported. A tripodal framework comprising the tris(2-(3′,5′-di-tertbutyl-2′-hydroxyphenyl)amino-phenyl) amine (H 6 Clamp) proligand was synthesized for comparison of its cerium complex with a potassium−cerium heterobimetallic complex of the 4,6-di-tert-butyl-2- [(2,6-diisopropylphenyl)imino]quinone ( dipp ap) proligand. Structural studies indicate differences in the cerium(III) cation coordination spheres, where Ce III (CH 3 CN) 1.5 (H 3 Clamp) (1-Ce(H 3 Clamp)) exhibits shorter Ce−O distances and longer Ce−N bond distances compared to the analogous distances in K 3 (THF) 3 Ce III ( dipp ap) 3 (2-Ce(ap)), due to the gross structural differences between the systems. Differences are also evident in the temperature-dependent magnetic properties, where smaller χT products were observed for 2-Ce(ap) compared to 1-Ce(H 3 Clamp). Solution electrochemical studies for the complexes were interpreted based on ligand-and metal-based oxidation events, and the cerium(III) oxidation of 2-Ce(ap) was observed to be more facile than that of 1-Ce(H 3 Clamp), behavior that was cautiously attributed to the rigidity of the encrypted 1-Ce(H 3 Clamp) complex compared to the heterobimetallic framework of 2-Ce(ap). These results contribute to the understanding of how ligand designs can promote facile redox cycling for cerium complexes of redox-active ligands, given the large contraction of cerium−ligand bonds upon oxidation.

Multi‐electron Oxidation of Ce(III) Complexes Facilitated by Redox‐Active Ligands

“…The CÀ O and C=N bond lengths are similar to those of previously published Th( dipp isq) 3 Cl, indicating single and double bond character respectively; [15] the CÀ O bond length of Th( dipp isq) 3 Cl is 1.306(5) Å while for 1-Ceisq, it is 1.294(5)-1.296(5) Å. The C=N bond for Th( dipp isq) 3 Cl is 1.342(5) Å as compared to 1.340(5)-1.351(4) Å for [14,15] CeÀ O single bonds average at about 2.45 Å and the CeÀ O bond for 1-Ceisq is less than that at 2.306(3)-2.329(3) Å, indicating an anionic Ce-O [17][18][19][20] CeÀ N bond of Ce-N=C complexes average at around 2.48 Å and the CeÀ N bond for 1-Ceisq is significantly longer than that at 2.577(3)-2.601(3) Å indicating a dative Ce-N. [19][20][21][22] From these crystallographic data, it can be inferred that there are three iminosemiquinone ligands around the cerium for 1-Ceisq similar to the published Th( dipp isq) 3 Cl complex. Thus, one electron oxidation of the amidophenolate has occurred three times to generate 1-Ceisq.…”

“…[16] Our group, along with Schelter and co-workers, recently reported the synthesis of Ce III ( dipp ap) 3 K 3 (2-Ceap), generated from reduction of CeCl 3 in the presence of iminoquinone ligand. [17] This species is paramagnetic due to the Ce III , 4f 1 centre. Structural data were reported and show that the potassium ions are situated within the tris(ligand) framework.…”

Ligand–Metal Complementarity in Rare-Earth and Actinide Chemistry

“…Cerium is unique among rare earths in being able to access the tetravalent oxidation state across a wide range of ligand types and under both aerobic and anhydrous conditions. In their contribution, the groups of Bart and Schelter explored the redox chemistry of cerium with the noninnocent amidophenolate ligand “Clamp” (H 6 Clamp = {N­(C 6 H 4 -2-NHC 6 H 2 -2-OH-3,5- t Bu) 3 }), which is heptadentate and anchored with a basal amine group, and also the simple bidentate ligand, “ap” (H 2 ap = {HOC 6 H 2 -2-(NHDipp)-4,6- t Bu}; Dipp = 2,6-di- iso -propylphenyl) . The groups found that Ce III complexes with both ligands exhibit metal- and ligand-centered redox events, which may help in the design of molecules that can engage in multielectron reactivity using metals, such as cerium, that are not conventionally able to do so.…”

Assembly of a trapped valent CeIII/IV–TCNQ complex through metal–ligand redox cooperativity

“…Among organometallic lanthanide complexes, cerium offers relatively mild and tunable Ce III/IV oxidation potentials, 19,20 and isolable tetravalent compounds. 21–26 Tris(cyclopentadienyl) cerium( iii ) complexes have previously been shown to be a useful platform to access organocerium( iv ) compounds: Cp 3 Ce–X, X = –halides, –alkoxides, and –aryloxides.…”