X-ray structure of a Ni(II)–tri-phenoxyl radical complex

International audienceThe diimino-diphenolato neutral square-planar Ni(II) complex, NiL 2 , is readily oxidised with 2 equiv. of Ag[SbF 6 ], to produce an unprecedented octahedral Ni(II) tris(phenoxyl) radical complex, [Ni(L •) 3 ]-[SbF 6 ] 2. This study reveals, for the first time, the X-ray structure of a metal–tri-phenoxyl radical complex. In the last two decades, inspired by the unique Cu(II)–tyrosyl radical moiety of the active site of galactose oxidase (GO) 1 [a fungal enzyme that catalyses the aerobic two-electron oxidation of a wide range of primary alcohols to their corresponding aldehydes], chemists have been successful in generating and characterising phenoxyl radical complexes of Cu(II) and other transition metals such as Fe(III), Zn(II), Co(II/III), and Ni(II). 2 However, isolated persistent phenoxyl radical complexes are still rare, 3 and only a few X-ray structures have been reported. 4–7 Thus, the isolation and exploration of transition metal compounds containing one (or more) phe-noxyl radical ligand(s) with the desired catalytic or magnetic properties still remain a significant challenge. In particular, compounds that possess two and/or three phenoxyl radical ligands are known to be highly unstable. 2f,8 In the continuous search for a suitable ligand framework capable of sustaining a phenoxyl radical state, we have recently designed 5,9 a versatile N,O-phenol-imidazole/pyrazole pro-ligand family that incorporates: (a) t-Bu protection of the phenol ortho-and para-positions, preventing radical coupling decomposition pathways, and (b) no other oxidisable position than the phenol(ate) moiety itself. These ligand frameworks have allowed tetracoordinated M(II)– (M = Cu, Zn, and Co) and octahedral Co(III)–mono-phenoxyl radical complexes to be isolated as air-stable crystalline powders. 5,9a,b Herein, we report, using the phenol-pyrazole pro-ligand LH 9c (Scheme 1), the synthesis, characterisation and X-ray structure of an unprecedented octahedral Ni(II) tri-(phenoxyl) radical complex, [Ni(L •) 3 ] 2+ (2 2+); produced by an unusual two-electron chemical oxidation of the parent Ni II L 2 phenolate complex (1) (Scheme 1). The reaction of LH with [Ni(H 2 O) 6 ][BF 4 ] 2 in methanol in a 2 : 1 ratio in the presence of triethylamine, affords a pale-green NiL 2 compound (1) (see the ESI †). The X-ray structure of 1 (Fig. 1, Tables 1 and SI1–3 †) is isostructural to that of neutral CuL 2 9c,d displaying a neutral centrosymmetric trans-N 2 O 2 square-planar geometry, resulting from the coordination of two N,O-ligands in their anionic forms. The Ni–O and Ni–N bond distances (1.869(2) Å and 1.850 (2) Å respectively) are as expected for Ni(II)-phenolato-imino complexes in an N 2 O 2 environment. 2 The planar structure of 1 is reinforced by two intramolecular N–H⋯O hydrogen bonds between the pyrazole N–H and the phenolate-O atoms (N⋯O distances of 2.717(2) Å, angle of 121°; Fig. 1). As expected for low-spin, d 8 , square planar Ni(II) ions, complex 1 is diamagnetic and exhibits a well resolved 1 H NMR spectrum in CDCl 3 , displaying one set of resonances for the two Scheme 1 † Electronic supplementary information (ESI) available. CCDC 1005503 and 1005504 of 1 and 2. For ESI and crystallographic data in CIF or other electronic format se

Nano-encapsulation: overcoming conductivity limitations by growing MOF nanoparticles in meso-porous carbon enables high electrocatalytic performance

MOFs uniquely combine metal-atom centers and developed organic-based structures. Both features are attractive for catalysis. However, their isolating nature prevents them from effective use in electrocatalysis processes. Modifying the chemical structure to gain electric conductivity often harms its natural advantages. In this study, Borenstein et al. present a new approach to overcoming the non-conductivity of MOF b growing MOF nanoparticles in a conductive carbon host. The host’s porosity controls the MOF nanoparticles’ size and their electric properties while preserving their structure. As a result, the composition efficiently electro-catalyzes carbon dioxide into formic acid at low overpotentials