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