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Redox Non-Innocence of Thioether Crowns: Spectroelectrochemistry and Electronic Structure of Formal Nickel(III) Complexes of Aza–Thioether Macrocycles

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Abstract

The NiII complexes [Ni([9]aneNS2-CH3)2]2+ ([9]aneNS2-CH3=N-methyl-1-aza-4,7-dithiacyclononane), [Ni(bis[9]aneNS2-C2H4)]2+ (bis[9]aneNS2-C2H4=1,2-bis-(1-aza-4,7-dithiacyclononylethane) and [Ni([9]aneS3)2]2+ ([9]aneS3=1,4,7-trithiacyclononane) have been prepared and can be electrochemically and chemically oxidized to give the formal NiIII products, which have been characterized by X-ray crystallography, UV/Vis and multi-frequency EPR spectroscopy. The single-crystal X-ray structure of [NiIII([9]aneNS2-CH3)2](ClO4)6⋅(H5O2)3 reveals an octahedral co-ordination at the Ni centre, while the crystal structure of [NiIII(bis[9]aneNS2-C2H4)](ClO4)6⋅(H3O)3⋅ 3H2O exhibits a more distorted co-ordination. In the homoleptic analogue, [NiIII([9]aneS3)2](ClO4)3, structurally characterized at 30 K, the Ni[BOND]S distances [2.249(6), 2.251(5) and 2.437(2) Å] are consistent with a Jahn–Teller distorted octahedral stereochemistry. [Ni([9]aneNS2-CH3)2](PF6)2 shows a one-electron oxidation process in MeCN (0.2 M NBu4PF6, 293 K) at E1/2=+1.10 V versus Fc+/Fc assigned to a formal NiIII/NiII couple. [Ni(bis[9]aneNS2-C2H4)](PF6)2 exhibits a one-electron oxidation process at E1/2=+0.98 V and a reduction process at E1/2=−1.25 V assigned to NiII/NiIII and NiII/NiI couples, respectively. The multi-frequency X-, L-, S-, K-band EPR spectra of the 3+ cations and their 86.2 % 61Ni-enriched analogues were simulated. Treatment of the spin Hamiltonian parameters by perturbation theory reveals that the SOMO has 50.6 %, 42.8 % and 37.2 % Ni character in [Ni([9]aneNS2-CH3)2]3+, [Ni(bis[9]aneNS2-C2H4)]3+ and [Ni([9]aneS3)2]3+, respectively, consistent with DFT calculations, and reflecting delocalisation of charge onto the S-thioether centres. EPR spectra for [61Ni([9]aneS3)2]3+ are consistent with a dynamic Jahn–Teller distortion in this compound.

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