Redox non-innocence of thioether crowns: Spectroelectrochemistry and electronic structure of formal nickel(III) complexes of aza-thioether macrocycles

The NiII complexes [Ni([9]aneNS2-CH3) 2]2+ ([9]aneNS2-CH3=N-methyl-1-aza- 4,7-dithiacyclononane), [Ni(bis[9]aneNS2-C2H 4)]2+ (bis[9]aneNS2-C2H 4=1,2-bis-(1-aza-4,7-dithiacyclononylethane) and [Ni([9]aneS 3)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)](ClO 4)6·(H3O)3· 3H 2O exhibits a more distorted co-ordination. In the homoleptic analogue, [NiIII([9]aneS3)2](ClO 4)3, structurally characterized at 30 K, the Nii-S distances [2.249(6), 2.251(5) and 2.437(2) Å] are consistent with a Jahn-Teller distorted octahedral stereochemistry. [Ni([9]aneNS 2-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-C 2H4)](PF6)2 exhibits a one-electron oxidation process at E1/2=+0.98 V and a reduction process at E 1/2=-1.25 V assigned to NiII/NiIII and Ni II/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. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.