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

The Ni^II complexes Ni(9]aneNS₂‐CH₃)₂]²⁺ (9]aneNS₂‐CH₃=N‐methyl‐1‐aza‐4,7‐dithiacyclononane), Ni(bis9]aneNS₂‐C₂H₄)]²⁺ (bis9]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 Ni^III products, which have been characterized by X‐ray crystallography, UV/Vis and multi‐frequency EPR spectroscopy. The single‐crystal X‐ray structure of Ni^III(9]aneNS₂‐CH₃)₂](ClO₄)₆?(H₅O₂)₃ reveals an octahedral co‐ordination at the Ni centre, while the crystal structure of Ni^III(bis9]aneNS₂‐C₂H₄)](ClO₄)₆?(H₃O)₃? 3H₂O exhibits a more distorted co‐ordination. In the homoleptic analogue, Ni^III(9]aneS₃)₂](ClO₄)₃, structurally characterized at 30 K, the Ni? 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 E_1/2=+1.10 V versus Fc⁺/Fc assigned to a formal Ni^III/Ni^II couple. Ni(bis9]aneNS₂‐C₂H₄)](PF₆)₂ exhibits a one‐electron oxidation process at E_1/2=+0.98 V and a reduction process at E_1/2=?1.25 V assigned to Ni^II/Ni^III and Ni^II/Ni^I 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(bis9]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.