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]aneNS(2)-CH3)(2)](2+) ([9]aneNS(2)-CH3=N-methyl-1-aza-4,7-dithiacyclononane), [Ni(bis[9]aneNS(2)-C2H4)](2+) (bis[9]aneNS(2)-C2H4=1,2-bis-(1-aza-4,7-dithiacyclononylethane) and [Ni([9]aneS(3))(2)](2+) ([9]aneS(3)=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]aneNS(2)-CH3)(2)](ClO4)(6)center dot(H5O2)(3) reveals an octahedral co-ordination at the Ni centre, while the crystal structure of [Ni-III(bis[9]aneNS(2)-C2H4)](ClO4)(6)center dot(H3O)(3)center dot 3H(2)O exhibits a more distorted coordination. In the homoleptic analogue, [Ni-III([9]aneS(3))(2)](ClO4)(3), structurally characterized at 30 K, the Ni-S distances [2.249(6), 2.251(5) and 2.437(2) angstrom] 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 (0.2M NBu4PF6, 293 K) at E-1/2 = +1.10 V versus Fc(+)/Fc assigned to a formal Ni-III/Ni-II couple. [Ni(bis[9]aneNS(2)-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% Ni-61-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]aneNS(2)-CH3)(2)](3+), [Ni(bis[9]aneNS(2)-C2H4)](3+) and [Ni([9]aneS(3))(2)](3+), respectively, consistent with DFT calculations, and reflecting delocalisation of charge onto the S-thioether centres. EPR spectra for [Ni-61([9]aneS(3))(2)](3+) are consistent with a dynamic Jahn-Teller distortion in this compound.