Stepwise transformation of a redox-active tetradentate ONNO ligand in the coordination sphere of tin(iv)†

New diorganotin(IV) complexes based on a redox-active tetradentate ONNO ligand, namely N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,2-phenylenediamine (LH₄), were synthesized. A methodology for the stepwise oxidation of tin(IV)-ONNO derivatives with a stoichiometric amount of p-benzoquinone was developed. This allowed the preparation of tin(IV) complexes containing the ligand in the tetraanionic doubly deprotonated form, in the dianionic state, and a controlled intraligand cyclization to be carried out. It has been found that the diorganotin(IV) compounds with the dianionic form of the ONNO ligand have a ground singlet spin state, but an increase in temperature leads to partial populating of the triplet state and a transition from the diamagnetic form of these complexes to the paramagnetic biradical ones. The UV-vis-NIR spectrum of L²⁻SnPh₂ shows an intense absorption band in the range of 600–1400 nm, corresponding to the intraligand charge transfer (ILCT). The coordination ability of the solvent strongly influences the position of this solvatochromic band. Cyclic voltammetry revealed that this compound undergoes two sequential reversible single-electron oxidations and two sequential reversible single-electron reductions. This cyclic voltammetry remains unchanged even after 100 cycles have been performed.