Ligand-Mediated Spin State Changes in a Cobalt-Dipyrrin-Bisphenol Complex

ABSTRACT: The influence of a redox-active ligand on spin changing events induced by coordination of exogenous donors is investigated within the cobalt complex [Co^II(DPP^•2‒)], bearing a redox-active DPP^2‒ ligand (DPP = dipyrrin-bis-(o,p-di-tert-butylphenolato) with a pentafluorophenyl moiety on the meso-position. This square planar complex was subjected to coordination of THF, pyridine, tBuNH₂ and AdNH₂ (Ad = 1‑adamantyl), and the resulting complexes were analyzed with a variety of experimental (XRD, NMR, UV-Vis, HRMS, SQUID, Evans’ method) and computational (DFT, NEVPT2-CASSCF) techniques to elucidate the respective structures, spin states and orbital compositions of the corresponding octahedral bis-donor adducts, relative to [Co^II(DPP^•2‒)]. This starting species is best described as an open-shell singlet complex containing a DPP^•2‒ ligand radical that is antiferromagnetically coupled to a low-spin (S = ½) cobalt(II) center. The redox-active DPP^n‒ ligand plays a crucial role in stabilizing this complex, and in its facile conversion to the triplet THF-adduct [Co^II(DPP^•2‒)(THF)₂] and closed-shell singlet pyridine and amine adducts [Co^III(DPP^3‒)(L)₂] (L = py, tBuNH₂ or AdNH₂). Coordination of the weak donor THF to [Co^II(DPP^•2-)] changes the orbital overlap between the DPP^•2‒ ligand radical π-orbitals and the cobalt(II) metalloradical d-orbitals, which results in a spin-flip to the triplet ground state without changing the oxidation states of the metal or DPP^•2‒ ligand. In contrast, coordination of the stronger donors pyridine, tBuNH₂ or AdNH₂ induces metal-to-ligand single-electron transfer, resulting in formation of low-spin (S = 0) cobalt(III)-complexes [Co^III(DPP^3‒)(L)₂] containing a fully reduced DPP^3‒ ligand, thus explaining their closed-shell singlet electronic ground states.