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

The influence of a redox-active ligand on spin-changing events induced by the 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 the coordination of tetrahydrofuran (THF), pyridine, tBuNH₂, and AdNH₂ (Ad = 1-adamantyl), and the resulting complexes were analyzed with a variety of experimental (X-ray diffraction, NMR, UV–visible, high-resolution mass spectrometry, superconducting quantum interference device, Evans’ method) and computational (density functional theory, 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 the 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.