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 , bearing a redox-active DPP ligand (DPP = dipyrrin-bis(,-di--butylphenolato) with a pentafluorophenyl moiety on the -position. This square-planar complex was subjected to the coordination of tetrahydrofuran (THF), pyridine, BuNH, and AdNH (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 . This starting species is best described as an open-shell singlet complex containing a ligand radical that is antiferromagnetically coupled to a low-spin ( = /) cobalt(II) center. The redox-active DPP ligand plays a crucial role in stabilizing this complex and in its facile conversion to the triplet THF adduct and closed-shell singlet pyridine and amine adducts (L = py, BuNH, or AdNH). Coordination of the weak donor THF to changes the orbital overlap between the 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 ligand. In contrast, coordination of the stronger donors pyridine, BuNH, or AdNH induces metal-to-ligand single-electron transfer, resulting in the formation of low-spin ( = 0) cobalt(III) complexes containing a fully reduced ligand, thus explaining their closed-shell singlet electronic ground states.