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 <b>[Co^II(DPP^•2‒)]</b>, bearing a redox-active <b>DPP^2‒</b> ligand (DPP = dipyrrin-bis-(<i>o,p</i>-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 <b>[Co^II(DPP^•2‒)]</b>. This starting species is best described as an open-shell singlet complex containing a <b>DPP^•2‒</b> ligand radical that is antiferromagnetically coupled to a low-spin (S = ½) cobalt(II) center. The redox-active <b>DPP^n‒</b> ligand plays a crucial role in stabilizing this complex, and in its facile conversion to the triplet THF-adduct <b>[Co^II(DPP^•2‒)(THF)₂]</b> and closed-shell singlet pyridine and amine adducts <b>[Co^III(DPP^3‒)(L)₂]</b> (L = py, tBuNH₂ or AdNH₂). Coordination of the weak donor THF to <b>[Co^II(DPP^•2-)]</b> changes the orbital overlap between the <b>DPP^•2‒</b> 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 <b>DPP^•2‒</b> 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 <b>[Co^III(DPP^3‒)(L)₂]</b> containing a fully reduced <b>DPP^3‒</b> ligand, thus explaining their closed-shell singlet electronic ground states.