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 the coordination of exogenous donors is investigated within the cobalt complex <b>[Co</b>^<b>II</b><b>(DPP·</b>^<b>2-</b><b>)]</b>, bearing a redox-active DPP^2- ligand (DPP = dipyrrin-bis(<i>o</i>,<i>p</i>-di-<i>tert</i>-butylphenolato) with a pentafluorophenyl moiety on the <i>meso</i>-position. This square-planar complex was subjected to the coordination of tetrahydrofuran (THF), pyridine, <i>t</i>BuNH₂, 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 <b>[Co</b>^<b>II</b><b>(DPP·</b>^<b>2-</b><b>)]</b>. This starting species is best described as an open-shell singlet complex containing a <b>DPP·</b>^<b>2-</b> ligand radical that is antiferromagnetically coupled to a low-spin (<i>S</i> = ¹/₂) cobalt(II) center. The redox-active DPP^<i>n</i>- ligand plays a crucial role in stabilizing this complex and in its facile conversion to the triplet THF adduct <b>[Co</b>^<b>II</b><b>(DPP·</b>^<b>2-</b><b>)(THF)</b>_<b>2</b><b>]</b> and closed-shell singlet pyridine and amine adducts <b>[Co</b>^<b>III</b><b>(DPP</b>^<b>3-</b><b>)(L)</b>_<b>2</b><b>]</b> (L = py, <i>t</i>BuNH₂, or AdNH₂). Coordination of the weak donor THF to <b>[Co</b>^<b>II</b><b>(DPP·</b>^<b>2-</b><b>)]</b> changes the orbital overlap between the <b>DPP·</b>^<b>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·</b>^<b>2-</b> ligand. In contrast, coordination of the stronger donors pyridine, <i>t</i>BuNH₂, or AdNH₂ induces metal-to-ligand single-electron transfer, resulting in the formation of low-spin (<i>S</i> = 0) cobalt(III) complexes <b>[Co</b>^<b>III</b><b>(DPP</b>^<b>3-</b><b>)(L)</b>_<b>2</b><b>]</b> containing a fully reduced <b>DPP</b>^<b>3-</b> ligand, thus explaining their closed-shell singlet electronic ground states.