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 [CoII(DPP•2‒)], bearing a redox-active DPP2‒ 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 [CoII(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 DPPn‒ ligand plays a crucial role in stabilizing this complex, and in its facile conversion to the triplet THF-adduct [CoII(DPP•2‒)(THF)2] and closed-shell singlet pyridine and amine adducts [CoIII(DPP3‒)(L)2] (L = py, tBuNH2 or AdNH2). Coordination of the weak donor THF to [CoII(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 [CoIII(DPP3‒)(L)2] containing a fully reduced DPP3‒ ligand, thus explaining their closed-shell singlet electronic ground states.