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 [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, t BuNH 2 , and AdNH 2 (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 = 1 / 2 ) 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, t BuNH 2 , or AdNH 2 ). 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, t BuNH 2 , or AdNH 2 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 ] conta...

Switching from Metal‐ to Ligand‐Based Oxidation in Cobalt Complexes with Redox‐Active Bisguanidine Ligands

“…Calculations were carried out without inclusion of the solvent (relative permittivity ϵ r of 1), and also with inclusion of a solvent with a relative permittivity ϵ r of 37.5 (close to the value of CH 3 CN), using the conductor‐like screening model (COSMO). Of course, it would be desirable to treat such open‐shell molecules (e. g. cobalt complexes[ 30 , 62 ]) with multireference methods (such as CASSCF). However, multireference calculations are not simple for molecules of that size.…”

“…Hence, dinuclear cobalt complexes with bridging tetraoxolene ligands were the first compounds showing redox‐induced intramolecular electron transfer (RIET), leading to the reduction of the metal atom upon overall oxidation of the complex [27–29] . Recently, it has been shown that the additional coordination of a ligand could trigger spin‐flip or metal‐to ligand single‐electron transfer in cobalt complexes with a dipyrrin‐biphenol ligand [30] …”

“…[ 27 , 28 , 29 ] Recently, it has been shown that the additional coordination of a ligand could trigger spin‐flip or metal‐to ligand single‐electron transfer in cobalt complexes with a dipyrrin‐biphenol ligand. [30] …”

Controlling Radical-Type Single-Electron Elementary Steps in Catalysis with Redox-Active Ligands and Substrates

“…Solvation of this complex in the weak sp 3 -hybrid donor THF quantitatively affords the octahedral complex [Co II (DPP •2– )(THF) 2 ], which has a triplet spin and again a low-spin cobalt(II) center and ligand-centered radical ( Figure 1 ). 112 The ligand-centered α-spin electron mainly resides in the strongly correlated antibonding combinations of the d yz orbital with the ligand pyrrole π-framework (d yz – Pyr L π ) and the complete ligand π-system (L π –d yz ). The latter is more delocalized over the ligand and bears the major amount of spin density to prevent electron–electron repulsion and thereby enhance radical stabilization.…”

“…Axial coordination of the stronger donors (L) pyridine (σ-donor, weak π-acceptor), t BuNH 2 , or AdNH 2 (σ-donors, Ad = 1-adamantyl) affords the octahedral complexes [Co III (DPP 3– )(L) 2 ], which all reside in the closed-shell singlet state ( Figure 1 ). 112 The major destabilization of the cobalt-centered d z 2 orbital due to coordination of these strong donors raises the energy of this orbital to such an extent that intramolecular metal-to-ligand single-electron transfer occurs. Consequently, cobalt adopts a low-spin +III oxidation state, and the ligand is reduced to the closed-shell trianionic state.…”