Characterization of the Ligand Field in Pseudo‐Octahedral Ni(II) Complexes of Pincer‐Type Amido Ligands: Magnetism, Redox Behavior, Electronic Absorption and High‐Frequency and ‐Field EPR Spectroscopy

High-frequency and -field electron paramagnetic resonance spectroscopy (HFEPR), magnetometry, and electronic absorption spectroscopy were used to determine the ligand field parameters in pseudo-octahedral Ni(II) complexes of diarylamido pincer-type amido ligands. These findings are discussed in the broader context of 3d transition metal coordination complexes of weak-field π-donor ligands.

Abstract

The description of π-donor amido moieties as ‘weak-field’ ligands can belie the influence of metal-ligand covalency on the overall ligand field of coordination complexes, which can in turn influence properties including the magnetic ground state and those of their excited states. In this contribution, the ligand fields of pseudo-octahedral Ni(II) complexes supported by diarylamido pincer-type amido ligands – three previously reported examples supported by asymmetric (2-R-phenanthridin-4-yl)(8-quinolinyl)amido ligands (R = Cl, CF₃, tBu; ^R L¹ ) along with a new congener bearing a symmetric bis(8-quinolinyl)amido ligand (BQA; L² ) – were investigated in two ways. First, high-frequency and -field electron paramagnetic resonance spectroscopy (HFEPR), SQUID magnetometry, and electronic absorption spectroscopy were used to determine the ligand field parameters. Second, the ability to electrochemically address ligand-based oxidations despite metal-centered SOMOs in the parent S=1 paramagnets was investigated, supported by time-dependent density functional theory (TDDFT) identification of strong intervalence charge-transfer (IVCT) transitions attributed to electronic communication between two N_amido moieties mediated by a Ni(II) bridge. These findings are discussed in the broader context of 3d transition metal coordination complexes of weak-field π-donor ligands.

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