Where is the unpaired electron density? A combined experimental and theoretical finding on the geometric and electronic structures of the Co(iii) and Mn(iv) complexes of the unsymmetrical non-innocent pincer ONS ligand

Abstract

The tridentate pincer ligand [L^ONS]³⁻ with the ONS donor set was generated in situ by cleaving the disulfane linkage of the pristine redox-active H₄Sar^(AP/AP) ligand during the complexation reaction with Co(II) and Mn(II) salts in the presence of air and Et₃N. X-ray crystal structure analysis of the Co complex (1) and Mn complex (2) revealed that both the complexes were neutral in charge and six-coordinate with the meridional coordination of the two pincer [L^ONS]ⁿ⁻ ligands. The Co ion was in the trivalent state, while the Mn ion was in the tetravalent state. Thus, the generated two pincer [L^ONS]³⁻ ligands were non-innocent and cumulatively were in trinegative and tetranegative charges for the respective complexes. The intraligand bond distances of the coordinating ligands in each complex were similar, implying the same oxidation/electronic structure of the two ligating units. Variable-temperature magnetic susceptibility measurements revealed an S = 1/2 ground state for each complex. X-band EPR measurements unambiguously established the presence of a ligand-based unpaired electron in complex 1, and in complex 2, the unpaired electron was at the Mn centre. DFT-based theoretical calculations suggested the three-electron oxidation of the two ligating units in complex 1. Two iminosemiquinone radicals were of opposite spins (α-spin and β-spin) and a thiyl radical in either α-spin or β-spin was delocalized between two sulfur atoms. Thus, the antiferromagnetic coupling among the two opposite spins provided an S = 1/2 ground state and resulted in the radical-based EPR spectrum. In complex 2, each ligating pincer unit contained an iminosemiquinone radical that interacted antiferromagnetically with the Mn(IV)-based three unpaired electrons. This rendered a doublet ground state with the residual electron density located at the Mn center.