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

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 H 4 Sar (AP/AP) ligand during the complexation reaction with Co( ii ) and Mn( ii ) salts in the presence of air and Et 3 N. 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. The geometry and electronic structures of the Co and Mn complexes of the pincer H 3 L ONS ligand composed of both hard and soft donor atoms at the coordinating sites are reported.