Investigation of metal identity on the structure and electronic properties of dinuclear Mn and Co complexes with triaryl tetradentate ligands

Abstract

We report dinuclear Mn(II) and Co(II) complexes supported by triaryl tetradentate ligands derived from o-phenylenediamide that are flanked by different metal-donor substituents (X = NMe₂ vs. SMe). Single-crystal XRD data revealed that the Mn complexes (Mn-1 and Mn-2) both possess Mn₂N₂ diamond cores with relatively similar bond distances, electronic structures, and magnetic properties regardless of the ligand identity. The Co complexes, by contrast, revealed dramatic substituent-dependent differences. Like Mn, the Co complexes were dinuclear, but their core structures varied from open (X = NMe₂; no μ-N bridging; Co-1a) to closed (X = SMe; intact Co₂N₂ diamond core; Co-2), with a second structure isolated with X = NMe₂ in between (semi-open core; Co-1b). Of the three structures, Co-2 had the shortest metal–metal distance of 2.4540(8) Å, just at the onset of that expected for Co–Co bonding. Evidence of an appreciable metal–metal interaction in Co-2 was revealed with a unique UV-vis absorption at 516 nm that was assigned to metal–metal charge transfer (MMCT). Moreover, magnetic measurements conducted on Co-2 revealed a magnetic moment of 1.2μ_B at room temperature, which was much lower than that of other Co and Mn complexes. Active space calculations corroborated the experimental observations and suggested that Co-2 possesses a weak metal–metal bond with a low effective bond order of 0.24. These findings, which are compared to those previously reported for Fe(II) and Cr(II) complexes with the same ligands, reveal the marked influence that metal identity has on the structures, magnetic properties, and metal–metal bonding within this family of triaryl tetradentate ligands.