Bis-Homoleptic Metal Complexes of a Tridentate Ligand with a Central Anionic Sulfonamide Donor

Redox-active manganese, iron, and nickel complexes of pyridin-2-ylsulfonyl-quinolin-8-yl-amide (psq) provide information for assessing the electronic and structural properties of this new tridentate ligand. Single-crystal X-ray structures show that psq coordinates in a meridional mode with a trigonal geometry for the central deprotonated sulfonamide N donor. With the structures described here, there are now five structures known for hexacoordinated bis-homoleptic complexes of psq. All show the same geometry. No fac isomer, although feasible, has been structurally characterized. The geometrical parameters for [M(psq)₂]^0/+ are surprisingly close to those for archetypical [M(terpy)₂]^2+/3+ (terpy =2,2':6',2″-terpyridine) complexes, with octahedral distortion parameters indicating a geometry that is slightly closer to a regular octahedral. The Fe(II) complex, however, bucks this trend, consistent with the magnetic susceptibility measurements indicating a high-spin S = 5/2 state, which stands in contrast to low-spin [Fe(terpy)₂]²⁺. This is rationalized by the trans secondary sulfonamide donors being weaker π acceptors compared to central terpy pyridine donors. An overall two-integer reduced charge for the complexes is consistent with the Co^II/Co^I, M^III/M^II M = Mn, Fe, Co, and Mn^IV/Mn^III redox events being ca. 600-900 mV more cathodic compared to the corresponding events for [M(terpy)₂]²⁺.