3d Metal Complexes Supported by a Bis(imidazolin‐2‐imino)pyridine Pincer Ligand – Synthesis, Structural Characterisation, and Magnetic Properties

Abstract

AbstractThe coordination chemistry of the tridentate pincer ligand TLtBu, which contains two electron‐rich imidazolin‐2‐imine moieties, has been explored. The reaction of TLtBu with metal dihalides gave complexes of the general formula [(TLtBu)MX2] (M = Mn, Fe, Co, Ni; X = Cl, Br). These complexes were characterized by X‐ray diffraction analysis, which showed them to have different geometries in the solid state. Although [(TLtBu)MnCl2] (1) is best described as distorted square‐pyramidal, the complexes [(TLtBu)FeCl2] (2) and [(TLtBu)CoCl2] (3) show distorted tetrahedral geometries with the TLtBu ligand bound in a κ2 fashion and one of its imidazolin‐2‐imine fragments not coordinated to the metal atom. [(TLtBu)NiBr]Br (4) crystallizes with a slightly distorted square‐planar cation. The reaction of the triflate (OTf) salts [M(OTf)2·nMeCN] (M = Mn, n = 1; M = Fe, Co, n = 2; OTf = CF3SO3) with TLtBu resulted in [(TLtBu)M(OTf)][OTf] (M = Mn, 5; M = Fe, 6) and [(TLtBu)Co(NCMe)][OTf]2 (7). Complexes 6 and 7 were characterized by X‐ray diffraction analysis, and both showed distorted square‐planar geometries. Magnetic measurements of 5, 6 and 7 showed them to possess a S = 5/2, 2 and 1/2 ground states, respectively. The bromide ions in complex 4 were exchanged by reaction with AgBF4 to afford [(TLtBu)Ni(NCMe)][BF4]2 (8), which showed a distorted square‐planar geometry in the solid state. The reactions of 1 and 6 with AgOTf resulted in the oxidation of the complexes and the formation of [(TLtBu)MnCl(OTf)][OTf] (9) and [(TLtBu)Fe(OTf)2][OTf] (10), respectively. Complexes 9 and 10 both display distorted square‐pyramidal geometries in the solid state. The ground state of 10 was found to be S = 5/2 by superconducting quantum interference device (SQUID) magnetometry measurements. The oxidation of 5, 6 and 7 with the iodosobenzene derivative (tBuSO2)C6H4IO was investigated. Although the reactions of 6 and 7 gave no evidence for the formation of transient oxo species, crystals were obtained from the reaction of 5. These crystals showed the formation of [TLtBu*Mn(OH)](OTf)2 (11), in which one of the tBu groups of the TLtBu ligand has undergone a C–H activation by a putative MnIV=O bond. The complex shows a distorted square‐pyramidal geometry in the solid state, and the carbon atom of the CH‐activated tBu group occupies the apical position.