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

The 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 kappa(2) 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)center dot 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 ((SO2)-S-tBu)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 Mn-IV= 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.