Trigonal Bipyramidal Geometry and Tridentate Coordination Mode of the Tripod in FeCl₂ Complexes with Tris(2-pyridylmethyl)amine Derivatives Bis-α-Substituted with Bulky Groups. Structures and Spectroscopic Comparative Studies

A series of dichloroferrous complexes with ligands derived from the tris(2-pyridylmethyl)amine tripod has been prepared and characterized. The X-ray crystal structures of the complexes [bis(2-bromo-6-pyridylmethyl)(2-pyridylmethyl)amine]Fe^IICl₂ ((Br₂TPA)Fe^IICl₂) and [bis(2-phenyl-6-pyridylmethyl)(2-pyridylmethyl)amine]Fe^IICl₂, ((Ph₂TPA)Fe^IICl₂) are reported. In these complexes, the tripod coordinates in the tridentate mode, with a substituted pyridyl arm dangling away from the metal. Both complexes have a trigonal bipyramidal iron center with two equatorial chloride ions. Their crystal structures are compared with those of the [tris(2-pyridylmethyl)amine]Fe^IICl₂ and [(2-bromo-6-pyridylmethyl)bis(2-pyridylmethyl)amine]Fe^IICl₂ complexes ((TPA)Fe^IICl₂ and (BrTPA)Fe^IICl₂, respectively) in which the ligand coordinates in the tetradentate mode. For all complexes, the metal to ligand distances are systematically above the value of 2.0 Å, and ¹H NMR displays paramagnetically shifted resonances with short relaxation times. This indicates that the iron is in a high-spin state. Electric conductivity measurements show that, for all complexes, the measured values lie within the same range, significantly below those expected for ionic complexes. Together with the analysis of the UV−visible and NMR data, this strongly suggests that the coordination mode of the tripod is retained in solution.