Controlled ligand distortion and its consequences for structure, symmetry, conformation and spin-state preferences of iron(ii) complexes

Abstract

The ligand-field strength in metal complexes of polydentate ligands depends critically on how the ligand backbone places the donor atoms in three-dimensional space. Distortions from regular coordination geometries are often observed. In this work, we study the isolated effect of ligand-sphere distortion by means of two structurally related pentadentate ligands of identical donor set, in the solid state (X-ray diffraction, ⁵⁷Fe-Mössbauer spectroscopy), in solution (NMR spectroscopy, UV/Vis spectroscopy, conductometry), and with quantum-chemical methods. Crystal structures of hexacoordinate iron(II) and nickel(II) complexes derived from the cyclic ligand L¹ (6-methyl-6-(pyridin-2-yl)-1,4-bis(pyridin-2-ylmethyl)-1,4-diazepane) and its open-chain congener L² (N¹,N³,2-trimethyl-2-(pyridine-2-yl)-N¹,N³-bis(pyridine-2-ylmethyl) propane-1,3-diamine) reveal distinctly different donor set distortions reflecting the differences in ligand topology. Distortion from regular octahedral geometry is minor for complexes of ligand L², but becomes significant in the complexes of the cyclic ligand L¹, where trans elongation of Fe−N bonds cannot be compensated by the rigid ligand backbone. This provokes trigonal twisting of the ligand field. This distortion causes the metal ion in complexes of L¹ to experience a significantly weaker ligand field than in the complexes of L², which are more regular. The reduced ligand-field strength in complexes of L¹ translates into a marked preference for the electronic high-spin state, the emergence of conformational isomers, and massively enhanced lability with respect to ligand exchange and oxidation of the central ion. Accordingly, oxoiron(IV) species derived from L¹ and L² differ in their spectroscopic properties and their chemical reactivity.