Reasons behind the Relative Abundances of Heptacoordinate Complexes along the Late First-Row Transition Metal Series

A series of transition metal complexes [ML¹] (H₂L¹ = 1,4,10-trioxa-7,13-diazacyclopentadecane-<i>N</i>,<i>N</i>′-diacetic acid, M = Co, Ni, Cu, or Zn) have been prepared and characterized. The X-ray structures of the [CoL¹] and [CuL¹] complexes reveal that the metal ions are seven-coordinated with a distorted pentagonal bipyramidal coordination. The five donor atoms of the macrocycle define the pentagonal plane of the bipyramid, while two oxygen atoms of the carboxylate groups coordinate apically. The [NiL¹] complex presents a very distorted structure with long Ni–O distances involving two oxygen atoms of the crown moiety [2.544(3) Å]. This distortion is related to the Jahn–Teller effect that is expected to operate in d⁸ pentagonal bipyramidal complexes. The spectroscopic characterization of the [ZnL¹] and [CuL¹] complexes using NMR and EPR and the theoretical calculation of the ¹³C NMR shifts and <i>g</i>- and <i>A</i>-tensors using DFT confirm that these complexes retain the pentagonal bipyramidal coordination in aqueous solution. The stability trend of the [ML¹] complexes (Co²⁺ > Ni²⁺ < Cu²⁺ > Zn²⁺), which is in contradiction with the Irving–Williams order, has been analyzed using DFT calculations (TPSSh functional). The free energy values calculated in the gas phase for [CoL¹]­(g) + [M­(H₂O)₆]²⁺(g) → [ML¹]­(g) + [Co­(H₂O)₆]²⁺(g) (M = Ni, Cu, Zn) reproduce fairly well the stability trend observed experimentally, the agreement being improved significantly upon inclusion of solvent effects. Our results indicate that the pentagonal bipyramidal coordination is particularly unfavorable for Ni²⁺, and thus preorganized ligands that favor this geometry such as L¹ are selective for Co²⁺ over Ni²⁺ cations