Formation and transmetalation mechanisms of homo- and heterometallic (Fe/Zn) trinuclear triple-stranded side-by-side helicates

Abstract

A novel linear hybrid tris-bidentate neutral ligand having 2,2′-bipyridine and two terminal triazolylpyridine coordination sites (L) was efficiently synthesized and explored in the synthesis of trinuclear triple-stranded homometallic side-by-side helicates L₃Fe₃(OTf)₆ (1) and L₃Zn₃(OTf)₆ (2), in which the three metal centers display alternating Λ and Δ configurations. Selective formation of the analogous heterometallic side-by-side helicate L₃Fe₂Zn(OTf)₆ (3) was achieved from a mixture of L, Fe(CH₃CN)₂(OTf)₂, and Zn(OTf)₂ (1:1:1) in acetonitrile at room temperature. Various analytical techniques, i.e., single-crystal X-ray diffraction and NMR and UV/vis spectroscopy, were used to elucidate the sequence of the metal atoms within the heterometallic helicate, with the Zn²⁺ at the central position. The formation of 3 was also achieved starting from either L₃Zn₃(OTf)₆ or L₃Fe₃(OTf)₆ by adding Fe(CH₃CN)₂(OTf)₂ or Zn(OTf)₂, respectively. ESI-MS and ¹H NMR studies elucidated different transmetalation mechanisms for the two cases: While a Zn²⁺-to-Fe²⁺ transmetalation occurs by the stepwise exchange of single ions on the helicate L₃Zn₃(OTf)₆ at room temperature, this mechanism is almost inoperative for the Fe²⁺-to-Zn²⁺ transmetalation in L₃Fe₃(OTf)₆, which is kinetically trapped at room temperature. In contrast, dissociation of L₃Fe₃(OTf)₆ at higher temperature is required, followed by reassembly to give L₃Fe₂Zn(OTf)₆. The reassembly follows an interesting mechanistic pathway when an excess of Zn(OTf)₂ is present in solution: First, L₃Zn₃(OTf)₆ forms as the high-temperature thermodynamic product, which is then slowly converted into the thermodynamic heterometallic L₃Fe₂Zn(OTf)₆ product at room temperature. The temperature-dependent equilibrium shift is traced back to significant entropy differences resulting from an enhancement of the thermal motion of the ligands at high temperature, which destabilize the octahedral iron terminal complex and select zinc in a more stable tetrahedral geometry.