Formation and Transmetalation Mechanisms of Homo- and Heterometallic (Fe/Zn) Trinuclear Triple-Stranded Side-by-Side Helicates

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.