Supramolecular Architectures with π‑Acidic 3,6-Bis(2-pyridyl)-1,2,4,5-tetrazine Cavities: Role of Anion−π Interactions in the Remarkable Stability of Fe(II) Metallacycles in Solution

The
comprehensive investigation reported herein provides compelling
evidence that anion−π interactions are the main driving
force in the formation of self-assembled Fe­(II)-templated metallacycles
with bptz [3,6-bis­(2-pyridyl)-1,2,4,5-tetrazine] in high yields. It
was demonstrated by X-ray crystallography, 1H NMR, solution
and solid-state MAS 19F NMR spectroscopies, CV and MS studies
that the anions [X]− = [BF4]−, [ClO4]− and the anions [Y]− = [SbF6]−, [AsF6]−, [PF6]− template molecular squares
[Fe4(bptz)4­(CH3CN)8]­[X]8 and pentagons [Fe5(bptz)5­(CH3CN)10]­[Y]10, respectively.
The X-ray structures of [{Fe4(bptz)4­(CH3CN)8}​​⊂​​BF4]­[BF4]7 and [{Fe5(bptz)5­(CH3CN)10}​⊂​2SbF6]­[SbF6]8 revealed that the [BF4]− and [SbF6]− anions occupy the π-acidic cavities, establishing close directional
F···Ctetrazine contacts with the tetrazine
rings that are by ∼0.4 Å shorter than the sum of the F···C
van der Waals radii (∑RvdW F···C
= 3.17 Å). The number and strength of F···Ctetrazine contacts are maximized; the F···Ctetrazine distances and anion positioning versus the polygon
opposing tetrazine rings are in agreement with DFT calculations for
C2N4R2···​[X]−···​C2N4R2 (R = F, CN; [X]− = [BF4]−, [PF6]−). In unprecedented
solid-state 19F MAS NMR studies, the templating anions,
engaged in anion−π interactions in the solid state, exhibit
downfield chemical shifts Δδ­(19F) ≈
3.5–4.0 ppm versus peripheral anions. NMR, CV, and MS studies
also establish that the Fe­(II) metallacycles remain intact in solution.
Additionally, interconversion studies between the Fe­(II) metallacycles
in solution, monitored by 1H NMR spectroscopy, underscore
the remarkable stability of the metallapentacycles [Fe5(bptz)5­(CH3CN)10]­[PF6]10 ≪ [Fe5(bptz)5­(CH3CN)10]­[SbF6]10 <
[Fe5(bptz)5­(CH3CN)10]­[AsF6]10 versus [Fe4(bptz)4­(CH3CN)8]­[BF4]8, given the inherent angle strain in five-membered rings.
Finally, the low anion activation energies of encapsulation (ΔG⧧ ≈ 50 kJ/mol), determined from
variable-temperature 19F NMR studies for [Fe5(bptz)5­(CH3CN)10]­[PF6]10 and [Zn4(bptz)4­(CH3CN)8]­[BF4]8, confirm
anion encapsulation in the π-acidic cavities by anion−π
contacts (∼20–70 kJ/mol).