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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

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posted on 19.02.2016, 21:14 by Helen T. Chifotides, Ian D. Giles, Kim R. Dunbar
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).