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

摘要

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 ~(19)F NMR spectroscopies, CV and MS studies that the anions [X]~- = [BF_4]~-, [ClO_4]~- and the anions [Y]~- = [SbF_6]~-, [AsF_6]~-, [PF_6]~- template molecular squares [Fe_4(bptz)_4(CH_3CN)_8]-[X]_8 and pentagons [Fe_5(bptz)_5(CH_3CN)_10][Y]_10, respectively. The X-ray structures of [{Fe_4(bptz)_4(CH_3CN)_8}is contained in BF_4][BF_4]_7 and [{Fe_5(bptz)_5(CH_3CN)_10}is contained in2SbF_6][SbF_6]_8 revealed that the [BF_4] and [SbF_6] anions occupy the π-acidic cavities, establishing close directional F…C_(tetrazine) contacts with the tetrazine rings that are by ～0.4 A shorter than the sum of the F…C van der Waals radii (ΣR_(vdW) F…C = 3.17 A). The number and strength of F…C_(tetrazine) contacts are maximized; the F…C_(tetrazine) distances and anion positioning versus the polygon opposing tetrazine rings are in agreement with DFT calculations for C_2N_4R_2-[X]~-…C_2N_4R_2 (R = F, CN; [X]~- = [BF_4]~-, [PF6]~-). In unprecedented solid-state ~(19)F MAS NMR studies, the templating anions, engaged in anion-π interactions in the solid state, exhibit downfield chemical shifts Δδ(~(19)F) ≈ 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 metallapentacydes [Fe_5(bptz)_5(CH_3CN)_(10)][PF_6]_(10) « [Fe_5(bptz)_5(CH_3CN)_(10)]-[SbF_6]_(10) < [Fe_5(bptz)_5(CH_3CN)_(10)][AsF_6]_(10) versus [Fe_4(bptz)_4(CH_3CN)_8][BF_4]_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 ~(19)F NMR studies for [Fe_5(bptz)_5(CH_3CN)_(10)][PF_6]_(10) and [Zn_4(bptz)_4(CH_3CN)_8][BF_4]_8, confirm anion encapsulation in the π-acidic cavities by anion-π contacts (～20-70 kj/mol).