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-pi interactions are the main driving force in the formation of self-assembled Fe(II)-templated metallacycles with bptz [3,6-bis(2-pyidyl)-1,2,4,5-tetrazine] in high yields. It was demonstrated by X-ray crystallography, (1)H NMR, solution and solid-state MAS F-19 NMR spectroscopies, CV and MS studies that the anions [X](-) = [BF4](-), [ClO4](-) and the anions [Y](-) = [SbF6](-), [AsF6](-), [PF6](-) template molecular squares [Fe-4(bptz)(4)(CH3CN)(8)][X](8) and pentagons [Fe-5(bptz)(5)(CH3CN)(10)][Y](10), respectively. The X-ray structures of [{Fe-4(bptz)(4)(CH3CN)(8)} [BF4][BF4](7) and [{Fe-5(bptz)(5)(CH3CN)(10)}subset of 2SbF(6)][SbF6](8) revealed that the [BF4](-) and [SbF6](-) anions occupy the pi-acidic cavities, establishing close directional F center dot center dot center dot C(tetrazine) contacts with the tetrazine rings that are by similar to 0.4 angstrom shorter than the sum of the F center dot center dot center dot C van der Waals radii (Sigma R-vdw(vdW) F center dot center dot center dot C = 3.17 angstrom). The number and strength of F center dot center dot center dot C(tetrazine) contacts are maximized; the F center dot center dot center dot C(tetrazine) distances and anion positioning versus the polygon opposing tetrazine rings are in agreement with DFT calculations for C(2)N4R(2)center dot center dot center dot[X](-)center dot center dot center dot C(2)N4R(2) (R = F, CN; [X](-) = [BF4](-), [PF(6)](-)). In unprecedented solid-state (19)F MAS NMR studies, the templating anions, engaged in anion-pi interactions in the solid state, exhibit downfield chemical shifts Delta delta(F-19) approximate to 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 (1)H NMR spectroscopy, underscore the remarkable stability of the metallapentacycles [Fe-5(bptz)(5)(CH3CN)(10)][PF6](10) << [Fe-5(bptz)(5)(CH3CN)(10)][SbF(6)](10) < [Fe-5(bptz)(5)(CH3CN)(10)][AsF6](10) versus [Fe-4(bptz)(4)(CH3CN)(8))[BF4](8), given the inherent angle strain in five-membered rings. Finally, the low anion activation energies of encapsulation Delta G(double dagger) approximate to 50 kJ/mol), determined from variable-temperature F-19 NMR studies for [Fe-5(bptz)(5)(CH3CN)(10)][PF6](10) and [Zn-4(bptz)(4)(CH3CN)(8)][BF4](8), confirm anion encapsulation in the p-acidic cavities by anion-pi contacts (similar to 20-70 kJ/mol).