TY - JOUR
T1 - Supramolecular architectures with π-acidic 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine cavities
T2 - Role of anion-π interactions in the remarkable stability of Fe(II) metallacycles in solution
AU - Chifotides, Helen T.
AU - Giles, Ian D.
AU - Dunbar, Kim R.
PY - 2013/2/27
Y1 - 2013/2/27
N2 - 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 (ΣRvdW 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).
AB - 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 (ΣRvdW 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).
UR - http://www.scopus.com/inward/record.url?scp=84874616819&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84874616819&partnerID=8YFLogxK
U2 - 10.1021/ja3082473
DO - 10.1021/ja3082473
M3 - Article
C2 - 23414294
AN - SCOPUS:84874616819
SN - 0002-7863
VL - 135
SP - 3039
EP - 3055
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 8
ER -