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 Helen T. Chifotides,* Ian D. Giles, and Kim R. Dunbar* Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States * S Supporting Information ABSTRACT: 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, 1 H 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 3 CN) 8 ]- [X] 8 and pentagons [Fe 5 (bptz) 5 (CH 3 CN) 10 ][Y] 10 , respec- tively. The X-ray structures of [{Fe 4 (bptz) 4 (CH 3 CN) 8 }⊂ BF 4 ][BF 4 ] 7 and [{Fe 5 (bptz) 5 (CH 3 CN) 10 }⊂2SbF 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 Å shorter than the sum of the F···C van der Waals radii (∑R vdW F···C = 3.17 Å). 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 2 N 4 R 2 ···[X] − ···C 2 N 4 R 2 (R = F, CN; [X] − = [BF 4 ] − , [PF 6 ] − ). 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 1 H NMR spectroscopy, underscore the remarkable stability of the metallapentacycles [Fe 5 (bptz) 5 (CH 3 CN) 10 ][PF 6 ] 10 ≪ [Fe 5 (bptz) 5 (CH 3 CN) 10 ]- [SbF 6 ] 10 < [Fe 5 (bptz) 5 (CH 3 CN) 10 ][AsF 6 ] 10 versus [Fe 4 (bptz) 4 (CH 3 CN) 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 3 CN) 10 ][PF 6 ] 10 and [Zn 4 (bptz) 4 (CH 3 CN) 8 ][BF 4 ] 8 , confirm anion encapsulation in the π-acidic cavities by anion−π contacts (∼20−70 kJ/mol). ■ INTRODUCTION Coordination-driven self-assembly, 1−4 a topic at the frontier of supramolecular chemistry, 5,6 harnesses the structural versatility of metal ions and the directionality of metal−ligand interactions to promote the spontaneous assembly of elegant supra- molecular achitectures, 1,3,7−9 endowed in some instances with unusual properties and ingenious applications 10−14 or intriguing host−guest behavior. 15−17 This fascinating synthetic strategy has led to a myriad of metallacyclic architectures, ranging from molecular triangles to cages, capsules, and various polyhedra 7 and to many examples of the most common nuclearity structures, namely, molecular squares. 1,3,4 In pioneering studies by Stang et al. 1 and Fujita et al. 3a transition metals with 90° L− M−L bonds (where L is a ligand), which are compatible with square angles, were incorporated as building blocks into the first square frameworks and, when combined with the appropriate organic linkers, led to an abundance of conforma- tionally stable molecular squares and rectangles. 1 In contrast to this situation is the challenge of constructing pentagons, due to the scarcity of 108° subunits and the incongruity between octahedral L−M−L and internal pentagon angles (108°), both of which contribute to their inherent lower stability as compared to squares. 18 These factors have contributed to a dearth of reported pentagonal architectures, with the exceptions of a handful of self-assembled pentameric metallacycles detected by mass spectrometry, 18,19 an anion-templated pentanuclear circular helicate, 20 a molecular pentafoil knot, 21 and very recently a remarkable unprecedented self-assembled pentagonal prism. 22 Apart from the directing elements imposed by the building blocks, the outcome of the self-assembly reactions can be controlled by external factors such as synthon concentration, solvent, counterions, or the presence of a template. A template is a directing element that induces or selectively stabilizes a desired assembly through noncovalent interactions, during a Received: August 24, 2012 Published: February 18, 2013 Article pubs.acs.org/JACS © 2013 American Chemical Society 3039 dx.doi.org/10.1021/ja3082473 | J. Am. Chem. Soc. 2013, 135, 3039−3055