Post-assembly Modi ﬁ cation of Tetrazine-Edged Fe II4 L 6 Tetrahedra

: Post-assembly modi ﬁ cation (PAM) is a powerful tool for the modular functionalization of self-assembled structures. We report a new family of tetrazine-edged Fe II4 L 6 tetrahedral cages, prepared using di ﬀ erent aniline subcomponents, which undergo rapid and e ﬃ cient PAM by inverse electron-demand Diels − Alder (IEDDA) reactions. Remarkably, the electron-donating or -with-drawing ability of the para -substituent on the aniline moiety in ﬂ uences the IEDDA reactivity of the tetrazine ring 11 bonds away. This e ﬀ ect manifests as a linear free energy relationship, quanti ﬁ ed using the Hammett equation, between σ para and the rate of the IEDDA reaction. The rate of PAM can thus be adjusted by varying the aniline subcomponent.

C ovalent post-assembly modification (PAM) has recently been developed into a useful tool for expanding chemical functionality in supramolecular architectures. To be most useful, a PAM reaction must proceed quantitatively under mild conditions as it is often not possible to purify supramolecular mixtures (e.g., by chromatography). Furthermore, the weaker, dynamic linkages that hold together many metallosupramolecular complexes 1 can be incompatible with the aggressive reagents associated with the formation of strong bonds under kinetic control. 2 Consequently, only a limited number of reactions have been successfully employed for supramolecular PAM, including olefin metathesis, 3 Williamson ether synthesis, 4 alkyne−azide cycloaddition, 5 acylation (acid anhydrides 6 and active esters 2b ), imine reduction, 7 Diels−Alder, 8 and nucleophile-isocyanate coupling. 2c, 8 The tetrazine-based inverse electron-demand Diels−Alder (IEDDA) reaction also satisfies the requirements of efficiency and mildness necessary for successful PAM. 9 However, this reaction has not yet been harnessed for PAM of a discrete supramolecular complex, although it has found widespread use as a synthetic bioconjugation reaction 10 and, more recently, for covalently modifying polymers 11 and metal−organic frameworks. 12 The IEDDA reaction between a 3,6-disubstituted 1,2,4,5tetrazine and an electron-rich dienophile is an ideal reaction for PAM of a metallosupramolecular complex: it is efficient, produces N 2 as an inert byproduct, does not interfere with metal−ligand coordination, and is compatible with a range of dienophiles. 9 Conveniently, the D 2h symmetry of the tetrazine ring allows it to be incorporated in place of the 1,4-disubstituted benzene moiety present in many existing supramolecular complexes. 13 We therefore envisaged that IEDDA reactions could be adapted to a range of existing supramolecular architectures, thus offering a general approach for the rapid and facile functionalization of supramolecular complexes through PAM.
Here we present a family of self-assembled metal−organic Fe II 4 L 6 tetrahedral cages that contain tetrazine moieties. These tetrazines react efficiently by IEDDA with alkyne or alkyneequivalent 9b dienophiles. The design of dialdehyde subcomponent A, based on an existing structural analogue, 14 integrates tetrazine rings into the framework of the cage. Strong electronic coupling is thus engendered between the central tetrazine cores and the aniline residues incorporated at the vertices. This coupling manifests through a linear free energy relationship (LFER) between the IEDDA rate constant and the Hammett parameter of the aniline para-substituent (σ para ). We thus demonstrate that the tetrazine moiety can serve as both a structural element of the supramolecular architecture and a useful reactive handle for introducing new chemical functionality.
Subcomponent A was efficiently synthesized from commercially available 5-bromo-2-iodopyridine in five steps (Scheme 1; see Supporting Information (SI), Section S3 for details). Cage 1a was then prepared by subcomponent self-assembly 15 of A with 4-fluoroaniline and iron(II) bis(trifluoromethane)sulfonimide (Fe(NTf 2 ) 2 ). ESI-MS confirmed the formation of an Fe II 4 L 6 complex in solution (SI, Section S4), and NMR spectra were consistent with the expected tetrahedral symmetry ( Figure 1b). Single-crystal X-ray diffraction analysis ( Figure 2a) confirmed the tetrahedral structure of 1a in the solid state.
We tested the reactivity of cage 1a toward norbornadiene (NBD) as a model IEDDA reaction. NBD, upon reacting with a tetrazine and after retro-Diels−Alder elimination of nitrogen ( Figure 1a, I and II), gives a dihydropyridazine intermediate that spontaneously rearomatizes via a second retro-Diels−Alder reaction to expel a molecule of cyclopentadiene. 9b The reaction between 1a and NBD (2 equiv per tetrazine) in CD 3 CN resulted in the clean formation of pyridazine-edged 2a within 4 h at 50°C, as verified by NMR and ESI-MS analyses (SI, Section S5), together with a stoichiometric quantity of cyclopentadiene ( Figure 1). By UV−vis spectroscopy, we found that the reaction kinetics were first-order in tetrazine (SI, Section S7), indicating that the six tetrazine sites on a given cage react noncooperatively. It is therefore remarkable that the conversion of 1a to 2a occurs cleanly and without degradation of the assembled complex.
During the course of PAM, desymmetrization of the cage signals was observed ( Figure 1c); however, persistence of the signals from protons H e and H f and the absence of signals of the free subcomponents suggested that the ligands do not dissociate from the cage during the IEDDA reaction sequence. Crossover experiments using a mixture of two tetrazine-edged cages (1a and 1f) revealed that the rate of ligand dissociation is much slower than the IEDDA reaction (SI, Figures S25−S27). We therefore conclude that the IEDDA reactions occur on the intact cage rather than as part of a dissociation−IEDDA−reassociation sequence. Although IEDDA reactions are known to proceed through distorted transitory bicyclic intermediates (structures I and II, Figure 1a), the distortion is in a direction that is mostly perpendicular to the long-axis of the ligand in 1a (SI, Figure S28), thus preventing strain-induced disassembly of the cage.
In order to probe both the solvent and substrate scopes of post-assembly modification, we prepared chloroform-soluble decylaniline-substituted cage 1i and investigated the IEDDA reaction using commercially available (1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCNM) as an alternative dienophile. The strained cycloalkyne of BCNM is reported to react cleanly with tetrazines to give ring-fused pyridazines, and its −OH group is a useful point of attachment for other functionalities. 16 As shown in Scheme 1, cage 1i was observed to react with BCNM in CDCl 3 at 25°C, yielding hexafunctionalized cage 3 as the major product of the reaction (SI, Section S6).

Journal of the American Chemical Society
Communication substituents with Hammett constants (σ para ) between −0.37 (−OH) and +0.06 (−F) (SI Section S5). 19 The reaction of NBD with each cage was chosen as a model system for analyzing the kinetics of IEDDA due to its moderate rate, clean conversion, and the high symmetry of the resulting product cages (2a−2h), which enabled facile characterization. NMR and mass spectra were consistent with the formation of pyridazine-containing Fe II 4 L 6 cages 2a−2h (SI, Section S5), and X-ray diffraction confirmed the tetrahedral structure of product cage 2e ( Figure  2b).
Pseudo-first-order rate constants (k obs ) for the reactions of cages 1a−1h with excess NBD (450 equiv) were determined by measuring the decrease in the MLCT absorption band of the tetrazine cages over the 2 h following the addition of NBD. We could not monitor the reaction directly using the weaker tetrazine n → π* band (λ lit. = 510−530 nm, ε lit. = 1000 M −1 cm −1 ) 20 as it overlapped substantially with the more intense MLCT absorptions (450−650 nm). However, we observed the intensity of the MLCT band of each tetrazine cage (λ max ≈ 630 nm) to decrease monoexponentially during the IEDDA reaction, consistent with the pseudo-first-order kinetics expected for the consumption of tetrazine. 21 This observation suggests that the IEDDA reaction of a tetrazine moiety directly influences the electronics of the adjacent tris(2-pyridylimine)iron(II) chromophore. Consequently, we were able to use the MLCT bands of cages 1a−1h as spectroscopic probes for the IEDDA reaction (see SI, Section S7 for details of kinetics analyses).
Measuring k obs values for the set of IEDDA reactions shown in Figure 3a revealed that these reactions follow the Hammett equation, whereby log 10 (k obs ) increased linearly with σ para : more electron-withdrawing substituents gave faster rates (Figure 3b). Weaker correlations were observed between log 10 (k obs ) and the Hammett resonance effect parameters (σ + or σ − ), 19 which is consistent with the absence of significant charge build-up at any center during the reaction ( Figure S58). 9a The positive Hammett reaction constant (ρ = +0.47) is consistent with a decrease in the tetrazine LUMO energy as the electron-withdrawing power of the substituent increases. Remarkably, the magnitude of ρ is comparable to values measured for IEDDA reactions in nonsupramolecular systems in which the substituent is much closer to the reaction site (five bonds 22 vs. 11 bonds in the present case). We attribute this sensitivity to the unimpeded conjugation pathway between the aniline para-substituent and the tetrazine ring, an effect also reflected in the influence of the tetrazine moiety over MLCT absorption, as noted above.
Chemical self-assembly has proven capable of generating architectures of great structural 23 and functional 24 complexity. The development of new methods to cleanly modify and transform these assemblies can expand the amount of chemical space accessible, 25 as each new reactive motif (tetrazene, in this study) may be incorporated into many previously reported structure types. Attractive features of our method are its clean "click" nature, 26 the ability to incorporate new functionality via BCNM, and the ability to adjust reaction rates through modular variation of the aniline subcomponent. Future work will seek to adapt this IEDDA methodology to other supramolecular architectures in order to modulate phenomena such as guest binding, reactivity, and structural transformations.

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.5b05080.