A caged tris(2-pyridylmethyl)amine ligand equipped with a C triazole –H hydrogen bonding cavity

A CAPPED BIOINSPIRED LIGAND BUILT FROM A TRIS (2-PYRIDYL - METHYL ) AMINE (TPA) UNIT AND SURMOUNTED BY A TRIAZOLE - BASED INTRAMOLECULAR H-BONDING SECONDARY SPHERE , WAS PREPARED . T HE RESULTING CAGE PROVIDES A WELL - DEFINED CAVITY COMBINING A HYDROPHOBIC NATURE WITH H-BONDING PROPERTIES . I TS COORDINATING PROPERTIES WERE EXPLORED USING Z N (II) AND C U (II) METAL IONS . The binding cavities found in metalloproteins govern reaction’s selectivity and efficiency. In their hydrophobic channels, destabilizing (like steric repulsion) and stabilizing (like hydrogen bonding, H-bonding) forces, allow for specific enzyme-substrate interactions, substrate positioning and activation/stabilization of highly reactive intermediates. 1 H-bonding is particularly important in metalloenzymes involved in dioxygen processing, such as copper-containing oxygenases and oxidases. The postulated oxidative active species of these systems is the highly reactive mononuclear cupric superoxide (Cu II -O 2 • - ). 2 Many efforts have been dedicated to the development of artificial Cu ligands able to generate and stabilize such metastable intermediate. Among them, the tris(2-pyridylmethyl)amine ( TPA ) ligand has been widely used as scaffold for mimicking the first coordination sphere in structural and functional models of copper, 3 but also iron, 4 mono-oxygenases. Interestingly, incorporating intramolecular H-bonding secondary spheres to the TPA ligand, was reported as an efficient strategy to stabilize either mononuclear hydroperoxo [(L)Cu II -OOH] + , 5 binuclear peroxodicopper [{(L)Cu II } 2 (O 22)], 6 or end-on superoxo [(L)Cu II -O 2 • - ] 7 copper-dioxygen

The binding cavities found in metalloproteins govern reaction's selectivity and efficiency.In their hydrophobic channels, destabilizing (like steric repulsion) and stabilizing (like hydrogen bonding, Hbonding) forces, allow for specific enzyme-substrate interactions, substrate positioning and activation/stabilization of highly reactive intermediates. 1 H-bonding is particularly important in metalloenzymes involved in dioxygen processing, such as copper-containing oxygenases and oxidases.The postulated oxidative active species of these systems is the highly reactive mononuclear cupric superoxide (Cu II -O2 •-). 2 Many efforts have been dedicated to the development of artificial Cu ligands able to generate and stabilize such metastable intermediate.Among them, the tris(2-pyridylmethyl)amine (TPA) ligand has been widely used as scaffold for mimicking the first coordination sphere in structural and functional models of copper, 3 but also iron, 4 mono-oxygenases.Interestingly, incorporating intramolecular H-bonding secondary spheres to the TPA ligand, was reported as an efficient strategy to stabilize either mononuclear hydroperoxo [(L)Cu II -OOH] + , 5 binuclear peroxodicopper [{(L)Cu II }2(O2 2-)], 6 or end-on superoxo [(L)Cu II -O2 •-] 7 copper-dioxygen intermediates.On another side, synthetic supramolecular chemistry is a powerful tool to build cage-like second coordination sphere around bioinspired catalysts. 8In particular, the archetypal TPA ligand has been equipped with well-defined cavities by mean of its covalent substitution, 9,10 or host-guest encapsulation into an H-bonded capsule. 11In this context, TPA-based hemicryptophanes are organic cages built from a bowl-shaped cyclotriveratrylene (CTV) cap, connected to the tripodal ligand via three linkers.We have recently demonstrated that TPA-hemicryptophanes displaying methylene or phenyl linkers, could respectively control the helical arrangement of the ligand, 12 and lead to enhanced oxidation catalysts. 13owever, the hydrophobic cavities found in such cages were devoid of H-bonding groups that could allow substrate positioning or intermediate stabilization.Despite these progresses, the preparation of TPA-based complexes combining hydrophobic cavities with intramolecular H-bonding units at their secondary sphere is still needed.Designing and discovering new methodologies to prepare such advanced model complexes is in fact crucial to better reproduce the key structural properties of metalloenzymes.Beside their connecting benefits, triazole bridges are particularly interesting due to their H-bonding donor properties.For instance, a triazolo organic cage has been recently reported as the most efficient chloride-binding receptor to date, by mean of Ctriazole-H H-bonding interactions. 14We therefore envisioned that the covalent substitution of the TPA by another C3 symmetrical cap, using triazole spacers, will represent an efficient strategy to construct functionalized cavities.In this communication, we report the preparation of unprecedented bioinspired complexes displaying a hydrophobic cavity offering three H-bonding triazoles, aiming at reproducing the metalloenzyme's functionalized hydrophobic channels.We design the hemicryptophane Hm-TriA-TPA where the archetypal TPA ligand is linked to a northern CTV cap, via three triazole bridges, resulting in an H-bond donor decorated cavity.Hm-TriA-TPA was prepared in an eight-step synthetic strategy (Scheme 1).The cage's walls were first prepared (Scheme 1a), before generating the southern TPA and the CTV cap in a final intramolecular macrocyclization closing the structure (Scheme 1b).The aryl propargyl ether derivative 2 was prepared in twosteps by alkylation of the starting vanillyl alcohol with propargyl bromide, followed by the protection of the resulting alcohol 1 with THP. 2 was then connected to the pyridine derivative 4 by a triazole link formed in a Cu-catalyzed azide-alkyne cycloaddition reaction (CuAAC).The CuAAC reaction between equimolar amounts of the propargyl 2, and the azide 4 precursors, catalyzed by CuSO4 (10 mol%) in the presence of the sodium ascorbate reducing agent (10 mol%), resulted in the formation of the triazole 5 in 83% yield.Precursor 7 was then prepared in two steps by reduction of 5 into the alcohol 6 followed by it mesylation.The addition of ammonia to 7 in the presence of Cs2CO3, at 90°C in THF, afforded the open TPA derivative 8 in 73% yield.Formation of Hm-TriA-TPA was finally achieved in a 29% yield, via the intramolecular cyclization of 8 in CH3CN, catalyzed by the Lewis acid scandium triflate [Sc(OTf3)], under diluted conditions.
The 1 H NMR spectrum of Hm-TriA-TPA indicates a C3 symmetrical structure on average, in CDCl3, at 298K (Fig. 1a).Identical, sharp and well-defined signals could be observed for the protons belonging to the northern CTV unit (Hh, Hi, Hj and Hk,k'), the -CH2-links (He,e' and Hg,g'), the Ctriazole-H bonds (Hf) and the southern TPA (Ha,a', Hb, Hc and Hd).2D-NMR experiments (see the ESI) were used to assign these resonances.Slow diffusion of Et2O to a CH2Cl2 solution of Hm-TriA-TPA afforded single crystals suitable for X-ray diffraction.The XRD structure of Hm-TriA-TPA confirms the endohedral functionalization of the TPA unit by the bowl-shaped CTV via the three triazole bridges having their Ctriazole-H bonds pointing toward the inside of the cavity (Fig. 1b).It should be noted that, in the X-ray structure of the cage, a pyridine unit of the TPA reside inside the cavity.This C1 symmetrical conformation observed in the solid state, contrasts with the symmetrical 1 H NMR spectrum of Hm-TriA-TPA in solution.Fast conformational exchanges between in-out orientation of the pyridines could explain this behavior in solution at 298K.We have then investigated the ability of our caged ligand to form metallo-complexes in solution via coordination at its TPA unit.Binding of the air-stable and diamagnetic zinc triflate salt Zn II (OTf)2, was monitored by 1 H-NMR in CD3CN at 298K (Fig. 2 and Fig. S6 ,ESI).The 1 H-NMR spectra of Hm-TriA-TPA in the presence of 0.5 equiv. of the zinc salt reveal two sets of signals for each protons of the cage, that could be attributed to the presence of Zn II (Hm-TriA-TPA)(OTf)2 and Hm-TriA-TPA in a 1:1 ratio (Fig. 2b).Interestingly, in the presence of a stoichiometric amount of the metal salt, the resonances belonging to the free cage fully disappear to the profit of the zinc complex signals, indicating full complexation.Upon metalation, a strong down-field shift occurs on the protons of the TPA's pyridines (Hb, Hc and Hd, ppm: 0,35-0,7 ppm) that remain equivalent.The triazole bridges appear less affected with a modest upfield-shift observed for the Ctriazole-H bond (Hf, ppm<0,03 ppm).Overall, the 1 H-NMR analysis of Zn II (Hm-TriA-TPA)(OTf)2 attests for retained C3 symmetry of the caged ligand (on average) with identical and sharp signals for every resonances.Altogether, these observations unambiguously confirm the coordination of the Zn II metal ion at the TPA unit with retention of the endohedral functionalization of the resulting complex. 12This was further supported by the optimized DFT (Density Functional Theory) structure that clearly reveals a C3 symmetrical caged Zn(II) complex in a trigonal bipyramidal geometry with an apical molecule of acetonitrile (Fig. 2e, Fig. S7, ESI).Finally, identical spectra were observed upon addition of a second equivalent of Zn II (OTf)2 (Fig. 2d), ruling out the possibility of a second metal-binding event occurring at the triazole crown.

Conclusions
In summary, the preparation of an organic cage where the canonical TPA ligand is surmounted by a Hbonding hydrophobic cavity offering three triazole units is described.We demonstrate that this cage can coordinate zinc(II) and copper(II) metal ions at its TPA unit with an endo-functionalization of the complex, in both solution and solid state.These are the first examples of bioinspired complexes equipped with a tristriazole decorated cavity mimicking the functionalized (H-bonding) hydrophobic channels of metalloenzymes.Finally, we found that the azidocopper(II) adduct [(L)Cu II -N3 -] can be prepared upon addition of N3 to the caged Cu(II) complex.Spectroscopic analysis of this structural analogue of the cupric superoxide intermediates [(L)Cu II -O2 •-], suggest stabilization of the azido adduct, within the Ctriazole-H based cavity.We envision that our strategy might finds applications toward the development of non-enzymatic catalysts able to stabilize reactive intermediates and/or control substrate positioning by their H-bonding hydrophobic cavity.Future work will focus on the use of TPA ligands equipped with our triazolefunctionalized cavity to generate, stabilize, and explore the reactivity of end-on superoxocopper(II) complexes upon dioxygen activation. 18

Figure 1 .
Figure 1.(a) 1 H NMR spectra (CDCl3, 400 MHz) of Hm-TriA-TPA along with (b) view of its X-ray crystal structure.Only the hydrogen atom belonging to the three triazole units have been included for clarity. 15

Figure 3 .
Figure 3. (a) Views of the X-ray crystal structure of Cu II (Hm-TriA-TPA)(OTf)2 (b) UV-vis monitoring of the formation of the azido adduct Cu II (Hm-TriA-TPA)(N3)(OTf), upon addition of NBu4N3 (0 to 1.0 equiv.) to a 0,5 mM solution of Cu II (Hm-TriA-TPA)(OTf) in acetone.(c) spectroscopic features (UV-vis and IR) of the azido complexes stabilized by Me3 TPA and Hm-TriA-TPA.These UV-vis data are consistent with the formation of the azido complex Cu II (Hm-TriA-TPA)(OTf)(N3). 2, 7, 11 In TPAcomplexes bearing intramolecular H-bonding donors, the potential presence of stabilizing interactions with the azido moiety has been associated to a blue-shift of the LMCT band, as well as a blue-shift of the (N-N) stretching frequency. 2,17Value of the LMCT band observed in case of Hm-TriA-TPA was therefore compared with the one of its corresponding open model ligand Me3 TPA devoid of intramolecular H-bonding groups (Fig 3c).Compared to the open TPA-based complex Cu II ( Me3 TPA)(OTf)(N3) ( max = 429nm, Fig. S10, ESI), the Cu II -N3 -LMCT bands in our triazolecontaining cage shift to higher energy ( max = 415nm).Finally, to further support the stabilization offer by Hm-TriA-TPA, the two azido complexes have been isolated and they antisymmetric N3 -IR stretch compared.Interestingly, Cu II (Hm-TriA-TPA)(OTf)(N3) displays a 5 cm -1 blue-shift of the (N-N) stretching frequency compared to the "base" ligand Me3 TPA (Fig. 3c, Fig. S11 and S12, ESI).These findings are therefore consistent with a bonding stabilization of the azido adduct within the triazole-containing cavity.