Interpenetrated cage assembly

In the previous research of the Clever group,^[28e] they have calculated the optimal Pd(pyridine)4−X−Pd(pyridine)4 distances for chloride, bromide and tetrafluoroborate anions equaling to 6.5 Å, 7.0 Å and 8.4 Å, respectively. As the comparison of the length of ligand L¹ with the lengths of the formerly studied systems shown in Figure 2.6, it’s able to predict this monomeric but not double cage behavior which is attributed to the fact that, the shorter carbazole-based ligand L¹ gives rise to a smaller cavity which does not allow for the interpenetrated cage unit in the presence of relatively large tetrafluoroborate counter anions.

Figure 2.4 ESI-FTICR mass spectra of a) monomeric cage [nBF4@Pd2L¹4]⁽⁴⁻ⁿ⁾⁺ with n = 0−2; b) double-cage {[3Br@Pd4L¹8](BF4)n}⁽⁵⁻ⁿ⁾⁺ with n = 0−1 and c) double-cage {[3Cl@Pd4L¹8](BF4)n}⁽⁵⁻ⁿ⁾⁺ with n = 0−2 (* = adducts with impurities, B = double-cage [(2Cl+Br)@Pd4L¹8]⁵⁺).

Furthermore, previously collected information about the ideal distance between the Pd(pyridine)4-planes and different anions, gave a clue to predict the dimerization from carbazole mono-cages in the presence of smaller anions such as chloride or bromide.^[28c,e]

Figure 2.5 a) Single-crystal X-ray structure of the monomeric cage [Pd2L¹4]⁴⁺ (counter anions = BF4−); b) DFT-optimized structure of [3Cl@Pd4L¹8]⁵⁺ and c) X-ray structure of the triple-catenane {trans-[(PdBr2)2L¹2]}3. Copyright © 2015 WILEY-VCH Verlag GmbH & Co.

As a fact, the formation of an interpenetrated species [3X@Pd4L¹8] in which three halide anions occupied all the three cavities was indicated by NMR spectroscopy (Figure 2.3c and d) and ESI mass spectrometry (Figure 2.4b and c) after adding 1.5 equivalents of bromide or chloride anions to the mono-cage [Pd2L¹4] solution and heating the reaction mixture for 5 h at 70 °C. On the basic of previous results in the Clever group, double-cage formation is characterized by a two-fold splitting of all ¹H NMR signals and distinctive signal shifts of the pyridine protons pointing inside the cage’s three pockets. From the high resolution ESI mass spectrum,

the interpenetrated cage was unambiguously identified as the species [3X@Pd4L¹8]⁵⁺, {[3X@Pd4L¹8]+BF4}⁴⁺

and {[3X@Pd4L¹8]+2BF4}³⁺.

Figure 2.6 Length of ligand L¹ from DFT calculation and the lengths of the formerly studied systems.^[30] *:

B3LYP/6-31G* DFT optimization of ligand (conformer with both pyridine-Ns in depicted endo position),

**: ωB97XD/def2-SVP DFT Model of the chloride-templated double-cage, ***: X-ray structures of the double-cages (in ** and *** the PdN4-plane-to-plane distances are given), #: special case: the bulky group attached to the backbone rather than the N-N distance prevents BF4− templation.^[28g]

From the analysis of NMR spectra, nevertheless, the double-cages are in equilibrium with the monomeric cage and the free ligand (Figure 2.3c and d). The ratio of the species [3X@Pd4L¹8] : [Pd2L¹4] : L¹ was found to be 5 : 3 : 10 for bromide and 5 : 5 : 2 for chloride^[31] by NMR spectroscopy. The dependent formation of double-cages upon different halide content was examined by titrating a solution of the halide anions (as their tetrabutylammonium salts in deuterated acetonitrile solution) in a range of 1.5 to 3.0 equivalents for bromide and 0.5 to 2.0 equivalents for chloride in an NMR tube containing the monomeric cage (Figure 2.7). The samples were heated to 70 °C for 5 h after each titration step prior to recording the NMR spectra. As shown in Figure 2.7a, the formation of bromide-templated double-cagereaches its maximum intensity at 1.75 equivalents, further addition of bromide leads to the decrease of the double-cage signal intensities accompanied by consumption of mono-cage (see characteristic signal in blue) and release of free ligand L¹ (see characteristic signal in red). As a similar result in Figure 2.7b, the formation of the desired chloride-templated cage reaches its maximum intensity at 1.0 equivalent. In the meantime, another double-cage B is formed as a minor component which is identified as mixed chloride/bromide species [(2Cl+Br)@Pd4L¹8](BF4)5 by high resolution mass spectrometry in Figure 2.8. From the simulation, there is another possible composition {[3Cl@Pd4L¹8]⁵⁺+CD3CN} (in red). However, more thorough evidence against the acetonitrile-containing/associated species is illustrated in (i) the distinct NMR signals of a minor double-cage species, (ii) a negative control experiment using CH3CN as a solvent instead of CD3CN and (iii) from the space filling model of the double cage which does not suggest the possibility of acetonitrile co-encapsulation inside the cavities. Further addition of chloride leads to the decrease of the double-cage signal intensities accompanied by consumption of mono-cage (see characteristic signal in blue) and release of free

ligand L¹ (see characteristic signal in red). Fascinatingly, the chloride-templated double-cage (“D” in the figure) reacts more readily with excess chloride than double-cage B as can be identified when comparing to the relative signal intensities in the spectra upon addition of 1.0 and 1.5 equivalents of bromide.

Figure 2.7 ¹H NMR titration (300 MHz, 298 K, CD3CN) of bromide a) and chloride b) into mono-cage [Pd2L¹4] (L = ligand, M = mono-cage).

Since the monomeric cage was obtained quantitatively and no free ligand was present in the solution, the constitution of the major component [3X@Pd4L¹8] must be accompanied by a partial ligand liberation by disassembly, even when stoichiometric amounts of halide were added. This phenomenon is in accordance with previously reported observations in the Clever group which showed that other halide-binding (but tetrafluoroborate-templated) double-cages release free ligand upon addition of excess amounts of halide anions.^[28b,e] It can be feasibly explained that halide binding inside the cage’s cavities competes with the direct

coordination to the square-planar Pd^II cations under displacement of up to two of the pyridine donors. The resulting PdX2(pyridine)2-motif is well known in supramolecular self-assembly.^[32]

Figure 2.8 ESI-FTICR-HRMS assignment of double-cage B in the experimental mass spectrum of the solubility of these neutral species in polar acetonitrile solution, the observed precipitates can be accounted to some extent.

Unfortunately, no X-ray structure of double cage [3X@Pd4L¹8] was obtained, a DFT model of the chloride-templated interpenetrated assembly was constructed according to the acquired spectroscopic results and the crystal structures of reported double-cages in the Clever group using a geometry refined optimization on the ωB97XD/def2-SVP level of theory (Figure 2.5b). Taking the Pd²⁺−Cl⁻−Pd²⁺ distance into account, the outcome of this calculation is in quite good agreement with previously published theoretical and experimental researches of relevant double-cage structures.^[26a,28]