Pb2+-Containing Metal-Organic Rotaxane Frameworks (MORFs)

The metal-organic rotaxane framework (MORF) structures with the advantage of mechanically interlocking molecules (MIMs) have attracted intense interest from the chemical community. In this study, a set of MORFs (i.e., MORF-Pb-1 and MORF-Pb-2) are constructed using Pb2+, a tetraimidazolium macrocycle (Texas-sized molecular box; 14+), and aromatic dicarboxylate (p-phthalate dianions (PTADAs; 2) or 2,6-naphthalene dicarboxylate dianions (3)) via a one-pot three-layer diffusion protocol. In particular, an unusual Pb…Pb weak interaction was shown in MORF-Pb-1 (charactered with distance of 3.656 Å).

In the structure of MORF-Pb-1, Pb 2+ cations locate in the same environment. As Figure 2a shows, Pb(1) coordinates with three p-phthalate dianions (2) in different local chemical environments. Pb 2+ has an outmost electronic closed shell as 5d 10 6p 2 and a reported ionic radius of 1.33 Å. [43] It was compared with the other closed outmost shell Zn 2+ cation (3d 10 , ionic radius of 0.74Å) in the presence of 1 4+ and 2. Swapping Pb 2+ with Zn 2+ , the same procedure resulted in [(1 4+ )2•(2)9•Zn6•12H2O]•2OH − •88.5H2O [44]. These structures showed that each Zn 2+ only binds with two p-phthalate dianions (2) and cannot cooperate with 2 inserting into the cavity of 1 4+ to form MIMs. All these findings show that the interactions between Pb 2+ and 2 and the further MORF-Pb-1 construction are highly metal cation dependant.    3a. The interpenetrated structure is further stabilized via intermolecular hydrogen bonding interactions (e.g., C(10A)-H(10A)…O(1) and C(10)-H(10B)…O(1B)) between 1 4+ and 2. Figure  3. The asymmetric monomer unit of MORF-Pb-1 contains one molecule of macrocycle 1 4+ , three Pb 2+ , three PTADA ligands in different local chemical environments, and two coordinated H2O solvent moieties. In particular, one molecule of free p-phthalic acid molecule is located in the channels of MORF-Pb-1 ( Figure 4). Monomer units can further construct a chain-shaped 1D polyrotaxane structure that extends infinitely in the direction of the crystallographic a-axis through the complexation between Pb(1) and 2 (highlighted with dark magenta colour) ( Figure 5). The pseudorotaxane unit [1 4+ •2•Pb 2 ] 6+ in the single crystal X-ray structure of The asymmetric monomer unit of MORF-Pb-1 contains one molecule of macrocycle 1 4+ , three Pb 2+ , three PTADA ligands in different local chemical environments, and two coordinated H 2 O solvent moieties. In particular, one molecule of free p-phthalic acid molecule is located in the channels of MORF-Pb-1 ( Figure 4).
Monomer units can further construct a chain-shaped 1D polyrotaxane structure that extends infinitely in the direction of the crystallographic a-axis through the complexation between Pb(1) and 2 (highlighted with dark magenta colour) ( Figure 5).  The asymmetric monomer unit of MORF-Pb-1 contains one molecule of macrocycle 1 4+ , three Pb 2+ , three PTADA ligands in different local chemical environments, and two coordinated H2O solvent moieties. In particular, one molecule of free p-phthalic acid molecule is located in the channels of MORF-Pb-1 ( Figure 4). Monomer units can further construct a chain-shaped 1D polyrotaxane structure that extends infinitely in the direction of the crystallographic a-axis through the complexation between Pb(1) and 2 (highlighted with dark magenta colour) ( Figure 5).  . The asymmetric monomer unit of MORF-Pb-1 contains one molecule of macrocycle 1 4+ , three Pb 2+ , three PTADA ligands in different local chemical environments, and two coordinated H2O solvent moieties. In particular, one molecule of free p-phthalic acid molecule is located in the channels of MORF-Pb-1 ( Figure 4). Monomer units can further construct a chain-shaped 1D polyrotaxane structure that extends infinitely in the direction of the crystallographic a-axis through the complexation between Pb(1) and 2 (highlighted with dark magenta colour) ( Figure 5). It is found that the distance between two neighbouring Pb 2+ on different 1D polyrotaxane is 3.656 Å (shown in Figure 6a), which is larger than the sum of two Pb 2+ ionic radii (1.33 Å). To the best of our knowledge, the maximum distance between two Pb metals in the lead cluster is 3.07 Å. [45,46] Herein, the weak Pb . . . Pb interaction is suggested to be similar to halogen bonds and to further stabilize the 2D array formed with polyrotaxanes in To the best of our knowledge, it is the first example of a halogen bond-like interaction shown in the Pb metal cation form. The characterization of these Pb . . . Pb interactions is under further investigation.  It is found that the distance between two neighbouring Pb 2+ on different 1D polyrotaxane is 3.656Å (shown in Figure 6a), which is larger than the sum of two Pb 2+ ionic radii (1.33Å). To the best of our knowledge, the maximum distance between two Pb metals in the lead cluster is 3.07 Å. [45,46] Herein, the weak Pb…Pb interaction is suggested to be similar to halogen bonds and to further stabilize the 2D array formed with polyrotaxanes in [1 4+ •(2)4•(2H + •2)•Pb2•2DMF•12H2O]. To the best of our knowledge, it is the first example of a halogen bond-like interaction shown in the Pb metal cation form. The characterization of these Pb…Pb interactions is under further investigation. It is noted that free p-phthalic acid (2H + •2) molecules are located in the channels of MORF-Pb-1. The distances between bezene on 2H + •2 and the planes on two neighbouring 1 4+ are less than 3.6 Å. The finding implies that the π-π donor-acceptor interactions further stabilized the 2D array. Furthermore, the 2D layers shown above are organized via strong π-π donor-acceptor interactions between neighbouring 1 4+ on different 2D layers. Finally, the 3D array of MORF-Pb-1 was achieved (Figure 7).  It is noted that free p-phthalic acid (2H + •2) molecules are located in the channels of MORF-Pb-1. The distances between bezene on 2H + •2 and the planes on two neighbouring 1 4+ are less than 3.6 Å. The finding implies that the π-π donor-acceptor interactions further stabilized the 2D array. Furthermore, the 2D layers shown above are organized via strong π-π donor-acceptor interactions between neighbouring 1 4+ on different 2D layers. Finally, the 3D array of MORF-Pb-1 was achieved (Figure 7). It is found that the distance between two neighbouring Pb 2+ on different 1D polyrotaxane is 3.656Å (shown in Figure 6a), which is larger than the sum of two Pb 2+ ionic radii (1.33Å). To the best of our knowledge, the maximum distance between two Pb metals in the lead cluster is 3.07 Å. [45,46] Herein, the weak Pb…Pb interaction is suggested to be similar to halogen bonds and to further stabilize the 2D array formed with polyrotaxanes in [1 4+ •(2)4•(2H + •2)•Pb2•2DMF•12H2O]. To the best of our knowledge, it is the first example of a halogen bond-like interaction shown in the Pb metal cation form. The characterization of these Pb…Pb interactions is under further investigation. It is noted that free p-phthalic acid (2H + •2) molecules are located in the channels of MORF-Pb-1. The distances between bezene on 2H + •2 and the planes on two neighbouring 1 4+ are less than 3.6 Å. The finding implies that the π-π donor-acceptor interactions further stabilized the 2D array. Furthermore, the 2D layers shown above are organized via strong π-π donor-acceptor interactions between neighbouring 1 4+ on different 2D layers. Finally, the 3D array of MORF-Pb-1 was achieved (Figure 7).
Monomer units can construct a chain-shaped 1D polyrotaxane structure through the complexation between Pb(4) and 3 (highlighted in green colour, Figure 10a). The 1D polyrotaxane further forms 2D frameworks bridged with the coordination between Pb(1) and 3 (highlighted in black colour). Furthermore, the 2D layers shown above are organized to form the final 3D array with the coordination between Pb(3,5) and 3 (highlighted in black and purple colour) ( Figure 11).  Monomer units can construct a chain-shaped 1D polyrotaxane structure through the complexation between Pb(4) and 3 (highlighted in green colour, Figure 10a). The 1D polyrotaxane further forms 2D frameworks bridged with the coordination between Pb(1) and 3 (highlighted in black colour). Furthermore, the 2D layers shown above are organized to form the final 3D array with the coordination between Pb(3,5) and 3 (highlighted in black and purple colour) ( Figure 11).

Reagents and Analytical Methods
For this study, all reagents were purchased commercially (Aldrich, Acros, or Fisher) and used without further purification. The single crystals used to obtain the X-ray diffraction structure grew as colourless plates with the .cif document available as a separate supporting information file providing details regarding the specific crystal used for the analysis, along with the structure in question. Diffraction grade crystals were obtained by slow evaporation from solution using a mixture of H2O/DMF as described below.
The data crystals were cut from a cluster of crystals and had the approximate dimensions given in the .cif document. The data were collected on a Saturn724+ (2 × 2 bin mode) or Mercury2 (2 × 2 bin mode) CCD diffractometer using a graphite monochromator with MoKα radiation. The structures were solved and refined by full-matrix least-squares on F 2 with anisotropic displacement parameters for the non-H atoms using SHELXL-2014. [49] The hydrogen atoms were calculated in ideal positions with isotropic displacement parameters set to 1.2 × Ueq of the attached atom (1.5 × Ueq for methyl hydrogen atoms).

Reagents and Analytical Methods
For this study, all reagents were purchased commercially (Aldrich, Acros, or Fisher) and used without further purification. The single crystals used to obtain the X-ray diffraction structure grew as colourless plates with the .cif document available as a separate supporting information file providing details regarding the specific crystal used for the analysis, along with the structure in question. Diffraction grade crystals were obtained by slow evaporation from solution using a mixture of H 2 O/DMF as described below.
The data crystals were cut from a cluster of crystals and had the approximate dimensions given in the .cif document. The data were collected on a Saturn724+ (2 × 2 bin mode) or Mercury2 (2 × 2 bin mode) CCD diffractometer using a graphite monochromator with MoKα radiation. The structures were solved and refined by full-matrix least-squares on F 2 with anisotropic displacement parameters for the non-H atoms using SHELXL-2014. [49] The hydrogen atoms were calculated in ideal positions with isotropic displacement parameters set to 1.2 × Ueq of the attached atom (1.5 × Ueq for methyl hydrogen atoms). The function, w (|Fo| 2 − |Fc| 2 ) 2 , was minimized. Definitions used for calculating R(F), Rw(F2), and the goodness of fit, S, are given below and in the .cif documents [49]. Neutral atom scattering factors and values used to calculate the linear absorption coefficient are from the International Tables for X-ray Crystallography (1992) [50]. All ellipsoid figures were generated using SHELXTL/PC [51]. Tables of positional and thermal parameters, bond lengths and angles, torsion angles, and figures and lists of observed and calculated structure factors are located in the .cif documents available from the Cambridge Crystallographic Data Centre (CCDC) via quoting ref. numbers 2084601 and 2084604. The document also contains details of crystal data, data collection, and structure refinement.

General One-Pot Three-Layer Diffusion Protocol
In a separate vial, Pb(NO 3 ) 2 (0.120 mL of 0.05 M solution in H 2 O) was added to form the first layer of three separate layers. A mixture of DMF and H 2 O (3 mL; 1/1, v/v) was added as the middle layer. A premixed mixture containing 1 4+ •4PF 6 − (0.120 mL of a 0.025 M solution in DMF), 2H + •2 or 2H + •3 (0.240 mL of a 0.05 M solution in H 2 O), and tetramethylammonium hydroxide pentahydrate (NMe 4 + •OH − •5H 2 O) (0.480 mL of a 0.05 M solution in H 2 O) was added as the upper layer. The final three-layer systems were set on the bench for two weeks, colourless crystals were cultivated from the clear solution, and these crystals proved suitable for an X-ray diffraction analysis.