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

Abstract

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).

1. Introduction

Metal-organic framework (MOF) materials have potential applications in a variety of fields, such as gas storage [1,2,3,4], heterogeneous catalysis [5], sensors [6], luminescence material [7], etc. Specifically, metal-organic rotaxane framework (MORF) structures involve mechanically interlocking molecules (MIMs; i.e., rotaxanes) [8,9,10,11,12] and attract interest of the chemical community. To date, crown ether [13,14,15,16,17,18], “blue box” (CBPQT⁴⁺) [19,20], cucurbiurils [21,22,23], octametallic metallacrown [24,25,26,27], and polyamide [28,29] have been utilized to produce a series of MORF structures. The Texas-sized molecular box ([cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene), 1⁴⁺) [30,31] was developed to construct MORFs cooperating with metal cations (e.g., Zn²⁺, Ag⁺, or lanthanide) and aromatic dicarboxylate anions (Figure 1).

Recently, Pb-containing materials have received widespread attention, including perovskite materials and novel lead batteries. The reported Pb-containing MOFs present different applications in gas storage [32,33,34], catalysis [35,36,37], electrode materials [38], high-energy density materials [39,40], luminescence materials [34,37,41], DNA probes [42], etc. Surprisingly, to the best of our knowledge, it is still a challenge to effectively construct MORF materials with Pb metal cation participation. Herein, we demonstrated that Pb²⁺ can cooperate with the Texas-sized molecular box (1⁴⁺) and aromatic dicarboxylate anions (e.g., p-phthalate dianions (PTADAs; 2) or 2,6-naphthalene dicarboxylate dianions (3)) to construct the first class of Pb²⁺-containing MORF structures.

2. Results and Discussion

A facile multicomponent self-assembly-based approach was developed to generate Pb²⁺-containing MORFs. It is a one-pot three-layer diffusion protocol. As detailed below, Pb²⁺ cation (Pb(NO₃)₂; 0.120 mL of 0.05 M solution in H₂O) was added as the first layer of three separate layers within a separate vial. A mixture of DMF and H₂O (3 mL; 1:1, v/v) was then added as the middle layer. A premixed solution (containing 1⁴⁺•4PF₆⁻ (0.0036 M), p-phthalic acid (2H⁺•2; 0.0143 M), and NMe₄⁺•OH⁻•5H₂O (0.0286 M) in a mixture of DMF and H₂O (0.84 mL; 1:1, v/v)) was then added as the upper layer. Standing for two weeks, [1⁴⁺•(2)₄•(2H⁺•2)•Pb₂•2DMF•12H₂O] (namely MORF-Pb-1) was obtained as single crystals suitable for X-ray diffraction analysis. When 2H⁺•2 was replaced by 2,6-naphthalene dicarboxylic acid (2H⁺•3) in the same protocol, crystalline samples of the [1⁴⁺•(3)₅•Pb₃•12.5H₂O] (MORF-Pb-2) were achieved. Single crystal analysis provided direct evidence for the formation of both Pb²⁺-containing MORF structures (

Table 1. X-ray crystallographic data summary of MORF-Pb-1 [1⁴⁺•(2)₄•(2H⁺•2)•Pb₂•2DMF•12H₂O] and MORF-Pb-2 [1⁴⁺•(3)₅•Pb₃•12.5H₂O].

	[14+•(2)4•(2H+•2)•Pb2•2DMF•12H2O]	[14+•(3)5•Pb3•12.5H2O]
CCDC No.	2084601	2084604
Description	prism	prism
Colour	colourless	colourless
From solution	DMF/H2O	DMF/H2O
Empirical formula	C84H94N12O34Pb2	C98H89N10O32.5Pb3
Mr	2230.09	2548.36
Crystal size (mm3)	0.11 × 0.07 × 0.04	0.21 × 0.12 × 0.10
Crystal system	triclinic	monoclinic
Space group	P -1	P 21/n
a [Å]	10.848(2)	20.504(4)
b [Å]	14.117(3)	20.005(4)
c [Å]	14.758(3)	24.046(5)
$\alpha$[deg]	100.95(3)	90
$\beta$[deg]	101.97(3)	101.23(3)
$\gamma$[deg]	90.31(3)	90
V/ [Å3]	2168.3(8)	9674(3)
d/[g/cm3]	1.711	1.75
Z	1	4
T[K]	173.15	153.15
R1, wR2 I > 2ó(I)	0.0462, 0.1398	0.0789, 0.1877
R1, wR2 (all data)	0.0485, 0.1426	0.1587, 0.2551
Quality of fit	1.008	1.006

).

2.1. Crystal Structure of [1⁴⁺•(2)₄•(2H⁺•2)•Pb₂•2DMF•12H₂O] (MORF-Pb-1)

In MORF-Pb-1, a chain-shaped 1D polyrotaxane structure [1⁴⁺•(2)₄•Pb₂]_n was constructed via the complexation between Pb(1) and 2 (highlighted in magenta colour). The 1D polyrotaxane further form a 2D and 3D array stabilized by intermolecular π-π donor–acceptor interactions and a possible unusual Pb…Pb weak interaction.

In the structure of MORF-Pb-1, Pb²⁺ cations locate in the same environment. As Figure 2a shows, Pb(1) coordinates with three p-phthalate dianions (2) in different local chemical environments. Pb²⁺ has an outmost electronic closed shell as 5d¹⁰6p² and a reported ionic radius of 1.33 Å. [43] It was compared with the other closed outmost shell Zn²⁺ cation (3d¹⁰, ionic radius of 0.74 Å) in the presence of 1⁴⁺ and 2. Swapping Pb²⁺ with Zn²⁺, the same procedure resulted in [(1⁴⁺)₂•(2)₉•Zn₆•12H₂O]•2OH⁻•88.5H₂O [44]. These structures showed that each Zn²⁺ only binds with two p-phthalate dianions (2) and cannot cooperate with 2 inserting into the cavity of 1⁴⁺ to form MIMs. All these findings show that the interactions between Pb²⁺ and 2 and the further MORF-Pb-1 construction are highly metal cation dependant.

One molecule of 2 threads through an individual 1⁴⁺ via C-H…π interactions between the bridge bezene planes on 1⁴⁺ and the aromatic ring of 2 highlighted in magenta in Figure 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 1⁴⁺ and 2.

The asymmetric monomer unit of MORF-Pb-1 contains one molecule of macrocycle 1⁴⁺, three Pb²⁺, 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 Pb²⁺ on different 1D polyrotaxane is 3.656 Å (shown in Figure 6a), which is larger than the sum of two Pb²⁺ 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 [1⁴⁺•(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 1⁴⁺ 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 1⁴⁺ on different 2D layers. Finally, the 3D array of MORF-Pb-1 was achieved (Figure 7).

2.2. Crystal Structure of [1⁴⁺•(3)₅•Pb₃•12.5H₂O] (MORF-Pb-2)

In MORF-Pb-2, a chain-shaped 1D polyrotaxane structure [1⁴⁺•(3)₅•Pb₆•H₂O]_n was constructed via the complexation between Pb(4) and 3 (highlighted in green colour). The 1D polyrotaxane further forms 2D and 3D frameworks bridged with the coordination between Pb(1) and 3 (highlighted in black colour) and Pb(3, 5) and 3 (highlighted in black and purple colour), respectively.

In the structure of MORF-Pb-2, Pb²⁺ has five different coordination modes, labelled as Pb(1, 2, 3, 4, or 5), different from [1⁴⁺•(3)₄•Zn₂•6H₂O] (MORF-Zn) containing 1⁴⁺, 3, and Zn²⁺, while Zn²⁺ coordinates with three 2,6-naphthalene dicarboxylate dianions (3) in different local chemical environments [47]. Meanwhile, Ag⁺ (4d¹⁰, ionic radius as 1.26 Å) can cooperate with 1⁴⁺ and 3 to form [1⁴⁺•(3)₃•Ag₂•16H₂O] [48]. In MORF-Ag, Ag⁺ coordinates with two 2,6-naphthalene dicarboxylate dianions (3) in different local chemical environments in [1⁴⁺•(3)₃•Ag₂•16H₂O] [48] (Figure 8).

The pseudorotaxane structure in [1⁴⁺•3•Pb₂]⁶⁺ (MORF-Pb-2) is also formed via π-π donor–acceptor interactions between 3 and 1⁴⁺. However, the structure of MORF-Pb-2, 1⁴⁺ is distorted and twisted into a “boat” configuration, while 1⁴⁺ adopts a more regular “box”-like configuration in MORF-Zn or MORF-Ag (Figure 9c; M = Zn²⁺ or Ag⁺).

The monomer unit of MORF-Pb-2 was found to include one molecule of macrocycle 1⁴⁺, five Pb²⁺, five dicarboxylate ligands 3 in different local chemical environments, and one coordinated H₂O solvent moiety (Figure 10).

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).

3. Materials and Methods

3.1. 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 F² 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.

3.2. General One-Pot Three-Layer Diffusion Protocol

In a separate vial, Pb(NO₃)₂ (0.120 mL of 0.05 M solution in H₂O) was added to form the first layer of three separate layers. A mixture of DMF and H₂O (3 mL; 1/1, v/v) was added as the middle layer. A premixed mixture containing 1⁴⁺•4PF₆⁻ (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 H₂O), and tetramethylammonium hydroxide pentahydrate (NMe₄⁺•OH⁻•5H₂O) (0.480 mL of a 0.05 M solution in H₂O) 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.

4. Conclusions

In summary, Pb²⁺-containing MORF structures (i.e., [1⁴⁺•(2)₄•(2H⁺•2)•Pb₂•2DMF•12H₂O] (MORF-Pb-1) or [1⁴⁺•(3)₅•Pb₃•12.5H₂O] (MORF-Pb-2)) were constructed using Pb²⁺, a tetraimidazolium macrocycle (1⁴⁺), and p-phthalate dianions (2) or 2,6-naphthalene dicarboxylate dianions (3) in a one-pot three-layer diffusion protocol, separately. Compared with smaller Zn²⁺ and Ag⁺ with the outmost electronic closed shell, Pb²⁺ has more diverse coordination modes and benefits for metal-organic polyrotaxane formation. In particular, an unusual Pb…Pb weak interaction (characterised with distance of 3.656 Å) was shown in MORF-Pb-1. Further construction, property, and usability studies of Pb²⁺-containing MORFs are currently being evaluated.