Subtle Ligand Effects on the Formation and Behavior of Inorganic Guest-Organic Host Supramolecular System Soon Sik Kwon,† Moon Soon Cha,† Ji Eun Lee,‡ Shim Sung Lee,‡ and Ok-Sang Jung*,† Department of Chemistry, Pusan National UniVersity, Pusan 609-735, Korea and Department of Chemistry, Gyeongsang National UniVersity, Jinju 660-701, Korea
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2073–2075
ReceiVed February 5, 2008; ReVised Manuscript ReceiVed May 21, 2008
ABSTRACT: An
unusual “cationic inorganic guests within anionic organic hosts” system was constructed. For [Pd(ppma)2]@2H2tma · 2H3tma (ppma ) N-(pyridine-2-yl)pyrimidin-2-amine; H2tma ) deprotonated trimesic acid; H3tma ) trimesic acid), one ppma is dissociated in methanol, whereas for [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O (dpa ) di(pyridine-2-yl)amine), [Pd(dpa)2]2+ species is retained the same solution. Thus, subtle differences between ppma and dpa species exist both in solid state and in solution. Guest-host supramolecular systems continue to receive a great deal of attention because they often provide invaluable insight into functional molecular materials such as separation devices, catalysts, chemical sensors, nonlinear materials, magnets, and solid-state reaction platforms.1–5 Control of weak interactions is a key step in the assembly of supramolecular systems, the realization of devices, and the hierarchical growth of informed systems.6–10 Thus, delicate energy differences induce significant structural transformation during supramolecular assembly processes and affect system behaviors.11 In particular, two-dimensional (2D) assembly potentially can open the way to applications such as organic network templates, porous materials, and adsorptions.12 Among the various supramolecular tectons, trimesic acid (H3tma, 1,3,5-C6H3(COOH)3) represents a system prototype useful for the formation of hexagonal topology hydrogen-bonded 2D networks providing open voids.13–15 Furthermore, the deprotonated carboxylate of H3tma generally can act as an anionic ligand to complete the metal coordination as well as compensate the charge.16–21 Thus, metal complexes and acid-base complexes in addition to trimesic acid itself have been prepared,22–24 but the construction and behavior of “an inorganic guest within an organic host” remains unexplored. With this understanding, one crucial aim of the present study was to explore the behavior of the included guest metal complex within the open organic framework of trimesic acid. In this paper, we report the subtle ligand effects on the formation and behavior of a supramolecular “an inorganic guest within an organic host” system.
Figure 1. Crystal structures of a single-layer motif of [Pd(ppma)2]@2H2tma · 2H3tma (top) and double-layer motif of [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O (bottom).
Reaction of PdCl2 with N-N (N-N ) N-(pyridine-2-yl)pyrimidin-2-amine (ppma); di(pyridine-2-yl)amine (dpa)) in a 2:1 molar * To whom correspondence should be addressed. Fax: 82-51-516-7421. E-mail:
[email protected]. † Pusan National University. ‡ Gyeongsang National University.
ratio in a mixture of H2O/EtOH, followed by treatment with H3tma, produced yellow crystals of [Pd(N-N)2]@2H2tma · 2H3tma suitable for X-ray single crystallography (see the Supporting Information). X-ray characterizations25 of single crystals have established that both skeletal structures have hydrogen-bonded 2D networks consisting of H2tma and H3tma (1:1), as shown in Figure 1. In contrast with the structure of H3tma,26 that of [Pd(ppma)2]@2H2tma · 2H3tma includes two trimesic unit hydrogenbonding synthons (I, dimer; and II, catemer in the 2:1 molar ratio).15
10.1021/cg800140s CCC: $40.75 2008 American Chemical Society Published on Web 06/07/2008
2074 Crystal Growth & Design, Vol. 8, No. 7, 2008
Communications To study the behavior of the “an inorganic guest within an organic host” in solution, the 1H NMR spectra were measured in MeOH-d4 (Figure 2). The NMR spectra show that the [Pd(ppma)2]@2H2tma · 2H3tma was converted into a unique species, [Pd(ppma)(H2tma)2] along with ppma and 2H3tma in methanol (Figure 2b). The appearance of two sets of ppma peaks in a 1:1 molar ratio indicates that the one ppma ligand dissociated from the metal complex. In the 1H NMR, H2tma and H3tma were not discriminated in the MeOH-d4 solution presumably owing to the fast exchange between H2tma and H3tma in the solution. Conductivity measurements of [Pd(ppma)2]@2H2tma · 2H3tma in the solution (60 mS/cm in MeOH) supported the behavior of the partial dissociated carboxylate in MeOH. To the best of our knowledge, this system is an unusual isomerism between the ppma and the carboxylate group of H2tma which are the moieties of the guest and the host, respectively. A more interesting feature is the dissociated ppma returning to its original coordination site palladium(II) when the single crystals are obtained from the ethanol solution, even though the organic skeletal structure is not restored because of the solubility difference between methanol and ethanol and the inadequate quantity of tma in the ethanol solution (see the Supporting Information). In contrast, for [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O, [Pd(dpa)2]2+ species is retained in methanol (Figure 2d), indicating that [Pd(dpa)2]2+ is more stable than [Pd(ppma)2]2+. Moreover, the reaction of [Pd(ppma)2]@2H2tma · 2H3tma with an equivalent quantity of dpa in chloroform produced the [Pd(dpa)2]2+ species (Figure 2c). The chemical shifts of the [Pd(dpa)2]2+ were positioned downfield relative to that of the known neutral compound, [Pd2+(dpa-)2],27 indicating that the present system exists as ionic species, [Pd(dpa)2]2+ rather than a neutral species [Pd2+(dpa-)2] in solution.
Figure 2. 1H NMR spectra of (a) a mixture of ppma and tma (1:1), (b) [Pd(ppma)2]@2H2tma · 2H3tma, (c) [Pd(ppma)2]@2H2tma · 2H3tma + 2dpa, and (d) [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O in MeOH-d4.
Thus, the organic skeleton consists of 48-, 40-, and 16-membered macrocyclic rings. The cationic [Pd(ppma)2]2+ complex is nestled in the 48-membered ring. The -NH moiety of the metal complex interacts with a carboxylic oxygen (IV, N · · · O distance ) 2.99 Å) of trimesic acid. The 2D sheets stack in an ABAB · · · sequence. For [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O, the organic skeleton is a 42-membered hexagon consisting of two synthons (I; III ) hydrogen bond through a water molecule), and [Pd(dpa)2]2+ is incorporated into each hexagon of the two-layer network. The two layer are interconnected through water molecules via hydrogen bonds (III). In the organic skeleton, the hydrogen bonds (O · · · · X distance of O-H · · · X) fall into the ranges of 2.5-3.0 Å. In this system, mixed trimesic acids of H2tma and H3tma along with the presence of [Pd(dpa)2]2+ seem to be attributable to the unusual motifs, in contrast with trimesic acid. This is an unprecedented supramolecular “cationic inorganic guest within an anionic organic host” assembly. Each palladium(II) ion is in a distorted square planar arrangement (N-Pd-N ) 86.50(7), 93.50(7)° for [Pd(ppma)2]2+; 86.89(6), 93.1(6)° for [Pd(dpa)2]2+) with four pyridine units (Pd-N ) 2.021(2), 2.031(2) Å for [Pd(ppma)2]2+; 2.022(1), 2.022(1) Å for [Pd(dpa)2]2+). The six-membered metallacycle consisting of Pd, N, N, C, C, and N is of boat form. The structural difference between [Pd(ppma)2]@2H2tma · 2H3tma and [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O can be attributed to the Lewis basicity difference between ppma and dpa together with the absence or the presence of crystalline water molecules. The skeletons of [Pd(ppma)2]@2H2tma · 2H3tma and [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O are thermally stable up to 340 and 300 °C, respectively. For [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O, solvate water molecules begin to evaporate at 140 °C (see the Supporting Information).
The driving force behind the formation of the unusual tma network is clearly the various hydrogen bonds. Furthermore, the guest cationic palladium(II) complex should affect the formation of the organic network, affording a different network contrasting with that of H3tma.26 Whereas it is difficult to predict the formation of inorganic-organic composite supramolecular system, the nonrigid organic framework has resulted in the realization of labile cationic metal complex. That is, the hydrogen bonds are dissociated in methanol, after which the deprotonated H2tma acts as a ligand or a counteranion. This system is a good example of competition between N-N (ppma, pma) and the carboxylate moiety of H2tma. Why is the ppma dissociated in solution? The six-membered metallacycle of [Pd(ppma)2]2+ is of the boat form, as revealed by X-ray structure, in contrast with the stable chair form. Furthermore, one Pd-N bond length of [Pd(ppma)2]2+ (2.031(2) Å) is longer than the corresponding bond of [Pd(dpa)2]2+ (2.022(1) Å), which in fact can be ascribed to the difference in Lewis basicity between ppma and dpa. Retention of the structure of [Pd(ppma)2]2+(ClO4-)2 (see the Supporting Information) in solution indicates that the dissociation of ppma in the [Pd(ppma)2]2+ species is induced by the carboxylate of H2tma. Thus, the most fascinating feature is that the unusual isomerism between the inorganic guest and organic host moiety occurs in solution.
Communications
Crystal Growth & Design, Vol. 8, No. 7, 2008 2075 Scheme 1 (4) (5) (6) (7) (8) (9) (10) (11) (12)
(13) (14) (15) (16) (17) (18)
In conclusion, the present reaction afforded an unprecedented supramolecular “a cationic inorganic guest nestled within an anionic organic host” system showing isomerism between the host and guest moiety. The solid structure and solution-behavior of the system exhibits the effects of the subtle difference between dpa and ppma. Further experiments will provide more detailed information on the enormous potentials, which include design of organic-inorganic hybrid materials, guest-host dynamics, hydrophilicity, stable storage, and intrachannel reactivity.
Acknowledgment. This work was supported financially by KRF2007-314-C00157 in Korea.
(19) (20) (21) (22) (23) (24) (25)
Supporting Information Available: X-ray data of [Pd(ppma)2]@2H2tma · 2H3tma, [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O, and [Pd(ppma)2] · 2H2tma · 2CH3OH (CIF); 1H NMR (Me2SO-d6) of [Pd(ppma)2](ClO4)2; IR spectra of [Pd(ppma)2]@2H2tma · 2H3tma, [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O, and [Pd(ppma)2] · 2H2tma · 2CH3OH;TGA and DSC curves of [Pd(ppma)2]@2H2tma · 2H3tma and [Pd(dpa)2]@2H2tma · 2H3tma · 2H2O (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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CG800140S
