Transformation Network Culminating in a Heteroleptic Cd6L6L'2 Twisted Trigonal Prism.

Transformations between three-dimensional metallosupramolecular assemblies can enable switching between the different functions of these structures. Here we report a network of such transformations, based upon a subcomponent displacement strategy. The flow through this network is directed by the relative reactivities of different amines, aldehydes, and di(2-pyridyl)ketone. The network provides access to an unprecedented heteroleptic Cd6L6L'2 twisted trigonal prism. The principles underpinning this network thus allow for the design of diverse structural transformations, converting one helicate into another, a helicate into a tetrahedron, a tetrahedron into a different tetrahedron, and a tetrahedron into the new trigonal prismatic structure type. The selective conversion from one host to another also enabled the uptake of a desired guest from a mixture of guests.

The development of new stimulus-induced transformations between self-assembled architectures is an important challenge in supramolecular chemistry.[1] Such transformations mimic the structural changes of biological molecules,[2] potentially leading to biomimetic functions, such as catalysts that may be modulated using cofactors, as with enzymes.[3] Differences in guest binding properties between interconvertible container molecules[4] also allow for the design of new functions, such as controllable guest uptake and release,[5] chemical purification,[6] reagent storage,[7] and drug delivery.[8] Although stimuli such as light,[5b,9] pH,[10] temperature,[11] solvent,[12] or concentration[13] can induce cages to respond in useful ways, linking these factors together to generate networks of transformations remains a challenging goal.[1a]

Complex metal–organic structures[14] made using subcomponent self-assembly[15] can transform due to the dynamic nature of both the coordinative (N → metal) and covalent (N=C) bonds that knit these structures together. The displacement of one subcomponent for another thus serves as a general strategy for structural interconversion.[16]

As different amines and aldehydes generate metal imine complexes that are more or less stable, the addition of new amines and aldehydes to existing structures can induce these structures to rearrange in well-defined ways through subcomponent substitution.[16] Electronic effects[17] and steric hindrance[18] can serve as driving forces for rearrangement. The presence of multiple distinct subcomponents may also lead to the formation of new heteroleptic assemblies.[19] The arrangements of the different subcomponents within these heteroleptic assemblies may bring about lower symmetries, helping to enable the binding of complex, low-symmetry substrates.[20]

Here we demonstrate the use of subcomponent displacement as an efficient method for bringing about structural transformations within a network of interconverting assemblies. This study led to the discovery of a new type of heteroleptic trigonal prismatic host having anion binding properties.[21] The transformations within the network also enabled selective guest uptake from solution.

We recently reported the use of the subcomponent di(2-pyridyl)ketone A. This ketone imparts improved acid resistance to its imine metal complexes, as compared to 2-formylpyridine, as a result of its free basic pyridyl units at the corners.[22]

Mixture of ketone A (6 equiv), n class="Chemical">triamine B (3 equiv), and cadmium trifluoromethanesulfonate (triflate, TfO–, 2 equiv) produced Cd2L3 triple helicate 1a (Figures and S3–S10). Complex 1a is the first member of a new family of helicates containing free basic pyridyl units and amine functionalities.

Figure 1

Network of transformations between architectures 1–8, showing the X-ray crystal structures of helicate FeII2L31b (a), which is the FeII analogue of CdII2L31a, and (b) TfO–⊂3. For the crystal structures, disorder, unbound counterions, hydrogen atoms, and solvent of crystallization are omitted for clarity.

Although numerous attempts to obtain the crystal structure of Cd2L31a were unsuccessful, we were able to obtain the crystal structure of its FeII analogue 1b, which was synthesized similarly (Figures S11–S18). As shown in Figure a, the two iron(II) centers of 1b, separated by a distance of 13.9 Å, have the same handedness, generating a structure with D3 symmetry. Only two of the amines from each residue of B condensed with ketone A, leading to the formation of C2-symmetric bis-bidentate ligands. Complex 1b thus contains six uncoordinated pyridyl units in total around its two vertices. The vertices of 1b are constrained due to the potential steric clash between these free pyridyl rings and the neighboring phenylene rings (Figure S85a).[22]

The addition of bis-amine C (3 equiv) to 1a led to the formation of new helicate 2, releasing subcomponent B (Figures , S19–S26, and S88). Similarly, addition of triamine F (3 equiv) to 1a produced helicate 4 (Figures S35–S42 and S90). As excesses of C and F were not added, we infer the driving forces for these conversions to have derived from the greater nucleophilicities of the incorporated amines.[16,23]

Analogously, aldehyde E (6 equiv) displaced ketone A from 2, resulting in the formation of more stable helicate 6 (Figures S51–S58 and S92). The release of steric encumbrance at the corners after conversion may provide the driving force for this transformation.[18] The incorporation of the long aliphatic chain in subcomponent E serves to improve the solubility of the corresponding architectures incorporating E (Figure ), in order to avoid solubility problems during structural transformation.

The addition of more electron-rich triamine D (4 equiv) to helicate 1a (2 equiv) resulted in conversion to tetrahedron 3 along with the release of less electron-rich B (6 equiv) (Figures S27–S34 and S89). In addition to the differing nucleophilicities of the amines,[16,23] this transformation may be favored by entropy, as more free particles are present in solution following the substitution reaction. The 19F NMR spectrum of 3 (Figure S29) reveals that a triflate anion resides inside its cavity, indicating that binding of the anion template may provide a further driving force for the transformation.[15d]

The crystal structure of tetrahedron 3 displays approximate T point symmetry (Figure b). Twelve uncoordinated pyridyl units derived from residues of A surround the four vertices of 3, in similar fashion to the structure of its ZnII analogue.[22] Consistent with the solution-phase 19F NMR spectrum (Figure S29), a triflate anion is encapsulated inside the cavity, occupying 54% of its cavity volume (158 Å3, Figure S87).

The addition of aldehyde E (12 equiv) and additional Cd(OTf)2 (1.33 equiv) to helicate 1a (1.33 equiv) resulted in its conversion into tetrahedron 5 (Figures S43–50 and S91). Similarly, the addition of E (12 equiv) and Cd(OTf)2 (1.33 equiv) to helicate 4 (1.33 equiv) gave tetrahedron 7 (Figures S59–S66 and S93), releasing subcomponent A in both cases. We infer the release of steric hindrance at the corners, as observed in the crystal structure of 1b (Figure S85), to have driven these transformations.[18] The greater nucleophilicity of triamine subcomponent F relative to triamine B(16,23) also enabled a tetrahedron-to-tetrahedron transformation from 5 to 7, following the addition of F (4 equiv) to 5 (Figure S94).

The network of transformations shown in Figure culminated in the generation of a new heteroleptic Cd6L6L′2 twisted trigonal prism 8 (Figures S67–S76), produced following the addition of the more nucleophilic (relative to B) subcomponent D (2 equiv) to 5 (1.5 equiv) (Figure S95). The composition of the new complex was confirmed by ESI-MS (Figure S75). The homoleptic tetrahedra 5 and 9 were not detected in solution (Figure S95), indicating that integrative self-sorting had occurred to form exclusively the heteroleptic structure.[24] This complex could be also prepared from the three subcomponents B, D, and E, together with cadmium triflate, in a molar ratio of 6:2:18:6. The 1H NMR peaks of 8 (Figures a and S67), which displayed the same DOSY diffusion coefficient, are sharp and numerous, consistent with the lower symmetry of the heteroleptic structure.[20]

Figure 2

(a) Subcomponent self-assembly of Cd6L6L′28 and the aromatic region of its 1H DOSY spectrum (400 MHz, 298 K, CD3CN). The three imine peaks of 8 are marked with solid black circles (●). For a full structural assignment, see Figure S67. (b, c) Crystal structure of 8 with (b) and without (c) the two bound triflate anions. Disorder, additional counterions, peripheral R groups on the E residues, hydrogen atoms, and solvent of crystallization are omitted for clarity. Only the N atoms of the free amines are colored blue, with the rest of symmetry-equivalent ligands being colored magenta, black, and cyan. CdII centers are yellow, which are connected by yellow lines to their nearest neighbors. Average CdII–CdII distances are labeled in (a).

Slow vapor diffusion of diethyl ether into an acetonitrile solution of 8 provided crystals suitable for X-ray crystallographic analysis. As shown in Figures and S86, Cd6L6L′28 was revealed to have a twisted trigonal prismatic structure. Six ligands (L, formed from imine condensation of B with 2 equiv of E) define the three twisted rectangular faces, with pairs of these ligands bridging two diagonally opposed cadmium centers with their pyridyl-imine arms. Each equivalent of B in 8 contains one unreacted aniline NH2 group, as was also observed for B in helicate 1b. Two ligands (L′, formed from imine condensation of D with 3 equiv of E) define the two triangular faces of the trigonal prism, which are twisted by 35 ± 3° relative to each other. All of the CdII centers of 8 show facial (fac) coordination and display the same handedness. The framework of 8 thus has approximate D3 point symmetry. The 1H NMR spectrum of heteroleptic 8 (Figure S67) in solution is consistent with its crystal structure.

The structure of heteroleptic 8 is stabilized by stacking interactions (3.4 Å) between the two central benzene rings of the two triangular faces; it thus lacks an internal cavity. Instead, two bowl-shaped cavities are defined by the two concave L′ ligands of the triangular faces, with each accommodating a triflate anion (Figure b).

A series of competition reactions (Figures S96–S102) showed that each of the homoleptic products 2–7 (Figure ) formed selectively in the presence of the subcomponent that was displaced during the formation of that structure in Figure , verifying that all transformations resulted in the formation of the thermodynamic product.

The guest binding properties of the helicates listed in Figure were not investigated in solution due to their small internal cavities and large apertures (Figure S85b). Tetrahedron 3 was observed to encapsulate its counteranion, triflate, in slow exchange on the NMR time scale (Figure S29), as noted above. Neutral molecules, such as cyclohexane, were found to bind within tetrahedron 5 in slow exchange (Figure S103), in similar fashion to the binding behavior observed for its zinc(II) analog.[25] Both tetrahedron 7 and trigonal prism 8 bound AsF6– and SbF6– in fast exchange on the NMR time scale (Figures S104–S108). Progressive addition of AsF6– or SbF6– to 8 resulted in clear shifts of the proton signals of the ligands defining the concave triangular faces (Figures S107 and S108), consistent with anions binding in the locations observed in the crystal structure (Figure b).

We hypothesized that the selective conversion from one host to another within the transformation network might enable selective guest uptake. To test this idea, we designed a system starting with helicate 1a and two different guests, triflate and cyclohexane, which are bound by tetrahedra 3 and 5, respectively. As shown in Figure , after addition of subcomponent D to the system, helicate 1a converted into 3, which took up the triflate guest selectively from solution (Figure S109). In contrast, the addition of E to 1a resulted in displacement of the ketone subcomponents, producing 5 and selectively binding cyclohexane (Figure S110).

Figure 3

Schematic representation of selective uptake of either triflate or cyclohexane upon structural transformations.

Schematic representation of selective uptake of either triflate or n class="Chemical">cyclohexane upon structural transformations.

Subcomponent displacement thus offers a versatile strategy for structural transformations between different complex assemblies, allowing selective guest uptake following structural transformation. A new type of M6L6L′2 twisted trigonal prism was produced using this strategy, the guest binding properties of which are of interest.[20] Studies to expand the generality of its formation with other tritopic or ditopic ligands and to achieve heteroleptic architectures with new guest binding properties are currently underway.