Multiporphyrinic Cages: Architectures and Functions - Chemical

Review pubs.acs.org/CR

Multiporphyrinic Cages: Architectures and Functions Stéphanie Durot, Julien Taesch, and Valérie Heitz* Laboratoire de Synthèse des Assemblages Moléculaires Multifonctionnels, Institut de Chimie de Strasbourg, CNRS/UMR 7177, 4, rue Blaise Pascal, 67000 Strasbourg, France encapsulation of even more than one guest molecule. The reactivity of the trapped molecules within the cavity was explored, leading to spectacular results in terms of selectivity and yield of reactions and the concept of nanoreactors emerged.3r,6 The interest of molecular chemists for hollow structures is also related to the active cavity of natural enzymes that clearly demonstrates the benefit to preorganize the active sites and the substrates in a confined environment for highly selective and efficient chemical transformations.7 Porphyrin derivatives and their metalated forms are currently used in natural processes for light harvesting, electron and energy transfer reactions, catalysis, or as oxygen transporter. They have attracted the attention of a broad range of researchers because their stable aromatic core can be functionalized on meso or CONTENTS pyrrolic β positions and the inserted metal can modulate their chemical, electronic, and photophysical properties. Chemists 1. Introduction 8542 have incorporated them in lots of different molecular 2. Covalent Multiporphyrinic Cages 8543 architectures for various fields of applications,8 like artificial 2.1. Stepwise Synthesis 8543 photosynthesis,9 molecular electronics,9m,10 molecular ma2.2. Templated Synthesis 8547 chines,11 catalysis,12 therapy,13 and surface engineering.14 The 3. Supramolecular Multiporphyrinic Cages 8550 design of molecular cages including porphyrins in the host 3.1. Cages Self-Assembled by Coordination structure opens several attractive opportunities: porphyrin, as a Bonds 8550 large structural element will delineate the molecular cavity; as an 3.1.1. Synthetic Strategies 8550 active component, its large π-delocalized core can stabilize π3.1.2. Axial Coordination of Zinc Porphyrins 8551 conjugated guest molecules inside the cavity, whereas its 3.1.3. Coordination of Ligands Appended to metalated form can coordinate various ligands within the cage; Porphyrins to External Bridging Comas a redox and photoactive component, it can participate directly plexes 8559 to the reactivity performed inside the structure. 3.2. Cages Self-Assembled by H-Bonds 8569 This Review will present an overview of the synthesis and 4. Multiporphyrinic Cages Based on Threaded applications of molecular containers containing more than one Components 8571 porphyrin in their framework and enclosing a three-dimensional 5. Conclusion and Outlook 8573 (3D) cavity. Only the multiporphyrinic molecular cages held Author Information 8574 together by more than two linkers will be considered, as these Corresponding Author 8574 multiple linkers restrict or prevent free rotation of the porphyrin Notes 8574 derivatives and in such molecular architectures an internal 3D Biographies 8574 void space exists (Figure 1a). The cyclic porphyrinic structures Acknowledgments 8575 with a two-dimensional (2D) cavity will not be reported as well as Dedication 8575 the cofacial bis-porphyrins covalently linked by one or two Abbreviations 8575 flexible or rigid linkers (Figure 1b). These tweezer or macrocyclic References 8575 structures have already been reported in excellent reviews, whereas the synthesis of 3D cages is nowadays a rapidly growing 1. INTRODUCTION field of research.9l,15 The enhanced rigidity and preorganization of 3D compounds and their protected inner cavity from the Since the first synthesis of cryptands and cavitands, molecular external medium is in favor of enhanced guest binding properties cages defined as hollow structures enclosing a three-dimensional and improved control over the reactivity. Nevertheless, some of cavity have experienced a large development in terms of their the methodologies developed to prepare these 2D porphyrinic synthesis and applications.1 Their construction based on compounds contributed to the synthetic strategies used for irreversible covalent bonds2 has broadened toward architectures 2j,3 multiporphyrinic cages, such as the template synthesis of self-assembled through metal−ligand interactions, hydrogen 2j,3s,4 bonds or other weak interactions, or prepared thanks to reversible covalent bonds.5 Their receptor properties were Received: November 26, 2013 Published: July 15, 2014 extended from small molecules to larger guests, enabling © 2014 American Chemical Society

8542

dx.doi.org/10.1021/cr400673y | Chem. Rev. 2014, 114, 8542−8578

Chemical Reviews

Review

most developed structures incorporate two cofacial porphyrins bridged by four linkers of various length and flexibility. With rigid linkers, the distance and relative orientation of the porphyrins is clearly defined, and these molecules are well adapted to study the electronic interactions between cofacial porphyrins, which is of fundamental importance regarding the specific properties of the porphyrinic derivatives involved in natural photosynthesis. They were also developed to mimic the receptor, redox, and catalytic activity of various hemoproteins. More flexible structures are also interesting because they enable one to develop adaptable receptors for guests of various sizes through an induced fit mechanism. The synthesis of covalent multiporphyrinic cages requires an important effort especially in the case of a stepwise synthesis (section 2.1), which starts usually with the preparation of a functionalized porphyrin ready to cyclize. Whatever the type of the chosen cyclization reaction, the closing step competes with the formation of oligomers that have to be limited by using high or pseudohigh dilution conditions. The final yield and amount of cage product can be disappointing due to the difficulties in isolating the compound from the side products. In the reported examples, yields ranged from 10% to 56%. As expected, the highest yields were obtained when the cyclization involves the lowest number of components and when it proceeds through linking short and rigid connectors through the benefit of preorganization once the first covalent link between porphyrins has been made.19 The use of alkyne metathesis, a dynamic covalent transformation whose reversible nature allows error checking and correction, represented a breakthrough in the field of covalent cages synthesis.19c The cage closure-step can also rely on the porphyrin ring synthesis, with the benefit of porphyrinogen ring formation proceeding under thermodynamic control, but this strategy is less convergent and was reported only twice, by Kagan et al.20 and Chen et al.21 Template-directed synthesis of covalent structure is an interesting way to organize the porphyrinic components and to provide a geometrical or topological control over the cyclization step (section 2.2). In the reported examples, large aromatic molecules organize two porphyrins through π interactions22 or through axial coordination of nitrogen-based ditopic ligand to zinc(II) porphyrins.23 The stability of the templated assembly and the adequate length and rigidity of the appended porphyrin chains to be closed allow for high yield synthesis of the desired 3D architecture. In case of templating cage formation involving more than two porphyrins, an appropriate multitopic template that affords a high degree of porphyrin preorganization has to be designed to efficiently cyclize the templated precursors, as described by the works of Inamota et al.24 and Youm et al.25

Figure 1. (a) Some representative 3D multiporphyrinic cages described in this Review; and (b) tweezer and macrocyclic porphyrinic structures that are not considered in this Review.

multiporphyrin cyclic arrays developed by Sanders and coworkers in the beginning of the 1990s.15a,b,16 The same group also demonstrated very promising chemical reactivity associated with the porphyrinic macrocycle when two reactants bind to the zinc porphyrins inside the macrocycle.17 More recently, a step toward intriguing bioinspired catalysts was crossed when cofacial porphyrinic macrocycles incorporating metal complexes as allosteric regulation sites were reported by the group of Mirkin.18 Coordination of effector ligands to the hemilabile transition metal complexes modulates the distance between the porphyrin catalytic sites and allows for an allosteric control of the catalytic activity within the macrocyclic structure. Exploring challenging synthetic pathways, mimicking the pocket structure of natural enzymes, stabilizing to a greater extent host−guest complexes, increasing the selectivity and performing more efficient reactivity, and exploring new fields of applications are among the motivations on going from twodimensional to three-dimensional porphyrinic architectures, as evidenced in the examples discussed below.

2.1. Stepwise Synthesis

The first covalent porphyrinic cage, named strati-bis-porphyrin, was designed by Kagan et al.20 in 1977 (Figure 2). The key step of its synthesis was the formation of the second porphyrin from a tetra-aldehyde porphyrinic derivative by using Adler and Longo’s methodology.26 The strati-bis-porphyrin 1 was obtained in 8% yield. This compound enabled the study of the electronic interactions between two porphyrins in close proximity, by analogy with the bacteriochlorophyll dimer that acts as the primary electron donor in natural bacterial photosynthesis. Double-decker porphyrin 2 reported by Neumann and Vögtle27 in 1988 is the second historical example (Figure 3). The synthetic strategy differs significantly from the one used in the work of Kagan and co-workers; the two porphyrins were

2. COVALENT MULTIPORPHYRINIC CAGES This section describes multiporphyrin 3D architectures assembled by covalent bonds. These cages were the first to be developed, their robustness and their characterizations by classical methods being their major assets. The simplest and 8543


Chemical Reviews

Review

Figure 2. strati-Bis-porphyrin reported by Kagan’s group.20

Figure 4. Examples of covalent containers 3a−d and 4 developed by Bruice and co-workers: (a) cofacial bis-porphyrins19a,29a−c,e and (b) spheroidal bis-porphyrin.30b

Figure 3. Covalent cage synthesized by Neumann and Vögtle.

reported to be the first water-soluble porphyrinic cage in 1991.29e Structures in chloroform were analyzed by 2D NMR spectroscopy, and the calculated structure (Figure 5) revealed that the interplanar distance between the porphyrin planes (5.17 Å in the case of cage 3c) depends on the steric bulk of the R substituent. Cofacial bis-cobalt(II) porphyrin dimers were also prepared and reported as the first metalloporphyrin dimers that catalyze the four electron electrochemical reduction of dioxygen to water.29a For example, in acidic medium, the quadruply bridge cationic dimer 3d was shown to catalyze more efficiently dioxygen reduction than the triply or doubly bridge dimers, possibly due to a decrease in the structure flexibility and an increase of the peripheral positive charges. The same group reported a more complex structure, the spheroidal bis-porphyrin 4 represented in Figure 4b.30 This molecule is composed of a porphyrinic double-decker whose external faces are capped by a bicyclo[2.2.2]octane derivative. Apart from its aesthetic value and the synthetic challenge that it represented (nine steps, 1% yield over the two last steps), this molecule was designed to mimic the function of heme-containing enzymes. Unfortunately, no data concerning its activity have been published.

27

formed in a first step, and then a six-component cyclization reaction involving four diacid chloride linkers was carried out in high dilution conditions. The 4-fold cyclization reaction afforded the porphyrinic double-decker in 18% yield. This compound was supposed to be the first optical switchable cage because each arm is functionalized with an azobenzene moiety. Unfortunately, no information about the azo unit photoisomerization was reported. In 1992, the same group described the synthesis of a similar cage that incorporates a 2,2′-bipyridine moiety28 in the four porphyrin connectors. In the early 1990s, Bruice and co-workers19a,29 developed a novel family of bis-porphyrins. Several synthetic pathways were reported to obtain cofacial bis(5,10,15,20-tetraphenyl)porphyrins quadruply linked via relatively short aza bridges (Figure 4a). The best yields (up to 38%) were obtained by a twocomponent cyclization reaction in high dilution conditions. Incorporation of N-methylpyrid-3-yliumsulfonamide groups in the linkages afforded the tetracationic dimer 3a, that was 8544


Chemical Reviews

Review

conditions by slow addition of para-xylylenedibromide to a solution of meso-tetrakis(4-(diphenylphosphino)-phenyl)porphyrin in DMF at 100 °C, to afford the corresponding cage in almost quantitative yield, which is quite remarkable. Another water-soluble porphyrinic cage, 6 (Figure 6b), was reported by Fujita21 and co-workers in 1999. In this molecule, the two porphyrins were linked together by four γ-cyclodextrins, these units providing the water solubility of the whole molecule. The dialdehyde derivative of the acetylated γ-cyclodextrin was condensed with 2 equiv of pyrrole in Lindsey’s conditions, affording the molecule in 3% yield after deacetylation of the cyclodextrin units. Shinkai and co-workers have developed several artificial receptors incorporating porphyrins for an allosteric control of guest binding.11m,32 In 2001, they reported a C3-symmetrical system 7 (Figure 7) incorporating three zinc(II) porphyrins Figure 5. Calculated structure (stereo view) of the most stable conformer of 3c (Quanta 3.2 (MSI), CHARMm 21.3): (a) side view and (b) top view.29a Reprinted with permission from ref 29a. Copyright 1993 American Chemical Society.

Märkl et al.31 developed an efficient synthesis of another watersoluble quadruply bridged bis-porphyrin, 5, that incorporates two phosphonium cations in each connector (Figure 6a). The porphyrin double-decker 5 was formed under high dilution

Figure 7. Tris-porphyrin covalent cage 7, functionalized with two ionophoric calix[3]arenes binding sites.33

capped by two homooxacalix[3]arenes.33 The system was designed to encapsulate C3-symmetrical guests molecules and to control the receptor properties by an external effector, thanks to the ionophoric sites present at the lower rim of the calixarenes. The design was effective; the zinc porphyrins coordinate strongly the tris(2-aminoethyl)amine (TREN) guest inside the capsule (Ka = 1.7 × 105 M−1 in CHCl3 at 25 °C), whereas addition of sodium ions enhanced the binding by about 3 orders of magnitude, showing evidence of a positive allosterism. In 2007, Osuka et al.34 published the synthesis of a hexaporphyrinic macrobicyclic cage molecule 8 with a C3symmetry (Figure 8). Each handle was constituted by a mesolinked bis-porphyrin, and this linkage was obtained thanks to a silver(I)-promoted direct meso−meso coupling, a reaction developed by the same group in 1997.35 The cage-closure step consisted of an intramolecular meso−meso coupling reaction of 9, which was carried out under high dilution in the presence of silver(I) hexafluorophosphate, as represented in Figure 8. The hexameric porphyrin cage 8 was obtained in 38% yield by

Figure 6. Two water-soluble cages developed by two different groups: (a) phosphonium-based cage by Märkl et al.31 and (b) γ-cyclodextrinbased cage by Fujita and co-workers.21 8545


Chemical Reviews

Review

Figure 8. meso-Linked diporphyrin 8 and fused-diporphyrin 10 compounds, developed by Osuka and co-workers.34

preparative GPC−HPLC. It was subsequently oxidized with Sc(OTf)3 and DDQ to afford 10 by converting the three meso− meso diporphyrins to three fused diporphyrins. Fuseddiporphyrin motifs enhanced the magnitude of the electronic interactions between the porphyrinic units in 10, and interesting nonlinear optical properties, such as two-photon absorption, are expected for such systems. In 2010, the same group reported the synthesis of a molecular barrel 11 that contained four Ni(II)porphyrins, each metalloporphyrin being doubly connected to two others by two 2,6pyridyl bridging moieties.36 This nanocontainer was synthesized via two consecutive Suzuki cross-coupling reactions (Figure 9). Starting from a Ni(II) tetraborylporphyrin derivative 12, a 4-fold pallado-catalyzed substitution allowed the introduction, on the pyrrolic positions, of ortho-bromopyridine in 50% yield; synthon 13 was then reacted with the same starting Ni(II) tetraborylporphyrin 12, leading to the desired molecular cage 11 in 10% yield. The final compound was characterized by MALDI-TOF mass spectrometry, 1H NMR, and X-ray crystallography. The crystal structure shows a concave structure of the porphyrinic loop with a diameter of around 14 Å (Figure 9). C60 was selected as a guest because its geometrical parameters are well adapted to the cavity shape and size of the porphyrin nanobarrel 11. Titration experiments using UV−visible (UV−vis) spectroscopy indicated that the nanobarrel 11 encapsulated C60 in a 1:1 complex with an association constant Ka = 5.3 × 105 M−1 in toluene at 25 °C. Remarkably, the crystal structure of the host− guest complex could be obtained, the diameter of the barrel did not significantly change upon insertion of C60, and an average

Figure 9. Synthesis of the molecular barrel 11 reported by Osuka et al., encapsulation of C60, and X-ray crystal structures of 11 and C60⊂11. Hydrogen atoms are omitted for clarity.36

distance of 3.6 Å of C60 within the porphyrin nanobarrel was determined. Zhang and co-workers19c reported a carefully designed rigid rectangular prism 14 in 2011 (Figure 10). The molecule 8546


Chemical Reviews

Review

Figure 10. (a) Porphyrinic molecular prism 14 reported by Zhang and co-workers19c and its energy-minimized structure (Amber 11.0, GAFF field) with methyl replacing hexadecyl groups for simplicity, (b) top view, (c) side view. Parts (b) and (c) were reprinted with permission from ref 19c. Copyright 2011 American Chemical Society.

incorporated two cofacial porphyrins meso-functionalized with four carbazoles and linked together by four ethynylene spacers. Carbazoles were chosen as the corners of the prism to provide an angle close to the ideal 90° required for a strainless rectangular prismatic structure. The key step in the synthesis of this cage was the alkyne metathesis reaction performed on a tetraethynylfunctionalized porphyrin by using a molybdenum-based catalyst developed by the same group. These conditions afforded the shape persistent covalent cage in 56% yield. According to computational calculations, the size of the cavity of 14 was defined by an interplanar porphyrin distance of 11.9 Å and a distance of 18.3 Å between two opposite ethyne linkers (Figure 10b,c). Notably, 14 shows one of the highest affinities for C60 and C70 in toluene at 23 °C (1.4 × 105 and 1.5 × 108 M−1, respectively) and a remarkably high selectivity (>1000) in the formation of an inclusion complex with C70 over C60. A reversible dissociation of C60 and C70 was also demonstrated based on porphyrin protonation/deprotonation. This reversible process allowed for a controlled-release of fullerene triggered by an acid− base stimulus and also for the separation of C70 from a fullerene mixture. In 2012, the group led by Li19b synthesized a flexible porphyrinic cage 15 using a copper-catalyzed azide alkyne coupling reaction in pseudo-high dilution conditions (Figure 11). The molecule was obtained in 55% yield after purification by standard column chromatography. In the X-ray structure of 15 (Figure 11b), obtained from a mixture of acetone and pyridine, two pyridine ligands coordinate to the zinc porphyrins outside the cage, and the distance between the two Zn atoms was 7.95 Å. The receptor properties of this cage were investigated by NMR and UV−vis. Titration experiments using tetrabutylammonium azide in acetone showed azide anion encapsulation through

Figure 11. (a) Encapsulation of an azide anion in the cage developed by Li et al.19b (b) Crystal structure of 15, with pyridine ligands coordinated to the Zn(II) porphyrins. Hydrogen atoms are omitted for clarity.

coordination to the two zinc(II) porphyrins as represented in Figure 11. This property is particularly interesting in view of the high toxicity of the azide.37 2.2. Templated Synthesis

Since the early 1960s, templated synthesis has greatly facilitated the synthesis of macrocycles, then that of cage molecules, and in the 1980s, they have made interlocked molecules synthetically accessible at a large scale.15a,38 Nevertheless, making large covalent 2D or 3D systems with a template-directed strategy that minimizes the number of synthetic steps is still challenging. In the case of porphyrin nanorings, as demonstrated by Anderson, the synthesis of the appropriate template can be tough work.10b Therefore, the elegant Vernier templating approach they developed to make very large nanorings of 12 and 24 porphyrins demonstrated the power of this methodology to make cyclic structures of increasing size and complexity.39 In the field of multiporphyrinic 3D structures, the use of a template-molecule to favor the cage closure step is a methodology that was exploited since the beginning of the 2000s. Template molecules organize the porphyrinic components in 3D complexes and convert the intermolecular ring closure reaction to an intramolecular one, increasing the rate and yield of the 8547


Chemical Reviews

Review

of the trispyridyl ligand used as a template and the porphyrinic precursors.25 The C3-symmetric template preorganized three rigidly linked butadiyne zinc porphyrin dimers on the edges and occupies the triangular faces of the final prism (Figure 13). A

desire product. Therefore, the interaction between the cage components and the chosen template must be strong and reversible. The template has to be removed easily after the final cage-closure step to have the benefit of a free cavity to encapsulate guest molecules for various applications in the field of supramolecular chemistry. In 2003, Konishi24 and co-workers have used a gold nanocluster, covered with pyridine-functionalized thiols, as a template to coordinate six alkene-functionalized zinc(II) porphyrins (Figure 12). The metathesis reaction closed the

Figure 13. Molecular prisms reported by Hupp and co-workers.25

trispyridyl-templated alkene metathesis reaction proceeded very efficiently to covalently link the porphyrin dimers functionalized with either two allyl or two pentenyl side chains. Two templates of different sizes were tested. The 2,4,6-trispyrid-4-yl-pyrazine (tpt) template afforded the smallest assembly 17a (n = 1) in 77% yield (Figure 13), whereas a larger cage 17b (n = 3) was obtained in 76% yield due to the preorganization induced with an extended C3-symmetrical trispyridyl ligand. Aida and co-workers developed in 2011 a two-step procedure to obtain 4-fold connected bis-porphyrins 18a,b (Figure 14).22 First, a cyclic structure 19a, based on a doubly linked porphyrinic dimer, was formed through a 2-fold fluoride-mediated Williamson reaction. Next, in the presence of a metallofullerene La⊂C82 that formed an inclusion complex with the cyclic structure, the two olefinic side chains of each porphyrin were subjected to a ring-closing alkene metathesis reaction. The yield of the cage 18a with La⊂C82 trapped inside was 20% when the reaction proceeded on a copper(II) porphyrin macrocycle, but it was increased to 40% for 18b when the cyclic precursor incorporated free-base porphyrins. This improvement is due to a 10-fold increase in the macrocycle 19b:La⊂C82 association constant in toluene at 25 °C in the case of the free-base porphyrinic structure. It must be noticed that the geometry of the host:La⊂C82 complexes had a profound influence on the supramolecular magnetic spin coupling between the Cu(II)porphyrin components and the metallofullerene. Indeed, the ferromagnetic coupling, observed for the copper(II) porphyrins

Figure 12. Templated covalent assembly of six porphyrins around a gold nanocluster.24

hexaporphyrinic molecule 16 with 12 double bonds in a remarkable yield of 50%. In this example, the gold nanocluster template is entrapped in the final structure and could not be removed from the cavity. Nevertheless, the ligand-functionalized thiols of the nanocluster could be exchange with 1-hexanethiol, leaving the cluster free to move inside the cage. Two porphyrinic trigonal prisms 17a,b were prepared in the group of Hupp in 2008, thanks to the good geometrical matching 8548


Chemical Reviews

Review

orthogonal binding sites, two zinc(II) porphyrins to coordinate nitrogen bearing ligands, and four peripheral pyridyl ligands to bind metals. The design of the molecule was made to prevent intramolecular zinc-pyridyl binding. The porphyrinic precursor of the cage was meso functionalized with four 2,6-dimethylphenyl groups to avoid π-stacking of the porphyrins in the final structure. Compound 2140 was subjected to a 4-fold Williamson reaction that enabled the allyl groups to be introduced. DABCO (1,4diazabicyclo[2.2.2]octane) preorganized the two zinc(II) porphyrinic precursors 22, to afford dimer 23. Intramolecular ring-closing metathesis reaction performed on 23 in mild conditions led to the closed structure 24 in 40% yield. The crystal structure of the DABCO-included cage 24 was resolved. It showed (Figure 16) a highly symmetric structure, the zinc

Figure 14. (a) Inclusion complex of endohedral metallofullerene into the bis-porphyrinic cages 18a,b developed by Aida and co-workers.22 (b,c) Geometry-optimized simplified structures based on BLYP DFT calculations (Gaussian 03 program) of (b) 19a and (c) 18a. Parts (b) and (c) were reprinted with permission from ref 22. Copyright 2011 American Chemical Society.

Figure 16. X-ray structure of the molecular cage 24 with a DABCO molecule coordinated to the zinc porphyrins.23 Hydrogen atoms are omitted for clarity.

cyclic structure sandwiching the La⊂C82 19a, turned to a ferrimagnetic spin coupling in the cage 18a. Applications in the field of stimuli responsive memory devices can be foreseen, based on paramagnetic porphyrinic architectures caging metallofullerene. In 2012, Heitz et al.23 reported the synthesis of a bis(zinc porphyrin) cage 20 based on a DABCO-templated alkene metathesis reaction (Figure 15). This molecule represented one of the few examples of a porphyrinic cage incorporating

porphyrins being in an eclipsed conformation separated by 7.05 Å. The overall dimensions of the molecule are 25 × 21 × 15 Å (width × depth × height), and the volume of the cavity is ca. 200 Å3. Removal of the DABCO under acidic conditions demetalated also the zinc porphyrins and afforded the molecular cage 20 with an empty cavity (Figure 16). In this example, the four polyether linkers are long and flexible, allowing the porphyrin−porphyrin

Figure 15. Synthesis of the porphyrin cage 20 reported by Heitz and co-workers.23,40 8549


Chemical Reviews

Review

3.1.2.2−3.1.2.4 are based on axial coordination of external bridging ligands to zinc porphyrin metal centers. In section 3.1.3, the coordination bonds involve no more the porphyrin metal center as a Lewis acid. Now, the appended donor ligands of the porphyrinic precursors coordinate to external bridging metal complexes. When these complexes are monometallic as described in subsection 3.1.3.1, they act as a bridging unit between the porphyrins by exchanging one or two of their weakly coordinating ligands for the stronger monotopic or chelating ligands that decorate the porphyrin component. Other coordination cages, described in subsection 3.1.3.2, involve bimetallic complexes connected by bridging ligands as the linkers of the porphyrin components. 3.1.1. Synthetic Strategies. The variety of kinetically labile transition metals and their predictable coordination geometries, the diversity of synthetically accessible multitopic ligands, and the high stability of the coordination complexes have resulted in a spectacular development in the coordination-driven selfassembly of 3D architectures in the past 20 years. Porphyrin is a rigid, symmetric, and versatile precursor for coordination-based assemblies. With a suitable choice of metal center, the Lewis acceptor metalloporphyrin enables axial coordinations of one or two ligands, and its meso positions oriented at 90° or 180° from each other can ideally connect one to four donor ligands to form a multitopic donor ligand panel with controlled orientation of the binding sites. As acceptor sites, Zn metalated porphyrins are easy to prepare and form five-coordinate square pyramidal complexes by axial coordination of N-donor ligand. They are by far the most popular ones in self-assembly because the Zn−N axial bond is highly directional (perpendicular to the porphyrin plane with tolerated deviation up to 10°), labile enough for the reaction to proceed under thermodynamic control, and provides very good stability to Zn−N coordination-driven multiporphyrinic structures.15e Therefore, simple self-coordination of ligands appended Zn-metalated multiporphyrins was investigated to obtain 3D architectures of well-defined geometry. More complex multicomponent assemblies that require ligands appended to porphyrinic components, external ligands, and metal acceptors were also explored. Predictable geometries of high symmetry can be conceived by the directional interaction of partially ligand-free metal acceptor with a complementary multitopic donor, and such strategy was defined as the directional bonding methodology.41 Alternatively, in the paneling approach, metals with easily exchangeable ligands can combine with a 2D multitopic porphyrinic component acting as a panel and spanning some or all of the faces of the final architecture.42 In the case of multicomponent self-assembly with different kinds of ligands, the use of a template to preorganize some of the components can efficiently drive the reaction toward the desired heteroleptic complex, or other factors like steric constraints can also lead to a selective assembly. With these methodologies, cages of various geometries like trigonal and tetragonal prisms, cubic systems, trigonal bipyramids, molecular boxes, and capsules were obtained in which the porphyrin core occupies the edge, face, or corner positions in the final structure. The versatility of the porphyrin building block acting as donor, acceptor, or both is illustrated in the case of the different chemical approaches reported to obtain a trigonal or tetragonal prismatic structure. Indeed, trigonal prims were obtained through four types of porphyrinic components. In a face-directed approach involving a two-component assembly, tetratopic porphyrins with 4′-pyridyl donor sites oriented toward the corner of the square porphyrinic panel combine with 90° metal acceptors and span three faces of

distance to vary from 5 to 15 Å according to the CPK-model and to coordinate, inside the cavity, guest molecules of variable size. The additional peripheral pyrid-3-yl functionalities can be useful to regulate the cage size and activity, which is an important aspect in the field of molecular recognition, sensing, or catalysis. Multifunctional nanoscale ensembles able to adjust their size for a specific activity will mimic the reactivity of biological systems, especially to allosteric enzymes. Examples of template-directed synthesis of covalent porphyrinic cages are still rare, whereas in most cases, the yields are improved, in comparison with the cyclization reaction based on the high dilution methodology. The cage closure step always relies on a ring-closing metathesis reaction, an efficient reaction performed in mild conditions compatible with the use of templates. The chosen templates are often pyridine-based ligands with the appropriate geometry, to take advantage of the axial coordination of pyridine to zinc porphyrins. This axial binding is also one of the most developed strategies to selfassemble supramolecular multiporphyrinic cages, as discussed in section 3.1.2.

3. SUPRAMOLECULAR MULTIPORPHYRINIC CAGES Discrete molecular assemblies of porphyrins can take advantage of multiple metal−ligand interactions or H-bonds to obtain 3D structures of high stability, and this section discusses separately those two types of architectures. Nature gives examples of diverse multiporphyrinic assemblies that are highly organized through noncovalent bonds, like in the photosynthetic reaction centers and in their associated light harvesting antennas. Self-assembly of multiporphyrinic cages relies on spontaneous and reversible noncovalent interactions of components. To access the desired assembly in high or quantitative yield, the self-assembly process has to be optimized by taking into account important factors related to the reaction conditions and the design of the components. The reaction must proceed under thermodynamic control, the interactions between the components must be reversible, allowing for the undesired products to form and dissociate, while the system reaches the thermodynamically most favored state, corresponding to the desired product. High stability of the final compound is crucial to prevent a random mixture of products at equilibrium, and it relies on the information encoded in the precursors. Therefore, a high complementarity of the components in terms of geometry (size, angle, dimensionality) and orientation of the interacting sites, as well as some rigidity, is required to form a discrete assembly. An appropriate ratio of the components to prevent free donor or acceptor sites is also essential to favor the desired selfassembly molecule as well as an adequate choice of solvent and temperature. Having fulfilled these conditions, the smallest discrete species will be entropically favored over oligomers, and the enthalpy factor will also be favorable to this species, because it corresponds to the least strained structure with the highest energy bond strength. 3.1. Cages Self-Assembled by Coordination Bonds

The coordination-driven self-assemblies are reported in two sections. In the first one, section 3.1.2, the coordination bonds are formed through axial coordination to the zinc porphyrin metal center. The simplest process described in subsection 3.1.2.1 is self-coordination. It involves only one component, a zinc porphyrin dimer or trimer with appended ligands, and the self-coordination of the appended donor sites to the zinc(II) centers. The other coordination cages reported in subsections 8550


Chemical Reviews

Review

the trigonal prim (Figure 17a). Because the ideal geometrical requirements for such prism would require one to combine a 60°

Figure 18. Representation of two coordination-driven strategies to obtain a porphyrinic trigonal prism. (a) Axial coordination of a linear ditopic donor (blue double arrow) to the metal centers of a C3symmetric Zn-porphyrin trimer acceptor (Zn, blue sphere; porphyrin, red losange); for the chemical structure of the compounds, see Figure 28.45 (b) A template three-component self-assembly involving a C3symmetric tritopic donor (tritopic ligand in blue, the U-shaped form represents a phenanthroline chelate), a linear bis-terdentate Zn porphyrin donor (Zn porphyrin, red losange; W-shaped form, terpyridine chelate), and a naked Cu(I) ion (blue sphere). The tritopic template is represented as a black triple arrow; for the chemical structure of the compounds, see Figure 57.46

Figure 17. Representation of two coordination-driven strategies to obtain a trigonal prism from a tetratopic porphyrinic ligand. (a) mesoTetra-4′-pyridylporphyrin (red arrows, 4′-pyridyl; red square, porphyrin) combines with a 90°-cis protected metal (blue sphere); for the chemical structure of the compounds, see Figure 54b.43 (b) meso-Tetra3′-pyridylporphyrin (red arrows, 3′-pyridyl; red square, porphyrin) combines with a 90°-cis protected metal (blue sphere); for the chemical structure of the compounds, see Figures 42 and 44.44

metal acceptor with the square porphyrinic panel, the structure was obtained thanks to the flexibility of the porphyrinic skeleton that allows structural distortions to obtain the smallest discrete species, as shown in the reported example (Figures 54b and 55a).43 When the donor sites on the porphyrinic panel were oriented toward the edges, by using 3′-pyridyl ligands, the prismatic structure was obtained with almost no porphyrinic distortion thanks to the adjustable direction of the coordination axes of the 3′-pyridyl ligands toward the metals. In the resulting molecular prism, the metals were located on the apical positions of the prism (Figure 17b).44 Also, based on a two-component process, a prism-like structure can be generated, but with the Zn-porphyrins located at the corners. It resulted from the axial coordination of linear ditopic ligands to the metal centers of the C3-symmetric Znporphyrin trimers (Figure 18a).45 Moreover, a challenging threecomponent approach with two different kinds of donors, a linear bis-terdentate Zn-porphyrin and a tris-bidentate ligand, that coordinate to a nacked copper(I) acceptor was successful to form selectively the prism incorporating six heteroleptic complexes (Figure 18b). This was due to the use of a template to gather the three porphyrinic components and to the high stability of the heteroleptic complexes with regard to the homoleptic ones, thanks to the steric constraints provided by the bidentate ligand.46 A tetragonal prism was designed through three strategies starting from a C4-symmetric tetratopic porphyrin that occupies the bottom and top faces in the final structure (Figure 19). One was based on the interaction of three components, a meso-tetra4′-pyridyl porphyrin panel, a linear ditopic ligand, and a 90° metal acceptor in a 2:4:8 ratio, and led to the desired structure (Figure 19a).43 Also, based on a three-component reaction, a more challenging pathway used a tetrahedral metal acceptor Cu(I), stabilized in the final structure by two different chelates disposed in two perpendicular planes (Figure 19b).47 This selfassembly process that selects the heteroleptic complexes is driven by the additional information encoded in the porphyrinic ligand,

Figure 19. Representation of three coordination-driven strategies leading to a porphyrinic tetragonal prism. (a) Self-assembly of a mesotetra-4′-pyridylporphyrin (4′-pyridyl ligand, red arrows; porphyrin, red square), a linear ditopic ligand (blue double arrow), and a 90°-cis protected metal (blue sphere); for the chemical structure of the compounds, see Figure 54a.43 (b) Self-assembly of a meso-functionalized porphyrin (porphyrin, red square functionalized with bidentate chelates as red U-shaped forms), a linear ditopic ligand (in blue, the arc-shaped form represents the bidentate chelate), and a naked Cu(I) ion (blue sphere); for the chemical structure of the compounds, see Figure 56.47 (c) Two meso-tetrakis(4′-carboxyphenyl)porphyrins (red square, porphyrin, red arrows, 4′-carboxyphenyl ligand) coordinate to four dimetallic clips (blue dumbbell); for the chemical structure of the compounds, see Figure 62.48

like in the present case, the steric hindrance. An alternative strategy, that reduces the number of possible self-assembled species at equilibrium, was a two-component assembly process that involved a dimetallic species as molecular clip to gather the two porphyrins in a face-to-face disposition (Figure 19c).48 3.1.2. Axial Coordination of Zinc Porphyrins. 3.1.2.1. With Appended Ligands: Self-Coordination. Selfcoordination is a simple one-component assembly process that was explored in the case of a Zn-porphyrin dimer or trimer with 8551


Chemical Reviews

Review

nitrogen-appended ligands. The geometry of the final product was controlled by the relative orientation of the coordinating axis of the ligand relative to the porphyrin plane and also by the relative orientation of the porphyrins in the covalently linked multiporphyrin precursor. Indeed, in the absence of spacer, meso−meso-linked porphyrins are not coplanar due to steric interactions between β-pyrrolic hydrogens, whereas connecting the porphyrins via an alkyne spacer allows them to be in a coplanar arrangement. A molecular capsule 25 (Figure 20) was assembled by Johnston and co-workers in 2001 by dimerization of two self-

Figure 20. Capsule 25, formed by self-coordination of the pyridine moieties of the central bis-pyridyl tetrazine linker to the Zn porphyrins.49

complementary gable zinc(II) bis-porphyrins 26.49 In this system, two porphyrin-appended norbornenyl blocks were covalently linked to a central s-tetrazine functionalized with two 4′-pyridyl ligands. The zinc(II) bis-porphyrin 26 was obtained after metalation of the peripheral porphyrins. By coordination of the pyridyl ligands of the central linker of one molecule of 26 to the Zn porphyrin moieties of another gable bisporphyrin 26, the three-dimensional cage molecule 25 was formed quantitatively. However, despite a quite large interior cavity (around 570 Å3), guest encapsulation could only be observed by mass spectroscopy.50 Osuka, Kim et al.,51 as well as Aida and co-workers52 developed the self-assembly of various molecular boxes from meso−meso-linked Zn(II) bis-porphyrins with two pyridineappended ligands (Figures 21−25).51,52 In 2002, Tsuda, Osuka et al. quantitatively self-assembled a box-shaped tetramer (27)4 of very high stability (at least 1025 M−3 from fluorescence titrations in chloroform at 25 °C) (Figure

Figure 21. meso−meso-Linked zinc bis-porphyrins 27, 28, and 29 used by Tsuda, Osuka et al. as building blocks for various boxes51a,b and tetramers (R)-(27)4 and (S)-(27)4, formed by self-sorting of 27.51a

21). Because of prohibited rotation around a direct meso−meso linkage, the two zinc(II) porphyrins are perpendicular in the bisporphyrin 27, which exists as two enantiomers, R and S.51a Only two stereoisomers formed by self-sorting of either four (R)-27 or four (S)-27, thanks to the necessary stereochemical complementarity of the four components of the cage. The cavity size was estimated to be 10 × 10 × 8 Å, from X-ray structures of compounds similar to the precursor. Extended cages measuring 14 × 14 × 8 Å3 for (28)4 and 18 × 18 × 8 Å3 for (29)4 were also obtained by Osuka, Kim et al., by inserting one or two phenyl groups between the zinc porphyrin and the pyridyl substituents (Figure 21).51b Optical resolution attested the homochiral nature of the chiral boxes (27)4, (28)4, and (29)4 assembled via a self-sorting process from their chiral 8552


Chemical Reviews

Review

porphyrin-dimer precursors. Various photophysical studies could evidence excitonic coupling and excitation energy migration processes within these three boxes. Excitation energy hopping rates in these well-defined perpendicular arrangements of porphyrins were well analyzed by the Förster-type incoherent energy hopping model. These supramolecular systems can therefore mimic the function of light harvesting realized by natural antenna complexes in photosynthesis. The same group studied related porphyrin boxes that insert ethynyl units either on the free meso positions or between the pyridyl porphyrin bonds of 28, to increase the excitation energy hopping within the self-assembled structure by enhancing the transition dipole moments of the dimers.51c Osuka, Kim et al. developed also the synthesis of meso−mesolinked bis-porphyrin functionalized with ligands other than pyridine, such as cinchomeronimide53 or 5-azaindole54 (Figure 22). The meso-cinchomeronimide appended zinc bis-porphyrin

(30out−out)5, the size of these aggregates being controlled by the dihedral angles imposed by the two pyridyl nitrogen atoms of each atropisomer. The X-ray structure of the pentamer (Figure 22b) confirmed the pentagonal cylindrical nature of the assembly, showing sides of ca. 11.5 Å and angles of ca. 108°. A very large association constant, above 1027 M−4 in chloroform, was estimated by UV−vis experiments. The self-sorting behavior of the meso−meso-(5-azaindol-2-yl) appended meso−meso-linked bis-porphyrin 31 led to a different result.54 In 31, the rotational barrier of the meso-linked ligands is smaller than in 30, allowing atropisomerization at room temperature, and, therefore, this dimer self-assembled to form a single fluorescent trimeric species (31in−in)3 (Figure 22c). Aida and co-workers also developed different tetrameric zinc porphyrin boxes, from alkynylene-bridged bis-porphyrin52 functionalized with 4′-pyridyl groups (Figure 23). Starting with

Figure 22. (a) Self-sorting assembly of meso-cinchomeronimide 30.53 (b) Top view (left) and side view (right) of X-ray crystal structure of the pentamer (R-30out‑out)5. Solvent molecules, hydrogen atoms, and mesoaromatic groups are omitted for clarity. (c) Self-sorting assembly of (meso-(5-azaindol-2-yl)-appended meso−meso-linked zinc(II) bis-porphyrin 31.54 Part (b) was reprinted with permission from ref 53. Copyright 2006 American Chemical Society.

Figure 23. Alkynylene-bridged bis-porphyrins 32, 33, and 34, and tetrameric zinc porphyrin box (32)4, obtained from self-coordination of planar monoalkynylene-bridged bis-porphyrin 32.52

a monoalkynylene-bridged bis-porphyrin 32, a self-assembly process led to a tetrameric zinc porphyrin box (32)4 in which 32 adopted exclusively a planar conformation.52a When a bisalkynylene-bridged zinc bis-porphyrin 33 was selected as the building block, a mixture of two conformationally isomeric tetramers (∥33)4 and (⊥33)4 was obtained, one analogous to (32)4 (Figure 23) in which the 33 component adopted a planar conformation (∥33) and the other similar to (27)4 (Figure 21) in which the two porphyrins of 33 were perpendicular (⊥33).52b,d A combination of NMR, absorption, and emission studies at different temperatures demonstrated that the major isomer of the cage was the one with a conformation similar to that of (27)4. The authors reported that such self-

30 is chiral because free rotation about the meso−meso bond is prohibited, and due to the restricted free rotation of the cinchomeronimide unit, each enantiomer exists as three stable atropisomers (in−in, in−out, and out−out) with respect to the orientation of the pyridyl nitrogen atoms. This bis-porphyrin exhibited an impressive self-sorting process upon self-assembly in terms of both stereoselectivity and conformational descrimination. Indeed, only three types of homochiral assemblies were formed: a trimer (30in−in)3, a tetramer (30in−out)4, and a pentamer 8553


Chemical Reviews

Review

assembled porphyrin boxes exhibited a new kind of solvatochromism, named “conformational” solvatochromism, because it could discriminate between benzene and CCl4 and also between the regioisomers of xylene. The ratio of the two conformers depended on electronic and steric effects of encapsulated nonpolar solvent molecules, as evidenced by 1H NMR and absorption spectroscopy. In a related study, Aida’s group reported the self-assembly of a tetraalkynylene-bridged zinc bis-porphyrin 34 consisting of a mixture of rotamers and in which the building block adopted, respectively, a perpendicular (⊥34) or a planar conformation (∥34). The tetragonal prism was formed as a mixture of a homochiral box (⊥34)4 and an achiral one (∥34)4 (Figure 24).52c These assemblies had an interesting chiroptical sensing

Figure 24. An equilibrium mixture of chiral and achiral multiporphyrinic boxes, in which the two zinc porphyrins in 34 adopted either a perpendicular (⊥34) or a coplanar orientation (∥34).52c

Figure 25. Three stereoisomers, (⊥35)4, (∥35)4, and (∥⊥35)4, of tubular boxes assembled from zinc tris-porphyrin 35.52b

behavior toward asymmetric hydrocarbons: in the presence of a chiral guest such as limonene, enantiomeric enrichment of the homochiral tetrameric assembly (⊥34)4 was observed, allowing the optical purity and absolute configuration of limonene to be determined. Furthermore, Aida and co-workers described self-coordination of a dialkynylene-bridged zinc tris-porphyrin 35, which gave tubular-shaped structures. The fluorescence spectrum showed bands assigned to three stereoisomers, (⊥35)4, (∥35)4, and (∥⊥35)4, each being characterized by different porphyrin− pyridine orientations, perpendicular or coplanar, relative to the other two porphyrins in the building block 35 (Figure 25).52b 3.1.2.2. With External Ditopic Ligands. Calixarenes are widely used receptors in host−guest chemistry. In a few cases, they have been functionalized with porphyrins to associate additional binding sites to the cavity-shape molecules.2n,55 In 2003, Hunter and co-workers functionalized the tetrapropoxycalix[4]arene upper rim with four porphyrins (Figure 26).56 This new building block 36 is well-adapted for the self-assembly of capsule-like structures because it can adopt a cone conformation with all of the porphyrin units in approximately the same plane. Detailed complexation studies of calix-tetrazincporphyrin 36 with DABCO were carried out using UV−vis (in CH2Cl2 at 22 °C) and 1H NMR titrations (in CDCl3 at 22 °C) experiments. They showed multiple equilibria: four complexes with different stoichiometries were formed. For example, the 2:4 complex (36)2·(DABCO)4 represented in Figure 26 is a dimeric cage assembly with a large internal cavity. It was formed quantitatively by addition of 2 equiv of DABCO to a 0.5 mM solution of 36. Building containers with compartments of different volumes and rigidities is particularly attractive for selective multiguest recognition processes. Reek and co-workers have developed an efficient strategy to build encapsulated catalysts.6c,57 They modified the coordination sphere of a rhodium-based catalyst with a self-assembled and protective multiporphyrin phosphite ligand (Figure 27).58 The authors prepared a self-assembled supramolecular complex 37

Figure 26. Calix[4]arene 36, functionalized with four zinc porphyrins and a cartoon drawing of its 2:4 complex with DABCO.56

(37: [Rh(acac)(CO)2(38)2(DABCO)3)]) in which two triporphyrin phosphite ligands 38, which form a stable 2:3 complex with DABCO ligands, acted as a supramolecular bidentate ligand for Rh, via the phosphorus donor atoms. The stoichiometry of the supramolecular complex 37 was determined by UV−vis titration experiments. Under H2/CO pressure, 37 converted into the active catalyst 39 (39: [Rh(CO)2H(38)2(DABCO)3)] in the hydroformylation of olefin. The authors could demonstrate a high regioselectivity in the hydroformylation of 1-octene in toluene (ratio of linear to branched aldehyde products l/b = 22.8 at 30 °C). Its activity (turnover frequency = 1.1 × 103 at 80 °C) was explained by the protective environment provided by the 8554


Chemical Reviews

Review

Figure 27. Supramolecular rhodium catalysts 37 and 39, formed by the multicomponent assembly of two porphyrinic phosphite ligands 38, one Rh complex, and three DABCO.58

supramolecular bidentate multiporphyrin phosphite ligand (38)2·(DABCO)3. A zinc tris-porphyrin 40 (Figure 28), constructed on a benzene-1,3,5-tricarboxamide core, was reported in 2008 by Ballester, Hunter, and co-workers, to self-assemble in the presence of ditopic ligands of various lengths. At micromolar concentrations, stable 2:3 double-decker molecular coordination cages were self-assembled by coordination of DABCO,45a−c 4,4′bipyridine (BIPY),45b or free-base 5,15-bis(4′-pyridyl)-10,20diphenylporphyrin (DPYP).45b The DABCO cage complex (40)2·(DABCO)3 featured a large central cavity, which could accommodate benzene-1,3,5-tricarboxamide derivatives as guest molecules in chloroform. The minimized structure (CAChe, MM3) of N,N′,N″-tris(phenyl)benzene-1,3,5-tricarboxamide⊂[(40)2·(DABCO)3] (Figure 28b,c) revealed the formation of six N−H···OC hydrogen bonds between the benzene-1,3,5-tricarboxamide of the guest and the ones of the host. In the case of the assembly (40)2·(41)3 with 40 at 10 μM concentration in dichloromethane, some 40 remained in solution, even in the presence of 20 equiv of 41, as shown by absorption titration.45d Cages incorporating metalated and freebase porphyrins in their framework can display interesting photophysical properties. Indeed, efficient photoinduced energy transfer (quantum yield Φet = 96%) was shown to occur in (40)2· (DPYP)3 from the Zn(II) tris-porphyrin base units to the freebase DPYP pillars. On the other hand, when 40 was associated with 41 as pillars of the prismatic assembly,45d fast photoinduced electron transfer occurred, from the zinc porphyrin units to the axially coordinated perylene bisimide units 41. The charge recombination rates are dependent on the solvents (k = 1.6 × 108 s−1 in toluene and 9.1 × 109 s−1 in dichloromethane), corresponding to a Marcus inverted region. 3.1.2.3. With External Tritopic Ligands. In 2000, Shinkai’s group reported nice examples of the formation of 1:1 complexes between a half-bowl-shaped homooxacalix[3]arene 42, bearing three pyridine pendant arms, and C3-symmetrical zinc trisporphyrins 43a or 43b (Figure 29).59 The simultaneous coordination of the pyridine units of 42 to Zn(II) in 43a or

Figure 28. (a) Zinc tris-porphyrin 40 and 2:3 double-decker molecular coordination cages assembled with DABCO,45a−c BIPY,45b DPYP,45b and perylene bisimide 41.45d (b) Top and (c) side views of the minimized structure (CAChe, MM3) of N,N′,N″-tris(phenyl)benzene1,3,5-tricarboxamide⊂[(40)2·(DABCO)3]. Nonpolar hydrogens and meso-phenyl groups have been omitted for clarity. Parts (b) and (c) were reprinted with permission from ref 45a. Copyright 2006 American Chemical Society.

Figure 29. Homooxacalix[3]arene 42, zinc tris-porphyrins 43a and 43b, and self-assembled molecular cages 44a and 44b.59

43b was characterized by the association constants Ka of 5.6 × 105 M−1 for 44a and of 8.5 × 105 M−1 for 44b in toluene at 25 °C. The authors ascribed the self-assembly of both molecular cages 44a and 44b to the flexibility of homooxacalix[3]arene 42. Host−guest experiments did not show evidence of inclusion of C60 in the inner cavity of the heterocapsule 44a, and, in the case of the larger capsule 44b, only a moderate association constant of 60 M−1 for C60 was determined by NMR in toluene-d8 at −30 °C. 8555


Chemical Reviews

Review

A collaboration between Hunter’s, Scrimin’s, and Tecilla’s groups resulted in an original work published in 2000.60 A benzene-1,3,5-tricarboxamide spacer was used to build a zinc trisporphyrin scaffold 45 (Figure 30).60 By axial coordination of

Figure 30. Zinc tris-porphyrin scaffold 45, TREN-derived compounds 46a−c, and, as an example, the self-assembled molecular cage 47a.60

various pyridine or N-methylimidazole functionalized tris(2aminoethyl)amine (TREN) ligands 46a−c to the zinc trisporphyrin platform 45, C3-symmetrical molecular cages 47a−c were formed as the sole products of the reactions. The highest association constant, Ka = 6.5 × 108 M−1 in dichloromethane at 25 °C, determined by UV−vis titration, was measured for the complex 47a, in which the components 45 and 46a have the best geometrical complementarity. It is noticeable that this cage 47a did not dissociate in the presence of up to a 2-fold excess of 46a. In 2006, Hupp and co-workers reported the formation of a sixor a nine-zinc porphyrin coordination prism (48)3·(50a)2 or (49)3·(50a)3, from a linear butadiyne linked zinc porphyrin dimer 48 or trimer 49, respectively, and a tritopic triethynylpyridylbenzene 50a (Figure 31).61 Their all-or-nothing formation was characterized by UV−vis spectroscopy, 1H NMR, and pulsefield-gradient NMR. In toluene, (49)3·(50a)3 was more stable than (48)3·(50a)2 in excess of 50a, in accordance with a greater number of coordination bonds. Furthermore, significantly different electron-density-weighted radii of gyration (Rg) (12.9 and 15.7 Å), as well as significant Zn−Zn distances, were obtained for the molecular assemblies, from small-angle X-ray scattering (SAXS). In 2008, Osuka, Wasielewski, and Hupp reported the synthesis of a related prism obtained from 48 and an extended tritopic ligand 50b (Figure 31) as well as self-assembled prisms, synthesized from a chlorophyll dimer 51 and 50a or 50b (Figure 32).62 Geometry-optimized models of the assemblies (51)3· (50a)2 and (51)3·(50b)2 showed the coplanarity of the chromophores of each dimer on the tetragonal faces of the prisms, represented in Figure 32.62 The different prismatic structures and their geometries were confirmed by small-angle Xray scattering experiment. Detailed spectroscopic investigations were realized to study through-space energy transfer within the assemblies, to determine the exciton hopping rates between the chromophore faces and the distance dependence of energy transfer in prisms of different sizes. In 2012, Iengo and co-workers reported the quantitative selfassembly of a multiporphyrin molecular cage from only two different types of components.63 A planar metallacycle 52 (Figure 33), composed of two zinc(4′-cis-dipyridyl)porphyrins connected to two cis-Ru complexes, was previously prepared64

Figure 31. Zinc porphyrin dimer 48 and trimer 49, tritopic pyridylappended benzenes 50a,b, and the resulting self-assembled trigonal prisms.61,62

and used to quantitatively assemble 2:2 sandwich complexes with diverse rigid bridging pyridyl-based ditopic ligands, including BIPY,64 DPYP,64 and perylene bisimide 41.65 Furthermore, the authors designed the tritopic pyridinic planar ligand tpt (2,4,6tripyridyl-1,3,5-triazine) to obtain 3D architectures by coordination to the zinc porphyrins of the metallacycle 52. In chloroform at room temperature, 52 (1 equiv) and tpt (2/3 equiv) self-assembled to afford the six-porphyrinic trigonal prism (52)3·(tpt)2 quantitatively. Its solid-state structure, represented in Figure 33, showed an inner cavity of ca. 280 Å3 and a total volume of ca. 940 Å3. The authors took advantage of the metallacycle 52 as a flat panel to construct cages with other geometries as described in section 3.1.2.4 (Figure 39). 3.1.2.4. With External Tetratopic Ligands. In 1998, Crossley and co-workers reported the assembly of a molecular capsule 53 (Figure 34) from two different components.66 Its formation from two zinc bis-porphyrins 54 and one tetraamine ligand of the appropriate length was proven by 1H NMR and UV−vis titration experiments. In this architecture, the tetraamine coordinated to the four zinc(II) metal centers of the two complementary bisporphyrins defines an upper and a lower 3D space. 8556


Chemical Reviews

Review

Figure 33. Panel 52, the resulting trigonal prism (52)3·(tpt)2, assembled from tpt and two different views of the X-ray structure of (52)3·(tpt)2.63 Color code: Ru(II) blue sphere, Zn(II) green sphere. Hydrogen atoms are omitted for clarity.

Figure 32. Zinc chlorophyll dimer 51 and optimized geometries (MM+, Hyperchem) of models without the ester side-chain for the prismatic cages (51)3·(50a)2 and (51)3·(50b)2.62 The tritopic ligands 50a and 50b are represented in green. Adapted with permission from ref 62. Copyright 2008 American Chemical Society.

5,10,15,20-Tetra(4′-pyridyl)porphyrin (TPYP) was recognized by many researchers as a rigid square carrying four binding sites for the coordination self-assembly of various supramolecular tetragonal prisms.43,63,67 In 2001, Osuka and co-workers reported the efficient synthesis of porphyrin pentamers Zn-55 and Ni-55 (Figure 35), which consisted of a central meso-tetraphenyl porphyrin, metaconnected to four etio-type zinc porphyrins.67a A supramolecular assembly of a porphyrin hexamer was achieved by cooperative coordination of the four pyridyl groups of either a 5,10,15,20tetra(4′-pyridyl)porphyrin (TPYP) or a 5,10,15,20-tetra(3′pyridyl)porphyrin (TMPYP) to the zinc porphyrin side walls of the pentamers Zn-55 or Ni-55 (Figure 35). The binding constants were all larger than 1.9 × 107 M−1 in chloroform at room temperature. The supramolecular assembly (Zn-55)· TPYP was studied by fluorescence spectroscopy and revealed that the fluorescence of this compound was effectively quenched by TPYP. This quenching was attributed to a two-step process, an intracomplex energy transfer from the peripheral Zn(II)porphyrin to the free-base TPYP, followed by electron transfer to the Zn(II)-porphyrin in the porphyrin hexameric complex. Supramolecular porphyrinic assemblies are therefore adapted to mimic the light harvesting and charge separation functions of natural photosynthetic systems. Nolte and co-workers have developed a very intriguing supramolecular catalyst based on a manganese porphyrin meso-

Figure 34. Dimerization of bis-porphyrin 54 by coordination to a tetraamine ligand.66

attached to a facially protective diphenylglycoluril unit to have a well-defined binding cavity.12c Bulky axial ligand binds to the manganese porphyrin outside the cavity of the cyclic structure and forces the oxidation reactions to proceed inside the cavity. Moreover, the authors demonstrated that this catalyst threaded onto a polybutadiene works as a processive catalyst, moving 8557


Chemical Reviews

Review

Figure 36. Bis(zinc porphyrin) receptor 56, the tetratopic ligand TPYP, and the self-assembly of the porphyrin-receptor complex (56)2· (TPYP).67b

Figure 35. Porphyrin pentamers Zn-55 and Ni-55 and their supramolecular assemblies with TPYP or TMPYP.67a

along the polymer chain during the oxidation reaction.11i,12c,68 Binding pockets derived from a capped porphyrin host structure are therefore appealing, and the authors developed an alternative strategy to construct such catalysts via a templated self-assembly. A tetratopic porphyrin-based ligand TPYP was selected as a template to assemble porphyrinic containers,67b and a diphenylglycoluril receptor 56 attached to two pendant zinc porphyrins was synthesized (Figure 36). In noncoordinating solvents, 56 adopted conformations in which one of the azacrown ether nitrogen atom was coordinated to one zinc(II) porphyrin unit. However, in the presence of TPYP, 56 formed a 2:1 complex (56)2·(TPYP). NMR studies showed that this coordination-assembled molecular host stabilized dihydroxybenzene guest molecules as other diphenylglycoluril-based receptors.69 This work validates the use of a porphyrin template to build other diphenylglycoluril-porphyrin receptors as potential supramolecular catalysts. An impressive work,70 in terms of both the number, the diversity of the self-assembled multiporphyrinic compounds, and the properties displayed by these architectures, was realized by Hupp and co-workers in 2008. They took advantage of the larger affinities of Zn(II) and Sn(IV) for nitrogen and oxygen Lewis bases, respectively, to build new molecular boxes. As an example, four butadiyne-linked zinc(II) porphyrin trimers 57 were elegantly combined with two meso-substituted trans-di(4′pyridyl) Sn(IV) porphyrin dimers 58a, to generate a symmetrical 16-porphyrin box structure (58a)2·(57)4 (Figures 37 and 38).70 Binding sterically crowded carboxylate ligands to Sn(IV) centers of the porphyrin dimer 58a forces the trans pyridine ligands in 58a to bind selectively to the external Zn(II)porphyrins of trimer 57 in the assembly (58a)2·(57)4.

Figure 37. Hupp’s building blocks: zinc(II) porphyrin trimer 57, meso4′-pyridyl-derivatized Sn(IV) porphyrin dimers 58a and 58b, and manganese porphyrin dimer 59.70,71

This box, (58a)2·(57)4, featured a large cavity 22 × 14 × 10 Å, from computational calculations, in which catalytically active manganese porphyrin dimer 59 was coordinated to the central zinc porphyrin of the trimer 57, to yield a supramolecular catalyst (58a)·(59)·(58a)·(57)4 (Figure 38).71 Various metalated porphyrins in 57, 58a, and 59 have a well-established function in the supramolecular box: Zn(II) porphyrins and Sn(IV) porphyrins act as structural elements of the container; Sn(IV) porphyrins limit the aperture of the box and the hollow space, 8558


Chemical Reviews

Review

Figure 39. Tetragonal prisms (52)4·(TPYP)2 and (52)4·(TPhPYP)2 assembled from panel 52 (represented in Figure 33) and tetrapyridylporphyrins (TPYP or TPhPYP), respectively.63

multitopic ligands to self-assemble cages with a controlled shape and size. Other interesting features of these structures are that metalation of the free-base porphyrins of the cruciform ligands is easily conceivable and the portals of the prisms may permit guest exchange, which should prevent product inhibition in future catalysis applications. In 2012, Osuka’s group synthesized a doubly bridged porphyrin−perylene−porphyrin compound 60 via Suzuki− Miyaura cross-coupling reaction (Figure 40).72 The bent structure permitted the formation of a 2:1 complex with TMPYP, as shown by UV−vis and NMR titrations. The supramolecular assembly (60)2·TMPYP featured unusual photochemical properties: the excited state of the Zn(II) porphyrin moiety was quenched via intracomplex electron transfer to the central free-base TMPYP. In this complex, the free energy for the photoinduced electron transfer reaction was indeed calculated to be more exothermic (−0.46 eV) than the expected photoinduced energy transfer reaction from the Zn(II) porphyrin to the free-base porphyrin (−0.15 eV). 3.1.3. Coordination of Ligands Appended to Porphyrins to External Bridging Complexes. 3.1.3.1. Monometallic Complexes. Self-assembly based on metal−ligand bond formation relies on a reaction that proceeds under thermodynamic control and that requires fast ligand substitution from the intermediate metal complexes, to reach the desired complex as the most stable product of the reaction. Therefore, kinetically labile Pd−pyridine bonds were extensively used for coordination-driven self-assembly. In 2000, Shinkai and co-workers prepared a new porphyrin building block 61 and its zinc complex 62, with four covalently linked 4-pyridyl moieties distant from the porphyrin core. Compound 61 or 62 was self-assembled through pyridyl−Pd coordination with 2 equiv of cis-Pd(II) complexes 63·(OTf)2 to afford the novel molecular capsule 64 or 65 (Figure 41).73 These cationic coordination cages with eight triflate counteranions were

Figure 38. Prismatic boxes (58a)2·(57)4, and the supramolecular catalyst boxes (58a)·(59)·(58a)·(57) 4 and (58b)·(59)·(58b)· (57)4.70,71

thanks to their axial coordinated ligands; and Mn(III) porphyrins function as catalysts and perform the reaction inside the cavity. In (58a)·(59)·(58a)·(57)4 (Figure 38), the manganese porphyrin dimer divided the cavity into two separate inner spaces. This supramolecular catalyst, tested in olefin epoxidation reaction, proved to be effective. The restrictive cavity was responsible for the selectivity of the reaction against a sterically hindered olefin. In addition, when a chiral carboxylate (N-acetyl-(D)-phenylalanine) was bound to the Sn(IV) porphyrins of the dimer 58b at the periphery of the box (58b)·(59)·(58b)·(57)4, enantioselective sulfoxidation, with a enantiomeric excess of 12%, was demonstrated. In 2012, Iengo and co-workers took advantage of square planar (tetra(4′-pyridyl)porphyrin derivative TPYP or TPhPYP (Figure 39) to bind the porphyrinic zinc centers of a flat metallacyclic panel 52 (Figure 33),63 that the authors have developed to form 2D and 3D structures (see section 3.1.2.3). Starting from a TPYP or the phenyl-extended TPhPYP ligand and 52 in an appropriate 2:4 ratio (Figure 39), the assembly process performed in chloroform at room temperature gave quantitatively two 10-porphyrinic parallelepipedic prisms of increasing sizes that were characterized by NMR experiments. In each prism, four dinuclear Ru-macrocycles constitute the walls and two tetrapyridylporphyrins the top and bottom faces of the prism. The authors showed the potential of their modular approach based on one type of metallomacrocycle 52 and various 8559


Chemical Reviews

Review

of 65, by coordination to both zinc-porphyrin faces of the capsule with a high association constant of Ka = 2.6 × 106 M−1 in chloroform at 25 °C. According to the authors, this was the first example of inclusion of a guest molecule in the cavity of a supramolecular porphyrinic cage by taking advantage of the axial coordination of two zinc porphyrin units. Starting from a chiral cis-Pd(II) complex 66, including a (R)(+)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (BINAP) ligand, a similar capsule 67 was assembled and presented a twisted structure induced by the chiral ligand (Figure 41).74 Chiral guest recognition as well as redox and photochemical reactions could be appealing applications for this supramolecular host. The pioneering work of Fujita’s group in the metal-directed self-assembly of macrocyclic structures contributed significantly to the field of noncovalent 3D hollow structures with and without porphyrins.3c,g,l,6e,44a,75 In 2001, a multiporphyrinic trigonal prism 68 featuring several interesting properties was assembled from 1 equiv of zinc(II) 5,10,15,20-tetra(3′-pyridyl)porphyrin (Zn-TMPYP) and 2 equiv of Pd(en)(NO3)2 (en = ethylenediamine)44a according to the face-directed self-assembly approach the group has developed (Figure 42).3c,42 The

Figure 40. Supramolecular capsule (60)2·TMPYP, self-assembled from 60 and TMPYP.72

Figure 42. Self-assembly of a porphyrin prism 68 in the presence of a cisPd(II) complex.44a

assembly process performed at 80 °C in a mixture of water and acetonitrile led to 68 in 96% yield. Because of the high positive charge of the assembly (12+), the nitrate salt 68 was soluble in water. The X-ray diffraction analysis confirmed the D3hsymmetry of the molecular cage in which the Pd centers occupy the apical positions of the prism. The nitrate ion, originally trapped inside the hollow framework, could be expelled by the inclusion of pyrene or other large aromatic guests like perylene or triphenylene, added in excess in CD3OD/D2O (1/1). The encapsulation of aromatic guests triggered a conformational

Figure 41. Porphyrins 61 and 62 with four appended pyridyl moieties, cis-Pd(II) complexes 63 and 66, 4,4′-trimethylenedipyridine guest tmdp, and coordination self-assembled molecular capsules 64, 65, and 67.73,74

soluble in both chloroform and dichloromethane. A 4,4′trimethylenedipyridine ligand tmdp could bind inside the cavity 8560


Chemical Reviews

Review

change of the porphyrinic host from a D3h to a C2-symmetry, as evidenced by 1H NMR experiments. Computational models suggested that the inner porphyrin faces arranged in a parallel way to optimize the strong π−π interactions with the pyrene molecule. Short peptide fragments, involving Ala-Ala-Ala sequence, were also efficiently encapsulated (Figure 43).76 The

Figure 43. Refined structures of (a) Ac-Ala-Ala-Ala-NH2⊂68 and (b) Ac-Gly-Gly-Ala-Ala-Ala-Gly-Gly-NH2⊂68 obtained from the crystal structure of 68 (represented as a thin ball and stick model) and the molecular dynamic-minimized structures (MacroModel 8.0 and OPLSAA force field) of oligopeptides Ac-Ala-Ala-Ala-NH2 and Ac-Gly-GlyAla-Ala-Ala-Gly-Gly-NH2, respectively (represented as a thicker ball and stick model and lighter CPK model). Reprinted with permission from ref 76a. Copyright 2006 American Chemical Society.

hydrophobic cavity favored oligopeptide folding, through CH−π interactions. Crystal structures of related host−guest complexes, with a chiral ancillary ligand bound to Pd(II), could also be resolved.76b A related work was reported by the group of Mukherjee in 2012.44b It described the prismatic porphyrinic structures obtained also from various cis-PdL complexes (L69 = N,N,N,Ntetra-methyleneethylene diamine, L70 = 1,2-diaminopropane, L71 = 2,2′-bipyridine) but in combination with the metal-free TMPYP (Figure 44). The three trigonal prisms 69−71 were obtained in high yields (92−98%). Remarkably, these symmetrical architectures were confirmed by NMR studies and, in the three cases, by single-crystal X-ray diffraction (Figure 45). Interestingly, despite the different conformations that could be adopted by the 3′-pyridyl moieties connected to the Pd center, the structures were consistent with all pyridyl-N atoms oriented toward the interior of the barrel in an all-endo conformation. Therefore, the wide hollow structures had narrow bottom and top apertures. As an example, the dimension of the cavity of 69 was 14 × 14 × 18 Å and the internal volume 1423 Å3, from the Xray structure (Figure 45). The ability of the highly microporous solid-state structure of 69 to store gas was also studied, and the N2-sorption diagram revealed that 42 mL g−1 of this gas was adsorbed into the cavities at standard pressure. Compound 69 was also a good adsorbent for water, MeOH, and EtOH vapors as attested by the solvent−vapor−adsorption isotherms measurements. These structures were appropriate for a postsynthetic modification of the self-assembled compounds 69−71 by zinc metalation of the free-base porphyrins. This reaction carried out under mild conditions afforded Zn-69, Zn-70, and Zn-71 without modification of the overall structures, as evidenced by 1H NMR. The nitrate salts of the highly charged free-base

Figure 44. Trigonal prisms 69−71 assembled from TMPYP and cisPd(II)L complexes.44b

porphyrins prisms and their zinc counterparts were soluble in water and in polar or protic solvents. Pt(II)−nitrogen bonds are kinetically less labile than Pd(II)− nitrogen ones; nevertheless, they were also effective to selfassemble coordination cages at room temperature or by heating the reaction medium. In 2008, Mukherjee’s group reported the self-assembly of porphyrin TPYP with cis-Pt(II) complexes 72. Hexagonal prism 73 was obtained in 94% yield from TPYP and 2 equiv of cis-[Pt(dppf)(OTf)2] in a mixture of dichloromethane/ nitromethane heated at 50 °C for 2 h. The triflate salt 73 was soluble in dichloromethane, methanol, and nitromethane. The large discrete open box 73 was the first one containing six faces occupied by a tetratopic unit (Figure 46).67c The hexagonal prismatic structure, confirmed by X-ray crystal diffraction studies (Figure 47), showed that the corners of each square face were occupied by a Pt(II), complexed with an ancillary dppf (dppf = 1,1′-bis(diphenylphosphino)ferrocene) ligand. It also featured a 8561


Chemical Reviews

Review

Figure 45. X-ray structures of trigonal prisms 69−71. Hydrogen atoms, counteranions, and solvent molecules are omitted for clarity. Reprinted with permission from ref 44b. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.44b

large internal cavity, with a diameter of 27.2 Å and a void volume estimated to be 43550 Å3, from the X-ray structure. This wide hexagonal box could be assembled due to the bulkiness of the dppf ligands. Without this steric hindrance, a trigonal prism is entropically favored, and was indeed reported by Stang, Zheng et al. in 2010 (Figure 54b).43 Metalation of the free-base porphyrin of the hexagonal prism 73 with Zn(II) occurred efficiently, without destruction of the host. The large cavity and the kinetically inert Pt complexes that characterize these structures should allow promising applications in host−guest chemistry, fluorescence sensing of metals ions, and zinc porphyrin-based molecular recognition. Beletskaya et al. reported a series of zinc tetraphenylporphyrins covalently linked to peripheral coordinating groups (hydroxy,77 cyano,78 azacrown ethers,79 or 3-butoxy-3-oxoprop-1-enyl80). Some preliminary studies showed that these appended ligands were coordinated to different metal ions (La(III), Pd(II), K+, etc.) to form face-to-face coordination dimeric cages. Bidentate nitrogen ligands, such as DABCO or 4,4′-bipyridine, could be trapped inside the cavity by axial ligation to both metalloporphyrins. In 2011, a new class of closed-face coordination cubic hosts was developed by Nitschke and co-workers81 through an efficient templated subcomponent self-assembly approach.82 The reaction of nickel(II) tetrakis(4′-aminophenyl)porphyrin Ni-74 with iron(II) complex Fe(OTf)2 and 2-formylpyridine in a 6:8:24 ratio resulted in the quantitative formation of a cubic structure, Ni-75, with 8 low spin Fe(II) ions on the corners and 6 porphyrins on the faces (Figure 48). In this elegant synthesis, Fe(II) both templated the formation of the imino-pyridine chelates and organized the cage assembly. The same reaction was also carried out with the free-base 2H-74 and the Zn porphyrin Zn-74. An X-ray structure of the cage Ni-75 was obtained. From this structure, an internal cavity of approximately 1340 Å3 and a Ni−Ni face-to-face distance of 15 Å, appropriate to incorporate large aromatic guest molecules, were determined. Indeed, the addition of excess coronene to 2H-75 or Ni-75 in DMF led to host−guest complexes incorporating precisely three coronene molecules. All of the solution studies were performed in DMF, apparently the only good solvent for the triflate salts of the

Figure 46. Large hexagonal prisms 73 and Zn-73, self-assembled from 72 and TPYP.67c

Figure 47. X-ray structure of the hexagonal prism 73: ball and stick (left) and CPK representations (right), Pt(II) is in green, Fe(II) violet, P red, N blue, black C. Triflate anions, free solvents, hydrogen atoms, as well as ferrocenyl units and phenyl groups are omitted for clarity in the CPK view. Reprinted with permission from Figure 2 of ref 67c. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.67c

assemblies. Solution studies also evidenced a higher affinity of Ni-75 for C70 over C60, the less spherical shape of the former 8562


Chemical Reviews

Review

Figure 48. X-ray structure of the closed-face coordination cube Ni-75 obtained from Ni-74 via a subcomponent self-assembly approach.81 Color code: Ni(II) green sphere, Fe(II) orange sphere.

being suitable to engage more π−π interactions with the porphyrinic faces. Furthermore, these cubic hosts are chiral, with either (Λ)8 or (Δ)8 configurations at the iron vertices, so that interesting chiral guest discrimination applications can be foreseen. In 2013, inspired by the self-assembled cubic cage reported by Nitschke, De Bruin and co-workers synthesized an enlarged M8L6 cubic cage Zn-76 to encapsulate pyridine-appended metalloporphyrins for valuable applications in catalysis.83 The Fe(II)-templated reaction between Zn-74 and 5-bipyridinealdehyde proceeded efficiently, leading to the cubic cage Zn-76 in 96% yield (Figure 49). Molecular modeling calculations (Figure 50) provided an average Zn−Zn distance of 19.5 Å between two opposite metalloporphyrins, suggesting that the cage could accommodate Zn-TPYP or Co-TPYP in which 15.5 Å separate two trans-pyridyl nitrogen atoms. It was indeed the case in DMF solution, as proven by NMR, DOSY, and mass experiments for Zn-TPYP coordinated inside Zn-76 or by EPR and mass spectroscopies for the Co-TPYP⊂Zn-76 complex. The authors could also demonstrate that Co-TPYP⊂Zn-76 was a catalytically active cubic cage complex. Indeed, Co-TPYP behaves as a catalyst in a “supramolecular flask” for metalloradical catalyzed transformations of diazo compounds (Figure 49). For example, the cyclopropanation of styrene realized within Co-TPYP⊂Zn-76 was more efficient (50% yield, turnover number 60 in 4 h) than for the nonencapsulated Co-TPYP (15% yield, turnover number 18 in 4 h). This result clearly showed the effective protection of the cage toward catalyst deactivation. In subsequent studies, replacing triflate with triflimidate N(Tf)2− counterions resulted in enhanced solubility of the cubic cage compound and its Co-TPYP encapsulated product in acetone, acetonitrile, and DMF and in some solvent mixtures like water/ acetone.84 In comparison with Co-TPYP, this catalyst was shown

Figure 49. Enlarged cubic cage Zn-76, encapsulation of Co-TPYP, and example of catalyzed cyclopropanation of styrene by Co-TPYP⊂Zn76.83

Figure 50. Molecular dynamic models (Spartan 08, MM SYBYL FF) of empty cage Zn-76 and of Co-TPYP⊂Zn-76.83 Fe ions are represented in orange, nitrogen atoms in dark blue, and Co-TPYP in pink. Hydrogen atoms, counteranions are omitted for clarity. Reprinted with permission from ref 83. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

to be able to accelerate cyclopropanation reactions in a sizeselective way. 8563


Chemical Reviews

Review

spanning six of the quadrilateral faces, are connected by three different kinds of six-coordinate Zn(II) complexes (Figure 51b,c). The presence of water as cosolvent of the reaction was important to stabilize this cage complex. The cavity, surrounded by the six porphyrins, has a trigonal bipyramidal geometry and an inner volume of 730 Å3, as revealed by X-ray analysis. This unsymmetrical cage was able to encapsulate up to two π acceptor molecules of 2,7-dinitro-9-fluorenone in an unsymmetric way, as attested by 1D and 2D NMR experiments in CD3CN. In another study of Shionoya and co-workers, the choice of tosylate (OTs−) as labile capping ligand in the Zn(II)-driven selfassembly of porphyrin ligand 77 and the use of C60 to template the reaction afforded a totally different finite structure.87 The obtained barrel complex 80 (80 = [C60⊂Zn8774(OH2)4(OTs)12]·(OTs)4) consists of four tetrakis(bipyridyl)porphyrin connected by eight heteroleptic Zn(II) complexes and incorporates one C60. Various studies were first performed to establish the conditions for such complex multicomponent self-assembly. Complexation of tetrakis(bipyridyl)porphyrin ligand 77 to Zn(OTs)2 (2 equiv) gave at equilibrium a mixture of a porphyrin tetramer [Zn8774X16]n+ as the major species and a trimer [Zn6773X12]m+ (X = OTs− or solvent) (Figure 52a). Encapsulation of C60 as a template in

Whereas formation of self-assembled complexes was mostly controlled by the metal-to-ligand ratio or the choice of a suitable template, Shionoya and co-workers demonstrated the critical role of the labile ligands bound to the metal or of its counteranions to control the outcome of the self-assembly. In 2013, they have reviewed several examples of self-assembled metallosupramolecular 3D systems of low symmetry obtained from symmetrical building blocks and one kind of naked metal ion.85 This desymmetrization self-assembly strategy is promising in terms of the different functionalities that could be associated with the complexes that have not the same coordination sphere in the final supramolecular structure. Shionoya and co-workers selected Zn(II), a metal center known for its versatile coordination sphere, and a C4-symmetric tetrakis(bipyridyl)porphyrin ligand 77 to drive the desymmetrization self-assembly under well-adjusted aqueous conditions.86 Indeed, when the ligand 77 (1 equiv) was heated with Zn(OTf)2 (2.5 equiv) in a mixture of aqueous−organic solvents (CHCl3/CH3OH/H2O, 10/10/1), a well-defined unsymmetrical supramolecular cage 78 (78 = [Zn11776(OH2)18]·(OTf)22) was obtained in 76% yield (Figure 51). This assembly was soluble in polar solvents such as acetonitrile or methanol. Its X-ray structure of D3 symmetry is consistent with an nonahedron in which the Zn porphyrins 77,

Figure 52. (a) C60-templated self-assembly of cage complex 79 from Zn(II) porphyrin 77 and AgOTs. (b,c) X-ray crystal structure of 79, viewed (b) from an S4 symmetry axis and (c) perpendicularly to an S4 axis. Zn-porphyrin ligands and Zn(II) ions are represented as a spacefilling model, with Zn(II) metal center in gray, and the other ones in orange and green. Other atoms are shown as a stick model. OTs−counteranions, free solvents, and hydrogen atoms of water coordinated to Zn(II) are omitted for clarity. (d) Coordination spheres of the two kinds of bis(bpy) Zn(II) complexes (Zn(II): left, orange sphere; right, green sphere), represented as stick models. Structures (b)−(d) were reprinted with permission from Figure 4 of ref 87. Copyright 2013 American Chemical Society.

Figure 51. (a) Self-assembly of cage complex 78 from Zn(II) porphyrin 77 and Zn(OTf)2, leading to three different kinds of six-coordinate Zn(II) complexes in the structure.86 (b,c) The X-ray crystal structure of 78 viewed (b) from a C2 symmetry axis and (c) from a C3 symmetry axis. Zn-porphyrins and Zn(II) ions are represented as a space-filling model, with Zn(II) ions coordinated to the porphyrins in gray, and the other ones in purple, orange, and green. Other atoms are shown as a stick model. OTf− anions, free solvents, and hydrogen atoms of water coordinated to Zn(II) are omitted for clarity. Parts (b) and (c) were reprinted with permission from Figure 3 of ref 86. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

CDCl3 /CD3OD = 1:1 was necessary to shift the equilibrium to the formation of the inclusion complex 79 (79 = [C60⊂Zn8774(OH2)4(OTs)12]·(OTs)4). It was isolated by recrystallization in 75% yield. No tetrameric porphyrin barrel was generated when Zn(OTf)2 was used instead of Zn(OTs)2 even in the presence of C60 as a template. This result enlightened 8564


Chemical Reviews

Review

the crucial role of the OTs− as counteranion in the assembly of 79 and the inability of the template to drive the formation of the barrel that would imply a change in the coordination geometry of Zn(II). The X-ray structure of 79 could be resolved (Figure 52b,c). Its architecture, with an overall pseudo S4 point group symmetry, consists of four porphyrin ligands 77, wrapped around a C60 guest molecule and connected through two kinds of zinc complexes, [Zn(bpy)2(OTs)2] and [Zn(bpy)2(OH2)(OTs)]+ (Figure 52d). In the inner cavity of 79, of estimated volume 810 Å3 from the X-ray structure, C60 seems to be trapped because four tosylate ions coordinated to two [Zn(bpy)2(OTs)2] close the bottom and up apertures of the tetrameric barrel. Furthermore, the coordination of tetrakis(bipyridyl)porphyrin ligand 77 toward other metals ions with flexible coordination geometry like Ag(I) was also investigated. When AgOTf was mixed with 77 in a 2:1 ratio in a mixture of CHCl3/CH3OH (1:1 v/v), a coordination cage 80a was obtained quantitatively (Figure 53).88 In the face to face porphyrin dimer 80a (80a =

four distorted square planar bis(bpy) Ag(I) complexes. Strong π−π interactions of guest 81 with both porphyrins situated at 3.1−3.2 Å and hydrogen bonds between the carbonyl oxygen atoms of the guest and the C−H of the pyridyl ligands contributed to stabilize the host−guest structure. 3.1.3.2. Multimetallic Complexes. The simplest coordination-driven self-assembly is a two-component process between porphyrinic ligands and preformed multimetallic complexes with one free binding site on the metals to enable the coordination of the porphyrinic ligands (Figures 59−62). More demanding is a three-component assembly in which a naked metal ion binds to two types of ligands, the multitopic porphyrin ligand and an additional ligand, because self-assembly involving selection is needed to form the expected cages incorporating heteroleptic complexes (Figures 56−57).46 Beside the seminal rational self-assembly of metalla-squares in the mid 1990s,89 Stang and co-workers also developed the selective self-assembly of 3D supramolecular architectures.3k,r The directional metal−ligand bond approach relies in particular on an adequate stoichiometry of the reacting building blocks and also on their complementarity in terms of geometry and symmetry. In 2010, Stang, Zheng et al. described the multicomponent assembly of a bis-porphyrin tetragonal prism 82 by simply heating in aqueous acetone cis-Pt(PEt3)2(OTf)2 83 with two types of coordinating species, sodium terephthalate and TPYP in an 8:4:2 ratio (Figure 54a).43 The triflate salt of the coordination assembly 82, obtained in 95% yield, was soluble in

Figure 53. (a) Silver-mediated self-assembly of cage complexes 80a and 80b from Zn(II) porphyrin 77 and encapsulation of acenaphthenequinone 81. (b) The X-ray crystal structure of the inclusion complex [81⊂80b(CH3OH)2]. 81 is represented as a space-filling model, and 80b is shown as a stick model. CF3CO2− anions and free solvents are omitted for clarity. (b) Reprinted with permission from Figure 1 of ref 88. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

[Ag4772]·(OTf)4), porphyrin 77 has a C4 symmetry, as shown by the 1H NMR spectrum. In solution, the structures of the porphyrin dimers obtained from Ag(I) salts with different counteranions (BF4−, CF3CO2−, CH3SO3−, NO3−) were essentially the same, on the basis of the similarities of the 1H NMR spectra. Encapsulation of different aromatic guests was investigated by UV−vis and emission titration experiments. The best guest was acenaphthenequinone 81, a π-electron-deficient aromatic compound. The binding constant Ka of 81 with 80a was determined to be 1.3 × 108 M−1 in CHCl3/CH3OH = 1:1 (v/v) at 300 K, by a competitive 1H NMR titration experiment. The Xray diffraction structure of the inclusion complex [81⊂80b(CH3OH)2] (80b = [Ag4772]·(CF3CO2)4) confirmed the expected cofacial orientation of the porphyrin rings, linked by

Figure 54. Self-assembly of multicomponent prisms from cis-Pt(PEt3)2(OTf)2 83, sodium terephthalate, and TPYP: (a) tetragonal prism 82, (b) trigonal prism 84, and (c) supramolecule-to-supramolecule transformation of trigonal prism 84 to tetragonal prism 82 with the platinum macrocyclic complex 85.43 8565


Chemical Reviews

Review

nitromethane. The driving force for such an assembly is the energy difference between homoleptic and heteroleptic Pt(II) cage complexes, in favor of the heteroleptic systems, as shown by computational simulation. To confirm this result, the authors prepared the homoleptic trigonal prism 84 from cis-Pt(PEt3)2(OTf)2 83 and TPYP in a 6:3 ratio in a mixture of deuterated dichloromethane and nitromethane (Figure 54b). The molecular modeling of 84 showed important distortions of the free-base porphyrins to allow a trigonal prism-like structure (Figure 55a). A supramolecule-to-supramolecule transformation

Figure 55. Molecular dynamics simulations (MMFF or MM2* force field in Maestro or Macromodel) of (a) trigonal prism 84 and (b) the encapsulated complex [triphenylene⊂86]. Reprinted with permission from ref 43. Copyright 2010 American Chemical Society.43

Figure 56. Tetragonal prism 89 assembled with Cu(I), porphyrin 87 with four appended bulky phenanthroline ligands, and ditopic ligand 88.47

in the same solvent mixture then was tested: the homoleptic trigonal prism 84 was heated with 2 equiv of the neutral triangle terephtalate platinum complex 85 and afforded selectively the heteroleptic assembly 82 in quantitative yield (Figure 54c). Pulsed-field-gradient spin−echo NMR measurements gave a hydrodynamic radius of 10.9 Å for this tetragonal cage, consistent with the radius of 12 Å obtained by computational study. The authors showed that a related tetragonal cage 86 (86 = [(Pt(PMe3)2)8(terephthalate)4(TPYP)2]·(OTf)8) was able to encapsulate triphenylene as a guest in 27% yield in aqueous acetone solution. The distance between the guest and porphyrin faces is ca. 3.8 Å, as shown by a computational simulation (Figure 55b). The HETPHEN (heteroleptic phenanthroline) complexation approach was reported by Schmittel in 1997.90 It consists of using one bulky phenanthroline and an unshielded one to construct dynamic heteroleptic copper(I) bis-phenanthroline assemblies in a quantitative yield. It was successfully used in 2013 to afford quantitatively the tetragonal prism 89 from a mixture in CDCl3 of a porphyrin linked to four 2,9-diaryl-1,10-phenantroline ligands 87, a dipyridinecarboxaldehyde precursor 88, and [Cu(CH3CN)4]·PF6 salt in 1:2:4 stoichiometric amounts (Figure 56).47 The cationic (8+) self-assembled structure has wide portals and an estimated distance of 7 Å between the porphyrins (from MM+ calculations in Hyperchem), which is suitable for accepting and releasing molecules. Host−guest studies demonstrated the encapsulation of C60 in toluene by 13C NMR, ESI−MS, and UV−vis titrations. The cofacial organization of the two zinc porphyrins in 89 optimized the

interactions with C60 leading to a high 1:1 association constant of 3.3 × 106 M−1 in toluene at 298 K. When the same experiments were performed with coronene as a flat aromatic guest molecule, the formation of a 1:1 complex was also observed, with a lower complexation constant of 1.1 × 104 M−1 in CH2Cl2 at 298 K. Therefore, a complete guest exchange of coronene with C60 was demonstrated after heating the solution of coronene⊂89 in toluene in the presence of 1 equiv of C60. A related work, based on the formation of heteroleptic complexes with sterically hindered 2,9-diarylphenanthroline ligands and terpyridines, known as the HETTAP concept (heteroleptic terpyridine and phenanthroline metal complexes)91 was reported by the same group in 2008.46 Selfassembly of a bulky tris-phenanthroline component 90 and a linear porphyrin bis-terpyridine conjugate 91 in the presence of copper(I) led initially to a mixture of the expected supramolar trigonal nanoprism 92 (92 = [Cu6(90)2(91)3]·6PF6) (Figure 57) and some oligomeric aggregates.46 In subsequent attempts and in the same reaction conditions, the use of an appropriate template (1,3,5-tris(4′-pyridyl)benzene tpyb or C60) to gather three porphyrin moieties 91 was decisive for complete conversion to the heteroleptic coordination 3D structures, tpyb⊂92 or C60⊂92, soluble in CD2Cl2/CD3CN (4:1) and CD3CN, respectively. Force-field modeling and detailed NMR studies suggested that the cage could distort to adapt itself to the encapsulated fullerene by twisting the prism about the C3 axis (Figure 58). Archimedean solids can serve as geometrical models to build different kinds of three-dimensional supramolecules with large 8566


Chemical Reviews

Review

Figure 57. Heteroleptic nanoprism tpyb⊂92, obtained by tpyb templated self-assembly of bulky tris-phenanthroline component 90 and bis-terpyridine zinc porphyrins 91 in the presence of Cu(I).46

Figure 59. Self-assembly of truncated tetrahedron 95 from its molecular components tris(platinum)triflate 93 and cis-bis(4′-pyridyl)porphyrin 94.92

complex 93 with 6 equiv of ditopic cis-bis(4′-pyridyl)porphyrin 94 in CH2Cl2/CHCl3/CH3OH yielded a truncated tetrahedron 95 in 87% yield. The porphyrins were located at the middle of the edges, whereas the planar platinum complexes laid on the faces of the highly symmetrical 3D structure 95, whose 12 positive charges were counterbalanced by triflate ions. In principle, this supramolecular-based Archimedean solid, possessing a large internal cavity, is capable of hosting guest molecules. Apart from the purely inorganic directional bonding approach involving coordination complexes with free acceptor sites, organometallic complexes and especially half-sandwich complexes were also extensively used as metal precursors to build 2D and 3D supramolecular architectures.93 With η5-cyclopentadienyl or η6-arene as ligand, three of the six coordination sites of an octahedral metal center are occupied. This type of ligand determined the accessibility of the remaining sites and the spatial organization of the incoming ligands. Connecting two halfsandwich complexes with different kinds of rigid bridging chelating ligands gave access to dinuclear arene precursors with one acceptor site on each metal. These molecular clips were useful to self-assemble 3D structures with a controlled cavity size,

Figure 58. Force field modeling (MM+, Hyperchem) of C60⊂92: (a) side view, (b) top view, and (c) arrangement of the porphyrin units around the C60 guest (in black). Color code: carbon, cyan; nitrogen, blue; bromine, yellow; copper, green; zinc, gray. Reprinted with permission from ref 46. Copyright 2008 American Chemical Society.46

voids.3k Stang and co-workers reported in 200092 a molecular truncated tetrahedron, formally derived from a tetrahedron by removing the four corners, prepared via a face-directed approach from planar tritopic 120° subunits and angular ditopic 90° corner units (Figure 59). Assembly of 4 equiv of tritopic tris(platinum) 8567


Chemical Reviews

Review

two helicoidal isomers. In both cases, the porphyrin planes are nearly cofacial but staggered, inducing helical chirality to the structures. From the X-ray structures, the external volumes of the boxes were estimated to be 800 Å3. The twisting of the two porphyrins allowed them to optimize the π-stacking of the macrocycles, which are separated by about 4 Å. In the same year 2008, related organometallic boxes 97−99 (Figure 61) were built in Therrien’s group by self-assembly from

from a reduced number of components. Incorporating several cationic organometallic complexes in a 3D structure provided also a high positive charge to the assembly in favor of water solubility. Such organometallic cages offer appealing features as multidrug therapeutic agents due to the potential of halfsandwich complexes to act as chemotherapeutic agents, the ability of the water-soluble cage to transport and deliver a hydrophobic drug, and the possibility to use the phototherapeutic activity of porphyrins incorporated in the cage framework.13d In 2008, several molecular boxes 96a−c incorporating organometallic complexes were prepared by Jin and co-workers from different half-sandwich Ir, Rh, and Ru complexes.94 By simply mixing stoichiometric amounts of the appropriate dichloro μ-oxalato binuclear bridging complexes with square TPYP panels in methanol in the presence of AgOTf, the desired cationic cages (Figure 60) were obtained as triflate salts in ca. 80% yield. The X-ray structures of the octanuclear Ir- and Rubased cages 96a and 96c were resolved as a racemic mixture of

Figure 61. Tetragonal prisms 97−99 consisting of bridged dimeric Ru complexes and two TPYPs.95

TPYP panels and 2,5-hydroxy-1,4-benzoquinolato-bridged diruthenium clips.95 These cationic octanuclear complexes, isolated as their triflate salts in good yields (ca. 80%), were soluble in various solvents (dichloromethane, acetone, acetonitrile, dimethylsulfoxide, and water). They strongly bind to DNA, via electrostatic interactions, but unfortunately their selectivity

Figure 60. (a) Organometallic boxes 96a−c consisting of four μ-oxalato dimeric Ir, Rh, and Ru complexes bridging two TPYPs; and (b) X-ray structure of 96a. Hydrogen atoms are omitted for clarity.94 The ligands are represented as a stick model, and Ir ions as a space-filling model (blue sphere). 8568


Chemical Reviews

Review

for quadruplex over duplex DNA was limited.96 Nevertheless, the organometallic boxes 97 and 98 presented very good cytotoxicity toward human Me300 melanoma cells, with IC50 of 5−10 μM (IC50 is defined as the concentration of the drug that leads to 50% inhibition of cell viability).97 They showed also good phototoxicities in A549 pulmonary and HeLa cervix cancer cell lines, at a concentration as low as 1 μM, upon laser light irradiation (DL50 between 2 and 7 J cm−2, DL50 being the light dose inducing 50% of mortality).97 Thus, these kinds of ruthenium boxes are promising candidates for both chemotherapy and photodynamic therapy of cancer. Cofacial Pd(II) porphyrins incorporated in a tetragonal prismatic cage were described in 2013 by Ribas and co-workers.48 In this work, the self-assembly procedure implied only two components: a 5,10,15,20-tetrakis(4′-carboxyphenyl)Pd(II) porphyrin 100 and a presynthesized dinuclear Pd(II) macrocycle, [Pd2101(OAc)2]2+, acting as a molecular clip (Figure 62). In [Pd2101(OAc)2]2+, the two Pd are in a square planar geometry, each coordinated to three N atoms of the macrocycle 101 and to a monodentate acetate anion. The self-assembly proceeded in high yield, 93%, by simply heating the two building blocks in DMF. The nanocage 102, isolated with eight triflate counteranions, was characterized by mono and two-dimensional

NMR experiments, and its X-ray crystal structure (Figure 63) could be determined despite the instability of the crystals. The

Figure 63. X-ray structure of cage 102: side (top) and top views (bottom). Hydrogen atoms are omitted for clarity. Reprinted with permission from Figure 3 of ref 48. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

overall structure has a D4h-symmetry, and the two parallel Pd porphyrins are 7.5 Å apart and held together by monocoordination of each carboxylate group to a Pd center. The capacity of the inner cavity to stabilize various flat guest molecules was tested. Surprisingly, neither the neutral π-electron-rich molecules like coronene, pyrene nor the electron-deficient cationic ones like paraquat showed to be complexed, based on UV−vis titrations in acetonitrile. Nevertheless, π-anionic planar complexes like [M(mnt)2]− (mnt = maleonitriledithiolate, M = Au, Pt, Pd, Ni (Figure 62)) were efficiently encapsulated with very high association constants (Ka ranging from 109 to above 1010 M−1 in acetonitrile at room temperature). Some metal−metal interactions between the encapsulated [M(mnt)2]− and the Pd(II) porphyrins of the host could be implied in the host−guest stabilization as suggested by DFT calculations. Cyclic voltammetry experiments were also performed on the host− guest complexes and showed a lower reduction potential for the guest [Ni(mnt)2]− in the presence of the host, also suggesting its stabilization upon inclusion in the cage. 3.2. Cages Self-Assembled by H-Bonds

Cages self-assembled by hydrogen bonds rely on weaker bonds energy (2−15 kcal mol−1) than the ones organized by coordination bonds (15−50 kcal mol−1). Stabilization of Hbonded structures depends strongly on the number, directionality, and spatial organizations of the H-bonds and on an adequate choice of H-bond donors and acceptors that selfassemble. The synthesis difficulties to obtain precursors with multiple hydrogen-bonding sites and the limited choice of non competitive solvent for the H-bonds that enable one to form soluble and stable structures might have limited their development, because only few examples of such structures have been reported.

Figure 62. Prismatic Pd(II) porphyrinic cage 102, self-assembled from Pd(II) porphyrins 100 and dinuclear Pd(II) clips [Pd2101(OAc)2]2+ and chemical structures of the π-anionic planar guest complexes [M(mnt)2]−.48 8569


Chemical Reviews

Review

Lehn and co-workers reported a pioneering work in the field of H-bonded self-assembled porphyrinic 2D structures as early as 1993. They described the self-assembly of a supramolecular macrocycle from two trans appended 5-alkyluracil porphyrins and two alkyltriaminopyrimidine complementary moieties.98 This result opened the way to the elaboration of H-bonded multiporphyrinic structures as 3D cages. In 1995, Ogoshi, Kuroda et al. reported the self-induced dimerization of meso-tetrakis(2-carboxy-4-nonylphenyl)porphyrin 103 in nonpolar solvents (CHCl 3 , CCl 4 , CHCl2CHCl2, benzene) (Figure 64).99 UV−vis, IR, NMR

coordination bonds, multiporphyrinic assemblies as described in subsequent work (Figure 65).101

Figure 65. Free-base porphyrins 105 and 106, functionalized with pyrazine derivatives, and nona- and heptadecameric porphyrin assemblies, 105·(1042)4 and 106·(1042)8.101 For clarity reasons, a simplified schematic representation of the cage 1042 described in Figure 64 is used.

Kuroda et al. took advantage of the free axial coordination site of Zn porphyrins in the stable supramolecular cage 1042 to build light-harvesting nona- and heptadecameric porphyrin assemblies, 105·(1042)4 and 106·(1042)8, respectively (Figure 65).101 In these assemblies in dichloromethane, the free-base porphyrin (105 or 106), functionalized with four or eight pyrazine derivatives via long alkyl chains, was surrounded by 8 or 16 zinc porphyrins due to inner coordination of the pyrazines inside the hydrogen-bonded cages 1042. The fluorescence of the central porphyrin 105 or 106 was strongly enhanced in these assemblies, 18 or 77 times, respectively, demonstrating the antenna effect resulting from their spatial arrangement. In 2000, a multitopic receptor (107)3·(108)6 was quantitatively self-assembled in the group of Reinhoudt from three bis(zincporphyrin)-functionalized dimelamines calix[4]arene subunits 107 and six 5,5-diethylbarbituric acids 108, due to the cooperative formation of 36 hydrogen bonds (Figures 66 and 67).102 In this structure, the six porphyrins are located outside the hexagonal prism, three below and three above the hexagonal faces, defining two external receptor sites. 1H NMR studies in CDCl3 at room temperature supported the 1:2 complexation of 1,3,5-tris(4′-pyridyl)benzene tpyb to the external porphyrin sites of the prism. The self-assembly of a library of three compounds, 107, 108, and 109, a dimelamine-functionalized calix[4]arene without porphyrin units, was also studied (Figure 68).102 The authors showed that the guest molecule tpyb could efficiently drive the equilibrium of the statistical mixture of homo- and heteromeric assembled capsules (1073‑n)·(109)n·(108)6 (n = 0−3), toward a 1:1 mixture of the homomeric hydrogen-bonded cages (107)3· (108)6 and (109)3·(108)6 in CDCl3 at room temperature (Figure 69). This work represents the first example of guest-

Figure 64. Free-base and zinc porphyrins 103 and 104, appended to four carboxylic acids and the H-bond self-assembled cages 1032 and 1042.99,100

spectroscopic data, and vapor pressure osmometry supported this phenomenon. Eight hydrogen bonds between the four pairs of carboxylic acids stabilized the dimeric system 1032. The same behavior was observed for zinc(II) porphyrin 104.100 The Hbond-stabilized dimer 1042 was able to incorporate pyrazine and 2-substituted pyrazine into its inner space, by coordination to the two zinc(II) ions, as shown by UV−vis (in dichloromethane) and 1 H NMR (in CDCl3) titrations. The authors showed that the long alkyl chain of the pyrazine guest molecule extended outside the cavity, through a portal of the cage 1042. Kuroda et al. used this property to self-assemble, through both H-bonds and 8570


Chemical Reviews

Review

Figure 66. Building blocks 107, 108, and 109 used by Reinhoudt to selfassemble H-bond cages.102

Figure 68. Self-assembly of a library of 107, 108, and 109 (chemical structures represented in Figure 66).102

ratio resulted in the self-assembly of the bis α4 atropisomer porphyrinic cages (110a−d)2·(111)4 in dry THF. Their diameters were estimated to be around 7−13 nm by dynamic light scattering. The authors also studied the self-organization of the cages on mica surfaces and observed by AFM the hierarchical organization of the robust supramolecular cages into nanometer thick films, due to interactions between the long decyl chains of the melamine units. These self-assembled porphyrin cages have demonstrated their potential as H-bond self-assembled materials with potential receptor functions due to the porphyrins they incorporate.

Figure 67. Multitopic receptor (107)3·(108)6 self-assembled from 107 and 108 (chemical structures represented in Figure 66).102

templated selection of a receptor in a dynamic noncovalent combinatorial mixture. In 2011, Drain et al.103 synthesized porphyrins 110a−d mesoappended with four rigid uracylic hydrogen-bonding units (Figure 70). In each case, mixtures of the four atropisomers (α4, αβ3, α2β2, αβαβ) were obtained with poor yields (

Multiporphyrinic Cages: Architectures and Functions - Chemical

Recommend Documents