Recent Developments in the Preparation and Chemistry of

Review pubs.acs.org/CR

Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination Timothy R. Cook*,† and Peter J. Stang*,‡ †

Department of Chemistry, University at Buffalo, State University of New York, 359 Natural Sciences Complex, Buffalo, New York 14260, United States ‡ Department of Chemistry, University of Utah, 315 S. 1400 E. Room 2020, Salt Lake City, Utah 84112, United States izations, and applied studies. Coordination-driven self-assembly has provided the basis for tremendous growth across many subdisciplines of chemistry, spanning fundamental investigations regarding the design and synthesis of new architectures to defining practical applications. These efforts are largely inspired by the observation that the complexity found in natural systems originates from exceedingly simple noncovalent interactions (H-bonding, hydrophobic/hydrophilic, dipole−dipole, etc.) between molecular subunits. Despite the simplicity of each interaction when considered on its own, architectures of unparalleled diversity, including helices (DNA), 2D surfaces (lipid membranes), micelles, viral capsids,1 etc., are realized CONTENTS when many such interactions act in concert. 1. Introduction A Metal−ligand bonding circumvents many of the difficulties 2. Structural Chemistry B associated with controlling the interactions that lack the 2.1. Edge- and Face-Directed Assembly B directional control afforded by the predictable and well-defined 2.2. Symmetry and Subcomponent Assembly L coordination geometries of transition metal ions. As such, the 2.3. Helicates and Metallocrowns M spontaneous formation of coordination bonds can be used as 2.4. Multicomponent/Self-Sorting O the impetus for self-assembly reactions between Lewis-basic 2.5. Hierarchical Self-Assembly T donor molecules and Lewis-acidic acceptor molecules. This 2.6. Post-Self-Assembly Modifications U donor/acceptor paradigm was used to great effect in the 2.7. Weak-Link Modifications W development of discrete metallacycles and cages, and remains at 2.8. Covalent Stabilization X the heart of all contemporary research efforts within the field. 2.9. Catenanes X Herein, we review the recent developments of coordination2.10. Host/Guest Structures Y driven self-assembly, with a focus on discrete architectures. 2.11. Self-Threaded Architectures AB Although relevant historical discoveries will be discussed, an 2.12. Large Assemblies AB emphasis is placed on results that have been reported in the 2.13. Crystal Engineering AD past three years. It is organized in a way that highlights both the 3. Functional Assemblies AD interdisciplinary nature of the field as well as its maturation 3.1. Electrochemistry and Redox-Active SCCs AD from studies that defined structural and synthetic method3.2. Photophysically Active SCCs AF ologies, to more recent efforts to expand structural complexity 3.3. Sensors AG and explore functional systems for a suite of applications. Key 3.4. Catalysis and Reactivity AI design principles will be introduced when relevant to a recent 3.5. Biological Studies AK discovery being discussed. Readers seeking additional back4. Conclusion AL ground knowledge on established design strategies and Author Information AM summaries of earlier discoveries are directed to our previous Corresponding Authors AM reviews on such topics.2−4 Notes AM Many fields of chemistry employ unique terminology for Biographies AM concepts that oftentimes appear in other related areas. Acknowledgments AM Unfortunately, this can lead to the introduction of multiple References AN terms for the same phenomenon or names whose literal Note Added in Proof AS interpretations may not match specific uses. The use of supramolecular coordination complexes (SCCs) herein refers

1. INTRODUCTION The continued development of molecular architectures of consequence to both longstanding and newly recognized problems demands ever evolving synthetic routes, character© XXXX American Chemical Society

Special Issue: 2015 Supramolecular Chemistry Received: October 2, 2014

A

DOI: 10.1021/cr5005666 Chem. Rev. XXXX, XXX, XXX−XXX

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2. STRUCTURAL CHEMISTRY Early efforts in the development of coordination-driven selfassembly reactions and the formation of SCCs were largely structural in nature. These studies focused on defining synthetic methodologies to achieve specific metallacycles and cages through various, now well-defined, assembly strategies including edge-directed, face-directed, symmetry-adapted, and weaklink approaches, all unified under the theme of “directional bonding”. Many of these methods are rooted in twocomponent assemblies, wherein one metal acceptor and one organic donor are used in specific ratios to form highly symmetric metallacycles or cages, with examples spanning many polygons and polyhedra.2−4 Although more recent studies have departed from these traditional two-component structural investigations, key advances continue to be made in the context of the structural and synthetic chemistry of SCCs. These advances oftentimes break the paradigm of one acceptor and one donor, providing increased structural complexities and novel designs through socalled multicomponent self-assembly. Alternatively, existing design methods continue to be employed toward new architectures or to provide scaffolds for post-assembly modifications.

to discrete metallacycles and metallacages obtained by selfassembly reactions. The precursors to these cages, grouped as Lewis-acid containing “acceptors” and Lewis-basic “donors”, overlap significantly with the library of building blocks used to synthesize coordination polymers (CPs), metal−organic frameworks (MOFs), etc. This overlap is no coincidence, as the difference between SCCs and CPs is simply a divergent or convergent self-assembly, leading to infinite or closed final structures.2,5 The use of metal−ligand bonding as the impetus for assembly joins a number of complementary techniques that use either noncovalent interactions or organic approaches to form 2D polygons, 3D polyhedra, and other nanoscopic materials of growing interest.6−8 A number of excellent reviews provide further reading and insight into specific areas of the chemistry of metallacycles and cages. Some of these reviews focus on a class of building blocks, either acceptors or donors. For example, Mishra and Gupta have summarized pyridine-amide-based assemblies.9 Chand and co-workers have provided a detailed investigation of Pd(II) SCCs.10 Mukherjee recently reviewed both Pd(II) and Pt(II) cages with an emphasis on template-free approaches.11 Mashima and co-workers reviewed the use of multinuclear complexes as components of supramolecular assemblies, with some examples of metallacycles and cages among other oligomeric designs.12 Other reviews emphasize classes of structures. Nitschke and co-workers recently published a tutorial review on 3D architectures with an emphasis on nonclassical structures (i.e., not Archimedean or Platonic solids).13 The prospects and future directions in self-assembly chemistry were the focus of a recent review by Ward and Raithby, with work in multicomponent assembly, host/guest chemistry, and light harvesting being used to illustrate challenges and limitations of current systems and to motivate new strategies.14 A review published in late 2013 by Trabolsi and co-workers discusses recent advances in coordinationdriven self-assembly with an emphasis on supramolecular transformations.15 A perspective recently written by Housecroft focuses on donors that coordinate through terpyridine moieties for both coordination polymers and discrete systems.16 Casini and Kükn have reviewed M2L4 type structures.17 A metalloligand design strategy was the subject of an extensive review by Kumar and Gupta, with discussions of both hydrogen bond and coordination bond style ligands.18 Computational methods are increasingly being applied toward the development of new self-assembled materials, with much effort placed on coordination-driven approaches, a theme explored in a recent review by Samori and co-workers.19 An excellent analysis of diffusion NMR techniques applied toward metallacycles and cages was recently written by Avram and Cohen.20 A careful review of design strategies by Young and Hay revealed that reproducing the ideal angles between edges and faces of a target polygon or polyhedron using molecular subunits is oftentimes hindered by distortions, primarily rotations of vertices about their symmetry axes, which alters the required angles from what a simple geometric approach would predict.21 Therefore, a new strategy was introduced using a de novo methodology that accounts for vertex rotation and targets the angular relationships between symmetry axes as the key design criteria. The authors rigorously explore existing structures in the context of these methods and offer access to a computation package to apply the de novo approach.

2.1. Edge- and Face-Directed Assembly

The edge-directed and face-directed approaches to directional bonding were largely developed using Pd(II) and Pt(II) as metal acceptors with bridging pyridyl-based ligands.22 To avoid divergent structures, the predictable square coordination environments about the metal centers were capped with amines or phosphines, a strategy still commonly used in contemporary work. A recent example of this is the [3 + 3] triangular metallacycle described by Otsubo, Kitagawa, and coworkers, containing tetramethylethylenediamine-capped Pd(II) nodes bridged by 4,4′-azopyridine (Figure 1).23 The same

Figure 1. Azo-containing triangular SCC formed via cis-capped edgedirected self-assembly.

strategy applies to 3D cages, as exemplified by the small prism described by Habata and co-workers.24 The ligands used as caps are by no means limited to these classes, however. Verpoort and co-workers described the synthesis and reactivity of a series of mono-, bi-, and trinuclear Pd(II) complexes with the Pd centers capped by Schiff-bases.25 Chang and workers used similar Schiff-base ligands in conjunction with oxalate and phosphine ligands to form metallacycles.26 Similarly, the library of ligands used as connecting units has expanded greatly. Even when pyridyl donors are still the Lewis-basic moieties, they are often found on increasingly elaborate cores, introducing new properties or structural behaviors.27 B


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The use of cis-capping ligands is common for Pd(II)- and Pt(II)-based assemblies. When trans-capping is used, it is typically for organometallic species containing a M−ethynyl or M−phenyl bond, connecting two or more metal centers to a central, rigid aromatic group to encode directionality. Thus, a single coordination site remains at each metal node. The use of trans-capping to yield acceptors with two open coordination sites for subsequent self-assembly chemistry is very rare. An example of this approach was provided by Clever and coworkers, using Pd(II) centers with pyridyl-based trans-chelating caps.28 These mononuclear centers were unique as they represented extremely small 180° acceptors, capable of linking bridging donors to form metallacycles via a single ion (Figure 2).

Figure 3. Combination of Cu2 acceptors with pseudohalide ligands furnishes tetranuclear metallacycles.

Although the conditions for SCC formation are typically mild, oftentimes occurring at room temperature or with moderate heating and with reactions times on the order of hours, there exists a bias in that problematic assemblies are likely to go unreported. It has long been recognized that efficient self-assembly necessitates soluble kinetic intermediates such that incorrectly oriented building blocks will undergo dissociation and reorientation as the system proceeds toward a single thermodynamic product. When these conditions are not met, such that a large number of insoluble or inert kinetic products are formed, a synthesis is susceptible to abandonment. Quintela, Peinador, and co-workers have demonstrated that microwave-assisted heating can greatly enhance the formation of SCCs, reducing reaction times from days to hours and improving yields.33 This was demonstrated on a series of dinuclear Pt(II)-based metallacycles (Figure 4), and is anticipated to be readily adaptable to other systems.

Figure 2. A [3 + 3] edge-directed assembly using trans-capped acceptors.

Pfeffer and co-workers developed a solvent-free synthesis of bis(pyridyl)-[5]polynorbornane ligands to act as edge donors with very rapid (5−10 min) reaction times.29 The resulting dipyridyl donors were evaluated in self-assembly reactions with Pd(II) and Pt(II) ions, with evidence for the formation of a variety of mono- and dinuclear products. Pfeffer and Clever have continued to investigate polynorbornane ligands for selfassembly.30 Dinuclear Cu2 molecular clips were used in self-assembly reactions with pseudohalide bridging ligands such as CN−, N3−, and a larger anionic donor, C(CN)3−, by Lescop and Réau to form small tetranuclear metallacycles (Figure 3).31 The Cuclips contained a N,P,N-pincer ligand containing two terminal pyridyl groups and a central phosphole, which bridged between the metal centers. All three metallacycles were structurally characterized using X-ray diffraction, confirming that a [2 + 2] assembly had occurred. The authors proposed that polymeric chains and other oligomeric species were avoided due to the steric bulk of the pincer ligands, and rigidity of the parallel coordination vectors remaining on the two Cu sites. These N,P,N−Cu clips were subsequently studied with other linear ditopic donors to form new metallacycles and “open” cages with a single bridging ligand.32

Figure 4. Representative metallacycle formed via microwave-assisted heating.

In a complementary study, McCready and Puddephatt explored self-assembly processes with the goal of selecting between kinetic and thermodynamic products, with an emphasis on the quantitative formation of one or the other.34 This proved possible by controlling the isomerization of mononuclear complexes of 1,4-di-2-pyridyl-5,6,7,8,9,10hexahydrocycloocta[d]pyridazine coordinated to Pt(II). Depending on which mononuclear isomer was present at the time of self-assembly, one of two so-called clamshell complexes was obtained. Eventual equilibration to the most open structure occurred, indicating that the smaller clamshell was an isolable kinetic product. A guanosine ligand containing a pendant terpyridine group self-assembled with Cu(II) ions to form rectangular metallacycles (Figure 5). Robertson and Vilar used Cu(NO3)2 in a 1:1 ratio with the organic donor to afford the [2 + 2] assembly as characterized by ESI-MS and X-ray diffraction to confirm stoichiometry and structure.35 Although the metal−metal C


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Figure 5. Dicopper metallacycle formed from an asymmetric guanosine-terpyridine donor.

separation was too far for direct electronic coupling, superexchange pathways were anticipated due to the π−π interactions originating from the terpyridine groups. Superexchange between metal sites was also explored by Reger, Ozarowski, and co-workers as part of their investigations of poly(pyrazolyl)methane-based ligands.36 Dinuclear metallacycles containing bridging hydroxide groups were synthesized with either Fe(II), Co(II), Ni(II), or Cu(II) using one of two dipyrazolyl ligands. The M−O−M angles in these complexes varied from 141° to 180°. As this angle approached linearity, the exchange interactions between metal sites were found to increase, further supported by computational studies. A comparison between traditional solvothermal methods and microwave heating in the construction of tetranuclear Co4 cubes was made by Zeng and Kurmoo, using 2-hydroxy-3methoxy-benzaldehyde or 2-hydroxy-3-ethoxy-benzaldehyde as ligands.37 These were mixed with cobalt nitrate and triethylamine and either microwaved to 120 °C for up to 80 min at 300 W, or heated for up to 4 days. For both cubes, the microwave method proved far more efficient. During ESI-MS analyses of the products, higher nuclearity species were observed, motivating the search for synthetic routes toward larger architectures. A simple dinuclear metallacycle containing two Zn(II) nodes bridged by two 1,3,4-oxadiazole-containing dipyridyl ligands was described by Dong and co-workers, obtained by mixing the donor with Zn(NO3)2 in acetonitrile (Figure 6).38 After 1

Figure 7. Suite of alkyl spaced Pd metallacycles via dithiocarbamate coordination.

structural information was obtained computationally, indicating that the number of methylene groups in the alkyl spacer determined which of two structure types the resulting SCCs would adopt, either a straight chain rectangle, or a variant with a significant kink at the center of each ligand. A similar bis(dithiocarbamate) motif was used by Höpfl and GarciaGaribay in the design of macrocyclic rotors with coordination to Sn(IV) sites.40 A smaller but still flexible imidazole-based ligand was used by Yang and co-workers to make dinuclear Pd2 metallacycles.41 Three different ligand variants were explored, all containing imidazole groups tethered to a central phenyl ring through a single methylene spacer. The ligands differ in their substitution of this central ring, either unfunctionalized, methyl functionalized, or tert-butyl and hydroxy functionalized. Self-assembly with Pd(NO3)2·H2O was performed in DMSO, resulting in ciscoordination to form [2 + 2] metallacycles. A trans-analogue was formed by using PdBr2 as a Pd(II) source. Seki and co-workers controlled the formation of both [2 + 2] and [3 + 3] assemblies using dipyrrin dimers.42 The terminal donor sites were linked through phenyl and ethynyl spacers, ultimately joined via ortho-substitution on a central benzene ring. Two or three such ligands were joined upon coordination to either Zn or Ni centers, forming distorted rhomboidal or trigonal metallacycles. The solid state structure revealed favorable packing alignments for electronic communication between SCCs. As such, conductivity was measured, revealing hole motilities of up to 0.11 cm2 V−1 s−1. Dipyrrin ligands were also employed by Pandey and co-workers to generate a suite of eight complexes, including one M2L2 metallacycle, when assembled with Cu(II) sources. The ligands for these syntheses contained one dipyrrin-site with a pendant pyridyl group (Figure 8).43 Successful self-assemblies of thermodynamically favored SCCs all share a commonality of avoiding kinetic traps, wherein incorrectly oriented building blocks precipitate or otherwise fail to undergo reversible bond formation, thus leading to undesirable products. Johnson and co-workers

Figure 6. Dinuclear Zn metallacycle with oxadiazole linkers.

week, crystals suitable for X-ray diffraction were collected, confirming the [2 + 2] assembly. A related Cu(II) species was made that formed coordination polymers rather than discrete species. A suite of seven dinuclear metallacycles containing Pd(II) nodes with homoleptic coordination to dithiocarbamates was reported by Höpfl and co-workers (Figure 7).39 These [2 + 2] assemblies differed in size and shape on the basis of the length of the alkyl spacer found in the alkyl spacers between the amine groups of the precursor to the ligand. Conversion to dithiocarbamates accompanied self-assembly, with CS2, KOH, and PdCl2 added in ethanol solution. In addition to a singlecrystal X-ray diffraction analysis of one of the metallacycles,

Figure 8. Two copper metallacycles with dipyrrin donors. D


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heterometallic system was realized by using a ditopic donor with a central 3-pyridyl-bian (bianbis(arylimino)acenaphthene). The addition of Ag(I) ions furnished a trapping of this second metal within the internal cavity of the metallacycle. The well-studied 2,4,6-tri(4-pyridyl)-1,3,5-triazine ligand was also used with these clips to form trigonal prisms. In collaboration with Han, Jin continued the use of Rh and Ir molecular clips to form metallacycles bridged by N,N-bis(4pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide ligands. Four different tetranuclear metallacycles were formed, differing in the metal used, and the bridging ligand in the molecular clip. X-ray crystallography confirmed the anticipated metallacyclic structure.49 An elaborate self-assembly of spoked, wheel-like architectures was developed by Newkome and co-workers, using 2,2′:6′,2″terpyridine (tpy) ligands and Zn(II) metal nodes.50 Both 2D and 3D variants were synthesized. Two types of tritpy ligands were used. The “spoke” ligands contained 120° angled coordination vectors, while the “rim” contained 60° displacement between vectors. These were combined with Zn(II) in a 2:6:12 ratio (spoke:rim:Zn), to afford the 3D wheel. It was also possible to use a hexa-tpy ligand as the sole spoke. Relatively simpler metallacycles were constructed by the same group using tpy-Ru-tpy dimers further linked through Fe(II) sites using one of two ditopic donors.51 The two tpy sites of the ligands were attached to a central benzene ring with phenyl spacers, connecting either with an ortho-arrangement to produce a 60° angle between coordination vectors, or with meta-substitution to give 120° directionality. When combined with Ru(II) sources, two such ligands were linked by a single metal ion. Cyclization then occurred upon introduction of Fe(II) ions. However, for both ligands, an unexpected second structure was found, with both systems furnishing tetrameric and hexameric metallacycles, despite the expectation for solely tetramers for the 60° and solely hexamers for the 120°. These extra species resulted from constitutional isomerism of the ligand. In collaboration with the Chujo group, Wesdemiotis and Newkome expanded their studies of typ-based metallacycles to include ligands built with central o-carborane moieties.52 Carborane-based ligands have also been employed by Jin and co-workers.53 The regioselective activation of carboxylate-functionalized carboranes enabled their use as ligands in metallacycle formations using an arene−Ir species activated by AgOTf. A pyrazine-bridge Ir2 complex was treated with the silver salt and then mixed with either ortho-, meta-, or para-substituted carborane dicarboxylates, with the ortho- and para-variants forming tetranuclear metallacycles with chelating Ir−B,O coordination. SCC formation was investigated by Jin and coworkers using mass spectrometry and X-ray crystallography.54 Bowman-James and co-workers used Pd(II) nodes to construct a hexanuclear ring, dubbed a “palladawheel”. The species contained six pincer complexes comprising a N,N′diphenyl-2,6-pyridinedicarboxamide ligand with Pd(II) ions (Figure 11).55 The parent monomer contained a bound acetonitrile, which dissociated in CHCl3 solution, resulting in the carbonyl oxygen of a second complex occupying its former site. This coordination was repeated five more times, resulting in a hexanuclear ring, as confirmed by X-ray diffraction. The wheel theme was taken even further by Schmittel and co-workers with their design of a supramolecular nanorotor using two different Zn-porphyrins bridged by dabco (Figure 12).56 In order to drive rotations, Cu(I) ions were incorporated

identified that chloride ions could act as catalysts for the selfassembly of simple M2L3 assemblies between As(III) and dithiolate ligands.44 Their studies revealed that the anion helps activate an intermediate As2L2Cl2 macrocycle en route to the formation of the target assembly. Tripycene moieties are suitable cores upon which to design tritopic ligands, as demonstrated by Das and co-workers.45 After coupling three pyridyl groups via amide linkages, selfassembly reactions with a 90° Pt(II) acceptor were explored. Although adamantanoids, double squares, and trigonal bipyramidal cages were all a possible structural outcome, mass spectrometry data supported only the stoichiometry of formation consistent with a trigonal bipyramidal architecture. A series of tetranuclear Re-based SCCs was discussed by Manimaran and co-workers wherein Re(I) nodes were bridged by oxamide and a linear dipyridyl donor.46 The oxamidatobridges were further functionalized with various alkyl groups (Figure 9). The four rectangular metallacycles thus formed

Figure 9. Tetranuclear Re-based SCCs with alkyl-functionalized bridging oxamide groups.

were all structurally characterized by X-ray diffraction, with the identity of the substituent on the oxamide ligands affecting the solubilities and emission properties of the SCCs. A mixed-metal SCC was synthesized by Lee and co-workers using an arene-Ru molecular clip and a tetrapyridyl metallocene ligand (Figure 10).47 The Fe-based metallocene served as a

Figure 10. Mixed Ru/Fe assembles may be obtained when Ru molecular clips assemble with metallocene donors.

tetratopic panel, with two such precursors being held cofacially, clipped together by four acceptors to furnish a tetragonal prismatic cage. Both Rh and Ir molecular clips were used by Jin and coworkers to make both metallacycles and cages, with the metal centers linked by 2,2′-bis(benzimidazole) ligands.48 As expected, when linear ditopic donors were used in conjunction with these clips, rectangular metallacycles were obtained. A E


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Pd(NO3)2, a twisted prismatic cage was obtained with helical features. In collaboration with Fujita, Yoneya and co-workers simulated the formation of M6L8 nanospheres using molecular dynamics methods (Figure 14).62 The cuboctahedral cage was modeled with Pd(II) ions and a tripyridyl ligand. Computational costs were minimized by selecting solvent models carefully and exploring formations in the absence of counterions, to reduce competition for coordination. The studies revealed notable changes to ligand exchange rates depending on the extent to which the SCCs had formed, and provide an interesting view of the self-assembly process. Given the rarity of kinetico-mechanistic insight into assembly formation relative to reports of new structures, experimental work such as the NMR and mass spectrometry studies of such systems are valuable.63,64 An experimental example of M6L8 cages was provided by Pan, Su, and co-workers using an interesting polypyridyl-Ru based donor ligand (Figure 15).65 Three terminal pyridyl donors were appended to the Ru complex, forming a trigonal panel that could assemble through homoleptic coordination to Pd(II) ions to generate a rhombododecahedral SCC with Pd(II) vertices and Ru sites centered in each face. These cages were used to protect photosensitive guests from decomposition by UV light. A second mixed-metal cage with the M6L8 skeletal structure was proposed by Williams and co-workers during their work with an octahedral aluminum(III) species that contained pendant pyridyl groups, allowing it to act as a tritopic donor.66 The initial Al monomer was chelated by three acetylacetonate groups, each functionalized with an pendant pyridyl donor. Mixing this “ligand” with Cu(ClO4)2 gave evidence of two species formed at different stoichiometries, proposed as the [Cu(AlL 3 ) 4 ] 2+ “monomer” and the [Cu6(AlL3)8]8+ cage. An analogous Pd(II) species was synthesized to facilitate NMR investigations that were consistent with the formation of single metallacages. Attempts to isolate the cage in the solid state resulted in the formation of polymeric species. Lützen and co-workers synthesized optically pure dipyridyl donors with chiral 2,2′-dihydroxy-1,1′-binaphthyl cores to use in the formation of M6L12 and M12L24 SCCs to determine if chiral molecular spheres could be obtained (Figure 16).67 Although rotation about the aryl−aryl bonds of the BINOL core introduces a degree of angular flexibility to these systems, selective assembly of either hexanuclear or dodecanuclear cages occurred, depending on the substitution pattern on the ligand core. ESI-MS and dynamic light scattering experiments were used to support the stoichiometries of formation of these cages, based on their anticipated sizes and nuclearities. Follow-up studies of BINOL-based ligands led to the synthesis of a unique M4L8 cage where four Pd(II) ions occupy the vertices of a stretched tetrahedron. Two different ligand conformations are present in the structure, bridging either the short Pd−Pd distance of 8.8 Å or the long separation of 14.0 Å. The two short bridge environments at either end of the structure generate pockets that encapsulate BF4− anions.68 Yang and coworkers have also employed ligands containing BINOL for the formation of chiral triangular metallacycles.69 A second type of chiral ligand scaffold was used by Lützen involving 4,12diethynyl-[2.2]paracyclophane cores functionalized with pyridyl donors. The orientation of the pyridyl coordination vectors rendered these ligands trans-chelators, as demonstrated by their binding to Pd(II) ions.70

Figure 11. X-ray structure of a Pd6 “wheel” linked by pyridinedicarboxamide ligands. Hydrogen atoms, solvents, and counterions omitted for clarity. Atom (color): Pd (yellow); C (gray); O (red); N (blue).

at four phenanthroline sites connected to one of the porphyrins via a series of phenyl and ethynyl spacers. The remaining porphyrin contained either trans-dipyridyl functionalities attached through an phenylethynyl spacer, or one pyridyl and one pyrimidine donor, attached with the same spacer. Stochastic oscillations were monitored by variable temperature NMR experiments, coupled with analyses of model complexes to estimate binding energies. These investigations pointed toward intrasupramolecular spinning rather than cleavage of the Zn-dabco bonding. Li and co-workers synthesized supramolecular cubes using terpyridyl donors with either Zn(II) or Cd(II) nodes.57 The tritopic ligands linked three terpyridyl sites to a central adamantyl moiety through phenyl spacers. Eight of these ligands occupied the vertices of a cubic structure, with 12 metal ions coordinated at the midpoint of each edge. The species were characterized with a host of NMR and MS experiments to confirm the stoichiometries of formation. Microscopy was also used to gain further structural insight. Structurally interesting 2D terpyridyl-based SCCs were also described by Li wherein ditopic and tritopic donors assembled into various sized metallacycles (ditopic) or a “hexagon wreath” where a central hexagonal core was fused with three smaller hexagonal metallacycles (Figure 13). This was achieved by using a central phenyl ring with meta-substitution to two tpy sites, encoding 120° directionality in the ditopic donor. The tritopic analogue was further substituted with a pendant terpyridine group, attached through phenylethynyl spacers. Self-assembly with Zn(II) furnished the metallacycles.58 The ability of Pd(II) to support a homoleptic environment of four pyridyl donors is the basis for many of Fujita’s molecular sphere constructs.59 The simplest of these MnL2n designs uses two metal centers bridged by four ligands. Using this approach, Fujita and co-workers have developed a rich library of spheres exhibiting interesting chemistry, including cubes (M6L12), cuboctahedra (M12L24), and rhombicuboctahedra (M24L48) and related species. The synthesis, characterization, and chemistry of these SCCs has been summarized.60 Chand and co-workers have synthesized such cages using a N,N′-(pyridine2,6-diyl)dinicotinamide ligand, which orients its two pyridyl groups with parallel coordination vectors.61 Upon mixing with F


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Figure 12. Nanorotors can be constructed from a central porphyrin core with pendant phenanthroline sites to house Cu(I) ions. Interactions between a second porphyrin-based donor and these metal sites provide the basis for stochastic oscillations. Adapted with permission from ref 56. Copyright 2013 American Chemical Society.

In 2012, Severin and workers developed the first examples of dicarboxylate-bridged arene−Ru building blocks as a variant of molecular-clip acceptors that have been used in a number of 2D and 3D SCCs.71 The resulting diruthenium species were readily incorporated into metallacycles and cages, with the increased flexibility of the bridging ligands enhancing the extent of conformational changes. An interesting class of clathrochelate-based ligands with terminal pyridyl groups was designed by the same group, expanding the library of ditopic donors for edge-directed assembly methods (Figure 17).72 These ligands contained central boronic-acid-capped Fe(II) tris(dioxime) moieties and were made in one-pot by mixing FeCl2 with the dioxime ligand and pyridyl-terminated boronic acid of desired length. This was demonstrated by a suite of six ligands, ranging in size from 1.5 to 5.4 Å. A model [4 + 4] square was also made using 90° Re acceptors to confirm that these ditopic donors were competent for self-assembly. Follow-up studies described Zn(II), Mn(II), and Co(II) clathrochelates suitable as donors for SCC formation.73 Other species have been made using ditopic

metalloligands, as in the case of a Pd6L12 cage with a clathrochelate core.74 New examples of SCCs made using these ligands are emerging, such as the mixed Ir/Fe species reported by Jin and co-workers.75 By using organometallic-based building blocks, a metal center can be embedded within a ligand, permitting heterobimetallic structures since a second, different metal may be used as an acceptor. For example, Ferrer, Engeser, and co-workers designed hexanuclear mixed Pd/Au and Pt/Au metallacycles by combining pyridyl-functionalized gold centers with 90° Pd or Pt acceptors.76 As a testament to the versatility of this approach, 24 such metallacycles were obtained, with some variants existing in equilibrium with higher nuclearity species. Ferrer, Torrente and co-workers have continued to explore mixed metal metallacycles using thiolate bridged Rh2 or Ir2 complexes with pendant pyridyl groups as donors in selfassembly reactions.77 The metalloligands were first formed by generating pyridine-4-thiolate by deprotonation with butyllithium. Addition of [Rh(μ-Cl)(cod)]2 or [Ir(μ-Cl)(cod)]2 led to isolation of the ditopic donor. Three different configurations G


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Figure 13. Formation of large “molecular wreaths” is enabled by assembling tritopic terpyridyl donors with Zn ions.

with three water molecules, two carbonyl oxygens, and one carboxylate coordinating. The discrete architectures undergo a high degree of further ordering in the solid state, due to hydrogen bonding and extensive π−π interactions. Sillanpää and co-workers synthesized and characterized six tetranuclear Cu-based SCCs while developing the chemistry of amine-bis(phenol) ligands. 79 The Cu 4 L 2 diamine-bis(phenoloate) complexes assembled upon mixing a Cu(II) source with one of two ligands, differing in the length of their alkyl bridges. All six variants were investigated by single-crystal X-ray diffraction, with the coordination environments about the Cu sites being influenced by the solvent used during selfassembly. The tetranuclear SCCs are relatively flexible, owing to the alkyl spacers found in the ligand. Although the “naked” metal ions, which require the donor ligands to occupy all available coordination sites, are most often used in the symmetry-adapted approach to SCC formation, there are examples of edge-directed assemblies using simple metal nodes, free of capping ligands. This subset of SCCs and related oligomeric species are covered at length in a recent review by Lindoy and co-workers.80 Expanding on the work covered in their review, Lindoy and co-workers explored the construction of Cu metallacycles that could act as precursors for higher order structures.81 These

Figure 14. Molecular dynamics may be used to simulate the selfassembly process, providing information about kinetic intermediates formed en route to a final thermodynamic product. Reprinted with permission from ref 62. Copyright 2012 American Chemical Society.

were anticipated for the orientation of the coordination vectors of the pyridyl groups, syn-exo, giving a clip-like geometry, synendo, approximating a 180° donor, and anti, resembling a 90° donor. Using cis-capped Pd(II) or Pt(II) acceptors, [2 + 2] metallacycles were obtained, with the ligand adopting the anticonfiguration. This geometry was also found when cis-capped Rh(I) and Ir(I) nodes were used. Heterometallic species were also constructed by Li and coworkers by using macrocyclic oxamide metalloligands containing Ni(II) centers in self-assembly reactions with Zn(II) ions.78 The resulting tetranuclear species contains Zn−O coordination, H


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Figure 15. Light-absorbing panels of a Pd6L8 cage protect guest molecules from photodecomposition. Reprinted with permission from ref 65. Copyright 2014 American Chemical Society.

Figure 17. X-ray (1.5 and 2.7 Å) and simulated structures (3.2, 3.4, 4.1, and 5.4 Å) of size-tunable clathrochelate dipyridyl ligands suitable for edge-directed self-assembly. Adapted with permission from ref 72. Copyright 2013 Royal Society of Chemistry.

dimethylpiperazine was used, two metallacycles were fused together, to give stacked rings. Depending on the ligands used and the stoichiometries present, a variety of related structures were formed, containing either five- or six-coordinate Cu centers. Two M4L4 squares assembled from [Cu(NH3)4]2+ and one of two bis(β-diketone) ligands, made by Yu and coworkers, further demonstrate this approach.82 Both square metallacycles were structurally characterized, revealing a loosely associated water molecule near each Cu site within the SCCs.

Figure 16. Calculated structures of Pd6 (top) and Pd12 (bottom) cages possessing O-symmetry obtained by DFT methods. Hydrogen atoms omitted for clarity. Atom (color): Pd (black); O (red); N (blue); C (gray). Adapted with permission from ref 67. Copyright 2014 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim.

species used one of two β-diketone-based ligands to form dinuclear metallacycles that were further subjected to exogenous heterocycles with Lewis-basic sites. When N,N′I


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Figure 18. Capped metallacalix[3]arene structures may be formed from three-component assembly with Rebased acceptors. Adapted with permission from ref 84. Copyright 2012 Elsevier.

Figure 19. X-ray structures of a jar-shaped metallacages formed from Re(CO)4 acceptors (green/blue) bridged by both imidazolate ligands (purple) and a tritopic donor with three benzimidazole sites (yellow). Ellipsoids drawn at the 30% probability level. H atoms omitted for clarity. Reproduced with permission from ref 89. Copyright 2014 American Chemical Society.

fragment.85 Analogues to the bicyclic calixarene structure could be formed as dinuclear assemblies containing a single Re2 motif by replacing the tritopic ligand with a ditopic variant. Similar self-assembly between Re2(CO)10, tetrahydrobenzoquinone, and the ditopic ligand afforded metallo-calixarene species.86 Similar structures could be obtained using Re2(CO)10 and flexible benzimidazole ligands and substituted salicylaldehydes.87 Simpler tetragonal prisms assembled from Re2(CO)10, tetrapyridyl donors, and either water or biimidazole were reported by Sathiyendiran and co-workers.88 When water was used, two Re2 faces were bridged by bridging hydroxy groups, with two such groups connecting each pair of Re sites. Alternatively, this bridging could be carried out by biimidazole moieties. The remaining three sites of each Re center were occupied by CO ligands. Both cages were studied by X-ray crystallography, confirming their tetragonal prismatic structural assignments. Virtually no twisting was observed between the tetragonal faces, with a high degree of π-interactions in the extended packing network. These intermolecular interactions also facilitated photoinduced [2 + 2] dimerization upon irradiation with UV light. Small molecular rectangles were formed when Re2(CO)10 was mixed with 1,1′-carbonyldiimidazole, resulting in a trinuclear species with three Re(CO)4 fragments bridged by

The use of ligands with both hard and soft donor sites provides an increased level of coordination complexity due to the differing coordination environments that may be adopted by metal centers. Constable, Housecroft, and co-workers employed ligands containing either pyridyl diamide or imine moieties along with phosphine donors.83 When these ligands were mixed with [Cu(NCMe)4][PF6], it was established that 1:1 metal−ligand complexes were formed, but the nuclearity of these complexes varied. In the solid state, the primary products were dimeric; however, for the imine ligand, helical hexanuclear constructs were also observed. An interesting M3L3L′-type metallacage, resembling a capped metallacalix[3]arene structure, was designed by Sathiyendiran and co-workers (Figure 18).84 The base of the structure contains three Re(CO)3 fragments linked by three anionic benzimidazolyl units. This trinuclear cluster is capped by a flexible 1,3,5-tris(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene ligand, with each of three arms coordinating to a different Re center. The structure of the cage was determined by singlecrystal X-ray diffraction. Related work by the same group described the formation of Re-based hexanuclear assemblies via treatment of Re2(CO)10 with chloranilic acid and 1,3,5-tris(2methylmercaptobenzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene. The resulting architecture contained two terminal Re2 sites adopting a calixarene-like structure, linked to a central Re2 J


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imidazolate ligands. Lu and co-workers showed that treatment of these triangles with a tritopic ligand containing three benzimidazole donors, 1,3,5-tris(benzimidazol-1-ylmethyl)2,4,6-trimethylbenzene), resulted in a supramolecular transformation to a jar-shaped SCC with each Re center losing a CO ligand to accommodate the additional N-donor (Figure 19).89 Similarly structured SCCs were reported by Sathiyendiran and co-workers containing either imidazole, benzimidazole, or benzotriazole as bridges between Re(CO)3 fragments capped with tritopic imidazole or benzimidazole-based donors.90,91 Exemplary SCCs continue to be useful not only as examples of design methods, but also as substrates for exploring and refining characterization methods. Engeser and Lützen carried out detailed mass spectrometry experiments on the well-known [{Pt(dppp)}4(4,4′-bpy)4](OTf)8 square, including electron capture dissociation and infrared multiphoton dissociation.92 Comparing results from these two methods revealed that the presence of bipyridine ligands is critical to allow for electron capture; radical cation formation could not occur solely at the metal nodes. The self-assembly process was studied on Au(111) surfaces by Liu and Lin, using cis- and trans-dipyridyl porphyrins along with 180° and 120° terpyridine ligands with Fe centers.93 The multicomponent systems investigated lacked the translational, rotational, and torsional freedom of solution-based selfassembly, as the building blocks were confined to the surface. As such, kinetic intermediates were much more effectively trapped for detection. Chain structures dominated at both low and high temperatures, being kinetically favored in cases of the former and thermodynamically in cases of the latter, due to entropic considerations. Porphyrins have been popular building blocks for the construction of metallacycles and cages. From a structural standpoint, they are attractive in that they can encode 180° or 90° angles as ditopic units, or can be substituted so as to serve as tetratopic tectons. This structural diversity has motivated their use in a number of designs.94 In 2012, Iengo and coworkers revealed a suite of three multiporphyrin metallacages, expanding upon face-directed self-assembly methods. The key molecular panel of their designs was a diporphyrin construct wherein two cis-dipyridyl porphyrins were fused via two bridging Ru-centers. When Zn(II) ions were present within the porphyrin subunits, they could be used as sites for subsequent coordination-driven self-assembly. This approach was demonstrated using one tritropic and two tetratopic polylpyridyl donors, resulting in prismatic assemblies with two cofacial donors bridged by the aforementioned diporphyrin panel. The trigonal prism was characterized by single-crystal Xray diffraction, confirming the [3panel + 2donor] stoichiometry of formation. Iengo and Tecilla also reported a porphyrin metallacycle with Re(I) nodes at each vertex (Figure 20).95 The bridging porphyrins were of the trans-A2B2 type, containing two pyridyl and substituted aryl-ethers, used to assist in solubilizing the assemblies. Both the free-base and Zn-metalated variants were formed, the latter capable of hosting a tetrapyridyl porphyrin via coordination to the four Zn sites. A recent review highlighted further investigations of the self-assembly of porphyrin metallacycles, specifically with Pd(II), Pt(II), Ru(II), and Re(I) metal nodes.96 An alternative M6L3 “molecular barrel” approach to multiporphyrin SCCs was used by Mukherjee and co-workers to construct three trigonal prismatic assemblies.97 These metal-

Figure 20. Re-based metallacycle with Zn-porphyrin edges can accommodate a tetrapyridyl porphyrin guest via Zn coordination (top). Emission titrations reveal spectral changes upon guest addition, consistent with the incorporation of the porphyrin (free guest: dashed line). Reproduced with permission from ref 95. Copyright 2013 Royal Society of Chemistry.

lacages used tetratopic 5,10,15,20-tetrakis(3-pyridyl)porphyrin as panels to be bridged by one of six Pd(II) nodes. To prevent further oligomerization, one of three N-donor based capping ligands was used to complete the square coordination geometry about each metal center. Although phosphine ligands are most commonly employed as capping groups, especially for Pt(II)- or Pd(II)-based SCCs, Cohen and co-workers synthesized a series of tetrahedra using tritopic ligands with diphenylphosphinyl fragments as the donors.98 These ligands occupied the faces of a tetrahedron, with either CuI, AgI, or AuI located at the vertices. ESI-MS data contained peaks corresponding to the fully intact assemblies, confirming that the SCCs remained intact in solution. A triflate analogue was also prepared to explore the ramifications of including a weakly coordinating anion instead of the iodide moieties, which tightly associated with the metal vertices of the cages. All four tetrahedra were subjected to single-crystal X-ray diffraction studies. Tunik and Koshevoy also used phosphine ligands as core structural elements in the construction of a suite of supramolecular metallacages with small Au3 clusters linked at central bridging imido groups (Figure 21).99 Either ditopic or tritopic phosphines were used, the former generating helical tubes, the latter assembling into decanuclear tetrahedra, as evidenced by K


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2.2. Symmetry and Subcomponent Assembly

The symmetry-adapted approach to SCC formation identifies the symmetry elements of target architectures, guiding the selection of building blocks for self-assembly (Figure 23).

Figure 21. X-ray structure of a tetragonal cage formed with Au3 clustered bridged by tritopic phosphine donors. Atom (color): Au (yellow); P (orange); C (gray); N (blue). Hydrogen atoms omitted for clarity.

X-ray diffraction. NMR and MS studies support that these structures are also maintained in solution. Trinuclear cages were formed by Mayer and co-workers by using tris(diphenylphosphino)cyclohexane ligands capable of a variety of coordination modes (Figure 22).100 When Ir(I),

Figure 23. Subcomponent self-assembly method encodes the symmetry elements of the faces or edges of a polygon in a ligand that forms from precursors during the assembly with metal acceptors, as illustrated by two tetrahedral Fe4 cages. Adapted with permission from ref 103. Copyright 2013 Royal Society of Chemistry.

These designs often use chelating ligands bound to homoleptic metal centers in the formation of 3D metallacages, eliminating the need for capping ligands. The ligands themselves oftentimes contain a large degree of internal twisting so as to properly orient their coordination vectors to occupy all available coordination sites. Ditopic ligands of this type are often found in M4L6 tetrahedra, as well as helical structures. Tritopic variants commonly appear in M4L4 cages. Nitschke and coworkers have developed an extension of this approach dubbed “subcomponent self-assembly” where the ligand precursors couple in the presence of metal ions to form bridging donors and ultimately the final assembly all in one pot.103 By no means is this approach limited to tetrahedra and helicates, however. Kwong and co-workers assembled a ditopic pyridylimine ligand in the presence of either Cd(II) or Mn(II) sources to form a complex M12L18 metallacage with a hexagonal prismatic structure.104 The stereochemistry alternates between Δ and Λ configurations at successive metal sites, evidenced by a single-crystal X-ray diffraction study. An unexpected S4symmetric cage was synthesized using a similar subcomponent self-assembly strategy by Mal, Rissanen, and co-workers.105 This strategy was recently used to carefully tune the pore size in a system capable of adopting either M4L4 tetrahedral structures or M2L3 helicates, based on the stoichiometry and reaction conditions used.106 A triamine was used as the core of a trichelating ligand, formed in one pot with 2-formylpyridine and an Fe(II) source. Host/guest chemistry could be enabled upon optimization of the size of the parent triamine (tuned via the number of phenyl and ethynyl spacers present), providing a

Figure 22. X-ray structure of a trigonal prismatic SCC containing three Pt(CN)2 sites linked by two tritopic phosphine donors. Hydrogen atoms omitted for clarity. Atom (color): Pt (gold); P (orange); C (gray); N (blue).

Rh(I), Pd(II), Pt(II), Ag(I), or Au(I) sources were introduced, two of the triphosphine ligands adopted trans-coordination to the square or linear metal nodes, as evidenced by single-crystal X-ray diffraction studies. Puddephatt and Nasser have also employed phosphine bridging ligands as structural components rather than caps. An initial investigation of a diphosphine ligand with a central diamide core provided some evidence for metallacyclic structures when treated with Au or Ag sources based on mass spectrometry data; however, the amide-containing diphosphine ligand was primarily used in the self-assembly of sheet-like structures with extensive hydrogen bonding.101 Follow-up studies investigated the coordination chemistry of this ligand with Au and Pt, revealing that the ability of the ligand to chelate with cis- or trans-configurations, bridge, and hydrogen bond provided a variety of architectures including mononuclear species, dinuclear metallacycles, and supramolecular polymeric chains.102 These species were often found as equilibrium mixtures in solution, as monitored by NMR methods. L


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cage suitable to accommodate a number of small molecule guests. Both smaller and larger cages lacked the appropriate pore sizes to either allow guest uptake, or prevent guest release, respectively. M4L6 cages are also competent hosts for anions, as indicated by recent work by Ward and co-workers.107 Such M4L6 cages can provide enhanced chiroptical response through an interesting subcomponent substitution reaction developed by Nitschke and co-workers.108 The ability of such cages to communicate stereochemical information was further explored on larger Fe4L6 cages revealing that even separations of over 2 nm between metal centers did not deter cooperativity.109 Further investigations of stereochemical control in such species were the topic of a recent review by the same group.110 The host/guest chemistry of tetrahedral cages was further studied by Custelcean and co-workers using urea-functionalized ligands that provide sites for anion binding.111 A number of techniques including X-ray diffraction, mass spectrometry, simulations, and photophysical characterizations provided insight into the self-assembly process and structural nature of the cages. By introducing solubilizing tBu groups onto the bridging ligand used, the uptake of various tetrahedral oxoanions was extensively investigated using a suite of NMR methods. A review on anion encapsulation by Custelcean was recently published, encompassing examples of host/guest systems as well as the resulting chemistry than can be induced by cage/anion interactions.112 As mentioned in the Introduction, there is significant overlap between the chemistry of MOFs and SCCs.2 In some cases a given system can be tuned between infinite and discrete structures, bridging both fields. For example, slight modifications to the angularity between the coordination vectors of a pyridyl-functionalized Co(III)-based building block made by Kumar and Gupta determined self-assembly outcomes of either metallacycles, coordination polymers, or chains.113 An asymmetric chelating ligand was explored by Mondal and co-workers, with two terminal chelating moieties, a pyridylpyrazole and a pyridylimine.114 Asymmetry was introduced by including a single amide group in the backbone of the ligand. Although some extended structures were formed, a discrete assembly was realized when Co(BF4)2 was used as an acceptor unit. X-ray diffraction confirmed a pentanuclear species, with two distinct Co environments: fac-tris-chelate and cis-tridentate. Li and co-workers used subcomponent assembly to link Ni(II) ions together with relatively short bridging imidazolate ligands (Figure 24).115 Two types of SCCs were formed, a Ni8L12X4 cubic cage and a Ni14L24 rhombic dodecahedral cage, the former converting to the latter in the presence of methylamine. The ligand precursors were 5-methyl-4-formylimidazole and one of three amines, 4-methoxybenzylamine, benzylamine, or 4-bromobenzylamine, which assembled with Ni(II) sources to give the SCCs under solvothermal conditions. The cages were all structurally characterized by X-ray diffraction. Examination of conditions yielding cubic or dodecahedral cages revealed that chloride and bromide anions heavily favored cubic structures. The steric bulk of the ligands also played a role, as the cubic structures possess tetrahedral and octahedral metal centers, in contrast to the square and octahedral centers of the dodecahedral species.

Figure 24. (a) Cubic cage containing Ni(II) nodes could be transformed into a rhombic dodecahedral cage upon addition of methylamine. (b) An initial solution of the cubic cage (vial 1) shows an immediate color change upon addition of methyl amine (vial 2) resulting in a pale yellow solution after 3 days (vial 3). (c) The final structure was characterized by X-ray diffraction. Atom (color): Ni (green); C (gray); N (blue); O (red). H atoms omitted. Reproduced with permission from ref 115. Copyright 2013 American Chemical Society.

Fe(II) sites. NMR and absorption experiments supported the existence of helicates in solution, and two solid state structures containing the M2L3 motif.116 Solvothermal methods were used by Journaux and co-workers to construct Co(II)-based M2L3 helicates under conditions more commonly found in MOF chemistry.117 The 3D layout of these dinuclear units in the solid state was heavily influenced by synthesis temperature, which was a prime determinant of the nature of the H-bond network dictating the extended structure. When 4,4′methylenebis(1-(2-pyridyl)-3,5-dimethylpyrazole is used to form helicates with either Co(II), Ni(II), or Cu(II) ions, the resulting structures contain unsaturated metal centers, with the remaining coordination sites occupied by either chloride ions or water molecules. Both the Co(II) and Ni(II) species contain two metals bridged by two ligands. The Cu(II) species, however, only contains a single bridging ligand, as determined by X-ray diffraction experiments performed by the same group.118 An asymmetric ligand containing catecholate and pyrazolyl moieties was designed by Metherell and Ward to form heterooctanuclear helicates due to the different metal affinities of the two donor sites (Figure 25).119 Upon combining the ligand with a mixture of Zn(II) and Ti(IV) salts in a 3:1:1 ratio, respectively, a Zn4Ti4L8 core with eight bridging methoxy groups was obtained, wherein the two metals were found as heterobimetallic dimers bridged by two methoxy groups each, and connected by the asymmetric ligand. This structure is a rare example of a one-pot assembly of cyclic heteronuclear architecture. A series of Cu-based SCCs, including a Cu2L4 helicate, was designed by Crowley and co-workers using a ligand based on a central pyridyl group with two terminal ethynylpyridines, all with parallel coordination vectors.120 Although solution-based characterization methods revealed similar properties, depending

2.3. Helicates and Metallocrowns

An M2L3 triple helicate was the sole structure type observed by Kruger and co-workers when employing a 4,4′-methylenebis(1-(2-pyridyl)pyrazole) ligand capable of spanning between M


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amino acids, exhibiting enhanced fluorescence in the presence of L-Ala and D-Ala, with the former providing a stronger signal. Amouri and co-workers designed two meso-helicates with M2L2 structures using a ligand with two terminal dipyridylamine groups linked through a series of phenyl and ethynyl groups.124 This ligand was treated with CoBF4 hydrate to generate the helical SCC with the remaining coordination sites of both Co centers completed by acetonitrile coordination. A similar Ni analogue was also structurally characterized. A suite of ferric helicates was described by Oshio and coworkers, including Fe5, Fe7, and Fe17 species (Figure 27).125 A

Figure 25. Asymmetric ligand with both catecholate and pyrazolyl donor sites forms cyclic heteronuclear structures when mixed with Zn(II) and Ti(IV) sources. The contains four dinuclear double helical units, with ligands shown in green/blue and purple/gold pairs. Atom (color): Zn (gray); Ti (cyan); O (red); C (black). H atoms omitted. Reproduced with permission from ref 119. Published 2013 by the Royal Society of Chemistry.

on the anion used, the solid state structures were determined to vary significantly. When BF4− was used, the Cu2L4 helicate was obtained. Other anions resulted in coordination polymers (tosylate) or Cu2L2 analogues (nitrate). The same group used an exofunctionalized variation of this ligand, with a pendant azide group, for pre-self-assembly “click chemistry” to generate decorated ligands that could be used for the self-assembly of Pd 2L 4 metallacages (Figure 26). 121 A follow-up study

Figure 27. A heptadentate polypyridyl ligand (top) used in the selfassembly of Fen cages (n = 5, 7, 17). An Fe5 cage is shown with Fe (orange), N (blue), O (red), C (gray), H (white).

polypyridine ligand containing pyrazole spacers was used for these syntheses. For the pentanuclear species, the central pyridyl groups of the ligand coordinated to three central Fe sites. Two terminal Fe nodes contained pyridyl-pyrazolate chelation. The Fe7 site was somewhat related, with similar chelated terminal Fe ions. The central sites, however, contained varied coordination environments, with both bridging hydroxy groups and chelation from the ligand. The final, elaborate Fe17 structure contained nine doubly deprotonated ligands which participated in forming a triple stranded helical structure. All three species were investigated by crystallography, with further studies on magnetic properties and electrochemical response. Hooley and co-workers explored Fe(II) helicates with pyridylimine donors containing a central diethynylbenzene core.126 A smaller ligand was also used to explore the effects of shortening the metal−metal distance in these structures on stereochemical control. The central phenyl ring of the primary ligands was subjected to a host of endo- and exofunctionalizations. Evidence for M2L3 helicates was found by ESI-MS, with further analysis by NMR indicating the presence of both homotopic ΔΔ/ΛΛ species and a mesotopic ΛΔ analogue. The helicate made from the control ligand did not show this mesotopic variant. Furthermore, endo-functionalization of the ligand with bulky groups appeared to increase selectivity for homotopic helicates, attributed to increased rigidity from the steric strain of accommodating the moieties within the internal cavities of the SCCs. A Bi-based system capable of cycling between M2L4, M2L3, and M2L2 states was described by the same group.127 The bridging ligand for these assemblies was a bis-hydrazone complex capable of tridentate binding to Bi(III) ions introduced as Bi(OTf)3. By monitoring the pyridyl and imine protons of the SCCs, evidence for conversion from M2L4

Figure 26. Pd2L4 cage with four pendant azide groups was used for “click-chemistry”, for instance to form a ferrocene-decorated structure characterized by X-ray diffraction. Solvents, counterions, and guest molecules omitted for clarity. Atom (color): Pd (yellow); Fe (orange); N (blue); C (gray).

constructed a suite of ligands using this method, with examples of alkyl, aryl, emissive, redox active, and biologically relevant pendant groups being installed. Pd2L4 cages were assembled using these ligands, and the resulting architectures were characterized with appropriate techniques based on the type of functionalization introduced.122 Although M2L3 helicates are the most common of this subclass of SCCs, alternative designs have been used to access other interesting helical structures. Cui and co-workers employed a dipyridyl functionalized salan ligand as the basis for twisted metallosalan subunits.123 When the ligand was combined with ZnCl2, an octanuclear species was obtained, containing two trigonal-bipyramidal Zn sites within each salan pocket, and a four membered Zn2O2 ring linking two such pockets. The cage was enantioselective in its interaction with N


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structures to M2L3 and ultimately M2L2 can be found as the mole fraction of Bi(III) in solution is increased, a testament to the versatile coordination modes of the metal center. In the M2L4 case, it is likely that each ligand coordinates via two atoms, whereas X-ray crystallography confirms that the M2L2 species contains the ligand in a tridentate form (Figure 28).

Figure 29. Pentanuclear Zn-bowl (top) adopts a concave shape based on its X-ray structure (bottom) and is proposed as an intermediate structure in the formation of metallacrowns. Atom (color): Zn (purple); O (red), N (blue), C (gray). Adapted with permission from ref 131. Copyright 2013 American Chemical Society.

characterized, with nitrate counterions for the former, and a suite of triflate, BF4−, nitrate, and acetate-based species for the latter. These species are proposed as intermediates in the formation of lanthanide metallacrowns and thus are expected to provide insight into the self-assembly chemistry of such constructs. Related investigations into the chemistry of Ni(II)-, Cu(II)-, and Zn(II)-based crowns and their ability to accommodate trivalent Eu, La, Nd, and Gd have been carried out by the same group.132 A magnesium-based metallocrown, unique in its use of a main-group element, was reported by Leung and co-workers wherein four Mg(II) ions were joined by a 2,3-pyrazyl-linked bis(thiophosphorano)methane ligand (Figure 30).133 The

Figure 28. Pyridylimine ligand (top) used to form a suite of interconverting M2L4, M2L3, and M2L2 structures resulting in the X-ray structure of an extended M2L2 network. One subunit has been isolated (bottom) showing two Bi centers bridged by two ligands. Atom (color): Bi (purple); N (blue); O (red); C (gray); H (white).

When a related linear ligand was used, polymeric species were found at low Bi concentrations, and a M3L3 triangle was formed under excess amounts of the metal. The same group explored M2L3 Fe-helicates with interesting post-assembly reactions with isocyanates.128 Helicates formed by fusing two mononuclear triscatecholate complexes together through three lithium bridges were synthesized by Albrecht and co-workers.129 The system exists in an equilibrium of monomers and dimers, with the dimer heavily favored in MeOH solvent. Chiral induction was induced by introducing enantiopure (R)-phenylethylammonium chloride, which preferentially interacted with the ΔR monomeric diastereoisomer. Thus, upon dimerization, the appearance of ΔΔ isomers was enhanced. Stereochemical control was also achieved by incorporating chiral ester groups onto the ligand itself. Dinuclear M2L2 complexes containing Ni(II) or Cu(II) metal nodes with amide-based pincer ligands with pendant quinoline groups were studied by Bowman-James and co-workers to evaluate the differences between the central phenyl and pyridyl rings in the helical structures.130 The ligands bind by chelation involving the quinoline groups with the deprotonated N atom of the amides. For the helicates containing central pyridyl rings, relatively close contact was found between the metals and the N atom, as short as 2.284(2) in the Ni species. A close analysis of the crystal structure of the Cu helicate containing the phenyl ring in place of the pyridyl ring indicated that the distances and angles observed were inconsistent with agostic interactions, and were instead attributed to hydrogen bonding. Pecoraro and co-workers were able to isolate tetranuclear and pentanuclear species that represent key M(II)-hydroximate intermediates with relevance to 15-metallacrown-5 architectures (Figure 29).131 Both Ni and Zn variants were structurally

Figure 30. Mg(II) metallocrown provides a rare example of a maingroup metallacycle assembly.

neutral ligand was deprotonated with n-Bu2Mg, resulting in metallacycle formation with each Mg(II) node bound by two sulfur atoms and two N atoms of the pyrazyl rings. The structure, confirmed by X-ray crystallography, was reportedly preserved in solution on the basis of 31P NMR analysis. 2.4. Multicomponent/Self-Sorting

Self-sorting and multicomponent assembly strategies are related efforts in the context of coordination-driven self-assembly. Even a simple two-component mixture of precursors A and B can give rise to random mixtures of products, for instance A2, B2, and AB. The two-component donor/acceptor paradigm obviates the homomeric species; however, if additional donors or acceptors are introduced, multiple metal−ligand interfaces become possible. If a system is designed such that a sole product is formed, containing all building blocks, it is said to be O


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Figure 31. Self-sorting in a Pd-based self-assembly system. Stoichiometrically controlled multicomponent self-assembly between a Pd acceptor, a tetratopic donor, and one of two ditopic donors selects for two different structure types based on the length of the ditopic donor.

used as pillars in the design contained either phenyl or alkyl spacers, with chain lengths up to eight carbons long (Figure 32).140 The SCCs could either be made with premetalated Zn-

a multicomponent self-assembly. On the other hand, if a system gives nonstatistical combinations of two or more SCCs from a mixture of building blocks, it is said to be self-sorting. Developing new strategies to control multicomponent assembly and self-sorting134 is an ongoing motivation of many research programs.135 When two different donors share the same Lewis-basic moiety, a risk arises that there will be no preference for ligation at a given metal acceptor, resulting in a mixture of self-assembly products. However, in some cases the size and shape complementarity between two donors enforces a single thermodynamic product. This most often occurs when the two donors differ in their number of binding sites; i.e., the use of two linear donors will give a mix of squares and rectangles, but a linear donor with a tetratopic panel may select for prisms since side products would give incomplete coordination at some metal centers. Mukherjee and co-workers provide an example of this stoichiometrically controlled multicomponent self-assembly with the formation of so-called molecular swings and boats, containing eight and six 90° Pd(II) acceptors, respectively, along with 1,2,4,5-tetrakis(1-imidazolyl)benzene as a tetratopic donor and either 4,4′-bpy or trans-1,2-bis(4-pyridyl)ethylene as linear donors.136 Both structures were determined by singlecrystal X-ray diffraction (Figure 31). The same group provided an example illustrating an important caveat when constructing multicomponent prisms from a polytopic panel donor and molecular clips.137 Although such two-component assemblies tend to favor single reaction products, if the size of the molecular clip matches the distance between two adjacent donor sites, a 2D product may form as the ratio of acceptor to donor matches that of the 3D prism. This was demonstrated using arene−Ru molecular clips whose lengths were modulated by the type of spacer ligand separating the metal centers. When these clips were combined with a tetraimidazole ligand, either 2D or 3D SCCs were selectively obtained. If a preference for heteroligation can be identified for a given metal center, multicomponent assembly becomes relatively straightforward as a mixture of two different types of donors will favor mixed coordination environments. For Pt(II) ions, Stang and co-workers identified a marked preference for Ptpyridyl-carboxylate coordination over homoligation. This selectivity is so strong that it can be used as the basis for multicomponent assemblies using nonrigid alkyl-based building blocks.138 This effect has been shown for Pd(II) ions by Mukherjee and co-workers.139 This strategy was used to form multicomponent cofacial porphyrin prisms with Pt(II) nodes. The dicarboxylate spacers

Figure 32. Multicomponent self-assembly of tetrapyridyl porphyrin with Pt(II) nodes and carboxylate spacers furnishes cofacial prisms.

tetrapyridyl porphyrin, or by assembling the prims first and then attempting metalation. Ten such prisms were constructed and characterized, with photophysical investigations revealing very little electronic communication between porphyrin units, resulting in virtually unaffected spectra. The presence of Pt(II) nodes did decrease quantum yields, attributed to enhanced intersystem crossing to nonradiative triplet states. Ghosh and co-workers studied a system comprising a tetratopic imidazolumn ligand that formed one of three molecular boxes depending on the N-alkyl group present.141 When the imidazolium contained either methyl or benzyl groups, treatment with Ag2O resulted in dinuclear Ag-carbene structures, with a single ligand wrapping around two Ag ions, forming linear N-heterocyclic carbene (NHC)−Ag−NHC motifs. When the alkyl group was ethyl, a tetranuclear box was instead obtained, with four such NHC−Ag−NHC centers. Hardie and co-workers have also used NHC ligands in the formation of metallo-cryptophanes capable of I2 uptake.142 Self-sorting can also be achieved by using a heterotopic ligand where two different binding sites prefer entirely different metal ions. By generating a donor with one end terminating P


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with a pyridyl group, and the other terminating with a pyridylimine fragment, coordination environments typically found in edge- and face-directed assembly can be combined with those found in symmetry-adapted methods, to give a single SCC containing two unique metal environments. Nitschke and co-workers have designed such cages, for instance combining Pt(II) ions, which readily coordinate four pyridyl centers, with Fe(II) ions, which prefer the pyridylimine ligands.143 The resulting Fe8Pt6L24 cubic cage was modeled theoretically and further supported by NMR, mass spectrometry, and elemental analysis data to confirm its stoichiometry of formation. A similar ligand containing a terpyridyl donor on one end and a pyridyl donor on the other was used to form metalloligands wherein Fe or Ru could bind to the polypyridyl ends, leaving the terminal pyridines to self-assemble with Pd(II) ions. Hanan and co-workers formed hexanuclear triangular metallacycles using this strategy, and studied the photophysical and electrochemical properties of the resulting multicomponent architecture (Figure 33).144

Nitschke and co-workers have shown that the presence or absence of a templating molecule, along with reaction conditions, can greatly influence the structure obtained in a self-assembly. When a tritopic pyridylimine ligand is generated in the presence of Fe(NTf2)2 in acetonitrile at 323 K, a M4L4 tetrahedron was obtained, requiring cyclohexane as a template in order to achieve a clean assembly. However, in the absence of any template, the same ligand generated in the presence of Fe(OTf)2 in a 50:50 (v/v) methanol/acetonitrile solution at 343 K produced M12L12 icosahedral capsules. This metallacage was structurally characterized, revealing three types of pores, with dimensions of ∼6.8, 3.4, and 1.1 Å.145 The templating effect of different anions was further investigated in a system capable of forming SCCs with T, D5, S4, or D2 symmetry, in addition to polymeric materials (Figure 34).146 The well-studied pyridylimine ligand formed by the coupling of p-toluidine and 6,6′-diformyl-3,3′-bipyridine was treated with a number of metal ions including Fe(II), Ni(II), Co(II), or Zn(II) in the presence of NO3−, BF4− ClO4−, OTf−, or NTf2−, resulting in the formation of M4L6 tetrahedra, M10L15 pentagonal prisms, M8L12 distorted cubes, and M6L8 circular helicates. This extensive study indicated that the larger metal ions could adopt a wider range of architectures, even small size differences between anion templates could manifest in large structural changes, and that the configuration at each metal center, in this case fac- or mer-coordination, is a critical determinant in the assembly outcome. Nitschke and co-workers designed a new pyridylimine ligand with the goal of construction of water-soluble SCCs.147 In doing so, they discovered methods to favor specific stereochemistry at the metal nodes, resulting in a selective assembly process. The subcomponent self-assembly used a 2,2′-bis(hydroxymethyl)benzidine precursor and delivered M4L6 tetrahedra when carried out in water at 50 °C for 20 h, and pentagonal M10L15 prisms when combined in a 9:1 mixture of MeOH and water at 20 °C for 20 h. It was possible to transform the prism to the tetrahedral cage by heating at 50 °C

Figure 33. Multicomponent assembly of Pd/Fe and Pd/Ru cages using pyridyl/terpyridyl donors. Adapted with permission from ref 144. Copyright 2014 Royal Society of Chemistry.

Figure 34. Templating effect of various anions determines the structural outcome of a subcomponent self-assembly with one of four metal ions and a pyridylimine ligand. Adapted with permission from ref 146. Copyright 2013 American Chemical Society. Q


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in water for 1 week. The reverse transformation occurred during a month-long crystallization. Anion-determinant selective self-assembly occurred in a study by Tao and co-workers wherein a series of Fe(II) complexes were made with [N(CN)2]− bridging ligands (Figure 35).148 The monomeric Fe(II) nodes were capped

Figure 36. When a 1,1′-binaphthyl-based ligand is used in selfassembly, self-sorting occurs to give homochiral Pd2 cages.

to self-assemble with [Pd(CH3CN)4](BF4)2, self-sorting into homochiral M2L4 cages resulted, as confirmed by a detailed analysis of the NMR spectra of the self-assembly mixture, free ligand, and solutions containing enantiomerically pure assembly products. A well-developed method to achieve self-sorting has been extensively documented by Schmittel and co-workers as part of their ongoing work with metal−phenanthroline complexes.152 Specific heteroligation environments were identified and used as the basis for selectivity during self-assembly. The recognition that heteroleptic phenanthroline coordination is favored when sterically encumbered ligands are mixed with unfunctionalized variants is the basis for HETPHEN selectivity. Similarly, heteroleptic terpyridine and phenanthroline selectivity defines the HETTAP approach. Finally, the combination of pyridyl and phenanthroline ligands results in a third, HETPYP153 strategy. These methods can be applied in various combinations to achieve noteworthy examples of self-sorting, multicomponent assembly, and drive supramolecular transformations. A combination of the HETPYP and HETPHEN approaches can be used to enforce self-sorting in systems containing as many as five components, as applied by Schmittel and coworkers in the construction of trapezoids and scalene triangles (Figure 37).154 Two different phenanthroline motifs were identified, with one being particularly suited for heteroligation with terpyridine about Zn(II) ions, forcing the other two to participate in heteroligation with simple pyridyl groups about Cu(I) sites. By employing either symmetrical or asymmetrical ditopic ligands containing these pyridyl, terpyridyl, or phenanthroline moieties, self-assembly with Zn(II) and Cu(I), complex metallacycles were obtained. This self-sorting was demonstrated to an impressive degree in the construction of a seven-component scalene quadrilateral (Figure 38). Both Cu(I) and Zn(II) sites were present, with the final structure containing a Zn-porphyrin with a pendant phenanthroline ligand.155 This ligand was bound to a Zn(II) center that also possessed a pyridyldiimine group. This group was part of a symmetric ditopic donor leading to a second Zn(II) center. This Zn(II) center also accommodated the same phenanthroline group; however, instead of being attached to a Zn-porphyrin, it terminated at a second type of phenanthroline ligand. The metallacycle was ultimate closed via Cu(I) coordination at this site with a pyridylimine ligand leading to a terminal pyridine that coordinated to the original Znporphyrin, as shown in Figure 38. Asymmetric pyridine/ terpyridine ligands have also been used by Jin and co-workers to form box-like metallacycles with multiple components.156 A Re(I) system using a bridging ligand with both terpyridyl and pyridyl donors was the basis of a recent manuscript by Hanan and co-workers.157

Figure 35. Anion-dependent self-selection in a Fe(II) self-assembly system. Reproduced with permission from ref 148. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

with tris(2-pyridylmethyl)amine ligands, leaving cis-coordination sites on the octahedral centers for subsequent coordination. Four different outcomes were observed depending on the anion present during assembly: squares, zigzag polymers, [2 + 2] dimers, and cocrystallized products with both squares and monomers. Squares were obtained when BF4− or ClO4− was used, the latter also resulting in polymeric species. A mix of polymers and dimers was found for B(Ph)4−, whereas SbF6− resulted in the co-crystal product. A related study of the effect of templating anions in the formation of either pentagonal or square Fe(II) metallacycles was described by Dunbar and co-workers.149 A 3,6-bis(2pyridyl)-1,2,4,5-tetrazine ligand was used to link Fe(II) ions, introduced as solvated salts with either BF4− or SbF6− counterions, the former resulting in tetranuclear squares, the latter generating pentagonal species. Detailed NMR investigations of the assembly process were performed, confirming the necessity of templating anions to form closed structures, and that significant interactions between the anions and the πsystems of the ligands occur. Larger anions were found to template the pentagonal assembly, while smaller anions favor square formation. Gardinier and co-workers explored the coordination chemistry of a series of isomeric bis[di(pyrazol-1-yl)methyl]1,1′-biphenyl ligands to investigate the role of ligand directionality in determining structure.150 Ligands containing 120° angularity were most promising for discrete, cyclic structures when assembled with Ag(I) ions, including an example of a large, 28-membered metallacycle. Detailed analyses of the twists and pivots possible for each ligand and the effects of these on the observed structures were given. Chiral self-sorting is particularly difficult as the system cannot take advantage of steric bulk or mismatches between the lengths or number of binding sites of precursors in order to avoid mixtures of products (Figure 36). Lützen and co-workers developed a 1,1′-binaphthyl ligand with terminal ethynylpyridines that could be prepared either as a racemic mixture or enantiomerically pure.151 When the racemic ligand was allowed R


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Figure 37. Exploiting preferences for heteroligation with phenanthroline and terpyridine donors with varying steric bulk enables self-sorting in multicomponent systems. Adapted with permission from ref 154. Copyright 2013 Royal Society of Chemistry.

Figure 38. Seven-component assembly is possible when a careful selection of metal ions with high selectivity for specific donor moieties. Various Ndonor ligands with different binding modes and steric bulk (top) were combined with Cu(I) and Zn(II) ions to form the quadrilateral. Adapted with permission from ref 155. Copyright 2013 American Chemical Society.

S


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2.5. Hierarchical Self-Assembly

The theme of orthogonal interactions in self-assembly, its relevance to self-sorting, and a detailed look at the unification of multiple noncovalent interactions to achieve unparalleled structural complexity is the focus of a recent review by Schmittel and co-workers.158 The control of noncovalent interactions is paramount in the design of molecular machinery.159 When these interactions are used for supramolecular polymerization, soft materials with interesting properties can be obtained. These topics drive much of the multicomponent and self-sorting chemistry introduced above, and provide the foundation for discussions of hierarchical selfassembly to follow.160 Yan and co-workers described the formation of [3]pseudorotaxane species wherein the final structure consisted of orthogonal interactions.161 As such, it was demonstrated that the order in which these interactions occurred did not affect the self-assembly. This degree of orthogonality is the basis for hierarchical schemes, wherein one type of interaction does not interfere with subsequent ordering and higher complexity. In this case, the interactions were metal−ligand coordination, using Pt(II) acceptors and pyridyl donors, and host/guest chemistry of a cryptand with bipyridinium guests. In addition to a one-pot assembly, coordination could precede guest encapsulation, or the inclusion complex could be generated followed by metal−ligand bond formation. Huang and Stang have collaborated in the development of a series of hierarchical self-assembly systems, using a mix of different orthogonal interactions. A combination of hydrogen bonding and metal−ligand coordination was used to construct rhomboids and hexagons with pendant 2-ureido-4-pyrimidinone (UPy) groups that could link the metallacycles into supramolecular polymers.162 The rhomboids formed linear chains that aggregated into long bundles, whereas the hexagons hydrogen-bonded into an extended honeycomb network. These cross-links resulted in a gel-like soft material when swelled in the presence of solvent, allowing for the formation of long fibers robust enough to be tied into knots without fracturing. A related system used the same UPy donor with a dendronized acceptor, to furnish rhomboids capable of polymerizing into 1D chains. Three generations of dendrimers were used, affecting the size of the supramolecular nanofibers.163 Metal−ligand coordination and amphiphilic interactions were unified in a third hierarchical system designed by the same groups (Figure 39). 164 Two 120° donors were synthesized, both with poly(ethylene glycol) functionalization at their exo-positions. One ligand contained a single long PEG chain, while the other contained three shorter chains tethered to a phenyl ring. An initial self-assembly generated rhomboids which could be isolated as discrete species in organic solvents. When placed in water, these rhomboids further ordered to give micelle structures, eventually evolving to nanofibers and sheets, with hydrophobic cores surrounded by aggregated PEG units. Microscopy revealed interconnected porous structures in the metallohydrogels formed at high concentrations. A complementary approach using hydrophobic alkyl chains was investigation by Yang and co-workers. A 120° donor was functionalized with three alkyl chains and a polyamide linker and used to construct hexagons. Reversible sol−gel transitions could be induced by modulating the presence of bromide ions.165

Figure 39. PEG-functionalized donor used in the construction of rhomboid metallacycles further orders into micelles and eventually fibrous supramolecular assemblies in water. Adapted with permission from ref 164. Copyright 2013 American Chemical Society.

A hexagon containing pendant crown either moieties was used to form an extended network upon the introduction of a bisammonium salt that acts as a short bridge between nearby host/guest sites.166 [2]Pseudorotaxane formation caused the formation of a supramolecular polymeric network of hexagonal metallacycles. At high concentrations, the resulting material undergoes thermal and cation-induced sol−gel transformations. A system using both H-bonding and metal−ligand coordination was studied by Marshall and Mendoza, wherein precursors were linked by both Pd(II)-pyridyl coordination and through a quadruple hydrogen bonding interface using 2ureido-4-[1H]-pyrimidinone (UPy).167 These two orthogonal interactions could be used to form square and triangle metallacycles. The product ratio was highly sensitive to the directionality of the building blocks. This system was an important demonstration of the unification of metal−ligand coordination with secondary noncovalent interactions to achieve a higher degree of structural complexity Yang and co-workers used metal−ligand coordination with crown-ether host/guest chemistry in the design of a hierarchical system with a central metallacyclic core threaded by dendronized species.168 A 120° donor with a pendant dibenzo-24-crown-8 group was self-assembled with a 120° diplatinum acceptor to furnish hexagonal metallacycles with three crown ethers along the periphery. Treatment with various generations of dendritic dibenzylammonium salts resulted in threading at each crown ether site, with no disruption to the initial metal−ligand coordination. T


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The same group has employed 60° acceptors modified with Fréchet-type dendrons.169 Three different triangles were made, differing by the generation of dendrons attached to the acceptor backbone. Similar dendronized rhomboids, where the dendrons were instead attached to a 120° donor, were also formed.170 A related system used dendronized donors with a pyrenefunctionalized acceptor in the self-assembly of dual-functionalized rhomboids.171 The same group made analogous donors where the O atoms of the dendrons were replaced with S atoms. These polysulfurated 120° donors were then combined with 60° acceptors to form rhomboid metallacycles.172 A simpler two-component system containing a dendronized 120° donor and a 60° diplatinum acceptor was investigated by the same group with a focus on aggregating the initially formed rhomboid metallacycles into supramolecular gels via interactions between the dendrons.173 The formation of gels occurred in a number of solvents, and conditions were identified to achieve reversible sol−gel phase transitions. The addition of Bu4NBr disrupted the supramolecular gel network, as evidenced by SEM images. The gel could be reformed upon sequestration of the bromide ions via addition of AgPF6. Dendronized hexagons formed via [3 + 3] assembly between the same donor and a 120° acceptor were also studied by Yang. Nanoscopic micelle-like structures proved capable of encapsulating fluorescent molecules, which could be released upon addition of Bu4NBr to disrupt the supramolecular structures.174 The Yang group’s extensive work on dendronized metallacycles and cages has prompted a recent review on the topic.175

Figure 40. X-ray structure of two Pt4 clusters linked by a ditopic amidine ligand. H atoms omitted for clarity. Atom (color): Pt (yellow); O (red); N (blue); C (gray).

subsequent coordination chemistry by binding secondary metal ions at these sites. Fujita and co-workers described such a system using an Pd12L24 cage built from a 120° donor with a central endo-oriented pyridyl group not used in selfassembly.181 These 24 sites could each bind a Ag(I) ion upon introduction of AgBF4, with ∼80% site occupancy determined by X-ray diffraction studies. When a similar exo-oriented pyridyl ligand was used, introduction of Ag(I) ions resulted in precipitation, likely due to polymerization of multiple spheres through Ag-bridges. When photoactive components are incorporated into a scaffold, it is possible to photoinduce changes to the shape and size of a metallacycle. This has been demonstrated by Yang and co-workers using hexagonal metallacycles containing bisthienylethene moieties,182 a functionality known to undergo reversible ring opening and closing, depending on the wavelength of light used for irradiation. Since bisthienylethene can be functionalized to orient two pyridyl groups with a 120° directionality, combinations with either a 120° acceptor or 180° provide routes to small and large hexagonal metallacycles, respectively. Irradiation of a rectangular metallacycle was used to induce a post-assembly modification of M-NHC SCCs by Hahn and coworkers (Figure 41).183 By linking two Ag or Au ions together with bis(imidazolium)-substituted stilbene bridges, the olefinic bonds of the donors were oriented for efficient [2 + 2] cycloadditions. This cyclization was triggered by illumination with 365 nm light and completed after 2.5 h. Preparation of the Au-containing species was achieved either before or after photolysis by simple transmetalation reactions with Au(tht)Cl (tht = tetrahydrothiophene). The same group has explored the formation of trigonal prisms from similar NHC ligands.184 A similar photoinduced cycloaddition was used by Jin and co-workers to enforce guest release of polyaromatic species from a metallacycle.185 A [2 + 2] SCC containing two arene-Ru clips bridged by 4,4′-dipyridylethene ligands. This metallacycle readily encapsulated pyrene-1-carboxaldehyde guests with 1:1 binding, as confirmed by X-ray diffraction. Upon formation of the host/guest complex, the emission of the pyrene-based molecule greatly diminished. However, upon irradiation, the central ethene groups of the bridging ligand underwent cycloaddition, changing the shape of the internal cavity and rendering the SCC unfit for host/guest binding. As such, solutions became emissive due to the presence of free pyrene-1carboxaldehyde. As part of their ongoing work in developing hierarchical selfassembly systems, Stang and Huang developed metallacycles containing stiff-stilbene ligands with phototunable directionality.186 When in the Z-configuration, the dipyridyl donor acts

2.6. Post-Self-Assembly Modifications

There have been a number of recent examples further establishing that self-assembly need not be the terminal synthetic step in the chemistry of SCCs. Post-assembly modifications, either involving significant structural changes176,177 or simply resulting in cage or metallacycle functionalization, remain an active area of research. These changes can be driven by ligand exchange, host/guest chemistry, subcomponent exchange, or transmetalation. The last approach was the subject of a recent review by Johnson and co-workers.178 Stang and co-workers expanded the library of post-assembly functionalizations to include copper-free chemistry.179 A 120° degree dipyridyl donor containing an amide linkage to a cyclooctyne moiety was used to construct both rhomboidal and hexagonal metallacycles containing two and three such ligands, respectively. The resultant SCCs could then be decorated with a number of pendant functionalities via copper-free Huisgen cycloadditions, demonstrated with three azide containing precursors. A stepwise substitution of the ligands of a tetranuclear Pt4 cluster was described by Mashima and co-workers,180 with evidence for intermediate mixed-ligand states (Figure 40). The initial mono(guanidinate)tetraplatinum complex was formed by mixing the parent Pt4 acetate cluster with guanidine for 16 h. Despite excess ligand, only monosubstitution occurred. Subsequent transformations could then be induced by treatment with amidine ligands. When ditopic amidine ligands were used, two Pt4 units could be fused together in a larger complex. Certain ligands may be designed so as to contain additional Lewis-basic moieties that, by virtue of the metal acceptor used or shape of the ligand, do not participate in self-assembly. SCCs formed with these ligands are therefore susceptible to U


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Figure 41. Irradiation of Ag- and Au-based metallacycles results in a post-self-assembly [2 + 2] cycloaddition. The Ag SCC can then undergo transmetalation or metal ion release. Adapted with permission from ref 183. Copyright 2013 American Chemical Society.

demonstrated by mass spectrometry data.191 Further studies showed that these species existed in dynamic equilibrium. Upon the introduction of hexamethylenetetramine (hmt), the system shifted to favor the exclusive formation of triangular metallacycles containing one molecule of hmt. X-ray diffraction and absorption experiments confirmed the 1:1 binding of triangle and guest, formed either by driving the equilibrium mixture to a single product, or via a one-pot assembly with the hmt added at the onset of self-assembly. A three-state supramolecular system was developed by Nitschke and co-workers, wherein the addition of specific anions induces complete structural reorganization.192 Since the new structure possesses unique properties from its predecessor, it interacts differently with subsequently introduced anions, thereby representing signal transduction within the system. The initial low-ordered state consists of disordered associations between Co(II) ions, introduced as cobalt triflimide hydrate, and pyridylimine-based ligands formed via the in situ coupling of p-toluidine with 6,6′-diformyl-3,3′-bipyridine, a donor used to great effect by Nitschke and others for the construction of metallacages. When triflate is added (or a triflate-based Co(II) is used initially), the system progresses to yield Co4L6 tetrahedra. This cage interacts further with percholorate anions to adopt a Co10L15 pentagonal prismatic structure, as confirmed by single-crystal X-ray diffraction. Structural reorganizations were also found in a triangular triple helicate system designed by Nitschke and co-workers comprising Zn(II) ions with a tritopic pyridylimine ligand formed via subcomponent assembly.193 A related double helicate could be formed either from a similar subcomponent

as a molecular clip, with two such groups bridged by a linear diplatinum donor. The discrete metallacycles thus formed can be transformed into a polymeric network upon irradiation with 387 nm light, inducing a cis/trans-isomerization to the Econfiguration. This isomerization is partially reversible. Upon irradiation with shorter wavelengths, larger metallacycles form due to mixed E- and Z-ligands in solution. Photoswitching of SCCs containing functionally similar azobenezene-derived sites, inducing cis/trans-isomerization, has been explored on Zn2 metallacycles by Pandey and co-workers.187 Newkome and Wesdemiotis have explored 2,2′:6′,2″terpyridine (tpy)−metal coordination for SCC formation to great effect.188,189 In 2012 they continued their work with this motif with the formation of rhomboidal and trigonal metallacycles using Zn(II) and Cd(II) ions.190 Three ligands were employed, two containing 120° directionality and one containing 60° directionality. The 120° ligands differed in that one was endo-functionalized with a third tpy group. When combined with Zn(II) or Cd(II), the unfunctionalized ligands gave a mixture of multicomponent rhomboids and homoligated triangles. However, when the functionalized 120° ligand was mixed with the 60°, and additional metal ion was bound at the center of the exclusively formed rhomboid;, that is, no triangles were observed. The internal bridge provided additional structural stability, allowing the mixture of building blocks to select for a single SCC. A similar selective assembly process was discovered by Quici and Armelao using Cu(II) ions and a bis-β-diketone ligand that, in the absence of guest, self-assembled into dimeric and trimeric metallacycles with two and three ligands, respectively, as V


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Figure 42. Preference of Cu(II) for heteroligation of one pyridyl and one terpyridine ligand was used to induce supramolecular transformations between rectangular metallacycles and open structures. Adapted with permission from ref 196. Copyright 2013 American Chemical Society.

sluggish ligand exchange kinetics due to the chelating nature of all the donors involved. A combination of HETPHEN and HETTAP was used in a related system capable of dynamic supramolecular transformations using an asymmetric ditopic ligand containing both phenanthroline and terpyridine-based donors.197 When mixed with Cu(I), trigonal metallacycles were obtained as the sole product. However, in the presence of Zn(II), the ligand delivered an equilibrium mixture of squares and triangles. When Zn(II) ions were added to the Cu(I) triangles, the system transformed to the square/triangle equilibrium. Subsequent sequestration of the Zn(II) with an exogenous terpyridine ligand prompted a reverse of the transformation, returning the Cu(I) triangles. Dinuclear metallacycles containing either Ag or Au nodes were formed with NHC linkers by Zhao and co-workers to investigate the functionalization of nanoparticles using SCCs.198 Citrate-protected Au nanoparticles underwent a ligand exchange reactions with the metallacycles, resulting in aggregated materials in the case of the Ag species and surfaceanchored materials for the Au variant.

assembly, or via subcomponent substitution wherein the bidentate ligands were replaced with tridentate donors. The resulting metallacage was a good host for 10 different guest molecules, with planar aromatic species like anthracene, pyrene, and fluorescein binding well. Since the initial cage was not a competent host for these guests, this post-assembly subcomponent substitution represents a triggered guest-binding. In one example by Nitschke and co-workers, the metal centers were removed entirely by reducing the parent cage. Initial subcomponent self-assembly with Zn used tris(2aminoethyl)amine rather than the more typical monoamine species when forming the pyridylimine groups. As a result, at each vertex, the pyridylimine groups were covalently linked. Subsequent reduction with sodium borohydride removed the Zn from the cage, leaving behind a covalent, water-soluble cage capable of interactions with various anions.194 A supramolecule-to-supramolecule transformation mimicking gene shuffling by virtue of its ability to occur catalytically as well as spontaneously was described by Schmittel and co-workers in 2012.195 The system consisted of triangular and rectangular metallacycles designed to rearrange to a third triangular metallacycle based on the preference for certain ligand pairs to coordinate to the same metal center, drawing upon the HETTAP and HETPHEN strategies introduced earlier. By using unfavorable ligand combinations in the initial SCCs, a clean transformation to a triangle with the proper ligand pairs at each metal node (either Cu or Zn) occurred over the course of 15 h at room temperature. The ligand exchanges necessary for the transformation to occur could be accelerated by the addition of 2-methylpyridine, expected to increase labilization without binding so strongly as to inhibit SCC formation. The catalyzed route shorted reaction times to ∼1 h. The related HETPYP strategy was used in a threecomponent system capable of reversibly transitioning between rectangular and open “rack” architectures (Figure 42).196 These structures contained three-coordinate Cu(I) centers with one phenanthroline and one pyridyl ligand. When a planar tripyridyl species was used, trigonal prisms were obtained. When simpler linear ditopic pyridyls were employed, the product depended on the ratio of building blocks and could be tuned after initial assembly had occurred. A 4:2:2 ratio of metal to the two ligands resulted in rectangles. When additional pyridyl ligands were added, an open rack was obtained, with two uncoordinated pyridyl sites. The structure could be closed once more by adding additional Cu(I) and the phenanthroline-based donor. The HETPYP method proved particularly advantageous for these types of post-assembly modifications relative to the HETPHEN and HETTAP approaches, which have more

2.7. Weak-Link Modifications

The wink-link approach developed by Mirkin is distinguished in its inclusion of hemilabile ligands that allow for flexible structure that may be cycled between open and closed configurations after an initial self-assembly has occurred. In 2012, the Mirkin group described the one-pot synthesis of heterobimetallic Pt/Fe constructs containing a central Fe terpyridine site flanked by two Pt centers.199 The electronic properties of the assemblies could be altered due to the hemilabile P,S-coordination about the Pt sites, while maintaining the fidelity of the Fe coordination. Related work demonstrated air-stable weak-link transformations using Pt(II)-based materials with hemilabile P,Sor P,N-ligands (Figure 43).200 The importance of halideinducted ligand rearrangement in the formation of observed

Figure 43. Weak-link modification strategy enables transformations from open structures to triple-layer complexes. Adapted with permission from ref 200. Copyright 2013 American Chemical Society. W


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blocks.202 This approach was demonstrated using molecular rectangles containing two dipyridyl-based ligands of various lengths defined by the number of ethynyl spacers between the heterocycles. Completing the remaining edges of the rectangles were acetylide-based ligands, forming relatively inert Pt− ethynyl bonds. It was observed that, in the presence of CuI, diethylamine, and an acetylene spacer, the Pt−pyridyl bonds are replaced by Pt−C, while the overall topology of the metallacycle is maintained. This fidelity was not maintained when the Pt precursor was simply mixed directly with the acetylene spacer under similar conditions. Instead, oligomeric products were observed, consistent with the requirement for reversibility during initial self-assembly. This covalent stabilization method provides a means to exploit the versatility of coordination-driven self-assembly without sacrificing the stability and robustness of the final product.

heteroligated complexes was probed using open and closed mononuclear structures. In the absence of an inner-sphere chloride ligand, mixtures of homoligated species did not generate heteroligated analogues. Formation of heteroligated species was observed over the course of 24 h for open variants containing one inner-sphere chloride. In addition to monomeric studies, Pt2 metallacycles were also synthesized and characterized, with the possibility of open, semiopen, and fully closed configurations accessible by the introduction or removal of halide ligands. An alternative approach to accessing open and closed constructs was demonstrated by Schmittel and co-workers using a triangular species containing a Zn-porphyrin center with a long phenyl-ethynyl arm terminating in an azabipyridine group (Figure 44).201 In the closed form, this terminal donor

2.9. Catenanes

The internal cavities often found at the core of metallacycles and cages have long been recognized as sites for host/guest interactions. Recently, Quintela and co-workers provided examples of inclusion and catenane complexes using Pd(II) or Pt(II) metal nodes and dizapyrenium ligands.203 The parent [2acceptor + 2donor] metallacycles hosted one of four aromatic guests in addition to one of two crown ethers. The solid state structures of two such inclusion complexes and one [2]catenane system were reported, with host/guest chemistry driven by π−π interactions, hydrogen bonding, and hydrophobic effects. The threading of macrocycles can be used as a structural element of SCC formation. Zhao and co-workers generated trinuclear silver clusters at the terminal ends of a deprotonated 1,3,5-triethynylbenzene molecule that subsequently interacted with azacalix[n]pyridines.204 When large enough macrocycles were used prisms could be obtained containing two cofacial trefoil-like panels held in place by three azacalix[n]pyridines. Tetragonal prisms using a macrocycle threading method have been prepared by Costas, Ribas, and co-workers wherein two tetracarboxylate porphyrins were held cofacially, each terminating at four Pd(II) ions.205 These metal centers were linked via a bis-triamine macrocycle that caps the three remaining coordination sites of each Pd(II). The linked Pd(II) species were isolated prior to the final self-assembly, making the design a variant in the library of molecular clip acceptors. The tetragonal prismatic SCC was structurally characterized and screened with a selection of small molecule guests, ranging from polyaromatics to small coordination compounds. A total of 10 molecular components from four unique species were capable of self-assembling into [3]catenanes as part of a multicomponent system developed by Li and Stang.206 The final assembly contained two bipyridinium ligands threaded with a dibenzo-24-crown-8 macrocycle, spanning two carboxylate-linked Pt(II) nodes. In addition to a one-pot approach, the pyridyl donors could first be threaded prior to selfassembly, or the crown ether could be added to the unthreaded metallacycle, both approaches furnishing the full [3]catenane structure, which was confirmed by single-crystal X-ray diffraction. A similar approach of threading crown ethers onto ligands containing bipyridinium moieties was used in follow-up work on the assembly of [4] and [7] molecular necklaces, wherein central metallacycles were threaded with either three or six secondary rings. Linear dipyridyl donors were used in both

Figure 44. Addition or removal of Cu(I) ions can induce transitions between open and closed structures in systems with a pendant bipyridine site. Adapted with permission from ref 201. Copyright 2013 Royal Society of Chemistry.

moiety wraps around and coordinates to the Zn center. When a source of Cu(I) is introduced along with an exogenous phenanthroline ligand, this arm detaches to chelate around the new metal ion. A related system was described without a covalent attachment of the azabipyridine. 2.8. Covalent Stabilization

In late 2011, Michl and co-workers developed a covalent stabilization method wherein an initially formed SCC can be used as a template to introduce significantly more inert building X


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reported by Lahtinen and co-workers. A tritopic ligand with three dabco functionalities was used to form tetrahedra capable of hosting anions, with preference for PF6− over OTf−.216 The inclusion complexes of [2 + 2] metallacycles containing either Pd(II) or Pt(II) nodes were explored by Quintela and Peinador.217 Naphthalene, carbazole, pyrene, and benzo[a]pyrene were all used as guest molecules, with an emphasis on exploring the size complementarity between the guests and the internal cavities of the metallacycles. Evidence was found for C−H/π interactions, stabilizing the inclusion complexes and giving predictable binding geometries. Their interest in the host/guest chemistry of Pd(II) and Pt(II) metallacycles manifested in a review published in mid-2014.218 Clever and co-workers described the interesting behavior and formation of interpenetrated double cages, templated by the presence of anions which then serve as guests in the final dimeric structure (Figure 46).219 These cages each contain two

cases, coordinated to either 60° acceptors (triangle) or 120° acceptors (hexagon). The orthogonality of threading versus metal−ligand bond formation allowed for either post- or preself-assembly host/guest formation.207 2.10. Host/Guest Structures

The host/guest chemistry of supramolecular constructs is arguably the most important feature of such architectures, providing the motivation for numerous fundamental and applied studies, ranging from explorations of how guests interact208−210 and how to modulate these interactions211 to practical approaches to drug delivery, catalysis, etc. Recent reviews and accounts on this broad subset of SCC chemistry include a accounts by Chifotides and Dunbar on anion−π interactions, which are at the heart of many host/guest systems.212 Frischmann and MacLachlan have provided a close look at metallocavitands, emphasizing bowl-shaped SCCs and their resulting host/guest chemistry.213 Huang and Lin studied the host/guest chemistry of a Znbased metallocavitands in late 2013, wherein a double hourglass shape is adopted by the self-assembly of eight Zn(II) ions with an acylthiosemicarbazide ligand.214 The SCC was well-suited to bind electron-rich aromatic molecules, with X-ray diffraction experiments showing 1:1 complexes for all the PhX (X = halide) variants save for PhF, which did not possess the binding strength to overcome the self-encapsulation of the 4chlorophenoxyl groups of the ligand. After working with β-diketonate linkers, Maverick and coworkers were motivated to explore the chemistry of multifunctional N-donors in an effort to construct more rigid SCCs (Figure 45).215 As such, they designed tetradentate bis-

Figure 46. Pd2L4 cages with homoleptic metal sites form interpenetrated double cages capable of encapsulating guest anions. The central cavity is well suited for large BF4− ions, while the two secondary cavities can accommodate halides. Adapted with permission from ref 219. Copyright 2013 Royal Society of Chemistry.

Pd(II) centers with homoleptic coordination to the pyridyl groups of ditopic ligands to give an overall {Pd2L4}2 core. These interpenetrated cages are interesting in that they create two secondary pockets that can each bind a second anion, to give three total guests within the structure. The host/guest chemistry of the system was extensively investigated by NMR methods. The threading behavior of the cages can be modulated by introducing sterically encumbered ligands. When bulky aryl groups are present, the central BF4− anion that is crucial for templating double-cage formation cannot access the internal cavity, leading to unthreaded species. However, in the presence of smaller Cl− anions, templated threading again occurs. The resultant cages’ secondary cavities show different binding affinities toward large anions relative to the standard BF4− templated cages.220 The same group studied similar cages containing redox-active ligands containing phenothiazine cores.221 When Cu(NO3)2· 3H2O was used as an oxidant, mono-oxygenation occurred. The harsher meta-chloroperbenzoic acid reagent resulted instead in dioxygenation. The ligand in any of these three redox states was competent for double-cage formation. Furthermore, when the

Figure 45. Cu2L2 metallacycle containing two metal sites linked by ditopic pyridyltriazole ligands serves as a suitable host for dabco guest molecules held in place by coordination. X-ray structure shown with counterions, with solvents of crystallization of H atoms omitted for clarity. Atom (color): Cu (gold); O (red); N (blue); C (gray).

(pyridyltriazole) ligands with differing numbers of aromatic spacers to assemble Cu metallacycles. When studying dabco encapsulation with their Cu2L2 species, a dabco-bridged dinuclear complex was isolated, prompting the exploration of larger cages. The inclusion of an extra phenyl ring in the ligand provided a suitable internal cavity to fully encapsulate dabco within the Cu2L2 cage. The coordination of dabco to Cu ions was used as the impetus for SCC formation in a system Y


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dynamic mixture of diastereomers in solution, adopting either T, C3, or S4 symmetric configurations. Eleven different anions were studied to determine their binding affinities and effects on the ratio of diastereomers in solution. In some cases, such as with halides, NO3−, or BF4−, evidence was found for the sole presence of T isomers. In this way the system adapted to enhance anion binding by cascading to the most favorable configuration. A ditopic 3,3′-diimino-4,4′-bipyridine bridging ligand with obtuse orientations of its coordination vectors was used by Nitschke and co-workers to construct a M8L12 metallacage approximating Th symmetry, due to alternating Λ and Δ configurations at each of the eight metal centers.232 NMR and electrochemical investigations indicated that ferrocene could be reversibly bound within the cage, whereas larger ferrocene derivatives were excluded. M4L4 cages were also designed by the same group as platforms to study spin crossover phenomena in SCCs.233 By replacing the common pyridylimine chelating moiety with a imidazolylimine tritopic ligand, two new tetrahedral cages were formed, differing in the identity of the fragment at the center of the tritopic ligand (either N or C−OH). These cages transitioned from low-spin to high-spin with increasing temperature. Furthermore, the presence of guest molecules reduced the temperature at which these transitions occurred. A system comprising small and large Fe4L6 tetrahedra was capable of chain-reaction anion exchange due to the different preferences of the two cages sizes for specific anions.234 Whereas the large cage could bind NTf2−, this species was too large for the small cage. With the large cage already encapsulating a guest, the addition of ClO4− led to selective uptake by the small cage. Once both tetrahedra were filled, the subsequent addition of PF6− led to displacement of ClO4−, due to its strong affinity for the small cage. The ejected ClO4− then displaced the NTf2− in the larger cage. Although the system was unoptimized, and as such the exchanges were not fully quantitative, these results show the potential complex exchange processes that may occur within SCCs. A similar Fe8 cubic cage was designed by Nitschke and coworkers wherein the tetratopic porphyrin panels were replaced with a Mo2 paddlewheel core bridged by carboxylates containing terminal pyridylimine moieties, thus serving as a new tetratopic ligand (Figure 47).235 The Mo−Mo subunits were found at the center of each face of the cube, with the metal−metal axes each oriented toward the very center of the cage. Host/guest chemistry was investigated with a suite of molecules, including the halides as well as ammonia, trimethylamine, trimethylphosphine oxide, and the ammonium salt of trifluoroacetate. Interestingly, affinities for the halides could be enhanced by prebinding secondary guests, as demonstrated by the 8-fold enhancement of iodide binding for the ammonia inclusion cage versus the free cube. Related cubes were further studied by the same group, revealing that two distinct sites can modulate guest binding. The Mo2 paddlewheels interacted with phosphine ligands, resulting in a reduction of binding affinities. The edges of the cubes could also be used for allosteric guest inhibition, by binding tetraphenylborate.236 In early 2013, Ward and Hunter continued to explore the host/guest chemistry of a Co8L12 cubic cage237 as part of ongoing investigations of such structures (Figure 48).238 The high-spin Co(II) nodes in the cage were bridged by pyrazolylpyridine chelating donors, and the resulting paramagnetism helped rather than hindered NMR analyses of host/guest

fully reduced cage was exposed to air for two month, monooxygenation occurred, even in single crystals. Chemical oxidation in solution also provided routes to obtain the oxidized cages. A follow-up study to these cages continued to look at phenothiazine-based ligands with an emphasis on cage formation under mixed-donor conditions. Despite the different lengths of ligands, mixtures resulted in statistic distributions of multicomponent cages, with the exception of the shortest ligand, which self-sorted into a homoligated cage.222 Their extensive studies of these so-called “banana-shaped” ligands were the focus of a recent review.223 Although many M2L4 structures result from the use of these ligands, more complex cages are also possible, including face-centered squarecuboids.224 The use of cyclotriveratrylene as the core of tripyridyl ligands has been explored by Fisher and co-workers in the construction of metallacages.225 One such cage, a [Pd6L8]12+ species, was recently used to study the uptake of the sodium salts of three alkyl sulfates of the octyl-, dodecyl-, and tetradecyl-variety. The stellated octahedral metallacage was capable of accommodating two guest molecules as evidenced by Job’s plot analyses based on NMR titrations. A measure of diffusion coefficients in the system showed that while the cage was invariant, the coefficients of the guest increased to a value similar to that of the cage, supporting encapsulation. Certain metallacycles and cages can be designed such that open coordination sites are preserved in the initial assembly. Subsequent coordination can then drive new chemistry. Such is the case for the pyrazolato-based cages made by Raptis and coworkers, in which Cu(I) and Au(I) ions interact with a bipyrazolyl ligand to form M6L3 species.226 These species were used in host/guest studies, specifically using a ligand with a 2,7naphthalene core between pyrazolyl groups. When solutions of the cages in the presence of S8 were precipitated by diffusion of cyclohexane, the resultant product contained a 1:1 host/guest complex, trapping S8 within the cage. Nitschke and co-workers explored the host/guest chemistry of two diastereomeric Cu8L4 tubular cages formed as a pair during self-assembly, due to the presence of C2 and C2v conformations of the donor building block.227 The resulting D4 and D2d cages each contained eight Cu(I) ions and four bridging ligands. The D4 tube interacted with two molecules of KAu(CN)2, forcing decomposition and extraction of a Cu ion from an exogenous cage to give a Cu(Au(CN)2)2− complex guest. The introduction of Ag(CN)2− did not give an analogous reaction; however, the central Cu(I) of the complex guest in the Au system could be replaced by Ag(I) when treated with AgBF4. Follow-up work by the same group generalized the strategy to form M8L4 tubes and further explored the host/ guest chemistry of these systems.228 The same group explored the possibility of modulating guest exchange within an M4L6 tetrahedron containing Fe(II) nodes and pyridylimine ligands.229 The bridging ligands in this study were functionalized with sulfonate groups capable of interacting with guanidinium cations, thereby blocking the pores of the cage. NMR studies of the uptake of cyclohexane were carried out, indicating that blocked cages were sluggish for guest uptake at all temperatures studied. This cage was also the subject of a detailed study of both the kinetics and thermodynamics of guest binding using NMR analyses.230 Similar M4L6 tetrahedra with undecorated pyridylimine ditopic donors spanning Fe(II) centers were investigated as a stimuli-responsive system.231 The initial cages existed as a Z


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easily detected. There were 19 different guest molecules tried, ranging in size from the small phenol to the large N,N-dibutylbenzenesulfonamide. Despite anticipated size complementary between the internal cavity and a number of the guests, only cumarin showed appreciable encapsulation. With this result, a series of isostructural molecules were then explored and used to probe H-bonding and contributions from aromatic centers. Certain metallacycles possess the proper dimensions to serve as hosts for fullerene molecules. When a suitable photophysical handle is present, this binding can be investigated spectroscopically. Such was the case for a metallacycle made by Chi and Stang,239 assembled by mixing cis-(dppf)Pd(OTf)2 with a 90° dipyridyl donor possessing a carbazole core. Although these precursors ideally form a square metallacycle, the solid state structure revealed a distortion, making the final metallacycle resemble a bowl. Fluorescence quenching upon the titration of C60 was taken as evidence for host/guest complex formation. A similar study was performed using a rectangular metallacycle designed by Chi and co-workers comprising two arene-Ru clips bridged by a dipyridyl donor containing a central diamide site. Two such rectangles were made, differing in the bridging ligand between the Ru sites in the clip. Fullerene binding was investigated using spectroscopic techniques, supporting a 1:1 interaction with both C60 and C70.240 Torres and co-workers designed metallacage SCCs wherein Pd(II) or Pt(II) nodes linked two tritopic, pyridyl-functionalized subphthalocyanine ligands.241 Since these ligands possessed a natural bowl shape, joining two such subunits furnished metallacages capable of hosting a variety of fullerene guests, including C60, C70, and C60-PCBM, a [6,6]-phenyl C61butyric acid methyl ester. These encapsulations were probed using absorption and emission techniques, along with NMR analyses, with C70 binding stronger than its smaller C60 counterpart. The binding of both C60 and C70 was studied with M2L4 and M2L2 SCCs constructed from dipyridyl donors with Hg(II) ions by Yoshizawa and co-workers (Figure 49).242 The ligand used for these species contained large aromatic surfaces, as both pyridyl groups were spaced from a central benzene ring through anthracene moieties. Self-assembly between this ligand and Hg(OTf)2 formed the M2L4 cage when a 2:1 ratio was used, and the M2L2 metallacycle with 1:1 conditions. Although the metallacycle was not capable of hosting fullerenes, the M2L4 cage could encapsulate both C60 and C70. The binding affinity of the smaller fullerene was higher, and allowed for the selective displacement of the larger guest. Addition of extra Hg(II) to either host/guest complex ejected the fullerene and formed the M2L2 metallacycle. Schmittel and co-workers used heteroleptic coordination at Cu(I) sites to construct a suite of tweezers, grids, and prisms containing Zn-porphyrins held cofacially.243 The ability of all three structures to encapsulate C60 was determined using spectroscopic methods, with Job plot analyses confirming a 1:1 binding ratio. Differences in binding affinities were ascribed in part to entropic penalties associated with forcing the porphyrin faces to remain strictly coplanar upon guest encapsulation. Whereas the prism was already rigid, the tweezers and grid were able to rotate their porphyrin units to a much more significant degree when free of guests. It was also determined that fullerene bound much more tightly than coronene, prompting a guest exchange study where the latter molecule was ejected, even without total cage disassembly.

Figure 47. Tetratopic donor (top) formed around a Mo2 paddlewheel center forms via subcomponent self-assembly of a cubic SCC (bottom) explored for host/guest chemistry with a suite of molecules including halides and amines. X-ray structure shown with anions, solvents of crystallization, and nonpolar H atoms omitted for clarity. Atom (color): Mo (teal); Fe (orange); N (blue); O (red); C (gray); H (white).

Figure 48. Molecular mechanics model of a Co8L12 cage playing host to 2H-chromen-2-one. Hydrogen bonding between cage and guest shown with dotted lines. Reprinted with permission from ref 237. Copyright 2013 American Chemical Society.

interactions, as the 1H NMR resonances were significantly spread out, allowing subsequent shifts upon guest binding to be AA


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ligands have a significant role in the outcome of an assembly process. Mukherjee and co-workers explored the chemistry of Pd-based prisms, revealing that when bulky amine-based chelators were used to cap the metal nodes, single cages were favored.250 When steric bulk was attenuated, evidence for interlocked prisms was found. In addition, when guest molecules were present in conditions that would normally favor interlocking, catenation was avoided. Hor and Jin described an interesting system capable of forming Borromean links with three metallacycles interlocked within a framework.251 This was achieved by modulating the length of a series of dipyridyl ligands that bridged two arene-Rh sites coupled through a Cu-based metallaligand. For smaller ligands, simple rectangular metallacycles were obtained. However, when the linkers were elongated, interpenetrated structures were observed, attributed to the system compensating for the weaker coordination bonds. The internal cavities of these species were also evaluated for acyl-transfer chemistry at the Cu centers, with the size of a given metallacycle having a significant effect on the rate of catalysis. Control over the nature of self-threading was exhibited in a system comprising Ag6L4 cages designed by Hong and coworkers, using tripyridyl donors linked through ester coupling to a alkyl backbone.252 When solid samples of the individual cage were covered with MeOH/CHCl3 mixtures for 24 h, threaded polycatenanes were formed. If lower concentrations of the cage are used, the cages join through metal−metal interactions rather than mechanical threading. This slower process takes up to 30 days in solution.

Figure 49. Hg-based metallacycles and tubes. The M2L4 cage is capable of host/guest interactions with C60 and C70.

Fullerene molecules may also act as templates for the formation of SCCs, as demonstrated by Shionoya and coworkers.244 An octanuclear cage was formed around a C60 template using a bpy-functionalized tetratopic porphyrin donor with Zn(II) nodes. Each Zn(II) site accommodated two bpy groups, with the remaining two coordination sites occupied by either water or OTs− anions. The same ligand was used to construct simpler cofacial dimers when the Zn(II) nodes and C60 template were replaced by simple Ag(I) ions.245 This small prism intercalated acenapthenequinone in its internal cavity. These two examples, one complex, one simple, highlight the theme of desymmetrization and how building blocks of lower symmetry oftentimes give rise to complex architectures, a topic of a recent review by Shionoya and co-workers.246 Another example of this uses the same porphyrin ligand to form a Zn11 SCC with three unique Zn coordination environments.247 Since Zn(II) is known to form mono-, bis-, and tris-chelated environments with bpy ligands, it was selected as a candidate to form complex assemblies from symmetric building blocks.

2.12. Large Assemblies

One pioneering achievement of directional bonding was the formation of a dodecahedron formed from 50 building blocks. This early example of the feasibility of coordination-driven selfassembly for the formation of large, discrete polyhedra has fueled a growing number of assemblies of high-nuclearity, in some cases encompassing main-group elements.253 In late 2011, Champness, Schröder, and co-workers synthesized a socalled nanosphere comprising 66 Cd(II) centers bridged by 28 μ3-hydroxide, 16 μ3-oxo, and 5 μ5-NO3− anions capped by an additional 20 tripodal ligands and 12 DMF molecules. The sphere was formed under solvothermal conditions that were a primary determinant of the structural outcome; lower temperatures or the use of CdCl2 instead of Cd(NO3)2·4H2O as a Cd(II) source resulted in nondiscrete products. Colquhoun and co-workers used cyclometalated 2-phenylpyridine chloro-bridged dimers of Pd(II) in self-assembly reactions with cyanuric acid to generate metallacages containing either 9, 10, or 12 metal centers.254 The Pd12 species was obtained by carrying out the assembly reaction in the presence of triethylamine as a base. Structural investigations revealed a chiral cage with T symmetry. Single crystals of a minor product were also observed, corresponding to a Pd10 species with an open face, suggested as a logical kinetic intermediate of the Pd12 cage. The ligand of the Pd starting material was then replaced with N,N-dimethylbenzylamine, and two analogous Pd10 and Pd12 cages were obtained. A final species was formed upon the replacement of the cyanuric acid with its trithio-variant. Cu12 and Cd16 cages were designed by Ward and co-workers using a ligand containing two chelating pyrazolyl-pyridine groups with a naphthalene-1,4-diyl spacer.255 The Cd-based cage contains significant interligand π-stacking which helps the cage maintain its structural integrity in solution. The species

2.11. Self-Threaded Architectures

If these interactions occur between two or more assemblies instead of with exogenous guests, structurally complex knots, catenanes, and other unique designs can be realized. In some instances, conditions can be selected to favor noncatenane threaded structures. In the case of rectangular metallacycles, this can be achieved by combining a diruthenium molecular clip that contains an extended π-system with a diethynyl-spaced bridging ligand.248 The resulting SCC, which appears to remain threaded in solution on the basis of ESI-MS characterization, is allegedly stabilized by π−π interactions among the donor and acceptor fragments. Stang and Chi have also demonstrated interlocked threading with 3D metallacages.249 A few factors governing the formation of interlocked cages have been explored, with evidence that the presence or absence of exogenous guest molecules and the steric bulk of capping AB


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comprises four Cd3 trigonal faces, with the remaining four Cd(II) ions acting as caps connecting three faces each through the bridging ligands. The corresponding Cu-based cage is smaller, with both six- and four-coordinate copper sites. Both species share M3L3 trigonal helical motifs as subunits. The formation of a large Co16 complex was investigated by Zeng, Kurmoo, and co-workers, using both X-ray crystallography and mass spectrometry experiments.256 The chiral Co centers in this large SCC were bridged by S,S-1,2-bis(1Hbenzimidazol-2-yl)-1,2-ethanediol ligands, with both fully and singly deprotonated forms occurring in the final structure. Six azide groups are also found among the bridging ligands. Smaller tetranuclear and dinuclear cobalt fragments were found during ESI-MS experiments, providing evidence for intermediate species that form en route to the fully assembled cage. Increased structural complexity may also be achieved by using initially formed SCCs as building blocks for higher order structures, drawing upon themes of hierarchical self-assembly. In 2012, Marchiò and co-workers described the formation of cyclic hexamers based on a thioether-functionalized bis(pyrazolyl)methane ligand and Ag(I) ions.257 Pulsed-gradient spin-echo NMR experiments provided evidence for the hexameric nature of the initial assembly product. In the solid state, the charge balancing anions coordinate to the Ag(I) centers, with the type of anion having an impact on the overall framework structure, resulting in one of two microporous 3D lattices. A high nuclearity metal−organic cluster containing 14 nickel ions was synthesized by Soo Lah and co-workers in a study of the conformational control of a tritopic carboxylate-based ligand with a central 1,3,5-triamidobenzene core,258 reminiscent of an earlier Cd66 nanosphere reported by Champness, Schröder, and co-workers.259 The ligand can adopt either a folded or extended form, depending on the configuration at each amide moiety. In the closed, folded form, the protonated ligand was mixed with Ni(NO3)2·6H2O in DMF under solvothermal conditions, discrete Ni14 species were obtained and structurally characterized, with the metal sites bridged by eight hydroxy groups and 20 carboxylate groups. Similar synthesis with the extended form of the ligand resulted in infinite frameworks. Wang and Dai have demonstrated the use of sulfonylcalix[4]arenes as components of large SCCs based on their ability to form tetranuclear complexes that could act as acceptors for selfassembly.260,261 By linking six such tetranuclear clusters together with 1,3,5-benzenetricarboxylate as a bridging ligand, octahedral metallacages were obtained. This approach was demonstrated using Ni(II), Co(II), and Mg(II) ions. The use of alternative bridging ligands was explored, revealing that both cis,cis-cyclohexane-1,3,5-tricarboxylate and 1,3,5-benzenetribenzoate could also be used to make cages. The behavior of such cages toward guest binding was the subject of a follow-up study by Wang and co-workers, with marked differences between solution and solid state interactions.262 A large dodecanuclear copper cage was reported by Ray and co-workers, formed by assembling 2,6-bis-[(3-hydroxy-propylimino)-methyl]-4-methyl-phenol with a Cu(II) source in the presence of two bases, NEt3 and NaN3 (Figure 50).263 Each ligand contained five donor sites, two imine N-atoms, a central phenol, and two terminal hydroxy groups. The structure of the Cu12 cage, as determined by X-ray crystallography, reveals that, for each ligand, one terminal hydroxy group remains unbound. The two imine N atoms each coordinate to neighboring Cu

Figure 50. X-ray structure of a cuboctahedral Cu12 cage. H atoms and solvents of crystallization omitted for clarity. Atom (color): Cu (gold); N (blue); O (red); C (gray).

sites, with a phenolate O atom bridge. The remaining hydroxy group also bridges to an adjacent Cu node. Two nitrate ions are found in a structural role, bridging between three Cu sites each. A large metallacycle containing 15 nickel sites was synthesized by Vittal and co-workers, using a glutamic acid Schiff-base ligand. The discrete rings formed upon selfassembly further ordered into 2D sheets. Further hydrophobic interactions were invoked to explain the staggered arrangements of these sheets into a 3D supramolecular structure.264 A cobalt thiacalix[4]arene was used as the basis for a large, Co32 SCC, one of the highest nuclearities to date. The structure was generated by Liao and co-workers using an in situ click reaction and self-assembly involving Co(AcO)2·4H2O, p-tertbutylthiacalix[4]arene, NaN3, and 1,3-dicyanobenzene.265 This results in a tetragonal prismatic arrangement with eight distinct “shuttle-cock” secondary building units at each vertex. The gas sorption properties of the cage were measured with N2, indicating that the structure is permanently porous, and efforts to identify new structures made using calixarene-based building blocks are ongoing.266 A relatively smaller Co20 calixarene nanocage has also been reported by Hong and co-workers.267 A recent Highlight by Dalgarno and co-workers summarizes these sorts of calixarene structures and related capsule-like species with an emphasis on metal−organic based designs.268 Interesting main-group-only cages were prepared by Dehnen and co-workers,269 assembled ionothermally from a mixture of [Ge4Se10]4− with SnCl4·5H2O in either 1-butyl-3-methylimidazolium (BMIm) or 1-butyl-2,3-dimethylimidazolium (BMMIm). The [Sn36Ge24Se132]24− polyanion was dubbed a “zeoball” owing to its zeolite-like composition. The cage is charge-balanced by the inclusion of either BMIm or BMMIm cations, depending on the synthetic conditions used. In some instances, polyoxometalates (POMs) may be used as components of larger SCCs, resulting in very high nuclearity structures.270 Mizuno and co-workers constructed sandwichtype constructs wherein two POM components were linked by Cu nodes.271 Three structure types were obtained, with either one, two, or four copper sites bridging the POM groups. By controlling the ratio of POM to Cu in solution, the system could be tuned between these three architectures. Mak and coworkers combined oxovanadates with Ag(I) ethynide clusters to synthesize one neutral and two anionic architectures that AC


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were investigated by X-ray diffraction.272 Niu and Wang273 and Zaworotko and co-workers274 have also used metal−organic coordination toward the construction of discrete POM assemblies. 2.13. Crystal Engineering

The diverse shapes and sizes of SCCs have prompted their use in crystal engineering, where the modularity of the selfassembly process allows for systematic investigations of ligand alterations. For instance, Chand and co-workers explored the effects of the presence of π-clouds in their capping ligands on the solid state structures of simple [2 + 2] assemblies of Pd(II) with dipyridyl bridging ligands.275 When a π-system is present, intermolecular interactions are introduced that result in rod-like stacking in the solid state. The same group explored trinuclear complexes that adopt octadecanuclear wheel-shaped structures in the solid state.276 Zhang and co-workers described an interesting cube-withina-cube coordination network wherein a large Na8 cube network surrounds a Mn8 species bridged by 12 acetate and 6 methoxy groups.277 This Na4Mn8(μ4-OMe)6(μ4-MeCO2)12·2MeCO2 species was formed from a solvothermal synthesis using Mn(CH3CO2)2, NaOH, CH3CN, and methanol. Although the central cube is necessarily part of the larger extended framework and therefore not isolable as a discrete species, the nested octahedra are a unique structure, including the rare presence of a square pyramidal five-coordination oxygen atom. A study of cages formed from Pd(II) ions and 1,3bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene by Wang and Lio revealed conditions to isolate M3L4, M2L4, or the cocrystallization of both species.278 The M3L4 species contains a central Pd(II) site from which two four ligands form the basis of two fused metallacycles. The M2L4 cage is simpler in design, sharing structural characteristics of many M2L4 species wherein homoleptic coordination at each Pd(II) center is observed. The mixed species is stabilized in part by π−π interactions and hydrogen bonding between the two discrete units.

Figure 51. Tetrathiafulvalene-based ligands impart redox-active properties to their corresponding SCCs. Adapted with permission from ref 279. Copyright 2012 American Chemical Society.

experimental support for 1:1 binding within its 13 Å cavity.281 Introducing 1,1′-bis(diphenylphosphino)ferrocene as a capping ligand for 90° acceptors, the same group explored mixed metal systems with their terathiafulvalene ligands. A M8L4 cage was assembled, permitting up to 16 reversible oxidations.282 Heterometallic SCCs, based on the use of (dppf)2M fragments (M = Pd, Pt), were also made with related tetrathiafulvalene donors (Figure 52).283 Two tetrapyridyl

3. FUNCTIONAL ASSEMBLIES 3.1. Electrochemistry and Redox-Active SCCs

The incorporation of redox-active ligands is an active pursuit in small molecule coordination chemistry and more recently in the study of metal−organic frameworks. Given the thematic overlap between these fields and SCCs, it follows that discrete assemblies incorporating such ligands may prove electroactive. Sallé and co-workers described one such cage using a tetratopic bis(pyrrolo)tetrathiafulvalene ligand280 (BPTTF) and Pt(II) nodes.279 A 6 equiv portion of the oft-employed Pt(PEt3)2(OTf)2 acceptor combined with three tetratopic ligands, resulting in an M6L3-type trigonal prism (Figure 51). The structure of the metallacage was determined by singlecrystal X-ray diffraction. Two oxidation events were observed in acetonitrile solution at higher potentials than those observed for the free ligand. In addition to electrochemical investigations, the cage was evaluated for its ability to accommodate the electron-poor tetrafluorotetracyano-p-quinodimethane molecule, with electronic absorption evidence for 1:1 binding. The BTTF core could also be used to make 2D assemblies when functionalized with two pyridyl groups with a 180° directionality. The same group used such a ligand to construct triangle and square metallacycles, both showing two reversible oxidation waves when investigated by cyclic voltammetry. The triangular metallacycle served as a host for C60, with

Figure 52. Multiple redox-active sites can be incorporated into the same SCC by using both donors and acceptors with appropriate functionalities, for example tetrathiafulvalene ligands with ferrocenecontaining Pd or Pt nodes. Adapted with permission from ref 283. Copyright 2013 American Chemical Society.

donors could be linked by four Pd(II) or Pt(II) nodes, providing six redox active sites across the SCC. Two oxidative waves were observed during cyclic voltammetry experiments, attributed to the tetrathiafulvalene core and the ferrocynl units. Although the ligand oxidation was not reversible, redox cycling did not reveal significant changes, suggesting that the core remained robust throughout oxidation. When arene-Ru clips were used as acceptor units, they could either form octanuclear cages with two redox-active ligands held cofacially, or tetranuclear constructs where the Ru-clips bridge adjacent pyridyl donors of a single ligand.284 AD


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A heterometallic Co/Fe cage was designed by Oshio and coworkers with a Co6Fe4 core bridged by cyanide ligands (Figure 53).285 When pyridinecarbaldehyde and phenylethylamine were

Figure 53. Mixed-metal Co/Fe cage exhibits electronic communication between metal sites, with electronic transitions induced by temperature changes. X-ray structure of the high-temperature structure reveals six cobalt centers and four iron centers. H atoms omitted for clarity. Atom (color): Fe (orange); Co (green); N (blue); C (gray). Figure 54. Tetrahedral cage shows multiple reversible redox waves associated with ligand-centered oxidations and reductions. Adapted with permission from ref 288. Copyright 2013 American Chemical Society.

combined in one-pot with Co(BF4)2·6H2O and (Et4N)3[Fe(CN)6], the dodecanuclear cage was formed. Each Co center was octahedral with four N atoms from the organic donors and two N atoms from the cyanide bridging ligands, leading to neighboring Fe sites. The Fe sites were homoleptic, with six Cbound cyanides. Electronic communication between metal centers was confirmed, with a temperature induced electron transfer occurring, allowing thermal control over the electronic and magnetic properties of the material. Small metallacycles containing two Cu(II) sites were synthesized and investigated by Pardo and co-workers, revealing a magnetic switching behavior upon oxidation.286 The metallacycles contained redox-noninnocent oligo-pphenylenebis(oxamato) bridging ligands designed to introduce electronic communication between metal centers. Upon reversible one-electron oxidation, a metallacyclic variant of Wurster’s blue was obtained, with the spin alignment of the two Cu(II) centers reversing from antiparallel to parallel. This system was the subject of a follow-up study employing a suite of experimental and theoretical techniques to elucidate the electronic structure and behavior of the SCCs. Adding substituents to the bridging organic moieties increased the extent of delocalization of the unpaired Cu electrons, thereby strengthening electronic communication between the two centers.287 Würthner and co-workers designed an M4L6 tetrahedron using the symmetry-adapted approach with Fe(II) vertices and a ditopic chelating ligand containing a perylene bisimide core (Figure 54).288 By incorporating this redox-active group into a metallacage using chelating donors, the resulting species was stable under dilute conditions and also possessed the ability to exhibit host/guest chemistry. The cage showed two reversible oxidation events along with five reversible reduction events. The first oxidation wave was metal centered, the second involved oxidation of the perylene bisimide. In the other direction, the reductions were ligand centered, first on the bisimides and then the bipyridines. Thus, the full tetrahedron

was capable of reversible 34-electron cycling. A related Zn(II) cage has also recently been reported.289 The redox-active component of an SCC does not need to be part of the structural backbone of the metallacycle or cage. Instead, a building block can be functionalized such that the internal or external walls of a given structure are decorated with moieties of interest. An endohedral functionalization strategy was used by Yang and co-workers to form hexagonal metallacycles containing either three or six ferrocenyl groups. Both variants showed reversible one-electron redox chemistry.290 The same group also designed rhomboidal metallacycles using a 60° donor with two ethynylpyridine moieties attached to a central phenanthrene, further functionalized through ester coupling to two ferrocene units.291 Two types of rhomboids were made: the first used a simple 120° degree diplatinum acceptor, to give a tetraferrocenyl SCC; the second used a diplatinum acceptor with a central benzene ring exofunctionalized with another ferrocene group. Rhomboids made from these two building blocks contained six total ferrocene sites. Electronic communication in the first rhomboid was minimal, with reversible oxidation waves observed with minor distortions suggesting weakly coupled centers. The second rhomboid showed additional peak separation in cyclic voltammetry experiments, indicative of the two different ferrocene environments present. An interesting M8L12 cube was synthesized by Ward and coworkers, containing both Ru and Cd metal nodes (Figure 55).292 Initially formed mononuclear Ru complexes with coordination from three different ligands with two terminal chelating sites were further assembled into a cube upon introduction of Cd(II) ions. Electrochemical analysis revealed that the charge of the cage could be cycled between 16+ and AE


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Interestingly, while the metallacage proved photoluminescent, solutions of a model complex of a mononuclear fragment under identical conditions, [Ir(ppy)2(PhCN)2]OTf, did not emit, suggesting that nonradiative dissociation pathways are avoided by the cage. Organic dyes are also attractive targets upon which to design building blocks for SCCs. Pistolis and co-workers have expanded the library of directional bonding building blocks to include those built around boron dipyrromethane dyes (BODIPY) (Figure 56).298 By replacing the fluorine atoms of

Figure 55. Four Ru subunits can be linked via coordination to Cd centers in a mixed/metal cubic structure. The Ru centers in the SCC were capable of reversible 1e− oxidations. Adapted with permission from ref 292. Copyright 2014 Royal Society of Chemistry. Figure 56. BODIPY-inspired acceptor building block useful for selfassembly of emissive SCCs.

20+, as all four Ru centers can be oxidized from Ru(II) to Ru(III), without cage degradation. Raja and co-workers designed a ditopic chelating ligand terminating with ferrocenyl groups that could be combined with Ga(III) or In(III) precursors to form M2L3 triple-stranded helicates.293 Coordination occurred at the enolate sites of the ligand, completing the distorted octahedral environments at the two metal centers. Cyclic voltammetry revealed a single oxidation wave corresponding to the Fc/Fc+ couple, with no observed electronic communication between the ligands, or within a given ligand. The quasireversibility of the wave was attributed to the high degree of conformational reorganization required to minimize repulsion upon oxidation.

a parent BODIPY core with rigid ethynyl moieties ultimately terminating at two Pt(II) centers, an acceptor can be obtained with the proper angularity to construct hexagonal metallacycles with six BODIPY fragments. The ethynyl spacer effectively shields the BODIPY sites from deleterious heavy-atom effects, resulting in an extremely emissive metallacycle with a quantum yield (Φ = 0.88) and molar absorption coefficient (77 900 M−1 cm−1) per dye unit virtually unchanged relative to those of the free ligand. A related system used BODIPY backbones for both donor and acceptor building blocks, with diplatinum and dipyridyl variants. The [2 + 2] self-assembly of these species resulted in highly emissive rhomboidal metallacycles that could play host to 1,3,6,8-tetrasulfopyrene, allowing for intrahost and guest-to-host energy transfer.299 Stang and co-workers have found that the intense emission of aniline-based 120° ligands gives rise to highly emissive metallacycles upon self-assembly with various organoplatinum acceptors. A series of rhomboid, hexagons, and smaller model fragments301 were synthesized and characterized revealing that, upon self-assembly, emission wavelengths are red-shifted relative to the free ligands.302 The most emissive assemblies were those containing endofunctionalized aniline species. Although the presence of the Pt(II) centers did diminish quantum yields, attributed to enhanced spin−orbit coupling facilitating nonradiative decay through triplet states, values as high as 28% were still observed. The emission of endoanilinebased rhomboids could be further tuned by simple modifications to the para-position of the central aniline group.300 The fluorescence wavelength was highly sensitive to the electron donating or withdrawing nature of the exo-group, and a suite of rhomboids with emission spanning the entire visible region was demonstrated (Figure 57). Metallacycles constructed from substituted carbazole ligands terminating in two terpyridine donors and Zn(II) ions were studied by Grimsdale and co-workers, with a focus on the effects of hydrogen bonding on assembly, and the tunable emission of the system (Figure 58).303 Two different ligands were made, a parent carbazole, and an N-alkylated carbazole containing a 12 carbon chain. Clean formation of pentagonal metallacycles were observed for the N−H ligand, whereas

3.2. Photophysically Active SCCs

The study of the photophysics of SCCs and related metal− organic materials294 is an emerging area of interest. In many cases, the properties of a metallacycle or cage are a direct result of the building blocks used in their construction, rather than a ramification of the assembly process. As such, efforts have been made to characterize the photophysical properties of common precursors. These studies often involve both experimental and theoretical investigations to elucidate the electronic structure of a given building block and correlate that structure to observed absorption and emission behaviors. Han and Stang have used such an approach on 0°, 60°, and 90° Pt-based acceptors.295 Expanded, related investigations on entire assemblies were also carried out, including squares, rectangles, and triangles containing either Pt-pyridyl, Pt-carboxylate, or mixed heteroligated Pt nodes.296 In all cases, the presence of Pt introduced a significant number of low-lying dark states, greatly reducing the emissive properties of the metallacycles relative to the free ligands. The incorporation of well-known chromophores or fluorophores as components of SCCs is attractive in that promising photophysical properties may be preserved and in some cases new phenomena may emerge. In late 2012, Lusby and co-workers targeted cyclometalated C,N−Ir(III) fragments for use as metal nodes.297 A panel-directed approach was selected for the synthesis of molecular capsules using the tritopic tricyanobenzene as a bridging ligand. When enantiopure [(Ir(ppy)2Cl)2] (ppy = 2-phenylatopyridine) was used as a precursor, homochiral truncated tetrahedra were obtained. AF


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different ligand types, a benzimidazole-based donor and a hydroxyphenol benzimidazole.304 Two species were made, differing by methyl substitutions of the central phenyl ring of the hydroxyphenol benzimidazole ligand. The methyl variant emitted at 482 nm, while the unfunctionalized parent species emitted at 514 nm. Oxygen efficiently quenched the emission of both species, revealing blue-shifted spectra that led to the discovery of a second emissive excited state with a short lifetime of

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