Constructing coordination nanocages: the ... - Springer Link

J Incl Phenom Macrocycl Chem (2015) 82:3–12 DOI 10.1007/s10847-015-0520-0

REVIEW ARTICLE

Constructing coordination nanocages: the metalloligand approach Li Li1 • Daniel J. Fanna1 • Nicholas D. Shepherd1 • Leonard F. Lindoy2 Feng Li1

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Received: 17 April 2015 / Accepted: 22 April 2015 / Published online: 3 May 2015 Ó Springer Science+Business Media Dordrecht 2015

Abstract The focus on this mini-review is on the use of metalloligands for the construction of discrete self-assembled, polyhedral metallo-cages. This metalloligand approach employs metal bound multifunctional ligand building blocks that display predetermined, well-defined electronic and structural preferences that facilitate the rational design and construction of both homo- and heteronuclear cages. The assembled cages exhibit a variety of defined topologies that, for example, range from trigonal bipyramidal to rhombododecahedral. Keywords Metalloligand Coordination cage Polyhedral Tripodal ligand Metallosupramolecular Self-assembly

work to synthesise, via self-assembly, a wide range of discrete homo- and hetero-nuclear and framework topologies [1, 2]. It has also been employed as one of a number of approaches for constructing self-assembled metallo-cage (or container) systems [3–5]. The focus on this mini-review is on studies of this latter type, with the coverage limited to small discrete cage systems; metalloligand systems based on clathrochelates [6], porphyrins [7–10] and related macrocyclic derivatives (for which the bound metal ion plays no direct role in influencing the steric properties of the metalloligand) are not discussed. Similarly systems that are organometallic in nature have not been included.

Polyhedral cage synthesis employing metalloligands Introduction The concept of a metalloligand, a ligand system that incorporates a primary coordinated metal that directs a secondary donor site, or sites, to be suitably oriented for coordination to a further metal centre, or centres, has long been exploited. For example, the concept has been put to This paper is dedicated to Jack Harrowfield and Jacques Vicens in celebration of their 70th birthdays. & Leonard F. Lindoy [email protected] & Feng Li [email protected] 1

School of Science and Health, University of Western Sydney, Locked Bag 1797, Penrith, NSW 2751, Australia

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School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia

In pioneering work, Raymond et al. rationally designed the bifunctional ligand 1 based on a phosphine-substituted catechol framework incorporating both hard (phenoxy) donors and a ‘soft’ (phosphine) donor (see Fig. 1) [11]. It was demonstrated that this ligand reacts with the ‘hard’ metal ions Ti(IV) or Sn(IV) under basic condition to produce the metalloligand 2 in which three catecholato units coordinate to the metal in an octahedral fashion, leaving the phosphine group free to participate in trans square planar coordination with PdBr2(PhCN)2 to yield the corresponding discrete cage 3. The structures of the Ti(IV) and Sn(IV)-containing cages were confirmed by X-ray analyses; each is a (achiral) triple mesocate exhibiting C3h symmetry, with the structures of the Ti(IV) and Sn(IV) derivatives shown in Fig. 2. In these cage systems both square-planar and octahedral metal sites are employed to define the overall final cage topology. A similar approach was also subsequently

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Fig. 1 Formation of the cage 3 from reaction of two molecules of the metalloligand 2 and three PdBr2 units [11]

Fig. 2 X-ray structures of a [Ti2L6Pd3Br6]4- (where H2L = 1) and b [Sn2L6Pd3Br6]4- (where H2L = 1); hydrogen atoms, counter ions and solvent molecules are omitted. Ti and Sn are silver, O is red, Br is dark red, P is orange and Pd is cyan [11]. (Color figure online)

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J Incl Phenom Macrocycl Chem (2015) 82:3–12

employed by other groups, with the results of these latter studies discussed below. In a related investigation based on 4-(3-pyridyl)catechol 4 (see Fig. 3), it was demonstrated that this bifunctional ligand reacts with TiO(acac)2 (acac = acetylacetonato) in the presence of strong base to form the tris-catecholato derivative 5 [12]. This species in turn reacts with PdCl2 (MeCN)2 to yield the corresponding cage 6. It was also shown that in the presence of TiO(acac)2, the cage exists in a (pH-influenced) dynamic equilibrium with a (TiII)2/ (PdII)2-metallacyclic ring species in which each octahedral Ti(IV) centre is bound to two catecholate ligand domains as well as to an acetylacetonato ligand. In a subsequent study, the group extended this chemistry to prepare other related cage derivatives generated by site-selective ligand exchange reactions [13]. In another study, the bifunctional fully-conjugated ligand 1-(4-pyridyl)butane-1,3-dione 7, incorporating an acetylacetone site was synthesised [14]. Following deprotonation, the resulting acetylacetonato group is suitable for coordination to a hard metal, while the softer pyridyl site is again available for coordination to a second (softer) metal. This potentially bifunctional ligand was then employed for the synthesis of the tris-ligand complex, [AlL3] 8 (where HL = 7). Two equivalents of this metalloligand react with three equivalents of ZnBr2 to yield the trigonal bipyramidal cage 9 (Fig. 4). In this structure, three Zn(II) ions occupy the triangular equatorial plane of the cage while an Al(III) centre is present in each axial position. Interestingly, both Al(III) sites have the same chirality within the one cage molecule, with both chiral forms existing in the crystalline state; that is, single crystals are racemic. As discussed later, the metalloligand ligand approach has also led to a range of more complex metallocages than the five-metal, trigonal bipyramidal systems discussed so far. In fact, when Pd(en)(NO3)2 (en = ethylenediamine) was substituted for ZnBr2 in the above reaction such a product forms; the bidentate ethylenediamine ligand is replaced by two pyridyl groups from two metalloligand ligands such that the larger cage, [Pd6(AlL3)8](NO3)12 (where HL = 7) 10, is generated. This product displays a distorted face-centred cubic geometry (Fig. 4). This selfassembled nanoscale structure consists of six Pd(II), eight Al(III) and twenty four 1-(4-pyridyl)butane-1,3-dionato ligands, with the Al(III) ions occupying the corners of the cube and Pd(II) ions capping each face. The different observed behaviour on substituting the Pd(II) connector for the Zn(II) connector in the above study, at least in part, seems likely to reflect the different coordination angles present in the respective bridges (distorted tetrahedral in the Zn(II) species to near orthogonal in the Pd(II) species) [15].


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Fig. 3 Synthesis of the Ti2Pd3 cage 6 via the threefold metalloligand 5 [12]

Fig. 4 Synthesis of the metalloligand and its cage derivatives. Al(III) centres are pink, Zn(II) centres are cyan, Pd(II) centres are brown [14]. (Color figure online)

The self-assembly of the trigonally preorganised Al(III)containing metalloligand 8 and its Ga(III) analogue 11 with the difunctional connector 12 has been reported by Stang et al. to form the corresponding trigonal bipyramidal [Pt(PEt3)2]3(metalloligand)2 cages (where metalloligand = 8 or 11) [16]. The formation of the respective

products is represented schematically Fig. 5. The synthesis involves the 2:3 assembly of 8 or 11 with 12 with replacement of the latter’s triflate groups by pyridyl nitrogens. No X-ray structures were obtained but the products were characterised using 1H and 31P NMR as well as high resolution mass spectrometry and the proposed trigonal

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Fig. 5 Schematic diagram of the formation of the trigonal bipyramidal cage systems derived from two metalloligands of type 8 or 11 and three connecting units of type 12 [16]

bipyramidal structures were simulated using molecular mechanics (MM2). Other related cages, incorporating diplatinum-organometallic connectors were also obtained in this study. As with many of the systems discussed so far, these cages appear to represent the smallest, least strained discrete assemblies that can form if each metal centre is to achieve its most common coordination number and geometry. In a related study, 3-(pyridin-4-ylmethyl)acetylacetonato) 13, a structural isomer of 7, was employed to form the corresponding neutral AlL3H2O complex 14 whose crystal structure (Fig. 6) once again shows that the pendant pyridyl arms are arranged in a diverging tripodal manner in which the nitrogen donors are too separated to bond simultaneously to a single metal centre [17]. The results from spectrophotometric and conductimetric titrations in which Cu(ClO4)2 solution was progressively added to a solution of [AlL3] in acetonitrile were in accord with the sequential formation of species with Cu:Al ratios of 1:4 and 3:4. The first of these was assigned to be a [Cu(AlL3)4]2? species in which one pyridyl nitrogen from each L is bound to a Cu(II) ion to yield a Cu(NPy)4 coordination geometry. The 3:4 species was proposed to be a [Cu6(AlL3)8]12? species and was assigned a symmetrical structure composed of eight triply bridging [AlL3] metalloligands and six square-planar Cu(II) tetrapyridyl units, with the Cu(II) centres forming an octahedral array. Parallel experiments in which Pd(NO3)2 was substituted for Cu(ClO4)2 also gave evidence for the formation of the analogous species [Pd6(AlL3)8]12?. Unfortunately suitable

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Fig. 6 X-ray structure of the metalloligand, [AlL3]H2O 14 where L = 13; the water solvate molecule is not shown [17]

crystals for X-ray diffraction studies were not able to be grown for either complex and hence no confirmation of their solid state structures was obtained. A further sub-category of three-fold metalloligands is derived from a single covalent tripodal ligand system incorporating primary and secondary metal coordination sites. On uptake of a metal ion the primary metal binding site results in a fixed (or semi-fixed) orientation of the secondary sites such that they are mutually divergent and suitably arranged for coordination to three different metals. This is illustrated by the pyrazol-1-yl/pyridyl ligand system 15 [18] shown in Fig. 7 where coordination of a Cu(I) ion in the primary binding site of 15 (to yield metalloligand 16) results in the coordination vectors of the pyridyl nitrogens (secondary donors) diverging such that binding to three different metal sites is promoted [19]. Further reaction of this preformed semi-rigid metalloligand 16 with Fe(NCS)2 in a CH2Cl2/CH3CN solvent mixture was shown to result in formation of the ‘‘nanoball’’ 17 with composition [{CuI(L)(CH3CN)}8{FeII(NCS)2}3.33 {FeII(NCS)-(CH3CN)}2.66](ClO4)2.66(CH3CN)n (L = 15). In this species the octahedral arrangement of the Fe(II) centres and the cubic arrangement of the Cu(I) centres results in an overall distorted rhombododecahedral geometry. This impressive metal-directed assembly reaction takes place in near quantitative yield over a 24-hour period, with the three-nanometre diameter product displaying temperature, light and guest induced magnetic switching behaviour.


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Fig. 7 Synthesis and X-ray structure of the cationic 8 CuI/6 FeII nanocage 17 (see text) derived from the metalloligand 16 and Fe(NCS)2. Iron centres are red, copper centres are pink [19]. (Color figure online)

The generality of the above synthesis was subsequently demonstrated by the same group through the preparation of new analogues of the above nanoball incorporating the following Cu(I)/divalent metal ion/anion combinations: Cu(I)/Cu(II)/ ClO4, Cu(I)/Cu(II)/BF4, Cu(I)/Zn(II)/ClO4, Cu(I)/Cd(II)/ClO4, Cu(I)/Fe(II)/ClO4 and Cu(I)/Mn(II)/ClO4 [20]. In this study it was shown that the properties of these new heteronuclear derivatives include solvent and gas (hydrogen) uptake. In a further study, it was anticipated that the low-spin Fe(III) anionic complex fac-[(Tp-)Fe(CN)3]- 18 [where Tp- = hydrotris(pyrazolyl)borate] would act as a metalloligand towards three metal centres to yield the corresponding cyanide-bridged species [21]. Indeed, reaction of 18 with Cu(II) perchlorate in acetonitrile-ethanol followed by diffusion of diethyl ether vapour into the solution yield ed crystals of ðTp Þ8 ðH2 OÞ6 CuII6 FeIII 8 ðCNÞ24 ðClO4 Þ4 12H2 O 2Et2 O: The X-ray structure showed that the heteronuclear cation approximates a face-centred cube in which eight Fe(III) ions, capped by Tp- ligands, occupy the

eight corners of the cube and are linked via bridging CNgroups to the six Cu(II) ions which are positioned just above the centre of each face of the cube (see 19 in Fig. 8). That is, metalloligand 18 uses its three cyanide groups to bridge Cu(II) sites, each of which also bind to a water molecule to yield a square pyramidal [Cu(CN)4(H2O)]2- coordination arrangement. Overall, the metallosupramolecular product incorporates a total of 14 paramagnetic metal sites and also displays single-magnet behaviour. The one-pot reaction of 4-formylimidazole with tris(2aminoethyl)amine in the presence of FeCl3 in a 3:1:1 ratio followed by addition of base yields the neutral Fe(III) metalloligand 20 whose structure is shown in Fig. 9 [22]. In this species the free imidazole nitrogen on each arm is again oriented in a divergent tripodal fashion. Reaction of 20 with Cu(II) perchlorate led to dark blue crystals of a discrete cage of composition [(CuII)6(FeIII)8L8](ClO4)12 (solvent)x. The X-ray structure of the heteronuclear polyhedral cage cation, 21, in this species is shown in Fig. 9.

Fig. 8 Assembly of the distorted face-centred cubic cationic cage from metalloligand 18 and Cu(II) perchlorate. Iron centres are purple, copper centres are green [21]. (Color figure online)

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Fig. 9 Synthesis of the discrete [(CuII)6(FeIII)8L8]12? cage derived from the uncharged metalloligand 20; anions and solvent molecules are not shown [22]

Each of the eight Fe(III) centres in 21 has an octahedral tris(imine-imidazolate) coordination sphere. Twenty-four Cu–N(imidazolate) bridges are present to form six distorted square pyramidal Cu(II) centres incorporating either bound water or ˚. DMF as axial ligands. The diagonal Fe–Fe distance is 14.4 A The Fe(III) centres in each molecule have the same chirality, with both enantiomers being present in equal amounts in each crystal so that, overall, the bulk sample is racemic. In many of the heteronuclear cage systems discussed so far, the hardness/softness concept [23] has played a definitive role in differentiating the positions of the heterometal centres in the final cage structure. However, using another strategy, Ward et al. [24] utilised the kinetic inertness of the fac and mer isomers of the octahedral species [RuIIL3](PF6)2, where L is the potentially bis-bidentate ligand 22, to achieve a similar outcome. Initially a 1:3 mixture of the fac- and mer-forms of the above complex was obtained using a two-step ligand reaction around a Ru(II) core. These kinetically inert Ru(II) complexes together with labile Cd(II) ions were then employed for the rational synthesis of a new heteronuclear [Ru4Cd4L12]16? cube-like cage in which the Ru(II) and Cd(II) ions alternately occupy different corners of the cube structure (Fig. 10). During assembly, the tris-chelate Ru(II) precursor species can be considered to define four of the vertices while the Cd(II) ions react with the pendent binding

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Fig. 10 Alternate arrangement of the Ru(II) and Cd(II) metal centres in the assembled Ru4Cd4L12 cube (where L is the potentially bisbidentate ligand 22) [24]


Fig. 11 Schematic diagram showing the mode of bridging of the hetero-metal sites by the bis-bidentate ligand 22. Ruthenium centres are orange, silver centres are grey [24]. (Color figure online)

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Fig. 12 X-ray structure of the cationic metalloligand, fac-[RuL3]2?, L = 23, with each ligand coloured differently for clarity [25]

domains to complete the cubic assembly (without scrambling of the two different metals present). In the resulting cage, the bis-bidentate ligand 22 spans each of the 12 edges (see Fig. 11 for the mode of bridging) such that both metal ion types achieve a tris(pyrazolyl-pyridyl) coordination geometry. The two metal centres at each end of the long diagonal have a fac-tris(pyrazolyl-pyridyl) coordination geometry whereas the remaining six metal centres have mer-tris(pyrazolyl-pyridyl) geometries. Thus, each metal type occupies one fac and three mer sites. This fac:mer ratio that occurs in the cage thus conveniently equates with the 1:3 ratio obtained in the initial synthesis of the trischelate Ru(II) precursor. Ward et al. [25] subsequently prepared pure fac[RuL3](PF6)2 for use as a metalloligand, (where L = 23 corresponds to the 1,4-phenylene linked analogue of 22).

Fig. 13 X-ray structure of the complex cation in [(RuL3)4Ag6] (PF6)14 (where L = 23). A and B indicate pyrazolyl-pyridine and phenyl units that are involved in p-stacking interactions [25]. Ag(I) is silver, Ru(II) is orange. (Color figure online)

The fac-[RuL3]2? cation again incorporates three pendant pyrazolyl-pyridine units, each suitably oriented for further coordination as bidentate chelates to different metal ion centres; the X-ray structure of this metalloligand is given in Fig. 12. The above metalloligand reacts with AgPF6 to yield an unusual heteronuclear [Ru4Ag6L12]14? cage which contains fac-octahedral, tris-chelate Ru(II) vertices and pseudo-tetrahedral Ag(I) bis-chelate edges to yield an

adamantane-like cage structure that displays tetrahedral symmetry. The X-ray structure of the above complex cation is given in Fig. 13. Clearly, the topology of the cage formed in this case depends on both the coordination preference of the Ag(I) ion coupled with the sole use of the fac-geometric isomer of [RuL3]2? as the metalloligand. Nitschke et al. have employed four-fold metalloligands as building blocks for the assembly of a heterometallic cubic cages [26, 27]. In the first of these studies [26], the square-planar Pt(II) complex 24 was initially prepared. In situ Schiff base condensation of this tetra-anilino derivative with 2-formyl pyridine in a 4:1 ratio in the

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presence of Fe(II) triflate yielded a product of type [Fe8Pt6L24]36?. This stoichiometry was expected since it was envisaged that the four-fold, square-planar symmetry of the Pt(II) precursor complex combined with the three-fold symmetry of the tris(pyridylimine)iron(II) moiety would promote formation of a cubic cage in which a Pt(II) complex occupies each of the six faces of the cube, while the eight corners are occupied by eight tris(pyridylimine)iron(II) moieties. Remarkably, for this system the same cage product was also generated in a one-pot synthesis starting from the respective sub-components. The ‘self sorting’ in this case involves the spontaneous assembly of 62 individual components. Unfortunately no crystal structure was able to be obtained of the assembled cage, but the assigned cubic structure was modelled using molecular mechanics.

Fig. 15 X-ray structure of the symmetric face-capped [Mo12Fe8L12]16? cage incorporating six dimolybdenum paddle wheels. Anions and solvents not shown [27]

In their subsequent study, Nitschke et al. [27] employed a related metalloligand approach to generate a further cubic Fig. 14 Schematic representation of the assembly of the cubic cage 27 via the four-fold paddle wheel metalloligand 26 [27]

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cage system—this time based on Mo(II) and Fe(II) metal centres. The reaction proceeds by the initial Schiff base condensation of 25 with four equivalents of 2-formylpyridine to yield the tetraimine paddle wheel structure 26 (Fig. 14). Further reaction of this product (six equivalents) and Fe(II) triflate (eight equivalents) results in assembly of the corresponding heteronuclear cubic cage of


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Fig. 16 Synthesis of the cationic dodecahedral cage {Pd6(RuL3)8]28? (30). Pd(II) is yellow, Ru(II) is orange [29]

stoichiometry [Mo12Fe8L12]16?; the role of one paddle wheel in forming the structure is illustrated schematically by 27. Crystals of the cage were grown from acetonitrile following slow ether diffusion into the solution. The X-ray structure (Fig. 15) revealed a symmetric face-capped cubic structure that, as expected, incorporates six dimolybdenum paddle wheels (Fig. 15). The Fe(II)–Fe(II) distance diagonally ˚ . The Fe(II) centres in each complex across the cage is 18.7 A ion are chiral, with both optical forms present in a single crystal so that the bulk material is racemic. The binding behaviour of this cage with a range of both neutral and anionic substrates has been explored in some detail [27, 28]. The stepwise assembly of a further [M6M0 8L8]28? cage has been reported by Li et al. [29]. In this case the moderately rigid ligand 28 was employed to synthesise the Ru(II)-containing octahedral complex 29 incorporating three peripheral 3-pyridyl groups. This trigonal species acts as a three-fold metalloligand towards Pd(II) to yield cages of the stoichiometry {Pd6(RuL3)8]X28(solvent)n (where X = BF4 or NO3 ]. In this case coordination of the pyridyl pendant groups to different square-planar Pd(II) ions occurs to give each of these centres a PdN4 coordination environment, resulting in formation of the cationic cage 30

whose X-ray structure is given in Fig. 16 and whose geometry is best described as distorted rhombododecahedral.

Final remarks There are now numerous reports of discrete metallo-cage systems that incorporate a single metal-ion type, often also involving a single ligand type [30, 31]. However, over recent years the construction of heteronuclear systems has also received increasing attention. In the design of such mixed metal systems commonly the individual metal ions were selected on the basis of their specific (different) properties, such as their different hardness or softness, preferred geometries or kinetic liability/inertness; these differences have enabled differentiation between the positions (and roles) of the respective metal centres in the final assembled cage structure. As outlined in the present discussion, the metalloligand approach has played a key role in the successful synthesis of a range of such new heteronuclear cage systems. The approach employs multifunctional building blocks with predetermined, well-defined electronic and structural preferences that clearly facilitate the rational design and

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construction of heteronuclear cages exhibiting a range of defined topologies, with often two different ligand types being present in the generated structure. Clearly, the additional complexity arising from the construction of both hetero-metal and hetero-ligand cage systems provides a continuing motivation for using the metalloligand approach to obtain new cage-like materials displaying unusual (and potentially useful) properties. Acknowledgments The authors thank the University of Western Sydney (UWS) for support.


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