Catherine E.
Housecroft
Department of Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland. E-mail: catherine.housecroft@unibas.ch
First published on 13th February 2014
4,2′:6′,4′′-Terpyridine (4,2′:6′,4′′-tpy) is one of the less well documented isomers of the well-established bis-chelating 2,2′:6′,2′′-terpyridine. The N-donors of the outer rings in 4,2′:6′,4′′-tpy subtend an angle of 120°, leading to a description of 4,2′:6′,4′′-tpy as a divergent ligand. Because it typically binds metal ions through the outer N-donors only, 4,2′:6′,4′′-tpy is an ideal linker for combination with metal nodes (often geometrically flexible d10 ions) in coordination polymers and metallomacrocyclic complexes. The facile functionalization of terpyridines in the 4′-position allows access to a suite of 4′-X-4,2′:6′,4′′-tpy ligands in which the 4′-substituent, X, can be selected to assist in directing the metal–ligand assembly process. This overview of recent advances in the chemistry of 4,2′:6′,4′′-tpy and its 4′-substituted derivatives looks at relationships within a series of chiral polymers, competition between the formation of metallocyclic complexes versus polymers, and the use of extended aryl systems to encourage the formation of coordination polymers in which π-stacking of arene domains dominates in the assembly process. Use of metal(II) acetates is key to the formation of paddle-wheel and larger cluster nodes that direct the assembly of both predetermined and unexpected architectures.
Oligopyridines9 are among the most popular and versatile of ligands. The simplest, bipyridine, possesses six isomers with varying directional properties (Scheme 1). 2,2′-Bipyridine (2,2′-bpy or bpy) is tailor-made to act as a chelating ligand, whereas the 4,4′-isomer is commonly used as a rigid, bridging linker in multinuclear complexes and coordination polymers. In contrast, the coordinating abilities of 2,3′-, 2,4′-, 3,3′- and 3,4′-bpy are less well investigated.1 Terpyridine possesses 48 isomers, of which the bis-chelating 2,2′:6′,2′′-terpyridine (tpy) is the best known. Synthetic routes to terpyridines allow tpy to be readily functionalized in the 4′-position. With the correct choice of substituent X, octahedral {M(4′-Xtpy)2} complexes (Scheme 2) are ideal building blocks for the assembly of coordination polymers.10,11 Whereas tpy is typically a ‘convergent’ ligand (i.e. the chelate effect favours a cis,cis-conformation when tpy binds a metal ion), 4,2′:6′,4′′-tpy presents a divergent set of donor atoms. Rotation about the inter-ring bonds (in red in Scheme 2) has no effect of the vectorial properties of the N,N′-donor set of 4,2′:6′,4′′-tpy. The ligand typically binds metal ions through the outer two N-donors, leaving the central donor uncoordinated. It is, therefore, highly suitable as a building block for coordination polymers.
This article focuses on systematic approaches to the assembly of coordination polymers built upon 4,2′:6′,4′′-tpy, and the structural diversity achieved through (i) functionalizing the ligand and (ii) varying the metal-containing domains (nodes). Underlying much of the discussion is the tenet that coordination polymer assembly is a matter of complementarity between the coordination requirements (geometry) of a metal centre and the spatial properties, coordinating ability and packing potential of the linking ligand. The first report of a coordination polymer involving 4,2′:6′,4′′-tpy appeared in 1998,12 and a search of the Cambridge Structural Database13 (CSD version 5.35) using Conquest v. 1.1514 reveals 62 coordination polymers and networks in which 4,2′:6′,4′′-tpy or 4′-X-4,2′:6′,4′′-tpy (X = various substituents) ligands bridge between two (or three if X is a donor such as a pyridyl substituent) metal centres. This is not a comprehensive account of these 62 structures, rather, in keeping with the spirit of a Dalton Transactions perspective review, it is a discussion of significant aspects arising from observations of the assembly processes using these divergent ligands.
In addition to its geometrical flexibility, zinc(II) has a propensity to associate with carboxylate anions to form {Zn2(μ-O2CR)4} motifs with ‘paddle-wheel’ structures (Scheme 3). These and related building blocks are popular choices as nodes in coordination polymers and MOFs because they impart directional control on the assembly process.15–17 In this review, we consider examples of the in situ assembly of discrete {Zn2(μ-OAc)4} units, and association with bridging ligands into vacant coordination sites (Scheme 3) to give one-dimensional polymers. This contrasts with the use of organic linkers bearing, typically, two terminal carboxylate groups which become an integral part of the paddle-wheel to generate three-dimensional MOFs.15,17 As Scheme 3 illustrates, a {Zn2(μ-O2CR)4} unit necessarily binds axial ligands that are disposed linearly with respect to one another. The consequences of choosing zinc(II) acetate versus zinc(II) halides for combination with 4,2′:6′,4′′-tpy ligands will become apparent in the following discussion. We will also comment on the effects of moving from first to third row d10 metals which introduces larger metal ions that can accommodate higher coordination numbers.
Table 1 summarizes the current status of one-dimensional [ZnCl2(4′-X-4,2′:6′,4′′-tpy)]n helical coordination polymers. Note that most structures are free of solvent of crystallization (see footnote to Table 1). Typically, crystallization results in an equal number of right-handed (P) and left-handed (M) helices in the same lattice, i.e. a rac- or heterochiral polymer, which is to be distinguished from a racemic conglomerate (a mixture of crystals, each of which contains one enantiomer). Table 1 lists two homochiral polymers, and for both, the corresponding heterochiral polymers have also isolated.19,25 The distance between the outer N-donors in 4,2′:6′,4′′-tpy is independent of whether the ligand is planar or twisted about the C–C bonds marked in red in Scheme 2, and so the distance between adjacent Zn2+ ions along a chain shows little variation (12.379(2) to 13.207(2) Å). However, the pitch of the helix is noticeably variable. M-[ZnCl2(4′-(4-MeC6H4)-4,2′:6′,4′′-tpy)]n is unique among the polymers in Table 1; it crystallizes in the P3121 space group with the helical chain generated by a 31-screw axis. Each turn in the helix contains three {ZnY2(tpy)} units and the helical pitch of 32.414(5) Å is significantly longer than those of the remaining polymers, each of which contains two {ZnY2(tpy)} units per helical-turn. For the latter, the data in Table 1 and Fig. 1 confirm a general relationship between the helical pitch and the N–Zn–N bond angle, and we consider below how this variation is associated with crystal packing.
Fig. 1 Scatter plot of N–Zn–N angle against helical pitch of the one-dimensional coordination polymers in Table 1 which contain two {ZnY2(tpy)} units per helical-turn; Y = Cl, black; Y = I, blue; Y = monodentate OAc, red. |
Coordination polymera | Space group | Number of {ZnY2(tpy)} units per turn | Angle N–Zn–N/° | Pitch of helixb (Zn⋯Zn/Å) | CSD refcode | Ref. |
---|---|---|---|---|---|---|
a Solvent molecules are omitted from formulae of NOGFOF, FAKRUG and the 4′-(4-anthracen-9-yl-C6H4) derivatives. b Pitch is measured from one Zn atom to the next which is the point the helix maps back onto itself. c Compound not yet entered in CSD v. 5.35. | ||||||
Heterochiral polymers | ||||||
rac-[ZnCl2(4,2′:6′,4′′-tpy)]n | P21/n | 2 | 98.6(2) | 14.078(2) | GAQYUS | 12 |
rac-[ZnCl2(4′-(4-MeC6H4)-4,2′:6′,4′′-tpy)]n | P21/n | 2 | 99.5(1) | 16.950(2) | LOCTED | 19 |
rac-[ZnCl2(4′-(4-EtC6H4)-4,2′:6′,4′′-tpy)]n | P21/n | 2 | 93.5(1) | 7.790(2) | NOGFOF | 19 |
rac-[ZnCl2(4′-(4-pyridyl)-4,2′:6′,4′′-tpy)]n | P21/n | 2 | 108.3(1) | 20.661(3) | AGUPEY | 23 |
rac-[ZnCl2(4′-(4-nC8H17OC6H4)-4,2′:6′,4′′-tpy)]n | P21/c | 2 | 104.0(1) | 19.3139(7) | AJURIG | 20 |
rac-[Zn(OAc)2(4′-Ph-4,2′:6′,4′′-tpy)]n | P21/n | 2 | 106.99(5) | 19.653(4) | CUXDOP | 24 |
rac-[Zn(OAc)2(4′-(4-anthracen-9-yl-C6H4)-4,2′:6′,4′′-tpy)]n | Pnna | 2 | 114.29(8) | 20.893(3) | 25 | |
rac-[ZnCl2(4′-Ph-4,2′:6′,4′′-tpy)]n | P21/n | 2 | 105.9(1) | 19.301(2) | FEPRUO | 18 |
rac-[ZnI2(4′-tBu-4,2′:6′,4′′-tpy)]n | P21/c | 2 | 105.0(1) | 18.158(1) | FAKRUG | 26 |
Homochiral polymers | ||||||
M-[ZnCl2(4′-(4-MeC6H4)-4,2′:6′,4′′-tpy)]n | P3121 | 3 | 100.3(2), 107.9 | 32.414(5) | NOGFIZ | 19 |
M-[Zn(OAc)2(4′-(4-anthracen-9-yl-C6H4)-4,2′:6′,4′′-tpy)]n | P212121 | 2 | 104.5(3) | 17.979(1) | 25 |
Of the polymers in Table 1, rac-[ZnCl2(4′-(4-EtC6H4)-4,2′:6′,4′′-tpy)]n stands out as possessing a very short helical pitch (7.790(2) Å).19 This follows from the interdigitation of adjacent chains of the same handedness. Fig. 2a views part of the lattice down the b-axis, parallel to which the helical chains run. Helices of a given chirality are interlocked, generating infinite pillars of π-stacked phenylpyridine domains of adjacent P- (or M-) chains (Fig. 2a and 2b). The lattice consists of alternating homochiral layers of P- or M-helical polymers, each layer lying parallel to the ab-plane. The factors that underlie homochiral versus heterochiral packing fascinate and challenge researchers,27 and it is noteworthy that the racemic polymers in the [ZnY2(4′-X-4,2′:6′,4′′-tpy)]n family (Table 1) include both those with homochiral layers (noted by Li and coworkers in 2008 as ‘very rare’19) and those in which P- and M-helices are always adjacent to one another. However, there appears to be no obvious trend that links the 4′-substituent to the mode of packing. In rac-[ZnCl2(4,2′:6′,4′′-tpy)]n (no 4′-substituent),12P- (or M-) helices associate through π-stacking between pyridine rings in adjacent chains of the same handedness forming homochiral two-dimensional sheets (Fig. 3a). A similar assembly is observed in rac-[ZnCl2(4′-(4-nC8H17OC6H4)-4,2′:6′,4′′-tpy)]n (Fig. 3b).20 In this case, the 4′-substituent is a long alkyl chain and CHalkyl chain⋯πphenyl and CHalkyl chain⋯πpyridine are dominant packing interactions within each homochiral sheet; the packing of polymer chains of the same handedness is illustrated in Fig. 3c.
The presence of homochiral sheets in the lattice is predicated upon a recognition event occurring between a helical chain of a given chirality with an adjacent chain of the same chirality. This occurs in three of the nine racemates in Table 1. In the remaining six, packing interactions occur only between P- and M-helices. Adjacent P- and M-chains in rac-[ZnCl2(4′-(4-MeC6H4)-4,2′:6′,4′′-tpy)]n engage in face-to-face π-interactions between pyridine rings (Fig. 4a); Li and coworkers comment that ‘the whole crystal presents a heterochiral packing’.19 Analogous behaviour is observed in rac-[ZnCl2(4′-(4-pyridyl)-4,2′:6′,4′′-tpy)]n (Fig. 4b).23 In rac-[ZnCl2(4′-Ph-4,2′:6′,4′′-tpy)]n18 and rac-[Zn(OAc)2(4′-Ph-4,2′:6′,4′′-tpy)]n,24P- and M-chains associate through π-stacking of tpy domains in adjacent helices. The change from chlorido to acetato ligands in the latter two polymers perturbs the structure only slightly. Note that each of the above four compounds has a similar 4′-aromatic substituent (phenyl, tolyl or pyridyl) and the polymers pack in similar, heterochiral fashions; each crystallizes with no solvent in the lattice. In rac-[ZnI2(4′-tBu-4,2′:6′,4′′-tpy)·1,2-Cl2C6H4]n, π-stacking between pyridine rings of tpy domains in adjacent P- and M-helices occurs (Fig. 5), interconnecting the chains throughout the lattice. The aromatic solvent molecules are intimately involved in π-stacking in the lattice, extending each double stack in Fig. 5 to a quadruple-decker arrangement.26
Fig. 5 Interlocking of P- and M-helices in rac-[ZnI2(4′-tBu-4,2′:6′,4′′-tpy)]n; face-to-face interactions of tpy domains are shown in space-filling representations. |
We have recently been examining the effects of introducing extended arene domains in the 4′-position of 4,2′:6′,4′′-tpy, with the supposition that enhancement of arene⋯arene π-stacking interactions should provide some degree of control over crystallization events. The isolation of crystals of both homo- and heterochiral [Zn(OAc)2(4′-(4-anthracen-9-yl-C6H4)-4,2′:6′,4′′-tpy)]n from the same crystallization tube25 illustrates how unpredictable the crystallization process can be. Serendipity is ever present, and we are a long way from being to drive these systems in a particular direction. The helical pitch in rac-[Zn(OAc)2(4′-(4-anthracen-9-yl-C6H4)-4,2′:6′,4′′-tpy)]n is ≈3 Å longer than in the enantiomerically pure M-helix (Table 1). Fig. 6 illustrates the packing of chains in the hetero- and homochiral structures. In both, face-to-face π-stacking between tpy and anthracene domains is dominant, but a comparison of Fig. 6a and 6b reveals the distinct ways in which chains of opposite handedness and of the same handedness associate with one another in heterochiral and homochiral coordination polymers, respectively.
In [{ZnCl2(4′-HCCC6H4-4,2′:6′,4′′-tpy)}6], each 4,2′:6′,4′′-tpy unit bridges two ZnCl2 units, and each Zn2+ ion is tetrahedrally sited. The conformation of the ring may be described as chair-like, but it is useful to note that this corresponds to an up/up/up/down/down/down arrangement of ligands (Fig. 7a).21 This same arrangement is found for [{ZnCl2(4′-(pentafluorobiphenyl-4-yl)-4,2′:6′,4′′-tpy)}6] but, in this case, this is one of two observed conformations; in the second (the minor form), the ligands are in an alternating up/down/up/down/up/down arrangement around the ring (a ‘barrel-like’ conformation, Fig. 7b). Both conformations were observed in the same crystallization experiment, suggesting little energy difference between them. The barrel conformation is also adopted by [{ZnCl2(4′-(biphenyl-4-yl)-4,2′:6′,4′′-tpy)}6], [{ZnBr2(4′-(biphenyl-4-yl)-4,2′:6′,4′′-tpy)}6], [{ZnBr2(4′-(pentafluorobiphenyl-4-yl)-4,2′:6′,4′′-tpy)}6] and [{ZnBr2(4′-(naphthalen-1-yl)phenyl-4,2′:6′,4′′-tpy)}6], each crystallizing in the trigonal space group R.28 The extended arene domains are the key to extremely efficient packing of the barrels into tubes. The interlocking of metallohexacycles through π-interactions between pyridine and arene domains produces a robust architecture (Fig. 8) and face-to-face π-interactions between tpy units in adjacent tubes further stabilize the lattice.
The organization of the pendant arene moieties (phenyl, pentafluorophenyl or naphthyl groups) and the inner diameter of the tubes suggested to us that the assembly should be amenable to capturing guests such as fullerenes. Indeed, crystallization of 4′-(naphthalen-1-yl)phenyl-4,2′:6′,4′′-tpy with ZnCl2 in the presence of C60 led to the host–guest complex shown in Fig. 9. Each C60 guest is embraced by six naphthyl units (green in Fig. 9), and the whole domain lies at the centre of another hexamer (orange in Fig. 9). Highly efficient arene⋯arene π-interactions operate between layers of the onion-like construction. Two features are particularly remarkable about the structure: (i) the fullerene molecule is crystallographically ordered, and (ii) the lattice is an ordered array in which a C60 molecule occupies every second cavity, despite there being room on steric grounds for complete occupation of cavities. We have suggested that the latter observation is closely linked to the manner in which the overall structure is assembled.28
Fig. 9 In [2{ZnCl2(4′-(naphthalen-1-yl)phenyl-4,2′:6′,4′′-tpy)}6·C60]·6MeOH·16H2O, guest molecules of C60 occupy every other cavity in the metallohexacyclic host. |
The rigidity of the tubes formed by interlocking of [{ZnY2(4′-arene-4,2′:6′,4′′-tpy)}6] metallohexacycles suggests that this family of complexes offers a rich opportunity for further explorations of host–guest chemistry.
Scheme 5 (a) Schematic representation of a [Zn2(μ-O2CR)4(4′-X-4,2′:6′,4′′-tpy)]n polymer (Zn–Zn = Zn2(μ-O2CR)4 unit); (b) proposed metallohexacycle (see text). |
In contrast to the structural variation of the helical polymers in Table 1, [M2(μ-OAc)4(4′-X-4,2′:6′,4′′-tpy)]n coordination polymers known to date are structurally related, both in the zigzag backbone of the polymer chain and in the packing of the chains in the crystal lattice. Space groups and cell dimensions are compared in Table 2. With the exception of [Zn2(μ-OAc)4(4′-(4-MeSC6H4)-4,2′:6′,4′′-tpy)]n,30 all crystallize in the C2/c space group with one half of the 4,2′:6′,4′′-tpy ligand in the asymmetric unit and the second half generated by a C2 axis. As a consequence, the tbutyl group in [Zn2(μ-OAc)4(4′-tBu-4,2′:6′,4′′-tpy)]n, is necessarily disordered.26 In [Zn2(μ-OAc)4(4′-(4-MeSC6H4)-4,2′:6′,4′′-tpy)]n (P21/c), the presence of an ordered MeS group is incompatible with a C2 axis through the 4′-(4-MeSC6H4)-4,2′:6′,4′′-tpy ligand. Taking this distinction into account, Table 2 shows that the unit cell dimensions of all coordination polymers are comparable.
Coordination polymera | Space group | a, b, c/Å | β/° | Distance d/Å (defined in Fig. 10c) | CSD refcode | Ref. |
---|---|---|---|---|---|---|
a Solvent molecules are omitted from formulae of CUXDUV. b Compound not yet entered in CSD v. 5.35. | ||||||
[Zn2(μ-OAc)4(4′-Ph-4,2′:6′,4′′-tpy)]n | C2/c | 26.1802(6) | 107.449(2) | 12.495 | CUXDUV | 24 |
15.2942(6) | ||||||
8.0478(2) | ||||||
[Zn2(μ-OAc)4(4′-(4-BrC6H4)-4,2′:6′,4′′-tpy)]n | C2/c | 26.0368(8) | 107.238(2) | 12.268 | CUXCOO | 30 |
15.0774(5) | ||||||
8.0056(3) | ||||||
[Zn2(μ-OAc)4(4′-(4-MeSC6H4)-4,2′:6′,4′′-tpy)]n | P21/c | 8.1338(4) | 90.457(3) | 12.092 | CUXCUU | 30 |
14.9020(7) | ||||||
25.2700(10) | ||||||
[Zn2(μ-OAc)4(4′-(biphenyl-4-yl)-4,2′:6′,4′′-tpy)]n | C2/c | 26.210(5) | 108.050(14) | 13.347 | RIJFUN | 31 |
16.151(2) | ||||||
8.3410(5) | ||||||
[Cu2(μ-OAc)4(4′-(biphenyl-4-yl)-4,2′:6′,4′′-tpy)]n | C2/c | 26.0528(9) | 108.113(2) | 13.346 | RIGJAU | 31 |
16.1512(9) | ||||||
8.2267(3) | ||||||
[Cu2(μ-OAc)4(4′-(pentafluorobiphenyl-4-yl)-4,2′:6′,4′′-tpy)]n | C2/c | 26.5522(13) | 107.038(3) | 13.942 | 32 | |
16.7313(9) | ||||||
8.0639(4) | ||||||
[Cu2(μ-OAc)4(4′-(biphenyl-4-yl)-4,2′:6′,4′′-tpy)]n.[Cu2(μ-OAc)4(4′-(pentafluorobiphenyl-4-yl)-4,2′:6′,4′′-tpy)]n | C2/c | 26.366(3) | 107.648(6) | 13.602 | 32 | |
16.393(2) | ||||||
8.1433(9) | ||||||
[Zn2(μ-OAc)4(4′-tBu-4,2′:6′,4′′-tpy)]n | C2/c | 26.316(6) | 105.687(17) | 12.088 | FAKROA | 26 |
14.8918(19) | ||||||
8.0849(17) |
The structural relationship between the coordination polymers stems from the dominant face-to-face π-interactions between 4,2′:6′,4′′-tpy domains of adjacent chains. Fig. 10 illustrates this for [Zn2(μ-OAc)4(4′-(4-BrC6H4)-4,2′:6′,4′′-tpy)]n.30 Zigzag chains nest with one another to generate planar sheets (Fig. 10b) in which each 4-bromophenyl group in one chain is accommodated in the V-shaped cavity formed by a 4,2′:6′,4′′-tpy unit in the adjacent chain (Fig. 10a). As the space-filling representation in Fig. 10a suggests, this cavity is big enough to accommodate larger substituents. For example, biphenyl units can be accommodated without significant moving apart of the zigzag chains. This statement is quantified by measuring the distance d defined in Fig. 10c; values are listed in Table 2 and show only a relatively small variation.
Fig. 10 Packing of zigzag chains in [Zn2(μ-OAc)4(4′-(4-BrC6H4)-4,2′:6′,4′′-tpy)]n. (a) Zigzag chains nest with one another to give sheets (blue and orange), and tpy domains (one is represented by a ^) in one sheet stack over tpy domains in the next sheet. See text for discussion of the Br (brown) atoms. (b) The sheets are flat. (c) The distance between central pyridine rings in each sheet (defined as d) varies only slightly with substituent X (see Table 2). |
The presence of biphenyl domains in [Zn2(μ-OAc)4(4′-(biphenyl-4-yl)-4,2′:6′,4′′-tpy)]n and [Cu2(μ-OAc)4(4′-(biphenyl-4-yl)-4,2′:6′,4′′-tpy)]n leads to inter-chain biphenyl⋯biphenyl stacking31 which augments the tpy⋯tpy interactions between sheets. Interestingly, introducing fluoro-substituents into the terminal phenyl ring of the biphenyl unit has little impact on the packing of the chains. Thus, on going from [Cu2(μ-OAc)4(4′-(biphenyl-4-yl)-4,2′:6′,4′′-tpy)]n to [Cu2(μ-OAc)4(4′-(pentafluorobiphenyl-4-yl)-4,2′:6′,4′′-tpy)]n, tpy⋯tpy stacking interactions are maintained, arene⋯arene πH⋯πH interactions are replaced by πH⋯πF, and H⋯H contacts are replaced by H⋯F interactions.32
The complex [Zn2(μ-OAc)4(4′-Ph-4,2′:6′,4′′-tpy)]n is unique among the series in Table 2 because it crystallizes as a solvate, viz. [Zn2(μ-OAc)4(4′-Ph-4,2′:6′,4′′-tpy)]n·0.3CH2Cl2. It is noteworthy that the partial occupancy CH2Cl2 molecules are located in sites that coincide with those occupied by the para-substituents of the phenyl rings in [Zn2(μ-OAc)4(4′-(4-ZC6H4)-4,2′:6′,4′′-tpy)]n (Z = Br, MeS or Ph).32 Undoubtedly, there has to be a limiting point at which the structure can no longer withstand the steric demands of this substituent. Indeed, we have reported that the reaction of zinc(II) acetate with 4′-(4-dodecyloxyphenyl)-4,2′:6′,4′′-tpy leads, not to a polymer, but to the discrete molecule [Zn2(μ-OAc)4(4′-(4-dodecyloxyphenyl)-4,2′:6′,4′′-tpy)2].33 We are currently undertaking a systematic study to better understand how the structure type shown in Fig. 10 responds to increasingly larger substituents appended to the 4′-position of 4,2′:6′,4′′-tpy.
Multiple strands in one-dimensional coordination polymers containing 4,2′:6′,4′′-tpy ligands are not restricted to the example above. Double-stranded chains are observed in [Cd2(OAc)4(4′-(biphenyl-4-yl)-4,2′:6′,4′′-tpy)2]n31 (Fig. 12a). The {Cd2(OAc)4} node does not mimic the paddle-wheel of its isoelectronic {Zn2(OAc)4} counterpart, but instead adopts the planar structure shown in Fig. 12b; the larger size of the Cd2+ ion permits a higher coordination number with respect to Zn2+. Extension to a trinuclear node is exemplified by the planar {Mn3(OAc)6} unit containing manganese(II) (Fig. 12c). These nodes are interconnected by 4′-(4-BrC6H4)-4,2′:6′,4′′-tpy bridging ligands in [Mn3(OAc)6(4′-(4-BrC6H4)-4,2′:6′,4′′-tpy)3]n to generate a triple-stranded one-dimensional polymer (Fig. 12d). Both multiply-stranded zigzag chains exhibit intra- and inter-chain face-to-face π-stacking of tpy domains, and packing characteristics of the ‘deep’ chains mimic those of the simpler single-stranded systems.
There are clearly points that relate the structures of the single, double, triple and quadruple-stranded coordination polymers. On the one hand, it is a trivial task to explain how the single strands are propagated from paddle-wheel motifs that are predictably linear nodes (Scheme 3). It is also straightforward to understand how the planar {Cd2(OAc)4} and {Mn3(OAc)6} nodes bind divergent 4,2′:6′,4′′-tpy linkers to produce double and triple-stranded polymers, respectively. However, it is difficult to rationalize why the {Zn5(OAc)10} motifs bind four ligands to give the oblique arrangement shown in Fig. 11, rather than five to generate a parallel arrangement akin to the double and triple-stranded assemblies.
Although crystallization of Zn(OAc)2·2H2O with 4′-(naphthalen-1-yl)phenyl-4,2′:6′,4′′-tpy (Scheme 4) typically gives [Zn2(μ-OAc)4(4′-(naphthalen-1-yl)phenyl-4,2′:6′,4′′-tpy)]n, a few crystals of the homochiral polymer 2[{Zn7(μ-OAc)10(μ4-O)2(4′-(naphthalen-1-yl)phenyl-4,2′:6′,4′′-tpy)}n]·CH2Cl2 (Fig. 13) have also been isolated.25 The nodes in this one-dimensional chain are {Zn7(μ-OAc)10(μ4-O)2} clusters (Fig. 13a), a building block that also appears in several other coordination polymers.34 The reasons behind this assembly and why chiral resolution is observed remain unknown. However, these findings constitute a word of caution in terms of drawing in-depth conclusions based on single-point structure determinations, and emphasize the need for powder diffraction data for bulk samples.
Switching to metal(II) acetates in place of halides results in [Zn2(μ-OAc)4(4′-X-4,2′:6′,4′′-tpy)]n coordination polymers being prevalent; the latter possess zigzag backbones and assemble into flat sheets which interact through π-stacking to give efficiently packed structures that are typically solvent free unless substituent X is relatively small. The tendency for metal acetate cluster formation leads to the assembly of a number of unexpected coordination polymers, several of which exhibit multiply-stranded chains which retain key elements of the packing characteristics of the single-stranded [Zn2(μ-OAc)4(4′-X-4,2′:6′,4′′-tpy)]n.
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