dalton 4 , 2 ’ : 6 ’ , 4 ’ ’-Terpyridines : diverging and diverse building blocks in coordination polymers and metallomacrocycles

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 d(10) 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.


Introduction
6][7] At this point, it is prudent to note the debate surrounding relevant terminology, viz.coordination polymer versus metal-organic framework (MOF). 8In this perspective review, the infinite structures described are termed coordination polymers, irrespective of their dimensionality.
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 Database 13 (CSD version 5.35) using Conquest v. 1.15 14 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.

The metal-based node
The d-block contains 30 metals with oxidation states and associated electronic configurations that dictate chemistry and coordination geometry of the metal ions.Our own studies of the assembly of coordination polymers tend to favour the use of d 5 or d 10 metal ions which are electronically spherically symmetric.The geometrical flexibility of d 5 or d 10 metal centres allows the coordination sphere to respond to environmental influences such as crystal packing interactions.For example, the angles within a 'tetrahedral' zinc(II) centre may lie well outside an ideal 109.5°.In a coordination polymer, distortions within the local coordination environment of the metal ion are transmitted through bridging ligands, and in the next section, we explore how this manifests itself in the pitch of a helical assembly.
In addition to its geometrical flexibility, zinc(II) has a propensity to associate with carboxylate anions to form {Zn 2 (μ-O 2 CR) 4 } motifs with 'paddle-wheel' structures (Scheme 3).][17] In this review, we consider examples of the in situ assembly of discrete {Zn 2 (μ-OAc) 4 } units, and association with bridging ligands into vacant coordination sites (Scheme 3) to give onedimensional 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. 15,17As Scheme 3 illustrates, a {Zn 2 (μ-O 2 CR) 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 Scheme 1 Isomer dependence of the directional metal-binding properties of bipyridine.
following discussion.We will also comment on the effects of moving from first to third row d 10 metals which introduces larger metal ions that can accommodate higher coordination numbers.
Table 1 summarizes the current status of one-dimensional [ZnCl 2 (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. 19,25The 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 Zn 2+ 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-[ZnCl 2 (4′-(4-MeC 6 H 4 )-4,2′:6′,4″-tpy)] n is unique among the polymers in Table 1; it crystallizes in the P3 1 21 space group with the helical chain generated by a 3 1 -screw axis.Each turn in the helix contains three {ZnY 2 (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 {ZnY 2 (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.
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 ZnCl 2 in the presence of C 60 led to the host-guest complex shown in Fig. 9.Each C 60 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 C 60 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. 28he rigidity of the tubes formed by interlocking of [{ZnY 2 (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.
In contrast to the structural variation of the helical polymers in Table 1, [M 2 (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   2 shows that the unit cell dimensions of all coordination polymers are comparable.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 [Zn 2 (μ-OAc) 4 (4′-(4-BrC 6 H 4 )-4,2′:6′,4″-tpy)] n . 30igzag 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.

A tendency to cluster
We have seen how the tendency for zinc(II) and copper(II) acetate to form paddle-wheel motifs dictates the linear relationship of the coordinated N-donors.2 X-Ray powder diffraction data for the bulk sample reveal that the polymer containing pentanuclear {Zn 5 (OAc) 10 } nodes is the dominant product.This polymer (Fig. 11) is significant for a number of reasons: (i) the 5 : 4 ratio of zinc atoms : tpy ligands which leads to a highly unusual network (Fig. 11a), (ii) the structure of the {Zn 5 (OAc) 10 } unit which, to the best of our knowledge, is unprecedented, and (iii) the assembly of a quadruple-stranded chain (Fig. 11b) that is a 'deep' version of the single-stranded chains exhibited by {Zn 2 (μ-OAc) 4 }-containing coordination polymers (Table 2).These 'deep' chains are stabilized by intrachain face-to-face stacking of pentafluorobiphenyl domains (Fig. 11b) and pack in an analogous manner to the singlestranded chains shown in Fig. 10.
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 {Cd 2 (OAc) 4 } and {Mn 3 (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 {Zn 5 (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.

Conclusions
This perspective review has considered a number of pertinent aspects of the coordination chemistry of the divergent 4,2′:6′,4″-terpyridine ligand.The ready functionalization of 4,2′:6′,4″-tpy in the 4′-position allows one to synthesize a range of ligands in which the 4′-substituent can be selected to assist in directing the metal-ligand coordination process.Combined with zinc(II) halides, 4′-X-4,2′:6′,4″-tpy ligands show a tendency to form one-dimensional coordination polymers which are often built up along a crystallographic screw axis.The pitch of the helical chains is variable and the factors that control this and the homo-or heterochiral packing of helical chains has  2).been discussed.Introducing extended aryl domains in the 4′-position facilitates the formation of metallohexacycles when 4′-(arene)-4,2′:6′,4″-tpy ligands react with ZnCl 2 or ZnBr 2 .This preference over polymer formation appears to driven by the interlocking of metallocycles through π-stacking which produces robust tube-like structures in the crystal lattice.The large void space in these tubes equips them to act as host materials.
Switching to metal(II) acetates in place of halides results in [Zn 2 (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 [Zn 2 (μ-OAc) 4 -(4′-X-4,2′:6′,4″-tpy)] n .

Fig. 10
Fig. 10 Packing of zigzag chains in [Zn 2 (μ-OAc) 4 (4'-(4-BrC 6 H 4 )-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 Table2).