Coordination-based assemblies

Scheme 6 The various self-assembly structures possible for simple f-d mixed coordination based compounds...

Fig. 7.16 Multi-step main-chain self-assembly to form a flexible, high molecular weight polymer. Step 1 dimerization of bifunctional unit via the self-complimentary ureidopyrimidinone end (top). Step 2 addition of a metal salt such as iron(ll) initiates metal coordination-based self-assembly of the trpy-functionalized ends to form extended polymer chains (bottom).

In 2004, Week and co-workers demonstrated that both mono- and multifunctional self-assembly could be employed simultaneously independently and reversibly on the same side-chain functionalized polymer [96]. A random terpolymer of poly(norbornene) was synthesized consisting of diaminopyridine (DAP) hydrogen-bonding receptors and a palladium-functionalized SCS-type pincer ligand for metal coordination-based self-assembly (Fig. 7.18). 

An interesting dimension of metal-coordinated self-assembly that is often ignored, or at least not exploited to its fullest extent, occurs when the resulting coordination complex is a charged species and, as such, in need of a counterion. This counterion itself presents yet another subtle instance of ionic self-assembly, which often is overshadowed by its partner, the coordination complex. The second multi-functional side-chain supramolecular polymer system is based on this simple but important concept [14, 106-111]. In 2003, Ikkala and coworkers reported a study in which they exploited (1) a side-chain functionalized polymer, poly(vinyl-pyridine), (2) metal-coordination self-assembly via a tridentate Zn2+ complex and (3) ionic self-assembly through functionalized counterions, i.e. dodecylbenzene-sulfonate ions, to form multiple self-assembled complexes which adopted a cylindrical morphology (Fig. 7.23) [112]. 

Kobayashi and co-workers have further extended the work of Dalcanale et al. [40, 41] to achieve selective self-assembly of a homo- or hetero-cavitand cage via metal coordination based on ligand tuning (Fig. 3) [42 -5]. The authors reported a series of tetra(4-pyridyl)-cavitand (6), tetrakis(4-pyridylethynyl)-cavitand (7), and tetrakis(4-cyanophenyl)-cavitand (8) molecules and demonstrated that addition of square-planar Pd(dppp)(OTf)2 complex (9) to cavitand 6-8 in 4 2 ratios respectively resulted in the formation of homo cavitand cages (7)2 Pd(dppp) 21 x+ 8(TfO ) (10)... 

Kobayashi K, Yamada Y, Yamanaka M, Sei Y, Yamaguchi K (2004) Complete selection of a self-assembling homo- or hetero-cavitand cage via metal coordination based on ligand tuning. J Am Chem Soc 126 13896... 

Work by the group of David Reinhoudt (Twente, Netherlands) has resulted in perhaps the only truly reversible, metal-coordination based dendrimer assembly strategy. Starting with the dendron core 14.4 Reinhoudt s group removed the chloride ligands with AgBp4 and coordinated the pyridyl functionality of... 

Fig. 6 Examples of stable dimer (a) and pentamer (b) prepared by eoordinative assembly of free-base m so-pyridylporphyrins and metalloporphyrins (Refs. [52,53]). (e) A triple-decker complex with 4,4 -bipyridine (Ref. [56]). (d) The coordinatively self-assembled receptor exhibiting strong binding of terephthalic acid derivatives in chloroform (Ref. [57]).

Through coor nation motif, a tetra-pyrazinyl porphyrin-based assembly with four dimeric [meso-tetrakis(2-carboxy-4-nonylphenyl)porphyrinato]-zinc(II) as the antenna moiety was constructed by Kuroda et al. [113]. The efficiency of energy transfer from the zinc porphyrin-pyrazine complex to the free base porphyrin has been determined to be 82%. Also for this system, the observed efficiency of energy transfer is not so high compared with those reported for covalently finked multi-porphyrin systems [114]. Himter s group has designed a pentameric porphyrin array, wtiich was built up by the complementary coordination of a tetra-pyridyl porphyrin and two zinc-porphyrin dimers [115]. Steady-state and time-resolved fluorescence spectroscopic studies revealed that the energy transfer efficiency of this array is around 70%. 

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