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3. The Electron Acceptor Site:

The secondary electron acceptors in PSII are quinone-based molecules, Qa, which remains in the vicinity of the reaction centre, and Qb, which diffuses out of PSII after being reduced (Fig. 2). Most artificial systems attempt to mimic Qa as it is synthetically easy to attach a quinone to a chromophore.17 A much more challenging approach, and one more directly related to Qb, is to link the chromophore to a receptor that binds quinone non-covalently. Very few systems of this sort exist,18 even though the diffusion of the reduced form of Qb away from the reaction centre is critical to avoid charge recombination. We are modelling quinone electron acceptors using metalloreceptors that bind substrate molecules non-covalently.

3.1 The design of receptors for the non-covalent binding of electron acceptors

Figure 10 a) Representation of metal-modified organic ligand; and, b) ligands assembled around metal ions.

The organic ligands contain three essential parts: metal-binding units, a rigid spacer, and hydrogen-bonding (HB) groups (Figure 10a). The spacers hold the metal binding units apart such that intramolecular complexation of one metal ion is not possible. Each ligand must bind to two metal ions. When two or more ligands are brought together by two metal ions, a pocket is formed, complete with sites for HB interactions. For example, a large, closed structure may be assembled by using metals with a trans-square planar (Fig. 10b) or a trigonal bipyramidal (not shown) coordination geometry.

A key advantage of our systems over other metal-organic complexes is the incorporation of HB sites into the pockets formed by metal complexation. The HB groups are introduced into the organic fragments before the metal-directed assembly takes place. Elaborate synthetic procedures would be required to make similarly functionalized pockets by conventional organic methodology. The self-assembly process, on the other hand, provides a multitude of structures due to the variety of metal ion coordination geometries available, i.e. different pocket sizes are generated by changing the metal ion. This offers an advantage over other microporous materials, such as zeolites, by allowing the incorporation of metal ions into precise positions in the structure. In addition to positioning the ligands, the metal ions may also introduce novel reactivity and interesting electrochemical and photochemical properties to the final structures.

3.2 The synthesis of ligands and complexes

An expedient rigid organic spacer is the pyridinediamido framework. The amide bonds provide H-bond donors and the external pyridines provide the metal-binding units. This ligand assembles with PdCl2 to form a metalloreceptor in 94% yield, as supported by X-ray diffraction studies (Fig. 11).

Figure 11 Synthesis and X-ray crystal structure of a dipalladium receptor (DMSO omitted).

Thus, metal-directed self-assembly is a viable approach to receptor synthesis. However, this charge neutral complex has limited solubility in common organic solvents. By using neutral ligands and forming charged complexes we were able to solubilize these complexes in a wide range of solvents (Fig. 12).

Figure 12 Cis and trans diplatinum receptors

In the presence of cis platinum macrocycle, we have shown that quinone is reduced to hydroquinone. We are interested in using quinone/hydroquinone as an electron trap, e.g., if the phosphine cone angles in trans complexes are increased, can quinone be non-covalently bound in the metallomacrocycles? As membrane and protein environments are known to modify the electro- and photochemical properties of molecules, we would like to do the same within this small metalloreceptor nano-environment. As supramolecular catalysis is rare,19 we are investigating the mechanism of this reduction more closely.

 

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