The solid-state behavior of six novel 6-substituted PnAO (propylene amine oxime) complexes (6-11) involving Tc(V), Co(III), and Cu(II) salts is reported. Each of the Tc complexes 6-8 has the C6-substituent located equatorially in a six-membered chelate ring involving a Tc=O unit which has the expected boat geometry. The C6-substituent therefore has little effect on the conformational behavior of the PnAO complex and thus provides an attractive site for further modification. The Co(III) complex 9 has the expected octahedral geometry, while the Cu(II) complexes 10 and 11 form square-based pyramids capped by water molecules. One Cu(II) system (10) contains two unique complexes in the asymmetric unit which are associated via multiple hydrogen bonds to a BF(4) anion, the remaining BF(4) anion being loosely hydrogen bonded to a coordinating water molecule. The cobalt and copper complexes 9-11 each exhibit a chair conformation for the six-membered chelate ring.
Whilst the synthesis of ferrocene-bridged molecular boxes is hampered by rotamer effects associated with the ferrocene unit, it is now shown that, with a degree of preorganisation, macrocyclisation can be an efficient and viable process for the synthesis of an asymmetric redox-active molecular box, without the expected formation of catenanes and higher order macrocycles.In recent years, there has been considerable interest in the incorporation of redox-active centres into binding sites for ionic or neutral species. 2 Complexation can alter the redox potential of the ligand allowing such receptors to act as electrochemical sensors. For example, Chen et al. have reported 3 the asymmetrical complexation of two different Group 2 metal cations (Ba 2+ and Mg 2+ ) in a ferrocenesubstituted bis-crown ether 1, and Beer et al. 4 have prepared a novel cobalticenium receptor for Cl -and Br -. In both cases there is an electrochemical response by the host to the presence of the guest. The recognition of neutral molecular species has also been an area of intense research in the last decade, and has increased the understanding of relatively weak p-p and hydrogen bonding interactions. This has led to the design, synthesis and characterisation of the benzoquinone receptor 2, 5 which is itself an important cofactor in the primary photoinduced charge-separation process in bacterial photosynthesis. Vögtle et al. 6 have recently shown that this receptor not only binds benzoquinone, but can also recognise other simple aromatic species which, when substituted with "end stoppers", afford simple rotaxanes, such as 3.The work reported in this communication concerns the synthesis of macrocycles incorporating a redox-active centre and investigates the requirement for preorganisation in such pathways, in effect producing molecular boxes by a combination of the two concepts discussed above.Our initial studies concerned the preparation of the symmetrical, bis-ferrocenophane 6. This has been successfully obtained, albeit in low yield, through the stepwise synthetic pathway outlined in Scheme 1.
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