Other metallocene receptors

Large differences in composition are, in principle, possible by changing the metal center in metallocenes combined with macrocyclic ethers or cryptands, but these were mostly limited to air- and water-stable compounds, which could be considered for further cation and anion sensing devices. So far ruthenocene, titanocene, mo-lybdocene, and tungstocene moieties associated with crown ether-type macrocycles have been described. 

Ruthenium-containing compounds are, in general, structurally similar to their iron analogs. Thus, coronands containing two polyoxa- and polyaza-oxa macrocycles, and from three to six polar atoms in each ring (oxygen and/or sulfur) have been synthesized and investigated [135-137]. 

The sulfur-containing receptors have little affinity for hard cations but can form extractable host-guest complexes with soft cations such as Ag+, Hg +, T1+, Pd +, and Pt +. Silver is found to interact strongly with sulfur [138, 139]. It was even possible to isolate 1 1 host-guest silver and mercury complexes in a large sulfur-containing macrocycle a linear HgC molecule is hosted in the cavity, but in the 1 2 complex a second mercury atom is externally attached to the ruthenium atom [140-142]. Palladium and platinum are also selectively complexed by polythia-coronands [143-148]. Some X-ray diffraction studies on palladium [147, 148] and platinum [143, 145] complexes established that the coordination of the metal occurs by interaction with the sulfur sites. 

A series of receptors containing two polyoxa- [149, 150] and aza-polyoxa [84a, 151-153] macrocycles connected by a ruthenocene unit, have been prepared and their behavior towards alkali metal cations investigated. In the 1 1 host-guest complexes formed the eation is held between the maerocycles and clear selectivity toward potassium is observed. 

Polyaza-oxa cryptands with one or two ruthenocene units have been reported [154, 155]. They form Mg + host guest complexes. 

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The synthesis of new molecular devices designed to sense and report the presence of a particular substrate is an area of analytical chemistry that is currently attracting intense interest. Metallocene units have previously been appended to calixarenes -" and other receptor molecules to generate conjugates that have been shown to be effective... 

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