Molecular Architecture. 2. Synthesis and Metal Complexation of Heptacyclic Terpyridyl Molecular Clefts

Methods are described for the synthesis of a series of functionalized derivatives of 9-butyl-1,2,3,4,5,6,7,8-octahydroacridine (<b>9</b>), a building block for several types of highly preorganized host compounds. A key intermediate is 5-benzylidene-9-butyl-2,3,5,6,7,8-hexahydroacridin-4(1<i>H</i>)-one (<b>23</b>), which can also be used in the syntheses of torands and hydrogen-bonding hexagonal lattice receptors. A tridentate cleft (<b>20</b>), consisting of 2,2‘;6‘,2‘‘-terpyridine imbedded in a heptacyclic framework, and a corresponding pentadentate diketone (<b>6</b>) were synthesized from <b>9</b> in five and seven steps, respectively. The picrate extraction method was used to estimate the solution stabilities of alkali metal complexes of heptacyclic terpyridyls <b>6</b> and <b>20</b>, which was also compared with a flexible terpyridyl (<b>37</b>). Alkali metal complexes of both heptacyclic terpyridyls showed relatively high <i>K</i><sub>s</sub> values, but low size selectivity. Pentadentate host <b>6</b> binds Na<sup>+</sup> and K<sup>+</sup> more strongly than do most hexadentate crown ethers; flexible tridentate analogue <b>37</b> failed to extract alkali metal picrates into chloroform. The complexation abilities of <b>6</b> and <b>20</b> are attributed to enforced orientation of functional group dipoles toward the center of the molecular cleft. Sodium and potassium picrate complexes of pentadentate cleft <b>6</b> were synthesized (1:1 stoichiometry), and a 2:1 complex of calcium triflate (<b>6</b><sub>2</sub>·Ca(CF<sub>3</sub>SO<sub>3</sub>)<sub>2</sub>) was also prepared.