Pseudocryptand Hosts for Paraquats and Diquats

H-bonding interaction
of acidic moieties (CH2OH, COOH)
at the 5- and 5′-positions of bis­(1,3-phenylene)-32-crown-10
(1) with di- or tritopic anions leads to enhanced formation
of inclusion complexes with N,N′-dialkyl-4,4′-bipyridinium
salts (“paraquats”, 2); the enforced folding
of the crown ethers into pseudocryptands thus leads to pseudo-pseudorotaxanes.
Strikingly, in the presence of the most effective anion (trifluoroacetate,
TFA), the apparent bimolecular association constants for crown–paraquat
complexation increase by more than an order of magnitude and approach
those for covalent cryptands derived from the crown ether. Even though
they may form pseudocryptands, the picolinate, nicotinate, and isonicotinate
diesters 6 of cis-(4,4’)-bis­(hydroxymethyl)­dibenzo-30-crown-10
do not exhibit enhanced binding of either diquat or paraquat relative
to the starting diol in contrast to the picolinate ester of isomeric
5,5′-bis­(hydroxymethyl)­bis­(m-phenylene)-32-crown-10,
which displayed a higher binding constant than the starting diol.
The results for the analogous reverse esters 7 derived
from cis-(4,4’)-dicarboxydibenzo-30-crown-10
and pyridylmethanols reveal weaker complexes with diquat than the
normal esters 6; however, surprisingly, two reverse esters 7 complex paraquat more strongly than isomers 6.