CONFORMATIONAL EFFECTS IN CROWN-ETHER COMPLEXATION OF LITHIUM PICRATEIN SOLVENTS OF LOW POLARITY

The binding of lithium picrate (LiPic) to isomers L(1) and L(2) of the crown ether tacyclo[16.2.1.1(3,6).1(8,11).1(13,16)]tetracosane (I) wa s studied spectrophotometrically in chloroform, chlorobenzene, and fou r ethereal solvents. In the latter, binding of L(1) to LiPic is weak, but L(2) yielded binding constants, K-c, of 60-7500 M(-1), depending o n the solvating power of the ether. In chloroform (with 1% ethanol), L (2) gives an inclusion complex with a spectrum identical to that of a separated ion pair, and K-c = 1.2 x 10(6) M(-1), while L(2) gives an e xternally solvated LiPic ion pair complex. Complexations are remarkabl y slow due to the difficulty in rearranging L(1) and L(2) to conformat ions in which all four oxygen atoms have the proper orientation for ef fective complexation of the Li ion. In CHCl3, the complexation rate co nstant for L(1) is 7 x 10(2) M(-1) s(-1) and the decomplexation rate c onstant is 6 x 10(-4) s(-1). In chlorobenzene there is spectrophotomet ric evidence for the presence of 2:1 and 4:1 LiPic-crown ether complex es.