STRUCTURAL, EQUILIBRIUM, AND KINETIC-STUDY OF THE COMPLEXATION OF SODIUM(I) BY THE CRYPTAND 4,7,13,16-TETRAOXA-1,10-DIAZABICYCLO[8.8.5]TRICOSANE, C22C5

The cryptate (4,7,13,16-tetraoxa-1,10-diazabicyclo[8.8.5]tricosane)sodium(I) perchlorate, [Na.C22C5]ClO4, crystallizes in the orthorhombic space group Pbca with unit cell dimensions a = 15.893 (3) angstrom, b = 15.782 (2) angstrom, c = 17.656 (3) angstrom, and V = 4428.5 angstrom 3 with Z = 8. The structure was refined by a full-matrix least-squares procedure to final R = 0.045 and R(w) = 0.054 for 1669 reflections with I greater-than-or-equal-to 2.5-sigma(I). The Na+ center is five-coordinate and lies in the same plane as the four oxygen atoms of C22C5, with the fifth coordination site occupied by a perchlorate oxygen atom above this plane completing a square-pyramidal coordination geometry. An unusual feature is that the two nitrogen atoms of C22C5 (which lie below the plane of the four oxygen atoms) are not within bonding distance of Na+. This contrasts with the structure of the closely related [Na.C221]+, (4,7,13,16,21-penta-oxa-1,10-diazabicyclo[8.8.5]tricosane)sodium(I), in which Na+ is in the center of the cryptand cavity and is within bonding distance of all five oxygen atoms and both nitrogen atoms, and illustrates the major structural effect of the replacement of an oxygen donor atom of [Na.C221]+ by a methylene moiety to give [Na.C22C5]+. This replacement also has a substantial effect in solution where, in acetonitrile, propylene carbonate, water, acetone, methanol, dimethylformamide, dimethyl sulfoxide, and pyridine, log (K/mol dm-3) = greater-than-or-equal-to 7, 1.8, 6.09, 5.41, 3.66, 3.15, and 6.41, respectively at 298.2 K, which are substantially smaller values than those characterizing [Na.C221]+. In methanol, the decomplexation kinetic parameters k(d)(298.2 K) = 41.0 +/- 1.7 s-1, DELTA-H(d) double-ended daggar = 55.1 +/- 1.1 kJ mol-1, and DELTA-S(d) double-ended daggar = -29.2 +/- 3.8 kJ mol-1 characterizing [Na.C22C5]+ indicate that [Na.C22C5]+ is several orders of magnitude more labile than [Na.C221]+. These characteristics of [Na.C22C5]+ are compared with those of related cryptates and are also discussed in terms of the reported greater efficiency of C22C5 as a membrane transport carrier for Na+ by comparison to C221.