SOLID-STATE CHEMISTRY OF POLYCARBOXYLATE CROWN-ETHER CATION COMPLEXES - COOPERATIVE BINDING OF WATER AND METAL-IONS BY FLEXIBLE CHORANDS

The solid-state structures of alkali-metal salt complexes of crown ether tetra- and hexa-carboxylic acids (18C6A4 and 18C6A6) have been determined. The sodium salt of 18C6A6 crystallizes as the trihydrate in orthorhombic space group P2(1)2(1)2(1) (a = 7.6868(12), b = 15.429(3), c = 21.975(2) angstrom, R = 0.031 for 5192 observed data); the potassium salt of 18C6A6 crystallizes as the the trihydrate in orthorhombic space group P2(1)2(1)2(1) (a = 7.6711(8), b = 15.709(2), c = 21.966(5) angstrom, R = 0.035 for 2521 observed data); the thallium(I) salt of 18C6A6 crystallizes as the dihydrate in orthorhombic space group P2(1)2(1)2(1) (a = 12.0331(10), b = 12.4333(12), c = 17.366(2) angstrom, R = 0.035 for 4952 observed data); and the cesium salt of 18C6A6 crystallizes as the dihydrate in monoclinic space group P2(1) (a = 7.708(3), b = 13.492(2), c = 11.9501(9) angstrom, beta = 94.49(2)-degrees, R = 0.029 for 4480 observed data). The sodium salt of 18C6A4 crystallizes as the dihydrate in monoclinic space group P2(1) (a = 11.4368(11), b = 7.7068(l 1), c = 12.6579(12) angstrom, beta = 98.515(8)-degrees, R = 0.035 for 3000 observed data); the thallium(I) salt of 18C6A4 crystallizes as the dihydrate in monoclinic space group P2(1)(a = 11.719(2), b = 7.668(2), c = 12.8403(13) angstrom, beta = 99.052(9)-degrees, R = 0.040 for 3981 observed data); and the cesium salt of 18C6A4 crystallizes as the dihydrate in monoclinic space group P2(1) (a = 11.787(2), b = 7.637(2), c = 13.001(2) angstrom, beta = 98.705(12)-degrees, R = 0.038 for 3428 observed data). Both chorands exhibit a cooperative relationship of mean cavity radius and guest-ion radius, which closely follows the effective ionic radii of the guests. The hexaacid is a preorganized host for monohydrated cations. It provides a trigonal hydrogen bonding receptor site for the water molecule, and up to nine oxygen donors for cation binding. Guests ranging in size from Na+ to Cs+ are accommodated without distortion of the macrocyclic framework. The tetraacid is similarly tolerant of a range of guests, but greater structural reorganization of the ligand is required to optimize contacts with fully dehydrated cations. Skeletal drawings of donor atoms, in conjunction with polar coordinate maps of torsion angles, reveal how the demands of the guest are accommodated by the host.