LITHIUM BINDER IN AQUEOUS-SOLUTION - SYNTHESIS AND CHARACTERIZATION OF THE NEW CAGE 4,10,15-TRIMETHYL-1,4,7,10,15-PENTAAZABICYCLO[5.5.5]HEPTADECANE(L) - PROTONATION AND LITHIUM COMPLEX-FORMATION - CRYSTAL-STRUCTURES OF [HL][BPH4] AND [LIL][BPH4]

The synthesis and characterization of the new azamacrocycle 4,10,15-trimethyl-1,4,7,10,15-pentaazabicyclo[5.5.5]heptadecane (L) are reported. The stepwise basicity constants have been determined by potentiometry (25-degrees-C, NaCl 0.15 mol dm-3). L behaves as biprotic base: log K1 = 11.8; log K2 = 10.0. The molecular structure of the monoprotonated salt [HL][BPh4] has been determined by single-crystal X-ray analysis. The compound crystallizes in a monoclinic unit cell (space group P2(1)/n) with lattice constants a = 9.92 (1) angstrom, b = 23.07 (1) angstrom, c = 15.769 (5) angstrom, and beta = 96.31 (5)-degrees, with Z = 4. Least-squares refinement converged at R = 0.096 (R(w) = 0.061) for 1497 unique reflections of I > 3-sigma (I). The structure of [HL]+ shows the five nitrogen atoms in endo configuration, indicating that the proton is inside the cage cavity. Geometrical considerations indicate that the proton is bound to the bridgehead nitrogen atom N(2). The cage L selectively encapsulates Li+, and the equilibrium for the formation of the inclusion complex [LiL]+ has been investigated by potentiometry (log K = 5.5) and Li-7 NMR techniques. The molecular structure of the complex [LiL][BPh4] has been determined by single-crystal X-ray analysis. The compound crystallizes in a monoclinic unit cell (space group P2(1)/c) with lattice constants a = 14.483 (4) angstrom, b = 12.762 (8) angstrom, c = 19.22 (3) angstrom, and beta = 93.42 (7)-degrees, with Z = 4. Least-squares refinement converged at R = 0.075 (R(w) = 0.054) for 2681 unique reflections with I > 3-sigma (I). Li+ is wholly enclosed in the cage cavity adopting a regular bipyramidal geometry with the Li-N distances in the 2.01-2.08-angstrom range. Cu(II) is also encapsulated by L; the electronic spectra of the complex show essentially the same features in the solid state and in solution and are diagnostic of a five-coordinate trigonal-bipyramidal structure.