A structural study of the complexation of the lithium ion by the cryptand 4,7,13-trioxa-1,10-diazabicyclo[8.5.5]icosane

Abstract

The crystal structure of the cryptate [LiL³]NCS, formed with the cryptand 4,7,13-trioxa-1,10-diazabicyclo[8.5.5]icosane (L³) has been determined by single-crystal X-ray diffraction techniques at 294 K and refined by least-squares methods to a conventional R value of 0.068 for 1 101 reflections. The crystal was of space group P2₁/n with a= 8.348(2), b= 24.798(7), c= 9.161(2)Å, β= 90.74(2)°, and Z= 4. The cryptate exists in the inclusive form in which the lithium ion resides in the cavity of L³ and is within bonding distances of the two nitrogens and the three oxygens of the cryptand. These bonding distances are in the range 2.00(2)–2.36(3)Å and the geometry about lithium is very irregular. The thiocyanate is not within bonding distance of lithium in [LiL³]NCS, in contrast to [NaL³(NCS)] which exists in the exclusive form and in which thiocyanate is within bonding distance of sodium. The kinetic and equilibrium aspects of [LiL³]⁺ and [NaL³]⁺, and their analogues with 4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]icosane in dimethylformamide solution are also discussed.