A structural study of the complexation of the sodium ion by the cryptands 4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]icosane and 4,7,1 3-trioxa-1,10-diaza bicyclo[8.5.5]icosane

Abstract

The crystal structures of the cryptates formed between sodium ion and 4,7,1 3,18-tetraoxa-1,10-diazabicyclo[8.5.5]icosane, [Na(L¹)(NCS)], and 4,7,13-trioxa-1,10-diazabicyclo[8.5.5]icosane, [Na(L³)(NCS)], have been determined by single-crystal X-ray diffraction methods at 295 K and refined by least-squares methods to conventional R values of 0.069 and 0.039 for 1 417 and 2 259 reflections respectively. The respective crystals were of space group Pcab with a= 14.686(3), b= 15.699(4), c= 16.522(5)Å, and Z= 8; and P2₁/n with a= 9.947(2), b= 15.681(4), c= 12.460(3)Å, β= 95.27(2)°, and Z= 4. Both [Na(L¹)(NCS)] and [Na(L³)(NCS)] exist in the ‘exclusive’ form in which Na⁺ is located 0.14 and 0.37 A respectively above the planes defined by the three oxygen atoms of the 15-membered cryptand rings. The ¹³C n.m.r. spectra of these cryptates are consistent with the retention of the exclusive structures in solution whilst the ¹³C n.m.r. spectra of the analogous Li⁺ cryptates indicate that they possess ‘inclusive’ structures in solution. Preliminary ²³Na n.m.r. data show the rate of Na⁺ dissociation from [Na(L¹)]⁺ to be significantly less than that from [Na(L³)]⁺ as indicated by dissociation rate constants (298.2 K) of 12.1 ± 0.2 and (2.88 ± 0.02)× 10⁴ s^–1 respectively determined in N,N-dimethylformamide solvent.