Complexation of lithium(I), sodium(I), silver(I) and thallium(I) by 4,7,13,16-tetraoxa-1,10-diazabicyclo[8.5.5]eicosane and 4,7,13-trioxa-1,10-diazabicyclo[8.5.5]eicosane in trimethyl phosphate. A potentiometric titration and 7Li and 23Na nuclear magnetic resonance study