Proton transfer in a phenol-based compartmental cryptate; solution and solid-state structural studies

Abstract

Under pH-neutral conditions, a potentially dinucleating azacryptand N[(CH₂)₂NCH–R–CHN(CH₂)₂]₃N L¹(R =m-C₆H₂OH-2-Me-5) was found to accommodate just one metal cation; ¹H NMR spectra showed that the second site is occupied by three protons transferred from phenol to imino N. Under basic conditions, the protons can be displaced by a second cation forming a dinuclear cryptate. X-Ray crystallographic structure determinations have been made for the complexes [PbL¹][BPh ₄]₂, two forms of [CdL¹][ClO₄]₂, a and b and [Cd₂(L¹– 3H)][ClO⁴]₂. In the first three complexes one metal ion is encapsulated within the macrocycle. However in all three cases the position is disordered over two equivalent sites; in the lead compound in the ratio 0.87:0.13 and in the cadmium compound a 0.66:0.34. In b the disorder is over two equivalent crystallographic positions so the ratio is 0.50:0.50. In the last complex two cadmium cations occupy the macrocycle. In all four structures the metal atoms are bonded to all three oxygen atoms and three imine atoms. In addition there is an interaction with a bridgehead nitrogen atom so that the metal co-ordination spheres can best be considered as seven-co-ordinate capped octahedra.