C-13 AND CD-113 NUCLEAR-MAGNETIC-RESONANCE EVIDENCE FOR A NOVEL TRANSANNULAR OSCILLATION OF CADMIUM(II) IN THE PENDANT ARM MACROCYCLIC COMPLEX [1,4,8,11-TETRAKIS(2-HYDROXYETHYL)-1,4,8,11-TETRAAZACYCLOTETRADECANE]CADMIUM(II)

A 13C NMR study of [1, 4, 8, 11-tetrakis(2-hydroxyethyl)-1, 4, 8, 11-tetraazacyclotetradecane]cadmium(II), [Cd(THEC)]2+, in CD3OD shows that the most probable structure for [Cd(THEC)]2+ incorporates the 1, 4, 8, 11-tetraazacyclotetradecane ring in the trans III configuration. In this structure Cd(II) is above the tetraaza plane and is trigonal-prismatically coordinated by four ring nitrogens and two hydroxyethyl pendant arms attached to either end of the same 1,3-diaminopropane moiety. The 13C CPMAS NMR spectrum of solid [Cd(THEC)]2+ is also consistent with this structure. Dynamic 13C NMR studies of natural abundance [Cd(THEC)]2+ and of [Cd(THEC)]2+ in which both carbons of each of the hydroxyethyl arms are 99 atom % enriched in 13C are consistent with a rapid oscillation of Cd(II) through the macrocyclic annulus of THEC. The 13C-enriched hydroxyethyl arms are characterized by 13C AB quartets under conditions of slow exchange but show a novel coalescence to a singlet under fast-exchange conditions consistent with the relative chemical shifts of the methylene carbons of the hydroxyethyl arm being reversed when it changes from the mono- to the bidentate coordination state. The pairwise exchange of the hydroxyethyl arm between the monodentate and bidentate environments is characterized by k(298.2 K) = 34200 ± 1800 s-1, ΔS* = 44.00 ± 0.56 kJ mol-1, and ΔS* = -10.6 • 2.2 J K-1 mol-1. 13C and 113Cd NMR spectra show that intermolecular THEC and Cd(II) exchange on [Cd(THEC)]2+ is a much slower process. © 1990, American Chemical Society. All rights reserved.