Carbon-13 and Cadmium-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.