A comparison of the solution behavior of Pt(II) complexes of N,N′- and N,N-ethylenediaminediacetate (edda and uedda)

Abstract

Abstract Pt(II) complexes of N , N ′-ethylenediaminediacetate (edda) and N , N -ethylenediaminediacetate (uedda) have been prepared from K 2 PtCl 4 by stepwise addition of the nitrogen backbone donors at pH ∼ 2.9 (50–60 °C, 60 h) and further coordination of the deprotonated carboxylate donors at pH ≥ 4 (65–75 °C, 24 h). Coordination of the glycinato donors was shown by 1 H and 13 C NMR and IR methods. The symmetrical edda ligands form 38.3% ( R , R )/( S , S )-[Pt(edda)] isomers and 61.7% meso ( R , S )/( S , R )-[Pt(edda)] isomers. All four forms of [Pt(edda)] undergo aquation of one in-plane glycinato donor in 72 h as detected by the appearance of a 13-line 1 H NMR pattern which may be deconvoluted into four AB glycinato sets. These results are indicative of pendant or ion-paired glycinato donor for [Pt(edda) (H 2 O)] which is placed either on the same side, or the opposite side, of the PtN 2 O 2 plane and coordinated glycinato donor. 195 Pt NMR shows that H 2 O is actually replaced by Cl − , i.e.[Pt(edda)Cl] − . The unsymmetrical [Pt II (uedda) X] (X = H 2 O, Cl − , OH − ) complex exhibits no major change over long time intervals (≥ 10 days, pD ∼ 6). The presence of a minor species at 15% abundance may be a similarly structured species as for [Pt(edda)(H 2 O)] with a pendant glycinato functionality. The major complex in solution is shown by the 1 H NMR with [NaCl] and [NaClO 4 ]-dependence studies to be [Pt(uedda)(H 2 O)] at low [Cl − ] and [Pt(uedda)Cl] − at 1.0 M Cl − . 195 Pt NMR confirms the formulation of X = H 2 O at low [Cl − ]. 1 H and 13 C NMR evidence supports one axially associated and one in-plane coordinated glycinato donor each for the major [Pt(uedda)(H 2 O)] complex. The 13 C NMR shows only one type of glycinato donor with a chemical shift of 189.3 ppm for the major species, and two types for the 15% species (185.6 and 170.5 ppm). The major species of [Pt(uedda)(H 2 O)] has only one type of carboxylate stretch in the IR spectra (1661 cm −1 ; shoulder feature at 1639 cm −1 ) which compares favorably with the fully-coordinated pair of glycinato donors of [Pt(edda)] (1640 cm −1 ). It is proposed that the structures of [Pt(uedda)(H 2 O] and [Pt(uedda)Cl] − are pseudo-square pyramids which illustrates the capacity of Pt(II) to adopt five-coordinate, 18-electron complexes when a suitable chelate ligand offers a fifth associable donor. These species are similar to the five-coordinate intermediates of ligand substitution reactions of typical square-planar Pt(II) complexes.