The structures of DPDP, CdDPDP and ZnDPDP as studied by X-ray diffraction and NMR

Abstract

It was of interest to investigate the nature and behaviour of MnDPDP (manganese(II) N, N′-dipyridoxylethylenediamine- N, N′-diacetate 5,5′-bis(phosphate)) in solution, especially with respect to complexation geometry and the existence of isomeric forms of the complex. Unfortunately, Mn(II) is paramagnetic and causes drastic broadening of the NMR resonances of magnetic nuclei in its vicinity; hence high-resolution data cannot be obtained on Mn(II)DPDP itself. The diamagnetic ions Zn 2+ and Cd 2+ are those, which would seem to most closely resemble Mn 2+, ZnDPDP and CdDPDP were therefore studied as reasonable analogues for MnDPDP. In the solid state, Cd(II) in CdDPDP is seven-coordinate, with one coordinated water molecule. The structure is different from MnDPDP, which is six-coordinate with no coordinated water molecules in the solid state. The coordination geometry in CdDPDP is that of a capped trigonal prism, and is similar to that reported for MnEDTA and MgEDTA. One of the carboxylate groups is particularly disordered, possibly indicating an incipient dissociation event. The NMR results for ZnDPDP indicate that the carboxylate groups spend a substantial percentage of the time free from the metal ion, a six-coordinate structure with one coordinated water molecule and one dissociated carboxylate group seems possible. A major α form and a minor β form of the complex have been observed in solution for both CdDPDP and ZnDPDP. The broadness of the α signal in the 113Cd spectrum indicates that the α structure is in fact a set of structures probably involving different numbers of complexing water molecules. The seven-coordinate structure found in the solid state, with one coordinated water molecule, and an eight-coordinate structure, with two water molecules, seem possible. MnDPDP is six-coordinate with no coordinated water molecules in the solid state. In solution, relaxation measurements show that the MnDPDP complex has one coordinated water molecule. It seems reasonable that MnDPDP in solution has a structure similar to the solid state structure of CdDPDP, that is, seven-coordinate with one coordinated water molecule. However, it cannot be excluded that the structure is six-coordinate with one coordinated water molecule and one dissociated carboxylate group.