Crystallographic and Computational Characterisation of the Potential PET Tracer 1,4,7-Triazacyclononane-1,4,7-tri(methylenephosphonato)gallium(III)

The macrocyclic chelator 1,4,7-triazacyclononane-1,4,7-tri(methylenephosphonic acid) (H6NOTP) has shown significant promise for use in positron emission tomography with the positron-emitting radionuclide Ga-68(3+). We report the X-ray crystal structure of the neutral complex 1,4,7-triazacyclononane-1,4,7-tri(methylenephosphonato)gallium(III) ([Ga(H3NOTP)]), which crystallised in the centrosymmetric space group P2(1)/n. The coordination geometry of the Ga3+ ion is distorted octahedral with three macrocyclic amine N donor atoms and three singly deprotonated phosphonato O donor atoms. Two discrete enantiomers, Delta(lambda lambda lambda)-SSS and ?(delta delta delta)-RRR, are present as a racemate within the unit cell. Intermolecular hydrogen bonds between phosphonic acid groups of adjacent complexes form a two-dimensional sheet network. Density functional theory (DFT) calculations supported the observation of complexes with low electric field gradient values with slight but significant deviations from ideal octahedral geometry. The DFT calculations were also used to investigate the influence of intermolecular interactions and ligand protonation states on the coordination environment and surface charge distribution.

Automated summary

The crystal structure of the gallium(III) complex 1,4,7-triazacyclononane-1,4,7-tri(methylenephosphonato)gallium(III) ([Ga(H3NOTP)]) was reported, showing a distorted octahedral coordination geometry with low electric field gradient values. The crystal structure contains two enantiomers in a racemate and exhibits a two-dimensional network formed by intermolecular hydrogen bonds.