Coordination chemistry of transition metal complexes of a novel pentadentate ligand

The pentadentate ligand N-(2-pyridylmethyl)-3-(bis(2-pyridylmethyl)amine)-propanamide (LH) and transition metal complexes hereof were synthesised. The negative charge of the deprotonated ligand was meant to stabilise metal ions in high oxidation states thus facilitating aerobic oxidation of the metal centre. 1H NMR studies and X-ray crystallography showed that the Co(III)–OAc complex of LH (1) adopts the same structure in solution as in the crystalline state. Complex 1 crystallised in space group P21/c (no. 14). Furthermore, the 1H NMR studies revealed dynamic behaviour of the ligand backbone. The six-membered chelate ring undergoes inversion with an estimated activation energy of 14.6 kJ mol−1. The Fe(II) complex of LH was studied by 1H NMR in the presence CN− ions to force the iron complex into a low spin state. But the cyanide ions competed effectively with the amide portion of LH so the ligand only coordinated the Fe(II) ion with three of its five nitrogen atoms. Interestingly, two isomeric Fe(II) complexes formed; one with a meridonal arrangement of the three nitrogen atoms, the other having a facial arrangement. Upon standing the mer complex rearranged to the fac complex. As the complex mixture turned paramagnetic upon exposure to air, it seems that the ligand L− promote aerobic oxidation of Fe(II).