Cooperative Interactions in the Second Coordination Sphere of Pyridazine/Pyridine Containing Polyazaheterocyclic Iron(II) Complexes Favor Protonation

The new pyridazine containing iron complexes, [ N , N , N′ , N′ ‐tetrakis(3‐pyridazylmethyl)propylenediamine]iron(II)(PF 6 ) 2 ( 1 ) and [ N , N′ ‐bis(2‐pyridazylmethyl)‐ N , N′ ‐bis(2‐pyridylmethyl)propylenediamine]iron(II) (PF 6 ) 2 ( 2 ) were synthesized and their reactivity towards protonation was compared to that of the analogous tetrapyridine complex [ N , N , N′ , N′ ‐tetrakis(2‐pyridylmethyl)propylenediamine]iron(II)(PF 6 ) 2 ( 3 ). The solution and solid‐state structures were confirmed by NMR and X‐ray crystallographic studies. For 1 – 3 , the ligands bind in a hexadentate fashion giving similar octahedral structures with an N 6 coordination environment. Across the series, the increasing number of pyridazines has only modest effects on the spectroscopic and electrochemical properties of the metal. Nevertheless, their reactivity towards protonation is drastically different. While 2 and 3 decompose in the presence of strong acids, 1 is able to be stably protonated as a result of cooperative second sphere interactions.