Towards catenanes using π-stacking interactions and their influence on the spin-state of a bis(2,2′:6′,2″-terpyridine)iron(ii) domain

Abstract

The ligand 6,6″-bis(4-methoxyphenyl)-4′-phenyl-2,2′:6′,2″-terpyridine (2) has been prepared and characterized; deprotection using pyridinium chloride leads to the formation of 6,6″-bis(4-hydroxyphenyl)-4′-phenyl-2,2′:6′,2″-terpyridinium chloride ([H3]Cl). Treatment of the latter with 3-(2-(2-bromoethoxy)ethoxy)prop-1-ene under basic conditions yields ligand 4 containing pendant, alkene-terminated chains. Whereas direct complexation of 4 with ruthenium(II) proved problematical, the homoleptic complexes [Fe(2)₂][PF₆]₂ and [Ru(2)₂][PF₆]₂ were prepared in good to moderate yields. In the solid state, both complexes exhibit multiple face-to-face π-stacking of arene and pyridine rings which influences the coordination geometry about the metal ion. Consequential weakening of the ligand field results in [Fe(2)₂][PF₆]₂ being high-spin. Variable temperature solution ¹H NMR spectroscopic studies confirm the iron(II) centre remains high-spin between 200 and 295 K. The paramagnetically shifted ¹H NMR spectrum exhibits signals in the range δ 109.7 to −66.5 ppm and has been fully assigned. Paramagnetic relaxation enhancement (PRE) has been used to correlate the observed proton line-widths to the distances of the protons from the metal centre and these are in good agreement with the Fe⋯H separations observed in the solid state. The [Fe(2)₂]²⁺ ion undergoes two dynamic processes (i) rotation of the pendant phenyl rings which is fast on the NMR timescale at 200 K, and (ii) twisting and sliding of the aromatic rings of the tpy and anisyl units which interconverts the two enantiomers of [Fe(2)₂]²⁺ at 295 K.