Spin crossover in Co(II) metallorods - replacing aliphatic tails by aromatic

Terpyridine ligands with ter- and quater-phenyl substituents at the 4' position provide bis(ligand)Co(II) complexes showing very different magnetic properties to those of their analogues with long-chain aliphatic substituents, with no evidence of re-entrant behavior involving multiple high- and low-spin species. Structure determinations of [Co(tptpy)(2)](BF4)(2)center dot CH3OH and [Co(qptpy)(2)](BPh4)2 center dot 3.5dmf center dot 2H(2)O (dmf = N,N-dimethylformamide) show the metal ion centres in both to be relatively distant and that the terpyridine embrace observed to be partially retained in their analogues is completely lost. Consideration of available structural and magnetic data for these and other Co(II) complexes of functionalised terpyridines and terpyridine itself provides evidence that spin crossover behaviour may be regulated by face-to-face contacts of the pyridyl units of the head groups.