Mutual influence of spacer length and noncoordinating anions on thermal and light-induced spin-crossover properties of iron(II)-α,ω-bis(tetrazol-1-yl)alkane coordination polymers

The influence of noncoordinating anions is analysed in an extension of our systematic investigations into iron(II) spin-crossover coordination polymers. We present here ditetrazole complexes of iron(II) tetrafluoroborate where the two tetrazole moieties are separated by alkylene spacers. The number of carbon atoms in the spacer (n) was varied between n=5-10, 12, and the complexes were compared with their ClO4-analogues. Pronounced magnetic and magneto-optical differences were found in the comparison of the two series. A chain-type arrangement in the complexes' crystal structures is proposed with decreased interactions between iron centres as n increases, leading to more gradual spin transitions. When n=5-7 the smaller tetrafluoroborate causes the spin transitions of the respective complexes to occur at higher temperatures than those of the equivalent perchlorate series. Both series exhibit a parity effect, but at n=8 the parity effect reverses for tetrafluoroborate. With increasing n, however, iron-iron interactions seem to fade away and a limiting value of T-SC=160 K is approached by both series. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).