Inter- and Intrachain Spin-Transition Processes in One-Dimensional Polynuclear Iron(III) Complexes of N,N′-Ethylenebis(acetylacetonylideneimine) Bridged by 1,3-Bis(4-pyridyl)propane and 1,4-Bis(imidazolyl)butane

Two one-dimenisonal (1D) polynuclear iron(III) complexes, [Fe-III(acacen)(bpyp)]BPh4 (1) and [Fe-III(acacen)(bimb)]BPh4 (2), were synthesized [acacen = N,N'-ethylenebis(acetylacetonylideneiminato); bpyp = 1,3-bis(4-pyridyl)propane; bimb 1,4-bis(imidazolyl)butane]. Complexes 1 and 2 show gradual spin-crossover (SCO) behavior over a wide. temperature range from 100 to 300 K and from 220 to higher than 350 K, respectively. The single-crystal X-ray structures of two complexes in the. high-spin (HS), low-spin (LS), and the midpoint between HS and LS States were determined. Crystal structures of both complexes consist of two molecular units (Fe1 and Fe2), where complex 1 consists of two crystallographically independent 1D chains and complex 2 consists of one 1D chain involving two molecular units, At the temperature midway between the HS and LS states, the Fe-N bond lengths in the two units are different from each other, At the midpoint, complex 1 consists of alternately arrayed independent HS and LS chains, whereas complex 2 consists Of alternately arrayed I IS and LS units within a chain. The results demonstrate two different SCO mechanisms of a 1D complex, that is, SCO occurs in chain-by-chain fashion for I and in adjacent Fe units within a chain for 2. ((C) Wiley-VCH Verldg GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)