Neville Suzanne M, Leita Benjamin A, Halder Gregory J, Kepert Cameron J, Moubaraki Boujemaa, Létard Jean-François, Murray Keith S
School of Chemistry, Monash University, Building 23, Clayton, VIC, Australia.
Chemistry. 2008;14(32):10123-33. doi: 10.1002/chem.200800886.
Three analogous one dimensional (1D) polymeric iron(II) spin crossover (SCO) materials containing the new ligand 4,6-bis(2',2''-pyridyl)pyrazine (bdpp) have been comprehensively characterised magnetically (thermal and light-induced) and structurally. Within this series are two polymorphs of the formula [Fe(NCS)(2)(bdpp)], 1 and 2 a, which differ magnetically in that phase 1 undergoes a full two-step SCO (T(1/2(1))=135 K and T(1/2(2))=90 K) whereas phase 2 a remains high spin (HS) over all temperatures. The central distinction between these two materials lies in the presence of intermolecular pi-pi interactions generated by the crystal packing in 1, which are absent in 2 a. The isostructural selenocyanate analogue of 2 a, [Fe(NCSe)(2)(bdpp)], 2 b, undergoes a full two-step SCO (T(1/2(1))=200 K and T(1/2(2))=125 K). Structural analyses of 1 and 2 b at a range of temperatures provide deep insight into their two-step SCO nature. Structural analysis of 1 at 25 K (1(LS-LS)), 123 K (1(LS-HS)) and 250 K (1(HS-HS)) reveals two distinct iron(II) centres at each temperature, with ordered, alternating HS and LS (low spin) sites at the intermediate plateau (IP) temperatures. In contrast, structural analysis of 2 b at 90 K (2 b(LS)), 150 K (2 b(LS/HS)) and 250 K (2 b(HS)) reveals one unique iron(II) centre at each temperature with an "averaged" LS/HS character at the IP temperature. Weak planes of diffuse scattering in the single-crystal X-ray diffraction patterns were observed for this phase at 90 and 150 K, indicating that 1D long range ordering of alternating HS/LS iron(II) centres occurs along the 1D coordination chains, but that there is no correlation between chains. The lack of observable diffuse scattering at 250 K suggests that the onset of the 1D structural ordering in the chain direction corresponds to the first step of the SCO and that this structural transition is electronically driven. The photomagnetic properties of both 1 and 2 b have been investigated and show approximately 62 and 53 % photo-excitation of a HS metastable state at low temperatures and T(LIESST) values of 55 and 49 K, respectively. Relaxation studies on the HS fraction in 2 b fitted well to a stretched exponential model with kinetic parameters indicative of weak cooperativity.
三种含有新型配体4,6 - 双（2',2'' - 吡啶基）吡嗪（bdpp）的类似一维（1D）聚合物铁（II）自旋交叉（SCO）材料已通过磁学（热致和光致）和结构方面进行了全面表征。该系列中包含分子式为[Fe(NCS)(2)(bdpp)]的两种多晶型物，1和2 a，它们在磁性上的差异在于，相1经历完整的两步SCO（T(1/2(1)) = 135 K且T(1/2(2)) = 90 K），而相2 a在所有温度下均保持高自旋（HS）状态。这两种材料之间的核心区别在于1中晶体堆积产生的分子间π - π相互作用的存在，而2 a中不存在这种相互作用。2 a的同构硒氰酸盐类似物[Fe(NCSe)(2)(bdpp)]，2 b，经历完整的两步SCO（T(1/2(1)) = 200 K且T(1/2(2)) = 125 K）。在一系列温度下对1和2 b进行的结构分析深入洞察了它们的两步SCO性质。在25 K（1(LS - LS)）、123 K（1(LS - HS)）和250 K（1(HS - HS)）下对1的结构分析表明，在每个温度下都有两个不同的铁（II）中心，在中间平台（IP）温度下具有有序的、交替的HS和LS（低自旋）位点。相比之下，在90 K（2 b(LS)）、150 K（2 b(LS/HS)）和250 K（2 b(HS)）下对2 b的结构分析表明，在每个温度下都有一个独特的铁（II）中心，在IP温度下具有“平均”的LS/HS特征。在90 K和150 K时，在该相的单晶X射线衍射图谱中观察到漫散射的弱平面，表明沿1D配位链发生了交替的HS/LS铁（II）中心的1D长程有序，但链之间没有相关性。在250 K时缺乏可观察到的漫散射表明，链方向上1D结构有序的开始对应于SCO的第一步，并且这种结构转变是由电子驱动的。对1和2 b的光磁性质都进行了研究，结果表明在低温下分别约有62%和53%的HS亚稳态光激发，且T(LIESST)值分别为55 K和49 K。对2 b中HS分数的弛豫研究很好地拟合了拉伸指数模型，其动力学参数表明存在弱协同性。
