Lapidus Saul H, Stephens Peter W, Fumanal Maria, Ribas-Ariño Jordi, Novoa Juan J, DaSilva Jack G, Rheingold Arnold L, Miller Joel S
Department of Physics & Astronomy, Stony Brook University, Stony Brook, New York 11794-3800, USA.
Dalton Trans. 2021 Aug 28;50(32):11228-11242. doi: 10.1039/d1dt02106k. Epub 2021 Aug 2.
To identify the genesis of the differing magnetic behaviors for the ferro- (FO) and metamagnetic (MM) polymorphs of [FeCp*][TCNQ] (Cp* = pentamethylcyclopentadienide; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) the low temperature (18 ± 1 K) structures of each polymorph were determined from high-resolution synchrotron powder diffraction data. Each polymorph possesses chains of alternating S = 1/2 [FeCp*]˙ cations and S = 1/2 [TCNQ]˙, but with differing relative orientations. These as well as an additional paramagnetic polymorph do not thermally interconvert. In addition, the room and low (<70 ± 10 K) temperature structures of the MM polymorph, MM and MM, respectively, differ from that previously reported at 167 K (-106 °C) MM structure, and no evidence of either phase transition was previously noted even from the magnetic data. This transition temperature and enthalpy of this phase transition for MM⇌MM was determined to be 226.5 ± 0.4 K (-46.7 ± 0.4 °C) and 0.68 ± 0.04 kJ mol upon warming, respectively, from differential calorimetry studies (DSC). All three MM phases are triclinic (P1[combining macron]) with the room temperature phase having a doubled unit cell relative to the other two. The lower temperature phase transition involves a small rearrangement of the molecular ions and shift in lattice parameters. These three MM and FO polymorphs have been characterized and form extended 1-D chains with alternating S = 1/2 [FeCp*]˙ cations, and S = 1/2 [TCNQ]˙ anions, whereas the fifth, paramagnetic (P) polymorph possesses S = 0 π-[TCNQ] dimers. At 18 ± 1 K the intrachain FeFe separations are 10.738(2) and 10.439(3) Å for the FO and MM polymorphs, respectively. The key structural differences between FO and MM at 18 ± 1 K are the 10% shorter interchain NN and the 2.8% shorter intrachain FeFe separation present for MM. Computational analysis of all nearest-neighbor spin couplings for the 18 K structures of FO and MM indicates that the intrachain [FeCp*]˙[TCNQ]˙ spin couplings (H = -2S·S) are the strongest (4.95 and 6.5 cm for FO and MM, respectively), as previously hypothesized, and are ferromagnetic due to their S = 1/2 spins residing in orthogonal orbitals. The change in relative [TCNQ]˙[TCNQ]˙ orientations leads to a computed change from the ferromagnetic interaction (0.2 cm) for FO to an antiferromagnetic interaction (-0.1 cm) for MM in accord with its observed antiferromagnetic ground state. Hence, the magnetic ground state cannot be solely described by the dominant magnetic interactions.
为确定[FeCp*][TCNQ]（Cp* = 五甲基环戊二烯基；TCNQ = 7,7,8,8 - 四氰基对苯二醌二甲烷）的铁磁（FO）和变磁（MM）多晶型物不同磁行为的起源，通过高分辨率同步辐射粉末衍射数据确定了每种多晶型物的低温（18 ± 1 K）结构。每种多晶型物都具有交替排列的S = 1/2 [FeCp*]˙阳离子和S = 1/2 [TCNQ]˙的链，但相对取向不同。这些以及另一种顺磁多晶型物不会发生热相互转化。此外，MM多晶型物在室温及低温（<70 ± 10 K）下的结构，分别为MM和MM，与之前报道的167 K（-106 °C）时的MM结构不同，甚至从磁性数据中也未发现任何相变的迹象。通过差示量热法（DSC）研究确定，MM⇌MM这种相变的转变温度和焓在升温时分别为226.5 ± 0.4 K（-46.7 ± 0.4 °C）和0.68 ± 0.04 kJ/mol。所有三种MM相均为三斜晶系（P1[上加横线]），室温相的晶胞相对于其他两个相翻倍。较低温度的相变涉及分子离子的小重排和晶格参数的变化。这三种MM和FO多晶型物已被表征，并形成具有交替的S = 1/2 [FeCp*]˙阳离子和S = 1/2 [TCNQ]˙阴离子的扩展一维链，而第五种顺磁（P）多晶型物具有S = 0的π - [TCNQ]二聚体。在18 ± 1 K时，FO和MM多晶型物的链内Fe - Fe间距分别为10.738(2) Å和10.439(3) Å。18 ± 1 K时FO和MM之间的关键结构差异在于，MM的链间N - N短10%，链内Fe - Fe间距短2.8%。对FO和MM在18 K结构的所有最近邻自旋耦合进行的计算分析表明，如先前假设的那样，链内[FeCp*]˙[TCNQ]˙自旋耦合（H = -2S·S）最强（FO和MM分别为4.95和6.5 cm），并且由于其S = 1/2自旋位于正交轨道中而呈铁磁性。[TCNQ]˙[TCNQ]˙相对取向的变化导致计算得出的相互作用从FO的铁磁相互作用（0.2 cm）变为MM的反铁磁相互作用（-0.1 cm），这与观察到的反铁磁基态一致。因此，磁性基态不能仅由占主导地位的磁相互作用来描述。
