Monroe Jeffrey C, Carvajal M Angels, Landee Christopher P, Deumal Mercè, Turnbull Mark M, Wikaira Jan L, Dawe Louise N
Carlson School of Chemistry and Biochemistry, Clark University, 950 Main Street, Worcester, MA 01610, USA.
Dept. Ciència de Materials i Química Física, & IQCTUB, Universitat de Barcelona, Martí i Franquès 1, Barcelona, E-08028, Spain.
Dalton Trans. 2022 Mar 22;51(12):4653-4667. doi: 10.1039/d1dt04219j.
The crystal structure and magnetic properties of two all-pyrazine-bridged antiferromagnetic spin ladders are reported. The complexes, -(bis(3-X-4-pyridone)(μ-pyrazine)copper(II)(-μ-pyrazine)diperchlorate (Cu(pz)(L) where L = 3-X-4-pyridone and X = Br (1) or Cl (2)), contain copper(II)-based ladders in which both the rung and rail bridges are pyrazine molecules bonded through the - orbital of the copper(II) ions. This structural scaffold is proposed to approach the isotropic spin-ladder regime. 1 and 2 crystallize in the monoclinic space group 2/. Due to the bulk of the 3-X-4-HOpy ligands, the ladders are well isolated in the -direction (1, 15.6 Å; 2, 15.5 Å). The ladders, which run in the -direction, are stacked in the -direction with the separation (1, 7.87 Å; 2, 7.82 Å) between copper(II) ions caused by the bulk of a semi-coordinate perchlorate ion coordinated in the axial position. Computational evaluation of magnetic couplings between Cu-moieties of 2 supports the experimentally proposed magnetic topology and agrees with an isolated isotropic spin-ladder ( = -4.04 cm (-5.77 K) and = -3.89 cm (-5.56 K)). These complexes introduce a convenient scaffold for synthesizing isotropic spin-ladders with modest superexchange interactions, the strength of which may be tuned by variations in L. The magnetic susceptibility down to 1.8 K, for both compounds, is well described by the strong-rung ladder model giving nearly isotropic exchange with ≈ ≈ -5.5 K (1) and -5.9 K (2) using the Hamiltonian. Theoretical simulations of the magnetic response of 2 using the isotropic ladder model are in excellent agreement with experiment. The measured magnetization to 5 T indicates a quantum-dominated magnetic spectrum. Again, calculated lower and saturation (4.3 and 24 T, respectively) critical fields for 2 are consistent with experimental measurements, and magnetization data at very low temperatures indeed suggest the presence of quantum effects. Further, the computational study of short- and long-range spin ordering indicates that a 2D-to-3D crossover might be feasible at lower temperatures. Analysis of the Boltzmann population corroborates the presence of accessible triplet states above the singlet ground state enabling the aforementioned 2D-to-3D crossover.
报道了两种全吡嗪桥联反铁磁自旋梯的晶体结构和磁性。配合物Cu(pz)(L)₂（其中L = 3-X-4-吡啶酮且X = Br (1) 或Cl (2)），即双(3-X-4-吡啶酮)(μ-吡嗪)铜(II)(-μ-吡嗪)二高氯酸盐，包含基于铜(II)的梯子，其中梯级和轨道桥都是通过铜(II)离子的π轨道键合的吡嗪分子。这种结构支架被认为接近各向同性自旋梯状态。1和2在单斜空间群P2₁/c中结晶。由于3-X-4-HOpy配体的体积较大，梯子在b方向上很好地隔离（1，15.6 Å；2，15.5 Å）。沿a方向延伸的梯子在c方向上堆叠，轴向位置配位的半配位高氯酸根离子的体积导致铜(II)离子之间的间距为（1，7.87 Å；2，7.82 Å）。对2的铜部分之间磁耦合的计算评估支持了实验提出的磁拓扑结构，并与孤立的各向同性自旋梯一致（J₁ = -4.04 cm⁻¹ (-5.77 K) 和J₂ = -3.89 cm⁻¹ (-5.56 K)）。这些配合物为合成具有适度超交换相互作用的各向同性自旋梯引入了一个便利的支架，其强度可通过L的变化进行调节。对于这两种化合物，低至1.8 K的磁化率都能很好地用强梯级梯子模型描述，使用哈密顿量给出近似各向同性交换，J₁ ≈ J₂ ≈ -5.5 K (1) 和 -5.9 K (2)。使用各向同性梯子模型对2的磁响应进行的理论模拟与实验结果非常吻合。测量到5 T的磁化强度表明是量子主导的磁谱。同样，计算得到的2的较低和饱和（分别为4.3和24 T）临界场与实验测量结果一致，并且非常低温度下的磁化数据确实表明存在量子效应。此外，对短程和长程自旋有序的计算研究表明，在较低温度下二维到三维的转变可能是可行的。玻尔兹曼布居分析证实了在单重基态之上存在可及的三重态，从而实现了上述二维到三维的转变。
