Organic Spin Clusters. A Dendritic-Macrocyclic Poly(arylmethyl) Polyradical with Very High Spin of <i>S</i> = 10 and Its Derivatives: Synthesis, Magnetic Studies, and Small-Angle Neutron Scattering

Synthesis and characterization of organic spin clusters, high-spin poly(arylmethyl) polyradicals with 24 and 8 triarylmethyls, are described. Polyether precursors to the polyradicals are prepared via modular, multistep syntheses, culminating in Negishi cross-couplings between four monofunctional branch (dendritic) modules and the tetrafunctional calix[4]arene-based macrocyclic core. The corresponding carbopolyanions are prepared and oxidized to polyradicals in tetrahydrofuran-d8. The measured values of S, from numerical fits of magnetization vs magnetic field data to Brillouin functions at low temperatures (T = 1.8−5 K), are S = 10 and S = 3.6−3.8 for polyradicals with 24 and 8 triarylmethyls, respectively. Magnetizations at saturation (Msat) indicate that 60−80% of unpaired electrons are present at T = 1.8−5 K. Low-resolution shape reconstructions from the small-angle neutron scattering (SANS) data indicate that both the polyradical with 24 triarylmethyls and its derivatives have dumbbell-like shapes with overall dimensions 2 × 3 × 4 nm, in agreement with the molecular shapes of the lowest energy conformations obtained from Monte Carlo conformational searches. On the basis of these shapes, the size of the magnetic anisotropy barrier in the polyradical, originating in magnetic shape anisotropy, is estimated to be in the milliKelvin range, consistent with the observed paramagnetic behavior at T ≥ 1.8 K. For macromolecular polyradicals, with the elongated shape and the spin density similar to the polyradical with 24 triarylmethyls, it is predicted that the values of S on the order of 1000 or higher may be required for “single-molecule-magnet” behavior, i.e., superparamagnetic blocking (via coherent rotation of magnetization) at the readily accessible temperatures T > 2 K.本文报道了有机自旋团簇的合成与表征工作，以及分别含有24个和8个三芳甲基（triarylmethyl）的高自旋聚芳甲基多自由基（poly(arylmethyl) polyradicals）。该类多自由基的聚醚前驱体通过模块化多步合成路线制备，最终以四个单功能支化（树突状）模块与四功能杯[4]芳烃基大环核心之间的根岸（Negishi）交叉偶联反应完成合成。将对应的碳聚阴离子制备得到后，在氘代四氢呋喃（tetrahydrofuran-d8）中将其氧化为多自由基。在低温（T=1.8~5 K）条件下，通过将磁化强度随磁场变化的数据与布里渊函数（Brillouin functions）进行数值拟合，测得含24个三芳甲基的多自由基的总自旋量子数S为10，含8个三芳甲基的多自由基的S值为3.6~3.8。饱和磁化强度（Msat）的测试结果表明，在T=1.8~5 K的温度区间内，体系中存在60%~80%的未成对电子。基于小角中子散射（small-angle neutron scattering, SANS）数据的低分辨率形貌重构结果显示，含24个三芳甲基的多自由基及其衍生物均呈哑铃状形貌，整体尺寸为2×3×4 nm，该结果与通过蒙特卡洛（Monte Carlo）构象搜索得到的最低能量构象的分子形貌一致。基于上述形貌，该多自由基中源于磁形状各向异性（magnetic shape anisotropy）的磁各向异性势垒的估算值处于毫开尔文量级，与在T≥1.8 K时观测到的顺磁行为相符。对于形貌呈伸长状、自旋密度与含24个三芳甲基的多自由基相近的高分子量多自由基，理论预测其若要表现出“单分子磁体（single-molecule-magnet）”行为——即在易达到的T>2 K温度下通过磁化相干旋转（coherent rotation of magnetization）实现超顺磁阻塞（superparamagnetic blocking）——其总自旋量子数S需达到1000及以上量级。