Dey Sourav, Rajaraman Gopalan
Department of Chemistry, Indian Institute of Technology Bombay Powai Mumbai 400076 India
Chem Sci. 2021 Oct 6;12(42):14207-14216. doi: 10.1039/d1sc03925c. eCollection 2021 Nov 3.
While the blocking barrier ( ) and blocking temperature ( ) for "Dysprocenium" SIMs have been increased beyond liquid N temperature, device fabrication of these molecules remains a challenge as low-coordinate Ln complexes are very unstable. Encapsulating the lanthanide ion inside a cage such as a fullerene (called endohedral metallofullerene or EMF) opens up a new avenue leading to several Ln@EMF SMMs. The CASSCF calculations play a pivotal role in identifying target metal ions and suitable cages in this area. Encouraged by our earlier prediction on Ln@CN, which was verified by experiments, here we have undertaken a search to enhance the exchange coupling in this class of molecules beyond the highest reported value. Using DFT and calculations, we have studied a series of Gd@C (30 ≤ 2 ≤ 80), where an antiferromagnetic of -43 cm was found for a stable Gd@C- cage. This extremely large and exceptionally rare 4f⋯4f interaction results from a direct overlap of 4f orbitals due to the confinement effect. In larger cages such as Gd@C and Gd@C, the formation of two centre-one-electron (2c-1e) Gd-Gd bonds is perceived. This results in a radical formation in the fullerene cage leading to its instability. To avoid this, we have studied heterofullerenes where one of the carbon atoms is replaced by a nitrogen atom. Specifically, we have studied Ln@CN and Ln@CN, where strong delocalisation of the electron yields a mixed valence-like behaviour. This suggests a double-exchange () is operational, and CASSCF calculations yield a value of 434.8 cm and resultant of 869.5 cm for the Gd@CN complex. These parameters are found to be two times larger than the world-record reported for Gd@CN. Further calculations reveal an unprecedented of 1183 and 1501 cm for Dy@CN and Tb@CN, respectively. Thus, this study offers strong exchange coupling as criteria for new generation SMMs as the existing idea of enhancing the blocking barrier crystal field modulation has reached its saturation point.
虽然“镝”单分子磁体的阻塞势垒（ ）和阻塞温度（ ）已提高到液氮温度以上，但由于低配位的镧系配合物非常不稳定，这些分子的器件制造仍然是一个挑战。将镧系离子封装在富勒烯等笼状结构中（称为内嵌金属富勒烯或EMF）开辟了一条通往多种Ln@EMF单分子磁体的新途径。CASSCF计算在识别该领域的目标金属离子和合适的笼状结构方面起着关键作用。受我们早期对Ln@CN的预测（已通过实验验证）的鼓舞，我们在此进行了一项研究，以提高这类分子中的交换耦合，使其超过已报道的最高值。使用DFT和 计算，我们研究了一系列Gd@C（30≤ ≤80），其中对于稳定的Gd@C- 笼状结构，发现反铁磁 为-43 cm 。这种极其大且异常罕见的4f⋯4f相互作用是由于限制效应导致4f轨道直接重叠产生的。在较大的笼状结构如Gd@C和Gd@C中，可察觉到形成了双中心单电子（2c-1e）Gd-Gd键。这导致富勒烯笼状结构中形成自由基，从而导致其不稳定。为避免这种情况，我们研究了其中一个碳原子被氮原子取代的杂富勒烯。具体而言，我们研究了Ln@CN和Ln@CN，其中电子的强离域产生了类似混合价的行为。这表明存在双交换（ ）作用，CASSCF计算得出Gd@CN配合物的 值为434.8 cm ，合成 为869.5 cm 。发现这些参数比报道的Gd@CN世界纪录值大两倍。进一步的 计算分别揭示了Dy@CN和Tb@CN前所未有的 值为1183和1501 cm 。因此，由于现有的通过晶体场调制提高阻塞势垒的想法已达到饱和点，本研究提供了强交换耦合作为新一代单分子磁体的标准。
