Are lanthanide-transition metal direct bonds a route to achieving new generation {3d-4f} SMMs?
Are lanthanide-transition metal direct bonds a route to achieving new generation {3d-4f} SMMs?
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DOI：
10.1039/d1dt02256c
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发表时间：
2021-10
影响因子：
4
通讯作者：
Abinash Swain;Asmita Sen;G. Rajaraman
中科院分区：
文献类型：
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作者：
Abinash Swain;Asmita Sen;G. Rajaraman
镧系单分子磁体因其在新兴技术中的潜在应用而受到广泛关注。这一领域的主要挑战之一是淬灭磁化量子隧穿（QTM），这通常会削弱磁化反转势垒。在所采用的几种策略中，已详细研究了增强交换耦合，大交换导致更强的QTM效应猝灭。然而，镧系元素受到深埋的4f轨道提供的弱交换的影响，并且许多试图增强{3d-4f}对中的交换耦合的尝试都没有超过大于30 cm-1的值。在这项工作中，使用DFT和从头计算CASSCF/RASSI-SO方法相结合，我们探索了镧系元素-过渡金属直接键作为工具，猝灭QTM效应。在这个方向上，我们已经模拟了[PyCp 2LnMCp（CO）2]（Ln = Gd（III）、Dy（III）和Er（III），M = V（0）、Mn（0）、Co（0）和Fe（I），这里PyCp 2 = [2，6-（CH 2C 5 H3）2C 5 H3 N]2-，使用[PyCp 2DyFeCp（CO）2]作为Nippe等人（C. P. Burns，X.杨，J.D. Wofford，N. S.布瓦内什，M. B。霍尔和M。Nippe，Angew.化学成分：2018，57，8144）。成键分析表明，在Gd-V和Gd-Mn的情况下，Ln-TM键具有π（V/Mndxy-π*CO）到5dz 2（Gd）的贡献，而在Gd-Co的情况下，4s（Co）到5dxy/5dyz（Gd），在所有情况下，过渡金属离子都以低自旋S = ½构型存在。B3LYP/TZV（Gd;对[PyCp 2GdMCp（CO）2]的CSDZ）计算得到JGd-V = -46.1 cm-1、JGd-Mn = -57.1 cm-1、JGd-Co = +55.3 cm-1、JGd-Fe+ = +13.9 cm-1、JGd-Vhs = -162.1 cm-1和JGd-Mnhs = -343.9 cm-1，并揭示了{3d-4f}络合物的创纪录的高J值。磁耦合的机制，揭示了强3d-4f轨道重叠在大多数情况下研究，导致反铁磁交换占主导地位的存在。当这些重叠较弱，3d Gd（5dz 2），电荷转移占主导地位，产生的Gd-Co/Gd-Fe+复合物的铁磁耦合。对各向异性Dy（III）和Er（III）配合物的计算表明，基态gzz轴分别沿着Cp-Ln-Cp轴和Ln-TM键。因此，Ln-TM键通过提供赤道连接和降低mJ = ±½态能量来阻碍Dy（III）的单离子各向异性，同时有助于增强Er（III）的轴性。当引入强{3d-4f}交换耦合时，实现了高达229 cm-1的创纪录高势垒高度。此外，交换偶联消除了QTM效应，并建议镧系元素-过渡金属直接键作为增强交换偶联的可行替代方案，以使{3d-4f}络合物回到高阻断SMM的竞争中。
Lanthanide based single-molecule magnets are gaining wide attention due to their potential applications in emerging technologies. One of the main challenges in this area is quenching quantum tunnelling of magnetisation (QTM), which often undercuts the magnetisation reversal barrier. Among the several strategies employed, enhancing exchange coupling has been studied in detail, with large exchanges resulting in stronger quenching of QTM effects. Lanthanides, however, suffer from weak exchanges offered by the deeply buried 4f orbitals and the numerous attempts to enhance the exchange coupling in the {3d-4f} pairs have not exceeded values larger than 30 cm-1. In this work, using a combination of DFT and the ab initio CASSCF/RASSI-SO method, we have explored lanthanide-transition metal direct bonds as a tool to quench QTM effects. In this direction, we have modelled [PyCp2LnMCp(CO)2] (Ln = Gd(III), Dy(III), and Er(III) and M = V(0), Mn(0), Co(0) and Fe(I) and here PyCp2 = [2,6-(CH2C5H3)2C5H3N]2- using [PyCp2DyFeCp(CO)2] as an example as reported by Nippe et al. (C. P. Burns, X. Yang, J. D. Wofford, N. S. Bhuvanesh, M. B. Hall and M. Nippe, Angew. Chem., Int. Ed. 2018, 57, 8144). Bonding analysis reveals a dative Ln-TM bond with a donation of π(V/Mndxy-π*CO) to 5dz2 (Gd) in the case of Gd-V and Gd-Mn and 4s(Co) to 5dxy/5dyz (Gd) for Gd-Co with the transition metal ion being found in the low-spin S = ½ configurations in all the cases. B3LYP/TZV (Gd;CSDZ) calculations on [PyCp2GdMCp(CO)2] yield JGd-V = -46.1 cm-1, JGd-Mn = -57.1 cm-1, JGd-Co = +55.3 cm-1, JGd-Fe+ = +13.9 cm-1, JGd-Vhs = -162.1 cm-1 and JGd-Mnhs = -343.9 cm-1 and unveiling record-high J values for {3d-4f} complexes. The mechanism of magnetic coupling is developed, which discloses the dominating presence of strong 3d-4f orbital overlaps in most of the cases studied, leading to antiferromagnetic exchange. When these overlaps are weaker and 3d to Gd(5dz2), charge transfer dominates, yielding a ferromagnetic coupling for the Gd-Co/Gd-Fe+ complexes. Calculations performed on the anisotropic Dy(III) and Er(III) complexes reveal that the ground state gzz axis lies along the Cp-Ln-Cp axis and the Ln-TM bonds, respectively. Thus the Ln-TM bond hinders the single-ion anisotropy of Dy(III) by offering equatorial ligation and lowering the mJ = ±½ state energy, and at the same time, helping in enhancing the axiality of Er(III). When strong {3d-4f} exchange couplings are introduced, record-high barrier heights as high as 229 cm-1 were accomplished. Furthermore, the exchange coupling annihilates the QTM effects and suggests the lanthanide-transition metal direct bond as a viable alternative to enhance exchange coupling to bring {3d-4f} complexes back in the race for high-blocking SMMs.