Carbonate-Bridged Lanthanoid Triangles: Single-Molecule Magnet Behavior, Inelastic Neutron Scattering, and Ab Initio Studies
Carbonate-Bridged Lanthanoid Triangles: Single-Molecule Magnet Behavior, Inelastic Neutron Scattering, and Ab Initio Studies
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DOI：
10.1021/acs.inorgchem.6b00108
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发表时间：
2016-06-06
影响因子：
4.6
通讯作者：
Boskovic, Colette
中科院分区：
文献类型：
--
作者：
Giansiracusa, Marcus J.;Vonci, Michele;Boskovic, Colette
对文献合成工艺的优化，以中等产率提供了五个类似物Na-11[{RE(OH2)}(3)CO3(PW9O34)(2)](1-RE；RE=Y、Tb、Dy、Ho和Er)的均一和结晶样品。相转移法分离了混合盐(Et4N)(9)Na-2[{RE(OH2)}(3)CO3(PW9O34)(2)](2-RE；RE=Y和Er)。这些化合物中的等结构多阴离子是由三价稀土离子的三角形排列组成的，这些三价稀土离子由一个Mu(3)-碳酸酯配体连接，并夹在两个三方Keggin{PW9O34}多金属氧酸盐配体之间。1-Dy、1-Er和2-Er的交流(AC)磁化率研究表明，在外加磁场的情况下，在最低测量温度下，异相磁化率的频率依赖性开始出现。1-Ho和1-Er的非弹性中子散射(INS)谱在Ho-III和Er-III离子的J=8和15/2基态自旋轨道多重态的最低晶场分裂态之间发生跃迁。对这两个类似物进行的互补从头计算可以很好地再现实验磁化率和低温磁化数据，并与实验INS数据合理地吻合。从头算结果表明，每个络合物中三个Ln(III)离子配位环境的微小差异导致了Cf裂解的差异，这些差异并不是不显著的。这一理论结果与交流磁化率观测到的多重弛豫过程和测量到的InS峰的宽度是一致的。从头计算还表明，M-J对每个Ln(III)离子的Cf分裂能级的贡献发生了实质性的混合。计算表明，1-HO中HO-III中心的Cf基态主要由小M-J贡献，而1-Er中Er-III中心的Cf基态主要由大M-J贡献组成，从而产生缓慢的磁弛豫。尽管在磁学和INS研究中都没有观察到分子内稀土-稀土磁性耦合的直接证据，但在从头计算的基础上，我们发现1-Er中的非共线磁轴与Er3+三角形共面，并且相对于该三角形的质心呈放射状排列，因此，我们认为该体系中没有磁耦合是由于偶极和反铁磁超交换相互作用相互抵消造成的。
Optimization of literature synthetic procedures has afforded, in moderate yield, homogeneous and crystalline samples of the five analogues Na-11[{RE(OH2)}(3)CO3(PW9O34)(2)] (1-RE; RE = Y, Tb, Dy, Ho, and Er). Phase-transfer methods have allowed isolation of the mixed salts (Et4N)(9)Na-2[{RE(OH2)}(3)CO3(PW9O34)(2)] (2-RE; RE = Y and Er). The isostructural polyanions in these compounds are comprised of a triangular arrangement of trivalent rare-earth ions bridged by a mu(3)-carbonate ligand and sandwiched between two trilacunary Keggin {PW9O34} polyoxometalate ligands. Alternating-current (ac) magnetic susceptibility studies of 1-Dy, 1-Er, and 2-Er reveal the onset of frequency dependence for the out-of phase susceptibility in the presence of an applied magnetic field at the lowest measured temperatures. Inelastic neutron scattering (INS) spectra of 1-Ho and 1-Er exhibit transitions between the lowest-lying crystal-field (CF) split states of the respective J = 8 and 15/2 ground-state spin-orbit multiplets of the Ho-III and Er-III ions. Complementary ab initio calculations performed for these two analogues allow excellent reproduction of the experimental magnetic susceptibility and low-temperature magnetization data and are in reasonable agreement with the experimental INS data. The ab initio calculations reveal that the slight difference in coordination environments of the three Ln(III) ions in each complex gives rise to differences in the CF splitting that are not insignificant. This theoretical result is consistent with the observation of multiple relaxation processes by ac magnetic susceptibility and the broadness of the measured INS peaks. The ab initio calculations also indicate substantial mixing of the M-J contributions to the CF split energy levels of each Ln(III) ion. Calculations indicate that the CF ground states of the Ho-III centers in 1-Ho are predominantly comprised of contributions from small M-J, while those of the Er-III centers in 1-Er are predominantly comprised of contributions from large M-J, giving rise to slow magnetic relaxation. Although no direct evidence for intramolecular RE RE magnetic coupling is observed in either magnetic or INS studies, on the basis of the ab initio calculations, we find noncollinear magnetic axes in 1-Er that are coplanar with the erbium triangle and radially arranged with respect to the triangle's centroid; thus, we argue that the absence of magnetic coupling in this system arises from dipolar and antiferromagnetic superexchange interactions that cancel each other out.