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Single-ion magnet behaviour in homoleptic Co(II) complexes bearing 2-iminopyrrolyl ligands
Inorganic Chemistry Frontiers ( IF 6.1 ) Pub Date : 2022-06-08 , DOI: 10.1039/d2qi00601d Patrícia S. Ferreira 1 , Ana C. Cerdeira 2 , Tiago F. C. Cruz 1 , Nuno A. G. Bandeira 3 , David Hunger 4 , Alexander Allgaier 4 , Joris van Slageren 4 , Manuel Almeida 2 , Laura C. J. Pereira 2 , Pedro T. Gomes 1
Inorganic Chemistry Frontiers ( IF 6.1 ) Pub Date : 2022-06-08 , DOI: 10.1039/d2qi00601d Patrícia S. Ferreira 1 , Ana C. Cerdeira 2 , Tiago F. C. Cruz 1 , Nuno A. G. Bandeira 3 , David Hunger 4 , Alexander Allgaier 4 , Joris van Slageren 4 , Manuel Almeida 2 , Laura C. J. Pereira 2 , Pedro T. Gomes 1
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In this report we present the structural and magnetic characterization of four distorted tetrahedral homoleptic Co(II) complexes bearing two 2-formiminopyrrolyl N,N′-chelating ligands, [Co{κ2N,N′-NC4H3-2-C(H)![[double bond, length as m-dash]](https://www.rsc.org/images/entities/char_e001.gif)
N(2,6-iPr2-C6H3)}2] (1), [Co{κ2N,N′-5-(C6H5)-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}2] (2), [Co{κ2N,N′-5-(2,6-Me2-C6H3)-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}2] (3) and [Co{κ2N,N′-5-(1-Ad)-NC4H2-2-C(H)N(1-Ad)}2] (Ad = adamantyl) (4), which display Single-Ion Magnet (SIM) behaviour. Static (dc) magnetic susceptibility measurements and high-field EPR spectroscopy showed a large and negative magnetic anisotropy with values of D = −69, −53, −48 and −52 cm−1 for complexes 1–4, respectively. These values are interpreted and reproduced by means of theoretical calculations (ab initio CASSCF/QD-NEVPT2 methods) where it was shown that the most important source of axial anisotropy stems from the first e → t2 electronic transition, in line with other tetrahedrally coordinated Co(II) complexes. Calculations on model systems show that the most favorable magnetostructural modification corresponds to a tetrahedral geometry with a strong distortion towards a trigonal based pyramid. Frequency-dependent (ac) magnetic susceptibility measurements show that the 5-substituted pyrrolyl ring derivatives 2–4 display slow relaxation of the magnetization at zero external magnetic field, whereas the 5-unsubstituted-2-iminopyrrolyl complex 1 requires the presence of a static magnetic field to exhibit this property. By applying a static magnetic field, the quantum tunnelling of magnetization (QTM) process is suppressed revealing large energy barriers (Ueff) for all the complexes studied, exhibiting values of 138, 106, 96 and 104 cm−1 for 1–4, respectively. These values are higher than the majority of tetracoordinated Co(II)-based SIMs reported in the literature. Despite large values of axial zero-field splitting, as determined by theory, the experimental energy barriers are considerably lower than expected for a pure Orbach process, indicating that other relaxation mechanisms are dominant in the range of temperatures studied.
中文翻译：
带有 2-亚氨基吡咯配体的均配 Co(II) 配合物中的单离子磁体行为
在本报告中，我们介绍了四个扭曲的四面体均配 Co( II ) 配合物的结构和磁性表征，这些配合物带有两个 2-甲亚氨基吡咯基N , N '-螯合配体 [Co{κ 2 N , N '-NC 4 H 3 -2- C(H)N(2,6- i Pr 2 -C 6 H 3 )} 2 ] ( 1 ), [Co{κ 2 N , N '-5-(C 6 H 5 )-NC 4 H 2 - 2-C(H) N(2,6- i Pr2 -C 6 H 3 )} 2 ] ( 2 ), [Co{κ 2 N , N '-5-(2,6-Me 2 -C 6 H 3 )-NC 4 H 2 -2-C(H )N(2,6- i Pr 2 -C 6 H 3 )} 2 ] ( 3 ) 和 [Co{κ 2 N , N ′-5-(1-Ad)-NC 4 H 2 -2-C( H)N(1-Ad)} 2 ] (Ad = 金刚烷基) ( 4)，显示单离子磁体 (SIM) 行为。静态 (dc) 磁化率测量和高场 EPR 光谱显示出大的负磁各向异性，配合物1-4的D值分别为-69、-53、-48 和 -52 cm -1。这些值通过理论计算（从头算CASSCF/QD-NEVPT2 方法）进行解释和再现，其中表明轴向各向异性的最重要来源源于第一个 e → t 2电子跃迁，与其他四面体协调钴( Ⅱ) 配合物。对模型系统的计算表明，最有利的磁结构修改对应于四面体几何形状，该几何形状对基于三角形的金字塔有强烈的扭曲。频率相关 (ac) 磁化率测量表明 5-取代的吡咯环衍生物2-4在零外部磁场下显示出缓慢的磁化弛豫，而 5-未取代的 2-亚氨基吡咯配合物1需要存在静态磁场来表现这种特性。通过施加静磁场，磁化量子隧穿 (QTM) 过程被抑制，显示所有研究的配合物的大能垒 ( U eff )，显示值为 138、106、96 和 104 cm-1分别代表1-4。这些值高于文献中报道的大多数基于四配位 Co( II ) 的 SIM。尽管理论确定的轴向零场分裂值很大，但实验能垒远低于纯奥尔巴赫过程的预期，表明其他弛豫机制在所研究的温度范围内占主导地位。
更新日期：2022-06-08
N(2,6-iPr2-C6H3)}2] (1), [Co{κ2N,N′-5-(C6H5)-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}2] (2), [Co{κ2N,N′-5-(2,6-Me2-C6H3)-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}2] (3) and [Co{κ2N,N′-5-(1-Ad)-NC4H2-2-C(H)N(1-Ad)}2] (Ad = adamantyl) (4), which display Single-Ion Magnet (SIM) behaviour. Static (dc) magnetic susceptibility measurements and high-field EPR spectroscopy showed a large and negative magnetic anisotropy with values of D = −69, −53, −48 and −52 cm−1 for complexes 1–4, respectively. These values are interpreted and reproduced by means of theoretical calculations (ab initio CASSCF/QD-NEVPT2 methods) where it was shown that the most important source of axial anisotropy stems from the first e → t2 electronic transition, in line with other tetrahedrally coordinated Co(II) complexes. Calculations on model systems show that the most favorable magnetostructural modification corresponds to a tetrahedral geometry with a strong distortion towards a trigonal based pyramid. Frequency-dependent (ac) magnetic susceptibility measurements show that the 5-substituted pyrrolyl ring derivatives 2–4 display slow relaxation of the magnetization at zero external magnetic field, whereas the 5-unsubstituted-2-iminopyrrolyl complex 1 requires the presence of a static magnetic field to exhibit this property. By applying a static magnetic field, the quantum tunnelling of magnetization (QTM) process is suppressed revealing large energy barriers (Ueff) for all the complexes studied, exhibiting values of 138, 106, 96 and 104 cm−1 for 1–4, respectively. These values are higher than the majority of tetracoordinated Co(II)-based SIMs reported in the literature. Despite large values of axial zero-field splitting, as determined by theory, the experimental energy barriers are considerably lower than expected for a pure Orbach process, indicating that other relaxation mechanisms are dominant in the range of temperatures studied.
中文翻译：
带有 2-亚氨基吡咯配体的均配 Co(II) 配合物中的单离子磁体行为
在本报告中，我们介绍了四个扭曲的四面体均配 Co( II ) 配合物的结构和磁性表征，这些配合物带有两个 2-甲亚氨基吡咯基N , N '-螯合配体 [Co{κ 2 N , N '-NC 4 H 3 -2- C(H)
N(2,6- i Pr 2 -C 6 H 3 )} 2 ] ( 1 ), [Co{κ 2 N , N '-5-(C 6 H 5 )-NC 4 H 2 - 2-C(H) N(2,6- i Pr2 -C 6 H 3 )} 2 ] ( 2 ), [Co{κ 2 N , N '-5-(2,6-Me 2 -C 6 H 3 )-NC 4 H 2 -2-C(H )N(2,6- i Pr 2 -C 6 H 3 )} 2 ] ( 3 ) 和 [Co{κ 2 N , N ′-5-(1-Ad)-NC 4 H 2 -2-C( H)N(1-Ad)} 2 ] (Ad = 金刚烷基) ( 4)，显示单离子磁体 (SIM) 行为。静态 (dc) 磁化率测量和高场 EPR 光谱显示出大的负磁各向异性，配合物1-4的D值分别为-69、-53、-48 和 -52 cm -1。这些值通过理论计算（从头算CASSCF/QD-NEVPT2 方法）进行解释和再现，其中表明轴向各向异性的最重要来源源于第一个 e → t 2电子跃迁，与其他四面体协调钴( Ⅱ) 配合物。对模型系统的计算表明，最有利的磁结构修改对应于四面体几何形状，该几何形状对基于三角形的金字塔有强烈的扭曲。频率相关 (ac) 磁化率测量表明 5-取代的吡咯环衍生物2-4在零外部磁场下显示出缓慢的磁化弛豫，而 5-未取代的 2-亚氨基吡咯配合物1需要存在静态磁场来表现这种特性。通过施加静磁场，磁化量子隧穿 (QTM) 过程被抑制，显示所有研究的配合物的大能垒 ( U eff )，显示值为 138、106、96 和 104 cm-1分别代表1-4。这些值高于文献中报道的大多数基于四配位 Co( II ) 的 SIM。尽管理论确定的轴向零场分裂值很大，但实验能垒远低于纯奥尔巴赫过程的预期，表明其他弛豫机制在所研究的温度范围内占主导地位。
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