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. 2021 May 12;12(1):2738.
doi: 10.1038/s41467-021-23057-4.

Water-oriented magnetic anisotropy transition

Sheng-Qun Su  1 , Shu-Qi Wu  1 , Masato Hagihala  2   3 , Ping Miao  2   4   5 , Zhijian Tan  2   4   5 , Shuki Torii  2 , Takashi Kamiyama  2 , Tongtong Xiao  6 , Zhenxing Wang  6 , Zhongwen Ouyang  6 , Yuji Miyazaki  7 , Motohiro Nakano  7 , Takumi Nakanishi  1 , Jun-Qiu Li  1 , Shinji Kanegawa  1 , Osamu Sato  8
Affiliations

Water-oriented magnetic anisotropy transition

Sheng-Qun Su et al. Nat Commun. .

Abstract

Water reorientation is essential in a wide range of chemical and biological processes. However, the effects of such reorientation through rotation around the metal-oxygen bond on the chemical and physical properties of the resulting complex are usually ignored. Most studies focus on the donor property of water as a recognized σ donor-type ligand rather than a participant in the π interaction. Although a theoretical approach to study water-rotation effects on the functionality of a complex has recently been conducted, it has not been experimentally demonstrated. In this study, we determine that the magnetic anisotropy of a Co(II) complex can be effectively controlled by the slight rotation of coordinating water ligands, which is achieved by a two-step structural phase transition. When the water molecule is rotated by 21.2 ± 0.2° around the Co-O bond, the directional magnetic susceptibility of the single crystal changes by approximately 30% along the a-axis due to the rotation of the magnetic anisotropy axis through the modification of the π interaction between cobalt(II) and the water ligand. The theoretical calculations further support the hypothesis that the reorientation of water molecules is a key factor contributing to the magnetic anisotropy transition of this complex.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Molecular structure and interactions of complex 1.
a Molecular structure of crystal 1 recorded at 190 K. b The plane of the coordinated water molecule and the molecular plane form the dihedral angle φ with a value of 83.99°. c The dihedral angles ψ and ω, which are related to nitrates, are 133.54° and 159.65°, respectively. d The entire structure is mainly stabilized by O–H···O hydrogen bonds and C–H–π interactions; the water molecule acts as a hydrogen bond donor. Maroon, Co; gray, C; blue, N; red, O; light gray, H.
Fig. 2
Fig. 2. Temperature dependence of the heat capacity (Cp) for 1.
The molar heat capacities under constant pressure (Cp) showed two sharp peaks accompanying the latent heat at 113.5 and 157.4 K (there is a shoulder peak at 165.3 K); and exhibited the super-cooling phenomenon. Blue dots, black circles and red circles represent the data obtained during the heating mode after the samples were cooled to 8.0, 110.9, and 160.9 K, respectively.
Fig. 3
Fig. 3. Variations in the orientation of the water molecule in complex 1.
The coordinated water molecule undergoes a reversible two-step rotation in response to the temperature, which can be induced by the direction of the hydrogen bonds, O–H···O, between the water and nitrates after the structural phase transition. The top images show the orientations of the water molecule at each temperature, where the corresponding dihedral angle, φ, changes from 61.71° at 70 K to 70.66° and 68.19° at 140 K, and to 85.30° at 190 K. Maroon, Co; gray, C; blue, N; red, O; light gray, H.
Fig. 4
Fig. 4. Determination of the magneto-crystalline anisotropy and its variations in single crystal 1.
a The shape, faces, and the x-, y-, and z-directions of the single crystal used in magnetic measurements, where the x-direction is parallel to the a-axis, the y-axis is perpendicular to the (010) plane, and the z-axis is perpendicular to the xy plane. The relationship between the x-axis and the molecular orientation in the crystal is shown. b Temperature dependence of the χMT (χMxT, χMyT, and χMzT) values for single crystal 1. c Left: resonance field versus microwave frequency (quantum energy) for the EPR transitions of 1, where the green, blue, and red lines correspond to the simulations using the best fit spin Hamiltonian parameters with the magnetic field H parallel to the X-, Y-, and Z-axes of the ZFS tensor, respectively. The vertical dashed line represents the frequency (120 GHz). Right: The HF-EPR spectrum with its simulations at 4.2 K and 120 GHz. d Angular dependence of the magnetic susceptibility measured at 5 and 190 K for the rotation along the x-, y-, and z-axes, where the solid lines represent the calculated values. e Experimental and ab initio calculated hard axis of the magnetization in crystal 1. The angle between the experimental hard axis and the a-axis changes from 49.8° at 5 K to 61.4° at 190 K. f Experimental and ab initio calculated χMxT curves in LTp and HTp. The changes in the calculated values after the phase transition are consistent with the experimental data. Maroon, Co; gray, C; blue, N; red, O; light gray, H.
Fig. 5
Fig. 5. Variations in the magnetic anisotropy parameters with the rotation of water and nitrates from ab initio calculations.
a Two molecular models (model 1, rotation of the water molecule; model 2, rotation of nitrates) based on the structure of LTp for calculations. b Magnetic susceptibility along the a-axis. c Deviation in the angle, σ, change with the rotation angle in the two models. d Schematic diagram of the change of the direction of the magnetic hard axis with the rotation of the water molecules. e Overlap between the d orbital centered on the cobalt and the p orbitals of the oxygen from the water and the nitrates in the highest fully occupied d orbital (dx′y′), where the red color corresponds to the regions where the phase of the wave function is positive, and the blue color corresponds to the regions where the phase of the wave function is negative. The reference coordinate axes are defined in Supplementary Fig. 14a. f Energy differences between the d orbitals and the dy′z′ orbital (ΔE) with respect to those of the initial state [ΔE(0°)] change with the rotation of water. Maroon, Co; gray, C; blue, N; red, O; light gray, H.
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