Crystal structures and magnetic properties of verdazyl-based complexes with transition metals

Verdazyl-based complexes, $\left[\mathrm{Zn}\right({\mathrm{hfac})}_2\left]\right(m\text{−}\mathrm{Py}\text{−}{\mathrm{V})}_2·{\mathrm{CH}}_3\mathrm{CN}$ and $\left[\mathrm{Cu}\right({\mathrm{hfac})}_2\left]\right[m\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}({p\text{−}\mathrm{F})}_2{]}_2$, were successfully synthesized. For $\left[\mathrm{Zn}\right({\mathrm{hfac})}_2](m\text{−}\mathrm{Py}\text{−}{\mathrm{V})}_2·{\mathrm{CH}}_3\mathrm{CN}$, molecular orbital (MO) calculations indicated the formation of a spin-$\frac{1}{2}$ antiferromagnetic (AFM) chain with fourfold magnetic periodicity consisting of three types of exchange interactions. We quantitatively explained the magnetic properties based on the expected spin model. For $\left[\mathrm{Cu}\right({\mathrm{hfac})}_2\left]\right[m\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}({p\text{−}\mathrm{F})}_2{]}_2$, MO calculations indicated the formation of a spin-$\frac{1}{2}$ spin lattice, topologically equivalent to distorted octagons arranged two-dimensionally. We reproduced the observed magnetic properties assuming a spin-$\frac{1}{2}$ AFM dimer of radical spins, with residual Cu spins exhibiting paramagnetic behavior. Furthermore, we evaluated the anisotropic $g$ values from the electron spin resonance absorption spectra, confirming that the low-temperature magnetic properties are mainly attributed to the Cu spins. These results demonstrate the realization of unconventional spin systems composed of organic radicals and transition metals and propose an effective method for spin arrangement design.

Figure 1

Molecular structure and spin-density distribution on radicals of (a) [Zn(${\mathrm{hfac})}_2]$($m$-Py-${\mathrm{V})}_2·{\mathrm{CH}}_3\mathrm{CN}$ and (b) [Cu(${\mathrm{hfac})}_2\left]\right[m$-Py-V-(${p\text{−}\mathrm{F})}_2{]}_2$. Hydrogen atoms are omitted for clarity. The purple and light green shapes correspond to positive and negative spin densities, respectively. The isodensity surface corresponds to a cutoff value of $0.001e{\text{Bohr}}^{-3}$. The circles at the center of $m$-Py-V represent spin $\frac{1}{2}$ on the radical. The thick line shows intramolecular exchange interaction $J_2$ originating from the overlapping of the $\pi$ orbitals on $m$-Py-V.

Figure 2

(a) Crystal structure of [Zn(${\mathrm{hfac})}_2]$($m$-Py-${\mathrm{V})}_2·{\mathrm{CH}}_3\mathrm{CN}$ forming a spin chain along the $c$ axis. The dashed lines indicate the N-N and N-C short contacts associated with the exchange interactions. Hydrogen atoms, ${\mathrm{CH}}_3\mathrm{CN}$ molecules, and Zn(${\mathrm{hfac})}_2$ moieties are omitted for clarity. The radical pair associated with $J_2$ corresponds to the two radicals in the same molecule. (b) Corresponding spin-$\frac{1}{2}$ antiferromagnetic (AFM) chain with fourfould magnetic periodicity. For $T\ll J_3/k_{\mathrm{B}}$, an effective spin-$\frac{1}{2}$ AFM alternating chain is formed by $J_1$ and $J_{\mathrm{eff}}$. (c) Crystal structure viewed along the $c$ axis. The broken line encloses the molecules comprising each spin chain structure.

Figure 3

Intermolecular radical pairs of [Cu(${\mathrm{hfac})}_2\left]\right[m$-Py-V-(${p\text{−}\mathrm{F})}_2{]}_2$ associated with exchange interactions (a) $J_{\mathrm{V}1}$ and (b) $J_{\mathrm{V}2}$. Hydrogen atoms and hfac moieties are omitted for clarity. The dashed lines indicate N-N and N-C short contacts. (c) Crystal structure forming a two-dimensional (2D) spin lattice in the $ac$ plane and (d) corresponding spin-$\frac{1}{2}$ spin model topologically equivalent to the distorted octagons arranged 2D. $S_{\mathrm{V}}$ and $S_{\mathrm{Cu}}$ represent the spins on the radical and ${\mathrm{Cu}}^{2+}$ ion, respectively. (e) Crystal structure in the $bc$ plane. The thick lines represent the 2D spin planes.

Figure 4

Temperature dependence of (a) magnetic susceptibility ($\chi =M/H$) and (b) $\chi T$ of [Zn(${\mathrm{hfac})}_2]$($m$-Py-${\mathrm{V})}_2·{\mathrm{CH}}_3\mathrm{CN}$ at 0.5 T. (c) Magnetization curve of [Zn(${\mathrm{hfac})}_2]$($m$-Py-${\mathrm{V})}_2·{\mathrm{CH}}_3\mathrm{CN}$ at 1.8 K. The open triangles denote raw data, and the open circles are corrected for the paramagnetic term due to the impurity. The solid lines represent the calculated results for the spin-$\frac{1}{2}$ antiferromagnetic (AFM) chain with fourfold periodicity. The broken line shows the calculated magnetization curve at 0.1 K, indicating an energy gap of 2.5 T.

Figure 5

Temperature dependence of (a) magnetic susceptibility ($\chi$ = $M/H$) and (b) $\chi T$ of [Cu(${\mathrm{hfac})}_2\left]\right[m$-Py-V-(${p\text{−}\mathrm{F})}_2{]}_2$ at 0.5 T. The solid and broken lines represent the calculated results for the spin-$\frac{1}{2}$ antiferromagnetic (AFM) dimer of $S_{\mathrm{V}}$ with paramagnetic contribution of $S_{\mathrm{Cu}}$ and those considering the effective interactions between $S_{\mathrm{Cu}}$ spins, respectively. The inset shows the expansion of the low-temperature region. (c) Magnetization curves at 1.8 and 4.2 K. The solid lines represent the Brillouin function for spin-$\frac{1}{2}$ with the averaged $g$ value.

Figure 6

Temperature dependence of electron paramagnetic resonance (ESR) absorption spectra of [Cu(${\mathrm{hfac})}_2\left]\right[m$-Py-V-(${p\text{−}\mathrm{F})}_2{]}_2$ at 34.14 GHz. The solid line represents the powder pattern simulation.