Magnetic coupling of guest metallocene molecules with SURMOF-2 host matrix

Metal-organic frameworks (MOFs) are crystalline and porous, molecular solids consisting of metal nodes and organic ligands. An interesting example is the MOF-2 system, where ${\mathrm{Cu}}^{2+}$ ions form antiferromagnetically coupled dimers to yield so-called paddlewheels. In the case of surface-anchored MOF-2 (SURMOF-2) systems the ${\mathrm{Cu}}^{2+}$ ions are connected via carboxylate and OH groups in a zipperlike fashion. This unusual coupling of the spin-1/2 ions within the resulting one-dimensional chains stabilizes a low-temperature ferromagnetic (FM) phase. In this study, the magnetic properties of two SURMOF-2 systems (Cu(bdc) and Cu(bpdc) with $\mathrm{bdc}=1, 4\text{-benzendicarboxylic}$ acid, $\mathrm{bpdc}=4$,4-biphenyldicarboxylic acid) were investigated using x-ray magnetic circular dichroism both in the absorption and in the scattering geometry. The presence of the FM phase in these SURMOF-2 systems is confirmed. Taking advantage of the element sensitivity of this technique. it was established that the magnetic signal originates from ${\mathrm{Cu}}^{2+}$ ions. After loading of SURMOF-2 with metallocene molecules, no remarkable changes of the magnetic properties of the host matrix were observed. However, the magnetic behavior of the guest molecules, as it turned out, is rather different. In the case of nickelocene loading, a polarization effect was found, resulting in ferromagnetic ordering of the guest molecules. However, in the case of manganocene derivatives, the polarization effect is not observed and these molecules remained in their paramagnetic state. The details of these effects are discussed.

Figure 1

Cu(bdc) (a) and Cu(bpdc) (b) SURMOF-2 structures (blue: copper, red: oxygen, brown: carbon, white: hydrogen). The zipperlike chain of ${\mathrm{Cu}}^{2+}$ is presented; Metallocene molecules: brown, carbon: white, hydrogen: magenta Me (Ni, Mn). Sketches of the host matrix and guest molecules are presented only for illustration.

Figure 2

X-ray absorption spectra [$\left(I^{+}+I^{-}\right)/2$, black lines] and corresponding XMCD signals ($I^{+}-I^{-}$, red-filled areas) measured on the Cu $L_{3,2}$ edges of Cu(bdc) (a) and Cu(bpdc) (b) SURMOF-2 in the grazing-incidence geometry. The magnetic field during the measurements was ±8 T, pointing parallel to the incident beam. Magnetization curves $\left(I^{+}+I^{-}\right)/\left(I^{+}-I^{-}\right)$ of Cu(bdc) (c) and Cu(bpdc) (d) SURMOF-2 measured at $T=8.4$ K for in-plane magnetization.

Figure 3

Temperature dependence of the XMCD signal measured at normal incidence on Cu(bdc) SURMOF-2 film.

Figure 4

Cu(bpdc)(100) Bragg peak measured: (a) in θ/2θ mode at the fixed photon energy corresponding to the maximum of the Cu $L_3$ edge and (b) at fixed θ/2θ angle positions while varying the photon energy around this absorption edge. The black curve was measured at zero field, the red and green curves at 8 and −8 T, correspondingly.

Figure 5

X-ray absorption spectra $\left(I^{+}+I^{-}\right)/2$ (black lines) measured at Cu (a) and Ni $L_{3,2}$ edges (b) on nickelocene-loaded Cu(bdc) samples in the grazing-incidence geometry. The corresponding XMCD signals $I^{+}-I^{-}$ are shown as red-filled area. Magnetic field during the XMCD measurements was ±8 T, parallel to the incident beam. Magnetization curves (points) of nickelocene-loaded Cu(bdc) measured at the Cu $L_{3,2}$ edges (c) and the Ni $L_{3,2}$ edges (d). The black curves in (c), (d) are the calculation using mean-field approximation for ferromagnetic coupling between Cu magnetic moments. The red curve in (d) is a fit by a simple model taking into account the coupling between Cu and Ni ions (for details see the text). Pure paramagnetic behavior of the Ni ions is presented by the blue curve (d).

Figure 6

X-ray absorption spectra $\left(I^{+}+I^{-}\right)/2$ (black lines) measured at the Cu $L_{3,2}$ edges (a) and the Mn $L_{3,2}$ edges (b) on Cu(bdc) in the grazing-incidence geometry. The corresponding XMCD signals $I^{+}-I^{-}$ are shown as red-filled area. The magnetic field during the XMCD measurements was ±8 T, parallel to the incident beam. Magnetization curves of Mn*-loaded Cu(bdc) measured at in-plane (black points) magnetization at $T=8.4$ K. Experimental curves were measured at the Cu $L_{3,2}$ edges (c) and the Mn $L_{3,2}$ edges (d). Solid line in (d) is an isotropic Brillouin function ($T=8.4$ K, $S=2$; the vertical scaling was chosen to fit the calculated curve with the corresponding experimental data).