Large Anisotropy Barrier in a Tetranuclear Single-Molecule Magnet Featuring Low-Coordinate Cobalt Centers

Abstract

The tetranuclear cobalt cluster compound [Co(μ-NPBu)][B(CF)] (Bu = tert-butyl) was synthesized by chemical oxidation of Co(NPBu) with [FeCp][B(CF)] and magnetically characterized to study the effect of electronic communication between low-coordinate metal centers on slow magnetic relaxation in a transition metal cluster. The dc magnetic susceptibility data reveal that the complex exhibits a well-isolated S = / ground state, which persists even to 300 K and is attributed to the existence of direct metal-metal orbital overlap. The ac magnetic susceptibility data further reveals that the complex exhibits slow magnetic relaxation in the absence of an applied field, and that the relaxation dynamics can be fit with a combination of Orbach, quantum tunneling, and Raman relaxation processes. The effective spin reversal barrier for this molecule is 87 cm, the largest reported to date for a transition metal cluster, and arises due to the presence of a large easy-axis magnetic anisotropy. The complex additionally exhibits waist-restricted magnetic hysteresis and magnetic blocking below 3.6 K. Taken together, these results indicate that coupling of low-coordinate metal centers is a promising strategy to enhance magnetic anisotropy and slow magnetic relaxation in transition metal cluster compounds.

An iron ketimide single-molecule magnet [Fe4(NCPh2)6] with suppressed through-barrier relaxation

“…This value is among the highest reported for an iron cluster to date, 15,17 and can be explained by strong electron delocalization via direct exchange. 1 A zero-eld 57 Fe Mössbauer spectrum of complex 1 taken at 90 K (Fig. 2c) reveals a sharp quadrupole doublet with an isomer shi of d ¼ 0.34 mm s À1 and a quadrupole coupling of DE Q ¼ 0.79 mm s À1 , in line with other mixed-valent Fe(II)/Fe(I) clusters.…”

“…There has therefore been much effort to increase ordering temperatures by maximizing the moments and magnetic anisotropies of SMMs. 1,2 However, it is oen the case that molecules with large U nevertheless do not show long lifetimes or signicant zero-eld magnetic hysteresis. 1,[3][4][5][6][7][8][9][10] In these cases, through-barrier relaxation mechanisms (i.e., quantum tunnelling or Raman relaxation) are typically to blame.…”

“…For example, Betley and coworkers isolated a series of Fe clusters, [ (18-crown-6) )3 ), with thermally-persistent spin ground states of S ¼ 11/2, 19/ 2, and 11, respectively, [14][15][16][17] while Long and co-workers reported a cobalt phosphiniminato complex, [Co 4 (N]P t Bu 3 ) 4 ] + , with a S ¼ 9/ 2 ground state. 1 Notably, direct exchange is thought to be responsible for the large spin ground states in these examples. That said, only a narrow range of ligands have been shown to promote direct exchange, including phosphiniminato, 1 Betley's hexaamide framework, [14][15][16][17][18] diphenylformamidinate (DPhF), 19 and guanidinate.…”

“…1 Notably, direct exchange is thought to be responsible for the large spin ground states in these examples. That said, only a narrow range of ligands have been shown to promote direct exchange, including phosphiniminato, 1 Betley's hexaamide framework, [14][15][16][17][18] diphenylformamidinate (DPhF), 19 and guanidinate. 20 As a result, the identication of new ligands that could promote these interactions remains an important goal for the eld.…”

NiII‐LnIII Heterometallic Complexes as Single‐Molecule Magnets

“…Soon many complexes, of varying nuclearity and geometry have been synthesized which contain various 3d metal ions such as V III , Mn II / III / IV , Fe I , Fe II , Fe III , Co II[14] and Ni II . The highest nuclearity cluster reported so far is a Mn III 84 cluster whereas the highest energy barrier was reported in [Co II Co I 3 (NPtBu 3 ) 4 ][B(C 6 F 5 ) 4 ] (U eff = 87 K with a large anisotropy barrier D = –12.34 cm –1 Figure ) . Several literature reports and reviews describe the interesting SMM properties of paramagnetic 3d metal ion clusters, some representative examples are shown in Figure .…”