Synthesis, magnetic properties, and STMspectroscopy of an unprecedented octanuclear chloro-bridged nickel(ii) double cubane

Abstract

Reaction of nickel(II) chloride hexahydrate with N-n-butyldiethanolamine H₂L (3) in the presence of LiH in anhydrous THF leads to the formation of the unique octanuclear chloro-bridged nickel(II) double cubane [({Ni^II₄(μ₃-OH)Cl₃(HL)₃}μ₂-Cl)₂] (4) in 57% yield. According to single crystal X-ray structure analysis, complex 4·4CH₂Cl₂ possesses a [({Ni₄(μ₃-OH)(μ₃-O)₃(OH)₃(N)₃(Cl)₃}μ₂-Cl)₂] core and crystallizes in the monoclinic space group P2₁/c with a = 18.292(2), b = 19.8972(5), c = 23.295(2) Å, β = 98.408(6)°, V = 8387.3(8) ų, and four molecules in the unit cell. The analysis of the SQUID magnetic susceptibility data identified 4 as a weakly coupled dimer (J₁ = 14.5 K, J₂ = −0.6 K) with a ground state of S = 0, resulting from two S = 4 states of each {Ni₄} subunits. Although complex 4 does not show an ac out-of-phase signal in a zero dc field at temperatures of 1.8 K and higher, low-temperature magnetization measurements revealed that complex 4 is a single-molecule magnet and shows hysteretic magnetization characteristics with typical temperature and sweep-rate dependencies. The eye-catching feature of complex 4 is the presence of two different blocking temperatures (0.9 K around zero field and 1.3 K at higher fields). The origin of this highly unusual behavior can be assigned to the dimer-nature of the interaction between the two S = 4 units. Furthermore STM and current imaging tunnelling spectroscopy (CITS) were performed on aggregates of 4 drop-coated on highly oriented pyrolytic graphite (HOPG) surfaces. CITS measurements show a strong contrast in the area of the nickel centers and a HOMO–LUMO gap of approximately 0.8 V.