Construction of magnet-type coordination polymers using high-spin {Ni4}-citrate cubane as secondary building units

Abstract

Three potassium(I)–nickel(II)–citrate coordination polymers, [K₄Ni₆(cit)₄(H₂O)₈]_n (1), [K₁₄Ni₁₇(cit)₁₂(H₂O)₃₃]_n·10nH₂O (2) and [K₈Ni₁₂(cit)₈(H₂O)₁₅]_n·2nH₂O (3), have been self-assembled in a solvothermal synthesis. Interestingly, these three polymers share the common {Ni₄(cit)₄}⁸⁻ cubane ({Ni₄}-cit-cub) secondary building units. The diverse ways of linking the {Ni₄}-cit-cubs and additional isolated octahedral Ni(II) ions lead to disparate magnetic exchange-coupling interactions, namely ferromagnetic for 1 and 2 and antiferromagnetic for 3. More importantly, the weak ferromagnetic interactions do not lead to long-range magnetic ordering above 2 K in 1 or 2, whereas the strong antiferromagnetic interaction in 3 leads to uncompensated magnetic moment due to the non-collinear alignment of the spins. Further magnetic characterization confirms the coexistence of spin-canted antiferromagnetism and spin glass behaviour in 3.