Polynuclear and Polymeric Gadolinium Acetate Derivatives with Large Magnetocaloric Effect

Two ferromagnetic μ-oxoacetate-bridged
gadolinium
complexes [Gd2(OAc)2(Ph2acac)4(MeOH)2] (1) and [Gd4(OAc)4(acac)8(H2O)4] (2) and two polymeric Gd­(III) chains [Gd­(OAc)3(MeOH)]n (3) and [Gd­(OAc)3(H2O)0.5]n (4) (Ph2acacH = dibenzoylmethane; acacH = acetylacetone)
are reported. The magnetic studies reveal that the tiny difference
in the Gd–O–Gd angles (Gd···Gd distances)
in these complexes cause different magnetic coupling. There exist
ferromagnetic interactions in 1–3 due to the presence of the larger Gd–O–Gd angles (Gd···Gd
distances), and antiferromagnetic interaction in 4 when
the Gd–O–Gd angle is smaller. Four gadolinium acetate
derivatives display large magnetocaloric effect (MCE). The higher
magnetic density or the lower MW/NGd ratio they have, the larger MCE they display.
Complex 4 has the highest magnetic density and exhibits
the largest MCE (47.7 J K–1 kg–1). In addition, complex 3 has wider temperature and/or
field scope of application in refrigeration due to the dominant ferromagnetic
coupling. Moreover, the statistical thermodynamics on entropy was
successfully applied to simulate the MCE values. The results are quite
in agreement with those obtained from experimental data.

本文报道了两种铁磁性μ-氧代乙酸根桥联钆配合物[Gd₂(OAc)₂(Ph₂acac)₄(MeOH)₂]（1）与[Gd₄(OAc)₄(acac)₈(H₂O)₄]（2），以及两种聚合态钆(III)链状配合物[Gd(OAc)₃(MeOH)]ₙ（3）和[Gd(OAc)₃(H₂O)₀.₅]ₙ（4）；其中Ph₂acacH为二苯甲酰甲烷，acacH为乙酰丙酮。磁性研究表明，上述配合物中Gd-O-Gd键角（Gd···Gd间距）的细微差异会引发不同的磁耦合行为：由于具备更大的Gd-O-Gd键角（Gd···Gd间距），配合物1~3均存在铁磁性相互作用；而当Gd-O-Gd键角较小时，配合物4则表现出反铁磁性相互作用。四种乙酸钆衍生物均展现出显著的磁热效应（magnetocaloric effect, MCE），这类配合物的磁密度越高，或每钆单元的分子量比（MW/NGd）越低，其磁热效应就越强。其中配合物4拥有最高的磁密度，展现出最大的磁热效应，数值可达47.7 J·K⁻¹·kg⁻¹。此外，得益于主导性的铁磁性耦合作用，配合物3具备更宽广的制冷适用温度与/或磁场范围。本研究将熵统计热力学方法成功应用于磁热效应数值的模拟，所得结果与实验数据吻合度极佳。