Cyclic M₂(RL)₂ Coordination Complexes of 5-(3-[<i>N</i>-<i>tert</i>-Butyl-<i>N</i>-aminoxyl]phenyl)pyrimidine with Paramagnetic Transition Metal Dications

5-(3-(N-tert-Butyl-N-aminoxyl)phenyl)pyrimidine (RL = 3NITPhPyrim) forms isostructural cyclic M2(RL)2 cyclic dimers with M(hfac)2 (M = Mn, Co, Cu; hfac = hexafluoroacetylacetonate). Mn2(hfac)4(RL)2 exhibits strong antiferromagnetic Mn−RL exchange, with weak ferromagnetic exchange (0.7 cm-1) between Mn−RL units that is consistent with a spin polarization exchange mechanism. The magnetic moment of Co2(hfac)4(RL)2 at higher temperatures is consistent with strongly antiferromagnetic exchange within the Co−NIT units and tends toward zero below 50 K at lower magnetic fields. Cu2(hfac)4(RL)2 shows more complex behavior, with no high-temperature plateau in χT(T) up to 300 K but a monotonic decrease down to about 100 K. The Cu(II)−nitroxide bonds decrease by 0.2−0.3 Å over the same temperature range, corresponding to a change of nitroxide coordination from axial to equatorial. This thermally reversible Jahn−Teller distortion leads to a thermally induced spin state conversion from a high-spin, paramagnetic state at higher temperature to a low-spin state at lower temperature. This spin state conversion is accompanied by a reversible solid-state thermochromic change between dull yellow-brown at room temperature and green at 77 K.5-(3-(N-叔丁基-N-氧自由基)苯基)嘧啶（简写为3NITPhPyrim）可与M(hfac)₂（M=Mn、Co、Cu；hfac=六氟乙酰丙酮（hexafluoroacetylacetonate））形成同构环状二聚体M₂(RL)₂。Mn₂(hfac)₄(RL)₂表现出强的Mn−RL反铁磁相互作用，且Mn−RL单元间存在弱铁磁交换（0.7 cm⁻¹），该结果与自旋极化交换机制相符。Co₂(hfac)₄(RL)₂的高温磁矩与Co−NIT单元内的强反铁磁交换一致，在低磁场下于50 K以下磁矩趋近于零。Cu₂(hfac)₄(RL)₂的行为更为复杂：在300 K以内的高温区间未出现χT(T)平台，而是随温度降低单调下降至约100 K。在此温度区间内，Cu(II)−硝酰基键长缩短0.2~0.3 Å，对应硝酰基配位模式从轴向转变为赤道向。这种热可逆的姜-泰勒（Jahn-Teller）畸变可诱导热致自旋态转换，即高温下为高自旋顺磁态，低温下转变为低自旋态。该自旋态转换伴随可逆的固态热致变色现象：室温下为暗黄褐色，77 K时变为绿色。