Department of Chemistry , University at Buffalo, State University of New York , Amherst , New York 14260 , United States.
Chemistry Department , State University of New York, College at Buffalo , 1300 Elmwood Avenue , Buffalo , New York 14222 , United States.
Inorg Chem. 2018 Jul 16;57(14):8364-8374. doi: 10.1021/acs.inorgchem.8b01022. Epub 2018 Jun 25.
Two macrocyclic complexes of 1,4,7-triazacyclononane (TACN), one with N-methyl imidazole pendants, [Fe(Mim)], and one with unsubstituted NH imidazole pendants, [Fe(Tim)], were prepared with a view toward biomedical imaging applications. These low-spin Fe complexes produce moderately paramagnetically shifted and relatively sharp H NMR resonances for paraSHIFT and paraCEST applications. The [Fe(Tim)] complex undergoes pH-dependent changes in NMR spectra in solution that are consistent with the consecutive deprotonation of all three imidazole pendant groups at high pH values. N-Methylation of the imidazole pendants in [Fe(Mim)] produces a complex that dissociates more readily at high pH in comparison to [Fe(Tim)], which contains ionizable donor groups. Cyclic voltammetry studies show that the redox potential of [Fe(Mim)] is invariant with pH ( E = 328 ± 3 mV vs NHE) between pH 3.2 and 8.4, unlike the Fe(III) complex of Tim which shows a 590 mV change in redox potential over the pH range of 3.3-12.8. Magnetic susceptibility studies in solution give magnetic moments of 0.91-1.3 cm K mol (μ value = 2.7-3.2) for both complexes. Solid-state measurements show that the susceptibility is consistent with a S = 1/2 state over the temperature range of 0 to 300 K, with no crossover to a high-spin state under these conditions. The crystal structure of Fe(Mim) shows a six-coordinate all-nitrogen bound Fe(III) in a distorted octahedral environment. Relativistic ab initio wave function and density functional theory (DFT) calculations on [Fe(Mim)], some with spin orbit coupling, were used to predict the ground spin state. Relative energies of the doublet, quartet, and sextet spin states were consistent with the doublet S = 1/2 state being the lowest in energy and suggested that excited states with higher spin multiplicities are not thermally accessible. Calculations were consistent with the magnetic susceptibility determined in the solid state.
我们制备了两种大环配合物，一种是具有 N-甲基咪唑侧基的 1,4,7-三氮杂环壬烷（TACN）配合物[Fe（Mim）]，另一种是具有未取代 NH 咪唑侧基的 1,4,7-三氮杂环壬烷（TACN）配合物[Fe（Tim）]，旨在将其应用于生物医学成像。这些低自旋 Fe 配合物在 paraSHIFT 和 paraCEST 应用中产生中等程度的顺磁位移和相对尖锐的 H NMR 共振。[Fe（Tim）]配合物在溶液中的 NMR 光谱随 pH 值发生变化，这与高 pH 值下所有三个咪唑侧基连续去质子化的情况一致。与含有可离解给体基团的[Fe（Tim）]相比，[Fe（Mim）]中咪唑侧基的 N-甲基化产生了一种在高 pH 下更容易解离的配合物。循环伏安研究表明，[Fe（Mim）]的氧化还原电位在 pH 3.2 到 8.4 之间与 pH 无关（E = 328 ± 3 mV 相对于 NHE），而 Fe（III）配合物 Tim 的氧化还原电位在 pH 3.3 到 12.8 之间发生了 590 mV 的变化。溶液中的磁化率研究给出了两个配合物的 0.91-1.3 cm K mol（μ 值 = 2.7-3.2）的磁矩。固态测量表明，在 0 到 300 K 的温度范围内，磁化率与 S = 1/2 状态一致，在这些条件下不会转变为高自旋状态。[Fe（Mim）]（OTf）的晶体结构显示了一个六配位的全氮配位 Fe（III）在扭曲的八面体环境中。对[Fe（Mim）]进行相对论从头算波函数和密度泛函理论（DFT）计算，一些带有自旋轨道耦合，用于预测基自旋态。双重态、四重态和六重态自旋态的相对能量与最低能量的双重态 S = 1/2 状态一致，并表明具有更高自旋多重性的激发态不能热激发。计算结果与固态中确定的磁化率一致。
