A family of pyridine, 2,2′-bipyridine, 1,10-phenanthroline, 3,6-bis(2-pyridyl)pyridazine ligands bearing two nitronyl nitroxide radicals was synthesized and characterized. The pincer ligands having two radicals in ortho positions relative to the nitrogen atoms form stable nickel(II) and copper(II) complexes. The pyridine based 2,6- and 2,5-biradicals crystallized in the monoclinic space group P2
1
/c while the bipyridine 5,5′-biradical crystallized in the monoclinic space group P2
1
/n (radical is 4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl). The highest dihedral angle between the radicals and the pyridine ring is found in the pyridine 2,6-biradical (ca. 47°), followed by the 2,5-biradical (ca. 38°) and then by the bipyridine 6,6′- (ca. 27°) and 5,5′-biradicals (ca. 24°). All the biradicals exhibit similar EPR spectra, in solution and at room temperature, with nine lines due to coupling of the unpaired electrons with four equivalent nitrogen nuclei (hyperfine coupling constant A
N
3.3-3.8 G). In some cases these nine lines are reduced to five lines (A
N
7.5-7.6 G) at low temperature due to a weakening of the spin-spin exchange interaction J which becomes smaller than the isotropic coupling constant. The χT product for all radicals measured at 300 K lies in the range of 0.70-0.77 emu K mol
–1
. For the pyridine 2,6- and 2,5-biradicals, magneto-structural correlation reveals through bond ferro- and antiferro-magnetic interaction (J/k
B
=+9.4 and –39 K, respectively) due to a favorable spin polarization effect. These intramolecular interactions persist when the radicals are dispersed in a polystyrene matrix. For the bipyridine 6,6′- and 5,5′-biradicals, the experimental data were fitted using a Bleaney-Bowers expression for two interacting spins and the values of J/k
B
=–24 to –1.9 K, respectively, are representative of an antiferromagnetic through-space interaction between two neighboring radicals. No intramolecular magnetic interactions were found in the oligopyridine based compounds. In all of these biradicals, both nitroxides are oxidized at the same potential (ca. +0.45 V vs. Ag/Ag
+
) by a two-electron quasi-reversible process to the nitrosonium dication and irreversibly reduced to the dianions between –1.80 and –1.40 V. In the Cu(II) and Ni(II) complexes of the bipyridine-6,6′- and phenanthroline-2,9-biradical reduction of the nitroxides to the hydroxylamine anions is quasi-reversible and is facilitated by ca. 1.20 V versus the
我们合成了一系列
吡啶、2,2′-联
吡啶、
1,10-菲罗啉、3,6-双(2-
吡啶基)
哒嗪配体,并对其进行了表征。钳状
配体的两个基相对于氮原子处于正交位置,形成稳定的
镍(II)和
铜(II)配合物。基于
吡啶的 2,6- 和 2,5- 双自由基在单斜空间群 P2 1 /c 中结晶,而双
吡啶的 5,5′- 双自由基在单斜空间群 P2 1 /n 中结晶(自由基为 4,4,5,5- 四甲基
咪唑啉 3- 氧 1-氧)。
吡啶 2,6-双烷基与
吡啶环之间的二面角最大(约 47°),其次是 2,5-双烷基(约 38°),然后是双
吡啶 6,6′-(约 27°)和 5,5′-双烷基(约 24°)。在溶液中和室温下,所有双
缩醛都显示出类似的 EPR 光谱,其中有九条线是由于未配对电子与四个等效氮核的耦合(超频耦合常数 A N 3.3-3.8 G)造成的。在某些情况下,由于自旋-自旋交换相互作用 J 变弱(小于各向同性耦合常数),这九条线在低温下减弱为五条线(A N 7.5-7.6 G)。在 300 K 温度下测量的所有自由基的 χT 乘积范围为 0.70-0.77 emu K mol -1 。对于
吡啶 2,6-和 2,5-双自由基,由于有利的自旋极化效应,磁结构相关性显示了通过键的
铁磁性和反
铁磁性相互作用(J/k B =+9.4 和 -39 K)。当自由基分散在聚
苯乙烯基体中时,这些分子内相互作用仍然存在。对于联
吡啶 6,6′- 和 5,5′- 双自由基,实验数据使用两个相互作用自旋的 Bleaney-Bowers 表达式进行拟合,J/k B =-24 至 -1.9 K 的值分别代表两个相邻自由基之间的反
铁磁性通空间相互作用。在低聚
吡啶化合物中没有发现分子内磁性相互作用。在所有这些双烷基化合物中,两种硝基氧化物在相同的电位(对 Ag/Ag + 的电位约为 +0.45 V)下通过双电子准可逆过程氧化成硝基锍二阳离子,并在 -1.80 和 -1.40 V 之间不可逆地还原成二阳离子。在联
吡啶-6,6′- 和
菲罗啉-2,9-双酰胺的 Cu(II) 和 Ni(II) 复合物中,亚硝基还原成
羟胺阴离子的过程是准可逆的,与 Ag/Ag + 相比,约为 1.20 V。