Roques Nans, Gerbier Philippe, Schatzschneider Ulrich, Sutter Jean-Pascal, Guionneau Philippe, Vidal-Gancedo José, Veciana Jaume, Rentschler Eva, Guérin Christian
Laboratoire de Chimie Moléculaire et Organisation du Solide, Université Montpellier 2, C.C.007, Place E. Bataillon, 34095 Montpellier Cedex 5, France.
Chemistry. 2006 Jul 17;12(21):5547-62. doi: 10.1002/chem.200501280.
Five bis(tert-butylnitroxide) diradicals connected by a silole (7 a-d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna2(1) with a = 20.752(5), b = 5.826(5), and c = 34.309(5) A. X-ray crystal structure determination, electronic spectroscopy, variable-temperature EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6-31+G*) were used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7 b, 7 c, and 7 d are quite stable both in solution and in the solid state, 7 a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoidal form and a monoradical species owing to the fact that they correspond to the open form of a pi-conjugated Kekulé structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topology. These interactions are best reproduced by means of a "Bleaney-Bowers" model that gives values of J = -142.0 cm(-1) for 7 a, -1.8 cm(-1) for 7 b, -1.3 cm(-1) for 7 c, -4.2 cm(-1) for 7 d, and -248.0 cm(-1) for 12. The temperature dependence of the EPR half-field transition in frozen dichloromethane solutions is consistent with singlet ground states and thermally accessible triplet states for diradicals 7 b, 7 c, and 7 d with DeltaE(T-S) values of 3.48, 2.09, and 8 cm(-1), respectively. No evidence of a populated triplet state was found for diradicals 7 a and 12. Similarities between the DeltaE(T-S) and J values (DeltaE(T-S) = -2 J) clearly show the intramolecular origin of the observed antiferromagnetic interaction. Analyses of the data with a "Karplus-Conroy"-type equation enabled us to establish that the silole ring, as a whole, allows a more efficient magnetic coupling of the two nitroxide radicals attached to its 2,5-positions than the thiophene ring. This superiority probably originates from the nonaromaticity of the silole which thus permits a better magnetic interaction through it. DFT calculations also support the experimental results, indicating that the magnetic exchange pathway preferentially involves the carbon pi system of the silole.
研究了通过硅杂环戊二烯（7 a - d）或噻吩（12）环作为耦合剂连接的五个双（叔丁基氮氧化物）双自由基。化合物12结晶于正交晶系空间群Pna2(1)中，a = 20.752(5)，b = 5.826(5)，c = 34.309(5) Å。利用X射线晶体结构测定、电子光谱、变温电子顺磁共振光谱、超导量子干涉仪测量和密度泛函理论计算（UB3LYP/6 - 31 + G*）来研究该系列分子中的分子构象和电子自旋耦合。虽然化合物7 b、7 c和7 d在溶液和固态中都相当稳定，但7 a和12会发生部分电子重排，形成抗磁性的醌型结构和单自由基物种，这是因为它们对应于π共轭凯库勒结构的开放形式。在固态中，磁性测量表明双自由基都是反铁磁耦合的，这与其拓扑结构预期一致。这些相互作用通过“Bleaney - Bowers”模型能得到最佳重现，该模型给出的J值为：7 a为 - 142.0 cm(-1)，7 b为 - 1.8 cm(-1)，7 c为 - 1.3 cm(-1)，7 d为 - 4.2 cm(-1)，12为 - 248.0 cm(-1)。在冷冻二氯甲烷溶液中，电子顺磁共振半峰跃迁的温度依赖性与双自由基7 b、7 c和7 d的单重基态和热可及三重态一致，其ΔE(T - S)值分别为3.48、2.09和8 cm(-1)。未发现双自由基7 a和12存在三重态的证据。ΔE(T - S)和J值之间的相似性（ΔE(T - S) = - 2J）清楚地表明了所观察到的反铁磁相互作用的分子内起源。用“Karplus - Conroy”型方程对数据进行分析，使我们能够确定，总体而言，硅杂环戊二烯环比噻吩环能更有效地使连接在其2,5 - 位的两个氮氧化物自由基发生磁耦合。这种优越性可能源于硅杂环戊二烯的非芳香性，从而使其能通过它实现更好的磁相互作用。密度泛函理论计算也支持实验结果，表明磁交换途径优先涉及硅杂环戊二烯环的碳π体系。
