Coordination-Induced Spin Crossover (CISCO) through Axial Bonding of Substituted Pyridines to Nickel-Porphyrins: σ-Donor versus π-Acceptor Effects
Coordination-Induced Spin Crossover (CISCO) through Axial Bonding of Substituted Pyridines to Nickel-Porphyrins: σ-Donor versus π-Acceptor Effects
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DOI：
10.1002/chem.201000603
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发表时间：
2010-09-03
影响因子：
4.3
通讯作者：
Herges, Rainer
中科院分区：
文献类型：
--
作者：
Thies, Steffen;Bornholdt, Claudia;Herges, Rainer
镍卟啉具有刚性的平面二次配位框架，为研究配位诱导的自旋交叉（CISCO）效应提供了一个很好的模型，因为一个或两个轴向配体与金属中心的键合导致了从S=0到S=1的自旋跃迁。在此，通过温度依赖性NMR光谱法测定镍卟啉（Ni-四（五氟苯基）卟啉）与不同的4-取代吡啶的反应的平衡常数K（1 S）和K（2），以及首次测定相应的热力学参数Δ H（1 S）、Δ H（2）、Δ S（1 S）和Δ S（2）。缔合常数K（1 S）和K（2）与4-取代吡啶的碱性相关（R：OMe>H>CO（2）Et>NO（2）），而Δ H（1 S）值呈现完全不同的顺序（OMe < H>CO（2）Et>NO（2））。4-硝基吡啶表现出最大的结合焓，然而，这是过度补偿了一个大的负结合熵。我们将大的缔合焓归因于Ni d（xz）和d（yz）轨道的电子反馈到吡啶的π轨道，而负的缔合熵则归因于更刚性的卟啉-吡啶络合物的振动熵和内旋转熵的降低.硝基和氰基吡啶配合物的反馈也证实了红外光谱，并显示了一个位移的N-O和C-N的振动，分别降低波数。与硝基-、氰基-和二甲基氨基吡啶的2：1络合物的X射线结构提供了返给的进一步指示。还观察到一个进一步的趋势：吡啶的碱性越强，K（1 S）相对于K（2）就越大。对于硝基吡啶，K（2）比K（1 S）大17倍，对于甲氧基吡啶，K（2）和K（1 S）几乎相等。
Nickel-porphyrins, with their rigid quadratic planar coordination framework, provide an excellent model to study the coordination-induced spin crossover (CISCO) effect because bonding of one or two axial ligands to the metal center leads to a spin transition from S=0 to S=1. Herein, both equilibrium constants K(1S) and K(2), and for the first time also the corresponding thermodynamic parameters Delta H(1S), Delta H(2), Delta S(1S), and Delta S(2), are determined for the reaction of a nickel-porphyrin (Ni-tetrakis(pentafluorophenyl)porphyrin) with different 4-substituted pyridines by temperature-dependent NMR spectroscopy. The association constants K(1S) and K(2) are correlated with the basicity of the 4-substituted pyridines (R: OMe>H>CO(2)Et>NO(2)) whereas the Delta H(1S) values exhibit a completely different order (OMe < H>CO(2)Et>NO(2)). 4-Nitropyridine exhibits the largest binding enthalpy, which, however, is overcompensated by a large negative binding entropy. We attribute the large association enthalpy of nitropyridine with porphyrin to the back donation of electrons from the Ni d(xz) and d(yz) orbitals into the pi orbitals of pyridine, and the negative association entropy to a decrease in vibrational and internal rotation entropy of the more rigid porphyrin-pyridine complex. Back donation for the nitro- and cyanopyridine complexes is also confirmed by IR spectroscopy, and shows a shift of the N-O and C-N vibrations, respectively, to lower wave numbers. X-ray structures of 2: 1 complexes with nitro-, cyano-, and dimethylaminopyridine provide further indication of a back donation. A further trend has been observed: the more basic the pyridine the larger is K(1S) relative to K(2). For nitropyridine K(2) is 17 times larger than K(1S) and in the case of methoxypyridine K(2) and K(1S) are almost equal.