Metal coordination by sterically hindered heterocyclic ligands, including 2-vinylpyridine, assessed by investigation of cobaloximes.
Metal coordination by sterically hindered heterocyclic ligands, including 2-vinylpyridine, assessed by investigation of cobaloximes.
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通过钴肟研究评估空间位阻杂环配体（包括 2-乙烯基吡啶）的金属配位。
DOI：
10.1021/ic051383a
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发表时间：
2006
影响因子：
4.6
通讯作者：
Marzilli,LuigiG
中科院分区：
文献类型：
--
作者：
Siega,Patrizia;Randaccio,Lucio;Marzilli,PatriciaA;Marzilli,LuigiG
迄今为止报道的具有最大体积取代吡啶的钴肟的结构和 1 H NMR 数据已获得。我们在非配位溶剂中分离出了配合物 CH3Co(DH)2L（甲基钴肟，其中 DH = 二甲基乙二肟的单阴离子），其中 L = 位阻 N-供体配体： 喹啉、4-CH3喹啉、2,4-(CH3)2吡啶和 2-R-吡啶（R = CH3、OCH3、CH2CH3、 CHCH2）。我们发现，在结构特征配合物中，Co−Naxbond 非常长。特别是，CH3Co(DH)2(4-CH3喹啉)具有比任何报道的甲基钴肟更长的Co−Naxbond (2.193(3) Å)。通过在一系列广泛的甲基钴肟上 Co−Naxdistance 与 CCA（计算锥角，CCA，体积的计算测量）发现的相当线性关系，长键的主要原因被明确地确定为 L 的空间体积。如果考虑 L 碱度（由 pKa 量化），则线性关系会改善。在 25 °C 无水 CDCl3 中，除 2-氨基吡啶加合物外的所有配合物均表现出 1 H NMR 谱，与 L 部分解离形成甲基钴肟二聚体一致。-20 °C 下的 1 H NMR 实验使我们能够定性评估 L 的相对结合能力，如下所示： 2,4-(CH3)2 吡啶 > 4-CH3 喹啉 ≈ 喹啉 ≈ 2-CH3吡啶 > 2-CH3吡啶 > 2-CH3CH2吡啶 > 2-CH2CH吡啶。 25°C 时 1 H NMR 信号的宽度表明配体交换速率具有相似的顺序。 2-氨基吡啶未解离的原因是 NH2 基团和肟 O 原子之间存在分子内氢键。相对于 Co−Naxbond 长度，2-乙烯基吡啶的结合比预期的弱，归因于该庞大配体与金属中心结合所需的 2-乙烯基的旋转，配位时 2-乙烯基吡啶信号的显着变化支持了这一结论。
Structural and1H NMR data have been obtained for cobaloximes with the bulkiest substituted pyridines reported so far. We have isolated in noncoordinating solvents the complexes CH3Co(DH)2L (methylcobaloxime, where DH = the monoanion of dimethylglyoxime) with L = sterically hindered N-donor ligands: quinoline, 4-CH3quinoline, 2,4-(CH3)2pyridine, and 2-R-pyridine (R = CH3, OCH3, CH2CH3, CHCH2). We have found that the Co−Naxbond is very long in the structurally characterized complexes. In particular, CH3Co(DH)2(4-CH3quinoline) has a longer Co−Naxbond (2.193(3) Å) than any reported for methylcobaloximes. The main cause of the long bonds is unambiguously identified as the steric bulk of L by the fairly linear relationship found for Co−Naxdistance vs CCA (calculated cone angle, CCA, a computed measure of bulk) over an extensive series of methylcobaloximes. The linear relationship improves if L basicity (quantified by pKa) is taken into account. In anhydrous CDCl3at 25 °C, all complexes except the 2-aminopyridine adduct exhibit1H NMR spectra consistent with partial dissociation of L to form the methylcobaloxime dimer.1H NMR experiments at −20 °C allowed us to assess qualitatively the relative binding ability of L as follows: 2,4-(CH3)2pyridine > 4-CH3quinoline ≈ quinoline ≈ 2-CH3pyridine > 2-CH3Opyridine > 2-CH3CH2pyridine > 2-CH2CHpyridine. The broadness of the1H NMR signals at 25 °C suggests a similar order for the ligand exchange rate. The lack of dissociation by 2-aminopyridine is attributed to an intramolecular hydrogen bond between the NH2group and an oxime O atom. The weaker than expected binding of 2-vinylpyridine relative to the Co−Naxbond length is attributed to rotation of the 2-vinyl group required for this bulky ligand to bind to the metal center, a conclusion supported by pronounced changes in 2-vinylpyridine signals upon coordination.