Metal Coordination by Sterically Hindered Heterocyclic Ligands, Including 2-Vinylpyridine, Assessed by Investigation of Cobaloximes
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- 作者：Patrizia Siega ; Lucio Randaccio ; Patricia A. Marzilli ; and Luigi G. Marzilli
- 刊名：Inorganic Chemistry
- 出版年：2006
- 出版时间：April 17, 2006
- 年：2006
- 卷：45
- 期：8
- 页码：3359 - 3368
- 全文大小：164K
- 年卷期：v.45,no.8(April 17, 2006)
- ISSN：1520-510X
文摘
Structural and 1H NMR data have been obtained for cobaloximes with the bulkiest substituted pyridines reportedso 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, CH=CH2). We have found that the Co-Nax bondis very long in the structurally characterized complexes. In particular, CH3Co(DH)2(4-CH3quinoline) has a longerCo-Nax bond (2.193(3) Å) than any reported for methylcobaloximes. The main cause of the long bonds isunambiguously identified as the steric bulk of L by the fairly linear relationship found for Co-Nax distance vs CCA(calculated cone angle, CCA, a computed measure of bulk) over an extensive series of methylcobaloximes. Thelinear relationship improves if L basicity (quantified by pKa) is taken into account. In anhydrous CDCl3 at 25 
C, allcomplexes except the 2-aminopyridine adduct exhibit 1H NMR spectra consistent with partial dissociation of L toform the methylcobaloxime dimer. 1H NMR experiments at -20 C allowed us to assess qualitatively the relativebinding ability of L as follows: 2,4-(CH3)2pyridine > 4-CH3quinoline 
quinoline 2-CH3pyridine > 2-CH3Opyridine> 2-CH3CH2pyridine > 2-CH2=CHpyridine. The broadness of the 1H NMR signals at 25 C suggests a similarorder for the ligand exchange rate. The lack of dissociation by 2-aminopyridine is attributed to an intramolecularhydrogen bond between the NH2 group and an oxime O atom. The weaker than expected binding of 2-vinylpyridinerelative to the Co-Nax bond length is attributed to rotation of the 2-vinyl group required for this bulky ligand to bindto the metal center, a conclusion supported by pronounced changes in 2-vinylpyridine signals upon coordination.
C, allcomplexes except the 2-aminopyridine adduct exhibit 1H NMR spectra consistent with partial dissociation of L toform the methylcobaloxime dimer. 1H NMR experiments at -20 C allowed us to assess qualitatively the relativebinding ability of L as follows: 2,4-(CH3)2pyridine > 4-CH3quinoline quinoline 2-CH3pyridine > 2-CH3Opyridine> 2-CH3CH2pyridine > 2-CH2=CHpyridine. The broadness of the 1H NMR signals at 25 C suggests a similarorder for the ligand exchange rate. The lack of dissociation by 2-aminopyridine is attributed to an intramolecularhydrogen bond between the NH2 group and an oxime O atom. The weaker than expected binding of 2-vinylpyridinerelative to the Co-Nax bond length is attributed to rotation of the 2-vinyl group required for this bulky ligand to bindto the metal center, a conclusion supported by pronounced changes in 2-vinylpyridine signals upon coordination.