制备了一系列咪唑啉鎓盐,其六元,七元和八元同系物以及相关的甲ami盐,并使用包围指示剂方法在DMSO中于25°C测定了其p K a值。考虑了每种类型的结构变化对每种盐的酸度的影响,尤其要注意环大小的重要性以及氮的空间和电子性质的影响-芳基取代基。还通过将环状体系与相应的甲ami盐进行比较,探讨了环状结构的影响,并指出了在后者情况下构象柔韧性的重要性。除了允许选择合适的碱基使这些物质去质子化外,预计提供的数据将有助于理解相应碳烯的亲核性和潜在的催化功效。
作者:Mary S. Collins、Evelyn L. Rosen、Vincent M. Lynch、Christopher W. Bielawski
DOI:10.1021/om1004226
日期:2010.7.12
Complexes of the type [(L)Ir(COD)Cl] and [(L)Ir(CO)(2)Cl] (L = N,N'-dimesityl-N,N'-dimethylformamidin-2-ylidene (3) and N,N'-bis(2,6-di-isopropylphenyl)-N,N'-dimethylformamidin-2-ylidene (4); COD = cis,cis-1,5-cyclooctadiene) were synthesized and studied in solution as well as in the solid state. While the acyclic diaminocarbene (ADC) ligand in [(3)Ir(COD)Cl] adopted a conformation in which the N-aryl substituents were anti with respect to the coordinated metal, the respective Ir carbonyl complex was prepared as separable isomers ([(anti-3)Ir(CO)(2)Cl] and [(amphi-3)Ir(CO)(2)Cl]). The ADC ligands in [(4)Ir(COD)Cl] and [(4)Ir(CO)(2)Cl] adopted exclusively amphi conformations, where one N-aryl substituent was oriented toward the coordinated metal and the other was oriented away. The Tolman electronic parameter (TEP) for anti-3 (2047.8 cm(-1)) was derived from the carbonyl stretching energy (v(CO)) of the aforementioned Ir(CO)(2)Cl complex and was found to be larger than the TEPs calculated for amphi-3 (2044.4 cm(-1)) and 4(2044.0 cm(-1)). Likewise, the oxidation potential of [(anti-3)Ir(CO)(2)Cl], as measured by cyclic voltammetry, was found to be significantly higher (1.57 V) than the analogous oxidation potentials measured for [(amphi-3)Ir(CO)(2)Cl] (1.26 V) and [(4)Ir(CO)(2)Cl] (1.24 V).