The relative ligand donor strengths of 10 pyrazole-derived ligands has been determined with great accuracy, making use of the interdependence between the donor strength of the co-ligand and the 13C NMR chemical shift of the iPr2-bimy carbene signal in trans-[PdBr2(iPr2-bimy)L] complexes (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; L = pyrazole-derived ligand). Even subtle variations in the substitution pattern of the pyrazole backbone up to three bonds away from the coordinating nitrogen could be detected reliably using this methodology. Alkylation experiments conducted on the pyrazoles using electrophiles of varied reactivity (ethyl bromide, ethyl iodide, and trimethyloxonium tetrafluoroborate) served as a benchmark to rank the pyrazoles in three groups of gradually increasing nucleophilicity, which correlated well with their determined donor strength.
通过利用协同
配体中给体强度与反式-[PdBr2(iPr2-bimy)L]配合物中iPr2-bimy卡宾信号的13C NMR
化学位移之间的相互依赖性,已精确测定了10种
吡唑衍生的
配体的相对供体强度(iPr2-bimy =
1,3-二异丙基苯并
咪唑啉-2-亚基;L =
吡唑衍生
配体)。即使
吡唑骨架上与配位氮原子相隔三个键的取代模式的微妙变化,也能可靠地使用这种方法检测到。通过使用不同反应活性的亲电试剂(
溴乙烷、
碘乙烷和四
氟硼酸三甲氧基甲基)对
吡唑进行烷基化实验,将
吡唑分为逐渐增强的亲核性的三个组,这与它们确定的供体强度高度相关。