[PCCP]- and [AsCCAs]-Pincer Complexes for the Activation and Functionalization of N2

在过去的几年里，我们一直专注于2,2 ' -二膦烷及其配位化学。后一种二膦化合物表现出分叉反应，这可以通过精心选择含金属的起始原料来控制：(A)与具有两个顺式开配位的氧化还原惰性金属片段反应，形成对芳基羰基羰基中离子羰基羰基化合物，称为CArY-MICs (Cyclic Aryl Ylidic Mesoionic carbenes)；(B)在与具有三个可接近的聚合物配位位的低价金属碎片反应时，开辟了第二反应途径，导致刚性[PCCP]钳形配合物。后一种钳子表现出有趣的反应性，特别是对小分子。在前期工作中，我们成功地利用[PCCP]钼配合物分裂了分子二氮。得到的氮络合物在其氮原子处被功能化。此外，对钼羰基的初步研究表明，[PCCP]配位配合物和CArY-MIC配合物可能相互转化，即配体-配合反应模式与这些配合物一致。在这些研究的基础上，我们提出了一个问题，即N2的活化和功能化是否可以与上述配体协同反应模式相结合，最终开辟新的合成循环来功能化N2。虽然预测这些反应性相当困难，但我们计划从钼和铬的n2配合物开始我们的研究，并通过[PCCP]配位的7族和8族金属（特别是Re和Ru）的n2配合物来补充我们的研究。因此，该项目的一个支柱是基于前n2配合物的合成和随后的深入反应性研究。鉴于配体-配合P-C键的形成和再打开主要由上述CArY-MIC和钳形配合物的固有稳定性控制，上述方法也存在风险，因为CArY-MIC的形成可能导致热力学汇。为了应对这种可能导致反应性丧失的情况，我们计划通过第二个支柱来补充我们的工作，这是基于相应的[AsCCAs]配体衍生物。由于As-ylides的稳定性降低（与P-ylides相比），只需采用后者[AsCCAs]配体平台，就可以避免CArY-MICs的形成。随后的[AsCCAs]钼螯合物被证明很容易切割N2，表明这种重要的反应模式不受正式取代P为As的影响。为了开发潜在的概念（即P取代As），需要在本项目的过程中制备和研究其他几种[AsCCAs]-配合的n2配合物（例如，中心有Re或Fe金属离子）。

Over the past years, we have focused on 2,2‘-diphosphinotolanes and their coordination chemistry. The latter diphosphines were found to exhibit bifurcated reactivities, which may be controlled by choosing the metal-containing starting material very thoughtfully: (A) Upon reaction with redox-inert metal fragments with two cis-positioned open coordination sites, P-ylidic carbenes are formed, which were termed CArY-MICs (Cyclic Aryl Ylidic Mesoionic Carbenes); (B) in reactions with low-valent metal fragments with three accessible mer-configured coordination sites, a second reaction pathway is opened up, which leads to rigid [PCCP] pincer complexes. The latter pincers exhibit interesting reactivities, in particular towards small molecules. In preliminary work, we succeeded in splitting molecular dinitrogen using a [PCCP] molybdenum complex. The resulting nitrido complex was then functionalized at its nitrogen atom. Additional preliminary studies, inter alia with molybdenum carbonyls, revealed that [PCCP]-coordinated complexes and CArY-MIC complexes may be interconverted, i.e. that ligand-cooperative reactivity patterns come into reach with these complexes. On the basis of these studies, we pose the question whether the activation and functionalization of N2 may be combined with the aforementioned ligand-cooperative reactivity patterns to ultimately open up new synthetic cycles to functionalize N2. Although it is fairly hard to predict these reactivities, we plan to start our investigation with molybdenum and chromium N2-complexes and complement our studies with [PCCP]-coordinated N2-complexes of selected group 7 and group 8 metals (in particular Re and Ru). One mainstay of this project is thus based on the synthesis of the former N2-complexes and on ensuing in-depth reactivity studies. Given that the ligand-cooperative P–C bond formation and re-opening is mainly controlled by the intrinsic stability of the aforementioned CArY-MIC and pincer complexes, the above approach also harbors a risk as the formation of CArY-MICs may lead into a thermodynamic sink. In order to encounter this scenario, which may result in a loss of reactivity, we plan to supplement our work by a second mainstay, which is based on the corresponding [AsCCAs] ligand derivatives. Due to the reduced stability of As-ylides (compared to P-ylides), the formation of CArY-MICs may be avoided, simply by employing the latter [AsCCAs] ligand platform. The ensuing [AsCCAs] molybdenum pincer complexes were shown to readily cleave N2, indicating that this important reactivity pattern is not influenced by formally replacing P for As. To exploit the underlying concept (i.e. the substitution of P for As), several other [AsCCAs]-coordinated N2-complexes (e.g. with a central Re or Fe metal ion) need to be prepared and studied over the course of this project.