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

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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.