Novel Cyclohexyl-Based POCOP-Pincer Ligands for C(sp3)-C(sp3) Bond Activation

Alkanes offer great and barely utilised potential for usage as raw materials in large-scale chemical processes. Their lack of reactivity, however, warrants methods of manipulating and transforming them in an efficient and mild way, which can potentially be realised by employing homogeneous catalysis using pincer complexes. A reliable procedure for the synthesis and diastereomer-separation of 2-methyl-1,3-cyclohexanediol was established, paving the way for the preparation of three novel cyclohexyl-based POCOP-pincer ligands, which have been characterised via 1H-, 31P{1H}- and 13C{1H}-NMR spectroscopy. These systems had been proposed as candidates for the ultimate goal of scission of unstrained, unactivated C(sp3)-C(sp3) bonds in an intermolecular reaction. Therefore, their cyclometallation behaviour was thoroughly studied under varying conditions employing i.a. different metallating agents. The results of these investigations revealed a trend of facile metal-induced ligand decomposition occurring with two of the ligands accompanied by a distinct lack of cyclometallation in all tested systems, clearly indicating an underlying issue, wherein bidentate coordination appears to not be feasible. While the desired bond cleavage has not been achieved, these studies provided an increased understanding of the general aspects of cyclometallation and ligand stability/flexibility as well as the influence these factors exert on each other. Further mechanistic investigations were initiated as to elucidate the potential of the proposed concept, affording novel insight into differences in reactivity between these POCOP-systems and their PCP-analogues.