Bor- und Kohlenstoff-basierte Liganden im Spannungsfeld von Donor- und Akzeptorfunktionalität

In this work, donor groups based on boron or carbon in the form of (R3P)2BH, (R3P)2C and [(R3P)2CH]+ were to be investigated. In transition metal complexes that were to be isolated with the donor groups of interest stabilized in pincer ligands, insight in the bonding situation of the different donor groups with emphasis on the donor-metal interactions was to be gained. The general reaction behaviour, especially in (de)protonation reactions, was to be determined and explained. Special attention was to be paid to the parallels towards amine-based ligands and pincer complexes. A virtually isostructural analogue of the CDP-based pincer ligand precursor was synthesized with a central (R3P)2BH-group by reaction of a borane with two equivalents of dppm. Starting with this boronium salt, a palladium-based pincer complex was isolated. By comparing structural, spectroscopic, and quantum chemical data with boron-based ligands identified as X- and Z-type, an assignment of the (R3P)2BH-group as an L-type donor was secured. Starting with the CDP-based nickel pincer complex from literature, the reactivities towards acids and bases were investigated in detail. Through base, first the deprotonation of one CH2- group in the backbone takes place, followed upon further addition of base by a not directly reversible dimerization process. The stability of the isolated complexes is clarified by quantum chemical means with the help of model complexes. If the starting compound is reacted with an acid containing a weakly coordinating anion instead, the protonation of the CDP-carbon atom can be achieved. X-ray data of the different isolated monomeric complexes only show minute differences in the bond lengths of the CDP-carbon atom to the nickel atom. By DFT calculations and QTAIM analysis, it can be demonstrated that the ability of the (R3P)2C-group to also act as π-donor is negligible in the present case. The calculated proton affinity of the CDP-based model complex is, in comparison to the amide-based model complex, notedly lower and hints at a good suitability for cooperative catalysis. By transferring the ligand systems to an iridium(+III)-based scaffold, it was possible to investigate the ligands in a comparable environment (scheme 12). Reactions with base show that protonated CDP-based [(R3P)2CH]+ donor groups are readily deprotonated, whereas amines and boron-based (R3P)BH-groups as donors primarily lead to reductive elimination on the metal centre. The reactivities are reflected by calculated proton affinities and confirm the preceding investigations with the nickel model systems. The employed synthesis of the iridium complexes proved to be versatile and enabled the addition of further pincer ligands for donor strength comparison. While donors based on the [(R3P)2CH]+-group were shown to be relatively weak, particularly the donor group based on (R3P)2BH is exceptionally strong for a neutral ligand. The CDP-based donor group is shown to be almost identical in donor ability. After further investigations by EDA, the donor strength in regards to the σ-contribution is lower due to partial π-interaction and thus, the ligand based on boron is to be regarded as the strongest neutral, solely σ-donating donor of all isolated complexes. Owing to the very good correlation of calculated and experimentally measured C–O-stretching frequencies, donor groups solely investigated by quantum chemical methods were added to the comparison. Thus it was possible to determine the donor strength of the boron-based ligands deprotonated analogue, the [(R2P)2B]– moiety as well as the heavy homologues of the (R2P)2C, [(R3P)2CH]+, (R3P)2BH and [(R3P)2B]– donors and further confirm the observed trends. Summing up, in this dissertation, an experimental comparison of the reactivities of formally isoelectronic donor groups was carried out and fortified by quantum chemical analyses. Additionally, a new ligand parameter, allowing the direct comparison of the central donor groups in pincer ligand scaffolds with anionic, neutral, and cationic charges, is presented.