Nonmetal coordination chemistry: P-adducts of electrophilic, terminal phosphinidene complexes

Abstract

This PhD thesis focused on the synthesis and reactions of neutral, donor-stabilized electrophilic, terminal phosphinidene complexes bearing only π-accepting coligands starting from very strained P-heterocycle complexes or acyclic donor-to-phosphinidene complex adducts. Highly strained 3-imino-azaphosphiridine chromium(0) and molybdenum(0) complexes deemed not to be suitable for experimental studies, and it was decided to examine the “direct” synthesis using Li/Cl phosphinidenoid complexes in reactions with promising donors. In doing so the scope of P-adducts could be expanded to a broader family of C-, N- and P-donors such as tBuNC, N-methylimidazole (N-MeIm), DMAP and trialkylphosphanes. The redox properties of the obtained donor-to-phosphinidene complex adducts were investigated by cyclic voltammetry. The nucleophilic character of P-adduct complexes was examined using an N-MeIm-to-phosphinidene complex adduct as a case in point and various Lewis acids such as labile [W(CO)5L] which gave a dinuclear complex. When tris(pentafluorophenyl)borane was added to the N-MeIm-to-phosphinidene complex adduct an analogous hetero-dinuclear phosphinidene complex did form which represents the first example ever of a µ2-phosphinidene ligand bridging a transition metal and a metalloid center. The proposed formation of a borane adduct with the donor still bound to P was further studied by using the sterically much less demanding borane BH3. As the reactions with Lewis acids gave products with a remarkable low thermal stability and high sensitivity, the reactivity with Brønsted-Lowry acids was tested next. The N-MeIm-to-phosphinidene complex adduct showed a selective reaction with water, alcohols and primary amines (as very weak acids) under formation of the formal O-H and N-H insertion products. Reactions of the N-MeIm-to-phosphinidene complex adduct with strong acids such as trifluoroacetic acid and hydrogen chloride selectively formed the respective formal insertion products. To get evidence for the primary reaction step, i.e. the protonation of phosphorus, the reactions with strong acids of weakly coordinating anions (WCA) were examined. The donor-to-phosphinidene complex adducts were also studied towards the characteristic coordination chemistry 1) dissociation, 2) ligand substitution and 3) transformation of the ligand to a modified ligand system and compared to transition metal complexes. First, the thermal dissociation of the P-donor bond to form the highly reactive electrophilic, terminal phosphinidene complexes was studied. However, for the N-donor-to-phosphinidene complex adducts the formation of white phosphorus was observed. The transient formation of the phosphinidene complex was confirmed by clear-cut trapping experiments with alkenes and 2,3-dimethyl-1,3-butadiene. Due to the lability of the N-MeIm-to-phosphinidene complex adduct, it was particularly suited for the study of donor substitution reactions which were performed with DMAP, tertiary phosphanes, tert-butylisocyanide and carbenes. Finally, the transformation of a donor molecule into a modified donor while keeping the Do-P connectivity was investigated. Therefore, the reactivity of an isocyanide-to-phosphinidene complex adduct towards primary amines was studied observing the formation of phosphaguanidine complexes for methylamine and isopropylamine and the bisaminocarbene adduct for tert-butylamine. The latter is very reminiscent to organometallic chemistry and represents an unprecedented example in p-block chemistry. The mechanism of the product formation was further investigated by theoretical calculations.