The formation of Lewis acid/base stabilised phosphanyltrielanes – A theoretical and experimental study

Theoretical investigations on the thermochemistry and the reaction mechanism of the formation of Lewis acid/base stabilised phosphanyltrielanes D · H2EPH2 · A (D = Lewis base, A = Lewis acid) were conducted. The reactions of EH3 · D with A · PH3 to form D · H2EPH2 · A and H2 (E = B, Al, Ga; D = NH3; A = BH3, Cr(CO)5) are all exothermic, regardless of whether donors and acceptors are present or absent. The lithium chloride elimination reactions between EH2Cl · D and A · PH2Li to give D · H2EPH2 · A and LiCl are endothermic for donor/acceptor stabilised compounds, if formation of gaseous LiCl is considered. If solid lithium chloride is considered all reactions are strongly exothermic. Studies of the transition state for H2-elimination reactions between EH3 · D and A · PH3 to yield D · H2EPH2 · A and H2 were only successful for E = Al, Ga. In these cases the reaction proceeds via a transition state featuring a five or six-coordinate group 13 element. Different donor molecules do not influence the activation energy of such H2-elimination reactions, but nevertheless they have an effect on the reaction energy. The synthesis of the Cr(CO)5 substituted phosphanyltrielanes [(CO)5Cr(H2PBH2 · NMe3)] (3a) and [(CO)5Cr(H2PAlH2 · NMe3)] (3b), as well as of the dinuclear complex [(CO)8Cr2(μ-HPBH2 · NMe3)2] (4) are described, the latter as a subsequent reaction product of the photolysis of 3a. All compounds were characterised spectroscopically and by X-ray structure analysis.

Graphical abstractMechanistic investigations on the reaction mechanism and thermochemistry of the formation reactions of Lewis acid/base stabilised phosphanyltrielanes via H2-elimination have been conducted. For the elements aluminium and gallium a five-coordinate transition state was determined, which gives an answer to the question why this reaction cannot be observed for boron containing molecules.Figure optionsDownload full-size imageDownload as PowerPoint slide