Synthesis and chemistry of P-bridged, bent and planar bis(N-heterocyclic) carbenes

Abstract

This PhD thesis focuses on the synthesis of new 7,8-dihydro-1,4-diphosphabarrelenes and their subsequent conversion into the corresponding P-bridged bis(imidazolium) salts and bis(N-heterocyclic) carbenes (bis(NHCs)).<br />New 7,8-dihydro-1,4-diphosphabarrelenes annulated to imidazole-2-diselones were synthesised via [4+2]-cycloaddition of a 1,4-diphosphinine bis(imidazole-2-selone) with non-functionalised internal or terminal alkenes or ethene, respectively. They possess weak P-σ-donor ability with lone pairs with large s-character as evidenced by the generation of different adducts and transition metal complexes. The formation of the 7,8-dihydro-1,4-diphosphabarrelenes is reversible and the 1,4-diphosphinine scaffold can be regained via thermal retro-[4+2]-cycloaddition.<br />An optimised reductive deselenisation allowed the formation of bent, P-bridged bis(imidazolium) salts under retention of the bridge motif. These salts show remarkable stability due to their highly aromatic, electron-withdrawing imidazolium units which prevent a thermal retro-[4+2]-cycloaddition to the respective 1,4-diphosphinine bis(imidazolium) salt and further lower the P-donor ability. However, they could be converted into the corresponding isolable bis(NHCs) via deprotonation with strong, weakly nucleophilic bases. Theoretical calculations on the frontier molecular orbitals (FMOs) revealed a set of close-lying MOs located at the carbene centres, the phosphorus centres or a mixture of both, making them potential tetratopic ligands with two sets of donor centres with different basicity. Thermal retro-[4+2]-cycloaddition of the bis(NHCs) transiently yields the desired 1,4-diphosphinine bis(NHC) which could be proven via trapping in a [4+2]-cycloaddition.<br />The new bent, P-bridged bis(NHCs) could easily be reacted with Ag(I) and Fe(0) reagents to give the corresponding dinuclear complexes and with a Rh(I) complex to yield a tetranuclear complex. In contrast to previously reported planar, P-bridged bis(NHCs), the bent bis(NHCs) could also be converted into main group adducts with derivatives from groups 13, 14 and 15. While a 1,4-diphosphinine bis(NHC) Fe(0) complex could be obtained from the bent bis(NHC) Fe(0) complex, no retro-[4+2]-cycloaddition was observed for any of the imidazolium-containing main group adducts.<br />Finally, the reaction of a planar, P-NEt₂-bridged bis(imidazolium) salt with boron trichloride surprisingly allowed access to a 1,4-diphosphinine bis(imidazolium) containing salt which showed unprecedented P-centred reactivity and the so far highest aromaticity in a 1,4-diphosphinine central ring.