Tricyclic 1,4-Diphosphinines: Local vs. Global Aromaticity.

1D and 2D NICS π . zz SOM ${{{\rm { NICS}}}_{{\rm { {\rm \pi}{}}}{\rm { .zz}}}^{{\rm { SOM}}}}$ (Sigma only model) calculations were performed on recently established tricyclic 1,4-diphosphinines as well as related benzene and pyrazine derivatives. The study was extended to evaluate the effect of the fused rings on the overall aromatic properties with a special focus on functional groups such as carbenes. The effect of non-aromatic heterocycles on the local ring current of the central ring is small, while aromatic heterocycles (e. g. NHC, imidazolium) lead to a global aromaticity. A higher sulfur content of the adjacent five-membered rings reduces the central ring current. The comparison to related tricyclic benzene and pyrazine derivatives showed that the 1,4-diphosphinine systems resemble more closely the situation in the benzene derivatives than the pyrazines. The effect of charged systems was studied using bis(TTF)-fused 1,4-diphosphinines and, according to NICS π . zz SOM ${{{\rm { NICS}}}_{{\rm { {\rm \pi}{}}}{\rm { .zz}}}^{{\rm { SOM}}}}$ values, the neutral form doesn't possess significant aromaticity but the tetracation resembles the global aromatic situation observed for other heterocycles in this study.

P-Centred redox reactions of a 1,4-dihydro-1,4-phosphasiline.

Tricyclic 1,4-dihydro-1,4-phosphasilines 3a,b were synthesized from Si(NR2)2-bridged imidazole-2-thione compounds 2a,b. Based on calculated FMOs of 3b, forecasting a possible P-selective P-N bond cleavage reduction, a redox cycle could be established using solutions of P-centred anionic derivative K[4b]. The cycle started with the oxidation of the latter to give the P-P coupled product 5b which could be chemically reduced by KC8 to yield K[4b], again. All new products have been unambiguously confirmed in solution and solid state.

Design of transition metal complexes containing a P-E radical motif.

Recently, we have synthesized phosphane W(CO)5 complexes containing a P-O-TEMP ligand motif as bench-stable precursors of thermally accessible phosphanoxyl complex radicals possessing a ligand with a P-O˙ group. In this work, extensive dispersion-corrected DFT calculations are used to explore both W(CO)5 and Fe(CO)4 phosphane complexes containing the P-E-TEMP ligands (E = O, S, NMe, and PMe) in order to reach thermally accessible radicals with a P-E˙ motif. Moreover, a more general single-electron transfer (SET) oxidation approach to synthesize such P-E˙ radicals via anionic precursors is disclosed. Furthermore, the tendencies for self-trapping and prototropic reactions of such radical complexes have been studied for the first time. Electronic structures and potential conversions of such P-E˙ radicals are discussed, thus paving the way to a broad range of transition metal radical complexes, including potential thermal radical initiators.

On metal coordination of neutral open-shell P-ligands focusing on phosphanoxyls, their electron residence and reactivity.

This feature article highlights the discovery and development of phosphanoxyl complex chemistry starting from (neutral) low-coordinate phosphorus radicals and the quest of metal ligation effects. We describe synthesis and reactions of precursors, namely 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) substituted phosphane tungsten(0) complexes. Trapping reactions of transient phosphanoxyl complexes, formed via thermal homolytic N-O bond cleavage, as well as their use in radical polymerisations are illustrated, thus revealing an interesting reactivity dichotomy. DFT calculations provide insight into thermal stabilities of precursors and the resulting spin density distributions (SDDs) in these reactive intermediates. Systematic studies on the dependance of the electron delocalisation in phosphanoxyl complexes have been performed examining different substitution pattern at phosphorus and different co-ligand combinations at the tungsten(0) center. Preliminary results on Mn and Fe complexes are reported.

1,3,2-Diheterophospholane complexes: access to new tuneable precursors of phosphanoxyl complexes and P-functional polymers.

Synthesis of a testbed of P-H functional diheterophospholane complexes (3 and 6a,b) with no or little steric bulk at the α-position was achieved using [NEt4][WH(CO)5] as a combined reductant and complexation reagent. Reaction with TEMPO leads to P-OTEMP substituted tungsten complexes (4 and 7a,b) possessing different thermostabilities towards N-O bond cleavage. The transient phosphanoxyl complexes obtained were used for the polymerisation of styrene and acrylonitrile. DFT calculations were performed on the formation of various open-shell complexes and Loewdin spin density distributions.