Synthesis, Isolation, and Structures of Multiple Bonded Compounds Containing Heavier Main-Group Elements

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Multiple bonds involving heavier main-group elements are normally unstable, because bond lengths between the two elements are long and because effective overlap of □-orbitals cannot be expected due to the wide orbital lobes. Furthermore, hybridization of orbitals is not effective in higher elements causing multiple bonds of such elements to be unstable. By introducing a very bulky group into the systems, however, we have been able to isolate unusual phosphorus-containing compounds, where the phosphorus atoms are in low coordination states.In this research project, protection groups, such as the 2,4,6-tri-t-butylphenyl group, have been utilized in the stabilization of low coordinated phosphorus compounds of coordination number 1 and 2. Compounds thus protected were isolated, characterized, and the structures were analyzed by multinuclear NMR spectroscopy and X-ray crystallography.1)Novel protection groups, which are useful in studies of substitution effects, both steric and electronic, have been developed for preparation of diphosphenes, phosphaallenes, diphosphinidenecyclobutenes, and phosphaalkynes.2)Cis-diphosphenes, phosphaallenes, thioxophosphines, and phosphaquinoid compounds were prepared and characterized by use of novel steric protection groups.3)Diphosphacyclobutenes and biradical species, diphosphacyclobutanediyls, were prepared and isolated by the reaction of phosphaalkynes with alkyllithium reagents.4)Sterically protected phosphaethenylcarbenoids were generated and the various reactivities were studied involving the formation of phosphaalkynes, diphosphabutatrienes, diphosphabutadienes, triphosphafulvenes, and diphosphafulvenes. The structures were analyzed mostly by X-ray crystallography.5)Various kinds of diphosphinidenecyclobutenes were prepared and the transition metal complexes were formed. The catalytic activities in the coupling reactions were investigated to show that they are effective catalysts.