Diversity of Structures and Bonding in Alkali Metal Ureaphosphanes

While organoelement compounds of lithium, sodium and potassium have been much studied for decades and consequently have found forests of applications, those of the heavier alkali metals, rubidium and caesium would barely manage to fill a tree. However, recently the literature has seen some little growth spurts with these metals, hinting at a possible fertile future in areas such as homogeneous catalysis provided more work is put into their fundamental development. Here we report the synthesis and crystal structures of lithium, rubidium and caesium derivatives of the ureaphosphane Ph₂PCH₂CH₂NHC(=O)NHPh, chosen because it offers O, N, P, and π-coordination sites. Though one may expect such alkali metal compounds to be essentially similar, the caesium complex has novel features where Cs⁺ engages in a side-on coordination to the C=O bond and in a weak bond to the P centre, both of which are absent in the Rb structure. Less surprisingly, the lithium derivative is tetrameric in contrast to the infinite networks of the rubidium and caesium structures. All alkali metal derivatives were made with deprotonating the ureaphosphane by a suitable base, including the sodium and potassium complexes though these two complexes could not be obtained in a crystalline form.