Synthesis and Characterization of 1,3-Diphosphapropene and Alkali-Metal 1,3-Diphosphaallyl Complexes and Unexpected 1,3-Rearrangement of a Cesium 1,3-Diphosphaallyl Complex to a Cesium Secondary Phosphanide

2004-11-08T00:00:00Z (GMT) by Stephen T. Liddle Keith Izod
Protonation of [{<i>t</i>-BuC(PMes)<sub>2</sub>}Li(THF)<sub>3</sub>] (<b>2</b>; Mes = 2,4,6-(CH<sub>3</sub>)<sub>3</sub>C<sub>6</sub>H<sub>2</sub>) with either <i>t</i>-BuCl or H<sub>2</sub>O yields the 1,3-diphosphapropene (<i>Z</i>)-MesPC(<i>t</i>-Bu)P(H)Mes (<b>3</b>) after recrystallization; treatment of <b>3</b> with <i>n</i>-butyllithium at −78 °C in THF regenerates <b>2</b>. Metathesis of <b>2</b> with MOR (M = Na, K, R = <i>t</i>-Bu; M = Rb, Cs, R = CH<sub>2</sub>CH(Et)CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>3</sub>) at −78 °C, followed by recrystallization in the presence of crown ethers, allows access to the 1,3-diphosphaallyl complexes [<i>t</i>-BuC(PMes)<sub>2</sub>][M(15-crown-5)<sub>2</sub>] (M = Na (<b>4</b>), K (<b>5</b>), Cs (<b>7</b>)) and [<i>t</i>-BuC(PMes)<sub>2</sub>][Rb(12-crown-4)<sub>2</sub>] (<b>6</b>), the latter of which exhibits a <i>tert</i>-butyl CH<sub>3</sub>···Rb contact in the solid state. An attempt to prepare [<i>t</i>-BuC(PMes)<sub>2</sub>][Cs(12-crown-4)<sub>2</sub>] (<b>8</b>) by metathesis resulted in ligand degradation to give, after recrystallization, the secondary phosphanide complex [Mes<sub>2</sub>P][Cs(12-crown-4)<sub>2</sub>] (<b>9</b>) in low yield. Studies reveal that the heavier alkali-metal complexes (<b>5</b>−<b>7</b>) are stable indefinitely in crystalline form under a dry nitrogen atmosphere, but are, as the group is descended, increasingly unstable in solution in the presence of the lithium alkoxide byproduct and may degrade to give mixtures containing secondary phosphanides, the known ditertiary diphosphane (Mes<sub>2</sub>P)<sub>2</sub> (<b>10</b>), <i>t</i>-BuC⋮P (<b>11</b>), and other minor unidentified phosphorus-containing compounds. DFT studies at the BL3YP/6-31G* level on the precursor compounds (<i>Z</i>)- and (<i>E</i>)-MesPC(<i>t</i>-Bu)P(Cl)Mes (<b>1a</b>,<b>c</b>) indicate that the <i>Z</i> isomer is more stable than the <i>E</i> isomer by 47.4 kJ mol<sup>-1</sup>, confirming the experimentally observed preference for the <i>Z</i> configuration of the PC bond.