A Convenient Synthetic Strategy toward Heavy Alkali Metal Bis(trimethylsilyl)phosphides: Crystal Structures of the Ladder-Type Polymers [A(thf)P(SiMe₃)₂]∞ (A = K, Rb, Cs)

A series of highly reactive heavy alkali metal phosphides was prepared by treating trimethylsilyl-substituted phosphines with alkali metal tert-butyl alcoholates. The compounds are formed in excellent yield and purity, and the side product can be easily removed in a vacuum. The high synthetic potential of this reaction route was further shown by utilizing excess alkali metal tert-butyl alcoholates in reaction with silyl substituted phosphines. In all cases, only monometalated products were isolated. In the course of this work the crystal structures of the bis(trimethylsilyl)phosphides [K(thf)P(SiMe₃)₂]∞, 1a, [Rb(thf)P(SiMe₃)₂]∞, 1b, and [Cs(thf)P(SiMe₃)₂]∞, 1c, were obtained. The compounds display polymeric ladder-type structures. Compounds 1a,b are isomorphous, while compound le displays a slightly altered local geometry. Despite small differences in local geometry, the coordination spheres for the phosphorus atoms and the alkali metal are fairly similar. The five coordinate phosphorus atoms are connected to three alkali metal centers in addition to two trimethylsilyl groups. The alkali metals are four coordinate with ligations to three phosphorus centers in addition to one thf oxygen donor. Compounds 1a-c were characterized using elemental analysis, NMR spectroscopy, and X-ray crystallography. Crystal data with Mo Kα (λ = 0.71073 Å) at 150 K are as follows: 1a, a = 6.4261(2) Å, b = 12.4119(2) Å, c = 21.5447(4) Å, V = 1718.41(7) ų, Z = 4, orthorhombic, space group P2₁2₁2₁, 3427 independent reflections, R1 (all data) = 0.0351; 1b, a = 6.5338(2) Å, b= 12.5664(3) Å, c = 21.5537(5) Å, V= 1769.70(8) ų, Z = 4, orthorhombic, space group P2₁2₁2₁, 4195 independent reflections, R1 (all data) = 0.0776; 1c, a = 11.3515(1) Å, b = 22.3445(3) Å, c = 7.2501(1) Å, β= 96.017(1)°, V = 1828.81(4) ų, Z = 4, monoclinic, space group P2₁/c, 4343 independent reflections, R1 (all data) = 0.0811.