EVIDENCE FOR A STRUCTURE-REACTIVITY RELATIONSHIP OF CROWDED SUBSTITUTED LITHIUM (FLUOROSILYL)PHOSPHANIDES

The molecular structures and the reactivity of the new solvated lithium (fluorosilyl)phosphanides 1a, 2a, and 2b has been investigated. The unusual solid state structures of 1a and 2b are established by X-ray crystallography. Whereas the lithium centre in la is bound to phosphorus and solvated by three molecules of THF, the electronical saturation of the lithium centre in 2b is achieved by a Li-P bond, tridentate chelatization (fluorine, phosphorus, nitrogen), and the additional coordination of one molecule of THF. The trigonal-planar geometry at phosphorus in la is unprecedented for derivatives of this type and reflects the steric demand around the phosphorus and the possibility of pi(p)/sigma(SiC)* hyperconjugation. The Si-P bond lengths [P-SiC(Phenyl) 2.186(2) and P-SiF 2.164(2) angstrom] are significantly shorter than in related pyramidally configurated derivatives (1b, c: P-SiC 2.21, P-SiF 2.18 angstrom). Both phosphorus atoms in 2b are pyramidally coordinated, but the intramolecular chelate coordination of the lithium centre causes much smaller bonding angles at the Si-P phosphorus atom [sum of bonding angles 238.4(2) (PSi), 312.5(1)degrees (PN)]. 1a as well as 2a, b show quite different reactivity as compared to 1b-d due to their unusual structures. Heating of solutions of la and 2a, b in toluene or hexane does not give the expected stable silylidenephosphanes (Si=P) by elimination of LiF. Instead, rearrangement reactions of the Si=P intermediates 6, 8 lead to the new heterocycles 5, 9. A single-crystal X-ray diffraction analysis of 5 has been performed. The Si2P2 cyclobutane ring is planar, and the NMe2 groups at the ring silicon atoms are cis-orientated. - Interestingly, 2b eliminates LiNiPr2 at 20-degrees-C in a kinetically controlled process to form the tetraphosphacyclobutane 7 as the major product. Presumably, the diphosphene 8 is an intermediate in this reaction.