Influence of Chain Length on the Structures and Dynamic Behavior of Alkyl-Tethered α,ω-Diphosphide Complexes of Lithium and Their Use in the Synthesis of P-Heterocyclic Stannylenes.

The alkyl-tethered α,ω-diphosphines (Dipp)PH(CH2) nPH(Dipp) ( n = 1 (3H), 2 (4H), 3 (5H), 4 (6H), and 5 (7H)) were prepared in good yield and characterized by multinuclear NMR spectroscopy [Dipp = 2,6- iPr2C6H3]. Treatment of 3H with 2 equiv of nBuLi and 2 equiv of TMEDA gives the diphosphide complex [CH2{P(Dipp)}2]Li2(TMEDA)2 (3Lia), which crystallizes as discrete monomers which do not exhibit temperature-dependent NMR behavior. Treatment of 4H-7H with 2 equiv of nBuLi in THF gives the diphosphides [[CH2{P(Dipp)}]2]2Li4(THF)2(OEt2)2 (4Li), [CH2{CH2P(Dipp)}2]Li2(THF)4 (5Li), [{CH2CH2P(Dipp)}2]Li2(THF)6 (6Li), and [CH2{CH2CH2P(Dipp)}2]2Li4(THF)6·PhMe (7Li) after crystallization. Compounds 4Li-7Li adopt either monomeric or dimeric structures in the solid state, depending on the length of the alkyl tether of the diphosphide ligand. In solution, compounds 4Li-7Li exhibit dynamic behavior: variable-temperature 31P{1H} and 7Li NMR spectroscopic studies indicate that this involves equilibria between monomeric and dimeric or higher oligomeric species with the nature of the equilibrium again depending on the length of the alkyl tether of the diphosphide ligand. The reactions between 3Li, 6Li, or 7Li and SnCl2 in THF give mixtures of products which could not be separated. In contrast, the reactions between 4Li or 5Li and 1 equiv of SnCl2 give the dimeric P-heterocyclic stannylenes [{CH2P(Dipp)}2Sn]2 (4Sn) and [CH2{CH2P(Dipp)}2Sn]2.1/2THF (5Sn), respectively. While compound 5Sn is isolated exclusively as the cis isomer, 4Sn is isolated as a mixture of cis and trans isomers in an approximate 5:1 ratio. The solid-state structures of trans-4Sn and cis-5Sn were obtained, and multinuclear NMR spectroscopy indicates that the dimeric structures of these compounds are maintained in solution. Compounds 4Sn and 5Sn represent the first P-heterocyclic stannylene dimers to be structurally characterized.