Synthesis of [LiPPh(SiMe 3)(tmeda)] 2 (from Mg(PHPh) 2(tmeda) or Li(PHPh)(tmeda)) and the X-ray structure of the trans-isomer (tmeda = N,N,N′,N′-tetramethylethylenediamine)

Abstract

Treatment of either (i) Mg(PHPh) 2(tmeda) with two equivalents of n-BuLi then with an excess of Me 3SiCl or (ii) Li(PHPh)(tmeda) with Me 3SiCl followed by n-BuLi yields [LiPPh(SiMe 3)(tmeda)] 2. Below ca. 0 and 5°C 31P and 7Li NMR spectroscopy shows that there is coupling of 7Li nuclei to two equivalent 31P nuclei, and vice versa. This is consistent with the dimeric structure found in the solid for the trans-isomer by an X-ray structure determination. The phosphido groups bridge two lithium centres to form Li 2P 2 rings; LiP (mean) 2.62 Å, LiN (mean) 2.16 Å, PLiP 94.5(6), 92.0(6)°, LiPLi 85.5(5), 88.0(5)°.