Synthesis and Structure of the Substituted Lithium Benzyl [Li(tmeda)2][C(PMe2)(SiMe3)C6H3-3,5-Me2]. The First Metal Phosphinomethanide Consisting of Solvent-Separated Ion Pairs

Z. Naturforsch. 55 b, 1114-1120 (2000); received September 18, 2000 Lithium Phosphinomethanides, Lithium Benzyls The lithium dimethylphosphinomethanide with trimethylsilyl and 3,5-dimethylphenyl sub­ stituents in a position [Li(tmeda)2]+[C(PMe2)(SiMe3)C6H3-3 ,5-Me2]~ (2 ) was synthesized from Me2P-CH(SiMe3)C6H3-3 ,5-Me2 (1) and Li''Bu/tmeda in hexane in 56% yield (tmeda = N, N, N', iV'-tetramethylethylenediamine). In the solid state it consists of solvent-separated ion pairs which probably is also the case in solution (crystal structure data: tetragonal, space group /4icd, a = b= 19.303(3), c = 34.905(7) Ä, Z = 16, R(F) = 0.054 for 2309 unique data and 299 refined parameters). The a-substituted lithium benzyl 2 is the first metal phosphinomethanide containing “free”, uncoordinated anions in the solid state. The lack of metal coordination of the anion is reflected in the bonding parameters at the planar carbanionoid benzylic carbon atom Cl (Cl-P 1.773(8), Cl-Si 1.846(8), Cl-C2(Ph) 1.442(9) A; C2-C1-P 125.3(5), C2-Cl-Si 127.1(5), P-Cl-Si 107.5(3)°; sum of the angles at Cl: 359.9°).