Dentritic Carbosilanes Containing Silicon-Bonded 1-[C6H2(CH2NMe2)2-3,5-Li-4] or 1-[C6H3(CH2NMe2)-4-Li-3] Mono-and Bis(amino)aryllithium End Groups: Structure of {[CH2SiMe2C6H3(CH2NMe2)-4-Li-3]2}2

Abstract

A useful synthetic procedure for the incorporation of the potentially multidentate monoanionic 1-[C6H2(CH2NMe2)2-3,5]- (=NCN) and 1-[C6H3(CH2NMe2)-4]- (=CN) ligands via the para-position on the periphery of carbosilane (CS) dendrimers has been developed. Lithiation of suitable brominated precursors [C6H3(CH2NMe2)2-3,5-Br-1], 1, and [C6H4(CH2NMe2)-4-Br-1], 2, in Et2O at -78 C with 2 equiv of t-BuLi gives the monolithiated aminoaryl compounds Li[C6H3(CH2NMe2)2-3,5], 1-Li, and Li[C6H4(CH2NMe2)-4], 2-Li, which can be subsequently treated in situ with the silicon chlorides ClSiMe2(CH2)nSiMe2Cl (n = 2 or 6) to yield the functionalized CS-ligand systems [CH2SiMe2C6H3(CH2NMe2)2-3,5]2, 3, and [(CH2)nSiMe2C6H4(CH2NMe2)-4]2 (for 4, n = 1; for 5, n = 3). The ligand systems 3-5 react with 2 equiv of t-BuLi to give the dilithiated derivatized carbosilane molecules [CH2SiMe2C6H2(CH2NMe2)2-3,5-Li-4]2 and [(CH2)nSiMe2C6H4(CH2NMe2)-4-Li-3]2 (7, n = 1) in good chemical yields. Quench reactions of the in situ prepared lithiated derivatives with either D2O, Me3SiCl/Me3SiOTf, or MeSSMe clearly showed that the CS-ligand systems have been quantitatively converted into their lithiated derivatives. In addition, the functionalized carbosilane dendrimers G0-SiMe2-(N)CN and G1-SiMe2-(N)CN (13-16), which can be prepared in high yield by using the same synthetic approach used for the model compounds, can also be quantitatively converted into highly aggregated, polylithium derivatives. An X-ray molecular structure determination of 7 revealed this compound to be a unique dimeric aggregate {[CH2SiMe2C6H3(CH2NMe2)-4-Li-3]2}2 with their respective 2-electron-4-center (4c-2e) anionic Cipso centers each bonded to a Li3 face. As a result the (Me)2SiCH2CH2Si(Me)2 CS backbone exhibits a nonideal trans conformation with a vicinal dihedral angle of -161.