Main Group Multiple C鈥揌/N鈥揌 Bond Activation of a Diamine and Isolation of A Molecular Dilithium Zincate Hydride: Experimental and DFT Evidence for Alkali Metal鈥揨inc Synergistic Effects

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• 作者：Ross Campbell ; Daniel Cannon ; Pablo Garc铆a-脕lvarez ; Alan R. Kennedy ; Robert E. Mulvey ; Stuart D. Robertson ; J枚rg Sa脽mannshausen ; Tell Tuttle
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：August 31, 2011
• 年：2011
• 卷：133
• 期：34
• 页码：13706-13717
• 全文大小：1176K
• 年卷期：v.133,no.34(August 31, 2011)
• ISSN：1520-5126

文摘

The surprising transformation of the saturated diamine (iPr)NHCH₂CH₂NH(iPr) to the unsaturated diazaethene [(iPr)NCH鈺怌HN(iPr)]^2- via the synergic mixture nBuM, (tBu)₂Zn and TMEDA (where M = Li, Na; TMEDA = N,N,N鈥?/i>,N鈥?/i>-tetramethylethylenediamine) has been investigated by multinuclear NMR spectroscopic studies and DFT calculations. Several pertinent intermediary and related compounds (TMEDA)Li[(iPr)NCH₂CH₂NH(iPr)]Zn(tBu)₂ (3), (TMEDA)Li[(iPr)NCH₂CH₂CH₂N(iPr)]Zn(tBu) (5), {(THF)Li[(iPr)NCH₂CH₂N(iPr)]Zn(tBu)}₂ (6), and {(TMEDA)Na[(iPr)NCH₂CH₂N(iPr)]Zn(tBu)}₂ (11), characterized by single-crystal X-ray diffraction, are discussed in relation to their role in the formation of (TMEDA)M[(iPr)NCH鈺怌HN(iPr)]Zn(tBu) (M = Li, 1; Na, 10). In addition, the dilithio zincate molecular hydride [(TMEDA)Li]₂[(iPr)NCH₂CH₂N(iPr)]Zn(tBu)H 7 has been synthesized from the reaction of (TMEDA)Li[(iPr)NCH₂CH₂NH(iPr)]Zn(tBu)₂3 with nBuLi(TMEDA) and also characterized by both X-ray crystallographic and NMR spectroscopic studies. The retention of the Li鈥揌 bond of 7 in solution was confirmed by ⁷Li鈥?sup>1H HSQC experiments. Also, the ⁷Li NMR spectrum of 7 in C₆D₆ solution allowed for the rare observation of a scalar ¹J_Li鈥揌 coupling constant of 13.3 Hz. Possible mechanisms for the transformation from diamine to diazaethene, a process involving the formal breakage of four bonds, have been determined computationally using density functional theory. The dominant mechanism, starting from (TMEDA)Li[(iPr)NCH₂CH₂N(iPr)]Zn(tBu) (4), involves the formation of a hydride intermediate and leads directly to the observed diazaethene product. In addition the existence of 7 in equilibrium with 4 through the dynamic association and dissociation of a (TMEDA)LiH ligand, also provides a secondary mechanism for the formation of the diazaethene. The two reaction pathways (i.e., starting from 4 or 7) are quite distinct and provide excellent examples in which the two distinct metals in the system are able to interact synergically to catalyze this otherwise challenging transformation.