title: Main group multiple C-H/N-H bond activation of a diamine and isolation of a molecular dilithium zincate hydride: experimental and DFT evidence for alkali metal-zinc synergistic effects.
abstract: The surprising transformation of the saturated diamine (iPr)NHCH(2)CH(2)NH(iPr) to the unsaturated diazaethene [(iPr)NCH═CHN(iPr)](2-) via the synergic mixture nBuM, (tBu)(2)Zn and TMEDA (where M = Li, Na; TMEDA = N,N,N',N'-tetramethylethylenediamine) has been investigated by multinuclear NMR spectroscopic studies and DFT calculations. Several pertinent intermediary and related compounds (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2) (3), (TMEDA)Li[(iPr)NCH(2)CH(2)CH(2)N(iPr)]Zn(tBu) (5), {(THF)Li[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)}(2) (6), and {(TMEDA)Na[(iPr)NCH(2)CH(2)N(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═CHN(iPr)]Zn(tBu) (M = Li, 1; Na, 10). In addition, the dilithio zincate molecular hydride [(TMEDA)Li](2)[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)H 7 has been synthesized from the reaction of (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2)3 with nBuLi(TMEDA) and also characterized by both X-ray crystallographic and NMR spectroscopic studies. The retention of the Li-H bond of 7 in solution was confirmed by (7)Li-(1)H HSQC experiments. Also, the (7)Li NMR spectrum of 7 in C(6)D(6) solution allowed for the rare observation of a scalar (1)J(Li-H) 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)NCH(2)CH(2)N(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.
authors.affiliation: WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, Scotland, United Kingdom.
authors.forename: Ross
authors.initials: R
authors.lastname: Campbell
authors.forename: Daniel
authors.initials: D
authors.lastname: Cannon
authors.forename: Pablo
authors.initials: P
authors.lastname: García-Álvarez
authors.forename: Alan R
authors.initials: AR
authors.lastname: Kennedy
authors.forename: Robert E
authors.initials: RE
authors.lastname: Mulvey
authors.forename: Stuart D
authors.initials: SD
authors.lastname: Robertson
authors.forename: Jörg
authors.initials: J
authors.lastname: Sassmannshausen
authors.forename: Tell
authors.initials: T
authors.lastname: Tuttle