Lithium–Olefin π-Complexes and the Mechanism of Carbolithiation: Synthesis, Solution Behavior, and Crystal Structure of (2,2-Dimethylpent-4-en-1-yl)lithium

Nature of the Short Rh-Li Contact between Lithium and the Rhodium ω-Alkenyl Complex [Rh(CH2CMe2CH2CH═CH2)2]. Inorg Chem 2021;60:8790-8801. [PMID: 34097392 DOI: 10.1021/acs.inorgchem.1c00737] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We describe the preparation of the cis-bis(η¹,η²-2,2-dimethylpent-4-en-1-yl)rhodate(I) anion, cis-[Rh(CH₂CMe₂CH₂CH═CH₂)₂]^-, and the interaction of this species with Li⁺ both in solution and in the solid state. For the lithium(diethyl ether) salt [Li(Et₂O)][Rh(CH₂CMe₂CH₂CH═CH₂)₂], VT-NMR and ¹H{⁷Li} NOE NMR studies in toluene-d₈ show that the Li⁺ cation is in close proximity to the d_z² orbital of rhodium. In the solid-state structure of the lithium(12-crown-4) salt [Li(12-crown-4)₂][Li{Rh(CH₂CMe₂CH₂CH═CH₂)₂}₂], one lithium atom is surrounded by two [Rh(CH₂CMe₂CH₂CH═CH₂)₂]^- anions, and in this assembly there are two unusually short Rh-Li distances of 2.48 Å. DFT calculations, natural energy decomposition, and ETS-NOCV analysis suggest that there is a weak dative interaction between the 4d_z² orbitals on the Rh centers and the 2p_z orbital of the Li⁺ cation. The charge-transfer term between Rh and Li⁺ contributes only about the 1/5 of the total interaction energy, however, and the principal driving force for the proximity of Rh and Li in compounds 1 and 2 is that Li⁺ is electrostatically attracted to negative charges on the dialkylrhodiate anions.

Synthesis and Characterization of Strontium N,N-Dimethylaminodiboranates as Possible Chemical Vapor Deposition Precursors

The reaction of SrBr₂ with 2 equiv of sodium N,N-dimethylaminodiboranate (DMADB; Na(H₃BNMe₂BH₃)) in Et₂O at 0 °C followed by crystallization and drying under vacuum gives the unsolvated strontium compound Sr(H₃BNMe₂BH₃)₂ (1). Before the vacuum-drying step, the colorless crystals obtained by crystallization consist of the diethyl ether adduct Sr(H₃BNMe₂BH₃)₂(Et₂O)₂ (2). If the reaction of SrBr₂ with 2 equiv of Na(H₃BNMe₂BH₃) is carried out in the more strongly coordinating solvent thf, the solvate Sr(H₃BNMe₂BH₃)₂(thf)₃ (3) is obtained. Treating the thf adduct 3 with 1,2-dimethoxyethane (dme), bis(2-methoxyethyl) ether (diglyme), or N,N,N',N'-tetramethylethylenediamine (tmeda) in thf affords the new compounds Sr(H₃BNMe₂BH₃)₂(dme)₂ (4), Sr(H₃BNMe₂BH₃)₂(diglyme) (5), and Sr(H₃BNMe₂BH₃)₂(tmeda) (6), respectively, in greater than 60% yields. Treatment of 3 with 2 equiv of the crown ether 12-crown-4 affords the charge-separated salt [Sr(H₃BNMe₂BH₃)(12-crown-4)₂][H₃BNMe₂BH₃] (7). Crystal structures of all the Lewis base adducts are described. Compounds 2-6 all possess chelating κ²-BH₃NMe₂BH₃-κ² groups, in which two hydrogen atoms on each boron center are bound to strontium. Compound 6 is dinuclear because each metal atom is also coordinated to one hydrogen atom on a BH₃NMe₂BH₃^- ligand that chelates to the neighboring metal center. Compound 7 possesses an unusual κ¹-BH₃NMe₂BH₃^- group owing to the near-complete encapsulation of the Sr atom by two 12-crown-4 molecules; the other BH₃NMe₂BH₃^- anion is a charge-separated counterion. When they are heated, the diglyme and tmeda compounds 5 and 6 melt without decomposition and can be sublimed readily under reduced pressure (1 Torr) at 120 °C. The diglyme and tmeda adducts are some of the most volatile strontium compounds known and are promising candidates as CVD precursors for the growth of strontium-containing thin films.

Computational Insight into 1,2-Diamine, -Diether, and -Amino Ether Chiral Ligand-Mediated Carbolithiation: A Case of Enantioinduction Reversal

trans-1,2-Cyclohexanediamine, -diether, and -amino ether were compared as chiral inducers in the asymmetric intramolecular carbolithiation of olefinic aryllithiums. Switching from diamine to ethereal ligands inverts the sense of asymmetric induction. This reversal of stereoselectivity was investigated through density functional theory calculations. High enantioselectivities observed with diether and amino ether ligands arise from favorable weak interactions between the ligand and the substrate. The relative efficiency of the three ligands and sense of stereoinduction for the most efficient diether and amino ether ligands prove to be foreseeable by modeling the reaction with the parent achiral 1,2-bidentate additives and comparing the diastereomeric transition states stemming from the two half-chair conformations of their lithium chelate.