Reversible Coordination of Boron–, Aluminum–, Zinc–, Magnesium–, and Calcium–Hydrogen Bonds to Bent {CuL₂} Fragments: Heavy σ Complexes of the Lightest Coinage Metal

A series of copper­(I) complexes bearing electron-deficient β-diketiminate ligands have been prepared. The study includes [{{ArNC­(CR₃)}₂CH}­Cu­(η²-toluene)_n] (Ar = Mes, R = F, <i>n</i> = 0.5, [<b>1</b>_<b>2</b><b>·tol</b>]; Ar = C₆F₅, R = Me, <i>n</i> = 1, [<b>2·tol</b>]; Ar = 2,6-Cl₂C₆H₃, R = H, <i>n</i> = 0.5, [<b>3</b>_<b>2</b><b>·tol</b>]). Reactions of [<b>1</b>–<b>3</b>_{<b><i>n</i></b>}<b>·tol</b>] with boranes, alanes, a zinc hydride, a magnesium hydride, and a calcium hydride generate the corresponding σ complexes ([<b>1–3·B</b>], <b>[3·B′</b>], [<b>3·Al</b>], [<b>3·Al′</b>], [<b>1–3·Zn</b>], [<b>1·Mg</b>], and [<b>1·Ca</b>]). These species all form reversibly, being in equilibrium with the arene solvates in solution. With the exception of the calcium complex, the complexes have all been characterized by single-crystal X-ray diffraction studies. In solution, the σ-hydride of the aluminum, zinc, magnesium, and calcium derivatives resonates between −0.12 and −1.77 ppm (C₆D₆ or toluene-<i>d</i>₈, 193–298 K). For the σ-borane complexes, the hydrides are observed as a single resonance between 2 and 3.5 ppm (C₆D₆, 298 K) and bridging and terminal hydrides rapidly exchange on the NMR time scale even at 193 K. Quantification of the solution dynamics by van’t Hoff analysis yields expectedly small values of Δ<i>H</i>° and negative values of Δ<i>S</i>° consistent with weak binding and a reversible process that does not involve aggregation of the copper species. The donor–acceptor complexes can be rationalized in terms of the Dewar–Chatt–Duncanson model. Density functional theory calculations show that the donation of σ-M–H (or E–H) electrons into the 4s-based orbital (LUMO or LUMO+1) of the copper fragment is accompanied by weak back-donation from a d_<i>xz</i>-based orbital (HOMO or HOMO–1) into the σ*-M–H (or E–H) orbital

Vinylic C–H Activation of Styrenes by an Iron–Aluminum Complex

The oxidative addition of sp2 C–H bonds of alkenes to single-site transition-metal complexes is complicated by the competing π-coordination of the CC double bond, limiting the examples of this type of reactivity and onward applications. Here, we report the C–H activation of styrenes by a well-defined bimetallic Fe–Al complex. These reactions are highly selective, resulting in the (E)-β-metalation of the alkene. For this bimetallic system, alkene binding appears to be essential for the reaction to occur. Experimental and computational insights suggest an unusual reaction pathway in which a (2 + 2) cycloaddition intermediate is directly converted into the hydrido vinyl product via an intramolecular sp2 C–H bond activation across the two metals. The key C–H cleavage step proceeds through a highly asynchronous transition state near the boundary between a concerted and a stepwise mechanism influenced by the resonance stabilization ability of the aryl substituent. The metalated alkenes can be further functionalized, which has been demonstrated by the (E)-selective phosphination of the employed styrenes

Vinylic C–H Activation of Styrenes by an Iron–Aluminum Complex

The oxidative addition of sp2 C–H bonds of alkenes to single-site transition-metal complexes is complicated by the competing π-coordination of the CC double bond, limiting the examples of this type of reactivity and onward applications. Here, we report the C–H activation of styrenes by a well-defined bimetallic Fe–Al complex. These reactions are highly selective, resulting in the (E)-β-metalation of the alkene. For this bimetallic system, alkene binding appears to be essential for the reaction to occur. Experimental and computational insights suggest an unusual reaction pathway in which a (2 + 2) cycloaddition intermediate is directly converted into the hydrido vinyl product via an intramolecular sp2 C–H bond activation across the two metals. The key C–H cleavage step proceeds through a highly asynchronous transition state near the boundary between a concerted and a stepwise mechanism influenced by the resonance stabilization ability of the aryl substituent. The metalated alkenes can be further functionalized, which has been demonstrated by the (E)-selective phosphination of the employed styrenes

Tunable Binding of Dinitrogen to a Series of Heterobimetallic Hydride Complexes

The reaction of [Ru­(H)₂(N₂)₂(PCy₃)₂] (<b>1</b>) with β-diketiminate stabilized hydrides of Al, Zn, and Mg generates a series of new heterobimetallic complexes with either H₂ or N₂ ligated to the ruthenium center. Changing the main-group fragment of the <b>M·Ru-N</b>_<b>2</b> (M = Al, Zn, Mg) complexes can subtly alter the degree of binding, and therefore activation, of the diatomic ligand, as evidenced by the ν_NN absorptions in the infrared data. Experimental and computational data rationalize this tunable binding; decreasing the electronegativity of the main group in the order Al > Zn > Mg infers greater ionic character of these <b>M·Ru-N</b>_<b>2</b> complexes, and this in turn results in greater destabilization of the frontier molecular orbitals of ruthenium and therefore greater Ru­(4d) → π*­(N₂) back-donation

Weakly Coordinated Zinc and Aluminum σ‑Complexes of Copper(I)

We report the synthesis and isolation of three new σ-complexes of Cu­(I) in which E–H (E = Al, Zn) σ-bonds are coordinated to copper. The addition of the main group hydride to a toluene-solvated Cu­(I) complex results in reversible ligand exchange, and the Cu­(I) σ-complexes have been crystallized. Experimental and computational data provide a wealth of evidence for weak binding of the E–H bond to Cu­(I), which can be ascribed to σ-donation from the E–H bond into the 4s orbital of copper and back-donation from copper into the E–H σ* orbital

A Highly Chemoselective, Zr-Catalyzed C–O Bond Functionalization of Benzofuran

The chemoselective C–O bond functionalization of benzofuran with an aluminum dihydride may be catalyzed by zirconocene dichlorides. The reaction proceeds with the formal addition of a C–O bond to, and elimination of dihydrogen from, aluminum. The product of C–O bond alumination reacts with benzaldehyde via insertion of the carbonyl into the newly formed Al–C bond

Rhodium Catalyzed, Carbon–Hydrogen Bond Directed Hydrodefluorination of Fluoroarenes

[Cp*RhCl­(μ-Cl)]₂ is reported as a highly efficient and selective precatalyst for the hydrodefluorination of perfluoroarenes using a hydrocarbon-soluble aluminum dihydride as the terminal reductant. Reactions are directed to cleave a C–F bond adjacent to an existing C–H bond with high regioselectivity (98.5–99%). A heterobimetallic complex containing an extremely rare Al–H–Rh functional group has been isolated and shown to be catalytically competent

Rhodium Catalyzed, Carbon–Hydrogen Bond Directed Hydrodefluorination of Fluoroarenes

[Cp*RhCl­(μ-Cl)]₂ is reported as a highly efficient and selective precatalyst for the hydrodefluorination of perfluoroarenes using a hydrocarbon-soluble aluminum dihydride as the terminal reductant. Reactions are directed to cleave a C–F bond adjacent to an existing C–H bond with high regioselectivity (98.5–99%). A heterobimetallic complex containing an extremely rare Al–H–Rh functional group has been isolated and shown to be catalytically competent

Catalytic and Stoichiometric Cumulene Formation within Dimeric Group 2 Acetylides

A series of β-diketiminate-supported magnesium and calcium acetylide complexes have been synthesized by σ-bond metathesis of magnesium <i>n</i>-butyl or magnesium and calcium amido precursors and a range of terminal acetylenes. The dimeric complexes have been characterized by NMR spectroscopy and X-ray diffraction analysis. The homoleptic bis­(amido) and dialkyl complexes [M­{X­(SiMe₃)₂}₂(THF)₂] (M = Ca, Sr; X = N, CH) have been assessed for the atom-efficient, catalytic head-to-head dimerization of donor-functionalized terminal alkynes into butatrienes and aryl-/silyl-substituted terminal acetylenes into 1,3-enynes. Deuterium labeling studies of the catalytic reactions are suggested to imply that triene formation requires concerted proton delivery and rearrangement via an adjacent methylene group at a bimetallic alkaline-earth species

Mild sp²Carbon–Oxygen Bond Activation by an Isolable Ruthenium(II) Bis(dinitrogen) Complex: Experiment and Theory

The isolable ruthenium­(II) bis­(dinitrogen) complex [Ru­(H)₂(N₂)₂(PCy₃)₂] (<b>1</b>) reacts with aryl ethers (Ar–OR, R = Me and Ar) containing a ketone directing group to effect sp²C–O bond activation at temperatures below 40 °C. DFT studies support a low-energy Ru­(II)/Ru­(IV) pathway for C–O bond activation: oxidative addition of the C–O bond to Ru­(II) occurs in an asynchronous manner with Ru–C bond formation preceding C–O bond breaking. Alternative pathways based on a Ru(0)/Ru­(II) couple are competitive but less accessible due to the high energy of the Ru(0) precursors. Both experimentally and by DFT calculations, sp²C–H bond activation is shown to be more facile than sp²C–O bond activation. The kinetic preference for C–H bond activation over C–O activation is attributed to unfavorable approach of the C–O bond toward the metal in the selectivity determining step of the reaction pathway