No abstract

The first [ECE]Ni(II) pincer complexes with E = Si(II) and E = Ge(II) metallylene donor arms were synthesized via C-X (X = H, Br) oxidative addition, starting from the corresponding [EC(X)E] ligands. These novel complexes were fully characterized (NMR, MS, and XRD) and used as catalyst for Ni-catalyzed Sonogashira reactions. These catalysts allowed detailed information on the elementary steps of this catalytic reaction (transmetalation → oxidative addition → reductive elimination), resulting in the isolation and characterization of an unexpected intermediate in the transmetalation step. This complex, {[ECE]Ni acetylide → CuBr} contains both nickel and copper, with the copper bound to the alkyne π-system. Consistent with these unusual structural features, DFT calculations of the {[ECE]Ni acetylide → CuBr} intermediates revealed an unusual E-Cu-Ni three-center-two-electron bonding scheme. The results reveal a general reaction mechanism for the Ni-based Sonogashira coupling and broaden the application of metallylenes as strong σ-donor ligands for catalytic transformations.

Experimental and theoretical charge density studies and molecular orbital analyses suggest that the complexes [Cp2Ti(PMe3)SiH2Ph2] (1) and [Cp2Ti(PMe3)SiHCl3] (2) display virtually the same electronic structures. No evidence for a significant interligand hypervalent interaction could be identified for 2. A bonding concept for transition-metal hydrosilane complexes aims to identify the true key parameters for a selective activation of the individual M-Si and Si-H bonds.

The nickel‐catalyzed decomposition of formic acid to yield molecular hydrogen and the nickel‐catalyzed hydrogenation of bicarbonate as a carbon dioxide mimic have been examined. Well‐defined nickel complexes modified by a PCP‐pincer ligand, especially nickel hydride and nickel formate complexes, revealed catalytic activity with turnover numbers of up to 626 (decomposition) and 3000 (hydrogenation). Thus, a formal hydrogen storage and release cycle performed by a well‐defined nickel catalyst was accomplished.

In general, C À H bonds can be considered chemically inert as a result of their strength, nonpolar nature, and low polarizability. Since the pioneering work of La Placa and Ibers in 1965, who reported the close approach of a CÀH bond to a transition-metal center, there have been many attempts to trace the microscopic control parameters of such C À H activation processes by metal atoms in general.[1] In particular, complexes containing side-on-coordinated (h 2 -CH) moieties next to a transition metal are the focus of intensive research as they allow the systematic study of the CÀH activation phenomenon in molecules and solids in their electronic ground states. Furthermore, M···H À C interactions (M = transition metal) play a key role in the performance of several industrially relevant catalytic processes, such as olefin polymerization.[2]In the course of a systematic analysis of such M···HÀC interactions, Brookhart and Green coined the expression agostic interactions to "discuss the various manifestations of covalent interactions between C À H groups and transitionmetal centers in organometallic compounds". [3a,b] In case of d 0 early-transition-metal alkyl or amido complexes, the strength of agostic interactions is mainly controlled by 1) the local Lewis acidity of the metal center, 2) the extent of negative hyperconjugative delocalization of the M À C/M À N bonding electrons, and 3) to a smaller degree by s(M ! H À C) donation.[3c, 4] For agostic late-transition-metal complexes, however, the control parameters are less clear. We therefore synthesized a variety of new Spencer-type [5] nickel alkyl cations 2 b-d by protonation of the corresponding olefin complexes 1 b-d to study the nature of their pronounced agostic interactions by combined experimental and theoretical charge density studies (Scheme 1).Re-examination of the classic Spencer-type complex [EtNi(dtbpe)]showed a fast rotation of the b-agostic methyl moiety in solution[6] and a systematic crystallographic disorder in the solid state, thus preventing a detailed investigation of the bonding properties of this agostic textbook example by experimental charge-density studies. We therefore replaced the ethylene moiety in 1 a by the sterically more demanding norbornyl (nbe) and dicyclopentadienyl (DCp) ligands. Protonation of 1 b-d yielded the agostic complexes 2 b-d, which all have a significantly reduced fluxional behavior in solution. Furthermore, single crystals of excellent quality could be obtained for 2 b-d, which even allowed an experimental charge-density analysis of 2 c.