Solvation Effects on the Structure and Stability of Alkali Metal Carbenoids.

s-Block metal carbenoids are carbene synthons and applied in a myriad of organic transformations. They exhibit a strong structure-activity relationship, but this is only poorly understood due to the challenging high reactivity and sensitivity of these reagents. Here, we report on systematic VT and DOSY NMR studies, XRD analyses as well as DFT calculations on a sulfoximinoyl-substituted model system to explain the pronounced solvent dependency of the carbenoid stability. While the sodium and potassium chloride carbenoids showed high stabilities independent of the solvent, the lithium carbenoid was stable at room temperature in THF but decomposed at -10 °C in toluene. These divergent stabilities could be explained by the different structures formed in solution. In contrast to simple organolithium reagents, the monomeric THF-solvate was found to be more stable than the dimer in toluene, since the latter more readily forms direct Li/Cl interactions which facilitate decomposition via α-elimination.

Efficient Pd-Catalyzed Direct Coupling of Aryl Chlorides with Alkyllithium Reagents.

Organolithium compounds are amongst the most important organometallic reagents and frequently used in difficult metallation reactions. However, their direct use in the formation of C-C bonds is less established. Although remarkable advances in the coupling of aryllithium compounds have been achieved, Csp2 -Csp3 coupling reactions are very limited. Herein, we report the first general protocol for the coupling or aryl chlorides with alkyllithium reagents. Palladium catalysts based on ylide-substituted phosphines (YPhos) were found to be excellently suited for this transformation giving high selectivities at room temperature with a variety of aryl chlorides without the need for an additional transmetallation reagent. This is demonstrated in gram-scale synthesis including building blocks for materials chemistry and pharmaceutical industry. Furthermore, the direct coupling of aryllithiums as well as Grignard reagents with aryl chlorides was also easily accomplished at room temperature.

Soft, Wet-Chemical Synthesis of Metastable Superparamagnetic Hexagonal Close-Packed Nickel Nanoparticles in Different Ionic Liquids.

The microwave-induced decomposition of bis{N,N'-diisopropylacetamidinate}nickel(II) [Ni{MeC(NiPr)2 }2 ] or bis(1,5-cyclooctadiene)nickel(0) [Ni(COD)2 ] in imidazolium-, pyridinium-, or thiophenium-based ionic liquids (ILs) with different anions (tetrafluoroborate, [BF4 ]- , hexafluorophosphate, [PF6 ]- , and bis(trifluoromethylsulfonyl)imide, [NTf2 ]- ) yields small, uniform nickel nanoparticles (Ni NPs), which are stable in the absence of capping ligands (surfactants) for more than eight weeks. The soft, wet-chemical synthesis yields the metastable Ni hexagonal close-packed (hcp) and not the stable Ni face-centered cubic (fcc) phase. The size of the nickel nanoparticles increases with the molecular volume of the used anions from about 5 nm for [BF4 ]- to ≈10 nm for [NTf2 ]- (with 1-alkyl-3-methyl-imidazolium cations). The n-butyl-pyridinium, [BPy]+ , cation ILs reproducibly yield very small nickel nanoparticles of 2(±1) nm average diameter. The Ni NPs were characterized by high-resolution transmission electron microscopy (HR-TEM) and powder X-ray diffraction. An X-ray photoelectron spectroscopic (XPS) analysis shows an increase of the binding energy (EB ) of the electron from the Ni 2p3/2 orbital of the very small 2(±1) nm diameter Ni particles by about 0.3 eV to EB =853.2 eV compared with bulk Ni0 , which is traced to the small cluster size. The Ni nanoparticles show superparamagnetic behavior from 150 K up to room temperature. The saturation magnetization of a Ni (2±1 nm) sample from [BPy][NTf2 ] is 2.08 A m2 kg-1 and of a Ni (10±4 nm) sample from [LMIm][NTf2 ] it is 0.99 A m2 kg-1 , ([LMIm]=1-lauryl-3-methyl- imidazolium). The Ni NPs were active catalysts in IL dispersions for 1-hexene or benzene hydrogenation. Over 90 % conversion was reached under 5 bar H2 in 1 h at 100 °C for 1-hexene and a turnover frequency (TOF) up to 1330 molhexane (molNi )-1 h-1 or in 60 h at 100 °C for benzene hydrogenation and TOF=23 molcyclohexane (molNi )-1 h-1 .

Atom-Precise Organometallic Zinc Clusters.

The bottom-up synthesis of organometallic zinc clusters is described. The cation {[Zn10](Cp*)6 Me}(+) (1) is obtained by reacting [Zn2 Cp*2] with [FeCp2][BAr4 (F)] in the presence of ZnMe2. In the presence of suitable ligands, the high reactivity of 1 enables the controlled abstraction of single Zn units, providing access to the lower-nuclearity clusters {[Zn9 ](Cp*)6} (2) and {[Zn8 ](Cp*)5 ((t) BuNC)3}(+) (3). According to DFT calculations, 1 and 2 can be described as closed-shell species that are electron-deficient in terms of the Wade-Mingos rules because the apical ZnCp* units that constitute the cluster cage do not have three, but only one, frontier orbitals available for cluster bonding. Zinc behaves flexibly in building the skeletal metal-metal bonds, sometimes providing one major frontier orbital (like Group 11 metals) and sometimes providing three frontier orbitals (like Group 13 elements).

Zn···Zn interactions at nickel and palladium centers.

The analogy between ZnR fragments and the hydrogen radical represents a fruitful concept in organometallic synthesis. The organozinc(ii) and -zinc(i) sources ZnMe2 (Me = methyl) and [Zn2Cp*2] (Cp* = pentamethylcyclopentadienyl) provide one-electron fragments ·ZnR (R = Me, Cp*), which can be trapped by transition metal complexes [L a M], yielding [L b (ZnR) n ]. The addition of the dizinc compound [Zn2Cp*2] to coordinatively unsaturated [L a M] by the homolytic cleavage of the Zn-Zn bond can be compared to the classic oxidative addition reaction of H2, forming dihydride complexes [L a M(H)2]. It has also been widely shown that dihydrogen coordinates under preservation of the H-H bond in the case of certain electronic properties of the transition metal fragment. The σ-aromatic triangular clusters [Zn3Cp*3]+ and [Zn2CuCp*3] may be regarded as the first indication of this so far unknown, side-on coordination mode of [Zn2Cp*2]. With this background in mind the question arises if a series of complexes featuring the Zn2M structural motif can be prepared exhibiting a (more or less) intact Zn-Zn interaction, i.e. di-zinc complexes which are analogous to non-classical dihydrogen complexes of the Kubas type. In order to probe this idea, a series of interrelated organozinc nickel and palladium complexes and clusters were synthesized and characterized as model compounds: [Ni(ZnCp*)(ZnMe)(PMe3)3] (1), [Ni(ZnCp*)2(ZnMe)2(PMe3)2] (2), [{Ni(CN t Bu)2(µ2-ZnCp*)(µ2-ZnMe)}2] (3), [Pd(ZnCp*)4(CN t Bu)2] (4) and [Pd3Zn6(PCy3)2(Cp*)4] (5). The dependence of Zn···Zn interactions as a function of the ligand environments and the metal centers was studied. Experimental X-ray crystallographic structural data and DFT calculations support the analogy between dihydrogen and dizinc transition metal complexes.

The σ-aromatic clusters [Zn3]⁺ and [Zn2Cu]: embryonic brass.

The triangular clusters [Zn3Cp*3](+) and [Zn2CuCp*3] were obtained by addition of the in situ generated, electrophilic, and isolobal species [ZnCp*](+) and [CuCp*] to Carmona's compound, [Cp*Zn-ZnCp*], without splitting the ZnZn bond. The choice of non-coordinating fluoroaromatic solvents was crucial. The bonding situations of the all-hydrocarbon-ligand-protected clusters were investigated by quantum chemical calculations revealing a high degree of σ-aromaticity similar to the triatomic hydrogen ion [H3](+). The new species serve as molecular building units of Cu(n)Zn(m) nanobrass clusters as indicated by LIFDI mass spectrometry.

Clusters [Ma(GaCp*)b(CNR)c] (M = Ni, Pd, Pt): synthesis, structure, and Ga/Zn exchange reactions.

Reactions of homoleptic isonitrile ligated complexes or clusters of d(10)-metals with the potent carbenoid donor ligand GaCp* are presented (Cp* = pentamethylcyclopentadienyl). Treatment of [Ni4(CNt-Bu)7], [{M(CNR)2}3] (M = Pd, Pt) and [Pd(CNR)2Me2] (R = t-Bu, Ph) with suitable amounts of GaCp* lead to the formation of the heteroleptic, tri- and tetranuclear clusters [Ni4(CNt-Bu)7(GaCp*)3] (1), [{M(CNt-Bu)}3(GaCp*)4] (M = Pd: 2a, Pt: 2b), and [{Pd(CNR)}4(GaCp*)4] (R = t-Bu: 3a, Ph: 3b). The reactions involve isonitrile substitution reactions, GaCp* addition reactions, and cluster formation reactions. The new compounds were investigated for their ability to undergo Ga/Zn exchange reactions when treated with ZnMe2. The novel tetranuclear Zn-rich clusters [Ni4GaZn7(Cp*)2Me7(CNt-Bu)6] (4) and [{Pd(CNR)}4(ZnCp*)4(ZnMe)4] (R = t-Bu: 5a, Ph: 5b) were obtained and isolated. The electronic situation and geometrical arrangement of atoms of all compounds will be presented and discussed. All new compounds are characterized by solution (1)H, (13)C NMR and IR spectroscopy, elemental analysis (EA), liquid injection field desorption ionization mass spectrometry (LIFDI-MS) as well as single crystal X-ray crystallography.