Tuning the Inorganic Core of a reduced Ni2Ge2 Cluster.

Tetranuclear cores (M-E)2 of transition metals (M) and tetrylenes (EII=Si, Ge, Sn) are key motifs in homogeneous and heterogeneous catalysis. They exhibit a continuum of M-M and E-E bonding within the inorganic core that leads to a variety of structures for which there are no specific synthetic methods. Herein, we report a series of highly reduced [Ni0GeII]2 squares solely stabilized by bulky terphenyl (C6H3-2,6-Ar2) ligands, for which we provide complementary and high-yielding syntheses. Reactivity studies with common Lewis bases (carbene and CO) evince that the structure of the (M-E)2 core can be transformed. We have investigated this core modification by computational means, offering a rationale to better understand the continuum of bonding across these clusters.

Nitrous oxide activation by picoline-derived Ni-CNP hydrides.

Oxygen atom transfer (OAT) from N2O to the Ni-H bond of proton-responsive picoline-derived CNP nickel complexes has been investigated both experimentally and theoretically. These Ni-CNP complexes efficiently catalyse the reduction of N2O with pinacolborane (HBpin) under mild conditions.

Broad-Scope Amination of Aryl Sulfamates Catalyzed by a Palladium Phosphine Complex.

Among phenol-derived electrophiles, aryl sulfamates are attractive substrates since they can be employed as directing groups for C-H functionalization prior to catalysis. However, their use in C-N coupling is limited only to Ni catalysis. Here, we describe a Pd-based catalyst with a broad scope for the amination of aryl sulfamates. We show that the N-methyl-2-aminobiphenyl palladacycle supported by the PCyp2ArXyl2 ligand (Cyp = cyclopentyl; ArXyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl) efficiently catalyzes the C-N coupling of aryl sulfamates with a variety of nitrogen nucleophiles, including anilines, primary and secondary alkyl amines, heteroaryl amines, N-heterocycles, and primary amides. DFT calculations support that the oxidative addition of the aryl sulfamate is the rate-determining step. The C-N coupling takes place through a cationic pathway in the polar protic medium.

A genuine germylene PGeP pincer ligand for formic acid dehydrogenation with iridium.

We report an iridium system constructed around a long-tethered PGeP ligand that facilitates access to the less common germylene form, so far unreported for an 'NHC-type' Ge ligand. Its bonding is substantiated by computational studies and we have demonstrated its use for the catalytic dehydrogenation of formic acid, highlighting the potential of this underdeveloped type of ligand.

Catalytic Nitrous Oxide Reduction with H2 Mediated by Pincer Ir Complexes.

Reduction of nitrous oxide (N2O) with H2 to N2 and water is an attractive process for the decomposition of this greenhouse gas to environmentally benign species. Herein, a series of iridium complexes based on proton-responsive pincer ligands (1-4) are shown to catalyze the hydrogenation of N2O under mild conditions (2 bar H2/N2O (1:1), 30 °C). Among the tested catalysts, the Ir complex 4, based on a lutidine-derived CNP pincer ligand having nonequivalent phosphine and N-heterocyclic carbene (NHC) side donors, gave rise to the highest catalytic activity (turnover frequency (TOF) = 11.9 h-1 at 30 °C, and 16.4 h-1 at 55 °C). Insights into the reaction mechanism with 4 have been obtained through NMR spectroscopy. Thus, reaction of 4 with N2O in tetrahydrofuran-d8 (THF-d8) initially produces deprotonated (at the NHC arm) species 5NHC, which readily reacts with H2 to regenerate the trihydride complex 4. However, prolonged exposure of 4 to N2O for 6 h yields the dinitrogen Ir(I) complex 7P, having a deprotonated (at the P-arm) pincer ligand. Complex 7P is a poor catalytic precursor in the N2O hydrogenation, pointing out to the formation of 7P as a catalyst deactivation pathway. Moreover, when the reaction of 4 with N2O is carried out in wet THF-d8, formation of a new species, which has been assigned to the hydroxo species 8, is observed. Finally, taking into account the experimental results, density functional theory (DFT) calculations were performed to get information on the catalytic cycle steps. Calculations are in agreement with 4 as the TOF-determining intermediate (TDI) and the transfer of an apical hydrido ligand to the terminal nitrogen atom of N2O as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC- or the P-CH2 pincer methylene linkers.

Electrophilic activation of alkynes promoted by a cationic alkylidene complex of Pt(II).

Pt(II) alkylidene 1a has been reacted with terminal alkynes to afford ylide complexes 3a-d, resulting from electrophilic activation of the CC bond and its insertion into the platinacyclic fragment of 1a that contains the carbene functionality. DFT calculations indicate that the observed regioselectivity is determined by the nucleophilic attack of the alkyne to the alkylidene carbon.

Ammonia-Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNNH Complexes.

Metal complexes incorporating proton-responsive ligands have been proved to be superior catalysts in reactions involving the H2 molecule. In this contribution, a series of IrIII complexes based on lutidine-derived CNNH pincers containing N-heterocyclic carbene and secondary amino NHR [R = Ph (4a), tBu (4b), benzyl (4c)] donors as flanking groups have been synthesized and tested in the dehydrogenation of ammonia-borane (NH3BH3, AB) in the presence of substoichiometric amounts (2.5 equiv) of tBuOK. These preactivated derivatives are efficient catalysts in AB dehydrogenation in THF at room temperature, albeit significantly different reaction rates were observed. Thus, by using 0.4 mol % of 4a, 1.0 equiv of H2 per mole of AB was released in 8.5 min (turnover frequency (TOF50%) = 1875 h-1), while complexes 4b and 4c (0.8 mol %) exhibited lower catalytic activities (TOF50% = 55-60 h-1). 4a is currently the best performing IrIII homogeneous catalyst for AB dehydrogenation. Kinetic rate measurements show a zero-order dependence with respect to AB, and first order with the catalyst in the dehydrogenation with 4a (-d[AB]/dt = k[4a]). Conversely, the reaction with 4b is second order in AB and first order in the catalyst (-d[AB]/dt = k[4b][AB]2). Moreover, the reactions of the derivatives 4a and 4b with an excess of tBuOK (2.5 equiv) have been analyzed through NMR spectroscopy. For the former precursor, formation of the iridate 5 was observed as a result of a double deprotonation at the amine and the NHC pincer arm. In marked contrast, in the case of 4b, a monodeprotonated (at the pincer NHC-arm) species 6 is observed upon reaction with tBuOK. Complex 6 is capable of activating H2 reversibly to yield the trihydride derivative 7. Finally, DFT calculations of the first AB dehydrogenation step catalyzed by 5 has been performed at the DFT//MN15 level of theory in order to get information on the predominant metal-ligand cooperation mode.

Dehydrogenative Double C-H Bond Activation in a Germylene-Rhodium Complex*.

Transition metal tetrylene complexes offer great opportunities for molecular cooperation due to the ambiphilic character of the group 14 element. Here we focus on the coordination of germylene [(ArMes2 )2 Ge :] (ArMes =C6 H3 -2,6-(C6 H2 -2,4,6-Me3 )2 ) to [RhCl(COD)]2 (COD=1,5-cyclooctadiene), which yields a neutral germyl complex in which the rhodium center exhibits both η6 - and η2 -coordination to two mesityl rings in an unusual pincer-type structure. Chloride abstraction from this species triggers a singular dehydrogenative double C-H bond activation across the Ge/Rh motif. We have isolated and fully characterized three rhodium-germyl species associated to three C-H cleavage events along this process. The reaction mechanism has been further investigated by computational means, supporting the key cooperative action of rhodium and germanium centers.

Tuning Activity and Selectivity during Alkyne Activation by Gold(I)/Platinum(0) Frustrated Lewis Pairs.

Introducing transition metals into frustrated Lewis pair systems has attracted considerable attention in recent years. Here we report a selection of three metal-only frustrated systems based on Au(I)/Pt(0) combinations and their reactivity toward alkynes. We have inspected the activation of acetylene and phenylacetylene. The gold(I) fragments are stabilized by three bulky phosphines bearing terphenyl groups. We have observed that subtle modifications on the substituents of these ligands proved critical in controlling the regioselectivity of acetylene activation and the product distribution resulting from C(sp)-H cleavage of phenylacetylene. A mechanistic picture based on experimental observations and computational analysis is provided. As a result of the cooperative action of the two metals of the frustrated pairs, several uncommon heterobimetallic structures have been characterized.

Structural Snapshots of π-Arene Bonding in a Gold Germylene Cation.

Heavier group 14 element cations exhibit a remarkable reactivity that has typically hampered their isolation. For the few available examples, the role of π-arene interactions is crucial to provide kinetic stabilization, but dynamic and structural information on those contacts is yet limited. In this study we have accessed the metalogermylenium cation [(PMe2 ArDipp2 )AuGe(ArDipp2 )Cl]+ (4+ ) (ArDipp2 =C6 H3 -2,6-(C6 H3 -2,6-iPr2 )2 ) that has been structurally characterized with three different non-coordinating counter anions. These studies provide for the first time dynamic information about the conformational rearrangement that characterizes π-arene bonding thorough a series of X-ray diffraction structural snapshots. Computational studies reveal the weak character of the π-arene bonding (ca. 2 kcal mol-1 ) that can be described as the donation from a πC=C bond toward the empty p valence orbital of germanium.

Metal-only Lewis Pairs of Rhodium with s, p and d-Block Metals.

Metal-only Lewis pairs (MOLPs) in which the two metal fragments are solely connected by a dative M→M bond represent privileged architectures to acquire fundamental understanding of bimetallic bonding. This has important implications in many catalytic processes or supramolecular systems that rely on synergistic effects between two metals. However, a systematic experimental/computational approach on a well-defined class of compounds is lacking. Here we report a family of MOLPs constructed around the RhI precursor [(η5 -C5 Me5 )Rh(PMe3 )2 ] (1) with a series of s, p and d-block metals, mostly from the main group elements, and investigate their bonding by computational means. Among the new MOLPs, we have structurally characterized those formed by dative bonding between 1 and MgMeBr, AlMe3 , GeCl2 , SnCl2 , ZnMe2 and Zn(C6 F5 )2, as well as spectroscopically identified the ones resulting from coordination to MBArF (M=Na, Li; BArF - =[B(C6 H2 -3,5-(CF3 )2 )4 ]- ) and CuCl. Some of these compounds represent unique examples of bimetallic structures, such as the first unambiguous cases of Rh→Mg dative bonding or base-free rhodium bound germylene and stannylene species. Multinuclear NMR spectroscopy, including 103 Rh NMR, is used to probe the formation of Rh→M bonds. A comprehensive theoretical analysis of those provides clear trends. As anticipated, greater bond covalency is found for the more electronegative acids, whereas ionic character dominates for the least electronegative nuclei, though some degree of electron sharing is identified in all cases.

Hydrogenation/dehydrogenation of N-heterocycles catalyzed by ruthenium complexes based on multimodal proton-responsive CNN(H) pincer ligands.

Ru complexes based on lutidine-derived pincer CNN(H) ligands having secondary amine side donors are efficient precatalysts in the hydrogenation and dehydrogenation of N-heterocycles. Reaction of a Ru-CNN(H) complex with an excess of base produces the formation of a Ru(0) derivative, which is observed under catalytic conditions.

Evidence for Genuine Bimetallic Frustrated Lewis Pair Activation of Dihydrogen with Gold(I)/Platinum(0) Systems.

Invited for the cover of this issue is the group of Joaquín López-Serrano and Jesús Campos at the Consejo Superior de Investigaciones Científicas and the University of Sevilla. The image depicts the importance of balancing the degree of frustration/interaction in the splitting of H2 by AuI /Pt0 . Read the full text of the article at 10.1002/chem.201905793.

Evidence for Genuine Bimetallic Frustrated Lewis Pair Activation of Dihydrogen with Gold(I)/Platinum(0) Systems.

A joint experimental/computational effort to elucidate the mechanism of dihydrogen activation by a gold(I)/platinum(0) metal-only frustrated Lewis pair (FLP) is described herein. The drastic effects on H2 activation derived from subtle ligand modifications have also been investigated. The importance of the balance between bimetallic adduct formation and complete frustration has been interrogated, providing for the first time evidence for genuine metal-only FLP reactivity in solution. The origin of a strong inverse kinetic isotopic effect has also been clarified, offering further support for the proposed bimetallic FLP-type cleavage of dihydrogen.

Ir-Catalyzed Atroposelective Desymmetrization of Heterobiaryls: Hydroarylation of Vinyl Ethers and Bicycloalkenes.

A highly regio-, diastereo-, and enantioselective, scalable Ir-catalyzed hydroarylation of electron-rich acyclic and tensioned cyclic olefins with heterobiaryls is described. The reaction of acyclic vinyl ethers, dihydrofuran, and norbornenes with a variety of aryl isoquinoline, quinazoline, and picoline derivatives takes place with simultaneous installation of central and axial chirality, reaching complete branched/linear or exo/endo ratios and excellent diastereo- and enantiomeric excesses when in situ formed [IrI/Tol-SDP] or [IrI/Tol-BINAP] complexes are used as the catalysts. Deuterium labeling experiments and a comprehensive computational study suggest that, despite fast double bond migratory insertion into Ir-H, the reaction proceeds through a modified Chalk-Harrod mechanism, starting with selectivity-determining insertion into Ir-CAryl. The regioselectivity is controlled by the electron-donating alkoxy group, whereas diastereo- and enantioselectivity have a complex origin, which depend on the relative orientation of the alkoxy group and the establishment of adequate π-π interactions between the biaryl and the phosphine.

From cyclic amines and acetonitrile to amidine zinc(ii) complexes.

A seemingly simple combination of [Zn(quin)2(H2O)] (quin- = quinaldinate) and a selected secondary cyclic amine, piperidine (pipe), pyrrolidine (pyro) or morpholine (morph), afforded in acetonitrile a number of products: anionic homoleptic quinaldinate, neutral heteroleptic quinaldinate/amine and quinaldinate/amidine complexes. The piperidine and pyrrolidine systems underwent reaction with acetonitrile to give amidines. The in situ formed piperidinoacetamidine (pipeam) or pyrrolidinoacetamidine (pyroam) coordinated to zinc(ii). Reactions with piperidine led to trans-[Zn(quin)2(pipe)2]·2CH3CN (1), [Zn(quin)2(pipe)]·cis-[Zn(quin)2(pipe)2] (2), pipeH[Zn(quin)3]·CH3CN (3), [Zn(quin)2(pipeam)]·CH3CN (4a), [Zn(quin)2(pipeam)]·2CHCl3 (4b), pipeamH[Zn(quin)3] (5) and pipeamH[Zn(quin)2(CH3COO)]·acetamide (6) (pipeH+ and pipeamH+ denote protonated amine or amidine). By analogy, [Zn(quin)2(pyro)2] (7), pyroH[Zn(quin)3]·CH3CN (8), pyroH[Zn(quin)2Cl] (9), [Zn(quin)2(pyroam)]·CH3CN·0.5pyroam·0.5H2O (10a), [Zn(quin)2(pyroam)]·2CHCl3 (10b), [Zn(quin)2(pyroam)]·CH2Cl2 (10c) and pyroamH[Zn(quin)3] (11) were obtained in the pyrrolidine reactions. The morpholine system allowed isolation of only two novel products, trans-[Zn(quin)2(morph)2] (12) and morphH[Zn(quin)3]·CH3CN (13). Importantly, no amidine could be isolated. Instead, in autoclaves at 105 °C morpholine degraded to ammonia, as confirmed by mass spectrometry of the gas phase. pyroamH[Zn(quin)3] exists in two polymorphs which differ in the binding modes of quinaldinate ligands. In 11triclinic, the metal ion of [Zn(quin)3]- features a five-coordinate environment, whereas that in 11monoclinic is surrounded by six donors. Stabilities of the [Zn(quin)3]- isomers were assessed with DFT calculations. The one with a six-coordinate zinc(ii) ion was found to be more stable than its five-coordinate counterpart. Favorable intermolecular interactions in the solid state stabilize both and reduce the energy difference between them. The calculations show the conversion of the five-coordinate [Zn(quin)3]- into its coordinatively saturated isomer to be an almost barrierless process.

Base-Promoted, Remote C-H Activation at a Cationic (η5-C5Me5)Ir(III) Center Involving Reversible C-C Bond Formation of Bound C5Me5.

C-H bond activation at cationic [(η5-C5Me5)Ir(PMe2Ar')] centers is described, where PMe2Ar' are the terphenyl phosphine ligands PMe2ArXyl2 and PMe2ArDipp2. Different pathways are defined for the conversion of the five-coordinate complexes [(η5-C5Me5)IrCl(PMe2Ar')]+, 2(Xyl)+ and 2(Dipp)+, into the corresponding pseudoallyls 3(Xyl)+ and 3(Dipp)+. In the absence of an external Brønsted base, electrophilic, remote ζ C-H activation takes place, for which the participation of dicationic species, [(η5-C5Me5)Ir(PMe2Ar')]2+, is proposed. When NEt3 is present, the PMe2ArDipp2 system is shown to proceed via 4(Dipp)+ as an intermediate en route to the thermodynamic, isomeric product 3(Dipp)+. This complex interconversion involves a non-innocent C5Me5 ligand, which participates in C-H and C-C bond formation and cleavage. Remarkably, the conversion of 4(Dipp)+ to 3(Dipp)+ also proceeds in the solid state.

Double asymmetric hydrogenation of conjugated dienes: a self-breeding chirality route for C2 symmetric 1,4-diols.

The synthesis and double asymmetric hydrogenation of (Z,Z)-1,3-diene-1,4-diyl diacetates is described. In this reaction C2/meso ratios up to 85 : 15 and enantioselectivities up to 97% ee have been achieved. As the hydrogenation products can be converted into chiral 1,4-diols, key starting materials for the preparation of the best catalysts used, this catalytic system enables a self-breeding chirality process.

Hydroboration of carbon dioxide with catechol- and pinacolborane using an Ir-CNP* pincer complex. Water influence on the catalytic activity.

Iridium complexes based on deprotonated lutidine-derived CNP* pincers 2a/2b selectively catalyzed the hydroboration of CO2 under mild conditions (1-2 bar CO2, 30 °C) to methoxyborane using HBcat (TOF up to 56 h-1) and to the formate level with HBpin (TOF up to 1245 h-1). Interestingly, an intriguing, positive water effect on the reaction rates has been observed. NMR spectroscopy and ESI-MS analysis of the hydroboration reactions have shown the formation of ligand-protonated [Ir(CNP)(CO)(BR2)H][B(R2)2] (R2 = catecholate, pinacolate) derivatives under catalytic conditions. Control experiments, however, have demonstrated that these derivatives are not catalytically competent species in the hydroboration of CO2.

Reactivity of a trans-[H-Mo[quadruple bond, length as m-dash]Mo-H] unit towards alkenes and alkynes: bimetallic migratory insertion, H-elimination and other reactions.

Complex [Mo2(H)2{µ-HC(NDipp)2}2(THF)2], (1·THF), reacts with C2H4 and PhCH[double bond, length as m-dash]CH2 to afford hydrido-hydrocarbyl and bis(hydrocarbyl) derivatives of the Mo[quadruple bond, length as m-dash]Mo bond. Reversible migratory insertion and ß-hydrogen elimination, as well as reductive elimination and other reactions, have been uncovered. PhC[triple bond, length as m-dash]CH behaves instead as a Brönsted-Lowry acid towards the strongly basic Mo-H bonds of 1·THF.