Square-Planar Anionic Pt0 Complexes.

A T-shaped Pt0 complex with a diphosphine-borane (DPB) ligand was prepared. The Pt→B interaction enhances the electrophilicity of the metal and triggers the addition of Lewis bases to give the corresponding tetracoordinate complexes. For the first time, anionic Pt0 complexes are isolated and structurally authenticated. X-ray diffraction analyses show the anionic complexes [(DPB)PtX]- (X=CN, Cl, Br, I) to be square-planar. The d10 configuration and Pt0 oxidation state of the metal were unambiguously established by X-ray photoelectron spectroscopy and DFT calculations. The coordination of Lewis acids as Z-type ligands is a powerful mean to stabilize elusive electron-rich metal complexes and achieve uncommon geometry.

Pd/Ni-Catalyzed Germa-Suzuki coupling via dual Ge-F bond activation.

Pd/Ni → Ge-F interactions supported by phosphine-chelation were found to trigger dual activation of Ge-F bonds under mild conditions. This makes fluoro germanes suitable partners for catalytic Ge-C cross-coupling and enables Germa-Suzuki reactions to be achieved for the first time.

Four-Electron Reduction of Dioxygen on a Metal Surface: Models of Dissociative and Associative Mechanisms in a Homogeneous System.

Two different four-electron reductions of dioxygen (O2) on a metal surface are reproduced in homogeneous systems. The reaction of the highly unsaturated (56-electron) tetraruthenium tetrahydride complex 1 with O2 readily afforded the bis(µ3-oxo) complex 3 via a dissociative mechanism that includes large electronic and geometric changes, i.e., a four-electron oxidation of the metal centers and an increase of 8 in the number of valence electrons. In contrast, the tetraruthenium hexahydride complex 2 induces a smooth H-atom transfer to the incorporated O2 species, and the O-OH bond is cleaved to afford the mono(µ3-oxo) complex 4 via an associative mechanism. Density functional theory calculations suggest that the higher degree of unsaturation in the tetrahydride system induces a significant interaction between the tetraruthenium core and the O2 moiety, enabling the large changes required for the dissociative mechanism.

Fluorosilane Activation by Pd/Ni→Si-F→Lewis Acid Interaction: An Entry to Catalytic Sila-Negishi Coupling.

A new mode of bond activation involving M→Z interactions is disclosed. Coordination to transition metals as σ-acceptor ligands was found to enable the activation of fluorosilanes, opening the way to the first transition-metal-catalyzed Si-F bond activation. Using phosphines as directing groups, sila-Negishi couplings were developed by combining Pd and Ni complexes with external Lewis acids such as MgBr2. Several key catalytic intermediates have been authenticated spectroscopically and crystallographically. Combined with DFT calculations, all data support cooperative activation of the fluorosilane via Pd/Ni→Si-F→Lewis acid interaction with conversion of the Z-type fluorosilane ligand into an X-type silyl moiety.

Counterion Dependence of Dinitrogen Activation and Functionalization by a Diniobium Hydride Anion.

We report the synthesis of anionic diniobium hydride complexes with a series of alkali metal cations (Li+ , Na+ , and K+ ) and the counterion dependence of their reactivity with N2 . Exposure of these complexes to N2 initially produces the corresponding side-on end-on N2 complexes, the fate of which depends on the nature of countercations. The lithium derivative undergoes stepwise migratory insertion of the hydride ligands onto the aryloxide units, yielding the end-on bridging N2 complex. For the potassium derivative, the N-N bond cleavage takes place along with H2 elimination to form the nitride complex. Treatment of the side-on end-on N2 complex with Me3 SiCl results in silylation of the terminal N atom and subsequent N-N bond cleavage along with H2 elimination, giving the nitride-imide-bridged diniobium complex.

Experimental and Theoretical Investigation of an SN2-type Pathway for Borate-Fluorine Bond Cleavage by Electron-Rich Late-Transition Metal Complexes.

B-F σ-bond activation of a fluoroborate has been experimentally achieved through reactions with electron-rich iridium(I) and palladium(0) complexes. The selectivity of B-F σ-bond cleavage by iridium complexes was improved through the high nucleophilicity of the iridium center, implying that a different pathway from that of well-accepted F- abstraction was in effect. The palladium(0) complex was found to promote exclusive B-F σ-bond cleavage even at ambient temperature. Density functional theory (DFT) calculations suggested that B-F σ-bond activation occurred through an SN2-type pathway, which is, to our knowledge, the first proposal of SN2-type borate-fluorine σ-bond cleavage mediated by a transition metal complex. The high feasibility of the SN2-type pathway appears to be attributed to the relatively low deformation energy of the transition state. It was also found that countercation Cs+ effectively stabilized the transition state and product by serving as a F- acceptor.

Palladium-Borane Cooperation: Evidence for an Anionic Pathway and Its Application to Catalytic Hydro-/Deutero-dechlorination.

Metal-Lewis acid cooperation provides new opportunities in catalysis. In this work, we report a new type of palladium-borane cooperation involving anionic Pd0 species. The air-stable DPB palladium complex 1 (DPB=diphosphine-borane) was prepared and reacted with KH to give the Pd0 borohydride 2, the first monomeric anionic Pd0 species to be structurally characterized. The boron moiety acts as an acceptor towards Pd in 1 via Pd→B interaction, but as a donor in 2 thanks to B-H-Pd bridging. This enables the activation of C-Cl bonds and the system is amenable to catalysis, as demonstrated by the hydro-/deutero-dehalogenation of a variety of (hetero)aryl chlorides (20 examples, average yield 85 %).

Modulation of the coordination geometries of NCN and NCNC Rh complexes for ambidextrous chiral catalysts.

A chirality switch between novel NCN pincer Rh complexes and a related double cyclometalated NCNC Rh complex containing secondary amino groups is described. Their catalytic abilities were determined in asymmetric alkynylation of ethyl trifluoropyruvate, and the change in the coordination geometry of the Rh catalysts affected the stereochemistry of the products.

Synthesis and characterisation of tetranuclear ruthenium polyhydrido clusters with pseudo-tetrahedral geometry.

Dicationic tetranuclear ruthenium octahydride [(Cp*Ru)4H8]2+ (5) with tetrahedral geometry was obtained by reaction of dinuclear ruthenium tetrahydride (Cp*Ru)2(µ-H)4 (1) with an excess of Brønsted acids, such as HBF4·OEt2, in toluene. Monocationic tetraruthenium heptahydride [(Cp*Ru)4H7]+ (7) was obtained by dropwise addition of a diluted acid to a rigorously stirred solution of 1 at ambient temperature. Dication 5 was converted into monocationic heptahydrido complex 7 in high yield by treatment with sodium methoxide or sodium hydride. The direct conversion of 5 into neutral hexahydrido complex (Cp*Ru)4H6 (8) was achieved in a highly efficient manner by treating 5 with LiAlH4 in tetrahydrofuran (THF). The conversion of 5 into 8 was reversible, and the addition of a Brønsted acid to 8 gave 5via the formation of 7 as an intermediate. Tetranuclear complex 8 was directly obtained from 1 by heating it in THF at 70 °C. Complex 8' and tetraruthenium tetrahydride (CpEtRu)4H4 (10'), where 8' and 10' possessed η5-C5EtMe4 ligands instead of Cp* ligands, were mutually related by the elimination/addition of dihydrogen. The structures of 5, 7, 8, and 10' were determined by X-ray diffraction, and the Ru4 core structure and the coordination mode of hydrido ligands were discussed based on density functional theory (DFT) calculations for model compounds where the methyl groups of Cp* ligands were replaced with hydrogen atoms.

Saturated Heavier Group 14 Compounds as σ -Electron-Acceptor (Z-Type) Ligands.

This review article describes the chemistry of transition-metal complexes containing heavier group 14 elements (Si, Ge, and Sn) as the σ-electron-acceptor (Z-type) ligands and discusses the characteristics of bonds between the transition metal and Z-type ligand. Moreover, we review the iridium hydride mediated cleavage of E-X bonds (E=Si, Ge; X=F, Cl), where the key intermediates are pentacoordinate silicon or germanium compounds bearing a dative M→E bond.

Cooperative Catalysis of Combined Systems of Transition-Metal Complexes with Lewis Acids: Theoretical Understanding.

The combination of transition-metal complexes and Lewis acids has been recently applied to several catalytic reactions, in which the Lewis acid plays a crucial role as a non-innocent additive to accelerate the reaction. In this review article, the reasons for the acceleration by the Lewis acid are discussed based on our recent theoretical studies. In the H-H σ-bond activation of a dihydrogen molecule by a nickel(0)-borane complex, the empty p orbital of the borane moiety interacts with the H-H σ bonding MO to form charge transfer (CT) from the dihydrogen molecule to the borane moiety to accelerate the reaction. In the B-F σ-bond activation of BF3 by a platinum(0)-bisphosphine complex, the second BF3 molecule interacts with the F atom that is dissociating from the B atom to stabilize the transition state and product by the CT from the F atom to the second BF3 . In this reaction, the substrate BF3 plays a crucial role as the Lewis acid to accelerate the activation of the B-F σ bond. In the nickel-catalyzed decyanative coupling of arylcarboxybenzonitriles with acetylenes, two molecules of the aluminum Lewis acid interact with the cyano N atom and the carbonyl O atom of the substrate to stabilize the transition state and intermediate. In the nickel-catalyzed alkylation of aromatic amides with alkenes, the Lewis acid enhances the para regioselectivity of alkylation by interacting with the carbonyl O atom. In the nickel-catalyzed carboxylation of sp3 carbon and sp carbon atoms with carbon dioxide, not the σ-bond activation but the insertion reaction of carbon dioxide into the metal-carbon bond is accelerated by the Lewis acid by interacting with the O atom of carbon dioxide, because the CT from the metal-carbon bond to carbon dioxide is enhanced by the interaction. This theoretical knowledge suggests that the combination of transition-metal complex and Lewis acid can broaden the application range of transition-metal complex as catalyst.

Transition-Metal-Mediated Cleavage of Fluoro-Silanes under Mild Conditions.

Si-F bond cleavage of fluoro-silanes was achieved by transition-metal complexes under mild and neutral conditions. The Iridium-hydride complex [Ir(H)(CO)(PPh3 )3 ] was found to readily break the Si-F bond of the diphosphine- difluorosilane {(o-Ph2 P)C6 H4 }2 Si(F)2 to afford a silyl complex [{[o-(iPh2 P)C6 H4 ]2 (F)Si}Ir(CO)(PPh3 )] and HF. Density functional theory calculations disclose a reaction mechanism in which a hypervalent silicon species with a dative Ir→Si interaction plays a crucial role. The Ir→Si interaction changes the character of the H on the Ir from hydridic to protic, and makes the F on Si more anionic, leading to the formation of H(δ+) ⋅⋅⋅F(δ-) interaction. Then the Si-F and Ir-H bonds are readily broken to afford the silyl complex and HF through σ-bond metathesis. Furthermore, the analogous rhodium complex [Rh(H)(CO)(PPh3 )3 ] was found to promote the cleavage of the Si-F bond of the triphosphine-monofluorosilane {(o-Ph2 P)C6 H4 }3 Si(F) even at ambient temperature.

Activation of Strong Boron-Fluorine and Silicon-Fluorine σ-Bonds: Theoretical Understanding and Prediction.

The oxidative addition of BF3 to a platinum(0) bis(phosphine) complex [Pt(PMe3)2] (1) was investigated by density functional calculations. Both the cis and trans pathways for the oxidative addition of BF3 to 1 are endergonic (ΔG°=26.8 and 35.7 kcal mol(-1), respectively) and require large Gibbs activation energies (ΔG°(≠)=56.3 and 38.9 kcal mol(-1), respectively). A second borane plays crucial roles in accelerating the activation; the trans oxidative addition of BF3 to 1 in the presence of a second BF3 molecule occurs with ΔG°(≠) and ΔG° values of 10.1 and -4.7 kcal mol(-1), respectively. ΔG°(≠) becomes very small and ΔG° becomes negative. A charge transfer (CT), F→BF3, occurs from the dissociating fluoride to the second non-coordinated BF3. This CT interaction stabilizes both the transition state and the product. The B-F σ-bond cleavage of BF2Ar(F) (Ar(F)=3,5-bis(trifluoromethyl)phenyl) and the B-Cl σ-bond cleavage of BCl3 by 1 are accelerated by the participation of the second borane. The calculations predict that trans oxidative addition of SiF4 to 1 easily occurs in the presence of a second SiF4 molecule via the formation of a hypervalent Si species.

Coordination and redox isomerization in the reduction of a uranium(III) monoarene complex.

Synthetic studies on the redox chemistry of trivalent uranium monoarene complexes were undertaken with a complex derived from the chelating tris(aryloxide)arene ligand ((Ad,Me) ArO)3 mes(3-) . Cyclic voltammetry of [{((Ad,Me) ArO)3 mes}U(III) ] (1) revealed a nearly reversible and chemically accessible reduction at -2.495 V vs. Fc/Fc(+) -the first electrochemical evidence for a formally divalent uranium complex. Chemical reduction of 1 indicates that reduction induces coordination and redox isomerization to form a uranium(IV) hydride, and addition of a crown ether results in hydride insertion into the coordinated arene to afford uranium(IV) complexes. This stoichiometric reaction sequence provides structural insight into the mechanism of arene functionalization at diuranium inverted sandwich complexes.

Recent developments in the coordination chemistry of multidentate ligands featuring a boron moiety.

Synergistic effects between a transition metal and an appropriate ligand are required to promote a desired catalytic reaction. Ancillary ligands, provided by the versatile functionality of certain elements, give rise to an almost infinite potential for catalytic applications. Recently, the study of the synergistic effect between transition metals and boron has become easy on account of the development of various rigid multidentate frameworks. In this Review, we mainly focus on the chemistry of σ-acceptor (Z-type) borane ligands, particularly the key achievements of their unique reactivity and catalytic applications. Conceptually, the unique character of σ-acceptor borane ligands provides a new strategy for developing remarkable reactivity and novel catalytic applications. This study discusses recent developments in the field in this context. The chemistry of boron-based multidentate ligands that involve a covalent M-B bond such as the boryl ligand (-BR2), in which a boron moiety serves as a strong electron-donating ligand, is also rapidly developing. The effect of the boryl ligand on a metal center is totally different from that of the borane (-BR3) ligand, and different boron-based functionalities confer opposing electronic properties to the metal center. The interesting character of boryl-based chelating ligands augments their unique coordination chemistry, which is also summarized in this context.

Si-C bond cleavage by hydride complexes of rhodium and iridium: comparison of Si-C(sp(2)) and Si-C(sp(3)) activation.

Single Si-C(R) (R = Ph, Me, Et) bond activation in {o-(Ph(2)P)C(6)H(4)}(2)Si(Me)(R) induced by Rh(H)(CO)(PPh(3))(3) was developed. The efficiency of Si-C(R) bond breaking reactions increased at 60 °C in the order Si-C(Et) < Si-C(Me) < Si-C(Ph) and strongly depended on the reaction temperature. Elevating the reaction temperature promoted Si-C(Me) over Si-C(Ph) bond activation, demonstrating that Si-C(Me) cleavage is entropically favored but enthalpically unfavored in comparison with Si-C(Ph) bond cleavage.

Facile synthesis of rhodium and iridium complexes bearing a [PEP]-type ligand (E = Ge or Sn) via E-C bond cleavage.

Rhodium and iridium complexes bearing a tridentate [PEP] type ligand ([PEP] = {o-(Ph(2)P)C(6)H(4)}(2)E(Me); E = Ge or Sn) were synthesized through the phosphine exchange reaction accompanied by selective E-C bond cleavage. The ligand precursors {o-(Ph(2)P)C(6)H(4)}(2)EMe(2) (E = Ge or Sn) were readily obtained in excellent yields by treating {o-(Ph(2)P)C(6)H(4)}(2)Li with 0.5 equivalents of Me(2)ECl(2). Tris(triphenylphosphine)rhodium(I) carbonyl hydride M(H)(CO)(PPh(3))(3) (M = Rh, Ir) cleaved one of the E-Me bonds of {o-(Ph(2)P)C(6)H(4)}(2)EMe(2) exclusively to afford the trigonal bipyramidal (TBP) complexes, [PEP]M(CO)(PPh(3)). Square-planar rhodium complexes [PEP]Rh(PPh(3)) were also prepared from the reactions of tetrakis(triphenylphosphine)rhodium(I) hydride Rh(H)(PPh(3))(4) with {o-(Ph(2)P)C(6)H(4)}(2)EMe(2). Further, the trans influence of group 14 elements E (E = Si, Ge, Sn) in [PEP]Rh(PPh(3)) is discussed in terms of the (1)J(Rh-P) coupling constants, indicating that E exhibited a stronger trans labilizing effect in the order Sn < Ge < Si.

Synthesis of iridium complexes bearing {o-(Ph2P)C6H4}(3)E type (E = Si, Ge, and Sn) ligand and evaluation of electron donating ability of group 14 elements E.

Trigonal bipyramidal (TBP) iridium(i) complexes {o-(Ph(2)P)C(6)H(4)}(3)EIr(CO) (E = Si: 1-Ir, Ge: 2-Ir, Sn: 3-Ir) comprising group 14 element E were synthesized and converted into the corresponding cationic iridium(III) complexes [{o-(Ph(2)P)C(6)H(4)}(3)EIr(H)(CO)][BF(4)] (E = Si: 4, Ge: 5, Sn: 6) bearing octahedral geometry by protonation using (Et(2)OH)(BF(4)). The origin of trans-labilizing abilities of E was investigated through structural analysis, IR and NMR spectroscopic analysis, and density functional theory calculations. Further, the electron-donating abilities of E were investigated through proton transfer reactions.

Insights into the mechanism of carbonate formation through reductive cleavage of carbon dioxide with low-valent uranium centers.

The low-valent U(III) complexes [((t-BuArO)3mes)U] and [((AdArO)3N)U] react with CO2 to form the bridging carbonate complexes [{((t-BuArO)3mes)U}2(mu-kappa2:kappa2-CO3)] and [{((AdArO)3N)U}2(mu-eta1:kappa2-CO3)]. Uranium(IV) bridging oxo complexes have been determined to be the intermediate in these transformations.