Stereoretentive elimination and trans-olefination of the dicationic dipalladium moiety [Pd2Ln]2+ bound on 1,3,5-trienes

Di- and trinuclear sandwich complexes of a cross-conjugated fulvene

We report di- and trinuclear palladium sandwich complexes of cross-π-conjugated fulvenes. Structural and theoretical analysis revealed that a fulvene sandwich framework holds a metal-metal bonded moiety, where the dinuclear and trinuclear bonds feature strong donation and back-donation, respectively. The trinuclear fulvene sandwich complex undergoes a unique reversible extrusion of a Pd atom from inside to outside the sandwich framework.

Bridging coordination of acenaphthylene to a Pd3 chain or a Pd4 sheet cluster

The coordination behaviour of multinuclear clusters to fused arene ligands is of continuous interest due to its relevance to metal catalysts supported by graphitic carbon materials. Herein, we report the bridging coordination behaviour of acenaphthylene to a Pd₃ or a Pd₄ cluster. A bis-acenaphthylene Pd₃ chain cluster and an acenaphthylene-COT Pd₄ sheet cluster were isolated, and the μ₃-π-coordination mode or the μ₄-oxidative π-addition mode of the acenaphthylene ligand in each cluster was elucidated by X-ray structure analysis.

Selective E to Z isomerization of 1,3-Dienes Enabled by A Dinuclear Mechanism

The E/Z stereocontrol in a C=C bond is a fundamental issue in olefin synthesis. Although the thermodynamically more stable E geometry is readily addressable by thermal Z to E geometric isomerization through equilibrium, it has remained difficult to undergo thermal geometric isomerization to the reverse E to Z direction in a selective manner, because it requires kinetic trapping of Z-isomer with injection of chemical energy. Here we report that a dinuclear PdI-PdI complex mediates selective isomerization of E-1,3-diene to its Z-isomer without photoirradiation, where kinetic trapping is achieved through rational sequences of dinuclear elementary steps. The chemical energy required for the E to Z isomerization can be injected from an organic conjugate reaction through sharing of common Pd species.

Thiophene and Selenophene Binding at a Pd3 Cluster Site

We report the triply bridging coordination of thiophene and selenophene to a trinuclear metal cluster site. The triply bridging coordination of selenophene at a Pd₃ site forms a unique spiro-type Pd₅ cluster. Furthermore, either thiophene or selenophene is accommodated stably at a Pd₃ site supported by a backside μ₃ -cyclooctatetraene ligand through the face-capping μ₃ -coordination. The Pd₃ site supported by the cyclooctatetraene ligand showed higher binding affinity of benzene over thiophene, although a Pd₂ site strongly favors thiophene over benzene.

Photo-induced translocation of a Pdn moiety (n = 2, 7) on a conjugated polyene ligand

The Pd₂-translocation on a tetraene ligand is switchable between the thermodynamic equilibrium state and the photo-stationary state. The Pd₇-translocation on a carotene ligand proceeds smoothly under photo-irradiation.

Contiguous multiple π-coordination of π-conjugated polyenes: bonding nature and charge delocalization behaviour of polyene–(palladium chain) sandwich clusters

We report the bonding nature and charge delocalization behaviour of a series of polyene–(palladium chain) sandwich clusters through systematic ¹³C NMR analysis and molecular orbital analysis.

A halide-free pyridinium-substituted η3-cycloheptatrienide-Pd complex

We report the synthesis and characterization of a novel 4-(dimethylamino)pyridinium-substituted η³-cycloheptatrienide-Pd complex which is free of halide ligands. Diacetonitrile{η³-[4-(dimethylamino)pyridinium-1-yl]cycloheptatrienido}palladium(II) bis(tetrafluoroborate), [Pd(C₂H₃N)₂(C₁₄H₁₆N₂)](BF₄)₂, was prepared by the exchange of two bromide ligands for noncoordinating anions, which results in the empty coordination sites being occupied by acetonitrile ligands. As described previously, exchange of only one bromide leads to a dimeric complex, di-μ-bromido-bis({η³-[4-(dimethylamino)pyridinium-1-yl]cycloheptatrienido}palladium(II)) bis(tetrafluoroborate) acetonitrile disolvate, [Pd₂Br₂(C₁₄H₁₆N₂)₂](BF₄)₂·2CH₃CN, with bridging bromide ligands, and the crystal structure of this compound is also reported here. The structures of the cycloheptatrienide ligands of both complexes are analogous to the dibromide derivative, showing the allyl bond in the β-position with respect to the pyridinium substituent. This indicates that, unlike a previous interpretation, the main reason for the formation of the β-isomer cannot be internal hydrogen bonding between the cationic substituents and bromide ligands.

A mechanistic insight into metal-cluster π-envelopment: a dual binding mode involving bent and planar ligand-conformers

We report a stepwise mechanism of the π-envelopment of a Pd₄ chain cluster. Bent- and planar binding modes of π-conjugated unsaturated hydrocarbon ligands are involved in the π-envelopment process.

Selective construction of Pd2Pt and PdPt2 triangles in a sandwich framework: carbocyclic ligands as scaffolds for a mixed-metal system

Sandwich time! The mixed-metal triangular-trinuclear sandwich complexes of Pd(2)Pt and PdPt(2) were selectively synthesized. The Pd(2)Pt and PdPt(2) triangles in a cycloheptatrienyl sandwich framework were identified by (31)P NMR analyses of the tris-triphenylphosphine complexes (see scheme).

Oxidative dinuclear addition of a PdI-PdI moiety to arenes: generation of μ-η3:η3-arene-Pd(II)2 species

We report the oxidative dinuclear addition of a Pd(I)-Pd(I) bond to arenes. The oxidative dinuclear addition products, which have a bi-π-allyl-type arene dipalladium(II) structure, were obtained from [2.2]paracyclophane, anthracene, tetracene, and pentacene. A systematic study of the reaction of [Pd(2)(CH(3)CN)(6)][BF(4)](2) with benzene and polyacenes showed that the larger polyacenes, tetracene and pentacene, afforded the oxidative dinuclear addition products, while benzene, naphthalene, and anthracene gave the π-sandwich Pd(I)-Pd(I) complexes.

Bimetallic catalysis involving dipalladium(I) and diruthenium(I) complexes

Dipalladium(I) and diruthenium(I) compounds bridged by two [{(5,7-dimethyl-1,8-naphthyridin-2-yl)amino}carbonyl]ferrocene (L) ligands have been synthesized. The X-ray structures of [Pd(2)L(2)][BF(4)](2) (1) and [Ru(2)L(2)(CO)(4)][BF(4)](2) (2) reveal dinuclear structures with short metal-metal distances. In both of these structures, naphthyridine bridges the dimetal unit, and the site trans to the metal-metal bond is occupied by weakly coordinating oxygen from the amido fragment. The catalytic utilities of these bimetallic compounds are evaluated. Compound 1 is an excellent catalyst for phosphine-free, Suzuki cross-coupling reactions of aryl bromides with arylboronic acids and provides high yields in short reaction times. Compound 1 is also found to be catalytically active for aryl chlorides, although the corresponding yields are lower. A bimetallic mechanism is proposed, which involves the oxidative addition of aryl bromide across the Pd-Pd bond and the bimetallic reductive elimination of the product. Compound 1 is also an efficient catalyst for the Heck cross-coupling of aryl bromides with styrenes. The mechanism for aldehyde olefination with ethyl diazoacetate (EDA) and PPh(3), catalyzed by 2, has been fully elucidated. It is demonstrated that 2 catalyzes the formation of phosphorane utilizing EDA and PPh(3), which subsequently reacts with aldehyde to produce a new olefin and phosphine oxide. The efficacy of bimetallic complexes in catalytic organic transformations is illustrated in this work.

DFT Modeling of Sandwich Complexes Involving Cationic Palladium Chains and Polyenic or Polycyclic Aromatic Hydrocarbons

DFT calculations are reported on a series of one-dimensional palladium complexes with general formula [Pd(m)(C(2n)H(2n+2))(2)](2+) (m = 2-4, n = 2-8, n > or = m), in order to model and analyze the bonding in the series of organometallic sandwich compounds recently reported by the group of T. Murahashi and H. Kurosawa. The bonding interactions are elucidated, and the frontier orbitals involved are described as a function of the haptotropic conformation of the metal atoms, either di-hapto or tri-hapto. In both cases, the driving force to the complex organization is a strong donation interaction from the pi system of the hydrocarbons to an orbital with appropriate phase and composition, delocalized over the metal chain, and depopulated by the double oxidation process. No net bonding interaction can be characterized along the metal string, and the metal-metal distances are mainly governed by the hapticities of adjacent atoms. The energy associated with the formation of a complex is calculated with respect to its fragments, assumed either isolated or solvated. The results emphasize the stabilizing role of a large delocalization of the positive charge transferred to the hydrocarbons. This delocalization extends to the hydrocarbon regions not directly in contact with palladium and highlights the importance of these "inactive" regions in complexes made from diphenyl polyenes or polycyclic aromatic hydrocarbons. Finally, the bonding pattern deduced from calculations has been utilized to consider the feasibility of novel sandwich architectures, whose computed energy balance eventually proves similar to that of already existing compounds.