Isolation and Structures of Polyarene Palladium Nanoclusters.

We report that surrounding coordination of neutral six-membered arene rings affords molecularly well-defined organotransition metal nanoclusters. With the use of [2.2]paracyclophane as the face-capping arene ligand, we have isolated two polyarene palladium nanoclusters, one consisting of a hexakis-arene ligand shell and a hexagonal close-packed Pd13 anticuboctahedron trichloride core, and the other consisting of an octakis-arene ligand shell and a non-close-packed Pd17 square gyrobicupola dichloride core, both with Pd-Pd direct bonding. The µ4-facial coordination mode of arene was discovered through the structural characterization of the Pd13 cluster. Their Pd13 and Pd17 cores, which are distinct from the previously identified face-centered-cubic Pd13 core surrounded by seven-membered cycloheptatrienyl, are explained by stereochemical and theoretical analyses.

Palladium cluster complex [Pd13(µ4-C7H7)6]2+ (C7H7 = tropylium) with an fcc-close-packed cuboctahedral Pd13 core and isomers: theoretical insight into ligand-control of the Pd13 core structure.

One of the challenging targets in today's chemistry is size-, shape- and metal-atom packing-controlled synthesis of nano-scale transition metal cluster complexes because key factors governing these features have been elusive. Here, we present a DFT study on a recently synthesized palladium cluster complex [Pd13(µ4-C7H7)6]2+ (named Cubo-µ4; C7H7 = tropylium) with an fcc-close-packed cuboctahedral Pd13 core and possible isomers. The stability decreases in the order Cubo-µ4 > [Pd13(µ3-C7H7)3(µ4-C7H7)3]2+ with an hcp-close-packed anticuboctahedral Pd13 core (Anti-µ3,4) > [Pd13(µ3-C7H7)6]2+ with a non-close packed icosahedral Pd13 core (Ih-µ3) > [Pd13(µ4-C7H7)6]2+ with an anticuboctahedral Pd13 core (Anti-µ4) > [Pd13(µ3-C7H7)6]2+ with a cuboctahedral Pd13 core (Cubo-µ3). This ordering disagrees with the stability of the Pd13 core. The key factor governing the stability and metal-atom packing manner of these Pd13 cluster complexes is not the stability of the Pd13 core but the interaction energy between the Pd13 core and the [(C7H7)6]2+ ligand shell. The interaction energy is mainly determined by the charge-transfer from the Pd13 core to the [(C7H7)6]2+ ligand shell and the coordination mode of the C7H7 ligand (µ3- vs. µ4-coordination bond). In the µ4-coordination, all seven C atoms of the C7H7 ligand interact with four Pd atoms of the Pd4 plane using two CC double bonds and one π-allyl moiety. On the other hand, in the µ3-coordination, one or two C atoms of C7H7 cannot form bonding interaction with the Pd atom of the Pd3 plane. Thus, the use of appropriate capping ligands is one of the key points in the synthesis of nano-scale metal cluster complexes.

Regio- and Diastereoselective Formal [2+2] Cycloaddition of Allenes with Amino-Functionalized Alkenes by Rare-Earth-Catalyzed C(sp2 )-H Activation.

The [2+2] cycloaddition of allenes with alkenes is of much interest and importance as a straightforward route for the construction of four-membered carbocycles but has remained much underexplored to date. Herein we report for the first time the intermolecular regio- and diastereoselective formal [2+2] cycloaddition of a wide range of allenes with amino-functionalized alkenes by half-sandwich rare-earth catalysts. The reaction proceeded through an allene C(sp2 )-H activation mechanism initiated by the site-selective deprotonation of the allene unit by a rare-earth metal alkyl species followed by alkene insertion into the resulting metal-allenyl bond and the subsequent intramolecular cycloaddition to an allene C=C bond. This protocol offers a unique route for the synthesis of a new family of cyclobutane and cyclobutene derivatives which were difficult to access previously.

Heterometallic d8 -d10 Coupling of Rh(I) and M(0) (M=Pd, Pt) in a Sandwich Framework of π-Conjugated Ligands.

A heterometallic M-M' bond formation is a key to construct atomically precise bimetallic clusters and materials. However, it is sometimes not straightforward to construct a heterometallic M-M' bond through conventional methods including redox condensation. Here, we found that a sandwich framework of π-conjugated unsaturated hydrocarbon ligands provides a unique coordination environment that facilitates unusual coupling of d8 RhI and d10 M0 (M=Pd, Pt). The molecular orbital analysis showed that the electron-accepting ability of the sandwich framework through back-donation allows the formation of a dσ-type Rh-Pd bond in a (d-d)18 electron system.

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.

Extended Open-Chain Polyenides as Versatile Delocalized Anion Ligands for Metal Chain Clusters.

Although small cyclic- and open-chain unsaturated hydrocarbon anions such as cyclopentadienide and open-chain pentadienide are used as the strongly electron-donating auxiliary ligands for metal complexes, more extended π-conjugated unsaturated hydrocarbon anions have rarely been used in coordination chemistry, despite their potential ability to serve as the multiply bridging π-ligands for metal clusters. This work reports isolation of metal chain clusters bearing the multi-dentate, open-chain extended unsaturated hydrocarbon anion ligands. The extended open-chain π-conjugated polyenyl ligands could effectively stabilize oxidized palladium chains, including an unprecedented [Pd4 ]4+ chain.

Exceeding Metal Capacity in Sandwich Complexes: Ligand-Unsupported Docking of Extra Metal Moieties at Edges of a Metal Sheet Sandwich Complex.

The ligand-unsupported accommodation of extra metal moieties in a sandwich complex is reported. Although it has been considered that the metal-capacity of a metal sheet sandwich complex is strictly limited by the size of cyclic unsaturated hydrocarbon ligands, the M-M edge bonds in a metal sheet sandwich complex provide a ligand-unsupported docking site for extra metal moieties, allowing expansion of metal-capacity in sandwich complexes. The metal sheet sandwich complex [Pd4 (µ4 -C8 H8 )(µ4 -C9 H9 )]+ , in which the ligand-based metal capacity is full in terms of the usage of all C=C moieties of the smaller carbocyclic ligand C8 H8 in coordination, can accommodate extra M0 {P(OPh)3 }2 (M=Pd, Pt) moieties without coordinative assistance by either the C9 H9 or the C8 H8 ligand.

Three-Dimensional Sandwich Nanocubes Composed of 13-Atom Palladium Core and Hexakis-Carbocycle Shell.

Coordination of cyclic unsaturated hydrocarbons to transition metal generally gives bis-ligated sandwich complexes, which are a fundamental class of organometallic compounds. This sandwich structure may be extended to a higher-order three-dimensional one when more than two carbocyclic ligands surround an agglomerate of many transition metal atoms. Here, we report synthesis of three-dimensional sandwich nanocube compounds containing a close-packed transition metal cluster core. Reduction of a [Pd3(µ3-C7H7)2]2+ with tetraarylborate under neat conditions afforded [Pd13(µ4-C7H7)6]2+ containing a cuboctahedral Pd13 core with an octahedral ligand-shell.

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

The contiguous multiple π-coordination (CMπC) bonds are formed at the molecular interface between the extended π-conjugated sp2-carbon framework and metal-metal moieties. However, the static properties including the bonding nature of the CMπC bonds have not been well-understood. Here we report the charge delocalization behavior and bonding nature of the polyene-(palladium chain) sandwich clusters. We observed the chain-length dependent 13C NMR chemical shifts of the terminal phenyl groups in [Pdm{Ph(CH[double bond, length as m-dash]CH)m+1Ph}2][B(ArF)4]2 (m = 3, 4, 5, 6), showing the efficient delocalization of positive charge over the polyene ligands. Furthermore, the detailed molecular orbital analysis of several model clusters provides insights into the bonding nature of the µ-η2:(η2:)nη2- and µ-η3:(η2:)nη3-poylene palladium chain sandwich clusters, where the former type is attributed to the corner-sharing Pd array and the latter to the edge-sharing/corner-sharing hybrid Pd array. The analogy of the µ-η3:(η2:)nη3-type with the carbonyl-bridge coordination is discussed. The Pd string in each mode of sandwich chain clusters has a weak M-M bonding interaction.

Bridging Coordination of Vinylarenes to Pd3 - or Pd4 Cluster Sites.

Though the M3 - or M4 face of palladium clusters may serve as the active binding site of substrates, it has been difficult to elucidate their substrate binding modes in solution. Here, it was proved that a soluble Pd3 - or Pd4 sheet cluster accommodates vinylarenes at its bridging coordination site in an unusual mode in palladium chemistry; that is, for the former, the coordination of the arene moiety precedes that of olefin moiety; for the latter, vinylarene coordinates to a distorted Pd4 sheet through oxidative π-addition of its styrene moiety.

Substrate Binding by a Parallel Metal Sheet Sandwich Complex: A Unique Role of an Additional Metal Atom.

A unique metal addition to the edge of a triangular palladium sandwich cluster is reported. While the edge coordination sites of a parallel sandwich complex are narrow because of two sterically encumbering π-ligands, association of an additional metal atom to a metal sheet gives a wider coordination site. It was proved that the additional metal atom plays a crucial role in binding of cyclopentadienyl by a Pd3 sheet sandwich complex.

σ-π Continuum in Indole-Palladium(II) Complexes.

The intrinsic features of (hetero-arene)-metal interactions have been elusive mainly because the systematic structure analysis of non-anchored hetero-arene-metal complexes has been hampered by their labile nature. We report successful isolation and systematic structure analysis of a series of non-anchored indole-palladium(II) complexes. It was revealed that there is a σ-π continuum for the indole-metal interaction, while it has been thought that the dominant coordination mode of indole to a metal center is the Wheland-intermediate-type σ-mode in light of the seemingly strong electron-donating ability of indole. Several factors which affect the σ- or π-character of indole-metal interactions are discussed.

Modulation of benzene or naphthalene binding to palladium cluster sites by the backside-ligand effect.

The backside-ligand modulation strategy to enhance the substrate binding property of Pd clusters is reported. The benzene or naphthalene binding ability of Pd3 or Pd4 clusters is enhanced significantly by the backside cyclooctatetraene ligand, leading to the formation of the first solution-stable benzene- or naphthalene Pd clusters. The present results imply that the ligand design of the metal clusters, especially for the backside ligand of the metal cluster site, is crucial to acquire a desired reactivity of metal clusters.

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.

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).

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.

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.

Selective dimerization of a trinuclear mixed-metal sandwich complex: construction of an axially chiral metal skeleton.

Certain metal sandwich complexes undergo dimerization through metal-metal bond formation. Here, we found that a reductive dimerization of mixed-metal Pd2Pt or PdPt2 sandwich complexes proceeds through selective Pt-Pt bond formation. A restricted rotation at the Pt-Pt bond of the PdPt2 dimer gave a unique axially chiral structure derived from a heterometal arrangement in a mixed-metal cluster.

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.

Square tetrapalladium sheet sandwich complexes: cyclononatetraenyl as a versatile face-capping ligand.

An unusual square tetrapalladium sheet sandwich complex, [Pd(4)(mu(4)-C(9)H(9))(mu(4)-C(8)H(8))][B(Ar(f))(4)] has been isolated and structurally characterized. X-ray structure analysis showed that the square palladium sheet is flanked by the nine-membered cyclononatetraenyl and eight-membered cyclooctatetraene ligands to form a square metal sheet molecular sandwich structure. The coordinative unsaturation of the complex was revealed by phosphine titration experiments.