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

Advances for Triangular and Sandwich-Shaped All-Metal Aromatics

Much experimental work has been contributed to all-metal σ, π and δ-aromaticity among transition metals, semimetallics and other metals in the past two decades. Before our focused investigations on the properties of triangular and sandwich-shaped all-metal aromatics, A. I. Boldyrev presented general discussions on the concepts of all-metal σ-aromaticity and σ-antiaromaticity for metallo-clusters. Schleyer illustrated that Nucleus-Independent Chemical Shifts (NICS) were among the most authoritative criteria for aromaticity. Ugalde discussed the earlier developments of all-metal aromatic compounds with all possible shapes. Besides the theoretical predictions, many stable all-metal aromatic trinuclear clusters have been isolated as the metallic analogues of either the σ-aromatic molecule's [H3]+ ion or the π-aromatic molecule's [C3H3]+ ion. Different from Hoffman's opinion on all-metal aromaticity, triangular all-metal aromatics were found to hold great potential in applications in coordination chemistry, catalysis, and material science. Triangular all-metal aromatics, which were theoretically proved to conform to the Hückel (4n + 2) rule and possess the smallest aromatic ring, could also play roles as stable ligands during the formation of all-metal sandwiches. The triangular and sandwich-shaped all-metal aromatics have not yet been specifically summarized despite their diversity of existence, puissant developments and various interesting applications. These findings are different from the public opinion that all-metal aromatics would be limited to further applications due to their overstated difficulties in synthesis and uncertain stabilities. Our review will specifically focus on the summarization of theoretical predictions, feasible syntheses and isolations, and multiple applications of triangular and sandwich shaped all-metal aromatics. The appropriateness and necessities of this review will emphasize and disseminate their importance and applications forcefully and in a timely manner.

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.

Activation of Dinitrogen Promoted by Adsorption of C6H6 on Fe2VC- Cluster Anions

The introduction of organic ligands is one of the effective strategies to improve the stability and reactivity of metal clusters. Herein, the enhanced reactivity of benzene-ligated cluster anions Fe₂VC(C₆H₆)^- with respect to naked Fe₂VC^- is identified. Structural characterization suggests that C₆H₆ is molecularly bound to the dual metal site in Fe₂VC(C₆H₆)^-. Mechanistic details reveal that the cleavage of N≡N is feasible in Fe₂VC(C₆H₆)^-/N₂ but hindered by an overall positive barrier in the Fe₂VC^-/N₂ system. Further analysis discloses that the ligated C₆H₆ regulates the compositions and energy levels of the active orbitals of the metal clusters. More importantly, C₆H₆ serves as an electron reservoir for the reduction of N₂ to lower the crucial energy barrier of N≡N splitting. This work demonstrates that the flexibility of C₆H₆ in terms of withdrawing and donating electrons is crucial to regulating the electronic structures of the metal cluster and enhancing the reactivity.

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.

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.

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.

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.

The partial dehydrogenation of aluminium dihydrides

The reactions of a series of β-diketiminate stabilised aluminium dihydrides with ruthenium bis(phosphine), palladium bis(phosphine) and palladium cyclopentadienyl complexes is reported.

The reactions of a series of β-diketiminate stabilised aluminium dihydrides with ruthenium bis(phosphine), palladium bis(phosphine) and palladium cyclopentadienyl complexes is reported. In the case of ruthenium, alane coordination occurs with no evidence for hydrogen loss resulting in the formation of ruthenium complexes with a pseudo–octahedral geometry and cis-relation of phosphine ligands. These new ruthenium complexes have been characterised by multinuclear and variable temperature NMR spectroscopy, IR spectroscopy and single crystal X-ray diffraction. In the case of palladium, a series of structural snapshots of alane dehydrogenation have been isolated and crystallographically characterised. Variation of the palladium precursor and ligand on aluminium allows kinetic control over reactivity and isolation of intermetallic complexes that contain new Pd–Al and Pd–Pd interactions. These complexes differ by the ratio of H : Al (2 : 1, 1.5 : 1 and 1 : 1) with lower hydride content species forming with dihydrogen loss. A combination of X-ray and neutron diffraction studies have been used to interrogate the structures and provide confidence in the assignment of the number and position of hydride ligands. ²⁷Al MAS NMR spectroscopy and calculations (DFT, QTAIM) have been used to gain an understanding of the dehydrogenation processes. The latter provide evidence for dehydrogenation being accompanied by metal–metal bond formation and an increased negative charge on Al due to the covalency of the new metal–metal bonds. To the best of our knowledge, we present the first structural information for intermediate species in alane dehydrogenation including a rare neutron diffraction study of a palladium–aluminium hydride complex. Furthermore, as part of these studies we have obtained the first SS ²⁷Al NMR data on an aluminium(i) complex. Our findings are relevant to hydrogen storage, materials chemistry and catalysis.

SAMBADENA Hyperpolarization of 13C-Succinate in an MRI: Singlet-Triplet Mixing Causes Polarization Loss

The signal enhancement provided by the hyperpolarization of nuclear spins of biological molecules is a highly promising technique for diagnostic imaging. To date, most 13C-contrast agents had to be polarized in an extra, complex or cost intensive polarizer. Recently, the in situ hyperpolarization of a 13C contrast agent to >20 % was demonstrated without a polarizer but within the bore of an MRI system. This approach addresses some of the challenges of MRI with hyperpolarized tracers, i. e. elevated cost, long production times, and loss of polarization during transfer to the detection site. Here, we demonstrate the first hyperpolarization of a biomolecule in aqueous solution in the bore of an MRI at field strength of 7 T within seconds. The 13C nucleus of 1-13C, 2,3-2H2-succinate was polarized to 11 % corresponding to a signal enhancement of approximately 18.000. Interesting effects during the process of the hydrogenation reaction which lead to a significant loss of polarization have been observed.

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.

Bimodal coordination of fused arenes to a Pd3 cluster site

We report bimodal coordination of fused arenes to a Pd₃ cluster site. The oxidative π-addition mode of anthracene and tetracene as well as the π-coordination mode of triphenylene and fluoranthene were identified.

All-metal aromatic cationic palladium triangles can mimic aromatic donor ligands with Lewis acidic cations

We present that cationic rings can act as donor ligands thanks to suitably delocalized metal-metal bonds. This could grant parent complexes with the peculiar properties of aromatic rings that are crafted with main group elements. We assembled Pd nuclei into equilateral mono-cationic triangles with unhindered faces. Like their main group element counterparts and despite their positive charge, these noble-metal rings form stable bonding interactions with other cations, such as positively charged silver atoms, to deliver the corresponding tetranuclear dicationic complexes. Through a mix of modeling and experimental techniques we propose that this bonding mode is an original coordination-like one rather than a 4-centre-2-electron bond, which have already been observed in three dimensional aromatics. The present results thus pave the way for the use of suitable metal rings as ligands.

A New Ir-NHC Catalyst for Signal Amplification by Reversible Exchange in D2 O

NMR signal amplification by reversible exchange (SABRE) has been observed for pyridine, methyl nicotinate, N-methylnicotinamide, and nicotinamide in D2 O with the new catalyst [Ir(Cl)(IDEG)(COD)] (IDEG=1,3-bis(3,4,5-tris(diethyleneglycol)benzyl)imidazole-2-ylidene). During the activation and hyperpolarization steps, exclusively D2 O was used, resulting in the first fully biocompatible SABRE system. Hyperpolarized (1) H substrate signals were observed at 42.5 MHz upon pressurizing the solution with parahydrogen at close to the Earth's magnetic field, at concentrations yielding barely detectable thermal signals. Moreover, 42-, 26-, 22-, and 9-fold enhancements were observed for nicotinamide, pyridine, methyl nicotinate, and N-methylnicotinamide, respectively, in conventional 300 MHz studies. This research opens up new opportunities in a field in which SABRE has hitherto primarily been conducted in CD3 OD. This system uses simple hardware, leaves the substrate unaltered, and shows that SABRE is potentially suitable for clinical purposes.

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.