Synthesis of palladium(II) 3d-metal(II) paddlewheel acetate-bridged heterodimetallic complexes: Unexpected catalysis by water molecules

Generic and facile mechanochemical access to versatile lattice-confined Pd(ii)-based heterometallic sites

Metal-organic frameworks (MOFs) show remarkable potential in a broad array of applications given their physical and chemical versatility. Classical synthesis of MOFs is performed using solution chemistry at elevated temperatures to achieve reversible metal-ligand bond formation. These harsh conditions may not be suitable for chemical species sensitive to high temperature or prone to deleterious reactions with solvents. For instance, Pd(ii) is susceptible to reduction under solvothermal conditions and is not a common metal node of MOFs. We report a generic and facile mechanochemical strategy that directly incorporates a series of Pd(ii)-based heterobimetallic clusters into MOFs as metal nodes without Pd(ii) being reduced to Pd(0). Mechanochemistry features advantages of short reaction time, minimum solvent, high reaction yield, and high degree of synthetic control. Catalytic performances of lattice-confined heterobimetallic sites are examined for nitrene transfer reactions and we demonstrate that the chemoselectivity for allylic amination versus olefin aziridination is readily tuned by the identity of the first-row metal ion in Pd(ii)-based heterobimetallic clusters.

Palladium(II)/Lewis Acid-Catalyzed Oxidative Olefination/Annulation of N-Methoxybenzamides: Identifying the Active Intermediates through NMR Characterizations

Although Pd(II)-catalyzed C-H activation in arenes has been widely successful in organic synthesis with many palladacycle compounds isolated as the intermediates in ligand-directed C-H activation, direct identification of the reaction intermediates such as the π-complex prior to the C-H activation is still not successful because of their instability. In the present study, we introduce a Pd(II)/LA (LA: Lewis acid)-catalyzed oxidative olefination/annulation reaction between N-methoxybenzamides and acrylates with oxygen as the oxidant source, in which two intermediates, including an unsymmetrical η⁶-complex and a palladacycle species without the proton releasing to the environment, were identified through NMR characterizations. The in situ formation of the heterobimetallic Pd(II)/LA species such as Pd(II)/Sc(III) may have enhanced the electrophilic properties of the Pd²⁺ cation, thus improving the stability of the π-complex, herein, an unsymmetrical η⁶-complex, and improving its catalytic efficiency. The observed insensitive electronic effect preferred the concerted metalation-deprotonation (CMD) mechanism for this C-H activation, and the detected palladacycle intermediate without the proton releasing to the environment offered an experimental clue to support the proposed CMD mechanism.

Paramagnetic Metal-Metal Bonded Heterometallic Complexes

Significant progress has been made in the past 10-15 years on the design, synthesis, and properties of multimetallic coordination complexes with heterometallic metal-metal bonds that are paramagnetic. Several general classes have been explored including heterobimetallic compounds, heterotrimetallic compounds of either linear or triangular geometry, discrete molecular compounds containing a linear array of more than three metal atoms, and coordination polymers with a heterometallic metal-metal bonded backbone. We focus in this Review on the synthetic methods employed to access these compounds, their structural features, magnetic properties, and electronic structure. Regarding the metal-metal bond distances, we make use of the formal shortness ratio (FSR) for comparison of bond distances between a broad range of metal atoms of different sizes. The magnetic properties of these compounds can be described using an extension of the Goodenough-Kanamori rules to cases where two magnetic ions interact via a third metal atom. In describing the electronic structure, we focus on the ability (or not) of electrons to be delocalized across heterometallic bonds, allowing for rationalizations and predictions of single-molecule conductance measurements in paramagnetic heterometallic molecular wires.

Templating metastable Pd2 carboxylate aggregates

Evaluation of the potential for metal-metal (M-M) cooperation to enable catalysis requires access to specific polynuclear aggregates that display appropriate geometry and size. In many cases, exerting synthetic control over the aggregation of simple metal salts is a challenge. For example, Pd(ii) acetate self assembles as a trimer (i.e. Pd3(OAc)6) both in the solid state and in solution and does not feature close Pd-Pd interactions. Related carboxylate-supported Pd2 aggregates (i.e. Pd2(OAc)4), which would feature close Pd-Pd interactions, are thermodynamically metastable in solution phase and thus largely unavailable. Here we demonstrate ion metathesis within pre-formed metal-organic frameworks (MOFs) to prepare metastable Pd2 tetracarboxylates sites. The newly synthesized materials are characterized by elemental analysis, PXRD, SCXRD, EXAFS, XANES, and gas adsorption analysis. In addition, the critical role of network solvation on the kinetics of ion metathesis was revealed by coupled TGA-MS and ICP-MS experiments. The demonstration of templated ion metathesis to generate specific metastable coordination sites that are inaccessible in solution phase chemistry represents a new opportunity to interrogate the chemistry of specific polynuclear metal aggregates.

Regulating the size and spatial distribution of Pd nanoparticles supported by the defect engineered metal–organic framework HKUST-1 and applied in the aerobic oxidation of cinnamyl alcohol

A series of novel Pd@DE-HKUST-1(Cu/Pd) catalysts with different pydc feeding ratios were successfully synthesized. The size regime and the spatial distribution of the Pd NPs can be controlled by the amount of framework incorporated pydc.

Bimetallic metal organic frameworks with precisely positioned metal centers for efficient H2 storage

We demonstrated that the ratio and position of two different metal ions, Pd and Cu, can be precisely controlled within MOFs through predesigned metal clusters. These MOF structures incorporating Pd-Cu paddle wheel units were synthesised simply by reacting Pd-Cu acetate metal clusters and tritopic organic linkers at room temperature. Pd-Cu open metal sites were found to be uniformly distributed throughout the MOFs with a ca. 1 : 1 ratio. The incorporation of Pd into the MOF structure also led to enhanced affinity towards H2 with Qst values up to 8.9 kJ mol-1.

Heterometallic Palladium(II)-Indium(III) and -Gallium(III) Acetate-Bridged Complexes: Synthesis, Structure, and Catalytic Performance in Homogeneous Alkyne and Alkene Hydrogenation

The reaction of Pd₃(OOCMe)₆ with indium(III) and gallium(III) acetates was studied to prepare new Pd^II-based heterometallic carboxylate complexes with group 13 metals. The heterometallic palladium(II)-indium(III) acetate-bridged complexes Pd(OOCMe)₄In(OOCMe) (1) and Pd(OOCMe)₄In(OOCMe)·MeCOOH (1a) were synthesized and structurally characterized with X-ray crystallography and extended X-ray absorption fine structure in the solid state and solution. A similar Pd-Ga heterometallic complex formed by the reaction of Pd₃(OOCMe)₆ with gallium(III) acetate in a dilute acetic acid solution, as evidenced by atmospheric pressure chemical ionization mass and UV-vis spectrometry, was unstable at higher concentrations and in the solid state. Complex 1 catalyzes the liquid-phase-selective phenylacetylene and styrene hydrogenation (1 atm of H₂ at 20 °C) in acetic acid, ethyl acetate, and N, N-dimethylformamide solutions, while no Pd metal was formed until alkyne and alkene hydrogenation ceased.

Role of Lewis acid additives in a palladium catalyzed directed C-H functionalization reaction of benzohydroxamic acid to isoxazolone

Metallic salts as well as protic additives are widely employed in transition metal catalyzed C-H bond functionalization reactions to improve the efficiency of catalytic protocols. In one such example, ZnCl₂ and pivalic acid are used as additives in a palladium catalyzed synthesis of isoxazolone from a readily available benzohydroxamic acid under one pot conditions. In this article, we present some important mechanistic insights into the role of ZnCl₂ and pivalic acid, gained by using density functional theory (M06) computations. Two interesting modes of action of ZnCl₂ are identified in various catalytic steps involved in the formation of isoxazolone. The conventional Lewis acid coordination wherein zinc chloride (ZnCl₂·(DMA)) binds to the carbonyl group is found to be more favored in the C-H activation step. However, the participation of a hetero-bimetallic Pd-Zn species is preferred in reductive elimination leading to C_aryl-N bond formation. Pivalic acid helps in relay proton transfer in C-H bond activation through a cyclometallation deprotonation (CMD) process. The explicit inclusion of ZnCl₂ and solvent N,N-dimethyl acetamide (DMA) stabilizes the transition state and also helps reduce the activation barrier for the C-H bond activation step. The electronic communication between the two metal species is playing a crucial role in stabilizing the C_aryl-N bond formation transition state through a Pd-Zn hetero-bimetallic interaction.

Mechanistic Insight into the Intramolecular Benzylic C-H Nitrene Insertion Catalyzed by Bimetallic Paddlewheel Complexes: Influence of the Metal Centers

The intramolecular benzylic C-H amination catalyzed by bimetallic paddlewheel complexes was investigated by using density functional theory calculations. The metal-metal bonding characters were investigated and the structures featuring either a small HOMO-LUMO gap or a compact SOMO energy scope were estimated to facilitate an easier one-electron oxidation of the bimetallic center. The hydrogen-abstraction step was found to occur through three manners, that is, hydride transfer, hydrogen migration, and proton transfer. The imido N species are more preferred in the Ru-Ru and Pd-Mn cases whereas coexisting N species, namely, singlet/triplet nitrene and imido, were observed in the Rh-Rh and Pd-Co cases. On the other hand, the triplet nitrene N species were found to be predominant in the Pd-Ni and Pd-Zn systems. A concerted asynchronous mechanism was found to be modestly favorable in the Rh-Rh-catalyzed reactions whereas the Pd-Co-catalyzed reactions demonstrated a slight preference for a stepwise pathway. Favored stepwise pathways were seen in each Ru-Ru- and Pd-Mn-catalyzed reactions and in the triplet nitrene involved Pd-Ni and Pd-Zn reactions. The calculations suggest the feasibility of the Pd-Mn, Pd-Co, and Pd-Ni paddlewheel complexes as being economical alternatives for the expensive dirhodium/diruthenium complexes in C-H amination catalysis.

Simultaneous introduction of various palladium active sites into MOF via one-pot synthesis: Pd@[Cu3−xPdx(BTC)2]n

Simultaneous introduction of various palladium active sites into MOF to form Pd@[Cu_3−xPd_x(BTC)₂]_n has been achieved and show good catalytic activity.

Hetero-bimetallic metal–organic polyhedra

Porous metal–organic polyhedra (MOPs), constructed from heterometallic Pd^II–M^II (M = Cu, Ni, Zn) paddlewheel nodes and 5-tert-butyl-1,3-benzenedicarboxylate organic links, were prepared in which the Pd^II ions preferentially line the inner surface of the cage molecules.

Recent Advances in the C-H-Functionalization of the Distal Positions in Pyridines and Quinolines

This review summarizes recent developments in the C-H-functionalization of the distal positions of pyridines, quinolines and related azaheterocycles. While the functionalization of the C2 position has been known for a long time and is facilitated by the proximity to N1, regioselective reactions in the distal positions are more difficult to achieve and have only emerged in the last decade. Recent advances in the transition metal-catalyzed distal C-H-functionalization of these synthetically-important azaheterocycles are discussed in detail, with the focus on the scope, site-selectivity and mechanistic aspects of the reactions.

Palladium-Catalyzed C8-Selective C-H Arylation of Quinoline N-Oxides: Insights into the Electronic, Steric and Solvation Effects on the Site-Selectivity by Mechanistic and DFT Computational Studies

We report herein a palladium-catalyzed C–H arylation of quinoline N-oxides that proceeds with high selectivity in favor of the C8 isomer. This site selectivity is unusual for palladium, since all of the hitherto described methods of palladium-catalyzed C–H functionalization of quinoline N-oxides are highly C2 selective. The reaction exhibits a broad synthetic scope with respect to quinoline N-oxides and iodoarenes and can be significantly accelerated to subhour reaction times under microwave irradiation. The C8-arylation method can be carried out on a gram scale and has excellent functional group tolerance. Mechanistic and density functional theory (DFT) computational studies provide evidence for the cyclopalladation pathway and describe key parameters influencing the site selectivity.

Preparation and characterization of supported bimetallic Pd(IV)-Co(III) model catalyst from organometallic single source precursor for aerobic oxidation of alcohols

The bimetallic paddlewheel catalyst precursor, [Pd(II)Co(II)(μ-OOCCH(3))(4)] H(2)O·2CH(3)COOH (1), prepared from [Pd(3)(μ-OOCCH(3))(6)] and [Co(OOCCH(3))(2)], was used as a single source precursor to prepare, after binding to a surface-hydroxylated silicon wafer and oxidation, the bimetallic oxides of Pd(IV)Co(III)/SiO(2) catalyst supported on a model planar (i.e., two-dimensional) silicon wafer. This catalyst catalyzes the aerobic oxidation of alcohols to its corresponding carbonyl compounds. The bimetallic tetracarboxylato catalyst precursor was bonded to the surface-hydroxylated silicon wafer by spin-coating and also by grafting. X-ray photoelectron spectroscopy (XPS) revealed that one of the four μ-acetato bridging ligands was substituted by Si-O fragments in a covalent bond formation process during grafting of 1 onto the wafer. In contrast, during the spin-coating process, all four acetato ligands remained intact during fixation on the silicon surface. Upon oxidation and workup, the grafted sample's Pd:Co ratio remained unchanged (1.0:1.3), whereas the spin-coated sample's Pd content decreased with respect to Co content. XPS determined binding energies were interpreted to imply that after oxidation in an oxygen/argon mixture of the grafted sample both Pd(II) and Co(II) were oxidized to produce PdO(2) (337.5 eV) and Co(III)(2)O(3) (781.1 eV) which most probably interacts with the silicon surface via Pd(IV)-O-Si and Co(III)-O-Si bonds. Solvent free aerobic oxidation of octadecanol to its corresponding carbonyl compound was achieved on this oxidized Pd(IV)Co(III)/SiO(2) model catalyst using molecular oxygen as oxidant under solvent-free conditions. The use of the single source catalyst precursor, 1, resulted in a Pd(IV)Co(III)/SiO(2) catalyst with superior catalytic activity toward the oxidation of octadecanol over a catalyst prepared from a physical mixture of the separate reactant compounds tripalladium(II) hexaacetate and cobalt(II) diacetate.