Integration of Paramagnetic Diruthenium Complexes into an Extended Chain by Heterometallic Metal-Metal Bonds with Diplatinum Complexes

Coligands Controlled Reactivities of Ruthenium(II) Precursors: Antiferromagnetically Coupled Ruthenium(III)-Phenoxyl versus Ruthenium(II)-Phenoxyl Forms

The study disclosed that the reactivities of [RuII (PPh3)3Cl2] and [RuII(PPh3)3(CO)(H)Cl] precursors toward a trimethoxyarylimino-phenol derivative are sensibly different. The former promotes methoxy demethylation reaction affording a [Phenolato-RuIII-Phenolato] unit, while the latter containing π-acidic CO and hydride as coligands leads to C-H activation reaction, generating a [Phenolato-RuII-Aryl] unit. Notably, the oxidized analogues of these two forms produce antiferromagnetically coupled [RuIII-phenoxyl] and paramagnetic [RuII-phenoxyl] forms, which exhibit diverse reactivities. Surprisingly, the magnetically coupled [RuIII-phenoxyl] form obtained from [Phenolato-RuIII-Phenolato] motif leads to coligand, PPh3 oxidation and undergoes dimerization, making a Ru-Ru bond (2.599(2) Å), while the [RuII-phenoxyl] form obtained from [Phenolato-RuII-Aryl] motif leads to C-C coupling and H abstraction reactions. The coupling reaction affords a 4,4'-dibenzosemiquinonate anion radical complex, but the H-abstraction of the phenoxyl form gives a [RuII-Phenol] complex. For comparison, [RuII(IQR 0)] and [RuII(ISQR·-)] complexes were also isolated, where IQR 0 and ISQR·- are p-R-o-iminobenzoquinone and p-R-o-iminobenzosemiquinonate anion radicals. However, they fail to promote any bond-formation reaction. The molecular and electronic structures of the ruthenium (II/III) complexes were confirmed by single-crystal X-ray crystallography, EPR spectroscopy, and DFT calculations.

On-Surface Synthesis of Multiple Cu Atom-Bridged Organometallic Oligomers

A metal-metal bond between coordination complexes has the nature of a covalent bond in hydrocarbons. While bimetallic and trimetallic compounds usually have three-dimensional structures in solution, the high directionality and robustness of the bond can be applied for on-surface syntheses. Here, we present a systematic formation of complex organometallic oligomers on Cu(111) through sequential ring opening of 11,11,12,12-tetraphenyl-1,4,5,8-tetraazaanthraquinodimethane and bonding of phenanthroline derivatives by multiple Cu atoms. A detailed characterization with a combination of scanning tunneling microscopy and density functional theory calculations revealed the role of the Cu adatoms in both enantiomers of the chiral oligomers. Furthermore, we found sufficient strength of the bonds against sliding friction by manipulating the oligomers up to a hexamer. This finding may help to increase the variety of organometallic nanostructures on surfaces.

Electronic Structure of Ru26+ Complexes with Electron-Rich Anilinopyridinate Ligands

Diruthenium paddlewheel complexes supported by electron-rich anilinopyridinate (Xap) ligands were synthesized in the course of the first in-depth structural and spectroscopic interrogation of monocationic [Ru₂(Xap)₄Cl]⁺ species in the Ru₂⁶⁺ oxidation state. Despite paramagnetism of the compounds, ¹H NMR spectroscopy proved highly informative for determining the isomerism of the Ru₂⁵⁺ and Ru₂⁶⁺ compounds. While most compounds are found to have the polar (4,0) geometry, with all four Xap ligands in the same orientation, some synthetic procedures resulted in a mixture of (4,0) and (3,1) isomers, most notably in the case of the parent compound Ru₂(ap)₄Cl. The isomerism of this compound has been overlooked in previous reports. Electrochemical studies demonstrate that oxidation potentials can be tuned by the installation of electron donating groups to the ligands, increasing accessibility of the Ru₂⁶⁺ oxidation state. The resulting Ru₂⁶⁺ monocations were found to have the expected (π*)² ground state, and an in-depth study of the electronic transitions by Vis/NIR absorption and MCD spectroscopies with the aid of TD-DFT allowed for the assignment of the electronic spectra. The empty δ* orbital is the major acceptor orbital for the most prominent electronic transitions. Both Ru₂⁵⁺ and Ru₂⁶⁺ compounds were studied by Ru K-edge X-ray absorption spectroscopy; however, the rising edge energy is insensitive to redox changes in the compounds due to the broad line shape observed for 4d transition metal K-edges. DFT calculations indicate the presence of ligand orbitals at the frontier level, suggesting that further oxidation beyond Ru₂⁶⁺ will be ligand-centered rather than metal-centered.

Paramagnetic one-dimensional chains containing high-spin manganese atoms showing antiferromagnetic interaction through -Pt-Rh-Rh-Pt- bonds

To exploit the magnetic interactions of multiple metals, a heterometallic one-dimensional (1D) chain containing three kinds of metals, Rh, Pt, and Mn, where [Rh₂(O₂CCH₃)₄] and [Pt₂Mn(piam)₄(NH₃)₄]²⁺ (piam = pivalamidate) are connected through unbridged Rh-Pt bonds to form -Rh-Rh-Pt-Mn-Pt- alignments was successfully synthesized. The Mn atoms are tetrahedrally coordinated by four oxygen atoms of the piam ligands, where the coordination geometries form a zigzag 1D chain. Each Mn atom is linked by -Pt-Rh-Rh-Pt-, with a Mn-Mn separation of 13.9 Å. In parent [Pt₂Mn(piam)₄(NH₃)₄](PF₆)₂, Mn adopts two coordination environments, octahedral and tetrahedral, both of which are Mn(+2) high-spin states. In EtOH, [Rh₂(O₂CCH₃)₄] selectively binds tetrahedral Mn to afford a 1D chain. Physical analysis of the 1D chain using electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) revealed that all metals are divalent, indicating five unpaired spin-localized electrons on the Mn atoms. Magnetic susceptibility measurements indicated antiferromagnetic intra-chain interactions between the Mn atoms in the 1D chain, where χT at 300 K was 5.33 cm³ K mol^-1 and gradually decreased to 1.65 cm³ K mol^-1 at 2 K. Theoretical fitting of the magnetic behavior showed weak exchange coupling (zJ = -0.43 cm^-1) between two high-spin Mn(+2) ions through diamagnetic Pt-Rh-Rh-Pt.

Improving the Solubility of Hexanuclear Heterometallic Extended Metal Atom Chain Compounds in Nonpolar Solvents by Introducing Alkyl Amine Moieties

The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interaction at the d _{z ²} orbital between two kinds of metal complex is useful for obtaining heterometallic one-dimensional (1D) chains as well as heterometallic metal string compounds (HMSCs). Platinum dinuclear complexes, [Pt₂(piam)₂(NH₂R)₄]X₂ (piam = pivalamidate, R = CH₃, C₂H₅, C₃H₇, or C₄H₉, X = anion), comprising σ* as HOMO were mixed with [Rh₂(O₂CCH₃)₄] comprising σ* as LUMO in solvents to afford single crystals of [{Rh₂(O₂CCH₃)₄}{Pt₂(piam)₂(NH₂R)₄}₂]X₄ (2-5). Single-crystal X-ray analyses revealed that 2-5 are hexanuclear complexes that are one-dimensionally aligned as Pt-Pt-Rh-Rh-Pt-Pt with metal-metal bonds, where the alkyl moieties at end Pt atoms obstruct further 1D extension. Complexes 2-5 appear as if they are cut off from an infinite chain [{Rh₂(O₂CCH₃)₄}{Pt₂(piam)₂(NH₃)₄}₂] _n (PF₆)_4n ·6nH₂O (1) aligned as -{Pt-Pt-Rh-Rh-Pt-Pt} _n -. The diffuse reflectance spectrum of 1 depicts broad shoulder bands, which are not present in the spectra of 2-5, proving that the infinite chain 1 forms a band structure. Compounds 4 and 5 with propyl or butyl moieties at amine ligands, respectively, are soluble in nonpolar solvents, such as CH₂Cl₂, without the dissociation of their hexanuclear structures. Taking advantage of their solubility, measurement of cyclic voltammetry in CH₂Cl₂ become possible, which shows the quasi-reversible oxidation and reduction waves at 4: E _ox = 0.86 V and E _red = 0.69 V and 5: E _ox = 0.87 V and E _red = 0.53 V.

Transition Metal Chain Complexes Supported by Soft Donor Assembling Ligands

The chemistry of discrete molecular chains constituted by metals in low oxidation states, displaying metal-metal proximity and stabilized by suitable metal-bridging, assembling ligands comprising at least one soft donor atom is comprehensively reviewed; complexes with a single (hard or soft) bridging atom (e.g., μ-halide, μ-sulfide, or μ-PR₂ etc.) as well as "closed" metal arrays (that fall in the realm of cluster chemistry) are excluded. The focus is on transition metal-based systems, with few excursions to cases combining transition and post-transition elements. Most relevant supporting ligands have neutral C, P, O, or S donor (mainly, N-heterocyclic carbene, phosphine, ether, thioether) or anionic donor (mainly phenyl, ylide, silyl, phosphide, thiolate) groups. A supporting-ligand-based classification of the metal chains is introduced, using as the classifying parameter the number of "bites" (i.e., ligand bridges) subtending each intermetallic separation. The ligands are further grouped according to the number of donor atoms interacting with the metal chain (called denticity in the following) and the column of the Periodic Table to which the set of donor atoms belongs (in ascending order). A complementary metal-based compilation of the complexes discussed is also provided in a concise tabular form.

Dirhodium complexes as electrocatalysts for CO2 reduction to HCOOH: role of steric hindrance on selectivity

A series of Rh₂(ii,ii) complexes were shown to electrocatalytically reduce CO₂ to HCOOH. Electrochemical and spectroelectrochemical studies reveal a correlation between catalytic selectivity and efficiency with the steric bulk at the axial sites afforded by the bridging ligands. Mechanistic studies point to the presence of a Rh₂(ii,i)-H hydride as a key intermediate in the catalytic cycle.

Modulation of Band Gaps toward Varying Conductivities in Heterometallic One-Dimensional Chains by Ligand Alteration and Third Metal Insertion

A heterometallic one-dimensional (1-D) chain consisting of multiple kinds of metals, Rh, Pt, and Pd, with direct metal-metal bonds was successfully obtained by mixing a Rh dinuclear complex and Pt-Pd-Pt trinuclear complex. The Pt-Pd-Pt trinuclear complex can be reversibly one-electron-oxidized or -reduced, where the electron paramagnetic resonance spectrum of the one-electron-oxidized one shows an axially symmetric signal with hyperfine splitting by two Pt and Pd, indicating that an unpaired electron is delocalized to the d _{z ²} orbital of Pt-Pd-Pt. Utilized with the highest occupied molecular orbital and lowest unoccupied molecular orbital interaction at the d _{z ²} orbital, simple mixing of the Pt-Pd-Pt trinuclear complex and Rh dinuclear complex in adequate solvents afforded heterometallic 1-D chains, which are aligned as -Rh-Rh-Pt-Pd-Pt-. Several physical measurements revealed that the metal oxidation state is +2. Diffuse reflectance spectra and theoretical calculations show that heterometallic 1-D chains have σ-type conduction and valence bands where π*(Rh₂) are lying between them, whose gaps become narrower than the prototype chains aligned as -Rh-Rh-Pt-Pt-Pt-Pt-. The narrower band gaps are induced by destabilization of the σ-type valence bands and accompanied by insertion of Pd ions because the d-orbital energy level of Pd is closer in value to Rh compared with Pt. Flash-photolysis time-resolved microwave conductivity measurements exhibited an increase in the product of charge carrier mobility and its generation efficiency (8.1 × 10^-5 to 4.6 × 10^-4 cm² V^-1 s^-1) with narrowing the band gaps, suggesting that the better conductivity is attributed to shorter metal-metal distances in 1-D chains. These results imply the possibilities of controlling band gap with ligand modification and third metal insertion in heterometallic 1-D chains to show various conductivities.

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.

Metal phosphonates incorporating metalloligands: assembly, structures and properties

This feature article summarizes the current status of metal–metalloligand phosphonates including the synthetic strategies, crystal structures and properties. Future challenges in this field are discussed.

Paramagnetic One-Dimensional Chain Complex Consisting of Three Kinds of Metallic Species Showing Magnetic Interaction through Metal-Metal Bonds

A heterometallic and paramagnetic one-dimensional (1-D) chain (3) aligned as -Rh(+2)-Rh(+2)-Pt(+2)-Co(+2)-Pt(+2)- with direct metal-metal bonds was obtained by HOMO-LUMO interactions at the σ* (d_z²) orbital between two kinds of complexes. The 1-D chains in 3 have relatively straight backbones because the raw material complexes, [Rh₂(O₂CCH₃)₄] and [Pt₂Co(piam)₄(NH₃)₄], are connected and stacked in a face-to-face fashion, where Co---Co are separated by about 13.3 Å with four different metals. Physical measurements revealed that 3 has a band gap between the σ-type conduction and valence bands, where d-orbitals of the Co ion with three unpaired electrons are laid among them. The magnetic behavior of 3 was investigated and found to be consistent with a complex interaction involving both zero-field splitting and Pauli paramagnetism attributed to band formation superimposed on relatively strong exchange coupling (zJ = -22.2 cm^-1) between two high-spin Co(+2) ions separated by the diamagnetic Pt-Rh-Rh-Pt.

One-dimensional complexes extended by unbridged metal-metal bonds based on a HOMO-LUMO interaction at the dz2 orbital between platinum and heterometal atoms

In this review, the crystal structures of seventeen heterometallic one-dimensional compounds 1-17 are shown, and their electronic structures are discussed based on the diffuse reflectance spectra and other physical measurements. Compounds 1-17 comprise two kinds of complexes with HOMO-LUMO interactions at σ-type (d_z²) orbitals between platinum and heterometal atoms, where two or three kinds of metal are regularly aligned through metal-metal bonds. In [{PtRh(piam)₂(NH₃)₂Cl_2.5}₂{Pt₂(piam)₂(NH₃)₄}₂]_n(PF₆)_6n·2nMeOH·2nH₂O (3, piam = pivalamidate), the platinum and rhodium atoms are aligned as -Pt-Rh-Pt-Pt-Pt-Pt-Rh-Pt-Cl- with a mixed valence, where an unpaired electron hops from one Rh atom to another. [{Rh₂(O₂CCF₃)₄}{Pt₂(piam)₂(NH₃)₄}₂]_n(CF₃CO₂)_4n·2nEtOH·2nH₂O (6) with -Pt-Pt-Rh-Rh-Pt-Pt- alignment has a conduction band (CB) and a valence band (VB) attributed to σ-type orbitals, exhibiting narrower gaps between CB and VB than other analogues. [{Ru₂(O₂CCH₃)₄}{Pt₂(piam)₂(NH₃)₄}₂]_n(PF₆)_4n·4nH₂O (14) with -Pt-Pt-Ru-Ru-Pt-Pt- and [{Rh₂(O₂CCH₃)₄}{Pt₂Cu(piam)₄(NH₃)₄}]_n(PF₆)_2n (15) with -Rh-Rh-Pt-Cu-Pt- are paramagnetic chains, where one or two unpaired electrons reside at each metal per repeating unit. Thus, this system is diverse in modulating the electronic structures, band structures and the expected physical properties based on the unique oxidation states and redox properties attributed to the metal-metal interaction.

Carbene insertion into N–H bonds with size-selectivity induced by a microporous ruthenium–porphyrin metal–organic framework

A stable and porous porphyrinic metal–organic framework Ru-PMOF-1(Hf) has been prepared and used for N–H insertion reactions with high efficiency and selectivity.