Efficient Electronic Communication of Two Ruthenium Centers through a Rigid Ditopic N‐Heterocyclic Carbene Linker

A ditopic benzobis(carbene) ligand precursor was prepared that contained a chelating pyridyl moiety to ensure co‐planarity of the carbene ligand and the coordination plane of a bound octahedral metal center. Bimetallic ruthenium complexes comprising this ditopic ligand [L₄Ru‐C,N‐bbi‐C,N‐RuL₄] were obtained by a transmetalation methodology (C,N‐bbi‐C,N=benzobis(N‐pyridyl‐N′‐methyl‐imidazolylidene). The two metal centers are electronically decoupled when the ruthenium is in a pseudotetrahedral geometry imparted by a cymene spectator ligand (L₄=[(cym)Cl]). Ligand exchange of the Cl^?/cymene ligands for two bipyridine or four MeCN ligands induced a change of the coordination geometry to octahedral. As a consequence, the ruthenium centers, separated through space by more than 10 Å, become electronically coupled, which is evidenced by two distinctly different metal‐centered oxidation processes that are separated by 134 mV (L₄=[(bpy)₂]; bpy=2,2′‐bipyridine) and 244 mV (L₄=[(MeCN)₄]), respectively. Hush analysis of the intervalence charge‐transfer bands in the mixed‐valent species indicates substantial valence delocalization in both complexes (delocalization parameter Γ=0.41 and 0.37 in the bpy and MeCN complexes, respectively). Spectroelectrochemical measurements further indicated that the mixed‐valent Ru^II/Ru^III species and the fully oxidized Ru^III/Ru^III complexes gradually decompose when bound to MeCN ligands, whereas the bpy spectators significantly enhance the stability. These results demonstrate the efficiency of carbenes and, in particular, of the bbi ligand scaffold for mediating electron transfer and for the fabrication of molecular redox switches. Moreover, the relevance of spectator ligands is emphasized for tailoring the degree of electronic communication through the benzobis(carbene) linker.     

Tuning the Magnetic Moment of [Ru2(DPhF)3(O2CMe)L]+ Complexes (DPhF=N,N′‐Diphenylformamidinate): A Theoretical Explanation of the Axial Ligand Influence

The magnetic behaviour of the compounds containing the [Ru₂(DPhF)₃(O₂CMe)]⁺ ion (DPhF^?=N,N′‐diphenylformamidinate) shows a strong dependence on the nature of the ligand bonded to the axial position. The new complexes [Ru₂(DPhF)₃(O₂CMe)(OPMe₃)][BF₄]?0.5 CH₂Cl₂ ( 1 ? 0.5 CH₂Cl₂) and [Ru₂(DPhF)₃(O₂CMe)(4‐pic)][BF₄] ( 2 ) (4‐pic=4‐methylpyridine) clearly display this influence. Complex 1 ?0.5 CH₂Cl₂ shows a magnetic moment corresponding to a S=3/2 system affected by the common zero‐field splitting (ZFS) and a weak antiferromagnetic interaction, whereas complex 2 displays an intermediate behaviour between S=3/2 and S=1/2 systems. The experimental data of complex 1 are fitted with a model that considers the ZFS effect using the Hamiltonian ?_D= S ? D ? S . The weak antiferromagnetic coupling is introduced as a perturbation, using the molecular field approximation. DFT calculations demonstrate that, in the [Ru₂(O₂CMe)(DPhF)₃(L)]⁺ complexes, the energy level of the metal–metal molecular orbitals is strongly dependent on the nature of the axial ligand (L). This study reveals that the increase in the π‐acceptor character of L leads to a greater split between the π* and δ* HOMO orbitals. The influence of the axial ligand in the relative energy between the doublet and quartet states in this type of complexes was also analysed. This study was performed on the new complexes 1 ?0.5 CH₂Cl₂ and 2 . The previously isolated [Ru₂(DPhF)₃(O₂CMe)(OH₂)][BF₄]?0.5 CH₂Cl₂ ( 3 ? 0.5 CH₂Cl₂) and [Ru₂(DPhF)₃(O₂CMe)(CO)][BF₄]?CH₂Cl₂ ( 4 ?CH₂Cl₂) complexes were also included in this study as representative examples of spin‐admixed and low‐spin configurations, respectively. The [Ru₂(DPhF)₃(O₂CMe)]⁺ ( 5 ) unit was used as a reference compound. These theoretical studies are in accordance with the different magnetic behaviour experimentally observed.     

Secondary Interactions in Phosphane‐Functionalized Group 4 Cycloheptatrienyl–Cyclopentadienyl Sandwich Complexes

Twice as reactive : The coordination chemistry of phosphane‐functionalized Zr and Hf cycloheptatrienyl–cyclopentadienyl complexes gives rise to unusual secondary interactions associated with the presence of Lewis acidic 16‐electron sandwich moieties. These structures can develop weak dative bonds as exemplified by the noncovalent Pd→Zr interaction in the heterobimetallic {Zr₂Pd} complex (see picture).

     

A Thermo‐ and Photo‐Switchable Ruthenium Initiator For Olefin Metathesis

A ruthenium carbene complex bearing azobenzene functionality is reported. The complex exists in the form of two isomers differing by the size of the chelate ring. Both isomers were isolated by applying kinetic or thermodynamic control during the synthesis and characterized by X‐ray diffraction analysis. The isomerization of the complex was studied by UV/Vis spectroscopy. The stable isomer was tested as a catalyst in olefin metathesis. The complex was activated at about 100 °C to promote ring‐closing and ring‐opening polymerization metathesis reactions. The activation took place also at room temperature under middle ultraviolet radiation.     

Unprecedented Efficient Structure Controlled Phosphorescence of Silver(I) Clusters Stabilized by Carba‐closo‐dodecaboranylethynyl Ligands

{Ag₂(12‐C≡C‐closo‐1‐CB₁₁H₁₁)}_n and selected pyridine ligands have been used for the synthesis of photostable Ag^I clusters that, with one exception, exhibit for Ag^I compounds unusual room‐temperature phosphorescence. Extraordinarily intense phosphorescence was observed for a distorted pentagonal bipyramidal Ag^I₇ cluster that shows an unprecedented quantum yield of Φ=0.76 for Ag^I clusters. The luminescence properties correlate with the structures of the central Ag^I_n motifs as shown by comparison of the emission properties of the clusters with different numbers of Ag^I ions, different charges, and electronically different pyridine ligands.     

Luminescent Heteropolynuclear Complexes of 3,5‐Dimethylpyrazolate [Pt2Au2M2(Me2pz)8] (M=Ag,Cu) Showing the Synergistic Effect of Three Transition Elements in the Excited State

Swap the coins! The Pt₂Au₂, Pt₂Au₂Cu₂, and Pt₂Au₂Ag₂ complexes of 3,5‐dimethylpyrazolate exhibit yellow‐green, orange, and sky‐blue luminescence, respectively (see figure). The emission energies of Pt₂Au₂M₂ complexes can be controlled by the change of the third coinage metal ions M. The Pt₂Au₂M₂ complexes take the cis configuration with respect to the Au₂M₂ plane.

     

Supramolecular Photoinduced Electron Transfer between a Redox‐Active Hexanuclear Metal–Organic Cylinder and an Encapsulated Ruthenium(II) Complex

By using redox‐active nickel(II) ions as the connect nodes, a hexanuclear metal–organic cylinder (Ni‐YL) was achieved through self‐assembly with a large cavity and an opening windows capable to accommodate guest molecules. The suitable cavity of Ni‐YL provides an opportunity to encapsulate the anionic ruthenium bipyridine derivative [Ru(dcbpy)₃] (dcbpy=2,2′‐bipyridine‐4,4′‐dicarboxylic acid) as the photosensitizer for light‐driven reactions. The host–guest behavior between Ni‐YL and [Ru(dcbpy)₃] was investigated by mass spectrometry, NMR spectroscopy, and computational studies, revealing an effective binding of the guest [Ru(dcbpy)₃] within the cavity of Ni‐YL. Optical experiments suggested a pseudo‐intramolecular photoinduced electron transfer (PET) process between the [Ru(dcbpy)₃] and the host Ni‐YL, leading to an efficient light‐driven hydrogen production based on this system. Control experiments with a mononuclear Ni complex as a reference photocatalyst and the inactive [Fe(dcbpy)₃] as an inhibitor for comparison were also performed to confirm such a supramolecular photocatalysis process.     

Controlling Metal–Ligand–Metal Oxidation State Combinations by Ancillary Ligand (L) Variation in the Redox Systems [L2Ru(μ‐boptz)RuL2]n,boptz=3,6‐bis(2‐oxidophenyl)‐1,2,4,5‐tetrazine,and L=acetylacetonate, 2,2′‐bipyridine,or 2‐phenylazopyridine

The new compounds [(acac)₂Ru(μ‐boptz)Ru(acac)₂] ( 1 ), [(bpy)₂Ru(μ‐boptz)Ru(bpy)₂](ClO₄)₂ ( 2 ‐(ClO₄)₂), and [(pap)₂Ru(μ‐boptz)Ru(pap)₂](ClO₄)₂ ( 3 ‐(ClO₄)₂) were obtained from 3,6‐bis(2‐hydroxyphenyl)‐1,2,4,5‐tetrazine (H₂boptz), the crystal structure analysis of which is reported. Compound 1 contains two antiferromagnetically coupled (J=?36.7 cm^?1) Ru^III centers. We have investigated the role of both the donor and acceptor functions containing the boptz^2? bridging ligand in combination with the electronically different ancillary ligands (donating acac^?, moderately π‐accepting bpy, and strongly π‐accepting pap; acac=acetylacetonate, bpy=2,2′‐bipyridine pap=2‐phenylazopyridine) by using cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance (EPR) spectroscopy for several in situ accessible redox states. We found that metal–ligand–metal oxidation state combinations remain invariant to ancillary ligand change in some instances; however, three isoelectronic paramagnetic cores Ru(μ‐boptz)Ru showed remarkable differences. The excellent tolerance of the bpy co ‐ ligand for both Ru^III and Ru^II is demonstrated by the adoption of the mixed ‐ valent form in [L₂Ru(μ‐boptz)RuL₂]³⁺, L=bpy, whereas the corresponding system with pap stabilizes the Ru^II states to yield a phenoxyl radical ligand and the compound with L=acac^? contains two Ru^III centers connected by a tetrazine radical‐anion bridge.     

A Mixed Ruthenium Polypyridyl Complex Containing a PEG‐Bipyridine Macroligand

Summary: An amino‐functionalized bipyridine ligand was prepared in order to serve as a bridging unit to an activated low‐molecular‐weight monomethyl ether of poly(ethylene glycol) (PEG). Coordination of a ruthenium(II ) phenantroline precursor onto the formed PEG‐containing bipyridine ligand yielded a metal‐containing polymer which shows interesting properties for solar cell applications.

A schematic of the described polymeric ruthenium(II ) complex and its absorption and emission properties.     

Synthesis,Structures, and Reactivities of Pincer‐Type Ruthenium Complexes Bearing Two Proton‐Responsive Pyrazole Arms

A new metal–ligand bifunctional, pincer‐type ruthenium complex [RuCl( L1‐H2 )(PPh₃)₂]Cl ( 1 ; L1‐H2 =2,6‐bis(5‐tert‐butyl‐1H‐pyrazol‐3‐yl)pyridine) featuring two proton‐delivering pyrazole arms has been synthesized. Complex 1 , derived from [RuCl₂(PPh₃)₃] with L1‐H2 , underwent reversible deprotonation with potassium carbonate to afford the pyrazolato–pyrazole complex [RuCl(L1‐H)(PPh₃)₂] ( 2 ). Further deprotonation of 1 and 2 with potassium hexamethyldisilazide in methanol resulted in the formation of the bis(pyrazolato) complex [Ru(L1)(MeOH)(PPh₃)₂] ( 3 ). Complex 3 smoothly reacted with dioxygen and dinitrogen to give the side‐on peroxo complex [Ru(L1)(O₂)(PPh₃)₂] ( 4 ) and end‐on dinitrogen complex [Ru(L1)(N₂)(PPh₃)₂] ( 5 ), respectively. On the other hand, the reaction of [RuCl₂(PPh₃)₃] with less hindered 2,6‐di(1H‐pyrazol‐3‐yl)pyridine ( L3‐H2 ) led to the formation of the dinuclear complex [{RuCl₂(PPh₃)₂}₂(μ₂‐ L3‐H2 )₂] ( 6 ), in which the pyrazole‐based ligand adopted a tautomeric form different from L1‐H2 in 1 and the central pyridine remained uncoordinated. The detailed structures of 1 , 2 , 3 , 3.MeOH , 4 , 5 , 6 were determined by X‐ray crystallography.     

Unsymmetrical Pincer‐Type Ruthenium Complex Containing β‐Protic Pyrazole and N‐Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere

A reaction of a 2‐(imidazol‐1‐yl)methyl‐6‐(pyrazol‐3‐yl)pyridine with [RuCl₂(PPh₃)₃] resulted in tautomerization of the imidazole unit to afford the unsymmetrical pincer‐type ruthenium complex 2 containing a protic pyrazole and N‐heterocyclic carbene (NHC) arms. Deprotonation of 2 with one equivalent of a base led to the formation of the NHC–pyrazolato complex 3 , indicating that the protic NHC arm is less acidic. When 2 was treated with two equivalents of a base under H₂ or in 2‐propanol, the hydrido complex 4 containing protic NHC and pyrazolato groups was obtained through metal–ligand cooperation.