Η2 Mode Research Articles

Template Synthesis of the Iron(III) Complex with the Ligands Based on Acylpyrazolonepyridines

The reaction of new bidentate ligand, 1-(5-hydroxy-1-methyl-3-(pyridin-2-yl)-1Н-pyrazol-4-yl)ethan-1-one (L), with iron(III) chloride affords the mononuclear iron(III) complex FeL₂Cl₃, which is characterized by XRD (CIF file CCDC no. 2309481). The intramolecular hydrogen bond between the protonated pyridyl and acetyl groups in ligand L, which exists in the crystal as a zwitterion, provides the formation of rarely met iron complexes in which the β-diketonate fragment coordinates via the η1 mode. A similar coordination mode along with a possibility of a more favorable η2 coordination provides new possibilities for the design of heteropolynuclear compounds of various structures used in the fabrication of molecular devices of data storage and processing.

Silver-π Interaction: A Diverse Approach to Hybrid Material and Its Efficacy in Electrocatalytic Reduction of Nitrate to Ammonia.

Inspired by the intriguing nature of the metal-π interaction in organometallic chemistry, a novel 1D hybrid material has been designed. Herein, a functionalized tellurium allyl macrocycle (TAM) acts as a molecular building block and is knit together via silver-π interaction to obtain Ag-TAM. Ag is coordinated to two allyl groups and a phenyl ring in η2 mode. Instead of the conventional polymerization strategy, a metal-π interaction is employed to interlink macrocycles. TAM and Ag-TAM showed electrocatalytic capability for the conversion of nitrate to ammonia. Ag-TAM showed an NH3 yield rate 2-fold greater than TAM with a high faradaic efficiency of 94.6% with good durability, proving that interlinking of macrocycles via metal-π interaction improves the catalytic activity. Detailed periodic density functional theory (DFT) calculations unveil novel mechanistic insights, suggesting cooperative catalysis between neighboring Ag sites and contributing to the enhanced efficiency.

Theoretical Implications of Bifunctional Ligands to Improve Uranium Extraction Performance

Extraction of uranium from seawater is considered to be an effective way to solve the shortage of uranium resources, and the development of efficient adsorption functional groups is the key to uranium extraction. In this work, the complexation of uranyl cations with a series of diamidoxime and bifunctional (amidoximate‐carboxylate, amidoximate‐phosphate) ligands has been probed by quantum chemical calculations. For most of the uranyl complexes, the amidoxime groups adopt η2 mode to uranyl cations. Based on bonding analyses, we found that the uranyl complexes with methyl‐substituted ligands (H2L') possess stronger covalent interactions than those with phenyl‐substituted ligands (H2L). Consequently, the uranyl complexes with H2L' are more stable in the extraction process according to thermodynamic analysis. The amidoximate‐phosphate bifunctional ligand (H2L3') has stronger extraction capacity to uranyl cations than other ligands, which is related to the relatively lower decomposition energy, and it shows selectivity in seawater for uranyl cations over vanadium ions, which may be a potential ligand for uranium extraction. Therefore, the introduction of synergistic functional groups, i.e. the bifunctional ligands, enhance the extraction properties of uranyl cations. This work improves understanding of synergistic ligands, and may contribute to design and development of efficient ligands for recovery of uranium from seawater.

Adsorption of acetonitrile on Pt(1 1 1) surface: Combining insights from experiment and theory

The extensive use of acetonitrile in various fields of organic synthesis and electrochemistry has provided widespread interest in studying the processes of acetonitrile adsorption on metal surfaces. In this work, the adsorption of acetonitrile and deuterated acetonitrile on thePt(111)surface was studied by polarization–modulation infrared reflection-absorption spectroscopy in the temperature range of 80–150 K. It was found that the adsorption and desorption of acetonitrile on platinum occurred in the molecular form. The possible configuration of adsorption with coordination through the nitrogen atom was proposed. To support the interpretation of obtained experimental data, density functional theory calculations were performed for acetonitrile adsorption considering 2 types of possible adsorption configurations: “bridge” with both N and C atoms bonded to the surface (η2(C,N) mode) and “top” with coordination through the nitrogen atom (N-top mode). Adsorption energies and frequency shifts of acetonitrile were calculated for different coverages ofPt(111)surface, including the second molecular layer. A joint experimental-theoretical study confirmed the formation of ice-like multilayers of acetonitrile onPt(111)under the considered conditions when C≡N bond is kept.

Reactions of Heterometallic Phosphinidene-Bridged MoMn and MoRe Complexes with Sulfur and Selenium: From Chalcogenophosphinidene- to Trithiophosphonate-Bridged Derivatives.

Reactions of [MoReCp(μ-PR*)(CO)6] with S8 were strongly dependent on experimental conditions (R* = 2,4,6-C6H2tBu3). When using 1 equiv of sulfur, complex [MoReCp(μ-η2:κ1S-SPR*)(CO)6] was slowly formed at 313 K, with a thiophosphinidene ligand unexpectedly bridging the dimetal center in the novel μ-κ1S:η2 coordination mode, as opposed to the μ-κ1P:η2 mode usually found in related complexes. The latter underwent fast decarbonylation at 363 K to give [MoReCp(μ-η2:η2-SPR*)(CO)5], with a six-electron donor thiophosphinidene ligand rearranged into the rare μ-η2:η2 coordination mode. Depending on reaction conditions, reactions with excess sulfur involved the addition of two or three S atoms to the phosphinidene ligand to give new complexes identified as the dithiophosphinidene-bridged complex [MoReCp(μ-η2:κ2S,S'-S2PR*)(CO)5], its dithiophosphonite-bridged isomer [MoReCp(μ-κ2S,S':κ2S,S'-S2PR*)(CO)5], or the trithiophosphonate-bridged derivative [MoReCp(μ-κ2S,S':κ2S,S'-S3PR*)(CO)5], all of them displaying novel coordination modes of their PRS2 and PRS3 ligands, as determined by X-ray diffraction studies. In contrast, the related MoMn complex yielded [MoMnCp(μ-η2:η2-SPR*)(CO)5] under most conditions. A similar output was obtained in reactions with gray selenium for either MoRe or MoMn phosphinidene complexes, which under different conditions only gave the pentacarbonyl complexes [MoMCp(μ-η2:η2-SePR*)(CO)5] (M = Re, Mn), these providing a new coordination mode for selenophosphinidene ligands.

Bonding Situation of σ-E-H Complexes in Transition Metal and Main Group Compounds.

The ambiguous bonding situation of σ-E-H (E=Si, B) complexes in transition metal compounds has been rationalized by means of Density Functional Theory calculations. To this end, the combination of the Energy Decomposition Analysis (EDA) method and its Natural Orbital for Chemical Valance (NOCV) extension has been applied to representative complexes described in the literature where the possible η1 versus η2 coordination mode is not unambiguously defined. Our quantitative analyses, which complement previous data based on the application of the Quantum Theory of Atoms in Molecules (QTAIM) approach, indicate that there exists a continuum between genuine η1 and η2 modes depending mainly on the strength of the backdonation. Finally, we also applied this EDA-NOCV approach to related main-group species where the backdonation is minimal.

Geometrically embedding dispersive Pt nanoparticles within silicalite-1 framework for highly selective ɑ, β-unsaturated aldehydes hydrogenation via oriented C = O adsorption configuration

The issue for thermodynamically favored C = C hydrogenation needs to be overcome in the hydrogenation of ɑ, β-unsaturated aldehydes to unsaturated alcohols. Herein, we employ confinement effect in the catalyst-designing engineering to orient the C = O adsorption by the zeolitic microchannel, where highly dispersed Pt nanoparticles are homogeneously well-embedded within the silicalite-1 framework (Pt@S-1). Comprehensive characterizations, such as HAADF-STEM, H2-TPR, FT-IR of CO adsorption and XPS are carried out to demonstrate the geometric, confinement and electronic properties of metal species. In the hydrogenation of ɑ, β-unsaturated aldehydes with suitable molecular size, Pt@S-1 displays preferential C = O hydrogenation. For example, Pt@S-1 gives conversion of 99.8% and meanwhile the aiming cinnamal alcohol selectivity of 98.7% in the hydrogenation of cinnamaldehyde, outperforming larger Pt nanoparticles ill-suitedly embedded within silicalite-1 (Pt@S-1-is) and highly dispersed Pt nanoparticles supported on the silicalite-1 surface (Pt/S-1). For the hydrogenation of smaller 3-methyl-2-butenal, though the confinement effect boosts the C = O hydrogenation to some extent, the synergetic electronic effect via introducing Fe species effectively increases the unsaturated alcohol selectivity. Investigations on the adsorption configuration by in-situ FT-IR spectroscopy clearly indicate that the unsaturated aldehyde with suitable molecular size is preferentially adsorbed on Pt sites of Pt@S-1 via the oxygen atom of C = O with on-top η1 mode, thus favoring high unsaturated alcohol selectivities, while the di-σCOη2 and η4 adsorption modes on Pt@S-1-is and Pt/S-1 are relatively adverse for prioritized C = O hydrogenation.

Functionalized polyarylether-based COFs for rapid and selective extraction of uranium from aqueous solution

Selective uranium extraction from aqueous solution is critical for both radioactive contamination remediation and uranium recovery from seawater. Herein, amidoxime (−AO) and carboxyl (–COOH) groups are equipped on a polyarylether-based covalent organic framework (COF) for fabrication of post-modified adsorbents, JUC-505-AO and JUC-505-COOH. The maximum adsorption capacities (Qm) of JUC-505-AO and JUC-505-COOH are 395 mg⋅g−1 and 464 mg⋅g−1, respectively. JUC-505-COOH shows preponderant uranium extraction efficiencies of 36–95% at low pH 2–4, and its removal performances are almost unaffected at high temperature ranging from 25 to 80 °C. Whereas, JUC-505-AO exhibits an impressive adsorption kinetics with the adsorption process equilibrates within 20 min (C0 = 20 mg⋅L-1), and marked sorption selectivity toward uranium against coexisting metal ions. JUC-505-AO also capable of extracting uranium with fast kinetics and capacity (Qm = 4604 μg⋅g−1) from real seawater. U L3 edge X-ray absorption spectra analysis reveals that uranyl ions in JUC-505-AO are anchored by coordinating with O and N from the oxime groups by η2 mode with the coordination numbers of 5 or 6. Meanwhile, HOH⋅⋅⋅O hydrogen bonds between the coordinated water of the hydrated uranyl ions and the oxygen atoms on the skeleton of JUC-505-AO were observed by density functional theory simulations.

Diversity of Coordination Architecture of Zinc Complexes with Diphenylarsinate

AbstractThe zinc complexes of diphenylarsinate [(Ph2AsO2)ZnCl] (1), [(Ph2AsO2)Zn(Ph2As(O)OH)Cl] (2) and [(Ph2AsO2)2Zn] (3) were prepared through the reaction of ZnCl2 with dihydroxodiphenylarsonium chloride [Ph2As(OH)2]Cl. Single‐crystal X‐ray diffraction analyses indicate that all three compounds form 1D polymer chain arrangements. Specifically, the [Ph2AsO2]− ligand coordinates with three Zn centres giving a syn,syn,anti μ3‐η1 : η2 mode in compound 1. A more diverse binding mode arised in compound 2: [Ph2AsO2]− shows a syn,anti μ2‐η1 : η1 bridging mode; while free ligand Ph2As(O)OH is bound to Zn through the oxygen atom of the As=O unit (monodentate). In addition, the hydrogen atom of Ph2As(O)OH forms a moderate hydrogen bond with the oxygen atom of [Ph2AsO2]−. Even though all [Ph2AsO2]− ligands also coordinate in a bridging mode in compound 3, a chain‐like arrangement with alternate single (anti,anti μ2‐η1 : η1) and triple (syn,syn μ2‐η1 : η1) bridging diphenylarsinate groups between tetrahedrally coordinated Zn atoms was identified.

Synthesis, characterization and photothermal conversion performance of three xanthene-functionalized dicopper complexes

For further elucidating the molecular absorption mechanisms in the photothermal conversion performance, three xanthene-functionalized dicopper coordination compounds have been isolated and structurally characterized by IR, Powder XRD, Solid-state UV, and TGA. They were prepared by solvothermal treatment the mixture of 9H-xanthene-9-carboxylic acid with Cu(CH3CN)4BF4 with/without the assistance of the auxiliary ligands {2,2′-bipyridine (bpy) or bis(diphenylphosphino)methane (dppm)}, namely, [Cu2(X9A)4(CH3CN)2] (1, X9A = xanthene-9-carboxylate), {[Cu2(X9A)(dppm)2(CH3CN)](BF4)∙(CH3CN)} (2), and {[Cu2(X9A)3(bpy)2](BF4)∙0.5(H2O)}2 (3). Single-crystal X-ray structural analysis and mass spectra revealed that the copper atoms in 2 were Cu(I), while in 1 and 3 were Cu(II) coordination compounds. The X9A ligands arranged in two types of coordination modes in 3: one in a bidentate bridging mode (μ2-η1, η1 mode) and two in an arrangement involving chelation and bridging mode (μ2-η1, η2 mode), while only simple bridging mode presented in 1 and 2. With the further stabilization of bpy ligands, compound 3 displayed excellent thermal stability, superior absorption ability as well as the best photothermal conversion performance (the highest temperature increment ~ 19 ℃ and equilibrium temperature ~ 32 ℃).

Solvothermal self assembly of three lanthanide(III)-succinates: Crystal structure, topological analysis and DFT calculations on water channel

Three new coordination polymers have been assembled by self-assembly of lanthanide(III) ions and succinic acid in dimethylformamide–water (DMF-H2O) mixed solvent having formulae; {[Ln(suc)1.5(H2O)2]•xH2O}n; (1), (2) and (3) [Where Ln = La (1), Pr (2) and Nd (3), suc = succinate dianion; x = 2 for CP 1, x = 1 for CP 2 and x = 4 for CP 3]. In all three coordination polymers (CPs) 1-3, the Ln(III) centres are ten coordinated with bicapped square antiprismatic geometry (SAPRS-10). Crystallographic analysis reveals that CPs 1-3 crystallize in triclinic system P¯1 space group. The linker succinic acid displays two different bonding modes: (μ2-κ4, η2:η2) and (μ4-κ6, η1:η2:η2:η1). All CPs 1-3 are isostructural and isomorphous. The two Ln centres in CPs 1-3 are bridged together through succinate anions via (μ4-κ6, η1:η2:η2:η1) mode forming dinuclear (Ln2O18) Secondary building unit (SBU) which are extended infinitely to generate one-dimensional chains. These 1D chains are further extended along the b-axis to form two-dimensional layered structures via (μ2-κ4, η2:η2) mode and finally three-dimensional network. In case of Nd, the quality of the crystal allows to locate a major number of lattice water molecules (x = 4) than in the other two complexes and, remarkably, the formation of water channels can be realized and studied. It has been analyzed using DFT calculations combined with the molecular electrostatic potential (MEP) surfaces, the quantum theory of “atoms-in-molecules” QTAIM and non–covalent interaction index (NCIplot) analyses.

Hydrophobic/Hydrophilic Interplay in 1,2,4‐Triazole‐ or Carboxylate‐Based Molybdenum(VI) Oxide Hybrids: A Step Toward Development of Reaction‐Induced Self‐Separating Catalysts

Abstract2‐(4H‐1,2,4‐triazol‐4‐yl)acetic acid (trglyH) and (dl)‐4‐methyl‐2‐(4H‐1,2,4‐triazol‐4‐yl)pentanoic acid (trleuH) were used as ligands for the development of MoO3 coordination hybrids for catalytic applications. Coordination polymers [Mo2O6(Htrgly)] ⋅ H2O (1) and [MoO3(trleuH)] ⋅ 0.5H2O (2) were prepared and structurally characterized. Compound 1 adopts a structure in which edge‐sharing MoO6 octahedra are organized in a ribbon motif via μ3‐O bridges. The Htrgly ligand exists as a zwitterion: −CO2− links two MoVI in a μ2‐η1 : η1 mode, while positively charged triazolium is left uncoordinated. In 2, Mo atoms are joined in a chain through μ2‐O. The more hydrophobic trleuH appears in a nonionized form and its [N−N] sites serve as a “clamp” for supporting the [−OMo(O)2−OMo(O)2(tr)2]n sequence. The complexes were explored for liquid phase catalytic epoxidation of cis‐cyclooctene, using H2O2 or tert‐butylhydroperoxide as oxidants. The hybrids showed good catalytic activity, and 2 behaved as a reaction‐induced self‐separating catalyst for olefin epoxidation with H2O2, presenting advantages of homogeneous and heterogeneous catalysis.

Triangular Cd(II)-Sm(III) Schiff Base Complex with Dual Visible and Near-Infrared Luminescent Responses to Nitro Explosives.

Two d-4f complexes [Zn2NdL2(OAc)2]·OH (1) and [Cd3Sm3L3(OAc)6(OH)3] (2) with a designed Schiff base ligand N,N'-bis(3-methoxysalicylidene)(binaphthyl)-1,4-diamine (H2L) were synthesized. The Schiff base ligands coordinate with metal ions by μ2(η1:η2:η1:η1:η2:η1) and μ2(η1:η2:η1:η1:η2:η1) modes in the complexes, which show typical lanthanide emissions. The triangular Cd-Sm complex 2 shows both visible and NIR luminescent responses to nitrobenzene explosive 2,4,6-trinitrophenol (PA).

Two dinuclear GdIII clusters based on isobutyric acid and nicotinic acid with large magnetocaloric effects

Two dinuclear GdIII cluster complexes with the formulas [Gd2(iba)6(bipy)2] (1) and [Gd2(nic)6(H2O)4] (2) (Hiba = isobutyric acid, Hnic = nicotinic acid and bipy = 2,2´-bipyridine) have been constructed by using gadolinium oxide as GdIII ion source under hydrothermal conditions. Although both 1 and 2 exhibit dinuclear structures, the coordination fashions are totally different. In 1, the carboxylate groups of Hiba take syn-syn-μ2-η1:η1, syn-syn-μ2-η1:η2 and syn-μ-η1:η1 modes and bipy acts as the chelating ligand. While the carboxylate groups of Hnic display syn-syn-μ2-η1:η1 and syn-μ-η1:η1 modes in 2 and two water molecules take part in coordination. Magnetic studies reveal weak ferromagnetic couplings and significant magnetic entropy changes with −ΔSmmax = 29.3 J kg−1 K−1 and 27.4 J kg−1 K−1 (ΔH = 7 T), respectively.

Dilithium complex incorporating phenyl bridged neutral pyrrolyl-arylamide ligand: Synthesis, characterization, reaction, and catalytic activity

The phenyl bridged neutral pyrrolyl-arylamide ligand with the formula 2-(2, 5-Me2C4H2N)C6H4NHSiMe3 (HL) was synthesized and characterized. Treatment of the ligand HL with equivalent of nBuLi or LiCH2SiMe3 afforded the lithiated complex [η5:(μ-η1:η1)-2-(2,5-Me2C4H2N)C6H4NSiMe3Li]2 (1) smoothly in good yield. The reaction of complex 1 with two equivalents of ErCl3(thf)3.5 and LiCH2SiMe3 produced novel trinuclear erbium-lithium cluster [η5:η1:(μ-η1:η1)-2-(2,5-Me2C4H2N)C6H4NSiMe2CH2ErCl]2LiCl(thf)2 (2) through sp3 C-H bond activation of methyl group and the resulting methylene group bonding two erbium centers in a rare μ2-η1:η1 mode. In addition, the dilithium complex 1 exhibited a high catalytic activity toward ring-opening polymerization of ε-caprolactone.

DFT investigations of linear Zn3-type complex with compartmental N/O-donor Schiff base: Synthesis, characterizations, crystal structure, fluorescence and molecular docking

A linear Zn(II) complex, [Zn3(LOMe-pn)2(ƞ1-NCS)2] (1) containing a N2O2O′2 donor Schiff base, (H2LOMe-pn = N, N-bis(3-methoxysalicylidene)-2, 2-dimethylpropane-1, 3 diamine) has been synthesized and structurally well characterized. SCXRD study reveals that in 1, the asymmetric section contains two identical discrete unit and each discrete unit contains three zinc metal ions (Zn1, Zn2 and Zn3) which are crystallographic independent, two deprotonated ligands [LOMe-pn]2- and [SCN−] ions which are linked with only terminal zinc ion in a η1 mode where terminal zinc atoms (Zn1/Zn3) are distorted square pyramidal while Zn2 is square antiprismatic. All computational calculations were performed by the DFT/M06 functional level and the 6-31G * basis set in the ground state. Complex optimized structure, HOMO-LUMO energy gap, Hirshfeld surface, MEP and NLO property was nicely explained with the help of DFT. Total dipole moment, average polarizability and first hyperpolarizability were calculated where α and βhyp values are well demonstrate the NLO property of complex (1). In addition, Bader’s “atoms-in-molecules” was analysed to delineate complex non-covalent interactions using the same level of theory. Experimental electronic spectra were explained using TD-DFT level of calculations with an IEFPCM solvent model. DMF solvent explore complex (1) is a fluorescent material with maximum emission at 479 nm at an excitation wavelength of 300 nm. Finally, molecular docking was executed with cytochrome P450 from B. megaterium and M. tuberculosis. Results of molecular docking and HOMO-LUMO energy gap can provide new insights in the development of antimycobacterial drugs and next-generation semiconductor devices.

Two Gd2 cluster complexes with monocarboxylate ligands displaying significant magnetic entropy changes

Two Gd2 cluster complexes with the formulas of [Gd2(dnba)6(phen)2] (1) and [Gd2(Hnsa)2(nsa)2(phen)2(H2O)2] (2) (Hdnba = 3,5-dinitrobenzoic acid, H2nsa = 5-nitrosalicylic acid and phen = 1,10-phenanthroline) respectively have been successfully synthesized under hydrothermal conditions. The GdIII ions of 1 are bridged by four carboxylates with syn-syn-μ2-η1:η1 and syn-syn-μ2-η1:η2 modes, while the GdIII ions of 2 are linked by two carboxylate ligands with only syn-syn-μ2-η1:η2 mode. Magnetic studies indicate that weak antiferromagnetic and ferromagnetic interactions exist between the adjacent GdIII ions within Gd2 clusters, respectively. Complexes 1 and 2 display distinct magnetocaloric effects induced by different magnetic interactions and magnetic densities. Notably, the maximum magnetic entropy changes (−ΔSmmax) of 1 and 2 reach 16.8 J kg−1 K−1 and 22.2 J kg−1 K−1 with ΔH = 7 T at 2 K and 3.5 K, respectively.

Syntheses, structures and hirshfeld surface analyses of two 3D supramolecules based on nitrogen-heterocyclic tricarboxylate ligand

A Co(II) complex, Co(H2dppc)2(H2O)2 (1) and a Cu(II) compex, Cu(H2dppc)2 (2) (H3dppc = 3-(2,4-dicarboxylate phenyl) -2-pyridinecarboxylic acid) have been synthesized and characterized by IR, elemental analysis, single-crystal X-ray diffraction and Hirshfeld analysis methods. The magnetic properties of the two complexes are also studied. For complex 1, the Co(II) ion is coordinated by two water molecules and two deprotonated H3dppc ligands in N-O chelate mode to form a mononuclear complex; for complex 2, the Cu(II) ion is coordinated by two deprotonated H3dppc ligands in μ2-η2:η1 mode to form a mononuclear complex; the two complexes are both connected into 3D supramolecules by intermolecular O-H⋯O and C-H⋯O hydrogen bonds. Hirshfeld surface analysis and 2D fingerprint plots are used to calculate the contribution of different intermolecular interactions. This work could provide a new strategy for the rational design and synthesis of supramolecules.

Synthesis and characterization of 2-t-butylimino-functionalized indolyl rare-earth metal amido complexes for the catalytic addition of terminal alkynes to carbodiimides: the dimeric complexes with the alkynide species in the μ-η1:η2 bonding modes.

A series of new rare-earth metal amido complexes bearing a 2-t-butylimino-functionalized indolyl ligand were synthesized via dehydrogenation of a secondary amine, and their reactivities and catalytic performances were investigated. The reactions of 2-(tBuNHCH2)C8H5NH (1) with rare-earth metal amides [(Me3Si)2N]3RE(μ-Cl)Li(THF)3 afforded the complexes [2-(tBuN[double bond, length as m-dash]CH)C8H5N]RE[N(SiMe3)2]2 (RE = Er (2), Y (3), Dy (4), Sm (5), Nd (6)) containing the bidentate ligand via dehydrogenation of the amine to the imine group. Complexes 2-6 exhibited an excellent catalytic activity for the addition of terminal alkynes to carbodiimides affording a series of corresponding N,N'-dialkyl-propiolamidines. The catalysts can also be efficiently used for the preparation of the sterically bulky N,N'-diaryl-propiolamidine. Furthermore, the dimeric complexes {(μ-η1:η2-RC[triple bond, length as m-dash]C)RE[2-(tBuN[double bond, length as m-dash]CH)C8H5N]2}2 (11: RE = Y, R = Ph; 12: RE = Yb, R = SiMe3), with the alkynyl ligand in the μ-η1:η2 haptic modes, were isolated in attempts to study the catalytic mechanism.

Diruthenium complexes having a partially hydrogenated bipyridine ligand: plausible mechanism for the dehydrogenative coupling of pyridines at a diruthenium site.

The reactions of the diruthenium tetrahydrido complex, Cp*Ru(μ-H)4RuCp* (1) (Cp* = η5-C5Me5), with pyridines were investigated in relation to the dehydrogenative coupling of 4-substituted pyridines. Complex 1 reacted with γ-picoline to yield the bis(μ-pyridyl) complex, {Cp*Ru(μ-H)(μ-4-MeC5H3)}2 (2a), with the elimination of dihydrogen. Complex 2a immediately reacted with the liberated dihydrogen to yield μ-η2-dihydrobipyridine (dhbpy) complex 4avia C-C bond formation between the two pyridyl groups, in which one of the pyridine rings underwent partial hydrogenation. The X-ray structure of 4a shows that the dhbpy moiety adopts a μ-η2 coordination mode at the Ru2 site. Complex 4a was reversibly converted to 5avia the elimination of dihydrogen in which the dhbpy moiety adopts a μ-η2:η2 mode. Although 5a was coordinatively saturated, 5a readily reacted with tBuNC to yield 6a. This was owing to the ability of the dhbpy ligand changing its coordination mode between the μ-η2:η2 and μ-η2 modes. This also causes the dehydrogenation from the dhbpy ligand to yield μ-η2:η2-bipyridine complex 7a at 140 °C. However, 7a was not shown to be an intermediate of the catalysis. The reaction of 1 with 1,10-phenanthroline afforded μ-η2-phenanthroline complex 8 containing two hydrides, which can be a model compound for the bipyridine elimination from the Ru2 site. Dynamic NMR studies suggested that 8 was isomerised to an unsaturated μ-N-heterocyclic carbene (NHC) complex. The unsaturated nature of the μ-NHC complex is likely responsible for the uptake of the third pyridine molecule to turn over the catalytic cycle.