Powerful Bis-facially Pyrazolate-Bridged Dinuclear Ruthenium Epoxidation Catalyst

A new bis-facial dinuclear ruthenium complex, {[Ru^II(bpy)]₂(μ-bimp)­(μ-Cl)}²⁺, <b>2</b>^<b>2+</b>, containing a hexadentate pyrazolate-bridging ligand (Hbimp) and bpy as auxiliary ligands has been synthesized and fully characterized in solution by spectrometric, spectroscopic, and electrochemical techniques. The new compound has been tested with regard to its capacity to oxidize water and alkenes. The <i>in situ</i> generated bis-aqua complex, {[Ru^II(bpy)­(H₂O)]₂(μ-bimp)}³⁺, <b>3</b>^<b>3+</b>, is an excellent catalyst for the epoxidation of a wide range of alkenes. High turnover numbers (TN), up to 1900, and turnover frequencies (TOF), up to 73 min^–1, are achieved using PhIO as oxidant. Moreover, <b>3</b>^<b>3+</b> presents an outstanding stereospecificity for both <i>cis</i> and <i>trans</i> olefins toward the formation of their corresponding epoxides due to specific interactions transmitted by its ligand scaffold. A mechanistic analysis of the epoxidation process has been performed based on density functional theory (DFT) calculations in order to better understand the putative cooperative effects within this dinuclear catalyst

Synthesis and Isomeric Analysis of Ru^II Complexes Bearing Pentadentate Scaffolds

A Ru^II-pentadentate polypyridyl complex [Ru^II(κ-N⁵-bpy2PYMe)­Cl]⁺ (<b>1</b>⁺, bpy2PYMe = 1-(2-pyridyl)-1,1-bis­(6–2,2′-bipyridyl)­ethane) and its aqua derivative [Ru^II(κ-N⁵-bpy2PYMe)­(H₂O)]²⁺ (<b>2</b>²⁺) were synthesized and characterized by experimental and computational methods. In MeOH, <b>1</b>⁺ exists as two isomers in different proportions, cis (70%) and trans (30%), which are interconverted under thermal and photochemical conditions by a sequence of processes: chlorido decoordination, decoordination/recoordination of a pyridyl group, and chlorido recoordination. Under oxidative conditions in dichloromethane, <i>trans</i>-<b>1</b>²⁺ generates a [Ru^III(κ-N⁴-bpy2PYMe)­Cl₂]⁺ intermediate after the exchange of a pyridyl ligand by a Cl^– counterion, which explains the trans/cis isomerization observed when the system is taken back to Ru­(II). On the contrary, <i>cis</i>-<b>1</b>²⁺ is in direct equilibrium with <i>trans</i>-<b>1</b>²⁺, with absence of the κ-N⁴-bis-chlorido Ru^III-intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure <i>trans</i>-[Ru^II(κ-N⁵-bpy2PYMe)­(H₂O)]²⁺ isomer (<i>trans-</i><b>2</b>²⁺), while the addition of a methyl substituent to a single bpy of the pentadentate ligand leads to the formation of a single cis isomer for both chlorido and aqua derivatives [Ru^II(κ-N⁵-bpy­(bpyMe)­PYMe)­Cl]⁺ (<b>3</b>⁺) and [Ru^II(κ-N⁵-bpy­(bpyMe)­PYMe)­(H₂O)]²⁺ (<b>4</b>²⁺) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [Ru^II(PY5Me₂)­X]^<i>n</i>+ system (X = Cl, <i>n</i> = 1 (<b>5</b>⁺); X = H₂O, <i>n</i> = 2 (<b>6</b>²⁺)), which also contains a κ-N⁵–Ru^II coordination environment, and to the newly synthesized [Ru^II(PY4Im)­X]^<i>n</i>+ complexes (X = Cl, <i>n</i> = 1 (<b>7</b>⁺); X = H₂O, <i>n</i> = 2 (<b>8</b>²⁺)), which possess an electron-rich κ-N⁴C–Ru^II site due to the replacement of a pyridyl group by an imidazolic carbene