New Aqua N-Heterocyclic Carbene Ru(II) Complexes with Two-Electron Process as Selective Epoxidation Catalysts: An Evaluation of Geometrical and Electronic Effects

Hydrogenation of α,β-Unsaturated Carbonyl Compounds over Covalently Heterogenized Ru(II) Diphosphine Complexes on AlPO4-Sepiolite Supports

In this work, the covalent immobilization of two ruthenium(II) complexes, i.e., [RuIICl (bpea){(S)(-)(BINAP)}](BF4), 1, and [RuIICl(bpea)(DPPE)](BF4), 2, where BINAP = 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl and DPPE = 1,2-bis(diphenylphosphino)ethane, have been obtained (AlPO4-Sepiolite@1 and AlPO4-Sepiolite@2) by using a N-tridentate ligand N,N-bis-(2-pyridylmethyl)ethylamine (bpea), linked to an amorphous AlPO4-Sepiolite (20/80) inorganic support. This AlPO4-sepiolite support is able to immobilize the double amount of ruthenium complex (1.65%) than the amorphous AlPO4 (0.89%). Both heterogenized complexes have been assessed as catalysts in the liquid phase hydrogenation of several substrates with carbonyl and/or olefinic double bonds using methanol as solvent, attaining good catalytic activity and high enantioselectivity (99%). The highest Turn Over Number (TON) value (748.6) was obtained over the [RuII Cl (bpea)(DPPE)](BF4) 2 catalyst, although the [RuIICl(bpea){(S)(-)(BINAP)}](BF4) 1 exhibits better reusability. In fact, the [RuIICl(bpea){(S)(-)(BINAP)}](BF4) immobilized on AlPO4-Sepiolite maintained the activity throughout 14 successive runs. Furthermore, some findings on hydrogenation mechanisms of the α,β-unsaturated carbonyl compounds over Ru catalysts have been also obtained.

Mononuclear ruthenium compounds bearing N-donor and N-heterocyclic carbene ligands: structure and oxidative catalysis

A new CNNC carbene-phthalazine tetradentate ligand has been synthesised, which in the reaction with [Ru(T)Cl₃] (T = trpy, tpm, bpea; trpy = 2,2';6',2''-terpyridine; tpm = tris(pyrazol-1-yl)methane; bpea = N,N-bis(pyridin-2-ylmethyl)ethanamine) in MeOH or iPrOH undergoes a C-N bond scission due to the nucleophilic attack of a solvent molecule, with the subsequent formation of the mononuclear complexes cis-[Ru(PhthaPz-OR)(trpy)X]ⁿ⁺, [Ru(PhthaPz-OMe)(tpm)X]ⁿ⁺ and trans,fac-[Ru(PhthaPz-OMe)(bpea)X]ⁿ⁺ (X = Cl, n = 1; X = H₂O, n = 2; PhthaPz-OR = 1-(4-alkoxyphthalazin-1-yl)-3-methyl-1H-imidazol-3-ium), named 1a⁺/2a²⁺ (R = Me), 1b⁺/2b²⁺ (R = iPr), 3⁺/4²⁺ and 5⁺/6²⁺, respectively. Interestingly, regulation of the stability regions of different Ru oxidation states is obtained by different ligand combinations, going from 6²⁺, where Ru(iii) is clearly stable and mono-electronic transfers are favoured, to 2a²⁺/2b²⁺, where Ru(iii) is almost unstable with regard to its disproportionation. The catalytic performance of the Ru-OH₂ complexes in chemical water oxidation at pH 1.0 points to poor stability (ligand oxidation), with subsequent evolution of CO₂ together with O₂, especially for 4²⁺ and 6²⁺. In electrochemically driven water oxidation, the highest TOF values are obtained for 2a²⁺ at pH 1.0. In alkene epoxidation, complexes favouring bi-electronic transfer processes show better performances and selectivities than those favouring mono-electronic transfers, while alkenes containing electron-donor groups show better performances than those bearing electron-withdrawing groups. Finally, when cis-β-methylstyrene is employed as the substrate, no cis/trans isomerization takes place, thus indicating the existence of a stereospecific process.

A Ruthenium(III)-Oxyl Complex Bearing Strong Radical Character

Proton-coupled electron-transfer oxidation of a Ru^II -OH₂ complex, having an N-heterocyclic carbene ligand, gives a Ru^III -O^. species, which has an electronically equivalent structure of the Ru^IV =O species, in an acidic aqueous solution. The Ru^III -O^. complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru-O bond showed a lower-energy Raman scattering at 732 cm^-1 and the Ru-O bond length was estimated to be 1.77(1) Å. The Ru^III -O^. complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C-H abstraction from the formyl group by the Ru^III -O^. complex bearing a strong radical character as the active species.

Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

Non-redox metal ions can affect the reactivity of active redox metal ions in versatile biological and heterogeneous oxidation processes; however, the intrinsic roles of these non-redox ions still remain elusive. This work demonstrates the first example of the use of non-redox metal ions as Lewis acids to sharply improve the catalytic oxygen atom transfer efficiency of a ruthenium complex bearing the classic 2,2'-bipyridine ligand. In the absence of Lewis acid, the oxidation of ruthenium(ii) complex by PhI(OAc)2 generates the Ru(iv)[double bond, length as m-dash]O species, which is very sluggish for olefin epoxidation. When Ru(bpy)2Cl2 was tested as a catalyst alone, only 21.2% of cyclooctene was converted, and the yield of 1,2-epoxycyclooctane was only 6.7%. As evidenced by electronic absorption spectra and EPR studies, both the oxidation of Ru(ii) by PhI(OAc)2 and the reduction of Ru(iv)[double bond, length as m-dash]O by olefin are kinetically slow. However, adding non-redox metal ions such as Al(iii) can sharply improve the oxygen transfer efficiency of the catalyst to 100% conversion with 89.9% yield of epoxide under identical conditions. Through various spectroscopic characterizations, an adduct of Ru(iv)[double bond, length as m-dash]O with Al(iii), Ru(iv)[double bond, length as m-dash]O/Al(iii), was proposed to serve as the active species for epoxidation, which in turn generated a Ru(iii)-O-Ru(iii) dimer as the reduced form. In particular, both the oxygen transfer from Ru(iv)[double bond, length as m-dash]O/Al(iii) to olefin and the oxidation of Ru(iii)-O-Ru(iii) back to the active Ru(iv)[double bond, length as m-dash]O/Al(iii) species in the catalytic cycle can be remarkably accelerated by adding a non-redox metal, such as Al(iii). These results have important implications for the role played by non-redox metal ions in catalytic oxidation at redox metal centers as well as for the understanding of the redox mechanism of ruthenium catalysts in the oxygen atom transfer reaction.

Crystal structure of cis,fac-{N,N-bis-[(pyridin-2-yl)meth-yl]methyl-amine-κ(3) N,N',N''}di-chlorido-(dimethyl sulfoxide-κS)ruthenium(II)

The reaction of di-chlorido-tetra-kis-(dimethyl sulfoxide)-ruthen-ium(II) with N,N-bis[(pyridin-2-yl)meth-yl]methyl-amine aff-ords the title complex, [RuCl2(C13H15N3)(C2H6OS)]. The asymmetric unit contains a well-ordered complex mol-ecule. The N,N-bis-[(pyridin-2-yl)meth-yl]methyl-amine (bpma) ligand binds the cation through its two pyridyl N atoms and one aliphatic N atom in a facial manner. The coordination sphere of the low-spin d (6) Ru(II) is distorted octahedral. The dimethyl sulfoxide (dmso) ligand coordinates to the cation through its S atom and is cis to the aliphatic N atom. The two chloride ligands occupy the remaining sites. The bpma ligand is folded with the dihedral angle between the mean planes passing through its two pyridine rings being 64.55 (8)°. The two N-Ru-N bite angles of the ligand at 81.70 (7) and 82.34 (8)° illustrate the distorted octa-hedral coordination geometry of the Ru(II) cation. Two neighboring molecules are weakly associated through mutual intermolecular hydrogen bonding involving the O atom and one of the methyl groups of the dmso ligand. One of the chloride ligands is also weakly hydrogen bonded to a pyridyl H atom of another molecule.

Polypyrrole-functionalized ruthenium carbene catalysts as efficient heterogeneous systems for olefin epoxidation

New Ru complexes containing the bpea-pyr ligand (bpea-pyr stands for N,N-bis(pyridin-2-ylmethyl)-3-(1H-pyrrol-1-yl)propan-1-amine), with the formula [RuCl2(bpea-pyr)(dmso)] (isomeric complexes 2a and 2b) or [Ru(CN-Me)(bpea-pyr)X)](n+) (CN-Me = 3-methyl-1-(pyridin-2-yl)-1H-imidazol-3-ium-2-ide; X = Cl, 3, or X = H2O, 4), have been prepared and fully characterized. Complexes 3 and 4 have been anchored onto an electrode surface through electropolymerization of the attached pyrrole group, yielding stable polypyrrole films. The electrochemical behaviour of 4, which displays a bielectronic Ru(IV/II) redox pair in solution, is dramatically affected by the electropolymerization process leading to the occurrence of two monoelectronic Ru(IV/III) and Ru(III/II) redox pairs in the heterogeneous system. A carbon felt modified electrode containing complex 4 (C-felt/poly-4) has been evaluated as a heterogeneous catalyst in the epoxidation of various olefin substrates using PhI(OAc)2 as an oxidant, displaying TON values of several thousands in all cases and good selectivity for the epoxide product.