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

Hai-Jie Liu; Marcos Gil-Sepulcre; Laia Francàs; Pau Nolis; Teodor Parella; Jordi Benet-Buchholz; Xavier Fontrodona; Jordi García-Antón; Nuria Romero; Antoni Llobet; Lluís Escriche; Roger Bofill; Xavier Sala

doi:10.1039/C6DT04729G

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

Hai-Jie Liu,^a Marcos Gil-Sepulcre,^a Laia Francàs,^b Pau Nolis,

^c Teodor Parella,^c Jordi Benet-Buchholz,^b Xavier Fontrodona,^d Jordi García-Antón,^a Nuria Romero,^a Antoni Llobet,^ab Lluís Escriche,*^a Roger Bofill*^a and Xavier Sala

*^a

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* Corresponding authors

^a Departament de Química, Facultat de Ciències, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Catalonia, Spain
E-mail: roger.bofill@uab.cat, lluis.escriche@uab.cat, xavier.sala@uab.cat
Fax: +34 93 581 24 77

^b Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Catalonia, Spain

^c Servei de Ressonància Magnètica Nuclear, Facultat de Ciències, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Catalonia, Spain

^d Serveis Tècnics de Recerca, Edifici P-II, Campus Montilivi, Universitat de Girona, 17071 Girona, Catalonia, Spain

Abstract

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.

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

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

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