Pyridine-Quinoline and Biquinoline-Based Ruthenium p-Cymene Complexes as Efficient Catalysts for Transfer Hydrogenation Studies: Synthesis and Structural Characterization

Abstract

Searching for new and efficient transfer hydrogenation catalysts, a series of new organometallic ruthenium(II)-arene complexes of the formulae [Ru(η⁶-p-cymene)(L)Cl][PF₆] (1-8) and [Ru(η⁶-p-cymene)(L)Cl][Ru(η⁶-p-cymene)Cl₃] (9-11) were synthesized and fully characterized. These were prepared from the reaction of pyridine-quinoline and biquinoline-based ligands (L) with [Ru(η⁶-p-cymene)(μ-Cl)Cl]₂, in 1:2 and 1:1, metal (M) to ligand (L) molar ratios. Characterization includes a combination of spectroscopic methods (FT-IR, UV-Vis, multi nuclear NMR), elemental analysis and single-crystal X-ray crystallography. The pyridine-quinoline organic entities encountered, were prepared in high yield either via the thermal decarboxylation of the carboxylic acid congeners, namely 2,2'-pyridyl-quinoline-4-carboxylic acid (pqca), 8-methyl-2,2'-pyridyl-quinoline-4-carboxylic acid (8-Mepqca), 6'-methyl-2,2'-pyridyl-quinoline-4-carboxylic acid (6'-Mepqca) and 8,6'-dimethyl-2,2'-pyridyl-quinoline-4-carboxylic acid (8,6'-Me₂pqca), affording the desired ligands pq, 8-Mepq, 6'-Mepq and 8,6'-Me₂pq, or by the classical Friedländer condensation, to yield 4,6'-dimethyl-2,2'-pyridyl-quinoline (4,6'-Me₂pq) and 4-methyl-2,2'-pyridyl-quinoline (4-Mepq), respectively. The solid-state structures of complexes 1-4, 6, 8 and 9 were determined showing a distorted octahedral coordination geometry. The unit cell of 3 contains two independent molecules (Ru-3), (Ru'-3) in a 1:1 ratio, due to a slight rotation of the arene ring. All complexes catalyze the transfer hydrogenation of acetophenone, using 2-propanol as a hydrogen donor in the presence of KOiPr. Among them, complexes 1 and 5 bearing methyl groups at the 8 and 4 position of the quinoline moiety, convert acetophenone to 1-phenylethanol quantitatively, within approximately 10 min with final TOFs of 1600 h^-1. The catalytic performance of complexes 1-11, towards the transfer hydrogenation of p-substituted acetophenone derivatives and benzophenone, ranges from moderate to excellent. An inner-sphere mechanism has been suggested based on the detection of ruthenium(II) hydride species.

Keywords: hydride species; hydrogenation; ketones; p-cymene; pyridine–quinoline ligands; ruthenium complexes; transfer hydrogenation.

Scheme 1

Synthesis and structures of organic ligands used in this work; hydrogen atom numbering is included for ¹H NMR assignment.

Figure 1

Molecular structure of 8-Mepq; ellipsoids are plotted at the 50% probability level. H atoms are omitted. Selected bond lengths (Å) and angles (°): N(1)–C(1) 1.376(5); N(1′)–C(1′) 1.365(5); N(2)–C(11) 1.346(6); N(1)–C(5) 1.321(5); N(1′)–C(5′) 1.326(5); N(2)–C(11) 1.346(6); C(1)–C(2) 1.416(6); C(1′)–C(2′) 1.423(6); C(5)–N(1)–C(1) 118.1(3); C(11)–N(2)–C(15) 117.3(4); C(5′)– N(1′)–C(1′) 118(3).

Figure 2

Molecular structure of 8,6′-Me₂pq; ellipsoids are plotted at the 50% probability level. H atoms are omitted. Selected bond lengths (Å) and angles (°): N(1)–C(1) 1.3718(15); N(1)–C(5) 1.372(5); N(2)–C(11) 1.347(5); N(2)–C(15) 1.346(5); C(1)–C(2) 1.415(5); C(5)–N(1)–C(1) 118.6(3); C(11)–N(2)–C(15) 117.7(3); N(1)–C(5)–C(4) 122.5(3).

Figure 3

Molecular structure of 4,6′-Me₂pq; ellipsoids are plotted at the 50% probability level. H atoms are omitted. Selected bond lengths (Å) and angles (°): N(1)–C(1) 1.3718(15); N(1)–C(5) 1.3257(16); N(2)–C(11) 1.3476(16); N(2)–C(12) 1.3452(16); C(1)–C(2) 1.4203(17); C(5)–N(1)–C(1) 117.27(10); C(11)–N(2)–C(12) 118.25(11); N(1)–C(5)–C(4) 123.26(10).

Scheme 2

General reaction scheme for the synthesis of the ruthenium(II) organometallic complexes 1–8 (a) and 9–11 (b), described in this work.

Figure 4

(a) Molecular structure of the complex cation of 1; (b) Molecular structure of the complex cation of 4. Hydrogen atoms and the PF₆⁻ anions are omitted for clarity. The ellipsoids were plotted at the 50% probability level.

Figure 5

(a) Molecular structure of the complex cation of 2; (b) Molecular structure of the complex cation of 6. Hydrogen atoms and the PF₆⁻ anions are omitted for clarity. The ellipsoids were plotted at the 50% probability level.

Figure 6

(a) Molecular structure of the complex cation [Ru(η⁶-p-cymene)(8,6′-Me₂pq)Cl]⁺ (Ru-3); (b) Molecular structure of the complex cation [Ru′(η⁶-p-cymene)(8,6′-Me₂pq)Cl]⁺ (Ru′-3). Hydrogen atoms and the PF₆⁻ anions are omitted for clarity. The ellipsoids were plotted at the 50% probability level.

Figure 7

A superposition of both independent molecules of 3 in the unit cell. Hydrogen atoms are omitted for clarity. The ellipsoids were plotted at the 50% probability level.

Figure 8

Molecular structure of the complex cation of 9. Hydrogen atoms, solvent molecules and the [Ru(p-cymene)Cl₃]⁻ complex anion are omitted for clarity. The ellipsoids were plotted at the 50% probability level.

Scheme 3

Catalytic transfer hydrogenation of benzophenone and acetophenone derivatives by catalysts 1–11, [Ru-Cat].

Figure 9

Time dependence of transfer hydrogenation of acetophenone by catalysts 1–5, 7-Cl and 8.