Carboxamide ruthenium(II) and manganese(II) complexes: structural, kinetic, and mechanistic studies in the transfer hydrogenation of ketones.
Authors:   , Kumah Robert Tettey 
Published: January 2024

The carboxamide ligands N-(benzo[d]thiazol-2-yl)pyrazine-2-carboxamide (HL1), N-(1H-benzo[d]imidazol-2-yl)pyrazine-2-carboxamide (HL2), were prepared by condensation of pyrazine-carboxylic acid and appropriate heteroaromatic amines. Reactions of HL1 and HL2 with ruthenium(II) precursors, [RuH(CO)Cl(PPh3)3] and [RuH2(CO)(PPh3)3] afforded the mononuclear complexes [RuL1(PPh3)2(CO)Cl] (Ru1), [RuL1(PPh3)2(CO)H] (Ru2), [RuL2(PPh3)2(CO)Cl] (Ru3), [RuL2(PPh3)2(CO)H] (Ru4). The solid-state structures of complexes Ru1, Ru2, and Ru4 reveal bidentate modes of coordination of the ligands and distorted octahedral geometries around the Ru(II) centre. The complexes formed active catalysts in the transfer hydrogenation of ketones and achieved turnover number (TON) up to 530 in 6 h. The ruthenium(II)–hydride complexes, Ru2 and Ru4, were capable of catalysing transfer hydrogenation of ketones reactions under base free reaction conditions and demonstrated higher catalytic activities compared to the corresponding non-hydride analogues (Ru1 and Ru3). An inner sphere monohydride mechanism involving dissociation of one PPh3 group was proposed from in situ 31P{1H} NMR spectroscopy studies. Dipicolinamide ligand system, N,N'-(1,4 phenylene)dipicolinamide (H2L3), N,N'-(1,2-phenylene)dipicolinamide (H2L4), N,N'-(4,5-dimethyl-1,2-phenylene)dipicolinamide (H2L5), N,N'-(4-methoxy-1,2-phenylene)dipicolinamide (H2L6) were synthesised following a similar protocol described for HL1 and HL2. Treatment of the ligands H2L3 and H2L4 with RuH(CO)Cl(PPh3)3 afforded bimetallic complexes [Ru2(H2L3)(PPh3)4(CO)2][2Cl] (Ru5), [Ru2(H2L3)(H)2(PPh3)4(CO)2] (Ru5b), [Ru2(HL4)(PPh3)3(CO)2Cl3] (Ru6) and a mononuclear complex [RuCl2L4(PPh3)2(CO)] (Ru7). The solid-sate structure of the dinuclear ruthenium(II) complexes confirmed a bidentate coordinate mode, with PPh3, CO, and chlorido auxiliary ligands occupying the remaining coordinating sites to afford distorted trigonal bipyramidal geometries (Ru5 and Ru6) while the mononuclear complex Ru7 adopted a distorted octahedral geometry around its ruthenium(II) atom. The reaction of the ligands H2L4-H2L6 with the [RuCl2-η6-p-cymene]2 precursor produces half-sandwich diruthenium complexes [{Ru(η6-p-cymene)}2-μ-Cl(L4)][Ru(η6-p-cymene)Cl3] (Ru8), [{Ru(η6-p-cymene)}2-μ-Cl(L4)][PF6] (Ru9), [{Ru(η6-p-cymene)}2-μ-Cl(L5)][PF6] (Ru10), and [{Ru(η6-p-cymene)}2-μ-Cl (L6)][PF6] (Ru11). The molecular structure of cationic complexes, Ru8-Ru11, was confirmed by single-crystal X-ray crystallography analysis. The complexes Ru8-Ru11 display a bidentate Npyridine ^ Namidate mode of coordination to give pseudo-octahedral geometry (piano-stool-like geometry). The ruthenium(II) complexes demonstrated remarkable enhanced catalytic activity (TON values up to 1.71 x 104) in the transfer hydrogenation of ketones at a very low catalyst loading of 2.75 x10-2 mol% (275 ppm). The dinuclear ruthenium(II) complexes showed higher catalytic activity compared to the corresponding mononuclear complex Ru5. The half-sandwich diruthenium complexes Ru8-Ru11 displayed relatively higher catalytic activity than the ruthenium complexes Ru5 and Ru6 bearing the PPh3 co-ligands. Monohydride inner-sphere catalytic cycle was proposed for the transfer hydrogenation of ketones catalysed by both Ru1 and Ru9, and the formation of the reactive intermediates was supported with low-resolution mass spectrometry data. The dinuclear ruthenium complexes of pyridine and pyrazine-carboxamide bearing quinolinyl motif were synthesised by reacting, N-(quinolin-8-yl)pyrazine-2-carboxamide, (HL7), 5-methyl-N-(quinolin-8-yl)pyridine-2-carboxamide, (HL8), 5-chloro-N-(quinolin-8-yl)pyridine-2-carboxamide, (HL9), and 2-pyrazine-carboxylic acid (HL10) with equimolar [RuCl2(η6-p-cymene)]2 to afford the dinuclear complexes [{Ru(η6-p-cymene)}2Cl3(L10)] (Ru12), [{Ru(η6-p-cymene)Cl}2(L7)] [PF6] (Ru13), [{Ru(η6-p-cymene)Cl}2(L8)][Ru(L8)Cl3] (Ru14), and [{Ru(η6-p-cymene)Cl}2(L9)][PF6] (Ru15), respectively. The solid-state structures of the dinuclear complexes Ru12 and Ru13 reveal a typical piano-stool geometry around the Ru(II) ions. The dinuclear ruthenium complexes Ru12-Ru15 were used as catalysts in the transfer hydrogenation of a broad spectrum of aldehydes and ketones and demonstrated excellent catalytic activity, TON values up to 4.8 x 104, using catalyst loading of 2.0 x10-3 mol% (20 ppm). The catalytic performance of the complexes was affected by the ligand architecture and the substituents on the pyridyl ring. Complexes Ru13-15 exhibited higher catalytic activities compared to the complex Ru12 which could be ascribed to the role of quinoline in stabilising the complexes. The pyridine and pyrazine motifs have a significant impact on the reactivity and the catalytic activity of the complexes. In-situ low-resolution ESI-MS analyses of the reactive intermediates aided in proposing a monohydride inner-sphere mechanism for the transfer hydrogenation of ketones catalysed by Ru15. To develop a more sustainable, environmentally compatible and cost-efficient protocol for transfer hydrogenation of ketones, a new catalytic system based on manganese(II) metal was synthesised. New manganese(II) complexes Mn1-Mn4, ligated on dipicolinamide ligands were synthesized by treating the N,N'-(1,4-phenylene)dipicolinamide (H2L3), N,N'-(1,2-phenylene)dipicolinamide (H2L4), N,N'-(4-methoxy-1,2-phenylene)dipicolinamide (H2L5) and N,N'-(4,5-dimethyl-1,2-phenylene)dipicolinamide (H2L6) with MnCl2.4H2O salt to afford dinuclear manganese(II) complexes [Mn2(H2L3)2Cl4] (Mn1), [Mn2(H2L4)2Cl4] (Mn2), [Mn2(H2L5)2(Cl)4] (Mn3) and [Mn2(H2L6)2Cl4] (Mn4). The solid-state structure of complex Mn2 showed a six-coordinate dinuclear complex with the two Mn(II) ions adopting a distorted octahedral environment surrounded by two tetradentate ligands and chlorido co-ligands, respectively. The Mn(II) complexes formed active catalysts in transfer hydrogenation of ketones to achieve TON values up to 5.12 x 104. The presence of electron-donating substituents -OCH3 and -CH3 in complexes Mn3 and Mn4 displayed minor effects in the transfer hydrogenation of ketones. The new carboxamide-manganese(II) complexes are among the most active manganese-based catalysts capable of hydrogenating a large scope of ketones ranging from aliphatic to aromatic ketones. A dihydride catalytic cycle has been proposed and supported with in-situ low-resolution mass spectrometry data.