Nitrogen- and phosphine-donor ruthenium(II/III) and palladium(II) complexes : synthesis and catalytic hydrogenation of ketones, alkenes and alkynes.

Abstract

The ligands, 2-(2-pyridyl)benzimidazole (L1), 2-(2-pyridyl)bezothiazole (L2), and 2-(2-pyridyl)bezoxazole (L3) were prepared from o-phenylenediamine, o-aminothiophenol and 2-aminophenol, respectively, following literature procedures While alkylation of L1 using bromopropane gave ligand 1-propyl-2-(pyridin-2-yl)-1H-benzoimidazole (L4). Ligands, 2,6-bis(3,5-dimethylpyrazolylmethyl)pyridine (L5) and 2,6-bis(3,5-diphenylpyrazolylmethyl)pyridine (L6) were prepared by phase transfer alkylation of 2,6-bis(chloromethyl)pyridine with one mole equivalents of the appropriate pyrazole. Reactions of ligands L1-L4 with RuCl₃ .3H₂O produced the corresponding compounds [RuCl₃(L1)] (1)], [RuCl₃(L2)] (2), [RuCl₃(L3)] (3)] and [RuCl₃(L4)] (7)], respectively. Likewise, reaction of ligands L1-L3 with RuCl₂(PPh₃)₃ gave [RuL1(PPh₃)₂Cl₂] (4), [RuL2(PPh₃)₂Cl₂] (5) and [RuL3(PPh₃)₂Cl₂] (6). Complexes 1-7 were characterized by mass spectrometry, elemental analysis, ¹H, ¹³C and ³¹P NMR spectroscopy and single X-ray analyses. They were then evaluated as catalysts for the transfer hydrogenation of ketones. The catalytic activity of the ruthenium(III) complexes, 1-3 and 7 is believed to be a result of their coordinative unsaturation while the high lability of the coordinated PPh₃ ligands are responsible for the catalytic activity of ruthenium(II) complexes, 4-6, in the transfer hydrogenation of ketones. Ruthenium(II) catalysts bearing PPh₃ ancillary ligands formed more active systems than the corresponding ruthenium(III) trichloride counterparts. Complexes bearing benzimidazole ligand L1 were more active than those of the respective benzothiazole (L2) and benzoxazole (L3). DFT studies show that the catalytic activities of the complexes depend on the dipole moment of the resultant catalysts.

The benzoazole complexes [PdCl₂(L1)] (8), [PdCl₂(L2)] (9), [PdCl₂(L3)] (10) and [PdClMe(L1)] (11) were prepared by reacting the corresponding ligands L1-L3 with either [PdCl₂(NCMe)₂] or [PdClMe(COD)]. Treatment of complex 8 with one molar equivalent of PPh₃ and PPh₃/NaBAr₄ produced the cationic complexes, [Pd(L1)ClPPh₃]Cl (12) and [Pd(L1)ClPPh₃]BAr₄ (13), respectively. Similarly, neutral pyrazolyl palladium complexes, [PdCl₂(L5)] (14), [PdCl₂(L6)] (15) and [PdClMe(L5)] (16) were prepared by reaction of ligands L5 and L6 with [PdCl₂(NCMe)₂] or [PdClMe(COD)]. The corresponding cationic complexes [Pd(L5)ClPPh₃]Cl (17) and [Pd(L5)ClPPh₃]BAr₄ (18) were prepared by reaction of complex 14 with equimolar amounts of PPh₃ and PPh₃/NaBAr₄, respectively. The synthesized compounds were characterized by a combination of ¹H, ¹³C and ³¹P-NMR spectroscopy, microanalyses and single X-Ray analyses for complexes 13 and 18. Ligands L1 and L5 bind to the Pd metal centre in a bidentate mode as confirmed by the molecular structures of complexes 13 and 18. Complexes 8-18 were applied as catalysts in the high pressure hydrogenation of alkenes and alkynes. The catalytic activities of complexes 8-18 were controlled by complex structure, catalyst concentration, hydrogen pressure, time and type of substrate. While the cationic benzoazole complexes [Pd(L1)PPh₃Cl]Cl (12) and [Pd(L1)PPh₃Me]BAr₄ (13) were generally less active compared to the neutral Pd(II) complexes, the cationic pyrazolyl palladium complexes [Pd(L5)ClPPh₃]Cl (17) and [Pd(L5)ClPPh₃]BAr₄ were relatively more active than the corresponding neutral complexes. Generally, the benzoazole complexes 8-13 were more active compared to the pyrazolyl complexes, 14-18.

Conjugated substrates were more reactive than non-conjugated substrates. The hydrogenation of aliphatic alkenes and alkynes were accompanied by isomerization reactions and higher activities were reported in the hydrogenation of alkynes compared to the corresponding alkenes. A mercury poisoning test on the complexes established that the catalytic systems were heterogeneous in nature. Further TEM analyses revealed that indeed the Pd(II) complexes 8-18 formed nanoparticles to produce heterogeneous hydrogenation systems. The kinetic of hydrogenation reactions indicated that the reactions are pseudo first order with respect to the catalyst and substrate. Catalyst loading, hydrogen pressure and temperature also influenced the activity of the catalysts. DFT studies further showed that the activity trend of complexes 8-18 depended on the average {(NPy +Nbz/pz)/2} bond lengths and the HOMO-LUMO energy gaps. The activities generally decreased with the increase in {(NPy +Nbz/pz)/2} bond lengths and decrease in the HOMO-LUMO energy gap. To bridge the gap between homogeneous and heterogeneous catalysis, water soluble ruthenium(II) complexes were synthesized and used in biphasic olefin hydrogenation reactions. Reactions of ligands L1-L3 with [η⁶-(2-phenoxyethanol)RuCl₂]₂ dimer afforded the respective cationic 2-(2-pyridyl)benzoazole)ruthenium(II) complexes: [η⁶-(2-phenoxyethanol)RuCl(L1)]Cl (19), [η⁶-(2-phenoxyethanol)RuCl(L2)]Cl (20) and [η⁶-(2-phenoxyethanol)RuCl(L3)]Cl (21) in high yields. The complexes were characterized by a combination of ¹H and ¹³C-NMR spectroscopy, microanalyses and X-ray crystallography for complexes 19-21.

Solid state structures of complexes 19-21 confirmed the bidentate coordination modes of ligands L1-L3 and the formation of cationic species through displacement of one chloride ligand from the Ru(II) coordination sphere. Complexes 19-21 were found to form active catalysts for the high pressure hydrogenation of alkenes and alkynes in toluene and biphasic media. Hydrogenation of alkynes produced a mixture of alkanes and alkenes. The complexes were recyclable and retained significant catalytic activities after six cycles. Reaction parameters such as substrate/catalyst ratio, temperature, and aqueous/organic ratio affected the catalytic trends.