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Cis-Locked Ru(II)-DMSO Precursors for the Microwave-Assisted Synthesis of Bis-Heteroleptic Polypyridyl Compounds.

ABSTRACT: We describe a synthetic strategy for the preparation of bis-heteroleptic polypyridyl Ru(II) complexes of the type [Ru(L1)₂(L2)]²⁺ (L1 and L2 = diimine ligands) from well-defined Ru(II) precursors. For this purpose, a series of six neutral, anionic, and cationic cis-locked Ru(II)-DMSO complexes (2-7) of the general formula [Y] fac-[RuX(DMSO-S)₃(O-O)]ⁿ (where O-O is a symmetrical chelating anion: oxalate (ox), malonate (mal), acetylacetonate (acac); X = DMSO-O or Cl^-; n = -1/0/+1 depending on the nature and charge of X and O-O; when present, Y = K⁺ or PF₆^-) were efficiently prepared from the well-known cis-[RuCl₂(DMSO)₄] (1). When treated with diimine chelating ligands (L1 = bpy, phen, dpphen), the compounds 2-7 afforded the target [Ru(L1)₂(O-O)]^0/+ complex together with the undesired (and unexpected) [Ru(L1)₃]²⁺ species. Nevertheless, we found that the formation of [Ru(L1)₃]²⁺can be minimized by carefully adjusting the reaction conditions: in particular, high selectivity toward [Ru(L1)₂(O-O)]^0/+ and almost complete conversion of the precursor was obtained within minutes, also on a 100-200 mg scale, when the reactions were performed in absolute ethanol at 150 °C in a microwave reactor. Depending on the nature of L1 and concentration, with the oxalate and malonate precursors, the neutral product [Ru(L1)₂(O-O)] can precipitate spontaneously from the final mixture, in pure form and acceptable-to-good yields. When spontaneous precipitation of the disubstituted product does not occur, purification from [Ru(L1)₃]²⁺ can be rather easily accomplished by column chromatography or solvent extraction. By comparison, under the same conditions, compound 1 is much less selective, thus demonstrating that locking the geometry of the precursor through the introduction of O-O in the coordination sphere of Ru is a valid strategic approach. By virtue of its proton-sensitive nature, facile and quantitative replacement of O-O in [Ru(L1)₂(O-O)]^0/+ by L2, selectively affording [Ru(L1)₂(L2)]²⁺, was accomplished in refluxing ethanol in the presence of a slight excess of trifluoroacetic acid or HPF₆.

SUBMITTER: Vidal A

PROVIDER: S-EPMC8154425 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Background?-Bromination of the side chain of aromatic ketones using NBS in the presence of p-toluenesulfonic acid (p-TsOH) in acetonitrile is very common. However, regioselective bromination of bis and tris(?-bromoacetophenones) with NBS in the presence of p-TsOH in acetonitrile under microwave irradiation is quite novel. The bis- and tris(?-bromoacetophenones) are used in synthesis of bis and tris(heterocycles). bis(heterocycles) have received a great deal of attention, because many biologically active natural and synthetic products have molecular symmetry. The use of the pressurized microwave irradiation is very advantageous to many syntheses and provide a large rate enhancement.ResultsBis and tris(?-bromoacetophenones) were obtained as single monobrominated derivatives in a shorter time than the conventional conditions. The results clearly demonstrate the better reactivity and selectivity of NBS/p-TsOH/CH3CN as a brominating mixture under microwave conditions. The reaction of bis and tris(?-bromoacetophenone) with 2-aminopyridine and 2-aminopyrimidine proceeded smoothly in a mixture of anhydrous ethanol and DMF under reflux or using 300 W/105°C/ 20 min microwave irradiation conditions to afford the corresponding bis(imidazo[1,2-a]pyridine), bis(imidazo[1,2-a]pyrimidine) and tris(imidazo[1,2-a]pyridine) derivatives in moderate to excellent yields. The carbonyl analogue of the targeted bis(imidazopyridines) could be synthesized by the reaction of N,N-dimethyl-N'-(pyridin-2-yl)formimidamide with bis(?-bromoacetophenone) in refluxing ethanol. The structures of the newly synthesized compounds were confirmed by their spectral data as well as their elemental analyses.ConclusionIn conclusion, selective ?-bromination of bis- and tris(acetophenones) has been accomplished efficiently utilizing NBS/p-TsOH/CH3CN under microwave irradiation. In addition, a facile synthesis of novel series of bis- and tris(imidazopyridine) and bis(imidazopyrimidine) derivatives.

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Cis-Locked Ru(II)-DMSO Precursors for the Microwave-Assisted Synthesis of Bis-Heteroleptic Polypyridyl Compounds.

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