Synthesis, crystal structures and catalytic activity of tetrakis(acetato)dirhodium(II) complexes with axial picoline ligands

Abstract

Three complexes were synthesized in high yields by reaction of Rh2(O2CCH3)4 with 2-picoline (1), 3-picoline (2) and 4-picoline (3), respectively, and characterized by elemental analysis, ESI+-MS, FT-IR and 1H NMR along with single-crystal X-ray structural analysis. All picoline ligands coordinate to the axial sites of Rh2(O2CCH3)4 via the pyridine nitrogen atoms, and interestingly, the coordination of 2-picoline in 1 is assisted by two intramolecular C–H⋯O hydrogen bonds formed between the methyl of 2-picoline and the oxygen atoms of Rh2(O2CCH3)4. Moreover, the intermolecular C–H⋯O interactions play the main role in the structural stacking of 1–3. Their catalytic activity was evaluated in the C–H insertion reactions for the preparation of 4-nitrobenzyl-(4R,5R,6S)-6-((R)-1-hydroxyethyl)-4-methyl-3,7-dioxo-1-azabicyclo [3.2.0] heptane-2-carboxylate, a key intermediate of Meropenem. The isolated yields for 1, 2 and 3 are 44%, 16% and 22%, respectively, significantly lower than the value of Rh2(O2CCH3)4 (73%), indicating that the axial ligands have negative but different influence on the catalytic activity. The activities of 1–3 are related to the displacement rate of the axial ligands, and essentially related to the Rh–N bond lengths which strong affect the displacement rate. Therefore, it is possible to tune the catalytic activity of Rh2(O2CCH3)4 by changing its axial ligands.