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

Three complexes were synthesized in high yields by reaction of Rh-2(O2CCH3)(4) with 2-picoline (1), 3-picoline (2) and 4-picoline (3), respectively, and characterized by elemental analysis, ESI+-MS, FT-IR and H-1 NMR along with single-crystal X-ray structural analysis. All picoline ligands coordinate to the axial sites of Rh-2(O2CCH3)(4) via the pyridine nitrogen atoms, and interestingly, the coordination of 2-picoline in 1 is assisted by two intramolecular C-H center dot center dot center dot O hydrogen bonds formed between the methyl of 2-picoline and the oxygen atoms of Rh2(O2CCH3)(4). Moreover, the intermolecular C-H center dot center dot center dot 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 Rh-2(O2CCH3)(4) by changing its axial ligands. (C) 2015 Elsevier B.V. All rights reserved.