New Set of Multicomponent Crystals as Efficient Heterogeneous Catalysts for the Synthesis of Cyclic Carbonates

Three new multicomponent
crystals 1a–1c of
Zn­(II), Mn­(II), and Co­(II), respectively, were synthesized by the
reaction of 2,6-bis­(hydroxymethyl)­pyridine, the respective metal salts,
and sodium benzoate in a 1:1:2 ratio. One component of these multicomponent
crystals 1a–1c is the dicationic 2,6-bis­(hydroxymethyl)­pyridine
metal complex and the other component is the dianionic tetrabenzoate
complex of the same metal. The complexes were fully characterized
by single-crystal X-ray structure determination. The X-ray structure
of these compounds 1a–1c reveals the formation
of 1D supramolecular chain parallel to the crystallographic b axis via H-bonding interactions between the dicationic
and dianionic parts of the respective compound. The Mn­(II) (1b) and Co­(II) (1c) complexes show antiferromagnetic
coupling between the two associated metal centers via the H-bonding
interaction pathway. All the three compounds 1a–1c were tested as heterogeneous catalytic systems for the successful
conversion of epoxides to cyclic carbonates in solvent-free condition
under approximately 10 bar of pressure of CO2 and temperature
ranging between 60 and 80 °C along with tetrabutyl ammonium bromide
acting as a cocatalyst. All the three compounds 1a–1c were found to have turnover number more than 1000 for the respective
epoxides except for the conversion of cyclohexene oxide to cyclohexene
carbonate.