Eugenol-Derived Bipyridine Ligand Scaffolds: Design, Metal Coordination, and Catalytic Applications in Cyclic Carbonate Synthesis

This work reports the synthesis of bis(4-allyl-2-methoxyphenyl)
[2,2′-bipyridine]-5,5′-dicarboxylate (bpyEu) (1) derived from the conjugation of 2,2′-5,5′-dicarboxylic
acid with the natural product eugenol. This hybrid bis-eugenol ligand
integrates π-conjugated bipyridine with a phenolic ether moiety.
After successful multigram synthesis of this bipyridine ligand 1, its metalation was explored by reacting with M(CO)5X, where M = Mn, Re and X = Br, Cl, resulting in the formation
of M(bpyEu)(CO)3X, where M = Mn, X = Br (2); M = Re, X = Cl (3). The hybrid ligand (1) and its transition metal complexes 2 and 3 were thoroughly characterized by different spectroscopic techniques,
such as NMR and Fourier transform infrared (FTIR) spectroscopy, which
confirmed their formation, and their molecular structures were established
using single-crystal XRD in the solid state and mass spectrometry
in solution. The electrochemical behaviors of 2 and 3 were further investigated by cyclic voltammetry. Compound 2 exhibited reversible reductions at −0.828 V and −1.493
V with an irreversible oxidation at +1.44 V, whereas complex 3 displayed a reversible reduction at −0.66 V and −1.08
V along with an irreversible metal-centered oxidation at +1.69 V.
Further, we used bis-eugenol-based ligand 1 as an organocatalyst
for the cycloaddition of CO2 with epoxides, which showed
excellent selectivity for cyclic carbonate synthesis without the need
for a cocatalyst. The catalytic activities of complexes 2 and 3 were also evaluated for cyclic carbonate synthesis,
which revealed that these metal complexes lacked the catalytic ability
for this transformation.