Stereochemical Influence of the Ligand on the Structure of Manganese Complexes: Implications for Catalytic Epoxidations

Manganese complexes of the ligand HphoxCOOR (R = H or Me) have been synthesized and characterized by
X-ray analysis, ESI-MS, ligand-field spectroscopy, electrochemistry, and paramagnetic 1H NMR. The ligands, chirally
pure or racemic, influence the structures of the complexes formed. Manganese(III) complexes of the ligand
HphoxCOOMe are square-pyramidal or octahedral with two ligands bound in a trans fashion in the solid state. The
racemic ligand (RS-HphoxCOOMe) as well as the enantiopure ligand (R-HphoxCOOMe) forms manganese complexes
with similar solid-state structures. Ligand-exchange reactions occur in solution giving rise to meso complexes as
confirmed by ESI-MS and deuteration studies. The manganese(III) complex of R-HphoxCOOH is octahedral, with
two dianionic ligands bound in a fac-cct fashion in a tridentate manner. The manganese(III) complex of RS-HphoxCOOH is also octahedral with two dianionic ligands now bound in a trans fashion in a didentate manner and
with two water molecules occupying axial sites. The paramagnetic 1H NMR spectra of the complexes have been
interpreted on the basis of the relaxation times with the help of the inversion-recovery pulse technique. The binding
of imidazole with the metal center depends on the chirality of the ligands in the metal complexes of HphoxCOOMe.
Imidazole coordination was found to occur with the metal complex that contains two ligands with the same chirality
(R and R) (R-1), while no imidazole coordination was found upon reaction with the metal complex that contains two
ligands with opposite chirality (R and S) (RS-1). Epoxidation reactions of various alkenes with H2O2 as the oxidant
reveal that the complexes give turnover numbers in the range of 10−35, the epoxide being the major product. The
catalytic activity depends on the additives used, and a clear base effect is observed. The turnover numbers have
been found to be higher in the complexes where no binding of N-Meim is observed. The latter fact unambiguously
shows that imidazole binding is not a prerequisite for higher turnover numbers, in contrast to the Mn−Schiff base
catalysts.