Metal complexes of chiral oligopyridines and chiral bridged oligopyridines : syntheses, characterizations and applications in catalytic epoxidation

Abstract

Oligopyridine, a subgroup of diimine ligands, is one of the most important classes of binding motif for metal complex formation. This thesis describes the synthesis of chiral oligopyridines and bridged chiral oligopyridine metal complexes, and their use in epoxidation. Two series of manganese(II)–oligopyridine complexes were synthesized by reacting MnCl2 with chiral terpyridines L1–3 and quaterpyridines L4–6. The complexes were characterized by elementary analysis and ESI-MS. The manganese mononuclear complexes were confirmed to be of formula [Mn(L)Cl2]. Their electronic absorption spectra in dichloromethane solution are characterized by the intraligand transition at 230−370 nm. Compared with the terpyridine complexes, quaterpyridine complexes generally show a relatively larger CD absorption. The results suggest there is probably more twisting of structure in the quaterpyridine complexes. Xray diffraction methods show [Mn(L2)Cl2] and [Mn(L6)Cl2] to have a distorted trigonal bipyramidal geometry and a distorted skewed-trapezoidal bipyramidal geometry, respectively. In the quaterpyridine, flipping between the pyridyl rings, with an overall twist of 40.3o, causes the manganese complex to bend like a “bowl”. The bending induces an asymmetric secondary structure which may account for the relatively strong signal in the CD experiment. The manganese–terpyridine complexes are active catalysts in the epoxidation of styrene with peracetic acid as oxidant, with yields up to 92% in 3 min at room temperature. The results show that bulkiness of the ligands play very important roles in the reactivity of the manganese complexes. On the other hand, although the manganese–quaterpyridine complexes are not active epoxidation catalysts with peracetic acid, they bring about a 10 times increase in rate of epoxidation of styrene by mCPBA with more than 80% conversion in 5 min. Iron–oligopyridine complexes were synthesized by reacting FeCl2 with chiral terpyridines L1–3 and quaterpyridines L4–6. The formulas of the iron mononuclear complexes were confirmed to be [Fe(L1)2Cl2] and [Fe(L)Cl2] (L2–5). By reacting L4 with FeCl3, an iron complex of formula [Fe(L4)Cl2]ClO4 was synthesized after metathesis. The complexes were characterized by elementary analysis and ESI-MS. The electronic absorption spectra of the iron(II) complexes in dichloromethane solution show dominant intraligand transition at 230−350 nm. Compared with the terpyridine complexes, the iron quaterpyridines show larger CD absorptions. The results suggest the quaterpyridine complexes may have more twisting of structure. Structural characterization by X-ray crystallography show [Fe(L3)Cl2] to have a distorted triganol bipyramidal geometry, and [Fe(L6)Cl2] and [Fe(L4)Cl2]ClO4 to be distorted octahedrals. In the structure of [Fe(L6)Cl2], there is flipping between the pyridyl rings of the quaterpyridine, likely accounting for the relatively strong signal in the CD experiment. The iron(II)–oligopyridine complexes are active catalysts in the epoxidation of styrene with oxone as oxidant. The terpyridine complexes give quantitative conversions of styrene with epoxide yields up to 87% in 5 min at room temperature. Under the same reaction condition, the quaterpyridine complexes are less reactive, giving up to only 80% conversion with about 70% yield in more than 30 min reaction time. New chiral bridged oligopyridine ligands were formed by linking chiral bipyridines with bis–phenyl spacers and their complexes were formed by reacting with iron(II) and cobalt(II) salts. With FeCl2, L7–9 formed dinuclear complexes with the formulas of [Fe2O(L7)Cl4] and [Fe2(L)Cl4] (L8–9). With Fe(ClO4)2, L7 formed complexes with the formula of [Fe2(L7)2](ClO4)4. When L7 was reacted with Co(OAc)2, a double-stranded helicate with formula [Co2(L7)2(MeOH)2(OAc)2](PF6)2 was formed. These complexes were characterized by ESI–MS, UV and elementary analysis. Circular dichroism analyses reveal the helical structures of these complexes. The structure of [Co2(L7)2(MeOH)2(OAc)2](PF6)2 was determined by X–ray crystallography. This octahedral complex is a two–metal, two–ligand double–helical structure. Using hydrogen peroxide as oxidant, the single helical iron complexes are active in epoxidation, and the best result was obtained by [Fe2O(L7)Cl4], which enabled epoxidation of styrene in 5 min, with good selectivity (82%). The Iron doublestranded helicate [Fe2(L7)2](ClO4)2 is shown to be inactive.