First‐Row Transition Metals Complexes with Fused Oxazolidine (FOX) Ligands

Abstract

In this comprehensive study, we report the syntheses and crystal structures of 6 fused oxazolidine (FOX) bicycles and 22 complexes with FOX ligands. The structures are compared to the few FOX complexes found in literature. For our investigations, we developed synthetic strategies to selectively synthesize chiral or achiral FOX bicycles with pyridyl substituents. The coordination chemistry of the chiral and achiral FOX compounds with first‐row transition metals is discussed in an in‐depth analysis of the crystal structures. We found that the binding modes of the ligands are dependent on the configuration of the FOX backbone and on the steric demand of the substituents which were introduced to the pyridyl groups. The configuration of the FOX ligands is retained during the coordination of metal(II) ions such as Mn2+, Fe2+, Co2+, Ni2+, Cu2+. Reactions of these metal ions with chiral FOX ligands lead to the formation of chiral complexes. In contrast, stronger Lewis acidic metal(III) ions, such as Fe3+ or Al3+, induce an isomerization of the chiral FOX ligands to their achiral diastereomers.

An Iron-Based Dehydration Catalyst for Selective Formation of Styrene

“…A similar difference was previously reported for the axial and equatorial M–X bonds in [M­(X) 2 (FOX)] (M­(X) 2 = Mn­(Br) 2 , Fe­(Cl) 2 , Fe­(Br) 2 , or Co­(Br) 2 ) . Methanol was found to preferentially occupy the axial positions in the complexes [Co­(Cl)­(CH 3 OH)­(FOX)]­[Cl] and [Ni­(Cl)­(CH 3 OH)­(FOX)]­[Cl] . Since the axial position is sterically less shielded, it should also provide more space for the relatively bulky cyclohexyl moiety.…”

“…This was surprising since the equatorial Fe–OTf bond length of 2.06 Å in A is much shorter than the axial Fe–OTf bond length of 2.24 Å. A similar difference was previously reported for the axial and equatorial M–X bonds in [M­(X) 2 (FOX)] (M­(X) 2 = Mn­(Br) 2 , Fe­(Cl) 2 , Fe­(Br) 2 , or Co­(Br) 2 ) . Methanol was found to preferentially occupy the axial positions in the complexes [Co­(Cl)­(CH 3 OH)­(FOX)]­[Cl] and [Ni­(Cl)­(CH 3 OH)­(FOX)]­[Cl] .…”

“…The tetradentate FOX ligand inhibits the aggregation of the iron ions in the presence of water and improves their solubility in organic media. It also favors the formation of octahedral high-spin complexes, such as A or its previously published chloride and bromide analogs, with two neighboring reactive sites …”

“…It also favors the formation of octahedral high-spin complexes, such as A or its previously published chloride and bromide analogs, with two neighboring reactive sites. 15 Reaction Optimization. NMR experiments showed that the conversion of 1 catalyzed by A occurs at relatively constant rates.…”

“…A similar difference was previously reported for the axial and equatorial M−X bonds in [M(X) 2 (FOX)] (M(X) 2 = Mn(Br) 2 , Fe(Cl) 2 , Fe(Br) 2 , or Co(Br) 2 ). 15 Methanol was found to preferentially occupy the axial positions in the complexes [Co(Cl 15 Since the axial position is sterically less shielded, it should also provide more space for the relatively bulky cyclohexyl moiety. However, the equatorial displacement appears to be thermodynamically favored in [Fe(OTf) 2 (FOX)] as the Fe−O bond strength is increased in this position.…”