Octahedral Werner complexes with substituted ethylenediamine ligands: a stereochemical primer for a historic series of compounds now emerging as a modern family of catalysts

Abstract

As reported by Alfred Werner in 1911-1912, salts of the formally D symmetric [Co(en)] (en = ethylenediamine) trication were among the first chiral inorganic compounds to be resolved into enantiomers, the absolute configurations of which are denoted Λ (left handed helix) or Δ (right handed helix). After a >100 year dormant period during which few useful reactions of these substitution inert complexes were described, carbon substituted derivatives have recently been found to be potent catalysts for enantioselective organic synthesis. This review systematically outlines the fascinating range of stereoisomers that can arise, such as conformers associated with the five membered chelate rings (λ/δ), alignment modes of the C-C bonds with the C symmetry axis (lel/ob), geometric isomers (fac/mer), and configurational diastereomers (R/S) arising from carbon stereocenters. These analyses demonstrate a profound stereochemical diversity that can be applied in catalyst optimization. Efforts are made to bridge the often orthogonal nomenclature systems inorganic and organic chemists employ to describe these phenomena.

Naked d-orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3+2] cycloaddition

“…In contrast to the classical symmetric octahedral Ni(II) catalysts (Fig. 1a), the Ni(II) catalyst reported here exhibits ( Λ )-chirality at the Ni(II) center1920212223, in which the coordination about the nickel atom is a distorted octahedron (Fig. 1b).…”

“…2b). In contrast to chiral nickel(II)–diamine catalysts identified so far7891011 (Supplementary Figs 3–5), the newly developed complexes are chiral-at-Ni(II) center1920212223, in which the initial C 2 -symmetry of the ligands on I and II is desymmetrized through formation of the Ni(II) complex. Mononuclear Ni(II) complex I has a distorted octahedral architecture; the atomic distance between Ni(II) and N(2) (2.109(1) Å) is longer than that of Ni(II)–N(1) (2.091(1) Å), and the N(2)–Ni(II)–O(4) angle is 155.99(5)°, which differs markedly from 180°; we show the longer Ni(II)–N(2) bond in pseudoapical position in Fig.…”

“…The presence of naked d -orbital interacting with the labile acetate ligand allows this acetate ligand to act as a Brønsted base to generate the ( Λ )-Ni(II)–enolate. Furthermore, the bifunctional catalytic motif that enables the merger of H-bonding activation with amine ligand and enolate formation provides a mechanistic basis to expand the potential utility of ligand-induced octahedral metal centrochirality1920212223. The operational simplicity and the easy tunability of the present catalytic system holds a vast potential for designing chiral chemospecific spaces with high predictability.…”

A Missing Structural Link in the Werner Series of Cobalt(III) Complexes: The Crystal Structure of Δ‐(–)589‐Tris(ethylenediamine)cobalt(III) Iodide Monohydrate

“…The correct configuration as shown in Figure is Δ(λλδ) according to IUPAC convention, or the lel 2 ob form of the three en ligands is present. This is opposite from the Λ‐[Co(en) 3 ](I) 3 · H 2 O enantiomer with (δδλ) configuration, meaning the lel 2 ob form of the three en ligands is present , . This is not unexpected as these complexes are supposedly non‐superimposable mirror images, and the assigned ligand configurations in the crystal structures indeed do show they are truly enantiomers of one another and not diastereomers …”

“…In his classic study, Alfred Werner, the father of coordination of chemistry, described the resolution of the D 3 symmetric [Co(en) 3 ] X 3 coordination compound . Regardless of the outer sphere anions (e.g., X = Cl – , Br – or I – ) the complex cation, [Co(en) 3 ] 3+ , can have absolute configurations denoted as either Δ (right‐handed helix) or Λ (left‐handed helix) . The resolution of these particular enantiomers was accomplished by a reaction with l ‐(+)‐tartaric acid to form a pair of diastereomers, Λ‐(+)‐[Co(en) 3 ]( l ‐(+)‐tartrate)Cl (s) and Δ‐(–)‐[Co(en) 3 ]( l ‐(+)‐tartrate)Cl (aq) , which have differing water solubilities.…”