Diastereoselective Assembly of Helicates Incorporating a Hexadentate Chiral Scaffold

Abstract

The condensation of enantiopure (1R,2R)‐(–)‐1,2‐diaminocyclohexane with two equivalents of 2,2′‐bipyridine‐6‐carbaldehyde followed by reduction with NaBH4 allows the facile synthesis of the enantiopure hexadentate ligand (R,R)‐3. Each tridentate metal‐binding domain in (R,R)‐3 exhibits a degree of flexibility arising from inversion at the amine nitrogen centre and rotation about N–C and C–C single bonds. This leads to the formation of either M‐[M{(R,R)‐3}]2+ or P‐[M{(R,R)‐3}]2+. The solid‐state structures of the copper(II), iron(II) and zinc(II) complexes of (R,R)‐3 all show a preference for M‐[M{(R,R)‐3}]2+. In solution, [Fe{(R,R)‐3}]2+ exists predominantly as one diastereoisomer (assumed to be the M‐form). The preference for the M‐ over P‐form is rationalized in terms of a favourable anti configuration of the NH and cyclohexane‐1,2‐diyl CH hydrogen atoms in the M‐form, and an unfavourable syn‐configuration in the P‐form. When (R,R)‐4, the Schiff base analogue of (R,R)‐3, combines with zinc(II) or silver(I), [2+2] double helicates with M‐chirality assemble in the solid state. With (S,S)‐4, iron(II) also assembles into a dinuclear, double helicate with M‐handedness. Complete stereoselectivity is observed in solution with NMR spectroscopic data indicating the presence of one diastereoisomer. The preference for a dinuclear over mononuclear helicate can be traced to a decrease in ligand flexibility on going from 3 (saturated backbone) to 4 (unsaturated imine backbone).

Synthesis and Characterization of a Zinc(II) Complex of Bispicen

“…In the zinc(II) complexes derived from the related ligands N,N -bis(2,2 -bipyridin-6-yl)methyl)-cyclohexane-1,2-diamine [23] and N,N -bis(quinolin-2-yl-)methyl)cyclohexane-1,2-diamine [24] also relatively strong Zn(II)-N bonds are generated, with similar bond lengths as those found in the present case.…”

Chiroptical Spectroscopic Studies on Lanthanide Complexes with Valinamide Derivatives in Solution

“…The luminescence properties and chiroptical spectra of these complexes in acetonitrile are quantitatively discussed with some spectral data of gadolinium complexes (abbreviated as S ‐/ R ‐EuL val ). For comparison of the photophysical behavior, hexadentate chiral complexes with Eu (abbreviated to EuL me ) were also used, which have two bipyridine moieties bridged by (+/−)2‐methyl‐ethylenediamine . Luminescence quantum yields and lifetimes were observed in solutions, and are discussed with regard to chiroptical behavior such as CD localized on the ligand, and CPL on the ff‐transitions.…”

From Mesocates to Helicates: Structural, Magnetic and Chiro‐Optical Studies on Nickel(II) Supramolecular Assemblies Derived from Tetradentate Schiff Bases

“…[16] Ligands must be chosen carefully to preparec ompounds of this kind because they must have the ability to link different metal centers in spite of chelating asingle cation. [17] Usually,a ll the coordination sites around the metal are filled by the ligands, resulting in double helicates ([M 2 L 2 ] n + )w hen the bis-bidentate ligands react with cations that prefer at etrahedral environment, or when the bis-tridentate ligands reactwith cations that prefer aoctahedral environment.W hen pyridyl/imine Schiff bases with an ethylene spacer are employed as ligands, systematic characterization of [M 2 L 2 ] n + helicates have been reported and, in both cases, the bidentate or tridentate units around the same cation have an ideal 908 angle between them.F urthermore, in both cases, the torsion angle subtended by the NCCN atoms of the flexible spacer typically lies around6 0 8,a ss hown in the analysiso ft he 30 reported structures with pyridyl/imine ligands and tetrahedral Cu I , [18][19][20][21][22][23][24][25][26][27][28][29] Ag I , [24,25,[30][31][32][33][34][35] or with bipyridyl/imine ligandsa nd octahedral Zn II ,C u II or Fe III cations, [28,36,37] Scheme 1(a) and (b). In this sense, the first-rowt ransition metals have been speciallys tudied, although structures with other transition metals or even quadruple helicates with rare earths have also been reported.…”