Chiral superstructures from homochiral Zn2+, Co2+, Fe2+-2,6-bis (aryl ethylimine)pyridine complexes

Received: 29 April 2019

DOI: 10.1002/chir.23115

Revised: 15 June 2019

Accepted: 6 July 2019

S P E C I A L I S S U E A R T I C L E

Chiral superstructures from homochiral Zn²⁺, Co²⁺, Fe²⁺‐

2,6‐bis (aryl ethylimine)pyridine complexes

Florina Dumitru^1,2| Arie van der Lee¹| Mihail Barboiu¹

¹Institut Européen des Membranes, UMR‐

Abstract

CNRS 5635, Université Montpellier,

We report the hierarchical supramolecular organization of metallosupra‐

ENSCM Place eugene Bataillon CC047,

molecular homochiral complexes 1‐Λ‐(S,S,S,S)‐M²⁺/1‐Δ‐(R,R,R,R)‐M²⁺and 2‐

Montpellier, France

²Faculty of Applied Chemistry and

Λ‐(S,S,S,S)‐M²⁺/2‐Δ‐ (R,R,R,R)‐M²⁺of M²⁺= Co²⁺, Fe²⁺, Zn²⁺metal ions

Materials Science, Department of

with chiral pseudo‐terpyridine‐type ligands: 1‐(S,S) or 1‐(R,R) = 2,6‐bis

Inorganic Chemistry, Physical Chemistry

and Electrochemistry, University

(naphthyl ethylimine)pyridine and 2‐(S,S) or 2‐(R,R) = 2,6‐bis (phenyl‐

ethylimine)pyridine. Circular dichroism measurements in solution were used

Politehnica of Bucharest, Bucharest,

Romania

to confirm the enantiomeric nature of all twelve complexes. For crystal

structures of 1‐ Λ‐ (S,S,S,S)‐M²⁺or 1‐Δ‐ (R,R,R,R)‐M²⁺complexes, absolute

Correspondence

Mihail Barboiu, Institut Européen des

configurations {Δ (or P), Λ (or M)} were confirmed by refinement of the

Membranes, UMR‐CNRS 5635, Université

Flack parameter x: −0.007 ≤ x ≤ 0.11 for the single crystals of 1‐Λ‐(S,S,S,S)‐

Montpellier, ENSCM Place eugene

M

²⁺/1‐Δ‐ (R,R,R,R)‐M²⁺, 2‐ Λ‐ (S,S,S,S)‐Fe²⁺, and 2‐Δ‐ (R,R,R,R)‐Co²⁺

.

Bataillon CC047, Montpellier, France.

Email: mihail‐dumitru.

barboiu@umontpellier.fr

KEYWORDS

aromatic interactions, chiral single crystals, imine, metallosupramolecular complexes, self‐assembly

Funding information

Agence Nationale de la Recherche, Grant/

Award Number: ANR‐15‐CE29‐0009

DYNAFUN

1 | INTRODUCTION

are obtained by condensation between aldehydes and

chiral primary amines, and, among the most frequently

Chiral symmetry breaking and transfer of chiral infor-

mation from molecular towards supramolecular level

through noncovalent interactions are topics of great

interest. Molecular and supramolecular chirality both

may be used as tools to assemble systems into dissym-

metric crystalline architectures based on selective chiral

packing.¹Chiral metallosupramolecular complexes are

of considerable interest because of their important appli-

cations as stereodynamic probes for chiral sensing,^2-5for

the preparation of chiral catalysts,^6,7and for the devel-

opment of multifunctional materials.⁸In the design

and the synthesis of chiral ligands that, upon coordina-

tion with metal ions, can induce high stereoselectivity at

a supramolecular level, Schiff bases with stereogenic

centres in their backbones have been extensively used

as powerful tools for the spontaneous generation of chi-

ral superstructures. Typically, these chiral Schiff bases

used chiral amines, one can include enantiomeric pairs

of R‐(+)‐1‐/S‐(−)‐1‐phenylethylamine and R‐(+)‐1‐/S‐

(−)‐1‐naphthylethylamine.^9-43

Herein, we report six enantiomeric pairs of M²⁺

=

Zn²⁺, Co²⁺, Fe²⁺mononuclear complexes: 1‐Λ‐(S,S,S,S)‐

²⁺/1‐Δ‐(R,R,R,R)‐M²⁺and 2‐Λ‐(S,S,S,S)‐M²⁺/2‐Δ‐(R,R,

R,R)‐M²⁺

where 1‐(S,S) or 1‐(R,R) are 2,6‐bis

M

,

(naphthylethylimine)pyridine and 2‐(S,S) or 2‐(R,R) =

2,6‐bis (phenylethylimine)pyridine, Schiff bases contain-

ing two stereogenic centres. The metal ions are used to

template the Schiff base formation from R‐(+)‐1‐/S‐(−)‐

1‐phenyl‐ethylamine,

R‐(+)‐1‐/S‐(−)‐1‐naphthylethy‐

lamine, and 2,6‐pyridine‐dicarbox‐aldehyde (Scheme 1).

The resulted pseudo‐terpyridines ligands orthogonally

wrap around the metal ion centres, positioning their four

stereogenic centres such that the metal ions are overally

surrounded by chiral coordination centres, as revealed

Chirality. 2019;1–13.

wileyonlinelibrary.com/journal/chir

1

2

DUMITRU ET AL.

SCHEME 1 One‐pot synthesis of homochiral complexes: A) 1‐Λ‐(S,S,S,S)‐M²⁺/1‐Δ‐(R,R,R,R)‐M²⁺and B) 2‐Λ‐(S,S,S,S)‐M²⁺/2‐Δ‐(R,R,R,

R)‐M²⁺; M²⁺= Zn²⁺, Fe²⁺, Co²⁺

by X‐ray crystal structures and circular dichroism (CD)

spectra.

2 | MATERIALS AND METHODS

The metal ion coordination by chiral molecular ligands

results in the formation of highly compact chiral supramo-

lecular homodimers: 1‐ Λ‐(S,S,S,S)‐M²⁺and 1‐‐Δ‐(R,R,R,

R)‐M²⁺stabilized by strong internal π‐π stacking interac-

tions between lateral aromatic arms and central pyridine

moiety. Further self‐assembly in the resolved solid‐state

homochiral metallosupramolecular domains is observed

in some cases with the formation of unique double‐

stranded monohelices with single handedness. The solid‐

phase homochirality is determined by a subtle interplay

of four directional orthogonal‐pseudo‐terpyridine coordi-

nation geometry and “locked” by weak interactions π‐π/

CH⋯π interactions between peripheral aryl rings.^8,44-46

Usually, such helical metallosupramolecular complexes

crystallize in distinct alternative P and M columns or layers

of Δ or Λ mirror enantiomers, but, overall, the crystals are

racemic. Intermolecular crystal packing is usually not dis-

criminating: A system of enantiomeric complexes evolves

towards solid‐phase homochirality if homochiral interac-

tions between molecules are more stable than heterochiral

interactions,⁴⁷but the greater stability of homochiral ver-

sus heterochiral interactions is a necessary but not a suffi-

cient condition for establishing solid‐phase homochirality.

There are a few previous examples of direct crystalliza-

tion of enantiopure helical supramolecular single

crystals,^48-50more often the crystal is racemic since homo-

chiral layers of opposite chirality could be present and

connected via different chirality inverting interactions.

Examples of homochiral supramolecular helices were

reported by us, in our previous work^51-57on complexes of

Zn²⁺, Co²⁺, Fe²⁺, Pb²⁺metal ions with bis (arene imine)

pyridines ligands, but these examples are exclusively based

on achiral ligands, and the resulted solid‐state chirality is

promoted by constitutional chiral affinity of supramolecu-

lar helices of the same handedness, interacting via their

van der Waals hypersurfaces.^51a

R‐(+)‐1‐/S‐(−)‐1‐phenylethylamine,

R‐(+)‐1‐/S‐(−)‐1‐

naphthylethyl‐amine, 2,6‐pyridinemethanol, MnO₂, Zn

(CF₃SO₃)₂, Fe (BF₄)₂·6H₂O, Co (BF₄)₂·6H₂O, and CD₃CN

were purchased from Aldrich and used as received. All

other reagents were obtained from commercial suppliers

and used without further purification. All organic solu-

tions were routinely dried over molecular sieves 4 Å.

2,6‐Pyridinedicarboxaldehyde was prepared by oxidation

of 2,6‐pyridinemethanol with activated MnO₂, according

to the procedure described in the literature.⁵⁸

Proton nuclear magnetic resonance (¹H‐NMR) spectra

were recorded on DRX 400 MHz Bruker Avance spec-

trometer, in CD₃CN, with the use of the residual solvent

peak as reference. Mass spectrometric studies were per-

formed in the positive ion mode using a quadrupole mass

spectrometer (Micromass, Platform 2+). Samples were

dissolved in acetonitrile and were continuously intro-

duced into the mass spectrometer at a flow rate of 10

mL/min through a Waters 616HPLC pump. The temper-

ature (80 °C) and the extraction cone voltage (V_c= 5‐10

V) were usually set to avoid fragmentations. The nota-

1

tions used for the assignments of the H‐NMR signals

are given below.

DUMITRU ET AL.

3

Ultraviolet‐visible (UV‐vis) absorbance spectra were

recorded using a Kontron Instruments Uvikon 923 spec-

trometer, in acetonitrile 10⁻⁴M to 10⁻⁵M, with acetoni-

trile as a reference. CD spectra were measured on a Jasco

J‐810 spectrometer, with a DC150 W xenon lamp. Mea-

surements were collected using a 1‐mm path‐length

quartz cuvette, and the standard parameters used were

as follows: bandwidth 2 nm, response time 1 second,

wavelength scan range 190 to 600 nm, data pitch 0.2

nm, scanning speed 50 nm·min⁻¹, and accumulation 5.

These data can be obtained free of charge from The Cam-

bridge Crystallographic Data Centre via www.ccdc.cam.

ac.uk/structures.

2.2 | General procedure for the synthesis

of homonuclear complexes

Homochiral complexes 1‐Λ‐(S,S,S,S)‐M²⁺/1‐Δ‐(R,R,R,R)‐

M²⁺and 2‐Λ‐(S,S,S,S)‐M²⁺/2‐Δ‐(R,R,R,R)‐M²⁺M²⁺

=

Zn²⁺, Fe²⁺, Co²⁺have been obtained by template reac-

tion between 2,6‐pyridine‐dicarboxaldehyde (0.148

mmol) and corresponding chiral amines R‐(+)‐1‐ or

S‐(−)‐1‐phenylethylamine and R‐(+)‐1‐ or S‐(−)‐1‐

naphthyl‐ethylamine in the presence of stoichiometric

amounts of Zn (CF₃SO₃)₂, Fe (BF₄)₂·6H₂O, Co

(BF₄)₂·6H₂O, in acetonitrile, in the molar ratio of alde-

hyde:amine:metal salt = 1:2:0.5. The reactions were per-

formed typically on a 10‐mg scale of ligand per millilitre

solvent. The reactants were dissolved in CD₃CN (1 mL)

and stirred overnight at 60 °C. These solutions were mon-

itored by ¹H‐NMR and ESI‐mass spectrometries. Layering

the solutions of complexes in acetonitrile with isopropyl

ether at room temperature resulted in a unique set of sin-

gle crystals suitable for X‐ray single‐crystal experiments.

2.1 | X‐ray single crystal diffraction

structure solution and refinement

Crystal evaluation and data collection were performed on

a Rigaku Oxford‐Diffraction Xcalibur‐I or a Gemini‐S dif-

fractometer with sealed‐tube Mo‐Kα radiation using the

CrysAlis Pro program (Table 1).⁵⁹The same program

was used for the integration of the data using default

parameters, for the empirical absorption correction using

spherical harmonics employing symmetry‐equivalent and

redundant data, and the correction for Lorentz and polar-

ization effects. The crystal structures were solved using

the ab initio iterative charge flipping method with

parameters described elsewhere⁶⁰using the Superflip

program,⁶¹and they were refined using full‐matrix least‐

squares procedures as implemented in CRYSTALS⁶²on

all independent reflections with I > 2σ(I). Special

attention was given to the determination of the absolute

structure of each compound. The compounds 1‐(R,R,R,

R)‐Co²⁺and 1‐(S,S,S,S)Co²⁺crystallize each in an enan-

tiomorphic space group. The structure solution was there-

fore done in each space group of the enantiomorphic pair,

and the space group was chosen on the basis of having

the Flack parameter close to 0.00. The other compounds

crystallize in non‐enantiomorphic Sohncke space groups,

and the structure was inverted if the Flack parameter was

found to be close to 1.0. All final Flack and Hooft param-

eters^61,63-67are very close to 0.00. Following an analysis

based on maximum likelihood estimation and Bayesian

statistics, the chance of having an enantiopure material

is in all cases 100%.⁶⁸The H atoms were all located in a

difference map but repositioned geometrically. They were

initially refined with soft restraints on the bond lengths

and angles to regularize their geometry (C─H in the

range 0.93‐0.98 Å) and U_iso(H) (in the range 1.2‐1.5 times

U_eqof the parent atom), after which the positions were

refined with riding constraints.⁶⁹In some cases, thermal

similarity restraints were used especially for solvent mol-

ecules. In one case, an acetonitrile solvent molecule was

refined as a rigid group. CCDC 1910805‐1910812 contains

the supplementary crystallographic data for this paper.

Complex 1‐Δ‐(R,R,R,R)‐Zn²⁺. Yellow crystals. ¹H‐

RMN (400 MHz, CD₃CN‐d₃, δ) 8.22 (s, 4H; CH═N),

7.82‐7.80 (d, J = 8 Hz, 4H; H^b), 7.70‐7.64 (m, J = 7.2

Hz, J = 8 Hz, 10H; H^a‐Hⁱ‐H^f), 7.62‐7.60 (t, J = 8 Hz,

4H; H^h), 7.56‐7.52 (td, J = 8 Hz, 4H; H^g), 7.27‐7.25 (d, J

= 7.6 Hz, 4H; H^e), 7.07‐7.03 (t, J = 7.6 Hz, 4H; H^d),

6.54‐6.52 (d, J = 6.4 Hz, J = 7.2 Hz, 4H; H^c), 4.76‐4.72

(q, J = 6.8 Hz, 4H; ─NCH─), 1.04‐1.03 (d, J = 6.4 Hz,

12H; CH₃). UV‐vis (acetonitrile 7.425·10⁻⁵M): λmax =

282 nm, 220 nm; MS (ESI, m/z): 473.36 (100) [Zn(1‐Δ‐

(R,R,R,R))₂]²⁺, 1095.72 [Zn(1‐Δ‐(R,R,R,R))₂](CF₃SO₃)⁺.

Complex 1‐Λ‐(S,S,S,S)‐Zn²⁺. Yellow crystals. ¹H‐

RMN (400 MHz, CD₃CN‐d₃, δ) 8.22 (s, 4H; CH═N),

7.82‐7.80 (d, J = 8 Hz, 4H; H^b), 7.70‐7.64 (m, J = 7.2

Hz, J = 8 Hz 10H, H^a‐Hⁱ‐H^f), 7.62‐7.60 (t, J = 8 Hz,

4H; H^h), 7.56‐7.52 (td, J = 8 Hz, 4H; H^g), 7.27‐7.25 (d, J

= 7.6 Hz, 4H, H^e), 7.07‐7.03 (t, J = 7.6 Hz, 4H, H^d),

6.54‐6.52 (d, J = 6.4 Hz, J = 7.2 Hz, 4H; H^c), 4.77‐4.72

(q, J = 6.8 Hz, 4H; ─NCH─), 1.04‐1.03 (d, J = 6.4 Hz

12H, CH₃). UV‐vis (acetonitrile 7.425·10⁻⁵M): λmax =

282 nm, 220 nm; MS (ESI, m/z): 473.36 (100) [Zn(1‐Λ‐

(S,S,S,S))₂]²⁺, 1095.78 [Zn(1‐Λ‐(S,S,S,S))₂](CF₃SO₃)⁺.

Complex 1‐Δ‐(R,R,R,R)‐Fe²⁺. Violet crystals. UV‐

vis (acetonitrile 7.425·10⁻⁵M): λ_max= 281 nm (35.57

cm⁻¹, MLCT d‐π*), 484 nm (20.67 cm⁻¹), 608 nm

(16.45 cm⁻¹), 705 nm (14.2 cm⁻¹). MS (ESI, m/z):

4

DUMITRU ET AL.

5

469.33 (100) [Fe(1‐Δ‐(R,R,R,R))₂]²⁺, 1025.75 [Fe(1‐Δ‐(R,

R,R,R))₂](BF₄)⁺.

Complex 2‐Λ‐(S,S,S,S)‐Co²⁺. Brown crystals. UV‐vis

(acetonitrile 7.425·10⁻⁵M): λ_max= 302 nm (33.06

cm⁻¹, MLCT d‐π*). MS (ESI, m/z): 370.76 (100)

[Co(2‐Λ‐(S,S,S,S))₂]²⁺, 828.77 [Co(2‐Λ‐(S,S,S,S))₂](BF₄)⁺.

Complex 1‐Λ‐(S,S,S,S)‐Fe²⁺. Violet crystals. UV‐vis

(acetonitrile 7.425·10⁻⁵M): λ_max= 281 nm (35.57

cm⁻¹, MLCT d‐π*), 484 nm (20.67 cm⁻¹), 608 nm

(16.45 cm⁻¹). MS (ESI, m/z): 469.46 (100) [Fe(1‐Λ‐(S,S,

S,S))₂]²⁺, 1025.88 [Fe(1‐Λ‐(S,S,S,S))₂](BF₄)⁺.

3 | RESULTS AND DISCUSSION

Complex 1‐Δ‐(R,R,R,R)‐Co²⁺. Brown crystals. UV‐

vis (acetonitrile 7.425·10⁻⁵M): λ_max= 281 nm (35.57

cm⁻¹, MLCT d‐π*). MS (ESI, m/z): 470.88 (100)

Mononuclear complexes 1‐Λ‐(S,S,S,S)‐M²⁺/1‐Δ‐(R,R,R,

R)‐M²⁺and 2‐Λ‐(S,S,S,S)‐M²⁺/2‐Δ‐(R,R,R,R)‐M²⁺were

obtained via template reactions, in the concentration

range of 10 mg ligand per 1 mL acetonitrile, from 2,6‐

pyridinedicarboxaldehyde (1 eq), R‐(+)‐1‐/S‐(−)‐1‐phen-

ylethylamine, and R‐(+)‐1‐/S‐(−)‐1‐naphthyl ethylamine

[Co(1‐Δ‐(R,R,R,R))₂]²⁺

, 1028.78 [Co(1‐Δ‐(R,R,R,R))₂]

(BF₄)⁺.

Complex 1‐Λ‐(S,S,S,S)‐Co²⁺. Brown crystals. UV‐vis

(acetonitrile 7.425·10⁻⁵M): λ_max= 281 nm (35.57 cm⁻¹

,

MLCT d‐π*). MS (ESI, m/z): 470.88 (100) [Co(1‐Λ‐(S,S,S,

(2 eq) and the corresponding metal ions, Zn²⁺, Fe²⁺

,

S))₂]²⁺, 1028.78 [Co(1‐Λ‐(S,S,S,S))₂](BF₄)⁺.

Co²⁺(0.5 eq). The Zn²⁺, Fe²⁺, Co²⁺metal ions used

to obtain complexes with in situ generated bis

(arylethylimine)pyridine ligands 1 and 2 are ions that pre-

fer octahedral coordination geometry and determine the

orthogonal orientation of the ligands similar to that

found in the mononuclear metal complexes with the

terpyridine ligands. The octahedral complexes of the

Zn²⁺, Fe²⁺, and Co²⁺ions are labile and suffer rapid equi-

libria in solution, and their stability can be correlated

with their electronic configuration. On the other hand,

the Schiff bases, bis (arylimine)pyridine ligands 1 and 2,

are strong field generators, π‐acceptors, with low‐energy

antibonding orbitals (LUMO, π*), and they form

complexes with metal ions stabilized by charge transfer

interactions metal(d)‐to‐ligand(π*) (MLCT).

Complex 2‐Δ‐(R,R,R,R)‐Zn²⁺. Yellow crystals. ¹H‐

RMN (400 MHz, CD₃CN‐d₃, δ) 8.56‐8.52 (t, J = 7.6 Hz,

J = 8 Hz, 2H; H^a), 8.30 (s, 4H; CH═N), 8.05‐8.03 (d, J

= 8 Hz, 4H; H^b), 7.20‐7.16 (t, J = 7.2 Hz, J = 7.6 Hz,

4H; H^e), 7.06‐7.02 (t, J = 8 Hz, J = 7.6 Hz, 8H; H^c),

6.59‐6.57 (d, J = 7.2 Hz, 8H, H^d), 4.37‐4.32 (q, J = 6.8

Hz, 4H; NCH─), 1.14‐1.13 (d, J = 6.8 Hz, 12H; CH₃).

UV‐vis (acetonitrile 7.425·10⁻⁵M): λ_max= 317 nm, 210

nm; MS (ESI, m/z): 373.24 (100) [Zn(2‐Δ‐(R,R,R,

R))₂]²⁺, 895.55 [Zn(2‐Δ‐(R,R,R,R))₂](CF₃SO₃)⁺.

Complex 2‐Λ‐(S,S,S,S)‐Zn²⁺. Yellow crystals. ¹H‐

RMN (400 MHz, CD₃CN‐d₃, δ) 8.56‐8.52 (t, J = 7.6 Hz,

J = 8 Hz, 2H; H^a), 8.30 (s, 4H; CH═N), 8.05‐8.03 (d, J

= 8 Hz, 4H; H^b), 7.20‐7.16 (t, J = 7.2 Hz, J = 7.6 Hz,

4H; H^e), 7.06‐7.02 (t, J = 8 Hz, J = 7.6 Hz, 8H; H^c),

6.59‐6.57 (d, J = 7.2 Hz, 8H, H^d), 4.37‐4.32 (q, J = 6.8

Hz, 4H; NCH─), 1.14‐1.13 (d, J = 6.8 Hz, 12H; CH₃).

UV‐vis (acetonitrile 7.425·10⁻⁵M): λ_max= 318 nm, 210

3.1 | NMR spectroscopy

nm; MS (ESI, m/z): 373.24 (100) [Zn(2‐Λ‐(S,S,S,S))₂]²⁺

895.69 [Zn(2‐Λ‐(S,S,S,S))₂](CF₃SO₃)⁺.

,

Only Zn²⁺ions, d¹⁰configuration, form diamagnetic

1

complexes that can be characterized by H‐NMR spec-

Complex 2‐Δ‐(R,R,R,R)‐Fe²⁺. Violet crystals. UV‐vis

troscopy. The H‐NMR spectra of the 1‐Λ‐(S,S,S,S)‐Zn²⁺

/

1

(acetonitrile 7.425·10⁻⁵M): λ_max= 325 nm (30.76 cm⁻¹

,

1‐Δ‐(R,R,R,R)‐Zn²⁺and 2‐Λ‐(S,S,S,S)‐Zn²⁺/2‐Δ‐(R,R,R,

R)‐Zn²⁺homonuclear complexes (Figures S1 and S2) con-

sist of a series of well‐defined peaks characteristic to the

1:2 symmetric metal‐ligand complex. In these spectra,

the H^apyridine protons signals of the ligand appear

shifted to the weaker magnetic field than the signals cor-

responding to these protons in similar free bis (arylimine)

pyridine ligands.^51-57Tridentate metal ion coordination

determine the ligands to adopt during the template

synthesis a cisoid conformation, corresponding to a

terpyridine (terpy) type coordination site. The conversion

of the all‐transoid conformer into the energetically

disfavoured all‐cisoid one upon metal complexation

occurs at the cost of conformational energy, which is

overcompensated by the interaction energy resulting

from metal ion binding.^51-57

MLCT d‐π*), 480 nm (20.83 cm⁻¹), 603 nm (16.58 cm⁻¹),

709 nm (14.09 cm⁻¹). MS (ESI, m/z): 369.27 (100)

[Fe(2‐Δ‐(R,R,R,R))₂]²⁺

,

825.67 [Fe(2‐Δ‐(R,R,R,R))₂]

(BF₄)⁺.

Complex 2‐Λ‐(S,S,S,S)‐Fe²⁺. Violet crystals. UV‐vis

(acetonitrile 7.425·10⁻⁵M): λ_max= 325 nm (30.76

cm⁻¹, MLCT d‐π*), 480 nm (20.83 cm⁻¹), 603 nm

(16.58 cm⁻¹), 709 nm (14.09 cm⁻¹). MS (ESI, m/z):

369.27 (100) [Fe(2‐Λ‐(S,S,S,S))₂]²⁺, 825.67 [Fe(2‐Λ‐(S,S,

S,S))₂](BF₄)⁺.

Complex 2‐Δ‐(R,R,R,R)‐Co²⁺. Brown crystals. UV‐

vis (acetonitrile 7.425·10⁻⁵M): λ_max= 300 nm (33.32

cm⁻¹, MLCT d‐π*). MS (ESI, m/z): 370.76 (100)

[Co(2‐Δ‐(R,R,R,R))₂]²⁺

,

828.46 [Co(2‐Δ‐(R,R,R,R))₂]

(BF₄)⁺.

6

DUMITRU ET AL.

bands at 608 nm and 705 nm were assigned to spin‐

3.2 | ESI‐MS spectrometry

3

forbidden transitions to triplet terms (³T_1g, T^2g). The bis

Electrospray ionization (ESI) conditions (T = 80 °C,

extraction cone voltage V_c= 10 V, 100% acetonitrile)

were set to avoid the fragmentation (dissociation) of the

complexes. ESI‐mass spectra of complexes in acetonitrile

solutions (10⁻⁴M) showed the formation of double‐

charged complex ions of [M(1‐Δ‐(R,R,R,R))₂]²⁺/[M(1‐Λ‐

(S,S,S,S))₂]²⁺and [M(2‐Δ‐(R,R,R,R))₂]²⁺/[M(2‐Λ‐(S,S,S,

S))₂]²⁺at MW/2 for all metal ions (Zn²⁺, Fe²⁺, and

Co²⁺). The primary coordination sphere for metal ions

(CN = 6, [MN₆]) remains intact in the solution; no substi-

tution reactions with solvent molecules or anions present

in the solution occur.

(arylethylimino)pyridine ligands, 1 and 2, are strong field

generators,

π acceptors, with antibonding orbitals

(LUMO, π*) low in energy, and they form complexes sta-

bilized by metal‐to‐ligand interactions (MLCT). MLCT

bands for octahedral Fe²⁺ion, d⁶occur in the low‐energy

domain approximately 30 kK and are very characteristic

to the red‐violet LS complexes of Fe²⁺with ligands simi-

lar to bis (arylethylimino)pyridines: bypiridine (bpy) and

phenanthroline (1,10‐phen), [Fe (bpy)₃]²⁺

, Fe(1,10‐

phen)₃]²⁺

.

Similar shifting of these MLCT bands and the brown

colour in 1‐Λ‐(S,S,S,S)‐Co²⁺/1‐Δ‐(R,R,R,R)‐Co²⁺and

2‐Λ‐(S,S,S,S)‐Co²⁺/2‐Δ‐(R,R,R,R)‐Co²⁺complexes are

observed and are due to the strong field behaviour of

bis (arylethylimino)pyridine ligands, 1 and 2, in a similar

manner to other Co²⁺‐complexes with related ligands:

[Co (bpy)₃]²⁺and [Co(1,10‐phen)₃]²⁺. For all Co²⁺

complexes, the spin‐allowed transitions are obscured by

MLCT bands.

3.3 | Electronic spectra

The information provided by the mass spectra, that in

solution are present only the [M(1‐Δ‐(R,R,R,R))₂]²⁺

/

[M(1‐Λ‐(S,S,S,S))₂]²⁺and [M(2‐Δ‐(R,R,R,R))₂]²⁺/[M(2‐Λ‐

(S,S,S,S))₂]²⁺complexes, namely, the primary coordina-

tion sphere of the metal ion is formed exclusively from

the nitrogen atoms of the tridentate ligands, allow assign-

ment of the d‐d transitions for Fe²⁺and Co²⁺complexes

(Table 2) from electronic UV‐Vis spectra in solution. In

octahedral coordination geometry, Fe²⁺metal ions can

present two spin states, depending on the strength of

the ligand field, with the following fundamental spectral

3.4 | CD spectra

The CD spectra measured in acetonitrile confirm the opti-

cal activity and enantiomeric nature of 1‐Λ‐(S,S,S,S)‐M²⁺

/

1‐Δ‐(R,R,R,R)‐M²⁺and 2‐Λ‐(S,S,S,S)‐M²⁺/2‐Δ‐(R,R,R,R)‐

M²⁺chiral complexes (Figure 1):

1

5

terms: A_1g(LS) and T_2g(HS). For the Co²⁺metal ion,

• In acetonitrile solution, the complexes retain their

chirality as the enantiomeric pairs present the same

absorption maxima with opposite signs. Their spectra

show negative or positive Cotton effects depending on

the sense of chirality.

• The absorption bands in the spectra of the 2‐Λ‐(S,S,S,

S)‐M²⁺/2‐Δ‐(R,R,R,R)‐M²⁺complexes are weaker in

intensity compared with those in the spectra of

in octahedral symmetry, the fundamental spectral terms

are E (LS) and T_1g(HS).

2

4

The UV‐Vis spectra of the 1‐Λ‐(S,S,S,S)‐Fe²⁺/1‐Δ‐(R,R,

R,R)‐Fe²⁺and 2‐Λ‐(S,S,S,S)‐Fe²⁺/2‐Δ‐(R,R,R,R)‐Fe²⁺

complexes have absorption maxima specific to the low‐

spin (LS) distorted octahedral geometries for Fe²⁺ions.

1

In these spectra, A_1g

assigned to the spin‐allowed transitions. The weaker

→ →

¹T_1gand A_1g¹T_2gare

TABLE 2 d‐d spin‐allowed transitions and MLCT bands for 1‐Λ‐(S,S,S,S)‐M²⁺/1‐Δ‐(R,R,R,R)‐M²⁺and 2‐Λ‐(S,S,S,S)‐M²⁺/2‐Δ‐(R,R,R,R)‐

²⁺, M = Co²⁺, Fe²⁺

M

1‐Δ‐(R,R,R,R)‐

1‐Δ‐(R,R,R,R)‐ 1‐Λ‐(S,S,S,S)‐

Fe²⁺Fe²⁺

2‐Δ‐(R,R,R,R)‐ 2‐Λ‐(S,S,S,S)‐

Fe²⁺Fe²⁺

Co²⁺/1‐Λ‐(S,S,S,S)‐ 2‐Δ‐(R,R,R,R)‐ 2‐Λ‐(S,S,S,S)‐

Complex

Co²⁺

Transitions, 35.57 (281 nm) 35.57 (281 nm) 30.76 (325 nm) 30.76 (325 nm)

kK

35.57 (281 nm)

MLCT d‐π*

33.32 (300 nm)

33.06 (303

nm) MLCT

1

¹A_1g→ T_2g

MLCT d‐π*

(MLCT d‐π*)

d‐π*

20.67 (484 nm) 20.67 (484 nm) 20.83 (480 nm) 20.83 (480 nm)

1

¹A_1g→ T_1g

16.45 (608 nm) 16.45 (608 nm) 16.58 (603 nm) 16.58 (603 nm)

3

¹A_1g→ T_1g

14.2 (705 nm)

14.09 (710 nm) 14.2 (705 nm)

3

¹A_1g→ T_2g

DUMITRU ET AL.

7

FIGURE 1 The CD spectra measured in acetonitrile solutions of (A) 1‐Λ‐(S,S,S,S)‐Zn²⁺/1‐Δ‐(R,R,R,R)‐Zn²⁺; (B) 2‐Λ‐(S,S,S,S)‐Zn²⁺/2‐Δ‐

(R,R,R,R)‐Zn²⁺; (C) 1‐Λ‐(S,S,S,S)‐Fe²⁺/1‐Δ‐(R,R,R,R)‐Fe²⁺; (D) 2‐Λ‐(S,S,S,S)‐Fe²⁺/2‐Δ‐(R,R,R,R)‐Fe²⁺; (E) 1‐Λ‐(S,S,S,S)‐Co²⁺/1‐Δ‐(R,R,R,

R)‐Co²⁺; and (F) 2‐Λ‐(S,S,S,S)‐Co²⁺/2‐Δ‐(R,R,R,R)‐Co²⁺complexes

1‐Λ‐(S,S,S,S)‐M²⁺/1‐Δ‐(R,R,R,R)‐M²⁺. This fact may

be correlated with the better π‐acceptor character of

the ligand 1 and, consequently, with the ability to

exert a stronger field in its complexes. In a series of

ligands with aromatic constituents, the antibonding

orbital (LUMO, π*) is at lower energy values as the

conjugate system is more extended; therefore, the

LUMO orbitals are energetically closer to the metal

orbital t_2g, and the overlapping π metal‐ligand is

strong. The result of this stronger interaction with

the π‐acceptor ligand is an increase in transition

energy.

chromophore‐pyridine.^70,71The same behaviour also

occurs in the CD spectra of Fe²⁺(Figure 1C,D) and

Co²⁺(Figure 1E,F) complexes.

3.5 | X‐ray single crystal diffraction

structures

The crystal structures of the complexes 1‐Λ‐(S,S,S,S)‐M²⁺

/

1‐Δ‐(R,R,R,R)‐M²⁺M²⁺= Co²⁺, Fe²⁺, Zn²⁺, 2‐Λ‐(S,S,S,

S)‐Fe²⁺and 2‐Δ‐(R,R,R,R)‐Co²⁺were determined from

crystals obtained from the acetonitrile/i‐propylether solu-

tions at room temperature. Labelled asymmetric units

and selected bond lengths and angles are described in

Tables S1 to S4. The molecular and the crystal packing

structures are presented in Figures 2–5.

• The absorption bands with maxima situated around

485 nm are present both in DC and UV‐vis spectra

of Fe²⁺complexes (Figure 1C,D). These absorption

bands were attributed to the spin‐allowed transition

1

¹A_1g→ T_1gfor Fe²⁺, d⁶, LS.

• For the Zn²⁺complexes (Figure 1A,B), in the absence

of crystal field stabilisation energy (CFSE) (d¹⁰), the

absorption bands are attributed to the allowed elec-

1‐Λ‐(S,S,S,S)‐Zn²⁺and 1‐Δ‐(R,R,R,R)‐Zn²⁺complexes

belong to the orthorhombic non‐centrosymmetric space

group P22₁2₁(#18), and the crystallographic data are

listed in Table S1. The asymmetric unit consists in both

cases of [1‐Λ‐(S,S,S,S)‐Zn]²⁺or [1‐Δ‐(R,R,R,R)‐Zn]²⁺cat-

1

tronic transitions L_aand B_bfrom the chromophore

1

moieties of the chiral ligands 1 and 2: L_a= 200 nm

1

−

and 206 nm, B_b= 267‐268 nm for phenyl chromo-

ions and two CF₃SO₃anions. The Zn²⁺‐N_Pyridineand

1

phore, L_a= 234 to 235 nm, B_b= 348 and 336 nm

for naphthyl chromophore.⁵⁷Compared with the

CD spectra of the constituent chromophores of

the ligands, the chiral amines R‐(+)‐1‐/S‐(−) ‐1‐

Zn²⁺‐N_iminedistances vary between 2.002⁴to 2.013⁴Å

and 2.2587¹⁷to 2.4055¹⁷Å, respectively.

1‐Λ‐(S,S,S,S)‐Fe²⁺and 1‐Δ‐(R,R,R,R)‐Fe²⁺complexes

crystallize in the orthorhombic non‐centrosymmetric

space group P2₁2₁2₁(#19) (Table S2). The asymmetric

unit consists in both cases of [1‐Λ‐(S,S,S,S)‐Fe]²⁺or

phenylethylamine

and

R‐(+)‐1‐/S‐(−)‐1‐

naphthylethylamine), these maxima are red‐shifted

1

−

(¹B_b= 269 nm_,261 nm‐phenyl, B_b= 336 nm_,282

[1‐Δ‐(R,R,R,R)‐Fe]²⁺cations, two BF₄anions, and two

nm‐naphthyl), probably because of the electronic

interactions (charge transfer or π‐π stacking)

acetonitrile molecules. The geometric parameters imply

that 1‐Λ‐(S,S,S,S)‐Fe²⁺and 1‐Δ‐(R,R,R,R)‐Fe²⁺complexes

8

DUMITRU ET AL.

FIGURE 2 Homochiral duplexes (top) and side view in stick (middle) and CPK (bottom) representation of the crystal packing of (A) 1‐Λ‐

(S,S,S,S)‐Zn²⁺and (B) 1‐Δ‐(R,R,R,R)Zn²⁺complexes. Green lines represent π‐π interactions

FIGURE 3 Homochiral duplexes (top) and side view in stick (middle) and CPK (bottom) representation of the crystal packing of (A) 1‐Λ‐

(S,S,S,S)‐Fe²⁺and (B) 1‐Δ‐(R,R,R,R)‐Fe²⁺complexes. Green lines represent π‐π interactions

−

are low spin at 175 K and the Fe‐N distances are of typical

values for LS complexes of Fe²⁺with pyridine‐type

ligands. The average Fe²⁺‐N_Pyridineand Fe²⁺‐N_iminedis-

tances are 1.8519¹⁷to 1.880²Å and 1.9457¹⁷to 2.1073¹⁸

Å, respectively.

and two BF₄anions (Table S3). The Co─N distances,

Co²⁺‐N_Pyridine= 1.848³to 1.925⁵Å and Co²⁺‐N_imine

=

2.017³to 2.318⁴Å, indicate that 1‐Λ‐(S,S,S,S)‐Co²⁺and

1‐Δ‐(R,R,R,R)‐Co²⁺are, as expected, low‐spin complexes

at 175 K.

1‐Λ‐(S,S,S,S)‐Co²⁺and 1‐Δ‐(R,R,R,R)‐Co²⁺crystals

belong to the enantiomorphic space group pair P6₁22/

P6₅22. The asymmetric unit consists in both cases of

[1‐Λ‐(S,S,S,S)‐Co]²⁺and [1‐Δ‐(R,R,R,R)‐Co]²⁺cations

2‐Λ‐(S,S,S,S)‐Fe²⁺and 2‐Δ‐(R,R,R,R)‐Co²⁺crystallize

in the orthorhombic non‐centrosymmetric space group

P2₁2₁2₁(#19) (Table S4). Each Fe²⁺/Co²⁺ion binds two

bis (phenylethylimine)pyridine ligands, 2, and has a

DUMITRU ET AL.

9

planes and present an octahedral coordination geometry.

Each ligand in the all‐cis configuration serves as a

tridentate ligand and coordinates meridionally to the

metal ion with one pyridine and two imine nitrogen

atoms. Continuous shape measurements analysis^72,73

showed that all transition metal ions, M²⁺= Zn²⁺

,

Fe²⁺, Co²⁺, display distorted octahedral coordination

environments (Table S5, Figures S3 and S4), and among

these, Fe²⁺complexes (LS, d⁶) are the less distorted from

ideal octahedron, whereas Co²⁺(LS, d⁷) with one elec-

tron in the antibonding e_gorbitals and Zn²⁺(d¹⁰) present

greater degrees of distortion.

For 1‐Λ‐(S,S,S,S)‐M²⁺/1‐Δ‐(R,R,R,R)‐M²⁺, whereas the

average Fe²⁺‐N_Pyridineand Co²⁺‐N_Pyridinedistances are

similar, 1.852/1.848 Å, the average Zn²⁺‐N_Pyridinedistance

is much longer: 2.002 Å. The M²⁺‐N_iminedistances are

progressively increasing as following: Fe²⁺‐N_imine

<

Co²⁺‐N_imine< Zn²⁺‐N_iminedistances of 1.946, 2.017, and

2.258 Å, respectively. These fairly different geometrical

parameters and the M²⁺coordination behaviour (ie,

distorted symmetry for Fe²⁺/Co²⁺and the lack of CFSE

for Zn²⁺) lead to slight differences in the spatial

disposition of lateral naphthyl arms. As an important

consequence, the double helix complexes are not

isostructural: They crystallize in different space groups

(P22₁2₁, P2₁2₁2₁, and P6₁22/P6₅22, respectively). This fact

is more evident for the crystals of enantiomeric pairs

1‐Λ‐(S,S,S,S)‐Fe²⁺/1‐Δ‐(R,R,R,R)‐Fe²⁺and 1‐Λ‐(S,S,S,S)‐

Co²⁺/1‐Δ‐(R,R,R,R)‐Co²⁺that are not isostructural,

although they have been synthesized under identical con-

ditions: same solvent system (CD₃CN/i‐propylether) and

same counterion (BF₄⁻).

FIGURE 4 Homochiral duplexes (A) 1‐Λ‐(S,S,S,S)‐Co²⁺and (B)

1‐Δ‐(R,R,R,R)‐Co²⁺; (C) side view and (D) top view in stick

representation of the crystal packing of complexes. Green lines

represent π‐π interactions

pseudo‐octahedral coordination geometry, but the com-

plexes did not crystallize similarly: Both complexes are

solvates, but 2‐Δ‐(R,R,R,R)‐Co²⁺crystallizes with only

one acetonitrile solvent molecule, whereas two

acetonitrile molecules are present in the structure of

The methyl‐CH^*chiral spacer in the structure of the

ligands is important for holding and stabilizing the

duplex formation by the internal π‐π stacking. As a gen-

eral rule, in the crystal, the two ligands are strongly

intertwined, stabilizing the duplex superstructures by

internal π‐π stacking interactions. The relative position

2‐Λ‐(S,S,S,S)‐Fe²⁺

.

In all duplex structures, the M²⁺metal ions are fully

coordinated by two ligands arranged into orthogonal

FIGURE 5 Homochiral duplexes (top) and side view of the crystal packing (bottom) of (A) 2‐Λ‐(S,S,S,S)‐Fe²⁺and (B) 2‐Δ‐(R,R,R,R)‐Co²⁺

complexes

10

DUMITRU ET AL.

of the duplex ligands allows a partial (1‐Λ‐(S,S,S,S)‐Fe²⁺

,

in the crystal. Long‐range 3D supramolecular structure

propagation is favoured when both partial internal over-

lapping and external π‐π stacking are present, leading to

the formation of robust single‐helical configurations or

tubular packed architectures. The use of multiple supra-

molecular interactions provides a very powerful platform

for the transfer of chiral information from molecular to

supramolecular level. The internal robustness of the

duplexes is mainly responsible for the transmission of

the supramolecular homochiral order and is reminiscent

with sliding biological processes along homochiral 3D

hypersurfaces occurring in the formation of chiral

biological relevant species at the nanolevel.^75-77

1‐Δ‐(R,R,R,R)Fe²⁺, 2‐Λ‐(S,S,S,S)‐Fe²⁺, and 2‐Δ‐(R,R,R,

R)‐Co²⁺) or a total (1‐Λ‐(S,S,S,S)‐Zn²⁺, 1‐Δ‐(R,R,R,R)‐

Zn²⁺, 1‐Λ‐(S,S,S,S)‐Co²⁺, and 1‐Δ‐(R,R,R,R)‐Co²⁺) inter-

nal overlap between the naphthyl or phenyl moieties

and the central pyridine moiety of a vicinal ligand via

π‐π stacking aromatic interactions with an average

centroid‐centroid distances of 3.4‐3.7 Å, corresponding

to van der Waals contacts: face‐to‐face π‐π,

heteroaromatic (py)‐aromatic interactions, and CH⋯π

(y‐interaction) (Figures S5 to S8).⁷⁴

Between peripheral aromatic rings (naphthyl for

1‐Λ‐(S,S,S,S)‐M²⁺/1‐Δ‐(R,R,R,R)‐M²⁺or

phenyl

for

2‐Λ‐(S,S,S,S)‐Fe²⁺and 2‐Δ‐(R,R,R,R)‐Co²⁺) intermolecu-

lar associations through CH⋯π (T‐interaction) with aver-

age centroid‐edge distances of 3.4‐3.7 Å are established

(Figures S5 to S8).⁷⁴

ACKNOWLEDGEMENTS

This work was supported by Agence Nationale de la

In the case of partial internal overlap, the external π‐π

stacking interactions are occurring between communicat-

ing duplex structures. It is resulting in the formation of

left‐handed 1‐Λ‐(S,S,S,S)‐Fe²⁺(Figure 3), 2‐Λ‐(S,S,S,S)‐

Fe²⁺) (Figure 5) or right‐handed 1‐Δ‐(R,R,R,R)‐Fe²⁺

(Figure 3), and 2‐Δ‐(R,R,R,R)‐Co²⁺(Figure 5), single

helix superstructures that are present in the crystal

structure.

In a different manner, in the crystals of 1‐Λ‐(S,S,S,S)‐

Zn²⁺, 1‐Δ‐(R,R,R,R)‐Zn²⁺, 1‐Λ‐(S,S,S,S)‐Co²⁺, and 1‐Δ‐

(R,R,R,R)Co²⁺complexes, the communication between

duplexes, which are mostly internally stacked, is

disrupted; each duplex being closely packed with two

neighbouring ones by weak van der Waals contacts

while the external π‐π stacking aromatic interactions

are completely suppressed in the frameworks (Figures 2

and 4).

Recherche ANR‐15‐CE29‐0009 DYNAFUN.

ORCID

Mihail Barboiu https://orcid.org/0000-0003-0042-9483

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How to cite this article: Dumitru F, van der Lee

A, Barboiu M. Chiral superstructures from

homochiral Zn²⁺, Co²⁺, Fe²⁺‐2,6‐bis (aryl

ethylimine)pyridine complexes. Chirality.

2019;1–13. https://doi.org/10.1002/chir.23115

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Chiral superstructures from homochiral Zn2+, Co2+, Fe2+-2,6-bis (aryl ethylimine)pyridine complexes

DOI: 10.1002/chir.23115

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