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Enantiopure versus Racemic Mixture in Reversible, Two‐Step, Single‐Crystal‐to‐Single‐Crystal Transformations of Copper(II) Complexes
Author(s) -
Ene Cristian D.,
Maxim Catalin,
Rouzières Mathieu,
Clérac Rodolphe,
Avarvari Narcis,
Andruh Marius
Publication year - 2018
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201800404
Subject(s) - chemistry , copper , crystallography , enantiopure drug , crystal structure , enantiomer , single crystal , schiff base , stereochemistry , molecule , medicinal chemistry , catalysis , enantioselective synthesis , organic chemistry
The reaction of chiral sodium complexes, 1 ∞ [Na( S ‐valmetH)]⋅H 2 O ( 1 ‐ S ) and 1 ∞ [Na( R ‐valmetH)]⋅H 2 O ( 1 ‐ R ), with copper(II) acetate affords chiral one‐dimensional coordination polymers with the formulas 1 ∞ [Cu( S ‐valmet)(H 2 O)] ⋅ H 2 O ( 2‐ S ) and 1 ∞ [Cu( R ‐valmet)(H 2 O)] ⋅ H 2 O ( 2‐ R ) ( R / S ‐valmetH 2 are Schiff base proligands resulting from the condensation reactions between o ‐vanillin and R / S ‐methionine). The copper ions are connected by the carboxylato groups belonging to the amino‐acid moieties, resulting in infinite chains showing syn‐anti out‐of‐plane bridging mode. The circular dichroism spectra of 1‐ S , 1‐ R , 2‐ S , and 2‐ R confirm their enantiomeric nature. Compounds 2‐ S and 2‐ R undergo a two‐step single‐crystal‐to‐single‐crystal transformation, with the elimination of the lattice and coordinated water molecules: 1 ∞ [Cu( S ‐valmet)(H 2 O)] ⋅ H 2 O ( 2‐ S )→ 1 ∞ [Cu( S ‐valmet)] ⋅ H 2 O ( 3‐ S ⋅H 2 O )→ 1 ∞ [Cu( S ‐valmet)] ( 3‐ S ) and 1 ∞ [Cu( R ‐valmet)(H 2 O)] ⋅ H 2 O ( 2‐ R )→ 1 ∞ [Cu( R ‐valmet)] ⋅ H 2 O ( 3‐ R ⋅H 2 O )→ 1 ∞ [Cu( R ‐valmet)] ( 3‐ R ), respectively. During these transformations, every pair of face‐to‐face chains present in 2‐ S (or 2‐ R ) has been “zipped up” into a chiral double chain through the removal of the aqua ligands and their replacement by the carboxylato oxygen atoms from the neighboring chain. Consequently, each carboxylato group now bridges three copper ions. The conversion of the single chains, 2‐ S and 2‐ R , into the double chains, 3‐ S and 3‐ R , is accompanied by a change of the strength of the exchange interactions between the copper ions: weak antiferromagnetic couplings are observed in compound 2‐ S ( J/k B =−1.23(5) K, H =−2 J Σ S i S i+1 ) and relatively strong in compound 3‐ S ( J/k B =−76.0(8) K). When the racemic mixture of the ligands, R,S ‐valmetH 2 , is employed, in the same experimental conditions, a racemic mixture of mononuclear compounds, [Cu( R,S ‐valmet)(H 2 O) 2 ] ⋅ H 2 O ( 4‐ RS ), is obtained. Compound 4‐ RS also undergoes a SCSC transformation with the elimination of the lattice and one of the coordinated water molecules, resulting in a racemic mixture of chiral chains, 1 ∞ [Cu( R ‐valmet)(H 2 O)] ⋅ 1 ∞ [Cu( S ‐valmet)(H 2 O)] ( 5‐ RS ). In this compound, the coupling of the copper(II) ions within the chains is weak and ferromagnetic ( J/k B =+0.10(2) K). These results prove that the chirality of the valmetH 2 ligands (optically pure or racemic mixture) plays a key role in the self‐assembly process of the copper(II) complexes.