Job/Unit: I50128 /KAP1 Date: 08-05-15 12:51:05 Pages: 12 FULL PAPER DOI:10.1002/ejic.201500128 Syntheses, Spectroscopy, and Structural Analyses of Dinuclear Chiral-at-Metal μ-Aqua-tetrakis[(R or S)-N-1- (Ar)ethylsalicylaldiminato]di-Λ- or -Δ-nickel(II) Complexes Mohammed Enamullah,* [a] Mohammad Abdul Quddus, [a] Mohammad Rezabul Hasan, [a] Gennaro Pescitelli, [b] Roberto Berardozzi, [b] Guido J. Reiß, [c] and Christoph Janiak* [c] Keywords: Schiff bases / Nickel / Diastereoselectivity / Configuration determination / Enantioselectivity Enantiopure dinuclear μ-aqua-tetrakis[(R or S)-N-1-(Ar)eth- ylsalicylaldiminato]di-Λ- or -Δ-nickel(II) [Ar = C 6 H 5 (R-1/S- 1), p-MeOC 6 H 4 (R-2/S-2), p-ClC 6 H 4 (R-3/S-3), p-BrC 6 H 4 (R- 4)] complexes have been synthesized from the reaction be- tween (R or S)-N-1-(Ar)ethylsalicylaldimine and nickel(II) acetate. Their CD spectra demonstrate chirality transfer from the ligands to the metal ions and a mirror-image relationship for the enantiomeric pair R-3/S-3 in thf. DSC analyses show that the complexes exhibit an irreversible dissociation to the mononuclear species, which in turn undergo decomposition. The 1 H NMR spectra reveal the presence of four salicylaldi- minates, one aqua ligand, and two molecules of methanol in each dinuclear complex. They also confirm the existence of a dinuclear-bridged aqua bis-octahedral Ni II complex in solu- Introduction Pseudo-octahedral six-coordinate tris- or bis-chelate Δ/Λ-metal complexes are classical textbook examples of metal-centered chirality. In these complexes, the asymmetry is due to the arrangement of an achiral or chiral ligand A ∧ A ( * ) around a metal M, leading to D 3 symmetry for Δ/ Λ-M(A ∧ A) 3 or C 2 symmetry for Δ/Λ-M(A ∧ A) 2 B 2 . [1–3] Use of enantiomeric chiral bidentate ligands leads to the forma- tion of diastereomeric complexes with possible absolute configurations of Λ(R,R,R), Δ(R,R,R), Λ(S,S,S), or Δ(S,S,S). [4–9] If the chiral ligand is racemic, the mixed forms Λ(R,S,S) and Δ(S,R,R) etc. may also be obtained in ad- dition. Moreover, in such chiral-at-metal complexes, the metal-centered configuration Δ or Λ (see Scheme 1) can be [a] Department of Chemistry, Jahangirnagar University, Dhaka 1342, Bangladesh E-mail: enamullah@juniv.edu http://people.juniv.edu/profile/enamullah [b] Department of Chemistry, University of Pisa, via Moruzzi 3, 56124 Pisa, Italy [c] Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität, Universitätsstr. 1, 40225 Düsseldorf, Germany E-mail: janiak@uni-duesseldorf.de http://www.janiak.hhu.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201500128. Eur. J. Inorg. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 tion. The IR spectra in CH 2 Cl 2 show a very strong band at around 2306 cm –1 due to the νO–H of the bridged aqua li- gand. The X-ray structures of R-2, S-2, R-3, and S-3 confirm the formation of dinuclear compounds comprising two nickel ions, four salicylaldiminates, and one aqua ligand with two molecules of methanol (or water in S-2). Each nickel ion is surrounded by two N,O-chelating salicylaldiminates, a bridging salicyl-O atom from the neighboring nickel ion, and a bridging aqua ligand in a distorted octahedral polyhedron. Analyses of the absolute structures reveal a diastereomeric induction of the R or S ligand giving a Λ or Δ configuration at the nickel atoms in R-2/3 or S-2/3 that is independent of the ligand substituents. induced by the R or S chirality of the ligand. [2–4,6–13] Intra- molecular noncovalent interactions acting within the chiral- at-metal chelate complexes lead to a free-energy difference between the diastereomers, and one of the diastereomers is generally dominant (termed stereochemical or dia- stereomeric induction). For example, the predominant for- mation of one diastereomer has been reported in a number of four-coordinate tetrahedral to distorted square-planar cases, including Λ-bis[(R)-N-(Ph)ethyl-X-Sal]Ni/Zn II (Ph = phenyl; Sal = salicylaldiminato; X = H, 5-NO 2 /OMe, 3,5- di-Cl), [6c,7a,7d] Δ-bis[(R)-N-(Ph)ethyl-X-Sal]Cu II (X = 4-Br, 3,5-di-Cl), [6a,6b] Λ-bis[(R)-N-(Ph)ethyl-X-Sal]Cu II (X = H, 3-OEt), [6b,6d] Δ-bis[N-(-menthyl)-Sal]Co II , [7b] Δ-bis[(R)-N- (naphthyl)ethyl-3,5-di-Cl-Sal]Cu/Ni/Zn II , [7c] Δ-bis[AP-(S)- pn or AP-(S,S)-dpen]Ni II (pn = enantiopure 1,2-diami- nopropane; AP = 2-hydroxyacetophenone), [8] and Λ-bis- [AP- or DA-(S,S)-chxn]Ni II (DA = dehydroacetic acid), [8] as evidenced by solid-state crystal-structure determinations. Some analogous complexes have shown absolute configura- tions (Δ vs. Λ) based only on CD spectral patterns in solu- tion. [5] In relation to this, some of us have reported an equi- librium of both Δ and Λ diastereomers with a dia- stereomeric ratio of around 20:80 in CDCl 3 for R,R ligands in Λ/Δ-configured bis[(R)-N-1-(Ar)ethylsalicylaldiminato- κ 2 N,O]Zn II , which compares with the enantiopure induc-