Synthesis, crystal structures and magnetic properties of dinuclear copper(II) compounds with NNO tridentate Schiff base ligands and bridging aliphatic diamine and aromatic diimine linkers

Luca Rigamonti; Alessandra Forni; Roberta Pievo; Jan Reedijk; Alessandro Pasini

doi:10.1039/C0DT01304H

Synthesis, crystal structures and magnetic properties of dinuclear copper(ii) compounds with NNO tridentate Schiff baseligands and bridging aliphatic diamine and aromatic diimine linkers†

Luca Rigamonti,*^a Alessandra Forni,*^b Roberta Pievo,*^cd Jan Reedijk^de and Alessandro Pasini^a

* Corresponding authors

^a Università degli Studi di Milano, Dipartimento di Chimica Inorganica, Metallorganica e Analitica ‘Lamberto Malatesta’, via Venezian 21, 20133 Milano, Italy
E-mail: luca.rigamonti@unimi.it
Tel: +39-0250314376

^b CNR-ISTM, via Golgi 19, 20133 Milano, Italy
E-mail: a.forni@istm.cnr.it

^c Max Planck Institute for Biophysical Chemistry, Am Faßberg 11, 37077 Göttingen, Germany
E-mail: roberta.pievo@mpibpc.mpg.de

^d Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands

^e Department of Chemistry, King Saud University, P.O. Box 2455, 11451 Riyadh, Saudi Arabia

Abstract

The synthesis and the characterization of new dinuclear copper(II) compounds of general formula [(L^a–d)₂Cu₂(μ-N–N)](ClO₄)₂ (1–6) with either neutral aliphatic diamine (N–N = piperazine, pip) or aromatic diimine (N–N = 4,4′-bipyridine, 4,4′-bipy) linker are reported. The copper ligands L⁻ (L^a− = (E)-2-((2-aminoethylimino)methyl)phenolate, L^b− = (E)-2-((2-aminopropylimino)methyl)-phenolate, L^c− = (E)-2-((2-aminoethylimino)methyl)4-nitrophenolate, L^d− = (E)-2-((2-aminoethylimino)methyl)4-methoxyphenolate) are NNO tridentate Schiff bases derived from the monocondensation of a substituted salicylaldehyde 5-G-salH (G = NO₂, H, OMe) with ethylenediamine, en, or 1,3-propylenediamine, tn. The crystal structures of compounds [(L^a)₂Cu₂(MeOH)₂(μ-4,4′-bipy)](ClO₄)₂ (1·2MeOH), [(L^b)₂Cu₂(MeOH)₂(μ-4,4′-bipy)](ClO₄)₂ (2·2MeOH), [(L^d)₂Cu₂(μ-4,4′-bipy)](ClO₄)₂ (4), [(L^a)₂Cu₂(μ-pip)](ClO₄)₂ (5) and [(L^b)₂Cu₂(μ-pip)](ClO₄)₂ (6) have been determined, revealing the preferred (e-e)-chair conformation of the bridging piperazine in compounds 5 and 6. The presence of hydrogen-bond-mediated intermolecular interactions, that involve the methanol molecules, yields dimers of dinuclear units for 1·2MeOH, and infinite zig-zag chains for 2·2MeOH. The temperature dependences of the magnetic susceptibilities χ_M(T) for all compounds were measured, indicating the presence of antiferromagnetic Cu–Cu exchange. For the compounds 2–4 with 4,4′-bipy, the coupling constants J are around −1 cm⁻¹, while in compound 1 no interaction could be detected. The compounds 5 and 6 with piperazine display higher Cu–Cu magnetic interactions through the σ-bonding backbone of the bridging molecule, with J around −8 cm⁻¹, and the coupling is favoured by the (e-e)-chair conformation of the diamine ring. The non-aromatic, but shorter, linker piperazine gives rise to stronger Cu–Cu antiferromagnetic couplings than the aromatic, but longer, 4,4′-bipyridine. In the latter case, the rotation along the C–C bond between the two pyridyl rings and the consequent non co-planarity of the two copper coordination planes play an important role in determining the magnetic communication. EPR studies reveal that the dinuclear species are not stable in solution, yielding the solvated [(L)Cu(MeOH)]⁺ and the mononuclear [(L)Cu(N–N)]⁺ species; it appears that the limited solubility of the dinuclear compounds is responsible for their isolation in the solid state.

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Synthesis, crystal structures and magnetic properties of dinuclear copper(ii) compounds with NNO tridentate Schiff baseligands and bridging aliphatic diamine and aromatic diimine linkers†

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Synthesis, crystal structures and magnetic properties of dinuclear copper(II) compounds with NNO tridentate Schiff base ligands and bridging aliphatic diamine and aromatic diimine linkers

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