Abstract
The mononuclear zinc(II) complex cis-[ZnL2(H2O)2] (1; L = 4-(pyridin-3-ylcarbamoyl)benzoate) was synthesized and characterized. By soaking crystals of 1 in a mixture of DMF-H2O solution containing a slight excess of Cu(NO3)2 × 3H2O a transmetalation reaction occurred affording the related copper(II) complex trans-[CuL2(H2O)2] (2). The structures of the compounds were authenticated by single crystal X-ray diffraction revealing, apart from a change in the isomerism, an alteration in the relative orientation of the chelating carboxylate groups and of the pyridine moieties. H-bond interactions stabilize both geometries and expand them into two-dimensional (2D) networks. The transmetalation was confirmed by SEM–EDS analysis. Moreover, the thermodynamic feasibility of the transmetalation is demonstrated by density-functional theory (DFT) studies. The catalytic activities of 1 and 2 for the oxidation of styrene and for the nitroaldol (Henry) C-C coupling reaction were investigated. The copper(II) compound 2 acts as heterogeneous catalyst for the microwave-assisted oxidation of styrene with aqueous hydrogen peroxide, yielding selectively (>99%) benzaldehyde up to 66% of conversion and with a turnover frequency (TOF) of 132 h−1. The zinc(II) complex 1 is the most active catalyst (up to 87% yield) towards the nitroaldol (Henry) coupling reaction between benzaldehyde and nitro-methane or -ethane to afford the corresponding β-nitro alcohols. The reaction of benzaldehyde with nitroethane in the presence of 1 produced 2-nitro-1-phenylpropanol in the syn and the anti diastereoisomeric forms, with a considerable higher selectivity towards the former (66:34).
Highlights
Transmetalation is commonly associated with an organometallic compound and involves a transfer of organic groups or ligands from one metal to another [1,2,3]
The synthesized compounds were characterized in terms of structural and optical properties by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) using a scanning electron microscope JEOL 7001 F (JEOL, Akishima, Tokyo, Japan) with Oxford light elements EDX detector
Compound 1 was synthesized by the hydrothermal reaction between Zn(NO3 )2 .6H2 O and HL in a DMF:H2 O solvent mixture (Scheme 1 and experimental section for synthetic details)
Summary
Transmetalation is commonly associated with an organometallic compound and involves a transfer of organic groups or ligands from one metal to another [1,2,3]. It provides a common and convenient method for preparing various organo-transition metal complexes, which are otherwise difficult to synthesize via direct metal-ligand self-assembly. Such phenomenon is well documented and has expanded to other types of materials [4,5,6,7,8,9,10,11]. The role of ligands in the process cannot be ruled out
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