Structural, spectral and magnetic studies of two Co(II)-N-heterocyclic diphosphonates based on multinuclear units

https://doi.org/10.1016/j.saa.2015.08.017Get rights and content

Highlights

  • Two Co(II)-diphosphonates with multinuclear units based on 2,2′-bipy/phen are gained by solvothermal technique.

  • Fluorescent bands of 1–2 are caused by the intraligand π*–π transition states of 2,2′-bipy/phen.

  • Research shows that 1 is antiferromagnetic, but 2 displays weak ferromagnetic property.

Abstract

Two examples of Co(II)-N-heterocyclic coordination polymers based on 1-hydroxyethylidenediphosphonic acid (H5L = CH3C(OH)(PO3H2)2), namely 0.5(H3NCH2CH2NH3)·[Co6(Cl2)(H3L)2(H2L)(HL)(2,2′-bipy)6] 1 and 2(NH4)·[Co3(HL)2(H2O)2(phen)2]·2(H2O) 2, have been solvothermally obtained by introducing the second ligands 2,2′-bipyridine/1,10-phenanthroline (2,2′-bipy/phen) and characterized by powder X-ray diffraction (PXRD), elemental analysis, IR, TG-DSC. The single-crystal X-ray diffractions show that compound 1 possesses a 0-D structure with hexa-nuclear cluster [Co6(O–P–O)8] built through single/double O–P–O bridges and compound 2 displays a 1-D ladder-like chain structure with magnetic topology building blocks [Co4(O–P–O)4]n. Then H-bonding and ππ stacking interactions further expand the two low-dimensional structures into three-dimensional supramolecular frameworks. Fluorescent measurements reveal that both the maximum emission peaks of 1–2 are centered at 423 nm, mainly deriving from intraligand π*–π transition state of N-heterocyclic ligand 2,2′-bipy/phen, respectively. Magnetism data indicate that 1 exhibits antiferromagnetic behavior within hexa-nuclear Co(II) clusters, while 2 shows weak ferromagnetic interactions in 1-D topology Co(II)-chain, showing promising potential as magnetic materials.

Introduction

As a new type of functional molecular materials, coordination polymers (CPs) not only have abundant space topology, but also have huge potential applications in gas storage, separation, optics, electron, magnetism, chiral separation and catalysis and other fields. The assembly of the coordination polymers based on phosphonates is currently of significant interest [1], [2], [3]. Any material properties mainly depend on their structures. So both the structure of organic ligand and the coordination behavior of metal ion are generally dominant for the self-assembly of phosphonate CPs. In addition we don't know enough about the synthetic rules and topology. It is means that the rational synthesis of CPs with the target structure is an extremely creative and intellectually challenging task. In our work, firstly, 1-hydroxyethylidenediphosphonic acid (H5L) with a flexible hydroxyl group is often used as building block in the construction of the phosphonates. The additional –OH group attached to the organic tether not only provides a possible hydrophobic/hydrophilic environment but also increases solubility of the resulting metal phosphonates. Importantly, it is more likely to behave as a bis(bidentate) chelating ligand using its four of six phosphonate oxygen atoms, which would be good for the construction low-dimensional CPs [4], [5]. Secondly, lots of auxiliary ligands with N-heterocycle as one assistant method have been applied widely during the self-assembly of the CPs. Polypyridyl ligands have been intensively used as ancillary building blocks in the phosphonate coordination chemistry [6], due to their interesting electronic, photonic and magnetic properties, as well as π-stacking ability and directional H-bonding when coordinating to transition metals. Although many chemists devoted to the study of self-assembly of the CPs by means of the mixed ligands [7], [8], [9], [10], [11], low-dimensional phosphonate CPs have not been well developed based on auxiliary N-heterocyclic ligands under the solvothermal condition. So 2,2′-bipy and phen have been selected as auxiliary ligands in our work. Finally, Co(II) ion is focused because of its unique high-spin d7 electron configuration and larger spin-orbital coupling effect for an octahedral Co(II) complex. Many great compounds of Co(II)-H5L have been resolved, but a few Co(II)-diphosphonate compounds with 2,2′-bipy/phen have been structurally determined [12]. By introducing the auxiliary ligands 2,2′-bipy/phen, we have succeeded in winning two new diphosphonate CPs, 0.5(H3NCH2CH2NH3)·[Co6(Cl2)(H3L)2(H2L)(HL)(2,2′-bipy)6] 1 and 2(NH4)·[Co3(HL)2(H2O)2(phen)2]·2(H2O) 2. In this paper, we study crystal structures, fluorescent and magnetic properties.

Section snippets

Materials and general methods

All reagents were purchased from commercial sources and used without further purification. Element analyses were performed on a Perkin-Elmer 2400 LS elemental analyzer. IR spectra were recorded from 4000 to 400 cm−1 with a Nicolet AVATAR360 instrument. The thermal gravimetric analyses (TG–DSC) were carried out with a NETZSCH STA 449F3 thermal analyzer with a heating rate of 10 K·min 1. Powder X-ray diffraction patterns (PXRD) were performed on an ARL X'TRA diffractometer using Cu-Kα radiation.

Description of the structures

Single-crystal X-ray diffraction analyses reveal that both 1 and 2 possess 3-D supramolecular architectures. The asymmetric unit of 1 (Fig. 1) contains six distinct Co(II) atoms, two H3L2− ligands, a H2L3− ligand, a HL4− ligand, two Cl ligands as well as six coordinated 2,2′-bipy molecules and a half of free ethylenediamine cation. Four centers Co(1), Co(2), Co(5) and Co(6) in 1 are all in a distorted octahedral coordination environment, while the remains of two other centers Co(3) and Co(4)

Conclusions

We have successfully obtained two low-dimensional Co(II)-N-heterocyclic-diphosphonate CPs under solvothermal condition. Both 1 and 2 display the 3-D supramolecular structures built by hexa-nuclear clusters for 1 and 1-D ladder-like chain containing tetra-nuclear clusters for 2 via H-bonding and ππ stacking interactions, respectively. Fluorescent studies at room temperature show emission bands are mainly derived from the ancillary ligands. The correlation of the structures and magnetic

Acknowledgments

The authors are grateful to the financial support from Jiangsu province college students innovation and entrepreneurship training programs, Open Fund of Jiangsu Key Laboratory for Chemistry of Low-dimensional Materials, Cultivation Fund of High Level Project of Huaiyin Normal University, the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province and National Natural Science Foundation of China (Projects nos. 201410323004Z, JSKC13127, 11HSGJBZ12, 12KJA15004,

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