Formation Mechanism Of Assembled Complexes Bridged By 1,3–bis(4–pyridyl)propane

Formation Mechanism of Assembled Complexes Bridged by 1,3–bis(4–pyridyl)propane

HarukaDotea, Hiroki Yasuharaa, Satoru Nakashimaa, b*

aDepartment of Chemistry, Graduate School of Science, Hiroshima University

1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan

bNatural Science Center for Basic Research and Development (N-BARD), Hiroshima University

1-4-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan

*E-mail: [anonimizat]

Abstract.Several types of crystals having different color (light blue, blue, and red) appeared in the synthesis of assembled complex of mixed crystals with three metals. The red crystal has 2D interpenetrated structure.The light-blue samples were [H2(bpp)][M(NCS)4] and they were built up spirally. The blue samples were M(NCS)2(bpp) and they formed 1D chain. [H2(bpp)][M(NCS)4] changed to 1D chain polymer (M(NCS)2(bpp)) by releasing HNCS from the cation and anion in a reaction vessel.

Keywords: Mixed crystal, Assembled complex, 1,3-bis(4-pyridyl)propane, Crystal formation

Introduction

Self-assembled coordination polymers containing transition metal ions and organic bridging ligands have attracted intensive interests because of their potential abilities for selective inclusion and transformation of ions and molecules [1]. There are various assembled structures [2-8]. We have studied iron complexes bridged by 1,3–bis(4–pyridyl)propane (bpp), which has three methylenes, by using single crystal X–ray diffraction analysis, Mössbauer spectroscopy, and SQUID measurements. Fe(NCX)2(bpp)2 (X = S, Se, and BH3) had a 2D interpenetrated structure and the NCBH3 complex showed a spin–crossover phenomenon [9]. When the benzene is enclathrated, the structure becomes 1D chain [10]. Recently, we discussed the spin state of the mixed crystals of iron with zinc or cobalt ion in the 2D interpenetrated structure for the assembled complexes bridged by bpp, and we revealed that NCSe complex had more low-spin state compared with pure Fe complex, suggesting chemical pressure effect[11]. In the middle of this study, we synthesized mixed crystals including zinc, cobalt, and iron. Several types of crystals appeared and the color changed in the reaction vessel. From the structure and color change we discussed the formation mechanism of the assembled complexes.

Experimental

Synthesis

Mixed crystals including three metals were obtained by solvent diffusion method.Solvent diffusion method:bottom layer; metals(FeSO4·7H2O(0.33 mmol) + ZnSO4·6H2O(0.33 mmol) + CoSO4·7H2O(0.33 mmol)), NaNCS(2mmol), and L-ascorbic acid in H2O( 20 ml ), middle layer; H2O ( 20mL )and cyclohexane( 20mL ), top layer; bpp( 2mmol ) in EtOH ( 1 mL )andcyclohexane( 19 mL ). Light-blue crystalsappeared about 4 hours after the setting and they decreased time to time. Precipitates are mixtures of light-blue crystals, blue crystals, and red crystals. Light-blue crystals are dissolved in acceton and they can be isolated.Anal. found (calc.)%: for {[H2(bpp)][M(NCS)4]}n , C, 41.67 (41.08); H, 3.21 (3.24); N 16.74 (16.91); S, 24.76 (25.81)(M = Fe0.06Zn0.87Co0.07 ) and for {M(NCS)2(bpp)}n, C, 47.79 (47.43); H, 3.64 (3.72); N 14.80 (14.75); S, 16.89 (16.89)(M = Fe0.01Zn0.95Co0.04 ).

Light-blue crystals were selectively obtained when the diluted HNO3 was added to the water phase. Single crystals appeared in the bottom.

ICP-AES analysis

The metal fraction, Fe : Zn : Co was determined by ICP-AES.ICP-AES was performed by SPS3510 madebySII NanoTechnology Inc. Calibration curves were drawn by measuring intensity of 1, 0.5, 0.1, and 0.01 ppm for iron and zinc standard solution and 0.994, 0.497, 0.099, and 0.050 ppm for cobalt standard solution and ultrapure water. Scan range of iron, zinc, and cobalt wasaround 238.204, 202.548, and 228.616 nm, respectively.

X-ray diffraction measurement

The powder X-ray diffraction patterns were measured on a Rigaku RAD-X system using Cu-Kα radiation and employing a scan rate of 5.0 °/min, step of 0.02 °, and region from 5.0 to 60.0 °. All measurements were performed under air at room temperature (RT).

Zn complexand light-blue crystal were obtained as a single crystal, and the structures were determined by single crystal X–ray structural analysis. The crystal data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. (CCDC :1060727 and 1433079)

Results and discussion

Figure 1 shows powder X-ray diffraction patterns of pure iron complex and mixed metal complexes. They revealed that the red crystal has 2D interpenetrated structure, which is similar to the structure of Fe(NCS)2(bpp)2. The light-blue and blue crystals showed new structure and structural change in the reaction vessel.

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Figure 2 shows ESI-MS of light-blue crystal on cationic peaks and anionic peaks (measurement and simulation). This result and elemental analysis reveal that the composition is [H2(bpp)][M(NCS)4]. Figure 3 shows ORTEP drawing and packing view of light-blue crystal. Crystal data, selected bond lenghts, and angles are shown in Table 1. Metalsare tetragonally coordinated by four NCS anions to become [M(NCS)4]2-. Bpp is doubly protonated to become [H2(bpp)]2+. The composition becomes [H2(bpp)][M(NCS)4], and this is mononuclear complex. This composition explains that this sample is more easily dissolved in the solvent compared with blue crystals and red crystals. The packing view in one plane is shown in Fig. 3(b). [H2(bpp)]2+ is surrounded by four [M(NCS)4]2-and [M(NCS)4]2- is also surrounded by four [H2(bpp)]2+ in the same plane. These [H2(bpp)]s are placed almost perpendicular to each other. This plane is stacked to each other to form crystal (Fig. 3(c)). This structure is similar to [4,4’-bipyH2][Zn(NCS)4][13]. Each sulfur atom of the [Zn(NCS)4]2- anion has a weak S…S contact with a sulfur atom from an adjacent [Zn(NCS)4]2- anion. The diatances of these contacts, being 3.4550(16) or 3.5058(14) Å,are slightly shorter than that in [4,4’-bipyH2][Zn(NCS)4][13](3.63 Å), and also slightly shorter than the sumof the van der Waals radii of two sulfur atoms, indicating weak interaction.[14] The frame constructed by weak S…S contact was built spirally. The sulfur atoms also form hydrogen bonds with the [H2(bpp)]2+ cations. The distances are 2.6591 (14), 2.7601 (13), 2.7224 (13), or 2.6913 (12) Å, which are slightly longer than those in [4,4’-bipyH2][Zn(NCS)4][13]( 2.404 (3) Å ).This is due to the difference that [H2(bpp)]2+ is placed on center between anion, while [4,4’-bipyH2]2+ approaches the one side of anion.

Blue sample could not be isolated as a single crystal but as an aggregation of very small crystals. It was confirmed that blue crystals and pure zinc complex (Zn(NCS)2(bpp)) have the same structure by comparing powder X-ray diffraction patterns (Fig. 4) and from the results of elemental analysis. It can be judged that the structure is controlled by the Zn metal. The structure of Zn(NCS)2(bpp) was determined by single X-ray structural analysis. Crystal data are shown in Table 2. Figure 5 (a) shows ORTEP drawing of pure zinc complex. Zn is tertragonally coordinated by one bpp and two NCS to form 1D chain. The 1D chain is shown in Fig.5 (b). This is not an ionic complex, but neutral 1D chain polymer.

The light-blue crystals are easily dissolved in acetone. The light-blue crystals changed to blue in a reaction vessel and in acetone solution. Usually, metals and ligands gather to each other to form stable assembled structure in the synthetic process of assembled complexes. In the present study, it was shown that the ionic crystals are firstly obtained and the crystal changed to more stable assembled structure. It is considered that at first [H2(bpp)]2+ and [M(NCS)4]2- are formed in water or EtOH and precipitate as [H2(bpp)][M(NCS)4]. This is not the most stable structure in a reaction vessel. This changes to 1D chain structure by releasing HNCS from the cation and anion.

Conclusion

We obtained several types of crystals in mixed crystals and successfully analyzed the structures of light-blue crystals and blue crystals. Firstly ionic crystals ( [H2(bpp)][Zn(NCS)4] ) appeared. The structure is built up spirally. And then, neutral 1D chain structure is formed by releasing HNCS from the cation and anion. We could pursue the change by using mixed crystals, because such change appears as the color change.

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