Structure and magnetism of heptanuclear complex encapsulated the octacyanomolybdate(IV) core

Inorganic Chemistry Communications 35 (2013) 58–60

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Structure and magnetism of heptanuclear complex encapsulated the octacyanomolybdate(IV) core Fan Yu a, b, Ai-hua Li a, b, Si-qian Hu a, b,⁎, Bao Li c,⁎⁎ a b c

Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University, Wuhan 430056, PR China School of Chemical and Environmental Engineering, Jianghan University, Wuhan, Hubei, 430056, PR China School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China

a r t i c l e

i n f o

Article history: Received 18 March 2013 Accepted 24 April 2013 Available online 2 May 2013 Keywords: Heptanuclear compound Cyanide Antimagnetic interaction Octacyanomolybdate

a b s t r a c t A new heptanuclear complex [{Cu(TPA)CN}6Mo(CN)2][ClO4]8 (1) (TPA = tris(2-pyridylmethyl)amine) was formed by the reaction of [Cu(TPA)] 2+ cations and K4Mo(CN)8. The octacyanomolybdate core is encapsulated by six mononuclear copper cations via cyano bridges. 1 Crystallizes in monoclinic space group P21/n with a = 22.30(2) Å, b = 27.13(3) Å, c = 24.35(2) Å, β = 94.10(2)°, and Z = 4. Variable temperature magnetic measurements have demonstrated that very weak antimagnetic interaction between the nearest paramagnetic Cu II centers at low temperature is exhibited, transferred by ―NC―Mo―CN― bridging units. © 2013 Published by Elsevier B.V.

The isolated or extended-array heteronuclear coordination complexes constructed by the utilization of poly-cyanometalates template have been attracted great attention mostly because of their exciting magnetic properties [1,2]. The types of discrete cluster have been being widely investigated due to their potential properties of “single molecular magnet,” which could be considered as the building components to frameworks that show bulk-like magnetism [3]. Reacting with polycyanidometallate tectons and versatile assembled complex cations with transitional metal ions and proper chelating ligands has been considered as one common synthesis route to build up isolated heterometal complexes [4]. The chelating ligands could not only block several coordination sites of the assembling cations but inhibit the formation of extended frameworks. Guided by this route, versatile cyanido-bridged bimetallic oligonuclear complexes have been assembled in recent years, which have been of excellent models for investigating the magnetostructural relationship mediated via the cyanide bridge [5]. Compared to the versatile bimetallic oligonuclear clusters constructed with hexacyanometalates tectons that have been characterized by X-ray crystallography, few ones with octacyanometalates have been reported [4,5]. Utilizing the octacyanometalates templates, several oligonuclear clusters with fascinating magnetic properties have been reported [6]. However, even with the excellent magnetostructural model for further understanding the magnetic interaction in thus heteronuclear SMMs, the other high nuclear Cu II–CN–MoIV systems more than three-nuclear ones have ⁎ Correspondence to: S-Q. Hu, School of Chemical and Environmental Engineering, Jianghan University, Wuhan, Hubei, 430056, PR China. Tel./fax: + 86 2784226930. ⁎⁎ Corresponding author. Tel./fax: +86 2787543532. E-mail addresses: [email protected] (S. Hu), [email protected] (B. Li). 1387-7003/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.inoche.2013.04.032

been rarely explored [7]. Herein, in this paper, we report another new hepta-bimetallic complex, [{Cu(TPA)CN}6Mo(CN)2][ClO4]8, formed by the self-assembly of octacyanidometallate template [Mo(CN)8] 4− and six assembled [Cu(TPA)] 2+ moieties, which exhibits very weak intracluster antimagnetic coupling between the nearest paramagnetic Cu(II) ions mediated through the ―NC―Mo―CN― bridging unit under low temperature region The synthesis of compound 1 was carried out by reaction of Cu(ClO4)2, TPA and K4Mo(CN)8 in a ratio of 5:5:1 in acetone–water solution by taking advantage of the original stepwise synthetic approach termed as “polynuclear complex as ligand.” Blue block-like crystals in about 15% yield were obtained by evaporation of the filtrate in 1 week. X-ray diffraction studies were performed at 293 K, and crystal data are gathered in Table S1. Complex 1 crystallizes in monoclinic space group P21/n. Fig. 1 shows the asymmetric unit of the heptanuclear cation in 1 that comprises of one complete [Mo(CN)8] 4− tecton and six assembling [Cu(TPA)]2+ moieties connected to central Mo ion via the cyano bridge, which is surrounded by eight ClO4− counter-anions and isolated aqua molecules. In the IR spectra (Fig. S1), complex 1 displays a band at the position of 2160 cm−1 due to the bridging υCN stretch of the hepta-nuclear cation, and a second band at 2129 cm−1 assigned to the terminal υCN stretch of the hepta-nuclear cation [8], which is consistent with the un-saturated coordination mode of [Mo(CN)8]4− templates characterized by the crystallographic studies. The Mo atom is coordinated by eight CN groups in slightly distorted dodecahedron with Mo―C distances ranging from 2.120(9) to 2.169(10) Å [9], and the Mo―C―N angles are almost linear, ranging from 176.0(6)° to 179.0(5)°. By contrast, very different to the range from 149.3(4)° to 172.5(6)° in 2, the angles of Cu-N-C(cyano) vary from 167.5° to

F. Yu et al. / Inorganic Chemistry Communications 35 (2013) 58–60

Fig. 1. ORTEP drawing of the structure of 1 at the 30% probability level showing the heptanuclear [Mo{Cu(TPA)}CN8]8+ cation of 1, H atoms have been omitted for clarity.

2.3

100

2.2

80

2.1

60

2.0

40

1.9

20

MT

χ -1 / M

120

cm-3 K -1 mol

140

2.4

3

/ cm K mol

-1

2.5

χ

174.2°, indicating the slight deviation from linearity. The large deviation might be ascribed to the steric hindrance originated from the bulk chelating ligands TPA on adjacent Cu centers and the presence of H-bonding interactions compared to the small size ligand tris(2-aminoethyl)amine [10]. The values of Mo―C bond distances, Mo―C―N angles, and Cu―N(cyano) bond distances are all consistent with typical ones in other heteropolynuclear complexes constructed by [Mo(CN)8] 4− and other Cu moieties [8–10]. The Mo―Cu distances in 1 of 5.17–5.23 Å are shorter than the ones reported for discrete molecules and lattice networks [1–3]. In MoCu6 cation, all of the Cu(II) centers are surrounded by five nitrogen atoms (four from TPA, and one from Mo-bound cyanide) with the range of Cu―N distance from 1.930(8) to 2.125(7) Å. The geometry of copper ions could be best described as a distorted trigonal bipyramid with the equatorial N atoms from three pyridinate rings of TPA ligands and axial N atoms containing aliphatic N atom and cyano N atom. The values of N(py)―Cu―N(cyano) angles are much larger than the ones of N(py)―Cu―N(amine) because of the steric effect caused by the five-membered chelating rings [11]. Further, the whole bimetallic heptanuclear cation are surrounded by eight perchlorate anions, which are located in the cavity of the adjacent cations. In addition, an important structural feature of 1 is that heptanuclear cation and counteranions are connected together by the elaborate H-bonding network, which seems to be aiding crystallization. The magnetic susceptibilities of 1 have been measured in an applied field of 5000 Oe over the temperature range 300–2 K, and the plots of χMT versus T were shown in Fig. 2. At room temperature, the χMT value of 2.22 cm3 K mol−1 is in agreement with the expected value of 2.25 cm3 K mol−1 for non-interact system with one Mo(II) (S = 0) and six Cu(II) ions (S = 1/2) calculated for g = 2.00 [12]. The χMT values keep constants as the temperature decreasing until to 20 K, and then decrease gradually and reach a minimum value of 2.16 cm3 K mol−1 at 2 K. This magnetic behavior is of a hint that weak intramolecular antimagnetic interaction between coordinated Cu(II) ions is exhibited, as observed in other related hepta-nuclear compounds. Fitting of data of the 1/χM vs. T in the range of 2–50 K using the Curie–Weiss law χM = C/(T − θ) gives the Curie constant C = 2.21 cm3 K mol−1 and the Weiss constant θ = −0.08 K. The small positive θ value also supports the presence of the weak antimagnetic interaction via the CuII–NC–MoIV–CN–CuII pathways [13]. The variable

59

0

1.8 0

50

100

150

200

250

300

T/K Fig. 2. Thermal variation of χMT for 1 at a magnetic field of 5000 G (empty square).

temperature magnetic data in the temperature range of 2–300 K for 1 were also simulated by a spin Hamiltonian (Eq. (1)) based on the summation of two independent trinuclear Cu(II)3 units [13]. In accordance with the exhibited weak ferromagnetism, the theoretical curve shows a good agreement with the magnetic data, as shown in Fig. 2, which gives the values of g = 2.00, J = −0.03 cm−1. As expected, very weak antiferromagnetic coupling is truly presented between the nearest Cu(II) in the heptanuclear cation. ^ ¼ −2J ½ðS S þ S S Þ þ ðS S þ S S Þ þ 6gβHS H 1 2 2 3 3 4 5 6

ð1Þ

Taking into consideration the characteristic of the heptanuclear compound, a photoswitchable behavior of the isolated cluster would be expected because of the potential electron transfer between Cu II and MoIV centers or spin transition of the central molybdenum atom [6,14]. Unfortunately, no any change of the magnetic properties after irradiation with a blue or green laser under standard photo-excitation conditions was observed. The absence of LIESST effect might be ascribed to the geometrical consideration of molybdenum. Empirically, it is thought that an intermediate geometry between square antiprism (D4d) and dodecahedron (D2d) was necessary to stabilize the long-lived photoinduced metastable state, for example in 2. The feature of the HOMO orbital composed of the d orbitals of the molybdenum atom tends to induce and stabilize the high-spin excited-state of MoIV ion (d2, S = 1) after irradiation [14]. In this way, two d orbitals (dz2 and dx2–y2) in the close energy would be singly occupied by one electron. As a result, the system could undergo a triplet-to-singlet conversion and potentially result in the long-lived MoV–CuI metastable state caused by the electron transfer from the molybdenum to one copper ion. However, in 1, the molybdenum atom does not adapt the intermediate geometry to stabilize the high-spin MoIV state, and therefore no change of the magnetic properties could be observed after light irradiation. In summary, a new heterometallic heptanuclear complex selfassembled around octacyanomolybdate (IV) by capping six cyano groups with [Cu(TPA)] 2 + moieties is obtained and its structure is characterized by X-ray crystallography. The whole crystal structure is stabilized by weak intermolecular interactions between the cations and ClO4− counter-anions. The magnetic study reveals that a very weak antimagnetic interaction between neighboring Cu(II) centers is clearly presented in the low temperature region, and the magnetic properties are consistent with the expected paramagnetic behavior in the high temperature region. Future work must be focused on the modification of the chelating ligands to construct the heteronuclear clusters and investigating the relationship between the proper intermediate geometry and photo-magnetic effect.

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Acknowledgements Professor Jun Tao in Xiamen University must be thanked for their guidance. We gratefully acknowledge the National Natural Science Foundation of China (nos. 21101066 and Wuhan Science and Technology Bureau (grant no. 201271031382) for generous financial support. Appendix A. Supplementary material Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.inoche.2013.04.032.

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