Synthesis, crystal structure and third-order non-linear optical property of heterobimetallic cluster compound [MoOICu3S3(2,2′-bipy)2]

Journal of Molecular Structure 690 (2004) 131–135 www.elsevier.com/locate/molstruc

Synthesis, crystal structure and third-order non-linear optical property of heterobimetallic cluster compound [MoOICu3S3(2,20-bipy)2] Yong Lia, Jing Lua, Jiqing Xua,1,*, Xiaobing Cuia,b, Yinghua Suna, Qingxin Yangc, Lingyun Panc a

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, No. 119 JieFang Road, ChangChun 130023, China b Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Science, Fuzhou 350002, China c College of Physics, Jilin University, ChangChun 130023, China Received 26 June 2003; revised 17 November 2003; accepted 19 November 2003

Abstract Nest-shaped cluster [MoOICu3S3(2,20 -bipy)2] (1) was synthesized by the treatment of (NH4)2MoS4, CuI, (n-Bu)4NI, and 2,20 -bipyridine  b ¼ 14:820ð3Þ A;  c ¼ 17:951ð4Þ (2,20 -bipy) through a solid-state reaction. It crystallizes in monoclinic space group P21 =n; a ¼ 9:591ð2Þ A;  b ¼ 91:98ð2Þ8; V ¼ 2549:9ð10Þ A  3 ; and Z ¼ 4: The nest-shaped cluster was obtained for the first time with a neutral skeleton containing A; 2,20 -bipy ligand. The non-linear optical (NLO) property of [MoOICu3S3(2,20 -bipy)2] in DMF solution was measured by using a Z-scan technique with 15 ns and 532 nm laser pulses. The cluster has large third-order NLO absorption and the third-order NLO refraction, its a2 and n2 values were calculated as 6.2 £ 10210 and 2 3.8 £ 10217 m2 W21 in a 3.7 £ 1024 M DMF solution. q 2004 Elsevier B.V. All rights reserved. Keywords: Crystal structure; Mo –Cu– S cluster; Non-linear optical property; Solid-state reaction

1. Introduction The design and synthesis of new cluster molecules with large macroscopic optical non-linearities represents an active research field in modern chemistry, physics, and materials science [1 – 4]. This is because, these clusters possess interesting electronic, biological, optical, structural, and catalytic properties and show promising potential as biological models of nitrogenases and various other metalloenzymes [5,6]. The Mo(W) – Cu(Ag) – S clusters in first aroused an extensive interest because of their special function related to biological process [7], which also show strong non-linear absorption and non-linear refraction (selfdefocusing or self-focusing effect) [8 –13]. However, in the well-known Mo(W) – Cu(Ag) – S clusters, the ligands attached to the copper (silver) atoms are only halogen, pyridine, PPh3 or their analogues showing strong affinities to Cu(I), which restricts the structures of such cluster * Corresponding author. Tel.: þ86-431-894-3977; fax: þ 86-431-8923907. E-mail address: [email protected] (J. Xu). 1 Fax: þ86-431-892-3907. 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.11.028

compounds to a narrow scope of structure types and so shows limitation in the advantage of easy structural modification. Therefore, in the present work, we chose 2,20 -bipy as the starting reagent and performed a solid-state reaction under low-heating temperature to gain a 2,20 -bipycontaining cluster. In order to further develop this active field and also as a part of our search for better non-linear optical (NLO) materials, herein we report the synthesis, crystal structure and the third-order NLO properties of one neutral nest-shaped [MoOS3Cu3I(2,20 -bipy)2] (1). To our knowledge, the nest-shaped cluster with 2,20 -bipy skeleton are found first on the Mo(W) – Cu(Ag) –S system. The thirdorder NLO susceptibilities xð3Þ and the third-order nonlinear hyperpolarizabilities g of this cluster are also reported.

2. Experimental 2.1. Starting materials All reactions and manipulations were conducted using Standard Schlenk techniques under an atmosphere of

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nitrogen. The compound (NH4)2MoS4 were prepared according to the literature [14]. The solvents were carefully dried and distilled prior to use and other chemicals were generally used as received from commercially available. 2.2. Physical measurements Infrared (IR) spectra (KBr Pellet) were recorded on a Perkin – Elmer spectrophotometer. Element analysis for carbon, hydrogen, and nitrogen were performed on a Perkin– Elmer 2400LS II elemental analyzer. Electronic spectra were measured on a Shimadzu UV-3100 spectrophotometer. 2.3. Z-scan measurements The NLO response of 1 dissolved in DMF was determined by a standard Z-scan set up with a Q-switched, frequency-doubled Nd:YAG laser. The pulse repetition rate is 10 Hz. The solution was contained in 5 mm thick quartz cell with a concentration of 3.7 £ 1024 M. 2.4. Synthesis of the nest-shaped cluster [MoOS3Cu3I(2,2 0 bipy)2] (1) A well-ground mixture of (NH4 )2 MoS4 (259 mg, 1.0 mmol), CuI (382 mg, 2.0 mmol), 2,20 -bipy (159 mg, 1.0 mmol) and (n-Bu)4NI (369 mg, 1.0 mmol) was heated in a sealed glass tube filled with N2 gas at 85 8C for 18 h. The mixture was extracted with the mixture of DMF (40 ml) and CH3OH (10 ml), then filtered. The red filtrate was allowed to evaporate in the atmosphere of nitrogen at 5 8C for 16 days, and red crystals were obtained. Yield: 88.8 mg (10.6%). The compound shows characteristic IR absorption peaks at 912 cm21 for n (Mo –Ot), 442 and 420 cm21 for n (Mo – Sbr).The results of elemental analyses are as follows. Anal. calcd for C20H16Cu3MoIN4OS3: C, 28.66%; H, 1.92%; N, 6.69%. Found: C, 28.63%; H, 1.94%; N, 6.68%. 2.5. Structure determination The red crystal of the title compound suitable for X-ray diffraction experiments was glued onto glass fiber with epoxy resin and mounted on a Siemens SMART CCD diffractometer by using graphite-monochromatized Mo Ka  for unit cell determination and radiation ðl ¼ 0:71073 AÞ data collection. The non-hydrogen atoms were found by direct methods (SIR88) [15], and the molecular structure was obtained by subsequent Fourier and difference Fourier syntheses. Intensities were collected for Lorentz-polarization effects and absorption correction by using the v=2u scan-technique. The structure was solved by using the direct method and refined by full-matrix least-squares against F 2 using the SHELXL 97 crystallographic software package [16]. All non-hydrogen atoms were refined anisotropically by the full-matrix least-squares method. The hydrogen atoms were

Table 1 Crystallographic data for cluster 1 Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group

C20H16Cu3I MoN4OS3 838.01 293(2) 0.71073 Monoclinic P21 =n

Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A b (deg)

9.591(2) 14.820(3) 17.951(4) 91.98(2)

˚ 3) Volume (A Z Calculated density (g cm23) Absorption coefficient (mm21) Fð000Þ Crystal size (mm3) u range for data collection (deg) Limiting indices Reflections collected/unique Data/restraints/parameters Goodness-of-fit on F 2 Final R indices ½I . 2sðIÞ R indices (all data) ˚ 23) Largest diff. peak and hole (e A

2549.9(10) 4 2.183 4.426 1608 0.24 £ 0.36 £ 0.52 1.78– 26.01 21 # h # 11; 21 # k # 18; 222 # l # 22 6636/5013 ½Rint ¼ 0:0531 5013/0/298 0.954 R1 ¼ 0:0431; wR2 ¼ 0:1018 R1 ¼ 0:0658; wR2 ¼ 0:1076 0.867 and 21.055

placed in their calculated positions, assigned fixed isotropic thermal parameters, and allowed to riding on their respective parent atoms. The data processing and structure refinement parameters are listed in Table 1.

3. Results and discussion 3.1. Crystal structure of cluster 1 The typical Mo(W) –Cu(Ag) – S cluster complexes can be classified into about 10 structural types [17], for example: linear, butterfly, cubane-like and nest-shape structures, etc. exemplified by [MoCu2 S4(PPh 3) 3]·0.8CH 2Cl 2 [18], [MoCu2OS3(PPh3)3] [19], [MoCu3S4X(PPh3)3] (X ¼ Cl, Br, I) [20 – 22] and [MoCu3OS3I(py)5] [11], respectively. However, the nest-shaped cluster 1 with 2,20 -bipy ligand was found first in Mo(W) –Cu(Ag) – S system. The ORTEP diagrams are shown in Fig. 1. Selected bond lengths and angles are listed in Table 2. The neutral skeleton of cluster 1 is composed of one Mo atom, one terminal O atom, three Cu atoms and three m3-S atoms to form a [MoOS3Cu3] aggregate assuming a nestshaped structure. The molybdenum atom is found in the center of a [MoOS3]22 tetrahedral moiety, with a terminal oxygen atom and three sulfur atoms for extensive coordination with three copper atoms. Within the core structure of the nest-shaped cluster 1, the S –Mo –S bond angle mean value [107.38(6)8] is obviously smaller than that

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the title compound acts as a bidentate ligand, linking Cu(1) and Cu(2) by the two N atoms in the 2,20 -bipy group. 3.2. Non-linear optical properties

Fig. 1. The perspective view of the structure of the neutral cluster 1.

of O – Mo – S [111.48(19)8]. Compared to the free [MoOS3]22 ion, the MoOS3 fragment constructs a crystallographic C3 symmetry through the Mo atom and O atom, with three S– Mo – S bond angles of 107.44(6), 107.23(6), and 107.46(6)8, respectively. The Mo –O bond length of ˚ is characteristic of a double bond, while the 1.699(4) A ˚, same three Mo – S bond distances mean value 2.2724(17) A are in the range of typical single bonds. In addition, weak interactions exist between the Mo atom and the three Cu atoms, while the Mo – Cu distances mean value ˚ ] in the cluster 1 are slightly shorter than [2.6536(17) A ˚ ] in [MoS3OCu3I(py)5] [11]. These three one [2.6958(6) A copper atoms are of two different kinds. Cu(3) adopts a trigonal coordination mode, surrounded by two S atoms from [MoOS3]22 and a terminal I atom; Cu(1) and Cu(2) adopt distorted tetrahedral geometry, while each Cu atom is coordinated by two m3-S atoms and one 2,20 -bipy unit, forming two almost equivalent [CuS2(2,20 -bipy)] unit. The N –Cu – N bond angles at these two [CuS2(2,20 -bipy)] unit range from 78.8(2) to 79.3(2)8. Compared with the tricoordinated Cu(3), the tetra-coordinated Cu(1) and Cu(2) adopts a more constrictive coordination and shows a stronger interaction with the central Mo atom. The 2,20 -bipy in

The UV –visible spectrum displayed in Fig. 2 shows that the first excited state is located at , 500 nm (3.0 eV) with a long tail up to 750 nm. When compound 1 is exposed to 532 nm laser radiation, the photon energy is sufficient to excite electrons from the ground state to this excited state. Hence, it is expected that the saturation in the excited state will lead to a non-linear refraction with a negative sign. The Z-scan data were shown in Fig. 3. We revealed the non-linear optical (NLO) properties of cluster 1 using a Z-scan technique [23]. The non-linear absorptive performances of the cluster solutions were measured by the Z-scan technique under an open-aperture configuration with a pulse width of 15 ns at 532 nm and 10Hz repetition rate shown in Fig. 3(a). The NLO absorption performances of this cluster be represented by the following equations: ðþ1 2 1 TðZÞ ¼ pffiffi ln½1 þ qðZÞ e2t dt pqðZÞ 21 qðzÞ ¼

ðþ1 ðþ1 0



0

a2

I0 exp½22ðr=v0 Þ2 2 ðt=t0 Þ2  1 þ ðz=z0 Þ2

1 2 exp2a0 L r dr dt a0

Bond angles O –Mo–S(3) O –Mo–S(1) S(3)–Mo– S(1) O –Mo–S(2) S(3)–Mo– S(2)

1.699(4) 2.2693(18) 2.2715(17) 2.2764(17) 2.4434(10) 2.049(6) 112.92(18) 110.63(19) 107.45(6) 110.90(19) 107.23(6)

Cu(2)–N(1) Cu(1)–N(2) Cu(1)–N(3) Cu(1)–S(1) Cu(1)–S(3)

S(1)–Mo– S(2) I –Cu(3)–Mo N(4)– Cu(2)–N(1) N(2)– Cu(1)–N(3)

ð2Þ

Here, TðzÞ represents the transmittance which is defined here as the ratio of the transmittance pulse energy and the incident pulse energy at 532 nm (with respect to focal point Z ¼ 0), Z is the distance of the sample from the focal point, I0 is the peak irradiation intensity at focus ðI0 ¼ 8:2 £ 108 W cm22 Þ; Z0 ¼ pv20 =l; where v0 is the spot radius of the laser pulse at focus and l is the laser wavelength, r is the radial coordinate, t is the time, and t0 is the pulse width. a0

Table 2 ˚ ) and bond angles (deg) for cluster 1 Selected bond lengths (A Bond lengths Mo–O Mo–S(3) Mo–S(1) Mo–S(2) I –Cu(3) Cu(2)–N(4)

ð1Þ

2.118(5) 2.020(5) 2.102(5) 2.2409(19) 2.2595(18)

107.47(6) 173.49(4) 78.8(2) 79.3(2) Fig. 2. UV –visible spectrum of cluster 1 dissolved in DMF.

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the optical path. By using our experimental values: l ¼ 532 nm; a0 ¼ 2:7 cm21 and L ¼ 0:5 cm; we obtained n2 ¼ 21:0 £ 10213 m2 W21 M21 : This figure is comparable to those found in clusters reported previously, for example, 2 1.7 £ 10 214 m 2 W 21 M 21 for (n-Bu4N) 2 [MoCu3OS3(NCS)3] [24]. A reasonably good fit between the experimental data and the theoretical curve was obtained, which suggests that the experimentally obtained NLO effects are effectively third-order in nature. The effective a2 value of 6.2 £ 10210 m W21 in 3.7 £ 1024 M DMF solution, were derived for the sample from the theoretical curve. In accordance with the observed a2 and n2 values, the modulus of the effective third-order susceptibility xð3Þ can be calculated by Eq. (4) vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2  2 ffi2 ! u  8 2 2    u  9 £ 10 10 n0 c a2   cn0  ð4Þ n2  lxð3Þ l ¼ t   þ   80p   2n

Fig. 3. Z-scan measurements of 1 dissolved in DMF with a concentration of 3.7 £ 1024 M. The circles are experimental data. (a) The data were collected under an open-aperture configuration. The solid curve is a theoretical fitting based on Eq. (1). (b) The data were obtained by dividing the normalized Z-scan measure under a closed aperture configuration by the normalized Z-scan data of (a). The solid curve is a theoretical fitting according to Eq. (3).

and a2 denote the linear and non-linear absorption coefficients, respectively. L is the optical path (L ¼ 5 mm in the current experiments). The NLO refractive behaviors, show in Fig. 3(b), were assessed by dividing the normalized Z-scan data obtained under the closed-aperture configuration by the normalized Z-scan data obtained under the open-aperture configuration. The data show that the cluster has a positive sign for refractive non-linearity. It is clear that the non-linear absorption is too small to be detected and there is a strong self-defocusing effect. The effective thirdorder NLO refractive indexes n2 of the clusters are given by n2 ¼

la0 DTv2p 0:812pI½1 2 expð2a0 LÞ

ð3Þ

where l is the laser wavelength, DTv2p is the difference between the valley and peak in the normalized transmittance. S is the transmittance of the aperture, and L is

where n is frequency of the laser light, 10 and c are the permittivity and speed of light in vacuum, respectively. n0 and n2 are the linear and non-linear refractive indices of the sample, respectively. For 3.7 £ 1024 M DMF solution, the xð3Þ value was calculated to be 2.1 £ 10211 esu. The corresponding modulus of the hyperpolarizabilities g of 1.7 £ 10229 esu was obtained from xð3Þ ¼ gNF 4 ; where N is the number density (concentration) of the clusters in the sample, F 4 ¼ 3:11 is the local field correction factor. Previous studies of the NLO properties of Mo(W) – Cu(Ag)– S clusters have covered a number of structural types, including nest-shaped and twin nest-shaped clusters. It is also interesting to compare the optical non-linearities of compound 1 with those of other different structural clusters; the values of these clusters are listed in Table 3. The NLO refractive properties and NLO absorptive properties of compound 1 may be compared to the behaviors of nestshaped and twin nest-shaped, which typically show effective self-defocusing and strong non-linear absorption effects, respectively. The title compound 1 has the same NLO properties as the studied nest-shaped clusters which may be due to the heavy-atom (Mo, Cu) effect. [MoOICu3S3R2] contains organic ligands, Phen (1,10-phenanthroline), bipy or bMe-bipy (4,4 0 -dimethyl-2,2 0 -bipyridinel). Table 3 Non-linear optical properties of clusters measured at 532 nm with nanosecond laser pulses Clusters

[MoOS3Cu3I(py)5] [WOS3Cu3I(py)5] [Et4N]4[Mo2O2S6Cu6I6]

Structure a2 (m W21) type

Nest Nest Twinnest [MoOS3Cu3I(2,20 -bipy)2] Nest

n2 (m2 W21)

Ref.

6.5 £ 10210 23.0 £ 10217 [11] 3.5 £ 10210 3.2 £ 10217 [11] 4.0 £ 10210 26.0 £ 10217 [12] 6.2 £ 10210 23.8 £ 10217 This work

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The difference of ligands may result in different NLO behavior. This very fact strongly implies that the heterothiometallic clusters may be designed and synthesized to obtain predictable and controllable NLO properties. 4. Supplementary data Supplementary data have been deposited with the Cambridge Crystallographic Centre, CCDC No. 210607. Copy of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: þ 44-1223-336033; e-mail: deposit@ ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk). Acknowledgements This research was supported by grants from the National Natural Science Foundation of China (No. 20271021 and No. 20333070) and the State Key Basic Research Project of China (No. 2001 CB 108906). References [1] T.J. Marks, M.A. Ratner, Angew. Chem. Int. Ed. Engl. 34 (1995) 155. [2] S.R. Marder, J.E. Sohn, G.D. Stucky, Materials for nonlinear optics, ACS Symposium Series No. 455, American Chemical Society, Washington, DC, 1991. [3] O. Keller, Nonlinear optical in solids, Proceedings International Summer School, Springer, Berlin, 1990. [4] M.H. Lyons, Materials for Nonlinear and Electrooptics Institute of Physics Conference Series No. 103, Bristol, 1989. [5] A. Muller, E. Diemann, H. Bogge, Angew. Chem. Int. Ed. Engl. 20 (1981) 934–935.

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