DALTON J. Chem. Soc., Dalton Trans., 1997, Pages 2357–2361 2357 Crystal structures and non-linear optical properties of cluster compounds [MAu2S4(AsPh3)2] (M 5 Mo or W)† He-gen Zheng,a Wei Ji,b Michael L. K. Low,b Genta Sakane,c Takashi Shibahara c and Xin-quan Xin *,a a State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing University, Nanjing, 210093, People’s Republic of China b Department of Physics, National University of Singapore, Singapore 119260, Republic of Singapore c Department of Chemistry, Okayama University of Science, Okayama 700, Japan The compounds [MAu2S4(AsPh3)2] (M = Mo 1 or W 2) were synthesized by reactions of [NEt4]2[MS4] (M = Mo or W), HAuCl4?4H2O and AsPh3 in CH2Cl2 solution.X-Ray crystallographic structure determinations show that the co-ordination of Mo(W) is slightly distorted from tetrahedral and those of the Au are distorted from trigonal planar. High non-linear susceptibilities of these gold-containing clusters were also observed for the first time.Z-Scan data measured with 532 nm nanosecond laser pulses showed that effective third-order non-linearities a2 = 7.9 × 1025 and 13 × 1025 dm3 cm W21 mol21 and n2 = 28.0 × 10210 and 19 × 10210 dm3 cm2 W21 mol21, respectively, for a 0.64 mmol dm23 solution of compound 1 and a 0.54 mmol dm23 solution of 2. The Mo(W)]Cu(Ag)]S cluster compounds have been studied extensively in the past two decades, because of their relevance to biological systems and catalytic processes.1,2 Recently, we have noticed that they also exhibit very interesting non-linear optical (NLO) properties.For example, strong NLO behaviour has been reported in nest-shaped clusters [NBun 4]2[MoCu3- OS3(NCS)3] and [NBun 4]2[MoCu3OS3BrCl2], a supracageshaped cluster [NBun 4]4[Mo8Cu12O8S24], and a twin nest-shaped cluster [NEt4]4[Mo2Cu6OS6Br2I4].3–5 Butterfly-shaped clusters [MCu2OS3(PPh3)n] (M = Mo or W, n = 3 or 4) and a half-open cage-shaped cluster [NEt4]3[W(CuBr)3OS3(m-Br)]?2H2O exhibit large NLO refraction.6,7 Cubane-like clusters [NBun 4]3- [MM93S4Br(X)] (M = Mo or W, M9 = Cu or Ag, X = Cl or I) possess strong NLO absorption.8 A very large optical limiting effect has been observed in a hexagonal prism-shaped cluster [Mo2Ag4S8(PPh3)4], which is about ten times larger than that observed in C60.9 In order to explore this field further, we have synthesized a series of new Mo(W)]Au]S cluster compounds. In this article we report the synthesis, characterization and NLO properties of gold-containing compounds with a linear structure, [MoAu2S4(AsPh3)2] 1 and [WAu2S4(AsPh3)2] 2.Experimental Materials Compounds [NEt4]2[MoS4] and [NEt4]2[WS4] were prepared according to a literature method.10 Other chemicals were of AR grade and used without further purification. Preparations [MoAu2S4(AsPh3)2] 1. Triphenylarsine (120.3 mg, 0.3931 mmol) dissolved in CH2Cl2 (5 cm3) was slowly added to HAuCl4?4H2O (81 mg, 0.1966 mmol) in absolute ethanol (5 cm3). The light yellow solution was stirred for 2 h and refrigerated at 5 8C overnight. The resulting colourless crystals were dissolved in CH2Cl2 (15 cm3) and [NEt4]2[MoS4] (47.64 mg, 0.0983 mmol) was added.After stirring for 1 h the red-black solution was filtered and PriOH (10 cm3) was added dropwise to † Non-SI unit employed: eV ª 1.60 × 10219 J. the top of the solution. The red crystals were obtained several days later (Found: C, 35.15; H, 2.4.Calc. for C36H30As2Au2- MoS4: C, 35.15; H, 2.45%). IR (KBr pellet, cm21): C]H in AsPh3, 734.1vs, 689.3vs; Au]P, 614.6w; Mo]Sb, 453.4vs. [WAu2S4(AsPh3)2] 2. The synthetic method was similar to that used for compound 1, [NEt4]2[WS4] being used instead of [NEt4]2[MoS4]. Yellow crystals were obtained (Found: C, 32.75; H, 2.32. Calc. for C36H30As2Au2S4W: C, 32.8; H, 2.3%). IR (KBr pellet, cm21): C]H in AsPh3, 737.5vs, 688.3vs; Au]P, 519.5w; W]Sb, 477.3vs, 442.2vs, 407.0w.X-Ray crystallography A red crystal of compound 1 was mounted in a glass capillary. All measurements were made on a Rigaku AFC6S diffractometer with graphite-monochromated Mo-Ka radiation (l = 0.7107 Å). The lattice parameters shown in Table 1 were refined using 21 reflections in the range 9.4 < q < 12.78. The data collection with w–2q scans between 3 and 258 resulted in 6949 intensity values, 4591 with I >1.50s(I) being used for the structure determination.The structure was solved by heavyatom Patterson methods11 and expanded using Fourier techniques. 12 The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included but not refined. The final cycle of full-matrix least-squares refinement converged with unweighted and weighted agreement factors R = 0.0299 and R9 = 0.0387. For compound 2, an orange crystal was mounted in a glass capillary for X-ray data collection. All measurements were made on a Mac Science MXC-18 diffractometer.The lattice parameters (Table 1) were refined using 39 reflections in the range 10.0 < q < 15.08. The data collection with w–2q scans between 3 and 308 resulted in 11 491 intensity values, 7676 with I >1.50s(I) being used for the structure determination. The structure was solved by direct methods13 and expanded using Fourier techniques. The refinement was based on F. An empirical absorption correction using the program DIFABS14 was applied. The data were corrected for Lorentz-polarization effects, and the final R = 0.0638 and R9 = 0.0859.The function minimised was Sw(|Fo| 2 |Fc|)2, where w = 1/s2(Fo). All calculations were performed using the TEXSAN15 crys-2358 J. Chem. Soc., Dalton Trans., 1997, Pages 2357–2361 3[MO2S2]22 2[MOS3]22 1 [MO4]22 4[MOS3]22 3[MS4]22 1 [MO4]22 Scheme 2 tallographic software package. Selected bond distances and angles are given in Tables 2 and 3. Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC).See Instructions for Authors, J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 186/499. Physical measurements Infrared spectra were recorded on a Fourier Nicolet FT-10SX spectrophotometer with pressed KBr pellets, electronic spectra with a Hitachi U-3410 spectrophotometer.Carbon and hydrogen analyses were performed on a PE-240C elemental analyser. Non-linear optical measurements The NLO properties of compounds 1 and 2 dissolved in CH2Cl2 were determined by using a standard Z-scan set up with a Q-switched, frequency-doubled Nd:YAG laser. The pulse repetition rate was 10 Hz. The details of the set-up can be found elsewhere.16 The solutions were contained in 1 mm thick quartz cells with concentrations of 6.4 × 1024 and 5.4 × 1024 mol dm23 for compounds 1 and 2, respectively. Results and Discussion Synthesis The compounds were synthesized from [NEt4]2[MS4] (M = Mo or W), HAuCl4?4H2O and AsPh3 in CH2Cl2 solution.When [NEt4]2[MO2S2] was used instead of [NEt4]2[MS4], the same compounds were obtained, as in Scheme 1. The transformation from [MO2S2]22 to [MS4]22 may take place as in Scheme 2. Therefore, the [MS4]22 anion reacts with [Au(AsPh3)]1 to give the products. However, an interesting fact is that [MoOS3- (AuPPh3){Au(PPh3)2}] was synthesized in poor yield by reaction of Cs2[MoOS3] and [Au(PPh3)Cl].17 Structures of [MAu2S4(AsPh3)2] (M = Mo 1 or W 2) Figs. 1 and 2 show the crystal structures of compounds 1 and 2, Figs. 3 and 4 the packings of the clusters in the solid state. The skeletons, consisting of one M, four m-S and two Au atoms, show linear structures with crystallographic C2v symmetry. The Au]Mo]Au and Au]W]Au angles are 178.51(3) and 178.27(2)8, respectively. The M (Mo or W) atom has essentially tetrahedral co-ordination and MS4 22 acts as a tetradentate ligand co-ordinating to two Au atoms through its four m-S atoms.Each Au atom is co-ordinated by two m-S atoms and one AsPh3 ligand, forming a planar trigonal geometry. The MS1S2Au1 and MS3S4Au2 (M = Mo or W) cores in compounds 1 and 2 are planar to within 0.0056 (0.0083) and 0.0125 (0.0136) Å, respectively. Their dihedral angle is 89.65 (89.73)8, which means that they are essentially perpendicular to each other.There are two types of structures in related linear compounds as depicted in Scheme 3; the main bond lengths are listed in Table 4, which reveals several structural trends. First, in all linear-shaped compounds MS2M92 (M = Mo or W; M9 = Cu, Scheme 1 [NH4]2[MO2S2] + HAuCl4•4H2O + AsPh3 (Ph3As)Au S M S S S Au(AsPh3) Ag or Au), each Au atom in compounds 1, 2, 6 and 10 is in a trigonal-planar co-ordination; one Cu(Ag) atom in 3–5 and 7–9 is tetrahedrally co-ordinated and the other is trigonally co-ordinated.However, the co-ordination modes of two Au atoms in the nest-shaped compound [MoOS3(AuPPh3){Au- (PPh3)2}] are the same as those observed in linear-shaped Mo(W)]Cu(Ag)]S cluster compounds. Secondly, the M]S bond lengths of four gold-containing linear compounds are similar to each other. Owing to the influences of the ligands, the Mo]Au, W]Au and Au]S bond lengths are different. The Au]As bond lengths are, of course, longer than corresponding Au]P distances.The explanation for this fact is that the covalent radius (1.21 Å) of As is longer than that (1.10 Å) of P. Thirdly, the Au]P bond length [2.272(2) Å for 6] trigonally coordinated in Mo]M9]L (M9 = Cu, Ag or Au) compounds 3, 4, Fig. 1 Crystal structure of [MoAu2S4(AsPh3)2] Fig. 2 Crystal structure of [WAu2S4(AsPh3)2] Fig. 3 Packing of [MoAu2S4(AsPh3)2] in the solid stateJ. Chem. Soc., Dalton Trans., 1997, Pages 2357–2361 2359 and 6 is between the Cu]P [2.210(5) Å] and Ag]P distances [2.380(4) Å], though atom covalent radii vary as Au > Ag > Cu, showing that the Au]P bond is stronger than the Cu]P and Ag]P.The same trend is observed in W]M9]S compounds 7, 8 and 10. Fourthly, M9]P, M9]S and M]M9 bond lengths in tetrahedral co-ordination are longer than those in trigonal coordination in compounds 3–5 and 7–9. However, the opposite trend is found in M]S bond distances. NLO properties of [MAu2S4(AsPh3)2] (M = Mo 1 or W 2) The similarity in the structures of the two compounds should lead to similar UV/VIS spectra, which is confirmed by Fig. 5. The red shift in the spectrum of compound 1 is expected since it contains one Mo atom instead of one W atom. The first absorption peaks are located at 500 (2.48) and 410 nm (3.02 eV) for compounds 1 and 2, respectively. Their Z-scan results are shown in Fig. 6, where the filled and open circles were measured Fig. 4 Packing of [WAu2S4(AsPh3)2] in the solid state Table 1 Crystal data and experimental parameters for complexes 1 and 2 * Formula M Crystal size/mm a/Å b/Å c/Å a/8 b/8 g/8 U/Å3 T/K Dc/g cm23 F(000) m(Mo-Ka)/cm21 2qmax/8 Scan speed/8 min21 No.observations [I >1.5s(I)] RR 9 Goodness of fit indicator Maximum, minimum peaks in final difference map/e Å23 1 C36H30As2Au2MoS4 1230.59 0.41 × 0.20 × 0.16 9.580(4) 10.753(4) 19.838(8) 88.12(4) 80.20(4) 67.39(3) 1857(1) 290.2 2.200 1152.00 102.55 50.0 2.0 4591 0.0299 0.0387 1.108 0.96, 20.71 2 C36H30As2Au2S4W 1318.50 0.50 × 0.41 × 0.30 9.572(2) 10.803(2) 19.816(4) 88.15(1) 80.30(2) 67.52(2) 1865.2(7) 295.2 2.348 1216.00 129.64 60.0 8.0 7676 0.0638 0.0859 0.935 6.85, 25.29 * Details in common: triclinic, space group P1� ; Z = 2; 407 variables; maximum shift in final cycle 0.00.with and without the aperture, respectively. To obtain the NLO parameters we employed a Z-scan theory which considers effective non-linearities of third-order nature only: a = a0 1 a2 I Scheme 3 Table 2 Selected bond distances (Å) and angles (8) for compound 1 Au(1)]Mo Au(1)]S(1) Au(2)]Mo Au(2)]S(3) Mo]S(1) Mo]S(3) Mo]Au(1)]As(1) Mo]Au(1)]S(2) As(1)]Au(1)]S(2) Mo]Au(2)]As(2) Mo]Au(2)]S(4) As(2)]Au(2)]S(4) Au(1)]Mo]Au(2) Au(1)]Mo]S(2) Au(1)]Mo]S(4) Au(2)]Mo]S(2) Au(2)]Mo]S(4) S(1)]Mo]S(3) S(2)]Mo]S(3) S(3)]Mo]S(4) Au(1)]S(2)]Mo Au(2)]S(4)]Mo 2.7837(7) 2.395(2) 2.7690(7) 2.378(2) 2.216(2) 2.214(2) 172.74(3) 49.93(5) 133.10(6) 174.86(3) 50.10(5) 125.23(6) 178.51(3) 55.80(6) 124.70(6) 123.01(7) 56.18(6) 109.07(10) 108.13(10) 111.86(9) 74.28(7) 73.72(7) Au(1)]As(1) Au(1)]S(2) Au(2)]As(2) Au(2)]S(4) Mo]S(2) Mo]S(4) Mo]Au(1)]S(1) As(1)]Au(1)]S(1) S(1)]Au(1)]S(2) Mo]Au(2)]S(3) As(2)]Au(2)]S(3) S(3)]Au(2)]S(4) Au(1)]Mo]S(1) Au(1)]Mo]S(3) Au(2)]Mo]S(1) Au(2)]Mo]S(3) S(1)]Mo]S(2) S(1)]Mo]S(4) S(2)]Mo]S(4) Au(1)]S(1)]Mo Au(2)]S(3)]Mo 2.3745(8) 2.392(2) 2.3715(8) 2.396(2) 2.213(2) 2.213(2) 49.97(5) 126.55(6) 99.89(7) 50.26(6) 134.31(6) 100.36(8) 55.86(6) 123.44(7) 125.34(6) 55.68(6) 111.65(9) 108.24(9) 107.92(10) 74.15(7) 74.06(7) Table 3 Selected bond distances (Å) and angles (8) for compound 2 Au(1)]W Au(1)]S(1) Au(2)]W Au(2)]S(3) W]S(1) W]S(3) W]Au(1)]As(1) W]Au(1)]S(2) As(1)]Au(1)]S(1) W]Au(2)]As(2) W]Au(2)]S(4) As(2)]Au(2)]S(4) Au(1)]W]Au(2) Au(1)]W]S(2) Au(1)]W]S(4) Au(2)]W]S(2) Au(2)]W]S(4) S(1)]W]S(3) S(2)]W]S(3) S(3)]W]S(4) Au(1)]S(2)]W Au(2)]S(4)]W 2.8103(4) 2.427(3) 2.7951(4) 2.400(3) 2.213(2) 2.213(3) 172.38(3) 49.55(6) 134.02(6) 174.63(3) 49.71(6) 125.34(6) 178.27(2) 55.70(7) 124.77(7) 122.93(7) 56.27(6) 108.6(1) 108.3(1) 112.08(10) 74.75(7) 74.02(7) Au(1)]As(1) Au(1)]S(2) Au(2)]As(2) Au(2)]S(4) W]S(2) W]S(4) W]Au(1)]S(1) As(1)]Au(1)]S(1) S(1)]Au(1)]S(2) W]Au(2)]S(3) As(2)]Au(2)]S(3) S(3)]Au(2)]S(4) Au(1)]W]S(1) Au(1)]W]S(3) Au(2)]W]S(1) Au(2)]W]S(3) S(1)]W]S(2) S(1)]W]S(4) S(2)]W]S(4) Au(1)]S(1)]W Au(2)]S(3)]W 2.3733(9) 2.406(3) 2.3698(9) 2.418(3) 2.217(2) 2.218(2) 49.32(6) 126.65(6) 98.86(8) 49.72(6) 135.12(7) 99.42(9) 56.28(7) 123.15(7) 125.11(7) 55.81(7) 111.96(10) 108.14(10) 107.9(1) 74.40(7) 74.47(8)2360 J.Chem. Soc., Dalton Trans., 1997, Pages 2357–2361 Table 4 Comparison of main bond distances (Å) a 13 4 5 627 8 9 10 11 Compound [MoAu2S4(AsPh3)2] [MoCu2S4(PPh3)3]?0.8CH2Cl2 [MoAg2S4(PPh3)3]?0.8CH2Cl2 [NEt4][MoAg(CuCN)S4(PPh3)2] [MoAu2S4(PPh3)2] [WAu2S4(AsPh3)2] [WCu2S4(PPh3)3]?0.8CH2Cl2 [WAg2S4(PPh3)3]?0.8CH2Cl2 [NEt4][WAg(CuCN)S4(PPh3)2] [WAu2S4(PMePh2)2] [MoOS3(AuPPh3){Au(PPh3)2}] M]Sb 2.214(2) 2.218(5) 2.198(5) * 2.215(5) 2.195(5) * 2.202(6) 2.189(5) * 2.214(2) 2.215(2) 2.224(8) 2.204(3) * 2.219(1) 2.195(5) * 2.202(5) 2.189(5) * 2.219(3) 2.261(2) 2.241(2) * M]M9b 2.7764(7) 2.642(3) 2.775(2) * 2.860(2) 3.030(2) * 2.622(3) 3.075(2) * 2.810(1) 2.8027(4) 2.670(3) 2.809(3) * 2.886(2) 3.056(2) * 2.638(3) 3.099(2) * 2.841(1) 2.838(1) 3.133(1) * M9]Sb 2.390(2) 2.220(5) 2.313(5) * 2.459(5) 2.572(5) * 2.209(7) 2.584(5) * 2.405(2) 2.413(3) 2.232(9) 2.333(3) * 2.476(6) 2.579(5) * 2.219(6) 2.596(5) * 2.429(3) 2.419(2) 2.644(2) * M9]L 2.373(8) 2.210(5) 2.303(5) * 2.380(4) 2.471(4) * 1.87(2) 2.484(5) * 2.272(2) 2.372(9) 2.209(8) 2.307(8) * 3.362(5) 2.460(1) * 1.82(2) 2.479(5) * 2.268(3) 2.277(2) 2.325(2) * Ref.This work 18 19 20 17 This work 19 19 21 22 17 a M = Mo or W; M9 = Cu, Ag or Au. b Average values. * The starred bond lengths are those when the Cu or Ag has tetrahedral co-ordination and the S or P atom is bonded to the Cu or Ag.and n = n0 1 n2 I, where a, a0 and a2 are the total, linear and non-linear absorption coefficients, n, n0 and n2 the total, linear and non-linear refractive indices and I is the light irradiance. The details of the theory can be found elsewhere.16 The good fits between the theory and the Z-scan data suggest that the observed non-linearities can be expressed effectively by thirdorder susceptibilities.The values of a2 and n2 extracted from the best fits are listed in Table 5. The modulus of the third-order molecular susceptibility was calculated from equation (1) where |g| = 1 NF4÷S9 × 108e0n0 2c2a2 4pw D2 1 Scn0 2n2 80p2 D2 (1) e0 and c are the permittivity and the speed of light in a vacuum, respectively, w is the angular frequency of the light, N the compound concentration, and F4 the local Lorentz field. In this expression all the units are SI except that N is in cm23 and |g| is in esu.Assuming that F4 = 3, we calculate that |g| = 3.0 × 10229 and 6.5 × 10229 esu (esu = 7.162 × 1013 m5 v22) for compounds 1 and 2, respectively. Note that such a large value is measured in the transparent region for compound 2, and is several orders of magnitude greater than those in well known NLO materials in the transparent part of their spectra (for example: 5.6 × 10235– Fig. 5 Electronic spectra of [MoAu2S4(AsPh3)2] (9.6 × 1025 mol dm23) (– – –) and [WAu2S4(AsPh3)2] (4.2 × 1024 mol dm23) (——) in CH2Cl2.Optical path 1 cm 8.6 × 10234 esu for Group 10 metal alkynyl polymers at 1064 nm,23,24 1 × 10232–1 × 10231 esu for metallophthalocyanines at 1064 nm25 and 7.5 × 10234 esu for C60 at 1910 nm).26 It is also interesting to compare these two new compounds with clusters that we have previously reported. Table 5 shows that compound 2 compares favourably with all the clusters in terms of figures of merit, a2/a0 and n2/a0. It should be emphasized that the Z scans reported here could not reveal the origins of the observed non-linearities.Excitedstate absorption and non-linear scattering are possible for the measured absorptive non-linearity. The change in the sign of the measured refractive non-linearity may give a hint as to the cause of the non-linear refraction. The signs of refractive nonlinearities for all the clusters, listed in Table 5, show that n2 alters from positive to negative as the ratio of the photon energy (hw) to that of the first absorption peak (hw0) approaches 1 : 1.The turning point is located at around hw/ hw0 ª 0.8 : 1, which is consistent with a recently developed theory on bound-electronic effects.27 Fig. 6 Z Scans of [MoAu2S4(AsPh3)2] (6.4 × 1024 mol dm23) and [WAu2S4(AsPh3)2] (5.4 × 1024 mol dm23) with 532 nm, 7 ns laser pulses. Optical path 1 mm. Incident energy of pulses 20 mJ. Transmittance of the aperture 0.34. The experimental data were measured with (d) and without (s) the aperture, respectively. The solid curves represent fits based on Z-scan theory.The Z scans of [WAu2S4(AsPh3)2] have been vertically displaced by 0.4 for clarityJ. Chem. Soc., Dalton Trans., 1997, Pages 2357–2361 2361 Table 5 NLO Parameters for clusters measured at photon energy hw = 2.33 eV Cluster [WCu2OS3(PPh3)4] a [MoCu2OS3(PPh3)3] a [Mo2Ag4S8(PPh3)4] b [NEt4]3[WOS3(CuBr)3(m-Br)]?2H2Oc [WAu2S4(AsPh3)2] d [NBun 4]2[MoCu3OS3(NCS)3] e [MoAu2S4(AsPh3)2] d [NBun 4]4[Mo8Cu12O8S24] f hw0/eV 4.85 4.80 4.75 3.55 3.02 2.50 2.48 2.43 hw/hw0 0.48 0.49 0.49 0.66 0.77 0.93 0.94 0.96 1023 a0/dm3 cm21 mol21 2.5 15 6.4 5.3 0.44 1.2 4.5 7.5 105 a2/dm3 cm W21 mol21 ª0 35 100 6.6 13 0.18 7.9 28 1010 n2/dm3 cm2 W21 mol21 6.7 68 120 12 19 21.7 28.0 223 108 a2a0 21/cm2 W21 ª0 2.3 16 1.2 29 0.15 1.8 3.7 103 |n2/a0|/cm3 W21 2.7 4.5 19 2.3 42 1.4 1.8 3.1 a Ref. 5. b Ref. 9. c Ref. 7. d This work. e Ref. 3. f Ref. 4. References 1 R. 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