Synthesis, Crystal Structure and MMCT Property of a New Mixed-valence Cyanide-bridged Binuclear Complex①

SUN Zhi-Ho WANG Yong MA Xio WANG Yn-Long HU Sheng-Min SHENG Tin-Lu② WU Xin-To

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Synthesis, Crystal Structure and MMCT Property of a New Mixed-valence Cyanide-bridged Binuclear Complex①

SUN Zhi-Haoa, bWANG Yonga, bMA XiaobWANG Yan-Longa, bHU Sheng-MinaSHENG Tian-Lua②WU Xin-Taoa

a(350002)b(100039)

A new mixed-valence cyanide-bridged complex, Br3Fe(-NC)RuBr(dppm)2(dppm = bis(diphenylphosphino)methane), was obtained through the reaction between trans-(dppm)2Cl- RuCN and FeBr3. Its crystal structure was characterized. Electronic absorption spectra indicate the existence of metal-to-metal charge transfer (MMCT) and this complex is Class II mixed valence complexes according to the classification of Robin and Day. Magnetic analysis shows it is paramagnetic.

cyanide-bridged, mixed-valence, binuclear, MMCT, crystal structure

1 INTRODUCTION

Electron transfer is ubiquitous in natural and artificial systems. To elucidate the fundamental fac- tors that govern electron transfer, extensive multi- disciplinary research efforts have been focused on mixed-valence complexes[1]. In this context, mixed- valence cyanide-bridged complexes have received increasing attention because of their metal-to-metal charge transfer (MMCT) property.This property has been intensively investigated by Neil G. Connelly[2, 3], Heinrich Vahrenkamp[4, 5], Luis M. Baraldo[6, 7]and so on. Metal centers, ancillary ligands and trans or cis geometry will all exert significant influence on the MMCT property. One of our research focuses is to investigate mixed-valence cyanide-bridged com- plexes and their MMCT and NLO properties. In our previous papers, we have reported a series of mixed- valence cyanide-bridged polynuclear complexes and their properties[8-13]. Herein we describe the synthesis and structure of a new mixed-valence binuclear complex, Br3Fe(-NC)RuBr(dppm)2, and its MMCT and magnetic properties are also re- ported.

2 EXPERIMENTAL

2. 1 Materials and measurements

UV-vis absorption spectra were recorded on a Perkin-Elmer Lambda 35 UV-vis spectrophotometer, and Infrared (IR) spectra on a Bruker Optics Vertex 70 FT-IR spectrometer with KBr pellets. Elemental analyses (C, H and N) were carried out on a Vario MICRO elemental analyzer. The magnetic suscepti- bilities were measured with the use of a Quantum Design MPMS-XL SQUID susceptometer under an applied magnetic field of 1000 Oe in a 2～300 K.

No further purification was made for all reagents purchased from commercial sources. Nitrogen atmosphere was used for all reactions through stan- dard Schlenk techniques.Trans-Ru(dppm)2Cl2was prepared according to a reported method[14].

2. 2 Synthesis of trans-(dppm)2ClRuCN

To a dichloromethane solution (5 mL) of trans- Ru(dppm)2Cl2(94.0 mg, 0.1 mmol), a water solution (5 mL) of 10 equiv of KCN (65.0 mg, 1 mmol) was added and the resulted mixture was stirred at 45 ℃ for 24 h. Then the water layer was discarded. The dichloromethane layer was washed with water (10 mL′3) and then dried with anhydrous magnesium sulfate. After that, the organic layer was filtered and the solvent was removed on a rotary evaporator to give the yellow powder. Pure yellow crystals of trans-(dppm)2ClRuCN (48.4 mg, 52% (based on trans-Ru(dppm)2Cl2)) were obtained by layering diethylether onto the dichloromethane solution of the product. Anal. Calcd. (%) for RuC51H44P4ClN: C, 65.77; H, 4.76; N, 1.50. Found (%): C, 65.59; H, 4.89; N, 1.38. IR (KBr, cm?1): 2073 (CoN).

2. 3 Synthesis of Br3Fe(μ-NC)RuBr(dppm)2 (1)

A mixed solution of trans-(dppm)2ClRuCN (93.1 mg, 0.1 mmol) in dichloromethane (10 mL) and FeBr3(59.1 mg, 0.2 mmol) in methanol (10 mL) was stirred at room temperature for 3 h. The solution was then filtered and the solvent was removed on a rotary evaporator. The gathered dark powder was dissolved in dichloromethane (10 mL) and filtered to get the filtrate. Dichloromethane was removed on a rotary evaporator and dark redpowder was obtained. Puredark red crystals of complex 1 (62.3 mg, 49% (based on trans-(dppm)2ClRuCN)) suitable for X-ray diffraction were obtained by layering diethylether onto the dichloromethane solution of the product. Anal. Calcd. (%) for RuFeC51H44P4Br4N: C, 48.18; H, 3.49; N, 1.10. Found (%): C, 48.47; H, 3.67; N, 1.19. IR (KBr, cm?1): 2013 (CoN).

2. 4 Crystal structure determination

The single-crystal data of complex 1 (0.45mm × 0.27mm × 0.10mm) were acquired with the use of Saturn724+ diffractometer equipped with graphite- monochromatic Mo(= 0.71073 ?) radiation using anscan mode at 123(2) K. A total of 56528 reflections were collected in the range of 2.70≤≤27.45o, out of which 11631 were independent withint= 0.0980 and 9815 were observed with> 2(). The structure was solved by direct methods with SHELXS-97[15]. Hydrogen atoms were generated geometrically. All the non-hydrogen atoms were located with successive difference Fourier syntheses and refined by full-matrix least-squares on2using SHELXL-97[16].The final= 0.0543 and= 0.1401 (= 1/[2(F2) + (0.0790)2+ 0.0000], where= (F2+ 2F2)/3),= 1.060, (Δ/)max= 0.002, (Δ)max= 2.738 and (Δ)min= ?1.419 e/?3.The selected bond lengths and bond angles are listed in Table 1.--

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

3 RESULTS AND DISCUSSION

3. 1 Synthesis

Both trans-(dppm)2ClRuCN and complex 1 are stable at room temperature in air. Trans- (dppm)2ClRuCN was obtained by the reaction of trans-Ru(dppm)2Cl2and KCN in dichloromethane and water. IR spectrum indication of the existence of CoN (2073 cm?1) and elemental analyses are consistent with (dppm)2ClRuCN. According to Winter[17], when the CN group which is a strongacceptor coordinates to [Ru(dppm)Cl]+, trans-isomer is thermodynamically more stable. This suggests the maintenance of trans mode during the preparation of trans-(dppm)2ClRuCN from trans-Ru(dppm)2Cl2.

During the formation of complex 1, one Br atom from FeBr3substituted the Cl atom of trans- (dppm)2ClRuCN when these two precursors reacted, leading to Br3Fe(-NC)RuBr(dppm)2instead ofBr3Fe(-NC)RuCl(dppm)2. This can be explained by the stronger nucleophilicity of Br-than Cl-. Because FeBr3is not soluble in dichloromethane, methanol was used in the preparation of complex 1.

3. 2 Structure description

The structure of complex 1 is shown in Fig. 1. Complex 1 crystallizes in the monoclinic21/space group. As depicted in Fig. 1, the ruthenium and iron atoms in 1 are linked through a cyanide bridge forming a binuclear structure.For the Fe center, each Fe atom is coordinated to three Br atoms and one N atom from the bridging cyanide. The coordination environment of iron atom is tetrahedral. For the Ru center, four P atoms from two dppm ligands, one Br atom and one CNgroup occupy the coordination sites of each Ru atom to form a distorted octahedron. The distortion should result from the short methylene bridge of the dppm ligand (71.67(3)°forP(1)-Ru(1)-P(2) and 71.88(3)°for P(3)-Ru(1)-P(4))[17]. The Ru-P bond distances vary from 2.361(8) to 2.377(8) ?, which are larger than that of trans-[Ru(dppm)2Cl2] (2.340(1)～2.367(1) ?)[18]. This elongation can be ascribed to the substitution of chlorine by CNgroup, aacceptor[17].The same reason can explain the elon- gation of Ru-Br bond (2.591(4) ?) in 1 compared to trans-[RuBrCl(dppe)2] (2.49(6) ?)[19](dppe = bis(diphenylphosphino)ethane). The Ru-C bond length of 1.923(3) ? is comparable to those of a simi-lar isonitrile complex, trans- [Ru(dppm)2Cl(CNtBu)]+PF6-(1.932(7) ?)[17]and another two corresponding complexes, Cp(dppe)RuCNFeCl3(1.927(6) ?) and Cp(PPh3)2RuCNFeCl3(1.935(4) ?)[20, 21]. The bond lengths of CoN and Fe-N are 1.170(4) and 1.937(3) ?, respectively, which fall in the same range with Cp(dppe)RuCNFeCl3and Cp(PPh3)2RuCNFeCl3(1.166(6), 1.902(5) ? and 1.161(5), 1.918(4) ?, respectively). The bond angles of Br-Ru-P and C-Ru-P are nearly 90°(84.17(2)～93.36(2)°and 88.15(9)～95.04(9)°, respectively) and the Ru-CoN-Fe array is essentially linear with the Ru-CoN and CoN-Fe angles being 175.9(3) and 175.4(3)°.

Fig. 1. Structure of complex 1 (Hydrogen atoms are omitted for clarity; probability is 30%)

3. 2 IR spectra

As expected, the precursor trans-(dppm)2ClRuCN and complex 1 both show a single cyanide band (2073 and 2013 cm?1, respectively). Owing to the withdrawal of charge of FeIII, theback-bonding from RuIIto the CoN bond is strengthened. This weakens the cyanide bond and leads to a shift to lower frequency of(CoN) of complex 1 compared to that of trans-(dppm)2ClRuCN.

3. 3 Electronic absorption spectra

The UV-vis spectra of complex 1 and its precur- sors trans-(dppm)2ClRuCN and FeBr3were recorded in CH2Cl2solution (CH3CN:CH2Cl2= 1:10 for FeBr3). As shown in Fig. 2, by comparison of the three spectra, the band atmaxof 283 nm can be assigned to dπ(Ru) →(P) metal-to-ligand charge transfer (MLCT). The shoulder peak at about 328 nm and the band at 431 nm may result fromtransition (t2→ e) of Ru[22, 23], Br → Fe(III) or(CN) → dπ(Fe(III)) ligand-to-metal charge transfer (LMCT) transition[9], or the combination of them. Compared with its precursors, the product exhibits a new band at 569 nm which should be MMCT from Ru(II) to Fe(III).

Fig. 2. UV-vis absorption spectra of complex 1 (solid), trans-(dppm)2ClRuCN (dot) and FeBr3(dash) in CH2Cl2

To investigate the extent of metal-metal coupling and the degree of electron delocalization in complex 1 which can be indicated by two parameters pro- vided by Hush[24], Haband α2respectively, we calcu- lated those parameters using equations listed below.

Hab=max, α = 2.06 × 10?2(max/d)(εmaxΔ1/2/max)1/2α2= 4.24 × 10?4(εmaxΔ1/2/maxd2)

max,max, Δ1/2and d are the peak location, extinction coefficient, the full width at half- maximum and distance between Ru and Fe, respectively. The value of Habis 1623 cm?1and that of α20.0085, suggesting the relatively weak magnitude of metal-metal coupling and electron delocalization, compared with Cp(dppe)RuCNFeCl3(Hab= 3332 cm?1and α2= 0.0271) and Cp(PPh3)2RuCNFeCl3(Hab= 2872 cm?1and α2= 0.0230). These data show that complex 1 is Class II mixed-valence compound, according to the classification of Robin and Day[25].

3. 4 Magnetic measurement

The variable-temperature magnetic susceptibility measurements of complex 1 were performed under an external magnetic field of 1000 Oe in the temperature range of 2～300 K. At 300 K, the observedmT value of 4.35 cm3K mol?1is consistent with the theoretical value of 4.38 cm3K mol?1for one uncoupled high spin Fe(III) ion (s = 5/2) and one diamagnetic low spin Ru(II) ion (s = 0). As shown in Fig. 3, with the decrease of temperature, themvalue increases andmT value keeps nearly constant. Based on a reported method[26], a tempera- ture-independent paramagnetism (TIP) of-0.00217 cm3mol-1was obtained by fitting the χmvalue using the functionm= C/(T-) +TIP. This also shows that the temperature dependence of molar magnetic susceptibility from 2 to 300 K obeys the Curie- Weiss law with C = 4.62 cm3mol?1K and=-0.08 K. These magnetic susceptibility data indicate com- plex 1 is paramagnetic.

Fig. 3. Temperature dependence ofm(squares) andmT (circles)of complex 1

4 CONCLUSION

A new binuclear cyanide-bridged complex, Br3Fe(μ-NC)RuBr(dppm)2, was obtained, in which there exists a MMCT from Ru(II) to Fe(III). The investigation shows that this binuclear cyanide- bridged complex is Class II mixed-valence com- pound according to the classification of Robin and Day. It is another evidence to prove that cyanide is an excellent bridging ligand to mediate the elec- tronic communication between metal centers through its-orbital.

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24 March 2014;

23 July 2014 (CCDC 992958)

① This work was supported by the 973 Program (2012CB821702, 2014CB845603) and the National Natural Science Foundation of China (21233009, 21203194 and 21173223)

. E-mail: tsheng@fjirsm.ac.cn