LIU Yang ZHU Xiao-Quan WU Xin-Tao SHENG Tian-Lu②

a (College of Chemistry,Fuzhou Uniνersity,Fuzhou 350002,China)

b (State Key Laboratory of Structural Chemistry,Fujian Institute of Research on the Structure of Matter,Chinese Academy of Sciences,Fuzhou 350002,China)

c (Uniνersity of Chinese Academy of Sciences,Beijing 100049,China)

ABSTRACT We have designed and synthesized a family of dinuclear cyanido-bridged complexes [PY5Me2Ru(μ-CN)Ru(dppe)CpMen][PF6]2 (PY5Me2=2,6-bis (1,1-bis(2-pyridyl)ethyl) pyridine,Cp=cyclopentadienyl,n=0,2[PF6]2;n=1,3[PF6]2;n=5,4[PF6]2) by using a mononuclear complex [PY5Me2Ru(μ-CN)][PF6] (1) as the precursor.All the three complexes have been fully characterized by including single-crystal X-ray diffraction analysis.The one-electron oxidation complexes 23+,33+ and 43+ obtained in situ all show a MMCT absorption band in the visible range.The MMCT energy increases as the redox potential of the N-terminal fragments decreases,and the redox potential decreases as the number of methyl groups on the cyclopentadiene of the cyanido-nitrogen coordinated Ru metal increases,supported by the TDF/TDDFT calculations.

Keywords:electron transfer,mixed-valence,metal to metal charge transfer (MMCT),cyanide bridge;DOI:10.14102/j.cnki.0254-5861.2011-3147

1 INTRODUCTION

The investigation on electron transfer process has attracted a lot of attention from chemists and physicists over the past decades[1-4],because understanding electron transfer process is very important in some critical issues such as designing artificial photosynthesis[5],exposing catalytic mechanisms[6],development of superconducting materials[4],design of molecular electronic devices[7,8],etc.Mixed-valence (MV) complexes are ideal simple models for investigating electron transfer process[9-13].Low-valent metals can be used as electron donors to transfer electrons to high-valent metal electron acceptor fragments.Using mixed valence model makes it easy to calculate the electron transfer rate and the activation energy of intervalence electron transfer[14,15].Among them,the investigation on dinuclear ruthenium is the most common[16-19],such as the Creutz-Taube ion[20].Various bridges can be used to connect the electron donors and acceptors,such as pyrazine[20],alkyne[21-31],cyanide bridges[16,32-54],naphthalene[55-58],the organic bridge with redox activity[59,60]and even mononuclear metals with multiple coordination sites[50,54].To date,it has been investigated how electron transfer process is influenced by the distance between the two metals[30],the energy barrier of electron transfer[54]and thecis-/trans-configuration.Cyanide is a short-range and asymmetric linear bridging ligand.The cyanide C-terminal metal feeds back much stronger electrons to the C≡N anti-orbital through the dπorbital than the cyanide N-terminal metal does.For this reason,the cyanide C-terminal metal is often used as electron-donor and the cyanide N-terminal metal as electron-acceptor.The metal center can effectively transfer electrons through the cyanide bridge.Therefore,cyanide bridge is often used as a bridging ligand to investigate the electron transfer process in asymmetric mixed-valence dinuclear compounds.

2 EXPERIMENTAL

2.1 Physical measurements

Vario MICRO elemental analyzer was used to detect the element content (C,H and N).Infrared (IR) spectra were collected using KBr pellets on a PerkinElmer Spectrum.UV-vis-NIR absorption spectroscopy was collected with the PerkinElmer Lambda 365 UV-vis-NIR spectrophotometer.The cyclic voltammetry curve was measured under argon by V3-Studio with methylene chloride as the solvent and 0.1 M (Bu4N) PF6as the promoting electrolyte at a scan rate of 100 mVs-1.The electrolytic cell consists of glassy graphite as working electrode,platinum as counter electrode and Ag/AgCl as the reference electrode.Ferrocene was used to calibrate the potential.The single-crystal X-ray diffraction data for complexes 2 [PF6]2,3 [PF6]2and 4 [PF6]2were collected on a Saturn 724+CCD diffractometer equipped with graphite-monochromatic MoKa(λ=0.71073 ?) radiation at 293 K.Complex 1 (PF6) was collected on a metal Jet D2+diffractometer with graphite-monochromatic GaKα(λ=1.3405 ?) radiation at 110 K.All structures were solved by intrinsic phasing methods using ShelXT-2018/3 and refined with ShelXL-2018/3[61],OLEX2[62]program package.The SQUEEZE program in the PLATON software was used[63].

2.2 Materials and synthesis

2.2.1 Materials

All operations were performed under argon atmosphere using standard Schlenk technology,unless otherwise specified.PY5Me2[64],CpRu(dppe)Cl[65],CpMeRu(dppe)Cl[51]and CpMe5Ru(dppe)Cl[66]were prepared by the previous literature.Methanol,ethanol and dichloromethane are 50 ppm super-dry solvents purchased from Adamas.All other raw materials were purchased commercially and used without further purification.

2.2.2 Synthesis

2.2.2.1 Preparation of PY5Me2RuCl2

At room temperature,RuCl3·3H2O (1.0 g,3.82 mmol) was added to 300 mL of anhydrous ethanol.The mixture was

Fig.1.Chemical structure of complexes 23+ (n=0),33+ (n=1) and 43+ (n=5)

Our group has carried out a systematic study on the regulation of electron transfer in cyanide bridged mixed-valence compounds by different methods,such as changing the electron donating ability of ligands,cyanide bridge orientation,cis-trans isomerism and so on[49,50,53,54].In our previous studies,we found that changing the electron-donating ability of the ligand in the electron acceptor fragment has a more impact on the behavior of MMCT than in the electron-donor fragment[53].To further investigate the influence of electron-acceptor fragment's electron-accepting ability on the behavior of MMCT,we have synthesized and characterized three complexes [(PY5Me2)Ru(μ-CN)Ru-(dppe)CpMen]2+(22+,n=0;32+,n=1;42+,n=5).As shown in Fig.1,all the one-electron oxidation complexes N3+(N=2,3,4),which are obtained in situ by gradually adding cerium ammonium nitrate in acetonitrile solution of the N2+complexes,exhibit a metal-to-metal charge transfer (MMCT) properties.The MMCT energy can be tuned by changing the number of methyl groups of cyclopentadiene located on the Ru center of the electron acceptor fragment,further supported by the time-dependent density functional theory (TD-DFT) calculations.refluxed at 88 °C for 6 hours,and then cooled to room temperature,to which PY5Me2(1.7 g,3.83 mmol) in 50 mL ethanol was added.The mixture was refluxed at 88 °C for 48 hours,cooled to room temperature and filtered to remove the precipitate.The filtrate was rotary evaporated to remove the solvent to obtain a pale yellow crude product,which was recrystallized with ethanol to obtain 1.4 g of pure product with a yield of 57.5%.

2.2.2.2 Preparation of [PY5Me2RuCN][PF6],1[PF6]

At room temperature,10 equivalents of KCN (1.02 g,15.7 mmol) were added to an aqueous solution of PY5Me2RuCl2(1.0 g,1.57 mmol).The mixture was refluxed at 110 °C for 2 hours,then cooled to room temperature,and an excess of NH4PF6was added,resulting in a light green precipitate.The precipitate was filtered and dried in vacuum,giving 1.02 g light green product of 1[PF6] (yield 90.8%).The yellow crystals of 1[PF6] suitable for X-ray diffraction single-crystal structure analysis were obtained by slow diffusion of anhydrous ether into the DMF solution of 1[PF6] (75 mg,yield 81%).

Anal.Calcd.(%) for C30H25N6F6PRu·2H2O:C,47.97;H,3.86;N,11.19.Found (%):C,47.69/47.85;H,4.02/4.04;N,11.00/11.01.IR (vCN,KBr pellet):2077 cm-1.

2.2.2.3 Preparation of [PY5Me2RuⅡCNRuⅡ(dppe)Cp][PF6]2,2[PF6]2

At room temperature,the compound PY5Me2RuCN [PF6] (50 mg,0.07 mmol) was added to 1 equivalent of CpRu(dppe)Cl (42 mg,0.07 mmol) in methanol (10 mL).The mixture was refluxed for 24 hours and cooled to room temperature.An excess of NH4PF6was added and stirred for ten minutes to obtain a red precipitate.This precipitate was filtered and washed with a small amount of methanol and ether,giving the product of 2[PF6]2.The yellow crystals of 2[PF6]2suitable for X-ray diffraction single-crystal structure analysis were obtained by slow diffusion of anhydrous ether into the dichloromethane solution of 2[PF6]2(69 mg,yield 75%).

Anal.Calcd.(%) for C61H54F12N6P4Ru2·CH3CN:C,51.30;H,3.89;N,6.69.Found (%):C,50.78;H,3.81;N,6.90.IR (νCN,KBr pellet):2093 cm-1.

2.2.2.4 Preparation of [PY5Me2RuⅡCNRuⅡ(dppe)MeCp][PF6]2,3[PF6]2

At room temperature,the compound PY5Me2RuCN [PF6] (50 mg,0.07 mmol) was added to 1 equivalent of Cp1Ru(dppe)Cl (43 mg,0.07 mmol) in methanol (10 mL).The mixture was refluxed for 24 hours and cooled to room temperature.Then an excess of NH4PF6was added and stirred for ten minutes to obtain a red precipitate which was filtered and washed with a small amount of methanol and ether,giving the product of 3[PF6]2.The yellow crystals of 3[PF6]2suitable for X-ray diffraction single-crystal structure analysis were obtained by slow diffusion of anhydrous ether into the dichloromethane solution of 3[PF6]2(75 mg,yield 81%).

Anal.Calcd.(%) for C62H56F12N6P4Ru2·CH3CN:C,51.89;H,3.99;N,6.62.Found (%):C,52.10/51.49;H,3.98/4.01;N,6.64/6.56.IR (νCN,KBr pellet):2091 cm-1.

2.2.2.5 Preparation of [PY5Me2RuⅡCNRuⅡ(dppe)Me5Cp][PF6]2,4[PF6]2

At room temperature,the compound PY5Me2RuCN [PF6] (50 mg,0.07 mmol) was added to 1 equivalent of Cp5Ru (dppe) Cl (47 mg,0.07 mmol) in methanol (10 mL),and the resulting mixture was refluxed for 24 hours and cooled to room temperature.An excess of NH4PF6was added and stirred for ten minutes to obtain a red precipitate.The preci-pitate was filtered and washed with a small amount of methanol and ether,giving the product of 4[PF6]2.The yellow crystals of 4[PF6]2suitable for X-ray diffraction single-crystal structure analysis were obtained by slow diffusion of anhydrous ether into the dichloromethane solution of 4[PF6]2(55 mg,yield 53%).

Anal.Calcd.(%) for C66H64F12N6P4Ru2·H2O:C,52.38;H,4.37;N,5.55.Found (%):C,52.75/52.62;H,4.78/4.89;N,5.63/5.53.IR (νCN,KBr pellet):2067 cm-1.

3 RESULTS AND DISCUSSION

3.1 X-ray structure determination

X-ray crystal structures of complexes 1[PF6] and 2[PF6]2～4[PF6]2are shown in Fig.2.The crystallographic data are summarized in Table 1.The space groups of compounds 1[PF6],2[PF6]2,3[PF6]2and 4[PF6]2arePbcm,P,P21/mandP21/n,respectively.For dinuclear complexes 2[PF6]2～4[PF6]2,the structures of their anions are similar.In the structure of each anion,the Ru(1) and Ru(2) centers are bridged through the cyanide ligand,and are six-and four-coordinated by five N atoms from the PY5Me2ligand and one cyanido-carbon atom and by cyclopentadiene,one cyanido-nitrogen atom and the two P atoms from the dppe ligand,respectively.Some selected bond lengths and angles are listed in Table 2.The Ru(1)-C(1)≡N(1)-Ru(2) arrange-ments of all dinuclear Ru complexes are close to a linearity with bond angles of almost 180°.For compounds 2 and 3,the bond lengths and angles are very similar,but compound 4 has undergone significant changes.First,the cyanide bond length increases due to the increase of electrons on the Ru(2)dorbital feedback to the cyanideπ* orbital,which reduces the cyanide bond level.The change trend of cyanide length is consistent with the results of the cyclic voltammetry and the infrared spectroscopy.Second,the bond lengths of Ru(1)-N(av.),Ru(2)-P(av.) and Ru-Cp become longer,which may be caused by the greater steric hindrance of the ligand as the methyl group increases.This indicates that the change of bond lengths between the metal center and ligand of such compounds not only depends on the electronic effect of the ligand,but also is affected by the steric effect of the ligand.

Table 1.Test Condition,Structure Refine and Crystallographic Data for Compounds 1[PF6],2[PF6]2,3[PF6]2 and 4[PF6]2

Table 2.Selected Bond Lengths (?) and Bond Angles (°) for Compounds 1～4

Fig.2.X-ray crystal structures of complexes 1～4 ([PF6]- ions,hydrogen atoms,acetonitrile and dichloromethane molecules have been removed for easy observation).Ru,dark blue;N,blue;P,wine red;C,grey

3.2 Electrochemistry

The cyclic voltammetry of the four compounds in dichloromethane all showed one reversible redox wave.The results are shown in Fig.3 and Table 2.The cyclic voltammetry of complexes 2[PF6]2,3[PF6]2and 4[PF6]2each shows one reversible redox wave at 0.342,0.267 and 0.234 V,respectively,which could be attributed to CpMen(dppe)RuII/CpMen(dppe)RuIIIand is higher than the similar monomer[67]based on the previous paper[53].The redox wave of the mononuclear complex 1[PF6]2exhibits one redox wave assigned to (PY5Me2)RuII/(PY5Me2)RuIII.However,the same redox wave for (PY5Me2)RuII/(PY5Me2)RuIIIin the three dinuclear complexes was not observed,which may be due to its too higher potential position.From Table 3,it can be found that with the increase of the number of methyl groups on cyclopentadiene the redox wave position moves to a lower potential from 2[PF6]2and 3[PF6]2to 4[PF6]2.

Table 3.Electrochemical Data (vs Cp2Fe+/0) for Complexes 1～4 in 0.10 M DCM Solution of Bu4NPF6 at a Scan Rate of 100 mV·s-1

Table 4.CN Stretching Frequencies for Complexes 1(PF6)～4(PF6)2

3.3 IR spectroscopy

Infrared spectroscopy is an excellent way to characterize cyanide bridged compounds.The contraction vibration of cyanide is easy to observe in infrared spectroscopy,and the cyanide stretchingν(CN) position can help us judge the connection rigidity and electrons feedback situation of the compounds.Compared with the position of the terminal cyanide signal of the mononuclear compound [PY5Me2RuCN]+(2077 cm-1),the position of the bridged cyanide band of the dinuclear compounds 22+(2093 cm-1) and 32+(2091 cm-1) has a blue shift due to the rigid restriction of the cyanide N-coordinated Ru on the movement of the bridging cyanide.But for compound 42+,the pentamethylcyclopentadiene ligand has a stronger electron-donating ability.It promotes the transfer ofdorbital electron of the nitrogen-terminal Ru fragment to theπanti-bond orbital of the cyanide bridge to form a feedbackπbond,which reduces the cyanide energy level and results in the absorption peak redshift.With the enhancement of the electron-donating ability of the ligand on the C-terminal metal,the redshift of the cyanide vibration frequency is often observed,but it is rare to reduce the cyanide vibration energy through the feedback electron from the N-terminal metal.

Fig.3.Cyclic voltammograms of 12+～42+ in a 0.10M dichloromethane solution of Bu4NPF6 at a scan rate of 100 mV·s-1 vs (Cp2Fe)+/0

3.4 UV-VIS-NIR spectroscopy

UV-VIS-NIR absorption spectroscopy is the most effective method for studying electron transfer.According to the strongest absorption position,absorption intensity and half-width of the MMCT absorption peak,the strength of the electron transfer of the compound can be investigated.In order to study the influence of the electron-donating ability of the acceptor terminal ligand on the MMCT,we used the acetonitrile solution of cerium ammonium nitrate to gradually oxidize the three dinuclear compounds to obtain mixed-valence compounds in situ.Their absorption spectra were measured,as shown in Figs.4 and 5.The absorption peak from 27500 cm-1to 22000 cm-1for each of the mixed-valence compounds 23+,33+and 43+obtained in situ is attributed to the metal-to-ligand electron transfer (MLCT) from Ru2+to the PY5Me2ligand[68],and the new absorption peak from 16000 cm-1to 9000 cm-1is attributed to the metal-to-metal electron transfer (MMCT)[53].For MLCT absorption peaks,the maxima absorption peaks of the three compounds 22+,32+and 42+before oxidation are basically the same,all at about 25000 cm-1.After one-electron oxidation,it can be observed that the positions of the MLCT maxima absorption peaks of the three dinuclear mixed valence Ru compounds 23+,33+and 43+each exhibits a significant blue shift,and the absorption intensity is also significantly weakened.This is because that from 22+,32+and 42+to 23+,33+and 43+the electron-donating ability of RuIIin the (PY5Me2)RuIIfragment weakens due to the electron withdraw effect of the CpMen(dppe)RuIII.

For the MMCT absorption peak,with the increase of the electron-donating ability of the substituted group from Cp,CpMe to CpMe5,the MMCT absorption peaks of the three mixed-valence compounds each shows a blue shift (10953 cm-1for 23+,11274 cm-1for 33+and 11442 cm-1for 43+),and the absorption intensity increases (2125M-1cm-1→2324 M-1cm-1→2786 M-1cm-1).This is because that the electron-accepting ability of RuIIIin the CpMen(dppe)RuIIIfragment decreases as the electron-donating ability increases from Cp,CpMe to CpMe5,resulting in the increase of the RuII→RuIIIMMCT energy from 23+,33+to 43+,strongly supported by the TD-DFT calculation.

The electronic coupling constant Habof complexes 23+～43+was calculated using the Hush-Mullikan equation (Eq.1)[9],with the results listed in Table 5.In the equation,v1/2is the bandwidth at half-intensity of the MMCT band maximumνmax,andεmaxandrABrepresent the molar extinction coefficient and the through space intermetallic distance,respectively.As shown in Table 5,the electronic coupling constantHabgradually increases from 23+,33+to 43+.This may be understood by the fact that as the number of methyl groups of Cp increases the mixed valence state [RuII-CN-RuIII] becomes more and more stable,resulting in the more and more MMCT energy from [RuII-CN-RuIII] to [RuIII-CN-RuII] from 23+,33+to 43+,and the Habincreases correspondingly.According to the calculation results,the one-electron oxidation products 23+,33+and 43+belong to the class II mixed valence compounds.

Table 5.MMCT Transition Energies and Electronic Coupling Constant for All the Mixed-valence Complexes

Fig.4.UV-Vis-NIR absorption spectra of complexes 2～4 oxidized by the addition of ammonium ceric nitrate in acetonitrile

3.5 DFT/TDDFT calculations

To continue studying the influence of electron acceptor orbital changes on the properties of MMCT,we performed TD-DFT calculations using B3LYP/lanl2dz[69,70]for the three MV compounds.As shown in Table 6,the spin electron density of the three MV compounds is mainly localized on RuIII,further indicating that the single-electron oxidation products of the three compounds belong to the class Ⅱ mixed-valence compounds.Due to the decrease of electron-accepting ability of the acceptor fragments from 23+,33+to 43+,the RuII→ RuIIIMMCT gets to be more difficult,resulting in an increase of the spin electron density on the acceptor metal center.For MMCT,the calculation results are basically consistent with the experimental data,as shown in Table 7.The metal donor HOMO orbitals and acceptor LUMO orbitals of the three MV compounds are shown in Fig.6.The major contribution for the MMCT absorption band of complex 23+comes from molecular orbital 248B to 251B.For compound 33+,the MMCT absorption peak is mainly derived from molecular orbital 252B to 255B.For compound 43+,the MMCT absorption peak mainly comes from the molecular orbitals from 268B to 271B.The LUMO orbitals of the three compounds are almost localized on Ru3+,while the HOMO ones are localized on Ru2+,which indicates that the three compounds all belong to the class II mixed-valence compounds,consistent with the measured UV-Vis-NIR absorption spectral results.

Table 6.Mulliken Spin Density of Mixed-valence Species

Table 7.Comparison of the Measured and the Calculated MMCT Energies of 23+,33+ and 43+

Fig.5.MMCT absorption spectra of complexes 2～4 oxidized by adding ammonium ceric nitrate in acetonitrile

Fig.6.Molecular orbital diagrams of HOMO-2 (248B) and LUMO (251B) for 2 (above),HOMO-2 (252B) and LUMO (255B) for 3 (middle),HOMO-2 (268B) and LUMO (271B) for 4 (bottom).The isosurface value is 0.02 au

4 CONCLUSION

In summary,we have synthesized and characterized a mononuclear Ru fragment and three cyanido-bridged dinuclear Ru compounds 22+,32+and 42+.All one-electron oxidation complexes 23+,33+and 43+obtained in situ show a MMCT absorption band in the NIR range.The MMCT energy increases as the number of methyl groups on the cyclopentadiene of the cyanido-nitrogen coordinated Ru metal increases,supported by the DFT/TDDFT calculations.Furthermore,the UV-vis-NIR absorption spectra and TDDFT calculations show that all the single-electron oxidation compounds belong to class Ⅱ mixed-valence compounds.This work shows that slight modifications to the ligand on the N-terminal metal center can tune the MMCT properties of the mixed valence compounds.