Syntheses,Structures and Electrochemical Properties of Two Tetranuclear Molybdenum(IV)Sulfur Nitrosyl Complexes

(Institute of Molecular Engineering andApplied Chemistry,Anhui University of Technology,Ma’anshan 243002,China)

Abstract：Reactions of ammonium molybdate tetrahydrate,ammonium thiocyanate,and hydroxylammonium chloride as a reducing agent in aqueous solution probably afforded a specie containing the Mo N--O fragment,which was further treated with ammonium polysulfide to result in isolation of two tetranuclear molybdenum(IV)sulfur nitrosyl complexes(NH4)4[Mo4(μ4-S)(μ3-S)2(μ-S2)4(η2-S2)(NO)4]·4H2O(1)and(NH4)4[Mo4(μ4-O)(μ-S2)6(NO)4](2).Two tetranuclear molybdenum(IV)complexes were characterized spectroscopically by ultraviolet spectrophotometer UV-3000 and infrared spectrophotometer FTS-40,and their structures were determined by single-crystal X ray diffraction,the results show that the two complexes containe nitrosyl group,their electrochemical properties were also investigated.

Key words：molybdenum(IV);molybdenum-sulfido complexes;nitrosyl;synthesis;structure;electrochemistry

Molybdenum is an indispensable element in some biological processes,and many biological proteins and enzyme activity centers contain molybdenum[1].Molybdenum compounds have broad application prospects in the fields of catalysis and materials science[2-3].It is well known that the activity centers of many chemical reaction catalysts and their changes are closely related to molybdenum sulfide compounds[4].Therefore,structural chemistry of molybdenum sulfide attracts many chemists to explore synthesis and reaction of different molybdenum-sulfide systems,for example,Müller firstly reported a tetranuclear molybdenum-sulfide nitrosyl complex(NH4)4[Mo4(NO)4S13]·2H2O in which the sulfide(S2)2-was found to be capable of being combined with the molybdenum atoms in the form of both end and bridge[5].Then,the syntheses and structures of such complexes have been extended,such as the binuclear molybdenum dithiolene complex[Cp*Mo(NO)S2]2reported by Nomura and his coworkers[6]and an octahedral cluster complex[Mo6S8(NO)4(PEt3)5]reported by Mizutani and coworkers[7].Herein we report syntheses,structures and electrochemical properties of two tetranuclear molybdenum(IV)sulfur nitrosyl complexes(NH4)4[Mo4(μ4-S)(μ3-S)2(μ-S2)4(η2-S2)(NO)4]·4H2O and(NH4)4[Mo4(μ4-O)(μ-S2)6(NO)4].

1 Experimental

Ammonium molybdate tetrahydrate and ammonium thiocyanate were purchased from Shanghai Chemical Technology Co.,Ltd and used as supplied.Ammonium polysulfide and hydroxylammonium chloride were purchased from Sinopharm Chemical Reagent Co.,Ltd and used as supplied.All elemental analyses were carried out using a Perkin-Elmer 2400 CHN analyzer.Electronic absorption spectra were obtained on a Shimadzu UV-3000 spectrophotometer.Infrared spectra were recorded on a Digilab FTS-40 spectrophotometer with use of pressed KBr pellets.Thermogravimetric analysis(TGA)was performed by using a Shimadzu DTG-60Hinstrument.Cyclic voltammetry was performed with on a CHI660D electrochemical workstation.A standard three-electrode cell was used with glassy carbon working electrode,a platinum counter electrode and Ag/AgCl reference electrode under an nitrogen atmosphere at 25℃formal potentials(Eo)were measured in CH2Cl2solutions with 0.1 mol/L[nBu4N]PF6as supporting electrolyte and reported with reference to the ferrocenium-ferrocene couple(Cp2Fe+/0).In the-1.5-+1.5 V region,a potential scan rate of 100 mV·s-1was used.

1.1 Synthesis of(NH4)4[Mo4(μ4--S)(μ3--S)2(μ--S2)4(η2--S2)(NO)4]·4H2O(1)

To a solution of(NH4)6Mo7O24·4H2O(208 mg,0.17 mmol)in aqueous solution(10 mL)was added NH4SCN(1.25 g,16.33 mmol)with stirring at room temperature.The resulting mixture was stirred at 60℃until a clear solution was obtained.After 5 min,a solution of NH2OH·HCl(175 mg,2.5 mmol)in distilled water(5 mL)was slowly added to the mixture solution,during which the color gradually changed from colorless to yellow.Then the mixture was heated at 80℃for 1 h,finally the color changed to yellow-green.The resulting solution was mixed with(NH4)2Sxsolution(10 mL)and then heated at 60℃in a glass round-bottom flask for 5 h.After that the mixture solution was cooled to room temperature and precipitated out black needle crystals identified as(NH4)2[Mo2(S2)6]·2H2O.The black needle crystals were collected by filtration,and then put the filtrate at 4 ℃ for three days to obtain red flake crystals of(NH4)4[Mo4(μ4-S)(μ3-S)2(μ-S2)4(η2-S2)(NO)4]·4H2O)(1).Yield：170 mg,53.7%(based on Mo).IR(KBr disc,cm-1),1 528(vs)and 1 401(vs);,3 442(s)and 3 130(s).

1.2 Synthesis of(NH4)4[Mo4(μ4-O)(μ-S2)6(NO)4](2)

NH4SCN(1.25 g,16.33 mmol)was added to an aqueous solution(10 mL)of(NH4)6Mo7O24·4H2O(208 mg,0.17 mmol),and the mixture was stirred for 5 min at room temperature and then further stirred at 60℃,till a clear solution was formed.A solution of NH2OH·HCl(175 mg,2.5 mmol)in distilled water(5 mL)was slowly added to the above solution,during which the color gradually changed from colorless to yellow.A clear yellow-green solution was formed after heating at 80℃for 1 h.To the resulting yellow-green solution was added ammonium polysulfide solution(10 mL)and then continued heating at 60℃in a glass round-bottom flask for 10 h.Black blockcrystalsof(NH4)4[Mo4(μ4-O)(μ-S2)6(NO)4](2)were directly obtained from the red filtrate at room temperature,and were isolated by filtration,washed with ethanol and diethyl ether,and dried in vacuo.Yield：111 mg,38.4%(based on Mo).IR(KBr disc,cm-1)：,1 565(vs)and 1 398(vs),,3 425(s)and 3 130(s).

1.3 X ray crystallography

Crystallographic data and experimental details for(NH4)4[Mo4(μ4-S)(μ3-S)2(μ-S2)4(η2-S2)(NO)4]?4H2O(1)and(NH4)4[Mo4(μ4-O)(μ-S2)6(NO)4](2)are summarized in Tab.1.Intensity data were collected on a Bruker SMART APEX 2000 CCD diffractometer using graphite-monochromated MoKαradiation(λ =0.710 73 ?)at 296(2)K.The collected frames were processed with the software SAINT[8].The data were corrected for absorption using the program SADABS[9].The structures were solved by Direct Methods and refined by full-matrix least-squares onF2using the SHELXTL software package[[10-12].All non-hydrogen atoms were refined anisotropically.All non-hydrogen atoms were refined anisotropically except for the solvent molecules due to disorder.Hydrogen atoms in the hydrone were treated as idealized contributions.

Tab.1 Crystallographic data and experimental details for[NH4]4[Mo4(μ4-S)(μ3-S)2(μ-S2)4(η2-S2)(NO)4]·4H2O(1)and[NH4]4[Mo4(μ4-O)(μ-S2)6(NO)4](2)

Crystal data(excluding structure factors)for the structures in this paper have been deposited with The Cambridge Crystallographic Data Centre as supplementary publication(CCDC-1849534-1849535).Copies of the data can be obtained free of chargeviahttp：//www.ccdc.cam.ac.uk/data_request/cif.

2 Results and discussion

Treatment of(NH4)6Mo7O24·4H2O with excess NH4SCN in the presences of NH2OH·HCl as a reducing agent and(NH4)2Sxas a sulfurizing agent in aqueous solution led to isolation of a tetranuclear molybdenum(IV)sulfur nitrosyl complex(NH4)4[Mo4(μ4-S)(μ3-S)2(μ-S2)4(η2-S2)(NO)4]·4H2O)(1)as red flake crystals in yield of 53.7%.(NH4)4[Mo4(μ4-O)(μ-S2)6(NO)4](2)was prepared by selective oxidation of the central μ4-S of complex 1 with oxygen in the air,isolated as black block crystals in a yield of 38.4%.The structures of both complexes 1 and 2 were established by X ray crystallography.Complex 1 crystallizes in space groupP21/cwith Z=4,with four[NH4]+cations and one[Mo4(NO)4S13]4-anion as shown in Fig.1.In[Mo4(NO)4S13]4-anion of complex 1,a sulfur atom in the center is bonded to four molybdenum atoms.The molybdenum atom is coordinated by μ4-S,μ3-S,μ-S2,η2-S2and NO groups.The central μ4-S atom is in an almost regular pentahedron environment presented by four molybdenum atoms,with Mo S bond lengths ranging from 2.581 8(10)to 2.710 0(10)?.The μ3-S atom is in an almost regular tetrahedron environment presented by three molybdenum atoms,with MoS bond lengths ranging from 2.446 9(10)to 2.522 3(10)?.The μ-S2Mo bond lengths range from 2.439 8(11)to 2.549 3(11)?.A similar kind of bonding of disulfide groups η2-S2to a molybdenum center(with a coordination number of 7)was observed in[(S2)2Mo(S2)2Mo(S2)]2-[13].In the function group,the bond lengths ofare 1.760,2.439 8 ?,respectively,and the bond angles of MoNO are in the ranges of 173.2(3)-178.1(3)o.Complex 2 crystallizes in space groupC2/cwithZ=4,with four[NH4]+cations and one[Mo4(NO)4OS12]4-anion as shown in Fig.2.In the[Mo4(NO)4OS12]4-anion of complex 2,an oxygen atom in the center is bonded to four molybdenum atoms.The molybdenum atom is coordinated by μ4-O,μ-S2and NO group.The central μ4-O is in an almost regular pentahedron environment presented by four molybdenum atoms,with MoO bond lengths being of 2.161 and 2.162 ?.The bond lengths of Moμ-Srange from 2.422 0(8)to 2.507 3(9)?,and the bond lengths of Mo--μ-S2range from 2.452 1(9)to 2.521 8(9)?.The bond angles of MoNO are 177.4(3)and 176.7(2)oin 2,similar to those in complex 1.

Fig.1 ORTEPdiagram of[Mo4(μ4-S)(μ3-S)2(μ-S2)4(η2-S2)(NO)4]4-anion in complex 1,showing the atom labeling scheme and 40%thermal ellipsoids

Fig.2 ORTEPdiagram of[Mo4(μ4-O)(μ-S2)6(NO)4]4-anion complex 2,showing the atom labeling scheme and 40%thermal ellipsoids

Complexes 1 and 2 were characterized spectroscopically by ultraviolet spectrophotometer UV-3000 and infrared spectrophotometer FTS-40,as shown in fig.3.The absorption peaks in the infrared spectra of both complexes 1 and 2 are approximately the same.Ab sorption peaks at 3 442.58 cm-1and 3 129.59 cm-1can be attributed to the NH of[NH4]+,and stretching vibrations at 1 527.83 cm-1and 1 400.76 cm-1are based on the symmetric and asymmetric oscillations of the nitrosyl groups.In Fig.4,The UV-vis spectrum of complex 1 shows intense absorption bands in the range of 200-560 nm,and the maximum at 450 nm.Complex 2 exhibits broad absorption in the range of 200-600 nm,with a maximum at 500 nm.These two absorption spectra have similar trends,which may be attributed to an intraligand transition and to ligand to metal charge transfers,compared with that of related oxo-molybdenum complexes.The thermo-gravimetric curves show in Fig.5 that the two complexes have similar variation in trend,experiencing three main weightlessness process：the first stage is between 25-130℃,compounds lose the crystal water;in the second stage,between 170-425℃,the skeleton collapses and the ligand is decomposed;The third stage is between 425-630℃,which should be the process of generating simple and stable metal oxides.In the cyclic volt-ampere curve of complex 1,anodic oxidation peak appeared at-0.73 V,and the cathode reduction peak appeared at-0.63 V.In the cyclic voltammetry curve of complex 2,anodic oxidation peak appeared at-0.86 V,whose cathode reduction peak appeared at-0.69 V.As shown in Fig.6,these cyclic voltammetry curves showed one obviously reversible couple,which is assigned as the metal-centered MoIV____MoVcouple.

Fig.3 IR absorption spectra of complexes 1 and 2

Fig.4 Solid-state UV/vis absorption spectra of complexes 1 and 2

Fig.5 TGAof complexes 1 and 2

Fig.6 CVs of complexes 1 and 2

3 Conclusion

These two tetranuclear molybdenum(IV)sulfur nitrosyl complexes(NH4)4[Mo4(μ4-S)(μ3-S)2(μ-S2)4(η2-S2)(NO)4]·4H2O(1)and(NH4)4[Mo4(μ4-O)(μ-S2)6(NO)4](2)were successfully isolated from ammonium molybdate tetrahydrate and ammonium thiocyanate in moderate yields,which is of significance to explore the catalytic effect and mechanism of some disproportionation reactions of olefin.