Sulphur-bridged rhenium(I) based molecular cubanes [Re4(CO)12(m3-SR)4] (1, R ¼ C4H9; 2, R ¼ C6H5; 3, R¼ C6H4CH3; 4, R ¼ CH2C6H5) have been synthesised in one-pot reaction via oxidative addition of dialkyl/diaryl disulphide, RSSR to low valent transition metal carbonyl, Re2(CO)10 with catalytic amount ofdimethyl formamide. The molecular cubanes 1e4 have been characterised by elemental analysis, NMR, IRand UVeVis absorption spectroscopic techniques. The molecular structures of 2 and 4 were determinedby single crystal X-ray diffraction analysis and the structural studies ascertain the presence of Re4S4distorted cube. The synthetic strategy offers a simple route for preparation of the class of sulphur-bridgedRe(I) based molecular cubanes.
Introduction also reported the selenato-bridged manganese cubane by reacting
Transition metal-sulphido compounds containing cubane-typeM4S4 core have drawn much attention in recent literature due totheir significant relevance to biological systems such as ferredoxins[1e5], hydrogenases [6,7] and nitrogenases [8]. Several transitionmetal-sulphido compounds of homo- and heterometallic cubaneshave been synthesised and characterised for their pertinence tometalloenzymes [9e21]. Apart from nature, this type of transitionmetal-sulphido cubanes have also been utilised in industrial pro-cesses as metal sulphide catalysts [22e28]. Mostly, transition metalcontainingmolecular cubanes were prepared by stepwise synthesisfrom pre-assembled smaller building blocks of di- and trinuclearsulphide complexes [29e37]. Rauchfuss et al. prepared rutheniumbased sulphido cubane [(MeC5H4)4Ru4S4] by thermolysis of(MeC5H4)Ru(PPh3)2SH in toluene medium [38]. Kanatzidis and co-workers prepared the platinum based cubanes K4[Pt4S22] bytreating K2PtCl4 with K2S4 in methanol under heating condition[39]. Earlier, Abel and co-workers reported the synthesis of methylthiolate-bridged rhenium cubane [Re(CO)3(SMe)]4 and analogousmanganese cubane [Mn(CO)3(SMe)]4 by the reaction of M(CO)5Br(M ¼ Mn, Re) and Me2Sn(SMe)2 [40e43]. Later, Villarreal et al.isolated alkyl/aryl thiolate-bridged manganese cubane[MnSR(CO)3]4 by treating [Mn(h5-C5H7)(CO)3] with alkyl/arylmercaptane at room temperature [44,45]. In addition, they have
: þ91 413 2656740.(Bala. Manimaran).
[Mn(h5-C5H7)(CO)3] with selenol at room temperature to yield[MnSeR(CO)3]4 [46]. Rhenium based anionic cubane(PPh4)4Re4S4(SCN)12 was synthesised by the heating reaction ofRe4S4Te4Cl16 with KSCN [47]. Mizobe et al. prepared rheniumcontaining heterometallic cubane [M2(ReL)2(m3-S)4] (M ¼ Ru(h5-C5Me5), PtMe3, Cu(PPh3); L ¼ S2C2(SiMe3)2) by treating dirheniumtetra sulphido complex with a series of group 8e11 metal com-plexes [48]. Saito and co-workers reported the synthesis ofrhenium and copper containing cubane [(Ph3P)2N][Re3(CuX)S4Cl6(PMe2Ph)3] by the reaction of Re3S7Cl7, dimethylphenylphos-phine and CuX2 (X ¼ Cl, Br) [49]. In literature, very few reports areavailable for the spontaneous formation of molecular cubanes viaself-assembly process to give thermodynamically favoured product[50,51]. Herein, we report on the spontaneous formation of[Re4(CO)12(m3-SR)4] (1e4) via oxidative addition of SeS bond acrossReeRe bond. The self-assembly of molecular cubanes 1e4 wasachieved in a facile one-pot reaction of rhenium carbonyl withdialkyl/diaryl disulphide in presence of catalytic amount ofdimethyl formamide.
Results and discussion
Synthesis of sulphur-bridged rhenium based molecular cubanes 1e4from rhenium carbonyl and dialkyl/diaryl disulphides
Synthesis of sulphur-bridged rhenium containing molecularcubanes [Re4(CO)12(m3-SR)4] (1e4) was accomplished in a one-pot
Scheme 1. Synthesis of sulphur-bridged molecular cubanes 1e4.
Table 1Crystallographic data and structure refinement of 2 and 4.
Compound 2 4
M. Karthikeyan, Bala. Manimaran / Journal of Organometallic Chemistry 769 (2014) 130e135 131
reaction via oxidative addition of dialkyl/diaryl disulphide (RSSR) tothe rhenium carbonyl (Re2(CO)10) with dimethyl formamide ascatalyst in mesitylene medium (Scheme 1). Dimethyl formamidefacilitated the removal of terminal carbonyl groups of rheniumcarbonyl via coordination through carbonyl oxygen of dimethylformamide [52]. Due to the labile nature of dimethyl formamidecoordination to rhenium metal centre, it further aided the forma-tion of molecular cubane [53]. Reaction of rhenium carbonyl withdialkyl/diaryl disulphide for the formation of molecular cubanewastuned by using different catalysts like triethyl amine, trimethylamine oxide and dimethyl formamide. The reaction in whichdimethyl formamide was used as a catalyst afforded better yield ofthe product in comparison with other catalysts. Moreover, thepreparative method involving dimethyl formamide as catalyst is asingle step one-pot process. Compounds 1e4 were isolated as airstable solids and soluble in common organic solvents. Molecularcubanes 1e4 were characterised by spectroscopic techniques andthe structural details of 2 and 4 were determined by single crystalX-ray diffraction analysis.
Empirical formula C36H20O12Re4S4 C40H28O12Re4S4Formula weight 1517.56 1573.74T (K) 293(2) K 150(2) KCrystal system monoclinic TetragonalSpace group P 21/c I 41/aa (Å) 11.56 18.98b (Å) 22.16 18.98c (Å) 16.57 24.42a (�) 90 90b (�) 107.96 90g (�) 90 90V (Å3) 4039.8(2) 8805.7(2)Z 4 8Dcalc (mg m�3) 2.495 2.374m (mm�1) 12.213 11.210h, k, l collected �13, 13; �23,
26; �19, 19�22, 21; �22,22; �29, 26
F(000) 2784 5822Crystal size (mm) 0.25 � 0.20 � 0.20 0.29 � 0.21 � 0.17Theta range for data
collection (�)2.61 to 25.00 3.03 to 25.00
Reflections collected 18697 20158Rint 0.0459 0.0260Completeness to
theta ¼ 25.00�99.8% 99.8%
Max. and min. transmission 0.087 and 0.067 0.2516 and 0.1394Data/restraints/parameters 7113/216/488 3879/0/271Goodness-of-fit on F2 1.139 1.054Final R indices
[I > 2sigma(I)]R1 ¼ 0.0453,wR2 ¼ 0.0950
R1 ¼ 0.0250,wR2 ¼ 0.0608
R indices (all data) R1 ¼ 0.0568,wR2 ¼ 0.1009
R1 ¼ 0.0349,wR2 ¼ 0.0626
Largest difference in peakand hole (e.�3)
2.167 and �2.135 1.197 and �0.979
Spectroscopic characterisation of 1e4
The IR spectra of compounds 1e4 in CH2Cl2 exhibited twocarbonyl stretches in the region of n(CO) 2030e1938 cm�1, char-acteristic of the fac-Re(CO)3 core [45], indicating higher symmet-rical nature of molecular cubanes in solution state. 1H NMRspectrum of compound 1 showed three multiplets and one tripletfor butyl group protons at around d 1.02e2.92 ppm. The 1H NMRspectra of compounds 2e4 displayed signals for aryl group protonsat around d 7.35e7.87 ppm. Compound 3 showed a singlet atd 2.43 ppm for methyl group protons of p-tolyl group and com-pound 4 displayed a singlet at d 3.98 ppm for methylene groupprotons present in benzyl moiety. The alkyl/aryl group protonsattached to sulphur moiety in 1e4 were shifted to down fieldwhen compared to free dialkyl/diaryl disulphides. The 13C NMRspectra of compounds 1e4 displayed a signal at aroundd 195.0e195.8 ppm for terminal carbonyl groups. 13C NMR spec-trum of 1 exhibited signals at d 41.0e12.6 ppm for butyl groupcarbons. Compounds 2e4 showed signals for aryl group carbons ataround d 129.3e139.7 ppm. Compound 3 displayed a signal atd 21.1 ppm for methyl carbon of p-tolyl group and compound 4showed a signal at d 46.1 ppm for methylene carbon present inbenzyl moiety (Table S1). UVeVis absorption spectra of 1e4 inCH2Cl2 showed intense bands in UV region at around lmax226e290 nm, and were attributed to ligand based p-p* transitions[54e56].
Structural characterisation of 2 and 4
Single crystals of compound 2 and 4 were obtained by slowdiffusion of hexane into concentrated solution of compound indichloromethane at 25 �C. Good quality single crystals of 2 and 4were subjected to X-ray diffraction studies. Details about datacollection, solution and refinement were summarised in Table 1 for2 and 4. The ORTEP diagram of 2 and 4 were depicted in Fig. 1 andFig. 2 respectively.
Crystal structure of [Re4(CO)12(m3-SC6H5)4] (2) adopted a dis-torted cubic structure, where rhenium and sulphur occupy alter-native corners of the cube. Each rhenium in fac-Re(CO)3 core isbonded to three phenyl sulphido groups and thereby rhenium
Fig. 1. Molecular structure of 2. The thermal ellipsoids are drawn at the 40% proba-bility level. Selected bond lengths (Å) and angles (
M. Karthikeyan, Bala. Manimaran / Journal of Organometallic Chemistry 769 (2014) 130e135132
centre has attained a distorted octahedral geometry. Each sulphidogroups is bonded to three rhenium metal centres and one phenylgroup. The Re2S2 ring is bent around the Re1eRe2 interconnectionforming a dihedral angle of 5.07
�between Re(1)-S(1)-Re(3) and
Re(1)-S(4)-Re(3) planes. The Re/Re distance in Re2S2 ring is
Fig. 2. Molecular structure of 4. The thermal ellipsoids are drawn at the 40% proba-bility level. Selected bond lengths (Å) and angles (
~3.99 Å. The ReeS and ReeC bond distances are in the range of2.506e2.509 Å and 1.913e1.946 Å respectively which are compa-rable to those of methyl thiolate cubane [Re(CO)3(SMe)]4 (ReeS:2.48e2.52 Å and ReeC: 1.88e1.92 Å) and rheniumemolybdenumheterocubane [Mo(S)Re3(CO)9(S4)] (ReeS: 2.505e2.515 Å andReeC: 1.892e1.963 Å) [43,57]. CeH/O hydrogen bonding wasobserved between C(33)-H(33) of phenyl group and C(20)-O(8) ofadjacent carbonyl group with a distance of 2.671 Å (Fig. 3) [58e60].
Crystal structure of [Re4(CO)12(m3-SCH2C6H5)4] (4) adopted adistorted cubic structure, where each rhenium and sulphur occupyalternative corners of the cube. Each rhenium in fac-Re(CO)3 coreis bonded to three benzyl sulphido groups and has a distortedoctahedral geometry around it. Each sulphido group is bonded tothree rhenium metal centres and one benzyl group. The Re2S2 ringis bent around the Re1eRe2 interconnection forming a dihedralangle of 17.10
�between Re(1)-S(1)-Re(1)#2 and Re(1)-S(1)#2-
Re(1)#2 planes. The Re/Re distance in Re2S2 ring is ~3.80 Å.The ReeS and ReeC bond distances are in the range of2.505e2.515 Å and 1.900e1.952 Å respectively that are alsocomparable to those of ReeS and ReeC bond distances in methylthiolate cubane [Re(CO)3(SMe)]4 and rheniumemolybdenum het-erocubane [Mo(S)Re3(CO)9(S4)] [43,57]. The two benzyl sulphidogroups were bent away from the Re2S2 ring. Intermolecular CeH… p interactions were observed in between C(4)eH(4) of CH2group of benzyl moiety and phenyl group of adjacent benzyl unitof cubane with a distance of 2.774 Å, which leads to zig-zagpacking arrangement [61,62] (Fig. 4). CeH/O hydrogen bondingwas viewed among C(9)-H(9) of benzyl group and C(13)-O(6) ofcarbonyl group of adjacent Re(CO)3 unit of cubane with a distanceof 2.685 Å.
Redox property of molecular cubanes 1e4
Redox properties of the molecular cubanes 1e4 were investi-gated by cyclic voltammetry in dichloromethane at 150mV s�1 scanrate. Molecular cubanes 1 and 4 consist of a quasi-reversible one-electron reduction at�1.09 and�1.15 V respectively followed by anirreversible one-electron reduction at �1.83 and �1.99 V respec-tively. The molecular cubanes 2 and 3 consist of irreversible onestep two-electron reduction at �1.50 and �1.40 V respectively,presumably due to the presence of rigid aryl groups attached tothiolate bridges. The redox potentials are comparable with litera-ture reports [57,63].
Experimental section
Instruments and materials
All the reactions were carried out under dry, oxygen-free N2atmosphere using standard Schlenk techniques. The starting ma-terials were purchased from SigmaeAldrich Chemicals. Rheniumcarbonyl, dibutyl disulphide, diphenyl disulphide and dibenzyldisulphide and p-tolyl disulphide were used as received. Dimethylformamide, mesitylene and other solvents were dried using liter-ature procedure prior to use [64]. IR spectra were taken on aThermo Nicolet 6700 FT-IR spectrometer. 1H and 13C NMR wererecorded on a Bruker Avance 400 MHz spectrometer. UVeVis ab-sorption spectra were recorded on a Shimadzu UV-2450 UVeVisspectrophotometer. Elemental analyses were performed usingThermo Scientific Flash 2000 CHNS analyzer. Cyclic voltammo-grams were obtained using Metrohm Autolab interface electro-chemical analyser consisting of three electrode configurations suchas; Pt working electrode (0.3 mm diameter), Pt auxiliary electrodeand Ag/AgCl reference electrode.
Fig. 3. Packing diagram of compound 2 viewed along b axis with CeH/O hydrogen bonding interactions.
M. Karthikeyan, Bala. Manimaran / Journal of Organometallic Chemistry 769 (2014) 130e135 133
Synthesis of [Re4(CO)12(m3-SR)4], general procedure
A mixture of Re2(CO)10 (0.1 mmol) and dialkyl/diaryl disulphide(0.1 mmol) were taken in a 50ml two neck Schlenk flask fitted witha reflux condenser. The systemwas evacuated and purged with N2.Dimethyl formamide (0.2 ml) and mesitylene (30 ml) were addedunder N2 atmosphere. The reaction mixture was heated to 130 �Cunder N2 for 10e12 h and allowed to cool to room temperature. Themesitylene was removed by vacuum distillation and the solidmixture was washed with hexane, chromatographed on silica gelcolumn using dichloromethane and hexane as eluent to give whitesolid of [Re4(CO)12(m3-SR)4] cubane.
Synthesis of [Re4(CO)12(m3-SC4H9)4] (1)
Amixture of Re2(CO)10 (65mg, 0.1mmol) and dibutyl disulphide(18 mg, 0.1 mmol) were taken in a 50 ml two neck Schlenk flaskfitted with a reflux condenser. The system was evacuated andpurged with N2. Dimethyl formamide (0.2 ml) and mesitylene(30 ml) were added under N2 atmosphere. The reaction mixturewas heated at 130 �C under N2 for 10 h and allowed to cool to roomtemperature. The solvent was removed by vacuum distillation and
Fig. 4. Packing diagram of compound 4 viewed along a axis with CeH/p interactions.
the reaction mixture was washed with hexane, chromatographedon silica gel column using dichloromethane and hexane as eluent(1:4) to give white solid of [Re4(CO)12(m3-SC4H9)4] (1). Yield: 50 mg,34% (based on Re2(CO)10). Anal. Calcd. for C28H36O12Re4S4: C,23.39%, H, 2.52%, S, 8.92%. Found: C, 24.18% H, 2.89% S, 8.63%. 1HNMR (400 MHz, CDCl3, ppm): 2.92 (m, 8H, H1, (butyl)), 1.64 (m, 8H,H2, (butyl)), 1.56 (m, 8H, H3, (butyl)), 1.02 (t, 12H, H4, (butyl),3J ¼ 7.6 Hz). 13C NMR (100 MHz, CDCl3, ppm): d 195.0 (CO group),41.0, 32.2, 20.3, 12.6 (butyl). UVeVis. {lmax
ab (CH2Cl2/nm)}: 227,248(sh), 280 (LIG). IR (CH2Cl2): n(CO) 2026 (s), 1938 (m) cm�1.
Synthesis of [Re4(CO)12(m3-SC6H5)4] (2)
Compound 2 was prepared using Re2(CO)10 (65 mg, 0.1 mmol),diphenyl disulphide (24 mg, 0.1 mmol) and dimethyl formamide(0.2 ml) in mesitylene (30 ml) by following the procedure adoptedfor 1. The product was isolated as a white solid of [Re4(CO)12(m3-SC6H5)4] (2). Yield: 48mg, 31% (based on Re2(CO)10). Anal. Calcd. forC36H20O12Re4S4: C, 28.49% H, 1.33% S, 8.45%. Found: C, 27.82% H,1.27% S, 8.23%. 1H NMR (400 MHz, CDCl3, ppm): d 7.87 (d, 8H, H2,(Ph), 3J ¼ 8.4 Hz), 7.57 (m, 8H, H3, (Ph)), 7.39 (m, 4H, H4 (ph)). 13CNMR (100MHz, CDCl3, ppm): d 195.5 (CO group),132.4,129.8,129.5,129.3 (ph). UVeVis. {lmax
Compound 3 was prepared using Re2(CO)10 (65 mg, 0.1 mmol),p-tolyl disulphide (24 mg, 0.1 mmol) and dimethyl formamide(0.2 ml) in mesitylene (30 ml) following the procedure adopted for1. The product was isolated as a white solid of [Re4(CO)12(m3-SC6H4CH3)4] (3). Yield: 58mg, 36%. Anal. Calcd. for C40H28O12Re4S4:C, 30.53%, H, 1.79%, S, 8.15%. Found: C, 30.13% H, 1.68% S, 8.08%. 1HNMR (400MHz, CDCl3, ppm): d 7.73 (d, 8H, H2, (p-tolyl), 3J¼ 8.0 Hz),7.35 (d, 8H, H3, (p-tolyl), 3J ¼ 8.0 Hz), 2.43 (s, 12H, CH3, (p-tolyl). 13CNMR (100 MHz, CDCl3, ppm): d 195.8 (CO group), 139.7, 130.5, 129.1,129.0 (ph), 21.1 (CH3, p-tolyl). UVeVis. {lmax
ab (CH2Cl2/nm)}: 229,250, 290(sh) (LIG). IR (CH2Cl2): n(CO) 2025 (s), 1938 (m) cm�1.
Synthesis of [Re4(CO)12(m3-SCH2C6H5)4] (4)
Compound 4 was prepared using Re2(CO)10 (65 mg, 0.1 mmol),dibenzyl disulphide (24 mg, 0.1 mmol) and dimethyl formamide(0.2 ml) in mesitylene (30 ml) following the procedure adopted for1. The product was isolated as a white solid of [Re4(CO)12(m3-SCH2C6H5)4] (4). Yield: 35mg, 22%. Anal. Calcd. for C40H28O12Re4S4:
M. Karthikeyan, Bala. Manimaran / Journal of Organometallic Chemistry 769 (2014) 130e135134
ab (CH2Cl2/nm)}: 227, 281 (LIG). IR (CH2Cl2): n(CO)2026 (s), 1938 (m) cm�1.
Crystal structure determinations
Crystals of 2 and 4 suitable for single crystal X-ray crystallog-raphy were grown by slow diffusion of hexane into concentratedsolution of compounds in dichloromethane. Single crystal X-raystructural studies were performed on Oxford Diffraction XCALIBUR-S CCD equipped diffractometer with an Oxford Instruments low-temperature attachment. Crystal data were collected at 150(2) Kusing graphite-monochromated Mo Ka radiation (la ¼ 0.71073 Å).The strategy for the data collection was evaluated by using theCrysAlisPro CCD software. The data were collected by the standard‘4-u scan’ techniques, and were scaled and reduced using CrysA-lisPro RED software. The structures were solved by direct methodsusing SHELXS-97 and refined by full matrix least squares withSHELXL-97 [65], refining on F2. The positions on all the atoms wereobtained by direct methods. All non-hydrogen atoms were refinedwith anisotropic displacement parameters. The hydrogen atomswere placed in geometrically constrained positions and refinedwith isotropic temperature factors, generally 1.2 � Ueq of theirparent atoms.
Conclusion
We have reported a facile route for the synthesis of rheniumbased sulphurebridged molecular cubanes from Re2(CO)10 anddialkyl/diaryl disulphide with dimethyl formamide as catalyst inone pot reaction. The molecular structure of compound 2 and 4have been elucidated by single crystal X-ray diffraction analysis toreveal the distorted cubic structure of Re4S4 core. This syntheticmethodology is an effective and simple route for the class ofcompounds.
Acknowledgements
We thank the Department of Science and Technology, Govern-ment of India and Council of Scientific and Industrial Research,Government of India for financial support. We are grateful to theCentral Instrumentation Facility, Pondicherry University forproviding spectral data.
Appendix A. Supplementary material
CCDC 971561 and 971562 contain the supplementary crystal-lographic data for this paper. These data can be obtained free ofcharge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Appendix B. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jorganchem.2014.07.016.
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