Multi-Ruthenocene Assemblies on an Organostannoxane Platform

Article pubs.acs.org/crystal

Multi-Ruthenocene Assemblies on an Organostannoxane Platform. Supramolecular Signatures and Conversion to (Ru−Sn)O2 Subrata Kundu,† Amit Chakraborty,† Kunal Mondal,‡ and Vadapalli Chandrasekhar*,†,§,# †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India § Tata Institute of Fundamental Research, Centre for Interdisciplinary Sciences, 21, Brindavan Colony, Narsingi, Hyderabad-500 075, India # National Institute of Science Education and Research, Institute of Physics Campus, School of Chemical Sciences, Bhubaneshwar-751005, India ‡

S Supporting Information *

ABSTRACT: The reaction of ruthenocene carboxylic acid (RcCOOH) with [n-BuSn(O)OH] n , (Ph 3 Sn) 2 O, and (PhCH2)3SnCl afforded hexameric compounds [RSn(O)OOCRc]6, R = n-Bu (1), Ph (2), and PhCH2 (3), respectively. These possess a prismane type Sn6O6 core which supports a hexa-ruthenocene periphery. Compounds [{n-Bu2Sn}2(μ3-O)OOCRc2]2 (4) and [n-Bu2Sn(OOCRc)2](5) were formed in the reaction of RcCOOH with n-Bu2SnO in 1:1 and 2:1 reactions, respectively. Compound [t-Bu2Sn(μ−OH)OOCRc]2 (6) is a dimer containing two ruthenocene units, and it was formed in the reaction of RcCOOH with (t-Bu2SnO)3 in a 3:1 ratio. Compounds 1−6 show an extensive supramolecular organization in the solid state as a result of several intermolecular interactions. Compound 1 could be converted quantitatively to a pure phase of the binary oxide, (RuSn)O2 at 400 °C.

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INTRODUCTION Multiferrocene assemblies have been attracting a lot of interest.1 This is due to the potential applications of such compounds as multielectron reservoirs, electrode modification materials, electrontransfer mediators, ion sensors, or as materials for electronic devices and its different type of supramolecular structure.2 Preparation of such compounds has been accomplished both by convergent or by divergent methodology.3 Although most of these compounds contain an organic focal point, use of inorganic scaffolds, albeit sparse, has been implemented: cylophosphazenes,4 siloxanes,5 a molybdenum chloride cluster,6 and organostanoxanes7 have been used. In contrast to ferrocenes, other multimetallocene assemblies are very few. Some of these include derivatives of (C5H5CoC5H5)+ and C5H5CoC4H4.8 In contrast to ferrocene, the heavier analogue viz., ruthenocene has been much less studied.8b This is surprising since available studies, though limited, indicate that ruthenocenecontaining biomolecules offer useful advantages over their ferrocene counterparts particularly in antibacterial and anticancer activity.9 In view of this, we were interested in exploring synthetic strategies that would allow ready assembly of multiruthenocene compounds. Previously, we have utilized the so-called stannoxane synthetic route to prepare multiferrocene derivatives.7b,10 The advantage of this protocol is its general robustness and a reasonable predictability about the type products being formed in a given reaction. Keeping this in mind, we explored the reactions of ruthenocene carboxylic acid (Rc-COOH) with various organotin © 2014 American Chemical Society

substrates, [BuSn(O)OH]n, (Ph3Sn)2O, (PhCH2)3SnCl, [nBu2SnO]n, and (t-Bu2SnO)3. The molecular and supramolecular structures of the various products obtained in these reactions, [n-BuSn(O)OOCRc]6 (1), [PhSn(O)OOCRc]6 (2), [PhCH2Sn(O)OOCRc] 6 (3), [{n-Bu 2 Sn} 2 (μ 3 -O)OOCRc 2 ] 2 (4), [n-Bu2SnOOCRc2] (5), and [t-Bu2Sn(μ−OH)OOCRc]2 (6) are discussed, herein. In addition, we also report the electrochemical behavior of these compounds (4-6) along with the conversion of [n-BuSn(O)OOCRc]6 (1), to (Ru−Sn)O2. This represents the first instance of the preparation of (Ru−Sn)O2 using a singlesource precursor.

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EXPERIMENTAL SECTION

All the reactions were performed under a dry nitrogen atmosphere by employing standard Schlenk techniques. All the solvents were purified by adopting standard procedures. [n-BuSn(O)OH]n (Aldrich), (Ph3Sn)2O, and [n-Bu2SnO]n (Aldrich) were used as such without any further purification. (PhCH2)3SnCl,11 (t-Bu2SnO)2,12 and RcCOOH [RcCOOH = (C5H5)Ru(C5H4)COOH]13 were synthesized according to the literature procedures. IR spectra were recorded as KBr pellets on a Bruker Vector 22 FT IR spectrophotometer operating from 400 to 4000 cm−1. Elemental analyses were carried out using a thermoquest CE instruments model EA/110 CHNS-O elemental analyzer. Received: November 19, 2013 Revised: December 20, 2013 Published: January 3, 2014 861

dx.doi.org/10.1021/cg4017323 | Cryst. Growth Des. 2014, 14, 861−870

Crystal Growth & Design

Article

Table 1. Crystal Data Collection and Refinement Parameters for 1−6 complex

1

2

3

completeness to θ (%) absorption correction data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)]

C90 H108O18Ru6Sn6 2796.45 100(2) triclinic, P1̅ a (Å):12.959(5) b (Å):13.832(5) c (Å):14.440(5) α (deg):69.617(5) β (deg):66.648(5) γ (deg):79.195(5) 2223.9(14), 1 1356 0.16 × 0.15 × 0.13 2.26 to 25.50 −15 ≤ h ≤ 14 −16 ≤ k ≤ 16 −17 ≤ l ≤ 7 11955/8082 [R(int) = 0.0234] 99 empirical 8082/21/544 1.052 R1 = 0.0393, wR2 = 0.0936

C144H132O18Ru6Sn6 3469.06 100(2) trigonal, R3̅ a (Å): 22.117 b (Å): 22.117 c (Å):24.215(5) α (deg): 90 β (deg): 90 γ (deg): 120 10258(2), 3 5112 0.080 × 0.070 × 0.066 1.84 to 25.48 −22 ≤ h ≤ 26 −26 ≤ k ≤ 25 −24 ≤ l ≤ 29 18451/4183 [R(int) = 0.0564] 98.9 empirical 4183/0/263 1.077 R1 = 0.0350, wR2 = 0.0813

R indices (all data)

R1 = 0.0485, wR2 = 0.1009

R1 = 0.0435, wR2 = 0.0859

largest difference peak and hole (e Å−3) complex

2.709 and −1.043 4

1.592 and −0.504 5

C108H96O18Ru6Sn6 3000.53 100(2) triclinic, P1̅ a (Å): 15.068(5) b (Å):15.086(5) c (Å): 15.680(5) α (deg):69.769(5) β (deg):68.998(5) γ (deg):71.416(5) 3044.1(17), 1 1452 0.14 × 0.13 × 0.12 1.76 to 25.50 --18 ≤ h ≤ 11 −18 ≤ k ≤ 14 −18 ≤ l ≤ 18 16521/11081 [R(int) = 0.0205] 97.7 empirical 11081/46/622 1.100 R1 = 0.0558 wR2 = 0.1613 R1 = 0.0645 wR2 = 0.1785 3.460 and −3.200 6

empirical formula formula weight (g/mol) temperature (K) crystal system, space group unit cell dimensions

volume (Å3), Z F(000) crystal size (mm3) θ collection range (°) limiting indices

feflections collected/unique

empirical formula formula weight (g/mol) temperature (K) crystal system, space group unit cell dimensions

completeness to θ (%) absorption correction data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)]

C76 H108O10Ru4Sn4 2060.75 100(2) triclinic, P1̅ a (Å): 12.481(5) b (Å): 13.445(5) c (Å): 13.744(5) α (deg):61.104(5) β (deg):72.618(5) γ (deg):84.290(5) 1924.2(13), 1 1020 0.16 × 0.14 × 0.12 4.08 to 25.02 −14 ≤ h ≤ 14 −15 ≤ k ≤ 14 −16 ≤ l ≤ 10 10004/6640 [R(int) = 0.0285] 97.8 empirical 6640/0/428 0.992 R1 = 0.0417, wR2 = 0.0983

R indices (all data)

R1 = 0.0534, wR2 = 0.1069

largest difference peak and hole (e Å−3)

1.725 and −0.836

volume (Å3), Z F(000) crystal size (mm3) θ collection range (deg) limiting indices

reflections collected/unique

C30H36O4Ru2Sn 781.44 100(2) orthorhombic, Pbcn a (Å): 21.612(5) b (Å): 9.698(5) c (Å): 26.478(5) α (deg): 90 β (deg): 90 γ (deg): 90 5550(3), 8 3088 0.17 × 0.16 × 0.14 2.30 to 25.50 −26 ≤ h ≤ 20 −11 ≤ k ≤ 10 −32 ≤ l ≤ 28 28483/5152 [R(int) = 0.0734] 99.7 empirical 5152/19/336 1.074 R1 = 0.0451 wR2 = 0.0995 R1 = 0.0626 wR2 = 0.1099 2.362 and −0.853

C38H56O6Ru2Sn2 1048.39 100(2) monoclinic, P21/n a (Å): 8.261(3) b (Å): 23.950(8) c (Å): 10.847(3) α (deg): 90 β (deg):111.639(5) γ (deg): 90 1994.9(11), 2 1040.0 0.09 × 0.08 × 0.07 4.38 to 51° −9 ≤ h ≤ 10, −27 ≤ k ≤ 29, −13 ≤ l ≤ 9 10530/3705 [R(int) = 0.0381] 99.1 empirical 3705/0/227 1.048 R1 = 0.0335 wR2 = 0.0828 R1 = 0.0384 wR2 = 0.0867 1.83 and −0.67

10 °C/min under nitrogen atmosphere. Cyclic voltammetric studies were performed on a BAS Epsilon electrochemical workstation using a platinum electrode, an Ag/AgCl reference electrode (3 M NaCl), and a

Melting points were recorded using a JSGW melting point apparatus and are uncorrected. TGA measurements were carried out using a Perkin-Elmer Pyris6 thermogravimetric analyzer at a heating rate of 862



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Scheme 1. Syntheses of 1 to 6

Cp ring), 5.18 (s, 8H, Cp ring). 119Sn NMR (186.51 MHz, CDCl3) (δ, ppm): −212(s), −219(s). IR (KBr, ν/cm−1): 3100 (w), 2957 (s), 2925 (s), 2857 (m), 1591 (s), 1575 (s), 1553 (s), 1467 (s),1381 (s), 1360 (m), 1261 (s), 1101 (s), 1024 (s),807 (s), 794 (s), 685 (m), 628 (w), 562 (m), 496 (s). ESI-MS: m/z (%) 1280.0551 [(n-Bu2Sn)3(O)2(RcCOO)2]+ (40). Anal. Calcd for C76H108O10Ru4Sn4: C, 44.29; H, 5.28. Found: C, 44.20; H, 5.31. [n-Bu2Sn(OOCRc)2](5). Quantities: [n-Bu2SnO]n (0.135 g, 0.540 mmol) and RcCOOH (0.30g, 1.08 mmol). Yield: 0.36 g (85%). Mp: 211 °C. 1H NMR (500 MHz, CDCl3, 25 °C) (δ, ppm): 0.92 (t, 6H, n-butyl CH3), 1.40−1.69 (m, 12H, n-butyl CH2’s), 4.59 (s, 10H, Cp ring), 4.71 (t, 4H, Cp ring), 5.17 (t, 4H, Cp ring). 119Sn NMR (186.51 MHz, CDCl3) (δ, ppm): −157(s). IR (KBr, ν/cm−1): 3100 (w), 2955 (s), 2856 (m),1591 (s), 1574 (s), 1467 (s), 1380 (s), 1331 (s), 1173 (s), 1101 (m), 807 (m) 496 (s). ESI-MS: m/z (%) 782.9855[5 + H]+ (20). Anal. Calcd for C30H36O4Ru2Sn: C, 46.11; H, 4.64. Found: C, 46.02; H, 4.71. Synthesis of [t-Bu2Sn(μ−OH)OOCRc]2 (6). Quantities: [tBu2SnO]3 (0.15g, 0.20 mmol) and RcCOOH (0.17 g, 0.60 mmol). Yield: 0.28 g (87%). Mp: 177 °C. 1H NMR (500 MHz, CDCl3, 25 °C) (δ, ppm): 1.36 (s, 36H, t-butyl CH3), 4.55 (s, 10H, Cp ring), 4.62 (s, 4H, Cp ring), 5.07 (s, 4H, Cp ring). 119Sn NMR (186.51 MHz, CDCl3) (δ, ppm): −270(s). IR (KBr, ν/cm−1): 3104 (m), 2928 (m), 2853 (s), 1596 (s), 2581 (s), 1473 (s), 1377 (s), 1330 (s), 1173 (s), 1161 (s), 1101 (s), 1019 (m), 996 (m),810 (s), 793 (s), 562 (m), 496 (s). ESI-MS: m/z (%) 793.1274[(t-Bu 2 Sn) 2 (OH) 2 L] + (100). Anal. Calcd for C 38 H 56 O6Ru2Sn2: C, 43.53; H, 5.38. Found: C, 43.01; H, 5.49. X-ray Crystal Structures of 1−6. Suitable crystals for single crystal X-ray diffraction studies were loaded on a Bruker AXS Smart Apex CCD diffractometer using a Mo Kα (λ = 0.71073 Å) sealed tube. All the structures were solved by direct methods using SHELXS-9714 and refined by full-matrix least-squares on F2 using SHELXL-97. The program SMART was used for collecting frames of data, indexing reflections, and determining lattice parameters, SAINT for integration of the intensity of reflections and scaling, SADABS for absorption correction, and SHELXTL for space group and structure determination and least-squares refinements on F2. Hydrogen atoms were fixed at calculated positions, and their positions were refined by a riding model. Hydroxyl protons were assigned from the electron density map and refined isotropically. Nonhydrogen atoms were refined with anisotropic displacement parameters. Details of the data collection and refinement parameters are given in Table 1. The crystallographic figures have been generated using Diamond 3.2g.15 Compounds 2 and 3 contained disordered solvent molecules that could not be modeled satisfactorily. These were removed by using PLATON/SQUEEZE. The squeeze method found the electron

platinum-wire counter electrode. All the measurements were performed using 1 mM concentration of the complexes in the presence of 0.1 M TBAPF6 (TBA = tetra-butyl ammonium) in dry dichloromethane. FESEM (Supra 400VP, Zeiss, Germany) was used to take the SEM image. The powder X-ray diffraction (XRD) measurements were conducted on a X’Pert Pro, PAN-analytical, Netherlands, X-ray system using Cu Kα radiation. Synthesis. A general procedure was applied to the synthesis of the metal complexes. A mixture of the metal precursor and RcCOOH were taken in 40 mL of toluene, and the mixture was heated under reflux for 6 h under a nitrogen atmosphere using a Dean−Stark apparatus. In the synthesis of complex 3, Dean−Stark apparatus was not used. In this reaction, one equivalent of triethylamine was added to remove the HCl formed in the reaction. The workup of each of these reactions was nearly similar. The reaction mixture was filtered, and the filtrate was concentrated to about 10 mL and kept for slow evaporation to afford X-ray quality crystals. Specific details of each reaction are given below. [n-BuSn(O)OOCRc]6 (1). Quantities: [n-BuSn(O)OH]n (0.30 g, 1.44 mmol) and RcCOOH (0.39 g, 1.44 mmol). Yield: 0.62 g (92%). Mp: 178 °C. IR (KBr, ν/cm−1): 3099 (m), 2955 (m), 2925 (m), 2856 (m), 1590 (s), 1574 (s), 1567 (s), 1381(s), 1353(s), 1331(s), 1173(s), 1101(m), 1025(m), 808(s), 793(s), 686(m), 562(m), 496(s). Anal. Calcd for C90H108O18Ru6Sn6: C, 38.65; H, 3.89. Found: C, 38.18; H, 3.97. [PhSn(O)OOCRc]6 (2). Quantities: (Ph3Sn)2O (0.30 g, 0.42 mmol) and RcCOOH (0.23 g, 0.84 mmol). Yield: 0.36 g (71%). Mp: 196 °C. IR (KBr, ν/cm−1): 3096 (m, br), 1520 (s), 1485 (s), 1432 (s), 1386 (s), 1359 (s), 1261 (s), 1181(s), 1100 (m), 807(s), 790(s), 727(s), 688(m), 550(m), 496(s). Anal. Calcd for C158H148O18Ru6Sn6: C, 51.94; H, 4.08. Found: C, 50.18; H, 3.89. [PhCH2Sn(O)OOCRc]6 (3). Quantities: (PhCH2)3SnCl (0.30 g, 0.70 mmol), RcCOOH (0.19 g, 0.70 mmol), and triethylamine (0.07 g, 0.70 mmol). Yield: 0.38 g (94%). Mp: 185 °C. IR (KBr, ν/cm−1): 3101 (m), 3022 (m), 2918 (m), 1569 (s), 1523 (s), 1484 (s), 1392 (s), 1359(m), 1194(m), 812(m), 757 (s), 694(m), 630(s), 511(s), 496(s). Anal. Calcd for C143H140O20Ru6Sn6: C, 49.11; H, 4.03. Found: C, 47.84; H, 3.88. [(n-Bu 2 Sn)(n-Bu 2 Sn)(μ 3 -O)OOCRc 2 ] 2 (4). Quantities: [nBu2SnO]n (0.30 g, 1.2 mmol) and RcCOOH (0.33 g, 1.2 mmol). Yield: 0.509 g (82%). Or (n-Bu3Sn)2O (0.15 g, 0.25 mmol) and RcCOOH (0.144 g, 0.52 mmol). Yield: 0.104 g (40%). Mp: 172 °C. 1H NMR (500 MHz, CDCl3, 25 °C) (δ, ppm): 0.92 (t, 24H, n-butyl CH3), 1.24−1.70 (m, 48H, n-butyl CH2’s), 4.59 (s, 20H, Cp ring), 4.71 (s, 8H, 863



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Figure 3. Supramolecular interaction between the two nearest molecules of 2. Bond distances (Å): C(6)−H(6)...O(1), 3.407(28) Å; H(6)...π(phenyl), 3.094(7) Å. Bond angles (deg): C(6)−H(6)−O(1), 151.429 (26). Five phenyl groups and all hydrogen atoms [except H(6)] are omitted for clarity.

monomeric compounds or coordination polymers where the carboxylate group bridges successive triphenyltin units.10a,d On the other hand, the reaction of (PhCH2)3SnCl with carboxylic acids does involve Sn−C bond cleavage.10b Remarkably, in both of these reactions, involving (Ph3Sn)2O or (PhCH2)3SnCl, Sn−C bond scission pertaining to two Sn−C bonds takes place. Interestingly, the reaction of (Ph3Sn)2O with FcCOOH (Fc = ferrocenyl) affords the expected product, Ph3SnO2CFc.10a Previously Sn−C bond scission involving the reactions of (Ph3Sn)2O have been reported in the following examples: [Ph2Sn(OH)OC(O){2,4,6(CF3)3C6H2}]216 and [(PhSn)6(OH)2(μ3-O)2(OEt)4{(ArO)PO3}4],17 where Ar = 2,6-i-Pr2−C6H3. Compounds 1−3, once crystallized, do not go into the solution again. The reactions of [n-Bu2SnO]n with RcCOOH affords the tetranuclear derivative 4, containing four ruthenocenes (Scheme 1). Interestingly, the reaction of (n-Bu3Sn)2O also affords 4 by a Sn−C bond cleavage (Scheme 1).

Figure 1. Molecular structure of 1. All the hydrogen atoms have been omitted for clarity. count to be 97.3 and 277 e in a volume of 362.6 and 745.6 Å3 in the solvent region in the unit cells of 2 and 3 respectively. The calculated electron counts show that two toluene molecules are present in the unit cell of 2. Five toluene and two water molecules are present in the unit cell of 3.

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RESULTS AND DISCUSSION Synthesis. The reaction of ruthenocene carboxylic acid with [n-BuSn(O)OH]n (1:1), (Ph3Sn)2O (2:1), and (PhCH2)3SnCl (1:1) to afford the hexameric compounds 1, 2, and 3, respectively (Scheme 1). In general, the reaction of carboxylic acids with monoorganotin precusor provides hexameric compounds. However, generally, such reactions with (Ph3Sn)2O afford either

Figure 2. Two-dimensional supramolecular structure of 1. Bond distances (Å): C(14)−H(14)···O(6), 3.238(9); C(25)−H(25)···O(2), 2.579(8). Bond angles (deg): C(14)−H(14)−O(6), 156.38 (26); C(25)−H(25)−O(2), 145.16(37). All n-Bu groups, two ruthenocene units, and all the hydrogen atoms except those involved in hydrogen bonding have been omitted for clarity. 864



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Figure 4. (A) HCP three-dimensional supramolecular frameworks. (B) Schematic drawing of the supramolecular hexagonal packing model.

hand, 1H and 119Sn NMR of 4−6 are consistent with their chemical structures (see experimental section). ESI−MS spectra of compounds 4−6 were taken in CHCl3. ESI-MS of 5 under positive ion ionization mode showed a peak at m/z 782.9855 which corresponds to [5 + H]+ (Figure S1 of the Supporting Information), indicating that the structural integrity of 5 is retained in solution. The ESI-MS of 4 and 6 reveal a breakdown of the structural integrity of these compounds. Peaks at 1280.0551 [(n-Bu2Sn)3(O)2L2]+ and 793.1274[(t-Bu2Sn)2 (OH)2L]+ were observed (Figures S2 and S3 of the Supporting Information). Single Crystal X-ray Diffraction Analyses of 1−6. The molecular structure of 1 is shown in Figure 1. The selected bond parameters of complex 1 are listed in the caption of Figure S4 of the Supporting Information. The other two hexamers (2 and 3) have similar structures and their bond parameters are given in the Figures S5−S6 and Tables S1−S3 of the Supporting Information. The molecular structures of 1−3 are very similar and correspond to the well-known drumlike structures known in the literature.18 The prismane-like cage comprises of an upper and lower Sn3O3 face; the sides of the cage consist of six Sn2O2 rings. The stannoxane core (Sn6O6) is held together by six substituted ruthenocene carboxylate ligands which bind to alternate tin atoms by an isobidentate coordination mode. Complex 1 forms a two-dimensional supramolecular assembly mediated by C−H···O interactions (Figure 2). Each stannoxane drum interacts with four neighboring drums involving four proton acceptor and four proton donor type C−H···O bonding. Thus, four bridging carboxylate oxygen atoms of every stannoxane moiety interact with the protons of the ruthenocene (cyclopentadienyl C−H) of four neighboring molecules. The resulting effect of these interactions is the generation of a highly symmetric supramolecular gridlike structure (Figure 2). There are two sets of hydrogen bonding: a stronger one, C(25)− H(25)···O(2) [2.579(8) Å] and a weaker one, C(14)−H(14)··· O(6) [3.238(9) Å]. Because of the stronger hydrogen bonding, the vertical intercentroid distance (13.832 Å) is relatively shorter than the horizontal intercentroid distance (15.103 Å) (Figure 2). Intermolecular interactions between the stannoxane drums in 2 occur through C−H···O and C−H···π interactions (Figure 3). Overall, each drum interacts with six of its neighbors, involving a total of six proton acceptor and four proton donor type of C−H···O bonds and six C−H···π interactions, resulting in a

Figure 5. Molecular structure of complex 4, (H atoms and part of the n-butyl groups are omitted for clarity). Bond distances (Å): Sn(1)−O(1), 2.236(4); Sn(1)−O(5), 2.040(4); Sn(1)−O(5)*, 2.158(4); Sn(1)− O(3), 2.650(5); Sn(2)−O(2), 2.245(4); Sn(2)−O(3), 2.171(4); Sn(2)− O(5), 2.031(4). Bond angles (deg): Sn(1)−O(5)−Sn(2), 137.98(19); Sn(1)−O(5)−Sn(1)*, 103.65(16); Sn(2)−O(5)−Sn(1)*, 118.34(17); O(1)−Sn(1)−O(5), 92.14(15); O(1)−Sn(1)−O(3), 124.932(13); O(5)−Sn(1)−O(5)*, 76.35(16); O(3)−Sn(1)−O(5), 142.95(13); O(3)−Sn(1)−O(5)*, 66.64(13); O(2)−Sn(2)−O(3)*, 167.43(17); O(2)−Sn(2)−O(5), 88.58(16); O(5)−Sn(2)−Sn(3)*, 78.91(16).

A dinuclear ruthenocene derivative, 5, was obtained in the reaction of [n-Bu2SnO]n with RcCOOH. Although structurally different, another dinuclear ruthenocene derivative, 6, was obtained in the reaction of (t-Bu2SnO)3 with RcCOOH (Scheme 1). The syntheses of 1−6 testify that barring minor variations the stannoxane synthetic strategy is quite robust and is generally insensitive to the nature of the carboxylic acid and depends more on the nature of the organotin precursor. With the use of this feature, 1−6, containing varying numbers of ruthenocene appendages on tin platforms could be assembled. ESI-MS/NMR studies on 1−3 could not be performed because of the insolubility of these products. On the other 865



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Figure 6. Two dimensional hydrogen-bonding framework of 4 mediated by intermolecular hydrogen bonding between the cyclopentadienyl hydrogen and the carboxylate oxygen. Bond distances (Å): H6···O4, 2.679(6); H13···O4, 3.046(38). Bond angles (°): C6−H6···O4, 130.52(36); C13−H13···O4, 131.274(41).

Figure 7. (a) Molecular structure of complex 5, (H atoms are omitted for clarity). (b) Intermolecular hydrogen bonding and tin−oxygen interaction. (c) One-dimensional hydrogen-bonding framework. (d) π···π stacking interaction of 5. Selected bond parameters of 5 are as follows. Bond distances (Å): Sn(1)−O(1), 2.482(4); Sn(1)−O(2), 2.132(4); Sn(1)−O(3), 2.110(4); Sn(1)−O(4), 2.609(4); Sn(1)−C(23), 2.114(7); Sn(1)−C(27), 2.118(7), O(4)···H(10), 3.069(39); Sn(1)···O(1), 2.954(40), O(2)···H(7), 2.498(38); O(3)···H(8), 2.898(39); O(2)···H(17), 2.855(38); O(3)···H(16), 2.614(39). Bond angles (deg): O(2)−Sn(1)−O(1), 56.05(14); O(3)−Sn(1)−O(1), 137.38(14); O(1)−Sn(1)−O(4), 167.97(13); O(3)−Sn(1)− O(2), 81.34(16); C(23)−Sn(1)−C(27), 142.76(21); O(4)−H(10)−C(10), 175.16(4); O(2)−H(7)−C(7), 138.12(38); O(3)−H(8)−C(8), 125.46(36); O(2)−H(17)−C(17), 122.32(67); O(3)−H(16)−C(16), 133.28(37).

In contrast to 2, complex 3 forms a two-dimensional supramolecular assembly by C−H···π interactions (Figure S7 of the Supporting Information).

hexagonal close packing arrangement (Figure 4). It may be recalled that [n-BuSn(O)(OOCCH2C5H4FeC5H5)]610e was reported to show a rangoli supramolecular architecture in the solid state. 866



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Figure 8. Molecular structure of complex 6 (H atoms are omitted for clarity). Bond distances (Å): Sn(1)−O(2), 2.184(3); Sn(1)−O(3), 2.036(3); Sn(1)−O(3)*, 2.173(3); Sn(1)−C(12), 2.164(4); Sn(1)−C(16), 2.183(4). Bond angles (deg): O(3)−Sn(1)−O(3)*, 70.33(13); C(12)−Sn(1)− C(16), 124.25(15); O(3)−Sn(1)−O(2), 85.42(11); O(3)*-Sn(1)−O(2), 155.39(10); Sn(1)−O(3)−Sn(1)*, 109.67(13).

Figure 9. Two dimensional hydrogen-bonding framework of 6 mediated by intermolecular hydrogen bonding between the ruthenocene hydrogen and the carboxylate oxygen. Bond distances (Å): H6···O1, 2.486(29); H3O···O1, 1.787(49). Bond angles (deg): C6−H6···O1, 122.809(24); O3−H3O··· O1, 162.72(47).

Table 2. Oxidation Potential Values of RuthenoceneneCentered Processes and a Comparison with Analogous Ferrocene Compoundsa complex

Ea (V)

[Ru2(η2-O2CRc)(dppe)2](PF6)19a 4 5 6 [n-BuSn(O)OOCC5H4FeC5H5]67b [PhCH2Sn(O)OOCC5H4FeC5H5]610b

0.64i 0.73i 0.75i 0.75i 0.72q 0.62q

a

Letters I and q indicate the irreversible and quasi reversible process, respectively.

compound are supported by a planar Sn4O2 core and adopt the familiar ladder framework. Two of the four ruthenocenes stay above this plane, and the other two are situated below this plane. Compound 4 organizes itself into a two-dimensional hydrogen-bonding array in the solid state due to the presence of intermolecular C−H···O interactions (Figure 6). A closer look reveals that each molecule (containing four ruthenocenes) interacts with four other neighboring molecules through proton donor interactions (cyclopentadienyl C−H). The oxygen atoms

Figure 10. Differential pulse voltammogram traces [current (μA) vs potential (V)] of 4−6.

The X-ray structure of the tetraruthenocene assembly 4 is shown in Figure 5. Some important bond parameters for this compound are listed in the caption of Figure 5 and all the bond parameters are given in Table S4. The molecular structure of 4 shows that the four ruthenocene moieties present in this 867



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Figure 9. The bridging hydroxide groups are involved in an intramolecular hydrogen bonding with the CO unit of the carboxylate ligands (Figure 8) to generate a six-membered ring on either side of the four-membered Sn2O2 ring. Each dimer (6) interacts with four neighboring molecules involving two proton acceptor and two proton donor type C−H···O (C6−H6···O1) interactions. The resulting effect of these interactions is the generation of a 2D hydrogen-bonded supramolecular structure (Figure 9). Electrochemistry. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out on 4−6 in dichloromethane, which contained 0.1 M tetrabutyl ammonium perchlorate. The ferrocene−ferrocenium (+1) couple was used for the calibration of the instrument. Ruthenocene and its derivatives compared to the corresponding ferrocene analogues, usually undergo an irreversible one-step, two-electron oxidation process in coordinating or weakly coordinating solvents and/or electrolyte.19 RcCOOH and complexes 1, 2, and 3 were not sufficiently soluble in a common organic solvent to allow us to perform electrochemical measurements. The cyclic voltammetry of 4−6 (Figure S9 of the Supporting Information) reveals an irreversible one-step oxidation event based on ruthenocene. On the basis of literature precedents, this event can be associated with a two-electron process. The oxidation potentials were determined by a DPV experiment (Figure 10 and Table 2), which are slightly higher than ruthenocene/ruthenocenium system (0.41 V)19f due to the presence of the electron withdrawing carboxylate group. These values are much closer to those observed for the previously reported complex, [{Ru2(η2O2CRc)(dppe)2}(PF6)].19a The electrochemical data suggests that in the complexes 4−6, individual ruthenocene units do not communicate with each other. This is due to the redox-inactive organostannoxane platform that supports these ruthenocene units. A similar phenomenon was also observed in previously reported hexaferrocene derivatives [n-BuSn(O)OOCC5H4FeC5H5]67b and [PhCH2Sn(O)OOCC5H4FeC5H5]6;10b both these complexes show quasi-reversible one-electron oxidation events in their cyclic voltammetry (Table 2). Conversion of 1 to the Binary Oxide (Ru−Sn)O2. Binary Ru−Sn oxides are potential candidates for many applications such as super capacitors, electrocatalyts, and electrode modification materials.20 At present, there are no synthetic methodologies available that can enable the use of single-source precursors for the generation of binary Ru−Sn oxides. In view of the fact that the hybrid organostannoxane−ruthenocene compounds offer a possibility to serve as single-source precursors

Figure 11. (a) XRD patterns of (Sn−Ru)O2.20c (b) XRD patterns of RuO2 and SnO2 [PCD no: 1818331(SnO2) and 1614535(RuO2)].

of the carboxylate moiety of the neighbors serve as the proton acceptors. The molecular structure of 5 along with its important metric parameters are given in Figure 7a. Compound 5 crystallizes in an orthorhombic space group (Pbcn) and contains a central tin, attached to which are two ruthenocene units through an anisobidentate carboxylate chelate rings. The two ruthenocene units are on the opposite side of the plane containing the tin and the carboxylate chelate (Figure 7a). The supramolecular structure of 5 can be understood in a stepwise manner. First, each molecule interacts with its nearest neighbors through Sn···O and O···H contacts to form a tetra-ruthenocene-containing tin dimer (Figure 7b). Further intermolecular C−H···O bonding (C7− H7···O2, C17−H17···O2, C8−H8···O3, and C16−H16···O3) generates a one-dimensional structure (Figure 7c). Next, such one-dimensional chains are interlinked through C−H···O bonding (O4···H10), affording a 2D structure (Figure S8a of the Supporting Information). Finally, π···π stacking culminates in a 3D structure (Figure 7d) and Figure S8b of the Supporting Information. Complex 6 crystallized as a discrete dimer in P21/n (monoclinic) space group. The two tin atoms present in this molecule are bridged by two hydroxide groups to generate a four-membered Sn2O2 ring (Figure 8). Each of the tins contains a unidentate [RcCOO]− ligand. Several hydrogen-bonding interactions are present in the crystal structure of 6. The various hydrogen-bonding parameters found in this compound are summarized in the caption of

Figure 12. Scanning electron microscopy images of (a) (Ru−Sn)O2 and (b) SnO2 and RuO2 (notice the clear demarcation of the individual oxide phases). 868



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for (Ru−Sn)O2 binary oxides, we investigated the thermal stability of 1−6 (Figure S10 of the Supporting Information), which revealed that all of these compounds have substantial char yields (36−53%) in the temperature range of 250 to 500 °C. As a representative example, we studied the decomposition of 1. After being heated to 400 °C under oxygen for 1 h, complex 1 gave a single phase product. The XRD pattern of this revealed a strong peak with an intermediate 2θ value of RuO2 and SnO2, suggesting that the obtained material is a solid solution of the two oxides (Figure 11a).20c The SEM of this is shown in Figure 12a, which confirms the conclusion obtained from PXRD experiments. On the basis of the literature,20e we heated the phase beyond 600 °C, at which point the two phases separated from the solid solution. Continuing the heating to 700 °C resulted in a clean separation of the two binary oxide phases (Figure 11b). The SEM of the mixed and separated phases is shown in Figure 12b.

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CONCLUSION Organostannoxane platforms can be utilized for supporting ruthenocene motifs. The modulation of the number of ruthenocenes can be achieved through the stannoxane synthetic route. We have been able to prepare compounds containing six, four, and two ruthenocene units. This is the first instance that such a synthetic modulation has been achieved with respect to ruthenocenes. The prominent supramolecular synthon in these compounds is generally the C−H···O interaction. However, other intermolecular interactions are also present, the cumulative effect of which is the realization of 2D and 3D supramolecular structures. The hexa-ruthenocene derivative, [n-BuSn(O)OOCRc]6 (1) has been shown to be a convenient single-source precursor for preparing the binary oxide, (RuSn)O2; the temperature of this synthesis is surprisingly low at 400 °C.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format, some structural diagrams, and bond parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+91) 512-259-7259. Fax: (+91) 521-259-0007/7436. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Department of Science and Technology, India, and the Council of Scientific and Industrial Research, India, for financial support. V.C. is thankful to the Department of Science and Technology for a J. C. Bose fellowship. S.K. and A.C. thank the CSIR, India for Senior Research Fellowships.

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REFERENCES

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Multi-Ruthenocene Assemblies on an Organostannoxane Platform

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