Article pubs.acs.org/Organometallics
Facile Formation of FePd Nanoparticles from Single-Source [1]Ferrocenophane Precursors Andrew D. Russell, George R. Whittell, Mairi F. Haddow, and Ian Manners* School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. S Supporting Information *
ABSTRACT: Mixed-metal FePd alloy nanoparticles (NPs) have been synthesized in moderate yield (ca. 55−60%) and at relatively low temperatures from single-source [1]ferrocenophane precursors. Thermolysis of [Fe(η5-C5H4)2Pd(PnBu3)2] (7) (1 h, 150 °C) and the new species [Fe(η5C5H4)2Pd{PnBu2(CH2)4PnBu2}] (9) (1 h, 190 °C) afforded crystalline, heterobimetallic FePd alloy NPs with diameters of ca. 4 nm (11) and 3.5 nm (12), respectively, together with mixtures of unidentified, mainly ligand-derived products. Both sets of particles were analyzed by high resolution transmission electron microscopy, which, in addition to providing particle size, determined the spacing between the lattice fringes to be 0.23 nm. Evidence for the formation of alloy nanoparticles, rather than a mixture of those comprising pure metals, was obtained by energy-dispersive X-ray analysis, which confirmed the presence of both Fe and Pd in a single particle. This assertation was further supported by wide-angle X-ray scattering of 11 and 12, which displayed broad reflections at 2θ = 40.58° and 40.09°, respectively, in good agreement with previous studies of FePd NPs. Atomic absorption spectroscopy was employed for bulk analysis of the particles and indicated that that the compositions of 11 and 12 were ca. Fe35Pd65.
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metallocenes (5, Scheme 1).18 For the ROP of heterobimetallic [n]metallocenophanes (or analogues with related π-ligands),19 this affords polymers with 100% incorporation of both metal centers into each repeat unit,20 thus affording materials with a high percentage metal content (e.g., 2). An alternative route to heterobimetallic materials involves the postfunctionalization of metal-containing polymers (e.g., to afford species 3, Figure 1).17b,21 To date, heterobimetallic [n]metallocenophanes (or related species) have been synthesized containing a variety of different metal combinations.20c,22 Despite the relative abundance of monometallic monomers, many species are unstrained and, therefore, cannot undergo ROP. Nonetheless, some species have been shown to afford metallic films directly from thermolysis of the monomeric precursor.23 We recently reported the synthesis, characterization, and reactivity of novel heterobimetallic [1]ferrocenophanes, bearing Ni (6), Pd (7), or Pt (8) in the bridge (Figure 2).24 Species 6, 7, and 8 proved resistant to both photolytic25 and anionic ROP;26 however, reactions of 6 and 7 with PhLi did result in stoichiometric ring-opening to yield molecular products. Further investigations identified 7 as a potential candidate for ROP, with differential scanning calorimetry (DSC) detecting an exothermic, first-order process at 145 °C, consistent with ringopening.27 The products from the thermolysis of 7 at 150 °C were analyzed by 31P and 1H NMR spectroscopy, which revealed broad signals consistent with a ring-opening process
INTRODUCTION Objects with nanoscale dimensions have potential applications in the fields of physics, electrical engineering, and cellular biology due to the unique physical properties that they exhibit.1 The combination of two or more metals into the mixed-alloy crystalline nanodomain of heterobimetallic NPs can afford species with enhanced magnetic,2 optical,3 and electronic4 properties, relative to their monometallic counterparts. Heterobimetallic NPs have thus been employed in a variety of applications, including catalysis,5 magnetic data storage,6,2a environmental remediation,7 and in fuel cell devices.4,8 Common methods for the synthesis of heterobimetallic nanoparticles include the co-reduction of metal salts,9 thermal decomposition,2a,10 and galvanic replacement reactions.11 Metallopolymers, where the metal centers are located in the main chain or pendant side group,12 are attractive precursors to both metal NPs,13 and magnetic ceramics.14 Thermolysis of metallopolymers can afford NPs that are stabilized in a ceramic matrix and are thus prevented from aggregating. Furthermore, polymeric precursors can often be cross-linked prior to pyrolysis, enabling ceramic yields to be maximized.14a,c Significantly, the propensity for metalloblock copolymers to form spatially segregated nanodomains, in both the bulk and solution state,15 facilitates the formation of nanoparticles in ordered arrays.16,14e The incorporation of a second metal center into the repeat unit of the polymer (e.g., 1, Figure 1)13d can facilitate heterobimetallic NP synthesis from polymeric singlesource precursors.17 Thermal treatment of [n]metallocenophanes (4) usually induces ring-opening polymerization (ROP) to afford poly© 2014 American Chemical Society
Received: June 26, 2014 Published: September 22, 2014 5349
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Figure 1. Heterobimetallic organometallic polymers.
ring-opened product had decomposed as a result of dissociation of the phosphine ligands. In order to explore this thermally-induced chemistry further and to resolve these ambiguities, we investigated the synthesis and characterization of the new palladium-bridged [1]ferrocenophane 9 (Figure 2), bearing a more strongly coordinating chelating diphosphine on the group 10 bridge. In addition, we describe detailed studies of the thermal reactivity of both the previously reported example 7, and the new species 9. Upon thermolysis, we now show that, rather than undergoing ROP, 7 and 9 afford heterobimetallic FePd nanoparticles. Characterization of the reaction products was achieved using high-resolution transmission electron microscopy (HRTEM), wide-angle X-ray scattering (WAXS), energydispersive X-ray (EDX) analysis, and atomic absorption spectroscopy (AAS).
Scheme 1. Ring-Opening Polymerization (ROP) of [n]Metallocenophanes
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RESULTS Synthesis and Characterization of [1]Ferrocenophane 9. Previous reports have suggested that the successful isolation of group 10 bridged [1]ferrocenophanes is highly dependent on the nature of the phosphine ligand(s) coordinated to the bridging metal.24b Isolation of the metalla[1]ferrocenophanes was exclusively achieved by employing tri(n-butyl)phosphine on the bridging group 10 metal precursor [Cl2M(PnBu3)2, M = Ni, Pd, Pt].28 We postulated that the properties of the ligand reduced the susceptibility of the MII precursor to reduction by a dilithioferrocene·tmeda complex (tmeda = N,N,N′,N′-tetra-
Figure 2. Structural metrics of [1]metallocenophanes (left) and group 10 bridged [1]ferrocenophanes 6, 7, 8, and 9.
taking place. However, upon precipitation, no polymer was isolated and free nBu3P was observed in the supernatant solution by 31P NMR spectroscopy. It appeared likely that the Scheme 2. Synthesis of [1]Ferrocenophane 9
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methylethylenediamine) during the ferrocenophane synthesis.24b Attempted ligand substitution with a variety of chelating diphosphines [e.g., bis(diphenylphosphino)ethane] was also unsuccessful, with either no reaction or a complex mixture of products resulting in each case.29 To explore these results further, we synthesized species 9, bearing diphosphine dbpb [dbpb = bis(di-nbutylphosphino)butane] at the bridging Pd. This ligand possesses similar steric and electronic properties to PnBu3 and, in contrast to the previous diphosphines employed, contains a (CH2)4 backbone that can accommodate the large P−M−P bond angles found in the group 10 bridged [1]ferrocenophanes.24b We anticipated that these features of the ligand might facilitate the isolation of the [1]ferrocenophane (9) in this case. The chelating effect of diphosphine dbpb would be expected to reduce the ease of ligand dissociation during thermolysis. The dbpb ligand was prepared by adaption of a literature procedure,30 first affording bis(di-chlorophosphino)butane (dcpb), followed by Grignard metathesis with MgClnBu to afford the target phosphine. Subsequent reaction of dbpb with [PdCl2(cod)] (cod = cis,cis-1,5-cyclooctadiene)31 afforded cis[Cl2Pd(dcpb)] (10) as a yellow solid in moderate yield (45%). The Pd bridged [1]ferrocenophane [Fe(η5-C5H4)2Pd(dcpb)] (9) was prepared through dropwise addition of a solution of 10 to a suspension of [Fe(η5-C5H4Li)2·tmeda] at −78 °C. 31P NMR spectroscopic analysis indicated complete conversion to a new phosphorus containing species, with a single resonance at δ = 14 ppm. Removal of the solvent in vacuo, followed by addition of dry hexanes and filtration through Celite, afforded a dark red solution that was concentrated in vacuo and cooled to −20 °C to afford 9 as a dark red, crystalline product in low yield (23%) (Scheme 2). The 1H NMR spectrum of 9 displayed similar resonances to those reported for the monodentate analogue,24a with signals assigned to the α- and β-protons of the Cp ring overlapping at δ = 4.65 and 4.64 ppm. Signals assigned to the phosphine ligand appeared at high field, with multiplets observed at δ = 1.83, 1.56, and 1.35 ppm assigned to the CH2 groups, in addition to a triplet at 0.94 ppm assigned to the terminal CH3 groups of the nBu chains. 13C and 31P NMR spectroscopic and elemental analyses were also consistent with the formation of 9. UV/vis spectroscopy (Figure S1, Supporting Information) of 9 revealed a red-shifted band II transition,32 which appears as a poorly resolved shoulder (ca. 510 nm) on a more intense absorption of higher energy [λmax (ε) = 407 nm (875 M−1 cm−1). These results are similar to those reported for 7, where band II (ca. 500 nm) also appeared as a shoulder.24b Structural characterization in the solid state was achieved through X-ray diffraction analysis of a single crystal of 9 obtained by cooling of a saturated hexane solution to −40 °C (Figure 3). Compound 9 crystallized in the space group Pna21 with one molecule per asymmetric unit. There were no short intermolecular interactions between molecules. The geometry at the Pd center in 9 is distorted square-planar, as indicated by the P−Pd−P angle [103.86(4)°], which is significantly increased relative to the 90° angle typically found for square planar complexes. The P−Pd−P angle, however, is smaller than that reported for 7 [109.21(5)°],24b this variation presumably resulting from the influence of the bidentate ligand, which constrains the size of this angle. Despite this difference, no significant change in the Cpipso−Pd−Cpipso (θ) angle is observed [9: θ = 79.69(16)°, 7: θ = 79.15(17)°] (Figure 2). The angle subtended by the two cyclopentadienyl rings (α) was
Figure 3. Solid-state structure of [Fe(η5-C5H4)2Pd{PnBu2(CH2)4PnBu2}] (9). Thermal ellipsoids shown at 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)−C(1) = 2.107(4); Pd(1)−C(6) = 2.111(4); Pd(1)−P(1) = 2.2740(12); Pd(1)−P(2) = 2.2733(13); Fe(1)−C(1) = 2.011(4); Fe(1)−C(2) = 2.001(6); Fe(1)−C(3) = 2.056(6); Fe(1)−C(4) = 2.064(6); Fe(1)−C(5) = 2.042(6); Fe(1)−C(6) = 2.014(4); Fe(1)−C(7) = 2.026(7); Fe(1)−C(8) = 2.067(6); Fe(1)− C(9) = 2.059(6); Fe(1)−C(10) = 2.017(6); P(1)−C(11) = 1.833(5); P(2)−C(14) = 1.857(9); C(6)−Pd(1)−C(1) = 79.69(16); P(1)− Pd(1)−P(2) = 103.86(4); Pd(1)−P(1)−C(11) = 118.81(15); Pd(1)− P(2)−C(14) = 118.7(3).
23.5(2)°, which is comparable to that of the monodentate analogue 7 [α = 24.1(2)°].24b The Fe−Cpcentroid distances [1.633(3) and 1.635(3) Å] for 9 are also comparable to those for 7, as are the β angles [9: β/β′ = 27.9(3)°/27.7(3)°; 7: β/β′ = 26.9(1)°/27.1(3)°]. Thermal Reactivity of [1]Ferrocenophanes 7 and 9. With compounds 7 and 9 in hand, the thermal behavior of these species was investigated by thermal gravimetric analysis (TGA) (7: Figure S3; 9: Figure S4 (Supporting Information)). Broadly similar TGA profiles were observed, with the first loss of mass observed at ca. 200 °C and the samples subsequently giving char yields of 39% (for species 7) and 36% (for species 9) at 350 °C.33 The analysis suggested incomplete volatilization of the organic material as the char yields obtained are significantly higher than the combined Fe and Pd content for each species (7 = 23%; 9 = 26%). Indeed, elemental analysis of the chars indicated the presence of substantial amounts of carbon (7: C 27.83%, H 1.25%; 9: C 23.86%, H 0.00%). DSC was conducted on species 9, and the result was compared to that reported for 7.24b For the latter species, a first-order endothermic process was observed at 140 °C, followed by a similar exothermic process at 145 °C, which was tentatively assigned to a ring-opening process in our preliminary study.34 Species 9 was found to exhibit different thermal behavior compared to 7, and the DSC thermogram displayed a small exotherm at 136 °C,35 followed by a endotherm at 163 °C and a broad exotherm at 181 °C (Figure S2, Supporting Information). 5351
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Figure 4. (a) TEM and (b) HRTEM images of FePd NPs (11) formed by thermolysis of 7 (150 °C, 1 h). (c) Particle size distribution determined by TEM. (d) EDX analysis of NPs. (e) EDX analysis of the carbon film on the copper TEM grid.
thermolysis for longer reaction times (150 °C, 16 h) did not result in a variation in the relative amounts of the products isolated. Similar observations were made from the thermolysis of species 9 at 190 °C (Tm + 10 °C), with a melt observed (after 5 min) and a brown/black gummy material obtained upon completion of the experiment (1 h). A 1H NMR spectrum obtained for the products before work up was similar to that obtained from the thermolysis of 7 (see the Supporting Information for details), and in this case, the 31P NMR spectrum displayed a single broad signal at ca. δ = 0 ppm. DLS studies on the products (THF, 25 °C) indicated the presence of particles with an increased diameter (9.7 nm) relative to that obtained from 7 (Figure S6, Supporting Information).38 Following a work up procedure similar to that employed for 7, a black powder (12) (9 mg vs. 75 mg of 9 used) and an orange gummy material (46 mg vs. 75 mg of 9 used) were ultimately afforded. Analysis of the orange gum by 1H NMR spectroscopy led to the detection of comparable signals to those observed previously for 7. The 31P NMR spectrum of the gum displayed multiple environments, one of which (δ = −31.6 ppm) could be assigned to nBu2P(CH2)4PnBu2. Characterization of Black Solids 11 and 12: Analysis of FePd Nanoparticles. Transmission electron microscopy (TEM) was conducted on the powder products (11 and 12) isolated from the thermolysis reactions. The TEM image obtained for 11 (Figure 4a) contained spherical, polydisperse nanoparticles with an average diameter of 4.00 ± 0.97 nm.39 Compound 11 was redispersed in THF (2 mg mL−1) and the resulting solution examined by DLS (Figure S7, Supporting Information). This technique afforded an average diameter of
To investigate the composition of the chars afforded from 7 and 9, bulk thermolysis of each species was conducted under conditions similar to those typically used for the thermal ROP of strained [n]ferrocenophanes. For species 7, thermolysis at 150 °C (Tm + 10 °C) afforded a dark red melt after 5 min, which turned brown/black after 15 min and remained relatively mobile for the duration of the thermolysis (1 h). Analysis of the products by 31P NMR spectroscopy (d8-THF) showed a broad signal at ca. −0.6 ppm, similar to that reported in our preliminary study.24b However, the position of the peak was also comparable to that previously reported for phosphinestabilized Pd NPs, in which a peak at δ = 2.89 ppm was observed in the 31P NMR spectrum and assigned to tri(octyl)phosphine coordinated to the particle surface.36 The 1 H NMR spectrum in the same solvent showed signals assigned to the nBu groups of the phosphine and to η5-C5Hx groups, in addition to confirming that no starting material (7) remained.37 THF was added to the product, and the resulting suspension was centrifuged to separate out a small amount of black solid. Dynamic light scattering (DLS) was conducted on the THF solution at 25 °C under an atmosphere of argon and suggested the presence of particles with an apparent hydrodynamic diameter of ca. 3.6 nm (Figure S5, Supporting Information). The solvent was then removed from the supernatant in vacuo, and the residual solid was washed with hexanes to afford a black powder (11) (11 mg vs. 95 mg of 7 used). Subsequent removal of the solvent from the washings afforded an orange gum (55 mg vs. 95 mg of 7 used), for which analysis by 31P NMR spectroscopy indicated the presence of multiple phosphorus environments. These included the same broad signal at ca. −0.6 ppm that was detected originally but also peaks assigned to PnBu3 (δ = −31.0) and OPnBu (δ = 43.3). Repeating the 5352
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Figure 5. (a) TEM image and (b) HRTEM image of FePd NP(s) 12, produced by the thermolysis of 9 (190 °C, 1 h). (c) Particle size distribution determined by TEM. (d) EDX analysis of NPs. (e) EDX analysis of the carbon film on the TEM grid.
analysis of 12 confirmed the presence of Fe and Pd (Figure 5d), while indicating the absence of these elements from other areas of the grid (Figure 5e). Analysis of 12 by WAXS afforded results similar to those obtained for 11, with a broad reflection at 2θ = 40.09° (Figure S11, Supporting Information) suggestive of mixed-alloy FePd NP formation.43 Analysis of the reflection with the Scherrer equation gave an average diameter of the crystalline domains in these NPs as 1.4 nm.45a Furthermore, bulk analysis of 12 by AAS confirmed both the presence of Fe and Pd and also the relative composition of the NPs (Fe33Pd67). From the results, the nature of the supporting phosphine can be concluded to have only a small effect on the size, shape, and elemental composition of the NPs formed from the thermolysis of Pd bridged [1]ferrocenophanes. Furthermore, to a first approximation, the relative yields of NPs isolated from the thermolysis reactions (ca. 55−60%) appear unaffected by the identity of the phosphine (see the Supporting Information for details). The chelate effect of the bidentate ligand does appear to alter the thermal properties of Pd bridged [1]ferrocenophanes, as indicated by the difference in temperature of the exothermic processes observed by DSC (7: 145 °C; 9: 181 °C). Significantly, formation of the particles in 11 and 12 occurs at lower temperatures than those reported for literature procedures of the synthesis of FePd NPs, which are typically in the range of 200−300 °C and require a chemical reductant.43d,49 Analysis of the orange gummy material isolated from the thermolysis experiments by NMR spectroscopy indicated that no starting material remained and that this appears to be mainly a complex mixture of ligand-derived products (see the Supporting Information). For example, 1H NMR resonances observed at δ = 4.45−3.85 ppm (from thermolysis of 7) and δ = 4.14−3.86 ppm (from thermolysis of 9) were assigned to η5C5Hx groups.50 These observations suggest the presence of
3.5 nm and is thus consistent with both the TEM analysis and the DLS data from the crude product from the thermolysis.40 HRTEM analysis was conducted on 11 (Figure 4b) and indicated that the particles formed were highly crystalline, with an average lattice fringe spacing of 0.23 nm.41 Analysis of the crystalline material was also performed by WAXS (Figure S9, Supporting Information),42 for which a broad reflection at 2θ = 40.58° was observed. The peak was indexed to the (111) reflection of chemically disordered cubic FePd NPs43 and appears at a slightly higher scattering angle than that observed for pure Pd NPs (2θ = 39.22°).44 Utilizing the Scherrer equation, the average size of the crystalline domains within the NPs was calculated as 1.5 nm.45 To investigate the composition of the NPs, energy-dispersive X-ray (EDX) analysis was performed within the transmission electron microscope (Figure 4d). This confirmed the presence of both Fe and Pd in the NPs and their absence in other areas of the grid (Figure 4e).46 Atomic absorption spectroscopy (AAS) was conducted to assess the composition of a bulk sample of NPs. The analysis corroborated the results from the EDX analysis, confirming the presence of both Fe and Pd in the particles, and revealed their composition as Fe38Pd62.47 The black powder (12) isolated from the thermolysis of species 9 (190 °C, 1 h) was also analyzed by TEM (Figure 5a), which revealed spherical NPs with an average diameter of 3.48 ± 0.45 nm, slightly smaller and more uniform in size than those observed for 11. DLS data were also obtained on 12 in THF (Figure S8, Supporting Information) and suggested a particle diameter of 5.6 nm, consistent with the TEM data obtained, but smaller than that determined (9.7 nm) for the crude thermolysis product from 9.48 Similar to the results for 11, HRTEM of 12 (Figure 5b) confirmed that the NPs were highly crystalline, with analysis of the fringes revealing an average lattice spacing of 0.23 nm. EDX 5353
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Samples were sealed in evacuated Pyrex tubes and wrapped in metal mesh as a precaution. Sealed samples should be handled with appropriate heat-proof gloves and face protection. Transmission Electron Microscopy. All samples were prepared by drop-casting one drop (ca. 10 μL) of the solution (1 mg mL−1) onto a carbon-coated copper grid that was placed on a piece of filter paper to remove excess solvent. Bright-field TEM images were obtained on a JEOL 1200EX II microscope operating at 120 kV and equipped with an SIS MegaViewIII digital camera. Images were analyzed using the ImageJ software package developed at the U.S. National Institute of Health. For the statistical particle size analysis, each TEM micrograph was analyzed completely; i.e., every nanoparticle in each image was counted in order to reduce subjectivity. From these data, the average nanoparticle diameter was calculated. For the calculations of the lattice fringes, every nanoparticle of which the fringes were clearly observed was analyzed. Wide-Angle X-ray Diffraction Analysis. For wide-angle X-ray scattering measurements, a solution (2 mg mL−1) was drop-cast onto a silicon wafer, the solvent was allowed to evaporate, and the process was repeated multiple (ca. 20−30) times. The data were collected with Cu Kα radiation (λ = 1.5418 Å) on a Bruker D8 Advance powder diffractometer fitted with a 2 mm fixed divergence slit, knife-edge collimator, and a LynxEye area detector. Data were collected between 5° and 70° 2θ in θ/2θ mode with a step width of 0.5°. Single Crystal X-ray Structure Determination. An X-ray diffraction experiment on single crystals of 9 was carried out at 100 K on a Bruker APEX II kappa diffractometer using Mo Kα radiation (λ = 0.71073 Å). Data collections were performed using a CCD area detector from a single crystal mounted on a glass fiber. Intensities were integrated54 from several series of exposures measuring 0.5° in ω or ϕ. Absorption corrections were based on equivalent reflections using SADABS.55 The structure was solved using SHELXS and refined against all Fo2 data with hydrogen atoms riding in calculated positions using SHELXL.56 Crystal structure and refinement data are given in Table S1 (Supporting Information). The crystal of 9 was a nonmerohedral twin with 2 components, which was refined against an HKLF 5 file for which racemic twinning was included, giving four components in the ratio 0.18699:0.27315:0.24855:0.29161. Dynamic Light Scattering (DLS). Different concentrations of the samples (2 mL) in THF were filtered through a membrane filter (0.2 μm pores) into an optical glass cuvette (10.0 mm path length) under an inert atmosphere. DLS was performed on a Malvern Instruments Zetasizer Nano S using a 5 mW He Ne laser (633 nm) at 25 °C. The correlation function was acquired in real time. Upon analysis with a function capable of modeling multiple exponentials, the diffusion coefficients for the component particles were extracted. These were subsequently expressed as effective hydrodynamic diameter, by volume, using the Stokes−Einstein relationship. Atomic Absorption Spectroscopy. Samples were dissolved in standard volumes of refluxing aqua regia [HCl: HNO3 (1:3)] and analyzed in a GBC Avanta Sigma Atomic Absorption Spectrometer equipped with a graphite AA furnace. Sensitivity of the instrument is certified to 0.1 μg mL−1. Synthesis of cis-PdCl2[PnBu2(CH2)4PnBu2] (10). A 100 mL Schlenk flask was charged with PdCl2COD (2.39 g, 8.37 mmol, 1.00 equiv). Dry and degassed toluene (20 mL) was added via a canula. PnBu2(CH2)4PnBu2 (2.9 g, 8.5 mmol, 1.01 equiv) was added dropwise via syringe to the resulting suspension. The solvent was removed in vacuo, the residue was dissolved in dry THF (50 mL), and the solution was filtered. Additional dry THF (190 mL) and dry hexane (20 mL) were added, and the product was placed at −60 °C to induce crystallization. Following solvent removal by canula and drying in vacuo, the yellow, crystalline product was isolated (Yield = 2.0 g, 45%). 1 H NMR (499.6 MHz, CDCl3, 25 °C): δ = 2.48−2.39 (m, 4H, CH2), 1.94−1.79 (m, 16H, CH2), 1.51−1.39 (m, 12H, CH2), 0.94 (br t, 12H, 3 J(H,H) = 7 Hz); 13C{1H} (100.5 MHz, CDCl3, 25 °C): δ = 28.2 (d, 1 J(P,C) = 33.2 Hz, CH2), 27.3 (d, 3J(P,C) = 2.0 Hz, CH2), 24.3 (d, 2 J(P,C) = 15.2 Hz, CH2), 21.8−21.3 (overlapping multiplets, CH2); 13.7 (s, CH3). 31P{1H} NMR (161. 8 MHz, CDCl3): δ = 31.8 ppm
some ferrocene-based species in the byproducts of the thermolysis reactions and could potentially explain the lower percentage of Fe than Pd in the NP products (ca. Fe35Pd65)
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SUMMARY Heterobimetallic FePd nanoparticles have been synthesized by thermolysis of single-source [1]ferrocenophane precursors 7 and 9, bearing either mono- or bidentate supporting phosphine ligand(s). Upon thermolysis (7: 150 °C, 1 h; 9: 190 °C, 1 h), both species 7 and 9 afforded spherical FePd nanoparticles with a diameter of ca. 3−4 nm in yields of 59% and 55%, respectively. 31P NMR spectroscopic analysis suggested that the particles are initially formed with supporting phosphine coordinated to the surface of the particles, which can be removed through a simple washing procedure.36 The temperatures employed in the formation of 11 and 12 are lower than those typically employed in the synthesis of FePd NPs (200− 300 °C), which also require a chemical reductant. Both sets of FePd NPs were found to be highly crystalline, as observed by both HRTEM and WAXS analysis. Analysis of the particles by EDX and AAS, in addition to comparison of the WAXS data with that reported in the literature, confirmed both the mixedmetal alloy nature of the particles, and their relative composition (ca. Fe35Pd65). Future work will focus on the syn thesis of n ew strain ed hetero bim etallic [n]metallocenophanes, with a view to the development of ROP routes to heterobimetallic polymers and block copolymers, and investigations of both species as precursors to metal alloy nanoparticles for catalytic and magnetic data storage applications.
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EXPERIMENTAL SECTION
Experimental Information. Unless otherwise stated, all reactions were carried out using standard Schlenk line techniques or in an MBraun glovebox under dry nitrogen or purified argon. Et2O, toluene, and hexane were dried via a Grubbs’ type solvent purification system51 and deoxygenated by three freeze−pump−thaw cycles. C6D6 was dried overnight over CaH2 and distilled under reduced pressure prior to use. d8-THF was dried over a Na/K alloy and distilled under reduced pressure prior to use. THF was dried over Na/benzophenone and distilled at 150 °C. NMR spectra were recorded at ambient temperature on Eclipse 300 (1H: 300.5 MHz, 11B 96.4 MHz) or Varian 500 (13C: 125.7 MHz) spectrometers, with all resonances referenced to residual protio solvent resonances [C6D6: δ 7.16 (1H NMR), δ 128.1 (13C NMR) ppm, d8-THF: δ 1.72 {CH2(3,4)} (1H NMR), δ 25.31 ppm {C(3,4)} (13C NMR)]. UV/vis absorption data were obtained on a Lambda 35 spectrometer employing standard quartz cells (1 cm path length) from 800 to 300 nm with a scan rate of 1 nm s−1. Elemental analysis was carried out by the Laboratory for Microanalysis at the University of Bristol (Model 3000 Euro EA Elemental Analyzer) using V2O5 to promote combustion. Li2[Fe(η5C5H4)2]·tmeda (tmeda = N,N,N′,N′-tetramethylethylenediamine) was prepared according to a literature procedure.52 [1]Ferrocenophane 7 was prepared according to a previously published protocol.24b dbpb [dbpb = bis(di-nbutylphosphino)butane] was prepared by adapting a literature procedure,30 [PdCl2(COD)] was prepared by a literature procedure.53 Differential scanning calorimetric (DSC) was performed on a TA Instruments DSC Q100 calorimeter at a heating rate of 10 °C/min. Thermal gravametric analysis (TGA) was performed on a TA instruments Q500 thermogravimetric analyzer at a heating rate of 10 °C/min, with a thermal couple calibrated using Curie points of Ni and alumel standards. Thermolysis Experiments. Thermolysis experiments were carried out in a Memert UNE 200 oven fitted with a rocking stage. The oven was calibrated according to DIN ISO 9001 under the observation of the certified quality assurance system, with certified errors of 0.1 °C. 5354
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Thermolysis of (η5-C5H4)2FePd(PnBu3)2 (7) at 150 °C for 16 h. A thermolysis tube was charged with (η5-C5H4)2FePd(PnBu3)2 (0.170 g, 0.245 mmol), sealed under vacuum, and heated at 150 °C for 16 h. The dark red crystalline material quickly turned into a dark red melt. After 10 min, the sample largely immobilized and appeared brown/ black and remained in this state for the duration of the thermolysis experiment (16 h). The sample was then removed from the oven and allowed to cool. The tube was opened, and the contents were dissolved in THF (25 mL). 31P{1H} NMR of the crude material did not display any identifiable signals. The sample was centrifuged, and the THF solution was decanted off to afford a small amount of black material (0.022 g). The THF was removed in vacuo to yield 0.110 g of black gummy material. The sample was then washed with hexanes and dried in vacuo to yield 0.020 g of a black solid. Yield [based on formula mass of Fe35Pd65 (136.49 g mol−1)] = 60%. Yield [based on formula mass of Fe50Pd50 (162.26 g mol−1)] = 50% Removal of the solvent from the hexane washings yielded 0.070 g of yellow gummy material. The relative amount of each product isolated from the reaction does not change after longer reaction times (16 h), suggesting that the reaction is complete after 1 h. Thermolysis of [(η5-C5H4)2Fe]Pd[P(nBu)2(CH2)4P(nBu)2] (9) at 190 °C for 1 h. A thermolysis tube was charged with [(η5C5H4)2Fe]Pd[P(nBu)2(CH2)4P(nBu)2] (0.075 g, 0.12 mmol), sealed under vacuum, and heated at 190 °C for 1 h. The dark red crystalline material quickly turned into a dark red melt. After 5 min, the sample was partially immobile, appeared brown/black in color and remained in this state for the duration of the thermolysis experiment (1 h). The sample was removed from the oven and allowed to cool. The tube was opened under an atmosphere of argon, and the contents were dissolved in THF (15 mL). The sample was centrifuged to isolate 0.016 g of an insoluble black solid. The solvent was removed in vacuo, and the residue was analyzed by NMR spectroscopy. 1 H NMR (300.53 MHz, C6D6): δ = 4.14−3.86 (br m, C5H4), 1.85− 1.20 (br m, nBu2P(CH2)4PnBu2), 1.03−0.80 ppm (br m, nBu2P(CH2)4PnBu2). 31P{1H} NMR (121.66 MHz, C6D6): δ = 0 ppm (s). DLS (THF, 25 °C): Measurements were performed at multiple concentrations (6 and 3 mg mL−1): Rh = 4.85 nm. The sample was then washed with hexanes and dried in vacuo to yield 0.009 g of a black solid. Yield [based on formula mass of Fe35Pd65 (136.49 g mol−1)] = 55%. Yield [based on formula mass of Fe50Pd50 (162.26 g mol−1)] = 46%. DLS (THF, 25 °C): Measurements were performed at multiple concentrations (2 and 1 mg mL−1): Rh = 2.8 nm. The hexane was removed from the washings in vacuo to yield 0.046 g of orange gummy material, which was analyzed by 1H and 31P NMR spectroscopy. 1H NMR (299.95 MHz, C6D6): δ = 4.40−3.85 (br m, η5-C5H4), 1.70−1.22 (br m, nBu2P(CH2)4PnBu2), 0.94−0.80 ppm (br s, nBu2P(CH2)4PnBu2). 31P{1H} NMR (121.66 MHz, C6D6): δ = 12.9 (s), −31.6 (s, nBu2P(CH2)4PnBu2), −35.4 ppm (s).
(s). Elemental analysis (%) calcd. for C20H44Cl2P2Pd: C 45.86; H 8.47; found C 46.39; H 8.40. Synthesis of [(η5-C5H4)2Fe]Pd[PnBu2(CH2)4PnBu2] (9). A solution of cis-PdCl2[P(nBu)2(CH2)4P(nBu)2] (1.84 g, 3.51 mmol, 1.05 equiv) in toluene (30 mL) was added dropwise to a suspension of Li2[Fe(η5-C5H4)]·tmeda (1.05 g, 3.35 mmol, 1 equiv) in toluene (30 mL) that was cooled to −78 °C. The resulting dark brown mixture was allowed to warm to room temperature overnight. The solvent was removed in vacuo, and the residual dark oil was dissolved in hexanes and filtered through Celite (1″ × 3″). The filtrate was then cooled to −20 °C, affording the product as dark red crystals (Yield = 0.50 g, 23%). 1H NMR (499.90 MHz, C6D6): 4.65 (m, 4H, Cp), 4.64 (m, 4H, Cp), 1.83 (m, 8H, CH2), 1.56 (m, 4H, CH2), 1.35 (br m, 20H, CH2), 0.94 (t, 12H, 3J(H,H) = 7.3 Hz); 13C NMR (125.7, C6D6): δ = 79.9 (t, 4 J(P,C) = 2.9 Hz, Cp); 74.2 (t, 4J(P,C) = 3.8 Hz, Cp), 56.8 (dd, 2 J(Ptrans,C) = 134.5 Hz, 2J(Pcis,C) = 31.5 Hz, Cpipso), 28.4−28.1 (overlapping multiplets, CH2), 25.5−25.3 (m, CH2), 29.3 (t, 1J(P,C) = 3.8 Hz, CH2CH2CH2CH2), 22.7 (dd, 2J(P,C) = 13.4 Hz, 3J(P,C) = 6.7 Hz, CH2CH2CH2CH2), 14.6 (s, CH3). 31P{1H} NMR (161.8 MHz, C6D6): δ = 13.3 (s). UV/vis (THF, 20 °C): λmax (ε) = 407 nm (875 M−1 cm−1). Elemental Analysis (%) calcd for C30H52FeP2Pd: C 56.57; H 8.23; found: C 57.10, H 8.10. Thermolysis of (η5-C5H4)2FePd(PnBu3)2 (7) at 150 °C for 1 h. A thermolysis tube was charged with (η5-C5H4)2FePd(PnBu3)2 (0.095 g, 0.14 mmol), sealed under vacuum, and heated at 150 °C for 1 h. The dark red crystalline material quickly turned into a dark red melt. After 10 min, the sample displayed some immobility and appeared brown/black and remained in this state for the duration of the thermolysis experiment (1 h). The sample was then removed from the oven and allowed to cool. The tube was opened, and the contents were dissolved in d8-THF (2 mL). The 1H NMR spectrum of the products displayed peaks assigned to residual phosphine and confirmed that no starting material remained. 1 H NMR (300.53 MHz, d8-THF): δ = 4.3−3.8 (br m, C5H4), 1.52− 1.26 (br m, PnBu3), 0.94−0.84 ppm (br m, PnBu3). 31P{1H} NMR (121.66 MHz, d8-THF): δ = −0.6 ppm (br s). DLS (THF, 25 °C): Measurements were performed at multiple concentrations (4 and 2 mg mL−1): Rh = 1.8 nm. The sample was centrifuged, and the THF solution was decanted off to afford a small amount of organic-solvent-insoluble black material (0.010 g). The THF was removed in vacuo to yield 0.068 g of black gummy material. The sample was then washed with hexanes and dried in vacuo to yield 0.011 g of a black solid (11). Yield [based on formula mass of Fe35Pd65 (136.49 g mol−1)] = 59%. Yield [based on formula mass of Fe50Pd50 (162.26 g mol−1)] = 50%. DLS (THF, 25 °C): Measurements were performed at multiple concentrations (2 and 1 mg mL−1): Rh = 1.75 nm. The solvent was removed from the hexane washings in vacuo to yield 0.055 g of a yellow gummy solid. No signals that could be assigned to the starting material were observed in the 1H NMR spectrum, although signals corresponding to η5-C5Hx were observed in addition to peaks assigned to the phosphine ligand scaffold. 1H NMR (400.19 MHz, C6D6): δ = 4.45−3.85 (br m, C5H4), 1.80−1.20 (br m, PnBu3), 1.0−0.8 ppm (br m, PnBu3). 31P NMR spectroscopic analysis showed the presence of multiple phosphorus environments, including signals assigned to the free phosphine. 31P{1H} NMR (162.00 MHz, C6D6): δ = 43.3 (s, O=PnBu3), 1.0 (br s) −31.0 ppm (s, nBu3P). UV/ vis (THF, 20 °C, 1 mg mL−1): λmax = 462 nm. WAXS analysis was conducted on the 0.010 g of black, organicsolvent-insoluble material isolated from the centrifugation of the thermolysis products (Figure S11). Although absolute assignment of chemical structure could not be made from the diffraction pattern, similarities were observed to the pattern reported for ferrocene (Figure S13, Supporting Information),57 with the strongest diffraction peak at a similar spacing to that assigned to the 110 planes in ferrocene. This suggests that this product may be an oligoferrocene material, such as an oligoferrocenylene, [Fe(η5-C5H4)2]x, which, based on the color, may have been partially oxidized during the thermolysis and/or work up procedure.
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ASSOCIATED CONTENT
S Supporting Information *
Additional figures and schemes and crystallographic details for species 9. 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.D.R. thanks EPSRC for funding. I.M. and G.R.W. thank the EU for funding. 5355
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Organometallics
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Article
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