Matti
Haukka
*,
Minna
Jakonen
,
Taina
Nivajärvi
and
Mirja
Kallinen
Department of Chemistry, University of Joensuu, Department of Chemistry, P.O. Box 111, FI-80101, Joensuu, Finland. E-mail: matti.haukka@joensuu.fi; Fax: +358-13-251 3390
First published on 12th May 2006
The use of iron-containing metal surfaces, Fe, Fe–Cr-alloy and stainless steel, for the synthesis of mixed metal Ru–Fe compounds has been studied. The studied process was reductive carbonylation of RuCl3 in the presence of a metal surface. Reactions were carried out in ethanol solutions under 10–50 bar carbon monoxide pressure at 125 °C using an autoclave. During the reaction the metal surface was oxidized, releasing iron into the solution and acting as a sacrificial source of iron. Under these conditions the corrosion of the metal surface was facile and produced a series of iron-containing species. In addition to the formation of most obvious iron(II) products, such as [Fe(H2O)6]2+ or [FeCl2(H2O)4] the use of the metal surface also provided a route to novel labile trinuclear [Ru2Cl2(µ-Cl)4(CO)6FeL2] (L = H2O, EtOH) complexes. The stability and reactivity of the [Ru2Cl2(µ-Cl)4(CO)6FeL2] complexes were further studied using computational DFT methods. Based on the computational results a reaction route has been suggested for the formation and decomposition of [Ru2Cl2(µ-Cl)4(CO)6FeL2].
The primary objective of the present work was to apply the sacrificial surface approach to the synthesis of mixed metal Ru–Fe–Ru carbonyl compounds. The studied process was carbonylative reduction of RuCl3 in the presence of an iron-containing metal surface. Ruthenium carbonyls are most commonly synthesized by reducing RuCl3 under a CO atmosphere.10–18 The pioneering work in this area was accomplished by Chatt and co-workers in the mid 1960's.10 These reactions are typically carried out in an alcoholic solution,10–19 either under elevated CO pressure or under reflux in a flow of CO.10–15,17,18 Zerovalent ruthenium carbonyl clusters have also been synthesized by CO reduction of [RuCl2(CO)3]2 in the presence of alkalicarbonyls.20 Several methods, quite different both mechanistically and in terms of efficiency, have been suggested to improve the reduction of ruthenium chlorides or carbonyl chlorides: for example, reduction in the presence of Zn,12 hydroxide ions13,14,21,22 or carbonates.23 Silver acetate has also been reported to be useful in enhancing the reduction of ruthenium halides.24 Another approach that has been used to improve the reduction is to use the surfaces to assist the reduction. Surface-mediated carbon monoxide reductions of ruthenium halides have been carried out primarily on oxide surfaces.25–28
In the present study the carbonylative reduction of ruthenium chloride was carried out in an alcoholic solution in the presence of iron-containing metal surfaces: iron, stainless steel or Fe–Cr-alloy. The goal was to take advantage of the iron released from the surface during the process. The effect of the reaction conditions on this process is discussed. The product distribution was analyzed, and the formation and stability of the new mixed metal products [Ru2Cl2(µ-Cl)4(CO)6FeL2] (L = H2O, EtOH) were further investigated by means of DFT calculations. Based on these results a reaction route for the decomposition of [Ru2Cl2(µ-Cl)4(CO)6FeL2] (L = H2O, EtOH) is also suggested.
All of the high-pressure (≥10 bar) syntheses were carried out in a 10 mL Berghof Autoclave equipped with a PTFE liner. In a typical experiment, a 100 mg sample of RuCl3·n(H2O) was introduced into the autoclave with 4 mL of solvent (ethanol or water) and one to three metal plates (total weight: 50–100 mg, size of the plates: 7 × 7 × 0.3–1.0 mm). The reactions were performed using iron, stainless steel, and Fe–Cr-alloy under 10, 20 and 50 bar of CO at 125 °C. The reaction times were 1, 3, 6, 17 and 20 h. Each reaction was repeated several times. After each reaction the autoclave was cooled down in an ice bath. The solid products were filtered off and the resulting solution was evaporated to dryness. The total yield of solid products was ca. 100–120 mg. The crystals for X-ray analysis were obtained directly from the ethanol solution during slow evaporation, or by crystallization from dichloromethane solution.
In ethanol at 50 bar CO pressures and longer reaction times (>3 h) the main product was Ru3(CO)12 (yield ca. 80% after 17 h reaction). Ru3(CO)12 was partially precipitated as an orange solid during the reaction and the remainder was recovered by evaporating the solution to dryness. In longer (>3 h) reactions at 50 bar CO pressure, variable amounts of [RuCl2(CO)3]2 were also formed, but this was always a minor product. When the reaction was conducted in water, the final carbonylation product was polymeric [Ru(CO)4]n, which was also obtained in high yield (75–80%, after 17 h reaction). These known products15,29,30 were identified by IR, elemental analysis, and X-ray diffraction techniques (single crystal or powder diffraction).
In ethanol at lower CO pressures (20 bar) and shorter reaction times (≤3 h) the product did not contain any solid material. Only minor amounts of zerovalent ruthenium carbonyls or [RuCl2(CO)3]2 were found, or none at all. The main components, colorless [RuCl3(CO)3]2[Fe(H2O)6] and the yellowish trinuclear [Ru2Cl2(µ-Cl)4(CO)6FeL2] (L = H2O, EtOH), were separated mechanically after crystallization and analyzed by IR, elemental analysis and single-crystal X-ray diffraction. Other minor species, RuCl2(H2O)(CO)3, FeCl2(H2O)4, and [RuCl3(CO)3][Fe(H2O)6], which were found in some reaction products, were characterized only by single-crystal X-ray diffraction.
The individual yields of [RuCl3(CO)3]2[Fe(H2O)6], [Ru2Cl2(µ-Cl)4(CO)6FeL2] (L = H2O, EtOH), RuCl2(H2O)(CO)3, FeCl2(H2O)4, or [RuCl3(CO)3][Fe(H2O)6] are not reported here since the reaction products were typically mixtures of the above species with variable ratios. The ratio varied even when the reactions were repeated using the same reaction conditions.
CCDC reference numbers 299525–299534.
For crystallographic data in CIF or other electronic format see DOI: 10.1039/b602834a
1 | 1b | 1c | 2 | 3 | 4 | 5 | 5b | 5c | 6 | |
---|---|---|---|---|---|---|---|---|---|---|
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2. | ||||||||||
Empirical formula | C6H12Cl6FeO12Ru2 | C6H16Cl6FeO14Ru2 | C6H16Cl6FeO14Ru2 | Cl2FeH8O4 | C3H2Cl2O4Ru | C10H12Cl6FeO8Ru2 | C6H4Cl6FeO8Ru2 | C6H6Cl6FeO9Ru2 | C6H4Cl6FeO8Ru2 | C3H12Cl5FeO9Ru |
M | 746.85 | 782.88 | 782.88 | 198.81 | 274.02 | 730.89 | 674.78 | 692.80 | 674.78 | 526.30 |
T/K | 100(2) | 150(2) | 100(2) | 120(2) | 120(2) | 120(2) | 120(2) | 120(2) | 140(2) | 120(2) |
λ/Å | 0.71073 | 0.71073 | 0.71073 | 0.71073 | 0.71073 | 0.71073 | 0.71073 | 0.71069 | 0.71073 | 0.71073 |
Crystal system | Triclinic | Triclinic | Triclinic | Monoclinic | Monoclinic | Monoclinic | Monoclinic | Triclinic | Monoclinic | Monoclinic |
Space group | P | P | P | P21/n | P21/n | P21/c | P21/n | P | P21/c | P21/n |
a/Å | 6.2958(6) | 6.2478(5) | 6.2244(1) | 5.8738(3) | 6.2164(3) | 9.2270(7) | 7.0444(4) | 6.9100(2) | 10.1193(2) | 11.7975(5) |
b/Å | 6.4156(4) | 7.1825(7) | 13.0253(2) | 7.1006(5) | 10.2373(5) | 10.9769(6) | 15.6274(3) | 11.7730(5) | 6.4008(1) | 9.4944(2) |
c/Å | 13.9122(14) | 14.7889(16) | 15.4469(4) | 8.4040(4) | 11.7124(5) | 11.0272(8) | 8.5003(7) | 12.5560(5) | 27.7787(8) | 15.1011(6) |
α/° | 87.513(5) | 92.023(4) | 70.985(1) | 90 | 90 | 90 | 90 | 103.407(2) | 90 | 90 |
β/° | 77.295(4) | 97.730(4) | 85.508(1) | 109.623(3) | 97.922(3) | 96.796(4) | 97.016(3) | 101.573(2) | 95.3390(9) | 107.635(2) |
γ/° | 80.322(6) | 115.042(5) | 89.077(1) | 90 | 90 | 90 | 90 | 95.605(2) | 90 | 90 |
V/Å3 | 540.36(8) | 592.65(10) | 1180.32(4) | 330.15(3) | 738.25(6) | 1109.03(13) | 928.75(9) | 962.20(6) | 1791.47(7) | 1611.99(10) |
Z | 1 | 1 | 2 | 2 | 4 | 2 | 2 | 2 | 4 | 4 |
ρ calc/Mg m−3 | 2.295 | 2.194 | 2.203 | 2.000 | 2.465 | 2.189 | 2.413 | 2.391 | 2.502 | 2.169 |
µ(Mo-Kα)/mm−1 | 2.832 | 2.594 | 2.605 | 3.017 | 2.797 | 2.745 | 3.267 | 3.160 | 3.387 | 2.693 |
R1a (I ≥ 2σ) | 0.0412 | 0.0321 | 0.0234 | 0.0214 | 0.0219 | 0.0308 | 0.0253 | 0.0300 | 0.0242 | 0.0251 |
wR2b (I ≥ 2 σ) | 0.0758 | 0.0857 | 0.0605 | 0.0509 | 0.0436 | 0.0619 | 0.0596 | 0.0616 | 0.0578 | 0.0567 |
1 | 4 | 5 | 5b | 5c | 6 | |
---|---|---|---|---|---|---|
Ru1–Cl1 | 2.4166(12) | 2.4178(8) | 2.4145(8) | 2.4316(9) | 2.4125(9) | 2.4281(6) |
Ru1–Cl2 | 2.4162(14) | 2.4224(8) | 2.4350(7) | 2.4275(9) | 2.4273(9) | 2.4173(6) |
Ru1–Cl3 | 2.4102(13) | 2.4176(8) | 2.4182(9) | 2.4073(9) | 2.3974(9) | 2.4220(6) |
Ru1–C1 | 1.910(6) | 1.895(4) | 1.907(3) | 1.898(4) | 1.914(4) | 1.907(3) |
Ru1–C2 | 1.892(6) | 1.907(4) | 1.915(3) | 1.914(4) | 1.921(4) | 1.918(2) |
Ru1–C3 | 1.937(6) | 1.905(3) | 1.918(4) | 1.930(4) | 1.919(4) | 1.898(3) |
Fe1–O4 | 2.153(3) | 2.043(2) | 2.089(2) | 2.0702(17) | ||
Fe1–O5 | 2.102(4) | 2.0571(16) | ||||
Fe1–O6 | 2.111(4) | 2.0680(17) | ||||
Fe1–O7 | 2.070(3) | 2.0069(17) | ||||
Fe1–O8 | 2.146(3) | |||||
Fe1–Cl1 | 2.5132(8) | 2.5036(7) | 2.4935(9) | 2.5145(10) | ||
Fe1–Cl2 | 2.5415(8) | 2.4624(7) | 2.5000(10) | 2.4573(10) | ||
Fe1–Cl4 | 2.5287(9) | 2.4994(10) | 2.2501(7) | |||
Fe1–Cl5 | 2.4912(9) | 2.4681(10) | 2.3038(6) | |||
Ru2–Cl4 | 2.4210(9) | 2.4201(9) | ||||
Ru2–Cl5 | 2.4164(8) | 2.4243(9) | ||||
Ru2–Cl6 | 2.4236(9) | 2.3990(10) | ||||
Ru2–C4 | 1.916(4) | 1.899(4) | ||||
Ru2–C5 | 1.907(4) | 1.900(4) | ||||
Ru2–C6 | 1.915(4) | 1.922(4) | ||||
Cl1–Ru1–Cl2 | 89.36(5) | 90.69(10) | 90.46(9) | 86.83(3) | 86.83(3) | 88.97(2) |
Cl1–Ru1–Cl3 | 89.12(4) | 90.23(3) | 89.85(3) | 91.20(3) | 87.79(3) | 89.94(2) |
C1–Ru1–C2 | 92.6(2) | 91.65(14) | 91.14(13) | 89.65(15) | 93.03(15) | 92.98(10) |
C1–Ru1–C3 | 92.0(2) | 91.38(13) | 92.63(13) | 92.08(14) | 93.92(15) | 88.54(10) |
O4–Fe1–O5 | 88.99(14) | 81.28(6) | ||||
O4–Fe–O6 | 90.61(13) | 88.64(7) | ||||
Cl1–Fe1–Cl2A | 98.93(3) | 96.07(2) | ||||
O7–Fe1–O8 | 87.21(10) | 87.79(11) | ||||
Cl1–Fe1–Cl4 | 169.75(3) | 173.77(4) | ||||
Cl2–Fe1–Cl5 | 93.11(3) | 90.08(3) | ||||
Cl4–Fe1–Cl5 | 81.96(3) | 84.91(3) | 97.74(2) | |||
Cl6–Ru2–Cl4 | 90.33(3) | 89.02(3) | ||||
Cl6–Ru2–Cl5 | 89.14(3) | 87.41(3) | ||||
C6–Ru2–C4 | 91.66(15) | 91.85(16) | ||||
C6–Ru2–C5 | 92.12(14) | 91.43(16) |
To facilitate an investigation in to the role of the metal surface and the effect of the reaction conditions, the carbonylation of RuCl3 in the presence of a plate, was repeated in an autoclave (equipped with a Teflon liner) at an elevated temperature (125 °C) under variable CO pressures (10–50 bar) using different reaction times (1–24 h). Each reaction was repeated several times in order to see whether the product distribution obtained under certain conditions was reproducible. Again, the metal surfaces were clearly corroded in all reactions (Fig. 1). Compared with heating under reflux in a CO stream, the use of an autoclave and high pressures accelerated and enhanced the carbonylation. As a result of the more efficient carbonylation conditions, the zerovalent Ru3(CO)12 was typically the dominant final reduction product at high pressures of CO (50 bar), although at least traces of [RuCl2(CO)3]2 could also be found even after 20 h of reaction. Both of these products crystallized directly from the reaction solution and were identified by IR and single-crystal X-ray diffraction. As might be expected, the formation of the Ru3(CO)12 was favored by the longer reaction times. Similarly, the formation of the blackish, powdery, inactivating layer on the metal surface was favored by long reaction times and high pressures. The solvent was found to have a strong effect on the final carbonyl product. We have previously reported that the reduction of RuCl3 under 50 bar of CO in water produces polymeric [Ru(CO)4]n instead of Ru3(CO)12.30 A similar solvent effect was also observed in reductions with metal surfaces. In water the polymeric [Ru(CO)4]n replaced Ru3(CO)12 as the dominant final carbonylation product.
Fig. 1 Pitting corrosion on a stainless steel surface after 3 h of reaction at 50 bar of CO (under × 500 magnification). |
At lower CO pressures (20 bar) the formation of Ru3(CO)12 and [RuCl2(CO)3]2 was strongly suppressed. After a three-hour reaction at 125 °C at 10 bar CO pressure, a yellow solution was obtained, but no significant formation of Ru(CO)12 or [RuCl2(CO)3]2 could be observed. The yellow solution was evaporated to dryness and the resulting solid product was dissolved in CH2Cl2 and crystallized by adding hexane slowly to the dichloromethane solution. Typically, the final precipitate contained a colorless crystalline material as the main product, with some larger yellowish crystals. Both materials were analyzed by means of single-crystal X-ray crystallography. The colorless product was identified as ionic [RuCl3(CO)3]2[Fe(H2O)6] (1) with Ru and Fe in oxidation state +2 (Fig. 2).
Fig. 2 Thermal ellipsoid plot of [RuCl3(CO)3]2[Fe(H2O)6] (1). The ellipsoids have been drawn at 50% probability level. The Ru and Fe moieties are held together by weak hydrogen bonding interactions. For example: O4–H4A: 0.92 Å, H4A⋯Cl1: 2.34 Å, O4⋯Cl1: 3.186(4) Å, Ru1–H4A⋯Cl1: 154.5° (symmetry transformations used to generate equivalent atoms: A: 1 − x, 1 − y, 1 − z). |
In some cases 1 was crystallized in other crystal systems with water of crystallization as [RuCl3(CO)3]2[Fe(H2O)]·2(H2O) (structures 1b and 1c). Structurally, [RuCl3(CO)3]2[Fe(H2O)6] is isomorhphous with [RuCl3(CO)3]2[Ru(H2O)6].41 Although formation of [RuCl3(CO)3]2[Ru(H2O)6] could be possible also during the reduction of RuCl3 with metal surfaces no evidence of this was observed. X-Ray analysis of a series of crystalline products showed only the presence of mixed metal 1, where the ruthenium and iron units are held together by weak hydrogen bonds. Although [RuCl3(CO)3]2[Fe(H2O)6] was obtained after only short reaction times and with relatively low pressures, some traces of it were found even after 20 h of reaction under 50 bar of CO. An additional aqueous iron species FeCl2·4(H2O), which can also be seen as trans(Cl)-[FeCl2(H2O)4] (2), was also found in some reaction products obtained at 20 bar of CO, 3 h reaction time and 125 °C using an iron plate. It was only a minor product and was not observed at all in most of the reactions. The structure of 2 was verified by means of single-crystal X-ray diffraction. The presence of aqueous iron species was unsurprising, since the formation of such moieties can be expected as a consequence of the corrosion process of an iron-containing metal surface. Even if the reaction is carried out in ethanol (99.9%) water is nevertheless available. Water is introduced to the system by the solvent as well as by the RuCl3·n(H2O). Furthermore, although the metal plates were washed with ethanol prior to use, they can also be a source of water. In addition to iron, partially reduced ruthenium also produced the neutral mononuclear aqua complex RuCl2(H2O)(CO)3 (3) as a minor side-product in some of the high pressure reactions performed at 20 bar of CO, 3 h reaction time and 125 °C using an iron plate. Like trans(Cl)-[FeCl2(H2O)4] this product was strictly a minor product, which was not found at all in most of the reactions.
The second component found in the reaction product, a yellowish crystalline material, was typically a mixture that contained two types of complex in a variable ratio: [Ru2Cl2(µ-Cl)4(CO)6Fe(CH3CH2OH)2] (4) and [Ru2Cl2(µ-Cl)4(CO)6Fe(H2O)2] (5). Both of these species were characterized crystallographically (Fig. 3 and 4). The formation of complexes 4 and 5 was somewhat more surprising than the formation of complexes 1–3, since trinuclear halide-bridged ruthenium compounds are relatively rare. To our knowledge, the only other known example of linear Ru–(µ-Cl)2–M–(µ-Cl)2–Ru mixed metal systems is [{cis-RuCl2(dppm)2}2Cu]+.46 In addition to this, a few linear homometallic trinuclear ruthenium compounds of the type Ru–(µ-Cl)3–Ru–(µ-Cl)3–Ru with three chloride bridges are also known to exist.47–51
Fig. 3 Thermal ellipsoid plot of [Ru2Cl2(µ-Cl)4(CO)6Fe(CH3CH2OH)2] (4). The ellipsoids have been drawn at 50% probability level. Intramolecular hydrogen bonding interactions: O4–H4: 0.85 Å, H4⋯Cl3A: 2.25(2) Å, O4⋯Cl3A: 3.073(3) Å, O4–H4⋯Cl3A: 162° (symmetry transformation used to generate equivalent atoms: A: −x + 1, −y, −z + 1). |
Fig. 4 Thermal ellipsoid plot of the linear [Ru2Cl2(µ-Cl)4(CO)6Fe(H2O)2] (5). The ellipsoids have been drawn at 50% probability level. Intramolecular hydrogen bonding interactions: O4–H4B: 0.85 Å, H4B⋯Cl3: 2.457(10) Å, O4⋯Cl3: 3.293(2) Å, O4–H4⋯Cl3: 168° (symmetry transformation was used to generate equivalent atoms A: 1 − x, −y, −z). |
Complex 4 was typically the dominant trinuclear product in low pressure reactions (10 bar) and the amount of 5 tended to increase with increasing pressure. However, traces of complex 5 were also found in the low pressure reactions. Structurally, compound 4 contains two Ru centers and an iron center in the oxidation state +2. The linear trinuclear Ru–Fe–Ru architecture is supported by intramolecular hydrogen bonds between the OH group of iron-coordinated ethanol and the terminal chloride ligands of Ru (Fig. 3). Complex 5 has two structural isomers: a linear one (5) and a bent one (5b, and 5c) (Fig. 5).
Fig. 5 (a): Thermal ellipsoid plot (50% probability level) of the bent [Ru2Cl2(µ-Cl)4(CO)6Fe(H2O)2]·(H2O) (5b). Hydrogen bonding: O8–H8B: 0.85 Å, H(B⋯O99: 2.01(2) Å, O8⋯O99: 2.779(4) Å, O8–H8B⋯O99: 151(4)°. (b): Thermal ellipsoid (50% probability level) plot of the bent [Ru2Cl2(µ-Cl)4(CO)6Fe(H2O)2] (5c). |
In most reactions the linear 5 was preferred over the bent 5b one. However, the use of pure iron surfaces and, in particular, powdered iron increased the amount of bent 5b. As shown in Fig. 4, the structure of linear 5 resembles closely that of 4 (Fig. 3). The linear arrangement is again supported by two intramolecular hydrogen bonds between the iron-bound water and the ruthenium coordinated chloride. In the isomer 5b the crystal structure contains water of crystallization, which is involved in the hydrogen bonding network and may thus support the bent arrangement in preference to the linear one. However, the water of crystallization is not the key to the bent structure. A stable bent molecule without water of crystallization (5c) was also found, isolated, and crystallized (Fig. 5(b)). In the case of the bent 5b and 5c, the bent structure, together with very weak intramolecular hydrogen interactions, permitted some variations in the orientation of the end groups [RuCl3(CO)3]. We compared the relative stabilities of the linear and bent isomers of 5 and 5c (or 5b) by means of DFT calculations in the gas phase (Scheme 1). The calculations suggested that, energetically, the isomers are almost identical. The energy of the bent 5b is only 4 kJ mol−1 lower than the corresponding value for the linear 5. It can therefore be expected that there is no energetic preference in the formation of either structural form, and isomerization from one form to another may be possible.
Scheme 1 |
The formation of trinuclear complexes 4 and 5 is less obvious. A reaction between the released Fe2+ ions and the partially reduced ruthenium species would provide the most straightforward reaction route. If this was in fact the mechanism, it should be possible to obtain 4 and 5 simply by adding Fe2+ ions to the reaction solution. In order to investigate this possibility, the reductive carbonylation of RuCl3 was repeated (at 125 °C, 20 bar, 3 h) by using dissolved FeCl2·n(H2O) as the iron source, instead of a metal surface. None of the reactions with dissolved FeCl2·n(H2O) produced 4 or 5. The only Fe(II)-containing species found were trans(Cl)-[FeCl2(H2O)4] (2) and ionic 1. In addition to the Fe(II) products, traces of a further oxidized iron(III) component, [RuCl3(CO)3][FeCl2(H2O)4]·2(H2O) (6), was also found. The Fe3+ product 6 was crystallized and characterized by X-ray diffraction (Fig. 6). No other Fe3+ products were found in any of the reactions. In reactions with metal surfaces the oxidation of the iron stopped at oxidation state +2 and no 6 was observed. If 4 and 5 are formed in solution as a result of a reaction between the dissolved Fe2+ species 1 or 2 and the ruthenium species, the extended reaction should eventually consume the mononuclear iron and ruthenium components, thus improving the yields of 4 and 5, but this was not observed. Such results may indicate a more active role for the metal surface, for example, in the surface-assisted formation of 4 and 5.
Fig. 6 Thermal ellipsoid plot of [RuCl3(CO)3][FeCl2(H2O)4]·2(H2O) (6). The ellipsoids have been drawn at 50% probability level. Intermolecular hydrogen bonding interactions: O5–H5B: 0.77 Å, H5B⋯Cl2: 2.40 Å, O5⋯Cl2: 3.158(2) Å, O5–H5B⋯Cl2: 170.4°, O6–H6A: 0.82 Å, H6A⋯Cl3: 2.33 Å, O6⋯Cl3: 3.151(2) Å, O6–H6A⋯Cl3: 178.2°. |
The energetics of the interconversions between products 1–5 was investigated using computational DFT methods. The results are summarized in Scheme 1.
The DFT calculations predict that the reaction of 4 with water (Scheme 1) leads to the replacement of the ethanol ligands by water. The total energy difference of −34 kJ−1 mol for the replacement reaction is not large, but it is favorable. According to the calculations, the presence of water should thus favor 5 over 4. In addition, both the linear and the bent isomers 5 and 5b should be more or less equally probable products. It should be noted that the gas phase calculations tend to overestimate the role of the intramolecular hydrogen bonds, and therefore the stability of 4 and 5, as a result of the absence of surrounding solvent and molecules. However, both 4 and 5 contain intramolecular hydrogen bonds, which should make the calculations more compatible when 4 and 5 are compared.
According to the DFT results, the amount of water is one of the key factors determining the distribution of the products. A suitable amount of water should favor 4, but an excess leads to a breakdown of the trinuclear framework of the [Ru2Cl2(µ-Cl)4(CO)6Fe(H2O)2] into mononuclear products, either as ionic [RuCl3(CO)3]2[Fe(H2O)6] (1) or as a mixture of neutral FeCl2(H2O)4 (2) and RuCl2(H2O)(CO)3 (3). Both fragmentation reactions [Ru2Cl2(µ-Cl)4(CO)6Fe(H2O)2] + 4 (H2O) → [RuCl3(CO)3]2[Fe(H2O)6] and [Ru2Cl2(µ-Cl)4(CO)6Fe(H2O)2] + 4 (H2O) → 2 RuCl2(H2O)(CO)3 + FeCl2(H2O)4 are favorable (Scheme 1), but the ionic products should clearly be more likely than the neutral products. Again, the absolute energy values obtained from the gas phase calculations should be interpreted with caution, especially in the case of the ionic products. The gas phase optimization overestimates the hydrogen bonding interactions between [RuCl3(CO)3]− and [Fe(H2O)6]2+ and therefore the stability of [RuCl3(CO)3]2[Fe(H2O)6] is also overestimated. Nevertheless, the breakdown of 4 to 1 is so facile that the calculations can be expected to give the right results on at least a qualitative level. Furthermore, computational results strongly support the experimental observations: the main product in all short (3 h) low pressure (10 bar) reactions is ionic 1, and only small amounts, if any, of the neutral RuCl2(H2O)(CO)3 (2) and FeCl2(H2O)4 are formed. Furthermore, when water was added to the reaction solution, products 4 or 5 were no longer obtained, and only aqueous mononuclear metal species 1–3 were found. In addition, the fact that the reaction time does not favor the formation of 4 or 5 is in agreement with the calculations. It seems very unlikely that the trinuclear 4 or 5 are formed in a reaction between 1–3. This may therefore mean that the presence of a metal surface is required to obtain these rather labile complexes. Most probably, there is a more straightforward reaction pathway to 1–3, but the breaking down of the trinuclear complexes certainly offers a facile route to these species.
Formation of 4 and 5 shows that the sacrificial surface approach can be used for the synthesis of new mixed metal Ru–Fe compounds. The oxidation process is driven by the potential differences between the Ru3+ ions and the Fe surface. This method can also be expanded to other metals. We are currently working with Ru–Co, Os–Fe and Os–Co systems. Scheme 1 also shows the main challenge involved in using the method: The mixed metal compounds are relatively labile and sensitive to moisture, which causes problems in the reproducibility of compounds 4 and 5. The sensitivity of moisture is also the main reason for variations in the product distributions.
Footnote |
† Electronic supplementary information (ESI) available: Thermal ellipsoid plots (at 50 % probability level) of 1b, 1c, 2, and 3. See DOI: 10.1039/b602834a |
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