The subtle effects of iron-containing metal surfaces on the reductive carbonylation of RuCl3

Article abstract of DOI:10.1039/b602834a

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(H ₂O)₄] the use of the metal surface also provided a route to novel labile trinuclear [Ru₂Cl₂(-Cl) ₄(CO)₆FeL₂] (L = H₂O, EtOH) complexes. The stability and reactivity of the [Ru₂Cl ₂(-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(-Cl)₄(CO)₆FeL ₂]. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602834a

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www.rsc.org/dalton | Dalton Transactions
The subtle effects of iron-containing metal surfaces on the reductive
carbonylation of RuCl3†
Matti Haukka,* Minna Jakonen, Taina Nivaja¨rvi and Mirja Kallinen
Received 24th February 2006, Accepted 4th May 2006
First published as an Advance Article on the web 12th May 2006
DOI: 10.1039/b602834a
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(l-Cl)4(CO)6FeL2] (L = H2O,
EtOH) complexes. The stability and reactivity of the [Ru2Cl2(l-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(l-Cl)4(CO)6FeL2].
of Co, Ni, Zn, and Cu9 are more recent examples of the successful
use of sacrificial electrodes in synthetic chemistry.
1. Introduction
A solid metal surface can be involved in chemical reactions in
various ways. A metallic material can serve as a redox agent or
catalyst, facilitating a reaction, or it can release metal species into
the system. The metal can be released as oxidized products via
corrosion processes or, in some cases, even as formally zerovalent
compounds. As a well-known example, Fe(CO)5, and especially
Ni(CO)4, can be obtained by the direct interaction between CO
and the metal. The controlled release of metal species from a
surface has been exploited in several chemical, biochemical and
synthetic chemistry processes. It has long been known that, for
example, under catalytic reaction conditions the walls of the
metallic reaction vessels can have an influence on the reaction.1
The released metal can even promote the catalysis processes.
However, in these processes the release of the metal is usually
not well controlled, and often the released metal compounds
are not completely characterized. The most common method of
taking advantage of controlled metal release from a bulk metal
is to use sacrificial electrodes. Sacrificial electrodes have been
applied, for example, to control the iron concentration in the feed
water of nuclear boiling water reactors,2 to release ions into the
water as a biocide3 or for electroflocculation in order to provide
a coagulating agent for the waste water.4,5 The electrochemical
synthesis of tetramethyl lead and some other organometallic
compounds was patented as long ago as the mid-1960s,6 while the
electrosynthesis of functionalized 2-arylpyridines,7 functionalized
organozinc compounds8 and also the aminopyridine derivatives
The primary objective of the present work was to apply the sac-
rificial 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(l-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(l-
Cl)4(CO)6FeL2] (L = H2O, EtOH) is also suggested.
Department of Chemistry, University of Joensuu, Department of Chem-
istry, P.O. Box 111, FI-80101, Joensuu, Finland. E-mail: matti.haukka@
joensuu.fi; Fax: +358-13-251 3390
† Electronic supplementary information (ESI) available: Thermal ellip-
soid plots (at 50 % probability level) of 1b, 1c, 2, and 3. See DOI:
10.1039/b602834a
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In ethanol at lower CO pressures (20 bar) and shorter re-
action 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(l-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.
2. Experimental
2.1. Materials
Ethanol (Primalco, 99.9%) and RuCl3·n(H2O) (Alfa Aesar,
p.a.) were used without further purification. Carbon monox-
ide (99.999%) was obtained from Aga. Metal plates: Iron
(Fe) foil (thickness: 0.25 mm, purity 99.5%, product number:
FE000400/11), stainless steel-17-7PH (thickness 0.3 mm, 76%
Fe, 17% Cr and 7% Ni, product number: FE280350/2), Fe–Cr-
R
alloyꢀ (iron/chromium) (thickness 1.0 mm 72.8% Fe, 22% Cr,
5% Al, 0.1% Y and 0.1% Zr, product number: FE080600/4) plates
were purchased from Goodfellow. Iron powder was obtained from
Merck.
The individual yields of [RuCl3(CO)3]2[Fe(H2O)6], [Ru2Cl2(l-
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.
2.2. Reactions
The reactions at low pressures under a CO flow were performed in a
round-bottom, two-neck flask. Carbon monoxide was introduced
into the system by bubbling the gas through the solution. In a
typical reaction, a 250 mg sample of RuCl3·n(H2O), 40 ml of
ethanol and ca. 100 mg of the metallic component (metal plates
or powder) were placed in the bottom of the reaction vessel. The
solution was heated under reflux overnight (17–20 h), after which
it was cooled to room temperature and filtered. The solution was
then evaporated to dryness for further analysis. Low-pressure
reactions with different metal plates were repeated under these
conditions ca. 10–20 times. The total yield of solid products
were ca. 200–230 mg. The crystals for X-ray analysis either were
obtained directly from the reaction solution or the dried product
was dissolved in CH2Cl2 and crystallized by adding hexane to the
dichloromethane solution.
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.
Analysis of products 1, 4, and 5.
[Ru(CO)3Cl3]2[Fe(H2O)6] (1). IR m(CO) (in CH2Cl2): 2141
(vs) cm−1, 2074 (vs) cm−1; m(CO) (in KBr): 2143 (s) cm−1, 2074
(sh) cm−1, 2060 (vs, br) cm−1. Calcd.: C 9.65%, H 1.62%, Found:
C 9.68%, H 1.56%.
[Ru2Cl2(l-Cl)4(CO)6Fe(CH3CH2OH)2] (4). IR m(CO) (in
CH2Cl2): 2143 (vs) cm−1, 2076 (vs) cm−1. Calcd.: C 16.43%, H
1.65%, Found: C 16.34%, H 1.81%.
[Ru2Cl2(l-Cl)4(CO)6Fe(H2O)2] (5). IR m(CO) (in KBr):
2149 (s), broad signal at ca. 2075 (vs) cm−1 or double-headed
signal at 2079 (vs) cm−1 and 2071 (vs) cm−1. IR m(CO) (in
CH2Cl2): 2142 (vs) cm−1, 2075 (vs) cm−1. No distinction could
be made between the linear and the bent isomers of 5. Calcd.
for [Ru2Cl2(l-Cl)4(CO)6Fe(H2O)2]·2(H2O): C 10.14%, H 1.13%,
Found: C 10.04%, H 1.38%.
2.3 X-Ray crystal structure determinations
The crystals were immersed in cryo-oil, mounted in a Nylon loop
and measured at a temperature of 100 K or 120 K. The X-ray
diffraction data were collected by means of a Nonius KappaCCD
˚
diffractometer using Mo-Ka radiation (k = 0.71073 A). The
Denzo-Scalepack program package31 was used for cell refinements
and data reductions. All of the structures were solved by direct
methods using the SHELXS97, SIR2004, or SIR9732–34 with the
WinGX graphical user interface.35 An empirical absorption cor-
rection was applied to all of the data (XPREP in SHELXTL v.6.14-
1).36 Structural refinements were carried out using SHELXL97.37
Both H2O and OH hydrogens were located from the difference
Fourier map in all structures. The H2O hydrogens were located
from the difference Fourier and restrained to ride on their parent
atom (Uiso = 1.5 Ueq (parent atom)) using the observed O–H
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).
˚
distance or refined with a restrained O–H distance (0.85 A). All
other hydrogens were placed in idealized positions and constrained
to ride on their parent atom. The crystallographic details are
summarized in Table 1. The selected bond lengths and angles are
shown in Table 2. The structures of 2 and 3 are known and have
been published previously by others.38–41
CCDC reference numbers 299525–299534.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b602834a
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2.4 Computational work
The full geometry optimization of the structures was conducted by
using the non-local hybrid density functional method B3PW91, as
implemented in the Gaussian03 program.42 The basis set was the
standard 6-31G* for all elements except Ru and Fe, for which
the Stuttgart–Dresden ECP basis set was used.43 In the case
of ruthenium, the ECP basis set was augmented by a p-type
polarization function with an exponent of 0.086.44 The Hessian
matrix was calculated analytically for all structures in order to
verify the location of the global minima (none of the optimized
structures had imaginary frequencies).
3. Results and discussion
3.1 Reductive carbonylation of RuCl3 in the presence of a
sacrificial metal surface
When RuCl3 is reduced by carbon monoxide in an alcoholic
solution, the originally dark solution first turns into a “ruthenium
red carbonyl solution”.10,11 If the reaction is continued, a yellow
solution is obtained. Mantovani and Cenini reported that the
reduction at low pressures produces a lemon-yellow solution
of uncharacterized ruthenium carbonyl chloride Ru(CO)nClm.12
When the reaction was repeated in the presence of an iron-
containing metal surface it followed the same steps. Under reflux
and in a stream of carbon monoxide the reaction solution first
turned slowly from black to dark red and finally to yellow
or greenish yellow. At this point a sample of the solution
was dried, dissolved in dichloromethane and analyzed using
IR spectroscopy. The IR spectrum typically showed a simple
pattern of two strong m(CO) frequencies at ca. 2141 cm−1and
2074 cm−1 (in CH2Cl2). The pattern resembles the spectrum of
the [RuCl2(CO)3]2 dimer (2144 cm−1and 2083 cm−1 in CH2Cl2),
but the bands are shifted towards lower wavenumbers. The higher
frequency band at 2141 cm−1 was sharp and the lower band
broader or double headed. This type of pattern is typical of a
symmetrical octahedral ruthenium tricarbonyl species of the type
[RuX3(CO)3] with facially arranged carbonyls.45 The spectrum
could be assigned to [RuCl3(CO)3]− ions. However, when the
reactions were repeated several times some variation was observed
in the spectra, indicating that the process was not completely
repeatable. Both the wavenumbers and the number of carbonyl
signals and also their relative intensities varied, suggesting the
presence of more than one metal carbonyl species with variable
ratios. Nevertheless, the reaction followed a similar path in all
reactions. In all cases, the metal surface was roughened and
corroded. The wearing-out of the metal surface indicates that it
had been involved in redox reaction and had released iron into the
solution. The nature of the corrosion was found to be dependent on
the solid material. Stainless steel tended to undergo typical pitting
corrosion, while the pure iron and Fe–Cr-alloy surfaces were worn
more uniformly. In some reactions a blackish powdery material,
rich in Ru, accumulated on the metal surface. The amount of
the blackish powder was also dependent on the metal surface
used, occurring especially in reactions with stainless steel. The
formation of a solid blackish side product of RuO2 during the
high pressure reductive carbonylation of RuCl3 had been reported
earlier by Bruce et al.19 Thus, RuO2 could also be formed in our
3214 | Dalton Trans., 2006, 3212–3220
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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1, 4, 5, 5b, 5c, and 6
1
4
5
5b
5c
6
Ru1–Cl1
Ru1–Cl2
Ru1–Cl3
Ru1–C1
Ru1–C2
Ru1–C3
Fe1–O4
Fe1–O5
2.4166(12)
2.4162(14)
2.4102(13)
1.910(6)
1.892(6)
1.937(6)
2.153(3)
2.102(4)
2.111(4)
2.4178(8)
2.4224(8)
2.4176(8)
1.895(4)
1.907(4)
1.905(3)
2.043(2)
2.4145(8)
2.4350(7)
2.4182(9)
1.907(3)
1.915(3)
1.918(4)
2.089(2)
2.4316(9)
2.4275(9)
2.4073(9)
1.898(4)
1.914(4)
1.930(4)
2.4125(9)
2.4273(9)
2.3974(9)
1.914(4)
1.921(4)
1.919(4)
2.4281(6)
2.4173(6)
2.4220(6)
1.907(3)
1.918(2)
1.898(3)
2.0702(17)
2.0571(16)
2.0680(17)
2.0069(17)
Fe1–O6
Fe1–O7
2.070(3)
Fe1–O8
Fe1–Cl1
Fe1–Cl2
Fe1–Cl4
Fe1–Cl5
Ru2–Cl4
Ru2–Cl5
Ru2–Cl6
2.146(3)
2.5132(8)
2.5415(8)
2.5036(7)
2.4624(7)
2.4935(9)
2.5000(10)
2.5287(9)
2.4912(9)
2.4210(9)
2.4164(8)
2.4236(9)
1.916(4)
2.5145(10)
2.4573(10)
2.4994(10)
2.4681(10)
2.4201(9)
2.4243(9)
2.3990(10)
1.899(4)
2.2501(7)
2.3038(6)
Ru2–C4
Ru2–C5
Ru2–C6
1.907(4)
1.915(4)
1.900(4)
1.922(4)
Cl1–Ru1–Cl2
Cl1–Ru1–Cl3
C1–Ru1–C2
C1–Ru1–C3
O4–Fe1–O5
O4–Fe–O6
Cl1–Fe1–Cl2A
O7–Fe1–O8
Cl1–Fe1–Cl4
Cl2–Fe1–Cl5
Cl4–Fe1–Cl5
Cl6–Ru2–Cl4
Cl6–Ru2–Cl5
C6–Ru2–C4
C6–Ru2–C5
89.36(5)
89.12(4)
92.6(2)
92.0(2)
88.99(14)
90.61(13)
90.69(10)
90.23(3)
91.65(14)
91.38(13)
90.46(9)
89.85(3)
91.14(13)
92.63(13)
86.83(3)
91.20(3)
89.65(15)
92.08(14)
86.83(3)
87.79(3)
93.03(15)
93.92(15)
88.97(2)
89.94(2)
92.98(10)
88.54(10)
81.28(6)
88.64(7)
98.93(3)
96.07(2)
87.21(10)
169.75(3)
93.11(3)
81.96(3)
90.33(3)
89.14(3)
91.66(15)
92.12(14)
87.79(11)
173.77(4)
90.08(3)
84.91(3)
89.02(3)
87.41(3)
91.85(16)
91.43(16)
97.74(2)
system. Nevertheless, a possible formation of metallic Ru cannot
be completely ruled out. Although the metal surface was clearly
corroded, no iron-oxo species were observed. The formation of the
blackish layer hindered the reductive carbonylation of RuCl3. If
the same metal particles were used again in a reaction without the
surface having been cleaned, the yellow solution took considerably
longer to form. Such results suggest that the metal surface also
takes part in the reduction of RuCl3.
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 reac-
tion 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
Fig. 1 Pitting corrosion on a stainless steel surface after 3 h of reaction
at 50 bar of CO (under × 500 magnification).
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
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in water produces polymeric [Ru(CO)4]n instead of Ru3(CO)12.30
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.
A
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,
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 ob-
tained, 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 crys-
tallized 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).
a
yellowish crystalline material, was typically
a
mixture
that contained two types of complex in
a
variable ra-
tio: [Ru2Cl2(l-Cl)4(CO)6Fe(CH3CH2OH)2] (4) and [Ru2Cl2(l-
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–(l-Cl)2–M–(l-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–(l-Cl)3–Ru–(l-Cl)3–Ru with three chloride bridges are also
known to exist.47–51
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 A, H4A · · · Cl1: 2.34 A, O4 · · · Cl1: 3.186(4) A,
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 forma-
tion 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
Fig. 3 Thermal ellipsoid plot of [Ru2Cl2(l-Cl)4(CO)6Fe(CH3CH2OH)2]
(4). The ellipsoids have been drawn at 50% probability level. Intramolecular
˚
˚
hydrogen bonding interactions: O4–H4: 0.85 A, H4 · · · Cl3A: 2.25(2) A,
◦
˚
O4 · · · Cl3A: 3.073(3) A, 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(l-Cl)4(CO)6Fe(H2O)2]
(5). The ellipsoids have been drawn at 50% probability level. Intramolecular
˚
hydrogen bonding interactions: O4–H4B: 0.85 A, H4B · · · Cl3: 2.457(10)
◦
˚
˚
A, O4 · · · Cl3: 3.293(2) A, O4–H4 · · · Cl3: 168 (symmetry transformation
was used to generate equivalent atoms A: 1 − x, −y, −z).
3216 | Dalton Trans., 2006, 3212–3220
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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).
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.
3.2 Formation of the products
The formation of the aqueous products [Fe(H2O)6]2+,
[FeCl2(H2O)], and [RuCl2(H2O)(CO)3] could be explained in terms
of the simple oxidation of the iron surface during the partial
reductive carbonylation of RuCl3. Carbon monoxide facilitates
the reaction further and stabilizes the ruthenium species. In such a
mechanism, an iron-containing surface acts as a sacrificial surface,
supplying iron ions to the solution. The corrosion of the metal is
enhanced by the chlorides from RuCl3, especially when stainless
steel with a protective oxide layer on its surface is used. It is known
that chloride and other aggressive anions can penetrate through
the oxide layer into the metal surface and launch the corrosion
process.52 The corrosion products tend to be deposited on the metal
surface, but as chloride also passes through the oxide layer it can
also penetrate through the corrosion deposit, thus accelerating the
corrosion even further. However, in the reduction conditions that
we used, the reduction of ruthenium also accelerates the release of
iron. This was seen clearly when the stainless steel surfaces were
exposed to an ethanol solution of NaCl under CO pressure (50 bar)
without Ru3+ ions. The extent of the corrosion was insignificant
even after 20 h of reaction at 125 ◦C.
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 reac-
tion 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
Fig. 5 (a): Thermal ellipsoid plot (50% probability level) of the bent
[Ru2Cl2(l-Cl)4(CO)6Fe(H2O)2]·(H2O) (5b). Hydrogen bonding: O8–H8B:
˚
˚
˚
0.85 A, H(B · · · O99: 2.01(2) A, O8 · · · O99: 2.779(4) A, O8–H8B · · · O99:
151(4)◦. (b): Thermal ellipsoid (50% probability level) plot of the bent
[Ru2Cl2(l-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
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Scheme 1
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(l-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 frag-
mentation reactions [Ru2Cl2(l-Cl)4(CO)6Fe(H2O)2] + 4 (H2O) →
[RuCl3(CO)3]2[Fe(H2O)6] and [Ru2Cl2(l-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 in-
teractions 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
Fig. 6 Thermal ellipsoid plot of [RuCl3(CO)3][FeCl2(H2O)4]·2(H2O)
(6). The ellipsoids have been drawn at 50% probability level. Inter-
˚
molecular hydrogen bonding interactions: O5–H5B: 0.77 A, H5B · · · Cl2:
◦
˚
˚
˚
2.40 A, O5 · · · Cl2: 3.158(2) A, O5–H5B · · · Cl2: 170.4 , O6–H6A: 0.82 A,
◦
˚
˚
H6A · · · Cl3: 2.33 A, O6 · · · Cl3: 3.151(2) A, O6–H6A · · · Cl3: 178.2 .
active role for the metal surface, for example, in the surface-assisted
formation of 4 and 5.
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
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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.
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An iron-containing metal surface can act as a sacrificial source for
iron in the synthesis of mixed metal [Ru2Cl2(l-Cl)4(CO)6Fe(L)2]
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Acknowledgements
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Germany, 1997.
Financial support in the form of a grant (grant no. 210265)
provided by the Academy of Finland is gratefully acknowledged.
The authors would also like to thank Dr Janne Ja¨nis for his help
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