Surface-mediated organometallic synthesis: High-yield and selective syntheses of neutral and anionic ruthenium carbonyl clusters by controlled reduction of silica-supported RuCl3 in the presence of Na2CO3 or K2C

Article abstract of DOI:10.1021/om970230v

[Ru₃(CO)₁₀Cl₂], [Ru₃(CO)₁₂], [H₄Ru₄(CO)₁₂], [H₃Ru₄(CO)₁₂]^-, [HRu₆(CO)₁₈]^-, and [Ru₆

Full text of DOI:10.1021/om970230v

Organometallics 1997, 16, 4531-4539
4531
Su r fa ce-Med ia ted Or ga n om eta llic Syn th esis: High -Yield
a n d Selective Syn th eses of Neu tr a l a n d An ion ic
Ru th en iu m Ca r bon yl Clu ster s by Con tr olled Red u ction
of Silica -Su p p or ted Ru Cl3 in th e P r esen ce of Na 2CO3 or
K2CO3
Dominique Roberto,* Elena Cariati, Elena Lucenti, Marco Respini, and
Renato Ugo
Dipartimento di Chimica Inorganica, Metallorganica e Analitica and Centro CNR, Universita`
di Milano, Via G. Venezian, 21, 20133 Milano, Italy
Received March 24, 1997X
[Ru3(CO)10Cl2], [Ru3(CO)12], [H4Ru4(CO)12], [H3Ru4(CO)12]-, [HRu6(CO)18]-, and [Ru6C(CO)16]2-
have been synthesized in good or high yields by one-step or two-step (with the surface species
[Ru(CO)3Cl2(HOSit)] as intermediate) one-pot controlled reduction of RuCl3 supported on
silica in the presence of alkali carbonates. The selectivity of the reaction is controlled by
the (i) nature and quantity of the alkali carbonate (Na2CO3 or K2CO3), (ii) manner by which
the alkali carbonate is deposited on the silica surface, (iii) gas-phase composition (CO, CO
+ H2, or CO + H2O), (iv) temperature, and (v) reaction time. The unusual selectivity can be
explained by the initial formation of [Ru(CO)x(OH)2]n (x ) 2, 3) which, under CO, aggregates
first to [Ru3(CO)12], key intermediate for the synthesis in situ of various ruthenium carbonyl
clusters on the silica surface.
In tr od u ction
work describes the efficient syntheses on the silica sur-
face of neutral ([Ru3(CO)10Cl2], [Ru3(CO)12], and [H4Ru4-
(CO)12]) and anionic ([H3Ru4(CO)12]-, [HRu6(CO)18]-,
and [Ru6C(CO)16]2-) clusters working in the presence
of Na2CO3 and K2CO3, respectively, and starting from
a simple material such as RuCl3.
Preparative organometallic chemistry mediated by
surfaces of inorganic oxides is a new area of synthesis
of metal carbonyl complexes and clusters, whereby the
surface of an inorganic oxide plays the role of the solvent
in conventional syntheses. It is usually characterized
by high yields and excellent selectivities.1 As a general
trend, strongly basic surfaces such as MgO favor the
formation of anionic metal carbonyl clusters1a-c while
uncharged carbonyl complexes and clusters are gener-
ated on the surface of rather neutral supports such as
SiO2.1d-f However, we found recently that both neutral
and anionic osmium carbonyl clusters can be synthe-
sized by controlled reduction of silica-supported R-[Os-
(CO)3Cl2]2 or OsCl3 in the presence of alkali carbo-
nates.1g,h We have reported that the reductive carbo-
nylation of silica-supported â-[Ru(CO)3Cl2]2 to [Ru3-
(CO)12] does not occur easily due to both the sublimation
of the starting material at temperatures higher than
100 °C and the difficulty of removing chloro ligands from
the ruthenium coordination sphere when working at
relatively low temperatures.1f Since in the case of os-
mium, addition of an alkali carbonate to the silica sur-
face favors removal of chloro ligands,2 we have extended
our investigation to the preparation from RuCl3 of
various ruthenium carbonyl clusters (generally best
prepared in solution starting from [Ru3(CO)12]3). This
Resu lts a n d Discu ssion
Syn t h esis of [R u 3(CO)12] a n d [R u 3(CO)10Cl2].
When RuCl3 supported on silica (2 wt % of Ru with
respect to SiO2) added with Na2CO3 (molar ratio
Na2CO3:Os ) 3:1; deposited from a water solution) is
heated at 100 °C under 1 atm of CO in the closed
cylindrical Pyrex vessel previously described,1e an or-
ange material sublimes onto the cold walls of the
reaction vessel. Extraction of the sublimate and the
silica powder with CH2Cl2 affords [Ru3(CO)12] in 38%
yield after 96 h. Better yields (56%) are achieved by
increasing the temperature (130 °C) and by adding some
water (Table 1). In both cases, the gray color of the
silica powder, after extraction of [Ru3(CO)12], suggests
some parallel reduction to metallic ruthenium.
As expected, a slight increase of the CO pressure
favors the above reductive carbonylation of RuCl3. By
working in an autoclave at 130 °C under 5 atm of CO
for 7 h, [Ru3(CO)12] is obtained in 69% yield (Table 1).
After extraction of [Ru3(CO)12], the infrared spectrum
of the orange silica powder shows a carbonyl band at
2013 cm-1 due to an unidentified carbonyl ruthenium
species bound to silica which cannot be converted to
[Ru3(CO)12] by further treatment under 5 atm of CO at
130 °C. By working with a higher ruthenium loading
(15 wt % of Ru with respect to SiO2), the reductive
X Abstract published in Advance ACS Abstracts, August 15, 1997.
(1) (a) Lamb, H. H.; Fung, A. S.; Tooley, P. A.; Puga, J .; Krause, R.;
Kelley, M. J .; Gates, B. C. J . Am. Chem. Soc. 1989, 111, 8367. (b)
Gates, B. C. J . Mol. Catal. 1994, 86, 95. (c) Kawi, S.; Xu, Z.; Gates, B.
C. Inorg. Chem. 1994, 33, 503. (d) Dossi, C.; Psaro, R.; Roberto, D.;
Ugo, R.; Zanderighi, G. M. Inorg. Chem. 1990, 29, 4368. (e) Roberto,
D.; Psaro, R.; Ugo, R. Organometallics 1993, 12, 2292. (f) Roberto,
D.; Psaro, R.; Ugo, R. J . Organomet. Chem. 1993, 451, 123. (g) Roberto,
D.; Cariati, E.; Psaro, R.; Ugo, R. Organometallics 1994, 13, 734. (h)
Roberto, D.; Cariati, E.; Ugo, R.; Psaro, R. Inorg. Chem. 1996, 35, 2311.
(2) Cariati, E.; Roberto, D.; Ugo, R., Submitted for publication in
Organometallics.
(3) Seddon, E. A.; Seddon, K. R. In The Chemistry of Ruthenium in
Topics in Inorganic and General Chemistry, Monograph 19, Clark, R.
J . H., Ed.; Elsevier: Amsterdam, 1984.
S0276-7333(97)00230-6 CCC: $14.00 © 1997 American Chemical Society

4532 Organometallics, Vol. 16, No. 21, 1997
Roberto et al.
Ta ble 1. Syn th esis of Neu tr a l Ru th en iu m Ca r bon yl Clu ster s on th e Silica Su r fa ce in th e P r esen ce of
a
Na 2CO3
[Ru3(CO)12
]
[H4Ru4(CO)12]
starting material
RuCl3/SiO2
RuCl3/SiO2
RuCl3/SiO2
Ru/SiO2, wt %
gas (P, atm)
CO (1)c
CO (5)
CO (5)
CO (1)
CO (1)
CO (1)
CO:H2 ) 1:3 (1)
H2 (1)
CO:H2 ) 1:3 (1)
CO:H2 ) 1:3 (1)
T °C
t, h
yield, % (mg)b
yield, % (mg)b
2
2
15
2
5
15
2
2
130
130
130
110e
110
110
110
110
110e
110
48
7
56 (312)
69 (218)
55 (100)
0
0
0
0
0
0
24
48
48
24
19
19
24
48
Ru(CO)3Cl2(HOSit)d
Ru(CO)3Cl2(HOSit)d
Ru(CO)3Cl2(HOSit)d,g,h
Ru(CO)3Cl2(HOSit)d
Ru(CO)3Cl2(HOSit)d
Ru(CO)3Cl2(HOSit)d,f,h
Ru(CO)3Cl2(HOSit)d,h
93 (223)
82 (295)f
49 (229)
0
0
0
0
88 (162)
39 (134)i
86 (298)
94 (324)
5 or 15
5 or 15
a
b
Molar ratio Na2CO3:Ru ) 3:1, deposited from a CH2Cl2 slurry. Actual weight of isolated cluster. c In the presence of added water
(ca. 16 wt % of H2O with respect to SiO2). Prepared on the surface by reductive carbonylation of silica-supported RuCl3.3 e Similar yields
d
are reached at 130 °C. f Similar yields are reached starting from pure â-[Ru(CO)3Cl2]2/SiO2. Na2CO3 deposited from water instead of
g
h
CH2Cl2; when Na2CO3 is deposited from a CH2Cl2 slurry, [Ru3(CO)10Cl2] (75%, 332 mg) is formed instead of [Ru3(CO)12]. By working
with a loading of 15 wt % of Ru with respect to SiO2, a mixture of silica-bound [Ru(CO)3Cl2(HOSit)] and silica supported â-[Ru(CO)3Cl2]2
i
is formed by reductive carbonylation of silica-supported RuCl3. Metallic ruthenium is formed in parallel.
Sch em e 1. Best Syn th eses, a t Atm osp h er ic P r essu r e, of Va r iou s Ru th en iu m Ca r bon yl Clu ster s on SiO2 in
th e P r esen ce of Na 2CO3 or K2CO3 Dep osited fr om a CH2Cl2 Slu r r y
carbonylation (5 atm of CO) gives a lower yield (55%)
of [Ru3(CO)12] even after 24 h (Table 1).
when too high metal loadings are used. In particular,
by working at atmospheric pressure and 100 °C, the
formation of [Ru(CO)3Cl2(HOSit)] from RuCl3 is sensi-
tive to metal loading, the optimal loading being in the
range 2-5 wt % of Ru with respect to SiO2.1e At higher
loading (10 wt %), yields are lower with parallel forma-
tion of poorly reactive [Ru(CO)2Cl2]n, due to the lack of
disponible surface silanol groups acting as electron
donors.1e,f However, we found that high yields of
tricarbonylRu(II) surface species (mixture of [Ru-
(CO)3Cl2(HOSit)] and silica-supported â-[Ru(CO)3Cl2]2)
can be reached even when the metal loading is high (15
wt % of Ru with respect to SiO2), by working under 1
atm of CO first at 110 °C for 48 h and then at 120 °C
for 24 h. Use of a slight CO pressure (5 atm) accelerates
the formation of tricarbonylRu(II) species, which is
completed in 24 h by working only at 110 °C.
The direct reductive carbonylation of silica-supported
RuCl3 occurs easily, but it does not give high yields of
[Ru3(CO)12], due to some by-reactions such as reduction
to ruthenium metal or to an unreactive ruthenium
carbonyl surface species. Therefore, to attain a high
yield synthesis of [Ru3(CO)12], we investigated a two-
step route corresponding to the following: (i) prepara-
tion in situ of silica-bound [Ru(CO)3Cl2(HOSit)] by
carbonylation of silica-supported RuCl3 at 110 °C un-
der 1 atm of CO,1e and then (ii) addition of a slurry of
Na2CO3 in CH2Cl2 (molar ratio Na2CO3:Ru ) 3:1),
followed by evaporation of the solvent and further
reaction with 1 atm of CO at 110 °C. With this two-
step approach the reductive carbonylation of RuCl3 (2
wt % of Ru with respect to SiO2) gives 93% yields of
[Ru3(CO)12] after 48 h (Table 1, Scheme 1).
Therefore the metal loading effect on the second step
of the silica-mediated synthesis of [Ru3(CO)12] was
investigated. Treatment of [Ru(CO)3Cl2(HOSit)] (5 wt
% Ru/SiO2) with a slurry of Na2CO3 (molar ratio
Na2CO3:Ru ) 3:1) in CH2Cl2, followed by evaporation
The possibility of working at high metal loadings is
relevant for the production of sizable amounts of reac-
tion products without using large amounts of silica.1e-h
In a few cases, lower yields and selectivities are reported

Surface-Mediated Organometallic Synthesis
Organometallics, Vol. 16, No. 21, 1997 4533
of the solvent and reaction with CO (1 atm) at 110 °C,
affords pure [Ru3(CO)12] in 82% yields after 48 h (Table
1). Similar yields are obtained by starting from silica-
supported â-[Ru(CO)3Cl2]2, prepared by impregnation
of silica with â-[Ru(CO)3Cl2]2 dissolved in CH2Cl2. After
extraction of [Ru3(CO)12], the infrared spectrum of the
silica powder shows carbonyl bands at 2049 (w) and
1979 (w) cm-1, similar to those reported for [Ru(CO)x-
(OH)2]n (x ) 2, 3).4 Therefore, an increase of the metal
loading from 2 to 5 wt % of Ru with respect to SiO2
slightly decreases the yields but does not affect selectiv-
ity. However, a further increase of the metal loading
up to 15 wt % leads to the formation of [Ru3(CO)10Cl2]
instead of [Ru3(CO)12] (Scheme 1).
(s), 2031 (vs), and 1996 (w) cm-1, in accordance with
the infrared spectra of the related clusters [Ru3(CO)10X2]
(X ) Br, I)5 and [Os3(CO)10Cl2]6. Also Ru-Cl infrared
absorptions (ν(Ru-Cl) in polyethylene 288 (m), 272 (m),
and 256 (s) cm-1) are similar to those reported for
[Os3(CO)10Cl2] (ν(Os-Cl) in Nujol 290 (m), 272 (m), and
253 (s) cm-1).6
The above-reported synthesis of [Ru3(CO)12] from
RuCl3 by a two-step process is more convenient than
the best known syntheses in solution. Yields (93%) are
much higher than that obtained by reductive carbony-
lation at atmospheric pressure of a 2-ethoxyethanolic
solution of RuCl3 (45-60% yields)7 or of a propanolic
solution of [Ru3O(O2CCH3)6(H2O)3](O2CCH3) (59% yield).8
They are comparable to yields reached in solution but
working under CO pressure (starting from a metha-
nolic solution of RuCl3 under 10-50 atm and from a
propanolic solution of [Ru3O(O2CCH3)6(H2O)3](O2CCH3)
under 3-4 atm, [Ru3(CO)12] has been obtained in 75-
95% yields9 and 80-85% yields,8 respectively). In addi-
tion, since excellent yields are also reached at relatively
high ruthenium loadings of the silica surface, sizable
amounts of [Ru3(CO)12] can be prepared by this surface-
mediated synthesis using relatively low amounts of
silica (Table 1).
Treatment of a mixture of [Ru(CO)3Cl2(HOSit)] and
silica-supported [Ru(CO)3Cl2]2 (15 wt % of Ru with
respect to SiO2), obtained by reductive carbonylation of
RuCl3, with a slurry of Na2CO3 (molar ratio Na2CO3:
Ru ) 3:1) in CH2Cl2, followed by evaporation of the
solvent and reaction with CO (1 atm) at 110 °C for 24
h, affords the new compound [Ru3(CO)10Cl2], which
sublimes in part on the cold walls of the reaction vessel.
Extraction of the sublimate and the silica powder with
CH2Cl2 under N2 affords this cluster in 75% yield (Table
1). Similar yields are obtained after 48 h, but in this
case, some [Ru3(CO)12] is formed as well (ca. 4% yield).
The different selectivity observed by increasing the
ruthenium loading (from 2-5 to 15 wt %, Scheme 1) and
therefore the alkali carbonate loading (molar ratio
Na2CO3:Ru ) 3:1) could be explained by a nonhomoge-
neous dispersion of Na2CO3 on the silica surface when
a slurry in CH2Cl2 is used for its deposition. This low
homogeneity of the Na2CO3 surface dispersion at high
loading leads to a lower surface basicity than that
expected for a 3:1 molar ratio of Na2CO3/Ru and,
therefore, to a more difficult removal of chloro ligands
from the ruthenium coordination sphere with formation
of [Ru3(CO)10Cl2] instead of [Ru3(CO)12]. In agreement
with this hypothesis, work with a 2-fold amount of
Na2CO3, reductive carbonylation (1 atm of CO, 110 °C)
of a mixture of [Ru(CO)3Cl2(HOSit)] and silica-sup-
ported [Ru(CO)3Cl2]2 (15 wt % of Ru with respect to
SiO2) gives pure [Ru3(CO)12] (72% yield after 48 h)
instead of [Ru3(CO)10Cl2]. In addition, when Na2CO3
is deposited on the silica surface by using a water
solution instead of a CH2Cl2 slurry, a better dispersion
of the base is reached and only [Ru3(CO)12] is formed
even when the reductive carbonylation is carried out
with a 3:1 molar ratio of Na2CO3/Ru (Table 1). Clearly,
also, the manner by which an alkali carbonate is
deposited on the silica surface influences the resulting
surface basicity and therefore the selectivity.
Syn th esis of [H4Ru 4(CO)12]. [H4Ru4(CO)12] may be
synthesized in good yields by reaction of silica-supported
[Ru3(CO)12] with H2 (1 atm) at 50 °C.1d Similarly,
treatment of [Ru3(CO)12] with 1 atm of H2 in octane
under reflux affords [H4Ru4(CO)12].10 This hydridocar-
bonyl cluster can also be prepared from the less expen-
sive RuCl3 in ethanol, but in low yields (10-30% yields
after 3 days at 75-100 °C) and under very high
pressures (40 atm of CO + 40 atm of H2).11
When RuCl3 supported on silica (2 wt % of Ru with
respect to SiO2) in the presence of Na2CO3 (molar ratio
Na2CO3:Ru ) 3:1) is heated at 130 °C for 48 h under 1
atm of CO + H2 (molar ratio 1:3), the initially grey-
brown silica powder color becomes pale grey, suggesting
the formation of metallic ruthenium. Extraction with
CH2Cl2 affords pure [H4Ru4(CO)12] but only in traces.
By working at 130 °C but under 10 atm of CO + H2
(molar ratio 1:3) for 48 h, only [Ru3(CO)12] is obtained
in 12% yield. Under these conditions, the surface
species characterized by a carbonyl band at 2013 cm-1
,
mentioned above, is also formed whereas no [H4Ru4-
(CO)12] is detected, suggesting that its formation is
inhibited under CO pressure even in the presence of H2
in the gas phase.
Such low yields and selectivity and in particular the
reduction to metal can be avoided by using a two-step
To the best of our knowledge, [Ru3(CO)10Cl2], which
was characterized by elemental analysis, mass spec-
trometry, and infrared spectroscopy (see Experimental
Section), had not been isolated up to now. We simply
suggested its formation in traces during the treatment
of silica-supported â-[Ru(CO)3Cl2]2 with CO + H2O in
the absence of alkali carbonates.1f However, the related
clusters [Ru3(CO)10X2] (X ) Br, I), prepared by reaction
of 3-X-propene (X ) Br, I) with [HRu3(CO)10X], are well
characterized.5 The infrared spectrum of [Ru3(CO)10-
Cl2] shows ν(CO), in CH2Cl2, at 2116 (w), 2088 (s), 2077
(5) Kampe, C. E.; Boag, N. M.; Knobler, C. B.; Kaesz, H. D. Inorg.
Chem. 1984, 23, 1390.
(6) Deeming, A. J .; J ohnson, B. F. G.; Lewis, J . J . Chem. Soc. A 1970,
897.
(7) (a) Dawes, J . L.; Holmes, I. D. Inorg. Nucl. Chem. Lett. 1971, 7,
847. (b) Mantovani, A.; Cenini, S. Inorg. Synth. 1976, 16, 47.
(8) J ames, B. R.; Rempel, G. L. Chem. Ind. 1971, 1036. J ames, B.
R.; Rempel, G. L.; Teo, W. K. Inorg. Synth. 1976, 16, 45.
(9) (a) Bruce, M. I.; Stone, F. G. A. Chem. Commun. 1966, 684. (b)
Bruce, M. I.; Stone, F. G. A. J . Chem. Soc. A 1967, 1238. (c) Eady, C.
R.; J ackson, P. F.; J ohnson, B. F. G.; Lewis, J .; Malatesta, M. C.;
McPartlin, M.; Nelson, W. J . H. J . Chem. Soc., Dalton Trans. 1980,
383.
(10) Knox, S. A. R.; Koepke, J . W.; Andrews, M. A.; Kaesz, H. D. J .
Am. Chem. Soc. 1975, 97, 3942.
(11) (a) J amieson, J . W. S.; Kingston, J . V.; Wilkinson, G. J . Chem.
Soc. D 1966, 569. (b) J ohnson, B. F. G.; J ohnston, R. D.; Lewis, J .;
Robinson, B. H.; Wilkinson, G. J . Chem. Soc. A 1968, 2856.
(4) Cariati, E.; Lucenti, E.; Pizzotti, M.; Roberto, D.; Ugo, R.
Organometallics 1996, 15, 4122.

4534 Organometallics, Vol. 16, No. 21, 1997
Roberto et al.
Ta ble 2. Syn th esis of An ion ic Ru th en iu m Ca r bon yl Clu ster s on th e Silica Su r fa ce fr om
[Ru (CO)3Cl2(HOSit)]a
-
-
-
2-
M2CO3 (molar ratio
M2CO3:Ru)
[HRu3(CO)11
]
[H3Ru4(CO)12
]
[HRu6(CO)18
]
[Ru6C(CO)16]
Ru/SiO2, wt %
gasb
CO
CO
CO
T, °C
t, h
yield, % (mg)c
yield, % (mg)c
yield, % (mg)c
yield, % (mg)c
2
5
15
5
5
18
5
K2CO3 (10:1)
K2CO3 (20:1)
K2CO3 (20:1)
K2CO3 (10:1)
K2CO3 (20:1)
Na2CO3 (3:1)
K2CO3 (10:1)
K2CO3 (30:1)
K2CO3 (30:1)
150
150
150
100
100
130
80
10
10
10
24
24
24
60
24
48
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
95 (246)
85 (298)
68 (149)
0
0
0
0
0
0
traces
CO/H2
CO/H2
CO/H2
CO
60 (288)d
58 (117)d
81 (439)e
42 (138)f
0
0
0
0
0
5
15
COg
80
80
61 (288)h
65 (102)i
COg
a
b
Prepared on the surface by reductive carbonylation of silica-supported RuCl3.3 One atmosphere; molar ratio CO:H2 ) 1:3. c Actual
weight of isolated cluster. [H4Ru4(CO)12] is also obtained in 10% yield (43 mg). e [H4Ru4(CO)12] is formed on the silica surface, but extraction
d
with a THF solution of [PPN]Br affords [PPN][H3Ru4(CO)12]; a similar yield is obtained by working at 110 °C. f [Ru3(CO)12] is also obtained
g
h
in 11% yield (36 mg). In the presence of added water (110 wt % of H2O with respect to SiO2). [Ru3(CO)12] is also obtained in 16% yield
(82 mg); similar yields of [Ru3(CO)12] and K[HRu6(CO)18] by using a 60:1 molar ratio of K2CO3/Ru. [Ru3(CO)12] is also obtained in 14%
i
yield (24 mg).
route similar to that above described for [Ru3(CO)12].
Thus, reductive carbonylation of silica-supported RuCl3
(2 wt % of Ru with respect to SiO2) under 1 atm of CO
at 110 °C affords [Ru(CO)3Cl2(HOSit)] which by addi-
tion of Na2CO3 (molar ratio Na2CO3:Ru ) 3:1; deposited
from a CH2Cl2 slurry) followed by heating at 110 °C for
19 h under 1 atm of CO + H2 (molar ratio 1:3) affords
[H4Ru4(CO)12] in 88% total yield (Table 1, Scheme 1).
The use of pure H2 as the gas phase affords this
hydridocarbonyl cluster in 39% yield only, due to
extensive reduction to ruthenium metal.
By working under 1 atm of CO + H2 (molar ratio 1:3)
with a higher metal loading (5 or 15 wt % of Ru with
respect to SiO2) [H4Ru4(CO)12] is obtained in 86 and 94%
yields after 24 and 48 h, respectively (Table 1). The
same yields are obtained at 130 °C. Similarly, when
silica-supported â-[Ru(CO)3Cl2]2 (15 wt % of Ru with
respect to SiO2; prepared by impregnation of silica with
a CH2Cl2 solution of â-[Ru(CO)3Cl2]2) is heated at 110
°C for 24 h under 1 atm of CO + H2 (molar ratio 1:3) in
the presence of Na2CO3 (molar ratio Na2CO3:Ru ) 3:1),
[H4Ru4(CO)12] is obtained in 86% yield (Table 1). The
above syntheses are the best known method to convert
RuCl3 directly into [H4Ru4(CO)12] with excellent yields
and under mild conditions (atmospheric pressure); in
addition, the possibility of working at high metal
loadings allows the production of sizable amounts of this
presence of a large excess of K2CO3 (molar ratio K2CO3:
Ru ) 20:1). Therefore, in order to attain excellent yields
in this case, it is necessary to avoid too high ruthenium
loadings.
Interestingly, when RuCl3 supported on silica (5 wt
% of Ru with respect to SiO2) in the presence of K2CO3
(molar ratio K2CO3:Ru ) 20:1) was directly heated
under 1 atm of CO at either 150 or 200 °C for 24-65 h,
we could not observe the formation of K2[Ru6C(CO)16],
while the infrared spectrum of the resulting silica
powder did not show the presence of carbonyl bands.
Even by working at 150 °C under 10 atm of CO for 24
h, only very low amounts of K2[Ru6C(CO)16] are obtained
(ca. 2% yield). Therefore, the one-step synthesis of
[Ru6C(CO)16]2- from silica-supported RuCl3 is not pos-
sible although, as shown above, excellent yields can be
reached by a two-step route.
[Ru6C(CO)16]2- is an interesting compound since it is
the key precursor for many carbonylruthenium clus-
ters.12-14 For example, it is a convenient starting ma-
terial for the synthesis of clusters bearing the Ru6C core
such as [H2Ru6C(CO)16],12 [Ru6C(CO)17]13,14 and [Ru6C-
(CO)14(C14H14)].14 Moreover, it has recently been re-
ported that [Ru6C(CO)16]2- reacts with [Ru3(CO)12] to
produce [Ru10C(CO)24]2-, opening a new route to high
nuclearity ruthenium clusters.15 The above-reported
two-step silica-mediated synthesis of [Ru6C(CO)16]2-
from RuCl3 via [Ru(CO)3Cl2(HOSit)] is a very attrac-
tive method of preparation of this important intermedi-
ate. In solution, the best routes to this cluster reported
so far require [Ru3(CO)12] as starting material (yields
60-90%).12-14,16 On the surface of MgO, the reduc-
tion of RuCl3 (H2, 300 °C, followed by treatment with
CO + H2, molar ratio 1:1, 1-10 atm, 225 °C) affords
[Ru6C(CO)16]2-, but yields are not reported.17
cluster without using too much silica.
2-
Syn th esis of [Ru 6C(CO)16
] . By increasing the
basicity of the silica surface by addition of K2CO3
2
instead of Na2CO3 (molar ratio K2CO3:Ru ) 10-30:1),
[Ru(CO)3Cl2(HOSit)] can be converted into anionic
ruthenium carbonyl clusters. Treatment of [Ru(CO)3-
Cl2(HOSit)] (2 wt % of Ru with respect to SiO2) with a
CH2Cl2 slurry of K2CO3 (molar ratio K2CO3:Ru ) 10:1)
followed by evaporation of the solvent and further
reaction with 1 atm of CO at 150 °C for 10 h affords
K2[Ru6C(CO)16], easily extracted with acetone under N2,
in excellent yield (95%; Table 2; Scheme 1). Working
under the same conditions but with a metal loading of
5 wt % of Ru with respect to SiO2, the yields of K2[Ru6C-
(CO)16] are somewhat lower (80% yield after 10 h)
although a slightly better yield (85%) is reached by
doubling the K2CO3 amount. Working with a 15 wt %
loading of Ru with respect to SiO2, yields of K2[Ru6C-
(CO)16] are even lower. After 10 h at 150 °C under 1
atm of CO, the latter cluster is obtained in 68% yield
only, along with traces of K[H3Ru4(CO)12], even in the
Syn th esis of [H 3Ru 4(CO)12]-. [H3Ru4(CO)12]- may
be prepared by deprotonation of [H4Ru4(CO)12] both in
solution18 and on the surface of basic inorganic oxides19
such as Al2O3 and MgO. However, to our knowledge,
(12) J ohnson, B. F. G.; Lewis, J .; Sankey, S. W.; Wong, K.; McPartlin,
M.; Nelson, W. J . H. J . Organomet. Chem. 1980, 191, C3.
(13) Hayward, C. T.; Shapley, J . R. Inorg. Chem. 1982, 21, 3816.
(14) Bradley, J . S.; Ansell, G. B.; Hill, E. W. J . Organomet. Chem.
1980, 184, C33.
(15) Chihara, T.; Komoto, R.; Kobayashi, K.; Yamazaki, H.; Matsura,
Y. Inorg. Chem. 1989, 28, 964.
(16) Han, S. H.; Geoffroy, G. L.; Dombek, B. D.; Rheinhold, A. L.
Inorg. Chem. 1988, 27, 4355.
(17) Lamb, H. H.; Krause, T. R.; Gates, B. C. J . Chem. Soc., Chem.
Commun. 1986, 821.

Surface-Mediated Organometallic Synthesis
Organometallics, Vol. 16, No. 21, 1997 4535
there is no reported synthesis of [H3Ru4(CO)12]- starting
directly from RuCl3 or â-[Ru(CO)3Cl2]2. We reported
that the osmium analogue can be prepared by reduction
on the silica surface (1 atm of CO, 150 °C) of [Os-
(CO)3Cl2]2 or [Os(CO)3Cl2(HOSit)] in the presence of
K2CO3,1g but by working under these conditions,
[Ru6C(CO)16]2- is obtained instead of [H3Ru4(CO)12]-,
as described above. This different reactivity is not
surprising since synthetic routes to clusters of ruthe-
nium and osmium often differ widely due to the different
balance of M-M and M-CO bond strengths for the two
kinds of metal clusters.20
with KOH in methanol/tetrahydrofuran and water/
tetrahydrofuran affords [HRu3(CO)11]- (100% yield) and
[Ru6(CO)18]2- (80% yield), respectively,9c showing a
remarkable effect of water on the reaction selectivity.
[HRu3(CO)11]- has also been prepared by reaction with
CO at 25 °C of [Ru(CO)3Cl2(THF)] supported on Al2O3.23
Because [HRu3(CO)11]- and [Ru6(CO)18]2- have never
been prepared directly from RuCl3, we studied their
synthesis from [Ru(CO)3Cl2(HOSit)], easily obtained on
silica from RuCl3.1e
Treatment of [Ru(CO)3Cl2(HOSit)] (5 wt % of Ru with
respect to SiO2) with K2CO3 (molar ratio K2CO3:Ru )
10:1) deposited from a CH2Cl2 slurry, followed by
heating at 80 °C for 60 h under 1 atm of CO, affords a
mixture of [Ru3(CO)12] and K[HRu3(CO)11] (Table 2).
Successive selective extractions with CH2Cl2 and ac-
etone under N2 give the neutral cluster (11% yield) and
the anionic cluster (42% yield), respectively. Similar
yields are obtained after 96 h. Attempts to obtain only
[HRu3(CO)11]- failed, because both an increase of the
temperature (100 °C) and of the surface basicity (molar
ratio K2CO3:Ru ) 20:1) gave mixtures of [Ru3(CO)12],
K[HRu3(CO)11], K[H3Ru4(CO)12], and K2[Ru6C(CO)16]
with ratios related to reaction time. Obviously it is more
convenient to first prepare [Ru3(CO)12] from RuCl3
supported on silica as described above and then to
convert it into [HRu3(CO)11]- in solution9c or on the
surface of basic inorganic oxides.21,22
The effect of water on the reduction (1 atm of CO) at
80 °C of [Ru(CO)3Cl2(HOSit)] (5 wt % of Ru with
respect to SiO2) in the presence of K2CO3 (molar ratio
K2CO3:Ru ) 10:1) was investigated. By working with
a large amount of water (110 wt % of H2O with re-
spect to SiO2), K[HRu6(CO)18] is formed instead of
K[HRu3(CO)11] (Table 2), but yields of the former anion
are low (10-13% after 24 h), the major product being
[Ru3(CO)12]. However, use of a 3-fold amount of K2CO3
favors the formation of K[HRu6(CO)18] (61% yield,
Scheme 1) at the expense of [Ru3(CO)12] (16% yield).
Further increase of the surface basicity (molar ratio
K2CO3:Ru ) 60:1) does not improve the yield of K[HRu6-
(CO)18], [Ru3(CO)12] still being present (13% yield). The
two clusters can be easily separated by successive
selective extractions with CH2Cl2, which extracts [Ru3-
(CO)12], and acetone, which extracts K[HRu6(CO)18].
Yields are not so good when working with higher
ruthenium loadings (Table 2). For example, with a 15
wt % loading of Ru with respect to SiO2, K[HRu6(CO)18]
and [Ru3(CO)12] are obtained in 40 and 4% yields,
respectively, after 24 h, although reaction for a further
24 h increases yields to 65 (K[HRu6(CO)18]) and 14%
([Ru3(CO)12]).
However it is known that, on the surface of Al2O3 and
MgO, [H3Ru4(CO)12]- is converted to [Ru6C(CO)16]2- by
treatment under CO + H2 (molar ratio 1:1) at 200 °C.19
Therefore, [H3Ru4(CO)12]- could be an intermediate
species in the silica-mediated synthesis of [Ru6C(CO)16]2-
described above. Treatment of [Ru(CO)3Cl2(HOSit)] (5
wt % of Ru with respect to SiO2) with a slurry of K2CO3
(molar ratio K2CO3:Ru ) 10:1) in CH2Cl2, followed by
evaporation of the solvent, drying under vacuum (10-2
Torr) for 3 h at 100 °C, and reduction for 24 h under 1
atm of CO + H2 (molar ratio 1:3) at lower temperatures
such as 100 °C, affords a mixture of [H4Ru4(CO)12] (10%
yield) and K[H3Ru4(CO)12] (60% yield, Table 2) which
can be separated by selective extraction under N2 from
the silica surface with CH2Cl2 and acetone, respectively.
The treatment under vacuum at 100 °C prior to reduc-
tion is necessary to avoid too much physisorbed water,
which favors a fast aggregation of [H3Ru4(CO)12]- to
[Ru6C(CO)16]2-. Similar selectivities and yields ([H4Ru4-
(CO)12], 10% yield, and K[H3Ru4(CO)12], 58% yield) are
obtained by using a 2-fold amount of K2CO3, where-
as both longer reaction time and higher temperatures
give some [Ru6C(CO)16]2- as byproduct. The unavoid-
able formation of some [H4Ru4(CO)12], even by working
with a highly basic silica surface, is probably due to its
rapid sublimation under reaction conditions. So in
order to reach a complete selectivity to [H3Ru4(CO)12]-,
an alternative method of extraction was investigated.
Treatment of [H4Ru4(CO)12] with [PPN]Cl in THF
gives [PPN][H3Ru4(CO)12] in 100% yield.18c Similarly
when silica-supported [H4Ru4(CO)12] (18 wt % of Ru
with respect to SiO2), prepared by the two-step route
from silica-supported RuCl3, is extracted under N2
with a stoichiometric amount of [PPN]Br in THF,
[PPN][H3Ru4(CO)12] is obtained in 81% total yield (Table
2, Scheme 1), corresponding to an excellent selectivity
to [H3Ru4(CO)12]- starting from a simple material such
as RuCl3. Obviously this selective method of extraction
can be used also for mixtures of [H4Ru4(CO)12] and
[H3Ru4(CO)12]- obtained from [Ru(CO)3Cl2(HOSit)] as
above described.
Syn th esis of [HRu 3(CO)11
Therefore, under basic conditions, the presence of ex-
cess water favors aggregation of trinuclear to hexanu-
clear clusters on the silica surface as in solution.9c How-
ever, whereas in solution water favors the formation of
-
]
a n d [HRu 6(CO)18]-.
Adsorption of [Ru3(CO)12] onto hydroxylated magnesia,21
zinc oxide,21 lanthanum oxide,21 or aluminum oxide22
yields [HRu3(CO)11]-, which can be extracted from the
surface with [PPN]Cl, whereas reaction of [Ru3(CO)12]
[Ru6(CO)18]2- 9c
on the silica surface a large excess of
,
water favors the formation of [HRu6(CO)18]-. Since
[HRu6(CO)18]- is usually prepared in solution by acidi-
fication of [HRu3(CO)11]- (50% yield) or [Ru6(CO)18]2-
(80% yield),9c its direct silica-mediated synthesis from
(18) (a) Koepke, J . W.; J ohnson, J . R.; Knox, S. A. R.; Kaesz, H. D.
J . Am. Chem. Soc. 1975, 97, 3947. (b) Inkrott, K. E.; Shore, S. G. J .
Am. Chem. Soc. 1978, 100, 3954. (c) Lavigne, G.; Kaesz, H. D. J . Am.
Chem. Soc. 1984, 106, 4647.
(19) Pierantozzi, R.; Valagene, E. G.; Nordquist, A. F.; Dyer, P. N.
J . Mol. Catal. 1983, 21, 189.
(20) Bailey, P. J .; Conole, G.; J ohnson, B. F. G.; Lewis, J .; McPartlin,
M.; Moule, A.; Powell, H. R.; Wilkinson, D. A. J . Chem. Soc., Dalton
Trans. 1995, 741.
(21) D’Ornelas, L.; Theolier, A.; Choplin, A.; Basset, J . M. Inorg.
Chem. 1988, 27, 1261.
(22) Darensbourg, D. J .; Ovalles, C. Inorg. Chem. 1986, 25, 1603.
(23) Bergmeister, J . J ., III; Hanson, B. E. J . Organomet. Chem. 1988,
352, 367.

4536 Organometallics, Vol. 16, No. 21, 1997
Roberto et al.
RuCl3, although not highly selective, is an attractive
way to prepare it under mild conditions and in good
yields.
Ru ) 3:1) can be extracted with acetone. After extrac-
tion, only weak carbonyl bands (2047 (w) and 1980 (w)
cm-1, in Nujol) remain on the silica surface whereas the
infrared spectrum of the acetone solution containing the
extracted ruthenium species is similar to that charac-
teristic of [Ru(CO)x(OH)2]n species (x ) 2, 3; ν(CO) )
2048 (s), 1974 (s), and 1944 (w, sh) cm-1).4,26 Evapora-
tion of this solution affords a powder that does not
present Ru-Cl infrared absorption bands, in accordance
with complete removal of the chloro ligands. Reaction
with HCl(aq) of this powder dissolved in ethanol affords
a mixture of [Ru(CO)2Cl2(EtOH)2] and fac-[Ru(CO)3-
Cl3]-, suggesting the presence of a mixture of dicarbonyl
and tricarbonyl species on the silica surface.4,26
Step s of th e Red u ction of Silica -Bou n d [Ru -
(CO)3Cl2(HOSit)] To Gen er a te Va r iou s Neu tr a l
a n d An ion ic Ca r bon yl Clu ster s. In order to under-
stand the surface organometallic chemistry involved in
the above-described syntheses of various ruthenium
carbonyl clusters, we investigated the (i) nature of the
species initially formed by reaction of silica-supported
â-[Ru(CO)3Cl2]2 or [Ru(CO)3Cl2(HOSit)] with alkali
carbonates, (ii) evidence for some key intermediates in
the nucleation process, and (iii) possible pathways for
the generation of the various neutral and anionic
ruthenium clusters.
The above observations would support the formation
of species such as [Ru(CO)x(OH)2]n (x ) 2, 3) by reaction
of â-[Ru(CO)3Cl2]2 or [Ru(CO)3Cl2(HOSit)] with alkali
carbonates on the silica surface under our reaction
conditions, as it occurs in the case of the reductive
carbonylation of R-[Os(CO)3Cl2]2 physisorbed on silica
in the presence alkali carbonates.2 These ruthenium
species are quite reactive, and they are easily converted
to [Ru3(CO)12] on the silica surface by working under
CO (1 atm) at 110 °C (Scheme 1). On the silica surface,
it is possible that the aggregation process occurs via
[HRu3(CO)10(OR)] (R ) H, Sit), as evidenced by the
reduction of the related silica-supported carbonylos-
mium(II) species to various osmium carbonyl clusters.27
However, attempts to stop the reduction of [Ru(CO)3-
Cl2(HOSit)], when detectable amounts of [HRu3(CO)10-
(OR)] (R ) H, Sit) are formed, failed due to the known
rapid reaction of the latter clusters with CO at room
temperature to give [Ru3(CO)12].24
In fact, the formation of [Ru3(CO)12] was observed at
50 °C under 1 atm of CO or CO + H2 (molar ratio 1:3)
in the presence of either a low surface basicity (molar
ratio Na2CO3:Ru ) 3:1) or a high surface basicity (molar
ratio K2CO3:Ru ) 10:1). After 24 h under these condi-
tions, a mixture of [Ru3(CO)12] and unreacted [Ru-
(CO)x(OH)2]n (x ) 2, 3) was present on the silica surface.
Therefore, [Ru3(CO)12] is the first detectable step of the
process of nucleation of surface carbonylruthenium(II)
entities to various ruthenium carbonyl clusters. It
follows that the observed selectivity (Scheme 1) must
be explained by involving [Ru3(CO)12] as the key inter-
mediate (Scheme 2).
When a slurry of silica, â-[Ru(CO)3Cl2]2, M2CO3 (M
) Na or K; molar ratio M:Ru ) 6-60:1), and CH2Cl2 is
stirred at room temperature, new surface ruthenium
carbonyl species are produced. Similar species are
obtained by starting from [Ru(CO)3Cl2(HOSit)], formed
in situ by reductive carbonylation of silica-supported
RuCl3.1e The infrared spectra of these surface species
are characterized by three carbonyl bands (2132-2135
(m), 2062-2065 (s), and 1999-2006 (m) cm-1, in Nujol)
which vary slightly, depending on the nature and
quantity of the added alkali carbonate. Lower frequen-
cies are observed when K2CO3 is used instead of
Na2CO3 and when the molar ratio of M2CO3/Ru in-
creases, that is, by increasing the surface basicity.2 The
carbonyl bands of these surface ruthenium species are
at lower frequencies than those of either silica-supported
â-[Ru(CO)3Cl2]2 (2144 (m) and 2080 (s) cm-1, in Nujol)
or [Ru(CO)3Cl2(HOSit)] (2140 (m) and 2070 (s) cm-1
,
in Nujol),1f but they are similar to those of a mixture of
silica-anchored species such as [Ru(CO)3(OSit)2]n (2138
(m), 2069 (s), 2014 (m) cm-1) and [Ru(CO)2(OSit)2]n
(2069 (s), 2005 (s) cm-1) obtained by thermal treatment
of silica-supported [Ru3(CO)12].24 This infrared evidence
suggests the removal of chloro ligands from the coordi-
nation sphere of ruthenium to afford new species similar
or related to [Ru(CO)x(OSit)2]n (x ) 2, 3).
However, from the behavior of some ruthenium sil-
anolate complexes, we know that the Ru-OSi bond gen-
erates easily, in the presence of water, the Ru-OH
bond.25 Therefore, any ruthenium silanolate species
that could be generated by reaction in the presence of
alkali carbonates of the silanol groups of the silica
surface with â-[Ru(CO)3Cl2]2 should easily hydrolyze to
the known compounds [Ru(CO)x(OH)2]n (x ) 2, 3),4
because water is present on the surface under our ex-
perimental conditions. In any case, by working with
high ruthenium loadings (15 wt % of Ru with respect
to SiO2), there would not be enough surface silanol
groups to convert all â-[Ru(CO)3Cl2]2 to [Ru(CO)3-
(OSit)2]n.1e At least in this latter case, some phys-
isorbed [Ru(CO)x(OH)2]n (x ) 2, 3) should be formed as
well.
By working with a low surface basicity (molar ratio
Na2CO3:Ru ) 3:1), we found that silica-supported
[Ru3(CO)12] can be easily converted to [H4Ru4(CO)12] in
the presence of H2 (Scheme 2). In the absence of H2
and on a more basic silica surface, [Ru3(CO)12] behaves
as on the surface of MgO19 or in basic solution.28
Treatment of silica-supported [Ru3(CO)12] (2 wt % of Ru
with respect to SiO2) with a slurry of K2CO3 in CH2Cl2
(molar ratio K2CO3:Ru ) 10:1), followed by evaporation
of the solvent and treatment under 1 atm of CO at 80
°C for 24 h, affords pure K[HRu3(CO)11], which can be
In agreement with this hypothesis, we found that the
surface species obtained by reaction of silica-supported
â-[Ru(CO)3Cl2]2 or [Ru(CO)3Cl2(HOSit)] (2 wt % of Ru
with respect to SiO2) with Na2CO3 (molar ratio Na2CO3:
(26) Lucenti, E. Doctoral Thesis, Universita` degli Studi di Milano,
Milano, Italy, 1995.
(27) Cariati, E.; Roberto, D.; Ugo, R. Gazz. Chim. Ital. 1996, 126,
339.
(28) (a) J ohnson, B. F. G.; Lewis, J .; Raithby, P. R.; Suss, G. J . Chem.
Soc., Dalton Trans. 1979, 1356. (b) Ungermann, C.; Landis, V.; Moya,
S. A.; Cohen, H.; Walker, H.; Pearson, R. G.; Rinker, R. G.; Ford, P. C.
J . Am. Chem. Soc. 1979, 101, 5922. (c) J ackson, P. F.; J ohnson, B. F.
G.; Lewis, J .; McPartlin, M.; Nelson, W. J . H. J . Chem. Soc., Chem.
Commun. 1979, 735.
(24) Zanderighi, G. M.; Dossi, C.; Ugo, R.; Psaro, R.; Theolier, A.;
Choplin, A.; D’Ornelas, L.; Basset, J . M. J . Organomet. Chem. 1985,
296, 127.
(25) Riva, L. Doctoral Thesis, Universita` degli Studi di Milano,
Milano, Italy, 1994.

Surface-Mediated Organometallic Synthesis
Organometallics, Vol. 16, No. 21, 1997 4537
sion of [H3Ru4(CO)12]- because when K[H3Ru4(CO)12]
supported on silica (2 wt % of Ru with respect to SiO2)
added with K2CO3 (molar ratio K2CO3:Ru ) 10:1) is
heated at 150 °C for 6 h under CO (1 atm), K2[Ru6C-
(CO)16] is obtained quantitatively. This thermal conver-
sion, which also occurs under N2, would explain why
K2[Ru6C(CO)16] is obtained in quantitative yield when
[Ru3(CO)12] supported on silica added with excess
K2CO3 is treated with CO at 150 °C. At this tempera-
ture, H2 is easily produced by the water gas shift reac-
tion, favoring the formation of K[H3Ru4(CO)12], which
is then thermally converted to K2[Ru6C(CO)16], as
reported above and as it occurs on Al2O3 and MgO.19
In summary, the remarkable selective syntheses of
various neutral and anionic ruthenium clusters de-
scribed in this work can be reasonably explained by the
series of organometallic interconversions reported in
Scheme 2, where [Ru3(CO)12] is the key intermediate
and where the selectivity to various clusters is controlled
by the basicity of the surface, the composition of the
gaseous phase (CO or H2 + CO), and the temperature.
Sch em e 2. P ossible P a th w a ys for th e Gen er a tion
of Va r iou s Ru th en iu m Ca r bon yl Clu ster s fr om
[Ru (CO)3Cl2(HOSit)]
extracted from the silica surface with CH3CN (Scheme
2). However, as on the surface of MgO,19 the reaction
continues when working with a more basic silica surface
(molar ratio K2CO3:Ru ) 30:1) since [Ru3(CO)12] (2 wt
% of Ru with respect to SiO2) reacts at 80 °C, under
CO (1 atm) and excess water, to give K[HRu6(CO)18].
By analogy with the redox condensation in THF of
Con clu sion
This work is another example of the potential of the
silica surface as an efficient, although unusual, reaction
medium which allows selective and high-yield syntheses
of both neutral and anionic metal carbonyl clusters
starting from very simple materials such as metal
chlorides. In particular, the observation that the sur-
face basicity is also controlled by the way of deposition
of the alkali carbonate suggests that we are in the
presence of a basic reagent (the alkali carbonate)
deposited on the silica surface and not of a reaction
between the silanol groups of the silica surface with the
basic reagent. In fact, by impregnation of the alkali
carbonate from a water solution instead of a CH2Cl2
slurry, a higher surface basicity is achieved, a result
that cannot be explained at all by a reaction with the
silanol groups but only by a more homogeneous disper-
sion of the basic reagent deposited on the surface. The
final result is the formation of a new basic phase (the
silica surface) characterized by a behavior comparable
to that of strong bases dissolved in solution or of strong
intrinsically basic surfaces such as MgO.
By working with this new basic medium we achieved
quite selective one-pot syntheses of various neutral and
anionic ruthenium carbonyl clusters, although in the
case of [HRu3(CO)11]-, both yields and selectivity are
not outstanding due to the great tendency of this cluster
to condensate with [Ru3(CO)12] or to react with H2. In
any case, the majority of the syntheses described in this
work is the best method to prepare various ruthenium
clusters starting from the simple material RuCl3, since
generally the best reported syntheses of these clusters
involve [Ru3(CO)12] as starting material. The possibility
of achieving, in the majority of cases, high yields and
selectivities even when working with high metal load-
ings of the silica surface gives to the syntheses described
in this work the potentiality of being used for the prep-
aration of good amounts of ruthenium carbonyl clusters
(Tables 1 and 2) by using only a few grams of silica
which, in any case, can be reused after the reaction.
[Ru3(CO)11]2- with [Ru3(CO)12] to give [Ru6(CO)18]2- 13
,
[HRu6(CO)18]- could be formed on the silica surface by
condensation of [HRu3(CO)11]- with some [Ru3(CO)12].
On the other hand, when silica-supported [Ru3(CO)12]
(2 wt % of Ru with respect to SiO2) is treated for 24 h
with CO + H2 (molar ratio 1:3, 1 atm) at 80 °C on a
basic silica surface (molar ratio K2CO3:Ru ) 10:1),
K[H3Ru4(CO)12] is obtained quantitatively. This latter
conversion possibly involves [H4Ru4(CO)12] and K[HRu3-
(CO)11] as intermediates. In fact, when [H4Ru4(CO)12]
does not sublime under reaction conditions, it is easily
deprotonated on such a basic silica surface, as confirmed
by a blank experiment carried out under N2 (Scheme
2). Besides, treatment at 80 °C for 24 h of K[HRu3(CO)11]
supported on silica added with K2CO3 (molar ratio
K2CO3:Ru ) 10:1) with CO + H2 (molar ratio 1:3) affords
pure K[H3Ru4(CO)12]. The latter transforms back to
K[HRu3(CO)11] when H2 is removed, the conversion
being complete after 24 h at 80 °C under 1 atm of CO.
Such a reversible interconversion between [HRu3(CO)11]-
and [H3Ru4(CO)12]- can be repeated and mimicks in-
terconversions of these ions in basic solution29 or on the
surface of MgO.30
As described above, attempts to prepare selectively
[HRu3(CO)11]- by reductive carbonylation (1 atm of CO,
molar ratio K2CO3:Ru ) 10:1) of [Ru(CO)3Cl2(HOSit)]
at 100 °C failed, a mixture of [HRu3(CO)11]-, [H3Ru4-
(CO)12]-, and some [Ru6C(CO)16]2- being obtained. The
formation of some [H3Ru4(CO)12]- is linked to the
presence in the gas phase of some H2, produced in situ
by a water gas shift reaction, as confirmed by gas chro-
matographic analysis which revealed the presence of
both H2 and CO2. This reaction is expected since it is
known that [HRu3(CO)11]- catalyzes the water gas shift
reaction in basic medium.21,31,32 The presence of some
[Ru6C(CO)16]2- is explained by a slow thermal conver-
(29) Bricker, J . C.; Nagel, C. C.; Bhattacharyya, A. A.; Shore, S. G.
J . Am. Chem. Soc. 1985, 107, 377.
(30) Chun, S. H.; Shay, T. B.; Tomaszewski, S. E.; Lawswick, P. H.;
Basset, J . M.; Shore, S. G. Organometallics 1997, 16, 2627.
(31) Bricker, J . C.; Nagel, C. C.; Shore, S. G. J . Am. Chem. Soc. 1982,
104, 1444.
Exp er im en ta l Section
Gen er a l Com m en ts. SiO2 (Aerosil 200 Degussa, with a
(32) Gross, D. C.; Ford, P. C. J . Am. Chem. Soc. 1985, 107, 585.
nominal surface area of 200 m2/g) was used as the support

4538 Organometallics, Vol. 16, No. 21, 1997
Roberto et al.
after treatment in vacuo (10-2 Torr) at 25 °C for 3 h.
RuCl3‚nH2O (41.2 wt % Ru) was purchased from Strem
Chemicals while â-[Ru(CO)3Cl2]2 was prepared according
to the literature.1e All the reaction products were identified,
after extraction from silica, by infrared, 1H NMR (when
appropriate), and mass spectroscopies, their spectra being
compared to those of pure samples. Their purity was con-
trolled by thin-layer chromatography (when possible) and
by elemental analysis. Gas analyses for the detection of H2
and CO2 were carried out on a Hewlett Packard 5890 gas
(c) F r om Silica -Su p p or ted Ru Cl3. Lower yields (38-
56%; actual weights of isolated cluster 0.150-0.312 g) of
[Ru3(CO)12] were obtained by operating as above (1 atm of CO;
100-130 °C; 48-96 h; see Table 1) but starting from RuCl3 (2
wt % of Ru with respect to SiO2) added with Na2CO3 (molar
ratio Na2CO3:Ru ) 3:1).
Syn th esis of [Ru 3(CO)12] fr om Silica -Su p p or ted Ru Cl3
u n d er 5 a tm of CO. Silica-supported RuCl3 (1.06 g of powder;
15 wt % of Ru with respect to SiO2; 0.086 g of Ru; 0.850 mmol
of Ru) added with Na2CO3 (0.270 g; 2.56 mmol; molar ratio
Na2CO3:Ru ) 3:1; deposited from a water solution) was placed
in a stainless steel autoclave containing a glass liner. The
autoclave was closed, purged twice with N2, evacuated, pres-
surized to 5 atm of CO, and heated at 130 °C for 24 h. After
the mixture cooled to room temperature, the pressure was
released and the autoclave opened. Extraction with CH2Cl2
of the final powder afforded pure [Ru3(CO)12] (0.100 g; 0.158
mmol; 55 % yield).
Syn th esis of [Ru 3(CO)10Cl2] fr om [Ru (CO)3Cl2(HOSit)].
A mixture of [Ru(CO)3Cl2(HOSit)] and silica-supported
â-[Ru(CO)3Cl2]2 (prepared as described above; 2.06 g of powder;
15 wt % of Ru with respect to SiO2; 0.222 g of Ru; 2.20 mmol
of Ru) added with Na2CO3 (0.702 g; 6.60 mmol; molar ratio
Na2CO3:Ru ) 3:1; deposited from a CH2Cl2 slurry) was heated
in the cylindrical vessel under CO (1 atm) at 110 °C for 24 h.
During the reaction, some product sublimed onto the cold part
of the reaction vessel. Extraction of the sublimate and the
silica powder with CH2Cl2 (ca. 200 mL) under CO or N2
afforded pure [Ru3(CO)10Cl2] (0.360 g; 0.550 mmol; 75% yield),
which was characterized by elemental analysis (Anal. Calcd:
C, 18.36; Cl, 10.84. Found: C, 18.38; Cl, 10.76), mass spec-
trometry (in the EI mass spectrum, there is the molecular ion
peak at m/e ) 653, followed by peaks corresponding to the
successive loss of 10 COs), and infrared spectroscopy (in
CH2Cl2, ν(CO) 2116 (w), 2088 (s), 2077 (s), 2031 (vs), and 1996
(w) cm-1; in polyethylene, ν(Ru-Cl) 288 (m), 272 (m), and 256
(s) cm-1). [Ru3(CO)10Cl2] decomposes in air and therefore must
be stored under CO or N2. When Na2CO3 was deposited from
a water solution, under the same reaction conditions, [Ru3-
(CO)12] (0.229 g; 0.359 mmol; 49% yield) was obtained instead
of [Ru3(CO)10Cl2].
Syn th esis of [H4Ru 4(CO)12] fr om [Ru (CO)3Cl2(HOSit)].
A mixture of [Ru(CO)3Cl2(HOSit)] and silica-supported
â-[Ru(CO)3Cl2]2 (prepared as described above; 1.71 g of powder;
15 wt % of Ru with respect to SiO2; 0.188 g of Ru; 1.86 mmol
of Ru) added with Na2CO3 (0.593 g; 5.58 mmol; molar ratio
Na2CO3:Ru ) 3:1; deposited from a CH2Cl2 slurry) was heated
in the cylindrical vessel at 110 °C for 24 h under CO/H2 (molar
ratio 1:3; 1 atm). During the reaction, some product sublimed
onto the cold part of the reaction vessel. Extraction of the
sublimate and the silica powder with CH2Cl2 (200 mL) under
N2 afforded [H4Ru4(CO)12] (0.298 g; 0.400 mmol; 86% yield).
When the reaction time is increased to 48 h, [H4Ru4(CO)12] is
obtained in 94% yield (0.324 g; 0.436 mmol). When pure H2
is used as the gas phase, [H4Ru4(CO)12] is obtained in only
39% yield (0.134 g; 0.181 mmol), after 19 h, due to some
reduction to ruthenium metal.
chromatograph equipped with a stainless steel Carbosieve
1
SII packed column (outside column diameter
/ in.; column
8
length 96 in.; oven program temperature 35 °C isotherm
for 7 min and then heated from 35 to 225 °C at a 32 °C/min
rate).
P r ep a r a tion of Silica -Su p p or ted Ru Cl3 w ith or w ith -
ou t Ad d ition of Alk a li Ca r bon a tes. A slurry of silica (9.98
g), RuCl3‚nH2O (41.2 wt % Ru; correct amount to have the
desired Ru loading, 2-15 wt % of Ru with respect to SiO2),
H2O (200 mL) and, when necessary, Na2CO3 or K2CO3 (correct
amount to have the desired molar ratio of M2CO3 to Ru) was
stirred overnight under N2 at 25 °C. The solvent was
evaporated under vacuum (10-2 Torr) using a water bath at
80 °C. The final powder was stored under N2.
P r ep a r a t ion of Silica -Su p p or t ed â-[R u (CO)3Cl2]2,
[R u 3(CO)12], or [H4R u 4(CO)12] w it h Ad d it ion of Alk a li
Ca r bon a tes. A slurry of silica (10.0 g), â-[Ru(CO)3Cl2]2,
[Ru3(CO)12], or [H4Ru4(CO)12] (correct amount to have the
desired Ru loading, 2-15 wt % of Ru with respect to SiO2),
Na2CO3 or K2CO3 (correct amount to have the desired molar
ratio of M2CO3/Ru), and CH2Cl2 (250 mL) was vigorously
stirred under N2 for 24 h at 25 °C. The solvent was evaporated
at 25 °C in vacuo (10-2 Torr), affording a powder which was
stored under N2.
P r ep a r a tion of [Ru (CO)3Cl2(HOSit)] w ith or w ith ou t
Ad d ition of Alk a li Ca r bon a tes. [Ru(CO)3Cl2(HOSit)] (2-5
wt % of Ru with respect to SiO2) was prepared by reductive
carbonylation (1 atm of CO, 110 °C, ca. 48 h) of silica-supported
RuCl3.1e With a loading of 15 wt % of Ru with respect to SiO2,
it was necessary to carry out the reductive carbonylation (1
atm of CO) first at 110 °C for 48 h and then at 120 °C for 24
h to obtain only tricarbonylRu(II) surface species as evidenced
by infrared spectroscopy (mixture of [Ru(CO)3Cl2(HOSit)] and
silica-supported â-[Ru(CO)3Cl2]2).1f By using a slight CO
pressure (5 atm), the conversion of RuCl3 (15 wt % of Ru with
respect to SiO2) to tricarbonylRu(II) surface species was
complete in 24 h only at 110 °C. The addition of alkali
carbonates to [Ru(CO)3Cl2(HOSit)] was made as described
above for supported â-[Ru(CO)3Cl2]2, but using [Ru(CO)3-
Cl2(HOSit)] instead of â-[Ru(CO)3Cl2]2.
Syn th esis of [Ru 3(CO)12] a t Atm osp h er ic P r essu r e. (a )
F r om [Ru (CO)3Cl2(HOSit)]. [Ru(CO)3Cl2(HOSit)] (pre-
pared as described above; 3.84 g of powder, 5 wt % of Ru with
respect to SiO2, 0.171 g (1.69 mmol) of Ru) added with Na2-
CO3 (0.501 g; 4.72 mmol; molar ratio Na2CO3:Ru ) 3:1;
deposited from a CH2Cl2 slurry as described above) was
transferred into the cylindrical Pyrex vessel (diameter 60 mm,
length 350 mm) previously described for the reductive carbo-
nylation of silica-supported metal chlorides at atmospheric
pressure.1e It was treated in vacuo (10-2 Torr) at 25 °C and
exposed to CO (1 atm). The bottom of the vessel (half of the
cylinder) was put into an oven and heated at 110 °C for 48 h.
During the reaction, some [Ru3(CO)12] sublimed onto the cold
part of the reaction vessel. Extraction of the sublimate and
the silica powder with CH2Cl2 (200 mL), followed by evapora-
tion of the solvent, gave [Ru3(CO)12] (0.295 g; 0.462 mmol; 82%
yield).
Syn th esis of K2[Ru 6C(CO)16] fr om [Ru (CO)3Cl2(HOSit)].
[Ru(CO)3Cl2(HOSit)] (4.19 g of powder; 5 wt % of Ru with
respect to SiO2; 0.187 g of Ru; 1.85 mmol of Ru) added with
K2CO3 (5.10 g; 37 mmol; molar ratio K2CO3:Ru ) 20:1;
deposited from a CH2Cl2 slurry) was heated in the closed vessel
at 150 °C for 10 h under CO (1 atm). Extraction of the final
silica powder with acetone (ca. 150 mL) under N2, followed by
evaporation of the solvent, afforded K2[Ru6C(CO)16] (0.298 g;
0.262 mmol; 85% yield). With higher loadings (15 wt % of Ru
with respect to SiO2), K2[Ru6C(CO)16] is isolated after 10 h in
only 68% yield (weight of isolated cluster 0.148 g).
(b) F r om Silica -Su p p or ted â-[Ru (CO)3Cl2]2. By operat-
ing as above but starting from silica-supported â-[Ru(CO)3Cl2]2
(5 wt % of Ru with respect to SiO2) instead of [Ru(CO)3Cl2-
(HOSit)], [Ru3(CO)12] was obtained in 83% yield (weight of
isolated cluster 0.332 g).
Syn th esis of K[H3Ru 4(CO)12] fr om [Ru (CO)3Cl2(HOSit)].
[Ru(CO)3Cl2(HOSit)] (5.50 g of powder; 5 wt % of Ru with
respect to SiO2; 0.246 g of Ru; 2.45 mmol of Ru) added with
K2CO3 (3.39 g; 24.6 mmol; molar ratio K2CO3:Ru ) 10:1;

Surface-Mediated Organometallic Synthesis
Organometallics, Vol. 16, No. 21, 1997 4539
deposited from a CH2Cl2 slurry) was transferred into the
cylindrical vessel, treated in vacuo (10-2 Torr) for 3 h at 100
°C, exposed to CO/H2 (molar ratio 1:3; 1 atm), and then heated
at 100 °C for 24 h. During the reaction, some [H4Ru4(CO)12]
sublimed onto the cold part of the reaction vessel. Extraction
of the sublimate and the silica powder with CH2Cl2 (ca. 100
mL) under N2, followed by evaporation of the solvent, afforded
pure [H4Ru4(CO)12] (0.043 g; 0.060 mmol; 10% yield). Further
extraction of the silica powder with acetone (ca. 150 mL) under
N2 afforded K[H3Ru4(CO)12] (0.288 g; 0.366 mmol; 60% yield).
Syn th esis of K[H3Ru 4(CO)12] fr om [Ru (CO)3Cl2(HOSit)]
via [H4Ru 4(CO)12]. A mixture of [Ru(CO)3Cl2(HOSit)] and
silica-supported â-[Ru(CO)3Cl2]2 (prepared as described above;
1.355 g of powder; 18 wt % of Ru with respect to SiO2; 0.172
g of Ru; 1.67 mmol of Ru) added with Na2CO3 (0.479 g; 4.52
mmol; molar ratio Na2CO3:Ru ) 3:1; deposited from a CH2Cl2
slurry) was heated as described above in the cylindrical vessel
at 110 or 130 °C for 24 h under CO/H2 (molar ratio 1:3; 1 atm).
During the reaction, some [H4Ru4(CO)12] sublimed onto the
cold part of the reaction vessel. Treatment of the sublimate
and the silica powder with a solution of [PPN]Br (0.216 g; 0.350
mmol) in degassed THF (200 mL) under N2 for 2 h, followed
by filtration and evaporation of the solvent, gave [PPN]-
[H3Ru4(CO)12] (0.439 g; 0.343 mmol; 81% yield).
Syn th esis of K[HRu 3(CO)11] fr om [Ru (CO)3Cl2(HOSit)].
[Ru(CO)3Cl2(HOSit)] (3.42 g of powder; 5 wt % of Ru with
respect to SiO2; 0.153 g of Ru; 1.51 mmol of Ru) added with
K2CO3 (2.09 g; 15.2 mmol; molar ratio K2CO3:Ru ) 10:1;
deposited from a CH2Cl2 slurry) was heated in the cylindrical
vessel at 80 °C for 60 h under CO (1 atm). Extraction of the
silica powder with CH2Cl2 (100 mL) under N2 afforded pure
[Ru3(CO)12] (0.036 g; 0.057 mmol; 11% yield). Further extrac-
tion of the silica powder with acetone (150 mL) under N2 gave
K[HRu3(CO)11] (0.138 g; 0.212 mmol; 42% yield).
mmol; 16% yield). Further extraction of the silica powder with
acetone (150 mL) under N2 afforded K[HRu6(CO)18] (0.288 g;
0.251 mmol; 61% yield). Under the same conditions but with
15 wt % of Ru with respect to SiO2 loading, K[HRu6(CO)18]
and [Ru3(CO)12] were obtained in 65 and 14% yields, respec-
tively, only after 48 h.
R ea ct ivit y of â-[R u (CO)3Cl2]2, [R u 3(CO)12], [H 4R u 4-
(CO)12], K[HRu 3(CO)11], or K[H3Ru 4(CO)12] on th e Silica
Su r fa ce Ad d ed w ith Alk a li Ca r bon a tes. A sample (4.0 g)
of the desired ruthenium carbonyl compound supported on
silica (2 wt % of Ru with respect to SiO2) added with Na2CO3
or K2CO3 (the correct amount to have the desired molar ratio
M2CO3:Ru) was prepared as described above. The sample was
transferred into the cylindrical vessel, treated in vacuo (10-2
Torr) at 25 °C, exposed to N2, CO, CO + H2 (molar ratio 1:3),
or H2 at atmospheric pressure, and heated at the desired
temperature (50-150 °C). The surface reactions were moni-
tored by infrared spectroscopy: samples were taken from
the glass vessel at room temperature and studied as Nujol
mulls. Extraction of the reaction products was carried out with
CH2Cl2 (in the case of neutral products) or with acetone (in
the case of anionic products).
R ever sib le In t er con ver sion b et w een K[H R u 3(CO)11
]
a n d K[H3Ru 4(CO)12] on th e Silica Su r fa ce Ad d ed w ith
K2CO3. When K[HRu3(CO)11] supported on silica added with
K2CO3 (molar ratio K2CO3:Ru ) 10:1) was heated at 80 °C for
24 h under CO + H2 (molar ratio 1:3), pure K[H3Ru4(CO)12]
was obtained (as shown by infrared spectroscopy of the silica
powder and extraction with acetone). Successive treatment
under vacuo (10-2 Torr) at 25 °C, followed by exposure to CO
and heating at 80 °C for 24 h, gave back K[HRu3(CO)11] (as
shown by infrared spectroscopy of the silica powder and
extraction with acetone). This reversible process was repeated
three times.
Syn th esis of K[HRu 6(CO)18] fr om [Ru (CO)3Cl2(HOSit)].
[Ru(CO)3Cl2(HOSit)] (5.592 g of powder; 5 wt % of Ru with
respect to SiO2; 0.248 g of Ru; 2.46 mmol of Ru) added with
K2CO3 (10.5 g; 75.8 mmol; molar ratio K2CO3:Ru ) 30:1;
deposited from a CH2Cl2 slurry) was transferred into the
cylindrical vessel. Degassed water (18 mL) was added under
N2. The slurry was rapidly treated in vacuo (10-2 Torr) at 25
°C, exposed to CO (1 atm), heated at 80 °C for 24 h, and finally
evaporated to dryness. Extraction of the silica powder with
CH2Cl2 (100 mL) under N2 afforded [Ru3(CO)12] (0.082 g; 0.129
Ack n ow led gm en t. We are grateful to Dr. Daniele
Delledonne (EniChem S.p.A., San Donato Milanese) for
gas analyses. E.C. thanks the CNR for a fellowship.
This work was supported by the Ministero dell’Universita`
e della Ricerca Scientifica e Tecnologica and by the
Consiglio Nazionale delle Ricerche (CNR, Roma).
OM970230V