A facile and benign synthesis of binuclear ruthenium(i) sawhorse complexes

Article abstract of DOI:10.1039/b002304n

Polymeric dicarbonylclichlororuthenium(n) [Ru(CO)₂Cl2]z is reduced by reaction with carboxylate salts (RCO₂ Na+) in alcoholic suspension. Dimeric ruthenium(i) sawhorse complexes [Ru(RCO₂)(CO)₂L]₂ (R = Me, Et, Bu', Ph) are formed on the addition of a monodentate ligand, L [L = pyridine (py) or 3-methylpyridine (Mepy)J. Characterisation of [Ru(MeCO²)(CO)₂(py)]₂> [Ru(MeCO₂)(CO)₂(Mepy)]₂, [Ru(EtCO₂)(CO)₂(py)]₂, [Ru(Bu'CO₂)(CO)₂(py)]₂ and [Ru(PhCO₂)(CO)₂(py)]₂ by IR and 'H NMR spectroscopy is reported. Structural elucidation by single crystal X-ray diffraction was achieved in each case revealing a dicarboxylate bridged diruthenium(i) complex with cisoid carbonyls and pyridine ligands approximately collinear with the ruthenium-ruthenium bond. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b002304n

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A facile and benign synthesis of binuclear ruthenium(I) “sawhorse”
complexes
Christopher M. Kepert,a Glen B. Deacon,a Leone Spiccia,*a Gary D. Fallon,a Brian W. Skeltonb
and Allan H. Whiteb
a Centre for Green Chemistry and Department of Chemistry, PO Box 23, Monash University,
Victoria 3800, Australia. E-mail: leone.spiccia@sci.monash.edu.au
b Department of Physical and Inorganic Chemistry, University of Western Australia, Nedlands,
Western Australia 6907, Australia
Received 23rd March 2000, Accepted 21st June 2000
Published on the Web 25th July 2000
Polymeric dicarbonyldichlororuthenium() [Ru(CO)2Cl2]n is reduced by reaction with carboxylate salts (RCO2ϪNaϩ)
in alcoholic suspension. Dimeric ruthenium() “sawhorse” complexes [Ru(RCO2)(CO)2L]2 (R = Me, Et, But, Ph) are
formed on the addition of a monodentate ligand, L [L = pyridine (py) or 3-methylpyridine (Mepy)]. Characterisation
of [Ru(MeCO2)(CO)2(py)]2, [Ru(MeCO2)(CO)2(Mepy)]2, [Ru(EtCO2)(CO)2(py)]2, [Ru(ButCO2)(CO)2(py)]2 and
[Ru(PhCO2)(CO)2(py)]2 by IR and 1H NMR spectroscopy is reported. Structural elucidation by single crystal X-ray
diffraction was achieved in each case revealing a dicarboxylate bridged diruthenium() complex with cisoid carbonyls
and pyridine ligands approximately collinear with the ruthenium–ruthenium bond.
The “sawhorse” di-µ-carboxylatotetracarbonyldiruthenium()
complexes ([Ru(RCO2)(CO)2L]2) have been studied for a variety
of reasons, apart from intrinsic interest. One example has been
the utilisation of the bis(pyridine) adduct (L = py, R = Me)
in the formulation of a dipole–dipole coupling model for
application in the analysis of the IR spectra of dinuclear
metal carbonyl complexes.1 Much of the interest surrounding
these compounds, however, may be attributed to their ability
to catalyse a variety of organic transformations. Phosphine
adducts of the ruthenium() dimeric species have demon-
strated activity as hydrogenation catalysts in the conversion of
dimethyl oxalate to methyl glycolate and ethylene glycol.2 Like-
wise, catalytic hydrogenation and hydroformylation of alkenes,
ketones and carboxylates have also been investigated.3 This has
led to attempts to correlate structural features with catalytic
properties. The structures of several analogous unidentate
dodecacarbonyl and the difficulties encountered in their total
synthesis from ruthenium trichloride have been major factors
restricting the application of these complexes.
An alternative but less extensively employed procedure has
entailed the reduction of µ3-oxo-triruthenium() acetate.8 The
process is summarised in Scheme 2. Solvated with dimethyl-
Scheme 2 A further synthetic procedure affording [Ru(MeCO2)-
(CO)2(L)]2.
formamide, this complex exists as a monovalent cation. Reduc-
tion with H2 afforded a brown residue, which was reacted with
various unidentate donors in purified deoxygenated methanol
to afford the complexes [Ru(RCO2)(CO)2(L)]2.8 In addition to
difficulties attributable to the air-sensitive nature of some
intermediates, reported yields were low and mixtures of
products obtained.8
We report here a benign and efficient procedure for the
synthesis of di-µ-carboxylatotetracarbonyldiruthenium() com-
plexes. This novel synthetic route is based upon the facile reduc-
tion of the readily prepared dicarbonyldichlororuthenium()
polymer9 in methanolic suspension in the presence of carb-
oxylates. It provides a convenient general procedure for
the formation of tetracarbonyldi-µ-carboxylatodiruthenium()
complexes. Not only is RuCl3ؒxH2O less than one fifth of the
cost of [Ru3(CO)12], but the synthesis of the latter from the
trichloride (autoclave, medium pressure)10 is much more
demanding than that of [Ru(CO)2Cl2]n (RuCl3ؒxH2O with
refluxing formic acid).9
phosphine complexes, [Ru(µ-MeCO2)(CO)2(PBun )]2,3 [Ru-
3
(µ-RCO2)(CO)2(PBut3)]2 (R = Me, Prn),3,4 [Ru(µ-MeCO2)-
3
5
(CO)2(PPri3)]2 and [Ru(µ-MeCO2)(CO)2(But2PH)]2 have been
determined by X-ray diffraction techniques. Additionally, use
of bidentate phosphines, arsines and thioethers have resulted
in the preparation of a plethora of ruthenium() polymers in
which dimeric tetracarbonyl units are linked by axial ligation.6
The synthesis of complexes incorporating the typical bis-
(µ-carboxylate)tetracarbonyldiruthenium() unit have generally
been based on the reaction of dodecacarbonyltriruthenium
with neat carboxylic acids for prolonged periods (6–12 h) at
high temperature.7 The products were subsequently reacted
with a series of unidentate ligands to yield a class of dimeric
compounds of formula [Ru(µ-RCO2)(CO)2(L)]2. The procedure
is summarised in Scheme 1. The high cost of ruthenium
Results and discussion
The IR spectrum of the precipitate resulting from the addition
of pyridine to a boiling solution of [Ru(CO)2Cl2]n and sodium
acetate revealed at least three strong, easily discernible carbonyl
Scheme 1 The most extensively used synthetic methodology for
[Ru(RCO2)(CO)2(L)]2.
DOI: 10.1039/b002304n
J. Chem. Soc., Dalton Trans., 2000, 2867–2873
This journal is © The Royal Society of Chemistry 2000
2867

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mixture to air are not usual conditions for the reduction of
ruthenium().
Since the synthesis was also carried out under an atmosphere
of nitrogen the possibility of reduction by methanol and con-
comitant oxidation to formate was excluded, as this requires
dioxygen. Successful replacement of methanol solvent by the
poor reductant ButOH provided further evidence against the
facilitation of RuII reduction by the alcohol solvent. Dispropor-
tionation to produce RuI and RuIII was ruled out since there
was no evidence of RuIII species in the electrospray mass spectra
of reaction mixtures containing methanol, MeCO2Na and
[Ru(CO)2Cl2]n. This possibility can also be discounted since the
yields of RuI dimers were in some cases in excess of fifty
percent. Furthermore, the reaction mixtures showed no change
in colour, which could be associated with RuIII complexes.
Carbon dioxide was evolved during the reaction, but was not
formed when either the ruthenium polymer or sodium acetate
was heated alone under the reaction conditions. Coordinated
carbonyls are unlikely to be the source of carbon dioxide since
similar yields of [Ru(MeCO2)(CO)2(py)]2 were obtained from
reactions under nitrogen and in air (an oxygen source for CO
oxidation). Further, similar yields were obtained from compar-
able reactions under CO and Ar, whereas a higher yield would
have been expected for the former if coordinated carbonyls
were the source of CO2. Careful work-up of the reaction mixture
indicated that the only ruthenium containing product detect-
able besides [Ru(MeCO2)(CO)2(py)]2 was the known Ru()
complex, [Ru(MeCO2)2(CO)2(py)2],11 which was present in trace
amounts. Accordingly coordinated carbonyl ligands appear to
be conserved. Thus, the acetate groups appear to be the source
of CO2. Detection of carbon dioxide equivalent to 110% (molar
basis) of the isolated dimer indicated that a two-electron oxid-
ation of the acetate was likely. Metal-ion oxidation of carb-
oxylates is well established.12 In addition to the initial oxidative
decomposition of the carboxylate to an alkyl radical and CO2,
further reduction of ruthenium() may be facilitated by reac-
tion of the resultant radicals to form esters, ethers or alkenes
which are established products of carboxylate oxidation.13
Determination of carbon dioxide from the reaction of [Ru(CO)2-
Cl2]n with sodium benzoate (PhCO2Na) under comparable con-
ditions established a similar stoichiometry. The amount of CO2
detected compared with the product {[Ru(PhCO2)(CO)2(py)]2}
was also 110%. Again, the formation of ruthenium dimers
and CO2 in near equimolar amounts suggests that a two-
electron reduction of carboxylate occurs, as exemplified by
the following sequence of reactions; for convenience the
Fig. 1 Projections of representative arrays of the [Ru(RCO2)(CO)2L]2
systems: (a) R = Me, L = py, showing the 2 × flat (N-base)2 disposition;
(b) R = Me, L = Mepy, (molecule 1; the other molecule is similar) show-
ing the 2 × upright (N-base)2 disposition of the nitrogen bases; (c)
R = Ph, L = py, showing the 1 × flat, 1 × upright (N-base)2 disposition.
(i) RuII ϩ RCO2 → Ru ϩ CO2 ϩ R
stretching frequencies. This implied the presence of more than
two carbonyl ligands in either a cis or fac orientation. The H
Ϫ
I
ؒ
1
NMR spectrum indicated the presence of pyridine and acetate
ligands in a 1:1 ratio. The composition Ru(MeCO2)(CO)2(py)
was consistent with microanalyses, indicative of reduction
of Ru() to Ru(). A monomeric composition, however, is
inconsistent with the IR spectrum. The ν(CO) values (Nujol)
2020, 1962, 1930 cmϪ1; correspond to those reported7 for
[Ru(MeCO2)(CO)2(py)]2.
II
(ii) Ru ϩ R ϩ RCO2Ϫ → RuI ϩ RCO2R
ؒ
II
I
ϩ
ؒ
(or possibly: Ru ϩ R ϩ RЈOH → Ru ϩ RЈOR ϩ H )
overall: 2 [Ru(CO)2Cl2] ϩ 4 RCO2Na ϩ 2 L →
[Ru(RCO2)(CO)2(L)]2 ϩ CO2 ϩ RCO2R ϩ 4 NaCl
An X-ray diffraction study confirmed the product to be a
dimeric Ru() complex ([Ru(MeCO2)(CO)2(py)]2) with four cis-
oid carbonyl and two µ-O,OЈ-acetate ligands in a “sawhorse”
configuration [Fig. 1(a)] capped by pyridine in a position such
that the ligated imine nitrogens approach the axis of the Ru–Ru
bond. A detailed description of this and the related struc-
tures of [Ru(MeCO2)(CO)2(Mepy)]2, [Ru(EtCO2)(CO)2(py)]2,
[Ru(ButCO2)(CO)2(py)]2 and [Ru(PhCO2)(CO)2(py)]2 (formed
by analogous reactions with propanoate, pivalate and benzoate
salts) is provided later (see Crystallography section). Reduction
of ruthenium() was not expected under the conditions of these
reactions, which were initially investigated as synthetic routes to
[Ru(RCO2)2(CO)2L2] complexes. The presence of a large excess
of a hard donor ligand and the exposure of the reaction
dicarbonyldichlororuthenium() polymer has been treated as a
monomer in yield calculations.
Negative electrospray mass spectroscopy of solutions formed
by the simultaneous dissolution of [Ru(CO)2Cl2]n and MeCO2-
Na in methanol and by the dissolution of [Ru(CO)2Cl2]n in
methanol followed by the addition of MeCO2Na gave similar
results. Both contained major peaks at m/z 498–513 {[Ru2-
(CO)4(MeCO2)(MeO)2Cl2]Ϫ}, 523–535 {[Ru2(CO)4(MeCO2)2-
(MeO)2Cl]Ϫ} and 547–559 {[Ru2(CO)4(MeCO2)3(MeO)2]Ϫ}.
These are not present in the spectrum of a solution formed by
dissolving [Ru(CO)2Cl2]n in methanol. This indicates that the
formation of the species responsible for these peaks is not
2868
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Table
1
Assignments of the 1H NMR spectra of [Ru(RCO2)-
for other µ-O,OЈ-carboxylate complexes.14,15 This family of
(CO)2(L)]2
compounds constitutes a typical example where the energy
separation between the symmetric and antisymmetric stretch-
ing modes for a bridging carboxylate is smaller than for that
of its ionic counterpart (e.g., ca. 164 cmϪ1 for ionic acetate).14
R, L
H2, H6
H3, H5
H4
Alkyl/phenyl
Me, py
Me, Mepy
8.74
7.43
7.83
7.63
2.06
2.06
8.56 H2
8.55 H6
8.75
7.32 H5
2.42 CH3
7.43
Crystallography
Et, py
7.83
2.29 CH2
1.05 CH3
1.05
7.85 o-H
7.27 m-H
7.39 p-H
Each of the complexes gave high quality crystals upon recrystal-
lisation (albeit twinned in a number of cases), and consequently
X-ray structural characterisation was undertaken. ORTEP16
representations of the molecular structures of [Ru(MeCO2)-
(CO)2(py)]2, [Ru(MeCO2)(CO)2(Mepy)]2, and [Ru(PhCO2)-
(CO)2(py)]2 are displayed in Fig. 1. Table 2 summarises the
geometric features of the ruthenium centres. The five complexes
are of analogous composition and structure, and are identical
in terms of stoichiometry (all unsolvated), connectivity and
stereochemistry. In the structures of the binuclear complexes,
[(L)(OC)2Ru(µ-O–CR–OЈ)2Ru(CO)2(L)], the pair of ruthenium
atoms are six-coordinate with similar coordination environ-
ments. The Lewis-base donor lies approximately collinear with
the ruthenium–ruthenium bond (which may be regarded as
defining an axis) and the pairs of carbonyl and µ-O,OЈ-
carboxylate donors are mutually cis in the plane (containing
the Ru–O and Ru–C bonds) equatorial to that axis. The total
array of the ruthenium atoms and their immediate neighbours
approximates 2m (C2v) symmetry, the two-fold axis lying nor-
mal to the Ru–Ru bond and contained in the mirror plane
which passes between and relates the two carboxylate groups,
and also contains the Ru–Ru bond. A second pseudo mirror
plane lies normal to the Ru–Ru bond. Further from the molecu-
lar core, the symmetry may be perturbed by the relative disposi-
tions of the carboxylate alkyl substituents, and by the relative
dispositions of the aromatic rings of the axially coordinated
ligands, which tend to lie within or bisected by the mirror plane.
The three possible combinations observed are illustrated in
Fig. 1. Despite the potential symmetry of the binuclear species,
[Ru(MeCO2)(CO)2(py)]2 is the only one of these molecules to
conform to any crystallographic symmetry element. The
binuclear unit comprises the asymmetric unit in the remainder
of the complexes with the exception of [Ru(MeCO2)(CO)2-
(Mepy)]2, in which two pseudo-symmetrically related molecules
comprise the asymmetric unit in a non-centrosymmetric space
group. In view of crystallographic difficulties (e.g., the twinned
nature of the crystals), interpretation of the geometries
obtained from the determination of the structure of [Ru-
(MeCO2)(CO)2(Mepy)]2 should be treated with appropriate
circumspection. {In regard to supramolecular dispositions,
it is of interest that the packing found in [Ru(MeCO2)(CO)2-
(py)]2, [Ru(MeCO2)(CO)2(Mepy)]2, [Ru(EtCO2)(CO)2(py)]2
and [Ru(ButCO2)(CO)2(py)]2 is approximately symmetrical,
quasi-2 approximately parallel to monoclinic b, and quasi-m
approximately normal to a.}
But, py
Ph, py
8.74
8.93
7.41
7.53
7.82
7.92
dependent upon the polymer structure and that the polymer
structure is likely to “break up” prior to the reduction of the
metal centre.
Combinations of reagents other than acetate and pyridine
have been examined in order to establish the generality of this
reaction. Addition of appropriate N-pyridine bases (L) to solu-
tions containing [Ru(CO)2Cl2]n and various carboxylate salts
(RCO2Na) resulted in the rapid formation of a bright yellow
precipitate which when recrystallised from CH2Cl2 afforded
crystalline samples of [Ru(RCO2)(CO)2(L)]2 complexes (R =
Me, L = py, Mepy; R = Et, But, Ph, L = py). The microanal-
yses, IR and NMR spectra for all the products were consistent
with their proposed formulation. The synthesis of complexes
[Ru(RCO2)(CO)2(L)]2 is represented in Scheme 3.
Scheme 3 The novel and facile synthesis of the ruthenium() “saw-
horse” dimers.
Spectroscopy
Assignments of the 1H NMR spectra of the pyridine com-
plexes are reported in Table 1. H NMR spectra of [Ru(Me-
1
CO2)(CO)2(py)]2 and [Ru(MeCO2)(CO)2(Mepy)]2 reflected the
symmetry of the complexes. The positions of the pyridine
resonances were typical, with very little variation among the
four pyridine complexes, and only slight changes in the Mepy
complex due to the inductive effects of the methyl substituent.
The spectra of [Ru(EtCO2)(CO)2(py)]2 and to a greater extent
[Ru(ButCO2)(CO)2(py)]2 exhibited broadened signals, hence
coupling constants could not be determined. Steric interactions
between the pyridine rings and the propanoate or the bulkier
pivalate groups may constitute a barrier to rotation which
was not completely overcome at room temperature. This
may engender slight inequivalence of protons on the two
pyridine rings which would otherwise experience identical
environments.
IR spectroscopy revealed intense bands in the region associ-
ated with absorptions due to carbonyl ligands in patterns
consistent with previous reports of these and similar com-
pounds.1,4,7 Assignment of νas(CO2) can be made to absorption
bands occurring within the range 1556–1574 cmϪ1. The identi-
fication of the symmetrical stretching mode (νs) was not as
straightforward, owing to the presence of a number of bands in
the region. Peaks in the vicinity of 1440–1450 cmϪ1 are the most
likely candidates in complexes incorporating alkyl carboxylate
bridges, although identification of νs(CO2) could not be
made for [Ru(EtCO2)(CO)2(py)]2. The average gap between the
νas(CO2) and νs(CO2) vibrational modes corresponds to ca. 125
cmϪ1 in agreement with previously reported spectra for these
types of complexes,7 and falls within the range reported
The geometries of the five pyridine and 3-methylpyridine
adducts are similar (Table 2), differing only for [Ru(ButCO2)-
(CO)2(py)]2 and [Ru(PhCO2)(CO)2(py)]2 in more general
features such as the dispositions of the pyridine rings, and some
slight twisting of the carboxylates, perhaps in response to the
different relative dispositions of tert-butyl substituents, whose
relation appears to be determined by gearing considerations.
Notwithstanding these observations, bond lengths and angles
among the five compounds may be regarded as archetypal.
About the ruthenium atoms, the most notable feature is the
enlargement of the angles between the cis-carbonyl and carb-
oxylate donors [e.g., O(1)–Ru–C(2)] in response to the smaller
angles between the pair of cis-carboxylate oxygen atoms [O(1)–
Ru–C(2) angles of 93–96Њ resulting from O(1)–Ru–O(2) angles
of ca. 84Њ], the latter persisting despite the presence of tert-butyl
groups adjacent to each other on adjoining bridging ligands in
the [Ru(ButCO2)(CO)(py)]2 complex [O(1)–Ru–O(2) of 83.95(8)
J. Chem. Soc., Dalton Trans., 2000, 2867–2873
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Table 2 Ruthenium environments in ruthenium() pyridine complexes. The five vertical values in each entry are for (a) [Ru(MeCO2)(CO)2-
(py)]2, (b) molecules i and ii of [Ru(MeCO2)(CO)2(3-Mepy)]2 (two crystallographically inequivalent molecules), (c) [Ru(EtCO2)(CO)2(py)]2,
(d) [Ru(ButCO2)(CO)2(py)]2 and (e) [Ru(PhCO2)(CO)2(py)]2. r(Å) is the ruthenium–ligand atom distance, other entries being the angles (Њ) subtended
by the pertinent atoms at the head of the row and column. The two values in each entry are for Ru(1,2), except for [Ru(MeCO2)(CO)2(py)]2 in which
the two Ru are related by symmetry. For (b) and (e), C(1) and O(1) correspond to the lowest numbered carbon and oxygen donor atoms attached to
each ruthenium of Fig. 1(b) and (c), respectively
Atom
Ru
r
C(1)
C(2)
O(1)
84.6(1)
83.8(3), 84.1(3)
83.6(3), 84.4(3)
82.5(2), 84.2(2)
84.06(6), 83.25(7)
83.77(6), 84.39(6)
O(2)
82.6(1)
84.3(3), 82.9(3)
83.7(3), 84.2(3)
83.9(2), 82.9(2)
N
(a)
2.672(1)
2.678(2)
2.678(2)
2.676(1)
2.6760(6)
2.6780(4)
92.6(2)
96.0(2)
162.7(1)
(b)i
(b)ii
(c)
(d)
(e)
95.8(5), 96.8(5)
95.2(5), 95.0(6)
95.2(3), 93.9(3)
94.7(1), 95.2(1)
94.8(1), 96.1(1)
92.8(5), 96.4(5)
93.9(5), 94.0(5)
94.7(3), 95.3(3)
93.6(1), 95.8(1)
94.7(1), 95.4(1)
163.7(4), 162.2(4)
163.5(4), 164.1(4)
161.5(2), 162.7(2)
84.03(5), 83.47(6) 163.73(6), 161.82(6)
84.17(6), 84.16(6) 161.46(8), 161.80(8)
C(1)
C(2)
O(1)
O(2)
N
(a)
1.836(6)
87.8(2)
88.4(9), 87(1)
89.7(9), 90.9(8) 176.3(8), 177.7(7)
88.5(4), 89.4(3) 177.3(4), 176.5(3)
90.3(1), 87.9(1) 177.3(1), 177.6(1)
88.3(2), 87.4(2) 177.4(1), 179.4(2)
176.6(2)
178.8(6), 178.9(6)
94.3(2)
99.2(2)
(b)i
(b)ii
(c)
(d)
(e)
1.78(2), 1.77(2)
1.83(2), 1.83(2)
1.840(8), 1.834(7)
1.836(3), 1.830(3)
1.834(4), 1.826(4)
93.6(7), 94.9(8)
93.0(8), 93.0(7)
94.4(3), 92.5(3)
93.5(1), 93.3(1)
93.7(1), 94.6(1)
94.3(7), 95.2(7)
97.3(6), 96.9(7)
98.4(4), 97.5(3)
96.0(1), 97.0(1)
100.2(1), 96.4(1)
(a)
1.846(6)
94.4(2)
177.5(2)
96.9(2)
(b)i
(b)ii
(c)
(d)
(e)
1.78(2), 1.81(2)
1.84(2), 1.83(2)
1.834(9), 1.829(7)
1.826(3), 1.826(3)
1.824(4), 1.833(4)
92.7(7), 93.2(7)
93.9(8), 91.4(7)
93.1(3), 93.7(3)
92.3(1), 94.0(1)
94.0(1), 92.9(1)
176.7(7), 177.7(7)
176.5(8), 175.9(7)
176.9(3), 177.5(3)
175.7(1), 178.6(1)
177.8(1), 178.1(1)
100.2(6), 97.2(7)
97.0(7), 96.3(6)
98.2(4), 97.8(3)
98.5(1), 98.1(1)
96.7(1), 98.4(1)
(a)
2.133(4)
83.4(1)
83.0(1)
(b)i
(b)ii
(c)
(d)
(e)
2.10(1), 2.12(1)
2.10(1), 2.12(1)
2.114(5), 2.118(5)
2.125(2), 2.117(2)
2.118(2), 2.141(3)
85.3(5), 84.6(5)
83.4(5), 84.8(5)
83.9(2), 84.3(2)
83.95(8), 84.72(9)
84.03(8), 85.13(8)
85.9(5), 83.8(5)
83.2(5), 83.3(5)
83.5(2), 83.7(2)
84.67(8), 84.1(1)
80.9(1), 83.0(1)
(a)
2.135(4)
θa 67.5(1)
84.1(1)
(b)i
(b)ii
(c)
(d)
(e)
2.13(1), 2.10(1)
2.11(1), 2.10(1)
2.121(5), 2.121(5)
2.121(2), 2.107(2)
2.134(2), 2.122(2)
71(1)
76(1)
73.1(4)
70.8(1)
84.9(2)
82.3(5), 83.1(6)
84.9(5), 84.7(5)
82.6(2), 83.6(2)
83.15(8), 82.43(9)
84.0(1), 81.71(9)
(a)
2.197(4)
(b)i
(b)ii
(c)
(d)
(e)
2.25(1), 2.21(2)
2.24(1), 2.21(1)
2.235(7), 2.224(7)
2.225(2), 2.224(3)
2.224(3), 2.195(3)
a θ (Њ) is the dihedral angle between the two carboxylate C2O2 planes.
and 84.72(9)Њ]. Ruthenium–nitrogen and –oxygen bonds appear
unperturbed by substituent effects, and variation in ruthenium–
ruthenium and –carbon bonds by any consequent trans-
influence is limited in extent. Ruthenium–carbonyl distances
vary little, being typically around 1.83 Å, while Ru–O and Ru–
N bond lengths are close to 2.12 and 2.22 Å, respectively. These
values match those in a related acetonitrile complex, [Ru(CF3-
CO2)(CO)2(NCMe)]2, in which the average Ru–N bond length is
2.18 Å, Ru–O is 2.133(3) Å, Ru–C is 1.83 Å, and the Ru–Ru
separation is marginally larger at 2.683(1) Å.17 The deviation in
the Ru–Ru bond distances in the pyridine and 3-methylpyridine
complexes is insignificant, with an average separation of
2.677(1) Å. Distances in the imine complexes are consistently
shorter than those reported for phosphine structural analogues
{[Ru(µ-O2CMe)(CO)2(Pri2PH)]2, [Ru(µ-O2CMe)(CO)2(But3P)]2,
[Ru(µ-O2CMe)(CO)2(But3P)]2, [Ru(µ-O2CMe)(CO)2(Bu2PH)]2
and [Ru(µ-O2CPrn)(CO)2(But3P)]2}, for which the average inter-
ruthenium distance is 2.735(2) Å.3 Conversely, distances found
in complexes with aqua and carbonyl or carboxylic acid ligands
occupying positions approximately trans to the Ru–Ru bond
are indicative of a small but consistent increase in Ru–Ru bond
order.18 Comparison of Ru–N bond distances of the dimers
with those in other pyridine complexes reveal that the separ-
ations of 2.20–2.25 Å in the five RuI imine complexes are
significantly longer than those observed in octahedral RuII
pyridine complexes {e.g., average of 2.12(1) Å in [Ru(py)6]2ϩ}.19
Apart from the larger cationic radius of RuI in comparison to
RuII, this elongation may be attributed to the considerable
trans-influence of a metal–metal bond, which probably con-
tributes to the lability of the pyridine in such species. Substitu-
ent effects on the ruthenium–ruthenium bond length have been
discussed in some detail.20
The packing motifs of [Ru(MeCO2)(CO)2(py)]2, [Ru(MeCO2)-
(CO)2(Mepy)]2, [Ru(EtCO2)(CO)2(py)]2 and [Ru(ButCO2)-
(CO)2(py)]2 seem to be largely determined by the packing
considerations of and interactions between the pyridine
moieties {e.g., an intermolecular contact of 3.566(8) Å is
observed between N(101) and C(104) in the structure of
[Ru(MeCO2)(CO)2(py)]2}. The introduction of a phenyl sub-
stituent in the benzoate/pyridine complex causes a perturbation
of the long range chain-like order observed in [Ru(MeCO2)-
(CO)2(py)]2 by enabling association of pairs of the dimer
through π-stacking of these groups. The low inter-
molecular separation of 3.594(9) Å for C(105) ؒ ؒ ؒ C(105Ј)
implies the existence of such interactions between the phenyl
rings.
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1
(yield: 0.60 g, 70%). IR and H NMR (CDCl3) data were in
agreement with those of the product from (i). A weak CO
absorption at 2067 cmϪ1 was also detected and may be attrib-
uted to an impurity of [Ru(MeCO2)2(CO)2(py)2],11 although
this was not detected by NMR spectroscopy. The filtrate was
evaporated to dryness and extracted with CH2Cl2 (50 ml). The
white residue was NaO2CMe containing no detectable metal
carbonyl species (IR). The CH2Cl2 extract was evaporated to an
oily yellow solid and attempted recrystallisation from MeOH–
H2O gave a small amount (ca. 5 mg) of [Ru(MeCO2)(CO)2(py)]2
(IR identification). The remaining material was isolated by
evaporation to dryness (yield: 0.10 g) and an IR spectrum (as a
CH2Cl2 solution) showed strong CO absorptions at 2059 and
1992 cmϪ1 consistent with the presence of predominantly
[Ru(MeCO2)2(CO)2(py)2].11
Conclusion
The new more benign synthetic procedure reported here
provides convenient access to di-µ-acetatotetracarbonyl-
ruthenium() complexes and should stimulate interest in the
further application of such complexes. Substitution reactions
involving the exchange of the pyridine ligands with mono-
dentate and bidentate ligands, to be reported in a subsequent
paper, lead to an even greater array of binuclear ruthenium()
complexes.
Experimental
Physical measurements
IR spectra were recorded using a Perkin-Elmer 1640 FTIR
spectrophotometer as Nujol mulls. 1H NMR spectra were
recorded in CDCl3 on a Bruker AM300 spectrometer at 300
MHz. Elemental microanalyses were performed by Chemical
and Micro Analytical Services (CMAS), Melbourne, Australia.
(iv) Under an argon atmosphere. A similar reaction con-
ducted under an argon atmosphere showed similar features and
1
gave the same product (yield: 0.66 g, 77%). IR and H NMR
(CDCl3) data were in agreement with those of the product from
(i). The presence of a trace of [Ru(MeCO2)2(CO)2(py)2] was
again indicated by a shoulder on the 2020 cmϪ1 ν(CO) absorp-
tion of the product. The filtrate was evaporated to dryness and
extracted with CH2Cl2 (50 ml). The remaining white solid
(NaO2CMe) showed no ν(CO) absorption in the IR spectrum.
The CH2Cl2 solution was evaporated to an oily yellow solid
(0.07 g) and an IR spectrum (as a CH2Cl2 solution) showed
strong ν(CO) absorptions at 2059, 1990 and 1964 cmϪ1 consist-
ent with the presence of a mixture of the main product and
[Ru(MeCO2)2(CO)2(py)2].11
Materials and reagents
HPLC grade methanol was used in preparation of complexes.
tert-Butyl alcohol was purified by crystallisation and dried
over sodium prior to use. Sodium pivalate and sodium prop-
anoate were prepared by reaction of the appropriate carboxylic
acids with sodium carbonate in aqueous media, with the
isolated salt recrystallised and dried by heating under vacuum.
All other chemicals were of reagent or analytical reagent
grade and were used without further purification. [Ru(CO)2-
Cl2]n was synthesised according to an existing method.9
(v) The repetition of (i) (above) with [Ru(CO)2Cl2]n (0.942
g, 4.1 mmol) and sodium acetate (2.85 g, 35 mmol) in tert-butyl
alcohol (50 ml) afforded [Ru(MeCO2)(CO)2(py)]2, as identified
by IR and 1H NMR spectroscopy (yield: 0.658 g, 54%).
Syntheses
Di-ꢀ-O,OЈ-acetatotetracarbonylbis(pyridine)diruthenium(I),
[Ru(MeCO2)(CO)2(py)]2. (i). [Ru(CO)2Cl2]n (0.66 g, 2.9 mmol)
was added to a solution of a large excess of sodium acetate
(2.80 g, 34 mmol) in degassed methanol (75 ml). The resultant
suspension was heated at a bath temperature of ca. 120 ЊC
under an atmosphere of nitrogen for a period of 2.5 h, during
which the reagents dissolved giving an orange solution. Intro-
duction of pyridine (0.70 ml, 8.7 mmol) resulted in an instant-
aneous change of the solution to bright yellow and the rapid
formation of a light yellow precipitate ensued. After 5 min con-
tinued heating, and cooling to Ϫ10 ЊC, [Ru(MeCO2)(CO)2(py)]2
was isolated as a yellow microcrystalline solid, washed with
ice-cold methanol (2 × 10 ml), and dried in air at 70 ЊC (yield:
0.358 g, 42%). The filtrate was evaporated to 10 ml, resulting in
a precipitation of a mixture of sodium acetate and [Ru(MeCO2)-
(CO)2(py)]2. Suspension of the mixture in the minimum volume
of CH2Cl2 and subsequent filtration was repeated until the
residual solid was colourless. The combined filtrate was added
to methanol (5 ml). Further evaporation afforded additional
[Ru(MeCO2)(CO)2(py)]2 (yield: 0.108 g, 13%). Total yield: 0.466
g, 55%. Found: C, 36.6; H, 2.5; N, 4.7. C18H16N2O8Ru2 requires
C, 36.6; H, 2.7; N, 4.7%. IR ν/cmϪ1: 2020s, 1962s, 1930s ν(CO);
1600w; 1568s νas(CO2); 1449s νs(CO2); 1352w, 1217m, 1070m,
1040w, 1012w, 754m, 697m. δH(300 MHz, CDCl3): 8.74 (4H, dt,
Tetracarbonyl-di-ꢀ-O,OЈ-propanoatobis(pyridine)diruth-
enium(I), [Ru(EtCO2)(CO)2(py)]2. Reaction of dicarbonyl-
dichlororuthenium() (0.650 g, 2.85 mmol) with sodium
propanoate (1.06 g, 11 mmol) as in (i) gave [Ru(EtCO2)-
(CO)2(py)]2 as yellow crystals (yield: 0.362 g, 41%). Found: C,
38.7; H, 3.1; N, 4.4. C20H20N2O8Ru2 requires C, 38.8; H, 3.3; N,
4.5%. IR ν/cmϪ1: 2016s, 1957s, 1932s ν(CO); 1601w, 1567s
ν(CO2); 1376m, 1303w, 1216m, 1152w, 1086w, 1069m, 1039w,
1009w, 894w, 811w, 751w, 688m. δH(300 MHz, CDCl3): 8.75
(4H, d, 3J = 5.0 Hz); 7.83 (2H, t, 3J = 7.8 Hz); 7.43 [4H, apparent
triplet (br), 3J ≈ 6.6 Hz]; 2.29 (4H, q, 3J = 7.6 Hz, CO2CH2CH3);
1.05 (6H, t, 3J = 7.5 Hz, CO2CH2CH3).
Tetracarbonyldi-ꢀ-O,OЈ-pivalatobis(pyridine)diruthenium(I),
[Ru(ButCO2)(CO)2(py)]2. The corresponding pivalate complex
[Ru(ButCO2)(CO)2(py)]2 was prepared by (i) (above) from
sodium pivalate (2.9 g, 23 mmol), which was reacted with
[Ru(CO)2Cl2]n (0.660 g, 2.89 mmol). Yield 0.497 g (0.74 mmol,
51%). Found: C, 41.5; H, 4.0; N, 4.0. C24H28N2O8Ru2 requires
C, 40.6; H, 4.3; N, 4.3%. IR ν/cmϪ1: 2014s, 1958s, 1933s ν(CO);
1600w, 1562s ν(CO2); 1447s, 1419m, 1361m, 1226w, 1216m,
1152w, 1066w, 1040w, 1010w, 937w, 806w, 756m, 696m. δH(300
MHz, CDCl3): 8.74 [4H, s (br)]; 7.82 [2H, s (br)]; 7.41 [4H, d
(br), 3J = 5.4 Hz]; 1.05 [18H, s, C(CH3)3CO2].
4
3
4
3J = 4.7, J ≈ 1.5 Hz); 7.83 (2H, tt, J = 7.6, J = 1.6 Hz); 7.43
(4H, ddd, 3J = 7.7, 3J = 4.7, 4J = 1.5 Hz); 2.06 (6H, s, CH3CO2).
(ii) The preparation was successfully repeated in air afford-
ing a similar yield of product.
Di-ꢀ-O,OЈ-benzoatotetracarbonylbis(pyridine)diruthenium(I),
[Ru(PhCO2)(CO)2(py)]2. Addition of pyridine to a solution
formed by the reaction of [Ru(CO)2Cl2]n (0.670 g, 2.9 mmol)
with sodium benzoate (1.10 g, 76 mmol) according to (i)
resulted in the precipitation of bright yellow [Ru(PhCO2)-
(CO)2(py)]2 (yield: 0.620 g, 59%). Found: C, 48.1; H, 2.8; N, 3.9.
(iii) Under a CO atmosphere. [Ru(CO)2Cl2]n (0.66 g, 2.9
mmol) was added to a solution of NaO2CMe (2.80 g, 34 mmol)
in 75 ml of MeOH. The mixture was purged with CO for 20 min
and then refluxed for 2.5 h. The initially yellow solution rapidly
changed to orange. After completion of the reaction, pyridine
(0.7 ml, 8.7 mmol) was added to the hot solution causing a
change to yellow and a yellow precipitate formed. After cooling
to room temperature, then to Ϫ20 ЊC overnight, the product
was collected, washed with cold MeOH and air-dried at 80 ЊC
C24H20N2O8Ru2 requires C, 47.1; H, 2.8; N, 3.8%. IR ν/cmϪ1
:
2027s, 1977s, 1954s, 1911m ν(CO); 1596s, 1560s, 1448s ν(CO2);
1409s, 1215m, 1182w, 1172w, 1156w, 1068m, 1037w, 1029w,
1008w, 942w, 855w, 815w, 755m, 695s, 668m. δH(300 MHz,
CDCl3): 8.93 (4H, d, 3J = 4.8 Hz); 7.92 (2H, t, 3J = 7.5 Hz); 7.85
J. Chem. Soc., Dalton Trans., 2000, 2867–2873
2871

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3
4
(4H, apparent doublet of triplets, J = 6.9, J ≈ 1.4 Hz); 7.53
[4H, apparent triplet (br), 3J ≈ 6 Hz]; 7.39 (2H, tt, 3J = 7.3,
4J = 1.4 Hz); 7.27 (4H, ddm, 3J = 7.3, 3J = 6.9, J ≈ 1 Hz).
after Lorentz-polarization and empirical absorption correc-
tions (transmission factors 0.6893–1.000) using the program
DIFABS.23 Structure solution was effected by use of DIR-
DIF9424 and refined with the teXsan22 crystallographic soft-
ware package.
Di-ꢀ-O,OЈ-acetatotetracarbonylbis(3-methylpyridine)-
diruthenium(I), [Ru(MeCO2)(CO)2(Mepy)]2. Addition of 3-
methylpyridine (0.70 ml) instead of pyridine to [Ru(CO)2Cl2]n
(0.462 g, 2.0 mmol) according to (i) resulted in the formation
of [Ru(CO)2(MeCO2)(Mepy)]2 as a yellow crystalline solid
(yield: 0.245 g, 41%). Found: C, 38.7; H, 3.2; N, 4.5. C20H20-
N2O8Ru2 requires C, 38.8; H, 3.3; N, 4.5%. IR ν/cmϪ1: 2021s,
1960s, 1928s (br) ν(CO); 1882m (sh), 1574s, 1441s ν(CO2);
1349m (sh), 1243w, 1197m, 1127w, 1104w, 1057m, 1023w, 814w,
792m, 697m, 650m. δH(300 MHz, CDCl3): 8.56 (2H, s); 8.55
(2H, d, 3J = 5.5 Hz); 7.63 (2H, d, 3J = 7.5 Hz); 7.32 (2H,
(b) 3-Methylpyridine/acetate adduct. C20H20N2O8Ru2,
M = 618.5, monoclinic, space group P21 (no. 4), a = 11.063(7),
b = 15.445(5), c = 15.110(4) Å, β = 111.57(4)Њ, V = 2401 Å3. Dc
(Z = 4) = 1.711 g cmϪ3; F(000) = 1224, µMo = 13.0 cmϪ1; speci-
men: 0.85 × 0.50 × 0.20 mm; A*min,max = 1.23, 1.44; 2θmax
50Њ; N = 4382, N0 = 3456; R = 0.051, Rw = 0.058; nν = 580,
|∆ρmax| = 1.4 e ÅϪ3
=
.
Variata. Material of this compound was badly twinned and
data were measured on one deconvoluted reciprocal lattice com-
ponent, a hemisphere being measured in mitigation and merged
(Rint = 0.094) in view of the quasi P21/c nature of the structure;
an ‘orthorhombic’ B-centred appearance is illusory. In the
refinement, zones hk0, hk13 were refined with separate scale
factors. In consequences of the above difficulties, derivative
geometries should be treated with due caution.
3
3
dd, J = 7.5, J = 5.5 Hz); 2.42 (6H, s, py-CH3); 2.06 (6H, s,
CH3CO2).
Carbon dioxide analysis
Detection of carbon dioxide from reactions performed under
identical conditions to those outlined in the preparation of
[Ru(MeCO2)(CO)2(py)]2 was effected by the precipitation of
barium carbonate from a saturated aqueous solution of barium
hydroxide on passage of the effluent nitrogen flow from the
reaction vessel. Carbon dioxide was not detected by this
method when either the ruthenium polymer or the acetate salt
were subjected to the same treatment, thus eliminating the
possibility that carbon dioxide was inadvertently introduced
into the system.
(c) Pyridine/propionate adduct. C20H20N2O8Ru2, M = 618.5,
monoclinic, space group P21/c (no. 14), a = 10.890(3), b =
15.706(2), c = 14.390(4) Å, β = 108.78(2)Њ, V = 2330 Å3, Dc
(Z = 4) = 1.763 g cmϪ3; F(000) = 1224, µMo = 13.4 cmϪ1; speci-
men: 0.38 × 0.15 × 0.35 mm; A*min,max = 1.21, 1.64; 2θmax = 50Њ;
N = 4095, N0 = 2712; R = 0.043, Rw = 0.061; nν = 290, |∆ρmax| =
0.89 e ÅϪ3
.
Variata. A hemisphere of data was measured and merged
(Rint = 0.049) in attempted mitigation of any undesirable con-
sequences of twinning.
(d) Pyridine/pivalate adduct. C24H28N2O8Ru2, M = 674.6,
monoclinic, space group P21/n (no. 14), a = 12.246(3), b =
13.808(2), c = 16.542(4) Å, β = 95.90(2)Њ, V = 2782 Å3, Dc
[Ru(MeCO2)(CO)2(py)]2. [Ru(CO)2Cl2]n [0.907 g, 4.0 mmol
(monomer assumed)] and excess sodium acetate (2.5 g) in
degassed methanol (75 ml) yielded BaCO3 0.219 g (1.11 mmol,
56%). Addition of pyridine (1.00 ml) afforded [Ru(MeCO2)-
(CO)2(py)]2 (0.582 g, 0.99 mmol, 49%) as a yellow crystalline
solid.
(Z = 4) = 1.610
A*min,max = 1.27, 1.46; 2θmax = 55Њ; N = 6503, N0 = 6148;
R = 0.041, Rw = 0.067; nν = 438, |∆ρmax| = 0.51 e ÅϪ3
g ; ;
cmϪ3 F(000) = 1352, µMo = 11.3 cmϪ1
.
(e) Pyridine/benzoate adduct. C24H20N2O8Ru2, M = 714.61,
monoclinic, space group C2/c (no. 15), a = 28.095(1),
b = 10.3742(4), c = 19.3657(7) Å, β = 102.774(1)Њ, V = 5504.6(3)
[Ru(PhCO2)(CO)2(py)]2. Reaction of [Ru(CO)2Cl2]n (0.976 g,
4.3 mmol) with an excess of sodium benzoate (3.0 g) in
degassed methanol (75 ml) gave BaCO3 0.267 g (1.4 mmol,
56% per Ru). [Ru(PhCO2)(CO)2(py)]2 (0.866 g, 1.21 mmol,
56%) precipitated upon the addition of pyridine (1.00 ml).
Å3. Dc (Z = 8) = 1.724 g cmϪ3; F(000) = 2832, µMo = 11.51 cmϪ1
;
2θmax = 56.6Њ; N = 6859, N0 = 3875; R = 0.035, Rw = 0.027;
nν = 361, |∆ρmax| = 0.59 e ÅϪ3
.
Variata. Data were measured at 123 K on a Nonius-Kappa
CCD diffractometer. Solution was achieved using the teXsan
crystallographic software package.22
Structure determination
Three room-temperature single-counter four-circle diffract-
ometer data sets were measured (graphite-monochromated
Mo-Kα radiation, λ = 0.71073 Å; 2θ–θ scan mode, 2θmax as
specified, T ≈ 295 K) in addition to one low-temperature data
set (T = 173 K) yielding N independent reflections, N0 with I >
3σ(I) being considered ‘observed’ and used in the full-matrix
least-squares refinement after Gaussian absorption correction.
A fifth data set was collected at 123 K with a Kappa-Nonius
CCD diffractometer, and was processed without absorption
correction. Anisotropic thermal parameter forms were refined
for the non-hydrogen atoms, (x, y, z, Uiso)H being included
constrained at estimated values. Conventional residuals R, Rw
on |F | are quoted at convergence. Neutral atom complex scat-
tering factors were used. Calculations were performed using
either the Xtal 3.4 program system21 or the teXsan crystallo-
graphic package.22
CCDC reference number 186/2057.
See http://www.rsc.org/suppdata/dt/b0/b002304n/ for crystal-
lographic files in .cif format.
Acknowledgements
Financial assistance in the form of an APA scholarship (to
C. M. K.) and grants from the Special Research Centre
for Green Chemistry (ARC) and Monash Research Fund
(L. S., G. B. D.) are gratefully acknowledged. We thank
Dr C. M. Forsyth for experimental assistance.
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.
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2872
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J. Chem. Soc., Dalton Trans., 2000, 2867–2873
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