Chloride labilization resulting from nucleophilic addition to cis-Ru(tpy)(CO)2Cl+PF6-: Synthesis and characterization of new CO2-bridged and formate complexes of ruthenium

Article abstract of DOI:10.1021/om970493p

Reaction of cis-Ru(tpy)(CO)₂Cl⁺PF₆^- (1; tpy = 2,2′:6′,2″-terpyridyl) with aqueous Na₂CO₃ in acetonitrile yields the CO₂-bridged complex cis,cis-(CH₃CN)(CO)(tpy)Ru(CO₂)Ru(tpy)-(CO) ₂²⁺2PF₆^- (3). Reaction of 1 with aqueous NaOCHO yields the corresponding η¹-formate complex 4; reaction of 4 with aqueous Na₂CO₃ also provides 3. Reaction of 3 with CO results in displacement of acetonitrile and formation of 5, cis,cis-(CO)₂(tpy)Ru(CO₂)Ru(tpy)(CO)₂ ²⁺2PF₆^-. Reaction paths leading to 3 and 4 from 1 are thought to occur by base-assisted trans labilization of chloride resulting from conversion of a π-acceptor ligand (CO) to a σ-donor (COOH). Support for proposed steps in the reaction paths leading to 3 and 4 was provided by a trapping experiment in which 1 reacts with cis-Ru(bpy)₂(CO)COOH⁺PF₆^- in the presence of aqueous Na₂CO₃ to give cis,cis-(CO)(bpy)₂Ru(CO₂)Ru(tpy)(CO)₂ ²⁺2PF₆^- (7; bpy = 2,2′-bipyridyl), which has been structurally characterized. Parallels between these reactions and the halide labilizations which occur with ruthenium or rhenium polypyridyl complexes during photochemical or electrochemical reductions of CO₂ are discussed.

Full text of DOI:10.1021/om970493p

Organometallics 1997, 16, 4421-4427
4421
Ch lor id e La biliza tion Resu ltin g fr om Nu cleop h ilic
-
Ad d ition to cis-Ru (tp y)(CO)2Cl+P F 6 : Syn th esis a n d
Ch a r a cter iza tion of New CO2-Br id ged a n d F or m a te
Com p lexes of Ru th en iu m
Dorothy H. Gibson,* Bradley A. Sleadd, Mark S. Mashuta, and
J ohn F. Richardson
Department of Chemistry and Center for Chemical Catalysis, University of Louisville,
Louisville, Kentucky 40292
Received J une 11, 1997X
Reaction of cis-Ru(tpy)(CO)2Cl+PF6- (1; tpy ) 2,2′:6′,2′′-terpyridyl) with aqueous Na2CO3
in acetonitrile yields the CO2-bridged complex cis,cis-(CH3CN)(CO)(tpy)Ru(CO2)Ru(tpy)-
(CO)22+2PF6- (3). Reaction of 1 with aqueous NaOCHO yields the corresponding η1-formato
complex 4; reaction of 4 with aqueous Na2CO3 also provides 3. Reaction of 3 with CO results
-
in displacement of acetonitrile and formation of 5, cis,cis-(CO)2(tpy)Ru(CO2)Ru(tpy)(CO)22+2PF6
.
Reaction paths leading to 3 and 4 from 1 are thought to occur by base-assisted trans
labilization of chloride resulting from conversion of a π-acceptor ligand (CO) to a σ-donor
(COOH). Support for proposed steps in the reaction paths leading to 3 and 4 was provided
by a trapping experiment in which 1 reacts with cis-Ru(bpy)2(CO)COOH+PF6- in the presence
-
of aqueous Na2CO3 to give cis,cis-(CO)(bpy)2Ru(CO2)Ru(tpy)(CO)22+2PF6 (7; bpy ) 2,2′-
bipyridyl), which has been structurally characterized. Parallels between these reactions
and the halide labilizations which occur with ruthenium or rhenium polypyridyl complexes
during photochemical or electrochemical reductions of CO2 are discussed.
In tr od u ction
Resu lts a n d Discu ssion
Syn t h esis of New Com p ou n d s. Reaction of
[Ru(CO)2Cl2]n with 2,2′:6′,2′′-terpyridine in refluxing
Ruthenium and rhenium carbonyl halide complexes,
especially chlorides and bromides, bearing polypyridyl
ligands are effective as photocatalysts and electrocata-
lysts for the reduction of CO2.1 Common to most of the
proposed catalytic cycles in these reactions are steps in
which halide ion dissociation follows electron transfer
to the complexes. Since carbon dioxide complexes have
not been observed in these reactions but are usually
proposed as intermediates, we have sought to prepare
model compounds whose properties might enhance the
understanding of steps in the catalytic cycles. We report
the characterization of the new complex cis-Ru(tpy)-
2
EtOH/H2O, followed by anion exchange using NH4PF6,
gave an off-white solid which has been characterized
1
by spectral data and elemental analysis. The H NMR
data showed one lower field doublet at δ 8.75 and a
series of multiplets between δ 8.49 and 7.67. The 13C
NMR data showed 8 resonances for 15 carbons in the
region between δ 158.08 and 125.29; a mirror plane
which bisects the terpyridyl ligand gives rise to internal
equivalence of the carbon (and proton) resonances.
Terminal carbonyl resonances in the product appear at
δ 194.94 and 187.31. DRIFTS data show νCO signals
at 2092 and 2029 cm-1; these data are consistent with
the formulation as the dicarbonyl cation cis-Ru(tpy)-
-
(CO)2Cl+PF6 (1; tpy ) 2,2′:6′,2′′-terpyridyl) and the
results of its reactions in the presence of several
nucleophiles, including weak aqueous bases.
-
(CO)2Cl+PF6 (1). Substitution of the hexafluorophos-
phate counterion by tetraphenylborate provided a crys-
talline material, cis-Ru(tpy)(CO)2Cl+BPh4- (1a ), which
has been structurally characterized (see discussion
below); the spectral properties of the cation portion of
1a are closely similar to those of 1.
In previous work, we have successfully employed
aqueous Na2CO3 as a reagent for the generation of
metallocarboxylic acids from metal carbonyl cations.3 In
our experience, and that of others,4 aqueous Na2CO3 is
sufficiently basic to provide OH- for nucleophilic attack
on a carbonyl ligand in a cationic metal complex but
not basic enough to deprotonate the resulting metallo-
carboxylic acid. Reaction of 1 with excess aqueous
X Abstract published in Advance ACS Abstracts, September 1, 1997.
(1) (a) Hawecker, J .; Lehn, J . M.; Ziessel, R. J . Chem. Soc., Chem.
Commun. 1983, 536. (b) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.;
Vining, T. J .; Meyer, T. J . J . Chem. Soc., Chem. Commun. 1985, 1414.
(c) Sullivan, B. P.; Conrad, D.; Meyer, T. J . Inorg. Chem. 1985, 24,
3640. (d) Hawecker, J .; Lehn, J .-M.; Ziessel, R. Helv. Chim. Acta 1986,
69, 1990. (e) Ishida, H.; Tanaka, K.; Tanaka, T. Organometallics 1987,
6, 181. (f) Bruce, M. R. M.; Megehee, E.; Sullivan, B. P.; Thorp, H.;
O’Toole, T. R.; Downard, A.; Meyer, T. J . Organometallics 1988, 7, 238.
(g) Sullivan, B. P.; Bruce, M. R. M.; O’Toole, T. R.; Bolinger, C. M.;
Megehee, E.; Thorp, H.; Meyer, T. J . In Catalytic Activation of Carbon
Dioxide; Ayers, W. M., Ed.; ACS Syms. Ser. 363; American Chemical
Society: Washington, D C, 1988; Chapter 6. (h) Ziessel, R. In Catalysis
by Metal Complexes: Photosensitization and Photocatalysis Using
Inorganic and Organometallic Compounds; Kalyanasundaram, K.,
Gra¨tzel, M., Eds.; Kluwer Academic: Dordrecht, The Netherlands, 1993
p 217. (i) Christensen, P.; Hammett, A.; Muir, A. V. G.; Timney, J . H.
J . Chem. Soc., Dalton Trans. 1992, 1455. (j) Yoshida, T.; Tsutsumida,
K.; Teratani, S.; Yasufuku, K.; Kaneko, M. J . Chem. Soc., Chem.
Commun. 1993, 631. (k) Stor, G. J .; Hartl, F.; van Outersterp, J . W.
M.; Stufkens, D. J . Organometallics 1995, 14, 115. (l) J ohnson, F. P.
A.; George, M. W.; Hartl, F.; Turner, J . J . Organometallics 1996, 15,
3374. (m) Klein, A.; Vogler, C.; Kaim, W. Oranometallics 1996, 15, 236.
(2) Colton, R.; Farthing, R. H. Aust. J . Chem. 1967, 20, 1283.
(3) Gibson, D. H.; Mehta, J . M.; Ye, M.; Richardson, J . F.; Mashuta,
M. S. Organometallics 1994, 13, 1070.
(4) Suzuki, H.; Omori, H.; Moro-oka, Y. J . Organomet. Chem. 1987,
327, C49.
S0276-7333(97)00493-7 CCC: $14.00 © 1997 American Chemical Society

4422 Organometallics, Vol. 16, No. 20, 1997
Gibson et al.
Na2CO3 did not provide the expected metallocarboxylic
acid, however. The reaction afforded a product whose
1H NMR spectrum showed a series of multiplets be-
tween δ 8.49 and 7.17 which are assigned to two sets of
terpyridyl protons, each displaying internal equivalence
characteristic of compounds having mirror planes which
bisect both terpyridyl ligands. The 13C NMR spectrum
showed 4 low-field signals at δ 204.25, 198.23, 195.38,
and 190.18, which have been assigned to three terminal
carbonyls and a carboxylate carbon, and 16 resonances
for 30 carbons in the region between δ 157.90 and
122.26, which are assigned to the terpyridyl carbons.
DRIFTS data showed νCO signals at 2069, 2013, and
1953 cm-1 and νOCO bands at 1503 and 1177 cm-1. The
reaction of 1 with base, treatment of complex 4 with
aqueous Na2CO3 in CH3CN also provides the CO2-
bridged complex 3, again in 77% yield. Also, 4 is readily
converted back to 1 by brief treatment with a catalytic
amount of NaOCHO and excess tetraethylammonium
chloride in aqueous CH3CN.
The CO2-bridged compound 3 is sparingly soluble in
CH2Cl2, which allowed us to explore the possibility of
displacing CH3CN with another ligand without competi-
tion from a coordinating solvent. When 3 was slurried
in CH2Cl2 and stirred for 30 min while CO was bubbled
into the mixture, the color of the solid component of the
reaction mixture changed from red to brown. Addition
of ether precipitated the remainder of a brown, light-
sensitive solid, which has been characterized by spectral
data (the compound is stable in acetone for a short time)
ν
OCO values for this compound are very similar to those
in the related CO2-bridged compound cis,cis-Ru(bpy)2-
-
(CO)(CO2)Ru(bpy)2(CO)2+2PF6 (2; bpy ) 2,2′-bipy-
1
and elemental analysis. The H NMR data showed a
ridyl),5 which shows νOCO signals at 1507 and 1176 cm-1
,
series of multiplets between δ 8.49 and 7.17 for the
terpyridyl protons; the pattern is very similar to that
for 3. The 13C NMR data showed five low-field peaks
at δ 199.64, 197.57, 195.34, 190.38, and 186.28 for four
terminal carbonyls and a carboxyl carbon. There are
16 resonances for the 30 remaining carbons at δ
157.90-122.26. DRIFTS data show νCO signals at 2071,
2054, 2016 and 1992 cm-1 and νOCO bands at 1533 and
1198 cm-1; the νOCO values for the carboxylate bridge
are indicative of a µ2-η2 bonding mode6 and are in
good agreement with those observed for 3 and 2.5 These
data support the formulation of this compound as
cis,cis-(CO)2(tpy)Ru(CO2)Ru(tpy)(CO)22+2PF6- (5), where
CH3CN in 3 has been replaced by CO. As expected,
when 5 was allowed to stand in acetonitrile, it reverted
to 3.
as well as other compounds with µ2-η2-type bridging CO2
ligands.6 Weak bands were also observed for the
coordinated nitrile (2313 and 2283 cm-1); these are
similar in position, and in intensity, to those reported7
for cis(CO),trans(CH3CN)-Ru(bpy)(CO)2(CH3CN)22+2PF6
.
-
These data, together with elemental analysis results,
support the compound’s formulation as the bimetallic
CO2-bridged dication cis,cis-(CH3CN)(CO)(tpy)Ru(CO2)-
-
Ru(tpy)(CO)22+2PF6 (3), where CO2 is bound between
the two metal centers in a µ2-η2 fashion and the aceto-
nitrile ligand is trans to the carboxyl ligand. The
product yield for 3 was 77%. Results of experiments
conducted under laboratory light were the same as those
from an experiment conducted in the dark.
Treatment of an acetonitrile solution of 1 with excess
aqueous sodium formate resulted in nearly quantitative
conversion to a salmon-colored solid which has been
characterized by spectral data and elemental analyses.
The 1H NMR spectrum revealed a pattern nearly
identical with that of 1 except for the addition of a
singlet at δ 7.70, which is assigned to the formate proton
(see Experimental Section). The 13C NMR spectrum
showed two low-field resonances, at δ 195.61 and 190.68,
as expected for a compound with cis terminal carbonyl
ligands. The lower field resonance is within 1 ppm of
the lowest field resonance for compound 1; the next
lowest field resonance differs from that in 1 by ca. 3
ppm. Since the carbonyl trans to formate (or Cl) would
be affected most by ligand substitution, the lowest field
resonance is assigned to the carbonyl trans to the
central terpyridyl nitrogen; the next lowest field reso-
nance is assigned to the carbonyl trans to the formate
ligand. The peak at δ 167.89 is assigned to the formate
carbon, and there are 8 resonances for the remaining
15 carbons in the region between δ 158.74 and 125.33.
DRIFTS data showed νCO bands at 2066 and 2004 cm-1
and νOCO signals at 1618 and 1291 cm,-1 the latter being
very similar to the νOCO values for cis-Ru(bpy)2-
Addition of aqueous Na2CO3 to a solution containing
-
equimolar amounts of 1 and Ru(bpy)2(CO)(COOH)+PF6
(6)9 in acetonitrile gave a mixture which, after analysis
of the NMR spectral data, showed approximately equal
amounts of 3, unreacted acid 6 and a new product which
has been characterized by spectral data, elemental
analysis, and X-ray structure determination (see below).
1
The H NMR spectrum for the new product showed one
low-field resonance at δ 9.48 and a series of multiplets
between δ 8.60 and 6.72, which are assigned to the
bipyridyl and terpyridyl protons. The 13C NMR spec-
trum showed 4 low-field signals at δ 202.72, 202.48,
198.73, and 190.36, which have been assigned to three
terminal carbonyls and a carboxylate carbon, and 35
resonances for the 35 inequivalent carbons in the region
between δ 158.28 and 122.91 of the bipyridyl and
terpyridyl ligands. DRIFTS data showed νCO signals at
2059, 2000 and 1940 cm-1 and νOCO bands at 1497 and
1186 or 1174 cm-1. Again, the stretching frequencies
for the carboxylate ligand are very similar to those for
2, 3, and 5. These data support the formulation of the
new compound as the CO2-bridged complex cis,cis-
-
(CO)(bpy)2Ru(CO2)Ru(tpy)(CO)22+2PF6 (7) with the
-
(CO)(OCHO)+PF6 (1621 and 1282 cm-1).8 All of the
CO2 ligand again bound in a µ2-η2 fashion. The yield of
7 was maximized by slow addition of a solution of 1 in
acetonitrile to a mixture containing acid 6, CH3CN,
water, and Na2CO3; the yield was 68% by this method.
Small amounts of 6 and 3 were also evident in the
reaction mixture. Compound 7 has been structurally
characterized (see discussion below).
data support the formulation of the new product as the
-
η1-formato complex cis-Ru(tpy)(CO)2(OCHO)+PF6 (4);
the product yield was 91%. In the same manner as the
(5) Gibson, D. H.; Ding, Y.; Sleadd, B. A.; Franco, J . O.; Richardson,
J . F.; Mashuta, M. S. J . Am. Chem. Soc. 1996, 118, 11984.
(6) Gibson, D. H. Chem. Rev. 1996, 96, 2063.
(7) Black, D. St.-C.; Deacon, G. B.; Thomas, N. C. Aust. J . Chem.
1982, 35, 2445.
(8) Sullivan, B. P.; Caspar, J . V.; J ohnson, S. R.; Meyer, T. J .
Organometallics 1984, 3, 1241.
(9) Toyohora, K.; Nagao, H.; Adachi, T.; Voshida, T.; Tanaka, K.
Chem. Lett. 1996, 27.

CO2-Bridged and Formate Complexes of Ru
Organometallics, Vol. 16, No. 20, 1997 4423
Ta ble 2. Selected Bon d Dista n ces (Å) a n d
An gles (d eg) for 1a
Bond Distances
Ru-Cl
2.3917(7)
2.092(2)
2.016(2)
2.097(2)
Ru-C(1)
Ru-C(2)
O(1)-C(1)
O(2)-C(2)
1.912(3)
1.873(3)
1.122(3)
1.126(3)
Ru-N(1)
Ru-N(2)
Ru-N(3)
Bond Angles
Cl-Ru-N(1)
Cl-Ru-N(2)
Cl-Ru-N(3)
Cl-Ru-C(1)
Cl-Ru-C(2)
N(1)-Ru-N(2)
N(1)-Ru-N(3)
N(1)-Ru-C(1)
Ru-C(2)-O(2)
86.97(6)
86.21(5)
88.77(5)
86.4(1)
178.39(9)
79.01(7)
157.14(7)
99.8(1)
N(1)-Ru-C(2)
N(2)-Ru-N(3)
N(2)-Ru-C(1)
N(2)-Ru-C(2)
N(3)-Ru-C(1)
N(3)-Ru-C(2)
C(1)-Ru-C(2)
Ru-C(1)-O(1)
93.1(1)
78.31(7)
172.6(1)
95.4(1)
102.30(9)
91.8(1)
92.0(1)
175.8(3)
178.3(3)
F igu r e 1. ORTEP diagram of 1a (cation only) with
thermal ellipsoids shown at the 50% probability level.
vents it from participating in idealized octahedral
geometry. As a consequence, the N(1)-Ru-N(3) bond
angle is 157.14(7)°, with the Ru-N(1) and Ru-N(3)
bond lengths slightly elongated (2.092(2) and 2.097(2)
Å, respectively) relative to Ru-N bonds in less con-
Ta ble 1. Su m m a r y of Cr ysta llogr a p h ic Da ta for
1a a n d 7
1a
7
formula
C41H31BClN3O2Ru
triclinic
P1h
C40.5H30F12N7.75O5P2Ru2
triclinic
P1h
14.362(4)
14.596(3)
12.895(4)
93.54(2)
114.80(2)
71.69(2)
2321(1)
cryst syst
space group
a, Å
b, Å
c, Å
10
strained bipyridyl complexes (in Ru(bpy)2Cl2 and
- 9
Ru(bpy)2(CO)(COOH)+O3SCF3
the average Ru-N
13.233(4)
13.374(6)
10.918(3)
113.00(3)
98.96(2)
78.97(3)
1738(1)
2
bond which is trans to a bpy nitrogen is 2.054 Å in
length). The Ru-N(2) bond (involving the central
terpyridyl nitrogen) in 1a is compressed at 2.016(2) Å;
the Ru-Cl bond length is 2.391(7)Å. More closely
related to 1a is the neutral complex cis-Ru(tpy)(CO)Cl2,
which has been structurally characterized by Deacon
et al.11 Here, the Ru-Cl bond is longer when this ligand
is trans to CO (2.438(3) Å) than when the relationship
is cis (2.420(3) Å). Again, the Ru-N bond of the central
nitrogen in the tpy ligand is shorter (1.969(7) Å as
compared to approximately 2.08 Å for the other two).
The Ru-C(2) bond in 1a (trans to Cl) is somewhat
shorter, at 1.873(3) Å, than that of Ru-C(1) (trans to
tpy), at 1.912(3) Å. The Ru-Cl bond distance (2.3917(7)
Å) is very similar to that in cis-Ru(bpy)2(CO)(Cl)+
(2.396(7) Å),12 even though a cis relationship of the
chloride and carbonyl ligands exists in this cation.
Compound 1a is cationic; the reduced electron density
about the ruthenium atom in 1a may be responsible for
the contraction of the Ru-Cl bond (which is trans to
CO in this compound) relative to the analogous Ru-Cl
bond in cis-Ru(tpy)(CO)Cl2.
R, deg
â, deg
γ, deg
V, Å3
Z
2
Dc, g/cm3
cryst dimens, mm
cryst descripn
µ(Mo KR), cm-1
abs cor
transmissn factors:
min/max
radiation (λ, Å)
diffractometer
monochromator
temp, °C
1.423
1.712
0.60 × 0.30 × 0.26 0.60 × 0.49 × 0.49
yellow prism
5.68
orange block
8.20
difabs
ψ scans
0.967/1.000
0.539/1.000
Mo KR (0.710 73)
Enraf-Nonius CAD4
graphite cryst
23(3)
scan range
scan speed, deg/min 1-5
max 2θ, deg
no. of unique rflns
collected
0.80 + 0.35 tan θ
0.85 + 0.35 tan θ
1-5
50.0
8139
50.0
6079
no. of rflns included 5571
6973
(Io > 3σ(Io))
no. of params
computer hardware
computer software
extinctn coeff
agreement factorsa
R
443
636
Silicon Graphics Iris Indigo
teXsan (msc)
[7.30(4)] × 10-7
[5.29(4)] × 10-7
The ORTEP diagram for the CO2-bridged complex 7
is shown in Figure 2 and clearly shows a µ2-η2-bound
CO2 ligand; crystallographic data are shown in Table
1, and selected bond distances and angles are given in
Table 3. Compound 7 exhibits slightly distorted octa-
hedral geometry about both metal centers. Bond angles
and distances for the tpy-containing portion of the
molecule do not deviate significantly from those in 1a .
The bpy-containing portion of the molecule displays
bond lengths and angles which are similar to those
reported by Tanaka et al.13 for the metalloester
Ru(bpy)2(CO)(C(O)OMe)+PF6-. The only other structur-
ally characterized, and non-metallacyclic, complex with
0.027
0.034
0.040
0.054
Rw
function minimized
GOF
∆/σ
∑w(|Fo| - |Fc|)2
2.93
0.02
2.95
0.01
weighting scheme
high peak in final
diff map, e/Åe
[σ2(Fo)]-1
0.50
0.39
a
2 1/2
R ) ∑||Fo| - |Fc||/∑(|Fo|); Rw ) [∑w(|Fo| - |Fc|)2/∑wFo
] .
Str u ctu r a l Ch a r a cter iza tion of 1a a n d 7. The
solid-state structures of 1a and 7 were established by
X-ray crystallography. The ORTEP diagram for 1a is
shown in Figure 1; crystallographic data are sum-
marized in Table 1. Selected bond lengths and angles
for 1a are shown in Table 2. Compound 1a exhibits
slightly distorted octahedral geometry about the metal
center, with cis terminal carbonyl ligands; the two
carbonyls and the chloride ligand comprise a meridional
plane which bisects the terpyridyl ligand. The bite
angle available for a terdentate terpyridyl ligand pre-
(10) Eggleston, D. S.; Goldsby, K. A.; Hodgson, D. J .; Meyer, T. J .
Inorg. Chem. 1985, 24, 4573.
(11) Deacon, G. B.; Patrick, J . M.; Skelton, B. W.; Thomas, N. C.;
White, A. H. Aust. J . Chem. 1984, 37, 929.
(12) Clear, J . M.; Kelly, J . M.; O’Connell, C. M.; Vos, J . G.; Cardin,
C. J .; Costa, S. R.; Edwards, A. J . J . Chem. Soc., Chem. Commun. 1980,
750.
(13) Tanaka, H.; Tzeng, B.-C.; Nagao, H.; Peng, S.-M.; Tanaka, K.
Inorg. Chem. 1993, 32, 1508.

4424 Organometallics, Vol. 16, No. 20, 1997
Gibson et al.
(O)OMe)Cl; it is one of the few transition metal com-
plexes having a chloride ligand trans to an acyl group
in an octahedral complex for which structural data are
available. This compound displays an even longer Ru-
Cl bond (2.496(3) Å) than the closely related dichloro
dicarbonyl complex cis,cis-Ru(bpy)(CO)2Cl2 (2.439(3)
Å).15 The lengthening of the Ru-Cl bond in the met-
alloester has been attributed to the trans effect of the
σ-donating acyl ligand, an effect well documented in
square-planar complexes.16 A long Ru-Cl bond is
present in cis-(CO)Ru(bpy)(CO)2(C(O)OMe)Cl, even
though the complex has two terminal carbonyl ligands
which can help to dissipate the additional electron
density on the metal center via back-bonding. Nucleo-
philic addition to the terminal carbonyl in 1 which is
trans to Cl would leave a species with only one terminal
carbonyl ligand, thus setting the stage for complete
dissociation of the chloride ligand.
The proposed pathways for the formation of the
formate complex (4) and CO2-bridged complex 3 from 1
are given in Scheme 1. The suggested initial step for
both transformations is nucleophilic addition of hydrox-
ide ion to the carbonyl ligand which is trans to chlorine,
resulting in intermediate A. Conversion of the π-ac-
ceptor CO to the σ-donating acyl group of the metallo-
carboxylic acid results in labilization of Cl- and initial
formation of a five-coordinate metallocarboxylic acid
intermediate, B (which may be quickly solvated). In the
reaction with aqueous NaOCHO, coordination of for-
mate ion (which is present in excess) at the position
trans to the carboxyl group in B would result in
hydroxide dissociation and generation of 4. Under the
more basic conditions provided by aqueous Na2CO3,
nucleophilic attack of a carboxylate oxygen from B (as
with other M-COOH, not deprotonated by this base)
at the position trans to COOH in a second B molecule
(or solvated B) would promote hydroxide dissociation
from the carboxyl group of the second molecule, in the
manner of the formate reaction. Deprotonation of the
carboxyl bridge and coordination of CH3CN would
provide the CO2-bridged complex 3.
The lability of the formate ligand in 4 toward dis-
sociation is evidenced by the ease of conversion of 4 back
to 1 by reaction with Cl-. Thus, the ease with which 4
can be converted to the CO2-bridged compound 3 by the
action of aqueous sodium carbonate is not surprising.
Several observations on the lability of the position
trans to the acyl carbon in 3 and 5 are worthy of note.
Compound 3 exhibits good solubility only in acetone or
CH3CN; however, dissolution of this compound in
acetone is followed by rapid decomposition to many
unidentifiable products. Attempts to return chloride to
that position, using excess Et4NCl in CH3CN solution
in the manner in which 4 can be converted back to 1,
were unsuccessful and resulted in decomposition of 3.
For compound 5 in the solid state, decomposition occurs
after several hours under laboratory lights, presumably
via loss of CO. Furthermore, dissolution of 5 in CH3CN
F igu r e 2. ORTEP drawing of 7 (cation only) with thermal
ellipsoids shown at the 50% probability level.
Ta ble 3. Selected Bon d Dista n ces (Å) a n d
An gles (d eg) for 7
Bond Distances
Ru(1)-N(4)
Ru(1)-N(5)
Ru(1)-N(6)
Ru(1)-N(7)
Ru(1)-C(1)
Ru(1)-C(3)
Ru(2)-O(2)
O(3)-C(3)
2.060(3)
2.116(3)
2.099(3)
2.183(3)
2.059(4)
1.835(5)
2.058(3)
1.151(5)
1.118(5)
Ru(2)-N(1)
Ru(2)-N(2)
Ru(2)-N(3)
Ru(2)-C(4)
Ru(2)-C(5)
O(1)-C(1)
O(2)-C(1)
O(4)-C(4)
2.078(3)
2.014(3)
2.087(3)
1.921(5)
1.886(4)
1.232(4)
1.297(5)
1.132(5)
O(5)-C(5)
Bond Angles
N(4)-Ru(1)-C(1)
N(4)-Ru(1)-C(3)
N(5)-Ru(1)-C(1)
N(5)-Ru(1)-C(3)
N(6)-Ru(1)-C(1)
N(6)-Ru(1)-C(3)
N(7)-Ru(1)-C(1)
N(7)-Ru(1)-C(3)
C(1)-Ru(1)-C(3)
O(2)-Ru(2)-N(1)
Ru(1)-C(3)-O(3)
Ru(2)-C(5)-O(5)
89.3(1)
96.8(2)
88.3(1)
172.9(1)
97.5(1)
91.0(2)
171.1(1)
100.3(2)
86.6(2)
84.3(1)
175.9(4)
177.2(4)
O(2)-Ru(2)-N(2)
O(2)-Ru(2)-N(3)
O(2)-Ru(2)-C(4)
O(2)-Ru(2)-C(5)
N(1)-Ru(2)-C(4)
N(1)-Ru(2)-C(5)
N(2)-Ru(2)-C(4)
Ru(1)-C(1)-O(1)
Ru(1)-C(1)-O(2)
O(1)-C(1)-O(2)
Ru(2)-O(2)-C(1)
Ru(2)-C(4)-O(4)
80.7(1)
86.8(1)
95.3(1)
175.6(1)
101.1(2)
92.6(2)
176.0(1)
124.6(3)
114.1(3)
121.3(4)
123.1(2)
172.5(4)
µ2-η2 binding of CO2 between two transition metals,
Cp(CO)(PPh3)Fe(CO2)Re(CO)4(PPh3) (Cp ) η5-cyclopen-
tadienyl),14 has angles and distances involving the CO2
bridge (C(1)-O(1) ) 1.226(3) Å; C(1)-O(2) ) 1.298(3)
Å; O-C-O ) 121.9(3)°) which are very similar to those
in 7 (see Table 3). Finally, the ruthenium-nitrogen
bond which is trans to the carboxyl carbon is unusually
long, at 2.183(3) Å; only Ru(bpy)2(CO)(CO2)‚3H2O dis-
plays a longer corresponding bond in the ruthenium
bipyridyl series of compounds, at 2.204(10) Å.12
Con sid er a tion s of Rea ction P a th s. Efforts to
remove the chloride ligand from 1 with silver or thal-
lium salts, or by thermal displacement with an aceto-
nitrile ligand, were unsuccessful. The labilization of the
Ru-Cl bond which occurs with 1 in the presence of
aqueous base appears to occur indirectly, as the result
of changes at the terminal carbonyl ligand trans to
chlorine. Recently, Pakkanen and co-workers15 have
structurally characterized cis-(CO)-Ru(bpy)(CO)2(C-
(15) Haukka, M.; Kiviaho, J .; Ahlgren, M.; Pakkanen, T. A. Orga-
nometallics 1995, 14, 825.
(16) See, for example: (a) Weaver, D. L. S. Inorg. Chem. 1970, 9,
2250. (b) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem.
Rev. 1973, 10, 335. (c) Bennett, M. A.; J ohnson, R. N.; Robertson, G.
B.; Tomkins, I. B.; Whimp, P. O. J . Am. Chem. Soc. 1976, 98, 3514. (d)
Anderson, O. P.; Packard, A. B. Inorg. Chem. 1978, 17, 1333. (e) Bardi,
R.; Piazzesi, A. M.; Del Pra, A.; Cavinato, G.; Toniolo, L. Inorg. Chim.
Acta 1985, 102, 99.
(14) Gibson, D. H.; Ye, M.; Richardson, J . F. J . Am. Chem. Soc. 1992,
114, 9716.

CO2-Bridged and Formate Complexes of Ru
Organometallics, Vol. 16, No. 20, 1997 4425
Sch em e 1
Sch em e 2
results in immediate loss of CO and conversion to 3, as
noted above.
Since formation of 3 from the reaction of 1 or 4 with
aqueous Na2CO3 suggested the possible intermediacy
of a metallocarboxylic acid, we decided to probe the
reactions of 1 further in the presence of a metallocar-
boxylic acid. The known17 acid cis-Ru(bpy)2(CO)-
(COOH)+PF6- (6), which can be prepared by the reaction
of cis-Ru(bpy)2(CO)22+(2PF6-)2 with aqueous Na2CO3,5
does not react further with this base. These properties
suggested its use as a potential trapping agent for
intermediate B in the reactions of 1 or 4 with aqueous
Na2CO3, thus providing support for this proposed active
species in the formation of the CO2-bridged compound
3. Initially, the yield of 7 from reaction between 1 and
6 was relatively low. The nucleophilicity of intermedi-
ate B is apparently comparable to that of 6, since only
half of 6 was consumed when this compound and 1 were
present in solution prior to the addition of base. The
problem was reduced, and the yield of 7 was enhanced,
by putting 6 and the aqueous base together before
adding compound 1. The proposed rationale for the
formation of 7 is shown in Scheme 2. The unsym-
metrical nature of 7 eliminated the potential for crys-
tallographically imposed disorder in the carboxylate
bridge, a problem which hindered structural refinement
of cis,cis-(CO)(bpy)2 Ru(CO2)Ru(bpy)2(CO)22+2PF6- (2).5
In 1983, Hawecker, Lehn, and Ziessel1d reported the
use of fac-Re(bpy)(CO)3Cl as a catalyst for the photoin-
duced reduction of CO2 and noted that halide dissocia-
tion occurred. These observations were followed by
additional studies from this group and from Meyer and
Sullivan et al.1c,f,g noting the generality of such reac-
tions. The latter group also reported that two sequen-
tial one-electron reductions of [M(bpy)2(L)Cl]n+ (M ) Ru,
Os; L ) Cl, pyridine, PR3, CO) in CH3CN resulted in
facile dissociation of Cl- and replacement by CH3CN.1c
This group also noted that the formate ligand in
Ru(bpy)2(CO)(OCHO)+PF6- was labilized under similar
electrochemical conditions.
subject of numerous studies and remain an active area
of investigation.1i-m The electron transfer step in
complexes involving R-diimine ligands coordinated to Ru
and Re centers has been regarded as a reduction of the
diimine ligand followed by halide dissociation from the
metal, as shown in eq 1. However, recent in situ FTIR
(R-diimine0)ReI(CO)3Cl + e- a
+L
{ }
[(R-diimine-)ReI(CO)3Cl]•-
The mechanisms by which these ligand labilizations
occur, and their role in CO2 reduction, have been the
[(R-diimine-)ReI(CO)3L]• + Cl- (1)
(17) Ishida, H.; Tanaka, K.; Morimoto, M.; Tanaka, T. Organome-
tallics 1986, 5, 724.
studies of the electrocatalyzed reductions of CO2 by fac-

4426 Organometallics, Vol. 16, No. 20, 1997
Gibson et al.
The clear yellow solution was cooled to room temperature, and
1.0 g (6.1 mmol) of NH4PF6 dissolved in a minimum amount
of water was added, followed by ca. 50 mL of water. The
resulting precipitate was collected by filtration and recrystal-
lized from CH3CN/ether to give 1.6 g (80% yield based on
Ru(CO)2Cl2 monomer) of a cream-colored solid, mp >240 °C.
Anal. Calcd for C17H11ClF6N3O2PRu: C, 35.77; H, 1.94.
Found: C, 35.87; H, 2.04. IR (DRIFTS, KCl): νCO 2092 and
2029 cm-1. 1H NMR (CD3CN): δ 8.75 (d, J HH ) 5.5 Hz), 8.49-
7.67 (m). 13C NMR (CD3CN): δ 194.94, 187.31, 158.08-125.29
(8 resonances for 15 carbons).
(b) The tetraphenylborate salt of this cation (1a ) was pre-
pared as above, using aqueous NaBPh4 as the anion exchange
reagent. Spectral data were in good agreement with the above
values, with the addition of three multiplets at 7.28, 6.98, and
6.81 ppm (integral ratio 2:2:1) for the tetraphenylborate anion.
Attem p ted Rem ova l of Ch lor id e fr om 1. (a) Under N2,
0.10 g (0.17 mmol) of 1 and 0.70 g (0.20 mmol) of TlPF6 were
dissolved in ca. 30 mL of dry acetonitrile and this solution was
refluxed for 24 h. IR spectral data obtained during this time
showed no evidence of reaction. Solvent was then removed
under vacuum, and the residue was analyzed by NMR
spectroscopy; the spectral data showed no evidence for conver-
sion of 1.
Re(bpy)(CO)3Cl have provided some clarification and IR
spectral data on intermediate reduced species.1k,l The
starting material shows IR bands (in CH3CN) for the
terminal carbonyls at 2020, 1914 and 1897 cm-1; after
reduction to the radical anion, the bands are lowered
to 1998, 1885, and 1867 cm.-1 The lowering of these
stretching frequencies clearly demonstrates that the
increased electron density is being shared by the car-
bonyl ligands even though the bipyridyl ligand may bear
most of it. Kaim et al.1m have described the dissociation
of halide as resulting from overlap of the π* orbital of
the diimine with the σ(Re-Hal) antibonding orbital as
well as ligand to metal electron transfer which deter-
mines the extent of activation; the diimine ligand is
proposed to serve as an “electron buffer”.
With many other metal carbonyl complexes, electron
transfer to the compounds has been shown to yield 19e
species which establish equilibrium with 18e metal acyl
radicals.18 Ligand dissociation can follow this conver-
sion, resulting in a 17e coordinatively unsaturated metal
radical, as shown in eq 2. In several cases, however,
(CO) M- • dO
-CO
(b) Similarly, 0.10 g of 1 and 0.04 g (0.20 mmol) of AgBF4
were placed in 30 mL of dry acetonitrile and the mixture was
refluxed for 24 h. Again, spectral data obtained during this
time, or after the reaction period, showed no evidence for
conversion of 1.
(c) Compound 1 0.10 g, 0.17 mmol) was dissolved in 20 mL
of acetonitrile and the solution was refluxed for 3 days. The
solution was concentrated to a small volume and then con-
centrated aqueous NH4PF6 was added (to effect anion exchange
if needed). The 1H NMR spectrum of the crude product showed
•
[M(CO)n]•
[M(CO)n-1
]
a
{
}
(2)
C
[
]
n-1
18e
+CO
19e
17e
the 18e acyl radical has been intercepted by H-atom
abstraction and conversion to a formyl complex.
We suggest that metal acyl radicals, present in small
concentrations, play an active role in the halide labili-
zations which occur under photochemical and electro-
chemical reactions with the diimine complexes. As in
our nucleophilic additions to compound 1, the driving
force for halide loss in these reactions is suggested to
be due, in part, to the conversion of a terminal carbonyl
ligand (π-acceptor) to a σ-donor acyl ligand.
only unreacted 1.
ci s,ci s-(C H 3C N )(C O )(t p y )R u (C O 2)R u (t p y )(C O )2
2+
-
2P F 6- (3). (a) A 0.30 g (0.53 mmol) amount of 1 was dissolved
in ca. 30 mL of CH3CN, and 1 mL of saturated aqueous Na2CO3
was added. The mixture was stirred for 2 h, and the solvent
was removed under vacuum. The residue was extracted with
CH3CN and filtered. Ether was added to the filtrate to effect
the precipitation of 0.23 g (77% yield) of a deep red solid, mp
>240 °C. Although this experiment was conducted under
laboratory lights, a similar experiment conducted in the dark
gave the same results after the same time.
Exp er im en ta l Section
Gen er a l d a ta . Reagent grade solvents dichloromethane,
diethyl ether, ethanol, acetone and acetonitrile were used as
received. Acetone-d6 and CD3CN were obtained from Cam-
bridge Isotope Laboratories. RuCl3‚nH2O was purchased from
Pressure Chemical Co., and 2,2′:6′,2′′-terpyridyl (tpy) was
purchased from Aldrich or Strem Chemicals, Inc.; Et4NCl
(monohydrate), TlPF6, and AgBF4 were obtained from Aldrich.
[Ru(CO)2Cl2]n2 and cis-Ru(bpy)2(CO)(COOH)+PF6- 17 were pre-
pared as described previously. Spectral data were obtained
on the following instruments: NMR, Bruker AMX-500; FTIR,
Mattson RS1. Diffuse-reflectance FTIR data were obtained
on the Mattson instrument with a DRIFTS accessory (Graseby
Specac Inc., “Mini-Diff”) as KCl dispersions.19 1H and 13C NMR
chemical shifts were referenced to residual protons in the
Anal. Calcd for C36H25F12N7O5P2Ru2: C, 38.34; H, 2.23.
Found: C, 38.42; H, 2.36. IR (DRIFTS, KCl): νCO 2069, 2013,
and 1953 cm-1; νOCO 1503 and 1177 cm-1; νCN 2313 and 2283
1
cm-1. H NMR (CD3CN): δ 8.49 (t, J HH ) 8.0 Hz), 8.37-7.17
(m). 13C NMR (CD3CN): δ 204.25, 198.23, 195.38, 190.18,
157.90-122.26 (16 resonances for 30 carbons).
(b) A 0.20 g (0.34 mmol) amount of 4 was dissolved in ca.
30 mL of CH3CN, and 1 mL of saturated aqueous Na2CO3 was
added. The mixture was stirred for 1.5 h, and the solvent was
removed under vacuum. The residue was extracted with
CH3CN and the extract filtered. Ether was added to the
filtrate to effect the precipitation of 0.15 g (77% yield) of a deep
red solid whose spectral properties were identical with those
of 3.
deuterated solvents. Melting points were obtained on
a
Thomas-Hoover capillary melting point apparatus and are
uncorrected. Elemental analyses were performed by Midwest
Microlab, Indianapolis, IN.
-
cis-Ru (tp y)(CO)2(OCHO)+P F 6 (4). 0.10 g (0.18 mmol)
-
cis-Ru (tp y)(CO)2Cl+P F 6 (1). (a) A 1.0 g (4.4 mmol)
amount of 1 was dissolved in ca. 20 mL of CH3CN, and 1 mL
of saturated aqueous Na(OCHO) was added. The mixture was
stirred for 1 h, and the solvent was removed under vacuum.
The residue was extracted with CH3CN and filtered. Ether
was added to the filtrate to effect precipitation of 0.09 g (91%
yield) of a salmon-colored solid, mp 210 °C dec.
amount of [Ru(CO)2Cl2]n and 1.1 g (4.7 mmol) of 2,2′:6′,2′′-
terpyridine were introduced into a flask containing ca. 60 mL
of H2O/EtOH (1:1, v:v) and the mixture was refluxed for 3 h.
(18) See: (a) Narayanan, B. A.; Kochi, J . K. Organometallics 1986,
5, 926. (b) Wayland, B. B.; Sherry, A. E.; Coffin, V. L. In Homogeneous
Transition Metal Catalyzed Reactions; Moser, W. R., Slocum, D. W.,
Eds.; Adv. Chem. Ser. 230; American Chemical Society: Washington,
DC, 1992; Chapter 17. (c) Astruc, D. Electron Transfer and Radical
Processes in Transition Metal Chemistry; VCH: New York, 1995;
Chapters 5 and 6, and references cited therein.
Anal. Calcd for C18H12F6N3O4PRu: C, 37.25; H, 2.08.
Found: C, 37.33; H, 2.09. IR (DRIFTS, KCl): νCO 2066 and
2004 cm-1; νOCO 1618 and 1291 cm-1
.
1H NMR (CD3CN): δ
8.76 (d, J HH ) 5.0 Hz), 8.48-7.63 (m). 13C NMR (CD3CN): δ
195.61, 190.68, 167.89, 158.74-125.33 (8 resonances for 15
carbons).
(19) Griffiths, P. W.; deHaseth, J . A. Fourier Transform Infrared
Spectroscopy; Wiley: New York, 1986; Chapter 5.

CO2-Bridged and Formate Complexes of Ru
Organometallics, Vol. 16, No. 20, 1997 4427
Con ver sion of 4 to 1. A 0.22 g (0.38 mmol) amount of 4
was dissolved in ca. 20 mL of acetonitrile; 0.40 g (2.41 mmol)
of Et4Cl (monohydrate) and 0.5 mL of saturated NaOCHO
were added, and the solution was stirred for 5 min. The
solution was then concentrated to 5 mL, and concentrated
aqueous NH4PF6 was added to effect anion exchange. 1H NMR
analysis of the resulting precipitate showed that it consisted
entirely of 1; the isolated yield of 1 after crystallization from
CH3CN/ether was 0.17 g (78%). Also, a similar experiment
conducted in the dark gave the same results. However, a
similar experiment conducted without NaOCHO afforded no
conversion of 4 to 1.
(SIR9220) and refined with anisotropic thermal parameters for
all non-hydrogen atoms. The calculated positions and thermal
parameters for the H atoms were kept constant with temper-
ature factors set to 1.2 times those of the carbon atoms to
which they are bonded. A final R index of 0.027 with Rw
)
0.034 was obtained for 433 variables. All computations were
performed using the teXsan21 package (Molecular Structure
Corp.).
X-r a y Cr ysta l Str u ctu r e of 7. A suitable crystal was
grown by slow diffusion of ether into a CH3CN solution of 7.
Data were collected on an Enraf-Nonius CAD4 diffractometer
at 23 °C; the crystallographic data are outlined in Table 1.
Selected bond distances and bond angles are shown in Table
3. Of 8139 unique reflections, 6976 were considered observed
(I > 3σ(I)). The structure was solved using direct methods
(SIR9220) and refined with anisotropic thermal parameters for
all non-hydrogen atoms except for one disordered PF6 anion;
the structure included a partial (3/4) occupancy CH3CN solvate
-
cis,cis-(CO)2(t p y)R u (CO2)R u (t p y)(CO)22+2P F 6 (5).
A
0.041 g (0.036 mmol) amount of 3 was added to ca. 30 mL of
CO-saturated CH2Cl2, and the mixture was stirred, with
continuous bubbling of CO, for 1.5 h. Ether was then added
to effect the precipitation of 0.033 g (83% yield) of a light brown
solid, mp 165 °C dec.
molecule. The PF6 disorder (rotational) was modeled using
Anal. Calcd for C35H22F12N6O6P2Ru2: C, 37.71; H, 1.99.
3
/
occupancy for four fluorine atoms which were refined
4
Found: C, 37.83; H, 2.09. IR (DRIFTS, KCl): νCO 2071, 2054,
1
anisotropically and completed using a second set of four
/
4
2016, and 1992 cm-1; νOCO 1533 and 1198 cm-1
.
1H NMR
occupancy fluorine atoms that were refined isotropically. The
calculated positions and thermal parameters for the H atoms
were kept constant with temperature factors set to 1.2 times
those of the carbon atoms to which they are bonded. A final
R index of 0.040 with Rw ) 0.054 was obtained for 636
variables. All computations were performed using the teX-
san21 package (Molecular Structure Corp.).
(acetone-d6): δ 8.71-8.66 (m), 8.55 (d, J HH ) 6.5 Hz), 8.46 (d,
J HH ) 5.5 Hz), 8.33-7.44 (m). 13C NMR (acetone-d6): δ 199.64,
197.57, 195.34, 190.38, 186.28, 158.02-123.35 (16 resonances
for 30 carbons).
-
cis,cis-(CO)(bp y)2Ru (CO2)Ru (tp y)(CO)22+2P F 6 (7). A
-
0.20 g (0.31 m m ol) am ou n t of cis-Ru(bpy)2(CO)(COOH)+PF6
(6) was dissolved in ca. 20 mL of CH3CN, followed by addition
of 1 mL of saturated aqueous Na2CO3. A 0.18 g (0.31 mmol)
amount of 1 was dissolved in a minimum amount of CH3CN,
and this solution was added dropwise over ca. 30 min to the
vigorously stirred mixture. The reaction mixture was allowed
to stir for an additional 1 h, and the solvent was removed
under vacuum. The resultant residue was extracted with
CH3CN and filtered. Ether was then added to the filtrate to
effect the precipitation of 0.25 g (68% yield) of a red crystalline
solid, mp 210 °C (dec).
Ack n ow led gm en t. Support of this work by the
United States Department of Energy, Division of Chemi-
cal Sciences (Office of Basic Energy Sciences), Office of
Energy Research, is gratefully acknowledged. The
X-ray equipment was purchased with assistance from
the National Science Foundation (Grant No. CHE-
9016978). Fellowship support to B.A.S. from the United
States Department of Education through a GAANN
grant is also gratefully acknowledged.
Anal. Calcd for C39H27F12N7O5P2Ru2: C, 40.18; H, 2.33.
Found: C, 40.19; H, 2.42. IR (DRIFTS, KCl): νCO 2059, 2000,
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, positional and isotropic thermal parameters,
anisotropic thermal parameters, H atom positional param-
eters, bond distances, and bond angles for 1a and 7 (29 pages).
Ordering information is given on any current masthead page.
and 1940 cm-1; νOCO 1497 and 1186 or 1174 cm-1
.
1H NMR
(CD3CN): δ 9.48 (d, J HH ) 5.5 Hz), 8.60-6.72 (m). 13C NMR
(CD3CN): δ 202.72, 202.48, 198.73, 190.36, 158.28-122.91 (35
inequivalent carbons).
X-r a y Cr ysta l Str u ctu r e of 1a . A suitable crystal was
grown by slow diffusion of ether into a CH3CN solution of 1a .
Data were collected on an Enraf-Nonius CAD4 diffractometer
at 23 °C; the crystallographic data are outlined in Table 1.
Selected bond distances and bond angles are shown in Table
2. Of 6079 unique reflections, 5571 were considered observed
(I > 3σ(I)). The structure was solved using direct methods
OM970493P
(20) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J . Appl. Crystallogr. 1994, 27,
435.
(21) teXsan: Single Crystal Structure Analysis Software, Version
1.6 (1993); Molecular Structure Corp., The Woodlands, TX 77381.