A study of unsaturated pyrazolate-bridged diruthenium carbonyl complexes

Article abstract of DOI:10.1021/om020445r

The reaction of Ru₃(CO)₁₂ with approximately 3 equiv of 3,5-di-tert-butylpyrazole, (dbpz)H, in hexane at 170 °C in a stainless steel autoclave afforded the unsaturated, pyrazolate-bridged diruthenium complex [Ru₂(CO)₅(dbpz)₃] (2). The spectroscopic and structural analysis indicated that this compound contains a vacant coordination site at one axial position trans to the Ru-Ru linkage. This compound reacted with CO at room temperature to give the saturated complex [Ru₂(CO)₆(dbpz)₂] (1b); its structure is analogous to that of the previously determined complexes [Ru₂(CO₆(pz)₂], pz = pyrazolate and 3,5-dimethyl pyrazolate, except that the equatorial CO ligands show a skew arrangement, an indication of steric constraints within the coordination sphere. Treatment of 2 with benzyl isocyanide or with pyridine in toluene leads to the formation of substitution products [Ru₂(CO)₄(L)(dbpz)₂], L = CNCH₂Ph and pyridine, in which the incoming donor ligand occupies an equatorial position at the saturated Ru center. The structural significance of this pyrazolate-bridged diruthenium complex 2 is discussed.

Full text of DOI:10.1021/om020445r

Organometallics 2002, 21, 4735-4742
4735
A Stu d y of Un sa tu r a ted P yr a zola te-Br id ged Dir u th en iu m
Ca r bon yl Com p lexes
Yi-Hwa Song,† Yun Chi,*,† Yao-Lun Chen,† Chao-Shiuan Liu,*,† Wei-Li Ching,†
Arthur J . Carty,*,‡ Shie-Ming Peng,§ and Gene-Hsiang Lee§
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan,
Steacie Institute for Molecular Sciences, National Research Council of Canada,
100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada, and Department of Chemistry and
Instrumentation Center, National Taiwan University,
Taipei 10764, Taiwan, Republic of China
Received J une 4, 2002
The reaction of Ru3(CO)12 with approximately 3 equiv of 3,5-di-tert-butylpyrazole, (dbpz)H,
in hexane at 170 °C in a stainless steel autoclave afforded the unsaturated, pyrazolate-
bridged diruthenium complex [Ru2(CO)5(dbpz)2] (2). The spectroscopic and structural analysis
indicated that this compound contains a vacant coordination site at one axial position trans
to the Ru-Ru linkage. This compound reacted with CO at room temperature to give the
saturated complex [Ru2(CO)6(dbpz)2] (1b); its structure is analogous to that of the previously
determined complexes [Ru2(CO)6(pz)2], pz ) pyrazolate and 3,5-dimethyl pyrazolate, except
that the equatorial CO ligands show a skew arrangement, an indication of steric constraints
within the coordination sphere. Treatment of 2 with benzyl isocyanide or with pyridine in
toluene leads to the formation of substitution products [Ru2(CO)4(L)(dbpz)2], L ) CNCH2Ph
and pyridine, in which the incoming donor ligand occupies an equatorial position at the
saturated Ru center. The structural significance of this pyrazolate-bridged diruthenium
complex 2 is discussed.
The chemistry of metal complexes with bridging
pyrazolate ligands has attracted great attention because
pyrazolate ligands are known to be strongly bond
bridging ligands and are capable of holding two metal
fragments in close proximity and in chemically stable
configurations.1 Such properties often enhance the
cooperative interaction between two metal atoms and
are of importance in homogeneous catalysis by bimetal-
lic systems.2
Since the first discovery of the diruthenium pyra-
zolate complexes [Ru2(CO)6(pz)2], pz ) pyrazolate and
3,5-dimethylpyrazolate, about a decade ago,3 relatively
few investigations have appeared in the literature, and
most of the documented studies deal with fundamental
chemical reactions such as oxidations and phosphine
substitutions.4 In contrast, studies of the related car-
boxylate complexes [Ru2(CO)6(O2CR)], R ) Me, Ph, have
attracted much more attention because of their latent
applications in the catalytic reactions.5 Apparently, the
activity of these bimetallic systems is provided by the
generation of a vacant coordination site on one metal
at the reaction temperature, which allows the organic
substrate to interact with the metal center and initiate
the catalytic cycle. Coincident with this result, it was
reported that the carboxylate complexes [Ru2(CO)6(µ-
O2CR)2] alone spontaneously form the polymeric com-
pound [Ru2(CO)4(µ-O2CMe)2]n at ambient temperature6
and in the presence of Lewis bases afford the corre-
sponding donor-stabilized complexes [Ru2(CO)4(µ-O2-
CMe)2(L)2] with L being an N-, P-, and S-donor ligand.7
* Corresponding authors.
(4) (a) Cabeza, J . A.; Landazuri, C.; Oro, L. A.; Belletti, D.;
Tiripicchio, A.; Tiripicchio Camellini, M. J . Chem. Soc., Dalton Trans.
1989, 1093. (b) Neumann, F.; Suess-Fink, G. J . Organomet. Chem.
1989, 367, 175. (c) Neumann, F.; Stoeckli-Evans, H.; Suess-Fink, G.
J . Organomet. Chem. 1989, 379, 151. (d) Shiu, K.-B.; Lee, W.-M.; Wang,
C.-L.; Wang, S.-L.; Liao, F.-L.; Wang, J .-C.; Liou, L.-S.; Peng, S.-M.;
Lee, G.-H.; Chiang, M. Y. Organometallics 1996, 15, 2979.
(5) (a) Frediani, P.; Bianchi, M.; Salvini, A.; Piacenti, F. J . Chem.
Soc., Dalton Trans. 1990, 3663. (b) Frediani, P.; Faggi, C.; Salvini, A.;
Bianchi, M.; Piacenti, F. Inorg. Chim. Acta 1998, 272, 141. (c) Funaioli,
T.; Fachinetti, G.; Marchetti, F. Chem. Commun. 1999, 2043. (d)
Salvini, A.; Frediani, P.; Piacenti, F. J . Mol. Catal. A 2000, 159, 185.
(6) Crooks, G. R.; J ohnson, B. F. G.; Lewis, J .; Williams, I. G. J .
Chem. Soc. (A) 1969, 2761.
(7) (a) Schumann, H.; Opitz, J .; Pickardt, J . J . Organomet. Chem.
1977, 128, 253. (b) Sherlock, S. J .; Cowie, M.; Singleton, E.; Steyn, M.
M. de V. Organometallics 1988, 7, 1663. (c) Hilts, R. W.; Sherlock, S.
J .; Cowie, M.; Singleton, E.; Steyn, M. M. d. V. Inorg. Chem. 1990, 29,
3161. (d) Matteoli, U.; Menchi, G.; Bianchi, M.; Piacenti, F.; Ianelli,
S.; Nardelli, M. J . Organomet. Chem. 1995, 498, 177. (e) Kepert, C.
M.; Deacon, G. B.; Spiccia, L.; Fallon, G. D.; Skelton, B. W.; White, A.
H. Dalton 2000, 2867.
† National Tsing Hua University.
‡ Steacie Institute for Molecular Sciences.
§ National Taiwan University.
(1) (a) La Monica, G.; Ardizzoia, G. A. Prog. Inorg. Chem. 1997, 46,
151. (b) Bushnell, G. W.; Fjeldsted, D. O. K.; Stobart, S. R.; Zaworotko,
M. J .; Knox, S. A. R.; MacPherson, K. A. Organometallics 1985, 4, 1107.
(c) Ardizzoia, G. A.; La Monica, G.; Maspero, A.; Moret, M.; Masciocchi,
N. Inorg. Chem. 1997, 36, 2321. (d) Tejel, C.; Bordonaba, M.; Ciriano,
M. A.; Edwards, A. J .; Clegg, W.; Lahoz, F. J .; Oro, L. A. Inorg. Chem.
1999, 38, 1108. (e) Barbera, J .; Elduque, A.; Gimenez, R.; Lahoz, F.
J .; Lopez, J . A.; Oro, L. A.; Serrano, J . L.; Villacampa, B.; Villalba, J .
Inorg. Chem. 1999, 38, 3085.
(2) (a) Contreras, R.; Valderrama, M.; Orellana, E. M.; Boys, D.;
Carmona, D.; Oro, L. A.; Lamata, M. P.; Ferrer, J . J . Organomet. Chem.
2000, 606, 197. (b) Carmona, D.; Ferrer, J .; Arilla, J . M.; Reyes, J .;
Lahoz, F. J .; Elipe, S.; Modrego, J .; Oro, L. A. Eur. J . Inorg. Chem.
2000, 159.
(3) (a) Cabeza, J . A.; Landazuri, C.; Oro, L. A.; Tiripicchio, A.;
Tiripicchio-Camellini, M. J . Organomet. Chem. 1987, 322, C16. (b)
Cabeza, J . A.; Ferna´ndez-Colinas, J . M. Coord. Chem. Rev. 1993, 126,
319.
10.1021/om020445r CCC: $22.00 © 2002 American Chemical Society
Publication on Web 10/01/2002

4736 Organometallics, Vol. 21, No. 22, 2002
Song et al.
Syn th esis of Com p lex 1b. A red-orange solution of [Ru2-
(CO)5(dbpz)2] (100 mg, 0.143 mmol) in 5 mL of CH2Cl2 was
first placed in a 10 mL sample vial which was capped with a
rubber septum. The vial was purged with a slow stream of
carbon monoxide, during which time a yellow solution was
rapidly obtained. The solution was then concentrated by
passing a relatively fast flow of CO. Addition of MeOH over
the top of the concentrated solution resulted in the formation
of the yellow crystalline complex [Ru2(CO)6(dbpz)2] (1b, 98 mg,
0.134 mmol) in 95% yield.
However, for the pyrazolate complexes [Ru2(CO)6(pz)2],
none of these reaction patterns have been observed, and
this reduced reactivity explains the fact that no catalytic
reactions have been reported.
In this study, we wish to report the synthesis and
structural characterization of two new pyrazolate com-
plexes, [Ru2(CO)6(dbpz)2] (1b) and [Ru2(CO)5(dbpz)2] (2),
dbpz ) 3,5-di-tert-butylpyrazolate. The first one is
structurally related to the pyrazolate complexes re-
ported in the literature and contains 34 valence elec-
trons, while the second shows a similar metal frame-
work, but with a vacant site at one of the axial positions,
and contains only 32 valence electrons. This experimen-
tal result provides strong structural and chemical
evidence for the reaction intermediate involved in
activation of substrates by the diruthenium pyrazolate
complexes. We hope that these discoveries will provide
an incentive for the further exploration of pyrazolate
complexes [Ru2(CO)6(pyrazolate)2] as possible catalysts
for organic synthesis.
Sp ectr a l Da ta of 1b. IR (CH2Cl2): ν(CO), 2088 (s), 2054
1
(s), 2011 (s), 1993 (m) cm-1. H NMR (400 MHz, CDCl3, 294
K): δ 5.84 (s, 2H, CH), 1.39 (s, 36H, Me). 13C NMR (100 MHz,
CDCl3, 298 K): δ 200.2 (4C, CO), 186.4 (2C, CO), 159.6 (4C,
CN), 102.3 (2C, CH), 31.5 (4C, CMe3), 31.2 (12C, Me).
Syn th esis of Com p lex 3a . The complex [Ru2(CO)5(dbpz)2]
(57 mg, 0.081 mmol) was first dissolved in 30 mL of toluene.
To this was added dropwise 10 µL of benzyl isocyanide (0.085
mmol) using a microsyringe. The mixture was stirred at room
temperature for 5 min, during which time the color changed
from red to light yellow. The temperature was then increased
to 40 °C, and stirring was continued for 4 h. For the workup,
the solvent was first removed in vacuo and the oily residue
separated by column chromatography (hexane/CH2CH2, 1:1),
giving a yellow-orange material. Recrystallization from a
mixture of CH2Cl2 and methanol at room temperature afforded
45 mg of orange crystalline solid [Ru2(CO)4(CNCH2Ph)(dbpz)2]
(3a , 0.057 mmol. 70%).
Exp er im en ta l Section
Gen er a l In for m a tion a n d Ma ter ia ls. Mass spectra were
obtained on a J EOL SX-102A instrument operating in electron
1
impact (EI) mode or fast atom bombardment (FAB) mode. H
and 13C NMR spectra were recorded on Varian Mercury-400
or INOVA-500 instruments; chemical shifts are quoted with
Sp ectr a l Da ta of 3a . MS (EI, 102Ru): m/z 762, (M - CO)+.
IR (CH2Cl2): ν(CN), 2202 (m); ν(CO), 2039 (m), 1989 (s), 1973
1
(m), 1914 (m) cm-1 1H NMR (400 MHz, CDCl3, 294 K): δ
.
respect to the internal standard tetramethylsilane for H and
13C NMR data. Elemental analyses were carried out at the
NSC Regional Instrumentation Center at National Cheng
Kung University, Tainan, Taiwan. The pyrazole ligand 3,5-
di-tert-butylpyrazole, (dbpz)H, was prepared according to the
method reported in the literature.8 All reactions were per-
formed in air using anhydrous solvents or solvents treated with
an appropriate drying reagent.
7.30-7.28 (m, 3H, C6H5), 6.93-6.91 (m, 2H, C6H5), 5.90 (s, 1H,
2
CH(pz)), 5.80 (s, 1H, CH(pz)), 4.90 (d, 1H, J HH ) 16.6 Hz, CH2),
2
4.70 (d, 1H, J HH ) 16.6 Hz, CH2), 1.39 (s, 9H, Me), 1.33 (s,
9H, Me), 1.27 (s, 9H, Me), 1.24 (s, 9H, Me). 13C NMR (100 MHz,
CDCl3, 298 K): δ 202.6 (1C, CO), 201.7 (1C, CO), 200.7 (1C,
CO), 181.3 (1C, CO), 160.6 (2C, CN(pz)), 160.4 (2C, CN(pz)), 147.7
(1C, CN), 131.6 (1C, C6H5), 129.0 (2C, C6H5), 128.5 (1C, C6H5),
126.7 (2C, C6H5), 99.9 (2C, CH), 48.5 (1C, CH2), 32.3 (3C, Me),
32.2 (3C, Me), 32.1 (1C, CMe3), 31.5 (1C, CMe3), 30.9 (6C, Me),
30.8 (1C, CMe3), 30.7 (1C, CMe3). Anal. Calcd for C34H45N5O4-
Ru2: C, 51.70; N, 8.87; H, 5.74. Found: C, 51.4; N, 8.65; H,
5.76.
Syn th esis of Com p lex 1a . A 160 mL stainless steel
autoclave was charged with 0.25 g of (dmpz)H (2.60 mmol),
0.5 g of Ru3(CO)12 (0.78 mmol), and 50 mL of hexane. The
autoclave was sealed, and the mixture was heated to 180 °C
for 24 h. The hexane solvent was then evaporated under
vacuum, giving a light yellow material. Recrystallization from
a mixture of CH2Cl2 and methanol at room temperature
afforded 0.47 g of a light yellow crystalline solid [Ru2(CO)6-
(dmpz)2] (1a , 0.84 mmol, 72%). Its spectral data are identical
to that reported in the literature.9
Syn th esis of Com p lex 3b. The complex [Ru2(CO)5(dbpz)2]
(100 mg, 0.143 mmol) was first dissolved in 30 mL of toluene.
To this was added 57 µL of pyridine (0.72 mmol) using a
microsyringe. The mixture was refluxed for 18 h, during which
time the color gradually changed from red to orange. The
solvent was then removed under vacuum to give an orange
material. Recrystallization from a mixture of CH2Cl2 and
methanol at room temperature afforded 102 mg of orange
crystalline solid [Ru2(CO)4(py)(dbpz)2] (3b, 0.136 mmol. 95%).
Sp ectr a l Da ta of 3b. MS (EI, 102Ru): m/z 724, (M - CO)+.
Syn th esis of Com p lex 2. A 160 mL stainless steel auto-
clave was charged with 0.85 g of (dbpz)H (4.7 mmol), 1.0 g of
Ru3(CO)12 (1.56 mmol), and 50 mL of hexane. The autoclave
was sealed and the mixture heated to 170 °C for 24 h. The
hexane solvent was then evaporated under vacuum, and the
residue was purified using vacuum sublimation at 110 °C and
50 mTorr, giving 1.18 g of a red-orange complex [Ru2(CO)5-
(dbpz)2] (2, 1.68 mmol, 75%). Single crystals suitable for X-ray
diffraction study were obtained by recrystallization from a
mixture of CH2Cl2 and methanol at room temperature.
Sp ectr a l Da ta of 2. MS (FAB, 102Ru): m/z 673 (M - CO)+.
IR (CH2Cl2): ν(CO), 2093 (s), 2026 (m), 2012 (s), 1998 (m), 1932
IR (CH2Cl2): ν(CO), 2024 (s), 1981 (s), 1947 (m), 1908 (m) cm-1
.
3
1H NMR (400 MHz, CDCl3, 294 K): δ 7.58 (t, 1H, J HH ) 7.6
3
Hz, CH(py)), 7.40 (d, 2H, J HH ) 5.2 Hz, CH(py)), 7.05 (t, 2H,
3J HH ) 7.1 Hz, CH(py)), 6.04 (s, 1H, CH(pz)), 5.72 (s, 1H, CH(pz)),
1.43 (s, 9H, Me), 1.41 (s, 9H, Me), 1.24 (s, 9H, Me), 1.22 (s,
9H, Me). 13C NMR (100 MHz, CDCl3, 298 K): δ 206.7 (1C, CO),
202.2 (1C, CO), 201.7 (1C, CO), 184.7 (1C, CO), 161.7 (1C,
CN(pz)), 161.4 (1C, CN(pz)), 161.1 (1C, CN(pz)), 160.3 (1C, CN(pz)),
155.6 (2C, CN(py)), 137.3 (1C, CH(py)), 124.9 (2C, CH(py)), 101.3
(1C, CH), 100.5 (1C, CH), 32.5 (3C, Me), 32.0 (3C, Me), 31.7
(1C, CMe3), 31.5 (1C, CMe3), 31.1 (3C, Me), 31.0 (3C, Me), 30.6
(1C, CMe3), 30.4 (1C, CMe3). Anal. Calcd for C31H43N5O4Ru2:
C, 49.52; N, 9.31; H, 5.76. Found: C, 49.61; N, 9.27; H, 5.81.
Syn th esis of Com plex 4a. An orange solution of [Ru2(CO)4-
(CNCH2Ph)(dbpz)2] (100 mg, 0.122 mmol) in 5 mL of CH2Cl2
was first placed in a 10 mL sample vial which was capped with
a rubber septum. The vial was purged with a slow stream of
carbon monoxide, during which time a yellow solution was
1
(m) cm-1. H NMR (400 MHz, CDCl3, 294 K): δ 5.87 (s, 2H,
CH), 1.39 (s, 18H, Me), 1.26 (s, 18H, Me). 13C NMR (100 MHz,
CDCl3, 298 K): δ 200.3 (2C, CO), 196.8 (2C, CO), 176.1 (1C,
CO), 161.0 (4C, CN), 100.6 (2C, CH), 32.1 (6C, Me), 31.5 (2C,
CMe3), 31.0 (6C, Me), 30.8 (2C, CMe3). Anal. Calcd for
C
27H38N4O5Ru2: C, 46.28; N, 8.00; H, 5.47. Found: C, 46.05;
N, 7.93; H, 5.46.
(8) (a) Renn, O.; Vananzi, L. M.; Marteletti, A.; Gramlich, V. Helv.
Chim. Acta 1995, 78, 993. (b) Fernandez-Castano, C.; Foces-Foces, C.;
J agerovic, N.; Elguero, J . J . Mol. Struct. 1995, 355, 265.
(9) Cabeza, J . A.; Oro, L. A. Inorg. Synth. 1997, 31, 217.

Pyrazolate-Bridged Diruthenium Carbonyl Complexes
Organometallics, Vol. 21, No. 22, 2002 4737
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t P a r a m eter s for Com p lexes 1b, 2, a n d 3a
2
1b
3a
empirical formula
fw
C
27H38N4O5Ru2
C
28H38N4O6Ru2
C
34H45N5O4Ru2
700.75
728.76
789.89
diffractometer
temperature
cryst syst
space group
a
b
c
â
Bruker SMART CCD
150(1) K
monoclinic
P2(1)/n
9.9430(3) Å
Nonius Kappa
150(1) K
monoclinic
Nonius Kappa
150(1) K
monoclinic
P2(1)/c
P2(1)/n
20.1549(2) Å
19.3637(2) Å
16.8350(2) Å
106.8017(4)
12.1266(2) Å
15.8058(3) Å
18.7928(4) Å
99.1720(8)°
20.8381(7) Å
14.8887(5) Å
95.790(1)°
volume, Z
density(calcd)
abs coeff
F(000)
3069.10(17) Å3, 4
1.517 Mg/m3
1.024 mm-1
6289.77(12) Å3, 8
1.539 Mg/m3
1.005 mm-1
3555.97(12) Å3, 4
1.475 Mg/m3
0.892 mm-1
1424
2960
1616
cryst size (mm3)
θ ranges
index ranges
0.30 × 0.30 × 0.12
1.69 to 27.5°
-12 < h < 11,
-26 < k < 27,
-18 < l < 19
24 580
7038 [R(int) ) 0.0366]
7038/0/343
1.064
0.40 × 0.32 × 0.10
2.10 to 27.50°
-26 < h < 26,
-25< k < 24,
-21 < l < 21
51 149
14435 [R(int) ) 0.0399]
14 435/0/722
1.244
0.30 × 0.25 × 0.17
1.87 to 27.50°
-15 < h < 15,
-20 < k < 20,
-24 < l < 24
27 604
8148 [R(int) ) 0.0533]
8148/0/419
1.174
no. of reflns collected
no. of ind reflns
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I>2σ(I)]
R indices (all data)
R1 ) 0.0293, wR2 ) 0.0579
R1 ) 0.0498, wR2 ) 0.0611
1.800 and -0.303 e‚Å-3
R1 ) 0.0391, wR2 ) 0.0952
R1 ) 0.0587, wR2 ) 0.1169
1.969 and -1.916 e‚Å-3
R1 ) 0.0431, wR2 ) 0.0981
R1 ) 0.0717, wR2 ) 0.1158
1.260 and -1.068 e‚Å-3
largest diff peak and hole
Ta ble 2. Selected Bon d Len gth s [Å] a n d An gles
[d eg] for Com p lex 2
obtained. The solution was then concentrated by passing a fast
flow of CO. The IR and 1H and 13C NMR analysis showed
quantitative conversion to the addition product [Ru2(CO)5-
(CNCH2Ph)(dbpz)2] (4a ), but the solution was found to revert
back to the starting material 3a within several seconds upon
purging with nitrogen.
Bond Lengths and Angles
Ru(1)-Ru(2)
Ru(1)-C(2)
2.6513(3)
1.947(3)
1.863(3)
2.146(2)
2.094(2)
1.380(3)
87.75(8)
172.95(9)
Ru(1)-C(1)
Ru(1)-C(3)
Ru(2)-C(5)
Ru(1)-N(3)
Ru(2)-N(4)
N(3)-N(4)
1.911(3)
1.908(3)
1.874(3)
2.131(2)
2.109(2)
1.381(3)
87.62(8)
Ru(2)-C(4)
Ru(1)-N(1)
Sp ectr a l Da ta of 4a . IR (CH2Cl2): ν(CN), 2186 (m); ν(CO),
Ru(2)-N(2)
2058 (s), 2014 (s), 1988 (m), 1978 (m), 1959 (m) cm-1. 1H NMR
N(1)-N(2)
N(1)-Ru(1)-N(3)
Ru(2)-Ru(1)-C(2)
N(2)-Ru(2)-N(4)
(400 MHz, CDCl3, 294 K): δ 7.30 (m, 3H, C6H5), 6.92 (m, 2H,
2
C6H5), 5.87 (s, 1H, CH), 5.79 (s, 1H, CH), 4.90 (d, 1H, J HH
)
16.7 Hz, CH2), 4.77 (d, 1H, 2J HH ) 16.7 Hz, CH2), 1.42 (s, 18H,
Me), 1.37 (s, 9H, Me), 1.26 (s, 9H, Me). 13C NMR (100 MHz,
CDCl3, 298 K): δ 203.9 (1C, CO), 202.5 (1C, CO), 200.6 (1C,
CO), 190.3 (1C, CO), 186.8 (1C, CO), 159.0 (2C, CN), 158.9
(2C, CN), 151.5 (1C, CN), 131.9 (1C, C6H5), 129.0 (2C, C6H5),
128.4 (1C, C6H5), 126.5 (2C, C6H5), 101.4 (2C, CH), 48.5 (1C,
CH2), 31.6-30.8 (4C, CMe3; 12C, Me).
Torsional Angles
C(1)-Ru(1)-Ru(2)-C(4) 0.42(13) C(3)-Ru(1)-Ru(2)-C(5) 2.25(13)
SHELXTL/PC program and refined using full-matrix least
squares. All non-hydrogen atoms were refined anisotropically,
whereas hydrogen atoms were placed at the calculated posi-
tions and included in the final stage of refinements with fixed
parameters. Crystallographic refinement parameters of com-
plexes 2, 1b, and 3a are summarized in Table 1, and the
selective bond distances and angles of these complexes are
listed in Tables 2-4, respectively.
Syn th esis of Com p lex 4b. An orange solution of [Ru2(CO)4-
(py)(dbpz)2] (50 mg, 0.067 mmol) in 5 mL of CH2Cl2 was first
placed in a 10 mL sample vial which was capped with a rubber
septum. The vial was purged with a slow stream of CO gas,
and a yellow solution was obtained. The solution was then
concentrated by purging the solution with a fast flow of carbon
Resu lts a n d Discu ssion
1
monoxide. The H and 13C NMR analysis showed quantitative
P a r en t P yr a zola te Com p lexes. It has been re-
ported that treatment of RuCl3‚nH2O with granular zinc
in the presence of CO and 3,5-dimethylpyrazole, (dmpz)H,
led to the formation of a double pyrazolate-bridged
ruthenium complex [Ru2(CO)6(dmpz)2] (1a ) in 85%
yield.9 Alternatively, the same reaction product can be
isolated in comparable yield via heating of a hexane
solution of Ru3(CO)12 and (dmpz)H in a stainless steel
autoclave (170 °C, 24 h). In this case, the reaction
conditions are akin to those used for the synthesis of
the diosmium pyrazolate complex [Os2(CO)6(hfpz)2], hfpz
) 3,5-bis(trifluoromethyl)pyrazolate, described in the
literature.10 Interestingly, both the literature report and
our experimental observation have revealed that com-
plex 1a does not lose CO even at elevated temperature
and under vacuum.
conversion to the expected addition product [Ru2(CO)5(py)-
(dbpz)2] (4b), but the solution was found to revert back to the
starting material 3b within several seconds upon re-purging
with nitrogen.
1
Sp ectr a l Da ta of 4b. H NMR (400 MHz, CDCl3, 294 K):
3
3
δ 7.53 (t, 1H, J HH ) 7.6 Hz, CH(py)), 7.40 (d, 2H, J HH ) 5.2
3
Hz, CH(py)), 7.03 (t, 2H, J HH ) 7.0 Hz, CH(py)), 5.99 (s, 1H,
CH), 5.70 (s, 1H, CH), 1.45 (s, 9H, Me), 1.43 (s, 9H, Me), 1.27
(s, 9H, Me), 1.21 (s, 9H, Me). 13C NMR (100 MHz, CDCl3, 298
K): δ 210.1 (1C, CO), 203.4 (1C, CO), 202.9 (1C, CO), 188.5
(1C, CO), 186.3 (1C, CO), 161.3 (1C, CN(pz)), 160.6 (1C, CN(pz)),
160.3 (1C, CN(pz)), 159.1 (1C, CN(pz)), 155.3 (2C, CN(py)), 136.8
(1C, CH(py)), 124.9 (2C, CH(py)), 102.3 (1C, CH), 101.4 (1C, CH),
31.8-31.1 (m, 4C, CMe3; 12C, Me).
X-r a y Cr yst a llogr a p h y. Single-crystal X-ray diffraction
data were measured on a Nonius Kappa or a Bruker SMART
CCD diffractometer using λ(Mo KR) radiation (λ ) 0.71073 Å).
The data collection was executed using the SMART program.
Cell refinement and data reduction were exercised using the
SAINT program. The structure was determined using the
(10) Chi, Y.; Yu, H.-L.; Ching, W.-L.; Liu, C.-S.; Chen, Y.-L.; T.-Y.,
C.; Peng, S.-M.; Lee, G.-H. J . Mater. Chem. 2002, 12, 1363.

4738 Organometallics, Vol. 21, No. 22, 2002
Ta ble 3. Selected Bon d Len gth s [Å] a n d An gles [d eg] for Com p lex 1b
Song et al.
Moelcule 1
Ru(1)-Ru(2)
Ru(1)-C(2)
2.6810(3)
1.887(3)
1.899(3)
1.965(3)
2.147(3)
2.126(2)
1.382(3)
87.74(9)
171.65(9)
Ru(1)-C(1)
Ru(1)-C(3)
Ru(2)-C(5)
Ru(1)-N(1)
Ru(2)-N(2)
N(1)-N(2)
1.978(3)
1.893(3)
1.893(3)
2.128(2)
2.153(2)
1.374(3)
Ru(2)-C(4)
Ru(2)-C(6)
Ru(1)-N(3)
Ru(2)-N(4)
N(3)-N(4)
N(1)-Ru(1)-N(3)
Ru(2)-Ru(1)-C(1)
N(2)-Ru(2)-N(4)
Ru(1)-Ru(2)-C(6)
87.89(9)
173.41(9)
Torsional Angles
Moelcule 2
C(2)-Ru(1)-Ru(2)-C(4)
9.30(14)
C(3)-Ru(1)-Ru(2)-C(5)
9.46(15)
Ru(3)-Ru(4)
Ru(3)-C(2A)
Ru(4)-C(4A)
Ru(4)-C(6A)
Ru(3)-N(7)
2.6758(3)
1.901(3)
1.899(3)
1.979(3)
2.159(2)
2.122(2)
1.378(3)
86.56(9)
172.15(9)
Ru(3)-C(1A)
Ru(3)-C(3A)
Ru(4)-C(5A)
Ru(3)-N(5)
Ru(4)-N(6)
N(5)-N(6)
1.970(3)
1.905(3)
1.884(3)
2.126(2)
2.153(2)
1.380(3)
Ru(4)-N(8)
N(7)-N(8)
N(5)-Ru(3)-N(7)
Ru(4)-Ru(3)-C(1A)
N(6)-Ru(4)-N(8)
Ru(3)-Ru(4)-C(6A)
86.76(9)
171.49(9)
Torsional Angles
C(2A)-Ru(3)-Ru(4)-C(4A)
18.07(14)
C(3A)-Ru(3)-Ru(4)-C(5A)
19.82(14)
Ta ble 4. Selected Bon d Len gth s [Å] a n d An gles [d eg] for Com p lex 3a
Bond Lengths and Angles
Ru(1)-Ru(2)
Ru(1)-C(2)
2.6444(4)
1.918(4)
1.852(4)
1.142(4)
2.161(3)
2.098(3)
1.382(4)
88.61(11)
173.35(10)
Ru(1)-C(1)
1.874(4)
1.859(4)
1.967(4)
1.427(5)
2.130(3)
2.133(3)
1.378(4)
86.92(11)
178.3(3)
Ru(2)-C(3)
Ru(2)-C(4)
Ru(1)-C(5)
N(5)-C(6)
Ru(1)-N(3)
Ru(2)-N(4)
N(3)-N(4)
N(2)-Ru(2)-N(4)
Ru(1)-C(5)-N(5)
C(5)-N(5)
Ru(1)-N(1)
Ru(2)-N(2)
N(1)-N(2)
N(1)-Ru(1)-N(3)
Ru(2)-Ru(1)-C(2)
Torsional Angles
C(2)-Ru(1)-Ru(2)-C(4)
10.21(17)
C(3)-Ru(2)-Ru(1)-C(5)
9.28(15)
However, during our attempts to synthesize the
corresponding di-tert-butyl-substituted pyrazolate metal
complex [Ru2(CO)6(dbpz)2] (1b), using the method of
direct treatment of Ru3(CO)12 with 3 equiv of 3,5-di-tert-
butyl-substituted pyrazole (dbpz)H, we obtained an
apparently different, orange-red crystalline material (2)
in excellent yield, which was then purified by vacuum
sublimation, followed by recrystallization from a mix-
ture of CH2Cl2 and methanol.
This complex was characterized using routine spec-
1
troscopic techniques such as FAB mass, IR, and H and
13C NMR spectral methods. The FAB mass spectrum
shows a parent peak at m/z ) 673, corresponding to the
species with a composition of Ru2(CO)4(dbpz)2, which is
two CO ligands less than that expected for the saturated
complex 1b. The IR spectrum exhibits four strong
terminal ν(CO) stretching bands at 2093 (m), 2026 (m),
2012 (s), and 1932 (m) cm-1, again distinctively different
F igu r e 1. ORTEP drawing of complex 2 with thermal
ellipsoids shown at the 50% probability level.
1
from that of the dmpz analogue 1a . Moreover, the H
NMR shows only one CH signal of the pyrazolate ligand
at δ 5.87, together with two tert-butyl signals at δ 1.39
and 1.26, indicating that the molecule does not have the
expected C2v symmetry. Finally, in addition to the
signals of the pyrazolate ligands, three CO signals at δ
200.3, 196.8, and 176.1 with an intensity ratio of 2:2:1
were observed in the 13C NMR spectrum. We concluded
that these spectral data are inconsistent with those of
the expected complex 1b, for which the possession of
two orthogonal mirror planes in the molecule would
produce a highly simplified spectral pattern with only
one signal for the tert-butyl groups in the 1H NMR
spectrum and two CO resonance signals with a 4:2 ratio
in the 13C NMR spectrum.
A single-crystal X-ray diffraction study was then
carried out to reveal the exact molecular structure. As
indicated in Figure 1, the structure of 2 consists of two
ruthenium metal atoms bridged by two pyrazolate
ligands, which are essentially orthogonal to one another.
It is notable that the first ruthenium atom Ru(1) adopts

Pyrazolate-Bridged Diruthenium Carbonyl Complexes
Organometallics, Vol. 21, No. 22, 2002 4739
an octahedral environment consistency of three carbon
atoms from terminal carbonyl ligands, two nitrogen
atoms from the bridging pyrazolate ligands, and the
other ruthenium atom. On the other hand, the second
ruthenium atom Ru(2) shows a distinctive, square
pyramidal geometry, which is apparently produced by
removal of the axial CO ligand at the position opposite
the Ru-Ru linkage. The Ru-Ru separation of 2.6513(3)
Å is comparable with those found in related binuclear
RuI-RuI complexes with two bridging ligands, such as
[Ru(µ-pyO)(CO)2(pyOH)]2, 2.670(1) Å with pyOH )
2-pyridone,11 [Ru(µ-dmpz)(CO)3]2,2 2.705(2) Å with dmpz
) 3,5-dimethylpyrazolate, and [Ru(µ-pz)(CO)2(PPh3)]2,12
2.732(1) Å with pz ) pyrazolate, showing a Ru-Ru
single-bond bonding interaction.
Moreover, for the octahedral Ru(1) metal center, the
Ru-N distances fall in the range 2.146(2)-2.131(2) Å,
but the Ru-CO distances exhibit a small variation in
bond length (1.991(3)-1.908(3) Å), with the equatorial
Ru-CO distance being slightly shorter. Nevertheless,
all these metal-ligand distances are significantly longer
than those of the respective Ru(2)-N distances to the
bridging pyrazolate ligands, 2.109(2) and 2.094(2) Å, and
the corresponding Ru-CO distances, i.e., 1.863(3) and
1.874(3) Å, of the Ru(2) metal atom. The shortening of
the metal-ligand distances can be attributed to the
relief of steric crowding, the lower coordination number
of Ru(2) compared to Ru(1), and the strengthening of
the ligand to metal σ-donation, required to compensate
for the coordination unsaturation at the Ru(2) site. To
our knowledge, the only other example of an unsatur-
ated Ru metal center in a binuclear or cluster complex
is in the spiked-triangular complex Ru4(CO)9(µ-PPh2)[µ4-
Ph2PCC(Ph)CC(Ph)], in which a phenyl group of the
PPh2 fragment partially protects the vacant site but is
not directly coordinated.13
After establishing the structure of 2, its reactivity was
then examined in an attempt to elucidate whether
unique chemical properties were associated with the
vacant coordination site. Surprisingly complex 2 did not
react with H2 gas in toluene solution even at elevated
temperatures; unreacted starting material was recov-
ered in high yield. This observation suggests that while
the vacant coordination site in 2 is incapable of activat-
ing dihydrogen, the pyrazolate ligands stabilize the two
Ru metal centers to decomposition. However, reaction
of 2 with carbon monoxide is instantaneous even at
room temperature, and the color of the solution fades
away within minutes by purging with CO, indicating
the formation of a CO adduct [Ru2(CO)6(dbpz)2] (1b).
The structure of 1b was then characterized according
to the IR and NMR spectral data and by a single-crystal
X-ray diffraction study. To minimize decomposition in
solution, a suitable crystal was obtained by layering a
CO-saturated MeOH solvent over a concentrated CH2-
Cl2 solution of 2 at room temperature under a CO
atmosphere.
F igu r e 2. (a) ORTEP drawing of the first molecule of
complex 1b and the corresponding atomic numbering
scheme. (b) ORTEP drawing and the respective space-
filling plot of the second molecule.
According to the single-crystal X-ray analysis, there
are two crystallographically independent molecules
within the asymmetric unit. An ORTEP plot of both
molecules is depicted in Figure 2, alone with a space-
filling plot of the second molecule to show the distortion
between the equatorial CO ligands. It is noteworthy that
(11) Andreu, P. L.; Cabeza, J . A.; Carriedo, G. A.; Riera, V.; Garcia-
Granda, S.; Van der Maelen, J . F.; Mori, G. J . Organomet. Chem. 1991,
421, 305.
(12) Sherlock, S. J .; Cowie, M.; Singleton, E.; Steyn, M. M. de V. J .
Organomet. Chem. 1989, 361, 353.
(13) Delgado, E.; Chi, Y.; Wang, W.; Hogarth, G.; Low, P. J .; Enright,
G. D.; Peng, S. M.; Lee, G. H.; Carty, A. J . Organometallics 1998, 17,
2936.

4740 Organometallics, Vol. 21, No. 22, 2002
Song et al.
most of the metal-ligand bond lengths and angles in
these two molecules are within four standard deviations
of each other. However, the first molecule displays a
slight skew arrangement (not the expected, staggered
arrangement observed in 2) with an average torsional
angle between the equatorial CO ligands being 9.38°,
while the second molecule has an even greater skew
distortion of 18.5°. A slightly longer metal-metal
distance is also observed for the first molecule (Ru(1)-
Ru(2) ) 2.6810(3) Å) with respect to that of the second
molecule (Ru(3)-Ru(4) ) 2.6758(3) Å), implying that the
torsional angle between the equatorial CO ligands as
well as the twisting of the pyrazolate fragments with
respect to the Ru-Ru vector is somehow inversely
related to the internal Ru-Ru separation.
A close inspection of the relative positions of the axial
CO ligands suggests an orientation toward the adjacent
tert-butyl substituents of the pyrazolate ligands by a
very small angle of 6.6-8.3° ( Ru(2)-Ru(1)-C(1) )
171.65(9)° and Ru(1)-Ru(2)-C(6) ) 173.41(9)°). This
structural information tends to suggest that there is
little physical contact between the axial CO ligands and
the adjacent tert-butyl substituents of the pyrazolate
ligands. However, as mentioned earlier, both pyrazolate
fragments are twisted with respect to the Ru-Ru vector,
and this motion may in part have reduced excessive
steric interaction with the axial CO ligands. If this is
the case, it is possible that the poor stability of 1b with
respect to CO elimination may be derived from a need
to compensate for steric repulsion between the tert-butyl
substituents and the axial CO ligands.
F igu r e 3. Space-filling plots of complex 2, showing (a) the
steric interaction between the axial CO ligand and the tert-
butyl substituents and (b) the environment around the
vacant coordination site.
For comparison, the space-filling plots of the unsatur-
ated complex 2, which are viewed from both of the axial
directions, are depicted in Figure 3 with all atoms drawn
to represent their van der Waals radii. The first space-
filling model clearly reveals that there is little steric
contact between the axial CO and the tert-butyl groups
of the pyrazolate ligands at its saturated Ru(CO)3
portion. However, on the opposite side of the molecule,
the second set of tert-butyl substituents have moved over
to the axial, vacant site on the Ru atom and cover at
least one-third of the void space above the coordination
site. This observation confirms that the dissociation of
an axial CO ligand from 1b must be enhanced by the
steric effect of the tert-butyl substituents.
yield, after chromatographic separation and recrystal-
lization. Curiously, the 1H NMR spectrum of 3a exhibits
two CH signals at δ 5.90 and 5.80 for the pyrazolate
ligands as well as two sets of doublets at δ 4.90 and
2
4.70 with coupling constant J HH ) 16.6 Hz. The latter
were identified as the diastereotopic methylene hydro-
gens of the benzyl group. These spectral data imply that
the isocyanide ligand has moved to an equatorial
position, showing that the mirror symmetry of the
previously observed reaction intermediate is no longer
retained.
The X-ray crystal structure was determined to reveal
the exact location of the incoming isocyanide ligand in
3a (Figure 4). Again, the two Ru atoms are bridged by
two dbpz ligands, which are located approximately
orthogonal to each other. The Ru-Ru distance, 2.6444(4)
Å, is similar to that of 2 and is consistent with the
presence of a metal-metal single bond. Moreover, both
Ru atoms are coordinated by two carbonyl ligands, while
one is further linked to an isocyanide ligand at the
equatorial position, and the second possesses the ex-
pected vacant axial site. The Ru-C distance to the
isocyanide ligand (1.967(4) Å) is slightly shorter than
that observed in the isocyanide cluster complex [Ru3-
(CO)11(CNBut)] (2.040(5) Å), where the metal atoms
show a different, zero oxidation state,14 but is longer
than that in the adjacent, equatorial CO ligand of the
same molecule (cf. Ru(1)-C(1) ) 1.874(4) Å), reflecting
the weaker π-accepting properties of the isocyanide.
This reduction in the π-acceptor properties is also
Rea ction s w ith Don or Liga n d s. Treatment of a
CDCl3 solution of 2 with a stoichiometric amount of
benzyl isocyanide led to an immediate color change from
red-orange to light yellow after mixing at room temper-
1
ature. The H NMR spectra of this solution indicated
the formation of several products, of which the major
component displays a sharp singlet at δ 5.02 for the
methylene group of the benzyl isocyanide. This observa-
tion suggests that the isocyanide donor ligand may have
coordinated to the vacant axial site on complex 2 during
the initial mixing of the reagents; otherwise, a spectral
pattern involving two nonequivalent doublets would be
expected. However, this isocyanide complex is rather
unstable and undergoes decomposition to several other
products upon extending the reaction time, prohibiting
the full characterization of this reaction intermediate.
However, on increasing the reaction temperature to
40 °C for 4 h, an isocyanide-substituted complex
[Ru2(CO)4(CNCH2Ph)(dbpz)2] (3a ) was isolated in 70%
(14) Bruce, M. I.; Matison, J . G.; Wallis, R. C.; Patrick, J . M.;
Skelton, B. W.; White, A. H. J . Chem. Soc., Dalton Trans. 1983, 2365.

Pyrazolate-Bridged Diruthenium Carbonyl Complexes
Organometallics, Vol. 21, No. 22, 2002 4741
Sch em e 1
F igu r e 4. ORTEP drawing of complex 3a with thermal
ellipsoids shown at the 50% probability level.
illustrated by the strengthening of the trans-pyrazolate
nitrogen atom bonding to the ruthenium atom (Ru(1)-
N(3) ) 2.130(3) Å). This distance is 0.03 Å shorter than
the Ru-N distance to the second pyrazolate (Ru(1)-
N(1) ) 2.161(3) Å), which is located trans to the CO
ligand.
Sch em e 2
The reaction between 2 and a second donor ligand,
pyridine, proceeded much slower at room temperature,
with no intermediate being observed even under condi-
tions of excess pyridine. However, upon increasing the
temperature to 110 °C for 18 h, an orange complex, [Ru2-
(CO)4(py)(dbpz)2] (3b), was isolated in 95% yield. The
identification of 3b was established by 13C NMR spec-
troscopy, in which four distinctive CO signals at δ 206.7,
202.2, 201.7, and 184.7 are observed, with a spectral
pattern nearly identical to that of the related isocyanide
complex 3a , thus allowing the unambiguous confirma-
tion of its molecular structure.
Since both complexes 3a and 3b show a similar type
of unsaturation with respect to the parent compound
2, it is reasonable to assume that these substituted
complexes would be converted from 32e species to 34e
complexes, upon addition of CO gas. Our control experi-
ments have not only confirmed this hypothesis but also
showed that the CO-addition products [Ru2(CO)5(L)-
(dbpz)2] (L ) NCCH2Ph, 4a ; L ) py, 4b) are far less
stable than the parent complex 1b, especially in the
absence of a CO atmosphere. As a result, although their
NMR spectra have been recorded, the instabilities of 4a
and 4b prohibited measurement of IR ν(CO) spectra due
to a rapid reversion to the starting complexes 3a and
4a during sample transfer.
of an axial CO ligand trans to the Ru-Ru linkage. This
reaction is quantitative in solution, and as a result, the
unsaturated complex 2 is the sole product obtained from
the direct treatment of Ru3(CO)12 and 3,5-di-tert-bu-
tylpyrazole, even when a large excess of carbon mon-
oxide is present in the sealed autoclave.
The structure of 1b is related to that of an isoelec-
tronic complex, [Os2(CO)6(thd)2] (5), thd ) 2,2-dimethyl-
3,5-heptanedionate,15 for which both of the osmium
atoms display an identical d7 electrical configuration.
The only difference is that the thd ligand in 5 is chelated
to each osmium atom, rather than serving as a bridging
ligand; thus free rotation along the Os-Os vector is
possible in the thd complex 5, producing an eclipsed
arrangement for all equatorial ligands. As a result, after
the removal of a CO ligand at the axial position, the
two [Os2(CO)5(thd)2] units couple to afford the dimer
complex [Os2(CO)5(thd)2]2 (6), utilizing the so-called
double dative interaction16 to produce a linear Os-Os‚
‚‚Os-Os array (Scheme 2). We speculate that such a
process is inaccessible for complex 2 because the tert-
butyl substituents near the vacant site block the dimer-
ization, and the double pyrazolate bridges within the
Str u ctu r a l P r op er ties. The structural relationship
of the complexes 1-4 is depicted in Scheme 1. It has
been suggested that the methyl derivative 1a is rela-
tively stable to CO dissociation upon increasing the
temperature. However, for the tert-butyl-substituted
derivative 1b, a spontaneous decarbonylation reaction
occurred at room temperature, giving complex 2 via loss
(15) Su, M.-D.; Liao, H.-Y.; Chu, S.-Y.; Chi, Y.; Liu, C.-S.; Lee, F.-
J .; Peng, S.-M.; Lee, G.-H. Organometallics 2000, 19, 5400.
(16) (a) Batchelor, R. J .; Davis, H. B.; Einstein, F. W. B.; Pomeroy,
R. K. J . Am. Chem. Soc. 1990, 112, 2036. (b) Wang, W.; Einstein, F.
W. B.; Pomeroy, R. K. Organometallics 1993, 12, 3079. (c) Liu, Y.;
Leong, W. K.; Pomeroy, R. K. Organometallics 1998, 17, 3387.

4742 Organometallics, Vol. 21, No. 22, 2002
Song et al.
Ru2 framework also prevent twisting with respect to the
Ru-Ru linkage.
upon switching to the more bulky and less reactive
pyridine ligand. Not surprisingly, the reaction is under
thermodynamic control, giving a ligand substitution
product upon extending the reaction time or increasing
the temperature, in which the incoming donor ligand
of the substitution products 3a and 3b resides at an
equatorial site of the saturated Ru metal center. The
mechanism leading to this type of substitution product
is unclear. From a range of possibilities, we prefer the
idea of having sequential addition and elimination,
which consists of the initial addition of ligand at the
vacant axial site, and then axial-to-equatorial rear-
rangement, followed by elimination of the second axial
CO ligand at the remote Ru atom. Moreover, the
progress of this substitution sequence implies that the
less π-accepting isocyanide and pyridine ligands have
a special tendency to remain at the relatively more
electron-rich metal atom, a phenomenon that has no
adequate explanation at this moment. Further studies
will be aimed at utilizing these congested pyrazolate
complexes as catalytic precursors.
Finally, the IR ν(CO) stretching vibrations of com-
plexes 1a and 1b have been utilized to assess the
possible contribution from the electronic effect, for which
a small, but notable difference in the ν(CO) stretching
vibrations (e3 cm-1) was recorded. The low-energy
shifting of 1b indicated that its CO ligands have
received an increased amount of back π-bonding from
the Ru metal atoms, showing a synergistic effect of the
tert-butyl substituents in this system.
Su m m a r y
The chemical and physical behavior of the parent
pyrazolate complexes [Ru2(CO)6(pz)2], pz ) pyrazolate
ligands, is strongly influenced by the nature of the alkyl
substituents on the pyrazolate ligands. Our observation
is consistent with a recent report on the monometallic
complexes [Cr(dbpz)3] and [Fe(dbpz)3],17 for which the
steric bulk of the tert-butyl substituents provides the
necessary stabilization for the novel η2-bonding mode.
Thus, we are confident that the same steric interaction
facilitates CO elimination from the dbpz complex 2,
giving one vacant coordination site at the axial position.
Upon addition of benzyl isocyanide, this strong donor
ligand coordinates immediately at the vacant coordina-
tion site to give an unstable addition product at room
temperature, but the addition product was not detected
Ack n ow led gm en t. Y.C. thanks the National Sci-
ence Council of the Republic of China (NSC 90-2113-
M-007-056) and the Chinese Petroleum Corporation,
Republic of China, for financial support (CPC 89-2119-
M-007).
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic file for complexes 2, 1b, and 3a . This material is
available free of charge via the Internet at http://pubs.acs.org.
(17) Gust, K. R.; Knox, J . E.; Heeg, M. J .; Schlegel, H. B.; Winter,
C. H. Angew. Chem., Int. Ed. 2002, 41, 1591.
OM020445R