X-ray structure and reactivity of (η4-tetraphenylcyclopentadienone)(CO)2 Ru(HOCHMe2): Unexpected stability of the neutral 2-propanol-ruthenium(0) complex with respect to β-hydride elimination

Article abstract of DOI:10.1021/om0103198

A neutral Ru(0)-alcohol complex, (η⁴-C₄Ph₄-CO)(CO)₂ Ru(HOCHMe₂) (7), was obtained by treating an acetone solution of (η⁴-C₄Ph₄CO)Ru(CO)₃ (6) with aqueous Na₂CO₃ at room temperature, followed by acidification with aqueous NH₄Cl at 0°C. Complex 7 is stable even at 90°C in toluene and showed catalytic activities in the transfer hydrogenation of acetophenone in 2-propanol and the racemization of optically active 1-phenylethanol.

Full text of DOI:10.1021/om0103198

3370
Organometallics 2001, 20, 3370-3372
X-r a y Str u ctu r e a n d Rea ctivity of
(η4-tetr a p h en ylcyclop en ta d ien on e)(CO)2Ru (HOCHMe2):
Un exp ected Sta bility of th e Neu tr a l
2-P r op a n ol-Ru th en iu m (0) Com p lex w ith Resp ect to
â-Hyd r id e Elim in a tion
Hyun Min J ung, Seung Tae Shin, Yu Hwan Kim, Mahn-J oo Kim,* and
J aiwook Park*
National Research Laboratory of Chirotechnology, Department of Chemistry, Division of
Molecular and Life Sciences, Pohang University of Science and Technology (POSTECH),
San 31 Hyoja Dong, Pohang 790-784, Republic of Korea
Received April 17, 2001
Summary: A neutral Ru(0)-alcohol complex, (η4-C4Ph4-
CO)(CO)2Ru(HOCHMe2) (7), was obtained by treating
an acetone solution of (η4-C4Ph4CO)Ru(CO)3 (6) with
aqueous Na2CO3 at room temperature, followed by
acidification with aqueous NH4Cl at 0 °C. Complex 7 is
stable even at 90 °C in toluene and showed catalytic
activities in the transfer hydrogenation of acetophenone
in 2-propanol and the racemization of optically active
1-phenylethanol.
Sch em e 1
Diruthenium carbonyl cyclopentadienone complex 1,
which is called the Shvo complex, has shown a broad
spectrum of catalytic activities with various organic
substrates.1 In particular, its intriguing catalytic activ-
ity for hydrogen transfer reactions have been demon-
strated in the Tischenko type disproportionation of
aldehydes to esters,1c hydrogenations and water-gas
shift type reductions of aldehydes and ketones,1e and
Oppenauer type oxidation of secondary alcohols.1h No-
tably, complex 1 catalyzes the transfer hydrogenation
of ketones in 2-propanol without the aid of additional
base1f,2 and has been adapted as an effective racemiza-
tion catalyst for the dynamic kinetic resolution of
secondary alcohols using lipases.3
aldehydes or ketones through transfer of hydride from
ruthenium and of a proton from the OH group.2,5 The
alcohol complex 4 has been postulated as a transient
intermediate in the reaction of 2 to give the product
alcohol and coordinatively unsaturated intermediate 3.
Herein we report the isolation of the unexpectedly stable
2-propanol complex 7. Also reported is the molecular
structure of 7 and its catalytic activity in hydrogen
transfer reactions.
It has been proposed that the dissociation of 1 to two
monomeric intermediates 2 and 3 is a key step in the
reversible hydrogen transfer reactions between alcohols
and ketones (Scheme 1).1f,4 In fact, 2 can be generated
by treating 1 with H2 or formic acid1a,g and hydrogenates
Neutral 2-propanol complex 7 was obtained in 45%
yield by treating an acetone solution of 6 with aqueous
sodium carbonate at room temperature for 1 h, followed
by quenching with aqueous ammonium chloride at 0 °C
(Scheme 2).6 This procedure is almost the same as that
reported for the Shvo complex (1), except for the
acidification temperature.1e In fact, 1 was produced
exclusively when treatment under basic conditions was
prolonged for more than 5 h or when the acidification
temperature was higher than 20 °C. Complex 7 was
formed to be stable enough to be separated by column
chromatography using silica gel. The molecular struc-
ture of 7 was elucidated by single-crystal X-ray diffrac-
(1) (a) Shvo, Y.; Goldberg, I.; Czarkie, D.; Reshef, D.; Stein, Z.
Organometallics 1997, 16, 133. (b) Menashe, N.; Salant, E.; Shvo, Y.
J . Organomet. Chem. 1996, 514, 97. (c) Menashe, N.; Shvo, Y.
Organometallics 1991, 10, 3885. (d) Shvo, Y.; Czarkie, D. J . Organomet.
Chem. 1989, 368, 357. (e) Shvo, Y.; Czarkie, D. J . Organomet. Chem.
1986, 315, C25. (f) Shvo, Y.; Czarkie, D.; J . Am. Chem. Soc. 1986, 108,
7400. (g) Blum, Y.; Czarkie, D.; Rahamim, Y.; Shvo, Y. Organometallics
1985, 4, 1459. (h) Almeida, M. L.; Beller, M.; Wang, G-.Z.; Ba¨ckvall,
J -.E. Chem. Eur. J . 1996, 2, 1533.
(2) Laxmi, Y. R. S.; Ba¨ckvall, J -.E. Chem. Commun. 2000, 611.
(3) (a) J ung, H. M.; Koh,J . H.; Kim, M-.J .; Park, J . Org. Lett. 2000,
2, 2487. (b) Kim, M-.J .; Lee, D.; Huh, E. A.; J ung, H. M.; Koh, J . H.;
Park, J . Org. Lett. 2000, 2, 2377. (c) J ung, H. M.; Koh, J . H.; Kim,
M-.J .; Park, J . Org. Lett. 2000, 2, Org. Lett. 2000, 2, 409 (d) Larsson,
A. L. E.; Persson, B. A.; Ba¨ckvall, J .-E. Angew. Chem., Int. Ed. Engl.
1997, 36, 121. (e) Persson, B. A.; Larsson, A. L. E.; Ray, M. L.; Ba¨ckvall,
J .-E. J . Am. Chem. Soc. 1999, 121, 1645. (f) Persson, B. A.; Huerta, F.
F.; Ba¨ckvall, J .-E. J . Org. Chem. 1999, 64, 5237.
(4) Mays, M. J .; Morris, M. J .; Raithby, P. R.; Shvo, Y.; Czarkie, D.
Organometallics 1989, 8, 1162.
(5) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana,
M. J . Am. Chem. Soc. 2001, 123, 1090.
10.1021/om0103198 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 07/12/2001

Communications
Organometallics, Vol. 20, No. 16, 2001 3371
Sch em e 2
Sch em e 3a
a
Legend: (A) (1) aqueous Na2CO3, (2) acetone, (3) aqueous
NH4Cl, 0 °C; (B) (1) Na2CO3/acetone-H2O, (2) 2-propanol, (3)
aqueous NH4Cl, 0 °C; (C) (1) Na2CO3/acetone-H2O, 20 °C; (2)
aqueous NH4Cl, 20 °C.
(2.190-2.257 Å), suggesting that the cyclopentadienone
ring is bonded to the Ru in an η4 coordination mode.9
In contrast to the most known alcohol complexes,10,11
7 is thermally stable even at 90 °C in toluene, as
indicated by the lack of any detectable change in the
1H NMR at that temperature over 2 h. However, 7
showed catalytic activity comparable to 1 in the race-
mization of optically active 1-phenylethanol and in the
transfer hydrogenation of acetophenone with 2-pro-
panol. The optical purity of (S)-1-phenylethanol (>99%
ee, 0.2 M) in toluene was changed to 45% ee (65% ee
with 2 mol % of 1) after heating with 4 mol % of 7 at 70
°C for 5 h, and 1-phenylethanol was obtained in 95%
yield (96% yield with 2 mol % of 1) by heating a solution
(0.2 M) of acetophenone in 2-propanol with 4 mol % of
7 at 70 °C for 5 h.
F igu r e 1. ORTEP drawing (50% probability) of the
structure of 7 at 203 K. Selected bond distances (Å):
C(1)-C(2) ) 1.463(4), C(1)-C(5) ) 1.472(4), C(2)-C(3) )
1.447(4), C(3)-C(4) ) 1.436(4), C(4)-C(5) ) 1.450(4),
Ru-C(1) ) 2.429(3), Ru-C(2) ) 2.239(3), Ru-C(3) )
2.202(3), Ru-C(4)
) 2.190(3), Ru-C(5) ) 2.257(3),
Ru-O(4) ) 2.189(2), C(1)-O(1) ) 1.259(3). Selected bond
angles (deg): C(1)-Ru-O(4) ) 82.91(9), O(1)-C(1)-Ru )
126.98(19).
To investigate the mechanism of the formation of 7,
we examined a number of related reactions as shown
in Scheme 3. A mixture of 1 and 7 in a 58:42 ratio was
obtained from mononuclear hydride complex 2 by se-
quential treatments with aqueous sodium carbonate,
excess acetone at room temperature, and aqueous
ammonium chloride at 0 °C. In contrast, only the Shvo
complex (1) was obtained in the reaction of 2 with
acetone at 0 °C. The alcohol complex 7 was also formed
along with 1 in a 50:50 ratio upon treatment of an
acetone solution of diruthenium complex 8 with aqueous
sodium carbonate, followed by adding 2-propanol at
room temperature and aqueous ammonium chloride at
0 °C. Interestingly, the naphthyl analogue 10 was
prepared from 9 in 51% yield without lowering the
acidification temperature to 0 °C.12,13 This result sug-
gests that the steric hindrance by two naphthyl groups
against forming a diruthenium complex analogous to
tion analysis and shows the coordination of 2-propanol
to the ruthenium through the hydroxyl oxygen (Figure
1).7 The hydroxyl proton appears to interact with the
carbonyl oxygen of the cyclopentadienone ring, but it is
not clear whether the interaction is intramolecular or
intermolecular in the crystal structure.8 In comparison
to 1, having an η5 coordination mode,1e 7 exhibits a
C(1)-O(1) bond length (1.259 Å) shorter by 0.027 Å and
a Ru-C(1) distance (2.429 Å) longer by 0.029 Å. In
addition, the Ru-C(1) distance is distinctly longer than
those of the other four Ru-ring carbon atom distances
(6) To a solution of 6 (700 mg, 1.23 mmol) in acetone (50 mL) was
added a saturated aqueous Na2CO3 solution (25 mL) at 20 °C. After it
was stirred at 20 °C for 1 h, the reaction mixture was cooled to 0 °C,
acidified by adding a saturated aqueous NH4Cl solution (50 mL), and
concentrated under reduced pressure. Then, the resulting residue was
extracted with CH2Cl2. After CH2Cl2 was removed, the crude product
was chromatographed on silica gel with CH2Cl2/ethyl acetate (6:1) to
give 7 (330 mg, 45%) and the Shvo complex (1; 305 mg, 46%).
Recrystallization of 7 from CH2Cl2-hexane afforded air-stable pale
yellow crystals. Mp: 166 °C dec. 1H NMR (CDCl3): δ 7.54 (d, J ) 6.96
Hz, 4H), 7.19-7.03 (m, 16H), 2.85 (sept, J ) 6.21 Hz, 1H), 2.44 (br s,
1H), 0.95 (d, J ) 6.42 Hz, 6H). 13C NMR (CDCl3): δ 201.32, 163.19,
132.99, 132.48, 131.95, 130.43, 128.06, 128.01, 126.71, 103.86, 83.59,
51.77, 24.92. IR (KBr, cm-1): ν(CO) 2006 (s), 1949 (s), 1601 (m). MS
(FAB, m/z): 602 (M+). Anal. Calcd for C34H28O4Ru: C, 67.87; H, 4.69.
Found: C, 67.82; H, 4.68.
(9) In comparison to (η4-C4Ph4CO)Ru(CO)3 (Ru(1)-C(1) ) 2.53 Å,
C(1)-O(1) ) 1.22 Å), the structure of 7 has some η5-coordination
character. The hydrogen bonding interaction would be responsible for
this character. Blum, Y.; Shvo, Y.; Chodosh, D. F. Inorg. Chim. Acta
1985, 97, L25.
(10) A hydroxybenzyl alcohol complex has been reported to be
isolated by recrystallization of the crude product from the reaction of
[RuH2(CO)(PPh3)3] with salicylaldehyde at the temperature of refluxing
toluene: Sahajpal, A.; Robinson, S. D.; Mazid, M. A.; Motevalli, M.;
Hursthouse, M. B. J . Chem. Soc., Dalton Trans. 1990, 2119.
(11) A few alcohol complexes with cationic metal centers are
known: (a) Milke, J .; Missling, C.; Su¨nkel, K.; Beck, W. J . Organomet.
Chem. 1993, 445, 219. (b) Song, J -.S.; Szalda, D. J .; Bullock, R. M.;
Lawrie, C. J . C.; Rodkin, M. A.; Norton, J . R. Angew. Chem., Int. Ed.
Engl. 1992, 31, 1233. (c) Agbossou, S. K.; Smith, W. W.; Gladysz, J . A.
Chem. Ber. 1990, 123, 1293. (d) Field, J . S.; Haines, R. J .; Sundermeyer,
J .; Woollam, S. F. Chem. Commun. 1990, 985. (e) Su¨nkel, K.; Urban,
G.; Beck, W. J . Organomet. Chem. 1985, 290, 231.
(7) Crystal data for 7: C34H28O4Ru. Mr ) 601.63, light yellow crystal,
size 0.25 × 0.30 × 0.40 mm3, monoclinic, a ) 12.09390(10) Å, b )
9.98720(10) Å, c ) 23.5615(2) Å, R ) 90°, â ) 94.4370(10)°, γ ) 90°,
space group P21/n, V ) 2837.33(4) Å3, Z ) 4, T ) 203(2) K, dcalcd
)
1.408 g/cm3, absorption coefficient 0.589 mm-1, Siemens SMART
diffractometer, λ ) 0.710 73 Å, scan mode ω (ω-scan width: 1.73-
24.13°). 11 160 reflections measured, giving 4450 unique data with
I > 2σ(I), R ) 0.0298, Rw ) 0.0837, GOF ) 1.123.
(8) Two sites are possible for the location of the hydroxyl proton in
the crystal structure. The distance to the carbonyl oxygen is 2.190 Å
for an intramolecular hydrogen bonding interaction, while that for an
intermolecular one is 1.843 Å.

3372 Organometallics, Vol. 20, No. 16, 2001
Communications
Sch em e 4
12 facilitates the production of alcohol complex 10 at
room temperature.
On the basis of the above results, a plausible pathway
for the formation of 7 from 6 is outlined in Scheme 4:
One of the carbonyl ligands in 6 is replaced with hydride
through the reaction with hydroxide to form intermedi-
ate 11, which is in equilibrium with 2. Complex 2 reacts
with acetone to give 2-propanol and 3. The coupling of
3 and 11 to give 12 competes with the base-induced
reaction between 3 and 2-propanol to form the anionic
alkoxide complex 13. Acidifying the mixture of 12 and
13 produces 1 and 7, respectively. The conversion of 13
to 12 is possible, and extending the reaction time and
increasing the temperature favor the formation of 12.14
In summary, we have isolated the neutral 2-pro-
panol-Ru(0) complex 7 that has been postulated as a
transient intermediate in hydrogen transfer reactions
catalyzed by the Shvo complex (1). Surprisingly, the
alcohol complex was found to be thermally stable with
(12) A solution of Ru3(CO)12 (281 mg, 0.44 mmol) and 2,5-bis(1-
naphthyl)-3,4-diphenylcyclopentadienone (640 mg, 1.32 mmol) in
benzene (120 mL) was heated to reflux for 48 h under an argon
atmosphere. After it was cooled to room temperature, the reaction
mixture was concentrated and purified by column chromatography on
silica gel with CH2Cl2/ethyl acetate to give a yellow solid (744 mg, 84%).
respect to dissociation or â-hydride elimination. Despite
such stability, 7 shows catalytic activity comparable to
that of 1 in hydrogen transfer reactions.
Ack n ow led gm en t. We are grateful for financial
support by MOST through the NRL program and by
KOSEF through the Center for Integrated Molecular
System. H.M.J . is grateful for his postdoctoral fellowship
through the Brain Korea 21 Project in 2001 by the
Korean Ministry of Education.
Recrystal1ization from CH2Cl2/hexane afforded
9 as pale yellow
crystals. Mp: 202 °C dec. 1H NMR (CDCl3): δ 7.93-7.82 (m, 6H), 7.50-
7.40 (m, 8H), 7.02-6.83 (m, 10H). 13C NMR (CDCl3): δ 194.63, 176.07,
134.60, 134.04, 132.88, 131.47, 130.22, 129.65, 129.11, 128.93, 128.07,
126.91, 126.20, 125.90, 109.25, 86.82. IR (KBr, cm-1): ν(CO) 2081 (s),
2023 (s), 2006 (s), 1654 (m). Anal. Calcd for C40H24O4Ru: C, 71.74; H,
3.61. Found: C, 71.51; H, 3.93.
(13) According to a procedure similar to that for
7 except for
acidification at room temperature, 10 (206 mg, 51%) and a Shvo-type
diruthenium complex (130 mg, 35%) were obtained from 9 (390 mg,
0.58 mmol). 10: mp 180 °C dec; 1H NMR (CDCl3) δ 8.26 (d, J ) 8.43
Hz, 2H), 7.83-7.76 (m, 4H), 7.55-7.34 (m, 8H), 6.94-6.80 (m, 10H),
3.27 (sept, J ) 6.36 Hz, 1H), 2.51 (br s, 1H), 1.32 (d, J ) 6.39 Hz, 6H);
13C NMR (CDCl3) δ 201.28, 165.01, 132.62, 131.83, 131.01, 129.13,
128.82, 127.97, 127.70, 126.75, 126.68, 126.23, 125.63, 103.97, 88.09,
52.47, 25.82; IR (KBr, cm-1) ν(CO) 2001 (s), 1939 (s), 1608 (m); MS
(FAB, m/z) 702 (M+). Anal. Calcd for C42H32O4Ru: C, 71.88; H, 4.60.
Found: C, 71.65; H, 4.52. Shvo-type diruthenium complex, {[2,5-bis-
(1-naphthyl)-3,4-diphenyl(η5-C4CO)]2H}Ru2(CO)4(µ-H): mp 198 °C dec;
1H NMR (CDCl3) δ 11.92 (br s, 1H), 7.84-7.55 (m, 14H), 7.35-7.24
(m, 12H), 6.96-6.82 (m, 22H), -18.95 (s, 1H); 13C NMR (CDCl3) δ
201.66, 157.26, 135.55, 133.68, 132.98, 131.02, 129.04, 128.61, 128.34,
127.80, 127.32, 126.53, 125.97, 125.82, 125.45, 105.00, 90.47; IR (KBr,
cm-1) ν(CO) 2032 (s), 2004 (m), 1975 (s), 1961 (m), 1542 (w); MS (FAB,
m/z) 1287 (M+ + 1). Anal. Calcd for C78H50O6Ru2: C, 72.88; H, 3.92.
Found: C, 73.04; H, 4.15.
Su p p or tin g In for m a tion Ava ila ble: Structural diagrams
with full atom labeling and tables of bond distances, angles,
anisotropic parameters, and atomic coordinates for 7. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM0103198
(14) Complex 12 was precipitated almost quantitatively upon
concentrating the reaction mixture before acidifying with aqueous
ammonium chloride. The precipitate was filtered, washed with water,
and recrystallized from CH2Cl2 and hexane to give orange crystals.
Mp: 184 °C dec. 1H NMR (CDCl3): δ 7.16-6.93 (m, 40H), -16.19
(s, 1H). 13C NMR (CDCl3): δ 202.38, 133.59, 132.40, 132.22, 132.28,
128.29, 127.78, 126.91, 101.29, 82. IR (KBr, cm-1): ν(CO) 2011 (s), 1980
(m), 1950 (s), 1935 (m), 1613 (br m). MS (FAB, m/z): 1108 (M+). Anal.
Calcd for C62H43NaO7Ru2: C, 66.18; H, 3.85. Found: C, 65.90; H, 3.61.