Synthesis and reactivity of hydroxycyclopentadienyl and aminocyclopentadienyl ruthenium alcohol complexes

Article abstract of DOI:10.1021/om020891e

The structure and reactivity of hydroxycyclopentadienyl and aminocyclopentadienyl ruthenium alcohol complexes were discussed. It was found that the rate of exchange of hydroxycyclopentadienyl ruthenium benzyl alcohol complex with 2-propanol at -46°C was (

Full text of DOI:10.1021/om020891e

Organometallics 2003, 22, 901-903
901
Communications
Syn th esis a n d Rea ctivity of Hyd r oxycyclop en ta d ien yl
a n d Am in ocyclop en ta d ien yl Ru th en iu m Alcoh ol
Com p lexes
Charles P. Casey,* Thomas E. Vos, and Galina A. Bikzhanova
Department of Chemistry, University of WisconsinsMadison, Madison, Wisconsin 53706
Received October 18, 2002
Sch em e 1
Summary: Cationic aminocyclopentadienyl and hydroxy-
cyclopentadienyl ruthenium alcohol complexes were
synthesized from reaction of the corresponding ruthen-
ium chloride with AgBF4 in the presence of an alcohol.
Exchange rates of free alcohol with hydroxycyclopenta-
dienyl and aminocyclopentadienyl ruthenium benzyl
alcohol complexes were rapid (t1/ 2 ) 5-10 min) at -47
°C.
Several years ago, Shvo discovered that the diruthen-
ium complex 1 was an efficient ketone hydrogenation
catalyst and determined that the active reducing species
was the ruthenium hydride 2 (Scheme 1).1 On the basis
of detailed mechanistic studies on the related active
reducing agent 3, including observation of primary
deuterium isotope effects for transfer of both OH and
RuH, we proposed a mechanism involving concerted
transfer of proton and hydride to aldehyde outside the
coordination sphere of the metal.2 Ba¨ckvall has proposed
an alternative mechanism involving Cp ring slippage
and aldehyde coordination prior to concerted transfer
of the hydrogens.3 In Ba¨ckvall’s mechanism, the alcohol
is “born” in the coordination sphere of the metal. In our
proposed mechanism, an alcohol complex may form
reversibly after the rate-determining step. Clearly,
information about the kinetic and thermodynamic sta-
bility of alcohol complexes is required to fully under-
stand the mechanism of aldehyde reduction. Park’s
reported isolation of a neutral 2-propanol ruthenium
complex piqued our interest. In the course of investigat-
ing its properties, we and Ba¨ckvall showed that Park’s
“alcohol complex” was in fact an amine complex.4
In an effort to prevent formation of bridging ruthen-
ium hydrides and to develop more active catalysts
related to Shvo’s complex 1, we prepared the N-
phenylamino-substituted cyclopentadienyl complex 5
and found that it reduced benzaldehyde rapidly in the
presence of 1 equiv of added acid (Scheme 2).5,6 Again,
Sch em e 2
the question of alcohol complex involvement arose.
Alcohol complexes have been observed as products of
some ionic hydrogenations of carbonyl compounds by
tungsten, rhenium, and molybdenum hydrides in the
presence of a strong acid.7-9 Other examples of metal
alcohol complexes have been reported,10 but there has
been no report of a ruthenium alcohol complex in the
reduction of ketones and aldehydes by hydroxycyclo-
pentadienyl ruthenium hydride. Here we report the in
situ synthesis of cationic hydroxycyclopentadienyl and
(6) Concurrently, Park made a similar aminocyclopentadienyl ru-
thenium complex which is a racemization catalyst for dynamic kinetic
resolution: Choi, J . H.; Kim, Y. H.; Nam, S. H.; Shin, S. T.; Kim, M.-
J .; Park, J . Angew. Chem., Int. Ed. 2002, 41, 2373.
(7) (a) 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. (b) Smith, K.-T.; Norton, J . R.; Tilset, M. Organometallics 1996,
15, 4515. (c) Magee, M. P.; Norton, J . R. J . Am. Chem. Soc. 2001, 123,
1778.
(8) (a) Voges, M. H.; Bullock, R. M. J . Chem. Soc., Dalton Trans.
2002, 759-770. (b) Song, J .-S.; Szalda, D. J .; Bullock, R. M. Organo-
metallics 2001, 20, 3337. (c) Bullock, R. M.; Voges, M. H. J . Am. Chem.
Soc. 2000, 122, 12594.
(1) Blum, Y.; Czarkie, D.; Rahamim, Y.; Shvo, Y. Organometallics
1985, 4, 1459.
(2) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana,
M. J . Am. Chem. Soc. 2001, 123, 1090.
(3) Persson, B. A.; Larsson, A. L. E.; Le Ray, M.; Ba¨ckvall, J .-E. J .
Am. Chem. Soc. 1999, 121, 1645-1650. Csjernyik, G.; EÄ ll, A. H.; Fadini,
L.; Pugin, B.; Ba¨ckvall, J .-E. J . Org. Chem. 2002, 67, 1657.
(4) Casey, C. P.; Bikzhanova, G. A.; Ba¨ckvall, J .-E.; J ohansson, L.;
Park, J .; Kim, Y. H. Organometallics 2002, 21, 1955-1959.
(5) Casey, C. P.; Vos, T. E.; Singer, S. W.; Guzei, I. A. Organome-
tallics 2002, 21, 5038.
(9) Bakhmutov, V. I.; Vorontsov, E. V.; Antonov, D. Yu. Inorg. Chim.
Acta 1998, 278, 122.
(10) (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) Field, J . S.; Haines, R. J .; Sundermeyer, J .;
Woollam, S. F. Chem. Commun. 1990, 985. (d) Agbossou, S. K.; Smith,
W. W.; Gladysz, J . A. Chem. Ber. 1990, 123, 1293. (e) Sahajpal, A.;
Robinson, S. D.; Mazid, M. A.; Motevalli, M.; Hursthouse, M. B. J .
Chem. Soc., Dalton Trans. 1990, 2119. (f) Su¨nkel, K.; Urban, G.; Beck,
W. J . Organomet. Chem. 1985, 290, 231.
10.1021/om020891e CCC: $25.00 © 2003 American Chemical Society
Publication on Web 01/31/2003

902 Organometallics, Vol. 22, No. 5, 2003
Communications
Sch em e 3
Sch em e 4
if the alcohol is bonded to the organometallic fragment
is to determine if the T1 times are correlated between
the alcohol fragment and organometallic fragment and
are different from the T1 times of the free alcohol in
solution.9 T1 is a measure of the spin-lattice relaxation
rate and is dependent on the tumbling rate, which
should be faster for free than for complexed 2-pro-
1
panol.13 As expected, the T1 time for the R-CH H NMR
resonance of 10 (1.0 s) was similar to that for the other
proton resonances of 10 (1.0-1.3 s), while the T1 time
for the free alcohol R-CH (3.7 s) was much longer at -10
°C.14
Ta ble 1. NMR of Ru th en iu m Alcoh ol Com p lexes
alcohol ligand
The rate of exchange of the (phenylamino)cyclopen-
tadienyl benzyl alcohol complex 11 with 2-propanol was
complex
1H NMR δ (J HH, Hz)
13C NMR δ
1
studied by H NMR spectroscopy at -47 °C. The rate
10
3.78 (oct, 6.0), HOCH(CH3)2
5.37 (d, 6.0) HOCH(CH3)2
4.76 (d, 4.5), HOCH2Ph
5.69 (t, 4.5), HOCH2Ph
4.46 (d, 4.5), HOCH2Ph
5.53 (t, 4.5), HOCH2Ph
3.71 (oct, 6.0), HOCH(CH3)2
4.44 (d, 6.0), HOCH(CH3)2
4.42 (d, 4.5), HOCH2Ph
5.79 (t, 4.5), HOCH2Ph
3.63 (oct, 6.5), HOCH(CH3)2
4.32 (d, 6.5), HOCH(CH3)2
76.1, HOCH(CH3)2
of exchange was nearly independent of the concentration
of 2-propanol, with a first-order rate constant for ap-
proach to equilibrium of (1.4 ( 0.5) × 10-3 s-1 (t1/2 ) 8
min) at -47 °C. The Keq value for alcohol exchange
showed a slight preference for the isopropyl alcohol
complex 10: Keq ) [10][HOCH2Ph]/[11][HOCHMe2] )
2.1 at -47 °C. These kinetic studies establish that
unimolecular dissociation of benzyl alcohol from 11 is
a rapid process, but the rate of this dissociation is slower
than the rate of production of benzyl alcohol from the
reaction of protonated 5 with benzaldehyde. Currently,
we are examining the reaction of protonated 5 with
benzaldehyde to determine if the alcohol is “born” inside
or outside of the coordination sphere of the metal.
The rate of exchange of hydroxycyclopentadienyl
ruthenium benzyl alcohol complex 12 with 2-propanol
at -46 °C was (2.2 ( 0.7) × 10-3 s-1 (t1/2 ) 5 min), as
determined by 1H NMR spectroscopy. The rates of
exchange of 11 and 12 were similar. The equilibrium
constant for alcohol exchange showed little dependence
on the structure of the alcohol: Keq ) [HOCH2Ph][15]/
[HOCHMe2][14] ) 1.08 at -10 °C. The rate of alcohol
exchange is similar to the rate of formation of benzyl
alcohol from reduction of benzaldehyde by hydroxycy-
clopentadienyl ruthenium hydride 3.
11
12
13
14
15
73.8, HOCH2Ph
74.1, HOCH2Ph
78.3, HOCH(CH3)2
74.1, HOCH2Ph
78.7, HOCH(CH3)2
cationic aminocyclopentadienyl ruthenium alcohol com-
plexes and the kinetics of alcohol exchange of these
complexes.
Aminocyclopentadienyl and hydroxycyclopentadienyl
ruthenium 2-propanol and benzyl alcohol complexes
were synthesized under mild conditions, starting with
the appropriate ruthenium chloride complex and silver
tetrafluoroborate in the presence of the alcohol (Scheme
3), and were characterized by NMR at low temperature.
{[2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)2(HOCHMe2)}-
BF4 (10) was synthesized by adding excess AgBF4 to a
CD2Cl2 solution of [2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru-
(CO)2Cl (7) and 2-propanol in an NMR tube at -20 °C.
When the NMR tube was shaken vigorously at -20 °C,
AgBF4 never completely dissolved, but a fine new
precipitate of AgCl formed and was allowed to settle.
1H and 13C NMR resonances for both free 2-propanol
and complex 10 were observed (Table 1). In the 1H NMR
spectrum of 10, exchange of the OH group is slow
enough that coupling (J ) 6 Hz) is seen to the adjacent
CH group.11 The R-CH proton resonance of the 2-pro-
panol complex 10 (δ 3.78) is shifted to lower frequency
than in the free alcohol (δ 4.05), while the R-CH carbon
resonance of 10 (δ 76.1) is shifted to higher frequency
than in the free alcohol (δ 66.0). Similar changes in
chemical shifts of R-CH carbon resonances have been
reported for other alcohol complexes.10 The OH reso-
nance of 10 (δ 5.37) is shifted to higher frequency than
in the free alcohol (δ 4.2).12 Another test to determine
Initial attempts to prepare a neutral alcohol complex
by deprotonation of 15 have been unsuccessful. Reaction
of 15 with pyridine at -78 °C led to the formation of
the pyridine complex [2,5-Ph2-3,4-Tol2(η4-C4CdO)]Ru-
(CO)2(NC5H5) (16) (Scheme 4). We are continuing to
pursue the synthesis of neutral alcohol complexes by
low-temperature deprotonations using bases that are too
(12) Comparable shifts to higher frequency have been observed for
other ionic alcohol complexes and have been attributed to hydrogen
bonding between the counteranion and the hydroxyl proton.7a
(13) (a) Sanders, J . K. M.; Hunter, B. K. Modern NMR Spectroscopy,
A Guide for Chemists; Oxford University Press: London, 1993. (b)
Abragam, A. The Principles of Nuclear Magnetism; Oxford University
Press: London, 1961.
(14) For 11, the T1 times were determined at -70 °C. Similar T1
values were seen for the benzyl protons for the alcohol ligand (0.48 s)
and for the other protons of 11 (0.4-0.8 s), and a somewhat longer T1
time was seen for the free alcohol (0.64 s). The T1 values were too
similar to be conclusive in correlating the alcohol fragment to the
organometallic compound by this method.
(11) Coupling between the OH and R-CH hydrogens has been seen
for tungsten alcohol complexes.10b

Communications
Organometallics, Vol. 22, No. 5, 2003 903
sterically crowded to complex to ruthenium, so that we
can determine the rate of dissociation of alcohols from
the neutral complexes.
Ack n ow led gm en t. Financial support from the De-
partment of Energy, Office of Basic Energy Sciences, is
gratefully acknowledged. We received grants from the
NSF (No. CHE-9629688) and NIH (No. I S10 RR04981-
01) for the purchase of NMR spectrometers.
Since alcohol dissociation from the neutral complexes
is anticipated to be substantially faster than from the
more electrophilic cationic complexes, it is very likely
that alcohol dissociation will be much faster than alcohol
production from reduction of aldehydes by hydroxycy-
clopentadienyl hydride complexes such as 3. This ren-
ders the task of determining whether alcohols are
produced inside or outside the coordination sphere of
the metal exceedingly difficult.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details and characterization data for 10-16, T1 mea-
surement of 10 and 11, exchange of 2-propan-d7-ol with 10,
and exchange of 2-propanol with 11. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM020891E