Protonated aminocyclopentadienyl ruthenium hydride reduction of benzaldehyde and the conversion of the resulting ruthenium triflate to a ruthenium hydride with H2 and base

Article abstract of DOI:10.1021/om020507d

Reaction of N-phenyl-2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienimine (6) with Ru₃CO₁₂ formed two isomers of {[2,5-Me₂-3,4-Ph₂(η⁵-C₄CNHPh)]Ru (CO)(μ-CO)}₂ (8-trans and 8-cis). Photolysis of 8 under a H₂ atmosphere led to the formation of the aminocyclopentadienyl ruthenium hydride [2,5-Me₂-3,4-Ph₂(η⁵-C₄CNHPh)]Ru( CO)₂H (9-H). 9-H reduced benzaldehyde slowly at 75 °C to give benzyl alcohol and 8. Protonation of 9-H with triflic acid produced {[2,5-Me₂-3,4-Ph₂(η⁵-C₄CNH₂ Ph)]Ru(CO)₂H}OTf (11-H), which reacted rapidly with benzaldehyde at -80 °C to give benzyl alcohol and [2,5-Me₂-3,4-Ph₂(η⁵-C₄CNHPh)]Ru( CO)₂OTf (9-OTf). Reaction of 9-OTf with H₂ and base led to the re-formation of 9-H. These reactions provide the transformations required for a catalytic cycle for hydrogenation of aldehydes.

Full text of DOI:10.1021/om020507d

5038
Organometallics 2002, 21, 5038-5046
P r oton a ted Am in ocyclop en ta d ien yl Ru th en iu m Hyd r id e
Red u ction of Ben za ld eh yd e a n d th e Con ver sion of th e
Resu ltin g Ru th en iu m Tr ifla te to a Ru th en iu m Hyd r id e
w ith H2 a n d Ba se
Charles P. Casey,* Thomas E. Vos, Steven W. Singer, and Ilia A. Guzei
Department of Chemistry, University of WisconsinsMadison, Madison, Wisconsin 53706
Received J une 26, 2002
Reaction of N-phenyl-2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienimine (6) with Ru3CO12
formed two isomers of {[2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)(µ-CO)}2 (8-tr a n s and 8-cis).
Photolysis of 8 under a H2 atmosphere led to the formation of the aminocyclopentadienyl
ruthenium hydride [2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)2H (9-H). 9-H reduced benzalde-
hyde slowly at 75 °C to give benzyl alcohol and 8. Protonation of 9-H with triflic acid produced
{[2,5-Me2-3,4-Ph2(η5-C4CNH2Ph)]Ru(CO)2H}OTf (11-H), which reacted rapidly with benzal-
dehyde at -80 °C to give benzyl alcohol and [2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)2OTf (9-
OTf). Reaction of 9-OTf with H2 and base led to the re-formation of 9-H. These reactions
provide the transformations required for a catalytic cycle for hydrogenation of aldehydes.
In tr od u ction
tandem with enzymatic conversion of one enantiomer
to a single enantiomer of an ester. This tandem process
accomplishes the dynamic kinetic resolution of second-
ary alcohols.
Homogeneous catalytic reduction of polar functional
groups mediated by transition-metal complexes has
emerged as an alternative to stoichiometric reduction
by metal hydrides such as LiAlH4 and NaBH4.1 Interest
in this area has been spurred by development of highly
efficient catalysts for the selective hydrogenation of
ketones and aldehydes.2 For example, Noyori’s chiral
catalyst [RuCl2{(S)-tolbinap}{(S,S)-1,2-diphenylethyl-
enediamine}] (1) hydrogenates ketones with high levels
of reactivity, chemoselectivity, and enantioselectivity.3
Recently, we investigated the mechanism of the
reaction of 4-H (R ) Tol) with benzaldehyde in detail8
and concluded that carbonyl reduction occurs outside
the coordination sphere of the metal (Scheme 1). Our
observation of kinetic deuterium isotope effects for both
OD and RuD of 4-H requires a mechanism involving
simultaneous transfer of H+ from OH and H- from RuH
to the aldehyde substrate. Noyori has proposed a similar
mechanism involving simultaneous transfer of an acidic
RuN-H and a hydridic Ru-H for reduction by 1 and
related compounds.9,10
A striking feature of the Shvo system is that the
reduction of benzaldehyde by 4-H is rapid at low
temperature (-40 to -10 °C) but catalysis with the
diruthenium complex requires elevated temperature
(80-150 °C). The dienone dicarbonyl intermediate
generated by hydrogen transfer is trapped by ruthenium
hydride 4-H to produce the kinetically stable diruthe-
nium complex 5 (R ) Tol). The slow step involves
reaction of the diruthenium complex with hydrogen.4
In an effort to develop a more reactive catalyst, we have
sought ways of blocking the formation of diruthenium
complexes analogous to 5.
Several years ago, Shvo discovered that the diruthe-
nium complex 2 (R ) Ph) was an efficient ketone
hydrogenation catalyst at 145 °C.4 The active reducing
agent in this catalytic system is the hydroxycyclopen-
tadienyl ruthenium hydride 3-H (R ) Ph). The presence
of an electronically coupled acidic OH group and a
ruthenium hydride in the same molecule appears to be
the key to the high reactivity of 3-H.5 Ba¨ckvall6 and
Park7 have used the Shvo catalyst 2 to interconvert
enantiomeric alcohols via reversible dehydrogenation in
(6) (a) Ba¨ckvall, J . E.; Pamies, O. J . Org. Chem. 2002, 67, 1261. (b)
Ba¨ckvall, J . E.; Huerta, F. F.; Minidis, A. B. E. Chem. Soc. Rev. 2001,
30, 321 and references therein.
(7) (a) Kim, M. J .; Choi, Y. K.; Choi, M. Y.; Kim, M. J .; Park, J . J .
Org. Chem. 2001, 66, 4736. (b) J ung, H. M.; Koh, J . H.; Kim, M. J .;
Park, J . Org. Lett. 2000, 2, 2487.
(8) (a) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.;
Kavana, M. J . Am. Chem. Soc. 2001, 123, 1090. (b) Casey, C. P.; Singer,
S. W.; Powell, D. R. Can. J . Chem. 2001, 79, 1002.
(9) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97 and
references therein.
(10) Noyori’s mechanism is consistent with DFT calculations: Alon-
so, D. A.; Brandt, P.; Nordin, S. J . M.; Andersson, P. G. J . Am. Chem.
Soc. 1999, 121, 9580.
(1) Fehring, V.; Selke, R. Angew. Chem., Int. Ed. 1998, 37, 1827 and
references therein.
(2) Ohkuma, T.; Noyori, R. Angew. Chem., Int. Ed. 2001, 40, 40 and
references therein.
(3) Ohkuma, T.; Ooka, H.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J .
Am. Chem. Soc. 1995, 117, 2675.
(4) Blum, Y.; Czarkie, D.; Rahamim, Y.; Shvo, Y. Organometallics
1985, 4, 1459.
(5) For our earlier attempts to develop heterobimetallic systems
capable of delivering an acidic and a hydridic hydrogen, see: (a) Casey,
C. P.; Wang, Y.; Tanke, R. S.; Hazin, P. N.; Rutter, E. W., J r. New J .
Chem. 1994, 18, 43. (b) Casey, C. P. J . Organomet. Chem. 1990, 400,
205.
10.1021/om020507d CCC: $22.00 © 2002 American Chemical Society
Publication on Web 10/11/2002

Protonated Aminocyclopentadienyl Ru Hydride
Organometallics, Vol. 21, No. 23, 2002 5039
Sch em e 1
Sch em e 2
methyl-substituted complex 7-H was 4 times faster than
by the aryl-substituted complex 4-H.12
N-Phenyl-2,5-dimethyl-3,4-diphenylcyclopenta-2,4-di-
enimine (6) was synthesized in 70% yield by heating
2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienone, aniline,
and TiCl4 in benzene.13 In the 1H NMR spectrum of
6, resonances at δ 1.28 and 2.01 were seen for the
methyl groups syn and anti to the N-phenyl group of
the imine.
{[2,5-Me2-3,4-P h 2(η5-C4CNH P h )]R u (CO)(µ-CO)}2
(8). Reaction of 6 and Ru3CO12 in refluxing MeOH for
1 day led to precipitation of 8-tr a n s as an orange
powder in 55% yield (Scheme 2). The structure of
8-tr a n s was established by X-ray crystallography (Fig-
ure 1; crystal data and structure refinement details are
given in Table 1).
Here we present an approach to preventing formation
of diruthenium complexes similar to 5 by introducing
sterically large -NPh groups on the Cp ring in place of
the less crowded -OH of 4-H. We anticipated that the
formation of a bridging PhN- - -H- - -NPh unit would be
greatly disfavored by steric interactions and by weaker
hydrogen bonding between the nitrogen centers com-
pared to that between oxygen centers. An additional
advantage of the -NHR catalysts is added flexibility
in catalyst design; changing the R group on the amine
will allow drastic alteration of the electronic and steric
features of the catalyst.
We have found that the -NHPh unit does prevent
formation of bridged complexes similar to 5 but that the
lower acidity of the CpNHPh unit reduces the activity
of the mononuclear hydride. A more active reducing
agent was obtained by protonating the nitrogen center
to give a CpNH2Ph+ complex which reduces benzalde-
hyde rapidly at low temperature. We have also found
means of regenerating the ruthenium hydride under
mild conditions and are poised to develop a new catalytic
system.
Spectroscopic studies are consistent with the solid
being pure 8-tr a n s, which dissolves to give solutions of
equilibrating mixtures of 8-tr a n s and 8-cis. The infra-
red spectrum of solid 8-tr a n s as a Nujol mull has only
two strong bands at 1946 and 1752 cm-1 for its terminal
and bridging CO’s. However, in CH2Cl2 solution, two
terminal CO bands at 2021 (s) and 1963 (vs) cm-1 and
a bridging CO band at 1769 cm-1 are seen for a mixture
1
of 8-tr a n s and 8-cis isomers. The H NMR spectrum
in CD2Cl2 at -70 °C shows a 5:1 mixture of isomers with
CpCH3 resonances at δ 1.78 and 1.87; at -40 °C, these
resonances are still sharp but in a 3:1 ratio; at room
temperature, a single sharp coalesced resonance is seen
at δ 1.87 for a rapidly interconverting mixture of
8-tr a n s and 8-cis. The 13C NMR spectrum in CD2Cl2
at -70 °C shows a 5:1 mixture of isomers and also
reveals an additional dynamic process that interchanges
the terminal and bridging CO’s of 8-tr a n s. Sharp
resonances are seen for the bridging (δ 256.1) and
terminal (δ 200.6) resonances of 8-cis, while broad (ω1/2
> 100 Hz) resonances are seen for the bridging (δ 258.6)
and terminal (δ 200.6) resonances of 8-tr a n s.
Resu lts
N-P h en yl-2,5-d im eth yl-3,4-d ip h en ylcyclop en ta -
2,4-d ien im in e (6). Since initial attempts to synthesize
N-phenyltetraarylcyclopenta-2,4-dienimine ruthenium
complexes were plagued by problems of low yield and
low solubility,11 we turned to the N-phenyl-2,5-dimethyl-
3,4-diphenylcyclopenta-2,4-dienimine ligand (6) at an
early stage. To determine whether the replacement of
two phenyl groups by two methyl groups greatly affected
the reactivity of ruthenium complexes, we measured the
relative rates of reduction of benzaldehyde by [2,5-Me2-
3,4-Ph2(η5-C4COH)]Ru(CO)2H (7-H) and by [2,5-Ph2-3,4-
Tol2(η5-C4COH)]Ru(CO)2H (4-H) in THF-d8 at -10 °C.
The stoichiometric reduction of benzaldehyde by the
The very rapid interconversion of the bridging and
terminal carbonyls of the trans isomer, the somewhat
slower interconversion of cis and trans isomers, and the
(12) kobs ) 0.50 × 10-3 s-1 at 0.8 M of benzaldehyde in THF-d8 at
-10 °C for [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2H (4-H); kobs ) 1.9 ×
10-3 s-1 at 0.8 M of benzaldehyde in THF-d8 at -10 °C for [2,5-Me2-
3,4-Ph2(η5-C4COH)]Ru(CO)2H (7-H).
(11) Park has made an N-isopropyltetraarylcyclopenta-2,4-dienimine
ruthenium chloride complex by heating Ru3(CO)12 and the correspond-
ing imine in CHCl3: Choi, J . H.; Kim, Y. H.; Nam, S. H.; Shin, S. T.;
Kim, M. J .; Park, J . Angew. Chem., Int. Ed. 2002, 41, 2373.
(13) When imine synthesis was attempted by dehydration of 2,5-
dimethyl-3,4-diphenylcyclopenta-2,4-dienone and aniline using azeo-
tropic distillation of toluene, only a 25% yield of 6 was obtained after
10 days.

5040 Organometallics, Vol. 21, No. 23, 2002
Ta ble 1. Cr ysta l a n d Str u ctu r e Refin em en t Da ta for 4-Cl, 8-tr a n s, 9-Cl, a n d 9-OTf
Casey et al.
4-Cl
8-trans
9-Cl
9-OTf
mol formula
C
33H25ClO3Ru
C54H44N2O4Ru2
C27H22ClNO2Ru‚
0.5CH2Cl2
571.44
C28H22F3NO5RuS
fw
606.05
987.05
642.60
color, shape
dimens (mm)
temp (K)
wavelength (Å)
cryst syst
space group
a (Å)
yellow, prism
0.46 × 0.32 × 0.24
133(2)
0.710 73
triclinic
orange, prism
0.43 × 0.34 × 0.26
173(2)
0.710 73
rhombohedral
R3h
30.726(2)
30.726(2)
12.3601(11)
90
90
120
10106.0(13)
9
1.460
yellow, prism
0.31 × 0.24 × 0.15
173(2)
0.710 73
monoclinic
P2/n
14.2002(6)
12.4984(6)
15.4984(7)
90
116.931(1)
90
2450.21(19)
4
1.549
0.883
1156
yellow, plate
0.38 × 0.28 × 0.08
100(2)
0.710 73
triclinic
P1h
P1h
11.1279(3)
12.1118(3)
12.1881(3)
105.520(2)
109.349(2)
106.600(2)
1360.92(6)
2
12.2553(11)
12.5502(11)
17.7033(16)
82.203(2)
74.237(2)
88.393(2)
2596.1 (4)
4
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
Fcalcd (Mg m-3
)
1.479
0.707
616
1.644
0.746
1296
abs coeff (mm-1
F(000)
)
0.721
4518
θ range (deg)
total no. of data
2.81-29.15
15 447
1.33-26.38
11 714
1.63-26.39
20 429
1.90-26.39
21 281
no. of unique data
abs cor
6467 (R(int) ) 0.0347)
4495 (R(int) ) 0.0424)
5012 (R(int) ) 0.0319)
10 497 (R(int) ) 0.0537)
empirical with SADABS
max and min transmission
no. of data/restraints/params
GOF
0.746 and 0.633
6467/0/343
1.028
0.835 and 0.747
4495/0/284
1.001
0.879 and 0.771
5012/0/295
1.057
0.943 and 0.765
10 497/0/707
1.026
R indices
R1) 0.0213
wR2 ) 0.0564
R1 ) 0.0473
wR2 ) 0.1031
R1 ) 0.0307
wR2 ) 0.0723
R1 ) 0.0483
wR2 ) 0.1187
Our synthesis of 8 demonstrates the success of our
strategy of employing sterically encumbered nitrogen-
substituted Cp ruthenium complexes to prevent forma-
tion of diruthenium complex A, which is analogous to
the kinetically sluggish diruthenium complex 2 of Shvo.
It should be noted that dimer 8 is an isomer of A.
[2,5-Me2-3,4-P h 2(η5-C4CNHP h )]R u (CO)2Cl (9-Cl)
fr om R ea ct ion of 8 w it h Ch lor in a t ed Solven t s.
Dimer 8 is stable to air as a solid and is stable for days
in solution under nitrogen in nonhalogenated solvents.
8 is stable in CH2Cl2 and CD2Cl2 for days in the dark
but reacts with CH2Cl2, CD2Cl2, or CHCl3 in ambient
light over 4-6 h to form the aminocyclopentadienyl
ruthenium chloride 9-Cl (Scheme 3). The structure of
9-Cl was determined spectroscopically and confirmed
by X-ray crystallography (Figure 2; crystal data and
structure refinement details are given in Table 1). This
chlorination is analogous to the photochemical conver-
sion of [CpRu(CO)2]2 (10) in halogenated solvents to
CpRu(CO)2X (X ) I, Cl, Br).15
F igu r e 1. X-ray structure of {[2,5-Me2-3,4-Ph2(η5-C4-
CNHPh)]Ru(CO)(µ-CO)}2 (8-tr a n s). Selected bond lengths
(Å) and angles (deg): Ru(1)-C(1), 2.297(4); Ru(1)-C(2),
2.301(4); Ru(1)-C(3), 2.233(4); C(1)-N(1), 1.414(5); Ru(1)-
Ru(1A) 2.7499(7); Ru(1)-C(27)-Ru(1A), 84.81(19); C(27)-
Ru(1)-C(27A), 95.2(2); C(26)-Ru(1)-C(27), 92.45(18).
[2,5-Me2-3,4-P h 2(η5-C4CNH P h )]R u (CO)2OTf (9-
OTf) was most readily prepared by reaction of 9-Cl with
1.2 equiv of AgOTf in CH2Cl2 at room temperature
(Scheme 3). The conversion from 9-Cl to 9-OTf was
observation of only the trans isomer in the solid state
has been seen for [CpRu(CO)2]2, [CpFe(CO)2]2, and other
related complexes.14
1
quantitative by H NMR spectroscopy. 9-OTf was also
(14) (a) Adams, R. D; Cotton, F. A. J . Am. Chem. Soc. 1973, 95, 6589.
(b) Cotton, F. A. Inorg. Chem. 2002, 41, 643 and references therein.
(c) Dyke, A. F.; Knox, S. A. R.; Mead, K. A.; Woodward, P. J . Chem.
Soc., Chem. Commun. 1981, 861.
(15) Eisenstadt, A.; Tannenbaum, R.; Efraty, A. J . Organomet.
Chem. 1981, 221, 317.

Protonated Aminocyclopentadienyl Ru Hydride
Organometallics, Vol. 21, No. 23, 2002 5041
Sch em e 3
Sch em e 4
Figu r e 2. X-ray structure of [2,5-Me2-3,4-Ph2(η5-C4CNHPh)]-
Ru(CO)2Cl (9-Cl). Selected bond lengths (Å) and angles
(deg): Ru-C(1), 2.375(2); Ru-C(2), 2.250(2); Ru-C(3),
2.182(2); Ru-Cl, 2.4362(6); N-C(1), 1.361(3); C(26)-Ru-
(1)-C(27), 92.75(11); C(26)-Ru(1)-Cl, 92.56(7).
prepared by addition of 2.1 equiv of triflic acid to the
dimer 8 in CH2Cl2 at room temperature. 9-OTf was
characterized by spectroscopy and by X-ray crystal-
lography (see the Supporting Information). Two equal-
character and η5-Cp binding. In the IR spectrum, two
CO stretching bands of equal intensity appeared at 2016
and 1954 cm-1. These bands occurred at frequencies
lower than those of the corresponding chloride 9-Cl
(2043, 1992 cm-1).
intensity IR bands were seen at 2043 and 1994 cm-1
.
The 13C NMR spectrum had a resonance for CO at δ
198.4.
In related work, the UV photolysis of [CpRu(CO)2]2
(10) under 10-20 atm of H2 produced CpRu(CO)2H in
[2,5-Me2-3,4-P h 2(η5-C4CNHP h )]Ru (CO)2H (9-H) is
an analogue of the hydroxycyclopentadienyl ruthenium
hydride 3-H, the active species in the catalytic reduction
of aldehydes to alcohols with the Shvo catalyst. It was
crucial to prepare 9-H and to determine whether
substitution of -NHPh for -OH still allowed ligand-
assisted hydride transfer. The critical 9-H complex was
synthesized by three different routes (Scheme 4).
UV photolysis of the dimer 8 in THF-d8 under 2 atm
of H2 led to the formation of hydride 9-H. The photolysis
was followed by 1H NMR spectroscopy; a ruthenium
hydride resonance grew in at δ -10.12 along with
resonances at δ 2.04 (CpCH3) and 7.13 (NH). 9-H was
formed in >95% yield and was used without further
purification. Solutions of 9-H in a variety of solvents
were easily prepared by evaporation of THF under high
vacuum followed by redissolving in a new solvent. 9-H
was also formed by reaction of 9-Cl with either lithium
triethylborohydride or sodium methoxide.
a reaction strongly inhibited by 2-5 atm of CO.16
A
mechanism involving CO dissociation from 10, oxidative
addition of H2, and reductive elimination of CpRu-
(CO)2H was proposed (Scheme 5). To determine whether
the conversion from 8 to 9-H might occur by a similar
mechanism, we studied possible CO inhibition. Side-by-
side tubes containing THF-d8 solutions of 8 under either
1 atm of H2 or 1 atm of a 1:1 mixture of H2 and CO
were photolyzed. Somewhat greater conversion to 9-H
was found in the presence of CO (63% vs 27% after 30
min) but was accompanied by the formation of a total
of 15% of two additional unidentified species with 1H
NMR methyl resonances at δ 1.96 and 1.89. The failure
to see CO inhibition of the formation of 9-H indicates
that it is formed by a different mechanism than the
conversion of 10 to CpRu(CO)2H. One possible mecha-
nism involves photochemical cleavage of the Ru-Ru
bond of 8 and reaction of monomeric species with H2.
Under 1 atm of H2, solutions of 9-H in THF-d8 were
stable for weeks at room temperature. In the absence
13C NMR and IR spectroscopy confirmed the structure
assignment of 9-H. In the 13C NMR spectrum of 9-H,
the resonance for the Cp ring carbon (C1) at δ 109.9 is
indicative of very little carbon-nitrogen double-bond
(16) Bitterwolf, T. E.; Linehan, J . C.; Shade, J . E. Organometallics
2000, 19, 4915.

5042 Organometallics, Vol. 21, No. 23, 2002
Casey et al.
Sch em e 5
Sch em e 6
between the pKa of the -NHPh units of hydride 9-H
and chloride 9-Cl is the same as the 5.5 pKa unit
difference between the acidities of the corresponding
-OH compounds 4-H and 4-Cl, then the pKa of the
-NHPh unit of hydride 9-H is estimated to be about
30.19
of H2, a solution of 9-H in THF-d8 underwent about 20%
decomposition over 36 h at 85 °C to produce an
unidentified black solid, but none of dimer 8. While
solutions of 9-H in THF-d8 were photostable, solutions
in CD2Cl2 under ambient light were converted to 9-Cl
within 1 h. Similar photoconversion of CpRu(CO)2H to
CpRu(CO)2Cl in halogenated solvents has been re-
ported.17
Low Acid ity of 9-H. For the aminocyclopentadienyl
ruthenium hydride 9-H to act as an effective reducing
agent for aldehydes, the -NH function needs to be acidic
enough to protonate the carbonyl oxygen as hydride is
being delivered from ruthenium. We therefore set out
to determine the acidity of 9-H and to compare its
acidity to that of the related hydroxycyclopentadienyl
ruthenium hydride 4-H, which we had measured in
CH3CN.8a When the base 1,1,3,3-tetramethylguanidine
(TMG, pKa ) 23.3 in CH3CN) was added to a solution
of 9-H in CH3CN, only a small decrease in the IR bands
of 9-H and no new IR peaks assignable to a deproto-
nated species were seen. This establishes that the pKa
value of the NHPh unit of 9-H is greater than 25.
The NHPh unit is therefore substantially less acidic
than the OH unit of the hydroxycyclopentadienyl ru-
thenium hydride 4-H (pKa ) 17.5 in CH3CN).8a To more
quantitatively assess the difference in acidity between
the -NHPh and -OH groups on Cp ruthenium systems,
we compared the acidities of the more acidic chloride
complexes 9-Cl and [2,5-Ph2-3,4-Tol(η5-C4COH)]Ru-
(CO)2H (4-Cl). Using Norton’s IR method for determi-
nation of the pKa of metal carbonyl hydrides in CH3CN,
TMG was added to a solution of 9-Cl in CH3CN; new
IR peaks at 1990 and 1920 cm-1 for the deprotonated
species grew in, and peaks at 2040 and 1991 cm-1 for
9-Cl diminished. The pKa of the amine group of 9-Cl
was determined to be 25.0 ( 0.2. Similarly, the pKa of
4-Cl in CH3CN was determined to be 12.0 ( 0.2 by
measuring the extent of deprotonation with pyridine
(pKa ) 12.3 in CH3CN). The NHPh group of 9-Cl is
therefore 13 pKa units less acidic than the OH group of
4-Cl.18 The pKa of the -OH unit of hydride 4-H is 5.5
units greater than that of chloride 4-Cl. If the difference
Slow Rea ction of 9-H w ith Ben za ld eh yd e. In
contrast to the rapid reaction of CpOH complex 4-H with
benzaldehyde at low temperature, the CpNHPh complex
9-H did not react with benzaldehyde at room temper-
ature. Slow reduction of benzaldehyde by 9-H in THF-
d8 was observed by 1H NMR spectroscopy at 75 °C
(Scheme 6). After 8.5 h, approximately 30% of 9-H had
reacted and benzyl alcohol (δ 4.53 for CH2O) was
formed. The rate of reduction of benzaldehyde by 9-H
is approximately 10 000 times slower than by 4-H.20 We
ruled out a fast and reversible hydrogenation of ben-
zaldehyde by reacting 9-H with benzaldehyde-d at room
temperature for 2 days and finding no hydrogen incor-
poration into the benzaldehyde and no deuterium
incorporation into 9-H. Also, no significant difference
in the rate of reduction of benzaldehyde by 9-H was seen
when the reaction was carried out under 1 atm of CO.
In toluene, where the rate of reaction of 4-H with
benzaldehyde is 50 times faster than in THF, 20%
reaction of 9-H with benzaldehyde to produce benzyl
alcohol (δ 4.36 for CH2O) was seen after 2 h at 75 °C.
Thus, the reaction of 9-H with benzaldehyde in toluene
is only about 2-3 times faster than in THF.
The greatly diminished reactivity of the CpNHPh
complex 9-H compared to that of the CpOH complex 4-H
is attributed to the much less acidic nature of the
CpNHPh unit and the necessity for simultaneous pro-
tonation of the aldehyde oxygen as hydride is delivered
from ruthenium. Protonation at nitrogen to give a
cationic CpNH2Ph+ complex provides an attractive way
to enhance the acidity of the NH group and to achieve
rapid reduction of benzaldehyde.
P r oton a tion of 9-H. There are two potential sites
for protonation of 9-H: the amine nitrogen to give an
(19) Because of the very low acidity of the -NHPh unit of 9-H, the
most acidic proton in the compound may be the RuH. The pKa of CpRu-
(CO)2H in CH3CN is 20.2: Moore, E. J .; Sullivan, J . M.; Norton, J . R.
J . Am. Chem. Soc. 1986, 108, 2257.
(20) Assuming a second-order rate law, 30% conversion of 9-H in
reaction with 0.8 M benzaldehyde corresponds to k2 ≈ 1.5 × 10-5 M-1
s-1. Careful kinetic studies of the reduction of benzaldehyde by 4-H
between 0 °C and -15 °C gave ∆Hq ) 12.0 ( 1.5 kcal mol-1 and ∆Sq
(17) Morandini, F.; Consiglio, G.; Lucchini, V. Organometallics 1985,
4, 1202.
(18) This is substantially larger than the 7 pKa unit difference
between the acidities of diphenylamine (25) and phenol (18) in
DMSO: Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
) -28 ( 5 eu; extrapolation to 75 °C gives k2 ≈ 1.6 × 10-1 M-1 s-1
.
While this estimate is crude, the calculated rate difference of 104 is
large.

Protonated Aminocyclopentadienyl Ru Hydride
Organometallics, Vol. 21, No. 23, 2002 5043
Sch em e 7
ammonium salt and ruthenium to give either a dihy-
dride or a dihydrogen complex. With excess acid, both
nitrogen and ruthenium might be protonated. Protona-
tion of 9-H with either triflic acid or HBF4‚OMe2 in CD2-
Cl2 at low temperature gave the ammonium complex
{[2,5-Me2-3,4-Ph2(η5-C4CNH2Ph)]Ru(CO)2H}+ (11-H ).
Triflic acid in CD2Cl2 was added to a solution of 9-H in
CD2Cl2 at -78 °C in a resealable NMR tube. The tube
was shaken and immediately placed in a precooled NMR
1
probe at -70 °C. In the H NMR spectrum, the N-Ph
protons moved to δ 7.5 from δ 6.71 and 6.69 in the
neutral complex 9-H; this chemical shift change is very
characteristic of protonation of NPh+ groups. The -NH2
resonance of 11-H appears at δ 11.2 as a broad singlet,
and RuH is a sharp singlet at δ -10.1.21
Interestingly, 9-H could not be appreciably protonated
in THF either by triflic acid or by excess HBF4‚OMe2.
Addition of THF to a CH2Cl2 solution of 11-H led to
deprotonation and re-formation of 9-H. Thus, while the
amine nitrogen of 9-H can be protonated by triflic acid,
the resulting ammonium complex 11-H is a very strong
acid capable of protonating THF. The high acidity of the
ammonium nitrogen would be expected to greatly
enhance the ability of 11-H to reduce aldehydes.
Protonation of the ruthenium triflate 9-OTf and the
ruthenium chloride 9-Cl by excess triflic acid in CD2-
Cl2 also occurred at the NHPh group to produce {[2,5-
Me2-3,4-Ph2(η5-C4CNH2Ph)]Ru(CO)2OTf}OTf (11-OTf)
and {[2,5-Me2-3,4-Ph2(η5-C4CNH2Ph)]Ru(CO)2Cl}OTf (11-
of 11-H was unstable above -25 °C and lost hydrogen
to form 9-OTf. At -25 °C, a solution of 11-H in CD2-
Cl2, formed by addition of triflic acid to 9-H, turned
cloudy and some solid precipitated.23 Upon being warmed
to room temperature, the precipitate redissolved and
9-OTf was observed by 1H NMR spectroscopy. A few
bubbles were seen, and a small resonance in the 1H
NMR spectrum at δ 4.6 was observed, consistent with
the evolution of H2. With only a slight excess of triflic
acid present, the more basic nitrogen of 9-H is proto-
nated while the somewhat less basic nitrogen of 9-OTf
is not protonated.
While we have not yet probed the mechanism of the
decomposition of 11-H, there are three distinct possible
mechanisms that appear plausible (Scheme 7). First, a
dihydrogen complex (B) might be formed from 11-H by
intramolecular proton transfer and then lose dihydro-
gen. Capture of the resulting vacant coordination site
by triflate would then produce 9-OTf. Second, the
dihydrogen complex B might form by a series of inter-
molecular proton transfers mediated by triflic acid and
then lose dihydrogen to give 9-OTf. Third, the dicationic
dihydrogen complex C might be generated by protona-
tion of 11-H at ruthenium with excess acid and then
lose dihydrogen and a proton to give 9-OTf.
Rea ction of 9-OTf w ith H2 To Gen er a te 9-H. The
development of a catalytic system for the reduction of
aldehydes based on the cationic 11-H requires a means
of reacting 9-OTf with H2 to regenerate at least small
equilibrium amounts of the active reducing agent 11-
H, which can be drained off by reaction with carbonyl
compounds. The rapid decomposition of the protonated
ruthenium hydride 11-H to dihydrogen and 9-OTf
suggests that the microscopic reverse, the conversion
of the triflate complex to ruthenium hydride complex
11-H under H2, may also be a facile process. To
determine whether the reaction of 9-OTf is kinetically
rapid enough to regenerate 11-H at room temperature,
we sought to trap 11-H by a very favorable deprotona-
1
Cl). H NMR spectroscopy at -70 °C showed shifts of
the NPh hydrogens to higher frequency (δ 7.5 from δ
7.0) upon protonation, just as seen for 11-H. Solutions
of 11-OTf and 11-Cl are stable at room temperature
for hours.
Red u ction of Ben za ld eh yd e w ith 11-H a t -80 °C.
In contrast to the very slow reaction of the neutral
ruthenium hydride 9-H with benzaldehyde at 75 °C,
reaction of 11-H with benzaldehyde occurred very
rapidly at -80 °C (Scheme 6). A solution of benzalde-
hyde in CH2Cl2 was added to a solution of 11-H in CD2-
Cl2 in a resealable NMR tube at -94 °C. The tube was
immediately placed in a precooled NMR probe at -80
1
°C, and a H NMR spectrum was obtained within 5 min.
The spectrum showed complete disappearance of 11-H.
The room-temperature NMR spectrum showed reso-
nances for benzyl alcohol at δ 4.69 (CH2O) and for 9-OTf
at δ 1.9 (CpCH3).
While 11-H reacted with benzaldehyde very rapidly
at -80 °C, no reaction between 11-H and cyclohexene
in CD2Cl2 was seen by 1H NMR spectroscopy up to -20
°C, where decomposition of 11-H to 9-OTf was observed.
Thus, 11-H rapidly adds hydrogen across the polar Cd
O double bond of benzaldehyde but does not react with
the unpolarized CdC double bond of cyclohexene.22
Similar reactivity was seen for the hydroxycyclopenta-
dienyl ruthenium hydride 4-H.
Loss of H yd r ogen fr om {[2,5-Me2-3,4-P h 2(η5-
C4CNH2P h )]Ru (CO)2H}OTf (11-H). The triflate salt
(23) We speculate that the precipitate may be the bridging ruthe-
nium hydride [{[2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)2}2(µ-H)]OTf.
The related BF4 salt was apparently formed when a solution of 11-H
in CD2Cl2 (formed by addition of excess HBF4‚OMe2 to 9-H) was
warmed above -25 °C. 1H NMR spectroscopy showed a bridging
hydride resonance at δ -16.5 (1 H), a methyl resonance at δ 2.06 (12
H), and resonances for an unprotonated N-Ph group at δ 7.0-7.2.
Similar hydride chemical shifts have been seen for other cationic
bridging diruthenium hydrides. Chinn, M. S.; Heinekey, D. M.; Payne,
N. G.; Sofield, C. D. Organometallics 1989, 8, 1824.
(21) No evidence for protonation at ruthenium was seen. This is
consistent with Morris’s estimate of the pK of CpRu(CO)2(η2-H2)+ in
CH2Cl2 of -6: Morris, R. H.; J ia, G. J . Am. Chem. Soc. 1991, 113, 875.
(22) In some cases, trisubstituted and tetrasubstituted alkenes that
are readily protonated can be reduced by strong acids and transition-
metal hydrides: Bullock, R. M.; Song, J . S. J . Am. Chem. Soc. 1994,
116, 8602.

5044 Organometallics, Vol. 21, No. 23, 2002
Casey et al.
Sch em e 8
4-H (pKa ) 17.5) reacted rapidly at -10 °C, and the
CpNH2Ph+ complex 11-H (pKa ≈ 1) reacted rapidly at
-80 °C. These rate enhancements are consistent with
our proposed concerted mechanism for aldehyde reduc-
tion, in which an acidic proton is transferred to the
aldehyde oxygen as hydride is transferred from ruthe-
nium. Clearly, the acidity of the CpOH or CpNHR group
plays a central role in carbonyl reductions.
tion with added base to produce the neutral hydride 9-H
(Scheme 8).
Mech a n ism of Ald eh yd e Red u ction . We have
proposed that the CpOH complex 4-H reacts with
aldehydes by a concerted mechanism (Scheme 1). The
simultaneous transfer of a proton from OH and hydride
from RuH is supported by a second-order kinetic rate
law, the absence of CO inhibition, and deuterium isotope
effects for both OD and RuD. A similar mechanism
would be anticipated for the neutral CpNHPh complex
9-H.
For the cationic CpNH2Ph+ complex 11-H, a stepwise
mechanism needs to be considered in addition to the
concerted mechanism because of the high acidity of the
ammonium group. The stepwise mechanism involves
reversible protonation of the aldehyde oxygen by the
ammonium group, followed by a hydride transfer from
ruthenium. Two-step mechanisms for “ionic hydrogena-
tion” have been proposed by Bullock24 and Norton25 for
reduction of ketones by the neutral metal hydrides
CpW(CO)3H, CpMo(CO)3H, and CpRe(NO)(PPh3)H, in
the presence of a strong Brønsted acid (CF3CO2H, CF3-
SO2H). In principle, the concerted and stepwise mech-
anisms for reduction of aldehydes by 11-H can be
distinguished by isotope effect measurements. Unfor-
tunately, the rate of aldehyde reduction by 11-H is too
fast to measure.
Reaction of 9-OTf with H2 and diisopropylethylamine
in CH2Cl2 at room temperature led to the clean forma-
tion of 9-H. The reaction was followed by IR spectros-
copy. No reaction between diisopropylethylamine and
9-OTf was seen until H2 was added. Upon addition of
100 atm of H2, IR peaks for 9-H (2014 and 1957 cm-1
grew in with a half-life of about 30 min.
)
The related reaction of 9-OTf with 1 atm of H2 in
1
THF-d8 was followed by H NMR spectroscopy. After a
few hours, the ratio 9-OTf:8:9-H was 25:60:15, as
measured by integration of the CpCH3 1H NMR reso-
nances. After 3 days, the equilibrium ratio 9-OTf:8:9-H
was <5:53:47. Apparently, THF is basic enough to drive
the equilibrium toward 9-H.
Discu ssion
Differ en t Str u ctu r a l Typ es for Cp OH a n d Cp N-
HP h Dir u th en iu m Com p lexes. We had determined
that reduction of aldehydes by the hydroxycyclopenta-
dienyl ruthenium hydride 4-H is very rapid and that
the slow step in aldehyde hydrogenations catalyzed by
the diruthenium bridging hydride 5 was its reaction
with H2. In an effort to develop more active hydrogena-
tion catalysts, we sought to prevent formation of a
kinetically sluggish bridging hydride. We turned to
CpNHPh systems with the anticipation that a sterically
congested bridging PhN- - -H- - -NPh unit would disfa-
vor formation of bridging hydride A. This strategy was
successful: instead of obtaining bridging hydride A, the
isomeric symmetric dimer 8 was obtained. The change
in the type of diruthenium compound formed in going
to -NHPh systems from -OH systems is attributed to
both steric destabilization of A and to weaker hydrogen
bonding between the nitrogen centers of A compared
with the oxygen centers of 5.
Another way of explaining why 5 and 8 have different
structures involves consideration of the hypothetical
monoanions of each complex. These structures have a
metal-metal bond and a single proton shared by the
CpO or CpNPh groups. For oxygen, the bridge should
resemble a carbonyl group hydrogen-bonded to an
alcohol (CdO- - -H-O); for nitrogen, the bridge involves
an imine hydrogen bonded to an amine (CdNR- - -H-
N). What is the most likely site of protonation for each
structure? For the much more basic imine, protonation
at N is expected and dimer 8 is formed. In contrast, the
less basic carbonyl oxygen is not readily protonated and
protonation of the Ru-Ru bond occurs to give the
bridging hydride complex 5.
Regen er a tion of Ru th en iu m Hyd r id e fr om 9-OTf
a n d H2. It is not clear whether base is merely driving
the equilibrium between 9-OTf and 11-H (Scheme 8)
or is playing a more active role. There are two ways that
a base might play an active role (Scheme 9). Reversible
dissociation of triflate, dihydrogen coordination, and
deprotonation of the dihydrogen complex by base could
produce 9-H. In related chemistry, a dihydrogen com-
plex was formed from reaction of Cp*Re(CO)(NO)OTf
and H2 at -78 °C.26 Alternatively, the base-promoted
elimination of triflic acid could produce the neutral
coordinatively unsaturated species D that could coor-
dinate dihydrogen and transfer a proton intramolecu-
larly to the imine nitrogen to produce 9-H.
P ossible Develop m en t of a Ca ta lytic Cycle. We
have demonstrated the feasibility of the two steps
needed for a catalytic cycle for aldehyde hydrogenation
(Scheme 10). We have shown that the cationic ruthe-
nium hydride 11-H rapidly reduces benzaldehyde to
benzyl alcohol at -80 °C and produces the triflate
9-OTf. This step requires strongly acidic conditions for
11-H to remain protonated. The existence of a pathway
for the regeneration of ruthenium hydride 11-H from
(24) (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) Voges, M. H.; Bullock, R. M. J . Am. Chem. Soc. 2000, 122,
12594.
(25) (a) Smith, K. T.; Norton, J . R.; Tilset, M. Organometallics 1996,
15, 4515. (b) Magee, M. P.; Norton, J . R. J . Am. Chem. Soc. 2001, 123,
1778.
E n h a n ced R ea ct ivit y of Acid ic Cp XH Com -
p lexes. Huge rate enhancements for reduction of ben-
zaldehyde by ruthenium hydrides were observed as the
acidity of the group attached to the Cp ring was
increased. The CpNHPh complex 9-H (pKa ≈ 30) reacted
slowly with benzaldehyde at 75 °C, the CpOH complex
(26) Chinn, M. S.; Heinekey, D. M.; Payne, N. G.; Sofield, C. D.
Organometallics 1989, 8, 1824.

Protonated Aminocyclopentadienyl Ru Hydride
Organometallics, Vol. 21, No. 23, 2002 5045
Sch em e 9
Sch em e 10
Exp er im en ta l Section
{[2,5-Me2-3,4-P h 2(η5-C4CNHP h )]Ru (CO)(µ-CO)}2 (8). A
suspension of Ru3(CO)12 (230 mg, 0.36 mmol) and 6 (387 mg,
1.15 mmol) in methanol (50 mL) was refluxed for 24 h.
Filtration gave 8 as a yellow-orange precipitate, which was
recrystallized from THF/pentane at -10 °C to give pure 8 (232
mg, 45%); mp 255-60 °C dec. IR (Nujol): 1947 (s), 1752 (s)
1
cm-1. H NMR (-75 °C, CD2Cl2, 500 MHz): major isomer (8-
tr a n s), δ 1.82 (s, Me), 5.27 (s, NH), 6.64 (d, J ) 7.5 Hz, NPh
ortho), 6.75 (t, J ) 7.5 Hz, NPh para), 7.10 (t, J ) 7.5 Hz,
NPh meta), 7.2-7.4 (aromatic); minor isomer (8-cis), δ 1.88
(s, Me), 5.01 (s, NH), 6.28 (d, J ) 7.5 Hz, NPh ortho), 6.74 (t,
J ) 7.5 Hz, NPh para), 7.17 (t, J ) 7.5 Hz, NPh meta), 7.2-
7.4 (aromatic). Ratio 8-tr a n s:8-cis: at -40 °C, 3:1; at -75 °C,
5:1. 13C{1H} NMR (-75 °C, CD2Cl2, 125 MHz): major isomer
(8-tr a n s), δ 10.8 (Me), 102.5 (C3, C4 of Cp), 105.7 (C2, C5 of
Cp), 111.1 (C1 of Cp), 114.7, 119.5, 128.4, 128.9, 130.1, 132.0
(ipso), 132.4, 145.6 (ipso), 200.7 (br s, terminal CO), 258.7 (br
s, bridging CO); 8-cis (CO resonances only), 200.7 (s, terminal
CO), 256.1 (s, bridging CO). MS (electrospray ionization, CH 2-
Cl2, MeOH): m/z (MNa+) calcd for C54H44O4N2Ru2Na, 1011.1,
found 1011.1. Anal. Calcd for C54H44N2O4Ru2 (987.08): C,
65.71; H, 4.49; N, 2.84. Found: C, 64.59; H, 4.32; N, 2.64.27
[2,5-Me2-3,4-P h 2(η5-C4CNHP h )]Ru (CO)2H (9-H). A solu-
tion of 8 (14.0 mg, 0.014 mmol) in THF-d8 (0.4 mL) in a
resealable NMR tube was degassed by three successive freeze-
pump-thaw cycles and placed under 1 atm of H2 at -78 °C.
The tube was irradiated with UV light (254 nm) for 30-40 h
reaction of H2 with 9-OTf was established by the
observation of its facile microscopic reverse. We know
this equilibrium is unfavorable for formation of 11-H
under strongly acidic conditions.
There is a second pathway for regeneration of a
ruthenium hydride. In the presence of base, we have
succeeded in adding H2 to the neutral ruthenium triflate
9-OTf to form the neutral ruthenium hydride 9-H.
Our favored explanation for this reaction is that an
equilibrium between (9-OTf + H2) and 11-H (Scheme
8) is driven by base toward the ruthenium hydride
9-H. We have not ruled out the possibility that the base
is required for a more active role in the formation of
9-H.
1
at room temperature. H NMR spectroscopy indicated nearly
quantitative formation of 9-H. IR (THF): 2016 (s), 1957 (s)
1
cm-1. H NMR (THF-d8, 500 MHz): δ -10.1 (s, RuH), 2.04 (s,
Me), 6.69 (d, J ) 7.5 Hz, NPh para), 6.71 (t, J ) 7.5 Hz, NPh
ortho), 7.01 (s, NH), 7.0-7.4 (aromatic). 13C{1H} NMR (THF-
d8, 125 MHz): δ 11.1 (Me), 101.8 (Cp), 106.1 (Cp), 109.9 (C1
of Cp), 113.8, 118.4, 127.8, 128.2, 129.4, 132.7 (ipso), 132.8,
148.2 (ipso), 202.7 (CO). Evaporation of THF under high
vacuum gave 9-H as an oily solid which could be redissolved
in a new solvent without decomposition. 9-H decomposed over
days under vacuum, and attempted recrystallizations of 9-H
were unsuccessful.
The cationic triflate 11-OTf which would be the major
species present under very acidic conditions is expected
to have a stronger ruthenium triflate bond than neutral
triflate 9-OTf. Consequently, 11-OTf is expected to be
less reactive toward H2 than 9-OTf. Problems might
then develop in choosing conditions for a catalytic cycle,
since aldehyde reduction requires strongly acidic condi-
tions and ruthenium hydride regeneration may require
less acidic conditions. If 11-OTf does not activate
hydrogen very well, an intermediate acidity might be
required to optimize catalysis. We are currently explor-
ing the development of hydrogenation catalysts based
on combination of triflic acid with the CpNHPh ruthe-
nium compounds reported here.
[2,5-Me2-3,4-P h 2(η5-C4CNHP h )]Ru (CO)2OTf (9-OTf). Tri-
flic acid (19 µL, 0.21 mmol) was added to a solution of 8 (100
mg, 0.10 mmol) in CH2Cl2 (6 mL). After 24 h, 3 mL of water
was added. The organic layer evaporated to give 9-OTf as a
yellow solid (100 mg, 77% yield). IR (CD2Cl2): 2043 (s), 1994
1
(s) cm-1. H NMR (CD2Cl2, 500 MHz): δ 1.90 (s, Me), 6.52 (br
s, NH), 7.0-7.4 (aromatic). 13C{1H} NMR (CD2Cl2, 125 MHz):
(27) While the carbon analysis was low, 1H and 13C NMR spectros-
copy indicated the compound was pure and an X-ray crystal structure
was obtained on crystals of 8-tr a n s formed upon slow evaporation of
a methanol solution.

5046 Organometallics, Vol. 21, No. 23, 2002
Casey et al.
δ 12.3 (Me), 82.2 (Cp), 102.0 (Cp), 119.7 (q, J C-F ) 315 Hz,
CF3), 129.9 (ipso), 125.4, 126.9, 129.2, 129.8, 130.2, 132.2, 136.5
(C1 of Cp), 139.5 (ipso), 198.4 (CO). 19F NMR (CD2Cl2, 282
MHz): δ -76.9. MS (electrospray ionization, CHCl3, MeOH):
m/z (M - OTf+) calcd for C27H22O2NRu 494.1, found 494.1.
Anal. Calcd for C28H22NO5RuS (642.61): C, 52.33; H, 3.45; N,
2.18. Found: C, 53.68; H, 3.98; N, 2.03.28 X-ray-quality crystals
were grown by slow evaporation of a CH2Cl2 solution.
9-OTf was also synthesized by addition of AgOTf (10 mg,
0.039 mmol) to a solution of 9-Cl (17 mg, 0.032 mmol) in
toluene-d8 (0.4 mL). The conversion was quantitative by NMR
after 24 h.
resealable NMR tube was evaporated to dryness under high
vacuum. CD2Cl2 (0.4 mL) was vacuum-transferred into the
tube. Triflic acid (3.1 µL, 0.035 mmol) was added to the solution
via syringe at -78 °C. The NMR tube was placed into a
precooled NMR probe at -70 °C. 1H NMR (-70 °C, CD2Cl2,
500 MHz): δ -10.13 (s, RuH), 2.22 (s, Me), 7.1-7.3 (aromatic
protons), 7.53 (m, 2H, NPh), 7.58 (m, 3H, NPh), 11.16 (br s,
NH2). 13C{1H} NMR (-70 °C, CD2Cl2, 125 MHz): δ 10.6 (Me),
97.2 (C1 of Cp), 98.4 (Cp), 106.9 (Cp), 121.7, 128.0, 128.1, 129.4
(ipso), 130.6, 130.7, 132.1 (br), 135.6 (ipso), 198.8 (CO). 19F
NMR (-70 °C, CD2Cl2, 470 MHz): δ -78.2.
9-OTf was also synthesized by addition of triflic acid to a
solution of 9-H in CH2Cl2 at -78 °C, followed by warming to
room temperature. The conversion was essentially quantitative
by NMR.
Ack n ow led gm en t. Financial support from the De-
partment of Energy, Office of Basic Energy Sciences, is
gratefully acknowledged. S.W.S. thanks the NIH (5T32
GM 08505) for support under a Chemistry Biology
Interface Training Grant. Grants from the NSF (Grant
No. CHE-9629688) and the NIH (Grant No. I S10
RR04981-01) for the purchase of NMR spectrometers
and from the NSF (Grant No. CHE-9105497) for the
purchase of X-ray instruments are acknowledged. We
thank Professor J aiwook Park for sharing his results
prior to publication.
{[2,5-Me2-3,4-P h 2(η5-C4CNH2P h )]Ru (CO)2OTf}OTf (11-
OTf). Triflic acid (6 µL, 0.068 mmol) was added to a solution
of 9-OTf (17.1 mg, 0.027 mmol) in CD2Cl2 (0.4 mL) in a
resealable NMR tube at -78 °C to give a solution of 11-OTf.
The NMR tube was placed into a precooled NMR probe at -70
°C. IR (CD2Cl2): 2082 (s), 2042 (s) cm-1 1H NMR (-70 °C,
.
CD2Cl2, 500 MHz): δ 2.19 (s, Me), 7.06 (d, J ) 7.5 Hz, CpPh
ortho), 7.31 (t, J ) 7.5 Hz, CpPh meta), 7.39 (t, J ) 7.5 Hz,
CpPh para), 7.59 (m, 2H, NPh), 7.74 (m, 3H, NPh), 11.37 (br
s, NH2). 13C{1H} NMR (24 °C, CD2Cl2, 125 MHz): δ 11.3 (Me),
95.6 (C1 of Cp), 102.3 (Cp), 107.0 (Cp), 122.6, 126.0 (ipso),
129.7, 131.1, 131.6, 132.3, 132.8, 134.3 (ipso), 192.9 (CO). 19F
NMR (-70 °C, CD2Cl2, 470 MHz): δ -76.4, -78.5, -77.2
(HOTf).
Su p p or tin g In for m a tion Ava ila ble: Text giving general
experimental methods, experimental details, and characteriza-
tion data for 4-Cl, 6, 7-H, 9-Cl, 11-Cl, figures giving original
1H and 13C NMR spectra of 8, 9-H, 9-Cl, and 9-OTf, text giving
details of the reaction of 9-OTf with H2, the reduction of
benzaldehyde by 7-H, 9-H, and 11-H, and the pKa determina-
tions of 4-Cl, 9-Cl, and 9-H, and tables giving X-ray crystal-
lographic data for 4-Cl, 8, 9-Cl, and 9-OTf. This material is
available free of charge via the Internet at http://pubs.acs.org.
{[2,5-Me2-3,4-P h 2(η5-C4CNH2P h )]Ru (CO)2H}OTf (11-H).
A solution of 9-H (17.1 mg, 0.035 mmol) in THF (0.4 mL) in a
(28) While the carbon and hydrogen analyses were slightly high,
9-OTf was shown to be pure by 1H and 13C NMR spectroscopy and
X-ray crystallography.
OM020507D