Catalytic silane alcoholysis based on the C5H5(CO)(PPh3)Fe+ moiety. NMR spectroscopic identification of key intermediates

Article abstract of DOI:10.1016/S1381-1169(97)00205-7

The Cp(CO)(PPh₃)Fe⁺ fragment has been shown to catalyze silane alcoholysis in the presence of the non-coordinating counterion. B Ar₄' (Ar'=3.5-(CF₃)₂C₆H₃). When ethanol is used,

Full text of DOI:10.1016/S1381-1169(97)00205-7

Ž
.
Journal of Molecular Catalysis A: Chemical 130 1998 107–119
q
ž / ž
/
Catalytic silane alcoholysis based on the C5H5 CO PPh3 Fe
moiety. NMR spectroscopic identification of key intermediates
1
)
Seok Chang, Eric Scharrer , Maurice Brookhart
Department of Chemistry, UniÕersity of North Carolina, Chapel Hill, NC 27599-3290, USA
Received 15 August 1997; accepted 10 September 1997
Abstract
q
Ž
.Ž
.
X
The Cp CO PPh3 Fe fragment has been shown to catalyze silane alcoholysis in the presence of the non-coordinating
counterion, B Ar4X y Ar s3,5- CF3 2C6H3 . When ethanol is used, catalyst deactivation rapidly occurs. However, when
phenol is utilized, catalytic activity continues until all of the phenol has been consumed. In each case, key intermediates in
the catalytic cycle, including resting states, have been observed using low temperature NMR spectroscopy. The modes of
catalyst deactivation when ethanol is used have also been determined. q 1998 Elsevier Science B.V.
Ž
Ž
.
.
1. Introduction
sumed to occur through the reaction of an alco-
hol with either a silyl hydride complex or an
h2-silane complex which leads to the formation
of a silyl ether and a dihydride complex or an
h2-H2 complex. H2 is then displaced by silane
In an attempt to find new methods to selec-
tively protect alcohols, a variety of silane alco-
holysis catalysts have been investigated
2
w
3–
x
Ž
Ž
.
.
10 . Wilkinson’s catalyst Rh PPh3 3Cl has
been shown to carry out this type of reaction by
Ž
Ž ..
to close the catalytic cycle Eq. 1 .
w
x
w
x
Ojima 11 and Corriu 12–14 . However, ter-
tiary silanes do not react and competitive hydro-
silation is a problem if an alcohol possessing a
double bond is utilized. Singer has had good
Ž .
1
Ž
. Ž
.
success using CO PPh3 2RuCl2 as a catalyst
2
as it is very active and no competing hydrosila-
w
x
tion is observed 15 . Other catalysts that are
Ž
.
.
Ž
pfbs
4
similarly selective include Rh2 pfb
While all of the catalyts described above are
neutral metal complexes, Crabtree has reported
.
ŽŽ
.
w
x
16,17 .
perfluorobutyrate , and CO MnBr
In all of the above cases, the catalysis is pre-
4
2
Ž
extraordinary rates of silane alcoholysis ca.
1
50,000 hy
using the cationic complex,
IrH2 THF PPh3 2 SbF6 18 . The tremendous
activity has been ascribed to the fact that two
.
)
Corresponding author.
w
Ž
. Ž
.
x
w x
2
1 Current address: Department of Chemistry, Franklin and Mar-
shall College, P.O. Box 3003, Lancaster, PA 17604-3003, USA.
2
w
x
solvent molecules can dissociate from the metal
For reviews of silane alcoholysis see Refs. 1,2 .
1381-1169r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
Ž
.
PII S1381-1169 97 00205-7

(
)
108
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
center allowing both alcohol and silane to coor-
dinate simultaneously which then react in an
intramolecular fashion. Attempts to detect inter-
mediates in the catalytic cycle were unsuccess-
ful; however, Crabtree was able to synthesize a
model compound in which the silane molecule
was bound to the metal center in a non-classical
fashion.
H2 evolution. The rate of evolution of H2 gas at
20 s corresponded to 7.8 TOrmin. However,
activity rapidly dropped and at 12 min, the rate
of H2 evolution corresponded to only 1 TOrmin
even though only ca. 50 equivalents of silane
had been consumed. After 20 min, no further
activity could be measured. The system under-
went a noticeable color change during the course
of the reaction. The solution was initially a
yellow-orange color, but after 5 min it turned
light green. To probe the details of this cataly-
sis, a series of variable temperature NMR exper-
iments were carried out as described below.
We have recently reported the generation and
characterization of cationic iron h2-silane com-
q
Ž
.Ž
.
Ž
.
plexes of the type Cp CO PR3 Fe HSiEt3
w
x
19 which are rapidly hydrolyzed by water. For
example, Cp CO PPh3 Fe HSiEt3 q, 1, is
Ž
.Ž
.
Ž
.
formed upon protonation of the neutral iron silyl
X
complex with H OEt2 2 BAr4X Ar s 3,5-
2.2. Variable temperature NMR studies of the
catalytic system
Ž
.
Ž
Ž
.
.
CF3 2C6H3 . Complex 1 reacts rapidly with
water to give a cationic iron h2-H2 complex, 2.
If excess silane is present, H2 is displaced at
room temperature by Et3SiH which reforms the
silane complex. This species is stable after the
solution has been ‘dried’ by this combination of
The following series of reactions were car-
ried out and monitored by variable temperature
NMR spectroscopy in an attempt to observe
catalytic intermediates and understand how cata-
lyst deactivation was occurring. During the cat-
alytic runs, there was an excess of triethylsilane
present as well as numerous species bearing a
triethylsilyl substituent, most of which pos-
sessed overlapping resonances in this region.
Nevertheless, the catalysis was monitored using
other resonances of the key intermediates and
these are shown in Table 1. All of the interme-
Ž
.
reactions Scheme 1 .
This sequence of steps is similar to those
proposed for systems known to carry out cat-
alytic silane alcoholysis and we therefore chose
to investigate the catalytic activity of these iron
systems in this regard. We report here the first
example of a silane alcoholysis catalyst in which
all of the intermediates in the cycle have been
detected by variable temperature NMR spec-
troscopy and modes of catalyst deactivation have
been identified.
Ž
diates were independently prepared see Section
.
3 and the resonances listed in Table 1 were
unique to these species and allowed their unam-
biguous indentification.
The sequence of reactions described below is
2. Results and discussion
summarized in Scheme 2.
Ž .
Ž
.Ž
.
1 Cp PPh3 CO FeCH3, 3, was protonated
2.1. Preliminary obserÕations
in an NMR tube in CD2Cl2 using H OEt2 2 BAr4X
Ž
.
at y788C which gave both the iron ether com-
Ž
plex, 4, identified by a Cp resonance at 4.72
Preliminary experiments were carried out to
test the catalytic activity of the iron triethylsi-
.
Ž
ppm , as well as the aquo complex, 5, Cp
lane complex, Cp CO PPh3 Fe HSiEt3 q, 1, in
the alcoholysis of Et3SiH. A solution of 1 was
treated with 1000 equivalents of Et3SiH and
200 equivalents of ethanol. Et3SiOEt and H2
were formed, and the course of the catalysis
could be qualitatively monitored by measuring
resonance at 4.62 ppm which is formed by
Ž
.Ž
.
Ž
.
.
reaction of the cationic iron fragment with trace
water present in the solution. The two species
were present in a 1:2 ratio.
Ž .
2 At y808C, 20 equivalents of Et3SiH
were added to the sample. Observation at y608C

(
)
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
109
Scheme 1. 1
w
x
indicated that the silane had displaced the Et2O
from the iron center to give the cationic silane
complex, 1. This species was identified by a
doublet at y16.6 ppm for the hydride reso-
nance and a Cp resonance at 4.76 ppm 19 . The
aquo complex remained present.
Ž .
3 The NMR tube was removed from the
probe and warmed briefly to room temperature.
Table 1
Characteristic 1H NMR shifts used to identify key species in the catalytic cycle

(
)
110
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
Scheme 2.

(
)
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
111
Ž
.
10 equivalents total . However, upon addition
Evolution of H2 was observed. This occurs via
the reaction of the water with the silane com-
plex. Hydrolysis of the silane complex is pre-
sumed to give initially the protonated silanol
and the neutral iron hydride, neither of which is
observed. Rapid proton transfer gives the dihy-
drogen complex, 2, which loses H2 at room
temperature. If the sample is recooled to y808C
as soon as gas evolution begins, then the iron
dihydrogen complex, 2, is the only organometal-
of 5 more equivalents of ethanol at y808C,
followed by warming to room temperature, some
gas evolution is evident and the solution turns
green. Observation at y808C indicates that no
Et3SiH remains; the iron dihydrogen complex,
2, was found to be present in a 1:1 ratio with
another organometallic species which was deter-
Ž
mined to be the iron ethanol complex, 7, Cp,
4.66 ppm . The latter species is presumed to be
.
Ž
lic species observed in solution Cp, 4.90 ppm,
formed by displacement of H2 by ethanol.
Clearly, if ethanol is able to displace H2 from
the metal center, this provides a pathway for
catalyst deactivation.
Given the potential importance of the previ-
ous result, we next examined the interaction of
the iron dihydrogen complex, 2, with ethanol
more closely. The neutral iron hydride complex,
Ž
.
.
Fe H2 , y10.8 ppm . It is important to note
that for each equivalent of water present, 2
equivalents of silane are consumed since the
initial hydrolysis product, Et3SiOH, can react
with another equivalent of silane complex to
Ž
.
give hexaethyldisiloxane vide infra .
Ž .
4 If the tube is rewarmed to 258C until all
Ž .Ž .
the H2 has evolved and then recooled to y808C,
three species are present in a 1:1:1 ratio: the
silane complex, 1, the neutral iron hydride com-
Cp CO PPh3 FeH, 6, was protonated at y808C
with H OEt2 2 BAr4X . Observation at y808C
showed that the iron dihydrogen complex, 2,
was the predominant species. However, a small
amount of the neutral iron hydride, 6, still re-
Ž
.
Ž
.
plex, 6, Cp, 4.40 ppm, H, y13.6 ppm and
triethylsilyldiethyloxonium BAr4Xy OCH2, 4.46
Ž
Ž .
Ž ..
Ž
.
ppm q , OCH2C H3, 1.48 t . The latter two
species are formed upon reaction of Et2O with
the silane complex.
mained 10:1 ratio . Upon addition of 5 equiva-
lents of ethanol at y808C, the Cp resonances
and the hydride resonances broadened signifi-
cantly which is suggestive of exchange between
the iron hydride and the dihydrogen complex.
Confirmation of this hypothesis was obtained by
cooling the sample to y1108C. At this tempera-
ture, the exchange process was frozen out and
both Cp and hydride resonances for each com-
plex were observed. The dihydrogen complex
was the major species, but there was a signifi-
cant amount of the iron hydride complex pre-
sent 2:1 ratio . Upon addition of 5 more equiv-
alents of ethanol to the NMR tube and observa-
tion at y1108C the iron hydride complex be-
came the predominant species 3:2 ratio .
Warming the solution to y1008C led to line
broadening and further warming led to coales-
cence at ca. y958C. Warming to 208C led to
formation of the ethanol complex, 7, as the only
Ž .
5 At y808C, 5 equivalents of ethanol were
added to the NMR tube. Observation at this
temperature indicated that the dihydrogen com-
plex, 2, was the only organometallic species
present. The ethanol reacts rapidly with the
silane complex giving a transient protonated
silyl ether and the neutral iron hydride complex.
Rapid proton transfer leads to formation of the
dihydrogen complex. Ethanol also reacts with
the triethylsilyldiethyloxonium tetraarylborate to
give Et2O and the protonated silyl ether which
can also protonate the iron hydride complex.
Ž
.
Ž .
Ž
.
6 If the sample is warmed to room tempera-
ture, gas evolution occurs as Et3SiH displaces
H2. Upon recooling to y808C, the silane com-
plex, 1, triethylsilyldiethyloxonium tetraarylbo-
rate, and the iron hydride complex, 6, are pre-
sent as observed previously.
Ž
.
observable organometallic species Scheme 3 .
This experiment provides evidence that
ethanol can disrupt the catalytic cycle in two
Ž .
7 The same sequence of events is repeated
upon addition of another 5 equivalents of ethanol

(
)
112
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
Ž
ways. It can deprotonate the iron dihydrogen
complex in a reversible fashion to give the iron
hydride complex and EtOHq2 . However, ethanol
also displaces H2 from the metal center to give
the ethanol complex.
During an actual catalytic run, bulk quantities
of silane and ethanol are added to the reaction
mixture and we sought to examine which species
were predominant under these conditions. The
catalyst system was prepared as before by pro-
species present ca. 2:1 ratio with respect to the
.
iron hydride . H2 was also evident in solution.
When the sample was warmed to 208C, com-
plete conversion to the ethanol complex oc-
curred. Although no iron dihydrogen complex
was observed, it is clear that the conversion of
the iron hydride to the ethanol complex must
occur through loss of H2 from the dihydrogen
complex.
This result suggests that at 258C in this cat-
alytic cycle, the iron hydride complex, 6, is the
resting state. Conversion to the dihydrogen
complex, 2, must occur followed by displace-
ment by silane in order to reenter the catalytic
cycle. It appears that once the ethanol complex
is formed, the catalyst is ‘dead’ as no further
reaction occurs.
Ž
.Ž
.
tonation of Cp PPh 3 CO FeCH 3 with
H OEt2 2 BAr4X followed by addition of 50
equivalents of Et3SiH which gave the species
Ž
.
Ž
.
described earlier Scheme 4 . At y808C, 30
equivalents of ethanol were added giving a very
pale green solution. Observation at y808C indi-
cated that the neutral iron hydride, 6, was the
major organometallic species present along with
From these results, we concluded that both
the basicity and the nucleophilicity of the alco-
hol are potentially important in deactivation
processes. In an effort to extend catalyst life-
times and improve activity phenol was exam-
ined.
Ž
a small amount of the ethanol complex, 7, 6:1
.
ratio . Presumably at this temperature, the
acidrbase reaction is the predominant reaction
pathway and the presence of a large excess of
ethanol favors the formation of the iron hydride
complex via deprotonation of the dihydrogen
complex. Upon briefly warming the tube to
room temperature, the solution turned a dark
green color. Reobservation at y808C indicated
that the ethanol complex, 7, was now the major
2.3. Triethylsilane alcoholysis with phenol
Ž
.Ž
.
Cp CO PPh3 FeCH3, 3,was protonated with
H OEt2 2 BAr4X at 08C in CH2Cl2 and 2500
Ž
.
Scheme 3.

(
)
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
113
Scheme 4.
equivalents of Et3SiH were added. The solution
was warmed to room temperature and H2 evolu-
tion occurred upon reaction of trace water with
the silane complex. Next, 2000 equivalents of
PhOH were added to the solution followed by
10,000 equivalents of both PhOH and Et3SiH
which led to vigorous gas evolution. Gas evolu-
tion was reasonably constant over the entire
course of the reaction. After 30 min, measure-
ment of H2 evolution yielded a rate of 80
Scheme 5.

(
)
114
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
Ž
TOrmin. The reaction was complete no gas
phenol does not deactivate the catalyst system
as was the case with ethanol.
.
evolution after 2 h and 40 min with an average
rate of 75 TOrmin over the entire reaction. No
phenol remained and Et3SiOPh was isolated in
high yields. It is clear from this result that the
To probe the reasons for the drastic differ-
ences between ethanol and phenol, the reaction
of the dihydrogen complex, 2, with phenol was
Scheme 6.

(
)
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
115
Ž
.
examined Scheme 5 . The dihydrogen complex
was prepared from protonation of the neutral
to completion. Subsequent NMR observation
showed the silane complex, 1, the neutral iron
hydride complex, 6, and the triethylsilyloxo-
nium species to be present.
1
iron hydride and observed by H NMR spec-
troscopy at y808C. Addition of 5 equivalents
of PhOH at y808C produced no change in the
spectrum. It was clear that no deprotonation,
reprotonation was occurring. No change in the
system was observed up to y208C, but at 08C,
a small resonance appeared at 4.62 ppm which
is the Cp resonance for the aquo complex, 5,
Scheme 6 summarizes the silane alcoholysis
activity of Cp CO PPh3 Fe HSiEt3 q. When
ethanol is used, catalytic activity rapidly dimin-
ishes, however when phenol is used, catalyst
turnover rates are constant until all of the phe-
nol has been consumed. It has been demon-
strated by using low temperature NMR spec-
troscopy that, upon treatment of the iron silane
complex with an alcohol, the silyl ether and the
h2-H2 complex are formed. However, the next
step in the reaction pathway is clearly depen-
Ž
.Ž
.
Ž
.
Ž
.
8:1 ratio . After 30 min at 208C, the conversion
of the dihydrogen complex to the aquo complex
Ž
.
had progressed 3:1 ratio , and after 1 h, the
aquo complex was the predominant species in
solution. This experiment showed that phenol
does not interact with the dihydrogen complex
in the same manner as ethanol; it does not
deprotonate the dihydrogen complex and it does
not displace H2 from the metal center. Since
neither of the deactivation pathways is avail-
able, high turnover numbers can be achieved
without catalyst deactivation. During a catalytic
run, all traces of water would be consumed by
reaction with the silane complex to form silox-
anes.
For comparative purposes, the silane alcohol-
ysis using phenol was examined by variable
temperature NMR spectroscopy. In order to
confirm that the above result was consistent
with the presence of a large excess of phenol,
the dihydrogen complex, 2, was first generated
and observed at y808C. Next, 50 equivalents of
PhOH were added. Observations were identical
to those made above. No deprotonation of the
dihydrogen complex occurred and upon warm-
ing to 208C, only a small amount of the aquo
complex formed. No phenol complex was ob-
served. The sample was next cooled to y808C
and 60 equivalents of Et3SiH were added. It
Ž
.
dent on the alcohol that is utilized Scheme 6 .
When ethanol is used, the dihydrogen complex,
2, is readily deprotonated to give the neutral
iron hydride complex, 6, which is the catalyst
resting state in the presence of excess ethanol.
When this species is reprotonated to give the H2
complex, displacement of H2 by ethanol ulti-
mately occurs and effectively terminates the
catalytic cycle. On the other hand, when phenol,
which is both less basic and less nucleophilic
than ethanol, is utilized, the activity of the
catalyst is high with no termination pathways
evident. Phenol neither deprotonates the dihy-
drogen complex, 2, nor displaces H2 from the
metal center. Under these conditions, the H2
complex is the resting state in the catalytic cycle
and in this case, the rate-determining step ap-
pears to be displacement of H2 by Et3SiH.
3. Experimental section
3.1. General
All manipulations were performed under an
atmosphere of dry, deoxygenated nitrogen using
Schlenk techniques with a double manifold vac-
uum line, or in a vacuum atmosphere dry box,
unless otherwise noted. Nitrogen gas was puri-
fied by passage through a columm of BASF
Ž
was allowed to warm until catalysis began gas
evolution observed and was then recooled to
.
y808C. NMR observation at this temperature
showed only the iron dihydrogen complex, 2, to
be present which indicates that this species is
the resting state of the catalytic cycle when
phenol is used. The catalysis was allowed to run
Ž
.
catalyst R3-11 heated to 1308C followed by a
1
column of activated molecular sieves. H chem-

(
)
116
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
Xy
ical shifts were referenced to residual protio
H OEt2 q2 BAr4 57.2 mg, 0.046 mmol was
placed in an ice bath. This mixture was dis-
solved in 3 ml of CH2Cl2 to give a green
Ž
.
Ž
.
solvent peaks CHDCl2 d 5.32 . 13C chemical
Ž
.
shifts were referenced to 13C solvent signals
Ž
.
Ž
.
CD2Cl2 d 53.8 . Methylene chloride was puri-
solution. Et3SiH 2500 equivalents, 11.2 ml
fied and rendered oxygen-free by distillation
under a nitrogen atmosphere using P2O5.
CD2Cl2 was dried over CaH2 and vacuum
transferred to a Kontes flask. Et3SiH was pur-
was syringed into the solution. When the ice
bath was replaced with a 258C water bath the
silane solution immediately became yellow and
vigorous evolution of H2 gas was observed.
PhOH 2000 equivalents, 5.6 ml of a 10 M
.
CH2Cl2 solution was syringed into the flask.
Then 10,000 equivalents 44.8 ml of Et3SiH
and 10,000 equivalents 28.2 ml of a 10 M
CH2Cl2 solution of PhOH was added to the
˚
Ž
chased from Aldrich and stored over 4 A molec-
ular sieves followed by degassing.
H OEt2 BAr4Xy 20,21 , Cp CO PPh3
Ž
Ž
.
.
w
Ž
x
Ž
.Ž
,
.
Ž
.
2q
3
w
x
w
.Ž
.
Ž
Fe CH 3
22 , Cp CO PPh3 FeH
and
Xy
Ph3CqBAr4 24 were prepared according to
published procedures. Et3SiOEtq2 BAr4Xy was
generated in situ using the procedure of Kira
x
.
solution. Evolution of H2 was monitored and at
30 min the rate of H2 evolution corresponded to
80 TOrmin. After 2 h and 40 min, H2 evolu-
tion ceased and upon workup, PhOSiEt3 was
obtained in a 99% yield. The average rate of the
catalysis over the entire reaction was 75 TO
w
x
25 .
)
3.2. G eneration of Cp CO PPh 3
(
)(
-
Xy
Fe HOEt qBAr4
(
)
1
miny . H NMR CD2Cl2 d 7.26 ppm t, 2H,
1
Ž
.
Ž
Ž
.Ž
.
.
Ž
. Ž
.
Cp CO PPh3 Fe CH3 10 mg, 0.023 mmol
Xy
q
.
Ž
.
Ž
.
Ž
Ž
meta , 6.96 t, 1H, para , 6.88 d, 2H, ortho ,
1.02 t, 9H, JHH s7.0 Hz, SiCH2C H3 , 0.78
q, 6H, JHH s7.0 Hz .
and Et2O H BAr4 1.1 eq., 26.2 mg, 0.026
mmol were added to an NMR tube and kept in
2
Ž
.
.
Ž
.
Ž
.
a y788C bath. CD2Cl2 0.7 ml was added at
y788C. 1H NMR observation at y408C showed
3.3.2. ROHsethanol
q
Ž
Ž
.Ž
Ž
.
that Cp CO PPh3 Fe OEt2
was present.
.
.
Ž
.
Ž
Et3SiH 600 eq., 2.7 ml was added to a
Ethanol 5 eq., 6.9 ml, 0.12 mmol was added at
y788C and the tube was warmed to 08C. Ob-
servation at this temperature showed that the
ethanol complex had formed. At y808C, the
two diastereotopic methylene protons exhibit
unique resonances as mutiplets at 2.93 and 2.85
ppm. Upon warming to 08C, the two sets of
resonances overlap and are observed as a broad
multiplet at 2.74 ppm. H NMR CD2Cl2, 08C
d 4.66 ppm s, C5 H5 , 2.74 m, 2H, C H2 , 0.54
Ž
.
CH2Cl2 or chlorobenzene 5 ml solution of
Ž
.Ž
.
.
Ž
.
.
Cp CO PPh3 FeCH3 12 mg, 0.028 mmol and
Xy
H OEt2 q2 BAr4
above. Then ethanol 500 equivalents, 0.8 ml
57.2 mg, 0.046 mmol as
Ž
Ž
Ž
.
was syringed into the flask and evolution of H2
was monitored. At 20 s, the rate of evolution
corresponded to 8 TOrmin. The solution took
on a green color within 5 min after the ethanol
was added and the rate of H2 evolution de-
creased over time. At 20 min, no measurable
activity was observed.
1
Ž
.
Ž
.
Ž
.
.
t, 3H, Js7.0 Hz, C H3 .
3.3. Catalysis of Et3SiH alcoholysis using
q
Xy
(
)(
)
(
)
Cp CO PPh3 Fe HSiEt3 qBAr4
3.4. Reaction of Cp CO PPh3 Fe H2
ethanol
with
(
)(
)
(
)
3.3.1. ROHsPhOH
Ž
.Ž
.
Ž
.
Cp CO PPh3 Fe–H 10 mg, 0.024 mmol
was added to an NMR tube and dissolved in 0.4
A
Schlenk flask charged w ith
Ž
.Ž
.
Ž
.
Cp CO PPh3 FeCH3 12 mg, 0.028 mmol and
ml CD2Cl2. The solution was cooled to y788C
Xy
and a solution of H OEt q2 BAr4
Ž
.
Ž
2 equiva-
3
Ž
.Ž
.
Cp CO PPh3 FeH was prepared by methods described in
.
lents, 48.6 mg, 0.048 mmol in 0.4 ml CD2Cl2
was added to the tube. Observation at y808C
w
x
Ref. 19 . An alternate route has been published previously: see
w
x
Ref. 23 .

(
)
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
117
Ž
.
Ž
.
showed that the dihydrogen complex was pre-
sent in a 10:1 ratio with respect to the iron
1:2 ratio . A sharp singlet at 0.18 ppm CH4
was also observed.
1 H NMR
q
Ž
Ž
.Ž
.
Ž
.
hydride complex. Addition of ethanol 5 equiva-
Cp CO PPh 3 Fe OEt 2 :
.
Ž
. Ž .
lents, 7.0 ml, 0.12 mmol at y788C led to the
CD2Cl2, y408C d 7.7–7.0 ppm m, aryl ,
Ž
.
Ž
m ult., 2H ,
observation of broadened Cp and hydride reso-
nances. Upon cooling to y1108C, the exchange
process was frozen out and the dihydrogen com-
plex was present in a 2:1 ratio with respect to
the iron hydride complex. Additon of 5 more
equivalents of ethanol shifted the equilibrium in
favor of the hydride complex which was now
4.72
s, C 5 H 5
,
3.30
X
Ž
Ž
. .
Ž
Fe O C H H CH 3
Fe O CH HCH3 2 , 0.67 t, 6H, Js7.0 Hz,
C H3 .
,
3.11 m ult., 2H ,
2
X
Ž Ž
. .
Ž
.
Ž .
Ž
b Next Et3SiH 20 equivalents, 74.4 ml,
.
0.46 mmol was added via a syringe to the
septum-capped tube at y788C and the tube was
warmed to y608C. The solution turned yellow.
Et3SiH displaces Et2O from the metal center to
Ž
.
the predominant species 3:2 ratio . Upon
warming the tube, the Cp resonances broadened
with coalescence at ca. y958C. Warming to
208C led to the formation of the ethanol com-
plex as the exclusive species.
g i v e
t h e
s i l a n e
c o m p l e x ,
Cp CO PPh3 Fe HSiEt3 q. The aquo complex
Ž
.Ž
.
Ž
.
is still present.
q
Ž
.Ž
.
Ž
Ž
.
Ž
pertinent reso-
Cp CO PPh3 Fe HSiEt3
1
1
3.5. H NMR studies of working catalyst solu-
.
.
nances : H NMR CD2Cl2, y808C d 4.76
tions
Ž
.
Ž
ppm s, C5 H5 , y16.6 d, 1H, JPH s29 Hz,
Ž
..
JSiH s67.3 Hz, Fe SiH .
The working catalyst system was examined
1
Ž .
c The sample was then warmed to room
by H NMR spectroscopy. A series of reagents
temperature which leads to hydrolysis of the
silane complex by water and subsequent loss of
H2. After gas evolution had stopped, NMR
analysis at y808C showed the presence of
Cp CO PPh3 FeH, Et2OSiEt3 BAr4 , and the
silane complex in a 1:1:1 ratio.
were added and spectra recorded at various
times and temperatures to detect intermediates
Ž
.
present in solution. Key intermediates, Cp CO -
q
q
Ž
.
Ž
.
.
Ž
.Ž
.
Ž
.
PPh3 Fe H 2
,
Cp CO PPh3 Fe OEt2 ,
Xy
q
Ž
.Ž
.
Cp CO PPh3 Fe HSiEt3 q, and Et3SiOEtq2
BAr4Xy have been independently generated and
Ž
.Ž
Ž
.
1
Ž
.Ž
.
Ž
.
Cp CO PPh3 FeH pertinent resonances : H
w
x
are described below or elsewhere 19,25 . In the
first experiment, ethanol is added stepwise to
the NMR tube after an excess of Et3SiH has
been added. In the second experiment, after
addition of 50 eq. of Et3SiH, 30 eq. of ethanol
is added at one time. In the third experiment,
silane alcoholysis using PhOH is examined.
Ž
.
Ž
NMR CD2Cl2, y808C d 7.7–7.0 ppm m,
.
Ž
.
.
Ž
aryl , 4.40 s, C5 H5 , y13.6 d, JHP s74.1 Hz,
1H, Fe–H .
1
Et2OSiEt3qBAr4X y pertinent resonances : H
Ž
.
Ž
.
.
Ž
NMR CD2Cl2, y808C d 4.46 ppm q, 4H,
q
Ž
.
JHH s7.1 Hz, CH3C H2 2O SiEt3 , 1.48 ppm
q
Ž
Ž
.
.
t, 6H, JHH s7.1 Hz, C H3CH2 2O SiEt3 .
( )
Ž .
3.5.1. 1
d H2 pressure in the tube was released. The
Ž .
Ž .Ž . Ž
Ž
a
Cp CO PPh3 Fe–CH3 10 mg, 0.023
tube was cooled to y808C and ethanol
equivalents, 6.9 ml, 0.12 mmol was added at
this temperature. Warming the sample to room
temperature until evolution of H2 started, fol-
lowed by immediate cooling at y808C led to
the observation of Cp CO PPh3 Fe H2
5
BAr4Xy 2 equivalents, 48
Ž
q
.
Ž
.
.
mmol and H OEt2
2
.
mg, 0.046 mmol were transfered to a 5 mm
NMR tube and kept in a y788C bath. CD2Cl2
Ž
.
0.7 ml was added at y788C and the tube was
q
Ž
.Ž
.
Ž
.
warmed to y408C until the solution turned
green. 1H NMR analysis at y408C showed
formation of the ether coordinated iron complex
as
the only organometallic species. A sharp singlet
at 4.55 ppm was observed for H2.
.q.
q
Ž
Ž
.Ž
.
Ž
Ž
.Ž
.
Ž
Ž
.
Ž
Cp CO PPh3 Fe OEt2 and the aquo com-
Cp CO PPh3 Fe H 2
pertinent reso-
1
q
Ž
.Ž
.
Ž
.
Ž
.
Cp, 4.62 ppm
.
.
plex Cp CO PPh3 Fe OH2
nances : H NMR CD2Cl2, y808C d 4.90

(
)
118
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
Ž
.
Ž
Ž
..
ppm s, C5 H5 , y10.71 br, Fe H2 .
complete conversion to the iron ethanol com-
plex had occurred.
Ž .
e Warming the tube back to room tempera-
ture resulted in vigorous evolution of H2. H2
pressure in the tube was released and when no
further gas evolution was observed, the tube
was recooled to y808C. The 1H NMR spectrum
( )
3.5.3. 3
Ž
.Ž
.
Ž
.
Cp CO PPh3 FeH 10 mg, 0.024 mmol was
weighed in an NMR tube in the dry box. The
tube was cooled to y808C and 0.3 ml of CD2Cl2
Ž
.Ž
.
.
at y808C showed Cp CO PPh 3 FeH,
Xy
was added. H OEt2 q2 BAr4
Ž
.
Ž
2 equivalents,
Xy
q
Et2OSiEtq3 BAr4 , Cp CO PPh3 Fe HSiEt3
to be present.
Ž
.Ž
Ž
.
.
49.3 mg, 0.049 mmol was dissolved in 0.4 ml
of CD2Cl2 and this was added via syringe to the
Ž .
f Repeating the same procedure using an-
other 5 equivalents of ethanol gave the same
result as above. After a total of ca. 15 equiva-
lents of ethanol had been added, the solution
turned green and the 1H NMR spectrum recorded
at y808C showed that the SiH resonance of
Et3SiH had disappeared indicating that all the
silane had been consumed. The water present
in solution accounts for the consumption of the
other 5 equivalents of silane. Cp CO -
PPh3 Fe HOEt
were present in a 1:1 ratio.
NMR tube. Observation at y808C showed
q
Ž
.Ž
.
Ž
.
Cp CO PPh 3 Fe H 2
to be the only
Ž
organometallic species present. PhOH 50
equivalents, 1.2 mmol, 121 ml of a 10 M
.
CH2Cl2 solution was added to the tube. No
reaction was observed to occur at y808C, but
upon warming to 208C, the aquo complex was
Ž
Ž
.
observed as a minor species 1:8 ratio . The
NMR tube was recooled to y808C and Et3SiH
.
Ž
Ž
.
Ž
.
60 equivalents, 1.4 mmol, 230 ml was added.
q
q
Ž
.
Ž
.
Ž
.Ž
.
.
and Cp CO PPh3 Fe H2
Upon warming the solution to 208C, silane alco-
holysis commenced and vigorous gas evolution
was observed. The tube was returned to the
probe at y808C and observation at this temper-
( )
3.5.2. 2
q
Ž
.Ž
.
Ž
.
ature showed that Cp CO PPh3 Fe H2
was
An NMR tube was charged with
the only organometallic species present. The
tube was removed from the probe and the catal-
ysis was allowed to continue. When gas evolu-
Ž
.Ž
.
Ž
.
Cp CO PPh3 Fe–CH3 10 mg, 0.024 mmol
Xy
and H OEt2 q2 BAr4 2 equivalents, 48.5 mg,
Ž
.
.
Ž
0.048 mmol and cooled to y808C. CD2Cl2
tion had stopped, NMR observation indicated
Ž
.
0.7 ml was added followed by 50 equivalents
Xy
that Cp CO PPh3 FeH, Et2OSiEtq3 BAr4
,
Ž
.Ž
.
Ž
.
of Et3SiH 186 ml, 1.2 mmol . The tube was
warmed to allow all of the trace moisture to
react with the silane complex after which the
q
Ž
.Ž
.
Ž
.
Cp CO PPh3 Fe HSiEt3 were present.
Ž
Ž
.Ž
.
species described earlier Cp CO PPh3 FeH,
Acknowledgements
Xy
.q.
q
Ž
.Ž
.
Ž
Et2OSiEt3 BAr4 , Cp CO PPh3 Fe HSiEt3
were found to be present. The tube was re-
cooled to y808C and 30 eq. of ethanol 42.7
ml, 0.73 mmol were added which gave a light
We thank the National Institutes of Health
Grant number GM 28938 for support of this
Ž
.
Ž
work.
.
green solution. Observation at this temperature
showed that the neutral iron hydride complex
was present along with the iron ethanol complex
References
w x
Ž
.
1
F. Lukevics, M. Dzintara, J. Organomet. Chem. 295 1985
265.
Ž
.
6:1 ratio . Upon warming the tube to room
temperature, the solution took on a dark green
color. Observation upon recooling to y808C
indicated that the iron ethanol complex was now
w x
Ž
.
2
J. Corey, in: G. Larson Ed. , Advances in Silicon Chemistry,
JAI Press, Greenwich, CT, 1991, p. 327.
w x
3
N.J. Archer, R.N. Haszeldine, R.V. Parish, J. Chem. Soc.
Dalton Trans. 1979 695.
R.N. Haszeldine, R.V. Parish, B.F. Riley, J. Chem. Soc.
Dalton Trans. 1980 705.
Ž
.
Ž
the major species present 2:1 ratio with respect
w x
4
.
Ž .
to the iron hydride complex . After 1 h at 208C,

(
)
S. Chang et al.rJournal of Molecular Catalysis A: Chemical 130 1998 107–119
119
w x
w
w
x
Ž
.
5
S.N. Blackburn, R.N. Haszeldine, R.V. Parish, J.H. Setchfi,
15 U. Oehmichen, H. Singer, J. Organomet. Chem. 243 1983
Ž
.
J. Organomet. Chem. 192 1980 329.
199.
w x
Ž
.
x
16 M.P. Doyle, K.G. High, V. Bagheri, R.J. Pieters, P.J. Lewis,
6
A.J. Chalk, J. Chem. Soc. Chem. Commun. 1970 847.
J.A. Davies, F.R. Hartlety, S.G. Murray, G. Marshall, J. Mol.
w x
Ž .
M.M. Pearson, J. Org. Chem. 55 1990 6082.
7
Ž
.
w
w
w
x
Ž
.
Cat. 10 1981 171.
17 T.B. Gregg, A.R. Cutler, Organometallics 13 1994 1039.
x Ž .
18 X. Luo, R.H. Crabtree, J. Am. Chem. Soc. 111 1989 2527.
x
19 E. Scharrer, S. Chang, M. Brookhart, Organometallics 14
w x
8
D.E. Barber, Z. Lu, T. Richardson, R.H. Crabtree, Inorg.
Ž
.
Chem. 31 1992 4709.
w x
Ž
.
Ž .
1995 5686.
9
W. Caseri, P.S. Pregosin, Organometallics 7 1988 1373.
w
w
w
w
w
x
Ž
.
w
x
10 T.C. Bedard, J.Y. Corey, J. Organomet. Chem. 428 1992
20 M. Brookhart, B.E. Grant, A.F. Volpe Jr., Organometallics
Ž
.
315.
11 1992 3920.
x
w
x
Ž
.
11 I. Ojima, T. Kogure, M. Nihonyanagi, H. Kono, S. Inaba, Y.
21 R. Taube, S.J. Wache, Organomet. Chem. 428 1992 431.
Ž
.
w x
22 P.M. Treichel, R.L. Shubkin, K.W. Barnett, D. Reichard,
Nagai, Chem. Lett. 1973 501.
x
Ž
.
.
.
Ž .
Inorg. Chem. 5 1966 1177.
12 R.J.P. Corriu, J.J.E. Moreau, J. Organomet. Chem. 114 1976
w
x
Ž
.
135.
23 D.L. Reger, E.C. Culbertson, J. Am. Chem. Soc. 98 1976
x
Ž
13 R.J.P. Corriu, J.J.E. Moreau, J. Organomet. Chem. 120 1976
2789.
w
w
x
x
Ž
.
337.
24 S.R. Bahr, P. Boudjouk, J. Org. Chem. 57 1992 5545.
25 M. Kira, T. Hino, H. Sakurai, J. Am. Chem. Soc. 114 1992
6697.
x
Ž
Ž
.
14 R.J.P. Corriu, J.J.E. Moreau, J. Organomet. Chem. 127 1977
7.