Iron hydride complexes bearing phosphinite-based pincer ligands: Synthesis, reactivity, and catalytic application in hydrosilylation reactions

Article abstract of DOI:10.1021/om2005589

Treatment of resorcinol-derived bis(phosphinite) ligands 1,3-(R ₂PO)₂C₆H₄ (R = ⁱPr and Ph) with Fe(PMe₃)₄ furnishes iron POCOP-pincer hydride complexes [2,6-(R₂PO)₂C₆H₃]Fe(H) (PMe₃)₂ (R = ⁱPr, 1a; R = Ph, 1b) with two PMe₃cis to each other. The isopropyl complex 1a undergoes ligand substitution upon mixing with CO to give [2,6-(ⁱPr ₂PO)₂C₆H₃]Fe(H)(PMe ₃)(CO). The kinetic product (2a) of this process contains a CO ligand trans to the hydride, whereas the thermodynamic product (2a′) has a CO ligand cis to the hydride. The displacement of PMe₃ in 2a by CO takes place at an elevated temperature, resulting in the formation of [2,6-( ⁱPr₂PO)₂C₆H₃]Fe(H)(CO) ₂ (3a). These new iron POCOP-pincer hydride complexes catalyze the hydrosilylation of aldehydes and ketones with different functional groups, and 1a is the most efficient catalyst for this process. Isotopic labeling experiments rule out the hydride ligand being directly involved in the reduction. The hydrosilylation reactions are more likely to proceed via the activation of silanes or carbonyl substrates after ligand (PMe₃, or CO in the case of 3a) dissociation from the iron center.

Full text of DOI:10.1021/om2005589

ARTICLE
pubs.acs.org/Organometallics
Iron Hydride Complexes Bearing Phosphinite-Based Pincer Ligands:
Synthesis, Reactivity, and Catalytic Application in Hydrosilylation
Reactions
Papri Bhattacharya, Jeanette A. Krause, and Hairong Guan*
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States
S Supporting Information
b
ABSTRACT: Treatment of resorcinol-derived bis(phosphinite)
i
ligands 1,3-(R2PO)2C6H4 (R = Pr and Ph) with Fe(PMe3)4
furnishes iron POCOP-pincer hydride complexes [2,6-
(R2PO)2C6H3]Fe(H)(PMe3)2 (R = iPr, 1a; R = Ph, 1b) with
two PMe3 cis to each other. The isopropyl complex 1a
undergoes ligand substitution upon mixing with CO to give
[2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)(CO). The kinetic pro-
duct (2a) of this process contains a CO ligand trans to the
hydride, whereas the thermodynamic product (2a0) has a CO
ligand cis to the hydride. The displacement of PMe3 in 2a by
CO takes place at an elevated temperature, resulting in the formation of [2,6-(iPr2PO)2C6H3]Fe(H)(CO)2 (3a). These new
iron POCOP-pincer hydride complexes catalyze the hydrosilylation of aldehydes and ketones with different functional groups,
and 1a is the most efficient catalyst for this process. Isotopic labeling experiments rule out the hydride ligand being directly
involved in the reduction. The hydrosilylation reactions are more likely to proceed via the activation of silanes or carbonyl
substrates after ligand (PMe3, or CO in the case of 3a) dissociation from the iron center.
’ INTRODUCTION
as a very efficient catalyst for the hydrogenation of ketones.10f
Noticeably, in both cases the iron center is supported by a
bis(phosphino)pyridine ligand that is typically synthesized from
2,6-bis(chloromethyl)pyridine and an expensive, pyrophoric sec-
ondary phosphine.13
Hydrido complexes of iron are fundamentally important for
their crucial roles in a wide variety of catalytic processes.1 Studies
on synthetic iron hydride complexes have shed light on the
mechanisms by which nitrogenase and hydrogenase enzymes
function.2,3 In homogeneous catalysis, iron hydride species are
often invoked as key intermediates in iron-catalyzed reactions.4
Recent efforts have been made to elucidate structureꢀreactivity
relationship of well-defined iron hydride complexes with the
objective to provide mechanistic bases for the rational design
of iron catalysis.5 Meanwhile, several well-characterized iron
hydride complexes have been directly employed as the catalysts
for a number of reactions.6
Transition metal complexes with pincer-type ligands, especially
those containing precious metals, have shown high reactivity for
both stoichiometric bond activations and catalytic transformations.7
One of the new focuses in this research area has been placed on the
chemistry of 3d metals, among which iron is particularly attractive
due to its relatively low cost and toxicity.8 Although various iron
compounds bearing either neutral9ꢀ11 or anionic12 pincer-type
ligands have been synthesized, examples of the corresponding
hydrido complexes are scarce in the literature.10d,f,12a,12i Of the
known iron pincer hydride complexes, only two have been reported
to be catalytically active. The first of such compounds is Chirik’s
(iPrPNP)FeH2(N2) (iPrPNP = 2,6-(iPr2PCH2)2(C5H3N)), which
can be used to catalyze the hydrogenation of olefins.10d The second
example is Milstein’s (iPrPNP)FeH(CO)Br, which has been shown
Taking into account not only the prices of metals but also the
costs of ligand synthesis, we have focused our studies on the
catalysis of first-row transition metal complexes with phosphinite-
based POCOP-pincer ligands as shown in Figure 1. This specific
type of pincer ligands is readily accessible via PꢀO bond forming
reaction of resorcinol or other diols with relatively inexpensive
ClPR2.14 The synthesis of POCOP-pincer complexes of nickel, in
particular, is also straightforwardly accomplished via cyclometala-
tion of the pincer ligands with simple metal salts such as NiCl2.
For catalytic applications, we and other groups have demon-
strated that these nickel pincer complexes are excellent catalysts
for the reduction of carbonyl functionalities,15 Michael addition of
amines and alcohols to acrylonitrile derivatives,14g,k,m Kharasch
addition of CCl4 to alkenes,14f,g and CꢀS cross-coupling reac-
tions.16 In stark contrast to nickel chemistry, POCOP-pincer
complexes of other first-row transition metals are exceedingly
rare, largely due to the inability of common metal salts to activate
the CꢀH bonds of the pincer ligands. To date, there are only two
established cobalt systems12i,17 and one iron system;12i however,
Received: June 28, 2011
Published: August 08, 2011
r
2011 American Chemical Society
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ARTICLE
Figure 1. Transition metal complexes with POCOP-pincer ligands.
Figure 2. Characteristic hydride resonance of 1a observed by 1H NMR
spectroscopy.
Scheme 1. Synthesis of an Iron Hydride Complex with a
POCOP-Pincer Ligand
expected, the 31P{1H} NMR spectrum showed three phosphorus
resonances: one doublet of doublets at 221.0 ppm for the
phosphinite phosphorus nuclei and a pair of doublets of triplets
at 13.7 ppm and 6.6 ppm for the two PMe3 ligands. The analogous
iron complex 1b, with phenyl substituents, was prepared in 69%
yield via a similar synthetic procedure. On the other hand, the
synthesis of complex 1c with a more bulky pincer ligand was
unsuccessful; the isolated material contained mainly the starting
materials along with a marginal amount of hydride species
suggested by 1H NMR spectroscopy. The characteristic triplet of
doublets at ꢀ14.20 ppm (2JPꢀH = 72.0 and 29.6 Hz) implied that
only one PMe3 remained coordinated as a result of a crowded iron
center.
the catalytic applications of these complexes have yet to be reported.
In this contribution, we describe our success of using Fe(PMe3)4
to promote the cyclometalation of POCOP-pincer ligands, which
leads to the synthesis of a series of new iron pincer hydride
complexes. We also report catalytic hydrosilylation of aldehydes
and ketones with these well-defined iron complexes. Our prelimin-
ary mechanistic studies of this catalytic process demonstrate the
pitfalls along the path to understanding the chemistry of metal
hydrides. One cannot always assume that a reduction reaction
catalyzed by a metal hydride must involve the insertion of an
unsaturated chemical bond into the metalꢀhydrogen bond.
In the X-ray crystal structure of 1a (Figure 3), iron is situated
in a distorted octahedral coordination sphere with P(1), P(2),
and P(3) leaning toward the hydride ligand; the P(x)ꢀFeꢀP(4)
angles [103.66(2)°, 99.42(2)°, and 97.38(3)°] are greater than
the 90° of a perfect octahedral geometry. The aromatic ring of the
pincer ligand is canted toward the neighboring PMe3 with a
’ RESULTS AND DISCUSSION
172.44(11)° angle for C(4) C(1)ꢀFe. The FeꢀP(4)
3 3 3
Synthesis and Structure of [2,6-(R2PO)2C6H3]Fe(H)(PMe3)2
(1aꢀc). One of the most efficient methods to synthesize transition
metal hydride complexes is via cyclometalation or ligand-directed
CꢀH bond activation with low-valent metal species.18,19 This
strategy has been applied to the synthesis of iron compounds
involving imine functionalities as the anchoring groups.20,21 Re-
lated chemistry with a pincer ligand has been described by Li and
co-workers for the reaction between Ph2PO(CH2)3OPPh2 and
Fe(Me)2(PMe3)4, where the postulated CꢀH bond activation
step takes place following methane elimination (Scheme 1).12i
Consistent with this mechanistic hypothesis, we found that mixing
1,3-bis(diisopropylphosphinito)benzene with an equimolar
amountof Fe(PMe3)4 atroom temperaturefurnished iron hydride
complex 1a in 67% isolated yield (eq 1). The 1H NMR spectrum
of 1a in THF-d8 confirmed the presence of the hydride ligand,
which appeared at ꢀ14.87 ppm as a well-resolved triplet of
doublets of doublets with coupling constants of 78.4, 47.6, and
22.0 Hz (Figure 2).22 This splitting pattern is in accordance with a
Cs symmetry molecule bearing one bis(phosphinite) pincer ligand,
one cis PMe3 (relative to the hydride), and one trans PMe3. As
[2.2583(7) Å] distance is longer than the FeꢀP(3) distance
[2.2167(7) Å], presumably due to the greater trans effect of the
hydride than the aryl ring.
Ligand Substitution Reactions. The FeꢀP bond distances of
1a suggested to us that the trans-PMe3 (with respect to the
hydride) would be more labile than the cis-PMe3. To test this
hypothesis, a THF solution of 1a was stirred under 1 atm of CO
at room temperature for 24 h. The substitution reaction pro-
ceeded quantitatively to form the mono-CO-substituted hydride
species 2a (eq 2), which exhibited a strong CO stretching band at
1928 cmꢀ1 and a set of low-field multiplets (216.7ꢀ217.1 ppm)
in the 13C{1H} NMR spectrum. The trans relationship between
CO and hydride is supported by fact that the hydride resonance
of 2a is significantly downfield-shifted (a triplet of doublets at
ꢀ9.58 ppm, in THF-d8) compared to that of 1a (ꢀ14.86 ppm).
The stereoconfiguration of 2a was further established by single-
crystal structure determination (Figure 4). Having a “slim” CO
ligand on iron appears to create more space on the hydride side.
The P(x)ꢀFeꢀC(22) angles of 2a [100.48(8)°, 97.86(8)°, and
95.60(8)°] are roughly 2° smaller than the corresponding
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ARTICLE
Figure 4. ORTEP drawing of [2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)-
(CO) (2a) at the 50% probability level. Selected bond lengths (Å)
and angles (deg): FeꢀH 1.40(2), FeꢀC(1) 2.013(2), FeꢀP(1)
2.2002(6), FeꢀP(2) 2.1888(7), FeꢀP(3) 2.2198(6), FeꢀC(22)
1.774(2), C(22)ꢀO(22) 1.157(3), C(1)ꢀFeꢀP(1) 79.43(7), C-
(1)ꢀFeꢀP(2) 77.58(7), P(1)ꢀFeꢀP(2) 149.81(3), C(1)ꢀFeꢀP(3)
176.78(6), C(1)ꢀFeꢀC(22) 87.62(10), P(1)ꢀFeꢀC(22) 100.48(8),
P(2)ꢀFeꢀC(22) 97.86(8), P(3)ꢀFeꢀC(22) 95.60(8), FeꢀC-
(22)ꢀO(22) 177.1(2).
Figure 3. ORTEP drawing of [2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)2
(1a) at the 50% probability level. Selected bond lengths (Å) and
angles (deg): FeꢀH 1.42(2), FeꢀC(1) 2.001(2), FeꢀP(1)
2.1724(6), FeꢀP(2) 2.1753(6), FeꢀP(3) 2.2167(7), FeꢀP(4)
2.2583(7), C(1)ꢀFeꢀP(1) 77.99(7), C(1)ꢀFeꢀP(2) 77.29(7), C-
(1)ꢀFeꢀP(4) 83.44(6), P(1)ꢀFeꢀP(4) 103.66(2), P(2)ꢀFeꢀP(4)
99.42(2), P(3)ꢀFeꢀP(4) 97.38(3), P(1)ꢀFeꢀP(3) 102.68(2), P-
(2)ꢀFeꢀP(3) 101.65(2).
P(x)ꢀFeꢀP(4) angles of 1a. The pincer aromatic ring is also
quantitative conversion after 7 d. The synthesis of 2a0 was more
conveniently carried out by heating a toluene solution of 2a at
80 °C for 12 h, although a small amount of precipitate formed
due to the decomposition of the iron species. Nevertheless, 2a0
was isolated in 83% yield after an appropriate workup procedure
(see Experimental Section). The hydride resonance of 2a0 (in
THF-d8) was found at ꢀ12.66 ppm as a triplet of doublets (JPꢀH
= 66.0 and 28.8 Hz), which is substantially upfield-shifted
compared to that of 2a (ꢀ9.58 ppm). The chemical shift is
consistent with a trans arrangement of the hydride and PMe3 in
2a0, because the less trans-influencing PMe3 should induce a
shorter FeꢀH bond and, hence, more shielding from the iron
canted toward CO in 2a, although the C(4) C(1)ꢀFe angle
3 3 3
has increased to 175.77(11)°.
1
center. In principle, the magnitude of heteronuclear 31Pꢀ H
coupling constants (2JP-M-H) can also provide valuable informa-
tion about the molecular structure, although it is very sensitive to
the nature of the metal center. Field and co-workers have shown
that in complexes RuRH(PP3) [PP3 = P(CH2CH2CH2P-
(CH3)2)3; R = H, Cl, and various alkenyl groups], the magnitude
of 2JP-M-H(trans) > 2JP-M-H(cis), whereas for the analogous FeRH-
The substituents on the phosphorus donors of the pincer
ligand greatly influence the rate of ligand substitution at the iron
center. In contrast to facile displacement of PMe3 from 1a, the
reaction of 1b under 1 atm of CO was markedly slower at room
1
temperature. In THF-d8, the H NMR spectrum showed that,
even after 4 d, only 2% of 1b was converted to 2b, as evidenced by
a triplet of doublets at ꢀ8.57 ppm (2JPꢀH = 55.2 and 50.4 Hz).
An attempt to accelerate this process by subsequently heating the
same sample at 60 °C resulted in a new triplet of doublets at
ꢀ11.20 ppm (2JPꢀH = 66.8 and 28.8 Hz). At that point, the
signals of 2b became barely observable. The upfield-shifted
resonance at ꢀ11.20 ppm suggested that the new hydride species
2b0 may no longer contain a trans-CO and could be a geometric
isomer of 2b through a PMe3/CO ligand swap. Unfortunately,
the substitution reaction at 60 °C remained sluggish; only 24%
NMR conversion was observed after 3 d, and therefore the
synthesis of 2b0 was not pursued.
(PP3) complexes, 2JP-M-H(cis) > 2JP-M-H(trans) 23 The latter results
.
are in agreement with our structural assignment of 2a0, as the cis
2
coupling constant JPMe3ꢀFe-H in 2a (52.0 Hz) is larger than
the trans coupling constant 2JPMe3ꢀFe-H in 2a0 (28.8 Hz). The IR
spectra of the two isomers revealed a significant difference
in the CO stretching frequencies, with the band of 2a0
being 25 cmꢀ1 lower. The red shift could be attributed to a
shorter FeꢀCO bond distance in 2a0, which would allow a more
effective π-back-donation from the iron center. Attempts to
validate this hypothesis by growing X-ray quality crystals of 2a0
were fruitless; however, the fact that the pincer aromatic ring
has a weaker trans effect than the hydride does support a
shorter FeꢀCO bond in 2a0. Moreover, the electron repulsion
between an occupied iron d orbital and a filled π orbital of the
pincer aromatic ring should also result in substantially more
The above-mentioned reactivity of 1b led us to suspect that 2a
might be the kinetic product of ligand substitution from 1a.
Indeed at 60 °C and in the absence of CO, 2a underwent a slow
isomerization to a new hydride species 2a0 (eq 3) with a nearly
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phosphorus arm, and irreversible dissociation of CO or PMe3
cannot be ruled out.
At 80 °C in toluene and in the presence of CO, the substitution
of the remaining PMe3 of 2a occurred, resulting in clean
formation of the dicarbonyl iron hydride 3a after 3 d (eq 5).
Key spectroscopic evidence for this compound included a
hydride resonance at ꢀ9.61 ppm as a triplet (2JPꢀH = 52.4 Hz,
1
in THF-d8) in the H NMR, two CO resonances at 213.1
2
2
ppm (triplet, JPꢀC = 13.8 Hz) and 214.7 ppm (triplet, JPꢀC
= 11.1 Hz) in the 13C{1H} NMR, and two strong CO stretching
frequencies (1993 and 1946 cmꢀ1) in the IR. Single crystals of 3a
were obtained from the recrystallization of the complex in
methanol, and the X-ray studies revealed two independent
molecules in the crystalline lattice with different orientations of
Figure 5. ORTEP drawing of [2,6-(iPr2PO)2C6H3]Fe(H)(CO)2 (3a)
at the 50% probability level. Selected bond lengths (Å) and angles (deg):
Fe(2)ꢀH 1.44(2), Fe(2)ꢀC(1B) 1.995(2), Fe(2)ꢀP(1B) 2.1843(6),
Fe(2)ꢀP(2B) 2.1885(6), Fe(2)ꢀC(22B) 1.794(2), Fe(2)ꢀC(23B)
1.772(2), C(22B)ꢀO(22B) 1.147(3), C(23B)ꢀO(23B) 1.150(3), C-
(1B)ꢀFe(2)ꢀP(1B) 80.15(7), C(1B)ꢀFe(2)ꢀP(2B) 80.03(7), P-
(1B)ꢀFe(2)ꢀP(2B) 158.16(3), C(1B)ꢀFe(2)ꢀC(22B) 92.04(9),
C(1B)ꢀFe(2)ꢀC(23B) 168.81(10), Fe(2)ꢀC(22B)ꢀO(22B) 178.8(2),
Fe(2)ꢀC(23B)ꢀO(23B) 177.8(2).
i
the Pr groups (only one molecule is shown in Figure 5). In
relation to the hydride ligand, the trans FeꢀC bond [1.794(2) Å]
is longer than the cis FeꢀC bond [1.772(2) Å], once again
reflecting the fact that the hydride exerts a greater trans effect
than the aryl ring. The trans CO bond distance [1.147(3) Å] is
shorter than the cis one [1.150(3) Å], which is probably due to
less π-back-donation from the iron center.26
back-donation into the π* orbital of CO in 2a0 (in a push-and-
pull manner).
Catalytic Studies. One conceivable catalytic application of
transition metal hydride complexes is for the reduction of
carbonyl functionalities.4,15,27 Our previous study of nickel
hydride complexes containing similar POCOP-pincer ligands
has shown that they are effective catalysts for the hydrosilylation
of aldehydes.15a On the basis of the observed stoichiometric
reactions, we have proposed a two-step mechanism involving the
insertion of an aldehyde into a NiꢀH bond followed by the
reaction of the resulting nickel alkoxide species with a silane to
regenerate the nickel hydride. Recently there has been consider-
able interest in developing iron-catalyzed hydrosilylation of
aldehydes and ketones.28,29 Although these catalytic systems
have not been subjected to thorough mechanistic investigations,
iron hydride species are quite likely to participate in the
catalytic cycles. We were thus curious to see whether the iron
hydride complexes reported here would catalyze similar reac-
tions. We found that in the presence of 1 mol % of 1a
the hydrosilylation of PhCHO with (EtO)3SiH in THF-d8 at
50 °C (eq 6) produced (EtO)3SiOCH2Ph quantitatively within 1
h (entry 2, Table 1). Interestingly, a similar reaction catalyzed
by an imido-molybdenum hydride complex has been reported to
yield a mixture of (EtO)3SiOCH2Ph, (EtO)2Si(OCH2Ph)2,
and (EtO)Si(OCH2Ph)3.30 Compared to 1a, other iron
pincer hydride complexes proved to be less reactive catalysts
The geometric isomerization of 2a is reminiscent of Milstein’s
iridium PCP-pincer system, where, upon heating, the cis-dihy-
dride complex is converted to the more stable trans-dihydride
complex (eq 4).24 The thermodynamic driving force for that
process as well as the isomerization reaction in eq 3 is probably
the reduced pπꢀdπ repulsion (when the CO π* orbital is aligned
with these orbitals), as described above. A computational study of
the iridium system with a truncated pincer ligand has supported a
trigonal twist mechanism for the isomerization reaction shown in
eq 4.25 Consistent with this nondissociative mechanism, the
isomerization of 2a was not inhibited by added PMe3 (see the
Supporting Information). When the reaction was performed
under 1 atm of CO, in addition to the expected 2a0, a doubly
CO-substituted hydride complex formed as the minor product
(vide infra). However, the percentage conversion of 2a was not
affected, suggesting that CO does not inhibit the isomerization
either. While these results appear to agree with the trigonal twist
mechanism, alternative pathways initiated by the reductive
elimination of the ArꢀH bond, the dissociation of the pincer
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Table 2. Catalytic Hydrosilylation of Aldehydes with 1aa
Table 1. Catalytic Activity of Iron Hydride Complexes in the
Hydrosilylation of PhCHOa
entry
[Fe]
silane
time (h)
conversion (%)b
1
no catalyst
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
PhSiH3
48
1
0
>99
>99
>99
92
2
1a
3
1b
4
4
2a
2a0
68
96
48
1
5
6
3a
6
7
1a
>99
0
8
no catalyst
1a
PhSiH3
48
2.5
48
24
9
Ph2SiH2
>99
0
10
no catalyst
Ph2SiH2
11
1a
Et3SiH
0
a Reaction conditions: PhCHO (0.50 mmol), silane (0.55 mmol), and
iron hydride complex (5.0 μmol) in 0.50 mL of THF-d8 at 50 °C.
b Determined by 1H NMR.
(entries 3ꢀ6). For complexes with an identical pincer ligand, the
catalytic activity followed the decreasing order of 1a > 2a > 2a0 >
3a. In addition to (EtO)3SiH, PhSiH3 and Ph2SiH2 were
suitable reagents for the hydrosilylation of PhCHO (entries 7
and 9), although more than one hydrosilylation product were
obtained in both cases due to multiple SiꢀH bonds in the silanes.
On the other hand, the catalytic reaction with Et3SiH did
not proceed at all (entry 11). To confirm that the reactions
were catalyzed by iron, control experiments were performed, and
there were no significant hydrosilylation products observed
(entries 1, 8, and 10).
a Reaction conditions: RCHO (2.0 mmol), (EtO)3SiH (2.2 mmol),
and 1a (0.020 mmol) in 2.0 mL of THF. All the aldehydes were
fully converted to the corresponding silyl ethers (monitored by TLC
and GC).
To further extend the utility of our iron catalysts, we explored
the catalytic hydrosilylation of aldehydes bearing different func-
tionalities (eq 7). With 1 mol % of 1a as the catalyst and
(EtO)3SiH as the reductant, F-, Me-, MeO-, and Me2N-sub-
stituted benzaldehydes (entries 2ꢀ7, Table 2) were reduced to
the corresponding alcohols in good yields following basic
hydrolysis of the initial products. Electron-donating groups
appear to make the hydrosilylation reactions sluggish (entries
3ꢀ5); however, there is almost no difference in terms of the
required reaction time for PhCHO and a benzaldehyde with an
electron-withdrawing group at the para-position (entry 2). Other
aromatic aldehydes such as 2-naphthaldehyde (entry 8) and
2-furaldehyde (entry 9) are viable substrates for our catalytic
system. For an aldehyde containing both CdO and CdC bonds,
only the CdO bond was reduced (entry 10).
Various ketones were also tested under our hydrosilylation
conditions (eq 8 and Table 3). In general, they are less reactive
than the aldehydes; for many ketone substrates, a higher
temperature of 80 °C was required to ensure full conversions.
Substituent effect on the hydrosilylation rate could potentially
provide some key mechanistic insights. Unfortunately, in this
case there are no obvious trends, as both electron-withdrawing
groups (entry 2) and electron-donating groups (entry 3) render
the ketones less reactive than the unsubstituted one (entry 1).
Functional groups such as MeO (entry 3), NH2 (entry 4), and
pyridyl group (entry 7) are tolerated under the catalytic
conditions. Aliphatic ketones (entry 6) can also be reduced,
but bulky ketones such as 20,40,60-trimethylacetophenone (entry 9)
resist the hydrosilylation.
Mechanistic Investigations. The catalytic cycles for transi-
tion metal catalyzed hydrosilylation of aldehydes and ketones
may involve the insertion of the carbonyl group into a me-
talꢀhydrogen bond.15a,29,31 To explore this mechanistic scenar-
io, we treated a solution of 1a in THF-d8 with 1 equiv of PhCHO
and heated the mixture to 50 °C. No appreciable reaction was
observed even after 2 d, suggesting that CdO insertion is
unlikely to happen under the catalytic conditions. No H/D
exchange between 1a and PhCDO at the same temperature
(monitored for 24 h) also ruled out the possibility of CdO
insertion being reversible and thermodynamically uphill. Like-
wise, there was no observable reaction between 1a and
(EtO)3SiH in THF-d8 when heated at 50 °C for 24 h. A very
recent study by Nikonov and co-workers uses a deuterium-
labeled silane to probe whether or not the metal hydride moiety
truly participates in the reduction.32 Such an approach works
only if the deuterium-labeled silane does not undergo fast H/D
exchange with the metal hydride. The commercially available
Ph2SiD2 fits this criterion, as there is no H/D exchange between
1a and Ph2SiD2 at 50 °C (within 8 h). Additionally, as shown in
Table 1 (entry 9), the use of Ph2SiD2 is still relevant to the
catalytic reactions. A stoichiometric reaction of PhCDO,
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cycle (in blue). Such a mechanism has been proposed in
hydrosilylation reactions catalyzed by high-valent rhenium oxo
complexes.35 Alternatively, as suggested by a recent report,30
perhaps the carbonyl substrate is activated first to form an η1- or
η2-carbonyl species,36 followed by the reduction with the silane
(in red). Distinguishing these two mechanistic pathways requires
detailed kinetic studies, which are ongoing in our laboratories.
Table 3. Catalytic Hydrosilylation of Ketones with 1aa
’ CONCLUDING REMARKS
When using a transition metal hydride complex to catalyze a
reduction reaction, one cannot intuitively assume that the
hydride ligand will directly participate in the reaction. In this
paper, we have prepared a new class of iron pincer hydride
complexes. These well-defined iron species are competent
catalysts for the hydrosilylation of aldehydes and ketones.
Mechanistic studies have ruled out the possibility of CdO
insertion into the ironꢀhydrogen bond. However, the hydride
ligand is not merely a spectator. As demonstrated in the ligand
substitution reactions, the hydride ligand exerts a strong trans
influence that facilitates the ligand dissociation and creates an
open coordination site for substrate activation.
’ EXPERIMENTAL SECTION
General Comments. All the organometallic compounds were
prepared and handled under an argon atmosphere using standard
glovebox and Schlenk techniques. Dry and oxygen-free solvents
(THF, pentane, diethyl ether, and toluene) were collected from an
Innovative Technology solvent purification system and used throughout
the experiments. Methanol was degassed by bubbling argon through it
for 15 min and then dried over molecular sieves. Tetrahydrofuran-d8
(99.5% D, packed in sealed ampules) was purchased from Cambridge
Isotope Laboratories, Inc. and used without further purification. Ben-
zene-d6 was distilled from Na and benzophenone under an argon
atmosphere. 1,3-(iPr2PO)2C6H4,14g 1,3-(Ph2PO)2C6H4,14b and 1,3-
a Reaction conditions: RCOR0 (2.0 mmol), (EtO)3SiH (2.2 mmol), and
1a (0.020 mmol) in 2.0 mL of THF (for reactions at 50 °C) or toluene
(for reactions at 80 °C). b For reactions that did not go to completion
within 2 d at 80 °C, NMR conversions are given in parentheses.
(tBu2PO)2C6H4 were prepared as described in the literature. All
14d
the aldehydes were freshly distilled or purified by recrystallization.
Preparation of Fe(PMe3)4. Fe(PMe3)4 waspreparedaccordingtoa
slightly modified procedure from the one described by Karsch.37
A
mixture of anhydrous FeCl2 (1.0 g, 7.9 mmol) and Mg turnings (1.5 g,
61.7 mmol) was vigorously stirred in a Schlenk flask under vacuum for at
least 1 h while being heated with a heat gun. Once the flask was cooled
to room temperature, a 1.0 M solution of PMe3 in THF (39.3 mL,
39.3 mmol) was added. The resulting mixture was stirred for 1 h at room
temperature as its color changed from green to dark yellow. The volatiles
were removed under vacuum, and the remaining solid was treated with
30 mL of pentane. After filtration, removal of pentane from the filtrate
under vacuum produced Fe(PMe3)4 as a yellow solid (2.38 g, 84% yield),
which was used for the subsequent synthesis without further purification.
Synthesis of [2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)2 (1a). At ꢀ78 °C,
1,3-(iPr2PO)2C6H4 (618 mg, 1.8 mmol) and Fe(PMe3)4 (650 mg,
1.8 mmol) were mixed in 30 mL of THF. The resulting mixture
was slowly warmed to room temperature and stirred for an additional
3 h, at which point the color of the solution changed from yellow to
dark brown. The volatiles were removed under vacuum, and the
yellowish residue was extracted with diethyl ether (20 mL ꢁ 2).
Removal of the solvent from the combined etherate solutions under
vacuum yielded the crude product, which was washed with methanol
(5 mL ꢁ 3) and dried under vacuum to produce a yellow powder
(667 mg, 67% yield). X-ray quality crystals of 1a were obtained from
Ph2SiD2, and 1a (1:1:1) in THF-d8 at 50 °C for 10 h produced a
mixture of Ph2SiD(OCD2Ph) and Ph2Si(OCD2Ph)2 in a 4:1
ratio with no deuterium incorporation into 1a. This result implies
that the hydride ligand is intact during the catalytic cycle. The
hydrosilylation reactions reported here are more likely to pro-
ceed via the dissociation of PMe3 (or CO in the case of 3a),
thereby creating a vacant coordination site at the iron center to
activate either the silane or the carbonyl substrate. The relative
catalytic reactivity of different iron hydride complexes seems to
support this mechanistic hypothesis, considering the fact that our
best catalyst, 1a, contains the most labile ligand. Additional
evidence comes from the effect of added PMe3 on the reaction
time needed for the catalytic hydrosilylation reaction. When the
concentrations of substrates and catalyst 1a were kept the same
in two experiments,33 the reaction without added PMe3 required
70 min to reach completion, while the one with added PMe3 (10
equiv with respect to 1a) took 4.5 h to complete, confirming that
PMe3 inhibits the catalysis. At the moment, the mechanistic
details after the dissociation of PMe3 remain unclear to us.
Possible catalytic cycles are illustrated in Scheme 2. The open
coordination site left by PMe3 dissociation may be occupied by a
silane to generate a η2-silane σ-adduct,34 and the subsequent
reaction with the carbonyl group would complete the catalytic
1
the recrystallization of the hydride complex in THF at ꢀ30 °C. H
NMR (400 MHz, THF-d8, δ): ꢀ14.87 (tdd, JPꢀH = 78.4, 47.6, and
22.0 Hz, FeH, 1H), 0.92 (d, JPꢀH = 5.6 Hz, PMe3, 9H), 0.95ꢀ0.98
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Scheme 2. Potential Catalytic Cycles of the Hydrosilylation Reactions Catalyzed by 1aa
a The square indicates the vacant coordination site.
(m, CHCH3, 6H), 1.12ꢀ1.17 (m, CHCH3, 6H), 1.33ꢀ1.37 (m,
CHCH3, 6H), 1.38ꢀ1.42 (m, CHCH3, 6H), 1.44 (d, JPꢀH = 6.0 Hz,
PMe3, 9H), 2.29ꢀ2.33 (m, CHCH3, 2H), 2.55ꢀ2.62 (m, CHCH3,
2H), 6.17 (d, JHꢀH = 7.6 Hz, Ar, 2H), 6.48 (t, JHꢀH = 7.6 Hz, Ar, 1H).
13C{1H} NMR (101 MHz, THF-d8, δ): 18.6 (s, CHCH3), 18.9
(s, CHCH3), 19.0 (t, JPꢀC = 2.0 Hz, CHCH3), 19.6 (s, CHCH3),
23.1 (dd, JPꢀC = 19.2 and 1.1 Hz, PMe3), 28.1ꢀ28.4 (m, PMe3), 35.2
(td, JPꢀC = 14.1 and 9.0 Hz, CHCH3), 35.6 (s, CHCH3), 102.9 (td,
JPꢀC = 3.3 and 3.3 Hz, Ar), 122.7 (d, JPꢀC = 3.2 Hz, Ar), 150.0ꢀ150.3
(m, Ar), 164.5 (td, JPꢀC = 9.8 and 4.3 Hz, Ar). 31P{1H} NMR (162
MHz, THF-d8, δ): 6.6 (dt, JPꢀP = 42.1 and 27.5 Hz, PMe3, 1P), 13.7
(dt, JPꢀP = 42.1 and 16.2 Hz, PMe3, 1P), 221.0 (dd, JPꢀP = 27.5 and
16.2 Hz, OPiPr2, 2P). Anal. Calcd for C24H50FeO2P4: C, 52.37; H,
9.16. Found: C, 52.29; H, 9.09.
spectrum of the isolated material (a sticky oil) in C6D6 showed
predominantly unreacted phosphinite ligand, and the H NMR spec-
1
trum contained a triplet of doublets at ꢀ14.20 ppm (2JPꢀH = 72.0 and
29.6 Hz), which constituted less than 5% of the isolated material.
Synthesis of [2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)(CO) (2a). Un-
der a carbon monoxide atmosphere (∼1 atm), a solution of 1a (840 mg,
1.53 mmol) in 25 mL of THF was stirred at room temperature for 24 h.
The resulting solution was filtered through a short pad of Celite, and the
solvent was removed under vacuum. The residue was washed with
methanol (3 mL ꢁ 2) and dried under vacuum to produce a pale yellow
solid (750 mg, 98% yield). X-ray quality crystals of 2a were obtained
from the recrystallization of the hydride complex in toluene/methanol.
1H NMR (400 MHz, THF-d8, δ): ꢀ9.58 (td, JPꢀH = 62.4 and 52.0 Hz,
FeH, 1H), 1.01ꢀ1.05 (m, CH3, 6H), 1.08ꢀ1.14 (m, CH3, 6H),
1.34ꢀ1.41 (m, CH3, 12H), 1.43 (d, JPꢀH = 7.6 Hz, PMe3, 9H),
[2,6-(Ph2PO)2C6H3]Fe(H)(PMe3)2 (1b). This was prepared in
1
69% yield by a procedure similar to that used for 1a. H NMR (400
2.30ꢀ2.38 (m, CH, 2H), 2.41ꢀ2.52 (m, CH, 2H), 6.23 (d, JHꢀH
=
MHz, THF-d8, δ): ꢀ13.61 (tdd, JPꢀH = 59.6, 54.0, and 19.9 Hz, FeH,
1H), 0.51 (d, JPꢀH = 5.6 Hz, PMe3, 9H), 1.14 (d, JPꢀH = 6.0 Hz, PMe3,
9H), 6.41 (d, JHꢀH = 7.6 Hz, Ar, 2H), 6.63 (t, JHꢀH = 7.6 Hz, Ar, 1H),
7.16ꢀ7.19 (m, Ar, 6H), 7.47ꢀ7.52 (m, Ar, 10H), 7.97ꢀ7.99 (m, Ar,
4H). 13C{1H} NMR (101 MHz, THF-d8, δ): 21.0 (d, JPꢀC = 20.9 Hz,
PMe3), 27.7 (d, JPꢀC = 21.1 Hz, PMe3), 103.8 (td, JPꢀC = 5.3 and 2.2 Hz,
Ar), 123.0 (d, JPꢀC = 2.3 Hz, Ar), 127.6 (t, JPꢀC = 3.9 Hz, Ar), 128.5 (t,
JPꢀC = 4.0 Hz, Ar), 129.4 (s, Ar), 130.6 (s, Ar), 131.9 (t, JPꢀC = 5.4 Hz,
Ar), 133.3 (t, JPꢀC = 6.6 Hz, Ar), 135.0 (t, JPꢀC = 13.3 Hz, Ar), 142.7 (t,
JPꢀC = 14.1 Hz, Ar), 149.9ꢀ150.3 (m, Ar), 163.8ꢀ164.0 (m, Ar).
31P{1H} NMR (162 MHz, THF-d8, δ): 3.9 (dt, JPꢀP = 31.3 and 31.3 Hz,
PMe3, 1P), 4.6 (dt, JPꢀP = 31.3 and 20.6 Hz, PMe3, 1P), 198.7 (dd, JPꢀP
= 31.3 and 20.6 Hz, OPPh2, 2P). Anal. Calcd for C36H42FeO2P4: C,
62.99; H, 6.17. Found: C, 62.70; H, 6.17.
7.6 Hz, Ar, 2H), 6.54 (t, JHꢀH = 7.6 Hz, Ar, 1H). 13C{1H} NMR (101
MHz, THF-d8, δ): 17.7 (s, CH3), 18.0 (t, JPꢀC = 2.7 Hz, CH3), 18.8 (t,
JPꢀC = 2.3 Hz, CH3), 25.8 (d, JPꢀC = 24.4 Hz, PMe3), 33.6 (t, JPꢀC = 13.2
Hz, CH3), 33.7 (t, JPꢀC = 8.6 Hz, CH3), 103.4 (t, JPꢀC = 5.2 Hz, Ar),
124.0 (s, Ar), 139.5ꢀ140.0 (m, Ar), 164.5 (t, JPꢀC = 8.5 Hz, Ar),
216.7ꢀ217.1 (m, CO). 31P{1H} NMR (162 MHz, THF-d8, δ): 10.7 (t,
JPꢀP = 14.9 Hz, PMe3, 1P), 227.7 (d, JPꢀP = 14.9 Hz, OPiPr2, 2P). IR (in
toluene): νCO = 1928 cmꢀ1. Anal. Calcd for C22H41FeO3P3: C, 52.60;
H, 8.23. Found: C, 52.98; H, 8.36.
Displacement of PMe3 from 1b by CO. In a J. Young valve
NMR tube, a solution of 1b (9.6 mg, 14 μmol) in ∼0.5 mL of THF-d8
was degassed using one freezeꢀpumpꢀthaw cycle and then mixed with
1 atm of CO. The progress of the reaction was monitored by both 1H
NMR and 31P{1H} NMR spectroscopy. After 4 d, approximately 2% of
1b (based on the 1H NMR integration of the hydride resonances) was
converted to 2b, which showed a hydride resonance at ꢀ8.57 ppm (td,
2JPꢀH = 55.2 and 50.4 Hz). The sealed NMR tube was then heated with a
60 °C oil bath, and the reaction was periodically monitored by NMR.
Attempted Synthesis of [2,6-(tBu2PO)2C6H3]Fe(H)(PMe3)2
(1c). The reaction of 1,3-(tBu2PO)2C6H4 (200 mg, 0.50 mmol) with
Fe(PMe3)4 (180 mg, 0.50 mmol) was carried out under the same
conditions as those used for the synthesis of 1a. The 31P{1H} NMR
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After 3 d, a new hydride species 2b0 grew in (24% NMR conversion)
Table 4. Crystal Data Collection and Refinement Parameters
1
with concomitant disappearance of 2b. Selected H NMR data (400
2
1a
2a
3a
MHz, THF-d8, δ) of 2b0: ꢀ11.20 (td, JPꢀH = 66.8 and 28.8 Hz).
31P{1H} NMR data (162 MHz, THF-d8, δ) of 2b0: 3.03 (t, JPꢀP = 30.1
Hz, PMe3, 1P), 188.87 (d, JPꢀP = 30.1 Hz, OPiPr2, 2P).
empirical formula
fw
C24H50O2P4Fe C22H41O3P3Fe C20H32O4P2Fe
550.37
150(2)
monoclinic
P21/n
502.31
454.25
Synthesis of [2,6-(iPr2PO)2C6H3]Fe(H)(CO)(PMe3) (2a0). Un-
der an argon atmosphere, a solution of 2a (600 mg, 1.2 mmol) in 20 mL
of toluene was heated at 80 °C for 12 h, during which time a small
amount of precipitate was formed. The reaction mixture was filtered
through a pad of Celite, and the solvent was removed under vacuum.
The product was further purified by washing with methanol (3 mL ꢁ 2)
and isolated as a pale gray crystalline solid (500 mg, 83% yield). 1H NMR
(400 MHz, THF-d8, δ): ꢀ12.66 (td, JPꢀH = 66.0 and 28.8 Hz, FeH, 1H),
0.85ꢀ0.90 (m, CH3, 6H), 1.06 (d, JPꢀH = 6.4 Hz, PMe3, 9H), 1.08ꢀ1.14
(m, CH3, 6H), 1.39ꢀ1.47 (m, CH3, 12H), 2.54ꢀ2.64 (m, CH, 4H), 6.27
(d, JHꢀH = 7.6 Hz, Ar, 2H), 6.58 (t, JHꢀH = 7.6 Hz, Ar, 1H). 13C{1H}
NMR (101 MHz, THF-d8, δ): 16.9 (t, JPꢀC = 2.7 Hz, CH3), 17.6 (s,
CH3), 18.1 (t, JPꢀC = 4.7 Hz, CH3), 18.8 (t, JPꢀC = 5.3 Hz, CH3), 20.7
(d, JPꢀC = 21.9 Hz, PMe3), 30.3 (td, JPꢀC = 14.9 and 8.8 Hz, CH), 36.1
temp, K
cryst syst
space group
a, Å
150(2)
monoclinic
P21/n
150(2)
triclinic
P1
10.3557(2)
15.3040(2)
18.1959(3)
90
10.6189(1)
14.7715(2)
15.9611(2)
90
10.7596(2)
14.0546(3)
14.9506(3)
82.846(1)
87.478(1)
89.989(1)
2241.05(8)
4
b, Å
c, Å
R, deg
β, deg
93.710(1)
90
91.729(1)
90
γ, deg
volume, Å3
2877.71(8)
4
2502.47(5)
4
Z
dcalc, g/cm3
λ, Å
μ, mmꢀ1
1.270
1.333
1.346
1.54178
6.438
1.54178
6.802
1.54178
6.924
(td, JPꢀC = 7.5 and 2.9 Hz, CH), 103.8ꢀ104.0 (m, Ar), 124.7 (d, JPꢀC
=
3.3 Hz, Ar), 145.6ꢀ146.2 (m, Ar), 164.5 (td, JPꢀC = 10.4 and 4.7 Hz,
Ar), 218.6ꢀ219.6 (m, CO). 31P{1H} NMR (162 MHz, THF-d8, δ): 7.2
(t, JPꢀP = 29.6 Hz, PMe3, 1P), 227.2 (d, JPꢀP = 29.6 Hz, OPiPr2, 2P). IR
(in toluene): νCO = 1903 cmꢀ1. Anal. Calcd for C22H41FeO3P3: C,
52.60; H, 8.23. Found: C, 52.65; H, 8.29.
no. of data collected
24 417
21 071
18 941
no. of unique data, Rint 5056, 0.0454
4436, 0.0486
1.040
7692, 0.0360
1.029
goodness-of-fit on F2
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
1.047
0.0347, 0.0888 0.0356, 0.0907 0.0338, 0.0871
0.0425, 0.0938 0.0423, 0.0951 0.0388, 0.0913
Effect of Added PMe3 or CO on the Isomerization of 2a. In a
glovebox under an argon atmosphere, a solution of 2a (30 mg,
0.060 mmol) in 1.5 mL of THF-d8 was evenly divided into three
portions and placed in three J. Young valve NMR tubes. The first
sample was degassed using one freezeꢀpumpꢀthaw cycle and then
mixed with 1 atm of CO. To the second sample tube, 5 equiv of PMe3
(10.3 μL, 0.10 mmol) was added. All three NMR tubes were then
sealed and heated with a 60 °C oil bath. The progress of the
isomerization was monitored by 31P{1H} NMR spectroscopy, and
the conversions were calculated from the integrations of the phos-
phorus resonances. A plot of NMR conversions as a function of time
is provided in the Supporting Information.
Effect of Added PMe3 on the Hydrosilylation of PhCHO
with (EtO)3SiH Catalyzed by 1a. To a solution of 1a (6.0 mg, 0.011
mmol) in 1.30 mL of THF-d8 was added PhCHO (111 μL, 1.1 mmol)
and (EtO)3SiH (221 μL, 1.2 mmol). The resulting mixture was evenly
divided into two portions and placed in two J. Young valve NMR tubes.
To one of them was added PMe3 (5.6 μL, 0.054 mmol), and then both
NMR tubes were placed in a 50 °C oil bath. The progress of the reaction
was monitored by 1H NMR spectroscopy.
General Procedure for the Catalytic Hydrosilylation of
Aldehydes. To a 25 mL Schlenk tube containing a solution of
1a (11 mg, 0.02 mmol) in 2 mL of THF were added an aldehyde
(2.0 mmol) and (EtO)3SiH (406 μL, 2.2 mmol). The reaction mixture
was stirred at 50 or 65 °C until there was no aldehyde left (monitored by
TLC and GC-MS). The reaction was then quenched by MeOH (1 mL)
and a 10% aqueous solution of NaOH (∼5 mL) with vigorous stirring at
50 °C for about 2 d. The organic product was extracted with Et2O
(10 mL ꢁ 3), dried over anhydrous MgSO4, and concentrated under
vacuum. The alcohol product was further purified using flash column
chromatography (eluted with 5ꢀ30% ethyl acetate in hexanes). The 1H
NMR and 13C{1H} NMR spectra of the primary alcohol products are
provided in the Supporting Information.
General Procedure for the Catalytic Hydrosilylation of
Ketones. Ketones were reduced following a similar procedure to the
one used for aldehydes except that toluene was used as the solvent for
the reactions at 80 °C. The 1H NMR and 13C{1H} NMR spectra of the
secondary alcohol products are provided in the Supporting Information.
X-ray Structure Determinations. Crystal data collection and
refinement parameters are summarized in Table 4. Intensity data were
collected at 150 K on a Bruker SMART6000 CCD diffractometer using
graphite-monochromated Cu KR radiation, λ = 1.54178 Å. The data
frames were processed using the program SAINT. The data were
corrected for decay, Lorentz, and polarization effects as well as absorp-
tion and beam corrections based on the multiscan technique. The
structures were solved by a combination of direct methods in SHELXTL
and the difference Fourier technique and refined by full-matrix least-
squares procedures. Non-hydrogen atoms were refined with anisotropic
Synthesis of [2,6-(iPr2PO)2C6H3]Fe(H)(CO)2 (3a). Under a
carbon monoxide atmosphere (∼1 atm), a solution of 1a (1.10 g,
2.0 mmol) in 30 mL of toluene was stirred at room temperature for 1 d
and then heated with an 80 °C oil bath for 3 d. The solution was
subsequently pumped to dryness, and the residue was extracted with
20 mL of toluene. After filtration, the toluene solution was concentrated
under vacuum and the crude product was subjected to recrystallization
from methanol, resulting in colorless crystals that were suitable for
1
X-ray diffraction studies (460 mg, 51% yield). H NMR (400 MHz,
THF-d8, δ): ꢀ9.61 (t, JPꢀH = 52.4 Hz, FeH, 1H), 1.03ꢀ1.08 (m, CH3,
6H), 1.12ꢀ1.18 (m, CH3, 6H), 1.36ꢀ1.45 (m, CH3, 12H), 2.44ꢀ2.53
(m, CH, 2H), 2.57ꢀ2.67 (m, CH, 2H), 6.35 (d, JHꢀH = 8.0 Hz, Ar, 2H),
6.67 (t, JHꢀH = 8.0 Hz, Ar, 1H). 13C{1H} NMR (101 MHz, THF-d8, δ):
17.2 (t, JPꢀC = 3.3 Hz, CH3), 17.3 (s, CH3), 17.7 (s, CH3), 17.9 (s, CH3),
33.4 (t, JPꢀC = 14.8 Hz, CH), 35.0 (t, JPꢀC = 11.5 Hz, CH), 104.8 (t,
JPꢀC = 5.6 Hz, Ar), 125.7 (s, Ar), 136.3 (t, JPꢀC = 17.8 Hz, Ar), 164.4 (t,
JPꢀC = 8.9 Hz, Ar), 213.1 (t, JPꢀC = 13.8 Hz, CO), 214.7 (t, JPꢀC = 11.1
Hz, CO). 31P{1H} NMR (162 MHz, THF-d8, δ): 230.5 (s). IR (in
toluene): νCO = 1993, 1946 cmꢀ1. Anal. Calcd for C20H32FeO4P2: C,
52.88; H, 7.10. Found: C, 53.12; H, 7.03.
Hydrosilylation of PhCHO Catalyzed by Various Iron
Hydride Complexes. In a J. Young valve NMR tube, PhCHO (51
μL, 0.50 mmol) and an iron hydride catalyst (5.0 μmol, 1 mol %) were
dissolved in 0.50 mL of THF-d8, followed by the addition of a silane
(0.55 mmol). The NMR tube was sealed and placed in a 50 °C oil bath.
The progress of the reaction was monitored by 1H NMR spectroscopy,
and the results are summarized in Table 1.
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displacement parameters. Each hydride was located directly from the
difference map and the position refined. The remaining H-atoms were
either located or calculated and subsequently treated with a riding
model. Compound 3a crystallized as two independent molecules in the
crystalline lattice with different orientations of the iPr groups.
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’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data in
b
CIF format, experimental details on the effect of added PMe3 or
CO on the isomerization of 2a, and characterization (1H NMR
and 13C{1H} NMR data) of the alcohol products. This material is
available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: hairong.guan@uc.edu.
’ ACKNOWLEDGMENT
We thank the National Science Foundation (CHE-0952083)
and the donors of the American Chemical Society Petroleum
Research Fund (49646-DNI3) for generous support of this
research, Prof. Hongjian Sun (Shandong University, China)
for helpful suggestions on the synthesis of Fe(PMe3)4, and
Prof. Mu-Hyun Baik (Indiana University) and Prof. Seth Brown
(University of Notre Dame) for stimulating discussions. P.B. is
grateful to the University of Cincinnati University Research
Council for a graduate student research fellowship. X-ray data
were collected on a Bruker SMART6000 diffractometer, which
was funded through an NSF-MRI grant (CHE-0215950).
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