Mechanistic aspects of hydrosilylation catalyzed by (ArN=)Mo(H)(Cl) (PMe3)3

Article abstract of DOI:10.1021/ic300010c

The reaction of (ArN=)MoCl₂(PMe₃)₃ (Ar = 2,6-diisopropylphenyl) with L-Selectride gives the hydrido-chloride complex (ArN=)Mo(H)(Cl)(PMe₃)₃ (2). Complex 2 was found to catalyze the hydrosilylation of carbonyls and nitriles as well as the dehydrogenative silylation of alcohols and water. Compound 2 does not show any productive reaction with PhSiH₃; however, a slow H/D exchange and formation of (ArN=)Mo(D)(Cl)(PMe₃)₃ (2_D) was observed upon addition of PhSiD₃. Reactivity of 2 toward organic substrates was studied. Stoichiometric reactions of 2 with benzaldehyde and cyclohexanone start with dissociation of the trans-to-hydride PMe₃ ligand followed by coordination and insertion of carbonyls into the Mo-H bond to form alkoxy derivatives (ArN=)Mo(Cl)(OR)(PMe₂)L₂ (3: R = OCH₂Ph, L₂ = 2 PMe₃; 5: R = OCH₂Ph, L₂ = ν²-PhC(O)H; 6: R = OCy, L₂ = 2 PMe ₃). The latter species reacts with PhSiH₃ to furnish the corresponding silyl ethers and to recover the hydride 2. An analogous mechanism was suggested for the dehydrogenative ethanolysis with PhSiH₃, with the key intermediate being the ethoxy complex (ArN=)Mo(Cl)(OEt)(PMe ₃)₃ (7). In the case of hydrosilylation of acetophenone, a D-labeling experiment, i.e., a reaction of 2 with acetophenone and PhSiD ₃ in the 1:1:1 ratio, suggests an alternative mechanism that does not involve the intermediacy of an alkoxy complex. In this particular case, the reaction presumably proceeds via Lewis acid catalysis. Similar to the case of benzaldehyde, treatment of 2 with styrene gives trans-(ArN=)Mo(H) (ν²-CH₂=CHPh)(PMe₃)₂ (8). Complex 8 slowly decomposes via the release of ethylbenzene, indicating only a slow insertion of styrene ligand into the Mo-H bond of 8.

Full text of DOI:10.1021/ic300010c

Article
pubs.acs.org/IC
Mechanistic Aspects of Hydrosilylation Catalyzed by
(ArN=)Mo(H)(Cl)(PMe3)3
Andrey Y. Khalimon,† Oleg G. Shirobokov,† Erik Peterson,† Razvan Simionescu,† Lyudmila G. Kuzmina,‡
Judith A.K. Howard,§ and Georgii I. Nikonov*,†
†Chemistry Department, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario L2S 3A1, Canada
‡N.S. Kurnakov Institute of General and Inorganic Chemistry, 31 Leninskii Prospekt, Moscow, 119991, Russia
§Chemistry Department, University of Durham, South Road, Durham DH1 3LE, United Kingdom
S
* Supporting Information
ABSTRACT: The reaction of (ArN=)MoCl2(PMe3)3 (Ar =
2,6-diisopropylphenyl) with L-Selectride gives the hydrido-
chloride complex (ArN=)Mo(H)(Cl)(PMe3)3 (2). Complex 2
was found to catalyze the hydrosilylation of carbonyls and
nitriles as well as the dehydrogenative silylation of alcohols and
water. Compound 2 does not show any productive reaction
with PhSiH3; however, a slow H/D exchange and formation of
(ArN=)Mo(D)(Cl)(PMe3)3 (2D) was observed upon addition
of PhSiD3. Reactivity of 2 toward organic substrates was
studied. Stoichiometric reactions of 2 with benzaldehyde and cyclohexanone start with dissociation of the trans-to-hydride PMe3
ligand followed by coordination and insertion of carbonyls into the Mo−H bond to form alkoxy derivatives
(ArN=)Mo(Cl)(OR)(PMe2)L2 (3: R = OCH2Ph, L2 = 2 PMe3; 5: R = OCH2Ph, L2 = η2-PhC(O)H; 6: R = OCy, L2 = 2
PMe3). The latter species reacts with PhSiH3 to furnish the corresponding silyl ethers and to recover the hydride 2. An analogous
mechanism was suggested for the dehydrogenative ethanolysis with PhSiH3, with the key intermediate being the ethoxy complex
(ArN=)Mo(Cl)(OEt)(PMe3)3 (7). In the case of hydrosilylation of acetophenone, a D-labeling experiment, i.e., a reaction of 2
with acetophenone and PhSiD3 in the 1:1:1 ratio, suggests an alternative mechanism that does not involve the intermediacy of an
alkoxy complex. In this particular case, the reaction presumably proceeds via Lewis acid catalysis. Similar to the case of
benzaldehyde, treatment of 2 with styrene gives trans-(ArN=)Mo(H)(η2-CH2CHPh)(PMe3)2 (8). Complex 8 slowly
decomposes via the release of ethylbenzene, indicating only a slow insertion of styrene ligand into the Mo−H bond of 8.
Ojima mechanism (for carbonyls),10 which starts with oxidative
INTRODUCTION
■
addition of silane to metal, substrate coordination, and
migration of the silyl (or the hydride ligand) followed by
either reductive C−H elimination (or Si−C for alkenes and O−
Si elimination for carbonyls). Heterolytic splitting of silanes on
the polar M−O bonds (or σ-bond metathesis) was suggested
for early metal systems.5a Recently, several groups provided
evidence for alternative mechanisms involving the addition of
carbonyls to an intermediate silylene complex (Gade−
Hoffmann mechanism),11 addition of a silane across the M
O double bond (Toste mechanism),6c,12 ionic hydrosilylation
(e.g., Dioumaev-Bullock,13 Abu-Omar et al.,14 Brookhart et
al.,15 and our group16).
Here we report a detailed study of catalytic hydrosylilation of
organic substrates mediated by an imido/hydride complex of
Mo(IV), as well as insights into the possible mechanism(s) of
these reactions. A preliminary communication has been
published.17
The development of efficient synthetic methodologies for
selective and efficient reduction of multiple carbon−carbon or
carbon-heteroatom bonds is a fundamental problem of organic
and organometallic chemistry.1 In this respect, the ability of
transition metal compounds to induce catalytic reactions of this
kind has been recognized and used extensively for many
years.1,2 The hydrosilylation methodology stands aside from
other catalytic reduction methods due to the availability,
stability and cost of silanes, as well as the safety, low toxicity,
and simplicity of experimental procedures.3 To date, however,
most practical protocols of hydrosilylation involve the
application of late transition metal catalyst which are very
efficient but suffer from the high cost and recognized toxicity.2
To circumvent these problems, the development of iron
catalysis4 and the use of early transition metal systems3,5 has
recently received significant attention. In this regard, catalysis
based on molybdenum is quite promising6,7 because of the low
cost and environmentally benign nature of this metal.8
Until very recently, the main mechanistic proposal for metal
catalyzed hydrosilylation had been based on the variations of
Chalk−Harrod mechanism9 (for alkenes and alkynes) and
Received: January 5, 2012
Published: March 21, 2012
© 2012 American Chemical Society
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Scheme 1. Reaction of (ArN=)Mo(PMe3)3 with HSiCl3 and Preparation of the Hydride Complex 2
Table 1. Selected Bond Distances (Å) and Angles (deg) for
Complex 1
RESULTS AND DISCUSSION
Preparation of Hydrido-chloride Complex (ArN=)Mo-
■
(H)(Cl)(PMe3)3 (2). We have previously found that the
bis(imido) complex (ArN=)2Mo(PMe3)3 reacts with HSiCl3
to give the mono(imido) derivative (ArN=)MoCl2(PMe3)3 (1)
and a silanimine dimer (ArN=SiHCl)2 (Scheme 1).18
Spectroscopic characterization of complex 1 has been reported
earlier and is now confirmed by the X-ray structure
determination. The complex adopts an octahedral geometry,
with one of the chloride substituents lying trans to the imido
group (Figure 1) and the second chloride occupies an
distances (Å)
angles (deg)
Mo1−N1−C1
Mo1−N1
1.753(4)
172.7(3)
179.28(13)
85.31(5)
94.95(13)
163.84(5)
90.45(5)
99.51(5)
Mo1−Cl1
Mo1−Cl2
Mo1−P1
Mo1−P2
Mo1−P3
N1−C1
2.5079(14)
2.5206(13)
2.5080(15)
2.5158(14)
2.4987(15)
1.405(6)
N1−Mo1−Cl1
Cl1−Mo1−Cl2
N1−Mo1−Cl2
P1−Mo1−P2
P1−Mo1−P3
P2−Mo1−P3
−1.1 ppm (2JP−P = 13.4 Hz) corresponding to two equivalent
phosphines and a triplet at δ −17.8 ppm (2JP−P = 13.4 Hz) for
1
the unique PMe3 group lying trans to the hydride. The H
NMR spectrum of 2 shows a downfield hydride signal at δ 5.31
(dt) coupled to two equivalent cis-PMe3 (2JH−P = 28.5 Hz) and
the unique trans-PMe3 (2JH−P = 51.9 Hz). The presence of the
hydride is also evident from the observation of a characteristic
Mo−H stretch at 1714 cm−1 in the IR spectrum. X-ray
diffraction analysis revealed an octahedral geometry, with the
chloride lying trans to the imido group and the hydride
occupying a site cis to the imido group. For the subsequent
discussion it is important to point out that the phosphine trans
to the hydride forms a significantly longer Mo−P bond than the
mutually cis-phosphines (2.572(1) Å vs 2.485(1) and 2.469(1)
Å, respectively).17
Catalytic Reactivity of Complex 2. Complex 2 has been
found to catalyze a variety of hydrosilylation reactions (Tables
2 and 3). Thus, the room temperature treatment of
benzaldehyde with PhSiH3 in the presence of 5.0 mol % of 2
results in the quantitative conversion of the substrate after one
day, affording a mixture of PhH2Si(OBn) and PhHSi-
21,22
(OBn)2
in 37 and 63% yield, respectively (Table 2, entry
1). An increase in temperature up to 50 °C leads to a faster
reaction, so that the full conversion of PhC(O)H was observed
after only 3 h (Table 2, entry 2). The use of PhMeSiH222c,23 or
poly(methylhydrosiloxane) (PMHS) instead of PhSiH3 results
in more sluggish reactions, with the conversion of benzaldehyde
being in the range 50−68% (Table 2, entries 3 and 4).
Figure 1. ORTEP plot of the molecular structure of complex 1
(hydrogen atoms are omitted for clarity).
equatorial position (cis- to ArN2¯) and coplanar with all three
PMe3 ligands. The imido moiety in 1 is almost linear (the
Mo1−N1−C1 angle is 172.7(3)°; Table 1), which indicates
that the ArN2¯ ligand acts as a 6e donor stabilizing the 18e
valence shell of Mo.19 Similar structural features were reported
for the analogous complex (tBuN=)MoCl2(PMe3)3.20
Complex 1 was found to be quite robust and, surprisingly,
did not show any reactivity with excess of NaBH4 as well as
MeLi/H2, Na(Hg)/H2, and KC8/H2. On the other hand,
addition of L-Selectride (1 equiv.) to 1 affords cleanly the
hydrido-chloride derivative (ArN=)Mo(H)(Cl)(PMe3)3 (2;
Scheme 1). Compound 2 was fully characterized by multi-
nuclear NMR, IR and X-ray diffraction analysis.17 The 31P
NMR spectrum of 2 exhibits two sets of signals: a doublet at δ
With acetophenone as the substrate, the activity of
hydrosilylation by PhSiH3 is much reduced,23 with the
complete conversion of the carbonyl being achieved only
after 11 days at room temperature (Table 2, entry 5). On the
other hand, the hydrosilylation of acetophenone with PMHS25
catalyzed by 2 requires heating (50 °C) and gives full
conversion after 48 h (Table 2, entry 6). Much faster reactions
with both PhSiH3 and PMHS were observed for the
cyclohexanone which was fully converted to the corresponding
silyl ethers within one hour (Table 2, entries 7−9).25,26 Low
catalytic activity of 2 was also found in the hydrosilylation of 1-
octyne (Table 3, entry 1), which leads to only 35% conversion
of the alkyne to form a mixture of PhH2SiCHCH(Hex),
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a
Table 2. Catalytic Hydrosilylation of Ketones and Aldehydes Mediated by Complex 2
d
e
e
entry
1
substrate
silane
T (°C)/time
products
substr. conv. (%)
yield (%)
PhC(O)H
PhSiH3
RT/24 h
PhH2Si(OBn)
PhHSi(OBn)2
PhH2Si(OBn)
PhHSi(OBn)2
PhMeHSi(OBn)
(BnO)xPMS
100
37
63
b
2
50/3 h
100
32
68
3
PhMeSiH2
PMHS
RT/24 h
50/48 h
68
50
68
c
4
50
5
PhC(O)Me
PhSiH3
RT/11 days
PhH2Si[OCH(Me)Ph]
PhHSi[OCH(Me)Ph]2
PhSi[OCH(Me)Ph]3
PhCH2CH3
100
54
23
12
11
c
6
PMHS
PhSiH3
50/48 h
[Ph(Me)CHO]xPMS
PhH2Si(OCy)
100
100
100
79
7
cyclohexanone
RT/35 min
PhHSi(OCy)2
21
c
8
PMHS
RT/20 min
50/1 h
(CyO)xPMS
50
50
c
9
(CyO)xPMS
100
100
a
b
c
d
Conditions: 5 mol % 2, C6D6, substrate/silane = 1/1, Csubst = 2.0 M. 3 mol % 2. 6 mol % 2. Reactions with PhSiH3 give Ph2SiH2 and SiH4
byproduct.24 Detected by H NMR using tetramethylsilane as a standard.
e
1
Table 3. Catalytic Hydrosilylation of Alkenes and Alkynes
and Alcoholysis/Hydrolysis of PhSiH3 by 2.
the 100% conversion of PhSiH3 after 3 h at room temperature
(Table 3, entry 4).
a
The last but not the least, the hydride complex 2 was found
to catalyze the addition of phenylsilane to nitriles, including a
rare example of a selective catalytic hydrosilylation of
benzonitrile to the corresponding silyl imine.7,16b,32,33 Thus,
the room temperature reaction of PhCN with one equivalent of
PhSiH3 and 5.0 mol % 2 leads to only stoichiometric (5%)
formation of PhH2Si(NCHPh) (Table 3, entry 6). An
increase of the reaction time (up to 13 days) and temperature
(up to 50 °C) results in full conversion of benzonitrile,
producing of a mixture of PhH2Si(NCHPh) and PhHSi(N
CHPh)2 in 76 and 24%, respectively (Table 3, entry 7). In
contrast, the hydrosilylation of acetonitrile with PhSiH3 affords,
after 6 days at 50 °C, EtN(SiH2Ph)233d,34 (45%, Table 3, entry
5). No monoaddition products PhH2Si(NCHMe) and
PhHSi(NCHMe)2 were seen by NMR at any step of the
reaction.
substr.
conv.
(%)
e
e
yield
c
entry substrate T (°C)/time
products
(%)
1
1-octyne RT/15 days
PhH2SiC(Hex)
35
4
CH2
PhH2SiCHC
7
H(Hex)
PhHSi[CHC
12%
H(Hex)]
[C(Hex)CH2]
PhHSi[CHC
12
H(Hex)]2
2
3
1-
hexene
RT/72 h and PhH2Si(Hex)
89
3
6
60/24 h
2-hexene
hexane
80
26
74
EtOH
RT/1 h
PhH2Si(OEt)
PhHSi(OEt)2
polysiloxane
EtN(SiH2Ph)2
100
b
d
4
5
6
7
H2O
RT/3 h
100
Stoichiometric Reactivity of Complex 2: Mechanistic
Aspects of Hydrosylilation. To shed light on the mechanism
of catalysis, stoichiometric reactions between complex 2 and
unsaturated organic molecules were studied. In our preliminary
communication, we suggested a possible mechanism of the
hydrosilylation of benzaldehyde (Scheme 2) on the basis of
studying individual steps under stoichiometric conditions.17 It
is important to mention that the hydrido-chloride complex 2
shows no visible reaction with PhSiH3 (1−2 equivs) over a
period of several days. No addition of silane to either the
molybdenum center or across the MoN bond18 in 2 was
observed. Such a behavior is in contrast with the previously
observed addition of R3SiH to the ReO bond in Re(O)2I-
MeCN
PhCN
50/6 days
45
5
45
5
RT/18 h and PhH2Si(NCHPh)
50/12 days
PhH2Si(NCHPh)
100
76
24
PhHSi(NCHPh)2
a
Conditions: 5 mol % of 2; silane: PhSiH3; C6D6; substrate/silane =
b
c
1/1; Csubst = 2.0 M. H2O/PhSiH3 = 5/1. Ph2SiH2 and SiH4 were also
observed.24 Conversion of PhSiH3. Detected by 1H NMR using
d
e
TMS as a standard.
PhH2SiC(Hex)CH2, PhHSi[CHCH(Hex)][C(Hex)
CH2], and PhHSi[CHCH(Hex)]2.27
Complex 2 turned out to be inactive in the hydrosilylation of
alkenes. Despite the fact that addition of PhSiH3 to 1-hexene in
the presence of 5.0 mol % 2 gives 86% conversion of the alkene,
the reaction affords mostly hexane,28 as well as a small amount
of isomerization product, 2-hexene (6%), was observed (Table
3, entry 2).29
12
(PPh3)2 and the H/Cl exchange observed for Re(O)-
Cl3(PCy3)2 and Re(NMes)Cl2(PPh3)2 by Abu-Omar et al.14a
However, 2 undergoes a slow H/D exchange (78% after 3
days) when reacted with 1 equiv. of PhSiD3. In the presence of
excess PhSiD3 (ca. 4.2 equivs), ∼ 93% of hydride in 2 is
substituted by deuteride after 1 day at room temperature
(Table 4).
Ethanolysis of PhSiH3 in the presence of 5.0 mol % 2 is fast
(Table 3, entry 3). After 1 h at room temperature, the reaction
is complete, affording a mixture of silyl ethers PhH2SiOEt
(26%) and PhHSi(OEt)2 (74%).22b,30 Interestingly, a similar
reaction of phenylsilane with excess water31 (ca. 5 equivs) is
more sluggish and leads to the formation of polysiloxane with
In contrast, we have found that 2 reacts easily with carbonyl
compounds (aldehydes and ketones) and alcohols. Thus, the
treatment of 2 with benzaldehyde affords the benzoxy
derivative (ArN=)Mo(Cl)(OCH2Ph)(PMe3)3 (3; Scheme 2).
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Scheme 2. Mechanism for the Hydrosilylation of PhC(O)H
with PhSiH3 Catalyzed by Complex 2
Figure 2. Dependence of the k of the reaction of 2 with PhC(O)H
(−5 °C) on the concentration of PhC(O)H. The plateau region
corresponds to the pseudo first order regime.
benzaldehyde and readdition to give an isomer where the Mo-
bound hydride is cis to the aldehyde ligand.38 This suggestion is
1
supported by a series of H−1H EXSY NMR experiments
which show a fast intermolecular exchange of 4 with
benzaldehyde and a slow intramolecular exchange of the
PMe3 ligands. No exchange of 4 with an external phosphine
Table 4. Change in the Integral Intensity of the Mo-H Signal
in the 1H NMR Spectrum of 2 upon Addition of PhSiD3 (4.2
equivs.)
37
was observed by H−1H EXSY NMR. Further fast migration
of the hydride to the cis-coordinated aldehyde would produce
the benzoxy ligand. Analogous hydride migration has been
shown previously by Abu-Omar et al. for the rhenium hydride
complex Re(O)(H)Cl2(PPh3)2.14a The smaller k(22 °C) = (2.1
0.1) × 10−4 s−1 of the formation of 3 from 4 than the rate
constant of aldehyde addition to 2 (k1(−5 °C) = (7.6 0.2) ×
10−4 s−1) agrees well qualitatively with the difference in the
trans-influence of phosphine and hydride ligands. However, we
cannot exclude the possibility that the rearrangement of 4 to 3
occurs via dissociation of phosphine at a rate too slow to be
determined by the EXSY experiment. The readdition of PMe3
on the aldehyde-phosphine or hydride-phosphine edge of the
complex would bring the hydride and η2-aldehyde into cis
position, suitable for hydride migration. This latter reaction
sequence is in accord with the slower rate of rearrangement of
4 into 3 in the presence of PMe3 than decomposition of 4 in
the absence of free phosphine.
1
time
Mo-H (%)
10 min
6 h
91
45
7
26 h
Monitoring the reaction by NMR revealed the initial formation
of intermediate trans-(ArN=)Mo(H)(Cl)(η2-OCHPh)-
(PMe3)2 (4, mixture of two isomers;35 Scheme 2). At −5 °C,
complex 4 is the sole reaction product. The η2-coordination of
benzaldehyde in 4 results in a significant upfield shift of the
1
OCH resonance to δ 5.77 ppm in the H NMR spectrum and
reduction of its CO stretch (1595 cm−1) in the IR
spectrum.36 Also, the η2-coordination of PhC(O)H results in
the nonequivalence of the mutually trans phosphine groups.
The latter give rise to a pair of coupled doublets at δ 1.43 and
−5.59 ppm (2JP−P = 109.3 Hz) in the 31P{1H} NMR spectrum.
At large aldehyde concentration, the reaction obeys the pseudo
first order kinetics (k1(−5 °C) = (7.6 0.2) × 10−4 s−1; Figure
2),37 consistent with the rate-limiting dissociation of the trans-
to-hydride PMe3 ligand, followed by fast addition of aldehyde.
Such an exclusive dissociation of the unique trans-PMe3 is in
accordance with the longest Mo−Ptrans distance (see above) and
is further supported by the observation of exchange of the trans
PMe3 with an external phosphine in the room temperature
31P−31P EXSY NMR experiment (Figure 3). Dissociation of
PMe3 from 2 in the reaction with benzaldehyde is reversible, as
addition of excess phosphine (ca. 10 equivs) to 4 regenerates
complex 2 and free benzaldehyde.
Another insertion of coordinated benzaldehyde into the
Mo−H bond is observed when 4 reacts with excess
benzaldehyde. In this case the sole product is the benzoxy
aldehyde complex (ArN=)Mo(Cl)(OCH2Ph)(η2-PhC(O)H)-
(PMe3)2 (5, Scheme 3). Alternatively, compound 5 could be
obtained by the reaction of 3 with excess PhC(O)H. The latter
reaction is reversible as the addition of a large excess of PMe3
1
(ca. 10 equivs) to 5 cleanly regenerates complex 3. The H
NMR spectrum of 5 shows two mutually coupled diaster-
eotopic OCH2Ph signals at δ 4.62 ppm and 5.14 ppm (2JH−H
=
12.2 Hz) and an upfield benzaldehyde proton at δ 5.37 ppm (d,
2JH−P = 3.9 Hz), diagnostic for the η2-coordination of
aldehyde.36 The 13C NMR spectrum of 5 displays a significant
upfield shift of the carbonyl signal (δ 90.7 ppm) additionally
suggesting the η2-coordination of PhC(O)H.
At large benzaldehyde concentrations (ca. 5−15 equiv.), the
formation of 5 obeys the first order kinetics in 4 with keff(22
°C) = (3.1 0.1) × 10−4 s−1. The reaction constant does not
depend on the concentration of benzaldehyde, suggesting the
In the absence of PMe3, the benzaldehyde adduct 4
decomposes within two hours to an intractable mixture of
products. However, in the presence of one equiv. of PMe3 it
slowly (within 5 h) rearranges at room temperature into the
benzoxy derivative 3. The reaction is first order on 4 (Figure 4)
and proceeds, most likely, via dissociation of the coordinated
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Figure 3. 31P−31P EXSY NMR spectrum (22 °C) for the mixture of 2 with PMe3.
However, EXSY experiments revealed a fast exchange of the
coordinated benzaldehyde with free PhC(O)H.37 Interestingly,
we also observed an exchange between the diastereotopic
protons of the benzoxy ligand in 5. This exchange occurs with a
large positive activation entropy ΔS‡ = 47.8 6.4 cal K−1 mol−1
(ΔH‡ = 31.3
1.9 kcal mol−1) (Figure 5), suggesting a
dissociatively activated process, most likely triggered by the
Figure 4. Kinetic profile for the rearrangement of 4 into 3 at 22 °C
(standard error: 0.068).
rate-limiting PMe3 dissociation (dissociation of benzaldehyde in
this case is suppressed), followed by coordination of PhC(O)H.
Such a dissociative mechanism for the reaction of 4 with excess
benzaldehyde is also consistent with the kinetic VT NMR
studies which reveal a large positive value of the entropy of
activation, ΔS‡ = 29.8 9.2 cal K−1 mol−1 (ΔH‡ = 30.8 2.7
kcal·mol−1, ΔG‡295.1 = 22.0 5.4 kcal·mol−1).37 Analogously to
complex 4, the exchange between the phosphine ligand in 5
and free PMe3 was too slow to be observed by EXSY NMR.
Figure 5. Eyring plot for the exchange of two enantiomers of 5.
Scheme 3. Preparation of Complex 5
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dissociation of the coordinated benzaldehyde. This process
generates the Cs symmetrical fragment (ArN=)Mo(Cl)-
(OCHaHbPh)(PMe3)2, in which both methylene protons
become equal, thus accounting for the Ha/Hb exchange.
Similar to the case of benzaldehyde, reaction of hydride 2
with one equivalent of cyclohexanone eventually gives the
cyclohexoxy derivative (ArN=)Mo(Cl)(OCy)(PMe3)3 (6;
Scheme 4). However, no η2-cyclohexanone intermediate,
analogous to 4, was observed upon monitoring the reaction
(the O1−Mo1−N1 angle is 175.99(11)°, Table 5), consistent
with the strong trans-influence of the ArN2− ligand39 and
Table 5. Selected Bond Distances and Angles for Complex 6
distances (Å)
Mo1−N1
Mo1−O1
Mo1−Cl1
Mo1−P1
Mo1−P2
Mo1−P3
angles (deg)
Mo1−N1−C1
N1−Mo1−O1
P1−Mo1−Cl1
P2−Mo1−P3
N1−Mo1−Cl1
N1−Mo1−P2
1.786(3)
1.984(2)
2.5462(8)
2.4762(9)
2.5524(9)
2.5072(9)
176.4(2)
175.99(11)
166.80(3)
168.64(3)
98.02(8)
90.88(8)
Scheme 4. Preparation of Complex 6
reduced steric repulsion in the molecule when the phosphines
are arranged in the meridian fashion. The chloride ligand is
occupying an equatorial position and is coplanar with all three
PMe3 ligands. The C1−N1−Mo1 angle in 6 is almost linear
(176.4(2)°), suggesting that the imido ligand donates six
19
electrons, stabilizing the formal 18e valence shell.
by NMR, suggesting that the rearrangement of such an η2-
adduct into 6 is much faster than the transformation of 4 into 3.
Furthermore, complex 6 showed no further reaction with
cyclohexanone: the adduct (ArN=)Mo(OCy)(Cl)(η2-O
C6H10)(PMe3)2 similar to 5 was not observed by NMR,
presumably because of steric constraint. Attempts to force the
ketone coordination by addition of an equivalent of BPh3 to 6
led to decomposition to a mixture of 1 and unidentified
products.
Complex 6 undergoes phosphine dissociation at room
temperature which was observed directly by 1H and 31P
NMR spectroscopy. Exchange of the PMe3 ligands in 6 with
free phosphine was also observed in the 2D 31P−31P and 1D 1H
EXSY NMR experiments.35 Judging by the peak intensities in
the 31P−31P EXSY NMR spectrum, the cis-to-chloride
phosphines dissociate easier than the trans-PMe3 ligand (Figure
7), consistent with the stronger phosphine trans-influence
compared with the chloride trans-influence. The intramolecular
1
PMe3 exchange was also observed by 1D H EXSY NMR and
most likely proceeds via dissociation of one of the cis-to-
chloride PMe3 ligands.
Complex 6 was characterized by NMR spectroscopy and X-
ray diffraction analysis (Figure 6). 6 has an octahedral geometry
with the cyclohexoxy ligand laying trans to the imido group
Both complexes 3 and 6 undergo fast reactions with PhSiH3
to furnish the corresponding hydrosylilation products and
regenerate the hydride-chloride 2, thus closing the possible
catalytic cycle. An NMR experiment between the benzoxy
complex 3 and the labeled silane PhSiD3 resulted in the
exclusive formation of (ArN=)Mo(D)(Cl)(PMe3)3 (2D). The
reactions between complex 3 and excess PhSiH3 in the
presence of large excess PMe3 is first order in the complex.
The linear dependence of keff on the PhSiH3 concentration
(Figure 8A) shows that the reaction has the first order in silane
too, i.e., the second order overall reaction. The 1/keff is
proportional to phosphine concentration (Figure 8B), which
indicates the PMe3 dissociation from 3 prior to the addition of
silane (Scheme 2).
Although these stoichiometric reactions establish that 2 is a
potent catalyst operating through the commonly accepted
hydride mechanism,40 the actual catalytic process can be more
complicated. Our recent studies on carbonyl hydrosilylation by
complex Tp(ArN=)Mo(PMe3)(H) suggested the possibility of
a nonhydride mechanism of reduction, when the metal-bound
hydride is not transferred to the substrate.7c Namely, we found
that a 1:1:1 reaction of Tp(ArN=)Mo(PMe3)(H), ketone and
the D-labeled silane PhSiD3 results in the complete retention of
the hydride on molybdenum and exclusive delivery of
deuterium to the substrate, which is at odds with the key
assumptions of the hydride mechanism. Our preliminary
investigation of other catalytic systems showed that the
nonhydride mechanism is not pertinent to molybdenum only
and is operative both for early and late metal systems.7c Guan et
al. has recently observed a similar result for an iron catalyst.4m
As to the 2-catalyzed hydrosilylation, attempted reaction of
benzaldehyde with PhSiD3 in the presence of a stoichiometric
Figure 6. ORTEP plot of the molecular structure of 6 (hydrogen
atoms are omitted for clarity).
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Figure 7. 31P−31P EXSY NMR spectrum (22 °C, tm = 0.5 s) of the mixture of 6 and PMe3.
Figure 8. Dependence of the k of the reaction of 3 with PhSiH3 at 22 °C on the concentration of (A) PhSiH3 (standard error 0.05) and (B) PMe3
(standard error 0.77).
Scheme 5. Alternative Mechanism for the Hydrosilylation of PhC(O)H with PhSiH3 Mediated by 2
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amount of 2 gives primarily 2D as the major product along with
a small amount of 2 and 1. This observation suggests that the
nonhydride mechanism is not the predominant reaction
pathway in the case of aldehydes, so that the mechanistic
proposal of Scheme 2 stands experimental verification.
2 and 2D is formed, which changes to the 2:3 ratio upon the
completion of the reaction. This result indicates that both the
hydride and nonhydride mechanisms can operate in this
system, most likely because of the reduced steric bulk of
cyclohexanone in comparison with acetophenone.
Mechanistic Aspects of Silane Alcoholysis Catalyzed
by 2. Analogously to the 2-catalyzed hydrosilylation of
benzaldehyde, the mechanism of ethanolysis of PhSiH3
mediated by 2 also likely involves the intermediacy of the
alkoxy complex (ArN=)Mo(Cl)(OEt)(PMe3)3 (8; Scheme 6).
The reaction between 2 and ethanol obeys the first order
kinetics in 2, with the keff being proportional to the ethanol
Another challenge for the mechanism outlined in Scheme 2
comes from the fact that under the catalytic conditions of
hydrosilylation the aldehyde is present in large excess (relative
to the catalyst), and therefore, the actual catalysis may involve
the intermediacy of complex 5 instead of 3 (Scheme 2). In this
case, the last step of the cycle, the reaction of 5 with PhSiH3,
could occur via either η2-PhC(O)H or PMe3 dissociation (see
above). Further addition of PhSiH3 would result in the
formation of the hydrosilylation product and the unsaturated
intermediate (ArN=)Mo(H)(Cl)(PMe3)2, which is instanta-
neously intercepted by the aldehyde to give 4 and then the
benzoxy derivative 5 (Scheme 5). Whichever of the benzoxy
complex 3 or 5 is the real intermediate, the final step of this
process should include a reaction of silane with the benzoxy
group. Our experiments suggest that this is a second-order
reaction, but we do not have firm data to speculate whether it
goes via the oxidative addition of the Si−H bond to metal to
give a Mo(VI) silyl hydride, via the Si−H coordination and
formation of a Mo(IV) silane σ-complex,41 or via an
intermolecular σ-bond metathesis or heterolytic silane splitting
on the Mo−O bond.5g,7b We have recently observed the first
Mo(VI) silyl hydride derivatives, as well as established the
heterolytic Si−H activation on the Mo-OR bond for the related
Cp(ArN=)Mo(OBn)(PMe3) system, thus confirming the
possibility of oxidative addition and heterolytic splitting
pathways, respectively.7b The small kinetic isotope effect for
the reaction of 3 with PhSiH3, kH/kD = 0.96 0.01 at 10 °C,
appears to favor the σ-bond metathesis or heterolytic silane
splitting over the silane oxidative addition pathway. However,
one can expect a small KIE for all three mechanisms of silane
addition if the preceding phosphine dissociation is the rate
limiting step. In the latter case, our experimental data do not
allow us to differentiate between the three possible pathways.
How general is this mechanistic proposal? It appears that it
applies to aldehydes only. The first observation is that
qualitatively the hydrosilylation of cyclohexanone is faster
than the hydrosilylation of benzaldehyde, although usually
aldehydes react faster than ketones. Second, no η2-coordinated
species has been observed in the case of ketones (cyclo-
hexanone and acetophenone). And third, our test for a
nonhydride mechanism (vide supra) suggests that this is the
prevailing pathway in the case of 2-catalyzed hydrosilylation of
acetophenone. Indeed, a 1:1:1 reaction of 2 with PhC(O)Me
and PhSiD3 at room temperature results overnight in the
formation of a mixture of 2 (80%) and deuteride complex 2D
(20%). The predominant incorporation of deuterium into the
hydrosilylation product PhCD(CH3)OSiD2Ph was also ob-
served by NMR. This result clearly indicates that insertion of
PhC(O)Me into the Mo−H bond of 2 (the hydride
mechanism40) is not the main pathway in this hydrosilylation.
Our tentative explanation of the nonhydride mechanism is that
it may proceed via a Lewis acid activation of the substrate.7c
The formation of the minor amount of 2D (20%) in the labeling
experiment with acetophenone can be accounted for by the
competing (minor) hydride pathway or a slow H/D exchange
between 2 and PhSiD3.
Scheme 6. Mechanism for the Ethanolysis of PhSiH3
Mediated by Complex 2
concentration (Figure 9, A).37 With excess ethanol (ca. 20
eqiuvs), the 1/keff is proportional to phosphine concentration
(Figure 9, B). This fact suggests that the reaction starts with a
reversible dissociation of phosphine, most likely that one
positioned trans to the hydride (confirmed by 31P−31P EXSY
NMR; see Figure 3), followed by addition of alcohol to
molybdenum (Scheme 6). Such a coordination of EtOH to the
metal probably acidifies the O−H bond sufficiently to allow for
proton transfer to the hydride to generate dihydrogen.
Dissociation of H2 and phosphine readdition furnishes the
ethoxy complex 8. Similar to the benzoxy derivative 3,42
complex 8 can react with silanes to give the alkoxysilane
product and regenerate the starting hydrido-chloride 2. In
contrast, previous mechanisms of silane alcoholysis implied the
activation of silane by an electrophilic metal center, making it
amenable for an attack by an external nucleophile (alcohol),43
similarly to the key step of the ionic hydrosilylation
mechanisms.13 Another conceivable mechanism of silane
alcoholysis by 2 could involve a direct proton transfer from
the alcohol to 2 to give a cationic dihydrogen complex as
observed in systems with “dihydrogen bonding”.44 However,
In the case of analogous reaction 2 with cyclohexanone and
PhSiD3 (in the 1:1:1 ratio), at 50% conversion a 3:2 mixture of
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Figure 9. Dependence of the k of the reaction of 2 with EtOH at 22 °C on the concentration of (A) EtOH (standard error 0.04) and (B) PMe3
(standard error 0.14).
Scheme 7. Reaction of Complex 2 with Styrene
the above kinetics (Figure 9) are not consistent with this
possibility. The difference between our system and the typical
system for dihydrogen bonding and protonolysis most likely
comes from the lower acidity of ethanol in comparison with
perfluoro alcohols, normally used for studying the dihydrogen
bonding.
Mechanistic Aspects of 2-Catalyzed Reactions of
Silanes and Alkenes. In contrast to reactions with
benzaldehyde and cyclohexanone, the addition of styrene to 2
does not lead to the expected insertion of PhCHCH2 into
the Mo−H bond. Rather, the adduct trans-(ArN=)Mo(H)-
(Cl)(η2-CH2CHPh)(PMe3)2 (9) is formed in a mixture with
the starting material. With an equivalent of BPh3 added, the
reaction proceeds to completion (by NMR) to give complex 9
and Ph3B·PMe3 (Scheme 7). No further transformation of 9
into the putative insertion product (ArN=)Mo(Cl)-
(CH2CH2Ph)(PMe3)3 was observed by NMR upon the
readdition of PMe3. The latter reaction resulted only in the
substitution of the η2-CH2CHPh ligand and the recovery of
the hydride complex 2.
at δ 4.08, 3.50, and 2.63 ppm (δ 3.82, 3.33, and 3.18 ppm for
1
the minor isomer), coupled in the H−13C HSQC to the 13C
NMR resonances at δ 60.5 and 49.6 ppm (δ 72.2 and 51.7 ppm
for the minor isomer). Complex 9 is thermally unstable in
solution and slowly (1 week) decomposes to give PhCH2CH3
and a difficult-to-characterize mixture of unidentified com-
pounds. The formation of ethylbenzene indicates that insertion
of the coordinated styrene ligand into the Mo−H bond is,
indeed, possible but is extremely slow.
CONCLUSIONS
■
The compound 2 has been found to catalyze a variety of
hydrosilylation reactions, including a rare example of the
hydrosilylation of nitriles. Possible catalytic cycles for the
hydrosilylation were proposed on the basis of studying
conceivable steps under stoichiometric conditions. Low
temperature VT NMR studies, as well as kinetic measurements
for the reactions of 2 with benzaldehyde and ethanol, indicate
that the hydrosilylation of carbonyls and silane alcoholysis
proceed via the initial substrate activation, but not silane
addition. The catalytic cycle of hydrosilylation of benzaldehyde
includes the intermediacy of the Mo(IV) benzoxy complex 5,
formed by the insertion of PhC(O)H into the Mo−H bond of
2. 5 reacts with PhSiH3 to afford the silyl ether and to recover
the hydride 2. On the other hand, a 1:1:1 D-labeling
experiment demonstrated that 2-catalyzed hydrosilylation of
more sterically demanding substrates, such as acetophenone,
proceeds via a nonhydride mechanism, which we tentatively
consider as a Lewis acid activation of the substrate. Analogously
to carbonyls, complex 2 reacts with styrene to form the η2-
styrene adduct 9, which slowly decomposes (presumably via
insertion of the coordinated styrene CH2CHPh into the
Mo−H bond) to form ethylbenzene. Intuitively, such products
of insertion of organic substrates into the Mo−H bond of 2
could serve as intermediates not only in hydrosilylation but also
in other catalytic processes, such as hydrogenation and/or
hydroboration reactions.46 These reactions are currently under
investigation in our laboratories and will be reported in due
course.
Compound 9 was characterized by IR and NMR spectros-
copy, which reveals the presence of two isomers (7:1, according
to 31P{1H} NMR). In the 1H NMR spectrum of 9, the hydride
ligand of the major isomer gives rise to a downfield signal at δ
6.84 ppm (multiplet), coupled in the 1H−31P HSQC spectrum
(at −18 °C) to two nonequivalent, mutually coupled 31P NMR
2
resonances at δ 0.4 ppm (d, JP−P = 111.0 Hz) and δ 1.0 ppm
2
(d, JP−P = 111.0 Hz). For the minor isomer of 9, the MoH
signal shows up as a doublet of doublets at δ 5.97 ppm (2JH−P
=
51.3 Hz, 2JH−P = 31.8 Hz). At −18 °C, the 31P NMR spectrum
of the minor isomer also contains two mutually coupled signals
at δ −2.2 ppm (d, 2JP−P = 103.7 Hz) and δ 0.8 ppm (d, 2JP−P
=
2
103.7 Hz). The large value of the JP−P for both isomers
indicates the trans-arrangement of the PMe3 ligands, consistent
with the structure depicted in Scheme 7. The presence of
hydride ligand is also evident from the observation of a Mo−H
1
stretch at 1757 cm−1 in the IR spectrum. In the H NMR
spectrum, the η2-coordination45 of styrene results in the
observation of three nonequivalent upfield shifted CH signals
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overlapping with signals of minor isomer); 5.77 (s, 1H, η2-O
EXPERIMENTAL SECTION
■
CHPh); 4.37 (bs, 1H, i-Pr, NAr); 4.12 (sept, 3JH−H = 6.9 Hz, 1H, i-Pr,
All manipulations were carried out using conventional inert
atmosphere glovebox and Schlenk techniques. Dry diethyl ether,
toluene, hexanes, and acetonitrile were obtained, using Innovative
Technology solvent purification columns, other solvents were dried by
distillation over appropriate drying agents. C6D6 and PhMe-d8 were
dried by distillation over the K/Na alloy. NMR spectra were obtained
with a Bruker DPX-300 and Bruker DPX-600 instruments (1H: 300
and 600 MHz; 2D: 92.1 MHz; 13C: 75.5 and 151 MHz; 29Si: 59.6 and
119.2 MHz; 31P: 121.5 and 243 MHz; 11B: 96.3 and 192.6 MHz). IR
spectra were measured on an ATI Mattson FTIR spectrometer.
Elemental analyses were performed by the “ANALEST” laboratory
(University of Toronto). PhSiCl3, (EtO)3SiH, SiMe4, PhMeSiCl2,
PMHS, PMe3, and BPh3 were purchased from Aldrich. PhSiH3 and
PhSiD3 were prepared by the reaction of PhSiCl3 with LiAlH4 and
LiAlD4, respectively. Organic substrates (benzaldehyde, acetophenone,
acetone, 1-hexene, styrene, benzonitrile, acetonitrile, 1-octyne, ethanol,
and propanol-2) were purchased from Sigma-Aldrich and used without
further purification. Preparation of (ArN=)MoCl2(PMe3)3 (1)18 was
reported previously. All catalytic and NMR scale reactions, as well as
kinetic experiments were done under nitrogen atmosphere using NMR
tubes equipped with Teflon valves. The structures and yields of all
hydrosilylated products were determined by NMR analysis using
tetramethylsilane as an internal standard. NMR analysis was performed
at room temperature unless stated otherwise.
2
NAr); 1.49 (d, JH−P = 9.6 Hz, 9H, PMe3, overlapping with signals of
minor isomer); 1.43 (bs, 6H, i-Pr, NAr, overlapping with signals of
minor isomer); 1.28 (d, 3JH−H = 6.9 Hz, 3H, i-Pr, NAr); 1.21 (m, 3H,
2
i-Pr, NAr); 1.16 (d, JH−P = 9.6 Hz, 9H, PMe3). 31P{1H} NMR (243
2
MHz; toluene-d8; −17 °C; δ, ppm): 1.43 (d, JP−P = 109.3 Hz, 1P,
2
PMe3); −5.59 (d, JP−P = 109.3 Hz, 1P, PMe3).
Minor Isomer of 4. 1H NMR (600 MHz; toluene-d8; 0 °C; δ, ppm):
o-H, m-H, and p-H signals of η2-OCHPh are obscured by the
corresponding signals of major isomer; 7.08 (m, 2H, m-H of NAr,
overlapping with signals of major isomer); 6.85 (m, 1H, p-H of NAr,
overlapping with signals of major isomer); 6.59 (dd, 2JH−P = 41 and 49
Hz, 1H, MoH); 5.17 (s, 1H, η2-OCHPh); 4.31 (bs, 1H, i-Pr, NAr);
3.46 (bs, 1H, i-Pr, NAr); 1.49 (m, 9H, PMe3, overlapping with signal of
major isomer); 1.43 (bs, 6H, i-Pr, NAr, overlapping with signal of
2
major isomer); 1.26 (m, 3H, i-Pr, NAr); 1.20 (d, JH−P = 7.8 Hz, 9H,
3
PMe3); 1.03 (d, JH−H = 6.9 Hz, 3H, i-Pr, NAr). 31P{1H} NMR (243
2
MHz; toluene-d8; −17 °C; δ, ppm): 0.77 (d, JP−P = 114.2 Hz, 1P,
2
PMe3); −5.56 (d, JP−P = 114.2 Hz, 1P, PMe3).
13C{1H} NMR (151 MHz; toluene-d8; −17 °C; both isomers; δ,
ppm): 150.5, 149.5, 148.2, 147.3, 146.8, 146.7, 144.7, 143.7 (o-C and i-
C of NAr and i-C of η2-OCHPh for both isomers); 129.5 (s, η2-O
CHPh, o-C, minor isomer); 128.6 (s, η2-OCHPh, p-C, major
isomer); m-C and p-C signals of η2-OCHPh of minor isomer are
overlapping with PhMe-d8 signals and corresponding signals of major
isomer; 128.0 (s, η2-OCHPh, o-C, major isomer); 126.7 (s, η2-O
CHPh, m-C, major isomer); 125.5 (s, m-C, NAr, overlapping signals
for both isomers); 123.1 and 123.0 (s, p-C, NAr for two isomers); 86.8
(bs, η2-OCHPh, major isomer); 76.9 (s, η2-OCHPh, minor
isomer); 27.0 (s, i-Pr, NAr, both isomers); 26.9 (s, i-Pr, NAr, major
isomer); 25.9 (s, i-Pr, NAr, both isomers); 23.4 (s, i-Pr, NAr, minor
Preparation of (ArN)Mo(H)(Cl)(PMe3)3 (2). A solution of L-
Selectride in THF (1.9 mL, 1.9 mmol, 1.0 M) was added at −30 °C to
a solution of 1 (1.05 g, 1.9 mmol) in 50 mL of toluene. The mixture
was stirred at −30 °C for 15 min, then allowed to reach ambient
temperature and stirred for additional two hours. Once the reaction
reached room temperature the color of the mixture changed from pale
green to brown and formation of precipitate was observed. The
mixture was filtered and the residue was extracted with 50 mL of
toluene. All volatiles were removed in vacuum to give a fine brown
solid of complex 2. Yield: 0.55 g, 65%. 1H NMR (300 MHz; C6D6; 25
1
isomer); 23.2 (s, i-Pr, NAr, major isomer); 17.2 (d, JC−P = 27.5 Hz,
1
PMe3, major isomer); 16.9 (d, JC−P = 24.9 Hz, PMe3, minor isomer);
1
16.8 (s, i-Pr, NAr, minor isomer); 13.7 (d, JC−P = 24.9 Hz, PMe3,
1
minor isomer); 13.5 (d, JC−P = 27.5 Hz, PMe3, major isomer). IR
3
2
(nujol, selected bands): 1771 cm−1 (s, Mo−H); 1595 cm−1 (s, CO).
B. RT Reaction. Benzaldehyde (5.2 μL, 0.051 mmol) was added at
room temperature to a solution of 2 (24.1 mg, 0.043 mmol) in 0.6 mL
of C6D6 in an NMR tube. NMR analysis after 10 min at room
temperature showed the selective formation of (ArN)Mo(H)(Cl)(η2-
PhC(O)H)(PMe3)2 (4). The reaction mixture was left at room
temperature for 5 h and was monitored by NMR spectroscopy, which
during this time revealed the rearrangement of 4 into 3.
An analogous room temperature reaction of 2 (27.9 mg, 0.049
mmol) with excess PhC(O)H (50.1 μL, 0.49 mmol) leads to the
formation of (ArN)Mo(Cl)(OCH2Ph)(η2-PhC(O)H)(PMe3)2 (5).
All attempts to isolate either 3 or 5 in the analytically pure form were
unsuccessful.
°C; δ, ppm): 1.19 (d, JH−H = 6.6 Hz, 4 CH3, NAr); 1.35 (d, JH−P
=
2
5.7 Hz, 9H, PMe3); 1.45 (vt, JH−P = 3 Hz, 18H, 2 PMe3); 4.37 (sept,
3JH−H = 6.9 Hz, 2H, 2 CH, NAr); 5.31 (dt, JH−P = 28.5 Hz, JH−P
=
1
1
51.9 Hz, 1H, Mo-H); 6.86 (m, 3H, NAr). 31P{1H} NMR (121.5 MHz;
2
C6D6; 25 °C; δ, ppm): −17.75 (t, JP−P = 13.4 Hz, 1P, PMe3); −1.10
(d, 2JP−P = 13.4 Hz, 2P, 2 PMe3). 13C{1H} NMR (75.5 MHz; C6D6; 25
1
1
°C; δ, ppm): 21.1 (vt, JC−P = 11.3 Hz, PMe3); 21.2 (d, JC−P = 15.9
Hz, PMe3); 24.2 (s, CH3, NAr); 26.6 (s, CH, NAr); 123.2, 124.9, 144.3
(all singlets, NAr aromatic carbons). IR (nujol, selected bands): 1714
cm−1 (s, Mo−H). Elem. Anal. (%) Calcd for C21H45ClMoNP3
(535.903): C, 47.07; H, 8.46; N, 2.61. Found: C, 46.47; H, 8.14; N,
2.77.
A. NMR Scale Reaction of 2 with Benzaldehyde. Low-
Temperature VT Reaction. PhC(O)H (5.2 μL, 0.051 mmol) was
added at −80 °C to a solution of 2 (24.1 mg, 0.043 mmol) in
0.6 mL of toluene-d8 in an NMR tube. The mixture was
immediately frozen in liquid N2 and the sample was placed into
the NMR machine precooled to −30 °C. The temperature was
dropped down to −50 °C and the reaction was monitored by
VT NMR spectroscopy. NMR analysis showed the selective
formation of trans-(ArN)Mo(H)(Cl)(η2-PhC(O)H)(PMe3)2
(4) at −5 °C. All attempts to isolate 4 were unsuccessful and
led to mixtures of 4, (ArN)MoCl2(PMe3)3 (1), (ArN)Mo(Cl)-
(OCH2Ph)(PMe3)3 (3; see the NMR scale generation and
characterization below), and unidentified decomposition
products.
Addition of excess PhSiH3 (ca. 10 equiv. to 2) at room temperature
to both 3 and 5 affords after 5 min PhH2Si(OBn) and PhHSi(OBn)2
and regenerates the complex 2.
1
3
3. H NMR (300 MHz; C6D6; δ, ppm): 7.40 (d, JH−H = 7.5 Hz,
2H, o-H, OCH2Ph); 6.86−7.26 (m, 6H, m-H and p-H of NAr and
OCH2Ph); 5.13 (s, 2H, OCH2Ph); 4.32 (bm, 2H, 2 CH, NAr); 1.58
(m, 6H, 2 CH3, NAr); 1.47 (d, 3JH−H = 6.3 Hz, CH3, NAr); 1.35 (bm,
27H, PMe3); 1.26 (d, JH−H = 6.3 Hz, CH3, NAr). 31P{1H} NMR
3
(121.5 MHz; toluene-d8; δ, ppm): 8.29 (t, 2JP−P = 15.8 Hz, 1P, PMe3);
9.50 (d, JP−P = 15.8 Hz, 2P, 2 PMe3). 13C{1H} NMR (75.5 MHz;
2
C6D6; δ, ppm): 151.4 (s, i-C, NAr); 147.5 (s, i-C, OCH2Ph); 144.7 (s,
o-C, NAr); 126.5, 125.3, 123.9 (m-C and p-C of NAr and p-C of
OCH2Ph); o-C and m-C signals of OCH2Ph are overlapping with
signal of C6D6; 71.5 (s, OCH2Ph); 27.2 (s, i-Pr, NAr); 22.8 (s, i-Pr,
NAr); 21.9 (vt, 1JC−P = 21.2 Hz, PMe3); 21.2 (s, i-Pr, NAr); 20.9 (s, i-
4. In solution, this complex exists as a mixture of two isomers,
depending on the orientation of coordinated benzaldehyde (5:1 ratio,
according to the 31P{1H} NMR spectrum).
1
Pr, NAr); 20.8 (s, i-Pr, NAr); 16.6 (d, JC−P = 20.4 Hz, PMe3).
Major Isomer of 4. 1H NMR (600 MHz; toluene-d8; 0 °C; δ, ppm):
5. 1H NMR (300 MHz; C6D6; δ, ppm): 1.02 (d, 2JH−P = 7.2 Hz, 9H,
PMe3); 1.15 (m, 3H, CH3, NAr); 1.19 (m, 3H, CH3, NAr); 1.28 (m,
3
2
7.71 (d, JH−H = 6.6 Hz, 2H, o-H, η2-OCHPh); 7.32 (dd, JH−P
=
35.0 Hz, 1H, MoH); 7.29 (t, 3JH−H = 6.6 Hz, 2H, m-H, η2-OCHPh);
6.99 (m, 1H, p-H, η2-OCHPh); 7.08 (m, 2H, m-H of NAr,
overlapping with signals of minor); 6.85 (m, 1H, p-H of NAr,
3H, CH3, NAr); 1.30 (d, JH−P = 7.9 Hz, 9H, PMe3); 1.41 (m, 3H,
2
CH3, NAr); 3.43 (sept, 3JH−H = 6.5 Hz, 1H, CH, NAr); 4.60 (d, 2JH−H
= 12.3 Hz, 1H, OCH2Ph); 4.83 (sept, 3JH−H = 6.5 Hz, 1H, CH, NAr);
4309
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Inorganic Chemistry
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5.13 (d, 2JH−H = 12.3 Hz, 1H, OCH2Ph); 5.37 (d, 3JH−P = 3.9 Hz, 1H,
NMR analysis showed full conversion of the starting material and
formation of a difficult-to-separate mixture of 3 and (ArN)-
MoCl2(PMe3)3 (1) (3:1, respectively, according to 31P{1H} NMR
analysis). Lowering the temperature of the experiment does not
improve selectivity of the reaction.
η2-PhC(O)H); 6.82−7.60 (m, 13 H, aromatic protons of NAr and 2
2
CPh).). 31P{1H} NMR (121.5 MHz; C6D6; δ, ppm): −5.3 (d, JP−P
=
290.3, 1P, PMe3); −6.9 (d, JP−P = 290.3, 1P, PMe3). 13C{1H} NMR
2
1
(75.5 MHz; C6D6; selected resonances; δ, ppm): 11.3 (dd, JC−P
=
1
NMR Scale Reaction of 2 with Styrene. Styrene (5.3 μL, 0.046
mmol) was added in one portion at room temperature to a solution of
2 (26.2 mg, 0.046 mmol) in 0.6 mL of C6D6 in an NMR tube. No
visual changes were observed. The mixture was left at room
temperature for 5 min and then the reaction was monitored by
NMR analysis for 24 h. Release of phosphine and formation of styrene
adduct (ArN)Mo(H)(Cl)(η2-CH2CHPh)(PMe3)2 (9) in a mixture
with the starting material was detected by NMR spectroscopy (2/9 =
1.3 by 31P NMR). No further conversion of 2 into 9 was observed after
additional 24 h. BPh3 (11.2 mg, 0.046 mmol) was added to the
mixture at room temperature. Immediate formation of the white
precipitate of Ph3B•PMe3 was observed. NMR analysis after 5 min at
room temperature showed full conversion of 2 and quantitative
formation of 9 as a mixture of two isomers (7:1 ratio). The mixture
was filtered, all volatiles were pumped off, and the residue was
extracted with 1 mL of hexane to give a brown oil. Yield: 13.2 mg,
51%. The product is not stable in solution at room temperature and
slowly (1 week) decomposes to give PhCH2CH3 and a difficult-to-
characterize mixture of unidentified compounds.
17.8 Hz, JC−P = 4.4 Hz, PMe3); 13.3 (dd, JC−P = 18.7 Hz, JC−P = 5.0
Hz, PMe3); 25.5 (s, CH, NAr); 26.6 (s, CH, NAr); 71.5 (s, OCH2Ph);
90.7 (s, η2-PhC(O)H).
Preparation of (ArN)Mo(Cl)(OCy)(PMe3)3 (6). Cyclohexanone
(64.0 mg, 0.650 mmol) was added to a toluene solution of 2 (350.0
mg, 0.650 mmol). The reaction mixture was left at room temperature
for 24 h, after which time the mixture was placed into freezer. The
product was crystallized at −78 °C, filtered off and dried in vacuum,
1
affording a light-green crystalline solid of 8. Yield: 120 mg, 51%. H
NMR (300 MHz; C6D6; δ, ppm): 0.72−1.53 (m, 8H, OCy and 12H, 4
2
2
CH3, NAr); 1.25 (d, JH−P = 7.3 Hz, 9H, PMe3); 1.29 (vt, JH−P = 2.9
Hz, 18H, 2 PMe3); 1.53−1.64 (m, 1H, OCy); 1.67−1.80 (m, 2H,
OCy); 1.92−2.04 (m, 2H, OCy); 3.1−4.3 (bm, 1H, CH, NAr); 4.07
(m, 1H, CH-O, OCy); 4.3−5.4 (bm, 1H, CH, NAr); 6.85−7.08 (m,
3H, NAr). 31P{1H} NMR (121.5 MHz; C6D6; δ, ppm): −12.2 (d,
2
2JP−P = 14.8 Hz, 2P, 2 PMe3); 7.0 (t, JP−P = 14.8 Hz, 1P, PMe3).
1
13C{1H} NMR (75.5 MHz; C6D6; δ, ppm): 17.3 (vt, JC−P = 9.4 Hz,
1
PMe3); 22.7 (d, JC−P = 20.7 Hz, PMe3); 26.3 (s, Cy); 27.2 (s, Cy);
39.8 (s, Cy); 76.1 (s, C-O, Cy); 124.3 (s, NAr); 128.7 (s, NAr); 151.4
IR (nujol, selected bands): 1757 cm−1 (s, Mo−H)
(s, i-C, NAr). 1H NMR (600 MHz; CD2Cl2; 258 K; δ, ppm): 0.84 (m,
Major Isomer of 9. 1H NMR (300 MHz; C6D6; δ, ppm): 7.61 (bd,
3JH−H = 6.9 Hz, 1H, o-H, CH2CHPh); 7.43 (bd, 3JH−H = 6.9 Hz, 1H,
o-H, CH2CHPh); 7.27 (bm, 4H NAr and m-H of CH2CHPh);
7.02 (bm, 1H, p-H, CH2CHPh); 6.84 (bm, 2H, NAr and Mo-H,
2
2
4H, OCy (γ)); 1.10 (d, JH−H = 6.9 Hz, 12H, NAr); 1.34 (vt, JH−P
=
2.6 Hz, 18H, 2 PMe3); 1.47 (d, 2JH−P = 7.7 Hz, 9H, PMe3); 1.55 (bm,
2
2H, OCy(δ)); 1.66 (bd, JH−H = 10.7 Hz, 2H, OCy(β)); 3.60 (bt,
2JH−H = 9.3 Hz, 1H, CH-O); 3.71 (b, 1H, CH); 4.55 (b, 1H, CH); 6.88
(b, 1H, p-Ar), 6.93 (m, 2H, m-Ar).
1
found by H−31P HSQC NMR); 4.08 (bm, 1H, CH2CHPh); 3.50
(bm, 1H, CH2CHPh); 3.33 (bm, 2H, 2 CH, NAr); 2.63 (broad dd,
13C NMR (150.9 MHz; CD2Cl2; 258 K; δ, ppm): 149.4 (s, i-Ar),
143.1 (s, o-Ar), 123.8 (s, m-Ar), 122.7 (s, p-Ar), 75.4 (s, HC-O), 38.7
(s, β-Cy and γ-Cy), 26.6 (s, CH (Ar)), 25.9 (s, CH (Ar)), 25.6 (s, Me
JH−H = 12.9 Hz, 1H, CH2CHPh); 1.40 (bm, 9H, PMe3, overlapping
3
with signal for minor isomer); 1.20 (bd, JH−H = 6.6 Hz, 6H, 2 CH3,
NAr); 1.12 (bm, 15H, 2 CH3 of NAr and PMe3). 1H{31P} NMR (300
1
(Ar)), 25.5 (s, δ-Cy), 25.1 (s, Me (Ar)), 22.1 (d, JC−P = 22.6 Hz,
3
MHz; C6D6; δ, ppm): 7.61 (d, JH−H = 6.9 Hz, 1H, o-H, CH2
PMe3), 16.7 (vt, JC−P = 9.3 Hz, PMe3)
3
CHPh); 7.43 (d, JH−H = 6.9 Hz, 1H, o-H, CH2CHPh); 7.29 (m,
X-ray. Single crystals for X-ray analysis formed in an NMR tube
contained the solution of 8 in C6D6. Elem. Anal. (%) Calcd. for
C27H55ClMoNOP3 (634.05): C, 51.15; H, 8.74; N, 2.21. Found: C,
50.89; H, 9.01; N, 2.06.
4H, NAr and m-H of CH2CHPh); 7.04 (m, 1H, p-H, CH2
CHPh); 6.84 (m, 2H, NAr and Mo-H, found by 1H−31P HSQC
NMR); 4.08 (dd, JH−H = 13.7 and 11.0 Hz, 1H, CH2CHPh); 3.50
3
(dd, JH−H = 11.0 and 3.9 Hz, 1H, CH2CHPh); 3.33 (sept, JH−H
=
NMR Scale Reaction of 2 with Ethanol. EtOH (1.2 μL, 0.02
mmol) was added in one portion at room temperature to a solution of
2 (10.1 mg, 0.019 mmol) in 0.6 mL of C6D6 in an NMR tube. Release
of the gas was observed almost immediately and the color of the
mixture slowly turned from brown to light green. The reaction mixture
was monitored by NMR spectroscopy for 16 h at room temperature
until all starting material was consumed. All volatiles were pumped off,
the oily residue was dried under vacuum, redissolved in fresh C6D6 and
checked with NMR analysis that showed the presence of only one
product (ArN)Mo(Cl)(OEt)(PMe3)3 (8). Complex 8 is stable in
solution at room temperature at least for a week. All attempts to isolate
8 in an analytically pure form gave a difficult-to-separate mixture of 8
and (ArN=)MoCl2(PMe3)3 (1). Addition of excess PhSiH3 to a
solution of 7 in C6D6 at room temperature gives, after 5 min, the
formation of PhH2Si(OEt) and recovers 2.
6.9 Hz, 2H, 2 CH, NAr); 2.63 (dd, JH−H = 13.7 and 3.9 Hz, 1H,
3
CH2CHPh); 1.40 (s, 9H, PMe3); 1.20 (d, JH−H = 6.9 Hz, 6H, 2
3
CH3, NAr); 1.15 (d, JH−H = 6.9 Hz, 6H, 2 CH3, NAr); 1.11 (s, 9H,
PMe3). 31P NMR (121.5 MHz; C6D6; selectively decoupled from
methyl groups; δ, ppm): −0.05 (d, 2JP−H = 39.2 Hz, 2 PMe3). 31P{1H}
2
NMR (243 MHz; toluene-d8; −18 °C; δ, ppm): 0.4 (d, JP−P = 111.0
2
Hz, PMe3); 1.0 (d, JP−P = 111.0 Hz, PMe3, major isomer). 13C{1H}
NMR (75.5 MHz; C6D6; δ, ppm): 149.0, 147.3, 146.6, 143.2 (s,
quaternary C of NAr and CH2CHPh); 135.7 (s, o-C, CH2
CHPh); 128.4, 126.8, 125.7, 124.2, 123.4 (s, m-C and p-C of NAr and
CH2CHPh); 60.5 (s, CH2CHPh); 49.6 (bs, CH2CCHPh);
28.4 (s, CH, NAr); 24.9 (s, CH3, NAr); 23.9 (s, CH3, NAr); 17.9 (d,
1
1JC−P = 12.8 Hz, PMe3); 17.5 (d, JC−P = 12.1 Hz, PMe3).
Minor Isomer of 9. Because of the low abundance of minor isomer,
1
we present only selected data. H NMR (300 MHz; C6D6; δ, ppm):
8. 1H NMR (300 MHz, C6D6 δ, ppm): 7.00 (m, 3H, m-H and p-H
5.97 (dd, 2JH−P = 51.3 Hz, 2JH−P = 31.8 Hz, 1H, Mo-H); 3.82 (m, 1H,
CH2CHPh, minor isomer); 3.18 (m, 2H, CH2CHPh, minor
isomer); 1.40 (bm, 9H, PMe3, overlapping with the signal for major
3
of NAr); 4.31 (bs, 2H, 2CH, NAr); 4.11 (q, JH−H = 6.9 Hz, 2H,
3
OCH2); 1.26 (m, 27H, 3PMe3); 1.19 (d, JH−H = 6.9 Hz, 6H, 2CH3,
3
3
1
isomer). H{31P} NMR (300 MHz; C6D6; δ, ppm): 5.97 (s, 1H, Mo-
NAr); 1.17 (d, JH−H = 6.9 Hz, 6H, 2CH3, NAr); 1.10 (t, JH−H = 6.9
Hz, 3H, OCH2CH3). 31P{1H} NMR (121.5 MHz; C6D6; δ, ppm): 8.19
H); 3.82 (dd, JH−H = 12.9 Hz, 1H, CH2CHPh); 3.20 (m, 1H,
CH2CHPh); 3.13 (dd, JH−H = 12.9 and 3.6 Hz, 1H, CH2CHPh);
1.37 (s, 9H, PMe3). 31P{1H} NMR (243 MHz; toluene-d8; δ, ppm):
2
2
(t, JP−P = 15.8 Hz, PMe3); −9.19 (d, JP−P = 15.8 Hz, 2PMe3).
13C{1H} NMR (75.5 Hz; C6D6; δ, ppm): 123.3 (s, NAr); 123.2 (s,
NAr); 64.4 (s, OCH2); 26.1 (s, CH, NAr); 25.6 (s, CH3, NAr); 25.1 (s,
CH31, NAr); 21.4 (d, 1JC−P = 23.1 Hz, PMe3); 20.7 (s, OCH2CH3); 16.8
(vt, JC−P = 11.6 Hz, PMe3).
2
2
−2.7 (d, JP−P = 104.0, PMe3); 0.6 (d, JP−P = 104.0, PMe3). 31P{1H}
NMR (243 MHz; toluene-d8; −18 °C; δ, ppm): −2.2 (d, 2JP−P = 103.7
Hz, PMe3); 0.8 (d, JP−P = 103.7 Hz, PMe3). H−13C HSQC NMR
(f1: 300 MHz; f2: 75.5 MHz; J = 145.0 Hz; C6D6; δ, ppm, selected
resonances): 72.2 (PhCHCH2); 51.7 (PhCHCH2).
2
1
NMR Scale Reaction of 2 with PhCH2OH. PhCH2OH (8.5 μL,
0.08 mmol) was added in one portion at room temperature to a
solution of 2 (8.6 mg, 0.016 mmol) in 0.6 mL of C6D6 in an NMR
tube. Immediate gas release and changing of color from brown to light
green was observed. The mixture was left at room temperature for 5 h.
General Procedure for Hydrosilylation Reactions Using 2. A
solution of organic substrate and silane (1:1 ratio), and tetramethylsi-
lane (5 mol %) in 0.6 mL of C6D6 was added in one portion at room
4310
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temperature to complex 2 (5−6 mol %). The mixture was immediately
transferred to an NMR tube. Depending on the substrate, the reaction
mixture was monitored by NMR analysis either at room temperature
or at 50 °C. Structures of all products were determined by NMR
spectroscopy. Conversion of organic substrates and yields of products
were calculated by NMR analysis using tetramethylsilane as the
internal standard.
General Procedure for Kinetic NMR Studies with 2. Reagents
were added at −50 °C to a solution of 2 (16.5 mg, 0.031 mmol) and
5.0 mol % P(o-Tol)3 in 0.6 mL of toluene-d8 in an NMR tube. After
addition the mixture was immediately frozen in liquid nitrogen and the
NMR tube was placed into precooled (−30 °C) NMR machine. For
benzaldehyde, depending on the experiment, the sample was slowly
warmed up to a desired temperature (−5 °C for the reaction of 2 with
PhC(O)H to produce 4, room temperature for the rearrangement of 4
into 3) and the reaction of 3 with PhSiH3 and PMe3 to form 2) and
monitored by 31P{1H} NMR analysis. The rate constants were
obtained from the integration of the 31P NMR signals of PMe3 ligands
(integrals were normalized to the integral of the standard, P(o-Tol)3),
resulting in the linear −ln[C]/time plots. An analogous procedure was
used for kinetic NMR studies in the case of ethanol.37
Table 6. Data Collection and Refinement Data for 1 and 6
compd
formula
1
6
C21H44Cl2MoNP3
570.32
C27H55ClMoNOP3
634.02
FW
color, habit
cryst size (mm3)
cryst syst
space group
a (Å)
green, plate
0.40 × 0.25 × 0.06
monoclinic
P2(1)/c
dark-green, plate
0.30 × 0.28 × 0.04
monoclinic
P21/c
17.819(2)
9.5317(11)
16.9762(19)
90.00/97.754(2)/90.00
2857.0(6)
4
12.6249(11)
19.6456(17)
13.7219(12)
90.00/102.5750(10)/90.00
3321.7(5)
b (Å)
c (Å)
α/β/γ (deg)
V (Å3)
Z
4
T (°C)
ρcalcd, g/cm3
F(000)
150(2)
153(2)
1.326
1.268
1192
1344
radiation
graphite monochromatized graphite monochromatized
MoKα (0.71073) MoKα (0.71073)
Experimental Details of EXSY NMR Studies. The EXSY NMR
spectra were acquired on a Bruker Avance AV600 spectrometer
equipped with a BBO-Z grad probe and VT accessory. 31P and 1H 2D
EXSY NMR spectra were recorded with a pulse sequence “noesygpph”
(2D homonuclear correlation via dipolar coupling, dipolar coupling
may be due to NOE or chemical exchange, phase sensitive, with
gradient pulses in mixing time)47 from Bruker pulse program library. A
μ (cm−1
)
0.822 0.639
transmission
factors
0.7344 and 0.9523
0.8315 and 0.9749
θmax (deg)
28.000
28.500
total no. of reflns 27785
34861
no. of unique
reflns
6892 [R(int) = 0.0723]
8402 [R(int) = 0.0337]
1
total of 2 scans per FID for H and 4 scans for 31P of 2K data points
no. with I ≥ nθ(I) 5066
7286
were used per time increment. A total of 256 time increments were
collected for both 1H and 31P. The FIDs were Fourier transformed to
generate and 1024 × 1024 data matrix. Two EXSY spectra were
recorded, one EXSY spectrum with a mixing time (tm) of between 100
and 500 ms, which contains the exchange cross peaks and a reference
EXSY spectrum with a mixing time 0 ms, which shows no exchange
cross peaks. All spectra were processed and analyzed using Bruker
no. of variables
253
307
R1
0.0666
0.1259
1.169
0.0433
0.1262
1.170
wR2
GOF
residual density,
max/min
0.737/−1.479
2.572/−0.436
1
Topspin 2.1 Pl4 software running on Windows XP. The series of H
1D EXSY NMR spectra were recorded using the ″selnogp″ (1D
NOESY using selective refocusing with a shaped pulse, dipolar
coupling may be due to NOE or chemical exchange)48 pulse sequence
from the Bruker library. Each spectrum was acquired using 16 scans
and 32 K data points with a spectral width of 20 kHz. The offset
frequency was always adjusted on resonance with the analyzed signal.
The acquired FIDs were processed using a line broadening of 0.3 Hz
and zero filled to 65 K points. A series of 5 to 8 1D EXSY spectra were
recorded with a mixing time ranging from 25 to 2000 ms optimized for
selected NMR data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gnikonov@brocku.ca. Phone: (+1)905 688-5550, ext.
3350.
■
Notes
1
each exchange rate and a H 1D spectrum used as a reference. T1
relaxation times were measured by inversion recovery method using
Bruker “t1ir” pulse program.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
X-ray Diffraction Study. Single crystals were coated with
polyperfluoro oil and mounted on the Bruker Smart APEX three-
circle diffractometer with CCD area detector at 150−153 K.49 The
crystallographic data and characteristics of the structure solution and
refinement are given in Table 6. The Bruker SAINT program50 was
used for data reduction. An absorption correction based on
measurements of equivalent reflections was applied (SADABS).51
■
This work was supported by NSERC (DG grant to G.I.N. and
USRA fellowship to E.P.), RFBR (grant to L.G.K.), and OGS
(graduate PhD fellowship for A.Y.K.). G.I.N. thanks CFI/OIT
for a generous equipment grant. We also thank the University
of Durham for X-ray facilities.
The structures were solved by direct methods52 and refined by full-
2
2
2
REFERENCES
matrix least-squares procedures, using ω(|Fo | − |Fc |) as the refined
function. All non-hydrogen atoms were found from the electron
density map and refined with anisotropic thermal parameters, whereas
all hydrogen atoms were calculate geometrically and refined using the
“riding” model. Relatively high residual density in the structure of 6
may be attributted to a partial twinning of the crystal, which we failed
to model completely.
■
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ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data in CIF format for the structure
determinations for complexes 1 and 6, kinetic plots, and
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