Selective Ni-Catalyzed Hydroboration of CO2 to the Formaldehyde Level Enabled by New PSiP Ligation

Article abstract of DOI:10.1021/acs.organomet.7b00497

The synthesis and characterization of group 10 metal pincer complexes supported by a new bis(indolylphosphino)silyl ligand are described, including the synthesis of Ni, Pd, and Pt hydride species. Solution NMR and single-crystal X-ray data revealed that a significant amount of structural variability is possible for such hydride complexes, particularly in the case of Ni, where terminal Ni-H as well as complexes involving η²-SiH coordination are both accessible and may even coexist, in ratios dependent on factors such as the nature of additional coligands, including N₂ from the reaction atmosphere, as well as solvent and temperature. Nickel and palladium hydride complexes of this new ligand were found to exhibit divergent selectivity in the catalytic hydroboration of CO₂ with pinacolborane (HBPin). While the Pd catalyst exhibited moderate activity for CO₂ hydroboration to the formate level, the analogous Ni species exhibited unprecedented selectivity (97%) for hydroboration of CO₂ to the formaldehyde level to provide the bis(boryl)acetal PinBOCH₂OBPin in high yield, under mild conditions. The HBPin-derived bis(boryl)acetal can be successfully isolated and utilized as a source of methylene for the formation of C-N and C-P bonds.

Full text of DOI:10.1021/acs.organomet.7b00497

Article
pubs.acs.org/Organometallics
Selective Ni-Catalyzed Hydroboration of CO to the Formaldehyde
2
Level Enabled by New PSiP Ligation
Luke J. Murphy, Helia Hollenhorst, Robert McDonald, Michael Ferguson,‡ Michael D. Lumsden,†
†
†
‡
,†
and Laura Turculet*
†Department of Chemistry, Dalhousie University, 6274 Coburg Road, P.O. Box 15000, Halifax, Nova Scotia, Canada B3H 4R2
‡X-Ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
*
S Supporting Information
ABSTRACT: The synthesis and characterization of group 10 metal pincer
complexes supported by a new bis(indolylphosphino)silyl ligand are
described, including the synthesis of Ni, Pd, and Pt hydride species.
Solution NMR and single-crystal X-ray data revealed that a significant
amount of structural variability is possible for such hydride complexes,
particularly in the case of Ni, where terminal Ni-H as well as complexes
2
involving η -SiH coordination are both accessible and may even coexist, in
ratios dependent on factors such as the nature of additional coligands,
including N2 from the reaction atmosphere, as well as solvent and
temperature. Nickel and palladium hydride complexes of this new ligand
were found to exhibit divergent selectivity in the catalytic hydroboration of
CO with pinacolborane (HBPin). While the Pd catalyst exhibited moderate activity for CO hydroboration to the formate level,
2
2
the analogous Ni species exhibited unprecedented selectivity (97%) for hydroboration of CO to the formaldehyde level to
2
provide the bis(boryl)acetal PinBOCH OBPin in high yield, under mild conditions. The HBPin-derived bis(boryl)acetal can be
2
successfully isolated and utilized as a source of methylene for the formation of C−N and C−P bonds.
INTRODUCTION
from a fundamental reactivity perspective. In the majority of
■
cases, the products of CO hydroboration are boryl formate
2
Significant effort has been made in recent years to develop
4
species (A, resulting from single reduction) or methoxyborane
species (C, resulting from triple reduction) that can be
efficient methods of utilizing CO as a C1 source for chemical
2
5
1
synthesis. Both transition-metal and main-group catalysts have
hydrolyzed to yield formic acid or methanol, respectively. By
been applied in this regard to achieve transformations such as
comparison, double reduction to the corresponding bis(boryl)-
the reduction of CO to formic acid, methanol, or methane, as
2
6
2
acetal (B) is relatively rare, and full reduction to methane
well as coupling reactions involving alkenes and alkynes. While
H represents the most attractive reductant from an atom
economy point of view, the hydroboration of CO has received
considerable attention and has been shown to be a versatile
7
using hydroboranes has remained elusive.
2
The selective formation of a bis(boryl)acetal product (B) by
2
a double CO reduction process is particularly intriguing, as it
2
represents a reduction of CO to the formaldehyde level.
2
reaction that can yield a variety of products along the reduction
3
Formaldehyde has been identified as a “missing link” in
pathway from CO to methane (Scheme 1). From a practical
2
homogeneous CO reduction catalysis that may serve as a
2
perspective, it must be noted that this reaction makes use of a
stoichiometric borane reductant that cannot be efficiently
regenerated. However, the hydroboration of CO is of interest
useful C1 synthon for the formation of various E−C bonds (E
6
b,8
=
main-group element).
The first report of double
2
hydroboration of CO was disclosed by Bontemps, Sabo-
2
6a
Etienne, and co-workers in the course of investigations
involving Ru-catalyzed CO2 reduction with pinacolborane
Scheme 1. Stepwise Hydroboration of CO to Boryl Formate
A), Bis(boryl)acetal (B), and Methoxyborane (C) Species
and Finally to Methane (Not Yet Reported)
2
(
(
HBPin). A number of reduction products were identified in
various proportions, including A−C and PinBOCH (OCOH),
2
where the last species results from a rare coupling of two
molecules of CO . When Fe(H) (dmpe) (dmpe = 1,2-
2
2
2
bis(dimethylphosphino)ethane) was used as a catalyst, it was
observed that, while the use of catecholborane (HBCat) as
reductant led to selective formation of the corresponding
Received: June 30, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00497
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Article
methoxyborane (C) in moderate yield, the use of HBPin and 9-
with the synthesis and characterization of group 10 pincer
complexes of this ligand. We focused on the preparation of
metal hydride derivatives, as these have been shown to be key
BBN (9-borabicyclo[3.3.1]nonane) as reductants favored the
6e
formation of the corresponding bis(boryl)acetals (B). The in
situ generated bis(boryl)acetal derived from 9-BBN was shown
to be an effective methylene transfer reagent that could be
utilized for the formation of C−N, C−O, and C−C bonds,
species for CO
hydride complexes of this new ligand are shown to exhibit
divergent selectivity in the catalytic hydroboration of CO with
reduction catalysis. Nickel and palladium
2
2
thereby providing new avenues for the utilization of CO in
HBPin. Interestingly, while the Pd catalyst is selective for the
formation of the corresponding boryl formate product, the
analogous Ni species leads almost exclusively (97% selectivity)
to the formation of the bis(boryl)acetal (B), in a manner that
was not achieved by use of previously reported group 10
catalysts, including those featuring bis(phosphino)silyl pincer
ligation. We further demonstrate that the HBPin-derived
bis(boryl)acetal can be successfully isolated and utilized as a
source of methylene for the formation of E−C bonds.
2
6
e
synthesis. Since these initial reports by Bontemps, Sabo-
Etienne, and co-workers, a handful of other CO hydroboration
2
studies have reported the generation of product mixtures
containing bis(boryl)acetals, but these studies have not focused
on optimizing the formation of this double-reduction
5
c,e,6d
product.
Our group has had significant interest in the reactivity and
catalytic applications of transition-metal pincer complexes,
including the utility of such complexes as catalysts for CO2
reduction processes. In this regard, we have previously reported
RESULTS AND DISCUSSION
■
Ligand Synthesis. In previous work, our group has
demonstrated that bis(phosphino)silyl PSiP pincer complexes
can facilitate a variety of challenging bond activation processes,
on the catalytic hydrosilylation of CO mediated by Pd and Pt
2
3
silyl pincer complexes supported by κ -(2-Cy PC H ) SiMe)
2
6
4 2
7c
(
Cy-PSiP) ligation developed in our group. Group 10 metal
11
including C−H and N−H bond oxidative addition, as well as
pincer complexes have also found application as effective
the reduction of carbon dioxide to methane using tertiary
catalysts for both single (formate level) and triple (methanol
7c
5a
silanes. In an effort to understand better the effects of PSiP
ancillary ligand modifications on the reactivity of the ensuing
complexes, we sought to prepare a new PSiP variant featuring a
bis(indolylphosphino) motif (Scheme 2). The tertiary silane
level) CO hydroboration processes. Guan and co-workers
2
were the first to report CO hydroboration to the methanol
2
t
level catalyzed by a POCOP (POCOP = 2,6-( Bu PO) C H )
2
2
6
3
pincer complex of Ni. A subsequent computational study
underlined the strong influence that both steric congestion at
the Ni center and the identity of the borane reductant have on
Scheme 2. Synthesis of Bis(indolylphosphino)silane Ligand
a
9
4b
Precursors and Single-Crystal X-ray Structure of 1
catalytic efficiency. More recently, Hazari and co-workers
reported on the hydroboration of CO with HBPin to the
2
formate level catalyzed by (Cy-PSiP)PdH. The system operates
under mild conditions and achieved the highest turnover
numbers to date for hydroboration of CO to the formate level.
2
While these examples demonstrate that tuning of parameters
such as catalyst structure and borane source can have a
significant effect on the selectivity of the hydroboration process,
no examples of related pincer species that are both productive
and highly selective for the double hydroboration of CO to the
2
10
formaldehyde level have been reported.
Given the relative rarity of catalyst platforms that can
selectively reduce CO2 to the formaldehyde level, the
development of new catalysts for this transformation is of
fundamental interest both from an organometallic reactivity
perspective and in an effort to develop useful transformations of
CO . In this context, we report herein the development of a
2
new bis(indolylphosphino)silyl PSiP ligand (Figure 1), along
a
Inset crystal structure shown with 50% thermal ellipsoids and with
selected hydrogen atoms omitted for clarity.
i
Ind
ligand precursor 1 (( Pr-PSiP )H) was readily prepared from
commercially available 3-methylindole using a synthetic
12
strategy analogous to that employed by Reek and co-workers
for the preparation of INDOLPhos ligands and was isolated as
a white solid in 53% overall yield (over three synthetic steps).
The X-ray crystal structure of 1 confirmed the connectivity in
this molecule (Scheme 2).
Synthesis of Group 10 Metal Complexes. Group 10
i
Ind
metal chloride complexes of the type ( Pr-PSiP )MCl (M = Ni
(2-Ni), Pd (2-Pd), Pt (2-Pt)) were prepared in high yield
(Scheme 3). Despite diligent efforts to obtain these complexes
Figure 1. Previously reported bis(phosphino)silyl group 10 metal
hydride complexes active in CO hydroboration catalysis and new
2
bis(indolylphosphino)silyl complexes of the type reported herein.
B
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Article
i
Ind
i
Ind
Scheme 3. Synthesis of ( Pr-PSiP )MCl (M = Ni, Pd, Pt)
Complexes
Scheme 4. Synthesis of ( Pr-PSiP )MH (M = Ni, Pd, Pt)
Complexes
and related derivatives (vide infra) solvent-free in analytically
pure form, in several cases the isolated compounds were found
to tenaciously retain solvent even after prolonged exposure to
vacuum. Nonetheless, spectroscopic and X-ray crystallographic
data (Figure 2) serve to unequivocally confirm the identity of
the target compounds. All three chloride complexes exhibit
distorted-square-planar coordination geometry about the metal
center with spectroscopic features comparable to those of
Table 1. Selected NMR Spectroscopic Data for Group 10
Complexes Supported by Pr-PSiP Ligation
i
Ind
a
1
29Si NMR (ppm)
compound
H NMR M−H (ppm)
−4.79
JSiH (Hz)
3
3
3
3
4
5
5
-Ni
-Ni*
-Pd
-Pt
59.8
n/a
b
b
,
bc
−2.92
0.15
−10.5
89
13
related PSiP complexes of this type. The synthesis of hydride
derivatives was pursued subsequently, with the goal of accessing
60.1
−101
−67
−69
−82
−81
4.23
64.1
species capable of CO reduction. Treatment of a cold toluene
2
−3.50
−5.44
−10.18
−24.1
6.3
solution of each metal chloride complex with 1 equiv of sodium
triethylborohydride led to the formation of the corresponding
hydride complexes (Scheme 4; M = Ni (3-Ni), Pd (3-Pd), Pt
a
b
65.7
a
Conditions unless specified otherwise: benzene-d , room temper-
6
(
3-Pt)). Unlike the related complex (Cy-PSiP)NiH which is
b
c
ature. Toluene-d , −80 °C. The sign of J could not be determined.
14
8
SiH
thermally sensitive, 3-Ni does not appear to decompose in
1
solution at temperatures up to 80 °C, as indicated by H and
3
1
P NMR analysis.
and 4.23 ppm (JPH = 18 Hz, JPtH = 900 Hz), respectively. The
latter resonance in particular is atypical for a late-transition-
metal hydride, as such resonances are most commonly
While the 31P and 13C NMR spectroscopic features of these
hydride complexes are generally similar to those of the chloride
1
29
15
29
precursors, the H and Si NMR data are intriguing (Table 1).
observed upfield of 0 ppm. By comparison, while the Si
NMR chemical shifts of these hydride species are not unusual
for transition-metal silyls, the large JSiH coupling constants
1
In particular, the H NMR spectra of 3-Pd and 3-Pt (benzene-
d6) feature metal hydride resonances at 0.15 ppm (JPH = 15 Hz)
Figure 2. Crystallographically determined structures of 2-Ni, 2-Pd, and 2-Pt with thermal ellipsoids shown at the 50% probability level. Hydrogen
atoms have been omitted for clarity. Selected interatomic distances (Å) and angles (deg): 2-Ni, Ni−Cl 2.2170(5), Ni−P(1) 2.1938(5), Ni−P(2)
2.1824(5), Ni−Si 2.1850(5), Cl−Ni−P(1) 96.58(2), Cl−Ni−P(2) 98.55(2), Cl−Ni−Si 167.82(2), P(1)−Ni−P(2) 154.54(2), P(1)−Ni−Si
83.542(18), P(2)−Ni−Si 85.881(18); 2-Pd, Pd−Cl 2.4255(5), Pd−P(1) 2.3156(4), Pd−P(2) 2.3216(4), Pd−Si 2.2483(5), Cl−Pd−P(1)
100.407(16), Cl−Pd−P(2) 95.083(16), Cl−Pd−Si 174.176(17), P(1)−Pd−P(2) 159.013(16), P(1)−Pd−Si 84.817(16), P(2)−Pd−Si 81.387(16);
2-Pt, Pt−Cl 2.432(3), Pt−P(1) 2.311(3), Pt−P(2) 2.283(3), Pt−Si 2.266(3), Cl−Pt−P(1) 94.76(12), Cl−Pt−P(2) 96.84(11), Cl−Pt−Si
175.29(13), P(1)−Pt−P(2) 159.88(10), P(1)−Pt−Si 83.67(9), P(2)−Pt−Si 85.95(9).
C
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Article
i
observed for 3-Pd (101 Hz) and 3-Pt (67 Hz) are intermediate
for 3-Ni also indicate the presence of a second ( Pr-
Ind
in magnitude between those anticipated for a one-bond J
PSiP )NiH species (3-Ni*), which appears to interconvert
SiH
16
1
31
(
typical range of ca. 150−200 Hz; cf. J = 252 Hz for 1a)
with 3-Ni. Complex 3-Ni* gives rise to a P NMR resonance
SiH
16
1
and a two-bond J (≤20 Hz). The magnitude of these JSiH
at 35.3 ppm and features a hydride H NMR resonance at
SiH
2
31
constants does not vary as a function of temperature (less than
−2.55 ppm ( J = 38 Hz; toluene-d ). Below −40 °C the
P
PH
8
2
Hz variation in the range of −80 to 60 °C), nor do we
NMR resonance for 3-Ni* begins to decoalesce, such that at
−80 °C it has decoalesced into two broad signals centered at
40.7 and 28.3 ppm, respectively. No such line-shape changes
were observed for the P NMR resonance corresponding to 3-
Ni, which persists in solution over this temperature range.
observe any other evidence of temperature dependence in the
1
29
31
H, Si, and P NMR features of 3-Pd and 3-Pt over this
31
temperature range (toluene-d8).
In an effort to gain further information regarding Si−H
coupling constants for 3-Pd and 3-Pt, we also sought to
29
Interestingly, while 3-Ni gives rise to a Si NMR resonance at
59.8 ppm, the 29Si resonance for 3-Ni* was observed at −10.5
ppm (−80 °C, toluene-d , J = 89 Hz). Despite repeated
determine the relative sign of the measured J by use of a
SiH
1
29
17
2
H− Si HECADE NMR experiment, wherein the J
SiH
8
SiH
constant for the Si−Me group in the ligand backbone was
efforts, we were unable to determine the sign of the J
SiH
used as a reference (Figure S8 in the Supporting Information).
constant observed for 3-Ni*, nor could we measure the
magnitude of JSiH for 3-Ni, including at low temperature.
The relative ratio of 3-Ni to 3-Ni* appears to be both
temperature and solvent dependent. As such, while the room-
temperature NMR spectra ( H, P, Si) of 3-Ni in benzene-d
show no evidence of a second hydride species in solution, a 3-
Ni:3-Ni* ratio of ca. 4:1 was observed for a room-temperature
toluene-d8 solution (by H and P NMR). When the
temperature is decreased to −20 °C, the amount of 3-Ni*
increases to afford a ca. 4:3 ratio of 3-Ni to 3-Ni*, and a ratio of
ca. 1:1 was observed at −80 °C. Somewhat similar behavior was
observed when low-temperature NMR data were collected on a
sample of 3-Ni in methylcyclohexane-d solution (Figure S6 in
2
1
While J constants are often positive, J constants are
SiH
SiH
16
1
known to be negative. Indeed, for 1 the J constant was
SiH
2
found to be opposite in sign to the J constant associated
SiH
1
31
29
with the Si−Me protons, which is consistent with the latter
6
being positive. In the cases of 3-Pd and 3-Pt the J constant
SiH
involving the metal hydride was observed to also be opposite in
2
1
31
sign to the J of the Si−Me protons, suggesting that the J
SiH
SiH
constants involving the Pd and Pt hydrides are negative.
Unlike our observations for 3-Pd and 3-Pt, the NMR
spectroscopic features of 3-Ni were found to exhibit significant
3
1
1
temperature dependence (Figure 3). While the P{ H} NMR
spectrum of 3-Ni features a sharp resonance at 65.6 ppm, the
14
1
H NMR spectrum of this complex (benzene-d ) features a
the Supporting Information). At 20 °C a 3-Ni:3-Ni* ratio of ca.
6
1 31
broad hydride resonance at −4.80 ppm. At low temperature
7:1 was observed ( H and P NMR). This ratio decreased
dramatically to 1:1 at 0 °C and decreased further to 1:10 at −20
°C. At −40 °C essentially only 3-Ni* is present in solution, and
(
−60 °C, toluene-d ) the latter resonance is resolved into an
8
2
apparent triplet with J = 47 Hz. Concurrent with these line-
shape changes, low-temperature NMR spectra ( H, P, Si)
PH
1
31
29
31
1
the P{ H} NMR signal for this complex is significantly
broadened. At −80 °C the P{ H} NMR resonance for 3-Ni*
has decoalesced to two broad signals at 39.8 and 28.0 ppm,
31
1
respectively. Finally, a degassed toluene-d solution of 3-Ni
8
showed only negligible formation of 3-Ni*, including at low
temperature. This is consistent with a formulation for 3-Ni*
that involves the coordination of N2 from the reaction
atmosphere (Scheme 5).
In considering the formulation of 3-Ni, 3-Pd, and 3-Pt in
solution, the NMR features observed (vide supra) suggest
several possibilities along the continuum between terminal
i
Ind
Scheme 5. Reactivity of ( Pr-PSiP )NiH
1
Figure 3. Variable-temperature (a) H NMR (hydride region) and (b)
3
1
1
P{ H} NMR spectra of 3-Ni (toluene-d ).
8
D
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Article
II
2
0
M −H and (η -SiH)−M complexes (Scheme 4), including
18,19
scenarios where such species are in equilibrium.
For 3-Pd
and 3-Pt, while no evidence of such an equilibrium process was
obtained, the large negative JSiH constants observed for the
hydride ligand are in keeping with a scenario where Si−H
1
6,20
bonding interactions are non-negligible.
On the other hand,
29
the downfield Si NMR chemical shifts for these complexes are
more typical of transition-metal silyl complexes rather than (η -
SiH)−M species. It is thus possible that the observed
phenomenon involving the J constants could result from the
2
13
SiH
trans disposition of the hydride and silyl groups in 3-Pd and 3-
2
1
1
29
Pt. While similar trends in H and Si NMR data were noted
3
for related PSiP Pt and Pd hydride complexes supported by κ -
2-Cy PC H ) SiMe ligation,
7c,13b
(
the solid-state structure of
2
6
4 2
(
Cy-PSiP)PtH has been confirmed to involve terminal Pt−H
21
coordination by neutron diffraction analysis. With respect to
3
1
31
29
-Ni, H, P, and Si NMR data provide clear evidence for a
temperature- and solvent-dependent equilibrium between two
Ni hydride species in solution. Moreover, the Ni hydride
species 3-Ni* that appears to increase in concentration at low
temperature involves the coordination of N from the reaction
2
atmosphere (vide supra) and gives rise to a significantly upfield
shifted 29Si NMR resonance relative to 3-Ni, 3-Pd, and 3-Pt.
On the basis of these data, we tentatively assign 3-Ni as a
Figure 4. Crystallographically determined structures of 4 and 5b with
thermal ellipsoids drawn at the 50% probability level. Solvent
molecules and most hydrogen atoms have been omitted for clarity.
Only one of the two crystallographically independent molecules of 4 is
shown. Selected interatomic distances (Å) and angles (deg): 4, Ni−
H(1) 1.50(3), Ni−P(1) 2.1816(7), Ni−P(2) 2.2245(8), Ni−Si
2.1745(8), Ni−N(3) 2.040(2), P(1)−Ni−P(2) 118.44(3), P(1)−
Ni−Si 89.98(3), P(1)−Ni−N(3) 106.67(7), P(2)−Ni−Si 88.42(3),
P(2)−Ni−N(3) 112.97(7), Si−Ni−H(1) 48.3(12), Si−Ni−N(3)
i
Ind
terminal Ni−H species formulated as ( Pr-PSiP )NiH and 3-
2
Ni* as an N adduct involving (η -SiH)−Ni coordination. The
2
interconversion between such structures in (Cy-PSiP)NiH(L)
L = N , CO) has been observed previously, and trends in Si
29
(
2
NMR data reported for such species are indeed consistent with
18c
our observations for 3-Ni and 3-Ni*. On the basis of the
NMR data available for 3-Ni*, we cannot definitively determine
whether in solution this complex is a monomeric species of the
1
39.99(7), N(3)−Ni−H(1) 93.7(12); 5b, Ni−H1 1.49(3), Ni−P(1)
i
2
Ind
2.1611(5), Ni−P(2) 2.1794(5), Ni−P(3) 2.1727(5), Ni−Si 2.2057(5),
Si−Ni−H1 175.6(10), P(1)−Ni−P(2) 119.10(2), P(1)−Ni--P(3)
126.58(2), P(1)−Ni−Si 88.024(19), P(1)−Ni−H(1) 87.9(10),
P(2)−Ni−P(3) 114.30(2), P(2)−Ni−Si 86.527(19), P(2)−Ni−H(1)
type [ Pr−P(η -SiH)P ]Ni(N ) or a related N -bridged dimer.
2
2
However, in the solid state, the IR spectrum of the Ni hydride
features a weak-intensity band at 2075 cm while the Raman
spectrum features a relatively intense band at 2073 cm , which
−1
−1
9
4.1(10), P(3)−Ni−Si 96.89(2), P(3)−Ni−H(1) 86.8(10).
is assigned to coordinated N . These data suggest that in the
2
solid state the Ni hydride is likely best formulated as the N -
2
i
2
Ind
PSiPInd ligand, which is consistent with C -symmetric structures
bridged dimer {[ Pr−P(η -SiH)P ]Ni} (μ-N ), which would
2
2
s
2
be anticipated to feature a Raman-active N ligand. This result
in solution. However, the substantial difference in J values
2
PP
2
2
is in agreement with the observations of Hazari and co-workers
for these two complexes ( J for 5a 6 Hz; J for 5b 111 Hz)
PP PP
in regard to the formulation of the related N adducts of (Cy-
PSiP)NiH.
suggests that the orientation of the PMe ligand relative to the
3
2
1
8c
i
Ind
29
Pr-PSiP phosphino donors is different in each case. The Si
and 31P NMR data for 5a are consistent with data previously
To investigate further the influence of an added donor ligand
i
2
on 3-Ni, the synthesis of complexes of the type ( Pr-
reported for the related complexes [Ph-P(η -SiH)P]Ni(PR )
3
Ind
3
18b,22
PSiP )NiH(L) (Scheme 5; 4, L = DMAP; 5, L = PMe )
(R = Ph, Me; Ph-PSiP = κ -(2-Ph PC H ) SiMe)
that
3
2
6
4 2
2
9
was pursued. Complexes 4 and 5 were both prepared by
treatment of 3-Ni with 1 equiv of DMAP and PMe3,
respectively. In the case of 4, the NMR spectroscopic data
were found to exhibit similarly upfield shifted Si NMR
resonances, large values of JSiH, and small values of J . By
2
PP
comparison, NMR data for 5b are similar to those reported for
2
9
obtained (e.g., Si chemical shift of −24.1 ppm, J = −69 Hz)
the trigonal-bipyramidal complex (Ph-PSiP)PtH(PPh ) that
SiH
3
18b
are closely aligned to those of 3-Ni* and support the
features a terminal hydride ligand coordinated trans to Si.
i
2
Ind
formulation of this complex as [ Pr-P(η -SiH)P ]Ni(DMAP)
in solution. Indeed, the solid-state structure of 4 determined by
X-ray crystallographic analysis is also in agreement with this
assignment (Figure 4).
On the basis of these data, we assign 5a as a PMe adduct
3
2
featuring η -SiH coordination, while 5b is assigned as a complex
of the type ( Pr-PSiP )NiH(PMe ) with a terminal hydride
i
Ind
3
ligand coordinated trans to Si. Thus, it appears the mixture of
5a and 5b generated upon treatment of 3-Ni with PMe3
represents a unique example of the Si−H oxidative addition
By comparison, in the case of 5, NMR analysis is consistent
with the existence of two Ni hydride species in solution in a ca.
2
9
2
2
6
:1 ratio (5a, major; 5b, minor). While the Si chemical shift of
.3 ppm (JSiH = −82 Hz) for 5a is relatively upfield, the minor
product (5b) and the η -SiH complex associated with “arrested
oxidative addition” (5a) existing simultaneously in solution as
2
9
constituent 5b gives rise to a Si NMR resonance at 65.7 ppm
an isomeric mixture. No conclusive evidence for a temperature-
1
(
JSiH = −81 Hz), which is more closely aligned with NMR data
dependent equilibrium involving 5a and 5b was obtained by H
obtained for 3-Ni. Complexes 5a and 5b both exhibit
equivalent phosphorus environments associated with the Pr-
and 31P NMR analysis (−80−80 °C), although broadening of
i
NMR features was observed at low temperature and slight
E
DOI: 10.1021/acs.organomet.7b00497
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
changes in the ratio of 5a to 5b were noted at elevated
temperatures. In addition, no evidence for chemical exchange
was obtained by selective excitation of the NiH resonances via
NOESY experiments (0.5 and 1 s mixing times; 80 °C). While
we were not able to separate 5a and 5b on a preparative scale, a
sample of X-ray-quality crystals obtained from the 5a/5b
mixture revealed a five-coordinate complex featuring an
approximate trigonal-bipyramidal coordination geometry at Ni
with Si and a terminal hydride ligand (H(1)) in the axial
positions, which is consistent with our formulation of 5b
Ni, 3-Pd, and 3-Pt, respectively, were assessed (Table 2). After
a 1 h reaction time, NMR analysis of the reaction mixtures
a
Table 2. Hydroboration of CO with HBPin
2
conversion (%)
entry
catalyst
7-A
7-B
7-C
(
Figure 4). Thus, it appears that a substantial amount of
b
1
2
3
4
5
3-Ni
3-Pd
3-Pt
2-Ni
<5
84 (1.8 mmol)
<5
<5
<5
0
i
Ind
structural variability is possible for ( Pr-PSiP )NiH(L)
complexes, where terminal hydride as well as complexes
involving η -SiH coordination are both accessible and may
even coexist, in ratios dependent on factors such as the nature
of L, as well as solvent and temperature. Furthermore, on
comparison of the NMR features of such bis(phosphino)silyl
group 10 hydride complexes (Table 1), the available data
suggest that such species generally feature relatively large,
39 (1.6 mmol)
6
<5
0
<5
0
2
NaEt BH
0
0
0
3
a
Reaction conditions: benzene-d , 4.2 mmol of HBPin, [HBPin] = 4.8
M, 1 atm of CO , 0.2 mol % of M (M = Ni, Pd, Pt) or NaEt BH vs
HBPin, room temperature, 1 h. Conversions were determined on the
basis of H NMR integration using C Me as an internal standard
6
2
3
1
6
6
2
b
negative JSiH values that can be associated with both η -SiH
interactions and trans-Si−M−H coordination. Indeed, the
NMR data obtained appear to suggest that in this specific
(average of two runs). TON = 420 based on moles of B−H reacted
per mole of Ni.
2
9
family of complexes the Si chemical shift may be a better
indicated negligible formation of products resulting from CO2
hydroboration in the case of 3-Pt. For 3-Pd, 39% conversion to
PinB(CO H) (7-A) was observed, with minimal formation of
other hydroboration products (Table 2, entry 2). By
comparison, for 3-Ni highly selective formation of the
bis(boryl)acetal 7-B was observed, as indicated by a character-
predictor of the extent of Si−H interaction, such that
2
complexes that feature an η -SiH interaction involving the
2
i
Ind
29
Pr-PSiP ligand also give rise to relatively upfield shifted Si
NMR resonances (Table 1).
To probe the ability of such group 10 hydride species to
1
reduce CO , 3-Ni was treated with ca. 1 atm CO (benzene-d ),
2
2
6
istic H NMR resonance at 5.51 ppm corresponding to the
6
a
which led to the quantitative generation of the Ni formate
complex 6-Ni (Scheme 5). While 6-Ni appears to decompose
upon attempted isolation, this complex was characterized in situ
spectroscopically. The H NMR spectrum of 6-Ni features a
characteristic singlet resonance at 8.71 ppm that corresponds to
methylene protons of the acetal. In an effort to establish the
necessity of 3-Ni for this reactivity, control experiments
utilizing 2-Ni as the Ni source were carried out under identical
conditions with no CO2 reduction products observed
spectroscopically after 1 h (Table 2, entry 4). Similarly,
1
13
the formate proton. This resonance correlates to a C NMR
NaEt BH alone was not effective as a catalyst (Table 2, entry
3
1
13
resonance at 168.8 ppm in a H− C HSQC experiment, which
is assigned to the carbonyl of the formate ligand. The 31P{ H}
NMR spectrum of 6-Ni features a singlet resonance at 34.6
5
).
1
Intrigued by the selectivity of 3-Ni for the reduction of CO2
to the formaldehyde level, we conducted further experiments to
optimize this reactivity (Table 3). We first established that,
under the initial screening conditions, increasing the reaction
ppm, which is consistent with its formulation as a C -symmetric
s
2
9
complex. Finally, the Si NMR spectrum of 6-Ni features a
resonance at 42.2 ppm. These data are in agreement with those
reported for the related Ni formate complex (Cy-PSiP)Ni-
Table 3. Hydroboration of CO with 3-Ni to the
Formaldehyde Level
2
1
4
a
(
HCO2).
In the case of Pd, treatment of 3-Pd with CO (ca. 1 atm,
2
benzene-d ) also appears to generate a formate complex (6-Pd)
6
as the major product; however, this reaction is not as clean as in
1
the case of Ni and 6-Pd could not be successfully isolated. H
Ni
[HBR2]
(mol/L)
time
(h)
conversion to
acetal (%)
NMR analysis of the reaction mixture indicates that 6-Pd gives
rise to a formate resonance at 9.26 ppm. Attempts to generate
an analogous Pt formate species by treatment of 3-Pt with CO2
led to the formation of multiple unidentified products from
which no pure material could be isolated. By comparison, the
entry (mol %)
HBR2
solvent
C D
b
1
2
3
4
5
6
7
8
9
0.2
0.02
0.2
0.2
0.2
0.2
0.2
0.2
0.2
HBPin
HBPin
HBPin
HBPin
HBPin
HBCat
9-BBN
HBPin
HBPin
4.8
4
1
1
1
1
1
1
1
1
97
6
6
6
6
6
6
6
6
4.8
C D
6
<5
84
21
<5
<5
<5
30
92
4.8
C D
6
0.96
0.096
4.8
C D
6
related Pd formate complex (Cy-PSiP)Pd(HCO ) is isolable,
2
C D
6
while the Pt derivative can be generated under a CO2
C D
6
atmosphere and readily eliminates CO to reform the parent
2
0.96
4.8
C D
6
7c
Pt hydride complex.
THF-d8
Catalytic Hydroboration of CO . Having observed facile
2
4.8
C D
6
12
insertion of CO by 3-Ni to afford a Ni formate complex, we
2
focused our attention on the development of catalytic CO2
reduction chemistry mediated by such hydride complexes.
Catalytic reactions were carried out in benzene solution at
a
Under 1 atm of CO . Conversions were determined on the basis of
H NMR integration using C Me as an internal standard (average of
two runs). Throughout, no indication of boryl formate and minimal
(≤5%) conversion to methoxyborane were observed. TON = 487
based on moles of B−H reacted per mole of Ni; 71% isolated yield.
2
1
6
6
b
room temperature and 1 atm of CO , with HBPin as the
2
reductant. In an initial screen, the catalytic performances of 3-
F
DOI: 10.1021/acs.organomet.7b00497
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
i
time to 4 h led to 97% conversion of HBPin to the targeted
acetal 7-B. This represents the highest selectivity reported to
date for hydroboration to the formaldehyde level with
negligible overreduction to the methoxyborane 7-C (ca. 1%
treatment with H N(2,6- Pr C H ) led to the formation of
2 2 6 3
i
1
CH N(2,6- Pr C H ) (86% conversion by H NMR after 24
2
2
6
3
h at room temperature), while treatment with HMeNPh led to
the formation of [Ph(Me)N] CH (79% conversion after 24 h
2
2
1
10
by H NMR analysis after 4 h). By comparison, in the only
at room temperature). We also observed net methylene transfer
to phosphorus upon treatment of 7-B with HPPh . While the
previous example of selective CO double hydroboration (with
2
2
9
-BBN), a yield of 85% bis(boryl)acetal and 8% methoxyborane
latter reaction leads exclusively to the generation of a
monophosphine intermediate, which we tentatively assign as
was obtained at full 9-BBN conversion by use of Fe-
6e
(
H) (DMPE) as the precatalyst. Decreasing the catalyst
PinBOCH
2
PPh
2
, treatment of this species (generated in situ)
PPh (78%
2
2
loading by a factor of 10 led to a dramatic dropoff in reactivity
Table 3, entry 2). We evaluated the effect of concentration on
with H NPh led to the formation of Ph(H)NCH
2
2
2
(
based on 7-B). This reactivity confirms the utility of 7-B as an
effective methylene transfer reagent.
the catalytic performance of 3-Ni by performing reactions at
lower concentrations of HBPin and once again found that the
conversion to the bis(boryl)acetal product decreased signifi-
cantly (Table 3, entries 4 and 5). We also examined the
reactivity of alternative boranes and found that HBCat and 9-
BBN each provided negligible conversion to any products
arising from CO hydroboration (Table 3, entries 6 and 7).
Finally, we explored the effect of varying the solvent by
performing the catalytic reaction in THF-d and cyclohexane-
d12 solution and found that while the use of THF resulted in
decreased formation of the bis(boryl)acetal product, the use of
cyclohexane resulted in 92% conversion to 7-B in 1 h at room
temperature (Table 3, entries 8 and 9).
Having optimized reaction conditions for the formation of 7-
B, we also found that this compound could be readily isolated
as an analytically pure, moisture-sensitive solid in 71% yield,
which may facilitate its utility as a synthon for subsequent
chemical manipulation. Samples of 7-B stored under a nitrogen
atmosphere proved stable in the solid state for several days.
While 7-B rapidly hydrolyzes to generate formaldehyde upon
CONCLUSION
■
In summary, a new bis(indolylphosphino)silyl PSiP ligand was
developed and utilized for the synthesis of group 10 pincer
complexes. Nickel, palladium, and platinum hydride complexes
supported by this ligand were prepared and characterized.
Solution NMR and single-crystal X-ray data revealed that a
significant amount of structural variability is possible for such
complexes, particularly in the case of Ni, where terminal
2
8
2
hydride as well as complexes involving η -SiH coordination are
both accessible and may even coexist, in ratios dependent on
factors such as the nature of additional coligands, including N2
from the reaction atmosphere, as well as solvent and
temperature.
While both the Ni and Pd hydride complexes were shown to
be capable of CO insertion to afford formate complexes, these
2
formate complexes were not isolable. However, both hydride
species were active for catalytic hydroboration of CO with
2
HBPin, displaying divergent selectivity. Thus, the Pd catalyst
exposure to water, in benzene-d solution under an inert
exhibited moderate activity in the hydroboration of CO to the
6
2
atmosphere 64% of the acetal was found to persist after 96 h at
room temperature (by H NMR integration relative to
ferrocene internal standard).
Bontemps, Sabo-Etienne, and co-workers have previously
formate level, whereas the analogous Ni species exhibited
unprecedented selectivity for hydroboration to the form-
aldehyde level. We further demonstrated that the HBPin-
derived bis(boryl)acetal product could be successfully isolated
and utilized as a source of methylene for the formation of C−N
and C−P bonds.
1
reported that in situ generated bis(boryl)acetal derived from
the hydroboration of CO with 9-BBN is an effective source of
2
formaldehyde that can undergo susbequent reactivity with
substrates such as anilines, amines, alcohols, and ylides to form
EXPERIMENTAL SECTION
General Considerations. All experiments were conducted under
N in a glovebox or using standard Schlenk techniques. Tetrahy-
■
6
e,23
C−N, C−O, and C−C bonds.
Intrigued by this reactivity,
we sought to study the reactivity of isolated 7-B in this regard.
Indeed, similar methylene transfer reactivity was observed for
the reaction of 7-B with anilines (Scheme 6), such that
2
drofuran and diethyl ether were distilled from Na/benzophenone
ketyl. Benzene, toluene, and pentane were first sparged with nitrogen
and subsequently dried by passage through a double-column (one
activated alumina column and one column packed with activated Q-5)
solvent purification system. All purified solvents were stored over 4 Å
molecular sieves. Deuterated solvents were degassed via three freeze−
pump−thaw cycles and stored over 4 Å molecular sieves. All reagents
were purchased from commercial suppliers and used without further
purification. The compound diisopropyl(3-methyl-2-indolyl)-
Scheme 6. Methylene Transfer from 7-B
12
phosphine was prepared according to a literature procedure. Unless
1
13
11
31
29
otherwise stated, H, C, B, P, and Si characterization data were
collected at 300 K, with chemical shifts reported in parts per million
1
13
29
11
downfield of SiMe (for H, C, and Si), BF ·OEt (for B), or 85%
4
3
2
31
H PO in D O (for P). Chemical shift ranges are cited to indicate
3
4
2
1
13
overlapping resonances. H and C NMR chemical shift assignments
13
1
13
1
1
are based on data obtained from C{ H}, C-DEPTQ, H− H
1
13
1
13
29
COSY, H− C HSQC, and H− C HMBC NMR experiments. Si
1
29
1
29
NMR assignments are based on H− Si HMBC, H− Si HMQC,
1
29
17
1
29
and H− Si HECADE experiments. The 2D H− Si HECADE
1
29
data were acquired using a value for the one-bond H− Si coupling of
1
80 Hz and a TOCSY mixing time of 60 ms; in order to ensure
adequate digital resolution in F2 for the measurement of coupling
constants, the acquisition time was adjusted to 1.14 s, resulting in a
G
DOI: 10.1021/acs.organomet.7b00497
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
digital resolution of 0.88 Hz/point. In experiments involving the
reactivity of the acetal 7-B, a 60 s relaxation delay was used when
determining reaction yields by NMR integration relative to a ferrocene
internal standard. Infrared spectra were recorded as thin films between
at room temperature for 20 min. The volatile components of the
reaction mixture were subsequently removed under vacuum. The
remaining residue was triturated with 3 × 2 mL of pentane and then
washed with pentane (ca. 1 mL) and dried under vacuum to afford 2-
Pd·C H (0.31 g, 88% yield) as a white solid. Despite prolonged
−1
NaCl plates at a resolution of 4 cm . Raman spectroscopy was
performed using a DXR Smart Raman Spectrometer. Elemental
analyses were performed by Canadian Microanalytical Service Ltd. of
Delta, British Columbia, Canada, or Galbraith Laboratories Inc. of
Knoxville, TN, USA.
5
12
exposure to vacuum, 2-Pd as prepared above was found to routinely
1
retain pentane (≤1 equiv). H NMR (300 MHz, benzene-d ): δ 7.62−
6
7.56 (4 H, H ), 7.25−7.11 (4 H, H ), 3.18 (m, 2 H, PCHMe ),
2.63 (m, 2 H, PCHMe ), 2.22 (s, 6 H, Indole-Me), 1.56 (m, 6 H,
arom
arom
2
2
i
Ind
(
Pr-PSiP )H (1). A precooled (−78 °C) solution of diisopropyl(3-
PCHMe ), 1.42 (m, 6 H, PCHMe ), 1.09 (m, 6 H, PCHMe ), 0.96 (m,
2
2
2
13
methyl-2-indolyl)phosphine (4.00 g, 16.2 mmol) in THF (ca. 100 mL)
was treated with BuLi (1.6 M in hexanes, 10.1 mL, 16.2 mmol),
6 H, PCHMe ), 0.54 (s, 3 H, SiMe). C NMR (75.5 MHz, benzene-
2
n
d6): δ 141.3 (apparent t, J = 7 Hz, Carom), 137.4 (Carom), 132.3
(apparent t, J = 25 Hz, Carom), 123.4 (CHarom), 121.6 (Carom), 121.2
(CHarom), 119.9 (CHarom), 117.2 (CHarom), 28.7 (apparent t, J = 13 Hz,
PCHMe ), 26.7 (apparent t, J = 13 Hz, PCHMe ), 20.4−19.5
added dropwise over the course of 10 min. The resulting reaction
mixture was stirred for 20 min at −78 °C and then warmed to room
temperature over the course of 1 h. The reaction mixture was once
2
2
again cooled to −78 °C, and neat Cl SiHMe (0.84 mL, 8.10 mmol)
(overlapping resonances, PCHMe ), 11.5 (Indole-Me), 7.0 (SiMe).
2
2
31
1
29
was added by syringe. The mixture was warmed to room temperature
over the course of 18 h with stirring, during which time a white
precipitate was observed. The volatile components of the reaction
mixture were subsequently removed under vacuum, and benzene (ca.
P{ H} NMR (121.5 MHz, benzene-d ): δ 38.5. Si NMR (59.6
6
MHz, benzene-d ): δ 41.8. Anal. Calcd for C H ClN P PdSi·C H
6
31 45
2
2
5
12
(2-Pd·C H ): C, 57.67; H, 7.66; N, 3.74. Found: C, 58.00; H, 7.02;
5
12
N, 3.58. Although these results in % H are outside the range viewed as
establishing analytical purity, they are provided to illustrate the best
values obtained to date. Crystals of 2-Pd suitable for X-ray diffraction
60 mL) was added to the remaining residue. The resulting slurry was
filtered through Celite. The filtrate solution was collected, and the
volatile components were removed in vacuo. The remaining residue
was triturated with pentane (4 × 3 mL) and subsequently washed with
analysis were grown from a concentrated Et O solution at −35 °C.
2
i
Ind
( Pr-PSiP )PtCl (2-Pt). A solution of 1 (0.25 g, 0.47 mmol) in
pentane (3 × 4 mL) to afford 1a (3.21 g, 74% yield) as an off-white
benzene (ca. 5 mL) was added to a solution of (Et S) PtCl (0.21 g,
2
2
2
1
solid. H NMR (500 MHz, benzene-d ): δ 7.56 (apparent d, 4 H, J = 8
0.47 mmol) in benzene (ca. 5 mL). Neat NEt (0.65 mL, 4.7 mmol)
6
3
Hz, Harom), 7.25−7.09 (5 H, SiH + Harom; the SiH resonance was
was added to the reaction mixture. The solution was subsequently
heated at 75 °C with stirring for 4 days, during which time a color
change to bright yellow was observed. The reaction mixture was
subsequently cooled to room temperature and was filtered through
Celite. The filtrate solution was collected, and the volatile components
were removed under vacuum. The remaining residue was washed with
1
29
identified at 7.19 ppm by the use of H− Si correlation spectroscopy),
2
.27 (s, 6 H, Indole-Me), 2.22 (m, 4 H, PCHMe ), 1.19 (m, 3 H,
2
3
3
SiMe), 1.04 (dd, 6 H, J = 7 Hz, J = 16 Hz, PCHMe ), 0.98 (dd,
HH
HP
2
3
3
3
6
H, J = 7 Hz, J = 17 Hz, PCHMe ), 0.77 (dd, 6 H, J = 7 Hz,
HH HP 2 HH
3
3
3
JHP = 13 Hz, PCHMe ), 0.67 (dd, 6 H, J = 7 Hz, J = 14 Hz,
2
HH
HP
13
1
PCHMe2). C{ H} NMR (126 MHz, benzene-d ): δ 143.3 (Carom),
pentane (2 × 3 mL) and dried under vacuum to afford 2-Pt·C H
5 12
6
1
36.1 (Carom), 133.4 (Carom), 123.2 (CHarom), 120.5 (CHarom), 118.9
(0.33 g, 84% yield) as an off-white solid. Despite prolonged exposure
to vacuum, 2-Pt as prepared above was found to routinely retain
pentane (≤1 equiv); other minor unidentified impurities are also
(CHarom), 115.5 (CHarom), 25.9 (apparent d, J = 9 Hz, PCHMe ), 25.2
2
(
apparent d, J = 9 Hz, PCHMe ), 22.1−21.4 (overlapping resonances,
2
31
1
1
PCHMe ), 11.1 (Indole-Me), 4.5 (SiMe). P{ H} NMR (202 MHz,
apparent in the H NMR spectrum of 2-Pt (see Figure S4a in the
2
29
1
benzene-d ): δ −8.5. Si NMR (100 MHz, benzene-d ): δ −27.3
Supporting Information). H NMR (300 MHz, benzene-d
6
): δ 7.80
6
6
1
(
J
= −271 Hz). Anal. Calcd for C H N P Si: C, 69.37; H, 8.64;
(m, 2 H, Harom), 7.58 (m, 2 H, Harom), 7.24−7.18 (4 H, Harom), 3.45
(m, 2 H, PCHMe ), 2.84 (m, 2 H, PCHMe ), 2.24 (s, 6 H, Indole-
Me), 1.53 (m, 6 H, PCHMe ), 1.41 (m, 6 H, PCHMe ), 1.12 (m, 6 H,
PCHMe ), 0.94 (m, 6 H, PCHMe ), 0.53 (s with Pt satellites, 3 H,
PtH = 22 Hz, SiMe). C NMR (75.5 MHz, benzene-d ): δ 141.2
SiH
31 46
2 2
N, 5.22. Found: C, 69.47; H, 8.89; N, 5.25.
2
2
i
Ind
(
Pr-PSiP )NiCl (2-Ni). A room-temperature solution of 1 (0.40 g,
.75 mmol) in benzene (ca. 3 mL) was added to a suspension of
NiCl (DME) (0.16 g, 0.75 mmol) in benzene (ca. 3 mL). Neat NEt3
2
2
0
2
2
3
13
J
2
6
(
1.05 mL, 7.5 mmol) was added, and the resulting mixture was stirred
(Carom), 137.4 (Carom), 132.3 (apparent t, J = 31 Hz, Carom), 123.4
(CHarom), 121.4 (Carom), 121.1 (CHarom), 120.1 (CHarom), 117.3
for 16 h at room temperature. The reaction mixture was subsequently
filtered through Celite. The filtrate solution was collected, and the
volatile components were removed under vacuum. The remaining
residue was triturated with pentane (3 × 5 mL) and then washed with
pentane (ca. 1 mL) and dried under vacuum to afford 2-Ni (0.43 g,
9
(CHarom), 28.0 (apparent t, J = 16 Hz, PCHMe ), 27.0 (apparent t, J =
2
16 Hz, PCHMe ), 20.1−19.2 (overlapping resonances, PCHMe ), 11.4
2
2
3
1
1
(Indole-Me), 5.2 (SiMe). P{ H} NMR (121.5 MHz, benzene-d ): δ
42.0 (s with Pt satellites, J = 2719 Hz). Si NMR (59.6 MHz,
benzene-d ): δ 22.2 (s with Pt satellites, J = 1506 Hz). Anal. Calcd
6
1
29
PtP
1
1
1% yield) as an orange-yellow solid. H NMR (300 MHz, benzene-
6
SiPt
d6): δ 7.64 (m, 2 H, Harom), 7.57 (m, 2 H, Harom), 7.21−7.15 (4 H,
for C H ClN P PtSi·C H (2-Pt·C H ): C, 51.57; H, 6.85; N, 3.34.
31 45 2 2 5 12 5 12
Harom), 2.95 (m, 2 H, PCHMe ), 2.72 (m, 2 H, PCHMe ), 2.24 (s, 6
Found: C, 52.37; H, 6.60; N, 3.29. Although these results in % C are
outside the range viewed as establishing analytical purity, they are
provided to illustrate the best values obtained to date. Crystals of 2-Pt
suitable for X-ray diffraction analysis were obtained from a
2
2
H, Indole-Me), 1.48 (m, 12 H, PCHMe ), 1.18−1.04 (12 H,
2
13
PCHMe ), 0.54 (s, 3 H, SiMe). C NMR (75.5 MHz, benzene-d ):
2
6
δ 141.6 (apparent t, J = 6 Hz, Carom), 138.0 (Carom), 133.2 (apparent t,
J = 27 Hz, Carom), 123.8 (CHarom), 121.6 (CHarom), 120.9 (Carom),
concentrated Et O solution at −35 °C.
2
i
Ind
1
20.4 (CHarom), 117.5 (CHarom), 28.7 (apparent t, J = 12 Hz,
PCHMe ), 27.5 (apparent t, J = 13 Hz, PCHMe ), 20.8 (apparent d, J
( Pr-PSiP )NiH (3-Ni). A cold (−35 °C) solution of 2-Ni (0.10 g,
0.16 mmol) in toluene (ca. 5 mL) was treated with NaEt BH (1.0 M
2
2
3
=
12 Hz, CHMe ), 20.3 (apparent d, J = 11 Hz, CHMe ), 12.2 (Indole-
in toluene, 0.16 mL, 0.16 mmol), which was added in a dropwise
manner via pipet. The resulting yellow solution was warmed to room
temperature over the course of 2 h with stirring. The volatile
components of the reaction mixture were subsequently removed under
vacuum. The remaining residue was triturated with 3 × 2 mL of
pentane and then extracted with pentane (ca. 10 mL). The combined
extracts were filtered through Celite, and the clear yellow filtrate
solution was collected. The filtrate was evaporated to dryness in vacuo,
and the remaining residue was subsequently washed with cold (−35
°C) pentane (3 × 0.5 mL) and dried under vacuum to afford a yellow
2
2
31
1
29
Me), 6.4 (SiMe). P{ H} NMR (121.5 MHz, benzene-d ): δ 36.0. Si
NMR (59.6 MHz, benzene-d ): δ 43.8. Anal. Calcd for
C H ClN P NiSi: C, 59.11; H, 7.20; N, 4.45. Found: C, 59.58; H,
6
6
31
45
2 2
7
.19; N, 4.12. Although these results in % C are outside the range
viewed as establishing analytical purity, they are provided to illustrate
the best values obtained to date. Crystals of 2-Ni suitable for X-ray
diffraction analysis were obtained from a concentrated Et O solution at
−
2
35 °C.
i
Ind
3
(
Pr-PSiP )PdCl (2-Pd). A solution of [(η -C H )PdCl] (0.085 g,
3 5 2
i
2
0
.23 mmol) in benzene (ca. 5 mL) was added to a solution of 1 (0.25
solid, which is formulated as the N -bridged dimer {[ Pr-P(η -
2
Ind
g, 0.47 mmol) in benzene (ca. 5 mL). The reaction mixture was stirred
SiH)P ]Ni} (μ-N ) (0.068 g, 70% yield) on the basis of IR and
2
2
H
DOI: 10.1021/acs.organomet.7b00497
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
i
2
Ind
Raman analysis. Raman data for isolated {[ Pr-P(η -SiH)P ]Ni} (μ-
solution was evaporated to dryness in vacuo, and the remaining
residue was washed with cold pentane (3 × 0.5 mL) and dried under
vacuum to afford 3-Pt·C H (0.086 g, 76% yield) as a tan solid.
2
−1
−1
N ) (solid state, cm ): 2073 (s). IR (thin film, cm ): 2075 (w).
Repeated attempts to obtain satisfactory elemental analysis for isolated
[ Pr-P(η -SiH)P ]Ni} (μ-N ) were unsuccessful.
2
5
12
i
2
Ind
{
Despite prolonged exposure to vacuum, 3-Pt as prepared above was
2
2
1
As discussed in the text (vide supra), upon dissolution in benzene
found to routinely retain pentane (≤1 equiv). H NMR (300 MHz,
{[iPr-P(η -SiH)P ]Ni} (μ-N ) generates 3-Ni quantitatively (by
2
Ind
benzene-d ): δ 8.17 (apparent d, J = 8 Hz, 2 H, Harom), 7.63 (m, 2 H,
2
2
6
1
2
NMR analysis). NMR data for 3-Ni: H NMR (300 MHz, benzene-
d6) δ 8.13 (apparent d, 2 H, Harom), 7.63 (m, 2 H, Harom), 7.28−7.18 (4
H, Harom), 2.59 (m, 2 H, PCHMe ), 2.39 (m, 2 H, PCHMe ), 2.33 (s,
Harom), 7.31−7.19 (4 H, Harom), 4.21 (t with Pt satellites, JPH = 18 Hz,
1
J
PtH = 900 Hz, 1 H, PtH), 2.64 (m, 2 H, PCHMe
), 2.46 (m, 2 H,
2
PCHMe ), 2.31 (s, 6 H, Indole-Me), 1.41 (m, 6 H, PCHMe ), 1.20 (m,
2
2
2
2
6
H, Indole-Me), 1.36−1.19 (12 H, PCHMe ), 1.01 (m, 6 H,
6 H, PCHMe ), 0.97−0.78 (15 H, PCHMe + SiMe; the SiMe
2
2
2
13
PCHMe ), 0.90 (s, 3 H, SiMe), 0.87−0.77 (6 H, PCHMe ), −4.79 (br
resonance was identified at 0.94 ppm by correlation spectroscopy).
NMR (75.5 MHz, benzene-d ): δ 141.0 (apparent t, J = 6 Hz, Carom),
137.5 (Carom), 136.7 (Carom), 122.9 (CHarom), 120.1 (CHarom), 119.8
CHarom), 118.6 (Carom), 116.6 (CHarom), 29.3 (apparent t, J = 16 Hz,
PCHMe ), 25.9 (apparent t, J = 18 Hz, PCHMe , 22.2 (PCHMe ),
C
2
2
1
s, 1 H, NiH); H NMR (300 MHz, −60 °C, toluene-d ) δ −4.91 (br t,
6
8
2
13
J
= 47 Hz, 1 H, NiH); C NMR (125.8 MHz, benzene-d ) δ 141.4
PH
6
(
(
1
apparent t, J = 6 Hz, Carom), 138.0 (apparent t, J = 26 Hz, Carom),
37.3 (Carom), 123.0 (CHarom), 120.2 (CHarom), 119.7 (CHarom), 118.0
2
2
2
0.4−18.8 (overlapping resonances, PCHMe ), 11.0 (Indole-Me), 7.2
(
(
1
Carom), 116.2 (CHarom), 29.0 (apparent t, J = 14 Hz, PCHMe ), 25.5
apparent t, J = 16 Hz, PCHMe , 21.6 (PCHMe ), 20.9 (PCHMe ),
2
2
2 31 1
PtC
2
2
2
1
29
9.3 (PCHMe ), 19.1 (PCHMe ), 11.2 (Indole-Me), 5.6 (SiMe);
6
PtP
2
2
1
3
1
1
29
P{ H} NMR (121.5 MHz, benzene-d ) δ 65.6; Si NMR (99.4
6
SiPt
6
31
46
2
2
5
12
5
12
MHz, benzene-d ) δ 59.8. Despite repeated efforts, we were unable to
measure the value of JSiH for 3-Ni.
6
are outside the range viewed as establishing analytical purity, they are
The reversible coordination of N2 to 3-Ni in solution is both
provided to illustrate the best values obtained to date.
temperature and solvent dependent, such that formation of the N2
i
2
Ind
i
2
[
Pr-P(η -SiH)P ]Ni(DMAP) (4). A solution of {[ Pr-P(η -SiH)-
adduct 3-Ni* can be observed at low temperature. Selected NMR data
Ind
1
2
P ]Ni} (μ-N ) (0.048 g, 0.039 mmol) in benzene (ca. 5 mL) was
2 2
for 3-Ni*: H NMR (300 MHz, 0 °C, toluene-d ) δ −2.92 (t, J
=
8
PH
1
treated with DMAP (0.009 g, 0.078 mmol) to produce a clear yellow
solution. The reaction mixture was allowed to stand at room
temperature for 14 h. The volatile components of the reaction
mixture were subsequently removed under vacuum, and the remaining
residue was washed with 2 × 1 mL of cold (−35 °C) pentane and
3
8 Hz, 1 H, NiH), a 3:2 ratio of 3-Ni to 3-Ni* was observed; H NMR
2
(
300 MHz, −40 °C, methylcyclohexane-d ) δ −2.58 (t, J = 38 Hz,
14
PH
1
H, NiH), near-quantitative formation of 3-Ni* was observed;
3
1
1
3
1
1
P{ H} NMR (121.5 MHz, 0 °C, toluene-d ) δ 35.2 (s); P{ H}
8
NMR (121.5 MHz, −80 °C, toluene-d ) δ 40.7 (br s, 1 P), 27.9 (br s,
1
Hz). Despite repeated efforts, we were unable to measure the sign of
JSiH for 3-Ni*.
8
dried in vacuo to afford 4 (0.055 g, 98% yield) as a bright yellow solid.
P); 29Si NMR (99.4 MHz, −80 °C, toluene-d ) δ −10.5 (|J | = 89
8
SiH
1
H NMR (500 MHz, benzene-d ): δ 8.71 (d, J = 7 Hz, 2 H, Harom),
6
8
.23 (d, J = 9 Hz, 2 H, Harom), 7.68 (d, J = 8 Hz, 2 H, Harom), 7.28 (t, J
8 Hz, 2 H, Harom), 7.21 (t, J = 8 Hz, 2 H, Harom), 5.75 (d, J = 7 Hz, 2
H, Harom), 2.76 (m, 2 H, PCHMe ), 2.65 (m, 2 H, PCHMe ), 2.51 (s,
=
(iPr-PSiP )PdH (3-Pd). A cold (−35 °C) solution of 2-Pd (0.11 g,
Ind
2
2
0.16 mmol) in toluene (ca. 10 mL) was treated with NaEt BH (1.0 M
3
3
6
H, Indole-Me), 2.03 (s, 6 H, NMe ), 1.35 (d, J = 2 Hz, 3 H,
2 HH
in toluene, 0.16 mL, 0.16 mmol), which was added in a dropwise
manner via pipet. The resulting solution was warmed to room
temperature over the course of 18 h with stirring. The volatile
components of the reaction mixture were subsequently removed under
vacuum, and the remaining residue was triturated with 3 × 1 mL of
pentane. The residue was then extracted with pentane (ca. 10 mL),
and the combined extracts were filtered through Celite. The filtrate
solution was evaporated to dryness in vacuo, and the remaining
residue was washed with cold pentane (3 × 0.5 mL) and dried under
2
SiMe), 1.22−1.09 (24 H, PCHMe ), −3.50 (t, J = 40 Hz, 1 H, Si-H-
2
PH
13
1
Ni). C{ H} NMR (125.8 MHz, benzene-d ): δ 156.0 (CHarom),
6
1
=
52.8 (Carom), 141.1 (apparent t, J = 6 Hz, Carom), 138.2 (apparent t, J
21 Hz, Carom), 135.8 (Carom), 122.3 (CHarom), 119.4 (CHarom), 119.1
(
CHarom), 116.1 (Carom), 115.2 (CHarom), 106.2 (CHarom), 38.1
(NMe ), 29.7 (apparent t, J = 13 Hz, PCHMe ), 27.9 (apparent t, J
2
2
=
6 Hz, PCHMe ), 21.1 (apparent t, J = 6 Hz, PCHMe ), 20.6
2
2
(
1
PCHMe ), 20.4 (PCHMe ), 20.1 (apparent t, J = 5 Hz, PCHMe ),
1.5 (Indole-Me), 10.5 (SiMe). P{ H} NMR (202.5 MHz, benzene-
2
2
2
31
1
vacuum to afford 3-Pd (0.086 g, 86% yield) as an orange-yellow solid.
29
1
d6): δ 28.6. Si NMR (99.4 MHz, benzene-d ): δ −24.1 (d, J
=
6
SiH
1
H NMR (300 MHz, benzene-d ): 8.06 (m, 2 H, Harom), 7.63 (m, 2 H,
6
−69 Hz). Anal. Calcd for C H N P NiSi: C, 63.60; H, 7.87; N, 7.81.
38
56
4 2
Harom), 7.28−7.17 (4 H, H ), 2.58 (m, 2 H, PCHMe ), 2.40 (m, 2
H, PCHMe ), 2.31 (s, 6 H, Indole-Me), 1.43 (m, 6 H, PCHMe ), 1.26
arom
2
Found: C, 63.35; H, 7.81; N, 7.57. X-ray-quality crystals of 4 were
2
2
obtained from a concentrated Et O solution at −35 °C.
2
i
2
Ind
i
Ind
(
m, 6 H, PCHMe ), 1.00−0.77 (15 H, PCHMe + SiMe; the SiMe
2
2
[ Pr-P(η -SiH)P ]Ni(PMe ) (5a) + ( Pr-PSiP )Ni(PMe )H (5b).
3 3
i 2 Ind
resonance was identified at 0.92 ppm by correlation spectroscopy),
A room-temperature solution of {[ Pr-P(η -SiH)P ]Ni} (μ-N )
2
2
2
13
1
0
.15 (br t, J = 15 Hz, 1 H, PdH). C{ H} NMR (75.5 MHz,
PH
(0.038 g, 0.031 mmol) in benzene (ca. 5 mL) was treated with
benzene-d ): δ 141.5 (apparent t, J = 7 Hz, Carom), 137.7 (apparent t, J
PMe (0.024 g, 0.31 mmol), which was added dropwise by syringe.
6
3
=
26 Hz, Carom), 137.4 (Carom), 123.0 (CHarom), 120.4 (CHarom), 119.7
CHarom), 119.0 (Carom), 116.7 (CHarom), 28.6 (apparent t, J = 14 Hz,
PCHMe ), 26.2 (apparent t, J = 14 Hz, PCHMe ), 22.3 (PCHMe ),
After standing for 2 h at room temperature, the reaction mixture was
evaporated to dryness under vacuum and the remaining residue was
triturated with 3 × 2 mL of pentane. The residue was subsequently
(
2
2
2
1
2
0.8 (PCHMe ), 19.9−19.4 (overlapping resonances, PCHMe ), 19.6
washed with pentane (3 × 2 mL) to afford a ca. 2:1 mixture (by H
2
2
31
1
31
(
PCHMe ), 11.2 (Indole-Me), 7.0 (SiMe). P{ H} NMR (121.5 MHz,
and P NMR) of isomers 5a and 5b as an off-white solid (0.029 g,
2
29
1
benzene-d ): δ 63.3. Si NMR (99.4 MHz, benzene-d ): δ 60.1 (J =
70% overall yield based on Ni). Selected NMR data for 5a: H NMR
6
6
SiH
−
101 Hz). Anal. Calcd for C H N P PdSi: C, 57.89; H, 7.21; N,
(500 MHz, benzene-d ) δ 2.38 (s, 6 H, Indole-Me), 1.48 (s, 3 H,
31
46
2
2
6
2
13
1
4
.36. Found: C, 57.52; H, 7.31; N, 4.03.
SiMe), −5.44 (td, J = 37 Hz, 22 Hz, 1 H, NiH); C{ H} NMR
PH
i
Ind
31 1
(
Pr-PSiP )PtH (3-Pt). A cold (−35 °C) solution of 2-Pt (0.11 g,
(125.8 MHz, benzene-d ) δ 11.7 (Indole-Me), 9.9 (SiMe); P{ H}
NMR (202.5 MHz, benzene-d ) δ 42.2 (d, J = 6 Hz, 2 P, P Pr ),
6 PP 2
6
2
i
0.14 mmol) in toluene (ca. 10 mL) was treated with NaEt BH (1.0 M
3
2
29
in toluene, 0.14 mL, 0.14 mmol), which was added in a dropwise
manner via pipet. The resulting solution was warmed to room
temperature over the course of 18 h with stirring. The volatile
components of the reaction mixture were subsequently removed under
vacuum, and the remaining residue was triturated with 3 × 1 mL of
pentane. The residue was then extracted with pentane (ca. 10 mL),
and the combined extracts were filtered through Celite. The filtrate
−23.0 (t, J = 6 Hz, 1 P, PMe ); Si NMR (99.4 MHz, benzene-d )
PP
3
6
1
δ 6.2 (J = −82 Hz). Selected NMR data for 5b: H NMR (500
SiH
MHz, benzene-d ) δ 2.40 (s, 6 H, Indole-Me), 1.25 (s, 3 H, SiMe),
6
2
13
1
−10.18 (dt, JPH = 49 Hz, 38 Hz, 1 H, NiH); C{ H} NMR (125.8
MHz, benzene-d ) δ 11.6 (Indole-Me), 7.5 (d, J = 6 Hz, SiMe);
P{ H} NMR (202.5 MHz, benzene-d ) δ 56.8 (d, JPP = 111 Hz, 2 P,
6
3
1
1
2
6
i
2
29
P Pr ), −19.2 (t, J = 111 Hz, 1 P, PMe ); Si NMR (99.4 MHz,
2
PP
3
I
DOI: 10.1021/acs.organomet.7b00497
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
benzene-d ) δ 65.8 (JSiH = −81 Hz). Anal. Calcd for C H N P NiSi:
mixture was stirred for 4 h at room temperature, during which time a
6
34 55
2 3
C, 60.81; H, 8.26; N, 4.17. Found: C, 60.56; H, 8.12; N, 4.01. X-ray-
precipitate separated from solution. Benzene-d (ca. 1 mL) was added
6
quality crystals of 5b were obtained from a concentrated Et O solution
to the reaction vessel to dissolve all reaction components, and the
conversion to 7-B was determined to be 96% (this represents one of
the two runs averaged to obtain 97% conversion to 7-B as shown in
2
at −35 °C.
i
Ind
Generation of ( Pr-PSiP )Ni(HCO ) (6-Ni). A room-temperature
2
i
2
Ind
1
solution of {[ Pr-P(η -SiH)P ]Ni} (μ-N ) (0.012 g, 0.010 mmol) in
Table 3, entry 1) on the basis of H NMR spectroscopic analysis
2
2
benzene-d (ca. 1 mL) was transferred to a J. Young NMR tube and
relative to the internal standard. The volatile components of the
reaction mixture were removed in vacuo, and the remaining white
residue consisting of 7-B was washed with 3 × 1 mL of cold (−35 °C)
pentane and dried under vacuum, thus affording an initial crop of 7-B.
The pentane washes were combined, from which additional 7-B was
collected by crystallization at −35 °C. Compound 7-B was isolated as
6
degassed via three freeze−pump−thaw cycles. CO (ca. 1 atm) was
2
introduced to the tube at ambient temperature. After 10 min at room
temperature, NMR analysis of the reaction mixture indicated the
1
quantitative formation of 6-Ni. H NMR (300 MHz, benzene-d ): δ
6
8
.71 (s, 1 H, HCO ), 7.62 (apparent d, J = 7 Hz, 2 H, Harom), 7.55
2
1
(
apparent d, J = 7 Hz, Harom), 7.17−7.11 (4 H, Harom), 2.76 (m, 2 H,
a white solid in 71% combined yield (0.45 g, 1.50 mmol). H NMR
PCHMe ), 2.56 (m, 2 H, PCHMe ), 2.23 (s, 6 H, Indole-Me), 1.38−
1
SiMe). C NMR (75.5 MHz, benzene-d ): δ 168.8 (HCO ), 140.9
(500 MHz, benzene-d ): δ 5.51 (s, 2 H, CH ), 1.01 (s, 24 H, CH ).
2
2
6
2
3
11
13
1
.27 (12 H, PCHMe ), 1.19−1.08 (12 H, PCHMe ), 0.67 (s, 3 H,
2
2
B NMR (160.5 MHz, benzene-d ): δ 22.8. C{ H} NMR (125.8
6
13
6
2
MHz, benzene-d ): δ 85.7 (CH ), 82.9 (CMe ), 24.6 (CMe ). Anal.
6 2 2 2
(
apparent t, J = 6 Hz, Carom), 137.0 (Carom), 131.9 (apparent t, J = 28
Calcd for C H B O : C, 52.05; H, 8.74. Found: C, 51.87; H, 8.60.
1
3
26
2
6
Hz, Carom), 123.2 (CHarom), 121.0 (CHarom), 120.4 (Carom), 119.7
i
Generation of CH N(2,6- Pr C H ). Compound 7-B (0.032 g,
2
2 6 3
(
CHarom), 116.7 (CHarom), 27.3 (apparent t, J = 12 Hz, PCHMe ), 26.3
2
0.11 mmol) and ferrocene (0.034 g, 0.18 mmol) were dissolved in
(
apparent t, J = 11 Hz, PCHMe ), 19.5−19.1 (overlapping resonances,
i
2
benzene-d6 (750 μL), and neat H N(2,6- Pr C H ) (20 μL, 0.11
2 2 6 3
31
1
PCHMe ), 11.4 (Indole-Me), 5.2 (SiMe). P{ H} NMR (121.5 MHz,
2
mmol) was added via syringe to the solution. The reaction mixture was
subsequently transferred to an NMR tube. After standing for 24 h at
2
9
benzene-d ): δ 34.6. Si NMR (99.4 MHz, benzene-d ): δ 42.2.
6
6
i
Ind
Generation of ( Pr-PSiP )Pd(HCO ) (6-Pd). A room-temper-
i
2
room temperature, 86% conversion to 2,6- Pr C H NCH was
2 6 3 2
ature solution of 3-Pd (0.015 g, 0.023 mmol) in benzene-d (ca. 1 mL)
1
6
observed, as determined by H NMR integration relative to the
ferrocene internal standard. H NMR (300 MHz, benzene-d ) δ: 7.25
(d, 1 H, J = 18 Hz, CH ), 7.09−7.01 (overlapping resonances, 3 H,
Harom), 6.85 (d, 1 H, J = 18 Hz, CH ), 2.96 (sept, 2 H, J = 7 Hz,
CHMe ), 1.11 (d, 6 H, J = 7 Hz, CHMe ). These NMR data are
consistent with those reported previously.
Generation of [Ph(Me)N] CH . Compound 7-B (0.026 g, 0.087
was transferred to a J. Young NMR tube and degassed via three
1
6
freeze−pump−thaw cycles. CO (ca. 1 atm) was introduced to the
2
2
HH
2
tube at ambient temperature. After 10 min at room temperature, NMR
2
3
HH
2
HH
analysis of the reaction mixture indicated the formation of 6-Pd as the
3
1
2
HH
2
major product. Selected NMR data for 6-Pd: H NMR (300 MHz,
6e
benzene-d ) δ 9.26 (s, 1 H, HCO ), 2.20 (s, 6 H, Indole-Me), 0.65 (s,
6
2
31
1
2
2
3
H, SiMe); P{ H} NMR (121.5 MHz, benzene-d ) δ 39.0.
6
mmol) and ferrocene (0.017 g, 0.091 mmol) were dissolved in
General Conditions for Catalytic Hydroboration of CO . For
2
benzene-d (750 μL), and neat HMeNPh (19 μL, 0.18 mmol) was
6
all catalytic runs using HBPin or HBCat, the total reaction volume was
kept a constant 872 μL (combined volume of borane and solvent).
Unless otherwise stated, reactions were performed at room temper-
ature for 1 h. If any precipitate was observed over the course of the
reaction, excess deutero solvent was added immediately prior to NMR
data acquisition, to ensure that all reaction components were
solubilized. For the more dilute reactions, the amounts of catalyst
and HBPin were decreased, and the volume difference was made up
added via syringe to the solution. The reaction mixture was
subsequently transferred to an NMR tube. After standing for 24 h at
room temperature, 79% conversion to [Ph(Me)N] CH was observed,
2
2
1
as determined by1 H NMR integration relative to the ferrocene
internal standard. H NMR (500 MHz, benzene-d ) δ: 7.18 (m, 4 H,
6
Harom), 6.79 (apparent t, 2 H, J = 8 Hz, Harom), 6.68 (d, 4 H, J = 8 Hz,
Harom), 4.33 (s, 2 H, CH ), 2.48 (s, 6 H, NMe). These NMR data are
2
6e
consistent with those reported previously.
with added benzene-d . Due to solubility issues, catalytic runs
6
Generation of PinBOCH PPh2 and Ph(H)NCH PPh . Com-
2
2
2
involving 9-BBN were performed at a concentration of 0.96 M in 9-
BBN monomer. In a typical catalysis run (Table 2, entry 1), 0.005 g
pound 7-B (0.026 g, 0.087 mmol) and ferrocene (0.022 g, 0.12 mmol)
were dissolved in benzene-d (750 μL), and neat HPPh (15 μL, 0.086
i
2
Ind
6
2
(
0.0041 mmol) of {[ Pr-P(η -SiH)P ]Ni} (μ-N ) (delivered as a 100
2 2
mmol) was added via syringe to the solution. The reaction mixture was
subsequently transferred to an NMR tube. After standing for 24 h at
room temperature, 82% conversion to PinBOCH PPh was observed,
i
2
Ind
μL aliquot of a 0.041 M stock solution of {[ Pr-P(η -SiH)P ]Ni} (μ-
2
N ) in benzene-d ) was added to 0.0054 g of C Me (the internal
2
6
6
6
2
2
standard) in a 1 dram vial. Benzene-d (162 μL) and HBPin (610 μL,
6
as determined by 1H NMR integration relative to the ferrocene
4.20 mmol) were added to the reaction mixture, and the combined
internal standard. Neat H NPh (20 μL, 0.22 mmol) was subsequently
2
mixture was subsequently transferred to a 250 mL Teflon-sealed
reaction vessel. The solution was degassed via three freeze−pump−
added to the reaction mixture. The resulting solution was allowed to
stand at room temperature for 48 h to obtain PhNHCH PPh in 78%
2
2
thaw cycles and subsequently exposed to ca. 1 atm of CO for 3 min at
2
yield relative to initial 7-B (>95% conversion relative to
ambient temperature, after which the reaction vessel was sealed. The
reaction mixture was stirred for 1 h at room temperature, during which
time a precipitate (subsequently identified as (PinBO) CH , 7-B)
1
PinBOCH PPh ), as determined by H NMR integration relative to
2
2
1
the ferrocene internal standard. NMR data for PinBOCH PPh : H
2
2
2
2
NMR (500 MHz, benzene-d ) δ 7.56−7.52 (m, 4 H, H ), 7.09−7.02
6
arom
separated from solution. Benzene-d (ca. 1 mL) was added to the
6
2
(
overlapping resonances, 6 H, H ), 4.74 (d, 2 H, J = 7 Hz, CH ),
.96 (s, 12 H, CH ). B NMR (160.5 MHz, benzene-d ): δ 22.7;
C{ H} NMR (125.8 MHz, benzene-d ) δ 136.8 (d, J = 13 Hz, Carom),
arom PH 2
reaction vessel to dissolve all reaction components, and the conversion
11
0
3
6
to (PinBO) CH (7-B) was determined to be 84% (average of two
2
2
13
1
1
6
runs) on the basis of H NMR spectroscopic analysis relative to the
1
33.9 (d, J = 19 Hz, CHarom), 128.8 (CHarom), 128.7 (d, J = 6 Hz,
internal standard.
1
CH ), 82.8 (CMe ), 65.4 (d, J = 13 Hz, CH ), 24.6 (CMe );
Synthesis and Isolation of (PinBO) CH2 (7-B). In a typical
arom
2
PC
2
2
2
31
1
P{ H} NMR (202.5 MHz, benzene-d ) δ −11.7. NMR data for
optimized catalysis run (Table 3, entry 1), 0.005 g (0.0041 mmol) of
6
1
i
2
Ind
PhNHCH PPh : H NMR (300 MHz, benzene-d ) δ 7.39 (m, 4 H,
{
0
[ Pr-P(η -SiH)P ]Ni} (μ-N ) (delivered as a 100 μL aliquot of a
2
2
6
2
2
i
2
Ind
Harom), 7.15−7.01 (overlapping resonances, 8 H, Harom), 6.74
(apparent t, 1 H, J = 8 Hz, Harom), 6.41 (d, 2 H, J = 8 Hz, Harom),
.041 M stock solution of {[ Pr-P(η -SiH)P ]Ni} (μ-N ) in benzene-
2 2
d ) was added to 0.0054 g of C Me (the internal standard) in a 1
6
6
6
dram vial. Benzene-d (162 μL) and HBPin (610 μL, 4.20 mmol) were
3.61 (m, 2 H, CH
2
), 3.48 (broad s, 1 H, NH), 1.01 (s, 12 H, CMe
P{ H} NMR (121.5 MHz, benzene-d ) δ −19.3. The NMR data for
Ph(H)NCH PPh are consistent with those reported previously.
2
);
6
31
1
added to the reaction mixture, and the combined mixture was
subsequently transferred to a 250 mL Teflon-sealed reaction vessel.
The solution was degassed via three freeze−pump−thaw cycles and
6
24
2
2
Crystallographic Solution and Refinement Details. Crystallo-
graphic data were obtained between 173(2) and 193(2) K on either a
Bruker D8/APEX II CCD or a Bruker PLATFORM/APEX II CCD
subsequently exposed to ca. 1 atm of CO for 3 min at ambient
2
temperature, after which the reaction vessel was sealed. The reaction
J
DOI: 10.1021/acs.organomet.7b00497
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
diffractometer equipped with a CCD area detector using graphite-
monochromated Mo Kα (λ = 0.71073 Å) radiation (except for 2-Ni
and 5b, where Cu Kα, λ = 1.54184 Å, radiation was used) employing
samples that were mounted in inert oil and transferred to a cold gas
stream on the diffractometer. Programs for diffractometer operation,
data collection, and data reduction (including SAINT) were supplied
by Bruker. Data reduction, correction for Lorentz−polarization, and
absorption correction were each performed. Structure solution was
achieved by either intrinsic phasing methods or Patterson search/
structure expansion. All structures were refined by use of full-matrix
0.03 Å) during refinement. The Ni−H groups in 4 and 5b were each
located in the difference map and refined isotropically. Otherwise, all
hydrogen atoms were added at calculated positions and refined by use
of a riding model employing isotropic displacement parameters based
on the isotropic displacement parameter of the attached atom.
Additional crystallographic details are available in the deposited CIFs
(CCDC 1558787−1558792).
ASSOCIATED CONTENT
Supporting Information
■
*
S
2
2
2
least-squares procedures (on F ) with R1 based on F ≥ 2σ(F ) and
o
o
wR2 based on Fo2 ≥ − 3σ(F ). Unless otherwise indicated, all non-
2
o
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00497.
hydrogen atoms were refined with anisotropic displacement
parameters. In the case of 1, disorder involving the isopropyl group
comprised of C(440−C(46) and the Si−Me group (C(1)) was
identified. Each of the carbon atoms in question was refined
anisotropically over two positions (A and B) with occupancy factors
of 0.5. In the case of both 2-Pt and 4, the crystal used for data
collection was found to display nonmerohedral twinning. In each case,
both components of the twin were indexed with the program
CELL_NOW (Bruker AXS Inc., Madison, WI, 2004). For 2-Pt, the
second twin component can be related to the first component by 180°
rotation about the [0,1,0] axis in both real space and reciprocal space.
For 4, the second twin component can be related to the first
component by 180° rotation about the [0.205,0,1] axis in real space
and about the [0,0,1] axis in reciprocal space. For each structure,
integrated intensities for the reflections from the two twin components
were written into a SHELXL-2014 HKLF 5 reflection file with the data
integration program SAINT (version 8.34A), using all reflection data
NMR spectra, including variable-temperature data (PDF)
Accession Codes
CCDC 1558787−1558792 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for L.T.: laura.turculet@dal.ca.
■
ORCID
(
exactly overlapped, partially overlapped, and nonoverlapped). The
refined value of the twin fraction (SHELXL-2014 BASF parameter) for
-Pt was 0.3(5), while for 4 the value was 0.1137(4). Disorder
Robert McDonald: 0000-0002-4065-6181
Michael Ferguson: 0000-0002-5221-4401
Laura Turculet: 0000-0002-2499-5343
2
involving the isopropyl groups was also identified for 2-Pt, such that
each of C(31)−C(33), C(41)−C(43), and C(44)−C(46) were
refined isotropically over two positions (A and B) with occupancy
factors of 0.5. C(44A) and C(44B) were refined with common
isotropic displacement parameters. C(35) was refined anisotropically
over two positions (A and B) with occupancy factors of 0.5. The
following pairs of distances were constrained to be equal (within 0.01
Å) during refinement: d(P(1)−C(31A)) = d(P(1)−C(31B));
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Natural Sciences and Engineering
Research Council (NSERC) of Canada, the Killam Trusts, and
Dalhousie University for their support of this work. We also
thank Prof. Georgii Nikonov (Brock University) for helpful
discussions.
d(P(2)−C(44A)) = d(P(2)−C(44B)). One equivalent of Et O was
2
located in the asymmetric unit for 2-Pt. Disorder involving this Et O
2
molecule was addressed in a satisfactory manner by refinement of the
non-hydrogen atoms over two positions (O(1SA), C(1SA)−C(4SA)
with an occupancy factor of 0.65, and O(1SB), C(1SB)−C(4SB) with
an occupancy factor of 0.35) with a common isotropic displacement
parameter. The following constraints were applied to distances within
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i
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Ind
[
Pr-P(η -SiH)P ]Ni(DMAP) (A and B) and 1.5 equiv of Et O were
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DOI: 10.1021/acs.organomet.7b00497
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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(
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3
3
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6
4
2
8
13
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2
(
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II
2
(
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2
6
4 2
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DOI: 10.1021/acs.organomet.7b00497
Organometallics XXXX, XXX, XXX−XXX