Catalytic properties of nickel bis(phosphinite) pincer complexes in the reduction of CO2 to methanol derivatives

Article abstract of DOI:10.1016/j.poly.2011.04.030

A new nickel bis(phosphinite) pincer complex [2,6-(R₂PO) ₂C₆H₃]NiCl (L^RNiCl, R = cyclopentyl) has been prepared in one pot from resorcinol, ClP(C₅H ₉)₂, NiCl₂, and 4-dimethylaminopyridine. The reaction of this pincer compound with LiAlH₄ produces a nickel hydride complex, which is capable of reducing CO₂ rapidly at room temperature to give a nickel formate complex. X-ray structures of two related nickel formate complexes L^RNiOCHO (R = cyclopentyl and isopropyl) have shown an "in plane" conformation of the formato group with respect to the coordination plane. The stoichiometric reaction of nickel formate complexes L^RNiOCHO (R = cyclopentyl, isopropyl, and tert-butyl) with catecholborane has suggested that the reaction is favored by a bulky R group. L^RNiOCHO (R = tert-butyl) does not react with PhSiH₃ at room temperature; however, it reacts with 9-borabicyclo[3.3.1]nonane and pinacolborane to generate a methanol derivative and a boryl formate species, respectively. The catalytic reduction of CO₂ with catecholborane is more effectively catalyzed by a more sterically hindered nickel pincer hydride complex with bulky R groups on the phosphorus donor atoms. The nickel pincer hydride complexes are inactive catalysts for the hydrosilylation of CO ₂ with PhSiH₃.

Full text of DOI:10.1016/j.poly.2011.04.030

Polyhedron 32 (2012) 30–34
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Catalytic properties of nickel bis(phosphinite) pincer complexes in the reduction
of CO2 to methanol derivatives
⇑
Sumit Chakraborty, Yogi J. Patel, Jeanette A. Krause, Hairong Guan
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221, USA
a r t i c l e i n f o
a b s t r a c t
A new nickel bis(phosphinite) pincer complex [2,6-(R2PO)2C6H3]NiCl (LRNiCl, R = cyclopentyl) has been
prepared in one pot from resorcinol, ClP(C5H9)2, NiCl2, and 4-dimethylaminopyridine. The reaction of this
pincer compound with LiAlH4 produces a nickel hydride complex, which is capable of reducing CO2 rap-
idly at room temperature to give a nickel formate complex. X-ray structures of two related nickel formate
complexes LRNiOCHO (R = cyclopentyl and isopropyl) have shown an ‘‘in plane’’ conformation of the for-
mato group with respect to the coordination plane. The stoichiometric reaction of nickel formate com-
plexes LRNiOCHO (R = cyclopentyl, isopropyl, and tert-butyl) with catecholborane has suggested that
the reaction is favored by a bulky R group. LRNiOCHO (R = tert-butyl) does not react with PhSiH3 at room
temperature; however, it reacts with 9-borabicyclo[3.3.1]nonane and pinacolborane to generate a meth-
anol derivative and a boryl formate species, respectively. The catalytic reduction of CO2 with catecholbo-
rane is more effectively catalyzed by a more sterically hindered nickel pincer hydride complex with bulky
R groups on the phosphorus donor atoms. The nickel pincer hydride complexes are inactive catalysts for
the hydrosilylation of CO2 with PhSiH3.
Article history:
Available online 4 May 2011
Keywords:
Nickel
Pincer complex
Carbon dioxide
Reduction
Methanol
Borane
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
has predicted that, if PhSiH3 is used instead, the reduction of CO2
will not occur [10]. To validate these computational results, we re-
Catalytic reduction of CO2 to liquid fuels such as methanol and
hydrocarbons is one of the key strategies to mastering carbon man-
agement [1]. Homogeneous systems of such a process are exceed-
ingly rare in the literature [2–4], despite the widespread use of
homogeneous catalysts in many other reactions. To make metha-
nol or its derivatives in particular, from CO2, one has to use an
appropriate reagent to abstract one of the two oxygen atoms in
CO2. Successful examples have involved the use of boron- [5] or
aluminum-based [6] frustrated Lewis acid–base pairs, largely due
to the oxophilic nature of these reagents. Simple silanes [7,8] and
boranes [9] may also be used, although a catalyst must be em-
ployed to facilitate the reduction. We have recently reported that
in the presence of a catalytic amount of LtBuNiH (Fig. 1), the reac-
tion of CO2 with HBcat affords CH3OBcat along with catBOBcat as
the oxygen-containing byproduct [9]. DFT calculations of our cata-
lytic system have implied that the steric environment around the
nickel center is critical to the efficiency of the catalysis [10]. The
choice of reductant is also crucial; the same computational study
port herein the synthesis of related nickel bis(phosphinite) pincer
complexes with different sizes of alkyl groups on the phosphorus
donor atoms, and the catalytic properties of these complexes in
the reduction of CO2 with various boranes and PhSiH3.
2. Results and discussion
2.1. Synthesis of LcPeNiH
We have previously prepared LRNiH (R = tBu and iPr) through
the reaction of LRNiCl with LiAlH4 in toluene [11]. Unfortunately,
using the same approach for the synthesis of LPhNiH and LMeNiH
produced intractable products. It is likely that the smaller R groups
do not provide enough steric protection to stabilize the hydride, or
perhaps in these complexes the more exposed pincer P–O bonds
are susceptible to cleavage by an adventitious base [12]. We were,
however, able to isolate LcPeNiH in 55% yield from the reaction
between LiAlH4 and LcPeNiCl, which was obtained following a
similar procedure described in the literature [13–17], or alterna-
tively from a one-pot synthesis as shown in Scheme 1. The 1H
NMR spectrum of LcPeNiH in C6D6 displays a characteristic hydride
resonance at À7.88 ppm as a triplet (2JP–H = 54.4 Hz), and the
31P{1H} NMR spectrum exhibits a singlet at 199.66 ppm.
Abbreviations: HBcat, catecholborane; DFT, density functional theory; tBu,
tert-butyl; iPr, isopropyl; cPe, cyclopentyl; DMAP, 4-dimethylaminopyridine;
9-BBN, 9-borabicyclo[3.3.1]nonane; HBpin, pinacolborane; TON, turnover number.
⇑
Corresponding author. Tel.: +1 513 556 6377; fax: +1 513 556 9239.
E-mail address: hairong.guan@uc.edu (H. Guan).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.04.030

S. Chakraborty et al. / Polyhedron 32 (2012) 30–34
31
toluene
RT, 1h
LRNiH + CO2 (1 atm)
LRNiOCHO
O
O
R = iPr, 82%
R = cPe, 77%
R2P
Ni
H
PR2
Scheme 2.
Fig. 1. Nickel bis(phosphinite) pincer hydride complexes (LRNiH, where R indicates
the substituents on the phosphorus atoms).
2.2. Synthesis and structures of LiPrNiOCHO and LcPeNiOCHO
Most catalytic systems for CO2 reduction involve the insertion
of CO2 into a metal-hydrogen bond [18–20]. Studies of well-de-
fined nickel hydride systems have shown that nickel formate com-
plexes are the insertion products [9,21–23], except in one case
where a more stable hydridonickel cyanate complex was isolated
as a consequence of the ancillary ligand participating in the reac-
tion [24]. Consistent with our previous observation of the reactiv-
ity of LtBuNiH [9], the reaction of CO2 with LiPrNiH or LcPeNiH at
room temperature gives the analogous nickel formate complex as
the sole product (Scheme 2). Although for the preparative scale
reaction, the solution of LRNiH was stirred under 1 atm of CO2 for
1 h to ensure a full conversion, NMR studies of small-scale reac-
tions suggested that the insertion was complete in several minutes.
Both LiPrNiOCHO and LcPeNiOCHO were characterized by NMR
spectroscopy, elemental analysis, IR spectroscopy, and X-ray crys-
tallography. Of particular interest is the orientation of the formato
groups in their crystal structures (Figs. 2 and 3), which show a rel-
atively small dihedral angle between the least-squares plane de-
fined by P1, P2, C1, Ni atoms and the OC(O)H plane [11.3(5)° in
LiPrNiOCHO; 16.8(5)° (top structure) and 6.6(7)° (bottom structure)
in LcPeNiOCHO]. To accommodate the ‘‘in plane’’ conformation, the
iPr and cPe groups are steered away from the OC(O)H moiety to
avoid the steric clash. In contrast, LtBuNiOCHO contains a ‘‘perpen-
dicular’’ formato group in the solid-state structure with a much lar-
ger dihedral angle [73.8(3)°] between the P1-P2-C1-Ni plane and
the OC(O)H plane [9]. However, in the solution the rotation of
the Ni–O3 bond is expected to be rapid. Restricted bond rotation
favoring the ‘‘in plane’’ conformer of LRNiOCHO would render the
two phosphorus nuclei inequivalent. We found that within the
temperature range of 23 to À50 °C, the 31P {1H} NMR spectra of
LiPrNiOCHO and LcPeNiOCHO in toluene-d8 showed singlet reso-
nances with no significant broadening.
Fig. 2. ORTEP drawing of LiPrNiOCHO at the 50% probability level. Hydrogen atoms
(except the formate hydrogen) are omitted for clarity.
less sterically hindered nickel center would be more susceptible
to the attack of HBcat and the subsequent insertion reactions, the
reaction of LiPrNiOCHO took 45 min to complete. Interestingly,
the reaction of LcPeNiOCHO finished within 10 min; however, the
NMR yield of CH3OBcat was merely 74%, implying a partial decom-
position of the nickel complex (or LcPeNiH, which is the intermedi-
ate during the reaction).
To expand the scope of reducing reagents that can be used for
the reduction of CO2, we focused on the stoichiometric reaction
of LtBuNiOCHO with other boranes and PhSiH3 under similar reac-
tion conditions (Scheme 4). 9-BBN behaves similarly to HBcat,
reducing the nickel formate complex to LtBuNiH and a methoxybo-
ryl species [25] within 1 h. In contrast, even with a large excess of
HBpin, only one equivalent of borane was consumed to yield LtBu-
NiH and presumably HCOOBpin on the basis of our previous study
of similar reactions with HBcat [9]. While this type of boryl formate
species is too unstable to be isolated, spectroscopic evidence to
support its formation includes 1H NMR resonances at 8.07 ppm
(singlet) and 0.88 ppm (singlet) with an expected integration ratio
of 1:12. DFT calculations have suggested that the reaction of
LtBuNiOCHO with PhSiH3 should be sluggish at ambient tempera-
ture with a high kinetic barrier of 42.6 kcal/mol [10]. Consistent
2.3. Stoichiometric reaction of LRNiOCHO (R = tBu, iPr, and cPe)
The regeneration of LRNiH from the reaction of LRNiOCHO with
a reducing reagent would close a potential catalytic cycle for the
reduction of CO2. To understand how the R groups influence the
reactivity of LRNiOCHO, we examined the NMR-scale reactions of
these complexes with 3 equivalents of HBcat in C6D6 (Scheme 3).
Under the same conditions (concentration and temperature), the
reaction of LtBuNiOCHO is the fastest with quantitative formation
of CH3OBcat within 10 min. Counterintuitive to the notion that a
O
O
O
O
HO
OH
PCl
LiAlH4
+
DMAP
THF, reflux
16 h
R2P
Ni
PR2
R2P
Ni
Cl
PR2
(R = cPe)
toluene, RT
24 h
2
2
H
+
LcPeNiH
55%
L
cPeNiCl
NiCl2
78%
Scheme 1.

32
S. Chakraborty et al. / Polyhedron 32 (2012) 30–34
LRNiH
(1 equiv)
3HBcat + CO2(1 atm)
(100 equiv)
CH3OBcat + catBOBcat
C6D6, RT
Scheme 5.
Table 1
Catalytic activity of nickel pincer hydride complexes in the reduction of CO2.a
Catalyst
Reductant
Time
TON
LtBuNiH
LiPrNiH
LcPeNiH
LtBuNiH
HBcat
HBcat
HBcat
45 min
2 h
12 h
48 h
100
100
30
b
PhSiH3
0
a
Reaction conditions: LRNiH (25
lmol), HBcat (2.5 mmol), and hexamethylben-
zene (62.5 mol) in 2 mL of C6D6 under 1 atm of CO2 at room temperature.
l
b
7.5 mmol of PhSiH3 was used.
complete reaction. In a typical catalytic study, HBcat and LRNiH
(with a ratio of 100:1) were mixed in C6D6 and then stirred under
1 atm of CO2 (Scheme 5). Hexamethylbenzene was used as an NMR
internal standard and the progress of the reaction was monitored
by 1H NMR spectroscopy. When LtBuNiH was used as the catalyst,
the reaction took 45 min to reach the maximum TON of 100 (based
on B–H, Table 1). The less sterically bulky nickel hydrides proved to
be less active catalysts. The same reaction catalyzed by LiPrNiH re-
quired 2 h to complete, whereas the catalytic reaction with LcPeNiH
did not go to completion within 12 h, giving a TON of 30. Fully
rationalizing the substituent effect on the catalytic activity is chal-
lenging due to the complexity of the reaction mechanism [9,10].
One explanation is that with a smaller R group, the interaction be-
tween LRNiH and HBcat is more favorable, resulting in a lower con-
centration of the catalytically active species LRNiH [9].
We also attempted to carry out catalytic hydrosilylation of CO2
using PhSiH3 as the reducing reagent and using LtBuNiH as the
catalyst (Table 1). Unfortunately, no reaction was observed at room
temperature even after 48 h. This result is consistent with the stoi-
chiometric reaction shown in Scheme 4 as well as the DFT studies
[10].
Fig. 3. ORTEP drawing of LcPeNiOCHO at the 50% probability level. Hydrogen atoms
(except the formate hydrogen) are omitted for clarity. Two independent molecules
in the crystal lattice are shown.
C6D6
LRNiOCHO + 3 HBcat
LRNiH + CH3OBcat + catBOBcat
RT
3. Conclusions
Scheme 3.
We have synthesized a new nickel hydride complex bearing a
bis(phosphinite) pincer ligand. This type of hydride complexes
with variable sizes of substituents (R groups) on the phosphorus
donor atoms undergoes rapid CO2 insertion to give nickel formate
complexes. Comparative studies of the stoichiometric reaction of
these formate species with HBcat have demonstrated that the for-
mation of CH3OBcat, the final product of this process, is favored by
bulky R groups. The outcome of the reaction is highly dependent
on the reducing reagent that is used. The reaction of a nickel for-
mate complex with 9-BBN produces a similar methanol derivative,
albeit at a slower rate. In contrast, HCOOBpin is the reduced prod-
uct when HBpin is provided as the hydrogen source. Consistent
with previous DFT calculations, PhSiH3 does not react with nickel
with the computational prediction, no appreciable reaction was
observed when LtBuNiOCHO was mixed with PhSiH3 for 16 h.
2.4. Catalytic reduction of CO2 with LRNiH (R = tBu, iPr, and cPe)
To compare the relative activity of LRNiH in catalyzing the
reduction of CO2, we chose HBcat as the hydrogen source mainly
for two reasons: (1) it has been established in the stoichiometric
studies as an efficient reductant for the conversion of nickel for-
mate complexes to a methanol derivative, and (2) the precipitation
of catBOBcat from C6D6 is often a good indication of a nearly
9-BBN
C6D6, RT, 1 h
tBuNiH +
tBuNiH + HCOOBpin
+
L
BOB
BOCH3
HBpin
C6D6, RT, 15 min
PhSiH3
LtBuNiOCHO
L
no reaction
C6D6, RT, 16 h
Scheme 4.

S. Chakraborty et al. / Polyhedron 32 (2012) 30–34
33
formate complexes at room temperature. The catalytic activity of
4.4. Synthesis of LiPrNiOCHO
nickel pincer hydride complexes for the reduction of CO2 with
HBcat correlates with the trend observed in the stoichiometric
reaction of nickel formate complexes, i.e., more bulky R groups
on the phosphorus atoms lead to a more reactive catalyst. The
hydrosilylation of CO2 with PhSiH3 under similar catalytic condi-
tions does not proceed at all.
At room temperature, a stream of CO2 was bubbled through the
solution of LiPrNiH (200 mg, 0.50 mmol) in 20 mL of toluene for 1 h.
Subsequent evaporation of the solvent under vacuum yielded
LiPrNiOCHO as a yellow powder (183 mg, 82%). X-ray-quality crys-
tals of LiPrNiOCHO were obtained by cooling a saturated pentane
solution of the complex to À30 °C. 1H NMR (400 MHz, C6D6, d):
1.16–1.21 (m, PCH(CH3)2, 12H), 1.30–1.36 (m, PCH(CH3)2, 12H),
4. Experimental
3
2.25–2.30 (m, PCH(CH3)2, 4H), 6.53 (d, ArH, JH–H = 7.6 Hz, 2H),
3
6.85 (t, ArH, JH–H = 7.6 Hz, 1H), 8.34 (s, NiOCOH, 1H). 31P{1H}
4.1. General considerations
NMR (162 MHz, C6D6, d): 183.29 (s). IR (in toluene, cmÀ1): 1605
(C@O). Anal. Calc. for C19H32O4P2Ni: C, 51.27; H, 7.25. Found: C,
51.49; H, 7.19%.
Unless otherwise stated, all the organometallic compounds
were prepared and handled under an argon atmosphere using
standard glovebox and Schlenk techniques. Carbon dioxide (tech-
nical grade), boranes, and PhSiH3 were purchased from commercial
sources and used without further purification. Dry and oxygen-free
solvents (THF, diethyl ether, toluene, and pentane) were collected
from an Innovative Technology solvent purification system and
used throughout the experiments. Methanol was degassed by bub-
bling argon through it for 15 min and then dried over molecular
sieves (4 Å). Benzene-d6 and toluene-d8 were distilled from Na
and benzophenone under an argon atmosphere. Both 1H NMR
and 31P{1H} NMR spectra were recorded on a Bruker Avance-
400 MHz NMR spectrometer. Chemical shift values in 1H NMR
spectra were referenced internally to the residual solvent reso-
nance (7.15 ppm for C6D6). The 31P{1H} NMR spectra were refer-
enced to an external 85% H3PO4 sample (0 ppm). LMeNiCl [12],
LPhNiCl [13], LiPrNiH [11], LtBuNiH [11], and LtBuNiOCHO [9] were
prepared as described in the literature.
4.5. Synthesis of LcPeNiOCHO
At room temperature, a stream of CO2 was bubbled through the
solution of LcPeNiH (100 mg, 0.20 mmol) in 20 mL of toluene for
1 h. Subsequent evaporation of the solvent under vacuum yielded
LcPeNiOCHO as a yellow powder (85 mg, 77% yield). X-ray-quality
crystals of LcPeNiOCHO were obtained by cooling a saturated pen-
tane solution of the complex to À30 °C. 1H NMR (400 MHz, C6D6,
d): 1.38–1.42 (m, CH2, 8H), 1.60–1.76 (m, CH2, 12H), 2.00–2.10
3
(m, CH2, 12H), 2.55–2.60 (m, PCH, 4H), 6.56 (d, ArH, JH–
3
H = 7.6 Hz, 2H), 6.86 (t, ArH, JH–H = 7.6 Hz, 1H), 8.39 (s, OCHO,
1H). 31P{1H} NMR (162 MHz, C6D6, d): 173.31 (s). IR (in toluene,
cmÀ1): 1605 (C@O). Anal. Calc. for C27H40P2O4Ni: C, 59.04; H,
7.34. Found: C, 59.06; H, 7.48%.
4.6. X-ray structure determinations
4.2. One-pot synthesis of LcPeNiCl
Crystal data collection and refinement parameters are summa-
rized in Table 2. Intensity data were collected at 150 K on a Bruker
SMART6000 CCD diffractometer using graphite-monochromated
Under an argon atmosphere, 60 mL of THF was added to a
Schlenk flask containing resorcinol (550 mg, 5.0 mmol), DMAP
(1.28 g, 10.5 mmol), anhydrous NiCl2 (648 mg, 5.0 mmol), and
chlorodicyclopentylphosphine (2.1 mL, 10.5 mmol). The resulting
mixture was refluxed for 16 h to give a deep brown solution along
with a purple/green precipitate, which was filtered through a short
column packed with silica. The filtrate was concentrated by reduc-
ing the volume under vacuum to 5 mL, followed by the addition of
anhydrous diethyl ether (ca. 10 mL) until a precipitate formed. The
solid was collected by gravity filtration and dried under vacuum to
give an orange/yellow powder (2.10 g, 78% yield). 1H NMR
(400 MHz, C6D6, d): 1.14–1.19 (m, CH2, 16H), 1.34–1.41 (m, CH2,
Cu Ka radiation, k = 1.54178 Å. The data frames were processed
Table 2
Summary of the crystallographic data.
LiPrNiOCHO
19H32O4P2Ni
445.10
150(2)
LcPeNiOCHO
Formula
Formula weight
T (K)
C
C27H40O4P2Ni
549.24
150(2)
k (Å)
Lattice
Space group
Unit cell dimensions
a (Å)
1.54178
triclinic
P1
1.54178
triclinic
P1
ꢀ
ꢀ
3
16H), 2.15–2.20 (m, PCH, 4H), 6.57 (d, ArH, JH–H = 8.0 Hz, 2H),
3
6.86 (t, ArH, JH–H = 8.0 Hz, 1H). 31P{1H} NMR (162 MHz, C6D6, d):
8.1207(2)
10.0328(2)
13.7821(3)
99.432(1)
99.873(1)
93.002(1)
1087.54(4)
2
8.6624(2)
14.8388(2)
20.8500(3)
83.670(10)
87.601(1)
89.795(1)
2661.38(8)
4
175.35 (s). Anal. Calc. for C26H39O2P2NiCl: C, 57.86; H, 7.28; Cl,
6.57. Found: C, 57.67; H, 7.25; Cl, 6.46%.
b (Å)
c (Å)
a
(°)
b (°)
4.3. Synthesis of LcPeNiH
c
(°)
V (Å3)
Under an argon atmosphere, the suspension of LcPeNiCl (200 mg,
0.37 mmol) and LiAlH4 (222 mg, 5.55 mmol) in 30 mL of toluene
was stirred at room temperature for 24 h. The resulting mixture
was filtered through a short plug of Celite to give a yellow solution.
Removal of the solvent under vacuum yielded the crude product as
a yellow solid, which was subjected to further purification by
washing it with a small amount (1 mL Â 2) of methanol (102 mg,
Z
q
l
(g cmÀ3
(mmÀ1
)
)
1.359
2.844
1.371
2.433
h (°)
3.31–67.90
9221
3743
0.0256
243
0.0352
0.0894
0.0447
0.0954
1.027
2.13–67.60
22268
9101
0.0536
622
0.0518
0.1227
0.0852
0.1394
No. reflections collected
No. unique reflections
Rint
No. variable parameters
R1 [I > 2
r
(I)]a
r
55% yield). 1H NMR (400 MHz, C6D6, d): À7.88 (t, NiH, JP–
2
wR2 [I > 2
(I)]b
R1 [all data]a
H = 54.4 Hz, 1H), 1.36–1.40 (m, CH2, 8H), 1.60–1.63 (m, CH2, 8H),
1.77–1.79 (m, CH2, 8H), 1.91–1.98 (m, CH2, 8H), 2.31–2.39 (m,
wR2 [all data]b
Goodness-of-fit (GOF) on F2
0.997
3
3
PCH, 4H), 6.87 (d, ArH, JH–H = 7.6 Hz, 2H), 7.05 (t, ArH, JH–
H = 7.6 Hz, 1H). 31P{1H} NMR (162 MHz, C6D6, d): 199.66 (s). Anal.
Calc. for C26H40O2P2Ni: C, 61.81; H, 7.98. Found: C, 61.12; H, 8.10%.
a
R1
wR2 = {
=
R
||Fo| À |Fc||/
R|Fo|.
w(F2o À F2c )2/
R .
w(F2o)2}1/2
b
R

34
S. Chakraborty et al. / Polyhedron 32 (2012) 30–34
using the program SAINT. The data were corrected for decay, Lorentz,
and polarization effects as well as absorption and beam corrections
based on the multi-scan technique. The structures were solved by a
combination of direct methods in SHELXTL and the difference Fourier
technique and refined by full-matrix least-squares procedures.
Non-hydrogen atoms were refined with anisotropic displacement
parameters. For LcPeNiOCHO, two independent molecules crystal-
lize in the lattice. A suitable two-component disorder model is ap-
plied for C25B/C25C (Fig. 3, bottom structure). Additional disorder
was observed in other cyclopentyl C-atoms; however, reasonable
multi-component disorder models were not obtained. The crystal-
lographic data for LiPrNiOCHO and LcPeNiOCHO have been depos-
ited with the Cambridge Crystallographic Data Centre (CCDC-
816918 and 816919).
4.11. Attempted catalytic hydrosilylation of CO2 with LtBuNiH
Under an argon atmosphere, a flame-dried 50 mL Schlenk flask
was charged with LtBuNiH (12.5 mg, 25
lmol), PhSiH3 (953 lL,
7.50 mmol), and 2 mL of C6D6. The mixture was degassed by a
freeze–pump-thaw cycle and placed under 1 atm of CO2. After
24 h, a small portion of the clear liquid layer (ca. 0.6 mL) was trans-
ferred into a J. Young NMR tube under an argon atmosphere. The
1H NMR spectrum of the solution showed the unreacted PhSiH3
with no evidence of forming a methanol derivative.
Acknowledgments
We thank the National Science Foundation (CHE-0952083) and
the donors of the American Chemical Society Petroleum Research
Fund (49646-DNI3) for support of this research. S. Chakraborty is
grateful to the University of Cincinnati URC for a graduate student
research fellowship. X-ray data were collected on a Bruker
SMART6000 diffractometer which was funded through an NSF-
MRI Grant (CHE-0215950).
4.7. Stoichiometric reaction of LRNiOCHO with HBcat
A screw-cap NMR tube was charged with LtBuNiOCHO or
LiPrNiOCHO (25
1.0 mg, 6.1
(8.1 L, 75
l
mol), hexamethylbenzene (internal standard,
mol), and 0.6 mL of C6D6. To this solution, HBcat
lmol) was added via a microliter syringe and the reac-
l
l
tion was monitored by 1H and 31P{1H} NMR spectroscopy. The
NMR yield was calculated by comparing the integration of CH3OB-
cat methyl resonance (3.34 ppm) with that of the internal standard
(2.12 ppm). Because the 1H NMR resonances of LcPeNiOCHO over-
lap with that of hexamethylbenzene, for the study of LcPeNiOCHO,
hexamethyldisilane (0.07 ppm) was used as the NMR internal stan-
dard. Control experiments showed that neither hexamethylben-
zene nor hexamethyldisilane interfered with the reactions.
Appendix A. Supplementary data
CCDC 816918 and 816919 contain the supplementary crystallo-
graphic data for LiPrNiOCHO and LcPeNiOCHO. These data can be ob-
tained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
4.8. Stoichiometric reaction of LtBuNiOCHO with boranes
References
A screw-cap NMR tube was charged with LtBuNiOCHO (12.5 mg,
[1] H. Arakawa, M. Aresta, J.N. Armor, M.A. Barteau, E.J. Beckman, A.T. Bell, J.E.
Bercaw, C. Creutz, E. Dinjus, D.A. Dixon, K. Domen, D.L. DuBois, J. Eckert, E.
Fujita, D.H. Gibson, W.A. Goddard, D.W. Goodman, J. Keller, G.J. Kubas, H.H.
Kung, J.E. Lyons, L.E. Manzer, T.J. Marks, K. Morokuma, K.M. Nicholas, R.
Periana, L. Que, J. Rostrup-Nielson, W.M.H. Sachtler, L.D. Schmidt, A. Sen, G.A.
Somorjai, P.C. Stair, B.R. Stults, W. Tumas, Chem. Rev. 101 (2001) 953.
[2] K. Tominaga, Y. Sasaki, M. Kawai, T. Watanabe, M. Saito, J. Chem. Soc. Chem.,
Commun. (1993) 629.
[3] T. Matsuo, H. Kawaguchi, J. Am. Chem. Soc. 128 (2006) 12362.
[4] A. Berkefeld, W.E. Piers, M. Parvez, J. Am. Chem. Soc. 132 (2010) 10660.
[5] A.E. Ashley, A.L. Thompson, D. O’Hare, Angew. Chem., Int. Ed. 48 (2009) 9839.
[6] G. Ménard, D.W. Stephan, J. Am. Chem. Soc. 132 (2010) 1796.
[7] T.C. Eisenschmid, R. Eisenberg, Organometallics 8 (1989) 1822.
[8] S.N. Riduan, Y. Zhang, J.Y. Ying, Angew. Chem., Int. Ed. 48 (2009) 3322.
[9] S. Chakraborty, J. Zhang, J.A. Krause, H. Guan, J. Am. Chem. Soc. 132 (2010)
8872.
25
12.5
(75
l
mol), hexamethylbenzene (internal standard, 2.0 mg,
mol), and 0.6 mL of C6D6. To this solution, 9-BBN or HBpin
mol) was added via a microliter syringe and the reaction
l
l
was monitored by 1H NMR and 31P{1H} spectroscopy. After 1 h
reaction with 9-BBN, LtBuNiH was the only nickel species observed
by 31P{1H} spectroscopy. The boron-containing products were
identified as the known B-methoxy-9-BBN [25] and a borate ester
[26]. The reaction of HBpin led to the quantitative formation of
LtBuNiH and HCOOBpin within 15 min.
4.9. Attempted stoichiometric reaction of LtBuNiOCHO with PhSiH3
[10] F. Huang, C. Zhang, J. Jiang, Z.-X. Wang, H. Guan, Inorg. Chem. 50 (2011) 3816.
[11] S. Chakraborty, J.A. Krause, H. Guan, Organometallics 28 (2009) 582.
[12] J. Zhang, C.M. Medley, J.A. Krause, H. Guan, Organometallics 29 (2010) 6393.
[13] V. Gómez-Benítez, O. Baldovino-Pantaleón, C. Herrera-Álvarez, R.A. Toscano, D.
Morales-Morales, Tetrahedron Lett. 47 (2006) 5059.
[14] V. Pandarus, D. Zargarian, Organometallics 26 (2007) 4321.
[15] M.A. Solano-Prado, F. Estudiante-Negrete, D. Morales-Morales, Polyhedron 29
(2010) 592.
A screw-cap NMR tube was charged with LtBuNiOCHO (12.5 mg,
25 lmol), PhSiH3 (3.2 lL, 25 lmol), and 0.6 mL of C6D6. The pro-
gress of the reaction was monitored by 1H and 31P{1H} NMR spec-
troscopy. No appreciable reaction was observed within 16 h.
4.10. Catalytic hydroboration of CO2 with LRNiH
[16] D. Morales-Morales, C.M. Jensen (Eds.), The Chemistry of Pincer Compounds,
Elsevier, Amsterdam, The Netherlands, 2007.
[17] D. Morales-Morales, Mini-Rev. Org. Chem. 5 (2008) 141.
[18] P.G. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 95 (1995) 259.
[19] W. Leitner, Angew. Chem., Int. Ed. Engl. 34 (1995) 2207.
[20] P.G. Jessop, F. Joó, C.-C. Tai, Coord. Chem. Rev. 248 (2004) 2425.
[21] D.J. Darensbourg, M.Y. Darensbourg, L.Y. Goh, M. Ludvig, P. Wiegreffe, J. Am.
Chem. Soc. 109 (1987) 7539.
[22] M.Y. Darensbourg, M. Ludwig, C.G. Riordan, Inorg. Chem. 28 (1989) 1630.
[23] T.J. Schmeier, N. Harari, C.D. Incarvito, J.A. Raskatov, Chem. Commun. 47
(2011) 1824.
[24] M.F. Laird, M. Pink, N.P. Tsvetkov, H. Fan, K.G. Caulton, Dalton Trans. (2009)
1283.
[25] G.W. Kramer, H.C. Brown, J. Organomet. Chem. 73 (1974) 1.
[26] R. Köster, Y.-H. Tsay, C. Krüger, J. Serwatowski, Chem. Ber. 119 (1986) 1174.
Under an argon atmosphere, a flame-dried 50 mL Schlenk flask
was charged with LRNiH (25
l
mol), HBcat (272
and 2 mL of C6D6. To this solution, hexamethylbenzene (10.0 mg,
62.5 mol) was added as an internal standard. The mixture was de-
lL, 2.50 mmol),
l
gassed by a freeze–pump-thaw cycle and placed under 1 atm of
CO2. At different time intervals, a small portion of the clear liquid
layer (ca. 0.6 mL) was withdrawn from the flask and transferred
into a J. Young NMR tube under an argon atmosphere. From the
1H NMR spectrum of the aliquot, turnover number (TON) was cal-
culated by comparing the integration of CH3OBcat methyl reso-
nance (3.34 ppm) with that of the internal standard (2.12 ppm).