Nickel-catalyzed release of H2 from formic acid and a new method for the synthesis of zerovalent Ni(PMe3)4

Article abstract of DOI:10.1039/c6dt01499b

Ni(PMe₃)₄ serves as a catalyst for the release of H₂ and CO₂ from formic acid. The capacity of Ni(PMe₃)₄ to achieve this transformation is linked to the ability of the PMe₃ ligand to induce decarboxylation, as illustrated by the observation that both Ni(py)₄(O₂CH)₂ and Ni(O₂CH)₂·2H₂O react with PMe₃ to afford Ni(PMe₃)₄; the latter transformation also provides a convenient method for the synthesis of a zerovalent nickel compound.

Full text of DOI:10.1039/c6dt01499b

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Nickel-catalyzed release of H2 from formic
acid and a new method for the synthesis of
Cite this: DOI: 10.1039/c6dt01499b
zerovalent Ni(PMe3)4†
Michelle C. Neary and Gerard Parkin*
Ni(PMe3)4 serves as a catalyst for the release of H2 and CO2 from formic acid. The capacity of Ni(PMe3)4
to achieve this transformation is linked to the ability of the PMe3 ligand to induce decarboxylation, as illus-
trated by the observation that both Ni(py)4(O2CH)2 and Ni(O2CH)2·2H2O react with PMe3 to afford
Ni(PMe3)4; the latter transformation also provides a convenient method for the synthesis of a zerovalent
nickel compound.
Received 19th April 2016,
Accepted 29th April 2016
DOI: 10.1039/c6dt01499b
www.rsc.org/dalton
Ni(PMe3)4, Pd(PMe3)4, and Pt(PMe3)4.10 Such compounds are
well-known to be highly reactive due to the strong σ-donor and
Introduction
The growing demand for energy has prompted much effort in weak π-acceptor character of PMe3, which results in metal
the utilization of fuels that are more sustainable than those centers that are classified as “electron rich”. For example,
based on fossil feedstocks.1 For example, hydrogen, which can Ni(PMe3)4 11–13 exhibits diverse reactivity, which includes ligand
be generated by renewable means, is a promising alternative exchange and Si–H, O–H, S–H, C–S, S–S, and C–halogen bond
because it can be used in either a fuel cell or a combustion cleavage transformations.14 In view of such reactivity, we have
engine.2 A problem with the use of hydrogen as a fuel, examined the ability of Ni(PMe3)4 to serve as a catalyst for the
however, is that current storage and transportation techniques release of H2 from formic acid. Significantly, Ni(PMe3)4 does
are inadequate.3 Therefore, much attention has been directed indeed catalyze this transformation (Scheme 1)15,16 and can
towards discovering both physical and chemical methods to decarboxylate formic acid at 80 °C with a turnover number
provide hydrogen on demand.4 Formic acid, in particular, has (TON) of 70 and an initial turnover frequency (TOF) of 1.7 h−1
.
garnered interest as a chemical medium for storing H2 This activity is, however, modest in comparison to other
because it is a liquid at room temperature and, as a conse- nonprecious metal catalysts; for example, [{P(CH2CH2PPh2)3}-
quence, is easy to handle and transport.5 The utilization of FeH(H2)][BPh4] possesses
a
TOF of 335 h−1 8a,17,18
TOF of 520 h−1 8b
effective catalysts to release H2 on demand; indeed, both hetero- [HN(CH2CH2PPri2)2]Fe(CO)(O2CH)H achieves
TOF of
geneous and homogeneous catalyst systems have been investi- 196 728 h−1 8g
and a bis(imino)pyridine aluminum complex,
,
formic acid as a storage medium for H2, however, requires [κ3-C5H3N(CH2PBut2)2]Fe(CO)H2 has
a
,
a
,
gated.6,7 In particular, there is much interest in the discovery of
[ . The only other
PhI2P]AlH(THF), has a TOF of 5200 h−1 8d
catalysts that utilize earth abundant nonprecious metals.8,9 example of a nickel compound, namely [κ3-C6H3(CH2PBut2)2]NiH,
Therefore, we describe here the ability of a nickel compound to that has recently been reported to release H2 from formic acid
serve as a catalyst for the release of H2 from formic acid.
has an initial TOF of 240 h−1 8c,19
. However, Ni(PMe3)4 can
achieve the catalytic conversion in absence of a base, whereas
[κ3-C6H3(CH2PBut2)2]NiH requires the presence of an amine
(either Et3N or OctNH2) to achieve catalysis.8c
The essential features of the mechanism of the catalytic
cycle are proposed to involve (i) formation of a nickel formate
species, (ii) decarboxylation of the nickel formate to afford
a nickel hydride intermediate, and (iii) release of H2. For
Results and discussion
Zerovalent trimethylphosphine compounds of the class
M(PMe3)n are known for a variety of transition metals,
e.g. Mo(PMe3)6, W(PMe3)6, Fe(PMe3)5, Os(PMe3)5, Co(PMe3)4,
Department of Chemistry, Columbia University, New York, NY 10027, USA.
E-mail: parkin@columbia.edu
†Electronic supplementary information (ESI) available: Cartesian coordinates
for geometry optimized structures. See DOI: 10.1039/c6dt01499b
Scheme 1
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Table 1 ΔG values of intermediates (kcal mol−1) relative to Ni(PMe3)4
+
example, one possible sequence, as illustrated in Scheme 2,
involves oxidative addition of formic acid to give a five-co-
ordinate formate-hydride, namely Ni(PMe3)3(O2CH)H,20 which
exists in equilibrium with four-coordinate Ni(PMe3)2(O2CH)H.21,22
Subsequent elimination of CO2 forms a dihydride inter-
mediate, Ni(PMe3)2H2,23,24 which releases H2 by either reduc-
tive elimination or by reaction with formic acid. Support for
one step in this type of mechanism is provided by Darens-
bourg’s proposal that Ni(PCy3)2(O2CH)H undergoes exchange
with 13CO2 by a mechanism that involves decarboxylation
and the formation of an unstable dihydride intermediate,
Ni(PCy3)2H2.25,26 In addition to the possibility of H2 release
occurring via Ni(PMe3)2H2, it is also possible that the release
occurs via five-coordinate Ni(PMe3)3H2, which is anticipated to
be in equilibrium with Ni(PMe3)2H2.21,27 Density functional
theory calculations illustrate that the proposed four-coordinate
square-planar28 and five-coordinate29 trigonal bipyramidal
intermediates (Fig. 1) are thermodynamically accessible from
Ni(PMe3)4 and HCO2H (Table 1) and thereby provide support
for the pathways illustrated in Scheme 2.
21(HCO2H)2 in the gas phase
x = 3
x = 2
Ni(PMe3)x(O2CH)H + (4 − x) PMe3
Ni(PMe3)xH2 + CO2 + (4 − x) PMe3
−3.4
8.6
−12.3
0.8
In view of the proposal that the catalytic mechanism for the
release of H2 from formic acid involves decarboxylation of a
formate intermediate, we sought to examine if coordination of
PMe3 to a well-defined nickel formate complex could induce
the transformation. Significantly, we observed that addition of
PMe3 to the pyridine complex, Ni(py)4(O2CH)2,30 results in
the release of CO2 and H2 at room temperature, with the
accompanying formation of Ni(PMe3)4 (Scheme 3). Further-
more, PMe3 also reacts with a suspension of Ni(O2CH)2·2H2O
(which, in the solid state, features an extended structure with
bridging formate ligands)31 in benzene to release CO2 and H2
at 60 °C and form Ni(PMe3)4 (Scheme 3).32,33
The ability of PMe3 to induce the room temperature decar-
boxylation of a nickel formate species is of considerable sig-
nificance in view of the fact that the reverse reaction, i.e. the
insertion of CO2 into a M–H bond, is more commonly
observed.34 However, despite the fact that many metal-hydride
compounds insert CO2 to afford a formate species, there are
several 13CO2 isotopic exchange studies that provide evidence
for the occurrence of the reverse transformation, as illustrated
by experiments involving [M(CO)5(O2CH)]− (M = Cr, Mo, W)34a
and Ni(PCy3)2(O2CH)H.25 Furthermore, some metal formate
compounds, e.g. Re(PMe3)4(CO)(O2CH),34c M(dppp)(CO)3(O2CH)
34f
(M = Mn, Re),35 CpRe(NO)(PPh3)(O2CH),36 Fe(dmpe)2(O2CH)2
CpMo(PMe3)2(CO)(O2CH)8f and [PR2OCOPR2]Ni(O2CH),34m,n are
known to dissociate CO2 at either elevated temperatures or
upon removal of the CO2 atmosphere.
Scheme 2
With respect to the mechanism of the PMe3 induced decar-
boxylation, a plausible sequence involves the initial co-
ordination of PMe3 to afford
a four-coordinate adduct,
Ni(PMe3)2(O2CH)2. Two decarboxylation events would sequen-
tially form Ni(PMe3)2(O2CH)H and Ni(PMe3)2H2, of which the
latter would be expected to release H2 in the presence of PMe3
to form Ni(PMe3)4, as illustrated in Scheme 4. Precedent for
this mechanism is provided by the observations that (i) the
tricyclohexylphosphine counterpart, Ni(PCy3)2(O2CH)H, is a
known compound and (ii) Ni(PCy3)2(O2CH)H undergoes facile
reversible decarboxylation to generate Ni(PCy3)2H2 (vide supra).25
Density functional theory calculations also indicate that
Fig. 1 Geometry optimized structures of proposed intermediates for
decarboxylation of formic acid (methyl group hydrogen atoms are
omitted for clarity).
Scheme 3
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Ni(py)4(O2CH)2 and Ni(O2CH)2·2H2O react with PMe3 to afford
Ni(PMe3)4. The latter transformation also provides a convenient
method for the synthesis of a zerovalent nickel compound.
Experimental section
General considerations
All manipulations were performed using a combination of
glovebox, high vacuum and Schlenk techniques under a nitrogen
or argon atmosphere.42 Solvents were purified and degassed by
standard procedures. NMR spectra were measured on Bruker
Scheme 4
this pathway is feasible. For example, decarboxylation of
Ni(PMe3)2(O2CH)2 is thermodynamically (ΔG) downhill by
2.4 kcal mol−1, while decarboxylation of Ni(PMe3)2(O2CH)H is
only uphill by 13.0 kcal mol−1; subsequent dissociation of H2
and formation of Ni(PMe3)4 from Ni(PMe3)2H2 is downhill by
7.7 kcal mol−1 (Scheme 4).37
1
300 DRX and Bruker Avance 500 DMX spectrometers. H NMR
spectra are reported in ppm relative to SiMe4 (δ = 0) and were
referenced internally with respect to the protio solvent impurity
(δ = 7.16 for C6D5H).43 13C NMR spectra are reported in ppm
relative to SiMe4 (δ = 0) and were referenced internally with
respect to the solvent (δ = 128.06 for C6D6).43 31P NMR chemi-
cal shifts are reported in ppm relative to 85% H3PO4 (δ = 0)
Finally, in addition to the PMe3 induced decarboxylation of
formate being of mechanistic interest, it is also of synthetic
relevance since it provides a very useful method for obtaining
Ni(PMe3)4, which was first reported in 1970 by Tolman12 and
by Klein and Schmidbaur.13 Specifically, Tolman described the
synthesis of Ni(PMe3)4 by the reaction of bis(1,5-cycloocta-
diene)nickel(0), Ni(COD)2, with PMe3,12 while Klein and
Schmidbaur described syntheses that involved (i) reduction of
Ni(II) salts by PMe3 in basic aqueous solution and (ii) the reac-
tion of (Me3P)2NiCl2 with aqueous-alcoholic solutions of either
KOH or NaOSiMe3;13 in both of the latter cases, Me3PO is
formed as a byproduct, while the reactions with KOH and
NaOSiMe3 are also accompanied by the formation of NiX2 (X =
OH, OSiMe3). Although a variety of other syntheses of
Ni(PMe3)4 have subsequently been reported, including (i) the
reaction of Ni(PMe3)2Me2 with PMe3,38 (ii) the reduction of
NiCl2 by Na(Hg) in the presence of PMe3,39 and (iii) the co-
condensation of nickel atoms with PMe3 via metal-vapor syn-
thesis techniques,40 the direct synthesis from Ni(O2CH)2·2H2O
offers several advantages. For example, the byproducts (CO2,
H2 and H2O) are easy to remove, whereas other methods gene-
rate byproducts, e.g. Me3PO, NiX2 (X = OH, OSiMe3), NaCl, and
Hg, that require additional steps for their removal. Further-
more, the direct synthesis from Ni(O2CH)2·2H2O (i) obviates the
need to prepare starting materials such as Ni(PMe3)2Me2 or
Ni(COD)2,41 (ii) does not require the use of mercury and an
alkali metal reductant, and (iii) does not require highly special-
ized metal-vapor synthesis equipment.
1
and were referenced electronically by using the H resonance
frequency of SiMe4.44 Chemicals were obtained from
Sigma-Aldrich [formic acid and mesitylene], Alfa Aesar
[Ni(O2CH)2·2H2O] and Cambridge Isotope Laboratories
[H13CO2H], and used as supplied. Ni(py)4(O2CH)2 was
obtained by the literature method.30
Computational details
Calculations were carried out using DFT as implemented in
the Jaguar 8.9 (release 15) suite of ab initio quantum chemistry
programs.45 Geometry optimizations were performed with the
B3LYP density functional using the 6-31G**++ (H, C, O, P) and
LACVP (Ni) basis sets, which were also used to determine
Gibbs free energy values at 1 atm and 298.15 K. The energies
of the optimized structures in the gas phase were also re-
evaluated by additional single point calculations on each opti-
mized geometry using the cc-pVTZ(-f)++ correlation consistent
triple-ζ (H, C, O, P) and LAV3P (Ni) basis sets. Cartesian coordi-
nates and energies of the geometry optimized structures are
provided in the ESI.†
Synthesis of Ni(PMe3)4 from Ni(O2CH)2·2H2O
A green suspension of Ni(O2CH)2·2H2O (500 mg, 2.71 mmol)
in benzene (ca. 30 mL) was cooled to −196 °C and treated with
PMe3 (2.0 mL, 20 mmol) via vapor transfer. The mixture was
heated overnight at 60 °C under a nitrogen atmosphere that
was open to a bubbler. Over this period, the mixture became
bright yellow and produced an orange oily deposit. The volatile
components were removed in vacuo, and the residue was
extracted into pentane (ca. 30 mL). The solvent was removed
from the pentane extract in vacuo to give Ni(PMe3)4 as a yellow
powder (639 mg, 65% yield), which was identified by 1H,
31P{1H} and 13C{1H} NMR spectroscopy.40
Conclusions
In summary, Ni(PMe3)4 has been shown to catalyze the conver-
sion of formic acid to H2 and CO2 via a mechanism that is
proposed to involve the decarboxylation of a nickel formate
intermediate, followed by elimination of H2 from a nickel di-
hydride species. The capacity of Ni(PMe3)4 to achieve this
Synthesis of Ni(PMe3)4 from Ni(py)4(O2CH)2
transformation is linked to the ability of PMe3 to induce de- A green suspension of Ni(py)4(O2CH)2 (150 mg, 0.322 mmol)
carboxylation, as illustrated by the observation that both in benzene (ca. 5 mL) was cooled to −196 °C and treated with
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PMe3 (0.20 mL, 2.0 mmol) via vapor transfer. The mixture was
stirred overnight at room temperature in a sealed ampoule.
Over this period, the mixture became bright yellow and pro-
duced an orange oily deposit. The volatile components were
removed in vacuo, and the residue was extracted into pentane
(ca. 6 mL). The solvent was removed from the pentane extract
in vacuo to give Ni(PMe3)4 as a yellow powder (51 mg, 44%
yield) which was identified by 1H, 31P{1H} and 13C{1H} NMR
spectroscopy.40
2 S. Satyapal, J. Petrovic, C. Read, G. Thomas and G. Ordaz,
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C. Würtele, W. H. Bernskoetter, N. Hazari and
S. Schneider, J. Am. Chem. Soc., 2014, 136, 10234–10237.
9 In addition, the reverse reaction, i.e. the hydrogenation of
CO2 by nonprecious metals, has also received attention.
See, for example: (a) M. S. Jeletic, M. L. Helm, E. B. Hulley,
M. T. Mock, A. M. Appel and J. C. Linehan, ACS Catal.,
2014, 4, 3755–3762; (b) M. S. Jeletic, M. T. Mock,
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J. F. Hull, D. J. Szalda, J. T. Muckerman, Y. Himeda and
E. Fujita, Inorg. Chem., 2013, 52, 12576–12586; (h) X. Yang,
Chem. Commun., 2015, 51, 13098–13101.
Decarboxylation of formic acid catalyzed by Ni(PMe3)4
Formic acid (5.0 μL, 0.13 mmol) was added to a solution of
Ni(PMe3)4 (5.0 mg, 0.014 mmol) in C6D6 (ca. 0.7 mL) containing
mesitylene (3.0 μL, 0.022 mmol) as an internal standard. The
sample was examined by NMR spectroscopy, thereby demon-
strating the immediate formation of a species tentatively
identified as [Ni(PMe3)4H]+ [1H NMR (C6D6): −17.76 [s, 1H,
NiH], 0.88 [s, 36H, Ni(PMe3)4]. 31P{1H} NMR (C6D6): −20.11 [s]].46
The mixture was then heated to 80 °C and monitored by
1H and 31P{1H} NMR spectroscopy. Most of the formic acid
was consumed after a period of 1 day, at which point another
portion of formic acid (5.0 μL, 0.13 mmol) was added, and the
mixture was heated again at 80 °C. A total of seven aliquots of
formic acid were added over a period of 14 days, corresponding
to a turnover number of 70. The initial turnover frequency was
1.7 h−1 (based on the first two hours), whereas the turnover
frequency over the entire experiment was 0.2 h−1. The for-
mation of H2 was observed by 1H NMR spectroscopy, while the
formation of CO2 was confirmed by examining the corres-
ponding reaction of H13CO2H with 13C{1H} NMR spectroscopy
(see below). During the course of the catalysis, Ni(PMe3)4 con-
verts to Ni(PMe3)3(CO) [1H NMR (C6D6): 1.09 [s]. 31P{1H} NMR
(C6D6): −18.56 [s]],47 and a small quantity of methyl formate
arising from disproportionation is also observed.
Decarboxylation of H13CO2H catalyzed by Ni(PMe3)4
H13CO2H (5.0 μL, 0.13 mmol) was added to a yellow solution
of Ni(PMe3)4 (5.0 mg, 0.014 mmol) in C6D6 (ca. 0.7 mL). The
sample was heated to 80 °C and monitored by 1H, 31P{1H} and
13C{1H} NMR spectroscopy, which demonstrated the formation
of H2 and 13CO2 over a period of 5 days. A trace quantity of
13CH3OH is also observed.
Acknowledgements
We thank the US Department of Energy, Office of Basic Energy
Sciences (DE-FG02-93ER14339) for support of this research.48
M. C. N. acknowledges the National Science Foundation for a
Graduate Research Fellowship under Grant No. DGE 11-44155.
10 See, for example: D. Rabinovich, R. Zelman and G. Parkin,
J. Am. Chem. Soc., 1992, 114, 4611–4621 and references therein.
11 H.-F. Klein, Angew. Chem., Int. Ed. Engl., 1980, 19, 362–375.
12 (a) C. A. Tolman, J. Am. Chem. Soc., 1970, 92, 2956–2965;
(b) S. D. Ittel, in Inorganic Syntheses, ed. A. G. MacDiarmid,
McGraw-Hill Inc., New York, 1977, vol. 17, pp. 117–124.
13 H.-F. Klein and H. Schmidbaur, Angew. Chem., Int. Ed.
Engl., 1970, 9, 903–904.
Notes and references
1 M. Z. Jacobson, Energy Environ. Sci., 2009, 2, 148–173.
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14 See, for example: (a) J. Grobe, R. Wehmschulte, B. Krebs 21 Four- and five-coordinate (Me3P)2NiX2 and (Me3P)3NiX2 com-
and M. Läge, Z. Anorg. Allg. Chem., 1995, 621, 583–596;
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release H2 and form Ni(PMe3)2(O2CH)2 (ΔG = −4.5 kcal mol−1),
which undergoes subsequent decarboxylation to regenerate
Ni(PMe3)2(O2CH)H (ΔG = −2.4 kcal mol−1).
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spectroscopic evidence for their existence has been
presented. See: (a) R. A. Schunn, Inorg. Chem., 1976, 15,
208–212; (b) C. S. Cundy, J. Organomet. Chem., 1974, 69,
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( j) Q. Niu, X. Zhang, S. Zhang, X. Li and H. Sun, Inorg. 24 The dimethyl counterparts, Ni(PMe3)2Me2 and Ni(PMe3)3Me2,
Chim. Acta, 2015, 426, 165–170.
are known. See: H.-F. Klein and H. H. Karsch, Chem. Ber.,
1972, 105, 2628–2636.
15 The formation of H2 was demonstrated by 1H NMR spec-
troscopy, while the formation of CO2 was demonstrated by 25 (a) D. J. Darensbourg, P. Wiegreffe and C. G. Riordan, J. Am.
13C{1H} NMR spectroscopic studies on the reaction employ-
ing 13C labeled formic acid.
Chem. Soc., 1990, 112, 5759–5762; (b) D. J. Darensbourg,
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16 Ni(PMe3)3(CO) is formed during the course of the catalysis,
which could be a consequence of either the formation of 26 Although Ni(PCy3)2H2 has not been isolated, palladium
CO via dehydration of formic acid or the reaction of a
nickel species with CO2. In this regard, Ni(PMe3)4 has been
reported to react with CO2 to form Ni(PMe3)3(CO). See:
N. Huang, X. Li, W. Xu and H. Sun, Inorg. Chim. Acta, 2013,
394, 446–451.
and platinum counterparts are known and have a trans-
arrangement of PR3 ligands.a,b Furthermore, the platinum
complex, Pt(PEt3)2H2, has been used as a catalyst for the
decarboxylation of formic acid.c (a) K. Kudo, M. Hidai and
Y. Uchida, J. Organomet. Chem., 1973, 56, 413–418;
(b) A. Immirzi and A. Musco, Inorg. Chim. Acta, 1977, 22,
L35–L36; (c) R. S. Paonessa and W. C. Trogler, J. Am. Chem.
Soc., 1982, 104, 3529–3530.
17 [{P(CH2CH2PPh2)3}FeH(H2)][BPh4] was first reported as a
catalyst for the dehydrogenation of formic acid in 1991, but
TON and TOF data were not given. See: C. Bianchini,
M. Peruzzini, A. Polo, A. Vacca and F. Zanobini, Gazz. 27 We note that it is also possible that the decarboxylation
Chim. Ital., 1991, 121, 543–549.
step could occur via five-coordinate Ni(PMe3)3(O2CH)H;
however, the availability of a vacant coordination site in
four-coordinate Ni(PMe3)2(O2CH)H would be expected to
facilitate the transformation.
18 Furthermore, an in situ mixture of Fe(BF4)2·6H2O and
[P(CH2CH2PPh2)3] can achieve a TOF of 9425 h−1 at 80 °C.
See ref. 8a.
19 For the heterogeneous decomposition of formic acid on 28 The four-coordinate compounds illustrated in Fig. 1,
nickel surfaces, see: (a) E. Iglesia and M. Boudart,
J. Catal., 1984, 88, 325–332; (b) R. J. Ruka, L. O. Brockway
and J. E. Boggs, J. Am. Chem. Soc., 1959, 81, 2930–2933.
20 The formation of Ni(PMe3)3(O2CH)H is proposed to
occur via a sequence that involves protonation of the
metal center rather than oxidative-addition of the
O–H bond. Specifically, a species tentatively identified as
[Ni(PMe3)4H]+ can be observed by 1H and 31P NMR spectro-
scopy upon treatment of Ni(PMe3)4 with formic acid at
room temperature. Precedent for such a protonation step is
provided by the report that Ni(PMe3)4 reacts with phenols
to afford [Ni(PMe3)4H][OAr].a Furthermore, diphosphine
variants, such as [(dmpe)2NiH]+, can be obtained by
protonation with mild acids such as [NH4]+.b,c (a) Ref. 14c;
namely Ni(PMe3)2(O2CH)H and Ni(PMe3)2H2, are rep-
resented with a trans-arrangement of PMe3 ligands. We
have also performed calculations on the cis-counterparts,
and these are higher in energy (ESCF) by 10.8 and 2.2
kcal mol−1, respectively (cc-pVTZ(-f)++ and LAV3P basis sets).
We also note that, in addition to palladium and platinum
dihydrides, M(PR3)2H2 (M = Pd, Pt), possessing a trans-
arrangement of PR3 ligands (ref. 26), four-coordinate nickel
compounds such as Ni(PMe3)2Me2 and Ni(PMe3)2(Me)Cl
also adopt trans structures (ref. 24). Nevertheless, although
the calculated energy of trans-Ni(PMe3)2H2 is lower than
that of the cis isomer, the difference is sufficiently small
that the latter would be accessible for the step involving
reductive elimination of H2.
(b) A. Miedaner, D. L. DuBois, C. J. Curtis and 29 D. M. Roddick and D. Zargarian, Inorg. Chim. Acta, 2014,
R. C. Haltiwanger, Organometallics, 1993, 12, 299–303; 422, 251–264.
(c) D. E. Berning, B. C. Noll and D. L. DuBois, J. Am. Chem. 30 D. V. Soldatov and J. Lipkowski, J. Struct. Chem., 1995, 36,
Soc., 1999, 121, 11432–11447.
979–982.
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33 The transformation is also noteworthy in regards to the
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37 Pathways involving tris(PMe3) complexes are also feasible,
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the United States Government or any agency thereof.
40 A. G. Avent, F. G. N. Cloke, J. P. Day, E. A. Seddon, K. R. Seddon
and S. M. Smedley, J. Organomet. Chem., 1988, 341, 535–541.
41 It should be noted that, although commercially available,
the synthesis of Ni(COD)2 involves the use of alkylmetal or
Dalton Trans.
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