Alkene hydrogenation catalyzed by nickel hydride complexes of an aliphatic PNP pincer ligand

Article abstract of DOI:10.1002/ejic.201200758

To investigate metal-ligand cooperativity as a strategy for promoting nickel-catalyzed alkene hydrogenation, cationic and neutral nickel(II) hydride complexes of the aliphatic pincer ligand PNHP^Cy {PNHP^Cy = HN[CH₂CH₂P(Cy)₂]₂} have been synthesized and characterized. Cationic hydride complex [(PNHP ^Cy)Ni(H)]BPh₄ (2) catalyzed the hydrogenation of styrene and 1-octene under mild conditions. Only low conversion was observed in the hydrogenation of 3,5-dimethoxybenzaldehyde using 2. The neutral hydride complex (PNP^Cy)Ni(H) (3) was also found to be an alkene hydrogenation catalyst. Mechanistic experiments suggest that for catalyst 2, the hydrogenation reaction proceeds through a pathway involving initial insertion of the alkene into the Ni-H bond. Contrary to the initial hypothesis, reactivity comparisons with the methyl-substituted hydride complex [(PNMeP^Cy)Ni(H)]BPh ₄ {PNMeP^Cy = (CH₃)N[CH₂CH ₂P(Cy)₂]₂} suggest that metal-ligand cooperativity is not involved in these rare examples of mild and homogeneous nickel hydrogenation catalysis. Copyright

Full text of DOI:10.1002/ejic.201200758

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
FULL PAPER
DOI: 10.1002/ejic.201200758
Alkene Hydrogenation Catalyzed by Nickel Hydride Complexes of an Aliphatic
PNP Pincer Ligand
Kalyan V. Vasudevan,[a] Brian L. Scott,[a] and Susan K. Hanson*[a]
Keywords: Homogeneous catalysis / Nickel / Hydride ligands / Hydrogenation / Earth-abundant metals / Metal–ligand
cooperativity
To investigate metal–ligand cooperativity as a strategy for
promoting nickel-catalyzed alkene hydrogenation, cationic
and neutral nickel(II) hydride complexes of the aliphatic
pincer ligand PNHPCy {PNHPCy = HN[CH2CH2P(Cy)2]2} have
been synthesized and characterized. Cationic hydride com-
plex [(PNHPCy)Ni(H)]BPh4 (2) catalyzed the hydrogenation
of styrene and 1-octene under mild conditions. Only low con-
version was observed in the hydrogenation of 3,5-dimeth-
tion catalyst. Mechanistic experiments suggest that for cata-
lyst 2, the hydrogenation reaction proceeds through a path-
way involving initial insertion of the alkene into the Ni–H
bond. Contrary to the initial hypothesis, reactivity compari-
sons with the methyl-substituted hydride complex
[(PNMePCy)Ni(H)]BPh4 {PNMePCy
=
(CH3)N[CH2CH2P-
(Cy)2]2} suggest that metal–ligand cooperativity is not in-
volved in these rare examples of mild and homogeneous
oxybenzaldehyde using 2. The neutral hydride complex nickel hydrogenation catalysis.
(PNPCy)Ni(H) (3) was also found to be an alkene hydrogena-
promise of metal–ligand cooperativity for increasing the ac-
tivity of nickel catalysts.
Introduction
A major challenge in sustainable chemistry is the replace-
ment of precious metal catalysts with earth-abundant metal
alternatives.[1–3] Designing substitutes for highly active and
versatile precious metal hydrogenation catalysts has proven
to be particularly challenging, perhaps due to a reduced
propensity of 1st-row transition metals to undergo two-elec-
tron oxidative addition and reductive elimination reactions.
Nickel complexes have been gaining increasing attention as
versatile catalysts with a high functional group tolerance,[4]
and have proven effective for a range of transformations,
including cross coupling reactions,[5,6] olefin polymeriza-
tion,[7,8] and hydrosilylation.[9–11] However, despite the at-
tractive features of nickel catalysts, examples of homo-
geneous nickel-catalyzed hydrogenations are extremely
scarce.[12–14]
Related work by DuBois, Bullock, and Caulton has ex-
plored an alternate strategy for hydrogen activation, that of
incorporating a base into the ligand scaffold to promote the
heterolytic activation of hydrogen.[16–19] Caulton and co-
workers found a NiII pincer complex [(PNPЈ)Ni]BArF
4
{PNPЈ = –N[SiMe2CH2P(tBu)2]2} which was proposed to
heterolytically cleave H2 through a pathway having NiIV di-
hydride character (Scheme 1).[18] This unusual example of
H2 activation by a T-shaped NiII center is thought to be
facilitated by a cooperative interaction between the metal
center and the highly basic amido nitrogen of the pincer
ligand. The concept of metal–ligand “bifunctional” cataly-
sis[20] has also been successfully employed in precious metal
catalysts for the hydrogenation of polar multiple
bonds.[21–26]
Very recently, Harman and Peters reported an example
of a diphosphane–borane nickel complex capable of styrene
hydrogenation at room temperature.[15] Hydrogen activation
was proposed to proceed through a cooperative metal–li-
gand interaction where the nickel(0) complex accepted a
proton and the borane on the ligand served as a hydride
acceptor. The few prior examples of homogeneous nickel-
catalyzed hydrogenations involved high temperatures
(Ͼ150 °C) or high pressures (50 atm),[12–14] suggesting the
Scheme 1. Activation of hydrogen at a nickel(II) center reported by
Caulton and co-workers.[18]
Inspired by these studies, we envisioned a pathway for
nickel-catalyzed alkene hydrogenation that would involve
metal–ligand cooperativity (Scheme 2). Heterolytic acti-
vation of hydrogen by a nickel–amido complex could gener-
ate a nickel(II) hydride complex. Insertion of a C=C bond
into the Ni–H would generate a nickel(II) alkyl complex.
[a] Chemistry and Materials Physics Applications Divisions, Los
Alamos National Laboratory,
MS J582, Los Alamos, NM 87545, USA
E-mail: skhanson@lanl.gov
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200758.
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
K. V. Vasudevan, B. L. Scott, S. K. Hanson
FULL PAPER
Finally, C–H bond formation from the nickel alkyl (proton-
ation of the alkyl ligand by the NH group) would release
alkane product and complete the catalytic cycle.
Scheme 2. Proposed metal–ligand cooperativity pathway for nickel-
catalyzed alkene hydrogenation.
To assess the potential of metal–ligand cooperativity for
promoting nickel-catalyzed hydrogenation, we have studied
nickel complexes of an aliphatic PNP pincer. Previous work
by Schneider, Abdur-Rashid, and others has demonstrated
the utility of this type of ligand for supporting a number of
different Ru, Ir, and Pd catalysts.[27–30] The aliphatic PNP
framework can stabilize both amido and amine derivatives,
and has been shown to enable the heterolytic activation of
hydrogen at Ru and Ir centers.[31,32] Herein, we describe the
Figure 1. (a) X-ray structure of complex 2-PF6 (molecular cation
shown, thermal ellipsoids at 50% probability, co-crystallized tolu-
ene and hydrogen atoms except for H1 and H2 omitted for clarity).
Selected bond lengths [Å] and angles [°]: Ni1–N1 1.978(2), Ni1–P2
2.155(1), Ni1–P1 2.157(1), N1–Ni1–P2 88.0(1), N1–Ni1–P1 87.5(1),
synthesis and characterization of nickel hydride complexes P2–Ni1–P1 171.4(1); (b) X-ray structure of complex 3 (one of two
crystallographically independent molecules, thermal ellipsoids at
of an aliphatic PNP pincer ligand. Both cationic and neu-
50% probability, non-hydridic hydrogen atoms omitted for clarity).
tral nickel hydride complexes have been found to catalyze
Selected bond lengths [Å] and angles [°]: Ni1–N1 1.876(2), Ni1–P2
alkene hydrogenation under mild conditions. Experiments
2.117(1), Ni1–P1 2.119(1), N1–Ni1–P2 89.4(1), N1–Ni1–P1 88.5(1),
to gain insight into the mechanism of these unusual exam-
ples of homogeneous nickel hydrogenation catalysts are also
presented.
P2–Ni1–P1 174.2(1).
Neutral hydride complex (PNPCy)Ni(H) (3) was prepared
by deprotonation of the cationic hydride complex 2 with
KH (Scheme 3). In the H NMR spectrum of 3 ([D6]benz-
1
Results and Discussion
ene), the hydride signal appears as a triplet at –17.32 ppm
(2JP-H = 62.8 Hz), shifted more than 2 ppm downfield from
the hydride signal of 2. The IR spectrum of 3 exhibits a Ni–
H stretch at 1811 cm–1. Single crystals of 3 were grown from
a concentrated diethyl ether solution. A single-crystal X-ray
diffraction analysis revealed two crystallographically inde-
pendent molecules in the unit cell of complex 3. The two
molecules display very similar bond lengths and one of the
two molecules is shown in Figure 1 (see Supporting Infor-
mation for details of the other crystallographically indepen-
dent molecule). The distance between the central nitrogen
of the pincer ligand and the nickel center of complex 3
[1.876(2) Å] is significantly shorter than the Ni1–N1 dis-
Synthesis of Nickel Hydride Complexes
Reaction of the aliphatic pincer ligand HN[CH2CH2P-
(Cy)2]2 (PNHPCy) with Ni(diglyme)Br2 in THF, followed
by recrystallization, afforded the cationic NiII
complex [(PNHPCy)Ni(Br)]Br (1) as orange crystals in good
yield. Complex 1 was characterized by NMR and IR spec-
troscopy as well as elemental analysis. The closely related
square-planar
complexes
[(PNHPiPr)Ni(Br)]Br
and
[(PNHPiPr)Ni(NCCH3)](BF4)2 have previously been re-
ported by Arnold and co-workers; the latter complex was
found to catalyze nucleophilic addition of piperidine to ace-
tonitrile.[33]
The cationic hydride complex [(PNHPCy)Ni(H)]BPh4 (2)
was subsequently prepared by reaction of complex 1 with
1
NaBH4 and NaBPh4 in MeOH. The H NMR spectrum
(CD3CN) of complex 2 shows a characteristic triplet hy-
dride signal at –19.59 ppm (2JP–H = 62.2 Hz), and the IR
spectrum of complex 2 shows a Ni–H stretch at 1886 cm–1.
The hexafluorophosphate derivative [(PNHPCy)Ni(H)]PF6
(2-PF6) was also prepared using an analogous procedure
and found to display very similar NMR features (see Exp.
Section). The X-ray structure of 2-PF6 is shown in Figure 1.
The distance between the central nitrogen of the pincer li-
gand and the nickel center [1.978(2) Å] is consistent with
nitrogen protonation.
Scheme 3. Synthesis of nickel hydride complexes 2 and 3.
2
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
Alkene Hydrogenation Catalyzed by Nickel Hydride Complexes
tance of cationic hydride complex 2 [1.978(2) Å]. For com- sphere of hydrogen (1 atm) with complex 2 (10 mol-%) in
plexes 2 and 3, the nickel hydride stretching frequency cor- [D8]THF solution, formation of ethylbenzene was observed
relates well with the strength of the trans-donor ligand, with in about 10% yield after 5 d at 80 °C, providing an initial
the observed Ni–H stretching frequency decreasing as the indication that complex 2 might be capable of hydrogena-
distance between the nickel center and the central nitrogen tion. When the hydrogen pressure was increased slightly to
on the pincer ligand decreases.
4 atm, complete conversion of styrene to ethylbenzene was
Previously, Liang and Ozerov have reported nickel hy- observed after 24 h at 80 °C (Scheme 5 and Table 1, Entry
dride complexes of amidodiphosphane pincer ligands.[34,35] 1).[37] At the end of the reaction, the only nickel species
1
The Ni1–N1 distance of neutral hydride complex 3 is detected in solution by H and 31P NMR spectroscopy was
shorter than the Ni–N distances of 1.918 Å and 1.920 Å for the cationic hydride complex 2. To test for the possible for-
the amidodiphosphane pincer complexes (P–N–PCy)Ni(H) mation of an active colloidal or nanoparticle Ni catalyst,[38]
and (P–N–PiPr)Ni(H) {P–N–PR = –N[(C6H5)-2-PR2]2} re- the hydrogenation of styrene using complex 2 (10 mol-%)
ported by Liang and co-workers, consistent with a more was conducted with stirring in the presence of excess Hg
basic amido nitrogen on the aliphatic pincer ligand.[36]
metal (390 equiv.). The hydrogenation of styrene was unaf-
A nickel(II) hydride analogue of 2 was also synthesized fected by the addition of Hg, suggesting that the active cata-
where the central nitrogen of the aliphatic PNP pincer li- lytic species is homogeneous.[39,40] Supporting this idea, the
gand was substituted with a methyl group. Complex hydrogenation reaction mixtures maintained a clear, pale
[(PNMePCy)Ni(H)]BPh4 (4) {PNMePCy
=
(CH3)N- yellow appearance throughout the reaction, with no evident
[CH2CH2P(Cy)2]2} was prepared by the reaction of the formation of nickel metal.
PNMePCy ligand with Ni(diglyme)Br2, followed by treat-
1
ment with NaBH4 and NaBPh4 (Scheme 4). The H NMR
spectrum of complex 4 shows a triplet signal corresponding
to the hydride ligand at –20.60 ppm (2JP-H = 61.2 Hz),
shifted slightly upfield from that of 2. The IR spectrum of
complex 4 exhibits a Ni–H stretch at 1878 cm–1. The X-ray
structure of complex 4 is shown in Figure 2. The distance
between the nickel and the central nitrogen on the pincer
ligand is 1.989(2) Å, nearly identical to that of 2.
Scheme 5. Hydrogenation of styrene using cationic nickel hydride
complex 2.
Table 1. Catalytic hydrogenation using cationic nickel hydride com-
plex 2.[a]
Scheme 4. Synthesis of nickel hydride complex [(PNMePCy)Ni-
(H)]BPh4 (4).
Figure 2. (a) X-ray structure of complex 4 (molecular cation shown,
thermal ellipsoids at 50% probability, co-crystallized THF and all
non-hydridic hydrogen atoms omitted for clarity). Selected bond
lengths [Å] and angles [°]: Ni1–N1 1.989(2), Ni1–P2 2.151(1), Ni1–
P1 2.156(1), N1–Ni1–P2 89.3(1), N1–Ni1–P1 89.3(1), P2–Ni1–P1
174.2(1).
Catalytic Hydrogenation Reactions
[a] Conditions: 80 °C, 4 atm H2, [D8]THF solvent. [b] Yields were
determined by 1H NMR spectroscopy (integration against an in-
Nickel hydride complexes 2–4 were evaluated as hydro-
genation catalysts. When styrene was heated under an atmo- ternal standard) and verified by GC–MS.
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
K. V. Vasudevan, B. L. Scott, S. K. Hanson
FULL PAPER
Cationic hydride complex 2 was then tested as a hydro- tion of 1-octene using 3 (10 mol-%), which afforded a mix-
genation catalyst for several other substrates. When 1-oc- ture of n-octane (76%) and internal octene isomers (24%)
tene was heated with complex 2 (10 mol-%) under 4 atm H2 after 24 h at 80 °C. No hydrogenation of 3,5-dimethoxybe-
([D8]THF solvent, 80 °C, 24 h), complete consumption of nzaldehyde or cinnamylaldehyde was observed using cata-
the 1-octene was observed, affording n-octane (70%) and lyst 3.
internal octene isomers (30%, arising from isomerization of
In an effort to explore the role of the N–H functionality
the 1-octene) (Table 1, Entry 2). Prolonging the reaction on the pincer ligand PNHPCy in promoting the reaction,
time to 48 h only slightly increased the yield of n-octane hydrogenation reactions were tested using the methyl-sub-
(76%). Internal octene isomers (24%) remained at the end stituted PNP nickel hydride complex [(PNMePCy)Ni(H)]-
of the reaction, indicating that the terminal 1-octene is hy- BPh4 (4). Surprisingly, complex 4 displayed similar activity
drogenated more rapidly than its internal isomers. The to complex 2 for alkene hydrogenation, affording ethylben-
more sterically hindered olefins 3,3-dimethyl-1-butene and zene in quantitiative yield upon the hydrogenation of styr-
α-methylstyrene were also hydrogenated under the same re- ene (10 mol-% catalyst, 18 h, 80 °C). The analogous reac-
action conditions (4 atm H2, 80 °C), albeit somewhat more tions with 1-octene and 3,5-dimethoxybenzaldehyde re-
slowly, affording neohexane (97%) and isopropylbenzene sulted in the respective generation of n-octane (50%) and
(48%) after 48 h (Table 1, Entries 3 and 4).
3,5-dimethoxybenzylalcohol (8%) after 18 h. The compar-
Lower conversions were observed in the attempted hy- able catalytic activities observed for nickel hydride com-
drogenation of aldehyde substrates using catalyst 2. Heating plexes 2 and 4 (with and without an available N–H group
a [D8]THF solution of 3,5-dimethoxybenzaldehyde under on the pincer ligand, respectively) provide a strong indica-
4 atm H2 with 2 (10 mol-%) afforded 3,5-dimethoxy- tion that a cooperative interaction involving the N–H of the
benzylalcohol in 5% yield (Table 1, Entry 5). The identity pincer ligand is not required for the catalytic hydrogenation.
of the 3,5-dimethoxybenzylalcohol was confirmed by spik-
ing the reaction mixture with authentic compound. No 3,5-
dimethoxybenzoic acid was detected in the reaction mixture
by H NMR spectroscopy, suggesting that the 3,5-dimeth-
oxybenzylalcohol formed from hydrogenation of the alde-
Investigating the Mechanism of Nickel-Catalyzed
Hydrogenation
1
hyde and not by aldehyde disproportionation. When the hy-
Having found both complexes 2 and 3 to be active hydro-
drogenation of cinnamylaldehyde was performed under the
genation catalysts, we performed further experiments in an
same reaction conditions (4 atm H2, 80 °C, 24 h), 3-phenyl-
attempt to gain insight into the reaction mechanisms of the
1-propanol was formed in 10% yield (Table 1, Entry 6).[37]
more active catalyst 2. First, the reaction of cationic hydride
No partially reduced intermediates were observed in the hy-
complex 2-PF6 was examined with 1-octene. When an ex-
drogenation of cinnamylaldehyde when the reaction was
cess of 1-octene was added to a [D6]benzene solution of 2-
1
monitored by H NMR spectroscopy.
PF6, complete conversion to the complex [(PNHPCy)-
The reactions catalyzed by 2 are unusual examples of hy-
Ni(CH2(CH2)6CH3)]PF6 (5-PF6) was observed after 4 d at
drogenation of C=O and C=C bonds mediated by a homo-
25 °C (Scheme 6). Complex 5-PF6 was isolated and charac-
geneous nickel complex under mild conditions. With the ex-
terized by NMR and IR spectroscopy. The 13C{1H} NMR
ception of the recent example of styrene hydrogenation re-
spectrum of 5-PF6 displays a characteristic signal for the
ported by Harman and Peters,[15] the few known homogen-
carbon bound to the Ni center, which appears as a triplet
eous nickel hydrogenation catalysts have utilized very high
(2JP-C = 21 Hz) at –2.2 ppm. Examination of the DEPT-135
temperatures and/or pressures.[12–14] A number of nickel
NMR spectrum of 5-PF6 confirmed that 1-octene inserted
pincer hydride complexes have been reported pre-
into the Ni–H bond of 2-PF6 in a 1,2-fashion, generating
viously,[34–36,41–45] and have been found to catalyze cross-
a primary octyl ligand (See Supporting Information). The
coupling and hydrosilylation reactions.[5,9,10] For example,
preferential formation of the primary octyl isomer of 5-PF6
the
related
hydride
complex
(POCOPtBu)-
is likely due in part to the large steric bulk of the cyclo-
Ni(H) [POCOPtBu = 1,3-(OPtBu2)2C6H3] was found to cat-
alyze both the hydrosilylation of aldehydes and ketones and
the catalytic reduction of CO2 with catecholborane.[9,46]
However, none of the previously reported nickel pincer hy-
dride complexes are known to catalyze hydrogenation reac-
tions.
hexyl-substituted phosphanes.[47]
In light of the promising reactivity observed with com-
plex 2, the reactivity of neutral hydride complex 3 was ex-
plored. Neutral hydride complex 3 also catalyzed hydrogen-
ation reactions, but did so less effectively than cationic hy-
dride complex 2. For instance, when a [D6]benzene solution
of styrene was heated under 4 atm H2 with 3 (10 mol-%),
30% yield of ethylbenzene was observed after 24 h at 80 °C.
A slightly higher conversion was observed in the hydrogena- Scheme 6. Insertion of 1-octene into the Ni–H bond of 2-PF6.
4
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
Alkene Hydrogenation Catalyzed by Nickel Hydride Complexes
Insertion of 1-octene into the Ni–H bond of 2 was found
In an attempt to gain further insight into the reactivity
to be a reversible reaction. When an isolated sample of of hydrogen at the cationic nickel(II) center, the deuteride
complex 5-PF6 was heated in [D6]benzene solution (80 °C) complex [(PNDPCy)Ni(D)]PF6 ([D2]-2) was prepared by re-
in the absence of 1-octene, 80% of complex 5-PF6 reacted action of 1 with NaBD4 and NaPF6 in CD3OD. When a
after 48 h, affording hydride complex 2-PF6 (80%), 1-oc- [D6]benzene solution of [D2]-2 was treated with H2 (1 atm),
1
tene (47%), and internal octene isomers (30%). To gain in- the formation of H-D gas was detected by H NMR spec-
sight into the product release step proposed in Scheme 2, troscopy within 10 min at room temperature (Scheme 8).
an isolated sample of complex 5 was heated in [D8]THF Resonances corresponding to the Ni-H and N-H signals
1
solution under hydrogen (4 atm).[48] After 2 h at 80 °C, 90% grew into the H NMR spectrum of complex 2 over the
conversion of 5 had occurred, affording cationic hydride course of 24 h (25 °C). The formation of H-D gas suggests
complex 2, 1-octene (64%), and n-octane (25%) and sug- a pathway for exchange that involves addition of H2 gas to
gesting that β-hydride elimination occurs more rapidly than the cationic nickel(II) center of [D2]-2, as opposed to a
alkane product release from 5.
pathway involving initial elimination of D2, followed by re-
To investigate the reactivity of a cationic nickel alkyl ana- action with H2, which would not be expected to form H-D.
logue where competing β-hydride elimination was not pos-
sible, the cationic methyl complex [(PNHPCy)Ni(CH3)]BPh4
(6) was prepared (Scheme 7). First, reaction of complex
(PNPCy)Ni(Br) (7) with methyllithium afforded neutral
complex (PNPCy)Ni(CH3) (8). The NMR spectra of com-
plex 8 display characteristic resonances for the Ni-CH3
Scheme 8. Reaction of [D2]-2 with H2.
group, which appears as a triplet at –0.49 ppm (3JP-H
=
1
8.8 Hz) in the H NMR spectrum and at –25.0 ppm (2JP-C
= 24 Hz) in the 13C{1H} NMR spectrum. Subsequent pro-
tonation of neutral nickel methyl complex 8 allowed for iso-
lation of cationic methyl complex [(PNHPCy)Ni(CH3)]BPh4
Finally, the stoichiometric reactions of cationic hydride
complex 2 were examined with styrene and 3,5-dimeth-
oxybenzaldehyde. When complex 2 was treated with
50 equiv. of styrene, approximately 85% conversion to a
1
(6) (see Exp. Section for details). The H NMR spectrum
1
new complex (9) was observed. H, 13C{1H}, and DEPT-
of complex 6 (CD2Cl2) shows a triplet signal for the Ni-
CH3 group at –0.58 ppm (3JP-H = 8.8 Hz). The Ni-CH3
group appears in the 13C{1H} NMR spectrum of 6 as a
triplet at –21.9 ppm (2JP-C = 24 Hz).
135 NMR spectra ([D8]THF) of complex 9 recorded in the
presence of excess styrene are consistent with its assignment
as the 1,2-insertion product [(PNHPCy)Ni(CH2CH2Ph)]-
BPh4 (9) (see Supporting Information for details). The
13C{1H} NMR spectrum of 9 generated in situ showed a
characteristic triplet signal for the carbon bound to the
nickel center at δ = 0.35 ppm (2JP-C = 20 Hz). In contrast,
no reaction occurred when an excess (10 equiv.) of 3,5-di-
methoxybenzaldehyde was added to a solution of complex 2
1
in [D8]THF (as determined by integration of the H NMR
spectrum against an internal standard), even upon heating
at 80 °C for 24 h. Thus, while both 1-octene and styrene
substrates readily inserted into the Ni–H bond of 2, the
formation of insertion products was found to be disfavored
for 3,5-dimethoxybenzaldehyde. The low hydrogenation ac-
tivities observed for 3,5-dimethoxybenzaldehyde and cinna-
mylaldehyde with catalyst 2 may be related to the low pro-
pensity of aldehyde substrates to insert into the Ni–H bond.
Scheme 7. Synthesis of cationic methyl complex 6.
Surprisingly, complex 6 was found to be relatively stable
when heated in [D6]benzene at 80 °C, with less than 5%
reacting after 48 h (as determined by integration against an
internal standard). No methane was detected in the 1H
NMR spectrum of the reaction mixture, indicating that
Conclusions
Rare new examples of homogeneous nickel alkene hydro-
C–H bond formation to generate methane from complex 6 genation catalysts have been uncovered in the cationic and
is not facile. The lack of methane formation observed upon neutral nickel hydride complexes of the aliphatic PNP
the thermolysis of 6 is inconsistent with the product release pincer ligand PNHPCy. Cationic hydride complex 2 was
step initially proposed in Scheme 2, and instead suggests found to catalyze the hydrogenation of several alkenes un-
that hydrogen could be involved in the product release step. der mild conditions (80 °C, 4 atm H2). Mechanistic experi-
A product release step involving hydrogenolysis of the cat- ments support a hydrogenation pathway involving alkene
ionic nickel(II) alkyl intermediate would also account for insertion into the Ni–H bond of 2 to generate a cationic
the catalytic activity observed for nickel hydride complex 4, nickel(II) alkyl complex. Subsequent product release likely
which lacks an available N-H group on the pincer ligand.
occurs by reaction of H2 at the cationic nickel(II) alkyl cen-
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
K. V. Vasudevan, B. L. Scott, S. K. Hanson
FULL PAPER
3402 cm–1. C28H53Br2NNiP2 (684.20): calcd. C 49.15, H 7.81, N
2.05; found C 49.61, H 7.89, N 1.96.
ter, as opposed to initial alkane release attributable to pro-
tonation of the alkyl ligand by the N-H group on the pincer,
followed by H2 addition (as proposed in Scheme 2). The
catalytic activity observed for the methyl-substituted nickel
hydride complex [(PNMePCy)Ni(H)]BPh4 (4) demonstrates
that a cooperative interaction involving the N-H moiety on
the pincer ligand is not necessary for the hydrogenation re-
action. Instead, the pincer ligand likely functions to pro-
mote hydrogenation by stabilizing the cationic nickel(II) hy-
dride center.
[(PNHPCy)Ni(H)]BPh4 (2): In
a vial, complex 1 (54.0 mg,
0.0818 mmol) was dissolved in MeOH (8 mL) by stirring at room
temp. In two portions was added NaBH4 (15.3 mg, 0.403 mmol).
The solution changed color from orange to a lighter yellow color
and bubbled vigorously. In a separate vial, NaBPh4 (32.6 mg,
0.0953 mmol) was dissolved in MeOH (1 mL). Once the bubbling
subsided, the solution of NaBPh4 was carefully layered on top of
the reaction mixture, and allowed to stand at room temp. for 2 h,
during which time light golden-colored crystals formed. The super-
natant was removed by pipette and the crystals washed with Et2O
Interest is growing in the development of effective hydro-
genation catalysts based on inexpensive, earth-abundant (2ϫ3 mL) and dried under vacuum; yield 60.9 mg (84%). 1H
NMR (400 MHz, CD3CN): δ = 7.29–7.25 (m, 8 H, BPh4), 6.99 (t,
J = 7.2 Hz, 8 H, BPh4), 6.84 (t, J = 7.2 Hz, 4 H, BPh4), 3.90 (br.
s, 1 H, NH), 3.25–3.11 (m, 2 H, PNP), 2.41–2.31 (m, 2 H, PNP),
2.19–2.00 (m, 6 H, PNP), 1.99–1.69 (m, 20 H, PNP), 1.48–1.18 (m,
metals. Metal–ligand cooperativity could enhance the reac-
tivity of earth-abundant metal catalysts and potentially
even enable new types of reaction mechanisms. However,
despite the fact that metal–ligand cooperativity has often
been proposed for precious metal catalysts, it can be diffi-
cult to definitively establish as the dominant reaction
mechanism. Future work will require differentiating metal–
ligand cooperativity from other ligand electronic and steric
effects of a ligand which profoundly influence reactivity.
2
22 H, PNP), –19.59 (t, JP-H = 62.2 Hz, 1 H, Ni-H) ppm. 13C{1H}
1
NMR (100 MHz, CD3CN): δ = 164.9 (q, JB-C = 49 Hz), 136.8 (s),
2
126.7 (q, JB-C = 3 Hz), 122.9 (s), 52.2 (vt, JP-C = 4 Hz), 34.3 (vt,
JP-C = 12 Hz), 33.5 (vt, JP-C = 14 Hz), 30.7 (m, 2 C), 29.8 (s), 29.2
(s), 27.6–27.2 (m, 4 C), 26.9 (s), 26.8 (s), 24.8 (vt, JP-C = 9 Hz) ppm.
31P{1H} NMR (162 MHz, CD3CN): δ = 56.4 (s) ppm. IR (thin
film): ν = 3187 (νN–H), 1886 (νNi–H) cm–1. C52H74BNNiP2 (844.63):
˜
calcd. C 73.95, H 8.83, N 1.66; found C 74.02, H 8.92, N 1.71.
[(PNHPCy)Ni(H)]PF6 (2-PF6): In a small vial, complex 1 (36 mg,
0.052 mmol) and NaBH4 (12 mg, 0.32 mmol) were suspended in
THF (3 mL). MeOH (1 mL) was added dropwise until the suspen-
sion began to bubble vigorously. The pale yellow mixture was al-
lowed to react at room temp. for 20 min, during which time the
bubbling ceased. At this time, KPF6 (16 mg, 0.087 mmol) was
added, and the reaction mixture stirred for 10 min. The solvent was
removed under vacuum, leaving an off-white residue. The residue
was extracted with benzene (2ϫ2 mL), and filtered through a glass
wool pipette. The benzene was removed under vacuum, leaving a
nearly colorless residue; yield 28 mg (80%). Crystals suitable for X-
ray diffraction were obtained by diffusion of Et2O into a toluene
Experimental Section
General: Unless specified otherwise, all reactions were carried out
under a dry argon atmosphere using standard glove-box and
Schlenk techniques. Deuterated solvents were purchased from
Cambridge Isotope Laboratories. [D6]Benzene and [D8]THF were
dried with Na metal, CD3CN and CD2Cl2 were dried with CaH2,
and CDCl3 was used as received. 1-Octene was dried with sodium
metal, and styrene was dried with CaH2. Anhydrous grade THF,
pentane, benzene, toluene, and Et2O were obtained from Aldrich
or Acros and stored over 4 Å molecular sieves. Bis[2-(dicyclohex-
ylphosphanyl)ethyl]amine (PNHPCy) was purchased from Strem
Chemical, and nickel(II) bromide 2-methoxyethyl ether complex
[Ni(diglyme)Br2] was purchased from Aldrich. 1H, 13C, and 31P
NMR spectra were obtained at room temp. with a Bruker
AV400 MHz spectrometer, with chemical shifts (δ) referenced to the
residual solvent signal (1H and 13C) or referenced externally to
H3PO4 (δ =0 ppm). GC–MS analysis was obtained with a Hewlett
Packard 6890 GC system equipped with a Hewlett–Packard 5973
mass selective detector. Elemental analyses were performed by At-
lantic Microlab of Norcross, GA.
1
solution of 2-PF6 at –20 °C. H NMR (400 MHz, [D6]benzene): δ
= 4.46 (br. s, 1 H, NH), 3.55–3.44 (m, 2 H, PNP), 2.22–1.91 (m, 6
H, PNP), 1.78–1.49 (m, 26 H, PNP), 1.28–1.03 (m, 18 H, PNP),
2
–19.33 (t, JP-H = 63.2 Hz, 1 H, Ni-H) ppm. 13C{1H} NMR
(100 MHz, [D6]benzene): δ = 52.6 (vt, JP-C = 5 Hz), 34.1 (vt, JP-C
= 13 Hz), 33.4 (vt, JP-C = 14 Hz), 30.5 (s), 30.0 (s), 29.3 (s), 28.8
(s), 27.4–27.0 (m, 4 C), 26.7 (s), 26.5 (s), 24.4 (vt, JP-C = 9 Hz) ppm.
31P{1H} NMR (162 MHz, [D6]benzene): δ = 55.2 (s), –142.6 (h,
1JF-P = 713 Hz) ppm. IR (thin film): ν = 3232 (νN–H), 1886 (νNi–
˜
H) cm–1.
[(PNHPCy)Ni(Br)]Br (1): In a vial, PNHPCy (91.5 mg, 0.197 mmol)
and Ni(diglyme)Br2 (65.0 mg, 0.185 mmol) were combined in THF
(4 mL), and the orange reaction mixture was stirred for 18 h at
room temp. MeOH (3 mL) was added, and then the mixture was
filtered through a Teflon syringe filter. The filter was washed with
MeOH (2 mL), the solutions combined, and the solvent removed
under vacuum. The resulting orange residue was recrystallized from
CH2Cl2/Et2O, affording orange crystals. The crystals were washed
with Et2O (2ϫ3 mL), and dried under vacuum; yield 117 mg
(87%). 1H NMR (400 MHz, CDCl3): δ = 6.90 (br. s, 1 H, NH),
3.21–3.12 (m, 2 H, PNP), 2.46–2.43 (m, 2 H, PNP), 2.35–2.16 (m,
10 H, PNP), 2.04–1.89 (m, 14 H, PNP), 1.82–1.58 (m, 12 H, PNP),
1.43–1.28 (m, 12 H, PNP) ppm. 13C{1H} NMR (100 MHz,
CDCl3): δ = 54.8 (vt, JP-C = 5 Hz), 34.3 (vt, JP-C = 11 Hz), 33.5
(vt, JP-C = 12 Hz), 29.5 (s), 28.5 (s, 2 C), 28.4 (s), 27.4–26.8 (m, 4
C), 26.2 (s), 26.0 (s), 21.9 (vt, JP-C = 9 Hz) ppm. 31P{1H} NMR
(PNPCy)Ni(H) (3): To a 15 mL thick-walled glass tube was added
2 (15 mg, 0.018 mmol) and KH (11 mg, 0.274 mmol). The solids
were suspended in toluene (7 mL), the vessel sealed, and the mix-
ture stirred at 80 °C for 15 h. The resulting brown suspension was
filtered through a PTFE syringe filter and the toluene removed
under vacuum, affording a yellow-green oil; yield 7.5 mg (80%). 1H
NMR (400 MHz, [D6]benzene): δ = 3.48–3.40 (m, 4 H, PNP), 2.18–
2.14 (m, 4 H, PNP), 2.04–2.00 (m, 4 H, PNP), 1.90–1.68 (m, 20 H,
2
PNP), 1.62–1.12 (m, 20 H, PNP), –17.32 (t, JP-H = 62.8 Hz, 1 H,
Ni-H) ppm. 13C{1H} NMR (100 MHz, [D6]benzene): δ = 59.7 (vt,
JP-C = 7 Hz), 34.9 (vt, JP-C = 12 Hz), 30.5 (vt, JP-C = 2 Hz), 29.2,
27.8–27.6 (m, 2 C), 27.4 (vt, JP-C = 9 Hz), 27.1 ppm. 31P{1H} NMR
(162 MHz, [D ]benzene): δ = 73.8 (s) ppm. IR (thin film): ν =
˜
6
1811 (νNi–H) cm–1.
PNMePCy: A 25 mL flask was charged with dicyclohexylphos-
(162 MHz, CDCl ): δ = 49.0 (s) ppm. IR (thin film): ν
=
phane (1.001 g, 5.05 mmol) and THF (9 g) and cooled to –20 °C.
˜
3
N–H
6
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
Alkene Hydrogenation Catalyzed by Nickel Hydride Complexes
To this cooled solution was added nBuLi (3.2 mL, 1.6 m in hexanes,
5.05 mmol) dropwise and the resulting solution stirred at reflux for
50 min, following which time the solution was cooled to –20 °C. A
(s), 28.6, 28.5, 28.0 (vt, JP-C = 7 Hz), 27.9 (vt, JP-C = 7 Hz), 27.5
(vt, JP-C = 4 Hz), 27.3 (vt, JP-C = 5 Hz), 26.7 (s), 26.6 (s), 23.5 (s),
2
23.1 (vt, JP-C = 10 Hz), 14.7 (s), –2.2 (t, JP-C = 21 Hz) ppm.
vial was charged with MeN(CH2CH2Cl)2·HCl (0.490 g, 2.52 mmol) 31P{1H} NMR (162 MHz, [D6]benzene): δ = 36.2 (s), –142.6 (h,
and THF (5 mL) and the suspension cooled to –20 °C. To the
cloudy suspension was added nBuLi (1.6 mL, 1.6 m in hexanes,
2.52 mmol) dropwise and the resulting faintly cloudy suspension
stirred for 20 min while warming to room temp. The resulting mix-
ture was cooled to –20 °C and added dropwise to the cooled flask
containing the lithium phosphide solution. The mixture was then
stirred at reflux for 4 h. The solution was subsequently cooled to
room temp. and the solvent volume reduced to 10 mL. Pentane
(10 mL) and degassed, deionized H2O (5 mL) were added. The
aqueous layer was removed and the organic phase washed with
degassed, deionized H2O (3ϫ3 mL). The combined organic phases
were dried with MgSO4, filtered, and all volatiles removed in vacuo,
1JF-P = 713 Hz) ppm. IR (thin film): ν = 3241 (νNi–H) cm–1.
˜
[(PNHPCy)Ni(CH3)]BPh4 (6): Complex 8 (24 mg, 0.045 mmol) was
dissolved in MeOH (2 mL). Upon addition of NaBPh4 (17 mg,
0.050 mmol), a pale yellow precipitate formed immediately. The so-
lid was allowed to settle, the supernatant removed by pipette, and
the solid washed with MeOH (2ϫ2 mL). The yellow solid was
dried under vacuum, and then recrystallized from hot toluene. The
yellow crystals were washed with pentane (2 mL), and dried under
1
vacuum; yield 23.5 mg (62%). H NMR (400 MHz, CD2Cl2): δ =
7.35 (m, 8 H, BPh4), 7.04 (t, J = 7.2 Hz, 8 H, BPh4), 6.89 (t, J =
7.2 Hz, 4 H, BPh4), 2.58–2.44 (m, 2 H, PNP), 2.02–1.97 (m, 6 H,
PNP), 1.90–1.64 (m, 22 H, PNP), 1.52–1.23 (m, 22 H, PNP), –0.58
1
affording a white solid. Yield 0.982 g (41%). H NMR (400 MHz,
3
(t, JP-H = 8.8 Hz, 3 H, Ni-CH3) ppm. 13C{1H} NMR (100 MHz,
[D6]benzene): δ = 2.78–2.70 (m, 4 H, NCH2), 2.29 (s, 3 H, N-CH3),
1.85–1.55 (m, 28 H, Cy and NCH2CH2), 1.29–1.14 (m, 20 H,
Cy) ppm. 31P{1H} NMR (162 MHz, [D6]benzene): δ = –7.3
(s) ppm.
1
CD2Cl2): δ = 164.6 (q, JB-C = 49 Hz), 136.5 (br. s), 126.2 (q,
2JB-C = 3 Hz), 122.4 (s), 52.1 (vt, JP-C = 5 Hz), 34.3 (vt, JP-C
=
11 Hz), 31.2 (vt, JP-C = 12 Hz), 29.7 (s), 29.5 (s), 29.1 (s), 28.6 (s),
27.7–27.5 (m, 2 C), 27.3 (vt, JP-C = 6 Hz), 27.2 (vt, JP-C = 6 Hz),
[(PNMePCy)Ni(H)]BPh4 (4): In
a
vial, PNMePCy (50.0 mg,
2
26.6 (s), 26.5 (s), 23.6 (vt, JP-C = 9 Hz), –21.9 (t, JP-C
=
24 Hz) ppm. 31P{1H} NMR (162 MHz, CD2Cl2): δ = 40.2 (s) ppm.
0.104 mmol) and Ni(diglyme)Br2 (37.0 mg, 0.104 mmol) were sus-
pended in THF (5 mL) and stirred for 2 h. The suspension was
filtered, the volume reduced to 2 mL and MeOH (2 mL) added. In
two portions was added NaBH4 (16.0 mg, 0.421 mmol). The solu-
tion changed color from deep orange-red to a lighter yellow color
with vigorous bubbling. In a separate vial, NaBPh4 (40.0 mg,
0.117 mmol) was dissolved in MeOH (1 mL). Once the bubbling
subsided, the solution of NaBPh4 was carefully layered on top of
the reaction mixture, and allowed to stand at room temp. for 12 h,
during which time a crop of golden-colored crystals formed. The
supernatant was removed by pipette and the crystals washed with
cold MeOH (2ϫ2 mL) and Et2O (2ϫ2 mL) and dried under vac-
uum; yield 49.0 mg (48%). 1H NMR (400 MHz, CD3CN): δ =
7.29–7.25 (m, 8 H, BPh4), 6.99 (t, J = 7.4 Hz, 8 H, BPh4), 6.84 (t,
J = 7.1 Hz, 4 H, BPh4), 2.90–2.60 (m, 4 H, PNP), 2.36 (s, 3 H, N-
CH3), 2.20–2.12 (m, 4 H, PNP), 2.12–2.05 (m, 2 H, PNP), 2.00–
1.85 (m, 4 H, PNP), 1.85–1.67 (m, 14 H, PNP), 1.60–1.40 (m, 2 H,
IR (thin film): ν = 3183 (νN–H) cm–1. C53H76BNNiP2 (858.66):
˜
calcd. C 74.14, H 8.92, N 1.63; found C 73.70, H 8.95, N 1.65.
(PNPCy)Ni(Br) (7): Complex 1 (105 mg, 0.159 mmol) and NaOCH3
(34 mg, 0.63 mmol) were combined in THF (3 mL). The suspension
was stirred at room temp. for 45 min, during which time the color
changed from orange to dark green. The solution was filtered
through a glass wool pipette and then solvent removed under vac-
uum. The dark green residue was treated with toluene (1 mL), and
the solvent removed under vacuum, affording a dark green oil.
Et2O (1 mL) was added, and the solvent removed under vacuum,
1
leaving a dark green microcrystalline solid; yield 83 mg (87%). H
NMR (400 MHz, [D6]benzene): δ = 2.71–2.59 (m, 6 H, PNP), 2.12–
2.07 (m, 4 H, PNP), 1.92–1.54 (m, 28 H, PNP), 1.30–1.12 (m, 14
H, PNP) ppm. 13C{1H} NMR (100 MHz, [D6]benzene): δ = 61.6
(vt, JP-C = 6 Hz), 33.9 (vt, JP-C = 11 Hz), 29.6 (s), 28.7 (s), 27.8
(vt, JP-C = 6 Hz), 27.6 (vt, JP-C = 5 Hz), 27.0 (s), 23.7 (vt, JP-C
=
2
PNP), 1.40–1.17 (m, 22 H, PNP), –20.60 (t, JP-H = 61.2 Hz, 1 H,
10 Hz) ppm. 31P{1H} NMR (162 MHz, CDCl3): δ = 58.8 (s) ppm.
C28H52BrNNiP2 (603.29): calcd. C 55.75, H 8.69, N 2.32; found C
55.88, H 8.84, N 2.37.
Ni-H) ppm. 13C{1H} NMR (100 MHz, CD3CN): δ = 164.9 (q,
2
1JB-C = 49 Hz), 136.8 (s), 126.7 (q, JB-C = 3 Hz), 122.8 (s), 62.3
(vt, JP-C = 5 Hz), 41.8 (s), 34.4 (vt, JP-C = 14 Hz), 33.5 (vt, JP-C
14 Hz), 31.7 (vt, JP-C = 2 Hz), 31.2 (vt, JP-C = 2 Hz), 30.2 (s), 29.0
(s), 27.6 (vt, JP-C = 7 Hz), 27.5–27.4 (m, 2 C), 27.2 (vt, JP-C
6 Hz), 27.0 (s), 26.7 (s), 23.3 (vt, JP-C = 8 Hz) ppm. 31P{1H} NMR
=
(PNPCy)Ni(CH3) (8): In
a small vial, complex 7 (41 mg,
0.068 mmol) was suspended in Et2O (2 mL). Methyllithium (50 μL
of a 1.6 m solution in Et2O, 0.08 mmol) was added dropwise at
room temp., until the color changed from dark green to bright
orange. The reaction mixture was allowed to stand for 5 min, and
then the solvent was removed under vacuum. The orange residue
was extracted with pentane (2ϫ2 mL), and then filtered through a
glass wool pipette. The pentane was removed under vacuum, af-
fording an orange oil; yield 33 mg (90%). 1H NMR (400 MHz,
[D6]benzene): δ = 3.31–3.23 (m, 4 H, PNP), 2.23–2.19 (m, 4 H,
PNP), 1.95–1.86 (m, 12 H, PNP), 1.76–1.14 (m, 32 H, PNP), –0.49
=
(162 MHz, CD CN): δ = 51.3 (s) ppm. IR (thin film): ν =
˜
3
1878 (νNi–H) cm–1. C53H76BNNiP2·THF: calcd. C 73.56, H 9.10, N
1.50; found C 73.80, H 9.00, N 1.65.
Isolation of [(PNHPCy)Ni(CH2(CH2)6CH3)]PF6 (5-PF6): Complex
2-PF6 (8.4 mg, 0.013 mmol) was dissolved in [D6]benzene (0.6 mL).
1-Octene (0.050 g, 0.45 mmol) was added and the resulting solution
was allowed to stand at room temp. for 4 d, after which time exami-
nation of the 1H and 31P NMR spectra of the reaction mixture
revealed that complete conversion to 5-PF6 had occurred. Addition
of pentane (2 mL) and Et2O (1 mL) afforded a pale tan precipitate,
which was washed with pentane (1 mL) and dried under vacuum;
3
(t, JP-H = 8.8 Hz, 3 H, Ni-CH3) ppm. 13C{1H} NMR (100 MHz,
[D6]benzene): δ = 59.7 (br. s), 33.7 (vt, JP-C = 11 Hz), 29.7 (s), 28.7
(s), 28.0 (vt, JP-C = 6 Hz), 27.7 (vt, JP-C = 5 Hz), 27.2 (s), 26.5
2
(vt, JP-C = 10 Hz), –25.0 (t, JP-C = 24 Hz) ppm. 31P{1H} NMR
1
yield 3.2 mg (31%). H NMR (400 MHz, [D6]benzene): δ = 3.42–
(162 MHz, [D6]benzene): δ = 59.2 (s) ppm.
3.22 (m, 3 H, N-H and PNP), 2.12–2.03 (m, 8 H, PNP), 1.91–1.56
(m, 28 H, PNP and octyl), 1.44–1.07 (m, 29 H, PNP and octyl),
0.76 (m, 2 H, octyl) ppm. 13C{1H} NMR (100 MHz, [D6]benzene):
General Procedure for Hydrogenation Reactions: In a Wilmad pres-
sure NMR tube, complex 2 (ca. 5 mg, 0.006 mmol) was dissolved
δ = 52.1 (vt, JP-C = 5 Hz), 35.2 (s), 34.8 (vt, JP-C = 10 Hz), 33.4 in [D8]THF (0.4 mL) containing hexamethylbenzene added as an
(vt, JP-C = 11 Hz), 32.6 (s), 32.3 (s), 30.4 (s), 30.2 (s), 29.9 (s), 29.2 internal standard. The appropriate substrate (0.06 mmol) was
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
7

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
K. V. Vasudevan, B. L. Scott, S. K. Hanson
FULL PAPER
added, and an initial 1H NMR spectrum was recorded immediately.
The solvent was frozen and the headspace of the tube was evacu-
ated. The tube was then submersed in a dewar containing liquid
nitrogen to a level just under the Teflon™ seal, and H2 (1 atm) was
added. The tube was sealed while still cold, and then warmed to
room temp., resulting in a pressure of approximately 4 atm (the
tube headspace was measured to be 2 mL, containing
ca. 0.34 mmol H2). The tube was heated at 80 °C and the reaction
monitored by 1H and 31P NMR spectroscopy. At the end of the
reaction, the product yields were determined by 1H NMR (integra-
tion vs. the internal standard) and verified by GC–MS (comparison
of retention time and mass to authentic samples). Hydrogenations
with catalyst 3 were conducted using an analogous procedure in
[D6]benzene solvent using 1,3,5-tri-tert-butylbenzene as an internal
standard.
[16] a) C. J. Curtis, A. Miedaner, W. W. Ellis, D. L. DuBois, J. Am.
Chem. Soc. 2002, 124, 1918–1925; b) M. R. DuBois, D. L. Du-
Bois, Acc. Chem. Res. 2009, 42, 1974–1982.
[17] a) S. E. Smith, J. Y. Yang, D. L. DuBois, R. M. Bullock, An-
gew. Chem. Int. Ed. 2012, 51, 3152–3155; b) M. O’Hagan, W. J.
Shaw, S. Raugei, S. Chen, J. Y. Yang, U. J. Kilgore, D. L. Du-
Bois, R. M. Bullock, J. Am. Chem. Soc. 2011, 133, 14301–
14312.
[18] T. He, N. P. Tsvetkov, J. G. Andino, X. Gao, B. C. Fullmer,
K. G. Caulton, J. Am. Chem. Soc. 2010, 132, 910–911.
[19] a) B. C. Fullmer, H. Fan, M. Pink, J. C. Huffman, N. P. Tsvet-
kov, K. G. Caulton, J. Am. Chem. Soc. 2011, 133, 2571–2582;
b) M. F. Laird, N. P. Tsvetkov, M. Pink, T. He, R. W. Buell,
K. G. Caulton, Inorg. Chim. Acta 2011, 374, 79–87.
[20] B. Askevold, H. W. Roesky, S. Schneider, ChemCatChem 2012,
4, 307–320.
[21]
T. Zweifel, J. V. Naubron, T. Büttner, T. Ott, H. Grützmacher,
Angew. Chem. 2008, 120, 3289; Angew. Chem. Int. Ed. 2008,
47, 3245–3249.
Supporting Information (see footnote on the first page of this arti-
cle): Additional experimental and crystallographic details and
NMR spectra.
[22]
[23]
T. Ikariya, Top. Organomet. Chem. 2011, 37, 31–53.
Y. Shvo, D. Czarkie, Y. Rahamim, D. F. Chodosh, J. Am.
Chem. Soc. 1986, 108, 7400–7402.
[24]
a) R. Noyori, C. A. Sandoval, K. Muniz, T. Ohkuma, Phil.
Trans. R. Soc. A 2005, 363, 901–912; b) N. Uematsu, A. Fujii,
S. Hashiguchi, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 1996,
118, 4916–4917; c) C. A. Sandoval, T. Ohkuma, K. Muniz, R.
Noyori, J. Am. Chem. Soc. 2003, 125, 13490–13503.
a) J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, Angew.
Chem. 2006, 118, 1131; Angew. Chem. Int. Ed. 2006, 45, 1113–
1115; b) E. Balaraman, B. Gnanaprakasam, L. J. W. Shimon,
D. Milstein, J. Am. Chem. Soc. 2010, 132, 16756–16758.
a) W. W. N. O, A. J. Lough, R. H. Morris, Organometallics
2012, 31, 2152–2165; b) W. W. N. O, A. J. Lough, R. H. Morris,
Organometallics 2011, 30, 1236–1252; c) P. Maire, F. Breher, H.
Schonberg, H. Grutzmacher, Organometallics 2005, 24, 3207–
3218.
Acknowledgments
This work was supported by Los Alamos National Laboratory
LDRD Early Career Award (20110537ER).
[25]
[26]
[1] Catalysis without Precious Metals (Ed.: R. M. Bullock), Wiley-
VCH, Hoboken, NJ, 2010.
[2] S. Enthaler, K. Junge, M. Beller, Angew. Chem. 2008, 120, 3363;
Angew. Chem. Int. Ed. 2008, 47, 3317–3321.
[3] J. I. van der Vlugt, Eur. J. Inorg. Chem. 2012, 363–375.
[4] D. W. Robbins, J. F. Hartwig, Science 2011, 333, 1423–1427.
[5] a) J. Breitenfeld, R. Scopelliti, X. Hu, Organometallics 2012,
31, 2128–2136; b) P. Ren, O. Vechorkin, K. von Allmen, R.
Scopelliti, X. Hu, J. Am. Chem. Soc. 2011, 133, 7084–7095; c)
O. Vechorkin, A. Godinat, R. Scopelliti, X. Hu, Angew. Chem.
2011, 123, 11981; Angew. Chem. Int. Ed. 2011, 50, 11777–
11781; d) O. Vechorkin, V. Proust, X. Hu, J. Am. Chem. Soc.
2009, 131, 9756–9766.
[6] L. G. Xie, Z. X. Wang, Angew. Chem. 2011, 123, 5003; Angew.
Chem. Int. Ed. 2011, 50, 4901–4904.
[7] a) M. R. Radlauer, M. W. Day, T. Agapie, Organometallics
2012, 31, 2231–2243; b) A. Berkefeld, S. Mecking, J. Am.
Chem. Soc. 2009, 131, 1565–1574; c) J. C. Jenkins, M. Brookh-
art, J. Am. Chem. Soc. 2004, 126, 5827–5842; d) D. V. Deubel,
T. Ziegler, Organometallics 2002, 21, 4432.
[8] a) W. Heyndrickx, G. Occhipinti, Y. Minenkov, V. R. Jensen,
Chem. Eur. J. 2011, 17, 14628–14642; b) A. Leblanc, E. Grau,
J. P. Broyer, C. Boisson, R. Spitz, V. Monteil, Macromolecules
2011, 44, 3293–3301.
[9] a) S. Chakraborty, Y. J. Patel, J. A. Krause, H. Guan, Polyhe-
dron 2012, 32, 30–34; b) S. Chakraborty, J. Zhang, J. A. Krause,
H. Guan, J. Am. Chem. Soc. 2010, 132, 8872–8873.
[10] S. Kundu, W. W. Brennessel, W. D. Jones, Inorg. Chem. 2011,
50, 9443–9453.
[11] a) J. Berding, J. A. van Paridon, V. H. S. van Rixel, E. Bouw-
man, Eur. J. Inorg. Chem. 2011, 2450–2458; b) B. L. Tran, M.
Pink, D. J. Mindiola, Organometallics 2009, 28, 2234–2243.
[12] I. M. Angulo, E. Bouwman, R. van Gorkum, S. M. Lok, M.
Lutz, A. L. Spek, J. Mol. Catal. A 2003, 202, 97–106.
[13] C. Gonzalez-Arellano, E. Gutierrez-Puebla, M. Iglesias, F.
Sanchez, Eur. J. Inorg. Chem. 2004, 1955–1962.
[14] A. Flores-Gaspar, P. Pinedo-Gonzalez, M. G. Crestani, M.
Munoz-Hernandez, D. Morales-Morales, B. A. Warsop, W. D.
Jones, J. J. Garcia, J. Mol. Catal. A 2009, 309, 1–11.
[15] W. H. Harman, J. C. Peters, J. Am. Chem. Soc. 2012, 134, 5080–
5082.
[27]
[28]
[29]
J. I. van der Vlugt, J. N. H. Reek, Angew. Chem. 2009, 121,
8990; Angew. Chem. Int. Ed. 2009, 48, 8832–8846.
See: S. Schneider, J. Meiners, B. Askevold, Eur. J. Inorg. Chem.
2012, 412–429 and references cited therein.
a) A. N. Marziale, E. Herdtweck, J. Eppinger, S. Schneider, In-
org. Chem. 2009, 48, 3669–3709; b) A. Friedrich, M. Drees,
J. S. auf der Gunne, S. Schneider, J. Am. Chem. Soc. 2009, 131,
17552–17553; c) J. Meiners, M. G. Scheibel, M. H. Lemee-Cail-
leau, S. A. Mason, B. Boeddinghaus, T. Fassler, E. Herdtweck,
M. M. Khusniyarov, S. Schneider, Angew. Chem. 2011, 123,
8334; Angew. Chem. Int. Ed. 2011, 50, 8184–8187.
a) W. Jia, X. Chen, R. Guo, C. Sui-Seng, D. Amoroso, A. J.
Lough, K. Abdur-Rashid, Dalton Trans. 2009, 8301–8307; b)
A. Acosta-Ramirez, M. Bertoli, D. G. Gusev, M. Schlaf, Green
Chem. 2012, 14, 1178–1188.
A. Friedrich, M. Drees, M. Kass, E. Herdtweck, S. Schneider,
Inorg. Chem. 2010, 49, 5482–5494.
a) X. Chen, W. Jia, R. Guo, T. W. Graham, M. A. Gullons, K.
Abdur-Rashid, Dalton Trans. 2009, 1407–1410; b) S. Bi, Q. Xie,
X. Zhao, Y. Zhao, X. Kong, J. Organomet. Chem. 2008, 693,
633–638.
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
S. S. Rozenel, J. B. Kerr, J. Arnold, Dalton Trans. 2011, 40,
10397.
L. C. Liang, P. S. Chien, Y. L. Huang, J. Am. Chem. Soc. 2006,
128, 15562–15563.
O. V. Ozerov, C. Guo, L. Fan, B. M. Foxman, Organometallics
2004, 23, 5573–5580.
L. C. Liang, P. S. Chien, P. Y. Lee, Organometallics 2008, 27,
3082–3093.
1
Yields were determined by H NMR spectroscopy (integration
against an internal standard) and verified by GC–MS; see Exp.
Section for details.
J. F. Sonnenberg, N. Coombs, P. A. Dube, R. H. Morris, J. Am.
Chem. Soc. 2012, 134, 5893–5899.
[38]
8
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
Alkene Hydrogenation Catalyzed by Nickel Hydride Complexes
[39]
[40]
[44] A. Rossin, M. Peruzzini, F. Zanobini, Dalton Trans. 2011, 40,
4447–4452.
[45] A. Castonguay, C. Sui-Seng, D. Zargarian, A. L. Beauchamp,
Organometallics 2006, 25, 602–608.
[46] S. Chakraborty, J. A. Krause, H. Guan, Organometallics 2009,
28, 582–586.
[47] D. L. Reger, Y. Ding, D. G. Garza, L. Lebioda, J. Organomet.
Chem. 1993, 452, 263–270.
G. M. Whitesides, M. Hackett, R. L. Brainard, J. P. M. Laval-
leye, A. F. Sowinski, A. N. Izumi, S. S. Moore, D. W. Brown,
E. M. Staudt, Organometallics 1985, 4, 1819–1830.
C. M. Hagen, J. A. Widegren, P. M. Maitlis, R. G. Finke, J.
Am. Chem. Soc. 2005, 127, 4423–4432.
[41] B. J. Boro, E. N. Duesler, K. I. Goldberg, R. A. Kemp, Inorg.
Chem. 2009, 48, 5081–5087.
[42] J. I. van der Vlugt, M. Lutz, E. A. Pidko, D. Vogt, A. L. Spek,
Dalton Trans. 2009, 1016–1023.
[43] T. J. Schmeier, N. Hazari, C. D. Incarvito, J. A. Raskatov,
Chem. Commun. 2011, 47, 1824–1826.
[48] See Supporting Information for details regarding the isolation
of 5.
Received: July 7, 2012
Published Online: ᭿
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
9

Job/Unit: I20758
/KAP1
Date: 30-08-12 15:40:34
Pages: 10
K. V. Vasudevan, B. L. Scott, S. K. Hanson
FULL PAPER
Earth-Abundant Metal Catalysts
Cationic and neutral nickel(II) hydride
complexes of the aliphatic pincer ligand
K. V. Vasudevan, B. L. Scott,
S. K. Hanson* ................................. 1–10
PNHPCy {PNHPCy
=
HN[CH2CH2P-
(Cy)2]2} have been prepared and were
found to be active alkene hydrogenation
catalysts. Mechanistic experiments suggest
that for cationic catalyst 2, hydrogenation
may proceed through a pathway involving
initial insertion of the alkene into the Ni–
H bond.
Alkene Hydrogenation Catalyzed by Nickel
Hydride Complexes of an Aliphatic PNP
Pincer Ligand
Keywords: Homogeneous catalysis
/
Nickel / Hydride ligands / Hydrogenation /
Earth-abundant metals / Metal–ligand co-
operativity
10
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0