Synthesis, characterization, and reactivity of nickel hydride complexes containing 2,6-C6H3 (CH2PR2) 2 (R = tBu, cHex, and tPr) pincer ligands

Article abstract of DOI:10.1021/ic8020194

The syntheses and full characterization of nickel hydrides containing the PCP "pincef"-type ligand, where PCP = 2,6-C₆H ₃(CH₂PR₂)₂ (R = tBu, cHex, and iPr), are reported. These Ni - H complexes are prepared by the conversion of ( ^RPCP)NiCI precursors into the corresponding nickel hydrides by use of appropriate hydride donors. Surprisingly, although the (^RPCP)NiCI precursors are quite similar chemically, the conversions to the hydrides were not straightforward and required different hydride reagents to provide analytically pure products. While NaBH₄ was effective in the preparation of pure (^tBuPCP)NiH, Super-Hydride solution (LiEt ₃BH in THF) was required to prepare either (^chexPCP)NiH or (^iPrPCP)NiH. Attempts to prepare a Ni - H from (^PhPCP) NiCI with a variety of hydride reagents yielded only the free ligand as an identifiable product. Two of the derivatives, tBu and cHex, have also been subjected to single crystal X-ray analysis. The solid-state structures each showed a classic, near-square planar arrangement for Ni in which the PCP ligand occupied three meridional ligand points with the Ni - H trans to the Ni - C bond. The resulting Ni - H bond lengths were 1.42(3) and 1.55(2) A for the tBu and cHex derivatives, respectively.

Full text of DOI:10.1021/ic8020194

Inorg. Chem. 2009, 48, 5081–5087 5081
DOI: 10.1021/ic8020194
Synthesis, Characterization, and Reactivity of Nickel Hydride Complexes
Containing 2,6-C6H3(CH2PR2)2 (R = tBu, cHex, and iPr) Pincer Ligands
Brian J. Boro,† Eileen N. Duesler,† Karen I. Goldberg,*,‡ and Richard A. Kemp*,†,§
†Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico
87131-2609, ‡Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, and
§Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106
Received October 21, 2008
The syntheses and full characterization of nickel hydrides containing the PCP “pincer”-type ligand, where PCP = 2,6-
C6H3(CH2PR2)2 (R = tBu, cHex, and iPr), are reported. These Ni-H complexes are prepared by the conversion of
(RPCP)NiCl precursors into the corresponding nickel hydrides by use of appropriate hydride donors. Surprisingly,
although the (RPCP)NiCl precursors are quite similar chemically, the conversions to the hydrides were not
straightforward and required different hydride reagents to provide analytically pure products. While NaBH4 was
effective in the preparation of pure (tBuPCP)NiH, Super-Hydride solution (LiEt3BH in THF) was required to prepare
either (cHexPCP)NiH or (iPrPCP)NiH. Attempts to prepare a Ni-H from (PhPCP)NiCl with a variety of hydride reagents
yielded only the free ligand as an identifiable product. Two of the derivatives, tBu and cHex, have also been subjected
to single crystal X-ray analysis. The solid-state structures each showed a classic, near-square planar arrangement for
Ni in which the PCP ligand occupied three meridional ligand points with the Ni-H trans to the Ni-C bond. The
˚
resulting Ni-H bond lengths were 1.42(3) and 1.55(2) A for the tBu and cHex derivatives, respectively.
Introduction
For many years there has existed significant interest in the
preparation and investigation of transition metal complexes
that contain rigid, chelated ligands. More specifically, com-
pounds that utilize anionic, tridentate “pincer”-type ligands
attached to the metal are currently under study by many
groups, and much of the work in this area has been summar-
ized in a recent book devoted to pincer chemistry.1 This high
catalysis can be found in the reports of alkane dehydroge-
level of interest is due largely to the ability to finely tune the
steric and electronic components of the pincer ligand in a
systematic fashion to alter the chemical and physical proper-
ties, and hence reactivity, of the metal center. These trans-
spanning pincer ligands have been shown to be highly
effective in preparing stable tridentate complexes of transi-
tion metals, including square-planar, d8 metal species.2,3
Within the broad class of pincer donor ligands, the PCP
pincers [where PCP = 2,6-C6H3(CH2PR2)2] are of particular
interest for use as ligands in catalysis.2,4 Recent illustrative
examples that demonstrate the usefulness of PCP pincers in
nation,5-10 olefin metathesis,11 Heck reaction chemistry,12-16
transfer hydrogenation,17 and Aldol condensation chemistry.18
One attractive feature of the PCP ligand family is the relative
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Gupta, M.; Jensen, C. M.; Kaska, W. C. Chem. Commun. 1997, 2273–2274.
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*To whom correspondence should be addressed. E-mail: rakemp@
unm.edu.
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r
2009 American Chemical Society
Published on Web 05/20/2009
pubs.acs.org/IC

5082 Inorganic Chemistry, Vol. 48, No. 12, 2009
Boro et al.
synthetic ease of R-group substitution on the pincer arms
yielding a variety of modified phosphine substituents attached
to the bridging atom. While many investigations utilize rela-
tively simple alkyl or aryl derivatives as part of the -PR2
moiety, significant changes in the electronic nature of the -PR2
group can easily be accomplished. To illustrate, alterations
such as using perfluoro-alkyl and aryl substituents19,20 or
π-accepting N-pyrrolyl groups21 have been reported. While
the most-commonly investigated PCP pincer ligands are based
on an aromatic sp2 phenyl ring backbone, it has also been
demonstrated that the carbon ligand component can be part of
an sp3 alkyl chain.22-26 While these saturated species are
indeed technically PCP pincer complexes, in this report we will
restrict ourselves to the more commonly used aromatic ring
pincers.
We have recently reported the first example of an unam-
biguous insertion of O2 into (tBuPCP)PdH to yield the X-ray
crystallographically characterized (tBuPCP)PdOOH pro-
duct,27 and we28 and others29,30 have followed this experi-
mental study with detailed computational studies of the
mechanism. As part of our search for new selective partial
oxidation catalysts that use O2 gas as the oxidant, we have
been interested in expanding the numbers and types of late
transition metal hydrides that contain PCP ligands, largely
focusing on Ni-H and Pd-H species.31 In our preliminary
investigations we were surprised to discover that there are no
pure, well-characterized constituents of the simplest member
of this family;the first-row (RPCP)NiH species;that have
been reported in the literature. Notably PNP analogues
(RPNP)NiH have recently been isolated32-34 and even
characterized by X-ray diffraction.35 Unfortunately, there
is no discussion of the crystallographic parameters or any
specific mention of the Ni-H bonds in the structural study.
There is only one example mentioned in the literature relating
to any aromatic, sp2 PCP-containing nickel hydride, that
being (tBuPCP)NiH reported in 1976 by Moulton and
Shaw.36 In this study, the authors characterized the com-
pound obtained by the NaBH4 reduction of (tBuPCP)NiCl, to
which they assigned the (PCP)NiH structure based on IR
spectroscopy. The authors were unable to observe the Ni-H
resonance by 1H NMR, and suggested an exchange process
was possibly responsible for the inability to observe the
resonance. Lastly, in related work, we note that an sp3
carbon-based PCP pincer complex of a nickel hydride has
recently been reported by Zargarian and Beauchamp.37
Similarly to the previous report from Moulton and Shaw,36
this (PCsp3P)NiH complex was not isolated as an analytically
pure material, but rather as a mixture of the hydride and the
free ligand. However, in this case the complex was unam-
biguously identified by NMR analysis. No structural data
were obtained.
As part of our reactivity studies into new pincer-containing
Group 10 hydrides we have recently prepared a number of
(RPCP)NiH complexes and have crystallographically char-
t
acterized two (RPCP)NiH derivatives containing either Bu
(the complex reported previously by Moulton and Shaw36) or
cHex substituents. The hydride complexes were prepared
from (RPCP)NiCl precursors and the choice of the specific
hydride reagent turned out to be critical to the effective
isolation of pure product samples. It is hoped that this report
of the preparations of high purity samples of these (RPCP)
NiH complexes and their characterization by X-ray crystal-
lography will allow for further exploration of their reactivity.
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Chem. Soc., Dalton Trans. 1982, 1225–1230.
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Experimental Section
General Procedures. All procedures and chemical manip-
ulations were carried out under Ar or vacuum using standard
inert atmosphere or Schlenk techniques. High-purity solvents
were either used as procured or were dried using standard
techniques, and after purification were stored under Ar or in
Ar-filled gloveboxes. All reagents were purchased from com-
mercial suppliers (Aldrich and Alfa) and used without further
purification. 1H, 31P, and 13C NMR spectra were obtained on a
Bruker AMX 250 MHz spectrometer. H and 13C spectra are
1
referenced to SiMe4, while 31P spectra are referenced to exter-
nal 85% H3PO4. Infrared data were obtained on a Bruker
Vector 22 instrument using Nujol mulls under an atmosphere
of flowing nitrogen gas. Elemental analyses were performed
by Columbia Analytical Services, Tucson, AZ. The starting
(RPCP)H ligands 1,3-C6H4[P(tBu)2]2,38 1,3-C6H4[P(cHex)2]2,39
1,3-C6H4[P(iPr)2]2,40 and 1,3-C6H4(PPh2)2 were prepared by
41
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K. I. J. Am. Chem. Soc. 2006, 128, 2508–2509.
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W. A.III; Oxgaard, J. Inorg. Chem. 2006, 45, 9631–9633.
(29) Chowdhury, S.; Rivalta, I.; Russo, N.; Sicilia, E. Chem. Phys. Lett.
2007, 443, 183–189.
literature methods. (PhPCP)NiCl (1d) was prepared as reported
previously.42
(36) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 11,
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Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5083
(
tBuPCP)NiCl (1a). (tBuPCP)NiCl (1a) was prepared with
stirred overnight. The volatiles were removed under vacuum,
and the product extracted in 15 mL of pentane. The slightly
cloudy golden solution was filtered, and the filtrate concen-
trated. Cooling to -35 °C gave the product as orange/brown
crystals. Yield: 0.055 g (70%). Anal. Calcd for C32H52NiP2:
C, 68.95; H, 9.40. Found: C, 68.94; H, 9.58. 1H NMR (C6D6, 250
MHz) δ 7.21 (m, 3H, ArH), 3.17 (vt, 4H, J*HP = 7.75 Hz,
ArCH2), 2.2-1.0 (m, 44H, cyH), -9.9 (t, 1H, 2JHP = 55.5 Hz,
slight modifications to the prior preparation.36 To an ethanol (10
mL) solution of (tBuPCP)H (1.02 g, 2.6 mmol) was added a
solution of NiCl2 (H2O)6 (0.60 g, 2.5 mmol) dissolved in 2 mL
3
of degassed water. The solution was heated to reflux. A golden-
yellow precipitate began to form only after 0.5 h. The solution
was stirred under low reflux overnight. After cooling, the product
was collected by filtration and washed with cold ethanol. 1a can
be recrystallized from a concentrated solution of pentane or
hexanes at -35 °C. Yield: 0.73 g (60%). 1H NMR (C6D6,
Ni-H). 13C{1H} NMR (C6D6, 62.9 MHz) δ 151.6 (vt, J*CP
=
27.0 Hz, Car-o), 125.0 (s, Car-p), 121.2 (vt, J*CP = 17.5 Hz, Car-m),
38.6 (vt, J*CP = 24.6 Hz, PCH2), 34.7 (vt, J*CP = 23.3 Hz,
3
250 MHz): δ 7.0 (t, 1H, JHH = 7.4 Hz, ArHp), 6.84 (d, 2H,
3JHH = 7.4 Hz, ArHm), 2.91 (vt, 4H, J*HP = 6.8 Hz, CH2), 1.40
(vt, 36H, J*HP = 12.7 Hz, CH3). 13C{1H} NMR (C6D6, 62.9
MHz) δ 155.7 (t, 2JCP = 16.7 Hz, Car-i), 153.0 (vt, J*CP = 25.5
Hz, Car-o), 125.2 (s, Car-p), 121.8 (vt, J*CP = 16.7 Hz, Car-m), 34.9
(vt,J*CP =13.4 Hz, PCH2), 34.3(vt, J*CP = 22.7Hz, PC(CH3)3),
29.8 (s, CH3). 31P{1H} NMR (C6D6, 101.3 MHz) δ 66.9.
C
cy-i), 29.3 (s, Ccy), 27.4-27.3 (m, Ccy), 26.8 (s, Ccy). The Car-I
was not observed. 31P{1H} NMR (C6D6, 101.3 MHz) δ 66.9
ppm. IR (Nujol mull) ν(NiH) 1727 cm-1
iPrPCP)NiH (2c). This compound was prepared and iso-
.
(
lated similarly to 2b, with the following quantities used: 1c
(0.066 g, 0.15 mmol), and Super-Hydride solution (0.2 mL, 0.2
mmol, 1 M solution in THF). The product crystallizes as yellow/
(
cHexPCP)NiCl (1b). The procedure followed was similar to
that of 1a using the following quantities of reagents: (cHexPCP)H
(1.0 g, 2.0 mmol) and NiCl2 (H2O)6 (0.50 g, 2.1 mmol). Yield:
0.78 g (66%). Anal. Calcd for C32H51ClNiP2: C, 64.94; H, 8.69.
brown needles. Yield: 0.33 g (55%). Anal. Calcd for
C20H36NiP2: C, 60.49; H, 9.14. Found: C, 60.50; H, 8.99. H
1
NMR (C6D6, 250 MHz) δ 7.24 (m, 3H, ArH), 3.15 (vt, 4H, J*HP
1
Found: C, 65.14; H, 8.81. H NMR (C6D6, 250 MHz) δ 7.11
= 7.75 Hz, ArCH2), 1.92 (m, 4H, CH), 1.23 (dvt, 12H, 3JHH
=
(t, 1H, ArH, 3JHH = 7.7 Hz), 6.95 (d, 2H, 3JHH = 7.7 Hz ArH),
2.86 (vt, 4H, J*HP = 7.75 Hz, ArCH2), 2.5-1.0 (m, 44H, cyH).
7.1 Hz, J*HP = 15.5 Hz, CH3), 0.97 (dvt, 12H, 3JHH = 6.9 Hz,
J*HP = 13.8 Hz, CH3), -9.9 (t, 1H, 2JHP = 55.6 Hz, Ni-H). 13
{1H} NMR (C6D6, 64.9 MHz) δ 151.6 (vt, J*CP = 27.8 Hz,
ar-o), 125.0 (s, Car-p), 121.3 (vt, J*CP = 17.2 Hz, Car-m), 37.9
C
13C{1H} NMR (C6D6, 62.9 MHz) δ 158.4 (t, JCP = 16.7 Hz,
2
C
ar-i), 152.9 (vt, J*CP = 27.4 Hz, Car-o), 125.3 (s, Car-p), 122.3
C
(vt, J*CP = 18.1 Hz, Car-m), 33.7 (vt, J*CP = 25.0 Hz, PCH2),
32.9 (vt, J*CP = 20.9 Hz, Ccy-i), 28.9 (s, Ccy), 27.4-27.3 (m, Ccy),
26.8 (s, Ccy). 31P{1H} NMR (C6D6, 101.3 MHz) δ 51.5.
(vt, J*CP = 24.6 Hz, PCH2 ), 25.1 (vt, J*CP = 23.7 Hz, PCH),
20.0 (s, CH3), 18.8 (s, CH3). 31P{1H} NMR (C6D6, 101.3 MHz) δ
78.2 ppm. IR (Nujol mull) ν(NiH) 1736 cm-1
.
(
iPrPCP)NiCl (1c). The procedure followed was similar to
X-ray Crystallography. Crystallographic data for 2a and 2b
were collected on a standard Bruker X8 APEX2 CCD-based
X-ray diffractometer equipped with an Oxford Cryostream 700
low temperature device and normal focus Mo-target X-ray tube
that of 1a using the following quantities of reagents: (iPrPCP)H
(0.785 g, 2.3 mmol) and NiCl2 (H2O)6 (0.55 g, 2.3 mmol). 1c was
isolated by crystallization from concentrated ethanol solution at
-35 °C. Yield: 0.54 g (54%). Anal. Calcd for C20H35ClNiP2:
˚
(λ = 0.71073 A) operated at 1500 W power (50 kV, 30 mA).
1
C, 55.66; H, 8.17. Found: C, 55.71; H, 7.95. H NMR (C6D6,
Crystals were mounted on nylon cryoloops obtained from
Hampton Research using Paratone-N oil. X-ray data collection
was performed at 228(2) K for both samples. The data collec-
tion and processing utilized the Bruker APEX2 suite of pro-
grams, and the SADABS program was used to correct for
Lorentz polarization effects and absorption.43 The two struc-
tures were solved by direct methods and refined by full-matrix
least-squares calculations based on F2 using the Bruker
SHELXTL (version 6.12) program.44 Non-hydrogen atoms
were refined anisotropically, while hydrogen atoms attached
to carbon were placed in calculated positions. The hydrogen
atoms attached to the Ni atoms in both structures were located,
and more extensive discussion on this topic is given in the text
below. Additionally, in the case of 2b it was apparent there were
disordered solvent molecules present, and so the data were
treated by SQUEEZE.45 Corrections of the data by SQUEEZE
3
250 MHz) δ 7.02 (t, 1H, JHH = 7.5 Hz, CarHp), 6.88 (d, 2H,
3JHH = 7.5 Hz, CarHm), 2.71 (vt, 4H, J*HP = 7.5 Hz, CH2), 2.08
(m, 4H, PCH), 1.44 (dvt, 12H, 3JHH = 7.7 Hz, J*HP = 15.9 Hz,
CH3), 0.95 (dvt, 12H, 3JHH = 6.9 Hz, J*HP = 14.0 Hz, CH3).
2
13C{1H} NMR (C6D6, 62.9 MHz) δ 158.2 (t, JCP = 16.6 Hz,
C
(vt, J*CP = 17.7 Hz, Car-m), 32.6 (vt, J*CP = 25.0 Hz, PCH2),
ar-i), 152.8 (vt, J*CP = 27.0 Hz, Car-o), 125.3 (s, Car-m), 122.4
23.5 (vt, J*CP = 21.2 Hz, PCH), 18.9 (s, CH3), 18.1 (s, CH3). 31
{1H} NMR (C6D6, 101.3 MHz) δ 60.2.
(
P
tBuPCP)NiH (2a). 2a was prepared by a slight modification
of the previous literature procedure.36 1a (0.08 g, 0.16 mmol), and
NaBH4 (0.06 g, 1.5 mmol) were mixed in a flask to which
15 mL of a degassed mixture of benzene and ethanol (1:1) was
added. The solution was stirred overnight and then another
0.20 g of NaBH4 was added. Stirring was continued for approxi-
mately3 h after which degassedwater was added. The solvent was
removed under vacuum, and the product extracted into benzene
(10 mL) and filtered. The solvent was again removed under
vacuum, and pentane was added. After filtering and concentrat-
ing the solution, the product crystallized as golden-brown blocks
at -35 °C. Yield 0.052 g (72%). Anal. Calcd for C24H44NiP2: C,
63.60; H, 9.78. Found: C, 63.79; H, 10.12. 1H NMR (C6D6, 250
MHz) δ 7.22 (m, 3H, ArH), 3.31 (vt, 4H, J*HP = 8.0 Hz, ArCH2),
1.26 (vt, 36H, J*HP = 12.9 Hz, CH3), -10.0 (t, 1H, 3JHP = 52.8
3
˚
(4 void spaces of 794 A with 34 electrons per void) were
consistent with two molecules of highly disordered THF solvent
with roughly 50% occupancy. Thermal ellipsoid plots were
prepared using the Diamond (version 3.1f) software.46 Crystal-
lographic data collection parameters and refinement data are
collected in Table 1.
Results and Discussion
Preparation and Characterization of (RPCP)NiCl
(1a-d) Species. To prepare the desired (RPCP)NiH
complexes, we first prepared the corresponding (RPCP)
NiCl precursors. The free ligand compounds of interest,
(RPCP)H, where R = tBu, cHex, iPr, and Ph, were already
Hz, NiH). 13C{1H} NMR (C6D6, 62.9 MHz) δ 152.7 (vt, J*CP
=
27.3 Hz, Car-o), 124.7 (s, Car-p), 121.0 (vt, J*CP = 17.3 Hz, Car-m),
38.0 (vt, J*CP = 21.5 Hz, PCH2), 33.6 (vt, J*CP = 16.3 Hz, PC
(CH3)3), 29.8 (s, CH3). 31P{1H} NMR (C7D8, 101.3 MHz) δ 99.8
ppm. IR (Nujol mull) ν(NiH) 1754 cm-1
(
.
cHexPCP)NiH (2b). A sample of 1b (0.083 g, 0.14 mmol)
(43) APEX2; Bruker AXS, Inc.: Madison, WI, 2007.
(44) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(45) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
(46) Brandenburg, K. Diamond, 3.1st ed.; Crystal Impact GbR: Bonn,
Germany, 1999.
was dissolved in 8 mL of toluene and cooled to 0 °C in an ice
bath. Super-Hydride solution (0.18 mL, 0.18 mmol, 1 M solu-
tion of LiEt3BH in THF) was added dropwise via syringe.
The solution was allowed to warm to room temperature and

5084 Inorganic Chemistry, Vol. 48, No. 12, 2009
Boro et al.
Table 1. Crystallographic Data and Parameters for Complexes 2a and 2b
Scheme 1. Preparation of Pincer-Containing Nickel Hydrides
2a
2b
empirical formula
Fw
T, K
C
453.24
228(2)
24H44NiP2
C32H52NiP2
557.39
228(2)
cryst size (mm)
cryst syst
space group
0.48 ꢀ 0.44 ꢀ 0.34
tetragonal
I4(1)cd
16.1519(4)
16.1519(4)
19.1801(5)
90
90
90
5003.8(2)
8
1.203
0.51 ꢀ 0.46 ꢀ 0.34
orthorhombic
Ccca
19.960(2)
26.455(3)
27.789(3)
90
90
90
14674(3)
16
1.009
˚
a, A
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
Volume, A
Z
calc. density, g/cm3
μ (Mo KR), mm-1
R1[I > 2σ(I)]a
wR2[I > 2σ(I)]b
0.910
0.0305
0.0766
0.631
0.0408
0.1153
of hydride employed previously with metal pincer halides
appear to be LiAlH4 and NaBH4. Analytically pure 2a
was prepared from (tBuPCP)NiCl in 72% yield using
NaBH4. The proton-decoupled 31P NMR spectrum of
the crude product indicated that 2a was the only
P-containing product present, and after recrystallization
from pentane, orange-brown single crystals suitable for
single-crystal X-ray analysis were obtained. Notably,
Moulton and Shaw were not able to obtain a pure sample
of 2a.36 With a pure sample of 2a in benzene-d6, the Ni-H
resonance clearly appears as a clean triplet at δ 10.0 ppm
(2JP-H = 52.8 Hz) in the 1H NMR spectrum. It is possible
that the impurities present in the earlier work caused an
exchange process to occur at room temperature, thus
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = { w(Fo2 - Fc2)2/ w(Fo2)2}1/2
.
reported in the literature,38-41 and a limited number of
(RPCP)NiCl salts were also known.36,42
Moulton and Shaw had previously synthesized (tBuPCP)
NiCl in 54% yield by reacting (tBuPCP)H directly with
NiCl2 6H2O in a mixed aqueous/ethanolic solution.36 We
followed this general synthetic procedure for the preparation
of 1b-d with the slight modifications mentioned in the
3
t
Experimental Section. While the Bu- and Ph-substituted
nickel chlorides are known compounds,36,42 the cHex and iPr
derivatives have not been reported previously. The new
compounds 1b and 1c were obtained in moderate yields
(54-66%) after recrystallization, and characterized by ele-
mental analysis and multinuclear NMR. In both cases, the
31P NMR resonance appears as a singlet in the same region
1
significantly affecting their H NMR spectrum. Addi-
tionally, the Ni-H stretch of high purity 2a was observed
in the IR spectrum at 1754 cm-1, slightly different from
the previously reported 1730 cm .
-1 36 It was also possible
to prepare 2a using a slight molar excess of Li(Et3BH)
(Super-Hydride solution) in THF as the source of hy-
dride, although 1H NMR analysis indicated that 2a was
not formed cleanly. Even with excess Li(Et3BH) the
reaction did not appear to go to completion, as indicated
by the continued presence of starting material 1a in the
crude product mixture. In solution, 2a reacted relatively
rapidly with air and was thus stored under inert atmo-
sphere.
Unlike 1a, we found it was not possible to prepare the
hydride with NaBH4 when using the cHex derivative 1b.
Reaction of 1b with NaBH4 at room temperature did not
produce 2b, but rather a broadened signal was present in
the 1H NMR spectrum that was consistent with a
[(cHexPCP )Ni][BH4] type complex containing bridging
hydrides.53 Under more forcing conditions of overnight
refluxing, a NaBH4 solution with 1b afforded only a small
amount (<5%) of the desired product as determined by
31P NMR, and the broadened signal seen previously in the
1H NMR spectrum at room temperature was also pre-
sent. Use of LiAlH4 at room temperature gave the
uncomplexed (cHexPCP)H ligand as the only identifi-
able product. Additionally, when K(sBu3BH) in THF
(K-Selectride solution) was used, we again obtained only
small amounts (<25%) of the desired product 2b, in
insufficient yields to warrant further investigation. How-
ever, changing the reductant to Super-Hydride solution in
THF/toluene at room temperature overnight, followed
by removal of solvents and crystallization from pentane
at -35 °C, afforded 2b in 70% yield. The orange-brown
1
as in 1a, and in the H NMR spectra the bridging CH2-
groups appear as a virtual triplet, indicating coupling to both
P arms. The strong coupling creating this virtual triplet is
indicative of the coordination of both arms of the (RPCP)-
ligand to the metal. While spectroscopic data were consistent
with square-planar (RPCP)NiCl structures with trans-span-
ning phosphines (as in 1a and 1d), we recently performed a
single crystal X-ray analysis on 1a to confirm its structure.47
Preparation and Characterization of (RPCP)NiH
(2a-c) Species. Our primary motive in preparing the
(RPCP)NiCl salts was to convert them to the correspond-
ing hydrides, (RPCP)NiH (Scheme 1). The conversions of
various other metal halides containing PCP ligands to
their corresponding hydrides appear in the literature and
can be quite facile.36,48-54 The two most common sources
(47) Boro, B. J.; Dickie, D. A.; Goldberg, K. I.; Kemp, R. A. Acta. Cryst.
E 2008, E64, m1304.
(48) Gusev, D. G.; Dolgushin, F. M.; Antipin, M. Y. Organometallics
2001, 20, 1001–1007.
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2003, 22, 4744–4749.
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tallics 2004, 23, 1766–1776.
(51) Goettker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem.
Soc. 2004, 126, 1804–1811.
(52) Kanzelberger, M.; Zhang, X.; Emge, T. J.; Goldman, A. S.; Zhao, J.;
Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 13644–13645.
(53) Gusev, D. G.; Dolgushin, F. M.; Antipin, M. Y. Organometallics
2000, 19, 3429–3434.
(54) Nemeh, S.; Jensen, C.; Binamira-Soriaga, E.; Kaska, W. C. Organo-
metallics 1983, 2, 1442–1447.

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5085
crystals obtained from pentane were suitable for single
Crystallographic Results. Solid-state X-ray diffraction
data exist for several PCP-containing transition metal
hydrides; however, there are no examples of (RPCP)NiH
complexes that have been structurally characterized. This
is a significant omission, as coordination complexes con-
taining Ni-H bonds have been invoked in a wide number
of academic and industrial catalytic processes.56-65 Inter-
estingly, a search of the Cambridge Structural Database
(CSD, November, 2008 update)66 indicates a paucity of
terminal hydrides attached to square-planar, tetra-coor-
dinate Ni ions, regardless of the other ligands present.
Fortunately, from concentrated pentane solutions held
at -35 °C we were able to grow single crystals of both 2a
and 2b suitable for X-ray analysis. We note that while the
analyzed crystal of 2a darkened significantly during the
data acquisition period and may have experienced degra-
dation while in the X-ray beam at -45 °C, the crystal was
of high enough quality to complete the analysis. A
summary of X-ray data and parameters for both com-
pounds is given in Table 1.
1
crystal X-ray diffraction. The H and 31P NMR spectra
were consistent with a square-planar (cHexPCP)NiH struc-
ture with trans-P arms. Again, as with 2a, the Ni-H
resonance of 2b was clearly observable in the H NMR
1
asa triplet atδ -9.9 ppm (2JP-H = 55.5 Hz). The single 31
P
{1H} signal was found at 66.6 ppm. Additionally, in the IR
spectrum the Ni-H stretch was observed at 1727 cm-1
As with 2a, 2b was stored under inert atmosphere as it was
.
reactive to air in solution.
For the Pr derivative 1c, the use of NaBH4 again
i
proved ineffective in converting the Ni-Cl bond to a
Ni-H bond. Ultimately, the experimental results ob-
tained using 1c mirrored closely those of 1b: the use of
Super-Hydride solution cleanly afforded 2c after recrys-
tallization as yellow-brown crystals in similar, but slightly
lowered, yield (55%). Unfortunately, we were unable to
grow single crystals of 2c that were of sufficient quality for
X-ray diffraction. However, we were able to show by
other spectroscopic methods that the conversion of 1c to
analytically pure 2c had occurred cleanly. Again, the 31P
NMR spectrum showed only a single resonance at 78.2
ppm, indicating the symmetrical nature of the two P(iPr)2
arms. In the 1H NMR spectrum, the iPr group was easily
identifiable, as the methyl group protons appear in this
t
The molecular structure of the Bu derivative 2a is
shown in Figure 1, which also includes the important
bond lengths and angles found in the structure. The over-
all structure of 2a is consistent with the classic Group 10
pincer structure, a slightly distorted square-planar coor-
dination geometry for Ni containing two trans-phosphine
arms. The other two coordination sites to Ni are filled
with the C(1) of the phenyl ring and the hydride, H(1)
(vide infra). The four coordinating atoms bound to nickel
are coplanar with Ni(1), and form a plane that is twisted
by approximately 8° from the plane formed by the six
aromatic ring carbons. This creates a C2 rotational axis in
2a through the H(1)-Ni(1)-C(1)-C(4) atoms, along the c
axis that is present. The P(1)-Ni(1)-P(1) angle is 173.54
(3)°, which is much closer to linearity than has previously
been seen in the related complexes (iPrPCP)NiX (X = -I,67
-OH,55 NH2,68 or OCH368), (cHexPCP)NiBr,69 (PhPCP)
NiBr,70 and (tBuPCP)NiCl47 in which the P-Ni-P angle
ranges from 165.1 to 169.7°. One might expect that as the
P-Ni-P angle increases toward linearity in this family of
compounds that the Ni-C bond would appreciably
shorten to accommodate this widening. However, the
(
iPrPCP)NiH structure as a doublet of virtual triplets, as
has been seen earlier by Campora in a (iPrPCP)Ni(ONO2)
complex.55 The methine proton is found in 2c as a
symmetrical multiplet. Upon P-decoupling, the doublets
of virtual triplets collapse to simple doublets. The Ni-H
resonance is observed as a triplet at δ -9.9 ppm (2JP-H
=
55.6 Hz). As well, the Ni-H bond stretch is found in the
IR at 1736 cm-1
.
Quite surprisingly, all attempts to convert the phenyl
derivative 1d to the corresponding hydride 2d were
unsuccessful. Use of any of the hydride reagents men-
tioned above led to immediate darkening of the solution
and apparent decomposition of 1d. This was somewhat
surprising as the reducing ability of the different re-
agents varied over a significant range. However, in no
case did we observe any product that could be shown to
contain both a PCP ligand and a nickel hydride. We
also utilized an additional hydride source, NaBH3CN
in THF solution, that did not appear to immediately
decompose 1d at room temperature. However, NMR
evaluation of the crude solution did not contain any
resonances that could be attributed to the nickel hy-
dride. Interestingly, a recent paper from Liang has
reported the X-ray structures of related (RPNP)NiH
compounds prepared by oxidative addition of the
(RPNP)H ligand to Ni(COD)2.35 Liang noted that while
the diphenylphosphino-substituted PNP nickel hydride
could be generated in solution and identified via spec-
troscopic means, it could not be isolated under the
conditions used, as the COD byproduct inserted into
the nickel hydride to form an Ni alkyl complex. Only
the PNP nickel hydrides prepared by Liang that
contained alkyl-substituted phosphines could be iso-
lated and characterized fully, results that are quite
similar to ours.
(56) Harmer, J.; Finazzo, C.; Piskorski, R.; Ebner, S.; Duin, E. C.;
Goenrich, M.; Thauer, R. K.; Reiher, M.; Schweiger, A.; Hinderberger,
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101, 1066–1067.
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269–272.
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1973, 174, 65–71.
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2255–2259.
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Milstein, D. Inorg. Chim. Acta 2004, 357, 4015–4023.
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Organometallics 2004, 23, 5653–5655.
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5086 Inorganic Chemistry, Vol. 48, No. 12, 2009
Boro et al.
Figure 1. Molecular structure of 2a (50% ellipsoids). The H atoms
˚
attached to carbon are omitted for clarity. Important bond lengths (A)
and angles (deg) are asfollows:Ni(1)-H(1)1.42(3), Ni(1)-P(1)2.1278(3),
Ni(1)-C(1) 1.920(2), C(1)-Ni(1)-H(1) 180.000(5), C(1)-Ni(1)-P(1)
86.768(14), P(1)-Ni(1)-H(1) 93.232(15), P(1)-Ni(1)-P(1) 173.54(3).
Figure 2. Molecular structure of 2b (50% ellipsoids). The H atoms
˚
attached to carbon are omitted for clarity. Important bond lengths (A)
and angles(deg)are asfollows: Ni(1)-H(1) 1.55(2), Ni(1)-P(1)2.1248(4),
Ni(1)-P(2) 2.1300(4), Ni(1)-C(1) 1.9263(14), C(1)-Ni(1)-H(1) 176.7
(8), P(1)-Ni(1)-P(2) 173.207(17), C(1)-Ni(1)-P(1) 86.41(5), C(1)-Ni
(1)-P(2) 86.90(5), P(1)-Ni(1)-H(1) 92.2(8), P(2)-Ni(1)-H(1) 94.5(8).
˚
Ni-C bond length in 2a is 1.920(2) A, the midpoint within
the range of Ni-C bonds found in these related (RPCP)
˚
chemically unrealistic value of 0.92 A for the Ni-H bond
NiX compounds, which has a range of Ni-C bond
length. We believe that the large residual electron density
is likely due to the Fourier series truncation effect because
of a lack of collected data,72 as the two residual electron
density peaks are located on a symmetry axis containing
the Ni atom. To solve this issue, we initially constrained
˚
lengths from 1.908-1.931 A. Additionally, this suggests
that there is no significant trans influence present, as the
two ligands with the largest difference in σ-donor
ability, -H and -OH, result in identical Ni-C bond lengths
trans to these ligands.71 The P(1)-Ni(1) bond length
˚
the Ni-H bond length to be 1.50 A, and then allowed the
˚
(2.1278(3) A) is somewhat shorter than the P-Ni bonds
Ni-H bond to vary during refinement. The value of the
Ni-H bond length resulting from this model treatment in
2a settled at 1.42(3) A, well within the range seen earlier
measured in the previously characterized (RPCP)NiX
pincer complexes listed above. The reduction in bond
˚
˚
length of ∼0.05 A in 2a can be attributed to the smaller
for other Ni-H bonds attached to four-coordinate Ni
atoms, and is consistent with the value obtained for 2b
(vide infra).
ligand (X = H) present in 2a relative to the other ligands
(X = -I, -Br, -Cl, -OH, -NH2, or -OCH3). This smaller
ligand explanation can also be utilized to help understand
why the P(1)-Ni(1)-P(1) bond angle widens and is sig-
nificantly closer to linearity in 2a than in the other related
compounds. Other bond angles within the primary coor-
dination sphere of Ni in 2a are similar to other (RPCP)NiX
species, and need not be discussed further. However,
location of the hydride attached to Ni proved somewhat
more challenging.
The molecular structure of the related cHex derivative
2b is shown in Figure 2, along with pertinent bond
distances and angles. The overall structural features of
2b appear largely similar to 2a; however, there are some
significant differences in details between the two com-
pounds. First, the molecular structure of 2b is of lower
symmetry than is seen in 2a, as 2b does not contain a C2
rotational axis or any other symmetry elements to inter-
relate the two halves of the molecule. Unlike 2a, the four
coordinating atoms [P(1), P(2), C(1), and H(1)] attached
to Ni(1) in 2b are not coplanar with Ni(1). This lack of
symmetry leads to slightly different Ni(1)-P(1) and Ni
In the search of the CSD (November, 2008 update)66
for prior structurally characterized, tetra-coordinated
nickel atoms containing a terminal hydride ligand we
found only six examples, none of which were closely
related structurally to 2a (see Supporting Information
for search details). The Ni-H bonds measured in these
˚
(1)-P(2) bond lengths [2.1248(4), 2.1300(4) A], as well as
unique P(1)-Ni(1)-H(1) and P(2)-Ni(1)-H(1) bond
angles [92.2(8), 94.5(8)°]. The Ni(1)-C(1) bond length
˚
compounds ranged from 1.327 to 1.654 A, a wide series
that indicates the variety of ligand environments and
ionic charges present within this small group of com-
pounds. In solving the structure of 2a, all H atoms were
refined in calculated positions with the exception of the
Ni-H bond. We initially allowed the hydrogen attached
to the nickel atom to vary in position and Uiso. However,
when doing this, two large residual electron density peaks
appeared, located on a symmetry axis around nickel, at a
˚
of 1.9263(14) A seen in 2b is very similar to the Ni-C
bond seen in 2a (1.920(2) A). As was seen above, the P
˚
(1)-Ni(1)-P(2) bond angle of 173.207(17)° of 2b is
opened up more toward linearity than has been seen with
the previously mentioned structures of (RPCP)NiX
(where X ¼ H). The C(1)-Ni(1)-H(1) bond angle also
approaches linearity at 176.7(8)°.
Unlike the situation above with 2a, in the case of 2b
there were no crystallographic issues dealing with locat-
ing the hydrogen atom attached to Ni. The Ni-H bond
˚
distance of approximately 0.8-0.9 A from Ni. The Uiso
value tended to be small and even negative, indicating
there was more electron density present than a hydrogen
atom should normally allow. We then fixed Uiso at a small
value (0.02), and this led to a final solution with a
˚
length was found to be 1.55(2) A, somewhat longer than
the one found in 2a, but quite reasonable relative to
(71) Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 3rd ed.; Pearson
(72) Crystal Structure Refinement:
A Crystallographer’s Guide to
Prentice Hall, 2004.
SHELXL; Muller, P., Ed.; Oxford University Press: New York, 2006.

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5087
Ni-H bond lengths found in previously studied Ni-H
compounds.
THF) was required to prepare the cHex- and iPr-containing
derivatives. Use of NaBH4 or LiAlH4, both common redu-
cing agents, yielded products consistent with overreduction
of the Ni. Attempts to reduce the (PhPCP)-derivative yielded
only the free ligand as an identifiable product.
Summary
We report herein the syntheses and initial single crystal
X-ray structural characterizations of nickel hydrides contain-
ing the trans-spanning RPCP-pincer ligand, where R = tBu
or cHex. The structures each showed a classic, near-square
planar arrangement for Ni in which the PCP ligand occupied
three meridional ligand points and the Ni-H was trans to the
Ni-C bond. The resulting Ni-H bond lengths were found to
Acknowledgment. This work was supported by the
Department of Energy (DE-FG02-06ER15765) via a
grant to R.A.K. and K.I.G. The Bruker X-ray diffract-
ometer was purchased via a National Science Foundation
CRIF:MU award to the University of New Mexico
(CHE-0443580). Sandia is a multiprogram laboratory
operated by Sandia Corporation, a Lockheed Martin
Company, for the United States Department of Energy
under Contract No. DE-AC04-94AL85000.
˚
be 1.42(3) and 1.55(2) A for the tBu and cHex derivatives,
respectively. The iPr derivative could also be prepared
cleanly, although its solid-state structure could not be deter-
mined. These complexes were prepared by the conversion of
the precursor (RPCP)NiX compounds in the hydrides by use
of suitable hydride donors. Surprisingly, although the
(RPCP)NiX precursors are quite similar, the conversions to
the hydrides were not straightforward and required different
hydride reagents to effect the transformations. The common
reducing agent NaBH4 was effective only in the (tBuPCP)-
containing complex. Super-Hydride solution (LiEt3BH in
Supporting Information Available: The search parameters
used and a listing of results obtained for previous Ni-H
containing complexes in the Cambridge Structural Database
are given. As well, crystallographic data in CIF format for 2a
and 2b. This material is available free of charge via the Internet
at http://pubs.acs.org.