Effects of phosphine-carbene substitutions on the electrochemical and thermodynamic properties of nickel complexes

Article abstract of DOI:10.1021/om500206e

Nickel(II) complexes containing chelating N-heterocyclic carbene-phosphine ligands ([NiL₂](BPh₄)₂, for which L = [MeIm(CH₂)₂PR₂], MeIm = 1- methylimidizolylidene, and R = Ph or Et) have been synthesized for the purpose of studying how this class of ligand affects the electrochemical and thermodynamic properties compared to the nickel bis-diphosphine analogues. The nickel complexes were synthesized and then characterized by X-ray crystallography, electrochemical methods, and thermodynamic studies, including DFT calculations. On the basis of the reduction potentials (E°), substitution of an NHC for one of the phosphines in a diphoshine ligand resulted in negative shifts in potential by 0.6 to 1.2 V relative to the corresponding nickel bis-diphosphine complexes. From computational studies of the nickel hydride complex of the phenyl-substituted phosphine-carbene ligand, the hydride donor ability was determined to improve by 32 kcal/mol relative to the estimated hydride donor ability for the analogous nickel complex of the chelating diphosphine ligand 1,3-bis(diphenylphosphino)propane. The free energy for addition of H₂ is presented, and the implications for catalysis are discussed. These quantitative results highlight the substantial effect that NHC ligands can have on the electronic properties of the metal complexes.

Full text of DOI:10.1021/om500206e

Article
pubs.acs.org/Organometallics
Effects of Phosphine−Carbene Substitutions on the Electrochemical
and Thermodynamic Properties of Nickel Complexes
Brandon R. Galan, Eric S. Wiedner, Monte L. Helm, John C. Linehan, and Aaron M. Appel*
Pacific Northwest National Laboratory Richland, Washington 99352, United States
S
* Supporting Information
ABSTRACT: Nickel(II) complexes containing chelating N-
heterocyclic carbene−phosphine ligands ([NiL2](BPh4)2, for
which L = [MeIm(CH2)2PR2], MeIm = 1-methylimidizolyli-
dene, and R = Ph or Et) have been synthesized for the purpose
of studying how this class of ligand affects the electrochemical
and thermodynamic properties compared to the nickel bis-
diphosphine analogues. The nickel complexes were synthe-
sized and then characterized by X-ray crystallography,
electrochemical methods, and thermodynamic studies, includ-
ing DFT calculations. On the basis of the reduction potentials
(E°), substitution of an NHC for one of the phosphines in a diphoshine ligand resulted in negative shifts in potential by 0.6 to
1.2 V relative to the corresponding nickel bis-diphosphine complexes. From computational studies of the nickel hydride complex
of the phenyl-substituted phosphine−carbene ligand, the hydride donor ability was determined to improve by 32 kcal/mol
relative to the estimated hydride donor ability for the analogous nickel complex of the chelating diphosphine ligand 1,3-
bis(diphenylphosphino)propane. The free energy for addition of H2 is presented, and the implications for catalysis are discussed.
These quantitative results highlight the substantial effect that NHC ligands can have on the electronic properties of the metal
complexes.
hydride.17 Several studies of ruthenium−NHC complexes have
INTRODUCTION
■
also been reported.18 Demonceau et al. correlated the reduction
The application of N-heterocyclic carbenes (NHCs) in
organometallic and coordination chemistry has undergone a
potentials of RuCl2(p-cymene)(NHC) complexes to the
efficacy for atom transfer radical polymerization (ATRP) of
renaissance1 since the isolation of the first stable, “free” carbene
methyl methacrylate (MMA).19 Electrochemical studies
by Arduengo in 1991,2 despite metal NHC complexes being
known since the late 1960s.3,4 Due to their strong σ-donating
properties, NHCs typically form stronger bonds to transition
performed by Xie et al. show that the potentials of the
Ru(III/II) couple for bridged carboranyl−cyclopentadienyl
ruthenium complexes are more negative when NHCs are
metal centers than the corresponding tertiary phosphine
employed as ancillary ligands then when compared to
analogues, resulting in more robust complexes, which are less
traditional nitrogen and phosphorus donors.20 Finally, Plenio
prone to decomposition via ligand dissociation.5−7 As such,
reported redox couples for a variety of NHC-modified second-
ongoing research has been dedicated to the modification of the
generation Grubbs and Hoveyda−Blechert metathesis cata-
NHC ligands and quantifying the electronic properties imparted
lysts.21,22
by NHCs to transition metal complexes.1,5,7−11
In more recent publications, there has been an effort in
Numerous studies, both experimental and theoretical, have
been reported for the use of techniques to quantify the
electron-donating ability of various NHC−transition metal
complexes.6,8,9,11−13 The most common method is observing
the ν(CO) of various [Ni(CO)3(NHC)] and [MCl(CO)-
(NHC)] (M = Ir or Rh) by IR spectroscopy, as reported by
Nolan,7,11 Crabtree,14 and others.6,15,16 Direct measurement of
the redox potentials using electrochemical methods, though
reported, has been underutilized in determining the electron-
donating ability of NHCs to transition metal centers. Roberts et
al. have recently shown that replacement of a PMe3 with IMes
(1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene) in CpW-
(CO2)2(L)H shifts the W•/− couple more negative, from
−1.12 V to −1.65 V versus ferrocene, resulting in a lower
BDFE of the W−H bond and increase in the pKa(MH) of the
synthesizing and studying mixed ligand systems, ligands that
contain both NHC and heteroatom donor functionalities,
particularly phosphine.23−36 One of the goals of our research is
developing an understanding of how to control thermochemical
properties of metal complexes through ligand variation, and
therefore the thermochemical effect of substitution of a
diphosphine ligand with a mixed ligand containing an NHC
is of great interest. In particular, nickel hydride complexes of
diphosphine ligands are insufficiently hydridic37,38 to convert
CO2 to formate,39−41 unlike the cobalt analogues.42 The
replacement of phosphines with NHC donors (Scheme 1) is
Received: February 28, 2014
Published: April 23, 2014
© 2014 American Chemical Society
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Scheme 1. Nickel Complexes of Diphosphine Ligands and
Newly Prepared Analogues Containing NHC Donors.
Scheme 3. Synthesis of Nickel Complexes from Chelating
Ligands
molar ratio of HLPh(BPh4) or HLEt(BPh4), NiCl2·6H2O, and
NaOAc·2H2O was combined in DMSO and heated to 70 °C
for 3 h. Removal of the DMSO by distillation followed by
washing the residue with distilled water and methanol afforded
expected to improve the hydride donor abilities of metal
complexes,17 but the extent to which the hydride donor abilities
of Ni−H complexes can be improved relative to nickel bis-
disphosphine complexes has not been previously determined.
We report the synthesis and characterization of two new
nickel(II) complexes with chelating ligands that contain both a
phosphine and an NHC. Electrochemical studies of these
complexes have been used to measure the effect of substitution
of diphosphine ligands with the chelating phosphine−carbene
ligands. To explain the reactivity of these new complexes, DFT
studies have been used to quantify the heterolytic and
homolytic bond strengths for one of the hydride complexes,
as well as free energy for H2 addition to the corresponding
Ni(II) complex.
pure [NiLR ](BPh4)2 as pale yellow, air stable solids.
2
2+
The 1H and 31P{1H} NMR spectra of [NiLPh
]
and
2
2+
[NiLEt
]
are typical of diamagnetic, low-spin, Ni(II)
2
complexes. Diastereotopic proton resonances corresponding
to the ethylene bridge are observed as multiplets with ranges of
4.2−4.8 ppm for the methylene adjacent to the C3-nitrogen of
the imidizolylidene and 1.9−2.2 ppm for the methylene
adjacent to the phosphorus atom. The ethylene bridging
2+
protons for [NiLPh
]
are shifted further downfield compared
2
to [NiLEt2]2+, consistent with the more electron-withdrawing
nature of the phosphine substituent. Each complex exhibits a
single resonance in the 31P{1H} spectrum with values of 22.5
and 23.1 ppm for the Ph and Et complexes, respectively. The
observation of a singlet in the 31P{1H} NMR spectrum of each
complex suggests that only one of the two possible isomers is
formed, specifically that the phosphorus atoms of both
chelating ligands occupy sites either cis or trans to one another.
X-ray Diffraction Studies. Single crystals of [NiLPh2]-
(BPh4)2 suitable for X-ray diffraction were obtained by vapor
diffusion of diethyl ether into a saturated solution of
[NiLPh2](BPh4)2 in acetonitrile. X-ray quality crystals of
[NiLEt2](BPh4)2 were grown by slow cooling of a hot, saturated
acetonitrile solution of [NiLEt2](BPh4)2 to room temperature.
The corresponding structures are shown in Figures 1 and 2,
along with selected bond distances and angles.
RESULTS
■
Synthesis of Ligand Precursors and Ni(II) Complexes.
The ligand precursors [MeIm(CH2)2PR2](BPh4) were pre-
pared in good yield using a slightly modified literature
procedure previously reported by Field et al.24 Alkylation of
1-methylimidizole with the appropriate dihaloalkane yielded the
desired 3-(2-bromoethyl)-1-methylimidazolium and 3-(2-chlor-
oethyl)-1-methylimidazolium salts (Scheme 2). Substitution of
Scheme 2. Synthesis of Phosphine−Carbene Ligand
Precursors.
Both structures show a slightly distorted square planar
geometry around the nickel(II) center. In each complex, ligands
the 3-(2-bromoethyl)-1-methylimidazolium bromide with
lithium diphenylphosphide in THF at room temperature
followed by anion exchange with sodium tetraphenylborate
(NaBPh4) gave the carbene precursor HLPh(BPh4).
When 3-(2-bromoethyl)-1-methylimidazolium bromide was
treated with LiPEt2, a significant amount of 1-methyl-3-
vinylimidazolium was observed as a major byproduct via an
elimination pathway. To avoid this elimination pathway, the
chloro-substituted equivalent was used (structure shown in
Figure S1). The treatment of 3-(2-chloroethyl)-1-methylimida-
zolium bromide with LiPEt2 followed by anion exchange with
NaBPh4 resulted in the clean isolation of the desired
substitution product HLEt(BPh4).
Figure 1. Thermal ellipsoid plot of the crystal structure of
[NiLPh2](BPh4)2. Thermal ellipsoids are shown at the 50% probability
level. Hydrogen atoms and the (BPh4)− anions are omitted for clarity.
Selected bond lengths (Å) and bond angles (deg): Ni1−P1 2.1935(5),
Ni1−P2 2.2019(5), Ni1−C15 1.9038(19), Ni−C33 1.9035(19); P1−
Ni1−P2 168.43(3), C15−Ni1−P1 86.95(6), C15−Ni1−P2 98.27(6),
C33−Ni1−P1 93.61(6), C33−Ni1−P2 80.66(6), C33−Ni1−C15
176.99(8).
Synthesis and Characterization of Ni(II) Complexes
[NiLR ](BPh4)2. The metalation of the HLR(BPh4) ligands was
2
performed using a one-pot procedure analogous to that
reported by Lee et al.,27 as shown in Scheme 3. A 2:1:2
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Figure 2. Thermal ellipsoid plot of the crystal structure of
[NiLEt2](BPh4)2. Thermal ellipsoids are shown at the 50% probability
level. Hydrogen atoms and the (BPh4)− anions are omitted for clarity.
Selected bond lengths (Å) and bond angles (deg): Ni1−P1 2.1988(9),
Ni1−P2 2.1931(9), Ni1−C1 1.872(3), Ni1−C11 1.889(3); P2−Ni1−
P1 164.74(4), C1−Ni1−P1 84.61(10), C1−Ni1−P2 93.36(9), C1−
Ni1−C11 178.48(13), C11−Ni1−P1 95.95(9), C11−Ni1−P2
86.48(9).
derived from HLPh(BPh4) and HLEt(BPh4) are coordinated to
nickel(II) by the phosphorus and carbene donor atoms, where
like donors are oriented trans to one another, consistent with
the observation of a single isomer in the 31P{1H} NMR spectra.
Deviations from an ideal square planar geometry occur along
the P−Ni−P bond angle. In complex [NiLPh2]2+, the P1−Ni1−
2+
P2 angle is 168.43(3)°, and in [NiLEt
]
the P1−Ni1−P2
2
angle is 164.74(4)°. Similar deviations have been observed in
analogous nickel complexes.27
Electrochemical Studies. Cyclic voltammograms were
obtained for both [NiLR ]2+ complexes in acetonitrile and are
2
2+
shown in Figure 3. Both of the complexes [NiLPh
]
and
2
Figure 3. Cyclic voltammograms of [NiLPh
]
2
(top) and [NiLEt
]
2
2+
2+
2+
[NiLEt
]
2
display two distinct and reversible reduction waves
(bottom) in 0.2 M [NBu4](PF6) supporting electrolyte in acetonitrile,
glassy carbon working electrode, and scan rate of 50 mV/s. Potentials
referenced with respect to the ferrocenium/ferrocene couple.
assigned to the Ni(II/I) and Ni(I/0) couples with peak-to-peak
separations, ΔEp, ranging from 60 to 70 mV, consistent with
the ΔEp value for the ferrocenium/ferrocene couple (66 mV)
under identical conditions, as shown in Table 1. The analogous
ΔEp values for the complexes and the internal reference suggest
that the couples for the complexes are electrochemically
reversible one-electron processes. A comparison of the Ni(II/I)
and Ni(I/0) couples is contained in Table 1, for the new
carbene-containing complexes as well as the analogous bis-
diphosphine complexes.40,41,43 Due to the strong electron-
donating abilities of the NHC moiety, the Ni(II/I) and Ni(I/0)
potentials have shifted to much more negative potentials.
Table 1. Electrochemical Data for [NiLPh 2+ and [NiLEt2]2+,
]
2
as Well as Literature Values for Related Complexes
a
a
complex
E1/2(II/I)
E1/2(I/0)
b
[Ni(dppp)2]2+
−0.19 (−)
−0.61 (74)
−1.38 (70)
−1.62 (60)
−0.91 (−)
−1.34 (71)
−1.69 (65)
−1.96 (65)
b
[Ni(depp)2]2+
[NiLPh
[NiLEt
]
2
2+
2+
]
2
a
Values in parentheses are ΔEp. Values in CH3CN with potentials
Thermodynamic Studies. To evaluate the possible use of
b
reported to the Cp2 Fe+/ 0 couple. dppp is 1,3-bis-
(diphenylphosphino)propane, and depp is 1,3-bis(diethylphosphino)-
propane. Potentials reported by Berning et al.40 and Curtis et al.43
2+
the [NiLPh
]
complexes for the catalytic hydrogenation of
2
CO2, the complexes were reacted with H2 in the presences of a
base to observe and quantify the generation of the hydride
complex, [HNiLPh2]+. At one atmosphere of H2 in the presence
of 10 equivalents of triethylamine (pKa of 18.8 in MeCN),44
hydride formation was not observed. When using stronger
bases, such as DBU (pKa of 24.3 in MeCN),44 the predominant
observed reaction was the formation of unidentified para-
magnetic species. This reactivity is distinct from that observed
for nickel bis-diphosphine complexes, for which the reaction
with H2 and a base produces the nickel hydride complex and a
protonated base.41
ligand LPh using the methods previously reported for bis-
diphosphine complexes of nickel.38 An isodesmic reaction was
+
constructed for hydride transfer from [HNiLPh
]
to [Ni-
2
(dmpp)2]2+ (eq 1, where dmpp is 1,3-bis(dimethylphosphino)-
propane) using a computational protocol that has been shown
to predict thermodynamic properties of [HNi(diphosphine)2]+
complexes within 2 kcal mol−1.38 By combining the reaction
free energy computed for eq 1 (−20.1 kcal/mol) and the
experimentally measured ΔG°H− of [HNi(dmpp)2]+ (61.2 kcal
To understand this reactivity, DFT calculations were
performed for nickel complexes of the phenyl-substituted
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mol−1),40 a ΔGH− of 41.1 kcal mol−1 was calculated for
[HNiLPh2]+.
of H2,45,46 a thermochemical cycle can be used to determine the
free energy for the addition of H2 to [NiLPh2]2+, as shown
through adding eqs 13−15 to give eq 16. The resulting value of
+28.4 kcal/mol at one atmosphere of H2 is consistent with the
addition of H2 being remarkably endergonic at any attainable
pressure.
Ph
[HNiL 2]+ + [Ni(dmpp)2]2+
Ph 2+
⇌ [NiL
]
+ [HNi(dmpp)2]+
(1)
2
This computed hydride donor ability and the experimentally
measured E1/2(II/I) and E1/2(I/0) values can be used in known
thermochemical cycles45−49 to calculate the solution homolytic
bond dissociation free energy (ΔG°H• = 46.9 kcal mol−1) and
the pKa value (23.7) for [HNiLPh2]+. The sum of eqs 2−4 is eq
5, which gives the homolytic bond dissociation free energy for
[HNiLPh2]+.
Ph 2+
Ph
+
[NiL
]
+ H− ⇌ [HNiL
]
2
−41.1 kcal/mol
(13)
2
Ph
Ph 2+
[HNiL 2]+ + H+ ⇌ [(H)2NiL
]
2
4.8 (−6.5 kcal/mol)
H2 ⇌ H+ + H− 76.0 kcal/mol
(14)
(15)
Ph
Ph 2+
[HNiL 2]+ ⇌ [NiL
]
+ H− +41.1 kcal/mol
(2)
Ph 2+
Ph 2+
2
[NiL
]
+ H2 ⇌ [(H)2NiL
]
2
+28.4 kcal/mol
(16)
2
Ph 2+
Ph
+
[NiL
]
+ e− ⇌ [NiL
]
2
2
The hydride donor ability, homolytic bond dissociation free
energy, pKa values, electrochemical potentials, and the free
energy for H2 addition were combined into the thermochemical
scheme shown in Scheme 4.
−1.38 V (+31.8 kcal/mol)
H− ⇌ H• + e− −26.0 kcal/mol
(3)
(4)
Ph
Ph
[HNiL 2]+ ⇌ [NiL 2]+ + H• +46.9 kcal/mol
(5)
Scheme 4. Thermochemical Scheme for [NiLPh
Related Complexes
]
and
2+
2
a
Similarly, the sum of eqs 6−9 is eq 10, which gives the pKa
value for [HNiLPh2]+.
Ph
Ph 2+
[HNiL 2]+ ⇌ [NiL
]
+ H− +41.1 kcal/mol
(6)
(7)
2
Ph 2+
Ph
+
[NiL
]
+ e− ⇌ [NiL
]
2
2
−1.38 V (+31.8 kcal/mol)
Ph
Ph
[NiL 2]+ + e− ⇌ NiL
−1.69 V (+39.0 kcal/mol)
2
(8)
(9)
H− ⇌ H+ + 2e− −79.6 kcal/mol
Ph
Ph
[HNiL 2]+ ⇌ NiL + H+ 23.7 (+32.3 kcal/mol)
2
(10)
An isodesmic reaction was also constructed for proton transfer
from [HNiLPh2]+ to Ni(dmpp)2 (eq 2) as a secondary check on
the computed bond energies. A pKa value of 23.1 was calculated
+
for [HNiLPh
]
by combining the reaction free energy
2
computed for eq 11 (−1.11 kcal mol−1) and the experimentally
measured pKa of [HNi(dmpp)2]+ (23.9).40 The pKa values of
[HNiLPh2]+ obtained by direct computation and by thermo-
chemical cycles employing the computed ΔGH− value differed
by only 0.6 pKa units (0.8 kcal mol−1), which is consistent with
the previously observed accuracy of this computational
protocol.38
a
Values in parentheses were determined computationally. Values in
brackets were determined using thermochemical cycles.
DISCUSSION
■
Ph
Ph
[HNiL 2]+ + Ni(dmpp)2 ⇌ NiL + [HNi(dmpp)2]+
Through the synthesis of [NiLPh
]
and [NiLEt2]2+, a
2+
2
2
(11)
comparison of nickel complexes of different chelating ligands
can be made, specifically for the purpose of quantifying the
effect of NHC ligands relative to phosphine ligands. These two
NHC-containing complexes both have a three-atom bridge
between the donor atoms and are thereby analogous to
[Ni(dppp)2]2+ and [Ni(depp)2]2+, respectively (where dppp is
1,3-bis(diphenylphosphino)propane and depp is 1,3-bis-
(diethylphosphino)propane; see Scheme 1). While the solid-
state structures of [Ni(dppp)2]2+ and [Ni(depp)2]2+ have not
been reported, the analogous structure for [Ni(dmpp)2]2+ has
been reported, both with acetonitrile bound and in the absence
A third isodesmic reaction was constructed for proton transfer
from [(H)2NiLPh2]2+, a formally Ni(IV) dihydride complex, to
[HNi(dmpp)2]+ (eq 12). Combination of the reaction free
energy for eq 12 (7.7 kcal mol−1) with the previously computed
pKa of [(H)2Ni(dmpp)2]2+ (−0.8)38 affords a pKa of 4.8 for
[(H)2NiLPh2]2+.
Ph 2+
[(H)2NiL
]
+ [HNi(dmpp)2]+
2
Ph
⇌ [HNiL 2]+ + [(H)2Ni(dmpp)2]2+
(12)
From the hydride donor ability of [HNiLPh2]+, the pKa value
for [(H)2NiLPh2]2+, and the free energy for heterolytic cleavage
of acetonitrile40 (where dmpp is 1,3-bis(dimethylphosphino)-
propane). Similar structures are observed for [NiLPh
]
and
2+
2
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[NiLEt2]2+, but acetonitrile is not observed to bind in either
structure, even though the crystals for each were grown in the
presence of acetonitrile. These results are consistent with a
complex of more strongly σ-donating ligands, which may result
in weaker binding of acetonitrile to the metal complexes.
The reduction potentials for the complexes containing
NHCs were found to be substantially more negative than the
diphosphine counterparts, as shown in Table 1. The Ni(II/I)
couples were observed to be 1.2 and 1.0 V more negative for
the complexes with phenyl substituents and ethyl substituents,
respectively, at phosphorus. Similarly, the Ni(I/0) couples were
observed to be 0.8 V more negative for the complexes with
phenyl substituents and 0.6 V more negative for the complexes
with ethyl substituents. The larger effect of the substitution on
the Ni(II/I) couple than the Ni(I/0) couple may be the result
of steric effects, as previously suggested for varying bis-
diphosphine complexes of nickel.40,50 Consistent with this
by 0.6−1.2 V relative to the analogous bis-diphosphine nickel
complexes. These results indicate that through systematic
functionalization, the thermochemical properties of nickel
complexes of chelating ligands can be controlled to a
remarkable extent, with a change of up to 1.4 V by exchanging
one phosphine of each ligand for an NHC donor as well as the
variation of the substituents at phosphorus. However,
thermodynamic studies of these complexes suggest that, while
the heterolytic bond strengths can be greatly influenced by
incorporating NHC donors, the effects on the free energy for
H2 addition as well as the homolytic bond dissociation free
energy will impede the use of these complexes as catalysts for
hydrogenation reactions.
MATERIALS AND METHODS
■
General Experimental Procedures. All manipulations were
carried out using standard Schlenk and glovebox techniques under
an atmosphere of nitrogen. All reagents were purchased from
commercial sources and used as received unless specified. Unless
noted otherwise, solvents were purified by passage through neutral
alumina using an Innovative Technology, Inc., PureSolv solvent
purification system. The imidazolium precursor (3-(2-bromoethyl)-1-
previous suggestion, steric effects appear to result in small
2+
differences in the structures of [NiLPh
]
and [NiLEt2]2+. The
2
Ni−C bond length is actually slightly shorter for the ethyl-
substituted complex than for the phenyl-substituted complex,
which is the reverse of the expected trend based on a purely
electronic argument.
methylimidazolium bromide)24 and lithium reagents LiPPh2 and
51
52
LiPEt2 were synthesized according to literature procedures.
In addition to the effects on the redox potentials, the
substitution of the phosphines for NHC ligands was expected
to have a substantial influence on the bond dissociation free
energies of the corresponding Ni−H complexes, especially for
the heterolytic bond cleavage reactions. Due to a lack of
reactivity of these complexes with H2, DFT calculations were
used in combination with experimental data to estimate the
heterolytic and homolytic bond dissociation free energies, as
shown in Scheme 4. These studies revealed that the
incorporation of NHC donors led to a remarkable improve-
ment in hydricity, with this heterolytic bond strength
decreasing by ∼32 kcal/mol relative to estimates for the
analogous [HNi(dppp)2]+ complex. This increase in the ability
of this complex to transfer a hydride was countered by a very
unfavorable free energy for the addition of H2 of +28.4 kcal/
mol at 1.0 atm. The extent to which this reaction is endergonic
is consistent with an inability of this complex to perform
hydrogenation reactions, as H2 addition will not occur at an
appreciable rate, even in the presence of a strong base.
Surprisingly, in addition to the influence of the NHC on the
hydride donor ability and the free energy for H2 addition, the
homolytic bond dissociation free energy for [HNiLPh2]+ was
estimated to be 47.1 kcal/mol, which is significantly lower than
typical nickel hydrides with diphosphine ligands. The relatively
weak homolytic bond dissociation free energy for the carbene-
containing complex is expected to result in bimolecular
homolytic loss of H2 from two equivalents of [HNiLPh2]+ in
solution. Given the inability to heterolytically cleave H2 due to
the large and unfavorable free energy for H2 addition as well as
the thermodynamic instability of [HNiLPh2]+ due to the weak
homolytic bond dissociation free energy, these complexes are
not expected to be effective catalysts for ionic hydrogenations,
such as the conversion of CO2 to formate.
Elemental analyses were performed by Atlantic Microlab, Inc.
(Norcross, GA, USA).
Instrumentation. All electrochemical experiments were performed
under an atmosphere of nitrogen in a 0.2 M tetrabutylammonium
hexafluorophosphate solution, [NBu4](PF6), in benzonitrile using a
CH Instruments 600 or 1100 series three-electrode potentiostat. The
working electrode was a glassy carbon disk (1 mm diameter), and the
counter electrode was a glassy carbon rod. The pseudoreference
electrode was a silver wire in electrolyte solution separated from the
working compartment by a Vycor (4 mm, BAS) frit. All potentials
were measured using Cp2Fe (ferrocene, 0 V) as an internal reference,
1
with all potentials reported vs the Cp2Fe+/0 couple. H and 31P NMR
spectra were recorded on a Varian 300 or 500 MHz spectrometer. The
1H spectra were referenced using residual solvent, and the 31P spectra
with respect to an 85% H3PO4 external standard. Crystals of
[NiLPh2](BPh4)2 and [NiLEt2](BPh4)2 suitable for X-ray structural
determinations were mounted in polybutene oil on a glass fiber and
transferred on the goniometer head to the precooled instrument.
Crystallographic measurements were carried out on a Bruker SMART
CCD 1K area detector diffractometer using Mo Ka radiation (λ =
0.71073 Å). All structures were solved by direct methods using
SHELXS-97 and refined with full-matrix least-squares procedures
using SHELXL-97.53 All non-hydrogen atoms are anisotropically
refined unless otherwise reported; the hydrogen atoms were included
in calculated positions as riding models in the refinement. Crystallo-
graphic data collection and refinement information can be found in the
Supporting Information.
Synthesis of [MeIm(CH2)2PPh2](BPh4) (HLPh(BPh4)). A slurry of
[MeIm(CH2)2Br](Br) (402 mg, 1.5 mmol) in 10 mL of THF was
treated dropwise with a yellow solution of LiPPh2 (315 mg, 1.6 mmol)
dissolved in 5 mL of THF. The resulting pale yellow solution was
stirred for 20 min at room temperature, after which the solvent was
removed by vacuum. The residue was dissolved in 20 mL of anhydrous
ethanol, and a NaBPh4 solution (513 mg, 1.5 mmol) in 10 mL of
EtOH was added. A white precipitate immediately formed, and the
slurry was stirred for 10 min, vacuum filtered, washed with EtOH (2 ×
5 mL) and ether (2 × 10 mL), and then dried under vacuum to give
the desired product (755 mg, 82%). Spectroscopic data were identical
to those reported by Messerle24 (see the Supporting Information).
Synthesis of [MeIm(CH2)2Cl](Br). A mixture of 1-methylimida-
zole (4.0 mL, 50 mmol) and 1-bromo-2-chloroethane (4.2 mL, 50
mmol) in 50 mL of THF was stirred at 60 °C under nitrogen for 48 h.
The resulting white precipitate was vacuum filtered, washed with ether
(2 × 10 mL), and dried under vacuum to give the desired compound
SUMMARY AND CONCLUSIONS
■
To probe the effects of NHCs on the redox properties of nickel
complexes, we synthesized and characterized nickel complexes
of chelating phosphine−carbene ligands that have not
previously been reported. The cyclic voltammetry data for
these complexes show redox potentials that are shifted negative
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Organometallics
Article
(6.89 g, 62% yield). The product may be recrystallized from hot
acetonitrile. H NMR (DMSO-d6, 20 °C): δ (ppm) 9.12 (1H, s, N-
C84H78B2N4NiP2: C, 78.46; H, 6.11; N, 4.49. Found: C, 78.33; H,
6.28; N, 4.49.
1
CHN), 7.82 (1H, s, Cim-Hbackbone), 7.75 (1H, s, Cim-Hbackbone), 4.56
(2H, t, J = 5.5 Hz, N-CH2-CH2-Cl), 4.08 (2H, t, J = 5.5 Hz, N-CH2-
CH2-Cl), 3.89 (3H, s, N-CH3). 13C{1H}NMR (DMSO-d6, 20 °C): δ
(ppm) 137.1 (s, N-CHN), 123.6 (s, CimCim), 122.4 (s, Cim
Cim), 50.2 (s, N-CH2-CH2-Cl), 43.2 (s, N-CH2-CH2-Cl), 35.9 (s, N-
CH3). Anal. Calcd for C6H10BrClN2: C, 31.96; H, 4.47; N, 12.42.
Found C, 31.85; H, 4.52; N, 12.44.
Synthesis of [MeIm(CH2)2Cl](BPh4). An aqueous solution of
[MeIm(CH2)2Cl](Br) (1.03 g, 4.6 mmol) in 10 mL of degassed water
was treated with 5 mL of aqueous NaBPh4 (1.56 g, 4.6 mmol). The
resulting white precipitate was stirred for 15 min, vacuum filtered,
washed with degassed water (2 × 10 mL), methanol (2 × 5 mL), and
ether (1 × 10 mL), and dried under vacuum to give the desired
compound as a fluffy white solid (1.73 g, 80%). X-ray quality crystals
were grown from vapor diffusion of ether into a saturated acetonitrile
solution. 1H NMR (DMSO-d6, 20 °C): δ (ppm) 9.12 (1H, s, N-CH
N), 7.78 (1H, m, Cim-Hbackbone), 7.71 (1H, m, Cim-Hbackbone), 7.16 (8H,
br s, B-Ph-H), 6.90 (8H, t, J = 7.5 Hz, B-Ph-H), 6.77 (4H, t, J = 7.0
Hz, B-Ph-H), 4.52 (2H, t, J = 5.5 Hz, N-CH2-CH2-Cl), 4.04 (2H, t, J =
5.0 Hz, N-CH2-CH2-Cl), 3.85 (3H, s, N-CH3). 13C{1H}NMR
(DMSO-d6, 20 °C): δ (ppm) 163.4 (q, JB‑C = 49.0 Hz, B-CPh),
137.1 (s, N-CHN), 135.5 (s, B-CPh), 125.3 (q, JB‑C = 2.7 Hz, B-CPh),
123.7 (s, CimCim), 122.4 (s, CimCim), 121.5 (s, B-CPh), 50.3 (s, N-
CH2-CH2-Cl), 43.1 (s, N-CH2-CH2-Cl), 35.8 (s, N-CH3).
[NiLEt2](BPh4)2. A similar procedure to that above was followed to
yield a bright yellow powder in 66% yield. 1H NMR (CD3CN, 20 °C):
δ (ppm) 7.31 (18H, m, Cim-Hbackbone and B-Ph-H), 7.22 (2H, d, J = 1.8
Hz, Cim-Hbackbone), 7.02 (16H, t, J = 7.4 Hz, B-Ph-H), 6.87 (8H, t, J =
7.2 Hz, B-Ph-H), 4.46 (2H, m, N-CH2-CH2-P-), 4.21 (2H, m, N-CH2-
CH2-P), 2.21 (2H, m, N-CH2-CH2-P), 2.15 (6H, s, N-CH3), 1.91 (2H,
m, N-CH2-CH2-P), 1.59−1.27 (8H, m, P-CH2-CH3), 0.82−0.75 (12H,
m, P-CH2-CH3). 31P{1H}NMR (CD3CN, 20 °C): δ (ppm) 23.1 (s).
Anal. Calcd for C68H78B2N4NiP2: C, 74.68; H, 7.19; N, 5.12. Found:
C, 74.25; H, 7.19; N, 5.19.
General Procedure for the Heterolytic Cleavage of H2 Using
[NiLPh 2+. A J. Young NMR tube containing a solution of
]
2
[NiLPh2](BPh4)2 (16.3 mg, 0.0127 mmol) and triethylamine (12.8
mg, 17.6 uL, 0.127 mmol) in 0.5 mL of CD3CN was degassed and
backfilled with one atmosphere of hydrogen gas. The reaction was
1
monitored over several days using H and 31P NMR spectroscopy;
however no [HNiLPh2]+ was observed during this time or even after
several months. A similar reaction was carried out using DBU as a base
but led to observation of unidentified paramagnetic products and a
limited amount of a hydride (<5%). Decomposition was observed with
stronger phosphine bases such as Verkade’s base.
COMPUTATIONAL DETAILS
■
All structures were optimized without symmetry constraints using the
B3P86 functional.54,55 The Stuttgart basis set with effective core
potential (ECP)56 was used for the Ni atom, whereas the 6-31G* basis
set57,58 was used for all other atoms with one additional p polarization
funtion [ξ(p) = 1.1] for the hydride atoms. Each structure was
confirmed by a frequency calculation at the same level of theory to be
a real local minimum on the potential energy surface without an
imaginary frequency. For each optimized geometry, harmonic
vibrational frequencies were calculated to obtain the zero-point energy
(ZPE) and thermal contributions (298 K and 1 atm) to the gas-phase
free energy. Solvation free energies in acetonitrile were calculated by
Synthesis of [MeIm(CH2)2PEt2](BPh4) (HLEt(BPh4)). A slurry of
[MeIm(CH2)2Cl](Br) (491 mg, 2.2 mmol) in 10 mL of THF was
treated dropwise with a yellow solution of LiPEt2 (419 mg, 2.2 mmol)
dissolved in 10 mL of THF and stirred for 1 h. The solvent was then
removed by vacuum, leaving a tacky, pale yellow solid that was
dissolved in 10 mL of degassed, distilled water. This solution was
treated with 10 mL of an aqueous solution of NaBPh4 (752 mg, 2.2
mmol), which immediately formed a white precipitate. The precipitate
was collected by vacuum filtration and dried under vacuum. The crude
product was recrystallized by diffusion of diethyl ether into a saturated
dichloromethane solution. The resulting fluffy white solid (909 mg)
was isolated by vacuum filtration and washed with diethyl ether (2 × 5
mL). A second crop of product was obtained (174 mg) from the
using the polarizabile continuum model (C-PCM)59,60 using Bondi61
atomic radii. Several conformations of [HNiLPh2]+ and [(H)2NiLPh
]
2
2+
were generated as a starting structure for geometry optimization, and
the lowest energy conformer was used in the isodesmic calculations.
The computed half-reaction free energies for hydride and proton
transfer from [HNi(dmpp)2]+ were previously reported,38 and the
half-reaction free energy for proton transfer from [(H)2Ni(dmpp)2]+ is
given in the Supporting Information. All of the calculations were
carried out with the program Gaussian 09.62
1
filtrate to give a total of 1.08 g (98%) of purified product. H NMR
(DMSO-d6): δ 9.08 (1H, s, N-CHN), 7.77 (1H, s, Cim-Hbackbone),
7.64 (1H, s, Cim-Hbackbone), 7.16 (8H, br s, B-Ph-H), 6.91 (8H, t, J =
7.3 Hz, B-Ph-H), 6.77 (4H, t, J = 7.1 Hz, B-Ph-H), 4.23 (2H, m, N-
CH2-CH2-P), 3.80 (3H, s, N-CH3), 1.96−1.86 (2H, m, N-CH2-CH2-
P), 1.42 (4H, q, J = 7.5 Hz, P-CH2-CH3), 0.99 (6H, m, P-CH2-CH3).
31P{1H}NMR (DMSO-d6, 20 °C): δ (ppm) −28.4 (s). 13C{1H}NMR
(DMSO-d6, 20 °C): δ (ppm) 163.3 (q, JB‑C = 49.4 Hz, B-CPh), 136.3
(s, N-CHN), 135.5 (s, B-CPh), 125.3 (q, JB‑C = 2.9 Hz, B-CPh), 123.5
(s, Cim= Cim), 122.2 (s, CimCim), 121.5 (s, B-CPh), 46.5 (d, J = 22.4
Hz, N-CH2-CH2-P), 35.7 (s, N-CH3), 26.3 (d, J = 17.3 Hz, N-CH2-
CH2-P), 17.6 (d, J = 11.6 Hz, P-CH2-CH3), 9.8 (d, J = 12.5 Hz, P-
CH2-CH3). Anal. Calcd for C34H40BN2P: C, 78.76; H, 7.78; N, 5.40.
Found: C, 79.01; H, 7.76; N, 5.39.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic cif files, a text file of all computed molecule
Cartesian coordinates in a format for convenient visualization,
thermal ellipsoid plot of the crystal structure of 3-(2-
chloroethyl)-1-methylimidazolium tetraphenylborate, a table
of computed energies, and spectroscopic data for HLPh(BPh4).
This material is available free of charge via the Internet at
http://pubs.acs.org.
General Synthesis of [NiLPh2](BPh4). A mixture of [MeIm-
(CH2)2PPh2)(BPh4)(HLPh) (627 mg, 1.02 mmol), NiCl2·6H2O (121
mg, 0.51 mmol), NaOAc·2H2O (138 mg, 1.02 mmol), and 5 mL of
DMSO was heated in a Schlenk flask at 70 °C for 2.5 h. The solution
began as pale green but became yellow at the end of the reaction time.
The solvent was distilled under vacuum at 70 °C to give a thick yellow
paste. The residue was washed with 25 mL of distilled water in air,
vacuum filtered, washed with distilled water (2 × 10 mL), methanol (2
× 10 mL), and ether (1 × 10 mL), and then dried under vacuum to
AUTHOR INFORMATION
Corresponding Author
*E-mail: aaron.appel@pnnl.gov.
■
Notes
1
give the product as a pale yellow powder (511 mg, 78%). H NMR
The authors declare no competing financial interest.
(CD3CN, 20 °C): δ (ppm) 7.84 (4H, m, P-Ph-H), 7.69−7.60 (6H, m,
P-Ph-H), 7.42 (4H, m, P-Ph-H), 7.31−7.24 (20H, m, P-Ph-H and B-
Ph-H), 7.01 (16H, t, J = 7.4 Hz, B-Ph-H), 6.94 (6H, m, B-Ph-H and
Cim-Hbackbone), 6.86 (8H, tt, J = 7.4 Hz, 1.4 Hz, B-Ph-H), 6.40 (2H, J =
1.8 Hz, Cim-Hbackbone), 4.89 (2H, m, N-CH2-CH2-P), 4.68 (2H, m, N-
CH2-CH2-P), 3.26 (6H, s, N-CH3), 3.06−2.93 (4H, m, N-CH2-CH2-
P). 31P{1H}NMR (CD3CN, 20 °C): δ (ppm) 22.5 (s). Anal. Calcd for
ACKNOWLEDGMENTS
■
B.R.G., E.S.W., J.C.L., and A.M.A. were supported by the U.S.
Department of Energy, Office of Basic Energy Sciences,
Division of Chemical Sciences, Geosciences & Biosciences.
M.L.H. was supported as part of the Center for Molecular
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