Stabilization of nickel complexes with Ni0H-N bonding interactions using sterically demanding cyclic diphosphine ligands

Article abstract of DOI:10.1021/om200709z

The series of complexes Ni(P^tBu₂N^R ₂)₂, [Ni(P^tBu₂N^R ₂)₂]BF₄, [HNi(P^tBu₂N ^R₂)₂]BF₄, and [Co(P ^tBu₂N^Ph₂)₂]BF ₄(P^tBu₂N^R ₂ = 1,5-dialkyl-3,7-tert-butyl-1,5-diaza-3,7-diphosphacyclooctane; alkyl = phenyl, benzyl) have been synthesized and characterized. Spectroscopic, electrochemical, and X-ray diffraction studies indicate these complexes are stable as a result of the tetrahedral arrangement of the two diphosphine ligands. Electrochemical oxidation of [HNi(P^tBu₂N^Ph₂) ₂]BF₄ results in rapid proton transfer from nickel at a rate faster than can be observed on the CV time scale. Double protonation of Ni(P^tBu₂N^Bn₂)₂forms the endo-endo, endo-exo, and exo-exo isomers of [Ni(P^tBu ₂N^BnHN^Bn)₂](BF₄) ₂, which were found to be more stable toward loss of H₂ than previously observed for similar complexes. The presence of Ni⁰HN hydrogen bonds at the endo protonation sites of [Ni(P^tBu ₂N^BnHN^Bn)₂](BF₄) ₂ results in significant differences in the Ni(I/0) oxidation potentials of each of the isomers. The differences in E_1/2(I/0) values correspond to bond free energies of 7.4 and 3.7 kcal/mol for the first and second Ni⁰HN hydrogen bonds of the endo-exo and endo-endo isomers, respectively. Computational studies give bond dissociation energies of the Ni⁰HN bonds that are within 1-2 kcal/mol of the experimentally determined values.

Full text of DOI:10.1021/om200709z

Article
pubs.acs.org/Organometallics
Stabilization of Nickel Complexes with Ni0···H−N Bonding
Interactions Using Sterically Demanding Cyclic Diphosphine Ligands
Eric S. Wiedner,† Jenny Y. Yang,† Shentan Chen,† Simone Raugei,† William G. Dougherty,‡
W. Scott Kassel,‡ Monte L. Helm,§,∥ R. Morris Bullock,*,† M. Rakowski DuBois,† and Daniel L. DuBois*,†
†Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington
99352, United States
‡Department of Chemistry, Villanova University, 800 E. Lancaster Avenue, Villanova, Pennsylvania 19085, United States
§Department of Chemistry, Fort Lewis College, Durango, Colorado 81301, United States
S
* Supporting Information
ABSTRACT: The series of complexes Ni(PtBu2NR )2, [Ni(PtBu2NR )2]BF4, [HNi(PtBu2NR )2]BF4, and [Co(PtBu2NPh2)2]BF4
2
2
2
(PtBu2NR = 1,5-dialkyl-3,7-tert-butyl-1,5-diaza-3,7-diphosphacyclooctane; alkyl = phenyl, benzyl) have been synthesized and
2
characterized. Spectroscopic, electrochemical, and X-ray diffraction studies indicate these complexes are stable as a result of the
tetrahedral arrangement of the two diphosphine ligands. Electrochemical oxidation of [HNi(PtBu2NPh2)2]BF4 results in rapid
proton transfer from nickel at a rate faster than can be observed on the CV time scale. Double protonation of Ni(PtBu2NBn
)
2
2
forms the endo-endo, endo-exo, and exo-exo isomers of [Ni(PtBu2NBnHNBn)2](BF4)2, which were found to be more stable toward
loss of H2 than previously observed for similar complexes. The presence of Ni0···HN hydrogen bonds at the endo protonation
sites of [Ni(PtBu2NBnHNBn)2](BF4)2 results in significant differences in the Ni(I/0) oxidation potentials of each of the isomers.
The differences in E1/2(I/0) values correspond to bond free energies of 7.4 and 3.7 kcal/mol for the first and second Ni0···HN
hydrogen bonds of the endo-exo and endo-endo isomers, respectively. Computational studies give bond dissociation energies of
the Ni0···HN bonds that are within 1−2 kcal/mol of the experimentally determined values.
INTRODUCTION
■
Nickel(II) complexes containing two cyclic diphosphine ligands
of the type shown in structure 1 have been found to function as
highly effective electrocatalysts for hydrogen oxidation and
proton reduction to produce hydrogen.1−4 The nature of the
substituents on the phosphorus atoms of the ligands controls
the hydricity of the metal.5−7 When the metal hydricity is
matched with the proton acceptor ability of the pendant
amines, the driving force for hydrogen addition to, or release
from, the complex can be tuned to achieve the desired direction
of the catalytic reaction. The proposed mechanism for the
catalytic reactions proceeds through Ni(I) and Ni(0)
intermediates and is also proposed to involve nickel
hydrides.1−4 The pendant amines of the ligands have been
shown to function as efficient proton relays. The role of the
bases in promoting rapid intramolecular proton exchange
between the amines and the metal center in intermediate stages
of the reaction, as well as intermolecular proton transfer
between the nickel complex and the solution during the course
of the catalytic cycles, has been discussed in previous
papers.8−13
Addition of H2 to selected hydrogen oxidation catalysts in
this class has led to the reduction of Ni(II) and formation of
diprotonated Ni(0) species, and these intermediates in the
catalytic cycle have been identified by multinuclear NMR
studies.10 We postulated that the introduction of steric bulk to
the phosphorus in the cyclic diphosphine ligands of the nickel
catalysts might enable us to spectroscopically detect and
characterize additional intermediates proposed for the catalytic
Received: July 29, 2011
Published: December 15, 2011
© 2011 American Chemical Society
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reactions. We report here the syntheses and spectroscopic and
electrochemical characterizations of Ni(0), Ni(I), and Ni(II)
hydride derivatives containing PtBu2NR2 ligands (R = Ph, Bn), as
well as additional important electrochemical characterizations
of the three isomers of [Ni(PtBu2NBnHNBn)2](BF4)2, a doubly
protonated Ni(0) complex in which two of the ligand nitrogen
atoms have been protonated. The results provide valuable
insights into factors that influence the stabilities of reduced
intermediates and the rates of the catalytic cycles.
Figure 1. UV−vis spectra of 1.0 mM [Ni(PtBu2NPh2)2]BF4 (red trace)
RESULTS
and 1.0 mM [Ni(PtBu2NBn2)2]BF4 (blue trace) in acetonitrile.
■
Synthesis and Characterization of Metal Complexes.
The reaction between Ni(COD)2 and 2−3 equiv of the ligand
which could belong to a d−d transition that is obscured by the
more intense charge transfer and π−π* bands. A d−d transition
for [Ni(PtBu2NBn2)2]BF4 that is blue-shifted relative to [Ni-
PtBu2NPh or PtBu2NBn affords the complexes Ni(PtBu2NPh
)
2
2
2
2
(PtBu2NPh2)2]BF4 is consistent with PtBu2NBn being a stronger
2
field ligand than PtBu2NPh2 as a result of inductive effects of the
NR groups.
Attempts were made to synthesize the Ni(II) complexes
[Ni(PtBu2NR )2](BF4)2 through two routes. In the first route, 2
2
equiv of PtBu2NR was added to [Ni(CH3CN)6](BF4)2; in the
2
second route, Ni(PtBu2NR )2 was oxidized with 2 equiv of
2
[FeCp2]PF6. However, all attempts resulted in a complex
mixture of products, as observed by 31P{1H} NMR spectros-
copy, and the mixture of products could not be purified. An
isoelectronic cobalt complex, [Co(PtBu2NPh2)2]BF4, was formed
and Ni(PtBu2NBn2)2 (eq 1). During the course of the reaction to
make Ni(PtBu2NPh2)2, an intermediate is observed that is
1
assigned as Ni(COD)(PtBu2NPh2) on the basis of its H NMR
14
by reduction of [Co(PtBu2NPh2)(CH3CN)3](BF4)2 with an
spectrum. The transformation of this intermediate into
Ni(PtBu2NPh2)2 is somewhat sluggish; therefore, the reaction
mixture is heated to 60 °C for ca. 24 h to facilitate complete
conversion to Ni(PtBu2NPh2)2. The conversion to Ni-
(PtBu2NBn2)2 is faster, and full conversion to Ni(PtBu2NBn
)
2
2
can be achieved at room temperature. Both compounds display
some light sensitivity in solution, and so the reactions were
conducted in the dark. A single resonance is observed in the
31P{1H} NMR spectrum at 20.11 ppm for Ni(PtBu2NPh2)2 and
at 15.45 ppm for Ni(PtBu2NBn2)2, as expected for tetrahedral
bis(diphosphine) nickel(0) complexes.
Both Ni(PtBu2NPh
) ) are oxidized by
and Ni(PtBu2NBn
2
2 2 2
[Fe(C5Me5)2]BF4 to afford the Ni(I) complexes [Ni-
(PtBu2NPh2)2]BF4 and [Ni(PtBu2NBn2)2]BF4, as shown in eq 1.
By using a slight excess of the nickel complex, [Ni(PtBu2NPh2)2]-
BF4 and [Ni(PtBu2NBn2)2]BF4 are easily purified by washing
with toluene followed by crystallization. Magnetic moments of
1.71 and 1.78 μB were determined for [Ni(PtBu2NPh2)2]BF4 and
[Ni(PtBu2NBn2)2]BF4, respectively, using the Evans method.
These values are close to the spin-only value of 1.73 μB
expected for a d9 metal complex containing a single unpaired
electron.
excess of zinc in the presence of 1 equiv of PtBu2NPh (eq 2).
2
Unlike other [Co(diphosphine)2]+ complexes,15,16 [Co-
(PtBu2NPh2)2]BF4 is paramagnetic with a magnetic moment of
2.85 μB, as determined by the Evans method. This value is close
to the spin-only value of 2.83 μB expected for an S =1 ground
state, which indicates that [Co(PtBu2NPh2)2]BF4 is a high-spin d8
complex that is significantly distorted from a square-planar
geometry. Two bands are observed in the UV−vis spectrum of
[Co(PtBu2NPh2)2]BF4 at 390 nm (ε = 501 M−1 cm−1) and 690
nm (ε = 93 M−1 cm−1), both of which are assigned to d−d
transitions (see Figure S1 in the Supporting Information).
The hydride complexes [HNi(PtBu2NPh2)2]BF4 and [HNi-
(PtBu2NBn2)2]BF4 were prepared by protonation of the Ni(0)
complexes Ni(PtBu2NPh2)2 and Ni(PtBu2NBn2)2 with slightly less
than 1 equiv of pyridinium tetrafluoroborate and 2,6-lutidinium
tetrafluoroborate, respectively, isolated as pure products, and
characterized spectroscopically (eq 3). The hydride ligands give
diagnostic 1H NMR signals at −12.46 ppm for [HNi-
(PtBu2NPh2)2]BF4 and at −11.63 ppm for [HNi(PtBu2NBn2)2]-
BF4. Coupling between the hydride and four phosphorus
atoms causes the hydride resonance to appear as a quintet,
Acetonitrile solutions of [Ni(PtBu2NPh2)2]BF4 are purple to
red, while [Ni(PtBu2NBn2)2]BF4 affords yellow solutions. The
UV−vis spectrum of [Ni(PtBu2NPh2)2]BF4 in CH3CN contains
two bands at 386 nm (ε = 1503 M−1 cm−1) and 515 nm (ε =
264 M−1 cm−1) which are assigned as metal-to-ligand charge
transfer and d−d transitions, respectively (Figure 1). A third,
very intense band at less than 360 nm is observed in the
spectrum of [Ni(PtBu2NPh2)2]BF4 and arises from π−π*
transitions within the phenyl groups of the ligands. A single
band is observed for [Ni(PtBu2NBn2)2]BF4 at 385 nm (ε = 1439
M−1 cm−1), which is assigned to a metal-to-ligand charge
transfer in analogy to [Ni(PtBu2NPh2)2]BF4 (Figure 1). The
charge transfer band of [Ni(PtBu2NBn2)2]BF4 does appear to
contain a small shoulder extending into the visible region,
1
which collapses to a singlet in the H{31P} NMR spectrum.
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Unlike other [Ni(PR NR′HNR′)2](BF4)2 complexes, 2A−2C
2
do not eliminate H2 to generate [Ni(PtBu2NBn2)2](BF4)2 but instead
decompose slowly to form the Ni(I) complex [Ni(PtBu2NBn2)2]-
BF4. For example, after 17 days a solution of 2A−2C consisted
of approximately 13% [Ni(PtBu2NBn2)2]BF4. This process is
attributed to decomposition of a trace amount (<1%) of the
hydride complex [HNi(PtBu2NBn2)2]BF4, which is consistently
observed by 31P{1H} NMR spectra to be in equilibrium with
2A−2C. Because of the equilibrium, small amounts of the
nickel hydride impurity can lead to larger amounts of the
Ni(I) complex as a decomposition product. In addition to
[Ni(PtBu2NBn2)2]BF4, a second paramagnetic product is
These data are consistent with those for other nickel hydride
complexes containing P2N2 ligands, which have been observed
and characterized in situ but not isolated.5,9,17 Complexes
[HNi(PtBu2NPh2)2]BF4 and [HNi(PtBu2NBn2)2]BF4 are stable
indefinitely in the solid state at −30 °C, but acetonitrile
solutions of [HNi(PtBu2NPh2)2]BF4 and [HNi(PtBu2NBn2)2]BF4
slowly decompose to form [Ni(PtBu2NPh2)2]BF4 and [Ni-
(PtBu2NBn2)2]BF4. The decomposition of HNiII into NiI in
1
observed in the H NMR spectrum of 2A−2C after partial
decomposition has occurred. The new compound displays a
broad resonance at 9.6 ppm in the 1H NMR spectrum, which is
slightly downfield from the tert-butyl resonance for [Ni-
(PtBu2NBn2)2]BF4 at 8.1 ppm. The same resonance is observed
when p-bromoanilinium tetrafluoroborate is added to [Ni-
(PtBu2NBn2)2]BF4, indicating that the Ni(I) complex has been
protonated on at least one of the pendant amines. Definitive
spectroscopic characterization of this protonated Ni(I) species
has remained elusive, due to its paramagnetic nature and lack of
distinctive bands in its UV−vis spectrum.
1
CD3CN solution was monitored by H NMR spectroscopy at
room temperature (24
2 °C), and after 7 days 64% of
[HNi(PtBu2NPh2)2]BF4 and 77% of [HNi(PtBu2NBn2)2]BF4
remained. Additional data on the rate of decomposition is
found in Figure S2 of the Supporting Information. No firm
conclusions can be drawn about the mechanism of this
transformation. It is clear, however, that [HNi(PtBu2NPh2)2]+
and [HNi(PtBu2NBn2)2]+ are sufficiently stable for isolation and
characterization and that the decomposition is slow enough
that it does not significantly affect the studies reported here.
Structural Studies. Yellow crystals of Ni(PtBu2NBn
)
2
2
suitable for an X-ray diffraction study were grown by slow
evaporation of a pentane solution. A drawing of the molecule is
shown in Figure 2a, and selected bond distances and angles are
given in Table 1. The molecule has a pseudotetrahedral
geometry with Ni−P bond lengths of 2.14−2.15 Å and
Treatment of Ni(PtBu2NBn
)
with 2.1 equiv of p-
2
2
cyanoanilinium tetrafluoroborate at room temperature afforded
the doubly protonated complex [Ni(PtBu2NBnHNBn)2](BF4)2
(2A−2C), which was not isolated but generated in situ (eq 4).
PtBu2NBn bite angles of 86.4 and 86.7°. The dihedral angle
2
(α) between the two planes formed by the two phosphorus
atoms of each diphosphine ligand and the Ni atom is 89°,
which is close to the 90° angle expected for an ideal tetrahedral
complex. These metrical parameters are very similar to those
18
observed for Ni(PPh2NMe
Ni(dppp)(PPh2NBn2).9
)
2
and for the PPh2NBn ligand of
2
2
Orange crystals of [Ni(PtBu2NBn2)2]BF4 were grown from a
solution of CH3CN and Et2O at −35 °C. The crystal consists of
a discrete [Ni(PtBu2NBn2)2]+ cation, a BF4 anion, and solvent
−
molecules. A drawing of the cation is shown in Figure 2b, and
selected bond distances and angles are given in Table 1. The
Ni−P bonds are all similar and range in length from 2.21 to
2.23 Å and are longer than the Ni−P bonds of Ni(PtBu2NBn
)
2
2
by 0.07−0.09 Å. The P−Ni−P bite angles of [Ni(PtBu2NBn2)2]-
BF4 (84.4, 85.3°) are slightly smaller than the corresponding
angles in Ni(PtBu2NBn2)2 (86.7, 86.4°) as a result of the greater
Ni−P bond length of [Ni(PtBu2NBn2)2]BF4.
The three observed isomers of [Ni(PtBu2NBnHNBn)2](BF4)2 are
assigned as endo-endo (2A), endo-exo (2B), and exo-exo (2C)
isomers by comparison of the observed 31P{1H} NMR
resonances to those for [Ni(PCy2NBnHNBn)2](BF4)2, which
has been characterized thoroughly by NMR spectroscopy.10 At
a reaction time of about 10 min, the isomer distribution
consists of 36% 2A, 29% 2B, and 35% 2C. After 2 h at room
temperature the solution reaches an equilibrium isomer
distribution of 74% 2A, 24% 2B, and 2% 2C that does not
change further over a period of days. In this system, the exo-exo
isomer 2C is the kinetic product that is formed at short reaction
times and is isomerized to the thermodynamically preferred
isomers 2A and 2B. This isomerization is likely facilitated by p-
cyanoaniline that is present in solution, since isomerization of
exo-exo isomers in related systems is known to be very slow.11
Experiments on the role of bases in accelerating isomerizations
in closely related Ni complexes are underway and will be
reported separately.
Green crystals of [Co(PtBu2NPh2)2]BF4 were grown by vapor
diffusion of Et2O into a CH3CN solution of the complex. The
crystals contain two crystallographically independent molecules
of [Co(PtBu2NPh2)2]BF4 in the asymmetric unit, one of which
lies on a crystallographic inversion center. A drawing of one of
the molecular cations is shown in Figure 2c, and selected bond
distances and angles are given in Table 1. The Co−P bonds
range in length from 2.24 to 2.26 Å and are about 0.03 Å longer
than the Ni−P bond lengths of [Ni(PtBu2NBn2)2]BF4, both of
which are formally in the 1+ oxidation state. This trend mirrors
that observed for the pairs [Co(PtBu2NPh2)(CH3CN)3]2+/
[Ni(PtBu2NPh2)(CH3CN)3]2+ and [Co(PtBu2NBn2)-
(CH3CN)3]2+/[Ni(PtBu2NBn2)(CH3CN)2]2+, where the Co−P
bond lengths are longer than the corresponding Ni−P bond
lengths by 0.03 Å.14 In accordance with the greater M−P bond
lengths of [Co(PtBu2NPh2)2]BF4, the observed P−Co−P bite
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Table 1. Selected Bond Distances (Å) and Angles (deg)
Ni(PtBu2NBn
)
2
2
Ni(1)−P(1)
Ni(1)−P(2)
P(1)−Ni(1)−P(2)
P(3)−Ni(1)−P(4)
2.1451(10)
2.1374(9)
Ni(1)−P(3)
Ni(1)−P(4)
2.1469(9)
2.1421(9)
89
a
86.70(4)
α
86.41(3)
[Ni(PtBu2NBn2)2]BF4
Ni(1)−P(1)
Ni(1)−P(2)
P(1)−Ni(1)−P(2)
P(3)−Ni(1)−P(4)
2.2289(15)
2.2292(16)
Ni(1)−P(3)
Ni(1)−P(4)
2.2124(17)
2.2113(15)
84
a
85.31(6)
84.42(6)
[Co(PtBu2NPh2)2]BF4
α
b
Co(1)−P(1)
Co(1)−P(2)
Co(1)−P(3)
P(1)−Co(1)−P(2)
P(3)−Co(1)−P(4)
P(5)−Co(2)−P(6b)
2.2597(8)
2.2522(8)
2.2606(8)
Co(1)−P(4)
Co(2)−P(5)
Co(2)−P(6)
2.2628(9)
2.2363(9)
2.2585(8)
85
a
a
82.71(3)
83.35(3)
83.13(3)
α(1)
α(2)
77
a
α is the dihedral angle between the two planes formed by the two
b
phosphorus atoms of each diphosphine ligand and the metal ion. The
asymmetric unit of [Co(PtBu2NPh2)2]BF4 contains two crystallo-
graphically independent molecular cations. Co(2) lies on an inversion
center; therefore, only the unique bonds and angles are listed.
pKa Measurements. Addition of benzylamine (BnNH2) to
a benzonitrile solution of [HNi(PtBu2NPh2)2]BF4 resulted in an
equilibrium between [HNi(PtBu2NPh2)2]BF4 and Ni-
(PtBu2NPh2)2, as determined by 31P{1H} NMR spectroscopy.
The resonances for Ni(PtBu2NPh2)2 were broader than those
observed in solutions of pure Ni(PtBu2NPh2)2, and the amount
of broadening increased with time. This broadening can be
attributed to degenerate electron exchange between Ni-
(PtBu2NPh2)2 and a trace amount of [Ni(PtBu2NPh2)2]BF4 that
is formed from decomposition of [HNi(PtBu2NPh2)2]BF4. Over
a time period of 1−2 h, decomposition to [Ni(PtBu2NPh2)2]BF4
is negligible, and the ratios of [HNi(PtBu2NPh2)2]BF4 and
Ni(PtBu2NPh2)2 were determined by integration of the 31P{1H}
NMR resonances. Determination of the ratios of BnNH2 and
[BnNH3]BF4 by mass balance allows an equilibrium constant of
Figure 2. X-ray crystal structure diagrams of (a) Ni(PtBu2NBn2)2, (b)
the molecular cation of [Ni(PtBu2NBn2)2]BF4, and (c) one of the two
molecular cations of [Co(PtBu2NPh2)2]BF4. Only the ipso carbons of
the phenyl rings and the quaternary carbons of the tert-butyl groups on
phosphorus are shown for clarity. Thermal ellipsoids are shown at 50%
probability.
1.5
0.3 to be calculated for the deprotonation of
[HNi(PtBu2NPh2)2]BF4. Previous studies have shown that
relative pKa values in acetonitrile and benzonitrile are the
same within experimental error.20 Therefore, the measured
+
equilibrium constant and a pKa value of 16.91 for BnNH3 in
acetonitrile21 were used to determine a pKa value of 16.7 0.4
for [HNi(PtBu2NPh2)2]BF4. Similar treatment of [HNi-
(PtBu2NBn2)2]BF4 with pyrrolidine resulted in a measured
equilibrium constant of 0.028 0.005. Using this equilibrium
constant and a pKa value of 19.56 for pyrrolidinium in
angles (82.7−83.4°) of [Co(PtBu2NPh2)2]BF4 are smaller than
the corresponding bite angles in [Ni(PtBu2NBn2)2]BF4. Dihedral
angles (α) of 85 and 77° are measured for the two molecular
cations of [Co(PtBu2NPh2)2]BF4. It is well-known that for d8
[M(diphosphine)2]n+ complexes α increases as the diphosphine
bite angle increases. In the case of [Co(PtBu2NPh2)2]BF4, the
small bite angle of 83° should result in a nearly square-planar
geometry at Co on the basis of electronic factors. This scenario
is observed for [Ni(PPh2NC6H4Me2)2]2+ (α = 24°), [Ni-
(PPh2NC6H4OMe2)2]2+ (α = 29°), and [Ni(PCy2NtBu2)2]2+ (α =
acetonitrile,21 a pKa value of 21.1
0.4 was determined for
[HNi(PtBu2NBn2)2]BF4. The pKa values for [HNi(PtBu2NPh2)2]-
BF4 and [HNi(PtBu2NBn2)2]BF4, along with the redox potentials
of Ni(PtBu2NPh2)2 and Ni(PtBu2NBn2)2, can be used to determine
homolytic bond dissociation free energies (ΔG°H•) of 53.0
0.5 kcal/mol for [HNi(PtBu2NPh2)2]BF4 and 52.8 0.5 kcal/
mol for [HNi(PtBu2NBn2)2]BF4 using well-known thermody-
namic cycles.22−25 These values are similar to those determined
for other [HNi(diphosphine)2]+ species.5,7,20,26,27
23°).12,19 The tert-butyl groups of the PtBu2NR ligand clearly
2
result in large steric effects in the coordination sphere of
[Co(PtBu2NPh2)2]BF4, as evidenced by the large dihedral angles
observed for this complex.
When Ni(PtBu2NBn2)2 is treated with a small excess of pyri-
dinium (pyH+), an equilibrium between [HNi(PtBu2NBn2)2]BF4
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and the endo-endo isomer of [Ni(PtBu2NBnHNBn)2](BF4)2 (2A)
is observed. The ratios of [HNi(PtBu2NBn2)2]BF4 and 2A were
determined by 31P NMR spectroscopy, and the ratios of pyH+
and py were determined by mass balance. These data allow an
equilibrium constant of 0.5 0.1 to be calculated for this reaction.
The measured equilibrium constant and the known pKa value of
12.53 for pyridinium in acetonitrile21 were used to determine a
pKa value of 12.2 0.4 for 2A. Using the equilibrium constants
for 2A with 2B and 2C, pKa values of 11.7 0.4 for 2B and 10.6
0.4 for 2C were determined.
Ep = +0.23 V at a scan rate of 100 mV/s, and this wave is
assigned to a Ni(II/I) couple and/or oxidation of the pendant
amine of the diphosphine ligand that is accompanied by rapid
decomposition. Decomposition of the analyte at the Ni(II)
oxidation state results in a diminished peak current for the
cathodic wave of the Ni(I/0) couple. Cyclic voltammograms of
Ni(PtBu2NBn2)2 are very similar to those of Ni(PtBu2NPh2)2,
except that both oxidation waves occur at more negative
potentials due to the greater electron donor ability of the
PtBu2NBn ligand. The cyclic voltammograms of the Ni(I)
2
analogues [Ni(PtBu2NPh2)2]BF4 and [Ni(PtBu2NBn2)2]BF4 were
recorded in acetonitrile, and these showed waves very similar to
Electrochemical Studies. Figure 3 shows a cyclic voltam-
mogram recorded on a benzonitrile solution of Ni(PtBu2NPh2)2.
those observed for Ni(PtBu2NPh
) ) in
and Ni(PtBu2NBn
2
2 2 2
benzonitrile. Slight shifts in potentials were observed for the
Ni(II/I) couples for the Ni(I) complexes recorded in
acetonitrile, and these are given in Table 2.
A cyclic voltammogram recorded on an acetonitrile solution
of [Co(PtBu2NPh2)2]BF4 is shown in Figure 4. Reversible
couples at −1.62 and −2.03 V are assigned to the Co(I/0) and
Co(0/−I) couples by analogy to other [Co(diphosphine)2]2+
complexes.15,29 Each of these reductions is a diffusion-
controlled process, as determined from the linear relationship
observed between the peak current and the square root of the
scan rate. An irreversible oxidation is observed at a peak
potential of −0.01 V and is assigned to an oxidation from Co(I)
to Co(II). Associated with this irreversible oxidation is a
cathodic wave at Ep = −0.93 V. These two waves are assigned
respectively to the oxidation of [Co(PtBu2NPh2)2]+ to [Co-
Figure 3. Cyclic voltammogram of a 2 × 10−3 M solution of
Ni(PtBu2NPh2)2. Conditions: scan rate 100 mV/s, 0.2 M NBu4OTf
benzonitrile solution, glassy-carbon working electrode.
(PtBu2NPh2)2]2+ followed by rapid dissociation of PtBu2NPh to
A reversible oxidation wave is observed at E1/2 = −1.03 V, and a
plot of the peak current (ip) versus the square root of the scan
rate for the wave is linear, indicating that this is a diffusion-
controlled process.28 This oxidation wave is assigned to the
Ni(I/0) couple. An irreversible oxidation wave is observed at
2
form [Co(PtBu2NPh2)(CH3CN)3]2+. The oxidation wave at Ep =
+0.33 V is tentatively assigned to the Co(III/II) couple of
[Co(PtBu2NPh2)(CH3CN)3]2+ and/or the oxidation of the
pendant amine of the diphosphine ligands.14
Table 2. Cyclic Voltammetry Data for Nickel and Cobalt Complexes
a
a
c
d
Ep(III/II)
ab
E1/2(II/I) Ep(II/I)
E1/2(I/0) (ΔEp) [ia/ic] Ep(I/0)
b
b
a
c
d
,
complex
(irr)
(irr)
(irr)
E1/2(0/−I) (ΔEp) [ia/ic]
ref
e
e
Ni(PtBu2NPh
)
+0.23 (irr)
−1.03 (69) [1.0]
this work
2
2
e
e
Ni(PtBu2NBn
)
2
−0.02 (irr)
−1.29 (76) [1.0]
this work
this work
2
[Ni(PtBu2NPh2)2]BF4
+0.18 (irr)
−1.02 (63) [1.0]
−1.29 (76) [1.0]
[Ni(PtBu2NBn2)2]BF4
[HNi(PtBu2NPh2)2]BF4
[HNi(PtBu2NBn2)2]BF4
[Ni(PtBu2NBnHNBn)2](BF4)2
endo-endo
[Ni(PtBu2NBnHNBn)2](BF4)2
endo-exo
−0.06 (irr)
this work
this work
this work
this work
f
f
−0.31 (irr)
−0.67 (irr)
g
−0.12 (q)
g
−0.29 (q)
this work
[Ni(PtBu2NBnHNBn)2](BF4)2 exo-exo
[Ni(PPh2NPh2)2(CH3CN)](BF4)2
[Ni(PPh2NBn2)2(CH3CN)](BF4)2
[Ni(PCy2NBn2)2](BF4)2
−0.61 (69) [1.5]
−1.02
−1.19
−1.28
−1.62 (62) [1.0]
−2.04
this work
−0.84
−0.94
−0.80
−0.01 (irr)
−0.58
5
14
5
[Co(PtBu2NPh2)2]BF4
−2.30 (74) [0.8]
−2.04
this work
[Co(PPh2NPh2)2(CH3CN)](BF4)2
28
12
[Co(dppe)2(CH3CN)](BF4)2
−0.70
−1.56
−2.03
a
b
+
Half-wave potential in volts versus the FeCp2 /FeCp2 couple in 0.2 M NEt4BF4/CH3CN solutions unless noted otherwise. Peak potential of
irreversible couples at a scan rate of 100 mV/s. Separation of cathodic and anodic peak potentials at a scan rate of 100 mV/s. Under these
conditions the reference couple (ferrocene, decamethylferrocene, or cobaltocenium) exhibited ΔEp values of 70 5 mV. Ratio of anodic and
cathodic peak currents at 100 mV/s. In 0.2 M NBu4OTf/PhCN solution. Oxidation is a formal Ni(III/II) oxidation. These waves become quasi-
reversible at scan rates above 10 V s−1.
c
d
e
f
g
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molecular proton transfer following oxidation of [HNi-
(PtBu2NBn2)2]BF4. Similar to the case for [HNi(PtBu2NBn2)2]BF4,
electrochemical oxidation of [HNi(PtBu2NPh2)2]BF4 exhibits an
irreversible oxidation wave at Ep = −0.31 V accompanied by a
reversible reduction wave at −1.02 V corresponding to the I/0
couple of Ni(PtBu2NPh2)2. The oxidation couples for both
[HNi(PtBu2NPh2)2]BF4 and [HNi(PtBu2NBn2)2]BF4 remain irre-
versible even at a scan rate of 100 V/s. Upon recording the
oxidation couple of either [HNi(PtBu2NPh2)2]BF4 or [HNi-
(PtBu2NBn2)2]BF4, the only return cathodic wave observed
occurs at the corresponding Ni(I/0) couple, which indicates
that proton loss from the oxidized nickel complex is rapid. As a
result, a kinetic potential shift is observed for oxidation of
[HNi(PtBu2NPh2)2]BF4 and [HNi(PtBu2NBn2)2]BF4, and the
observed half-wave potentials were found to be dependent on
the scan rate. For every 10-fold increase in scan rate, the half-
wave potentials of [HNi(PtBu2NPh2)2]BF4 and [HNi-
(PtBu2NBn2)2]BF4 shift to more positive potentials by
approximately 30 mV.28
Figure 6a shows cyclic voltammograms recorded on a
solution of [Ni(PtBu2NBnHNBn)2](BF4)2 (2A−2C) at equilibrium.
Three oxidations are observed on the anodic scan at potentials
more negative than the irreversible II/I couple of the parent
complex Ni(PtBu2NBn2)2. These oxidations are assigned to the
I/0 couples of the isomers 2A−2C. The largest peak current is
observed for the oxidation at Ep = −0.12 V, which is assigned to
the endo-endo isomer 2A, as the equilibrium mixture of
isomers contains 74% of 2A, as discussed previously. The next
largest peak current is observed for the oxidation at Ep = −0.29 V,
which is assigned to the endo-exo isomer (2B), and the smallest
peak current is observed at Ep = −0.59 V and is assigned to the
exo-exo isomer (2C). The total measured anodic current
consists of 70% 2A, 23% 2B, and 7% 2C, which is close to
the equilibrium distribution of isomers as measured by
31P{1H} NMR spectroscopy. At slow scan rates, the oxidations
of 2A and 2B at −0.12 and −0.28 V are irreversible with no
observable cathodic current on the return sweep. At scan rates
of 10 V/s and higher, these couples begin to show quasi-
reversible behavior (Figure 6b). In contrast, the oxidation of 2C
at E1/2 = −0.61 V is partially reversible at low scan rates
(100 mV/s), as evidenced by the presence of a cathodic current
upon scan reversal when only the oxidation of 2C is recorded
(Figure 6c). The irreversible nature of the waves at −0.12 and
−0.28 V suggest that proton loss from the oxidized products
2A+ (endo-endo) and 2B+ (endo-exo) is more rapid than for
2C+ (exo-exo).
Figure 4. Cyclic voltammogram of 2 × 10−3 M solution of
[Co(PtBu2NPh2)2]BF4. Conditions: scan rate 100 mV/s, 0.2 M
NEt4BF4 acetonitrile solution, glassy-carbon working electrode.
Figure 5a shows a cyclic voltammogram recorded on a
0.5 mM solution of [HNi(PtBu2NBn2)2]BF4 in acetonitrile. At a
scan rate of 100 mV/s, an irreversible oxidation of [HNi-
(PtBu2NBn2)2]BF4 is observed at Ep = −0.67 V, and this wave is
accompanied by a reduction wave at Ep = −1.31 V
corresponding to the Ni(I/0) couple of Ni(PtBu2NBn2)2. Cyclic
voltammograms recorded on 1.0−2.0 mM solutions of
[HNi(PtBu2NBn2)2]BF4 in acetonitrile show two additional
quasi-reversible waves at E1/2 = −0.61 V and E1/2 = −0.32 V
as the concentration of [HNi(PtBu2NBn2)2]BF4 is increased.
These waves correspond to isomers 2C and 2B, respectively,
as discussed in the next paragraph. The absence of 2A and
2B in voltammograms recorded at low concentrations of
[HNi(PtBu2NBn2)2]BF4 suggests that they are formed via inter-
Two additional cathodic peaks are observed at Ep = −0.88 V
and Ep = −1.24 V in voltammograms of 2A−2C (Figure 6a).
The reduction at Ep = −1.24 V is assigned to the I/0 couple of
[Ni(PtBu2NBn2)2]BF4 that is shifted to a slightly more positive
potential through an EC mechanism, where the chemical
reaction following the electrochemical oxidation is protonation
of Ni(PtBu2NBn2)2 in the diffusion layer. The reduction at Ep =
−0.88 V is assigned to a monoprotonated Ni(I) species,
[Ni(PtBu2NBnHNBn)(PtBu2NBn2)]1+, that forms through proton
loss upon oxidation of 2A−2C in the diffusion layer. These
features can be reproduced by titration of small amounts of
p-anisidinium tetrafluoroborate (pKa = 11.86)21 into acetoni-
trile solutions of [Ni(PtBu2NBn2)2]BF4 (Figure 7). The
monoprotonated species is not observed when only 2C is
oxidized (Figure 6c), which indicates that the monoproto-
nated species forms from 2A and/or 2B.
Figure 5. (a) Cyclic voltammogram of a 0.5 mM solution of
[HNi(PtBu2NBn2)2]BF4. (b) Cyclic voltammograms of a solution of
[HNi(PtBu2NBn2)2]BF4 at varying concentrations. Conditions: scan rate
100 mV/s, 0.2 M NEt4BF4 acetonitrile solution, glassy-carbon working
electrode.
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DFT calculations show that the calculated free energies for
the diprotonated Ni(0) isomers increase in the order 2A < 2B
< 2C, in agreement with the experimental equilibrium isomer
distribution (Table 3). For the corresponding oxidized species
Table 3. Calculated Relative Free Energies, Redox
Potentials, and d(Ni)→σ*(N−H) Interactions for Isomers
2A−2C
ΔG
ΔE1/2(I/0)
(kcal/mol)
(V)
d(Ni)→σ*(N−H)
(kcal/mol)
Ni(0)
Ni(0) Ni(I) calcd exptl
endo-endo
(2A)
0.0
8.4
0.53
0.49
15.4
endo-exo (2B)
exo-exo (2C)
2.4
3.9
4.2
0.0
0.24
0.00
0.32
0.00
8.1
0.0
the stability order is reversed: 2A+ > 2B+ > 2C+ (Table 3). The
higher free energy of 2A+ and 2B+ suggests that these species
are more likely to be deprotonated to form monoprotonated
species than 2C+, as experimentally observed. Reassured by
these findings, we calculated the relative Ni(I/0) redox
potentials for the three isomers. Our calculations are able to
reproduce the shift toward more positive E(I/0) redox
potentials on passing from isomer 2C to isomer 2A (Table 3)
that was observed experimentally. The difference between
theoretical prediction and experiment is within the accuracy
of the adopted computational protocol (about 0.15 V).30,31
A natural bond orbital (NBO) analysis32−39 of three species
2A−2C indicates a significant charge transfer from occupied
metal d orbitals to the antibonding σ*(N−H) orbital
when N−H is oriented toward the metal center. The mag-
nitude of the hydrogen-bond-like d(Ni)→σ*(N−H) inter-
action has been estimated according to the NBO pertur-
bative framework,38 and these are reported in Table 3. As can
be seen in Figure 8, there is a linear correlation between the
Figure 6. Cyclic voltammograms of a 2 × 10−3 M solution of
[Ni(PtBu2NBnHNBn)2](BF4)2 (2A−2C) showing (a) 2A−2C at scan
rate 100 mV/s, (b) 2A−2C at scan rates 5, 10, and 25 V/s, and (c) 2C
at scan rate 100 mV/s. Conditions: 0.2 M NEt4BF4 acetonitrile
solution, glassy-carbon working electrode.
Figure 8. Correlation between the hydrogen-bond-like d(Ni)→
σ*(N−H) interaction and the calculated redox potential for isomers
2C−A (from left to right).
d(Ni)→σ*(N−H) interaction energy and the shift in the
relative redox potential, which suggests that the Ni···HN
hydrogen bond is the cause of the shift in the redox potential,
as opposed to conformational differences in the ligands of the
different isomers.
Figure 7. Successive linear voltammograms of a 2 × 10−3 M solution
of [Ni(PtBu2NBn2)2]BF4 in acetonitrile at increasing concentrations of
p-anisidinium tetrafluoroborate. Conditions: scan rate 100 mV/s,
0.2 M NEt4BF4 acetonitrile solution, glassy-carbon working
electrode.
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products have been recently characterized for′the decomposition
of a related Ni(II) hydride containing PR NR ligands.17
DISCUSSION
■
2
2
Syntheses, Structures, and Reactivity. The syntheses of
The addition of 2 equiv of an acid (p-cyanoanilinium) to
Ni(PtBu2NBn2)2 in acetonitrile solution generates a mixture of
diprotonated Ni(0) isomers, 2A−2C, as shown in eq 4. Although
the exo-exo isomer 2C is a kinetic product that is formed at
short reaction times, after 2 h at 20 °C, the isomer distribution
reaches equilibrium with 74% 2A, 24% 2B, and 2% 2C.
Analogous isomers have been synthesized and identified in
the PtBu2NR ligands (R = Ph, Bn) and their reactions with
2
[M(CH3CN)6]2+ (where M = Ni, Co) have been reported
recently.14 These complexes with sterically demanding t-Bu
groups on the phosphines differ from other P2N2 complexes
that we have studied in that the Ni(II) and Co(II) products do
not accommodate two of the P2N2 ligands; only derivatives of
the formula [M(PtBu2NR )(CH3CN)x]2+ (x = 2, 3) have been
2
the addition of hydrogen to [Ni(PCy2NBn2)2]2+,10 [Ni-
isolated and characterized. The nickel derivatives were found to
be inactive as catalysts for the electrochemical oxidation of
hydrogen or reduction of protons.
5
(PCy2NBu2)2]2+,19 and [Ni(PPh2NBn2)2]2+
and in the
protonation of [Ni(PPh2NBn2)2]2+.13 Details of intramolecular
proton transfer in these isomer mixtures and intermolecular
proton exchange between isomers have been studied by NMR
techniques.11 Each of the previously studied systems displays
catalytic activity for hydrogen oxidation or proton reduction,
and H2 elimination from the endo-endo isomer is facile. In the
catalytically inactive system studied here, the diprotonated
isomers are quite stable to H2 elimination over a period of days,
permitting more detailed electrochemical characterizations of
these systems, as discussed below.
In the present work we have prepared and characterized
Ni(0) derivatives of the formula Ni(PtBu2NR )2. The tetrahedral
2
structures of these complexes reduce steric interactions and
permit coordination of two P2N2 ligands. These derivatives
provide an entry into the syntheses of additional Ni(P2N2)2
complexes that are analogous to proposed intermediates in the
catalytically active systems. For example, oxidation of Ni-
(PtBu2NR )2 with [Fe(C5Me5)2]+ results in the preparation of
2
rather rare examples of Ni(I) derivatives, [Ni(PtBu2NR )2]+,
2
Electrochemical Studies. In earlier studies, several nick-
which have been characterized by magnetic measurements and
elemental analyses. X-ray crystallographic data for complexes in
′
el(0) complexes of the formula Ni(PR NR )2 have been
2
2
two oxidation states, Ni(PtBu2NBn
)
2
and [Ni(PtBu2NBn2)2]+,
characterized electrochemically, and typically two reversible one-
electron oxidations are observed that are assigned to the Ni(I/0)
and Ni(II/I) couples. The new Ni(0) complexes described here
2
have been obtained and compared. The Ni−P bonds for the
Ni(I) complex (2.21−2.23 Å) are longer than the Ni−P bonds
of the Ni(0) derivative by 0.07−0.09 Å, and they are similar to
display a reversible Ni(I/0) couple at −1.03 V for Ni(PtBu2NPh
)
2 2
′
those observed for Ni(II) complexes containing PR NR
2
2
and at −1.29 V for Ni(PtBu2NBn2)2, but the corresponding Ni(II/
I) waves are irreversible with Ep at +0.23 V for Ni(PtBu2NPh2)2 and
−0.02 V for Ni(PtBu2NBn2)2; these peak currents are shifted to
values much more positive than those observed for the reversible
Ni(II/I) couples for related complexes, which occur in the region
of −0.8 to −0.9 V (see Table 2). The cyclic voltammograms ob-
served for the corresponding Ni(I) derivatives [Ni(PtBu2NPh2)2]BF4
and [Ni(PtBu2NBn2)2]BF4 are very similar to those described for
Ni(PtBu2NPh2)2 and Ni(PtBu2NBn2)2. The irreversible nature of
the II/I oxidation wave and its anodic shift are attributed to the
rapid decomposition of the Ni(II) complex and are consistent
ligands. An increase in the Ni−P bond length upon oxidation
of nickel by one electron suggests that the Ni(I) cation
possesses a weaker metal-to-ligand π bond. For a d9 complex
with local D2 symmetry at nickel, a Jahn−Teller distortion is
expected to remove the degeneracy of the dxy and dxz orbitals.
This distortion is manifested in [Ni(PtBu2NBn2)2]BF4 in the
observed dihedral angle of 84°, which is significantly less than
the 89° angle observed for the d10 complex Ni(PtBu2NBn2)2.
Protonation of the Ni(0) complexes results in the formation of
Ni(II) hydride derivatives [HNi(PtBu2NR ) ]BF4, which have
been isolated and characterized by NMR spectroscopy and cyclic
2 2
with our inability to synthesize stable [Ni(PtBu2NR )2]2+ com-
2
voltammetry. Although nickel hydride complexes with other
plexes. The hydride complexes [HNi(PtBu2NR )2]+ also display
′
2
PR NR ligands have been identified spectroscopically,5,9,17
2
irreversible oxidation waves at −0.31 V for [HNi(PtBu2NPh2)2]BF4
and −0.67 V for [HNi(PtBu2NBn2)2]BF4 (scan rate 100 mV/s), and
these waves remain irreversible at a scan rate of 100 V/s, indicating
that proton loss from the oxidized products is very rapid. The
formation of isomers 2A and 2B following oxidation of
[HNi(PtBu2NBn2)2]BF4 is also consistent with rapid intermolecular
proton transfer following oxidation of the hydride complex.
The relative kinetic stabilities of the diprotonated Ni(0)
isomers [Ni(PtBu2NBnHNBn)2]2+ (2A−2C) have allowed us to
obtain for the first time cyclic voltammetric data on this type of
isomeric mixture. The cyclic voltammograms show three distinct
Ni(I/0) oxidation waves with relative intensities that roughly
correspond to the equilibrium distribution of the isomers. The
exo-exo isomer is the easiest to oxidize, and a reversible wave is
observed with E1/2 = −0.59 V. The d10 Ni(0) centers in the
endo-exo (2B) and endo-endo (2A) isomers are stabilized
by interactions with the hydrogen of the protonated amine(s),
and this shifts the potentials for the Ni(I/0) oxidation to more
positive values, with E1/2 values of quasi-reversible waves at
−0.28 (2B) and −0.12 V (2A). The data permit an evaluation
of the strength of the hydrogen-bonding interactions in
isomers 2A and 2B.
2
[HNi(PtBu2NPh2)2]BF4 and [HNi(PtBu2NBn2)2]BF4 are the first
such hydrides to be isolated. The equilibrium constants for the
reactions of each hydride complex with either benzylamine or
pyrrolidine were determined by NMR spectroscopy and used
to determine the pKa values for [HNi(PtBu2NPh2)2]BF4 and
[HNi(PtBu2NBn2)2]BF4 of 16.7 and 21.1, respectively, in
benzonitrile solution. These values are very similar to those
determined previously for [HNi(PPh2NPh2)2]+ (16.3) and
[HNi(PCy2NBn2)2]+ (21.0), generated in the same solvent.5
The Ni(II) hydride products are stable indefinitely in the solid
state at −30 °C but decompose in solution. At room
temperature, the hydride complexes reacted slowly in acetonitrile
solutions to form the Ni(I) derivatives [Ni(PtBu2NPh2)2]BF4 and
[Ni(PtBu2NBn2)2]BF4 and presumably H2, although the gaseous
product was not detected in these NMR scale experiments. The
homolytic bond dissociation free energies of 53.0 kcal/mol for
[HNi(PtBu2NPh2)2]BF4 and 52.8 kcal/mol for [HNi(PtBu2NBn2)2]-
BF4, together with the bond dissociation free energy of H2
(103.6 kcal/mol in acetonitrile),22 indicate that loss of H2 should
be unfavorable for these two complexes by about 2 kcal/mol in
the presence of 1.0 atm of H2 but favorable in the absence of H2.
The elimination of H2 and the formation of Ni(0) and Ni(II)
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The difference in the E1/2 values for 2C, which has no
Ni0···HN bonds, and 2B, which has a single Ni0···HN bond,
is 0.32 V, which corresponds to 7.4 kcal/mol. Assuming that
the small difference in the local geometry at the Ni center
between isomers has little influence on the redox potential, this
difference gives an estimate of the free energy of the Ni0···HN
bond. Similarly, the free energy of the second Ni0···HN
bond formed by isomerization from 2B to 2A is 3.7 kcal/mol.
These values correlate well with the Ni0···HN bond strengths
derived from computational studies. The experimentally
observed redox potentials for the Ni(I/0) couples also agree
very well with the theoretical values calculated for isomers
2A−2C. Hydrogen bonds involving transition metals and acids
have been previously reported,40−42 and in some cases these
bond enthalpies have been estimated to range from 3 to
7 kcal/mol. Perhaps the M···HX bonds most relevant to these
without disproportionation to [NiII(PtBu2NR )2]2+. Electro-
2
chemical investigations of [HNiII(PtBu2NR )2]+ indicate that
2
proton transfer from nickel to nitrogen is very rapid, indicating
that this is not a rate-limiting process in corresponding catalytic
systems.
Double protonation of Ni(PtBu2NBn2)2 led to the formation of
[Ni(PtBu2NBnHNBn)2]2+, which can be observed as a mixture of
endo-endo (2A), endo-exo (2B), and exo-exo (2C) isomers in
analogy to related systems. Hydrogen release from [Ni-
(PtBu2NBnHNBn)2]2+ is uphill due to the high energy of
[NiII(PtBu2NR )2]2+, and so this complex could be characterized
2
electrochemically. The E1/2(I/0) potentials of 2A−2C were
found to differ significantly as a result of Ni···HN bonding in
the endo isomers, with the oxidation potential increasing with
the number of Ni···HN bonds. From these potentials, the
Ni···HN bond of 2B was calculated to have a free energy of
7.4 kcal/mol, and the second Ni···HN bond of 2A was
calculated to have a free energy of 3.7 kcal/mol. These values
correspond very well with those derived from computations.
These studies demonstrate the importance of hydrogen
bonding in catalytic Ni systems for H2 oxidation and
production, as Ni···HN bonding has been shown to significantly
affect the I/0 half-wave potentials of species protonated at a
pendant amine. These differences in potential can ultimately
affect the observed overpotential, and so efficient proton
movement between endo and exo sites of the pendant amines
is likely important for minimizing catalytic overpotentials. In a
more general sense, these studies provide insight into how
hydrogen bonding between a metal center in a low oxidation
state and an acid located in the second coordination sphere can
influence catalytic reactions in synthetic homogeneous catalyst
systems, and by extension they suggest the possibility of similar
interactions in enzymes where measuring such interactions
directly are likely to be more difficult.
43
studies are those of [(HN(CH2CH2PPh2)3)Ni(CO)]+ and
Co(CO)3(PPh2(o-C6H4CH2NHMe2)),44 as both complexes
possess an intramolecular M···HN bond at a d10 metal center.
The large stabilization energy resulting from hydrogen
bonding (∼11 kcal/mol) in 2A suggests that such intra-
molecular hydrogen bonding between the metal and nitrogen
play important roles in determining the relative stability of
intermediates and heights of activation barriers in catalytic
′
2
reactions of closely related [Ni(PR NR )2](BF4)2 complexes.
2
Ni(II), Ni(I), and Ni(0) oxidation states are known to be
involved during the catalytic oxidation and production of H2,
and it is of interest to consider how changes in oxidation
states of the metal might affect Ni···HN bonding during such a
cycle. For example, protonation of the Ni(II) complex
[Ni(PPh2NBn2)2]2+ with strong acids such as 2,6-dichloroanili-
nium results in the formation of the tetracation [NiII(PPh
-
2
NBnHNBn)2]4+, in which only the exo-exo isomer analogous
to 2C is observed.13 Because the Ni(II) site is much more
electron-deficient compared to a Ni(0) center, formation of a
NiII···HN bond is not favorable. As a result, the endo-endo and
endo-exo isomers are not stable for this oxidation state. The
observation that the oxidation of 2C to 2C+ is more reversible
than the oxidations of 2A and 2B suggests that the Ni(I)
intermediates 2A+ and 2B+ are less stable than 2C+. This is
likely due to the loss in Ni···HN hydrogen bonding in the Ni(I)
oxidation state. Thus Ni0···HN bonding likely stabilizes Ni(0)
complexes during the catalytic cycles and plays a major role in
determining the geometries of these complexes. Such
interactions likely play a lesser role in higher oxidation states,
although N···HN bonding (as opposed to Ni···HN bonding)
does occur in the higher oxidation states. These interactions
can have significant consequences for catalyst performance. For
example, if during the oxidation of H2, the oxidation occurs
from the endo-endo isomer 2A, which would be expected to be
the kinetic isomer resulting from H−H bond cleavage, the
overpotential would be approximately 0.5 V larger than if the
oxidation occurs from the exo-exo isomer 2C. Thus, hydrogen
bonding between the protonated pendant base in the second
coordination sphere and the metal center can have important
consequences in terms of catalytic rates and overpotentials.
EXPERIMENTAL SECTION
■
General Experimental Procedures. 1H and 31P{1H} NMR
spectra were recorded on a Varian Inova spectrometer (500 MHz
1
1
for H) at 25 °C unless otherwise noted. All H chemical shifts have
been internally calibrated to the monoprotio impurity of the
deuterated solvent. The 31P{1H} NMR spectra were referenced to
external phosphoric acid at 0 ppm. Magnetic moments were
determined by Evans method45−47 in CD3CN solution. UV−vis
spectra were recorded on a Shimadzu UV-2401 PC spectrometer using
UV Probe (version 1.10) software. Elemental analyses were performed
by Atlantic Microlab, Inc. Electrospray ionization (ESI) mass spectra
were collected at the Indiana University Mass Spectrometry Facility on
a Waters/Micromass LCT Classic using anhydrous solvents and inert-
atmosphere techniques.
Electrochemical measurements were performed using a CH
Instruments 660C potentiostat equipped with a standard three-
electrode cell. Experiments were performed in a glovebox using a 4 mL
glass vial as the cell, or on a Schlenk line using an oven-dried 4−5 mL
conical vial fitted with a polysilicone cap having openings sized to
closely accept each electrode. For each experiment performed on a
Schlenk line, the cell was assembled and used under a flow of nitrogen
that was bubbled through dry acetonitrile. The working electrode (1
mm PEEK-encased glassy carbon, Cypress Systems EE040) was
polished using grade 3 alumina polishing gamal (Fisher A446−100)
and then rinsed with neat acetonitrile. A glassy-carbon rod (Structure
Probe, Inc.) was used as the counterelectrode, and a silver wire
suspended in a solution of Et4NBF4 (0.2 M) in acetonitrile and
separated from the analyte solution by a Vycor frit (CH Instruments
112) was used as a pseudo reference electrode. Ferrocene,
decamethylferrocene (E1/2 = −0.50 V in CH3CN, −0.55 V in
PhCN), or cobaltocenium hexafluorophosphate (E1/2 = −1.33 V) was
SUMMARY AND CONCLUSIONS
■
A series of nickel complexes containing bulky PtBu2NR2 (R = Ph,
Bn) ligands was synthesized. The [NiII(PtBu2NR )2]2+ state was
2
shown to be high in energy as a result of the large t-Bu group
on phosphorus. Therefore, the [NiI(PtBu2NR )2]+ and [HNiII-
2
(PtBu2NR )2]+ complexes could be isolated and characterized
2
152
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Organometallics
Article
used as an internal standard, and all potentials are referenced to the
ferrocenium/ferrocene couple at 0 V.
(257.6 mg, 0.290 mmol, 1.1 equiv) in toluene (10 mL). THF (1 mL)
was added to the mixture, which was stirred until there was not any
visible ferrocenium salt present (ca. 1 day). The solvent was removed
in vacuo and the solid rinsed with toluene (3 × 3 mL). The remaining
solid was crystallized by vapor diffusion of Et2O into CH2Cl2 to afford
[Ni(PtBu2NPh2)2]BF4 (197.9 mg, 0.203 mmol, 79%) as purple-red
crystals. 1H NMR (500 MHz, CD3CN): δ 62.48 (br s, Δν1/2 = 265 Hz,
8H, PCHAHXN), 13.43 (br s, Δν1/2 = 92 Hz, 8H, PCHAHXN), 9.10
(br s, Δν1/2 = 77 Hz, 36H, C(CH3)3), 7.48 (t, 8H, J = 5.7 Hz, ArH),
6.80 (d, 8H, J = 8.1 Hz, ArH), 6.51 (td, 4H, J = 7.2 Hz, 2.2 Hz, ArH).
μeff (CD3CN) = 1.71 μB. Anal. Calcd for C48H72BF4N4NiP4: C, 59.16;
H, 7.45; N, 5.75. Found: C, 59.17; H, 7.61; N, 5.73.
Methods and Materials. All manipulations were carried out
under N2 using standard vacuum-line, Schlenk, and inert-atmosphere
glovebox techniques. Benzonitrile was degassed and dried over 5 Å
molecular sieves. All other solvents were purified by passage through
neutral alumina using an Innovative Technology, Inc., Pure Solv
solvent purification system. NMR solvents were purchased from
Cambridge Isotopes and were dried, degassed, and distilled prior to
use. Tetraethylammonium tetrafluoroborate was twice recrystallized
from CH3CN/Et2O and dried in vacuo at room temperature. Tetra-n-
butylammonium triflate was recrystallized from CH2Cl2/hexane at
−30 °C and dried in vacuo at room temperature. Anilinium and
pyridinium salts were prepared by reaction of the parent base with
1.5 equiv of HBF4·Et2O, and then the crude salts were recrystallized
from CH3CN/Et2O. Benzylamine was degassed via four consecutive
freeze−pump−thaw cycles. Pyrrolidine was dried over CaH2 and
distilled prior to use. [Fe(C5Me5)2]BF4 was prepared by oxidation of
Fe(C5Me5)2 with AgBF4 in toluene. Zinc dust (<10 μm) was
purchased from Aldrich, and Ni(COD)2 was purchased from Strem
Chemicals. PtBu2NPh2, PtBu2NBn2, and [Co(PtBu2NPh2)(CH3CN)3]-
(BF4)2 were prepared according to literature procedures.14
Computational Details. All structures were optimized without
symmetry constraints using the B3P8648,49 functional. The Stuttgart
basis set with effective core potential (ECP)50 was used for the Ni
atom, whereas the 6-31G* basis set51,52 was used for all of the other
atoms with one additional p polarization funtion (ξ(p) = 1.1) for the
proton on pendant amines. The optimized structures were confirmed
by frequency calculations. For each species, the gas-phase free energy
was calculated at 298 K in the harmonic approximation. Solvation free
energy in acetonitrile was calculated by using the polarizable
continuum model C-PCM model53,54 using Bondi55 atomic radii.
Intramolecular interactions were analyzed in terms of natural bond
orbitals (NBO).32−39 This computational setup has been shown to
describe well the H2 chemistry of the Ni(P2N2)2 complexes.56 All of
the calculations were carried out with the program Gaussian 09.57
Syntheses. Ni(PtBu2NPh2)2. In the dark, solid Ni(COD)2 (101.5 mg,
0.37 mmol) was added to a cold (ca. −35 °C) suspension of
PtBu2NPh2 (431.0 mg, 1.0 mmol) in THF (16 mL). After it was stirred
overnight at ambient temperature, the mixture was heated in a 60 °C
oil bath for 24 h in the dark. Upon cooling, the mixture was filtered
and the filtrate was concentrated to dryness. The residue was rinsed
[Ni(PtBu2NBn2)2]BF4. In the dark, a solution of Ni(PtBu2NBn2)2 (501.4
mg, 0.531 mmol, 1.1 equiv) in toluene (15 mL) was added to solid
[Fe(C5Me5)2]BF4 (200.0 mg, 0.484 mmol, 1.0 equiv). THF (3 mL) was
added to the mixture, which was stirred until there was not any visible
ferrocenium salt present (ca. 2 days). The mixture was concentrated
slightly to remove THF, and then the mixture was filtered and the solid
rinsed with toluene (2 × 3 mL). The remaining solid was dissolved in a
3:1 mixture of Et2O and MeCN, and then the solution was cooled to
−35 °C. The orange crystals that formed were rinsed with ether and
dried in vacuo to afford [Ni(PtBu2NBn2)2]BF4 (428.3 mg, 0.416 mmol,
1
86%). H NMR (500 MHz, CD3CN): δ 8.11 (br s, Δν1/2 = 133 Hz,
36H, C(CH3)3), 7.25 (t, 8H, J = 7.0 Hz, ArH), 7.20 (t, 4H, J = 7.1 Hz,
ArH), 7.01 (d, 8H, J = 7.0 Hz, ArH), 4.58 (br s, Δν1/2 = 79 Hz, 8H). μeff
(CD3CN) = 1.78 μB. Anal. Calcd for C52H80BF4N4NiP4: C, 60.60; H,
7.82; N, 5.44. Found: C, 60.66; H, 7.97; N, 5.49.
[Co(PtBu2NPh2)2]BF4. Solid zinc dust (307 mg, 4.7 mmol, 23 equiv)
was added to a brown solution of [Co(PtBu2NPh2)(CH3CN)3](BF4)2
(154.8 mg, 0.201 mmol) in CH3CN (8 mL). After it was stirred for
21 h, the mixture was filtered and the green filtrate was added to solid
PtBu2NPh (80.6 mg, 0.194 mmol, 1 equiv). The green mixture was
2
stirred for 45 min, during which time the added ligand dissolved
completely. The solvent was removed in vacuo, and then the residue
was dissolved in CH2Cl2 (5 mL) and passed through a small plug of
Celite inside of a glass pipet. After the Celite was rinsed with
additional CH2Cl2 (2 mL), the combined filtrate was concentrated to a
volume of ca. 4 mL. To this solution was added hexane (15 mL), and
then the precipitate was collected by filtration and dried in vacuo to
afford [Co(PtBu2NPh2)2]BF4 (184.4 mg, 0.189 mmol, 97%) as a green
1
powder. The H NMR spectrum was analyzed using the line-fitting
feature of MestReNova (version 6.0.4), and the listed peak
integrations were obtained directly from the areas of the best-fit
with CH3CN (3 × 5 mL) and dried in vacuo to afford Ni(PtBu2NPh
)
2
2
(274.2 mg, 0.31 mmol, 84%) as a yellow powder. 1H NMR (500 MHz,
lines. H NMR (500 MHz, CD3CN): δ 78.49 (br s, Δν1/2 = 314 Hz,
1
C6D6): δ 7.22 (t, 8H, 3JHH = 7.6 Hz, ArH), 7.06 (d, 8H, 3JHH = 7.9 Hz,
8H PCHAHXN), 9.28 (br s, Δν1/2 = 315 Hz, 6H, PCHAHXN), 8.06 (s,
9H, ArH), 7.99 (br s, Δν1/2 = 188 Hz, 34H, C(CH3)3), 7.17 (s, 8H,
ArH), 6.00 (s, 4H, ArH). μeff (CD3CN) = 2.85 μB. ESI-MS: obsd
{[Co(P2N2)2] − 2H}+ at m/z 885.3890, predicted 885.3883.
3
2
ArH), 6.84 (t, 4H, JHH = 7.2 Hz, ArH), 3.46 (d, 8H, JHH = 12.6 Hz,
2
PCHAHBN), 3.36 (d, 8H, JHH = 12.6 Hz, PCHAHBN), 1.06 (d, 36H,
3JHP = 11.5 Hz, C(CH3)3). 31P{1H} NMR (C6D6): δ 20.11 (s). Anal.
Calcd for C48H72N4NiP4: C, 64.94; H, 8.18; N, 6.31. Found: C, 65.04;
H, 8.15; N, 6.37.
[HNi(PtBu2NPh2)2]BF4. In the dark, a solution of pyridinium
tetrafluoroborate (25.0 mg, 0.150 mmol) in CH3CN (2 mL) was
added to a stirred solution of Ni(PtBu2NPh2)2 (160.8 mg, 0.181 mmol,
1.2 equiv) in THF (6 mL). After 1.5 h, the mixture was concentrated to
dryness. The residue was washed with Et2O (1 × 10 mL, 3 × 4 mL) to
afford [HNi(PtBu2NPh2)2]BF4 (110.9 mg, 0.114 mmol, 76%) as a yellow
powder. 1H NMR (500 MHz, CD3CN): δ 7.93 (m, 8H, ArH), 7.15 (m,
The complex Ni(COD)(PtBu2NPh2) could be identified by NMR
spectroscopy as the major product if the reaction was conducted at
1
room temperature. H NMR (300 MHz, C6D6): 4.11 (br s, 4H, C
CH−C), 3.41 (m, 4H, PCHAHBN), 3.21 (m, 4H, PCHAHBN), 2.45
(m, 4H, COD), 2.20 (m, 4H, COD), 1,12 (m, 18H, C(CH3)3).
31P{1H} NMR (C6D6): δ 21.03 (s).
2
8H, ArH), 6.95 (m, 4H, ArH), 3.68 (d, 8H, JHH = 13.7 Hz,
2
Ni(PtBu2NBn2)2. In the dark, solid Ni(COD)2 (0.4071 g, 1.48 mmol)
PCHAHBN), 3.58 (d, 8H, JHH = 13.7 Hz, PCHAHBN), 1.20 (d, 36H,
3JHP = 12.3 Hz, C(CH3)3), −12.46 (quintet, 1H, 2JHP = 12.8 Hz, NiH).
was added to a cold (ca. −35 °C) solution of PtBu2NBn (1.3099 g,
2
31P{1H} NMR (CD3CN):
δ 26.64 (s). Anal. Calcd for
2.96 mmol) in THF (20 mL). The solution was stirred for 4 days at
ambient temperature, and then the solution was concentrated to
dryness. The residue was rinsed with CH3CN (2 × 10 mL) and dried
in vacuo to afford Ni(PtBu2NBn2)2 (1.2746 g, 1.35 mmol, 91%) as a
yellow powder. 1H NMR (500 MHz, C6D6): 7.24−7.09 (m, 20H,
ArH), 3.58 (s, 8H, NCH2Ph), 2.83 (dd, 8H, 2JHH = 11.3 Hz, 2JHP = 3.2
C48H73BF4N4NiP4: C, 59.10; H, 7.54; N, 5.74. Found: C, 59.38; H,
7.73; N, 5.74.
[HNi(PtBu2NBn2)2]BF4. In the dark, a solution of 2,6-lutidinium
tetrafluoroborate (28.2 mg, 0.145 mmol) in CH3CN (2 mL) was
added to a stirred solution of Ni(PtBu2NBn2)2 (165.4 mg, 0.175 mmol,
1.2 equiv) in THF (6 mL). After it was stirred for 45 min, the mixture
was concentrated to dryness. The residue was washed with Et2O (4 ×
2 mL) to afford [HNi(PtBu2NBn2)2]BF4 (113.4 mg, 0.110 mmol, 76%)
2
Hz, PCHAHBN), 2.41 (d, 8H, JHH = 11.3 Hz, PCHAHBN), 1.07 (d,
3
36H, JHP = 11.4 Hz, C(CH3)3). 31P{1H} NMR (C6D6): δ 15.45 (s).
Anal. Calcd for C52H80N4NiP4: C, 66.17; H, 8.54; N, 5.94. Found: C,
66.89; H, 8.57; N, 5.86.
1
as a yellow powder. H NMR (500 MHz, CD3CN): δ 7.35 (m, 12H,
[Ni(PtBu2NPh2)2]BF4. In the dark, solid [Fe(C5Me5)2]BF4 (106.6 mg,
ArH), 7.15 (m, 8H, ArH), 3.73 (s, 8H, NCH2Ph), 2.80 (d, 8H, 2JHH
12.4 Hz, PCHAHBN), 2.61 (d, 8H, JHH = 12.4 Hz, PCHAHBN), 1.00
=
0.256 mmol, 1.0 equiv) was added to a solution of Ni(PtBu2NPh
)
2
2
2
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Table 4. Crystallographic Data for Ni(PtBu2NBn2)2, [Ni(PtBu2NBn2)2]BF4, and [Co(PtBu2NPh2)2]BF4
Ni(PtBu2NBn
)
2
[Ni(PtBu2NBn2)2]BF4·2CH3CN
1.5[Co(PtBu2NPh2)2]BF4·CH3CN
2
emp formula
mass (amu)
temp (K)
wavelength (Å)
cryst syst
space group
a (Å)
C52H80N4NiP4
C56H86BF4N6NiP4
1112.71
C74H111B1.50Co1.50F6N7P6
943.79
100(2)
0.710 73
monoclinic
C2/c
1503.13
100(2)
100(2)
0.710 73
0.710 73
monoclinic
P2/n
triclinic
P1
̅
38.5337(7)
12.8379(2)
20.9599(4)
90
13.4155(15)
13.7624(15)
19.175(2)
86.220(3)
70.099(3)
77.714(3)
3252.6(6)
2
14.876(4)
11.517(3)
43.545(12)
90
b (Å)
c (Å)
α (deg)
β (deg)
97.2350(10)
90
91.718(7)
90
γ (deg)
V (Å3)
10286.1(3)
8
7457(4)
4
Z
calcd density (Mg/m3)
1.219
1.136
1.339
abs coeff (mm−1
)
0.539
0.445
0.528
F(000)
4064
1186
3184
cryst size (mm3)
color and morphology
θ range (deg)
0.10 × 0.05 × 0.03
yellow block
1.67−33.22
72 824
0.17 × 0.08 × 0.06
orange block
1.65−25.35
58 463
0.20 × 0.16 × 0.02
green plate
1.46−33.34
99 242
no. of rflns collected
no. of indep rflns
19 636 (R(int) = 0.1838)
1.04
11 914 (R(int) = 0.0688)
1.05
28 678 (R(int) = 0.1168)
1.10
Tmax/Tmin
no. of data/restraints/params
19 636/0/562
1.039
11 914/48/730
1.048
28 678/0/885
1.016
a
GOF on F2
b
R index (I > 2σ(I))
R1
0.0701
0.0845
0.0942
0.2735
0.0666
0.1093
wR2
b
R index (all data)
R1
0.2493
0.1286
0.1207
0.2999
0.1575
0.1339
wR2
c
weighting coeff
a
b
0.0248
0.1741
0.0419
0
0
15.7368
largest diff peak, hole (e Å−3
a
)
0.539, −0.579
2.548, −0.714
3.066, −2.518
b
GOF = S = [∑w(Fo2 − Fc2)/(M − N)]1/2, where M is the number of reflections and N is the number of parameters refined. R1 = ∑||Fo| − |Fc||/
c
∑|Fo|; wR2 = [∑w(Fo − Fc2)/∑w(Fo )2]1/2. w−1 = [σ2(Fo ) + (ap)2 + bp], where p = (Fo + 2Fc2)/3.
2
2
2
2
3
3
(d, 36H, JHP = 11.7 Hz, C(CH3)3), −11.63 (quintet, 1H, JHP = 11.3
Hz, NiH). 31P{1H} NMR (CD3CN): δ 21.78 (s). Anal. Calcd for
C52H81BF4N4NiP4: C, 60.54; H, 7.91; N, 5.43. Found: C, 60.24; H,
8.09; N, 5.49.
analyte was determined from the measured Keq and the pKa of the
reference. The listed error is three times the standard deviation for all
of the measurements. [HNi(PtBu2NPh2)2]BF4: [HNi(PtBu2NPh2)2]BF4
was combined with 0.8, 1.0, and 1.3 equiv of benzylamine to give Keq
1.5 0.3 and pKa = 16.7
0.4. [HNi(PtBu2NBn2)2]BF4: [HNi-
(PtBu2NBn2)2]BF4 was combined with 3.3, 6.1, and 14.7 equiv of
=
Protonation of Ni(PtBu2NBn
) with p-Cyanoanilinium To form
2 2
[Ni(PtBu2NBnHNBn)2](BF4)2 (2A−2C). A solution of p-cyanoanilinium
tetrafluoroborate (6.6 mg, 0.032 mmol, 2.1 equiv) in ca. 0.7 mL of
CD3CN was added to solid Ni(PtBu2NBn2)2 (14.4 mg, 0.015 mmol,
1.0 equiv) in a J. Young NMR tube, resulting in a clear yellow solution
upon mixing. The solution was degassed by three consecutive freeze−
pump−thaw cycles, and the reaction was monitored by NMR
spectroscopy. 31P{1H} NMR: δ 29.53 (s, 2A, endo-endo), 25.91 (s,
2B, endo-exo), 2.59 (s, 2B, endo-exo), −1.51 (s, 2C, exo-exo).
pKa Measurements. Using solutions of known concentrations,
aliquots of the analyte and the acid or base reference were combined in
an NMR tube to produce a total volume of 0.7 mL with analyte
concentrations of <15 mM and reference concentrations of <50 mM.
The samples were analyzed by 31P{1H} NMR spectroscopy after 1 h,
and the ratios of the nickel species present were determined by
integration. The NMR spectra were recorded again 1 h later, and the
ratios were the same within experimental error. The ratios of acid and
base were determined by mass balance after taking into account the
self-association of the acid−base pair.58 Each measurement was
performed twice to verify consistency, and equilibrium constants (Keq)
were determined from these measurements. The pKa value of each
pyrrolidine to give Keq = 0.028
0.005 and pKa = 21.1
0.4.
Ni(PtBu2NBnHNBn)2](BF4)2 (2A): Ni(PtBu2NBn2)2 was combined with
1.3 and 2.5 equiv of pyridinium tetrafluoroborate to give Keq = 0.50
0.10 and pKa = 21.1 0.4.
X-ray Diffraction Studies. For each of the crystal structure
studies, a single crystal was mounted using NVH immersion oil onto a
nylon fiber and cooled to the data collection temperature of 100(2) K.
Data were collected on a Bruker-AXS Kappa APEX II CCD
̈
diffractometer with 0.710 73 Å Mo Kα radiation. Unit cell parameters
were obtained from 90 data frames at 0.3° ϕ (Ni(PtBu2NBn2)2 and
[Co(PtBu2NPh2)2]BF4) or 36 data frames at 0.5° ϕ ([Ni(PtBu2NBn2)2]-
BF4) from three different sections of the Ewald sphere. The data sets
were treated with SADABS absorption corrections based on redundant
multiscan data (G. Sheldrick, Bruker-AXS, 2001). All non-hydrogen
atoms were refined with anisotropic displacement parameters. All
hydrogen atoms were treated as idealized contributions. Additional
details of the structural studies are given in Table 4.
Crystals of Ni(PtBu2NBn2)2 suitable for an X-ray diffraction study
were grown from slow evaporation of a pentane solution at room
154
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Organometallics
Article
temperature. The systematic absences in the diffraction data were
consistent with the centrosymmetric, monoclinic space group C2/c.
The molecule is located on a general position, yielding Z = 8 and
Z′ = 1.
(3) Rakowski DuBois, M.; DuBois, D. L. C. R. Chim. 2008, 11, 805−
817.
(4) DuBois, D. L.; Bullock, R. M. Eur. J. Inorg. Chem. 2011, 1017−
1027.
Crystals of [Ni(PtBu2NBn2)2]BF4 suitable for an X-ray diffraction
study were grown from a CH3CN/Et2O solution at −35 °C. The
systematic absences in the diffraction data were consistent with the
(5) Fraze, K.; Wilson, A. D.; Appel, A. M.; Rakowski DuBois, M.;
DuBois, D. L. Organometallics 2007, 26, 3918−3924.
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triclinic space group P1. The asymmetric unit contains one
̅
−
[Ni(PtBu2NBn2)2]+ cation, one BF4 anion, and two molecules of
acetonitrile solvent located on general positions, yielding Z = 2 and
Z′ = 1. The BF4− anion and one molecule of acetonitrile solvent were
each disordered over two positions. These positions were located from
the difference map and restrained to be equivalent using SADI and
EADP commands to stabilize the refinement.
Crystals of [Co(PtBu2NPh2)2]BF4 suitable for an X-ray diffraction
study were grown from vapor diffusion of Et2O into CH3CN. The
systematic absences in the diffraction data were consistent with the
centrosymmetric, monoclinic space group P2/n. The asymmetric unit
−
contains 1.5 [Co(PtBu2NPh2)2]+ cations, 1.5 BF4 anions, and one
molecule of acetonitrile solvent, yielding Z = 4 and Z′ = 1.5. There is
some residual density around the cobalt that sits on the special
position. This is likely an artifact of some disorder about the symmetry
element and does not appear to be related to any accuracy issues with
the model.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving crystallographic data for Ni(PtBu2NBn2)2,
[Ni(PtBu2NBn2)2]BF4, and [Co(PtBu2NPh2)2]BF4 and figures
giving the UV−vis spectrum for [Co(PtBu2NPh2)2]BF4, kinetic
plots for the decomposition of [HNi(PtBu2NPh2)2]BF4 and
[HNi(PtBu2NBn2)2]BF4, NMR spectra of 2A−2C, and additional
cyclic voltammetry plots of [HNi(PtBu2NPh2)2]BF4 and [HNi-
(PtBu2NBn2)2]BF4. This material is available free of charge via
the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: morris.bullock@pnnl.gov (R.M.B.); daniel.dubois@
pnnl.gov (D.L.D.).
(19) Yang, J. Y.; Chen, S. T.; Dougherty, W. G.; Kassel, W. S.;
Bullock, R. M.; DuBois, D. L.; Raugei, S.; Rousseau, R.; Dupuis, M.;
Rakowski DuBois, M. Chem. Commun. 2010, 46, 8618−8620.
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1999, 121, 11432−11447.
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Notes
∥Sabbatical visitor at Pacific Northwest National Laboratory,
2010−2011.
ACKNOWLEDGMENTS
■
We thank the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Biosciences and Geosciences, for support of work by E.S.W.,
R.M.B., M.R.D., and D.L.D. Research by J.Y.Y., S.C., S.R., and
M.L.H. was supported as part of the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center funded by
the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences, under FWP 56073. Computational
resources were provided at the National Energy Research
Scientific Computing Center (NERSC) at Lawrence Berkeley
National Laboratory. Pacific Northwest National Laboratory is
a multiprogram national laboratory operated for the U.S.
Department of Energy by Battelle.
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