Two pathways for electrocatalytic oxidation of hydrogen by a nickel bis(diphosphine) complex with pendant amines in the second coordination sphere

Article abstract of DOI:10.1021/ja400705a

A nickel bis(diphosphine) complex containing pendant amines in the second coordination sphere, [Ni(P^Cy₂N^t-Bu ₂)₂](BF₄)₂ (P^Cy ₂N^t-Bu₂ = 1,5-di(tert-butyl)-3,7-dicyclohexyl- 1,5-diaza-3,7-diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni^II complex gives three isomers of the doubly protonated Ni⁰ complex [Ni(P^Cy ₂N^t-Bu₂H)₂](BF₄) ₂. Using the pK_a values and Ni^II/I and Ni ^I/0 redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(P^Cy₂N^t-Bu ₂)₂]²⁺ was determined to be -7.9 kcal mol ^-1. The catalytic rate observed in dry acetonitrile for the oxidation of H₂ depends on base size, with larger bases (NEt₃, t-BuNH₂) resulting in much slower catalysis than n-BuNH₂. The addition of water accelerates the rate of catalysis by facilitating deprotonation of the hydrogen addition product before oxidation, especially for the larger bases NEt₃ and t-BuNH₂. This catalytic pathway, where deprotonation occurs prior to oxidation, leads to an overpotential that is 0.38 V lower compared to the pathway where oxidation precedes proton movement. Under the optimal conditions of 1.0 atm H₂ using n-BuNH₂ as a base and with added water, a turnover frequency of 58 s^-1 is observed at 23 C.

Full text of DOI:10.1021/ja400705a

Subscriber access provided by Binghamton University | Libraries
Article
Two Pathways for Electrocatalytic Oxidation of
Hydrogen by a Nickel Bis(diphosphine) Complex with
Pendant Amines in the Second Coordination Sphere
Jenny Y Yang, Stuart E Smith, Tianbiao Liu, William G Dougherty, Wesley A Hoffert,
William Scott Kassel, Mary Rakowski DuBois, Daniel L. DuBois, and R. Morris Bullock
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja400705a • Publication Date (Web): 30 Apr 2013
Downloaded from http://pubs.acs.org on May 15, 2013
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Two Pathways for Electrocatalytic Oxidation of Hydrogen by a
Nickel Bis(diphosphine) Complex with Pendant Amines in the
Second Coordination Sphere
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
‡§
‡
‡
Jenny Y. Yang* , Stuart E. Smith, Tianbiao Liu, William G. Dougherty,
†
Wesley A.
‡
‡
‡
‡
Hoffert, W. Scott Kassel,
† M. Rakowski DuBois, Daniel L. DuBois, and R. Morris Bullock*
‡
Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, P.O. Box 999,
K2ꢀ57, Richland, WA 99352
†
Department of Chemistry, Villanova University, Villanova, Pennsylvania 19085
§
Current address: Department of Chemistry, University of California Irvine, Irvine, California
92697, United States
eꢀmail: jyy@caltech.edu, morris.bullock@pnnl.gov
Abstract: A nickel bis(diphosphine) complex containing pendant amines in the second
coordination sphere, [Ni(PCy2Nt-Bu2)2](BF4)2 (PCy2Nt-Bu2 = 1,5ꢀdi(tertꢀbutyl)ꢀ3,7ꢀdicyclohexylꢀ1,5ꢀ
diazaꢀ3,7ꢀdiphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of
hydrogen to the NiII complex gives three isomers of the doubly protonated Ni0 complex
[Ni(PCy2Nt-Bu2H)2](BF4)2. Using the pKa values and NiII/I and NiI/0 redox potentials in a
thermochemical cycle, the free energy of hydrogen addition to [Ni(PCy2NtꢀBu2)2]2+ was
determined to be −7.9 kcal molꢀ1. The catalytic rate observed in dry acetonitrile for the oxidation
of H2 depends on base size, with larger bases (NEt3, tꢀBuNH2) resulting in much slower catalysis
than nꢀBuNH2. The addition of water accelerates the rate of catalysis by facilitating
deprotonation of the hydrogen addition product before oxidation, especially for the larger bases
NEt3 and tꢀBuNH2. This catalytic pathway, where deprotonation occurs prior to oxidation, leads
to an overpotential that is 0.38 V lower compared to the pathway where oxidation precedes
1
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 38
1
2
3
4
5
6
7
proton movement. Under the optimal conditions of 1.0 atm H2 using nꢀBuNH2 as a base and
with added water, a turnover frequency of 58 sꢀ1 is observed at 23 °C.
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Introduction
The widespread use of renewable energy will require improved methods of energy
storage because of the intermittent nature of energy production from solar and wind power.1
Chemical fuels such as hydrogen are ideal because of their high energy density by weight, but
electrocatalysts are necessary for both hydrogen production by proton reduction and subsequent
oxidation of hydrogen in a fuel cell. Currently, the most efficient and fastest catalyst for
hydrogen oxidation is platinum, but its high cost and low abundance limit its general use.2ꢀ5 The
utility of hydrogen as a chemical fuel and as a reductant to generate other chemical fuels has
generated a great deal of research interest in catalyst discovery for hydrogen production. Many
molecular catalysts for hydrogen production have been discovered using abundant, inexpensive
metals such as nickel,6ꢀ16 cobalt,17ꢀ25 iron,26ꢀ33 and molybdenum.34,35 In contrast, there have been
few reported homogeneous catalysts for the reverse reaction, oxidation of hydrogen, which is
necessary for efficient fuel utilization.36ꢀ52
Hydrogenase enzymes catalyze both the production and oxidation of hydrogen at high
rates and efficiency using active sites composed of the abundant metals iron and nickel.53,54 The
[FeFe] hydrogenase enzyme incorporates a pendant amine in the active site that is believed to be
properly positioned to facilitate formation and cleavage of the HꢀH bond. We have reported a
series of catalysts based on a nickel bis(diphosphine)
2+
R
R
framework [Ni(PR NR 2)2] , where P N 2 is an
´
´
2
2
eightꢀmembered diphosphine ligand with two
pendant amines, and R and R denote the substituents
´
on the phosphorous and nitrogen, respectively.55ꢀ58
The nonꢀcoordinating tertiary amines are positioned
2
ACS Paragon Plus Environment

Page 3 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
to stabilize binding of H2, serve as a proton acceptor to facilitate the heterolytic cleavage of H2,
and mediate protonꢀcoupled electron transfer steps in the catalytic cycle. The complex
[Ni(PCy2NtꢀBu 2)2](BF4)2 was recently reported to electrocatalytically oxidize hydrogen with a
high turnover frequency (TOF).50
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In this work we report detailed studies of this catalyst that reveal two separate catalytic
pathways which differ by 0.38 V in overpotential. This difference can be traced to the sequence
of proton movement and electron transfer in the catalytic cycle. In addition, we now report the
effect of water in accelerating the catalytic rate for hydrogen oxidation, and we trace this
acceleration as kinetically favoring the pathway in which deprotonation occurs prior to
oxidation. Thus, the addition of water results in both a reduction in overpotential and a higher
catalytic rate. Also, the free energy of heterolytic cleavage of H2 by [Ni(PCy2NtꢀBu2)2](BF4)2, has
been calculated from the experimentally determined hydride and proton acceptor energies, as
well as electrochemical data. From this thermochemical data and previously reported data we
have determined the effect of varying the functional groups of the ligand on the thermodynamic
properties of the nickel complex, and the relationship between driving force for hydrogen
addition and catalytic rate. These detailed studies provide mechanistic insights into the role of
managing thermodynamic properties and proton movement in significantly reducing the
overpotential while increasing the rate in hydrogen oxidation electrocatalysis
Results
Synthesis and Characterization of Ni0(PCy2Nt-Bu2)2 and [NiI(PCy2Nt-Bu2)2](BF4). The Ni0
complex Ni(PCy2NtꢀBu2)2 was prepared by reaction of Ni(COD)2 (COD = 1,5ꢀcyclooctadiene)
with two equivalents of the ligand PCy2NtꢀBu2 (1,5ꢀdiꢀ(tertꢀbutyl)ꢀ3,7ꢀdicyclohexylꢀ1,5ꢀdiazaꢀ3,7ꢀ
diphosphacyclooctane)50 under nitrogen at −40° C in tetrahydrofuran. The yellow airꢀsensitive
powder was isolated and characterized by 31P and 1H NMR spectroscopy. The 31P59 NMR
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 38
1
2
3
4
5
6
spectrum in C6D6 exhibits a singlet at 7.41 ppm, typical of tetrahedral (diphosphine)2Ni0
complexes.12,18,60
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. The structure of Ni(PCy2NtꢀBu2)2. Thermal ellipsoids are shown at the 80%
probability level. For clarity, only the carbons of the cyclohexyl rings directly bound to
the phosphorous atoms are shown.
Crystals of Ni(PCy2NtꢀBu 2)2 were grown by evaporation of a pentane solution, and the structure
was confirmed by a single crystal Xꢀray diffraction study. The structure is shown in Figure 1,
and selected bond distances and angles are shown in Table 1. The complex has a distorted
tetrahedral geometry with all four Ni—P bond distances nearly equal at 2.13ꢀ2.14 Å. The
P─Ni─P bond angles for the diphosphine ligand are about 84°, which is significantly smaller
than the 109° expected for an ideal tetrahedron. The smaller bite angle observed for the PCy2Ntꢀ
Bu
2 ligand is attributed to constraints imposed by the two sixꢀmembered chelate rings in the
backbone of the cyclic ligand. Two of the sixꢀmembered rings have boat conformations,
directing the pendant base towards the metal center (Ni•••N distances are 3.399 and 3.483 Å),
and the other two are in a chair form (both Ni•••N distances are 3.83 Å). The dihedral angle
between the two planes formed by the two phosphorous atoms of each diphosphine ligand and
the Ni atom is 84.66°, close to the 90° angle expected for a tetrahedral complex. We previously
4
ACS Paragon Plus Environment

Page 5 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
reported an alternate preparation by reduction of the corresponding NiII complex with hydrogen
in the presence of a base;50 the yield and purity of the complexes prepared by the two methods
are comparable.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Selected bond distances (Å) and angles (°) for Ni(PCy2Nt-Bu 2)2 and [Ni(PCy2Nt-
2H)](BF4)2, Isomer B.
Bu
Ni(PCy2NtꢀBu
2.1417(4)
2.1436(4)
3.399
)
2 2
Isomer B
2.1394(10)
2.1418(9)
3.870
NiꢀP1
NiꢀP3
Ni•••N1
Ni•••N2
Ni•••N3
Ni•••N4
P1ꢀNiꢀP2
P3ꢀNiꢀP4
3.834
3.821
3.483
3.307
3.830
3.828
83.897(15)
85.75(3)
85.08(4)
85.75(3)
The NiI complex [Ni(PCy2NtꢀBu2)2](BF4) was synthesized by reaction of the corresponding
NiII and Ni0 complexes.50 The E1/2 of the NiII/I and NiI/0 couples for the complex are separated by
0.65 V. This large potential difference leads to clean and quantitative comproportionation of the
NiII and Ni0 complexes to generate the orange airꢀsensitive NiI complex. The magnetic moment
was measured using the Evans method.61ꢀ63 In an acetonitrile solution, eff for [NiI(PCy2Ntꢀ
ꢀ
Bu
2)2](BF4) is 1.90 ꢀB at 293 K. This value is close to the expected spinꢀonly value for a S = ½
NiI center, assuming g = 2.0 (0.375 emu⋅K⋅mol−1). However, late transition metal ions like Nin+
often exhibit g values slightly larger than 2.0 due to spinꢀorbit coupling, so the measured
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 38
1
2
3
4
5
6
7
8
9
susceptibility value is consistent with a S = ½ NiI ion. The cyclic voltammogram of the complex
in acetonitrile displays two reversible oneꢀelectron couples that match the halfꢀwave
potentials for the NiII/I and NiI/0 couples for the corresponding NiII complex (at −0.77 V and
−1.42 V, respectively, versus the Cp2Fe+/0 couple).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Products of the Addition of Hydrogen to [Ni(PCy2Nt-Bu 2)2](BF4)2
Addition of dry hydrogen to a solution of [Ni(PCy2NtꢀBu2)2]2+ in anhydrous CD3CN leads
to a rapid reaction at room temperature, forming the doubly protonated Ni0 complex [Ni(PCy2Nt-
Bu
2H)2](BF4)2. The reaction goes to completion as observed by 31P{1H} NMR spectroscopy.
Protonation of the pendant amines can occur endo or exo to the metal, as shown in Scheme 1,
leading to three isomers. Previous studies on the three closely related isomers of
[Ni(PCy2NBn2H)2]2+ (with benzyl (Bn) on nitrogen) have established their structures by 1H, 31P,
and 2H NMR spectroscopy.7,64
The first isomer formed after heterolytic cleavage of hydrogen by [Ni(PCy2NtꢀBu2)2]2+ is
the endoꢀendo isomer, A, which is observed in the 31P{1H} NMR spectrum at 14.7 ppm as the
sole product. The broad singlet resonance is consistent with a tetrahedral Ni0 complex with a
Scheme 1
6
ACS Paragon Plus Environment

Page 7 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
protonated amine in each cyclic ligand. Isomer B is observed in the 31P{1H} NMR spectrum after
an hour, and continues to increase over several hours. Isomer B exhibits two broad singlets of
equal intensity at 13.3 and −11.9 ppm in the 31P{1H} NMR spectrum. The two resonances are
consistent with two inequivalent ligands, and are expected because of the different protonation
sites of each ligand. One ligand is protonated on the side of the amine towards the metal (endo),
and the other ligand is protonated on the opposite side (exo), forming a NꢀHꢀN “pinched”
hydrogen bond between the two amines on the same ligand. In a tetrahedral coordination, the
two phosphorous atoms on the ligand protonated exo would be expected to show inequivalence
due to the endo protonation of the other ligand. However, we have observed rapid intramolecular
proton transfer at room temperature of the endo proton to the other nitrogen on the same ligand.64
This leads to the single broad resonance observed for the exoꢀprotonated ligand at room
temperature. The population of isomer B increases relative to isomer A over the next few hours,
becoming about 60% after 6 hours. Over the 13 hours that the solution was monitored, isomer C
slowly appeared in the 31P{1H} NMR spectrum as a singlet at −10.53 ppm. A singlet is
consistent with a tetrahedral complex that is protonated in an exo position on both diphosphine
ligands, as each phosphorus atom is equivalent. The concentration of isomer C by integration of
the 31P{1H} NMR spectrum is small (4% after 13 hours) but increases over time. After 13 hours,
the concentration of isomer C was still increasing, while the concentration of isomer A and B
were still declining. These three isomers are similar to those observed previously for the
analogous nickel complex with benzyl substituents on the nitrogen, which display similar
resonances in the 31P{1H} NMR spectrum.64
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 38
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Two views of the structure of the cation of [Ni(PCy2Nt-Bu2H)2](BF4)2, isomer B.
Thermal ellipsoids are shown at the 50% probability level. For clarity, only the carbons of the
cyclohexyl rings directly bound to the phosphorous atoms are shown. Hydrogens are not
shown.
When the same experiment is performed in the presence of 10 equivalents of water with
respect to [Ni(PCy2NtꢀBu 2)2]2+, an equilibrium distribution of isomers A, B, and C is attained
much more rapidly. Isomer B is observed within the first few minutes of hydrogen addition. The
amount of isomer A, initially the predominant isomer, rapidly decreases in concentration
concomitant with an increase in isomer C. The solution reaches equilibrium after about 5 hours.
The equilibrium concentration of the isomers was determined by integration of the 31P{1H}
NMR spectrum, and is 8% A, 16% B, and 73% C.
The addition of hydrogen to [Ni(PCy2NtꢀBu 2)2]2+ at low temperature was examined by 31P
NMR spectroscopy to investigate the possibility of observable intermediates (e.g. a NiII
dihydrogen complex)65,66 before the formation of isomer A. Hydrogen was bubbled through a
solution of [Ni(PCy2NtꢀBu 2)2]2+ in nꢀbutyronitrile cooled to −112 °C. The sample was then
inserted into an NMR probe preꢀcooled to −100 °C, and a 31P{1H} NMR spectrum was collected.
The only resonances observed corresponded to [Ni(PCy2NtꢀBu 2)2]2+ and isomer A. As the
temperature was warmed to room temperature, the 31P{1H} NMR spectrum showed complete
8
ACS Paragon Plus Environment

Page 9 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
conversion of [Ni(PCy2NtꢀBu 2)2]2+ to isomer A, thus giving no experimental evidence for
intermediacy of the dihydrogen complex [Ni(H2)(PCy2NtꢀBu 2)2]2+.
7
8
9
Crystals of isomer B were grown from diffusion of diethyl ether into an acetonitrile
solution of [Ni(PCy2NtꢀBu2)2]2+ that had been purged with hydrogen and cooled to −45 °C. The
structure of isomer B is shown in Figure 2. Although the protons were not located, the ꢀ
orientations of the sixꢀmembered rings formed by the ligand and metal support the structure
suggested by 31P{1H} NMR spectroscopy and shown in Scheme 1, and selected bond distances
and angles are given in Table 1. The complex has a distorted tetrahedral geometry with all four
NiꢀP bond distances nearly equal at 2.14 ꢀ 2.15 Å, similar to the previously examined Ni0
complexes in our laboratory, which have bond distances ranging from 2.11ꢀ2.15 Å.18,48,60 The Pꢀ
NiꢀP bond angles for the two diphosphine ligands are 85.75° and 85.01°, slightly larger than
those observed for Ni(PCy2NtꢀBu 2)2. The ligand that is protonated in the “pinched” exo position
has both rings in the chair conformation. With the two lone pairs directed toward each other, the
distance between the two nitrogens is 2.69 Å, consistent with a NꢀHꢀN hydrogen bond. These
observations are only expected with a proton interacting with both N atoms of the “pinched”
ligand. The PCy2NtꢀBu2 ligand protonated in the endo position has one sixꢀmembered ring in the
chair conformation and the other sixꢀmembered ring in the boat conformation. The distance
between the protonated nitrogen and the metal is 3.31 Å, significantly shorter than the Ni•••N
distances of 3.40 and 3.48 Å observed for Ni(PCy2NtꢀBu 2)2. This is again consistent with a
hydrogen bond between the proton on N3 and the Ni0.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
When crystals were grown using the same solvents, but at room temperature, isomer C
was obtained. The crystals cracked under the cold stream during single crystal Xꢀray analysis,
but a structure was obtained from the data collected. All four of the sixꢀmembered rings on the
ligand are in chair conformations, which led to our assignment of this structure as isomer C. The
structure is reported in the Supporting Information. The complex also adopts a distorted
tetrahedral structure with all four Ni—P bond distances at 2.13 Å. It is interesting that
9
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 38
1
2
3
4
5
6
7
8
9
protonation of the ligands on isomers B and C do not result in a significant shortening of the Niꢀ
P bond compared to the Ni0 complex Ni(PCy2NtꢀBu 2)2, since as a result of the protonation, the
ligand becomes cationic. The PꢀNiꢀP bond angles for the diphosphine ligands are 86.07°, again
slightly larger than those observed for Ni(PCy2NtꢀBu 2)2. The protonated ligands adopt “pinched”
exo positions with an average distance between the two disordered nitrogen atoms of 2.63 Å,
consistent with NH•••N hydrogen bonds.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Protonation of Ni(PCy2Nt-Bu2)2. Buffered solutions consisting of a mixture of the base 7ꢀmethylꢀ
−
1,5,7ꢀtriazabicyclo[4.4.0]decꢀ5ꢀene (MTBD) and its protonated form, [HꢀMTBD]+BPh4 , were
added to Ni(PCy2NtꢀBu 2)2 in benzonitrile; (Ni(PCy2NtꢀBu 2)2 is not soluble in acetonitrile). Under
these conditions, an equilibrium is observed (eq. 1) for protonation of Ni(PCy2NtꢀBu2)2 to give the
nickel hydride complex [NiH(PCy2NtꢀBu2)2]+. The two species are observed in the 31P{1H} NMR
spectrum at 13.0 and 7.3 ppm, respectively; no other resonances are observed. Most of the 1H
NMR spectrum of this solution was obscured by the protons of the benzonitrile solvent, but a
broad resonance observed at −13.5 ppm is attributed to the hydride resonance of [NiH(PCy2Ntꢀ
Bu
2)2]+. The ratios of these two complexes were measured by integration of the 31P{1H} NMR
spectrum, and an equilibrium constant of Keq = 9 (±6) was calculated from this data. This
equilibrium constant and a pKa value of 25.5 for [HꢀMTBD]BF4 in acetonitrile67 were used to
determine a pKa1 value of 24.6(±0.3) for [NiH(PCy2NtꢀBu2)2]+ (Scheme 2). Previous studies have
shown that relative pKa values for similar complexes in acetonitrile and benzonitrile are the same
within experimental error.68 As a cross check, the same experiments were repeated using a
buffered solution of 1,8ꢀdiazabicyclo[5.4.0]undecꢀ7ꢀene (DBU) and its protonated analog [Hꢀ
DBU]BF4. With this buffer, and using a pKa of 24.34 for [HꢀDBU]BF4 in acetonitrile,67 a pKa1
10
ACS Paragon Plus Environment

Page 11 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
value of 24.7 was determined for [NiH(PCy2NtꢀBu2)2]+ in good agreement with the value obtained
using MTBD/[HꢀMTBD]+.
7
8
9
Scheme 2. Protonation equilibria for Ni(PCy2NtꢀBu 2)2 and [NiH(PCy2NtꢀBu2)2]+. The structures of
isomers A, B, and C are shown in Scheme 1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The acidity of hydrogen addition products (pKa2 in Scheme 2), AꢀC, were measured by
the addition of hydrogen to [Ni(PCy2NtꢀBu2)2]2+ in acetonitrile, followed by addition of the base 4ꢀ
dimethylaminopyridine (DMAP) to give an equilibrium between the three isomers and the nickel
hydride. The concentrations of the Ni complexes were determined by 31P{1H} NMR
spectroscopy, and the concentrations of the protonated and unprotonated DMAP were measured
by 1H NMR spectroscopy. The chemical shift of the DMAP resonances is a weighted average of
the values for the respective pure acid and base because of rapid exchange on the NMR time
scale. The equilibrium constants between the hydride and isomers A, B, and C, are KA,eq = 240
(± 20), KB,eq = 70 (± 10) and KC,eq = 31 (± 6), respectively. Using a pKa value of 17.95 for [Hꢀ
DMAP]BF4 in acetonitrile,67 the pKa values of isomers A, B, and C are 15.6, 16.1 and 16.5,
respectively. Weighting the pKa values of the isomers with their relative concentrations gives an
average pKa2 of 16.5. These measurements were also crossꢀchecked by performing similar
experiments with 2, 6ꢀlutidinium (pKa of 14.13 in acetonitrile)67 which gave similar pKa values
(see Experimental Section).
11
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 38
1
2
3
4
5
6
7
8
9
Thermodynamic Data for Hydrogen Addition to [Ni(PCy2Nt-Bu 2)2](BF4)2. The pKa1
value of [NiH(PCy2NtꢀBu2)2]+ and the redox potentials of [Ni(PCy2NtꢀBu 2)2]2+, together with the
known value in acetonitrile for the twoꢀelectron reduction of a proton to a hydride,69 were used
in the thermodynamic cycle shown in Scheme 3 to determine the hydride donor ability (ꢁG°Hꢀ)
of 61.3 kcal/mol for [NiH(PCy2NtꢀBu2)2]+.8 The value of ꢁG°Hꢀ for this complex is comparable to
that of [NiH(PCy2NBn2)2]+, which has cyclohexyl substituents on the phosphorous and benzyl
groups on the amine (ꢁG°Hꢀ = 60.7 kcal/mol), and is larger (indicating a weaker hydride donor)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 3. Determination of the Hydride Donor Ability of [NiH(PCy2NtꢀBu2)2]+
than analogous complexes with phenyls on the phosphorous (ꢁG°Hꢀ = 57.1–59.0 kcal/mol).8
Previous studies have shown that the steric size of the substituent on the phosphorous is an
important factor influencing hydride donor ability, and in some cases they can override
electronic effects.70,71
The hydride donor ability (ꢁG°Hꢀ) of 61.3 kcal/mol for [NiH(PCy2NtꢀBu2)2]+ and the pKa2
value of 16.5 (weighted average for the three isomers, as described above) for the doubly
protonated Ni0 complex can be used along with the free energy for the heterolytic cleavage of
Scheme 4. Determination of the Free Energy of H2 addition to [Ni(PCy2NtꢀBu2)2]2+
.
12
ACS Paragon Plus Environment

Page 13 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
hydrogen in acetonitrile69,72 to determine the free energy of −7.9 kcal/mol (Scheme 4) associated
with the heterolytic addition of hydrogen to [Ni(PCy2NtꢀBu2)2]2+. This is significantly more
negative than the previously studied hydrogen oxidation complex, [Ni(PCy2NBn2)2]2+, which has a
free energy of hydrogen addition of −3.1 kcal/mol.8,48
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Electrochemical Studies and Catalytic Oxidation of H2. The cyclic voltammogram of
[Ni(PCy2NtꢀBu2)2](BF4)2 in benzonitrile exhibits NiII/I and NiI/0 redox couples at −0.81 V and −1.45
V (ꢁEp = 82 mV and 60 mV at a scan rate of 50 mV/s), respectively, versus the Cp2Fe+/0 couple.
In acetonitrile, the NiII/I couple
Figure 3. Cyclic voltammogram of a 1.8 mM solution of [Ni(PCy2NtꢀBu2)2](BF4)2 in acetonitrile
under nitrogen (black), under 1 atm hydrogen (red), and 8 hours after the addition of 10
equivalents of water (blue). Conditions: scan rate = 50 mV/s, 0.2 M Bu4NPF6, glassy carbon
working electrode. Potentials are referenced to the Cp2Fe+/0 couple.
for [Ni(PCy2NtꢀBu2)2]2+ is observed at −0.77 V vs Cp2Fe+/0 and is shown as the black trace in
Figure 3. Purging this solution with hydrogen results in an immediate color change of this
solution from red to colorless and is associated with the formation of isomer A, which persists
13
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 14 of 38
1
2
3
4
5
6
7
8
9
for hours as the major product in the absence of water. A cyclic voltammogram of a solution of
[Ni(PCy2NtꢀBu 2)2]2+ purged with hydrogen is shown in Figure 3 as the red trace. The NiI/0 couple
is quasireversible, and occurs at E1/2(I/0) = −0.36 V (ꢁEp = 116 mV and icathodic/ianodic = 1.3 at a
scan rate of 50 mV/s). As discussed above, the addition of water to solutions of A promotes its
isomerization to endoꢀexo (B), and exoꢀexo (C), isomers. Ten equivalents of water with respect
to the Ni complex were added to a solution to facilitate isomerization of A to form B and C, and
an NMR spectrum acquired of the solution after 8 hours confirmed the presence of all three
isomers. The cyclic voltammogram of this solution displayed two NiI/0 quasireversible and one
irreversible oxidation couples assigned to the three isomers (see the blue trace in Figure 3). The
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
quasireversible oxidation couple with E1/2(I/0) = −0.48 V (est. ꢁEp = 55 mV and icathodic/ianodic
=
1.5 at a scan rate of 50 mV/s) is assigned to isomer B, and the irreversible couple with Ep(I/0) =
−0.75 V is assigned to isomer C. The oxidation potential of isomer A is 120 mV positive of the
oxidation potential of isomer B, and 530 mV positive of the potential for isomer C. Similar
separations of the redox potentials were found previously for [Ni(PtꢀBu2NBn2H)2]2+.73
Effect of Water on the Catalytic Rate of Hydrogen Oxidation. Recent results in our
R
2+
laboratory with related [Ni(PPh2N ) ] complexes demonstrate increases in the rate of
´
2 2
electrocatalytic proton reduction upon addition of water to acetonitrile solutions.9,12,15,74 These
rate accelerations range from 20%12 to over a factor of 60.15 To determine whether water might
have an effect on the opposite reaction, oxidation of hydrogen, we examined [NiI(PCy2Ntꢀ
Bu
2)2](BF4) for changes in catalytic activity upon addition of water. Water is significantly more
miscible with acetonitrile than benzonitrile, so the former was used in these studies. The E1/2
potentials of the NiII/I and NiI/0 couples in acetonitrile are observed at −0.77 V and −1.42 V,
respectively.
14
ACS Paragon Plus Environment

Page 15 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
The cyclic voltammogram of [NiI(PCy2NtꢀBu2)2] in acetonitrile under an atmosphere of
nitrogen (black trace) is shown in Figure 4. The cyclic voltammogram of the same solution under
one atmosphere of hydrogen is shown as the red trace in Figure 4. Upon oxidation to NiII, the
complex reacts with hydrogen to form the doubly protonated Ni0 complex, which displays an
oxidation wave at Ep = −0.36 V, as discussed above. On the return scan, the wave associated
with the (II/I) couple of [NiI(PCy2NtꢀBu2)2]+ is absent. Addition of NEt3 results in an increase in
the anodic current observed at −0.77 V. The plateau shape of this wave and the magnitude of the
current enhancement indicate that this is a catalytic wave corresponding to the oxidation of H2.
No further increase in catalytic current was observed above a concentration of 17 mM NEt3
(green trace). Recent experimental measurements of open circuit potentials at a platinum
electrode provide an accurate evaluation of the thermodynamic potentials for oxidation and
production of hydrogen under different conditions.75 Based on these measurements, the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Cyclic voltammogram of a 1.0 mM solution of [NiI(PCy2NtꢀBu2)2](BF4) in acetonitrile
under nitrogen (black), under hydrogen (red), with 17 mM NEt3 (green), 17 mM NEt3 and 300
mM H2O (light blue), and 83 mM NEt3 and 860 mM H2O (dark blue). Conditions: scan rate =
50 mV/s, 0.2 M Bu4NPF6, glassy carbon working electrode, 23 °C. Potentials are referenced to
the Cp2Fe+/0 couple.
15
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 16 of 38
1
2
3
4
5
6
7
8
9
reversible potential for H2 oxidation (1.0 atm) in the presence of 17 mM NEt3 in acetonitrile is
−1.31 V, indicating that the overpotential (as measured at the halfꢀheight of the catalytic wave)
under these conditions is about 0.54 V. However, we did not attempt to minimize the
overpotential by studying bases of varying strength.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Subsequent cyclic voltammograms recorded with increasing concentrations of water (44
mM to 860 mM) displayed an increase in the catalytic current at −0.77 V. Successive increases
in the concentrations of NEt3 and water led to an increase in current until the concentration was
83 mM NEt3 and 860 mM H2O (see Supporting Information). Higher concentrations of water
and NEt3 did not result in any further increase in current, indicating that the catalyst is operating
under substrate concentration independent conditions (zeroꢀorder in base). Changes in catalyst
concentration from 0.3ꢀ0.8 mM do not result in changes in the measured icat/ip, indicating the
catalyst is not operating under hydrogenꢀlimited conditions. The oxidative current (icat) for the
NiII/I couple under 1.0 atmosphere of hydrogen, 83 mM NEt3, and 860 mM H2O is 22 times
larger than the current (ip) observed for the NiII/I couple in the absence of hydrogen, base, and
ꢀ
ꢆ
ꢋꢌꢍꢎ
ꢁꢂꢃ
ꢏꢐꢑ
ꢅ ꢇ.ꢈꢈꢉꢊ
(2)
ꢀ
ꢒꢓ
ꢄ
water. The ratio (icat/ip) of the catalytic current under substrate (base) independent conditions
divided by the peak current under nonꢀcatalytic conditions can be used to calculate (equation 2) a
turnover frequency of 45 sꢀ1 under 1.0 atm H2. This value is close to the value of 50 sꢀ1 reported
previously using triethylamine as the base in benzonitrile, although the prior study was not
performed under rigorously dry conditions.50 In equation 2, R is the gas constant, T is the
temperature in K, F is the Faraday constant,
υ is the scan rate in V/s, and kobs is the observed rate
constant, or turnover frequency.6,13,76ꢀ78
In addition to the increase in current observed for the wave at ꢀ0.77 V, it can also be seen
that there is enhanced current at ꢀ0.36 V in the presence of NEt3 with no added water (green trace
in Fig. 4), indicating a second catalytic pathway for H2 oxidation that involves Ni0 isomer A,
16
ACS Paragon Plus Environment

Page 17 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
which oxidizes at a much more positive potential. This pathway is disfavored upon addition of
water.
8
9
Effect of Base Size on Catalytic Rate of Hydrogen Oxidation. We also examined the
effect of base size on the catalytic rate by using tꢀbutylamine and nꢀbutylamine. Their conjugate
acids have nearly identical pKa values of 18.1 and 18.3, respectively in acetonitrile; for
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+
comparison the pKa value of HꢀNEt3 is 18.8 (see Table 2).67 This allows for a comparison of the
effect of base size, because the thermodynamic driving force for catalysis is nearly the same with
all of these amine bases. A plot of icat/ip as a function of the concentration of nꢀbutylamine is
shown in Fig. 6. The value of icat/ip increases with [nꢀBuNH2] at low concentrations, and
becomes independent of base concentration at higher concentrations. Without added water, the
Figure 5. Cyclic voltammogram of a 0.62 mM solution of [Ni(PCy2NtꢀBu2)2](BF4)2 in
acetonitrile under nitrogen (black), under hydrogen (red), varying amounts of nꢀBuNH2 (green
and light blue traces), and 101 mM nꢀBuNH2 and 128 mM H2O (dark blue). Conditions: scan
rate = 50 mV/s, 0.2 M Bu4NPF6, glassy carbon working electrode, 23 °C. Potentials are
referenced to the Cp2Fe+/0 couple.
17
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 18 of 38
1
2
3
4
5
6
7
8
9
maximum turnover frequency obtained using nꢀBuNH2 as the base under 1.0 atmosphere of
hydrogen is 50 sꢀ1, which is significantly larger than that observed for tꢀBuNH2 or NEt3. All of
the rates increase upon addition of water, but the rate increase is most pronounced for the
sterically hindered bases, tꢀBuNH2 and NEt3, which increase by more than an order of
magnitude. As shown in Table 2, the fastest rate observed for the catalyst is in the presence of
both nꢀBuNH2 and water, 58 sꢀ1, but this rate is only slightly (20%) faster than that observed in
the absence of water.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2. Turnover frequencies (TOF, sꢀ1) at 23 °C for electrocatalytic oxidation of hydrogen (1
atm) by [Ni(PCy2Nt-Bu2)2]2+ using three amines in dry acetonitrile, and in acetonitrile with added
water.
a
Base
pKa
TOF (sꢀ1), dry
TOF (sꢀ1), with H2O (mM H2O)
nꢀBuNH2
t-BuNH2
NEt3
18.3
18.1
18.8
50
2
58 (128)
43 (360)
45 (860)
3
apKa values (MeCN solvent) are for those of the corresponding conjugate acids of the bases
Figure 6. Plot of icat/ip vs. [nꢀBuNH2] for oxidation of H2 by an 0.62 mM solution of [Ni(PCy2Ntꢀ
Bu
18
2)2](BF4)2 in acetonitrile. Conditions: scan rate = 50 mV/s, 0.2 M Bu4NPF6 as supporting
electrolyte, glassy carbon working electrode. Red diamonds are for experiments with 219 mM H2O.
ACS Paragon Plus Environment

Page 19 of 38
Journal of the American Chemical Society
1
2
3
4
5
Reactivity of [Ni(PCy2Nt-Bu2)2]2+ with Carbon Monoxide.
6
7
8
9
It is important to study the reaction of our catalysts with CO because the hydrogen used
in fuel cells is typically generated by steam reforming of methane, and is contaminated with CO,
which poisons platinum catalysts even at parts per million levels. The complex [Ni(PCy2Ntꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Bu
2)2]2+ in acetonitrile reacts rapidly with CO, turning from red to orange. The 31P{1H} NMR
spectrum displays resonances at 43 and −15 ppm (Figure 6). This reaction proceeds to
completion under 1 atmosphere of CO, with no starting material or other products observed in
the 31P{1H} NMR spectrum. The previously studied hydrogen oxidation catalyst
[Ni(PCy2NBn2)2]2+ displayed an intermediate with similar 31P NMR resonances (57 and −4 ppm)
upon reaction with CO, and was characterized by a single crystal Xꢀray diffraction study.79 The
Figure 7. 31P{1H} NMR spectrum of [Ni(PCy2Nt-Bu2)2]2+ in acetonitrile under one atmosphere
of nitrogen (black), under one atmosphere of CO (blue), and under one atmosphere of
hydrogen (red). The proposed structure of the CO complex is shown in the figure, with the
cyclohexyls on the phosphines excluded for clarity.
19
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 20 of 38
1
2
3
4
5
6
7
8
9
proposed reaction product of [Ni(PCy2NtꢀBu 2)2]2+ with CO (shown in Figure 7) is expected to be
structurally similar. We were unable to obtain an Xꢀray crystal structure of the CO adduct.
Although large single crystals of [Ni(CO)(PCy2NtꢀBu 2)2]2+ can be grown from diffusion of Et2O
into acetonitrile, the crystals were sensitive to visible light, likely resulting from CO loss.
Although [Ni(PCy2NtꢀBu 2)2]2+ binds CO, H2 binding is favored. When a solution of
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
[Ni(CO)(PCy2NtꢀBu 2)2]2+ is purged with hydrogen, the hydrogen addition product is the only
species seen in the 31P NMR spectrum, with no evidence of decomposition. Exposure of
[Ni(PCy2NtꢀBu 2)2]2+ to a 1:1 mix of H2 and CO for 10 minutes gives the same result; only the
hydrogen addition product is observed, indicating a strong preference for H2 binding over CO.
Discussion
2+
Previous studies have shown that [Ni(PR NR 2)2] complexes are very active catalysts for
´
2
H2 production and oxidation.6ꢀ13,15,16,48,50,74 Systematic variations of the R substituents on
phosphorus and the R´ groups on N have been used to control the hydride donor/acceptor ability
of the Ni center and the basicity of the pendant base, respectively, for hydrogen production
catalysts.12,15,74 As shown in Scheme 4, increasing the hydride donor ability (decreasing ꢁG°Hꢀ)
and decreasing the pKa value (increasing the acidity) of the protonated pendant amine will favor
H2 elimination. These studies demonstrated that an increase in the driving force for H2
R
2
R
2+
´
elimination from [Ni(P N H)2] complexes correlates with an increase in the catalytic rate for
H2 production.79
We have previously reported that the complex [Ni(PCy2NBn2)2]2+ (Bn = benzyl) is a
catalyst for hydrogen oxidation, with a turnover frequency of 10 sꢀ1 under one atmosphere of
hydrogen.6 In an effort to increase the catalytic rate of H2 oxidation for this class of catalyst, we
sought to extend the approach used for H2 production catalysts to H2 oxidation catalysts. This
led us to prepare NiII complexes containing the PtꢀBu2NBn2 ligand. It was thought that the bulky tꢀ
Bu groups of this ligand would lead to [Ni(PtꢀBu2NBn2)2]2+ complexes with large tetrahedral
20
ACS Paragon Plus Environment

Page 21 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
distortions from a nominally square planar geometry. Such tetrahedral distortions are correlated
with a strong hydride acceptor ability for these complexes. However, the steric strain introduced
by the interligand interactions between the tꢀBu groups resulted in the formation of [Ni(Ptꢀ
2NBn2)(CH3CN)2]2+ containing only one diphosphine ligand, and this complex is not an active
Bu
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
catalyst for either H2 oxidation or production.18
An alternative approach to preparing faster H2 oxidation catalysts was to increase the
basicity of the pendant amine. To achieve this goal we prepared the complex [Ni(PCy2NtꢀBu2)2]2+
as communicated previously,50 and showed that it is a very fast catalyst for hydrogen oxidation
under one atmosphere of hydrogen. In this paper, we provide a quantitative evaluation of the
difference in the driving forces for H2 addition to [Ni(PCy2NBn2)2]2+ and [Ni(PCy2NtꢀBu2)2]2+ (Table
3), report the first Xꢀray diffraction studies of an H2 addition product of this class of H2 oxidation
catalyst, discuss the energetics of addition of hydrogen to the NiII complex, and explore the role
of the pendant base and water in transferring a proton from the metal center to a base in solution.
Table 3. Experimentally Measured Thermodynamic Values and Catalytic Turnover Frequencies
for [Ni(PCy2NR ) ]2+.
2 2
a
b
a
H2
R
ΔGH-
Benzyl8 −60.7
t-Bu −61.3
pKa (NR)
ΔG0
TOF (s-1)c
13.4
16.5
−3.1
−7.9
10
45
ain kcal/mol, bin acetonitrile, averaged between isomers, cusing Et3N as a base, under wet
conditions
21
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 22 of 38
1
2
3
4
5
6
7
8
9
We previously reported the synthesis and structural characterization of the NiII complex,
[Ni(PCy2NtꢀBu2)2]2+, which exhibited a tetrahedrally distorted square planar geometry.50 In this
work, we report the synthesis of the corresponding Ni0 and NiI complexes. Thus the complete
series of complexes of the oxidation states NiII, NiI, and Ni0, has now been isolated and
characterized. The structural studies of [Ni(PCy2NtꢀBu2)2]2+ and Ni(PCy2NtꢀBu2)2 together with
previous studies of [Ni(P tꢀBu2NBn2)2]+ and Ni(P tꢀBu2NBn2)2 indicate that there are two major
structural changes that occur upon reduction of these complexes, an increase in the dihedral
angle between the two diphosphine ligands by approximately 60° upon reduction from NiII to
NiI, and a contraction of the NiꢀP bond distance by nearly 0.1 Å upon reduction from NiI to Ni0.
The reaction of [Ni(PCy2NtꢀBu2)2]2+ with H2 results in the formation of the doubly
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
protonated Ni0 product that exists in the form of three isomers, A, B, and C, as shown in
Scheme 1. Similar isomers have been reported previously in the reaction of [Ni(PCy2NBn2)2]2+
with H2, and their structures were deduced by extensive 1H, 31P, and 15N NMR studies.7,64
However, we had previously been unable to grow suitable crystals for Xꢀray diffraction studies
to provide additional structural information. The Xꢀray diffraction study of isomer B, (Figure 2),
is of special interest because it exhibits both endo protonation and exo protonation of the ligands.
The distance between the two N atoms of the exo protonated or “pinched” ligand is 2.69 Å,
consistent with a N•••HꢀN hydrogen bond. DFT calculations performed on the analogous
complex with the PCy2NBn2 ligand gave a calculated NꢀN distance of 2.61 Å.64 These calculations
also suggest that the bridging proton is located in an asymmetric position, being closer to one N
than the other. However, the triplet pattern observed for the proton resonance for the 15N labeled
[Ni(PCy2NBn2H)2]2+ complex is consistent with a dynamic process in which this proton moves
back and forth between the two N atoms of the pinched ligand. The NiꢀN3 distance for the endo
protonated ligand of isomer B in Figure 2 is 3.31 Å, compared to the NiꢀN2 and NiN3 distances
of 3.83 and 3.48 Å for the N atoms of the rings in the boat conformer of the Ni(0) complex,
Ni(PCy2NtꢀBu 2)2. This is consistent with shortening of the NiꢀN distance due to the presence of a
22
ACS Paragon Plus Environment

Page 23 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Ni0•••HꢀN hydrogen bond. NMR studies on intramolecular proton exchange for isomer B of the
complex [Ni(PCy2NBn 2)2]2+ indicates facile proton movement from an endo position on one N
atom of the endo P2N2 ligand to the other N atom, via a NiH intermediate.64 This is also observed
in the singly protonated complex [Ni(dppp)(PPh2NBn2H)]+, where the endoꢀproton undergoes
rapid exchange (104 sꢀ1 at 10° C) between the two amines through a NiH intermediate.48
The NiII complex [Ni(PCy2NtꢀBu2)2]2+ reacts with hydrogen to produce a doubly protonated
Ni0 complex. The kinetic product of addition of H2 to [Ni(PCy2NtꢀBu 2)2]2+ in the absence of water
is isomer A, which has both protons endo with respect to the metal. This isomer rearranges
through intermolecular proton transfers over several hours to form isomers B and C, which is
similar to the behavior observed for the related complex [Ni(PCy2NBn2)2]2+ upon reaction with
hydrogen.64,80 Addition of water facilitates this intermolecular exchange, resulting in a more
rapid approach to equilibrium, which is achieved in approximately five hours. The observed
equilibrium indicates that these species are nearly of equal energy (within 1.3 kcal/mol of each
other at room temperature). The driving force for H2 addition to complex [Ni(PCy2NtꢀBu 2)2]2+ is
determined by the hydride and proton acceptor abilities of this complex as shown in Scheme 4.
The hydride acceptor ability (−ꢁG°Hꢀ) was experimentally determined from the pKa value of
[HNi(PCy2NtꢀBu2)2]+ and the NiII/I and NiI/0 potentials of [Ni(PCy2NtꢀBu 2)2]2+ to be ꢀ61.3 kcal/mol,
very close to the ꢀ60.7 kcal/mol measured for [Ni(PCy2NBn2)2]2+.8 However, replacing the benzyl
substituent on the pendant amine with a tꢀbutyl group increases the pKa from 13.4 to 16.5. This
corresponds to a more negative free energy (ꢁG°H2) for addition of H2 to [Ni(PCy2NtꢀBu2)2](BF4)2,
from −3.1 kcal/mol to −7.9 kcal/mol. Thus replacing the benzyl substituent on the pendant amine
with a tꢀBu substituent increases the driving force for H2 addition by 4.8 kcal/mol, more than the
1.8 kcal/mol difference expected on the basis of the pKa values of protonated benzylamine (16.8)
and protonated tꢀbutylamine (18.1) in acetonitrile. This correlates with an increase in catalytic
rate, as shown in Table 3.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
23
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 24 of 38
1
2
3
4
5
The cyclic voltammograms of the three nearly isoenergetic isomers A, B, and C show
6
7
different oxidation potentials, as shown in Figure 3. Each of these isomers has two hydrogen
8
9
bonds. For isomer C there are two
hydrogen bond and one Ni•••HꢀN hydrogen bond, and for isomer A there are two Ni•••HꢀN
hydrogen bonds. Theoretical calculations on [Ni(PCy2HNBn)2]2+ indicate that the
•••HꢀN
N•••HꢀN hydrogen bonds, for isomer B there is one N•••HꢀN
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N
hydrogen bonds and the Ni•••HꢀN hydrogen bonds are of roughly equal energy.73 Isomer C is the
easiest to oxidize (most negative redox potential), followed by isomers B and A. The dramatic
differences in oxidation potentials for these three conformers is due to the presence of hydrogen
bonds between the Ni and the endo proton of the pendant amine. The Ni•••HꢀN hydrogen bond
stabilizes the HOMO of the Ni0, which results in a higher oxidation potential for the metal. For
isomer A, two Ni•••HꢀN hydrogen bonds are broken upon oxidation of Ni0 to NiI. For isomer B,
one Ni•••HꢀN bond is broken upon oxidation to Ni(I), and for isomer C, no Ni•••HꢀN hydrogen
bonds are broken upon oxidation. As a result the relative stability of the oxidized isomers is
expected to be C+ (with two
N N
•••HꢀN bonds) > B+ (with one •••HꢀN bond) > A+ (with no
N•••HꢀN bonds). Other examples of hydrogen bonds with transition metals have been reported,
with bond enthalpies estimated to be between 3ꢀ7 kcal/mol.81ꢀ85 Similar trends in electrochemical
data have also been observed experimentally in the analogous complex [Ni(PtꢀBu2NBn2H)2]2+.73
The magnitude of the oxidation potential differences in that complex was consistent with DFT
calculations of the free energies of each of the species.73 The difference in the oxidation potential
between isomer B and C is 410 mV, which corresponds to an energy of 9.4 kcal/mol for the
Ni•••HꢀN bond in isomer B. The oxidation potential for isomer A is 530 mV more positive than
that for isomer C, suggesting that the two Ni•••HꢀN hydrogen bonds in isomer A are worth 12.2
kcal/mol. The difference in oxidation potentials resulting from the Ni•••HꢀN hydrogen bonds
may have implications for the operating potential (and overpotentials) of these catalysts.
24
ACS Paragon Plus Environment

Page 25 of 38
Journal of the American Chemical Society
1
2
3
4
5
Scheme 5. Proposed mechanism for electrocatalytic oxidation of H2, showing that deprotonation
6
7
followed by oxidation occurs at a more negative potential (lower overpotential) than oxidation
8
9
followed by proton transfer. R groups on P and R´ groups on N not shown.
2+
H
N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N
P
H2
N
P
P
Ni
N
N
2+
P
P
P
-e-, -0.36 V
N H
N
Ni
A
N
P
P
Base
+
H
N
3+
H
Ba
se
N
+
N
P
-e-
H
N
P
P
Ni
N
P
P
P
N
P
P
Ni
N H
+
N
N
-e-, -0.75 V
Base
N
P
P
N
P
P
Ni
+
H
Ba
se
2+
H
N
N
N
P
P
N
P
P
+
Ni
H
Ba
se
Base
N
During the catalytic cycle (Scheme 5) the kinetic product of H2 addition to [Ni(PCy2NtꢀBu 2)2]2+ is
isomer A. Oxidation of isomer A at −0.36 V will lead to cleavage or weakening of the Ni•••HꢀN
bond, facilitating the boat to chair conformational change that may be required for the
intermolecular transfer of a proton to the bulky NEt3 base in solution. In addition, the proton on
the pendant amine will be much more acidic after oxidation of Ni0 to NiI. Because the proton on
the pendant base is more sterically accessible and more acidic, NEt3 can readily deprotonate this
doubly protonated NiI species in two sequential steps to form the unprotonated NiI species,
25
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 26 of 38
1
2
3
4
5
6
7
8
9
which is rapidly oxidized to regenerate [Ni(PCy2NtꢀBu 2)2]2+. Thus the catalytic wave observed in
the presence of NEt3 at ꢀ0.36 V (Figure 4) results from oxidation of the doubly protonated Ni0
species, A, followed by proton transfer.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
An alternate catalytic pathway observed in the presence of NEt3 involves the direct
deprotonation of isomer A. This deprotonation reaction will lead to the formation of the nickel
hydride complex, as shown in Scheme 5. Subsequent oxidation of [HNi(PCy2NtꢀBu2)2]+ would
likely occur at potentials positive of the NiII/I couple, but rapid proton transfer from this oxidized
hydride species will shift this potential in a negative direction such that it can approach the
potential of the Ni(II/I) couple, as observed, leading to the catalytic wave observed at
approximately ꢀ0.75 V in Figure 4. Thus in the presence of NEt3 in “dry” solvent, two
competing pathways for catalysis are observed, and they occur at different potentials. For the
catalytic wave observed at −0.75 V, deprotonation of A is followed by oxidation (Scheme 5). In
the catalytic wave observed at −0.36 V, oxidation of A occurs before deprotonation. The 0.38 V
more positive oxidation potential for the wave at ꢀ0.36 V results in a corresponding increase of
that magnitude in the overpotential for hydrogen oxidation. These results clearly indicate the
importance of the sequence and pathway for proton movement in determining overpotentials,
An increase in catalytic current is observed at ꢀ0.75 V upon addition of water, as shown
in Figure 4. Water facilitates the deprotonation of the hydrogen addition product by making the
endo proton more accessible. The use of sterically hindered amines (NEt3 and t-BuNH2) as the
base results in slow catalysis in the absence of water, which increases by an order of magnitude
upon addition of water. However, the use of the smaller base nꢀbutylamine gives fast catalysis in
the absence of water, and only a slight increase with addition of water (see Figure 5). Therefore,
it appears that in the absence of water, deprotonation and subsequent catalysis at the lower
overpotential is hindered by steric interference and this effect is overcome by the presence of
water.
26
ACS Paragon Plus Environment

Page 27 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
We propose two possible avenues for the acceleration by water (Scheme 6). One
possibility is that water forms a hydrogen bond with the endo proton, weakening the Ni•••HꢀN
hydrogen bond (Scheme 6a). This would stabilize a boatꢀtoꢀchair conformational change of the
sixꢀmembered ring containing the endo proton, creating easier access for deprotonation by
Scheme 6. Two possible mechanisms for acceleration by water.
(a)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(b)
the base. A similar boatꢀtoꢀchair conformational change is required for the intermolecular proton
transfer required to isomerize A to B or C, and our NMR spectroscopic and theoretical studies
have established that water facilitates that isomerization.80 Alternatively, water may form a
hydrogen bond network that allows deprotonation of a distal, more accessible proton from water,
which deprotonates the NꢀH, thus relieving the steric requirements for deprotonation (Scheme
6b). At this time, we are unable to distinguish whether one effect is dominant, or if it is a
combination of both, but we are continuing experimental and theoretical studies to determine the
contributions for the influence of water on the catalytic rate.
27
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 28 of 38
1
2
3
4
5
6
7
8
9
In the presence of excess NEt3 and water, or in the presence of excess nꢀBuNH2, the
catalytic rate is independent of the concentration of base, and the observed catalytic potential is
near that of the NiII/I couple of [Ni(PCy2NtꢀBu2)2]2+. These data suggest that under these
conditions, the catalytic rate for H2 oxidation does not depend on the rate of intermolecular
proton transfer, suggesting that H2 addition may be the rateꢀlimiting step. In this case an increase
in the driving force for H2 addition is expected to result in faster catalysis. Indeed, the TOF for
[Ni(PCy2NtꢀBu 2)2]2+ (with a driving force for H2 addition of ꢀ7.9 kcal/mol) under 1.0 atm of H2 of
45 sꢀ1 is significantly faster than the TOF of 10 sꢀ1 reported previously for [Ni(PCy2NBn2)2]2+ (with
a driving force of ꢀ3.1 kcal/mol).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Conclusions
[Ni(PCy2NtꢀBu2)2]2+ is the fastest reported molecular catalyst reported for hydrogen
oxidation, an essential reaction for the conversion of chemical energy to electrical energy in
hydrogen fuel cells. [Ni(PCy2NtꢀBu2)2]2+ binds hydrogen preferentially over CO, a common
impurity in hydrogen produced from steam reforming of methane which poisons platinum, the
catalyst used in most lowꢀtemperature hydrogen fuel cells. The fast rate of the Ni catalyst
reported here is attributed to the high driving force for heterolytic cleavage of hydrogen. One of
the products of hydrogen addition has been characterized by Xꢀray crystallography, providing
confirmation of the structure proposed on the basis of NMR studies.
The rate of hydrogen oxidation catalysis is accelerated by the addition of water. Our
NMR and electrochemical studies demonstrated that water is important for fast intermolecular
proton transfer. The kinetic product of hydrogen addition has the protons on the pendant amines
in an endo position facing the metal. In this position, the protons are stabilized by a Ni•••HꢀN
hydrogen bond that decreases the electron density of the metal, resulting in a more positive
oxidation potential. Cleavage of the Ni•••HꢀN hydrogen bond and removal of the proton from the
second coordination sphere is one observed pathway for catalysis. This pathway is facilitated by
28
ACS Paragon Plus Environment

Page 29 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
water or small bases such as nꢀbutylamine. An alternative catalytic pathway involves the
oxidation of the doubly protonated Ni0 complex prior to deprotonation. This pathway requires a
more positive oxidation potential, which results in an increase in the overpotential. With large
external bases, water is critical for fast proton removal and therefore fast catalytic rates as well as
operating at a more modest potential. These results provide valuable insight into the different
catalytic cycles possible for hydrogen oxidation and the importance of the sequence of proton
and electron transfer steps for fast catalysis and low overpotentials.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Experimental Section
1
31
General Experimental Procedures. H, and P{1H} NMR spectra were recorded on a
1
1
Varian Inova spectrometer (500 MHz for H) at 20 °C unless otherwise noted. All H chemical
shifts have been internally calibrated to the monoprotio impurity of the deuterated solvent. The
31P NMR spectra were proton decoupled and referenced to external phosphoric acid.
All electrochemical experiments were carried out under an atmosphere of nitrogen, or
hydrogen when indicated, in 0.2 M Bu4NBF4 acetonitrile solutions. Cyclic voltammetry
experiments were performed with a CH Instruments model 660C potentiostat. Ferrocene was
used as an internal standard, and all potentials are referenced to the Cp2Fe+/0 couple.86,87
Synthesis and Materials. All reactions and manipulations were performed under an N2
atmosphere using standard Schlenk techniques or a glovebox unless otherwise indicated.
Solvents were dried using an activated alumina column. Triethylamine was dried over CaH2 and
vacuum transferred, while tertꢀbutylamine and nꢀbutylamine were dried over potassium
hydroxide. Bu4NBF4 was dried at 100° C under vacuum for 12 hours. PCy2NtꢀBu and [Ni(PCy2Ntꢀ
Bu
2)2](BF4)2 was prepared using a literature method.50 Ni(COD)2 was purchased from Strem
ꢀ
Chemicals. The acid salt [HꢀDMAP]+BF4 was synthesized by reaction of the corresponding base
with a stoichiometric amount of HBF4 in diethyl ether and isolated by filtration.88 [Hꢀ
ꢀ
MTBD]+BPh4−, [HꢀDBU]+BPh4 were synthesized by adding a stoichiometric amount of aqueous
29
ACS Paragon Plus Environment