Catalytic Proton Coupled Electron Transfer from Metal Hydrides to Titanocene Amides, Hydrazides and Imides: Determination of Thermodynamic Parameters Relevant to Nitrogen Fixation

Article abstract of DOI:10.1021/jacs.6b08009

The hydrogenolysis of titanium-nitrogen bonds in a series of bis(cyclopentadienyl) titanium amides, hydrazides and imides by proton coupled electron transfer (PCET) is described. Twelve different N-H bond dissociation free energies (BDFEs) among the various nitrogen-containing ligands were measured or calculated, and effects of metal oxidation state and N-ligand substituent were determined. Two metal hydride complexes, (η⁵-C₅Me₅)(py-Ph)Rh-H (py-Ph = 2-pyridylphenyl, [Rh]-H) and (η⁵-C₅R₅)(CO)₃Cr-H ([Cr]^R-H, R= H, Me) were evaluated for formal H atom transfer reactivity and were selected due to their relatively weak M-H bond strengths yet ability to activate and cleave molecular hydrogen. Despite comparable M-H BDFEs, disparate reactivity between the two compounds was observed and was traced to the vastly different acidities of the M-H bonds and overall redox potentials of the molecules. With [Rh]-H, catalytic syntheses of ammonia, silylamine and N,N-dimethylhydrazine have been accomplished from the corresponding titanium(IV) complex using H₂ as the stoichiometric H atom source. The data presented in this study provides the thermochemical foundation for the synthesis of NH₃ by proton coupled electron transfer at a well-defined transition metal center.

Full text of DOI:10.1021/jacs.6b08009

Subscriber access provided by CORNELL UNIVERSITY LIBRARY
Article
Catalytic Proton Coupled Electron Transfer from Metal Hydrides
to Titanocene Amides, Hydrazides and Imides: Determination
of Thermodynamic Parameters Relevant to Nitrogen Fixation.
Iraklis Pappas, and Paul J. Chirik
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b08009 • Publication Date (Web): 09 Sep 2016
Downloaded from http://pubs.acs.org on September 13, 2016
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 12
Journal of the American Chemical Society
Catalytic Proton Coupled Electron Transfer from Metal Hydrides to Titanocene
Amides, Hydrazides and Imides: Determination of Thermodynamic Parameters
Relevant to Nitrogen Fixation.
1
2
3
4
5
6
7
8
9
10
Iraklis Pappas and Paul J. Chirik*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544.
pchirik@princeton.edu
RECEIVED DATE (automatically inserted by publisher)
bis(phosphine) tungsten compound in the presence of strong
acid.6 Shilov also reported catalytic ammonia and hydrazine
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
ABSTRACT: The hydrogenolysis of titanium-nitrogen
bonds in a series of bis(cyclopentadienyl) titanium amides,
hydrazides and imides by proton coupled electron transfer
(PCET) is described. Twelve different N-H bond
dissociation free energies (BDFEs) among the various
nitrogen-containing ligands were measured or calculated
and effects of metal oxidation state and N-ligand
substituent were determined. Two metal hydride
synthesis at 150 atm of N2 with up to 10,000 turnovers, using
Ti(OH)3,phosphine/phospholipid mixtures in the presence of
7
catalytic Mo(III).
More recently, Schrock and Nishibayashi demonstrated that
molybdenum coordination compounds, upon treatment with
sources of protons and electrons, are active catalysts for the
reduction of N2 to ammonia.8 In the latter examples, ligand
modifications have produced turnover numbers in excess of
50. Peters and coworkers have applied a similar approach,
using [H(OEt2)2][BArF4] and KC8 for iron9 and cobalt10
catalyzed N2 fixation and most recently Nishibayashi has used
the same proton and electron sources with pincer iron
complexes, (η5-C5Me5)(py-Ph)Rh-H (py-Ph
=
2-
pyridylphenyl, [Rh]-H) and (η5-C5R5)(CO)3CrH ([Cr]R-H,
R= H, Me) were evaluated for formal H-atom transfer
reactivity and were selected due to their relatively weak M-
H bond strengths yet ability to activate and cleave
molecular hydrogen. Despite comparable M-H BDFEs,
disparate reactivity between the two compounds was
observed and was traced to the vastly different acidities of
the M-H bonds and overall redox potentials of the
molecules. With [Rh-H], catalytic synthesis of ammonia,
silylamine and N,N-dimethylhydrazine have been
accomplished from the corresponding titanium(IV)
complex using H2 as the stoichiometric H-atom source. The
data provided in this study provides the thermochemical
foundation for the synthesis of NH3 by proton coupled
electron transfer at a well-defined transition metal center.
11
compounds for up to four turnovers of ammonia.
It is instructive to evaluate the energetic efficiency these
reactions compared to the Haber-Bosch reaction, nitrogenase
enzymes or other hypothetical N2 fixation schemes using H2 as
the source of protons and electrons. The Bordwell equation
allows computation of the effective bond dissociation free
energy of the reagent pair, e.g. the acid and electron source, as
a function of pKa and redox potential (eq 1):
BDFE = 1.37(pKa) + 23.06Ered + C (1)
As shown in Figure 1, each reported acid-reductant
combination used in catalytic ammonia synthesis generates a
relatively weak effective BDFE and hence high chemical over
potential for each of these reagents. Likewise, nitrogenase
enzymes, by virtue of the stoichiometry of the catalytic
reaction requiring eight protons and electrons coupled with
consumption of 16 equivalents of ATP operates with an
Introduction
The synthesis of NH3 from its elements, N2 and H2, is a
long-standing challenge of both fundamental and practical
importance.1 In 2015, global ammonia production was in
excess of 146 million metric tons - approximately 80% of
which was used as crop fertilizer.2 Nitrogen fixation schemes
that are compatible with renewable rather than steam reformed
and hence fossil fuel derived hydrogen3 as well as operate in
bath conditions are key components of carbon neutral
ammonia synthesis.
12
estimated overpotential of 117 kcal/mol. While current
molecular catalysts are triumphs of chemical synthesis,
coordination chemistry, ligand design and illustrate the
sophistication within reach of homogenous catalysis, they also
highlight a fundamental challenge with using molecular
catalysts for ammonia synthesis – strong acids and reductants
are not necessarily good mimics of H2. Nevertheless,
successful molecular catalysts with molybdenum, iron and
cobalt have been described for ammonia synthesis using
combinations of strong acids and reductants as the source of
N-H bonds.13 Equation 1 can be applied to specific acids and
reductants and allows computation of an effective bond
dissociation free energy (BDFE) for each combination in a
given solvent.14
Molecular catalysts for the fixation of N2 to NH3 are
attractive not only for batch ammonia synthesis but also for
developing the fundamental coordination chemistry and ligand
design concepts for N-H bond formation. The “Chatt cycle” –
a hypothetical and pedagogical scheme for the stepwise
conversion of coordinated dinitrogen to ammonia4 has
provided the blueprint for subsequent catalyst development.
Early demonstrations by Chatt and Hidai with phosphine-
ligand molybdenum(0) dinitrogen compounds treated with
mineral acids were valuable in illustrating the potential of this
concept but were limited to stoichiometric NH3 generation
because the metal served as the sole source of the reducing
equivalents.5 Picket later reported use of a mercury pool
cathode as an external reductant and demonstrated ammonia
synthesis by electrolysis (-2.6
V
vs. Fc/Fc+) of
a
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 12
1
2
3
4
5
6
7
8
9
operate by PCET or HAT22 as pyridinyl radicals are known to
have relatively weak N-H bonds (BDFE ≈ 35 kcal/mol).
Molecular Pre-Catalysts
iPr
23
iPr
Our laboratory has recently reported the application of
PCET to catalytic ammonia synthesis via hydrogenation by
treatment of a titanocene(IV) amide complex with (η5-
iPr
N
N
N
N
N
iPr
iPr
N
HIPT
P
Mo
P
N
N
P
PR2
PR2
N
R2P
Fe
B
HIPT
iPr
N
R
R
N
Mo
N N
Mo
N
N
P
N
C5Me5)(py-Ph)RhH ([Rh]-H; py-Ph = 2-pyridylphenyl),24
a
N
N
N
N
P = tBu2
P
R = iPr
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
compound first described by Norton and coworkers (Scheme
1).25 During the course of these investigations, rare
experimental measures of N-H BDFEs were obtained for the
addition of a hydrogen atom to parent amido (M-NH2) ligands
Acid:
Reductant:
[LutH][BArF24
Cp*2Cr
]
[LutH][OTf]
Cp2Cr
[H(OEt2)2][BArF24]
KC8
enzyme
N2
+
8H+
+
8e-
2NH3
+
H2
+
16Pi
Biological N2 Fixation:
16 ATP
16 ADP
in
a
family of group
4
metallocene complexes.26
Hydrogenation of the Ti-NH2 bond does not occur in the
absence of rhodium hydride catalyst, highlighting both the
high kinetic barrier of the background reaction and the utility
of the PCET-HAT approach.
Key to expanding these initial observations into a general
method for ammonia synthesis and breaking otherwise strong
BDFE (kcal/mol) = 1.37(pKa)+ 23.06(Ered)+ C
ΔΔ
G
(NH3)
BDFE
f
Acid
Reductant
(kcal/mol) (kcal/mol)e
44.2a
41.2b
6.5c
0d
Cp2Co
Cp*2Cr
[LutH][OTf]
[LutH][BArF24
HOTs
26.4
]
44.4
e- (-2.6 V)
KC8
252.6
291.6
[H][BArF24
]
ΔG°(H ) = 48.6 kcal/mol
f
bonds to transition metals is obtaining
a
thorough
Figure 1. Top: Molecular catalysts for reduction of N2 to NH3 using
sources of protons and electrons. Bottom: Effective BDFE values for
various acid/reductant combinations and calculated overpotentials for
understanding of the BDFEs of the N-H bonds in various
coordinated nitrogen ligands – particularly diazenides, imides
and amides, found along the N2 fixation cycle. While PCET
has been implied in a number of examples,27 detailed
thermodynamic bracketing and estimation of thermochemical
parameters for N−H bond formation and cleavage is rare and
has typically been confined to metal imido and nitrido
species.28 A systematic body of experimentally determined N-
H BDFEs as a function of type of nitrogen ligand (nitride,
imide, amide, ammine), metal, and supporting ligand is
currently absent from the literature and
a
NH3 formation. pKa (MeCN) = 14.4, Ered (vs. Fc/Fc+) = -1.32V.Error!
Bookmark not defined.c bpKa (MeCN) = 14.4, Ered (vs. Fc/Fc+) = -1.45V.Error!
Bookmark not defined.d
cpKa (MeCN) = 8.4, Ered (Hg-pool cathode, vs.
Fc/Fc+) = -2.6 V.Error! Bookmark not defined.a dpKa (Et2OH+, DMSO) = -3.2,
Ered (vs. Fc/Fc+) estimated as -3.0V.15 eChemical overpotential
calculated as 6(48.6-BDFE).
From a thermodynamic perspective, efficient nitrogen
fixation occurs when the effective BDFE of the acid-reductant
pair approaches the gas phase free energy value of the
hydrogen atom, H• (ΔGf°(H•) = 48.6 kcal/mol). In systems
where the effective BDFE of an acid-reductant pair exhibit
modest deviations from 48.6 kcal/mol, it is useful to calculate
a chemical overpotential defined as ΔΔGf(NH3). This value is
often significant and ranges between 29.6 and 291.6 kcal/mol
for the reagents used with known molecular nitrogen fixation
catalysts. Such an analysis highlights the challenge of using
strong external acids and reductants for N2 fixation and
underscores the unique role of dihydrogen as the most energy
efficient source of protons and electrons. Unfortunately
treatment of molecular dinitrogen compounds with H2 often
leads to N2 displacement, in rare instances where the
dinitrogen ligand is sufficiently activated, N2 hydrogenation to
form N-H bonds has been observed.
or potent electrophiles are often required to remove the
functionalized nitrogen ligand from the coordination sphere of
the transition metal and the conditions are typically
Scheme 1. Top: Catalytic hydrogenolysis of a Ti-N bond resulting in
the formation of a reduced titanium complex and free ammonia.
Bottom: Hypothetical scheme for the catalytic formation of NH3 via
PCET mediated by a rhodium co-catalyst.
SiMe3
SiMe3
Cl
Rh
N
H
Ti Cl
+ NH3
Ti
NH2
5 mol%
4 atm H2
5 days, 23 ºC, THF
SiMe3
1-(Cl)
SiMe3
1-(Cl)NH2
92% (18 turnovers)
16
R
,
17 18
X
However, strong acids
M
Rh
N
NH2
R
N2/H2
H2
PCET
,
19 20
incompatible with catalytic turnover.
R
Release of coordinated nitrogen ligands to free ammonia
or hydrazine is anticipated to be thermodynamically favored;
it is the high kinetic barrier that limits reactivity. As a
consequence, proton coupled electron transfer (PCET) and
related hydrogen atom transfer (HAT) reactions are attractive
alternatives for ammonia synthesis by hydrogenation as
reaction trajectories may be accessed that avoid high-energy
intermediates and open an energetically accessible pathway
for product release.21 It is possible that the known molecular
molybdenum catalysts reported by Schrock and Nishibayahsi
that rely on a pyridinium acid and metallocene reductants,
M
X
R
Rh
N
H
NH3
would provide useful guiding principals for rational catalyst
design. Here we describe a systematic study of PCET with a
series of titanocene and zirconocene complexes bearing imide,
hydrazide and amide ligands and describe the mechanistic and
thermodynamic parameters for productive N-H bond
formation. Two structurally and thermodynamically distinct
metal hydrides were explored and experimental and
ACS Paragon Plus Environment

Page 3 of 12
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
computational data for N-H BDFEs in 12 different complexes
along a hypothetic nitrogen fixation cycle are also reported.
There are a number of important aspects to note concerning
the similarities and differences between [Rh]-H, [Cr]-H, and
[Cr]*-H. The most obvious distinction is the dramatic change
in the acidity of the three metal hydrides with both [Cr]-H,
and [Cr]*-H being more than 10 orders of magnitude more
acidic than [Rh]-H. The [M]0/- redox couple for the chromium
complexes are anodically shifted by approximately 1V
compared to [Rh]-H. Comparing [Cr]-H to [Cr]*-H, [Cr]*-H
is more basic by 3 pKa units and more reducing 140 mV,
consistent with expectations from permethylation of the
cyclopentadienyl ring. Importantly, the reduced acidity and
cathodic shift offset and there is no significant change in the
overall [M]-H BDFE between the two chromium compounds.
The calculated and experimentally determined BDFE values
for all three metal hydride compounds are in the range of 50 –
60 kcal/mol. Assuming that the theoretical minimum value for
a hydrogen atom donor derived from H2 is equal to the free
energy of formation of of H• (48.6 kcal/mol), [M]-H BDFEs
higher than 48.6 kcal/mol decrease the thermodynamic
feasibility of the hypothetical catalytic cycle shown in Scheme
1. As such, [Rh]-H is a more desirable hydrogen atom donor
than [Cr]-H and [Cr]*-H. However, thermodynamic
assessment of the relative bond strengths alone may not be an
accurate predictor of reactivity as kinetic preferences may
dominate.
Results and Discussion
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 identity and thermodynamic properties of the hydrogen
atom transfer reagent are essential for enabling successful
PCET for N-H bond formation to a nitrogen-containing ligand.
Detailed thermochemical properties, typically represented as
thermochemical square schemes, provide the means for
determining the overall thermodynamics of a PCET reaction
and the extent to which proton transfer and electron transfer
are anticipated to be coupled.21,29 To realize catalytic PCET
where the protons and electrons are ultimately derived from
H2, transition metal compounds capable of promoting catalytic
homolytic H2 cleavage followed by net atom transfer to form
an N-H bond were evaluated.
[Rh]-H was an excellent starting point for these studies due
to its ease of synthesis, the availability of sound
thermochemical data24 and proven capability in the catalytic
hydrogenation of TEMPO• (2,2,6,6-tetramethylpiperidine 1-
oxyl), and titanocene amides to release free ammonia. The
30
chromium hydride, (η5-C5H5)(CO)3CrH ([Cr]-H) was also
of interest due to its well established thermodynamic
properties31 and its known compatibility with sensitive and
reactive group 4 metallocenes. Norton32 and others33 have also
demonstrated [Cr]-H as a catalyst for radical cyclization and
hydrogenation reactions that proceed via PCET. Rational
modifications of the cyclopentadienyl ligand in [Cr]-H also
modulate the thermochemical properties, solubility and
crystallinity with (η5-C5Me5)(CO)3CrH ([Cr]*-H) serving as
Our studies on PCET as applied to N-H bond formation
relevant to nitrogen fixation commenced with titanocene
compounds. This platform was chosen due to the historical
35
significance of titanium complexes to ammonia synthesis, the
expected chemical inertness and modularity of the
cyclopentadienyl ligands and the ability of sterically hindered
metallocene environment to support rare examples of
monomeric imides, hydrazides and amides in some cases in
different oxidation states.
the most well-known example.34 Table
2 reports the
thermochemical properties for [Rh]-H, [Cr]-H, and [Cr]*-H.
Although gas-phase [M]-H BDFEs have been calculated and
reported previously,32c the values listed in Table 1 have been
recalculated in THF for consistency with other calculated
BDFE values reported in this work.
The titanocene(III) N,N-dimethyl hydrazide(-1) compound,
(η5-C5Me4SiMe3)2TiNHNMe2 (1-NHNMe2), was prepared by
straightforward salt metathesis by treatment of (η5-
C5Me4SiMe3)2TiCl (1-Cl) with LiNHNMe2. Oxidation of this
compound by chlorine atom transfer with (C6H5)3CCl cleanly
Table 1. Thermochemical data for the metal hydrides used in this
yielded
the
related
chloride
derivative,
(1-(Cl)NHNMe2).
(η5-
a
b
study. pKas are reported in MeCN solvent and THF (italicized) Ered
reported for the [M]0/- couple in MeCN and THF (italicized) vs.
Fc/Fc+ cCalculated in MeCN solvent using equation 1 with C = 54.9
kcal/mol21 dDFT calculated gas-phase BDFE. See supporting
C5Me4SiMe3)2Ti(Cl)NHNMe2
Representations of the solid state structures of both
compounds are presented in Figure 2 and establish rare η1
coordination36 of the hydrazide(-1) ligand. The majority of
titanium hydrazide(-1) complexes exhibit side-on η2
coordination37 and serve to highlight the unique steric
environment imparted by the substituted cyclopentadienyl
ligands. Importantly this coordination mode allows for more
direct analogies to be drawn to 1-NH2 and 1-(Cl)NH2 in
subsequent thermochemical studies. Attempts to prepare (η5-
e
information for details. In MeCN solution the phosphazene base
(tert-butylimino)tris(pyrrolidino)- phosphorane was required to
deprotonate [Rh]-H. The pKa of this base in THF is 20.2.24 f See ref
24. g See ref 31 for values in MeCN. Values in THF have been re-
determined (see text).
Rh
H
Cr
H
Cr
H
C5Me4SiMe3)2TiNHNH2
(1-NHNH2)
and
(η5-
CO
CO
CO
N
CO
C5Me4SiMe3)2Ti(Cl)NHNH2 (1-(Cl)NHNH2) by analogous
methods to the syntheses of 1-NHNMe2 and 1-(Cl)NHNMe2
were unsuccessful, yielding mixtures of unidentifiable
titanium products.
To gain insight into the PCET N-H bond forming reactivity
of complexes with titanium-nitrogen multiple bonds, the
titanium imido compound (η5-C5Me4SiMe3)2TiNSiMe3
(1=NSiMe3) was prepared. Our group previously reported the
CO
CO
[Rh]-H
[Cr]-H
[Cr]*-H
a
13.3 (7.8)g
30.3 (20.2)e
-1.85 (-1.98)f
16.1 (10.1)g
-0.83 (-0.83)g
57.8 kcal/mol
53 kcal/mol
pKa
Ered (V)b
-0.69 (-0.67)g
57.3 kcal/mol
52 kcal/mol
BDFE (exp)c 52.3 kcal/mol
BDFE (calc)d 55 kcal/mol
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 12
1
2
3
4
5
6
7
8
9
reactivity of 1=NSiMe3 with H2 and described facile 1,2
demonstrating successful catalytic hydrogenolysis of
a
addition to form (η5-C5Me4SiMe3)2Ti(H)NHSiMe3 (1-
(H)NHSiMe3).38 However, the thermochemistry of 1=NSiMe3
relevant to PCET was not examined. Additionally, 1-
(H)NHSiMe3 proved unreactive towards H2 and did not
release free silylamine.
titanium nitrogen double bond with a TON of 19 with the
[Rh]-H catalyst. Repeating the hydrogenation of 1=NSiMe3
with 20 mol% [Cr]-H (entry 11) produced a different outcome
with a 36% yield of H2NSiMe3 (quantified as NH4Cl)
corresponding to a TON of 1.8. While observation of a TON
greater than 1 is consistent with PCET from [Cr]-H followed
by catalyst regeneration. The dramatically reduced efficiency
of [Cr]-H compared to [Rh]-H is notable and suggests that
the two catalysts may operate by different mechanisms.
The catalytic hydrogenolysis of the titanium hydrazido
complexes, 1-NHNMe2 and 1-(Cl)NHNMe2, was also
explored. The observed reactivity paralleled the chemistry
observed with 1-(Cl)NH2 and 1-NH2; treatment of 1-
(Cl)NHNMe2 with 5 mol% [Rh]-H under 4 atm of H2 yielded
87% H2NNMe2 (TON = 17). However, H2NNMe2 was not
observed when the Ti(III) hydrazide, 1-NHNMe2, was
subjected to catalytic conditions. When 20 mol% [Cr]-H was
employed as the hydrogen atom donor, 1-(Cl)NH2NMe2
afforded 33% H2NNMe2 (TON = 1.7) while 1-NHNMe2
afforded substoichiometric quantities of NH2NMe2 (15%,
TON = 0.75).
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. ORTEP representations of the solid state structures of 1-
NHNMe2 (left) and 1-(Cl)NHNMe2 (right) at 30% probability
ellipsoids. Hydrogen atoms, except those attached to N(1), have been
omitted for clarity.
Table 2. Catalytic hydrogenolysis of various complexes using metal
hydride reagents in the presence of 4 atm H2.a [Rh] = (η 5-C5Me5)(py-
With this family of compounds in hand, the viability of
catalytic PCET with H2 and catalytic [Rh]-H, [Cr]-H and
[Cr]*-H were evaluated (Table 2). Each experiment was
conducted with 5 mol% of [Rh]-H and 10 mol% of each of
the chromium reagents, [(η5-C5H5)(CO)3Cr]2 ([Cr]2) and [(η5-
C5Me5)(CO)3Cr]2 ([Cr]*2). Hydrogenation of the dimeric
chromium precursors yielded [Cr]-H and [Cr]*-H producing
a final loading of 20 mol% in each case. Each reaction was
stirred for 5 days and following vacuum transferring of the
volatiles, was analyzed for ammonia by the indophenol
method 39 (entries 1-11) or NH2NMe2 by quantitative 1H NMR
spectroscopy (entries 12-15). For each entry, a corresponding
control reaction without catalyst was performed; in all
controls, the formation of NH3 or NH2NMe2 was not observed.
As was previously reported, [Rh]-H is a competent catalyst
for the hydrogenolysis of the amide ligand in 1-(Cl)NH2
(entry 1, 92% NH3, TON = 18) but not in 1-NH2 or (η5-
C5Me4SiMe3)2Zr(Cl)NH2 2-(Cl)NH2 (entries 4 and 7, 0%
NH3).24 Repeating the hydrogenation of 1-(Cl)NH2 with [Cr]-
H or [Cr]*-H (entries 2-3) produced only 37% NH3 (TON =
1.9) and 34% NH3 (TON = 1.7), respectively, despite the
higher catalyst loading compared to [Rh]-H. Although the
apparent yields of NH3 are low, the observation of TONs
greater than 1 indicate that [Cr]-H and [Cr]*-H are
regenerated under catalytic conditions. The reactions of [Cr]-
H and [Cr]*-H with 1-NH2 and 2-(Cl)NH2 (entries 5-6, 8-9)
yielded 15-19% NH3 (TON: 0.85 - 0.95), establishing
stoichiometric reactivity between the chromium reagents and
the metal amides.
Ph)RhH, 5 mol%. [Cr] = [(η5-C5H5)Cr(CO)3]2, 10 mol%. [Cr]* = [(η5-
b
C5Me5)Cr(CO)3]2, 10 mol%.
Determined by spectrophotometry
1
c
(entries 1-11) and quantitative H NMR (entries 12-15). Calculated
as the number of mols of NR2H divided by the number of mols of
monomeric metal hydride.
SiMe3
SiMe3
X
X
cat. [M-H]
H
M
M
NR2
THF, 23 °C, 5 d
4 atm H2
NR2
SiMe3
SiMe3
% NR2-Hb TON (NR2H / [Cat])c
[M-H]a
Entry Complex
1
2
3
4
5
6
7
8
9
1-(Cl)NH2
[Rh]
[Cr]
92
37
34
0
17
19
0
18
15
95
36
87
33
0
18
1.9
1.7
0
0.85
0.95
0
0.90
0.75
19
[Cr]*
[Rh]
[Cr]
[Cr]*
[Rh]
[Cr]
[Cr]*
[Rh]
[Cr]
1-NH2
2-(Cl)NH2
10 1=NSiMe3
11
1.8
17
1.7
0
12 1-(Cl)NHNMe2 [Rh]
13
[Cr]
[Rh]
[Cr]
14 1-NHNMe2
15
15
0.75
The titanium imido complex, 1=NSiMe3, was completely
consumed in the presence of 5 mol% of [Rh]-H under 4 atm
of H2 (entry 10). Analysis of the volatiles by 1H NMR
spectroscopy established formation of H2NSiMe3. Quantitation
by the indophenol method following hydrolysis to ammonia
established near quantitative (>95%) formation of NH3,
The data in Table 2 establishes important trends for
hydrogenolysis of titanium-nitrogen bonds by proton coupled
electron transfer. All of the titanium(IV) compounds
underwent catalytic Ti-N hydrogenolysis (TON > 1) using
[Rh]-H, [Cr]-H, and [Cr]*-H. Among these, [Rh]-H was the
ACS Paragon Plus Environment

Page 5 of 12
Journal of the American Chemical Society
1
2
3
4
most efficient hydrogen atom donor (TONs ≈ 20), supporting
Similar reactivity was observed with 1-NHNMe2 (Scheme
5
a pathway involving PCET from [Rh]-H to the Ti-NR2 ligand
followed by regeneration of the metal-hydride from activation
of H2 by the resulting metalloradical. By comparison, [Cr]-H
and [Cr]*-H are inefficient hydrogen atom donors (TONs ≈ 2)
- though observation of TONs greater than 1 indicates
catalytic regeneration of the metal hydrides. All Ti(III)
compounds proved unreactive towards [Rh]-H and
hydrogenolysis products were not detected. However, when
[Cr]-H and [Cr]*-H were used, variable amounts of product
were formed from the Ti(III) complexes. Importantly, the low
yields of NR2H (TONs < 1) demonstrate stoichiometric rather
than catalytic reactions between the chromium hydrides and
the Ti(III) complexes with little to no regeneration of the
metal hydride catalyst.
To gain insight into poor catalytic efficiency of [Cr]-H and
[Cr]*-H compared to [Rh]-H, a series of stoichiometric
studies were performed. Given the similar reactivity of [Cr]*-
H and [Cr]-H under catalytic conditions and their similar
thermochemical properties (Table 1), the two compounds were
used interchangeably in these experiments and were selected
based on the ability to observe or isolate relevant
intermediates.
2). Addition of [Cr]*-H to a toluene solution of 1-NHNMe2
resulted in an immediate color change from purple to red. The
volatiles of the reaction were analyzed by 1H NMR
spectroscopy using 1,3,5-trimethoxybenzene as an internal
reference and found to contain NH2NMe2 (76%). Analysis of
the nonvolatile organometallic residue by EPR spectroscopy
identified the concomitant formation of 1-[Cr]* and the
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
featureless NMR spectrum confirmed no other byproducts.
Figure 3. ORTEP representations of the solid-state structures of 1-
[Cr]* at 30% probability ellipsoids. Hydrogen atoms have been
omitted for clarity.
Scheme 2. Reactivity of a Ti(III) amide and a Ti(III) hydrazide (-1)
with chromium hydride reagents.
SiMe3
SiMe3
Rn
H
Reaction of both 1-NH2 and 1-NHNMe2 with [Cr]*-H are
best viewed as straightforward protonations with release of
ammonia or substituted hydrazine with concomitant capture of
the chromium anion by titanium. Because the titanium remains
in the +3 oxidation state both before and after the reaction,
proton transfer is facile but electron transfer does not occur.
Thermochemical measurements were conducted to
understand the lack of electron transfer reactivity. The pKa of
Rn
Ti NHR1
Cr
+
Ti
O
C
Cr
- NH2R1
OC
CO
CO
OC
CO
SiMe3
SiMe3
1-NH2: R1 = H
1-NHNMe2: R1 = NMe2
1-[Cr]: Rn = 5 H
1-[Cr]*: Rn = 5 Me
[Cr-H]: Rn = 5 H
[Cr-H]*: Rn = 5 Me
[(η5-C5Me4SiMe3)2Ti(NH3)][BArF ] (ArF = C6H3-3,5-(CF3)2,
Initial stoichiometric studies comparing metal reagents were
conducted using 1-NH2 (Scheme 2). Addition of [Cr]*-H to a
toluene solution of 1-NH2 resulted in an immediate color
change from purple to red. The volatile products were
analyzed by the indophenol method, establishing formation of
ammonia in 91% yield. The organometallic product of the
4
[1-NH3]+) was previously determined as 16.2 in THF.24
Although the reported pKa of [Cr]*-H in CH3CN is reported
at 16.1, this value has been re-determined in THF by
equilibration with collidinium (pKa = 8.1) and 1,8-bis(
dimethylamino)naphthalene (‘DMAN,’ pKa = 11.1)40 and
found to be 10.1. Thus, in the reaction medium (THF), [Cr]*-
H is six orders of magnitude more acidic than [1-NH3]+. The
redox potential of the [Cr]*(0/-) couple was also re-determined
in THF solution as -0.83V.41 Because the Ti(III)/Ti(II) redox
couple for [1-NH3]+/0 is -2.0V,42 electron transfer from
reaction was identified as the S = 1/2 (µeff,
= 1.7 µB)
23 °C
titanocene, (η5-C5Me4SiMe3)2Ti(µ-CO)Cr(CO)2(η5-C5Me5) (1-
[Cr]*) as determined by single crystal X-ray diffraction
(Figure 3). The presence of a chromium carbonyl subunit in
the product was confirmed by infrared spectroscopy with
observation of a strong and diagnostic band centered at 1638.7
cm-1. This band was successfully reproduced by full molecule
DFT calculations, where a vibrational mode was identified at
1620 cm-1, assigned as the µ-CO stretch.
SiMe3
SiMe3
Cl
Cl
+
1/2
Ti
+
1/2
1/2
CO
Ti
O
C
Cr
CO
Cr
SiMe3
CO
OC
The room temperature X-band EPR spectrum of 1-[Cr]* in
toluene displays a single feature at giso = 1.974 with a
hyperfine interaction to the 47Ti nucleus (I = 5/2, Aiso = 49.3
MHz). No 53Cr (9.5%, I=3/2) hyperfine was observed,
consistent with a titanium-based SOMO. This view of the
electronic structure was also confirmed by DFT calculations
where spin density was located principally on the titanium (see
Figure S14).
CO
SiMe3
1-Cl2
SiMe3
1-[Cr]*
[Cr]*-H
- NH3
Cl
M
[Cr]*
NH2
SiMe3
Cl
SiMe3
(M = Ti)
(M = Zr)
1-(Cl)NH2
2-(Cl)NH2
Zr
O
CO
CO
C
Cr
SiMe3
2-(Cl)[Cr]*
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 12
1
2
3
4
5
6
7
8
9
{[Cr]*}- to [1-NH3]+ is highly endergonic while proton
transfer is highly exergonic. As a result, protonation but not
electron transfer occurs.
Scheme 4. Proposed reaction pathway which accounts for the formation of NH3 and the pseudocatalytic regeneration of chromium hydride
reagents under H2.
Scheme 3. Divergent reactivity of 1-(Cl)NH2 and 2-(Cl)NH2 with
[Cr]*-H.
SiMe3
SiMe3
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
2 [Cr]R-H
Cl
Cl
Ti
2
Ti
2
CO
- 2 NH3
NH2
O
CO
C
Cr
H2
SiMe3
1-(Cl)NH2
SiMe3
- 1-(Cl)2
SiMe3
SiMe3
[Cr]
[Cr]R
[Cr]R-H
1-(Cl)[Cr]R
Rn
Ti
O
C
Cr
Ti
CO
O
C
CO
OC
CO
Cr
[Cr]R-H
SiMe3
SiMe3
SiMe3
SiMe3
-NH2SiMe3
1-[Cr]R
2 [Cr]R-H
NHSiMe3
1-[Cr]R
2
Ti NSiMe3
2
2
Ti
CO
O
CO
C
Cr
SiMe3
SiMe3
1-(NHSiMe3)[Cr]R
1=NSiMe3
2-(Cl)[Cr*]
1-(Cl)[Cr]
1-[Cr]2
1-(NHSiMe3)[Cr]
Figure 4. ORTEP representations of the solid state structures of mechanistically-relevant heterobimetallic complexes. Left to right: 2-(Cl)[Cr]*, 1-
(Cl)[Cr], 1-[Cr]2, 1-(NHSiMe3)[Cr]. Hydrogen atoms except that attached to N1 have been omitted for clarity.
The stoichiometric reactivity of [Cr]*-H with 1-(Cl)NH2
and 2-(Cl)NH2 was also examined (Scheme 3). Addition of
[Cr]*-H to a toluene solution of 2-(Cl)NH2 resulted in an
immediate color change from colorless to deep red. Analysis
of the volatile products of the reaction established 93%
formation of NH3 while analysis of the non-volatile
organometallic components established formation of the Cs
[Cr]*-H to 2-(Cl)NH2 is thermodynamically inaccessible
while stoichiometric protonation is facile.
The reactivity of [Cr]*-H with 1-(Cl)NH2 exhibited more
complex behavior (Scheme 3). Upon addition of [Cr]*-H to a
benzene-d6 solution containing 1-(Cl)NH2 and 1,3,5
trimethoxybenzene as an internal standard, an immediate color
1
change from yellow to dark red was observed. Analysis by H
symmetric
zirconocene,
(η5-C5Me4SiMe3)2Zr(Cl)(µ-
NMR
spectroscopy
established
formation
(η5-
CO)Cr(CO)2(η5-C5Me5) (2-(Cl)[Cr]*, 89%). This product was
identified by a combination of X-ray diffraction (Figure 4) and
by solid-state (KBr) infrared spectroscopy, which exhibits a
diagnostic band at 1530.4 cm-1. This feature was assigned as a
bridging carbonyl based on DFT frequency calculations (νDFT
= 1545 cm-1). As expected based on the data in Table 2, 2-
(Cl)[Cr]* was unreactive toward H2, consistent with the
observation that [Cr]*-H was not regenerated. The pKa of
C5Me4SiMe3)2TiCl2 (1-(Cl)2) in 42% yield. The volatiles of
the reaction were analyzed by NMR spectroscopy and 83% of
the expected NH3 was identified. Exposure of a benzene-d6
solution of the non-volatile products to 4 atm of H2 resulted in
regeneration of [Cr]*-H in 46% yield. In a separate reaction,
the non-volatile products were analyzed by IR spectroscopy
and exhibited spectroscopic signatures diagnostic with the
formation of 1-[Cr]*. The observed reaction products can be
rationalized by the following sequence shown in Scheme 4: (i)
initial protonation of 1-(Cl)NH2 by [Cr]*-H to yield the
putative bimetallic intermediate, (η5-C5Me4SiMe3)2Ti(Cl)(µ-
[(η5-C5Me4SiMe3)2Zr(Cl)(NH3)][BArF ] ([2-(Cl)NH3]+) in
4
THF was previously determined as 14.7 indicating that the
protonation of 2-(Cl)NH2 by [Cr]*-H is highly favored. The
Zr(IV)/Zr(III) redox couple for [2-(Cl)NH3](+/0) is -2.0 V,
making subsequent electron transfer from {[Cr]*}-
energetically inaccessible. Analogous to 1-NH2, PCET from
CO)Cr(CO)2(η5-C5Me5)
(1-(Cl)[Cr*]);
1-(Cl)[Cr*]
(ii)
form
ligand
redistribution
from
to
(η5-
C5Me4SiMe3)2Ti[(µ-CO)Cr(CO)2(η5-C5Me5)]2 (1-[Cr]*2) and
1-(Cl)2; and (iii) disproportionation of 1-[Cr]*2 to 1-[Cr]* and
ACS Paragon Plus Environment

Page 7 of 12
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
[Cr]*. The formation of [Cr*], a molecule that forms [Cr]*-H
kcal/mol. Thus, the thermodynamic trap arising from a highly
upon treatment with H2, explains to pseudo-catalytic behavior
observed in Table 2. However, generation of 0.5 equivalents
of 1-[Cr]* per molecule of NH3 limits the overall catalytic
performance to two turnovers. This reaction sequence also
explains the observation of turnover numbers approaching 2
for the reactions of 1-(Cl)NH2 and 1-(Cl)(NHNMe2) with
[Cr]-H and [Cr]*-H under catalytic conditions.
exergonic protonation step is operative for [Cr]R-H but not for
[Rh]-H.
This comparison highlights the importance of the hydrogen
atom donor and the hydrogen atom acceptor to have both
ΔpKa and ΔE1/2 properly matched to avoid thermodynamically
favored side reactions. The experimental observation of
pseudocatalytic regeneration of [Cr]R-H is likely a result of
the instability of 1-(Cl)[Cr]R with respect to the formation 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
To support to the reaction sequence proposed in Scheme 4,
direct evidence for the formation of 1-(Cl)[Cr]* was sought.
Addition of Ph3CCl as a potential chlorine atom donor to 1-
[Cr]* instead resulted in formation of a mixture of 1-Cl2 and
1-[Cr]* as judged by 1H NMR spectroscopy and
decomposition studies. It is likely that the sterically
encumbered pentamethylcyclopentadienyl ligand contributes
to the observed instability of 1-(Cl)[Cr]* and ligand
redistribution results. To support this hypothesis, the
chlorination procedure was repeated with 1-[Cr] in place of 1-
[Cr]* (Scheme 5). Addition of (C6H5)3CCl to a concentrated
pentane solution of 1-[Cr] at -35 °C followed by solvent
evaporation resulted in isolation of red crystals of (η5-
C5Me4SiMe3)2Ti(Cl)(µ-CO)Cr(CO)2(η5-C5H5), 1-(Cl)[Cr].
The solid state structure of 1-(Cl)[Cr] was determined by
X-ray diffraction (Figure 4) and the overall coordination
geometry is typical of a bent metallocene and is structurally
analogous to (η5-C5Me5)2Ti(CH3)(µ-CO)Mo(CO)2(η5-C5H5),
reported by Stucky and coworkers from the reaction of (η5-
1-[Cr]R . The two π-acidic µ-CO ligands coordinated to the Ti
2
center in 1-[Cr]R would be expected to attenuate the
2
otherwise inaccessible Ti(IV)/Ti(III) reduction potential and
thus allow for electron transfer from the chromium anion and
formation of 1-[Cr]R and [Cr]R (Scheme 4).
Scheme 5. Low temperature reaction of 1-[Cr] with (C6H5)3CCl
(Ph3C-Cl) to afford the thermally unstable complex, 1-(Cl)[Cr].
SiMe3
SiMe3
Cl
Ph3C-Cl
Ti
O
C
Cr
Ti
CO
-35 ºC
-[Ph3C]
O
CO
C
OC
CO
Cr
SiMe3
SiMe3
1-[Cr]
1-(Cl)[Cr]
The stoichiometric reactivity of 1=NSiMe3 with [Cr]-H
was also examined because of the observed TON of 1.8 under
catalytic conditions. Addition of [Cr]-H to a pentane solution
1=NSiMe3 resulted in a color change from red to orange. Slow
evaporation of this solution over 6 hours furnished crystals
suitable for X-ray diffraction in 75% yield. The solid-state
C5Me5)(η6-C5Me4CH2)Ti(CH3) with (η5-C5H5)(CO)3MoH.43
A
diagnostic solid-state infrared band was observed at 1602.7
cm-1, a value successfully reproduced (νCO = 1598 cm-1) by the
DFT computed frequency spectrum and assigned as the µ-CO
stretching mode. In benzene solution, 1-(Cl)[Cr] proved
unstable with respect to ligand redistribution, converting to a
mixture of 1-(Cl)2, 1-[Cr], and [Cr]2 over the course of
structure
of
(η5-C5Me4SiMe3)2Ti(NHSiMe3)(µ-
(1-(NHSiMe3)[Cr]) established
CO)Cr(CO)2(η5-C5H5)
protonation of the imido fragment with subsequent
coordination of the anionic chromium complex through a
bridging µ-CO ligand. The 1H NMR spectrum of 1-
(NHSiMe3)[Cr] exhibits the number of peaks consistent with
Cs molecular symmetry with the N-H resonance located at
9.39 ppm. The KBr infrared spectrum exhibits medium and
intensity band at 3320.0 cm-1 assigned to the N-H stretch
while a stronger feature was observed at 1630.5 cm-1 for the
µ-CO vibration. Isolated 1-(NHSiMe3)[Cr] was unreactive
towards 4 atm H2 over a period of 5 days as neither formation
of NH2SiMe3 nor [Cr]-H was observed.
1
minutes at 23 ºC as determined by a combination of H NMR
and IR spectroscopies as well as decomposition studies.
Attempts at obtain the 1H NMR spectrum and elemental
analysis of 1-(Cl)[Cr] have been unsuccessful due to this
instability. Nevertheless, the observation of 1-(Cl)[Cr] and the
instability of both 1-(Cl)[Cr]* and 1-(Cl)[Cr] with respect to
ligand redistribution and disproportionation supports the
hypothesis that these compounds intermediates on the route to
the regeneration of [Cr]*-H and [Cr]-H, as shown in Scheme
4.
The observed reactivity of 1-(Cl)NH2 with [Cr]*-H is an
instructive example for application of proton coupled electron
transfer to N2 functionalization. The BDFEs for [Cr]*-H (60.7
kcal/mol) and 1-(Cl)NH3 (61.2 kcal/mol) in THF suggests that
PCET from [Cr]*-H to the titanocene amide ligand should be
thermodynamically feasible. However, the thermochemical
properties of the chromium hydride reveal that ΔG for proton
transfer is –6.7 kcal/mol while ΔG for subsequent electron
transfer is +6.2 kcal/mol.42 A similar analysis holds for the
reaction with [Cr]-H (ΔGPT = -9.9 kcal/mol; ΔGET = 9.9
kcal/mol) Therefore, ammonia release via protonation and
formation for 1-(Cl)[Cr]R is the preferred reaction pathway. In
examining the thermochemistry for the reaction of 1-(Cl)NH2
with [Rh]-H in THF, ΔG for the proton transfer is 7.1
kcal/mol while ΔG for subsequent electron transfer is -20.3
Addition of
a second equivalent of [Cr]-H to 1-
(NHSiMe3)[Cr] produced an unexpected outcome.
Monitoring the reaction by 1H NMR spectroscopy in the
presence of 1,3,5 trimethoxybenzene as an internal standard
revealed formation of NH2SiMe3 in 76% yield. No resonances
for a diamagnetic titanium product were observed. Treatment
of the mixture of NMR silent compounds with 4 atm of H2 and
analysis resulted in formation of [Cr]-H in 42% yield as
1
judged by H NMR spectroscopy and integration versus the
internal standard. In a separate reaction, an IR spectrum of the
non-volatile products prior to treatment with H2 was collected
and revealed a strong band at 1595.7 cm-1 (DFT calculated νCO
= 1592 cm-1), consistent with the formation of 1-[Cr]. In an
attempt to identify intermediates en route to formation of 1-
[Cr], the addition of two equivalents of [Cr]-H to a
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 12
1
2
3
4
5
concentrated pentane solution of 1=NSiMe3 at -35 ºC resulted
In the absence of N2H4, addition of TEMPO• to 1-Cl, produced
in deposition of small crystals suitable for X-ray diffraction
identified as (η5-C5Me4SiMe3)2Ti[(µ-CO)Cr(CO)2(η5-C5H5)]2
(1-[Cr]2) (Figure 4). The solid state structure of 1-[Cr]2
contains two chromium fragments each with an isocarbonyl
ligand coordinated to titanium. Full molecule DFT
calculations support a titanium(IV) oxidation state assignment
with two anionic isocarbonyl-linked chromium subunits
(Figure S15). The solid-state KBr infrared spectrum of 1-[Cr]2
exhibits two strong bands at 1607.4 cm-1 and 1573.1 cm-1.
These are in good agreement with the calculated frequency
spectrum, which predicts two bands corresponding to µ-CO
stretching modes at 1590 cm-1 and 1559 cm-1. Attempts to
no reaction. This observation suggests that the putative
titanocene(III) intermediate, 1-(Cl)NH2NH2 is formed and
undergoes H-atom abstraction but the resulting parent
titanocene(IV) chloro hydrazide, 1-(Cl)NHNH2 is unstable.
Because direct experimental evidence for 1-(Cl)NH2NH2 was
not obtained, full molecule DFT calculations were conducted
to gain insight into the N-H BDFE. A computed value of 61
kcal/mol was obtained, suggesting little perturbation upon
dimethylation of the hydrazine. Similar measurements were
attempted for the putative titanocene(II) hydrazine compound,
1-NH2NMe2.
Attempts to prepare the compound by formation of the N-H
bond by addition of [Rh]-H to 1-NHNMe2 produced no
reaction. In the absence of an insurmountable kinetic barrier,
this experiment establishes an upper limit on the BDFE of 52
kcal/mol, consistent with the DFT-computed value of 39
kcal/mol. A similar value of 42 kcal/mol was calculated for
the corresponding parent hydrazide complex, 1-NH2NH2,
again highlighting little influence on nitrogen substitution on
the N-H BDFE of the coordinated hydrazine. Perhaps
unsurprisingly, ammonia and κ1-hydrazine ligands have
indistinguishable N-H BDFEs when coordinated to
titanocenes. This value may change upon a shift in hapticity to
κ2, however attempts to optimize such structures resulted in
reversion to κ1-hapticity.
Imido (Ti=NR) and hydrazido(2-) (Ti=NNH2) complexes of
titanocenes in the +3 and +2 oxidation states were also of
interest. With this family of compounds, formation of an N-H
bond is accompanied by reduction of a Ti=N π-bond. Addition
of TEMPO-H to a THF-d8 of 1=NSiMe3 resulted in formation
of TEMPO• and a new EPR-active titanocene(III) compound
identified as (η5-C5Me4SiMe3)2Ti(NHSiMe3) (1-NHSiMe3).
The titanocene imide, 1=NSiMe3 proved unreactive toward
2,4,6-tBuPhOH, establishing an experimental N-H BDFE
value in the range of 65 – 77 kcal/mol. The calculated N-H
BDFE of 68 kcal/mol is in good agreement with the
experimentally determined range. To experimentally
determine the N-H BDFE in the parent imido compound (η5-
C5Me4SiMe3)2Ti(NH) (1=NH), hydrogen atom abstraction
from 1-NH2 was explored using 2,4,6-tBuPhO•. No reaction
was observed between these two reagents over the course of
hours at 23 ºC. The lack of reactivity could be attributed to a
high kinetic barrier, although this possibility seems unlikely
given the facile H-atom abstraction chemistry observed with
2,4,6-tBuPhO• and other titanocenes. Thus, an experimental
lower limit on the N-H BDFE is 77 kcal/mol. An upper limit
on the N-H BDFE proved inaccessible as suitable non-
coordinating reagents with known BDFEs in compatible
solvents are unavailable. Nevertheless, the calculated N-H
BDFE of 1-NH2 (79 kcal/mol) is in good agreement with the
experimental observations. Both experimental and calculated
data indicate a ~10 kcal/mol difference in the BDFEs of 1-
NHSiMe3 and 1-NH2, highlighting the potential sensitivity of
the thermodynamic parameters to silicon-containing
substituents.
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
1
obtain a H NMR spectrum and elemental analysis of 1-[Cr]2
were unsuccessful likely owing to the thermal instability of the
compound.
Experimental
Bracketing
and
Calculation of N-H BDFEs.
Given the paucity of data on N-H BDFEs in coordination
compounds24,26 and the importance of these values for rational
design of ammonia synthesis and oxidation catalysts, a
combined experimental and computational study of N-H
BDFEs in model titanocene and zirconocene complexes was
conducted. The sterically demanding bis(cyclopentadienyl)
ligand platform is ideally suited for this purpose as it enables
preparation and isolation of metallocene complexes in the +3
and +4 oxidation and supports a variety of nitrogen-containing
ligands, including rare “parent” derivatives where the
44
substituents are exclusively hydrogen.
Our previous communication reported the N-H bond
dissociation free energies of NH3 coordinated to metallocenes
with Ti(II), Ti(III), and Zr(III).24 The N-H BDFE of 1-
(Cl)NH3 was calculated as 61 kcal/mol, consistent with the
experimentally bracketed value of 52 - 65 kcal/mol. Notably
substitution of the titanium(III) center with zirconium(III)
plummets the N-H BDFE to 41 kcal/mol while reduction of
Ti(III) to Ti(II) has a similar effect as the N-H BDFE in 1-NH3
is 42 kcal/mol. The calculated values are in good agreement
with experimental estimates of 35-48 kcal/mol. Analysis of the
thermochemical data reveals that the difference in N-H
BDFEs are driven principally by differences in redox
potentials of the various metals. In cases where the redox
couples have large and negative potentials such as Zr(IV)-
Zr(III) and Ti(III)-Ti(II), weaker N-H BDFEs result.
With this foundation in hand, measurement of N-H BDFEs
of other nitrogen-containing ligands relevant to a N2 fixation
scheme was conducted (Figure 5). Titanocene hydrazides were
studied and exhibit similarities to the corresponding amide
complexes. Addition of TEMPO• to a THF-d8 solution
containing 1-Cl in the presence of one equivalent of
NH2NMe2 cleanly yielded 1-(Cl)NHNMe2 and TEMPO-H, as
judged by 1H NMR spectroscopy and setting an upper limit of
65 kcal/mol for the N-H BDFE of the coordinated hydrazine.
Performing an analogous experiment with N2H4 also
produced TEMPO-H along with free cyclopentadiene ligand
and an unidentified high-symmetry organometallic complex.
ACS Paragon Plus Environment

Page 9 of 12
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Reaction:
Amides
Hydrazides (-1)
SiMe3
SiMe3
SiMe3
Cl
SiMe3
Cl
SiMe3
Cl
SiMe3
Cl
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
Cl
Cl
- H
H
NMe2
H
H
H
H
M
M
Ti
Zr
Ti
Ti
N
N NH2
N
N
N
ΔG = BDFE(N-H)
NR2
H
H
H2
H2
R2
SiMe3
M(III)
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
M(IV)
1-(Cl)NH2-H
61 kcal/mol
2-(Cl)NH2-H
1-(Cl)NH2NMe2-H
58 kcal/mol
1-(Cl)NH2NH2-H
61 kcal/mol
---
BDFE (calc)
BDFE (exp)
41 kcal/mol
52 - 65 kcal/mol
35 - 48 kcal/mol
52 - 65 kcal/mol
Reaction:
Amides
Hydrazides (-1)
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
H
H
H
NMe2
H
NH2
H
- H
ΔG = BDFE(N-H)
M
N
M
NR2
Ti
N
Zr
N
Ti
N
Ti N
R2
H2
H2
H
H
SiMe3
SiMe3
M(III)
SiMe3
SiMe3
SiMe3
SiMe3
M(II)
BDFE (calc)
BDFE (exp)
1-NH2
42 kcal/mol
35 - 48 kcal/mol
2-NH2
1-NH2NMe2
39 kcal/mol
< 52 kcal/mol
1-NH2NH2
42 kcal/mol
---
31 kcal/mol
---
Reaction:
Imides
Hydrazides (-2)
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
H
H
H
H
NMe2
H
NH2
- H
ΔG = BDFE(N-H)
M
N
R
M NR
Ti
Ti
N
N
Ti
N
Ti N
SiMe3
H
SiMe3
SiMe3
M(IV)
SiMe3
SiMe3
SiMe3
SiMe3
M(III)
BDFE (calc)
BDFE (exp)
1-N(SiMe3)-H
68 kcal/mol
65 - 77 kcal/mol
1-NH-H
1-NHNMe2
71 kcal/mol
1-NHNH2
67 kcal/mol
---
79 kcal/mol
> 77 kcal/mol
65 - 77 kcal/mol
Figure 5. Summary of experimentally determined and calculated N-H bond dissociation free energies (BDFEs) in titanocene and zirconocene
complexes. Key (Left to right): Reaction: Definition of the N-H BDFEs described in this work and associated redox state changes for metallocene
complexes. Amides: N-H BDFEs for ammonia coordinated to either M(III) (top) or M(II) metal centers. Loss of H• results in the formation of the
corresponding M(IV) or M(III) metal amide complex, respectively. Hydrazides (-1): N-H BDFEs for hydrazines coordinated to either Ti(III) (top)
or Ti(II) metal centers. Loss of H• results in the formation of the corresponding Ti(IV) or Ti(III) metal hydrazide(-1) complex, respectively. Imides
/ Hydrazides (-2): N-H BDFEs for Ti(III) amides (top) and for Ti(III) hydrazides (bottom). Loss of H• results in the formation of the
corresponding Ti(IV) imido (top) or Ti(IV) hydrazido (bottom) complex.
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
(Cl)NHNH2, the impact of methyl substituents on the N-H
Titanocene(III) hydrazides(1-) (Ti-NHNR2) were the final
class of compounds examined. Hydrogen atom abstraction
from 1-NHNMe2 is expected to furnish the titanium
BDFEs is minimal and the experimentally determined BDFE
values for 1-(Cl)NHNMe2 and 1-NHNMe2 appear to be valid
for the parent hydrazide(1-) analogues.
hydrazide(2-)
complex,
(η5-C5Me4SiMe3)2Ti(NNMe2)
(1=NNMe2). However, no reaction was observed upon
treatment of 1-NHNMe2 with TEMPO•, while treatment with
2,4,6-tBuPhO• produced 2,4,6-tBuPhO-H in 93% yield along
with a mixture of unidentifiable titanium products. The
instability of the putative complex, 1=NNMe2, is consistent
with observations reported by Mountford and coworkers on
the bonding of bis(cyclopentadienyl) titanium hydrazide (2-)
complexes and their instability.45,46 The differential reactivity
with TEMPO• and 2,4,6-tBuPhO• supports an experimental
BDFE of 65 – 77 kcal/mol, in good agreement with the
calculated value of 71 kcal/mol. The hypothetical parent
hydrazide complex, (η5-C5Me4SiMe3)2Ti(NHNH2) (1-
NHNH2), was calculated to have an N-H BDFE of 67
kcal/mol. As was observed with 1-(Cl)NHNMe2 and 1-
Conclusions
A family of bis(cyclopentadienyl) titanium and zirconium
amide, hydrazide and imide complexes have been studied for
N-H bond formation and ultimately catalytic amine or
hydrazine synthesis by proton coupled electron transfer using
H2 as the stoichiometric H-atom source. A combination of
chemical bracketing studies coupled with DFT methods
allowed determination of N-H bond dissociation free energies
in 12 different complexes. In all cases examined, there is a
significant lowering the N-H BDFE upon coordination to the
transition metal. The impact of the metal identity, metal and
nitrogen oxidation states on the N-H BDFE have been
systematically determined where metals with relatively low
ACS Paragon Plus Environment

Journal of the American Chemical Society
redox potentials (e.g. Ti(III)-Ti(IV)) favor the highest values.
Page 10 of 12
22
1
2
3
4
5
6
7
8
Catalytic NH3, NH2SiMe3 and NH2NMe2 synthesis has been
demonstrated using [Rh-H] as the H2 activation and H-atom
transfer catalyst. Although the chromium hydrides, [Cr-H]
and [Cr]*-H have comparable M-H BDFEs to [Rh-H], the
mismatched pKa and reduction potentials associated with
these reagents and the group 4 metallocene substrates resulted
in low turnover due to catalyst deactivation. The outcomes of
these studies demonstrate the value of N-H BDFEs in guiding
the rational design and modification of both metal and ligand
environments to generate complexes capable of catalytic
ammonia synthesis, ideally from molecular N2 and H2.
a) Munisamy, T.; Schrock, R. R. Dalton Trans. 2012, 41, 130. b)
Nishibayashi, Y. Inorg. Chem., 2015, 54, 9234.
23
While experimental data for the N-H BDFEs of parent pyridnyl radicals
is not available, the N-H BDFEs of Hantzsch ester derivatives have been
determined to be 35 – 38 kcal/mol. See Cheng, J.; Lu, Y.; Zhu, X.; Sun,
Y.; Bi, F.; He, J. J. Org. Chem. 2000, 65, 3853.
24 Pappas, I.; Chirik, P. J. J. Am. Chem. Soc. 2015, 137, 3498.
25 (a) Hu, Y.; Norton, J. R. J. Am. Chem. Soc. 2014, 136, 5938. (b) Hu, Y.;
Li, L.; Shaw, A. P.; Norton, J. R.; Sattler, W.; Rong, Y. Organometallics
2012, 31, 5058.
26
Related bond strengths have been calculated for (η5-C5H5)2Ti(Cl)NR2-H
but experimental thermochemical parameters were not determined. See
Paradas, M.; Campaña, A. G.; Jiménez, T.; Robles, R.; Oltra, J. E.;
Buñuel, E.; Justicia, J.; Cárdenas, D. J.; Cuerva, J. M. J. Am. Chem. Soc.
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
2010, 132, 12748.
27
(a) Mankad, N. P.; Mu, P.; Peters, J. C. J. Am Chem. Soc. 2010, 132,
4083. (b) Bowman, A. C.; Bart, S. C.; Heinemann, F. W.; Meyer, K.;
Chirik, P. J. Inorg. Chem. 2009, 48, 5587. (c) Fantauzzi, S.; Gallo, E.;
Caselli, A.; Ragaini, F.; Casati, N.; Macchi, P.; Cenini, S. Chem.
Commun. 2009, 3952. (d) King, E. R.; Betley, T. A. Inorg. Chem. 2009,
48, 2361. (e) Chomitz, W. A.; Arnold, J. Chem. Commun. 2008, 3648. (f)
Avenier, F.; Gouré, E.; Dubourdeaux, P.; Sénèque, O.; Oddou, J. L.;
Pécaut, J.; Chardon-Noblat, S.; Deronzier, A.; Latour, J. M. Angew. Chem.
Int. Ed. 2008, 47, 715. (g) Ni, C.; Fettinger, J. C.; Long, G. J.; Brynda, M.;
Power, P. P. Chem. Commun. 2008, 6045. (h) Badiei, Y. M.; Dinescu, A.;
Dai, X.; Palomino, R. M.; Heinemann, F. W.; Cundari, T. R.; Warren, T.
H. Angew. Chem. Int. Ed. 2008, 47, 9961. (i) Lucas, R. L.; Powell, D. R.;
Borovik, A. S. J. Am. Chem. Soc. 2005, 127, 11596. (j) Shay, D. T.; Yap,
G. P. A.; Zakharov, L. N.; Rheingold, A. L.; Theopold, K. H. Angew.
Chem. Int. Ed. 2005, 44, 1508. (k) Kogut, E.; Wiencko, H. L.; Zhang, L.;
Cordeau, D. E.; Warren, T. H. J. Am Chem. Soc. 2005, 127, 11248. (l)
Thyagarajan, S.; Shay, D. T.; Incarvito, C. D.; Rheingold, A. L.;
Theopold, K. H. J. Am Chem. Soc. 2003, 125, 4440. (m) Jensen, M. P.;
Mehn, M. P.; Que, L. Angew. Chem. Int. Ed. 2003, 42, 4357. (n) Siewert,
I. Chem. Eur. J. 2015, 21, 15078.
Acknowledgment. We thank the Director of the Office of Basic
Energy Sciences, Chemical Sciences Division, U. S. Department of
Energy (DE-FG-02-05ER15659) for financial support. We also thank
Prof. Robert Knowles (Princeton) for helpful discussions.
Supporting Information Available: Experimental procuedures, full
characterization data including NMR, EPR and infrared spectra, DFT
output and crystallographic data in CIF format. is available free of
charge via the Internet at http://pubs.acs.org.
References.
1 K. Tamaru, in Catalytic Ammonia Synthesis. ed. J. R. Jennings, Plenum,
New York, 1991.
2
(a) Nitrogen (fixed) – Ammonia; U.S. Geological Survey, Mineral
28 (a) Milsmann, C.; Semproni, S. P.; Chirik, P. J. J. Am. Chem. Soc. 2014,
136, 12099. (b) Wiese, S.; Mcafee, J. L.; Pahls, D. R.; Mcmullin, C. L.;
Cundari, T. R.; Warren, T. H. J. Am Chem. Soc. 2012, 134, 10114. (c)
Cowley, R. E.; Holland, P. L. Inorg. Chem. 2012, 51, 8352. (d) Iluc, V.
M.; Miller, A. J. M.; Anderson, J. S.; Monreal, M. J.; Mehn, M. P.;
Hillhouse, G. L. J Am Chem Soc 2011, 133, 13055. (e) Iluc, V. M.;
Hillhouse, G. L. J. Am. Chem. Soc. 2010, 132, 15148. (f) Scepaniak, J. J.;
Young, J. A; Bontchev, R. P.; Smith, J. M. Angew. Chem. Int. Ed. 2009,
48, 3158. (g) Nieto, I.; Ding, F.; Bontchev, R. P.; Wang, H.; Smith, J. M.
J. Am Chem. Soc. 2008, 130, 2716. (h) Cowley, R. E.; Bontchev, R. P.;
Sorrell, J.; Sarracino, O.; Feng, Y.; Wang, H.; Smith, J. M. J. Am. Chem.
Soc. 2007, 129, 2424. For an example of separated PCET to diiron
diazenides, hydrazides, and amides, see (1) Li, Y. Y.; Wang, B.; Luo, Y.;
Yang, D.; Tong, P.; Zhao, J.; Luo, L.; Zhou, Y.; Chen, S.; Cheng, F.; Qu,
J. Nat. Chem. 2013, 5, 320.
Commodity Summaries, U.S. Government Printing Office: Washington
DC, 2016. (b) Tracking Industrial Energy Efficiency and CO2 Emissions;
International Energy Agency (IEA): Paris, 2007. p 82-3.
3
Appl, M. Ammonia, 2. Production Process In Ullmann’s Encyclopedia
of Industrial Chemistry; 2012; pp. 140–225.
4
(a) Peters, J. C.; Mehn, M. P. In Activation of Small Molecules:
Organometallic and Bioinorganic Perspectives; 2006. (b) Chatt, J.;
Richards, R. L. J. Organomet. Chem. 1982, 239, 65.
5
(a) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589.
(b) Hidai, M. Coord. Chem. Rev. 1999, 185-186, 99.
6
(a) Pickett, C. J.; Ryder, K. S.; Talarmin, J. J. Chem. Soc., Dalt. Trans
1986, 1453. (b) Pickett, C. J.; Talarmin, J. Nature 1985, 317, 652.
7
(a) Shilov, A. E. Russ. Chem. Bull. Int. Ed. 2003, 52, 2555. (b)
Bazhenova, T. A.; Shilov, A. E. Coord. Chem. Rev. 1995, 144, 69–145.
8
(a) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Tanaka, H.; Yoshizawa,
29 Tilset, M. In Electron Transfer in Chemistry; 2001; pp. 766–713.
30 (a) Keppie, S. A.; Lappert, M. F. J. Organomet. Chem. 1969, 19, P5. (b)
Fischer, E. O. Inorg. Synth. 1963, VII, 136. (c) King, R. B.; Stone, F. G.
K.; Nishibayashi, Y. Chem. Sci. 2015, 6, 3940. (b) Kuriyama, S.;
Arashiba, K.; Nakajima, K.; Tanaka, H.; Kamaru, N.; Yoshizawa, K.;
Nishibayashi, Y. J. Am. Chem. Soc. 2014, 136, 9719. (c) Arashiba, K.;
Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2011, 3, 120. (d) Yandulov, D.
A. Inorg. Synth. 1963, VII, 99.
31
(a) Hartung, J.; Norton, J. R. In Catalysis Without Precious Metals;
2010; pp. 1–24. (b) Tilset, M. J. Am. Chem. Soc. 1992, 114, 2740. (c)
Parker, V. D.; Handoo, K. L.; Roness, F.; Tilset, M. J. Am. Chem. Soc.
1991, 113, 7493. (d) Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1989,
111, 6711. (e) Jordan, R. F.; Norton, J. R. J. Am. Chem. Soc. 1982, 104,
V; Schrock, R. R. Science 2003, 301, 76.
9
(a) Ung, G.; Peters, J. C. Angew. Chem. Int. Ed. 2015, 54, 532. (b)
Creutz, S. E.; Peters, J. C. J. Am Chem. Soc. 2014, 136, 1105. (c)
Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84.
10
Del Castillo, T. J.; Thompson, N. B.; Suess, D. L. M.; Ung, G.; Peters,
1255.
32
(a) Estes, D. P.; Norton, J. R.; Jockusch, S.; Sattler, W. J. Am. Chem.
J. C. Inorg. Chem. 2015, 54, 9256.
11
Soc. 2012, 134, 15512. (b) Smith, D. M.; Pulling, M. E.; Norton, J. R. J.
Am. Chem. Soc. 2007, 129, 770. (c) Tang, L.; Papish, E. T.; Abramo, G.
P.; Norton, J. R.; Baik, M.-H.; Friesner, R. A.; Rappé, A. J. Am. Chem.
Soc. 2003, 125, 10093.
Kuriyama, S.; Arashiba, K.; Nakajima, K.; Matsuo, Y.; Tanaka, H.;
Ishii, K. Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2016, 7, 12181.
12
van der Ham, C. J. M.; Koper, M. T. M.; Hetterscheid, D. G. H. Chem.
Soc. Rev. 2014, 43, 5183.
33
13 Macleod, K. C.; Holland, P. L. Nat. Chem. 2013, 5, 559.
(a) Bullock, R. M.; Samsel, E. G. J. Am. Chem. Soc. 1990, 112, 6886.
14
(b) Miyake, A.; Kondo, H. Angew. Chem. Int. Ed. 1968, 7, 631.
34 (a) Baird, M. C.; Jaeger, T. J. Organometallics 1988, 7, 2074. (b) Leoni,
O.; Landi, A.; Pasquali, M.; A, E. S. S.; Leoni, O.; Landi, A.; Pasquali, M.
J. Organomet. Chem. 1987, 321, 365.
Waidmann, C. R.; Miller, A. J. M.; Ng, C.-W. A.; Scheuermann, M. L.;
Porter, T. R.; Tronic, T. A.; Mayer, J. M. Energy Environ. Sci. 2012, 5,
7771.
15 Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
16 Fryzuk, M. D.; Johnson, S. A. Coord. Chem. Rev. 2000, 200, 379.
35 Vol’pin, M. E.; Shur, V. B. Nature 1996, 209, 1236.
36
17 Fryzuk, M. D.; Love, J. B.; Rettig, S. J. Science 1997, 275, 1445.
(a) Schwarz, A. D.; Onn, C. S.; Mountford, P. Angew. Chem. Int. Ed.
18
2012, 51, 12298.
a) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527. b)
(b) Janssen, T.; Severin, R.; Diekmann, M.; Friedemann, M.; Haase, D.;
Pool, J. A.; Bernskoetter, W. H.; Chirik, P. J. J. Am. Chem. Soc. 2004,
Saak, W.; Doye, S.; Beckhaus, R. Organometallics 2010, 29, 1806.
126, 14326.
37
19
(a) Pietryga, J. M.; Jones, J. N.; MacDonald, C. L. B.; Moore, J. A.;
a) Chirik, P. J. Dalton Trans. 2007, 1, 16. b) Semproni, S. P.; Chirik, P.
Cowley, A. H. Polyhedron 2006, 25, 259. (b) Ubn, D. U.; Eselsberg, O.;
Leigh, G. J.; Walker, D. G. J. Organomet. Chem. 1987, 323, C29. (c)
Dilworth, J. R.; Latham, I. A.; Leigh, G. J.; Huttner, G.; Jibril, I. J. Chem.
Soc., Chem. Commun. 1983, 1368.
J. J. Am. Chem. Soc. 2013, 135, 11373.
20
Sodium amalgam and silyl chlorides are an exception and compatible.
Examples of catalytic N2 silylation using this approach are well-
documented. See for example: (a) Siedschlag, R. B.; Bernales, V.;
Vogiatzis, K. D.; Planas, N.; Clouston, L. J.; Bill, E.; Gagliardi, L.; Lu, C.
C. J. Am. Chem. Soc. 2015, 137, 4638. (b) Imayoshi, R.; Tanaka, H.;
Matsuo, Y.; Yuki, M.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y.
Chem. Eur. J. 2015, 21, 8905.
38
(a) Hanna, T. E.; Lobkovsky, E.; Chirik, P. J. Eur. J. Inorg. Chem.
2007, 2007, 2677. (b) Hanna, T. E.; Keresztes, I.; Lobkovsky, E.;
Bernskoetter, W. H.; Chirik, P. J. Organometallics 2004, 23, 3448.
39 Marbach, P.; Chaney, L. Clin. Chem. 1961, 130.
21 Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961.
ACS Paragon Plus Environment

Page 11 of 12
Journal of the American Chemical Society
40
1
2
3
For a comprehensive table of pKa values in THF, see Rodima, T.;
Kaljurand, I.; Pihl, A.; Mäemets, V.; Leito, I.; Koppel, I. a. J. Org. Chem.
2002, 67, 1873.
41
Notably, our calculation of the [Cr]*-H BDFE in THF using a pKa
value of 10.1, Ered value of -0.83V, and correction constant in THF of 66
kcal/mol yields 60.7 kcal/mol – in good agreement with the literature
value of 58 kcal/mol in MeCN. There may be error in this value due to ion
pairing effects and uncertainty in the correction constant for THF.
Concerning the correction constant in THF, see (a) Tilset, M. In Electron
Transfer in Chemistry; 2001; pp. 766–713. (b) Cappellani, E. P.; Drouin,
S. D.; Jia, G.; Maltby, P. A.; Morris, R. H.; Schweitzer, C. T. J. Am.
4
5
6
7
8
9
Chem. Soc. 1994, 116, 3375.
42
In ref 21 we reported the peak cathodic potentials due to the observed
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 irreversibility.
A comparison of the peak cathodic
potentials amongst the structurally similar metallocene complexes was
sufficient to establish trends. For the present purposes, we have estimated
the equilibrium reduction potentials of [1-NH3]+, [1-(Cl)NH3]+, and [2-
(Cl)NH3]+ as -2.0 V, -1.1 V, and -2.0 V, respectively, using the known
pKa values, calculated BDFEs, and correction constant in THF of 66.
43
Hamilton, D. M.; Willis, W. S.; Stucky, G. D. J. Am. Chem. Soc. 1981,
103, 4255.
44 For examples relevant to N2 fixation, see reference 24 and (a) Anderson,
J. S.; Moret, M. E.;Peters, J. C. J. Am. Chem. Soc. 2013, 135, 534. (b)
Yandulov, D. V; Schrock, R. R. Inorg. Chem. 2005, 44, 1103.
45
(a) Tiong, P. J.; Groom, L. R.; Clot, E.; Mountford, P. Chem. Eur. J.
2013, 19, 4198. (b) Selby, J. D.; Feliz, M.; Schwarz, A. D.; Clot, E.;
Mountford, P. Organometallics 2011, 30, 2295.
46 For examples of stable titanium hydrazide (-2) complexes, see also (a)
Tiong, P. J.; Nova, A.; Schwarz, A. D.; Selby, J. D.; Clot, E.; Mountford,
P. Dalton Trans. 2012, 41, 2277. (b) Selby, J. D.; Manley, C. D.; Feliz,
M.; Schwarz, A. D.; Mountford, P. Chem. Commun. 2007, 4937. (c)
Parsons, T. B.; Hazari, N.; Cowley, A. R.; Green, J. C.; Mountford, P.
Inorg. Chem. 2005, 44, 8442.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 12
1
2
3
4
5
6
7
8
9
For Table of Contents Only
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
Catalytic PCET
N-H BDFEs
N
N
[MnNR]
M2
N2
M2
M1
H2
H
pKa
H
3H2
N2
M1
H
[MnNR-H]+
[Mn-1NR-H]
E1/2
2 NH3
ACS Paragon Plus Environment