Ammonia Synthesis by Hydrogenolysis of Titanium-Nitrogen Bonds Using Proton Coupled Electron Transfer

Article abstract of DOI:10.1021/jacs.5b01047

The catalytic hydrogenolysis of the titanium-amide bond in (η⁵-C₅Me₄SiMe₃)₂Ti(Cl)NH₂ to yield free ammonia is described. The rhodium hydride, (η⁵-C₅Me₅)(py-Ph)RhH (py-Ph = 2-phenylpyridine), serves as the catalyst and promotes N-H bond formation via hydrogen atom transfer. The N-H bond dissociation free energies of ammonia ligands have also been determined for titanocene and zirconocene complexes and reveal a stark dependence on metal identity and oxidation state. In all cases, the N-H BDFEs of coordinated NH₃ decreases by >40 kcal/mol from the value in the free gas phase molecule.

Full text of DOI:10.1021/jacs.5b01047

Communication
pubs.acs.org/JACS
Ammonia Synthesis by Hydrogenolysis of Titanium−Nitrogen Bonds
Using Proton Coupled Electron Transfer
Iraklis Pappas and Paul J. Chirik*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
*
S Supporting Information
9
10
N−H and N−C bond formation with appropriately
11
ABSTRACT: The catalytic hydrogenolysis of the tita-
substituted zirconocene and hafnocene complexes. One
significant challenge in this chemistry is removing the
functionalized nitrogen ligand from the coordination sphere
of the azaphilic early transition metal. Strong acids or silylating
agents are often required for liberation of the desired product
5
nium−amide bond in (η -C Me SiMe ) Ti(Cl)NH to
5
4
3 2
2
yield free ammonia is described. The rhodium hydride,
η -C Me )(py-Ph)RhH (py-Ph = 2-phenylpyridine),
5
(
5 5
serves as the catalyst and promotes N−H bond formation
via hydrogen atom transfer. The N−H bond dissociation
free energies of ammonia ligands have also been
determined for titanocene and zirconocene complexes
and reveal a stark dependence on metal identity and
oxidation state. In all cases, the N−H BDFEs of
and generate metal products incompatible with catalytic
12
turnover. Alternative methods, such as PCET, for M−NR
n
bond hydrogenolysis are attractive as this linkage is typically
13
unreactive toward H2.
To explore whether PCET was a viable strategy for liberation
of ammonia from the coordination sphere of a group 4
coordinated NH decreases by >40 kcal/mol from the
3
value in the free gas phase molecule.
metallocene, the sterically protected chlorotitanocene and
5
-
zirconocene parent amide complexes, (η -C Me SiMe ) M-
5
4
3 2
(
Cl)NH (1-(Cl)NH ), M = Ti; 2-(Cl)NH , M = Zr) were
2 2 2
he interconversion of ammonia, NH , with its constituent
3
prepared and crystallographically characterized (see Supporting
Information (SI)). Initial experiments focused on the
determination of the pK and redox potential of the protonated
complexes to provide an experimental measure of the N−H
BDFE of coordinated NH as a function of metal. Treatment of
T
elements N and H , is a long-standing challenge in
2 2
chemical synthesis and is attractive for the development of
alternative energy schemes with reduced fossil fuel depend-
a
1
encies and carbon footprints. Batch rather than continuous
3
ammonia synthesis at low pressure, for example, may prove
more compatible with renewable hydrogen and also enable
F
F
either 1-(Cl)NH or 2-(Cl)NH with HBAr (Ar = C H -3,5-
2
2
4
6
3
5
2
(CF
Me
([2-(Cl)NH
graphically characterized and readily characterized via H NMR
in polar noncoordinating solvents such as CD Cl ; stronger
donor media such as THF resulted in NH dissociation.
The acidities of both cationic ammonia complexes were
determined in 2-methyl-THF (MeTHF) solution as NH
3 2
) ) yielded the ammonia complexes, [(η -
NH to serve as a fuel or H storage medium. Proton coupled
3
2
F
+
3
C
5
4
SiMe
3
)
2
M(Cl)NH ]BAr
] ), M = Zr). These compounds were crystallo-
3
3
4 3
([1-(Cl)NH ] ), M = Ti,
electron transfer (PCET), a pathway operative in enzymatic
ammonia synthesis, is an attractive strategy for N−H bond
formation under mild conditions. Molecular molybdenum
catalysts such as those independently reported by Schrock
and Nishibayashi rely on pyridinium acids in conjunction with
+
4
1
5
2
2
6
3
strong reducing agents to affect ammonia synthesis and may
7
3
involve PCET reactions for N−H assembly. Elucidation and
dissociation was not observed in this solvent and pK (where
pK represents the ion-pair acidity) values for an array of bases
are known in THF solution. Titrations with an appropriate
base were monitored by UV−vis spectroscopy as the acid and
conjugate base pair for both titanium and zirconium
compounds were readily discerned. Both complexes were
rapidly deprotonated by pyrrolidine (pK , THF = 15.3) and
yielded 1-(Cl)NH or 2-(Cl)NH as the sole organometallic
products. No reaction was observed with NEt (pK , THF =
1
ip
optimization of these pathways may ultimately result in
catalysts with longer lifetimes that operate with reduced
overpotential.
ip
14
For such an approach to be feasible, a detailed understanding
of N−H bond dissociation free energies (BDFEs) must be
determined. Despite considerable efforts over the past five
decades in homogeneous N2 reduction, surprisingly few
thermochemical data are available for intermediates relevant
to ammonia synthesis. Here we describe experimental and
computational determination of N−H BDFEs as a function of
the transition metal and oxidation state for titanocene and
zirconocene complexes and demonstrate the utility of PCET
reactions in ammonia synthesis and breaking strong M−N
bonds in early transition metal compounds.
ip
2
2
3
ip
4.1). A competition experiment was also performed whereby
+
[1-(Cl)NH ] was mixed with 2-(Cl)NH and allowed for the
3
2
determination of a ΔpK of −0.3 between the two compounds.
Individual titration experiments with pyrrolidine yielded pK
values for [1-(Cl)NH ] and [2-(Cl)NH ] of 15.0 and 14.7,
respectively.
ip
ip
+
+
3
3
Dating to Vol’pin and Shur’s seminal work on ammonia
synthesis, group 4 metallocenes have a rich N2 fixation
8
chemistry. Our laboratory has since demonstrated an array
Received: January 30, 2015
of N cleavage and functionalization reactions coupled to
2
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b01047
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
+
Cyclic voltammetry was also carried out on MeTHF
tion of [1-NH ] with 1-(Cl)NH established a ΔpK of 1.2
3
2
ip
+
+
solutions of [1-(Cl)NH ] and [2-(Cl)NH ] with 0.10 M
with the titanium(III) complex being more basic. A cyclic
voltammogram of [1-NH ] exhibited an irreversible wave with
3
3
n
F
+
[
Bu N][BAr ] as the electrolyte. Irreversible waves were
4
4
3
observed with cathodic peak potentials at −1.60 V (Ti) and
a peak cathodic peak potential at −2.60 V. The observed
+
−
2.32 V (Zr) vs Fc/Fc ; the observed irreversibility is a result
irreversibility is likely a result of N−H oxidative addition upon
19
of NH dissociation upon reduction (vide infra). Chemical
reduction to Ti(II).
3
reduction of both compounds was accomplished using
stoichiometric 0.5% Na(Hg) (E = −2.36 V vs Fc/Fc ) and
resulted in the isolation of the titanocene(III) and zirconocene-
It is instructive to compare the pK and electrochemical data
of [1-(Cl)NH ] , [2-(Cl)NH ] , and [1-NH ] . While selected
3 3 3
ip
0
+
+
+
+
examples of measurements of N−H BDFEs have been reported
(
III) chloride complexes, 1-Cl and 2-Cl, respectively. Both
for the interconversion of metal-imido to metal-amido
1
2
compounds were characterized by solution EPR spectroscopy
compounds, much less is known about M−NH
2
derivatives
at 23 °C and exhibited isotropic signals with giso values of 1.953
en route to NH . This absence of data is due, in part, to the
3
(
Ti) and 1.937 (Zr). 2-Cl was also independently synthesized
paucity of parent amido compounds. For titanocenes and
zirconocenes in this study, absolute BDFEs cannot be
determined because of the irreversible electrochemical behavior
and the lack of a known correction value for the H atom in 2-
and characterized by X-ray diffraction and is a rare example of a
15
monomeric Zr(III) complex. Formation of free ammonia
obtained from the reduction reaction was confirmed by
16
3b
indophenolic titration.
MeTHF solution. To overcome this limitation, relative
Experiments were also conducted to evaluate the coordina-
tion of ammonia to the metallocene(III) chloride complexes.
BDFEs among 1-(Cl)NH , 2-Cl(NH ), and 1-NH are
3 3 3
20
nevertheless useful and can be calculated according to eq 2:
Monitoring the addition of excess NH gas to 1-Cl or 2-Cl by
3
BDFE = 1.37(pK ) + 23.06E + C
ip
red
(1)
(2)
X-band EPR spectroscopy at 23 °C produced new features at
giso = 1.979 (Ti) and 1.970 (Zr) that restored to the original
values upon evacuation. Formation of similar aquo complexes
ΔBDFE = 1.37ΔpK + 23.06ΔE
red
ip
5
17
of (η -C H ) TiCl have also been previously oberved.
Table 1 reports the compilation of physical data measured
for the metallocene amide compounds. The data assume similar
5
5 2
5
F
Addition of [(η -C H ) Fe][BAr ] to MeTHF solutions of
5
5 2
4
1
-Cl and 2-Cl in the presence of excess NH gas furnished [1-
3
+
+
(
Cl)NH ] and [2-(Cl)NH ] , respectively as determined by
3 3
Table 1. Summary of Physical Data for Titanocene and
Zirconocene Amide Complexes (BDFEs reported in kcal/
mol)
1
H NMR spectroscopy. These experiments complete a
sequence of reversible proton and electron transfer events
linking the metal−amide complexes with ammonia (Scheme 1).
a
b
c
d
compd
Epc (V)
pKip
ΔBDFE (exptl) BDFE (calcd)
[
[
[
1-(Cl)NH ]+
−1.60
−2.32
−2.60
15.0
14.7
16.2
−
−17
−21
61
41
42
3
Scheme 1. Transformations Relevant to Proton Coupled
Electron Transfer (PCET) for Titanocene and Zirconocene
Complexes
2-(Cl)NH ]+
3
1-NH ]+
3
a
+
Peak cathodic potential reported relative to Fc/Fc in Me−THF
solution with 0.10 M [ Bu N][BAr ] as the electrolyte. Ion-pair
n
F
4
b
4
acidity determined by UV/vis titration against standards in Me−THF.
c
Defined as BDFE(2-(Cl)NH ) − BDFE(1-(Cl)NH ) and BDFE(1-
3
3
d
NH ) − BDFE(1-(Cl)NH ) using eq 2. Calculated free energy
change for the reaction of [M]NH −H → [M]NH + H .
3
3
•
2
2
ion-pairing effects and utilize peak cathodic potentials (E ) due
pc
to the observed electrochemical irreversibility. Given the
structural similarity of the compounds and the constant solvent
medium, it is likely these assumptions have little impact on the
relative N−H BDFE values. Also reported in Table 1 are full
molecule DFT-computed N−H BDFEs for each compound.
Several trends are evident. Coordination of ammonia to
titanium or zirconium reduces the BDFE by >40 kcal/mol
compared to the gas phase bond strength of 99.4 kcal/mol of
3b
free ammonia. Similar weak E−H bonds of 49.4 and 65.0
have been computed for the coordination of H O and NH to
2
3
5
21
(
η -C H ) TiCl, respectively. The BDFEs vary within the
5 5 2
series where changing the titanium oxidation state from 4+ to
3+ greatly reduces the N−H bond strength. Likewise, moving
from Ti(IV) to Zr(IV) has a similar bond-weakening impact.
Importantly, the pKip values within the series exhibit little
variance; the observed decrease in BDFE as a function of metal
or oxidation state is therefore dominated by the change in
redox potential. It is also apparent from the calculated BDFE
The impact of the metal oxidation state on the proton and
electron transfer properties of amide complexes was also
18
examined. The titanocene(III) amide complex, 1-NH2, was
prepared, crystallographically characterized, and treated with
F
HBAr 4 and yielded the cationic titanium(III) ammonia
5
F
4
+
complex, [(η -C Me SiMe ) TiNH ][BAr ] ([1-NH ] ). De-
5
4
3 2
3
3
protonation of this compound required the stronger base, 1,8-
values that the titanium(IV) derivative, 1-(Cl)NH , is the most
promising for liberation of ammonia by PCET.
2
diazabicycloundec-7-ene (DBU, pK , THF = 17.8). Equilibra-
ip
B
DOI: 10.1021/jacs.5b01047
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
H atom abstraction experiments were conducted to validate
Scheme 2. Generation of Ammonia from Catalytic
these findings. Addition of NH gas to a benzene-d solution
containing 1-Cl or 2-Cl and 1 equiv of TEMPO (2,2,6,6-
tetramethylpiperidine 1-oxyl) resulted in immediate formation
Hydrogenolysis of 1-(Cl)NH in the Presence of 5 mol %
Rh−H
3
6
2
of the chlorometallocene amide compounds, 1-(Cl)NH or 2-
2
(
Cl)NH , and TEMPO-H (eq 3) quantitatively as determined
2
(
Cl)NH with 4 atm of H in the presence of 5 mol % of Rh−
2
2
H yielded 92 ± 6% of NH over the course of 5 days,
3
corresponding to 18 turnovers based on rhodium. Rhodium
loadings as low as 1 mol % were also effective but proved
irreproducible. Repeating the catalytic hydrogenations with 2-
1
(Cl)NH
reaction.
2 2
and 1-NH and 5 mol % of Rh−H produced no
by H NMR spectroscopy. Control experiments whereby NH3
gas was added to a benzene-d of TEMPO produced no
reaction. As the BDFE of TEMPO-H in benzene is 65.2 kcal/
mol, and the chlorometallocene amide complexes are
generated in greater than 95% yield, these experiments establish
an N−H BDFE of <63 kcal/mol upon coordination of
ammonia to Ti(III) or Zr(III). Similar experimental verification
has been provided from observation of HAT chemistry from
6
Comparison of redox potentials eliminates the possibility
that the catalytic hydrogenolysis of the Ti−NH bond in 1-
3
b
2
(
Cl)NH proceeds via initial electron transfer. The oxidation
2
potential of Rh−H in THF has been reported as −0.38 V vs
+
Fc/Fc , which is well below the potential of −2.8 V required to
reduce 1-(Cl)NH (see SI). Likewise, the pK of [Rh]-H is
2
ip
23
n
5
∼20, which is well above the pKip of 1-(Cl)NH
The relatively high barriers for both initial proton and initial
electron transfer from Rh−H to 1-(Cl)NH implicate
concerted hydrogen atom transfer as the most likely pathway
for catalytic ammonia formation from 1-(Cl)NH2.
In summary, we have demonstrated the utility of proton
coupled electron transfer as a method for the liberation of
ammonia from the coordination sphere of an early transition
metal. In the presence of an appropriate metal catalyst,
hydrogen can be used as the stoichiometric H atom source
providing an attractive method for product release following
small molecule activation and functionalization.
2
(vida supra).
addition of OctNH to (η -C H ) TiCl in the context of
epoxide ring opening.
2
5
5 2
2
1
2
Attention was then devoted to ammonia synthesis from 1-
Cl)NH2 using PCET. Stirring a THF-d8 solution of 1-
Cl)NH under 4 atm of H for 5 days at 23 °C produced no
(
(
2
2
change, highlighting the strength of the Ti−NH interaction
2
and lack of a kinetic pathway for hydrogenolysis. We reasoned
that addition of an appropriate metal complex with an M−H
bond strength less than 61 kcal/mol would enable PCET and
liberate ammonia from the coordination sphere of the titanium.
Additionally, if the metal complex were chosen such that the
weak M−H bond could be regenerated under an atmosphere of
H , the PCET could be rendered catalytic. Norton and co-
workers have recently reported (η -C Me )(py-Ph)RhH (Rh−
H; py-Ph = 2-pyridylphenyl), an attractive PCET reagent as the
2
ASSOCIATED CONTENT
Supporting Information
Experimental details; computational details; electrochemical
5
■
5
5
*
S
2
2
Rh−H BDFE was measured to be 52.3 kcal/mol.
+
+
+
characterization of [1-(Cl)NH ] , [2-(Cl)NH ] , [1-NH ] ,
and 1-(Cl)NH ; EPR spectra of 1-(Cl)NH , 2-(Cl)NH , 1-
3
3
3
Significantly, the Rh−H complex was demonstrated to be
2
3
3
operative for catalytic PCET under an atmosphere of H .
2
+
NH , [1-NH ] , and 2-Cl; crystallographic data for 1-(Cl)NH ,
2
3
2
The feasibility of this approach was evaluated in a series of
stoichiometric experiments whereby 1 equiv of Rh−H was
added to each of the metallocene amides, 1-(Cl)NH , 2-
+
+
+
2
-(Cl)NH , 1-NH , [1-(Cl)NH ] , [2-(Cl)NH ] , [1-NH ] ,
2 2 3 3 3
and 2-Cl in cif format. This material is available free of charge
2
via the Internet at http://pubs.acs.org.
(
Cl)NH , and 1-NH , in THF-d solution. As expected from
2 2 8
the physical measurements, both 2-(Cl)NH2 and 1-NH2
produced no reaction even upon heating to 60 °C for 24 h.
AUTHOR INFORMATION
Corresponding Author
pchirik@princeton.edu
Notes
■
1
Monitoring the reaction with 1-(Cl)NH2 by H NMR
*
spectroscopy established the disappearance of the Rh−H
resonance at −12.5 ppm along with the peaks for the
diamagnetic titanium complex over the course of 6 h. Analysis
of the reaction volatiles at this time point established a 98%
yield of free ammonia while treatment of the organometallic
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
residue with HCl resulted in 95% recovery of 1-Cl , implicating
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.
2
paramagnetic 1-Cl as the sole product following ammonia
5
release. The fate of the rhodium is likely paramagnetic [(η -
C Me )(py-Ph)Rh] as addition of H to the reaction mixture
5
5
2
following hydrogen atom transfer cleanly regenerated Rh−H,
implicating its formation.
REFERENCES
■
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DOI: 10.1021/jacs.5b01047
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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