Ammonia Activation, H2 Evolution and Nitride Formation from a Molybdenum Complex with a Chemically and Redox Noninnocent Ligand

Article abstract of DOI:10.1021/jacs.7b03070

Treatment of the bis(imino)pyridine molybdenum η⁶-benzene complex (^iPrPDI)Mo(η⁶-C₆H₆) (^iPrPDI, 2,6-(2,6-iPr₂C₆H₃N CMe)₂C₅H₃N) with NH₃ resulted in coordination induced haptotropic rearrangement of the arene to form (^iPrPDI)Mo(NH₃)₂(η²-C₆H₆). Analogous η²-ethylene and η²-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η²-coordinated ligand followed by N-H bond activation, bis(imino)pyridine modification, and H₂ loss. A dual ammonia activation approach has been discovered whereby reversible M-L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation. Significantly, the weakened N-H bonds in (^iPrPDI)Mo(NH₃)₂(η²-C₂H₄) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH₃ oxidation.

Full text of DOI:10.1021/jacs.7b03070

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Communication
Ammonia Activation, H2 Evolution and Nitride Formation from a
Molybdenum Complex with a Chemically and Redox Non-Innocent Ligand.
Grant W Margulieux, Mate J. Bezdek, Zoë R. Turner, and Paul J. Chirik
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b03070 • Publication Date (Web): 17 Apr 2017
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Ammonia Activation, H2 Evolution and Nitride Formation from
a Molybdenum Complex with a Chemically and Redox Non-
Innocent Ligand.
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Grant W. Margulieux, Máté J. Bezdek, Zoë R. Turner, and Paul J. Chirik*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
Supporting Information Placeholder
use of a more chemically inert terpyridine ligand, a non-classical mo-
ABSTRACT: Treatment of the bis(imino)pyridine molybdenum
lybdenum(I)
ammine
complex,
η6-benzene complex
(
iPrPDI)Mo(η6-C6H6)
(
iPrPDI, 2,6-(2,6-
[(PhTpy)(PPh2Me)2Mo(NH3)][BArF24
]
(
PhTpy, 4’-Ph-2,2’,6’,2”-
terpyridine; ArF24 = [C6H3-3,5-(CF3)2]4) was isolated and a remarka-
bly low N-H bond dissociation free energy of 45.8 kcal/mol was meas-
ured.12 This significant bond weakening (NH3(g) = 99.5 kcal/mol)13
iPr2C6H3N=CMe)2C5H3N) with NH3 resulted in coordination in-
duced haptotropic rearrangement of the arene to form
(
iPrPDI)Mo(NH3)2(η2-C6H6). Analogous η2-ethylene and η2-
cyclohexene complexes were also synthesized and the latter crystallo-
graphically characterized. All three compounds undergo loss of the η2-
coordinated ligand followed by N-H bond activation,
bis(imino)pyridine modification and H2 loss. A dual ammonia activa-
tion approach has been discovered whereby reversible M-L cooperativ-
ity and coordination induced bond weakening likely contribute to
dihydrogen formation. Significantly, the weakened N-H bonds in
enables spontaneous H2 evolution upon mild heating (BDFEN-H
<
ΔG°(HŸ) = 48.6 kcal/mol).13 Discovery of transition metal complexes
that promote multiple potentially reversible N-H bond activation
events remains a significant challenge in ammonia oxidation. Here we
describe a family of bis(imino) pyridine molybdenum bis(ammine) η2-
benzene and alkene complexes that promote H2 evolution from am-
monia as well as reversible N-H bond activation that combines metal-
ligand cooperativity and bond weakening by coordination. This strate-
gy enabled synthesis of a terminal molybdenum nitride from NH3 by a
series of hydrogen atom abstraction events, key steps in ammonia oxi-
dation.
(
iPrPDI)Mo(NH3)2(η2-C2H4) enabled hydrogen atom abstraction and
synthesis of a terminal nitride from coordinated ammonia, a key step in
NH3 oxidation.
The diamagnetic bis(imino)pyridine molybdenum benzene com-
plex,
The synthesis of ammonia from its elements, H2 and N2 and the re-
verse process, liberation of dihydrogen from NH3, are key steps for
developing NH3 as a potential carbon-free fuel and hydrogen carri-
er.1,2,3 While considerable effort has been devoted to the understanding
and discovery of molecular catalysts for ammonia synthesis,4 by com-
parison much less is known about H2 formation following NH3 activa-
tion. One challenge is overcoming the σ-donating ability of ammonia
and its propensity to act as a supporting ligand in coordination chemis-
try. Organometallic and coordination compounds are known to pro-
mote N-H bond cleavage by oxidative addition,5,6 deprotonation,7 co-
operative chemistry between the ligand and the metal8 or reactions
with multiple transition metals.9 Main group compounds, specifically
nucleophilic carbenes,10a frustrated Lewis pairs,10b diarylstannylenes,10c
germylenes10d as well as constrained phosphorous(III)10e,10f and sili-
con(II) sites10g are also known to promote ammonia activation. In
most of these cases, formation of H2 is not observed following N-H
bond cleavage. One notable exception is a report from Wolczanski and
coworkers describing H2 formation following oxidative addition of
NH3 and α-hydrogen abstraction from the resultant tantalum amido-
hydride.6
(
iPrPDI)Mo(η6-C6H6) (1-(η6-C6H6); iPrPDI, 2,6-(2,6-
iPr2C6H3N=CMe)2C5H3N) was isolated in 88% yield by straightfor-
ward reduction of (iPrPDI)MoCl3 with 1% Na(Hg) in benzene solu-
tion. Benzene coordination was confirmed by observation of a singlet
at 3.56 ppm in the benzene-d6 1H NMR spectrum. There was no evi-
dence for arene exchange even upon heating to 80 ºC for days. The
solid-state structure of 1-(η6-C6H6) was determined by X-ray diffrac-
tion and the bond distances of the bis(imino)pyridine chelate (Cimine
-
Nimine = 1.332(1) and 1.344(1); Cipso-Cimine = 1.418(1) and 1.410(1) Å)
are consistent with a closed shell dianionic chelate arising from strong
π-backbonding, suggesting the Mo(II) oxidation state is most appro-
priate.14
Molybdenum complexes bearing redox-active bis(imino)pyridine
and terpyridine ligands have recently been shown to exhibit a rich am-
monia activation chemistry, including H2 formation. Addition of NH3
to the bis(imino)pyridine molybdenum dinitrogen complex,
[{(iPrBPDI)Mo(N2)}2(µ2,η1,η1-N2)]
(
iPrBPDI,
2,6-(2,6-
iPr2C6H3N=CPh)2C5H3N) resulted in sequential N-H bond activa-
tions ultimately forming bridging parent imido ligands (µ-NH) along
with conversion of the one imines on the chelate to a terminal aryl
imido.11 Ammonia serves as a source of hydrogen during the modifica-
tion of the bis(imino)pyridine ligand and is irreversible. In contrast,
Figure 1. Comparison of selected ammonia activation methods high-
lighting the strategy reported in this work.
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Addition of two equivalents of NH3 gas to a thawing benzene solu-
tion of 1-(η6-C6H6) resulted in formation of a Cs symmetric com-
pound identified as (iPrPDI)Mo(NH3)2(η2-C6H6) (1-(NH3)2(η2-
C6H6); Figure 2). Observation of three distinct resonances at 3.45,
violet product at 23 ºC (Figure 2a). The time for conversion was de-
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pendent on the identity and amount of alkene; complete conversion
was observed with the benzene compound in benzene-d6 over the
course of 2 hours at 23 ºC. With the corresponding ethylene com-
pound, consumption of the diamagnetic starting material required 72
hours; for cyclohexene derivative, 36 hours was needed. Addition of 10
equiv of ethylene to 1-(NH3)2(η2-C2H4) extended the half life of
the complex to 240 hours, supporting a pathway where alkene dissocia-
tion is necessary to facilitate ammonia activation. X-ray diffraction
established the identity of the product of each reaction as
1
5.22 and 5.53 ppm together with 2-D H NMR experiments confirmed
the unusual η2 hapticity of the arene ring (see SI for details). Distinct
NH3 environments were observed at -1.31 and -1.91 ppm, consistent
with a static η2 benzene ligand, although exchange with free benzene
was observed over the course of 90 minutes at 23 ºC. While Harman
and coworkers have reported examples of η2-coordinated molyb-
denum complexes of naphthalene and other heterocycles,15a to our
knowledge 1-(NH3)2what(η2-C6H6) is the only monometallic
example with benzene.15b-f
9
(
iPrPIA)Mo(NH2)2 (2-(NH2)2; iPrPIA = 2-(2,6-iPr2C6H3N=CMe), 6-
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(2,6-iPr2C6H3N-CHMe)C5H3N), arising from NH3 activation and
hydrogen transfer to the imine carbon of the iPrPDI chelate. Formation
of two amide ligands leaves one hydrogen atom unaccounted in the
final molybdenum product. Collecting the volatiles of the reaction by
Toepler pump and combustion in a CuO burn tube confirmed for-
mation of 0.5 equiv of hydrogen gas (93%), demonstrating H2 evolu-
tion accompanies ammonia activation en route to 2-(NH2)2.
Chelate modification in 2-(NH2)2 was confirmed by X-ray diffrac-
tion (Figure 2d), where the newly introduced hydrogen atom on the
modified iPrPIA backbone as well as those residing on the amide lig-
ands were located and freely refined. A solid-state magnetic moment of
1.89(2) µB was measured at 23 ºC, consistent with an S = ½ ground
state. Accordingly, the X-band EPR spectrum of 2-(NH2)2 exhibits an
isotropic signal (giso = 1.97) with hyperfine coupling to both 95Mo (I
=5/2, 15.9%) and 97Mo (I = 5/2, 9.6%) spin active nuceli (Ai-
(
95/97Mo) = 139 MHz, Figure 3b), suggesting that the singly-occupied
so
molecular orbital of 2-(NH2)2 is principally molybdenum based.
Protonation studies were conducted with 2-(NH2)2 to establish the
fidelity of the new formed C-H bond in the iPrPIA ligand. Addition of
one equivalent of lutidinium triflate ([LutH]OTf) resulted in isolation
of a red paramagnetic powder identified as
(
iPrPDI)Mo(NH3)2OTf
(1-(NH3)2(OTf)). Apart from lutidine, no other organic or gaseous
byproducts were detected during the course of the reaction. One-
electron oxidation of in situ prepared 1-(NH3)2(η2-C6H6) with
[(η5-C5H5)2Fe][OTf] provided an alternative route to 1-
(NH3)2(OTf). The metrical parameters obtained from X-ray diffrac-
tion, magnetic and EPR spectroscopic data support the description of
1-(NH3)2(OTf) as a low-spin Mo(III) complex. Remarkably, this
protonation reaction resulted in reconstitution of both ammonia lig-
ands in addition to regenerating of the bis(imino) pyridine chelate,
where [LutH]OTf and the iPrPIA backbone C-H bond served as the
hydrogen sources. While the initial site of protonation and the se-
quence of N-H bond forming and C-H bond cleavage steps remain
unknown, the observed reactivity is a rare reversal of the previously
demonstrated chelate modification arising from M-L ligand coopera-
tivity in ammonia N-H bond activation.11
Figure 2. (a) Coordination, activation and hydrogen release from
bis(imino)pyridine molybdenum compounds with η2-benzene, cyclo-
hexene, and ethylene ligands. Solid-state structures of (b) 1-(η6-
C6H6), (c) 1-(NH3)2(η2-C6H10) and (d) 2-(NH2)2 at 30% prob-
ability ellipsoids. Most hydrogen atoms were omitted for clarity.
Because 1-(NH3)2(η2-C6H6) proved too reactive for isolation in
the solid state, related alkene complexes were synthesized. Addition of
ethylene or cyclohexene to 1-(NH3)2(η2-C6H6) resulted in the
formation of C2v and Cs symmetric products, respectively, correspond-
ing to complexes of the type (iPrPDI)Mo(NH3)2(η2-alkene) (alkene =
C2H4, 1-(NH3)2(η2-C2H4); C6H10, 1-(NH3)2(η2-C6H10)). The-
se compounds were also synthesized in a single step from sequential
addition of ammonia and the appropriate alkene to 1-(η6-C6H6).
The single-crystal X-ray structure of the cyclohexene derivative con-
firms formation of a six-coordinate molybdenum complex with trans-
ammine ligands. The bond distances of the bis(imino)pyridine are
similar to those in 1-(η6-C6H6) and support a Mo(II) oxidation state
assignment.
Each (iPrPDI)Mo(NH3)2(η2- alkene) complex was kinetically unsta-
ble and underwent conversion to the same paramagnetic, NMR-silent
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ment of in situ prepared 1-(NH3)2(η2-C2H4) with one equiv of
(a)
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[(η5-C5H5)Cr(CO)3], a complex mixture of products was obtained,
likely arising from undesired participation of the carbonyl ligands of the
chromium dimer or hydride product.19 By contrast, addition of three
equiv of tBu3ArOŸ to a benzene solution of 1-(NH3)2(η2-C2H4)
resulted in rapid formation of tBu3ArOH (BDFEO-H = 77 kcal/mol)20
as well as a red-orange, paramagnetic product identified as the terminal
molybdenum nitride, (iPrPDI)Mo(N)(η2-C2H4) (1-(N)(η2-C2H4))
isolated in 61% yield after recrystallization (Figure 4a). This result
demonstrates that each N-H BDFE in the sequence of HAA events
during the course of the reaction is sufficiently weak (<77 kcal/mol)
for abstraction to take place using 2,4,6-tri-tert-butylphenoxyl radical.
(b)
(c)
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Figure 3. (a) Reconstitution of the bis(imino)pyridine and ammonia
ligands by protonation and solid-state structure of 1-(NH3)2OTf at
30% probability ellipsoids. Most hydrogen atoms were omitted for
clarity. X-band EPR spectra of (b) 2-(NH2)2 and (c) 1-(NH3)2OTf
recorded at 23 ºC in toluene solution (see SI for fitting parameters).
The observed reversible chelate modification chemistry and hydro-
gen evolution from the bis(imino) pyridine molybdenum ammine
complexes prompted computation of the thermodynamics of N-H
bond activation during the conversion of 1-(NH3)2(η2-C2H4) to 2-
(NH2)2. With 1-(NH3)2(η2-C2H4), an ammine N-H BDFE of 51.0
(58.8) kcal/mol was computed using density functional theory (DFT),
where the two values presented were obtained from generation of low-
and high-spin products following H-atom abstraction. Notably, these
values weaken substantially to a range between 22.5 and 49.4 kcal/mol
upon dissociation of the olefin (Figure S14), suggesting that the puta-
tive 5-coordinate intermediate, 1-(NH3)2 is a non-classical ammine
complex,12 being thermodynamically unstable towards H2 loss, in this
case also subject to PDI ligand backbone modification. This is corrobo-
rated by the finding that the product complex 2-(NH2)2 features
considerably stronger DFT-computed amide N-H bonds (58.3 (66.4)
kcal/mol (basal); 66.4 (60.3) kcal/mol (apical)) as well as a PIA ligand
backbone C-H bond with a BDFE of 67.4 (60.2) kcal/mol (Figure
S14). These computational results suggest that the phenomenon of
bond weakening by coordination precedes N-H bond activation, set-
ting the thermodynamics in place to favor of the unique reactivity ob-
served and is consistent with the experimentally observed inhibition of
ammonia activation by excess arene or alkene.
Figure 4. (a) Nitride formation from coordinated ammonia and
solid-state structure of 1-(N)(η2-C2H4) at 30% probability ellip-
soids. Hydrogen atoms were omitted for clarity. (b) X-band EPR spec-
trum of 1-(N)(η2-C2H4) recorded at 23 ºC in toluene solution (see
SI for fitting parameters). (c) Spin density plot for 1-(N)(η2-C2H4)
obtained from a Mulliken population analysis in a full-molecule gas
phase DFT calculation at the B3LYP level of theory.
The solid-state structure of 1-(N)(η2-C2H4) was determined by
X-ray diffraction and confirms ethylene coordination (d(C=C) =
1.410(4) Å) as well as formation of the terminal nitride ligand
(d(Mo≡N) = 1.653(2) Å). The Mo≡N stretch was located at 1036
cm-1 in the solid-state infrared spectrum (KBr pellet) and shifts to 1004
cm-1 upon isotopic labeling with 15N. These values are comparable to
those reported by Cummins for Mo[N(tBu)Ar]3 (Ar = 3,5-C6H3Me2)
(ν(Mo≡N = 1042 cm-1; Mo≡15N = 1014 cm-1).21 The distortions to the
bond distances of the bis(imino)pyridine support one electron reduc-
tion to the radical anionic form of the ligand.22 Both magnetic meas-
urements and the X-band EPR spectrum recorded at 296 K established
an S = ½ ground state. The observed giso of 2.00 with a relatively small
A(95/97Mo) value of 21 MHz supports a ligand- rather than metal-based
SOMO (Figure 4b). The DFT computed spin density (Figure 4c) also
supports this electronic structure depiction of low-spin Mo(IV) with a
principally bis(imino)pyridine-based SOMO arising from one electron
reduction. These results highlight the versatility of the
bis(imino)pyridine molybdenum platform to accommodate both neu-
tral NH3 and terminal nitride coordination along with electronic ad-
justments of the redox-active chelate. Observation of the ligand-
centered radical in 1-(N)(η2-C2H4) is also unusual as most second
and third row transition metal complexes of bis(imino)pyridine ligands
adopted a closed-shell two-electron reduced form.23
The computationally predicted weak N-H bonds in 1-(NH3)2(η2-
C2H4) suggested that the synthesis of molybdenum nitrides from
coordinated ammonia by hydrogen atom abstraction (HAA) may be
plausible. Such a sequence is key for oxidation of ammonia but by and
large has remained elusive experimentally. Recently Mock and cowork-
ers reported abstraction of all three H-atoms in coordinated ammonia
in a cyclopentadienyl molybdenum compound by treatment with 2,4,6-
tri-tert-butylphenoxyl radical (tBu3ArOŸ).16 A molybdenum alkylimido
product was isolated from this sequence rather than the terminal ni-
tride. Nitrogen-carbon bond formation occurs with the phenoxy radi-
cal reagent at some point during the HAA sequence; the terminal ni-
tride was ultimately synthesized by subsequent reduction with KC8.
Direct synthesis of a terminal metal-nitride from coordinated ammonia
remains elusive by HAA and was targeted with 1-(NH3)2(η2-
C2H4).8e,17 The dimeric chromium complex, [(η5-C5Me5)Cr(CO)3]2,
was chosen initially for HAA because the bond strength of the resulting
metal-hydride product (BDFECr-H = 62 kcal/mol) should provide suffi-
cient thermodynamic driving force for HAA.18 However, upon treat-
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Am. Chem. Soc. 2014, 136, 4640. (f) Robinson, T. P.; De Rosa, D. M.; Al-
In summary, molybdenum ammine complexes have been synthe-
sized with both a chemically- and redox-active bis(imino)pyridine
ligand that promotes the reversible N-H activation of NH3 upon alkene
dissociation Coordination induced bond weakening of the ammine
ligands was observed, that in combination with ligand cooperativity,
enables H2 loss and provides a coordination environment that supports
both ammonia and nitride ligands. Efforts are now directed at nitride
coupling and N2 evolution24 to complete the ammonia oxidation se-
quence.
dridge, S.; Goicoechea, J. M. Angew. Chem. Int. Ed. 2015, 54, 13758. (g) Jana,
A.; Schulzke, C.; Roesky, H. W. J. Am. Chem. Soc. 2009, 131, 4600.
(11) Margulieux, G. W.; Turner, Z. R.; Chirik, P. J. Angew. Chem. Int. Ed.
2014, 53, 14211.
(12) Bezdek, M. J.; Guo, S.; Chirik, P. J. Science 2016, 354, 730.
(13) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961.
(14) Russell, S. K.; Bowman, A. C.; Lobkovsky, E.; Wieghardt, K.; Chirik, P. J.
Eur. J. Inorg. Chem. 2012, 535.
(15) (a) Meiere, S. H.; Keane, J. M.; Gunnoe, T. B.; Sabat, M.; Harman, W. D. J.
Am. Chem. Soc. 2003, 125, 2024. (b) For related examples of benzene coordi-
nation in bimetallic molybdenum complexes, see: Carrasco, M.; Curado, N.;
Maya, C.; Peloso, R.; Rodríguez, A.; Ruiz, E.; Alvarez, S.; Carmona, E. Angew.
Chem., Int. Ed. 2013, 52, 3227; (c) Kajita, Y.; Ogawa, T.; Matsumoto, J.; Ma-
suda, H. Inorg. Chem. 2009, 48, 9069. For related examples of arene coordina-
tion in molybdenum complexes supported by terphenyl-type diphosphine
ligands see: (d) Buss, J. A.; Agapie, T. J. Am. Chem. Soc. 2016, 138, 16466; (e)
Henthorn, J. T.; Lin, S.; Agapie, T. J. Am. Chem. Soc. 2015 , 137, 1458; (f)
Buss, J. A.; Edouard, G. A.; Cheng, C.; Shi, J.; Agapie, T. J. Am. Chem. Soc.
2014, 136, 11272.
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Supporting Information
Experimental details, characterization data, NMR spectra, and crystal-
lographic information. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
(16) Bhattacharya, P.; Heiden, Z. M.; Wiedner, E. S.; Raugei, S.; Piro, N. S.;
Kassel, W. S.; Bullock, R. M.; Mock, M. T. J. Am. Chem. Soc. 2017, 139, 2916.
(17) For examples of terminal transition metal nitride synthesis from coordinat-
ed ammine ligands employing other oxidants see (a) Eikey, R. A.; Abu-Omar,
M. M. Coord. Chem. Rev. 2003, 243, 83 and references therein; (b) Leung, S.
K. Y.; Haung, J. S.; Liang, J. L.; Che, C. M.; Zhou, Z. Y. Angew. Chem., Int. Ed.
2003, 42, 340.
(18) (a) Kiss, G.; Zhang, K.; Mukerjee, S. L.; Hoff, C. D.; Roper, G. C. J. Am.
Chem. Soc. 1990, 112, 5657; (b) Tang, L.; Papish, E. T.; Abramo, G. P.; Nor-
ton, J. R.; Baik, M.-H.; Friesner, R. A.; Rappé, A. J. Am. Chem. Soc. 2003, 125,
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(19) Pappas, I.; Chirik, P. J. J. Am. Chem. Soc. 2016, 138, 13379.
(20) Mader, E. A.; Manner, V. W.; Markle, T. F.; Wu, A.; Franz, J. A.; Mayer, J.
M. J. Am. Chem. Soc. 2009, 131, 4335.
(21) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.;
Kim, E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem.
Soc. 1996, 118, 8623.
Corresponding Author
pchirik@princeton.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
We thank the U. S. Department of Energy, Office of Science, Basic
Energy Science (grant DE-SC000066498). M.J.B. thanks the Natural
Sciences and Engineering Research Council of Canada for a predoctor-
al fellowship (PGS-D). Z.R.T thanks the US-UK Fulbright Commis-
sion for a fellowship. We thank Brian A. Schaefer for assistance with
fitting the EPR spectra of 1-(NH3)2OTf and 2-(NH2)2.
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Bowman, A. C.; Milsmann, C.; Bill, E.; Lobkovsky, E.; Weyhermüller, T.;
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(23) Schoffel, J.; Rogachev, A. Y.; George, S. D.; Burger, P. Angew. Chem. Int.
Ed. 2009, 48, 4734.
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