Pyridine(diimine) Chelate Hydrogenation in a Molybdenum Nitrido Ethylene Complex

Article abstract of DOI:10.1021/acs.organomet.9b00095

Addition of H₂ gas to the aryl-substituted pyridine(diimine) molybdenum nitride (^iPrPDI)Mo(N)(C₂H₄) ([1-(N)(η²-C₂H₄)]; ^iPrPDI = 2,6-(2,6-iPr₂-C₆H

Full text of DOI:10.1021/acs.organomet.9b00095

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Pyridine(diimine) Chelate Hydrogenation in a Molybdenum Nitrido
Ethylene Complex
́
́
Mate J. Bezdek and Paul J. Chirik*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
S
* Supporting Information
ABSTRACT: Addition of H2 gas to the aryl-substituted
pyridine(diimine) molybdenum nitride (iPrPDI)Mo(N)-
(C2H4) ([1-(N)(η2-C2H4)]; iPrPDI = 2,6-(2,6-iPr2-C6H3N
CMe)2C5H3N) in the presence of the rhodium hydride
precatalyst (η5-C5Me5)(py-Ph)Rh(H) ([Rh−H]; py-Ph = 2-
phenylpyridine) resulted in partial hydrogenation of the
central pyridine of the iPrPDI chelate to yield (iPrTHPDI)-
Mo(N)(C2H4) ([2-(N)(η2-C2H4)]; iPrTHPDI = 2,6-(2,6-
iPr2-C6H3NCMe)2C5H6N). The product, [2-(N)(η2-C2H4)], was structurally and spectroscopically characterized, and its
electronic structure was examined by density functional theory (DFT). The stepwise addition of H atoms from [Rh−H] to [1-
(N)(η2-C2H4)] was computed to be thermodynamically viable by DFT.
nitially prepared as modular terpyridine mimics, pyridine-
(diimines) (PDIs) have emerged as a privileged ligand class
Scheme 1. Selected PDI Ligand Modifications: (a) Imine C-
Alkylation;18 (b) Pyridine N-Alkylation;19 (c) Double Imine
C-Alkylation;20 (d) H-Atom Migration;21 (e)
I
in homogeneous catalysis, particularly with Earth-abundant
transition metals.1 Following independent reports from
Brookhart,2 Bennett,3 and Gibson4 describing high-activity
PDI iron(II) and cobalt(II) dihalide catalyst/methylaluminox-
ane (MAO) activator combinations for ethylene polymer-
ization,5 PDI ligands have since been applied in the
cycloaddition and hydrofunctionalization of olefins,6 carbonyl
hydrosilylation,7 and C−H functionalization8 as well as small-
molecule activation including N2,9 NH3,10 O2,11 and CO2.12
The rich reaction chemistry observed with transition-metal,
main-group,13 and f-block14 elements bearing PDI ligands can
often be traced to their chemical and electronic versatility,
owing to their relative ease of synthesis15 and the accessibility
of low-lying π* orbitals of a2 and b2 symmetry.16 As a
consequence, PDI ligands can exhibit both redox activity and
noninnocence by either engaging in reversible 1e− redox
chemistry with a metal center or serving as traditional π-acids
with a high degree of covalency, respectively.17 PDI ligands are
also chemically noninnocent and can undergo alkylation,18−20
H atom migration,21 oxidation,11a deprotonation, and,
consequently, dimerization chemistry upon treatment with
nucleophiles and bases (Scheme 1a−g).18,22,23 While ligand
modification chemistry arising from electrophilic sites and
acidic C−H bonds can be detrimental to the catalytic
performance of PDI complexes, these features can also be
leveraged to achieve productive reactivity such as the activation
of inert small molecules by metal−ligand cooperation (Scheme
1h).9d Therefore, identifying new ligand modification pathways
continues to be of interest both in the context of mechanistic
investigations and for the development of next-generation
catalysts featuring PDI ligands.
Deprotonation−Dimerization;22b,c (f) Alkylation−
Dimerization;18,22a,23 (g) Oxidation;11a (h) Metal−Ligand
Cooperation;9d (i) the Pathway Reported in This Work
structures. For example, addition of NH3 to the aryl-
substituted PDI dinitrogen complex [{(iPrBPDI)Mo(N2)}2-
(μ2,η1,η1-N2)] (iPrBPDI = 2,6-(2,6-iPr2-C6H3N
CPh)2C5H3N) resulted in imine NimineCimine bond cleavage
and conversion to a terminal aryl imide, where ammonia
Molybdenum complexes supported by PDI ligands exhibit
ligand modification chemistry as well as unusual electronic
Received: February 13, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00095
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
served as the irreversible source of hydrogen (Scheme 1h).9d
More recently, our group reported the PDI molybdenum
nitride [(iPrPDI)Mo(N)(C2H4)] ([1-(N)(η2-C2H4)]; iPrPDI =
2,6-(2,6-iPr2-C6H3NCMe)2C5H3N), which can be prepared
from coordinated NH3 and exhibits a ligand-centered singly
occupied molecular orbital (SOMO).10 While pyridine-
(diimine)-based radicals are commonplace with first-row
transition metals, PDIs more frequently adopt a closed-shell,
dianionic form with second- and third-row metals,24,25
highlighting the unusual electronic structure of [1-(N)(η2-
C2H4)]. Here we describe an extension of these studies and
report a distinct PDI modification pathway involving partial
hydrogenation of the pyridine ring (Scheme 1i). The
molecular and electronic structures of the molybdenum
complex bearing the hydrogenated chelate were determined.
As part of our laboratory’s ongoing efforts to explore the
proton-coupled electron transfer (PCET)26 reactivity of
molybdenum complexes supported by potentially redox active
ligands,10,27 the reactivity of [1-(N)(η2-C2H4)] with the
known hydrogen atom donor [(η5-C5Me5)(py-Ph)Rh(H)]
([Rh−H]; py-Ph = 2-phenylpyridine)28 was examined.
Addition of 3 equiv of the rhodium hydride [Rh−H] to [1-
(N)(η2-C2H4)] in THF solution resulted in a color change
from orange-red to green over the course of 15 min at room
C2H4)] exhibits the number of resonances consistent with Cs
molecular symmetry, as indicated by four distinct iPr methyl
resonances at 1.43, 1.19, 1.01, and 0.98 ppm. The partial
hydrogenation of the iPrPDI chelate was identified by the lack
of diagnostic downfield C−H resonances assignable to the 3-,
4-, and 5-pyridine positions along with the concomitant
appearance of signals in the aliphatic region (2.01−1.21 ppm).
Also consistent with Cs molecular symmetry was the
observation of a doublet of doublets in the 13C NMR spectrum
of [2-(N)(η2-C2H4)] at 51.71 ppm assigned to the ethylene
carbons (1JC−H = 156.7, 149.2 Hz). Analysis of [2-(15N)(η2-
C2H4)], prepared from hydrogenation of [1-(15N)(η2-C2H4)],
by 15N{1H} NMR spectroscopy revealed a singlet at +1040
ppm, consistent with the presence of a terminal molybdenum
nitride.29
The solid-state structure of [2-(N)(η2-C2H4)] was deter-
mined by single-crystal X-ray diffraction and confirmed
coordination of ethylene and nitride ligands (d(CC) =
1.420(3) Å; d(MoN) = 1.644(2) Å) as well as the partial
hydrogenation of the central pyridine of the iPrPDI chelate.
Ligand reduction was also evidenced by structural distortion
with the lowering of the 4-pyridine carbon below the chelate
plane by 0.654 Å (Figure 1b), a geometry expected from the
generation of three sp3 carbon centers in [2-(N)(η2-C2H4)].
Remarkably, reduction of the ethylene and imine ligands was
not observed despite ample precedent for these functionalities
to undergo Rh-catalyzed hydrogenation.30
1
temperature. Monitoring the reaction by H NMR spectros-
copy revealed the formation of a diamagnetic product
identified as [(iPrTHPDI)Mo(N)(C2H4)] ([2-(N)(η2-
C2H4)]; iPrTHPDI = 2,6-(2,6-iPr2-C6H3NCMe)2C5H6N),
the product of three H-atom additions from [Rh−H] to the
iPrPDI chelate in [1-(N)(η2-C2H4)] (Figure 1). The benzene-
d6 1H NMR spectrum of blue (λmax = 625 nm) [2-(N)(η2-
The unusual chelate modification in [2-(N)(η2-C2H4)]
prompted determination of the electronic structure of the
complex and assignment of the redox state of the new ligand.
While the diamagnetism of [2-(N)(η2-C2H4)] limits the
number of experimental observables for an electronic structure
determination, DFT computations were nevertheless carried
out to investigate the oxidation state of the molybdenum and
the corresponding redox state of the iPrTHPDI ligand.
Calculations were conducted at the B3LYP level of theory
and used to construct a qualitative molecular orbital diagram
for [2-(N)(η2-C2H4)]. As shown in Figure 2, the highest
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of [2-(N)(η2-C2H4)]
are iPrTHPDI ligand based π* orbitals of a′′ and a′ symmetry,
respectively, supporting a closed-shell, anionic description for
iPrTHPDI.31 HOMO-7 and HOMO-8 exhibit molybdenum dyz
Figure 1. (a) Partial hydrogenation of the pyridine(diimine) chelate
in [1-(N)(η2-C2H4)] by stoichiometric or catalytic reaction with
[Rh−H]. (b) Solid-state structure of [2-(N)(η2-C2H4)] with
ellipsoids at 30% probability. Hydrogen atoms (except those
connected to C4, C5, and C6) have been omitted for clarity. Selected
bond distances (Å): Mo1−N1 2.215(2), Mo1−N2 2.171(2), Mo1−
N3 2.214(2), Mo1−N4 1.644(2), C34−C35 1.420(3), N1−C2
1.326(2), C2−C3 1.423(2), C3−C4 1.501(3), C4−C5 1.524(3),
C5−C6 1.526(3), C6−C7 1.497(3), C7−C8 1.419(3), N3−C8
1.323(2).
Figure 2. Molecular orbital illustrations and populations for [2-
(N)(η2-C2H4)] obtained from a spin-restricted DFT calculation at the
B3LYP level of theory. The z axis is defined as the MoN vector.
iPrTHPDI symmetry designations are from the Cs point group. L =
ligands.
B
DOI: 10.1021/acs.organomet.9b00095
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Organometallics
Communication
and dxz character engaged in π-bonding with nitrogen py and px
orbitals, respectively, consistent with a nitrido MoN triple
bond. Finally, HOMO-1 exhibits significant ethylene π*
character, consistent with the elongated ethylene (CC)
bond length of 1.420(3) Å observed in the solid-state structure
of [2-(N)(η2-C2H4)]. This value is statistically indistinguish-
able from the ethylene (CC) bond length of 1.425(6) Å
reported for the cationic molybdenum ethylene complex
[(PhTpy)(PPh2Me)2Mo(C2H4)][BArF]24 (PhTpy = 4′-Ph-
2,2′,6′,2″-terpyridine, ArF24 = [C6H3-3,5-(CF3)2]4) that
exhibits EPR spectroscopic features consistent with a Mo(III)
metallacyclopropane.27d Furthermore, the CH coupling
constants (1JC−H = 156.7, 149.2 Hz) observed for coordinated
ethylene in the 13C NMR spectrum of [2-(N)(η2-C2H4)] are
in the range reported for methylene carbons of ethyl (147.3
Hz)27d and “alkylidene-like” η2-vinyl (160 Hz)32 ligands of
molybdenum. Therefore, the electronic structure of the
complex is likely best described as a Mo(VI) metal-
lacyclopropane nitride supported by a singly reduced iPrTHP-
DI ligand. In this electronic structure description, (iPrTHPDI)−
is isoeletronic with anionic dipyridylazaallyl ligands which
exhibit a rich coordination chemistry with a host of transition
metals.33 Localized orbital bonding analysis (LOBA)34 was
performed by DFT and also supports a 6+ oxidation state
assignment at the molybdenum in [2-(N)(η2-C2H4)].
Figure 3. Plausible H-atom addition pathways in [1-(N)(η2-C2H4)].
Table 1. DFT-Computed C−H BDFEs for Sequential H-
Atom Addition in [1-(N)(η2-C2H4)]
a
complex
[3-H1]
pyridine position level of theory C−H BDFE
3
B3LYP
50
52
48
50
60
59
58
57
32
32
69
70
94
96
PW6B95
B3LYP
[4-H1]
4
PW6B95
B3LYP
[3,4-H2]
3
PW6B95
B3LYP
4
Because the complex [2-(N)(η2-C2H4)] was the only
product observed by 1H NMR spectroscopy from the reaction
of [1-(N)(η2-C2H4)] with [Rh−H], the fate of the Rh
complex was investigated by EPR spectroscopy. A single
isotropic signal at g = 2.070 was observed at the completion of
the reaction in toluene solution at room temperature,
consistent with the formation of the paramagnetic Rh(II)
complex [(η5-C5Me5)(py-Ph)Rh] ([Rh]), the product of H-
atom loss from [Rh−H].35 Because [Rh] is known to react
with H2 gas to regenerate the rhodium hydride starting
material, [Rh−H] was next employed in catalytic amounts
using H2 gas as the terminal reductant. Indeed, conducting the
reaction using 5 mol % of [Rh−H] under 4 atm of H2 in THF
generated [2-(N)(η2-C2H4)] in quantitative NMR yield after
72 h at room temperature.36 To investigate whether related
molybdenum iPrPDI complexes exhibit similar reactivity,
[(iPrPDI)MoCl3], [(iPrPDI)Mo(η6-C6H6)], and [(iPrPDI)Mo-
(CO)4] were treated with 25 mol % of [Rh−H] and stirred
under 4 atm of H2. However, in each case, no reaction was
observed for up to 6 days at room temperature, and intractable
product mixtures were obtained upon heating to 60 °C for 18
h. These results suggest that [1-(N)(η2-C2H4)] is uniquely
suited for well-defined hydrogenation chemistry with [Rh−H]
and is likely due to the stability of the molybdenum−nitrido−
ethylene core. Consequently, reactivity takes place at the
iPrPDI ligand, resulting in a Mo(VI) oxidation state in the final
product.
PW6B95
B3LYP
PW6B95
B3LYP
PW6B95
B3LYP
[3,5-H2]
3/5
3/5
4
[2-(N)(η2-C2H4)]
PW6B95
a
Values computed in kcal mol−1 in the gas phase.
mol−1 (BDFERh−H = 52 kcal mol−1).38 In contrast, consecutive
formation of C−H bonds at the 3- and 5-pyridine positions
was computed to be endergonic by ∼20 kcal mol−1 leading to
intermediate [3,5-H2] (BDFEC−H = 32 kcal mol−1) and is
likely disfavored (Figure 3, dashed arrow). These computa-
tional results suggest that PCET from [Rh−H] to the
pyridine(diimine) chelate in [1-(N)(η2-C2H4)] is thermody-
namically feasible for each H-atom addition step. However, it is
important to note that the initial site of H-atom addition to [1-
(N)(η2-C2H4)] is unknown, as reaction intermediates were not
observed during the course of the hydrogenation. Therefore,
the structures depicted in Figure 3 can, in principle, be
accessed by intramolecular hydrogen atom transfer in analogy
to intramolecular alkyl group migrations observed in PDI
ligands.39 In addition, the possibility that a PCET pathway is
circumvented altogether by pyridine (C−C) insertion into the
rhodium−hydrogen bond cannot be rigorously excluded.
In summary, partial hydrogenation of a pyridine(diimine)
chelate was demonstrated in a molybdenum nitride and the
thermodynamics of a plausible C−H bond forming PCET
pathway were examined. The transformation described herein
provides unique insight into the hydrogenation chemistry of a
molybdenum nitride bearing a redox-active ligand.
Given that the coordinatively saturated, 18e− complex [Rh−
H] is an effective H-atom donor that is known to react by
PCET,35,37 DFT computations were conducted in order to
probe the thermodynamics of a plausible C−H bond forming
PCET pathway leading to the formation of [2-(N)(η2-C2H4)].
As presented in Figure 3 and Table 1, sequential addition of H
atoms from [Rh−H] was computed at both the B3LYP and
PW6B95 levels of theory to be approximately thermoneutral or
exergonic for plausible PCET pathways, forming the
intermediates [3-H1], [4-H1], and [3,4-H2] with C−H bond
dissociation free energies (BDFEs) in the range of 48−60 kcal
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00095.
■
S
C
DOI: 10.1021/acs.organomet.9b00095
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Experimental details, characterization data including
Communication
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NMR spectra of complexes, and computational methods
and results (PDF)
Computed coordinates for [1-(N)(η2-C2H4)], [3-H1],
[4-H1], [3,4-H2], [3,5-H2], and [2-(N)(η2-C2H4)]
(XYZ)
Accession Codes
CCDC 1894932 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for P.J.C.: pchirik@princeton.edu.
■
ORCID
́
́
Mate J. Bezdek: 0000-0001-7860-2894
Paul J. Chirik: 0000-0001-8473-2898
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
under Award DE-SC0006498. M.J.B. thanks the Natural
Sciences and Engineering Research Council of Canada for a
predoctoral fellowship (PGS-D) and Princeton University for a
Porter Ogden Jacobus Honorific Fellowship. We thank Dr.
́
Istvan Pelczer (Princeton) for assistance with the acquisition of
15N NMR data.
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