Coordination-Induced N-H Bond Weakening in a Molybdenum Pyrrolidine Complex: Isotopic Labeling Provides Insight into the Pathway for H2Evolution

Article abstract of DOI:10.1021/acs.organomet.0c00471

The synthesis and characterization of a cationic molybdenum pyrrolidine complex are described that exhibits significant coordination-induced N-H bond weakening. The N-H bond dissociation free energy (BDFE) of the coordinated pyrrolidine in [(PhTpy)(PPh2Me)2Mo(NH(pyrr))][BArF24] ([1-NH(pyrr)]+PhTpy = 4′-Ph-2,2′,6′,2″-terpyridine, NH(pyrr) = pyrrolidine, ArF24 = [C6H3-3,5-(CF3)2]4) was determined to be between 41 and 51 kcal mol-1 by thermochemical analysis and supported by a density functional theory (DFT) computed value of 48 kcal mol-1. The complex [1-NH(pyrr)]+ underwent proton-coupled electron transfer (PCET) to 2,4,6-tri-tert-butylphenoxyl radical, as well as spontaneous H2 evolution upon gentle heating to furnish the corresponding molybdenum pyrrolidide complex [(PhTpy)(PPh2Me)2Mo(N(pyrr))][BArF24] ([1-N(pyrr)]+). Thermolysis of the deuterated isotopologue [1-ND(pyrr)]+ still produced H2 with concomitant incorporation of the isotopic label into the pyrrolidide ligand in the product [(1-N(pyrr-dn)]+ (n = 0-2), consistent with an H2 evolution pathway involving intramolecular H-H bond formation followed by an intermolecular product-forming PCET step. These observations provide the context for understanding H2 evolution in the nonclassical ammine complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+) and are supported by DFT-computed reaction thermochemistry. Overall, these studies offer rare insight into the H2 formation pathway in nonclassical amine complexes with N-H BDFEs below the thermodynamic threshold for H2 evolution and inform the development of well-defined, thermodynamically potent PCET reagents.

Full text of DOI:10.1021/acs.organomet.0c00471

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Article
Coordination-Induced N−H Bond Weakening in a Molybdenum
Pyrrolidine Complex: Isotopic Labeling Provides Insight into the
Pathway for H2 Evolution
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Mate J. Bezdek, Istvan Pelczer, and Paul J. Chirik*
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ABSTRACT: The synthesis and characterization of a cationic
molybdenum pyrrolidine complex are described that exhibits
significant coordination-induced N−H bond weakening. The N−H
bond dissociation free energy (BDFE) of the coordinated
pyrrolidine in [(PhTpy)(PPh2Me)2Mo(NH(pyrr))][BArF24] ([1-
NH(pyrr)]+; PhTpy = 4′-Ph-2,2′,6′,2″-terpyridine, NH(pyrr) =
pyrrolidine, ArF24 = [C6H3-3,5-(CF3)2]4) was determined to be
between 41 and 51 kcal mol−1 by thermochemical analysis and supported by a density functional theory (DFT) computed value of
48 kcal mol−1. The complex [1-NH(pyrr)]+ underwent proton-coupled electron transfer (PCET) to 2,4,6-tri-tert-butylphenoxyl
radical, as well as spontaneous H2 evolution upon gentle heating to furnish the corresponding molybdenum pyrrolidide complex
[(PhTpy)(PPh2Me)2Mo(N(pyrr))][BArF24] ([1-N(pyrr)]+). Thermolysis of the deuterated isotopologue [1-ND(pyrr)]+ still
produced H2 with concomitant incorporation of the isotopic label into the pyrrolidide ligand in the product [(1-N(pyrr-dn)]+ (n =
0−2), consistent with an H2 evolution pathway involving intramolecular H−H bond formation followed by an intermolecular
product-forming PCET step. These observations provide the context for understanding H2 evolution in the nonclassical ammine
complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+) and are supported by DFT-computed reaction thermochemistry.
Overall, these studies offer rare insight into the H2 formation pathway in nonclassical amine complexes with N−H BDFEs below the
thermodynamic threshold for H2 evolution and inform the development of well-defined, thermodynamically potent PCET reagents.
epoxides, olefins, and enamines.8,9 For instance, Cuerva19,20
INTRODUCTION
■
21
and later Gansauer demonstrated that (η5-C5H5)2TiCl2, in
̈
Coordination of a ligand to a transition metal, a foundational
principle in inorganic chemistry, fundamentally alters the
bonding properties of the ligand including lowering the
homolytic X−H (X = C, N, O) bond dissociation free
energies (BDFEs).1 Termed coordination-induced bond
weakening, this phenomenon is key to understanding the
thermochemistry of proton-coupled electron transfer (PCET)
processes2−4 in transition-metal complexes and has implica-
tions in biology,5−7 organic synthesis8,9 as well as energy
science.10−12 In contrast to the extensive thermochemical data
available for organic molecules, systematic reports of analogous
BDFE data for ligand X−H bonds (X = C, metal, N, O, P)
have only recently emerged and remain scarce by compar-
ison.1,13−17
combination with a manganese reductant in THF/water
mixtures, forms [(η5-C5H5)2Ti(H2O)m(THF)n]+ complexes
and promote the reductive ring opening of epoxides enabled by
the O−H bond weakening in water by over 60 kcal mol−1
(Scheme 1a, left). Similarly, Flowers22 and Mayer23 reported
that SmI2(H2O)n mixtures are effective for achieving
thermodynamically challenging reductions of anthracene and
enamines, respectively, with an estimated O−H BDFE of 26
kcal mol−1 resulting from the coordination of H2O to
samarium(II) (Scheme 1a, right). Recently, Nishibayashi
applied SmI2(H2O)n to catalytic nitrogen fixation, where
SmI2(H2O)n was used to deliver H atom equivalents to a
reduced molybdenum dinitrogen precatalyst, ultimately
furnishing free ammonia.24 Interest therefore remains in the
development of well-defined, yet thermodynamically potent
PCET reagents with low X−H BDFEs.
In rare instances, coordination-induced bond weakening has
been demonstrated or proposed to have a dramatic impact on
X−H BDFEs that are quantified in terms of a 1e− redox couple
(E°), X−H pKa, and the solvent-specific H+ standard reduction
potential (CG; eq 1):18
Received: July 11, 2020
BDFEX−H = 1.37pKa + 23.06E° + CG
(1)
Metal−ligand combinations with low X−H BDFEs are
desirable in synthetic chemistry with application as potent H
atom donors to enable reductive PCET with substrates such as
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would eliminate this competing process, as the resulting N,N-
dialkyl amide lacks a site for exchange (Scheme 1c).
Pyrrolidine was selected as a representative secondary amine,
as coordination to the cationic terpyridine bis(phosphine)
molybdenum(I) complex would likely result in N−H bond
weakening on the order of that observed for ammonia and
should give rise to analogous H2 evolution to yield the
corresponding secondary alkyl amido product (Scheme 1c). In
addition, pyrrolidide α-(C−H) bonds are well poised to
undergo β-H elimination, thereby reporting on potential Mo−
D intermediates generated during dehydrogenation.
Scheme 1
Here we describe the synthesis and characterization of the
targeted cationic molybdenum pyrrolidine complex [(PhTpy)-
(PPh2Me)2Mo(NH(pyrr))][BArF24] ([1-NH(pyrr)]+; NH-
(pyrr) = pyrrolidine) and explore its electronic structure,
thermochemical properties, and H2 evolution reactivity.
Deuterium labeling studies with the N−D isotopologue are
reported that provide insight into the H2 evolution pathway in
[1-NH(pyrr)]+ and provide a context for understanding the
dehydrogenation reactivity of the related ammine complex [1-
NH3]+.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Molybdenum
Pyrrolidine Complex [1-NH(pyrr)]+. Our studies com-
menced with the synthesis of the terpyridine bis(phosphine)
molybdenum pyrrolidine complex [1-NH(pyrr)]+. Stirring a
benzene solution containing [(PhTpy)(PPh2Me)2MoCl] ([1-
Cl])40 and 1 equiv of both Na[BArF24] and pyrrolidine at
room temperature for 18 h furnished a dark green solid in 85%
yield identified as [1-NH(pyrr)]+ (Figure 1a). The solid-state
infrared spectrum (KBr) of [1-NH(pyrr)]+ exhibits an
isotopically sensitive N−H vibration at 3246 cm−1 that shifts
to 2400 cm−1 in the d1 isotopologue [1-ND(pyrr)]+,
consistent with the coordination of pyrrolidine upon chloride
Our laboratory recently reported the synthesis of an isolable
nonclassical ammine complex, [(PhTpy)(PPh2Me)2Mo-
(NH3)][BArF24] ([1-NH3]+; PhTpy = 4′-Ph-2,2′,6′,2″-terpyr-
idine, ArF24 = [C6H3-3,5-(CF3)2]4), that exhibits a remarkably
low N−H BDFE of 46 kcal mol−1 in the coordinated ammonia,
below the thermodynamic threshold for spontaneous hydrogen
evolution (ΔGf°(H•) = 49 kcal mol−1, gas phase).25 The
ammine complex [1-NH3]+ is a competent PCET reagent that
has been shown to deliver H atom equivalents to acceptors
such as styrene as well as the molybdenum imido and ethylene
complexes [(PhTpy)(PPh2Me)2Mo(NtBuAr)][BArF24] ([1-
(NtBuAr)]+; tBuAr = 4-tert-butyl-C6H4) and [(PhTpy)-
(PPh2Me)2Mo(C2H4)][BArF24] ([1-(C2H4)]+), to furnish
ethylbenzene or the corresponding molybdenum amido and
ethyl complexes [1-(NHtBuAr)]+ and [1-(CH2CH3)]+,
respectively (Scheme 1b, Path A).26,27 In contrast to
SmI2(H2O)n, mild thermolysis of [1-NH3]+ at 60 °C resulted
in H2 evolution with concomitant generation of the
corresponding molybdenum amido complex [1-NH2]+
(Scheme 1b, Path B). While thermochemical studies suggest
that PCET from [1-NH3]+ occurs either by electron transfer
followed by proton transfer or by a concerted pathway,26,27 to
date there has been little experimental study into the pathway
for H2 evolution. Gaining insight into the mechanism of H2
loss from [1-NH3]+ is therefore of interest to guide the
development of kinetically stable yet thermodynamically
potent PCET reagents as well as to provide design principles
for bond activation in the context of NH3 oxidation13l,28−36
and catalysis involving metal−ligand cooperation.37−39
Following the discovery of [1-NH3]+, important questions
immediately arose concerning the molecularity of the H2
evolution step. While attempts were made to address this
question with deuterium-labeling experiments, the exchange-
able N−H bonds present in both the molybdenum ammine
starting material and amido product complicated an inter-
pretation of the results.25 Coordination of a secondary amine
Figure 1. (a) Synthesis of [1-NH(pyrr)]+ by chloride abstraction. (b)
X-band EPR spectrum of [1-NH(pyrr)]+ recorded in toluene glass at
8 K. Collection and simulation parameters: microwave frequency
9.381 GHz, power 2.0 mW, modulation amplitude 4.0 G; gx = 2.020,
gy = 2.005, gz = 1.968. (c) DFT-computed spin density plot for [1-
NH(pyrr)]+ obtained from Mulliken population analysis in the gas
phase at the B3LYP level of theory.
B
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abstraction. The formally Mo(I) complex [1-NH(pyrr)]+ has
an S = 1/2 ground state, as evidenced by a solid-state magnetic
moment of 1.7(2) μB (Guoy balance, 23 °C). The EPR
spectrum of [1-NH(pyrr)]+ was collected in toluene glass at 8
K and exhibits a rhombic signal that was readily simulated with
the g values gx = 2.020, gy = 2.005, and gz = 1.968 (Figure 1b).
This EPR spectrum closely resembles the rhombic signal
observed for [1-NH3]+ at 7 K (see the Supporting
Information), suggesting that the electronic structures of the
parent ammine and pyrrolidine complexes are analogous.
Accordingly, the DFT-computed Mulliken spin density plot for
[1-NH(pyrr)]+ supports a molybdenum-centered singly
occupied molecular orbital (SOMO, Figure 1c) in agreement
with the electronic structure previously determined for [1-
NH3]+.25
Determination of the N−H BDFE of Coordinated
Pyrrolidine in [1-NH(pyrr)]+. Efforts were next directed at
experimentally establishing the N−H BDFE in [1-NH(pyrr)]+.
Homolytic cleavage of the pyrrolidine N−H bond in [1-
NH(pyrr)]+ was accomplished by addition of 1 equiv of 2,4,6-
tri-tert-butylphenoxyl radical (tBu3ArO•) to generate the
diamagnetic molybdenum(II) pyrrolidide complex [(PhTpy)-
(PPh2Me)2Mo(NC4H8)][BArF24] ([1-N(pyrr)]+) in quanti-
tative yield within 30 min at room temperature (Scheme 2a,
of [1-NH(pyrr)]+ at 60 °C for 48 h furnished [1-N(pyrr)]+ in
98% yield with the concomitant formation of H2, as confirmed
by Toepler pump experiments (81% yield of H2). The
spontaneous dehydrogenation reactivity observed42 establishes
the upper bound of the N−H BDFE in [1-NH(pyrr)]+ as the
free energy of H• formation from H2 in benzene solution (∼51
kcal mol−1).1
The low upper bound for the N−H BDFE in [1-
NH(pyrr)]+ complicates an estimation of its absolute value,
given the lack of reference reagents near the thermodynamic
threshold for H2 evolution. Therefore, the N−H BDFE was
instead estimated using a thermochemical square scheme
(Scheme 2b). Because the electrochemistry of [1-NH(pyrr)]+
proved intractable, the cyclic voltammogram of [1-N(pyrr)]+
was collected to probe the thermodynamics of accessing the
one-electron-reduced pyrrolidide complex [1-N(pyrr)] that in
turn enables estimation of the E° and pKa terms in Scheme 2b.
The cyclic voltammogram (CV) of [1-N(pyrr)]+ was collected
in THF solution and exhibits two reversible anodic waves with
E
1/2 = −0.63 and 0.08 V vs Fc/Fc+ (Fc = [Cp2Fe]) assignable
to oxidations to yield [1-N(pyrr)]2+ and [1-N(pyrr)]3+,
respectively (Figure 2, black trace). In addition, a quasi-
Scheme 2. Synthesis of [1-N(pyrr)]+ and Thermochemical
Square Scheme Defining the N−H BDFE in [1-NH(pyrr)]+
Figure 2. Cyclic voltammograms of [1-N(pyrr)]+ (2.0 mM in THF,
black trace) and [1-N(pyrr)]+ in the presence of 1.0 equiv of [TBD-
H][BArF24] (2.0 mM, red trace) using a glassy-carbon working
electrode, a platinum-wire counter electrode, a silver-wire reference
electrode, 0.2 M [(n-Bu)4N][PF6], and a scan rate of 100 mV s−1 in
THF at 23 °C versus Fc/Fc+. TBD = triazabicyclodecene.
reversible cathodic wave was observed in the CV of [1-
N(pyrr)]+ at E1/2 = −2.10 V that corresponds to a reduction to
yield the neutral pyrrolidide complex [1-N(pyrr)] and was of
interest for estimating the N−H BDFE in [1-NH(pyrr)]+.
Attempts to synthesize [1-N(pyrr)] by one-electron
reduction of [1-N(pyrr)]+ afforded intractable product
mixtures, and as a result, the lower bound for the pKa of [1-
NH(pyrr)]+ was estimated by in situ protonation experiments
in the electrochemical cell. Upon collection of the CV of [1-
N(pyrr)]+ in the presence of 1 equiv of [TBDH][BArF24]
(TBD = triazabicyclodecene), the cathodic wave became
irreversible with a concomitant positive shift of the peak
cathodic potential from Epc = −2.17 V (no [TBDH][BArF24])
to −2.13 V (1.0 equiv of [TBDH][BArF24]) (Figure 2, red
trace). These observations are consistent with an EC
mechanism involving fast electron transfer (E) followed by
rate-limiting proton transfer (C) from [TBDH][BArF24] to in
situ generated [1-N(pyrr)].43,44 Evaluation of the peak
potential as a function of added acid allowed the determination
top). The benzene-d6 1H NMR spectrum of [1-N(pyrr)]+
exhibits the number of resonances consistent with a C2v-
symmetric molecule in solution with diagnostic pyrrolidide α-
and β-(C−H) signals at 4.04 and 1.75 ppm, respectively, and a
single 31P{1H} peak at 11.95 ppm that is consistent with trans-
PPh2Me ligands coordinated to cationic molybdenum(II)
amido.25,26 While the H atom abstraction reactivity with
tBu3ArO• establishes the upper bound of the N−H BDFE in
[1-NH(pyrr)]+ as that of the O−H bond in tBu3ArOH (77
kcal mol−1),41 the pyrrolidide product [1-N(pyrr)]+ was also
accessed by thermal dehydrogenation of the pyrrolidine
complex (Scheme 2a, bottom). Heating a benzene-d6 solution
C
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of the bimolecular proton transfer rate constant as kPT = 1.3 ×
103 M−1 s−1, a rare quantification of amido ligand protonation
kinetics (Figure 3; see the Supporting Information for
logues of H2, rapid H/D exchange equilibrates protons and
deuterons in the water product and the resulting H2O:HDO
ratio is expected to reflect the overall deuterium content,
including D2. Accordingly, the 1H NMR spectrum of the
combustion product established a H2O:HDO ratio of 6:1 and
therefore an overall H2:(HD + D2) ratio of 6:1 prior to
combustion (Figure S12; see the Supporting Information for
experimental details). Importantly, these results demonstrate
that <5% of D2 is evolved during the dehydrogenation of [1-
ND(pyrr)]+ and imply substantial deuterium incorporation in
the molybdenum pyrrolidide product denoted as [1-N(pyrr-
dn)]+ in Figure 4a.
To determine the fate of the deuterium label following
dehydrogenation of [1-ND(pyrr)]+, the quantification and
location of the deuterium content in the molybdenum
pyrrolidide product [1-N(pyrr-dn)]+ was pursued. Notably,
the 2H NMR spectrum of [1-N(pyrr-dn)]+ established
significant deuterium incorporation in the α-pyrrolidide
Figure 3. (a) Cyclic voltammograms of [1-N(pyrr)]+ (2.0 mM in
THF) in the presence of 0 M (black trace) to 1.2 mM (red trace) of
[TBD-H][BArF24] using a glassy-carbon working electrode, a
platinum-wire counter electrode, a silver-wire reference electrode,
0.2 M [nBu4N][PF6], and a scan rate of 100 mV s−1 in THF at 23 °C
versus Fc/Fc+. (b) Plot of peak cathodic potential (Epc) for the [1-
N(pyrr)]+/[1-N(pyrr)] wave as a function of ln([TBD-H]+). The
slope is equal to 0.0132 with R2 = 0.999. (c) Plot of kobs obtained as a
function of [TBD-H]+ concentration. The slope yields kPT = 1.3 × 103
M−1 s−1 with R2 = 0.996.
derivation). While the quasi-reversibility of the cathodic wave
in [1-N(pyrr)]+ precludes the extraction of equilibrium
parameters for the EC reaction, the observation of the rapid
protonation of in situ generated [1-N(pyrr)] by the weak acid
[TBDH][BArF24] implies that the pKa of this acid (21.0 in
THF)45 is likely a satisfactory lower bound for the pKa of [1-
NH(pyrr)]+.46 These data, in combination with the E° value
for the [1-N(pyrr)]+/[1-N(pyrr)] couple (E1/2 = −2.10 V),
define the lower bound for the N−H BDFE in [1-NH(pyrr)]+
as 41 kcal mol−1 in THF according to eq 1.47,48 Together with
the observation of spontaneous H2 evolution from the
complex, an experimental bound for the N−H BDFE of 41−
51 kcal mol−1 is obtained for [1-NH(pyrr)]+, in close
agreement with a DFT-computed value of 48 kcal mol−1
(gas phase). These data establish [1-NH(pyrr)]+ as a rare
example of a nonclassical amine complex that is thermody-
namically unstable with respect to H2 evolution.
Figure 4. (a) Isotope labeling experiment and pyrrolidide ligand
liberation in [1-N(pyrr-dn)]+. Legend: (a) for the reaction conditions,
see the Supporting Information. (b) Pyrrolidide α- and β-carbon
region of the 13C{1H} NMR spectrum (benzene-d6, 23 °C) of [1-
N(pyrr-dn)]+ exhibiting isotopic perturbation of the resonances. (c)
Quantitative 13C{1H} NMR spectrum (benzene-d6, 23 °C) of a
mixture containing pyrrolidine (50%), pyrrolidine-2-d1 (40%),
pyrrolidine-2,5-d2 (8%) and pyrrolidine-2-d2 (2%) liberated from
[1-N(pyrr-dn)]+ by protonolysis. The red symbols represent a fit of
the data, and the blue symbols show the deconvolution of the
individual peaks.
Isotopic Labeling Experiments. We next turned our
attention to deuterium labeling experiments to gain insight
into the pathway of H2 evolution from [1-NH(pyrr)]+. Mildly
heating the ND isotopologue [1-ND(pyrr)]+ at 60 °C for 48 h
1
and analyzing the evolved gases by H NMR spectroscopy
revealed the generation of H2 and HD gases in a 9:1 ratio. To
probe the amount of D2 formed, the evolved gases were passed
over CuO at 200 °C for 30 min, collected, and analyzed as the
isotopologues of H2O. Unlike the nonexchangeable isotopo-
D
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position (4.04 ppm), corroborated by 13C{1H} NMR spec-
troscopy, which revealed an isotopic perturbation of both α-
and β-pyrrolidide carbon resonances. In addition to the
expected triplet for the α-pyrrolidine carbons in [1-N(pyrr)]+
at 71.44 ppm (3JC−P = 11.6 Hz), a broad peak was observed at
71.05 ppm that corresponds to a deuterated α-pyrrolidide
carbon in [1-N(pyrr-d1)]+ that is broadened due to coupling
to deuterium as well as two equivalent phosphorus atoms in
the trans-PPh2Me ligands (Figure 4b). Additionally, three
distinct singlets were observed at 26.75, 26.63, and 26.51 ppm
corresponding to the pyrrolidide β-carbon signals of [1-
N(pyrr)]+, [1-N(pyrr-d1)]+ and [1-N(pyrr-d2)]+, respectively,
with isotopic shifts arising due to remote deuteration at the α-
pyrrolidide carbon (Figure 4b).
carbon signals in [1-N(pyrr-dn)]+ (26.75−26.51 ppm) by
quantitative 13C NMR spectroscopy established that the ratio
d0:(d1 + d2) pyrrolidide isotopologues remained constant over
the course of the dehydrogenation of [1-ND(pyrr)]+ (3−48 h,
60 °C), indicating that pyrrolidide ligand deuteration proceeds
more rapidly than [1-N(pyrr-dn)]+ formation. To eliminate the
possibility that the deuterium label in pyrrolidine-1-d1
scrambles rapidly upon coordination, a large excess (50
equiv) of PPh2Me was added to [1-ND(pyrr)]+ to displace
the pyrrolidine ligand, which enabled probing of its isotopic
composition at various stages of the dehydrogenation reaction.
Accordingly, [1-ND(pyrr)]+ was treated with 50 equiv of
PPh2Me and heated for 6 h at 60 °C, after which time a new
paramagnetic molybdenum complex was formed, as judged by
EPR spectroscopy (giso = 2.011, Figure S23), tentatively
assigned as the cationic terpyridine tris(diphenylmethyl-
phosphine) molybdenum complex [(PhTpy)(PPh2Me)3Mo]-
[BArF24] ([1-PPh2Me]+) (Scheme 3, top). Concurrently,
Because coupling to the two equivalent phosphorus atoms in
PPh2Me ligands complicated the assignment and quantification
of the pyrrolidide isotopomers and isotopologues present in
[1-N(pyrr-dn)]+, the pyrrolidide ligand was liberated from the
coordination sphere of molybdenum by protonolyis and
analyzed as free pyrrolidine. Accordingly, treatment of [1-
N(pyrr-dn)]+ with excess HCl generated [(PhTpy)-
(PPh2Me)2Mo(Cl)][BArF24] ([1-Cl]+) and pyrrolidinium-dn
Scheme 3. Displacement of the Pyrrolidine Ligand in [1-
ND(pyrr)]+ at Various Time Intervals
1
chloride (n = 0−2), as judged by H NMR spectroscopy.
Further treatment of the reaction mixture with the base TBD
and vacuum transfer of the volatiles enabled exclusive isolation
of pyrrolidine-dn (n = 0−2). By this method, quantitative
13C{1H} NMR spectroscopy and mass spectrometric analysis
established the presence of pyrrolidine (50%), pyrrolidine-2-d1
(40%), pyrrolidine-2,5-d1,d1 (8%) and pyrrolidine-2-d2 (2%)
by comparison of the acquired spectrum to those of
independently synthesized pyrrolidine isotopologues (Figure
4c; for syntheses of pyrrolidine isotopologues, see the
Supporting Information). Control experiments involving
addition of DCl to [1-N(pyrr)]+ ruled out modification of
the isotopic content of the pyrrolidide ligand backbone as a
result of treatment with acid. These results establish that [1-
N(pyrr-dn)]+ contains α-(d0−d2) isotopologues at the
pyrrolidide ligand, including two distinct d2 isotopomers.
The mechanistic significance of these isotopologues is
discussed below.
pyrrolidine-1-d1 was detected as the exclusive diamagnetic
2
product by quantitative 13C{1H} and H NMR spectroscopy,
Experiments were conducted to examine the effect of added
phosphine as well as the reversibility of both the deuterium
scrambling and H2 evolution processes. Addition of 10 equiv of
PPh2Me to [1-NH(pyrr)]+ inhibited the dehydrogenation
reaction with ∼10% diamagnetic [1-N(pyrr)]+ observed after
heating at 60 °C after 1 day and ∼30% after 4 days. EPR
spectroscopic analysis established [1-NH(pyrr)]+ as the sole
paramagnetic compound during the course of this experiment,
demonstrating that 10 equiv of added phosphine does not
decompose the starting material. Addition of 4 atm of D2 to
[1-N(pyrr)]+ produced no detectable quantities of [1-N(pyrr-
dn)]+ after 60 °C of heating at 5 days. These data suggest that
phosphine dissociation is likely necessary for the observed N−
H/D bond activation/H2 evolution reactivity and that the
overall H2 evolution reaction is irreversible.
The relative time scales of hydrogen evolution and
pyrrolidide ligand deuteration in [1-N(pyrr-dn)]+ were ex-
plored next. Observation of an isotopic perturbation of
resonances in the pyrrolidide β-carbon signals in [1-N(pyrr-
dn)]+ (Figure 4b) provided a convenient spectroscopic handle
for probing the degree of pyrrolidide deuteration over the
course of the dehydrogenation reaction. Accordingly, monitor-
ing the relative areas of isotopically shifted β-pyrrolidine
demonstrating that deuterium incorporation into the pyrroli-
dine backbone in [1-ND(pyrr)]+ does not take place upon
coordination to molybdenum at room temperature. Instead,
the deuterium scrambling processes are likely tied to both H2
evolution and [1-N(pyrr-dn)]+ formation that proceed upon
thermolysis.
Next, the deuterium distribution in the pyrrolidine ligand at
an early thermolysis stage was examined to probe the
reversibility of the N−H/D activation reaction. Accordingly,
the pyrrolidine dehydrogenation reaction was carried out to
partial conversion (60 °C, 3 h, ∼10% conversion) followed by
displacement of the pyrrolidine ligand by addition of a large
excess of PPh2Me (50 equiv). Only traces (<2%) of the α-
deuterated pyrrolidine were observed by quantitative 13C{1H}
NMR spectroscopy with the observation of predominantly
pyrrolidine-1-d1 (Scheme 3, bottom). This is in contrast to the
observation of significant deuterium incorporation into the
pyrrolidide ligand of the dehydrogenation product [1-N(pyrr-
dn)]+ at an identical time point, with a d0:(d1 + d2) pyrrolidide
isotopologue ratio of ∼1:1 in [(1-N(pyrr-dn)]+ observed after
3 h at 60 °C (Scheme 3, bottom). When they are taken
together, these data suggest that N−H/D activation in
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coordinated pyrrolidine in [1-ND(pyrr)]+ is likely irreversible
and that deuterium incorporation into the pyrrolidide ligand
necessarily accompanies formation of the product [1-N(pyrr-
dn)]+.
To probe whether the H2 evolution reaction is intra-
molecular in nature, a key crossover experiment was conducted
wherein [1-NH(pyrr)]+ and [1-ND(pyrr)]+ were mixed in a
1:1 ratio and heated in benzene solution at 60 °C for 48 h
(Scheme 4).49 The evolved gases were collected, analyzed, and
The resulting molybdenum azametallacyclopropane complex
then undergoes an irreversible, bimolecular PCET reaction
with the molybdenum(I) pyrrolidine starting material or any of
the preceding intermediates with weak Mo−H bonds to
generate the molybdenum(II) pyrrolidide product and
accounts for the generation of pyrrolidide-d2 isotopomers
when the reaction involves N−D or Mo−D bonds (see below
for computational evidence for low Mo(III)−H BDFEs). This
step likely proceeds in analogy to the reported reactivity of [1-
NH3]+ undergoing reductive PCET to the related molybde-
num imido and ethylene complexes [1-(NtBuAr)]+ and [1-
(C2H4)]+ to furnish [1-NH2]+ as well as the aryl amido and
ethyl complexes [1-(NHtBuAr)]+ and [1-(CH2CH3)]+,
respectively.26,27 It is important to note that an intermolecular
PCET event must occur to account for the net 1H+/1e−
change between the S = 1/2 [1-NH(pyrr)]+ starting material
and S = 0 [1-N(pyrr)]+ product. However, deuterium
scrambling and H2 evolution likely precede product-forming
PCET to account for the observation of constant pyrrolidide
isotopologue ratios in [1-N(pyrr-dn)]+ as a function of
reaction progress as well as the low deuterium content in the
evolved gases.
Scheme 4. Crossover Experiment between [1-NH(pyrr)]+
and [1-ND(pyrr)]+ and Subsequent Pyrrolidide Ligand
Liberation by Protonolysis
a
For the reaction conditions, see the Supporting Information.
found to contain H2 and HD + D2 in a 14:1 ratio. The
pyrrolidide ligand was liberated at the conclusion of the
reaction by the protonolysis procedure described previously
and found to contain pyrrolidine (74%), pyrrolidine-2-d1
(23%), pyrrolidine-2,5-d1,d1 (3%), and pyrrolidine-2-d2
(<1%). Because the relative percentages of H2 and pyrrolidine
isotopologues are approximately half of the values observed in
the dehydrogenation of [1-ND(pyrr)]+ where no [1-NH-
(pyrr)]+ was added, these results demonstrate no deuterium
crossover and support intramolecular deuterium scrambling
and H2 evolution pathways.
While the pathway proposed in Scheme 5 accounts for the
fate of the majority (∼80%) of the deuterium label in the
2
pyrrolidide ligand and evolved gases, H NMR spectroscopy
revealed a minor amount of deuterium incorporation both into
o-phenyl (∼6%) and methyl (∼6%) positions of the PPh2Me
ligands in [1-N(pyrr-dn)]+ (Figure S5). Although the low
deuterium content in the PPh2Me ligands prevented
identification of specific isotopomers and isotopologues by
quantitative 13C NMR spectroscopy, these results suggest that
a minor phosphine ligand deuteration pathway is operative but
is likely less facile than N−H/D bond activation upon
thermolysis of [1-ND(pyrr)]+ (Scheme S1).
The insights gained from the isotopic labeling experiments
provide context for the H2 evolution reaction in the
nonclassical ammine complex [1-NH3]+. A key experimental
connection between the ammine and pyrrolidine systems is the
similarly nonstatistical amount of HD observed when [1-
NH3]+ and [1-ND3]+ are mixed in a 1:1 ratio and heated
(H2:HD = 9:1),25 suggestive of an intramolecular H2 evolution
mechanism. In analogy to [1-NH(pyrr)]+, Scheme 6 presents a
proposed pathway for [1-NH3]+ that involves an intra-
molecular H2 evolution step and an intermolecular product-
forming PCET reaction. Computational analysis of the BDFEs
Proposed H2 Evolution Pathway in Amine Complexes
with Weak Bonds. A proposed H2 evolution pathway that is
consistent with the experimental results is presented in Scheme
5. Following phosphine dissociation, N−H(D) oxidative
Scheme 5. Proposed H2 Evolution Pathway and Pyrrolidide
Ligand Deuterium Incorporation Resulting in Formation of
[1-N(pyrr-d1)]+
Scheme 6. Proposed H2 Evolution Pathway in [1-NH3]+ and
DFT-Computed NH/Mo−H BDFEs of Intermediates at the
B3LYP Level of Theory
addition takes place and is followed by pyrrolidide β-H
elimination50,51 and reinsertion processes that account for
scrambling of the deuterium label principally into the
pyrrolidide 2- and 5-positions. No deuterium incorporation
was observed in the pyrrolidide 3- and 4-positions of the
product, suggesting that deuterium scrambling does not
involve “chain walking” in the proposed imine intermediate.
Intramolecular H2 evolution proceeds by reductive elimination
from a putative molybdenum dihydride imine intermediate.
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of the intermediates demonstrates that phosphine dissociation
does not have a significant effect on the ammine N−H BDFE,
wherein an approximately thermoneutral N−H oxidative
addition event (ΔG° ≈ −1 kcal mol−1) generates a proposed
molybdenum amido hydride intermediate with a weak Mo−H
BDFE of 48 kcal mol−1, near the thermodynamic threshold for
H2 evolution. This is likely a key feature of these molybdenum
complexes, as exergonic ammonia N−H oxidative addition
would generate a strong M−H bond from which H2
elimination is expected to be highly endergonic.52 Key
literature precedent for the subsequent 1,2-H2 elimination
step is Wolczanski’s observation of H2 evolution from
(silox)3Ta(NH2)(H) (silox = tBu3SiO), a complex that was
prepared by oxidative addition of ammonia.53
The final step of the pathway in Scheme 6 is intermolecular
PCET between the starting ammine complex [1-NH3]+ and
the proposed “parent” imido intermediate [1NH]+ to
generate 2 equiv of the amido product [1-NH2]+. This
PCET step is predicted to be exergonic by 21 kcal mol−1,
driven by coordination-induced bond weakening in [1-NH3]+
(BDFEN−H = 47 kcal mol−1) relative to the amido N−H bond
formed (BDFEN−H = 68 kcal mol−1). As mentioned previously,
this step has direct precedent in that our group demonstrated
that the aryl imido complex [1-(NtBuAr)]+ reacts with [1-
NH3]+ to generate the corresponding amido complexes [1-
(NHtBuAr)]+ and [1-NH2]+, a reaction that was shown to
have a lower overall driving force in comparison to the reaction
involving a “parent” amido.26 In contrast to H2 evolution, the
PCET reactivity of [1-NH3]+ has been previously found to
proceed independently of added phosphine26,27 and is
therefore proposed to take place between bis(phosphine)
molybdenum complexes in Scheme 6. Because multiple
intermediates in Scheme 6 were computed to contain weak
(<50 kcal mol−1) N−H and Mo−H bonds, PCET from these
complexes to the parent imido is also thermodynamically
feasible. Also important to note is that the proposed pathway
can be generalized to account for the observation of H2
evolution upon coordination of water and hydrazine to the
cationic terpyridine bis(phosphine) molybdenum complex
reported in our original study,25 each likely proceeding
through formally Mo(III) oxo and hydrazido intermediates,
respectively.
Overall, the proposed mechanism presented is consistent
with our originally reported crossover experiment whereby a
1:1 mixture of [1-NH3]+ and [1-ND3]+ were heated and a
nonstatistical amount of HD gas was observed,25 likely due to
the intramolecular H2 evolution step shown in Scheme 6. A
recent computational study by Bandeira et al. on [1-NH3]+ is
noteworthy and establishes the kinetic feasibility of ammonia
N−H oxidative addition to yield a seven-coordinate Mo(III)
amido hydride complex.54 However, pathways involving
phosphine dissociation and imido formation were not
reported. While the possibility of phosphine dissociation
taking place subsequent to N−H bond activation cannot be
rigorously excluded, the experimental data support a
unimolecular H2 evolution pathway. In addition, it is worth
noting that the proposed pathways in Schemes 5 and 6
represent a combination of classical organometallic steps as
well as PCET enabled by coordination-induced bond
weakening during H2 evolution, likely a unique feature of
cationic terpyridine bis(phosphine) molybdenum complexes.
CONCLUDING REMARKS
■
The synthesis and characterization of the cationic molybde-
num pyrrolidine complex [1-NH(pyrr)]+ supported by
terpyridine and bis(phosphine) ligands are described. Sig-
nificant weakening of the pyrrolidine N−H bond was
demonstrated upon coordination to the molybdenum center
that enables spontaneous H2 evolution reactivity, with the N−
H BDFE in [1-NH(pyrr)]+ being experimentally determined
to be in the range 41−51 kcal mol−1. These findings are
supported by the DFT-computed value of 48 kcal mol−1.
Deuterium labeling experiments were conducted, and the
results support a dehydrogenation mechanism involving
intramolecular H2 evolution at a single molybdenum site
followed by a product-forming intermolecular PCET reaction.
These investigations have examined the H2-evolution
reactivity of coordinated amines exhibiting exceptionally
weak N−H bonds that may compete with PCET in the
presence of H atom acceptors. Specifically, the coordinatively
saturated complexes [1-NH3]+ and [1-NH(pyrr)]+ behave as
thermodynamically potent reductive PCET reagents similar to
(η5-C5H5)2TiCl(THFx/H2Oy) and SmI2(H2O)n, whereby
phosphine dissociation is the likely gateway to the observed
dehydrogenation reactivity. While the phenomenon of
coordination-induced bond weakening provides the overall
thermodynamic driving force for H−H bond formation, it is
likely the access to molybdenum hydride intermediates with
low Mo−H BDFEs that opens the kinetic pathway for H2
evolution upon phosphine dissociation. These insights will aid
the application of coordination-induced bond weakening for
the synthesis of complexes with exceptionally weak bonds,
both in settings where H2 evolution reactivity may be desirable
and for cases where it is to be avoided.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00471.
■
sı
DFT-computed coordinates of complexes (XYZ)
General considerations, preparation of molybdenum
complexes, characterization data including NMR and
EPR spectra of complexes, electrochemical data,
independent synthesis of pyrrolidine isotopologues and
corresponding NMR spectra, deuterium labeling experi-
ments and associated data, and computational methods
and results (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Paul J. Chirik − Department of Chemistry, Princeton University,
Princeton, New Jersey 08544, United States; orcid.org/
0000-0001-8473-2898; Email: pchirik@princeton.edu
Authors
́
́
Mate J. Bezdek − Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States;
orcid.org/0000-0001-7860-2894
́
Istvan Pelczer − Department of Chemistry, Princeton University,
Princeton, New Jersey 08544, United States; orcid.org/
0000-0002-7806-6101
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00471
G
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ization, and C-H bond activation kinetics. Science 2010, 330, 933−
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by the U.S. Department of
Energy, Office of Science, Basic Energy Science (DE-
SC0006498). M.J.B. thanks the Natural Sciences and Engineer-
ing Research Council of Canada for a predoctoral fellowship
(PGS-D) and Princeton University for a Porter Ogden Jacobus
Honorific Fellowship. We thank Ken Conover (Princeton
University) for assistance with the acquisition of quantitative
13C-NMR spectra.
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