Insights into Formate Oxidation by a Series of Cobalt Piano-Stool Complexes Supported by Bis(phosphino)amine Ligands

Article abstract of DOI:10.1021/acs.inorgchem.1c00563

A series of (cyclopentadienyl)cobalt(III) half-sandwich complexes (1-4) supported by bidentate bis(phosphino)amine ligands was synthesized and characterized by NMR spectroscopy, X-ray crystallography, and cyclic voltammetry. The CoIII-hydride complex 4-H bearing the bis(cyclohexylphosphine) ligand derivative was successfully isolated via protonation of the neutral reduced CoI complex 5 with a weak acid. Experimental and computational methods were used to determine the thermodynamic hydride accepting ability of these CoIII centers and to evaluate their reactivity toward the oxidation of formate. We find that the hydride accepting ability of 1-4 ranges from 71 to 74 kcal/mol in acetonitrile, which should favor a highly exergonic reaction with formate through direct hydride transfer. Formate oxidation was demonstrated at elevated temperatures in the presence of stoichiometric quantities of 4, generating carbon dioxide and the CoIII-hydride complex 4-H in 72% yield.

Full text of DOI:10.1021/acs.inorgchem.1c00563

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Article
Insights into Formate Oxidation by a Series of Cobalt Piano-Stool
Complexes Supported by Bis(phosphino)amine Ligands
Andrew W. Cook, Thomas J. Emge, and Kate M. Waldie*
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ABSTRACT: A series of (cyclopentadienyl)cobalt(III) half-
sandwich complexes (1−4) supported by bidentate bis-
(phosphino)amine ligands was synthesized and characterized by
NMR spectroscopy, X-ray crystallography, and cyclic voltammetry.
The CoI I I -hydride complex 4-H bearing the bis-
(cyclohexylphosphine) ligand derivative was successfully isolated
via protonation of the neutral reduced CoI complex 5 with a weak
acid. Experimental and computational methods were used to
determine the thermodynamic hydride accepting ability of these
CoIII centers and to evaluate their reactivity toward the oxidation
of formate. We find that the hydride accepting ability of 1−4
ranges from 71 to 74 kcal/mol in acetonitrile, which should favor a highly exergonic reaction with formate through direct hydride
transfer. Formate oxidation was demonstrated at elevated temperatures in the presence of stoichiometric quantities of 4, generating
carbon dioxide and the CoIII−hydride complex 4-H in 72% yield.
Formic acid is a particularly attractive candidate for this
purpose, and there have been significant advances in both the
catalytic dehydrogenation of formic acid and the electro-
catalytic oxidation of formate.1,5−8 However, current hetero-
geneous systems that exhibit the best performance are
generally based on precious metals, such as Ir, Ru, Pt, and
Pd, which is cost prohibitive for large scale implementa-
tion.1,5−8 Additionally, catalyst poisoning by carbon monoxide,
the product of formic acid dehydration, is problematic due to
the poor selectivity of heterogeneous systems. Homogeneous
catalysts based on first-row transition metals may ameliorate
these issues by enhancing selectivity while using lower-cost
metals and maintaining high efficiency. Indeed, several formic
acid oxidation catalysts based on Fe and Mn have been
recently reported that utilize tripodal phosphine9 or
phosphine-based PNP pincer ligands,10−13 while the only
first-row transition metal catalyst reported for the electro-
catalytic oxidation of formate is based on Ni with phosphine
ligands containing pendent amine groups.14
INTRODUCTION
■
The storage of renewable energy, such as solar and wind, in
chemical bonds offers a promising means of addressing the
intermittency of these power sources. Hydrogen has been the
subject of intense investigation in this regard given its carbon-
free nature and unsurpassed gravimetric energy density (Figure
1).1 However, the low volumetric energy density of gaseous
hydrogen necessitates its pressurization or liquification, which
will require significant infrastructure development for its safe
transport and on-board storage.2 As such, liquid fuels that can
be reversibly (de)hydrogenated to serve as H2 storage carriers
or used directly in non-H2 fuel cells are especially appealing.3,4
The proposed mechanism for formate oxidation generally
involves a metal−hydride complex as a key intermediate, which
is generated via hydride transfer from formate to the metal
Received: February 24, 2021
Figure 1. Volumetric energy density versus specific energy of select
chemical fuels. Modified from refs 3 and 4.
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(Scheme 1).8,10−24 Subsequent protonation of the metal−
hydride intermediate creates a scheme for formic acid
were carried out according to the literature procedure for
related complexes (Scheme 2).49 Briefly, oxidation of [CpCo-
Scheme 1. Proposed Mechanism for the Thermal (Red
Path) or Electrochemical (Blue Path) Oxidation of Formate
Scheme 2. Synthesis of Bis(phosphino)amine Cobalt
Complexes 1−4
(CO)2] with I2 in diethyl ether yields the CoIII half-sandwich
di-iodide complex [CpCo(CO)I2] as a dark powder. This Co
precursor was dissolved in acetonitrile, and the phosphine
ligand (PNP, 1 equiv) was added as a solid (PNP = Medppa)
or as a toluene solution (PNP = Phdppa, Bndppa, Medcpa) to
generate [(PNP)CpCoI]I as a dark solution in situ. Subsequent
halide abstraction with AgPF6 (2.2 equiv) affords complexes
1−4 as moderately air stable orange powders in decent yields
after 16 h. Complexes 1−4 are very soluble in acetonitrile and
insoluble in dichloromethane and less polar solvents.
dehydrogenation, while oxidative deprotonation of the
metal−hydride would enable an electrocatalytic cycle (Scheme
1). Notably, a common feature in many of the aforementioned
oxidation catalysts is the presence of phosphine ligands that
incorporate an amine10,13,14 or amide functionality.11,12
Mechanistic studies have shown that these pendent proton-
active groups play an important role in the intramolecular
movement of hydrogen as well as stabilizing key transition
states.10−14,25,26 Thus far, ligand architectures based on
methylene or ethylene connections between the P and N
atoms have been explored, while bis(phosphines) bridged
solely by an amine group have yet to be investigated. Such
bis(phosphino)amine ligands have proven useful in organo-
metallic catalysis with first-row transition metals for cross-
coupling and cyclization reactions,27−29 hydrogen evolu-
tion,30−32 and ethylene polymerization.33−36
1
The H NMR spectra of 1−4 in MeCN-d3 (Figures S1, S5,
S9, and S13) are diamagnetic, with the chemical shift of the Cp
resonance being nearly invariant with respect to the identity of
the PNP ligand, ranging between 5.72 and 5.83 ppm. These
data are in good agreement with previously reported
octahedral, low-spin d6 CoIII half-sandwich complexes with
bidentate nitrogen or phosphine ligands.45,49−56 Complexes 1
and 4 both exhibit a triplet resonance at δ 3.02 and 2.09 ppm
with 3JPH = 10 and 8 Hz, respectively, which are assigned to the
N-methyl group on the ligand backbone. A similar triplet signal
is observed in the 1H NMR spectrum of 3 at δ 4.66 ppm with
3JPH = 13 Hz and is assigned to the methylene protons of the
N-benzyl group. The 31P{1H} NMR spectra (Figures S2, S6,
S10, and S14) of 1−4 reveal a downfield singlet at ca. 78 ppm
for 1−3 and 94 ppm for 4 from the PNP ligand, as well as a
characteristic septet at −144.63 ppm from the [PF6]− anion.
We attempted to synthesize the CoI and CoIII−hydride
analogs of these complexes following reported proce-
dures;44,57,58 however, only [(Medcpa)CpCo] (5) and
[(Medcpa)CpCoH][PF6] (4-H) were isolable. To generate
5, [CpCo(CO)2] and Medcpa (1 equiv) were dissolved in
toluene and refluxed for 4 h to give a brown solution (Scheme
3a). Workup of the reaction mixture gives red crystals of 5 in
63% yield. Protonation of 5 with NH4PF6 (1 equiv) in
tetrahydrofuran immediately produces a bright yellow solution
−
The thermodynamic hydricity (ΔGH ) of formate in
acetonitrile is 44 kcal/mol,37−39 and thus a metal complex
must have a greater hydride accepting ability in order to favor
exergonic formate oxidation. Given the large hydride affinities
typically observed for CoIII-based hydrogen evolution cata-
lysts,39−46 we posited that the CoIII center in [(PNP)CpCo-
(MeCN)]2+ “piano-stool” complexes (Cp = cyclopentadienyl,
PNP = bis(phosphino)amine) should act as a hydride acceptor
and provide a large thermodynamic driving force for the
oxidative release of CO2 from formate. Herein, we report the
synthesis and characterization of a series of CoIII complexes 1−
4 bearing bis(phosphino)amine ligands and assess their
efficacy for formate oxidation. The thermodynamic hydricity
of the corresponding CoIII−hydride complexes is determined
using their pKa values and the reduction potentials of 1−4. We
demonstrate that complex 4 with a cyclohexylphosphine-
substituted ligand facilitates hydride transfer from formate at
elevated temperatures. Additionally, independent preparation
of the CoI (5) and CoIII−hydride (4-H) analogs of 4 coupled
with theoretical computations provide insights into the
equilibria taking place during this reaction.
Scheme 3. Synthesis of (a) 5 and (b) 4-H
RESULTS AND DISCUSSION
■
Synthesis. The bis(phosphino)amine ligands used in this
study, bis(diphenylphosphino)methylamine (Medppa), bis-
(diphenylphosphino)phenylamine (Phdppa), bis-
(diphenylphosphino)benzylamine (Bndppa), and bis-
(dicyclohexylphosphino)methylamine (Medcpa), were synthe-
sized according to literature procedures.35,36,47,48 The
syntheses of the bis(phosphino)amine Co complexes 1−4
B
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Table 1. Selected Bond Lengths and Angles for 1−5
1
2
3
4
5
bond lengths (Å)
Co1−P (avg.)
Co1−N1
P1−P2
2.20
2.21
2.20
2.22
2.11
2.834(2)
2.603(1)
1.69
2.866(3)
2.615(1)
1.70
2.853(2)
2.578(8)
1.67
2.855(6)
2.621(2)
1.69
2.825(2)
2.522(1)
1.71
N1−P (avg.)
bond angles (deg)
P1−Co1−P2
ΣN
72.4
358.9
72.8
359.8
71.6
359.7
72.3
359.6
73.8
360
Figure 2. Single crystal X-ray structures of 1, 2, 4, and 5 as well as one independent molecule of 3 shown with 50% probability ellipsoids. Hydrogen
atoms, the second molecule of 3, hexafluorophosphate anions, and cocrystallized solvent molecules are omitted for clarity.
(Scheme 3b), from which 4-H was obtained as pale-yellow
crystals in 81% yield. Complex 5 is soluble in benzene, toluene,
tetrahydrofuran, and dichloromethane; very sparingly soluble
in diethyl ether and acetonitrile; and insoluble in hexanes. 4-H
is soluble in acetonitrile, partially soluble in tetrahydrofuran,
and insoluble in chlorinated and less polar solvents. Attempts
to isolate the CoI states of 1−3 in a similar manner led to an
intractable mixture of products. The addition of a weak acid,
such as NH4PF6, to these mixtures did result in the formation
of a small amount of the corresponding CoIII−hydride, as
observed by 1H NMR, but these species underwent
decomposition in solution to give unidentified products and
cyclopentadiene.
consistent with the increased charge of 4-H versus 5.
Additionally, a new triplet signal appears at −14.02 ppm
2
with JPH = 68 Hz, which is diagnostic of the CoIII−hydride.
The 31P{1H} NMR spectrum of 4-H (Figure S21) shows that
the Medcpa ligand resonance is shifted further downfield to
118.15 ppm and again shows the characteristic septet at
−144.92 ppm from the [PF6]− anion, consistent with the
monocationic charge of 4-H. These chemical shifts are all in
good agreement with similar half-sandwich CoIII−hydride
complexes.44,57,58
Characterization. Solid-State Molecular Structure. Crys-
tals suitable for X-ray crystallographic characterization were
obtained through slow diffusion of diethyl ether into a
saturated acetonitrile solution at −35 °C. A summary of
selected bond lengths and angles is presented in Table 1, and
the structures of complexes 1−5 are given in Figure 2.
Complexes 1, 2, and 4 crystallize as the acetonitrile solvates 1·
MeCN, 2·MeCN, and 4·MeCN with one independent
molecule in the unit cell, whereas 3 crystallizes as the
acetonitrile solvate 3·0.25MeCN with two independent
molecules of 3 in the unit cell. 5 crystallizes without solvation.
Complexes 1−4 are nearly isostructural, with each complex
exhibiting a three-legged “piano-stool” geometry around the
Co center with a planar η5-cyclopentadienyl ligand, bidentate
phosphine ligand, and monodentate acetonitrile ligand. To the
best of our knowledge, there are only two other structurally
1
The H NMR spectrum of 5 (Figure S17) is diamagnetic,
which is consistent with a low-spin CoI formulation. Given the
increased electron density and decreased overall charge of the
reduced species 5, the Cp and methyl resonances are
significantly shifted upfield compared to 4, at 4.84 and 2.15
ppm, respectively. This shielding effect upon reduction of the
metal center has been previously observed for other half-
sandwich metal complexes.44,55,57,59 The 31P{1H} NMR
spectrum of 5 (Figure S18) reveals a resonance due to the
Medcpa ligand at 104.65 ppm, slightly downfield compared to
1
that of the CoIII complex 4. The H NMR spectrum of 4-H
(Figure S20) shows a downfield shift for both the Cp and
methyl resonances to 5.16 and 2.62 ppm, respectively,
C
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characterized CoIII half-sandwich complexes containing a
bidentate phosphine ligand with either a methylene or amine
bridge: [CpCo(dppm)I]I (dppm = bis(diphenylphosphino)-
methane) and [(C5Me4H)CoH(Medppa)][PF6].51,58 The
Co−P bond distances and P−Co−P bond angles are very
similar to these reported structures; however, the P−P
distances are intermediate between [CpCo(dppm)I]I
(2.690(0) Å) and [(C5Me4H)CoH(Medppa)][PF6]
(2.569(2) Å). Additionally, the P−N bond distances are
somewhat shorter than what has been observed for the free
bis(phosphino)amine ligands and are consistent with
[(C5Me4H)CoH(Medppa)][PF6].58,60−62 The sum of the
angles about the ligand nitrogen center is nearly 360° in 1−
4. This nearly trigonal planar geometry at the nitrogen suggests
that the amine nucleophilicity is significantly reduced
compared to trialkylamines.
Figure 3. Cyclic voltammograms of 1 (red trace), 2 (green trace), 3
(blue trace), and 4 (purple trace) in acetonitrile (1 mM [Co] and 0.1
M [NBu4][PF6]; scan rate = 100 mV/s).
Unlike the dicationic CoIII−acetonitrile complexes, the
neutral CoI species 5 exhibits a two-legged “piano-stool”
geometry about the Co center where the metal is ligated by
only the η5-cyclopentadienyl and the bidentate phosphine
ligands. To the best of our knowledge, this is the first example
of a structurally characterized two-legged “piano-stool” Co
complex containing a bis(phosphino)amine ligand. The Co−P
bond distances are slightly contracted compared to the
oxidized complex 4, which is consistent with a greater extent
of π-backbonding from the reduced metal center.54,63 Similarly,
the P−N bonds are slightly elongated, suggesting modest
weakening of these bonds. The P−Co−P angle and the sum of
the angles about the nitrogen in the ligand backbone do not
vary significantly between 4 and 5.
a clear shift in the reduction potentials when the phosphine
substituents are changed from phenyl in complex 1 to
cyclohexyl in complex 4; thus, the electronic structure is
more significantly dictated by the electron donating ability of
the phosphine. This effect has been previously found by Artero
R
R
and co-workers for a series of [CpCo(P2 N2 ′)I]I complexes
R
R
(P2 N2 ′ = 1,5-diaza-3,7-diphosphacyclooctane; R = Cy, Ph;
R′ = Bn, Ph) complexes.45 To our surprise, the first reduction
event for 4 occurs at a slightly more positive potential (−0.53
V vs Fc/Fc+) than for 1−3, despite the greater electron
donating ability of cyclohexylphosphine. However, the second
reduction process exhibits the expected significant cathodic
shift compared to complexes 1−3.
Electrochemistry. Cyclic voltammetry (CV) measurements
of complexes 1−4 were collected in acetonitrile using
[NBu4][PF6] as supporting electrolyte and referenced against
the ferrocene/ferrocenium (Fc/Fc+) redox couple (Table 2,
Thermodynamic Hydricity. On the basis of the H2
evolution activity of various cobalt catalysts, CoIII−hydrides
are generally considered to be weakly hydridic, although the
hydride donating ability of CoIII−hydride complexes has rarely
been quantified. One example is the related compound
a
Table 2. Cyclic Voltammetry Data for 1−4
−
[CpCo(dppe)H][PF6] with ΔGH = 71.5 kcal/mol, which
b
c
b
c
E1/2
ΔEp
E1/2
ΔEp
was calculated based on the work of Dempsey and co-
workers.39,44 To establish the potential utility of 1−4 for
formate oxidation via a hydride transfer route, the hydride
accepting ability of 1−4 was determined via the potential-pKa
method (Scheme 4).64 Note that the hydride accepting ability
d
d
complex
(V)
(mV)
ia/ic
(V)
(mV)
ia/ic
1
2
3
4
−0.59
−0.55
−0.57
−0.53
90
90
90
80
1.04
1.06
1.03
1.08
−0.90
−0.82
−0.89
−1.22
70
75
70
70
1.04
1.01
1.01
1.12
a
Conditions: 1 mM [Co] in 0.1 M [NBu4][PF6] in acetonitrile, glassy
Scheme 4. Thermochemical Cycle for the Hydricity of 1-H−
4-H
carbon working electrode, Pt counter electrode, Ag/Ag+ reference
b
electrode, 100 mV/s. E1/2 = (Epa + Epc)/2 where Epa and Epc are
anodic and cathodic peak potentials, respectively. Potentials versus
c
d
Fc/Fc+. ΔEp = Epa − Epc. ia = anodic peak current, ic = cathodic peak
current. Determined from the CV shown in Figure 3.
Figure 3). Each complex displays two, reversible reduction
events, E1 and E2, at approximately −0.5 V and −0.8 to −1.2 V
vs Fc/Fc+, which are assigned to the CoIII/II and CoII/I couples
based on previously reported Co half-sandwich systems with
bidentate phosphine ligands.44,45,54,57 The ratio of the anodic
to cathodic peak currents is approximately one for both redox
couples of 1−4, confirming the reversibility of these electro-
chemical events. Additionally, the linear dependence of both
the anodic and cathodic peak currents on the square root of
the scan rate (ν1/2) confirms the freely diffusing nature of these
species in solution (Figures S51, S54, S57, and S60).
of the CoIII−acetonitrile complexes is equal to −ΔGH of the
−
analogous CoIII−hydrides. The standard free energies for eqs 1
and 2 are obtained from the E1 and E2 reduction potentials of
1−4, measured by CV (Table 2). The standard free energy for
the two-electron reduction of H+ to H− (Scheme 4, eq 4) is
79.6 kcal/mol in acetonitrile.64 To complete the thermochem-
ical cycle for hydricity determination, we sought to measure
the pKa of the CoIII−hydride complexes (Scheme 4, eq 3).
Neither E1 nor E2 varies considerably between 1−3, which
suggests that the amine substituent only marginally contributes
to the electronic structure of these species. In contrast, there is
D
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Spectrophotometric titration of an appropriate base into a
solution containing a metal−hydride complex has been used to
accurately determine the pKa of several metal−hydrides.44
Here, metered addition of DBU (DBU = 1,8-
diazabicyclo[5.4.0]undec-7-ene, pKa = 24.31 in acetonitrile)65
to an acetonitrile solution containing 0.9 mM of 4-H resulted
in deprotonation of the hydride complex and the production of
its conjugate base 5, as indicated by an increase in the
absorbance at 350 and 461 nm (Figure S48). These data were
used to obtain a pKa = 23.15 for 4-H, and subsequently using
electro-oxidation14 often utilize an amide or amine function-
ality to assist in the hydride transfer step. Given this precedent,
we probed the stoichiometric reaction of 1−4 with formate.
Treatment of 1−3 with one equivalent of formate (added as
the soluble biformate salt, [NBu4][HCO2]·HCO2H) in
MeCN-d3 at 25 °C immediately resulted in a color change
from orange to dark brown with concomitant deposition of an
intractable dark powder and disappearance of all resonances
associated with 1−3 in the 1H NMR spectra. Additionally, new
signals at 6.56, 6.47, and 2.97 ppm appear, consistent with
noncoordinated cyclopentadiene. On the other hand, the
reaction of 4 with [NBu4][HCO2]·HCO2H (1.25 equiv) in
MeCN-d3 at 25 °C does not result in the formation of solids
and the 1H NMR immediately reveals new diamagnetic signals
(Figures 4, S34, and S35, see Supporting Information for
−
eq 6 we obtain ΔGH = 71.0 kcal/mol for 4-H in acetonitrile.
Unfortunately, the hydride analogs of 1−3 could not be
isolated due to instability (vide supra); however, an
approximate limiting value for their pKa was estimated by
generation of the Co−hydride on an NMR scale followed by
rapid deprotonation in situ with an appropriate base (Scheme
5, see Supporting Information for Experimental Details). The
Scheme 5. In Situ Generation and Deprotonation of CoIII−
Hydrides 1-H−4-H
1
Figure 4. (a) Reactivity of 1−4 with formate. (b) H NMR of the
pKa of 4-H was also estimated in this manner to ensure the
veracity of these measurements. Briefly, 1−4 were reduced
with 2 equiv of cobaltocene in MeCN-d3, followed by in situ
protonation with NH4PF6 (1 equiv) to generate the CoIII−
hydride, which was confirmed by 1H NMR (Figures S26, S28,
S30, and S32). To establish an upper pKa limit for these
complexes, 1 equiv of base with a known pKa in acetonitrile
was quickly added to the sample. Treatment of 1-H, 2-H, and
3-H with triethylamine (1 equiv, pKa = 18.83 in acetonitrile)65
led to rapid and quantitative deprotonation of the hydride,
reaction of 4 with formate: 4 (black), and formation of 4-OCHO
(blue; after formate addition at 25 °C), 6 (green; after 1 h at 65 °C),
and 4-H (red; after 13.5 h at 65 °C). Formate added as
[NBu4][HCO2]·HCO2H.
Experimental Details). Specifically, we observe a slight upfield
shift of the Cp and methyl resonances, as well as a new triplet
signal (4JPH = 5 Hz) at 7.67 ppm that is assigned to the proton
on a coordinated formate ligand. This proposed formate
adduct, 4-OCHO, is surprisingly stable at 25 °C, showing
almost no change over 24 h by 1H NMR spectroscopy.
However, heating a solution of 4-OCHO at 65 °C results in
the disappearance of this species over 90 min and the
appearance of broad paramagnetic resonances. Diamagnetic
signals from 4-H gradually grow in over the course of 13.5 h
with the simultaneous decay of the paramagnetic product. The
−
giving an upper limit for the hydricity according to eq 6: ΔGH
= 71.0, 73.8, and 71.7 kcal/mol for 1-H, 2-H, and 3-H,
respectively. Deprotonation of 4-H was obtained by the
−
addition of 1 equiv of DBU, which gives ΔGH = 72.5 kcal/mol
and is in excellent agreement with our spectrophotometrically
determined value (vide supra). We note that the relatively small
variation in these estimated hydricity values is unsurprising
given the similarity of the first reduction potentials for 1−4 and
the strong correlation between E1 of the parent metal complex
and the thermodynamic hydricity of the corresponding hydride
species.39
final conversion to 4-H was determined to be 72
5% by
integration of the Cp, methyl, and hydride signals versus an
internal standard (hexamethyldisiloxane). In a larger scale
experiment, a solution of 4 and [NBu4][HCO2]·HCO2H (1
equiv) in MeCN was heated to 60 °C in a sealed flask. Analysis
of the flask headspace by gas chromatography after 5 h showed
CO2 production with 74 10% conversion, confirming that
the formation of 4-H proceeds with formate decarboxylation
(see Supporting Information for Experimental Details).
Notably, negligible CO2 production is observed in the absence
of 4 under otherwise identical conditions. These data show
that 4 is capable of facilitating formate oxidation via hydride
Reactivity with Formate. The measured hydride accept-
ing ability of 1−4 is sufficiently large to provide considerable
thermodynamic driving force for the oxidation of formate and
formation of the corresponding Co−hydride. However, the
kinetics of hydride transfer to a first-row transition metal can
often be sluggish despite favorable reaction energetics. To
overcome this kinetic barrier, first-row transition metal
catalysts for formic acid dehydrogenation8,10−13 or formate
E
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transfer and that decomposition of the metal complex is
minimal under these conditions. However, the rate of the
stoichiometric reaction of 4 with formate is slow, even at
elevated temperatures. Given these shortcomings, 4 was not
explored further as a catalyst for formate oxidation.
Mechanistic Considerations. A proposed mechanism for
the formation of a paramagnetic species and delayed onset of
4-H production is shown in Scheme 6. Initial coordination of
Information), and its 1H NMR spectrum in MeCN-d3 matches
the paramagnetic species observed during formate oxidation
(Figures 4b and S24). After consumption of 4-OCHO,
comproportionation is no longer accessible, and instead the
equilibrium will favor disproportionation of 6 followed by
protonation of 5 with formic acid. These reversible equilibria
account for the decrease in the concentration of 6 and the
accumulation of 4-H at later times.
Density functional theory (DFT) was employed to further
validate our measurements of the thermodynamic hydride
accepting ability of 1−4 as well as the mechanism of formate
oxidation. The bond lengths and angles of the calculated
structures of 1−4 were found to be in good agreement with the
single crystal X-ray structures (Table S7). To mitigate the
difficulties of accurately calculating the absolute free energy of
the hydride anion, we used the isodesmic reactions shown in
Scheme 7 to obtain computed hydricity values for 1-H−4-H.
Scheme 6. Proposed Reaction Steps and Calculated Free
Energies for the Conversion of 4 to 6 and 4-H (S =
acetonitrile)
Scheme 7. Isodesmic Reaction for Determining Hydricity of
1-H−4-H
These results are summarized in Table 3: for all complexes,
there is good agreement between the computed hydricities and
our experimentally determined values.
Table 3. Measured and Calculated Hydricities of 1-H−4-H
at 25°C
hydricity (kcal/mol)
c
complex
measured
calculated
a
1-H
2-H
3-H
4-H
71.0
72.9
72.6
73.3
71.7
a
73.8
a
71.7
b
71.0
a
formate is rapid (Scheme 6a), and decarboxylation of 4-
OCHO would release CO2 and generate 4-H (Scheme 6b);
however, this step is slow, allowing for excess formate or 4-
OCHO to quickly deprotonate 4-H as soon as it is generated
(Scheme 6c). While the estimated pKa of formic acid in
acetonitrile (20.9)66 is lower than that of 4-H, we note that this
value is not accurately established, and the deprotonation
equilibrium will be initially driven forward by the relatively
high concentration of base compared to the small amount of 4-
H. This deprotonation yields formic acid and the neutral CoI
complex 5, which is not observed during the reaction likely due
to its limited solubility in acetonitrile. The presence of both
CoIII (as 4 and/or 4-OCHO) and CoI species will result in
rapid comproportionation to yield the paramagnetic CoII
complex, [(Medcpa)CpCo][PF6], 6 (Scheme 6d). This
process should be favorable based on the difference in the
CoIII/II and CoII/I reduction potentials (Keq ∼ 5 × 1011).
Complex 6 was independently synthesized via one electron
reduction of 4 with KC8 (1 equiv, see Supporting
Hydricity derived from the in situ deprotonation of the Co−hydride
b
complex. Hydricity derived spectrometrically from the titration of 4-
H with DBU. Hydricity calculated from the isodesmic reaction
c
shown in Scheme 7. Level of theory: M06-L/def2-TZVPP (Co)/def2-
TZVP (C, H, N, P)/def2-TZVPD (C, H, O of formate).67−70
Our calculations predict that the decarboxylation of formate
by 4 to yield CO2 and 4-H is overall exergonic by 27.5 kcal/
mol (Scheme 6). In good agreement with our proposed
reaction sequence and experimental results, initial formation of
4-OCHO is thermodynamically favorable by 8.3 kcal/mol, and
subsequent hydride transfer generation of 4-H and CO2 is
downhill by 19.2 kcal/mol. Once 4-H is generated, it can be
deprotonated by either 4-OCHO or free formate (Scheme 6c).
While both of these reactions are predicted to be slightly
endergonic at room temperature, we expect both deprotona-
tion equilibria to be readily accessible at the elevated reaction
temperature used experimentally, and the low initial concen-
trations of 4-H will shift these equilibria toward product
F
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Inorg. Chem. XXXX, XXX, XXX−XXX

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pubs.acs.org/IC
Article
formation. The subsequent comproportionation of the CoIII
and CoI complexes is calculated to be favorable (Scheme 6d)
and is consistent with the initial accumulation of paramagnetic
6 observed during the reaction (Figures 4b and S34).
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.
Despite the large predicted thermodynamic driving force for
formate oxidation, elevated temperatures are required to
induce the hydride transfer step with 4, suggesting that a
large kinetic barrier hinders this reaction. It is also clear from
our studies that the instability of 1−3 and their associated
hydride complexes in the presence of formate precludes their
use as formate oxidation catalysts. However, with complex 4,
we have demonstrated all key steps to establish an electro-
catalytic cycle for formate oxidation (Scheme 1): formate
decarboxylation via hydride transfer to the metal, rapid
deprotonation of the resulting CoIII−hydride by an excess of
formate, and facile electrochemical oxidation of CoI to
regenerate 4 at the electrode. The improved stability of 4
under these reaction conditions may be due to greater steric
protection from the bulky cyclohexyl groups, as well as the
lower acidity of 4-H that may disfavor proton transfer to the
Cp ligand and subsequent decomposition to release cyclo-
pentadiene.71 Future investigations will further probe the
detailed mechanism of formate decarboxylation at 4 using
computational studies and will explore alternative ligand
designs at cobalt that maintain similar thermodynamic
properties as these bis(phosphino)amine systems but provide
greater stability of the metal complex under electrocatalytic
conditions and mediate faster reactivity for hydride transfer.
AUTHOR INFORMATION
■
Corresponding Author
Kate M. Waldie − Department of Chemistry and Chemical
Biology, Rutgers, The State University of New Jersey, New
Brunswick, New Jersey 08903, United States; orcid.org/
0000-0001-6444-6122; Email: kate.waldie@rutgers.edu
Authors
Andrew W. Cook − Department of Chemistry and Chemical
Biology, Rutgers, The State University of New Jersey, New
Brunswick, New Jersey 08903, United States; orcid.org/
0000-0002-8850-8946
Thomas J. Emge − Department of Chemistry and Chemical
Biology, Rutgers, The State University of New Jersey, New
Brunswick, New Jersey 08903, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00563
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
CONCLUSION
■
ACKNOWLEDGMENTS
In summary, we have synthesized and structurally charac-
terized a series of CoIII half-sandwich complexes supported by
bis(phosphino)amine ligands. The thermodynamic hydride
accepting ability of these compounds was determined through
spectroscopic and DFT computational methods. On the basis
of these values, all complexes in this series were predicted to
favor hydride transfer from formate and the oxidative release of
CO2 with high exergonicity. Indeed, with heating, stoichio-
metric hydride transfer from formate to 4 was observed,
concomitant with the extrusion of CO2. It was hypothesized
that the pendent amine group in the bis(phosphino)amine
ligands would facilitate facile formate oxidation akin to other
amine-functionalized phosphine ligands. However, the kinetic
barrier for this step remains prohibitively high, indicating the
poor capacity of the bis(phosphino)amine ligand system to
assist in achieving rapid reactivity. We are further exploring the
role that pendent amine groups may play for facilitating facile
and rapid hydride transfer with other ligand architectures as we
continue to investigate formate electro-oxidation with first-row
transition metal compounds.
■
The authors thank Rutgers, The State University of New
Jersey, for generous funding support. The authors acknowledge
the Office of Advanced Research Computing (OARC) at
Rutgers, The State University of New Jersey, for providing
access to the Amarel cluster and associated research computing
resources that have contributed to the results reported here.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00563.
■
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NOTE ADDED AFTER ASAP PUBLICATION
■
This article was published ASAP on April 27, 2021. Schemes 1,
4, 6, and 7 have been updated and the corrected version was
reposted on April 28, 2021.
I
https://doi.org/10.1021/acs.inorgchem.1c00563
Inorg. Chem. XXXX, XXX, XXX−XXX