Electrocatalytic Oxidation of Alcohol with Cobalt Triphosphine Complexes

Spencer P. Heins, Patrick E. Schneider, Amy L. Speelman, Sharon Hammes-Schi ﬀ er,*

Letter

Electrocatalytic Oxidation of Alcohol with Cobalt Triphosphine

Complexes

and Aaron M. Appel *

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sı Supporting Information

ABSTRACT: Coordination of the tridentate ligand bis(2-diphenylphosphinoethyl)-

phenylphosphine (P ) to cobalt forms [(CH CN) Co P ](BF ) (Co P ). In the

4 2

presence of the Bro

alcohol (BnOH) to benzaldehyde at an applied potential of −630 mV vs Fc with a

nsted base Pr EtN, Co P electrocatalytically oxidizes benzyl

2 3

+/0

TON of 19.9. In a noncatalytic reaction with excess BnOH and Pr EtN, Co P is

reduced by one electron to [(CH CN) Co P ]BF (Co P ) with concomitant formation

of half an equivalent of benzaldehyde. This stoichiometric oxidation of BnOH suggests

electron transfer occurs between intermediate cobalt species and starting Co P .

Kinetics and computational studies support an unfavorable alcohol binding

preequilibrium step followed by favorable deprotonation of the bound alcohol.

KEYWORDS: alcohol oxidation, catalysis, electrocatalysis, catalyst design, electrocatalyst, electrooxidation, cobalt

5,26

major challenge for global energy markets is the eﬃcient

potentials to generate acidic protons,

making them

storage and use of energy from intermittent renewable

candidates for electrocatalysis. We therefore targeted a cobalt

complex of linear triphos, P₃(P₃bis(2-

sources. To address the needed scale, energy can be stored in

the form of chemical bonds, such as those in alcohols. These

fuels are attractive for energy storage because they are liquids

and thereby oﬀer high energy densities and safety advantages

diphenylphosphinoethyl)phenylphosphine) for electrocatalytic

oxidation of alcohols, focusing in this work on benzyl alcohol

as an initial target.

over gaseous fuels. Eﬃcient extraction of this energy

necessitates development of electrocatalysts for oxidation of

alcohols (eq 1).

The complexes [(CH CN) Co P ](BF ) (Co P ) and

3 2 3 4 2 3

[(CH CN) Co P ]BF (Co P ) were synthesized according

3 2 3 4 3

to Scheme 1. Both complexes were characterized by NMR

spectroscopy, cyclic voltammetry (CV), X-ray diﬀraction, UV−

vis spectroscopy, and elemental analysis. The crystal structures

of Co P and Co P (Figure 1) reﬂect square-pyramidal (τ =

Despite a long history of molecular catalysts for alcohol

Scheme 1. Synthesis of Triphos Cobalt Complexes

−7

oxidation,

the development of electrocatalysts for this

transformation has been limited. Catalysts based on precious

−12

metals, including Ru,

Ir, and Pd, have been extensively

studied. Only three catalysts derived from earth-abundant ﬁrst-

row transition metals (Fe, Ni, Cu ) have been reported.

We seek to broaden the range of catalysts based on

nonprecious metals by developing a novel electrocatalyst for

the oxidation of alcohol as a critical step toward the utilization

of renewable liquid fuels. In this pursuit, we have emphasized

the synthetic tunability and the generation of acidic metal

hydrides that can be deprotonated by alkyl amines.

Received: February 18, 2021

Revised: April 18, 2021

Published: May 13, 2021

Cobalt complexes have been used for alkylation reactions

18−21

involving alcohol dehydrogenation

of aldehydes and ketones.

and the hydrogenation

More importantly, Co−H

complexes supported by phosphine donors are oxidized at mild

2−24

2021 American Chemical Society

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Letter

conversion to Co P , isolable Co hydride complexes

41−43

supported by pincer ligands have been reported

and are

known to undergo one-electron reduction. β-Hydride elimi-

nation from Ir alkoxide complexes has been observed, and

supporting mechanistic studies on β-eliminations from Rh

and Ir

44,46,47

alkoxides are also known. Furthermore, isotopic

labeling experiments for Co-catalyzed acceptorless dehydro-

genations implicate hydride species formed through β-

elimination.

During the stoichiometric oxidation of BnOH, multiple

reaction pathways are possible after the β-hydride elimination.

Figure 1. Crystal structures of Co^I^IP₃(left) and Co^IP₃(right).

First a net hydrogen atom transfer reaction between two

Hydrogen atoms and BF counterions are omitted for clarity.

equivalents of HCo P is possible, analogous to the reactivity

of bis-diphosphine hydride complexes, [HM(dppe) ] (M =

^.⁰³⁾^aⁿ^d^dⁱ^s^t^o^r^t^e^d^t^rⁱ^g^oⁿ^a^l^-^bⁱ^p^y^r^a^mⁱ^d^a^l⁽^τ5 = 0.9)

Co, Rh, Ir ). This pathway should form Co P and the

coordination environments, respectively. Metric parameters

III

dihydride complex [(H) CoP (CH CN) ] ((H) Co P , n =

0−1), which is expected to be diamagnetic. However, no

hydridic resonances are observed in the H NMR spectrum of

aﬃliated with both structures are normal compared with

25,28−31

related Co complexes,

and detailed structural parame-

ters are listed in the Supporting Information.

the reaction mixture. Alternatively, conversion of HCo P to

CVs of Co P feature three redox couples (Figure S1). The

26,49

Co P and 1/2 H is possible.

However, gas chromato-

cathodic reversible wave (E₁= −0.78 V vs Fc , ΔE = 64

mV) is assigned to the Co couple and is the feature expected

to be relevant to electrocatalytic oxidation of alcohols. A

second cathodic wave (E_p_/₂= −1.74 V vs Fc , ΔE = 210

mV) is assigned to the Co couple, and an anodic wave (E

II/I

graphic analysis of the reaction headspace revealed no

detectible H . Most importantly, neither of these pathways

provides the necessary 2:1 product ratio.

I/0

Scheme 3 illustrates the proposed explanation for the

III/II

experimentally observed 2:1 ratio of Co P to aldehyde. In the

+0.11 V vs Fc ) is assigned to the Co

couple. The

second cathodic wave is irreversible, and the anodic wave has a

large peak to peak separation of 308 mV, consistent with

quasireversibility that likely results from solvent coordination

Scheme 3. Proposed Pathway for the Stoichiometric

Oxidation of BnOH

that is coupled to electron transfer. Additionally, the

electrochemical behavior of Co P is consistent with that of

the Co P complex (Figure S4).

Stoichiometric reactivity studies were undertaken with

Co P to determine its eﬃcacy as a catalyst for benzyl alcohol

(

BnOH) oxidation. Treating Co P and BnOH in acetonitrile-

d with Pr EtN immediately caused a color change from green

to red (Scheme 2). H NMR spectroscopy conﬁrmed nearly

Scheme 2. Stoichiometric Oxidation of BnOH

ﬁrst step (Scheme 3a), one equivalent of Co P reacts with

BnOH and base, generating an alkoxide species BnOCo P₃

which proceeds to release the product aldehyde and a putative

HCo P₃(Scheme 3b). Presumably, this process occurs

through β-hydride elimination (vide supra).

Oxidation of HCo P by the parent complex Co P should

be energetically favorable (Scheme 3c). Thermochemical data

from Ciancanelli and co-workers showed that the (III/II)

couple for the cationic complex [HCo(dppe) ] is 130 mV

quantitative formation of Co P and benzaldehyde in a 2:1

negative of the (II/I) couple of the parent [Co(dppe) ]

ratio. The reduction to Co is not surprising as alcohols have

complex. Similar behavior is expected in the present system,

suggesting that the cationic HCo P can undergo electron

transfer with the parent Co P , generating Co P and

HCo P₃(Scheme 3c). The latter species should be

been used to reduce Rh and Ir complexes. Since oxidation

of BnOH requires the loss of two electrons, the 2:1 product

ratio suggests an electron transfer occurs from a second

equivalent of Co P in solution. Treating Co P with BnOLi

also produces Co P₃and benzaldehyde in a ∼2:1 ratio

III

25,26

suﬃciently acidic to be deprotonated by Pr EtN,

a second equivalent of Co P (Scheme 3d).

forming

(Scheme S1).

To test for electrocatalytic activity, two types of experiments

were performed. First, CV studies were performed in the

presence of alcohol and base. A cathodically scanned CV of

We propose that BnOH oxidation proceeds through a

hydride complex [HCoP (CH CN) ] (HCo P , n = 0−2)

that is formed by β-hydride elimination from an alkoxide

ligand. This hydride complex is then susceptible to oxidation.

Monomeric Co alkoxide complexes are known, but all lack β-

with only one exception. While we were

unable to isolate or directly observe HCo P because of rapid

II/I

Co P in the presence of BnOH displayed the reversible Co

couple (Figure S31). Co P was then generated in situ by

addition of Pr EtN. Reduction to Co P was conﬁrmed by an

anodic scan across the Co couple. However, no current

enhancement was observed under these conditions, suggesting

5−39

II/I

hydrogens,

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Letter

that the electrocatalysis is slower than the voltametric time

scale, even at low scan rates (Figures S36 −S37). Second,

controlled-potential electrolysis was performed (Scheme 4).

Scheme 4. Electrocatalytic Oxidation of BnOH

[

BnOH] = 536 mM, [ Pr EtN] = 619 mM, [Co P ] = 0.51 mM,

0 2 0 3

2.25 h, T = 25 °C.

The applied potential (E ) for the electrolysis experiment was

app

II/I

set to 150 mV positive of the observed Co couple (E_a_p_p

630 mV vs Fc ). After 2.25 h, the current plateaued and a

total charge of 105.1 C was passed yielding a turnover number

TON) of 19.9 (Figure S32). Quantitative H NMR

spectroscopy conﬁrmed the production of benzaldehyde

TON = 19.3; 97% Faradaic eﬃciency) but did not show

−

(

Figure 2. Percent conversion of 4-ﬂuorobenzyl alcohol to 4-

(

ﬂuorobenzaldehyde in the presence of Co P

as a function of base (a) and alcohol (b) concentration. Percent

conversion is calculated as [Aldehyde]

3 2

and FeCp* at 500 s

any resonances corresponding to Co P , suggesting catalyst

^/^[^A^l^d^e^h^y^d^e^]max where

decomposition during electrolysis. Correspondingly, CVs taken

following the electrolysis revealed over a 90% loss in current of

500 s

[

Aldehyde]_m_a_x= 1/2 [FeCp * ] + [Co P ] . Further details are

2 0 3 0

II/I

provided in the Supporting Information .

the Co couple (Figure S33).

Kinetics studies using the chemical oxidant FeCp* BF

were then undertaken to determine the factors inﬂuencing the

contain solvent-dependent terms, possibly leading to nonzero

52−54

rates of alcohol oxidation by Co P . NMR-scale experiments

intercepts in percent conversion versus [substrate] plots.

showed that Co P catalytically converts benzyl alcohol to

Furthermore, H NMR spectroscopic analyses of the reaction

benzaldehyde (TON = 8.8, See Supporting Information).

Because of overlapping product and ligand H resonances, 4-

mixture indicate catalyst decomposition over time, and UV−

vis experiments show that Co P slowly decomposes in the

ﬂuorobenzyl alcohol was chosen as a model substrate, and the

presence of Pr EtN (Figures S17 −S18). Despite these

rates of alcohol oxidation were determined using F NMR

spectroscopy.

complications, the kinetics studies are consistent with pre-

equilibrium binding of alcohol and deprotonation occuring in

the rate-limiting transition state or in a pre-equilibrium

process.

Catalytic reactions were performed at 25 °C in CD CN in

the presence of varying amounts of alcohol, Pr EtN, Co P ,

and FeCp*₂(Table S2 ) under pseudo-ﬁrst order conditions

DFT calculations were performed to gain insight into the

thermodynamic landscape of the reaction. Note that barriers

were not calculated, and therefore the calculations do not

identify kinetically favored reaction pathways but rather

provide information about the structures and relative free

energies of potential intermediates. Figure 3 illustrates the

calculated free-energy proﬁle for the oxidation of BnOH to

benzaldehyde. Ligand substitution by BnOH is thermodynami-

cally unfavorable by +9.4 and +13.5 kcal/mol for axial (1′-

HOBn ) and equatorial (1′-HOBn ) isomers, respectively,

with FeCp*₂as the limiting reagent (TON = 5−10). A 3-

max

fold increase in [FeCp* ] (38−114 mM) led to a minor (1.3-

fold) change in % conversion (58−75%, Figure S20),

suggesting a zero-order dependence on [FeCp* ] . Because

the kinetic traces exhibit signiﬁcant deviation from linearity

Figures S21 −S23 ), the data could not be ﬁt to obtain k_o_b_s

values. Instead, percent conversion to 4-ﬂuorobenzaldehyde at

and [Co P ] resulted in a linear increase in conversion to

00 s was examined as a proxy for rate. Increasing [ Pr EtN]

2 0

product after 500 s, indicating ﬁ rst-order dependencies on base

which is consistent with the lack of observed alcohol adducts

and catalyst (Figures 2 and S20). Saturation behavior was

and the saturation behavior. Calculations suggest that

observed for alcohol across the range of [Alcohol] = 140−

deprotonation of bound BnOH by Pr EtN is favorable,

140 mM (Figure 2), consistent with pre-equilibrium alcohol

leading to the alkoxides 2′-OBn and 2′-OBn via reactions

binding.

Several factors can potentially explain the nonideal behavior

observed in the full kinetic traces and the nonzero intercepts in

that are exergonic by −1.0 and −9.7 kcal/mol, respectively.

Calculations suggest CH CN loss forming 3′-OBn prior to

−

4′-H . Generation of an intermediate 16e complex (3′-

plots of percent conversion vs [ Pr EtN] and [Co P ]

OBn ) is expected for a β-elimination process, but a concerted

3 0

(

Figures 2 and S20). First, based on the stoichiometric

process cannot be ruled out. The optimized geometry of the

reactivity studies described above, Co P can act as both a

catalyst and an oxidant. In addition, alcohol coordination to

Co, presumably accompanied by solvent dissociation, is

lowest free energy hydride species 4′-H reveals a bent P₃

ligand (∠P1−Co−P3 = 157°), relieving the steric interaction

of the apical η -aldehyde and lowering the energy of the d

σ* orbital that accepts the hydride ligand.

x ‑y

necessary for Pr EtN to deprotonate benzyl alcohol (O−H

pK (CH CN) ∼ 28). In acetonitrile, the rate laws for both

From 4′-H , exchange of PhCHO for CH CN is favorable

associative and dissociative ligand substitution most likely

by more than 10 kcal/mol, leading to Co hydride, 5′-H. The

386

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Letter

Figure 3. Computed free energies (in kcal/mol) for BnOH oxidation. Primes indicate calculated states; L = CH CN; pK values are given as

+/0

absolute values assuming that the pK value for the [ Pr EtNH] is equal to that of [Et NH] ; E values are reported versus Fc (E₁_/₂= 0 V) by

1/2

II/I

+/0

assigning the experimental potential for the Co couple (−0.781 V versus Fc ). The DFT calculations were computed at the BP86/6-31G**

51,52

level with the LANL2DZ pseudopotential for cobalt and the SMD solvation model.

See the Supporting Information for details.

+/0

potential required to oxidize 5′-H to 6′-H is −1.00 V vs Fc

and includes solvent association to generate an acidic hydride

Scheme 5. Proposed Catalytic Cycle for BnOH Oxidation

(

pK (calc) = 8.5). The pK of 5′-H was calculated to be 18.8,

further supporting the mechanism of oxidation followed by

deprotonation. The computed oxidation potential and pK of

′-H are in excellent agreement with experimental studies by

Ciancanelli and are consistent with our reactivity studies

showing that an electron transfer occurs between an

intermediate hydride species and starting Co P (Scheme 3).

Deprotonation leading to 7′ (Co P ) provides the thermody-

namic impetus (ΔG° = −17.5 kcal/mol for 5′-H to 7′) that

drives BnOH oxidation. Overall, the calculations are consistent

with the experimental observations of unfavorable alcohol

binding, reactivity with alkyl amine base, and electron transfer

from intermediate(s) such as 5′-H.

Scheme 5 illustrates a proposed catalytic cycle for BnOH

oxidation based upon the kinetic and theoretical studies

presented herein. The ﬁrst step involves coordination of BnOH

to Co P . The saturation kinetics, lack of an observable

alcohol adduct, and computational results all suggest alcohol

binding is unfavorable. Deprotonation of coordinated alcohol

generates an alkoxide complex. This reaction sequence is

Controlled potential electrolysis experiments demonstrate

supported by computations, the ﬁrst-order dependence on

that Co P electrocatalytically oxidizes benzyl alcohol with a

+/0

base, and the reactivity of Co P with BnOLi. Calculations

TON = 19.9 at an applied potential of −630 mV vs Fc .

Stoichiometric reactivity studies indicate that electron transfer

between Co P and an intermediate cobalt species leads to a

2:1 ratio of Co P to aldehyde. Kinetics studies suggest the

suggest the bound alkoxide can undergo β-hydride elimination,

producing aldehyde and HCo P .

HCo P represents the point of divergence for the

stoichiometric and catalytic pathways. Under catalytic

reaction is ﬁrst order in Pr EtN and Co P and saturates in

2 3

conditions HCo P is oxidized by [FeCp* ] or the electrode;

alcohol, consistent with an unfavorable alcohol binding pre-

under stoichiometric conditions electron transfer between an

equilibrium.

equivalent of Co P and HCo P can occur. In both scenarios,

To our knowledge, Co P is the ﬁrst reported molecular

III

an acidic HCo P complex is produced that can be readily

electrocatalyst based on cobalt for the oxidation of alcohols.

Although its TOF is lower than those of other ﬁrst-row

transition metal complexes, the number of catalytic turnovers

observed under bulk electrolysis conditions is higher (20

turnovers compared with 2−3 for Fe(PNP)(CO)(H) and

deprotonated yielding Co P . Lastly, the cycle is closed by

oxidation of Co P to Co P .

In summary, the Co P complex is capable of oxidizing

BnOH under chemical and electrochemical conditions.

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Jiang, Z.; Kuznetsov, V. L.; Littau, K. A.; Di Marco, L.; Taylor, S. R.

Letter

15,16

Ni(P N )(L ) ).

Since the thermodynamic potentials for

REFERENCES

■

oxidation of alcohols are not known in organic solvents, it is

diﬃcult to rigorously evaluate overpotentials. However, the

^o^v^e^r^p^o^t^eⁿ^tⁱ^a^l^s^o^f^C^o^P3 and Ni(P N )(L ) should be

approximately equivalent based on their similar catalytic

potentials and the use of trialkylamine bases in both systems.

In contrast, catalysis occurs ∼400 mV more positive using

Cu(bpy)/TEMPO, and a much stronger base (phospha-

zene) is required for Fe(PNP)(CO)(H), resulting in higher

driving forces and thereby overpotentials for both of these

systems. Our present studies demonstrate the feasibility of

developing new electrocatalysts for alcohol oxidation based on

ﬁrst-row transition metals, with opportunities for improving

catalyst performance.

(

1) Pearson, R. J.; Eisaman, M. D.; Turner, J. W. G.; Edwards, P. P.;

G. Energy Storage via Carbon-Neutral Fuels Made from CO , Water,

and Renewable Energy. Proc. IEEE 2012 , 100 , 440 −460.

(2) Meerwein, H.; Schmidt, R. New Method for the Reduction of

Aldehydes and Ketones. Liebigs Ann. Chem. 1925, 444, 221−238.

(3) Verley, A. The Exchange of Functional Groups between Two

Molecules. The Passage of Ketones to Alcohols and the Reverse. Bull.

Soc. Chim. Fr. 1925 , 37 , 871 −874.

4) Ponndorf, W. Der Reversible Austausch Der Oxydationsstufen

Zwischen Aldehyden Oder Ketonen Einerseits Und Primaren Oder

Sekundaren Alkoholen Anderseits. Angew. Chem. 1926 , 39 , 138 −143.

5) Oppenauer, R. V. Dehydration of Secondary Alcohols to

https://pubs.acs.org/doi/10.1021/acscatal.1c00781.

Ketones. I. Preparation of Sterol Ketones and Sex Hormones. Recl.

Trav. Chim. Pays-Bas 1937, 56, 137 −144.

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

(6) Schultz, M. J.; Sigman, M. S. Recent Advances in Homogeneous

Transition Metal-Catalyzed Aerobic Alcohol Oxidations. Tetrahedron

2006, 62, 8227−8241.

7) Conley, B. L.; Pennington-Boggio, M. K.; Boz, E.; Williams, T. J.

Discovery, Applications, and Catalytic Mechanisms of Shvo ’s Catalyst.

Chem. Rev. 2010, 110, 2294−2312.

Synthetic and experimental procedures, characterization

of new compounds, details of X-ray crystal structures,

kinetic data and analysis, additional electrochemical

data, and computational details and methods (PDF)

Crystallographic data for [(CH CN) CoP ](BF )

4 2

8) Catalano, V. J.; Heck, R. A.; Immoos, C. E.; Ohman, A.; Hill, M.

G. Steric Modulation of Electrocatalytic Benzyl Alcohol Oxidation by

[Ru(Trpy)(R dppi)(O)] Complexes. Inorg. Chem. 1998, 37, 2150−

2157.

(CIF )

(9) Liu, Y. P.; Zhao, S. F.; Guo, S. X.; Bond, A. M.; Zhang, J.; Zhu,

G.; Hill, C. L.; Geletii, Y. V. Electrooxidation of Ethanol and

Methanol Using the Molecular Catalyst [{Ru O (OH) (H O) }(γ -

Crystallographic data for [(CH CN) CoP ]BF (CIF )

0 ‑

SiW O ) ] . J. Am. Chem. Soc. 2016, 138, 2617−2628.

36 2

AUTHOR INFORMATION

Corresponding Authors

Aaron M. Appel − Center for Molecular Electrocatalysis,

Paciﬁc Northwest National Laboratory, Richland,

Washington 99352, United States; orcid.org/0000-0002-

604-1253; Email: aaron.appel@pnnl.gov

Sharon Hammes-Schiﬀer − Department of Chemistry, Yale

University, New Haven, Connecticut 06520, United States;

orcid.org/0000-0002-3782-6995 ;

■

(10) Moyer, B. A.; Thompson, M. S.; Meyer, T. J. Chemically

Catalyzed Net Electrochemical Oxidation of Alcohols, Aldehydes, and

Unsaturated Hydrocarbons Using the System (Trpy)(Bpy)Ru-

Parella, T.; Stoeckli-Evans, H. Synthesis, Structure, and Redox and

OH ) /(Trpy)(Bpy)RuO . J. Am. Chem. Soc. 1980, 102, 2310−

312.

11) Rodríguez, M.; Romero, I.; Llobet, A.; Deronzier, A.; Biner, M.;

Catalytic Properties of a New Family of Ruthenium Complexes

Containing the Tridentate Bpea Ligand. Inorg. Chem. 2001, 40,

(

150−4156.

Email: sharon.hammes-schi ﬀ er@yale.edu

12) Waldie, K. M.; Flajslik, K. R.; McLoughlin, E.; Chidsey, C. E.

D.; Waymouth, R. M. Electrocatalytic Alcohol Oxidation with

Ruthenium Transfer Hydrogenation Catalysts. J. Am. Chem. Soc.

Authors

Spencer P. Heins − Center for Molecular Electrocatalysis,

Paciﬁc Northwest National Laboratory, Richland,

Washington 99352, United States

Patrick E. Schneider − Department of Chemistry, Yale

University, New Haven, Connecticut 06520, United States

Amy L. Speelman − Center for Molecular Electrocatalysis,

Paciﬁc Northwest National Laboratory, Richland,

Washington 99352, United States

(

017, 139, 738−748.

13) Bonitatibus, P. J.; Rainka, M. P.; Peters, A. J.; Simone, D. L.;

Doherty, M. D. Highly Selective Electrocatalytic Dehydrogenation at

Low Applied Potential Catalyzed by an Ir Organometallic Complex.

Chem. Commun. 2013, 49, 10581−10583.

(14) Serra, D.; Correia, M. C.; McElwee-White, L. Iron and

Ruthenium Heterobimetallic Carbonyl Complexes as Electrocatalysts

for Alcohol Oxidation: Electrochemical and Mechanistic Studies.

Organometallics 2011, 30, 5568−5577.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.1c00781

(15) McLoughlin, E. A.; Matson, B. D.; Sarangi, R.; Waymouth, R.

M. Electrocatalytic Alcohol Oxidation with Iron-Based Acceptorless

Alcohol Dehydrogenation Catalyst. Inorg. Chem. 2020, 59, 1453−

1460.

Notes

The authors declare no competing ﬁnancial interest.

(16) Weiss, C. J.; Wiedner, E. S.; Roberts, J. A. S.; Appel, A. M.

Nickel Phosphine Catalysts with Pendant Amines for Electrocatalytic

Oxidation of Alcohols. Chem. Commun. 2015 , 51 , 6172 −6174.

ACKNOWLEDGMENTS

■

17) Badalyan, A.; Stahl, S. S. Cooperative Electrocatalytic Alcohol

S.P.H. acknowledges Dr. Peter Dunn, Dr. Morris Bullock and

Dr. Eric Wiedner for helpful discussions. This work was

supported as part of the Center for Molecular Electrocatalysis,

an Energy Frontier Research Center funded by the U.S.

Department of Energy, Oﬃce of Science, Basic Energy

Sciences. P.E.S. is also supported by a National Science

Foundation Graduate Research Fellowship Grant No. DGE-

Oxidation with Electron-Proton-Transfer Mediators. Nature 2016,

35, 406−410.

18) Deibl, N.; Kempe, R. General and Mild Cobalt-Catalyzed C-

(19) Rösler, S.; Ertl, M.; Irrgang, T.; Kempe, R. Cobalt-Catalyzed

Alkylation of Unactivated Amides and Esters with Alcohols. J. Am.

Chem. Soc. 2016, 138, 10786−10789.

Alkylation of Aromatic Amines by Alcohols. Angew. Chem., Int. Ed.

2015, 54, 15046−15050.

752134.

388

ACS Catal. 2021, 11, 6384−6389

ACS Catalysis

Letter

20) Daw, P.; Chakraborty, S.; Garg, J. A.; Ben-David, Y.; Milstein,

(38) Olmstead, M. M.; Power, P. P.; Sigel, G. Mononuclear

Cobalt(II) Complexes Having Alkoxide and Amide Ligands: Syn-

thesis and X-Ray Crystal Structures of [Co(Cl)(OC-tert -Bu ₃ )₂

D. Direct Synthesis of Pyrroles by Dehydrogenative Coupling of Diols

and Amines Catalyzed by Cobalt Pincer Complexes. Angew. Chem.,

Int. Ed. 2016, 55, 14373−14377.

Li(THF)

], [Li(THF)4.5 ][Co{N(SiMe

)

}(OC-tert -Bu

) )(OC-tert -Bu ) }]. Inorg. Chem. 1986, 25, 1027−

3 2 3 2

) ], and [Li-

3 2

(21) Zhang, G.; Hanson, S. K. Cobalt-Catalyzed Acceptorless

{Co(N(SiMe

Alcohol Dehydrogenation: Synthesis of Imines from Alcohols and

1033.

Amines. Org. Lett. 2013, 15, 650−653.

(39) Bryndza, H. E.; Tam, W. Monomeric Metal Hydroxides,

Alkoxides, and Amides of the Late Transition Metals: Synthesis,

Reactions, and Thermochemistry. Chem. Rev. 1988, 88, 1163−1188.

(40) Tubbs, K. J.; Szajna, E.; Bennett, B.; Halfen, J. A.; Watkins, R.

W.; Arif, A. M.; Berreau, L. M. Mononuclear Nitrogen/Sulfur-Ligated

Cobalt (II) Methoxide Complexes: Structural, EPR, Paramagnetic H

NMR, and Electrochemical Investigations. Dalton Trans. 2004, 2398−

2399.

22) Zhang, G.; Scott, B. L.; Hanson, S. K. Mild and Homogeneous

Cobalt-Catalyzed Hydrogenation of C = C, C = O, and C = N Bonds.

Angew. Chem., Int. Ed. 2012, 51, 12102−12106.

(23) Xu, R.; Chakraborty, S.; Yuan, H.; Jones, W. D. Acceptorless,

Reversible Dehydrogenation and Hydrogenation of N-Heterocycles

with a Cobalt Pincer Catalyst. ACS Catal. 2015, 5, 6350−6354.

(24) Zhang, G.; Vasudevan, K. V.; Scott, B. L.; Hanson, S. K.

Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation

(41) Kuriyama, S.; Arashiba, K.; Tanaka, H.; Matsuo, Y.; Nakajima,

K.; Yoshizawa, K.; Nishibayashi, Y. Direct Transformation of

Molecular Dinitrogen into Ammonia Catalyzed by Cobalt Dinitrogen

Complexes Bearing Anionic PNP Pincer Ligands. Angew. Chem., Int.

Ed. 2016, 55, 14291−14295.

and Dehydrogenation Reactions. J. Am. Chem. Soc. 2013, 135, 8668−

25) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; DuBois, M. R.

681.

Comprehensive Thermodynamic Characterization of the Metal-

Hydrogen Bond in a Series of Cobalt-Hydride Complexes. J. Am.

Chem. Soc. 2002, 124, 2984−2992.

42) Wu, S.; Li, X.; Xiong, Z.; Xu, W.; Lu, Y.; Sun, H. Synthesis and

Reactivity of Silyl Iron, Cobalt, and Nickel Complexes Bearing a

PSiP]-Pincer Ligand via Si −H Bond Activation. Organometallics

013, 32, 3227−3237.

43) Merz, L. S.; Blasius, C. K.; Wadepohl, H.; Gade, L. H. Square

[

(26) Marinescu, S. C.; Winkler, J. R.; Gray, H. B. Molecular

Mechanisms of Cobalt-Catalyzed Hydrogen Evolution. Proc. Natl.

Acad. Sci. U. S. A. 2012, 109, 15127−15131.

Planar Cobalt(II) Hydride versus T-Shaped Cobalt(I): Structural

Characterization and Dihydrogen Activation with PNP −Cobalt

Pincer Complexes. Inorg. Chem. 2019 , 58 , 6102 −6113.

(27) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,

G. C. Synthesis, Structure, and Spectroscopic Properties of Copper-

(II) Compounds Containing Nitrogen −Sulphur Donor Ligands; the

(44) Blum, O.; Milstein, D. Mechanism of a Directly Observed.

Crystal and Molecular Structure of Aqua[1,7-Bis(N-Methylbenzimi-

dazol-2 ′yl)-2,6-Dithiaheptane]Copper(II) Perchlorate. J. Chem. Soc.,

Dalton Trans. 1984, 1349−1356.

Beta-Hydride Elimination Process of Iridium Alkoxo Complexes. J.

Am. Chem. Soc. 1995, 117, 4582 −4594.

45) Pamies, O.; Bäckvall, J. Studies on the Mechanism of Metal-

(28) Jiang, F.; Wei, G.; Huang, Z.; Lei, X.; Hong, M.; Kang, B.; Liu,

Catalyzed Hydrogen Transfer from Alcohols to Ketones. Chem. - Eur.

H. Syntheses and Crystal Structures of Two Thiolato-Organo-

J. 2001, 7, 5052−5058.

phosphino Cobalt(II) Complexes, (Et N)[Co(SC H ) (PPh ] and

(46) Ritter, J. C. M.; Bergman, R. G. A Useful Method for Preparing

[

Co(S-m -C H CH ) (Ph PCH CH P(Ph)CH CH PPh )]. J. Coord.

6 4 3 2 2 2 2 2 2 2

Iridium Alkoxides and a Study of Their Catalytic Decomposition by

Iridium Cations: A New Mode of β -Hydride Elimination for

Coordinativeiy Saturated Metal Alkoxides. J. Am. Chem. Soc. 1998,

Chem. 1992, 25, 183−191.

29) Mason, R.; Scollary, G. R. The Crystal Structure of Two

Cobalt(I) Complexes Containing the Polyphosphine Ligands PhP-

(CH )NPPh } (n = = 2, 3). Aust. J. Chem. 1977 , 30 , 2395 −2406.

47) Zhao, J.; Hesslink, H.; Hartwig, J. F. Mechanism of β -Hydrogen

20, 6826−6827.

{

2 2

30) Wei, G.; Hong, M.; Huang, Z.; Liu, H. Stereochemistry of

Elimination from Square Planar Iridium(I) Alkoxide Complexes with

Mixed Thiolate and Ditertiary Phosphine Cobalt(II) Complexes.

Crystal Structures of [Co{Ph P(CH ) PPh }(SPh) ] and [Co-

Labile Dative Ligands. J. Am. Chem. Soc. 2001, 123 , 7220 −7227.

(48) Pilloni, G.; Schiavon, G.; Zotti, G.; Zecchin, S. Electrochemistry

{

Ph PCH CH P(Ph)CH CH PPh }(SPh) ]. J. Chem. Soc., Dalton

2 2 2 2 2 2 2

of Coordination Compounds XV. Paramagnetic Hydrido Complexes

of Cobalt(II), Rhodium(II) and Iridium(II). J. Organomet. Chem.

Trans. 1991, 3145−3148.

(31) Chen, J. F.; Li, C. Enol Ester Synthesis via Cobalt-Catalyzed

Regio- and Stereoselective Addition of Carboxylic Acids to Alkynes.

977, 134, 305−318.

49) Monfette, S.; Turner, Z. R.; Semproni, S. P.; Chirik, P. J.

Org. Lett. 2018, 20, 6719−6724.

Enantiopure C 1-Symmetric Bis(Imino)Pyridine Cobalt Complexes

for Asymmetric Alkene Hydrogenation. J. Am. Chem. Soc. 2012, 134,

(32) Böttcher, A.; Takeuchi, T.; Hardcastle, K. I.; Meade, T. J.; Gray,

H. B.; Cwikel, D.; Kapon, M.; Dori, Z. Spectroscopy and

Electrochemistry of Cobalt(III) Schiff Base Complexes. Inorg. Chem.

50) Weiss, C. J.; Das, P.; Miller, D. L.; Helm, M. L.; Appel, A. M.

561−4564.

(

997, 36, 2498−2504.

Catalytic Oxidation of Alcohol via Nickel Phosphine Complexes with

33) Giordano, G.; Crabtree, R. H.; Heintz, R. M.; Forster, D.;

Pendant Amines. ACS Catal. 2014, 4, 2951−2958.

Morris, D. E. Di-μ -Chloro-Bis(η -1,5-Cyclooctadiene)-Dirhodium(I).

Inorg. Synth. 1990, 28, 88−90.

(51) Ugur, I.; Marion, A.; Parant, S.; Jensen, J. H.; Monard, G.

Rationalization of the p K Values of Alcohols and Thiols Using

34) Van Der Ent, A.; Onderdelinden, A. L.; Schunn, R. A.

Atomic Charge Descriptors and Its Application to the Prediction of

Chlorobis(Cyclooctene)Rhodium(I) and-Iridium(I) Complexes.

Amino Acid pK ’s. J. Chem. Inf. Model. 2014, 54, 2200−2213.

Inorg. Synth. 1990, 28, 90−92.

(52) Cheeseman, T. P.; Odell, A. L.; Raethel, H. A. Trans-Effect

(

35) Jayasundara, C. R. K.; Sabasovs, D.; Staples, R. J.;

Order for Alkene, Alkyne, Phosphine, Arsine, Stibine, and Sulphide

Oppenheimer, J.; Smith, M. R.; Maleczka, R. E. Cobalt-Catalyzed

C-H Borylation of Alkyl Arenes and Heteroarenes Including the First

Selective Borylations of Secondary Benzylic C-H Bonds. Organo-

metallics 2018, 37, 1567−1574.

Ligands from Studies of Diethylamine Exchange Reactions of L,PtCl ,

[

C]NHEt in Various Solvents. Chem. Commun. 1968 , 1496 −1498.

53) Richens, D. T. Ligand Substitution Reactions at Inorganic

L.; Groysman, S. Synthesis and Characterization of a Stable High-

Centers. Chem. Rev. 2005, 105, 1961−2002.

54) Langford, C. H.; Gray, H. B. Ligand Substitution Processes; W. A.

Benjamin, Inc.: New York, 1966.

(36) Bellow, J. A.; Stoian, S. A.; Van Tol, J.; Ozarowski, A.; Lord, R.

(

Valent Cobalt Carbene Complex. J. Am. Chem. Soc. 2016, 138, 5531−

5534.

(37) Bellow, J. A.; Yousif, M.; Fang, D.; Kratz, E. G.; Cisneros, G. A.;

Groysman, S. Synthesis and Reactions of 3d Metal Complexes with

the Bulky Alkoxide Ligand [OC Bu Ph]. Inorg. Chem. 2015, 54,

5624−5633.

389