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Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen Elliott Hulley, Neeraj Kumar, Simone Raugei, and R. Morris Bullock ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01751 • Publication Date (Web): 05 Oct 2015 Downloaded from http://pubs.acs.org on October 11, 2015
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ACS Catalysis
Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen Elliott B. Hulley,†, ‡ Neeraj Kumar,‡ Simone Raugei, R. Morris Bullock* Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, P. O. Box 999, K212, Richland, WA 99352, USA ABSTRACT: Oxidation of H2 (1 atm) is catalyzed by the manganese electrocatalysts [(P2N2)MnI(CO)(bppm)]+ and [(PNP)MnI(CO)(bppm)]+ (P2N2= 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane; PNP = (Ph2PCH2)2NMe); bppm = (PArF2)2CH2, and ArF = 3,5-(CF3)2C6H3). In fluorobenzene solvent using 2,6-lutidine as the exogeneous base, the turnover frequency for [(P2N2)MnI(CO)(bppm)]+ is 3.5 s-1 with an estimated overpotential of 700 mV. For [(PNP)MnI(CO)(bppm)] in fluorobenzene solvent using N-methylpyrrolidine as the exogeneous base, the turnover frequency is 1.4 s-1, with an estimated overpotential of 880 mV. Density functional theory calculations suggest that the slow step in the catalytic cycle is proton transfer from the oxidized 17electron manganese hydride, [(P2N2)MnIIH(CO)(bppm)]+, to the pendant amine. The computed activation barrier for intramolecular proton transfer from the metal to the pendant amine is 20.4 kcal/mol for [(P2N2)MnIIH(CO)(bppm)]+ and 21.3 kcal/mol for [(PNP)MnIIH(CO)(bppm)]. The high barrier appears to result from both the unfavorability of the metal-to-nitrogen proton transfer (thermodynamically uphill by 9 kcal/mol for [(P2N2)MnIIH(CO)(bppm)]+ due to a mismatch of 6.6 pKa units), as well as the relatively long manganese-nitrogen separation in the MnIIH complexes.
KEYWORDS: Hydrogen, oxidation, electrocatalysis, proton transfer, quantum chemistry, manganese
Sun and co-workers reported a [FeFe]-hydrogenase model complex that oxidizes H2.9
Introduction Design of efficient electrocatalysts for the production and oxidation of H2 based on earth-abundant metals1 (e.g., Fe, Ni, Co and Mn) is a prominent challenge for the widespread utilization of H2 as a sustainable, carbon-neutral energy storage vector for renewable energy applications. Molecular electrocatalysts are attractive, as they allow the rational design of ligands of a transition metal complex to create complexes with tailored catalytic properties and performance. Substantial efforts have been dedicated to design ligands with specific electronic, steric and acid/base properties. Incorporation of an amine base in the second coordination sphere of a metal center can facilitate fast heterolytic scission of the H-H bond and proton delivery to/from the catalyst. Our laboratory has developed a series of earth-abundant metal electrocatalysts that feature pendant amines in the secondary coordination sphere to facilitate the kinetics of key steps in the formation and oxidation of the H-H bond.2 Extensive efforts have been devoted to the design of molecular electrocatalysts for the reduction of protons to generate H2,3 but there are fewer studies that have focused on the opposite reaction, the oxidation of hydrogen. Rauchfuss and co-workers reported an iron catalyst that oxidizes H2 using a chemical oxidant,4 and recently found iron complexes that mimic the active site of the [FeFe]-hydrogenase by carrying out the bidirectional production and oxidation of H2.5 Ogo and coworkers prepared bimetallic complexes that oxidize H2,6 a functional mimic of the [NiFe]-hydrogenase,7 and a NiFe catalyst that shows very high activity for oxidation of H2.8 Wang,
Our studies on Ni(II) complexes showed that H2 binding can be thermodynamically unfavorable in the oxidation of H210 as a consequence of the loss of translational and rotational degrees of freedom, which is not sufficiently counterbalanced by the enthalpy of H2 binding to the Ni(II) center.11 Thus, we sought to develop platforms based on more electrophilic Fe(II)12 and Mn(I)13 centers, which have been shown to form stable H2 adducts. In our studies on Fe complexes,14-17 electrocatalytic oxidation of H2 has been studied experimentally and theoretically. These studies have shown that when H2 binding becomes more facile, proton transfer to and from the pendant amine can become rate limiting.16,17 In our initial studies of Mn-based systems, we found that Bn I + complexes such as [(PPh 2 N 2 )Mn (H2)(CO)(dppm)] [where P Ph N Bn is 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,72 2 diphosphacyclooctane, and dppm is (PPh2)2CH2], with pendant amines in the diphosphine ligand, form stable dihydrogen complexes; no evidence was obtained for heterolytic cleavage of H2.18 Related Mn complexes with electron-withdrawing substituents on the diphosphine heterolytically cleave H2 with very high rates.19,20 These initial results, along with the wide range of oxidation states offered by Mn, makes Mn complexes promising for the development of new families of H2 oxidation electrocatalysts. We report here a detailed electrochemical and computational analysis of the oxidation of H2 by the Mn(I) Bn I + complex [(P Ph 2 N 2 )Mn (CO)(bppm)] , (where bppm is F F (PAr 2)2CH2, and Ar is 3,5-(CF3)2C6H3), and the related com-
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plex [(PNP)MnI(CO)(bppm)]+ (where PNP is (Ph2PCH2)2NMe). These complexes are shown in Figure 1.
RESULTS AND DISCUSSION Generation and Oxidation of MnH Species Bearing Pendant Amines Under Electrocatalytic Conditions
Bn N P Bn
N
ArF
P P ArF
Mn
P Ph
Ph Ph
ArF
Ph
C
Me
P
N
ArF Ph Mn
P Ph
ArF
P
C
O
O
[(κ3-P 2N 2)MnI ]+
[(κ2-PNP)MnI ]+
In the presence of a suitable exogenous amine base, the cationic Mn complexes [(κ 3-P2N2)MnI]+ and [(κ 2-PNP)MnI]+ rapidly bind and heterolytically cleave H2, generating intermediate Mn-H/N-H complexes that are deprotonated to generate neutral Mn hydrides, as shown in Eq. 1 for the reaction of [(κ 3-P2N2)MnI]+.
ArF
P ArF ArF
I
Figure 1. Mn complexes studied as electrocatalysts in this paper.
R
P P
N P Bn
H2
R
N
P P
R
H
Mn
H H
P P
C
Ph P P
H2
Bn
H
ArF
N
Mn Ph
C
N
ArF
H
O
(P 2N 2)MnIH
P P
ArF ArF ArF
(1)
Under the electrocatalytic conditions described below, with excess amine base present, the neutral MnH is the resting state prior to oxidation by the electrode. Isolated manganese hydride complexes (P2N2)MnIH and (PNP)MnIH function as electrocatalysts for the oxidation of H2, but the absence of a reversible MnIH/MnIIH redox couple prevented us from determining the rate of catalytic turnover due to the need for a well-defined reversible redox couple as a current reference (see below). As we discovered in our previous studies20 on H2 binding and oxidation, MnIIH species of this type are intrinsically unstable, and rapid proton- and electron-transfer reactions lead to regeneration of MnI species (Figure 3).
Mn
N H
Mn
Ph
N
ArF P P ArF
[(κ3-P 2N 2)MnI ]+
N
P P
Ph
P
N
N
ArF
O
R
Mn
Bn
Bn
Bn I + Our previous studies19,20 of [(P Ph 2 N 2 )Mn (CO)(bppm)] showed that the pendant amine can coordinate to the metal center, yielding a κ3-complex that is in equilibrium with the κ2complex (Figure 2). Experiments carried out in the presence of H2 showed that the κ2-complex can reversibly bind, via a Mn(H2) complex, and heterolytically split H2 (Figure 2) yielding an N-protonated Mn(I) hydride. 1H NMR spectroscopic experiments subsequently showed that proton/hydride exchange occurs at a rate of at least 104 s-1 at -95 °C.19
N
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Mn
Bn Figure 2. Dissociation of the pendant amine in [(PPh 2 N2 I + )Mn (CO)(bppm)] , and heterolytic H2 cleavage. The bppm and CO ligands, and Ph substituents on P are omitted.
R
R
R N
N H P P
In this paper, we report the oxidation of H2 by the removal of the two protons and two electrons from the proton-hydride intermediate, with the regeneration of the cationic Mn(I) complex. Along with the electrochemical characterization, we investigated the detailed mechanism for oxidation of H2 by density functional theory (DFT) calculations. We found that although binding of H2 is reversible and fast, proton transfer is a difficult step that represents a bottleneck for catalysis. The major barrier for deprotonation comes from the intramolecular proton transfer from the Mn center to the pendant amine, which occurs prior to the removal of the second proton by an exogenous base. This slow rate appears to be a consequence of the mismatch between the pKa of the metal hydride and the protonated pendant amine, as well as the relatively long distance between the Mn and the nitrogen of the pendant amine.
N
H
H [ox] – e–
Mn
P P
MnI(H)
P P
Mn
[MnII(H)]+
R
Mn
[Mn 0(NH)]+
R N
N
H H
1/ 2
P P [MnI ]+
Mn
+
1/ 2
P P
Mn
[MnI(H)(NH)]+
Figure 3. Stoichiometric one-electron oxidation of MnH species bearing pendant amines results in a formal H-atom transfer from one Mn complex to another. For both (P2N2)MnIH and (PNP)MnIH, an irreversible oxidation wave occurs around –0.2 V vs. Cp*2Fe+/0 (Cp* = η5C5Me5) at 50 mV/s scan rate, indicating a chemical process that depletes the electrochemically generated MnIIH before the return scan. The cyclic voltammogram for the MnIH/MnIIH redox wave of (P2N2)MnIH in fluorobenzene (PhF) is shown in Figure 4 at variable scan rates.
2
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Figure 4. Cyclic voltammograms of (P2N2)MnIH in PhF with 0.1 M [NBu4][B(C6F5)4] at varying scan rates from 25 mV/s (light blue) to 3200 mV/s (dark blue) The redox wave at –0.616 V is the Cp*2Fe+/0 couple used for internal reference. Although this wave does become reversible at fast scan rates (ic/ia > 0.8 for scan rates greater than 3 V/s and 1 mM (P2N2)MnIH), the reversibility is concentration dependent; 2 mM (P2N2)MnIH solutions gave irreversible waves, even at scan rates >10 V/s. The reversibility of oxidation of (P2N2)MnIH also appears to depend on impurities in the starting material and solvent, as it was difficult to obtain consistent ic/ia values at lower concentrations of (P2N2)MnIH (
