Cobalt Complexes Containing Pendant Amines in the Second

30 Jun 2014 - Ping WangGuangchao LiangM. Ramana ReddyMelissa LongKandria DriskillChristian LyonsBruno DonnadieuJohn C. BollingerCharles Edwin ...
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Cobalt Complexes Containing Pendant Amines in the Second Coordination Sphere as Electrocatalysts for H2 Production Ming Fang,† Eric S. Wiedner,† William G. Dougherty,‡ W. Scott Kassel,‡ Tianbiao Liu,† Daniel L. DuBois,† and R. Morris Bullock*,† †

Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352, United States ‡ Department of Chemistry, Villanova University, 800 East Lancaster Avenue, Villanova, Pennsylvania 19085, United States S Supporting Information *

ABSTRACT: A series of heteroleptic 17e cobalt complexes, [CpCo II (P tBu 2 N Ph 2 )](BF 4 ), [Cp C6F5 Co II (P tBu 2 N Ph 2 )](BF 4 ), and [CpC5F4NCoII(PtBu2NPh2)](BF4) (where PtBu2NPh2 = 1,5-diphenyl-3,7-ditert-butyl-1,5-diaza-3,7-diphosphacyclooctane, CpC6F5 = C5H4(C6F5), and CpC5F4N = C5H4(C5F4N)) were synthesized, and the structures of all three were determined by X-ray crystallography. Electrochemical studies showed that the CoIII/II couple of [CpC5F4NCoII(PtBu2NPh2)]+ appears 250 mV positive of the CoIII/II couple of [CpCoII(PtBu2NPh2)]+ as a result of the strongly electron withdrawing perfluoropyridyl substituent on the Cp ring. Reduction of these paramagnetic CoII complexes by KC8 led to the diamagnetic 18e complexes CpCoI(PtBu2NPh2), CpC6F5CoI(PtBu2NPh2), and CpC5F4NCoI(PtBu2NPh2), which were also characterized by crystallography. Protonation of these neutral CoI complexes led to the CoIII hydrides [CpCoIII(PtBu2NPh2)H](BF4), [CpC6F5CoIII(PtBu2NPh2)H](BF4), and [CpC5F4NCoIII(PtBu2NPh2)H](BF4), and crystal structures of each of these cobalt hydrides were determined. The cobalt complex with the most electron withdrawing Cp ligand, [CpC5F4NCoII(PtBu2NPh2)]+, is an electrocatalyst for production of H2 using [p-MeOC6H4NH3][BF4] (pKaMeCN = 11.86), with a turnover frequency of 350 s−1 and an overpotential of 0.86 V at Ecat/2. A pKa value of 15.6 was measured in CH3CN for [CpC5F4NCoIII(PtBu2NPh2)H], which was used in conjunction with electrochemical measurements to obtain thermodynamic data for cleavage of the Co−H bond.

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decomposition involving dissociation of the PR2NR′2 ligand. More recently, we prepared a series of [Co I I (P n C ‑ P P h 2 2 N P h 2 )(CH 3 CN)] 2 + complexes, where PnC‑PPh22NPh2 is a tetradentate phosphine containing two (diphenylphosphino)alkyl groups linked to a central P2N2 moiety.42 Introduction of the tetraphosphine ligand in [Co(PnC‑PPh22NPh2)(CH3CN)]2+ affords a significant enhancement in the acid stability and electrocatalytic turnover frequencies (up to 18000 s−1) of the catalysts but is accompanied by a large increase in the overpotential (930− 1270 mV).43 Cobalt complexes of the type CpCo(diphosphine), reported by Koelle and Ohst in 1986, are among the first reported molecular electrocatalysts for H2 production.44 Electrocatalytic H2 production using CpCoI(diphosphine) complexes requires reduction of the intermediate [CpCoIII(diphosphine)H]+, which occurs at very negative potentials (−1.39 to −1.94 V vs Cp2Fe+/0) and leads to large overpotentials.44 Recent work

INTRODUCTION Electrocatalysts that facilitate conversion between electrical energy and chemical energy are needed for a secure energy future. Generation of electricity from renewable sources such as solar and wind is attractive, but the intermittent nature of these energy sources requires storage of the energy due to the mismatch of supply and demand. The H−H bond in hydrogen is particularly attractive, as it has a high energy density by weight, and since H2 can be generated by reduction of protons.1 Earth-abundant metals2−6 are being sought as catalysts rather than precious metals, and several classes of cobalt catalysts for production of H2 have been shown to be promising.3,5,7−34 Inspired by hydrogenase enzymes,35−37 incorporation of pendant bases in the second coordination sphere of the metal center has been shown to enhance the catalytic performance by accelerating proton movement.38,39 Our group has previously reported molecular cobalt complexes that function as electrocatalysts for the production of H2. The [Co(PR2NR′2)(CH3CN)3]2+ (PR2NR′2 is a 1,5-diaza3,7-diphosphacyclooctane ligand)40−43 complexes catalyze the formation of H2 by reduction of protons, with turnover frequencies of 90−160 s−1 and overpotentials of 360−430 mV,43 but these complexes are prone to acid-induced © XXXX American Chemical Society

Special Issue: Catalytic and Organometallic Chemistry of EarthAbundant Metals Received: April 30, 2014

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Scheme 1. Synthesis of Cobalt Complexes

Figure 1. Molecular structures of the complexes (a) [CoII]+, (b) [CoII]+C6F5, and (c) [CoII]+C5F4N, with thermal ellipsoids at the 30% probability level. Hydrogen atoms and the BF4− anion are not shown.

from our group demonstrated that the iron complex CpC6F5Fe(PtBu2NBn2)H functions as an electrocatalyst for H2 oxidation,45−47 and we reasoned that a related [CpC6F5Co(PR2NR′2)]+ complex might afford an improvement in H2 production over the CpCo(diphosphine) catalysts. Specifically, incorporation of a proton relay in the PR2NR′2 ligand could facilitate delivery of protons to cobalt, which might increase the rate and decrease the overpotential of catalysis. Additionally, the overpotential of catalysis could be lowered through use of electron-withdrawing CpC6F5 and CpC5F4N ligands that were originally developed by Deck.48,49

74% yield, as shown in Scheme 1. A byproduct of this reaction was [Cp2Co]+, which was detected by cyclic voltammetry (20%, Figure S1 in the Supporting Information) and was removed by recrystallization of the crude product. Several broad signals were observed in the 1H NMR spectrum of [CoII]+ between 12.2 and −24.5 ppm and were assigned to [CoII]+, consistent with a paramagnetic (d7) CoII center. No signal was observed in the 31P{1H} NMR spectrum of this paramagnetic complex. Brown X-ray-quality crystals of [CoII]+ were obtained by recrystallization of the crude product. A single-crystal X-ray crystallographic analysis showed that [CoII]+ adopts a twolegged piano-stool geometry (Figure 1a). Bond distances and angles are given in Table 1. The P−Co−P angle of [CoII]+ is 86.65(2)°, which is much smaller than the P−Co−P angle of 98.49° in [CpCoII(PEt3)2]+,50 which has two phosphines rather than one chelating diphosphine. The Co−P bond distances of

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RESULTS Synthesis and Characterization of CoII Complexes. Addition of NaCp·DME to the known complex 4 1 [CoII(PtBu2NPh2)(CH3CN)3](BF4)2 in acetonitrile at room temperature afforded [CpCoII(PtBu2NPh2)](BF4) ([CoII]+) in B

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Synthesis and Characterization of CoI Complexes. Addition of 1 equiv of potassium graphite (KC8) to a CH3CN solution of [CoII]+ at room temperature cleanly afforded the corresponding 18e CoI complex CpCoI(PtBu2NPh2) (CoI), which was isolated in 86% yield. The diamagnetic complex CoI was fully characterized by 1H and 31P{1H} NMR spectroscopy and elemental analysis. The 31P{1H} NMR spectrum of CoI in C6D6 showed a singlet at 56.5 ppm that was broadened (Δν1/2 = 92 Hz) due to coupling between phosphorus and the quadrupolar 59Co nucleus (I = 7/2, 100%). X-ray-quality crystals of CoI were grown by vapor diffusion of Et2O into a concentrated THF solution of CoI. X-ray crystallographic analysis showed that the overall structure of CoI has a two-legged piano stool geometry which is similar to that of [CoII]+ (Figure 2a). Unlike the case for [CoII]+, the Cp ring of CoI is not tilted away from the pendant amine that is in a boat conformation; a dihedral angle of 87° was measured between the Cp ring and the P−Co−P plane. The 2.1076(4) and 2.1103(4) Å Co−P distances in CoI are about 0.1 Å shorter than those in [CoII]+, while the P−Co−P angle of 84.509(16)° in CoI is 2° smaller than in [CoII]+. Contraction of the Co−P bond distances and P−Co−P bond angle upon reduction from CoII to CoI has been observed previously in the X-ray structures of cobalt phosphine complexes.50,51 Similar to the synthesis of CoI, KC8 was used to reduce both [CoII]+C6F5 and [CoII]+C5F4N to afford CpC6F5CoI(PtBu2NPh2) (CoIC6F5) and CpC5F4NCoI(PtBu2NPh2) (CoIC5F4N). These diamagnetic complexes were fully characterized by 1H, 31P{1H}, and 19F NMR spectroscopy and elemental analysis. The 19 F NMR spectrum of CoIC6F5 in C6D6 displayed three resonances at −143.2, −165.1, and −167.2 ppm in a 2:2:1 ratio, while two resonances were observed for CoIC5F4N at −92.6 and −142.4 ppm in a 1:1 ratio in C6D6 solution. However, no resonances could be detected in the 31P{1H} NMR spectrum of CoIC5F4N recorded in CD3CN, and the 1H NMR spectrum of CoIC5F4N recorded in CD3CN showed broadened signals for the PCH2N protons of the PtBu2NPh2 ligand (Δν1/2 = 39 Hz, 136 Hz at 25 °C) as well as those for the CpC5F4N ring (Δν1/2 = 240 Hz at 25 °C) (Figure S2, Supporting Information). The signals for the CpC5F4N ring sharpened slightly as the temperature was raised to 45 °C (Δν1/2 = 120 Hz at 45 °C). One possible explanation of this fluxional behavior is ring slippage52 of CpC5F4N in CH3CN solution. Substitution of Cp with electron-withdrawing groups has been shown to favor the ring-slippage process.53,54

Table 1. Selected Bond Distances (Å) and Angles (deg) for Co Complexes

complex CoI CoIC6F5 CoIC5F4N [CoII]+ [CoII]+C6F5 [CoII]+C5F4N [CoIIIH]+ [CoIIIH]+C6F5 [CoIIIH]+C5F4N

Co−P1

Co−P2

P1−Co−P2

dihedral angle (P− Co−P to Cp)

2.1076(4) 2.1201(5) 2.1134(5) 2.1310(5) 2.1315(5) 2.2061(6) 2.2006(14) 2.2023(4) 2.1651(11) 2.1629(7) 2.1744(6)

2.1103(4) 2.1152(5) 2.1181(5) 2.1273(5) 2.1310(5) 2.1884(6) 2.2053(14) 2.2081(4) 2.1717(11) 2.1750(7) 2.1745(6)

84.509(16) 84.75(2) 84.83(2) 84.617(18) 84.737(18) 86.65(2) 86.80(5) 87.157(15) 85.95(4) 85.39(3) 86.41(2)

87.06 82.28 82.79 80.22 80.18 68.23 66.12 64.95 64.12 66.06 65.24

2.2061(6) and 2.1884(6) Å in [CoII]+ are slightly shorter than the Co−P distances of both [CoII(PtBu2NPh2)(CH3CN)3]2+ (2.2242(5) and 2.2504(5) Å) 41 and [CpCo II (PEt 3 ) 2 ] + (2.233(1) and 2.227(1) Å).50 The PtBu2NPh2 ligand forms two six-membered rings, one of which is in a boat conformation and the other in a chair conformation. In contrast to the previously reported structure of [CpCoII(PEt3)2]+,50 the Cp ring in [CoII]+ is not perpendicular to the P−Co−P plane; it has a dihedral angle of 68° between the Cp ring and the P−Co−P plane (Figure 1a). Similar to the synthesis of [Co II ] + , treatment of [CoII(PtBu2NPh2)(CH3CN)3]2+ with NaCpC6F5 or NaCpC5F4N in acetonitrile at room temperature afforded [CpC6F5CoII(PtBu2NPh2)]+ ([CoII]+C6F5) and [CpC5F4NCoII(PtBu2NPh2)]+ ([CoII]+C5F4N). In contrast to the synthesis of [CoII]+, only trace amounts ( [CoIIIH]+ > CoI. The Cp ring of [CoIIIH]+ is tilted from the P−Co−P plane, with a dihedral angle of 64°. The yellow hydride complexes [CpC6F5CoIII(PtBu2NPh2)H](BF4) ([CoIIIH]+C6F5) and [CpC5F4NCoIII(PtBu2NPh2)H](BF4) ([CoIIIH]+C5F4N) were also synthesized by protonation of CoIC6F5 and CoIC5F4N (Scheme 1). Triplet resonances corresponding to a hydride ligand were observed in the 1H NMR spectra of [CoIIIH]+C6F5 at −15.09 ppm (2JPH = 61 Hz) and [CoIIIH]+C5F4N at −15.31 ppm (2JPH = 61 Hz), and the 31 1 P{ H} spectra of [CoIIIH]+C6F5 and [CoIIIH]+C5F4N displayed singlets at 64.0 and 65.0 ppm. The 19F NMR spectra of [CoIIIH]+C6F5 and [CoIIIH]+C5F4N displayed resonances for the C6F5 and C5F4N fluorine atoms and for the BF4− anion. X-rayquality crystals of [CoIIIH]+C6F5 were grown at room temperature from a CH3CN solution of [CoIIIH]+C6F5 that was layered with Et2O. The hydride ligand in [CoIIIH]+C6F5 was located from the difference map and refined freely. Subsequent X-ray diffraction studies showed that the bond distances and angles of [CoIIIH]+C6F5 and [CoIIIH]+C5F4N are similar to those of [CoIIIH]+ (Figure 4 and Table 1). While [CoIIIH]+ and [CoIIIH]+C6F5 are stable in CH3CN at room temperature, yellow acetonitrile solutions of [CoIIIH]+C5F4N slowly decompose at room temperature to afford a blue solution. This reaction was monitored by 1H NMR spectroscopy, with the resonances corresponding to [CoIIIH]+C5F4N gradually decreasing in intensity while new broad resonances that have chemical shifts similar to those of paramagnetic [CoII]+C5F4N appeared concurrently (Figure S5, Supporting Information). No resonance due to H2 in CD3CN (expected at 4.57 ppm) could be identified due to overlapping resonances. A bimolecular reaction of [CoIIIH]+C5F4N to eliminate dihydrogen was initially considered as a plausible reaction pathway. Surprisingly, when the decay of [CoIIIH]+C5F4N was monitored by 1H NMR spectroscopy in CD3CN, a first-order dependence on [CoIIIH]+C5F4N was observed, with a pseudo-first-order rate constant of 8.2 × 10−6 s−1 (Figure S6, Supporting Information). While the mechanism of this first-order Co−H bond cleavage is unclear, the rate of the reaction is sufficiently slow that it is not likely to be involved in electrocatalytic H2 production. Electrochemical Studies. Cyclic voltammograms of [CoII]+, [CoII]+C6F5, and [CoII]+C5F4N in acetonitrile displayed both CoIII/II and CoII/I redox couples, as shown in Figure 5, and the half-wave potentials (E1/2) of each redox couple vs Cp2Fe+/0 (the reference couple for all potentials in this paper) are given in Table 2. The cyclic voltammogram of CoIC5F4N in acetonitrile was identical with that of [CoII]+C5F4N. The cathodic and anodic peak currents (ipc and ipa) vary linearly

Figure 3. UV−vis spectra of CoIC5F4N in hexane, toluene, and acetonitrile.

similar phenomenon has been observed for the MLCT charge transfer bands of ferrocene complexes bearing electronwithdrawing substituents.62,63 This behavior is characteristic of positive solvatochromism, which results from stabilization of a polarized excited state by polar solvents.64,65 Synthesis and Characterization of CoIIIH Complexes. The complex CoI was protonated with p-bromoanilinium tetrafluoroborate (pKaMeCN = 9.43)66 in a THF/CH3CN D

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Figure 4. Molecular structures of complexes (a) [CoIIIH]+, (b) [CoIIIH]+C6F5, and (c) [CoIIIH]+C5F4N represented as thermal ellipsoid drawings at the 30% probability level. Hydrogen atoms are not shown, except for the hydride in [CoIIIH]+C6F5. BF4− anions are not shown.

couple (65−70 mV), except for the CoIII/II couple of [CoII]+, which displayed limited chemical reversibility (ΔEp = 113 mV at 100 mV s−1) (Table 2). For [CoII]+C5F4N, an additional irreversible wave with a cathodic peak potential of −2.61 V is tentatively assigned to the CoI/−I couple, since the peak current was roughly double that of the CoII/I wave (Figure S8, Supporting Information). The CoIII/II and CoII/I couples shift to more positive potentials as the CpR ligand becomes more electron withdrawing. For example, transitioning from [CoII]+ to [CoII]+C6F5 affords a positive shift of 180 mV in the CoII/I couple, while [CoII]+C5F4N displays a further positive shift of 120 mV in comparison to [CoII]+C6F5. Cyclic voltammograms of [CoIIIH]+, [CoIIIH]+C6F5, and [CoIIIH]+C5F4N recorded at 0.1 V s−1 each showed an irreversible reduction wave, with cathodic peak potentials (Epc) of −1.96, −1.73, and −1.61 V, respectively. These potentials are approximately 700 mV more negative than the CoII/I couples of the corresponding CoII complexes [CoII]+, [CoII]+C6F5, and [CoII]+C5F4N, and they are assigned to the CoIII/IIH couple on the basis of values reported for related complexes.44 Similar to the trend observed for the CoII complexes, these CoIII/IIH potentials shifted positively on moving from unsubstituted Cp in [CoIIIH]+ to the more electron-withdrawing Cp C6F5 and Cp C5F4N ligands in [CoIIIH]+C6F5 and [CoIIIH]+C5F4N. For each of these CoIIIH complexes, a new oxidation wave is observed on the return sweep at a potential corresponding to the CoII/I couple, which suggests that CoIIH undergoes either a bimetallic or monometallic Co−H bond cleavage reaction (Scheme 2). In the bimetallic pathway, two CoIIH groups react to afford two CoI atoms and 1 equiv of H2, corresponding to a net homolytic cleavage of the Co−H bond. In the monometallic pathway,

Figure 5. Cyclic voltammograms of [CpCoII(PtBu2NPh2)]+ ([CoII]+), [CpC6F5CoII(PtBu2NPh2)]+ ([CoII]+C6F5), and [CpC5F4NCoII(PtBu2NPh2)]+ ([CoII]+C5F4N). Conditions: υ = 0.1 V s−1, 1 mM cobalt complex, 0.2 M nBu4NPF6 supporting electrolyte acetonitrile solution, 1 mm diameter glassy-carbon working electrode. The wave at 0.0 V is the Cp2Fe+/0 couple, used as an internal reference potential.

Table 2. Electrochemical Characterization of Cobalt Complexes in Acetonitrile Solution (0.2 M nBu4NPF6) at 22 °C E1/2a (ΔEp)b R

Cp substituent R=H R = C6F5 R = C5F4N

Co

III/II

−0.51 (113) −0.35 (67) −0.26 (69)

CoII/I

Epc(CoIII/IIH)c

−1.25 (65) −1.07 (66) −0.95 (66)

−1.96 −1.73 −1.61

a

Half-wave potential (V) versus the Cp2Fe+/0 couple. bSeparation of the cathodic and anodic peak potentials (mV) at a scan rate of 0.1 V s−1. The Cp2Fe+/0 couple displayed a ΔEp value of 64−74 mV under these conditions. cCathodic peak potential of irreversible CoIIIH reduction at a scan rate of 0.1 V s−1.

Scheme 2. Possible Mechanisms for Cleavage of the CoII−H Bond

with the square root of the scan rate (υ1/2), indicating that the electron transfers are diffusion-controlled68 (Figure S7, Supporting Information). A peak-to-peak separation (ΔEp) consistent with a reversible wave was measured for each redox E

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−1.61 V for [CoIIIH]+C5F4N. Only [CoII]+C5F4N was further studied for catalysis, since the catalytic wave of [CoII]+C5F4N does not significantly overlap with the current for direct reduction of [DMF(H)]+ at a glassy-carbon electrode. The stability of [CoII]+C5F4N under catalytic conditions was tested by periodically recording a cyclic voltammogram on a solution of [CoII]+C5F4N (1 mM) and [DMF(H)]+ (10 mM) in acetonitrile (0.2 M nBu4NPF6) over 100 min. The catalytic current at −1.53 V gradually lost intensity and was accompanied by an increase in current at −1.28 V (Figure S13, Supporting Information), consistent with electrocatalysis mediated by [Co(CH3CN)6]2+.40 The intensity of the CoIII/II redox couple of [CoII]+C5F4N at −0.26 V also decreased over the same time period, which indicates that the composition of the bulk solution was changing. One possible explanation is that [CoII]+C5F4N converts to [Co(CH3CN)6]2+ due to loss of the PtBu2NPh2 ligand and decomposition in the presence of acid. Decomposition of [CoII]+C5F4N was not observed when the weaker acid p-anisidinium (pKaMeCN = 11.86)66 was employed for the catalytic studies (Figure 7). Variable-scan-rate studies

CoIIH reacts with a trace protic impurity, such as H2O, to afford CoII and H2 via heterolytic cleavage of the Co−H bond, and the resulting CoII is rapidly reduced to CoI, since the applied potential is much more negative than the potential of the CoII/I couple. To provide further information on the mechanism of Co−H bond cleavage, cyclic voltammograms of [CoIIIH]+ were recorded at varying scan rates. Support for a bimetallic reaction of CoIIH was obtained by plotting Epc vs log υ, which affords a slope of 18 mV over a scan rate range of 0.1−3 V s−1 (Figure S9, Supporting Information). A slope of 20 mV is expected for an ideal bimolecular reaction following an electron transfer (ErCi mechanism).68,69 Additionally, the reduction wave for [CoIIIH]+ was observed to be partially reversible over a scan rate range of 1−10 V s −1 (Figure S10a, Supporting Information), which indicates that CoIIH is not completely consumed by Co−H bond cleavage prior to its reoxidation on the return anodic sweep. The ratio ipa/ipc of the CoIII/IIH couple decreased as the concentration of [CoIIIH]+ was increased (Figure 6), consistent with an increase in the rate of Co−H

Figure 6. Plot of ipa/ipc ratio for the CoIII/IIH couple versus concentration of [CoIIIH]+ at different scan rates. The switching potential was −2.26 V.

bond cleavage at higher concentrations and a bimetallic reaction of CoIIH. Similar results were observed in cyclic voltammograms recorded on [CoIIIH]+C6F5 (Figures S11 and S12, Supporting Information). The ipa/ipc ratio measured for [CoIIIH]+ depends on the concentration of cobalt, the second-order rate constant for Co−H bond cleavage (k2), the scan rate, and the difference between the potential of the CoIII/IIH redox couple and the switching potential. A value of k2 = 3(1) × 104 M−1 s−1 was estimated for [CoIIIH]+ using the experimental parameters and a working curve calculated by Lasia.70 Similar experiments were performed for [CoIIIH]+C6F5, affording a value of 2(1) × 105 M−1 s−1 for k2. The error estimates given for k2 reflect only the scatter between ipa/ipc values measured at different scan rates and concentrations and do not reflect the precision of the cyclic voltammetry technique, systematic errors in the experimental data, or the accuracy of the derived working curve; therefore, taking those into account will lead to a larger uncertainty. Electrocatalytic H2 Production. Complexes [CoII]+, [CoII]+C6F5, and [CoII]+C5F4N were initially tested for electrocatalytic H2 production with [DMF(H)]+ (pKaMeCN = 6.1)71 as the acid in CH3CN. [DMF(H)]+ was previously used in the study of electrocatalytic production of H2 with both [Ni(P 2 N 2 ) 2 ] 2+ and [Co(P nC‑PPh2 2 N Ph 2 )] 2+ catalysts in our group.38,39,43 Each complex showed a significant current increase near the corresponding CoIIIH reduction potentials: i.e., −1.96 V for [CoIIIH]+, −1.73 V for [CoIIIH]+C6F5, and

Figure 7. Catalytic H2 production in CH3CN using 0.2 mM [CpC5F4NCoII(PtBu2NPh2)](BF4) ([CoII]+C5F4N) as catalyst and panisidinium as the acid. Conditions: scan rate 5 V s−1, 0.2 M n Bu4NPF6 supporting electrolyte acetonitrile solution, 1 mm diameter glassy-carbon working electrode.

indicated that the catalytic current (icat) becomes independent of the scan rate at 5 V s−1 and above (Figure S14, Supporting Information); therefore, all catalytic studies were performed at υ = 5 V s−1. A plot of icat versus concentration of [CoII]+C5F4N was linear (Figure S15, Supporting Information), which indicates that the reaction is first-order in [CoII]+C5F4N. Under steady-state conditions in which the catalytic wave displays a “plateau” shape, the ratio of icat to the peak current measured in the absence of acid (icat/ip) can be used to calculate the observed catalytic rate constant (kobs) in units of s−1 at 298 K (eq 1).68,72−75 A plot of kobs vs [acid] is approximately linear kobs

⎛ i ⎞2 = 1.94υ⎜⎜ cat ⎟⎟ ⎝ ip ⎠

(1)

over a 0−10 mM range of p-anisidinium, which indicates a firstorder dependence on p-anisidinium under these conditions (Figure 8). A second-order rate constant of 3 × 104 M−1 s−1 F

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solutions of pure CoIC5F4N, and the amount of broadening increased with time, possibly due to small amounts of paramagnetic Co(II) decomposition products. Nonetheless, equilibrium was reached within 1 h, and a pKaMeCN value of 15.6(0.1) was determined for [CoIIIH]+C5F4N on the basis of the NMR integrations. The pKa value of [CoIIIH]+C5F4N can be combined with the potentials of the CoIII/II, CoII/I, and CoIII/IIH redox couples to generate the thermochemical diagram illustrated in Scheme 3.43,78−80 In this diagram, the CoIII−H and CoII−H bond strengths are given in terms of their acidities (pKa), homolytic bond dissociation free energies (BDFE, ΔG°H•), and hydride donor abilities (ΔG°H−). Oxidation of [CoII]+C5F4N and hydride donation from [CoIIIH]+C5F4N are each coupled to coordination of an acetonitrile ligand to produce the 18-electron complex [CpC5F4NCoIII(PtBu2NPh2)(CH3CN)]2+. The uncertainty in the thermochemical values is expected to be less than 1.5 kcal mol−1 due to the precision of the experimentally measured pKa value (±0.1 pKa, ±0.2 kcal mol−1) and redox potentials (±10− 30 mV, ±0.2−0.7 kcal mol−1).

Figure 8. Plot of the pseudo-first-order rate constant, kobs, vs concentration of p-anisidinium. Conditions: 0.2 mM [CoII]+C5F4N, scan rate 5 V s−1, 0.2 M nBu4NPF6 supporting electrolyte acetonitrile solution, 1 mm diameter glassy-carbon working electrode.

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was determined from the slope of the plot of kobs versus concentration of p-anisidinium. The deviation from linearity at low concentrations of p-anisidinium is consistent with partial depletion of acid at the electrode, though this effect is relatively minor. At acid concentrations greater than 10 mM, the reaction rate becomes zero order in acid concentration (Figure 8). Using eq 1 and the experimentally observed icat/ip = 6.0, a kobs value of 350 s−1 was determined in this region. No change of the catalytic wave was observed when H2O (0.56 M) was added to a solution containing 0.2 mM [CoII]+C5F4N and 14 mM panisidinium (Figure S16, Supporting Information). Hydrogen production was confirmed by gas chromatographic analysis of the headspace gas obtained from a bulk electrolysis experiment; the current efficiency was determined to be 97% ± 5% . Following a recent recommendation from Appel and Helm,76 a catalytic run was conducted with [CoII]+C5F4N and a 1/1 buffer of p-anisidinium and anisidine in order to accurately determine the overpotential for electrocatalytic H2 production. The TOF measured in the acid concentration independent region with a 1:1 buffer was 360 s−1, which is the same within experimental error as the TOF measured using acid only. The overpotential at Ecat/2 was determined (eq 2) to be 860 mV from the difference between the potential at half-current of the catalytic wave (Ecat/2 = −1.59 V) and the thermodynamic potential for reduction of 1/1 p-anisidinium/anisidine (EBH+ = −0.73 V) measured experimentally by Roberts.77 Overpotential = |E BH + − Ecat/2|

DISCUSSION

Synthesis and Characterization. In the most common method reported for the synthesis of CpRCo(PR3)2 complexes, a cyclopentadienyl cobalt precursor, e.g., CpCoI(CO)2 or [CpCoIII(CO)I]I, is treated with the desired phosphine ligand.81−88 This method works well for synthesizing complexes with different phosphine ligands but is not well-suited to variation of the CpR ligand, as each precursor must be prepared individually in a multistep synthesis. In the method reported here, [CoII(PtBu2NPh2)(CH3CN)3]2+ is readily synthesized in a high-yielding reaction between [Co II (CH 3 CN) 6 ]2+ and PtBu2NPh241 and then is treated with NaCpR to afford [CoII]+, [CoII]+C6F5, and [CoII]+C5F4N. This method is versatile for synthesizing complexes with different CpR groups, as the NaCpR starting materials can be readily synthesized. The tendency for formation of the cobaltocene byproducts decreases as the electron-withdrawing ability of the cyclopentadienyl ligands increases, as observed in cyclic voltammograms of the crude products (Figure S1, Supporting Information). A critical factor for the success of these reactions is that the bulky PtBu2NPh2 ligand prevents formation of the bis(diphosphine) byproduct [CoII(PtBu2NPh2)2(CH3CN)]2+.40,41 A general synthetic method was used for the synthesis of all complexes in this study for all three CpR derivatives. The CoII complexes were reduced by potassium graphite (KC8) to cleanly generate the corresponding CoI complexes CoI, CoIC6F5, and CoIC5F4N. Protonation of these CoI complexes afforded the CoIIIH complexes [CoIIIH]+, [CoIIIH]+C6F5, and [Co III H]+C5F4N . The 31P{ 1H} NMR spectra of the Co I complexes showed singlets that were shifted to higher field

(2)

Thermochemical Measurements. Addition of benzylammonium (pKaMeCN = 16.91)66 to a benzonitrile solution of CoIC5F4N resulted in an equilibrium between CoIC5F4N and [CoIIIH]+C5F4N, as determined by 19F NMR spectroscopy. The resonance for CoIC5F4N was broader than those observed in

Scheme 3. Thermochemical Data for CoIC5F4N and Related Species in Acetonitrile Solution, Showing Relationships among E1/2, pKa, ΔG°H‑, and BDFE Values

G

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by approximately 10 ppm in comparison to their corresponding CoIIIH complexes. Although [CpRCoIII(diphosphine)H]+ complexes have previously been characterized in solution,44,67,89 to the best of our knowledge, only one X-ray diffraction study of such a species67 has previously been performed. X-ray crystallography studies on a series of CoI, CoII, and CoIII complexes provides a comparison of these piano-stool complexes in different oxidation states. The Co−P distances of the CoI complexes CoI, CoIC6F5, and CoIC5F4N and of CoII complexes [CoII]+, [CoII]+C6F5, and [CoII]+C5F4N agree well with values previously reported for CpCoI(diphosphine) and [CpCoII(diphosphine)]+ complexes.50,86,87,90,91 For all the Co complexes studied here, the Co−P bond lengths and the P− Co−P bond angles were found to increase in the order CoI < CoIIIH < CoII. According to previous investigations with other CpCo(diphosphine) complexes, Co−P bond distances are typically shorter in Co(I) than in Co(II) complexes.50,92 This shortening of Co−P bond distances has been attributed to additional Co−P(π) interactions in which dπ electrons in Co back-donate to the P−C σ* orbital.93 The general structural features of these cobalt complexes are similar to those of the analogous iron complexes reported recently by our group.45,46 In particular, [CoIIIH]+C6F5 is isoelectronic with CpC6F5FeII(PtBu2NBn2)H, which is an electrocatalyst for oxidation of H2.46 The average Co−P bond length of [CoIIIH]+C6F5, 2.1690(10) Å, is slightly longer than the average Fe−P bond length of (CpC6F5)FeII(PtBu2NBn2)H, 2.1511(13) Å. Similarly, the P−Co−P bond angle in [CoIIIH]+C6F5, 85.39(3)°, is larger than the P−Fe−P angle in CpC6F5FeII(PtBu2NBn2)H, 82.70(3)°. Thermodynamic Studies. It is generally accepted that CoIIIH complexes are important intermediates for electrocatalytic reduction of protons, yet many CoIIIH complexes of catalytic relevance are challenging to isolate and characterize due to their high reactivity.10,94−97 While several previous thermodynamic studies have reported pKa values and homolytic bond dissociation free energies (BDFE) for CoIIIH complexes,43,44,79 to the best of our knowledge, no experimental studies have measured the thermodynamics for all three Co−H cleavage modes (transfer of hydride, hydrogen atom, or proton) for a single CoIIIH complex. One limiting factor in previous thermodynamic studies has been the absence of a reversible CoIII/II redox couple, which would permit calculation of the hydride donor ability (ΔG°H−) of CoIIIH when used in a thermochemical cycle with the pKaMeCN value of CoIIIH and the CoII/I redox couple. When they are recorded in acetonitrile solution, cyclic voltammograms of [CoII]+C6F5 and [CoII]+C5F4N display reversible CoIII/II couples, thus making these complexes suitable for determination of the thermodynamics of all three modes of Co−H bond cleavage. The large ΔG°H− value of [CoIIIH]+C5F4N (73 kcal mol−1) indicates that this complex is a very poor hydride donor. By using ΔG°H− in a thermochemical cycle, the free energy for protonation of [CoIIIH]+C5F4N to produce H2 can be calculated, as shown in Scheme 4. From this thermochemical cycle, it can be predicted that an acid with a pKaMeCN of 2.2 would be required for thermoneutral protonation of [CoIIIH]+C5F4N to produce H2 (ΔG°rxn = 0 kcal mol−1). This result is consistent with prior conclusions that H2 formation via protonation of CoIIIH is viable only in the presence of strong acids.10,97,98 Several studies have computed free energies for protonation of CoIIIH species of the cobaloxime family of electrocatalysts,98,99 which permit values of ΔG°H− for the CoIIIH species to be

Scheme 4. Thermochemical Cycle for Calculating the Free Energy for Protonation of CoIIIH To Afford H2

determined from Scheme 3 for comparison to [CoIIIH]+C5F4N. In this manner, a ΔG°H− value of 75 kcal mol−1 can be calculated for HCoIII(dmgBF2)2(CH3CN) (dmg = dimethylglyoxime),98 which is comparable to the ΔG°H− value of [CoIIIH]+C5F4N (72 kcal mol−1). A much larger ΔG°H− value of 91−99 kcal mol −1 was computed for [HCo III(pngH)(CH3CN)]+ (pngH = N2,N2′-propanediylbis(2,3-butanedione2-imine-3-oxime)), though in this study the computed protonation step was not coupled with acetonitrile coordination to the resulting 16-electron species, [CoIII(pngH)(CH3CN)]2+, which would increase the hydride donor ability (smaller ΔG°H−) of [HCoIII(pngH)(CH3CN)]+ by an amount equal to the free energy of acetonitrile coordination.99 The poor hydride donor ability of [CoIIIH]+C5F4N (ΔG°H− = 73.0 kcal mol−1) contrasts sharply with the strong hydride donor ability of CoIIHC5F4N (ΔG°H− = 41.9 kcal mol−1). Use of a thermochemical cycle similar to Scheme 4 indicates that protonation of CoIIHC5F4N will be thermodynamically favorable using an acid with a pKa value up to 25. For the electrocatalytic studies of [CoII]+C5F4N using p-anisidinium (pKaMeCN = 11.86), protonation of CoIIHC5F4N to make H2 is expected to be downhill by −17.9 kcal mol−1. The homolytic bond dissociation free energy of H2 in acetonitrile is 103.6 kcal mol−1;80 thus, under 1 atm of H2 pressure a bimetallic reaction between two cobalt hydride species to form H2 will be thermodynamically favorable if the BDFE of the cobalt hydride is less than 51.8 kcal mol−1. A BDFE of 53.0 kcal mol−1 was measured for [CoIIIH]+C5F4N, which suggests that this complex might be unstable with respect to formation of H2 in the absence of an H2 atmosphere. The gradual decomposition of [CoIIIH]+C5F4N over days is consistent with the net loss of H2, as predicted from the BDFE value. However, a first-order dependence on the concentration of [CoIIIH]+C5F4N was determined for the decomposition of [CoIIIH]+C5F4N, implying that the reaction mechanism is more complex than the direct bimolecular reaction of [CoIIIH]+C5F4N. A much smaller BDFE of 37.8 kcal mol−1 was determined for [CoIIH]C5F4N, indicating that bimetallic formation of H2 from this complex is favorable by 28 kcal mol−1. Cyclic voltammograms of [Co III H] + , [Co III H] + C6F5 , and [Co III H] + C5F4N displayed only partial chemical reversibility even at a scan rate of 50 V s−1, which is in accord with the predicted thermodynamic instability of the CoIIH complexes that result from reduction. Second-order rate constants of 3(1) × 104 M−1 s−1 and 2(1) × 105 M−1 s−1 were determined for the bimetallic reaction of CoIIH and [CoIIH]C6F5, respectively, which may be compared to the second-order rate constant estimated for the bimetallic reaction of CpCoII(dppv)H (dppv = Ph2PCH CHPPh2) to generate H2 (k > 5 × 103 M−1 s−1).44 We are not aware of other second-order rate constants that have been reported in the literature for the bimetallic reaction of a CoIIH species, though Kaim has recently proposed that electrochemical oxidation of CoI(CO)2(dippf)2H (dippf = 1,1′bis(diisopropylphosphino)ferrocene) is followed by homolytic H

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Scheme 5

slowest protonation step. Exo protonation of CoI (step 2exo) is expected to be kinetically preferred over endo protonation of CoI (step 2exo),43,101 yet exo-CoI represents a catalytically inactive isomer, since the protonated pendant amine is not near the Co center. At higher concentrations of acid, isomerization from exo-CoI to endo-CoI, requiring two intermolecular proton transfers, is expected to become rate limiting for catalysis, consistent with the observation that catalysis becomes pseudozero-order in p-anisidinium at high concentrations of acid. In principle, the bimetallic two molecules of CoIIH could be a competitive pathway for H2 formation in these systems. However, icat was found to have a linear dependence on the concentration of [CoII]+C5F4N, indicating that the ratedetermining step for catalysis is first-order in catalyst. While the second-order rate constant for the bimetallic reaction of [CoIIH]+C5F4N was not measured, the rate constant for [CoIIH]+C6F5 (2 × 105 M−1 s−1) indicates that the maximum catalytic TOF for a bimetallic pathway would be approximately 40 s−1 at the catalyst concentrations employed for catalysis (0.2 mM). Therefore, the bimetallic pathway is slower than the observed TOF (350 s−1) and thus is not likely to contribute significantly to the observed catalytic response. Introduction of the electron-withdrawing CpC6F5 and CpC5F4N ligands resulted in positive shifts of 230 and 350 mV of the CoIIIH reduction potentials, respectively, relative to the Cp derivative, which corresponds to a reduction in overpotential for each of these catalysts. However, the potential for reduction of [CoIIIH]+C5F4N (Epc = −1.61 V) is still very negative relative to the thermodynamic potential for reduction

H 2 f o rm a t i o n t hr o u g h a bi m e t a l l i c r e act i on of [CoII(CO)2(dippf)2H]+.100 Catalytic H2 Production. The proposed mechanism for electrocatalytic H2 production by [CoII]+C5F4N is shown in Scheme 5. Reduction of CoII to CoI (step 1) is followed by protonation at a pendant amine that is located either exo or endo with respect to cobalt, forming either exo-CoI (step 2exo) or endo-CoI (step 2endo). Formation of endo-CoI is followed by a rapid intramolecular proton transfer from nitrogen to cobalt to form CoIIIH (step 3), which must be further reduced to CoIIH (step 4). Similar to the case for CoI, protonation of CoIIH can occur either exo or endo with respect to the hydride ligand, resulting in either exo-CoIIH (step 5exo) or endo-CoIIH (step 5endo). The endo-CoIIH isomer can form an H−H bond by heterocoupling of the proton from the pendant amine and the hydride on the metal, followed by or in concert with H2 loss to generate CoII (step 6) and complete the electrocatalytic cycle. Catalytic H2 production was observed to have a first-order dependence on acid at low concentrations of p-anisidinium, indicating that one of the protonation steps (2endo or 5endo) is significantly faster than the other. In a prior computational study of [CoII(P2C‑PPh22NPh2)(CH3CN)]2+, the endo-CoIIH isomer was found to be approximately 3 pKa units less acidic than the endo-CoI isomer due to a favorable N−Hδ+···Hδ−−Co dihydrogen-bonding interaction between the proton on the pendant amine and the hydride ligand.43 A similar pKa trend is expected for the endo-CoIIH and endo-CoI isomers of [CoII]+C5F4N, which suggests that the protonation step with the lowest thermodynamic driving force (2endo) is also the I

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of 1/1 p-anisidinium/p-anisidine, and the resulting overpotential of 860 mV for this catalyst is large. Reduction of [CoIIIH]+C5F4N produces CpC5F4NCoII(PtBu2NPh2)H, a highenergy 19e species. The maximum turnover frequency of [CoII]+C5F4N (350 s−1) using p-anisidinium (pKaMeCN = 11.86) is very similar to the maximum turnover frequencies previously reported for [CoII(PR2NPh2)(CH3CN)3]2+ complexes (R = Ph, TOF = 90 s−1; R = tBu, TOF = 160 s−1) using the stronger acid pbromoanilinium (pKaMeCN = 9.43).40,41 This observation is consistent with the pendant amines of [CoII]+C5F4N being more basic than the pendant amines of [CoII(PR2NPh2)(CH3CN)3]2+ due to the decreased molecular charge; thus, similar catalytic rates can be achieved using acids with different pKa values. A larger turnover frequency of 1000 s−1 was observed for [CoII(P2C‑PPh22NPh2)(CH3CN)]2+ using [(DMF)H]+ (pKaMeCN = 6.1) as the acid, and the turnover frequency increased to 18000 s−1 upon addition of H2O.43 In a prior study on [NiII(PR2NR′2)2]2+ electrocatalysts for H2 production, endo protonation has been proposed to be faster with [(DMF)H]+ than with substituted anilinium acids due to steric effects.102 An initial hypothesis of this study was that incorporation of proton relays into a [CpRCoII(diphosphine)]+ complex would enhance its activity for electrocatalytic H2 production. Meaningful quantitative comparisons of [CoII]+C5F4N with other cobalt catalysts that do not contain pendant amines are challenging to make, since most catalysts are studied under substantially different experimental conditions and kinetic regimes. However, qualitative comparisons between catalysts can be made in some cases and can provide insight into the role of the pendant amine in catalysis. Electrocalytic H2 production by [CpCoII(dppv)]+, which is related to [CoII]+C5F4N but does not contain a pendant amine, displays a second-order dependence on the acid concentration with CF3COOH as the acid in propylene carbonate solution.44 The second-order acid dependence of [CpCoII(dppv)]+ contrasts with the firstorder acid dependence observed for [CoII]+C5F4N and suggests that the presence of a pendant amine significantly accelerates the delivery of protons to the CoIIH intermediate. A first-order dependence on acid concentration was observed for the wellknown cobaloxime catalyst CoII(dmgBF2)2, and a second-order catalytic rate constant of 7 × 10−1 M−1 s−1 was measured using tosic acid (pKaMeCN = 8.6)66 in acetonitrile solution.10 A much larger second-order rate constant of 3 × 104 M−1 s−1 was measured for [CoII]+C5F4N using the weaker acid p-anisidinium (pKaMeCN = 11.86),66 which is again consistent with the pendant amines of [CoII]+C5F4N enhancing proton delivery.

anisidinium as the acid. Comparison of the catalytic activity of [CpC5F4NCoII(PtBu2N2Ph)]+ to that of the related complex [CpCoII(dppv)]+, which does not contain a pendant amine, highlights the beneficial role of proton relays in achieving fast rates of H2 production.

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EXPERIMENTAL SECTION

General Experimental Procedures. 1H and 31P{1H} NMR spectra were recorded on a 500 MHz spectrometer (at 25 °C unless otherwise noted). All 1H chemical shifts have been internally calibrated to the monoprotio impurity of the deuterated solvent. The 31P{1H} NMR spectra were referenced to external phosphoric acid at 0 ppm. Phosphorus peaks are broadened due to coupling with the quadrupolar 59 Co nucleus (I = 7/2, 100%). Elemental analyses were performed by Atlantic Microlab (Norcross, GA). Electrochemical studies were conducted using a CH Instruments 620D potentiostat and a standard three-electrode cell. All electrochemical measurements and electrode manipulations were carried out in a nitrogen-filled glovebox at ambient temperature (22 °C). The working electrode was a 1 mm diameter glassy-carbon disk encased in polyether−ether−ketone (PEEK; ALS), cleaned using a polishing pad (Buehler MicroCloth) loaded with diamond paste (Buehler MetaDi II 0.25 μm), and lubricated with ethylene glycol; then the electrode was rinsed with CH3CN. A fresh portion of the polishing pad was used for each polishing operation. The counter electrode was a 3 mm diameter glassy-carbon rod (Alfa Aesar). The reference electrode was a silver wire (Alfa Aesar; 1 mm diameter, 99.9%) anodized for 5 min in aqueous HCl (6 M, Aldrich), washed with water and acetone, dried, and suspended in a glass tube containing CH3CN (0.2 M nBu4NPF6) and fitted with a porous Vycor disc. Ferrocene was used as an internal standard, and all potentials are referenced to the Cp2Fe+/0 couple at 0 V. Methods and Materials. All manipulations were carried out under N2 using standard vacuum-line, Schlenk, and inert-atmosphere glovebox techniques. Benzonitrile was degassed and dried over 3 Å molecular sieves. Acetonitrile (CH3CN; Alfa-Aesar, anhydrous, aminefree), dichloromethane (CH2Cl2; Fisher, not stabilized), and THF, toluene, and diethyl ether (Et2O; VWR, not stabilized) were purified by sparging with nitrogen and passage through neutral alumina using a solvent purification system (PureSolv, Innovative Technologies, Inc.). Acetone (Fisher, reagent) and ethylene glycol (anhydrous, Aldrich) were used as received. Tetrabutylammonium hexafluorophosphate (nBu4NPF6) was prepared from nBu4NI and NH4PF6 (Aldrich) and purified by crystallization from saturated acetone solution.103 Ferrocene (Aldrich) was purified by sublimation. Acetonitrile-d3 (Cambridge Isotope Laboratories) was vacuum-distilled from P2O5. C6D6 (Cambridge Isotope Laboratories) was dried over NaK and vacuum-distilled before use. Benzylammonium tetrafluoroborate was synthesized by slow addition of HBF4·Et2O to an Et2O solution of ∼5% excess base. The precipitate was collected by filtration, washed with excess Et2O, and recrystallized from CH3CN/Et2O in a glovebox. Tetrabutylammonium hexafluorophosphate was recrystallized twice from CH3CN/Et2O and dried under vacuum at room temperature. Anilinium salts were prepared by reaction of the parent base with 1.5 equiv of HBF4·Et2O, and then the crude salts were recrystallized from CH3CN/Et2O. Protonated dimethylformamide, [(DMF)H]OTf, was prepared by the method of Favier and Duñach.104 [Co(PtBu2NPh2)(CH 3 CN) 3 ](BF 4 ) 2 , 1 0 5 NaCp·DME, 1 0 6 NaCp C 6 F 5 , 4 8 and NaCpC5F4N 47,49 were synthesized according to literature procedures. Syntheses. [CpCo(PtBu2NPh2)]BF4 ([CoII]+). Solid NaCp·DME (12 mg, 0.067 mmol) was added to a stirred brown solution of [Co(PtBu2NPh2)(CH3CN)3](BF4)2 (50 mg, 0.065 mmol) in CH3CN (3 mL), causing an immediate color change to red. The reaction mixture was stirred for 20 min and then filtered through Celite, and the solvent was removed under vacuum. The dark red solid was extracted with THF and filtered through Celite, and then the solvent was removed under reduced pressure. The resulting solid was dissolved in CH3CN (ca. 2 mL), and then Et2O was added dropwise until the solution became cloudy. The solution was then cooled

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CONCLUSION The sterically hindered PtBu2NPh2 ligand was used to synthesize a series of [CpRCoII(PtBu2N2Ph)]+ (R = H, C6F5, C5F4N) complexes using a synthetic route shorter than that previously reported for similar complexes. Stable complexes were isolated in three different cobalt oxidation states, including three rare CoIIIH species that were fully characterized, including structure determinations by X-ray crystallography. Thermochemical measurements on [CpC5F4NCoII(PtBu2N2Ph)]+ led to the first experimental determination of all three CoIII−H bond cleavage modes, providing insight into the chemical reactivity of these species. [CpC5F4NCoII(PtBu2N2Ph)]+ was found to be an electrocatalyst for production of H2, with a maximum turnover frequency of 350 s−1 and an overpotential of 860 mV using pJ

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overnight at −35 °C to afford [CoII]+ (30 mg, 0.048 mmol, 74%) as a dark brown microcrystalline solid. X-ray-quality single crystals were obtained by slow evaporation of a concentrated THF solution of [CoII]+ under a steady stream of dinitrogen. 1H NMR (CD3CN): δ 12.2 (br s, Δν1/2 = 676 Hz), 7.5 (br s, Δν1/2 = 33 Hz), 7.1 (br s, 4H, Δν1/2 = 164 Hz), 2.6 (br s, 18H, Δν1/2 = 148 Hz, C(CH3)3), −0.7 (br s, 2H, Δν1/2 = 111 Hz), −2.1 (br s, 4H, Δν1/2 = 403 Hz), −24.5 (br s, Δν1/2 = 863 Hz); most peaks except for the tBu peak of this paramagnetic complex could not be assigned unambiguously. Anal. Calcd for C29H41BCoF4N2P2·0.5CH3CN: C, 55.79; H, 6.63; N, 5.42. Found: C, 56.16; H, 6.76; N, 5.70. See Table 2 for electrochemical data. CpCo(PtBu2NPh2) (CoI). KC8 (22 mg, 0.16 mmol) was added to a stirred solution of [CoII]+ (100 mg, 0.160 mmol) in THF (7 mL), causing an immediate color change from dark red to yellow. The reaction mixture was stirred for 5 min, the solution was then filtered through Celite, and THF was removed under reduced pressure to afford CoI (74 mg, 0.14 mmol, 86%) as a brown powder. X-ray-quality single crystals were obtained by diffusing Et2O vapor into a concentrated THF solution of CoI at room temperature. This reaction could be performed using [CoII]+ that was either isolated or generated in situ. Anal. Calcd for C29H41CoN2P2: C, 64.68; H, 7.67; N, 5.20. Found: C, 64.72; H, 7.63; N, 5.04. 1H NMR (C6D6): δ 7.22 (t, 3JHH = 7.8 Hz, 4H, Ar meta H), 6.84 (d, 3JHH = 8.3 Hz, 4H, Ar ortho H), 6.79 (t, 3JHH = 7.2 Hz, 2H, Ar para H), 4.60 (t, 3JPH = 1 Hz, 5H, C5H5), 3.36 (dt, 2JHH = 13.1 Hz, 2JPH = 3.0 Hz, triplet due to virtual coupling, 4H, PCH2N), 3.22 (d, 2JHH = 13.1 Hz, 4H, PCH2N), 0.98 (m, 18H, C(CH3)3). 31P{1H} NMR (C6D6): δ 56.5 (br s, Δν1/2 = 92 Hz). [CpCo(PtBu2NPh2)H](BF4) ([CoIIIH]+). THF (1 mL) was added to a solid mixture of CoI (37 mg, 0.069 mmol) and p-bromoanilinium tetrafluoroborate (21 mg, 0.081 mmol). The resulting olive solution was stirred, and an olive precipitate began forming within 1 min. CH3CN (1.5 mL) was added to the slurry, causing the solid to dissolve and affording a brown solution. Pentane vapor was diffused into this solution at room temperature for 2 days to afford large brown plates of [CoIIIH]+ (33 mg, 0.053 mmol, 77%). X-ray-quality single crystals were grown by layering pentane on a concentrated THF solution of [CoIIIH]+. Anal. Calcd for [CoIIIH]+·0.5THF, C31H46BCoF4N2O0.5P2: C, 56.21; H, 7.00; N, 4.23. Found: C, 56.56; H, 7.24; N, 4.14. 1H NMR (CD3CN): δ 7.35 (t, 3JHH = 7.6 Hz, 2H, Ar meta H), 7.30 (t, 3 JHH = 7.6 Hz, 2H, Ar meta H), 7.10 (d, 3JHH = 8.3 Hz, 2H, Ar ortho H), 7.02 (d, 3JHH = 8.3 Hz, 2H, Ar ortho H), 6.98 (t, 3JHH = 7.3 Hz, 1H, Ar para H), 6.92 (t, 3JHH = 7.3 Hz, 1H, Ar para H), 5.27 (s, 5H, C5H5), 3.81 (m, 4H, PCH2N), 3.64 (m, 2H, THF), 3.58 (m, 4H, PCH2N), 1.80 (m, 2H, THF), 1.37 (m, 18H, C(CH3)3), −15.25 (t, 1H, 2JPH = 57 Hz, CoH). 31P{1H} NMR (CD3CN): δ 67.7 (br s, Δν1/2 = 59 Hz). See Table 2 for electrochemical data. [CpC6F5Co(PtBu2NPh2)](BF4) ([CoII]+C6F5). Solid NaCpC6F5 (29 mg, 0.11 mmol) was added to a stirred brown solution of [Co(PtBu2NPh2)(CH3CN)3](BF4)2 (70 mg, 0.091 mmol) in CH3CN (3 mL), causing an immediate color change to red. The reaction mixture was stirred for 10 min, and then the solvent was removed. The resulting brown solid was washed with water (10 mL) to remove NaBF4 and washed with toluene (2 × 10 mL), and then the solid was dissolved in CH3CN and this solution filtered through Celite. The filtrate was concentrated to ca. 2.5 mL under reduced pressure, and then vapor diffusion of Et2O into the CH3CN solution afforded brown crystals of [CoII]+C6F5 (40 mg, 0.051 mmol, 56%). Anal. Calcd for C35H40BCoF9N2P2: C, 53.12; H, 5.09; N, 3.54. Found: C, 53.13; H, 5.16; N, 3.66. See Table 2 for electrochemical data. CpC6F5Co(PtBu2NPh2) (CoIC6F5). A solution of NaCpC6F5 (33 mg, 0.13 mmol) in CH3CN (3 mL) was added to a stirred dark brown solution of [Co(PtBu2NPh2)(CH3CN)3](BF4)2 (100 mg, 0.130 mmol) in CH3CN (3 mL). The reaction mixture was stirred for 5 min, and KC8 (18 mg, 0.15 mmol) was added to the reaction mixture, causing an immediate color change to red and formation of a red precipitate. After this mixture was stirred for 5 min, the solvent was removed under vacuum, and then the resulting solid was extracted in toluene (1 mL) and the solution filtered through Celite. The filtrate was cooled overnight at −35 °C, affording dark red crystals of CoIC6F5 (60 mg,

0.086 mmol 66%). Hexane (ca. 1 mL) was added to the mother liquor of this solution, and this mixture was cooled to −35 °C for 3 days to afford X-ray-quality crystals of Co I C6F5 . Anal. Calcd for C38.5H44CoF5N2P2 (CoIC6F5·0.5Tol): C, 61.60; H, 5.91; N, 3.73. Found: C, 61.50; H, 5.90; N, 3.92. 1H NMR (C6D6): δ 7.13 (t, 3JHH = 7.5 Hz, 4H, Ar meta H), 6.75 (t, 3JHH = 6.5 Hz, 2H, Ar para H), 6.70 (d, 3JHH = 7.8 Hz, 4H, Ar ortho H), 5.00 (s, 2H, C5H4C6F5), 4.70 (s, 2H, C5H4C6F5), 3.20 (m, 8H, PCH2N), 0.95 (m, 18H, C(CH3)3). 31 1 P{ H} NMR (C6D6): δ 50.4 (br s, Δν1/2 = 75 Hz). 31P{1H} NMR (PhCN): δ 54.4 (br s, Δν1/2 = 80 Hz). 19F NMR (C6D6): δ −143.2 (d, 3 JFF = 20 Hz, 2F, Ar ortho F), −165.1 (t, 3JFF = 20 Hz, 2F, Ar meta F), −167.2 (t, 3JFF = 20 Hz, 1F, Ar para F). [CpC6F5Co(PtBu2NPh2)H](BF4) ([CoIIIH]+C6F5). p-Bromoanilinium tetrafluoroborate (17 mg, 0.065 mmol) was added to a stirred red solution of CoIC6F5 (40 mg, 0.057 mmol) in THF (1 mL), causing an immediate color change to yellow. After 20 min, THF was removed under vacuum and the resulting yellow powder was dissolved in CH3CN. Et2O was layered on the CH3CN solution, which was cooled overnight to −35 °C to afford yellow crystals of [CoIIIH]+C6F5 (21 mg, 0.027 mmol, 47%). X-ray-quality single crystals were grown by diffusing Et2O vapor into a concentrated CH3CN solution of [CoIIIH]+C6F5 for 2 days. 1H NMR (CD3CN): δ 7.37 (t, 3JHH = 7.7 Hz, 2H, Ar meta H), 7.15 (t, 3JHH = 7.7 Hz, 2H, Ar meta H), 7.11 (d, 3 JHH = 8.5 Hz, 2H, Ar ortho H), 7.02 (t, 3JHH = 7.3 Hz, 1H, Ar para H), 6.87 (d, 3JHH = 8.4 Hz, 2H, Ar ortho H), 6.78 (t, 3JHH = 7.4 Hz, 1H, Ar para H), 5.67 (s, 2H, C5H4C6F5), 5.60 (s, 2H, C5H4C6F5), 3.96 (m, 2H, PCH2N), 3.88 (d, 2JHH = 14.4 Hz, 2H, PCH2N), 3.71 (m, 2H, PCH2N), 3.48 (d, 2JHH = 14.4 Hz, 2H, PCH2N), 1.96 (s, 0.7H, CH3CN), 1.40 (m, 18H, C(CH3)3), −15.09 (t, 1H, 2JPH = 61 Hz, CoH). 31P{1H} NMR (CD3CN): δ 64.0 (br s, Δν1/2 = 50 Hz). 19F NMR (CD3CN): δ −133.9 (m, 2F, C5H4C6F5), −147.4 (s, 4F, BF4), −152.5 (m, 1F, C5H4C6F5, para), −159.2 (m, 2F, C5H4C6F5). Anal. Calcd for C35.6H41.9CoF5N2.3P2 ([CoIIIH]+C6F5·0.3CH3CN): C, 61.60; H, 5.91; N, 3.73. Found: C, 61.50; H, 5.90; N, 3.92. See Table 2 for electrochemical data. [CpC5F4NCo(PtBu2NPh2)](BF4) ([CoII]+C5F4N). A solution of NaCpC5F4N (25 mg, 0.10 mmol) in THF (2 mL) was added to a stirred brown solution of [Co(PtBu2NPh2)(CH3CN)3](BF4)2 (70 mg, 0.090 mmol) in CH3CN (3 mL), causing an immediate color change to purple. The reaction mixture was stirred for 2 h, and the solvent was removed. The black solid was washed with water (2 × 10 mL) to remove NaBF4 and washed with toluene (2 × 10 mL), and then the solid was dissolved in CH3CN and filtered through Celite. The filtrate was concentrated to ca. 3 mL under reduced pressure, and then vapor diffusion of Et2O into the purple CH3CN solution afforded black crystals of [CoII]+C5F4N (43 mg, 0.055 mmol, 61%) after 2 days. 1H NMR (CD3CN): δ 7.90 (br s, Δν1/2 = 60 Hz), 4.50 (br s, Δν1/2 = 220 Hz), 0.16 (br s, Δν1/2 = 555 Hz), −1.04 (br s, Δν1/2 = 880 Hz). Anal. Calcd for C34H40BCoF8N3P2: C, 52.73; H, 5.21; N, 5.43. Found: C, 52.57; H, 5.24; N, 5.57. See Table 2 for electrochemical data. CpC5F4NCo(PtBu2NPh2) (CoIC5F4N). A solution of NaCpC5F4N (34 mg, 0.14 mmol) in THF (3 mL) was added to a stirred dark brown solution of [Co(PtBu2NPh2)(CH3CN)3](BF4)2 (100 mg, 0.130 mmol) in CH3CN (5 mL), causing an immediate color change to purple. The reaction mixture was stirred for 5 min, and KC8 (20 mg, 0.15 mmol) was added to the reaction mixture, causing an immediate color change to blue. The solvent was removed under reduced pressure, and then the resulting solid was dissolved in toluene (10 mL) and the solution filtered through Celite. The filtrate was concentrated to ca. 2 mL under reduced pressure, and then vapor diffusion of pentane into the toluene solution afforded dark purple crystals of CoIC5F4N (70 mg, 0.10 mmol 78%) after 4 days. 1H NMR (C6D6): δ 7.12 (t, 3JHH = 7.7 Hz, 4H, Ar meta H), 6.78 (t, 3JHH = 7.3 Hz, 2H, Ar para H), 6.64 (d, 3JHH = 8.2 Hz, 4H, Ar ortho H), 5.00 (s, 2H, C5H4C5F4N), 4.74 (s, 2H, C5H4C5F4N), 3.16 (m, 8H, PCH2N), 0.91 (m, 18H, C(CH3)3). 1H NMR (CD3CN): δ 7.08 (t, 3JHH = 7.0 Hz, 4H, Ar meta H), 6.79 (d, 3 JHH = 7.8 Hz, 4H, Ar ortho H), 6.57 (unresolved t, 2H, Ar para H), 5.21 (s, 2H, Δν1/2 = 240 Hz, C5H4C5F4N), 4.30 (s, 2H, Δν1/2 = 250 Hz, C5H4C5F4N), 3.58 (m, 8H, PCH2N), 1.29 (m, 18H, C(CH3)3). K

dx.doi.org/10.1021/om5004607 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

P{1H} NMR (C6D6): δ 53.8 (br s, Δν1/2 = 70 Hz). 19F NMR (C6D6): δ −92.6 (m, 2F, C5F4N), −142.4 (m, 2F, C5F4N). Anal. Calcd for C34H40CoF4N3P2: C, 59.39; H, 5.86; N, 6.11. Found: C, 59.53; H, 5.98; N, 6.04. See Table 2 for electrochemical data. [CpC5F4NCo(PtBu2NPh2)H](BF4) ([CoIIIH]+C5F4N). p-Bromoanilinium tetrafluoroborate (10 mg, 0.039 mmol) was added to a stirred blue solution of CoIC5F4N (30 mg, 0.044 mmol) in THF (5 mL). The solution was stirred for 20 min, and a yellow precipitate formed. Et2O (15 mL) was added to the slurry. After the mixture was stirred for an additional 20 min, the blue mother liquor was removed by pipet. The remaining yellow powder was washed with Et2O (3 × 15 mL) and dried under vacuum to afford [CoIIIH]+C5F4N (17 mg, 0.022 mmol, 50% yield). 1H NMR (CD3CN): δ 7.36 (m, 2H, Ar meta H), 7.12 (m, 4H, Ar ortho and meta H), 7.03 (m, 1H, Ar para H), 6.86 (d, 3JHH = 8.2 Hz, 2H, Ar ortho H), 6.76 (t, 3JHH = 7.2 Hz, 1H, Ar para H), 5.80 (s, 2H, C5H4C5F4N), 5.69 (s, 2H, C5H4C5F4N), 3.98 (m, 2H, PCH2N), 3.90 (m, 2H, PCH2N), 3.73 (m, 2H, PCH2N), 3.46 (m, 2H, PCH2N), 1.42 (m, 18H, C(CH3)3), −15.31 (t, 1H, 2JPH = 61 Hz, CoH). 31P{1H} NMR (CD3CN): δ 65.0 (br s, Δν1/2 = 50 Hz). 19F NMR (CD3CN): δ −92.0 (m, 2F, C5F4N), −139.5 (m, 2F, C5F4N), −151.0 (s, 4F, BF4). Anal. Calcd for C34H41BCoF8N3P2: C, 52.67; H, 5.33; N, 5.42. Found: C, 52.73; H, 5.47; N, 5.09. See Table 2 for electrochemical data. Catalytic Hydrogen Production. A typical procedure is illustrated here. A 10 mM Co stock solution was prepared by diluting [CoII]+C5F4N (7.7 mg, 0.010 mmol) to 1.0 mL with CH3CN. A 0.10 M acid stock solution was prepared by diluting [p-MeOC6H4NH3]BF4 (21.1 mg, 0.10 mmol) to 1.0 mL with CH3CN. A 0.20 M electrolyte stock solution was prepared by diluting nBu4NPF6 (774.9 mg, 2.0 mmol) to 10 mL with CH3CN. Cyclic voltammograms were recorded following (1) addition of 0.980 mL of electrolyte stock solution, (2) addition of 0.020 mL of [CoII]+C5F4N stock solution, (3) addition of ferrocene (one crystal,