Bridgehead Hydrogen Atoms Are Important: Unusual Electrochemistry

Article pubs.acs.org/Organometallics

Bridgehead Hydrogen Atoms Are Important: Unusual Electrochemistry and Proton Reduction at Iron Dimers with Ferrocenyl-Substituted Phosphido Bridges Carolina Gimbert-Suriñach,*,† Mohan Bhadbhade,‡ and Stephen B. Colbran*,† †

School of Chemistry and ‡Mark Wainwright Analytical Centre, University of New South Wales, Sydney, New South Wales 2052, Australia S Supporting Information *

ABSTRACT: The diphosphido-bridged diiron clusters syn-[{μ2-P(CH2Fc)H}2Fe2(CO)6] (2a) and anti-/syn-[{μ2-P(CH2Fc)Me}2Fe2(CO)6] (3), containing covalently linked ferrocenyl (Fc) groups, were synthesized in order to explore the effect of having a redox-active ligand bound to a Fe2P2 core as in the covalently linked Fe4S4-{μ2-S(Cys)}-Fe2S2 cofactor of [FeFe]hydrogenases. The X-ray crystal structure of 2a shows an Fe−Fe bond length of 2.630(1) Å and confirms that the two P−H bonds of the bridging 1°-phosphido groups are parallel and are separated by 2.683(2) Å. Cyclic voltammetry and spectroelectrochemistry studies revealed that 2a, unusually, undergoes one-electron reduction at −2.18 V (vs Fc+/Fc) to give the anion [{μ2-P(CH2Fc)}{μ2-P(CH2Fc)H}Fe2(CO)6]− ([2a − H]−), which was independently obtained by deprotonation of 2a with excess 1,8-diazabicycloundec-7-ene (DBU). The reduction proceeds through the radical anion 2a′•− intermediate, which was detected by X-band EPR spectroscopy in situ during electrolysis. The formation of [2a − H]− from the 2a′•− radical formally equates to loss of a hydrogen atom from the bridging P−H group. The result suggests that a new low-energy route for evolution of molecular hydrogen is available in Fe2E2 dimers with bridgehead hydrogen atomsi.e. dimers with hydrogen directly bonded to the bridging nonmetal atoms (E = P, S). In contrast to the one-electron reduction behavior of 2a, the mixture of dimers 3 exhibited a two-electron reduction at −2.11 V (vs Fc+/Fc) that afforded 32−. Both dimers catalyze the reduction of protons from p-toluenesulfonic acid, with 2a exhibiting higher catalytic currents at lower overpotential.

1. INTRODUCTION Multielectron reductions such as conversion of carbon dioxide to formate or methanol, molecular nitrogen to ammonia, or protons to molecular hydrogen are important in a world with burgeoning energy demand and dwindling resources.1 Multielectron reactions, however, are sluggish: catalysts are needed to circumvent the inherently high activation barriers for the uncatalyzed reactions. Biology employs enzymes containing multimetallic cluster centers to catalyze such multielectron reduction reactions, achieving very high efficiencies under mild conditions. Prominent examples are the [FeFe]-hydrogenases, which are excellent proton reduction catalysts with turnovers of 6000−9000 mol of H2/mol catalyst.2 The cofactor of [FeFe]hydrogenases is a unique iron−sulfur cluster comprised of Fe2S2 and Fe4S4 subunits linked through a μ2-S(cysteinyl) bridge (Figure 1A). The Fe2S2 dimer is the active site for proton binding and hydrogen evolution, while the Fe4S4 tetranuclear cluster is responsible for electron mediation. Cooperativity between both subsites is proposed to be © 2012 American Chemical Society

responsible for the high catalytic rate and efficiency at which [FeFe]-hydrogenases operate.3 Most of the biomimetic models of the [FeFe]-hydrogenase cofactor are based on the dithiolate structure shown in Figure 1B (top),2b−e although a few diphosphido analogues are also known (Figure 1B, bottom).4 Particular effort has recently been expended to reproduce the (CH2)2NH bridge of the natural cofactor (Figure 1A),2 which is thought to be crucial as a proton relay during reduction. 5 Although the models synthesized so far accurately mimic the Fe2S2 subunit, most of them lack a group mimicking the Fe4S4 cluster; that is, a group able to supply/accept electrons during reduction of protons/oxidation of molecular hydrogen. Considering the importance of electronic communication between the Fe2S2 and the Fe4S4 clusters in the protein,3 the synthesis and study of new iron cluster cores (Fe2X2) linked to suitable redox-active Received: November 15, 2011 Published: April 27, 2012 3480

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Figure 1. (A) Structure of the [FeFe]-hydrogenase cofactor. X is likely to be NH. (B) Biomimetic models reported in the literature. (C) Compounds targeted in this paper.

Scheme 1. Synthesis of Compounds 1−3 (Fc = Ferrocene)

in the dimers with 1°-phosphido bridges might facilitate proton reduction. Protonation of bridging sulfur atoms (μ-S) in analogous dithiolato dimers have been suggested as a plausible step toward proton reduction catalysis.9 Dance has also proposed that an analogous buildup of protonated sulfido (μSH) bridges is key to reduction of molecular nitrogen and the hydrogenase activity exhibited by the FeMoco cofactor of nitrogenase.10

groups may afford new insights and produce new [FeFe]hydrogenase analogues with enhanced efficiencies. There are only a few reports of [FeFe]-hydrogenase mimics with a Fe2X2 core attached to a redox-active group.6 Relevant examples are {μ2,μ2-S(CH2)3S}Fe2(CO)5(mppf) (mppf = (C 5 H 5 )Fe(C 5 H 4 PPh2 )) 6c and {μ 2 ,μ2 -SCH 2 N(Bn)CH 2 S}Fe2(CO)3(FcP*)(dppv) (FcP* = (C5Me5)Fe(C5Me4CH2PEt2); dppv = cis-C2H2(PPh2)2).6d In both cases, the presence of the redox-active ferrocenyl or nonamethylferrocenyl moiety influences the oxidative response of the cluster and either facilitates access to the “fully oxidized” FeII2S2 species6c or enhances the catalytic activity toward hydrogen oxidation.6d Also relevant are complexes with Fe2S2 groups covalently linked to photosensitizers such as [Ru(bipy)3]2+ and Zn(porphyrinato) moieties to photoinject electrons into the Fe2S2 core for proton reduction.7 A recent theoretical investigation finds that electronic communication between a Fe2S2 core and attached redox centers should facilitate proton reduction.6b The present work describes the synthesis and study of [FeFe]-hydrogenase analogues containing phosphido bridges with ferrocenyl substituents (Figure 1C), which have been incorporated not only for their potential role as electron donor (or acceptor) groups but also as an internal electrochemical standard.8 Moreover, we were intrigued as to whether hydrogen atoms directly bonded to the bridging phosphorus atoms (μ-P)

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. Treatment of the stable 1°-phosphine (ferrocenylmethyl)phosphine (FcCH2PH2)11 with iron carbonyl proved a convenient, straightforward methodology to covalently link ferrocenyl groups to a Fe2P2 core.12 The best results were obtained after heating a mixture of FcCH2PH2 and excess of Fe(CO)5 in toluene at 90 °C, which afforded the targeted dimer {μ2P(CH2Fc)H}2Fe2(CO)6 (2) as a mixture of syn and anti isomers in 40% yield (2a,b in reaction 2, respectively, in Scheme 1). A minor byproduct also formed and was identified by (+)-ESI mass spectrometry and 31P{1H} NMR spectroscopy as the trimer (μ3-PCH2Fc)2Fe3(CO)9. μ3-Phosphinidenecapped clusters are well-known for Fe,12c,e Ru,13 and Os.13b,14 The formation of 2 presumably proceeds through the mononuclear complex (FcCH2PH2)Fe(CO)4 (1), which dimerizes by consecutive oxidative addition−reductive elimi3481

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nation steps. This hypothesis is supported by the isolation of monomer 1 as the predominant product when similar reaction mixtures were heated for short times (reaction 1, Scheme 1). Complex 1 is a highly air-sensitive red oil that was always isolated with small amounts of 2a,b and was characterized by FTIR and NMR spectroscopy. When a solution of 1 was heated to 90 °C, conversion to 2a,b was observed, accompanied by precipitation of a considerable amount of intractable solid. However, the conversion from 1 to 2a,b was considerably improved by adding excess Fe(CO)5 (5 equiv). These experiments reveal that an excess of iron carbonyl assists the synthesis of 2a,b. Dimer 2 has three possible isomers that differ in the relative positioning of the hydrogen and ferrocenylmethyl substituents on the phosphido bridges, but only 2a,b were obtained under the conditions used (Scheme 1). Dimer 2b was identified by NMR spectroscopy and is characterized by two doublets (2JP−P′ = 157 Hz) at δ 83.6 and 90.3 in the 31P{1H} NMR spectrum for the two nonequivalent phosphorus atoms. The symmetric, less hindered compound 2a was the major and most stable isomer and was isolated free from 2b by recrystallization. It was fully characterized by FTIR and NMR spectroscopy, (+)-ESI mass spectrometry, and X-ray crystallography. The FTIR spectrum of compound 2a shows three carbonyl bands at 2053, 2016, and 1976 cm−1 in THF, which have a profile very similar to that exhibited by other analogous compounds of the formula (μ2-PHR)2Fe2(CO)6 (entries 4−7, Table 1). The νC̅ O values for 2a, which are the lowest in the series, reveal the strong electron-donating character of the FcCH2 substituent. A similarly strong electron donation was observed and thoroughly studied for related Fe2S2 dimers with (CH3)2SnCH2 substituents to the thiolate bridges.15 The 31P NMR spectrum of dimer 2a shows a characteristic second-order multiplet at δ 91.8 for the AA′XX′ spin system of the two P−H groups. Crystals of 2a suitable for diffraction studies were grown from a hexane−dichloromethane solution at room temperature. Figure 2 shows the molecular structure of 2a, which exhibits the near (noncrystallographic) C2 symmetry. The Fe2P2 core displays a butterfly geometry with an angle between the Fe2P wings of 77.5°. The distance between iron atoms, 2.630(1) Å, is consistent with a metal−metal single bond and is within the range found in similar Fe2P2 compounds.16 For the phosphido bridges, the acute Fe−P−Fe′ angles are 73.12(6) and 73.45(6)°. The structure confirms that 2a is the syn isomer, with both phosphido P−H bonds lying adjacent and parallel. The distance between the bridgehead phosphorus atoms (P···P′) is 2.683(2) Å. The FcCH2 substituents are aligned away from each other, on the same side of the Fe2(CO)6 core. Treatment of 2a with methyllithium at −78 °C gave the reactive dilithio intermediate Li2·{μ2-P(CH2Fc)}2Fe2(CO)6, the infrared spectrum of which shows carbonyl bands at lower values compared to those for the parent compound 2a (entries 20 and 7, respectively, in Table 1) as expected. This intermediate was not isolated but was treated with methyl iodide to give a 1:1 mixture of the symmetric isomers {μ2P(CH2Fc)Me}2Fe2(CO)6 (3a,b) (reaction 3, Scheme 1). They could not be separated and, therefore, were characterized as a mixture. The 31P{1H} spectrum of dimers 3a,b exhibits two singlets at δ 132.6 and 134.2, whose relative intensities varied upon recrystallization. No evidence for the formation of asymmetric isomer, which would have nonequivalent phosphido bridges, was detected by NMR spectroscopy. In the infrared spectrum of the mixture of 3a,b, three carbonyl bands are

Table 1. Comparative FTIR Data over the Carbonyl Stretching Region for Complexes 1−3 and Their Reduction Products and for Other Relevant Compounds R1

entry

R2

solvent

ν̅CO/cm−1

ref

1

(PH2R )Fe(CO)4 heptane 2063, 2028, 1989, 1955, 1920 heptane 2061, 2024, 1988, 1983, 1918 THF 2052, 1974, 1944

1

Ph

2

Me

3

FcCH2

4

Ph

H

5

Me

H

6

t

Bu

H

7

FcCH2

H

8

Ph

Ph

benzene

9

Ph

Me

pentane

10

Ph

−(CH2)3−

THF

11 12

Me FcCH2

Me Me

DME THF

a

Ph

b

14

FcCH2

15

Ph

16

Ph

17

Me

18c

FcCH2

19

FcCH2

20d

FcCH2

13

(μ2-PR1R2)2Fe2(CO)6 heptane 2063, 2027, 2000, 1996, 1984, 1974 heptane 2063, 2025, 1992, 1982 cyclohexane 2058, 2019, 1983, 1975, 1966 THF 2053, 2016, 1976 2055, 2015, 1992, 1960 2054, 2015, 1989, 1969, 1961 2050, 2015, 1980, 1960, 1950 2044, 2008, 1960 2044, 2006, 1965

[(μ2-PR1R2)2Fe2(CO)6]− −(CH2)3− THF 1990, 1945, 1910, 1880 H/− THF 2006, 1963, 1926, 1914, 1895 [(μ2-PR1R2)2Fe2(CO)6]2− Ph THF 1930, 1905, 1845, 1825, 1800 −(CH2)3− THF 1910, 1890, 1835, 1820, 1805 Me DME 1909, 1879, 1810, 1800 Hc THF 1912, 1896, 1828, 1810 Me THF 1912, 1883, 1816, 1800 THF 1974, 1926, 1889, 1876, 1858

12b 12b this work 12b 12b 12e this work 12d 12f 4c 12a this work 4c this work 16 4c 12a this work this work this work

a Radical anion [{μ2,μ2-PPh(CH2)3PPh}Fe2(CO)6]•−. bData for [HDBU][{μ2-P(CH2Fc)}{μ2-P(CH2Fc)H}Fe2(CO)6]. cThis species was only characterized by its IR spectrum: the profile and energies of its carbonyl bands are consistent for [{μ2-P(CH2Fc)H}2Fe2(CO)6]2−. d Data for Li2·{μ2-P(CH2Fc)}2Fe2(CO)6.

Figure 2. View of the molecular structure of syn-{μ2-P(CH2Fc)H}2Fe2(CO)6 (2a). Thermal ellipsoids at the 50% probability level at 150 K are shown, and carbon-bound hydrogen atoms are omitted for clarity. 3482

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Figure 3. Cyclic voltammetry of 2a obtained with a 1 mm diameter glassy-carbon working electrode, platinum auxiliary electrode, and Ag/AgCl reference electrode: (A) 2a (2 mM) in THF−[(n-Bu)4N][PF6] (0.4 M), 100 mV s−1; (B) 2a (1.5 mM) in THF−[(n-Bu)4N][PF6] (0.2 M), 50−500 mV s−1; (C, D) 2a (2 mM) in THF−[(n-Bu)4N][PF6] (0.2 M), 500 mV s−1.

Scans to negative potential at lower scan rates (200 mV s−1), a second small anodic peak appeared at ∼−1.49 V (E5) (Figure 3B), which grew compared to E4 as the scan rate was raised or, more dramatically, as the temperature was lowered to 195 K (see Figure S2, Supporting Information). These results suggest that E5 is due to the oxidation of the primary reduction product 2a•− (eq 1) and that this radical anion undergoes a rapid chemical step to generate a new species, 2a′•−, which is oxidized at E4. EPR spectroscopy results (see below) suggest 2a′•− is a radical species. We then tested whether the oxidation at E4 regenerated 2a as follows. CVs of 2a were recorded in which the cathodic peak at E3 was traversed and the return positive sweeps were switched before and after the anodic peak at E4. In CVs switched after E5 but before E4 (Figure 3C), the cathodic current at E3 dropped in subsequent scans, whereas if E4 was traversed (Figure 3D), the cathodic current at E3 remained unchanged. These results confirmed that dimer 2a, which is reduced to 2a•− at E3, was fully replenished by traversing peak E4 but not peak E5. The electrochemistry can thus be summarized by a classic ECEC “square scheme” shown in Scheme 2;21 the primary reduction

[{μ2 P‐(CH 2Fc)H}2Fe2(CO)6 ] + e− → [{μ2 ‐P(CH 2Fc)}{μ2 ‐P(CH 2Fc)H}Fe 2(CO)6 ]− + 1/2 H 2

(2)

Brønsted base 1,8-diazabicycloundec-7-ene (DBU) (see eq 3). Compound [H-DBU][2a − H] was characterized by FTIR and [{μ2 ‐P(CH 2Fc)H}2Fe 2(CO)6 ] + DBU ⇄ [H‐DBU][{μ2 ‐P(CH 2Fc)}{μ2 ‐P(CH 2Fc)H}Fe 2 (CO)6 ]

(3)

31

P{1H} NMR spectroscopy, by (−)-ESI mass spectrometry, and by cyclic voltammetry (see Figure S4, Supporting Information). The CV of [2a − H]− shows, in addition to the ferrocene-centered oxidation, only a broad anodic peak at −0.40 V and a cathodic peak at −3.0 V that merges with the cathodic discharge for the DBU−electrolyte solution, but none of the peaks E3, E4 or E5 are observed (Figure S4B, Supporting Information). Therefore, [2a − H]− is not a species observed in the CVs of 2a, confirming that [2a − H]− is not 2a′•− in Scheme 2 but forms from 2a′•− on the longer time scale of the electrolyses (seconds). Formation of the species [2a − H]− upon one-electron reduction of 2a requires the homolytic fission of the P−H bond to give half a molecule of H2 (formally loss of a hydrogen atom) (eq 2) and therefore represents a novel pathway for hydrogen evolution. Given that dimer 2a is easily recovered by reprotonation of [2a − H]− with weak acid, the loss of a bridgehead (E−H) hydrogen atom upon reduction represents a previously unconsidered pathway for hydrogen evolution catalyzed by Fe2E2 dimers (E = P, S). This is discussed further below. CVs of 3 are presented in Figure 4 and reveal a different electrochemical reduction response in comparison to that of 2a. The peak current for the apparently irreversible broad reduction wave observed at E6 = −2.11 V is close to that for the two-electron ferrocenyl couple oxidation (E9, E10) at +0.05 V, e.g., ipc (E6) ∼ 0.76 ipc (E10), irrespective of scan rate. These results are indicative of a two-electron reduction to give 32−, the anticipated Fe0Fe0 dianion (eq 4). FTIR and EPR spectroelec-

Scheme 2. ECEC Mechanism Proposed for the Reduction of 2a

E 6 :[{μ2 ‐P(CH 2Fc)Me}2Fe2(CO)6 ] + 2e− 3

product 2a•− reacts within the time scale of the experiment to give the new species 2a′•−. In the return positive sweep, oxidation of 2a′•− at E4 regenerates 2a. As a further test of this interpretation, digital simulations of the CVs were performed. Although the results are insufficient to quantitatively define the thermodynamic and kinetic parameters, the good fits obtained for E(2a/2a•−) = −2.00 V and E(2a′/2a′•−) = −1.52 V provide a qualitative measure of the validity of our interpretation of the electrochemistry (see Figure S3 in the Supporting Information). Spectroelectrochemical experiments presented below reveal that the monoanion [{μ 2 -P(CH 2 Fc)}{μ 2 -P(CH 2 Fc)H}Fe2(CO)6]− ([2a − H]−) is the ultimate product of the reduction of 2a, equating overall to eq 2. The formation of [2a − H]− was confirmed by isolation of the same species after deprotonation of 2a by excess of the weakly-nucleophilic

→ [{μ2 ‐P(CH 2Fc)Me}2Fe2(CO)6 ]2 − 32 −

(4)

trochemical studies support the formation of 32− and indicate that the radical anion 3•− is an intermediate species during reduction, which undergoes one-electron reduction at a potential more positive than the starting dimer 3. Similar inversion of consecutive reduction potentials leading to overall two-electron reduction processes has been observed for other dithiolato19d and diphosphido-bridged dimers.4c,18,22 As noted above, the broad profile observed for wave E6 is indicative of slow electron transfer kinetics and structural change during reduction. We suggest that the bulkier phosphido bridges in dimer 3 compared to those in dimer 2a increase intramolecular steric strain and, thereby, should favor more rapid opening of 3484

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Figure 4. Cyclic voltammetry of 3 obtained with a 1 mm diameter glassy-carbon working electrode, platinum auxiliary electrode, and Ag/AgCl reference electrode: (A) 1 mM in THF−[(n-Bu)4N][PF6] (0.4 M), 100 mV s−1; (B) 2 mM in THF−[(n-Bu)4N][PF6] (0.2 M), 100−500 mV s−1.

electron reduction of the dimers to afford the dianion [{μ2P(CH2Fc)Me}2Fe2(CO)6]2− (32−).12a,16 The CO stretching bands observed for the species generated from 2a are higher than those for 32− by ∼80−110 cm−1 (Figures 5A and 6A, respectively), revealing that they are for species with different charges. We first considered the possibility that the species observed after reduction of 2a was the radical anion 2a•−. Of note, the FTIR spectrum obtained after reduction of 2a is similar to that simulated by Best and coworkers for the radical anion [{μ2,μ2-PPh(CH2)3PPh}Fe2(CO)6]•− (5•−), the putative intermediate in the twoelectron reduction of 5 (compare entries 13 and 14, Table 1),4c and also to the FTIR spectrum for the related dithiolatebridged radical anion [(μ2-pdt)Fe2(CO)6]•− (pdt = propane1,3-dithiolate).23 However, as already noted, deprotonation of 2a with excess DBU affords diamagnetic [2a − H]−, which has an FTIR spectrum identical with that of the product of reductive electrolysis of 2a (compare parts A and D of Figure 5), thus confirming that [2a − H]− is the ultimate reduction product (eqs 2 and 3). We considered the possibility that [2a − H]− is formed by deprotonation of 2a by extraneous species generated at the negative potential of the reductive electrolyses. This possibility is discounted for several reasons. First, the solutions were rigorously anhydrous and oxygen free (see the Experimental Section) and negligible electrolysis current was observed in blank runs without 2a present. Second, the electrolyses of 2a were clean and rapid, with the current rising only when negative potentials corresponding to wave E3 for the reduction of 2a were reached. Third, the EPR spectoelectrochemistry, discussed in the next paragraph, suggests an intermediate radical species is generated from 2a during electrolysis. The X-band EPR spectrum acquired in situ during reductive electrolysis of 2a using an EPR spectroelectrochemical cell is shown in Figure 5B. A broad triplet at giso = 2.021 is observed. Simulation is consistent with hyperfine couplings of 13.8 G and 1−2 G due to the two equivalent P−H groups. Coupling to the exo-oriented methylene protons is not observed and must be less than 0.2 G22 (see Figure S6, Supporting Information). Given that conversion of 2a•− to 2a′•− is rapid (see above), the latter species should be that observed in the EPR spectrum. The EPR spectrum suggests 2a′•− is a radical and retains two equivalent P−H groups. A possible scenario for the chemical step following formation of 2a•− is coordination of a tetrahydrofuran molecule to the 17-electron Fe(I) center in radical 2a•− to afford a 19-electron Fe(I) center in 2a′•−, which would retain the equivalence of the P−H groups. The

the butterfly geometry upon reduction. More rapid structural change is associated with overall two-electron reduction behavior.19a Two irreversible oxidation waves appeared beyond the ferrocene oxidation at ∼+0.72 and ∼+1.26 V (Supporting Information). The latter is associated with the one-electron oxidation of the Fe2P2 core.6c,17a,b The presence of trace iodide, carried over from the preparation of 3, is possibly responsible for the peak at ∼+0.72 V and for the peak intensity of ipa (E9) being slightly higher than that of ipc (E10) (see Figure S1B, Supporting Information). After traversing E6, the return positive sweep displayed a prominent coupled oxidation process at E7 = −1.56 V and a weak second oxidation wave at E8 = −0.91 V. Oxidations E7 and E8 did not appear unless E6 was traversed first, and therefore, they arise from oxidation of the reduced species produced at E6: i.e., from oxidation of 32− and 3•−, respectively (eqs 5 and 6). E7 :[{μ2 ‐P(CH 2Fc)Me}2Fe2(CO)6 ]2 − 32 −

→ [{μ2 ‐P(CH 2Fc)Me}2Fe2(CO)6 ]•− + e− 3•−

(5)

E 8:[{μ2 ‐P(CH 2Fc)Me}2Fe2(CO)6 ]•− 3•−

→ [{μ2 ‐P(CH 2Fc)Me}2Fe2(CO)6 ] + e− 3

(6)

Digital simulations were performed to support the assignment of waves E7 and E8. A close fit between experimental and simulated CVs was obtained for E(3•−/32−) = −1.88 V and E(3/3•−) = −2.11 V (see Figure S5, Supporting Information), consistent with potential inversion of the successive reductions as described above. FTIR and EPR Spectroelectrochemistry. FTIR spectroelectrochemistry experiments of dimers 2a and 3 were performed to identify the reduction products. For 2a, clean conversion to a new species with lower carbonyl bands at 2006, 1963, 1926, 1914, and 1895 cm−1 was observed (with isosbestic points at 2008, 2000, and 1968 cm−1; see Figure 5A). The same experiment using 3 showed new bands growing at 1912, 1883, 1816, and 1800 cm−1 (with an isosbestic point at 1937 cm−1; see Figure 6A). The behavior observed for 3 is analogous to that observed for the dimers 4−64a,c,12a and is indicative of two3485

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Figure 5. (A) FTIR spectroelectrochemistry of 2a (4 mM) in THF−[(n-Bu)4N][PF6] (0.4 M), −2.1 V; (B) X-band EPR spectrum of 2a′•− obtained in situ during electrolysis of 2a (2 mM) in THF−[(n-Bu)4N][PF6] (0.4 M) at 295 K, −2.5 V; (C) FTIR spectrum at maximum conversion of [2a − H]− to a new reduction product upon electrolysis at −3 V in THF−[(n-Bu)4N][PF6] (0.4 M) (the asterisk indicates a solvent subtraction artifact); (D) FTIR in THF of [H-DBU][{μ2-P(CH2Fc)}{μ2-P(CH2Fc)H}Fe2(CO)6] ([2a − H]−) (purple solid line) and Li2·{μ2-P(CH2Fc)}2Fe2(CO)6 ([2a − 2H]2−) (black dotted line).

Figure 6. (A) FTIR spectroelectrochemistry of 3 (2 mM) in THF−[(n-Bu)4N][PF6] (0.4 M), −2.6 V; (B) X-band EPR spectrum of 3•− obtained in situ during electrolysis of 3 (1 mM) in THF−[(n-Bu)4N][PF6] (0.4 M) at 295 K, −2.6 V.

distinct from those for dilithiated Li2·{μ2-P(CH2Fc)}2Fe2(CO)6 (compare parts C and D of Figure 5). The peaks are comparable in profile and energy to those observed for 32− (Figure 6A), thus suggesting the reduction product is 2a2−. Given the conversion occurs with significant decomposition and is incomplete, we are reluctant to speculate further about this reduction of [2a − H]−. EPR spectra recorded in situ during electrolysis of a solution of 3 at −2.6 V showed a signal at giso = 1.999 consistent with detection of the intermediate radical anion 3•− (Figure 6B).22 A simulation of the spectrum is consistent with overlapping signals from the anti/syn symmetric isomers 3a•− and 3b•− (see Figure S7, Supporting Information). Detection of the one-

formation of 19-electron organometallic radical species from the corresponding 17-electron species by coordination of a solvent molecule is a well-accepted step, which is generally thermodynamically favorable.24 The simulation of the CV gives Keq = 5 × 104, which corresponds to ΔGo = −27 kJ mol−1 for the chemical step (see Figure S3, Supporting Information). The reduction of electrogenerated [2a − H]− at −3 V was also investigated by FTIR spectroelectrochemistry. Incomplete conversion to a new species with carbonyl bands at 1912, 1896, 1828, and 1810 cm−1, consistent with the formation of a doubly reduced species, was observed accompanied by significant decomposition (Figure 5C). Significantly, the carbonyl peaks of the new species generated after electrolysis of [2a − H]− are 3486

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Figure 7. (A) Cyclic voltammograms of solutions of 2a (1.5 mM) in THF−[(n-Bu)4N][PF6] (0.2 M), with 0, 0.9, 3.9, 6.0, and 12.0 mM HOTs, obtained with a 1 mm diameter glassy-carbon working electrode, platinum auxiliary electrode, and Ag/AgCl reference electrode, 100 mV s−1. (B) Plot of ipc/[cat.] of C1 and C3 vs [HOTs]. (C) Cyclic voltammograms of solutions of 3 (1.6 mM)) in THF−[(n-Bu)4N][PF6] (0.2 M), with 0, 1.7, 3.8, 5.4, and 7.2 mM [HOTs], obtained with a 1 mm diameter glassy-carbon working electrode, platinum auxiliary electrode, and Ag/AgCl reference electrode, 100 mV s−1. (D) Plot of ipc/[cat.] of C2 and C4 vs [HOTs] and their corresponding linear fits.

electron-reduction product 3•− during electrolysis supports production of the radical anion 3•− as an intermediate for the two-electron reduction and is consistent with a potential inversion19d of the 3/3•− and 3•−/32− couples; i.e., the second couple is positive of the first. Rapid (millisecond time scale) reduction of 3•− explains why the two-electron-reduction product 32− is the only species observable by FTIR spectroelectrochemistry and is in agreement with the overall two-electron-reduction behavior found in the CV experiments. 2.3. Proton Reduction Catalysis. Reduction of 2a in the presence of small amounts of p-toluenesulfonic acid (HOTs) increased the current and broadened and slightly shifted the reduction wave at E3 = −2.18 V (Figure 7A). At higher concentrations of acid, two well-defined catalytic waves, C1 and C2, were observed. The first catalytic wave, C1, occurred ∼70 mV positive of the parent reduction wave E3, and its peak current saturated beyond [HOTs] ≈ 12 mM (Figure 7A,B). The second catalytic wave, C2, moved to negative potential with increasing concentration of acid; e.g., C2 ≈ −2.31 V at [HOTs] = 3.9 mM but C2 ≈ −2.50 V at [HOTs] = 12 mM. It was not possible to reach the plateau point for the peak current of C2, because at higher concentrations of acid, wave C2 started to merge with the reduction wave for the reduction of HOTs at the glassy-carbon electrode. The oxidation wave E4 ≈ −0.71 V was quenched upon acid addition, indicating that the radical 2a′•− was completely consumed by protonation within the CV time scale. The absence of peaks for proton reduction in the same potential range for analogous experiments performed using p-toluenesulfonic acid alone (Figure S8A, Supporting

Information), suggests that C1 and C2 are due to electrocatalytic reduction of protons catalyzed by dimer 2a at process C1 and by a reduced derivative of 2a at process C2. Similar experiments were carried out using 3. As depicted in Figure 7C, two catalytic waves were also observed after addition of HOTs. The first, C3 ≈ −2.02 V, appeared ∼90 mV positive of the reduction wave E7, and the peak current saturated beyond [HOTs] ≈ 3.8 mM (Figure 7B,C). The potential of the second process shifted from C4 ≈ −2.45 V at [HOTs] = 1.7 mM to C4 ≈ −2.57 V at [HOTs] = 7.2 mM; the latter potential remains positive of that observed for the reduction of HOTs at the same working electrode in the absence of 3 (see Figure S8B, Supporting Information). The broad (re)oxidation wave at E7 ≈ −1.56 V in return sweeps was quenched by acid, indicative of consumption of 32− during catalytic reduction. Catalytic overpotentials could not be calculated, as the pKa of HOTs in THF is not available. However, the catalytic peak potentials (Ecat.) provide a measure of the relative overpotential required for the catalysis of hydrogen evolution.25 For dimers 2a and 3, the first catalytic waves C1 and C3 appear at −2.11 and −2.02 V, respectively. At a concentration of ∼3.8−3.9 mM HOTs, the second catalytic waves C2 and C4 are at −2.31 and −2.49 V, respectively, and reveal a difference of 180 mV in overpotential between the two species. These Ecat values are comparable to those obtained for the compounds 4 and 5 under the same experimental conditions as reported by Best and co-workers;4a,c the former dimer exhibits one catalytic wave at −2.26 V,4a and the latter dimer shows two catalytic waves at ∼−2.11 and ∼−2.24 V.4c The plots shown in Figure 7B,D give 3487

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Scheme 3. Mechanisms for Proton Reduction at Fe2X2 Centers: (A) ECEC; (B) EECC; (C) EECCEE

the formation of [2a − H]− is shown in Scheme 4. After reduction and rearrangement of 2a, the radical intermediate

a qualitative assessment of the relative catalytic rates of proton reduction by 2a and 3. At limiting current, the first catalytic process for 2a at C1 is at least 5 times faster than that for 3 at C3, as revealed by the plots of ipc/[cat.] vs [HOTs] shown in Figure 7B.20 The linear plots in Figure 7D show slopes of (1.16 ± 0.02) × 10−6 for catalysis by 2a at wave C2 and (1.15 ± 0.05) × 10−6 for catalysis by 3 at wave C4, suggesting that the rates of hydrogen production at the second catalytic reduction waves are the same within experimental error. Several experimental and theoretical studies have concentrated on the mechanism of proton reduction at a Fe2X2 center.4a,c,23,26 One of the most supported pathways is that corresponding to an ECEC mechanism (e.g., PI in mechanism A, Scheme 3),23,26 but other pathways including EECC4c or EECCEE4a mechanisms have also been proposed (e.g., PI in mechanisms B and C, Scheme 3). The catalytic profiles for both 2a and 3 are similar to that reported for the analogous rigid phosphido-bridged compound {μ2,μ2-PPh(CH2)3PPh}Fe2(CO)6 (5)4c in that hydrogen evolution happens after primary reduction of the starting cluster and also occurs at a second catalytic wave that appears at more negative potentials. Such behavior can be explained by the ECEC and EECC mechanisms A and B in Scheme 3, respectively. This behavior differs from that of the more flexible dimer (μ2-PPh2)2Fe2(CO)6 (4),4a in which the addition of acid shifts the potential of the primary reduction wave but no electrocatalytic response is obtained. In the case of dimer 4, further reduction was necessary to observe hydrogen evolution and the resting state of the catalyst is a reduced species of the starting dimer (mechanism C, Scheme 3). Best and co-workers had pointed out the dependence of the core geometry of Fe2P2 clusters on electrocatalytic proton reduction.4c The presence of the bulky ferrocenyl groups in compounds 2a and 3 may limit the conformational freedom of these dimers, thus influencing their catalytic response. The reduction chemistry of the iron dimer catalyst (one- vs two-electron reduction) is also important, because it will determine the most likely early steps in the mechanism for catalysis of dihydrogen evolution. Dimers 3−5 each undergo two-electron reduction and, therefore, are more likely to follow an EECC-type mechanism (e.g., B or C in Scheme 3). On the other hand, it has been demonstrated that 2a undergoes oneelectron reduction to afford the anion [2a − H]− with loss of one bridgehead hydrogen atom. One plausible mechanism for

Scheme 4. Possible Mechanism for Dihydrogen Evolution Catalyzed by 2a

2a′•− is protonated in acid solution at the 18-electron Fe(0) center to form a neutral iron hydride species, which can undergo reductive elimination of dihydrogen accompanied by spontaneous electron addition to generate monoanion [2a − H]−. Protonation of [2a − H]− regenerates 2a and closes the catalytic cycle. Alternatively, bimolecular elimination of dihydrogen from 2a′•− could also account for the formation of [2a − H]−. Either mechanism involves homolytic fission of a bridgehead P−H bond and affords dihydrogen. As demonstrated above, the different electrochemistry shown by 2a and 3 can strongly influence the proton reduction pathway. A different mechanism could explain the enhanced activity observed for 2a at C1 compared to that of 3 at C3 (Figure 7B). Studies to investigate this hypothesis and to gain further insights into the mechanism of proton reduction induced by 2a and 3 are underway.

3. SUMMARY AND CONCLUSIONS The synthesis and electrochemistry of dimers 2a and 3 and the anion [2a − H]− have been described. The two ferrocenyl groups provide an internal electrochemical standard8 that allows unambiguous assignment of the number of electrons involved in the primary reduction processthe reduction wave for 2a at E3 = −2.18 V corresponds to a one-electron process, while that for 3 at E6 = −2.11 V corresponds to a two-electron process. One- versus two-electron reduction is largely 3488

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26.8 Hz, CH2P), 68.4 (d, JC−P = 2.6 Hz, C2AC2BCCH4), 68.4 (C2AC2BCCH4), 69.4 (C5H5), 83.6 (d, JC−P = 6.3 Hz, C2AC2BCCH4), 212.7 (d, JC−P = 20.7 Hz, CO). IR (THF): νC̅ O (cm−1) 2052 (m), 1974 (m), 1945 (vs). 4.3. Synthesis of {μ2-P(CH2Fc)H}2Fe2(CO)6 (2). To a toluene solution (2 mL) of (ferrocenylmethyl)phosphine (476 mg, 2.05 mmol) was added iron pentacarbonyl (1.6 mL, 12.2 mmol), and the mixture was heated to 90 °C for 72 h. The resulting brown suspension was evaporated to dryness and the residue extracted with dichloromethane (50 mL × 3). The extracts were filtered through Celite and concentrated to give a mixture of 2 (40%) and (μ 3 PCH2Fc)2Fe3(CO)9 (9%). The mixture was redissolved in dichloromethane (15 mL) and the solvent evaporated slowly to ca. 2 mL, affording orange crystals of 2 that were filtered. The filtrate was concentrated again and more crystals formed, which were collected and combined with the first batch (146 mg, 0.20 mmol, 20%). A third recrystallization using a mixture of dichloromethane/pentane afforded bright orange crystals of the major compound, 2a. Analytical data for isomer 2a are as follows. 1H NMR (300 MHz, CDCl3): δ (ppm) 1.78−3.11 (AA′XX′ complex second-order coupling, 2H, PH), 3.09− 3.14 (m, 4H, CH2P), 4.12 (s, 10H, C5H5), 4.13−4.15 (m, 8H, C5H4). 31 P{H} NMR (121.5 MHz, CDCl3): δ (ppm) 91.8. 13C{1H} NMR (75.5 MHz, CDCl3): δ (ppm) 27.9 (dd, JC−P = 13.8 and 13.8 Hz, CH2P), 68.2 (dd, JC−P = 1.4 and 1.4 Hz, C2AC2BCCH4), 68.6 (C2AC2BCCH4), 69.3 (C5H5), 85.0 (dd, JC−P = 3.1 and 3.4 Hz, C2AC2BCCH4), 212.1 (dd, JC−P = 5.0 and 5.1 Hz, CO). IR (THF) ν̅CO (cm−1) 2053 (m), 2016 (vs), 1976 (s). HR-(+)-ESI-MS: m/z 741.8449 found, 741.8440 calcd for [C28H24Fe4O6P2]+. Anal. Calcd for C28H24Fe4O6P2: C, 45.33; H, 3.26. Found: C, 45.99; H: 3.29. Isomer 2b: 31P{1H} NMR (121.5 MHz, CDCl3) δ (ppm) 83.6 (d, JP−P = 157 Hz), 90.3 (d, JP−P = 157 Hz). 4.4. Synthesis of {μ 2-P(CH 2Fc)Me} 2 Fe 2 (CO) 6 (3). To a tetrahydrofuran solution (2 mL) of compound 2a (54.5 mg, 0.073 mmol) cooled to −78 °C was added methyllithium (135 μL, 1.6 M solution in diethyl ether, 0.29 mmol) dropwise to give a bright red solution, followed by 18 μL of methyl iodide (0.29 mmol), causing another color change from red to dark orange. After it was stirred for 30 min at −78 °C, the reaction mixture was warmed to room temperature and stirred for 1 h and the solvent and volatiles were evaporated to dryness. The orange residue was extracted with dichloromethane (10 mL × 2), and the extracts were filtered through Celite and evaporated to give an orange solid identified as 3 (36.7 mg, 0.048 mmol, 65%). Compounds 3 can be further recrystallized using diethyl ether. Analytical data for a mixture of symmetric isomers 3 are as follows. 1H NMR (500 MHz, C6D6): δ (ppm) 0.97 (t, JH−P = 5.7 Hz, 6H, CH3 isomer 1), 1.56 (t, JH−P = 5.4 Hz, 6H, CH3 isomer 2), 3.00 (s broad, 4H, CH2 isomer 2), 3.12 (s broad, 4H, CH2 isomer 1), 3.93 (s, 4H, C5H4 isomer 1), 3.95 (s, 4H, C5H4 isomer 1), 3.98 (s, 5H, C5H5 isomer 1), 4.05 (s, 5H, C5H5 isomer 2), 4.08 (s, 4H, C5H4 isomer 2), 4.09 (s, 4H, C5H4 isomer 2). 31P{1H} NMR (242.9 MHz, C6D6): δ (ppm) 132.6 (isomer 1), 134.2 (isomer 2). 13C{1H} NMR (150.9 MHz, CDCl3): δ (ppm) 12.6 (t, JC−P = 7.6 Hz, CH3 isomer 1), 19.4 (t, JC−P = 15.8 Hz, CH3 isomer 2), 34.4 (s broad, CH2 isomer 2), 38.6 (t, JC−P = 12.8 Hz, CH2 isomer 1), 67.5 (s broad, C2AC2BCCH4 isomer 1), 67.7 (s broad, C2AC2BCCH4 isomer 1), 68.3 (s broad, C2AC2BCCH4 isomer 2), 68.37 (C5H5 isomer 1), 68.41 (s broad, C2AC2BCCH4 isomer 2), 68.5 (C5H5 isomer 2), 82.3 (C2AC2BCCH4 isomer 1), 82.6 (C2AC2BCCH4 isomer 2), 211.6 (s broad, CO isomer 2), 211.9 (s broad, CO isomer 1); IR (THF): ν̅CO (cm−1) 2044 (m), 2006 (vs), 1965 (s). HR-(+)-ESI-MS: m/z 769.8733 found, 769.8753 calcd for [C30H28Fe4O6P2]+. Anal. Calcd for C30H28Fe4O6P2: C, 46.80; H, 3.67. Found: C, 46.43; H: 3.36. 4.5. Electrochemical and Spectroelectrochemical Studies. Cyclic voltammetric measurements were performed in a conventional one-compartment three-electrode cell using a computer-controlled Pine Instrument Co. AFCBP1 bipotentiostat as described in detail elsewhere.29 Cyclic voltammograms were measured, and are presented, without compensation for solution iR drop. The cyclic voltammogram for ferrocene standard was always measured under identical experimental conditions, and only minor variations from ideal behavior

determined by the relative time scales of the electrochemical experiment and the structural rearrangements that accompany electron addition.19d,27 The results suggest that structural change is faster for dimer 3, which has the bulkier phosphido bridges, suggesting release of steric strain may be important. The two-electron reduction response observed for 3 proceeds through the intermediate radical anion 3•−, detected by EPR spectroelectrochemistry during electrolysis, indicating a potential inversion for the consecutive one-electron transfer steps. This behavior is similar to that observed for other (μ2PR2)2Fe2(CO)6 dimers described in the literature.4c,18,19d,22 In contrast, the one-electron reduction of dimer 2a gives the radical 2a•−, which is proposed to bind solvent within the CV time scale to afford 2a′•−. Species 2a′•− was observed by EPR spectroscopy. On the longer electrolysis time scale, anion [2a − H]− forms from 2a′•−, a process that (formally) equates to loss of a bridgehead hydrogen atom. Electrocatalytic responses, due to the reduction of protons to give molecular hydrogen, were observed for 2a and for 3 upon addition of p-toluenesulfonic acid. Dimer 2a showed higher peak current densities (faster rate) than 3, at one of the lowest Ecat. (high efficiency) values found for (μ2-PR2)2Fe2(CO)6 dimers under the same experimental conditions.4a,c For dimer 2a, a proton reduction mechanism involving homolytic cleavage of bridgehead P−H is possible. We propose that after reduction and rearrangement of 2a, radical anion 2a′•− protonates in the presence of acid at iron and then reductively eliminates dihydrogen, as shown in Scheme 4. This process involves homolytic cleavage of both a bridgehead P−H and the adjacent Fe−H bonds. Although this mechanism neatly accounts for all of our observations, we cannot at present discount a bimolecular process where two P−H bonds from two different 2a′•− species cleave to generate dihydrogen. The homolytic cleavage of a bridgehead E−H bond (E = S, P) upon reduction of Fe2E2 dimers is intriguing. It may be more widely observed during reductive evolution of dihydrogen catalyzed by Fe2E2 dimers than has been previously appreciated.

4. EXPERIMENTAL SECTION 4.1. Methods and Equipment. All experiments were carried out under an inert atmosphere using standard Schlenk techniques. (Ferrocenylmethyl)phosphine was synthesized as previously described.11 Methyl iodide was purified as described in the literature.28 All other reagents were used as received. Tetrahydrofuran was dried over, and distilled from, Na/benzophenone. All other solvents were taken from an Innovative Technology Pure Solvent Dispenser prior to use. NMR spectra were recorded on Bruker DPX 300, Bruker Avance III 400, and Bruker Avance III 500 spectrometers. A Bruker EMX10 EPR spectrometer was used to record EPR spectra. FTIR spectra were recorded using a Thermo-Nicolet AVATAR 370 FTIR spectrometer. HR-(+)-ESI mass spectra were acquired using an Orbitrap Velos XL spectrometer. Elemental analyses for C, H, and N were performed in the Microanalytical Unit of the Research School of Chemistry, Australian National University, Canberra, Australia. 4.2. Synthesis of (FcCH2PH2)Fe(CO)4 (1). To a toluene solution (1 mL) of (ferrocenylmethyl)phosphine (212 mg, 0.92 mmol) was added iron pentacarbonyl (1.2 mL, 9.13 mmol), and the mixture was heated to 90 °C for 20 h. The resulting bright red solution was filtered through Celite and evaporated to dryness to give a brown-red oil (357.5 mg) identified as compound 1 (75%), which could not be separated from byproduct 2 (25%). 1H NMR (500 MHz, CDCl3): δ (ppm) 3.02 (dt, JH−P = 7.2 Hz and JH−H = 6.6 Hz, 2H, CH2P), 4.17 (s, 5H, C5H5), 4.18 (m, 4H, C5H4), 4.81 (dt, JH−P = 180.1 Hz and JH−H = 6.6 Hz, 2H, PH2). 31P{1H} NMR (202.5 MHz, CDCl3): δ (ppm) −13.7. 13C{H} NMR (125.7 MHz, CDCl3): δ (ppm) 25.5 (d, JC−P = 3489

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were observed. All potentials in this paper are referenced relative to the ferrocene/ferrocenium couple. The peak-to-peak separation of the cathodic and anodic waves for ferrocene was always less than 75 mV (cf. 59 mV in theory). FTIR spectroelectrochemistry experiments were performed in a conventional solution IR cell that had been modified to fit a Pt-gauze working electrode sandwiched between the NaBr windows, a Pt-gauze auxiliary electrode in a separate compartment outside of the light path, and the same reference electrode as for the CVs positioned adjacent to the working electrode through a modified solution port. EPR spectroelectrochemistry experiments were performed in a cell consisting of a platinum-wire working electrode, a glass-shrouded platinum-wire auxiliary electrode, and a silver-wire (pseudo)reference electrode, all confined in a standard 3.5 mm quartz ESR tube. Great care was taken to ensure the spectroelectrochemical experiments proceeded under water- and dioxygen-free conditions. All equipment and the electrolyte were oven-dried overnight. The THF solvent was distilled and collected from a purple solution of sodium benzophenone ketyl under high-purity dinitrogen immediately before use. All electrolysis experiments were conducted on freshly prepared solutions that were made up and sealed in the spectroelectrochemical cells in a dinitrogen-filled glovebox (operating with