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. 2024 Oct 28;146(44):30204–30211. doi: 10.1021/jacs.4c09007

A Carborane-Derived Proton-Coupled Electron Transfer Reagent

Enric H Adillon 1, Jonas C Peters 1,*
PMCID: PMC11544690  PMID: 39466817

Abstract

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Reagents capable of concerted proton–electron transfer (CPET) reactions can access reaction pathways with lower reaction barriers compared to stepwise pathways involving electron transfer (ET) and proton transfer (PT). To realize reductive multielectron/proton transformations involving CPET, one approach that has shown recent promise involves coupling a cobaltocene ET site with a protonated arylamine Brønsted acid PT site. This strategy colocalizes the electron/proton in a matter compatible with a CPET step and net reductive electrocatalysis. To probe the generality of such an approach a class of C,C′-diaryl-o-carboranes is herein explored as a conceptual substitute for the cobaltocene subunit, with an arylamine linkage still serving as a colocalized Brønsted base suitable for protonation. The featured o-carborane (PhCbPhN) can be reduced and protonated to generate an N–H bond with a weak effective bond dissociation free energy (BDFEeff) of 31 kcal/mol, estimated with measured thermodynamic data. This N–H bond is among the lowest measured element–H bonds for analyzed nonmetal compounds. Distinct solid-state crystal structures of the one- and two-electron reduced forms of diaryl-o-carboranes are disclosed to gain insight into their well-behaved redox characteristics. The singly reduced, protonated form of the diaryl-o-carborane can mediate multi-ET/PT reductions of azoarenes, diphenylfumarate, and nitrotoluene. In contrast to the aforementioned cobaltocene system, available mechanistic data disclosed herein support these reactions occurring by a rate-limiting ET step and not a CPET step. A relevant hydrogen evolution reaction (HER) reaction was also studied, with data pointing to a PT/ET/PT mechanism, where the reduced carborane core is itself highly stable to protonation.

Introduction

Proton-coupled electron transfer (PCET) reagents can facilitate challenging substrate reductions via pathways that are efficient compared with stepwise ET-PT pathways.1 Such reagents have been demonstrated to be useful tools for selective hydrogen atom delivery (or abstraction) with organic and inorganic substrates.2

Systems that mediate reductive PCET transformations have largely utilized the well-behaved redox properties of transition metal and lanthanide complexes (Scheme 1A).3−7 To access PCET reactivity, the reduced form of these systems (e.g., Co(II) or Sm(II)) can be coupled to a Brønsted acid via covalent linkage, coordination, or substrate preorganization to form a reactive net hydrogen atom equivalent that can be delivered to substrates. Redox potential and acid pKa govern the effective X–H bond dissociation free energy (BDFEX–H, X = C, N, O) as in eq 1,8 allowing H atom reactivity to be measured and tuned via the individual proton and electron transfer equilibria (pKa and E°, respectively, where Cg is a solvent-dependent constant).

graphic file with name ja4c09007_m001.jpg 1

Scheme 1. (A) Widely Adopted PCET Reagents Derived from Metal Reductants; (B) An Aniline-Appended Cobaltocene ePCET Mediator; (C) Exploring the Reactivity of a Main-Group Reductant with An Appended Brønsted-Acid.

Scheme 1

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While weakening the BDFEX–H can promote net H atom transfer reactivity, it can in turn predispose a reagent toward the hydrogen evolution reaction (HER); the latter is thermodynamically favorable when the BDFEX–H is less than half that of the dihydrogen bond (BDFEH–H = 104 kcal/mol).3 Strategies for mitigating HER are central to reductive PCET reagent design.4,9

One strategy for promoting PCET reactions in a fashion compatible with electrocatalysis involves covalent tethering of a Brønsted acid to a reductant, as illustrated in Scheme 1B.10 Our lab has shown that attaching an anilinium fragment to cobaltocene (left) yields CpCoCpNH+ (right), a complex with a weak BDFEN–H. This species operates as an electrocatalytic PCET (ePCET) mediator under controlled potential electrolysis conditions, effecting ketone and olefin reductions,11 ketyl-olefin cyclizations,12 and with transition metal cocatalysts, alkyne semihydrogenation13 and nitrogen reduction to ammonia.14 The turnover-limiting PCET step can be concerted, as evidenced by zero order acid concentration dependence on the catalytic rate but still a substantial primary kinetic isotope effect (KIE).

Given the observed PCET reactivity of aniline-modified cobaltocene, we sought to explore an alternative system wherein the cobaltocene redox relay is replaced by another redox subunit, focusing herein on a main group cluster. We selected the o-carborane platform for its synthetic malleability and its compatibility with strongly reducing anionic states.15,16 In particular, o-carboranes—icosahedral clusters comprised of ten boron atoms and two adjacent carbon atoms—are reported to have two one-electron redox features between −1 and −3 V (all potentials reported in acetonitrile referenced to the ferrocene/ferrocenium couple).17 Whereas the potentials of o-carboranes can be modulated by over 2 V by changing the carbon substituents, the use of reduced carboranes as stoichiometric reductants (whether in ET or PCET steps) has not to our knowledge been previously explored.

Results and Discussion

Structure of Reduced Carboranes

As an entry point for reductive carborane chemistry, we first determined the structures of diaryl-o-carboranes following one- and two-electron reduction. Samples of the anionic and dianionic compounds were generated according to prior reports by reducing diphenyl-o-carborane (Ph2Cb) with potassium metal; one equivalent yields the radical anion (Ph2Cb–) and excess affords the closed-shell dianion (Ph2Cb2–).18 The radical nature of [Ph2Cb]K is supported by Evans method and a CW-EPR spectrum featuring a broad resonance with no resolvable hyperfine interactions centered at g = 2.026 (Figure S36). [Ph2Cb]K2 has diagnostic 1H, 11B, and 13C NMR signals consistent with the formation of a closed-shell species (Figure S2–S4).

The mono- and dianions were characterized by single crystal XRD as their respective K(18-crown-6) salts (crown = c hereafter). The structure of [Ph2Cb]K(18-c-6) is shown in Figure 1 (top) with the neutral Ph2Cb for reference and counterions omitted for clarity.19 Comparing these two structures, the largest change is a lengthening of the C–C bond between the two adjacent carbon atoms of the icosahedron core (which lengthens from 1.733 to 2.374 Å). In addition, the C–C bond length between the core carbon atoms and the aryl ipso-atoms shortens considerably (1.504 to 1.470 Å).20 The lengthening of C–C bond is consistent with previous theoretical studies and related structures.15,21−27

Figure 1.

Figure 1

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X-ray Crystal Structures of Ph2Cb and [Ph2Cb]K(18-crown-6) with a cartoon of the SOMO highlighting its σ* and π bonding character.

The structural parameters suggest that the SOMO is chiefly comprised of the tangential p-orbitals of the two adjacent carbon atoms of the cluster (Figure 1, bottom). The cluster carbon atoms engage in a σ* interaction leading to the observed breaking of the C–C bond upon reduction to the radical anion. Because the carbon p-orbitals are aligned with the aryl π* orbitals, they form a partial π bond that distributes negative charge away from the o-carborane core. For the dianionic Ph2Cb2–, the structure follows the Wade’s rules prediction of a nido-icosahedron (Figure S52).28

Thermochemistry of PhCbPhNH

Through incorporation of a Brønsted base on the aryl substituents, we envisaged forming a compound which could be reduced at the carborane core and protonated at the base to yield a net H atom donor. 1-(4-N,N-dimethylaniline)-2-phenyl-o-carborane (PhCbPhN) is a previously reported carborane which contains an appended base and was readily prepared.15 Chemical reduction of PhCbPhN with one equivalent of potassium metal and two equivalents of benzo-15-c-5 yielded the radical anion as red needles. XRD analysis provides a structure featuring an icosahedron core structure similar to Ph2Cb– (Figure 2A).

Figure 2.

Figure 2

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(A) square scheme of PhCbPhNH with the X-ray crystal structure of [PhCbPhN]K(b-15-c-5)2 (bottom left). (B) UV–vis spectra of [PhCbPhN]K(b-15-c-5)2 (1 mM) in THF at −80 °C with [4-CNPhNH3]OTf titrant (0–1 eq in 0.2 eq increments) and (inset) TBD titrant (0–1 eq in 0.2 eq increments). (C) Cyclic voltammograms of PhCbPhN (1 mM, blue) with [4-CNPhNH3]OTf (10 mM, gray) and [PhCbPhNMe3]OTf (1 mM, red) in acetonitrile with nBu4PF6 (200 mM) supporting electrolyte. The scans are swept cathodic, then anodic at 100 mV/s.

UV–vis spectroscopy was used to study the protonation of PhCbPhN to furnish PhCbPhNH. [PhCbPhN]K(b-15-c-5)2 (b-15-c-5 = benzo-15-crown-5) (blue trace) in THF solution cooled to −80 °C was protonated by addition of up to one equivalent of 4-cyanoanilinium triflate ([4-CNPhNH3]OTf), forming a new species assigned as PhCbPhNH (Figure 2B). Addition of triazabicyclodecene (TBD) as base produces a spectrum consistent with PhCbPhN–, suggesting that the protonation is reversible (Figure 2B, inset). Addition of the same acid to Ph2Cb– does not yield any new features, suggesting that incorporation of the aniline moiety allows for reversible protonation of the carborane species (Figure S68).

Next, we set out to determine the effective BDFEN–H of PhCbPhNH by measuring the PT and ET equilibria depicted along the edges of the square scheme in Figure 2A. PhCbPhN was analyzed by cyclic voltammetry to determine its reduction potential and the shift in potential upon protonation (Figure 2C). First, the voltammogram of PhCbPhN (blue trace) shows two reversible features at −1.67 and −1.80 V corresponding to the formation of the radical anionic and dianionic carboranes. Upon addition of acid to generate PhCbPhNH, we observe an anodically shifted irreversible multielectron wave (gray trace), suggesting that formation of PhCbPhNH is coupled to a fast chemical step. To estimate the reduction potential of [PhCbPhNH]+/0, the N-methylated analog 1-(4-N,N,N-trimethylanilinium)-2-phenyl-o-carborane triflate ([PhCbPhNMe3]OTf) was prepared and isolated by addition of methyl triflate to PhCbPhN. Its voltammogram (red trace) contains two reversible features at −1.43 and −1.59 V which are anodically shifted relative to PhCbPhN. Since the onset of the cathodic waves corresponding to reduction of [PhCbPhNMe3]+/0 and [PhCbPhNH]+/0 overlay, we deduce that the ([PhCbPhNMe3]+/0E1/2 is a good approximation of the [PhCbPhNH]+/0E1/2.

The pKa of PhCbPhNH+ was determined by titration with 2-chloroanilinium triflate in CD3CN and quantification of the equilibrium constant by 1H NMR spectroscopy (see S8). The pKa was measured to be 8.6—much lower than N,N-dimethylaniline (pKa = 11.5)—demonstrating the strong electron-withdrawing nature of neutral o-carborane.29

With the [PhCbPhNH]+/0E1/2 and the PhCbPhNH+ pKa in hand, the PhCbPhNH BDFEN–H can be estimated to be 31 kcal/mol. This is a very low BDFEN–H, especially for a nonmetal compound. To our knowledge, only one nonmetal compound, the nicotinamide radical cation, has a weaker experimentally measured value for a condensed phase element-hydrogen BDFE (BDFEC–H = 26 kcal/mol).3,30,31 Akin to the nicotinamide radical cation, the o-carborane’s driving force for H atom loss can be linked to restoring aromaticity upon oxidation by reforming the icosahedral core and breaking Ccluster–Caryl π-bond. Relatedly, restoring aromaticity has been invoked by our laboratory to rationalize the very weak C–H bond of ring-protonated cobaltocenes.32

Reactivity of PhCbPhNH

The weak BDFEN–H of PhCbPhNH indicates that it should serve as a potent H atom donor but may also be disposed to HER. To probe its reactivity, we first studied its stability in solution in the absence of a substrate. PhCbPhNH was generated in THF at −40 °C by protonation of [PhCbPhN]K(b-15-c-5)2 with [Ph3PH]OTf. Its concentration was monitored by UV–vis spectroscopy (Figure S71). The behavior of its absorbance at 550 nm reflects first-order decay with t1/2 = 7.9 min which we ascribe to HER (vide infra).

Next, we studied the reactivity of PhCbPhNH toward model organic substrates. To do so, PhCbPhNH was preformed by mixing [PhCbPhN]K(b-15-c-5)2 and [Ph3PH]OTf in THF solution at −40 °C, followed by substrate addition. We chose the substrates acetophenone and diphenylfumarate as they had been previously shown to react with CpCoCpNH+, in addition to azoarenes which may also be reduced via PCET pathways.33−35 The results are summarized in Scheme 2. In each case, the only observed boron-containing product by 11B NMR spectroscopy is PhCbPhN, as expected if PhCbPhNH serves as a net H atom donor (Figure S13).

Scheme 2. Reduction of model organic substrates by PhCbPhNH plotted as a function of their calculated redution potential.

Scheme 2

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Diphenylfumarate, nitrotoluene, and azobenzene were reduced in good yield to diphenyl succinate, toluidine, and diphenylhydrazine, respectively, suggesting that the transfer of reducing equivalents from PhCbPhNH to these substrates can outcompete HER. However, for acetophenone and electron-rich azoarenes (Ar = p-tolyl, p-MeOPh), little to none of the reduced products were formed despite the substantial driving force for the first net H atom transfer. We note that similar substrate classes have been assessed for reactions with bismuth- and phosphorus-derived reductants.36,37

To probe the mechanism through which reduction by PhCbPhNH operates, we performed a competition experiment between azobenzene and para-substituted azoarenes, extracting the relative rates from starting material conversion to generate linear free-energy relationships (Figure 3). Should azoarene reduction by PhCbPhNH be occurring through a CPET step rather than ET-PT steps, we would expect that the relative rate (kx/kH) should be fairly insensitive to substitution on the azoarenes. Previous Hammett analyses of reactions occurring through CPET steps returned shallow slopes, implying that a decrease in ET-driving force is compensated by an increase in PT-driving force.10,11,38,39

Figure 3.

Figure 3

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(A) Azoarene reduction competition experiment using PhCbPhNH as the reductant. (B) Rate-driving force analysis for an ET step.

Plotting the relative rates against the substituent Hammett parameter, a large positive slope (ρ = 2.5 or 2.8 for σ+ and σ, respectively) is observed (Figure S61–S62). For general comparison, in this case ρ is substantially larger than that observed for reductive CPET from CpCoCpNH+ to acetophenone (ρ = −0.55) or fumarate (ρ = 0.44) as substrates.40 These analyses support a rate-limiting step involving a substantial negative charge accumulation on the substrate in the transition state relative to the ground state.

To further disentangle whether azoarene reduction occurs through a rate-limiting ET or CPET step, we plotted the relative rates against the driving force for ET and net H atom transfer. The relative rates plotted against the driving force (Marcus plots) for ET (ΔE) or CPET (ΔGCPET) yield excellent (R2 = 0.99) and moderate (R2 = 0.70) correlations, respectively (Figure 3B, S66).41,42 These plots demonstrate that the reduction potential, not the BDFE, most accurately trends with the observed rates. Also considering that the Hammett analysis yields a large and positive slope, we deduce that these data support azoarene reduction by PhCbPhNH proceeding by a rate-limiting ET step rather than a CPET step.

The stepwise ET-PT reactivity of PhCbPhNH contrasts the apparent CPET-type reactivity of CpCoCpNH+ studied on different classes of substrates (e.g., acetophenones, fumarates, as well as N2 and phenylmethylpropriolate via tandem catalysis).10−14 This difference is noted despite shared thermochemical properties (they have the same pKa (8.6, in MeCN) and similar shifts in potential upon protonation (ΔE = +240 mV vs +140 mV, respectively); the carborane system is nevertheless moderately more reducing (E1/2 = −1.43 V vs −1.21 V, respectively). The absence of observed CPET-type reactivity for PhCbPhNH may be due to the large degree of structural reorganization that can be anticipated between PhCbPhNH and PhCbPhN (vide supra). Previous theoretical studies on CPET reactivity point to reorganization energy and vibronic coupling as factors that can gate CPET.43

Ph2Cb– Hydrogen Evolution Reaction

For a better understanding of the competing HER from reduced diaryl-o-carboranes, we sought to understand the mechanism through which H2 is formed. Combining [Ph2Cb]K(18-c-6) with [Ph3PH]OTf yields Ph2Cb (only observed boron-containing product by 11B NMR) and H2 (measured at 78% yield). We considered two plausible pathways through which Ph2Cb– could combine with acid to form H2 (Scheme 3A). First, Ph2Cb– could be protonated, generating a neutral intermediate that could then be rapidly reduced and protonated to generate hydrogen (PT/ET/PT). This mechanism is analogous to the proposed HER mechanism for cobaltocenes.32,44 Second, we considered that a disproportionation reaction could generate Ph2Cb2– which could then be twice protonated to yield hydrogen (ET/PT/PT). In either case, we deduced that the first PT must be rate limiting since no intermediates are observed upon addition of acid to a THF solution of [Ph2Cb]K(18-c-6) at −40 °C by UV–vis spectroscopy (Figure S68).

Scheme 3. (A) Plausible HER Mechanisms for Ph2Cb– with Relevant Measured and Computed Thermochemical Parameters of Intermediates; (B) Regioselectivity of the Rate Determining PT Step for Ph2Cb– and Cp*2Co.

Scheme 3

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A distinguishing feature of these two HER mechanisms is the carborane order in the rate law, where a PT-limited mechanism would have a first-order carborane dependence while a disproportionation mechanism would have a second-order carborane dependence. The decay of [Ph2Cb–] in the presence of excess acid fits first-order kinetics, suggesting that HER is first-order in carborane and that the disproportionation pathway is (if operative) a minor contributor (Figure S68). We note that at low conversion—and thus high [Ph2Cb–]—the fit to the data deviates from ideal first order, providing circumstantial evidence that another pathway, perhaps a disproportionation pathway, may be rate contributing at early time points.

Using [Ph3PD]OTf as the acid, a kinetic isotope effect of 2.0 is observed, consistent with a rate-limiting protonation step (Figure S69–S70).

Notably, when PhCbPhN– is used as the reductant, the rate of HER decreases 3-fold compared to Ph2Cb–, reflecting the lower driving force of PhCbPhNH for HER.10

Next, we set out to determine the site-selectivity of the rate-limiting PT step through an isotopic labeling experiment using the deuterated acid [Ph3PD]OTf as a reporter of the protonation site. 11B NMR analysis demonstrates clean formation of Ph2Cb (Figure S34). In the 2H NMR spectrum, no signals could be detected in the aromatic or carborane B–H regions (Figure S35). These data suggest that the carborane is not protonated on sites where we expect deuterium incorporation, namely the ortho-, meta-, and para-phenyl positions. Rather, PT presumably occurs at sites where deuterium is cleaved in a later step, either at the ipso-phenyl position, or possibly on the icosahedral core. These latter two cases are not easily distinguished by isotopic labeling of the acid.

To better identify the likely protonation site, we turned to density functional theory to assess which sites are thermodynamically plausible. We determined the relative energies of four cage-protonated and six phenyl-protonated isomers whose energies and structures can be found in the Supporting Information (Figure S79). For all isomers considered, protonation of the phenyl groups is much more favorable; with p-H-Ph2Cb as a reference (ΔGrel = 0), the phenyl protonated isomers vary modestly (ΔGrel = 0 to 7 kcal/mol) and all feature a carborane core which has reverted to an ideal icosahedron. In contrast, cage protonation isomers are much higher in energy (ΔGrel = +29 to +38 kcal/mol) featuring distorted icosahedra, suggesting that cage protonation is unlikely.

Although the most stable computed isomer is protonated at the para-position, this scenario is inconsistent with the 2H NMR data. Instead, the ipso-protonated isomer is higher in energy (ΔGrel = +7 kcal/mol) but also consistent with the 2H NMR data, suggesting that ipso-phenyl may be the kinetic protonation site. If true, this would not be surprising considering that the structural data show that the carborane core dissipates charge directly to the ipso carbon through a π-bonding interaction, rendering it modestly basic. More broadly, irreversible proton transfer with kinetic control, rather than thermodynamic control, has been established in other systems that invoke a relatively higher energy tautomer as an intermediate.45 Regardless, the calculated isomer energies make a compelling case that the driving force for Ph2Cb– protonation stems from restoring the carborane aromaticity despite breaking aromaticity of the phenyl groups.

We calculated thermochemical parameters of downstream intermediates to understand the feasibility of the latter steps of the PT/ET/PT HER mechanism (Scheme 3A). Once Ph2Cb– is protonated, its reduction potential shifts by +0.80 V, a similar potential shift to decamethylcobaltocene (Cp*2Co) protonation at the cyclopentadienyl ligand. The follow-up ET step from another equivalent of Ph2Cb– is thus exergonic, forming a hydridic C–H bond. The calculated C–H hydricity is 38 kcal/mol, akin to that of aluminum hydride.46 The final PT generates H2 and Ph2Cb, the observed products.

The HER mechanism most consistent with our experimental and computational data, PT/ET/PT, mirrors that of the proposed HER mechanism of Cp*2Co (Scheme 3B). In both cases, rate-limiting protonation of an aromatic group anodically shifts the potential and generates an intermediate with a weak C–H bond. The magnitude of the anodic potential shift is less for Ph2Cb (+0.80 V) than Cp*2Co (+1.29 V), suggesting weaker coupling in Ph2Cb–. Notably, Ph2Cb and Cp*2Co have different structures and yet appear to generate hydrogen by a similar mechanism. The aromatic groups used in each case to stabilize their reducing states appear to open a pathway for HER. In both cases, appending a Brønsted base (an aniline group) mitigates HER, although less so for PhCbPhNH than CpCoCpNH+.

Conclusions

Herein, we have introduced the synthesis and study of PhCbPhNH, a diaryl-o-carborane radical anion with an appended Brønsted acid that acts as a net H atom donor for fumarate, azoarene, and nitrotoluene reductions. The study hence provides a new view of carboranes as candidates for mediating PT/ET transformations. Through crystallographic analysis of one- and two-electron reduced diaryl-o-carboranes, we better understand their interesting redox properties. The PhCbPhNH reagent has one of the weakest measured element-hydrogen bonds of nonmetal compounds. Mechanistic study of azoarene reduction points toward a rate-limiting ET step. These data highlight a key consideration in the design of new CPET reagents: the mechanism of net H atom transfer may depend on both thermodynamic parameters (ΔGET, ΔGPT, electrostatic/electronic coupling) as well as kinetic parameters (reorganization energy, vibronic coupling), which are not easily predicted. The proposed PT/ET/PT mechanism for HER herein implicates the aromatic groups as the site of protonation, suggesting that the anionic icosahedral core is robust to strong acid. Continued studies of compounds with unusually weak net element-H bonds will inform improved understanding of H atom transfer reactivity toward more selective, lower overpotential systems for catalytic reductions.

Acknowledgments

This work was supported by the Department of Energy Basic Enegy Sciences (DE-SC0019136). E.H.A. acknowledges the Resnick Sustainability Institute for support via a Cross-Resnick Fellowship and the Department of Defense for support via a National Defense Science and Engineering Graduate Fellowship. The Beckman Institute at Caltech Supports the X-ray Crystallography Facility. JCP acknowledges the Resnick Sustainability Institute for enabling facilities and the Dow Next Generation Fund for support of Caltech’s EPR facility.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c09007.

  • DFT Coordinates (TXT)

  • The Supporting Information is available free of charge at Experimental procedures, characterization data, kinetic data, details of computational thermochemistry, X-ray crystallographic data (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja4c09007_si_001.txt (101.2KB, txt)
ja4c09007_si_002.pdf (8.4MB, pdf)

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