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Title Steps Toward CO2 Reduction to Methanol via Electrochemical Cascade Catalysis

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Author Mercer, Ian Patrick

Publication Date 2020

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Steps Toward CO2 Reduction to Methanol via Electrochemical Cascade Catalysis

THESIS

submitted in partial satisfaction of the requirements for the degree of

MASTER OF SCIENCE

in Chemistry

Ian Patrick Mercer

Dissertation Committee: Professor Jenny Y. Yang, Chair Professor William J. Evans Professor Andy S. Borovik

2020

Table of Contents

Acknowledgements iv

Abstract v

Chapter 1: Methyl Formate and Formaldehyde Reduction by Metal Hydrides 1

Chapter 2: Synthesis of Group 8 Metal Complexes of LDMA for the Study of Outer Sphere Interactions 25

Conclusion 33

Experimental 34

References 37

iii

Acknowledgements

I would like to express my sincere appreciation to my committee, especially my chair, Professor Jenny Y. Yang. Her commitment to science, mentoring, and ‘doing the right thing’ will forever inspire me.

Financial support was provided by the University of California, Irvine and US Department of Energy, Office of Science, Office of Basic Energy Sciences Awards DE-SC0012150 and DE- 0000243266.

Abstract of the Thesis

Steps Toward CO2 Reduction to Methanol via Electrochemical Cascade Catalysis

Ian Patrick Mercer

Master of Science in Chemistry

University of California, Irvine, 2020

Professor Jenny Y. Yang, Chair

Research on two independent projects is presented:

(1) Homogeneous cascade catalysis has been used for the hydrogenation of CO2 to methanol as it allows for rational tuning of catalyst reactivity and lower reaction temperatures compared to heterogeneous catalysis. Still, temperatures and pressures far above ambient conditions are required for hydrogenation. Performing cascade CO2 reduction electrochemically could result in milder reaction conditions. To this end, a series of bis(diphosphine) metal hydrides was synthesized. Their reactivity towards the reduction of methyl formate and formaldehyde, the final two intermediates in a proposed reaction pathway between CO2 and methanol, was investigated in alcohol solvents and acetonitrile (or acetonitrile substitutes). In alcohol solvents, transesterification

+ of methyl formate and protonation of the metal hydride [HPt(dmpe)2] outcompetes any possible hydride transfer to methyl formate. Because the hydride donor ability, or hydricity, of the metal hydrides tested is known in acetonitrile, studying reactivity in acetonitrile allowed for the bracketing of previously unreported methyl formate and formaldehyde hydride affinity values. The hydride affinity of methyl formate and formaldehyde were bracketed between 26.4 – 34.0 kcal/mol

v and 49.9 – 57.6 kcal/mol, respectively. This result represents an important step in the rational construction of an electrochemical cascade pathway for CO2 reduction to methanol.

(2) Concerted proton-electron transfer (CPET) pathways can sometimes lead to lower energetic pathways for small molecule oxidation reactions. There is evidence to suggest that incorporating hydrogen bond donors/acceptors in the outer sphere of catalysts can promote CPET. To investigate this idea, syntheses of complexes containing the ligand N,N′-bis((6-(dimethylamino)pyridin-2- yl)methyl)-N,N′-dimethylethane-1,2-diamine (LDMA) were explored. This ligand features pendant dimethyl amine groups that appear to promote CPET water oxidation in previous work with cobalt.

Substituting cobalt with group 8 metals (Fe, Ru, Os) may impart enhanced water oxidation reactivity and/or stability of oxidized products (key for verifying a CPET mechanism for water oxidation). Thus, the attempted syntheses of group 8 complexes of LDMA are discussed with only

DMA DMA DMA the iron L complex, with purported structure [FeL (CH3CN)2][OTf]2 or FeL (OTf)2, yielding clean product and spectroscopic characterization by 1H NMR.

Chapter 1

Methyl Formate and Formaldehyde Reduction by Metal Hydrides

1.1 Introduction

Growing global energy demands continue to spur increased extraction and consumption of fossil fuels.1,2 Due to the environmental impact of these resources, production of chemical fuels using renewable energy sources is highly desirable to promote a sustainable carbon-neutral energy economy. Thus, there is intense interest in “closing the loop” on CO2, where the byproduct of fuel combustion (CO2) is utilized as the carbon source for fuel formation and the production of industrially important chemical precursors, like methanol.3-4

Although there exist numerous examples of selective and highly efficient catalysts for the

5-6 hydrogenation of CO2 to formic acid/formate, direct CO2 reduction to simple alcohols remains a challenge for homogeneous catalysis. Thus, an attractive approach for CO2 conversion to methanol involves indirect hydrogenation, meaning CO2 derivatives serve as an intermediate between CO2 and methanol. In the first example of indirect homogeneous hydrogenation of CO2 to methanol, demonstrated by Huff and Sanford7 in 2011, a three-step cascade catalysis reduction pathway (Scheme 1.1) proceeds as follows: hydrogenation of CO2 to formic acid (step A), followed by Lewis acid-catalyzed esterification to methyl formate (step B), and a final hydrogenation step to yield two equivalents of methanol (step C). Using this approach, they report a 75% yield of methanol with a TON of 21 after 16 hours at 135°C. Other examples of indirect or

8-12 cascade CO2 reduction to methanol have followed in recent years, further improving methanol selectivity and yield and broadening the array of intermediate compounds that can bridge CO2 and

1 methanol. This approach has even been combined with CO2 capture methods as a means of

13-16 utilizing captured CO2.

Scheme 1.1. Reaction pathway for conversion of CO2 + 3 H2 to CH3OH + H2O outlined by Huff and Sanford.7 Each catalyst is optimized for a particular reaction step, characteristic of a cascade- style catalysis.

Unfortunately, to achieve high yields of methanol, hydrogenation reactions typically require the use of high temperatures (75-160°C), high pressures (upwards of 30 atm H2/CO2), and long reaction times (>16 hours), decreasing the energy efficiency of the process. A more sustainable route to methanol could likely be accomplished via electrochemical reduction of CO2 as reactions could be run at more moderate temperatures and pressures. Operating at ambient pressure also allows for probing of various kinetic and thermodynamic aspects of this pathway, something not as easily accomplished under high pressure. Thus, our new reaction pathway to methanol can be visualized in Scheme 1.2 whereby H2 has been replaced by successive hydride and proton transfer reactions, the former catalyzed by electrochemically-generated metal hydride complexes.

Scheme 1.2. Proposed reaction pathway for electrochemical CO2 reduction to methanol.

To investigate the thermodynamic landscape of this reaction pathway, I synthesized a series of bis(diphosphine) metal hydride complexes across a wide range of hydricities, or hydride donor strengths (eq 1, ΔGH-). These metal hydride complexes were used to bracket the hydride acceptor

2 ability, or hydride affinity, of intermediates methyl formate and formaldehyde via NMR-scale reactivity studies. To my knowledge, these hydride affinity values have not been reported.

Additionally, I discuss the attempted esterification and amidation of tetrabutylammonium formate

(Scheme 1.2, Step B), possible reasons why these reactions failed, and how to improve conversion in future experiments. By building the thermodynamic and kinetic groundwork for indirect CO2 reduction to methanol, we can work towards developing a rational approach to catalyst selection and optimization of reaction parameters that improve product yield, selectivity, and reaction rate.

1.2 Results and Discussion

1.2.1 Synthesis of bis(diphosphine) complexes of Pt, Ni, and Rh

The syntheses for complexes [Pt(dmpe)2][PF6]2 (dmpe = 1,2- bis(dimethylphosphino)ethane), [Ni(dmpe)2][BF4]2, [Ni(dhmpe)2][BF4]2 (dhmpe = 1,2- bis(dihydroxymethylphosphino)ethane), [Rh(dmpe)2][BF4], and [Rh(dppb)2][BF4] (dppb = 1,2- bis(diphenylphosphino)benzene) have been previously published.17-20 Typically, synthesis

n+ involves addition of a stoichiometric amount of diphosphine to either [M(CH3CN)6] or

n+ [M(COD)2] (COD = 1,5-cyclooctadiene) in acetonitrile, followed by salt metathesis where appropriate. These complexes were used to synthesize the respective metal hydride complexes

[HPt(dmpe)2][PF6], [HNi(dmpe)2][BF4], [HNi(dhmpe)2][BF4], HRh(dmpe)2, HRh(dppb)2, all of

17,18,21-23 which have been previously prepared. Complexes [Pt(dmpe)2][OTf]2 and

[HPt(dmpe)2][OTf] were prepared by a method similar to the PF6⁻ analogs but with NH4OTf as the metathesis salt (Scheme 1.3). Details of their synthesis and spectroscopic characterization are listed in the Experimental section. The identity and purity of all complexes listed above were

3 confirmed by 31P{1H} and 1H NMR spectroscopy. The metal hydrides chosen are well studied and have known hydricity values (in acetonitrile) that span a range of 31 kcal/mol.24 Diphosphine-type ligands allow the use of 31P NMR spectroscopy to track the conversion of the metal hydride species during hydride transfer reactions. Since integration of the M-H 1H NMR resonance is not always quantitative, due to proton exchange with the NMR solvent, the 31P NMR resonances also aids in quantifying hydride transfer.

Scheme 1.3. Synthesis of [Pt(dmpe)2][OTf]2 and [HPt(dmpe)2][OTf].

1.2.2 Reaction of [HPt(dmpe)2][OTf] and methyl formate in alcohol solvents

Ideally, cascade catalysis is performed in a single vessel with a single catalyst in a single solvent. Although these conditions would likely be broken in the course of developing our cascade pathway, my initial aim was to study methyl formate reduction (to MeO⁻/MeOH) using a proven

CO2 reduction electrocatalyst and an ideal solvent. The ideal solvent for the cascade would be a simple alcohol, like methanol or ethanol, because esterification of formate to formate ester

(Scheme 1.2, Step B) would be most favorable with an excess of minimally-hindered alcohol.

[Pt(dmpe)2][PF6]2 was chosen as the hydride transfer catalyst since it has been well studied by the

25 Yang group and is known to perform selective CO2 conversion to formate, albeit in acetonitrile.

The hypothesis is that this catalyst may be able to carry out CO2 reduction (and formate ester reduction) in protic solvents as the free energy for hydride transfer can become more favorable in solvents capable of hydrogen bonding.18

Scheme 1.4. Equilibrium reaction depicting reduction of methyl formate to methoxide by a bis(diphosphine) metal hydride complex.

To test this hypothesis, a stoichiometric reaction of [HPt(dmpe)2][PF6] (1) (ΔGH- = 41.4 kcal/mol) and methyl formate (2:1, respectively) was performed in methanol-d4 and reactivity was observed by 1H and 31P{1H} NMR spectroscopy at various timepoints. Unfortunately,

[Pt(dmpe)2][PF6]2 (2) readily precipitates out of methanol-d4 upon hydride transfer, skewing the equilibrium of the reaction and convoluting the reduction reaction kinetics. To address this issue, the salt [HPt(dmpe)2][OTf] was chosen as a replacement because it is expected to have greater solubility in protic solvents versus the PF6⁻ salt.

As stated previously, [HPt(dmpe)2][OTf] (3) and its synthetic precursor

[Pt(dmpe)2][OTf]2 (4) were prepared following literature procedures used to make the PF6⁻ analogs, with minor changes to account for the water solubility of the triflate salts. Although not a novel structure, crystals were grown and an X-ray crystal structure was obtained for

[Pt(dmpe)2][OTf]2 (CSD Refcode: NIWZOI) confirming the identity of the intended product. A crystal structure for [HPt(dmpe)2][OTf] or elemental analysis confirmation of either species has yet to be obtained. However, characterization by 31P and 1H NMR spectroscopy for both compounds match well to their PF6⁻ counterion companions indicating clean conversion to product with negligible impurities. Both 3 and 4 are readily soluble in methanol and ethanol as expected.

An NMR-scale, slightly excess reaction between [HPt(dmpe)2][OTf] (3) and methyl formate (2.5:1, respectively) was carried out in ethanol-d6, in order to detect any methanol produced as a result of methyl formate reduction. Benzene, in equal molar concentration to methyl formate, was used as an internal standard. Following addition of 3, findings suggest that

5 transesterification of methyl formate by ethanol-d6 (Scheme 1.5) is outcompeting any possible hydride transfer to methyl formate. 1H NMR shows formation of methanol with complete loss of the methyl group of methyl formate, yet only a 17% decrease in the signal of the formyl-H (Figure

1.1b).† There is also a small but significant shift in the formyl-H signal (8.09 ppm to 8.06 ppm) which I would expect following conversion of methyl formate to (deuterated) ethyl formate. The

31P{1H} NMR spectrum at this timepoint shows negligible conversion of 3 to 4, solidifying that no hydride transfer occurred. Interestingly, control experiments show that methyl formate is stable to transesterification for up to 16 hours in ethanol-d6. It is not clear why transesterification of methyl formate occurs so much faster in the presence of 3 than in its absence. In any case, an explanation for this behavior was not sought. Although the formyl-H signal (of methyl formate) decays about 50% (–0.29H) over the course of the next 24 hours, formate appears in solution (8.57 ppm) and increases by about the same amount (+0.34H), likely indicating further esterification by trace H2O present in the NMR solvent (Figure 1.1c). Evidence of hydride transfer is now apparent in the 31P{1H} NMR with the appearance of 4; however, the methanol 1H signal is unchanged; thus, the hydride is likely transferring to solvent or methanol to form H2. [HPt(dmpe)2][OTf] is stable to protonation for up to 24 hours in ethanol-d6 so methanol is more likely the cause of protonation.

Scheme 1.5. Arrow-pushing mechanism depicting transesterification of methyl formate by ethanol-d6 to yield (deuterated) ethyl formate and methanol.

† The initial 17% decrease in the formyl-H integration may be due to evaporation of the highly volatile methyl formate (bp=32°C) upon mixing of the ‘methyl formate only’ solution with [HPt(dmpe)2][OTf]. This experimental error was corrected in proceeding experiments of this sort.

Figure 1.1. 1H (left) and 31P{1H} (right) NMR spectra of methyl formate (11 mM) and benzene (11 mM, internal standard) in ethanol-d6 at timepoints before (a) and after (b-c) addition of 1 [HPt(dmpe)2][OTf] (28 mM). All integrations relative to 6H of benzene.

To counteract the effect of transesterification, which made it difficult to quantify the progress of the reaction, the same reaction was performed in CD3OD. Methyl formate reduction would then be inferred from the disappearance of the formyl-H signal and concurrent conversion

+ 2+ of [HPt(dmpe)2] to [Pt(dmpe)2] . Indeed, addition of 3 yields an initial 14% decrease in the formyl-H signal (Figure 1.2b). And although conversion to 4 is not quantitative (only 9% conversion, expected: 14%), it is plausible that we are observing methyl formate reduction.

However, from 0.5 to 3 hours after addition, there is no further change in the formyl-H signal, yet conversion to 4 jumps to over 55% (Figure 1.2c). Beyond 3 hours, once again methyl formate converts to formate due to residual H2O and the presence of 4 continually rises in solution (Figure

1.3). Control experiments show a nearly identical rate of hydride transfer when [HPt(dmpe)2][OTf]

7 is alone in CD3OD, sans methyl formate (Figure 1.4). The instability of 3 suggests that methanol is too acidic (or 3 is too hydridic) and is protonating the hydride complex. The presence of H-D in solution (clearly evident at the 21-hour timepoint) provides further support for this claim (Figure

1.5). This instability in CD3OD likely explains the elevated instability of 3 in ethanol-d6 when methyl formate is present as transesterification affords a significant amount of methanol that could then protonate 3. In any case, a change of solvent or catalyst (or both) was necessary to continue my investigation of methyl formate reduction.

Figure 1.2. 1H (left) and 31P{1H} (right) NMR spectra of methyl formate (9 mM) and benzene (9 mM, internal standard) in CD3OD at timepoints before (a) and after (b-c) addition of 1 [HPt(dmpe)2][OTf] (20 mM). All integrations relative to 6H of benzene.

Figure 1.3. 1H (left) and 31P{1H} (right) NMR spectra of methyl formate (9 mM) and benzene (9 mM, internal standard) in CD3OD at timepoints before and after addition of [HPt(dmpe)2][OTf] (20 mM).

31 1 Figure 1.4. P{ H} NMR spectra of [HPt(dmpe)2][OTf] in CD3OD in the presence (left) and absence (right) of methyl formate at various timepoints.

1 Figure 1.5. H NMR spectrum of [HPt(dmpe)2][OTf] + methyl formate (2:1) in CD3OD after 21 hours at RT. H-D is observed as a triplet centered at 4.52 ppm (JH-D = 42 Hz).

1.2.3 Hydride transfer reactions of HPt and HRh complexes to methyl formate in acetonitrile

Acetonitrile was the obvious next solvent of interest because both [HPt(dmpe)2][PF6] (1) and [Pt(dmpe)2][PF6]2 (2) are exceptionally stable in acetonitrile. Additionally, working in acetonitrile makes it easier to develop a quantitative thermodynamic understanding of the methyl formate reduction pathway because the hydricity values for a majority of metal hydrides have been measured solely in acetonitrile. Thus, by choosing metal hydrides with known hydricity values and reacting them with methyl formate and formaldehyde, in acetonitrile, we can calculate, or at the very least bracket, the hydride acceptor values of these intermediates.

An NMR-scale, 2:1 stoichiometric reaction of 1 and methyl formate was performed in

1 31 1 CD3CN. After 3 days at room temperature, H and P{ H} NMR spectra displayed no change from the spectra taken at t0. After an additional 7 days, however, methoxide (3.27 ppm) is detected in solution in small quantity (5% conversion) (Figure 1.6b). Evidence of hydride transfer is present in the 31P{1H} spectrum with conversion of 1 to 2, however at only about half the expected value

(2% conversion, expected: 5%). This deviation could be due to integration error from poor signal- to-noise or a competing reaction that also produces methoxide. As we’ve seen previously,

+ [HPt(dmpe)2] seems to promote the transesterification of methyl formate in ethanol. A similar reaction could be occurring in CD3CN to yield a formamide product and methoxide. Curiously, the methyl signal of methyl formate decreases after 14 days at room temperature but the formyl-H signal actually increases slightly (Figure 1.6c). However, this notion of competing amidation is somewhat dispelled by the next timepoint (14 days @ RT + 4 days @ 60°C) as the formyl-H and methyl signals finally decrease in tandem and no possible formamide resonance is detected (Figure

1.6d). Thus, it is plausible that 1 is hydridic enough to reduce methyl formate. However, methanol formation is slow and utilizing a catalyst that takes several days to react is not practical for catalysis. Thus, a stronger hydride donor was needed in hopes of increasing the rate of reaction.

Figure 1.6. 1H (left) and 31P{1H} (right) NMR spectra of methyl formate (11 mM) and benzene (11 mM, internal standard) in CD3CN at timepoints after addition of [HPt(dmpe)2][PF6] (23 mM) (a-d). All 1 integrations relative to 6H of benzene. Relative integration values for 31P{1H} for the 14 days RT + 4 days 60°C timepoint were not determined due to noise and interference from impurities.

The complex HRh(dppb)2 (5) was a reasonable choice because of its significantly lower hydricity (ΔGH- = 34.0 kcal/mol) and bis(diphosphine) architecture. Synthesis involved reaction of two equivalents of dppb with [Rh(COD)2][BF4] in acetonitrile to yield [Rh(dppb)2][BF4] (6).

Stirring 6 for 48 hours in toluene with excess LiAlH4 produced 5 with >90% yield. Due to poor solubility of 5 in acetonitrile, the NMR-scale reactivity study with methyl formate was performed in benzonitrile. The hydricity difference of 5 in benzonitrile (versus acetonitrile) is estimated to be less than 0.8 kcal/mol,23 thus any results obtained should be applicable to acetonitrile with marginal error. Still, solubility of 5 in benzonitrile is relatively low, and thus, the preferred molar ratio of 2:1 HRh(dppb)2:methyl formate was sacrificed slightly to achieve a minimum

13 concentration of methyl formate for quantification by NMR spectroscopy in non-deuterated solvent. This error is likely of little consequence, however, as no methyl formate reduction is apparent, even after 7 days at room temperature (Figure 1.7c). Methanol is expected to appear as a quartet and doublet (2.66 & 3.52 ppm, respectively, relative to the acetonitrile standard set to

1.94 ppm); however, no matching resonances are observed. 31P{1H} NMR shows slow conversion of 5 to 6, indicating that 5 may be reacting with benzonitrile, though no control experiment has been conducted to confirm this assumption and a possible mechanism is unclear. The inactivity of

+ 5 toward methyl formate effectively proves that [HPt(dmpe)2] is not hydridic enough to reduce methyl formate. It also allows us to bracket the methyl formate hydride acceptor value with a lower bound of 34.0 kcal/mol.

Figure 1.7. 1H (left) and 31P{1H} (right) NMR spectra of methyl formate (2.6 mM) and acetonitrile (2.6 mM, internal standard) in benzonitrile at timepoints after addition of HRh(dppb)2 (< 5.3 mM) (a-c). All 1H chemical shifts were referenced to acetonitrile set at 1.94 ppm. Peaks at 1.72 ppm and 2.27 ppm are unknown impurities.

To determine an upper bound for methyl formate reduction, I took another step down on the hydricity scale to arrive at HRh(dmpe)2 (7) (ΔGH- = 26.4 kcal/mol). Addition of 7 to methyl formate (2:1, respectfully) was conducted in THF-d8 due to undesirable side reactions with acetonitrile. The hydricity of 7 was derived from reactions in THF and should be a reasonable estimate for the hydricity in acetonitrile according to DuBois, et. al.22 Within 30 minutes of

HRh(dmpe)2 addition, a significant reduction in the methyl and formyl-H signals of methyl formate (-88% and -85%, respectively) is observed, along with the appearance of methoxide in solution (Figure 1.8b). Both 7 and methyl formate are stable in THF-d8 for multiple days, ruling out the possibility of methoxide formation via esterification or decomposition. By the 4-hour timepoint, essentially all methyl formate and HRh(dmpe)2 have been consumed, yet methoxide conversion sits at only 55% (Figure 1.8c). The remaining methoxide is, as of yet, unaccounted for.

The 31P{1H} NMR spectra display very peculiar behavior. Instead of converting to the oxidized

+ species, [Rh(dmpe)2] (8), upon hydride transfer, as expected, loss of 7 in solution appears to correspond to the rise of three other signals in the 31P{1H} spectrum: the first (23.0 ppm) is comprised of two overlapping doublet of triplets; the second (-9.3 ppm) is comprised of two overlapping doublet of pentets; the last (-46.7 ppm) is a far upfield singlet (Figure 1.9). The doublet

n+ of triplets may correspond to a octahedral complex of the form cis-[Rh(dmpe)2X2] as the two pairs of inequivalent phosphorous atoms may produce one of the doublet of triplets. The other, less intense and inlayed, set could be a result of proton exchange with THF-d8, with the heavier deuterium atoms incurring a slight chemical shift in the 31P signal. I do not wish to speculate on the identity or general structure of the doublet of pentets or singlet; however, I will point out that the coupling constants for the triplets and pentets are very similar (Jtriplet = 28.4 Hz, Jpentet = 28.7

Hz) meaning these two species may have similar phosphorus environments. Crystals were obtained

15 from evaporation of the reaction solution, but a structure could not be determined from X-ray diffraction due to their poor quality. Although the deviation to unknown products following hydride transfer may present an issue for electrochemical catalysis, HRh(dmpe)2 promotes rapid reduction of methyl formate to methoxide. With this result, the hydride affinity of methyl formate has been successfully bracketed in acetonitrile and falls between 26.4 and 34.0 kcal/mol, a fairly narrow range of 7.6 kcal/mol. And from the lack of reactivity I see between HRh(dppb)2 and methyl formate, I would predict the actual value to be at least a few digits below 34.0 kcal/mol.

Figure 1.8. 1H (left) and 31P{1H} (right) NMR of methyl formate (16 mM) and benzene (16 mM, internal standard) in THF-d8 at timepoints before (a) and after (b-c) addition of HRh(dmpe)2 (30 mM). All 1 integrations relative to 6H of benzene.

31 1 Figure 1.9. P{ H} NMR spectrum of 2:1 HRh(dmpe)2 : methyl formate in THF-d8 after 15 hours at RT. Inlays show expanded regions of spectrum with accompanying coupling constants for doublet of triplets and doublet of pentets shown (HRh(dmpe)2: Jdoublet = 140.0 Hz).

1.2.4 Hydride transfer reactions of HPt and HNi complexes to formaldehyde in acetonitrile

With the hydride affinity of methyl formate bracketed in acetonitrile, I chose to do the same for formaldehyde. Formaldehyde is the intermediate that forms following single hydride transfer to methyl formate, with concurrent liberation of methoxide (Scheme 1.6). Hydride transfer to formaldehyde is expected to form another equivalent of methoxide. The hydride affinity of formaldehyde is predicted to be significantly greater than that of methyl formate because formaldehyde is never isolated or observed in studies involving homogeneous CO2 hydrogenation to methanol.

Scheme 1.6. Equilibrium reaction depicting reduction of methyl formate to formaldehyde and methoxide by a bis(diphosphine) metal hydride complex.

In separate experiments, [HPt(dmpe)2][PF6] (1) (ΔGH- = 41.4 kcal/mol) and

[HNi(dmpe)2][BF4] (9) (ΔGH- = 49.9 kcal/mol) were added to an equimolar formaldehyde solution

1 31 1 in CD3CN,* and reactivity was observed by H and P{ H} NMR spectroscopy at various timepoints. Both experiments show similar results but over different timescales (Figures 1.10 and

1.11). In the 1H NMR spectra, the formaldehyde singlet signal (9.60 ppm) disappears with the rise of methoxide and methyl formate in solution. In the experiment with 1, complete loss of formaldehyde occurs within 30 mins of hydride addition (Figure 1.10b), whereas in the experiment with 9, it takes at least 3-4 days (Figure 1.11d). This difference in rates is not so surprising considering the fairly large difference in hydricities between the two complexes. The formation of methyl formate as a product was, initially, surprising however. This observation may suggest that methoxide, produced as a result of hydride transfer to formaldehyde, can attack formaldehyde to form methyl formate, with loss of hydride. These reactions continue until all formaldehyde is consumed leaving methoxide and methyl formate in equilibrium (eq 2). From the 31P{1H} NMR spectra, evidence of some hydride transfer is apparent in each experiment; however, neither shows

2+ 2+ full conversion of metal hydride, 1 and 9, to oxidized species, [Pt(dmp)2] and [Ni(dmpe)2] , respectively. This result likely indicates that the reaction between methoxide and formaldehyde is fast and may outcompete the reaction between formaldehyde and these two metal hydride species.

In both reactivity experiments, equilibrium settles at a methyl formate to methoxide ratio of

* The formaldehyde solution was prepared by vigorously stirring a 0.5% (m/v) paraformaldehyde suspension at 75°C for 12-18 hours. For reactivity studies with [HPt(dmpe)2][PF6] and [HNi(dmpe)2][BF4], the solution was not filtered and a small amount of paraformaldehyde was transferred to the J-Young NMR tube. In the reactivity study of [HNi(dhmpe)2][BF4] with formaldehyde (see pg. 21-22), suspended paraformaldehyde was filtered away.

1 approximately 1.9:1 (based on integration of -CH3 protons in the H spectrum of the final timepoint). Control experiments indicate that formaldehyde slowly decomposes in CD3CN; however, no methoxide or methyl formate is observed as a result (Figure 1.12). Most importantly, from these results we can set an upper bound on the hydride affinity of formaldehyde at 49.9 kcal/mol.

Figure 1.10. 1H (left) and 31P{1H} (right) NMR spectra of formaldehyde (4.3 mM) and benzene (10 mM, internal standard) in CD3CN at timepoints before (a) and after (b-d) addition of 1 [HPt(dmpe)2][PF6] (4.4 mM). All integrations relative to 6H of benzene. Area of impurities accounted for in the integration values of [HPt]+.

Figure 1.11. 1H (left) and 31P{1H} (right) NMR spectra of formaldehyde (2.5 mM) and benzene (10 mM, internal standard) in CD3CN at timepoints before (a) and after (b-e) addition of 1 31 1 [HNi(dmpe)2][BF4] (2.6 mM). All H integrations relative to 6H of benzene. P{ H} NMR was not taken at the 14 day timepoint due to an instrument malfunction. Singlets at 21.5 ppm and 18.2

20 ppm in the 31P{1H} spectrum are of unknown identity.

Figure 1.12. 1H NMR spectra of formaldehyde (13.4 mM) and benzene (10 mM, internal standard) 1 in CD3CN at various timepoints All integrations relative to 6H of benzene.

In hopes of finding a lower bound, an analogous experiment with the complex

[HNi(dhmpe)2][BF4] (11) (ΔGH- = 57.6 kcal/mol) was performed. Following addition of 11 to a

1 formaldehyde solution in CD3CN, the H NMR spectra show the formaldehyde signal decay by

90% over 19 hours, yet no methoxide or methyl formate is observed in solution (Figure 1.13). The

31P{1H} NMR spectrum displays no evidence of hydride transfer as no conversion of 11 to

2+ [Ni(dhmpe)2] (12) or any other species is observed. Given more time it is possible that 11 would have effected hydride transfer; however, the tendency for formaldehyde to decompose/polymerize means the lower bound of our bracket is somewhat of a kinetic one. Even so, I would confidently say that a hydricity of at least 57.6 kcal/mol is needed to reduce formaldehyde. Combined with the

21 previously described upper bound gives the range 49.9 – 57.6 kcal/mol, a spread of 7.7 kcal/mol.

Indeed, our hypothesis was correct with the formaldehyde bracket and methyl formate bracket

(26.4 – 34.0 kcal/mol) differing by nearly 24 kcal/mol (based on midpoint values). Thus, any hydride donor that can reduce methyl formate should be hydridic enough to reduce formaldehyde.

+ + However, from what I observed in reactivity studies with [HPt(dmpe)2] and [HNi(dmpe)2] , the hydride donor may compete with methoxide for attack on formaldehyde.

Figure 1.13. 1H (left) and 31P{1H} (right) NMR spectra of formaldehyde (6.6 mM) and benzene (10 mM, internal standard) in CD3CN at timepoints before (a) and after (b-d) addition of 1 [HNi(dhmpe)2][BF4] (6.6 mM). All integrations relative to 6H of benzene

1.2.5 Attempted conversion of formate to formate esters and formamides

Although most of the focus of this thesis has been on methyl formate and formaldehyde reduction, the transformation of formate to something more easily reduced, like a formate ester or formamide, is potentially a larger roadblock in our cascade pathway as it is likely to be the rate- limiting step. For experimental studies that have performed similar CO2 to methanol cascade catalysis using hydrogenation catalysts, the barrier of formic acid esterification or amidation is typically aided by the high temperature and pressures that accompany hydrogenation reactions

However, the desire to perform catalysis electrochemically necessitates ambient pressures and mild(er) temperatures which puts us at a considerable disadvantage. Additionally, working with metal hydride complexes limits the pH of our solution (least we threaten protonating the electrocatalyst), and thus restricts formate primarily to its basic, anionic form and not the more easily reduced conjugate acid. Fortunately, Lewis acids are known to catalyze the esterification and amidation of carboxylic acids and can achieve high conversion at mild temperatures and short reaction times (a few hours).26-28 Thus, conversion of formate to formate esters or formamides was investigated using a small group of simple alcohols and amines and two Lewis acids, Sc(OTf)3

29 and Al(OTf)3, of moderate strength. Sc(OTf)3, in particular, has been utilized in aforementioned indirect CO2 to methanol hydrogenation studies. In each case, 0.3 mmol tetrabutylammonium

(TBA) formate was reacted with an equal molar or excess amount of alcohol or amine in the presence of 5 mol %, 10 mol %, or a saturated amount of Lewis acid and >10% (v/v) molecular sieves (to remove any H2O that may be produced). Results are summarized in Figure 1.14 and are poor. The highest conversion of 5.6% was only achieved after 18 hours in boiling ethanol.

Figure 1.14. Reaction conditions and conversion results for attempted esterification and amidation of TBA formate. DIPA = diisopropylamine, DMEDA = dimethylethylenediamine, DMA = dimethylamine

Even at high temperatures and excess substrate (in some cases), the mere presence of a

Lewis acid is not enough. Likely, formate must pass through its conjugate acid form to undergo esterification or amidation, thus an acid source with a pKa comparable to formic acid may be

- needed to boost conversion. Utilizing stronger Lewis acids, like BCl3 or [B(C6F5)4] , might aid in more rapid and complete conversion. Additionally, reactions where protonation of formate generates a nucleophilic amine should be able to elicit rapid amidation with elimination of water, for example, the decomposition of ammonium formate to formamide.30 Although the reaction time is reportedly short (0.5 – 1 hr), the acidity of ammonium formate and high reaction temperature required (>150°C) may be destabilizing to the Lewis acid or metal hydride catalysts, thus further experimentation is required.

Chapter 2

Synthesis of Group 8 Metal Complexes of LDMA for the Study of Outer Sphere Interactions

2.1 Introduction

Facile and energy efficient catalysis requires minimization of intermediate and transition state energies. This is especially important for fuel-forming and utilization redox reactions that involve multi-electron and multi-proton processes. Oftentimes, the most efficient pathway for these processes is to couple proton and electron transfer steps. Proton-coupled electron transfers

(PCET) are ubiquitous in chemical and biological processes and are implicit in a range of areas, from hydrogen atom transfer reactions to catalytic water splitting in photosynthetic plants.31-34

Under the umbrella of PCET, one mechanism that is particularly relevant to small molecule activation catalysis involves transfer of the proton and electron “in tandem” (i.e. in a single kinetic step) and is commonly termed concerted proton-electron transfer (CPET).**,35 By this mechanism, redox reactions proceed without the formation of high energy, charged intermediates associated with step-wise electron transfer (ET) or proton transfer (PT) (Figure 2.1). To direct small molecule activation toward CPET, it is crucial to study the catalyst design features that influence this mechanism. Especially important are outer coordination sphere functional groups that, with proper positioning and pKa, can act as a proton donor/acceptor proximal to the redox-active site or facilitate hydrogen bonding networks for proton shuttling.

** It should be noted that there is no standardized convention for the nomenclature of PCET reactions, and even for the concerted mechanism, there exists several terms to demarcate this pathway, including coupled proton electron transfer (CPET), concerted proton electron transfer (CPET, CEP, EPT), and electron transfer proton transfer (ET-PT).

Figure 2.1. Square scheme depicting oxidation of metal-bound H2O via step-wise electron and proton transfer events (perimeter of square) and concerted proton-electron transfer (diagonal).

The careful incorporation of pendant functional groups into numerous synthetic systems has shown significant PCET enhancement and has been attributed to the promotion of a CPET mechanism.36-38 Previous work in our group involved the synthesis a cobalt precatalyst,

DMA DMA [CoL (CH3CN)2][BF4]2, (L = N,N′-bis((6-(dimethylamino)pyridin-2-yl)methyl)-N,N′- dimethylethane-1,2-diamine) (Figure 2.2a). The complex contains dimethyl amine groups in the secondary coordination sphere and displays electrochemical activity towards the oxidation of H2O to O2 with excellent conversion (99% Faradaic yield). Conversely, the analogous complex without

H these pendant bases, [CoL (CH3CN)2][BF4]2, had no activity (Figure 2.2c). It was proposed that this reactivity is due to a CPET mechanism for water oxidation and likely stems from the proper positioning and pKa of the pendant bases, which can facilitate proton transfer through hydrogen bonds to cobalt-bound water molecules (Figure 2.2b). Confirmation of a CPET mechanism

DMA requires isolation of the oxidized species, CoL (OH)2, so that reduction potential and pKa values that define the square scheme can be obtained. Isolation of the hydroxide species was not possible, however, as an oxo- or hydroxide-bridging cobalt dimer was formed following oxidation of the

26 catalytic species.39 By investigating precatalysts with group 8 metal centers (Fe, Ru, Os) I hope to observe enhanced catalytic activity for water oxidation compared to the cobalt catalyst. I predict

DMA n-1 DMA n-2 improved stability of the hydroxide species, either [ML (OH)(L)] or [ML (OH)2] , for the 2nd and 3rd row metals as these are known to support higher oxidations states. Isolating the hydroxide species would allow for the measurement of reduction potential and pKa values and construction of the square scheme for the complex, providing evidence for the mechanism of water oxidation. Herein, challenges associated with the synthesis, characterization, and purification of the iron, osmium, and ruthenium complexes of LDMA are discussed.

DMA Figure 2.2. (a) Representative structure of [CoL (CH3CN)2][BF4]2. (b) ORTEP of DMA [CoL (H2O)2][BF4]2. Dashed gray lines indicate hydrogen bonding. (c) Cyclic H voltammograms of 1 μM solutions of [CoL (CH3CN)2][BF4]2 (left) and DMA [CoL (CH3CN)2][BF4]2 (right) after subsequent additions of H2O (0 – 5000 equivalents).

2.2 Results and Discussion

2.2.1 Attempted synthesis and partial characterization of Fe, Os, and Ru LDMA complexes

Attempted synthesis of the iron LDMA complex involved first forming the dinuclear iron

DMA 40 complex Fe2L Cl4 reported by Hoffert (Scheme 2.1). Addition of AgOTf to a suspension of

DMA DMA Fe2L Cl4 (pale yellow solid) in acetonitrile with available L in solution immediately yields a white solid (likely AgCl) and an orange solution. Filtering the solution and removing the solvent

II DMA in vacuo yields a pale orange solid which is believed to be [Fe L (CH3CN)2][OTf]2 or

II DMA 1 Fe L (OTf)2. Crude product from both of the two attempted syntheses give H NMR spectra with only the ligand resonances and expected solvent impurities present (Figure 2.3). There is a noticeable shift of methyl, methylene, and downfield pyridine-H resonances (Hb – Hf) to more downfield values relative to those of uncoordinated LDMA. These shifts may indicate metalation as electron density from the ligand is donated to the metal upon coordination. The shift is most prominent for protons in close proximity to coordinating atoms. However, the 1H NMR spectrum, by itself, is not enough to identify the product. Unfortunately, confirmation of a chemical structure by mass spectrometry, X-ray crystallography, or elemental analysis has not yet been obtained which has delayed plans to electrochemically characterize this product and test its water oxidation activity. Substituting triflate with a less-coordinating counterion may make crystallization easier.

Jacobsen, et. al. were successful in determining a X-ray crystal structure of a similar compound,

H H DMA 41 [FeL (CH3CN)2][SbF6]2 (L = L without pendent amines) utilizing a SbF6 counterion.

DMA Scheme 2.1. Synthesis pathway to purported product [FeL (CH3CN)2][OTf]2 or DMA FeL (OTf)2.

1 DMA Figure 2.3. H NMR spectrum (500MHz, CD3CN) of uncoordinated L (top) and crude DMA DMA product of Fe2L Cl4 + 4 AgOTf + L reaction from Scheme 2.1 (bottom).

IV DMA III DMA Scheme 2.2. Synthesis pathways of [Os L Cl2][ClO4]2 and [Ru L Cl2][ClO4]

IV DMA III DMA Syntheses for the preparation of [Os L Cl2][ClO4]2 and [Ru L Cl2][ClO4] were adapted from literature procedures (Scheme 2.2).42,43 However, there have been significant difficulties purifying and characterizing the chloride counterion precursor complexes, likely due to the suspected paramagnetic nature of both complexes (d4 and d5 metals centers for Os and Ru,

29 respectfully). For the crude products produced from the reaction of Os and Ru starting materials with ligand, 1H NMR spectra show characteristic resonances belonging to LDMA and expected solvent impurities, as well as some unidentified impurities (Figures 2.4 & 2.5). Downfield shifting of pyridyl protons and methyl protons (Ha – Hc) of the pendent amines may suggest metalation (as explained previously); however, methyl and methylene resonances on the ligand backbone (Hd –

Hf) appear to be missing or diminished in signal. Paramagnetic effects could be distorting these

IV DMA signals, especially given their proximity to coordinating atoms. For [Os L Cl2][Cl]2, mass spectrometry has not yet shown evidence of metalation as the ligand appears as the only major

DMA + III DMA identifiable peak (m/z = 357.2 ; [L + H ]). Analysis of [Ru L Cl2][Cl] by mass spectrometry reveals sets of peaks each with a pattern that matches the known isotope abundance of ruthenium; however, none of these peaks match the intended product or any obvious side products. Confirmation of product identities by elemental analysis has not been successful. For either metalation reaction, crude products typically present as oils and efforts to further purify these products have been largely unsuccessful, making crystallization difficult. Conversion to the

⁻ more easily crystallizable ClO4 salt has been complicated by the crude products’ solubility in water, likely due to hydrogen bonding available at the pendent amine sites. Metathesis with bulky,

⁻ F non-coordinating counterions like BPh4 or [BAr 4]⁻ or with chloride abstractors (various silver salts) may induce precipitation out of organic solvents to yield purer and easily crystallizable solids. Investigating other synthetic routes to osmium and ruthenium LDMA complexes may be a good idea too, especially those that use starting materials with open coordination sites (ex.

DMA RuCl3·xH2O or OsCl3·xH2O) as these may be more easily ligated by L .

1 DMA Figure 2.4. H NMR spectrum (500MHz, CD3CN) of uncoordinated L (top) and DMA “[OsL Cl2][Cl]2” crude product (bottom)

1 DMA Figure 2.5. H NMR spectrum (500MHz, CD3CN) of uncoordinated L (top) and DMA “[RuL Cl2][Cl]” crude product (bottom).

Conclusion

In Chapter 1, I proposed a three-step cascade catalysis pathway for the reduction of CO2 to methanol. This pathway is similar to previous work in homogeneous CO2 hydrogenation to methanol but utilizes electrochemically-generated metal hydride complexes as the reduction catalysts. A series of previously reported bis(diphosphine) metal hydride complexes was synthesized and used to perform hydride transfer reactions to intermediates methyl formate and formaldehyde. These reactions were tracked by 1H and 31P{1H} NMR spectroscopy at various timepoints and reactivity on a 0-to-1 scale (no reactivity vs reactivity) was used to bracket the hydride acceptor values of methyl formate and formaldehyde. This work represents, to my knowledge, the first approximation of these value, in acetonitrile. To probe the second step of the pathway, the esterification and amidation of formate was attempted. Despite the assistance of moderate Lewis acids, high reaction temperatures, and excess of substituent, formate displayed poor conversion to intended formate ester and formamide products. A hypothesis for this poor conversion was discussed and remedies were proposed, including addition of Brønsted acids, stronger Lewis acids, use of formate salts that undergo facile self-amidation.

In Chapter 2, I discussed the importance of investigating catalyst design features that promote small molecule oxidation via CPET mechanisms. The ligand LDMA features pendant amine groups that are believed to promote water oxidation via CPET in previous work with a cobalt complex. I hypothesized that analogous complexes with group 8 metals, especially 2nd and

3rd row ruthenium and osmium, would enhance the stability of the oxidized complex (necessary for confirmation of a CPET mechanism). While 1H NMR suggests metalation of LDMA to all three metals, products could not be confirmed by X-ray crystallography or EA, preventing electrochemical characterization and investigation of their water oxidation activity.

Experimental

General Methods: All synthesis and manipulations of metal complexes were carried out in a glovebox or utilizing standard Schlenk techniques under inert atmosphere of nitrogen. Solvents used during inert atmosphere synthesis and/or manipulations were degassed by sparging with argon and dried by passing through columns of neutral alumina or molecular sieves. All deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. Deuterated acetonitrile and methanol used for NMR characterization were degassed and stored over activated 3 Å molecular sieves prior to use. Deuterated tetrahydrofuran was degassed and stored over sodium metal prior to use. Deuterated ethanol was packaged in glass ampoules and was used without further degassing or drying. All solvents and reagents were purchased from commercial vendors and used without

21 17 further purification unless otherwise noted. [HPt(dmpe)2][PF6] (1), [Pt(dmpe)2][PF6]2 (2),

23 23 22 22 HRh(dppb)2 (5), [Rh(dppb)2][BF4] (6), HRh(dmpe)2 (7), [Rh(dmpe)2][BF4] (8),

17 17 18 [HNi(dmpe)2][BF4] (9), [Ni(dmpe)2][BF4]2 (10), [HNi(dhmpe)2H][BF4] (11),

18 [Ni(dhmpe)2][BF4]2 (12) , N,N′-bis((6-(dimethylamino)pyridin-2-yl)methyl)-N,N′-

DMA 44 DMA 40 dimethylethane-1,2-diamine (L ), and Fe2L Cl4 were prepared following literature procedures.

Physical Methods: 1H, 31P, and 19F nuclear magnetic resonance (NMR) spectra were collected at room temperature, unless otherwise noted, on a Bruker DRX 500 MHz or Bruker AVANCE 600

MHz spectrometer. Chemical shifts reported in δ notation in parts per million (ppm). 1H spectra were referenced to TMS at 0 ppm via the residual proteo solvent resonances. 31P spectra were referenced to H3PO4 at 0 ppm within Bruker’s Topspin 3.2 software, which derives the chemical shifts from the known frequency ratios (Ξ) of the 31P standard to the lock signal of the deuterated solvent.45 1H spectra used in time-dependent reactivity studies were obtained using a d1 time of

60s due to long T1 relaxation time of the formyl proton of methyl formate. 31P spectra used in determining relative concentration were obtained using at least 64 scans (on 600 MHz instrument) or at least 256 scans (on 500 MHz instrument) to ensure quantitative integration within an approximate 10% error. Automatic shimming, Fourier transformation, and automatic spectrum phasing were performed using Bruker’s Topspin software. Spectra were worked up and figures were generated using MestReNova 6.0.2 software. Peak integrations were performed either manually or using the peak fitting functionality within MestReNova. Electrospray ionization (ESI) mass spectrometry was performed using a Waters ESI LC-TOF Micromass LCT

3 premier mass spectrometer fitted with a Leap Technologies CTC Analytics autosampler.

Synthesis:

[Pt(dmpe)2][OTf]2 (3)

Synthesis adapted from literature procedure.17 Under nitrogen atmosphere, a suspension of

Pt(COD)Cl2 (112 mg, 0.299 mmol) in CH3CN (5 mL) was treated with a solution of dmpe (90 mg,

0.60 mmol) in CH3CN. The resulting mixture was stirred for 16 hours at room temperature, and the volatiles were removed under vacuum, leaving a white solid. This solid was suspended in acetonitrile (5 mL) and a solution of NH4OTf (50 mg, 0.30 mmol) in min. acetonitrile was added before stirring for 24 hours at room temperature. Suspension was filtered and solvent was removed in vacuo to yield a white solid which was washed with diethyl ether (4 x 2 mL) and dried under vacuum (128 mg, 50% yield). Crystals were grown by slow diffusion of diethyl ether into a

1 concentrated solution of the complex in acetonitrile/THF. H NMR (600 MHz, CD3CN): 2.12 (m,

3 31 1 4 H, PCH2CH2P), 1.81 (m, 12 H, JPt-H = 10.2 Hz, P-CH3). P{ H}NMR (600 MHz, CD3CN):

1 19 – 33.6 (s, JP-Pt = 1086 Hz). F NMR (600 MHz): -79.3 (s, OTf).

[HPt(dmpe)2][OTf] (2)

21 Synthesis adapted from literature procedure. Under nitrogen atmosphere, [Pt(dmpe)2][OTf]2 (196 mg, 0.249 mmol) was dissolved in acetonitrile (10 mL). With the acetonitrile solution stirring, 19 mg (0.51 mmol, 2 equiv.) NaBH4 was added as a solid. The mixture was stirred for 12 hours, filtered, and the filtrate was dried under vacuum to afford a very pale yellow solid. The crude material contained only [HPt(dmpe)2][OTf] and excess NaBH4. The desired [HPt(dmpe)2][OTf] was extracted with 5 mL C6H5Cl, and filtered away from the insoluble white NaBH4. The solvent was removed from the filtrate under vacuum, and the white powder was washed with diethyl ether

(3 x 2 mL) and dried under vacuum to yield the purified product (68 mg, 40% yield). 1H NMR

3 (600 MHz, CD3CN): 1.79 (m, 4 H, PCH2CH2P), 1.60 (m, 12 H, JPt-H = 11.3 Hz, P-CH3). -11.5

2 1 31 1 1 (quintet, 1H, JP-H = 30 Hz, JPt-H = 334 H, Pt-H). P{ H} NMR (600 MHz, CD3CN): -7.01 (s, JP-

19 – Pt = 1093 Hz). F NMR (600 MHz): -79.3 (s, OTf).

DMA DMA Product from reaction of Fe2L Cl4 + 4 AgOTf + L in acetonitrile (Scheme 2.1)

*Note: Be advised. Confirmation of this product’s chemical structure has not been made by X-ray

DMA crystallography or elemental analysis. Under nitrogen atmosphere, Fe2L Cl4 (120 mg, 0.197 mmol) was suspended in acetonitrile (5 mL). To this suspension was added a solution of LDMA (70 mg, 0.197 mmol) in acetonitrile (1 mL) followed by a solution of AgOTf (202 mg, 0.788 mmol) in minimum acetonitrile added dropwise. Following addition of AgOTf, an off-white solid precipitated out of solution almost immediately. The reaction solution was allowed to stir for 1 hour at room temperature. Suspension was filtered to collect the filtrate and the solvent was removed in vacuo. The resulting dull orange solid was washed with diethyl ether (3 x 2 mL) and

1 dried under vacuum (84 mg, 60% yield). H NMR (500 MHz, CD3CN): δ 7.48 (m, 2H, Hc), 6.52

(m, 4H, Hb), 3.85 (s, 4H, Hf), 3.01 (s, 12H, Ha), 2.98 (s, 4H, Hd), 2.41 (s, 6H, He).

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