Rhenium(I) Phosphazane Complexes for Electrocatalytic CO2

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Rhenium(I) Phosphazane Complexes for Electrocatalytic CO2 Reduction Matthew R. Crawley,† Karthika J. Kadassery,† Amanda N. Oldacre,† Alan E. Friedman,‡ David C. Lacy,*,† and Timothy R. Cook*,† †

Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States Department of Materials, Design, and Innovation, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States

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‡

S Supporting Information *

ABSTRACT: A library of 11 rhenium phosphazane complexes was synthesized and characterized for fundamental studies of electrochemical CO2 reduction. Because they contain chelating phosphorus donors, these complexes prevent deleterious dimerization of the precatalyst similarly to their monodentate analogues. These chelating species open a coordination site without complete ligand loss, which is hypothesized to increase catalyst longevity. Furthermore, the phosphazane ligands provided sites for synthetic modifications to control electronic structure, steric bulk, and install hydrogen bond donors. The effects of these parameters on the formation of CO from CO2 were investigated in the context of Faradaic efficiencies, catalytic current responses (icat/ip), and catalyst longevity. Full structural characterization provided insight into the differences in catalytic performance between the complexes, including a dirhenium variant. Re(PNPH)(bpy)(CO)2OTf (bpy = 2,2′-bipyridine, PNPH = (Ph)2PNHP(Ph)2) showed high catalytic performance with an (icat/i p)2 value of 795 and was active for >24 h with a 94% Faradaic efficiency. This library of electrocatalysts establishes a new class of tunable Re complexes from which a deeper mechanistic understanding of the CO2 reduction reaction emerges and underscores the efficacy of exploiting ligand-based participation in catalytic processes.

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INTRODUCTION

bipyridine; see Figure 1), and its activity toward both photocatalytic and electrocatalytic reduction of CO2.11,12 More recently, an impressive body of Re-based catalysis has been put forth by Kubiak,7,13−18 Ishitani,19−23 and recently by Nippe24 and Angeles-Boza, among many others.25−27 These

Reduction of carbon dioxide into either hydrocarbon fuels or chemical feed stock presents an attractive strategy to address growing global energy consumption.1 Of great industrial relevance is the production of carbon monoxide from carbon dioxide. The Fischer−Tropsch process,2 hydroformylation,3 water−gas shift reaction,4 Monsanto process,5 and the Cativa process6 all use CO as a chemical feedstock for the production of value-added and commercially desirable products. Current CO production is dominated by intensive methane reforming or coal gasification; in addition to being energetically inefficient, these approaches suffer from 20 to 50% loss of available carbon to CO2.2 As such, there is a critical need for energy efficient and highly selective routes to the production of CO under mild conditions. A variety of transition metal complexes have been reported to perform homogeneous electrocatalytic CO2 reduction summarized in recent reviews.7−10 Rhenium-based bipyridine systems are distinguished for high selectivity toward reducing CO2 over protons, typically via a two-proton two-electron process to furnish CO and H2O.7,10 Nearly all contemporary rhenium-bipyridine catalysts are based on the seminal report by Lehn and coworkers of Re(CO)3(bpy)Cl (bpy = 2,2′© XXXX American Chemical Society

Figure 1. Contrasting previous Re(I)-based systems to the work presented here, our system offers greater versatility in terms of tunability without the added synthetic challenge of covalent bpy modification. Received: February 28, 2019

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DOI: 10.1021/acs.organomet.9b00138 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics systems share themes of the use of a tricarbonyl-diimine coordination sphere and covalent modifications to the bpy backbone as a means to tune physicochemical properties. A second modification route is to substitute the X-type halide with an L-type phosphine ligand, which reduces the tendency for these complexes to dimerize to [Re(bpy)(CO)3]2 during the catalytic cycle.28,29 These modifications have provided a deeper mechanistic understanding of CO2 reduction; however, accessing more complex pendant groups on the bipyridyl ligand for further tuning requires multistep syntheses which are often low yielding and require demanding purifications.17,30 Exchanging the bpy ligand for a more modular platform such as an oxazoline ligand provided excellent catalytic activity with (icat/ip)2 values as high as 85, although there was a marked loss in Faradaic efficiency for CO production during controlled potential electrolysis experiments (55%).27 These systems collectively reveal that electron-withdrawing groups lead to a more facile reduction of the bipyridyl ligand that initiates catalysis. While this provides a seeming beneficial reduction of overpotential, it comes at the cost of reducing the nucleophilicity of the rhenium center, which ultimately inhibits the critical attack on CO2 at the onset of the catalytic cycle.18 Rhenium-phosphazane chemistry is largely unexplored, and no Re-phosphazane-bpy complexes have been previously reported.31−37 Outstanding challenges remain in the design of Re-based catalysts that can be readily tuned with simple synthetic modifications. Phosphazane ligands are thus attractive for two reasons. First, their syntheses are modular and offer tunability both in terms of electronic structure and well as steric bulk. Tunability extends to both the direct coordination sphere of the metal with phosphorus R-groups along with more distant modifications with nitrogen R′groups.38,39 Second, the chelating nature of these ligands may improve long-term catalyst stability by inhibiting decomposition pathways associated with ligand dissociation that plague systems with monodentate phosphine ligands. Herein, we report the synthesis, characterization, and catalytic reactivity studies of 11 isolated Re(I) phosphazane complexes. The complexes show excellent stability in ambient conditions and are easily synthesized and isolated. The use of a second chelating L-type ligand further enables catalyst tunability and introduces a site for ligand participation. Electrochemical experiments informed the operative mechanism of CO2 reduction to CO and H2O, and CPE experiments support our hypothesis that the use of bidentate auxiliary ligands promotes catalyst longevity.

Scheme 1. Synthetic Pathway to Rhenium(I) Dicarbonyl Phosphazanes

Spectroscopic Characterization. A breadth of techniques, including 1-D 1H and 31P{1H} along with 2-D COSY NMR techniques, FT-IR, FT-HR-MS, and single crystal X-ray diffraction, were used to characterize the complexes. For the tricarbonyl complexes, each 1H NMR spectrum exhibited a multiplet at ∼7.5 ppm corresponding to the 20 protons associated with the phenyl substituents of the phosphazane ligands. The remaining protons of the Nsubstituents were located upfield. The 31P{1H} spectra contained a single resonance, indicating that both phosphorus nuclei are chemically and magnetically equivalent (see Figures S1−12). The phosphazane-diimine complexes are chiral and less symmetric than the corresponding tricarbonyls. The reduction in symmetry resulted in each of the eight protons of the diimine ligand, as well as the protons of the phenyl substituents, to occupy unique chemical environments. In the case of Re(PNPH)(bpy)(CO)2OTf, the NMR exhibits the easily identified α-proton of the bpy moiety at 9.64 ppm as well as the N−H peak at 8.50 ppm characterized by the broad line shape. Beyond these, there is significant overlap of the remaining peaks (see Figure 2); however, the integrations of all observed peaks sum to the expected 29 protons when the identifiable α-proton resonance is integrated to 1. All subsequent rhenium phosphazane diimine dicarbonyl complexes exhibited similar splitting and integration patterns. Coordination of the bpy ligand breaks the symmetry of the two phosphorus nuclei; as a result, the 31P{1H} NMR spectra of the mononuclear dicarbonyl complexes exhibited two doublets of the two phosphorus nuclei coupling to one another with a typical JP−P of 11−13 Hz. In the case of the dinuclear p-tdpb bridged complex, there are four doublets corresponding to the mixture of the two possible diastereomers, the enantiomeric pairs ΔΔ/ΛΛ and ΔΛ/ΛΔ. COSY NMR experiments were conducted to give a deeper insight into atomic connectivity. In Figure 3, signals corresponding to the easily identified pyridyl α-proton and the protons of bpy were assigned and differentiated from those of phenyl protons of the phosphazane ligand. These COSY spectra enable the assignment of the overlapping peaks in the 1D spectra (see Figure S25 for fully assigned spectrum). For example, these COSY experiments reveal the location of the signal for the second α-proton, which was located significantly upfield of its congener at 7.88 ppm. This assignment is based on two pieces of evidence: first, a very similar cross-peak pattern to the other α−β coupling was observed (see cross-

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RESULTS Synthesis and Characterization. The syntheses of six novel rhenium-phosphazane-diimine-dicarbonyl complexes were carried out by treating the freshly prepared Re(PNPR)(CO)3OTf salts with the desired diimine ligands (either bpy or phen) (Scheme 1). A description of the synthesis of the tricarbonyl precursors is included in the Supporting Information. Thermolysis of the Re−CO bond to form the dicarbonyl species required significantly more forcing conditions as compared to those of the tricarbonyl complexes from Re(CO)5Cl. Refluxing o-dichlorobenzene (ODB, bp: 180 °C) allowed for sufficient temperature to cleave the CO ligand, affording the desired Re(PNPR)(diimine)(CO)2OTf complex. This procedure was effective to form all of the dicarbonyl complexes reported here with yields between 60 and 76%. B


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Organometallics

spectra of the subsequent modified phosphazane diimine complexes show similar off-diagonal coupling patterns. NMR alone is unable to provide an adequate explanation for the considerable upfield shift of the second α-proton; however, solid-state structural results, discussed below, are able to provide a satisfactory explanation. Fourier transform infrared spectroscopy (FT-IR) was used to monitor changes in symmetry as well as the number of carbonyl ligands. All tricarbonyl complexes show idealized Cs symmetry about the metal center with the mirror plane containing the Re−Cl and one Re−CO bond vector and bisecting the angle between Re−P and Re−CO bond vectors of the meridional plane. Symmetry analysis predicts three IR active modes (two a′ and a single a″ mode), and indeed, there are three observed CO stretching frequencies observed for all tricarbonyl complexes (see Figures S30−S34). Upon coordination of the diimine ligand, the symmetry is reduced to C1 that results in two IR active CO stretches (two a modes). IR spectra of all dicarbonyl complexes exhibit two sharp peaks in the CO stretching region, consistent with the loss of a carbonyl ligand. Figure 4 shows the IR spectrum of

Figure 2. Compiled NMR spectra of all dicarbonyl complexes. An asterisk (*) indicates residual CHCl3. With the exception of Re(PNPH)(bpy)(CO)2OTf (spectrum acquired in CDCl3), all spectra were acquired in CD2Cl2.

Figure 4. FT-IR spectrum of Re(PNPH)(bpy)(CO)2OTf performed neat of the bulk powder using an attenuated total reflection attachment.

Re(PNPH)(bpy)(CO)2OTf; the two carbonyl stretches were observed at 1937 and 1866 cm−1 along with the broad N−H stretch centered at 3170 cm−1 of the PNPH ligand. All other dicarbonyl IR spectra can be found in the Supporting Information. For discussion of characterization by mass spectrometry along with the associated mass spectra, see the Supporting Information. Crystallographic Details. A complete discussion of the refinement of the tricarbonyl complexes, supplemental crystallographic discussion, and crystallographic details tables can be found in the Supporting Information. The model of Re(PNPH)(bpy)(CO)2OTf refined superbly against the empirical data with a final R1 of 0.0175 and GooF of 1.048. This complex is chiral and crystallized in a noncentrosymmetric space group, Pna21. This is not a Sohncke space group (a space group lacking both mirror and inversion symmetry), and therefore, the crystals contained a racemic mixture of both the Δ and Λ enantiomers (one related to the other through either the n or a glide).40 The Flack parameter

Figure 3. COSY NMR spectrum of Re(PNPH)(bpy)(CO)2OTf performed in CD2Cl2 with a 300 MHz instrument and 8 transients at 256 points in F1.

peaks centered at [7.67, 7.88]); second, based on this assignment, both β-protons have very similar chemical shifts (7.56 and 7.67 ppm). Additionally, integrations of the 1D spectrum in CD2Cl2 further corroborate the assignment. COSY C


Article

Organometallics of 0.004(3) confirms the correct “handedness” was solved in the asymmetric unit (Λ). Structurally, the PNPH ligand retains the sp2 nitrogen (deviation of 0.006 Å from planarity), which can be seen in Figure 5. The triflate counterion was 2.22 Å

compiled in Table 1. Inspection of the bpy-centered reductions reveals that phosphazane functionalization has a minor effect as they differ by only ∼10 mV across the series of complexes studied here. Generally, the CVs of the PNPR dicarbonyl complexes exhibit two quasi-reversible reductions and two oxidation events. The first reduction is a bpy-centered event that is reversible for only the PNPH complex. The remaining bpycentered reductions are quasi-reversible based on the separation of Eox and Ered and the ratio of the current responses. Similarly, the second reduction event for the PNPHbpy complex is quasi-reversible. For the phen species (Figure 7B), several reduction events were observed. Differential pulsed voltammetry (DPV) was used to clarify the redox events that overlapped in the CV. It was found that four discrete reduction events occurred (Figure S50). The CV of (p-tdpb)[Re(bpy)(CO)2OTf]2 showed similar overlap, and DPV revealed two reductions beyond the initial bpy-centered event (Figure S51). The second reduction event of the PNPAr complexes was irreversible. As such, the potential of maximum current response (Epc) was reported in place of an E1/2 value. The large, irreversible response observed when scanning anodically was attributed to a phosphazane-centered oxidation. The basis of this assignment is that a similar irreversible response is present in the CV of Re(PNPH)(CO)3Cl at similar potentials. DPV showed that there are two separate oxidation events (Figure S52). This suggests electronic communication between the two P−N−P groups through the central phenylene spacer. A subsequent, quasi-reversible oxidation was observed at even more positive potentials when PNPH was used (Figures 7A and B), which has been attributed to the ReI/II couple. To test stability on the CV time-scale, 50 cycles were performed at a scan rate of 100 mV/s. All six dicarbonyl complexes are stable during cycling with no major changes in features or current response (see Figure S53). Electrocatalytic CO2 Reduction Studies. The CVs for each complex change dramatically upon addition of CO2 in terms of peak shape as well as current response. The reversibility of the bpy-centered reduction is significantly diminished, and a second large irreversible wave appears just beyond −2 V vs Fc/Fc+. The inclusion of a proton source (e.g., 2,2,2-trifluoroethanol, or TFE) caused additional increase in current and a change in the line shape. These changes are all summarized in Figure 8, where the red curve represents the CV of the metal complex in dry, CO2-free acetonitrile, the blue curve upon addition of CO2, and the black curve with further addition of TFE. The marked increase in current is similar to what others observed in the literature, where CO2 is catalytically reduced to CO and water.18,24 Importantly, the new waves from CO2 and acid are stable to cycling, indicating a degree of stability that was confirmed later in controlled potential electrolysis experiments. To ensure that the current response was due to homogeneous catalysis and was not the result of the formation of a heterogeneous film on the surface of the glassy carbon working electrode, a series of CVs was acquired under catalytic conditions. The working electrode was removed and moved to a fresh solution of supporting electrolyte, CO2, and TFE, and another CV was acquired. No significant current response was observed. The effects of acid on the catalytic current vary from complex to complex (Figure 9). Specifically, the current

Figure 5. ORTEP of Re(PNPH)(bpy)(CO)2OTf with probability level set at 50%. Hydrogen atoms with the exception of H3 and H3A and triflate counterion were removed for clarity. Selected atoms were labeled for discussion in the main text.

from the N−H of the PNPH ligand, suggesting a hydrogen bonding interaction. A T-shaped π−π interaction was observed between one of the α-protons of the bpy and one of the phenyl substituents of the PNPH ligand.41 The α-proton (H3A) was located 2.61 Å from the mean plane generated by the carbon atoms of the phenyl ring (see Figure 5). This close contact leads to the disparity between NMR chemical shifts of the two α-protons (H3A and H12). H3A sits in the π-cloud of the phenyl ring of the phosphazane ligand, leading to considerably greater shielding. This also indicates that the solid-state structure is representative of the solution-state conformation, a commonality shared by all of the dicarbonyl complexes. Data collections for the remaining three phosphazane complexes (phenyl, tolyl, and 4-bromophenyl functionalized, collectively referred to as PNPAr) were carried out at 250 K, and it was discovered that below 200 K, the crystals underwent an irreversible phase change that resulted in a marked shift in crystal symmetry. Low temperature data sets revealed the same atomic connectivity; however, space group assignment was problematic for data sets collected at 250 K. Examination of the 250 K data sets, particularly the toly- and 4-bromophenyl functionalized phosphazane complexes which are essentially isostructural, reveals that the triflate counterions reside in pseudochannels running along the crystallographic c-axis (see Figures S47 and S48). We contend that changes in the ordering of the triflate counterions within these channels results in the change of the crystal symmetry at low temperature. The PNPH dicarbonyl complexes (either with bpy or phen) did not show the same behavior. We attribute this to weak hydrogen bond interactions between the N−H of the PNPH ligand and the triflate counterion which enforces a favored conformation (Figure 6). Electrochemical Analysis and Reactivity Studies. Cyclic voltammetry (CV) was used to interrogate the electrochemical properties of each complex. The full CV of each of the dicarbonyl complexes with potential catalytic activity are found in Figure 7. In general, all dicarbonyl complexes presented exhibited a bpy-centered reduction at an average of −1.726(8) V vs Fc/Fc+. Reduction potentials of these events and potentials of maximum current response are D


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Organometallics

Figure 6. Compiled ORTEP of Re complexes. Probability level set at 50%. Hydrogen atoms, solvent molecules of crystallization, and outer sphere triflate counterions were omitted for clarity.

enhancement in the presence of CO2 is only modestly altered for the PNPPh and PNPtolyl with the addition of TFE. The PNP4‑BrPh complex reduced CO2 with the smallest overpotential of the PNPAr series (vide infra) and has the largest current enhancement in the presence of TFE. Given the modest current enhancement for tolyl and Ph derivatives, we were surprised that (p-tdpb)[Re(bpy)(CO)2OTf]2 demonstrated similar current enhancements to the 4-BrPh. This suggests the possibility of multimetal center cooperativity, which is a point of ongoing research in our lab. Changing from the electron-rich PNPtolyl to the electrondeficient PNP4‑BrPh ligands resulted in a ∼100 mV anodic shift in current response (Figure 9). This positive shift was even more pronounced when PNPH was used as the phosphazane ligand, affording a further 100 mV positive shift in the onset catalytic current. We also studied the influence of acid concentration in our system. Several concentrations of TFE were used with all other factors held constant. The results of the concentration studies were plotted and are shown in Figure 10. Each of the PNPAr

complexes showed a decrease in activity as a function of TFE concentration, while the PNPH complex exhibited an initial increase in activity followed by a drop off as acid concentration increased; similar trends were seen in systems that contain Hbond donors.24,42 The performance of (p-tdpb)[Re(bpy)(CO)2OTf]2 was largely unaffected by changes in acidity with only a minor decrease. In all cases, changes in proton concentrations lead to small shifts in the Ecat/2 values. Re(PNPH)(bpy)(CO)2OTf was subject to controlled potential electrolysis (CPE). A gastight custom H-cell was used (see Experimental Section for full details), and the headspace was periodically subjected to GC analysis. The charge vs time plot was linear through the first 12 h of analysis and trailed off slightly through 24 h. GC analysis informed that the primary gaseous product was carbon monoxide, typical of rhenium-diimine-based systems. Excellent selectivity was observed; GC analysis showed that hydrogen production was near zero (0.0065% (v/v) or 4 × 10−7 moles produced), and Faradaic efficiency for CO production was 94.4(1)%. E


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Organometallics

Table 1. Reduction Potentials of Select Redox Features of Rhenium(I) Phosphazane Complexes complex Re(PNPH)(bpy) (CO)2OTf Re(PNPH)(phen) (CO)2OTf Re(PNPPh)(bpy) (CO)2OTf Re(PNPtolyl)(bpy) (CO)2OTf Re(PNP4‑BrPh)(bpy) (CO)2OTf (p-tdpb)[Re(bpy) (CO)2OTf]2

E1/2a bpy

E1/2a red. 2

Emaxab PNP ox.

−1.74

−2.08

0.845

1.09

−1.73

−1.94

0.839

1.09

−1.72

−2.22

0.956

−1.72

−2.23c

0.939

−1.71

−2.15c

0.965

−1.72

−2.04c

0.910, 0.986

E1/2aReI/II

a

All potentials are referenced to the ferrocene couple. bEmax refers the potential of peak current response for the irreversible phosphazane based oxidation. cThese values are also potentials of peak current response given the irreversible nature of the peaks in the CV.

catalytic current responses. Therefore, in this case, icat/ip is an approximation and proportional to kcat. The general agreement in the current literature is that icat/ip, or in some cases (icat/ip)2, provides a useful comparison between different systems using data acquired by CV.44 Using the methodology put forth by Appel and Helm,45 overpotentials were calculated using the thermodynamic reduction potential of CO2/CO calculated by Savéant (−1.36 V vs Fc/Fc−+). The results are outlined in Table 2. The catalytic performance metrics prompt three observations: First, the phosphazane complex with the most electronrich substituent (PNPtolyl) possessed the largest overpotential, and the electron deficient PNP4‑BrPh complex had the lowest. Second, the dinuclear complex showed excellent performance, second only to Re(PNPH)(bpy)(CO)2OTf, dwarfing its mononuclear analogues. Third, the PNPH complex exhibits superior performance over all other functionalized mononuclear phosphazane complexes in terms of overpotential as well as icat/ip values. As a point of comparison, the (icat/ip)2 value of Lehn’s catalyst (Re(bpy)(CO)3Cl) is 491 under identical conditions, whereas the value for Re(PNPH)(bpy)(CO)2OTf is 795.44 In systems where the bpy moiety was replaced with oxazoline groups, the highest (icat/ip)2 observed was 85, and these enhancements came at the cost of Faradaic efficiencies of CO production (∼55%).27 Imidazole functionalized bpy units show greater performance with ((icat/ip)2 values of 144 and Faradaic efficiencies of 73%.24 The best performing Re-based catalysts exhibit (icat/ip)2 values of 1150, 967, and 900 with corresponding Faradaic efficiencies of 100, 96, and 100% (with the addition of PhOH as a proton source).18 We propose that one of the phosphazane ligand arms dissociates upon the second reduction event (P atom trans to the carbonyl ligand due to the trans-labeling nature of CO ligands), liberating a coordination site for CO2 to interact with the metal center through the formation of a metallocarboxylate species. One string of evidence that supports this hypothesis is that the current response due to the bpy centered reduction remains unchanged in both CVs of the complex and when CO2 and acid are present. It is not until the second reduction event that the onset of catalytic current occurs. This suggests that an EECC mechanism is at play with two electron transfer events to the catalyst, followed by the dissociation of

Figure 7. Cyclic voltammograms (CV) of rhenium phosphazane dicarbonyl complexes. All CVs were performed in acetonitrile at 1.0 mM of the respective rhenium complex for mononuclear complexes and 0.5 mM for (p-tdpb)[Re(bpy)(CO)2OTf]2 and 100 mM TBAPF6 at a scan rate of 100 mV/s and at room temperature.

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DISCUSSION To compare these new catalysts with others in the literature, we used the ratio of current under catalytic conditions (icat) to that under noncatalytic conditions (ip).16,18,24 Eq 143 relates icat/ip to kcat (intrinsic catalytic rate constant):18 icat 1 = ip 0.446

RT n′kcat nFν

(1)

where R is the universal gas constant, F is Faraday’s constant, T is temperature, n is the number of electron transfer events per catalytically active site, n′ is the number of catalytically active sites required per turnover, and ν is the scan rate. A crucial note is that eq 1 was developed for ideal S-shaped F


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Organometallics

Figure 9. Comparison of onset potential and peak current potentials of Re(PNPR)(bpy)(CO)2OTf complexes. All CV were acquired at 100 mV/s in acetonitrile with all complexes at 1.0 mM at room temperature.

Figure 10. Plot of icat/ip vs [TFE]. With the exception of Re(PNPH)(bpy)(CO)2OTf, all show inhibition of catalytic activity as a function of TFE concentration.

With regard to the far greater enhancement of PNPH (N−H functionalized) over the other functionalities, there are two possible explanations: the first is that upon dissociation, the ligand rotates clear of the coordination site such that the Rgroup of the nitrogen hinders the approach of CO2. The second possibility for enhancement is that the N−H group of PNPH is able to stabilize the CO2-bound species, leading to the far greater activity. Within the literature, there are other examples of hydrogen bond donors that have led to enhancement of catalytic activity.17,24,46−48 We expect the Hbonding to be the dominant effect over steric considerations, especially considering the high activity of the dinuclear catalyst, which contained the largest phosphazane. Support for the latter hypothesis comes from a Hammett plot exploring the relationship between rate constants extracted using eq 1 and the functionalization of the phosphazane ligands. The rate constants are related through the Hammett parameter σ,49 indicating that a resonance or induction effect from the phosphazane ligand exists (Figure 11). The positive Hammett ρ value suggests a buildup of negative charge in the transition state, consistent with the formation of a metallocarboxylate during the rate-limiting step. However, the PNPH species falls far outside of the Hammett relationship governing the aryl catalysts, consistent with a mechanistic shift when a hydrogen bond donor is present.

Figure 8. CV of dicarbonyl rhenium(I) phosphazane complexes under various conditions: a solution of the respective complex (1.0 mM for mononuclear complexes and 0.5 mM for the dinuclear complex to normalize for rhenium concentration) and TBAPF6 (100 mM) sparged with N2 (red), CO2 (blue), and CO2 and TFE (280 mM) (black). All CV were acquired with a scan rate of 100 mV/s with 100 mM of TBAPF6 as a supporting electrolyte in acetonitrile at room temperature.

the one phosphazane arm.18 The third step, CO2 binding, is the slow step, or prior to the rate-determining step, as evidenced by scan rate studies under catalytic conditions. With increasing scan rate, the reversibility of the initial bpy-centered reduction increases. The fourth step is a proton induced CO bond cleavage to afford water and CO, the latter of which was confirmed to be the major product by controlled potential electrolysis (vide infra). G


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Organometallics Table 2. Compiled Metrics of Catalytic Performance of Complexes Synthesized in This Worka ligand H

PNP PNPPh PNPtolyl PNP4‑BrPh p-tdpbd

icat/ip c

28.2(6) 3.9(1) 4.3(4) 8.8(5) 15(1)

(icat/ip)2

kcat (s−1)

795 15 19 78 225

1.23(7) × 10 24(1) 29(6) 1.2(2) × 102 3.5(5) × 102

3

ηb (V)

Ecat/2 (V)e

0.68 0.77 0.78 0.73 0.69

−2.04 −2.13 −2.14 −2.09 −2.05

a General note: all measurements were acquired in acetonitrile with 100 mM TBAPF6, acid conc 270 mM TFE, and 1.0 mM catalyst at room temperature with a scan rate of 100 mV/s. bOverpotentials were calculated in the method outlined by Appel and Helm.45 cThe maximum icat/ip in this case was when [TFE] = 790 mM. dThe concentration of the dinuclear complex was reduced to 0.5 mM to normalize by rhenium concentration so a useful comparison to the mononuclear complexes can be made. eReferenced to Fc/Fc+.

with monodentate ancillary ligands.15,29 To this same end, the bulky phosphazane ligand hinders the approach of two Re0 species, further shutting down this decomposition pathway. This is particularly relevant given the mechanism proposed by Turner in which a bis(monophosphite) catalyst was used which required the dissociation and subsequent reassociation of a phosphite ligand.28 When compared to other notable systems, Kubiak’s tert-butyl functionalized Re(4,4′-tert-butylbpy)(CO)3Cl showed activity through 5 h, and Lehn’s catalyst loses activity even more rapidly.13 In contrast, our catalysts exhibit long stability with 24 h CPE experiments, exhibiting no sign of decomposition.

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CONCLUSIONS We reported the synthesis and complete characterization of a series of rhenium-phosphazane-diimine-dicarbonyl catalysts for CO2 reduction. These are the first heteroleptic rhenium phosphazane bpy complexes. We hypothesized that the use of phosphazane ligands would provide a means to tune electronic structures and enable mechanistic insight. The parent PNPH system provided a site for hydrogen bond donation, which has been implicated in stabilizing the key metallocarboxylic acid intermediate formed during CO2 reduction. Indeed, this catalyst showed a greatly enhanced response relative to the other phosphazane systems. The rate of CO2 reduction for the parent system does not fall on the Hammett plot from the other substituents used in this study, further supporting a mechanistic pathway in which the secondary coordination environment plays a large role in catalyst performance. Finally, the use of the chelating phosphazane unit provided additional benefit of enabling the necessary ligand dissociation step while not completely dissociating from the complex. This aspect manifested itself in high catalyst stability (>24 h maintaining 94% Faradaic efficiency for CO) compared to some of the best molecular CO2 reduction catalysts. As a final note, the dinuclear system reported exhibits properties that hint at multicenter cooperativity, prompting future studies into the role of multinuclear CO2 reduction catalysts.

Figure 11. Linear relationship between the potential of maximum catalytic response and the Hammett substituent constant. The fit does not include the PNPH data point.

We contend that the PNPAr complexes are rate-limited by the binding of CO2. This is supported by the following observations: First, the positive Hammett ρ is consistent with the formation of a metallocarboxylate. Second, the icat/ip values were nearly independent of acid concentration (Figure 10), suggesting that proton induced C−O bond cleavage occurs after the rate-determining step, and therefore, [H+] will not appear in the rate law. The stark difference when the PNPH phosphazane was used can be explained by a mechanistic shift to C−O bond cleavage becoming the rate-determining step. The acid concentration dependence observed for Re(PNPH)(bpy)(CO)2OTf was similarly observed when Lehn’s catalyst was evaluated under identical conditions, suggesting a similar operative pathway. It has been shown that in the Re(bpy)(CO)3Cl system, the rate limiting step is C−O bond cleavage.50,51 Furthermore, the divergence of the PNPH complex from the linear trend found in the Hammett relationship further supports this mechanistic shift. Hence, we favor the latter hypothesis that the H-variant stabilizes the carboxylate intermediate, thereby accelerating the rate of reaction. A key difference with previous systems that stabilize CO2 adducts with H-bonds and our work is that the inclusion of the phosphazane additionally stabilizes the complex toward decomposition, which has also been demonstrated using macrocyclic ligand scaffolds.52,53 Namely, the origin of Re(PNPH)(bpy)(CO)2OTf’s excellent long-term stability under catalytic conditions is attributed to the chelate effect of the bidentate phosphazane ligand. Because these ancillary ligands never fully dissociate, deleterious side reactions that would otherwise result from monodentate ligand dissociation are avoided. For instance, Re−Re dimerization is a known deactivation pathway of catalysts

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EXPERIMENTAL SECTION

General Information. Unless otherwise noted, all manipulations were carried out under an inert dinitrogen atmosphere, either in a glovebox (Vigor, United States) or using standard Schlenk techniques. Solvents were purified using a Grubbs style solvent system (Pure Process Technology). 1H NMR spectra were acquired on Varian 300, 400, or 500 MHz spectrometers; 31P{1H} spectra were acquired on a 300 MHz NMR spectrometer. Chemical shifts (δ) were reported in parts per million (ppm) and referenced against the residual proteosolvent peak in 1H experiments and 85% H3PO4 in 31P{1H} experiments. Re(CO)5Cl was purchased from Acros Organics; 2,2′bipyridine and 1,10-phenanthroline (phen) were purchased from H


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Organometallics Aldrich and used as received without further purification. Bis(diphenylphosphino)amine (PNPH),54 PNPPh,39 PNPtolyl,39 PNPBr,39 and p-tdpb38 were synthesized according to literature protocols. orthoDichlorobenzene was dried over 3 Å molecular sieves for 2 days prior to use. Headspace analysis was performed on a PerkinElmer Clarus 580 GC (thermal conductivity detector, Ar carrier gas). Samples were infused by electrospray ionization using a home-built ESI sprayer into a Bruker SolariX 12T FTMS (12 T FT ICR) system equipped with a dual source. In a typical experiment, 20 spectra were collected from 300 to 3000 m/z at high resolving power (Δ = 150 000 at m/z 622, where Δ is the peak full width at half-maximum peak height). Internal calibration mass accuracy results in a RMS mass error typically less than 500 ppb.

Synthesis of Re(PNP4‑BrPh)(CO)3Cl. 68.63 mg (0.1897 mmol) of Re(CO)5Cl and 128.67 mg (0.23811 mmol) of (4-BrPh)N(PPh2)2 were suspended in 10.0 mL of toluene, reaction conditions and work up were the identical to Re((Ph)N(PPh2)2)(CO)3Cl. Yield: 87% (140.36 mg, 0.16102 mmol)). 1H NMR (CD2Cl2, 300 MHz): δ 7.68−7.34 (20H, m, Ph), 7.19 (2H, d, J = 9.0 Hz), 6.60 (2H, d, J = 9.0 Hz); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 53.14. FT-IR (neat, cm−1) 2027 (CO, sharp), 1946 (CO, sharp), 1898 (CO, sharp).

Synthesis of (p-tdpb)[Re(CO)3Cl]2. 305.0 mg (0.8432 mmol) of Re(CO)5Cl and 334.0 mg (0.3953 mmol) p-tdpb were suspended in 60 mL of THF and refluxed overnight. The volume of THF was reduced to approximately 20 mL by rotary evaporation. The resulting suspension was cooled to −20 °C for 1 h to promote product crystallization. The mother liquor was decanted, and the colorless crystalline solid was dried in vacuo. Yield 73% (419.12 mg, 0.28781 mmol). (Note: This yield reflects isolated yield of the pure compound, if the mother liquor volume is again reduced and placed back in the freezer, a second crop of solid is afforded; however, this crop shows impurities in the 1H and 31P{1H} NMR (possible contamination with monosubstitution).) 1H NMR (CDCl3, 300 MHz): δ 7.55−7.30 (32H, m), 7.24−7.16 (8H, m), 6.38 (4H, s); 31 1 P{ H} NMR (CDCl3, 121 MHz) δ 51.79. FT-IR (neat, cm−1) 2027 (CO, sharp), 1947 (CO, sharp), 1901 (CO, sharp). Preparation of Re(PNPH)(CO)3OTf. In a vial, 147.08 mg (0.213 mmol) of Re(PNPH)(CO)3Cl was dissolved in the minimal amount of THF (ca. 10 mL) in a nitrogen-filled glovebox. In a separate vial, 63.10 mg of AgOTf (0.246 mmol) was dissolved in 5.0 mL of THF. The AgOTf solution was then added to a stirred solution of Re(PNPH)(CO)3Cl, which resulted in the immediate precipitation of white AgCl. The mixture was stirred overnight in the dark. The cloudy white suspension was then filtered through a short bed of Celite slurried in THF in air. Volatiles were removed by rotary evaporation to give an off-white solid. The solid was resuspended in CH2Cl2 (5 mL) and filtered a second time through glass wool to remove any remaining AgCl. The solid was washed with 2.0 mL of CH2Cl2 and the filtrate added to 20 mL of hexanes; this induced the precipitation of Re(PNPH)(CO)3OTf as a white powder. The solvent volume was reduced by half, and the solid was collected by filtration and dried in vacuo before immediate use without characterization. Preparation of Re(PNPPh)(CO)3OTf. Analogously to the synthesis of Re(PNPH)(CO)3OTf, 94.68 mg (0.1234 mmol) of Re(PNPPh)(CO)3Cl was treated with 38.10 mg (0.1483 mmol) of AgOTf in 5 mL of THF, and the resulting solid was immediately carried forward to the reaction with bpy without further workup or purification. Preparation of Re(PNPtolyl)(CO)3OTf. Analogously to the synthesis of Re(PNPH)(CO)3OTf, 125.79 mg (0.16102 mmol) of Re(PNPtolyl)(CO)3Cl was treated with 47.62 mg (0.1853 mmol) of AgOTf in 5 mL of THF, and the resulting solid was immediately carried forward to the reaction with bpy without further workup or purification. Preparation of Re(PNP4‑BrPh)(CO)3OTf. Analogously to the synthesis of Re(PNPH)(CO)3OTf, 123.22 mg (0.14564 mmol) of Re(PNP4‑BrPh)(CO)3Cl was treated with 42.90 mg (0.1670 mmol) of AgOTf in 5 mL of THF, and the resulting solid was immediately carried forward to the reaction with bpy without further workup or purification. Preparation of (p-tdpb)[Re(CO)3OTf]2. 198.74 mg (0.13647 mmol) of (p-tdpb)[Re(CO)3Cl]2 was treated with 77.07 mg (0.3000 mmol) of AgOTf in 50 mL of 1:1 CH2Cl2:THF mixture and allowed to stir overnight with the exclusion of light. AgCl was removed via filtration, and the volatiles were removed using rotary

Synthesis of Re(PNPH)(CO)3Cl. Re(CO)5Cl (500.12 mg, 1.3827 mmol) and PNPH (595.7 mg, 1.546 mmol) were suspended in 65 mL of toluene in a nitrogen filled glovebox. The reaction mixture was sealed under an inert atmosphere and removed from the glovebox. Using standard Schlenk technique, a water-cooled reflux condenser was attached and the reaction mixture brought to reflux. The reaction mixture was initially a white suspension; as temperature approached 90 °C, it transitioned to clear and colorless, and after approximately 15 min at reflux, a cloudy white solid began to precipitate. After 4 h, the reaction mixture was removed from the heat and filtered hot in air. The white solid was washed with hexanes and dried in vacuo. Yield: 75% (712.00 mg, 1.03 mmol). 1H NMR (CD2Cl2, 300 MHz): δ 7.66−7.44 (20H, m, Ph), 5.93 (1H, s, N−H); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 27.09. HR-ESI-MS: m/z [M − Cl−]+ 656.07029 (calcd for C27H21NO3P2Re+[M − Cl−]+, 656.05497). FT-IR (neat, cm−1) 3147 (N−H, broad), 2029 (CO, sharp), 1945 (CO, sharp), 1898 (CO, sharp).

Synthesis of Re(PNPPh)(CO)3Cl. In an analogous manner to the synthesis of Re(PNPH)(CO)3Cl, 97.23 mg (0.2688 mmol) of Re(CO)5Cl and 129.87 mg (0.28142 mmol) of (Ph)N(PPh2)2 were suspended in 10.0 mL of toluene. All other reaction conditions were the same as the analogous PNPH complex. At the conclusion of a 4-h reflux, a modified isolation technique was used. The reaction mixture was cooled to room temperature in air, and the toluene was removed via rotary evaporation. The residue was redissolved in the minimal amount (∼1−2 mL) of CH2Cl2, and hexanes were added to form a white precipitate. The white solid was collected via vacuum filtration and washed with 3 × 10 mL of hexanes. The solid was dried in vacuo. Yield: 68% (141.24 mg, 0.18411 mmol). 1H NMR (CD2Cl2, 300 MHz): δ 7.68−7.34 (20H, m, Ph), 7.17 (1H, t, J = 7.5 Hz), 7.06 (2H, t, J = 7.5 Hz), 6.74 (2H, d, J = 9 Hz); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 52.27. FT-IR (neat, cm−1) 2027 (CO, sharp), 1962 (C O, sharp), 1914 (CO, sharp).

Synthesis of Re(PNPtolyl)(CO)3Cl. 97.67 mg (0.2700 mmol) of Re(CO)5Cl and 145.51 mg (0.30601 mmol) of (tolyl)N(PPh2)2 were suspended in 10.0 mL of toluene; reaction conditions and work up were the same as those for Re((Ph)N(PPh2)2)(CO)3Cl. Yield: 60% (125.79 mg, 0.16590 mmol)). 1H NMR (CD2Cl2, 300 MHz): δ 7.68−7.34 (20H, m, Ph), 6.87 (2H, d, J = 9.0 Hz), 6.62 (2H, d, J = 9.0 Hz), 2.22 (3H, s); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 51.81. FT-IR (neat, cm−1) 2024 (CO, sharp), 1959 (CO, sharp), 1890(CO, sharp). I


Article

Organometallics evaporation. The colorless residue that remained was suspended in ∼1 mL of CH2Cl2 and added to 15 mL of hexanes, which immediately induced precipitation of a white solid. The suspension was rotovaped to half volume, and the solid was collected by filtration. The solid was used immediately without further purification or characterization.

Synthesis of Re(PNPH)(phen)(CO)2OTf. In the manner previously described in the synthesis of Re(PNPH)(bpy)(CO)2OTf, 153.3 mg (0.1905 mmol) of Re(PNPH)(CO)3OTf was treated with 54.9 mg (0.305 mmol) of 1,10-phenanthroline in 15.0 mL of ODB. Yield: 76% (141.4 mg, 0.1477 mmol). 1H NMR (CD2Cl2, 300 MHz): δ9.99 (1H, d, J = 6.0 Hz), 8.59 (1H, d, J = 9.0 Hz), 8.45 (1H, s, N−H), 8.25− 8.07 (4H, m), 8.01 (1H, d, J = 9.0 Hz), 7.97−7.88 (1H, m), 7.82 (1H, d, J = 9.0 Hz), 7.78−7.51 (7H, m), 7.48−7.30 (6H, m), 7.11 (1H, dd, J = 6, 9 Hz), 6.87 (1H, t, J = 6.0 Hz), 6.60 (2H, t, J = 6.0 Hz), 6.21 (2H, dd, J = 6, 12 Hz).); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 41.35 (d, JP−P = 10.9 Hz), 30.80 (d, JP−P = 10.9 Hz). HR-ESI-MS: m/z [M − OTf−]+ 808.14603 (calcd for C38H29N3O2P2Re+ [M − OTf−]+ 808.12889). FT-IR (neat, cm−1) 3134 (N−H, broad), 1922 (CO, sharp), 1852 (CO, sharp).

Synthesis of {(p-tdpb)[Re(CO)3(NCCH3)]2}OTf2,(Re2−ACN2). (ptdpb)[Re(CO)3Cl]2 (48.69 mg, 0.03344 mmol) and AgOTf (21.32 mg, 0.08298 mmol) were suspended in 10 mL of acetonitrile with the exclusion of light, air, and water. (Note: (p-tdpb)[Re(CO)3Cl]2 is poorly soluble in acetonitrile at room temperature and slowly goes into solution as ligand exchange occurs.) The suspension was stirred and brought to reflux for 48 h, after which a cloudy white/pink suspension was produced. The suspension was filtered through a Celite plug, and the volatiles were removed in vacuo. The residue was redissolved in a minimal amount of CH2Cl2 and added to diethyl ether to induce precipitation. The white, air-stable solid was collected and dried in vacuo. Vapor diffusion of diethyl ether into a solution of the title compound in CH2Cl2 afforded colorless, clear crystals suitable for SC-XRD. Yield: 65% (38.07 mg, 0.02156 mmol). 1H NMR (CD2Cl2, 300 MHz): δ 7.72−7.55 (16H, m), 7.52−7.28 (24H, m), 6.49 (4H, s), 1.97 (6H, s); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 54.99. ESI-MS: m/z [M − OTf − ] + 1614.7 (calcd for C65H50F3N4O9P4SRe+ [M − OTf−]+ 1615.1), [M − 2OTf−]2+ 733.2 (calcd for C64H50N4O6P4Re2 [M − 2OTf−]2+ 733.1).

Synthesis of (Re(PNPPh)(bpy)(CO)2OTf). In the manner previously described in the synthesis of Re(PNPH)(bpy)(CO)2OTf, 213.67 mg (0.24259 mmol) of Re((Ph)N(PPh2)2)(CO)3OTf was treated with 47.01 mg (0.3010 mmol) of 2,2′-bipyridine in 20 mL of ODB. In place of diethyl ether, hexanes was used to layer, as attempts with ether proved fruitless. Yield: 67% (164.29 mg, 0.16283 mmol). 1H NMR (CD2Cl2, 300 MHz): δ 9.44 (1H, d, J = 6.0 Hz), 8.22 (1H, d, J = 9.0 Hz), 8.10−8.00 (2H, m), 7.96−7.83 (3H, m), 7.76−7.63 (5H, m), 7.59−7.38 (10H, m), 7.34−7.23 (1H, m), 7.10−6.86 (6H, m), 6.71−6.53 (4H, m); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 62.15 (d, JP−P = 12.1 Hz), 53.69 (d, JP−P = 13.3 Hz). HR-ESI-MS: m/z [M − OTf−]+ 860.17042 (calcd for C42H33N3O2P2Re+ [M − OTf−]+ 860.16023). FTIR (neat, cm−1) 1949 (CO, sharp), 1886 (CO, sharp).

Synthesis of Re(PNPH)(bpy)(CO)2OTf. In air, 62.00 mg (0.077 mmol) of Re(PNPH)(CO)3OTf was suspended in ortho-dichlorobenzene (ODB) (7 mL) and sparged with N2 for 30 min. After degassing, 17.19 mg of 2,2′-bipyridine (2,2′-bpy) (0.110 mmol) was added as a solid with positive pressure of N2 and stirring. A watercooled reflux condenser was then equipped, and the reaction was heated with a sand bath to 220 °C. The reaction mixture transitioned from cloudy to clear and colorless to yellow to orange as the temperature increased. The reaction mixture was gently refluxed for 7 h, after which the reaction mixture was transferred into a nitrogen atmosphere glovebox. The deep orange solution was added to 50 mL of stirred hexanes to afford a bright yellow solid. Stirring was ceased, and the solid was allowed to settle. The faint yellow hexanes/ODB solution was decanted so that only the bright yellow solid remained. The yellow solid was redissolved in 4.0 mL of CH2Cl2, and the deep orange solution was layered with ∼100 mL of diethyl ether and allowed to stand for 1 week in the glovebox, at which time deep yellow blocks had crystallized. A small sample was used for SC-XRD and the remainder was collected by filtration and dried in vacuo. Yield: of Re(PNPH)(bpy)(CO)2OTf was 60% (43.11 mg, 0.0462 mmol). 1H NMR (CDCl3, 500 MHz): δ 9.64 (1H, d, J = 5.0 Hz, α-H), 8.50 (1H, s, N−H), 8.21 (1H, d, J = 10.0 Hz), 8.11−8.01 (3H, m), 7.92 (1H, d, J = 5.0 Hz), 7.84 (1H, d, J = 5.0 Hz), 7.71−7.47 (9H, m), 7.39−7.28 (6H, m), 7.07−7.01 (1H, m), 6.95−6.88 (2H, m), 6.72−6.67 (1H, m), 6.66−6.59 (2H, m); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 41.38 (d, JP−P = 10.9 Hz), 30.98 (d, JP−P = 10.9 Hz). HR-ESI-MS: m/z [M − OTf−]+ 784.14526 (calcd for C36H29N3O2P2Re+ [M − OTf−]+ 784.12887). FT-IR (neat, cm−1) 3170 (N−H, broad), 1937 (CO, sharp), 1866 (CO, sharp).

Synthesis of (Re(PNPtolyl)(bpy)(CO)2OTf). In the manner previously described in the synthesis of Re(PNPH)(bpy)(CO)2OTf, 49.29 mg (0.05507 mmol) of Re((tolyl)N(PPh2)2)(CO)3OTf was treated with 11.51 mg (0.07370 mmol) of 2,2′-bipyridine in 5 mL of ODB. Yield: 65% (36.62 mg, 0.03579 mmol). 1H NMR (CD2Cl2, 300 MHz): δ 9.45 (1H, d, J = 6.0 Hz), 8.20 (1H, d, J = 9.0 Hz), 8.10−7.98 (2H, m), 7.95−7.83 (3H, m), 7.77−7.59 (5H, m), 7.60−7.37 (10H, m), 7.29 (1H, t, J = 7.5 Hz), 7.06−6.90 (3H, m), 6.80 (2H, d, J = 9.0 Hz), 6.64−6.50 (4H, m), 2.16 (3H, s); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 62.40 (d, JP−P = 10.9 Hz), 53.78 (d, JP−P = 10.9 Hz). HR-ESI-MS: m/z [M − OTf−]+ 874.20313 (calcd for C43H35N3O2P2Re+ [M − OTf−]+ 874.17589). FTIR (neat, cm−1) 1945 (CO, sharp), 1874 (CO, sharp). J


Article

Organometallics

solution of 100 mM TBAPF6 was sparged for 30 min with CO2 generated from dry ice and added via syringe to the H-Cell through septa on the cell (20 mL to both the counter electrode and working electrode sides). Additionally, 0.2 mL of 2,2,2-trifluoroethanol (dried on 3 Å molecular sieves for at least 24 h) was added via syringe to both compartments. Electrodes were installed through gastight septa with the working electrode and reference on the working side of the cell, and the counter electrode on the counter side. The cell was then sealed and removed from the Schlenk line and attached to a Bio-Logic SP-300 or SP-200 potentiostat/galvanostat. The working electrode was held at a potential of −1.85 V vs Ag/AgNO3 (1.95 vs Fc/Fc+). Headspace analysis was periodically performed to analyze the composition of the gases in the cell using gas chromatography. Single Crystal X-ray Diffraction Studies. With the exception of Re(PNPPh)CO3Cl and Re(PNPtolyl)(bpy)(CO)2OTf, X-ray diffraction experiments were conducted on a Bruker D8 Venture diffractometer in a fixed-chi geometry equipped with a Photon-100 CMOS area detector and a molybdenum X-ray tube (Mo Kα 0.7107 Å) with a graphite monochromator. An Oxford cryostat was used to maintain the specified temperature (see Table S2) for the entirety of data collection. Datum crystals were mounted using MiTeGen MicroLoops affixed with a drop of N-Paratone oil. For the exceptions, data were collected at Northwestern University as part of the ACA Summer Course for Chemical Crystallography using Bruker diffractometer in a kappa geometry equipped with an APEX2 CCD detector and a molybdenum sealed tube source with Triumph monochromator and copper I μs microfocus. All crystals were run with the molybdenum source. A series of φ and ω scans were used to cover reciprocal space. Unless otherwise stated, data collection was performed at low temperature to minimize thermal motion. Integration was performed using SAINT and scaling using a multiscan method via SADABS, both within the APEX3 software suite.56 XPREP was use to assign a space group on the basis of E2 statistics, systematic absences, and the successful modeling and refinement of each structure.

Synthesis of (Re(PNP4‑BrPh)(bpy)(CO)2OTf). In the manner previously described in the synthesis of Re(PNPH)(bpy)(CO)2OTf, 63.48 mg (0.06615 mmol) of Re((4-BrPh)N(PPh2)2)(CO)3OTf was treated with 16.75 mg (0.1072 mmol) of 2,2′-bipyridine in 7 mL of ODB. Yield: 69% (49.31 mg, 0.04533 mmol). 1H NMR (CD2Cl2, 300 MHz): δ 9.40 (1H, d, J = 6.0 Hz), 8.25 (1H, d, J = 9.0 Hz), 8.13−8.01 (2H, m), 7.94−7.80 (3H, m), 7.77−7.64 (5H, m), 7.63−7.40 (10H, m), 7.31 (1H, t, J = 7.5 Hz), 7.16−6.98 (4H, m), 6.91 (1H, t, J = 7.5 Hz), 6.67−6.46 (4H, m); 31P{1H} NMR (CD2Cl2, 121 MHz) δ 63.24 (d, JP−P = 14.5 Hz), 54.15 (d, JP−P = 13.3 Hz). HR-ESI-MS: m/z [M − OTf−]+ 938.09017 (calcd for C42H32BrN3O2P2Re+ [M − OTf−]+ 938.06894). FTIR (neat, cm−1) 1949 (CO, sharp), 1886 (CO, sharp).

Synthesis of (p-tdpb)[Re(bpy)(CO)2OTf]2. In the manner previously described in the synthesis of Re(PNPH)(bpy)(CO)2OTf, 112.00 mg (0.064946 mmol) of (p-tdpb)[Re(CO)3OTf]2 was treated with 24.76 mg (0.1585 mmol) of 2,2′-bipyridine in 10 mL of ODB. Yield: 68% (74.50 mg, 0.04416 mmol). 1H NMR (CD2Cl2, 400 MHz): δ 9.40 (2H, d, J = 4.0 Hz), 8.19−8.11 (2H, m), 8.02 (2H, t, J = 8.0 Hz), 7.96 (2H, t, J = 8.0 Hz), 7.85 (2H, t, zJ = 8.0 Hz), 7.77− 7.59 (10H, m), 7.52−7.39 (12H, m), 7.36−7.21 (14H, m), 7.03−6.93 (4H, m), 6.90 (2H, t, J = 8.0 Hz), 6.50−6.38 (4H, m), 6.25 (4H, s); 31 1 P{ H} NMR (CD2Cl2, 121 MHz) δ 63.69 (d, JP−P = 10.9 Hz), 63.43 (d, JP−P = 12.1 Hz), 55.52 (d, JP−P = 10.9 Hz), 55.36 (d, JP−P = 10.9 Hz). HR-ESI-MS: m/z [M − 2OTf−]2+ 821.15998 (calcd for C78H60N3O4P4Re2+ [M − 2OTf−]2+ 821.13712). FT-IR (neat, cm−1) 1946 (CO, sharp), 1875 (CO, sharp). Cyclic Voltammetry Experiments. All experiments were conducted using a Bio-Logic SP-300 or SP-200 potentiostat/ galvanostat. A three-electrode configuration was used with a glassy carbon working electrode (CH Instruments), platinum wire counter electrode, and a nonaqueous Ag/AgNO3 reference electrode (BASi). Tetrabutylammonium hexafluorophosphate (TBAPF6) was recrystallized a minimum of three times from absolute ethanol, dried for at least 24 h under vacuum, and used as a supporting electrolyte. To polish the working electrode, 0.05 μm of alumina powder slurried in water was used, after which the electrode was washed with deionized water, acetone, and allowed to air-dry. Unless otherwise noted, all CVs were performed in dry acetonitrile with 100 mM TBAPF6 and under an atmosphere of N2 (either sparged or in an N2 atmosphere glovebox). For solvents sparged with CO2, dry ice was allowed to sublime in a flask connected to a acetonitrile bubbler, which was connected to the electrochemical apparatus with a metal needle.55 Solutions were sparged for 10 min before CV were acquired. Controlled Potential Electrolysis (CPE). All CPE experiments were conducted in a custom-built gastight H-cell from Adams & Chittenden Scientific Glass. A graphite felt working, a platinum mesh counter, and a nonaqueous Ag/AgNO3 reference electrode were used. The two compartments were separated by a fine glass frit. The general procedure was as follows: 8.0 mg (0.0086 mmol) of Re(PNPH)(bpy)(CO)2OTf was massed out and added to the H-cell along with a stir bar to the working side of the H-cell attached to a Schlenk line, and the apparatus was placed under an inert nitrogen atmosphere. A

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00138. NMR, IR, MS, supporting CV, DPV, CPE, and crystallographic information tables (DOCX) Optimized structures of: Re(PNPH)(bpy)(CO)2+, Re(PNPH)(bpy)(CO)2•, Re(PNPH)(bpy)(CO)2−, Re(PNPtolyl)(bpy)(CO)2+, Re(PNPtolyl)(bpy)(CO)2•, and Re(PNPtolyl)(bpy)(CO)2− (XYZ) Accession Codes

CCDC 1849766−1849771, 1849774−1849777, 1860810, and 1866062 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: dclacy@buffalo.edu. *E-mail: trcook@buffalo.edu. ORCID

Matthew R. Crawley: 0000-0002-2555-9543 Karthika J. Kadassery: 0000-0003-2065-1356 Amanda N. Oldacre: 0000-0001-8873-7186 Alan E. Friedman: 0000-0002-4764-8168 K


Article

Organometallics

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David C. Lacy: 0000-0001-5546-5081 Timothy R. Cook: 0000-0002-7668-8089 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.R.C. thanks SUNY Fredonia for use of their X-ray diffractometer and the ACA Summer Course for Chemical Crystallography for data collection on two of the structures presented in this work as well as the knowledge gained by attending. T.R.C. thanks the University at Buffalo for startup support. The project described was supported by Award S10RR029517 from the National Center for Research Resources for an FTMS instrument.

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Rhenium(I) Phosphazane Complexes for Electrocatalytic CO2

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