Synthesis and Characterization of Heterobimetallic Iridium–Aluminum

Jan 22, 2018 - Department of Chemistry, The University of Memphis, 3744 Walker Avenue, Smith Chemistry Building, Memphis, Tennessee 38152, United Stat...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Synthesis and Characterization of Heterobimetallic Iridium− Aluminum and Rhodium−Aluminum Complexes Timothy P. Brewster,*,‡ Tan H. Nguyen,‡ Zhongjing Li,‡ William T. Eckenhoff,† Nathan D. Schley,⊥ and Nathan J. DeYonker‡ ‡

Department of Chemistry, The University of Memphis, 3744 Walker Avenue, Smith Chemistry Building, Memphis, Tennessee 38152, United States † Department of Chemistry, Rhodes College, 2000 N. Parkway, Memphis, Tennessee 38112, United States ⊥ Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States S Supporting Information *

ABSTRACT: We demonstrate the synthesis and characterization of a new class of late-transition-metal−aluminum heterobimetallic complexes via a novel synthetic pathway. Complexes of this type are exceedingly rare. Joint experimental and theoretical data sheds light on the electronic effect of ligands containing aluminum moieties on late-transition-metal complexes.

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INTRODUCTION Investigation of the exciting chemistry of heterobimetallic systems has dramatically increased over the last several years. Generally, researchers have focused on systems involving two transition metals, hoping to exploit the unique properties of each metal for tandem activation of chemical bonds. This has been successful in a wide range of stoichiometric and catalytic systems.1−7 In contrast, we seek to explore heterobimetallic complexes containing a transition metal and aluminum, a main group metal. Unlike their well-established transition-metal−boron analogues,8−23 aluminum-containing compounds remain relatively unexplored.24−34 Aluminum is particularly interesting, as it is the most electropositive Group 13 element and a strong Lewis acid in organometallic systems. Of the few known heterobimetallic aluminum species, many employ early transition metals.35−39 These complexes have been explored for olefin polymerization, in which the tethered aluminum species serves as a surrogate for methylaluminoxane (MAO). Utilization of an electron-rich metal with Lewis acidic aluminum opens up reactivity not accessible with the known early-metal systems. First, the transition metal and aluminum could function in a manner similar to a frustrated Lewis pair.23,24,28,40 In this type of system, the transition metal would function as a Lewis base and work in tandem with the pendant aluminum center to activate chemical bonds. This has been observed previously in a few aluminum-based and boron-based systems.16,23,24,28 Second, a dative bond between the transition metal and aluminum could form. These “Z-type” interactions, in which a transition metal serves as a formal two-electron σdonor to a Lewis acid ligand,41 have only been observed in a few aluminum compounds.24−26,30−33,42,43 Recently, this architecture was proven to be catalytically useful for hydrosilylation of CO2.24 © XXXX American Chemical Society

Due to the paucity of known late-metal aluminum heterobimetallic species, it is not well established what effect the aluminum-based ligand has on the electronic nature of the transition metal and its subsequent reactivity in either a Z-type complex or a more traditional heterobimetallic complex. Additionally, it is not always obvious which type of compound will be formed from given precursors. A precedent for catalytic relevance, combined with the need for structural information, suggests that development of additional heterobimetallic complexes of late transition metals and aluminum, Z-type or otherwise, would be of great interest to the catalysis community. Before this family of complexes can be reliably and predictably used for catalytic applications, aluminumcontaining ligands must be better understood, and new, reliable synthetic protocols must be developed. Here we report the synthesis of iridium(I) and rhodium(I) complexes featuring aluminum-based ligands. Prior to this work, fewer than ten such compounds were reported in the literature.26,31,32,42,44−46 The newly synthesized complexes have been synthesized via a novel synthetic route, thoroughly characterized, and investigated spectroscopically and computationally to determine the electronic effect of the aluminumbased ligand on the transition-metal center.

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RESULTS AND DISCUSSION Synthesis. Synthesis of the target iridium−aluminum and rhodium−aluminum complexes could be envisioned to proceed through two distinct strategies. First, the aluminum-based ligand could be fully assembled and introduced to an appropriate transition-metal precursor in the final synthetic step. This route has been employed for most known latetransition-metal−aluminum heterobimetallics.25,28,30 AlternaReceived: October 10, 2017

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DOI: 10.1021/acs.inorgchem.7b02601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

reported by Johnson and coworkers.49 They reported the presence of broad 1H NMR signals in the aromatic region, consistent with 1H NMR data obtained in our lab for 3 (see Supporting Information). They were able to obtain X-ray data for the triflate salt and found the solid-state structure to be the N-bound pyridone isomer.49 Similar to our rhodium complex 4, a hydrogen bond could be located between the acidic pyridone proton and the associated counterion. The broad spectrum we obtained for complex 3, in combination with obtained crystallographic data for both N-bound (Johnson lab) and Obound (complex 4) pyridone isomers led us to conclude that 3 exists in solution at room temperature as an equilibrium mixture of isomers, tentatively assigned as 3a and 3b (Scheme 2). Interestingly, two distinct signals are observable in the 31P NMR spectrum of 3 at room temperature, which further suggests that a mixture of these two isomers is likely present. 31 P chemical shifts are consistent with Vaska-type Ir(I) complexes. Cooling of the NMR sample of 3 to −40 °C resolves the broadened 1H NMR signal from the pyridone ligand into two distinct species, consistent with the conclusion of fast exchange. Analysis of NMR line width broadening of two triplets at δ 5.34 ppm (major species) and δ 5.41 ppm (minor species) at the coalescence temperature of 293 K yields an estimated barrier to exchange of 14 kcal/mol (see Supporting Information).41 Infrared data (benzene solution) is also consistent with the presence of multiple isomers in the product mixture 3. A broad signal is present in the carbonyl region with a maximum at 1968 cm−1 and a shoulder at 1985 cm−1, consistent with two isomers present. Overlapping carbonyl signals attributable to the pyridone CO stretch are observed as a broad peak centered at 1828 cm−1. Computationally derived stretching frequencies agree with the experimental trend, with the O-bound major product computed to be ωCO = 2043 cm−1 and the N-bound isomer computed to be ωCO = 2045 cm−1. Further computational investigation of this equilibrium was carried out. Calculations were undertaken using B3LYP hybrid density functional theory (DFT) with a modified LANL2DZ basis set/effective core potential on Rh/Ir,50−52 the standard LANL2DZ basis set/effective core potential on P/Al, and 631G(d′) basis sets on H/C/N/O atoms with the SMD implicit solvation model with standard benzene parameters53 (see Experimental Section for further details). Initially, we investigated the possibility of exchange between O-bound and N-bound 2-pyridone. A hydrogen bond to the nitrate counterion is assumed in the calculations based on the X-ray structure of 4. Surprisingly, the calculated difference in relative free energies of the O-/N-bound isomers are nearly the same for the Ir complex (3) and the Rh complex (4). The O-bound isomer is more stable than the N-bound isomer by 6.9 and 7.7 kcal/mol for 3 and 4, respectively. Despite the semiquantitative nature of DFT and uncertainty in the global minimum conformation of the nitrate anion in the geometry optimizations, we would expect to observe much smaller free energy differences in O-/N-bound isomers of complex 3 if a simple thermodynamic explanation was reasonable. From computation, it is thus unclear the exact nature of the solution equilibrium. It is evident from elemental analysis that the mixture is an isomerization. No hydride signals are visible, even at low temperature, suggesting against the possibility of oxidative addition of the O−H/N−H bond to generate an Ir(III) species. It is possible that solvent effects from toluene or residual dichloromethane are complicating the system in a way

tively, a transition-metal complex bearing a substituent to bind to aluminum could be assembled first and subsequently reacted with an aluminum precursor. Due to the sensitivity of aluminum compounds and the ease of synthesis of aluminum−oxygen bonds,47 the second synthetic route was chosen for this work. A schematic of the general synthetic strategy is shown in Scheme 1. Scheme 1. General Synthetic Strategy of Heterobimetallics

Synthesized transition-metal complexes were derived from Vaska’s complex, Ir(PPh3)2(Cl)(CO) (1) and its rhodium analogue Rh(PPh3)2(Cl)(CO) (2).48 Abstraction of the chloride ligand from 1 and 2 with AgNO3 in the presence of 2-pyridone yielded [Ir(PPh3)2(2-pyridone)(CO)][NO3] (3) and [Rh(PPh3)2(2-pyridone)(CO)][NO3] (4), respectively (Scheme 2). Rhodium complex 4 can be isolated cleanly as a Scheme 2. Synthesis of Transition-Metal Nitrate Precursors

single pyridone isomer. This species has been fully characterized by NMR and IR spectroscopies. X-ray crystallography confirms the geometrical assignment (Figure 1). The X-ray

Figure 1. ORTEP representation of 4. Ellipsoids shown at 50% probability. Hydrogen atoms omitted for clarity.

structure of 4 clearly displays pyridone ligated via oxygen with slight dearomatization of the pyridone ligand [alternating C−C bond lengths of 1.423(3), 1.366(3), 1.405(3), and 1.351(3) Å]. The N−H proton was located in the density map and shown to engage in a hydrogen bonding interaction with the nitrate counterion. Characterization of complex 3 proved to be less straightforward. Analogous complexes [Ir(PPh3)2(2-pyridone)(CO)][BF4] and [Ir(PPh3)2(2-pyridone)(CO)][OTf] were recently B

DOI: 10.1021/acs.inorgchem.7b02601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry not easily simulated by DFT. It is also possible that difficult-tomodel anion effects account for a significant portion of this discrepancy. In spite of the computed free energies, due to the low barrier to exchange, we believe the equilibrium is most likely exchange between N-bound (3a) and O-bound (3b) 2pyridone isomers. Alcohols are well-known to react with alkylalanes to release one equivalent of alkane and form an aluminum alkoxide species.47 We were hopeful that this would also be the case in the presence of our transition-metal complexes. To test this, the isomeric mixture 3 and pure species 4 were reacted with excess AlEt3 and Al(iBu)3 in THF at room temperature (Scheme 3).

Figure 2. Measured (top, black) and modeled (bottom, navy) 1H NMR signal for two diastereotopic methylene protons in 5. Parameters: δ 0.284 ppm, δ 0.323 ppm, 2JHa−Hb = 14 Hz, 3JHA−methyl = 8.1 Hz, 3JHB−methyl = 8.1 Hz, line width: 0.8 Hz.

Scheme 3. Synthesis of Aluminum−Group 9 Bimetallics

variable parameters of chemical shift and coupling constant, to generate predicted coupling patterns consistent with those observed experimentally (Figure 2). A geminal coupling constant of 14 Hz between diastereotopic protons of chemical shifts δ 0.284 and δ 0.323 ppm is found to best describe the experimental data for 5. Similar analysis to determine chemical shifts and coupling constants have been carried out for 6−8, with modeled numbers reported in the synthetic details of the Experimental Section. The NMR spectra of minimized structures of complexes 5−8 have also been modeled using DFT. Computations predict geminal couplings of 15.1−15.7 Hz, consistent with the experimental value. Structural confirmation was achieved upon obtaining X-ray quality crystals of 5 and 6 from analytically pure samples. ORTEP representations of the X-ray crystal structures can be found in Figure 3. The aluminum center is clearly bound to

To our delight, within minutes, iridium complexes Ir(PPh3)2(μN,O-2-pyridone-[Al(Et)2(NO3)])(CO) (5) and Ir(PPh3)2(μN,O-2-pyridone-[Al(iBu)2(NO3)])(CO) (6) and rhodium complexes Rh(PPh3)2(μ-N,O-2-pyridone-[Al(Et)2(NO3)])(CO) (7) and Rh(PPh 3 ) 2 (μ-N,O-2-pyridone-[Al(iBu)2(NO3)])(CO) (8) formed in moderate-to-good yield (Scheme 3). Complexes 5−8 were characterized by NMR and IR spectroscopies. The 1H NMR spectra of complexes 5−8 contain several diagnostic features to support this structural assignment in bulk solution. First, from integration, the alane species clearly has lost one equivalent of its alkane substituent, suggestive of a successful reaction. Second, the methylene protons (Figure 2), clearly visible between δ 0.3 and δ 0.5 ppm, become diastereotopic when the aluminum center is tetrahedral and is bound to two identical alkyl groups, as suggested by the structures shown in Scheme 3. The coupling patterns for the methylene protons can be effectively modeled,54 based on

Figure 3. ORTEP representations of X-ray crystal structures of 5 (left) and 6 (right). Ellipsoids shown at 50% probability. Hydrogen atoms omitted for clarity.

both the pyridone (via oxygen) and to nitrate. Interestingly, aluminum prefers to form a bond to nitrate over forming a Ztype interaction with the iridium center. Pyridone bond lengths in complexes 5 and 6 [1.372(4)−1.409(4) Å] are consistent with an aromatic pyridine ring. A slight elongation of the carbon oxygen bond [1.276(2) in 4 vs 1.313(4) in 5 and 1.309(3) in 6] is also consistent with an aromatized pyridine ring bearing a C−O single bond as the dominant resonance contributor to the overall structure. X-ray quality crystals of rhodium analogues 7 and 8 could not be obtained for analysis. This is likely due to the fact that 7 and 8 slowly decompose in benzene solution, even under inert atmosphere (t1/2 = C

DOI: 10.1021/acs.inorgchem.7b02601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry approximately 5 days). In contrast, the iridium species 5 and 6 can be kept in solution for over a month with no decomposition observable by 1H NMR spectroscopy. To fully understand why bridged heterobimetallic complexes form rather than Z-type complexes with outer-sphere nitrate anions, the minimized structure of cation 9 (anion omitted), featuring a Z-type interaction between Ir and Al (Figure 4), was

Scheme 4. Synthesis of 10 and 11

Figure 4. Calculated Z-type cation 9.

(10d) from the 2-pyridone. DFT data suggests that the major product, bearing a hydride resonance at δ −16.1 ppm, corresponds to 10c, in which the hydride is trans to N. This isomer is calculated to be lower in free energy than 10d, in which the hydride is trans to O, by 1.7 kcal/mol. The relative downfield shift of the hydride signal in the major isomer is also consistent with calculated 1H NMR spectra for these two isomers (δ −5.1 and δ −6.3 ppm for 10c and 10d, respectively). Elemental analysis of the bulk material is consistent with the assignment of the four species as a mixture of constitutional isomers. X-ray crystallographic data was collected for a crystal grown from a saturated solution of 10 in dichloromethane layered with pentane. The obtained structure is consistent with an iridium(I) N-bound isomer 10a (Figure 5), though there is a

computed. A model was then set up to probe the interaction of complex cation 9 with NO3−. In this study, the calculated metal−aluminum bond distance from 9 was used as an initial structural guess, and the system was then allowed to relax in the presence of an added NO3− moiety. If the nitrate counterion is placed in the coordination sphere of the aluminum moiety, a Ztype complex is not obtained upon geometry optimization. Rather, complex 5 is formed. Interestingly, a Z-type complex can theoretically be obtained as a local minimum if the NO3− counterion is included but is placed far from the aluminum center. The Ir−Al bond remains intact (2.680 Å), but the optimized geometry is 37 kcal/mol higher in free energy than that of complex 5. In benzene solution, where ion pairing is likely substantial, it is unlikely that a nitrate moiety would exist far from the aluminum coordination sphere. Therefore, the thermodynamic product is formed exclusively, as observed experimentally. In an attempt to force the formation of a bond between aluminum and iridium, we sought out an alternative anion that would not interact with either metal center. We selected the commonly used noncoordinating anion tetrakis(3,5trifluoromethylphenyl)borate (BArF24)55 due to its relative inertness in the presence of Lewis acids when compared to other noncoordinating anions such as PF6−. Reaction of 1 and 2 with NaBArF24 in the presence of 2-pyridone yielded complexes with elemental compositions consistent with the chemical formulas [Ir(PPh3)2(2-pyridone)(CO)][BArF24] (10) and [Rh(PPh3)2(2-pyridone)(CO)][BArF24] (11) (Scheme 4). As with the nitrate salt 4, 11 was cleanly isolated as the Obound isomer and fully characterized by NMR and IR spectroscopies. Attempts to obtain X-ray quality crystals of 11 were unsuccessful. Interestingly, the solution behavior of complex 10 was dramatically different from that of the analogous nitrate complex 3. The 1H NMR spectrum of 10 displayed sharp signals corresponding to four separate constitutional isomers suggesting that 10 does not rapidly exchange at room temperature. Signals can be located that are tentatively assigned to the N-bound and O-bound isomers 10a and 10b, respectively. Additionally, two hydride resonances are observed at δ −16.04 (58% of material) and δ −20.87 ppm (6% of material) in CD2Cl2 consistent with two iridium(III) species derived from oxidative addition of the N−H or O−H bond. We tentatively assign these two species as isomers, in which the hydride is either trans to nitrogen (10c) or trans to oxygen

Figure 5. ORTEP representation of crystal of 10a obtained on recrystallization of isomeric mixture 10. A single component of the disorder model of the pyridone ligand is shown for clarity. Ellipsoids shown at 50% probability. Hydrogen atoms omitted for clarity.

good amount of crystallographic disorder observed in the coordinated 2-pyridone ligand. The observed disorder is primarily a 180° rotation of the pyridone ligand, suggesting that there is no hydrogen bonding interaction between the O− H proton (not located in density map) and BArF24 counterion. Long carbon−oxygen bonds [1.345(9) or 1.337(12) for the respective components of the modeled disorder] are consistent with enhanced single bond character and a fully aromatized pyridine. This isomeric form is directly analogous to that reported by Johnson and coworkers for the related triflate salt.49 The hydrogen bond accepting ability of the counterion D

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Inorganic Chemistry may explain the observation of hydride signals in the 1H NMR spectrum of mixture 10 that are absent in the low-temperature spectrum of 3 and not reported by Johnson. Given the success of reacting both isomeric mixture 3 and pure species 4 with aluminum precursors to generate a single product, we proceeded to react isomeric iridium mixture 10 and pure rhodium species 11 in the same manner (Scheme 5).

Electronic Effect of Aluminum Ligands. To facilitate usage of our aluminum-containing ligands in rational design of future catalysts, we then sought to understand the overall electronic effect the ligands have on the transition-metal center. Addition of aluminum to the isomeric mixture 3 (1985, 1968 cm−1) causes a shift in the measured metal−carbonyl stretching frequency to 1961 cm−1 for 5 and 1967 cm−1 for 6. Similarly, the measured stretching frequency for rhodium species 4 (1962 cm−1) was observed to red shift when the aluminum moiety is added. The maximum absorbance from 7 is observed at 1955 cm−1 and from 8 at 1959 cm−1. In agreement with experimental results, a red shift occurs in computed ωCO values when Al is present (relative to the 2-pyridone starting materials 3a, 3b, and 4). For example, on moving from 3b to 5, the calculated stretching frequency red shifts from 2043 to 2038 cm−1. Additional electronic information was derived from cyclic voltammetry (CV). CV experiments were conducted in acetonitrile solution using tetrabutylammonium tetraphenylborate (NBu4BPh4) as the supporting electrolyte and the ferrocenium/ferrocene couple as an internal standard. Though a coordinating solvent, acetonitrile was found not to react (by NMR spectroscopy) with any of our complexes to generate solvent-coordinated species that would complicate electrochemical analysis. The results of our electrochemical experiments can be found in Figures 7 and 8 for the iridium and rhodium complexes, respectively.

Scheme 5. Attempted Synthesis of Z-Type Complexes

Unfortunately, in all cases, at room temperature, the reaction mixture rapidly turned from yellow to dark red, and only decomposition products were detectable by NMR spectroscopy. Similarly, low-temperature syntheses at 0, −40, and −78 °C yielded intractable complex reaction mixtures with no unequivocally identifiable or separable products. We were, however, able to obtain an X-ray crystal structure of a decomposition product from the reaction of 11 with triethyl aluminum. This structure is shown in Figure 6 and is

Figure 6. ORTEP representation of X-ray structure obtained on crystallization of decomposition product of reaction of 11 with AlEt3. Ellipsoids are shown at 50% probability. Hydrogen atoms and BArF24− have been omitted for clarity. Arene rings on triphenylphosphine ligands have been truncated to show only carbon bound to phosphorus for clarity.

Figure 7. Cyclic voltammetry of 3, 5, and 6 vs Fc+/Fc internal standard. Voltammogram obtained from 5 μmol of complex dissolved in 5.0 mL of acetonitrile. 0.445 mmol of (NBu4)(BPh4) used as supporting electrolyte. Scan rate: 0.5 V/s.

highly instructive as to why the desired Z-type complexes are kinetically unstable, even in the presence of noncoordinating anions. The structure displays a pseudo-C 2 -symmetric rhodium−aluminum−aluminum−rhodium dimer, in which each transition-metal center is bridged to aluminum by a single 2-pyridone ligand, and the aluminum atoms are bridged by two 2-pyridone ligands. To form this structure, 2-pyridone must dissociate from a neighboring rhodium center, generating an equivalent of rhodium decomposition product. Stabilizing a high-energy Z-type complex therefore requires a stronger bond between the transition metal and the bridging ligand. Ongoing efforts in our laboratory are pursuing alternative ligand structures that will ensure that this criterion is met.

For all iridium and rhodium complexes, an irreversible reduction wave can be seen between −1.8 and −2.1 V vs Fc+/ Fc (Figures 7 and 8). Though irreversible, measurement of Epc, defined as the peak maximum for the cathodic wave, is instructive in determining a relative trend. We believe this signal corresponds to a transition-metal/2-pyridone-based reduction. Control experiments with NBu4NO3 as supporting electrolyte exclude reduction of nitrate being responsible for the cathodic wave. Moving from isomeric mixture 3 to aluminum ethyl species 5 to aluminum isobutyl species 6 corresponds to a movement in Epc from −1.98 to −2.01 to −2.06 V vs Fc+/Fc. Similarly, moving from rhodium complex 4 to aluminum ethyl species 7 to aluminum isobutyl species 8 corresponds to a shift in Epc from −1.89 to −1.92 to −1.97 V. In both cases, the presence of E

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stored over molecular sieves in the glovebox prior to usage. Toluene was dried over calcium hydride and stored over molecular sieves in the glovebox. Deuterated solvents were purchased from Cambridge Isotope Laboratories, dried over molecular sieves, and stored in the glovebox prior to use. Ir(PPh3)2Cl(CO) and Rh(PPh3)2Cl(CO) were synthesized according to methods established in the literature.48 NaBArF24 was synthesized by the method of Bergman55 and purified using the modification reported by Peters.56 All other reagents and solvents used were commercially available and used without further purification unless specified. Aluminum reagents and 2-pyridone were stored in a nitrogen glovebox prior to use. 1H and 13C NMR spectra were recorded on a 400 MHz JEOL spectrometer and referenced to the residual solvent peak.57 Additional NMR data were obtained on a 400 MHz JEOL spectrometer and referenced to an appropriate external standard (31P: H3PO4 in D2O (0.00 ppm); 19F: NaBArF24 in acetone-d6 (−62.12 ppm)). Infrared spectra were recorded on a PerkinElmer Frontier FT-IR spectrometer. Spectra were measured in air-free solution cells (benzene) prepared in the glovebox. Elemental analysis was performed by Atlantic Microlabs, Inc. and is reported as the average of duplicate runs. Synthetic Methods. [Ir(PPh3)2(CO)(2-pyridone)][NO3] (3). In the glovebox, 1.91 g (2.45 mmol) of Ir(PPh3)2Cl(CO) was weighed to a 100 mL Schlenk flask. Separately, 450 mg (2.67 mmol) of AgNO3 was weighed to a vial. The solids were then sealed and removed from the glovebox. The Ir complex was then suspended in 75 mL of 2:1 (v/v) dichloromethane/benzene. AgNO3 was suspended in 5 mL of 1:1 acetone/dichloromethane and added to the Schlenk flask via pipet against a flow of nitrogen. The suspension was stirred overnight at room temperature in the absence of light. After 18 h, the suspension was returned to the glovebox and filtered through Celite. To the crude product was added 256 mg (2.67 mmol) of 2-pyridone. The solution was stirred in the glovebox at room temperature. After 2 h, the stirring was stopped, and the solvent volume reduced until a precipitate was observed (approximately 10 mL of solvent). A volume of 50 mL of pentane was added to precipitate the remaining product. The solid was collected by filtration and washed three times with 3 mL of benzene. The product was then recrystallized by layering a saturated solution of crude material in dichloromethane with pentane to yield pure material. The compound likely exists in solution as an exchanging mixture of Nbound and O-bound isomers at room temperature. Yield: 1.21 g (60%). 1H NMR (400 MHz, CD2Cl2) δ 7.45 (m, 13H), 7.31−7.12 (m, 20H), 6.93−6.65 (br m, 1H), 6.13 (br s, 1H), 5.85 (br s, 1H). 13C NMR (101 MHz, CDCl3) δ 175.14, 164.09, 145.91, 139.08, 134.34 (t, J = 5.5 Hz), 130.98, 130.54 (t, J = 27.2 Hz), 128.68 (t, J = 5.1 Hz), 115.33, 114.32. 31P NMR (162 MHz, CD2Cl2) δ 29.82, 28.55. IR (benzene solution): νCO‑carbonyl 1968 cm−1, shoulder visible at 1985 cm−1, νCO‑pyridone 1828 cm−1. Elemental Analysis: Calculated C 55.93, H 3.91, N 3.11; Measured C 55.98, H 4.01, N 3.20. Rh(PPh3)2(CO)(2-pyridone)][NO3]·C6H6 (4). In the glovebox, 1.73 g (2.50 mmol) of Rh(PPh3)2Cl(CO) was weighed to a 100 mL Schlenk flask. Separately, 450 mg (2.67 mmol) of AgNO3 was weighed to a vial. The solids were then sealed and removed from the glovebox. The Rh complex was then suspended in 75 mL of 2:1 (v/v) benzene/ dichloromethane. AgNO3 was suspended in 5 mL of 1:1 acetone/ dichloromethane and added to the Schlenk flask via pipet against a flow of nitrogen. The suspension was stirred overnight at room temperature in the absence of light. After 18 h, the suspension was returned to the glovebox and filtered through Celite. To the crude product was added 260 mg (2.70 mmol) of 2-pyridone. The solution was stirred in the glovebox at room temperature. After 2 h, the stirring was stopped, and the solvent volume reduced until a precipitate was observed (approximately 10 mL of solvent). A volume of 50 mL of pentane was added to precipitate the remaining product. The solid was collected by filtration, washed three times with 3 mL of benzene, and dried in vacuo. X-ray quality crystals were obtained by layering a dichloromethane solution of the compound with pentane. Yield: 1.51 g (67%). 1H NMR (400 MHz, CD2Cl2) δ 12.10 (s, 1H, N−H), 7.64 (dddd, J = 8.3, 7.1, 5.6, 1.5 Hz, 13H), 7.52−7.38 (m, 20H), 7.27 (s, 1H), 6.41 (d, J = 9.0 Hz, 1H), 6.18 (t, J = 6.5 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 190.01 (d, J = 71.3 Hz), 164.58, 134.24 (t, J = 6.7

Figure 8. Cyclic voltammetry of 4, 7, and 8 vs Fc+/Fc internal standard. Voltammogram obtained from 5 μmol of complex dissolved in 5.0 mL of acetonitrile. 0.445 mmol of (NBu4)(BPh4) used as supporting electrolyte. Scan rate: 0.5 V/s.

the Al(R2)(NO3) unit caused the reduction potential to shift in the negative direction. Though the IR and CV shifts represent very small changes, they are universally consistent with the aluminum ligand functioning as a slightly stronger donor than 2-pyridone itself. This is counterintuitive. One would expect aluminum to function as an electron-withdrawing group, which would in turn decrease electron donation from pyridone into the transition metal. However, the presence of the bound nitrate anion likely negates this effect, transforming the L-type ligand 2-pyridone into the X-type [(2-pyridone)(AlR2)(NO3)]−. This overall anionic character likely leads to the net electron-donating ability of the aluminum-based ligand system in spite of the acidity of aluminum.

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CONCLUSION We have demonstrated the synthesis and characterization of a new class of late-transition-metal−aluminum heterobimetallic complexes. The modular synthetic route, in which the aluminum moiety is functionalized in the final synthetic step, represents a new route to access these unique heterobimetallics. Bridged complexes containing an aluminum-bound nitrate anion have been isolated and fully characterized spectroscopically and electrochemically to determine the electronic effect that the aluminum-containing ligands impart on the transition metal. Computational studies support the experimental characterization of the nitrate species. Analogous Z-type complexes were computationally found to be high in energy and were not isolable, even in the presence of noncoordinating counteranions. The understanding of electronic effects of metal-containing ligands is essential for rational design of future catalysts based on a heterobimetallic platform. To further this goal, development of late-transition-metal−aluminum systems, which can be isolated with both coordinating and noncoordinating anions is currently being undertaken in our laboratory.

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EXPERIMENTAL SECTION

Procedures were performed in a nitrogen-filled Inert Technologies glovebox or using standard Schlenk techniques unless otherwise specified. Extra dry benzene, pentane, tetrahydrofuran, diethyl ether, and dichloromethane were purchased from commercial suppliers and F

DOI: 10.1021/acs.inorgchem.7b02601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Hz), 133.97, 131.26 (t, J = 22.9 Hz), 130.77, 130.51, 129.13, 128.72 (t, J = 5.1 Hz), 128.43. 31P NMR (CD2Cl2) δ 33.19 (d, 1JP−Rh = 132.8 Hz). IR (benzene solution): νCO‑carbonyl 1962 cm−1, νCO‑pyridone 1824 cm−1. Elemental Analysis: Calculated C 64.73, H 4.64, N 3.15; Measured C 65.61, H 4.67, N 3.11. [Ir(PPh3)2(CO)(2-pyridone-AlEt2)][NO3] (5). In the glovebox, 200 mg (0.220 mmol) of [Ir(PPh3)2(CO)(2-pyridone)][NO3] was suspended in 5 mL of THF. A volume of 60 μL (49 mg, 0.436 mmol) of AlEt3 was added via microliter syringe. The yellow solution was stirred for 10 min at room temperature, turning to a golden yellow. The solvent was removed in vacuo, and the residual oily solid was washed with excess pentane until the washings remained colorless. X-ray quality crystals were obtained on recrystallization of a saturated benzene solution layered with pentane. Yield: 161 mg (74%). 1H NMR (400 MHz, C6D6) δ 7.73 (d, J = 7.6 Hz, 12H), 7.10 (t, J = 7.6 Hz, 12H), 6.99 (t, J = 7.5 Hz, 6H), 6.45 (d, J = 5.7 Hz, 1H), 6.27 (d, J = 7.4 Hz, 1H), 6.21 (d, J = 8.6 Hz, 1H), 5.25 (t, J = 6.4 Hz, 1H), 1.50 (t, J = 8.1 Hz, 6H), 0.32 (dq, J = 14.0 Hz, 8.1 Hz, 2H), 0.28 (dq, J = 14.0 Hz, 8.1 Hz, 2H). 13C NMR (101 MHz, C6D6) δ 165.58, 146.24, 137.63, 134.40 (t, J = 6.4 Hz), 131.05 (t, J = 27.0 Hz), 130.64, 130.52, 128.49 (t, J = 5.2 Hz), 116.88, 112.74, 10.02, 0.13. 31P NMR (162 MHz, C6D6) δ 28.68. IR (benzene solution): νCO‑carbonyl 1961 cm−1, νCO‑pyridone 1815 cm−1. Elemental Analysis: Calculated C 56.03, H 4.50, N 2.84; Measured C 54.31, H 4.59, N 2.73. This analysis is consistent with a formulation of [Ir(PPh3)2(CO)(2-pyridone-Al(Et)(OH))][NO3] (Calculated C 54.26, H 4.14, N 2.88) suggesting partial hydrolysis of the aluminum alkyl during characterization. Notably, NMR data is inconsistent with this assignment, as two equivalent aluminum alkyl groups are observed in the 1H spectrum. Hydrolysis of alkyls is expected to be rapid and facile, and thus, this elemental analysis discrepancy is not unexpected. [Ir(PPh3)2(CO)(2-pyridone-Al(iBu)2)][NO3] (6). In the glovebox, 160 mg (0.176 mmol) of [Ir(PPh3)2(CO)(2-pyridone)][NO3] was dissolved in 24 mL of THF. A volume of 100 μL (79 mg, 0.396 mmol) of Al(iBu)3 was added via microliter syringe. The yellow solution was stirred for 20 min at room temperature, turning to a golden yellow. The solvent was removed in vacuo, and the residual oily solid was washed with 25 mL of pentane in three portions (the final washing should be nearly colorless) yielding a yellow powder. X-ray quality crystals were obtained on recrystallization of a saturated toluene solution layered with pentane. Yield: 72 mg (39%). 1H NMR (400 MHz, C6D6) δ 7.70−7.60 (m, 12H), 7.08−7.02 (m, 12H), 6.94 (t, J = 7.5 Hz, 6H), 6.42−6.34 (m, 1H), 6.31−6.19 (m, 2H), 5.15 (td, J = 6.2, 1.7 Hz, 1H), 2.15 (hept, J = 6.6 Hz, 2H), 1.20 (dd, J = 6.5, 2.7 Hz, 12H), 0.26 (dd, J = 13.8, 6.7 Hz, 2H), 0.17 (dd, J = 13.8, 7.4 Hz, 2H). 13C NMR (101 MHz, C6D6) δ 176.04, 165.97, 146.64, 137.93, 134.72 (t, J = 6.4 Hz), 131.43 (t, J = 27.0 Hz), 130.91 (d, J = 14.3 Hz), 128.84 (t, J = 5.2 Hz), 117.35, 113.01, 29.15, 28.84, 26.72, 24.85. 31P NMR (162 MHz, C6D6) δ 28.71. IR (benzene solution): νCO‑carbonyl 1967 cm−1, νCO‑pyridone 1823 cm−1. Elemental Analysis: Calculated C 57.63, H 5.03, N 2.69; Measured C 55.45, H 4.79, N 2.73. This analysis is consistent with a formulation of [Ir(PPh3)2(CO)(2pyridone-Al(iBu)(OH))][NO3] (Calculated C 55.14, H 4.43, N 2.80) suggesting partial hydrolysis of the aluminum alkyl during characterization. Notably, NMR data is inconsistent with this assignment, as two equivalent aluminum alkyl groups are observed in the 1H spectrum. Hydrolysis of alkyls is expected to be rapid and facile, and thus, this elemental analysis discrepancy is not unexpected. [Rh(PPh3)2(CO)(2-pyridone-AlEt2)][NO3] (7). In the glovebox, 200 mg (0.277 mmol) of [Rh(PPh3)2(CO)(2-pyridone)][NO3] was dissolved in 15 mL of THF. A volume of 70 μL (59 mg, 0.510 mmol) of Al(Et)3 was added via microliter syringe. The yellow solution was stirred for 10 min at room temperature, turning to a dark golden yellow. The solvent was removed in vacuo, and the residual oily solid was washed with pentane in 10 mL portions until the washings were colorless, yielding a yellow powder. X-ray quality crystals were obtained on recrystallization of a saturated toluene solution layered with pentane. Yield: 153 mg (62%). 1H NMR (400 MHz, C6D6) δ 7.70−7.57 (m, 12H), 7.03 (m, 12H), 6.94 (m, J = 6.8 Hz, 6H), 6.49− 6.41 (m, 1H), 6.29 (ddd, J = 8.8, 6.9, 2.0 Hz, 1H), 6.14 (d, J = 8.6 Hz,

1H), 5.24 (ddd, J = 7.0, 5.7, 1.2 Hz, 1H), 1.46 (t, J = 8.1 Hz, 6H), 0.28 (dq, J = 14.0 Hz, 8.1 Hz, 2H) 0.24 (dq, J = 14.0 Hz, 8.1 Hz, 2H). 13C NMR (101 MHz, C6D6) δ 165.46, 146.58, 137.67, 134.21 (t, J = 6.7 Hz), 131.69 (t, J = 22.7 Hz), 130.48, 130.38, 128.56 (t, J = 5.1 Hz), 115.76, 112.84, 10.09, 0.18. 31P NMR (162 MHz, C6D6) δ 34.41 (d, JP−Rh = 132.6 Hz). IR (benzene solution): νCO‑carbonyl 1955 cm−1, νCO‑pyridone 1808 cm−1. Elemental Analysis: Calculated C 61.62, H 4.95, N 3.12; Measured C 60.10, H 5.00, N 2.96. This analysis is consistent with a formulation of [Rh(PPh3)2(CO)(2-pyridone-Al(Et)(OH))][NO3] (Calculated C 59.74, H 4.56, N 3.17) suggesting partial hydrolysis of the aluminum alkyl during characterization. Notably, NMR data is inconsistent with this assignment, as two equivalent aluminum alkyl groups are observed in the 1H spectrum. Hydrolysis of alkyls is expected to be rapid and facile, and thus, this elemental analysis discrepancy is not unexpected. [Rh(PPh3)2(CO)(2-pyridone-Al(iBu)2)][NO3] (8). In the glovebox, 200 mg (0.277 mmol) of [Rh(PPh3)2(CO)(2-pyridone)][NO3] was dissolved in 15 mL of THF. A volume of 140 μL (110 mg, 0.554 mmol) Al(iBu)3 was added via microliter syringe. The yellow solution was stirred for 20 min at room temperature, turning to a dark golden yellow. The solvent was removed in vacuo, and the residual oily solid was washed with pentane in 10 mL portions until the washings were colorless, yielding a yellow powder. X-ray quality crystals were obtained on recrystallization of a saturated toluene solution layered with pentane. Yield: 122 mg (47%). 1H NMR (400 MHz, C6D6) δ 7.66−7.47 (m, 12H), 7.08−7.02 (m, 12H), 6.95 (t, J = 7.4 Hz, 6H), 6.50−6.42 (m, 1H), 6.38−6.23 (m, 2H), 5.25−5.17 (m, 1H), 2.18 (hept, J = 6.6 Hz, 2H), 1.21 (dd, J = 6.5, 2.2 Hz, 13H), 0.29 (dd, J = 13.8, 6.7 Hz, 2H), 0.21 (dd, J = 13.8, 7.4 Hz, 2H). 13C NMR (101 MHz, C6D6) δ 165.57, 146.71, 137.66, 134.22 (t, J = 6.7 Hz), 131.74 (t, J = 22.8 Hz), 130.50, 130.38, 128.58 (t, J = 5.1 Hz), 115.94, 112.79, 28.88, 28.53, 26.44, 24.53. 31P NMR (162 MHz, C6D6) δ 34.26 (d, JP−Rh = 133.0 Hz). IR (benzene solution): νCO‑carbonyl 1959 cm−1, νCO‑pyridone 1816 cm−1. Elemental Analysis: Calculated C 63.03, H 5.50, N 2.94; Measured C 62.18, H 5.52, N 2.78. [Ir(PPh3)2(CO)(2-pyridone)][BArF24] (10). In the glovebox, 390 mg (0.500 mmol) of Ir(PPh3)2Cl(CO) and 48 mg of (0.505 mmol) 2pyridone were weighed to a 200 mL Schlenk flask. The solids were then suspended in 75 mL of dichloromethane. A total of 443 mg (0.500 mmol) NaBArF24 was then added. The suspension was stirred overnight at room temperature. The resulting suspension was filtered through Celite, and the solvent was removed from the filtrate in vacuo to yield the desired product. The product is obtained as a mixture of four inseparable Ir(I) and Ir(III) isomers. Crystals suitable for X-ray crystallography were obtained by layering a solution of the isomeric mixture in dichloromethane with pentane. Combined yield: 790 mg (89%). NMR spectra of the mixture are available in the Supporting Information. Elemental Analysis: Calculated C 52.19, H 2.78, N 0.82; Measured C, 51.73 H 3.03, N 0.85. [Rh(PPh3)2(CO)(2-pyridone)][BArF24] (11). In the glovebox, 345 mg (0.500 mmol) of Rh(PPh3)2Cl(CO) and 48 mg of (0.505 mmol) 2pyridone were weighed to a 200 mL Schlenk flask. The solids were then suspended in 50 mL of dichloromethane. A total of 443 mg (0.500 mmol) NaBArF24 was then added. The suspension was stirred overnight at room temperature. The resulting suspension was filtered through Celite, and the solvent was removed from the filtrate in vacuo to yield the desired product. Yield: 717 mg (89%). 1H NMR (400 MHz, CD2Cl2) δ 8.58 (br, 1H), 7.69 (br, 8H), 7.53 (br m, 16H), 7.44 (t, J = 7.3 Hz, 6H), 7.41−7.32 (m, 12H), 7.15 (m, 1H), 6.83 (br s, 1H), 6.19 (t, J = 7.0 Hz, 1H), 6.15 (d, J = 9.0 Hz, 1H). 13C NMR (101 MHz, CD2Cl2) δ 161.78 (q, J = 50.1 Hz), 143.05, 134.97−134.62 (m), 134.12 (br, s), 131.97, 131.36, 130.23 (t, J = 24.1 Hz), 129.38 (qq, J = 103.1 Hz, 3.3 Hz), 128.98 (t, J = 5.2 Hz), 124.62 (q, J = 272.5 Hz), 119.95, 109.93, 117.51 (dt, J = 7.9, 3.9 Hz). Carbonyl not located in spectrum. 31P NMR (162 MHz, CDCl3) δ 31.07 (d, JP−Rh = 124.9 Hz). 19 F NMR (376 MHz, CDCl3) δ −59.77. Elemental Analysis: Calculated C 55.08, H 2.94, N 0.87; Measured C 55.31, H 3.10, N 0.97. Attempted Syntheses of Heterobimetallics from 10 and 11. Several routes were attempted to synthesize Z-type Ir−Al and Rh−Al G

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of Rh and Ir have been replaced by a split of the optimized 5p/6p function, respectively. Computations with phosphorus and aluminum used the standard LANL2DZ basis set/ECP combination. All carbon, nitrogen, oxygen, and hydrogen atoms utilized the 6-31G(d′) basis set.63 Spherical harmonic d functions were used throughout, i.e., there are five angular basis functions per d function. The Hessian of the energy was computed at all stationary points to designate them as either minima or transition states (first-order saddle points). Zeropoint energies (ZPE) and thermal enthalpy/free energy corrections were computed at 1 atm and 298.15 K. Aqueous solvation energies were computed using the SMD model with standard cavity parameters for benzene.53 Isotropic absolute shielding values were computed using the same level of theory specified above, which provided 1H and 13 C NMR chemical shifts (in ppm) relative to the computed values of trimethylsilane. Spin−spin coupling constants were also computed using the same level of theory specified above. The simulated NMR spectra were produced with an in-house program using a Gaussian broadening of 0.025 ppm.

complexes from 10 and 11. In no case was clean product obtained. Representative syntheses are shown. 1. In the glovebox, 160 mg of 11 (0.01 mmol) was dissolved in 10 mL of benzene at room temperature. A volume of 27 μL (2 equiv) AlEt3 was added. The solution turned dark red immediately. Solvent was removed in vacuo after 30 s of stirring. The products were then washed with pentane until washings were colorless, and then, the products were dried and analyzed by NMR spectroscopy. 2. In the glovebox, 160 mg of 11 (0.01 mmol) was suspended in 10 mL of hexane at room temperature. A volume of 27 μL (2 equiv) AlEt3 was added. The solution turned dark red immediately. Solvent was removed in vacuo after 30 s of stirring. The products were then washed with pentane until washings were colorless, and then, the products were dried and analyzed by NMR spectroscopy. 3. In the glovebox, 160 mg of 11 (0.01 mmol) was dissolved in 10 mL of toluene at room temperature and cooled in a liquidnitrogen chilled cold well. A volume of 27 μL μL (2 equiv) AlEt3 was added. The solution turned dark red immediately. Solvent was removed in vacuo after 30 s of stirring. The products were then washed with pentane until washings were colorless, and then, the products were dried and analyzed by NMR spectroscopy. 4. In the glovebox, 160 mg of 11 (0.01 mmol) was dissolved in 2 mL of toluene at room temperature and removed from the glovebox. The solution was chilled in a controlled temperature bath (−40 °C). A precooled solution of 27 μL (2 equiv) of AlEt3 in 0.5 mL of toluene was added. Solvent was removed in vacuo after 30 s of stirring. The products were then washed with pentane until washings were colorless, and then, the products were dried and analyzed by NMR spectroscopy. Reactions at −40 °C appeared to give the best results. 5. In the glovebox, 160 mg of 11 (0.01 mmol) was dissolved in 2 mL of toluene at room temperature and removed from the glovebox. The solution was chilled in a controlled temperature bath (−40 °C). A precooled solution of 27 μL (2 equiv) of AlEt3 in 0.5 mL of toluene was added. Cold, dry pentane (approximately 50 mL) was added to precipitate the desired material. Solvent was then removed from the mixture at the reaction temperature via syringe. The products were then washed with prechilled pentane until washings were colorless, and then, the products were dried and analyzed by NMR spectroscopy. Electrochemical Methods. All electrochemical measurements were performed in a glovebox with a dry nitrogen atmosphere using a CH Instruments 600E potentiostat. In a typical experiment, 250 mg of tetrabutylammonium tetraphenylborate (TBA-BPh4) (0.445 mmol) was dissolved in 5.0 mL of dry acetonitrile along with ∼2 mg of ferrocene and 5 μmol of the substrate under investigation. Experiments were performed with a glassy carbon electrode polished with 0.05 μm alumina, a platinum wire, and a standard calomel electrode. Final data was then adjusted with the Fc/Fc+ couple at 0.0 V. X-ray Crystallography. General Methods: A suitable crystal was selected for analysis and mounted in a polyimide loop. All measurements were made on a Rigaku Oxford Diffraction Supernova EosS2 CCD with filtered Mo Kα radiation at a temperature of 100 K. With use of Olex2,58 the structure was solved with the ShelXT structure solution program using Direct Methods and refined with the ShelXL refinement package using Least Squares minimization.59 For further details, see Supporting Information. Computational Details. All computations were performed using the Gaussian09 software package.60 Gas-phase energies, optimized geometries, and unscaled harmonic vibrational frequencies were obtained using density functional theory (DFT).61 The pure hybrid B3LYP functional was used with default grid parameters.62 The basis set for rhodium and iridium was the Hay and Wadt basis set and effective core potential (ECP) combination (LANL2DZ)50,51 as modified by Couty and Hall,52 where the two outermost p functions

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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.inorgchem.7b02601. Spectra of newly synthesized compounds, X-ray data, and computational coordinates (PDF) Accession Codes

CCDC 1578955−1578959 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, or by emailing [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 Author

*E-mail: [email protected]. ORCID

Timothy P. Brewster: 0000-0002-7654-4106 William T. Eckenhoff: 0000-0003-2511-578X Nathan D. Schley: 0000-0002-1539-6031 Nathan J. DeYonker: 0000-0003-0435-2006 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. CHE-1531466. The authors gratefully acknowledge the University of Memphis for startup funding. Computational work was performed using resources at the University of Memphis High-Performance Computing Facility and the Computational Research on Materials Institute at the University of Memphis (CROMIUM). T.P.B., T.H.N., and Z.L. thank Prof. Kensha Clark for helpful discussions in generating the final version of this manuscript.

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REFERENCES

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