Evaluation of a Kinetic Model Based on NMR Spectroscopic Kinetic Measurements

Since the reaction of hydride 12 with CO2 to hydroxycarbonyl 16 represents the first report of a so-called abnormal CO2 insertion into a transition metal hydride bond, investigation of the mechanism of this transformation was performed.^[156] Photochemical isomerization of formate 15 to 16 is ruled out based on photolysis experiments on in situ formed 15. Heating a solution of 16 in THF shows formation of hydroxide 18 in small quantity, rendering thermal decarbonylation feasible. No formation of Ni^II hydride 12 can be observed, indicating that neither formation of 16 from 12 nor the reverse reaction, decarboxylation of 16, proceeds thermally.

Figure 86: ³¹P{¹H} NMR spectra of a THF-d8 solution of 16 (top) before and (bottom) after heating to 70°C for 1 day.

Density functional theory was utilized to probe the thermodynamic data of CO2 insertion into the nickel hydride bond of 12, giving either formate 15 by normal or hydroxycarbonyl 16 by abnormal CO2 insertion (Figure 87).²³ Solvent effects have been accounted for by the conductor-like screening model (COSMO,

 = 7.25 for THF) using the outlying charge corrected values. An initial benchmark study on truncated model compounds having methyl instead of tert-butyl substituents on the phosphorus atoms shows that the TPPS functional gives the least deviation from coupled cluster (DLPNO-CCSD(T)) single point ground state energy calculations.

Formation of formate 15 from hydride 12 and CO2 is computed to be almost thermoneutral with

RG⁰(298 K) = +1.7 kcal∙mol^-1. Since the experiment does show successive conversion of 12 to 15, this value can be seen as the error of the DFT experiment (Chapter 2.3.1). The obtained barrier of

G^‡eff(298 K) = +25.1 kcal∙mol^-1 is in good agreement with the slow reaction observed, since it is at the upper energetic limit of thermodynamically feasible barriers at room temperature according to the Eyring-Polanyi equation. The transition state structure TS15 shown in Figure 87 is typical for an innersphere CO2

23 Computational analysis was performed by Dr. Markus Finger.

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insertion mechanism with an ²-CO2 coordination mode, showing an already present nickel carbon interaction.^[245] The hydride ligand is best described as bridging µ-H, since it is positioned above the nickel carbon bond.

Figure 87: Thermodynamic data on thermal CO2 insertion from hydride 12 to formate 15 via normal or to hydroxycarbonyl 16 via abnormal CO2 insertion with the optimized transition state structure TS15 and TS16

(D3(BJ)-TPSS/def2-TZVP// D3(BJ)-RI-J-PBE/def2-SVP(Cosmo:THF)).

Thermal formation of hydroxycarbonyl 16 proceeds via an even higher transition state TS16 which is best described as a nickel carboxylate since no nickel oxygen interaction is present and the CO2 fragment is clearly bend. While the CO2 moiety resembles the structure of product 16, the former hydride is located apical showing interaction to both oxygen and nickel. The high barrier of G^‡eff(298 K) = +35.6 kcal∙mol^-1 prevents formation of 16 on a thermal pathway at ambient conditions and the overall uphill reaction with

RG⁰(298 K) = +7.3 kcal∙mol^-1 shows that 16 is only accessible viaphotochemical conditions.

To further explore the conversion of hydride 12 under photochemical conditions, the reaction progress was monitored NMR spectroscopically. To provide reproducibility of the obtained results, the experiments for kinetic investigation are performed under specific conditions (e.g. position of the reaction vessel with respect to the light source). This might result in differences in time scale of the performed reaction with respect to previously discussed experiments. The negligible underlying thermal reactivity of 12 with CO2

allows for exact kinetic measurement since the progress of the reaction can essentially be stopped upon interrupting photolysis (Figure 88). The arrayed data show the reaction progress monitored by ¹H{³¹P} NMR spectroscopy over two hours with the first spectrum representing measurement of the reaction mixture after 10 minutes of photolysis (λexc. > 305 nm). Initially, clean conversion of 12 to 16 can be monitored, with increasing concentration of 17 at prolonged reaction times. While the OH resonances of hydroxycarbonyl 16 and hydrocarbonate 17 give rise to a single broad signal, unresolved fine structure is visible, suggesting conditions close to resonance of both signals due to chemical exchange. In agreement, the signal’s chemical shift  varies with the ratio of both compounds.

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Figure 88: ¹H{³¹P} NMR spectra of photolysis (λexc. > 305 nm) of a solution of 12 in THF-d8 at p(CO2) = 1 atm showing the reaction progress between (top) t = 10 min and (bottom) t = 120 min (*denotes THF-d8; ^†denotes

TMS2O).

By integration against the internal standard TMS2O, the concentrations of all involved compounds can be determined resulting in the plot shown in Figure 89a. Importantly, the concentration of 16 undergoes a maximum at t = 80 min, suggesting a follow-up reaction of photoproduct 16 to hydrocarbonate 17 and limiting the yield of 16 to 76% under these conditions. The obtained experimental data can be fitted as reactions which are first-order in starting material 12 and 16, respectively. Assuming constant CO2

concentration (c(CO2) = 0.34 M at 298.15 K and p(CO2) = 1 atm^[314], c0(12) = 9.7 mM) in solution and a constant photon flux, a satisfactory fit of the experimental data can be obtained over three half lifes T1/2 with the first-order rate constants kobs1 = (2.9±0.08)∙10^-2 min^-1 and kobs2 = (4.4±0.1)∙10^-3 min^-1 (Figure 89a).²⁴ The plot of ln(c(12)) over t for 3×T1/2 further shows decent agreement with a first-order reaction in 12 for the conversion of the starting material (Figure 89b).

24 COPASI software is used for the simulation.^[133]

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Figure 89: (a) Plot of c vs. t for the photolysis (λexc. > 305 nm) of 12 in THF-d8 determined by integration of ¹H{³¹P}

NMR spectra and fit of the experimental data by pseudo first-order model reactions. (b) Plot ln(c(12)) vs t and linear fit over 3∙T1/2.

The conversion of hydroxycarbonyl 16 to hydrocarbonate 17 was further explored by photolysis of a solution of isolated 16 under identical conditions as utilized in photolysis of 12 (λexc. > 305 nm, p(CO2) = 1 atm). Hydroxycarbonyl 16 shows strong absorption above  = 305 nm with an absorption maximum at

 = 320 ( = 1.9∙10⁴M^-1cm^-1) (Figure 57c). Clean conversion of 16 to 17 is observed upon irradiation at p(CO2) = 1 atm, however minor reactivity is also monitored without irradiation by a light source (Figure 90a). While thermal decarbonylation of 16 to hydroxide 18 is observed upon prolonged heating to 70°C (Chapter 2.3.1), reversible decarbonylation might also be present at room temperature. This might serve as explanation for the underlying thermal conversion of 16 to 17, considering the large excess of CO2 under the given experimental conditions compared to liberated carbon monoxide by decarbonylation of 16. As discussed in Chapter 2.3.6, formation of carbonyl 20^X and carboxylate 19^M from hydroxycarbonyl 16 by dehydration is feasible. In the presence of CO2, formation of bicarbonate would result, which is shown to substitute the carbonyl ligand in 20^X (Chapter 2.3.6). Photochemical acceleration of the reaction could be rationalized based on the photochemical CO loss of 20^X (Chapter 2.3.5).

Figure 90: ³¹P{¹H} NMR spectra of photolysis (λexc. > 305 nm) of a solution of 16 in THF-d8 at (a) p(CO2) = 1 atm (top: initial reaction mixture; middle: 18 h under CO2 atmosphere; bottom: 18 h photolysis

(λexc. > 305 nm) under CO2 atmosphere) and (b) p(Ar) = 1 atm.

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Hydroxide 18 can be identified as initial product for both the thermal and photochemical decarbonylation of 16 by photolysis under argon atmosphere (Figure 90b). The absence of 18 in the photolysis of 12 under CO2 atmosphere is therefore explained by the rapid CO2 insertion to give hydrocarbonate 17 under the experimental conditions.

Figure 91: (a) Plot of c(12) vs. t for the photolysis (λexc. > 305 nm) of 12 at different applied CO2 pressures. (b) Plot of initial rates d(c(12))/dt vs. p(CO2).

To probe the order of the reaction of 12 to 16 in CO2 pressure and photon flux, initial rate measurement at different CO2 pressures and light source output currents were performed. The photon flux correlates linearly to the output current of the light source, as determined by actinometry (Chapter 2.3.6). Measurements at 1, 5 and 10 atm CO2 pressure were performed using Wilmad medium wall precision pressure/vacuum valve NMR tubes. At the first measurement (t = 10 min) a minor difference between the three applied pressures can be observed (Figure 91). With increasing reaction progress, the experiments give nearly identical results.

Comparison of the initial rates for consumption of starting material 12 over 10 minutes further shows no linear correlation between pCO2) and the initial rate (Figure 91b). Accordingly, the reaction of 12 to 16 is derived to be zero-order in CO2 under the investigated conditions.

Upon changing the output current of the Xe arc light source, an influence on the rate of the reaction is observed. As shown in Figure 92, lowering the output current results in a drop of the observed rate for consumption of the starting material. In contrast to the observation made upon variation of p(CO2), the difference between c(12) at t = 10 min for the individual experiments continues to grow with increasing reaction time. A plot of the initial rates derived at t = 10 min over the output current of the light source indicates a first-order reaction in photons (Figure 92b). While the data obtained with I = 3–6 A show minor variation from linearity, the results at I = 7 A and I = 7.5 A result in a significant error of the linear fit.

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Figure 92: (a) Plot of c(12) vs. t for the photolysis (λexc. > 305 nm) of 12 under CO2 atmosphere (p(CO2) = 1 atm) at different output currents. (b) Plot of initial rates d(c(12))/dt vs. output current I.

The order of the photochemical reaction from 12 to 16 in starting material, CO2 and photons give important information about the reaction’s mechanism and allow for the formulation of a simplified mechanistic picture (Scheme 52). The reaction is first-order in starting material c(12) and photon flux while it is zero-order in c(CO2). Since the thermal reactivity of 12 with CO2 is examined and does not suggest any reactivity aside from insertion to formate 15, it is reasonable to assume the photochemical step to be located prior to CO2 activation on the reaction coordinate. Since photoexcitation proceeds on the femtosecond timescale and is therefore faster than diffusion controlled processes by several orders of magnitude, the photoexcitation certainly is not the rate-determining step (RDS).^[152] However, photoexcitation and relaxation to the ground state can be formulated as an equilibrium in the sense that excitation and relaxation represent conversion pathways between ground state 12 and excited state 12* with, even though unknown, assignable rate constants.

Scheme 52: Mechanistic picture for the conversion of 12 to 16.

The RDS is assigned to the formation of a persistent species PS which is necessary to fulfill the requirement of zero-order in c(CO2). The classification persistent in this context means a lifetime in the nanosecond timescale, allowing bimolecular reactivity which is necessary for CO2 activation.^[315,316] Since PS forms from the excited state 12* which is in equilibrium with starting material 12, the overall reaction is first-order in c(12) and photon flux. Following the RDS, CO2 activation takes place at PS and results in product formation. As pointed out, this mechanistic picture is a simplification. Vibrational cooling (VC) and internal

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conversion (IC) are likely to occur after excitation and before formation of PS, however these processes occur on very fast timescales (ca. 10^–12 s).^[152] Further, they rather represent photophysical than -chemical processes since they do not change connectivity of atoms, even though they are crucial in influencing bond strength and therefore reactivity. Similarly, conversion of PS to 16 might involve several steps and is at this point reduced to a single transformation. Whether PS represents a long-lived excited state on the triplet hypersurface or a photoproduct cannot be differentiated based on the data available at this point, but will be addresses in the following. In photochemical proton reduction by [IrH(bpy)Cp*]PF6, population of the long-lived excited state by intersystem crossing (ISC) actually represent the rate-determining step.^[275]

To determine the quantum yield Φ for photochemical CO2 activation by 12 in THF, a procedure similar to the one discussed previously for H2 activation on 20^BArF was conducted. According to eq. (56)(59), photolysis of a solution of 12 in a cuvette at p(CO2) = 1 atm resulting in a quantum yield of Φ410 = 9.0%

(see Chapter 2.12 for detailed description of the experimental procedure). While this value for  shows that consumption of 12 does not proceed via a radical chain mechanism which would result in  > 1, the overall photochemical process is rather efficient. Again, the determined quantum yield has to be regarded as a lower limit, since the absorbance of the starting material and therefore the number of absorbed photons decreases with reaction progress. While Ishitani reports a quantum yield of Φ480 = 13% for photocatalytic CO2

reduction by [IrH(tpy)(ppy)]⁺, a quantum yield for a stoichiometric experiment is not reported.^[212] Miller’s report on photochemical proton reduction by [IrH(bpy)Cp*]⁺ states a quantum yield close to 1, which however strongly varies with the concentration of the photocatalyst due to the bimolecular mechanism.^[275]

Figure 93: UV-vis spectra of 12 and 14^BArF in THF.

Similarly, the quantum yield Φ410 for the conversion of 14^BArF to 20^BArF in the presence of 1 eq NEt3 at p(CO2) = 1 atm in THF is determined as Φ410 = 3.6%. Therefore, the photochemical conversion of 14^BArF to 20^BArF is approximately half as efficient as the conversion of 12 to 16. Assuming that 14^BArF does not convert directly to the product but via the identical photochemical reaction, this result can be understood by

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comparison of the absorption spectra of compounds 12 and 14^BArF (Figure 93). Both 12 (410(12) = 1.1∙10³M

-1 cm^-1) and 14^BArF (410(14^BArF) = 5.5∙10²M^-1 cm^-1) show absorption at  = 410 nm, however only 12 undergoes productive photochemistry. Accordingly, the absorption of 14^BArF contributed to the number of absorbed photons NAbs but does not result in formation of 20^BArF, resulting in a decrease of the overall quantum yield.

2.4.2 Photochemical Excitation, Transient Spectroscopy and Luminescence Spectroscopy of