Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. In particular, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of wellestablished and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally, we confirm that by varying the base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of the reaction medium, on its own capable of improving the catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation.

The ultrafast photochemistry of the [Cr­(NCS)6]3– complex upon excitation to the 4T2 ligand-field (LF) state was studied in dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) in a wide temporal range (100 fs to 9 ms) by a combination of femtosecond and nanosecond transient absorption spectroscopy techniques and supported by quantum-chemical DFT/TD-DFT calculations. The initially excited 4T2 state undergoes intersystem crossing to the vibrationally hot 2E state with time constants of 1.1 ± 0.2 and 1.8 ± 0.1 ps in DMSO and DMF, respectively. Vibrational relaxation occurs in the same time scale and takes 1–5 ps. A major part of the [Cr­(NCS)6]3– complex in the 2E state undergoes intersystem crossing to the ground state with time constants of 65 ± 5 and 85 ± 5 ns in DMSO and DMF, respectively. A minor part of electronically excited [Cr­(NCS)6]3– undergoes irreversible photochemical decomposition. In DMSO, the photolysis of the [Cr­(NCS)6]3– complex results in single or double isothiocyanate ion release followed by the coordination of the solvent molecules with a time constant of 1 ± 0.2 ms.

Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. Specifically, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of well-established and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally we confirm that by varying base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of reaction medium, on its own capable of improving catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation.

Homogeneous catalytic hydrogenations often operate in dynamic conditions. For example, in hydrogenation of esters reaction medium changes its polarity and becomes protic as reaction proceeds. As a result, the nature of the catalytic and reactive species in catalysis can change throughout the reaction, making it impossible to draw a static picture describing the catalyst performance. Herein we report on the molecular origins of such a complexity in the catalytic hydrogenation of esters. Using a new bis-N-heterocyclic carbene manganese (I) pincer catalyst we perform operando FTIR spectroscopy and kinetic studies to reveal the highly dynamic nature of the Mn intermediates formed in the catalytic mixture. Furthermore, we identify persistent inhibition phenomena caused by the reversible interaction of catalytically competent species with alcohol products. Pronounced strongly in Mn-promoted hydrogenation, this inhibitory pathway can principally affect any transition metal catalyst in ester hydrogenation. Finally, we show that the catalyst inhibition can be suppressed by using basic alkoxide promotors common for the majority of polar hydrogenations. We provide the first experimental and computational evidence that alkoxide promotion can alter the shape of the free energy surface and render catalyst inhibition unfavorable.At large, it implies a marked change to the values of the standard free energies of the key steps in the catalytic cycle that are universally assumed to be constant. While allowing to promote transformations that would otherwise be thermodynamically prohibited, these phenomena make a case for reconsideration of a traditional static viewpoint on homogeneous hydrogenation reactions.

Ultrafast excited-state dynamics of CuBr3 – complex was studied in acetonitrile and dichloromethane solutions using femtosecond transient absorption spectroscopy with 18 fs temporal resolution and quantum-chemical DFT calculations. Upon 640 nm excitation, the CuBr3 – complex is promoted to the ligand-to-metal charge transfer (LMCT) state, which then shortly undergoes internal conversion into the vibrationally hot ligand field (LF) excited state with time constants of 30 and 40 fs in acetonitrile and dichloromethane, respectively. The LF state nonradiatively relaxes into the ground state in 2.6 and 7.3 ps in acetonitrile and dichloromethane, respectively. Internal conversion of the LF state is accompanied by vibrational relaxation that occurs on the same time scale. Based on the analysis of coherent oscillations and quantum-chemical calculations, the predominant forms of the CuBr3 – complex in acetonitrile and dichloromethane solutions were revealed. In acetonitrile, the CuBr3 – complex exists as [CuBr3(CH3CN)2]−, whereas three forms of this complex, [CuBr3CH2Cl2]−, [CuBr3(CH2Cl2)2]−, and [CuBr3(CH2Cl2)3]−, are present in equilibrium in dichloromethane.