Herein, we describe the catalytic hydrogenation of CO2 to formate with (PNP)­Mn–H (PNP = 2,6-bis­(di-tert-butylphosphinomethyl)­pyridine; Mn = Mn­(CO)2). Contrary to the established mechanism for CO2 hydrogenation, mechanistic studies indicate that CO2 does not insert into the Mn–H bond of (PNP)­Mn–H to give the formate species, (PNP)­Mn–OCHO. The lack of reactivity is confirmed by thermochemical studies that show that (PNP)­Mn–H is not sufficiently hydridic to reduce CO2. Deprotonation of the hydride to give [(*PNP)­Mn–H] – (* indicates the deprotonated ligand) enhances the hydricity by ∼17 kcal·mol–1 and hence should be sufficiently hydridic to hydrogenate CO2. This reactivity is not observed, and CO2 instead binds to the backbone to generate another anionic hydride species [(CO 2 -PNP)­Mn–H]. The formate is lost only from this species, through hydride transfer to an external CO2. These findings are unexpected because substrate binding to the backbone of catalysts that can undergo metal–ligand cooperativity (MLC) is thought to be detrimental to catalysis; this work suggests that alternative mechanisms should be considered. The enhanced hydricity observed upon deprotonation may be broadly applicable to systems capable of undergoing MLC. Moreover, this work shows an example of how thermochemical analysis can be used to advance mechanistic understanding in (de)­hydrogenation catalysis.

The partial chemical activation of water by reactive radicals was examined computationally for small clusters of chlorine and water, Cl • (H 2 O) n=1−4 . Using an automated isomer-search procedure, dozens of unique, stable structures were computed. Among the resulting structural classes were intact, hydrated-chlorine isomers, as well as hydrogen-abstracted (HCl)(OH)(H 2 O) n−1 configurations. The latter showed increased stability as the degree of hydration increased, until n = 4, where a new class of structures was discovered with a chloride ion bound to an oxidized water network. The electronic structure of these three structural classes was investigated, and spectral signatures of this hydration-based evolution were connected to these electronic properties. An ancillary outcome of this detailed computational analysis, including coupled-cluster benchmarks, was the calibration of cost-effective quantum chemistry methods for future studies of these radical−water complexes.

The copper hydroxide ion, CuOH+, serves as the catalytic core in several recently developed water-splitting catalysts, and an understanding of its chemistry is critical to determining viable catalytic mechanisms. In spite of its importance, the electronic structure of this open-shell ion has remained ambiguous in the literature. In particular, computed values for both the thermodynamics of hydration and the vibrational signatures of the mono- and dihydrates have shown prohibitively large errors compared to values from recent experimental measurements. In this work, the source of this discrepancy is demonstrated to be the propensity of this ion to exist between traditional Cu­(I) and Cu­(II) oxidation-state limits. The spin density of the radical is accordingly shown to delocalize between the metal center and surrounding ligands, and increasing the hydration serves to exacerbate this behavior. Equation-of-motion coupled-cluster methods demonstrated the requisite accuracy to resolve the thermodynamic discrepancies. Such methods were also needed for spectral simulations, although the latter also required a direct simulation of the role of the deuterium “tag” molecules that are used in modern predissociation spectroscopy experiments. This nominally benign tag molecule underwent direct complexation with the open-valence metal ion, thereby forming a species akin to known metal–H2 complexes and strongly impacting the resulting spectrum. Thermal populations of this configuration and other more traditional noncovalently bound isomers led to a considerable broadening of the spectral lineshapes. Therefore, at least for the CuOH+(H2O)0–2 hydrates, these benchmark ions should be considered to be delocalized radical systems with some degree of multireference character at equilibrium. They also serve as a cautionary tale for the spectroscopy community, wherein the role of the D2 tag is far from benign.

Chemical activation of water by a single chlorine atom was examined computationally for clusters of chlorine radicals and water in a size regime just prior to internal hydration of water/ions, Cl • (H 2 O) n=4−8,17 . This investigation follows a recent analysis of this radical−molecule interaction [Christensen et al. J. Phys. Chem. A 2019, 123, 8657] for n = 1−4, which demonstrated that n = 4 marked a transition in which an oxidized-water structural motif became viable, albeit high in energy. Thousands of unique isomers were computed in the present analysis, which resulted in three structural classes of isomers, including intact hydrated chlorine, hydrogen-transferredThe electronic structures of these classes were investigated, along with harmonic vibrational signatures that probed the degree of water-network perturbations and generated experimentally verifiable computational predictions. The main outcome of this analysis is that the charge-transferred isomers were stabilized considerably upon increased hydrationleading to an energetic crossover with the hydrogen-transferred formsbut the degree of hydration was surprisingly still not sufficient to achieve crossover between the intact chlorine−water complexes and these charge-separated configurations.

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