Thermodynamics of H⁺/H^•/H^–/e^– Transfer from [CpV(CO)₃H]⁻: Comparisons to the Isoelectronic CpCr(CO)₃H

Abstract

Hydrogen atom (H•) donors generated from H2 facilitate the atom efficient reduction of small molecule substrates. However, generating H• donors with X–H bond dissociation free energies (BDFEs) below 52 kcal mol–1 is especially challenging because they thermodynamically favor the bimolecular evolution of H2. We have recently proposed that [CpV­(CO)3H]− catalyzes the conversion of H2 into a proton, an electron, and a hydrogen atom in the presence of a sacrificial base. In order to understand the driving force for H• transfer, the free energies of H+/H•/H–/e– transfer from [CpV­(CO)3H]− have been evaluated using solution phase techniques and state-of-the-art quantum chemical calculations. Thermochemical cycles have been constructed in order to anchor the computational values against experimental observations. This facilitates a quantitative comparison of the thermodynamic driving force for H+/H•/H–/e– transfer between isoelectronic anionic/neutral hydrides of the same row (the corresponding values are already available for CpCr­(CO)3H). The overall charge greatly influences the thermodynamics of transferring H+, H–, and e– (i.e., [CpV­(CO)3H]− is a much weaker acid, a stronger hydride donor, and a stronger reductant than CpCr­(CO)3H); there is almost no change in the thermodynamics of H• transfer (V–H BDFE 54.7 kcal mol, Cr–H BDFE 57.0 kcal mol–1). In MeCN, the one electron oxidation of [CpV­(CO)3H]− (−0.83 V vs Fc/Fc+) generates CpV­(CO)3H, which spontaneously evolves H2. The resulting CpV­(CO)3 is trapped as the solvent adduct CpV­(CO)3(MeCN). Because H• transfer is now coupled to metal–solvent binding, the V–H bond is substantially weakened for CpV­(CO)3H (V–H BDFE 36.1 kcal mol–1), amounting to a strategy for obtaining very reactive H atoms from H2.

Catalytic hydrogen atom transfer to alkenes: a roadmap for metal hydrides and radicals

“…WF reactions oen require an oxidant, such as TBHP or O 2 , consistent with the requirement for higher oxidation states. Second, the SF hydrides are supported by strong-eld CO and phosphine ligands, giving a low-spin electronic conguration, while the WF metal hydrides for catalytic HAT 23 These studies indicate that manipulating the oxidation states and redox potentials of MHAT catalysts (both SF and WF) is a promising area for continued study. It is likely that modication of the geometry will also be inuential because this changes the relative energies of different spin states and oxidation states.…”

“…A recent experimental study in a SF system showed the complementary trend, that the oxidation from vanadium( i ) [CpV(CO) 3 H] – to vanadium( ii ) CpV(CO) 3 H decreases the V–H BDE from 55 to 36 kcal mol –1 . 23 These studies indicate that manipulating the oxidation states and redox potentials of MHAT catalysts (both SF and WF) is a promising area for continued study. It is likely that modification of the geometry will also be influential because this changes the relative energies of different spin states and oxidation states.…”

“…Metal hydride species have been a longstanding topic of study in organometallic chemistry, and bond dissociation enthalpies (BDE) are known for many M–H bonds in classic 18-electron organometallic hydride complexes. 22 Measured BDE values are generally 52 kcal mol –1 or greater, 22 , 23 which renders them thermodynamically stable with respect to H 2 formation, and slow for MHAT.…”

Systematic Trends in the Electrochemical Properties of Transition Metal Hydride Complexes Discovered by Using the Ligand Acidity Constant Equation

“…For example, VH­(Cp)­(CO) 3 , which is produced by oxidation of the anion [VH­(Cp)­(CO) 3 ] − is proposed to be completely unstable because it is rapidly trapped by MeCN to produce VHCp­(CO) 3 (MeCN) with a very weak V–H bond (BDFE 36.1). Therefore, it evolves H 2 as in eq . The oxidation of a dihydride or polyhydride complex might result in the intramolecular formation of an H–H bond to generate a dihydrogen ligand ,, as first proposed in the oxidation of ReH 7 (PPh 3 ) 2 .…”

Benchmark Study of Electrochemical Redox Potentials Calculated with Semiempirical and DFT Methods

“…Elaborate wave function theory methods such as CCSD­(T) typically yield high accuracy for electronic energies but are not computationally feasible for larger molecules or in extensive screening studies. , Density functional theory (DFT)-based methods, however, can be routinely applied for systems with a few hundred atoms and offer a good compromise between the computational cost and accuracy. There are many examples of DFT computations of redox potentials for organic , and organometallic − compounds. Combined WFT/DFT schemes such as the connectivity-based hierarchy (CBH-Redox) model by Raghavachari et al provide cost-effective alternatives to DFT.…”