Temperature dependence of the acid–base equilibrium constants of substituted pyridine N-oxides in acetonitrile

Temperature and Solvent Effects on H2 Splitting and Hydricity: Ramifications on CO2 Hydrogenation by a Rhenium Pincer Catalyst

The catalytic hydrogenation of carbon dioxide holds immense promise for applications in sustainable fuel synthesis and hydrogen storage. Mechanistic studies that connect thermodynamic parameters with the kinetics of catalysis can provide new understanding and guide predictive design of improved catalysts. Reported here are thermochemical and kinetic analyses of a new pincer-ligated rhenium complex (^tBuPOCOP)Re(CO)₂ (^tBuPOCOP = 2,6-bis(di-tert-butylphosphinito)phenyl) that catalyzes CO₂ hydrogenation to formate with faster rates at lower temperatures. Because the catalyst follows the prototypical "outer sphere" hydrogenation mechanism, comprehensive studies of temperature and solvent effects on the H₂ splitting and hydride transfer steps are expected to be relevant to many other catalysts. Strikingly large entropy associated with cleavage of H₂ results in a strong temperature dependence on the concentration of [(^tBuPOCOP)Re(CO)₂H]^- present during catalysis, which is further impacted by changing the solvent from toluene to tetrahydrofuran to acetonitrile. New methods for determining the hydricity of metal hydrides and formate at temperatures other than 298 K are developed, providing insight into how temperature can influence the favorability of hydride transfer during catalysis. These thermochemical insights guided the selection of conditions for CO₂ hydrogenation to formate with high activity (up to 364 h^-1 at 1 atm or 3330 h^-1 at 20 atm of 1:1 H₂:CO₂). In cases where hydride transfer is the highest individual kinetic barrier, entropic contributions to outer sphere H₂ splitting lead to a unique temperature dependence: catalytic activity increases as temperature decreases in tetrahydrofuran (200-fold increase upon cooling from 50 to 0 °C) and toluene (4-fold increase upon cooling from 100 to 50 °C). Ramifications on catalyst structure-function relationships are discussed, including comparisons between "outer sphere" mechanisms and "metal-ligand cooperation" mechanisms.

Solution chemistry of a water-soluble eta2-H2 ruthenium complex: evidence for coordinated H2 acting as a hydrogen bond donor

The ability of an eta2-H2 ligand to participate in intermolecular hydrogen bonding in solution has long been an unresolved issue. Such species are proposed to be key intermediates in numerous important reactions such as the proton-transfer pathway of H2 production by hydrogenase enzymes. We present the synthesis of several new water-soluble ruthenium coordination complexes including an eta2-H2 complex that is surprisingly inert to substitution by water. The existence of dihydrogen hydrogen bonding (DHHB) was experimentally probed by monitoring the chemical shift of H-bonded Ru-(H2) complexes using NMR spectroscopy, by UV-visible spectroscopy, and by monitoring the rotational dynamics of a hydrogen-bonding probe molecule. The results provide strong evidence that coordinated H2 can indeed participate in intermolecular hydrogen bonding to bulk solvent and other H-bond acceptors.

Detection of hydrogen bonding in solution: A 2H nuclear magnetic resonance method based on rotational motion of a donor/acceptor complex

The effect of hydrogen bonding on the rotational correlation time of an H-bond acceptor, pyridine N-oxide-d(5), in various solvents was investigated using the (2)H spin-lattice relaxation time (T(1)). The results demonstrate a linear relationship between viscosity and measured rotational correlation times, an example of Stokes-Einstein-Debye behavior. The results also clearly demonstrate reduced rotational rates for the probe in hydrogen bonding solvents in comparison to solvents incapable of forming hydrogen bonds with the probe. The utility of this observation was exploited to estimate the association constant (K(a)) through an NMR titration procedure. These results are presented as a new technique that can be applied to the characterization of hydrogen bonding in similar systems.