Redox inactive Lewis acidic cations are thought to facilitate the reactivity of metalloenzymes and their synthetic analogues by tuning the redox potential and electronic structure of the redox active site. To explore and quantify this effect, we report the synthesis and characterization of a series of tetradentate Schiff base ligands appended with a crown-like cavity incorporating a series of alkali and alkaline earth Lewis acidic cations (1M, where M = Na, K, Ca, Sr, and Ba) and their corresponding Co(II) complexes (2M). Cyclic voltammetry of the 2M complexes revealed that the Co(II/I) redox potentials are 130 mV more positive for M = Na and K and 230-270 mV more positive for M = Ca, Sr, and Bacompared to Co(salen-OMe) (salen-OMe = N,N'-bis(3-methoxysalicylidene)-1,2-diaminoethane), which lacks a proximal cation. The Co(II/I) redox potentials for the dicationic compounds also correlate with the ionic size and Lewis acidity of the alkaline metal. Electronic absorption and infrared spectra indicate that the Lewis acid cations have a minor effect on the electronic structure of the Co(II) ion, which suggests the shifts in redox potential are primarily a result of electrostatic effects due to the cationic charge.

The free energy of hydride donation (hydricity) for [HNi(DHMPE)2][BF4] (DHMPE = 1,2-bis(dihydroxymethylphosphino)ethane was experimentally determined versus the heterolytic cleavage energy of hydrogen in acetonitrile, dimethyl sulfoxide, and water to be 57.4, 55.5, and 30.0 kcal/mol, respectively. This work represents the first reported hydricity values for a transition metal hydride donor in three different solvents. A comparison between our values and the hydricity of hydrogen and formate reveals a narrowing in the range of values with increasing solvent polarity. The thermochemical values also reveal solvation effects that impact the overall thermodynamic favorability of hydride generation from hydrogen and transfer to carbon dioxide. The quantitative solvation effects described herein have important consequences to the design and reactivity of catalysts for transformations that have hydride transfer steps throughout synthetic chemistry.

Two isostructural, nonclassical Co(H 2 ) complexes are prepared from their Co(N 2 ) precursors using tris(phosphino)silyl and tris(phosphino)borane ancillary ligands. Comproportionation of CoBr 2 and Co metal in the presence of TPB (tris-(o-diisopropylphophinophenyl)borane) gives (TPB)-CoBr (4). One-electron reduction of 4 triggers N 2 binding to give (TPB)Co(N 2 ) (2-N 2 ) which is isostructural to previously reported [SiP 3 ]Co(N 2 ) (1-N 2 ) ([SiP 3 ] = tris-(odiisopropylphosphinophenyl)silyl). Both 1-N 2 and 2-N 2 react with 1 atm H 2 to generate thermally stable H 2 complexes 1-H 2 and 2-H 2 , respectively. Both complexes are characterized by a suite of spectroscopic techniques in solution and by X-ray crystallography. The H 2 and N 2 ligands in 2-H 2 and 2-N 2 are labile under ambient conditions and the binding equilibria are observable by temperature-dependent UV/vis. A van't Hoff analysis allows for the ligand binding energetics to be determined (H 2 : ΔH o = −12.5(3) kcal mol −1 and ΔS o = −26(3) cal K −1 mol −1 ; N 2 : ΔH o = −13.9(7) kcal mol −1 and ΔS o = −32(5) cal K −1 mol −1 ).

Electrocatalytic activity of a water-soluble nickel complex, [Ni(DHMPE)] (DHMPE = 2-bis(di(hydroxymethyl)phosphino)ethane), for the hydrogen evolution reaction (HER) at pH 1 is reported. The catalyst functions at a rate of ∼10 s (k) with high Faradaic efficiency. Quantification of the complex before and after 18+ hours of electrolysis reveals negligible decomposition under catalytic conditions. Although highly acidic conditions are common in electrolytic cells, this is a rare example of a homogeneous catalyst for HER that functions with high stability at low pH. The stability of the compound and proposed catalytic intermediates enabled detailed mechanistic studies. The thermodynamic parameters governing electron and proton transfer were used to determine the appropriate reductants and acids to access the catalytic cycle in a stepwise fashion, permitting direct spectroscopic identification of intermediates. These studies support a mechanism for proton reduction that proceeds through two-electron reduction of the nickel(II) complex, protonation to generate [HNi(DHMPE)], and further protonation to initiate hydrogen bond formation.

The torsion between the central benzene ring and the outer aromatic rings in 1,3,5-tri-p-(tetrazol-5-yl)phenylbenzene (H3TPB-3tz) and the absence of such strain in 2,4,6-tri-p-(tetrazol-5-yl)phenyl-s-triazine (H3TPT-3tz) are shown to allow the selective synthesis of noncatenated and catenated versions of expanded sodalite-type metal-organic frameworks. The reaction of H3TPB-3tz with CuCl2.2H2O affords the noncatenated compound Cu3[(Cu4Cl)3(TPB-3tz)8]2.11CuCl2.8H2O.120DMF (2), while the reaction of H3TPT-3tz with MnCl2.4H2O or CuCl2.2H2O generates the catenated compounds Mn3[(Mn4Cl)3(TPT-3tz)8]2.25H2O.15CH3OH.95DMF (3) and Cu3[(Cu4Cl)3(TPT-3tz)8]2.xsolvent (4). Significantly, catenation helps to stabilize the framework toward collapse upon desolvation, leading to an increase in the surface area from 1120 to 1580 m2/g and an increase in the hydrogen storage capacity from 2.8 to 3.7 excess wt % at 77 K for 2 and 3, respectively. The total hydrogen uptake in desolvated 3 reaches 4.5 wt % and 37 g/L at 80 bar and 77 K, demonstrating that control of catenation can be an important factor in the generation of hydrogen storage materials.