Catalyst self-assembly accelerates bimetallic light-driven electrocatalytic H2 evolution in water

Hydrogen evolution is an important fuel-generating reaction that has been subject to mechanistic debate about the roles of monometallic and bimetallic pathways. The molecular iridium catalysts in this study undergo photoelectrochemical dihydrogen (H2) evolution via a bimolecular mechanism, providing an opportunity to understand the factors that promote bimetallic H-H coupling. Covalently tethered diiridium catalysts evolve H2 from neutral water faster than monometallic catalysts, even at lower overpotential. The unexpected origin of this improvement is non-covalent supramolecular self-assembly into nanoscale aggregates that efficiently harvest light and form H-H bonds. Monometallic catalysts containing long-chain alkane substituents leverage the self-assembly to evolve H2 from neutral water at low overpotential and with rates close to the expected maximum for this light-driven water splitting reaction. Design parameters for holding multiple catalytic sites in close proximity and tuning catalyst microenvironments emerge from this work.

Visible light induced formation of a tungsten hydride complex

When irradiated with blue light in the presence of a Lewis base (L), [CpW(CO)₃]₂ undergoes metal-metal bond cleavage followed by a disproportionation reaction to form [CpW(CO)₃L]⁺ and [CpW(CO)₃]^-. Here, we show that in the presence of pyridinium tetrafluoroborate, [CpW(CO)₃]^- reacts further to form a metal hydride complex CpW(CO)₃H. The rection was monitored through in situ photo ¹H NMR spectroscopy experiments and the mechanism of light-driven hydride formation was investigated by determining quantum yields of formation. Quantum yields of formation of CpW(CO)₃H correlate with I^-1/2 (I = photon flux on our sample tube), indicating that the net disproportionation of [CpW(CO)₃]₂ to form the hydride precursor [CpW(CO)₃]^- occurs primarily through a radical chain mechanism.

Molecular reorientation of CD(4) in gas-phase mixtures

Spin-lattice relaxation times were measured for the deuterons in CD(4) in pure gas and in mixtures with the following buffer gases: Ar, Kr, Xe, HCl, N(2), CO, CO(2), CF(4), and SF(6). Effective collision cross sections sigma(theta, 2) for the molecular reorientation of CD(4) in collisions with these ten molecules are obtained as a function of temperature. These cross sections are compared with the corresponding cross sections sigma(J) obtained from (1)H spin-rotation relaxation in mixtures of CH(4) with the same set of buffer gases. Various classical reorientation models typically applied in liquids predict different ratios of the reduced correlation times for the reorientation of spherical tops. The Langevin model comes closest to predicting the magnitude of the sigma(theta, 2)/sigma(J) ratio that we obtain for CD(4).