Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions.

Proton transfer reactions involving transition metal hydride complexes are prevalent in a number of catalytic fuel-forming reactions, where the proton transfer kinetics to or from the metal center can have significant impacts on the efficiency, selectivity, and stability associated with the catalytic cycle. This review correlates the often slow proton transfer rate constants of transition metal hydride complexes to their electronic and structural descriptors and provides perspective on how to exploit these parameters to control proton transfer kinetics to and from the metal center. A toolbox of techniques for experimental determination of proton transfer rate constants is discussed, and case studies where proton transfer rate constant determination informs fuel-forming reactions are highlighted. Opportunities for extending proton transfer kinetic measurements to additional systems are presented, and the importance of synergizing the thermodynamics and kinetics of proton transfer involving transition metal hydride complexes is emphasized.

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.

Reversible oxidative-addition and reductive-elimination of thiophene from a titanium complex and its thermally-induced hydrodesulphurization chemistry.

The masked Ti(ii) synthon (Ketguan)(η6-ImDippN)Ti (1) oxidatively adds across thiophene to give ring-opened (Ketguan)(ImDippN)Ti[κ2-S(CH)3CH] (2). Complex 2 is photosensitive, and upon exposure to light, reductively eliminates thiophene to regenerate 1 - a rare example of early-metal mediated oxidative-addition/reductive-elimination chemistry. DFT calculations indicate strong titanium π-backdonation to the thiophene π*-orbitals leads to the observed thiophene ring opening across titanium, while a proposed photoinduced LMCT promotes the reverse thiophene elimination from 2. Finally, pressurizing solutions of 2 with H2 (150 psi) at 80 °C leads to the hydrodesulphurization of thiophene to give the Ti(iv) sulphide (Ketguan)(ImDippN)Ti(S) (3) and butane.

Synthesis and characterization of rhodium, iridium, and palladium complexes of a diarylamido-based PNSb pincer ligand.

A new diarylmido-based pincer proto ligand (iPrPNHSbPh) with one -PPri2 and one -SbPh2 side donor has been synthesized. Three complexes of its amido form were prepared using standard metalation techniques: (iPrPNSbPh)PdCl, (iPrPNSbPh)RhCO, and (iPrPNSbPh)Ir(COE), where COE = cis-cyclooctene. These complexes were compared with their previously reported analogs incorporating a -PPh2 side donor in place of -SbPh2. The -SbPh2 donor arm is less donating towards the metal and is less strongly trans-influencing, based on the structural and IR spectroscopic analysis of the Rh complexes. The redox potential of the Pd complexes is only marginally affected by the change from -PPh2 to -SbPh2. (iPrPNSbPh)Ir(COE) proved to be a slower and less selective catalyst in the dehydrogenative borylation of terminal alkynes (DHBTA) than its -PPh2 analog.