Caltech Research Summary

My research here at Caltech as a joint postdoc between Prof Harry Gray and Prof Nate Lewis has focused on the synthesis of novel hydrogen (H2) generating catalysts, and their covalent attachment to silicon semiconductor surfaces. The work has been conducted under the umbrella of the Chemical Center for Innovation - Solar Fuels (CCI-Solar), also under the guidance of CCI-Solar PI’s Bruce Brunschwig (surface chemistry & characterization) and Jay Winkler (kinetics & mechanism). This research project came about as a synergistic result of the two research groups’  interests: Prof Gray has a longstanding interest in the synthesis and electronic properties of novel late transition metal complexes, and Prof Lewis has an established publication record in the electrochemistry of silicon semiconductor liquid/solid junctions. So naturally, a joint position was ideal to investigate the covalent attachment of small molecule catalysts to silicon surfaces.

Project #1: C-C Coupling Methodology on Passivated Si(111) Surfaces
The first challenge in this project was to develop a robust means of making C-C bonds on a Si(111) surface. Although there are some known strategies to covalently attach small molecules to Si-R(111), these methods either lead to incomplete surface passivation (hydrosilylation) or rely on semi-labile functional groups (amide, ester linkages) which are unlikely to persist under the acidic aqueous conditions necessary for H2 generation. For this reason we have developed a Pd-catalyzed Heck reaction to reliably make C-C bonds on the Si(111) surface.

The primary means of characterizing the functionalized Si surfaces are X-ray Photoelectron Spectroscopy (XPS); surface FTIR and HATR/GATR; and photo/electrochemical methods.

Publication (in preparation): L. E. O'Leary, M. J. Rose, T. X. Ding, E. Johansson, B. S. Brunschwig
and N. S. Lewis. Heck Couplings of Small Molecules to Mixed Methyl/Thienyl Monolayers at
Low Defect Density Si(111).

Project #2:
H2-Generating Catalyst/Ligand Modification for Covalent Attachment to Si(111)
The next logical step is to synthesize catalyst ligands bearing a pendant olefin functional group to participate in the robust Pd-mediated C-C coupling. We have derivatized a number of PNP, P2N2 and dmg (dimethylglyoxime) type ligands with pendant olefins and are pursuing ligand attachement, metallation and electrochemical characterization of the catalyst-functionalized surfaces.

The primary means of characterizing the ligands and complexes are the usual complement of techniques of interest to the inorganic chemist: 1H/13C/31P/19F NMR, X-ray crystallography, EPR, UV/vis, SQUID and electrochemistry.

Publication (in preparation): M. J. Rose, L. E. O’Leary, J. R. Lattimer, B. S. Brunschwig
and N. S. Lewis. Covalent Attachment of Small Molecule Catalysts to Planar Si(111)
Semiconductor Surfaces for Photoelectrochemical Hydrogen Generation.

Project #3: Perfluorinated Diglyoxime Iron Complexes for H2 Electrocatalysis
One important challenge in H2 catalysis is substitution of earth-abundant first-row transition metals for Platinum. Much work has been done in recent years regarding several families of Cobalt and Nickel molecular catalysts, motivated by the 1000x more abundant metals (and correspondingly 1000x cheaper). Ultimately, it is desirable to utilize the
most abundant transition metal - iron. Iron is of course present in all naturally occurring hydrogenase enzymes (Fe-Fe, Ni-Fe and mono-Fe Hases).

We have synthesized a perfluorinated version of a diglyoxime ligand, which allows for generation of H2 from iron complexes at moderate overpotentials (~-0.8 V vs SCE; dmgBF2-Co system = -0.6 V vs SCE). This is a remarkable finding as, to date, all small molecule Fe-Hase mimics operate at approximately ~-1.5 V vs SCE, a value far too negative for coupling to a visible light-driven process.

Publication: M. J. Rose, H. B. Gray and J. R. Winkler. Electrocatalytic Dihydrogen (H2)
Evolution at Low Overpotentials from Fluorinated Diglyoxime Iron Complexes.
J. Am. Chem. Soc. 2012, 134, 8310-8313.

Other Projects:
I have pursued several other projects related to the goals stated above. The synthesis of novel Cobalt- and Nickel-Phosphine complexes has been of particular interest:

         Q. Dong, M. J. Rose, W.-Y. Wong and H. B. Gray. Dual Coordination Modes of Ethylene Linked NP2 Ligands           with Cobalt(II) and Nickel(II) Iodides. Inorg. Chem. 2011, 50, 10213-10224.

M. J. Rose, D. E. Bellone and H. B. Gray. Structural, Magnetic and  Spectroscopic Characterization of a High-Spin  Iodo Co(I) Complex Derived from a Tripodal Phosphine Ligand. Submitted.

         M. J. Rose, R. M. Mercado, B. S. Brunschwig, H. B. Gray. Electrocatalytic Dihydrogen Evolution from           Perfluorinated Cobalt Glyoximes in Aqueous Solution. Work in progress.