The Hollis Group designs and develops next-generation organometallic ligands and complexes for many applications, which often requires the development of new synthetic methodologies. Access to new molecules and materials is required to solve many of the technological challenges facing society, such as improving energy-efficiency, direct conversion of solar energy to useful forms, and more cost-effective access to medicines. These goals are reached by developing efficient, scalable syntheses of molecules with interesting properties.
Basic Synthetic Methodology Development
A few years ago, the Hollis Group developed the seminal methodologies that allow access to a new class of organometallic pincer complexes – CCC-NHC ligands, which contain two donor N-heterocyclic carbenes (NHCs) with no spacer between the central aryl donor and the NHC donors. The core of the synthesis starts with a Cu catalyzed aryl amination that we routinely perform on the 120 g scale. It is followed by an alkylation by simple nucleophilic substitution.
Cu Catalyzed Core Synthesis –
This metalation/transmetalation strategy has proven highly successful as the Hollis group has now prepared examples from almost every group in the transition metal series.
Materials for Light-based Interactions – Emission and Absorption: OLEDs and Photovoltaics
The societal challenges that 21st century science is addressing require fundamental advances – the development of technologies that currently are not in existence. To sustain any kind of technologically-advanced society (energy-driven society) beyond the lifetime of our great-grandchildren major scientific-breakthroughs are required. At present, “No economic activity is yet sustainable…” [Duma12]. Many different avenues require exploration and, eventually, exploitation in full-scale economic application to bridge the time between the current state of the world and a carbon-neutral, fully-renewable energy economy. Reaching such a goal will solve many of the challenges we face, world-wide, that are driven by competition for limited resources. While a multi-pronged approach to a sustainable future is required with emphases in many areas, we are focused on developing, fully characterizing and engineering materials for improving photovoltaic (PV) efficiency, an approach to utilizing our greatest source of renewable energy (solar energy capture). Much fundamentally new science must be developed. The CCC-NHC Pt complexes depicted below absorb UV light and emit blue, a much needed color for OLEDs, and are phtostable for extended periods of time.
Organometallic Complexes for Catalytic Activation of Strong Bonds (Small Molecules and Unactivated C-H bonds)
The ability to fix (convert to usable, high-value chemicals) nitrogen (N2) and carbon dioxide (CO2) are critically important chemical research frontiers. Converting nitrogen to ammonia for fertilizer is crucial for feeding the world. We must be able to efficiently sequester (chemically capture) CO2 emissions in power generation to fuel a technologically-advanced, energy-driven society while averting the looming global warming situation. The development of high energy organometallic complexes capable of these feats is an area of rapidly expanding research. Access to low valent versions of materials with the CCC-NHC ligand architecture are predicted to be robust and capable these conversions. Similar chemical reactivity parameters are required to convert unactivated C-H bonds into useful starting materials for the chemical industries.
Organometallic Complexes for Catalytic Organic Transformations and Asymmetric Catalysis
The development of more efficient, catalytic methodologies for preparing organic compounds leads to more cost-effective synthetic procedures for the preparation of pharmaceutically-active compounds for the treatment of human disease thereby alleviating human suffering. Complexes prepared for the first time in the Hollis research group have been demonstrated to be effective for the formation of C-N bonds (hydroamination), C-Si (hydrosilylation), C-C (Michael addition) and C-B bonds (Michael addition).
- Helgert TR, Zhang X, Box HK, Denny JA, Valle HU, Oliver AG, Akurathi G, Webster CE, Hollis TK. Extreme π-loading as a design element for accessing imido ligand reactivity. A CCC-NHC pincer tantalum bis(imido) complex: Synthesis, characterization, and catalytic oxidative amination of alkenes. Organometallics 2016;35(20):3452-60.
- Perera GS, Yang G, Nettles CB, Perez F, Hollis TK, Zhang D. Counterion effects on electrolyte interactions with gold nanoparticles. J Phys Chem C 2016;120(41):23604-12.
- Reilly SW, Webster CE, Hollis TK, Valle HU. Transmetallation from CCC-NHC pincer zr complexes in the synthesis of air-stable CCC-NHC pincer co(iii) complexes and initial hydroboration trials. Dalton Trans 2016;45(7):2823-8.
- Reilly SW, Akurathi G, Box HK, Valle HU, Hollis TK, Webster CE. β-Boration of α,β-unsaturated carbonyl compounds in ethanol and methanol catalyzed by CCC-NHC pincer rh complexes. J Organomet Chem 2016;802:32-8.
- Clark WD, Akurathi G, Valle HU, Hollis TK. Crystal structure of tris(dimethylamido-κN)bis(dimethylamine-κN)zirconium(IV) iodide. Acta Crystallographica Section E: Crystallographic Communications 2016;72.
- Clark WD, Leigh KN, Webster CE, Hollis TK. Experimental and computational studies of the mechanisms of Hydroamination/Cyclisation of unactivated α,ω-amino-alkenes with CCC-NHC pincer zr complexes. Aust J Chem 2016;69(5):573-82.
- Valle HU, Akurathi G, Cho J, Clark WD, Chakraborty A, Hollis TK. CCC-NHC pincer zr diamido complexes: Synthesis, characterisation, and catalytic activity in Hydroamination/Cyclisation of unactivated amino-alkenes,-alkynes, and allenes. Aust J Chem 2016;69(5):565-72.
- Tyson GE, Tokmic K, Oian CS, Rabinovich D, Valle HU, Hollis TK, Kelly JT, Cuellar KA, McNamara LE, Hammer NI, Webster CE, Oliver AG, Zhang M. Synthesis, characterization, photophysical properties, and catalytic activity of an SCS bis(N-heterocyclic thione) (SCS-NHT) pd pincer complex. Dalton Trans 2015;44(32):14475-82.
- Perera GS, Nettles CB, Zhou Y, Zou S, Hollis TK, Zhang D. Direct observation of ion pairing at the Liquid/Solid interfaces by surface enhanced Raman spectroscopy. Langmuir 2015;31(33):8998-9005.
- Reilly SW, Box HK, Kuchenbeiser GR, Rubio RJ, Letko CS, Cousineau KD, Hollis TK. 1,4-addition of aryl boronic acids to α,β-unsaturated ketones catalyzed by a CCC-NHC pincer rhodium complex. Tetrahedron Lett 2014;55(49):6738-42.
- Howell TO, Huckaba AJ, Hollis TK. An efficient synthesis of bis-1,3-(3′-aryl-N-heterocycl-1′-yl) arenes as CCC-NHC pincer ligand precursors. Org Lett 2014;16(9):2570-2.
- Helgert TR, Hollis TK, Oliver AG, Valle HU, Wu Y, Webster CE. Synthesis, characterization, and X-ray molecular structure of tantalum CCC-N-heterocyclic carbene (CCC-NHC) pincer complexes with imidazole- and triazole-based ligands. Organometallics 2014;33(4):952-8.
- Clark WD, Cho J, Valle HU, Hollis TK, Valente EJ. Metal and halogen dependence of the rate effect in hydroamination/ cyclization of unactivated aminoalkenes: Synthesis, characterization, and catalytic rates of CCC-NHC hafnium and zirconium pincer complexes. J Organomet Chem 2014;751:534-40.
- Huckaba AJ, Hollis TK, Reilly SW. Homobimetallic rhodium NHC complexes as versatile catalysts for hydrosilylation of a multitude of substrates in the presence of ambient air. Organometallics 2013;32(21):6248-56.