People - [ Faculty ]
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).
Tyler O. Howell, Aron J. Huckaba, and T. Keith Hollis*. "An Efficient Synthesis of bis-1,3-(3'-arly-N-heterocycl-1'-yl)arenes as CCC-NHC Pincer Ligand Precursors." Organic Letters, 2014, Article ASAP. DOI:10.1021/ol5007407.
Theodore R. Helgert, T. Keith Hollis*, Allen G. Oliver, Henry U. Valle, Yunshan Wu, and Charles Edwin Webster. "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, 134(4) 2355-2366, 2014. DOI: 10.1021/om401063e.
Wesley D. Clark, Joon Cho, Henry U. Valle, T. Keith Hollis*, Edward J. Valente. “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 (http://www.sciencedirect.com/science/article/pii/S0022328X13007985) .” Journal of Organometallic Chemistry, 751, 2014, pp 534-540. DOI: 10.1016/j.jorganchem.2013.11.001( http://dx.doi.org/10.1016/j.jorganchem.2013.11.001)
Aron J. Huckaba, T. Keith Hollis*, Sean W. Reilly. “Homobimetallic Rhodium NHC Complexes as “Versatile Catalysts for Hydrosilylation of a Multitude of Substrates in the Presence of Ambient Air.” Organometallics, 2013, 32 (21), pp 6248–6256. DOI: 10.1021/om400452q
Aron J. Huckaba, Bei Cao, T. Keith Hollis,* Henry U. Valle, Yunshan Wu, Nathan Hammer, John Kelley, “Benzimidazolyl Platinum CCC-NHC Pincer Complexes: Synthesis, Characterization, and Photostability of a Blue-Green Emitter,” Dalton Trans. 2013, 42, 8820-8826, DOI:10.1039/C3DT50438G.
Huckaba, A. J.; Hollis, T. K.; Howell, T. O.; Valle, H. U., Wu, Y. " Synthesis and Characterization of a 1,3-Phenylene-Bridged N.Alkyl Bis(benzimidazole) CCC-NHC Pincer Ligand Precursor: Homobimetallic Silver and Rhodium Complexes and the Catalytic Hydrosilylation of Phenylacetylene,” Organometallics, 2013, 32, 63-69, DOI: dx.doi.org/10.1021/om3008037.
Wesley D. Clark, Ginger E. Tyson, T. Keith Hollis,* Edward J. Valente, Henry U. Valle, “Toward Molecular Rotors: Tetra-N-Heterocyclic Carbene Ag(I)-Halide Cubane-type Clusters,” Dalton Transactions 2013, 42(20), 7338 – 7344, DOI: 10.1039/C3DT32862G.
Xiaofei Zhang, Bei Cao, Edward J. Valente, T. Keith Hollis* “Synthesis, Characterization, Photoluminescence, and Simulations of a CCC-NHC Supported Pt2Ag2 Mixed-Metal Cluster Containing a PtAg2 Metallo-Cyclopropane,” Organometallics, 2013, 32, 752-761, DOI: dx.doi.org/10.1021/om300330y.
Theodore R. Helgert, T. Keith Hollis,* Edward J. Valente “Synthesis of Titanium CCC-NHC Pincer Complexes and Catalytic Hydroamination of Unactivated Alkenes,” Organometallics, 2012, 31(8), 3002-3009, DOI:10.1021/om2010436.
Xiaofei Zhang, Ashley M. Wright, Nathan J. DeYonker, T. Keith Hollis,* Nathan I. Hammer,* C. Edwin Webster,* Edward J. Valente “Synthesis, Air-stability, Photo-bleaching, and DFT Modeling of Blue Light-Emitting Platinum CCC-N-Heterocyclic Carbene Pincer Complexes,” Organometallics 2012, 31(5), 1664-1672, DOI: 10.1021/om2010436.
Joon Cho, T. Keith Hollis,* Edward J. Valente, Jaclyn M. Trate “CCC-N-Heterocyclic Carbene Pincer Complexes: Synthesis, Characterization and Hydroamination Activity of a Hafnium Complex,” Journal of Organometallic Chemistry, 2011, 696, 373-377, DOI:10.1016/j.jorganchem.2010.10.004.
Eike B. Bauer, Gurusamy Thangavelu Senthil Andavan, T. Keith Hollis,* Ramel J. Rubio, Joon Cho, Glenn R. Kuchenbeiser, Theodore R. Helgert, Christopher S. Letko, Fook S. Tham “Air and Water Stable Catalysts for Hydroamination/Cyclization. Synthesis and Application of CCC-NHC Pincer Complexes,” Organic Letters, 2008, 10, 1175-1178.
Joon Cho, T. Keith Hollis,* Theodore R. Helgert, Edward J. Valente “An Improved Method for the Synthesis of Zirconium (CCC-N-Heterocyclic Carbene) Pincer Complexes and Applications in Hydroamination,” Chemical Communications, 2008, 5001 – 5003, DOI: 10.1039/b805174g.
Gurusamy Thangavelu Senthil Andavan, Eike B. Bauer, Christopher S. Letko, T. Keith Hollis,* Fook S. Tham “Synthesis and Characterization of a Free Phenylene Bis(N-Heterocyclic Carbene) and its di-Rh complex. Catalytic Activity of the di-Rh and CCC-NHC Rh Pincer complexes in Intermolecular Hydrosilylation of Alkynes,” Journal of Organometallic Chemistry, 2005, 690, 5938-5947.
Ramel J. Rubio, Gurusamy Thangavelu Senthil Andavan , Eike B. Bauer, T. Keith Hollis,* Joon Cho, Fook S. Tham, Bruno Donnadieu “Toward a General Method for CCC N-Heterocyclic Carbene Pincer Synthesis: Metallation and Transmetallation Strategies for Concurrent Activation of Three C-H Bonds,” Journal of Organometallic Chemistry, 2005, 690, 5353-5364.
Victor C. Vargas, Ramel J. Rubio, T. Keith Hollis, Martha E. Salcido “An Efficient Route to 1,3-Di-N-Imidazolylbenzene. A Comparison of Monodentate vs. Bidentate Carbenes in Pd Catalyzed Cross Coupling,” Org. Lett. 2003,5, 4847-4849.