As the global population continues to expand, one of the major challenges facing today’s society is the ever increasing demand for energy, especially in light of the limited access and rising costs of fossil fuels. A promising candidate for a sustainable energy source involves the use of hydrogen, and recent work has shown that ammonia-borane (H3NBH3) shows great potential as a hydrogen storage material. In the present proposal, we plan to generate novel zwitterionic copper complexes containing unique arylspiroboronate ester ligands for their potential to act as catalysts for the dehydrogenation of ammonia-borane. Mechanistic and in-depth catalytic studies will be conducted by my undergraduate students in collaboration with Dr. R. Tom Baker at the University of Ottawa. This work will be published in due course as it is bound to provide fundamental insight into the organometallic chemistry of boron chemistry which, in turn, will provide direction and fuel for the next thrust of my research. Funding for this project will allow me to train several undergraduate students for future careers in industry or graduate school.
Addition of 3,5-di-tert-butylcatechol (butcat) to solutions of H3B.SMe2 gave the novel diboron species B2(butcat)3 in moderate to high yields. This compound reacts with Tl(acac) to give butcatB(acac) and Tl(Bbutcat2). Attempts to abstract the chlorides from [(dppb)Rh(mu-Cl)]2 (where dppb = 1,4-bis(diphenylphosphino)ethane) using Tl(Bbutcat2) led to the unusual dimer [(dppb)Rh(mu-Cl)2(mu-Tl)Rh(dppb)][Bbutcat2] which contains an unsymmetrical Rh-Tl-Rh bridge. We have found that addition of Tl(Bbutcat2) to (PPh3)3CuCl lead to ionic complexes where the arylspiroboronate ester does not bind directly to the copper center. We are therefore in the process of using other organic soluble, yet less bulky, catechol derivatives (4-methylcatechol for example) to generate novel copper complexes. Initial studies confirm that these copper complexes are active in the dehydrogenation of ammonia-borane. In the mean time we have also been looking at alternative synthetic methodologies to these complexes and have recently generated new iridium complexes from this work. We have found that addition of B2cat3 (cat = 1,2-O2C6H4) to Ir(acac)(dppb) (where dppb = 1,4-bis(diphenylphosphino)butane) gave the novel arylspiroboronate ester iridium complex Ir(eta-6-catBcat)(dppb), the first example of an iridium compound containing a coordinating [Bcat2]- anion. This iridium zwitterionic complex is a remarkably selective catalyst precursor for the hydroboration of unhindered vinylarenes using pinacolborane. We are also looking at these complexes for their ability to generate dihydrogen. While several rhodium complexes have been made, the analogous ruthenium complexes are proving more problematic to isolate. We will hire more students next summer to complete these projects and hope to write at least two more papers from our initial studies. Our research will continue to explore cheap and environmentally catalytic systems to generate dihydrogen as a green fuel.
ACS Narrative Report 2011