DEVELOPMENT OF CATALYTICALLY ENHANCED COMPLEX ALUMINUM HYDRIDES AS VEHICULAR HYDROGEN STORAGE MATERIALS. For decades, hydrogen has been targeted as the utopian fuel of the future due to its abundance and environmental friendliness. A major difficulty in the utilization of hydrogen as a fuel is the problem of onboard hydrogen storage. High pressure and cryogenic hydrogen storage systems are impractical for vehicular applications due to safety concerns and volumetric constraints. This has prompted an extensive effort to develop solid hydrogen storage systems. Unfortunately, despite decades of extensive effort no material has been found which as has the combination of a high gravimeteric hydrogen density, adequate hydrogen dissociation energetics, and low cost required for commercial vehicular application. The discharging of a high, 5.5, weight percentage of hydrogen from sodium aluminum hydride, NaAlH₄, is thermodynamically favorable at moderate conditions. However, this process is characterized by very slow kinetics and reversibility only under severe conditions. Thus NaAlH₄ was not explored as a rechargeable hydrogen carrier until 1996 when Bogdanovic discovered that titanium doping catalyzes the reversible dissociation of hydrogen from the hydride. This finding represented a breakthrough in the application of this low cost material to hydrogen storage. We have subsequently developed significantly improved dehydriding/rehydriding catalysts. Sodium aluminum hydride containing our optimized catalyst undergoes rapid dehydriding at temperatures as low as 100^oC and contains 4.5 wt % hydrogen which can be repeatedly cycled. These findings suggest the application of these materials as hydrogen carriers for onboard fuel cells. Our current research efforts are aimed at elucidating the mechanism of the catalytic process and devlopment of improved catalysts and complex aluminum hydrides for vehicular hydrogen storage applications.

ALIPATHIC DEHYDROGENATION REACTIONS CATALYZED BY IRIDIUM P-C-P PINCER COMPLEXES. The selective functionalization of aliphatic groups is one of the great unsolved problems of organic chemistry. One promising approch to this problem has been the development of soluble transition metal complexes which activate alkane C-H bonds under moderate conditions. Several such complexes have been found to catalyze dehydrogenation of alkanes to alkenes. However, the utility of these reactions has been limited by very slow rates, low numbers of turnovers, catalyst instability at the reaction conditions, and the requirement of a sacrificial hydrogen acceptor. We recently found that the P-C-P pincer complex, IrH₂{C₆H₃-2,6-(CH₂PBu^t₂)₂} (1) is a highly active catalyst for a wide variety of aliphatic dehydrogenations.

The P-C-P pincer framework renders the metal center reactive with saturated hydrocarbons but restricts its access to the ligand P-C bonds. As a result, the complexes also have unprecedented, long term stabilities under catalytic conditions. We have found 1 to be the first reported homogeneous catalyst for: the "acceptorless" dehydrogenation of linear alkanes to alpha olefins; dehydrogenation of cycloalkanes to arenes; the dehydrogenation of ethylbenzene to styrene; and the highly regioselective dehydrogenation of amines to imines thus ilustrating that this type of complex can transverse previously inaccessible catalytic pathways. Our current research in this area involves the synthesis of hydrido iridium complexes with modified pincer ligands and studing the effect of this fine-tuning of the metals's coordination environment on its catalytic activity.

C-C BOND FORMING REACTIONS CATALYZED BY PALLADIUM P-C-P PINCER COMPLEXES. We have recently synthesized the novel palladium PCP complex, PdCl{C₆H₃-2,6-(OPPrⁱ₂)₂} (2). This complex has proven to be a highly active catalyst for Heck coupling reactions

including regio- and stereoselectivity couplings of disubstituted alkenes to trisubstituted alkenes and high yield couplings utilizing a wide variety of aryl chlorides. We are currently exploring the activity of 2 and related complexes as catalysts for other C-C bond forming reactions.