New Approaches to Functionalizing Metal-Coordinated N2

Catalytic conversion of N2 into value-added products is the challenge of attack on a refractory molecule. We develop herein ideas, using product thermodynamics, to gauge which derivatizing reagents hold special promise. Since N2 is an especially stable molecule, it must be paired with an endothermic molecule to make the overall reaction exothermic. A number of unusual nitrogen heterocycle products are targeted in this way, based on derivatization with alkynes, with alkenes, or with allenes. Since the late transition elements (M) have M–heteroatom bonds which are readily subject to hydrogenolysis, aldehydes and ketones are also identified as potential traps. It is proposed that advantages arise from targeting organonitrogen products retaining an N N bond, including double bonds. While fundamentally thermodynamic in character, such advantages may also translate into lowering of barriers in elementary processes, hence to improved rates. The ideas are general in nature, and thus invite testing among newly formed N2 complexes of any metal. A current goal of global importance is the conversion of diatomic nitrogen to value-added organonitrogen compounds. While improved, less-energy intensive hydrogenation of coordinated N2 (“nitrogen fixation”, forming NH3) is an attractive, if challenging goal owing to the importance of agricultural fertilizers, there are broader goals for organonitrogen compounds, including other fertilizers (e.g., urea), but also compounds useful in pharmaceutical and electronic materials applications. The purpose of this Essay is to show that, in the absence of available experimental thermochemical data, it can be useful to go “prospecting”, using density functional theory (DFT) calculations, for organic molecules with which to “derivatize” N2. Our goal is to design new reactions for the conversion of N2 into chemicals based on a thermodynamic concept: high energy reagents serve as fuel to convert the very stable, refractory molecule N2. The challenge of hydrogenation of coordinated, hopefully electron rich N2 to NxHy compounds may be due in part to the fact that diatomic H2 is not very electrophilic. Indeed, earliest efforts in this quest have involved first protons (targeting NH3) and carbon electrophile reagents (alkyl halides, etc.). A solution to this problem is proton-coupled electron transfer (PCET), which is the alternating delivery of electrons and protons, instead of H2. [21–29] This approach however creates the challenge of preventing the protons from combining with the electrons to simply generate H2 and Schrock and Yandulov have nicely solved this problem by delivering the electrons with [Cp*2Cr] (Cp*= h-C5Me5) and using a weak and highly sterically hindered acid: an anilinium cation. From this success, H clearly has a demonstrated ability to react with one metal nitride in the Schrock PCETreduction of N2 to ammonia on molybdenum. However, this reaction is highly species-dependent, since another molybdenum-bound terminal nitride is found to be less Bronsted basic than even a neighboring coordinated tertiary phosphine. It is a truism of catalysis that one must only attempt reactions that are exergonic (DG8< 0), hence thermodynamically favorable to permit high conversion into product. When chosen well, this thermodynamic preference is what makes it possible to convert alkynes into arenes ... even with beach sand! The thermodynamics makes alkyne polymerizations sometimes dangerously explosive whereas olefin polymerizations are harder to accomplish, and less exergonic, since the entropy change is unfavorable. Likewise there are many cyclizations of diynes and enynes, but fewer with dienes. The Huisgen “click reaction” cyclizing alkyne and azide to a triazole is easily accomplished, since it is hugely exergonic. On the negative side, hydrogenation of N2 to ammonia is not very exothermic, so, coupled with an unfavorable DS8, this approach to ammonia production is thermodynamically challenging. The limited quantity of experimental organonitrogen thermodynamic data is a hindrance to establishing what reactions are exergonic; some body of data exists, but less than desired. With the broadening availability of density functional methods, which give chemically useful accuracy even for larger molecules and those outside the experimental database, this situation is improved. We apply this approach herein to an important problem, that of choosing reagents [*] Dr. J. G. Andino, S. Mazumder, Dr. K. Pal, Prof. K. G. Caulton Department of Chemistry, Indiana University 800 East Kirkwood Avenue, Bloomington, IN 47405 (USA) E-mail: caulton@indiana.edu