Continued investigations of the catalytic reduction of N₂ to NH₃ by molybdenum triamidoamine complexes

Abstract

A study of the effects of employing different solvents and the introduction of dihydrogen during the catalytic reduction of dinitrogen to ammonia with [HIPTN 3N]Mo complexes was completed. During a catalytic reaction, the use of different solvents including fluorobenzene, THF, and toluene did not produce catalytic turnover of ammonia from dinitrogen. The acid source used was [CollH][BAr' 4] which was soluble in each of these solvents and dihydrogen was generated. It was found that dihydrogen acts as a powerful inhibitor of the catalytic reduction of dinitrogen to ammonia. In hopes of isolating species relevant to the proposed structures of [HIPTN 3N]MoH2, where an amide arm has become unbound from the metal center, [HIPTN 3N]MoPMe 3 was investigated. [HIPTN3N]MoPMe3 can be synthesized cleanly and oxidized to [HIPTN 3N]MoPMe 3[BAr'4] by treatment with H[(Et 20) 2][BAr'4]. An interesting species, [HIPTN 3N]Mo(H)PMe 3 was developed which was unstable in vacuo. The JPH value for the molybdenum hydride was 111 Hz, which is higher than a typical M(H)PMe3 JPH. [HIPTN 3N]W(H)PMe 3 was synthesized in an effort to elucidate the nature of the metal phosphine and metal hydride binding. The JPH was found to be 117 Hz for [HIPTN 3N]W(H)PMe 3. The JWH 12.5 Hz and Jwp = 119 Hz values for [HIPTN 3N]W(H)PMe 3 suggested that the hydride and phosphine were bound to the tungsten center, and that the phosphine and hydride have some interaction. New ligand systems based on a C, symmetric framework were also explored. [(TerPh)HIPT 2N3N]Mo complexes were synthesized but provided no catalytic reduction of dinitrogen to ammonia. The decomposition of [(TerPh)HIPT 2N3N]MoN 2H was found to be slower (t1/2 = 15 hours) than previously investigated C, symmetric [N3N] complexes (< 10 minutes). A new electron donating symmetric ligand based on the [HIPTN 3N]Mo was synthesized containing a methoxy group, [pMeOHIPTN 3N]Mo. This complex was shown to reduce dinitrogen to ammonia catalytically (6.5 equiv.). Other symmetric species, [DTBATN3N]Mo and [DTBAN3N]MoN, were developed and found to be crystalline but not catalytically active in the reduction of dinitrogen to ammonia. Thesis Supervisor: