Activation of nitrogen heterocycles towards the fundamental reactions of hydrodenitrogenation catalysis: Transition metal mediated carbon-nitrogen bond cleavage, hydrogenation, and ring degradation.
AuthorGray, Steven Daniel.
Committee ChairWigley, David
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractTreatment of the η(N,C)-pyridine complex [η²(N,C)-2,4,6-NC₅ᵗBU₃H₂]TA(DIPP)₂Cl (DIPP = 2,6-diisopropylphenoxide) with LiBEt₃H affords the C-N bond scission product (DIPP)₂Ta(=NCᵗBu=CHCᵗBu=CHCHᵗBu) (4). Reaction of [η²(N,C)-2,4,6-NC₅ᵗBU₃H₂]TA(DIPP)₂Cl with the appropriate alkyl lithium or Grignard reagent provides the alkyl derivatives [η²(N,C)-2,4,6-NC₅ᵗBU₃H₂]TA(DIPP)₂R [R = Me (5); Et (6); ⁿPr (7); ⁿBu (8); Ph (9); CH₂SiMe₃ (10)] in high yield. The molecular structure of the ethyl complex, [η²(N,C)-2,4,6-NC₅ᵗBU₃H₂]TA(DIPP)₂Et (6) has been determined. Upon thermolyzing complexes 5 - 9, a metal-ta-pyridine ligand alkyl migration is effected and the C-N bond cleavage products (DIPP)₂Ta(=NCᵗBu=CHCᵗBu=CHCHᵗBuR) [R = Ph (11); Me (12); Et (13); ⁿPr (14); ⁿBu (15)] are formed. Kinetic and mechanistic studies of the 5 → 12 conversion, indicate that methyl migration is strictly intramolecular; thus a formal endo attack on the η²(N,C)-pyridine ligand has occurred. While (DIPP)₂Ta(=NCᵗBu=CHCᵗBu=CHCHᵗBu) (4) and (DIPP)₂Ta(=NCᵗBu=CHCᵗBu=CHCᵗBuPh) (11) are indefinitely stable and are structurally characterized, the β-hydrogen containing alkyl derivatives, (DIPP)₂Ta(=NCᵗBu=CHCᵗBu=CHCᵗBuCHR') [R' = H (12); Me(13); Et (14); ⁿPr (15)] decompose to provide the metallapyridine complex, [(DIPP₂Ta(μ- NCᵗBu=CHCᵗBu=CH)₂ (16) and the t-butyl-substituted alkenes ᵗBuCH=CHR', respectively. The structure of 16 is reported. Labelling studies reveal the source of the ᵗBuCH=CH₂ in the 5 → 12 → 16 conversion and the decomposition of 12 to 16 and ᵗBuCH=CH₂ is proposed to proceed via the eight-membered, ring-expansion isomer, (DIPP₂)Ta(=NCᵗBu=CHCᵗBu=CHCᵗBuCH₂) (17). An acetonitrile adduct of this intermediate, (DIPP)Ta(=NCᵗBu=CHCᵗBu=CHCᵗBuCH₂)(MeCN)₂ (17-MeCN), has been trapped. An overall mechanism for the decomposition (DIPP)₂Ta(=NCᵗBu=CHCᵗBu=CHCᵗBuCH₂R') [R' = H (12); Me(13); Et (14); ⁿPr (15)] to [(DIPP)₂Ta(μ-NCtBu=CHCᵗBu=CH)₂ (16) and ᵗBuCH=CHR', based on these observations, is proposed. The heterocyclic complexes [η¹(N)-QUIN]Ta(Oar)₃Cl₂ (18) and [η ¹(N)-6MQ]- Ta(Oar)₃Cl₂ (19) (QUIN = quinoline, and 6MQ = 6-methylquinoline) are prepared from Ta(Oar)₃Cl₂(OEt₂) and QUIN or 6MQ. [η¹(N)-6MQ]Ta(OAr)₂Cl₃ (20) is prepared similarly from Ta(OAr)₂Cl₃(OEt₂). Upon rapid, two-electron reduction of these complexes, an η¹(N) → η²(N,C) bonding rearrangement is effected and the thermally sensitive, d² species [η²(N,C)-QUIN]Ta(Oar)₃ (21), [η²(N,C)-6MQ]Ta(Oar)₃ (22), and [η²(N,C)-6MQ]Ta(OAr)₂- Cl(OEt₂) (25) can be isolated. The PME₃ adducts [η²(N,C)-QUIN]Ta(Oar)₃(PMe₃) (23) and [η²(N,C)-6MQ]Ta(Oar)₃(PMe₃) (24) can be prepared by simple coordination of PMe₃ to the base-free compounds 21 and 22. The quinoline ligand of [η²(N,C)-QUIN]Ta(Oar)₃ (21) is readily hydrogenated under mild reaction conditions to afford 1,2,3,4- tetrahydroquionline. When Ta(Oar)₂Cl₃(OEt₂) is reduced by one electron in the presence of QUIN or 6MQ, the d¹ bis(ligand) complexes [η¹(N)-6MQ]₂Ta(Oar)₂Cl₂ (26) and [η¹(N)- QUIN]₂Ta(OAr)₂Cl₂ (27) can be isolated. The relevance of these studies with respect to industrial hydrodenitrogenation (HDN) catalysis is discussed.