Activation of nitrogen heterocycles towards the fundamental reactions of hydrodenitrogenation catalysis: Transition metal mediated carbon-nitrogen bond cleavage, hydrogenation, and ring degradation.

Abstract

Treatment 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.