Syntheses and characterization of compounds of niobium and tantalum with phenylimido and phenoxide ligation.

Abstract

The metallacyclopentadiene (DIPp)₂CITa(CCMe₃=CHCH=CCMe₃) (α, α') (DIPP=2,6-diisopropylphenoxide) is isolated from the 2 electron reduction of Ta(DIPP)₂CI₃(OEt₂) in the presence of excess Me₃CC=CH. The α, α' isomer represents the kinetic product of this reaction since it can be thermolyzed to produce (DIPP)₂CITa(CCMe₃=CHCCMe₃=CH) (α, β’). The reactivity of these two isomers toward the addition of ethylene and alkynes has been investigated and found to be driven primarily by steric effects with the less hindered α, β’ isomer showing a more comprehensive chemistry. The rearrangement of α, α' to α, β’ was followed through several kinetic and mechanistic studies. This data support a mechanism involving dissociation of the a,a' isomer to the metallacyclopropene, (DIPP)₂CITa(HC=CCMe₃), and free Me₃CC=CH. These intermediates then recouple to form the thermodynamic product, the α, β’ isomer. The reaction of TA(DIPP)₃Cl₂(OEt₂) with quinoline or 6-methylquinoline (HC) produces the ligand exchange product (η¹-HC)Ta(DIPP)₃CI₂, the 2 electron reduction of either of these products yields [η²-(N,C)-HC]Ta(DIPP)₃ which can be isolated as the phosphine complex [η²-(N,C)-HC]Ta(DIPP)₃(PMe₃) upon reaction with trimethylphosphine. The reaction of either [η²-(N,C)-quinoline]Ta(DIPP)₃ or [η²-(N,C)quinoline] Ta(DIPP)₃(PMe₃) with hydrogen both in the presence and absence of Pd/C produces 1,2,3,4-tetrahydroquinoline and 2,2'-bisquinoline. The production of these compounds implicates the η²-(N,C) bonding mode in selective hydrogenation of heterocycles and ortho functionalization. The reaction of Nb(NEt₂)₂CI₃ with two equivalents of LiNHMes (Mes=2,4,6- C₆H₂Me₃) followed by addition of pyridine produces Nb(NMes)₂CI(py)₂. The reaction of Nb(NEt₂)₂CI₃ with 6 equivalents of LiNHMes produces the first tris imido complex of group 5, [(THF)₂Li][Nb(NMes)₃(NHMes)]. Comparable chemistry is seen with Ta. [(THF)₂Li][Nb(NMes)₃(NHMes)] reacts with lithium alkyls under kinetic conditions to produce [(THF)2Li][Nb(NMes)₃(R)] (R=Me, ⁿSu, ᵗSu, Me₃SiCH₂). Under thermodynamic conditions, however, the reaction of [(THF)₂Li][Nb(NMes)₃(NHMes)] with ᵗBuLi yields the first tetrakis imido complex of group 5, [(THF)₄Li₃][Nb(NMes)₄]