The Synthesis and Coordination Chemistry of Bulky Beta-Triketimines and Application of their Nickel(II) Derivatives as Ethylene Polymerisation Catalysts

Abstract

The reaction of imidoyl chlorides IMC1-9 with lithium β-diketiminates Li(BDKx) (x = iPr, iPr2, tBu, Me3, iPr2/OMe) yields β-triketimines L1-16 which represent a new class of facially-coordinating neutral tridentate nitrogen ligands. The synthetic route is highly modular, allowing for wide variation of substitution patterns and thus fine control over the steric demands of the ligands. In all but two examples (L11 and L12) the β-triketimines exist in solution either solely as their enamine-diimine tautomers, or as equilibrium mixtures of the two. The processes of solution-phase equilibration have been studied by NMR spectroscopy. The molecular structures of L1, L10 and L11 are presented, and for each case a single differing tautomer or isomer is observed exclusively. The majority of the ligands yield fac-(L)M(CO)3 adducts upon reaction with group 6 metal carbonyls, except in cases of extreme steric bulk. The β-triketimines are relatively weak sigma-donors, as determined by the CO stretching frequencies in (L)M(CO)3. The molecular structures of the isomorphous pair (L1)M(CO)3 (M = Cr, Mo) reveal porous infinite network arrangements generated by aryl-aryl and CH---OC interactions. The direct reaction of β-triketimines with ZnCl2 in most cases gives [(L)ZnCl]2[Zn2Cl6] complexes, whilst in the presence of either NaBArF4 or AgOTf the corresponding [(L)ZnCl][BArF4] and [(L)ZnCl][OTf] species are obtained. The complexes invariably feature four-coordinate, cationic zinc centres, except in the case of [(η4-L16)ZnCl][BArF4] where a five-coordinate complex is formed by additional coordination of a single ligand methoxy group. The reaction of [(L)ZnCl][BArF4] with TlOEt yielded not the desired zinc alkoxides, but novel [(L)Tl][BArF4] complexes, the solid-state structures of which display thallophilic interactions. The direct reaction of L8 and L10-12 with NiBr2(DME) gives the corresponding four-coordinate [(L)NiBr]2[NiBr4] complexes, whereas the products obtained with weakly-coordinating anions are dependent on the ligand bulk and the size of the anion. With the large BArF4- ion the formation of the five-coordinate dimeric [{(L)Ni(μ-Br)}2][BArF4]2 (L = L1, L3, L5, L7, L8, L12) is strongly favoured, except for the bulkiest ligands where the four-coordinate [(L)NiBr][BArF4] (L = L10, L11) are obtained. The smaller triflate ion generally favours the formation of [(L)NiBr][OTf] (L1, L8, L10-12) except in the case of [{(L)Ni(μ-Br)}2][OTf]2 (L = L3, L5). L15 and L16 act invariably as tetradentate donors to Ni(II), yielding the five-coordinate [(η4-L)NiBr][X] (X = BArF4, OTf) and [(η4-L16)NiBr]2[NiBr4]. The very bulky L6 coordinates in a bidentate mode to Ni(II), giving the four-coordinate [(η2-L6)NiBr(THF)][BArF4] and (η2-L6)NiBr2. [{(L)Ni(μ-Br)}2][BArF4]2 (L = L3, L5, L7), [{(L)Ni(μ-Br)}2][OTf]2 (L = L3, L5), [(η2-L6)NiBr(THF)][BArF4] and (η2-L6)NiBr2 show moderate catalytic activity for the polymerisation of ethylene in combination with MAO. All other complexes tested are inactive, either due to steric factors or the inability of certain β-triketimines to support catalytically active square-planar Ni(II) species. A proposed mechanism for catalyst activation is presented. Elastomeric polyethylene of very low crystallinity is obtained, with branching rates in the range 43-84 branches per 1000 C. 13C NMR spectroscopy reveals the presence of all short-chain branches from methyl to hexyl, as well as longer branches and pairs of branches.

Date of Award	1 Aug 2011
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Francis Mair (Supervisor)