Ethylene oligomerization promoted by chromium complexes bearing pyrrolide-imine-amine/ether tridentate ligands.

Chromium(iii) complexes [CrCl2(L)(THF)] based on monoanionic tridentate ligands [, L = {2-(C4H3N-2'-CH[double bond, length as m-dash]N)C2H4NHPh}; , L = {5-tert-butyl-2-(C4H2N-2'-CH[double bond, length as m-dash]N)C2H3NHPh}; , L = {2-(C4H3N-2'-CH[double bond, length as m-dash]N)C2H4OPh}] have been prepared. Complexes and were converted into the monomeric acetonitrile adducts [CrCl2(L)(NCMe)] [, L = {2-(C4H3N-2'-CH[double bond, length as m-dash]N)C2H4NHPh}; , L = {5-tert-butyl-2-(C4H2N-2'-CH[double bond, length as m-dash]N)C2H3NHPh}] by reaction with acetonitrile at room temperature. All Cr complexes were characterized by IR spectroscopy, elemental analysis, magnetochemistry for , and by X-ray crystallography for and . Upon activation with methylaluminoxane (MAO), chromium precatalysts and showed good activity in ethylene oligomerization (TOF = 47.0-57.0 × 10(3) (mol ethylene)(mol Cr)(-1) h(-1) at 80 °C), producing mostly oligomers (93.0-95.6 wt% of total products). On the other hand, under identical oligomerization conditions, /MAO behaved as a polymerization catalyst generating predominantly polyethylene (73.0 wt%). However, the catalytic behavior of the precatalyst can be adjusted by varying the MAO-to-Cr ratio. Thus, the use of 500 equiv. causes a dramatic shift from polymerization to ethylene oligomerization, eventually producing mainly lighter α-olefin fractions [α-C4 (68.7 wt%) and α-C6 (19.2 wt%)]. A further increase in the amount of MAO (1000 equiv.) leads to a more balanced distribution of oligomers, with a drastic decrease in the α-C4 and increase in the α-C8 fractions.

Aluminum and zinc complexes based on an amino-bis(pyrazolyl) ligand: synthesis, structures, and use in MMA and lactide polymerization.

The coordination chemistry of bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine (1, LH) with aluminum- and zinc-alkyls has been studied. Reaction of 1 with AlR3 affords the adducts [LH] x AlR3 (R = Me, 2; Et, 3), which undergo alkane elimination upon heating to yield the amido complexes [L]AlR2 (R = Me, 4; Et, 5). Reaction of LiO(iPrO)C=CMe2 with 2 proceeds via N-H deprotonation to give Li[L]AlMe3 (6), while the former enolate adds to 4 to generate [Me2C=C(OiPr)OLi] x [L]AlMe2 (7). Similarly, the 1:1 reaction of ZnEt2 with 1 gives [LH] x ZnEt2 (9), which is transformed into [L]ZnEt (10) upon heating. When an excess of ZnEt2 was used in the latter reaction, the bimetallic complex [L]ZnEt x ZnEt2 (11) was isolated beside 10. Performing the same reaction in the presence of O2 traces yielded selectively the dinuclear ethyl-ethoxide complex [L]Zn2Et2(mu-OEt) (12), which was alternatively prepared from the reaction of 10 and ZnEt(OEt). Zinc chloride complexes [LH] x ZnRCl (R = Et, 13; p-CH3C6H4CH2, 14) and [L]ZnCl (15) were prepared in high yields following similar strategies. Ethyl abstraction from 10 with B(C6F5)3 yields [L]Zn+EtB(C6F5)3- (16). All complexes have been characterized by multinuclear nuclear magnetic resonance (NMR), elemental analysis, and single-crystal X-ray diffraction studies for four-coordinate Al complexes 2, 4, and 6 and Zn complexes 9-12 and 14. Aluminate species 6 and 7 initiate the polymerization of methyl methacrylate, and the monomer conversions are improved in the presence of neutral complexes 2 or 4, respectively; however, these methyl methacrylate (MMA) polymerizations are uncontrolled. Polymerization of rac-lactide takes place at 20 degrees C in the presence of zinc ethoxide complex 12 to yield atactic polymers with controlled molecular masses and relatively narrow polydispersities.

1,5-Diphenyl-4,8-bis(3-phenylpyrazol-1-yl)pyrazabole.

The six-membered B2H4 ring of the title compound, C36H30B2N8, adopts a slightly distorted boat conformation, with the terminal B substituents in a trans orientation. One 3-phenylpyrazolyl group is in an equatorial position, whereas the second is in an axial position with respect to the plane defined by the B atoms.