Contents 1. Introduction 3031 1.1. Definition of the β-Diketiminato Ligand and Scope of This Review 3031 1.2. Some Significant Developments 3032 2. Preparation of β-Diketimines and Their Metal Complexes 3034 2.1. Synthesis of a β-Diketimine from a β-Diketone or 1,1,3,3-Tetraalkoxypropane 3034 2.2. Synthesis of a Metal β-Diketiminate by Nitrile or Isonitrile Insertion into a Metal−Alkyl Bond 3034 2.3. Various Other Synthetic Routes to Metal β-Diketiminates 3035 2.4. Other Synthetic Routes to β-Diketimines 3035 3. Bonding Modes and Structural Features of β-Diketiminatometal Complexes 3037 4. Discussion of Specific β-Diketiminatometal Complexes 3039

The composition of methylalumoxane (MAO) and its interaction with trimethylaluminum (TMA) have been investigated by a combination of chemical, spectroscopic, neutron scattering, and computational methods. The interactions of MAO with donor molecules such as THF, pyridine, and PPh 3 as a means of quantifying the content of "free" and "bound" TMA have been evaluated, as well as the ability of MAO to produce [Me 2 AlL 2 ] + cations, a measure of the electrophilic component likely to be involved in the activation of single-site catalysts. THF, pyridine, and diphenylphosphinopropane (dppp) give the corresponding TMA−donor ligand complexes accompanied by the formation of [Me 2 AlL 2 ] + cations. The results suggest that MAO contains not only Lewis acid sites but also structures capable of acting as sources of [AlMe 2 ] + cations. Another unique, but still unresolved, structural aspect of MAO is the nature of "bound" and "free" TMA. The addition of the donors OPPh 3 , PMe 3 , and PCy 3 leads to the precipitation of polymeric MAO and shows that about one-fourth of the total TMA content is bound to the MAO polymers. This conclusion was independently confirmed by pulsed field gradient spin echo (PFG-SE) NMR measurements, which show fast and slow diffusion processes resulting from free and MAO-bound TMA, respectively. The hydrodynamic radius R h of polymeric MAO in toluene solutions was found to be 12 ± 0.3 Å, leading to an estimate for the average size of MAO polymers of about 50−60 Al atoms. Small-angle neutron scattering (SANS) resulted in the radius R S = 12.0 ± 0.3 Å for the MAO polymer, in excellent agreement with PFG-SE NMR experiments, a molecular weight of 1800 ± 100, and about 30 Al atoms per MAO polymer. The MAO structures capable of releasing [AlMe 2 ] + on reaction with a base were studied by quantum chemical calculations on the MAO models (OAlMe) n (TMA) m for up to n = 8 and m = 5. Both −O−AlMe 2 −O− and −O−AlMe 2 −μ-Me− four-membered rings are about equally likely to lead to dissociation of [AlMe 2 ] + cations. The resulting MAO anions rearrange, with structures containing separated Al 2 O 2 4-rings being particularly favorable. The results support the notion that catalyst activation by MAO can occur by both Lewis acidic cluster sites and [AlMe 2 ] + cation formation.

Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)n (AlMe3)m. The thermodynamically favored pathway of MAO formation is followed up to n=8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co-catalysts show the decisive role of the associated TMA in forming the catalytically active sites. Catalyst activation can take place either by Lewis-acidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO-derived AlMe2(+) cation. Thermodynamics suggest that activation through AlMe2(+) transfer is the dominant mechanism because sites that are able to release AlMe2(+) are more abundant than Lewis-acidic sites. The model catalyst system is demonstrated to polymerize ethene.

We have carried out a systematic computational study on olefin polymerization by metallocene/borate catalysts, using three metallocenes: Cp 2 ZrMe 2 (Cp), rac-SiMe 2 -bis(1-(2-Me-(4-PhInd))-ZrMe 2 (4-PhInd), and rac-SiMe 2 -bis(1-(2-Me-(4,5-BenzInd))ZrMe 2 (4,5-BenzInd). Detailed reaction pathways, including the structure of the catalytically active ion pair, anion displacement, chain propagation, and chain termination steps, are reported for ethene homopolymerization, alongside with investigation of ethene−propene copolymerization reactions. Initially, all catalysts form inner-sphere ion pairs ([L 2 ZrMe] + −[B(C 6 F 5 ) 4 ] − ) with a direct Zr−F interaction, which is weak enough to be displaced by the incoming monomer. In comparison to Cp, the bulky and electron-rich 4-PhInd and 4,5-BenzInd show higher barriers for anion displacement but lead to relative stabilization of the resulting π complexes. 4-PhInd enables the most feasible propene uptake, and both catalysts suppress the chain termination reactions relative to Cp. The borate counterion is shown to have a minor influence after the catalyst activation step. ■ INTRODUCTION Single-site α-olefin polymerization catalysis is an industrially important application of Group 4 organometallic complexes, 1 particularly of metallocenes. The metallocene complexes need an activator to form a catalytically active [L 2 MMe] + [A] − ion pair. 2 The catalytic properties of the resulting ion pair are highly dependent on both the ligand framework of the metallocene cation [L 2 MMe] + and the structure of the counterion [A] − . 2−4 Typical activators used in the process i n c l u d e m e t h y l a l u m i n o x a n e ( M A O ) , 5 , 6 t r i s -(pentafluorophenyl)borane (B(C 6 F 5 ) 3 ), 7 and organoborates such as [CPh 3 ] + [B(C 6 F 5 ) 4 ] − , the last giving rise to the weakly coordinating tetrakis(perfluoroaryl)borate counterion [B-(C 6 F 5 ) 4 ] − . 7,8Regarding catalyst properties, the stability of the ion pair plays a key role. 9,10 Experimental data suggest that weak coordination of the counterion is usually beneficial for catalytic activity. 11 On the other hand, the counterion needs to stay close to the metallocene cation in order to compensate for its positive charge. 12 Optimally, the counterion provides the needed stabilization for the electron-deficient metallocene cation but is easily displaced by the incoming monomer.The overall mechanisms of catalyst activation are not precisely understood. In the case of MAO, two mechanisms have been proposed: (1) abstraction of a leaving group from the precatalyst by a Lewis acidic site of MAO 2 and (2) abstraction of an AlMe 2 + end group from the MAO by the precatalyst followed by dissociation of AlMe 3 by the incoming monomer. 13,14 Recent computational studies have suggested mechanism 2 to dominate by thermodynamic considerations. 15 In any case, all computational studies dealing with the MAO activator suffer from its elusive structure, thereby requiring the use of model systems. In that respect boron activators, formin...

The influence of methylaluminoxane (MAO) catalyst activators of different concentrations and preparative histories on the performance of 1-hexene polymerisations was investigated by kinetic methods, using rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2 as the standard catalyst precursor. Fast sampling and analysis allow the time dependence of monomer conversion and the growth of the number-average polymer molecular weight to be determined at a sufficiently short timescale to make this a feasible method for routine catalyst evaluation. Differences in productivity, polymer molecular weight and active species count are shown to be primarily a linear function of the trimethylaluminium concentration. The results in toluene and heptane as solvents are compared; the data show that the inferior performance in heptane is due to a substantially lower active species concentration.