Nickel diimine complexes

DFT/MM calculations on ethylene polymerization by nickel diimine complexes have been applied within Car-Parrinello molecular dynamics simulations [40, 41]. A first set of calculations was used to refine the computed energy barrier for the termination step. The enthalpy barrier computed in the calculations described above was 18.6 kcal/mol, a value which decreased to 14.8 kcal/mol at 25 °C in the molecular dynamics calculation, in better agreement with experiment [40]. A second study analyzed the capture of the olefin by the catalyst [41], and found that this process, which has no en-thalpic barrier, has an entropic barrier. 

Figure 4-26. Unconstrained MD simulations for methyl acrylate bound to the palladium/nickel diimine complex through the oxygen (top-right/bottom-right) and the C=C (top-left/bottom-left) functionalities. The three panels in each of the four graphs represent variations in the metal-carbon (top two panels) and metal-oxygen (bottom panel) distances. The simulations were carried out at 300 K for all the systems, and 700 K for the local minima, as indicated...

Figure 10 Precursors for soluble polymerization catalysts ansa-zirconocene complex (left), constrained-geometry titanium complex (middle) and nickel diimine complex (right).

The general catalytic performance of these metal complexes in polymerization of olefins was screened by the following standard procedure The complexes (50 or 100 pmol) were activated with 100 mole equivalents of methylalumoxane (MAO) in toluene solution. The polymerization reaction was carried out at a temperature of 30°C, during which ethene was added with a flow of 40 L h"t After 4.5 h, the mixture was quenched with methanol, the solid polymer isolated, washed and dried. For benchmarking a nickel diimine complex [12a] with 2,6-(di-isopropyl) phenyl substituents at the imine nitrogen atoms (133) was also included. Tab. 3.2 shows the activity and polymer data. 

Wang, Q. and Liu, R 2005. Dual bimodal polyethylene prepared by intercalated silicate with nickel diimine complex. Journal of Polymer Science, Part A Polymer Chemistry 43 5506-5511. 

Coates and coworkers employed the chiral, Ca-symmetric nickel diimine complex 70 (Figure 9.9) to control polymer microstructure for living polymerization of a-olefins (Rose et al., 2006). The ability of nickel diimine catalysts to undergo successive ) -hydride eliminations/reinsertions may lead to regioirregular polymers with less branching than expected (Ittel et al., 2000). Careful tailoring of reaction conditions (low temperatures and... 

Merna, J., Host alek, Z., Peleska, J., and Roda, J. (2009) Living/controlled olefin polymerization initiated by nickel diimine complexes The effect of ligand ortho substituent buUciness. Polymer, 50,5016-5023. 

McLain et polymerized cyclopentene by late transition metal catalysts using MAO and borate-activated nickel and palladium diimine complexes. The nickel diimine complexes produce crystalline materials showing a ds-1,3 enchainment with a melting point of 240-330 °C. The hydroligomers were mainly atactic. Palladium catalysts gave pure atactic polymers. It is also possible to polymerize substituted cyclopentenes such as 3-methyl- or 3-ethyl-cyclopentene. 

In a series of studies of the spectroscopy and photochemistry of nickel(O) -a-diimine complexes, the structural differences among the complexes NiL2 and Ni(CO)2L (L Q-diimine) have been examined by means of molecular orbital calculations and electronic absorption Raman resonance studies.2471, 472 Summing up earlier work, the noninnocence of a-diimine ligands with a flat — N=C—C=N— skeleton in low-valent Ni chemistry and the course of substitution reactions of Ni° complexes with 1,4-diaza-1,3-dienes or a,a -bipyridine have been reviewed.2473... 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

Nickel(HI) complexes of formula [Ni(N—N)3](Q04)3 with N—N = 2,2 -bipyridyl and 1,10-phenanthroline and substituted derivatives have been isolated as products of the electrolysis in acetonitrile of the corresponding nickel(II) salts. Electrode potentials for the NPVNi11 couples in MeCN (0.1MNaClO4) were in the range 1.51-1.82 V.3079 The same diimine complexes have also been formed in strong acidic solutions.3080,3081... 

Electrochemical studies (62, 64,106,137) have shown that nickel(III) complexes with macrocycles, peptides, and diimine ligands are rel-... 

As mentioned above, reactions of this type have been widely used in the synthesis of macrocyclic ligands. Indeed, some of the earliest examples of templated ligand synthesis involve thiolate alkylations. Many of the most important uses of metal thiolate complexes in these syntheses utilise the reduced nucleophilicity of a co-ordinated thiolate ligand. The lower reactivity results in increased selectivity and more controllable reactions. This is exemplified in the formation of an A -donor ligand by the condensation of biacetyl with the nickel(n) complex of 2-aminoethanethiol (Fig. 5-78). The electrophilic carbonyl reacts specifically with the co-ordinated amine, to give a complex of a new diimine ligand. The beauty of this reaction is that the free ligand cannot be prepared in a metal-free reac-... 

Figure 5-78. The nickel(n) complex of 2-aminoethanethiol reacts smoothly with biacetyl at nitrogen to give a diimine ligand.

Unfortunately, this macrocycle cannot be prepared as a free ligand by this method. The starting diimine 6.10 could apparently be prepared from 2-aminoethanethiol and biacetyl. However, we saw in Fig. 5-79 that the direct reaction of 2-aminoethanethiol with 1,2-dicarbonyls leads to a range of cyclic and acyclic products, rather than to products such as 6.10. However, we also saw in Fig. 5-78 that the nickel(n) complex (6.12) of the 6.12 could be obtained if the reaction was conducted in the presence of an appropriate salt. 

Magnetochemical measurements for the solid phosphorus-containing zinc, iron, nickel, and cobalt tris-diiminates showed that the first two are diamagnetic and low-spin complexes. The cobalt and nickel(II) complexes proved to be high-spin clathrochelates with magnetic moments of 4.91BM (S=3/2) and 3.11BM (S=l), respectively [92, 93]. 

Following the pioneering studies of Keim [19] and Fink [20] in the 1970s and 1980s, mainly Brookhart and coworkers have reported on the development of palladium(II) and nickel(ll) diimine complexes (Fig. 2.4, F, G) that polymerize ethene to high molecular weight polymers that have a branched microstructure [21]. 

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