Ni diimine complexes

The results of the MD simulations clearly demonstrate that the insertion starting from the higher energy isomers of the ethylene-chelate complexes in which the chelating bond has been broken have much smaller activation barriers, that are comparable to those observed in ethylene homopolymerization. This, however, does not explain the differences in the copolymerization activity of Pd and Ni-diimine complexes, as the barriers for the ethylene insertion into Ni-alkyl bond are smaller (14.2 kcal/mol) than those for Pd-alkyl bond (16.8 kcal/mol). Thus, it may be concluded that the ethylene insertion following the insertion of the polar monomer is not a crucial factor for the diimine catalyst copolymerization activity. It is the initial poisoning of the catalyst by formation of the... 

The steric environment for iron complexes with tridentate pyridine-bis(imine) ligands is similar to that for the Ni diimine complexes, except that the two metal-bound halogen atoms, which are presumably transformed into a polymerization site upon activation, are located in the plane perpendicular to the Fe-N3 plane (9, Figure 6.2), while they are in the Ni-N2 plane of the square planar Ni complexesIn the iron complex-catalyzed polymerizations, the propylene monomer is inserted in a highly regioregular 2,1 -fashion and exclusively yields 1 -propenyl chain ends. The polypropylene that is produced is prevailingly isotactic (up to 67% mmmm at —20 C 69% mm at 0 °C) irrespective... 

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. 

In early studies, reaction of the Ni(n) complexes (59) and (60) of the trans and cis (diimine) isomers of the Curtis macrocycle with nitric acid yielded the tetraimine species (294) and (295), respectively. There is strong evidence that these reactions proceed via Ni(m) intermediates... 

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... 

Ligands such as aniline (an), 1,2-diaminobenzene (dab, o-phenylenediamine) and 2,2 -dia-minobiphenyl886 887 are classed separately, not because their ability to bind to a central metal is any less than the ligands discussed previously, but because of their potential non-innocent behavior888 with respect to internal redox reactions. Indeed, the dark blue complex isolated from the air oxidation of Con/dab in aqueous ethanol (a conventional route to yellow Co(diamine)3+ systems) has been shown to have structure (117) with five-coordinate Co11.888 Related diimine complexes have been reported for Ni11889 as well as the conventional Ni(dab)(+,890 Co(dab)3+ 891 and Pt(dab) + 892 systems. 

We will start by discussing the calculations on ethylene polymerization. In this case, the issues are mostly polymer length and the formation of linear or branched polymers. The first DFT/MM applications on this topic were published near simultaneously in 1997 and 1998 by the groups of Ziegler [36] and Morokuma [37, 38] on the reaction of ethylene polymerization catalyzed by cationic diimine Ni(II) complexes of the type (ArN=C(R)-C(R)=NAr)-Ni-CFl3+. Shortly before this, these species (one of them is... 

Fig. 2 DFT model (left) and full DFT/MM (right) descriptions commonly used in DFT/MM calculations of olefin polymerization catalyzed by Ni(II) diimine complexes...

A related topic that was already discussed in these first DFT/MM works is that of branching. The scheme shown in Fig. 1 would always produce always a linear polymer if ethylene was used as olefin. But a simple process of / C-H oxidative addition/reductive elimination, coupled with olefin rotation, can produce a branched polymer, as shown in Fig. 4. Calculations on the branching process for cationic diimine Ni(II) complexes [36, 37] indicated a small increase between 0.9 and 2.5 kcal/mol in the barrier for this process, associated with the introduction of the bulky substituents in the catalysts. 

In general, oc-olefins can be made by this process using a Ni[II] complex of an oc-diimine as a catalyst, where the Ni[II] complex is obtained using Ni[0], Ni[I] or Ni[II] precursors. 

O-complexes and the chelate opening prior to the ethylene insertion that seems to be responsible for differences in catalytic activity of the Ni- and Pd-diimine complexes. 

Square-planar metallo(diimine)(dithiolene) complexes generally display intense, solvatochromatic absorptions in the visible region of the spectrum that are not found in the corresponding metallo-bis(dithiolene) or metallo-bis (diimine) complexes. Futhermore, the LLCT transition energy does not vary appreciably as a function of the metal ion. Extended Hiickel calculations on Ni, Pt, and Zn metallo(diimine)(dithiolene) complexes indicate that the HOMO is comprised almost entirely of dithiolene orbital character (Figure 2), while the LUMO was found to possess essentially all diimine n orbital character (112, 252, 268). In stark contrast to the spectra of square-planar Ni and Pt metallo (diimine)(dithiolene) complexes, the psuedo-tetrahedral complexes of Zn possess extremely weak LLCT transitions. Now, it is of interest to discuss the differences in LLCT intensity as a function of geometry from a MO point of view. This discussion should help to explain important orientation-dependent differences in photoinduced electron delocalization and charge separation. 

The pooled encoded format was also used by Boussie and co-workers (179, 180) to prepare and screen Pd- and Ni-diimine ligand complexes as olefin polymerization catalytic systems. The successful application of deconvolutive or encoding methods to medium-large SP catalytic system pool libraries should become routine in the future. Careful validation of the synthetic scheme, the screening, and the detection methods will ensure the applicability of HTS of SP catalytic pool libraries to each specific transformation investigated. 

Square coplanar bonds are formed only in complexes with certain polydentate ligands or combinations of ligands such that steric factors prevent a tetrahedral configuration. They include the phthalocyanine (low-spin), the bis-salicylaldehyde diimine complex (a) (in which the maximum deviation of any atom from the mean plane of the complex is 0 6 A and Co-O(N) is 1-85 A), and the ion (b) in the salt [Co(mnt)2] [N(C4H9)4] 2. The magnetic moment of this salt (3-92 BM) corresponds to 3 unpaired electrons (d" ). The Ni((ii) compound is isostructural. Ions [M(mnt)2] are formed by Ni(m) and Cu(iii). The similar ion formed by... 

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