Nickel complexes 3-diimines

Mixed-ligand complexes such as nickel dithiolene diimine can be prepared via ligand substitution reactions (Eq. 6) (52). 

The UV-vis spectra of phosphorus-containing cobalt(II) and nickel(II) diiminates were obtained both in solution and in the solid state (the [CoPcc(BF)](BF4) and [NiPcc(BF)](BF4) complexes were diluted in the monocrystal of zinc clathrochelate). Pure crystals of... 

The complex [Ni(2,3-Me2[14]-l,3-diene-l,4,8,lI-N4)] [ZnCU] is square planar and low-spin. The visible spectra show bands near 21.3 kK (characteristic of square planar nickel(II)), near 26.1 kK (due to the imine functions), and near 35.1 kK. The infrared spectra of all of the nickel complexes prepared show absorptions near 3195 and 1595 cm assignable to the N—H stretching vibration and to the symmetric imine vibration, respectively. A strong sharp band also occurs near 1210 cm and is characteristic of the a-diimine function. The NMR spectrum of the perchlorate complex in nitromethane shows a methyl singlet at 2.33 ppm. The ligand can be hydrogenated on nickel(II) with Raney nickel and hydrogen to produce the fully saturated macrocyclic complex [Ni(2,3-Me2[14]-ane-1,4,8,1 1-N4] ... 

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]. 

Not only palladium, but many more non-metallocene late (and early) transition metal catalysts for the coordination polymerization of ethene and 1-olefins were reported [11]. Among the most significant findings in this area are the disclosures of novel highly active and versatile catalysts based on (i) bidentate diimine [N,N] nickel and palladium complexes [12], (ii) tridentate 2,6-bis(imino)pyridyl [N,N,N] iron and cobalt complexes [13], and (iii) bidentate salicyl imine [N,O] nickel complexes [14]. 

Nickel Complexes with N-Hetaryl 1,2-Diimine Ligands... 

Another unusual optical property is solvatochromism (27, 28). Most inorganic complexes have small solvent-dependent absorption spectra. Pronounced solvatochromism has not been extensively characterized for inorganic complexes in solution with the exception of a recent report on the electronic spectra of a variety of dipolar dithiolene a-diimine nickel complexes (29). 

Czv-Symmetric Catalysts. Syndiotactic polymers have been formed using metallocene catalysts where the polymer chain end controls the syndiospecificity of olefin insertion. Resconi has shown that Cp 2MCl2 (M = Zr. Hf) derived catalysts produce predominantly syndiotactic poly(l-butene) with an approximate 2 kcal/mol preference for syndiotactic versus isotactic dyad formation." At —20 °C. Cp 2HfCl2/MAO produces poly(l-butene) with 77% rr triads. Pellecchia had reported that the diimine-ligated nickel complex 30 forms moderately syndiotactic polypropylene at —78 °C when activated with MAO ([rr] = 0.80)." " Olefin insertion was shown to proceed by a 1.2-addition mechanism." in contrast to the related iron-based systems which insert propylene with 2.1-regiochemistry. ... 

Platinum complexes show none of the catalytic activity found in palladium and nickel complexes. This chemical inertness makes platinum a useful model for the more active catalysts. There has been a suggestion that platinum complexes of chiral a-diimines might lead to stereoselective olefin polymerization. Chiral camphor-based ligands have been employed in palladium complexes for ethylene polymerization, but there was no mention of stereoselectivity in hexene polymerizations. 

The active species for the PBI complexes is not as well characterized as in the nickel and palladium systems. It is assumed to be a cationic alkyl complex formed by reaction of the dihalo precatalyst with a cocatalyst such as methylaluminoxane (MAO). The resulting active species polymerizes ethylene at unusually high rates to form linear high-density polyethylene. Even at ethylene pressures as low as 1 atm, the polymerization is extremely exothermic and the crystalline polymer product rapidly precipitates from solution. Computational chemistry is proving to be of utility in understanding the mechanistic aspects of this chemistry. - Lower barriers to insertion, relative to the nickel a-diimine complexes, support the higher activity. 

Most recently, Grubbs group demonstrated that some neutral salicylaldimi-nato nickel(II) complexes, whose skeleton structure appears as lb in Fig. 1, show catalytic activities rivaling those of the diimine complexes [9]. This potentially opens the door to a new class of catalysts as the active sites derived from these nickel complexes are neutral, thus reducing the ion-pairing problems encountered in the current catalysts. 

Frontier with the subject of this paragraph, the use of early and late metal complexes combinations for reactor blending during ethylene polymerization can also be mentioned. In such a process, known since the 1980s, the early and late catalysts polymerize ethylene independently to afford an intimate mixture of polyethylene PE) chains of different structure. Although in this case there is no cooperative effect of the two metals from a molecular or mechanistic point of view, the beneficial simultaneous use of the two catalysts, such as a dichlorozirconocene and a diimine nickel complex (Scheme 58) ([147] and references therein), is found in the bulk physicochemical properties of the obtained PE. 

Fujii, K., Ishihama, Y, Sakuragi, T., Ohshima, M., Kurokawa, H., and Miura, H. 2008. Heterogeneous catalysts immobilizing a-diimine nickel complexes onto fluorotetrasilicic mica interlayers to prepare branched polyethylene from only ethylene. 

Correia, S. G. Marques, M. M. Ascenso, J. R. Ribeiro, A. R G Gomes, P. T. Dias, A. R. Blais, M. Rausch, M. D. Chien, J. C. W. Polymerization with TMA protected polar vinyl copolymers. II. Catalyzed hy nickel complexes containing alpha-diimine type hgands. J. Polym. Set, Part A Polym. Chem. 1999, 37, 2471-2480. 

FIGURE 18.8 Diimine nickel complexes used for CHD polymerization (An = acenaphthyl). 

Correia SG, Marques MM, Ascenso JR, Ribeiro AFG, Gomes PT, Dias AR, Blais M, Rausch MD, Chien JCW (1999) Polymerization with TMA-protected polar vinyl comonomers. II. Catalyzed by nickel complexes containing a-diimine-type ligands. J Polym Sci A Polym Chem 37 2471... 

As with iron(II), O Reilly et reported that nickel complexes with an a-diimine ligand (Ni-9), which is an analog of the precursor for a coordination polymerization catalyst, efficiently worked for controlling the radical polymerization of styrene. When coupled with 1-phenylethyl bromide as the initiator, the styrene polymerization with Ni-9 provided well-controlled molecular weights and MWDs (Mw/Mn= 1.15). Neutral Ni(II) acetylides (Ni-10 and Ni-11) were used for the polymerization of DMAEMA and MMA in conjunction with an organic halide as the initiator by Sun et Although judicious conditions, such as concentra-... 

Syndiotactic polypropylene is formed by the nickel(II)-diimine complex 67 (M=Ni) at low temperature ([rrrr]=0.80 at -78 °C,0.65 at 0 °C) [204,205]. Polymerization proceeds by 1,2-insertion and the stereochemistry is regulated under chain-end control. On the other hand, isotactic polypropylene can be prepared using the iron complexes 68 (M=Fe [mmmm]=0.55-0.67 at -20 °C) despite the low molecular weight of the polymer [206]. Polymerization proceeds via a 2,1-insertion mechanism by chain-end control. 

Complexes with pyridine-2,6-diimine ligands, five-coordinate [NiX2(174)] (X = C1, Br) or six-coordinate [Ni(174)2]X2, were usually assumed to have innocent neutral ligands. The X-ray structure and spectroscopic characteristics of [Ni(174)2](PF6) confirm that the complex contains the neutral ligand ([174] ) and a divalent nickel ion.579 The cyclic voltamogram of this complex in CH2C12 shows three reversible one-electron-transfer processes at = 1.19 V, —1.30 V, and — 1.82V (vs. Fc+/Fc), of which the first is a one-electron oxidation, while the other two correspond to two successive one-electron reductions. The spectroscopic data allow one to assign these processes as follows ([174]1 is a one-electron reduced radical form of [174] ) [Nini(174)°2]3+ [NiII(174)02]21 - " [NiI(174)°2]+ = " [NiI(174)°(174)1 ]°. 

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... 

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