Ligands diimine-based

Cai et al. (71) examined the use of dinuclear copper complexes as catalysts in the cyclopropanation reaction. Their ligand design, based on the success exhibited by the Aratani system, incorporates a diimine aryloxide. A comparison of the mononuclear catalyst 99 with the corresponding dinuclear catalyst 100 showed certain modest benefits conferred by the latter, Eq. 52. The authors note that these catalysts are effective at ambient temperature but isolated yields are higher at 50°C with no loss in enantioselectivity. 

Killian et al. [21] described the well-performing a-diimine-based catalysts after the development of a procedure for the living polymerization of a-olefins based on Ni a-diimine catalysts and application of this procedure to the synthesis of diblock and triblock poly(a-olefins) (Scheme 3.11). Moreover, they found that the ratio of 1,2—versus 2,1-insertion is sensitive to the nature of the a-diimine ligand [21]. 

Diimine-based ligands such as bipyridine have been used widely as ligands for transition metals. Their conjugated structure makes them attractive candidates for incorporation directly into a conjugated polymer backbone. In this configuration, metal centers that are coordinated by the diimine are strongly electronically coupled to the polymer. 

The solvatochromic behaviour of the absorption spectra of complexes of this type illustrated in Fig. 3.9 is a further proof of the charge-transfer nature of the lowest electronic transition. Differently to thiolate or catecholate complexes, the lowest excited state in the bis-acetylide complexes Pt(diimine)(C = C-R)2 (R = 4-X-CeH4 ) mentioned above is a charge-transfer from the largely Pt based HOMO to the Pt/diimine-based LUMO. Consequently, the effect of electron donating properties of the acetylide ligand has much lesser influence on the redox and optical properties of the complexes [29, 30, 51] if compared to thiolate ligands. 

The first examples of highly active olefin polymerization catalysts based on late transition metals were nickel and palladium complexes containing bulky diimine ligands.310 312 For example, complex (120) was found to polymerize ethylene with an activity of ll,000gmmol h bar A range of PE materials with molecular weights up to 106 and... 

Fig. 8 Teraryl-substituted a-diimine ligand-based Ni(II) polymerization catalysts...

A specific goal we have in mind is to design cyclophane-based a-diimine ligands for overcoming the low thermal stability of the regular a-diimine catalyst systems. 

The formation of diimine systems by Schiff -base-type condensation of suitable aldehydes and primary amines has been widely applied. Those reported are mostly strong field systems and their relevance to the spin crossover field is generally in systems of the kind [Fe(diimine)2(NCS)2]. The effect of the incorporation of substituents likely to hinder coordination has been studied. Robinson and Busch noted a fundamental difference at room temperature in the electronic properties of the [Fe N6]2+ derivatives of 2-pyridi-nalmethylhydrazone and 2-pyridinal-dimethylhydrazone, those of the former being low spin and those of the latter high spin [49]. The temperature-dependence of the magnetism of the latter complex was not reported but may well be of interest. However, spin crossover [Fe(diimine)3]2+ systems have been characterised for systems where the incorporation of appropriate substituents has reduced the ligand field. 

The complex [MnL](PF6)2, where L is the pendant arm ligand (184), also based on a N5 diimine macrocycle, was obtained directly by a template procedure." . The coordination geometry is pentagonal bipyramidal with the pendant amino groups occupying the axial sites. The aqueous HNMR relaxation rate enhancement in the presence of this complex is small in comparison to... 

Figure 2 Diagrammatic summary of selected structural, substituent, and solvent effects on rate constants (kj, at 298 K) for base hydrolysis of low spin iron(II)-diimine complexes. Ligand abbreviations not appearing in the list at the end of this chapter are apmi = (73) with = Me BOH cage = (78) with X = OH ...

Fe(gmi)3] in glycol-water and a range of other binary aqueous solvent mixtures. These results, along with further results for AV for base hydrolysis of [Fe(phen)3] " and of [Fe(bipy)3] " in alcohol-water mixtures, have permitted the construction of a scheme combiniim solvent and ligand effects on AF for base hydrolysis of a range of diimine-iron(II) complexes. ... 

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