1.2- Diimines metal complexes

DIAZA-l,3-BUTADIENE (a-DIIMINE) METAL COMPLEXES 13.5.3.1 Introduction... 

As noted above, MLCT excited states of diimine metal complexes are both better reductants and oxidants than the ground-state species. This property has been exploited by Gray, Barton, and others for the study of proteins, DNA, and other biological molecules. Flash/quench experiments were first developed to provide a high driving force method to measure rates of electron transfer in proteins. In these experiments an excited diimine complex, typically a member of the Ru(bipy)2(diimine) + family of complexes, is oxidatively quenched with Ru(NH3)6 +, Co(NH3)5CP+, or methyl viologen, or reductively quenched with / -methoxy-A,A-dimethylaniline to yield a highly active redox species. 

Breu, J., and C.R.A. Catlow. 1995. Chiral recognition among tris(diimine)-metal complexes. 4. Atomistic computer modeling of a monolayer of [Ru(bpy)3]2+ intercalated into a smectite clay. Inorg. Chem. 34 4504-4510. 

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

There has been considerable interest in the study of multinuclear metal complexes with bridging sulfide ligands.381 The first examples of triangulo palladium(II) are the complexes containing substituted 2,2 -bipyridine and triply bridging sulfide ligands, namely [Pd3(diimine)3(M3-S)2][C104]2382... 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

The scope of this chapter will be to focus on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides and carbon dioxide. Although there are numerous such well-characterized metal complexes that catalyze this transformation, we will focus this chapter on recent contributions involving metal salicylaldimine (salen) and derivatives thereof [6, 7]. Some of the alternative catalysts systems are very active and selective for copolymer production. Most notably among these are the zinc p-diiminates reported by Coates and coworkers [8, 9]. These systems have been reviewed in detail elsewhere [10]. 

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