A-diimine metal complexes

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

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]. 

The hexahydropyrimidine (58), formed from l-phenylpropane-l,2-dione and propane-1,3-diamine, is an excellent precursor for the a-diimine macrocyclic complexes (60), presumably via the amino ketone (59) (Scheme 36).126 In this case, intramolecular cyclization of (59) to (58) is reversible, so that the metal ion can exert a thermodynamic template effect in formation of the complex (60). This represents a further example of a long-known phenomenon in which a metal ion can stabilize an a-diimine structure by virtue of the formation of stable five-membered chelate rings. Many 2-hydroxy- or 2-mercapto-amines undergo reaction with a-dicarbonyl compounds to yield heterocyclic compounds rather than a-diimines. However, in the presence of suitable metal... 

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. 

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. 

Rate-determining steps leading to I(IV) and I(III) are postulated, with subsequent rapid reduction of intermediate iodine species. An induction period followed by oxidation of the [Fe(phen)3] complex is a feature of the reaction of that complex with bromate ions. The rates of the corresponding reactions with CI2 and affected by Cl", Br", or hydrogen ion. The oxidations occur via the one-electron transfer steps and an analysis of the data and those for other metal ion reductants has been made using a Marcus theory approach. The kinetics of the peroxodisulfate oxidation of two [Fe(II)(a-diimine)3] complexes have been investigated in binary aqueous-solvent mixtures.The rate data have been dissected into initial and transition state energies. Comparisons between the relative contributions to these parameters for redox and substitution reactions remain a topic of interest. 

A series of complexes of platinum(ll) containing cyanide and a-diimine ligands have been prepared that have intense emissions in the 560-715-nm range. These complexes have the formulae Pt(LL)(CN)2 or [Pt(LL)2][Pt(CN)4], and they all have a close intermetallic separation between the platinums of approximately 3.3 A. The ambient temperature emission maxima and lifetimes are collected in Table 3.6. This emission results from excitation due to a metal to ligand charge transfer (MLCT) transition of type da dz ( )) n (a-diimine). Metal-metal... 

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... 

The location of the induced unsaturation in the macrocyclic system is metal-ion dependent. This is illustrated by the examples given in Figure 8.2. In the Fe(n) complex, the imine functions form as conjugated pairs (Dabrowiak, Lovecchio, Goedken Busch, 1972 Goedken Busch, 1972) - such a-diimine species have long been known to have a special affinity for Fe(ii). In contrast, Ni(n) promotes formation of a product in which the respective imine functions are in electronically isolated positions (Curtis, 1968 1974). 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

There is a small solvent dependence for [Ru(L)3]2+complexes which depends on the structure of L. The emission spectra of the Ru(bpy)32+ is mildly sensitive to media, the Ru(phen)32+ much less so, and the Ru(Ph2phen)32+ is virtually media independent. This decreasing sensitivity to solvent perturbation is a consequence of the excitation being localized in the metal a-diimine portion of the complex (-N=C-C=N).(27) The more extended the complex, the greater the shielding of the excited portion and the smaller the solvent perturbations of the emission spectrum. In particular, the bulky phenyl groups are extremely effective at shielding the excited state from environmental perturbations. 

---