Kinetically block metal complexes

Crystal field theory can bring together structures, magnetic properties and electronic properties, and we shall expand upon the last two topics later in the chapter. Trends in CFSEs provide some understanding of thermodynamic and kinetic aspects of J-block metal complexes (see Sections... 

Inhibition of a catalytic reaction by impurities present may take place and sometimes this may have a temporary character. If it is permanent one cannot be mistaken in the kinetic measurements. Impurities that are more reactive than the substrates to be studied may block the catalyst if they react according to a scheme like that of Figure 3.7. Only after all inhibitor has been converted the conversion of the desired substrate can start. Another type of deactivation that may occur is the formation of dormant states, which is very similar to inhibition. Either the regular substrate or an impurity may lead to the formation of a stable intermediate metal complex that does not react further. There are examples where such intermediates can be rescued from this dormant state for instance by the addition of another reagent such as dihydrogen (Chapter 10, dormant states in propene polymerisation). 

Chiral sulfoxides have emerged as versatile building blocks and chiral auxiliaries in the asymmetric synthesis of pharmaceutical products. The asymmetric oxidation of prochiral sulfides with chiral metal complexes has become one of the most effective routes to obtain these chiral sulfoxides.We have recently developed a new heterogeneous catalytic system (WO3-30% H2O2) which efficiently catalyzes both the asymmetric oxidation of a variety of thioethers (1) and the kinetic resolution of racemic sulfoxides (3), when used in the presence of cinchona alkaloids such as hydroquinidine 2,5-diphenyl-4,6-pyrimidinediyl diether [(DHQD)2-PYR], Optically active sulfoxides (2) are produced in high yields and with good enantioselectivities (Figure 9.3). ... 

In this article the design, synthesis and d-block metal ion chemistry of some more recent examples of covalently-linked, macrocyclic ligand systems are discussed. The use of macrocyclic rings in such systems is not surprising given that the resulting macrocyclic complexes often exhibit both enhanced kinetic and thermodynamic stabilities and hence tend to retain their integrity under a variety of conditions - a lesson that nature knows well. 

Cyclam, with its 14-membered ring, has been demonstrated to complex with a very wide range of metal ions (and especially d-block metal ions) and tends to generate metal complexes that are especially kinetically and thermodynamically stable [22], In particular instances it is also known to aid the stabilisation of less common oxidation states such as Ni(III), Cu(III), Ag(II) and Ag(III) [25,26],... 

Low-spin rf transition metal complexes are classical examples of kinetically inert complexes. When injected into mice, species such as [ColNHsle] , [Fe(l,10-phen)3]2+, [Ru(bipy)3]2+, and [Os(terpy)s] rapidly cause convulsions, paralysis, and death by respiratory failure. They produce a curariform block at the neuromuscular junction, consistent with inhibition of acetylcholine esterase. The d isomers of [Rulphen)] " and [Os(phen)] + are 1.5-2 times more potent than the I isomers (9,10). These inert complexes are excreted largely unchanged from the body. 

Other transition metal complexes with cis dichloro ligands have been tested for antitumor activity. The palladium analog cis-[PdCl2(NH3)2] is inactive, prohahly due to the high kinetic lability of Pd(II) compared to Pt(II), and isomerization is facile this process is blocked in the chelated en complex, which is active (16). More inert metal ions such as Rh(III) and Ru(III) also have active analogs (Table VI), and it is likely that some ruthenium complexes will soon enter clinical trials (31-33). 

Organolanthanide complexes differ from late d-block transition metal complexes in several aspects. They are electrophilic, kinetically labile and lack conventional oxidative addition/reductive elimination pathways in their reactions. They have alternative mechanisms to perform catalytic transformations and are being increasingly used in homogeneous catalysis. The hydrophosphination reaction was proposed to proceed through the cycle depicted in... 

Anions of common strong acids, such as C104, S04, CF, NOa , etc. exhibit as a rule only weak complexing interactions, if any. Nevertheless, even weak complexation may be of importance in electrode kinetics if the complex ion undergoes electrode reaction more easily than the free metal ion, as is often the case, especially with chlorides. In such cases, the complex takes the role of an electroactive species, as already discussed for the hydroxo complexes. Thus, e.g., nickel can hardly be anodically dissolved at all if chloride ions are not present in the solution. In sulfate electrolytes, the oxidation product (some oxygen-containing species) forms a passive film and further dissolution is blocked soon after an anodic overpotential is imposed upon the electrode. The phenomenon of passivity is discussed elsewhere (cf. Volume 4). At this point, one should note that passivity is absent in the presence of chlorides. 

Stabilization of metal d-electrons has been employed previously to explain thermodynamic and kinetic data for 6-coordinate hexa-aqua divalent transition metal complexes from the first-row of the d-block. Kinetic data for the dissociation of one lattice water from M +(H20)6 were analyzed by postulating a 5-coordinate square pyramidal product [i.e., M (H20)5] that was not allowed to distort. The... 

Therefore, as Mn2(CO)io was never employed in polymerizations of main chain fluorinated monomers, or with inactivated alkyl halides or with perfluoroalkyl iodides, we decided to assess its scope and limitations as photo-coinitiator and to demonstrate that such LjjMt-MtLjj photolyzable transition metal complexes afford the initiation of VDF polymerization directly from a variety of regular and (per)fluorinated alkyl halides (Cl, Br, I) even at rt, thus opening up novel synthetic avenues for the photome-diated synthesis of block and graft copolymers based on FMs. Second, we also set to kinetically explore such polymerization and investigate the possibility of Mn2(CO)io-mediated IDT-VDF-CRP. Third, we aimed to demonstrate the first examples of the synthesis of well-defined PVDF-block copolymers. 

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