Isomerization of complexes

Scheme 3 Photoinduced cis-trans isomerization of complexes with C = N linkages...

During their studies on the metal catalyzed cis-trans isomerizations of complexes of the type MX2L2, Nelson et al. (4) observed the phosphine exchanges shown in Eq. (29). The ratios of mixed to symmetrical species for... 

The kinetics of isomerization of complexes of type [Al(/ -diketonato)a] were studied earlier and now isomerization involving the cis- and trans-lsomevs of a related complex (15) has been studied. The trans (Ci) isomer on dissolving in CCI4 is converted into an equilibrium mixture with the cis (C3) isomer the rates of isomerization and the equilibrium positions have been determined. The intramolecular process relates kinetically to similar processes involving classical / -diketonato-ligands. 

Multiple Time Scales Kinetics. A homogeneous catalytic system will exhibit precatalytic reactions, reactions associated with the intermediates and hence product formation, and reactions associated with deactivation. The fastest elementary steps will occur on a vibrational time scale of 10 -10 s. The isomerization of complexes and ligand dissociations may occur on NMR time scales of approximately 10 -10 s. Solvation kinetics and solvent exchange also commonly occur on an NMR time scale (22). The turnover frequencies of many practical organic syntheses are approximately 10 to 10 s . Deactivation, when present in useful synthetic reactions, typically occurs on a time scale of 1000 s or longer. Therefore, the events associated with homogeneous catalysis occur on multiple time scales. 

In the case of the isomerization of complex II, the energy of the transition state with the -coordinated ligand (Figure 3.9a) will increase compared to the energy for complex I because the methyl and ethyl groups are close to the axial... 

Isomerism in general has been reviewed in The isomerism of complex compounds R. G. Wilkins and M. J. G. Williams in Modern Coordination Chemistry, R. G. Wilkins and J. Lewis (eds.). Interscience, New York, 1960. Although old, this remains a useful review. 

Autocatalysis as an isomerization mechanism finds support in a number of recent publications. Tetrahydrofuran(thf) has no detectable catalytic effect on the isomerization of complex 15, but a slow geometry change in... 

The most important feature of yet another investigation of nitrito to nitro isomerization of complexes [M(NH3)5(ONO)] ", with M = Rh and Ir (vide infra) as well as Co, is the discovery of a hitherto undetected base catalysed pathway [equation (33)]. 

The dinuclear complex [(H3N)5Ru(/i-N2)Ru(NH3)5] has an aquation rate constant >10 s at room temperature, much faster than that for [Ru(NH3)5(N2)]. The tetranuclear cation [Ru4(OH)4] undergoes depolymerization, presumably giving initially [Ru2(OH)2], much more slowly. At 298 K the depolymerization rate constant is 2.5xl0 s = 63kJmorO. Linkage isomerization of complexes [Ru(NCS) -(SCN)6 ] n = 1-4), produced by reacting ruthenium trichloride with potassium thiocyanate in aqueous solution, under various conditions has been discussed in qualitative terms. ... 

It is significant that the diaryl complexes are not isomerized by free phosphine, as this supports the view that the isomerization of complexes containing halogen occurs through an ionic intermediate, e.g. 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

Isomerization of double bonds in vitamin D analogs such as calciferol by oxidation and reduction has been carried out via the formation of the tt-allylpalladium complex 334 with PdCl2(PhCN)2 in 70% yield, followed by hydride reduction to afford 335[295],... 

As supporting evidence, rapid isomerization of the ds- and maui-Tr-allylpal-ladium complexes 27 and 28 is catalyzed by Pd(Ph3P)4 in THF even at -15 C to give a 45 55 equilibrium mixture from either 27 or 28[29-31].. Actually, in the intramolecular reaction of soft nucleophiles of 29 and 30, a trans-ds mi.xttire (31 and 32) (1 1) was obtained from /raiw-allylic acetate 29. On the... 

Deviation includes, in fact, the summation of steric and electronic effects, and basicity is somewhat a useful predictor for properties of complex dyes (solvent sensitivity, isomeric forms of trinuclear dyes) and gives also semiquantitative data for color structure relation (atomic)... 

The first identified complexes of unsubstituted thiazole were described by Erlenmeyer and Schmid (461) they were obtained by dissolution in absolute alcohol of both thiazole and an anhydrous cobalt(II) salt (Table 1-62). Heating the a-CoCri 2Th complex in chloroform gives the 0 isomer, which on standirtg at room temperature reverses back to the a form. According to Hant2sch (462), these isomers correspond to a cis-trans isomerism. Several complexes of 2,2 -(183) and 4,4 -dithiazolyl (184) were also prepared and found similar to pyridyl analogs (185) (Table 1-63). Zn(II), Fe(II), Co(II), Ni(II) and Cu(II) chelates of 2.4-/>is(2-pyridyl)thiazole (186) and (2-pyridylamino)-4-(2-pyridy])thiazole (187) have been investigated. The formation constants for species MLr, and ML -" (L = 186 or 187) have been calculated from data obtained by potentiometric, spectrophotometric, and partition techniques. 

Similar to IFP s Dimersol process, the Alphabutol process uses a Ziegler-Natta type soluble catalyst based on a titanium complex, with triethyl aluminum as a co-catalyst. This soluble catalyst system avoids the isomerization of 1-butene to 2-butene and thus eliminates the need for removing the isomers from the 1-butene. The process is composed of four sections reaction, co-catalyst injection, catalyst removal, and distillation. Reaction takes place at 50—55°C and 2.4—2.8 MPa (350—400 psig) for 5—6 h. The catalyst is continuously fed to the reactor ethylene conversion is about 80—85% per pass with a selectivity to 1-butene of 93%. The catalyst is removed by vaporizing Hquid withdrawn from the reactor in two steps classical exchanger and thin-film evaporator. The purity of the butene produced with this technology is 99.90%. IFP has Hcensed this technology in areas where there is no local supply of 1-butene from other sources, such as Saudi Arabia and the Far East. 

C-19 dicarboxyhc acid can be made from oleic acid or derivatives and carbon monoxide by hydroformylation, hydrocarboxylation, or carbonylation. In hydroformylation, ie, the Oxo reaction or Roelen reaction, the catalyst is usually cobalt carbonyl or a rhodium complex (see Oxo process). When using a cobalt catalyst a mixture of isomeric C-19 compounds results due to isomerization of the double bond prior to carbon monoxide addition (80). 

The nickel or cobalt catalyst causes isomerization of the double bond resulting in a mixture of C-19 isomers. The palladium complex catalyst produces only the 9-(10)-carboxystearic acid. The advantage of the hydrocarboxylation over the hydroformylation reaction is it produces the carboxyUc acids in a single step and obviates the oxidation of the aldehydes produced by hydroformylation. 

Spectroscopy at variable temperatures enables us to reveal linkage isomerism of adsorption, when certain molecule fonu with the same site two or more complexes with different geometry and chemical properties. The most studied so far is the case of CO in zeolites, when besides the usual C-bonded complexes with the cations or OH-groups, energetically unfavorable O-bonded complexes ai e formed. 

Unlike reactive diatomic chalcogen-nitrogen species NE (E = S, Se) (Section 5.2.1), the prototypical chalcogenonitrosyls HNE (E = S, Se) have not been characterized spectroscopically, although HNS has been trapped as a bridging ligand in the complex (HNS)Fc2(CO)6 (Section 7.4). Ab initio molecular orbital calculations at the self-consistent field level, with inclusion of electron correlation, reveal that HNS is ca. 23 kcal mof more stable than the isomer NSH. There is no low-lying barrier that would allow thermal isomerization of HNS to occur in preference to dissociation into H -1- NS. The most common form of HNS is the cyclic tetramer (HNS)4 (Section 6.2.1). 

Partial hydrogenation of pyrrole derivatives and partial dehydrogenation of pyrrolidines afford /I -pyrrolines (80-82). However, because of the complex nature of the reaction, it is of little preparative value. The same is true for isomerization of /) -pyrrolines to /) -pyrrolines (83). A photodehydrogenation of 2,6-dimethylpiperidine (26) has been observed recently, affording 2,6-dimethyl-3,4,5,6-tetrahydropyridine (27) in a good yield (84)-... 

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