Transition metals substitution

Whereas most of the known silylamines containing both a silicon halogen and nitrogen hydrogen unit undergo rapid condensation with HC1 elimination [11], transition metal substitution of the silicon provides unexpected high thermal stability. Compounds of this type are available via reaction of the dichlorosilyl complexes 18a-c with bulky primary amines to generate 19a-c (Eq. (4)). 

Analogously to linear silicon halides, SigMenX does not react with Na[Co(CO>4]. However Na[Co(CO)3PPh3] and Na[Fe(CO)2cp] result in the respective transition metal-substituted silicon rings after reaction with SigMei X or 1,4-Si6Mei0X2. 

Various catalytic systems for H202- and 02-based oxidations catalyzed by POMs have been developed. Typical examples are listed in Table 13.1. The systems can be classified into four groups according to the stmctures of POMs (1) mixed-addenda POMs, (2) transition-metal-substituted POMs, (3) POMs, and (4) lacunary POMs. In this chapter, liquid-phase homogeneous oxidations by POMs with H202 and 02 are described according to the above classification. 

Figure 13.3 Application of transition-metal-substituted polyxometalates as biomimetic catalysts.

Polyoxoacids, defined, 41 117,118 Polyoxometalates defined,41 117,118 transition-metal-substituted, 41 224-225 Polyoxomolybdates, 41 193 Polyoxotetramelhyleneglycol, synthesis, 41 156-157... 

The use of transition-metal-substituted aluminophosphates (AIPOs) as catalysts for free-radical auto-oxidations of hydrocarbons, a type IIaRH reaction, has been explored by several groups [60-62]. In a series of elegant contributions, Thomas... 

In the following, a review of the traditional and novel concepts of catalytic combustion for GTs is addressed, with emphasis on the requirements and challenges that the different applications open to catalysis. The most relevant characteristics of PdO-supported catalysts and of transition metal-substituted hexaaluminates (which have been most extensively considered for lean combustion applications) are described, along with those of noble metal catalysts adopted in rich combustion systems. 

The ample field of transition metal substituted polyoxometallates (POM), which are often other precursors for peroxo derivatives in catalytic processes, will be discussed in brief. 

Reactions of cyclopentadienyl- and (pentmethylcyclopentadienyl)iron dicarbonyl 2-alkynyl complexes as well as cyclopentadienylmolybdenum tricarbonyl 2-alkynyl complexes with 4,5-diphenyl-3,6-dihydro-l,2-dithiin 1-oxide 111 were shown to yield transition metal-substituted five-membered ring thiosulfinate esters 112 in moderate to excellent yields (Scheme 27) <19910M2936, 1989JA8268>. These reactions are formal [3-1-2] cycloadditions. When... 

Goals and five limitations in conjunction with the development of selective catalytic homogeneous oxidation systems are evaluated. Systems are presented that address several of the problems or goals. One involves oxidation of alkenes by hypochlorite catalyzed by oxidatively resistant d-electron-transition-metal-substituted (TMSP) complexes. A second involves oxidation of alkenes by H2O2 catalyzed by specific TMSP complexes, and a third addresses functionalization of redox active polyoxometalate complexes with organic groups. 

Transition-metal substituted or modified zeolites are currently receiving increasing attention as gas-phase heterogeneous partial oxidation catalysts, because they offer the... 

The possibilities of POMs compositions and structures are virtually enormous, and the number of these molecules actually prepared and characterized continues to grow unabated. However, examination of their short electrochemical studies, which are becoming a usual part of their characterization, shows that a limited selection of representative groups of POMs is sufficient for a description of the main electrochemical behaviors of this class of chemicals. Therefore, a-Keggin- and Dawson-type heteropolyanions of phosphotungstate, silicotungstate, phosphomolybdate, and silicomolybdate mixed addenda heteropolyanions, and transition-metal substituted heteropolyanions including sandwich-type derivatives were selected for the present article. 

This section is separated from the former one, even though mixed addenda POMs are mostly, if not exclusively, synthesized from the appropriate lacunary precursors and can be viewed normally as substituted POMs. At least two reasons can be invoked to justify such dichotomy first, a substitutionally labile position is available on the transition metal substituted into the POMs framework, which is not the case with mixed addenda compounds second, these transition metal centers are usually the active sites for catalytic and electrocatalytic reactions and might deserve special attention. 

In addition to structural studies, one of the main incentives to synthesize and characterize these transition metal-substituted complexes is the remark that they might... 

Scheme 5 General electrochemical behavior of transition metal-substituted ROMs (taken from Ref. 8).

Transition metal-substituted POMs (TM-SPs) can be considered as the largest subclass of polyanions [6]. Within the class of TMSPs, the sandwich-type compounds represent the largest family. In short, sandwich-type POMs are formed by the fusion of two trivacant a-XW9 034" ... 

Co2 +-Substitution at the addenda atoms gives catalysts for the epoxidation of olefins in the presence of aldehyde [293). PWM-Co is the most active among the mono-transition-metal-substituted polyanions the order of activity is PWn-Co > -Mn 2= -Fe 2= -Cu > -Ni. Here, PWll(M + )0(379", ) (M = Co2 +, Cu2+, Fe3 +, Ni2 +, Mn2 + ) is denoted by PWn M. The same order was observed for the oxidation of isobutyraldehyde, suggesting that the oxidation of aldehyde to give peracid is an important step in the reaction. It has been reported that substitution of V5+ for Mo6+ in PMo O3 gives a good catalyst for epoxidation and the Baeyer-Villiger reaction [294). 

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