Five-coordinate trapping

Protonation reactions of the related dimethyl(hydrido)platinum(IV) complex TpMe2PtMe2H (58) leading to rapid methane reductive elimination have also been reported (86). This protonation was shown to occur exclusively at the pyrazole nitrogen, presumably forming a five-coordinate Pt(IV) intermediate. This species should undergo C-H coupling, and while a Pt(II) methane complex is not observed, trapping with... 

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

If the hydroxide ion accelerates reactions by proton abstraction rather than by direct attack, it might be supposed that it would be possible to trap the five-coordinate intermediate by addition of large amounts of anion other than hydroxide. One system for which this is possible is the base hydroly.sis of [Co(NH2R)5Xl (X = OSO CFj) with Nj as the trapping agent ... 

Three-c(H)rdinate carbenium ion and five-coordinate carbonium ion intermediates satisfactorily account for many of the acid-catalyzed reactions of hydrocarbons at high temperatures. Yannoni et al. have characterized the structure and dynamics of several carbenium ions trapped in (noncatalytic) solids at low temperatures [32,94,95), but lifetimes of such ions on active surfaces at higher temperatures would preclude NMR observation in all but special cases. Maciel observed triphenyl carbenium ion on alumina 196). The alkyl-substituted cyclopentenyl ions discussed earlier are also special ions they are commonly observed products in conjunct polymerization reactions of olefins in acidic solutions. The five member ring cannot easily rearrange to an aromatic structure, and ions like I and II are apparently too hindered to be captured by the framework to form alkoxy species. 

Reacting la,b with MeLi in HMPA as active solvent and in the presence of MeOH as trapping agent, the attack of a methyl anion at the silicon atom of the silacyclobutane is the first reaction step and gives a pentacoordinated silicon anion (Scheme 2). Such five-coordinated species are discussed as intermediates during the ring opening polymerization of silacyclobutenes, -butanes, and -pentenes [2]. Furthermore, five-coordinated silicon species are well described to be stable compounds [3]. 

Thermolysis of either 4a or 4b in the presence of ethylene led to isolable products 13a and 13b in which efficient trapping of B (the orthometalated analog ofB) or B, respectively, with ethylene occurs as shown in Scheme 7. Analysis of the rates of reaction of 4a with respect to ethylene concentration showed that ethylene can bind to the five-coordinate Pt(lV) species and inhibit C-C reductive elimination. These results support direct C-C coupling from the five-coordinate complexes and establish the validity of these species as functional models for the intermediates... 

Reductive elimination of ethane from five-coordinate Pt(IV) alkyl complexes has also led to the generation of three-coordinate complexes that have shown catalytic activity in the hydroarylation of olefins. In contrast to the f-Bu or Ph substituted pypyr ligands which underwent facile cyclometalation and trapping with ethylene (Scheme 7), when the Me-substituted ( pypyr)PtMe3 (4c) was heated in benzene solvent under a pressure of ethylene, ethyl benzene product was produced with a TON of 26 [94]. Other combinations of arenes and olefins were also observed to yield hydroarylation products when ( pypyr)PtMe3 complex 4c was used as a catalyst precursor. Presumably C-C reductive elimination of ethane is followed by C-H activation of the arene, reductive elimination of methane, and then... 

An associative or mechanism involving attack by the anions at the tungsten center was considered unlikely, and a concerted process involving initial attack at a carbonyl carbon was proposed (Scheme 13). The alternative initial addition of X to a carbonyl ligand followed by loss of COX and rapid pick up of X" was eliminated by the failure to trap the five-coordinate [W(CO)5]. On the basis of these and earlier studies it seems probable that most reactions of anionic nucleophiles (N3, halides, pseudohalides) with [M(CO)6] (M = Cr, Mo, or W) complexes involve interaction at a carbonyl carbon. 

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