Rhenium complexes reactive intermediates

The analogous coordination behavior of hydridotris(pyrazolyl)borate and tetrakis(pyrazolyl)-borate has been demonstrated by numerous reactions. Reduction of the rhenium(VII) complex [Re03 B(pz)4] with PPh3 in THF gives a reactive dimeric intermediate which reacted with monodentate and bidentate protic substrates to yield products with tridentate B(pz)4 ligands... 

In this reaction scheme, the formation of the pentacyano complex is a relatively fast reaction, with rate constants of about 116 and 2.9 Af" sec" for the molybdenum (20°C) and tungsten (25°C) complexes, respectively, whereas the formation of the octacyano complex from the pentacyano complex is a relative slow reaction, with a half-life of several minutes at a cyanide ion concentration of 1 Af for both the molybdenum and the tungsten complexes. The formation of the octacyano complex from the pentacyano complex is third order in the cyanide ion concentration 155,156). This suggests that the rate-determining step is the reaction of the heptacyano complex with cyanide ions. It seems, however, that the pentacyano complex is a necessary intermediate in the synthesis of the octacyano complex. This proposed reaction scheme makes it possible for the first time to explain why the octacyano complex of rhenium(V), which is also a d species, is still unknown in spite of several attempts (and claims of success) by different groups in the past (see Section IIA) to synthesize this complex The reactive complexes [Re0(H20)(CN)4]" and [ReO(OH)(CN)4] do not exist at a pH > 8, at which there are enough free cyanide ions since the values of [Re0(H20)(CN)4]" are only 1.4 and 4.2. The formation ofthe intermediate [ReOtCNlg] (see Scheme 6) is thus not possible. Thus one cannot proceed beyond the tetracyano complex in this way. 

Sutton has reported a pentamethylcyclopentadienyl rhenium dinitrogen complex that activates benzene (Eq. 29) [114]. The intermediate involved is similar to that described above in studies by Bergman [92]. In the absence of a reactive solvent, cyclometallation was observed. 

The nucleophilic-electrophilic/Schrock-Fischer distinctions have been extremely useful throughout the development of metal-carbene chemistry because they provide a way to categorize metal carbenes and rationalize their reactivity patterns [6]. Yet, as an increasing variety of complexes are studied, it is becoming clear that these classifications represent only the prototypical complexes that were inihally discovered. We now know of many examples with intermediate characteristics and reactivity profiles, such as electrophilic species that lack heteroatom stahilizahon and even complexes like (Cp)(CO)2Re=CHR that display ambiphilic reachvity, meaning that this rhenium carbene reacts with both nucleophiles and electrophiles (Eq. 4.1) [7]. 

Now, one must wonder if there is any limit to the ability of metals to bond in a stable way to other o bonds, including those in non-reactive molecules like alkanes. In fact, evidence for an intermediate methane complex has been found at low temperature in the reductive elimination of methane from a cationic rhenium methyl hydride [34]. The ab initio theoretical study of the intermolecular process of oxidative addition of a methane C - H bond has led to the location of transition states where the bond is partially broken [35]. The same results have been fond for intramolecular oxidative additions which are related to agostic interactions. In fact, agostic interaction itself is a kind of non-oxidative coordination [15,36]. For unsaturated substrates like ethylene, the activation of a C - H bond seems to follow an intermolecular path, without any previous coordination of the double bond. A feasible explanation consists here of the fact that metal orbitals suitable for ethylene coordination are the same as those which are responsible for oxidative addition, thus making the processes competitive [37]. 

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