Dinuclear elimination

The kinetic acidity (rate constant for metal-to-metal proton exchange) also decreases down a column (Cr>Mo W) in the periodic table. This parallels the order of rates we have observed for the dinuclear elimination of methane from... 

This work was supported by NSF Grant No. CHE79-20373 and by the Colorado State University Regional NMR Center, funded by Grant No. CHE78-18581. The authors are grateful to Dr. Bruno Longato and Robin Edidin for preliminary work on dinuclear elimination rates and to the Alfred P. Sloan Foundation for a fellowship to J. R. N. 

With Os(CO)4H2 and other complexes to be discussed later, dinuclear elimination is kinetically first order. The detailed mechanism for Os(CO)4H2 appears (19) to be ... 

In view of the structural similarity between the Os(CO)4 units in Os(CO)4H2 and Os3(CO)i2 noted above, it is interesting to compare the Os(CO)4H2 thermolysis rate, for which we said carbonyl dissociation is rate determining, with the known rate of dissociative carbonyl exchange for Os3(CO)i2 (20). The exchange rate per Os(CO)4 unit extrapolated to 125.8°C is 25 X 10-5 sec-1 the rate of Os(CO)4H2 thermolysis at that temperature is 6 X 10-5 sec-1. The similarity of these numbers is final evidence that carbonyl dissociation from Os(CO)4H2 does occur in the rate-determining step. A vacant coordination site apparently is needed before the actual dinuclear elimination step can occur. 

The diagnostic reaction, thermolysis of a Os(CO)4(D)CH3 and Os(CO)4-(H)CD3 mixture, yields CD4 and CH4 as well as CH3D and CD3H. Therefore, this is classified as a dinuclear elimination reaction. Appropriately labeled starting materials are easily obtained by using CF3C02D or CD30S02F at the appropriate place in tfre synthesis. The rate of the elimination reaction. 

Dinuclear elimination processes are possible only when at least one of the ligands to be eliminated is a hydride. This is supported by our observations (dinuclear elimination from Os(CO)4(H)CH3 ana Os(CO)4H2 but not from Os(CO)4(CH3)2) and is explained reasonably by the unique ability of a hydride to bridge a pair of transition metal atoms. Tne interaction of Os-H with a vacant coordination site on another Os to form a dinuclear species appears to be an essential part of the dinuclear elimination process. 

Combination of both of the above elements in a single molecule such as Os(CO)4(H)CH3 gives rise to facile dinuclear elimination. The dihydride is capable of dinuclear elimination but must rely on the comparatively hign-energy process of carbonyl dissociation to provide the necessary vacant coordination site. The dimethyl compound has the necessary vacant site easily available but no hydride to interact with it. The hydridomethyl compound has both elements and is uniquely unstable. 

We conclude that our working hypothesis is valid, and that when ML4 is a sufficiently unstable fragment, both dinuclear elimination and metal-carbon bond homolysis can occur instead of simple reductive elimination of R-R from ML RR. We conclude further that the involvement of a hydride ligand is necessary for dinuclear elimination from such ML RR. On that basis, we propose the above general mechanism to explain the instability of hydridoalkyls of this type. 

The above conclusions and suggestions, if valid, require than an alkyl carbonyl (because it can easily generate a vacant coordination site) and a hydride should be able to carry out a facile dinuclear alkane elimination. Thus we predict that dinuclear eliminations between Os(CO)4H2 and Os(CO)4(CH3)2 should occur more rapidly than the decomposition of either compound separately, and we have confirmed this prediction. 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

Carbon-carbon bond formation by reductive elimination from Ni, Pd, or Pt complexes is widespread. In many cases it is presumed to occur as the final step in a catalytic cycle, whereby the organic product is expelled from the metal center, but in others it is a well-defined, mechanistically studied reaction. Elimination takes place from Ni, Pd, and Pt complexes in their - - 2 or + A oxidation states, and it may be promoted by thermolysis, by photolysis, or by nucleophilic attack at the metal center. The reaction may proceed by heterolytic or homolytic metal-carbon bond cleavage, reductive elimination, or dinuclear elimination, and more than one mechanism may operate. 

Strong evidence for a dinuclear rhodium species as the active catalyst in hydro-formyladon of styrene was reported for the silica gel-supported species in uation 22 (941 Relative amounts of diphosphine and tetraphosphine rhodium species were determined from UV/visible spwtra. The rates of hydroformylation decreased markedly as % site isolation on the silica surface increased. All of the supported cat ysts were less active than the corresponding soluble catalysts, which were not site isolated. The results were interpreted by a dinuclear elimination step in the mechanism of hydroformylation (Equation 23). 

Dinuclear elimination reactions of metal alkyls (Equation 2.4, L = all other ligands) have been studied less intensely than, for example, similar reactions of metal amides (Equation 2.5). The following characteristics are common to known dinuclear elimination reactions of metal alkyls (i) a site of unsaturation must be present cis to the alkyl ligand and (ii) the other reagent must be a hydrido-complex. 

Square-planar complexes Pt(CO)(PR3)Cl2 (R = Ph or Bu) are transformed to the dinuclear [R COPt(PR3)Cl]2 by the action of HgRj (R = Me or Ph) under mild conditions 82). These reactions are thought to proceed through the oxidative addition to Pt(II) of HgRj, migration of R onto CO, and elimination of R HgCl. 

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

A five-membered methanide auracycle [Au(C6F5)2(SPPli2CFl2PPh2)]C104 (25) is described with the monosulfonated dppm, obtained after chlorine elimination of [Au (C6F5)2Cl(SPPh2CH2PPh2)j with a silver salt. After deprotonation of the methylene group (26) and later coordination of additional metal centers affords dinuclear and trinuclear methanide derivatives (27, 28) [210]. 

Sources of catalytically active palladium(O) typically arise from ligand dissociation from coord-inatively more saturated Pd° complexes871-880 or from reduction of a Pd11 species.353,881 Another route to catalytically active (P—P)Pd fragments is the dissociation of the dinuclear complexes [(//-P—P)Pd]2.882 Complexes [(/r-dcpm)Pd]2 and [(/r-dtbpm)Pd]2 were obtained from the reductive elimination of ethane from dimethylpalladium(II) complexes (dippm = bis(diisopropylphosphino)methane dcpm = bis(dicyclohexylphosphino)methane dcpm = bis(di-t-butylphosphino)methane).883... 

Thermolysis of the dinuclear [Au2(C=CBut)2(/i-PPh2CH2PPh2)] in refluxing toluene leads to the intermolecular elimination of IIC CBu1 and formation of complex (378).2228... 

W.L. Gladfelter, University of Minnesota The thermolysis of arachno-[(CO)H(PMe3)2(IrB8H12)], in which H2 is evolved, is an example of a dinuclear reductive elimination. Have you observed any dependence of the rate on the PMe3 concentration Also, do the compounds undergo H/D exchange when placed in a D2 atmosphere ... 

In the case of polynuclear metal cluster SCO complexes in the solid state, there will be intra-cluster, as well as inter-cluster cooperativity. To eliminate inter-cluster effects totally, studies must be made in dilute solutions. Williams et al. have done just this for a dinuclear [Fe(II)2L3] helicate complex which does not contain a good superexchange pathway between the Fe(II) centre but, rather, three flexible bis-bidentate ligands. A very broad, two step, SCO was observed (LS-LS<->LS-HS<->HS-HS) and fitted to a model for negative cooperativity in which subtle structural changes around each Fe oc-... 

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

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