Cationic hydrido complexes

With strongly basic trialkylphosphines water may be oxidatively added to give cationic hydrido complexes, as in the case of the water-soluble ligand P(CH2CH2OH)3 3... 

Since metal hydrides and metal alkyls are often intermediates of catalytic reactions of unsaturated hydrocarbons, reactions of these species with O2 are of interest. Cationic hydrido complexes of Ir, Rh, Ru and Os react with molecular oxygen to insert oxygen between the metal and the hydrido ligand, equations (53)-(56). 

Preliminary Results In the 1980s, Crabtree reported that cationic hydrido complexes [IrH2(solvent)(PPh3)2]BF4 (1) could oxidize alkane with a hydrogen acceptor, such as terf-butylethylene (tbe), into olefins [96-101]. The tbe was used as a sacrificial hydrogen acceptor to overcome the thermodynamic barrier reaction. For example, the transfer of hydrogen from COA to tbe resulted in the formation of cyclooctene (COE) and tert-butylethane (tba), which is thermodynamically favored by 6 kcalmol" (Scheme 2.14). 

It should be noted that this mechanism explains the acid-catalyzed labilization of Fe(CO)5 (9) and the preferential exchange of three CO groups in (Ph3P)Fe(CO)4 in the presence of strong acids (10). Such iron(O) complexes apparently do not form stable adducts with proton acids but are in equilibrium with small amounts of cationic hydrido complexes. 

It has been postulated that these cycloheptenes must be formed via a 7r-allylruthenium intermediate (Scheme 59). The cyclization is initiated by activation of the allylic C-H bond to form the 7r-allylruthenium 234. The 1-exo-dig carboruthenation of the alkynoate 234 produces the hydrido-ruthenium enolate 235. Equilibration of 235 followed by reductive elimination gives the corresponding cycloheptenes 237 and regenerates the cationic ruthenium complex. 

The Ir(III)-amido-hydrido complex 43 was isolated by reacting the electron-rich neutral phosphine-cyclooctene complex 42 according to Equation 6.13 [7] (note the cis arrangement of the hydride and amide function). Likewise, the cationic complex 44 reacted in neat aniline to afford a 50/50 mixture of the N—H (45) and C—H (46) activation products that was isolated as a light orange powder (Equation 6.14). Compound 45 was separated from 46 and purified in 49% overall yield. 

First attempts to isolate monocarbene-hydrido complexes by oxidative addition of A -(2-pyridyl)imidazolium cations to Pd° with utilization of the chelate effect of the donor-functionalized carbene ligand failed and only the dicarbene complexes such as 29 were isolated [112]. The iridium hydrido complex 30 was obtained in the oxidative addition of an W-(2-pyridylmethyl)imidazolium cation to iridium(I) (Fig. 11) [113]. This reaction proceeds most likely via the initial coordination of the nitrogen donor which brings the imidazolium C2-H bond in close proximity to the metal center. No reaction was observed with Rh under these conditions. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

It is deduced from infrared data (206) that iron pentacarbonyl reacts with bromine at — 80° C to form the cationic halogeno carbonyl [Fe(CO)sBr]+Br (206), and the covalent Br—CO—Fe(CO)4Br. From similar infrared data, however, it is concluded that the product is a seven-coordinate non-ionic complex Fe(CO)5Br2 (76) The reaction of Os(CO)3(PPh3)2 with hydrogen chloride produces a cationic hydrido carbonyl, which may be isolated, although it readily reverts to a neutral dichloride by attack of the associated chloride anion (52). 

In this manner, the cationic dihydrogen complex 13 can be deprotonated to the catalytically active Ru(II) hydrido complexes. The undesired isomerization of 13 to the dihydride complex 14 can be avoided by lowering the reaction temperature and changing the solvent to THF. 

It has been noted above that the weakly solvated cation [Rh(diphos)(MeOH)2]+ can be obtained by hydrogenation of [Rh(diphos)(C7Hs)][BF4]. If the former compound is allowed to react with an equimolar quantity of [IrH5(PPrj)2] the di-p-hydrido complex (140) is formed (equation 330). The presence of base merely increases the yield. Only the heavy atoms have been located by X-ray crystallography.1347... 

The electrochemical reduction of a series of cationic iron(II) hydrido complexes trans-[FeH(L)(dppe)2]+ (L = N2, C5H5N, PhCN, CH CHCN, MeCN, P(OMe)3, P(OEt)3, CO) has been shown to proceed via transient d1 iron(I) species which lose one of the neutral ligands. Further reduction of the resulting five-coordinate intermediates yields stable rf8 iron(O) anionic hydrides.233... 

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