Transition metal hydrides basicity

One must always keep in mind that in aqueous solutions the transition metal hydride catalysts may participate in further (or side) reactions in addition to being involved in the main catalytic cycle. H and P NMR studies established that in acidic solutions [RhCl(TPPMS)3] gave cis-fac-and ci5-7 er-[RhClH2(TPPMS)3] [86,88], while in neutral and basic solutions these were transformed to [RhHX(TPPMS)3] (X = H2O or Cl ) [86]. Simultaneous pH-potentiometiic titrations revealed, that deprotonation of the dihydride becomes significant only above pH 7, so this reaction of the catalyst plays no important role in the pH effects depicted on Figs. 3.2.a and 3.2.b. 

TABLE 8.2. Basicity Factors Characterizing Hydridic Hydrogens in Boron and Transition Metal Hydrides... 

At the time of the study by Dessy et al. (7), little was known about the Brpnsted basicity of MLn in a quantitative sense. Therefore, what role, if any, was played by the parameter H in the Edwards equation was unclear. Recently, a number of pKa values were measured for transition metal hydrides, both neutral, HMLn, and cationic, HMLn+. The solvents used were chiefly methanol (12) and acetonitrile (13, J. R. Norton and J. Sullivan, personal communication). 

Transition metal hydrides, which are weakly basic as isolated molecules, are expected to display acidic properties in solution. With an appropriate choice of solvent we are thus able to induce Umpolung of the acid-base behavior of certain transition metal hydrides. The break-even point of a TMH in water would be reached with 3.7. This relatively low value indicates that most transition metal hydrides will dissociate protons in water. 

In the previous section it was demonstrated that transition metal hydrides may change their naturally basic character at the hydride site into acid behavior in solution. The latter aspect however strongly depends on the nature of the solvent. Those with only minor solvation properties are expected to retain the original features of the hydrides to a great extent, and thus allow them to react as bases. 

The main bond characteristics of dihydrogen bonding is anticipated to be of electrostatic nature. Thus, the ability of L M-H complexes to interact with protic compounds is expected to be directly related to the basicity of transition metal hydrides, as we have defined them above. This in our mind provides an excellent additional method to quantify the basicity of TMH complexes by the strength of the L MH—HX bonding. 

In our group we have applied all three methods to study dihydrogen bonding of transition metal nitrosyl hydrides [25-27] These studies led to an overall consistent picture of the general phenomenon of dihydrogen bonding, and of the basic character of transition metal hydrides. 

Figure 2. Scale of basicity factors (EJ) for some transition metals hydrides and organic bases.

The investigation carried out has shown that transition metal hydrides as well as boron hydride and fullerenes are promising as basic materials for high-performance hydrogen accumulators. 

Some transition metal hydrides are also strong bases [7t-Cp2ReH] (Eq. 2-25) has a basicity comparable to that of ammonia. 

Another route to productive CT photochemistry can involve proton transfer reactions between A and D +, which are favored in the redox pair arising from the enhanced acidity of D + (relative to D) and basicity of A (relative to A). Numerous examples of these reactions exist in the organic literature [240-241], and such a pathway should be particularly important for transition-metal hydrides with significantly enhanced acidities of their (metastable) cation radicals [242]. Thus irradiation of the EDA complex of fumaronitrile (as acceptor) with the hydridic donor (CP2M0H2) [35] leads to CT hydro-metallation. 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

A basic hydride ligand should be present. This is usually realized for electropositive early transition metals or late transition metals in low oxidation states supported by electron-donating ligands. The metals are preferably from the second and third transition series to ensure strong covalent bonding. 

How does the anionic alkyl of the original trialkylaluminum or of the dialkylaiuminum chloride, which has sufficient anionic character to undergo anionic hydride exchange or CH3OT reaction, form a catalyst which becomes cationic under certain polymerization conditions No studies of this have been reported. One possibility is an internal oxidation-reduction reaction that converts an anionic alkyltitanium trichloride to a cationic alkyltitanium trichloride (Equation 10). Basic and electrophilic catalyst components would determine the relative contributions of the anionic and cationic forms. This type of equilibrium or resonance structures could also explain the color in transition metal compounds such as methyltitanium trichloride (73). 

An early review by Koelle on transition metal catalyzed proton reduction nicely developed the various chemical steps involved in hydrogen evolution including metal hydride formation, hydride acidity (basicity) and protonation and requisite redox potentials.284 The complexes review here have little structural relevance to the hy-drogenase active sites but many show promising catalytic activity. More recently... 

A similar mechanism might operate in the activation of an azolium salt by a transition metal compound forming the metal carbene complex. However, since a basic substituent on the metal (acetate, alkoxide, hydride) usually reacts with the H -proton, the proton is removed from the reaction as the conjugate acid and reductive elimination does not occur. 

Section 3 is devoted to the complexes of hydride, alkyl, alkenyl, and silyl ligands. Hydrido see Hydride Complexes of the Transition Metals) and alkyl complexes of most transition metals, and in particular those of nickel, are often involved in many catalytic processes of conunercial importance therefore, an in-depth understanding of the fundamental reactivities of these complexes is crucial to expanding their practical apphcations. Silyl complexes are involved in the transformations of organosilicon compounds, and for this reason their basic reactivities and structural properties are of interest. 

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