Proton transfers, transition metal-complexes

Figure 10.10 Energy profiles of proton transfer to a hydride ligand of a transition metal complex in solution AEi = + 3 to 4kcal/mol, AE2 = — 5 to — 7 kcal/mol, A 3= + 10 to 14 kcal/mol, and A 4 = —7 kcal/mol the energy is a function of the proton-hydride distance, varying from an initial state (2.5 A) to the final product (0.9 A) conversion of the intimate ion pair to the solvent-separated ion pair is shown as a function of the H+- O" distance. (Reproduced with permission from ref. 29.)...

CgQ with this Zr complex, a red solution is formed, unlike the green solution ofr transition metal complexes of Cjq. The structure of the air-sensitive Cp2ZrClC5oH was confirmed by NMR spectroscopy. The hydrogen transferred from the Zr to CgQ resonates at 5 = 6.09, a typical value for fullerenyl protons [83]. Hydrolysis of Cp2ZrClC5oH with aqueous HCl provides access to the simplest hydrocarbon C5QH2 (30, Scheme 7.14). Spectroscopic characterization of CggH2 showed that the compound is the isomerically pure 1,2-addition product. 

Some intermolecular reactions involving the coordinated ligands (e.g., hydrogen or proton transfer) may be fast enough to compete with the excited state decay. However, except for a few cases [e.g., Ru(bpy)2(CN)2 34 and Ru(bpy)2 (bpy-4,4 -(COOH)2 )2 + 35)] reactions of this kind have not yet been well documented for transition metal complexes, although they are very common for organic molecules19, 36,37. ... 

Another possible two-electron mechanism involves the direct transport of two electrons from a mononuclear transition metal complex to a substrate (S). Such a transport alters sharply the electrostatic states of the systems and obviously requires a substantial rearrangement of the nuclear configuration of ligands and polar solvent molecules. For instance, the estimation of the synchronization factor (asyn) for an octahedral complex, with Eq. 2.44 shows a very low value of asyn = 10 7to 10 8 and, therefore, a very low rate of reaction. The probability of two-electron processes, however, increases sharply if they take place in the coordination sphere of a transition metal, where the reverse compensating electronic shift from the substrate to metal occurs. Involvement of bi- and, especially, polynuclear transition metal complexes and clusters and synchronous proton transfer in the redox processes may essentially decrease the environment reorganization, and, therefore, provide a high rate for the two- electron reactions. 

The imido groups of transition metal complexes undergo a wide range of reactions including electrophilic attacks by protons, NR transfer, and many other reactions.154... 

For example, many catalytic cycles involve the transfer of protons. Common intermediates are carbenium ions and carbanions, and the catalysts include soluble and solid acids and bases and enzymes. The catalytic cycles may be similar, whether the proton donor (or acceptor) is a soluble molecule or ion or a functional group on a surface. Similarly, catalysis proceeding via organometallic intermediates may involve soluble transition metal complexes, metalloenzymes, or metal surfaces. Catalysis by metals is, however, much more complicated than acid-base catalysis, and the analogies between soluble metal complexes and surfaces cannot yet be developed beyond a few selected examples. 

We note however that the reactivity summarized in Table 17.1 for transition metal acceptors fundamentally differs from their organic counterparts in one important aspect - the proton and electron accepting sites for the reactions in Table 17.1 are distinct. It has been well documented that transition metal complexes that are capable of abstracting hydrogen atoms from substrates do not need to have unpaired spin density at the abstracting atom [52]. With the unpaired spin residing mainly at the transition metal center, upon completion of a PCET event, the electron is transferred to the metal M") while the proton comes to... 

---