Oxygen exchange metal centers

The proton transfer processes described above induce interesting effects on the geometry of these metal complexes upon protonation (see also Section II). If it is assumed that the equatorial cyano ligands form a reference plane and are stationary for any of these distorted octahedral cyano oxo complexes, the protonation/deprotonation process as illustrated in Scheme 3 is responsible for the oxygen exchange at the oxo sites. This process effectively induces a dynamic oscillation of the metal center along the O-M-O axis at a rate defined by kmv, illustrated in Fig. 15. This rate of inversion is determined by the rate at which the proton is transferred via the bulk water from the one... 

Under selected conditions, the observed rate of oxygen exchange in these complexes can primarily be a function of the protonation rate, i.e., the inversion of the metal center, since the exchange on an oxo at any M = O site must proceed though the bottleneck of the dioxo species which is further discussed in Section V. 

The oxygen exchange on the different oxo complexes for the W(IV), Mo(IV), Tc(V), Re(V), and Os(VI) metal centers were studied as described below in more detail. A summary of the rate constants thus obtained are given in Table V. 

It has previously been concluded that even in strong acidic solution, the dioxotetracyanoosmate(VI) complex cannot be protonated to form the oxo aqua complex or even the corresponding hydroxo oxo complex. The pA i and pKa2 values have been estimated to be substantially less than -1, which is also supported by the relationship between pKa values and 170 and 13C chemical shifts (Table II). Extreme slow kinetic behavior, as expected in the case of a +6 charged metal center for a dissociative activation exchange process, has been observed, with only an upper limit for the oxygen exchange determined (Table II). 

In summary, it is clear from the above-discussed aspects that it was possible by multinuclear NMR (oxygen-17, nitrogen-15, carbon-13, and technetium-99) to successfully study the very slow cyanide exchange and the slow intermolecular oxygen exchange in these oxocy-ano complexes and correlate them both with the proton-transfer kinetics. Furthermore, the interdependence between the proton transfer and the actual dynamic inversion of the metal center was clearly demonstrated. 

In the present work, the Jacobsen s catalyst was immobilized inside highly dealuminated zeolites X and Y, containing mesopores completely surrounded by micropores, and in Al-MCM-41 via ion exchange. Moreover, the complex was immobilized on modified silica MCM-41 via the metal center and through the salen ligand, respectively. cis-Ethyl cinnamate, (-)-a-pinene, styrene, and 1,2-dihydronaphtalene were used as test molecules for asymmetric epoxidation with NaOCl, m-CPBA (m-chloroperoxybenzoic acid), and dimethyldioxirane (DMD) generated in situ as the oxygen sources. 

The increase of the coordination number by exchanging one metal center by two oxygen or nitrogen donor atoms in compounds 3 leaves the shift range between 600 and 1500 ppm. 

Heterobimetallic catalysis mediated by LnMB complexes (Structures 2 and 22) represents the first highly efficient asymmetric catalytic approach to both a-hydro and c-amino phosphonates [112], The highly enantioselective hydrophosphonylation of aldehydes [170] and acyclic and cyclic imines [171] has been achieved. The proposed catalytic cycle for the hydrophosphonylation of acyclic imines is shown representatively in Scheme 10. Potassium dimethyl phosphite is initially generated by the deprotonation of dimethyl phosphite with LnPB and immediately coordinates to the rare earth metal center via the oxygen. This adduct then produces with the incoming imine an optically active potassium salt of the a-amino phosphonate, which leads via proton-exchange reaction to an a-amino phosphonate and LnPB. 

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