PHIP with Immobilized Metal Complexes

As demonstrated above, PHIP can be a very useful tool in the studies of homogeneous catalytic processes that involve activation of molecular hydrogen. However, as most of the industrial catalytic processes are heterogeneous, it would be highly desirable to extend PHIP to the hypersensitive NMR studies of heterogeneous catalysts and catalytic reactions. The requirements for PHIP observations imposed on the catalysts and reaction mechanisms are rather stringent (see Section 7.2). Therefore, an obvious first step in this direction was to address immobilized metal 

What was not known at the time when these first PHIP experiments with heterogeneous catalysts were performed was the fact that metal clusters and particles can produce PHIP effects as well. In fact, it was widely believed that the mechanism of catalytic hydrogenation on metal surfaces was incompatible with the requirement of the pairwise H2 addition to a substrate. Therefore, the possibility cannot be excluded that the immobilized Rh complexes used in the early studies were precatalysts rather than the actual catalysts, especially in some of the gas - solid hydrogenations. Extended catalyst activation periods may have resulted in the reduction of supported metal complexes and the production of nanoparticulate Rh catalysts. However, the main conclusion that PHIP effects can be produced in heterogeneous processes is still valid. 

Similar liquid-solid styrene hydrogenation studies were reported by Gutmann et cd. [91]. They used Wilkinson s complex immobilized on the mesoporous 

Vaska s complex, [Ir(CO)(PPh3)2Cl] immobilized onto phosphine-modified silica showed very low activity in Uquid-phase hydrogenations. In gas-phase hydrogenations of propylene at about 70 °C, significant product yield and some weak PHIP effects were observed. In contrast, gas-phase propyne hydrogenation at about 110 C provided very little product but the signal enhancement for 

Silica-immobilized tridentate Rh complex [Rh(COD)(sulfos)]-Si02 (sul-fos=-03S(C6H4)CH2C(CH2PPh2)3) was the first example of a cationic Rh 

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Schiff base complex immobilized on a metal-organic framework (MOF) material, designated as lRMOF-3-SI-Au, was shown to be stable under reactive conditions [102,103]. It was used for the gas-phase hydrogenation of propyne and propylene, and PHIP effects were successfully observed for the reaction products (propylene and propane, respectively), with stereoselective syn addition of H2 observed in the hydrogenation of propyne to propylene (Figure 7.7). The reactions were carried out at 130 °C, with no evidence of the metal complex reduction under these conditions. The reaction yields were relatively low, and the signal enhancement factors provided by PHIP were moderate (about 16), which could be the result of an enhanced nuclear spin relaxation of the reaction products in contact with the porous matrix of the MOF support. 

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