Reactivity of H2 Complexes

The solid state reactivity of H2 complexes has not been extensively studied, but significantdifferencesoccurfor RuH2(H2)2(PCy3)2, which is stable to 70°C, whereas... 

The use of NHCs has also found application in the catalytic hydrogenation of olefins. By the simple ligand exchange reaction of [Ir(COD)2(py)2]PF6 with SIMes in toluene, Nolan and coworkers have prepared the SIMes analog (27) of Crabtree s catalyst (26) [67]. The reactivity of this complex was tested for catalytic activity in the hydrogenation of several olefins. While the complex did show activity, it was less efficient than Crabtree s catalyst at ambient temperature and atmospheric H2 pressure. However, the SIMes complex did display greater activity at 60 psi of H2 pressure and 50 °C. 

The vibrationally excited states of H2-OH have enough energy to decay either to H2 and OH or to cross the barrier to reaction. Time-dependent experiments have been carried out to monitor the non-reactive decay (to H2 + OH), which occurs on a timescale of microseconds for H2-OH but nanoseconds for D2-OH [52, 58]. Analogous experiments have also been carried out for complexes in which the H2 vibration is excited [59]. The reactive decay products have not yet been detected, but it is probably only a matter of time. Even if it proves impossible for H2-OH, there are plenty of other pre-reactive complexes that can be produced. There is little doubt that the spectroscopy of such species will be a rich source of infonnation on reactive potential energy surfaces in the fairly near future. 

In the case of tantalum, the treatment under H2 at 150 °C of a mixture of [(=SiO)Ta(= CHtBu)(CH2tBu)2] and [(= SiO)2Ta(= CHtBu)(CH2tBu)] is transformed into mainly one surface species [(=SiO)2Ta-H] (Scheme 13) [52]. However, when the temperature is increased step by step to 450 °C, this surface complex is gradually converted to [(=SiO)3Ta] and (Si - H) [53]. The gradual reactivity of [(= SiO)2Ta - H] with adjacent silox-ane bridges also speaks for an heterogeneity of the sihca surface. 

In the presence of 10% H20 but no C02, the same orange species accumulated in the solution and electrocatalysis was slow, with H2 being generated with a current efficiency of c. 85% (only a tiny amount of H2 was observed under the same conditions in the absence of the complex). In the presence of C02 the water reduction reaction was completely inhibited, showing that the orange species is less reactive towards water than COz and hence is a highly specific catalyst for the conversion of C02 to CO. 

Remarkably high reactivity of cationic alkyl complexes of Group 4 metals with 1-alkenes has been observed in gas-phase reactions [129]. Typical ionic species such as TiCl2Me+ react with ethylene, and the insertion followed by H2 elimination gives rise to a cationic allyl complex TiCl2C3H5, which does not react further with ethylene. 

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