Hydrogen bonding hydride ligands

Abstract Organic syntheses catalyzed by iron complexes have attracted considerable attention because iron is an abundant, inexpensive, and environmentally benign metal. It has been documented that various iron hydride complexes play important roles in catalytic cycles such as hydrogenation, hydrosilylation, hydro-boration, hydrogen generation, and element-element bond formation. This chapter summarizes the recent developments, mainly from 2000 to 2009, of iron catalysts involving hydride ligand(s) and the role of Fe-H species in catalytic cycles. 

Shvo and coworkers prepared a bimetallic complex in which the two metals are joined by a bridging hydride as well as by an O-H-O hydrogen bond joining the two substituted Cp ligands [70]. Shvo used this versatile catalyst precursor for hydrogenation of C=C and C=0 bonds at 145 °C under 34 bar H2 (Eq. (41))... 

Hydrogenation of CO2 can also be carried out in water [10c, 15]. In this case, the substrate is not CO2 but HCO3, and insertion of HCO3 into the Ru-H bond is involved in the catalytic cycle. In our opinion, this step can be seen as a nucleophilic attack of the hydride ligand to the positively charged C atom of HCO3. ... 

When the protonated amine-arm does not participate in the reaction (TS1-I ), the reaction is very endothermic. However, the activation barrier decreases to 2.6 kcal/mol and the endothermicity of the reaction becomes very small (only 0.2 kcal/mol), if the protonated amine-arm participates in the reaction (TS1-II). In this transition state the O-H distance is 1.709A, typical of a hydrogen bond. The origin of the acceleration was explained in terms of the enhancement of polarization of CO2 induced by the protonated amine arm, which increases the positive charge in the carbon atom. This polarization favors the electrostatic interaction between hydride and C02, and since this polarization increases the contribution of the C p orbital in the it orbital of C02, it favors also the charge-transfer from the H ligand to the 71 orbital of C02. In other paths starting with direct interaction between C02 and Ru the effect of the protonated amine was much smaller. 

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