Heterolytic Activation of the H-X (X=H,Si,B, and C) Bonds inSmall Molecules

Abstract

The protonation reaction of trans-[Ru(H)(P(OMe)?)(dppe)?][OTf] 2.2 with HOTf led to the formation of 3 new dihydrogen complexes trans-[Ru(?²?H?)(P(OMe)???(OH)?)(dppe)?] (x = 0, 1, 2) which are characterized by their short T? values and the observation of substantial J(H,D) in the corresponding HD isotopomers. The protonation also takes place at the trans phosphite to give new phosphite P(OH)?(OMe)??? (x = 1 and 2) dihydrogen complexes, via the single and double C–O bond cleavage of P(OMe)?. These dihydrogen complexes lose H? to give a 16?electron, coordinatively unsaturated, highly electrophilic complex [Ru(P(OMe)(OH)?)(dppe)?][OTf]? 2.8. The complex 2.8 activates the H–H bond in molecular hydrogen in a heterolytic fashion at room temperature to give trans-[Ru(H)(P(OMe)(OH)?)(dppe)?][OTf] 2.10 and generates HOTf. The HOTf brings about the C–O bond cleavage of the phosphite P(OMe)(OH)? to give yet another 16?electron, coordinatively unsaturated, highly electrophilic, unprecedented example of a ruthenium phosphorous acid complex [Ru(P(OH)?)(dppe)?][OTf]? 2.11 which has been isolated and structurally characterized. The electrophilic complex 2.8 also activates the Si–H bond of the silane (Me?EtSiH) in a heterolytic fashion. We have studied for the first time the heterolytic activation of all the H–X (X = H, SiH?, H?B–PH? and CH?) bonds using a single electrophilic, coordinatively unsaturated, dicationic ruthenium complex [Ru(P(OH)?)(dppe)?][OTf]? 2.11. This complex activates the H–H (in H?(g)), Si–H (in silanes), and the B–H (in borane–Lewis base adducts) bonds in a heterolytic fashion, whereas the heterolytic activation of the C–H bond in methane does not take place. The DFT calculations carried out at B3LYP/LANL2DZ level using a model system [Ru(P(OH)?)(H?PCH?CH?PH?)?][OTf][Cl] 3.1 in order to get an insight into the variation in experimental reactivity showed that the activation barriers for the heterolysis of the H–H, the Si–H, and the B–H bonds are low whereas that for the C–H bond is quite high. The high barrier in the case of C–H bond activation of methane is due to the enormous stretching that the C–H bond has to undergo in order to avoid the unfavorable non?bonded H···H interactions. The reaction of complex [Ru(P(OH)?)(dppe)?][OTf]? 2.11 with PhCH=NCH? gave a new coordinatively unsaturated, electrophilic complex [Ru(P(O)(OH)?)(dppe)?][OTf] 4.1 with a phosphonate ligand. This reaction is a demonstration of abstraction of hydrogens of the P(OH)? group by a base. The reaction of [Ru(P(OH)?)(dppe)?][OTf]? 2.11 with ??Al?O? shows an indication of a small fraction of the metal complex 2.11 anchored onto ??Al?O? but further studies are required to establish this with certainty. The complex [Ru(P(O)(OH)?)(dppe)?][OTf] 4.1 activates the H–H bond in H?(g) in a heterolytic fashion. We have synthesized the first examples of tris(pyrazolylmethane) sulfonate (Tpms) complexes of iridium which are analogous to well?known tris(pyrazolyl)borate (Tp) systems. The catalytic hydrogenation of sterically bulky alkene, 3,3?dimethyl?1?butene, has been achieved using [Ir(H)?(PPh?)(Tpms)] 5.3. The DFT calculations carried out at the B3LYP/LANL2DZ level using the model complex [Ir(H)?(PH?)(Tpms)] 5.3a support our proposed mechanism of hydrogenation. The flexibility of the Tpms ligand in facilitating the generation of coordinatively unsaturated complexes under milder conditions makes these complexes potentially useful for other catalytic reactions like activation and functionalization of silanes and alkanes.