Hydridic-protonic bonding

Scheme 3.4 The heterolytic splitting of dihydrogen at Ru(ll) to give a hydridic-protonic bond, as proposed by Chu et al. [55] in the mechanism of the homogeneous hydrogenation of carbon dioxide.

An alternative pathway to eq 2-4 might be the classical dihydride route, eq 5-6. Here the oxidative addition of the dihydrogen ligand results in a seven-coordinate dihydride species. Reaction 6 has A=7/6 and might also result in the production of a hydridic-protonic bonded intermediate. 

Independently in 1993-1994, our group [74] and Crabtree s group [75] identified a much shorter, 1.7-1.8 A hydridic-protonic bond or dihydrogen bond [76]... 

Figure 6. The existence of an equilibrium between [Ir(D—DpyS-ri )(H" HpyS-Ti )(PCy3)2]BF4 and the unobserved q -dideuterium (or q2.djhydrogen) complex [Ir(q2. D2)(q -Spy)(H- HpyS-q )(PCy3)2]BF4 is proposed to explain the deuteration by D2(g) of hydridic-protonic bonds in the complex [Ir(H -HpyS-q )2(PCy3)2]BF4...

Since that time there have been many reports of intramolecular hydridic-protonic bonds [78-86]. Recently an intermediate with an intramolecular RuH—HN interaction has been implicated in the catalytic asymmetric reduction of ketones [87] and another, in the reduction of CO2 to formic acid [83]. In the last process, the RuH—HN bond is proposed to form via the heterolytic splitting of dihydrogen (see Scheme 4, Section 1.9). 

Scheme 2. The synthesis of tricyclohexylphosphine complexes of Ir(III) that contain hydridic-protonic bonds involving the p3aidinethione ligand.

Pyridinethione-iridium triphenylphosphine system and the effect of the ancillary ligands on the hydridic-protonic bond strength... 

Some unique structural types involving hydridic-protonic bonds were observed in triphenylphosphine complexes of iridium(III) [84, 88]. The reaction of IrH3(PPh3)3 with HBF4 in the presence of pyridinethione affords a dihydrido complex with a single hydridic-protonic contact, [IrH(H -HpyS-ii )(PPh3)3](BF4) (Scheme 3). 

The hydridic-protonic bond is weak in this complex as judged by the Ir-N distance and Av(NH) values from IR steric crowding by the three bulky PPhj... 

The observation of intermolecular hydridic-protonic bonds in complexes in solution is more challenging because often these energies are of the same magnitude as the 5 kcal/mol energy penalty for working against entropy at room temperature (eq. 26, AS<0). Proton transfer and subsequent loss of Hj to form M-X is a common reaction that must be avoided to observe eq 26. 

We first showed that hydridic-protonic bonds between ion pairs of anionic hydrides and potassium(aza-crown) cations that have one N-H hydrogen bond donor can be readily observed in solution and the solid state [16,17]. The anionic... 

Figure 10. A chain of [K(HNQNH)]+ cations and [IrH4(P Pr3)2] anions held together by hydridic-protonic bonds.

The X-ray structures of these crystalline salts indicate that short intermolecular hydridic-protonic bonds between the hydrides of the anion and the NH moieties of the cation cause the self-assembly of one-dimensional chains of pentagonal bipy-ramidal [MH5(P Pr3)2] anions and [K(diaza-18-crown-6)]+ cations (Figure 11). 

As in the case of the [K(QNH)]+ salts, the more basic ruthenium anion makes the stronger hydridic-protonic bonds as evidenced by the NMR and infrared spectral characteristics of the ruthenium and osmium salts. 

An exciting and mysterious observation is the rapid H/D exchange kinetics caused by these hydridic-protonic bonds. The complex [K(HNQNH)]+... 

Figure 13. The deuteration of the hydridic-protonic bonds in [K(HNQNH)]+ [RuH5(P Pr3)2]- in THF by reaction with D2 (1 atm).

Custelcean and Jackson have demonstrated that anionic boron hydrides can also serve as hydrogen bond acceptors to assemble novel lattices held together by hydridic-protonic bonds [107, 108]. Cations with hydrogen bond donors include A -[2-(6-aminopyridyl)]acetamidine and triethanolamine(sodium). The former is an NH donor while the latter is an OH donor. These scientists provided evidence for the topochemical conversion of the hydridic-protonic bonds into B-N and B-0 covalent bonds, respectively, when the materials are heated and dihydrogen is driven from the H-H bonds. 

Another ionic mechanism could involve the hydrogenation of polar C=E bonds from hydridic-protonic bonds (eq 27-29 where R, could be H, alkyl or aryl and E could be O, NR, CRCOSiRj). 

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