Oxidation and Protonation of Transition Metal Hydrides:  Role of an Added Base as Proton Shuttle and Nature of Protonated Water in Acetonitrile

The Cp 2 Fe + oxidation and the protonation of CpMoH(CO) 2 L (L: PPh 3, 1; PMe 3, 2 ) in MeCN have been investigated. In the dry solvent, the oxidation of both compounds consumes 1 mol of oxidant/mol of hydride with production of [CpMo(CO) 2 L(MeCN)] + (L: PPh 3, [ 3 ] +; PMe 3, [ 4 ] + ) and H 2 . The stoichiometry changes toward the consumption of 2 mol of oxidant in the presence of excess water when the oxidizing equivalents are added rapidly, either chemically or electrochemically. However, 1 oxidizing equiv is again sufficient to consume the hydride material completely under conditions of slow oxidation. Under comparable conditions, the more basic 2 leads to a lower [ox]/M−H stoichiometry. Protonation of 1 and 2 with HBF 4 ·Et 2 O in dry MeCN results in rapid H 2 evolution and formation of [ 3 ] + and [ 4 ] +, respectively, whereas the presence of excess water suppresses the H 2 evolution and gives rise to protonated water. However, this process is followed by slow and irreversible delivery of the proton back to 1 or 2 to afford [CpMoH 2 (CO) 2 L] +, which ultimately decomposes to [ 3 ] + or [ 4 ] + and H 2 . The dihydride complex is too unstable to be isolated, even when the protonation of 1 or 2 is carried out in a noncoordinating solvent. The proton delivery is faster for the more basic 2 and slower for the less basic 1 . Thus, water operates as a “proton shuttle”, whose speed depends on the basicity difference between the hydride complex and water. The identity of the protonated water in MeCN as [H(H 2 O) 3 ] + is suggested by an independent 1 H-NMR experiment in CD 3 CN.