Dihydrogen Bonding and Proton Transfer from MH and OH Acids to Group 10 Metal Hydrides [( t Bu PCP)MH] [ t Bu PCP = κ 3 ‐2,6‐( t Bu 2 PCH 2 ) 2 C 6 H 3 ; M = Ni, Pd]

The interactions of HA acids {indole, fluorinated alcohols, phenols, and [CpW(CO)3H] (2)]} with the title hydrides [(tBuPCP)MH] [M = Ni (1a), Pd (1b)] have been studied by a combination of spectroscopic (variable-temperature IR, NMR, UV/Vis) and computational (DFT/M06, AIM) methods in THF and toluene. The formation of the dihydrogen bond (DHB) 1···HA is the first step in a process leading to proton transfer and H2 evolution. The DHBs of 1 with 2 are much weaker than those of 1 with NH or OH acids, but the former complexes are more reactive. Kinetic studies of the reactions of 4-(4′-nitrophenylazo)phenol and [CpWH(CO)3] with 1b in THF gave activation enthalpies ΔH≠ = 9.2 ± 0.4 and 6.5 ± 1.5 kcal mol–1 and activation entropies ΔS≠ = –30 ± 1 and –35 ± 6 cal mol–1 K–1, respectively. Calculations revealed the (η2-H2)-like TS (ΔE≠ = 9.3 kcal mol–1) for the reaction of 1b with p-nitrophenol and the [Pd(η2-H2)]+OAr– complex as a local minimum at around 5 kcal mol–1 above the DHB adduct. In the reaction of 1b with 2, both the acidic WH and hydridic NiH or PdH bonds undergo heterolytic cleavage (ΔE≠ = 7.4 kcal mol–1) to yield the unusual µ,η1:1-H2 end-on complex. The µ,η1:1-H2 molecule transforms easily into the more stable η2-H2 side-on tautomer, which eventually gives the bimetallic product after H2 evolution.