Dihydrogen Bonding and Proton Transfer from MH and OH Acids to Group 10 Metal Hydrides [(tBuPCP)MH] [tBuPCP = ?3-2,6-(tBu2PCH2)2C6H3; 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 ?H? = 9.2 ± 0.4 and 6.5 ± 1.5 kcal mol-1 and activation entropies ?S? = -30 ± 1 and -35 ± 6 cal mol-1 K-1, respectively. Calculations revealed the (?2-H2)-like TS (?E? = 9.3 kcal mol-1) for the reaction of 1b with p-nitrophenol and the [Pd(?2-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 (?E? = 7.4 kcal mol-1) to yield the unusual ?,?1:1-H2 end-on complex. The ?,?1:1-H2 molecule transforms easily into the more stable ?2-H2 side-on tautomer, which eventually gives the bimetallic product after H2 evolution.