Synthesis and spectroscopy of binuclear phosphine bridged palladium hydrides: Pd2HX3[dppm]2 (X=Br, I; dppm=bis[diphenylphosphino]methane)

Reactions of orange–red dichloromethane solutions of Pd2X2dppm2 (X=Br, I; dppm=bis{diphenylphosphino}methane) with aqueous, concentrated HBr or HI at ambient temperature yields dark green solids which analyze for Pd2HX3dppm2 (1a X=Br and 1b X=I). No reaction is observed between Pd2X2dppm2 and aqueous, concentrated HCl. Line shape analysis of dynamic 31P-NMR spectra for 1a and 1b over a 100 °C range indicates that in each case a system involving two sets of chemically equivalent 31P nuclei, mutually coupled, is exchanging 31P environments via initial exchange with a less populated intermediate system in which all four 31P nuclei are equivalent. From lineshape analysis of the 31P spectra, activation parameters for the rates going to the symmetrical intermediates are as follows: 1a: ΔG†(−78°)=7.8±0.2 kcal mol−1, ΔH†=7.5 kcal mol−1, ΔS†=−1.3 e.u. and for 1b: ΔG†(−78°)=9.7±0.3 kcal mol−1, ΔH†=8.9 kcal mol−1, ΔS†=−3.9 e.u. Similar analysis of dynamic 1H spectra for 1b over a 80 °C range reveals two exchanging Pd–H sites with activation parameters for the exchange: ΔG†(−78°)=9.5±0.3 kcal mol−1, ΔH†=8.2 kcal mol−1, ΔS†=−6.9 e.u. Compounds 1a and 1b decompose to a 1:2 mixture of Pd2X2dppm2 and PdX2dppm in solution with the evolution of hydrogen. Compound 1b reacts with PPh3 yielding [HPPh3+][I−] and Pd2I2dppm2 while reaction of 1b with KOH also yields Pd2I2dppm2. Decomposition of 1b is unchanged in the presence of styrene with no evidence for the formation of iodoethylbenzene or ethylbenzene.