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

Article abstract of DOI:10.1016/j.jorganchem.2003.09.017

Reactions of orange-red dichloromethane solutions of Pd₂ X₂dppm₂ (X=Br, I; dppm=bis{diphenylphosphino} methane) with aqueous, concentrated HBr or HI at ambient temperature yields dark green solids which analyze for Pd₂HX₃ dppm₂ (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: Δ G ^?(-78°)=7.8±0.2 kcal mol^-1, Δ H ^?=7.5 kcal mol^-1, Δ S ^?=-1.3 e.u. and for 1b: Δ G ^? (-78°)=9.7±0.3 kcal mol^-1, Δ H ^?=8.9 kcal mol^-1, Δ S ^?=-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: Δ G ^?(-78°)=9.5±0.3 kcal mol^-1, Δ H ^?=8.2 kcal mol^-1, Δ S ^?=-6.9 e.u. Compounds 1a and 1b decompose to a 1:2 mixture of Pd₂X₂dppm₂ and PdX ₂dppm 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 Pd₂I₂dppm ₂. Decomposition of 1b is unchanged in the presence of styrene with no evidence for the formation of iodoethylbenzene or ethylbenzene.