Homogeneous CO Hydrogenation: Ligand Effects on the Lewis Acid ...

D. Mechanism Studies on the Reduction of [1-E 2] + .
Reaction of [1-E 2][BF 4] with [HPt][PF 6] at -40 °C.
A J-Young NMR tube was charged with a ~0.2 mL C 6D 5Cl solution of 22.4 mg (0.0213 mmol)
[1-E 2][BF 4]. The solution was frozen in a liquid nitrogen cooled cold well in the glove box. A
~0.1 mL layer of C 6D 5Cl was added, and the tube was cooled to freeze the solvents again.
Finally, a layer of ~0.3 mL C 6D 5Cl solution of 13.6 mg (0.0213 mmol) [HPt][PF 6] was added,
and the solution frozen. The tube was sealed, removed from the glove box, and inserted into a
dry ice / acetone bath to keep the contents of the tube frozen. Meanwhile, the NMR probe was
cooled to -40 °C. The tube was allowed to just start to thaw, at which point it was shaken
vigorously for a few seconds, and then inserted into the -40 °C probe. NMR spectra were
obtained at various times and temperatures, with the temperature slowly being raised to 0 °C, at
which point the reaction was complete. In separate experiments a broad 31 P signal at -5.9, slightly
downfield of [HPt][PF 6] (δ -7.2) was always observed (sometimes an additional sharp peak for
[HPt][PF 6] was observed), appearing to be an intermediate of hydride transfer from Pt to [1-E 2] + .
A set of two doublets at δ 1.15 (J PP = 71 Hz) and δ 4.5 (J PP = 75 Hz) were also observed, which
disappeared at roughly the same rate as the broad hydride signal.
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