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

the experimental values of c were not 6, but varied from roughly 5.75-5.85. The molecules were
assumed to be roughly spherical, and no further corrections were applied.
The PGSE NMR experiments were carried out in two phases, one with unknown
aggregation, the other assumed to be strictly monomeric, and the size of the species compared.
First, an NMR tube was charged with a C 6D 5Cl solution of 2-E 2 and (PPh 3) 2Re(<strong>CO</strong>) 3Br (6)
(internal standard of similar size and shape). The 2-E 2 solution was prepared either A) in situ (by
treating [1-E 2][BF 4] with 1 equiv NaHBEt 3), or B) by dissolution of crudely isolated 2-E 2 (by
precipitation of a C 6H 5Cl solution as prepared in situ, collection on a frit, and drying under
vacuum). 1 H and 31 P{ 1 H} NMR experiments confirmed that 2-E 2 and 6 were the major species. A
1 H PGSE NMR experiment was carried out on this mixture, under which conditions the
speciation of 2-E 2 was in question. After the first PGSE experiment was complete, the tube was
returned to a nitrogen-filled glove box, and an excess of pyridine (3-12 equiv) was added by
syringe, and the tube sealed. 1 H and 31 P{ 1 H} NMR experiments confirmed that 2-E 2 had been
converted to the bis(pyridine) adduct 2-E 2•(pyridine) 2, which has a significantly downfield-
shifted formyl resonance, and a single 31 P resonance (see spectroscopic details elsewhere in the
SI). There was no change in chemical shift or lineshape of 6 upon addition of pyridine,
confirming that no interaction occurs with the internal standard. The excess pyridine appeared
close to the expected shift of free pyridine in C 6D 5Cl, 17 indicating any exchange of bound and
free pyridine must be slow on the NMR time-scale. A second 1 H PGSE NMR experiment was
then carried out, and in these conditions the Re formyl was assumed to be strictly monomeric.
The data was processed as described above, providing D t values for all species in the
presence and absence of pyridine. Initial measurements provided a ratio of hydrodynamic
volumes of 2-E 2:6; then excess pyridine was added to the NMR tube, and the experiment was
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