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

epeated. If 2-E 2 were strictly monomeric in solution, the ratio would be expected to increase,
since pyridine binding would increase the hydrodynamic volume; if 2-E 2 were strictly dimeric,
the second ratio should be close to 0.5 times the first, as the monomer should have roughly half
the hydrodynamic volume of the dimer. The values of the ratio were 2.34(7) before pyridine
addition and 1.6(1) after; the decrease by a factor of 0.69(5) could reflect a monomer-dimer
equilibrium, giving a time-averaged PGSE value. 18 Variable-temperature NMR experiments
showed a subtle sharpening of the 1 H and 31 P resonances of 2-E 2 at low temperatures, but
warming the solution led to decomposition, confounding complete analysis.
The uncertainty in the hydrodynamic volumes obtained from PGSE NMR experiments is
significant; therefore no quantitative analysis was undertaken. The qualitative results are quite
certain, however: the intermediate values for the speciation of 2-E 2 are reproducible under two
different conditions (two runs of each experiment A and B) and are inconsistent with a strictly
monomeric solution structure (in which case 2-E 2•(pyridine) 2:6 would be expected to be slightly
larger than 2-E 2:6). Furthermore, the observed reactivity of 2-E 2 is also consistent with a
monomer-dimer equilibrium.
C. Reactivity Studies of 2-E 2.
Reaction of 2-E 2 with pyridine.
As part of a diffusion NMR experiment (see details of NMR experiment elsewhere in this SI),
26.0 mg (0.0247 mmol) [1][BF 4] was treated with 24.7 µL (0.0247 mmol) NaHBEt 3 (1.0 M in
toluene) by syringe in ~0.6 mL C 6D 5Cl to form carbene 2-E 2. 21.6 mg (0.0247 mmol)
(PPh 3) 2Re(<strong>CO</strong>) 3Br was added as an internal standard, and an NMR was taken. Roughly 45
minutes later, 6.0 µL (0.0741 mmol, 3 equiv) pyridine was added by syringe, and the reaction
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