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

Reaction of 2-M 2 with pyridine.
A vial was charged with 39.6 mg (0.0245 mmol) [1-M 2][B(C 6F 5) 4], 24.0 mg (0.0490 mmol)
tetraheptylammonium tetrafluoroborate, and ~0.6 mL C 6D 5Cl. Solid [HPt][PF 6] (15.7 mg,
0.0245 mmol) was added in portions to the C 6D 5Cl solution with stirring, and the contents were
added to a J-Young NMR tube. The tube was mixed by rotary motor overnight, at which point
NMR spectroscopy showed clean conversion to boroxycarbene 2-M 2. The NMR tube was
returned to the glovebox, and the contents of the tube were poured onto an internal standard, 20
mg (0.0228 mmol) (PPh 3) 2Re(<strong>CO</strong>) 3Br. The mixture was filtered into a J-Young NMR tube, and
NMR studies were carried out (there was no reaction or interaction with the Re-Br). Later,
pyridine (6 µL, 0.0735, 3 equiv) was added. Instead of the expected B-O bond cleavage and
downfield shift of the Re-CHO peak, a doublet at essentially unchanged chemical shift was
observed. The 1 H NMR resonances sharpened and were consistent with two inequivalent ligands
(two Ph 2PCH 2BR 2 groups). Two sharp 31 P NMR resonances were observed. Even with addition
of a few drops (~100 equiv) pyridine, no significant spectral change was observed. Based on
comparisons to the boroxycarbene spectra, the product is assigned as intramolecularly
coordinated boroxycarbene with the second pendent borane binding pyridine. The spectral
characteristics are quite similar to the low temperature spectra of 2-M 2 in the absence of
pyridine, suggesting an analogous structure. 1 H NMR (C 6D 5Cl, 300 MHz): δ 1.4-2.5 (br, m,
Ph 2PCH 2B(C 8H 14)), 1.83 (d, 13.7 Hz), 2.50 (d, 11.2 Hz), 6.9-7.1 (m, pyridine and Ph), 7.27 (br t,
J = 7.01, pyridine), 7.39 (m, 6H, Ph), 7.58 (m, 6H), 8.61 (br, pyridine), 13.66 (d, J = 10.3 Hz,
Re-CHO). 31 P{ 1 H} NMR (C 6D 5Cl, 121 MHz): δ 2.56 (d, J PP = 96.0 Hz, 1P), 7.41 (d, J PP = 95.8
Hz, 1P).
S67