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

emoved from the cold bath and allowed to just start to thaw, at which point the tube was shaken
vigorously, and re-frozen. The tube was then inserted into the pre-cooled (– 40 °C) probe while
still frozen, and was allowed to thaw in the probe. Upon thawing, a single Re-containing species
was observed by multinuclear NMR experiments, characterized spectroscopically as the anionic
bis-boroxycarbene species [9] – . The species was quite stable at – 40 °C, but as the probe was
warmed to 0 °C the intensity of the signals of [9] – steadily decreased until no Re-containing
products were present in solution. Ejecting the NMR tube showed large amounts of pale yellow
precipitates; extraction with THF-d 8 proved these solids to be the ultimate double-reduction
product [3-E 2] – . Spectroscopic characterization of [9] – : 1 H NMR (C 6D 5Cl, 500 MHz, – 40 °C, all
peaks were broad): δ -0.15 (4H), 0.39 (4H), 0.66 (2H), 1.42 (2H), 1.57 (4H), 1.73 (16H), 1.98
(2H), 2.34 (2H), 2.64 (4H), 3.07 (4H), 7.58 (10H), 7.62 (10H), 16.43 (2H). 31 P{ 1 H} NMR
(C 6D 5Cl, 202 MHz, – 40 °C): δ 10.0 (s). 13 C{ 1 H} NMR (C 6D 5Cl, 126 MHz, – 40 °C): δ 13.35
(br), 25.57, 26.02, 28.02 (br), 30.29, 38.23, 39.71, 130.79, 132.11, 142.82 (br m; other aromatic
peaks are obscured by toluene and C 6D 5Cl peaks), 198.31 (m, Re-<strong>CO</strong>), 291.57 (Re-CHO).
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