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

Synthesis of mer,trans-(PPh 3) 2Re(<strong>CO</strong>) 3(CHO) (2-Ph 2).
An alternative route was followed: a 20 mL scintillation vial was charged with 95 mg (0.083
mmol) trans-[(PPh 3) 2Re(<strong>CO</strong>) 4][BPh 4] and 5 mL THF. With stirring, 83 µL (0.083 mmol)
LiHBEt 3 (1.0 M in THF) was added dropwise, during which time the colorless solution turned
yellow. The reaction was stirred for 5 minutes, filtered, and the solvents concentrated to 1 mL
under vacuum. A layer of 2 mL pentane was added, and the mixture placed in the freezer at -35
°C. A yellow powder formed overnight, which contained a mixture of 2-Ph 2 and LiBPh 4.
Another 10 mL pentane was added to the mother liquor of the first crystallization, which upon
cooling yielded a second crop of powder, which yielded ~30 mg pure 2-Ph 2 (45% yield). As
previously reported, the product reacts with CH 2Cl 2 and was sparingly soluble in most solvents;
an IR spectrum (KBr/nujol mull) matched the literature values well. 4
Single crystals of the BEt 3 adduct 2-Ph 2•BEt 3 were obtained in a separate reaction by
adding 10 equiv BEt 3 (1.0 M in hexanes) to a THF solution of 2-Ph 2 (prepared as above), which
was then layered 1:1 with pentane and cooled to -35 °C. Long colorless needles suitable for XRD
needles grew over a few days. See below for full crystallographic details.
Titration of 2-Ph 2 with t Bu(CH 2) 2B(C 8H 14).
The equilibrium constant for adduct formation between formyl 2-Ph 2 and a t Bu(CH 2) 2B(C 8H 14)
(Scheme S1) was carried out by titration, with monitoring by 1 H NMR. Due to the insolubility of
2-Ph 2, an internal standard was used to quantify the concentration. A sample of 15.2 mg (0.0185
mmol) was weighed and 0.6 mL THF-d 8 was added. The slurry was filtered into a J-Young NMR
tube, and a 2.8 mg crystal (0.0106 mmol) 18-crown-6 was added to the tube. An initial NMR
S13