Bimetallic Hydroformylation Catalysis: In Situ Characterization of a Dinuclear Rhodium(II) Dihydrido Complex with the Largest Rh–HNMR Coupling Constant
✍ Scribed by Rhonda C. Matthews; Donna K. Howell; Dr. Wei-Jun Peng; Spencer G. Train; Dr. W. Dale Treleaven; Prof. George G. Stanley
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 557 KB
- Volume
- 35
- Category
- Article
- ISSN
- 0044-8249
No coin nor oath required. For personal study only.
✦ Synopsis
T = 25 "C, t = 3.5 h) also showed that complex 1 is able to perform more than 36 000 (yield 99.9 YO) without loss of activity.
In order to rule out the participation in the catalysis of free, solvated [Rh(PPh,),]+ or similar cationic species formed by dissociation of complex 1, two independent experiments were made. First. the addition of four equivalents of Cs-2 did not alter the rate of reaction, which suggests that the common ion effect is absent. Second, hydrogenation experiments with added D,O provided further evidence to eliminate this hypothesis. If [Rh(PPh,),]+ or a related species were present, partially deuterated hexane would be found.["' By GC-MS analysis it was shown that the hexane formed in the reaction was not deuterium-enriched.
The first e.w-monothiocarborane rhodium complex has been synthesized, and it catalyzed the hydrogenation of 1 -hexene eight times faster than Wilkinson's catalyst. The thioether group directly bonded to the cluster's open face enables discrimination between upper and lower belt B-H groups, leading to the formation of a B(l I)-H-Rh bond. This supports the initial hypothesis that the favoring of an exo-nido upper-belt form would readily enhance the hydrogenation activity of a rhodacarborane molecule.
Experimental Procedure
I : [RhCI(PPh,),J (203 mg. 0.22 mmol) was added to a solution of Cs[7-SPh-S-Me-7.X-C2B,H,,] (90 mg. 0.23 mmol) in toluene/ethanol (811, 28 mL) under N, atmosphere. The solution was stirred at room temperature for 18 h until a red precipitate formed. T H F was added until the red solid dissolved to give an orange solution and a white precipitate of CsCI. which was removed by filtration through Celite The orange filtrate was concentrated in vacuo to 5 mL, and ethanol (20 mL) was added.
The resulting red solid was isolated by filtration. washed with ethanol (2 x 5 mL) and ether (2 x 5 mL). and dried in vacuo. Yield: 176 mg (91 O h ) . If a lower volume of toluene is used in the synthesis, the product precipitates as an orange solid which, upon stirring. is slowly converted into the red crystalline form. IR(KBr): i. = 2527 (s. BH). 21 12 cm ~ ' (B -H -Rh); ' H( B} NMR (300 MHz. CD,CI,, 25 "C. TMS):