Sequential Dehydrogenative Borylation/Hydrogenation Route to Polyethyl-Substituted, Weakly Coordinating Carborane Anions
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- 作者：Eduardo Molinos ; Simon K. Brayshaw ; Gabriele Kociok-Kö ; hn ; Andrew S. Weller
- 刊名：Organometallics
- 出版年：2007
- 出版时间：April 23, 2007
- 年：2007
- 卷：26
- 期：9
- 页码：2370 - 2382
- 全文大小：401K
- 年卷期：v.26,no.9(April 23, 2007)
- ISSN：1520-6041
文摘
Treatment of Rh(PPh3)2(1-H-closo-CB11H11) with ethene results in dehydrogenative borylation to formthe vinyl-borate complex Rh(PPh3)2(1-H-7/12-(H2C=CH)-closo-CB11H10) as a mixture of 7- and 12-isomers. Further dehydrogenative borylation does not occur; this is accounted for by the strong bindingof the vinylcarborane to the {Rh(PPh3)2}+ fragment through C=C and B-H interactions. Addition of H2results in hydrogenation of the vinyl group and the quantitative formation of the B-ethylcarborane complexRh(PPh3)2(1-H-7/12-(Et)-closo-CB11H10). The crystal structure of the norbornadiene adduct of one theisomers, [Rh(PPh3)2(nbd)][1-H-12-(H2C=CH)-closo-CB11H10], has been determined. Addition of etheneto the complex Rh(PPh3)2(1-H-12-Br-closo-CB11H10), in which the 12-position on the cage is blocked,results in only one isomer: Rh(PPh3)2(1-H-7-(CH2=CH)-12-Br-closo-CB11H9). Sequential addition ofethene/H2 to Rh(PPh3)2(1-H-7/12-(Et)-closo-CB11H10) results, after six cycles, in the pentaethyl-substitutedcomplex (characterized as the nbd salt) [Rh(PPh3)2(nbd)][1-H-2,4,8,10,12-(Et)5-closo-CB11H6]. The solid-state structure shows that the antipodal boron vertex, two lower pentagonal belt vertices, and two upper-belt vertices have been functionalized, with no two adjacent vertices on the same pentagonal beltsubstituted. The degree of ethylation can be controlled. Replacing the hydrogen on the cage carbon witha bulkier substituent (methyl or SiiPr3) affords products in which only three B-H vertices have beensubstituted, and the solid-state structure of Rh(PPh3)2(1-Me-7,11,12-(Et)3-closo-CB11H8) shows that theantipodal boron vertex and two lower pentagonal belt vertices have undergone dehydrogenative borylation.Mechanistic insight into the dehydrogenative borylation comes from addition of D2 to a CH2Cl2 solutionof Rh(PPh3)2(closo-CB11H12), which results in H/D exchange of the B-H vertices, suggesting that themetal fragment reversibly inserts into a B-H bond of the cage anion to form a boryl species. Attemptsto observe intermediates in the actual hydroboration process by addition of ethene to Rh(PPh3)2(1-H-closo-CB11H11) resulted in the observation of the tris(ethene) complex [Rh(PPh3)2(
mages/gifchars/eta.gif" BORDER=0 >2-C2H4)3][1-H-closo-CB11H11], which has been characterized crystallographically as the [closo-CB11H6Br6] salt.