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Sequential Dehydrogenative Borylation/Hydrogenation Route to Polyethyl-Substituted, Weakly Coordinating Carborane Anions

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journal contribution
posted on 03.04.2020, 17:44 by Eduardo Molinos, Simon K. Brayshaw, Gabriele Kociok-Köhn, Andrew S. Weller
Treatment of Rh(PPh3)2(1-H-closo-CB11H11) with ethene results in dehydrogenative borylation to form the 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 binding of the vinylcarborane to the {Rh(PPh3)2}+ fragment through CC and B−H interactions. Addition of H2 results in hydrogenation of the vinyl group and the quantitative formation of the B-ethylcarborane complex Rh(PPh3)2(1-H-7/12-(Et)-closo-CB11H10). The crystal structure of the norbornadiene adduct of one the isomers, [Rh(PPh3)2(nbd)][1-H-12-(H2CCH)-closo-CB11H10], has been determined. Addition of ethene to 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 of ethene/H2 to Rh(PPh3)2(1-H-7/12-(Et)-closo-CB11H10) results, after six cycles, in the pentaethyl-substituted complex (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 belt substituted. The degree of ethylation can be controlled. Replacing the hydrogen on the cage carbon with a bulkier substituent (methyl or SiiPr3) affords products in which only three B−H vertices have been substituted, and the solid-state structure of Rh(PPh3)2(1-Me-7,11,12-(Et)3-closo-CB11H8) shows that the antipodal 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 solution of Rh(PPh3)2(closo-CB11H12), which results in H/D exchange of the B−H vertices, suggesting that the metal fragment reversibly inserts into a B−H bond of the cage anion to form a boryl species. Attempts to 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)22-C2H4)3][1-H-closo-CB11H11], which has been characterized crystallographically as the [closo-CB11H6Br6] salt.