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Counterion dependence on the synthetic viability of NHC-stabilized dichloroborenium cations
S. Muthaiah, D.C.H. Do, , D. Vidović
Published in
Volume: 32
Issue: 22
Pages: 6718 - 6724
The synthetic viability of several N-heterocyclic carbene stabilized dichloroborenium cations [NHC·BCl2]+ (NHC = (RC)2(NR′)2C; 1, R = R′ = Me; 2, R = H, R′ = iPr; 3, R = Me, R′ = iPr; 4, R = H, R′ = tBu; 5, R = H, R′ = 2,6-iPr 2-C6H3) in the presence of Cl-, AlCl4-, OTf- (Tf = O2SCF 3), NTf2-, and [BArCl 4]- (ArCl = 3,5-Cl2-C 6H3) was investigated. None of the target borocations could be synthesized in the presence of Cl-, as only neutral NHC·BCl3 compounds were observed. On the other hand, it was not surprising that all targeted cations were synthetically viable in the presence of AlCl4- but a different degree of interion interaction was evident from 11B NMR experiments. This was confirmed by X-ray analyses of [1·BCl2]+, [2·BCl 2]+, and [3·BCl2]+ in the presence of AlCl4- counterions, as the degree of cation-anion interaction was dependent on the steric encumbrance of the corresponding NHCs. Apart from [4·BCl2]+, no borocation was synthetically viable when OTf- and NTf 2- were used as the counterions. Finally, we were able to show that only [4·BCl2]+ could be synthetically viable without the counterion stabilization effect(s) as the preparation of [4·BCl2][BArCl4] was achieved. Even though the presence of [3·BCl2][BArCl4] was detected, this compound appeared not to be thermally stable, as it decomposed in solution after 48 h. The thermal stability of [4·BCl 2]+ and instability of [3·BCl2] + in the presence of [BArCl4]- was attributed to the presence and absence, respectively, of very weak intraion (agostic) interactions in these two borocations. © 2013 American Chemical Society.
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