Urea – boron trihalide adducts. IV. Ionic–covalent equilibria in the tetramethylthiourea–BF₃ and tetramethylselenourea–BF₃ systems and the influence of B₂F₇⁻ formation

Abstract

. 54,1121 (1976). Tetramethylthiourea and tetramethylselenourea form 1 :I BF3 adducts which are partially converted to ionic structures (donor)2BF2+.BF4-in solution. The degree of ionization is small except at low temperatures when BF3 is in excess; ionization is essentially complete at -90 "C when a sultcient excess of BF3 is present. Formation of B2F7-from BF4-and BF3 at low temperatures is responsible for this. Hence under these conditions the 1 :1 adduct is replaced by (donor)2BF2+.B2F7-. The existence of ionic adducts is further confirmed by the identification of BF3CI-and BF2C12-in the mixed-halogen system tetramethylthiourea-BF3-BCI3. On confirme I'existence des complexes ioniq~~es par l'identification de BF3CI-et BF2C12-dans la systkme halogCnCs mixtes tCtramCthylthiourCe-BF3-BCI3.Le fait que I'on observe la non-equivalence des groupes mCthyles dans le spectre rmn B des tempkratures plus basses que -70 "C indique qu'il y a empgchement B la rotation autour des liens centraux C-N de la tCtramCthylthiourCe coordonnCe.[Traduit par le journal]

Metal‐Free C−H Borylation of N‐Heteroarenes by Boron Trifluoride

“…This inspired us to look for additives, which could promote the borylation reaction further. Considering that formation of borocations in benzene cannot be completely excluded (Scheme 3), we turned out attention to tetramethylthiourea (TMTU) that proved assisting the autoionization of BF 3 [17] . Accordingly, we found that the addition of small (5 mol %) amounts TMTU ( 5 a ) to both BF 3 ⋅SMe 2 / 4 a and BF 3 ⋅SMe 2 / 4 d pairs increases the rate of 2 a formation (Figure 3).…”

Assessing the effects of covalent, dative and halogen bonds on the electronic structure of selenoamides

“…B(C 6 F 5 ) 3 (B). To the best of our knowledge, only two examples of Lewis pairs between a CQSe moiety and boron have been characterized, 27,33 of which only one at the solid state. Here, we suspended an equimolar amount of B and 1 in n-pentane, obtaining a solid, which was recrystallized from CH 2 Cl 2 /n-pentane.…”

“…The C-N bond of selenoamides has been known to be partially double since 1969. 30 This implies that the C-Se bond is also partially double and, indeed, its length can be influenced by the coordination of CQSe to a metal center 29,31 (even if not all the metals have an appreciable effect 32 ) or a boron trihalide 33 and, by weaker secondary interactions, as hydrogen bonding. 34 Therefore, it can be supposed that DH ‡ r will vary in the presence of different Lewis acids (LAs).…”