Diboron Tetrafluoride

Stable Lewis Base Adducts of Tetrahalodiboranes: Synthetic Methods and Structural Diversity

“…As previously observed, the BÀBd istance is governed by the stericso ft he diphosphine spacer,w hereas B 2 Br 4 (dppa iPr ) (dppa iPr = bis(diphenylphosphine)isopropylamine) and B 2 Br 4 (dppa PhF )( dppa PhF = bis(diphenylphosphine)-p-fluoroaniline), with their small amino spacer, display identical BÀBb ond lengths (1.714 (7) and 1.713(3) ,r espectively), similart ot hose of the lesss terically demandingB 2 Br 4 (PR 3 ) 2 adducts (R 3 = Me 3 , Et 3 ,M e 2 Ph:B ÀB1 .708(7)-1.718(5) ), B 2 Br 4 (dppf) (dppf = 1,1'bis(diphenylphosphine)ferrocene), with its large ferrocene spacer,d isplays am uch longerB ÀBb ond of 1.752 (6) .F urthermore, the (P,B,B,P)t orsion angle widensw ith the size of the spacer,f rom dppa PhF (22.6(1)8)o ver dppa iPr (32.7(3)8)t o dppf (50.5(5)8), because the molecule strives to reach as taggered gauche conformation with minimal ligand repulsion. The effect of the ligand electronicsi ss een in the two dppa analogues, in whicht he BÀBr bonds of the more electron-withdrawing dppa PhF derivative( 2.058(2), 2.068(2) )a re slightly longer than in the dppa iPr derivative (2.027(5)-2.042(5) ) whereas the BÀPb onds in the former (1.956(2) )a re significantly shorter than in the latter (1.980(5), 1.995 (5) ).…”

“…In diethyle ther at À78 8C, for example, B 2 F 4 and B 2 Cl 4 form isolable bis-adducts with the solvent, which upon warming decompose to the mono-etherates, which in turn decompose by ether cleavage (Scheme 1a). [5,6] With SH 2 and PH 3 ,B 2 Cl 4 forms bis-adducts at À78 8C, which decompose upon warming, liberating H 2 and PH 4 Cl, respectively (Scheme 1b,c). [11] With weaker phosphine donors, such as PCl 3 and PBr 3 ,a dduct formation with B 2 Cl 4 and B 2 Br 4 was found to be reversible in solution (Scheme 1d).…”

“…First obtained in 1925 in poor yields from the reduction of BCl 3 by a zinc arc discharge, B 2 Cl 4 is a highly pyrophoric liquid that decomposes rapidly at 0 °C. The optimization and scaleup of its synthesis by Schlesinger allowed for its use as a precursor for the convenient solution‐phase syntheses of B 2 F 4 , B 2 Br 4 , and later B 2 I 4 by halogen exchange. Although B 2 F 4 is stable up to moderately elevated temperatures, B 2 Br 4 has to be stored below −40 °C and B 2 I 4 below 0 °C under the exclusion of light.…”

“…Although these adducts are in general electronically more stable than their B 2 X 4 precursors, the early days of this chemistry were marked by recurring problems of ligand lability and adduct decomposition. In diethyl ether at −78 °C, for example, B 2 F 4 and B 2 Cl 4 form isolable bis‐adducts with the solvent, which upon warming decompose to the mono‐etherates, which in turn decompose by ether cleavage (Scheme a) . With SH 2 and PH 3 , B 2 Cl 4 forms bis‐adducts at −78 °C, which decompose upon warming, liberating H 2 and PH 4 Cl, respectively (Scheme b, c) .…”

sp2–sp3diboranes: astounding structural variability and mild sources of nucleophilic boron for organic synthesis

“…Similar results were reported nine years later by the same group using tetrafluorodiborane(4), resulting in a ''crystalline, non-volatile solid at 0 1C'' claimed to be the corresponding monoetherate (1b). 16 In the same year, Wartik and Apple reported the isolation of a white solid (2) at À78.5 1C from the treatment of B 2 Cl 4 with dimethylsulfide in a 1 : 1 molar ratio. However, like Schlesinger's results, these are far from conclusive as to the presence of a neutral monoadduct of the diborane(4).…”