¹¹B NMR of Diborane and Derivatives

Abstract

The interest of theoretical chemists in diborane is paralleled by many attempts of NMR spectrocopists to provide a complete set of NMR data for the diborane molecule. The pure sample gives a complex ¹ H and a first order ¹¹ B NMR spectrum, which have both been completely analyzed using various techniques (e.g., isotopic enrichment, variable temperature,¹ H {¹¹ B} double resonance, etc.) [68/58; 72/53]. From ¹ H and ¹¹ B NMR spectra the difference in BH coupling was shown to reflect the presence of terminal (H _t ) and bridge hydrogens (H _b ). For a general discussion of ¹¹ B chemical shifts the δ¹¹ B value of diborane is of great importance since diborane is the basic compound with three-center two-electron bonds. The ¹¹ B resonance occurs at higher field as compared to most of the trivalent boron compounds; this can be interpreted as the result of the increase in coordination number of boron from three to four and/or an increased overall charge density at boron. J(¹¹ B¹¹ B) values have been used to check on the nature of BB bonds [76/50] and the results are in excellent agreement with Lipscomb’s description of multiple center bonding using fractional bonds [75/10]; the B-B coupling in diborane was found close to zero, which proves a very low BB bond order. It has be shown that in the case of diborane, where the two boron atoms are linked by two BHB bridges [65/20; 68/55], that the electron density along the internuclear BB axis is very low [69/63; 70/53; 75/70], in accordance with a BB bond length larger than in compounds with direct BB bonds [73/70; 76/55; 74/3; 71/70].