Observations on Alkylboranes

Abstract

Some synthesis and exchange reactions of diborarle and its allcyl derivatives and of the higher boron hydrides were reviewed as baclrground for new work.In pursuit of an idea prorripted by knowledge of the deuterium and BI0 exchanges found for the reactive and unstable higher boron hydrides and BeH11, mono-and di-methyldih y d r~~e n t a b o r a n e arid methyltetraborane ha\-e been prepared by direct reaction between the parent higher hydrides and 1,2-dimethyldiborane or monomethyldiborane. The identity and the reactions of each product have been delineated by gas chromatographic separation, gas density, and mass, infrared, and 1l.m.r. spectrotnetry. The H' n.ni.r. spectrum of dihydro~e n t a b o r a n e has been reinterpreted and used to decide that monomethyldihydrope~ltaborane is probably a B 111 derivative which displays stereolsomerism.T o confine the topic within proportioils suitable for a symposium this paper concerns mostly alkylation reactions in which carbon atoms are transferred fronl sites in one boron coillpound to sites in another, thus excluding allnost conlpletely the extensive material on hq droborntion.As a preliminary, coilsider some facts about the boron hydrides. They occur in two types, the open pqramidal and the closed pqrainidal structures, with the commonest members, diborane, tetraborane-10, pentaborane-11, as examples of the first class and pentaborane-9 and decaborane-14 of the second. The former are the labile, reactive hydrides, the latter the ones of lesser reactivity. Exchange and interconversion reactions show the labile hydrides to maintain effective steady-state concentrations of reactive f r a g m e n t s~ the more stable ones do not (1, 2).Diboraile is convertible to the other hydrides by pyrolysis (3) and by interactions such as that with tetraborane (4):Three boron and deuterium exchanges-exemplify models for alkylation reactions: (a) self-exchange of diborane, (b) diborane exchange with pentaborane-9, and (c) diborane exchange with pentaborane-11. The first is 3/2 order involving successively dissociation of diborane and a borane-diborane displacement reaction with E* = 22 kcal. Deuterium exchanges three times faster than boron. Similarly, the second is 1/2 order in diborane (dissociation) and first order with respect to pentaborane-9, but without boron exchange, E+ = 27 kcal. Since the third, pentaborane-11 -diborane exchange, is 1/2 order with respect to the higher hydride and first order in diborane the dissociation is that of pentaborane-11 and the displacement is borane-diborane. With these varying patterns 'Presented at the Synzposium on Organometallic Compounds, Vancouver, British Columbia, September 4-6, 1962, sponsored

X-ray and Synchrotron FTIR Studies of Partially Decomposed Magnesium Borohydride

“…At the same time, the stretching modes of terminal B-H in the pentaboranes are shifted to higher frequencies with respect to the modes 1,2 in Table 4 (>2548, and 2460 cm −1 ), one of the ν(B-H-B) is located below 2000 cm −1 (clearly absent in Figure 3a); a few prominent lower frequency modes do not correspond well with features 9-11 from Table 4 (1164, 1043, 951, 899 cm −1 for B 5 H 11 [39] and 1045-1035, 900 cm −1 for B 5 H 9 [40]). In B 4 H 10 , the most prominent structural feature is the dual pair of bridges at 2120 cm −1 holding the molecule together [39]. The intensity of these modes is as strong as ν(B-H), which is certainly not the case for the weak features 5,6.…”

“…Some boranes and borane anions, such as B 5 H x (x = 9, 11) have strong IR absorptions in the region 1620-1400 cm −1 assigned to B-H bending modes [39][40][41]. At the same time, the stretching modes of terminal B-H in the pentaboranes are shifted to higher frequencies with respect to the modes 1,2 in Table 4 (>2548, and 2460 cm −1 ), one of the ν(B-H-B) is located below 2000 cm −1 (clearly absent in Figure 3a); a few prominent lower frequency modes do not correspond well with features 9-11 from Table 4 (1164, 1043, 951, 899 cm −1 for B 5 H 11 [39] and 1045-1035, 900 cm −1 for B 5 H 9 [40]). In B 4 H 10 , the most prominent structural feature is the dual pair of bridges at 2120 cm −1 holding the molecule together [39].…”