Polyborane formation

Transition metals such as It were used to control polyborane formation. Metal-assisted borane condensation is demonstrated by the formation of [ara A) o-2,5- Cp ltH]2B4Hs] 791 by the reaction of two BHs THF moieties with the metal centers of [ Cp IrH 2(/r-H)B2Hs] 792. Mild thermolysis of 791, which contains separated B2H4 fragments, results in H2 elimination and the formation of the [ /i ( i-l,2- Cp"lr 2(/t-H)B4H7] 793. 

This section reviews the developments in the chemistry of monoborane complexes of the transition metals especially borohydride and hydridoborate complexes. Although such complexes are not strictly metallaboranes in the sense that they are not cluster species, they are included here as they share many similarities with polyborane species of the transition metals such as three-center two-electron bonding. Additionally, as will be shown in Section 3.04.3.1 borohydride species can also be intermediates in the formation of larger M By clusters. In this chapter, three-coordinate monoborane species, which are best considered as cr-complexes between a transition metal and HBR2 or metal-boryl (M-B) species, are not considered. 

In general the deprotonation of a polyborane B H +m (to = 4,6) leads to the anions [BnHn+m-i] or [B H +m 2]2 by removal of one or two protons from a BHB 3c2e bridge with formation of a B-B single bond. Cluster expansion with a BH3 unit, usually offered as diborane in diethyl ether or tetrahydrofuran, produces borane anions [B +1H +m+2] or [B +1H +m+1]2 and these in turn on protonation give the polyboranes Bn+iHn+m+-. These may be stable species. However, in most cases they loose H2 which results in a cluster expansion by one BH unit. Several examples of this method are described in the following sections. 

At the same time, interaction of the neutral base 1 with polyboranes or boron trifluoride does occur even at low temperatures (Scheme 17)179. NMR monitoring showed that while 1 with diborane or BF3 gave salts of similar type, 144 and 145, interaction between 1 and decaborane resulted in proton transfer, and reaction with pentaborane led to the formation of ionic compound X+ B9II 4, whose cationic part (X+) remained unidentified. 

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