Carboranes metallacarboranes

SPh boron detenuination in form of complex with Azomethine-H is the reliable microanalytical method for boronorganic compounds, carboranes, metallacarboranes, etc. Before boron determination the samples should be melted with KOH at 900°C. 

In Chapter 15 we observed that the 18-eleciron rule was adequate for predicting stabilities of small organometallic clusters. In this chapter we have seen that Wade s rules allow us to make predictions about borune structures based on the number of framework electrons. These rules al.so are adequate for most carboranes, metallacarboranes, and other heteroboranes.Furthermore, organometallic clusters that are not derived from boranes can be dealt with in a similar fashion. More sophisticated extensions are required for complex larger clusters. 9... 

Many novel cluster compounds have now been prepared in this way, including mixed metal clusters. Further routes involve the oxidative fusion of dicarbon metallacarborane anions to give dimetal tetracarbon clusters such as (103) and (104) O (jjg insertion of isonitriles into inetallaborane clusters to give monocarbon meiallacarboranes such as (105) and the reaction of small ii/t/o-carboranes with alane adducts such as Et3NAlH3 to give the commo species (106) ... 

Metal-vapor synthesis also prepares metallaboranes and metallacarboranes with oxidation of the metal. Thermally generated Ni, Co and Fe atoms react with the nido-carborane 2,6-C2B7H, and either cyclopentadiene, toluene, mesitylene, or... 

Serendipitous insertion of CsHs- into the metallaborane Cp 2Cr2B4Fl8 gave the triple-decker sandwich 122 in low yield.140 This compound can be described as an eight-vertex Cr2C2B4 c/ara-metallacarborane cluster, and its preparation is notable as one of the few examples of metallacarborane synthesis from a metallaborane precursor (rather than by the usual metal insertion into a carborane), an approach first demonstrated by Grimes and co-workers in the 1970s.34,141,141a... 

Our discussion will be restricted to metallacarboranes in which the r/-block metal is incorporated into the polyhedral framework of the carborane. No attempt will be made to cover those compounds where the transition metal atom is in a bridging group linking several carborane polyhedra together or is involved solely as a member of a substituent group. Since much of the insight into the chemistry of the metallacarboranes is obtained from structural and bonding... 

Tc-carborane complex reported to date. A new method for the preparation of Re- and "mTc-metallacarboranes in water under mild reaction conditions was later developed by the same investigators.673... 

The stability of the liquid carboranes depends on the substituents R at carbon and boron. The axial (endo) hydrogen atom is acidic and involved in 3c2e bonding to one of the basal boron atoms. In the 1H NMR it exhibits a high field shift near S = —1.4. Deprotonation of 55 with potassium or Bu Li leads to the anion (55-H)-, which is isolobal with C5H5. Reactions of 55 and (55-H) with appropriate metal complexes lead to metallacarboranes with sandwich structures [67, 69],... 

The material reviewed in this Chapter hitherto has focused on metallacarboranes in which the metal atom is a vertex in an icosahedral cage framework. Until recently, monocarbollide metal compounds with core structures other than 12 vertexes were very rare since suitable carborane precursors were not readily available." However, Brellochs recent development of the reaction of decaborane with aldehydes to give 10-vertex monocarboranes permits a considerable expansion in this area of boron cluster chemistry. As a consequence, several intermediate-sized monocarboranes are now easily accessible and we have recently begun to exploit the opportunities that these present. In particular, we have focused thus far on complexes derived from the C-phenyl-substituted species [6-Ph- zJo-6-CBgHii] It is clear from these initial studies that a wealth of new chemistry remains to be discovered in this area, not only from among the metal derivatives of PhCBg car-boranes such as those discussed in this section, but also in the metal complexes of other newly available carboranes. 

Many structures are possible for the smaller metallacarboranes and various synthetic strategies are available. Especially noteworthy is the occurrence of triple- and tetradecker sandwich compounds (214). The polyhedral expansion synthetic strategy can also be used with small carboranes (212). For instance, the small metallacarborane f/ojo-l,l,l-(CO)3-l,2,3-FeC2B4H6 [3276140-3] (212) is obtained from C2B4Hg upon treatment with Fe(CO)5. 

Main Group Element Carborane Derivatives. Main group element carborane derivatives have been reviewed (231). Only a few alkaline-earth element metallacarborane derivatives have been characterized. The icosahedral beryllacarborane, f/ojo-3-[(CH3)3N]-3,l -BeC2B9H11, shown in Figure 24a, has been prepared via the reaction of /< -7,8-C2B9H13 and Be(CH3)2 [0(C2H3)2]2 followed by reaction of the diethyletherate product and trimethylamine (234). 

In addition to that said above, complexes with closopolyhedral hydroborate metal-containing fragments are known, for example 564, obtained through the interaction of tetramethylammonium salt of nido-B9H12 anion and nickelocene [957], An up-to-date covering of carborane and metallacarborane structures and properties is given in Ref. 184f. 

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