Other Compounds of Boron

For (95), long range isotope effects have been elaimed on H, J( T1 H) 

Figuet, M.T. Averbuch-Pouchot, A.D. d Hardemare and O. Jaqayes, Eur. J. Inorg. Chenc, 2001,2089. 

Factors affecting the ability of potentially chelating amine ligands to form chelated fluoroboron cations have been explored by and NMR spectroscopy/and NMR spectroscopy has been compared for detection of the neutron capture therapy agent, borocaptate sodium/ The uptake of BSH in M2R melanoma cells has been monitored by B NMR spectroscopy/ 

1329 J Groh, G. Elter, M. Noltemeyer, H.-G. Schmidt and A. Meller, Main Group Met. Chem., 2001, 23, 709. 

Schnieder and P. Lonnecke, J. Chem. Soc., Dalton Trans., 2001, 3459. 

7977 I I Padilla-Martinez, F.J. Martinez-Martinez, A. Lopez-Sandoval, K.I. Giron-Castillo, M.A. Brito, and R. Contreras, Eur. J. Inorg. Chem., 1998,1547. 

7979 H. Hommer, H. Noth, J. Knizek, W. Ponikwar, and H. Schwenk-Kircher, Eur. J. 

7975 N. Andrade-Lopez, R. Cartas-Rosado, E. Garcia-Baez, R. Contreras, and H. Tla-huext, Heteroat. Chem., 1998, 9, 399 Chem. Abstr., 1998,129, 136 213). 

---

In this case the covalency of boron is brought up to four because the donor molecule supplies the necessary electrons. The adduct formed, trimethylamine-borane, is a stable white solid. Other compounds of a similar kind are known, all derived from the simple structure H3N -> BH3. This compound is isoelectronic with ethane, i.e. it contains the same number of electrons and has the same shape. 

Lewis acids are defined as molecules that act as electron-pair acceptors. The proton is an important special case, but many other species can play an important role in the catalysis of organic reactions. The most important in organic reactions are metal cations and covalent compounds of metals. Metal cations that play prominent roles as catalysts include the alkali-metal monocations Li+, Na+, K+, Cs+, and Rb+, divalent ions such as Mg +, Ca +, and Zn, marry of the transition-metal cations, and certain lanthanides. The most commonly employed of the covalent compounds include boron trifluoride, aluminum chloride, titanium tetrachloride, and tin tetrachloride. Various other derivatives of boron, aluminum, and titanium also are employed as Lewis acid catalysts. 

Details of other uses of boron compounds are noted at appropriate places in the text. 

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

It is now recognized that iminoboranes are not merely unique intermediates but rather are members of an independent class of stable compounds which has its own chemistry such as the aminoboranes, borazines, or other examples of boron-nitrogen compounds. 

Similar icosahedral Bu groups are found in other modifications of boron and also in the compound BUC , in which there are linear C groups.70... 

Among tile other inorganic compounds of boron are the following ... 

The great number of compounds of boron is due to the readiness with which boion atoms fonn, to some extent, chain structures with other boion atoms, and, to a far greater extent, cyclic compounds, both with other boron atoms, mid with atoms of carbon, oxygen, nitrogen, phosphorus, arsenic, the halogens, and many other elements. Examples of Lhcm arc shown below, beginning with the two pentaboranes B5 Hc> and Bs Hu ... 

When considering binary compounds of boron, it should be kept in mind that boron does not always behave as if the atom forms compounds in which the octet rule is obeyed. For example, compounds formed with scandium and titanium are ScB2 and TiB2, but other... 

Other new compounds of boron with extended structures include Rb2B2Se7, Tl2B2Se7, CssBgSeio, and Tl3B3Seio, which contain infinite chains, as well as BaB2Sc6 and Ba2B4Sei3, which contain layers. ... 

In contrast to other transition metal compounds of boron such as borides or metallaboranes, the class of transition metal complexes of boron, e.g. borane (I), boryl (II), and borylene (Ilia, b) complexes (Figure I), are characterized by two-center-two-electron bonds between the metal center and boron. 

Boron s chemistry is so different from that of the other elements in this group that it deserves separate discussion. Chemically, boron is a nonmetal in its tendency to form covalent bonds, it shares more similarities with carbon and silicon than with aluminum and the other Group 13 elements. Like carbon, boron forms many hydrides like silicon, it forms oxygen-containing minerals with complex structures (borates). Compounds of boron have been used since ancient times in the preparation of glazes and borosilicate glasses, but the element itself has proven extremely difficult to purify. The pure element has a wide diversity of allotropes (different forms of the pure element), many of which are based on the icosahedral Bj2 unit. 

Accordingly, carbon takes its place with the three groups of elements mentioned earlier as the only representative yet known of a fourth group (the compounds of boron and silicon being still too little known). Its simplest combinations with elements of the three other groups are ... 

Boron — (Ar. Buraq, Pers. Burah), B at. wt. 10.811(7) at. no. 5 m.p. 2075°C b.p. 4000°C sp. gr. of crystals 2.34, of amorphous variety 2.37 valence 3. Boron compounds have been known for thousands of years, but the element was not discovered until 1808 by Sir Humphry Davy and by Gay-Lussac and Thenard. The element is not found free in nature, but occurs as orthoboric acid usually in certain volcanic spring waters and as borates in borax and colemanite. Ulexite, another boron mineral, is interesting as it is nature s own version of fiber optics. Important sources of boron are the ores rasorite (kern-ite) and tincal (borax ore). Both of these ores are found in the Mojave Desert. Tincal is the most important source of boron from the Mojave. Extensive borax deposits are also found in Turkey. Boron exists naturally as 19.9% B isotope and 80.1% B isotope. Ten other isotopes of boron are known. High-purity crystalline boron may be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on... 

The other exceptions to this rule involve compounds of boron and beryllium, which form compounds like BeHj and BF3, in which there are four and six electrons, respectively, in their completed valence shells. However, the octet rule applies to all of the other elements in the first and second periods of the Periodic Table, on whose compounds we will now focus our attention. 

Existence of a nitrogen-rich boron nitride denoted as h BN was suggested by Yoo et ai, who observed appearance of a low-density hexagonal compound after laser heating of boron in excess of nitrogen in a diamond cell [123], This compound was synthesized both in the stability fields of conventional hBN (2GPa, 1300 K) and of cubic BN (15 GPa, 1800 K). h BN is highly transparent and can be recovered as white polycrystals, which do not convert to other forms of boron nitride at ambient conditions [123]. Unfortunately, chemical composition of the compound obtained was not determined quantitatively. 

Phenolics can be chemically modified during synthesis by the use of substituted monomers or monomer mixtures. After synthesis additional modification can occur by electrophilic ring substitution, nucleophilic hydro l group capping, and reactions with compounds of boron, phosf orous, silicon, and titanium. Furthermore, phenolics can be physical modified by formation of polymer blends, interpenetrating polymer networks, and foam or by using fibers, fillers or other additives. 

After Frankland s discovery of zinc methyl and ethyl a number of other compounds of metals (and semi-metals) with alkyl radicals came to light aluminium, antimony,arsenic (see Bunsen, p. 284), beryllium, bismuth, boron, cadmium, lead, and magnesium. ... 

---