Boron transition-metal complex anions

Various neutral and, especially, anionic boron species are well known as ligands in transition metal complexes. In the majority of cases, however, one or more H, N or other basic atom of the boron compounds is coordinated to the transition metal (e.g. Chapters 13.8, 13.6 and 19). In the last 20 years, however, several complexes containing boron-transition metal bonds have been prepared with ligands such as sBX, -BX2 or BX3 (Table 14). 

Cyclic organoboranes containing mnltiple boron atoms within the ring stracture have been extensively stndied as precursors to (multidecker) transition-metal complexes and to nonclassical organoboranes (see Boron Metallacarbabo-ranes and Boron Polyhedral Carboranes). Earlier efforts have been thoroughly reviewed. Recently, a series of triboracyclopentane derivatives were reported by Bemdt and Schleyer (Scheme 6). " Reduction of these boracy-cles gave the nonclassical bishomoaromatic anions (28) and homoaromatic dianions (29). Related trishomoaromatic dianions obtained via reduction of 1,3,5-triboracyclohexane derivatives with lithium were studied by Siebert and Schleyer. i 5> ... 

Catecholborane and its derivatives had a pivotal role in establishing the class of boryl complexes, since they proved to be the most versatile starting materials for the synthesis of such compounds. This was demonstrated in many works especially by Baker, Hartwig, Marder, Norman, and Smith. ia In the course of our investigations we aimed at boryl complexes with different substituents attached to boron, and we could show that boranes with B-N bonds are also very well suited for the synthesis of boryl complexes. Over the last three years a wide variety of products was obtained by salt elimination reactions between anionic transition metal complexes and haloboranes. Examples include aminoboryl- 4, 5, diborane(4)yl- 6, 7, and rj -borazine complexes 8 (Figure 4). 

There is a variety of chemical species whose constitution cannot be adequately described in terms of electron pair bonds between pairs of cores. Examples are the boron hydrides, the transition metal complexes of hydrocarbons and their anions, such as Tt-allyl nickel or ferrocene, or the hypothetical intermediate of the limiting Sjy mechanism 88). In these cases the molecular structure can be adequately represented by the model concepts of imdlicenter bonds and fractional bond orders. 

Like the alkali metal, boron and aluminium hydride compounds, hydroorganosilanes are so polarised that the electron cloud is concentrated on the hydrogen atom, which aids the formation of the hydride anion. Combinations of hydroorganosilanes with organic and inorganic acids or with certain catalysts (the transition metal complexes, the peroxides, etc.), are especially suitable for the reduction of organic compounds. The relative ease of hydride anion formation follows this series [537, 538] ... 

Reactions between anionic species containing one or more group-IIIB elements (particularly boron) and complexes of transition-metal halides are used to produce an immense number of ionic boron-containing compounds. For this reason a strong selection factor must be made. 

Covalently bonded sandwiches having five or six decks 78, 79 have been constructed via the reactions of C2B3-endcapped anions with transition metal cations 78,85 the hexadeckers vary in metal composition, in the nature of the substituents on boron (X), in the number of protonated metal centers, and in the number of electrons associated with the metals. Some of these complexes are diamagnetic, but most are paramagnetic with one or two unpaired electrons, in most cases extensively delocalized over the stack as shown by ESR and electrochemical studies.85... 

Boron. Earlier papers reviewed the properties of the borabenzene anion and discussed its potential aromaticity.130 182 A few examples have been shown in Schemes 31 and 53. The borabenzene anion (borinate) 141, with R = phenyl or methyl, gives complexes with transition metals such as Co (142) and Mn (143) (Scheme 61).183 Aza- and polyaza-... 

Lippard, S.J., Ucko, D.A. 1968. Transition metal borohydride complexes. II. Th reaction of copper(I) compounds with boron hydride anions. Inorg Chem 7 1051-1058. 

Boron-bonded p -borazine complexes of transition metals have been prepared by two different approaches (a) nucleophilic substitution of B,fi, fi"-trichloro-borazine with an anionic metal carbonyl reagent and (b) oxidative addition of a B-Br bond of 5,5, B"-tribromoborazine to a zerovalent group 10 complex (see examples in Scheme 9.2). 

It is not always easy to deduce the mechanism of a polymerization. In general, no reliable conclusions can be drawn solely from the type of initiator used. Ziegler catalysts, for example, consist of a compound of a transition metal (e.g., TiCU) and a compound of an element from the first through third groups (e.g., AIR3) (for a more detailed discussion, see Chapter 19). They usually induce polyinsertions. The phenyl titanium triisopropoxide/aluminum triisopropoxide system, however, initiates a free radical polymerization of styrene. BF3, together with cocatalysts (see Chapter 18), generally initiates cationic polymerizations, but not in diazomethane, in which the polymerization is started free radically via boron alkyls. The mode of action of the initiators thus depends on the medium as well as on the monomer. Iodine in the form of iodine iodide, I I induces the cationic polymerization of vinyl ether, but in the form of certain complexes DI I (with D = benzene, dioxane, certain monomers), it leads to an anionic polymerization of 1-oxa-4,5-dithiacycloheptane. 

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