Transition metals boron complexes

Interactions between non-halogen-containing IIIB compounds and transition-metal complexes are found in 6.5.3. Most of these IIIB compounds are boron-containing heterocycles. A series of interesting sandwich compounds, including triple- and tetradecked complexes, are synthesized by methods in 6.5.2.1-6.5.3. 

Boron in heterocycles is an electron acceptor, and the following neutral carbocy-cles form transition-metal complexes via basic metal centers ... 

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. 

In relation to the mechanistic proposal, an interesting reactivity of (boryl)(silyl)platinum(n) complex has been reported.223 The complex is prepared by the reaction of silylborane with Pt(cod)2 complex via oxidative addition (Scheme 46). The (boryl)(silyl)platinum complex undergoes insertion of alkynes at the B-Pt bond to give (/3-borylalkenyl)(silyl)platinum(n) complex in high yield. Importantly, the insertion takes place regioselectively, with Pt-G bond formation at the internal. -carbon atom. This result may indicate that the boron-transition metal bond is more prone to undergo insertion of unsaturated molecules. 

Other unsaturated boron heterocycles, such as borazines and borabenzenes, form transition metal complexes with the expected nido geometry, as exemplified by compounds (Et3N3B3Et3)Cr(CO)3 (123) and (CBH5BPh)Mn(CO)s (110) (Fig. 29). 

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). 

Compounds of transition metal complexes possessing a nonbonding electron pair with boranes (BX3) can be regarded classically as boron complexes with a transition metal ligand. In a broader sense, however, boranes can be classified as acceptor ligands.154,155 Thus, coordination of a borane results in a decrease of the electron density on the metal atom. In the case of carbonyl complexes this effect is reflected in the increase, by 20-100 cm-1, of the CO stretching frequency.154-156 It follows from the foregoing that stable coordination of boranes is... 

Hydroboration. Although hydroboration seldom requires a catalyst, hydrobora-tion with electron-deficient boron compounds, such as boric esters, may be greatly accelerated by using transition-metal catalysts. In addition, the chemo-, regio- and stereoslectivity of hydroboration could all be affected. Furthemore, catalyzed hydroboration may offer the possibility to carry out chiral hydroboration by the use of catalysts with chiral ligands. Since the hydroboration of alkynes is more facile than that of alkenes the main advantage of the catalytic process for alkynes may be to achieve better selectivities. Hydroboration catalyzed by transition-metal complexes has become the most intensively studied area of the field.599... 

An important part of the inorganic chemist s contribution has been the preparation and characterization of transition metal complexes with unsaturated boron rings as ligands. They have also settled the geometry of a number of compounds using X-ray methods. 

Ferrocene, bis(cyclopentadienide)iron, was the first transition metal complex with aromatic ligands, and its discovery induced extensive research on complexes of different transition metals and different aromatic ligands. It is therefore not surprising that borinate ion complexes of this type are known. Some complexes with five-membered heterocycles were mentioned in Section 1.21.7. In this section borinate complexes are considered in greater detail because of their formal relationship to benzene. An extensive review on transition metal complexes with boron heterocycles has recently been published (80MI12100). 

The first borinate-transition metal complex to be prepared was actually the first known derivative of borin. Bis(cyclopentadienide)cobalt (94) reacts with organic halides and was analogously found to react with boron halides in a redox reaction to give (95), followed by an insertion to yield (cyclopentadienide)(borinato)cobalt (97) (72CB3413). The product composition depends on the ratio of reactants. Compound (97) is the main product (80% yield when R = Ph, X = Br) when the molar ratio between (94) and the boron halide is 2.5 1. A second and slower insertion occurs to give (28) when (97) is treated with another equivalent of the boron halide (Scheme 13). Compounds (28), (29) and (97) have one electron more than predicted by the 187r-electron rule for transition metal complexes. They are red in colour and, of course, paramagnetic. The mixed complexes (97) are thermally labile, in contrast to (28) and (29), which can be heated to 180 °C and sublimed at 90 °C. Their ionization potentials are low and the complexes are sensitive to air. 

The significant interest that the prospects of transition metal complexes of boron received over the past few years is not only due to a hitherto unknown type of metal-boron linkage. Boryl complexes in particular became a highly rewarding target due to their potential application for the functionalisation of hydrocarbons. They are well known to be key intermediates in the metal catalysed hydroboration and related reactions,58 65... 

Since the epoxidation step involves no formal change in the oxidation state of the metal catalyst, there is no reason why catalytic activity should be restricted to transition metal complexes. Compounds of nontransition elements which are Lewis acids should also be capable of catalyzing epoxidations. In fact, Se02, which is roughly as acidic as Mo03, catalyzes these reactions.433 It is, however, significantly less active than molybdenum, tungsten, and titanium catalysts. Similarly, boron compounds catalyze these reactions but they are much less effective than molybdenum catalysts 437,438 The low activity of other metal catalysts, such as Th(IV) and Zr(IV) (which are weak oxidants) is attributable to their weak Lewis acidity. 

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