Boron heterocycles metal 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 ... 

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). 

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). 

A combination of a ir-homocyclic or 77-heterocyclic metal moiety, a carbonylmetal fragment (CO) M, and an electron-poor boron heterocycle should lead to unsymmetrical triple-decker complexes. The first example (72) was reported by Grimes and co-workers (91), obtained from the reaction of [ (CH3—C BJIJjFeFy with [(C5H5)Co(CO)2] under UV... 

G. E. Herberich, G. Greis, and H. F. Heil, Novel Aromatic Boron Heterocycle as Ligand in a Transition Metal rc-Complex, Angew. Chem. Int. Ed. Engl. 9, 805-806 (1970). 

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

In the past decade, X-ray crystallography has become a more routine method of characterization. Consequently, there are many structures of boron-containing heterocycles. The heterocycles relevant to this chapter are separated into structures which do not contain transition metals and the transition metal complexes of the Cp-like 1,2-azaborolyl, 1,2-thiaborolyl, and 1,2-oxaborolyl rings. 

Thermodynamic aspects of 1,3-diborolanes, 2,3-dihydro-l//-l,3-diboroles, 1,3-azaborolidines, 2,3-dihydro-l,3-thia-boroles 2,3-dihydro-l//-l,3-stannaboroles, or 2,3-dihydro-l//-l,3-silaboroles are only sparsely mentioned. It has been found that the 127t-electron antiaromatic heterocycle 23 is stabilized by electron delocalization via the boron atom (cf. compound 9) <2002ZN1125>. Noteworthy is the comparison between the 8jt-electron antiaromatic 2,3-dihydro-l,3-benzothiaborole 24 or 4jt-electron antiaromatic 2,3-dihydro-l,3-thiaborole 26 and the corresponding lOtt-electron 25 or 67t-electron 27 aromatic lithium compounds, the latter forming stable Jt-coordinated transition metal complexes. 

---