Boron Donors

The complex LiBH4(TrNCH2)3 gave IR bands at 2185 and 2378 cm due to the bridging and terminal (respectively) B-H groups of the unit (37).  

The zwitterionic species (ri -C5H4SiMe3)(CNxylyl)Nb(p-H)(BF3) had IR bands due to vBF at 1050 cm and vCN at 2084 cm. 7 rans-Cl(diphos)2Ru(. i-Cl)Ag(p-F-BF3) gave characteristic sphtting of the vB-F bands in the IR spectrum - showing the coordination indicated.  

---

The series of neutral B-containing carbocycles as complex ligands within the theme of metal-boron donor-acceptor relationships concludes with the 4,5-dihydro-borepines. l-Phenyl-4,5-dihydroborepine is able to substitute pyridine, acetonitrile or CO... 

Group III Donors. Boron donor ligands. The unusual tris(pyrazolyl)borato-complex [RuL3(CO)2X] [X = Cl, Br, or I L = (16a)] has been prepared ... 

Figure 2.37 Complexes with phosphorus or boron donor ligands bridging three or four gold atoms...

Bridging boron-donor ligands, in W-CO and W-CNR with boron, 5, 653 Bridging hydrides... 

Group III. Boron donor ligands. The preparation of a range of metal complexes of B6H10 has been reported (equation 42a) following preliminary publications.177-179... 

Reactivity studies of two-coordinate chelate group 13 species have Just begun and appear particularly interesting because such species may exhibit both a Lewis acid character (due to the sextet configuration at the metal center) and a Lewis base one (due the lone pair of electrons at the metal center) see Lewis Acids Bases). For instance, the two-coordinate Ga chelate HC(CMeNAr)2 Ga reacts with B(C6F5)3 to form the gallium-boron donor-acceptor complex HC(CMeNAr)2 Ga... 

Scheme 4.16 Effect of water on the coupling reaction of benzimidazole with aryl boron donors.

In terms of atom-economy a very attractive strategy for accessing arylboronic acids is the direct boronylation of arenes through a transition metal promoted C-H functionalization. In addition to the catalyst, a suitable boron donor is required, and both diboronyl esters and dialkoxyboranes are very appropriate in this role. The concept of this type of direct borylation was first demonstrated on alkanes using photochemical... 

Conversely, in 4 -cyanostilbene-4-boronic acid 60, the electron-withdrawing cyano moiety is the acceptor group. When boron is sp hybridised and therefore also an acceptor, no excited-state ICT is feasible. On re-hybridisation of boron to its sp form, the boron becomes a donor group, allowing ICT to occur between the boron donor and cyano acceptor, redshifting the emission wavelength of the sp species. 

Al-B co-deposition was carried out with pure Al as Al donor and B4C as boron donor. The results presented in Fig. 10.10 show the formation of a 100 pm thick coating composed of Fe2Al5 with a thin FeAl layer in the interdiffiision zone. This coating contains some pores and cracks. No significant effect of the boron addition could be observed. 

Boron achieves a covalency of three by sharing its three outer electrons, for example BFj (p. 153). By accepting an electron pair from a donor molecule or ion, boron can achieve a noble gas configuration whilst increasing its covalency to four, for example H3N->BCl3. K BF4. This is the maximum for boron and the second quantum level is now complete these 4-coordinate species are tetrahedral (p. 38). 

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. 

The monomers are electron pair acceptors, and donor molecules are often able to split the dimeric halide molecules to form adducts thus, whilst the dimeric halides persist in solvents such as benzene, donor solvents such as pyridine and ether appear to contain monomers since adduct formation occurs. Aluminium halides, with the one exception of the fluoride, resemble the corresponding boron halides in that they are readily hydrolysed by water. 

The covalently bonded oxygen atom still has two lone pairs of electrons and can act as an electron pair donor. It rarely donates both pairs (to achieve 4-coordination) and usually only one donor bond is formed. A water molecule, for example, can donate to a proton, forming H30, and diethyl ether can donate to an acceptor such as boron trifluoride ... 

Concerning my research during my Dow years, as I discuss iu Chapter 4, my search for cationic carbon intermediates started back in Hungary, while 1 was studying Friedel-Crafts-type reactions with acyl and subsequently alkyl fluorides catalyzed by boron trifluoride. In the course of these studies I observed (and, in some cases, isolated) intermediate complexes of either donor-acceptor or ionic nature. 

The reactions described so far can be considered as alkylation, alkenylation, or alkynylation reactions. In principle all polar reactions in syntheses, which produce monofunctional carbon compounds, proceed in the same way a carbanion reacts with an electropositive carbon atom, and the activating groups (e.g. metals, boron, phosphorus) of the carbanion are lost in the work-up procedures. We now turn to reactions, in which the hetero atoms of both the acceptor and donor synthons are kept in a difunctional reaction produa. 

Compounds containing fluorine and chlorine are also donors to BF3. Aqueous fluoroboric acid and the tetrafluoroborates of metals, nonmetals, and organic radicals represent a large class of compounds in which the fluoride ion is coordinating with trifluoroborane. Representative examples of these compounds are given in Table 5. Coordination compounds of boron trifluoride with the chlorides of sodium, aluminum, iron, copper, 2inc, tin, and lead have been indicated (53) they are probably chlorotrifluoroborates. 

Sihcon carbide can be doped using boron [7440-42-8] to provide acceptor levels within the band gap (0.3 eV above the valence band), thus making it a -type conductor, or nitrogen can be added to provide donor levels and n-ty e conduction (0.07 eV) below the conduction band. 

Neutral compounds such as boron trifluoride and aluminum chloride form Lewis acid-base complexes by accepting an electron pair from the donor molecule. The same functional groups that act as lone-pair donors to metal cations can form complexes with boron trifluoride, aluminum chloride, and related compounds. 

Table 8-11. Enthalpy Change for Boron Trifluoride Complex Formation with Donors al 25°C...

---