Boron dimer

Instead of the borohydrido pinacol ester one can also use the boron dimer. Several metals, intermediates as the one shown in Figure 19.10 have been isolated. They may contain 1-3 Bpin units and 3-1 hydrides. For the mechanism... 

Deutsch PW, Curtiss LA, Pople JA. Boron dimer dissociation energy and ionization potentials. Chem Phys Lett. 1990 174 33-36. 

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. 

Dimerization. A coumarin dimer is formed by prolonged exposure of coumarin to sunlight or uv radiation. Photodimerization is also catalyzed by boron trifluoride (30). 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

Similar possibilities arise for 10-atom clusters. Thus, dimerization of the c/oso-CtBj claster l,5-Me2C2B3Et3 (56) by means of K metal then I2 in thf yields the classical adaniantane derivative Me4C4B6Et6 (f) when this is heated to 160° the mdd-tetracaibadecaborane cluster (g) is obtained rapidly and quantitatively. It will be noted that in (f) all four C atoms are 4-coordinate and all six B atoms are 3-coordinate, whereas in (g) the three C atoms in the C3 triangular face are 5-coordinate while the boron atoms are variously 4, 5 or 6 coordinate. 

The boron trihalides are volatile, highly reactive, monomeric molecular compounds which show no detectable tendency to dimerize (except perhaps in Kr matrix-isolation experiments at 20K). In... 

Boron trichloride, a colorless, reactive gas of BC13 molecules, behaves chemically like BF3. However, the trichloride of aluminum, which is in the same group as boron, forms dimers, linked pairs of molecules. Aluminum chloride is a volatile white solid that vaporizes at 180°C to a gas of Al2Cl6 molecules. These molecules survive in the gas up to about 200°C and only then fall apart into A1C13 molecules. The Al,CI6 molecule exists because a Cl atom in one AlCI, molecule uses one of its lone pairs to form a coordinate covalent bond to the Al atom in a neighboring AICI molecule (33). This arrangement can occur in aluminum chloride hut not boron trichloride because the atomic radius of Al is bigger than that of B. 

The 2 1 reaction of 9-BBN with a series of dicarboxylic acids, namely oxalic acid, malonic acid, 2,2-dimethylmalonic acid, and succinic acid, in dimeth-oxyethane gives in some cases dimeric and in other cases macrocyclic (acyloxy)diorganoboranes. This has been proved by IR spectroscopy (all C = O groups are bidentate), B-NMR 5 = 10 ppm) and X-ray crystallography [47]. With oxalic acid two structures are possible (IV and V), of which the first with a five-membered boron heterocycle instead of a four-membered one is the more probable formulation (Fig. 13). 

While 1,2- and 1,3-alkanediols form only monomeric 1,3,2-dioxaborolanes and 1,3,2-dioxaborinanes with triethylborane, 1,4- and 1,5-alkanediols give mainly dimeric macrocyclic boronates 41 and 42, which are obtained in yields of 70-75% after purification by vacuum distillation. As lateral products oligo-and polymeric molecules are formed. The main products have a 14- and 16-membered heterocyclic ring, respectively (Fig. 15) [84]. 

The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. 

The phenyl ether oxygen atoms allow the two borazaphenanthrene rings to pivot with respect to each other, therefore this dimeric boronic acid anhydride can potentially exist in two isomeric forms, either face-to-face or helical (Fig. 18). In the face-to-face form the boron atoms of the bis(borazaphe-nanthrene) moieties have syn-orientation, while they have approximate anti-orientation in the helical form. Compound 68 has been characterized by X-ray crystallography in the helical form [109]. The dimensions of the cavity can be described by the transannular C C contacts between the carbon atoms in 2-position of the phenyl ether units, which have values of 5.12 and 6.21 A. 

Fig. 18. The dimeric boronic acid anhydride 68 can potentially exist in a face-to-face or a helical form. X-ray crystallography proves the helical conformation...

However, in this structure, boron does not have an octet. It has an empty p orbital, (very similar to a carbocation, except there is no positive charge here). Therefore, borane is very reactive. In fact, it reacts with itself to give dimeric structures, called diborane ... 

Borane, BH3, having only six valence electrons on boron, is an avid electron pair acceptor. Pure borane exists as a dimer in which two hydrogens bridge the borons. 

A novel zinc porphyrin dimer (52) with a boronic ester linkage was prepared by Shinkai (Fig. 18) (65). The authors introduced an additional... 

Fig. 18. Cooperative closed dimer assembled using boronic esters as well as zinc-pyridine coordination.

Both the monomer and dimer were composed most likely of isomers with some protons residing on ring nitrogens and with the NHSiMe moiety replacing the amino group on some of the boron ring atoms in accordance with the Scheme 1. This assumption is supported by the liquid nature of the product and its infrared spectrum which exhibited two broad bands centered at 3430 and 3530 cm The... 

To remove volatile impurities, the naterial was heated in vacuo at 135 °C. This treatment resulted in further condensation. Thus, the distillation residue was composed of a mixture of doubly-bridged dimers and tetramers, x = 1 and 2 as determined from the molecular weight, boron and nitrogen analyses and the volatile condensibles produced. 

Potential Non-Cvcllc Precursors of Preceramic Polymers. Boranes such as bis(trimethylsilyl(aminotrimethylsilylaminochloroboranes can be viewed as monomers for preceramic polymer and, ultimately, boron nitride production. Intermolecular dehydrohalogenation of this borane would be thus expected to yield either the dimer or the polymeric system. 

The configuration of dimers of BH3, BR3, AIH3, AIX3, AIR3, etc. may be connected to the HO and LU extensions of monomers. Literature is available with regard to the knowledge of the HO and LU of boron hydride 97> and aluminum hydride and related compounds 98). 

---