Boron ring systems planar structures

Figure 46 The planar structures of the boron ring systems, borazine (71) and boroxine (72), with nonexistent aromaticities and of triphosphatriborin (73) with an appreciable degree of aromaticity. (Adapted from ref. 95.)...

Aromaticity is no doubt one of the most interesting features of small boron clusters. Possessing planar structures, these boron clusters were characterized as highly aromatic systems, on the basis of various indices such as nucleus-independent chemical shift (NICS), electron localization function (ELF), resonance energy (RE), the presence of ring current induced by an external magnetic field, and the classical Hilckel rule of (4IV -]- 2) valence electrons, etc... [8-10, 27]. In recent reports [20, 26], we found that all small boron clusters B with n <20 have an aromatic character, irrespective of their numbers of valence electrons. 

Boroxines have trimeric, cyclic structures with planar rings of alternating boron and oxygen atoms. The alkyl groups are also in the plane of the ring. Boron can also be incorporated into numerous carbon ring systems. 

A six-membered-ring system consisting of one boron, three carbon, and two oxygen atoms is found in the structure of meso-pcntane-2,4-diol borate (82). i The 5 carbon atoms are close to planarity, and the boron atom is... 

These systems are pertinent to the present review because, depending on the number and position of the boron centers, B—C multiple bonding can be involved. The 1,2-dihydroborete (87) possesses a nonplanar borete ring with a fold angle of 148.8° along the B—C3 diagonal, and its structure can fruitfully be compared with the planar 1,2-dihydro- 1,2-diborete (88).87... 

Compounds with the molecular formula (CgH4N2)3BX, where X = F or Cl, have been previously isolated from reactions of halogenoboranes and phthalodinitrile. The structure of the chlorine compound has now been determined and shows a phthalocyanine-like structure (31).179 The system is bowl-shaped, in contrast to the planar phthalocyanine structure. The boron atom is co-ordinated to three N atoms (at 1.467 A, i.e. short) and one Cl (1.863 A). The tt-electrons in the 14-membered ring form a quasi-aromatic conjugated Tr-system. 

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