Structure determination boron

In 1970, the synthesis of the orange-red sandwich cation 1 from cobaltocene and PhBCl2 (1) marked a further starting point in the chemistry of boron metal compounds. The presence of a planar benzenoid C5H5B ligand moiety in 1 was deduced from XH and UB NMR data (1). This was made ironclad by two X-ray structure determinations which revealed typical centrosymmetric sandwich structures for the 19-e complexes Co(C5H5BOMe)2 (6) (21,22) and Co(C5H5BMe)2 (7) (22) as shown in Fig. 1. 

The value of methylation studies in structural determination of carbohydrates is well known. Methylation of sucrose has generally been achieved by the use of dimethyl sulfate-sodium hydroxide,34,35 methyl iodide-silver oxide-acetone,20 sodium hydride-methyl io-dide-N,N-dimethylformamide,35 or diazomethane-boron trifluoride etherate.36,37 The last method (already applied to monosaccharides38,39) has been found particularly useful for sucrose, because it proceeds without concomitant migration of acyl groups. The reaction of 2,3,6,T,3, 4, 6 -hepta-0-acetylsucrose (21) and 2,3,4,6,1, 3, 4 -hepta-O-acetylsucrose (22) with diazomethane in dichloromethane in the presence of a catalytic proportion of boron trifluoride etherate for 0.5 h at —5° gave the corresponding 4-methyl (23) and 6 -methyl (24)... 

Vinylboronates are generally less reactive than vinylzirconocenes towards various electrophiles and hence selective reactions of the latter should be possible. It was found that selective cleavage of the carbon—zirconium bond in 45 by N-halosuccinimides provides (a-haloalkenyl)boronic esters 53 in excellent chemical yields and with complete re-gioselectivity (Scheme 7.17) [54], An X-ray crystal structure determination of 45 confirmed the configuration of the four-coordinate Zr complex, with two cyclopentadienyl rings, Cl, and C(sp2) as the four ligands (Fig. 7.5) [54,126]. 

In a series of papers some thieno fused boron compounds have been subjected to structure determination by X-ray methods. Thus the bis(dithienoborepinyl) ether (26 R = C10H6BS2O) has dimensions close to those of the isoelectronic dithienotropylium ion (74ACS(B)998>. The borepin ring is almost planar, and the B—C bond length was found to be 1.533 A. 

Mass spectral data have frequently been used in the structural determination of boron heterocycles. One paper has been devoted to the mass spectra of some six-membered boron-nitrogen systems. It was concluded that the spectra could be interpreted analogously to their hydrocarbon counterparts. In all cases the molecular peak was the base peak of the spectrum (68T6755). Doubly charged molecular ions, a feature typical of aromatic compounds, are often encountered. It should be noted, however, that some certainly non-aromatic aminoboranes give such doubly charged ions as well. 

The electrical conductivity of these alloys is metallic i.e., inverse proportional to the temperature between 4 to 300 K and is about 7 x 106 (Qm) 1 at room temperature. The crystal structure determination [7] shows the B atomic arrangement to consist of coplanar 4B clusters (isolated from one another) with three B atoms at the vertices and one B at the center of a triangle as shown in Fig. 5. This isolated 4B atomic arrangement has no precedent either in metal-borides [14] or boron hydrides [15]. 

The ability of boron to coordinate to three and four oxygen atoms enables a wide range of theoretical structural entities to be formulated. An interesting history of early postulates in which only three-coordinate boron was considered has been outlined by Bokii and Kravchenko (56). Several authoritative reviews of the structural chemistry of borates have been published (56, 77, 173, 182, 381, 408, 429), but the most recent and reliable is that by Christ and Clark (80). In their paper they have considered structures determined from X-ray or neutron diffraction studies of single crystals, rather than include interpretations of spectroscopic or dehydration characteristics of bo-... 

The structure determination of crystalline borates is well advanced, and it is hoped that the present enthusiasm will continue. There are many candidates for further investigation, particularly where spectroscopic or dehydration characteristics have suggested a particular boron anion. For example, the borate 2ZnO - 3B203 - 3.5H20 loses its water at temperatures in excess of 260°C, implying that its formula water is in the form of hydroxyl groups. 

The reaction of the porphyrin ligand, TTP (tetra-p-tolylporphyrin) with BF3-OEt2 leads to the oxide fluoride complex, B2OF2(TTP) and the structure has been established using H, 13C, nB and 19F NMR spectroscopy and FAB mass spectrometry [9], The structure contains a B—O—B bridge in which each boron is bonded to fluorine and to a nitrogen of TTP. The structure of the diboron complex was confirmed by an X-ray crystal structure determination of the tetrakis-(p-chlorophenyl)porphyrin (TpCiPP) derivative. 

A single-crystal X-ray structural determination of Co(C5HsBMe)2 (Figure 15) revealed a planar sandwich where the cobalt atom is displaced towards C(4)by 0.28 A. The Co-B distance of2.283(5) A is considered a bonding distance. The upheld shift of the "B NMR signal and the diminished nucleophilicity of the boron atom are consistent with this interpretation. 

N,0 heterocyclic carbenes have also been shown to form adducts with triorganoboron complexes. Triphenyl-boron carbene adducts (62) were isolated from the reaction of an isocyanide adduct of BPhs with methanol (equation 6), in the presence of a catalytic amount of KF. An X-ray structural determination showed that (62) consists of a tetrahedrally coordinated boron atom with all four B C bond distances equal (within experimental error). 

Characterization of all cluster compounds has been carried out primarily by single crystal x-ray structure determination. In some cases the exact structural details are ambiguous, for example cluster charge can be difficult to determine due to i) possible mixed valence of the vanadium centres, ii) difficulty in location of hydroxy protons and iii) the possibility that waters of crystallization may infact be hydroxonium ions. In some structures boron hydroxy protons are well located and imply that all trigonal B-O termini around these clusters are protonated, since the reaction pH (7-9) is similar in most cases. 

In the context with our studies on silylboranes we also investigated silylborates having a tetracoordinated boron atom. The crystal structure determination of the previously reported LiB(SiMe3)45 (see Figure 5) revealed a most interesting result the B-Si bond lengths are shorter than in the silylboranes. This unusual feature is also observed in the silylborate Li(Ph3SiBH3). A typical example is shown in fig. 6. 

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