Structural Characteristics of Metal Borates

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 most common units found in borate structures are shown in Fig. 1. Many of the known borate structures can be rationalized by assigning appropriate charges, protons, or hydroxyl groups (which change the coordination of boron from trigonal to tetrahedral) to these basic units. Thus the B03 unit can become BOj -, B(OH)3, or B(0H)4, respectively. Polymerization by elimination of one water molecule between two hydrated units results in chain formation, and further water elimination gives sheets or networks. These operations are illustrated in Fig. 2 for the triborate ring unit. 

Chain Bj04(0H) and sheet B Of /3-Metaboric acid-II 448) CsBjOs 

NaJB dOH),] 3HjO Kemite (82, 90, 144) Chain BjOdOH) - NaJB O OH),l (289) 

Of recent interest has been the determination of structures containing partially hydrated polyborate ions such as found in Na3B305(0H)2 (94), K3 or RbJB OHlJ 2H 0 (319, 458), and row-eite (Ca n OHlJ O/OIDd) (299), in which oxygen atoms arranged tetrahedrally around boron atoms are protonated and the triangular oxygens are not. 

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Up to March 2006 the molecular structures of about 40 alkylidyne complexes ligated by poly(pyrazolyl)borates had been reported, and structural data for the crystallographically characterised alkylidyne-metal complexes are compiled in Table II. Of these, Tp- and Tp -ligated complexes represent more than 80%, with one third of the total known structures being bi- and polymetallic complexes. Structural features of alkylidyne complexes in general have been discussed elsewhere and, accordingly, an attempt to restrict this discussion to features which are unique or characteristic of the scorpionate alkylidyne complexes has been made. 

As illustrations. Tables 2 and 3 list assignments for the H-NMR spectra of two important bryostatins and two bryostatin acetates while Fig. 5 lists assignments for the C-NMR spectrum of bryostatin 12. Fig. 6 shows the ultraviolet spectra, while mention of other ultraviolet, infrared, and mass spectral characteristics has already been made in Section 5. However an illustration of the application of mass spectrometry to the structure elucidation of bryostatins 5-7 will be instructive (27). The solution phase secondary ion mass spectra of these three bryostatins were quite revealing when new techniques for detecting molecular ions of hitherto refractory substances were utilized. By employing an alkali metal iodide such as sodium iodide or silver tetrafluoro-borate or thallium tetrafluoroborate in sulfolane the molecular ions... 

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