Borate complexes structures

Fig. 4.3 Ranges of isomer shifts observed for Fe compounds relative to metallic iron at room temperature (adapted from [24] and complemented with recent data). The high values above 1.4-2 mm s were obtained from Co emission experiments with insulators like NaCl, MgO or Ti02 [25-28], which yielded complex multi-component spectra. However, the assignment of subspectra for Fe(I) to Fe(III) in different spin states has never been confirmed by applied-field measurements, or other means. More recent examples of structurally characterized molecular Fe (I)-diketiminate and tris(phosphino)borate complexes with three-coordinate iron show values around 0.45-0.57 mm s [29-31]. The usual low-spin state for Fe(IV) with 3d configuration is 5 = 1 for quasi-octahedral or tetrahedral coordination. The low-low-spin state with S = 0 is found for distorted trigonal-prismatic sites with three strong ligands [30, 32]. Occurs only in ferrates. There is only one example of a molecular iron(VI) complex it is six-coordinate and has spin S = 0 [33]...

Recently, Lipton et al. [25] have used zinc-67 NMR to investigate [Zn(HB(3,5-(CH3)2pz)3)2] complexes which have been doped with traces of paramagnetic [Fe(HB(3,4,5-(CH3)3pz)3)2]. The low-temperature Boltzmann enhanced cross polarization between XH and 67Zn has shown that the paramagnetic iron(II) dopant reduces the proton spin-lattice relaxation time, Tj, of the zinc complexes without changing the proton spin-lattice relaxation time in the Tip rotating time frame. This approach and the resulting structural information has proven very useful in the study of various four-coordinate and six-coordinate zinc(II) poly(pyrazolyl)borate complexes that are useful as enzymatic models. 

Although the poly(pyrazolyl)borate complexes of iron(II) have been well known for many years, [1] it is only recently that the complexes with the tris(l-pyrazolylmethane ligand, HC(pz)3, [45-48] have been studied in detail. It should be noted that poly(pyrazolyl)methane ligands, such as the tris(l-pyrazolylmethane ligand, are neutral, whereas the poly(pyrazolyl)bo-rate ligands, such as the tris(l-pyrazolyl)borate ligand, HB(pz)3", are monoanions. As a consequence, the metal(II) poly(pyrazolyl)methane complexes are dications and often have quite different properties from those of the analogous metal(II) poly(pyrazolyl)borate molecular complexes. But, in spite of these differences there are often very close structural similarities between the dicationic complexes and the neutral complexes. Therefore the study of the pyrazolylmethane complexes will parallel that of the borate complexes discussed above. 

Solid-phase borate complexation coupled with RP-HPLC has been employed for the measurement of polyhydroxyflavones in human blood plasma, vegetables and redwine. The chemical structures of polyhydroxyflavones included in the investigation are shown in Fig. 2.87. Vegetables were homogenized, centrifuged and the supernatant was applied for analysis. Human plasma was heparinized before analysis. The outer skins of onion were... 

Ghanges in Mn—N bond distances taken from, F. Hossain, Synthesis, Structure and Characterization of Low-spin Mn(III)-Bisfpoly pyrazolyl)borate] Complexes, M.S. Thesis Indiana University-Purdue University Indianapolis, Indianapolis, IN 2004... 

The introduction of steel cord into the manufacture of automobile tyres led to the need to find additives which would promote the adhesion of steel to rubber. A cobalt complex known commercially as Manobond C-16 has been used for this purpose.70,71 Manobond C-16 is a borate complex with the composition (RC02CoO)3B. The R groups are described as having an average of 21 carbon atoms, five of which are methyl groups. 2 No further structural details seem to be available. 

Since 1979 Chen s group has turned its attention to borates. They recognized that borate compounds have numerous structural types since borate atoms may have either three or four-fold coordination. This complex structural nature of borate compounds leads to a great variation in the selection of structural types... 

The SOD activity measurements using the xanthine-xanthine oxidase nitro blue tetrazolium (NBT) method [45] indicated that a concentration of 0.75 xM of [Mn(OBz)(H-3,5-/-Pr2pz)(L 1)] 1 and 0.8 iM of the related [Mn(OBz)(Ll)] 2 complex exert the same SOD activity as one unit of native SOD. The high SOD activity of the tris(pyrazolyl)borate complexes was attributed to their structural similarities to the MnSOD active site [45],... 

Poly(pyrazolyl)borate complexes of Sn(II) and Pb(II) 92SL469. Synthesis, structure, and properties of vicinal hydroxy-, mercapto-, and hydroselenoaldimines of the pyrazole series 92MI22. 

The solid-state structure of the heteroleptic compound [Tp Bal (HMPA)2] is shown in Fig. 21.111 The steric bulk of the poly(pyrazolyl)bo-rate ligand is apparent in this figure, effectively blocking oligomerization from occurring. This is even more apparent in the bis(3,5-dimethylpoly (pyrazolyl)borate) complex Ba[HB(3,5-Me2pz)3]2 shown in Fig. 22, where two poly(3,5-dimethylpyrazolylborate) ligands have sterically saturated the coordination sphere of the barium center. The shortest intermolecular distance is 5.23 A.112... 

Interestingly, while no rearranged / -allylplatinum complex could be detected in solution for either the borate or phosphate salts, the crystals of the tetraphenylborate salt have the -allyI structure whereas the crystals of the hexafluorophosphate salt have the rf-allyl structure . This reflects differences in stabilization of the complex structure due to crystal packing, and thus strongly suggests that the rearrangement is a low energy... 

Numerous physical studies of poly(l-pyrazolyl)borate complexes have been carried out, some as part of structure determinations, others to study different phenomena inherent in the complexed transition metal. 

A large number of poly(l-pyrazolyl) borate complexes had their structure determined by X-ray crystallography these are listed in Table 8. It should be remembered, however, that in some instances the structure in solution will have a different denticity of the poly(l-pyrazolyl)borate ligand than that found in the crystal. 

Table 8. Listing of Poly(l-pyrazolyl)borate Complexes of Known Crystal Structures...

In general, borates are structurally complex, since the boron atoms can be in 3 and/or 4 coordination and oligomer, ring, and chain polymers are all found (Christ and Clark, 1977 Wells, 1975). We shall not attempt to describe fully the complexity of these structures but will concentrate on the fundamental polyhedral units. The molecular geometric and electronic structures of these materials can be studied using many of the site-specific spectroscopies previously discussed. The bulk properties of the materials also change, of course, depending upon the molecular structure. 

Thus, there are numerous structural types of anhydrous borates. However, it is possible to reduce some of these complex structures to various combinations of the three types of the basic structural units mentioned above FSU, CSU, and CRP. 

It was anticipated that the amino alcohol would form one of the borate complexes of the type 3-6 with bi-2-naphthol in the presence of boric acid. Indeed, this has been observed. The racemic amino alcohol 8 was resolved to obtain 8-lR,2R in >99% ee in 3 repetitive operation shown in Scheme 7.15 The IR spectrum indicated the presence of hydroxy group in the borate complex involved in this reaction (Scheme 7). However, the solid was not suitable for X-ray crystal structure analysis. Fortunately, however, the corresponding racemic methyl ether 9 gave better results in this resolution method (Scheme 8).15 Also, X-ray crystal structure analysis indicated that the borate complex formed in this reaction is the Bronsted acid salt of the type 5.15 Scheme 8... 

It was of interest to examine the use of chiral bi-2-naphthol to resolve the racemic amino alcohol 11 that can be prepared starting from pyroglutamic acid (Scheme 9). Preliminary studies revealed that die resolution can be readily carried out using B(OH)] and chiral bi-2-naphthol (Scheme 9).17 Further optimisation of the results and structural analysis of the borate complex involved in this reaction are under way. 

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