Tetrahedral, borate complex

Boronic acid derivatives have been shown to inhibit -lactamases [168]. The boronic acid function, which is isoelectronic with a protonated carboxyl group, is presumed to block enzymatic activity by forming a stable tetrahedral borate complex with the active serine residue. Boronic acids 319, structurally related to penicillin G and methicillin, were synthesised in three steps from dibutyl iodomethaneboronate [169] (Scheme 97). They are inhibitors of P-lactamase I from B. cereus. There are presently no reports of boronic acid derivatives acting as inhibitors of bacterial D,D-peptidases. 

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]...

Properties of nickel poly(pyrazol-l-yl)borate complexes such as solubility, coordination geometry, etc., can be controlled by appropriate substituent groups on the pyrazol rings, in particular in the 3- and 5-positions. Typical complexes are those of octahedral C symmetry (192)°02-604 and tetrahedral species (193). In the former case, two different tris(pyrazolyl)borate ligands may be involved to give heteroleptic compounds.602,603 Substituents in the 5-position mainly provide protection of the BH group. Only few representative examples are discussed here. 

Speculation is made about why epi-inositol flips into the triaxial form to form a tridentate complex with borate, but complexes without flipping with metal ions. The distances between the three axial oxygen atoms are similar to those between the three oxygen atoms in the ax-eq-ax sequence. However, in the triaxial borate complexes (3) the bonds are tetrahedral if one were formed at the ax-eq-ax oxygen atoms, the bonds would be considerably distorted. However, the angles have much less effect on complex formation with cations which does not involve covalent bonds. 

Isotope labeling experiments indicate that the B—O bond is broken and not the C—O bond in the formation of the diol-boric acid complexes (18). This indicates that the initial step in the mechanism may be an attack on the boron atom by an oxygen of the diol, followed by the release of water. This could occur without developing any charge separation. If such a mechanism were correct, it would seem that an attack on the boron atom would be easier for trigonal boric acid than for the tetrahedral borate anion (Figure 1). 

Guaran is a polygalactomannan containing T-OH groups. It is well known that the tetrahedral borate ions readily form tetrahedral complexes with the cw-OH groups. Thus the borate immobilized polymer (boron content 0.7 mmol/g) was used for enantioseparation of racemates such as 1,2- or 1,3-dihydroxy compounds or a-hydroxy acids.This appears to be the first example of the use of boron as a complexing ion in CLEC. 

Tetrahedral borate or boronate complexes have been shown to be involved in enzyme inhibition. Serine proteases were proposed to be inhibited by boric acid [6], and simple borates have been patented as protease stabilizers in liquid detergent formulations [7, 8]. 

At this relatively high ligand concentrations predominantly one-to-two complexes are formed. This ist not surprising since the tetrahedral borate anion should allow the attachment of even large polyol molecules... 

He et al. (2011) synthesized a pH-responsive hydrogel based on bor-onate-catechol complexation using 1,3-benzenediboronic acid and catechol end groups present on 4-arm PEG catechol (cPEG) (Fig. 8.5). Under basic aqueous conditions, the 1,3-benzenediboronic acid establishes a tetrahedral borate ester with the catechol end moieties of 4-arm PEG catechol and forms a three-dimensional, pH-responsive smart hydrogel. 

Unfortunately, the evaluation of rate constants for the substitution reactions of tetrahedral borates with acidic ligands is complicated by reaction pathways, which are kinetically indistinguishable due to proton ambiguity. There is therefore a comparative paucity of data for complexation with the tetrahedral... 

Scheme 16 As the reaction kinetics of tetrahedral borates with most ligands cannot be followed due to the problems associated with kinetically indistinguishable pathways and proton ambiguity, studies have examined the biruling of boric acid with bidentate ligands in a 1 2 complex. By considering the change from a 1 1 to a 1 2 complex a tetrahedral structure is enforced at boron. This allows parallels to be made with the complexation of other tetrahedral boronate anions. Yoshimura s proposed transition state is depicted here, illustrating the complexation of boric acid with chromo tropic acid. ...

A highly fluorinated bis(pyrazolyl)borate ligand, dihydridobis(3,5-bis(trifluoromethyl)pyrazo-lyl)borate has been synthesized. The zinc triflate complex was prepared from the potassium salt of the ligand and exhibits distorted tetrahedral coordination which allows comparison of the amount of distortion, in relation to the electronic properties of substituents, with the methylated analog.165... 

The substituted tris(2-thioimidazolyl)borate ligands with l-R-2-thioimidazole groups where R = tBu and CgH4-/>-CH(CH3)2 yielded tetrahedral complexes on reaction with zinc salts. The... 

The hydridotris(pyrazolyl)borate (Tp) ligand has become well known and well established as a formal analogue of the cyclopentadienyl (Cp) ligand (43). Unlike Cp, however, when appropriately substituted, Tp can become what has been described (44, 45) as a tetrahedral enforcer ligand, producing complexes that are constrained to be tetrahedral even when other factors might allow octahedral coordination. 

A reaction looked at earlier simulates borate inhibition of serine proteinases.33 Resorufin acetate (234) is proposed as an attractive substrate to use with chymotrypsin since the absorbance of the product is several times more intense than that formed when the more usual p-nitrophcnyl acetate is used as a substrate. The steady-state values are the same for the two substrates, which is expected if the slow deacylation step involves a common intermediate. Experiments show that the acetate can bind to chymotrypsin other than at the active site.210 Brownian dynamics simulations of the encounter kinetics between the active site of an acetylcholinesterase and a charged substrate together with ah initio quantum chemical calculations using the 3-21G set to probe the transformation of the Michaelis complex into a covalently bound tetrahedral intermediate have been carried out.211 The Glu 199 residue located near the enzyme active triad boosts acetylcholinesterase activity by increasing the encounter rate due to the favourable modification of the electric field inside the enzyme and by stabilization of the TS for the first chemical step of catalysis.211... 

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