Tris borate complexes

CHROMIUM ALKYLS WITH NITROGEN- AND OXYGEN-LIGANDS 3.1. Tris(pyrazolyl)borate complexes... 

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. 

There has been particular recent interest in zinc nitrate complexes as coordination models for bicarbonate binding in carbonic anhydrase. The mono- or bidentate coordination modes have been studied with tris-pyrazolyl borate complexes and can be rationalized in the context of the enzyme activity.433 Caution in this comparison is introduced by ab initio calculations on these model systems demonstrating both monodentate and bidentate coordination energy minima for nitrate binding to zinc 434... 

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. 

Other buffers that have been used for continuous, native electrophoresis are Tris-glycine (pH range 8.3 to 9.5),19 Tris-borate (pH range 8.3 to 9.3),26 and Tris-acetate (pH range 7.2 to 8.5).27 Borate ions26 can form complexes with some sugars and can therefore influence resolution of some glycoproteins. 

The formation of a ruthenium vinylidene is proposed as the key intermediate in the regioselective addition of hydrazine to terminal alkynes [55]. This reaction, which proceeds via addition of the primary amino group of a 1,1-disubstituted hydrazine followed by deamination, provides an unprecedented access to a variety of aromatic and aliphatic nitriles. The tris(pyrazolyl)borate complex RuCl(Tp)(PPh3)2 gave the best catalytic activity in the absence of any chloride abstractor (Scheme 10.17). 

Stereoselective reduction of 3-ketogibberellin adds.1 The 3-ketogibberellin acid 1 is reduced by this borohydride almost entirely to the 3/(-alcohol. Reduction with lithium tri-.wr-hutylhorohydride proceeds with the opposite stereoselectivity. The dilfcrence is considered lo be the result of differences in size between potassium and lithium borate complexes with the carboxylic acid group. 

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],... 

A comparison of the carbonyl frequencies of a tris(pyrazolyl borate) complexes of copper [TpBr3Cu(CO), v(CO) = 2105cm 1] provides good evidence for a lower electron density at silver [TpBr3Cu(CO), v(CO) = 2157cm 1].14a... 

It is pertinent, in this connection, to mention that Angyal and McHugh have shown that cyclitols possessing 1,3,5-cfs-hydroxyl groups form tri-dentate complexes with borate (VII — VIII). Equilibrium constants for... 

The brown paramagnetic tris(pyrazolato)borate complex (Tp )Co(02) is formed on exposure of (Tp )Co (CO) to 02 and exists in equilibrium with an unusual green, diamagnetic bis-02 dimer (0—0 1.35 A).23... 

For H2 oxidative addition to Rh1 or Ir1 to take place, a square-planar geometry is required. Where this is not the case, as with the tbp tris(pyrazolyl)borate complex (21-VII), dissociation of a pyrazolyl arm precedes the H2 addition step27 ... 

The oxidative addition of alkanes to Rh1 and Ir1 species CpML is very facile. In the case of the tris(pyrazolyl)borate complex Tp Rh(CO)2 [Tp = HB(2,4-Me2pyr)3], the lifetimes of the intermediates were determined by ultrafast time-resolved infrared spectroscopy. In this case de-chelation of one of the pyrazolyl arms was found to precede the C—H oxidative addition step. The proposed intermediates for R—H addition and their lifetimes are shown in Fig. 21-1.127... 

This methane based tris-NHC ligand system behaves very similar to its boron based analogue. The most obvious difference is of course its neutral charge compared with the monoanionic borate ligand. As a consequence, the corresponding silver complex [ Agj HC(MeBenzIm)3 )2](BF )3, although supposedly isostructural with the borate complex... 

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