Meridional geometry

Pincer ligands, that is, tridentate Hgands that enforce meridional geometry upon complexation to transition metals, result in pincer complexes which possess a unique balance of stability versus reactivity [3]. Transition-metal complexes of bulky, electron-rich pincer ligands have found important appHcations in synthesis, bond activation, and catalysis [4, 5]. Among these, pincer complexes of Pr-PNP (2,6-bis-(di-iso-propylphosphinomethyl)pyridine), Bu-PNP (2,6-bis-(di-terPbutyl-phosphinomethyl)pyridine), and PNN ((2-(di-tert-butylphosphinomethyl)-6-diethyl-aminomethyl)pyridine), PNN-BPy (6-di-tert-butylphosphinomethyl-2,2 -bipyridine) ligands exhibit diverse reactivity [6-8]. These bulky, electron-rich pincer ligands can stabilize coordinatively unsaturated complexes and participate in unusual bond activation and catalytic processes. 

A recent example demonstrated that modification of the central atom of the pincer with acidic and basic functionality was possible with a mononuclear PPP pincer iridium(I) complex (Figure 3.18) [76]. In contrast to a meridional geometry typical of pincer ligands, the phosphide ligand in 30 is pyramidalized and, accordingly, affords a facial coordination mode. The geometry about the central phosphoms atom is consistent with predominantly a-bonding character of the Ir-P unit. 

Base hydrolysis of optically active /n r-[Co(dien)(l,3-diaminopropan-2-oOCl] " results in complete racemization. Such a stereochemical result has not been reported before for base hydrolysis of cobalt(III) complexes. However the dien ligand remains 100% in the meridional geometry. The optically active hydroxo analog racemizes at least 10" times more slowly. These three observations can be accommodated readily in the classical tt stabilization hypothesis of Basolo and Pearson. 

In this chapter, we have successfully developed bifunctional chiral rhodium complexes bearing chiral phebox ligands that can be used in catalytic asymmetric reactions. The N,C,N meridional geometry with the rhodium-carbon covalent bond is the key character in the phebox complexes. The metal-phebox cooperative bifunctionality significantly contributes reactivity and selectivity in the catalytic asymmetric reactions. Furthermore, the prototype of the bifunctional catalyst can be explained to a wide range of asymmetric catalytic reactions promoted by the Lewis acids, hydrides, enolates, and bory active species. Their diversity further broadens the range of opportunities for asymmetric catalysis. 

Bis(pyrazolylethyl)ether derivatives (106) have been coordinated to zinc providing an N20 donor set. The structural data shows that the ligand coordinates in a meridional rather than facial geometry limiting the application for the modeling of N20 zinc enzyme sites, (derivatives R = i-Pr or Me).161... 

Fig. 7. Geometry of a helical structure (A) and the form of its diffraction pattern (B). In (A), the pitch (P) of the helix is like the wavelength of a sine wave. The radius (r) of the helix is like the amplitude of the sinewave. The subunit axial translation (h) is the rise along the helix axis from one monomer to the next. If there is not a whole number of monomers in one turn of the helix (said to be a non-integral helix), then there may be a longer repeat (C). In the case illustrated C = 2P. Dimensions in the helix in (A) have their counterparts in the diffraction pattern illustrated in (B), but dimensions in (B) are reciprocal to those in (A). Meridional reflections occur at positions m/h from the equator, where m is an integer. Each of these positions is the center of a so-called helix cross consisting of layer lines, which are n/P up or down from the meridional peaks, where n is another integer. All of the resulting layers of intensity can be related to orders of 1/C, where C is the repeat of the helix and l is the layer line number.

The majority of interest in cobalt(III) complexes of terpy has centered on the [Co(terpy)2] cation, although a few 1 1 complexes have been reported. A crystal structural analysis of the complex [Co(terpyXC03XOH)]-4H20, obtained by the reaction of [Co(terpy)2] with aqueous carbonate, has revealed the expected distorted octahedral geometry about the metal ion, with a bidentate chelating carbonate and meridional terpy ligands 287). The... 

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