Organometallic complexes thermodynamic stability

It was soon recognized that in specific cases of asymmetric synthesis the relation between the ee of a chiral auxiliary and the ee of the product can deviate from linearity [17,18,72 - 74]. These so-called nonlinear effects (NLE) in asymmetric synthesis, in which the achievable eeprod becomes higher than the eeaux> represent chiral amplification while the opposite case represents chiral depletion. A variety of NLE have been found in asymmetric syntheses involving the interaction between organometallic compounds and chiral ligands to form enantioselective catalysts [74]. NLE reflect the complexity of the reaction mechanism involved and are usually caused by the association between chiral molecules during the course of the reaction. This leads to the formation of diastereoisomeric species (e.g., homochiral and heterochiral dimers) with possibly different relative quantities due to distinct kinetics of formation and thermodynamic stabilities, and also because of different catalytic activities. 

The tetrameric Zr complexes [Zr(0H)2(H20)2L]4X8 (L = bipyridyl, phenanthroline, various Schiff-bases X = Cl, NCS) were the presumed products from the reaction of ZrOCl2 with various heterocyclic bidentate ligands.336 The interaction of H20 with Zr0(C104)2 or Zr0(N03)2 was probed by NMR spectroscopy.337,338 In the latter case, the cation [Zr4(0H)8(H20)i6]8+ (80) was proposed as the product.338 The related trimer [Zr02Ci2H8(/u2-0H)]3(//3-0)Li5(THF)g(H20)5 (81) was isolated from the hydrolysis product of an organometallic precursor. X-ray structural data confirmed the planarity of the six-membered Zr3(/r2-OH)3 core.339 In related work, protometric studies of Zr hydroxide complexes have probed thermodynamic stability,340 while FT IR and theoretical investigations have addressed the details of laser-ablated group IV metal atoms that... 

In a very general sense, Stephenson has defined the term bioprobes as. functional molecules or devices that provide information about biological systems. The high kinetic and thermodynamic stability of many organo-metallic complexes, in addition to their electronic and spectroscopic properties, have spurred their use in numerous sensor applications. Among those are sensors which involve biomolecules, or which detect biomolecules. In this chapter, only a few selected examples are presented as an introduction to the field. Organometallic biosensors are comprehensively summarized in four chapters in a recent book on bioorganometallic chemistry. A more... 

Many other species are stabilized in 18-electron organometallic complexes car-benes and carbynes, enyls and polyenyls (XL ligands), o-xylylene (o-quinodime-thane), trimethylenemethane, benzyne, norbornadiene-7-one, cyclohexyne, 1,2-di-hydropyridines (intermediates in biological processes), thermodynamically unfavorable organic tautomers such as vinyl alcohols [less stable by 14 kcafrmol (58.5 kJ mol ) than their aldehyde tautomers], aromatic anions resulting from deprotonation in juxta-cyclic position such as tautomers of phenolates and benzylic carbanions. All these species have a specific reactivity that can lead to synthetic applications in the same way as cyclobutadiene above. 

Interest in iridium hydrocarbyl complexes has been fueled by the higher thermodynamic stability of both Ir-C and Ir-H bonds in comparison to related compounds of rhodium. Therefore, iridium alkyl and aryl complexes have been frequently and successfully used as kinetically inert models for a variety of rhodium-catalyzed reactions allowing to collect intriguing mechanistic information on important industrial processes and organometallic reactions of academic interest. 

Platinum (II) complexes of the type (COD)PtRX (COD = jj -l,5-cyclooctadiene R, X = alkyl, aryl, halide) are known for many combinations of R and X, and then-reaction chemistry is well developed see Platinum Organometallic Chemistr. The lability of the COD-Pt interaction renders these compounds convenient sources of organoplatinum fragments for coordination to Lewis bases such as phosphines, according to equation (27). In an effort to better understand the factors that influence the stability of bis(phosphine)platinum(II) species, a thermodynamical study of the reaction in equation (27) was undertaken for a series of monodentate and bidentate phosphine ligands of varying steric and electronic character. 

The thermodynamic constants have been measured and they show that it is a complex process, including the solvent (in this case water) and possibly ammonium salts (i.e. the counter ion plays a part in the racemization). In organometallic and coordination chemistry, because of the numerous dative bonds involved, the question of racemization must be studied to ensure that the product obtained as an enantiomer does not racemize over time. This can happen in solution, where the solvent can act as a ligand and thereby participate in the process, but it is also true of the solid state in the presence of an external agent, such as temperature or exposure to light. We say that an enantiomer has configurational stability if it does not racemize under given conditions. 

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