Metal enolates molecular structure

Figure 4.11 Molecular structures of typical crown-ether complexes with alkali metal cations (a) sodium-water-benzo-I5-crown-5 showing pentagonal-pyramidal coordination of Na by 6 oxygen atoms (b) 18-crown-6-potassium-ethyl acetoacetate enolate showing unsymmelrical coordination of K by 8 oxygen atoms and (c) the RbNCS ion pair coordinated by dibenzo-I8-crown-6 to give seven-fold coordination about Rb.

Cp 2Sm(jU-H)]2, (188), affords very high-molecular-weight PMMA with very low polydispersities (typically < 1.05).453-456 At — 95 °C the polymer formed is highly syndiotactic (95% rr triad). Isolation and X-ray analysis of (189), the 1 2 complex of (188) and MMA, provides strong support for the participation of a metal-enolate as the active site. (189) behaves in an identical manner to the hydride precursor, converting 100 equivalents MMA to polymer with Mn= 11,000 and Mw/Mn= 1.03.457 The successful structural characterization of (189) provides support for intermediates proposed earlier.458,459... 

Molecular structure of metal enolates TABLE 2. IR data for uranyl fi-ketoenolates [U02(dik)2l... 

Metal enolate solutions consist of molecular aggregates (6) such as dimers, trimers and tetramers in equilibrium with monomeric covalently bonded species (7), contact ion pairs (8) and solvent-separated ion pairs (9), as shown in Scheme 1. The nature of the metal cation, the solvent and, to a degree, the structure of the enolate anion itself may significantly influence the extent of association between the anion and the metal cation. In general, the factors that favor loose association, e.g. solvent-separated ion pairs, lead to an increase in the nucleophilicity of the enolate toward alkylating agents and also its ability to function as a base, i.e. to participate in proton transfer reactions. 

To an inorganic chemist one of the most fascinating features of the chemistry of acetylacetone and other congeneric diketones is the variety of ways in which these -diketones and especiahy their enolate anions can bond to metallic or metalloidal atoms to give varied molecular structures. Attention here will be focused where possible on acetylacetonates which most often can be thought of as protot5rpes for other /ff-ketoenolate complexes. 

Molecular orbital calculations on fluorinated butadienes and hexatrienes were used to model the effects of fluorination on the properties of poly(acetylene). Like poly(acetylene), "head-to-head" poly(fluoro- acetylene), (-CH=CF-CF=CH-), is predicted to adopt a planar, all trans structure, but poly(difluoro-acetylene) favors a non-planar skewed chain conformation. "Head-to-tail" poly(fluoroacetylene), (-CH=CF-CH=CF-) is predicted to favor a nearly planar cis structure stabilized by intramolecular CF-HC hydrogen binding. Calculations on 2-fluoroethanol and on both 2-fluoroacetaldehyde enol and its alkali metal (Li, Na, K) enolates reveal moderately strong intramolecular CF—HO hydrogen bonds(1.9 and 3.2 kcal/mol, respectively) and even stronger intramolecular coordination of CF to alkali metal cations (9-12 kcal/mol). 

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