Alkali metal complexes 1.3- diketones

So pronounced is the chelating tendency of the diketonate anion that even alkali metal complexes may be isolated, as illustrated by RbiKCFjCOCHCOCFjljNa], in which sodium is surrounded by a trigonal prismatic array of donor oxygen atoms. For complexes derived from dibenzoylmetbane, stabilities in dioxane-water are in the order Li > Na > K > Cs. For divalent metal ions formation constants increase in the order Ba > Sr > Ca > Mg > Cd > Mn > Pb > Zn > Co, Ni, Fe > Cu, and for higher valent metal ions the first formation constants for chelates are in the order Fe + > Ga " " > Th" > In " " > Sc " " > Y " " > Sm " > Nd " > More recently, such stu-... 

Alkali metals may also form stable complexes for which parent ions are observed in the mass spectra species such as NaHo(C8Hjo02F3)4+ (m/e = 968) (108) and metal jS-diketones (109) have been observed. In the latter case, the 1 1 complexes have spectra complicated by polymeric... 

The coordination of /3-diketonates and related species to alkali and alkaline earth cations has long been recognized. Combes first prepared Be(acac)2 in 1887,255 and the contribution of Sidgwick and Brewer concerning the nature of dihydrated alkali metal /3-diketonates plays a seminal role in the development of this area of coordination chemistry.19 A review concerning the structure and reactivity of alkali metal enolates has been written, 6 and sections on alkali and alkaline earth /3-diketonates can be found within more generalized accounts of /3-diketone complexes.257,258... 

The (3-diketonate chelate complexes are very stable and exhibit properties which are rather typical of aromatic systems. Many of their reactions such as halogenation, alkylation and acylation can be compared with those of the P-diketonate anions associated with alkali metal cations. However, complexes of transition and other metals add to the stability of the system, so that quite vigorous reaction conditions can be employed. In most of the work carried out on P-diketonate chelates, the modified ligand has not been removed from the metal ion, but this can usually be effected if desired. 

Collman18 described numerous ligand reaction methods for preparing methylene-substituted 0-diketone complexes. Often these materials cannot be prepared by other techniques because of the lack of stability either of the ligand or its alkali metal salt. A particularly useful procedure leading to the introduction of a functionally active substituent has been reported.14... 

The fi-diketonates have been extensively studied, particularly since some of the fluorinated derivatives give complexes that are volatile and suitable for gas-chromatographic separation. The preparation of /3-diketonates by conventional methods invariably gives hydrated or solvated species such as [Ln(acac)3]-QHsOH SHzO. Anhydrous species obtained by vacuum dehydration appear to be polymeric, not octahedral. The neutral /3-diketonates can complex further to give anionic species such as the eight-coordinate thenoyltrifluoroacetate [Nd(TTA)4] The alkali metal salts of [Ln(/3-dike)4] are sometimes appreciably volatile and can be sublimed. 

Bidentate /3-diketonates usually have symmetric structure, and many crystal structures show sets of equal M—O, C—C and C—O bonds. Alkali metal enolate structures are symmetric as well as those for the Pd, Rh and A1 enolates. Few structures show unequal M—O distances these enolate complexes (M = Ge, Sn and Sb) are asymmetric as the metal atom is not located at equal distances from the nearest oxygen atoms . 

The stability constants of the 1 1 alkali metal picrate complexes of diketone 18. heptacyclic terpyridyl 27 and flexible terpyridine 28 were estimated by hquid-liquid extraction, and the resulting log fj values for Na" " and are also given in Fig. 4. The affinities of 18 and 27 for these alkali metals are remarkably high for hosts containing only five or three ligand atoms, respectively. In water-saturated chloroform, pentadentate host 18 binds Na" and K more strongly than do most crown ethers. Compared to naphtho-18-crown-6 (26), 18 binds Na " 4000 times better, and K 40 times better. Even tridentate host 27 extracts alkali metal picrates into chloroform, whereas flexible model system 28 is ineffective under the same experimental conditions. The potent complexation... 

Few stable complexes of alkali metals have been reported. Complexes of the alkali metals with j -diketone derivaties, o-nitrophenol, and sali-cylaldehyde have been described in the literature.The sodium compounds ranged from hydrated ionic salts such as Na (acac)-2 H2O to adducts like Na(Sal) HSal (HSal = Salicylaldehyde). PfeiflFer and others have reported the isolation of a complex of sodium perchlorate with 1,10-phenan-throline. Brady and Badger have presented evidence for chelation in sodium salts of o-hydroxybenzaldehydes. More recently, Popp and Joesten have isolated and characterized a number of alkali metal and alkaline earth metal salts with the ligand octamethylpyrophosphoramide (OMPA). Gentile and co-workers have also reported complexes of diacetamide with alkali and alkaline earth metal salts. 

The role of the alkali metal in complex metal hydride reductions is critical, - os and complexing this metal with a macrocyclic co-ordinator curtails the reaction. A full report on the stereoselectivity of the cathodic reduction of cycloalkanoncs to cycloalkanols has been published. The electrochemical reduction of 1,3-diketones yields cyclopropane-1,2-diols (27), often in good yields. ... 

The volatility of the j8-diketonate complexes can also be increased by formation of the tetrakis complexes, because the latter are mononuclear. The first volatile heterobimetallic compound was the ionic Cs[Y(hfac)4], which Lippard (1966) sublimed without decomposition both in air and in vacuum. Addition of alkali metal acetylacetonate to [R(acac)3] complexes results in the formation of volatile M[R(acac)4] complexes (M = Li, Na, Cs) (Zaitseva... 

Homoleptic and chloro /3-diketonates have been reported for both metals. They derive from the reaction of /3-diketones (or their alkali derivatives) with either M or M halide complexes. They are generally monomeric and thus paramagnetic their structures have been proposed on the basis of UV or ESR data and less often X-ray data. 

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