Complexes of 3-Diketones

A number of /3-diketones form metal chelate rings of Type A  

The i fMO) of acac complexes are most interesting since they provide direct information about the M-O bond strength. Using the metal isotope technique, Nakamoto et al. assigned the MO stretching bands of acetylacetonato 

Complexes of the M(acac)2X2-iype may take the cis or trans structure. Although steric and electrostatic considerations would favor the trcns-isomer, the greater stability of the cis-isomer is expected in terms of metal-ligand TT-bonding. This is the case for Ti(acac)2F2, which is cis with two r (TiF) at 633 and 618 cm . In the case of Re(acac)2Cl2, however, both forms can be isolated the Irans-isomer exhibits (ReO) and v ReCl) at 464 and 309 cm respectively, while each of these bands splits into two in the ci.s-isomer [472 and 460 cm for /(ReO) and 346 and 333 cm for i/(ReCl) in the infrared For VO(acac)2L, where L is a substituted pyridine, cis- and /ram-isomers are expected. According to Caira et al., these structures can be distinguished 

Vibrational spectra of acetylacetonato complexes have been studied by many other workers. Only references are cited for the following Raman spectra of tris(acac) complexes, infrared spectra of acac complexes of rare-earth metals, relationships between CH and CH3 stretching frequencies and C—H spin-spin coupling constants, and relationships between p(C—O), p(C C), and C NMR shifts of CO groups.  

The following dimeric bridging structure has been proposed for [CoBr(acac)]2  

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Under this section are considered lanthanide complexes of /3-diketones and their adducts, but work specifically in the area of lanthanide shift reagents is dealt with in Section 39.2.9. Of course, the majority of lanthanide shift reagents are lanthanide /3-diketonates, and when they function as shift reagents they do so by forming adducts in solution. Furthermore, interest in shift reagents has directly stimulated a considerable amount of fundamental research on lanthanide /3-diketonates and their adducts. Much of this fundamental work was, therefore, carried out in the early 1970s. AH this means that the division between this section and Section 39.2.9 is not entirely clear cut. 

These occur particularly in solution, but in the solid state also coexistence of the different stereochemistries is possible in either case, complicated magnetic behaviours as functions of concentration, preparative details and temperature arise. Complexes of (3-diketones, substituted alkylenediamines and salicylaldiamines are characteristic sources of these phenomena. 

With few exceptions, Ni11 complexes of /3-diketones are green, octahedral and paramagnetic (ji 3.2 BM), whereas Ni" complexes of monothio-/3-diketones are brown, square-planar and diamagnetic.190 However, they form green, octahedral and paramagnetic adducts [NiL2-2B] (B = py 2B = phen, bipy).214... 

Thermodynamic data for the interaction of Eu(fod>3 with the cobalt(III) complexes of /3-diketones [99]. 

Europium, and to a lesser extent terbium, complexes of /3-diketones have been studied in solution and in the solid state by means of their fluorescence (luminescence) spectra. As explained further in Section 39.2.10, it is possible to relate the splitting of the Do—> F transitions of Eu " to the symmetry of the emitting complex, and studies of circularly polarized luminescence (CPL) spectra can give related information. Thus a study of EuCls and complexes of Eu with hexafluoroacetylacetone and four other /8-diketones in methanol or DMF showed that while EUCI3 itself had axial symmetry in solution, the complexes had orthorhombic symmetry. The emission spectra of solutions of adducts of Eu(dpm)3 with PhsPO or borneol have been studied at low temperatures where conformal lability is reduced the Ph3PO adduct has uniaxial symmetry but the bulky, less symmetrical bomeol molecule confers lower symmetry on its adducts. ... 

Lanthanide /3-diketonates are complexes of /3-diketone ligands (1,3-diketones) with lanthanide ions. These complexes are the most popular and the most intensively investigated luminescent lanthanide coordination compounds (see P-Diketonate). Three main types of lanthanide(III) /3-diketonate complexes have to be considered fra complexes, Lewis base adducts of the tris complexes (ternary lanthanide /3-diketonates), and tetrakis complexes. Most Eu +/3-diketonate complexes show an intense visible luminescence, but many /3-diketonates are not good ligands to sensitize the luminescence of Tb + ions. NIR luminescence can be... 

Metal-containing smectic materials are of considerable interest because, as they carry metal atoms or ions, they can be effectively studied by X-ray diffraction techniques. In addition, their unique shapes and physical properties make them ideal candidates for the study of biaxiality and conductivity in smectic liquid crystals. One class of materials that stands out are the copper(II) complexes of )3-diketones. These materials can exhibit columnar as well as smectic mesophases. 

For example gold(III) complexes with /3-diketonates or alkylsiloxydes, are likewise employed as precursors for CVD. 73,177 Another promising result in the field of homogeneous catalysis is the discovery that gold(III) alkoxo complexes promoted the condensation of benzaldehyde with compounds containing an active methylene group.1775... 

PPhs, the active oxidant being probably a Os =0 species. trani-Dioxoosmium(VI) complexes of Schiff base ligands obtained from the condensation of /3-diketones and 1,2-diaminoethane are also known. The X-ray structure of tra 5-[0s (0)2 (BA)2en ] has been determined. The 0s=0 distances are 1.753 A and 1.736 A (average) for the two crystallographically independent... 

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... 

A series of [M(02)F3(diket)] complexes has been prepared by the reaction of /3-diketones with [M(02)F4(H20)] . 3H and 19FNMR spectra indicate that the products exist as a mixture of geometrical isomers in MeCN. The reaction of oxalic acid with [Nb(02)F4(H20)] yielded [Nb2(02)2F6(C204)]z. A bridging oxalato group and a coordination number of seven for the metal were proposed. [M(02)F3(phen)], [M20(02)F4(bipy)2] and [M(02)F3(0AsPh3)2] were obtained when the neutral ligands were allowed to react with [M(02)F4(H20)] . 

Metal complexes with fluoro-/3-diketones have been comprehensively reviewed.1585 The introduction of electron-withdrawing groups in the chelate ring increases the Lewis acidity strength of the ML2 complexes, and consequently the bis adducts of the fluoro-/3-diketonato complexes are more stable than the corresponding complexes with /3-diketones. As an example of a nickel complex with 1,1,1-trifluoroacetylacetone which does not have a counterpart in the nickel acetylacetonate complexes we can mention the hexanuclear complex Ni6Lio(OH)2(H20)2.1586... 

The main problem here is to obtain suitable coordination compounds that can be vaporized at reasonably low temperatures at which they are thermally stable. To this end a great deal of work has been published on the gas chromatography of (3-diketone complexes such as (61 R2 = H) 93 Volatile complexes of the rare earths with dipivaloylmethane (61 R1 = R3 = CMe3 R2 = H) were first prepared in 1965 but while the separation of pairs of neighbouring metals (e.g. Eu and Sm) has been reported, the complete separation of the whole group has not been successfully achieved. Attempts to use columns longer than 70-75 cm result in peak-broadening and inefficient separation.94... 

The MS fragmentation patterns of dithio-/J-diketonato complexes differ from those of both /J-diketonato and monothio-/J-diketonato complexes. The mass spectra of bivalent dithioacetyl-acetonato complexes are characterized by a molecular ion peak M and a much stronger peak for L+ due to the formation of the 3,5-dimethyl-l,2-dithiolium ion another peak at tn/e 96 is due to the loss of HS from the dithiolium cation. The mass spectra of nickel complexes of dithio-/ -diketones are dominated by fragmentation products of the uncomplexed oxidized ligand, which is consistent with the low stability of dithio-/J-diketones and the greater stability of the 1,2-dithiolium... 

Template reactions between malonaldehydes and diamines in the presence of copper(II), nickel(II) or cobalt(II) salts yield neutral macrocyclic complexes (equation 15).99-102 Both aliphatic102 and aromatic101 diamines can be used. In certain cases, non-macrocyclic intermediates can be isolated and subsequently converted into unsymmetrical macrocyclic complexes by reaction with a different diamine (Scheme ll).101 These methods are more versatile and more convenient than an earlier template reaction in which propynal replaces the malonaldehyde (equation 16).103 This latter method can also be used for the non-template synthesis of the macrocyclic ligand in relatively poor yield. A further variation on this reaction type allows the use of an enol ether (vinylogous ester), which provides more flexibility with respect to substituents (equation 17).104 The approach illustrated in equation (15), and Scheme 11 can be extended to include reactions of (3-diketones. The benzodiazepines, which result from reaction between 1,2-diaminobenzenes and (3-diketones, can also serve as precursors in the metal template reaction (Scheme 12).101 105 106 The macrocyclic complex product (46) in this sequence, being unsubstituted on the meso carbon atom, has been shown to undergo an electrochemical oxidative dimerization (equation 18).107... 

Generally, zinc reductions of /3-diketones under Clemmensen conditions have been known to afford a complex mixture. However, when treated with Zn—AcaO—HC1 gas (0°, 2 hr), 1-acetyl-1-methylcyclo-hexanone (69) was readily converted into the diacetoxycyclopropane 70 in more than 80% yields, as shown below (32). 

It is less well known that the synthetic methods of (3-diketonate complexes have been elaborated, where ligands of the type discussed are bound with the metal through a C-donor atom 850 [6,11,12,14,85 88], Such mercury complexes, for example 851, were obtained as a result of transformation (4.20) [89] ... 

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