Rare-earth beta-diketonates [chapter]

Koen Binnemans
2005 Handbook on the Physics and Chemistry of Rare Earths  
Contents 112 K. BINNEMANS plication of these compounds as electroluminescent materials in organic light emitting diodes (OLEDs), as volatile reagents for chemical vapor deposition or as catalysts in organic reactions. Three main types of rare-earth β-diketonate complexes have to be considered: tris complexes, Lewis base adducts of the tris complexes (ternary rare-earth β-diketonates) and tetrakis complexes. The neutral tris complexes or tris(β-diketonates) have three β-diketonate ligands for
more » ... h rare-earth ion and they can be represented by the general formula [R(β-diketonate) 3 ]. Because the coordination sphere of the rare-earth ion is unsaturated in these six-coordinate complexes, the rare-earth ion can expand its coordination sphere by oligomer formation (with bridging β-diketonates ligands), but also by adduct formation with Lewis bases, such as water, 1,10-phenanthroline, 2,2 -bipyridine or tri-n-octylphosphine oxide. It is also possible to arrange four β-diketonate ligands around a single rare-earth ion and in this way tetrakis complexes or tetrakis(β-diketonates) with the general formula [R(β-diketonate) 4 ] − are formed. These complexes are anionic and the electric neutrality is achieved by a counter cation. The cation can be an alkali-metal ion (Li + , Na + , K + , Cs + , Rb + ), but more often it is a protonated organic base (pyridinium, piperidinium, isoquinolinium, . . .) or a quaternary ammonium ion (Et 4 N, But 4 N, Hex 4 N, . . .). Although hundreds of different rare-earth β-diketonate complexes have been described in the literature, only few of them have been thoroughly investigated. The most popular luminescent rare-earth complex is [Eu(tta) 3 (phen)], where tta is the conjugated base of 2thenoyltrifluoroacetone (Htta) and phen represents 1,10-phenanthroline. The most often used rare-earth β-diketonate complexes are however the [R(fod) 3 ] and the [R(thd) 3 ] complexes, where fod is the conjugate base of 6,6,7,7,8,8, and thd is the conjugate base of 2,2,6,6-tetramethyl-3,5-heptanedione (Hthd). These compounds were originally developed as NMR shift reagents, but they have now found a wide use as volatile precursors for chemical vapor deposition, and the [R(fod) 3 ] complexes are also used as Lewis acid catalysts in organic reactions. The popularity of the rare-earth β-diketonates is not reflected in an extensive review literature. Only two major reviews on rare-earth β-diketonates have been written in the past, namely the contribution of Forsberg in Gmelin Handbuch der anorganischen Chemie (Forsberg, 1981) and the book of Wenzel on NMR shift reagents (Wenzel, 1986) . Other reviews had a wider scope, such as the book of Mehrotra on metal β-diketonates (Mehrotra et al., 1978) , or were focused on one application such as NMR shift reagents or catalysis (see sections 9 and 13 of this Chapter). Properties of rare-earth β-diketonate complexes have been discussed in several of the previous chapters in this Handbook, but the information is fragmentary because these chapters were not focused on rare-earth β-diketonates as such. Thompson (1979) gives in Chapter 25 an overview of rare-earth coordination compounds with organic ligands and mentions shortly the rare-earth β-diketonates. Weber (1979) describes in Chapter 35 the rare-earth lasers, including the rare-earth chelate lasers. NMR shift reagents are discussed in Chapter 38 (Reuben and Elgavish, 1979) . Long (1986) gives in Chapter 57 an overview of earlier work on β-diketonate complexes as catalysts in organic reactions. Shen and Ouyang (1987) review in Chapter 61 stereospecific polymerization by rare-earth coordination catalysts, and some of these catalysts are rare-earth β-diketonates. RARE-EARTH BETA-DIKETONATES 113 RARE-EARTH BETA-DIKETONATES 119 mentioned above that 100% of the molecules of dibenzoylmethane are in the enol form. The lower the polarity of the solvent, the higher is the percentage of the enol form. In CCl 4 , 94% of the acetylacetone molecules are present in the enol form, whereas in acetonitrile this value is reduced to 36%. The amount of enol form decreases with increasing temperatures. As the degree of enolization increases, the acidity of the enol proton decreases (Hammond et al., 1959) . In the case of unsymmetrically-substituted β-diketones, two different enol forms are possible. Lowe and Ferguson (1965) have shown that benzoylacetylacetones (with different substituents in the para-position of the phenyl group) are enolized towards the phenyl group. When the β-diketone is deprotonated, the proton is removed from the α-carbon (if the βdiketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol form). The acidity of the β-diketone depends on the substituents. Electron-withdrawing groups increase the acidity, whereas electron-donating groups decrease it. Because of the presence of the two carbonyl groups, the proton on the α-carbon is quite acidic and it can be removed by relatively weak bases. Examples of bases that are used for deprotonation of β-diketones are ammonia, sodium hydroxide, piperidine or pyridine. A much stronger base is needed to remove a second proton. The negative charge of the β-diketonate ligand is delocalized, as it is in the rare-earth β-diketonates, which form six-membered chelate rings. Many β-diketones are commercially available at reasonable low prices, so that the synthesis of the rare-earth β-diketonates can often be restricted to the synthesis of the complexes, without the need to bother about the ligand synthesis. Only in the case that exotic β-diketones are needed or when new β-diketones are designed, the worker in the field of rare-earth βdiketonates has to synthesize the β-diketones himself/herself. The classic method for the synthesis of β-diketones is the Claisen condensation between a deprotonated methylketone and an ethyl or a methyl ester (Reid and Calvinyields vary from 20 to 80%. For instance, benzoyltrifluoroacetone can be prepared by reaction between ethyl trifluoroacetate (1 eq.) and acetophenone (1 eq.) in dry diethyl ether, in the presence of sodium methoxide (1.05 eq.) as the base (Reid and Calvin, 1950) . In the general procedure, the ester is added dropwise to the suspension of sodium methoxide in diethyl ether, followed by dropwise addition of the ketone. The ketone is added as the last component, in order to avoid self-condensation. The β-diketone is isolated by acidic workup. Solid β-diketones are purified by recrystallization. Liquid β-diketones can be purified by steam distillation or by vacuum distillation. Some authors purify the crude product by converting it first into the corresponding copper(II) complex. The copper(II) chelate is subsequently purified, for instance by recrystallization, and the pure β-diketone is obtained after decomposition of the copper(II) complex by a diluted aqueous sulfuric acid solution or by hydrogen sulfide. Instead of sodium methoxide, other bases can be used, for instance sodium ethoxide, sodium amide or sodium hydride (Paskevich et al., 1981) . Dry diethyl ether can be replaced by benzene, toluene, dimethoxyethane or dimethyl sulfoxide (DMSO). Ternary rare-earth β-diketonates contain one or two additional ligands besides the β-diketonate ligands. These ligands act as Lewis bases, and form adducts with tris β-diketonate complexes because of the tendency of the rare-earth ion to expand its coordination sphere 120 K. BINNEMANS Fig. 4 . Lewis bases that form adducts with rare-earth tris β-diketonates. Abbreviations: bipy = 2,2 -bipyridine; phen = 1,10-phenanthroline; terpy = 2,2 ,6 ,2 -terpyridyl; bath = bathophenanthroline or 4,7-diphenyl-1,10-phenanthroline; Hpbm = 2-(2-pyridyl)benzimidazole; tppo = triphenylphosphine oxide; tbpo = tri-n-butylphosphine oxide; topo = tri-n-octylphosphine oxide; tbp = tributylphosphate; dmso = dimethylsulfoxide. and to achieve a coordination number higher than six (typically eight or nine). Because the trivalent rare-earth ions are hard Lewis acids, the tris β-diketonate complexes form preferentially complexes with oxygen-donor or nitrogen-donor Lewis bases. An overview of Lewis bases that are often found in rare-earth β-diketonate complexes is given in fig. 4 . Two very RARE-EARTH BETA-DIKETONATES 121 popular Lewis bases are the N -donor ligands 1,10-phenanthroline (phen) and 2,2 -bipyridine (bipy), because the resulting europium(III) complexes show often an intense luminescence. These N-donor ligands can be modified by substituents on the heterocycle ring. For instance 2,9-dimethyl-1,10-phenanthroline can be used instead of 1,10-phenanthroline (Holz and Thompson, 1993) . New types of Lewis bases are the imidazo[4,5-f]1,10-phenanthroline, such as 3-ethyl-2-(4 -dimethylaminophenyl)-imidazo[4,5-f]1,10-phenanthroline or 2-(4dimethylaminophenyl) imidazo[4,5-f]1,10-phenanthroline (Bian et al., 2002) . Another new type of Lewis bases are 1,4-diaza-1,3-butadiene derivatives (Fernandes et al., 2004) . Synthetic strategies Although rare-earth β-diketonates have been known for more than one century, reliable synthetic procedures have been described only in 1964 (Bauer et al., 1964; Melby et al., 1964) . Many of the earlier reported synthetic procedures gave impure compounds or compounds with an ill-defined composition. Moreover, much of these studies have been performed by physicists who neglected to fully characterize the materials they were measuring on. Several of the older papers on rare-earth β-diketonates describe tris complexes which are in reality either hydrates, hydroxy bis(β-diketonates), tetrakis complexes or polymeric materials. Without precautions, only tris chelates of β-diketonates with bulky substituents (e.g. [R(thd) 3 ] or [R(fod) 3 ]) can be obtained easily in anhydrous form. A number of earlier workers have used the so-called "piperidine method" described by Crosby et al. (1961) . According to this method, the tris chelates were prepared by addition of piperidine to a solution of the corresponding rare-earth chloride and the β-diketone in water, ethanol or methanol. A modified procedure for the synthesis of dibenzoylmethanate complexes (Whan and Crosby, 1962) mentions that an alcoholic solution of the rare-earth chloride and an 25% excess of Hdbm was treated with piperidine and part of the solvent was evaporated to precipitate the rare-earth β-diketonate complex. The experimental procedure further mentions that is was necessary to heat the crude product for a prolonged time in vacuo at 125-150 • C to drive off an "extra mole of chelating agent". Experimental methods such as these were found very difficult to reproduce. It is not easy to get rid of all the excess of the β-diketone. Much of the confusion in the earlier works was caused by the fact that it was not realized that the rare-earth ions can have coordination numbers higher than six. In two often-cited papers that were published in the Journal of the American Chemical Society, Bauer et al. (1964) and Melby et al. (1964) give experimental procedures for the synthesis of the adducts of the tris and tetrakis complexes. The careful reader will not only notice that these papers not only follow each other in the same issue of JACS, but that moreover they have been received the same date (July 17, 1964) at the editorial office of the journal. Bauer et al. (1964) discusses three different methods to prepare rare-earth tetrakis β-diketonate complexes, and the authors also describe the synthesis of [Tb(tta) 3 (phen)]. The different synthetic routes to the tetrakis complexes differ mainly in the type of base that has been used. The β-diketone and the rare-earth chloride are first dissolved in hot ethanol at a 4:1 molar ratio. In the case of the complexes with piperidinium as the counter ion, piperidine is a base strong enough to deprotonate the β-diketone, and no 122 K. BINNEMANS other base has to be used. In the case of complexes with tetrapropylammonium counter ions, tetrapropylammonium hydroxide was used as the base and as provider for the counter ion. For the synthesis of complexes with N -hexadecylpyridinium counter ions, the counter ions was provided by N-hexadecylpyridinium chloride, and a 2.0 N NaOH solution was used as the base. In the case of [Tb(tta) 3 (phen)], a 2.0 N NaOH solution was used as the base as well. 1,10-Phenanthroline was added, after the precipitated NaCl was filtered off. It should be noticed that in all cases, the rare-earth chloride salts was added to the β-diketone, before addition of the base. Melby et al. (1964) describe the synthesis of several hydrated tris complexes, Lewis base adducts of tris complexes and tetrakis complexes. The synthetic procedures vary from compound to compound. In contrast to the work of Bauer et al. (1964 ), Melby et al. (1964 add the rare-earth after deprotonation of the β-diketone by an appropriate base. The latter authors use not only hydrated rare-earth chloride salts as starting reagents, but hydrated rareearth nitrate salts as well. Lewis base adducts of the tris complexes can prepared by dissolving the anhydrous tris complex and the Lewis base in a suitable solvent (1:1 ratio for a bidentate Lewis base and a 1:2 ratio for a monodentate Lewis base). When adducts of bidentate Lewis bases are prepared, one can use hydrated tris complexes as the starting material, because the bidentate ligands will expel the water molecules out of the first coordination sphere (Melby et al., 1964) . Charles and Ohlmann (1965a) prepared adducts of [R(dbm) 3 ] with monodentate Lewis bases (e.g. dmf, dmso, pyridine, pyridine-N -oxide, piperidine) by dissolving anhydrous [R(dbm) 3 ] in an excess of the Lewis base, either as the pure liquid or as a solution in toluene. The adducts were isolated by evaporation of the excess of liquid or by precipitation with petroleum ether. Mattson et al. (1985) had difficulties in obtaining pure Lewis base adducts of [Eu(fod) 3 ], due to the high solubility of [Eu(fod) 3 ] in the usual organic solvents and the tendency of the corresponding Lewis base adducts to form an oil, and they had to carefully select the solvent system and the working conditions. For instance, [Eu(fod) 3 (phen)] was obtained by mixing of equimolar quantities of [Eu(fod) 3 ] and 1,10-phenanthroline in a small volume of hexane. Crystals of the adduct were formed upon slow evaporation of the solvent at room temperature. The solid was recrystallized from heptane. [Eu(fod) 3 (dmso)] was made in ethyl acetate, [Eu(fod) 3 (bipy)] in chloroform. Most of the rare-earth β-diketonate complexes are prepared using the chloride as the rareearth salt, although other studies mention the corresponding nitrate salt. Because of the greater coordinating power of nitrate ions in comparison with chloride ions, there is the possibility that if nitrate salts are used as a reagent for the synthesis of rare-earth β-diketonate complexes, the resulting complex contains a coordinating nitrate group. An example is the triboluminescent complex [Eu(NO 3 )(tta) 2 (tppo) 2 ] (Zhu et al., 1993) . Melby et al. (1964) report that [Eu(tta) 3 (tppo) 2 ] could only be prepared starting from europium(III) chloride, not from europium(III) nitrate. Attempts to synthesize this compound from europium(III) nitrate, gave [Eu(tta) 2 (tppo) 2 (NO 3 )], even in the presence of an excess of triphenylphosphine oxide. [R(acac)(terpy)(NO 3 ) 2 (H 2 O) n ] complexes were prepared by Fukuda et al. (2002) . Lyle and Witts (1971) made a critical examination of different methods that have been used previously by other workers to prepare tris and tetrakis β-diketonate complexes of europium(III). They mention that the molar ratios in which the β-diketone, the base and the europium(III) salts are mixed, give only a rough guide to the stoichiometry of the reaction product. Although it is generally recognized that a β-diketone:base:europium(III) ratio of 3:3:1 favors the formation of the tris complex, and a ratio 4:4:1 favors the formation of the tetrakis complex, unexpected results are possible. If water is not excluded from the reaction mixture, the tris complexes are invariable hydrated (monohydrate, dihydrate or even trihydrate), except when very bulky β-diketone ligands are used. In favorable cases, the hydrated tris complexes can be converted in the anhydrous forms (Belcher et al., 1969a) . Dibenzoylmethanate complexes [R(dbm) 3 ]·H 2 O tend to decompose by hydrolysis on heating, and give a basic product, e.g. [R(dbm) 2 (OH)] (Ismail et al., 1969) . Further studies have shown that the [R(dbm) 2 (OH)] compounds are polymeric. Differences from the predicted stoichiometries can be expected when a bidentate Lewis base is present in solution and when not enough base is used for the deprotonation of the β-diketone ligands. For instance, Wang et al. (1994c) obtained a compound with composition [Eu(dbm) 3 (bipy)](Hdbm) when dibenzoylmethane and 2,2 -bipyridine were mixed with Eu(NO 3 ) 3 ·6H 2 O in dry ethanol, in the absence of a base. The crystal structure of the compound shows that the neutral Hdbm molecule is non-coordinating. Preparation of the anhydrous tris acetylacetonate complexes is very difficult (Liss and Bos, 1977) . The general synthetic procedures will lead to hydrated complexes. Attempts to remove hydration water by vacuum drying often lead to partial hydrolysis, especially in the case of the heavy lanthanides. Koehler and Bos (1967) prepared the pure anhydrous tris acetylacetonate complexes of dysprosium(III), holmium(III) and erbium(III) by the reaction of the corresponding rare-earth hydride with purified acetylacetone. Later on, also pure anhydrous acetylacetonates of gadolinium(III), terbium(III) and yttrium(III) could be obtained by this method (Przystal et al., 1971) , but not complexes of lanthanum(III), neodymium(III) and europium(III) (Liss and Bos, 1977) . Liss and Bos (1977) were able to synthesize the anhydrous tris acetylacetonates by very careful vacuum drying. Dehydration was carried out in a vacuum of 10 −5 -10 −6 Torr at 60-80 • C over periods of 5-8 days. This procedure lead to amorphous anhydrous tris acetylacetonates of lanthanum(III), neodymium(III), samarium(III), europium(III), gadolinium(III) and terbium(III). Crystalline [Nd(acac) 3 ], [Eu(acac) 3 ] and [Gd(acac) 3 ] were obtained by recrystallization from acetylacetone of the amorphous anhydrous acetylacetonates, under anhydrous conditions. Other methods to obtain anhydrous acetylacetonate complexes are discussed further in this section. More than 95% of the rare-earth β-diketonate complexes described in the literature have been prepared by the metathesis reaction between the sodium or ammonium salt of a βdiketone and a rare-earth salt (chloride or nitrate) in water or ethanol as the solvent. In most cases, these methods work well, especially when the pH of the reaction mixture is controlled during the synthesis. Sometimes other synthetic routes have to be used, for instance when strictly anhydrous complexes are needed, or when complexes are wanted that are free of contaminating anions or cations. Complexes can be obtained by direct synthesis between a rareearth metal and a β-diketone in an 1:3 molar ratio in an inert solvent (for instance toluene). Hydrogen gas is evolved and a tris β-diketonate complex is formed: The tris β-diketonate complexes prepared in this way are free from contaminating ions such as sodium or chloride ions. It is evident that this method can be used only when the func-124 K. BINNEMANS 126 K. BINNEMANS 144 K. BINNEMANS
doi:10.1016/s0168-1273(05)35003-3 fatcat:t2yfytg4dbbwjedpiq5lvrxlsq