Metal alkoxides heteroleptic

HETEROLEPTIC METAL ALKOXIDES CONTAINING CARBOXYLATO-, p-DIKETONATO, AND AMINOALKOXIDE LIGANDS... 

Metal aUcoxides and alkyls are useful starting materials, particularly for the preparation of some unsolvated P-diketones that require anhydrous conditions (see Section 3.3). Addition of /3-diketonate ligands to metal alkoxides has also been used to produce heteroleptic metal complexes that are less reactive than the homoleptic metal alkoxides this simplifies the use of such complexes in chemical vapor deposition applications. ... 

Although metal alkoxides were successfully used as a synthon for the synthesis (see Scheme 9) of more interesting homo- and heteroleptic metal alkyls, they have not attained the same importance as their sterically hindered aryloxide analogues. This finding might be due to the general solubility of both the products [i.e., desired metal alkyls and alkali metal (generally lithium) alkoxides] in hydrocarbon solvents. This limitation has made a cleaner separation of the products more difficult. 

Most of the metal alkoxides of interest for electrooptical ceramics are solids (less often liquids) that can be purified by recrystallization, sublimation, or distillation. They are all moisture sensitive, and handling under an inert atmosphere and with anhydrous solvents is thus required. Their unequivocal characterization and formulation are best achieved by x-ray diffraction studies (on monocrystals). Studies on solutions (molecular weight data, nuclear magnetic resonance, NMR, with H, or metal nuclei) are a means either to establish whether the solid-state structure is retained or, in the absence of x-ray data, to establish the molecular structure and eventually stereolability [48]. Mass spectrometry provides information on the stability of the oligomers or the het-erometallic species in the vapor phase. The information gained by infrared spectroscopy is limited the technique is mostly useful for the identification of solvates M(OR) (ROH)x (vOH absorption 3400-3100 cm-l or of chemically modified (heteroleptic) alkoxides (probe for the vCO stretching of P-diketonate or carboxylate ligands, for instance). 

In addition to the above, alcoholysis or transesterification reactions of metal alkoxides themselves have been widely used for obtaining the targeted homo- and heteroleptic alkoxide derivatives of the same metal. Since the 1960s, the replacement reactions of metal dialkylamides with alcohols has provided a highly convenient and versatile route (Section 2.9) for the synthesis of homoleptic alkoxides of a number of metals, particularly in their lower valency states. 

Exchange reactions between two different metal chlorides and alkoxides have been employed for the synthesis of a wide variety of homo- and heteroleptic metal alkoxides. For example, Bradley and Hill" observed that a mixture of excess titanium tetrachloride and silicon dichloride diethoxide at O C deposited a crystalline product which was characterized as titanium trichloride monoethoxide ... 

Table 2.20 presents some data on the NMR spectra of diamagnetic homo- and heteroleptic metal alkoxides. In comparing chemical shifts it should be kept in mind that the 8 values of some metal alkoxides are subject to specific solvent effects. As expected the 8 values of the protons on a-carbon atoms are shifted considerably down-field relative to protons on /5-carbons. Since the H/ C chemical shift is the resultant of several contributing factors, there are no obvious correlations with metal oxidation state, atomic radius, or co-ordination number. However, metal NMR studies have proved to be of considerable importance in indicating the coordination environment of the central metal atom. 

Metal alkoxides react readily with a wide variety of hydroxylic reagents such as water, " alcohol, " alkanolamines," carboxylic acids,j6-diketones and yd-ketoesters, Schiff bases, oximes, hydroxylamines, " and glycols, " to afford homo- and heteroleptic derivatives involving metals in novel coordination states and geometries. The reactions of metal alkoxides with monofunctional reagents (LOH) can be illustrated by Eq. (2.182) ... 

The characteristic ability of metal alkoxides to undergo facile exchange reactions with hydroxylic reagents has been extensively utilized for preparing homo- and heteroleptic phenoxides of a large number of elements by the general reaction (Eq. 2.198) ... 

Like alcohols, silanols have been found to react with metal alkoxides to form the corresponding homo- and heteroleptic metal siloxides (Eq. 2.199). 

R" = alkoxy or aryloxy group) contains a reactive hydroxy gronp, which is prone to react readily with metal alkoxides to yield alcohol(s) and a variety of interesting (in terms of compositions, stmctures, volatility, reactivity, and appUcabiUty) homo- and heteroleptic derivatives of a wide variety of metals and metaUoids. Typical general reactions are shown in Eqs (2.218) and (2.219) ... 

The reactions of metal alkoxides with a wide range of oximes (Eq. 2.244) provide a convenient and versatile route for the synthesis of a variety of interesting homo- and heteroleptic oximate derivatives ... 

Although many routes for the synthesis of metal complexes of Schiff bases and P-ketoamines are available, " the facile reactivity of metal alkoxides has been utilized for the synthesis of homo- and heteroleptic Schiff base and -ketoamine derivatives of metals and metalloids with advantage. The most interesting lesnlts concern the Schiff bases and /6-ketoamines shown in Fig. 2.15, and other ligands obtained by snitably modifying them. 

The interactions of early 3d transition metal alkoxides with lithium alkyls, Grignard reagents, or dimethylzinc yield interesting homo- and heteroleptic alkyls as shown in... 

The most important ways to prepare heteroleptic complexes are (1) the reaction of chemical modification, (2) the interaction of an alkoxide with a carboxylate, p-diketonate, or other derivative of the same or the other metal, and (3) reactions of alcoholysis of derivatives initially not containing alkoxoli-gands. 

The reactions between alkoxides of different metals between themselves and sometimes with metal halides was found to yield homoleptic or heteroleptic chloro heterobimetallic alkoxide under suitable condition (cf., Section II.C.2). 

Homo- and heteroleptic Zr or Hf alkoxide complexes have been employed to prepare mixed metal species incorporating Ba, Sr, Al, Tl, Sn, Pb, transition, or lanthanide metals. This area has been reviewed thoroughly.390-392... 

Although single crystal X-ray studies presented considerable difficulties in the earlier stages, the development of more sophisticated X-ray diffraction techniques has led to the structural elucidation of a number of homo- and heteroleptic alkoxides and actual identification of many interesting metal oxo-aUcoxide systems (Chapter 4). 

One of the characteristic features of metal(loid) alkoxides is their ability to exchange alkoxo groups with alcohols, and this has been widely exploited for the synthesis of new homo- and heteroleptic alkoxide derivatives of various s-, d-, f-, and j)-block elements... 

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