Metal aryloxides ligand coordination

Table 1. Monomeric A1R1R2R3 complexes incorporating simple, monodeprotonated aryloxide ligands (Rl) and, where appropriate, Lewis base (L). R2, R3 = other ligands, C.N. = coordination number of the metal... 

REACTIONS OF METAL ALKOXIDES AND ARYLOXIDES 15.3.5.1 Ligand Coordination... 

The use of unsubstituted or 4-methyl phenols resulted in the formation of cluster compounds [58]. However, 2,6-di(fcrt-butyl) substituted aryloxide ligands allowed the isolation of mononuclear 3-coordinate homoleptie complexes of the lanthanide elements, the coordination mode of which was first demonstrated with the N(SiMe3)2 ligand [59], The 2,6-substitution pattern is very effective because the alkyl groups are directed towards the metal center and impose a steric coordination number onto the metal which is comparable to the Cp ligand (Cp 2.49 OC6H3rBu2-2,6 2.41) [60],... 

Unusual coordination around potassium was observed in the 2-dimensional layered structure of the ate complex K[Cp2Nd(/i-OC6H3Me2-2,6)2] n [208]. No close K O contact was found, however, aryloxide ligands bridge the metal centers through arene interactions as in homoleptic ate complexes [69]. 

Alkoxide and aryloxide ligands are excellent ligands for the actinides. As a result, these ligands have been studied extensively in the coordination chemistry and reactivity of tri-, tetra-, penta-, and hexavalent actinides. The alkoxides and aryloxides can be synthesized by a variety of routes the two most popular routes include direct reaction of actinide halides with alkali metal salts of the alkoxide or aryloxide of interest and protonolysis of actinide amides by alcohols. 

Sterically crowded multidentate, macrocyclic aryloxide ligands tend to engender different chemistry as a result of the constrained and more rigid disposition of their aryloxide moieties in the macrocyclic structure. The linking group between the individual aryloxide imits within the macrocycle may also participate in synergetic metal coordination. 

Undoubtedly the most active homogeneous catalyst systems are the well-defined metal alkylidene complexes synthesised in the laboratories of Schrock, Basset and Grubbs. First examples are the complexes I [29] and II [30]. The bulkiness of imido and aryloxide ligands probably slows down dimerization of these electron-deficient organometallic complexes to inactive complexes and prevents to some extent the coordination of the functional group to the tungsten atom [37]. 

Mechanistic studies show that the first reaction proceeds via a migratory insertion reaction. In contrast the second was shown to proceed via initial displacement of aryloxide by CO followed by nucleophilic attack at Ihe coordinated carbonyl ligand. In the case of the bis(aryloxycarbonyl), subsequent thermolysis under CO leads to formation of a metal dicarbonyl, CO2, ArCOaAr and deoxygenation of one of the initial aryloxide ligands. A competing reaction is elimination of phenol and formation of a metallolactone. 

The mononuclear aryloxide chemistry of these metals spans a wide range of formal oxidation states. This is particularly true for tungsten, where the aryloxide ligand has been coordinated to this metal centre in its highest oxidation state, e.g. [WIOCeHiMe-dle]," low valent carbonyl compounds, e.g. [NEt4][W(OPh)(CO)5]," and intermediate redox... 

The alkoxides and aryloxides, particularly of yttrium have excited recent interest. This is because of their potential use in the production of electronic and ceramic materials,in particular high temperature superconductors, by the deposition of pure oxides (metallo-organic chemical vapour deposition, MOCVD). They are moisture sensitive but mostly polymeric and involatile and so attempts have been made to inhibit polymerization and produce the required volatility by using bulky alkoxide ligands. M(OR)3, R = 2,6-di-terr-butyl-4-methylphenoxide, are indeed 3-coordinate (pyramidal) monomers but still not sufficiently volatile. More success has been achieved with fluorinated alkoxides, prepared by reacting the parent alcohols with the metal tris-(bis-trimethylsilylamides) ... 

In the context of bidentate ligands, and in a similar vein to the polyfunctional aryloxides discussed above [141, 142, 146, 147], the chelation of a metal centre by deprotonated j5-diketones is a recurrent feature of monomeric organooxide complexes of aluminium. The employment of such ligands results in the observation of simple hexa-coordinate complexes which incorporate three [RC(0)C(H)C(0)R ] moieties (R = R = Me [177,178], CF3 [179], Ph [180, 181] R = f-Bu, R = CF3 [182]). More recently, the cocrystallisation of M(acac)3 (M = Al, Cr) has allowed the crystallographic study of metal disorder in a series of solid solutions of stoichiometry Ali, cCi x(acac)3 (x = 0.02-0.91) [183, 184]. Chelation of the metal centre similar to that reported in monomeric ] -diketonates has also been noted in the presence of... 

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