Siloxides ligands

Tris(ferf-butoxy)siloxide molecular precursors of V(IV) and V(V) can be prepared via simple silanolysis reactions. For example, OV[OSi(O Bu)3]3 was obtained in 85% yield by reaction of OVCI3 with excess HOSi(O Bu)3 in the presence of pyridine [79]. Although crystals of sufficient quality for an X-ray structural analysis of 0V[0Si(0 Bu)3]3 were not obtained, its identity was confirmed by various spectroscopic and analytical techniques. Additionally, ( BuO)3VOSi(O Bu)3 and ( BuO)2V[OSi(O Bu)3]2 were obtained via reaction of V(0 Bu)4 with 1 and 2 equiv of HOSi(O Bu)3, respectively, in toluene at 80 °C [80] (Eq. 5). Both (fBu0)3V0Si(0 Bu)3 and CBu0)2V[0Si(0 Bu)3]2 are monomeric in the solid state, and possess only monodentate siloxide ligands... 

OSi Bus) ligands [84], However, Bradley has reported the less sterically hindered, homoleptic siloxide Ta(OSiMe3)5 [85]. 

The zinc tris(ferf-butoxy)siloxide complex Zn[0Si(0 Bu)3]2 2 was prepared from the reaction of ZnMe2 with HOSi(O Bu)3 [107]. This complex was structurally characterized as an asymmetric dimer with four - OSi(O Bu)3 ligands, each exhibiting a unique coordination mode ( fi - , r] ... 

Fig. 5 The molecular structure of Bu0B[0Si(0 Bu)3]2 generated from crystallographic data, with all hydrogen atoms and the siloxide ligand methyl groups omitted for clarity...

Successful strategies for generating complexes of the di(terf-butyl)phosphate ligand primarily focus on the use of H0P(0)(0 Bu)2 as a reagent. As with the related siloxide species, all synthetic manipulations must be performed under inert conditions to avoid hydrolysis of the M - O - P linkages. Complexes of the - 02P(0 Bu)2 ligand are useful precursors to M/P/0 oxide materials. 

The - 0B[0Si(0 Bu)3]2 ligand has provided species of the form L M OB[OSi (0 Bu)3]2 x that are viable molecular precursors to M/B/Si/0 materials. However, the chemistry of this ligand appears to be sensitive to the ancillary ligands on the associated metal, with ligand transfer to the boron center (sometimes accompanied by siloxide transfer from boron to the metal) being a primary pathway for decomposition [64,90]. 

Organozinc Compounds Bearing Alkoxides, Siloxides, Boryloxides, Complex Oxides, and Related Ligands... 

The polarity of the Al-C bond allows easy derivatization of the five-coordinate aluminum alkyls by alkane elimination (Figure 9). For example, Salen aluminum alkyls LAlMe could be converted to dimeric or polymeric Salen aluminum phosphinates [LAl 02P(H)Ph ] 98 (n = 2 or oo, depending on the Salen ligand backbone)98,99 by reaction with phenyl phosphinic acid, Salen aluminum siloxides LA10SiPh3 by reaction with triphenyl silanol,96 or Salen aluminum alkoxides LAIOR by reaction with an alcohol.100... 

The properties of siloxide as ancillary ligand in the system TM-O-SiRs can be effectively utilized in molecular catalysis, but predominantly by early transition metal complexes. Mono- and di-substituted branched siloxy ligands (e.g., incompletely condensed silsesquioxanes) have been employed as more advanced models of the silanol sites on silica surface for catalytically active centers of early TM (Ti, W, V) that could be effectively used in polymerization [5], metathesis [6] and epoxidation [7] of alkenes as well as dehydrogenative coupling of silanes [8]. 

A comprehensive study by Wolczanski [4] and Feher [3] groups has led to the conclusion that siloxide as ancillary ligands show an unusual ability to stabilize the reduced early-TM centers. A combination of high reduction potential and high electrophilicity is responsible for the unusually high reactivity in low-valent siloxide derivatives. 

Scheme 7.4 Formation of surface rhodium siloxide complexes containing phosphine and hydride ligands.

The heteroleptic B-type ligand sphere offers more steric flexibility. However, chelation by simple donor-functionalities can yield strongly-bonded ligand-metal moieties which are kinetically rather stable, as is shown, for example, in benzamidinate [179] or siloxide complexes [180]. 

Scheme 5. Siloxide ligands employed in lanthanide chemistry...

Use of less basic triphenylsiloxide ligands also allowed the isolation of mononuclear complexes (Table 1) [46,47]. In addition to the silylamide route, anhydrous nitrates and isopropoxides were employed as synthetic precursors (Eqs. 1,2). The siloxide bridges in the solvent-free dimeric systems Ln2(OSiPh3)6 are readily disrupted by donor solvents like THF, OP Bu3 or DME. The m s-THF adducts of lanthanum, cerium (Fig. 5) and yttrium adopt an approximately /ac-octahedral geometry. 

X-ray structure analysis revealed a 7-coordinate rare-earth metal center with two asymmetrically / -coordinating tetramethylaluminate ligands, an asymmetrically / -coordinating siloxide ligand and one methyl group of a trimethylaluminum donor molecule (Fig. 28). Such heteroleptic complexes can be regarded as molecular models of covalently bonded alkylated silica surface species. Moreover, isoprene was polymerized in the presence of 1-3 equivalents of diethylaluminum chloride, with highest activities observed for (Cl) (Ln) ratios of 2 1 (Table 12) (Fischbach et al., 2006, personal communication) [150]. 

Most of the knowledge about aluminate and alkylaluminum coordination stems from X-ray crystallographic studies. The basic idea of this section is to compile a rare-earth metal aluminate library categorizing this meanwhile comprehensive class of heterobimetallic compounds. Main classification criteria are the type of homo- and heterobridging aluminate ligand (tetra-, tri-, di-, and mono alkylaluminum complexes), the type of co-ligand (cyclopen-tadienyl, carboxylate, alkoxide, siloxide, amide), and the Ln center oxidation state. In addition, related Ln/Al heterobimetallic alkoxide complexes ( non-alkylaluminum complexes) are surveyed. Emphasis is not put on wordy structure discussions but on the different coordination modes (charts) and important structural parameters in tabular form. An arbitrary collection of molecular structure drawings complements this structural report. 

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